**3. Results and discussion**

### **3.1. Catalyst precursor characterization**

*Nitrogen physisorption (N2 -physisorption—NP)*. Nitrogen adsorption-desorption measurements were performed to determine the effect of the support and preparation parameters on the BET surface area, pore volume and mean pore diameter of prepared catalyst precursor samples. The obtained results are summarized in **Table 4**.

**Table 4** reveals that specific surface area (*S*BET) was greatly enhanced after deposition Ni2+ precipitates onto the macroporous diatomite and/or perlite support. The prepared samples have BET surface areas, which are an order of magnitude larger than the surface area of the supports. The catalyst precursors have significantly different values of the pore volume and mean diameter of pores compared to the starting support materials (**Tables 1** and **4**). It has been shown in numerous studies that improved textural properties of the supported metal catalysts are related to the metal-support interaction (MSI) [49, 71–73]. This phenomenon was also observed in the nickel catalysts supported on diatomite and/or silica [54, 71]. The authors explained an extremely enhanced BET surface area by forming supported intermediate phases layered structure-nickel hydrosilicates as a result of nickel-support interaction. Nepouite-like (1:1 nickel hydrosilicate) is the main Ni2+ phase formed in Ni/SiO<sup>2</sup> samples prepared by the PD method on silica with a high surface area [56]. In the case of Ni-Mg/D precursors, it is reasonable to assume that the improved textural properties of the precursors are the results of MSI, as elucidated by IR and XRD patterns discussed later in this paper. Ni/SiG precursors showed different behavior. A considerable decrease in the BET surface area was observed in the Ni/SiG-A and Ni/SiG-B precursors formed by precipitating the Ni2+ species onto micro- (SiG-A) and mesoporous (SiG-B) supports initially having high surface areas. It is well known that the supports with low specific surface areas favor the formation of nickel hydroxide or hydroxyl-carbonate phases whatever the preparation method is, while supports with high specific surface areas allow the growth of hydrosilicate to occur [74]. It is likely that the high nickel loadings in those precursors (above 40 wt%, **Table 3**)


\*Data for reduced-passivated samples: (Ni-Mg/D)Rp, (Ni-Mg-Ag1.55/D)Rp and (Ni-Mg-Ag5.88/D)Rp; (Ni/SiG-A)Rp, (Ni/SiG-B)Rp and (Ni/SiG-C)Rp.

a S: specific surface area-BET method.

b Smeso and Vmeso: contribution of the mesopores.

c Dmean: mean pore diameter.

then pressurized with pure hydrogen to the operating pressure (0.16 MPa). During the experiments, the heat flow, hydrogen uptake and reactor temperature and pressure were monitored by instruments interfaced to the reactor PPV system. For each run, the soybean oil batch was partially hydrogenated to a final IV of 90. The composition of fatty acids in the original soybean oil and hydrogenated products was analyzed by the capillary gas chromatographic method. Experiments were performed on a Schimadzu GC-9A equipped with flame ionization detector (FID). Chromatographic conditions were as follows: HP-88 capillary column (100 m × 0.25 mm, 0.20 μm film thickness, Agilent), oven temperature of 180°C, detector and injector temperature of 240°C. Injection was carried out in the splite mode at a splite ratio of 1:4. The injection volume

method II.D.19 [70] for preparation and CG analysis of fatty acids methyl esters was used to convert fatty acids, taken out at predetermined time intervals from the catalytic reactor, into

were performed to determine the effect of the support and preparation parameters on the BET surface area, pore volume and mean pore diameter of prepared catalyst precursor samples.

**Table 4** reveals that specific surface area (*S*BET) was greatly enhanced after deposition Ni2+ precipitates onto the macroporous diatomite and/or perlite support. The prepared samples have BET surface areas, which are an order of magnitude larger than the surface area of the supports. The catalyst precursors have significantly different values of the pore volume and mean diameter of pores compared to the starting support materials (**Tables 1** and **4**). It has been shown in numerous studies that improved textural properties of the supported metal catalysts are related to the metal-support interaction (MSI) [49, 71–73]. This phenomenon was also observed in the nickel catalysts supported on diatomite and/or silica [54, 71]. The authors explained an extremely enhanced BET surface area by forming supported intermediate phases layered structure-nickel hydrosilicates as a result of nickel-support interaction.

Nepouite-like (1:1 nickel hydrosilicate) is the main Ni2+ phase formed in Ni/SiO<sup>2</sup>

prepared by the PD method on silica with a high surface area [56]. In the case of Ni-Mg/D precursors, it is reasonable to assume that the improved textural properties of the precursors are the results of MSI, as elucidated by IR and XRD patterns discussed later in this paper. Ni/SiG precursors showed different behavior. A considerable decrease in the BET surface area was observed in the Ni/SiG-A and Ni/SiG-B precursors formed by precipitating the Ni2+ species onto micro- (SiG-A) and mesoporous (SiG-B) supports initially having high surface areas. It is well known that the supports with low specific surface areas favor the formation of nickel hydroxide or hydroxyl-carbonate phases whatever the preparation method is, while supports with high specific surface areas allow the growth of hydrosilicate to occur [74]. It is likely that the high nickel loadings in those precursors (above 40 wt%, **Table 3**)

*-physisorption—NP)*. Nitrogen adsorption-desorption measurements

min−1. The IUPAC

samples

was 2 μL. Helium was used as the carrier gas at a constant flow rate of 1.2 cm3

142 New Advances in Hydrogenation Processes - Fundamentals and Applications

their corresponding methyl esters.

**3. Results and discussion**

*Nitrogen physisorption (N2*

**3.1. Catalyst precursor characterization**

The obtained results are summarized in **Table 4**.

**Table 4.** Nitrogen physisorption data for dried and selected reduced-passivated catalyst precursor samples.

mitigate the effect of MSI leading to the formation of bulk nickel hydroxyl-carbonate covering the hydrosilicate layer formed in MSI. On the other hand, the precipitation of Ni2+ species onto the SiG-C support does not affect the BET surface area of the Ni/SiG-C precursor (see **Tables 1** and **4**).

The nitrogen adsorption-desorption isotherms are presented in **Figure 5**. Comparative plots have been constructed for all supports and systems of catalyst precursors.

In general, the experimental N2 adsorption-desorption curves of samples closely resemble a type IV isotherm characteristic for mesoporous solids according to the IUPAC classification, with exception of the samples of D and SiG-A that appear to be extensively macroporous (isotherm type II) and microporous (isotherm type I). In the case of catalyst precursors of type Ni-Mg/D (**Figure 5a** and **d**) after saturation of micropores, nitrogen uptake monotonically increases with *p/p*<sup>0</sup> values due to sorption in the sample larger pores. After filling of mesopores, the uptake remains essentially invariant with *p/p*<sup>0</sup> values. The slope of curves is small, indicating a low content of mesopores having a wide pore size distribution (PSDs). In contrast, the catalyst precursors prepared on SiG and PF supports had sharper slope of nitrogen adsorption curves, indicating mesopores present with narrow PSDs.

It is well known that the occurrence of the capillary hysteresis loop depends on the pore sizes. In N2 -physisorption, the isotherms of the samples with the smallest pore size do not exhibit hysteresis, while the samples with the smaller pore sizes exhibit isotherms with narrow hysteresis.

**Figure 5.** Experimental N2 adsorption-desorption isotherms at -196°C for dried and reduced-passivated samples: (a) D, Ni-Mg/D; (b) SiG, Ni/SiG; (c) PF, Ni-Mg/PF; (d) (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp, (Ni/SiG)Rp. Units and shifts along Y axis are chosen for convenience.

Wider capillary hysteresis loops are observed in the nitrogen isotherms on the samples with larger pore sizes. Besides, the shapes of the capillary hysteresis loops vary from a "triangle" to a well-pronounced "parallelogram". The results obtained for N2 -physisorption showed that samples differ in the shape and types of hysteresis. As can be observed, the precursors Ni-Mg/D, Ni-Mg-Ag/D and Ni-Mg/PF are characterized by capillary hysteresis loops with shape of a triangle (**Figure 5a** and **c**) unlike the precursor Ni/SiG having capillary hysteresis loops shape that look like a parallelogram (**Figure 5b**). The hysteresis type of the samples cannot be classified into any types of IUPAC classification and mostly resembles the H3 type corresponding to the mesoporous solids with a broad distribution of the pore sizes. Despite the expected absence of hysteresis for the diatomite support, a narrow loop of H1 type in the IUPAC classification was observed (**Figure 5a**).

Thermal treatment of reduced samples has not changed the pore structure significantly, preserving their mesoporosity (**Figure 5d**).

*Mercury porosimetry (Hg-porosimetry—MP)*. The relevant mercury porosimetry experimental data are summarized in **Table 5**.

**Table 5** indicates a large total pore volume and porosity for the dried catalyst precursor of Ni/ SiG and Ni-Mg/PF systems. In the case of Ni-Mg/D catalyst precursor, the pore volume and porosity, as shown in **Table 5**, are obviously different from those mentioned above. The total pore volume and porosity are, in general, significantly lower and the pores are significantly smaller in diameter (Dmean and/or Dav). The differences between the prepared precursors may be associated with differences in pore structure characteristics of supports and the nature of nickel precursor salt.


\*Data for reduced-passivated samples: (Ni-Mg/D)Rp, (Ni-Mg-Ag1.55/D)Rp and (Ni-Mg-Ag5.88/D)Rp.

a Acquisition data obtained by Milestone 200 and Pascal softwares.

b Pore volume.

c Porosity calculated from bulk density of dried sample as measured in porosimeter.

ᵈBulk density.

Wider capillary hysteresis loops are observed in the nitrogen isotherms on the samples with larger pore sizes. Besides, the shapes of the capillary hysteresis loops vary from a "triangle" to a

Ni-Mg/D; (b) SiG, Ni/SiG; (c) PF, Ni-Mg/PF; (d) (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp, (Ni/SiG)Rp. Units and shifts along Y axis

adsorption-desorption isotherms at -196°C for dried and reduced-passivated samples: (a) D,

ples differ in the shape and types of hysteresis. As can be observed, the precursors Ni-Mg/D, Ni-Mg-Ag/D and Ni-Mg/PF are characterized by capillary hysteresis loops with shape of a triangle (**Figure 5a** and **c**) unlike the precursor Ni/SiG having capillary hysteresis loops shape that look like a parallelogram (**Figure 5b**). The hysteresis type of the samples cannot be classified into any types of IUPAC classification and mostly resembles the H3 type corresponding to the mesoporous solids with a broad distribution of the pore sizes. Despite the expected absence of hysteresis for the diatomite support, a narrow loop of H1 type in the IUPAC classification was


well-pronounced "parallelogram". The results obtained for N2

144 New Advances in Hydrogenation Processes - Fundamentals and Applications

observed (**Figure 5a**).

**Figure 5.** Experimental N2

are chosen for convenience.

e Pore diameter: Dmean—computed from the corresponding PSD curves.

f Pore diameter: Dav—average pore diameter calculated according to the Dav=4Vp/SBET assuming cylindrical pore shape.

**Table 5.** Mercury porosimetry data for dried and selected reduced-passivated catalyst precursor samplesa .

The experimental PSDs data are presented in the form of cumulative pore diameters distribution curves in **Figure 6**. The data are cumulated from larger pore diameters measured to the smallest diameter limit set by the pressuring capacity of the instrument. According to the hysteresis curves (not shown), the main part of mercury remains in the pores after the measurement, indicating the presence of ink bottle-like pores.

**Figure 6.** Cumulative and derivative PSDs for the SiG supports and catalyst precursor systems: (a) Ni-Mg/D, Ni-Mg-Ag/D; (b) SiG, Ni/SiG; (c) Ni-Mg/PF; (d) (Ni-Mg-Ag/D)Rp.

The derivate distribution function (dV/dlogD) is represented as insert (see **Figure 6a**–**d**). It should be noted that the pressurization data from mercury intrusion yields information about the size of the opening of pores and/or voids and does not reflect the pore size behind the "neck". It is apparent that the deposition of Ni2+ precipitates onto surface of SiG supports leading to a shift in the PSD curves towards the larger pore diameters. The explanation for this effect appears to lie in different microstructural arrangements of supported Ni2+ species in dried samples compared to the starting supports (**Figure 6b**). Thermal treatment has led to opening of smaller pores and a slight displacement of PSDs to larger pore diameters (**Table 5**, **Figure 6d**).

Combined nitrogen physisorption and mercury porosimetry studies showed that all catalyst precursors had good textural properties, namely a high specific surface area and a well-developed porous structure, containing mesopores stable to thermal treatments. Mesoporosity (pore width of 2–50 nm) is preferable for application that involves the liquid phase since it provides a balance between good diffusion rates of reactants and useful in-pore effects.

*IR analysis*. The IR method was employed to identify the Ni2+ species in the precursor samples. The identification of Ni2+ species by IR analysis is made by the comparison with reference bulk compound—basic nickel carbonate (BNC). **Figure 7** shows infrared and Fourier transform infrared spectra of the studied samples. The 2000–400 cm−1 region has been presented in order to obtain a better representation.

**Figure 7.** IR spectra of the supports and catalyst precursor samples: (a) D, Ni-Mg/D, Ni-Mg-Ag/D; (b) SiG, Ni/SiG; (c) PF, Ni-Mg/PF; (d) (Ni-Mg-Ag/D)Rp. Units and shifts along Y-axis are chosen for convenience.

The derivate distribution function (dV/dlogD) is represented as insert (see **Figure 6a**–**d**). It should be noted that the pressurization data from mercury intrusion yields information about the size of the opening of pores and/or voids and does not reflect the pore size behind the "neck". It is apparent that the deposition of Ni2+ precipitates onto surface of SiG supports leading to a shift in the PSD curves towards the larger pore diameters. The explanation for this effect appears to lie in different microstructural arrangements of supported Ni2+ species in dried samples compared to the starting supports (**Figure 6b**). Thermal treatment has led to opening of smaller pores and a slight displacement of PSDs to larger pore diameters

**Figure 6.** Cumulative and derivative PSDs for the SiG supports and catalyst precursor systems: (a) Ni-Mg/D, Ni-Mg-

Combined nitrogen physisorption and mercury porosimetry studies showed that all catalyst precursors had good textural properties, namely a high specific surface area and a well-developed porous structure, containing mesopores stable to thermal treatments. Mesoporosity

(**Table 5**, **Figure 6d**).

Ag/D; (b) SiG, Ni/SiG; (c) Ni-Mg/PF; (d) (Ni-Mg-Ag/D)Rp.

146 New Advances in Hydrogenation Processes - Fundamentals and Applications

The infrared spectra of the D support and dried Ni-Mg/D catalyst precursors are shown in **Figure 7a**. The IR spectrum of the D support having silica as its main constituent ingredient shows antisymmetric stretching vibration band at around 1090 cm−1 and symmetric stretching vibration band at around 800 cm−1 characteristic for the Si-O-Si bonds [75]. The band at around 470 cm−1 is associated with O-Si-O bond bending vibrations. The absorption band at around 1630 cm−1 can be assigned to the vibration of adsorbed molecular water.

IR spectra of precursors are similar to the spectrum of diatomite in the OH-stretching region containing vibration bands of silica-free hydroxyl groups, hydrogen-bonded hydroxyl groups and adsorbed molecular water (not shown in **Figure 7a**). The main absorption broad band in the spectrum of diatomite (1090 cm−1) in the IR spectra of the precursors appeared to be composed of multiple bands around 1100 cm−1. It can be observed the presence of three bands: IR band characteristic of silica at 1090 cm−1 was reduced to a shoulder, IR band characteristic of silicate-type species connected/interacted with carbonate rich BNC species appeared at 1065 cm−1 and shoulder appeared around 1000 cm−1 which could be attributed to silicate-type species connected/interacted with hydroxide rich BNC species [54]. The presence of intercalated anionic species in the Ni2+ precipitates is attested by the existence of bands at around 1630 cm−1 and at around 1385 cm−1 may be attributed to adsorbed molecular water and carbonate ions. By comparing spectra of precursors and diatomite, it can be observed that IR spectra of precursors contain a band at 650.6 cm−1 (**Figure 7a**) which does not exist in the IR spectrum of the diatomite support. The appearance of a new phase can be attributed to nickel hydrosilicates arisen from the interaction between nickel and the diatomite support. It is apparent that this method of preparation leads to extensive interaction between Ni ions and silica, presumably because under alkaline conditions the silica have a tendency to dissolve. Characterization studies have shown that Ni2+ precipitates on silica as layered nickel hydrosilicate [45–49, 53–57, 68, 72, 74]. In support of this claim is the fact that samples are prepared by a precipitation-deposition method, which leads to the formation of supported nickel hydrosilicates.

The IR spectra of SiG supports and Ni/SiG precursor samples are shown in **Figure 7b**. It is obvious that the IR spectra of these samples resemble those prepared with diatomite support (**Figure 7a** and **b**). This result was expected, having in mind the proportion of the SiO<sup>2</sup> component in the chemical composition of diatomite and SiG supports. In addition, a band at 972 cm−1 can be observed in all three types of SiG support. The band at around 970 cm−1 has been widely used to characterize the incorporation of metal ions in the silica framework as the stretching Si-O vibration mode perturbed by the neighboring metal ions. A new band at 662.8 cm−1 attributable to nickel hydrosilicates can also be observed in all Ni/SiG precursor samples (**Figure 7b**).

FT-IR spectra of the perlite support and Ni-Mg/PF precursors are presented in **Figure 7c**. The spectrum of perlite support in the region from 400 to 2000 cm−1 shows the presence of the main absorption structures, an intense band at about 1045 cm−1 with a shoulder at about 1200 cm−1. These two bands are attributed to Si-O-Si and Si-O-M anti-symmetric stretching vibrations, where M can be Al or Si. A further group of three bands of medium intensity is present at lower wavelengths: 795, 730 and 575 cm−1. The band at 795 cm−1 is assigned to symmetric stretching of Si-O-Si, at 730 cm−1 to bending Si-O-Al and at 575 cm−1 to symmetric stretching of Si-O-R [76]. Water molecule deformation vibrations at around 1630 cm−1 are also registered. The well-expressed bands at 1387 and 1489 cm−1 in the IR spectrum of precursors are attributed to the presence of an additional carbonate containing phase, most probably located on the surface of the support. Comparing bands gained in synthesized precursors are apparent evidence of the created Ni2+ species on the support surfaces. On the reference sample spectra, existence of broad antisymmetric band at 688 cm−1 is evident. This band also exists in spectra of precursors, although slightly shifted towards lower wavenumbers with minimum at around 658 cm−1 (**Figure 7c**). Shift indicates a new type of interaction with the support, compared to the reference material and may be attributed to the Ni-O-Si vibrations [77]. It can also be stated that no evidence of structural change among the dried precursors can be acknowledged.

around 470 cm−1 is associated with O-Si-O bond bending vibrations. The absorption band at

IR spectra of precursors are similar to the spectrum of diatomite in the OH-stretching region containing vibration bands of silica-free hydroxyl groups, hydrogen-bonded hydroxyl groups and adsorbed molecular water (not shown in **Figure 7a**). The main absorption broad band in the spectrum of diatomite (1090 cm−1) in the IR spectra of the precursors appeared to be composed of multiple bands around 1100 cm−1. It can be observed the presence of three bands: IR band characteristic of silica at 1090 cm−1 was reduced to a shoulder, IR band characteristic of silicate-type species connected/interacted with carbonate rich BNC species appeared at 1065 cm−1 and shoulder appeared around 1000 cm−1 which could be attributed to silicate-type species connected/interacted with hydroxide rich BNC species [54]. The presence of intercalated anionic species in the Ni2+ precipitates is attested by the existence of bands at around 1630 cm−1 and at around 1385 cm−1 may be attributed to adsorbed molecular water and carbonate ions. By comparing spectra of precursors and diatomite, it can be observed that IR spectra of precursors contain a band at 650.6 cm−1 (**Figure 7a**) which does not exist in the IR spectrum of the diatomite support. The appearance of a new phase can be attributed to nickel hydrosilicates arisen from the interaction between nickel and the diatomite support. It is apparent that this method of preparation leads to extensive interaction between Ni ions and silica, presumably because under alkaline conditions the silica have a tendency to dissolve. Characterization studies have shown that Ni2+ precipitates on silica as layered nickel hydrosilicate [45–49, 53–57, 68, 72, 74]. In support of this claim is the fact that samples are prepared by a precipitation-deposition method, which leads to the formation of supported

The IR spectra of SiG supports and Ni/SiG precursor samples are shown in **Figure 7b**. It is obvious that the IR spectra of these samples resemble those prepared with diatomite support (**Figure 7a** and **b**). This result was expected, having in mind the proportion of the SiO<sup>2</sup> component in the chemical composition of diatomite and SiG supports. In addition, a band at 972 cm−1 can be observed in all three types of SiG support. The band at around 970 cm−1 has been widely used to characterize the incorporation of metal ions in the silica framework as the stretching Si-O vibration mode perturbed by the neighboring metal ions. A new band at 662.8 cm−1 attributable to nickel hydrosilicates can also be observed in all Ni/SiG precursor

FT-IR spectra of the perlite support and Ni-Mg/PF precursors are presented in **Figure 7c**. The spectrum of perlite support in the region from 400 to 2000 cm−1 shows the presence of the main absorption structures, an intense band at about 1045 cm−1 with a shoulder at about 1200 cm−1. These two bands are attributed to Si-O-Si and Si-O-M anti-symmetric stretching vibrations, where M can be Al or Si. A further group of three bands of medium intensity is present at lower wavelengths: 795, 730 and 575 cm−1. The band at 795 cm−1 is assigned to symmetric stretching of Si-O-Si, at 730 cm−1 to bending Si-O-Al and at 575 cm−1 to symmetric stretching of Si-O-R [76]. Water molecule deformation vibrations at around 1630 cm−1 are also registered. The well-expressed bands at 1387 and 1489 cm−1 in the IR spectrum of precursors are attributed to the presence of an additional carbonate containing phase, most probably

around 1630 cm−1 can be assigned to the vibration of adsorbed molecular water.

148 New Advances in Hydrogenation Processes - Fundamentals and Applications

nickel hydrosilicates.

samples (**Figure 7b**).

The IR spectra of precursors after the reduction treatment are presented in **Figure 7d**. The thermal treatment produced the elimination of adsorbed molecular water and carbon dioxide. The absence of silanol groups, as attested by disappearance of the anti-symmetric band at 980 cm−1 is clearly evident. Besides, a low intense band at 668 cm−1 indicates the presence of nonreduced silicate species in smaller amounts than in the dried samples (**Figure 7a** and **d**).

From the above results and with the available information in the literature, it could be concluded that during the deposition reaction under alkaline conditions, the silica as a constitutive component of all studied supports reacts with the basic nickel carbonate precipitate and generates the new supported nickel hydrosilicate phase containing Si-O-Ni linkages.

*Powder X-ray diffraction (XRD)*. Powder XRD analyses were performed on the dried and reduced-passivated catalyst precursors to identify the phases present in the precursor samples at various stages of synthesis. **Figure 8** shows the XRD diffractograms obtained for dried and reduced-passivated precursor samples.

The diffractogram of the diatomite support (**Figure 8a**, curve 1) shows reflections characteristic of amorphous silica (silica halo peak centered at two-theta around 21°) and the well-crystallized quartz (Q) phase (two-theta = 26.6°; JCPDS 46-1045). Typical diffractograms of dried precursor samples exhibited only broad and asymmetrical bands attributable to ill-defined and badly crystallized nickel hydrosilicates (**Figure 8a**, curves 2–6). Besides, the XRD patterns of bulk BNC disappear in the patterns of precursors. The observed phenomenon proves that an interaction occurs between BNC and silica from the support. The formation of surface lamellar hydrosilicates in the preparation of silica supported nickel catalysts was postulated on the basis of several techniques of characterization (IR, TPR, XPS and Extended X-ray Absorption Fine Structure-EXAFS). X-ray diffraction was also employed to corroborate the presence of nickel hydrosilicate. The XRD studies showed that the nickel hydrosilicates are formed under various conditions at relatively low temperatures (under 100°C) [48, 71, 78–81]. As it is well known, when the nickel salt is precipitated with Na2 CO3 in the presence of silica, the precipitated Ni2+ phase is silica supported-BNC with composition of Ni(OH)*<sup>x</sup>* (CO3 ) *y* /SiO<sup>2</sup> ×*z*H2 O that varies considerably with the changes in the precipitation conditions [41]. This interaction leads to the formation of badly ill-crystallized layered nickel hydrosilicate compounds, identified as nepouite-like (1:1 nickel hydrosilicate; Ni3 (OH)<sup>4</sup> (Si2 O5 )) and/or talc-like structure (2:1 nickel hydrosilicate; Ni3 (OH)<sup>2</sup> (Si2 O5 ) 2 ). The formed phases have been identified as nickel antigorite [48, 71, 78, 81], nickel chrysotiles [82], nickel montomorillonite [48, 71], nickel palygorskite [54], serpentine [83], or orthosilicate-type [54].

**Figure 8.** XRD diffractograms of supports, dried and reduced-passivated precursor samples: (a) D, Ni-Mg/D; (b) Ni-Mg-Ag/D; (c) Ni/SiG, (Ni/SiG)Rp; (d) (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp.

Samples modified with silver (Ni-Mg-Ag/D, **Figure 8b**) have slightly altered XRD spectra. In addition to XRD peaks characteristic to the nickel hydrosilicates, two new peaks can be observed at 32.2 and 39.4° attributable to the α-Ag<sup>2</sup> CO3 phase (JCPDS file 31-1237). Moreover, it is observed that the modification with silver contributes to the further amorphization of the dried precursor samples [84].

The XRD patterns of the dried and reduced-passivated silica gel supported Ni catalyst precursors and reference material-BNC are presented in **Figure 8c**. The XRD patterns of three types of silica gel supports had characteristic reflections of amorphous silica (not shown). The insert in **Figure 8c** represents a comparison between the XRD patterns of the dried precursors of the Ni/SiG system and the reference BNC material. The absence of the diffraction line at 16.3° of the reference material-BNC sample and appearance of a new broad reflection at around 23° in the spectra of this catalyst precursor system represented a substantial difference between the bulk reference material and the supported Ni2+ phase, present in the samples of this catalyst precursor system. The turbostratic structure of nickel hydrosilicate [48, 54] predetermined the ill-organized reflections of the Ni/SiG system of the precursor. Moreover, the nickel hydrosilicate phase exhibits different degrees of crystallization, more pronounced in the Ni/SIG-B sample. It is obvious that the usage of different silica gel types affects the crystallinity of the deposited Ni containing phase. Note that, the registered high background below two-theta values of 15° in XRD diffractograms of all samples indicates advanced amorphization of the observed phase.

The XRD patterns of the reduced precursor samples at 430°C (**Figure 8c**) display reflections located at two-theta, typical for nickel metal (Ni0 ) (JCPDS file 00-004-0850). The peaks of lower intensity between two-theta from 32 to 40° indicate the presence of nickel hydrosilicate in all reduced samples, but it is better represented in the sample Ni/SiG-B.

In the case of the reduced catalyst precursors of (Ni-Mg/D)Rp and (Ni-Mg-Ag/D)Rp systems, typical XRD spectra showed common peaks corresponding to nickel metal (Ni0 ) and silver metal (Ag0 ) (**Figure 8d**). The layered structure of the nickel hydrosilicate phase was also registered. The experimental conditions for the reduction step was selected in order to establish the relation between the reduction time and the reduction temperature (selected temperature of 430°C) was assumed to be very important. Despite the obvious reduction in intensity of peaks caused by prolonged dwell time (5h) in the selected temperature, reflections corresponding to nickel hydrosilicates are still visible. This shows that the reduction temperature of 430°C used for reduction of the dried precursor with H<sup>2</sup> was not sufficient to reduce all the nickel hydrosilicate species to the nickel metal (Ni0 ) and silica for these two systems of catalyst precursors.

The results concerning the influence of the preparation stage and nature of the support and the modifier clearly illustrate the feature of the supported Ni2+ phase and demonstrate that XRD measurements may offer an effective tool to identify the nickel species and their interaction with the support in differently supported and modified nickel-based catalyst precursors.

*Hydrogen temperature programmed reduction (H2 -TPR)*. The objective of H2 -TPR experiments was to determine the reducibility as well as the optimum reduction temperature for prepared catalyst precursor systems. In conjunction with IR and XRD data, TPR profiles were also useful in determining the type of Ni2+ phases present in the catalyst precursors and may be indicative of the actual activity of the final reduced metal catalyst. The H<sup>2</sup> -TPR profiles of all prepared catalyst precursors are given in **Figure 9**.

Samples modified with silver (Ni-Mg-Ag/D, **Figure 8b**) have slightly altered XRD spectra. In addition to XRD peaks characteristic to the nickel hydrosilicates, two new peaks can be

**Figure 8.** XRD diffractograms of supports, dried and reduced-passivated precursor samples: (a) D, Ni-Mg/D; (b) Ni-Mg-

it is observed that the modification with silver contributes to the further amorphization of the

The XRD patterns of the dried and reduced-passivated silica gel supported Ni catalyst precursors and reference material-BNC are presented in **Figure 8c**. The XRD patterns of three types of silica gel supports had characteristic reflections of amorphous silica (not shown). The insert in **Figure 8c** represents a comparison between the XRD patterns of the dried precursors of the Ni/SiG system and the reference BNC material. The absence of the diffraction line at 16.3° of the reference material-BNC sample and appearance of a new broad reflection at around 23° in the spectra of this catalyst precursor system represented a substantial difference between the bulk reference material and the supported Ni2+ phase, present in the samples of this catalyst

CO3

phase (JCPDS file 31-1237). Moreover,

observed at 32.2 and 39.4° attributable to the α-Ag<sup>2</sup>

Ag/D; (c) Ni/SiG, (Ni/SiG)Rp; (d) (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp.

150 New Advances in Hydrogenation Processes - Fundamentals and Applications

dried precursor samples [84].

The influence of the nature of precursor salts of nickel on the reducibility of prepared samples is reported in **Figure 9a**. These results demonstrate rather well the differences between the Ni2+ species formed in the case of diatomite supported nickel-based catalyst precursors. A peak due to the reduction of the Ni2+ phase, which corresponds to the BNC (**Figure 9b**–**d**—insert) was seen only in the sample prepared from the sulfamate salt of nickel (NiS -Mg/D). Among the prepared catalyst precursors, the smallest proportion of the Ni2+ phase from BNC can be seen in the NiA-Mg/D sample. The high reduction temperature needed for the samples prepared from the acetate nickel precursor salt is obtained by the presence of difficult to reduce nickel hydrosilicates, which is the form in which nickel precipitates are deposited during synthesis. Consequently, layered nickel hydrosilicates whose thermal decomposition starts above 450°C [47] appear to be the main nickel species present in this sample. The stronger interaction of nickel and diatomite support hinders reduction of samples. This leads to a shift in the *T*max value of peaks corresponding to the reduction of Ni2+ phases interacting with the support from 320°C over Ni<sup>S</sup> -Mg/D to 462°C over NiA-Mg/D. The reduction extent (R, %) of the Ni2+ supported phase at 430°C (**Figure 9a**) increases with the following sequence: NiA-Mg/D (24.8%) < Ni<sup>F</sup> -Mg/D (26.4%) < Ni-Mg/D (43.1%) < NiC-Mg/D (54.5%) < Ni<sup>S</sup> -Mg/D (75.8%).

**Figure 9.** TPR profiles normalized to the sample weight and fitted with Gaussian deconvolution peaks for all prepared catalyst precursor systems: (a) Ni-Mg/D; (b) Ni/SiG; (c) Ni-Mg/PF; (d) Ni-Mg-Ag/D.

The reduction properties of Ni containing SiG-A, SiG-B and SiG-C catalyst precursors are shown in **Figure 9b**. The interpretation of the TPR profiles of the precursor samples is accomplished by comparing them with the profile of the reference BNC sample. The comparison is supposed to clarify the support role in the studied solids. Indeed, the experiments revealed a quite different reduction behavior of the formed Ni2+ species. The higher reducibility of the unsupported BNC is attested by the single low temperature peak in the region 220–310°C which is assumed to represent the full reduction of bulk Ni2+ ions to the nickel metal. In contrast, multiple reduction peaks with poorly resolved maxima characterize the TPR profiles of the precursors indicating a complex interaction between the Ni2+ species and SiG supports. Two most distinguishable peaks at 320 and 428°C can be observed in the Ni/SiG-A sample, alongside a shoulder at 540°C. The reduction peak at 320°C can be attributed to the reduction of BNC species on Ni/SiG-A. All the reduction temperatures above this temperature can be directly associated with the different type of interaction between the nickel species and the supported material. The existence of a broader peak at 428°C and the shoulder at 540°C is caused by the strong interaction between the Ni2+ supported phase and the SiG-A framework which points to the existence of hydrosilicate species. The TPR profile of the Ni/SiG-C sample resembles that of the Ni/SiG-A sample with a clearly observed difference in the contribution from low temperature (BNC) and high temperature Ni2+ hydrosilicate species. Finally, the absence peak at 320°C and no reduction at all up to 350°C in Ni/SiG-B sample shows that BNC species does not exist, while three peaks at around 462, 525 and 624°C can be attributed to Ni2+ hydrosilicate species [85]. It may be summarized that the TPR profiles of the Ni/SiG system evidenced a variety of interaction strength depending on the type of the SiG support resulting in the formation of varying amounts of different Ni2+ species.

[47] appear to be the main nickel species present in this sample. The stronger interaction of nickel and diatomite support hinders reduction of samples. This leads to a shift in the *T*max value of peaks corresponding to the reduction of Ni2+ phases interacting with the support from

ported phase at 430°C (**Figure 9a**) increases with the following sequence: NiA-Mg/D (24.8%) <

The reduction properties of Ni containing SiG-A, SiG-B and SiG-C catalyst precursors are shown in **Figure 9b**. The interpretation of the TPR profiles of the precursor samples is accomplished by comparing them with the profile of the reference BNC sample. The comparison is supposed to clarify the support role in the studied solids. Indeed, the experiments revealed a quite different reduction behavior of the formed Ni2+ species. The higher reducibility of the unsupported BNC is attested by the single low temperature peak in the region 220–310°C

**Figure 9.** TPR profiles normalized to the sample weight and fitted with Gaussian deconvolution peaks for all prepared

catalyst precursor systems: (a) Ni-Mg/D; (b) Ni/SiG; (c) Ni-Mg/PF; (d) Ni-Mg-Ag/D.


152 New Advances in Hydrogenation Processes - Fundamentals and Applications



320°C over Ni<sup>S</sup>

Ni<sup>F</sup>

Reducibility of the Ni-Mg/PF precursors studied by TPR is shown in **Figure 9c**. Clearly, the profiles show almost the same tendency, since both precursors have almost the same *T*max and slightly anti-symmetric profile toward higher temperature. The low temperature reduction at 313 and/or 314°C for Ni-Mg/PF-2 and Ni-Mg/PF-1, respectively implies the presence of bulk Ni2+ easily reduced species and the weak interaction between nickel and perlite support. By comparing TPR profiles of precursors with reference material (**Figure 9c**—insert) the shifting of the *T*max of about 40°C towards higher temperatures is apparent. A reasonable postulation for a lower temperature reduction of the Ni-Mg/PF system is that it contains the majority of Ni2+ species nucleated as BNC and anchored to the oxygen containing groups of the perlite support after deposition. Clearly, reduction of BNC proceeds easily. Based on the entire temperature range, it can be stated that all Ni2+ species in this system are reducible under 430°C since no temperature profiles above this temperature have been observed. A complete reduction of the Ni2+ species does not necessarily mean that the catalyst will be active, due to the possibility of nickel crystallite growth through aggregation which therefore decreases the number of active sites on the support, resulting in overall decrease of catalyst activity.

The influence of the silver modifier on the reducibility of Ni-Mg-Ag/D catalyst precursors is displayed in **Figure 9d**. TPR measurements clearly showed the differences in reducibility of Ni-Mg-Ag/D samples. The TPR profile of the sample Ni-Mg-Ag5.88/D displays five peaks. It is obvious that there are two different areas where hydrogen is consumed. The first is the low temperature region (LTR) between 210 and 440°C (TPR peaks maxima at 318 and 398°C) and the second is high temperature region (HTR) from 440 to 700°C (TPR peaks maxima at 485, 535 and 591°C). In the LTR area, the reduction of easily reducible silver (Ag<sup>+</sup> ) and nickel (Ni2+) phases occurs. The higher TPR peak at LTR is in agreement with XRD, which also revealed the presence of more bulk-like silver upon increasing Ag loading (**Figure 8d**). Hydrogen consumption at HTR is normally attributed to hardly reducible Ni2+ phases nickel hydrosilicates. When the Ag loading is decreased, the reduction profiles become less resolved (Ni-Mg-Ag1.55/D and Ni-Mg-Ag0.16/D) suggesting that the reduction occurs in a single unresolved step. Obviously, increasing Ag loading in the precursors shifts the onset temperatures of the initial reduction to lower temperatures (**Figure 9d**). In addition, the reduction was completed at lower temperatures with the precursors of higher Ag loading.

Although the nickel particles of samples do not easily sinter because of the strong interaction with support [49, 72, 73], XRD and H2 -chemisorption result (discussed later in the paper) have shown that the silver loading has an impact on the reduction ability of modified catalyst precursors. The increase in the silver content leads to larger nickel particles in the Ag modified catalyst precursors, Ni-Mg-Ag/D, which displayed easier nickel reduction. In the literature, it is widely accepted as influence of reduction ability of supported metal catalysts on the particle size of the active metal: the lower the reduction ability, the smaller the nickel metal particles [56, 86]. Such a conclusion could be an explanation for better reduction ability of the catalyst precursors with higher Ag loadings.

TPR is a favorable technique for studying the impact of co-metal modifier and support effects on the ability of the reduction of supported metal catalysts [87]. The effect of adding silver on the ability of transition metals reduction is not sufficiently studied in the literature. Richardson and co-workers [88] showed the positive role of silver oxide to promote a better understanding of nickel oxide reduction. A higher degree of nickel oxide reduction in the presence of silver was interpreted by easier nucleation of the nickel clusters, which is rate determining for the reduction, according to Coenen [89]. In bimetallic silver-based catalysts, the higher the Ag loading, the deeper the reduction occurs. In the system with nickel, reduced silver forms metal particles that act as foreign nuclei for subsequent growth and reduction of nickel crystallites. The more silver cations (Ag+ ) were introduced in the catalyst, the more silver nuclei formed for Ni crystallites growth and more nickel was reduced.

The TPR results demonstrate rather well the differences between Ni compound formed in the case of diatomite, silica gel and perlite. In the case of the Ni-Mg/PF system, the sharp peak at about 310°C has been identified as being due to the reduction of bulk Ni2+ species. The Ni-Mg/D and Ni/SiG systems are difficult to reduce and are comparable in reduction characteristics to Ni hydrosilicates. Significantly however for the Ni-Mg-Ag/D system, reduction is much more facile due to easier nucleation of the nickel crystallites in the presence of silver. In addition, it has been shown that the nature of the nickel precursor salt has a profound effect on the reducibility of Ni-Mg/D catalyst precursors.

*Hydrogen chemisorption (H2 -chemisorption)*. Conventional supported metal catalysts are prepared by *in situ* reduction of a metal salt. The metallic surface is formed by particles of ultimate size and the rates of structure-dependent reactions depend on the size of these particles. Therefore, the catalytic activity of metal particles formed is strongly related to their size and shape. Previous work of our group showed that supported nickel nanoparticles can be obtained in the Ni/Diatomite and Ni/Water glass catalyst systems synthesized by the DP method [90]. They were found as active catalysts in partial hydrogenation of soybean oil. As to the silica support, it is known not to give rise of nickel precipitates in different oxidation states and allows a better approach of the particle size effect in the behavior of supported nickel catalysts [91].

The chemisorption of a gas on a catalyst surface, such as hydrogen chemisorption, is commonly used as a suitable method for the determination of the size of active metal surface area in supported metal catalytic systems [92]. The active metal surface may be measured under suitable conditions taking into account the peculiarity of the system being tested. Hydrogen chemisorption method consists of the use of a hydrogen molecule, which chemisorbs selectively on the metal and not on the support. Assuming a given stoichiometry for this surface reaction, it is possible to obtain an estimate of the metal surface area and of the average metal particle size. Thus, in the H2 -chemisorption studies, the measured value of the active metal surface is dependent on the stoichiometry of the hydrogen adsorption, which in turn depends on the metal-support interaction, modifiers and preparation method [93]. The estimation of the metal crystallite size from hydrogen uptake requires the assumptions to be made regarding metal crystallite morphology. However, it should be noted that the results of chemisorption for supported Ni catalysts in the literature are not always in agreement, mainly for two reasons: the first is that adsorption of H<sup>2</sup> on supported nickel catalysts involves simultaneous physical adsorption, chemisorption, reduction of Ni compound, activated adsorption and hydrogen spillover; the second is the existence of several forms of chemisorbed hydrogen bonded to surface, subsurface, edge and vertex Ni atoms.

resolved (Ni-Mg-Ag1.55/D and Ni-Mg-Ag0.16/D) suggesting that the reduction occurs in a single unresolved step. Obviously, increasing Ag loading in the precursors shifts the onset temperatures of the initial reduction to lower temperatures (**Figure 9d**). In addition, the reduction was

Although the nickel particles of samples do not easily sinter because of the strong interaction

shown that the silver loading has an impact on the reduction ability of modified catalyst precursors. The increase in the silver content leads to larger nickel particles in the Ag modified catalyst precursors, Ni-Mg-Ag/D, which displayed easier nickel reduction. In the literature, it is widely accepted as influence of reduction ability of supported metal catalysts on the particle size of the active metal: the lower the reduction ability, the smaller the nickel metal particles [56, 86]. Such a conclusion could be an explanation for better reduction ability of the

TPR is a favorable technique for studying the impact of co-metal modifier and support effects on the ability of the reduction of supported metal catalysts [87]. The effect of adding silver on the ability of transition metals reduction is not sufficiently studied in the literature. Richardson and co-workers [88] showed the positive role of silver oxide to promote a better understanding of nickel oxide reduction. A higher degree of nickel oxide reduction in the presence of silver was interpreted by easier nucleation of the nickel clusters, which is rate determining for the reduction, according to Coenen [89]. In bimetallic silver-based catalysts, the higher the Ag loading, the deeper the reduction occurs. In the system with nickel, reduced silver forms metal particles that act as foreign nuclei for subsequent growth and reduction

The TPR results demonstrate rather well the differences between Ni compound formed in the case of diatomite, silica gel and perlite. In the case of the Ni-Mg/PF system, the sharp peak at about 310°C has been identified as being due to the reduction of bulk Ni2+ species. The Ni-Mg/D and Ni/SiG systems are difficult to reduce and are comparable in reduction characteristics to Ni hydrosilicates. Significantly however for the Ni-Mg-Ag/D system, reduction is much more facile due to easier nucleation of the nickel crystallites in the presence of silver. In addition, it has been shown that the nature of the nickel precursor salt has a profound effect

by *in situ* reduction of a metal salt. The metallic surface is formed by particles of ultimate size and the rates of structure-dependent reactions depend on the size of these particles. Therefore, the catalytic activity of metal particles formed is strongly related to their size and shape. Previous work of our group showed that supported nickel nanoparticles can be obtained in the Ni/Diatomite and Ni/Water glass catalyst systems synthesized by the DP method [90]. They were found as active catalysts in partial hydrogenation of soybean oil. As to the silica support, it is known not to give rise of nickel precipitates in different oxidation states and allows a better

approach of the particle size effect in the behavior of supported nickel catalysts [91].

*-chemisorption)*. Conventional supported metal catalysts are prepared


) were introduced in the catalyst, the more

completed at lower temperatures with the precursors of higher Ag loading.

154 New Advances in Hydrogenation Processes - Fundamentals and Applications

silver nuclei formed for Ni crystallites growth and more nickel was reduced.

with support [49, 72, 73], XRD and H2

catalyst precursors with higher Ag loadings.

of nickel crystallites. The more silver cations (Ag+

on the reducibility of Ni-Mg/D catalyst precursors.

*Hydrogen chemisorption (H2*

Hydrogen chemisorption results for nickel dispersion, nickel surface area and nickel crystallite size are summarized in **Table 6**.

Although some discussion concerning the adequacy of this procedure can be found in the literature [55, 71, 92–95], the values thus obtained are useful from a comparative point of view. **Table 6** shows that a broad range of crystallite sizes is obtained as a consequence of the nickel salt precursors and modifiers used, metal loading and support type in each case. By using both static and dynamic methods under selected conditions of TPR and H2 -chemisorption experiments, the overall dispersion degree does not exceed 13% and crystallite size is lower than 23 nm (excepting the sample Ni/SiG-A). It is known that for the nickel loadings higher than 30 wt% Ni, an important decrease in dispersion is observed [55]. This fact is due to that higher nickel loadings favor agglomeration of particles. Moreover, this agglomeration process is also favored by the weak interaction between the metal and the surface of the support. Hydrogen chemisorption results showed that the particle sizes of nickel metal (Ni0 ) in the samples from the different precursors salts may be correlated with their reducibility (see **Table 6** and **Figure 9a**) and textural properties (**Table 4**).

The addition of Ag (hydrogen does not chemisorb onto silver) to the Ni-Mg/D system decreased its chemisorption capacity. The cause of decreased hydrogen adsorption can be a result of blocking of the nickel active site by silver atoms, electronic interactions between Ni and Ag atoms that affect the hydrogen binding to the surface Ni and changes in the stoichiometry of hydrogen adsorption on Ni surfaces due to structure sensitivity [94]. The estimates of the crystallite size from hydrogen chemisorption are also compared with the values determined from X-ray diffraction methods line broadening (**Table 6**). The mean size of nickel particle deduced by the static H2 -chemisorption method was confirmed by the XRD method. The fact that the ratio of these two values is close to unity may be taken as added support for the assumed geometric model. The hydrogen chemisorption results for the samples of the Ni-SiG system are in agreement with their NP measurements (**Tables 1** and **4**). The lowest dispersion of Ni/SiG-A samples is most likely caused by steric hindrances (microporous SiG-A support). In such a case, metal distribution on the external surface of the support is to be preferred, with consequent lower dispersion (the nickel content being almost the same in each sample (Ni/SiG-A, Ni/SiG-B and/or Ni/SiG-C, **Table 6**).


a Each entry represents a different run.

b Hydrogen chemisorbed.

c Ni metal surface area; SNi was calculated assuming Nisurf/Hchs = 1 and mean surface area of Ni atom equal to 6.5 × 10−20 m2 . ᵈMean Ni particle or crystallite size: dNi chs—H<sup>2</sup> -chemisorption method (surface mean value).

ᵉMean Ni particle or crystallite size: dNi XRD – XRD line broadening method (volume mean value); Ni crystallite geometric model: hemispherical crystallites attached to support with their equatorial plane.

fComputed assuming spherical model.

ᵍNumber of accessible Ni atoms-exposed fraction.

hDispersion.

<sup>i</sup>Chemisorption method: static.

j Chemisorption method: dynamic.

**Table 6.** Chemisorption experiments on the selected samples of Ni-Mg-Ag/D, Ni/SiG and Ni-Mg/PF systemsᵃ.

Comparison was made between Ni-Mg/PF-1 and Ni-Mg/D samples. A smaller metal surface area and a larger Ni crystallite size can be observed in Ni-Mg/PF-1 and attributed to the rapid aggregation of nickel crystallites formed in the reduction stage. It is likely that the reason for this behavior is the weak interaction between the PF support and the Ni surface. For this sample, a crystallite size of nickel calculated assuming a spherical model which is suitable for the supported catalysts with weak metal-support interaction represents a more realistic result (**Table 6**). On the contrary, a larger nickel metal surface area and a smaller nickel particle size is observed on the Ni-Mg/D sample due to the strong anchoring effect of D support. This anchoring restricts the migration of nickel particles hence prevents the formation of large nickel particles that did not sinter on the mild reduction at 430°C.

NP measurements (**Tables 1** and **4**). The lowest dispersion of Ni/SiG-A samples is most likely caused by steric hindrances (microporous SiG-A support). In such a case, metal distribution on the external surface of the support is to be preferred, with consequent lower dispersion (the nickel content being almost the same in each sample (Ni/SiG-A, Ni/SiG-B and/or Ni/SiG-C, **Table 6**).

**)**

Ni/SiG-Ai 43.7 123 9.6 46.7 1.5 1.4 Ni/SiG-Bi 45.5 571 44.8 10.0 6.9 6.7 Ni/SiG-C<sup>i</sup> 43.5 1073 84.0 5.3 12.9 12.6

NiA-Mg/Dj 36.6 259 20.3 22.2 3.1 3.0 NiC-Mg/Dj 36.2 437 34.2 13.1 5.3 5.1



Ni metal surface area; SNi was calculated assuming Nisurf/Hchs = 1 and mean surface area of Ni atom equal to 6.5 × 10−20 m2

ᵉMean Ni particle or crystallite size: dNi XRD – XRD line broadening method (volume mean value); Ni crystallite geometric


**dNi chsd (nm)**

**Nisurf × 10−20 <sup>g</sup> (atNi acc gNi −1)**

) 9.0 8.7

) 8.5 8.3

) 6.5 6.3

) 6.2 6.0

) 6.9 6.8

**Dh (%)**

.

**Nickel metal properties (Ni0**

**SNi c (m2 gNi −1)**

**Sample code Ni** 

Ni<sup>F</sup>

NiS

Each entry represents a different run.

fComputed assuming spherical model.

Chemisorption method: static.

Chemisorption method: dynamic.

ᵈMean Ni particle or crystallite size: dNi chs—H<sup>2</sup>

ᵍNumber of accessible Ni atoms-exposed fraction.

Hydrogen chemisorbed.

a

b

c

i

j

hDispersion.

**(wt%)**

**H2-chs b (μmol gNi −1)**

156 New Advances in Hydrogenation Processes - Fundamentals and Applications

Ni-Mg/Di 36.3 744 58.2 7.7 (7.1e

Ni-Mg-Ag0.16/Di 35.9 709 55.5 8.1 (8.3<sup>e</sup>

Ni-Mg-Ag1.55/Di 35.2 538 42.1 10.7 (11.7e

Ni-Mg-Ag5.88/Di 33.5 513 40.2 11.2 (13.6<sup>e</sup>

Ni-Mg/PF-1<sup>i</sup> 30.2 576 45.1 9.9 (14.9f

model: hemispherical crystallites attached to support with their equatorial plane.

Comparison was made between Ni-Mg/PF-1 and Ni-Mg/D samples. A smaller metal surface area and a larger Ni crystallite size can be observed in Ni-Mg/PF-1 and attributed to the rapid aggregation of nickel crystallites formed in the reduction stage. It is likely that the reason for this behavior is the weak interaction between the PF support and the Ni surface. For this sample, a crystallite size of nickel calculated assuming a spherical model which is suitable for the supported catalysts with weak metal-support interaction represents a more realistic result (**Table 6**). On the contrary, a larger nickel metal surface area and a smaller nickel particle size is observed on the Ni-Mg/D sample due to the strong anchoring effect of D support.

**Table 6.** Chemisorption experiments on the selected samples of Ni-Mg-Ag/D, Ni/SiG and Ni-Mg/PF systemsᵃ.

The hydrogen chemisorption study showed that the size of nickel nanoparticles obtained in the studied catalyst precursor systems depended on the nature of precursor nickel salt from which they are formed, the kind and loading of metal modifier and the type of support used.

*X-ray photoelectron spectroscopy (XPS)*. In the previous part of this work, it was pointed out that two major factors that affect the reduction of Ni2+ supported phase are: (i) interaction between precipitating nickel precursors and the catalyst support and (ii) the dispersion nickel metal phase (Ni0 ) arising by reduction of the deposited Ni2+ phase. These factors are in turn affected by various examined parameters including the nature of the support, the use of modifier and the choice of nickel precursor salt. Useful confirmatory evidence concerning the interaction between the Ni2+ supported phase and support including the structural changes occurring during preparation, drying and reduction and the dispersion of nickel metal phase can be obtained by photoelectron spectroscopy. XPS is virtually surface sensitive (a few atomic layers) and a quantitative instrumental method particularly suitable for the evaluation of surface character of supported nickel catalysts [96].

The XPS results discussed in this work will be restricted to the cases of Ni/SiG, Ni-Mg/D and Ni-Mg-Ag/D systems. The XPS spectra of the Ni 2p and Ag 3d peaks for the systems under investigation are shown in **Figure 10**.

On investigating the influence of the support characteristics on the strength of the interaction between Ni containing species and the support, we chose the Ni/SiG precursor system keeping in mind that the interaction between the Ni2+ species and the support is commonly accepted depending on the characteristics of the support. By choosing such a system containing the samples prepared without addition of the modifier with almost the same Ni loadings (**Table 3**), we hoped to increase the value of any comparison one may make. The 2p peaks in the nickel spectrum were used to characterize the chemical state of nickel. The intensity of the Ni 2p signal was obtained by integration over the binding energy (BE) range of 850–890 eV to include the double excitation, shake-up and shake-off peaks. It is known that the chemical forms of nickel have certain characteristics, which serve to identify their presence [97]. The shape of the peaks also contains information. The separation and intensity of the shake-up satellite of the Ni 2p3/2 core level can be helpful in identifying a particular species.

As expected for dried samples, nickel in these precursors is present in the Ni2+ oxidation state—Ni 2p3/2 peak is a doublet structure (splitting a few electronvolts). The nickel 2p core level, as seen in **Figure 10a**, is similar in shape for all samples, however, the binding energies of the Ni 2p level vary from each other. Since XPS is surface sensitive, the differences in binding energies of the XPS peaks indicate that the nickel species on the surface are changed. The XPS data in **Table 7** show that the binding energies of the Ni 2p3/2 level for the Ni/SiG-A sample and for the reference material (BNC) are the same. It means that the aggregates of BNC are situated on the surface of nickel hydrosilicates located on the precursor SiG-A [72, 98]. The chemical shift of the Ni 2p3/2 peak toward higher binding energy values for the Ni/SiG-B sample is assigned to the stronger interaction between the Ni2+ species and the SiG-B support. On the contrary, the observed shift toward lower binding energies for the Ni-SiG-C sample suggests weakening interaction between the Ni2+ supported phase and the SiG-C support [68]. **Table 7** reveals the variations of the Ni/Si ratio suggesting the different dispersion of the Ni2+ species in analyzed samples, as previously shown in the H2 chemisorption results of the corresponding precursor samples (see **Table 6**).

**Figure 10.** Ni 2p3/2 XPS spectra of supported nickel catalyst precursors: (a) dried Ni/SiG; (b) dried Ni-Mg/D, Ni-Mg-Ag/D; (c) reduced passivated (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp; (d) reduced passivated (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp (deconvoluted).

The XPS spectra for diatomite supported nickel-based catalyst precursors are presented in **Figure 10b**–**d**. The Ni 2p core level signal of dried Ni-Mg/D and Ni-Mg-Ag/D precursor samples consist of a single Ni 2p3/2 peak centered at around 855 eV (Ni-Mg/D and Ni-Mg-Ag5.88/D) assigned to Ni2+ (**Figure 10b**). The existence of a second component at a higher BE (860.9-863.5) could be due to the presence of hardly reducible Ni2+ species, as previously noted in the discussion of the corresponding TPR curves. Ag 3d XPS spectra for dried Ni-Mg-Ag/D catalyst precursors are depicted in **Figure 10b**—insert. There are two peaks, a result of spin-orbit splitting, designated Ag 3d5/2 and Ag 3d3/2, respectively, corresponding to the strongest photoelectron lines. These peaks are observed at 367.3 eV and 373.1 eV, respectively, shifted to lower BE values in relation to metallic Ag and could be assigned to the Ag+ oxidation state [99]. Silver modification provokes a shifting of the Ni 2p3/2 peak toward higher BE values. The observed shift could be due to the interaction between the components in the dried samples, which is more intense for the sample with the lowest Ag loading.


**Table 7.** X-ray photoelectron spectroscopy data for Ni-SiG catalyst precursors—dried samples.

After H2 reduction, a shoulder appears on the low binding energy side of the Ni 2p3/2 peak (**Figure 10d**). The shoulder can be deconvoluted and the binding energy is that of the nickel metal (851.9 eV). The other peaks observed on the reduced-passivated samples at higher BE values assigned to Ni2+ also appear. The presence of Ni2+ species after H2 reduction has also been confirmed by IR and XRD (**Figures 7d** and **8d**). It is worth mentioning that the increase of Ag 3d5/2 binding energy after reduction treatment with the respect to the dried precursors (**Figure 10c** and **b**—insert). Although it is known that silver oxides are quite unstable and the two silver oxides, Ag2 O and AgO, decompose below the temperature of 230°C, even in the oxygen atmosphere [100, 101] the results of the Ag 3d and O 1s (not shown here) XPS spectra seem to suggest the presence of silver (I) oxide on the surface of reduced-passivated samples.

The XPS study of Ni/SiG, Ni-Mg/D and Ni-Mg-Ag/D precursor samples confirm the formation of surface species with different strength of interaction and different dispersion of the supported nickel species.

#### **3.2. Partial hydrogenation of soybean oil (SBO)**

for the Ni/SiG-B sample is assigned to the stronger interaction between the Ni2+ species and the SiG-B support. On the contrary, the observed shift toward lower binding energies for the Ni-SiG-C sample suggests weakening interaction between the Ni2+ supported phase and the SiG-C support [68]. **Table 7** reveals the variations of the Ni/Si ratio suggesting the different dispersion of the Ni2+ species in analyzed samples, as previously shown in the H2

The XPS spectra for diatomite supported nickel-based catalyst precursors are presented in **Figure 10b**–**d**. The Ni 2p core level signal of dried Ni-Mg/D and Ni-Mg-Ag/D precursor samples consist of a single Ni 2p3/2 peak centered at around 855 eV (Ni-Mg/D and Ni-Mg-Ag5.88/D) assigned to Ni2+ (**Figure 10b**). The existence of a second component at a higher BE (860.9-863.5) could be due to the presence of hardly reducible Ni2+ species, as previously noted in the discussion of the corresponding TPR curves. Ag 3d XPS spectra for dried Ni-Mg-Ag/D catalyst precursors are depicted in **Figure 10b**—insert. There are two peaks, a

**Figure 10.** Ni 2p3/2 XPS spectra of supported nickel catalyst precursors: (a) dried Ni/SiG; (b) dried Ni-Mg/D, Ni-Mg-Ag/D; (c) reduced passivated (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp; (d) reduced passivated (Ni-Mg/D)Rp, (Ni-Mg-Ag/D)Rp

(deconvoluted).

chemisorption results of the corresponding precursor samples (see **Table 6**).

158 New Advances in Hydrogenation Processes - Fundamentals and Applications


#### *3.2.1. Activity of Ni-Mg/D and Ni-Mg-Ag catalysts in partial hydrogenation of SBO*

Hydrogenation overall activity was monitored by the decay of the iodine value which indicates the level of unsaturation of double bonds (C=C). Activity (A) was calculated from the following equation:

 *<sup>A</sup>* <sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_ H2c *t* ⋅ *m*oil ⋅ Nilcc (1)

where H2c is the hydrogen consumption for decay in iodine value; *t* is the reaction time; *m*oil is mass of oil; and Nilcc is the nickel loading in the catalyst charge for hydrogenation run (in grams).

The results of SBO hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in the slurry pilot-plant reactor system (**Figure 4**) are presented in **Table 8**.


a Process parameters: 160°C (temperature); 0.16 MPa (H<sup>2</sup> pressure); 750 rpm (stirring rate).

b Ni loading in catalyst charge for hydrogenation run (catalyst concentration: 0.05 wt%, i.e., 2.5 g catalyst/5000 g oil). c Iodine value determined by Wijs method.

dHydrogen consumption for decrease of iodine value by *ca* 40–41 units.

e Rate of hydrogen consumption per unit mass of oil.

**Table 8.** Soybean oil hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in slurry pilot plant reactor system catalytic activity test runs results.

The obtained results showed clearly the influence of the silver addition on the catalyst activity. Under the same process conditions, Ni-Mg/D and silver modified catalysts exhibited different activities toward SBO hydrogenation (see **Table 8**). By comparing the results of catalytic test runs over Ni-Mg/D and Ni-Mg-Ag0.16/D catalysts, it can be observed that the activity of the sample without silver is slightly higher. The modification by silver inhibits hydrogenation activity, this effect being more obvious as the Ag loading is higher. From these results, the hydrogenation activity for the studied catalyst (**Table 8**) increases in the following order: Ni-Mg-Ag5.88/D < Ni-Mg-Ag1.55/D < Ni-Mg-Ag0.16/D < Ni-Mg/D.

The observed differences in the activity of the studied catalysts could be attributed to nickel dispersion and different textural properties of the catalysts. From the chemisorption results, the silver-modified Ni catalyst sample with high loading (Ni-Mg-Ag5.88/D) demonstrated 6.0% nickel dispersion and an average nickel crystallite size of ≈ 11 nm. A higher nickel dispersion and a smaller nickel crystallite size were obtained for the Ni-Mg/D catalyst sample (**Table 6**). Besides, among the studied catalysts Ni-Mg catalyst sample had the highest SBET surface area (**Table 4**). This indicates that the role of catalyst texture and dispersion of the active phase is critical in assessing the catalytic efficiency. In considering an explanation for the diminished hydrogenation activity of silver modified nickel catalyst it can be also assumed a physical blocking of nickel active sites or even changes in the morphology of nickel metal particles by the silver modifier. It is difficult to discriminate between these different possibilities. Apart from the effect of co-metal blocking of the surface nickel atoms, it should also be noted that an electronic effect has been taken into account [94]. It must be noted that the electronic properties of very small particles—nanoparticles should be different from those of large particles at least for two reasons. The first relates to the differences in the fraction of the total atoms that are present on the surface. The second is the incomplete coordination of the surface atoms from those in the bulk. Based on the assumption that catalytic activity of a metal is related to its electronic properties, it seems reasonable that the activity would vary with the crystallite size. However, a clearer understanding of the factors responsible for the crystallite size effects will require more information on the properties of nanoparticles.
