**3.1. Catalysts characterization**

**Table 1** presents the Brunauer-Emmett-Teller (BET) surface area and micro-, supermicro-, meso- and macropore volumes (*S*BET, *V*micro, *V*sm, *V*meso and *V*macro, respectively) of the γ-Al2O3and AC supports. It can be observed that the activated carbon (AC) includes almost the same amounts of the four types of pores, with a large proportion of pore volume in the range of supermicro-, micro-, meso- and macropores, the so-called transport pores while γ-Al2O3 is a mesoporous solid having a poor contribution of supermicro-, micro- and macropores.


**Table 1.** BET surface area and pore volume distribution of the supports [16].

**Table 2** contains the catalyst notation used according to the precursor salt, metal loading or reduction temperature used during the catalysts preparation. Besides for all of the studied catalysts, dispersions (D), XPS results and activity values (turnover frequency (TOF)) calculated considering a kinetic of zero order are reported in **Table 2**.


**Table 2.** Catalysts naming convention, precursor salt, metal loading, reduction temperature, dispersion values, XPS BE of Pd 3d5/2, Cl/Pd or NPd atomic ratios and TOF values for all of the catalysts.

As it can be seen in **Table 2**, the dispersion values for Pd(5%)/Al catalyst decrease as the reduction temperature increases owing to the agglomeration of Pd particles. Also, at the same reduction temperature (573 K), palladium supported on AC presents slightly higher dispersion than when alumina is used as support possibly because of the high surface area of AC [21] or to the different porosity of the supports. Besides, a decrease in the metal loading, using the same precursor salt, produces a significant increase in the dispersion value as the palladium active sites are most exposed on the catalyst surface. On the other hand, at the same reduction temperature, the change of precursor salt produces a higher dispersion when PdCl2 is used because of complex oxychlorinated species formation during the calcination process. These species, Pdδ+OxCly, present a stronger interaction with the support than that showed by PdO, thus improving the metal dispersion [22].

Additionally, as nickel monometallic catalysts do not consume hydrogen during the chemisorption analysis, the dispersion value of bimetallic Pd-Ni/Al catalyst was calculated taking into account only the Palladium active sites. It can be noted that the dispersions of PdN(0.4%)/Al and Pd-Ni/Al catalysts are very similar, confirming that nickel in the bimetallic catalyst does not chemisorb H2 during the analysis. Last but not least, the dispersion values in **Table 2** are in total accordance with those reported by other authors [23].

In **Table 2**, Pd 3d5/2 BE and the Cl/Pd or N/Pd superficial atomic ratios obtained by XPS technique are also listed. Low-loaded monometallic catalysts, Pd(0.4%)/Al and PdN(0.4%)/Al, present the Pd 3d5/2 peak with a binding energy equal to 334.9 eV that corresponds to Pd° [24], whereas high-loaded monometallic catalysts, Pd(5%)/Al\_373, Pd(5%)/Al and Pd(5%)/AC, displayed peak values of BE at 337.0, 336.5 and 336.8 eV, respectively. These higher binding energy values indicate that the metal is electron-deficient (Pdδ+) possibly because of the presence of nonreduced Pd oxychloride species formed during the calcination process [22] or to non-reduced Pd species stabilized by neighbouring Cl atoms [23, 25]. The XPS BE results indicate that the reduction temperature influences the electronic state of palladium in Pd(5%)/Al catalysts: the one reduced at 373K has a peak at 337.0 eV, whereas that reduced at 573 K presents a peak shifted to 336.5 eV. Therefore, the low reduction temperature used generates a more electrondeficient Pd (337.0 eV) with a high Cl superficial content. On the other hand, high-loaded monometallic catalysts present Pd with a slightly higher electron deficiency (Pdδ+ species) and a higher concentration of superficial Cl when Pd is anchored on GF-45.

In the case of the Pd-Ni/Al bimetallic catalyst after the deconvolution of the Pd 3d5/2 BE, two signals can be seen at 334.2 eV (54% of all of the metal species, atomic basis) and 335.3 eV (46 at/at%) palladium species. These values suggest the presence of two type of Pd species in simultaneous, represented by Pdδ- (electron-rich species) and slightly electron-deficient palladium species (Pdδ+, with δ close to 0), respectively. The former could be attributed to the formation of metallic bonds or alloy, occurring at low temperatures [26, 27]. Additionally, for Pd-Ni/Al catalyst, the BE of Ni 2p3/2 peak appears at 856.4 eV, which is attributed to electrondeficient species (Nin+, with *n* close to 2) probably corresponding to different interactions between nickel and aluminium (from the support) [28], or to the formation of intermediate Pd-Ni-Al2O3 surface species [12].

The XPS spectra of high-loaded monometallic catalysts prepared from chlorine precursors show a peak at ca. 198.5 eV that corresponds to C1 2p3/2 The peak was associated to surface chloride species [24] that were not completely eliminated after reduction. Besides, neither Cl nor N was detected by XPS on the surface for the low-loaded monometallic catalysts.

The TPR profiles of palladium mono- and bimetallic catalysts are shown in **Figure 2**. In this figure, it can be seen that all the prepared catalysts present a main reduction peak at low temperatures, between 259 and 358 K, that can be attributed to the reduction of palladium oxidized species (PdOx) to Pd° [29]. Besides, the low-loaded catalysts present the reduction peak shifted to lower temperatures, indicating that Pd species are more easily reduced than those present on high-loaded catalysts. These shifts to higher temperatures are due to different types of the metal-support interactions.

when alumina is used as support possibly because of the high surface area of AC [21] or to the different porosity of the supports. Besides, a decrease in the metal loading, using the same precursor salt, produces a significant increase in the dispersion value as the palladium active sites are most exposed on the catalyst surface. On the other hand, at the same reduction temperature, the change of precursor salt produces a higher dispersion when PdCl2 is used because of complex oxychlorinated species formation during the calcination process. These species, Pdδ+OxCly, present a stronger interaction with the support than that showed by PdO,

Additionally, as nickel monometallic catalysts do not consume hydrogen during the chemisorption analysis, the dispersion value of bimetallic Pd-Ni/Al catalyst was calculated taking into account only the Palladium active sites. It can be noted that the dispersions of PdN(0.4%)/Al and Pd-Ni/Al catalysts are very similar, confirming that nickel in the bimetallic catalyst does not chemisorb H2 during the analysis. Last but not least, the dispersion values in **Table 2** are

In **Table 2**, Pd 3d5/2 BE and the Cl/Pd or N/Pd superficial atomic ratios obtained by XPS technique are also listed. Low-loaded monometallic catalysts, Pd(0.4%)/Al and PdN(0.4%)/Al, present the Pd 3d5/2 peak with a binding energy equal to 334.9 eV that corresponds to Pd° [24], whereas high-loaded monometallic catalysts, Pd(5%)/Al\_373, Pd(5%)/Al and Pd(5%)/AC, displayed peak values of BE at 337.0, 336.5 and 336.8 eV, respectively. These higher binding energy values indicate that the metal is electron-deficient (Pdδ+) possibly because of the presence of nonreduced Pd oxychloride species formed during the calcination process [22] or to non-reduced Pd species stabilized by neighbouring Cl atoms [23, 25]. The XPS BE results indicate that the reduction temperature influences the electronic state of palladium in Pd(5%)/Al catalysts: the one reduced at 373K has a peak at 337.0 eV, whereas that reduced at 573 K presents a peak shifted to 336.5 eV. Therefore, the low reduction temperature used generates a more electrondeficient Pd (337.0 eV) with a high Cl superficial content. On the other hand, high-loaded monometallic catalysts present Pd with a slightly higher electron deficiency (Pdδ+ species) and

In the case of the Pd-Ni/Al bimetallic catalyst after the deconvolution of the Pd 3d5/2 BE, two signals can be seen at 334.2 eV (54% of all of the metal species, atomic basis) and 335.3 eV (46 at/at%) palladium species. These values suggest the presence of two type of Pd species in simultaneous, represented by Pdδ- (electron-rich species) and slightly electron-deficient palladium species (Pdδ+, with δ close to 0), respectively. The former could be attributed to the formation of metallic bonds or alloy, occurring at low temperatures [26, 27]. Additionally, for Pd-Ni/Al catalyst, the BE of Ni 2p3/2 peak appears at 856.4 eV, which is attributed to electrondeficient species (Nin+, with *n* close to 2) probably corresponding to different interactions between nickel and aluminium (from the support) [28], or to the formation of intermediate Pd-

The XPS spectra of high-loaded monometallic catalysts prepared from chlorine precursors show a peak at ca. 198.5 eV that corresponds to C1 2p3/2 The peak was associated to surface chloride species [24] that were not completely eliminated after reduction. Besides, neither Cl

nor N was detected by XPS on the surface for the low-loaded monometallic catalysts.

thus improving the metal dispersion [22].

Ni-Al2O3 surface species [12].

in total accordance with those reported by other authors [23].

20 New Advances in Hydrogenation Processes - Fundamentals and Applications

a higher concentration of superficial Cl when Pd is anchored on GF-45.

For the low-loaded catalysts, Pd(0.4%)/Al, PdN(0.4%)/Al and Pd-Ni/Al, the profiles in **Figure 2** show an inverted peak between 335 and 339 K, which could be assigned to the decomposition of the β-PdH phase that is formed during the reduction of the PdOx particles at low temperatures [22, 29, 30].

The TPR profiles for the high-loaded catalysts prepared from PdCl2 salt (Pd(5%)/Al and Pd(5%)/AC) show a second broad peak between 400 and 600 K, which is attributed to the reduction of Pdδ+OxCly species [19, 22].

**Figure 2.** TPR profiles for Pd(5%)/Al, Pd(5%)/AC, Pd(0.4%)/Al, PdN(0.4%)/Al and Pd-Ni/Al.

The TPR profile of the bimetallic catalyst Pd-Ni/Al is also presented in **Figure 2**. Up to 500 K, the reduction profile is very similar to the monometallic PdN(0.4%)/Al catalyst, having its main reduction peak at 286 K corresponding to PdOx reduction. The decomposition of the β-PdH phase is also present at lower temperature, 307 K; the shift of this signal suggests that the decomposition of the β-PdH phase is more easily accomplished in the bimetallic catalyst. Furthermore, as shown in **Figure 2**, the bimetallic catalyst has a second peak at 621 K, which is attributed to the reduction of NiO species to Ni° [31–33]. It is well known that nickel monometallic catalysts prepared from nitrate salts are reduced at temperatures between 600 and 1000 K when the contact between NiO and alumina is intimate [33–35]. The patterns of reduction depend on the nature of the metal-support interactions, which can be modified by the calcination temperature employed during the preparation of the monometallic nickel catalysts [34]. Besides, a broad peak is also present in this profile with a maximum at 1000 K, which is attributed to the reduction of nickel aluminates, NiAl2O4, showing a strong metalsupport interaction [35, 36]. According to the calculated degree of reduction, determined by TPR, the bimetallic catalyst has a low percentage of reduced Ni (7%) and Pd (74%). This suggests the presence of strong Pd-Ni intermetallic interaction in the catalyst; however, the interaction of Pd and Ni with the support cannot be neglected.

When palladium-supported catalysts are used during the alkyne hydrogenations, the β-phase hydride acts as a hydrogen source that promote over hydrogenation to obtain mainly the alkane, decreasing the selectivity to the alkene. The disappearance of the β-PdH phase is very important because it could impact directly on the activity and selectivity [37]. These authors state that the disappearance of the β-PdH phase considerably decreases alkynes hydrogenation rate to alkanes, thus increasing the selectivity to alkenes formation. For the prepared monoand bimetallic catalysts, the palladium β-phase Pd hydride is not present as it is proved by the TPR profiles at the pretreatment reduction temperature adopted.

According to XPS and TPR characterizations, it can be concluded that after pretreatment Pd° is present in the low-loaded monometallic catalysts, while Pdδ+OxCly species are formed in the high-loaded Pd catalysts. On the other hand, on the bimetallic catalyst, two kinds of palladium species (Pdδ+, with δ close to 0, and Pdδ−) and Nin+ (with *n* close to 2) are present on the surface.
