**4. Probing interfacial reactions by ATR-FTIR investigations**

#### **4.1. Metal oxide-water interface**

Considering its relevance to semiconductor photocatalysis, water splitting, and other impor‐ tant applications, the interaction of water with metal oxide surfaces, especially TiO2, has been the focus of several experimental and theoretical investigations over the last decades [3, 4, 6, 8, 34–36]. Molecular, dissociated, and undissociated states of water adsorbed at a solid surface have been suggested. In addition to that, a mixture of these adsorption states is possible.

ATR-FTIR spectroscopy is one of the suitable techniques to investigate the adsorption of water molecules on a metal oxide surface under a wide range of conditions [37–39]. From many perspectives, numerous experimental and theoretical water adsorption studies have been conducted by means of ATR-FTIR spectroscopy [17, 37, 40, 41].

Figure 3 depicts the typical spectra of water adsorbed on TiO2 (anatase/rutile Evonik-Degussa Aeroxide TiO2 P25) [38]. The broad absorption band at around 3600–2800 cm-1 and the small peak at 3696 cm-1 are well-known to be the stretching vibration modes of the H2O molecules, which have complex interactions through hydrogen bonds, and the end part of polymerically chained H2O molecules without hydrogen bonds, respectively. The broad band contains not only the components of the H2O molecules with different numbers of hydrogen bonds but also the Fermi resonance attributed to the overtone absorption of the bending mode δ (H2O) at 1637 cm-1. Therefore, it is difficult to analyze the detailed adsorption state of the polymerically chained H2O molecules on metal oxide surfaces only from FTIR (mid-infrared) measurements [38]. However, based on the information obtained from such IR spectra, ATR-FTIR spectro‐ scopy has been used for the characterization and identification of intermediate mechanisms involved in environmental interfaces [42], mainly during photocatalytic oxidation processes induced at the TiO2-water interface [37,40].

Prior to coating the ATR crystal, a spectrum of the blank ATR crystal is collected for spectral processing. Mainly, two different approaches can be used for the spectral processing. The first one is the normalization of the spectra of the ligand to that of the matrix (solvent at the pH of interest in the liquid phase or dispersant in the gas phase) from which a spectrum is collected. The probe molecule is then introduced and the corresponding spectrum is collected. The spectrum of the probe molecule is then referenced to the background spectrum (solvent/ dispersant). The second approach is as follows: after preparing the thin film, a spectrum of the solvent at the pH of interest (or of the dispersant in the gas phase) is collected; the probe molecule is introduced and a spectrum is collected; the single beam spectrum of both solvent/ dispersant and of the probe molecule in the solvent/dispersant is referenced to the blank ATR crystal to obtain the absorbance spectra of each. Subsequently, the absorbance spectrum of the solvent/dispersant is subtracted from the spectrum of the probe molecule. To collect spectra for the probe molecule alone the same experimental process is used but without the nanopar‐

208 Emerging Pollutants in the Environment - Current and Further Implications

**4. Probing interfacial reactions by ATR-FTIR investigations**

conducted by means of ATR-FTIR spectroscopy [17, 37, 40, 41].

induced at the TiO2-water interface [37,40].

Considering its relevance to semiconductor photocatalysis, water splitting, and other impor‐ tant applications, the interaction of water with metal oxide surfaces, especially TiO2, has been the focus of several experimental and theoretical investigations over the last decades [3, 4, 6, 8, 34–36]. Molecular, dissociated, and undissociated states of water adsorbed at a solid surface have been suggested. In addition to that, a mixture of these adsorption states is possible.

ATR-FTIR spectroscopy is one of the suitable techniques to investigate the adsorption of water molecules on a metal oxide surface under a wide range of conditions [37–39]. From many perspectives, numerous experimental and theoretical water adsorption studies have been

Figure 3 depicts the typical spectra of water adsorbed on TiO2 (anatase/rutile Evonik-Degussa Aeroxide TiO2 P25) [38]. The broad absorption band at around 3600–2800 cm-1 and the small peak at 3696 cm-1 are well-known to be the stretching vibration modes of the H2O molecules, which have complex interactions through hydrogen bonds, and the end part of polymerically chained H2O molecules without hydrogen bonds, respectively. The broad band contains not only the components of the H2O molecules with different numbers of hydrogen bonds but also the Fermi resonance attributed to the overtone absorption of the bending mode δ (H2O) at 1637 cm-1. Therefore, it is difficult to analyze the detailed adsorption state of the polymerically chained H2O molecules on metal oxide surfaces only from FTIR (mid-infrared) measurements [38]. However, based on the information obtained from such IR spectra, ATR-FTIR spectro‐ scopy has been used for the characterization and identification of intermediate mechanisms involved in environmental interfaces [42], mainly during photocatalytic oxidation processes

ticle thin layer [30].

**4.1. Metal oxide-water interface**

**Figure 3.** FT-IR (MIR) absorption spectra of TiO2 (Evonik-Degussa Aeroxide TiO2 P25) in air (a) and after evacuation at room temperature for 1 h (b) (Reprinted with permission from Takeuchi M, Martra G, Coluccia S, Anpo M. Investiga‐ tions of the Structure of H2O Clusters Adsorbed on TiO2 Surfaces by Near-Infrared Absorption Spectroscopy. Journal of Physical Chemistry B; 109(15):7387–91. Copyright (2005) American Chemical Society).

Starting from the hypothesis that adsorbed H2O changes its conformation due to the coadsorption of cyclohexane on TiO2 (anatase, Sachtleben Hombikat UV100), Almeida et al. [40] have shown with the help of additional DFT (Density Functional Theory) calculations, yielding the adsorption energy and the structure of the water molecule at different hydration levels (Figure 4), that at least three layers of water are formed during the adsorption process. The first layer includes only chemisorbed H2O molecules. The second hydration level includes physisorbed (H-bonded) H2O molecules on surface OH sites, and the highest hydration level contains an additional adsorbed water layer. The dissociative chemisorption of water is assumed to be energetically favored. In addition to that, dissociative chemisorption of water generates at least two different Ti-OH groups. At least one of these two new OH sites contains an oxygen atom originally originating from the TiO2 lattice structure [40]. This finding allowed the authors to provide a spectral and structural interpretation of the mode of adsorption of cyclohexanone on the hydrated TiO2 surface [40].

Besides of that, several research reports have identified and specified the different bending modes and structures of water on the TiO2 surface during, before, and after UV light irradia‐ tion. It has been reported that UV irradiation induces a structural ordering of the adsorbed water layer [43], or results in an increase in the amount of surface OH groups, thus increasing the hydrophilicity of the TiO2 surface [44]. Mendive et al. [37] have revealed by ATR-FTIR studies that the disaggregation of particle agglomerates plays an important role in UV illuminated aqueous TiO2 nanoparticulate systems.

However, it should be noted here that the exact nature of the adsorption of water is still a matter of discussion in the field of metal oxide (especially of TiO2) photocatalysis. This is a

**Figure 4.** Adsorption energies and structures of H2O on TiO2 (100), (101), and (001) facets, at different hydration levels (Reprinted with permission from Almeida A, Calatayud M. Combined ATR-FTIR and DFT Study of Cyclohexanone Adsorption on Hydrated TiO2 Anatase Surfaces. Journal of Physical Chemistry C; 115(29):14164–14172. Copyright (2011) American Chemical Society).

consequence of the diverse possibilities of interpretation arising from the combination of experimental results obtained by ATR-FTIR spectroscopy and by other techniques. Obviously, there is not yet a general consensus on the mechanism of adsorption of water on TiO2.

#### **4.2. Interactions of probe molecules with the metal oxide surface**

ATR-FTIR spectroscopy yields important insight into the surface speciation of probe molecules adsorbed on nanomaterials [30]. Chemical or inner sphere adsorption is generally studied when it is expected that the probe molecule is able to coordinate with the metal ions of the substrate covering the ATR crystal [15].

Investigations of the interaction of a large number of ligands on metal oxide, metal hydroxide, and metal oxyhydroxide systems have been performed employing ATR-FTIR spectroscopy [26]. The objective of these investigations is to obtain an insight into the chemical nature of these interactions, being either qualitative such as the mode of adsorption and the surface speciation, or quantitative such as the kinetics and the surface coverage.

It is worth to note that TiO2 nanoparticles are much more extensively used as substrates as compared with other metal oxides. The adsorption of organic compounds bearing common functional groups such as acids [23, 45, 46], amino acids [47], phenolic compounds [11, 48], and a few complex heteroaromatic compounds [49–52] has been studied in detail (cf. Table 3). As an example, a typical ATR-FTIR spectrum of an aqueous solution of the herbicide imazapyr in the absence and presence of a TiO2 layer is presented in Figure 5. The reliability of infor‐ mation obtained from the IR spectra is dependent mainly upon the correct assignment of the vibrational modes by comparison with published spectroscopic data [15, 30]. Mudunkotuwa et al. have presented a summary of several common IR absorption band frequencies (Table 2) [30]. Furthermore, the infrared spectral data collected for coordination compounds [53] are very useful when interpreting the spectra of adsorbates, which mostly resemble those of ligands of coordination compounds [15]. In addition to that, the interpretation of the increase in the intensities of the bands of functional groups, as well as the shifting of these bands either to the blue or to the red spectral regions also provide important information concerning the type of interaction between adsorbate and surface. The interpretation of IR bands is very helpful for a qualitative analysis, e.g., concerning the points of interactions, the modes of adsorption, and the molecular speciation, respectively.

consequence of the diverse possibilities of interpretation arising from the combination of experimental results obtained by ATR-FTIR spectroscopy and by other techniques. Obviously, there is not yet a general consensus on the mechanism of adsorption of water on TiO2.

**Figure 4.** Adsorption energies and structures of H2O on TiO2 (100), (101), and (001) facets, at different hydration levels (Reprinted with permission from Almeida A, Calatayud M. Combined ATR-FTIR and DFT Study of Cyclohexanone Adsorption on Hydrated TiO2 Anatase Surfaces. Journal of Physical Chemistry C; 115(29):14164–14172. Copyright

ATR-FTIR spectroscopy yields important insight into the surface speciation of probe molecules adsorbed on nanomaterials [30]. Chemical or inner sphere adsorption is generally studied when it is expected that the probe molecule is able to coordinate with the metal ions of the

Investigations of the interaction of a large number of ligands on metal oxide, metal hydroxide, and metal oxyhydroxide systems have been performed employing ATR-FTIR spectroscopy [26]. The objective of these investigations is to obtain an insight into the chemical nature of these interactions, being either qualitative such as the mode of adsorption and the surface

It is worth to note that TiO2 nanoparticles are much more extensively used as substrates as compared with other metal oxides. The adsorption of organic compounds bearing common functional groups such as acids [23, 45, 46], amino acids [47], phenolic compounds [11, 48], and a few complex heteroaromatic compounds [49–52] has been studied in detail (cf. Table 3).

**4.2. Interactions of probe molecules with the metal oxide surface**

210 Emerging Pollutants in the Environment - Current and Further Implications

speciation, or quantitative such as the kinetics and the surface coverage.

substrate covering the ATR crystal [15].

(2011) American Chemical Society).

**Figure 5.** ATR-FTIR spectra of 8×10-3 mol L-1 imazapyr aqueous solution at pH 3 (dashed lines); and 2×10-3 mol L-1 imazapyr aqueous solution in contact with a TiO2 film (solid lines). Reference spectra were of water in contact with the bare ZnSe prism and of the bare TiO2 film respectively [54].

As mentioned above, the complexity of the obtained IR spectra usually requires the combina‐ tion of different techniques to enable their interpretation. Generally, the deductions resulting from the analysis of the IR spectra have to be supported by the results of other experimental techniques and/or by theoretical calculations. Several experimental and theoretical studies on the adsorption of aliphatic mono- and di-carboxylic acids on metal oxide surfaces have been performed [46]. It is assumed that the binding of carboxylates at the solid metal oxide surface occurs in several ways such as physisorption through electrostatic attraction and hydrogen bonding, and chemisorption in different modes including monodentate, bridged bidentate, and chelating bidentate adsorbed structures [55–57]. These different binding modes can be


**Table 2.** IR absorption frequencies of common organic functional groups (Adapted from [30] with permission of The Royal Society of Chemistry).

distinguished in an infrared spectrum by the difference Δνa-s of the frequencies of the asym‐ metric and the symmetric mode of the carboxylate stretching vibration. By comparing the Δνa-s of free aqueous carboxylate, Δνa-s (free), to the Δνa-s (adsorbed) values measured in transition metal complexes, the following correlations were found [46, 56, 58]:

Δνa-s (adsorbed) > Δνa-s (free): monodentate coordination

Δνa-s (adsorbed) < Δνa-s (free): bidentate chelating or bridging

Δνa-s (adsorbed) << Δνa-s (free): bidentate chelating, unless short metal-metal bonds are present

DFT calculations have been performed by Vittadini et al. for several possible adsorption conformations of formic acid and sodium formate on the anatase surface to support the interpretation of ATR-FTIR spectra measured of formic acid adsorbed on the TiO2 surface [59]. The comparison of the calculated results with this experimental information enabled the identification of seven different surface species (see Figure 6). On the hydrated surface, both HCOOH and HCOONa preferentially form inner-sphere adsorption complexes. HCOOH as monodentate adsorbate dissociates due to the interaction with a nearby water molecule, while HCOONa prefers a bridging bidentate structure [59].

Mono-carboxylic acids, i.e., formic and acetic acid, were found to bind on ZrO2 and Ta2O5 surfaces in both protonated and deprotonated carboxylic acid forms indicating a bridging bidentate adsorption. Under the experimental conditions of this work no adsorption of formic acid onto TiO2 and Al2O3 was observed [46].

**Figure 6.** Possible configurations for HCOOH and HCOO species bound to metal cations (Reprinted with permission from Vittadini A, Selloni A, Rotzinger FP, Grätzel M. Formic Acid Adsorption on Dry and Hydrated TiO2 Anatase (101) Surfaces by DFT Calculations. Journal of Physical Chemistry B; 104(101):1300–1306. Copyright (2000) American Chemical Society).

distinguished in an infrared spectrum by the difference Δνa-s of the frequencies of the asym‐ metric and the symmetric mode of the carboxylate stretching vibration. By comparing the Δνa-s of free aqueous carboxylate, Δνa-s (free), to the Δνa-s (adsorbed) values measured in transition

**Table 2.** IR absorption frequencies of common organic functional groups (Adapted from [30] with permission of The

Δνa-s (adsorbed) << Δνa-s (free): bidentate chelating, unless short metal-metal bonds are present

DFT calculations have been performed by Vittadini et al. for several possible adsorption conformations of formic acid and sodium formate on the anatase surface to support the interpretation of ATR-FTIR spectra measured of formic acid adsorbed on the TiO2 surface [59]. The comparison of the calculated results with this experimental information enabled the identification of seven different surface species (see Figure 6). On the hydrated surface, both HCOOH and HCOONa preferentially form inner-sphere adsorption complexes. HCOOH as monodentate adsorbate dissociates due to the interaction with a nearby water molecule, while

Mono-carboxylic acids, i.e., formic and acetic acid, were found to bind on ZrO2 and Ta2O5 surfaces in both protonated and deprotonated carboxylic acid forms indicating a bridging bidentate adsorption. Under the experimental conditions of this work no adsorption of formic

metal complexes, the following correlations were found [46, 56, 58]:

**Vibrational Mode Wavenumber (cm-1)**

212 Emerging Pollutants in the Environment - Current and Further Implications

1730–1720 1620–1590 1410–1390 1630 1571 2920 1442–1438 1400–1200 730–720 3300–3100

1700–1600 (Amide I) 1580–1510 (Amide II) 1400–1200 (Amide III)

ν(C=O) νasym(COO- )

νsym(COO- )

δasym(NH3 <sup>+</sup> )

δsym(NH3 <sup>+</sup> )

νasym(CH2) νsc(CH2) νw(CH2) νr(CH2) νsym(NH)

νsym(C=O)major + νsym(C-N)minor νsym(C-N) + δ (N-H)out of phase νsym(C-N) + δ (N-H)in phase

Royal Society of Chemistry).

Δνa-s (adsorbed) > Δνa-s (free): monodentate coordination

HCOONa prefers a bridging bidentate structure [59].

acid onto TiO2 and Al2O3 was observed [46].

Δνa-s (adsorbed) < Δνa-s (free): bidentate chelating or bridging

Dicarboxylic acids adsorb much more strongly to oxide surfaces than mono-carboxylic acids due to both electrostatic and chemical interactions.

Oxalic acid is one of the most investigated molecules in this regard [18, 45, 58, 60, 61]. Based on a series of spectra recorded at varying different experimental parameters (concentration, pH, and ionic strength), and supported by the comparison of these spectra with those of the aqueous [Fe(Ox)y]z complex, Hug et al. [18] described several surface complexes formed during the adsorption of oxalic acid at the TiO2 P25 surface. The obtained data strongly support the assumption that oxalate forms specific inner-sphere coordination complexes with surface Ti4+ sites. These complexes are formed through bidentate bridging or monodentate bending modes.

Mendive et al. have published several papers presenting experimental results of their inves‐ tigation of the TiO2-oxalic acid system using both pure anatase and rutile phases. In addition to that, data of quantum chemical calculations using Modified Symmetrically Orthogonalized Intermediate Neglect of Differential Overlap (MSINDO) have been presented to yield a complete insight into the TiO2-oxalate system [23, 24, 62, 63]. A detailed analysis of the experimental ATR-FTIR data and the data obtained from theoretical calculations (IR spectra (Figure 7) and calculated bending energies) has led to the suggestion of different adsorbate structures of oxalic acid either on anatase or on rutile nanoparticles (Figure 8). By comparison between both TiO2 phases (anatase and rutile), the difference as well as the similarity in the adsorption of oxalate can be explained either by the mode of adsorption, the structure of the surface complexes, the surface speciation of either TiO2 phases, or the adsorption energies.

Young et al. [45] have published the results of an ATR-FTIR study focused on the adsorptiondesorption kinetics of oxalic acid on the anatase TiO2 surface. The measured spectra were not found to be well resolved. However, based on the absorbance versus time behavior, the authors were able to extract the pseudo-first-order rate constants corresponding to the three expected adsorbed species of oxalic acid at the TiO2 surface.

Furthermore, Mendive et al. [61] have proposed the mechanism of the photocatalytic degra‐ dation of oxalic acid with the help of the above mentioned experimental and theoretical investigations [24, 63]. The possible pathways for the formation of oxalic acid photoproducts, as well as the role of the TiO2 surface as active surface have been discussed in detail [61]. An example of the proposed degradation pathways of the oxalic acid surface complexes is depicted in Figure 9.

**Figure 7.** Experimental and calculated FTIR spectra of oxalic acid on anatase (Reproduced from [23] with permission of the PCCP Owner Societies).

Intermediate Neglect of Differential Overlap (MSINDO) have been presented to yield a complete insight into the TiO2-oxalate system [23, 24, 62, 63]. A detailed analysis of the experimental ATR-FTIR data and the data obtained from theoretical calculations (IR spectra (Figure 7) and calculated bending energies) has led to the suggestion of different adsorbate structures of oxalic acid either on anatase or on rutile nanoparticles (Figure 8). By comparison between both TiO2 phases (anatase and rutile), the difference as well as the similarity in the adsorption of oxalate can be explained either by the mode of adsorption, the structure of the surface complexes, the surface speciation of either TiO2 phases, or the adsorption energies.

Young et al. [45] have published the results of an ATR-FTIR study focused on the adsorptiondesorption kinetics of oxalic acid on the anatase TiO2 surface. The measured spectra were not found to be well resolved. However, based on the absorbance versus time behavior, the authors were able to extract the pseudo-first-order rate constants corresponding to the three expected

Furthermore, Mendive et al. [61] have proposed the mechanism of the photocatalytic degra‐ dation of oxalic acid with the help of the above mentioned experimental and theoretical investigations [24, 63]. The possible pathways for the formation of oxalic acid photoproducts, as well as the role of the TiO2 surface as active surface have been discussed in detail [61]. An example of the proposed degradation pathways of the oxalic acid surface complexes is

**Figure 7.** Experimental and calculated FTIR spectra of oxalic acid on anatase (Reproduced from [23] with permission of

adsorbed species of oxalic acid at the TiO2 surface.

214 Emerging Pollutants in the Environment - Current and Further Implications

depicted in Figure 9.

the PCCP Owner Societies).

**Figure 8.** Adsorbed structures of oxalic acid on anatase (A) (Reproduced from [24] with permission of the PCCP Own‐ er Societies) and Rutile (B) (Reproduced from [63] with permission of the PCCP owner Societies) in equilibrium in the dark. A scheme of every structure is provided. Ti, O, H and C atoms are represented by large light, dark, small light and dark-dashed spheres respectively.

As mentioned in the introduction of this chapter, the direct evidence for the formation of structurally different surface complexes is an important step in the understanding of metal oxide photocatalysis, especially of TiO2 photocatalysis. This is due to the fact that the reactivity and the pathways for product formation are determined by the structures of the formed surface species during the dark adsorption. Recently, Montoya et al. [11] have investigated the interaction of the TiO2 surface with three probe molecules, *e.g.*, formic acid, benzene, and phenol employing ATR-FTIR spectroscopy. Based upon the analysis of the IR spectra (Figure 10), assumptions have been made concerning the physisorption of benzene (no changes have been observed in the spectra with and without the TiO2 layer), the strong chemisorption of formic acid, and also the role of the solvent (water or acetonitrile) for the adsorption mode of phenol. Based on these results, the authors provided an insight into the mode of interaction of the probe molecules with the TiO2 surface (chemisorption or physisorption) (Figure 11). In addition to that, they discussed the photocatalytic oxidation mechanism induced either by the

**Figure 9.** Possible photocatalytic degradation pathways of species adsorbed on anatase (Reprinted from Oxalic Acid at the TiO2/Water Interface under UV(A) Illumination: Surface Reaction Mechanisms, Cecilia B. Mendive, Thomas Bred‐ ow, Jenny Schneider, Miguel Blesa, Detlef Bahnemann. Journal of Catalysis 2015, 322:60-72, Copyright (2015), with per‐ mission from Elsevier).

reaction of surface trapped holes with the adsorbate (direct pathway) or the reaction of photocatalytically generated ˙OH radicals with the physisorbed molecules (indirect pathway). The authors concluded that formic acid is directly oxidized due to its strong chemisorption onto the TiO2 surface, while physisorbed benzene is indirectly oxidized. For phenol the authors suggested a combination of both pathways [10, 11].

a) 100 mM Phenol in H2O

b) 1 Min. c) 40 Min. d) 60 Min.

Figure 10. ATR-FTIR spectra of: A) a 100 mM solution of acetonitrile dissolved phenol, in the absence (a) and in the presence of an anatase film under different TiO2-Phenol contact times (b-d); B) a 100 mM solution of water dissolved phenol (pH 3) in the absence (a) and in the presence of a TiO2 anatase film, under different TiO2-Phenol contact times (b-d); C) a 3365 mM solution of acetonitrile dissolved benzene in absence (a) and presence of a TiO<sup>2</sup> anatase film under different TiO2-Benzene contact times (b-c) (Reprinted with permission from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimental Validation. Journal of Physical Chemistry C; 28(118):14276–

1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 Wavenumber (cm-1)

1377

1500 1450 1400 1350 1300 1250 1200 1150 1100 Wavenumber (cm-1)

> b) 1 Min. c) 60 Min.

1269

a b

a) 3365 mM Benzene in ACN

1192

1242

c d

14290. Copyright (2014) American Chemical Society).





0,03

0,08

Absorbance

0,13

0,18

0,23

0,28

C)

1543

0

a

1377

1481

b

a

1396

c

d

1477

1501

B)

0,01 0,02 0,03 0,04 0,05 0,06 0,07

Absorbance

1169 1153

1165

Wavenumber (cm-1)

1366

1362

b

1269

a) 100 mM Phenol in ACN

b) 1 Min. c) 40 Min. d) 60 Min. c d

1223

1223

a

1261

reaction of surface trapped holes with the adsorbate (direct pathway) or the reaction of photocatalytically generated ˙OH radicals with the physisorbed molecules (indirect pathway). The authors concluded that formic acid is directly oxidized due to its strong chemisorption onto the TiO2 surface, while physisorbed benzene is indirectly oxidized. For phenol the authors

**Figure 9.** Possible photocatalytic degradation pathways of species adsorbed on anatase (Reprinted from Oxalic Acid at the TiO2/Water Interface under UV(A) Illumination: Surface Reaction Mechanisms, Cecilia B. Mendive, Thomas Bred‐ ow, Jenny Schneider, Miguel Blesa, Detlef Bahnemann. Journal of Catalysis 2015, 322:60-72, Copyright (2015), with per‐

a) 100 mM Phenol in ACN

1366

a) 100 mM Phenol in H2O

b) 1 Min. c) 40 Min. d) 60 Min.

1362

b) 1 Min. c) 40 Min. d) 60 Min.

1500 1450 1400 1350 1300 1250 1200 1150 1100 Wavenumber (cm-1)

1500 1450 1400 1350 1300 1250 1200 1150 1100 Wavenumber (cm-1)

> b) 1 Min. c) 60 Min.

1269

b

1269

c d

1223

1223

1165

1169 1153

1192

a

a b

a) 3365 mM Benzene in ACN

1242

c d

1261

Figure 10. ATR-FTIR spectra of: A) a 100 mM solution of acetonitrile dissolved phenol, in the absence (a) and in the presence of an anatase film under different TiO2-Phenol contact times (b-d); B) a 100 mM solution of water dissolved phenol (pH 3) in the absence (a) and in the presence of a TiO2 anatase film, under different TiO2-Phenol contact times (b-d); C) a 3365 mM solution of acetonitrile dissolved benzene in absence (a) and presence of a TiO<sup>2</sup> anatase film under different TiO2-Benzene contact times (b-c) (Reprinted with permission from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimental Validation. Journal of Physical Chemistry C; 28(118):14276–

1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 Wavenumber (cm-1)

1377

14290. Copyright (2014) American Chemical Society).

suggested a combination of both pathways [10, 11].

216 Emerging Pollutants in the Environment - Current and Further Implications

A) 1485






0,03

0,08

Absorbance

0,13

0,18

0,23

0,28

C)

1543

0

0,01 0,02 0,03 0,04 0,05 0,06 0,07

Absorbance

0,015

a

1477

1501

B)

d

1474

a

1377

1481

b

a

1396

c

d

1501

0,035

0,055

0,075

Absorbance

mission from Elsevier).

0,095

0,115

0,135

Figure 10. ATR-FTIR spectra of: A) a 100 mM solution of acetonitrile dissolved phenol, in the absence (a) and in the presence of an anatase film under different TiO2-Phenol contact times (b-d); B) a 100 mM solution of water dissolved phenol (pH 3) in the absence (a) and in **Figure 10.** ATR-FTIR spectra of: A) a 100 mM solution of acetonitrile dissolved phenol, in the absence (a) and in the presence of an anatase film under different TiO2-Phenol contact times (b-d); B) a 100 mM solution of water dissolved phenol (pH 3) in the absence (a) and in the presence of a TiO2 anatase film, under different TiO2-Phenol contact times (b-d); C) a 3365 mM solution of acetonitrile dissolved benzene in absence (a) and presence of a TiO2 anatase film under different TiO2-Benzene contact times (b-c) (Reprinted with permission from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimental Validation. Journal of Physical Chemistry C; 28(118):14276–14290. Copyright (2014) American Chemical Society).

the presence of a TiO2 anatase film, under different TiO2-Phenol contact times (b-d); C) a 3365 mM solution of acetonitrile dissolved benzene in absence (a) and presence of a TiO<sup>2</sup> anatase film under different TiO2-Benzene contact times (b-c) (Reprinted with permission In addition to the presented examples, several experimental and theoretical studies have been performed concerning the adsorption and various photocatalytic reactions by means of the ATR-FTIR technique. Table 3 presents a survey of published data on the adsorption as well as

from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimental Validation. Journal of Physical Chemistry C; 28(118):14276–

14290. Copyright (2014) American Chemical Society).


0,015

a

d

1474

1501

A) 1485

0,035

0,055

0,075

Absorbance

0,095

0,115

0,135

**Figure 11.** Interaction modes of benzene, formic acid and phenol, model organic compounds with the TiO2 surface (Re‐ printed with permission from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimen‐ tal Validation. Journal of Physical Chemistry C; 28(118):14276–90. Copyright (2014) American Chemical Society).

the photooxidation of aqueous organic compounds on metal oxide surfaces studied by means of ATR-FTIR spectroscopy.


The Relevance of ATR-FTIR Spectroscopy in Semiconductor Photocatalysis http://dx.doi.org/10.5772/60887 219


the photooxidation of aqueous organic compounds on metal oxide surfaces studied by means

**Figure 11.** Interaction modes of benzene, formic acid and phenol, model organic compounds with the TiO2 surface (Re‐ printed with permission from Montoya JF, Atitar FM, Bahnemann DW, Peral J, Salvador P. Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct-Indirect (DI) Model: II) Experimen‐ tal Validation. Journal of Physical Chemistry C; 28(118):14276–90. Copyright (2014) American Chemical Society).

Photo-Oxidation [69]

[75] [76]

[48]

Adsorption

**Liquid Phase Solid (TiO2)**

**Liquid Phase Solid (TiO2)**

Ti Ti O

Ti Ti O

> Adsorption Adsorption Adsorption Adsorption

Adsorbate or reactant Material Study Ref Acetic acid Rutile Adsorption [64] Acetate Rutile Adsorption [65] Acrylic acid P25 Adsorption [66] Poly(Acrylic acid) Hematite Adsorption [67] L-α-alanine P25 Adsorption [68]

**Solid (TiO2) Liquid Phase**

**c) d)**

**a) b)**

Ti Ti O

H HO

**O**

Ti Ti O O C H

218 Emerging Pollutants in the Environment - Current and Further Implications

p-Arsanilic acid Iron-(Oxyhydr)Oxides Adsorption [70],[71] Aspartic acid TiO2 (synthesis) Adsorption [72] Benzoic acid / benzoate Aluminum Hydroxide Adsorption [73] Boric acid Hydrous Ferric Oxide Adsorption [74]

Au/TiO2

Catechol P25 Photo-Oxidation

Cr2O3 MnO2 Fe2O3

TiO2 (synthesis)

4-Chlorocatechol P25 Adsorption [77] Citric acid Rutile Adsorption [64]

of ATR-FTIR spectroscopy.

Amino acid P25


**Table 3.** Selection of previously published ATR-FTIR studies concerning the adsorption and photooxidation of common ligands on metal oxides surfaces.
