Two different TiO2/SiO2 compounds containing TiO2 nanodomains dispersed over SiO2 were investigated applying the AEIR method at the adsorption equilibrium of NH3 and H2O from 300 to 723 K, particularly for the measurement of the individual heats of adsorption of the different species on Lewis acidic sites (LAS) and Brønsted acidic sites (BAS) as evaluation of the strength of the sites. It revealed two types of NH3 adsorption sites: the first ones could correspond either to NH3 species H-bonded to free OH groups or to coordinated weak LAS (named L1). The second ones (L2) were attributed to strongest LAS similar to those present at the surface of TiO2 nanocrystallites. They also correspond to the stronger adsorption sites of H2O. Two types of Brønsted acid sites (BAS) were additionally evidenced by the AEIR method and proposed to be specifically located on the Si-O-Ti bridging bonds at the TiO2/SiO2 interface. The heats of adsorption of the different adsorbed species provided by the AEIR method were consistent with literature data on average values of the heats of adsorption of NH3 and H2O from microcalorimetry measurements. The surface acidity of the two compounds in the presence of H2O was determined using NH3-H2O coadsorption. At T ≥ 473 K, the NH3 species on the L2 sites were not significantly displaced from the surface whatever the partial pressure of H2O studied in agreement with the Temkin competitive model using the individual heats of adsorption of the NH3 and H2O species. This model also revealed the presence of a small amount of H2O species adsorbed on L2 sites allowing H2O dissociation or/and hydrolysis of SiOTi or TiOTi bridges, leading to the formation of a much higher amount of BAS. Therefore, this original work combining the AEIR method and the Temkin competitive model provided new insights for understanding water effects on acidic oxide catalysts.

Alkaline-earth phosphates efficient in the dehydration of lactic acid to acrylic acid were previously shown to contain a surface mono/dihydrogen phosphate amorphous layer composed of M2+ cations and both P=O and POH groups. In this work, acidic properties of such a layer were determined combining Fourier transform infrared (FTIR) spectra achieved at the dehydrated state and under water vapor and density functional theory (DFT) simulations of nondefective and defective MPOH structure. The FTIR spectra of adsorbed pyridine and lutidine revealed the presence of moderate Lewis acid sites (LAS) and of POH groups interacting by H-bonding without significant protonation. DFT calculations were key to interpret FTIR spectra after adsorption of NH3: when solely adsorbed, NH3 interacts with the LAS on both the nondefective surface and the defective surface, whereas the POH for which H points up toward the gas phase are reoriented downward. Bronsted acid sites (BAS) were shown to form under water vapor. This phenomenon was shown by DFT to arise from a more acidic character of H2PO4- species for the nondefective surface and casual formation of nondefective surface leading to higher amount of H2PO4- species, which are more acidic BAS.

The present study is dedicated to the characterization (identification, heats of adsorption, and coverages) of the adsorbed species formed by the adsorption and coadsorption of NH3 and H2O on two SiO2 solids. Adsorption equilibrium infrared spectroscopy allowed us (a) to show that NH3 and H2O are mostly adsorbed on free SiOH groups via H bonds and (b) to determine their individual heats of adsorption: 53 and 49 kJ/mol, whatever be their coverages (Langmuir adsorption model), for NH3ads and H2Oads, respectively. These values consistent with the microcalorimetry literature data explain that their coverages are decreased upon NH3-H2O coadsorption, considering a competitive Langmuir model. However, the temperature-programmed adsorption equilibrium procedure achieved from MS data indicated that a minor amount of other NH3 species (not detected using Fourier-transform infrared) is more strongly adsorbed and that hydrolysis of SiOSi siloxane by H2O could occur in parallel. NH3-H2O coadsorption leads to the formation of NH4+ species, which involves H2O adsorbed species. Both NH3 and H2O are not adsorbed above 450 K, which means that the SiO2 contribution to the characterization of the acidity of metal oxide catalysts supported on SiO2 using NH3 as the probe molecule in the presence of H2O is negligible above this temperature.

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOx to N-2 with NH3 in excess O-2 on V2O5/WO3/TiO2 catalysts (NH3-SCR process). In the temperature range of interest for NH3-SCR (T >=approximate to 473 K) and for three TiO2-based solids (sulfated and sulfate-free TiO2 supports and a sulfated 0.7% V2O5/9% WO3/TiO2 catalyst), FTIR spectroscopy and volumetric measurements with a mass spectrometer are used to study the impacts of the NH3H2O coadsorption on the coverages of (a) the molecularly adsorbed NH3 species and (b) the molecularly and dissociated H2O species on Lewis and Bronsted sites. Whatever the solid, it is shown that NH3 dominates the molecular coadsorption on the Lewis sites. However, this does not prevent the dissociative H2O chemisorption on a small amount of Lewis acidic sites, leading to an increase in the amount of OH groups. On the two sulfated solids, these OH groups increase the amount of adsorbed NH4+ species as compared to the NH3 adsorption equilibrium. For the sulfate-free TiO2 solid having weak Bronsted sites, the switch between the NH3 adsorption equilibrium to the NH3H2O coadsorption equilibrium is associated to the production of a small amount of NH3 due to the displacement of NH3ads-L species by H2O dissociation (competitive adsorption). It is shown that these experimental data are consistent with an original development of a competitive Temkin model (named Temkin-C), taking into account the individual heats of adsorption of NH3 and H2O species at different coverages in the absence of competition. The EMA and Temkin-C model developed in the present study can be applied to all solids having a significant IR transmission offering a method to study the surface acidity during realistic experimental conditions (in the presence of H2O), which is of interest for different catalytic processes such as NH3-SCR and alcohol dehydration.

The present study is a part of an experimental microkinetic approach of the selective reduction of NOx to N-2 with NH3 in excess of O-2 on V2O5/WO3/TiO2 catalysts (NH3-selective catalytic reduction (NH3-SCR) reaction). Water is always present either in the reactive gas mixtures representative of industrial processes or produced by the reaction. This suggests that H2O may modify the coverage of the pivotal adsorbed NH3 intermediate of the reaction by either a competitive adsorption or reactions (i.e., formation of NH4+). In the temperature range of interest for NH3-SCR (T >= approximate to 423 K), Fourier transform infrared spectroscopy and volumetric measurement using a mass spectrometer are used to study the impacts of the NH3-H2O coadsorption on the coverages of adsorbed NH3 (molecular adsorption) and H2O (molecular and dissociative adsorption) species on two sulfated solids: a 0.7% V2O5/9% WO3/TiO2 NH3-SCR catalyst and its TiO2 support. Regardless of the solid, it is shown that at the NH3-H2O coadsorption equilibrium, (a) NH3 dominates the adsorption on the Lewis sites (i.e., the introduction of NH3 at the H2O adsorption equilibrium displaces H2Oads-L species at the benefit of NH3ads-L species) and (b) the introduction of H2O at the NH3 adsorption equilibrium increases significantly the amount of adsorbed NH4+ species. This is ascribed to the H2O dissociation, which is operant on a small number of sites forming new Bronsted sites without a strong impact on the amount of Lewis sites. The surface composition of the solids has a limited impact on the coverages during the NH3-H2O coadsorption except on the fact that the NH4+ species is more stable on the NH3-SCR catalyst. In Part 6 of the present study (10.1021/acs.jpcc.8b05847), it is shown that the present experimental data are consistent with the mathematical formalism of a competitive Temkin model (named Temkin-C) developed without major approximations. The experimental procedure (present study) and the mathematical Temkin-C formalism (Part 6) can be applied for all solids having a significant IR transmission, thus offering a method to study the surface acidity during realistic experimental conditions (in the presence of H2O), which is of interest for different catalytic processes such as NH3-SCR and alcohol dehydration.

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOₓ to N₂ with NH₃ in excess O₂ on V₂O₅/WO₃/TiO₂ catalysts (NH₃-SCR process). In the temperature range of interest for NH₃-SCR (T ≥≈ 473 K) and for three TiO₂-based solids (sulfated and sulfate-free TiO₂ supports and a sulfated 0.7% V₂O₅/9% WO₃/TiO₂ catalyst), FTIR spectroscopy and volumetric measurements with a mass spectrometer are used to study the impacts of the NH₃–H₂O coadsorption on the coverages of (a) the molecularly adsorbed NH₃ species and (b) the molecularly and dissociated H₂O species on Lewis and Bronsted sites. Whatever the solid, it is shown that NH₃ dominates the molecular coadsorption on the Lewis sites. However, this does not prevent the dissociative H₂O chemisorption on a small amount of Lewis acidic sites, leading to an increase in the amount of OH groups. On the two sulfated solids, these OH groups increase the amount of adsorbed NH₄⁺ species as compared to the NH₃ adsorption equilibrium. For the sulfate-free TiO₂ solid having weak Bronsted sites, the switch between the NH₃ adsorption equilibrium to the NH₃–H₂O coadsorption equilibrium is associated to the production of a small amount of NH₃ due to the displacement of NH₃ₐdₛ₋L species by H₂O dissociation (competitive adsorption). It is shown that these experimental data are consistent with an original development of a competitive Temkin model (named Temkin-C), taking into account the individual heats of adsorption of NH₃ and H₂O species at different coverages in the absence of competition. The EMA and Temkin-C model developed in the present study can be applied to all solids having a significant IR transmission offering a method to study the surface acidity during realistic experimental conditions (in the presence of H₂O), which is of interest for different catalytic processes such as NH₃-SCR and alcohol dehydration.

The present study is a part of an experimental microkinetic approach of the selective reduction of NOₓ to N₂ with NH₃ in excess of O₂ on V₂O₅/WO₃/TiO₂ catalysts (NH₃-selective catalytic reduction (NH₃-SCR) reaction). Water is always present either in the reactive gas mixtures representative of industrial processes or produced by the reaction. This suggests that H₂O may modify the coverage of the pivotal adsorbed NH₃ intermediate of the reaction by either a competitive adsorption or reactions (i.e., formation of NH₄⁺). In the temperature range of interest for NH₃-SCR (T ≥ ≈423 K), Fourier transform infrared spectroscopy and volumetric measurement using a mass spectrometer are used to study the impacts of the NH₃-H₂O coadsorption on the coverages of adsorbed NH₃ (molecular adsorption) and H₂O (molecular and dissociative adsorption) species on two sulfated solids: a 0.7% V₂O₅/9% WO₃/TiO₂ NH₃-SCR catalyst and its TiO₂ support. Regardless of the solid, it is shown that at the NH₃-H₂O coadsorption equilibrium, (a) NH₃ dominates the adsorption on the Lewis sites (i.e., the introduction of NH₃ at the H₂O adsorption equilibrium displaces H₂Oₐdₛ₋L species at the benefit of NH₃ₐdₛ₋L species) and (b) the introduction of H₂O at the NH₃ adsorption equilibrium increases significantly the amount of adsorbed NH₄⁺ species. This is ascribed to the H₂O dissociation, which is operant on a small number of sites forming new Bronsted sites without a strong impact on the amount of Lewis sites. The surface composition of the solids has a limited impact on the coverages during the NH₃-H₂O coadsorption except on the fact that the NH₄⁺ species is more stable on the NH₃-SCR catalyst. In Part 6 of the present study (10.1021/acs.jpcc.8b05847), it is shown that the present experimental data are consistent with the mathematical formalism of a competitive Temkin model (named Temkin-C) developed without major approximations. The experimental procedure (present study) and the mathematical Temkin-C formalism (Part 6) can be applied for all solids having a significant IR transmission, thus offering a method to study the surface acidity during realistic experimental conditions (in the presence of H₂O), which is of interest for different catalytic processes such as NH₃-SCR and alcohol dehydration.

The present experimental microkinetic approach is dedicated to the interpretation using the Temkin formalism of the experimental evolutions of (a) the coverage of adsorbed CO and hydrogen species and (b) the rate of the CH4 production during the increase in the temperature for x% CO/H-2 gas mixtures (x =1, 10(-2) and 10(-3), P-T = 1 atm) on a reduced 2.9% Pt/Al2O3 catalyst (Pt dispersion D approximate to 0.26). FTIR spectroscopy (a) shows that three adsorbed CO species: linear, bridged and threefold coordinated CO species (denoted L, B and 3FC respectively) are formed at 300 K and (b) provides the evolution of the coverage of the dominant L CO species (IR band at 2080 cm(-1) at 300 K) for the three CO/H-2 gas mixtures in the 300-740 K temperature range. These data support the development of a rigorous (without approximations) competitive adsorption model associated with a reaction based on the Temkin formalism (denoted Temkin-C.R model). This model provides (a) the theoretical coverages of the L CO and hydrogen species and (b) the theoretical rate of the CH4 production during the CO/H-2 reaction as a function of the partial pressures of the reactants and the reaction temperature which are compared to the experimental data. This shows that the L CO species which dominates the CO adsorption is not the active adsorbed species at the beginning of the CH4 production (T approximate to 475 K for 1% CO/H-2). It is shown that the active species is the 3FC CO species which represents 1/7 of the total amount of adsorbed CO species. These conclusions are consistent which literature data on Pt particles indicating that a small number of sites/adsorbed CO species are involved at the beginning of the CO/H-2 reaction. Moreover, as a contribution to the debate on the paradox of kinetics on heterogeneous surfaces, the Temkin-C.R model is compared to models based on the Langmuir formalism mainly used in kinetic studies. This reveals the clear advantage of the Temkin-C.R model for the representation of experimental data in large ranges of experimental conditions. (C) 2017 Elsevier Inc. All rights reserved.

The understanding of water effects on solid acid catalysts is a key issue in developing efficient processes to produce olefins by bio-alcohols dehydration. In this work, the effects of water on TiO2/SiO2 catalysts for the gas phase conversion of isobutanol into linear olefins have been unraveled, using for the first time in situ acidity measurements achieved with a flowing NH3 probe and water vapor containing gas mixtures at adsorption equilibrium in the temperature range of the reaction. Such compounds, prepared by grafting titania onto mesoporous silica, contain well-dispersed TiO2 amorphous clusters anchored by Ti-O-Si linkages, leading to much higher catalytic activity than TiO2 and SiO2. They yielded only dehydration products, among which 30% were linear butenes. Furthermore, their activity was significantly improved by addition of water into the feed, whatever the contact time. Acidity measurements indicated that TiO2/SiO2 mixed oxides were mostly of Lewis type after activation at 450 degrees C. However, in situ FTIR acidity measurements showed that addition of H2O to NH3/He gas mixture has no influence on the number of Lewis sites, while weak Bronsted sites were formed on other sites. This formation, due to a shift of equilibrium depending on both the temperature and the H2O partial pressure, corresponds to the hydrolysis of Ti-O-Si bonds, generating OH acidic groups and enhancing catalytic activity under water vapor. The novel type of acidity measurements used in this work appear powerful and can be applied to other acidic heterogeneous catalysts. (C) 2016 Elsevier Inc. All rights reserved.

The present study is dedicated to the development of a Temkin model for competitive chemisorption (denoted Temkin-C) which can be applied in large range of experimental conditions (partial pressures and temperatures). It is based on experimental data from the adsorption/reaction of x% CO/H-2 gas mixtures (x = 1, 10(-2) and 10(-3), total pressure 1 atm.) on a reduced 2.9% Pt/Al2O3 catalyst for two platinum dispersions (D approximate to 0.6 and 0.26) in the temperature range 300-740 K. FTIR spectroscopy (a) shows that three adsorbed CO species: linear, bridged and threefold coordinated CO species (denoted L, B and 3FC respectively) are formed at 300 K and (b) provides the evolution of the coverage of the dominant L CO species (IR band at 2080 cm(-1) at 300 K) for the three CO/H-2 gas mixtures in the 300-740 K temperature range. These data support the rigorous development of a Temkin-C model providing the theoretical coverage of the L CO and hydrogen species in the absence of the CH4 production. The comparison of the theoretical and experimental evolutions of the coverage of the L CO species shows that (a) the L CO species dominates the competitive chemisorption with the hydrogen species and (b) the heats of adsorption of the two species are not significantly modified by their co-adsorption. Moreover, in line with the debate dedicated to the paradox of kinetics on heterogeneous surface (briefly the catalytic activity of a heterogeneous surface can be well represented by assuming an homogeneous surface), the Temkin-C model is compared to models based on the Langmuir formalism currently used in the literature dedicated to kinetic studies. This reveals the clear advantage of the Temkin-C model for the representation of experimental data in large range of experimental conditions. In Part 2, the Temkin-C model is extended by considering that the L CO species is the adsorbed intermediate species of the CH4 formation from the CO/H-2 reaction at T > 500 K. (C) 2017 Elsevier Inc. All rights reserved.

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x = 2 or 5 wt %) in the 300–713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as “B”), which are the dominant species on Pd0 particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as “L”). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3, in parallel to the formation of two strongly adsorbed B CO species, two ...

1. Introduction Environmental regulations on NOx emissions of mobile sources are becoming increasingly demanding. The NOx storage-reduction (NSR) catalytic technology is one of the solution lean-burn engines. NSR catalysts work in cyclic gas-composition conditions. During the first step (lean phase), NSR materials store emitted NOx as nitrates until the surface reaches the saturation threshold. A pulse of fuel post-injection then triggers the short second step (rich phase), during which the reducing conditions lead to nitrate decomposition and release as well as subsequent NOx reduction into N2 and surface regeneration. Ceria is an interesting material since it plays a double function first as a high specific surface area support [1] for platinum group metal (PGM) dispersion and secondly as a NOx trap at low temperature until 350°C. Below 200°C, the NOx storage capacity of ceria is higher than that of barium which is commonly used as a storing agent [2]. In addition, ceria can limit the thermal sintering of Pt nanoparticles [3]. This study aims to deeply characterize the impact of the nature of dopant (Zr and Pr) on the redox properties of ceria-based oxides. 2. Experimental/methodology Different compositions of commercial ceria-based oxides were provided by the Solvay Special Chem Company : CeO2, Ce0.49Zr0.51O2 (CZ)) and Ce0,80Pr0,20O2 (CP20). All samples have been calcined at 800°C for 2 h. In-situ X-Ray Diffraction measurements were carried out in an atmosphere-controlled Anton Paar XRK 900 reactor chamber, either under air, N2 or H2, using a Versatile Panalytical X’Pert Pro MPD Diffractometer equipped with a diffracted beam graphite monochromator (Cu Kα radiation) and a 1-dimensional multistrip detector (X’Celerator). Diffractograms were collected at several temperatures from 25 to 750°C and crystallographic parameters were determined by the Rietveld method. Surface and bulk reducibility was characterized by Temperature-Programmed Reduction (TPR) in H2 from room temperature up to 850°C. Oxygen-Temperature-Programmed Desorption (TPD) experiments were also performed to quantify the surface reactivity of the different oxides toward oxygen. Oxygen was adsorbed at 500°C while the desorption was followed in He from RT up to 800°C with an heating ramp of 10°C/min. In-situ Raman spectroscopy was used to determine structural/electronic defects and surface oxygen species of the samples after reducing/oxidizing treatments. Spectra were recorded with a LabRam HR Raman spectrometer (Horiba-Jobin Yvon) at low temperature after oxidation or reduction at 525°C. Three exciting wavelengths were used (514, 633 and 785 nm) and the laser spot was focused using a ×50 long working distance objective. The in-situ Raman spectra were recorded using a THMS600 cell (Linkam) between -196°C and 525°C under 10 % H2-N2, N2 and 10% O2-N2. 3. Results and discussion XRD patterns have evidenced the cubic structure (Fm-3m) for CeO2 and CP20 while CZ also contains a t’ tetragonal phase. The lattice parameters were extracted from in-situ patterns in air, N2 and H2 as a function of the temperature. The increase of the lattice parameter with respect to the one measured in air at a given temperature was used to estimate the bulk reducibility. Figure 1a clearly shows that CP20 is reduced in pure H2 from 300°C, instead of 400°C for CZ and above 700°C for CeO2. These results are in good agreement with TPR experiments which evidenced that the surface reduction of doped ceria is promoted by the insertion of Pr. By the same way during O2-TPD experiments, a low O2 desorption peak below 400°C was only observed on CP20. In addition, the capacity of the ceria oxide to chemisorb oxygen was strongly improved in the presence of Pr. Beyond confirming the XRD structural characterization, Raman mappings evidenced a high homogeneity at the micrometer scale. Furthermore, structural defects and oxygen species were observed with this technique. In particular, Raman spectra of CP20 oxide highlighted a defects band at 570 cm-1 (Figure 1B) which can be explained by a resonance effect involving a particular defect stabilized at the oxidized state. The reduction of cations contained in such defect modifies its electronic properties inhibiting the resonance effect. Concerning CZ support, a typical band of Ce3+ was visible after reduction whereas peroxo species were observed after re-oxydation contrarily to the other solids. Figure 1. (A) Variation of the fluorite lattice parameter as a function of temperature in pure H2 (B) Evidence of a new defects band enhanced by Raman Resonance effect for CP20. 4. Conclusion Structure, surface and bulk properties of three different ceria oxides have been deeply investigated in both oxidized and reduced states. The role of the dopants, i.e. Pr and Zr, on the redox properties in cycling lean/rich conditions encountered in NSR processes will be discussed. 5. References [1] Z. Say, E.I. Vovk, V.I. Bukhtiyarov, E. Ozensoy, Appl. Catal. B Environ. 142-143 (2013) 89–100. [2] E. Rohart, V. Belliere-Baca, K. Yokota, V. Harle, C. Pitois, Top. Catal. 42-43 (2007) 71–75. [3] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, J. Catal. 242 (2006) 103–109.

The effect of a reduction step on the catalytic activity of Pd supported doped ceria catalysts was investigated for the propane combustion. Two different oxides based on ceria zirconia (CZ), as a reference three-way catalyst support, and Gadolinia doped ceria (GDC), a mixed ionic electronic conductor, have been used to support Pd nanoparticles. The samples were characterized by H2–CO chemisorption, temperature programmed reduction, and XPS. In addition, the reduction step in H2 was in situ observed by environmental transmission electron microscopy. It was found that the catalytic activity of Pd supported on ceria-based supports can be strongly promoted by a reduction step whereas it does not change on alumina. This effect was attributed, in particular on GDC, to the reduction of a surface interaction phase, PdxCeO2−δ, which induces a re-dispersion of metallic Pd nanoparticles.

The present study is a part of an experimental microkinetic approach of the removal of NOx from coal-fired power plants by reduction with NH3 on V2O5/WO3/TiO2 catalysts (NH3-selective catalytic reduction, NH3-SCR). It is dedicated to the characterization of the heats of adsorption of molecularly adsorbed H2Oads species formed on sulfate-free and sulfated TiO2 supports. Water, which is always present during the NH3-SCR, may be in competition and/or react (formation of NH4+) with the adsorbed NH3 species controlling the coverage of the adsorbed intermediate species of the reaction. Mainly, an original experimental procedure named adsorption equilibrium infrared spectroscopy (AEIR) previously used for the adsorption of NH3 species on the same solids is adapted for the adsorption of H2O. At T-a = 300 K and for P-H2O <= 1 kPa, three main H2Oads species are formed (associated with a minor amount of dissociated H2O species) on the two TiO2 solids. The species are identified by the positions of their IR bands in the 3750-3000 cm(-1) range. Considering the decreasing order of stability, they are (a) coordinated to strong (L-2) and weak (L-1) Lewis sites and denoted H2Oads-L2 and H2Oads-L1, respectively, and (b) hydrogen bonded to the H2Oads-L species and on O2-/OH sites of the solids (denoted H2Owads). The three species have a common well-defined delta H2O IR band at a position in the range 1640-1610 cm(-1) according to the total coverage of the surface. According to the AEIR method, the evolution of the intensity of this IR band during the increase in the adsorption temperature Ta in isobaric condition provides the evolution of the average coverage of the three species and then to their individual heats of adsorption as a function of their coverage. It is shown that there are no significant differences on the two TiO2 solids. In particular, the heat of adsorption of the H2Oads-L2 species varies from similar to 114 to 61 kJ/mol at low and high coverages respectively, indicating that it can be present in the experimental conditions of the NH3-SCR. In a forthcoming article, the competitive chemisorptions and reaction between adsorbed H2O and NH3 species are studied and modeled on the TiO2 supports and model and commercial V2O5/WO3/TiO2 catalysts.

The adsorption of x% CO/He (x = 1 and 2) on a reduced 10 wt % Co/Al2O3 catalyst at an adsorption temperature of 560 K > T-a > 420 K leads to the formation of a linear CO species on Co degrees sites (denoted L-Co degrees species) with an infrared (IR) band at approximate to 2030 cm(-1) at full coverage. This adsorption is associated with the formation of carbon that is implicated in the reconstruction of the surface of the cobalt particles, leading to the progressive transformation of the L-Co degrees species into a new linear CO species on Co degrees C sites of the reconstructed surface with an IR band at approximate to 2060 cm(-1). An experimental microkinetic approach of the reconstruction process via the L-Co degrees -> L-Co degrees C transformation reveals the impact of different kinetic parameters such as the reaction temperature, the CO partial pressure, and the presence of hydrogen. During the reconstruction the coverages of the L-Co degrees and L-Co degrees C species remain very high (>0.95) due to their high heats of adsorption determined by the Adsorption Equilibrium Infra Red spectroscopy method. It is shown that the rate of the reconstruction process is controlled by that of the disappearance of the L-Co degrees species via its dissociation into adsorbed elemental carbon and oxygen species according to a pseudo first kinetic order elementary step involving a small amount of free cobalt sites (approximate to 4 X 10(12) sites/cm(2) of cobalt particles). The impact of the reaction temperature on the L-Co degrees -> L-Co degrees C transformation indicates that (a) the activation energy of the L-Co degrees dissociation which controls the reconstruction process is approximate to 125 kJ/mol and (b) the formation of elemental carbon via the disproportionation of the L-Co degrees species seems unlikely. These two conclusions are consistent with literature data on density functional theory (DFT) calculations. For adsorption temperature >560 K, the formation of superficial graphitic like species overlaps the reconstruction process. In the presence of hydrogen, the reconstruction is observed for H-2/CO ratios <3 but not for higher values (i.e., H-2/CO = 10).

IR spectra in transmission mode are used to measure the heats of adsorption at different coverages θ:E(θ), of two linearly adsorbed CO species formed on Co2+ and Co° sites (noted LCo2+ and LCo°) of reduced x% Co/Al2O3 (wt%, x≤10) catalysts according to the adsorption equilibrium infrared spectroscopy procedure developed previously. For LCo2+ species characterized by an IR band at 2151cm−1, ELCo2+(θ) varies linearly with θ from ELCo2+(1)=45kJ/mol to ELCo2+(0)=52kJ/mol. These values are modified by the presence of neither Co° particles nor carbon deposition (from CO disproportionation reaction) nor H2 using CO/H2 gas mixtures. During the CO adsorption at high temperatures (i.e. 538K), C deposition on the surface and in the bulk of the cobalt particles modifies the Co° adsorption sites. This leads to a transformation LCo° (IR band at 2020cm−1)→LCo°C (IR band at 2060cm−1) where LCo°C denotes a linear CO species formed on Co° sites modified by the C deposition. The heat of adsorption of the LCo°C species varies linearly with its coverage from ELCo°C(1)=93kJ/mol to ELCo°C(0)=165kJ/mol. In the presence of H2 with a ratio H2/CO=10, the C deposition is strongly decreased and the LCo° species dominates the surface of the cobalt particles. However, in these conditions its hydrogenation into CH4 disturbs its adsorption equilibrium in a large coverage range and only ELCo°(1)=108kJ/mol has been determined. For a ratio H2/CO=3, the carbon deposition cannot be prevented leading to the formation of the LCo°C species. It is shown that the ELCo°C(θ) values are not modified by the presence of adsorbed hydrogen. The heats of adsorption of the different adsorbed CO species on Co° sites are consistent with some literature data on DFT calculations and experimental values obtained on model cobalt surfaces (i.e. single crystals). In particular, they confirm DFT calculations which indicate that the C deposition on reduced cobalt particles decreases slightly the heats of adsorption of the linear CO species adsorbed on Co° sites.

IR spectra in diffuse reflectance mode are quantitatively exploited to determine the heats of adsorption of two linearly adsorbed CO species formed on Fe2+ and Fe° sites (denoted LFe2+ and LFe° CO species) of reduced x% Fe/Al2O3 (wt%, x=1 and 5) catalysts according to the AEIR procedure developed previously using the IR transmission mode. The IR transmission properties of iron containing catalysts are limited particularly for high iron loadings favoring the use of the diffuse reflectance mode. The heats of adsorption of the LFe2+ CO species linearly vary with the coverage of the sites from ELFe2+(1)=45kJ/mol to ELFe2+(0)=66kJ/mol at coverage 1 and 0, respectively. These values are modified by the presence of neither Fe° sites nor carbonaceous adsorbed species. The heats of adsorption of the LFe° CO species on a C-free iron surface (for adsorption temperature Ta<456K), linearly vary with its coverage from ELFe°(1)=79kJ/mol to ELFe°(0)=105kJ/mol at high and low coverages. For Ta>456K, the CO dissociation overlaps the CO adsorption equilibrium leading to a C-containing iron surface. The presence of carbonaceous species has no significant impact on the heat of adsorption of the LFe° species at high coverages whereas it increases significantly that at low coverages: ELFe°(0)=120kJ/mol. Considering reduced iron supported catalysts, the AEIR method is particularly useful because it allows the determination of the individual heats of adsorption of two adsorbed CO species that can be simultaneously present on the surface for high iron loadings. The study confirms that quantitative exploitations of DRIFT spectra are available for well designed experimental conditions offering an alternative for solid catalysts with low IR transmission properties.

A new synthesis pathway toward hydrogen peroxide has been investigated using non-thermal plasma. This work is aimed at studying the activation of oxygen/hydrogen mixtures by a cylindrical dielectric barrier discharge. An experimental device has been especially developed for this application, it mainly differs from other cylindrical discharges in that the liquid ground electrode, and subsequently the reactor, can be regulated in temperature. The formation of hydrogen peroxide is reported (1) in a gas phase discharge and (2) in surface discharge. The gas phase discharge, characterized by an empty discharge gap, lead to a low activation of O 2 into O 2 /H 2 mixtures and poor selectivity toward H 2 O 2 . The modification of the discharge into a surface discharge, by introducing in the gap fibrous materials, considerably improves the efficiency of the process. The influence of the temperature on H 2 O 2 formation is discussed and correlated to the formation of a water layer on fibre surface. This layer appears to be a crucial point into H 2 O 2 plasma synthesis. The presence of TiO 2 on the fibre surface is reported as improving the stabilisation of hydrogen peroxide. The formation of a complex between H 2 O 2 and TiO 2 is suggested and discussed. The formation of H 2 O 2 in the gas phase or in the aqueous condensed phase is finally discussed. The investigation of the influence of the reactant gas composition and the presence or not of water, lead to the conclusion that (1) both H 2 and O 2 are required to achieve the synthesis reaction; (2) H 2 O 2 is formed in the gas phase and then solubilised and/or stabilised in the water layer. A global reaction pathway is finally proposed to summarize the synthesis reaction.