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The procedure applied for the L species on Pt/Al2O3 is used to prove that there is a linear relationship between the IR band of the L species absorbed on reduced 4.7%Cu/Al2O3 solid and its amount on the surface. The conclusion confirms that quantitative exploitations of the FTIR spectra recorded after adsorption of CO can be reasonably reformed at least under adsorption equilibrium.

Abstract: Catalytic partial oxidation (CPO) of methane to syngas has received considerable interest recently as a way to utilize remote natural gas resources. Despite this interest, the mechanism of methane CPO is not completely understood. Investigation of the catalytic ignition of methane CPO can provide insight into the mechanism of CPO, particularly on the role of the chemical and physical state of the noble metal catalyst. In this work, ignition of methane CPO and methane activation on Rh/Al2O3 catalysts was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Different catalysts were studied in this research: fresh, aged, sintered and oxidized catalysts. The ignition temperature when the CH4 to O2 ratio was two is lowest on reduced fresh catalysts. The ignition temperature decreased with an increase in oxygen concentration in the reactant mixture, which is the opposite trend previously noted for Pt/Al2O3. To explain these results, methane activation on different catalysts was explored and the oxidation state of rhodium was characterized by using CO as a probe molecule. Methane activation was found to occur at different temperatures depending on the catalyst state. As the reactant mixture flowed over the catalyst and the temperature was raised towards the ignition temperature, the oxidation state of the catalyst changed and an oxidized rhodium state, Rh n+ (1≤ n ≤3), progressively formed. In addition, a greater amount of Rh n+ was found when the oxygen concentration in the feed was higher. From these results, it is hypothesized that ignition of methane CPO on Rh/Al2O3 is potentially related to the amount of the higher oxidation state Rh. Different Rh sites play different roles in the heating process during methane activation. Rh0 sites covered by O2 are responsible for C2 species formation at low temperatures while bare Rh0 sites are preferred for CO production. Rh n+ sites are also presumed active for CH4 dissociation. They are then progressively reduced into lower oxidation state of Rh under CH4/He environment with increasing temperature. [Copyright &y& Elsevier]

Abstract: The impact of Pt dispersion (denoted by D) of Pt/Al2O3 catalysts on the turnover frequency (TOF) of the CO/O2 reaction and on the elementary steps involved in the catalytic reaction is studied using transient experiments with either a mass or a FTIR spectrometer as a detector. Similar to literature data, it is observed that TOF decreases with an increase in D. The aim of the present study is to correlate this observation with the modifications of the surface elementary steps of two kinetic models, M1 and M2, developed previously for . The elementary steps considered are (1) adsorption of CO as a linear CO species (denoted by L and involved in models M1 and M2) and as a bridged CO species (denoted by B); (2) the oxidation by O2 of the adsorbed CO species; and (3) the reduction by CO of the strongly adsorbed oxygen species (denoted by Osads) involved in model M2. It is shown that D has no significant impact on the heat of adsorption of the L CO species and on the mechanism of the reduction of Osads species by CO. For oxidation of the L CO species by O2, it is shown that there is an induction period in the CO2 production for not observed for a lower dispersion. This is explained by considering that the rate of formation of the Pt sites, which adsorb O2 during oxidation, depends on D. These sites, which represent a small fraction of the Pt0 sites, are associated with the B CO species. It is shown that the heat of adsorption of the B CO species increases with an increase in D. It is suggested that it is the removal of the B CO species (by desorption and oxidation) that controls the induction duration. [Copyright &y& Elsevier]

This article is the final part of a study on the CO/O2 reaction over a 2.9% Pt/Al2O3 in line with the microkinetic approach of the heterogeneous gas–solid catalysis. Mainly, the kinetic parameters of each elementary step of two kinetic models (Models M1 and M2) determined previously are used to explain the evolution of the coverage of the adsorbed CO intermediate species (a strongly adsorbed linear CO species on Pt0, denoted by L) as well as the turnover frequency (TOF in s−1) during light-off tests (increase in the reaction temperature Tr) using 1% CO/x% O2/He gas mixtures with x⩽50. Model M1 involves a L-H elementary step between L CO species and a weakly adsorbed oxygen species (Owads). It is operative (a) whatever Tr in excess CO and (b) only at low Tr values in excess O2. Model M2 involves a L-H elementary step between a L CO and a strongly adsorbed oxygen species: Osads is operative at high Tr values in excess O2. It is shown that the switch for M1 to M2, at the ignition process, during the heating stage occurs for a high CO conversion (>60%) at a specific Tr value (denoted by Ti) depending on the oxygen partial pressure. Similar to the observations on Pt single crystals, it is shown that the ignition process is associated with a surface-phase transformation from a Pt surface mainly covered by L CO species (denoted by Pt–CO) to a Pt surface mainly covered by Osads (denoted by Pt–O). The Pt–CO → Pt–O transformation is due to an oxidative removal of the adsorbed L CO species into CO2 and not to a competitive chemisorption. The high CO conversion associated with the Pt–CO → Pt–O transformation indicates that mass-transfer processes contribute to the ignition process. During a cooling stage from Tr>Ti, the switch from M2 to M1 (extinction process) is associated with the surface-phase transformation Pt–O → Pt–CO at a reaction temperature Te. [Copyright &y& Elsevier]

Transient experiments using mass and FTIR spectroscopy as detectors are performed at 300 K with a reduced 2.9% Pt/Al2O3 catalyst to study the reduction of strongly adsorbed oxygen species (denoted Osads) formed by O2 chemisorption using several y% CO/z% Ar/He mixtures (y and z in the range 0.5–10). During the first seconds of the reaction C mass balances reveal that the CO consumption is mainly due to the formation of a strongly adsorbed CO species identified as a linear CO species (denoted L) interacting with the Osads species (IR band at 2084 cm−1). The evolution of the CO2 production rate with time on stream presents different profiles according to the reaction temperature: decreasing exponential at Tr<273 K and peak profiles for Tr&ges;300 K. The CO2 production at Tr<273 K is in agreement with a kinetic model considering two elementary steps: the adsorption of the L CO species without competition with Osads followed by a L–H elementary step (denoted S3b): Osads+L→CO2ads, with a rate constant k3b=ν3bexp(−E3b/RT) and E3b=65 kJ/mol at θOsads&ap;1. For Tr>300 K, mass transfer processes contribute to the apparent CO2 production rate. At high θOsads values, they compete with the surface reactions for 273 K K and finally dominate the CO2 production at Tr>360 K. However, kinetic studies can be performed at Tr>300 K after a significant decrease in θOsads due to the increase in E3b: E3b=110 kJ/mol at θOsads=0.4. Several conclusions of the present study are in very good agreement with the reduction of Osads species on Pt single crystals using a CO molecular beam under UHV conditions. [Copyright &y& Elsevier]

In previous studies, it has been shown that the heats of adsorption at several coverages of each adsorbed CO species (linear, bridged, and threefold coordinated) on a 2.9% Pt/Al2O3 catalyst can be obtained from the change in the IR band area of each adsorbed CO species with the adsorption temperature Ta (in the 300–800 K range) at a constant CO partial pressure Pa. In the present study, this procedure is used to reveal the eventual impact of the Pt dispersion (range 0.44–0.75) on the heats of adsorption of the linear CO species adsorbed on several Pt/Al2O3 catalysts. It is shown that the Pt dispersion has no strong impact on this parameter at several coverage values. [Copyright &y& Elsevier]

The adsorption of CO (1% CO/He mixture) at 300 K on a 2.9% Pt/Al2O3 catalyst leads to the detection of a strong IR band at 2075 cm−1 associated with weak and broad IR bands in the range 1900–1700 cm−1 ascribed to linear (denoted L) and multibound (bridged and threefold coordinated) CO species, respectively. In parts I and II of the present study it was shown that the L CO species is oxidized at T<350 K according to the surface elementary step denoted S3, L+Owads→CO2, where Owads is a weakly adsorbed oxygen species formed without competition with the L CO species by the dissociative chemisorption of O2. In the present study the coverage of the L CO species as well as the conversion of CO into CO2 are determined during the increase in the reaction temperature Tr from 300 to 740 K (lighting-off tests) using several 1% CO/x% O2/He mixtures, with x≤0.5. In an excess of CO (x<0.5), it is shown that the experimental curves θL=f(Tr) can be fitted by a kinetic model by considering that θL is determined by the equilibrium between the rates of adsorption, oxidation, and desorption of the L CO species. The parameters used in the kinetic model are those previously determined by studying (a) the adsorption equilibrium of the L CO species in the temperature range 300–740 K and (b) its oxidation by O2 at T<350 K (in the absence of CO). The evolution of the experimental turnover frequency (TOFex) during the increase in Tr is determined and compared to the theoretical TOFth for low CO conversions. It is shown that TOFex=f(Tr) is in very good agreement with a Langmuir–Hinshelwood mechanism via step S3, with the kinetic parameters used to fit the curves θL=f(Tr). [Copyright &y& Elsevier]

Experiments in the transient regime using a mass spectrometer as a detector are performed at T=300 K on a 2.9% Pt/Al2O3 catalyst to study the oxidation of the linearly adsorbed CO species (denoted L) with several x% O2/z% Ar/He mixtures (x and z in the range 0.5–4). CO and O2 adsorption measurements show that the L CO species and a strongly adsorbed oxygen species (denoted Osads) formed by the dissociative chemisorption of O2 are adsorbed on the same sites of the freshly reduced Pt particles. L CO and Osads species have high heats of adsorption: 115 and 175 kJ/mol, respectively, at full coverage of the Pt surface and do not desorb in helium at a temperature lower than 350 K. Moreover, it is shown that at 300 K a preadsorbed L CO species is not displaced by oxygen adsorption but is converted into CO2 by a weakly adsorbed oxygen species (denoted Owads) according to the elementary step (denoted S3): L+Owads→CO2 (rate constant, k3). This confirms the conclusion of a previous study performed using FTIR spectroscopy. O and C mass balances during the transient regime reveal (a) that during the first seconds of the transient the oxygen consumption is mainly due to the formation of the Owads species and that its adsorption equilibrium is rapidly attained and (b) that an Osads species is adsorbed for each L CO species removed by oxidation. This leads to a Pt surface where the coverage of the LCO species decreases while that of the Osads species increases with the duration of the oxidation. However, it is shown that the rate of the reaction (denoted S3a), L+Osads species, is significantly lower than that of step S3 and does not contribute to the CO2 formation in the presence of O2. A comparison is presented with the literature data on Pt single crystals (UHV studies) and supported Pt catalysts. [Copyright &y& Elsevier]