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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]

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]