Transient kinetics of carbon monoxide oxidation by oxygen over supported palladium/ceria/zirconia three-way catalysts in the absence and presence of water and carbon dioxide

The transient kinetics of CO oxidation by O2 over alumina-supported Pd/CeO2/ZrO2 three-way catalysts is described in the absence and presence of H2O and CO2 in the feed. Experiments were carried out in a laboratory fixed-bed reactor at 573 K, while periodically switching between a feed of 1 vol% CO in He and a feed of 0.5 vol% O2 in He with a frequency of 1/30 Hz. Separate experiments were performed with 14 vol% water in both feeds, and, as is the case with real engine exhaust gas, with 14 vol% water and 14 vol% CO2 in both feeds. The presence of water largely enhanced the reaction rate, while it was inhibited by CO2. A transient kinetic model has been developed for this catalyst, based on the experimental data. It was found that the reaction in the absence of H2O and CO2 proceeds via the same elementary steps as reported for a Pt/Rh/CeO2/¿-Al2O3 catalyst (R.H. Nibbelke, A.J.L. Nievergeld, J.H.B.J. Hoebink, G.B. Marin, Appl. Catal. B 19 (1998) 245). Only the rate coefficient for CO desorption from Pd was significantly lower, in line with literature data. Additional elementary steps were combined with the above model in order to describe quantitatively the effect of water. The present study also shows that bulk diffusion of oxygen in ceria plays a major role, when the rate is enhanced by water. The inhibition effect of CO2 could be described by a lower amount of oxygen-storage sites

Kinetics of the steady-state acetylene oxidation by oxygen over a Pt/Rh/CeO\u3csub\u3e2\u3c/sub\u3e/γ-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e three-way catalyst

\u3cp\u3eThe steady-state kinetics of acetylene oxidation has been studied in the framework of automotive exhaust gas catalysis over a commercially available three-way catalyst. Experiments under cold-start conditions have been carried out in a laboratory fixed-bed reactor, which can adequately be described by the developed elementary step model and rate parameters.\u3c/p\u3

An investigation of the oxygen pathways in the oxidative coupling of methane over MgO based catalysts

The oxidative coupling of methane to ethane and ethene has been investigated by admitting pulses of pure methane, pure oxygen, and mixtures of methane and oxygen to MgO, Li/MgO, and Sn/Li/MgO at temperatures ranging from 923 to 1073 K in a Temporal Analysis of Products (TAP) set-up. Moreover, pulses of oxygen followed by pulses of either methane, ethane, ethene, or carbon monoxide were applied to study the role of both adsorbed oxygen and surface lattice oxygen in the reaction mechanism. Two types of reversibly adsorbed oxygen are present on Sn/Li/MgO. The first type is strongly adsorbed oxygen, which desorbs from the surface on a time scale of 3 min at 973 K. This type of oxygen does not seem to be reactive toward methane. The second type of oxygen consists of weakly adsorbed oxygen species with a time scale of desorption amounting to 4 s at 973 K. The weakly adsorbed oxygen species are involved in the direct conversion of methane to carbon dioxide. Surface lattice oxygen is also interacting with the admitted reductants. The percentage of surface lattice oxygen reactive in the methane conversion is less than 0.1% of a theoretical monolayer on MgO at 1023 K. This value amounts to 27% for Li/MgO and 44% for Sn/Li/MgO at the same temperature. On Li/MgO and Sn/Li/MgO two different types of surface lattice oxygen are present. The first is active in methyl radical formation, while the second is involved in the direct conversion of methane to carbon dioxide. Weakly adsorbed oxygen and the second type of surface lattice oxygen are also involved in the nonselective reaction paths of ethane and ethene as well as in the consecutive oxidation of carbon monoxide. Strongly adsorbed oxygen is not involved in these reactions. The observations are consistent with the Lunsford mechanism [Ito, T., Wang, J.-X., Lin, C.-H., and Lunsford, J. H.,J. Am. Chem. Soc.107, 5062 (1985)] for the generation of methyl radicals over MgO-based catalysts. The increasing activity toward methane due to the addition of lithium and moreover tin to MgO can be explained by an increase in the amount of reactive surface lattice oxygen

Kinetics of the steady state acetylene oxidation by oxygen over a Pt/Rh/CeO2/gamma-Al2O3 three-way catalyst

The steady-state kinetics of acetylene oxidation has been studied in the framework of automotive exhaust gas catalysis over a commercially available three-way catalyst. Experiments under cold-start conditions have been carried out in a laboratory fixed-bed reactor, which can adequately be described by the developed elementary step model and rate parameters

A mechanistic study of the Fischer-Tropsch synthesis using transient isotopic tracing. Part-1: Model identification and discrimination

A steady state isotopic transient kinetic anal. (SSITKA) of the Fischer-Tropsch synthesis over Co/Ru/TiO2 catalyst is reported by using 13CO and D2. Besides a qual. interpretation of the transients, model identification and discrimination are mainly based on numerical modeling. From this, it is concluded that two single-C species are present on the catalyst surface, Ca,ads and Cb,ads. These species both participate in the formation of methane and of higher hydrocarbons. The heterogeneity of the catalyst surface is limited to these two single-C species. Only one type of chain-growth site is present in a low concn. compared to the surface concns. of COads, Ca,ads, and Cb,ads. The H-content of Ca,ads and Cb,ads is assessed by comparing the simulated transient for the incorporation of the D-labeling into methane with expts. In case the stepwise hydrogenations of Cads to CH4 are irreversible, Ca,ads and Cb,ads are H-free species. In case these reactions are reversible, the H-exchange between Cads, CHads, CH2,ads, and CH3,ads is fast compared to the net formation of methane, and the H-content of Ca,ads and Cb,ads cannot be assessed. The most probable mechanism for the Fischer-Tropsch reaction resulting from this study is used in a next paper (Part 2) for the quantification of the kinetic parameters. In that paper, a systematic anal. of these parameters yields detailed mechanistic insight into the Fischer-Tropsch synthesis reaction. [on SciFinder (R)

Methanol from natural gas: proven and new technologies

The role of heterogeneous and homogeneous reactions in the oxidative coupling of CH4 over Li/MgO was investigated by carrying out expts. at reduced pressure (60 Pa). The radical reactions occurring in the gas phase play an essential role in the C2+ selectivity of the process. A reaction model of consecutive reactions is confirmed. The selectivity of the active catalyst is mainly due to very high rates of Me radical prodn. which couple in the gas phase

Ethylene from natural gas: Proven and new technologies

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