Studies of partial oxidation and combustion over heterogeneous catalysis by transient experiments

Detta är en avhandling från Secretary, Dept. of Chem. Eng. II, P.O. Box 124, SE-22100 Lund

Sammanfattning: Transient experiments are one of many techniques available to study heterogeneous catalysis and the catalysts used. The results obtained by transient experiments contain valuable information that is helpful in explaining reaction mechanism and interaction at the catalytic surface. The work that was performed covers areas that reach from the selective (ammoxidation) and complete oxidation of propane, over simulation of step- and pulse-transients to the total combustion of methanol at low temperatures. The results that were obtained for the ammoxidation of propane on an Al-Sb-V-W-oxide catalyst show that adsorbed acrolein reacts with a short-lived NHx-species to give acrylonitrile. This NHx-species is probably a –NH2 group and is also responsible for the formation of N2, N2O and NO. Lattice oxygen species (O2-) are involved in such reactions, as well as weakly adsorbed oxygen species (O-, O2-). The weakly adsorbed oxygen species are responsible for the formation of degradation products from the NHx-species and the formation of carbon oxides (CO + CO2). It is suggested that limiting the amount of weakly adsorbed oxygen species at the catalyst surface can be achieved by using a high propane/oxygen ratio in the process feed coupled with propane recirculation. Step- and pulse-transients were simulated using Matlab®. Comparison of the results obtained by both methods showed that the complexity of result evaluation favoured step-transients especially when considered in respect to the rate limiting step of a reaction. Step-transients give qualitative information about possible reaction mechanisms and rate limiting steps without the need for modelling. TAP pulse-transients on the other hand, have a high time resolution and make it possible to detect short-lived reaction intermediates. Therefore, TAP pulse-transients are valuable for the detection and identification of reaction intermediates, while step-transients are more valuable for the analysis of reaction mechanism in regard to the rate limiting step. The promotion of the propane combustion over Pt/Al2O3 by traces of SO2 was studied at 200°C. From the results it is suggested that different reaction mechanisms operate in the absence and presence of SO2 in the feed gas. When SO2 is not present, the initial step for the combustion of propane is the abstraction of hydrogen and the formation of propene. If SO2 is present in the feed gas, it becomes adsorbed on the catalyst surface and forms sulphate species the first step of the major pathway for the total oxidation of propane then involves the breaking of a C-C bond of propane instead of H-abstraction. Platinum on Al2O3, TiO2 and MgO supports were used for the low temperature combustion of methanol in a waste gas with a trace of NH3. The results show that in the presence of NH3 Pt/MgO is more superior for the combustion of methanol then Pt/Al2O3 and Pt/TiO2. In the case of Pt/Al2O3 and Pt/TiO2, it is seen that ammonia and methanol compete for the same adsorption sites. It is suggested that spillover of adsorbed species from the support surface to the platinum surface contribute to the reaction. Furthermore, the metal-support interaction plays an important role in the sensitivity of the catalysts towards deactivation. Catalysts with 0.1, 1.0 and 3.0 wt% Pt supported on Al2O3 were examined and characterised for the low temperature combustion of methanol in the absence and presence of NH3. The results indicate that the methanol combustion proceeds in the absence of NH3 at sites that are mainly located at the metal-support interface. In the presence of NH3 these sites are poisoned and the combustion proceeds at Pt surface sites.

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