Estudo das propriedades fisico-quimicas de catalisadores a base de CeO2 e Pd-CeO2 modificados por alcalinos I

AUTOR(ES)
DATA DE PUBLICAÇÃO

1995

RESUMO

The objective of this work was to study the physico-chemical and catalytic properties of ceO2 and Pd-CeO2 systems. Cerium oxides of high (C400) and low (C850) surface area and cerium oxides doped with sodium and barium compounds were used in this study. It was also analysed the influence of residual contaminants such as carbonates and nitrates on the properties of CeO2 e Pd-CeO2 systems. The redox properties of these solids were extensively studied by conventional temperature programmed reduction (TPR), TPR coupled with mass spectroscopy (TPR-MS) and TPR associated with TG/DTA analysis. Furthermore, temperature programmed oxidation (TPO) and magnetic measurements of reduced samples were used for better evaluation of the reduction degrees of the catalysts. The catalytic activity of these systems was evaluated in alkane oxidation reactions and ethanol conversion (inert and oxidant atmosphere ). The presence of contaminants on the catalyst surface introduces important changes in TPR profiles. These contaminants are essentially carbonates (adsorbed ftom air) and nitrates (introduced during the impregnation of alkaline elements and palladium) which remain on the catalysts even after calcination up to 450°C. Nitrates are reduced at approximately the same temperature as the cerium oxide surface so that the hydrogen consumption takes in account both effects. In this case, the presence of sodium helps to stabilize these contaminants. The results of mass spectrometry analysis show that carbonate species are eliminated as CO at temperatures around 800°C under hydrogen atmosphere. Barium can easily associate with these contaminants, "cleaning" the surface of cerium oxide and favoring its reduction. In Pd-CeO2 catalysts, these contaminants are less stable and they are removed during pre-treatment steps or at low temperature during the TPR. The support is easially reduced in the presence of palladium. However, the magnitude of this effect depends on the presence of sodium and the impregnation sequence of the metal and the additive. The accumulation of hydrogen on CeO2 and Pd-CeO2 catalysts during TPR was well characterized by TPR-MS experiments. This effect was attributed to formation of hydride species on the surface of cerium oxide

ASSUNTO(S)

catalisadores

ACESSO AO ARTIGO

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