The influence of surface structure on the catalytic activity of alumina supported transition metal oxides: A study of carbon monoxide and methane oxidation.

摘要

The projects described in this thesis examine the effect of rare earth and transition metal oxide promoters on the structure and reactivity of base metal oxide/alumina catalysts for total oxidation reactions. The research involves the preparation and testing of alumina supported single and mixed metal oxide catalysts designed for total oxidation reactions. The catalysts were prepared with various active phase loadings, promoter contents, and precursors to control the chemical state and structure of supported phases. The identification and quantification of various supported phases have been determined using surface (XPS), bulk (XRD, ESR, FTIR, TGA, BET), and microprobe (TEM) techniques. The information derived from these techniques was correlated with CO and CH{dollar}sb4{dollar} oxidation activities to identify active sites and to examine the effect of promoter on the structure and reactivity of the catalysts.; The effect of crystallinity on the photoreduction of Ce(IV) species during XPS analysis of CeO{dollar}sb2{dollar} samples and Ce/Al{dollar}rmsb2Osb3{dollar} catalysts has been determined. The crystallinity of the samples was controlled by varying the calcination temperature or by using different Ce precursors (ammonium Ce(IV) nitrate and Ce(IV) methoxyethoxide). XPS, XRD, and TEM results indicated that the amorphous cerium oxide was reduced more extensively than the crystalline material during XPS analysis.; XRD, XPS, and ESR results indicated that an isolated copper surface phase, an interacting copper surface phase, and large CuO crystallites are formed on Cu/Al{dollar}rmsb2Osb3{dollar} catalysts, depending on Cu content. The crystalline CuO and isolated copper surface phase has been assigned to the active site for CO and CH{dollar}sb4{dollar} oxidation, respectively. Cu/Al{dollar}rmsb2Osb3{dollar} catalyst prepared using Cu(II) ethoxide showed higher Cu dispersion, less crystalline CuO, and lower oxidation activity for CO and CH{dollar}sb4{dollar} than the catalyst derived from Cu(II) nitrate. For Cu/Ce/Al{dollar}rmsb2Osb3{dollar} catalysts, Ce had little effect on the dispersion and crystallinity of the copper species. However, Cu addition decreased the Ce dispersion and increased the amount of crystalline CeO{dollar}sb2{dollar}. Cerium addition dramatically increased the CO oxidation activity, but it had little effect on CH{dollar}sb4{dollar} oxidation. This indicated that cerium strongly interacted with crystalline CuO, but not with copper surface phase.; The characterization of Cr/Al{dollar}rmsb2Osb3{dollar} catalysts indicated that Cr was present as a highly dispersed Cr{dollar}sp{lcub}6+{rcub}{dollar} surface phase, Cr{dollar}sp{lcub}3+{rcub}{dollar} clusters, and large Cr{dollar}rmsb2Osb3{dollar} crystallites, depending on Cr content. The active phase for CH{dollar}sb4{dollar} oxidation has been assigned to a Cr(III)-Cr(VI) interaction species. For the Cu/Cr/Al{dollar}rmsb2Osb3{dollar} catalysts, Cu addition decreased the dispersion of the chromium phase by reacting selectively with a dispersed Cr{dollar}sp{lcub}3+{rcub}{dollar} species to form CuCr{dollar}rmsb2Osb4{dollar}. The Cu dispersion also decreased with increasing Cr loading due to the formation of CuO and CuCr{dollar}rmsb2Osb4{dollar}. These phases contributed to the catalytic activity of CO oxidation with increasing Cr content up to Cr/Al = 0.054. For further Cr addition, the catalytic activity decreased due to the decrease in catalyst dispersion or to encapsulation of the active site by excess Cr species.