Mixed oxide

About: Mixed oxide is a research topic. Over the lifetime, 5224 publications have been published within this topic receiving 115567 citations.

Characterization of High Surface Area Zr−Ce (1:1) Mixed Oxide Prepared by a Microemulsion Method

Abstract: AZ r-Ce mixed oxide with ca. a 1:1 atomic ratio is prepared by a microemulsion method and studied by X-ray diffraction, transmission electron microscopy, and Raman, X-ray photoelectron (XPS) and electron paramagnetic resonance (EPR) spectroscopies. The results show the formation of a high surface area material (SBET ) 96 m2 g-1) constituted by homodispersed particles of a major pseudocubic phase t′′ (as shown by Raman); the stabilization of the latter phase, instead of the normally more stable tetragonal phase t′, is probably due to the small crystallite size (ca. 5 nm). XPS indicates a moderate degree of surface enrichment in cerium. An EPR study is carried out on the superoxide species formed on the material by O2 adsorption after outgassing at temperatures up to Tv ) 773 K; this shows that the reduced surface centers thermally formed on this mixed oxide are similar to those found on pure ceria but are generated more easily than on the latter, thus evidencing a surface redox reactivity higher than that of the CeO2 single oxide.

Significant Promotion Effect of Mo Additive on a Novel Ce–Zr Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3

Abstract: A novel Mo-promoted Ce-Zr mixed oxide catalyst prepared by a homogeneous precipitation method was used for the selective catalytic reduction (SCR) of NO(x) with NH3. The optimal catalyst showed high NH3-SCR activity, SO2/H2O durability, and thermal stability under test conditions. The addition of Mo inhibited growth of the CeO2 particle size, improved the redox ability, and increased the amount of surface acidity, especially the Lewis acidity, all of which were favorable for the excellent NH3-SCR performance. It is believed that the catalyst is promising for the removal of NO(x) from diesel engine exhaust.

Activity and structure-sensitivity of the water-gas shift reaction over CuZnAl mixed oxide catalysts

Abstract: The activity and structure-sensitivity of the water-gas shift (WGS) reaction over Cu Zn Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary hydroxycarbonate precursors contained hydrotalcite, aurichalcite and/or rosasite phases. The decomposed precursors contained CuO, ZnO, ZnAl 2 O 4 , and Al 2 O 3 . The relative proportion of these phases depended on both the chemical composition of the sample and the calcination temperature employed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequency of the eighteen samples tested was essentially the same (0.2–0.3 s −1 ) irrespective of changing the copper metal surface area between 3 and 35 m 2 /g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r 0 (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed oxides with a high copper dispersion is therefore required for obtaining more active catalysts. It was found that the value of the metallic copper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content in the precursor, the higher the copper metal dispersion in the resulting catalyst and, as a consequence, the higher the catalyst activity. Ternary Cu/ZnO/Al 2 O 3 catalysts exhibited a substantially faster WGS activity than binary Cu/ZnO catalysts. The addition of aluminium, although inactive for the WGS reaction, is required for improving the catalyst performance.

Catalysis Science of Bulk Mixed Oxides

Abstract: Bulk mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, electrocatalysts for solid oxide fuel cells, and solid oxide electrolyzers for the production of hydrogen), but fundamental understanding of their structure–performance relationships have lagged in the literature. The absence of suitable surface composition and surface structural characterization techniques and methods to determine the number of catalytic active sites, with the latter needed for determination of specific reaction rates (e.g., turnover frequency (1/s)), have hampered the development of sound fundamental concepts in this area of heterogeneous catalysis. This Perspective reviews the traditional concepts that have been employed to explain catalysis by bulk mixed oxides (molybdates, vanadates, spinels, perovskites, and several other specific mixed oxide systems) and introduces a modern perspective to the fundamental surface structure–activity/selectivity relationships for bulk mixed oxide cataly...