Structures and Properties of Zirconia-Supported Ruthenium Oxide Catalysts for the Selective Oxidation of Methanol to Methyl Formate

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• 作者：Weizhen Li ; Haichao Liu ; Enrique Iglesia
• 刊名：Journal of Physical Chemistry B
• 出版年：2006
• 出版时间：November 23, 2006
• 年：2006
• 卷：110
• 期：46
• 页码：23337 - 23342
• 全文大小：121K
• 年卷期：v.110,no.46(November 23, 2006)
• ISSN：1520-5207

文摘

The effects of RuO_x structure on the selective oxidation of methanol to methyl formate (MF) at low temperatureswere examined on ZrO₂-supported RuO_x catalysts with a range of Ru surface densities (0.2-3.8 Ru/nm²).Their structure was characterized using complementary methods (X-ray diffraction, Raman and X-rayphotoelectron spectra, and reduction dynamics). The structure and reactivity of RuO_x species change markedlywith Ru surface density. RuO_x existed preferentially as RuO₄^2- species below 0.4 Ru/nm², probably as isolatedZr(RuO₄)₂ interacting with ZrO₂ surfaces. At higher surface densities, highly dispersed RuO₂ domains coexistedwith RuO₄^2- and ultimately formed small clusters and became the prevalent form of RuO_x above 1.9 Ru/nm².CH₃OH oxidation rates per Ru atom and per exposed Ru atom (turnover rates) decreased with increasing Rusurface density. This behavior reflects a decrease in intrinsic reactivity as RuO_x evolved from RuO₄^2- toRuO₂, a conclusion confirmed by transient anaerobic reactions of CH₃OH and by an excellent correlationbetween reaction rates and the number of RuO₄^2- species in RuO_x/ZrO₂ catalysts. The high intrinsic reactivityof RuO₄^2- structures reflects their higher reducibility, which favors the reduction process required for thekinetically relevant C-H bond activation step in redox cycles using lattice oxygen atoms involved in CH₃OH oxidation catalysis. These more reactive RuO₄^2- species and the more exposed ZrO₂ surfaces on sampleswith low Ru surface density led to high MF selectivities (e.g. ~96% at 0.2 Ru/nm²). These findings provideguidance for the design of more effective catalysts for the oxidation of alkanes, alkenes, and alcohols by thesynthesis of denser Zr(RuO₄)₂ monolayers on ZrO₂ and other high surface area supports.