Reversible Structure Transformation of Antimony Oxides on SiO₂Relevant to Selective Catalytic Oxidation of Ethanol

Abstract

Sb/SiO2 catalysts were prepared by chemical vapor deposition of Sb(OEt)3 on a SiO2 surface followed by water vapor admission and calcination. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectra, and X-ray absorption fine structure and by catalytic performance for selective oxidation of ethanol. At low loadings (2−4 wt %) Sb(OEt)3 reacted with surface OH groups on the SiO2 surface to form surface-attached Sb(III) species. At high Sb loadings (>5 wt %), in addition to the attached Sb(III) species, crystalline Sb2O3 species were formed after exposure to water vapor. The amount of crystalline Sb2O3 species increased with Sb loading, and 30−50% of Sb species existed as crystalline Sb2O3 at 7−12 wt %. Most of the Sb(III) species remained trivalent after calcination. On the other hand, the crystalline Sb2O3 was oxidized to Sb(V) and spread on the SiO2 surface. The spread antimony oxide species were active for selective catalytic oxidation of ethanol through a redox mechanism. When the spread antimony oxide species were reduced with ethanol, they were reversibly transformed to crystalline Sb2O3 species. This paper shows a dynamic structure change that occurs under the ethanol selective oxidation reaction conditions.

Studies on the SbOx species of SbOx/SiO2 catalysts for methane-selective oxidation to formaldehyde

“…2a and 3. It is also in line with the conclusion that, on Sb 2 O 3 /SiO 2 catalysts prepared from Sb(OEt) 3 , most attached Sb in SbOx species remain trivalent after the calcination at 500 8C while the crystalline Sb 2 O 3 can be oxidized to Sb 5+ oxidic entities [14].…”

“…The trend for CO and CO 2 reflects that the depression of deep oxidation towards CO 2 is favored on aggregated SbOx species on the catalysts with high loading of antimony oxides. This is in line with the fact that excess antimony oxides of antimony-containing catalysts can restrict complete oxidation of hydrocarbons [14][15][16]. Formaldehyde is more reactive toward the formation of over-oxygenated byproducts under same reaction conditions than methane itself.…”

“…Excess antimony of antimony-containing mixed oxide catalysts has been found to be a clue to high selectivity to partially oxidized products [14]. It was found that the complete oxidation of hydrocarbons can be restricted on excess antimony [15,16].…”

“…Golunski studied bulk antimony oxides in different phases and found that both Sb 3+ and Sb 5+ species are involved in the overall process of trapping and spilling oxygen during selective oxidation [17]. Few studies have been done to elucidate the intrinsic nature of supported antimony oxides [14]. It was found that Sb 2 O 3 can be spontaneously dispersed on ZSM-5 in the calcination of the mixture of Sb 2 O 3 and ZSM-5 [18,19].…”

“…Matsuzawa et al studied the Sb 2 O 3 /SiO 2 catalysts, which were prepared from Sb(OEt) 3 , and found that the SbOx species changed dynamically in structure under the ethanol selective oxidation conditions [14]. Li et al studied the antimony species on Beta zeolite and found that the zeolite containing antimony species do not behave like bulk Sb 2 O 5 , but the state of aggregation of antimony species in Sb/Beta is still unknown [3].…”