Molecular structures of supported metal oxide catalysts under different environments

Abstract

The use of in situ Raman spectroscopy to study the molecular structures of supported metal oxide catalysts under different environments is reviewed. The molecular structures under ambient (hydrated) and dehydrated conditions are presented. The effect of moisture at elevated temperatures is also presented and discussed with regard to its implications for catalytic phenomena. The molecular structural transformations during C 2 -C 4 lower alkane (LPG) oxidation, methane oxidation, methanol oxidation and selective catalytic reduction of NO with NH 3 reaction conditions are presented. In situ spectroscopy during catalytic reaction with simultaneous activity/selectivity measurement ('operando' spectroscopy) is emphasized owing to its contribution to the fundamental understanding of catalytic performance. The reducibility of the different surface metal oxide species, the relevance of surface coverage (surface monomeric vs polymeric species) and the specific oxide support are discussed when LPG, methane, methanol or hydrogen is the reducing agent. In situ Raman spectroscopy provides molecular-level information about the surface metal oxide species: structures, stability and transformations under different environments. In many cases, the use of complementary spectroscopic techniques results in a more complete understanding of the molecular structure-activity/selectivity relationships for supported metal oxide catalysts.

Effect of synthesis method on stabilized nano-scaled Sb–V–O catalysts for the ammoxidation of propane to acrylonitrile

“…Weak Raman bands near 1024 cm )1 are present in almost all catalysts, these bands are typical of the V = O mode of tetrahedral surface VO x species [32,33]. This band is affected by hydration, which confirms that it belongs to surface dispersed VO x species.…”

Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V⋯V coordination number and distance and the point of zero charge of the support oxide

“…[21] Based on this diagram and the PZC of the silica surface Wachs and co-workers proposed that upon hydration silica-supported vanadium catalysts consist of a mixture of V 2 O 5 , V 10 O 26 (OH) 2 and VO(OH) 3 , whereas for alumina-supported vanadium oxide catalysts VO 3 (OH) and V 2 O 7 were predicted to be present. [4,5,22,23] The hydrated structure is also influenced by the vanadium oxide loading, and thus the vanadium oxide species are in a more polymerized state at high vanadium oxide concentrations. Vanadium oxide species are acidic and thus lower the pH of the solution above the support surface, implying that the effect of the support on the molecular structure will be the most pronounced at low vanadium oxide loadings.…”

Niobiosilica Materials as Attractive Supports for Sb–V–O Catalysts

“…The shape of the spectra of samples with and without niobium is quite different, indicative that the effect partially oxidizes under reaction conditions, since Sb(V) and Sb(III) are present in Sb 2 O 4 oxide [7]. The Raman band at the 1010-1020 cm -1 range is assigned to the terminal V=O bond of dispersed VOx structures [24]. The Raman spectra for the corresponding sample calcined at lower temperature, 0.5VSb/NbSiOx(813), is similar but it does not exhibit the Sb 2 O 4 bands.…”