Molybdenum Oxide on Fe₂O₃ Core–Shell Catalysts: Probing the Nature of the Structural Motifs Responsible for Methanol Oxidation Catalysis

Abstract

A series of MoO x -modified Fe 2 O 3 catalysts have been prepared in an attempt to make core−shell oxidic materials of the type MoO x /Fe 2 O 3 . It is conclusively shown that for three monolayers of Mo dosed, the Mo stays in the surface region, even after annealing to high temperature. It is only when the material is annealed above 400 °C that it reacts with the iron oxide. We show by a combination of methods, and especially by XAFS, that at temperatures above 400 °C, most of the Mo converts to Fe 2 (MoO 4 ) 3 , with Mo in a tetrahedral structure, whereas below that temperature, nanocrystalline MoO 3 is present in the sample; however, the active catalysts have an octahedral MoO x layer at the surface even after calcination to 600 °C. This surface layer appears to be present at all temperatures between 300 and 600 °C, and it is the nanoparticles of MoO 3 that are present at the lower temperature that react to form ferric molybdate, which underlies this surface layer. It is the MoO x layer on the Fe 2 (MoO 4 ) 3 underlayer that makes the surface active and selective for formaldehyde synthesis, whereas the iron oxide surface itself is a combustor. The material is both activated and improved in selectivity due to the dominance of the methoxy species on the Mo-doped material, as opposed to the much more stable formate, which is the main intermediate on Fe 2 O 3 .

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

“…This confirms that VO x is present in the surface layers. CO production is observed at higher temperatures, reaching its peak at approximately 230 °C; similar behaviour was observed for previous Mobased systems, though formaldehyde production peaks at higher temperatures for MoO x /Fe 2 O 3 [18,19]. It is suggested that isolated cation sites are responsible for CO generation [18,20,24,25]; surface VO x is present in sufficient quantity to preclude multiple neighbouring Fe sites ( : this can be ascribed to the shell-core process, which is known to enhance surface area [18,19].…”

“…This behaviour has similarities to that seen for Mo-based shell-core catalysts reported previously. However, 3 ML VO x /Fe 2 O 3 is a poorer catalyst than 3 ML MoO x /Fe 2 O 3 , achieving only 55% selectivity to formaldehyde at 50% conversion compared to 89% selectivity at 50% conversion for MoO x /Fe 2 O 3 [18]. Similar catalytic behaviour is observed upon repeated usage, although studies of prolonged usage and catalyst longevity have yet to be performed.…”

“…This shell-core approach has previously been studied in MoO 3 /Fe 2 O 3 catalysts for methanol oxidation, where it was seen that high selectivity to formaldehyde can be maintained at high methanol conversions [16][17][18][19][20]. The validity of the shell-core model when applied to molybdena-based catalysts was clearly established, in addition to its applicability to the methanol oxidation reaction [18][19][20][21].…”

“…The validity of the shell-core model when applied to molybdena-based catalysts was clearly established, in addition to its applicability to the methanol oxidation reaction [18][19][20][21]. Mo-based catalysts were investigated since current industrial catalysts for methanol oxidation include iron molybdate, but also because of the high formaldehyde selectivity (though poor activity) of MoO 3 .…”

Reduction Kinetics of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y]

“…Nanostructured iron molybdenum alloys and their oxides exhibit desirable chemical, electrical, and mechanical properties showing promise for applications in gas sensing [1][2][3][4], energy storage [5][6][7], and catalysis [8][9][10][11][12]. Of particular interest, are Fe-Mo nanoalloys used for catalytic chemical vapor deposition of carbon nanotubes (CNTs).…”