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Al-doped Fe2O3as a support for molybdenum oxide methanol oxidation catalysts

Al-doped Fe2O3as a support for molybdenum oxide methanol oxidation catalysts
Al-doped Fe2O3as a support for molybdenum oxide methanol oxidation catalysts

We have made high surface area catalysts for the selective oxidation of methanol to formaldehyde. This is done in two ways-(i) by doping haematite with Al ions, to increase the surface area of the material, but which itself is unselective and (ii) by surface coating with Mo which induces high selectivity. Temperature programmed desorption (TPD) of methanol shows little difference in surface chemistry of the doped haematite from the undoped material, with the main products being CO2 and CO, but shifted to somewhat higher desorption temperature. However, when Mo is dosed onto the haematite surface, the chemistry changes completely to show mainly the selective product, formaldehyde, with no CO2 production, and this is little changed up to 10% Al loading. But at 15 wt% Al, the chemistry changes to indicate the presence of a strongly acidic function at the surface, with additional dimethyl ether and CO/CO2 production characteristic of the presence of alumina. Structurally, X-ray diffraction (XRD) shows little change over the range 0-20% Al doping, except for some small lattice contraction, while the surface area increases from around 20 to 100 m2 g-1. Using X-ray absorption spectroscopy (XAS) it is clear that, at 5% loading, the Al is incorporated into the Fe2O3 corundum lattice, which has the same structure as α-alumina. By 10% loading then it appears that the alumina starts to nano-crystallise within the haematite lattice into the γ form. At higher loadings, there is evidence of phase separation into separate Al-doped haematite and γ-alumina. If we add 1 monolayer equivalent of Mo to the surface there is already high selectivity to formaldehyde, but little change in structure, because that monolayer is isolated at the surface. However, when three monolayers equivalent of Mo is added, we then see aluminium molybdate type signatures in the XANES spectra at 5% Al loading and above. These appear to be in a sub-surface layer with Fe molybdate, which we interpret as due to Al substitution into ferric molybdate layers immediately beneath the topmost surface layer of molybdena. It seems like the separate γ-alumina phase is not covered by molybdena and is responsible for the appearance of the acid function products in the TPD.

1463-9076
18911-18918
Bowker, Michael
c9ab10a5-d144-4533-bf6d-2fa16b669565
Hellier, Pip
c3bd090a-2f8d-45ec-8ee6-781c034eb9b9
Decarolis, Donato
aa002823-ac92-4afe-aa03-83562110dd3c
Gianolio, Diego
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Mohammed, Khaled M.H.
f50a275b-4588-43b0-ba9c-5f108f23d3fc
Stenner, Alex
da5ad573-4396-4d91-9090-11eb97661d6d
Huthwelker, Thomas
65a9fbba-ce44-4d97-a7d8-57567e622e23
Wells, Peter P.
bc4fdc2d-a490-41bf-86cc-400edecf2266
Bowker, Michael
c9ab10a5-d144-4533-bf6d-2fa16b669565
Hellier, Pip
c3bd090a-2f8d-45ec-8ee6-781c034eb9b9
Decarolis, Donato
aa002823-ac92-4afe-aa03-83562110dd3c
Gianolio, Diego
5b316f7d-f314-4337-954e-8c0ce8e38223
Mohammed, Khaled M.H.
f50a275b-4588-43b0-ba9c-5f108f23d3fc
Stenner, Alex
da5ad573-4396-4d91-9090-11eb97661d6d
Huthwelker, Thomas
65a9fbba-ce44-4d97-a7d8-57567e622e23
Wells, Peter P.
bc4fdc2d-a490-41bf-86cc-400edecf2266

Bowker, Michael, Hellier, Pip, Decarolis, Donato, Gianolio, Diego, Mohammed, Khaled M.H., Stenner, Alex, Huthwelker, Thomas and Wells, Peter P. (2020) Al-doped Fe2O3as a support for molybdenum oxide methanol oxidation catalysts. Physical Chemistry Chemical Physics, 22 (34), 18911-18918. (doi:10.1039/d0cp01192d).

Record type: Article

Abstract

We have made high surface area catalysts for the selective oxidation of methanol to formaldehyde. This is done in two ways-(i) by doping haematite with Al ions, to increase the surface area of the material, but which itself is unselective and (ii) by surface coating with Mo which induces high selectivity. Temperature programmed desorption (TPD) of methanol shows little difference in surface chemistry of the doped haematite from the undoped material, with the main products being CO2 and CO, but shifted to somewhat higher desorption temperature. However, when Mo is dosed onto the haematite surface, the chemistry changes completely to show mainly the selective product, formaldehyde, with no CO2 production, and this is little changed up to 10% Al loading. But at 15 wt% Al, the chemistry changes to indicate the presence of a strongly acidic function at the surface, with additional dimethyl ether and CO/CO2 production characteristic of the presence of alumina. Structurally, X-ray diffraction (XRD) shows little change over the range 0-20% Al doping, except for some small lattice contraction, while the surface area increases from around 20 to 100 m2 g-1. Using X-ray absorption spectroscopy (XAS) it is clear that, at 5% loading, the Al is incorporated into the Fe2O3 corundum lattice, which has the same structure as α-alumina. By 10% loading then it appears that the alumina starts to nano-crystallise within the haematite lattice into the γ form. At higher loadings, there is evidence of phase separation into separate Al-doped haematite and γ-alumina. If we add 1 monolayer equivalent of Mo to the surface there is already high selectivity to formaldehyde, but little change in structure, because that monolayer is isolated at the surface. However, when three monolayers equivalent of Mo is added, we then see aluminium molybdate type signatures in the XANES spectra at 5% Al loading and above. These appear to be in a sub-surface layer with Fe molybdate, which we interpret as due to Al substitution into ferric molybdate layers immediately beneath the topmost surface layer of molybdena. It seems like the separate γ-alumina phase is not covered by molybdena and is responsible for the appearance of the acid function products in the TPD.

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Accepted/In Press date: 7 May 2020
e-pub ahead of print date: 29 May 2020
Published date: 14 September 2020

Identifiers

Local EPrints ID: 444891
URI: http://eprints.soton.ac.uk/id/eprint/444891
ISSN: 1463-9076
PURE UUID: 1b2c8a95-3b78-4e40-97c7-81b5b4b99499
ORCID for Peter P. Wells: ORCID iD orcid.org/0000-0002-0859-9172

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Date deposited: 10 Nov 2020 17:30
Last modified: 18 Feb 2021 17:05

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Contributors

Author: Michael Bowker
Author: Pip Hellier
Author: Donato Decarolis
Author: Diego Gianolio
Author: Khaled M.H. Mohammed
Author: Alex Stenner
Author: Thomas Huthwelker
Author: Peter P. Wells ORCID iD

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