Segregation and reactivity in bi-cationic oxide catalysts

The in uence of segregation on shell-core selective oxidation catalysts has been investigated in this thesis in three main areas. Novel shell-core catalysts, namely VOx/Fe2O3 catalysts, are selective to formaldehyde during reaction with methanol, indicating that the core Fe2O3 is sufficiently segregated, while VOx remains at the surface. Significant structural understanding has been gained, suggesting VO4 tetrahedra constitute the active site at the surface. With the soundness of the shell-core model further confirmed by VOx/Fe2O3, other reactions of interest can be investigated with shell-core catalysts. While VOx and MoOx/Fe2O3 function well as shell-core catalysts, benefitting from greater surface area and amenability to analysis, NbOx/Fe2O3 catalysts are unable to achieve the necessary segregation during calcination, resulting in exposed Fe2O3 at the surface, worsening their selectivity. This is attributed to the notably higher Tamman temperature of NbOx which is never reached during calcination, preventing the spreading of the NbOx across the Fe2O3 required for shell-core formation. The properties of Al dopants in Fe2O3 cores have also been examined. The addition of up to 20 wt% Al in Fe2O3 can increase surface area fourfold, enhancing catalytic activity in turn; however, detrimental effects on catalyst selectivity are seen for higher Al loadings, indicating a degree of structural disruption. It is now known that Al can only occupy sites in the Fe2O3 structure at low Al loadings, above which the impetus to phase separate increases. Magnetocatalysis has also been investigated using shell-core catalysts based on zincdoped cobalt ferrite. Clear evidence of selectivity manipulation by applying an external field during reaction has been obtained. Additionally, strong indications of internal magnetocatalytic effects have been observed, which are those effects on selectivity caused by magnetisation changes in the ferrite support. Overall, these studies have further emphasised the importance of multicomponent catalysts, and the need to carefully control catalyst speciation

VOx/Fe2O3 Shell-Core Catalysts for the selective oxidation of methanol to formaldehyde

Efficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VOx/Fe2O3 shell–core catalysts with varying VOx coverage have been produced to investigate the stability of VOx monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VOx overlayer has formed; by 500 °C a sandwich layer of FeVO4 arises between the VOx shell and the Fe2O3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell–subshell–core system of VOx/FeVO4/Fe2O3, possess good catalytic activity and selectivity to formaldehyde

Al-doped Fe2O3 as 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

Investigation of MoOx/Al2O3 under cyclic operation for oxidative and non-oxidative dehydrogenation of propane

A MoOx/Al2O3 catalyst was synthesised and tested for oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane in a reaction cycle of ODP followed by DP and a second ODP run. Characterisation results show that the fresh catalyst contains highly dispersed Mo oxide species in the +6 oxidation state with tetrahedral coordination as [MoVIO4]2− moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2− is present during the first ODP run of the reaction cycle and is reduced to MoIVO2 in the following DP run. The reduced species are partly re-oxidised in the subsequent second ODP run of the reaction cycle. The partly re-oxidised species exhibit oxidation and coordination states that are lower than 6 but higher than 4 and are referred to as MoxOy. These species significantly improved propene formation (relatively 27% higher) in the second ODP run at similar propane conversion activity. Accordingly, the initial tetrahedral [MoVIO4]2− present during the first ODP run of the reaction cycle is active for propane conversion; however, it is unselective for propene. The reduced MoIVO2 species are relatively less active and selective for DP. It is suggested that the MoxOy species generated by the reaction cycle are active and selective for ODP. The vibrational spectroscopic data indicate that the retained surface species are amorphous carbon deposits with a higher proportion of aromatic/olefinic like species

Methanol oxidation over shell-core MO_x/Fe₂O₃ (M = Mo, V, Nb) catalysts

We present a comparison of Mo, V and Nb oxides as shell materials atop haematite cores used for selective methanol oxidation. While Mo and V both yield high selectivity to formaldehyde, Nb does not. Very different reactivity patterns are seen for Nb, which mainly shows dehydrogenation (to CO) and dehydration (to DME), indicating the lack of a complete shell, while Raman spectroscopy shows that the Mo and V formation process is not followed by NbOx. We suggest this is due to the large differences in mobility within the solid materials during formation, NbOx requiring significantly higher (and deleterious) calcination temperatures to allow sufficient mobility for shell completion.</p

Safe handling of diazo reagents through inline analytics and flow chemistry

Through the use of flow chemistry in multistep processes, dangerous but synthetically useful diazo reagents can be made accessible for large scale applications. The generation, isolation and use of diazo compounds can be performed continuously, therefore never accumulating large quantities of highly energetic material. The use of inline analytics via infrared spectroscopy is key in developing these processes

VOx/Fe2O3 shell–core catalysts for the selective oxidation of methanol to formaldehyde

Efficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VOx/Fe2O3 shell–core catalysts with varying VOx coverage have been produced to investigate the stability of VOx monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VOx overlayer has formed; by 500 °C a sandwich layer of FeVO4 arises between the VOx shell and the Fe2O3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell–subshell–core system of VOx/FeVO4/Fe2O3, possess good catalytic activity and selectivity to formaldehyde

Safe handling of diazo reagents through inline analytics and flow chemistry

Through the use of flow chemistry in multistep processes, dangerous but synthetically useful diazo reagents can be made accessible for large scale applications. The generation, isolation and use of diazo compounds can be performed continuously, therefore never accumulating large quantities of highly energetic material. The use of inline analytics via infrared spectroscopy is key in developing these processes

Ensemble effects on methanol oxidation to formaldehyde on ferric molybdate catalysts

The properties of Mo-doped iron oxide are compared with those of the single oxides of Fe and Mo, and with stoichiometric ferric molybdate for the selective oxidation of methanol. It is found that Mo oxide segregates to the surface of the iron oxide at low loadings, while at higher loadings, but below the stoichiometric ratio, presents layers of ferric molybdate at the surface. The relationship between bulk loading and surface Mo is explored, and it is concluded that the reactivity is dominated by ensemble effects. Simple modelling indicates that four or more Fe cation ensembles are required to combust methanol to CO 2, ensembles of two Mo cations are required for selective oxidation to formaldehyde, whereas it seems that isolated single sites of either Fe or Mo produce CO.</p