Structure analysis of supported disordered molybdenum oxides using pair distribution function analysis and automated cluster modelling.

Troels Lindahl Christiansen,Emil T S Kjær,Kirsten M Ø Jensen,Anton Kovyakh,Martin Høj,Morten L Röderen,Tom Vosch

doi:10.1107/s1600576719016832

Abstract

Molybdenum oxides and sulfides on various low-cost high-surface-area supports are excellent catalysts for several industrially relevant reactions. The surface layer structure of these materials is, however, difficult to characterize due to small and disordered MoO x domains. Here, it is shown how X-ray total scattering can be applied to gain insights into the structure through differential pair distribution function (d-PDF) analysis, where the scattering signal from the support material is subtracted to obtain structural information on the supported structure. MoO x catalysts supported on alumina nanoparticles and on zeolites are investigated, and it is shown that the structure of the hydrated molybdenum oxide layer is closely related to that of disordered and polydisperse polyoxometalates. By analysing the PDFs with a large number of automatically generated cluster structures, which are constructed in an iterative manner from known polyoxometalate clusters, information is derived on the structural motifs in supported MoO x .

Highlights

Nanostructured metal oxides of, for example, molybdenum, chromium or vanadium supported on cheap high-surface-area supports, e.g. titania, alumina or zeolites, are attractive candidates in many areas of heterogeneous catalysis (Zaera, 2013; Shiju & Guliants, 2009; Munnik et al, 2015; Banares, 1999; Zhang et al, 2013)
X-ray total scattering, where diffuse scattering arising from disordered atomic arrangements is included in the data treatment, has over the past decade evolved into a powerful technique for structural characterization of nanomaterials with limited structural order
We use a new method for analysing the obtained differential pair distribution function (d-pair distribution function (PDF)) of disordered nanoclusters, where the data are fitted with a large number of cluster models, which are automatically built on the basis of known metal oxido cluster structures

Summary

Introduction

Nanostructured metal oxides of, for example, molybdenum, chromium or vanadium supported on cheap high-surface-area supports, e.g. titania, alumina or zeolites, are attractive candidates in many areas of heterogeneous catalysis (Zaera, 2013; Shiju & Guliants, 2009; Munnik et al, 2015; Banares, 1999; Zhang et al, 2013). X-ray total scattering, where diffuse scattering arising from disordered atomic arrangements is included in the data treatment, has over the past decade evolved into a powerful technique for structural characterization of nanomaterials with limited structural order. We use a new method for analysing the obtained d-PDFs of disordered nanoclusters, where the data are fitted with a large number (thousands) of cluster models, which are automatically built on the basis of known metal oxido cluster structures. By studying metal oxido clusters and fitting this large number of structures to the d-PDF, we can extract information on the structural motifs present in the disordered molybdenum oxide systems. We first demonstrate this method for nanostructured MoOx on -Al2O3. The structures, which are hydrated from atmospheric air, contain similar motifs to known polyoxometalate structures found in solution, they are not monodisperse and include a much larger degree of structural disorder

Molybdenum oxide supported on c-Al2O3

Molybdenum oxides supported on zeolites

Conclusions

Material preparation

Micro-Raman spectroscopy

Funding information