Molybdate-based double perovskite materials in methane dry reforming

Abstract

A series of SrMMoO (M = Ni, Co and (Ni,Co)) compounds was tested as representative model systems to highlight the capabilities of double perovskite structures as precursor materials for methane dry reforming (DRM) applications. Pretreatments in either pure hydrogen or dry reforming CO/CH mixtures exclusively yield partial decomposition of the initial double perovskite structures through exsolution of small Ni or CoO particles and the associated formation of additional crystalline compounds, such as SrMoO or SrCO (in DRM mixtures). The formation of a defective SrMoO transient phase has been revealed by in situ X-ray diffraction measurements in a pure hydrogen atmosphere. The main difference between the Ni- and Co-containing Sr molybdate perovskite structures is the much stronger oxidation propensity of exsolved Co, most likely by oxygen supply from the partially intact double perovskite structure. For SrNiMoO, the resulting metallic Ni-double perovskite interface is highly DRM active without strong coking, both if a pre-reduction step in hydrogen is carried out before the DRM experiment or if SrNiMoO is directly decomposed in the DRM mixture. Despite partial decomposition, the corresponding SrCoMoO structure is not active under DRM operation, most likely due to the in situ formation of small exsolved CoO particles, while Ni is exsolved in its metallic state. Different strategies to improve the catalytic activity, including hydrogen by-mixing, enhanced A-site deficiency or co-alloying with Ni have been followed, but only the latter has a beneficial effect on improving the DRM activity at compositions of SrNiCoMoO. In SrNiCoMoO, the substitution of Co by Ni suppresses the oxidation propensity of Co and during DRM yields the exsolution of Co-rich Ni–Co alloy nanoparticles. We also reveal a strong response of molybdenum as the B’ site cation to reduction and DRM treatment, causing the formation of reduced MoO phases accompanying the exsolution process.