Linking Bulk and Surface Structures in Complex Mixed Oxides

The interface between a solid catalyst and the reacting medium plays a crucial role in the function of the material in catalysis. In the present work, we show that the surface termination of isostructural molybdenum-vanadium oxides is strongly linked to the real structure of the bulk. This conclusion is based on comparing (scanning) transmission electron microscopy images with pair distribution function (PDF) data obtained for (Mo,V)O_x and (Mo,V,Te,Nb)O_x. Distance-dependent analyses of the PDF results demonstrate that (Mo,V,Te,Nb)O_x exhibits stronger deviations from the averaged orthorhombic crystal structure than (Mo,V)O_x in the short and intermediate regimes. These deviations are explained by higher structural diversity, which is facilitated by the increased chemical complexity of the quinary oxide and in particular by the presence of Nb. This structural diversity is seemingly important to form intrinsic bulk-like surface terminations that are highly selective in alkane oxidation. More rigid (Mo,V)O_x is characterized by defective surfaces that are more active but less selective for the same reactions. In line with machine learning interatomic potential (MLIP) calculations, we highlight that the surface termination of (Mo,V,Te,Nb)O_x is characterized by a reconfiguration of the pentagonal building blocks, causing a preferential exposure of Nb sites. The presented results foster hypotheses that chemical complexity is superior for the performance of multifunctional catalysts. The underlying principle is not the presence of multiple chemically different surface centers but instead the ability of structural diversity to optimally align and distribute the elements at the surface and, thus, to shape the structural environment around the active sites. This study experimentally evidences the origin of the structure-directing impact of the real structure of the bulk on functional interfaces and encourages the development of efficient surface engineering strategies toward improved high-performance selective oxidation catalysts.