Interfacial effects of Cu/Fe3O4 in water-gas shift reaction: Role of Fe3O4 crystallite sizes

Copper-promoted iron oxides are commonly utilized in high-temperature water-gas shift (WGS) reactions. Although the promotional effect of Cu has been previously documented, the nature of the active sites for Cu/ Fe3O4 remains debated and the role of the crystallite size of Fe3O4 has not been tackled. In this study, a wide range of Cu/Fe3O4 catalysts were designed by hosting Cu species onto Fe3O4 of two contrasting crystallite sizes (approximately 10 and 160 nm) and by utilizing different preparation methods (ammonia evaporation and deposition-precipitation). The evaluation of their catalytic WGS performances revealed a remarkable and previously undocumented phenomenon: the addition of Cu resulted in universal activity enhancements only for small Fe3O4 crystals, while the larger analogues displayed decreased activity. This peculiar phenomenon can be explained by the creation of Cu-Fe3O4 interfaces present in the iron oxides of small crystallite sizes. Thorough characterizations, such as advanced electron microscopic, in situ powder X-ray diffractions, in situ diffuse reflectance infrared Fourier transform spectroscopy, and computational studies, suggest strong electronic interactions between the different metal species, facilitated reductions of the individual oxides, and more favorable CO activation at the interfacial sites, all accounting for the promoted activity.