Metal Phosphides and Sulfides in Heterogeneous Catalysis: Electronic and Geometric Effects

The efficiency of a heterogeneous catalyst depends on the nature and the number of catalytically active sites, but these are often inhomogeneous. One possible solution is to construct site-isolated catalysts in which most, if not all, of the sites are structurally uniform and well-defined. Metal phosphides and sulfides form with distinct crystal structures based on a range of component stoichiometries. Hence, incorporating active components (i.e., the catalytically active metal) into this structure can regulate geometric arrangements in a more reproducible manner where nonmetal atoms act as spacers around metal atoms to create isolated sites. A d-metal/p-block element strategy can provide several advantages compared with other methods which generate discrete active sites, including convenient synthesis, good process economics, and improved catalyst stability. Interestingly, the metal atoms in these systems still show typical catalyst traits associated with the metal which opens up many possible catalytic applications. This Review presents several interesting synthesis methods for preparing metal phosphides and sulfides and aims to draw links between geometric structure/electronic properties and enhanced catalytic performance (i.e., enhanced activity, selectivity, and stability) for both petrochemical and fine chemical processes. With precise knowledge of metal phosphide and sulfide structures/active sites, we envision the development of practically useful d-metal/p-block element catalysts from powder formulations to more industrial type pellets. It should, however, be noted that these materials are not without additional complexities. For example, metal phosphides and sulfides can have complex surface chemistry and the operating environment can induce structural evolution. These factors also need to be carefully considered.