(464f) Single Metal Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Identification of the active catalytic site and design of catalysts with 100% atom efficiency has been a long-standing goal in heterogeneous catalysis. A promising approach to reaching this goal through the controlled preparation of isolated single-atom heterogeneous catalysts has emerged in the recent literature. For catalytic metals, atomic dispersion affords better utilization, different (often better) selectivity than the extended metal, and new prospects for low-cost and green process development. Isolated supported metal atoms may be viewed as species bonded to a support, the latter serving as a ligand. An analogy between a homogeneous and a heterogeneous single-site catalytic center can thus be made. Single atom sites catalyze some, but not all reactions. It is crucial to understand the mechanisms behind catalysis by supported single metal atoms, as this will guide the new, improved catalyst designs.

In this presentation, suitably stabilized catalytic sites as single metal atoms/cations on various supports will be showcased drawing examples from a variety of reactions, including the low-temperature water-gas shift reactions; methanol and ethanol dehydrogenation and steam reforming reactions; the direct methane conversion to oxygenates; and selective hydrogenation reactions on single-atom alloys. Reaction mechanisms involving single metal atoms/cations often transcend support structure and composition, thus allowing flexibility in the choice of the support. A unique “signature” of the metal (Au, Pt, Pd, Ni, etc.) at the atomic state is preserved, distinct however from the corresponding extended metal catalyst. Single-atom alloys offer a unique approach towards rational catalyst design, one that combines surface science, catalysis and theory in a most efficient way. Model surfaces and nanoparticles that can host isolated atoms in the surface layers behave similarly in escaping the linear scaling relationships and allowing for rational fine-tuning of activity and selectivity. Good stability is imparted by the strong metal-metal bonds between the host and the minority metal, and atomic dispersion can be maintained at high temperatures. Resistance to CO poisoning and to coking are additional advantages of these promising materials, as will be shown in the presentation drawing examples from alkyne and alkadiene hydrogenation and alkane dehydrogenation. Novel synthesis methods and the stability of single-atom metal catalysts in various supports and reaction environments will be discussed.