Maria Flytzani-Stephanopoulos

Abstract:

Novel catalyst designs aiming at the development of energy-efficient, low-cost and sustainable processes are of great interest for applications to fuel and chemical production, and to environmental pollution abatement. 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 recent literature. For catalytic metals, atomic dispersion affords better utilization, different (often better) selectivity than the extended metal, as well as 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 new, improved catalyst designs. In this presentation, suitably stabilized catalytic sites as single metal atoms/cations on various supports will be showcased by drawing examples from a variety of reactions: 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.
A new class of single-atom heterogeneous catalysts will be presented, namely single-atom alloys that comprise of catalytically active elements like Pt, Pd and Ni alloyed in a more inert host metal like Cu, Au or Ag at the single-atom limit. 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 allow for the rational fine-tuning of activity and selectivity. Good stability is imparted by the strong metal-metal bonds between the host, the minority metal, and atomic dispersion. This can be maintained at high temperatures. Resistance to CO poisoning and coking are additional advantages of these promising materials, as will be shown in the presentation where examples will be drawn 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.