Single-Site vs Cluster Catalysis in High Temperature Oxidations.

The behavior of single Pt atoms and small Pt clusters for high temperature oxidation reactions has been investigated using CHA zeolite. Subtle changes in the atomic structure of the active sites are responsible for drastic changes in performance driven by specific interactions between the metal, the gas-phase components, and the support - in our case, a well-defined, non-reducible macro-ligand. The high-stability of these molecular sites in CHA is a key to intrinsic structure-performance descriptions of elemental steps such as O2 dissociation, and subsequent oxidation catalysis of industrial interest, such as the combustion of methane, propane, and CO. Whereas single Pt-atoms and Pt nanoparticles larger than ~ 1 nm are unable to activate, scramble and desorb two O2 molecules in isotopic exchange experiments at moderate T (i.e. 200⁰C), clusters < 1 nm can do so catalytically, but are unstable against oxidative fragmentation under reaction conditions, leading to catalyst deactivation. For oxidation processes that require the activation of rather inert molecules at high T, like alkanes, catalytic combustion is attributed to stable single Pt atoms when the support is CHA, generated in situ in the O2 stream. The alkane combustion activity is facilitated when the reacting alkane includes weaker C-H bonds (e.g. propane vs methane), and the C-H cleavage is inferred to be rate-determining, which is less sensitive to changes in the active site size compared to the effect on the O2 scrambling. When the combustion involves a strong reductant such as CO, in contrast, the catalysis is dominated by metal clusters and nanoparticles, and not single Pt atoms, as the latter do not form stable mononuclear carbonyl species in the zeolite. The great stability of single Pt atoms for methane combustion, and of small Pt clusters for CO oxidation, including upon regeneration and/or steaming, open new opportunities for the system in a variety of high temperature oxidation scenarios.