Among the most notable and influential aspects of heterogeneous catalysis research in the past several decades have been the development and widespread utilization of in situ and operando characterization tools. Interrogation of chemical processes on surfaces at operating temperatures and pressures has yielded extraordinary amounts of fundamental insight into the contributions of specific active sites to overall reaction outcomes. However, given these remarkable advances, by comparison there is now a conspicuous knowledge gap regarding chemical events in the near-surface gas phase – especially their relationship to correlate events occurring on catalyst surfaces.<br/>Here, we present results from recent investigations of the gas phase above catalyst surfaces in operating conditions. Specifically, the near-surface region (defined as a region hundreds of microns to millimeters above a surface) is interrogated through a novel combination of optical spectroscopy, Raman spectroscopy, and molecular-beam mass spectrometry with universal species detection capability. These techniques are supplemented by operando diffuse reflectance FTIR spectroscopy. Efforts focus on understanding oxidative coupling reactions, which are known to be influenced by chemical events occurring both on the surface and in the gas phase. Emphasis is placed on conversion of methanol and methane over supported metal and metal alloy surfaces (eg, Ag, Pd, Pt, AuPd). The included figure shows representative results indicating that during methanol oxidation with Pd-based catalysts, the near-surface gas phase is populated by a wide diversity of species – including methoxymethanol, a rarely observed C2 ether-alcohol that is highly reactive in catalytic conditions.<br/>This presented approach is extended to include well-defined, site-isolated (often atomically dispersed) supported metal catalysts. The ultimate goal of this research is to relate local composition/structure of (uniform) active sites to the local, gas-phase environment (including unstable or reactive species, such as radicals).

Understanding oxidative coupling reactions through operando characterization of surface sites and the near-surface gas phase

ACS Spring 2022

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