Oxide-supported Pt-group single atoms and clusters in the subnanometer size regime maximize the metal utilization and have shown extraordinary catalytic properties for many reactions including selective hydrogenation. Establishing relations between the metal nuclearity and electronic and catalytic properties is crucial for catalyst design. Here, we varied the nuclearity of Pt supported on TiO2 from single atoms to subnanometer clusters to larger nanoparticles to develop such relations for acetylene hydrogenation. We show that, in contrast to the low selectivity on large Pt nanoparticles, in the subnanometer size regime, Pt exhibits remarkably high selectivity to ethylene. Through a combination of X-ray photoelectron spectroscopy and calorimetry, we demonstrate that the origin of high selectivity is the decreased electron density on Pt and destabilization of C2H4 as the Pt nuclearity decreases. However, as the Pt nuclearity decreased, the activity for H2 activation and acetylene hydrogenation decreased, indicating a trade-off between activity and selectivity. The results show that, while different properties emerge in the subnanometer regime, Pt supported on TiO2 appears to be bound by similar scaling and Brønsted–Evans–Polanyi relationships as on metal surfaces.

CeO2-supported Pt single-atom catalysts have been extensively studied due to their relevance in automobile emission control and for the fundamental understanding of CeO2-based catalysts. Though CeO2-supported Pt nanoparticles are often more active than their single-atom counterparts, the former could easily redisperse to Pt single atom under oxidizing diesel conditions. Therefore, to maximize the reactivity of every Pt atom, it is important to fully understand the reaction mechanism of CeO2-supported Pt single atoms. Here, we report a CO oxidation study on a Pt/CeO2 single-atom catalyst, where we can account for all of the neighbors using in situ and operando spectroscopy techniques and microcalorimetric measurements. Coupled with density functional theory calculations, we present a comprehensive picture of the dynamics of the surface species, the role of surface intermediates, and explain the observed reaction kinetics. We started with a catalyst containing exclusively single atoms and used in situ/operando spectroscopy to provide evidence for their stability during the reaction and to identify the Pt1 complexes before and during the reaction and their binding to CO. The results reveal that in the precatalyst, Pt is present as Pt­(O)4 on the CeO2(111) step edge sites, but during CO oxidation, we find that two Pt1 complexes coexist, representing two states of the same active site in the reaction cycle. The dominant state/complex remains Pt­(O)4, which adsorbs CO very weakly as shown by CO microcalorimetry. The second, minority state/complex, Pt­(CO)­(O)3 is generated through the reaction of Pt­(O)4 with CO, and CO is bound strongly to Pt1. Labile oxygen adatoms from the CeO2 surface play a major role in the regeneration of Pt­(O)4 either directly from Pt­(O)3 or by reaction with the strongly adsorbed CO in Pt­(CO)­(O)3. We show that the formation of an oxygen vacancy and generation of a labile O* are not barrierless, which explains the long lifetime of Pt­(CO)­(O)3 and its detectability despite being a minority complex. The results help to develop a comprehensive view of the dynamic evolution of Pt1 complexes along the reaction cycle and provide mechanistic insights to guide the design of Pt-based single-atom catalysts.

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CO oxidation on Pt group metals is affected by the metal size and reducibility of the oxide support. Here, we report that Ir supported on MgAl2O4, traditionally considered non-reducible, exhibits...