Single-atom catalysts have attracted attention because of improved atom efficiency, higher reactivity, and better selectivity. A major challenge is to achieve high surface concentrations while preventing these atoms from agglomeration at elevated temperatures. Here we investigate the formation of Pt single atoms on an industrial catalyst support. Using a combination of surface sensitive techniques such as XPS and LEIS, X-ray absorption spectroscopy, electron microscopy, as well as density functional theory, we demonstrate that cerium oxide can support Pt single atoms at high metal loading (3 wt % Pt), without forming any clusters or 3D aggregates when heated in air at 800 °C. The mechanism of trapping involves a reaction of the mobile PtO2 with under-coordinated cerium cations present at CeO2(111) step edges, allowing Pt to achieve a stable square planar configuration. The strong interaction of mobile single-atom species with the support, present during catalyst sintering and regeneration, helps explain the sinter resistance of ceria-supported metal catalysts.

In this work, we compare the CO oxidation performance of Pt single atom catalysts (SACs) prepared via two methods: (1) conventional wet chemical synthesis (strong electrostatic adsorption–SEA) with calcination at 350 °C in air; and (2) high temperature vapor phase synthesis (atom trapping–AT) with calcination in air at 800 °C leading to ionic Pt being trapped on the CeO2 in a thermally stable form. As-synthesized, both SACs are inactive for low temperature (<150 °C) CO oxidation. After treatment in CO at 275 °C, both catalysts show enhanced reactivity. Despite similar Pt metal particle size, the AT catalyst is significantly more active, with onset of CO oxidation near room temperature. A combination of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and CO temperature-programmed reduction (CO-TPR) shows that the high reactivity at low temperatures can be related to the improved reducibility of lattice oxygen on the CeO2 support.

Since the discovery that ceria is an active catalyst for selective hydrogenation of alkynes, there has been much debate on the catalytic mechanism. In this work, we propose, based on density functional theory (DFT) investigations, a mechanism that involves the heterolytic dissociation of H at oxygen vacancies of CeO(111), facilitated by frustrated Lewis pairs consisting of spatially separated O and Ce sites. The resulting O-H and Ce-H species effectively catalyze the hydrogenation of acetylene, avoiding the overstabilization of the CH* intermediate in a previously proposed mechanism. On the basis of our mechanism, we propose the doping of ceria by Ni as a means to create oxygen vacancies. Interestingly, the Ni dopant is not directly involved in the catalytic reaction, but serves as a single-atom promoter. Experimental studies confirm the design principles and demonstrate much higher activity for Ni-doped ceria in selective hydrogenation of acetylene. The combined results from DFT calculations and experiment provide a basis to further develop selective hydrogenation catalysts based on earth-abundant materials.

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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.