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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 techniquesmore » 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.« less

Dopants enhance the catalytic properties of ceria. However, conventional techniques for synthesizing doped ceria have limitations in terms of structural homogeneity, surface area, and catalytic activity of the resulting oxide. Use of toxic and corrosive chemicals presents further challenges. The sol-gel method described in this work provides a facile approach for incorporating high concentrations of dopants in a uniform, high surface area structure, yielding excellent catalytic activity. Addition of polyvinylpyrrolidone (PVP) complexing agent prevents the segregation of cerium and dopant atoms during synthesis. Surface areas up to 179 m²/g are achieved, which represents a substantial improvement over doped ceria producedmore » through coprecipitation, solution combustion, or melt-synthesis methods. The resulting powders exhibit dramatically improved CO oxidation activity (T₉₀ = 132 °C for 3.2 wt% Cu-CeO₂ compared to 274 °C for a 2 wt% Pt-Al₂O₃ reference catalyst). First principles calculations suggest a Mars Van Krevelen mechanism, which is facilitated by dopants causing oxygen vacancies.« less

Ammonia decomposition catalyzed by Ru nanoparticles supported on carbon nanotubes offers an efficient way for CO_x-free hydrogen generation. To understand the catalytic mechanism, the two most important elementary steps of ammonia decomposition, namely the initial cleavage of the NH₂–H bond and the nitrogen recombination, have been studied using density functional theory on a carbon nanotube deposited with Ru_x (x = 1, 2, 6, and 13) clusters. The results indicate the reaction steps are catalyzed at Ru sites with barriers significantly lower than those on Ru(0001), but the barriers have a strong dependence on the size of the cluster. Finally, itmore » is also found that Ru sites at the interface with the carbon nanotube are more active, showing a strong interfacial effect due apparently to facile charge transfer from the carbon nanotube to interfacial metal atoms.« less

Metallic nickel is known to be an active, but not a selective hydrogenation catalyst for conversion of alkynes to alkenes. On the other hand, nickel oxide is not active. Recently, we have demonstrated that nickel doped into ceria provides an inexpensive catalyst for selective hydrogenation of acetylene in the presence of ethylene. Here, we evaluate various synthesis methods to achieve optimal selective hydrogenation performance. We examined incipient wetness impregnation, coprecipitation, solution combustion, and sol-gel synthesis to study how the method of preparation affects catalytic structure and behavior. Sol-gel synthesis, coprecipitation, and solution combustion synthesis methods favor nickel incorporation into themore » ceria lattice, while incipient wetness impregnation creates segregated nickel species on the ceria surface. For hydrogenation of acetylene, these nickel surface species lead to poor ethylene selectivity due to ethane and oligomer formation. However, when nickel is incorporated into the ceria lattice, ethane formation is prevented even while achieving 100 % conversion of acetylene. Coke formation is also significantly reduced on these catalysts compared to conventional nanoparticle counterparts. Finally, we conclude that sol-gel synthesis provides the optimal method for creating a uniform dopant distribution within the high surface area ceria.« less