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.

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.

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.

Pt/CeO2 single-atom catalysts have recently attracted increasing interest due to excellent thermal stability, high atom efficiency, and high activity in catalysis. In this study, by means of density functional theory (DFT) calculations, we systematically compare the stability and CO oxidation reactivity of Pt single atoms supported on CeO2(111) (Pt/CeO2) and Ga-doped CeO2(111) (Pt/Ga–CeO2). It was found that the formation of an oxygen vacancy (OV) is very facile near a surface Ga-doping site (Pt/Ga–CeO2–OV). Significantly, the stability of Pt single atoms anchored on the Ga site was enhanced compared with those on the bare ceria surface. In addition, our DFT results suggest a CO oxidation mechanism on Pt/Ga–CeO2–OV that differs from that on Pt/CeO2. In particular, the OV site plays an important role in activating the oxygen molecule, which then reacts with CO preadsorbed on Pt. The calculated energy barrier on Pt/Ga–CeO2–OV is about 0.43 eV lower than that on the undoped catalyst, suggesting an enhanced reactivity for CO oxidation. Experiments on CO oxidation and in situ diffuse reflectance infrared Fourier transform spectroscopy are performed to corroborate the results obtained from the DFT calculations, and a good agreement is achieved. The combination between calculations and experiments sheds light on the influence of support doping on atomically dispersed Pt/CeO2 catalysts.

Converged differential and integral cross sections are reported for the H + O2 --> OH + O reaction on an improved potential energy surface of HO2(X2A'') using a dynamically exact quantum wave packet method and Gaussian weighted quasi-classical trajectory method. The complex-forming mechanism is confirmed by strong forward and backward scattering peaks and by highly inverted OH rotational state distributions. Both the quantum and classical results provide strong evidence for nonstatistical behavior in this important reaction.

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. It 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.

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A global potential-energy surface for the first excited electronic state of NH(2)(A(2)A(')) has been constructed by three-dimensional cubic spline interpolation of more than 20,000 ab initio points, which were calculated at the multireference configuration-interaction level with the Davidson correction using the augmented correlation-consistent polarized valence quadruple-zeta basis set. The (J=0) vibrational energy levels for the ground (X(2)A(")) and excited (A(2)A(')) electronic states of NH(2) were calculated on our potential-energy surfaces with the diagonal Renner-Teller terms. The results show a good agreement with the experimental vibrational frequencies of NH(2) and its isotopomers.