Identifying the origin of carbon deposition in reactions, such as dry reforming of methane (DRM) over cobalt (Co) nanocatalysts, is an important yet challenging issue in heterogeneous catalysis. In this study, we used density functional theory (DFT) calculations to investigate the surface reactions of CO2 with C* at the flat and step sites of face-centered cubic (FCC) Co [(111), (110), (100), (211), and (221)], which represent the major surfaces of Co nanoparticles. The results were being compared with Ni nanoparticles. Hereby, we identified that the high degree of preference for C–C coupling over CO* formation, especially on Co(111), serves as the origin of carbon graphitization. This finding is similar to that on Ni(111). Furthermore, we reported for the first time that the significant difference between Co and Ni is due to CO2 activation, which is far more favored on Co than on Ni, accounting for the lower carbon deposition on Co. On the other hand, within the investigated surfaces of Co, step and less common surfaces, namely, Co(211), Co(221), and Co(100), do not favor C–C coupling. Based on our findings, we proposed that high-index-facet, surface-modified, and/or promoted Co nanoparticles be used for DRM to restrict C–C coupling.

The interactions of CO 2 with terrace, step, and defect or kink sites on Pt surfaces were investigated using temperature-programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations. Desorption peaks of CO 2 on Pt(997) were observed at ∼79, 88−89, ∼92 , and ∼103 K and were respectively assigned to desorption of CO 2 from multilayer CO 2 (amorphous CO 2 ), CO 2 from terrace, CO 2 from step, and CO 2 from defect sites. The defect sites, step sites, and terrace sites were saturated in that order before multilayer adsorption occurred. The adsorption energies of CO 2 on the terrace, step, and defect sites were estimated to be around −0.23, −0.28, and −0.34 eV, respectively. The experimentally measured adsorption energies of CO 2 on Pt were successfully reproduced using the optB86b-vdW, rev-vdW-DF2, and PBE-D2 functionals, and the actual adsorption energies were found to be between those calculated with rev-vdW-DF2 and optB86b-vdW. Additionally, it was found that CO 2 adsorption is energetically more stable at higher CO 2 coverage than at lower coverage because of CO 2 −CO 2 lateral attractive interactions.

The reaction mechanism of the CH3OH synthesis by the hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Thus, the objective of this study is to clarify the reaction mechanism of the CH3OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOOa on Cu(111). We exposed HCOOa on Cu(111) to atomic hydrogen at low temperatures of 200–250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOOa species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOOa species to atomic hydrogen, were detected in the TPD studies. Therefore, H2COOa is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH3OH synthesis via H2COOa from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOOa to H2COOa is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO2 on Cu surfaces.

The Cu–Zn surface alloy has been extensively involved in the investigation of the true active site of Cu/ZnO/Al2O3, the industrial catalyst for methanol synthesis which remains under controversy. The challenge lies in capturing the interplay between the surface and reaction under operating conditions, which can be overcome given that the explicit dynamics of the system is known. To provide a better understanding of the dynamic of Cu–Zn surface at the atomic level, the structure and the formation process of the Cu–Zn surface alloy on Cu(997) were investigated by machine-learning molecular dynamics (MD). Gaussian process regression aided with on-the-fly learning was employed to build the force field used in the MD. The simulation reveals atomistic details of the alloying process, that is, the incorporation of deposited Zn adatoms to the Cu substrate. The surface alloying is found to start at upper and lower terraces near the step edge, which emphasize the role of steps and kinks in the alloying. The incorporation of Zn at the middle terrace was found at the later stage of the simulation. The rationalization of alloying behavior was performed based on statistics and barriers of various elementary events that occur during the simulation. It was observed that the alloying scheme at the upper terrace is dominated by the confinement of Zn step adatoms by other adatoms, highlighting the importance of step fluctuations in the alloying process. On the other hand, the alloying scheme at the lower terrace is dominated by direct exchange between the Zn step adatom and the Cu atom underneath. The alloying at the middle terrace is dominated by the wave deposition mechanism and deep confinement of Zn adatoms. The short propagation of alloyed Zn in the middle terrace was observed to proceed by means of indirect exchange instead of local exchange as proposed in the previous scanning tunneling microscopy (STM) observation. The comparison of migration rate and activation energies to the result of STM observation is also made. We have found that at a certain distance from the surface, the STM tip significantly affects the elementary events such as vacancy formation and direct exchange.

Understanding the nature of active sites is a nontrivial task, especially when the catalyst is sensitively affected by chemical reactions and environmental conditions. The challenge lies on capturing explicitly the dynamics of catalyst evolution during reactions. Despite the complexity of catalyst reconstruction, we can untangle them into several elementary processes, of which surface diffusion is of prime importance. By applying density functional theory−kinetic Monte Carlo (DFT−KMC) simulation employed with cluster expansion (CE), we investigated the microscopic mechanism of surface diffusion of Cu with defects such as steps and kinks. Based on the result, the energetics obtained from CE have shown good agreement with DFT calculations. Various diffusion events during the step fluctuations are discussed as well. Aside from the adatom attachment, the diffusion along the step edge is found to be the dominant mass transport mechanism, indicated by the lowest activation energy. We also calculated time correlation functions at 300, 400, and 500 K. However, the time exponent in the correlation function does not strictly follow the power law behavior due to the limited step length, which inhibits variation in the kink density.