Single noble metal atoms and ultrafine metal clusters catalysts tend to sinter into aggregated particles at elevated temperatures, driven by the decrease of metal surface free energy. Herein, we report an unexpected phenomenon that noble metal nanoparticles (Pd, Pt, Au-NPs) can be transformed to thermally stable single atoms (Pd, Pt, Au-SAs) above 900 °C in an inert atmosphere. The atomic dispersion of metal single atoms was confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. The dynamic process was recorded by in situ environmental transmission electron microscopy, which showed competing sintering and atomization processes during NP-to-SA conversion. Further, density functional theory calculations revealed that high-temperature NP-to-SA conversion was driven by the formation of the more thermodynamically stable Pd-N structure when mobile Pd atoms were captured on the defects of nitrogen-doped carbon. The thermally stable single atoms (Pd-SAs) exhibited even better activity and selectivity than nanoparticles (Pd-NPs) for semi-hydrogenation of acetylene.

A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO 2 to specific hydrocarbons and oxygenates is to determine the detailed reaction mechanism including kinetics and product selectivity as a function of pH and applied potential for known systems. To accomplish this, we apply ab initio molecular metadynamics simulations (AIMμD) for the water/Cu(100) system with five layers of the explicit solvent under a potential of −0.59 V [reversible hydrogen electrode (RHE)] at pH 7 and compare with experiment. From these free-energy calculations, we determined the kinetics and pathways for major products (ethylene and methane) and minor products (ethanol, glyoxal, glycolaldehyde, ethylene glycol, acetaldehyde, ethane, and methanol). For an applied potential (U) greater than −0.6 V (RHE) ethylene, the major product, is produced via the Eley-Rideal (ER) mechanism using H 2 O + e -. The rate-determining step (RDS) is C-C coupling of two CO, with ΔG ‡ = 0.69 eV. For an applied potential less than −0.60 V (RHE), the rate of ethylene formation decreases, mainly due to the loss of CO surface sites, which are replaced by H*. The reappearance of C 2 H 4 along with CH 4 at U less than −0.85 V arises from *CHO formation produced via an ER process of H* with nonadsorbed CO (a unique result). This *CHO is the common intermediate for the formation of both CH 4 and C 2 H 4 . These results suggest that, to obtain hydrocarbon products selectively and efficiency at pH 7, we need to increase the CO concentration by changing the solvent or alloying the surface.reaction mechanism | electrocatalysis | copper | QM metadynamics | free-energy reaction barriers

Energy and environmental concerns demand development of more efficient and selective electrodes for electrochemical reduction of CO2 to form fuels and chemicals. Since Cu is the only pure metal exhibiting reduction to form hydrocarbon chemicals, we focus here on the Cu (111) electrode. We present a methodology for density functional theory calculations to obtain accurate onset electrochemical potentials with explicit constant electrochemical potential and pH effects using implicit solvation. We predict the atomistic mechanisms underlying electrochemical reduction of CO, finding that (1) at acidic pH, the C1 pathway proceeds through COH to CHOH to form CH4 while C2 (C3) pathways are kinetically blocked; (2) at neutral pH, the C1 and C2 (C3) pathways share the COH common intermediate, where the branch to C-C coupling is realized by a novel CO-COH pathway; and (3) at high pH, early C-C coupling through adsorbed CO dimerization dominates, suppressing the C1 pathways by kinetics, thereby boosting selectivity for multi-carbon products.

We propose and validate with quantum mechanics methods a unique catalyst for electrochemical reduction of CO 2 (CO 2 RR) in which selectivity and activity of CO and C 2 products are both enhanced at the borders of oxidized and metallic surface regions. This Cu metal embedded in oxidized matrix (MEOM) catalyst is consistent with observations that Cu 2 O-based electrodes improve performance. However, we show that a fully oxidized matrix (FOM) model would not explain the experimentally observed performance boost, and we show that the FOM is not stable under CO 2 reduction conditions. This electrostatic tension between the Cu + and Cu 0 surface sites responsible for the MEOM mechanism suggests a unique strategy for designing more efficient and selective electrocatalysts for CO 2 RR to valuable chemicals (HCOx), a critical need for practical environmental and energy applications.electrochemical reduction of CO 2 | Cu metal embedded in oxidized matrix | density functional theory | CO 2 activation | CO dimerization E lectrochemical reduction of CO2 (CO2RR) to valuable chemicals is an essential strategy to achieve industrial-scale reduction of the carbon footprint under mild conditions and to provide a means of storing electrical power from intermittent renewable sources into stable chemical forms (1). Cu is the prototype electrocatalyst for CO2RR, because it is the only pure metal that delivers appreciable amounts of methane and ethylene plus minor alcohol products (2-7), but it suffers from high overpotentials and very significant hydrogen evolution reactions (HERs). Consequently, tremendous efforts are being made to develop more efficient and selective electrocatalysts, for example by surface modification (8) and by nanoparticle (9, 10) and nanowire (11) engineering.We examine here the mechanism by which Cu2O-based electrodes are observed to improve both efficiency and selectivity for C2 products (12-15), which also suppresses HERs by severalfold. Because Cu2O is subject to reduction (back to Cu metal) under CO2RR conditions, the improved performance was initially attributed to Cu metal surface morphology (8,16). But a more recent experiment (15) showed that Cu + sites can survive on the Cu surface for the course of CO2RR. Importantly, a Cu sample that is first oxidized and then reduced using an H2 plasma leads to performance substantially worse than that of the oxidized sample, despite both having similarly roughened surfaces. This provides solid evidence that surface Cu + plays an essential role in promoting the efficiency and selectivity of CO2RR. However, experiments have provided no clue about how surface Cu + affects the mechanisms of CO2RR. Moreover, no previous theoretical efforts have elucidated its role.To understand the promising results achieved with Cu2O-based electrodes, we investigated three distinct models aimed at unraveling the role of surface Cu + in shaping the free energy profiles of two key steps for CO2RR. Here we carry out quantum mechanics (QM) calculations at constant potential by using our ...

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Heteroatom-doped Fe-NC catalyst has emerged as one of the most promising candidates to replace noble metal-based catalysts for highly efficient oxygen reduction reaction (ORR). However, delicate controls over their structure parameters to optimize the catalytic efficiency and molecular-level understandings of the catalytic mechanism are still challenging. Herein, a novel pyrrole-thiophene copolymer pyrolysis strategy to synthesize Fe-isolated single atoms on sulfur and nitrogen-codoped carbon (Fe-ISA/SNC) with controllable S, N doping is rationally designed. The catalytic efficiency of Fe-ISA/SNC shows a volcano-type curve with the increase of sulfur doping. The optimized Fe-ISA/SNC exhibits a half-wave potential of 0.896 V (vs reversible hydrogen electrode (RHE)), which is more positive than those of Fe-isolated single atoms on nitrogen codoped carbon (Fe-ISA/NC, 0.839 V), commercial Pt/C (0.841 V), and most reported nonprecious metal catalysts. Fe-ISA/SNC is methanol tolerable and shows negligible activity decay in alkaline condition during 15 000 voltage cycles. X-ray absorption fine structure analysis and density functional theory calculations reveal that the incorporated sulfur engineers the charges on N atoms surrounding the Fe reactive center. The enriched charge facilitates the rate-limiting reductive release of OH* and therefore improved the overall ORR efficiency.

A national priority is to convert CO 2 into high-value chemical products such as liquid fuels. Because current electrocatalysts are not adequate, we aim to discover new catalysts by obtaining a detailed understanding of the initial steps of CO 2 electroreduction on copper surfaces, the best current catalysts. Using ambient pressure X-ray photoelectron spectroscopy interpreted with quantum mechanical prediction of the structures and free energies, we show that the presence of a thin suboxide structure below the copper surface is essential to bind the CO 2 in the physisorbed configuration at 298 K, and we show that this suboxide is essential for converting to the chemisorbed CO 2 in the presence of water as the first step toward CO 2 reduction products such as formate and CO. This optimum suboxide leads to both neutral and charged Cu surface sites, providing fresh insights into how to design improved carbon dioxide reduction catalysts. T he discovery of new electrocatalysts that can efficiently convert carbon dioxide (CO2) into liquid fuels and feedstock chemicals would provide a clear path to creating a sustainable hydrocarbon-based energy cycle (1). However, because CO2 is highly inert, the CO2 reduction reaction (CO2RR) is quite unfavorable thermodynamically. This makes identification of a suitable and scalable catalyst an important challenge for sustainable production of hydrocarbons. We consider that discovering such a catalyst will require the development of a complete atomistic understanding of the adsorption and activation mechanisms involved. Here the first step is to promote initiation of reaction steps.Copper (Cu) is the most promising CO2RR candidate among pure metals, with the unique ability to catalyze formation of valuable hydrocarbons (e.g., methane, ethylene, and ethanol) (2). However, Cu also produces hydrogen, requires too high an overpotential (>1 V) to reduce CO2, and is not selective for desirable hydrocarbon and alcohol CO2RR products (2). Despite numerous experimental and theoretical studies, there remain considerable uncertainties in understanding the role of Cu surface structure and chemistry on the initial steps of CO2RR activity and selectivity (3, 4). To reduce CO2 to valuable hydrocarbons, a source of protons is needed in the same reaction environment (2), with water (H2O) the favorite choice. Thus, H2O is often the solvent for CO2RR, representing a sustainable pathway toward solar energy storage (1). However, we lack a comprehensive understanding of how CO2 and H2O molecules adsorb on the Cu surface and interact to first dissociate the CO2 (5, 6). An overview of the various surface reactions of CO2 on Cu(111) is reported in Fig. 1, illustrating the transient carbon-based intermediate species that may initiate reactions.Previous studies using electron-based spectroscopies observed physisorption of gas-phase g-CO2 at 75 K, whereas a chemisorbed form of CO2 was stabilized by a partial negative charge induced by electron capture (CO δ− 2 ) (Fig. 1A) (7, 8). The same experiments showed th...

The oxygen evolution reaction (OER) is critical to solar production of fuels, but the reaction mechanism underlying the performance for a best OER catalyst, Fe-doped NiOOH [(Ni,Fe)OOH], remains highly controversial. We used grand canonical quantum mechanics to predict the OER mechanisms including kinetics and thus overpotentials as a function of Fe content in (Ni,Fe)OOH catalysts. We find that density functional theory (DFT) without exact exchange predicts that addition of Fe does not reduce the overpotential much. However, DFT with exact exchange predicts dramatic improvement in performance for (Ni,Fe)OOH, leading to an overpotential of 0.42 V and a Tafel slope of 23 mV/decade (dec), in good agreement with experiments, 0.3-0.4 V and 30 mV/dec. We reveal that the high spin [Formula: see text] Fe(IV) leads to efficient formation of an active O radical intermediate, while the closed shell [Formula: see text] Ni(IV) catalyzes the subsequent O-O coupling, and thus it is the synergy between Fe and Ni that delivers the optimal performance for OER.