Single-Atom Catalysts for Hydrogen Activation

DFT Study of the Mechanism of Selective Hydrogenation of Acetylene by Rhodium Single-Atom Catalyst Supported on HY Zeolite

The selectivity of acetylene hydrogenation by the Rh single-atom catalyst (SAC) supported on HY zeolite was investigated using density functional theory (DFT) and a 5/83T quantum mechanics/molecular mechanics (QM/MM) embedded cluster model. The calculated activation barrier (ΔG≠) for the oxidative addition of dihydrogen to the Rh metal center (15.9 kcal/mol) is lower in energy than that for the σ-bond metathesis of dihydrogen to the Rh-C bond (22.7 kcal/mol) and the Rh-O bond (28.4 kcal/mol). The activation barriers of the oxidative addition of subsequent dihydrogen molecules are significantly higher than that of the oxidative addition of the first dihydrogen molecule. These findings align with the experimentally observed activity and selectivity of the atomically dispersed Rh catalyst supported on HY zeolite. Natural bond orbital (NBO), molecular orbital (MO) and fuzzy bond order analyses were used to examine the interaction between the Rh metal center and acetylene versus ethylene ligands. The occupancies of the Rh lone pairs, π-bonding and π*-antibonding orbitals of acetylene and ethylene are consistent with the expected stronger interaction between the Rh metal center and acetylene compared to ethylene on the HY zeolite support.

Exceptional CO2 Hydrogenation to Ethanol via Precise Single-Atom Ir Deposition on Functional P Islands

The thermocatalytic hydrogenation of CO2 to ethanol has attracted significant interest because ethanol offers ease of transport and substantial value in chemical synthesis. Here, we present a state-of-the-art catalyst for the CO2 hydrogenation to ethanol achieved by precisely depositing single-atom Ir species on P cluster islands situated on the In2O3 nanosheets. The Ir1-Px/In2O3 catalyst achieves an impressive ethanol yield of 3.33 mmol g-1 h-1 and a turnover frequency (TOF) of 914 h-1 under 1.0 MPa (H2/CO2=3 : 1) at 180 °C, nearly 8 times higher than that of the unmodified Ir1/In2O3 catalyst. Additionally, at a more industrially relevant pressure of 5.0 MPa, the TOF of the Ir1-Px/In2O3 catalyst can reach up to 2108 h-1, surpassing previously reported catalysts. Combined in situ characterization and theoretical studies reveal that the hydrogenation process is significantly enhanced by the Ir1-Px entities. Specifically, the Ir atom facilitates CO2 activation and C-C coupling, while the surrounding P island exhibits exceptional H2 dissociation ability. These three steps have been found crucial for the CO2 hydrogenation reaction. This discovery opens new opportunities for the regulation of the microenvironment of current catalysts by providing essential chemical functionalities that enhance intricate and complex reaction processes.

From Single Atoms to Clusters: Unraveling the Structural Evolution of Pt/CeO2 for Enhanced CO Oxidation

The structural evolution of catalysts and the identification of active sites are critical yet challenging aspects of heterogeneous reactions. In this work, we investigate the structural evolution of Pt/CeO2 catalysts during CO oxidation by using theoretical calculations, focusing on the influence of initial catalyst states on the resulting active sites and reactivity. Our findings reveal that under the reaction conditions, single Pt atoms gradually aggregate into Pt clusters. When single Pt atoms are substituted for surface Ce atoms (Ptin), the resulting small clusters (Ptn) are exclusively formed based on Ptin. However, when both Ptin and surface-adsorbed Pt atoms (Ptad) coexist, additional small surface-adsorbed clusters (Ptnad) are generated. An increase in the Ptad/Ptin ratio leads to a higher proportion of clusters at the active sites, which correlates with enhanced CO oxidation activity as the number of clusters increases. This study underscores the importance of understanding catalyst evolution and active site dynamics under the reaction conditions, providing theoretical insights for the rational design of more efficient catalysts.

Theoretical Investigation of Single-Atom Catalysts for Hydrogen Evolution Reaction Based on Two-Dimensional Tetragonal V2C2 and V3C3

Developing stable and effective catalysts for the hydrogen evolution reaction (HER) has been a long-standing pursuit. In this work, we propose a series of single-atom catalysts (SACs) by importing transition-metal atoms into the carbon and vanadium vacancies of tetragonal V2C2 and V3C3 slabs, where the transition-metal atoms refer to Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. By means of first-principles computations, the possibility of applying these SACs in HER catalysis was investigated. All the SACs are conductive, which is favorable to charge transfer during HER. The Gibbs free energy change (ΔGH*) during hydrogen adsorption was adopted to assess their catalytic ability. For the V2C2-based SACs with V, Cr, Mn, Fe, Ni, and Cu located at the carbon vacancy, excellent HER catalytic performance was achieved, with a |ΔGH*| smaller than 0.2 eV. Among the V3C3-based SACs, apart from the SAC with Mn located at the carbon vacancy, all the SACs can act as outstanding HER catalysts. According to the ΔGH*, these excellent V2C2- and V3C3-based SACs are comparable to the best-known Pt-based HER catalysts. However, it should be noted that the V2C2 and V3C3 slabs have not been successfully synthesized in the laboratory, leading to a pure investigation without practical application in this work.

The Role of Frustrated Lewis Pair in Catalytic Transfer Hydrogenation of Furfural using Nickel Single-Atom Catalysts: A Theoretical Study

The burgeoning field of frustrated Lewis pair (FLP) heterogeneous catalysts has garnered significant interest in recent years, primarily due to their inherent ability to activate H-source molecules, thereby facilitating hydrogenation reactions. In this study, non-precious transition metal atoms were anchored onto several models of pyridinic nitrogen incorporated graphene sheet. Theoretical calculations substantiated energy barriers as low as 0.10 eV for isopropanol activation, thereby positioning these catalysts as highly promising candidates for catalytic transfer hydrogenation of furfural. Electronic structure analyses revealed that the H-O bond breakage in isopropanol molecules was significantly facilitated by the presence of FLP sites within the catalysts. Notably, both Ni-C2N and Ni-N6-C demonstrated exceptional potential as selective catalysts for the hydrogenation of furfural into furfuryl alcohol, exhibiting remarkably low barriers of only 0.65-0.72 eV for the rate-determining steps. The findings in this study are helpful to design FLP containing single atom catalysts for hydrogenation reactions.

Electrochemical hydrogenation and oxidation of organic species involving water

Fossil fuel-driven thermochemical hydrogenation and oxidation using high-pressure H2 and O2 are still popular but energy-intensive CO2-emitting processes. At present, developing renewable energy-powered electrochemical technologies, especially those using clean, safe and easy-to-handle reducing agents and oxidants for organic hydrogenation and oxidation reactions, is urgently needed. Water is an ideal carrier of hydrogen and oxygen. Electrochemistry provides a powerful route to drive water splitting under ambient conditions. Thus, electrochemical hydrogenation and oxidation transformations involving water as the hydrogen source and oxidant, respectively, have been developed to be mild and efficient tools to synthesize organic hydrogenated and oxidized products. In this Review, we highlight the advances in water-participating electrochemical hydrogenation and oxidation reactions of representative organic molecules. Typical electrode materials, performance metrics and key characterization techniques are firstly introduced. General electrocatalyst design principles and controlling the microenvironment for promoting hydrogenation and oxygenation reactions involving water are summarized. Furthermore, paired hydrogenation and oxidation reactions are briefly introduced before finally discussing the challenges and future opportunities of this research field.