Defective Graphene on the Transition-Metal Surface: Formation of Efficient Bifunctional Catalysts for Oxygen Evolution/Reduction Reactions in Alkaline Media

DFT-assisted low-dimensional carbon-based electrocatalysts design and mechanism study: a review

Low-dimensional carbon-based (LDC) materials have attracted extensive research attention in electrocatalysis because of their unique advantages such as structural diversity, low cost, and chemical tolerance. They have been widely used in a broad range of electrochemical reactions to relieve environmental pollution and energy crisis. Typical examples include hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Traditional "trial and error" strategies greatly slowed down the rational design of electrocatalysts for these important applications. Recent studies show that the combination of density functional theory (DFT) calculations and experimental research is capable of accurately predicting the structures of electrocatalysts, thus revealing the catalytic mechanisms. Herein, current well-recognized collaboration methods of theory and practice are reviewed. The commonly used calculation methods and the basic functionals are briefly summarized. Special attention is paid to descriptors that are widely accepted as a bridge linking the structure and activity and the breakthroughs for high-volume accurate prediction of electrocatalysts. Importantly, correlated multiple descriptors are used to systematically describe the complicated interfacial electrocatalytic processes of LDC catalysts. Furthermore, machine learning and high-throughput simulations are crucial in assisting the discovery of new multiple descriptors and reaction mechanisms. This review will guide the further development of LDC electrocatalysts for extended applications from the aspect of DFT computations.

Rare Earth Metal Anchored into Nitrogen-Doped Graphene for CO2 Electrocatalytic Reduction to C1 Products

Single-atom catalysts (SACs) are attracting global attention due to their 100% atomic utilization rate and unique properties. Rare-earth-based SACs have shown great potential in the field of electrocatalysis in recent years. In this study, the catalytic performance of four rare earth metals (REMs) anchored into N-graphene for the CO2RR is systematically studied by density functional theory. The calculation results of formation energy show that all REM@N6-G compounds have favorable stability. In addition, the Gibbs free energy calculation results of all possible elementary reactions show that the *OCHO pathway is the optimal hydrogenation pathway for all catalysts, and they have the same potential determining step (*OCHO + e- + H+ → *HCOOH). Meanwhile, the products of the CO2RR on these catalysts are different, and the product on REM@N6-G (REM = La, Pr, and Nd) is CH4, while the product on Ce@N6-G is CH3OH. In particular, Nd@N6-G exhibits the best catalytic activity in this work, with a very low limiting potential of -0.38 V. These results may guide the development of rare-earth-based SACs for CO2RR.

The rational design of high-performance graphene-based single-atom electrocatalysts for the ORR using machine learning

In this work, high-performance two-dimensional (2D) graphene-based single-atom electrocatalysts (ZZ/ZA-MN_xC_y) for the oxygen reduction reaction (ORR) were screened out using machine learning (ML). A model was built for the fast prediction of electrocatalysts and two descriptors valence electron correction (VEc) and degree of construction differences (DC) were proposed to improve the accuracy of the model prediction. Two evaluation criteria, high-performance catalyst retention rate r_R and high-performance catalyst occupancy rate r_O, were proposed to evaluate the accuracy of ML models in high-performance catalyst screening. The addition of VEc and DC in the model could change the mean absolute error (MAE_test) of the test set, the coefficient of determination (R²_test) of the test set, r_O, and r_R from 0.334 V, 0.683, 0.222, and 0.360 to 0.271 V, 0.774, 0.421, and 0.671, respectively. The partially screened potential high-performance ORR electrocatalysts such as ZZ-CoN₄ and ZZ-CoN₃C₁ were also further investigated using a Density Functional Theory (DFT) method, which confirmed the accuracy of the ML model (MAE = 0.157 V, R² = 0.821).

Design of γ-Alumina-Supported Phosphotungstic Acid-Palladium Bifunctional Catalyst for Catalytic Liquid-Phase Citral Hydrogenation

This study primarily addresses the development of dynamic, selective and economical metal–acid (bifunctional) catalysts for one-pot menthol production by citral hydrogenation. Specifically, various metals such as Pd, Pt, Ni, Cs and Sn were doped over alumina support. Additionally, bifunctional composite catalysts were also prepared with the impregnation of heteropoly acids and Pd precursors over alumina support. Analytical techniques (e.g., BET, PXRD, FT-IR, pyridine adsorption and amine titration methods) were applied for characterization of the most efficient and selective catalysts (e.g., Al2O3 and PTA-Cat-I). Similarly, most of the essential operational variables (e.g., loading rate of metal precursor, type of heteropoly acid, temperature, gas pressure and reaction time) were examined during this study. The experimental data shows that the bifunctional catalyst (PTA-Cat-I) produced 45% menthol at full citral substrate conversion (r = 0.038 mmoles.min−1) in liquid-phase citral hydrogenation (at optimized operating conditions: 70 °C, 0.5 MPa and 8 h). However, the heteropoly acid-supported bifunctional catalysts (e.g., PTA-Cat-I, PMA-Cat-I, SMA-Cat-I and STA-Cat-I) resulted in cracking and the dehydration of isopulegol/menthol by the generation of side products (e.g., 4-isopropyl-1-methyl, cyclohex-1-ane/ene); therefore, menthol yield was extensively diminished. On the other hand, non-acidic catalysts (e.g., Cat-I, Cat-II, Cat-III, Cat-IV and Cat-V) readily promoted hydrogenation reactions. The optimum menthol yield occurred due to the presence of strong Lewis and weak Bronsted acid sites. Mass transfer and reaction rate were substantially diminished due to acidity strength, heteropoly acid type and blockage of pores by the applied bifunctional catalysts.

Multifunctional Electrocatalysis on Single-Site Metal Catalysts: A Computational Perspective

Multifunctional electrocatalysts are vastly sought for their applications in water splitting electrolyzers, metal-air batteries, and regenerative fuel cells because of their ability to catalyze multiple reactions such as hydrogen evolution, oxygen evolution, and oxygen reduction reactions. More specifically, the application of single-atom electrocatalyst in multifunctional catalysis is a promising approach to ensure good atomic efficiency, tunability and additionally benefits simple theoretical treatment. In this review, we provide insights into the variety of single-site metal catalysts and their identification. We also summarize the recent advancements in computational modeling of multifunctional electrocatalysis on single-site catalysts. Furthermore, we explain each modeling step with open-source-based working examples of a standard computational approach.

Rational Design of Self-Supported CuO x -Decorated Composite Films as an Efficient and Easy-Recycling Catalyst for Styrene Oxidation

The applications of graphene-based materials in catalysis are limited by their strong tendency to aggregate, which may lead to a decrease in active sites. Herein, we propose a facile and controllable strategy to fabricate a series of heterogeneous catalysts with a unique nanostructure wherein CuO _x -decorated reduced graphene oxide (rGO) sheets are incorporated into a solid matrix composed of poly(vinylpyrrolidone) (PVP) and carboxymethyl cellulose (CMC). The resultant materials are self-supported films and could be directly used as catalysts for the liquid-phase oxidation of styrene without the requirement for extra substrates. The employment of PVP-CMC (PC) as the support for CuO _x -decorated rGO sheets successfully inhibits their aggregation. Benefiting from the dispersion of copper species, these films exhibit good catalytic activity and recyclability under mild reaction conditions. Especially, they can be conveniently removed from the reaction mixture by tweezers due to their structural stability. For catalyzing multiple reactions with high efficiency and facile recyclability, this study offers a universal strategy to design heterogeneous catalysts based on graphene materials and provides a promising platform.

Unraveling the single-atom electrocatalytic activity of transition metal-doped phosphorene

The development of single-atom catalysts (SACs) for chemical reactions of vital importance in the renewable energy sector has emerged as an urgent priority. In this perspective, transition metal-based SACs with monolayer phosphorous (phosphorene) as the supporting material are scrutinized for their electrocatalytic activity towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) from first-principle calculations. The detailed screening study has confirmed a breaking of the scaling relationship between the ORR/OER intermediates, resulting in various activity trends across the transition metal series. Groups 9 and 10 transition metal-based SACs are identified as potential catalyst candidates with the platinum single atom offering bifunctional activity for OER and HER with diminished overpotentials. Ambient condition stability analysis of SACs confirmed a different extent of interaction towards oxygen and water compared to pristine phosphorene, suggesting room for improving the stability of phosphorene via chemical functionalization.

Contrasting Oxygen Reduction Reactions on Zero- and One-Dimensional Defects of MoS2 for Versatile Applications

Oxygen reduction reaction (ORR) is a key microscopic process in many electrochemical applications of materials, where the requirements of their ORR performances may vary strikingly, for example, during the uses of MoS₂ as an electrocatalyst and anticorrosion/lubricating coating in aqueous/humid environments, ORR should be activated and inhibited, respectively. To reveal a complete ORR profile of MoS₂, using first-principles calculations, we examine the stabilities of various possible zero-dimensional point defects on the surface and one-dimensional edge defects and comprehensively explore the ORR activities on pristine MoS₂ surface and those defects in acid/alkaline solutions. It is found that the ORRs on the pristine surface and surfaces with point defects always require large overpotentials (>1.0 V), indicating a defect-immune resistance of the planar MoS₂ surface against the ORR. However, the ORR overpotentials on edge defects can reach as low as 0.66 V, corresponding to a relatively high activity close to that of the prototypical catalyst Pt (overpotential ∼0.45 V). Such contrasting ORR behaviors of point and edge defects are also understood in depth by analyzing the underlying thermodynamic and electronic-structure mechanisms. This work not only quantitatively explains the performances of MoS₂ in both galvanic corrosion and electrochemical catalysis but also provides a useful structure-ORR map that can facilitate adapting the realistic MoS₂ to versatile electrochemical applications.

Silicon-doped graphene edges: an efficient metal-free catalyst for the reduction of CO2 into methanol and ethanol

Si doped graphene as a metal-free catalyst to convert CO₂ to methanol and ethanol with high selectivity and activity.