Surface-Functionalized Boron Arsenide as a Photocathode for CO₂ Reduction

Boron arsenide (BAs) has recently emerged as a promising semiconductor for both heat dissipation and device fabrication in future electronics and optoelectronics. In particular, a proper band gap value (∼1.85 eV) combined with a large carrier mobility makes BAs a potential photoelectrode material for solar energy harvesting. In this work, we employ spin-polarized density functional theory calculations to investigate the possibility of using surface-functionalized BAs as a photocathode for solar-driven CO2 reduction. With a focus on the BAs (110) facet in aqueous solution, we first examine the band-edge positions and show a large thermodynamic driving force for CO2 reduction by photoexcited electrons from the conduction band. We further study the catalytic activity of BAs functionalized with a cocatalyst, pyridine (Py), by describing the reaction pathway of a hydride-transfer process from adsorbed 2-pyridinide (2-PyH–*, a Py-derived catalytic intermediate) to CO2. We find that both the 2-PyH–* formation and the CO2 reduction step are thermodynamically and kinetically favorable and predict that Py-functionalized BAs is a promising candidate for heterogeneous CO2 photoreduction

Theoretical Study on Improving the CO₂ Reduction Performance of the Cu₂ZnGeS₄ Photoelectrode via Doping and Surface Engineering

Cu2ZnGeS4 (CZGS) and Cu2ZnSnS4 (CZTS) represent an important kesterite-structure family of photovoltaic materials due to low cost and toxicity. The upshift of CZGS’s conduction band minimum (CBM) compared to that of CZTS has also drawn attention recently for a potential application of CZGS in a photo-electro CO2 reduction reaction (CO2RR), which is, however, limited by a relatively large band gap and a low selectivity of the system. In this study, we employ ab initio atomic-scale models to explore the possibility of adjusting the electronic structure and catalytic selectivity of the CZGS (112) facet via doping and surface engineering approaches. We demonstrate that the substitution of Cu with Fe or Cr dopants can significantly shift up the valence band maximum position without much influence on the CBM level, thus leading to a smaller band gap while maintaining the strong reduction driving force of CZGS. Moreover, we reveal that different surface terminations of the CZGS (112) facet exhibit different selectivities toward CO2RR against the hydrogen evolution reaction (HER). We propose two possible surface terminations that could thermodynamically suppress hydrogen adsorption, providing guidance for the experimental treatment of the CZGS photoelectrode surface applied for CO2RR

Unraveling the Complete Mechanism of the NH₃‑Selective Catalytic Reduction of NO over CeO₂

CeO2-based oxides, with promising redox properties, exhibit application potential for the selective catalytic reduction (SCR) of nitrogen oxide (NOx) with NH3 (NH3-SCR). Despite decades of research, the underlying mechanisms governing the SCR activity remain unclear, and the catalytic paths of fast SCR (Fast_SCR) and standard SCR (Std_SCR) on the CeO2 surfaces are still under debate. Understanding the complete SCR reaction mechanism is crucial for the design and synthesis of efficient SCR catalysts. We perform density functional theory (DFT) simulations, synthesize CeO2 model catalysts for in situ spectroscopy experiments (in situ drifts, in situ Raman, in situ NAP-XPS, and in situ EPR) and SCR activity evaluation experiments to reveal the complete mechanism for NH3-SCR over CeO2. We find that the Std_SCR and the fast-SCR mechanisms share the same NO reduction path but go through two different adsorbed-hydrogen (H*) removal processes. For the NO reduction reaction, NH3 dissociation to NH2* and H* is catalyzed by the coupled [O* + Ovac] species. The NH2* then combines with NO to generate the NH2NO active intermediate, which further dissociates to N2 and H2O. In the Fast_SCR H* removal process, NO2 reacts with H* and *NH3 to generate H2O and NH2NO. For the Std_SCR, the catalytic species of O* is consumed to complete the H* removal. Our experimental–theoretical joint study further provides design principles of oxide catalysts for NO removal based on the atomic-level understanding of the catalytic mechanisms

Theoretical Study on Improving the CO₂ Reduction Performance of the Cu₂ZnGeS₄ Photoelectrode via Doping and Surface Engineering

Cu2ZnGeS4 (CZGS) and Cu2ZnSnS4 (CZTS) represent an important kesterite-structure family of photovoltaic materials due to low cost and toxicity. The upshift of CZGS’s conduction band minimum (CBM) compared to that of CZTS has also drawn attention recently for a potential application of CZGS in a photo-electro CO2 reduction reaction (CO2RR), which is, however, limited by a relatively large band gap and a low selectivity of the system. In this study, we employ ab initio atomic-scale models to explore the possibility of adjusting the electronic structure and catalytic selectivity of the CZGS (112) facet via doping and surface engineering approaches. We demonstrate that the substitution of Cu with Fe or Cr dopants can significantly shift up the valence band maximum position without much influence on the CBM level, thus leading to a smaller band gap while maintaining the strong reduction driving force of CZGS. Moreover, we reveal that different surface terminations of the CZGS (112) facet exhibit different selectivities toward CO2RR against the hydrogen evolution reaction (HER). We propose two possible surface terminations that could thermodynamically suppress hydrogen adsorption, providing guidance for the experimental treatment of the CZGS photoelectrode surface applied for CO2RR

Oxidation State of GaP Photoelectrode Surfaces under Electrochemical Conditions for Photocatalytic CO₂ Reduction

Illuminated GaP electrodes selectively reduce CO2 to CH3OH in aqueous solution. To understand the photoelectrocatalytic mechanism, knowledge of the GaP surface atomic structure in contact with water under relevant electrochemical conditions is essential. However, there remains a debate about the oxidation state of GaP, i.e., whether oxide species are present at the surface. To address this issue, we use density functional theory to investigate the adsorption of oxide species on GaP(110), a stable and active facet for CO2 reduction. We predict that GaP(110) indeed could be oxidized at the standard reduction potential for CO2 to CH3OH. However, we find that unoxidized GaP(110) is stable under illumination, as it corresponds to a highly reducing condition induced by photoexcited electrons. We conclude that an oxidized GaP electrode is very likely unstable thermodynamically under photoelectrochemical conditions for CO2 reduction, and therefore, the relevant GaP/water interface for catalysis is indeed the unoxidized one

Unraveling the Complete Mechanism of the NH₃‑Selective Catalytic Reduction of NO over CeO₂

CeO2-based oxides, with promising redox properties, exhibit application potential for the selective catalytic reduction (SCR) of nitrogen oxide (NOx) with NH3 (NH3-SCR). Despite decades of research, the underlying mechanisms governing the SCR activity remain unclear, and the catalytic paths of fast SCR (Fast_SCR) and standard SCR (Std_SCR) on the CeO2 surfaces are still under debate. Understanding the complete SCR reaction mechanism is crucial for the design and synthesis of efficient SCR catalysts. We perform density functional theory (DFT) simulations, synthesize CeO2 model catalysts for in situ spectroscopy experiments (in situ drifts, in situ Raman, in situ NAP-XPS, and in situ EPR) and SCR activity evaluation experiments to reveal the complete mechanism for NH3-SCR over CeO2. We find that the Std_SCR and the fast-SCR mechanisms share the same NO reduction path but go through two different adsorbed-hydrogen (H*) removal processes. For the NO reduction reaction, NH3 dissociation to NH2* and H* is catalyzed by the coupled [O* + Ovac] species. The NH2* then combines with NO to generate the NH2NO active intermediate, which further dissociates to N2 and H2O. In the Fast_SCR H* removal process, NO2 reacts with H* and *NH3 to generate H2O and NH2NO. For the Std_SCR, the catalytic species of O* is consumed to complete the H* removal. Our experimental–theoretical joint study further provides design principles of oxide catalysts for NO removal based on the atomic-level understanding of the catalytic mechanisms

Surface-Functionalized Boron Arsenide as a Photocathode for CO₂ Reduction

Boron arsenide (BAs) has recently emerged as a promising semiconductor for both heat dissipation and device fabrication in future electronics and optoelectronics. In particular, a proper band gap value (∼1.85 eV) combined with a large carrier mobility makes BAs a potential photoelectrode material for solar energy harvesting. In this work, we employ spin-polarized density functional theory calculations to investigate the possibility of using surface-functionalized BAs as a photocathode for solar-driven CO2 reduction. With a focus on the BAs (110) facet in aqueous solution, we first examine the band-edge positions and show a large thermodynamic driving force for CO2 reduction by photoexcited electrons from the conduction band. We further study the catalytic activity of BAs functionalized with a cocatalyst, pyridine (Py), by describing the reaction pathway of a hydride-transfer process from adsorbed 2-pyridinide (2-PyH–*, a Py-derived catalytic intermediate) to CO2. We find that both the 2-PyH–* formation and the CO2 reduction step are thermodynamically and kinetically favorable and predict that Py-functionalized BAs is a promising candidate for heterogeneous CO2 photoreduction

2‑Pyridinide as an Active Catalytic Intermediate for CO₂ Reduction on p‑GaP Photoelectrodes: Lifetime and Selectivity

The active intermediate responsible for pyridine (Py)-catalyzed reduction of CO₂ on a p-GaP photoelectrode is currently under debate. Exploration of the proposed intermediates’ available pathways for further reaction may yield a deeper understanding of the CO₂ reduction mechanism that will be essential to designing better cocatalysts in such photoelectrochemical systems. Adsorbed 2-pyridinide (2-PyH^–*) was recently proposed by Carter and co-workers to be an intermediate that facilitates hydride transfer (HT) to CO₂ to produce formate. However, the lifetime of 2-PyH^–*, most likely controlled by the rate of 2-PyH^–* protonation to form adsorbed dihydropyridine (DHP*), is still in question. In this work, we provide evidence for the transient existence of 2-PyH^–* on a p-GaP surface by comparing the activation energy for HT to CO₂ to those predicted for 2-PyH^–* being protonated to form either DHP* or Py* + H₂ via a hydrogen evolution reaction (HER). We predict that 2-PyH^–* situated next to an adjacent surface hydroxide (OH^–*) will be the most effective intermediate leading to CO₂ reduction on p-GaP. Predicted high barriers of HER (via either 2-PyH^–* or H^–*) also explain the high selectivity toward CO₂ reduction observed in experiments

Balancing Competing Reactions in Hydride Transfer Catalysis via Catalyst Surface Doping: The Ionization Energy Descriptor

Hydride transfer (HT) is ubiquitous in catalytic reduction reactions. In heterogeneous electrocatalysis, the hydride donor could be a molecular catalytic intermediate adsorbed on an electrode surface. The stability and hydride-donating capability of such an intermediate may determine overall catalytic efficiency. Here, we report how to fine-tune a hydride donor’s performance via doping an electrode surface. For semiconductor electrodes, we find that the ionization energy of the surface dopant can serve as a good descriptor for both the stability and hydride-donating capability of the catalytic intermediate adsorbed on the doped site. For the specific case of CO2 reduction on p-GaP, where adsorbed 2-pyridinide (2-PyH–*) was predicted to be the most likely hydride-donating species, we predict that its catalytic performance should be particularly enhanced by substituting Ga with Ti on the electrode surface; Sc, Al, and V surface dopants also could be worthy of further investigation