Activity and Mechanism Mapping of Photocatalytic NO2 Conversion on the Anatase TiO2(101) Surface

Activity Trend and Selectivity of Electrochemical Ammonia Synthesis in Reverse Artificial Nitrogen Cycle

Ammonia is important for modern agriculture and food production as it is a major source of fertilizer. Electrochemical ammonia synthesis (EAS) with sustainable energy generated electricity and decentralized reactors has been considered as environmentally friendly process. Several nitrogen sources have been considered and intensively studied in experiments and computations. Recently, it has been proposed and demonstrated that nitrogen oxides (NOx ) electroreduction for selective ammonia production is feasible. Fundamental insights on experimental observation are necessary for more rational design of catalysts and reactors in the future. In this concept, we review the theoretical and computational insights of electrochemical nitrogen oxides reduction, particularly, the activity trend over diverse transition metal catalysts and products selectivity at varying potentials. Finally, we address the opportunities and challenges in the reverse artificial nitrogen cycle, as well as fundamental issues in electrochemical reaction modelling.

Unraveling oxygen vacancy site mechanism of Rh-doped RuO2 catalyst for long-lasting acidic water oxidation

Exploring durable electrocatalysts with high activity for oxygen evolution reaction (OER) in acidic media is of paramount importance for H₂ production via polymer electrolyte membrane electrolyzers, yet it remains urgently challenging. Herein, we report a synergistic strategy of Rh doping and surface oxygen vacancies to precisely regulate unconventional OER reaction path via the Ru-O-Rh active sites of Rh-RuO₂, simultaneously boosting intrinsic activity and stability. The stabilized low-valent catalyst exhibits a remarkable performance, with an overpotential of 161 mV at 10 mA cm^-2 and activity retention of 99.2% exceeding 700 h at 50 mA cm^-2. Quasi in situ/operando characterizations demonstrate the recurrence of reversible oxygen species under working potentials for enhanced activity and durability. It is theoretically revealed that Rh-RuO₂ passes through a more optimal reaction path of lattice oxygen mediated mechanism-oxygen vacancy site mechanism induced by the synergistic interaction of defects and Ru-O-Rh active sites with the rate-determining step of *O formation, breaking the barrier limitation (*OOH) of the traditional adsorption evolution mechanism.

Fundamental Limit of Selectivity in Photocatalytic Denitrification over Titania

Although photocatalytic decomposition of NO (deNO) into N₂ and O₂ is low-cost and non-polluting, it has a low NO conversion efficiency. Establishing the activity and selectivity trend among active sites is an important base to explore and improve the deNO processes. Because the experimental performances are determined by the reaction rate, it is worthwhile to investigate the kinetic limiting steps calculated by comparative microkinetic modeling. We found that, without illumination, N₂ production is inactive over various TiO₂ surfaces/sites, but photogenerated holes can break the scaling relation of the dark condition by weakening O₂* adsorption, leading to a significant increase in deNO activity on defective titania surfaces. However, the low N₂ selectivity can be attributed to the small strength of N₂O adsorption. In contrast, the N₂ selectivity is enhanced in Ti-modified zeolite because of a stronger N₂O* adsorption. We demonstrate here that the reaction phase diagram analysis can clearly establish a global picture of reaction activity and selectivity over various catalytic sites. In combination with microkinetic modeling, it can effectively determine the kinetic limits, providing insights to improve the design of photocatalysts.

Predictive Theoretical Model for the Selective Electroreduction of Nitrate to Ammonia

The electrochemical reduction of nitrate (eNO₃RR) emerges as a promising route for decentralized ammonia synthesis. However, the competitive production of nitrite at low overpotentials is a challenging issue. Herein, using the combination of density functional theory and microkinetic modeling, we show that the selectivity for NH₃ surpasses that of NO₂^- at -0.66 V_RHE, which nicely reproduced the experimental value on titania. NH₂OH* → NH₂* is the kinetically controlling step at a low overpotential for NH₃ generation, while NO₂* → HNO₂ has the highest barrier to producing nitrite. Based on these mechanistic insights, we suggest that ΔG₁ (NH₂OH* → NH₂*) - ΔG₂ (NO₂* → HNO₂) can serve as a descriptor to predict the S(NO₂^-)/S(NH₃) crossover potential. Such a model is verified by the experimental results on Ag, Cu, TiO_2-x, Fe₃O₄, and Fe-MoS₂ and can be extended to the Au catalyst. Thus, this work sheds light on the rational design of catalysts that are simultaneously energy-efficient and selective to NH₃.

Dynamics of Heterogeneous Catalytic Processes at Operando Conditions

The rational design of high-performance catalysts is hindered by the lack of knowledge of the structures of active sites and the reaction pathways under reaction conditions, which can be ideally addressed by an in situ/operando characterization. Besides the experimental insights, a theoretical investigation that simulates reaction conditions-so-called operando modeling-is necessary for a plausible understanding of a working catalyst system at the atomic scale. However, there is still a huge gap between the current widely used computational model and the concept of operando modeling, which should be achieved through multiscale computational modeling. This Perspective describes various modeling approaches and machine learning techniques that step toward operando modeling, followed by selected experimental examples that present an operando understanding in the thermo- and electrocatalytic processes. At last, the remaining challenges in this area are outlined.