No abstract

Minimizing the use of precious metal catalysts is important in many applications. Single-atom catalysts (SACs) have received much attention because all of the metal atoms can be used for surface reactions. However, SACs cannot catalyze some important reactions that require ensemble sites. Here, Rh catalysts were prepared by treating 2 wt % Rh/CeO hydrothermally at 750 °C for 25 h. Nearly 100% dispersion was obtained, but the surface Rh atoms were not isolated (denoted as ENS). They catalyzed the oxidation of CH or CH at low temperatures, but these oxidations did not occur on the Rh SAC. When the simultaneous oxidation of CO, CH, and CH was performed, the T (temperature at conversion 20%) for CO oxidation increased significantly from 40 °C for sole CO oxidation to 180 °C on SAC due to the competitive adsorption of hydrocarbons. However, T increased much less on ENS, from 60 to 100 °C. ENS exhibited superior activity for low-temperature oxidation. During hydrothermal treatment for 25 h, the Rh size initially increased from 2.3 to 6.7 nm then decreased to 0.9 nm. The surface hydroxyl groups formed on the catalyst surface help detach Rh atoms from Rh clusters, while preventing the reaggregation of dispersed Rh atoms into Rh clusters. This fully dispersed catalyst would have maximum atom-efficiency while catalyzing various surface reactions.

The electrochemical N 2 reduction reaction has attracted interest as a potential alternative to the Haber−Bosch process, but a significantly low conversion efficiency and a significantly low ammonia production rate stimulate the need for alternatives. Here, we represent the electrochemical reduction of nitric oxide (NO) on a nanostructured Ag electrode in combination with a rationally designed electrolyte containing the EDTA−Fe 2+ metal complex (EFeMC), which results in an ∼100% efficiency for NH 3 with a current density of 50 mA/cm 2 at −0.165 V RHE , without any degradation in catalytic activity or product selectivity up to 120 h. Economic analysis using itemized cost estimation predicted that the synthesis of ammonia from NO reduction in an EFeMC-designed electrolyte can be market competitive at an electricity price of $0.03 kWh −1 with a current density of >125 mA/cm 2 . Therefore, this approach opens an entirely new avenue of renewable electricitydriven ammonia synthesis.

Recently, many experimental and theoretical efforts are being intensified to develop high-performance catalysts for electrochemical CO 2 conversion. Beyond the catalyst material screening, it is also critical to optimize the surrounding reaction medium. From vast experiments, inclusion of room-temperature ionic liquid (RTIL) in the electrolyte is found to be beneficial for CO 2 conversion; however, there is no unified picture of the role of RTIL, prohibiting further optimization of the reaction medium. Using a state-of-the-art multiscale simulation, we here unveil the atomic origin of the catalytic promotion effect of RTIL during CO 2 conversion. Unlike the conventional belief, which assumes a specific intermolecular coordination by the RTIL component, we find that the promotion effect is collectively manifested by tuning the reaction microenvironment. This mechanism suggests the critical importance of the bulk properties (e.g., resistance, gas solubility and diffusivity, viscosity, etc.) over the detailed chemical variations of the RTIL components in designing the optimal electrolyte components, which is further supported by our experiments. This fundamental understanding of complex electrochemical interfaces will help in the development of more advanced electrochemical CO 2 conversion catalytic systems in the future.

Single-atom catalysts (SACs) have emerged as promising materials in heterogeneous catalysis.P revious studies reported controversial results about the relative level in activity for SACs and nanoparticles (NPs). These works have focused on the effect of metal atom arrangement, without considering the oxidation state of the SACs.H ere,w e immobilized Pt single atoms on defective ceria and controlled the oxidation state of Pt SACs,f rom highly oxidized (Pt 0 : 16.6 at %) to highly metallic states (Pt 0 :8 3.8 at %). The Pt SACs with controlled oxidation states were then employed for oxidation of CO,C H 4 ,o rN O, and their activities compared with those of Pt NPs.T he highly oxidized Pt SACs presented poorer activities than Pt NPs,w hereas metallic Pt SACs showed higher activities.The Pt SACreduced at 300 8 8Cshowed the highest activity for all the oxidations. The Pt SACs with controlled oxidation states revealed ac rucial missing link between activity and SACs.