Hengshan Qiu

Strain commonly exists in nanomaterials and may significantly influence the physico-chemical properties and local reactivities. Enhanced oxygen adsorption was reported to be induced by subsurface Ar bubbles [Gsell et al., Science 280, 717–720 (1998)] due to the alteration of local surface strain. In this work, we show a reverse effect wherein post-oxygen exposure induces the desorption of Ar from the subsurface of sputter-annealed Cu(100). The multiple desorption peaks, by the combined analysis of multiple techniques, are ascribed to the structural evolution (surface roughening and recrystallization) and strain release during oxygen exposure and subsequent annealing. Notably, the quantity of desorbed subsurface Ar exhibits a correlation with oxygen exposure. The thermal behaviors of subsurface Ar as well as the surface reactivities can thus be rationally regulated by the modification of the surface, demonstrating a new strategy to tune the physico-chemical properties of subsurface species.

Plasma-assisted ammonia synthesis is generally perceived as a promising alternative to the Haber-Bosch process under milder conditions. However, the synergistic effect in plasma catalysis has not yet been thoroughly understood. Herein, the plasma-assisted ammonia synthesis over Ni-based catalysts was investigated on a double dielectric barrier discharge (DDBD) coupled temperature-programmed surface reaction system. It is found that the chemical enhancement of the synergistic effect possibly originates from the adsorption of plasma-activated nitrogen species on the surface, which suggests a Langmuir-Hinshelwood (L-H) process. The physical promotion is sourced from the catalyst-modulated discharge. The overall performance is integrally affected by (I) temperature-dependent plasma chemistry in gas phase, (II) decomposition of NH₃ (>600 K), and (III) thermal catalysis (>700 K). This study offers fundamental understandings and theoretical insights on plasma-assisted ammonia synthesis over Ni-based catalysts.

Modification of copper-based catalysts by incorporating a second metal is prevailing in developing high-performance catalysts for CO2 hydrogenation. In particular, the insight into how the reaction is influenced is key to understanding the nature of the strategy. Herein, we show that both intermediates and reaction pathways of CO2 over Cu(100) are conspicuously regulated by Ni deposition and CO2 pressure. CO2 exposure to Cu(100) at room temperature mainly yields surface oxygen and gas phase CO, whereas the deposited Ni functions in two ways: either to stabilize the surface carbonate species or to dissociate CO, leading to surface carbon and oxygen deposition. Interestingly, the pathways depend strongly on the pressure of CO2, which essentially induces surface roughening and alters the competition of CO/CO2 to react with Ni. Density functional theory simulations reveal that both factors have a notable influence on the adsorption/desorption kinetics and the stability of intermediates, hence leading to varied pathways.

Copper-based catalysts are widely applied in catalytic CO conversion. Despite their importance in determining catalytic performance, the interaction of CO with different copper states has often been the subject of debate. Herein, we discuss the preparation of four different CuO_x states (namely, fully oxidized and reduced (O-573 and R-573, respectively) and 800 and 1000 K annealed (A-800 and A-1000, respectively)) through in situ treatments, comparing CO adsorption behaviors with vacuum-transmission IR spectroscopy. CO only weakly adsorbed on the A-1000 and R-573 surfaces, whereas it led to the creation of trace amounts of surface oxygen vacancies over the O-573 surface, providing adsorption sites for the subsequent CO. Meanwhile, the produced CO₂ re-adsorbed on the catalyst to form carbonate species. The reduction process was notably promoted over A-800 due to the presence of abundant surface oxygen vacancies, demonstrating the key role of Cu^δ+ in oxygen vacancies, rather than merely its chemical state, in dominating the interaction of CO with CuO_x.

Single atom alloys (SAAs) often bring new chemistry in heterogeneous catalysis and well-defined structure for the study of structure–activity relationship (SAR). However, the existing pressure gap causes the reported SARs quite divergent. Herein, we have studied CO activation over Ni/Cu(100) SAAs in ultrahigh vacuum (UHV) and millibar range. While the Ni SAAs formed on Cu(100) significantly enhance the CO adsorption strength under UHV conditions, the CO treatment at elevated pressure leads to notable surface carbon and oxygen deposition through surface reaction. Density functional theory calculations revealed that either dissociation or disproportionation is thermodynamically forbidden for the coverage of CO less than 5/16 ML. However, these two reaction pathways can be opened at higher CO coverages due to the elevated energy state involving repulsion between adsorbed CO. This work uncovers the initial activation process of CO and demonstrates one typical cause for the pressure gap in surface science study as well.

High-efficiency utilization of CO₂ facilitates the reduction of CO₂ concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO₂ to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO₂ to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO₂ activation, the recent research advances in the catalytic hydrogenation of CO₂ to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.