Xuze Guan

Single‐atom catalysts have garnered significant attention due to their exceptional atom utilization and unique properties. However, the practical application of these catalysts is often impeded by challenges such as sintering‐induced instability and poisoning of isolated atoms due to strong gas adsorption. In this study, we employed the mechanochemical method to insert single Cu atoms into the subsurface of Fe₂O₃ support. By manipulating the location of single atoms at the surface or subsurface, catalysts with distinct adsorption properties and reaction mechanisms can be achieved. It was observed that the subsurface Cu single atoms in Fe₂O₃ remained isolated under both oxidation and reduction environments, whereas surface Cu single atoms on Fe₂O₃ experienced sintering under reduction conditions. The unique properties of these subsurface single‐atom catalysts call for innovations and new understandings in catalyst design.

Single‐atom catalysts have garnered significant attention due to their exceptional atom utilization and unique properties. However, the practical application of these catalysts is often impeded by challenges such as sintering‐induced instability and poisoning of isolated atoms due to strong gas adsorption. In this study, we employed the mechanochemical method to insert single Cu atoms into the subsurface of Fe₂O₃ support. By manipulating the location of single atoms at the surface or subsurface, catalysts with distinct adsorption properties and reaction mechanisms can be achieved. It was observed that the subsurface Cu single atoms in Fe₂O₃ remained isolated under both oxidation and reduction environments, whereas surface Cu single atoms on Fe₂O₃ experienced sintering under reduction conditions. The unique properties of these subsurface single‐atom catalysts call for innovations and new understandings in catalyst design.

Rigorous comparisons between single site- and nanoparticle (NP)-dispersed catalysts featuring the same composition, in terms of activity, selectivity, and reaction mechanism, are limited. This limitation is partly due to the tendency of single metal atoms to sinter into aggregated NPs at high loadings and elevated temperatures, driven by a decrease in metal surface free energy. Here, we have developed a unique two-step method for the synthesis of single Cu sites on ZSM-5 (termed Cu _S /ZSM-5) with high thermal stability. The atomic-level dispersion of single Cu sites was confirmed through scanning transmission electron microscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance spectroscopy. The Cu _S /ZSM-5 catalyst was compared to a CuO NP-based catalyst (termed Cu _N /ZSM-5) in the oxidation of NH ₃ to N ₂ , with the former exhibiting superior activity and selectivity. Furthermore, operando XAFS and diffuse reflectance infrared Fourier transform spectroscopy studies were conducted to simultaneously assess the fate of the Cu and the surface adsorbates, providing a comprehensive understanding of the mechanism of the two catalysts. The study shows that the facile redox behavior exhibited by single Cu sites correlates with the enhanced activity observed for the Cu _S /ZSM-5 catalyst.

The Mars-van Krevelen mechanism (MvK) is a widely recognized model for describing the role of lattice oxygen in catalysis. Following the MvK mechanism, the formation and conversion of surface oxygen vacancy (VO) are considered as the key steps. CeO2-ZrO2 (CZ) mixed oxides are the typical catalyst support in MvK mechanism. They have the unique property of hosting remarkable amount of VO without significant change in lattice structure, offering O storage and release capability that maintains the required concentration of active O on the catalytic surface. In this regard, the rate of VO formation and conversion directly affect their catalytic performance. In this work, we obtained the VO formation and conversion kinetics by measuring the rate of the Ce4+ reduction and oxidation via operando energy dispersive Extended X-ray Absorption Fine Structure (EDE). The main conclusions are: 1) VO formation is 10 times faster than VO conversion; 2) VO formation rates are comparable with the CO oxidation rates, thereby serving as the rate-determining step in CO oxidation; 3) Pd and Cu serve as catalysts for VO formation by significantly improving its rate by 50 times at 250 C by weakening the metal-O bonding strength, whereas the activation energy have been reduced to 58.4 kJ/mol and 36.5 kJ/mol, respectively. Our method in measuring and analysing partial reaction rates within a turnover is therefore important for all chemical reactions.

Selective catalytic oxidation (SCO) of NH3 to N2 is one of the most effective methods to eliminate NH3 emissions. However, achieving high conversion over a wide operating temperature range while avoiding over-oxidation to NOx is a significant challenge. Commercial Pt catalysts have full NH3 conversion at low temperatures but suffer from low N2 selectivity. To mitigate this challenge, it is crucial to limit the over-oxidation of NH3. In this study, we developed a bi-metal surface catalyst with improved Pt atom efficiency. The PtSCuO/Al2O3 catalyst consists of Pt atoms on the surface of CuO nanoparticles. It achieves full NH3 conversion at 250 °C with WHSV of 600 mlNH3h-1g-1 which is 50 °C lower than the commercial Pt/Al2O3, and maintains high N2 selectivity through all temperature window (150-450 °C). At 200 °C, PtSCuO/Al2O3, exhibited a staggering 30-fold increase in activity compared to the commercial Pt/Al2O3. Operando XAFS studies reveal that the surface Pt atoms in PtSCuO/Al2O3 not only enhance the redox ability of Cu species but also alter the redox rate of Cu species, which improves the while reaction rate of the NH3-SCO reaction. Moreover, the synergistic effect of Pt sites and Cu sites in PtSCuO/Al2O3 contributes to improved selectivity via the internal selective catalytic reduction (i-SCR) route.

The selective reduction of NO with NH3 (SCR) is an important reaction for emission control. The mobility of a metal cation in a confined space is a classic problem in transport phenomena. When the metal cation is an active centre of a chemical reaction, its mobility will be decisive to its catalytic performance, such as the Cu in chabazite (CHA) zeolite catalyst for selective catalytic reduction of NO (SCR) in diesel exhaust. Here we identify electron paramagnetic resonance (EPR) as an effective tool to quantify Cu2+ at different locations of CHA, and study the mobility of hydrated Cu2+. The Cu2+ transfers from one adjacent Al3+ site (1AlCu) to two adjacent Al3+ site (2AlCu) during dehydration. In the hydrothermal aging process, the transition is recorded from CuOx clusters to isolated Cu2+ sites and from Cu at 8-O member ring to 6-O member ring. The result indicates that the Cu – framework (fw) O interaction is thermodynamic drive for the migration of Cu species. For SCR, this means that the reduced Cu-Ofw interaction will lead to mobile Cu species in the catalytic circle.

Cu-exchanged zeolite is an efficient catalyst to remove harmful nitrogen oxides from diesel exhaust gas through the selective catalytic reduction (SCR) reaction. The SCR performance is structure dependent, in which a Cu with one adjacent framework Al (1AlCu) has lower activation energy in oxidative half-cycle than Cu with two adjacent framework Al (2AlCu). Using a combination of operando X-ray absorption spectroscopy, valence to core - X-ray emission spectroscopy and density functional theory calculations, here we showed that 1AlCu proceeds with nitrate mechanism, in which side-on coordination of O2 at a CuI(NH3)xOfw (fw = framework) is the rate-limiting step in the oxidation half-cycle. As a result, the CuI(NH3)xOfw at 1AlCu can easily yield a transient CuIINOx intermediate upon breaking of Cu-Ofw after interaction with NO. In the meantime, 2AlCu has high barriers for Cu-Ofw bond breaking and proceeds with dimer mechanism. Our results show the coexisting of both dimer and nitrate mechanism, in particular at high Cu loadings, in which controlling the strength of the Cu-Ofw coordination is key for the O-O split in the nitrate pathway.

CeO2 offers excellent oxygen storage capacity for catalysis and energy applications. It accommodates a surprising number of oxygen vacancies (VO), yet the driving force and origin of these VO are rarely discussed. Through a combination of in situ techniques and resonant inelastic X-ray scattering (RIXS), here, we discovered that upon formation of the VO, the remaining Ce4+-O2- bond energy increases by at least 0.2 eV. Such an increase in the bond energy provides an additional enthalpy change (ΔH) in the reaction of VO formation, stabilizes the reduced CeO2 phase and explains why CeO2 can host an extensive array of VO. This work demonstrates that RIXS is effective in probing metal-oxygen bond energy, possibly under working conditions, which is an ideal method to quantify electronic structures with material performance.