Dynamics and Heterogeneities of Atomically dispersed Active Centres

Recent advances in heterogeneous catalysis have demonstrated that atomically dispersed metal species on oxide supports can act as catalytic active sites. While this progress has deepened mechanistic understanding, it has also highlighted unresolved challenges, including active-site heterogeneity, dynamic structural evolution under reaction conditions, and the coexistence of multiple types of active sites within a single catalyst. This PhD thesis looks into these issues through rational catalyst design combined with time-resolved operando spectroscopy and theoretical simulations. To stabilize atomically dispersed metal species and suppress sintering associated with reducible supports, a penta-coordinate Al3+ enriched alumina (Al2O3-x) was developed via a one-pot synthesis strategy. This amorphous alumina exhibits a high fraction of pentacoordinated Al sites (up to ~40%), a large specific surface area (~420 m2 g-1), enabling stable atomic dispersion of metal species through simple incipient wetness impregnation. Owing to its thermal stability and strong metal anchoring capability, Al2O3-x was employed as a model support throughout this thesis. Methane oxidation over Pd-based catalysts was used as a model reaction to disentangle the roles of Pd nanoparticles (NPs), clusters, and single atoms. Operando and time-resolved spectroscopic studies revealed that conventional Pd/γ-Al2O3 undergoes significant active-site evolution and deactivation, whereas Pd/Al2O3-x maintains structural and catalytic stability under prolonged reaction conditions. At steady state, Pd/γ-Al2O3 contains both sintered PdO nanoparticles and atomically dispersed Pd, demonstrating that multiple active-site motifs can coexist even when not explicitly designed. In contrast, Pd/Al2O3-x predominantly stabilizes Pd as clusters and single atoms. Second-resolved operando quick X-ray absorption spectroscopy under alternating reaction atmospheres enabled direct comparison of redox dynamics, revealing that Pd clusters are intrinsically more active than Pd nanoparticles. Operando FT-IR spectroscopy showed that carbonate species accumulate on PdO NPs, suppressing activity, while single-atom Pd is mainly deactivated by strong water adsorption. Density functional theory (DFT) calculations confirmed that single-atom Pd can intrinsically activate methane with barriers comparable to PdO NPs, but suffers from slow water desorption, explaining its limited water tolerance. This thesis further demonstrates that atomically dispersed active sites themselves exhibit substantial heterogeneity. Using CO2 hydrogenation over atomically dispersed Zn on Al2O3-x, a “site density effect” is identified, whereby increasing Zn site density promotes cooperative interactions between adjacent Zn atoms. Operando spectroscopy combined with microkinetic modeling shows that such interactions lower the activation barrier for formate hydrogenation, enhancing both activity and methanol selectivity. Overall, this work highlights the importance of controlling active-site distribution and density, and provides a framework for resolving active-site heterogeneity in heterogeneous catalysis.