Conversion of biomass-derived molecules involves catalytic reactions under harsh conditions in the liquid phase (e.g., temperatures of 250 °C and possibly under either acidic or basic conditions). Conventional oxide-supported catalysts undergo pore structure collapse and surface area reduction leading to deactivation under these conditions. Here we demonstrate an approach to deposit graphitic carbon to protect the oxide surface. The heterogeneous catalysts supported on the graphitic carbon/oxide composite exhibit excellent stability (even under acidic conditions) for biomass conversion reactions.

Conversion of biomass-derived molecules involves catalytic reactions under harsh conditions in the liquid phase (e.g., temperatures of 250 °C and possibly under either acidic or basic conditions). Conventional oxide-supported catalysts undergo pore structure collapse and surface area reduction leading to deactivation under these conditions. Here we demonstrate an approach to deposit graphitic carbon to protect the oxide surface. The heterogeneous catalysts supported on the graphitic carbon/oxide composite exhibit excellent stability (even under acidic conditions) for biomass conversion reactions.

PdZn mixed oxides are precursors for the formation of intermetallic PdZn phases, which show improved catalytic performance for methanol steam reforming. In this work we have prepared mixed oxides (Pd_xZn_1-xO) that span a range of compositions around the tetragonal PdZn 1:1 L₁₀ phase (x = 0.25, 0.5 and 0.75). We find that Pd⁺² can be isomorphously substituted into hexagonal ZnO and likewise Zn⁺² can also be substituted within tetragonal PdO. Our results show that the mixed oxide has a composition Pd_0.75Zn_0.25O within the tetragonal PdO lattice with a slight contraction in unit cell volume. The results are relevant for understanding the enhanced sensing properties of ZnO and the nature of the oxide precursors for the synthesis of intermetallic PdZn nanoparticles for heterogeneous catalysis.

Aberration-corrected transmission electron microscopy and high-angle annular dark field imaging was used to investigate the surface structures and internal defects of CeO₂ nanoparticles (octahedra, rods, and cubes). Further, their catalytic reactivity in the water–gas shift (WGS) reaction and the exposed surface sites by using FTIR spectroscopy were tested. Rods and octahedra expose stable (111) surfaces whereas cubes have primarily (100) facets. Rods also had internal voids and surface steps. The exposed planes are consistent with observed reactivity patterns, and the normalized WGS reactivity of octahedra and rods were similar, but the cubes were more reactive. In situ FTIR spectroscopy showed that rods and octahedra exhibit similar spectra for OH groups and that carbonates and formates formed upon exposure to CO whereas for cubes clear differences were observed. These results provide definitive information on the nature of the exposed surfaces in these CeO₂ nanostructures and their influence on the WGS reactivity.

Diesel oxidation catalysts (DOCs), which decrease the amount of harmful carbon monoxide (CO), nitrogen oxide (NO), and hydrocarbon (HC) emissions in engine exhaust, typically utilize Pt and Pd in the active phase. There is universal agreement that the addition of Pd improves both the catalytic performance and the durability of Pt catalysts. However, the mechanisms by which Pd improves the performance of Pt are less clear. Because these catalysts operate under oxidizing conditions, it is important to understand these catalysts in their working state. Herein, we report the microstructure of PtPd catalysts that are aged in air at 750 °C. After 10 h of aging, EXAFS and XANES analysis show that the Pt is fully reduced but that almost 30 % of the Pd species are present as an oxide. HRTEM images show no evidence of surface oxides on the metallic PtPd particles. Instead, the PdO is present as a separate phase that is dispersed over the alumina support. Within the metallic particles, Pt and Pd are uniformly distributed and there is no evidence of core–shell structures. Therefore, the improved catalytic performance is likely associated with the co-existence of metallic Pt and Pd on the catalyst surface.

This work is directed at investigating the contribution of metal particle sintering to catalyst deactivation in close-coupled automotive catalysts that are aged at elevated temperatures. We focus on the evolution of metal particle sizes in Pd/Al₂O₃ under conditions typically used for accelerated aging of automotive exhaust catalysts (10 mol % H₂O at 900 °C). By using multiple analytical techniques (transmission electron microscopy, X-ray diffraction, chemisorption, and CO oxidation) we can determine the role of support surface area collapse (encapsulation) versus metal particle sintering. The final dispersion (% metal atoms exposed) after sintering for 96 h ranged from 1.94 to 0.86 % for metal loadings ranging from 0.1 to 7.0 wt % (a 70-fold variation). Thus, it appears that metal loading (over the range studied) has only a limited effect on the final dispersion in the sintered catalyst. The sintering kinetics were found to obey a relationship dⁿ−=kt for which the exponent n is approximately 2.0 and d is the number average particle diameter at time t. This relationship and the fact that metal particle size continues to grow with time are both consistent with Ostwald ripening as the dominant mechanism. Furthermore, no limiting (equilibrium) particle size was achieved within the sintering times studied here (up to 200 h). These results have important implications for the design of thermally stable automotive catalysts.