The Sintering of Supported Pd Automotive Catalysts

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−dn 0 =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.