(304f) 3D Constructed Metal Oxide Nano-Array Based Monolithic Catalysts With Ultrahigh Materials Usage Efficiency and Catalytic Performance Tunability

The monolithic catalysts and reactors represent a widely used device in chemical, mechanical, automotive, pharmaceutical and biotechnology applications. Several major issues, however, stand in the way of the monolithic catalysts development. Specifically, the wash-coating process with indispensable use of binder leads to mediocre uniformity and random distribution of catalytic layer. Besides, a high amount of materials loading is usually required to guarantee a stable catalytic performance, which indicates relatively low materials utilization efficiency. In this work, a new type of monolithic catalyst constructed with three dimensional nanowire arrays has been fabricated to mitigate the above mentioned problems by a facile hydrothermal method. Various metal oxide nanowires including ZnO, TiO₂, Co₃O₄ and CeO₂have been successfully grown on the ceramic cordierite and stainless steel honeycomb substrate. The hierarchical three dimensional nanowire arrays demonstrate excellent thermal and mechanical robustness. Specifically, the nanowire based honeycombs are able to maintain their morphology and geometry after 800 °C annealing for 100 hours and 10 days of rapid air flow with flow rate of 50 L/min. After deposition of platinum nanoparticles, the catalytic honeycombs exhibit remarkable CO oxidation performance, which is used as a probe reaction to test the catalytic activity. The materials utilization efficiency has been greatly improved by 10-40 times compared with the traditional wash-coated monolithic honeycomb of similar catalytic performance. The high efficiency with reduced materials deposition can be ascribed to the ordered nanoscale geometry, which facilitates the gas diffusion and allows for better active sites exposure. In addition, the 3D constructed metal oxide nanowire array catalyst provides geometrically well-defined platform to study the correlation between catalytic layer structure and the catalytic performance. We elucidate here that by controlling the nanowire shape and geometry it is possible to tune the catalytic performance. ZnO nanowires with various aspect ratios and thus different geometries such as nanoplate, nanorod and long nanowires have been demonstrated to achieve different catalytic performance. The synergic interaction between ZnO (0001) plane and Pt nanoparticles is found to be responsible for the catalytic activity difference. With predominant (0001) crystal facets involved, the ZnO nanoplate supported Pt catalyst exhibits the best catalytic performance. The ZnO nanowires with 5 mm in length give better activity than the 1 mm ZnO nanowires with the same Pt loading amount, which implies the longer nanowires will enhance the dispersion of Pt on the nanowire surface. All Pt-decorated ZnO nanowire based catalysts demonstrate better catalytic performance compared with ZnO powder supported Pt catalyst. The well-defined nanostructure array monolithic catalysts represent a new and effective model platform for bridging catalytic nanomaterials science and engineering with the practical industrial catalysis.

Reference

[1] Y. Guo^†, Z. Ren^†, (^†equally contributed) W. Xiao, C. Liu, H. Sharma, H. Gao, A. Mhadeshwar and P. X. Gao, Nano Energy, 2013, in press.

[2] P. X. Gao, Y. Guo, Z. Ren and Z. Zhang, US non-provisional patent filed, (2012)