Mass Transport in Metal Oxide Nanoarray based Monolithic Catalysts: An Experimental-Theory Study
Xingxu Lu, University of Connecticut
The conversion efficiency of the monolithic three-way catalysts (TWC) is bounded by the kinetic resistance of the catalytic reactions at low temperatures, internal mass transfer resistance over the intermediate temperatures and the external mass transfer resistance at high temperatures.1 The state-of-the-art washcoat configured monolithic catalysts have been intensively studied to enhance the catalytic activity and reduce the external mass transfer resistance. However, the internal mass transfer resistance is reluctant to be reduced because it requires the increase in the ratio of macropores while maintaining the high specific surface area and the mechanical and thermal stability of the catalysts.2 In order to mitigate this challenge, a novel three-dimensional (3D) nanoarray-based monolithic catalyst configuration has been developed by integrating hierarchically arranged mesoporous metal oxide nanoarrays onto the channel surfaces of commercial honeycomb monoliths in the past decade.3-4 Compared with the conventional particulate washcoat formed catalysts, the 3D nanoarray-based monolithic catalysts show advantages in terms of the enhanced material utilization efficiency, higher mechanical robustness as well as tunable structures and catalytic performance.5 In this work, the mass transfer coefficients and controlling regimes during the oxidation of CH4 and C2H4 over the Pt/TiO2 nanoarray based monolithic catalysts were experimentally measured and compared with the washcoat formed counterparts. First, we determined the global kinetics of the oxidation of CH4 and C2H4 over both types of catalysts. Then, using the low-dimensional model developed by Joshi et al.,6 we quantified the relative contributions of the chemical kinetics, internal mass transfer and external mass transfer as functions of various catalyst design and operating parameters. The analysis indicates that the nanoarray based monolithic catalysts show decreased internal mass transfer resistance compared to the washcoat based catalysts, which originates from the ordered nanoarray configuration and decreased catalyst layer thickness, which facilitate the diffusion of the reactant gas molecules through the nanoarray layers. Furthermore, the mesoporous TiO2 nanoarrays provide sufficient specific surface area and catalytically active sites for reactions, and the adherent interface between the in-situ grown nanoarrays and substrate surfaces guaranteed the mechanical robustness of the monolithic catalysts. This study sheds light on the design of novel monolithic catalysts for various gas phase reactions.
(1) Joshi, S. Y.; Harold, M. P.; Balakotaiah, V. Overall mass transfer coefficients and controlling regimes in catalytic monoliths. Chemical Engineering Science 2010, 65, 1729-1747.
(2) Santos, H.; Costa, M. Influence of the three way catalytic converter substrate cell density on the mass transfer and reaction resistances. Chemical Engineering Science 2014, 107, 181-191.
(3) Guo, Y.; Ren, Z.; Xiao, W.; Liu, C.; Sharma, H.; Gao, H.; Mhadeshwar, A.; Gao, P.-X. Robust 3-D configurated metal oxide nanoarray based monolithic catalysts with ultrahigh materials usage efficiency and catalytic performance tunability. Nano Energy 2013, 2, 873-881.
(4) Lu, X.; Tang, W.; Gao, P.-X. Nanostructured TiO2 Support Effect on Hydrothermal Stability of Platinum based Catalysts. Microscopy and Microanalysis 2018, 24, 1642-1643.
(5) Weng, J.; Lu, X.; Gao, P.-X. Nanoarray Integrated Structured Catalysts: A New Paradigm upon Conventional Wash-Coated Monolithic Catalysts? Catalysts 2017, 7, 253.
(6) Joshi, S. Y.; Harold, M. P.; Balakotaiah, V. Low-dimensional models for real time simulations of catalytic monoliths. AIChE Journal 2009, 55, 1771-1783.