Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for clean fuel applications: Carbon nanofibers as dispersant for the mixed oxide particles

Florian Huber
1Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

Zhixin Yu
1Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

John Walmsley
SINTEF Materials and Chemistry, N-7465 Trondheim, Norway

Hilde Venvik
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

De Chen
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

Anders Holmen
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

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     Last modified: January 14, 2006

Abstract
Mixed oxide catalysts containing Cu, Ce and Zr are widely used in environmental heterogeneous catalysis. Examples include reactions relevant in hydrogen production (methanol steam reforming, water-gas shift, selective CO oxidation) for fuel cell applications, methanol synthesis and selective catalytic NO reduction for automotive emission control. In these catalytic processes, copper - in its reduced state - is typically held as the active catalyst component in the mixed metal oxide materials. The ceria-zirconia support enhances catalytic activity and stability via metal-support interactions and/or improved dispersion of the active metal component. A large and stable catalyst surface as well as a homogeneous metal distribution are prerequisites for a high catalytic activity. Nanocrystalline Cu-Ce-Zr mixed oxides were prepared by homogeneous alkalinization of nitrate precursors resulting in small primary nanoparticles (3 – 5 nm as determined by XRD) and high surface area (up to 170 m2/g). Preparation parameters such as stirring, organic additives and drying conditions were correlated with pore structure and surface area of the final mixed oxides, as determined by N2 adsorption-desorption. The homogeneity of the mixed oxide samples was investigated by XRD, STEM/EDX and XAS, addressing the issue ‘single phase product/solid solution’, i. e. whether oxide phases show uniform, average, composition or separate, distinct, compositions. Nanocomposites composed of the mixed oxide and carbon nanofibers (CNF) were subsequently prepared by the alkalinization route. CNF can be used as dispersant for the mixed oxide particles, potentially improving catalyst stability against sintering. CNF might also change the local reaction environment by adsorbing and activating reactant molecules, affecting catalytic activity thereby. CNF with parallel oriented graphitic layers, activated in HNO3, were used as dispersant. The water-gas shift reaction was used as test reaction to investigate the effect of CNF as dispersant in the mixed oxide catalyst. The catalyst activity was measured between 200 and 300 °C. The short-term stability was tested at 300 °C, close to the Huettig temperature of copper (325 °C), above which copper atoms become mobile leading to enhanced sintering. A conventional, home-made Cu-Zn-Al mixed oxide catalyst was used for comparison. The catalysts show promising activity and stability for typical reformate gas feed applications with copper being stabilized in the mixed oxide structure.