Chem. Mater. 2005, 17, 3935-3943
3935
N2O Temperature-Programmed Oxidation and EXAFS Studies on the Dispersion of Copper in Ceria-Supported Nanocatalysts A. Tscho¨pe,* J. Markmann, P. Zimmer, and R. Birringer FB Physik, Geba¨ude 43, UniVersita¨t des Saarlandes, 66041 Saarbru¨cken, Germany
A.V. Chadwick Centre for Materials Research, School of Physical Sciences, UniVersity of Kent, Canterbury, Kent CT2 7NR, U. K. ReceiVed December 13, 2004. ReVised Manuscript ReceiVed April 20, 2005
The dispersion of copper in ceria-supported catalysts was investigated. By variation of the copper loading in a series of CuxCe1-xO2-x samples, it was possible to separate the oxidation of the copper component and the cerium oxide support during N2O temperature-programmed oxidation (TPO). Quantitative analysis confirmed that the copper component was completely reduced by hydrogen and completely oxidized by N2O at 300 °C. Extended X-ray absorption fine structure (EXAFS) studies on as-prepared powders as well as on annealed samples were performed under reducing/oxidizing conditions at elevated temperatures. Structural refinement during simulation suggested that in the oxidized state, copper was mostly present as isolated ions, whereas small clusters of metallic copper were found under reducing conditions. A wetting/de-wetting transition of the copper component on the ceria surface was suggested to cause this change in morphology. Annealing at high temperatures, which resulted in a substantial decrease in the specific surface area of the catalyst, also induced the precipitation of CuO as a second phase, as confirmed by X-ray diffraction. The enhanced redox activity of dispersed copper in the as-prepared samples as compared to that of the copper bulk phase in annealed samples was evident from in situ EXAFS measurements.
1. Introduction The promoting effect of cerium oxide, when used as support for precious metals in the three-way catalytic converter, has been intensively investigated in the past.1 Recently, excellent catalytic activity was also found for the ceria-supported base metal catalyst Cu/CeO2.2-7 The catalytic activity of Cu/CeO2 in CO oxidation is higher than with any other combination of base metal catalyst or oxide support, and it is even comparable to the activity of more expensive precious metal catalysts.3,8 Various investigations revealed high catalytic activity of Cu/CeO2 in further reactions, such as SO2 reduction by CO,2,4 the low-temperature water-gas shift reaction,6 methanol steam reforming,7 and hydrocarbon oxidation.3,4,7,9 The activity in the latter reaction in combina* Corresponding author tel.: +49 681 302 5187, fax: +49 681 302 5222, e-mail:
[email protected].
(1) Trovarelli, A. Catal. ReV.sSci. Eng. 1996, 38, 439. (2) Liu, W.; Sarofim, A. L.; Flytzani-Stephanopoulos, M. Appl. Catal., B 1994, 4, 167. (3) Liu, W.; Flytzani-Stephanopoulos, M. J. Catal. 1995, 153, 304. (4) Tscho¨pe, A.; Liu, W.; Flytzani-Stephanopoulos, M.; Ying, J. Y. J. Catal. 1995, 157, 42. (5) Martı´nez-Arias, A.; Ferna´ndez-Garcı´a, M.; Conesa, J. C. J. Catal. 1999, 182, 367. (6) Flytzani-Stephanopoulos, M. MRS Bull. 2001, 885. (7) Liu, Y.; Hayakawa, T.; Suzuki, K.; Hamakawa, S.; Tsunoda, T.; Ishii, T.; Kumagai, M. Appl. Catal., A 2002, 223, 137. (8) Tscho¨pe, A.; Schaadt, D.; Birringer, R.; Ying, J. Y. Nanostruct. Mater. 1997, 9, 423. (9) Park, P. W.; Ledford, J. S. Catal. Lett. 1998, 50, 41.
tion with the ionic conductivity of ceria is assumed to be the reason for the superior properties of Cu/CeO2-YSZ cermets used as anode material in solid-oxide fuel cells for direct combustion of hydrocarbons.10,11 Motivated by the exceptionally high catalytic activity of this particular combination of copper and cerium oxide, catalytic studies have been complemented by characterization of the structure, chemical state, and reactivity of this material. Electron paramagnetic resonance (EPR) measurements revealed that a significant fraction of copper ions is dispersed atomically at the surface and also as ion pairs and small aggregates, whereas the amount of copper dissolved in the cerium oxide lattice is negligible.5,12,13 Microscopic particles of the CuO tenorite phase were detected by X-ray diffraction and scanning transmission electron microscopy only when the copper concentration was above a critical value of ca. 15 atom %, suggesting that the copper component was highly dispersed on the cerium oxide surface at lower concentrations (
