A reactive oxide overlayer on rhodium nanoparticles during CO oxidation and its size dependence studied by in situ ambient-pressure X-ray photoelectron spectroscopy.

Carbon monoxide oxidation is one of the most studied heterogeneous reactions, being scientifically and industrially important, particularly for removal of CO from exhaust streams and preferential oxidation for hydrogen purification in fuel-cell applications. The precious metals Ru, Rh, Pd, Pt, and Au are most commonly used for this reaction because of their high activity and stability. Despite the wealth of experimental and theoretical data, it remains unclear what is the active surface for CO oxidation under catalytic conditions for these metals. Herein we utilize in situ synchrotron ambient pressure X-ray photoelectron spectroscopy (APXPS) to monitor the oxidation state at the surface of rhodium nanoparticles (Rh NPs) during CO oxidation and demonstrate that the active catalyst is a surface oxide, the formation of which is dependent on particle size. The amount of oxide formed and the reaction rate both increase with decreasing particle size. Many single-crystal CO oxidation studies over rhodium suggest that the reaction is structure-insensitive and that the oxide formation decreases the reaction rate. However, recent advances in synthetic techniques and in-situ experimentation have revealed that the oxidation state and stoichiometry of the surface oxide greatly affects CO oxidation rates. At low temperatures or low O2/CO ratios, CO strongly adsorbs onto the catalyst surface and inhibits O2 adsorption. At high temperatures or high O2/CO ratios, the catalyst surface becomes saturated with oxygen atoms and the reaction proceeds more rapidly. It has been demonstrated that small palladium nanoparticles are more active for CO oxidation than larger particles and single crystals, whereas the opposite is reported for platinum. For Rh NPs, no particle size effect was observed for supported rhodium catalysts, but a strong particle size dependence was observed for CO desorption, dissociation, and transient CO oxidation over electron-beam-prepared Rh NPs that were precovered with oxygen. For this investigation we have prepared small, polymerstabilized Rh NPs with a narrow size distribution and studied CO oxidation; polymer stabilized NP syntheses enable control of NP size, shape, and/or composition for reaction studies. The turnover frequency (TOF) for CO oxidation at 200 8C increases five-fold, and the apparent activation energy decreases from 27.9 kcalmol 1 to 19.0 kcalmol 1 as the particle size decreases from 11 nm to 2 nm. APXPS of 2 nm and 7 nm Rh NP films during CO oxidation at about 1 Torr provides the first in-situ measurement of the oxidation state of Rh NPs during CO oxidation and demonstrates that smaller particles are more oxidized than larger particles during reaction at 150–200 8C. A surface oxygen species is also observed during CO oxidation that is not present when heating in O2 alone, possibly indicating a unique active oxide phase on Rh NPs. This oxide phase may alter the relative bonding geometries of CO and/or oxygen on the rhodium surface, thereby lowering the activation energy for the reaction. The synthesis of monodisperse Rh NPs by polyol reduction using poly(vinylpyrrolidone) (PVP) as a capping agent and [Rh(acac)3] as a rhodium precursor [17] was extended to smaller sizes by the addition of sodium citrate. Using this approach, Rh NPs of 3.5 nm (3.6 0.5 nm), 2.5 nm (2.5 0.4 nm), and 2 nm (1.9 0.3 nm) were formed by increasing the amount of sodium citrate. Monolayer films of these particles were then prepared in a Langmuir–Blodgett (LB) trough and characterized with transmission electron microscopy (TEM) and XPS. Figure 1a–c shows TEM images of the NPs, with insets of size distribution histograms taken from 100 particles. Figure 1 f shows X-ray photoelectron spectra for the Rh 3d peak of the as-synthesized (no pretreatment) particles after LB deposition onto a silicon wafer. The ratio of oxidized rhodium to reduced rhodium clearly increases as the particle size decreases. The three samples of small Rh NP (2, 2.5, and 3.5 nm) LB films and two previously synthesized samples, 7 nm (7.1 [*] M. E. Grass, D. R. Butcher, Dr. J. Y. Park, Dr. Y. Li, Dr. H. Bluhm, Dr. K. M. Bratlie, Dr. T. Zhang, Prof. G. A. Somorjai Department of Chemistry; University of California, Berkeley Chemical and Materials Sciences Divisions Lawrence Berkeley National Laboratory; Berkeley, CA 94720 (USA) Fax: (+1) 510-643-9668 E-mail: [email protected]