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Anchoring Pt-3d Bimetallic Surfaces on Tungsten Carbide

Michael P. Humbert and Jingguang G. Chen. Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716

Bimetallic surfaces often exhibit catalytic properties that are not present on either of the parent metal surfaces. To gain insight into these novel properties, the electronic and chemical properties of Pt(111) modified by 3d transition metals were studied using a combination of experimental methods and theoretical modeling[1, 2]. Temperature programmed desorption (TPD) experiments were performed with various probe molecules, and density functional theory (DFT) was utilized as a tool to provide insight into the interaction between these molecules and the model surfaces. The overall objective was to correlate the electronic and chemical properties of bimetallic surfaces.

For the 3d transition metals studied (Fe, Co, Ni, Cu), deposition on Pt(111) at 300 K results in the formation of a surface configuration, 3d-Pt-Pt(111), where the first atomic layer is concentrated in the 3d metal. In contrast, when the Pt(111) substrate is held at 600 K during deposition, the 3d metal diffuses into the bulk resulting in a subsurface configuration, Pt-3d-Pt(111), where the second atomic layer is enriched in the 3d metal and the surface layer in Pt. The general results of the TPD experiments on these surfaces are that hydrogen binds weaker on Pt-3d-Pt(111) surfaces and stronger on 3d-Pt-Pt(111) in comparison to Pt(111)[2]. This is consistent with DFT calculations, which further show that several hydrocarbons, including ethylene and cyclohexene, also bind strongly to 3d-Pt-Pt(111) and weakly to Pt-3d-Pt(111)[1, 2]. These results help to explain the unique hydrogenation activity and selectivity on the Pt-3d-Pt(111) surface as determined by TPD experiments with cyclohexene and acrolein, respectively.

From these results it was evident that Pt-3d-Pt(111) surfaces are active for hydrogenation reactions, with Pt-Ni-Pt(111) having the highest activity for cyclohexene and acrolein hydrogenation[2]. With this insight, and the idea that WC has demonstrated similar electronic properties to Pt[3, 4], it was desired to replace bulk platinum in the Pt-Ni-Pt(111) surface with WC. Using WC to replace Pt has two important benefits. The obvious one is cost. Perhaps more important, however, is the fact that WC is an effective diffusion barrier layer[5]. This property prevents the diffusion of Pt and Ni into the bulk of a tungsten carbide substrate at elevated temperatures, which is crucial to applications in heterogeneous catalysis. TPD experiments have been performed on WC and Pt and/or Ni-modified WC substrates, and these results are compared to those on Pt(111) surfaces. In doing so, it was found that the Pt-Ni-WC and Pt-Ni-Pt(111) surfaces have qualitatively similar chemical properties.

[1] J. G. Chen, C. A. Menning, M. B. Zellner, Surf. Sci. Rep. 63 (2008) 201.

[2] M. P. Humbert, L. E. Murillo, J. G. Chen, ChemPhysChem in press (2008).

[3] R. B. Levy, M. Boudart, Science 181 (1973) 547.

[4] H. H. Hwu, J. G. Chen, Chem Rev 105 (2005) 185.

[5] P. Gouy-Pailler, Y. Pauleau, Journal of Vacuum Science & Technology A 11 (1993) 96.