Theoretical Density Functional Analysis of Maleic Anhydride Chemisorption on Pd(111), Re(0001), and Bimetallic PdML/Re(0001) and PdML/Mo(110) Pseudomorphic Overlayers

Abstract

Nonlocal density functional theory (DFT) calculations were performed to analyze the di-σ, π, and atop chemisorption modes of maleic anhydride on the close-packed Pd(111), Re(0001), and pseudomorphic monolayers of Pd on Re(0001) [PdML/Re(0001)] and Pd on Mo(110) [PdML/Mo(110)] surfaces. The DFT-computed binding energies for maleic anhydride in the atop, π, and di-σ modes on Pd(111) are −28, −34, and −83 kJ/mol, respectively. The calculated adsorption energy and vibrational frequencies for di-σ adsorption are in good agreement with the UHV-TPD and HREELS data of Xu and Goodman (Langmuir 1996, 12, 1807−1816). The atop, π, and di-σ adsorptions of maleic anhydride on Re(0001) are significantly stronger than on Pd(111), with binding energies of −38, −210, and −200 kJ/mol, respectively. The π adsorption mode of maleic anhydride on Re(0001), in particular, is much stronger than that on Pd(111), due to additional interactions of the carbonyl groups of maleic anhydride with the Re(0001) surface. Adsorption energies for maleic anhydride on the bimetallic Pd/Re(0001) and Pd/Mo(110) surfaces are 10−20 kJ/mol weaker than on monometallic Pd(111). This decrease in the adsorption energy is explained on the basis of formal chemisorption theory. Using the analysis of Hammer and Nørskov (Nature 1995, 376, 238, and In Chemisorption and Reactivity on Supported Clusters and Thin Films; Lambert, R. M., Pacchioni, G., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; pp 285−351), the di-σ adsorption energies have been correlated to the location of the d band center for the clean metal surface. Calculations indicate that the strong Pd−Re and Pd−Mo interactions in the bimetallic surfaces shift the d band center for the surface Pd layer away from the Fermi level. This weakens the interaction of the olefinic group of maleic anhydride with the bimetallic PdML/Re(0001) and PdML/Mo(110) surfaces. The only exception is that for π-bound maleic anhydride on the PdML/Mo(110) surface, where the larger Pd−Pd spacing, relative to Pd(111), allows for better overlap of the carbonyl group π-orbitals with the metal d-orbitals. The π-mode binding energy on the PdML/Mo(110) surface is, therefore, stronger than the corresponding binding energy on Pd(111). The results are consistent with the experimental observations of Xu and Goodman (Langmuir 1996, 12, 1807−1816).