DOI: 10.1002/cctc.201000190 Hydrogenation Reactions on Au/TiC(001): Effects of Au C Interactions on the Dissociation of H 2 Elizabeth Florez, [a, b] Tatiana Gomez, [a, c] Ping Liu, [d] JosØ A. Rodriguez, [d] and Francesc Illas* [a, e] Hydrogenation reactions are very important in many areas of the chemical and petrochemical industries. [1, 2] There is a con- tinuous search for new hydrogenation catalysts and materials that can adsorb and dissociate H 2 in an efficient way. [2–4] Metal carbides have received attention as potential substitutes for noble metal catalysts, displaying good performance for catalytic processes in which molecular hydrogen is present as a reactant, such as hydrocarbon synthesis from CO and H 2 , hy- drogenation of benzene, hydrogenolysis of alkanes, alcohol synthesis, and hydrotreating. [5–7] In this sense, a fundamental question becomes how to improve the catalytic activity of these systems. Recent studies have shown that Au nanopar- ticles supported on TiC(001)—hereafter referred to simply as Au/TiC(001)—are excellent catalysts for the hydrogenation of olefins and the hydrodesulfurization (HDS) of thiophene. [8] The adsorption strength of thiophene on TiC(001) is very weak, and the molecule desorbs at temperatures below 200 K. However, in spite of the poor desulfurization performance of TiC(001) or Au(111), Au/TiC(001) systems display a surprising HDS activity which is even higher than that of conventional Ni/MoS x cata- lysts. [8] The Au–TiC(001) interactions produce a substantial po- larization of electron density around the Au nanoparticle, [9, 10] which may facilitate the dissociation of H 2 and provide the H atoms necessary for the HDS of thiophene or the hydrogena- tion of olefins. It has also been suggested that, although low- coordinated Au atoms facilitate hydrogen dissociation, [11] both the shape and size of gold particles are important. [3, 12] Herein we investigate the adsorption and dissociation of H 2 on Au/ TiC(001) surfaces using calculations based on density function- al theory (DFT) carried out on realistic models of Au/TiC(001). Small Au clusters in contact with TiC(001) exhibit chemical properties not seen in the gas phase or for Au supported on oxide surfaces, making Au/TiC an excellent catalyst for hydrogenation processes. Moreover, we present evidence of the interplay between Au nanoparticles and the underlying TiC surface. The good agreement between the theory and experi- mental results from previous studies [8, 9, 13] firmly supports the conclusions of the present work. Density functional calculations for the interaction of atomic hydrogen with TiC(001) indicate that H prefers to bond to the top C position, with a rather large adsorption energy of 2.6 eV, and remains almost neutral. H 2 adsorbs molecularly on TiC(001) with the molecule center on top of a C atom and the H H axis along the diagonal connecting C atoms (Figure 1). The molecule is highly activated with the H atoms negatively charged by 0.19 e and almost dissociated with an internuclear distance of 1.73 , compared to 0.74  for the free molecule, and a stretching frequency of 1199 cm 1 , much smaller that the value for the isolated molecule, which, in our calculation, is 4477 cm 1 (close to the experimental value of 4395 cm 1 ). [14] For this molecularly adsorbed species, the adsorption energy (E ads ) with respect to isolated H 2 and clean TiC(001) is 0.48 eV, meaning that at moderate H 2 pressure and low temperature the surface should be covered by molecular hydrogen. There are two other more stable configurations, nearly equivalent in energy but these correspond to fully dissociated H 2 with H atoms on top of nearest-neighbor C atoms (E ads = 0.89 eV) and hence separated by 3.09  and the other with H atoms separated by 6.13  and above C atoms along the diag- onal containing C atoms but separated by one C atom without H(E ads = 0.92 eV). Two reaction pathways have been identi- fied for H 2 dissociation on clean TiC(001). The most favorable has a modest energy barrier (E act ) of 0.52 eV, large enough to prevent H 2 spontaneous dissociation unless the system is heated. In this pathway (Figure 2), one of the H atoms remains above the C atom where the molecule is located initially and the other migrates to the nearest-neighbor C atoms, leading Figure 1. Structure of molecular hydrogen adsorbed on TiC(001). [a] Dr. E.Florez, T. Gomez, Prof. F. Illas Departament de Química Física & IQTCUB, Universitat de Barcelona C/Martí i Franqus 1, 08028 Barcelona (Spain) Fax: (+ 34) 24-402-1231 E-mail : francesc.illas@ub.edu Homepage: http://www.ub.edu/cmsl/xino.html [b] Dr. E. Florez Instituto de Química, Universidad de Antioquia (Colombia) [c] T. Gomez Departamento de Química, Facultad de Ecología y Recursos Naturales Universidad AndrØs Bello, Republica 275, Santiago (Chile) [d] Dr. P. Liu, Dr. J. A. Rodriguez Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) [e] Prof. F. Illas Institució Catalana de Recerca i Estudis AvanÅats (ICREA) 08010 Barcelona (Spain) ChemCatChem 2010, 2, 1219 – 1222  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1219