Trinuclear tantalum clusters grafted to hydroxylated silica surfaces: A density-functional embedded-cluster study

Abstract

To identify the coordination modes of bare and hydrogenated trinuclear tantalum species on hydroxylated silica, we computationally examined models of Ta 3 H n (n = 0, 3, 5-9) species grafted to a β cris tobalite surface. Ta 3 H n clusters are bound to the surface by substitution of hydrogen atoms of vicinal (…O ) 3 SiOH and geminal (…O ) 2 Si(OH) 2 groups via three and six, respectively, Ta-O bonds of ~193 pm on average, in both types of models. The maximum Ta-O coordination number of non hydrogenated Ta 3 spe cies to a silica surface is seven for the second type model surface; the additional Ta-O bond is due to an oxy gen atom located in a bridging position to Ta-Ta bond. In the latter case, the mean Ta-O bond distance to one of =Si(O-) 2 group is increased by 15 pm. For the complexes bound via vicinal silanol groups, each addi tional unit of hydrogen loading on the metal elongated the average Ta-Ta distance by ~2 pm, covering a range of 258-277 pm. For the most stable hydrogenated trimers, Ta 3 H 9 , the desorption energies of hydrogen atoms are relatively high, above 70 kJ/mol. The average Ta-Ta distances increase by ~19 pm on going from the com plex (≡SiO-) 3 Ta 3 H 9 to complex (≡SiO-) 3 Ta 3 and by ~5 pm when the hydrogen loading is increased by one unit for (=Si(O-) 2 ) 3 Ta 3 H n complexes, reaching the maximum value 319 pm when n = 9. The desorption energies of hydrogen atoms for the most stable tantalum trimer species grafted to the surface by geminal sil anol groups, (=Si(O-) 2 ) 3 Ta 3 H 7 , are rather low, less than 40 kJ/mol.