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

To identify the coordination modes of bare and hydrogenated trinuclear tantalum species on hydroxylated silica, we computationally examined models of Ta₃H _n (n = 0, 3, 5-9) species grafted to a β-cris-tobalite surface. Ta₃H _n clusters are bound to the surface by substitution of hydrogen atoms of vicinal (...O-)₃SiOH and geminal (...O-)₂Si(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 Ta₃ species to a silica surface is seven for the second type model surface; the additional Ta-O bond is due to an oxygen 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 additional 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, Ta₃H₉, 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 complex (=SiO-)₃Ta₃H₉ to complex (=SiO-)₃Ta₃ and by ∼5 pm when the hydrogen loading is increased by one unit for (=Si(O-)₂)₃Ta₃H _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 silanol groups, (=Si(O-)₂)₃Ta₃H₇, are rather low, less than 40 kJ/mol.