Adsorption and Dehydrogenation of Ortho-Carborane on the Pt(111) Surface

The surface chemistry of 1,2-closo-dicarbadodecaborane (ortho-carborane), C2B10H12, on Pt(111), was studied with reflection–absorption infrared spectroscopy (RAIRS), temperature programmed desorption, and X-ray photoelectron spectroscopy (XPS). This molecule has a cage-like structure in which boron and carbon atoms occupy the vertices of a slightly distorted icosahedron with 2 C−H and 10 B−H radially directed bonds. At submonolayer coverages at 85 K, the RAIRS spectrum of carborane displays strong B−H stretching vibrations near 2600 cm−1 and a weak C−H stretch at 3090 cm−1 that are close in value to those of the isolated molecule. This indicates molecular adsorption at low temperature. At 85 K and low coverages, the B−H stretch peaks of carborane are unusually narrow with full widths at half-maxima as low as 4.4 cm−1. Comparison of calculated spectra for various assumed orientations reveals that the molecule is oriented with its permanent dipole moment and the C−C bond parallel to the surface. The molecule is stable on the surface up to 250 K, where it is transformed into a new intermediate with a strongly red-shifted B−H stretch vibration at 2507 cm−1. This intermediate is stable up to 400 K, above which no B−H stretch vibrations are observed. Hydrogen is released in stages as the carborane monolayer is heated from 85 to 800 K, indicating the formation of partially hydrogenated surface intermediates. From XPS measurements of the B 1s peak area as a function of annealing temperature, the boron coverage steadily decreases as the boron cage structure is disrupted and boron atoms diffuse into the bulk of the crystal.