Adsorption of boron on molybdenum(100) and its effect on chemisorption of carbon monoxide, ethene, propene, and 3,3,3-trifluoropropene

The interaction of boron with Mo(100) and its effect on surface reactivity have been investigated for the first time. Boron-covered surfaces can be prepared by adsorption and decomposition of diborane (B₂H₆) at 300 K. The B(1s) binding energy (BE) measured by XPS increases approximately linearly with boron coverage from 186.9 eV at 0.2 monolayer to 187.6 eV at 1.1 monolayers (saturation coverage), indicative of coverage-dependent B-B interactions. The absence of multiple B(1s) peaks or peak broadening suggests the overlayer grows by uniformly filling the available surface sites. After annealing to 1073 K and above, two distinct phases are formed: a low-coverage phase (∼0.2 monolayer) with B(1s) BE = 186.8 eV and a three-dimensional, B-rich, surface "boride" phase (MoB₂) with B(1s) BE = 188.2 eV. The assignment of the latter phase is established by comparison of the B(1s) BE with the literature value for MoB₂. The B(1s) signal disappears upon heating above 1700 K, indicating loss of surface boron. Adsorption of carbon monoxide, ethene, propene, and 3,3,3-trifluoropropene on B-covered surfaces was studied. Comparison of the results with data for adsorption of the same molecules on carbon- and oxygen-modified Mo(100) suggests that the B adatoms are not located in 4-fold hollow sites on the surface.