Dimethyl Methylphosphonate Decomposition on Titania-Supported Ni Clusters and Films: A Comparison of Chemical Activity on Different Ni Surfaces

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The thermal decomposition of dimethyl methylphosphonate (DMMP) has been studied in ultrahigh vacuum by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) on Ni clusters and films deposited on TiO2(110). The four different Ni surfaces under investigation consisted of small Ni clusters (5.0 ± 0.8 nm diameter, 0.9 ± 0.2 nm height) deposited at room temperature and quickly heated to 550 K, large Ni clusters (8.8 ± 1.4 nm diameter, 2.3 ± 0.5 nm height) prepared by annealing to 850 K, a 50 monolayer Ni film deposited at room temperature, and a 50 monolayer Ni film annealed to 850 K. The morphologies of the Ni surfaces were characterized by scanning tunneling microscopy (STM). TPD experiments show that CO and H2 are the major gaseous products evolved from the decomposition of DMMP on all of the Ni surfaces, and molecular DMMP and methane desorption were also observed. The product yields for CO and H2 were highest for reactions on the small Ni clusters and unannealed Ni film and lo...