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 TiO 2 (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 H 2 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 H 2 were highest for reactions on the small Ni clusters and unannealed Ni film and lowest for reactions on the large clusters and annealed film. Furthermore, XPS experiments demonstrate that the unannealed Ni surfaces decompose a greater fraction of DMMP at room temperature. The loss of activity for the annealed surfaces is not caused by a reduction in surface area because the annealed surfaces have approximately the same surface area as the small clusters. CO adsorption studies suggest that the loss of activity upon annealing cannot be completely due to a decrease in surface defects, such as step and edge sites, and we propose that a TiO x moiety is responsible for blocking active sites on the annealed Ni surfaces. In comparison to the TiO 2 surface, the small Ni clusters are more chemically active because a greater fraction of DMMP decomposes at room temperature, and the total amount of DMMP decomposition is also higher on the small Ni clusters. Although DMMP decomposes on TiO 2 to produce gaseous methyl radicals, methane, and H 2 , the activity of the substrate surface itself appears to be quenched in the presence of the Ni clusters and films. However, the TiO 2 support plays a significant role in providing a source of oxygen for the recombination of atomic carbon on Ni to form CO, which desorbs above 800 K.

Scanning tunneling microscopy studies of Cu islands grown on TiO 2 (110) demonstrate that these islands disappear from the STM images after exposure to oxygen gas (60-2000 Langmuir). Based on X-ray photoelectron spectroscopy experiments, the disappearance of the Cu islands cannot be explained by the loss of Cu from the surface or by a dramatic change in the electronic properties of the islands. The adsorption of oxygen appears to weaken the Cu-Cu bond, allowing two-dimensional (2D) islands to form on the surface at the expense of the existing three-dimensional (3D) islands. Thermodynamically, the conversion of 3D to 2D islands is favored by the lower surface free energy of the oxidized Cu compared to that of copper itself. This effect has also been observed for Ni islands, but the rate of island disappearance is slower even though Ni is more easily oxidized than Cu.