AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS1-ThM

Paper SS1-ThM1
Decomposition of Dimethyl Methylphosphonate on TiO@sub2@(110)-Supported Ni Clusters of Different Sizes

Thursday, November 18, 2004, 8:20 am, Room 210B

Session: Metal Oxide and Clusters III: Supported Cluster Formation and Reactivity
Presenter: D.A. Chen, University of South Carolina
Authors: D.A. Chen, University of South Carolina
S. Ma, University of South Carolina
J. Zhou, Oak Ridge National Laboratory
Correspondent: Click to Email
The thermal decomposition of dimethyl methylphosphonate (DMMP) on Ni clusters deposited on TiO@sub2@(110) has been studied in ultrahigh vacuum by TPD and XPS. Ni cluster sizes were characterized by STM; the small Ni clusters (5.0±0.8 nm diameter, 0.9±0.2 nm height) were deposited at room temperature while the large clusters (8.8±1.4 nm diameter, 2.3±0.5 nm height) were prepared by room temperature deposition followed by annealing to 850 K. TPD experiments show that CO and H@sub2@ were the major gaseous products evolved from the decomposition of DMMP, and molecular DMMP and methane desorption were also observed. The product yields for CO and H@sub2@ were higher for reaction on the small Ni clusters. Furthermore, XPS experiments demonstrate that the small Ni surfaces decompose a greater fraction of DMMP at room temperature. The loss of activity for the large annealed clusters is not caused by a reduction in surface area, which does not change substantially before and after annealing. Since CO adsorption studies suggest that the loss of activity cannot be completely due to a decrease in surface defects upon annealing, we propose that a TiO@subx@ moiety is responsible for blocking active sites on the annealed Ni surfaces. Low ion scattering studies show that the Ni clusters are not completely encapsulated after heating to 850 K, but these experiments are also consistent with partial encapsulation. Although DMMP decomposes on TiO@sub2@ to produce gaseous methyl radicals, methane and H@sub2@, the activity of the substrate surface itself appears to be quenched in the presence of the Ni clusters. However, the TiO@sub2@ 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.