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

Abstract

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.

Thermal desorption of dimethyl methylphosphonate from MoO3

“…surface as PO x ; this species begins to form at 323 K and is often seen in studies of DMMP decomposition on metal oxides [18,20,21]. The lack of a resolvable corresponding O 1s component is reasonable considering the small amount of PO x compared to the bulk oxygen.…”

“…Several DMMP fragments and likely products were monitored, and the results are plotted in Figure 4. The parent ion (124 amu) and another characteristic fragment (79 amu, either PO 3 + or PO 2 CH 4 + ) [18,19] of DMMP show that desorption begins just below 250 K and has a maximum around room temperature, suggesting that DMMP does not strongly adsorb to MoO 3 . These mass spectrometry signals reach baseline levels again at around 450 K. Signals from methyl (15 amu) and methoxy (31 amu), follow the same trend as the parent ion, suggesting no decomposition.…”

Characterization of Pt–Au and Ni–Au Clusters on TiO2(110)

“…Experiments were conducted on a rutile TiO 2 (110) crystal (1 cm 9 1 cm 9 0.1 cm, Princeton Scientific Corp.) in two ultrahigh vacuum (UHV) chambers, which have been previously described in detail [10,23,29,30,[32][33][34][35]. LEIS and STM experiments were carried out in the first chamber, which has a base pressure of B5 9 10 -11 Torr, and is equipped with a fine focus ion gun (Omicron, ISE100) and hemispherical analyzer (EA125) for LEIS experiments, as well as a variable-temperature STM (Omicron, [10,23,30,32].…”

“…During this treatment, the crystal becomes an n-type semiconductor through the loss of oxygen from the crystal. Furthermore, titania is a support that is known to participate in catalytic reactions by providing a source of oxygen for reactions on the metal particles [10,[23][24][25]. Because metals like Pt [26][27][28][29][30] and Ni [23,25] interact strongly with titania while Au [31] does not, it may be possible to tune the extent of cluster-support interactions by changing the bimetallic compositions.…”

Nano-structured Solids and Heterogeneous Catalysts for the Selective Decontamination of Chemical Warfare Agents

“…In fact, compounds containing phosphorus and sulfur atoms are typical side-products that coordinate and bind irreversibly to the catalytically active sites of the nanoparticles, causing a gradual deactivation of the metal catalyst. Chen et al reported that small Ni nanoparticles (with main diameters of 5.0 nm) supported on TiO 2 show higher effi ciency than larger particles (8.8 nm) or annealed Ni fi lms in the decomposition of dimethylmethylphosphonate, DMMP (a widely used simulant for organophosphorus nerve agents) into CO and H 2 at room temperature [ 20 ]. However, the reaction left high amounts of carbon and phosphorus adsorbed on the metal catalyst surface.…”