3446
J . Phys. Chem. 1992, 96, 3446-3452
explain the higher quantum efficiency of K2Cr207with respect to CaCr04.40 It can also be applied in the present case since the excitation is localized in a Cr-O or V-O bond. Such a bond has only one nondegenerate vibrational mode.
Conclusions The systems V/Si02, V/A1203, and Cr/Si02 show at low temperature a rather efficient luminescence. This was not expected because an important requirement for efficient luminescence is the presence of stiff surroundings of the luminescent complex which can counteract the expansion in the excited state. A surface clearly does not offer such stiff surroundings. Luminescence spectroscopy indicates one luminescent oxovanadium (-chromium) species on the silica surface and more than one species on the alumina surface. These results confirm the
results from other characterization techniques, such as Raman spectroscopy. The luminescence properties of the tetrahedral monomeric surface species, which are strongly distorted, are different from those of the corresponding regular tetrahedral complexes. This is ascribed to the localization of the excited state in a short metal-oxygen bond. Hence, in addition to information on the molecular structure of these surface complexes, luminescence spectroscopy can also give information on the electronic structure of these complexes.
Acknowledgment. We thank Dr. M. A. Vuurman of the Inorganic Chemistry Department of the University of Amsterdam for a valuable discussion and for sharing some of his results prior to publication.
Catalytic Oxidation of Phosphorus on MOO, As Studied by Infrared Spectroscopy Dilip K. Paul, Ling-Fen Rao, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: October 31, 1991; In Final Form: December 27, 1991)
The decomposition and oxidation of phosphine has been studied on an MoO3/AI2O3supported catalyst using transmission IR spectroscopy and mass spectroscopy in the temperature range 300-800 K. Phosphine decomposes on Moo3 at 573 K and is oxidized to a surface species containing the H P = O moiety, exhibiting a characteristic H-P mode at 2490 cm-I and a P=O mode at -1 100 cm-l. Further oxidation at 673 K under 0 2 ( g ) produces a surface species (HO),P==O(a) which has probably migrated to the support. The ( H O ) , F O species is characterized by an 0-H stretching mode at 3672 cm-l; it is desorbed from the surface at temperatures near 773 K. The two sequential oxidation steps have been performed for several cycles using additional PH, adsorbate. These observations suggest that the MoO3/AI20, catalyst may be effective for the continuous catalytic oxidation of organophosphorus compounds.
I. Introduction The oxidation of phosphorus on surfaces is of fundamental importance in the development of heterogeneous catalytic methods for environmental protection from phosphorus-containing compounds such as those used in pesticides, herbicides, and chemical warfare agents.l It has been shown that certain metals such as Pt,2 Pd,) and Mo4 are somewhat effective in promoting the catalytic oxidation of an organophosphorus compound, dimethyl methylphosphonate (DMMP). In recent surface science studies on single crystals of Pd3 and M o , ~it has been shown that the key to continued catalytic oxidation activity is the attainment of temperatures where surface phosphorus is readily oxidized away leaving clean metal or metal oxide sites available for continued catalytic chemistry. In model experiments on single crystals in an ultrahigh-vacuum environment, the critical temperatures for phosphorus oxidation were found to be approximately 900 K (M0(110)~)and 1000 K (Pd(111)3). This paper is concerned with the catalytic oxidation of adsorbed phosphorus by an A1203-supportedMOO, catalyst. The fundamental premise of our approach is that the key rate-controlling step in the catalytic destruction of phosphorus-containing compounds will be the elemental phosphorus catalytic oxidation step. Therefore this basic catalytic reaction should first be studied under controlled conditions. Phosphorus is supplied to the MOO, catalyst by phosphine gas, and measurements of partially oxidized phosphorus surface species and their interconversions are made using transmission IR spectroscopy. It has been found that at (1) Ekerdt, J. G.;Klabunde, K. J.; Shapley, J. R.; White, J . M.; Yates, J. T.,Jr. J . Phys. Chem. 1988, 92, 6182. (2) Henderson, M. A.; White, J. M. J . Am. Chem. SOC.1988, 110, 6939. (3) Guo,X.;Ycshinobu, J.; Yates, J. T., Jr. J . Phys. Chem. 1990, 94,6439. ( 4 ) Smentkowski. V . S.; Hagans, P.;Yates, J . T., Jr. J . Phys. Chem. 1988. 92, 6351
1.5 Torr oxygen pressure, on MOO,, the onset of adsorbed phosphorus oxidation may be detected by IR spectroscopy at 673 K. This is a much lower and more practical oxidation temperature than that measured in the single-crystal Mo( 1 10) experiment^.^ This work suggests that a low-temperature catalytic oxidation process for organophosphorus compounds can be designed using MOO,-based catalysis. 11. Experimental Section
The stainless steel ultrahigh-vacuum IR cell used in these studies is shown in Figure 1 and has been described in more detail earlier.s The cell is equipped with CaF2 optical windows sealed into 2.75-in.-diameter conflat flanges, allowing transmission IR measurements in the ~ 1 O O O - c mspectral ~' range. In the center of the cell is a tungsten grid, which consists of closely spaced square openings of 0.22-mm width photoetched in tungsten foil of 0.0254-mm thickness. Inside each of the openings in the grid, the high area catalyst is supported in intimate contact with the tungsten foil frames surrounding each grid opening. This permits excellent thermal contact between the powdered catalyst and the tungsten grid, while also permitting high transmission efficiency for infrared radiation. The tungsten grid is rigidly held between Ni clamps which serve both as electrical connections and as heat-transfer connections to a reentrant dewar which supports the Ni clamps on their electrical feedthroughs; these clamps then support the tungsten grid. The grid containing the catalyst can be heated using an electronic controller, and temperatures are measured with a thermocouple welded to the tungsten grid. The upper temperature limit exceeds 1500 K with this cell, and rapid and reproducible ( 5 ) Basu, P.; Ballinger. T.H.; Yates, J. T., Jr. Reu. Sci. Insrrum. 1988, 59, 1321.
0022-365419212096-3446%03.00/0 0 1992 American Chemical Society
Catalytic Oxidation of Phosphorus on Moo3
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3447
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(6) Ballinger, T. H.; Yates, J. T., Jr. J . Phys. Chem. 1991, 95, 1694. (7) Goldwasser, J.; Fang, S. M.; Houalla, M.; Hall, W. K. J . Curd. 1989, 115, 34. (8) Killeffer, D. H.; Linz, A.; Pauling, L. Molybdenum Compounds: Their Chemistry and Technology;Interscience Publishers: New York, 1952; p 32.
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Figure 1. Design of wide-temperature-range infrared cell.
temperature programming of the catalyst can be achieved using the controller. The cell is also capable of operation at low sample temperatures near 120 K. The sample was heated to the designated temperature in 2-5 min. The cell is attached to a bakeable all-metal gas handling system equipped with a liquid nitrogen cooled zeolite sorption pump and a 30 L/s ion pump. A quadrupole mass spectrometer in the vacuum system may be used for gas analysis as a function of the temperature of the catalyst in the cell. The mass spectrometer is connected to the IR cell by means of a stainless steel bellows. The base pressure of the vacuum system is typically
