STUDIES OF THE KINETICS AND MECHANISM OF GAS-SURFACE REACTIONS ON METAL AND METAL OXIDE SURFACES.

摘要

The coadsorption of SO(,2) and CO on Pt is studied by using Flash Desorption Mass Spectrometry (FDMS) and Auger Electron Spectroscopy (AES). We find that: (1) SO(,2) chemisorbs on clean Pt with an initial sticking probability of unity and with at least three first order, coverage independent binding states with E(,d) of 7, 11 and 23 kcal mole('-1) respectively. (2) CO adsorbs on clean Pt with an initial sticking probability of 0.6 and with at least two first order and coverage dependent binding states with E(,d) around 18 and 22 kcal mole('-1) respectively. (3) In the coadsorption experiments, the surface reaction proceeds between the adsorbed species to form a complex which dissociates to form CO(,2) and sulfur. The sulfur diffuses into the bulk of the Pt foil. The complex formation modifies the surface bonding. The E(,d) for CO is decreased while the E(,d) of the two low temperature binding states for SO(,2) are increased. (4) The CO(,2) FDMS shows that there are at least two first order and coverage dependent binding states with E(,d) around 13 and 16 kcal mole('-1) respectively. (5) When the surface is saturated with SO(,2) ((theta) = 1) adsorption of CO is completely inhibited. However, if the surface is saturated with CO ((theta) = 0.67) appreciable amounts of SO(,2) can still be adsorbed. The observed reduction in Pt activity is simply due to blocking of the sites. (6) The kinetics of the surface reacton for this system are characteristic of a Langmuir-Hinshelwood mechanism. The rate-determining step is the first order desorption of CO(,2) from a surface complex. (7) The energy change for the formation of the complex {('O) (,S) ('O) (,C) ('O)} (ad.) from the SO(,2)(ad.) and CO(ad.) molecules is calculated to be (DELTA)E = -31 kcal mole('-1).;Moreover, the stability of the AlO(,2) molecule is studied by using a 60(DEGREES) sector magnetic mass spectrometer. We find that: (11) The AlO(,2) molecules exist as a minor vapor species evaporating from the Al(,2)O(,3) surface. The atomization enthalpy of AlO(,2) is found to be.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;This leads to a heat of formation for AlO(,2) of.;In addition, the Reaction Probability technique is used to study the kinetics and mechanism of the dissociation reaction of In(,2)O on W and on Al(,2)O(,3). We find that: (8) In atoms chemisorb on both a clean W surface and an oxidized W surface with an initial sticking probability of unity and with a first order, coverage dependent binding state. The E(,d) decreases with increasing coverage up to (theta) = 0.25 and (theta) = 0.125 for W(clean) and W(oxidized) respectively. In the zero coverage limit, the E(,d) are 92 and 65 kcal mole('-1) for In on W(clean) and on W(oxidized) respectively. (9) In(,2)O is chemisorbed on an oxidized W surface with a first order, coverage dependent binding state. At high coverage, the interaction among the In(,2)O molecules and the surface is rather complicated. The adsorbed In(,2)O molecules can either desorb from the surface as In(,2)O or undergo dissociation to form In and O atoms. The O atoms react with W immediately to form tungsten oxides. The desorption of In is the rate-determining step of this system. (10) In atoms also chemisorb on Al(,2)O(,3) with an E(,d) of 56 kcal mole('-1) in the limit of zero coverage. In(,2)O is chemisorbed on Al(,2)O(,3). The E(,d) of desorption in the zero coverage limit is 44 kcal mole('-1). The desorption of In is the rate-determining step of the In(,2)O on Al(,2)O(,3) system.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).