Showing papers on "Chemisorption published in 1975"

Catalytic Decomposition of Nitric Oxide on Zirconia by Electrolytic Removal of Oxygen

Abstract: Zirconia stabilized with 8 mole per cent of scandia has a very high oxygen ion conductivity and can "pump" oxygen from oxygen‐bearing gases thus decomposing them In this study the rate of decomposition of the air‐pollutant species, nitric oxide, to harmless species (via the reaction: ) was found to be markedly catalyzed when a potential above 1V was applied across a zirconia disk coated with either a porous platinum or porous gold electrode It is known that platinum can both form oxides and catalyze the decomposition of ; whereas, gold does neither The catalytic decomposition of on platinum metal is inhibited by , such behavior being attributed to preferential chemisorption of over as well as the possible formation of an inhibiting platinum oxide surface The original rationale for this investigation was the possibility that the decomposition of might be enhanced if were electrolytically "pumped" away from a platinum electrode deposited on zirconia, keeping the platinum oxygen‐free However it was not anticipated that at high potentials dissociation rates a thousandfold that on nonporous platinum electrodes occur in the presence of either a platinum or gold porous electrode No reactivity at all was observed on a nonporous gold electrode These results suggest that catalysis occurs mainly on a surface other than the platinum or gold, namely, on the zirconia surface itself It is proposed that F‐centers on the zirconia surface formed by the applied potential are primarily responsible for the observed enhanced catalysis

Reflectance spectra of surface states in magnesium oxide and calcium oxide

Abstract: The ultraviolet and visible reflectance spectra (52 000–15 000 cm–1) of high surface area MgO (250–300 m2 g–1) and CaO (100–200 m2 g–1) have been studied in vacuo and in the presence of O2, N2O, CO2 and H2O. The effect of outgassing MgO and CaO at increasing temperatures (773–1073 K) is to develop u.v. absorption bands in the range 30 000–50 000 cm–1. The absorption can be progressively destroyed by chemisorption, and also by sintering. The bands are attributed to excitons bound in surface states. In vacuo the outgassed oxides exhibit a fluorescence which is readily quenched by exposure to a low pressure of oxygen. Absorption bands occur at 37 000 and 46 000 cm–1 for MgO and at 35 500 and 44 500 cm–1 for CaO. With each oxide the chemisorption of gases erodes the lower energy band preferentially, and a similar effect occurs on sintering. The two bands are ascribed to exciton absorption at surface oxide ions in different states of coordinative unsaturation, the lower energy exciton band corresponding to the lower coordination.The influence of gases on the exciton spectra of outgassed MgO and CaO is shown to depend on their chemical reactivity, increasing in the sequence O2, N2O, CO2 and H2O. The effects with each gas are reversible on outgassing. Exposure of CaO to O2 gives rise to an absorption band at 23 500 cm–1, tentatively attributed to an O–3 adsorbed species.The surface states observed on CaO can be produced on MgO by adsorbing Ca ions from solution and heating to 1073 K.

Interactions on metal surfaces

Abstract: Theory of electronic properties of surfaces.- Theory of Chemisorption.- Chemisorption: Aspects of the experimental situation.- Desorption phenomena.- Photoemission and field emission spectroscopy.- Low energy electron diffraction (LEED) and Auger methods.- Concepts in heterogeneous catalysis.

Chemisorption of atomic hydrogen on the silicon (111) 7 × 7 surface

Abstract: The chemisorption of atomic hydrogen on the silicon (111) 7 \ifmmode\times\else\texttimes\fi{} 7 surface has been studied using ionneutralization spectroscopy and ultraviolet-photoemission spectroscopy with the help of low-energy electron diffraction and work-function measurement. Both spectroscopies showed that the dangling-bond surface state disappears when the clean surface is exposed to atomic hydrogen. Chemisorbed hydrogen produces two sharp peaks in the surface density of states at approximately -10 and -12 eV from the vacuum level. These results are in good quantitative agreement with the recent theoretical works by Appelbaum and Hamann and by Pandey.

Dynamics of activated chemisorption: Methane on rhodium

Abstract: Chemisorption of methane has been examined on highly perfect rhodium surfaces by a combination of field emission and molecular beam techniques in order to explore the mechanism of activation. It has been found that dissociative adsorption can be initiated on rhodium at 245 K just by excitation of the gas. Quantitative rate studies as a function of gas temperature have established that the CH4 molecule must overcome an activation barrier of 7 kcal mole−1 for reaction to occur. For gas temperatures 600 < TG < 710 K, the efficiency of chemisorption per impact is at least an order of magnitude smaller for CD4 than for CH4 and a factor of ? 3 smaller for CH2D2. This suggests that translational and rotational motion are not directly involved in passing over the barrier. Vibrational excitation of methane molecules appears to be the significant step in the reaction at the surface. Transition state theory fails to account for the significantly smaller rates observed for CD4 than for CH4. However, Slater’s dynamica...

Chemisorption Bond Length and Binding Location of Oxygen in a p ( 2 × 1 ) Overlayer on W(110) Using a Convergent, Perturbative, Low-Energy-Electron-Diffraction Calculation

Abstract: A convergent perturbation scheme known as the layer-doubling method is used to determine the chemisorption position and bond length of an oxygen $p(2\ifmmode\times\else\texttimes\fi{}1)$ overlayer on W(110) surface. The use of perturbation treatment allows accurate and economical determination of surface crystallography on this strong-scattering material. Using eight phase shifts and 89 beams in the calculation, the oxygen atoms are found to occupy threefold-coordinated binding sites with a bond length of 2.08 \AA{}.

Density Functional Theory of Chemisorption on Metal Surfaces.

Abstract: : The density functional theory of chemisorption is developed by means of a self-consistent linear response formalism. Any chemisorbed species which can be represented as an external charge distribution perturbing the metallic surface can be studied with this approach. The formalism is applied to hydrogen chemisorbed on a tungsten surface. Theoretical results for the ionic desorption energy, adatom vibrational frequency, relative size of the dipole moment, resonance levels associated with the adsorbate, and the question of dissociation agree well with experimental measurements. The calculated level width is too large. Differential scattering cross sections for chemisorbed hydrogen are compared with those of the isolated atom. The rather satisfactory overall agreement of our theory with experimental results suggests that the linear response formalism may have a wider usefulness.