Chemisorption bond energies of Lewis acids and bases on oxygen modified Mo(100) surfaces

Deffeyes, J.E.; Smith, A. Horlacher; Stair, Peter C.

doi:10.1016/0039-6028(85)90850-7

Cited by 27 publications

(3 citation statements)

References 14 publications

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“…The total amount of CO desorption increases with decreasing oxygen coverage. This can be attributed to the dissociative adsorption of CH 3 on bare metal patches. , This interpretation is in line with the decreasing yield of β-CO with increasing θ O . β-CO results from the reaction of atomic carbon with atomic oxygen on metallic molybdenum .…”

Section: Resultssupporting

confidence: 66%

Surface Chemistry of Methyl Radicals on O/Mo(100) Surfaces

Kim

Stair

1999

J. Phys. Chem. B

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The chemistry of CH3 radicals on oxygen-modified Mo(100) surfaces (O/Mo(100)) has been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Gas-phase CH3 radicals were produced by pyrolysis of azomethane and dosed on O/Mo(100) at a surface temperature of 320 K. In TPD, O/Mo(100) with θO = 1.4 monolayer (ML) produces exclusively CH4 and CO, but O/Mo(100) with θO = 0.9 and 0.4 ML produce significant amounts of C2+ alkenes in addition to CH4 and CO. HREELS shows that the CH3 groups are bound to surface Mo atoms, not to surface oxygen. On 1.4 ML-O, the CH3 groups are stable at 320 K and have a symmetry lower than C 3 v . On 0.9 ML-O and 0.4 ML-O, some CH3 groups decompose to methylene groups, which react with intact CH3 groups to form surface alkyl groups. The surface species at 320 K appear to be controlled by the preadsorbed oxygen coverage, depending on whether θO < 1 ML or θO > 1 ML. CH4 is formed via hydrogenation of CH3 groups by surface hydrogen that is a product of CH3 decomposition. C2+ alkene products are formed by β-hydrogen elimination of surface alkyl groups. When atomic iodine is coadsorbed on O/Mo(100), the alkene yield in TPD is significantly reduced.

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Section: Resultssupporting

confidence: 66%

Surface Chemistry of Methyl Radicals on O/Mo(100) Surfaces

Kim

Stair

1999

J. Phys. Chem. B

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“…The studies conclude that at low temperatures propene on both surfaces forms a di-σ complex bonded to two metal atoms that dehydrogenates to form propylidyne at roughly room temperature. Temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS) of propene and trifluoropropene on oxidized Mo(100) − indicated that backbonding contributes to the chemisorption of the trifluoropropene but not the propene. The desorption energy of trifluoropropene was ∼3 kcal/mol higher than that of propene.…”

Section: Introductionmentioning

confidence: 99%

Orientation of Physisorbed Fluoropropenes on Cu(111)

Street

Gellman

1997

J. Phys. Chem. B

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Fourier transform infrared reflection absorption spectra (FT-IRAS) have been obtained for adsorbed monolayers of propene and a set of fluorinated propenes (CFH2CHCH2, CF3CHCH2, and CF3CFCF2) on the Cu(111) surface. These species interact weakly with the copper surface. The FT-IRAS spectra indicate that the molecules lie flat with the plane of the molecule defined by the CCC atoms essentially parallel to the surface regardless of the degree of fluorination. Quantitative analysis of the orientation of hexafluoropropene determines the tilt of the molecular plane with respect to the surface to be 11°. The results for propene indicate a molecular tilt of 20°. Where the ν(CC) mode can be identified in the spectra of the adsorbed molecules, there is no perturbation of its frequency, which indicates that there is little backdonation between the metal and the π* orbital of the propene.

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“…Since it is proposed above that methane forms via a carbon−carbon double-bond dissociation reaction to form carbenes, the variation in methane yield may be related to this dissociation probability. It has been suggested that, within the context of the Dewar−Chatt−Duncanson model, alkenes bond on Mo(100) primarily by donation of electrons from π-orbitals to the metal rather than via back-donation into vacant π* orbitals. , That is, alkenes behave like π-donors on Mo(100). Corroborative evidence for this idea comes from the effect on the energetics of alkene desorption, , where the desorption activation energies increase with the addition of oxygen to the surface in accord with the above proposal.…”

Section: Discussionmentioning

confidence: 99%

Surface and Catalytic Chemistry of Small Hydrocarbons on Oxidized Molybdenum

Bartlett

1998

Langmuir

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It is shown that model oxides grown on metallic substrates catalyze propylene metathesis to form ethylene and butene with an activity that mimics that of supported catalysts for reaction below ∼650 K. Another reaction regime is found for olefin metathesis above ∼650 K, where the reaction proceeds with a much higher activation energy of ∼60 kcal/mol. Unfortunately, alkenes do not react to any detecable extent on the model oxide surfaces in ultrahigh vacuum. However, the high-temperature (>650 K) metathesis rate is found to be affected by the presence of oxygen overlayers, which also modify the chemistry of alkenes on Mo(100) in ultrahigh vacuum. It is found that methane is formed by reaction of alkenes on O/Mo(100). The only other products detected are ascribed to either hydrogenation or total thermal decomposition into carbon and hydrogen. It is shown, using iodine-containing molecules to graft hydrocarbon fragments onto the surface, that alkenes can dissociate, forming carbenes which react to yield methane. This chemistry is in accord with that found catalytically at high temperatures, where the product distribution from the reaction of ethylene is well described by a Schulz-Florey distribution and the product distribution from propylene is well described by copolymerization of carbenes and methyl carbenes.

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Chemisorption bond energies of Lewis acids and bases on oxygen modified Mo(100) surfaces

Cited by 27 publications

References 14 publications

Surface Chemistry of Methyl Radicals on O/Mo(100) Surfaces

Surface Chemistry of Methyl Radicals on O/Mo(100) Surfaces

Orientation of Physisorbed Fluoropropenes on Cu(111)

Surface and Catalytic Chemistry of Small Hydrocarbons on Oxidized Molybdenum

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