K. T. Queeney

Dinitrosyl intermediate for N2O formation from reaction of NO on Mo(110)

The adsorption and subsequent reaction of nitric oxide (NO) on Mo(110) has been studied by temperature programmed reaction, electron energy loss, and infrared reflectance absorbance spectroscopies. The predominant reaction pathway for a saturated NO overlayer is dissociation to atomic nitrogen and oxygen; in fact, for low NO coverages, dissociation is the only reaction and largely takes place below 300 K. At NO coverages above 65% of θsat, evolution of N2O, N2, and NO is also observed at low temperature. Temperature programmed reaction of isotopically mixed overlayers demonstrates that N2O formation occurs via reaction of two intact NO molecules, suggestive of a dimeric surface intermediate. Electron energy loss and infrared spectroscopies identify three ν(NO) features which are assigned to three distinct species; the frequencies of the ν(NO) peaks in the infrared spectrum of a saturated 14NO overlayer at 100 K are 1860, 1821, and 1720 cm−1. The 1860 and 1720 cm−1 features are assigned to monomeric NO. Ba...

Spectroscopic evidence for perturbed NO dimers on oxidized Mo(110)

We report herein the first direct experimental evidence of a nitric oxide dimer (NO)2 which is significantly perturbed from gas- and condensed-phase (NO)2 by bonding of one of the nitrosyls to a metal surface. Infrared reflectance absorbance spectroscopy of isotopically mixed overlayers is used to identify the formation of this species from NO adsorption on oxidized Mo(110) and to characterize it as a second-layer NO[ν(14NO)=1871 cm−1] bound to a surface nitrosyl [ν(14NO)=1728 cm−1], The spectroscopic signature of this species is a small (∼8 cm−1) splitting of ν(NO) of each of the nitrosyls upon isotopic mixing, rather than the appearance of three different frequencies for νs(NO) and νa(NO). The formation of such a dimer on oxidized Mo(110) does not result in N–N bond formation, in contrast to the evolution of both N3 and N2O via a dinitrosyl intermediate on the same surface. This result suggests that, on surfaces which interact strongly enough with NO to form chemisorbed nitrosyls stable above room tempe...

Competing pathways for methoxy decomposition on oxygen-covered Mo(110)

The reactions of methanol (CH3OH) are investigated on a range of oxygen overlayers on Mo(110), with θO from ∼0.5 to >1 ML, using a combination of vibrational spectroscopies and temperature-programmed reaction. Infrared spectroscopy identifies a common, tilted methoxy intermediate at high temperature on all overlayers studied; electron energy loss spectroscopy shows that this intermediate decomposes to deposit oxygen exclusively in high-coordination sites. While C–O bond scission to evolve gas-phase methyl radicals is the only reaction observed for methoxy on highly oxidized Mo(110), on the surface oxygen overlayers competition between dehydrogenation and methyl evolution is highly sensitive to oxygen coverage. The enhanced selectivity for hydrocarbon formation from methanol reaction on oxygen-modified Mo(110) relative to the clean surface is attributed to inhibition of dehydrogenation pathways rather than to marked changes in the C–O bond potential of methoxy.

Structure and reactivity of thin-film oxides and metals

The structure and reactivity of thin-film oxides of molybdenum and Co metal supported on oxidized Mo(110) is discussed. Reactions of interest in heterogeneous oxidation catalysis, in particular hydrocarbon oxidation is the focus of the work. A combination of electron energy loss, infrared, and X-ray photoelectron is used to characterize the structures of the oxides and Co films. Oxidation conditions are used to control the nature of the oxygen coordination sites available as well as the thickness and morphology of the oxide. Accordingly, the reactivity of specific types of oxygen coordination sites was investigated. In the case of the Co overlayers, thermal treatment was used as a means of varying the structure and morphology of the metal supported on the oxidized Mo. Oxygen bound to Mo(110) in low-symmetry, high-coordination sites was found to play an important role in the hydrocarbon oxidation process. For example, gaseous methyl radicals selectively add to oxygen in these sites, but not to terminal oxygen. In the microscopic reverse of methyl radical oxidation, vacancies at high-coordination sites are necessary for methanol reaction to methoxy to occur. The site-specific oxidation chemistry is modeled in selected cases using first-principles electronic structure calculations. The reactions of alcohols on various Co thin films were also investigated. The selectivity for alcohol reaction is altered by electronic and structural modification of the film. The reactions of ethanol and methanol were used to illustrate these principles.