Abstract: On 100 K rutile TiO2(1 1 0) surfaces with 3.5 ± 0.5% surface bridge-bonded oxygen vacancies (BBOV’s), propene formation by dehydration of 2-propanol was monitored by temperature programmed desorption (TPD). The BBOV’s were either filled or unfilled when the 2-propanol was dosed. Propene desorption rates exhibit two local maxima, nominally at 350 (LT) and 570 K (HT). The former is not altered by filling BBOV’s while the latter is reduced by factors of 2.5 and 5 when the vacancies are pre-filled with water (H–OH) and alcohol (R–OH), respectively. The HT process is attributed to a reaction of 2-propoxy groups located on surface BBO’s. To account for much of the HT C3H6 yield when BBOV’s are filled before (CH3)2CHOH dosing, we propose a model whereby, during TPD, vacancies form and are filled with 2-propoxy. The factor of two difference between titrating BBOV’s with H2O and alcohols is attributed to stoichiometry; twice as many OH groups form on surface BBO rows when H2O is used so twice as many vacancies are created when OH recombines.
Notes: Role of Hydroxyls and Bridge-bonded Oxygen Vacancies in Dehydration of 2-Propanol on TiO2(110)
Abstract: Our study of a (WO3)3/TiO2(110) model mixed oxide catalyst shows that this system is an extremely efficient for dehydration of alcohols, effectively lowering the energy barrier as much as possible for an endothermic reaction without yielding a significant concentration of other side products.
Abstract: The activation energies for alkene formation via dehydration of alcohols on bridge-bonded oxygen (BBO) vacancy sites of TiO2(110) is found to correlate with the inductive electron donating effect of alcohol alkyl groups as measured by the Taft parameter. Based on this correlation we conclude that the reaction involves a single transition state that undergoes concerted rupture of the C–O bond of the alkoxide and a C–H bond of the alkyl group attached to the β-carbon.
Notes: Alkyl group-inductive effect on Ov-mediated dehydration of alcohol on TiO2(110)
Abstract: A comparison of the reactivity of chemisorbed oxygen with chemisbrbed CO on Ru(109) and Ru(001) surfaces to produce CO2 shows that the atomically stepped Ru(109) surface provides active oxygen species, whereas the Ru(001) surface is inactive. Using Auger spectroscopy and low-energy electron diffraction, it is found that the activity of Ru(109) is caused by the local high oxygen coverages achieved at the Ru step sites where the O/Ru ratio is 1.3 +/- 0.3. The O/Ru(109) surface, saturated with CO, produces an inhomogeneous CO layer as observed by infrared spectroscopy, whereas O/Ru(001) produces a single terminally bound CO species.
Abstract: Dosed on rutile TiO2(110) at 100 K, the thermal chemistry of 2-propanol in three forms-C3H7OH, C3D7OD, and C3H7OD-was characterized using temperature-programmed desorption. Only 2-propanol, propene, and water desorb with no evidence for acetone. The propene forms and desorbs by two paths, a heretofore unreported low-temperature path extending from 300 to 450 K and, concurring with prior work, a high-temperature path peaking between 570 and 580 K. Both paths exhibit isotope effects. The high-temperature path is interpreted in terms of decomposition of 2-propoxy species located on bridging oxygen atom rows. The low-temperature path is attributed to 2-propanol dehydration on undercoordinated Ti4+ ions of the Ti4+ rows. The low-temperature path characteristics vary with the long-range order and bridge-bonded oxygen atom vacancy concentration.
Notes: Surface Chemistry of 2-Propanol on TiO2(110): Low and High temperature dehydration, isotope effects, and influence of local surface structure
Abstract: The dissociation of CO molecules on a stepped Ru surface in the presence of Li promoter atoms is probed by reflection-absorption infrared spectroscopy (RAIRS). We find that even at 85 K, Li adsorbate atoms cause the rupture of the C-O chemical bond. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The Li-promoted CO dissociation on flat and atomically stepped Ru surfaces has been investigated by thermal desorption spectroscopy (TDS) and infrared reflection-absorption spectroscopy (IRAS). Li-induced dissociation of CO is observed to be greatly enhanced on stepped Ru(109) compared to Ru(001) from the scrambling of isotopic CO molecules as well as from the intensity reduction of the CO stretching mode. Interestingly, on Ru(001), Li produces a highly red-shifted CO stretching mode at similar to 1820 cm(-1), implying the formation of Li,CO, complexes. On the other hand, the preferential adsorption of Li by the atomic steps on Ru(109) impedes the formation of such complexes. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The interaction of Li with Ru was investigated on Ru(001) and Ru(109) using work function measurement, temperature-programmed desorption and low-energy electron diffraction. Strong depolarization effects for Li atoms at atomic step sites are observed, causing the dipole moment of a chemisorbed Li atom at the limit of zero coverage to decrease from 10 D to 6 D. In contrast to the (n x n) incommensurate Li structures observed on Ru(001), the Ru(109) surface does not produce any ordered Li structures. Despite these differences, the thermal desorption kinetics for Li from Ru(001) is virtually identical to that from Ru(109) at all coverages. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Adsorption of NH3 on the Si(111)7x7 surface below room temperature has been investigated by high-resolution core-level photoemission. The N 1s spectra reveal intriguing details of temperature-dependent adsorption and dissociation behaviors by clearly resolving out NH3, NH2, and NH species. The initial adsorption occurs through the dissociation into not only NH2 but also NH with a substantial fraction even down to 70 K, which is in clear contrast with that on Si(001)2x1. The saturation of this initial adsorption is shown to be related to the depletion of restatom sites from the Si 2p spectra. In addition to the initial dissociation, NH3 molecular adsorption occurs below 120 K. At that low temperature, the total coverage of the dissociated species increases substantially, well beyond the saturation coverage above 120 K, as the coverage of the molecular adsorbates increases. From these results, it is concluded that the adsorption of NH3 on Si(111)7x7 proceeds through two largely different processes; (i) an initial active dissociation into both NH2 and NH involving Si restatoms and (ii) a low-temperature high-coverage dissociation involving an NH3 precursor state.
