Interaction of Alcohols with Surface Hydroxyls on Chromium(III) Oxide

Kittaka, Shigeharu; Umezu, Tomoiki; Ogawa, Hiroshi; Maegawa, Hirofumi; Takenaka, Tohru

doi:10.1021/la970969+

Cited by 11 publications

(9 citation statements)

References 24 publications

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“…The reason that the thickness calculated from the ATR-IR data is lower than that from the QCM data might be due to slight temperature difference between the environment-controlled chamber and the ATR-IR unit. The observed trend of the n-propanol film thickness as a function of partial pressure is consistent with the general characteristic of the alcohol adsorption isotherm observed for other systems [23][24][25]. However, it should be noted that the thickness data shown in figure 2 are measured for gold (in QCM) and Ge (in ATR-IR) substrates.…”

Section: Resultssupporting

confidence: 86%

Reduction of adhesion and friction of silicon oxide surface in the presence of n-propanol vapor in the gas phase

et al. 2005

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The equilibrium adsorption of gas phase alcohol molecules has been proposed as a new means of in-use anti-stiction and lubrication for MEMS devices. Adhesion and friction of silicon oxide surfaces as a function of n-propanol vapor pressure in the ambient gas were invesitigated using atomic force microscopy. As the vapor pressure increases, the adsorbed n-propanol layer thickness increases. The adhesion and friction significantly decrease with very little addition of n-propanol vapor.

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Section: Resultssupporting

confidence: 86%

Reduction of adhesion and friction of silicon oxide surface in the presence of n-propanol vapor in the gas phase

et al. 2005

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“…Similar conclusions about these dual adsorption pathways and the evolution of water from the metal oxide surfaces have been drawn by Suda et al, Farneth et al., − Busca et al, and others. ,, These studies and the present results clearly indicate that quantitative chemisorption measurements must either measure the methoxylated surface species directly by IR spectroscopy (CeO 2 , ZnO,MgO, MoO 3 , SiO 2 , and ZrO 2 51 ), or measure the water desorbed upon adsorption if gravimetric methods are used. − , However, the absence of Lewis-bound water after methanol chemisorption on virtually all catalysts tested in the present investigation suggests that higher temperature methanol chemisorption (110 °C) forces the water equilibrium toward the vapor phase and allows for the assumption of complete water loss (relevant to gravimetric methods).…”

Section: Discussionsupporting

confidence: 89%

“…Finally, a number of related studies have quantitatively measured the adsorption isotherms of methanol on metal oxides using IR, calorimetric, gravimetric, and volumetric methods, although not necessarily for the purpose of active site determinations. Generally, the isotherms were of either the Langmuir type (Al 2 O 3 , Cr 2 O 3 , MgO, β-MoO 3 , SiO 2 , TiO 2 , and ZnO), the Elovich/Temkin type (α-MoO 3 31-33 and ZnO 37 ), or BET type II (MgO and TiO 2 ). Other isotherm and saturation coverages are found elsewhere for CeO 2 , MoO 3 , , heteropoly-Mo, TiO 2 , , and ZrO 2 .…”

Section: Introductionmentioning

confidence: 99%

Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 1. Supported Metal Oxide Catalysts

2001

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Methanol oxidation over metal oxide catalysts is industrially important for the production of formaldehyde, but knowledge about the intrinsic catalysis taking place is often obscured by a lack of knowledge as to the number of active sites present on the catalyst surface. In the present study, the number of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methanol chemisorption and in-situ infrared titration techniques performed at an experimentally optimized temperature of 110 °C. It was found that a steric limitation of about 0.3 methoxylated surface species (e.g., strongly Lewis-bound CH3OHads and dissociatively adsorbed -OCH3,ads, which are the reactive surface intermediates in methanol oxidation) exists per active deposited metal oxide metal atom across all supported metal oxides. Hence, the use of methanol chemisorption for counting active surface sites is more realistic than other site-counting methods for the kinetic modeling of methanol oxidation, where during steady-state reaction the departure of the actual coverage of methoxylated surface intermediates from the maximum saturation surface coverage is of critical importance. Methanol oxidation turn-over frequencies (TOF ) methanol molecules converted per second per active surface site) calculated using these new methanol chemisorption surface site densities increased by a factor of ∼3 the TOFs estimated in previous studies using the total number of deposited metal oxide metal atoms. Nevertheless, the support effect observed previously (TOFs for MoO3 and V2O5 supported on oxides of Zr ∼ Ce > Ti > Al . Si) remains virtually unchanged as a general trend in the present study and correlates with the support cation electronegativity via bridging M-O-Support bonds. The methanol chemisorption technique may now be used with confidence to search for similar ligand effects in bulk metal oxides, where counting active sites has traditionally been very difficult (subject of part 2, Burcham, L.

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“…This dual-pathway methanol chemisorption mechanism was similarly found to occur in supported metal oxide catalysts (see part 1 7 of the present two-paper series 7 ), and has been investigated on other metal oxides by Suda et al, Farneth et al, − Busca et al, and others. ,, It was also shown in part 1 7 that the intact, surface Lewis-bound methanol species (species I) is generally stable to high temperatures (at least 100−200 °C) on Lewis-acidic surfaces even under vacuum. The adsorption temperature of 110 °C employed in part 1 7 and in the present study, which is well above the boiling point of methanol at and below 1 atm, ensures that methoxylated surface species I is, in fact, a strongly Lewis-bound surface moiety and is not due to weakly physisorbed and perhaps multilayered methanol present at room temperature.…”

Section: Discussionsupporting

confidence: 67%

Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 2. Bulk Metal Oxide Catalysts

2001

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Bulk metal oxide catalysts, especially bulk mixed-metal molybdates such as Fe2(MoO4)3, often exhibit high methanol oxidation activity and selectivity. However, the difficulties involved in determining active surface site densities on these catalysts have, in the past, generally prevented side-by-side comparisons of their intrinsic activities, or turn-over frequencies (TOFs). In the present study, high temperature (110 °C) methanol chemisorption and in-situ infrared spectroscopy have been employed to directly and quantitatively determine the number of active metal oxide surface sites available for methanol oxidation. The IR spectra indicate that methanol chemisorption on these catalysts produces both associatively adsorbed, intact Lewis-bound surface methanol species (CH3OHads, species I) on acidic sites, as well as dissociatively adsorbed surface methoxy species (−OCH3, species II) on less acidic or basic sites. In fact, the Lewis acidity of bulk mixed-metal molybdates relative to the methanol probe molecule was found to decrease as follows: Fe2(MoO4)3, NiMoO4 (species I predominates) > MnMoO4, CoMoO4, ZnMoO4, Al2(MoO4)3 > Ce(MoO4)2 > Bi2Mo3O12 > Zr(MoO4)2 (species II predominates). It also appears that Mo cations are the primary methanol chemisorption sites in many of the bulk mixed-metal molybdates, including commercially important Fe2(MoO4)3. By quantifying the surface concentrations of the adsorbed methoxylated reaction intermediates from the IR spectra, it was then possible to normalize the catalytic methanol oxidation activities for the calculation of TOFs. The methanol oxidation TOFs of bulk molybdates were shown to be relatively similar to those of model supported catalysts with the same co-cation (e.g., MoO3/NiO vs NiMoO4)possibly due to the formation of a “monolayer” of surface molybdenum oxide species on the surfaces of the bulk metal molybdates. In addition, the bulk mixed-metal molybdates were found to exhibit the same ligand effect as that discovered previously in supported metal oxide catalysts, in which the TOF generally decreases with increasing ligand cation electronegativity due to electronic variations in localized M−O−Ligand bonds.

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Interaction of Alcohols with Surface Hydroxyls on Chromium(III) Oxide

Cited by 11 publications

References 24 publications

Reduction of adhesion and friction of silicon oxide surface in the presence of n-propanol vapor in the gas phase

Reduction of adhesion and friction of silicon oxide surface in the presence of n-propanol vapor in the gas phase

Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 1. Supported Metal Oxide Catalysts

Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 2. Bulk Metal Oxide Catalysts

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