Methanol chemisorption

Tadjeddine and co-workers have used SFG [Guyot-Sionnest and Tadjeddine, 1990 Eisenthal, 1992 Richmond, 2002 Vidal et al., 2002, 2004, 2005] to study the adsorbed CO produced from a variety of solution species, including methanol [Vidal et al., 2002, 2005]. With BB-SFG, we studied the electrochemical kinetics of methanol chemisorption as surface CO, as shown in Fig. 12.13. We used apolycrystal-line Pt electrode and 0.1 M H2SO4 electrolyte with 0.1 M methanol. Figure 12.13a-d characterize the potential-dependent SFG spectra obtained under the voltammetric... 

Entina VS, Petrii OA, Rysikova VT. 1967. On the nature of products of methanol chemisorption on Pt + Ru electrode surface. Elektrokhimiya 3 758-761. 

Bagotzky et al. [5] have also reported some kinetic studies for methanol chemisorption on polycrsytalline platinum. Given that the current transient... 

Both these concerns were addressed by the development of modified IR techniques. In the technique of Subtractively Normalised Fourier Transform IR Spectroscopy (SNIFTIRS) or Potential Difference IR (SPAIRS or PDIR) [37], the increased stability and sensitivity of Fourier Transform IR is exploited, allowing usable spectra to be obtained by simple subtraction and ratioing of spectra obtained at two potentials without the need for potential modulation or repeated stepping. A second technique which does not call for potential modulation, but actually modulates the polarisation direction of the incoming IR beam is termed Photo-elastically Modulated Infra-Red Reflectance Absorption Spectroscopy (PM-IRRAS) this was applied to the methanol chemisorption problem by Russell and co-workers [44], and Beden s assignments verified, including the potential-induced shift model for COads. 

Aity quantitative inteqjretation is still difficult to make, since the mass variations result from several coupled processes replacement of adsorbed hydrogen, water molecules and supporting anions by strongly adsorbed species from methanol chemisorption, reorganization of the double layer, formation of oxygenated species at the platinum surface, etc. 

In most of the catalysts composed of copper zinc and copper-pyrochlore a correlation between the copper surface area, the amount of formates located on the copper sites after CO2 or methanol chemisorption, and the catalytic activity can be found, but in most cases the relation is not strictly linear. The promoting effect of ZnO on Cu-LaZr catalysts cannot be ascribed to an enhancement of copper coverage or formate formation. Zinc plays therefore rather a positive role in the formate hydrogenation than its formation. 

Conclusions about a rate limiting stage of methanol chemisorption on the micropo-rous Xerogel were performed by Mertens and Fripiat [62] from the comparison of a self-diffusion coefficient of physically adsorbed methanol (D = 1.5 10 cm s at 25°C or 1.5 10 cm - s at 150°C) with a range of its reaction rate constants on the... 

Surface-science studies using copper single-crystal surfaces of (110) and (310) orientation onto which ZnO islands had been deposited indicate that CO and CO2 chemisorption can be used to identify the metal and the oxide sites, respectively. Methanol chemisorption produces both formate and methoxy species. The concentration of formate is enhanced by the presence of ZnO-copper interfaces, implicating these species as a reaction intermediate. 

Further reported examples include electrocatalytic processes and their intermediates [313, 314]. Formate could be identified as an active intermediate of methanol electrooxidation at a polycrystalline platinum electrode [315]. Water molecules coadsorbed during methanol adsorption on platinum were identified as those species that react subsequently with COad that was formed as a result of methanol chemisorption [316]. The high sensitivity of SEIRAS allows mapping of two-dimensional spectra (for selected examples, see [285]). Finally, two-dimensional correlation analysis of electrochemical reactions becomes possible [317]. 

Another consequence of the changes in Pt electronic structure caused by the presence of Ru (i.e., creation of electron deficiency on Pt by decreasing the Fermi level density of states and reducing the Pt-Pt distance) is the increased rate of dissociative methanol adsorption [92]. Thus, in addition to H2O activation (according to the bifunctional mechanism [93]) Ru plays a significant role with respect also to methanol chemisorption and surface diffusion of COad. 

Burcham, L., Briand, L. and Wachs, I. (2001). Quantification of Active Sites for the Determination of Methanol Oxidation Turnover Frequencies Using Methanol Chemisorption and In Situ Infrared Techniques. 2. Bulk Metal Oxide Catalysts, Langmuir, 17, pp. 6175-6184. 

Investigation of the Nature and Number of Surface Active Sites of Supported and Bulk Metal Oxide Catalysts through Methanol Chemisorption... 

The spectroscopic investigations performed by Sleight and coworkers [14-17] demonstrated that methanol chemisorption on M0O3 at room temperature results in a combination of molecular and dissociative adsorption mechanisms. The first mechanism can be considered as a physical adsorption since the methanol molecules adsorb intact on the surface. Dissociative adsorption is a chemisorption process that involves the formation of metal-methoxy (M-OCH3) groups. Further,... 

Surface methoxy groups are the intermediate reaction species in the production of partially oxygenated reaction products (formaldehyde, methyl formate, methylal, etc.) during methanol selective oxidation on catalysts containing transition metal oxides (oxides of vanadium, molybdenum, chromium, etc.). Therefore, the knowledge of the amount of surface methoxy species formed during methanol chemisorption is the key for the determination of the number of surface active sites... 

Methanol chemisorption experiments have also been carried out at atmospheric pressure in a specially adapted thermal gravimetric analyzer (TGA) microbalance coupled with a PC for temperature and weight monitoring [38]. The system allowed a controlled flow of high purity gases air for pretreatment, helium and a mixture of 2000 ppm methanol in helium for adsorption experiments. 

Section 11.4.1 to Section 11.4.4 discuss the stoichiometry of adsorption, kinetic parameters, reactivity, and surface morphology of several catalytic materials that have been obtained through the application of methanol chemisorption to determine Ns and TOP values. 

Monolayer supported metal oxide catalysts possess a two-dimensional overlayer of an active metal oxide that is molecularly dispersed over a high surface area support. Usually, all the metal atoms deposited onto the oxide support are considered as the number of surface active sites. However, the application of methanol chemisorption as surface intermediate methoxy species on monolayer supported molybdenum. 

Surface Molybdenum Oxide Structures and Methanol Chemisorption Data... 

The number and nature of the active surface sites and the catalytic activity of bulk metal molybdates and vanadates were also investigated through methanol chemisorption [51-53]. These materials proved to be equally or more active and stable than the industrial catalyst Mo03/Fe2(Mo04)3 in formaldehyde production [54-56],... 

The previous sections discussed the applications of methanol chemisorption as a tool to describe the surfaces of oxide catalysts. Other molecules, such as isopropanol and formic acid, have also been used as chemical probes to measure the number of surface active sites of metal oxide catalysts but to a lesser extent. The ability of isopropanol to distinguish between surface redox and acid sites and the observation that it adsorbs as a stable monolayer of surface isopropoxy species over oxide materials, also makes this molecule a suitable surface chemical probe. 

The production of CO during alcohols desorption experiments is attributed to the readsorption and further oxidation of primary products (e.g., the readsorption and further oxidation of formaldehyde to CO during methanol chemisorption and TPSR over oxide catalysts). Similarly, the production of CO could be attributed to the oxidation of propylene that is unable to desorb out of the surface due to the presence of an aqueous layer. 

Briand, L.E., Earneth, W.E., and Wachs, I.E. Quantitative determination of the number of active sites and turnover frequencies for methanol oxidation over metal oxide catalysts. I. Fundamentals of the methanol chemisorption technique and application to monolayer supported molybdenum oxide catalysts. Catal Today 2000, 62, 219-229. 

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