Catalyst methanol oxidation reaction mechanism

Fig. 15.12 Methanol oxidation on Pd-Al O (mean particle size 6 nm) and Pd(lll) [27, 50, 75]. (a) In situ steady-state PM-IRAS and SFG spectra, shown together with the CHjOH conversion as monitored by gas chromatography, (b) Comparison of Pd3d XP spectra obtained before upper) and after (lower) the methanol oxidation reaction indicated a partial oxidation of Pd nanoparticles during the reaction, whereas Pd(lll) remained metalhc adapted in part from [75] with permission. Copyright (2007) The PCCP Owner Societies, (c) PM-IRAS (p-s) surface vibrational spectra measured during CH OH decomposition on Pd(lll) at 300 K, with the various species indicated. The time-dependent evolution of CH O (as observed by PM-IRAS) and of CH (values deduced from XPS) upon methanol decomposition at -lO mbar suggests a correlation between the two species adapted in part from [27] with permission. Copyright (2005) American Chemical Society, (d) Suggested mechanism of CH OH decomposition and oxidation on Pd catalysts reprinted [50] with permission. Copyright (2007) Elsevier...

Bewick et al." identified CO as the species that acts as a catalytic poison and inhibits further oxidation of methanol on Pt electrodes. The reactive intermediate is a formate species, HCOO that generates asynunetric COO vibration around 1300 cm, leading to an increase in the methanol oxidation current after CO oxidation. "Recently, water molecules were detected adsorbed on the Ru sites on Ru and Pt-Ru (but not on Pt) catalysts, and were assigned as the oxy gen donor to the methanol adsorbates that promote methanol oxidation."" This was considered as directly supporting the bi-functional mechanism of Pt-Ru catalysts for the methanol-oxidation reaction. ... 

Very few electrode materials have been shown to be capable of adsorbing methanol in acidic media, and of these only Pt-based materials display a high enough sta-bihty and activity to be attractive as catalysts. The overall reaction mechanism for methanol oxidation is (Eq. 9-34) ... 

The bi-functional mechanism, although simple, can explain very well the promoted MOR activity of Pt-Ru alloy catalysts. This mechanism is also well adapted by other binary alloys such as Pt-Sn [48]. It has been identified that CO does not bind to the Sn sites, with the result that OH can more easily adsorb on the Sn sites without competition from CO. The synergetic effect on Pt and Sn sites gives rise to Pt-Sn, a very active CO electrooxidation catalyst. However, the strong adsorption of OH species on Sn sites, particularly at high potentials, makes the Pt-Sn catalyst inferior to the Pt-Ru catalyst for the methanol oxidation reaction. 

Pt-based alloys have been developed for many years to improve the catalytic activity of the oxygen reduction reaction and the methanol oxidation reaction, and reduce Pt loading in the catalyst layers of the PEM fuel cell cathode and anode. Great progress has been made in recent years in terms of alloy activity screening, alloy mechanism discovery, and alloy stability investigation. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

Currently, low-temperature CO oxidation over Au catalysts is practically important in connection with air quality control (CO removal from air) and the purification of hydrogen produced by steam reforming of methanol or hydrocarbons for polymer electrolyte fuel cells (CO removal from H2). Moreover, reaction mechanisms for CO oxidation have been studied most extensively and intensively throughout the history of catalysis research. Many reviews [4,19-28] and highlight articles [12, 29, 30] have been published on CO oxidation over catalysts. This chapter summarizes of the state of art of low temperature CO oxidation in air and in H2 over supported Au NPs. The objective is also to overview of mechanisms of CO oxidation catalyzed by Au. 

Ai85,86 is discussed on p. 114. Agarwal et al.102 as well as Sharma et al.103 studied this reaction using silica-supported V2Os-alkali metal sulphate catalysts. A two-step oxidation-reduction mechanism gave the best description of the process. The activity increased with increasing atomic number of the added alkali metal for which no interpretation was offered. In an electron microscopic study of these catalysts Sharma et al.103 showed that K2 S04 and V2 05 are present as separate phases but that the sulphate causes the presence of a larger amount of V2 05 in the form of needle-like crystals which appear to be more active for the methanol oxidation. A similar result was obtained by these authors for catalytic oxidation of toluene over these catalysts.104... 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

Turco M, et al. Production of hydrogen from oxidative steam reforming of methanol - II. Catalytic activity and reaction mechanism on Cu/ZnO/Al2C>3 hydrotalcite-derived catalysts. J Catal. 2004 228(l) 56-65. 

Bronkema and Bell (2007) analyzed the Raman bands of surface methoxy species and of supported vanadia to elucidate the mechanism of methanol oxidation to formaldehyde. In their detailed investigation, insight from Raman spectroscopy was combined with information from EXAFS and XANES spectroscopies. The authors discussed the reaction pathways in the presence and absence of 02, and identified the roles of various lattice oxygen sites. Formaldehyde was found to decompose to H2 and CO in the absence of 02 (Bronkema and Bell, 2007). Similar observations were reported by Korhonen et al. (2007) for methanol conversion on supported chromia catalysts. 

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