Infrared methanol oxidation

Zhu Y, Uchida H, Yajima T, Watanabe M. 2001. Attenuated total reflection-Fourier transform infrared study of methanol oxidation on sputtered Pt film electrode. Langmuir 17 146-154. 

Secondly, compared to methanol oxidation, the rates of reaction decrease significantly upon oxidation of HMF for the Pt and Pd catalysts. In the case of Pt this decrease is drastic under the mild conditions applied here, indicating a strong interaction of HMF with the Pt surface. In the case of Pd the decrease is less pronounced and for Pd/Al-N even an increase in rate of reaction is found (entries 3 and 15). Probably the interaction of HMF with Pd is less strong, especially in the case of highly dispersed Pd. In order to study the interaction of HMF with noble metals, DRIFT (diffuse reflectance infrared fourier transform) measurements are in progress. 

However, after 1980 the development of in situ infrared spectroscopic techniques have allowed the direct identification of adsorbed intermediates. In terms of methanol oxidation on R, electrochemically modulated infrared reflectance spectroscopy (EMIRS) led to the unambiguous identification of adsorbed CO as the poisoning species. Two adsorbed CO species were identified as being responsible for the poisoning phenomena (i) a linearly bonded species (IR absorption band around 2060 cm ) and (ii) a bridge-bonded species (a small band around 1850-1900 cm ). These results were... 

The use of an infrared microscope enables the investigation of the surface of rather small electrodes. The resulting miniaturization of the necessary electrochemical cell allows its operation as a fiow cell in thin layer arrangement [242]. Combined with a rapid-scan FTIR spectrometer, acquisition of infrared spectra during electrode potential scans at a rate of d /dr = 200 mV-s are possible. The time resolution is equivalent to one complete spectrum recorded every 2.6 mV. The formation of various reaction intermediates of methanol oxidation in alkaline solution at a platinum electrode could be assigned to specific electrode potential ranges. 

The controlled deposition of ruthenium on well-defined surfaces, such as Pt(hkl) [95-103] and Au hkl) [38-40], has been characterized by electrochemical measurements, Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), XPS and STM measurements. The interest in these studies is mainly concentrated on the ruthenium modification of a platinum surface because of its extreme importance in electrocatalysis. It has been demonstrated that a ruthenium-deposited Pt( 111) substrate showed an extremely high activity in methanol oxidation compared to ruthenium-deposited Pt(hkl) electrodes with other crystallographic orientations [98, 99]. 

Liao LW, Liu SX, Tao Q et al (2011) A method for kinetic study of methanol oxidation at Pt electrodes by electrochemical in situ infrared spectroscopy. J Electroanal Chem 650 233-240... 

Several reaction schemes have been proposed to explain tee formation of all byproducts during methanol oxidation over Mo-Fe catalysts. Edwards et al [5] and Machiels [6] have suggested reactional mechanisms for tee formation of formaldehyde and by-products. However intermediate speeies proposed in such mechanisms have not been identift by any spectroscopic or other techniques. More recently Busca [7] on basis of infrared studies of surface intermediates species has proposed a rake-type mechanism for methanol oxidation over oxide catalysts. This mechanism account for the formation of formaldehyde and byproducts. 

Since the oxidation of methanol to CO includes six electrons, the reaction process must involve several steps with several products or intermediates. The results from mass spectral measurements, high performance liquid chromatography (HPLC), and gas chromatography (GC) absorbance have showed that H O, HCOOH, HCOOCH, and CO were all produced during the oxidation of methanol on Pt in acid solutions. These species were formed initially but eventually became CO [92], In addition, some other adsorbed species such as (CHO)ads or (COOH)ads were identified by infrared reflectance spectroscopy or Fourier transform infrared reflectance spectroscopy. The detailed reaction mechanism of methanol oxidation on a Pt electrode is shown in Figure 1.20 [94]. 

Pt-Ru alloy [158-160] or Ru modified Pt surfaces [161] are known to be prominently effective catalysts for methanol oxidation. Watanabe and Motoo [87] introduced a bifunctional mechanism into electrochemical interfaces in that Ru in Pt-Ru alloy surfaces supplies OH moieties to promote CO oxidation on neighboring Pt surfaces. In this context, it is crucial to elucidate water or hydroxide that are anticipated to adsorb on Ru in the alloy or modified Pt electrode surfaces. Nevertheless, there have been substantial uncertainties on this point because of experimental constraints in vibrational spectroscopy relevant for this purpose, e.g., poor sensitivity or uncompensated interruption by bulk solution species in IRAS (infrared reflection absorption spectroscopy). Watanabe and co-workers recently reported that water molecules were detected on the Pt-Ru alloy surfaces, a finding... 

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. 

Methanol oxidation was investigated recently by IRAS (Infrared Reflection Absorption Spectroscopy) and TPD (Temperature Programmed Desorption)... 

Burcham, L.J. The Origin of the Ligand Effect in Supported and Bulk Metal Oxide Catalysts In Situ Infrared, Raman, and Kinetic Studies During Methanol Oxidation. Ph.D. thesis, Lehigh University, Bethlehem, PA, 1999. 

Quantification of active sites for the determination of methanol oxidation turnover frequencies using methanol chemisorption and in situ infrared techniques. 2. BaUc metal oxide 43 catalysts. Lawgmajr 2001,17, 6175-6184. 

Burcham, L.J. and Wachs, I.E. The origin of the support effect in supported metal metal oxides catalysts in situ infrared and kinetic studies during methanol oxidation. Catal. Today 1999, 49, 467—484. 

The mechanism of methanol oxidation on Pt-based catalysts has been studied for several decades [1-14]. Complex parallel and series reaction pathways in which several adsorbed species and soluble intermediates were involved in methanol oxidation were proposed by Bagotzky et al. [2]. The in situ application of infrared spectroscopy during methanol oxidation showed that adsorbed CO is formed on the Pt surface [15]. However, other adsorbed intermediates are still not identified. Formaldehyde, formic acid, methyl formate, and dimethoxy methane have been identified as soluble intermediates [8, 10, 16-18]. The quantitative analysis of methanol oxidation products changing with various parameters can help us better understand the mechanism of methanol oxidation and identify reactirai pathways. This can be achieved by online quantitative differential electrochemical mass spectrometry (OEMS), which will be discussed in Sect. 3. 

These conclusions from the infrared reflectance spectra recorded with Pt and Pt-Ru bulk alloys were confirmed in electrocatalysis studies on small bimetallic particles dispersed on high surface area carbon powders.Concerning the structure of bimetallic Pt-Ru particles, in situ Extended X-Ray Absorption Fine Structure (EXAFS>XANES experiments showed that the particle is a true alloy. For practical application, it is very important to determine the optimum composition of the R-Ru alloys. Even if there are still some discrepancies, several recent studies have concluded that an optimum composition about 15 to 20 at.% in ruthenium gives the best results for the oxidation of methanol. This composition is different from that for the oxidation of dissolved CO (about 50 at.% Ru), confirming a different spatial distribution of the adsorbed species. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Electrochemical infrared spectroscopy can be used on all kinds of electrodes and for all substances that are IR active. It is particularly useful for the identification of reaction intermediates, and has been used extensively for the elucidation of the mechanisms of technologically important reactions. A case in point is the oxidation of methanol on platinum, where linearly bonded = C = O (i.e., CO bonded to one Pt atom) has been identified as an intermediate Figs. 15.7 and 15.8 show EMIRS [6c] and IRRAS [8] spectra of this species. Near 2070 cm-1 the EMIRS spectrum shows the typical form produced by a peak that shifts with potential. This shift can be followed in the IRRAS spectrum... 

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