Infrared methane adsorbed

The spectra of methane, adsorbed at 90° K., showed a weak band at 2,899 cm.", in addition to a strong band (vt) at 3,006 cm. h This weak band was assigned to the I l symmetrical breathing frequency of methane, which is normally observed only in the bulk state in the Raman spectrum at 2,916 cm. h No over-all dipole change is associated with the vi vibration consequently, it is forbidden in the infrared spectra of liquid and gaseous methane. The appearance of this band is a direct measure of the... 

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

In situ infrared observations show that the primary species present during the reduction of NO by CH4 over Co-ZSM-5 are adsorbed NO 2 and CN. When O2 is present in the feed NO2 is formed by the homogeneous and catalyzed oxidation of NO. In the absence of O2, NO2 is presumed to be formed via the reaction 3 NO = NO2 + N2O. The CN species observed are produced via the reaction of methane with adsorbed NO2, and transient response studies suggest that CN species are precursors to N2 and CO2. A mechanism for the SCR of NO is proposed (see Figure 10). This mechanism explains the means by which NO2 is formed from adsorbed NO and the subsequent reaction sequence by which adsorbed NO2 reacts with CH4 and O2 to form CN species. N2 and CO or CO2 are believed to form via the reaction of CN with NO or NO2. CH3NO is presumed to be formed as a product of the reaction of CH4 with adsorbed NO2. The proposed mechanism explains the role of O2 in facilitating the reduction of NO by CH4 and the role of NO in facilitating the oxidation of CH4 by O2. 

The mechanism of C02 reduction to methane at Cu electrodes has been proposed by various groups [72-74], most of which involved the splitting of adsorbed CO followed by the hydrogenation of surface C atoms. When DeWulf et al. used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy to study the reaction [72], they observed surface-bound carbenes (Cu CH2) as an intermediate in the system. Likewise, others used both in situ infrared (IR) reflection absorption spectroscopy and surface-enhanced Raman spectroscopy to observe the initial product of C02 reduction on Cu [74]. Typically, two different linearly bound CO species were identified and attributed to adsorption on either surface defect sites or terraces. 

Extremely interesting infrared studies of physically adsorbed molecules were carried out by Sheppard and Yates (52). These workers studied the spectra of methane, ethylene, acetylene, and hydrogen on porous glass. They found that the perturbing effects of surface forces made it possible to detect bands which are found in the Raman spectra but are not observed in the normal infrared spectra. This indicates that the degree of symmetry of the adsorbed molecule is less than in the gaseous state because of the one-sided nature of the surface forces. This effect was discovered independently by Karagounis and Peter (52a) in studies 1,3,5-trichlorobenzene physically absorbed on silica. 

The infrared spectrum of physically adsorbed methane had a band at 2899 cm.-1 which is not present in the equivalent path-length of either the gas or the liquid. This corresponds to the symmetrical C—H stretching vibration which produces a Raman band at 2916 cm.-1. The spectrum of adsorbed ethylene shows an extra weak shoulder at 3010 cm.-1 which was assumed to be the normally infrared inactive v vibration in which all four hydrogens are vibrating in phase and which produces a Raman band at 3019 cm.-1. 

Sheppard and Yates (62) also studied the degree of rotation in physically adsorbed methane. The shape of infrared stretching bands is modified by rotation of the molecule so they calculated the band shapes expected for (1) no free rotation, (2) one degree of rotational freedom—rotation about an axis perpendicular to the surface—and (3) three degrees of rotational freedom. Comparison of the shapes of the calculated and observed bands... 

V vs. RHE in agreement with previous studies by DBMS, infrared spectroscopy and gas chromatography. Moreover, Iwasita and Pastor have detected some traces of methane at low potentials. From prolonged electrolysis experiments and in situ infrared reflectance spectroscopy, the main reaction products and intermediates involved in ethanol electro-oxidation on a platinum catalyst have been determined CO and CH3CO species as adsorbed intermediates, CH3CHO as intermediate and final product, CH3COOH, CO2 and CH4 as final products. 

The infrared bands of adsorbed CO may be related to the activity in the electrochemical reduction. Ni and Fe electrodes reduce CO to methane, ethylene and ethane electrochemically with the current efficiency 3 % at a constant current density 2.5 mA / cm (the electrode potentials are around -1.5 V) in 0.1 M KHC03.[8,11] The Cu electrode effectively reduces CO to hydrocarbons with the current efficiency 50 % at 2.5 mA / cm (the potential is -1.4 V).[4,5,11] Thus the activity order in CO reduction is Cu > Ni Fe. The linear CO is more easily reduced than bridged one on the Ni electrode. The order of the electrochemical activity of metals in CO reduction roughly agrees with the reverse order of the adsorption strength of CO. 

In their now classic study of the effect of surface forces on adsorbed molecules, Sheppard and Yates (26) found that some of the Raman-active vibrations of methane, ethylene, and hydrogen appeared in the infrared when these materials were adsorbed on silica. The frequency shifts for the molecule on going from the gas phase to the adsorbed phase were all rather small, indicating that no chemical change in the species was brought about by the adsorption. 

Figure 3. Infrared spectra show the formation of gas phase products when the catalysts were exposed to methane at pressure of 30 bar and temperature of 703 K for 2 h (a) Fe203, (b) Mo/Fe = 1.0, (c) Mo/Fe = 1.7, (d) Mo/Fe = 5.0, and (e) M0O3. The spectra of the catalysts at 0 min (when methane first added) have been subtracted. 1300 cm [24-27], and (4) an adsorbed formaldehyde identified by a small peak at 1608 cm" [25, 27], and a shoulder at 2782 cm l [28], In addition to the peaks mentioned above, there exists some other features in the IR spectra around 20(X)-1700 cm 1 region that have not been assigned. These bands may result from the reduction of the catalyst surface during reaction.

The results from the infrared studies and from the GC analysis show that the reaction of methane with the ferric molybdate catalysts gives methanol, formaJdehyde, carbon dioxide, and carbon monoxide as final products. The IR spectra also indicate the formation of methoxy, surface dioxymethylene, surface formate species, and adsorbed formaldehyde. Based on these observations, a mechanism was proposed to account for all intermediates and final products and is shown in Figure 5. Since the surface structure of the catalysts is not known, the surface is represented by a straight line in the scheme. 

Fig. 40. Infrared spectra of methane, CH4, adsorbed on Na-A the spectra obtained at 10 and 10 mbar (adopted from [599])...

Adsorbed carbon monoxide can serve as a useful infrared probe of surface composition in bimetallic colloids if both metals bind CO. This is exemplified in the infrared spectrum of CO on a PVP stabilized colloidal alloy CutgPdjT, [38] Carbon monoxide adsorbs readily onto these PdCu particles (co. 45 A) in dichloro-methane at 25 °C, as shown by the infrared absorption spectrum in Figure 6-28. By comparing this to the IR spectrum of CO on a pure palladium colloid of similar size [34] in Figure 6-27d, it can be clearly seen that CO occupies both palladium and copper sites. Whereas the bands at 2046 cm" and 1936 cm" are in the frequency ranges found for linear and bridged CO on the pure palladium particles, the new band at 2089 cm corresponds to CO on surface copper atoms, thus demonstrating that both metals are present at the surfaces of the particles. 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

Methane is one of the best-understood adsorbates in zeolites. By the concomitant application of infrared spectroscopy, inelastic neutron scattering and theoretical studies the interaction of this molecule and its dynamic behaviour have been carefully investigated by Cohen de Lara et al. [114-122]. Chapter 4 deals in detail with neutron scattering studies on this molecule. 

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. 

---