Formate, adsorbed, methanol synthesis

This is illustrated by the TPD spectra of formate adsorbed on Cu(lOO). To prove that formate is a reaction intermediate in the synthesis of methanol from CO2 and H2, a Cu(lOO) surface was subjected to methanol synthesis conditions and the TPD spectra recorded (lower traces of Fig. 7.13). For comparison, the upper traces represent the decomposition of formate obtained by dosing formic acid on the surface. As both CO2 and H2 desorb at significantly lower temperatures than those of the peaks in Fig. 7.13, the measurements represent decomposition-limited desorptions. Hence, the fact that both decomposition profiles are identical is strong evidence that formate is present under methanol synthesis conditions. 

In the methanol synthesis, a fast hydrogenation of the adsorbed formaldehyde would overcome arguments invoking any thermodynamic limitation to its formation. Formaldehyde adsorbed through both the oxygen and the carbon ends has been characterized in homogeneous catalysis (43), on oxide surfaces (44) and more recently on ruthenium metal (45). 

Coadsorption of reactants and subsequent thermal decomposition of the surface complexes formed have been used to resolve the mechanisms in several studies (81-84). Mutual enhancement of the adsorbed amounts of the reactants is indicative of their interaction, and if the adsorption of separately admitted components is negligible, the stoichiometry of the adsorbed complex can be determined. Further evidence for the formation of an adsorbed complex, employed in a mechanistic study of methanol synthesis over ZnO (84), is obtained by thermal decomposition of the adsorbed complex if the reactants appear simultaneously at one temperature upon thermal desorption from a coadsorbed layer, but if each reactant adsorbed separately gives a thermal desorption peak at a different temperature, the existence, although not necessarily the structure or com-... 

The activity of supported Pt catalysts for methanol synthesis from C0-H2 is considerably enhanced when the metal is supported on oxides which exhibit themselves appreciable activity for MeOH synthesis. Furthermore it is found that the rate of methanol formation on Pt-supported catalyst is increased when Th02, Ce02 were mechanically mixed with the Pt catalyst. Such behaviour is typical for bifunctional catalysts. It has already been reported that Th02, Ce02 adsorb carbon monoxide without dissociation. Such activated CO can be hydrogenated to form a formyl species, the formyl species interacting with lattice oxygen will produce a formate intermediate. 

The prediction obtained by including the non-activated formation of a surface carbonate species having a heat of formation of 22 k cal mol" is that, at steady state, the surface will be totally carbonated and that no reaction will proceed. This is an important conclusion that the carbonate species having a heat of formation of 22 k cal mol" will adsorb totally on the surface with the effect of poisoning its activity. It cannot act merely as a spectator. The carbonate species once formed must therefore be removed by further reaction, ie by hydrogenation to methanol. It is a corollary of this work then that methanol synthesis must always accompany the forward and... 

A known theory states that the active sites of the Cu-ZnO catalysts for methanol synthesis are metallic Cu which is not dependent on any synergy effect with metal oxides or supports. According to their reports, at the first step of the reaction scheme, the COj component adsorbs on the partially oxidized Cu as a symmetric carbonate, and then this is hydrogenated to a formate species. Also it is claimed that the role of CO is to keep the Cu in a highly reduced state therefore, the oxygen coverage of the Cu is a function of the CO2/CO ratio. In this thesis, the electronic nature of the active site during the reaction seems to be a little indefinite. 

Thus the question of what is the actual carbon source in methanol synthesis can not be unambiguously answered. Two mechanisms have been suggested to explain the formation of methanol on the heterogeneous catalyst. In the first mechanism (Eq. 8-5), adsorbed CO reacts on active copper centers of the surface with dissocia-tively adsorbed hydrogen in a series of hydrogenation steps to give methanol. 

Besides the purely electronic effects that we have discussed up to now, the promoter can also form direct chemical bonds with the adsorbate. An example is the influence of alkali metal cations on the synthesis of methanol with copper catalysts. Sodium and potassium hydroxide can react with CO under relatively mild conditions to form alkali metal formates, which are hydrogenated to methanol by hydrogen dis-sociatively adsorbed on copper. 

Bond Order Conservation-Morse Potential (BOC-MP) formalism is used to study the change in activation energy for different elementary steps involving all possible chemical species on any metallic smrface in order to identify optimum catalysts for the production of methane, methanol or ethylene from synthesis gas. All possible steps are analyzed for the methanation reaction. It is assumed that the reaction occurs on pure metallic surfaces, thus, the influence of the support or promoters is not taken into account. The method suggests that Ni is a more selective for methanation reaction, in agreement with the fact that Ni/SiOz is the real catalyst used industrially. On the other hand on the surface of Fe or W the reaction may residt in the formation of adsorbed C, also in agreement with experiment. 

Similar IR experiments were performed to establish the reaction mechanism for methanol oxidation on unsupported and sihca-supported vanadia, which are more selective catalysts for formaldehyde synthesis than vanadia-titania. The formation of methoxy groups from methanol dissociative or condensative adsorption was determined while it was established that formaldehyde (directly adsorbed or produced by methoxy group oxidation) mainly adsorbs in the form of dioxymethylene species, stable only at relatively low temperatures. It was concluded that dioxymethylene can react with methanol at low conversion to give rise to dimethoxymethane while it preferentially desorbs as formaldehyde at higher conversions and temperatures. The weakness of the adsorption of formaldehyde was considered to be the key feature of catalysts allowing high selectivity in formaldehyde synthesis. 

Bielahski et al. [34,35] performed a detailed study of methanol adsorption-desorption on dehydrated H4SiWi2O40 through IR spectroscopy and thermogravimetric analysis in order to obtain evidences on the mechanism of methyl-tert-butyl ether (MTBE) synthesis. The authors demonstrated that up to 16 molecules of methanol are adsorbed per Keggin unit at 18°C. The formation of dimethyl ether and water were observed upon heating the catalyst up to 100°C although methanol is not completely desorbed out of the structure even at 250°C. 

Formate (HCOO) species adsorbed on a copper-based catalyst are a key intermediate in the industrial synthesis of methanol from CO2 and H2. INS spectra were recorded for a number of different copper surfaces spectra were taken before and after formic... 

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