Formate intermediates

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

Onishi, H. and Iwasawa, Y. (1994) STM-imaging of formate intermediates adsorbed on a TiO2(110) surface. Chem. Phys. Lett., 226, 111-114. 

Scheme 5-6 Proposed dinuclear mechanism of ates 3 and 4), but one-at-a-time intermediates Pt-catalyzed acrylonitrile hydrophosphination at 5 and 6 were also proposed. Several mononu-low [P(CH2CH2CN)3. The main pathway illus- clear intermediates (see Scheme 5-5) have been trates pairwise P-C bond formation (intermedi- removed for clarity...

The water oxidation of CO to C02 via a formate intermediate is well documented, and realistic pathways for reaction (75) at a single metal site are available (437, 438), for example ... 

Scheme 17.5 Return pathways from the formate intermediate to the starting hydride.

Yoneda and coworkers—bicarbonate, carbonate, and formate intermediates. 

King and coworkers—formate intermediates and a kinetic approach over Cr and other Group VIb carbonyl catalysts. Like Pettit and coworkers,18 34 King et a/.25,3 J 42,43,54,59,138,155 also studied water-gas shift on mononuclear carbonyls of iron and, in a detailed investigation, indicated that Scheme 7 of Pettit et al,34 was... 

However, for Group Ylb transition metal complexes (M = Cr, Mo, and W) in basic solution, their studies led them to propose a different type of catalytic cycle involving the decomposition of formate intermediate.25,33,54 Below (Scheme 18a) is an example for Mo, but one can readily replace Mo with Cr or W. 

Edwards and Schrader—IR investigations support common formate intermediate in water-gas shift and methanol synthesis over Cu/ZnO. Edwards and Schrader,234 using careful reduction procedures (95%N2/5%H2), were able to obtain direct evidence by infrared spectroscopy of the formation of active OH groups on Cu/ZnO, that formed surface formates on the surface of the zinc phase (1576, 1381, 1366, 2970, and 2878 cm-1, respectively for OCO asymmetric, OCO symmetric, and C-H stretching bands) upon exposure to CO. In the presence of CO and H20, the formate intensity initially increased, followed by the production of C02, indicative of water-gas shift. A carbonyl band was also observed at 2093 cm-1. The authors... 

Campbell and coworkers287,297 also published work on methanol synthesis. In that case, they also found that the active site is metallic copper, but they suggest that the mechanism in that case does involve formate intermediates, and that the ratedetermining step is likely the hydrogenation of the formate species to methanol. 

The authors use some arguments to rule out the formate intermediate before proceeding to the discussion regarding the redox mechanism. The basis was not the activation energy barrier for OCOH formation, as it is quite low relative to other possible pathways. They rule out the OCOH as an intermediate on the basis of its proposed decomposition selectivity, which they indicate will always preferentially decompose back to CO and OH. And the isomerization to a true formate species (i.e., with a C-H bond) they indicate cannot occur by reacting OH on Cu(110) with CO. Yet intense formate bands have been observed by infrared spectroscopy upon... 

In 2007, Meunier et al460 quantified reactive exchange rates of formates observed by DRIFTS over Au/Ce(La)02 relative to the C02 product exchange and identified that not all formates at the working temperature (155 °C and 220 °C) could be implicated as the main reaction intermediates. Those found at close to the rim of the metal particles were found to be more reactive than those further away. The authors concluded that fast formate intermediates or another kind of interfacial adduct too low in concentration and not observable by DRIFTS, or a redox mechanism, were still open possibilities. Formate surface concentrations observable by DRIFTS were estimated by a calibration curve developed using Na-formate as a reference. 

Yamamoto, Ichikawa, and coworkers—mesoporous material and zeolite supported Au and Pt catalysts—formate intermediates observed with Au/zeolites. 

The water-gas-shift (WGS) reaction (HzO + CO -> H2 I C02) on MgO, ZnO, and Rh/CeOz is another example of a surface catalytic reaction that is assisted by gas-phase molecules. It is known that the WGS reaction proceeds via surface formate intermediate (HCOO-), which can be monitored by FT-IR. The behavior of the surface intermediates (HCOO-) (Cat-X in Figure 8.1a) is remarkably influenced by weakly coadsorbed water molecules (A in Figure 8. lb). The characteristic aspect of the WGS reactions on ZnO and Rh/Ce02 are as follows ... 

In the reverse WGS reaction, hydrogen promoted both decomposition paths of the formate to H2+C02 and H20+CO, and the decomposition selectivity did not change. Thus, the mechanism of hydrogen promotion is different from that of electron donors in the WGS reaction. COz not only blocks the adsorption sites of H2 but also suppresses the decomposition of the formate intermediate. The rate constant for the steady-state reaction is higher than that obtained from the formate decomposition in vacuum, but it is smaller than that for the formate decomposition under the ambient H2. As a result, the reverse WGS reaction proceeds with a balance of H2 promotion and C02 suppression. 

On the alloy surface the reaction proceeded both via the anhydride and formate intermediates (117). As the copper concentration was increased, the formate species dominated the reaction, until at 63% copper the CO/COj ratio was less than 0.1. This change was due to the decrease in the amount of anhydride formed with increasing copper and the corresponding increase in formate. Since only the anhydride decomposition produced CO, the relative amount of anhydride formed could be determined as a function of surface composition. This relationship is shown in Fig. 21 the anhydride concentration fell as the fourth power of the nickel concentration, suggesting the requirement of four nickel atoms for its stabilization. This value agreed with the earlier determination for the saturation density of anhydride intermediates on Ni(llO) (99). 

The CO 2 was produced via the formate intermediate, but the mechanism of its formation is not entirely clear. It was observed to form 138) from H2CO according to the reaction... 

Although other experimental evidence also exists for a formate to methoxy mechanism, Baiker argues that formate intermediates are not involved in the reduction of carbon oxides to methanol on Zr02-based catalysts.8,35,624 Rather, formates are intermediates in methanation (see Section 3.2.1). 

The possible role of nickel formate as an intermediate in the breakdown of formic acid on nickel has been extensively discussed (3, 232, 240b, 244) this is another catalytic reaction in which there is compensation behavior (Table III, R). While the observed obedience to Eq. (2) does not identify the reaction mechanism, it is probably significant that catalytic activity becomes apparent in the temperature range of onset of salt instability. Again it may be envisaged that the temperature dependence of effective concentration of nickel formate intermediate may vary with reaction conditions. 

Scheme 3 forms a catalytic cycle for the water-gas shift reaction (63) employing [Rh2(/i-CO)(CO)2(dpm)2] in the presence of acid as a catalyst (62). It should be reiterated that alternative cycles might be written which do not involve formate intermediates. For example, a possible mechanism for catalysis of the water-gas shift reaction involving the binuclear metal species, [Pt2H2( -HXdpm)2]+, is outlined below (Scheme 4) (64). We have critically discussed the role of formate versus carboxylic acid intermediates in homogeneous catalysis of the water-gas shift reaction by mononuclear metal catalysts elsewhere (34).

C02 directly coordinated to the Ru center to afford Ru(H)2(ri2-C02)(PMe3)3. The Ru-(q -formate) intermediate was then produced via C02 insertion, instead of a nucleophilic attack of the H ligand to C02 in the presence of water. As a result, the activation barrier was higher, the exothermicity lower, and a back reaction could occur. Hence, this is an example where the formation of a metal-C02 complex is not favorable to the further reaction. 

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