Formate species HCOO

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. ... 

Fein et al. [65] also investigated the TOFs of bulk metal oxides toward formic acid oxidation through the dissociative chemisorption of the HCOOH to surface formate species HCOO-M. The authors obtained similar structure-activity relationships as observed for methanol and isopropanol. 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

They focused their research on answering the question as to whether catalysis proceeds via formate anion as an intermediate, such that dissociation of a CO ligand is the first step in the mechanism Cr(CO)6 -> Cr(CO)5 + CO followed by nucleophilic attack by formate anion i.e., from CO + OH- -> HCOO-) to produce the formate species Cr(CO)5 + HCOO- > Cr(CO)5(OOCH) , according to King et al.5 in Scheme 18 or whether a metallocarboxylic acid forms upon... 

Chen et al. [35] studied the photocatalytic decomposition of formic acid over Pt/Ti02- The catalyst was prepared by impregnation of H2PtCl, on titania P25. The catalyst was illuminated by a 5 Hz, 70 mW, and 355 nm pulse laser. It is shown that the molecularly adsorbed formic acid is transformed to formate species. The HCOO species are oxidized to the corresponding radical. The latter decompose on electron holes to carbon dioxide HCOO + h —> CO2. + I/2H2. The addition of water vapor accelerates the reaction and promotes the formation of hydrogen. The formation of labile radicals could be a reason that photocatalytic degradation pathways forming formate species are effective for mineralization. 

On the other hand, the adsorption of formic acid at a gold electrode gives two bands, one around 1325 cm" and the other one around 1720 cm" The first one can be attributed, for the same reasons as above, to a formate adsorbed species (HCOO)ads, and the second one to adsorption of molecular formic acid, because 1720 cm is the wavenumber of the carbonyl stretching mode of formic acid. 

Figure 52.3. Adsorbed acetaldehyde (CHsCHOad), adsorbed acetate (CHsCOO ad), and adsorbed formate (HCOO ad) IR relative intensities during the first 20 min of photocatalytic oxidation for the Pt/Ti02 and Ti02 catalysts. IR intensities were obtained by measuring the peak height of bands corresponding to intermediate species directly from the difference spectra presented in Fignre 52.2.

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

The natural assumption made by a large number of researchers in the field of electrochemical C02 reduction was that the intermediate was C02, as postulated by Haynes and Sawyer (1967). The observation of oxalate as a major product in addition to, or in competition with, the formation of CO, CO, HCOj and HCOO , increased the attention focused on the reactive intermediate and the mechanisms by which it reacted. However, controversy has arisen over whether the subsequent reaction of the CO 2 was via dimerisation (the EC mechanism) or via attack on another C02 molecule (the ECE mechanism). In addition, the existence of such species as CO 2 (ads) and HCOO (ads) have also been suggested but, as we shall see, these are not now thought to play a major role on simple metals. 

Formic acid did not adsorb on clean Ag(llO) above 180 K. In order to obtain the adsorbed species, the surface was predosed with oxygen to produce adsorbed oxygen atoms. Surface species could then be stabilized by reacting formic acid with surface oxygen to produce water and the formate (102). Subsequent heating of the surface produced decomposition near 425 K. Only CO2 and H2 were observed as products from the HCOO intermediate. Additionally, some back reaction to reform HCOOH occurred between HCOO(a) and the H(a) liberated by the decomposition. The rate constant for the decomposition was... 

Figure 1.9. The local aligned-bridge adsorption sites of the formate (HCOO-) species on Cu(110) and Cu(100). Also shown is the cross-bridge site on Cu(100) originally proposed as a new type of surface bond but subsequently shown to be incorrect.

E5-2 When 13C-labelled formaldehyde, 13CH20, is fed to live cultures of bacteria in an NMR spectrometer, the metabolism of the label can be followed by 13C NMR. Many bacterial species produce roughly equal amounts of formate (HCOO ) and methanol (CH3OH). This is reminiscent of the purely chemical Cannizzaro reaction in which a hydride ion (H ) is transferred directly from one formaldehyde molecule to another. The accompanying 61 MHz deuterium NMR spectra are of methanol that results from the metabolism of deuterium-labelled formaldehyde, CD2O, by Escherichia coli and Pseudomonas putida. What do they tell us about possible Cannizzarase enzymes in those organisms ... 

A least squares plot of F](x) vs. formate concentration gave a value of 16.3 for the intercept K and a slope of 197 which is in good agreement with the value of 192 for K.2 derived from F2 (x) The slope of 2 00 is zero and thus the major species present in solution are Eu(HCOO)+, Eu(HCOO)2+ and Eu3+. 

On the other hand, the concentrations of the reactants, bridging formate R4 and weakly adsorbed formic add, in Path 2 were sufficient at the surface because no special sites are required for this path and one of the reactants, R4, is the most stable species. Furthermore, high HCOOH coverage by increasing HCOOH pressure increases the coadsorption state in such a manner that a weakly adsorbed HCOO H is located adjacent to a R4 species. Thus Path 2 via the transition state activated in a concerted manner by three Ti4+ ions (Figure 18.5) should be the most plausible dehydrogenation pathway under the reaction conditions [14]. 

Formation of CO2 anion radical and subsequent reduction to formate were studied by photoemission and polarization measurements with Hg electrodes as well as the capacitance and potential decay measurements at Sn and In electrodes all these measurements agreed that very low fraction of the electrode is covered by the adsorbed species. Schififrin investigated reduction of CO2 anion radical formed in a photoemission measurement with a Hg electrode, and showed that the potential of the reduction does not depend on pH. He thus concluded that H2O is the proton donor in the formate formation from CO2 ", as confirmed later by Hori and Suzuki. They demonstrated that the electrode potential is constant in the pH range 2 to 8 at a constant partial current density of HCOO formation 0.5 mA cm at a Hg electrode. 

Table 3 shows that CO formation takes place with relatively lower overpotentials than HCOO formation. Among all the electrodes, Au electrode reduces CO2 to CO at remarkably low cathodic potential, -1.14 V at 5 mA cnf Tliis fact strongly suggests that CO is produced by a mechanism different from HCOO formation which may proceed with free CO 2 intervening. Hori et al. suggested that intermediate species CO 2 " is fonned on the Au electrode and greatly stabilized by adsorption, leading to decrease of overpotential. 

Platinum electrodes do not give products continuously in CO2 reduction in aqueous media under 1 atm as shown in Table 3. Platinum electrodes initially reduce CO2 to reduced 002 . The entity of the reduced CO2 is CO strongly adsorbed on the Pt electrode, as revealed by Beden et al. by means of infrared spectroscopy. Tills fact is later confirmed by other workers. " In addition to linearly bonded CO as the major adsorbed species, small amounts of bridged and multibonded CO, COH and HCOO species are also detected on Pt electrode surface. The presence of reduced CO2 on Pt electrode practically inhibits further reduction of CO2 in aqueous media. The formation of reduced CO2 proceeds as below in the potential region in which adsorbed hydrogen is stably present. 

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