Adsorption of formic acid

Smith PE, Ben-Dor KF, Abruna HD. 2000. Poison formation upon the dissociative adsorption of formic acid on bismuth-modified stepped platinum electrodes. Langmuir 16 787-794. 

In this section, we will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

Similar ideas can be applied to formaldehyde oxidation. For bulk formaldehyde oxidation, we found predominant formic acid formation under current reaction conditions rather than CO2 formation. Hence, it cannot be ruled out, and may even be realistic, that formaldehyde is first oxidized to formic acid, which can subsequently be oxidized to CO2. The steady-state product distribution at 0.6 V is much more favorable for such a mechanism as in the case of methanol oxidation. On the other hand, because of the high efficiency of COad formation from formaldehyde, this process is likely to proceed directly from formaldehyde adsorption rather than via formation and re-adsorption of formic acid. Alternatively, the second oxygen can be introduced via formaldehyde hydration to methylene glycol, which could be further oxidized to formic acid and finally to CO2 (see the next paragraph). 

Hartnig C, Grimminger J, Spohr E. 2007a. Adsorption of formic acid on Pt(lll) in the presence of water. J Electroanal Chem 607 133-139. 

Pastor E, Castro CM, Rodriguez JL, Gonzalez S. 1996. On-line mass spectrometric studies on the interaction between organic adlayers on platinum. Part 1. Consecutive adsorption of formic acid and propargyl alcohol. J Electroanal Chem 404 77-88. 

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

Galvanostatic Transient Technique. Breiter (4) measured the adsorption of formic acid (HCOOH) on platinum in the solution of perchloric acid (HC104) using... 

Figure 2 Adsorption of formic acid on ZnO(lOlO) in the monodentate B (left), and bridge (right) modes.

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

The TiOiCllO) surface facilitates both molecular and dissociative adsorption of formic acid. The dissociative adsorption of formic acid (to form surface formates) is mediated by surface oxygen anions low-energy electron diffraction studies have shown that formate is ordered into (2x1) domains with formates bridging the surface titanium cations. This ordered layer is disrupted on heating formate may recombine with surface hydroxyl groups to desorb formic acid (reverse of reaction 2) or it may decompose to form the dehydration products CO and H2O, as well as small amounts of CO2 and H2 [43]. 

It has to be said that carbon monoxide species can be formed from a dissociative adsorption of formic acid, formaldehyde, methanol, ethylene glycol, etc. and are species that are formed as those of the first type of distribution. This suggests that the surface structure is an open structure, since dissociative adsorption of the organic molecule requires adjacent free platinum sites and that at the electrochemical-environment interface, once carbon monoxide is formed, there is almost no mobility at all. 

This idea seems to be a good starting point for the description of the kinetics of the dehydrogenation on most metals. Assuming an irreversible adsorption of formic acid on the metal surface and an irreversible decomposition of the adsorbed intermediate, we arrive at the following reaction scheme ... 

Rienacker and Hansen (3a), and also Suhrmann and Wedler (47, 48), have tried to gather information on the mechanism of the decomposition of HCOOH on evaporated nickel films by measuring the change in electrical resistance during reaction, and also the changes occurring upon adsorption of formic acid and of possible reaction products. 

The first infrared evidence of the possible occurrence of formate ions as reaction intermediates on the surface of metals resulted from the research of Hirota and his colleagues (46, 56), who showed formate ions to be present on powders of silver, copper, nickel, palladium, rhodium, platinum, and zinc, after adsorption of formic acid at room temperature. Moreover, in the far-infrared region they observed bands at 410 and 130 cm-1, which is an indication of bonding between metal atoms and formate ions via oxygen atoms [Hirota and Nakai (57)]. 

The Adsorption of Formic Acid on Nickel at Higher Temperatures... 

In our laboratory also Scholten applied the infrared technique to A1203, preheated to 450°C, using a cell which could be heated to induce reaction, but remained at room temperature when the absorption spectrum was being measured. After addition of formic acid at 200°C up to a coverage of two per cent, he observed the symmetric and antisymmetric C-0 stretching-vibration band and the C-H band of the formate ion (Fig. 26). Furthermore it was observed that no shift in the position of the stretching-vibration band of the OH groups occurred upon adsorption of formic acid. 

In accordance with these investigations, Fahrenfort and Hazebroek (58) found that no specific absorption band appeared after adsorption of formic acid on silica. 

Energy of activation values calculated from rate data between 75 and 150°C ranged from 22.7 to 24.4 kcal/mole with the minimum at 90°C. This agrees well with the value of 22.8 kcal/mole for formic acid decomposition on nickel powder between 125 and 150°C. The interpretation of the infrared data was further supported by calorimetric measurements. Thus, the heat of adsorption of formic acid at monolayer coverage was 18 kcal/mole which compares favorably with the heat of formation for mole of Ni (OOCH)2, 13 kcal. 

Apart from the decomposition products, formic acid itself can alter the resistance. As Schwab 4) found, the activation energy of the decomposition of formic acid with Hume-Rothery alloys increases if the Brillouin zone is filled up with electrons by changing the composition of the alloys. He concluded that on adsorption of formic acid, that is to say, in its activation, electrons pass over to the catalyst. This electron transfer would make the resistance decrease if formic acid does not decompose. 

According to former investigations (2), aU precautions of modern vacuum technique were considered as far as the evaporation of the film, the preparation, and influence of formic acid vapor are concerned. Figure 5 shows the behavior of the resistance of the film cooled down to 90° K (S) on adsorption of formic acid. At a pressure of formic acid of only 10 mm. Hg, R spontaneously decreases about 1.0% and at 10 mm Hg it has decreased by... 

Fig. 5. Change of resistance of a transparent nickel film with the adsorption of formic acid vapor. The film is cooled down to —183°.

Fig. 6. Change of resistance of a transparent nickel film with the adsorption of formic acid vapor at room temperature. C = trap near the cell cooled down to —183°. W = trap near the cell rewarmed to room temperature. P = gas formed by the decomposition is pumped off.

The pressure increase-time curves have a shape which can be interpreted on the basis of a Langmuir-Hinshelwood mechanism as resulting from a reaction which is retarded by adsorption of the products. If the reaction is unimolecular on the surface, and strong adsorption of formic acid is assumed, an integrated rate equation is obtained of the form... 

If the reaction is bimolecular on the surface, and strong adsorption of formic acid is again assumed, the integrated equation... 

Measurements in the formic acid system were reported by Lawrence and Parsons. The interfacial tension in formic acid (39.90 fiH cm at the electrocapillary maximum) is the highest of any non-aqueous solvent so far investigated. The work of adhesion is correspondingly low in contrast to the high value in formamide (Table 7.1.1). Preferential adsorption of formic acid from aqueous solutions occurs to a similar extent at both the (uncharged) mercury-solution interface and the air-solution interface. The formic acid dipole appears to be preferentially oriented... 

Persson P, OJamae L (2000) Periodic Hartree-Fock study of the adsorption of formic acid on ZnO(lOlO). Chem Phys Lett 321(3.4) 302-308... 

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