Formic acid dissociative adsorption

Figure 7.13 Variation of the CO stripping charge formed from formic acid dissociative adsorption as a function of adatom coverage for a Pt(lll) electrode modified with Bi and Se, as indicated, in 0.5 M H2SO4 solution.

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

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. 

Comparable patterns are followed by other organic substances such as formaldehyde and formic acid. All these substances are dissociatively adsorbed on platinum [4] and it was suggested that they build the same adsorption product [35]. 

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. 

The coordinative and/or dissociative adsorption of various probe molecules has been used to characterize the surface properties of Ti02) which finds applications as a catalyst, photocatalyst, and sensor. Among the molecules used as probes, we mention CO (37, 38, 563-576), C02 (563, 565, 577), NO (578,579), water (580,581), pyridine (582,583), ammonia (584,585), alcohols (586, 587), ethers (including perfluoroethers) (588), ozone (589), nitrogen oxide (590), dioxygen (591), formic acid (592-594), benzene (584), benzoic acid (595), and chromyl chloride (596). 

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... 

SFG spectra of CO adsorbed on nickel have been reported 116,118,416,417), as have spectra characterizing NH3 adsorption/dissociation on Fe(l 1 1) 418). UHV SFG investigations of formic acid decomposition on NiO(l 1 1) were also reported 419,420). Investigations of ruthenium surfaces 147,148,157,421-425) and of CO adsorbed on Ir(l 1 1) are also available 426). 

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]. 

The break-point temperature in dehydration (above which the rate was temperature insensitive) matched the maximum temperature for dehydrogenation, suggesting that a common intermediate exists for each reaction, and that the product selectivity is determined by interactions with other molecules and the surface. Above 650 K, the catalytic dehydration channel dominates, but the rate-determining step changes above 700 K. Below 700 K, the reaction rate is nearly independent of the partial pressure of formic acid (ca. 0.2 order). Above 700 K, the rate of the reaction is essentially independent of temperature, implying that reaction is limited by formic acid adsorption and dissociation thus, above 700 K, the rate becomes first-order with respect to the partial pressure of formic acid. Higher pressures of formic acid over the crystal surface should therefore increase the transition temperature - this behavior was observed by Iwasawa and coworkers, and the turnover frequency for catalytic dehydration approached the collision frequency of formic acid at high... 

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. 

Morrow and Cody studied the adsorption of NHa and pyridine on Pt/Si02. For NH3, no evidence was found for dissociation up to 250 °C, as shown by isotopic exchange. To the contrary, pyridine adsorbs by dissociation of the a-C-H bond. The interaction of the N atom with a nearby Pt probably forces the molecule to lie perpendicular to the surface. Other molecules have been studied including ethylene,cyclohexanol, cyclohexanone, and cyclohexane ° on Pt/Al203 and formic acid and ethanoic... 

On the other hand, carbon-supported Pt catalysts for the electrooxidation of formic acid are severely poisoned by the adsorbed CO intermediate of the reaction [14—16], although platinum is at present the best-known catalyst for the dissociative adsorption of small organic molecules. It has been demonstrated [17, 18] that PtRu and PtPd alloys can diminish this CO poisoning effect to some extent. 

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