Formic acid decomposition dehydrogenation

Selectivity of metal or oxide for formic acid decomposition dehydrogenation rate rate of total decomposition... 

Values and standard deviations of parameters that define compensation effects, calculated using methods described in Appendix II, Reactions C, cracking D, dehydrogenation E, exchange F, formic acid decomposition H, hydrogenation O, oxidation. 

Formic acid decomposition has been studied on the (110), (001), and (100) surfaces of Ti02 [23-25,40-51]. The degree to which surface reducibility influences the reaction paths e.g., dehydrogenation vs. dehydration unimolecular reactions vs. bimolecular ones) will be explored in more detail... 

Formic acid can decompose either by dehydration, HCOOH — H2O + CO (AG° = —30.1 kJ/mol AH° = 10.5 kJ/mol) or by dehydrogenation, HCOOH H2 + CO2 (AG° = —58.6 kJ/mol AH° = —31.0 kJ/mol). The kinetics of these reactions have been extensively studied (19). In the gas phase metallic catalysts favor dehydrogenation, whereas oxide catalysts favor dehydration. Dehydration is the predominant mode of decomposition ia the Hquid phase, and is cataly2ed by strong acids. The mechanism is beheved to be as follows (19) ... 

PCSs obtained by dehydrochlorination of poly(2-dilorovinyl methyl ketones) catalyze the processes of oxidation and dehydrogenation of alcohols, and the toluene oxidation207. The products of the thermal transformation of PAN are also catalysts for the decomposition of nitrous oxide, for the dehydrogenation of alcohols and cyclohexene274, and for the cis-tnms isomerization of olefins275. Catalytic activity in the decomposition reactions of hydrazine, formic acid, and hydrogen peroxide is also manifested by the products of FVC dehydrochlorination... 

Grenoble and coworkers229 reported an important influence of the support on the water-gas shift activity of various metal catalysts. For example, the rate increased an order of magnitude when Pt was supported on alumina versus silica. Turnover numbers for alumina-supported metal catalysts decreased in the order Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir, whereby the activity varied by 3 orders of magnitude, suggesting a correlation between activity of the metal and the heat of adsorption. To describe these differences in activity, the authors used a bifunctional model, involving chemisorption of water on alumina and CO on the metal, followed by association of the CO with the water to form a formic acid-like formate species, with subsequent decomposition via dehydrogenation on the metal sites (Scheme 55). 

Dehydrogenation of formic acid on metals and alloys, decomposition of... 

This is the reason why, for example, the zero order formic acid dehydrogenation may easily be measured on bulk metal catalysts at 200-300°C. whereas the approximately first order ethanol dehydrogenation requires highly activated porous metals of large specific surface in order to become measurable under the same conditions. The same has been shown for the decomposition of formaldehyde, acetic acid, and hydrazine hydrate. In these cases, the fractional surface coverage is simply 1000 times lower than in the case of a zero order reaction. 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

Krupay and Ross (272b), in a study of the decomposition of formic acid on manganese (II) oxide, demonstrate that manganese (II) formate is produced during reaction and discuss the probable role of this participant in the catalytic process. The reported Arrhenius parameters (log A, E) for the dehydration and dehydrogenation reactions were (28.7, 132) and (24.9, 87), respectively both points were close to the compensation line (Table V, K) characteristic of the breakdown of formic acid on oxides. 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Like formic acid, methanol decomposition has also been used to probe the acid-base properties of metal oxides [70]. However, methoxide decomposition is dependent on surface structure in much the same way as formate decomposition. For example, methanol undergoes parallel dehydration and dehydrogenation reactions on the same crystal surface of zinc oxide [25]. Once again, product selectivity ratios may not necessarily serve as a diagnostic of acid-base properties alone. 

The formate, formed by oxidative dehydrogenation of the acid, is quite stable and doesn t decompose until 480 K. This decomposition is a classical first-order case with a decomposition activation energy of 130 kJ mol-1 and a normal value pre-exponential of 1013 s-1. The great ability of the TPD technique is the separation of the individual steps in the reaction in temperature. It is clear that the step proceeding over the highest barrier in this case is the formate decomposition, and that in a catalytic oxidation of formic acid the most abundant surface intermediate is likely to be the formate with its decomposition being rate determining. 

It has been thought that the acid-base character is an intrinsic property of oxide substrates. The selectivity in the catalytic decomposition reaction of formic acid has been used to scale the acid-base property dehydration over acidic oxide and dehydrogenation over basic oxide, though this classification is over simplified vide infra. 

As it appeared that the main reactions on different catalytic systems had many features in common, the decomposition of formic acid will in the following chapters be discussed along the lines given by the two main reaction paths. Thus the dehydrogenation both on metals and oxides will be treated in the first part and the second part will be devoted to the dehydration reaction. As the latter reaction is also largely catalyzed in the liquid phase by strong acid, a short discussion of this phenomenon has been included. 

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