Oxidation of hydrogen on platinum

The oxidation of hydrogen catalyzed by finely divided platinum was first observed by Dobereiner in 1823 [75] and prompted Berzelius to introduce the term catalysis [76]. One might expect that in this case the mechanism is comparably simple as with carbon monoxide oxidation. Both reactants are known to first adsorb dissociatively. While Oad forms the known 2x2 structure presented several times before, with adsorbed H a problem arises The maximum coverage was found to be one, but no ordered structures could be identified with LEED nor could the adsorbates be identified by STM. This is caused by the fact that this surface is energetically very smooth, and the first vibrational excitations need only a few kj/mol [77-80]. As a consequence, at not extremely 

Since OHad can also be experimentally identified, the mechanism of this reaction was believed to proceed along the steps (l)-(5), as shown in Fig. 6.25. However, the actual mechanism is more complicated [81]. H2O desorbs at about 170K [82], and the net activation energy for the reaction was found to be much smaller below this temperature (0.13 eV) [83] than above room temperature ( 0.7eV) [84]. In addition, at low temperatures, an induction period was found [83]. This problem could be solved by combined application of STM and vibrational spectroscopy [81]. It turned out that reaction step (6), H20ad + Oad —20Had,isvery important as long as adsorbed H2O is present, that is, below 170K in UHV. 

The progress of the reaction is illustrated in the STM images of Fig. 6.26. If an O-covered Pt(l 11) surface is exposed to a constant H2 pressure of 8 x 10 mbar at 131K, the O atoms appearing as dark dots are continuously replaced by bright patches with internal structure that are identified with ordered OHad islands. In the further progress of the reaction, the latter are transformed into fuzzy patterns of adsorbed H2O, which then desorb. These 

FIGURES.26. A series of STM images showing the progress of reaction between O atoms adsorbed on Pt(l 11) and H2 (a-e). Fraction of the surface covered by OHad as a function of time (f) [81]. 

FIGURE 6.27. Large-Scale STM images from the H2 -I- O2 reaction on Pt(l 11), illustrating the propagation of an OHa wave [85]. 

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In the presence of Pb(II) ions in sulfuric acid, potential oscillations have been observed for galvanostatic oxidation of hydrogen on platinum electrode [129]. This behavior has been attributed to ad-sorption/oxidation/desorption processes of lead on the platinum surface. Lead at high values of coverage is oxidized to insoluble PbS04, which blocks the Pt surface. 

In this example the exclusion of the intermediates could be effected, essentially in one way only, since the column of stoichiometric numbers ( (2) is obtained from the column (2) by multiplication by an arbitrary factor this holds for any other suitable column of stoichiometric numbers. However, this is not always the case—even when the reaction is described by one overall equation. Thus, a possible mechanism of oxidation of hydrogen on platinum is (25)... 

The concepts which we develop above were used in the dissertation by Buben, written under the guidance of Frank-Kamenetskii at the Institute of Chemical Physics in 1940. As Buben showed, together with the difference between D and n, also significant is the role of thermal diffusion which acts, as a rule, in the same direction, i.e., increasing Tr compared to (9) in the case when the light component is deficient, DZ > k so that in accordance with (9) Tr > Tc. In contrast, when Tr < Tc anyway, thermodiffusion decreases the flow of the heavy component to the heated surface and causes further decrease of Tr. Direct experiments by Buben, who studied the oxidation of hydrogen on platinum wires, showed that for a content of 2-5% H2 in air,... 

It should be said that at present the available literature concerning the kinetic models which account for the topochemical character of catalyst surface processes is limited, but reference can be made to refs. 119 and 120. In ref. 119, a kinetic model for the oxidation of hydrogen on platinum is... 

E. Fridell, U. Westblom, M. Alden, and A. Rosen, Spatially resolved laser-induced fluorescence imaging of OH produced in the oxidation of hydrogen on platinum, J. CataL 725 92 (1991). 

Innocente AF, Angelo ACD. Electrocatalysis of oxidation of hydrogen on platinum ordered intermetalhc phases Kinetic and mechanistic studies. J Power Sources 2006 162(1) 151-159. 

E. Lamy-Pitara, S.E. Mouahid, and J. Barbier, Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum, Electrochim. Acta 45, 4299-4308 (2000). 

The electrochemical oxidation of bromoalkanes on platinum in anhydrous hydrogen fluoride displaces bromine by fluorine.27... 

Together with his colleagues in the laboratory he studied problems of the crystallization of nitroglycerin, oxidation of hydrogen on a platinum catalyst, and oxidation of CO on manganese catalysts. The first and the third problems were of practical significance. 

FIGURE 14-4 Current-voltage curve from a linear potential sweep for deposition of hydrogen on platinum from 0.5 M H2SO4 in ultrapurified water. A, formation of chemisorbed hydrogen atoms B, oxidation of hydrogen atoms C, surface oxidation of platinum D, reduction of platinum oxide. (From Conway, Angerstein-Kozlowska, Sharp, and Criddle. )... 

Navarro-Suarez AM, Hidalgo-Acosta JC, Fadini L, Feliu JM, Suarez-Herrera MF (2011) Electrochemical oxidation of hydrogen on basal plane platinum electrodes in imidazolium ionic liquids. J Phys Chem C 115 11147-11155... 

Adsorption and oxidation of CO on platinum electrodes have been the subject of numerous investigations, due to the importance of this molecule in the electrochemical processes that occur in fuel cells. The potential at which CO is oxidized depends on the potential at which it was previously adsorbed. It depends also on the coverage of the surface and is sensitive to the structure (i.e., the process is different according to the superficial crystallographic orientation). Furthermore, various authors have suggested the possibility of CO being reduced, at least partially, in the presence of coadsorbed hydrogen. 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

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