Platinum electrocatalytic hydrogenation

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). 

Cathodic surfaces of finely divided platinum, palladium and nickel have a low hydrogen overvoltage and the dominant electrochemical reaction is the generation of a layer of hydrogen atoms. The electrocatalytic hydrogenation of aldehydes and ketones can be achieved at these surfaces. Cathodes of platinum or palladium black operate in both acid solution [203] and in methanol containing sodium methoxide [204], The carbonyl compound is converted to the alcohol. Reduction of 4-tert-butylcyclohexanone is not stereoselective, however, 1,2-diphenylpropan-l-one is converted to the / reo-alcohol. 

Commercial catalytic hydrogenations of unsaturated compounds use Raney nickel or—less commonly—Pt catalyst supported on active carbon. Electrocatalytic hydrogenation can be performed at platinized platinum or other platinum-metal electrodes. Adsorbed hydrogen atoms are the active reactant in catalytic as well as in electrocatalytic hydrogenation. 

A different way of making electrolytic reduction is through electrocatalytic hydrogenation, which is a kind of indirect electrolysis. Protons are reduced and the key intermediate is Me(H) (with Me being platinum, palladium, rhodium, or nickel), and the potential determining step is the formation of this reactive intermediate. Selective reductions may be performed by this method, and the potential used for the formation of Me(H) is often less negative than that required for the direct electron transfer to the reducible substrate. Hence, electrolysis occurs with a lower energy consumption. The selectivity of the reaction may depend on the support for the catalyst. 

In addition to these different types of alloys, some studies were also devoted to alternatives to platinum as electrocatalysts. Unfortunately, it is clear that even if some catalytic activities were observed, they are far from those obtained with platinum. Nickel tungsten carbides were investigated, but the electrocatalytic activity recorded for methanol oxidation was very low. Tungsten carbide was also considered as a possible alternative owing to its ability to catalyze the electrooxidation of hydrogen. However, it had no activity for the oxidation of methanol and recently some groups showed that a codeposit of Pt and WO3 led to an enhancement of the activity of platinum. ... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Takasu Y, Eujii Y, Yasuda K, Iwanaga Y, Matsuda Y. 1989. Electrocatalytic properties of ultra-fine platinum particles for hydrogen electrode reaction in an aqueous solution of sulfuric acid. Electrochim Acta 34 453-458. 

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

The electrochemical hydrogenation of double bonds can be performed either electrocatalytically at Raney nickel, palladium, or platinum modified electrodes [32] or via electron transfer under Birch conditions to the intermediate anion radical [33]. Examples are given in the dihydrogenation of phthalic acid (Eq. 22.15) and the hydrogenation of heteroaromatic compounds (Eq. 22.14). 

Jones, Anne K. Sillery, Emma Albracht, Simon P. J. Armstrong, Fraser A. Direct comparison of the electrocatalytic oxidation of hydrogen by an enzyme and a platinum catalyst. Chemical Communications (Cambridge, UK) 2002 (8) 866-867. 

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

MVD of ruthenium on Pt(llO) has been shown to provide an ideal system for the study of the promotion of electrocatalytic reactions on a well-characterized Pt-Ru alloy surface [85,86]. In transfer studies, XPS, LEISS, and LEED have been used to characterize the Pt(110)-Ru alloy system, and TPD and stripping voltammetry used to investigate the chemisorption behavior of CO, and the promotion of CO electro-oxidation as a function of incorporated ruthenium. The facile incorporation of ruthenium in the relatively open-packed Pt(110)-(1 x 2) surface provided an ideal model for the alloy system. It is also interesting to note also that the clean Pt(llO) surface exhibits the highest hydrogen oxidation currents of the three basal planes of platinum [108]. 

On the other hand, Chen et al. developed polypyrrole film electrodes containing nanodispersed platinum particles and investigated their catalytic properties for the electro-oxidation of hydrogen [13]. They confirm the remarkable electrocatalytic activity of these PPy/Pt films compared to bulk platinum electrodes, as the film thickness increases to 5 pm. 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

Similarly, the difficulty for electrocatalytic, electrogenerative hydrogenation of alkenes on platinum parallels the strength of gas phase adsorption of the substrate (55) acetylene > ethylene > propylene > cyclopropane. Palladium is a more active electrocatalyst for ethylene reduction than platinum (55), in agreement with adsorption strength on each metal. Selectivity and reduction rate of substituted alkenes also depends on adsorption... 

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