Palladium electrocatalytic hydrogenation

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. 

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. 

The preparative use of nitrile reduction has been primarily for the synthesis of amines [1,2,152-157]. This is done in acidic medium and gives fair to good yields. Electrocatalytic hydrogenation of nitriles is also sometimes efficient [158] in these cases, nickel or palladium black, deposited on the cathode, promotes reduction by electrolyti-cally generated hydrogen atoms. 

I.M.F. De Oliveira, J.C. Moutet, and S. Hamar-Thibault, Electrocatalytic hydrogenation activity of palladium and rhodium microparticles dispersed in alkylammonium- and pyridi-nium-substituted polypyrrole films, J. Mater. Chem., 2,167 173 (1992). 

Cyr A et al (2000) Electrocatalytic hydrogenation of lignin models at Raney nickel and palladium-based electrodes. Can J Chem 78 307-315... 

The reduction of 2- and 4-cyanopyridine to the aminomethylpyridine was mentioned in Part I. 3-Aminomethylpyridine has been prepared from 3-cyanopyridine in an electrocatalytic reduction in aqueous hydrochloric acid solution, using an electrode consisting of a thinly deposited layer of palladium black on graphite. The reduction proceeds with electrolytically generated hydrogen not via an electron transfer to the substrate.418... 

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). 

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... 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

As shown in Fig. 1, palladium is mainly consumed as an autocatalyst now[l, 2]. Palladium is also an important nohle metal element widely applied in the development of electrocatalytic materials [46, 48, 72]. ft is widely used as a promoter for electroless deposition of metals on various substrates. Palladium is also known for its extraordinary ability to absorb a large amount of hydrogen. The growth of palladium thin layers on various substrates in UHV has been investigated in detail [3-8]. Electrodeposition of palladium is widely used, and a number of commercially available electroplating baths of palladium for different purposes have been developed [56-58, 61, 72]. However, the rapid increase in the... 

The electrocatalytic behavior of the thin palladium layers deposited on Au(hkl) surfaces for hydrogen adsorption/absorp-tion [77, 80], oxygen reduction [79], oxide formation/reduction [80], copper UPD [33, 77], electrochemical oxidation of formic acid [84] as well as formaldehyde [80] has been investigated in detail. These electrocatalytic activities depended significantly on the surface structure and thickness of the ultrathin palladium layers [80, 84]. 

Cai, X.-R, Kalcher, K., Koelbl, G., Neuhold, C.G., Diewald, W., and Ogorevc, B. (1995) Electrocatalytic reduction of hydrogen peroxide on a palladium-modified carbon paste electrode. Electroanalysis, 7, 340- 345. 

Babaei, A., Jiang, S.P. Li, J. Electrocatalytic promotion of palladium nanoparticles on hydrogen oxidation onNi/GDC anodes of SOFCs via spillover. J. Electrochem. Soc. 156 (2009), pp. B1022-B1029. 

Huang JS, Wang DW, Hou HQ, You TY (2008) Electrospun palladium nanoparticle-loaded carbon nanofibers and their electrocatalytic activities towards hydrogen peroxide and NADH. Adv Fund Mater 18(3) 441 48. doi 10.1002/adfm.200700729... 

Lin, Z., Ji, L., Woodroof, M.D., Medford, A.J., Shi, Q., Krause, W., and Zhang, X., Electrocatalytic Interaction of Nano-Engineeied Palladium on Carbon Nanofibers with Hydrogen Peroxide and y -NADH , Journal of Solid State Electrochemistry, 15, 1287-1294, 2011. 

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