Study of the Mechanism of the Hydrogen Evolution Reaction at Raney Nickel Electrodes in the Presence of Organic Compounds

Abstract

975trocatalyst may be due to the combined effects of the Raney type Pt-Cr-Cu alloy and of the base metal oxide. A shortterm stability test indicated nearly stable performance for 20 h after little loss of performance in the first 10 h of cell operation. REFERENCES 1. ABSTRACTThe hydrogen evolution reaction (HER) has been studied at Raney nickel electrodes in the absence and presence of several organic compounds. A poisoning effect of nonreducible organic substrates on the kinetics of the HER was studied. It was found that only pyridine had a strong inhibiting effect on the HER. A compound having one reducible double bond, trans-cinnamic acid, was used as a model in a study of the mechanism of the electrocatalytic hydrogenation (ECH) reaction and its influence on the HER. It was established that the HER proceeds via the Volmer-Hyrovsky reaction mechanism, and the slowest step is the Heyrovsky reaction. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.198.30.43 Downloaded on 2015-03-16 to IP

Electrocatalytic hydrogenation of conjugated enones on nickel boride, nickel, and Raney nickel electrodes

“…The conversion rate increases from 67% (entry 1) to 84% (entry 9) as the current density decreases from 250 to 50 mA dm-' (geometric area), which shows that a decrease of current density favors hydrogenation over hydrogen evolution (Scheme 1). Assuming that the mechanism of the HER in methanol-water at pH 5.3-8.3 on nickel boride electrodes is the same as in aqueous alkaline solutions, that is, a Volmer-Heyrovsky mechanism (reactions [I] and [5]) (14,15), the competition between hydrogenation and hydrogen evolution (Scheme 1) becomes a competition between a chemical process (reactions [2]- [4]) and an electrochemical process (reaction [5] q h e electrodes were prepared by pressing the powder at p = 800 MPa. 'See footnote e of Table I.…”

The influence of surfactants on the electrocatalytic hydrogenation of organic compounds in micellar, emulsified, and hydroorganic solutions at Raney nickel electrodes

“…One explanation for an increase of the 819 ratio from 2.1 at pH 2 to 7.5 at pH 12 could be the slow hydrogenation of the enolate at pH 1 2 .~ Epirnerization of alcohol 9 to alcohol 8 (through dehydrogenation-hydrogenation) is very unlikely since 8 with the hydroxy group axial should be less stable than 9 with the hydroxy group equatorial (38c). 111 aqueous ernitlsions without any surfactant, the ECH of (-)-cawone (4) proceeded with a current efficiency (77%), conversion rate (92%), and yield of ketones (25%) and alcohols (61%) ( Table 4, entry 6) similar to those observed in hydroorganic solutions (Table 4, entries [1][2][3][4][5]. Under these conditions, lirnonene (1) was not electrohydrogenated (see above) and the greater facility of electrohydrogenation of 4 can be attributed to its higher solubility in water.…”

“…10 and entries 5 to 1 of Table 6) because of a decrease in the rate of hydrogenation (eqs. [3]- [5]) competing with hydrogen evolution (eq. [2]).…”

“…However, cathode has significant advantages over the chemical catalytic the competing hydrogen evolution reaction (HER) (reactions hydrogenation (2): (i) the chemisorbed hydrogen is generated [2]) can drastically lower the current efficiency of ECH and in in situ at low overpotential (reaction [I]) and its concentration the worst cases can even prevent the hydrogenation of the may be controlled by monitoring the current density; (ii) the organic compound (reactions [3] - [5]). On highly porous…”

“…[2]) mechanism both in aqueous (4) and in aqueous-methanolic (5) solutions. Furthermore, in the presence of an ECH reaction, the Heyrovsky reaction is the rate-limiting step of the HER, which implies that the surface coverage by adsorbed hydrogen increases with the potential (6). The relative rates of hydrogenation (reactions [3]- [5]) and hydrogen evolution (reactions [2]) depend on several parameters among which, for a given electrode, the strength of the bond to be hydrogenated and the local concentration of the organic substrate adsorbed on the catalyst are particularly important (2).…”