Electrocatalytic hydrogenation and oxidation of model coal-derived compounds chemisorbed at smooth polycrystalline Pt: a survey

Abstract

The chemisorption and electrocatalytic reactivity of six model coal-derived compounds at smooth polycrystalline platinum electrodes in aqueous solutions have been investigated. The compounds studied were aniline (AN), hydroquinone (HQ), 3,6-dihydroxypyridazine (DHPz), 2,3-dihydroxypyridine (DHPy), 2,5-dihydroxythiophenol (DHT), and thiophene (TP). Electrocatalytic hydrogenation and oxidation using thin-layer electrochemical cells were carried out at −0.25 V [Ag/AgCl (1 M Cl −) reference] and 1.18 V, respectively, in 1 M H 2SO 4 in the absence of bulk (unadsorbed) species. Electrocatalytic hydrogenation was characterized by the average number of hydrogen atoms n H reacted per chemisorbed molecule, and anodic oxidation was investigated in terms of the effective number of electrons n ox transferred during the oxidation reaction. The results indicate that: (i) Hydrogenation of benzene to cyclohexane, which is a function of the initial adsorbate orientation, is a more facile process than hydrogenolysis of the aromatic ring. (ii) Hydrogenation of the aromatic ring proceeds more readily than cleavage of the C-O bond even if the oxygen heteroatom is not part of the aromatic ring. (iii) Hydrogenative cleavage of the C-S bond occurs more readily and with greater specificity if the sulfur heteroatom is not part of the aromatic framework. (iv) Hydrogenative cleavage of the C-N bond also takes place more easily and with higher specificity if the nitrogen heteroatom is not part of the aromatic ring. (v) Anodic oxidation is dependent upon orientation or mode of attachment, if the orientational states of a given molecule are vastly different from one another, as in η 6-HQ and 2,3-η 2-HQ; if the alternative orientations are similar, as in S-η 1 and tilted DHT, n ox may be independent of initial orientation. (vi) For a given mode of surface attachment, the extent of anodic oxidation is a function of the structure of the aromatic ring. (vii) In general, hydrogenation is limited to the functional group directly bonded to the Pt surface; no such limitation applies to catalytic oxidation.