Electrocatalytic activation of small molecules

Abstract

Electrochemsitry and electrocatalysis are useful techniques for energy conversion and energy storage applications. In this project, the electrocatalytic activation of small molecules, H₂O, methanol and ethanol, and CO₂, has been studied as potential methods for energy storage and conversion. A hexaniobate Lindqvist ion assisted Co and Ni nanostructure deposition method has been developed. Efficient catalytic activity towards water oxidation has been observed with high TOF values obtained in alkaline 1 M NaOH solution and even one cycle of potential for film deposition generating significant catalytic activity. CoIV and NiIV may be responsible for the high catalytic activities as results showed by FTAC voltammetry. Four cobalt phosphonate coordination polymers were also studied and showed catalytic activity for water oxidation in both phosphate buffer and water-saturated ionic liquid [BMIM][PF₆]. The catalytic activity shows a structural dependence and presumably due to the different intrinsic catalytic activities of these catalysts towards water oxidation reaction or/and their different rates of decomposition to form the well-known CoOx water oxidation catalysts. Voltammetric studies of the Ru-containing water oxidation molecular catalyst, [{Ru₄O₄(OH)₂(H₂O)₄}(-SiW₁₀O₃₆)₂]10- have been carried out in aqueous media over a wide range of pH (2 to 12) and ionic strength conditions. Well defined voltammograms in the buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO3, NaNO3, KNO3, Ca(NO3)₂, Mg(NO3)₂, MgSO₄ or Na₂SO₄). Analysis of the reversible potentials reveals that (i) K+ has a significantly stronger interaction with the one-electron oxidized form 1(1) than the other cations investigated; (ii) proton transfer may not necessarily be coupled to all electron transfer steps to generate active water oxidation catalysts; (iii) the four-electron oxidized form, 1(4), is capable of oxidizing water under all conditions studied and (iv) under some conditions, the three-electron oxidized form, 1(3), also exhibits considerable activity. These electrochemical knowledge obtained for the molecular water oxidation catalyst [1(-SiW₁₀O₃₆)₂]10- have been also applied to the electrooxidation of methanol and ethanol. In both aqueous and alcohol media, the one-electron oxidized form 1(1) is able to catalyze methanol and ethanol under all conditions, and the 1(0) showed some catalytic activity especially for ethanol oxidation under some conditions. Methanol and ethanol were oxidized into aldehyde and acid through two- and four-electron transfer pathways, respectively, as shown by analysis of products after bulk electrolysis. One Cu and two Ni macrocyclic complexes have been studied for electrocatlytic CO₂ reduction. N,N´-ethylenebis(acetylacetoniminato) complexes of nickel(II) and copper(II) both showed catalytic activity for CO₂ reduction in organic and ionic solvents, and also for proton reductions. C₂O₄2- was formed after bulk electrolysis as shown by spectroelectrochemical data. A novel square-planar nickel(II) Schiff base complex, [Ni(L´)]PF6 (where L´ is 2,4,9,11,11-pentamethyl-2,3,4 triaza-1-one-4-amine), has been synthesized and shown with electrocatalytic activity for CO₂ reduction with oxalate and bicarbonate as the reduction products after bulk reductive electrolysis under a CO₂ atmosphere.