Value Added Coversion of Carbon Dioxide to Alcohol Fuels

Selective conversion of CO2 to alcohols has several redeeming features especially in view of the fact that large amounts of these chemicals are manufactured world-wide. However the current feedstocks for alcohol production (mainly via steam-reforming) are petroleumbased with a sizeable carbon footprint. Further, the high operating pressures and temperatures needed for steam reforming translates to correspondingly high capital investment and a long energy pay-back period. Therefore mild methods for producing alcohols from environmentally problematic feedstocks such as CO2 take on added importance. It is also worth noting that in addition to applicability as transportation fuels, alcohols are commodity chemicals that have wide-ranging use in plastics manufacture, paint formulations, for making resins and electric insulation, in the pharmaceutical sector, in anti-freeze fluids, and as laboratory solvents. In this talk, both electrocatalytic and photoelectrocatalytic (PEC) methods for converting CO2 to alcohols will be described as milder process alternatives to steam reforming. First the use of Pt/C/TiO2 nanocomposites will be discussed whereby CO2 is shown to be efficiently converted to methanol and isopropanol in the presence of a pyridine co-catalyst [1]. Comparison with the behavior of a Pt disk electrode of comparable geometric are will be presented and it will be shown how the product distribution is different in the two cases. The process effeciiency is also greatly enhanced with the use of a naocomposite film while drastically reducing the amount of Pt in the latter case. The second approach centers on the use of an inorganic semiconductor (specifically p-type) to photocatalytically drive the reduction of CO2 without the use of a co-catalyst or an expensive noble metal such as Pt. Many inorganic p-type semiconductors have been deployed in the literature [2] for PEC reduction of CO2 including: CdTe, GaP, GaAs, InP, Si, and FeS2. However with the exception of the last compound (pyrite) and elements such as Si and P, all the other component elements in these semiconductors are either toxic (As, Cd) or not earth-abundant (Te, Ga, In). Selective photoconversion of CO2 to methanol without the use of a co-catalyst also is not commonplace in these prior studies, and in the isolated such cases, methanol was only photogenerated at very high overpotentials. Two-electron reduction products such as CO and formic acid have been reported in some cases in place of the (more difficult) sixelectron conversion to methanol. With the use of semiconductor colloids such as CdS or ZnS, photocatalytic reduction of CO2 yields mostly formate and/or formaldehyde as the products. In the second part of this talk, solar photoelectrosynthesis of methanol will be described on hybrid CuO/Cu2O semiconductor nanorod arrays for the first time at potentials ~800 mV below the thermodynamic threshold value and at Faradaic efficiencies of ~95%. The CuO/Cu2O nanorod arrays were prepared on Cu substrates by a two-step approach consisting of the initial thermal growth of CuO nanowires followed by controlled electrodeposition of p-type Cu2O crystallites on their walls. No co-catalysts (such as pyridine, imidazole or metal cyclam complexes) were used contrasting with earlier studies on this topic using ptype semiconductor photocathodes. The roles of the core/shell nanorod electrode geometry and the copper oxide composition were established by varying the time of electrodeposition of the Cu2O phase on top of the CuO nanorod core. The use of electrochemical quartz crystal microgravimetry (EQCM) to unravel the mechanistic aspects of CO2 photoreduction on copper oxide photocathosed will be presented [3]. Finally methanol photogeneration was confirmed by GC-MS analyses of product evolved at -0.2 V vs. SHE, i.e., at an underpotential relative to the standard potential of CO2/CH3OH. This last feature is an important virtue of the p-type semiconductor based photoreduction approach. In contrast the (“dark”) electrocatalytic process counterpart for CO2 reduction suffers from the electrical energy cost incurred from the need for considerable overpotentials to overcome the kinetic barrier associated with this process. This cost is simply circumvented in the solar PEC process from the energy inherent in sunlight.