Production of CO from CO2 over mixed-metal oxides derived from layered-double-hydroxides

Abstract

Carbon monoxide generation from the splitting of CO2 is of greater importance now than ever before due to continuous increase in CO2 levels in the atmosphere and concerns on the sustainability of energy and environment. This paper examines the conversion of CO2 to CO via thermochemical redox looping of metal oxides. Oxides of Mg/Fe and Mg/Al/Fe prepared from layered double hydroxide (LDH) were explored to aid in the conversion of CO2 to CO. The prepared metal oxides from LDH offered superior stability of porous metal structure and sustained redox capability at moderate to high temperatures required for conversion. The formations of these metal oxides from LDH were analyzed using thermogravimetric analysis (TGA). The results showed 635 µmol CO/g of Mg/Fe oxides as compared 557 µmol CO/g from Mg/Al/Fe. Continuous operation resulted in some sintering effects with deteriorated redox performance. The redox capability of Mg/Fe oxide over multi-cycle operations was lower as compared to Mg/Al/Fe oxides, which showed significant thermal stability while maintaining high CO yield and redox capability over extended redox cycles. The specific LDH derived metal oxides produced CO three times higher than those reported previously in the literature using Zr-doped ceria and other perovskites per unit mass of the oxygen carriers. While the surface area was reduced in Mg/Al/Fe the yield after the first cycle remained unchanged; both the surface area and yield decreased in Mg/Fe oxides. Mixed-metal oxide prepared from co-precipitation provided highly porous Mg/Al/Fe oxide compared to Mg/Fe or Mg/Al. High porosity enhanced the surface oxidation of Mg/Al/Fe that contributed to more than 85% oxidation from fast surface reactions. After two cycles, the redox rates changed little with further increase in the number of cycles to reveal surface area stability of Mg/Al/Fe. Multiple cycle operation suggested superior redox kinetics of Mg/Al/Fe than Mg/Fe. These LDH derived mixed metal (Mg/Al/Fe) oxide provided high and sustained CO production rates compared to those reported in the current literature used other materials so that the examined materials have good potential to serve as oxygen carrier for CO2 splitting to CO.