Sensitization of La modified NaTaO3 with cobalt tetra phenyl porphyrin for photo catalytic reduction of CO2 by water with UV–visible light

Abstract

Lanthanum modified sodium tantalate, Na(1−x)LaxTaO(3+x), in conjunction with cobalt (II) tetra phenyl porphyrin (CoTPP) as sensitizer, has been explored for photo catalytic reduction of carbon dioxide (PCRC) with water. HOMO and LUMO energy level characteristics/redox potentials for ground (S0) and excited states (S1 singlet) of CoTPP have been calculated by Density Functional Theory (DFT). HOMO and LUMO energy levels enable sensitization of the tantalate, a typical wide band gap semi-conductor, with visible light. Visible light absorption by CoTPP results in the direct transfer of photo generated electrons to the conduction band of the tantalate, in addition to the intrinsic UV light excitation. Besides, sensitization also retards charge carrier recombination rate, as indicated by the photo luminescence spectral data for the pristine and sensitized Na(1−x)LaxTaO(3+x). A co-operative effect of these factors contributes towards nearly 3 fold increase in apparent quantum yield value for PCRC with the 1% w/w CoTPP/tantalate composite vis-à-vis pristine tantalate. After 20h of irradiation, rate of methanol formation remains constant with pristine and sensitized tantalates, while the rate of formation of ethanol increases on sensitization, indicating multi electron reduction process. Chemical composition and structural characteristics of the composite are preserved even after 20h of irradiation.