Assessing the effect of light intensity and light wavelength spectra on the photoreduction of formic acid using a graphene oxide material

Photocatalysis has been a topic of interest in recent years for both, oxidation and reduction reactions, and although there is a broad variety of research regarding photocatalytic materials and the reaction itself, studies on reactor design and related phenomena, radiation transfer and its direct impact on reaction extent specifically, are usually neglected. From this end, the present work focuses on the elucidation of the effect of light intensity and wavelength spectra in the visible light region during the photoreduction reaction of formic acid using graphene oxide as a promising catalyst. By using formic acid, one of the main intermediaries in the photoreduction of carbon dioxide, the possibility of methanol production is evaluated without the thermodynamic constraints presented by carbon dioxide. A graphene oxide material, synthetized through a modified Hummer’s method, is assessed for the reduction of formic acid evaluating four different light sources (red, green, blue and white). An analysis of energy balances in the reaction set-up allows the determination of both the energy absorbed by the GO photocatalyst and isoactinity conditions at studied radiative operating conditions. At an isoactinity environment, the adsorption rate of formic acid and production rate of methanol are then evaluated, relating them to the absorbed energy achieved at the wavelength spectra and light intensities evaluated; IR spectroscopy is utilized to follow formic acid concentration as well as methanol production. The largest initial reaction rate (ca. 57%) relates to the use of the red wavelength at its largest intensity. Reaction rates at larger times start to be apparent being affected by adsorption, reaction and radiation conditions. The maximum conversion, 14%, is attained by using the white wavelength spectra at its lowest intensity. Thus, higher intensities will not necessarily yield higher conversions, nor the highest reaction rates. This, in turn, poses the necessity of quick, reliable assessments for whichever catalyst used in this type of reactions that leads to the correct election of operating conditions that maximize the product yield. Independent evaluation for every wavelength within the visible spectra and assessing carbon dioxide photoreduction are future steps into the elucidation of solar fuel production feasibility.