Hysteresis phenomena and rate fluctuations under conditions of glycerol photo-reforming reaction over CuOx/TiO2 catalysts

Abstract

The production of hydrogen by photocatalytic reforming of aqueous solutions of glycerol has been investigated over CuOx/TiO2 catalysts of variable metal loading (0.01–2.8wt.% Cu) prepared by the equilibrium deposition filtration (EDF) method. It has been found that the EDF method results in the formation of highly dispersed copper(II) species, which interact strongly with the TiO2 surface and retain their characteristics upon heating at temperatures of 450°C. Results of photocatalytic performance tests obtained under simulated solar light irradiation show that glycerol is photo-reformed efficiently over CuOx/TiO2 catalysts toward H2 and CO2 with intermediate formation of acetone, acetaldehyde and formic acid. The rate of hydrogen production increases significantly with increase of Cu loading from 0.01 to ca. 1.0wt.% Cu and is enhanced by a factor of 10 with increase of initial glycerol concentration from 0.37mM to 1M. Results obtained with the use of low initial glycerol concentrations in solution show that CO2 evolves in the gas phase immediately after illumination whereas production of H2 exhibits a hysteresis and appears at the reactor effluent only after prolonged exposure to light. Under these conditions, the rates of both H2 and CO2 production were found to “fluctuate” with irradiation time. This behavior has been attributed to the light-induced periodic change of the oxidation state of deposited copper species and the different reactivity of these species for H2 and CO2 evolution. To our knowledge, this is the first example of a photocatalytic system that exhibits such dynamic characteristics.