Mechanisms of Simultaneous Hydrogen Production and Formaldehyde Oxidation in H₂O and D₂O over Platinized TiO₂

Abstract

The simultaneous photocatalytic degradation of formaldehyde and hydrogen evolution on platinized TiO2 have been investigated employing different H2O/D2O mixtures under oxygen-free conditions using quadrupole mass spectrometery (QMS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The main reaction products obtained from the photocatalytic oxidation of 20% formaldehyde were hydrogen and carbon dioxide. The ratio of evolved H2 to CO2 was to 2/1. The HD gas yield was found to be dependent on the solvent and was maximized in a H2O/D2O mixture (20%/80%). The study of the solvent isotope effect on the degradation of formaldehyde indicates that the mineralization rate of formaldehyde (CO2) decreases considerably when the concentration of D2O is increased. On the basis of the ATR-FTIR data, the formaldehyde in D2O is gradually converted to deuterated formic acid during UV irradiation, which was confirmed by different band shifting. An additional FTIR band at 2050 cm–1 assigned to CO was detected and was found to increase during UV irradiation due to the adsorption of molecular CO on Pt/TiO2. The results of these investigations showed that the molecular hydrogen is mainly produced by the reduction of two protons originating from water and formaldehyde. A detailed mechanism for the simultaneous hydrogen production and formaldehyde oxidation in D2O is also presented.