Kinetic studies on photoreduction of methyl orange at semiconductor-electrolyte interfaces in SiC and TiO{sub 2} suspensions

We have performed kinetic photo-experiments on aqueous suspensions of SiC and TiO{sub 2} powders using the reduction of methyl orange as a probe. An effective combination of these materials is expected to provide a larger range of redox potentials for oxidative and reductive photoprocesses when compared to the redox potentials provided by either of these individual materials. We have found that etching SiC with 50% HF solution for a period of two hours is necessary before any photoreduction occurs. Investigation of SiC powders with transmission electron microscopy has confirmed that the HF-treatment removes an amorphous SiO{sub 2} layer from the surface. The initial rate of photoreduction of methyl orange (to hydrazine) is found to decrease with increasing pH (for the range 4.4 - 9.0) This observation can be related to the relative unavailability of a specific form of methyl orange at higher pH. The photoreduction of methyl orange by SiC is slower by an order of magnitude than that by TiO{sub 2}. Nevertheless, it can be used to remove Cr{sup +6} from aqueous solution in the presence of Na{sub 2}EDTA. Our observations indicate that the redox processes on SiC most likely occur via processes similar to those known for TiO{submore » 2}.« less