Insights into the promotion effect of macrocycle molecule on HCOOH electro-oxidation

Abstract

Macrocycle molecules were found to be highly promotive to HCOOH electro-oxidation on Pt surface since 2007. But it is still not known how the macrocycle molecules induce the promotion effect. The difficulty is that it’s very hard to obtain the coverage of macrocycle molecules during the reaction. In this paper, the under-potential deposited hydrogen (UPD-H) is used as the probe to measure the coverage of macrocycle molecules on Pt surface. Subsequently, the kinetics of adsorption and the promotion effect of the macrocycle molecule (copper phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid (CuPcTsA)) is systematically studied. We find that the coverage of CuPcTsA increases very fast at the beginning 500s during the adsorption, and saturates after 1000s. Usually, the activity of CuPcTsA modified Pt electrode increases with the increasing coverage of CuPcTsA under a certain CuPcTsA concentration during the range 3–25μM. But this behavior follows different curves for different CuPcTsA concentrations. The selectivity (direct pathway to CO intermediate pathway) also increases with the increasing coverage. Surprisingly, the selectivity from different CuPcTsA concentrations follows a same curve. The order of the reaction selectivity is ∼1 and ∼3 for the CuPcTsA coverage lower and higher than 0.34. The desorption is not a simple reverse process of the adsorption, and has some memory of its initial state, where the desorption starts.