The electrocatalytic reduction of carbon dioxide to methane, one of the possible paths towards reducing greenhouse gas concentration and generating fuels sustainably, is a challenging reaction to catalyse as it can yield several other partially reduced compounds. Therefore, designing effective catalysts for the CO2-to-CH4 transformation requires an in-depth mechanistic understanding of the reaction conditions. Recently, a Cu(ii) phthalocyanine complex showed attractive activity and selectivity properties. Weng et al. now report an in situ study and show that this complex undergoes a morphological transformation that is responsible for the high activity recorded.
The researchers carry out X-ray absorption spectroscopy, cycling the electrochemical potential between the open circuit voltage (~0.80 V) and the voltage where the maximum catalytic activity occurs (–1.06 V). They observe the appearance of Cu(i) and then Cu(0) peaks. The peaks disappear as the potential is cycled back to less reducing conditions. Morphological analysis and theoretical calculations show the presence of Cu–Cu metallic bonds and the formation of Cu clusters of ~2 nm at –1.06 V. It is likely that these clusters are stabilized by the phtalocynanine ligands. Weng et al. therefore conclude that the superior catalytic performance of the copper complex is due to the reversible formation of the Cu(0) cluster.