September 26, 2014
Understanding interactions between CO2 reduction and H2 evolution
CO poisons Cu, but promotes Mo.
The hydrogen evolution reaction (HER) is one of the most fundamental and important electrocatalytic reactions. However, a precise understanding of the HER in a complex reaction environment is still lacking. In a recent paper, we show that besides the catalyst species and morphology, the presence of co-adsorbates can strongly effect the HER in a non-trivial manner. These sorts of co-adsorbates, like CO, can exist at high coverages during CO2/CO reduction, which leads to a strong interaction with the competition between reactions.
Specifically, we have studied the effect of CO adsorbates on the catalysis activity of HER by both theoretical and experimental methods. Generally, one thinks of CO as a poison to the HER process, and this readily explains experimentally observed delays in HER current observed in the presence of CO on Cu catalysts. However, the co-adsorbates have a dual effect. In the first effect, the CO blocks reaction sites, which causes a fractional decrease in the catalysts effectiveness. However, in a second effect, the CO changes the binding properties of the catalyst surface, fundamentally changing the catalytic activity of the remaining exposed sites. This effect can either act to poison or promote catalysis, and the energetic changes should result in exponential, rather than fractional, changes to current density. Our DFT calculation results suggest that coadsorbed CO on a transition metal surface during HER will weaken the binding strength between H and the surface. We can readily understand this by examining the volcano plot to the right: this leads to a shift of HER catalytic activity, which provides a relationship between H binding strength and the corresponding catalytic activity of different metal species. For metals on the right-hand side of the volcano plot, this will result in a lower HER activity; while for metals on the left-hand side, this will result in a higher HER activity. Therefore, metals on the right-hand side, such as Cu, will be experience solely a poisoning effect, while metals far on the left-hand side, like Mo, will exhibit a mixed effect of promotion (due to binding strength differences) and poisoning (due to site blocking). Following this, we performed electrochemical experiments on Cu and Mo, two representative metals from far left- and right-hand side, which gave a consistent result with the above theory.
This article suggests a dual effect of CO, a common co-adsorbate, on HER catalysis. It provides insight into the outstanding performance of copper and gold in CO2 reduction and suggests the right-hand region of the HER volcano may be more desirable when designing high-selectivity CO2 reduction catalysts. It also intriguingly suggests that, in the search for better HER catalysts, in certain situations one may want to deliberately ``poison'' the catalyst surface rather than scrupulously avoid it.
Read the article at ACS Catalysis.
