The electrochemical reduction of CO2 enables carbon-neutral fuels and chemicals to be produced using intermittent renewable electricity. Commercial implementation of this technology would facilitate EU policy goals associated with the mitigation of global climate change by providing a means of utilizing CO2 as an industrial feedstock. Unfortunately, monometallic Cu is the only electrocatalyst that exhibits a significant Faradaic efficiency for hydrocarbon and alcohol formation and its voltage efficiency is not sufficient to make the process economically viable.1 This issue is compounded by the fact that electrocatalyst discovery efforts over the past 30 years have failed to identify alternative materials for catalyzing this reaction, let alone with superior activity. However, these studies have revealed that the reduction of a CO intermediate is the rate-determining step of the reaction. Thus, the unique ability of Cu to catalyze CO2 reduction to hydrocarbons and alcohols has been attributed to its moderate CO adsorption energy, which is unique among transition metals.2 The unique CO adsorption energy of Cu is thought to be the result of its unique d-band structure.3 Thus, if the d-band structure of another metal could be modified to resemble Cu then it might also exhibit the ability to catalyze CO2 reduction to hydrocarbons and alcohols. Furthermore, systematic electronic structure tuning provides a viable route toward the discovery of electrocatalysts with superior activity. Such a discovery could enable this process to be economically viable. Electronic modifications of this magnitude can be induced through intermetallic bonding between electronically dissimilar metals in what are known as intermetallic alloys.4 Intermetallic alloys have received considerable interest as advanced catalysts for thermally catalyzed reactions but have not received concomitant interest in the electrocatalysis community. However, several Cu-free intermetallic alloys have been shown to exhibit Cu-like electronic structures, chemical reactivities, and catalytic activities for a variety of thermally catalyzed reactions, such as methanol synthesis and steam reforming.5
The CO2RR VALCAT project sought to investigate the role of systematic valence electronic structure modifications on the CO reduction activity of Cu-free intermetallic alloys. The goals of the project were to:
1. Identify the electrocatalyst properties required to catalyze CO reduction. The unique ability of Cu to catalyze CO reduction is likely a result of its unique valence electronic structure. This hypothesis was explored by synthesizing Cu-free intermetallic alloys with nearly identical valence electronic structures and measuring their electrocatalytic activity for CO reduction.
2. Demonstrate the impact of systematic electronic structure modifications on the CO reduction activity of transition metals. The d-band portion of the valence band density of states of a transition metal can be systematically modified via the formation of a strong heteronuclear bonds with electronically dissimilar metals, such as those found in intermetallic alloys. We will investigate the extent to which surface reactivity can be tuned through systematic d-band modifications and will demonstrate the impact such modifications have on catalytic activity and selectivity.
3. Discover novel electrocatalysts for CO reduction with superior activity to Cu. The strong heteronuclear bonds characteristic of intermetallic alloys result in periodic structural order. Co-locating different metal sites results in the formation of catalytically active motifs that can enhance the stability of transition states though bidentate binding to metal sites with disparate chemical reactivity, as are present in many enzymes.
References
1. Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 14 (1985).
2. Peterson, A. A.; Nørskov, J. K. J. Phys. Chem. Lett. 3 (2012).
3. Nilsson, A.; Pettersson, L. G. M.; Hammer, B.; Bligaard, T.; Christensen, C. H.; Nørskov, J. K. Catal. Lett. 100 (2005).
4. Bligaard, T.; Nørskov, J. K. Electrochimica Acta 52 (2007).
5. Iwasa, I.; Masuda, S.; Ogawa, N.; Takezawa, N. Appl. Catal. A 125 (1995).
