Tuning CO2 to CO Conversion on Metal-Doped Carbon Catalysts

Abstract

One of the significant challenges faced during electrochemical CO2 reduction (ECO2RR) is the low selectivity of the products obtained. The best example is polycrystalline Cu, which can electrochemically produce hydrocarbons and alcohols but with poor selectivity.1, 2 To date, high selectivity has been achieved only towards CO and formate on Au and Sn surface, respectively but scalability and commercialization are limited owing to their cost and stability. Metal-nitrogen-carbon (MNC) materials are comparable to Au or Ag catalysts, albeit with their lower overpotentials, higher mass activity and high selectivity toward CO.3 , 4 During the pyrolysis of N-containing carbon molecules, different chemical functionalities such as pyridinic, pyrrolic and graphitic N atoms can be formed, each behaving as a product-specific active site. 5 It is still debatable whether pyrdinic or pyrrolic N is responsible for CO2RR activity; thus, it remains unclear how to favor CO2 reduction over hydrogen evolution.In this work, we aimed to synthesize C2N-like covalent organic frameworks, with 8-10 Å pore size, tailored to host dual atoms coordinated to nitrogen. Such sites are expected to favor C-C coupling and produce multi-carbon products during ECO2RR. A step-by-step synthetic approach was employed to optimize the CO2RR:HER ratio through: Tuning carbon:nitrogen ratio using two different synthetic approaches.Reducing nitrogen in the carbon matrix through pyrolyzing at different temperatures (700, 800 and 900 oC),Doping phosphorus and nitrogen into the carbon matrixTuning CO2RR product distribution through different metals (Fe and Ni) doping. EXAFS and STEM studies revealed the presence of a mixture of single and dual atomic sites. XPS and ICP results showed that Fe and Ni loadings <2 wt% could be obtained in the C2N materials. Higher hydrogen evolution, owing to the higher N-content, was observed at a lower pyrolysis temperature which decreased at a pyrolysis temperature of 900 oC. Both Fe-NC and Ni-NC selectively produced CO, thus suppressing FE H2 to <5%. Fe-NC also exhibited lower overpotential for CO production compared to its Ni-NC counterpart. A small percentage of ethanol and 2-propanol (FE<5%) was observed indicating the favoured C-C coupling on the dual-atomic sites. Further optimization of the metal-N coordination environment may allow for an improved selectivity for >C2 products. References K. P. Kuhl, E. R. Cave, D. N. Abram and T. F. Jaramillo, Energy Environ. Sci., 5, 7050 (2012). S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo and I. Chorkendorff, Chem. Rev., 119, 7610 (2019). A. S. Varela, N. Ranjbar Sahraie, J. Steinberg, W. Ju, H.-S. Oh and P. Strasser, Angew. Chem. Int. Edit., 54, 10758 (2015). A. S. Varela, W. Ju, A. Bagger, P. Franco, J. Rossmeisl and P. Strasser, ACS Catal., 9, 7270 (2019). Y. Lum and J. W. Ager, Nat. Catal., 2, 86 (2019).