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Van Marum Colloquium: Catalyst Design at Extremely Small Sizes: From CO2 Reduction to Ammonia Production

Monday 22 July 2024
Gorlaeus Building
Einsteinweg 55
2333 CC Leiden


Electrocatalysis is a promising approach to address various environmental and energy challenges including greenhouse gas mitigation, renewable fuel generation, and electrifying hard-to-abate chemical sectors. Electrochemical CO2 reduction reaction (CO2RR), for instance, could contribute to lowering atmospheric CO2 levels while it utilizes intermittent renewable electricity to generate value-added chemical feedstock and fuels. The practical design of electrocatalytic systems, capable of effectively converting CO2 into desired products with high selectivity and at a high reaction throughput, depends on the development of efficient catalysts.

In the Electrocatalysis Lab at McGill University, we investigate catalysts with extremely small particle sizes. We are focused on Cu, In- and Sn-based catalysts with particle sizes from 0.5 nm to 200 nm. We observed intriguing switches in reaction selectivity both with changes in particle size and when catalysts were covered with an extremely thin layer of another metal. Our research is supported through extensive density functional theory (DFT) computations to determine the most stable configuration and crystallinity for particles at each size, and to calculate the reaction energetics. In addition, ex-situ, and in-situ X-ray absorption spectroscopy (XAS) were utilized to shed light on the catalyst structure and its evolution under the reaction conditions. We first observed that, decreasing the Cu particle size from 200 nm to less than 1 nm, changes the reaction selectivity of the CO2RR from ethylene to methane. Using Cu thin film with 0.5 nm thickness, we were able to produce methane with a maximum Faradaic efficiency of 85% and a maximum partial current density of 1.2 A/cm² in highly alkaline electrolytes.

On the other hand, we observed that while thick film of 200 nm Cu or In/Sn results in a selective production of ethylene or formate, respectively, adding a thin layer of 1 nm In/Sn on top of the thick Cu catalyst, changes the selectivity towards methane. This is counter intuitive, as one expects to observe either ethylene or formate with those catalysts, while the methane becomes the dominant product, with Faradaic efficiency surpassing 50%. This unique behavior suggests a significant impact of the interfacial electric field on the CO2RR pathway.

At the last part of this talk, I am going to discuss our observation of enhancing the nitrate reduction reaction (NO3RR) to ammonia over an almost inactive material such as nickel. By modulating its electronic structure through implementing a more negative element such as phosphorous (NiP), we first suppressed the hydrogen evolution reaction (HER). However, due to insufficient proton supply, nitrite selectivity increased instead of ammonia. We then reconstructed the surface to balance the electronic and local atomic properties, to provide sufficient proton for ammonia production instead of nitrite, while suppressing the HER. Consequently, an ammonia Faradaic efficiency of 89% was achieved along with a 2- and 8-fold enhancement in ammonia production rate per active site, compared to NiP and Ni, respectively.

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