PhD Oral Qualifying Examination : Spectroscopic investigations in single-atom catalysed C1-conversion

Speaker Max Joshua Huelsey (Supervisor: Dr Yan Ning)

Host Department of Chemical and Biomolecular Engineering

Date/Time 12 Jun - 12 Jun, 1.00pm

Venue E5-02-32 , Faculty of Engineering, National University of Singapore

Synopsis

Research on single-atom catalysts (SACs), or atomically dispersed catalysts, has been quickly gaining momentum over the past few years. Although the unique electronic structure of singly dispersed atoms enable uncommon—sometimes exceptional—activities and selectivities for various catalytic applications, fundamental knowledge of single-atom catalysts relies vastly on quantum theoretical calculations. At the same time, the conversion of C1 building blocks such as CO, CH4 and CO2 lies in the centre of interest mainly due to the shale gas boom and the detrimental effects of CO2 and CH4 on our climate. Despite their small size, those molecules are inherently challenging to activate and thus new advances in catalysis science are crucial to tackle those emerging needs. Besides the CO oxidation reaction, small molecule activation on single-atom catalysts remains widely unexplored and the high dispersion render those catalysts ideal for (in-situ) spectroscopic investigations as supposedly all metal atoms contribute to the catalytic reaction whereas most atoms are buried below the surface in the case of nanoparticle catalysts.

In this proposal, I will present my approach to in-depth investigations of our current single-atom platforms. The first is based on a previously reported rhodium single-atom catalyst on phosphotungstic acid for the CO oxidation reaction. Due to the precise structure and extreme dispersion of isolated atoms on the heteropoly acid ‘islands’, massive changes in the X-ray absorption spectroscopy experiments can be observed during exposure to various gas atmospheres at different temperatures. We find evidence for a Mars-van Krevelen mechanism in the CO oxidation where the heteropoly acid donates one oxygen atom from an adjacent site to generate one molecule CO2. The reoxidation of the heteropoly acid only occurs at relatively high temperature and thus represents the rate-limiting step in this reaction – well in accordance with previous kinetic studies. We further elucidate the electronic state and structure of the catalyst under steady-state conditions, solidifying our understanding of the rate-determining step in the catalytic mechanism. Additionally, in-situ DRIFT spectroscopy consolidates our proposed reaction cycle.

Secondly, I would like to present preliminary data on the single-atom catalysed liquid-phase CO2 hydrogenation to formic acid. It appears that especially positively charged single-atom catalysts do not show a promising activity in the CO2 activation and hydrogenation. One strategy to overcome this limitation is by using single-atom alloys and indeed, promising activity was found and further investigated. On the longer term, we want to attempt the gas phase hydrogenation of CO2 to CO or preferably methanol. A combination of in-situ DRIFT, X-ray absorption and X-ray photoelectron spectroscopy will then reveal the surface coverage and electronic state of the catalyst under reaction conditions.

Although this thesis work is intended to be based primarily on the elucidation of active sites of single-atom catalysts and the changes in structure and oxidation state during reaction conditions, the synthesis of new catalysts must be targeted first as single-atom catalytic systems for C1 conversion are very rare so far. Based on the knowledge that we obtain from in-situ spectroscopy, we hopefully can be guided in the intelligent design of enhanced catalytic systems thus forming a fruitful synergy between synthesis and characterisation.

Biography