An Investigation of the Reaction Mechanism of the Direct Electrochemical Propylene Oxidation to Propylene Oxide with Electrochemical Mass Spectroscopy

Abstract

Propylene oxide is a high-value precursor in the chemical industry with a large variety of applications and a market growing 4-5% annually. Currently, more than 10% of the propylene is used for the production of propylene oxide. The traditional methods for producing propylene oxide are chlorohydrin and hydroperoxide methods. These methods are costly, environmentally pollutant, and inefficient with a high proportion of by-products[1]. Renewable energy sourced direct electrocatalytic propylene oxidation to propylene oxide stands out as a way to replace the traditional methods and overcome the mentioned disadvantages. However, there has been a very limited number of studies about this topic in the last few decades starting from the 1960s. The experimental systems in these studies were significantly different from each other in terms of electrochemical cells and test methods, temperature, reactants, catalysts, etc[2–7]. Additionally, due to the limitation of available characterization and product detection methods the conclusions could not be well established. The majority of the performed studies suffered from difficulties in detecting and accurately quantifying propylene oxide due to its high volatility and hydrolysis to propylene glycol. Likewise, recent results from Winiwarter et al. revealed that there is propylene glycol formation on Pd catalysts at high potentials above 1.0 V vs. RHE in 0.1 M HClO4 and the selectivity towards dehydrogenation products decreases however, due to the lack of sensitive and fast product analysis method they could not quantify the propylene oxide even though they could detect it [8]. Moreover, there is only a limited number of studies investigating the reaction mechanism for electrochemical propylene oxidation towards propylene oxide. The theoretical study done by Li et al. proposed propylene oxide formation on PdO2(110) at 1.23 V vs RHE where the oxygen vacancies formed on the catalyst therefore they suggest that propylene reacts with lattice oxygen. Additionally, they suggest the competing reaction pathway to epoxidation as dehydrogenation of –CH3 results in acrolein, CO, and CO2[9]. Contrarily, Otsuka suggested a tentative Langmuir-Hinselwood mechanism for electrochemical propylene oxidation to propylene oxide[10]. In conclusion, the reaction mechanism for electrochemical propylene epoxidation on Pd is still not well understood. Therefore, we focused on discovering the reaction mechanism since it is crucial to design a rational catalyst. In this work, we are investigating in-situ electrocatalytic propylene oxidation to propylene oxide on different catalysts, starting from Pd. The unique electrochemical mass spectroscopy device that we are using enables in-situ detection and quantification of propylene oxide and also competing dehydrogenation pathway products acrolein and CO2 proposed by the theoretical study of Li et al.[9]. In addition to that, this device enables us to detect these products with great time resolution, so we are able to see the differences in product distribution very quickly when we change the applied potential. With these measurements, we offer an insight into the reaction pathway towards the electrochemical formation of propylene oxide and extend our investigations to other catalysts besides Pd. We believe that this study will help to understand the reaction mechanism for different catalysts leading to a great impact on rational catalyst design for electrochemical propylene oxidation to propylene oxide. [1] T.A. Nijhuis, M. Makkee, J.A. Moulijn, B.M. Weckhuysen, Ind. Eng. Chem. Res., 45, (2006), doi:10.1021/ie0513090. [2] J.O.M. Bockris, H. Wroblowa, E. Gileadi, B.J. Piersma, Trans. Faraday Soc., 61, (1965), doi:10.1039/tf9656102531. [3] T.C. Chou, J.C. Chang, Chem. Eng. Sci., 35, (1980). [4] K. Scott, C. Odouza, W. Hui, Chem. Eng. Sci., 47, (1992),doi:10.1016/0009-2509(92)87158-M. [5] V.M. Schmidt, E. Pastor, J. Electroanal. Chem., 401, (1996), doi:10.1016/0022-0728(95)04299-7. [6] H. Wise, L.L. Holbrook, J. Catal., 298, (1975). [7] G.R. Stafford, Electrochim. Acta., 32, (1987),doi:10.1016/0013-4686(87)80024-5. [8] A. Winiwarter, L. Silvioli, S.B. Scott, K. Enemark-Rasmussen, M. Sariç, D.B. Trimarco, P.C.K. Vesborg, P.G. Moses, I.E.L. Stephens, B. Seger, J. Rossmeisl, I. Chorkendorff, Energy Environ. Sci., 12, (2019), doi:10.1039/c8ee03426e. [9] H. Li, C.S. Abraham, M. Anand, A. Cao, J.K. Nørskov, J. Phys. Chem. Lett., 13, (2022), doi:10.1021/acs.jpclett.2c00257. [10] K. Otsuka, T. Ushiyama, I. Yamanaka, K. Ebitani, J. Catal., 157, (1995),doi:10.1006/jcat.1995.1310.