Tuning Catalyst Electronic Structure & O2 Activity for OCM Reactions

Natural gas serves as an excellent bridge fuel as we transition to a low carbon economy. However, the high C-H bond strength and the directionality of the sp³ orbital in the methane molecule make it difficult for methane to form bonds with other species or catalytic surfaces. Hence, high temperatures and effective catalysts are required to activate and transform methane into useful chemicals, products, or electricity. [1]

Tuning heterogeneous catalytic activity by electrochemical methods provides enhanced control of catalyst redox state and nanocatalyst activity. Michaels and Vayenas [2] demonstrated the viability of electrochemical process for dehydrogenation of ethylbenzene to styrene. The presence of an electrical potential can dramatically alter the energy profile of the chemical reaction by lowering the activation barrier for the forward reaction. [1] The oxygen content of non-stoichiometric oxide catalysts can be controlled electrochemically, thus altering catalytic activity. We are developing electrochemical micro reactors to investigate oxidative coupling of methane reactions with better selectivity. By designing cell capacitance and resistivity, the reactor can be switched at high rates and this may change the methane conversion selectivity. The micro reactors are fabricated using ceramic additive manufacturing. We have made progress in the additive manufacturing process for lanthanum manganite, YSZ and Pt for use in the micro reactors. The ceramic additive manufacturing process includes characterization of the starting powder material, milling of powder to attain a uniform particle size, mixing with vehicle and thinner resins, characterization of the rheological properties and sintering of the printed material. The material is also characterized post-sintering and this helps in fine-tuning the sintering temperature parameters.

There is a large body of literature regarding the use of lanthanum perovskites as catalysts for the conversion of methane to CO. Due to their high activity and thermal stability, perovskites have been gaining importance as catalysts for hydrocarbon oxidation over active noble metals such as Pt and Pd which are expensive and do not resist operating temperatures above 850 K. Ciambelli et. al [3] conducted catalytic methane combustion experiments over AFeO₃ (A = La, Nd, Sm) and LaFe_1−xMg_xO₃ perovskites in a quartz down flow reactor electrically heated in a three zones tube furnace. Their tests showed that all AFeO₃ samples gave methane conversion with 100% selectivity to CO₂ below 973 K. The order of activity towards methane combustion was La > Nd > Sm. Further experiments involve the study of the thermodynamic stability and redox behavior of our printed materials using high temperature cyclic voltammetry techniques, heterogeneous conversion of methane to CO using our printed La(M)MnO_x as catalysts and printing and characterization of ceria and rare earth doped ceria films as the electrolyte/catalyst support. The heterogeneous catalysis experiments occur under both steady-state polarization conditions and high frequency switching, investigating the influence of the rapidly changing catalyst activity on the product species.

References:
(1) Gür, T. M. Energy Combust. Sci. 2016, 54, 1–64.
(2) Michaels, J. N.; Vayenas, C. G. J. Catal. 1984, 85 (2), 477–487.
(3) Ciambelli, P.; Cimino, S.; De Rossi, S.; Lisi, L.; Minelli, G.; Porta, P.; Russo, G. Appl. Catal. B Environ. 2001, 29 (4), 239–250.