Lashina Elena A, Vinokurov Zakhar S, Saraev Andrey A, Kaichev Vasily V
Deparment of Catalyst Research, Boreskov Institute of Catalysis, Novosibirsk 630090, Russia.
J Chem Phys. 2022 Jul 28;157(4):044703. doi: 10.1063/5.0097251.
An experimental study of the catalytic oxidation of methane over Pd foil in a flow reactor revealed that regular temporal oscillations in the reaction rate can arise at atmospheric pressure under methane-rich conditions. CO, CO, H, and HO were detected as products. The oscillations of partial pressures of products and reactants in the gas phase were accompanied by oscillations of the catalyst temperature. According to an operando x-ray diffraction and mass-spectrometry study, the oscillations originate due to spontaneous oxidation and reduction of palladium; the high active catalyst surface is represented by metallic palladium, and the transition to the low-active state is accompanied by the formation of the PdO phase. In addition, it was detected that carbon dissolves in near-surface layers of palladium to form the PdC phase. To describe the oscillations in the oxidation of methane, a 17-step reaction mechanism and a corresponding kinetic model were developed. The mechanism considers direct dissociative adsorption of methane and oxygen, pyrolytic activation of methane, oxidation and reduction of palladium, and direct formation and desorption of CO, CO, H, and HO. Numerical solutions from the mathematical model of the continuously stirred-tank reactor qualitatively reproduce experimentally observed oscillatory dynamics. We have also developed a model, which considers the reversible diffusion of adsorbed oxygen and carbon atoms into the Pd bulk that allows us to explain the long induction period preceding the appearance of the oscillations. Mathematical modeling shows that the concentrations of dissolved oxygen and carbon atoms also oscillate under reaction conditions.
在流动反应器中对钯箔上甲烷催化氧化进行的一项实验研究表明，在大气压下富甲烷条件下，反应速率会出现有规律的时间振荡。检测到的产物有一氧化碳、二氧化碳、氢气和水。气相中产物和反应物分压的振荡伴随着催化剂温度的振荡。根据原位X射线衍射和质谱研究，振荡源于钯的自发氧化和还原；高活性催化剂表面由金属钯表示，向低活性状态的转变伴随着PdO相的形成。此外，检测到碳溶解在钯的近表面层中形成PdC相。为了描述甲烷氧化中的振荡，开发了一个17步反应机理和相应的动力学模型。该机理考虑了甲烷和氧气的直接解离吸附、甲烷的热解活化、钯的氧化和还原以及一氧化碳、二氧化碳、氢气和水的直接形成和解吸。连续搅拌釜反应器数学模型的数值解定性地再现了实验观察到的振荡动力学。我们还开发了一个模型，该模型考虑了吸附的氧和碳原子向钯体相的可逆扩散，这使我们能够解释振荡出现之前的长诱导期。数学建模表明，在反应条件下，溶解的氧和碳原子的浓度也会振荡。
