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Toluene oxidation on metal-oxide catalysts: Theoretical modeling

Removal of reactive organic species from the environment becomes a standard in modern society. In many reactions catalytic centers are used to enable reactions which would otherwise be energetically demanding. Apart from the activity of the catalyst, the material availability and the cost of production of the catalyst should also be taken into account. Here, several relatively inexpensive materials are investigated as potential catalysts for toluene degradation. Interaction of toluene in oxygen atmosphere has been investigated by modeling the most abundant fraction in several powder catalysts as investigated in toluene degradation experiments. The repeated unit cell has been formed from MnO2, Mn2O3, Fe2O3, NiO and CuO crystal slab consisting of approximately 1000 atoms from crystal surrounded by vacuum layer to which three or four toluene and oxygen molecules were added. For calculation, the reactive force-field method was used as implemented in the ReaxFF code developed and by Duin et al [1]. Although a semiempirical method, this code has shown considerable success in modeling binding, due to the implemented bond order changes depending on the interatomic distances [2]. Temperature controlled Berendsen thermostat (NVT) with damping constant of 0.1 fs is used for 2.5fs molecular dynamics (MD) calculation on the initially energy minimized crystal structure at 500K. Toluene is then added and additional 25ns MD calculation is performed (100000 steps of 0.25fs) at 500K. Additionally, calculations involving temperature rise from 500K to 700K, followed by 6.25 fs calculation at 700K and then temperature drop back to 500K were performed keeping all the other parameters as in constant temperature calculations. These were done in an attempt to model the much longer time available for reactions under experimental conditions more realistically. Unfortunately the experimental times of the order of 1s are not accessible to the MD model due to time and size limitations. Binding and/or toluene reactions are estimated by the net amount of free toluene molecules as a function of time. Structures investigated showed markedly different behavior which seems to be related to the reactivity of the oxide species as well as the toluene binding energy. The largest catalytic activity is in this investigation observed for Fe2O3 and MnO2, while CuO and Mn2O3 did not show any toluene adsorption or degradation during the simulation time. While Fe2O3 activity seems to be the result of large toluene binding energy (heat of formation of toluene-crystal surface complex), MnO2 structure additionally offers crystal surface oxygen (by means of an Eley- Rideal reaction mechanism) and show the contribution to the activity from surface dynamics and structure (including oxygen diffusion, defects, etc.) [3]. References 1) van Duin, A. C. T., Dasgupta, S., Lorant, F. & Goddard, W. a. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 105, 9396– 9409 (2001). 2) Senftle, T. P., Hong, S., Islam M., Kylasa, S. B., Zheng, Y., Shin, Y. K., Junkermeier C., EngelHerbert, R., Janik, M. J., Aktulga, H. M., Verstraelen, T., Grama, A. & van Duin, A. C. T.: The ReaxFF reactive force-field: development, applications and future directions. Computational Materials 2, 15011 (2016) 3) M. Misono (Ed.), Studies in Surface science and catalysis 176: Heterogenous catalysis of mixed oxides, Perovskite and heteropoly Catalysts, Elsevier, Amsterdam- Oxford 2013.

reaxff, md calculation, metal-oxide catalyst

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