Abstract
First-principles kinetic Monte Carlo (1p-kMC) simulations for CO oxidation on two RuO2 facets, RuO2(110) and RuO2(111), were coupled to the computational fluid dynamics (CFD) simulations package MFIX, and reactor-scale simulations were then performed. 1p-kMC coupled with CFD has recently been shown as a feasible method for translating molecular scale mechanistic knowledge to the reactor scale, enabling comparisons to in situ and online experimental measurements. Only a few studies with such coupling have been published. This work incorporates multiple catalytic surface facets into the scale-coupled simulation, and three possibilities were investigated: the two possibilities of each facet individually being the dominant phase in the reactor, and also the possibility that both facets were present on the catalyst particles in the ratio predicted by an ab initio thermodynamics-based Wulff construction. When lateral interactions between adsorbates were included in the 1p-kMC simulations, the two surfaces, RuO2(110) and RuO2(111), were found to be of similar order-of-magnitude in activity for the pressure range of 1 × 10-4 bar to 1 bar, with the RuO2(110) surface-termination showing more simulated activity than the RuO2(111) surface-termination. Coupling between the 1p-kMC and CFD was achieved with a lookup table generated by the error-based modified Shepard interpolation scheme. Isothermal reactor scale simulations were performed and compared to two separate experimental studies, conducted with reactant partial pressures of ≤0.1 bar. Simulations without an isothermality restriction were also conducted and showed that the simulated temperature gradient across the catalytic reactor bed is <0.5 K, which validated the use of the isothermality restriction for investigating the reactor-scale phenomenological temperature dependences. The approach with the Wulff construction based reactor simulations reproduced a trend similar to one experimental data set relatively well, with the (110) surface being more active at higher temperaures; in contrast, for the other experimental data set, our reactor simulations achieve surprisingly and perhaps fortuitously good agreement with the activity and phenomenological pressure dependence when it is assumed that the (111) facet is the only active facet present. The active phase of catalytic CO oxidation over RuO2 remains unsettled, but the present study presents proof of principle (and progress) toward more accurate multiscale modeling from electrons to reactors and new simulation results.
| Original language | English |
|---|---|
| Pages (from-to) | 5002-5016 |
| Number of pages | 15 |
| Journal | ACS Catalysis |
| Volume | 8 |
| Issue number | 6 |
| DOIs | |
| State | Published - Jun 1 2018 |
Funding
We set the relative surface areas of the (110) and (111) facets in the reactor to 16% (111) based on a published ab initio Wulff construction that was calculated with the intention of comparing to the Rosenthal data set.70 We have chosen this ratio as Wulff construction thermodynamic calculations revealed that under the high temperature oxidative pretreatment by Rosenthal et al.,45 the particle surface would be expected to expose this ratio. This finding is supported by the good agreement between simulated particle shapes and electron microscopy images. In contrast, Aßmann et al.44 employed a commercial catalyst without the same pretreatment and thus have increased odds of exposing different facets or different ratios of facets relative to the Wulff construction. The total mass of catalyst (including the inert diluent) in the reactor and the particle diameters were set to values in line with experiment (particle diameters of 5 and 10 μm for the Aßmann et al. and Rosenthal et al. works, respectively).44,45 We set the specific surface area to 1 m2/g for the Rosenthal model (equivalent to the experimental value) and to 0.39 m2/g for the Aßmann model (this was 30% of the experimental value and was chosen to match the conversion at 2:1 CO:O2 feed and 423 K). This discrepancy could be due to the amount of actual bulklike RuO2 in the reactor. The Rosenthal et al. experiments45 were conducted with well-characterized bulk-like microscale polycrystalline RuO2 generated by high-temperature oxidative treatment of a commercial catalyst. On the other hand, the Aßmann et al. experiments44 employed commercial material directly in the reactor.
Keywords
- computational fluid dynamics
- interpolation
- kinetic Monte Carlo
- kmos
- mfix
- multiscale
- ruthenium
- ruthenium oxide