Article pubs.acs.org/IECR
Microkinetic Modeling of Ethane Total Oxidation on Pt Nageswara Rao Peela,† Jonathan E. Sutton,† Ivan C. Lee,‡ and Dionisios G. Vlachos*,† †
Department of Chemical and Biomolecular Engineering, Center for Catalytic Science and Technology, and Catalysis Center for Energy Innovation, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States ‡ Sensors and Electron Devices Directorate, US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States S Supporting Information *
ABSTRACT: Catalytic total oxidation is important in several applications. However, associated models are rather empirical. In this study, a microkinetic model is developed for ethane total oxidation, under fuel-lean conditions on a Pt catalyst using input from density functional theory and Brønsted−Evans−Polanyi linear free energy relations. Reaction orders and the apparent activation energy estimated from the model are in good agreement with experimental values. The inclusion of oxygen coverage effects on the activation of ethane changes the rate-determining step from thermal dehydrogenation to oxidative dehydrogenation of ethane. A significant portion (30%) of the reaction flux proceeds via oxygen insertion reactions to C2 hydrocarbons (CH2C*).
1. INTRODUCTION Total oxidation of hydrocarbons over catalysts is encountered in various applications, including pollution abatement,1−4 partial oxidation,5 and portable power generation in combination with thermoelectrics.6 Noncatalytic (thermal) combustion of smaller hydrocarbons requires high temperatures, which limit potential applications, exhibits rather narrow flammability limits, and results in emissions of CO and NOx. Catalytic combustion is an attractive alternative since it overcomes the limitations of homogeneous combustion. As such, it has widely been studied experimentally, with Pt exhibiting the best activity and stability for hydrocarbons larger than methane.1,2,7−11 Despite a large number of experimental studies devoted to this chemistry, the reaction mechanism is still not well understood. Kinetic models are largely limited to power law7,11−13 and, in a few cases, Langmuir−Hinshelwood ones.14,15 These empirical kinetic models have inherent limitations as summarized in the review by Salciccioli et al.16 In contrast, microkinetic modeling is a powerful tool to understand a reaction mechanism and interpret experimental data,16,17 provide insights into catalyst selection16 and design of reactors.18,19 Unlike its predecessors, microkinetic model can be predictive. Reliable parametrization of the microkinetic model is though essential in ensuring reliable predictions. Ethane total oxidation has been studied experimentally by several researchers on metal wires, foils and supported catalysts2,11,12,20−22 over noble and non-noble metals.2,23 The apparent activation energy of this reaction on alumina supported Pt catalyst is in the range of 16−20 kcal/mol23,24 and on Pt foils is slightly higher, 23−27 kcal/mol.11,12,22 Most of the above-mentioned values are derived from power law kinetics or ignition temperature measurements. Initial C−H bond scission is often the rate-determining step (RDS) in hydrocarbon cracking. Less is known about the RDS in catalytic oxidation; yet, recent experimental work on methane oxidation proposed that the RDS is also C−H bond activation at relatively high oxygen partial pressures.25 In general, the role of oxygen in activating hydrocarbons remains © 2014 American Chemical Society
rather elusive. On the basis of DFT data alone, Chen and Vlachos reported thermal dehydrogenation of methane as the RDS of methane oxidation and steam reforming.26 However, adsorbed O and coverage effects may have a substantial effect on the kinetics but our understanding of these issues is rather limited. In the microkinetic model of Deshmukh and Vlachos on methane oxidation, the dominant pathway to form CO2 entails complete dehydrogenation to form C and finally insertion oxidation to CO and then CO2,27 with the dissociative adsorption of methane being the RDS. In theoretical studies of ethane oxidation, the thermal dehydrogenation was the RDS.28,29 This first generation of microkinetic models was rather limited, for example, models missed elementary steps (in some examples numerous reactions) and their parameters were either fitted to experimental data and lacked physical significance or estimated via the rather empirical bond-order conservation method. The limitations are exacerbated from recent experimental work of Iglesia and co-workers who proposed three regimes in methane20,25 and ethane20 oxidation, with oxidative dehydrogenation (ODH) being the RDS in one of them. In view of discrepancies in pathways reported in literature, there is a need to develop a microkinetic model that includes coverage effects. In this study, we develop a microkinetic model for ethane total oxidation on Pt at relatively high oxygen concentrations and provide insights into the mechanism. The parameters of important steps are estimated using DFT and the rest using first-principles based semiempirical method (FPSEM).We also study the effect of oxygen coverage on the RDS. To the best of our knowledge, this is the first first-principles based microkinetic model for ethane total oxidation. Received: Revised: Accepted: Published: 10051
January 31, 2014 May 29, 2014 May 30, 2014 May 30, 2014 dx.doi.org/10.1021/ie5004587 | Ind. Eng. Chem. Res. 2014, 53, 10051−10058
Industrial & Engineering Chemistry Research
Article
2. METHODS 2.1. Density Functional Theory (DFT) Calculations. The energetics used in this work were obtained using the SIESTA DFT code30 with Troullier−Martins norm-conserving scalar relativistic pseudopotentials.31 A double-ζ plus polarization (DZP) basis set and the Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation (GGA) functional32 were utilized with a mesh cutoff of 200 Ry. All slabs were four layers thick, with the two bottom layers frozen in their bulk positions. Metal slabs were separated by a 15 Å of vacuum to avoid interactions between neighboring cells. A Pt lattice constant of 4.02 Å was utilized.26 A 2 × 2 unit cell of Pt(111), with a 5 × 5 × 1 Monkhorst−Pack mesh, was utilized for most of the calculations. On the basis of preliminary MKM results, the thermal and ODH of ethane were found to be important reactions (as identified by sensitivity analysis) under the reaction conditions used. Therefore, vibrational frequencies (Table S1 of the Supporting Information (SI)) were calculated, in a 3 × 3 unit cell and a 5 × 5 × 1 Monkhorst−Pack mesh, for ethane adsorbate and for the transition states (at 0 and 0.67 ML O coverage) for these two sensitive reactions in order to obtain temperature corrected activation energies. While calculating the vibrational frequencies, the O atoms on the surface are also relaxed. This kind of hierarchical refinement, performing more accurate calculations for only the sensitive reactions, reduces considerably computational time.16 We included the zero-point energy and finite temperature corrections for the stable species in the model. To reduce the already substantial computational burden of parametrizing such a large kinetic model, we have not included these corrections for insensitive reactions, as these corrections would have a negligible effect on the model responses. The transition states (TS) were searched using a constrained optimization scheme.33−35 More details can be obtained from our previous study.36 DFT calculations were also used to estimate the adsorbate−adsorbate interactions and are shown in Supporting Information Table S2. The effect of O coverage on the barriers of O2* dissociation reaction and on CH4 ODH reaction were taken from the literature.25 According to Chin et al.,25 the O2 dissociation barrier is very low (
