Modeling of transient studies on the reaction kinetics over catalysts with lattice oxygen mobility: Dry reforming of CH4 over a Pt/PrCeZrO catalyst
Graphical abstract
Introduction
Transient methods for studying the catalytic reaction kinetics are known to be much more efficient than the conventional steady-state approaches in revealing some specific features of a reaction mechanism and providing information about the intrinsic reaction kinetics with considerable reduction of the number of experimental runs and time consumed. Development of the computational tools for transient kinetic investigation in combination with the experimental studies permits to obtain more information about the catalyst surface state and the reaction rates. As a result, the kinetic data required for the reactor design and process optimization are elucidated. These approaches have been applied successfully to the gas-solid systems where the catalytic properties are mainly controlled by the chemical transformations at the catalyst surface [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
The character of transients of a complex catalytic reaction depends on the processes that determine the alterations of the catalytic surface. Besides the chemical reaction stages, some physical processes such as the surface and bulk diffusion could influence the surface state. These processes have different time scales, and their impact on transient behavior of the system can be rather complex. A number of studies have revealed that for the oxide catalysts the bulk oxygen mobility can play an important role in the system dynamics [4], [10], [11], [12], [13], [14], [15], [16]. Near-surface oxygen mobility of solid catalysts strongly affects not only the character of transient regimes, but activity, selectivity, and stability of the catalysts performance because the surface state and, respectively, the reaction rates are determined both by transformations of the reactive surface species and the oxygen transport to the surface from the bulk layers of catalyst particle.
For example, the impact of oxygen mobility on the catalyst properties was studied for nanocrystalline Pt/LnCeZrO catalysts doped with rare-earth elements (Ln = Gd, Pr, La) in partial oxidation of methane, methane steam/dry reforming, and autothermal reforming of acetone into syngas [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Due to strong interaction of supported Pt with doped ceria-zirconia oxides, after pretreatment of catalysts in O2 it is mainly present as Pt2+ and Pt4+ cations (XPS data [18]) possessing a high ability to activate CH4 as revealed by TAP (Temporal Analysis of Products) [20] and transient [14], [16], [19], [20] studies. It was shown that the bulk oxygen mobility is important for stable performance of these catalysts in the frame of the bifunctional reaction mechanism. In this case, the fuel molecules are activated on the Pt sites producing CxHyOz species, while the molecules of oxidants dissociate on the partially reduced sites of the support producing reactive oxygen species. The latter rapidly migrate to Pt sites via the surface/near-surface diffusion and consume CxHyOz species thus producing syngas and preventing coking. The rates of near-surface (as well as along domain boundaries) or bulk oxygen diffusion estimated for doped ceria-zirconia oxides with supported Pt by analysis of the oxygen isotope exchange data/chemical transients are high enough (oxygen diffusion coefficients DV in the range of 10−12 ÷ 10−14 cm2/s at 600–700 °C [14], [15], [16], [24], [25], [26], [27]) to provide the required rates of oxygen transfer to the metal-support interface.
Experiments with single channels of honeycomb corundum substrate loaded with these active components (gas stream contacted only with the internal part of the channel) were carried out to elucidate dynamics of the catalysts interaction with reaction media, basic features of mechanism and kinetics of these reactions [16], [17], [19], [20], [21], [22], [23], [28], [29]. Similar to the well-known approach based on the annular reactor design [30], these experiments with single channels allowed to minimize the impact of heat- and mass transfer on kinetic transients, thus providing reliable data on the main features of reaction mechanism and kinetics. On the bases of these experiments, Pt/Pr0.3Ce0.35Zr0.35Ox active component was selected as the most promising for detailed studies due to a high lattice oxygen mobility required to prevent coking in CH4 dry reforming in realistic feeds.
This work aims at development of the mathematical description of catalytic gas-solid systems under unsteady conditions taking into account the lattice oxygen mobility and its application for the numerical investigation of transients of complex catalytic reaction. The quantitative estimation of the rates of lattice oxygen mobility and catalytic steps becomes possible when the mathematical model of the system behavior reflects the processes in the gas phase, on the catalyst surface, and in the catalyst bulk.
Modeling studies with software developed are performed for two cases. To understand the impact of lattice oxygen mobility on the transient system behavior, firstly the theoretical analysis of the process dynamics is fulfilled for red-ox reaction with a model kinetic scheme. Then the response regimes for CH4 dry reforming over Pt/CeZrPrO oxide catalyst are studied. The experimental data are analyzed by modeling, computational transient runs are performed to clarify the factors that control catalytic properties under unsteady conditions and evaluate the rates of bulk oxygen diffusion as well as main catalytic stages.
Section snippets
Mathematical description
The mathematical model was developed to describe dynamics of a catalytic reaction on the oxide catalyst in the flow reactor. The case under study is when one of the reagents in the gas mixture is oxidant, and the catalytic surface state along the length of the catalyst bed during kinetic transients is determined by the reaction proceeding on the active surface sites and the oxygen diffusion inside the near-surface layers of the catalyst.
The model is constructed on the following assumptions: (1)
Catalyst characteristics
Pr0.3Ce0.35Zr0.35Ox mixed oxide was prepared via Pechini route, and Pt was supported by the incipient wetness impregnation as described in [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. These samples were characterized by using XRD, TEM, neutronography, EXAFS, Raman, UV–Vis, O2 TPD, FTIRS of adsorbed CO and oxygen isotope exchange, their reactivity was estimated by H2 and CH4 TPR, pulse reduction by CO and CH4, reoxidation by pulses of O2 and CO2
Catalysts characteristics
For Pr0.3Ce0.35Zr0.35Ox mixed oxide nanocrystalline structure formed of plate-like domains with typical sizes in the range of 10–15 nm (thickness ∼ nm) was demonstrated by TEM, the most developed faces being of the (1 1 1) type. These domains are stacked into platelets with typical sizes up to 100 nm [14]. X-ray diffraction patterns correspond to single phase fluorite-like solid solution with the cubic structure. Neutron diffraction patterns are described by the tetragonal P42/nmc space group
Conclusion
The complex catalytic reaction occurring under unsteady conditions is appropriately described using a mathematical model and implementation of the software developed in this work. Processes accounted by the model include convective gas transport, reactions on the catalyst surface and diffusion of oxygen-containing species from the subsurface of support to the surface. A red-ox reaction model describes the dynamics of the catalytic surface reaction in the flow reactor. Oxygen mobility in
Acknowledgements
This work was conducted within the framework of budget projects No. 0303-2016-0013 and No. 0303-2016-0017 for Boreskov Institute of Catalysis. Support by the European Commission in the 7th Framework Programmes OCMOL (GA 228953) and BIOGO (GA 604296), Ministry of the High Education and Science of the Russian Federation, and SB RAS Interdisciplinary Integration Project no. 80 is gratefully acknowledged.
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