Microkinetic Investigation of the Transient Methanation of Carbon Dioxide on Ni Catalysts

Der Power-to-Gas (PtG) Prozess bietet die Chance erneuerbare Energien in Form von synthetischem Erdgas zu speichern. Im PtG-Prozess wird zunächst H2 mittels Wasserelektrolyse erzeugt, welches anschließend mit CO2 an einem Ni-Katalysator zu CH4 umgesetzt wird. Die fluktuierend anfallenden erneuerbaren Energien erzwingen einen dynamischen Betrieb, was die Entwicklung von toleranten Katalysatoren und Reaktoren erfordert, welche effizient unter den Bedingungen arbeiten. Reaktoren für die CO2 -Methanisierung werden mithilfe geeigneter Reaktormodelle ausgelegt, wobei stationäre kinetische Ansätze verwendet werden, bei denen alle Schritte eines Mechanismus in einer analytischen Gleichung zusammengefasst werden. Die Simulation des periodischen Betriebs eines mikrostrukturierten Reaktors auf der Grundlage einer solchen Kinetik zeigt eine starke Variation des Temperaturprofils und der Produktivität. Um die transienten Phänomene auf der Katalysatoroberfläche korrekt zu beschreiben ist es erforderlich Mikrokinetiken zu verwenden, wobei jeder Schritt des Reaktionsmechanismus berücksichtigt wird. Die Entwicklung einer Mikrokinetik für die CO2 -Methanisierung auf Ni-Katalysatoren erfolgt mit einer Kombination aus ab-initio Berechnungen und experimentellen Methoden. Zunächst werden verschiedene Ni/SiO2 und Ni/γ-Al2O3 Katalysatoren hergestellt und in dynamischen Methanisierungsexperimenten auf ihre Aktivität untersucht. CO2 zeigt vielfältige Wechselwirkungen mit den Ni-Katalysatoren, welche durch temperaturprogrammierten Desorptionsexperimenten (TPD) untersucht wurden. Diese TPD-Experimente zeigen, dass das Ni/SiO2 System für weitere kinetische Untersuchungen verwendet werden muss, da CO2 an basischen Zentren auf dem γ-Al2O3 Träger adsorbiert. Die Ni-Kristalle auf dem Katalysator bestehen aus einer Vielzahl an Kristallflächen, wohingegen mikrokinetische Modelle üblicherweise nur für eine einzelne Ebene ermittelt werden. Das Desorptionsspektrum von CO2 wurde mit einem mikrokinetischen Modell, basierend auf ab-initio Parametern und unter Berücksichtigung der Form eines realen Ni-Kristalls, reproduziert. Die Zusammensetzung des Ni-Kristalls aus den vier wichtigsten Ebenen wurde anhand einer Wulff-Konstruktion bestimmt. Kinetischen und thermodynamische Parameter des Models wurden mit Dichtefunktionaltheorie (DFT) Methoden berechnet. Die Unsicherheiten in den ab-initio Parametern werden in einer globalen Unsicherheitsanalyse bis zu den Simulationsergebnissen fortgepflanzt. Kombinationen an Parametern werden identifiziert, welche die Experimente mit guter Genauigkeit wiedergeben können. Diese Untersuchung zeigt, dass sich das TPD-Profil aus den einzelnen Kristallflächen zusammensetzt. Ni(111) trägt signifikant zum Desorptionsprofil bei und wird für die Entwicklung einer Mikrokinetik der CO2 -Methanisierung verwendet. Mikrokinetiken werden nicht von Hand mit DFT-Berechnungen erstellt, sondern mithilfe des "Reaction Mechanism Generator (RMG)", einer Software zur automatischen Konstruktion von Reaktionsnetzwerken. Dadurch werden alle möglichen Reaktionspfade berücksichtigt und der Mechanismus ist frei von den Erwartungen des Forschenden. Aufgrund der beträchtlichen Unsicherheit in den DFT-basierten Parametern wird diese direkt in der Generierung der Mechanismen berücksichtigt. 5000 mögliche Methanisierungsmechanismen werden in dem Bereich der Unsicherheiten generiert und analysiert. Alle erzeugten Mechanismen werden mit dynamischen Experimenten aus einem differentiellen Festbettreaktor und Berty-Reaktor in einem multiskalen Modell verglichen. Es existierten Kombinationen von ab-intio Modellparametern, welche die dynamischen Experimente mit bemerkenswerter Genauigkeit beschreiben können. Die Einbindung von Unsicherheiten in die automatische Generierung von Mechanismen und der multiskalen Modellierung liefert tiefgreifende Einblicke in den Reaktionsmechanismus der CO2 -Methanisierung auf Ni(111).Production of CH4 in the Power-to-Gas (PtG) process offers the chance to store renewable energies while producing sustainable natural gas. In the PtG process, the renewable energies are used to produce H2 via water electrolysis, which is subsequently converted with CO2 to CH4 over a Ni catalyst. The fluctuating nature of the renewable energy source imposes a transient operation, requiring the design of tolerant catalysts and reactors that can efficiently function under these conditions. CO2 methanation reactors are designed with reactor models, where closed-form rate expressions are used, which lump all elementary steps of a mechanism into an analytical equation. The simulation of the periodic operation of a microstructured reactor with such kinetics predicts ample variation in the temperature profile and productivity. This highlights the necessity for accurate and predictive kinetic approaches to describe the transient behavior of the methanation catalyst. Transient phenomena on the catalyst surface can only be quantified by detailed microkinetic models, where each elementary step of the methanation mechanism is considered. The investigation of the microkinetics for the CO2 methanation on Ni is accomplished with a combination of ab-initio electronic structure calculations and experimental methods. First, Ni/SiO2 and Ni/γ-Al2O3 catalysts were produced and screened for activity in transient methanation experiments to determine suitable catalysts for the development of the microkinetics. The interaction of CO2 with supported catalysts is challenging and was investigated with temperature-programmed desorption (TPD) experiments. These TPD experiments show that the Ni/SiO2 catalyst needs to be used because CO2 interacts with basic sites on the γ-Al2O3 support, overshadowing the interaction of CO2 with the Ni crystal. The Ni crystals on the support consist of a variety of exposed crystal facets, whereas microkinetic models are usually derived for a single crystal facet. This gap was bridged by comparing CO2 -TPD experiment with a first-principles-based microkinetic model considering the combination of the four most important Ni facets via a Wulff construction of the crystal. Energetic properties of the microkinetic model were derived with state-of-the-art density functional theory (DFT) methods. Propagation of the uncertainty in the DFT-derived parameters to the output of the model in a global uncertainty analysis revealed feasible sets with reasonable agreement with the data. This combination of experiments and multiscale modeling reveals that the multiple desorption peaks can be attributed to desorption from different Ni facets. Ni(111) contributes considerably to the desorption profile and is further considered in the development of a full microkinetic model for the CO2 methanation. Microkinetic models are not created by hand with DFT, but by using the Reaction Mechanism Generator (RMG), a software for the automated construction of reaction networks. Therefore, it is ensured that all the possible methanation chemistry is considered and the discovered reaction mechanism is not biased. Uncertainty quantification is directly included in the mechanism generation procedure because of the considerable uncertainty in DFT-derived parameters. 5,000 possible mechanisms within the uncertainty range were generated and analyzed. All generated microkinetics were compared to transient methanation experiments from a differential fixed-bed and a Berty reactor in a multiscale modeling approach. Feasible sets of ab-initio model parameters exists, which can describe the experiments with remarkable accuracy. This approach identifies the limitations of current DFT methods in elucidating the mechanism. The combination of uncertainty quantification in automated mechanism generation and multiscale modeling provides deep insights into the CO2 methanation mechanism on Ni(111)

Non-idealities in lab-scale kinetic testing: a theoretical study of a modular Temkin reactor

The Temkin reactor can be applied for industrial relevant catalyst testing with unmodified catalyst particles. It was assumed in the literature that this reactor behaves as a cascade of continuously stirred tank reactors (CSTR). However, this assumption was based only on outlet gas composition or inert residence time distribution measurements. The present work theoretically investigates the catalytic CO2 methanation as a test case on different catalyst geometries, a sphere, and a ring, inside a single Temkin reaction chamber under isothermal conditions. Axial gas-phase species profiles from detailed computational fluid dynamics (CFD) are compared with a CSTR and 1D plug-flow reactor (PFR) model using a sophisticated microkinetic model. In addition, a 1D chemical reactor network (CRN) model was developed, and model parameters were adjusted based on the CFD simulations. Whereas the ideal reactor models overpredict the axial product concentrations, the CRN model results agree well with the CFD simulations, especially under low to medium flow rates. This study shows that complex flow patterns greatly influence species fields inside the Temkin reactor. Although residence time measurements suggest CSTR-like behavior, the reactive flow cannot be described by either a CSTR or PFR model but with the developed CRN model

Modeling the dynamic power-to-gas process: coupling electrolysis with CO2 methanation

The dynamic operation of a power-to-gas plant powered by wind energy is theoretically studied by coupling an empirical model of an alkaline water electrolyzer with a 1D heterogeneous model of a methanation reactor. H 2 produced by the electrolyzer follows the wind power profile, but operation in the part-load range can raise safety concerns. The dynamically generated methane quality comes close to the required value for injection into the gas grid, if the stoichiometric ratio is controlled. To satisfy the gas quality at all times, it is necessary to design a more tolerant reactor

NaWuReT Colloquium: From PhD Student to Assistant Professor – Early Career Chemical Engineers in Academia

The Nachwuchs Reaktionstechnik (NaWuReT) are early-career scientists from the ProcessNet Division Reaction Engineering. In autumn 2021, they organized an online colloquium with international early-career scientists from the chemical engineering community. Five guests were invited to give a scientific talk and provide insights into their career paths. The guests gave advice and emphasized the main challenges and opportunities during their early careers. Crucial points are networking, guidance, mentoring, as well as funding acquisition and the personal work-life balance

Spray-dried Ni catalysts with tailored properties for CO2 methanation

A catalyst production method that enables the independent tailoring of the structural properties of the catalyst, such as pore size, metal particle size, metal loading or surface area, allows to increase the efficiency of a catalytic process. Such tailoring can help to make the valorization of CO2 into synthetic fuels on Ni catalysts competitive to conventional fossil fuel production. In this work, a new spray-drying method was used to produce Ni catalysts supported on SiO2 and Al2O3 nanoparticles with tunable properties. The influence of the primary particle size of the support, different metal loadings, and heat treatments were applied to investigate the potential to tailor the properties of catalysts. The catalysts were examined with physical and chemical characterization methods, including X-ray diffraction, temperature-programmed reduction, and chemisorption. A temperature-scanning technique was applied to screen the catalysts for CO2 methanation. With the spray-drying method presented here, well-organized porous spherical nanoparticles of highly dispersed NiO nanoparticles supported on silica with tunable properties were produced and characterized. Moreover, the pore size, metal particle size, and metal loading can be controlled independently, which allows to produce catalyst particles with the desired properties. Ni/SiO2 catalysts with surface areas of up to 40 m2 g−1 with Ni crystals in the range of 4 nm were produced, which exhibited a high activity for the CO2 methanation

Automated Generation of Microkinetics for Heterogeneously Catalyzed Reactions Considering Correlated Uncertainties

The study presents an ab-initio based framework for the automated construction of microkinetic mechanisms considering correlated uncertainties in all energetic parameters and estimation routines. 2000 unique microkinetic models were generated within the uncertainty space of the BEEF-vdW functional for the oxidation reactions of representative exhaust gas emissions from stoichiometric combustion engines over Pt(111) and compared to experiments through multiscale modeling. The ensemble of simulations stresses the importance of considering uncertainties. Within this set of first-principles-based models, it is possible to identify a microkinetic mechanism that agrees with experimental data. This mechanism can be traced back to a single exchange-correlation functional, and it suggests that Pt(111) could be the active site for the oxidation of light hydrocarbons. The study provides a universal framework for the automated construction of reaction mechanisms with correlated uncertainty quantification, enabling a DFT-constrained microkinetic model optimization for other heterogeneously catalyzed systems

Linking Experimental and Ab-initio Thermochemistry of Adsorbates with a Generalized Thermochemical Hierarchy

Enthalpies of formation of adsorbates are crucial parameters in the microkinetic modeling of heterogeneously catalyzed reactions, since they quantify the stability of intermediates on the catalyst surface. This quantity is often computed using density functional theory, as more accurate methods are computationally still too expensive, which means that derived enthalpies have a large uncertainty. In this study, we propose a new error cancellation method to compute the enthalpies of formation of adsorbates more accurately from DFT through a generalized connectivity-based hierarchy. The enthalpy of formation is determined through a hypothetical reaction that preserves atomistic and bonding environments. The method is applied to a dataset of 60 adsorbates on Pt(111) with up to 4 heavy (non-hydrogen) atoms. Enthalpies of formation of the fragments required for the bond balancing reactions are based on experimental heats of adsorption for Pt(111). Thus, the proposed methodology creates an interconnected thermochemical network of adsorbates that combines experimental with ab-initio thermochemistry in a single thermophysical database

Spray-dried Ni catalysts with tailored properties for CO2 methanation

A catalyst production method that enables the independent tailoring of the structural properties of the catalyst, such as pore size, metal particle size, metal loading or surface area, allows to increase the efficiency of a catalytic process. Such tailoring can help to make the valorization of CO2 into synthetic fuels on Ni catalysts competitive to conventional fossil fuel production. In this work, a new spray-drying method was used to produce Ni catalysts supported on SiO2 and Al2O3 nanoparticles with tunable properties. The influence of the primary particle size of the support, different metal loadings, and heat treatments were applied to investigate the potential to tailor the properties of catalysts. The catalysts were examined with physical and chemical characterization methods, including X-ray diffraction, temperature-programmed reduction, and chemisorption. A temperature-scanning technique was applied to screen the catalysts for CO2 methanation. With the spray-drying method presented here, well-organized porous spherical nanoparticles of highly dispersed NiO nanoparticles supported on silica with tunable properties were produced and characterized. Moreover, the pore size, metal particle size, and metal loading can be controlled independently, which allows to produce catalyst particles with the desired properties. Ni/SiO2 catalysts with surface areas of up to 40 m2 g−1 with Ni crystals in the range of 4 nm were produced, which exhibited a high activity for the CO2 methanation

Automatic Mechanism Generation Involving Kinetics of Surface Reactions with Bidentate Adsorbates

New features have been added to the open-source Reaction Mechanism Generator (RMG) that enhance its ability to handle multidentate adsorbates. New reaction families and improved thermophysical estimation routines have been added, based upon ab-initio data from 26 reactions involving CxOyHz bidentate adsorbates with two heavy atoms on Pt(111). Non-oxidative dehydrogenation of ethane over Pt(111) is used as a case study to demonstrate the effectiveness of these new features. RMG not only discovered the pathways from prior literature, but it also uncovered new elementary steps involving abstraction reactions. Various mono- and bimetallic catalysts for this process were screened using linear scaling relations within RMG, where a unique mechanism is generated for each catalyst. These results are consistent with prior literature trends, but they add additional insight into the rate determining steps across the periodic table. With these additions, RMG can now explore more intricate reaction mechanisms of heterogeneously catalyzed processes for the conversion of larger molecules, which will be particularly important in fuel synthesis