Microkinetic Model Research Articles

Carbon-anchoring synthesis of Pt1Ni1@Pt/C core-shell catalysts for stable oxygen reduction reaction

Proton-exchange-membrane fuel cells demand highly efficient catalysts for the oxygen reduction reaction, and core-shell structures are known for maximizing precious metal utilization. Here, we reported a controllable “carbon defect anchoring” strategy to prepare Pt1Ni1@Pt/C core-shell nanoparticles with an average size of ~2.6 nm on an in-situ transformed defective carbon support. The strong Pt–C interaction effectively inhibits nanoparticle migration or aggregation, even after undergoing stability tests over 70,000 potential cycles, resulting in only 1.6% degradation. The stable Pt1Ni1@Pt/C catalysts have high oxygen reduction reaction mass activity and specific activity that reach 1.424 ± 0.019 A/mgPt and 1.554 ± 0.027 mA/cmPt2 at 0.9 V, respectively, attributed to the optimal compressive strain. The experimental results are generally consistent with the theoretical predictions made by our comprehensive microkinetic model which incorporates essential kinetics and thermodynamics of oxygen reduction reaction. The consistent results obtained in our study provide compelling evidence for the high accuracy and reliability of our model. This work highlights the synergy between theory-guided catalyst design and appropriate synthetic methodologies to translate the theory into practice, offering valuable insights for future catalyst development.

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory.

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.

Glucose oxidation on gold in alkaline solution: A DEMS and microkinetic modeling study

The mechanism and kinetics of the electrochemical oxidation of glucose on gold surfaces in 0.1 M NaOH solution are investigated by rotating disk electrode and differential electrochemical mass spectrometry coupled to microkinetic modeling. For this purpose, the glucose mass-transport parameters (solution viscosity and density, glucose diffusion coefficient) were determined in the relevant conditions. Koutecký–Levich analysis of rotating disk electrode current-potential curves revealed that the glucose oxidation reaction (GOR) follows a one-electron mechanism at 0.6 V vs RHE, while H2 production could be evidenced by DEMS in this potential range. This suggests that the glucose oxidation selectively produces gluconate at 0.6 V vs RHE through an electrochemical oxidative dehydrogenation (EOD) mechanism. A microkinetic model of the current-potential curves was developed, taking into account the hydrogen electrode reactions kinetics, the dissociative adsorption/desorption of glucose, and the glucose electrooxidation on gold surfaces. The dissociative adsorption of glucose was found to be the rate-determining step of the reaction. It is also revealed that the release of anodic H2 through the Tafel step is triggered by the consumption of the adsorbed reaction intermediates. The gluconate electrooxidation into further products has to be considered for potential above 0.7 V vs RHE.

A Comprehensive Investigation on Catalytic Behavior of Anaerobic Jar Gassing Systems and Design of an Enhanced Cultivation System

The rapid and reliable diagnosis of anaerobic bacteria constitutes one of the key procedures in clinical microbiology. Automatic jar gassing systems are commonly used laboratory instruments for this purpose. The most critical factors affecting the cultivation performance of these systems are the level of residual oxygen remaining in the anaerobic jar and the reaction rate determined by the Pd/Al2O3 catalyst. The main objective of the presented study is to design and manufacture an enhanced jar gassing system equipped with an extremum seeking-based estimation algorithm that combines real-time data and a reaction model of the Pd/Al2O3 catalyst. The microkinetic behavior of the palladium catalyst was modeled through a learning-from-experiment methodology. The majority of microkinetic model parameters were derived from material characterization analysis. A comparative validation test of the designed cultivation system was conducted using conventional gas pouches via six different bacterial strains. The results demonstrated high cell viability, with colony counts ranging from 1.26 × 105 to 2.17 × 105 CFU mL−1. The favorable catalyst facets for water formation on Pd surfaces and the crystal structure of Pd/Al2O3 pellets were identified by X-ray diffraction analysis (XRD). The doping ratio of the noble metal (Pd) and the support material (Al2O3) was validated via energy-dispersive spectroscopy (EDS) measurements as 0.68% and 99.32%, respectively. The porous structure of the catalyst was also analyzed by scanning electron microscopy (SEM). During the reference clinical trial, the estimation algorithm was terminated after 878 iterations, having reached its predetermined termination value. The measured and modelled reaction rates were found to converge with a root-mean-squared error (RMSE) of less than 10−4, and the Arrhenius parameters of ongoing catalytic reaction were obtained. Additionally, our research offers a comprehensive analysis of anaerobic jar gassing systems from an engineering perspective, providing novel insights that are absent from the existing literature.

Selective direct deoxygenation of m-cresol on Heusler alloy catalysts via precise control of electronic structure: An integrated density function theory and microkinetic modeling study

Directional designing a catalyst with high direct deoxygenation selectivity of Phenols during hydrodeoxygenation (HDO), remains a formidable challenge in cost-effectively converting biomass to bio-fuel and chemicals. Herein, we evaluated for the first time the catalytic performance of Heusler alloy surfaces for HDO process of m-cresol at the atomic scale. The direct deoxygenation (DDO) route of m-cresol on the Co2FeGa (110) surface at 573.15 K exhibited relatively low kinetic (1.20 eV) and thermodynamic (−0.04 eV) barriers, leading to high selectivity (near 100 %) for toluene production. The degree of rate control (DRC) analysis revealed that direct dehydroxylation constituted the rate-limiting elementary reaction. Additionally, by tailoring the Ga and Ge ratio on the Co2FeGa1-xGex (110) surface, an almost linear relationship (R2 = 0.95) was discovered between activation energy of C-OH and the d-band center energy of the Fe element in the unit cell. Both the activation energy of C-OH and d-band center energy of the Fe decreased with increasing Ge/Ga substitution ratio. This research unveils for the first time the catalytic performance of Heusler alloy for the selective direct deoxygenation of m-cresol, but also suggest the possibility tuning d-band center of Heusler alloys to precisely tailor the selectivity of DDO during HDO process.

Rational screening of metal single-atom-doped ZSM-5 to promote ring-opening cracking of cycloalkanes to light olefins

The ring-opening cracking of cycloalkanes to light olefins poses a formidable challenge due to the sluggish activation of the C–C bond. In this study, we realize the rational screening of various single metal doped M−ZSM−5 catalysts (M = Mn, Co, Ni, Cu, Zn, Zr, Ru, Rh, Pd, Ag, Pt) to promote the ring-opening cracking of cycloalkanes to light olefins. Our calculation results show that methyl cyclopentane, a model compound of cycloalkanes, tends to adsorb in the form of a “half-chair” configuration in the intersecting channels of the M−ZSM−5, which is caused by the drastic distortion of the five-membered ring, accompanied by large changes in the bond angles. Based on the host–guest interaction between the active site and hydrocarbon in ring-opening cleavage, we reveal that the binding energies of metals to H and C can respectively quantify the metal’s ability to activate C–H and C–C bonds in cycloalkanes. By solving the microkinetic model, we plot activity-selectivity TOF volcano diagrams, where Mn, Ru, Rh-ZSM-5 within the EH range of −2.5 to −3.5 eV and the EC range of −6 to −8 eV can efficiently induce the ring-opening reaction of cycloalkanes without excessive dehydrogenation leading to aromatic hydrocarbon formation. The calculation shows good agreement with catalytic experimental tests and in situ DRIFT spectra results, providing a series of potential candidate catalysts for the ring-opening cracking of cycloalkane-rich diesel fuel.

Heron: Visualizing and Controlling Chemical Reaction Explorations and Networks.

Automated and high-throughput quantum chemical investigations into chemical processes have become feasible in great detail and broad scope. This results in an increase in complexity of the tasks and in the amount of generated data. An efficient and intuitive way for an operator to interact with these data and to steer virtual experiments is required. Here, we introduce Heron, a graphical user interface that allows for advanced human-machine interactions with quantum chemical exploration campaigns into molecular structure and reactivity. Heron offers access to interactive and automated explorations of chemical reactions with standard electronic structure modules, haptic force feedback, microkinetic modeling, and refinement of data by automated correlated calculations including black-box complete active space calculations. It is tailored to the exploration and analysis of vast chemical reaction networks. We show how interoperable modules enable advanced workflows and pave the way for routine low-entrance-barrier access to advanced modeling techniques.

Fleeting-Active-Site-Thrust Oxygen Evolution Reaction by Iron Cations from the Electrolyte.

Oxygen evolution reaction (OER) is key to sustainable energy and environmental engineering, thus necessitating rational design of high-performing electrocatalysts that requires understanding the structure-performance relationship with a possible dynamic nature under working conditions. Herein, we uncover a novel type of OER mechanisms thrust by the fleeting active sites (FASs) dynamically formed on Ni-based layered double hydroxides (Ni-LDHs) by Fe cations from the electrolyte under OER potentials. We employ grand-canonical ensemble methods and microkinetic modeling to elucidate the potential-dependent structures of FASs on Ni-LDHs and demonstrate that the fleeting-active-site-thrust (FAST) mechanism delivers superior OER activity via the FAST intramolecular oxygen coupling pathway, which also suppresses the lattice oxygen mechanism, leading to improved operando stability of Ni-LDHs. We further reveal that introducing only trace-level loadings (10-100 ppm) of FASs on Ni-LDHs can significantly boost and govern the catalytic performance for OER. This underscores the crucial importance of considering the novel FAST mechanism in OER and also suggests the electrolyte as a key part of the structure-performance relationship as well as an effective design strategy via engineering the electrolyte.

Selective methanol production via CO2 reduction on Cu2O revealed by micro-kinetic study combined with constant potential model

High efficiency electrocatalysis of greenhouse gas CO2 to liquid fuel methanol attracts great attentions that allows energy transformation from renewable electric energy to storable chemical energy. Catalytically converting CO2 to methanol using green hydrogen is a promising pathway to achieve carbon neutrality. In this work, the catalytic activity and product selectivity of the electrochemical CO2 reduction reaction on Cu2O were studied using a micro-kinetic model based on Marcus charge transfer theory. The constant potential model under the grand-canonical ensemble enables the accurate description of binding energies and ground-state charge transfer upon surface adsorption as a function of electrode potential. Firstly, the potential-dependent activation energies and rate constants of all possible elementary reactions involved in CO2 reduction are calculated. Then, the surface coverage and its derivative can be obtained in a steady-state approximation. Finally, the total reaction rate and partial reaction rate can be deduced, allowing for direct comparison with catalytic activity (current density) and product selectivity (Faradaic efficiency). It is found that CH3OH is the dominant product on the Cu2O(111) surface, while CO is the main product on the Cu2O(110) surface. Based on the derived total and partial reaction rates under the steady-state approximation, it is demonstrated that Cu2O(111) exhibits the highest electrocatalytic activity and methanol selectivity at U = −0.6 V vs. SHE. This research presents a methodology for predicting apparent electrocatalytic performance from microscopic reaction energy calculations, which can be applied to other important electrocatalytic reaction mechanisms and performance predictions.

The Performance of Ethylene Partial Oxidation on Group IB Metal Stepped Surfaces

Group IB metal catalysts, particularly Ag, Au, and Cu, exhibit particular selectivity for ethylene oxide (EO) formation, while Ag demonstrating the highest performance so far. Previous studies have explored the EO formation mechanism on (100) and (111) surfaces of group IB metals, but the reaction mechanism on the (211) facet (analogous to the edge sites of a catalyst particle) remains poorly understood. Herein, we fill in the knowledge gap by analyzing ethylene partial oxidation to EO on the (211) surfaces of Ag, Au, and Cu through density functional theory (DFT) calculations, scaling relationship analysis, and microkinetic modeling. Our study demonstrates that the (211) surface decreases the energy barrier for the dissociation of oxygen molecules into oxygen atoms, while unfavorable for the production of EO. Therefore, we should preserve an appropriate concentration of (211) surface on the nanoparticles when designing catalysts for the ethylene epoxidation reaction.

Real-Time Simulation of the Reaction Kinetics of Supported Metal Nanoparticles.

A common issue with supported metal catalysts is the sintering of metal nanoparticles, resulting in catalyst deactivation. In this study, we propose a theoretical framework for realizing a real-time simulation of the reactivity of supported metal nanoparticles during the sintering process, combining density functional theory calculations, microkinetic modeling, Wulff-Kaichew construction, and sintering kinetic simulations. To validate our approach, we demonstrate its feasibility on α-Al2O3(0001)-supported Ag nanoparticles, where the simulated sintering behavior and ethylene epoxidation reaction rate as a function of time show qualitative agreement with experimental observation. Our proposed theoretical approach can be employed to screen out the promising microstructure feature of α-Al2O3 for stable supported Ag NPs, including the surface orientation and promoter species modified on it. The outlined approach of this work may be applied to a range of different thermocatalytic reactions other than ethylene epoxidation and provide guidance for the development of supported metal catalysts with long-term stability.

Can ZnO/Cu catalyst provide promising activity for glycerol direct dehydrogenation? A combined density functional theory and coverage-dependent microkinetics study

Non-oxidative dehydrogenation (NODH) reaction of glycerol is a perfect atom economical technical route to produce higher-value 1,3-dihydroxyacetone (DHA). Cu-based catalyst (especially ZnO/Cu system), is known as active species for alcohol dehydrogenation, which may be also promising one for glycerol NODH. In this study, we combine coverage-dependent free energy profile using first-principle calculation, and microkinetic model, to investigate the NODH of glycerol on the ZnO/Cu(111) surface, and a detailed comparison is made with Cu (111) surface. The coverage-dependent microkinetic model takes into account the lateral adsorbate–adsorbate self-interactions and cross-interactions, and their effect on binding energy of both intermediate and transition state. Besides, it guarantees the reaction kinetics is based on the coverage self-consistent between surface model and microkinetic result under practically reaction conditions. Compared with coverage-dependent kinetics simulation (20 %–30 %), coverage-independent model overestimates the DHA selectivity on Cu (111) (over 90 %). Our coverage-dependent kinetics simulation illustrates that both glycerol conversion and DHA selectivity are most determined by the first dehydrogenation step (OH bond scission) of glycerol on Cu (111). However, when ZnO cluster adsorbed on the Cu (111) surface, ZnO cluster promotes the electron transferring between glycerol and Cu sites, which accelerates OH bond scission process of glycerol. Under coverage-dependent microkinetic model, the turnover frequency and selectivity of DHA on ZnO/Cu(111) get much improvement compared with Cu (111). Finally, the superior of ZnO/Cu(111) is further proved by continuous stirred tank reactor simulation, where NODH of glycerol need shorter residence times or lower temperature to reach 100 % conversion, as well as keep higher DHA selectivity.

Unraveling the mechanism of CH3CH2OH dehydrogenation on m-ZrO2(111) surface, Au13 cluster, and Au13 cluster/m-ZrO2(111) surface: A DFT and microkinetic modeling study

The dehydrogenation of CH3CH2OH to produce CH3CHO and H2 is crucial for generating valuable chemicals. This study uses density functional theory (DFT) and microkinetic modeling to elucidate the reaction pathways on the m-ZrO2 (111) surface, Au13 cluster, and Au13 cluster/m-ZrO2(111) surface. Dehydrogenation on both the m-ZrO2(111) surface and Au13 cluster occurs via two key steps. The first step involves the cleavage of the OH bond in CH3CH2OH, forming a CH3CH2O moiety and an OH bond with the lattice oxygen on m-ZrO2 (111) surface or with a low-coordination Au atom in the Au13 cluster, respectively; while the formation of H2 takes place in the second step; however, the results microkinetic modeling render low values for the corresponding rate constants for this reaction path. Although the Au13cluster/m-ZrO2(111) surface introduces an additional step, where the H atom migrates from the m-ZrO2 (111) surface to the Au13 cluster, we shown that the relative energy of the three transition states is similar, the activation barriers are lower, and the rate constants are favorable for the dehydrogenation of CH3CH2OH. These results demonstrate the potential of the Au13 cluster supported on m-ZrO2(111) for efficient and selective CH3CHO and H2 production, providing valuable insights for advanced catalytic system design.

Uncertainty Quantification of Linear Scaling, Machine Learning, and Density Functional Theory Derived Thermodynamics for the Catalytic Partial Oxidation of Methane on Rhodium.

Accurate and complete microkinetic models (MKMs) are powerful for anticipating the behavior of complex chemical systems at different operating conditions. In heterogeneous catalysis, they can be further used for the rapid development and screening of new catalysts. Density functional theory (DFT) is often used to calculate the parameters used in MKMs with relatively high fidelity. However, given the high cost of DFT calculations for adsorbates in heterogeneous catalysis, linear scaling relations (LSRs) and machine learning (ML) models were developed to give rapid estimates of the parameters in MKM. Regardless of the method, few studies have attempted to quantify the uncertainty in catalytic MKMs, as the uncertainties are often orders of magnitude larger than those for gas phase models. This study explores uncertainty quantification and Bayesian Parameter Estimation for thermodynamic parameters calculated by DFT, LSRs, and GemNet-OC, a ML model developed under the Open Catalyst Project. A model for catalytic partial oxidation of methane (CPOX) on Rhodium was chosen as a case study, in which the model's thermodynamic parameters and their associated uncertainties were determined using DFT, LSR, and GemNet-OC. Markov Chain Monte Carlo coupled with Ensemble Slice Sampling was used to sample the highest probability density (HPD) region of the posterior and determine the maximum of the a posteriori (MAP) for each thermodynamic parameter included. The optimized microkinetic models for each of the three estimation methods had quite similar mechanisms and agreed well with the experimental data for gas phase mole fractions. Exploration of the HPD region of the posterior further revealed that adsorbed hydroxide and oxygen likely bind on facets other than Rhodium 111. The demonstrated workflow addresses the issue of inaccuracies arising from the integration of data from multiple sources by considering both experimental and computational uncertainties, and further reveals information about the active site that would not have been discovered without considering the posterior.

Integration of Facet‐Dependent, Adsorbate‐Driven Surface Reconstruction into Multiscale Models for the Design of Ni‐Based Bimetallic Catalysts for Hydrogen Oxidation

AbstractNi‐based bimetallic catalysts (NiM) show promise to replace expensive Pt‐based catalysts for hydrogen oxidation reaction (HOR). However, the effect of dopant and reaction conditions on the adsorbate‐driven surface reconstruction of NiM nanoparticles remains largely unexplored. Here, we use a multiscale modeling approach – integrating density functional theory, kubic harmonics interpolation, and microkinetic modeling – to investigate the interplay between dopant, reaction conditions, facet, adsorbate type, adsorbate coverage, in situ surface structure, and performance for NiM nanoparticles during HOR. Clear periodic trends appear in dopant effects on key adsorption energies, with dopants showing 7‐fold greater effects when located in the surface as compared to subsurface. Multi‐faceted nanoparticle models showed a non‐uniform correlation between O* coverage and surface reconstruction. HOR performance was facet‐dependent, with the highest performance occurring at reactive fronts formed between regions of high O* and OH* coverage. The nanoparticle averaged performance showed promotional effects for nearly all dopants compared to pure Ni, with the best‐performing dopants located preferentially in the surface layer (e. g. Au, Pd, Ag). Taken together, this work emphasizes the importance of understanding the interplay between reaction conditions, surface reconstruction, and HOR performance for NiM nanoparticles, enabling researchers to both predict and control the working nanoscale catalyst structure.

A DFT and microkinetic modeling study of pressure effects on electroreduction reduction of CO2 to ethanol

The CO2 electroreduction reaction (CO2RR) is a promising way to convert surplus renewable electricity into valuable low-carbon fuel. Ethanol is one of the most beneficial products because of its high energy density, ready transport, and extensive use. The CO2 partial pressure (PCO2) significantly impacts ethanol formation. This work explores the mechanism of pressure dependence in ethanol formation on the Cu2O/Cu(111) surface via the density functional theory (DFT) and microkinetic model calculations. DFT results show that *CO is the major adsorbate on the catalyst surface, and its coverage is the descriptor of pressure. Initially, increasing the coverage of *CO promotes the C–C coupling to ethanol, while the excess coverage shifts the potential determining step (PDS) from the *CHO formation to the *CHO–*CO addition, prejudicing the ethanol formation. The degree of selectivity control (DSC) results show that *CO stability positively affects ethanol production, and the effects decrease with the pressure increase. The stabilities of *H and *CH3CH2OH exhibit the negative effects, which increase with the pressure. They lead to the volcano plot of the pressure dependence on ethanol formation. The mechanistic insights gained from this work provide relevant guidelines to optimize the reaction pressure conditions for the CO2RR to ethanol reaction.

Mechanism of methanol synthesis from CO2 hydrogenation over Rh16/In2O3 catalysts: A combined study on density functional theory and microkinetic modeling

In this study, the hydrogenation of carbon dioxide (CO2) to methanol (CH3OH) over Rh16/In2O3 catalyst was studied through Density Functional Theory (DFT) and microdynamics modeling. The spontaneous dissociation mechanisms of H2 and CO2 adsorption at the Rh16/In2O3 interface were investigated. The oxygen vacancies in In2O3 enhanced the adsorption process. Bader charge analysis revealed a marginal positive charge on Rh16, elucidating the critical insights into the electronic characteristics and catalytic activity. The study established the RWGS+CO-Hydro pathway as the predominant mechanism for methanol synthesis, characterized by a sequential transformation of intermediates: CO2*→COOH*→CO*+OH*→HCO*→CH2O*→CH2OH*→CH3OH*. Furthermore, Degree of Reaction Rate Control (DRC) analysis conducted in the range of 373–873 K and 10–2 to 103 bar identified two principal kinetic phenomena: at lower temperature and higher pressure, the conversion of CO* + H* to HCO* significantly impacted the overall reaction rate. Conversely, at higher temperature, the step from CH2O* + H* to CH3O* was dominate.

Microkinetic Model to Rationalize the Lifetime of Electrocatalysis: Trade-off between Activity and Stability.

Electrocatalysts which can operate for several years are required to produce hydrogen and commodity chemicals in an environmentally friendly manner. However, designing materials with long operational lifetimes is challenging, due to the lack of a conceptual framework to predict catalytic lifetimes quantitatively. Here, we report a microkinetic equation which quantifies the lifetime of an electrocatalyst undergoing dissolution. This equation was obtained by taking advantage of the fact that catalysis is much faster than deactivation, which allows the ordinary differential equations to be solved via the quasi steady-state approximation. All chemical reactions were modeled as irreversible, first-order elementary reactions. Under this assumption, the catalytic rate correlates linearly with the deactivation rate, leading to a trade-off relationship between activity and stability. Our model was supported by the correlation between theoretical and experimental lifetimes (r2 = 0.86) of a manganese oxide electrocatalyst during the oxygen evolution reaction.

CO oxidation on IrO2(110) surfaces

We investigated the oxidation of CO on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed reaction spectroscopy (TPRS), density functional theory (DFT) calculations and microkinetic simulations. Adsorbed CO on the s-IrO2(110) surface generates CO and CO2 peaks near 545 K during TPRS, and only about 38 % of the CO oxidized to CO2 when the initial CO layer was saturated. Pre-adsorbed O-atoms, so-called on-top oxygen (Ot), promote the oxidation of CO adsorbed on IrO2(110). On the Ot-covered surface, CO oxidation by Ot atoms produces a CO2 TPRS peak at ∼370 K, and all of the initially adsorbed CO oxidizes to CO2 when the initial Ot coverage is greater than the CO coverage. In agreement with the TPRS results, DFT calculations predict that the barrier is about 100 kJ/mol lower for CO oxidation by an Ot atom than a lattice O-atom of IrO2(110). A microkinetic model, parameterized with energy barriers computed using DFT, accurately reproduces the CO and CO2 TPRS traces only after CO binding energies are lowered to values determined using a hybrid exchange-correlation functional and the barrier for CO molecules to fill bridging O-vacancies is lowered. The simulations predict that O-vacancies play an important role in mediating the CO oxidation kinetics on s-IrO2(110), and thereby demonstrate the importance of future spectroscopic studies aimed at characterizing the nature of the surface CO and O species involved in reaction. This study provides new insights for understanding CO oxidation on IrO2(110), and provides evidence that several elementary steps can be involved in governing this chemistry.

CO2 Hydrogenation on Ru Single-Atom Catalyst Encapsulated in Silicalite: a DFT and Microkinetic Modeling Study.

The critical levels of CO2 emissions reached in the past decade have encouraged researchers into finding techniques to reduce the amount of anthropogenic CO2 expelled to the atmosphere. One possibility is to capture the produced CO2 from the source of emission or even from air (i.e., direct air capture) by porous materials (e.g., zeolites and MOFs). Among the different usages of captured CO2, its conversion into light fuels such as methane, methanol, and formic acid is essential for ensuring the long-awaited circular economy. In the last years, single-atom catalysts encapsulated in zeolites have been considered to this purpose since they exhibit a high selectivity and activity with the minimum expression of catalytic species. In this study, a detailed mechanism composed by 47 elementary reactions, 42 of them in both forward and reverse directions and 5 of them that correspond to the desorption of gas products just forwardly studied), has been proposed for catalytic CO2 hydrogenation over Ru SAC encapsulated in silicate (Ru1@S-1). Periodic density functional theory (DFT) calculations along with microkinetic modeling simulations at different temperatures and pressures were performed to evaluate the evolution of species over time. The analysis of the results shows that carbon monoxide is the main gas produced, followed by formic acid and formaldehyde. The rate analysis shows that CO(g) is formed mainly through direct dissociation of CO2 (i.e., redox mechanism), whereas COOH formation is assisted by OH. Moreover, the Campbell's degree of rate control analysis suggests that the determining steps for the formation of CO(g) and CH2O(g) gas species are their own desorption processes. The results obtained are in line with recent experimental and theoretical results showing that Ru1 SACs are highly selective to CO(g), whereas few atom clusters as Ru4 increase selectivity toward methane formation.