Kinetics Of Heterogeneous Reactions Research Articles

Basics of the reaction kinetics on metallic alloys.

Kinetics of heterogeneous catalytic reactions are often complicated by various factors, and from the perspective of statistical physics the development of the corresponding models is frequently challenging especially in the case of practically important alloy catalysts. To extend the basics in this area, I use a generic kinetic model of N_{2} formation inthe NO reaction with such species as CO or H_{2}. The focus is onthe reaction on the surface of a bimetallic alloy with low integral fraction of one of the metals so that the alloy is formed only at the surface. The main goal is to illustrate the specifics of the reaction kinetics and to clarify the accuracy of various approximations which are often inevitable for the analysis of such systems. The key results are as follows. (i) In the baseline one-metal case with the Langmuir-type equations, the model predicts the existence of the optimal adsorbate binding energy corresponding to the maximal reaction rate. (ii) With suitable substitution of the symbols, the equations employed for a uniform surface [item (i)] can be used for the mean-field description of reaction on the surface of a random alloy. In particular, the maximum in the dependence of the reaction rate on the binding energy is converted to the maximum with respect to the alloy composition. (iii) The reaction kinetics on the random-alloy surface have been described exactly. The corresponding maximum in the reaction rate is lower and somewhat smeared compared to that predicted in the mean-field approximation for an alloy or for the optimal monometallic catalyst. (iv) The reaction kinetics have been scrutinized also in the case of two-dimensional (2D) segregation of metal atoms at the surface in the limits of slow and rapid adsorbate diffusion between the spots. The kinetics are shown to be qualitatively different in these limits and also compared to the random-alloy case. (v) The thermodynamic criteria for 2D segregation of metal atoms at the surface have been derived as well.

(Keynote) Unified Quantum Theory of Electrochemical Kinetics Based on Coupled Ion-Electron Transfer

A general theory of coupled ion–electron transfer (CIET) is presented, which unifies quantum kinetics of electron transfer (ET) with classical kinetics of bond-breaking ion transfer (IT) in a general framework of non-equilibrium thermodynamics [1]. In the limit of large reorganization energy, the theory predicts Marcus kinetics of “electron-coupled ion transfer” (ECIT). In the limit of large ion-transfer energies, the theory predicts generalized Butler–Volmer kinetics of “ion-coupled electron transfer” (ICET), where the charge transfer coefficient and exchange current are connected to microscopic properties of the electrode/electrolyte interface. In the ICET regime, the reductive and oxidative branches of Tafel’s law are predicted to hold over a wide but finite range of overpotentials, bounded by the ion-transfer energies for oxidation and reduction, respectively. The probability distribution of transferring electron energies in CIET smoothly interpolates between a shifted Gaussian distribution for ECIT (as in the Gerischer–Marcus theory of ET) to an asymmetric, fat-tailed Meixner distribution centered at the Fermi level for ICET. The latter may help interpret asymmetric line shapes in x-ray photo-electron spectroscopy (XPS) and Auger electron spectroscopy (AES) for oxidized metal surfaces in terms of shake-up relaxation of the ionized atom and its image polaron by ICET. In the limit of large overpotentials, the theory predicts a transition to inverted Marcus ECIT, leading to a universal reaction-limited current for metal electrodes, dominated by barrierless quantum transitions. Uniformly valid, closed-form asymptotic approximations are derived that smoothly transition between the limiting rate expressions for ICET and ECIT for metal electrodes, using simple but accurate mathematical functions. The theory is applied to lithium intercalation in lithium iron phosphate (LFP) and found to provide a consistent description of the observed current dependence on overpotential, temperature and concentration, learned from x-ray images [2]. CIET theory thus provides a critical bridge between quantum electrochemistry and electrochemical engineering.[1] M. Z. Bazant, Unified quantum theory of electrochemical kinetics by coupled ion-electron transfer, Faraday Discussions 246, 60-124 (2023).[2] H. Zhao, H. D. Deng, A. E. Cohen, J. Lim, Y. Li, D. Fraggedakis, B. Jiang, B. D. Storey, W. C. Chueh, R. D. Braatz, and M. Z. Bazant, Learning heterogeneous reaction kinetics from X-ray movies pixel-by-pixel, Nature 621, 289-294 (2023). Figure 1

Intensification of valorization of cooked rice water through energy-efficient synthesis of drop-in biofuel (butyl levulinate): engine performance, emission profile, and environmental impact assessments.

For the first time, an energy-efficient and eco-friendly technology for the conversion of abundantly available kitchen waste, specifically waste cooked rice water (WCRW) to drop-in- biofuels, namely, butyl levulinate (BL), has been explored. The synthesis of BL was accomplished employing butyl alcohol (BA) and WCRW in an energy-efficient UV (5W each UVA and UVB)-near-infrared (100W) irradiation assisted spinning (120rpm) batch reactor (UVNIRSR) in the presence of TiO2-Amberlyst 15 (TA15) photo-acidic catalyst system (PACS). The optimal 95.81% yield of BL (YBL) could be achieved at 10 wt% catalyst concentration, 60°C reaction temperature, 80min time, and 1:10 WCRW: BA concentration as per Taguchi statistical design. Moreover, additional combination of different PACS such as TiO2-Amberlyst 16, TiO2-Amberlyst 36, and TiO2-Amberlite IRC120 H rendered 86.72% YBL, 90.04% YBL, and 93.47% YBL, respectively, proving superior efficacy compared to individual activity of the acidic catalysts and photocatalysts. The heterogeneous reaction kinetics study for TA15 PACS suggested Langmuir-Hinshelwood model to be the best fitted model. A significant 63.33% energy could be saved by UVNIRSR as compared to conventional heated reactor at the optimized experimental condition using PACS TA15. An overall alleviation in environmental pollution with 59.259% reduction in GWP, 15.254% decline in terrestrial ecotoxicity, 18.238% diminution in marine ecotoxicity, 17.25% decrease in ozone formation affecting human health, 5.865% reduction in human non-carcinogenic toxicity, 18.65% diminution in ozone formation affecting terrestrial ecosystem, 55.17% significant decrease in terrestrial acidification, and 25.619% mitigation in fresh water ecotoxicity could be observed. Furthermore, BL-biodiesel-diesel blends (3% BL, 7% biodiesel, and 90% diesel) exhibited significant reduction (25.45% and 36%, respectively, for CO and HC) in harmful engine exhaust emissions demonstrating environmental sustainability of the overall process.

Catalytic kinetics in nanoconfined space of acidic micro/mesoporous materials

Kinetics of heterogeneous acid catalyzed reactions is considered for micro-mesoporous materials taking into account nonuniformity of active sites, located in different pores. The methodology for deriving rate equations is developed accounting for reactions in single site nanopores, in macropores/external surfaces of crystals, in narrow nanopores with Gemini sites, as well as in more wider nanoconfined pores with limited pore occupancy. Contribution of such nanoconfined pores was considered using the acid site density as a kinetic descriptor for activity and selectivity. The rate equation wasderived for a mechanism with two kinetically significant steps,assuming thatthethe intermediate sizepores accommodateonly two molecules.The kinetic treatment accounts for different reactivity of acid sites and lateral interactions.Modification of zeolites to form mesopores was discussed from the kinetic viewpoint highlighting how such treatment influences activity and selectivity.Finally, the reactant and product shape selectivity is discussed in a quantitative way.

Application of the kinetic polynomial idea to describe catalytic hydrogenation of propene

AbstractThe kinetics of heterogeneous catalytic reactions is a topic of theoretical and practical importance that combines theoretical and experimental efforts to achieve a deeper insight into the process. Theoretical aspects are concerned with determination of the process mechanism, whereas in practical applications kinetic experiments are applied to assist reactor design and scaling up of various processes. These approaches overlap; basis of the assumed mechanism that consists of many elementary steps, it is possible to find a kinetic equation for which precision is verified by comparison with experimental data. The method most often applied requires finding a single step that has the strongest influence on the process rate. This “classical approach” fails if the rate of two or more steps has comparable values, the precision of the determined kinetic rate becomes only average or even low. Such accuracy was observed, among others, for the gas‐phase hydrogenation of propene. The reaction is easy to carry out and proceeds under mild conditions; the byproducts are not observed. It suggests that there cannot be a single dominating effect step on the process rate. In this work, the application of the polynomial kinetic idea to the gas‐phase hydrogenation of the propene process realized in practice is tested. An attempt of obtaining a handy and precise relationship, without insignificant parameters was made. To realize this, the theoretical form of the polynomial kinetic was derived, and then, using statistical analysis of estimated polynomial parameters, the kinetic relationship was simplified. The final version of the kinetic polynomial and some selected kinetic equations taken from the literature were compared with respect to precision. The differences were significant: the precision of anticipation of the kinetic rate by the polynomial kinetic was 5% higher than for the power law and 12% higher than for the LHHW kinetic.

Heterogeneous Electron Transfer with Redox Active Insulating Solid

For more than a century, electron transfer kinetics at electrode/electrolyte interface has been studied for dissolved redox active species at conductors (heterogeneous) or amongst dissolved redox species (homogeneous). However, a third major case is almost unexplored: dissolved redox species exchanging charge with redox active insulators. The electron transfer with redox active insulators such as Lithium peroxide (Li2O2), Lithium sulfide (Li2S), Sulfur (S8), Lithium carbonate (Li2CO3) are salient feature and central difficulty of next generation ‘beyond intercalation’ batteries such as metal-air (O2), metal-CO2 and metal-sulfur (Li-S) batteries.Redox mediation is the key process in electrochemistry with insulators. Mediators shuttle charge between the electron-conducting electrode and the surface of the insulating storage material to drive the discharge or charge reaction by reducing or oxidizing the surface of the redox active insulator. Mediation is now widely accepted to be key to high efficiency, rate capability, and lifetime. It may on the one hand involve external mediators that are added as an electrolyte additive and which have been studied in a wide variety for metal-O2 cells and a few examples for metal-sulfur. On the other hand, there is inherent mediation, where soluble intermediates act as mediators, such as lithium polysulfides in lithium-sulfur batteries. However, surprisingly little attention has been devoted to quantitatively assessing heterogeneous reaction kinetics between mediator and storage material. Work so far clearly shows that the kinetics of this electron transfer step is overall rate limiting and decisive to understand and to control reaction mechanisms towards improving energy efficiency, rate capability, capacity, and cycle life, which underscores the urgency to measure them and to identify governing factors.Here we systematically assess heterogeneous electron transfer at redox active insulators and identify governing factors. Particular systems we present to investigate are twofold. First, mediated kinetics of (su)peroxide oxidation. Our previous work has strikingly discovered decreasing electron transfer rate as the driving force (mediator redox potential) increases beyond a certain value, which represents a first example of Marcus inverted-region behaviour for heterogeneous reactions1. We are leveraging the predictive value of this finding regarding possibilities to impact maximum kinetics and the occurrence of a separate kinetic parabola for singlet oxygen evolution. Second, mediated kinetics of S8 reduction and Li2S oxidation2. Major tools to assess kinetics include following species concentration over time using UV-Vis-NIR spectroscopy and Scanning Electrochemical Microscope (SECM). Doing so requires deconvoluting spectra of mixtures of mediator oxidation/reduction states even if pure substance spectra cannot be obtained. Particularly, obtaining kinetics in S chemistry requires assessing dynamic equilibria of multiple species, for which we develop chemo-metrics tools. The results afford detailed insights into kinetics, thermodynamics, and reaction mechanisms, allowing them to influence in an informed way.References K. Petit, E. Mourad, C. Prehal, C. Leypold, A. Windischbacher, D. Mijailovic, C. Slugovc, S. M Borisov, E. Zojer, S. Brutti, O. Fontaine, S. A. Freunberger, Nature chemistry, 13, (2021)465–471 (https://doi.org/10.1038/s41557-021-00643-z)Cao, X. Shen, A. Wang, F. Yu, Y. Wu, S. Shi, S. A. Freunberger, Y. Chen, Nature Catalysis, 5, (2022) 193–201 (https://doi.org/10.1038/s41929-022-00752-z)

Heterogeneous catalysts, reaction kinetics, and reactor designs for methanol production from carbon dioxide: A critical review

AbstractThe process of preparing methanol from carbon dioxide is one of the ways to solve the environmental problems caused by greenhouse gases, the problem of fossil energy depletion, and the problem of fuel exhaust emissions. However, after decades of development, the process of preparing methanol from carbon dioxide is still unable to achieve large‐scale industrialization. This critical review sharply points out the problems and obstacles that need to be solved urgently on the industrialization road of carbon dioxide methanol preparation process and summarizes its progress. The problems faced by the methanol production industry from carbon dioxide are first the thermodynamic constraints, second the lower reaction rate and conversion effect, and finally the energy loss. In order to solve these problems, this paper first introduces the active sites, structural effects, and dynamic changes of copper‐based catalysts, as well as the unique reaction mechanism and doping modification of indium‐based catalysts. Zinc and zirconium promoters and some metal oxide supports can form unique interactions with active components to improve the catalytic performance of the catalyst. Next, various kinetic models and applications of methanol production from carbon dioxide are summarized, which is an important bridge linking laboratory and industrialization; the advantages and disadvantages of fixed bed reactor and paddle reactor were compared. Finally, the full text is summarized and prospected.

Phase Behavior and Viscosity in Biomass Burning Organic Aerosol and Climatic Impacts.

Smoke particles generated by burning biomass consist mainly of organic aerosol termed biomass burning organic aerosol (BBOA). BBOA influences the climate by scattering and absorbing solar radiation or acting as nuclei for cloud formation. The viscosity and the phase behavior (i.e., the number and type of phases present in a particle) are properties of BBOA that are expected to impact several climate-relevant processes but remain highly uncertain. We studied the phase behavior of BBOA using fluorescence microscopy and showed that BBOA particles comprise two organic phases (a hydrophobic and a hydrophilic phase) across a wide range of atmospheric relative humidity (RH). We determined the viscosity of the two phases at room temperature using a photobleaching method and showed that the two phases possess different RH-dependent viscosities. The viscosity of the hydrophobic phase is largely independent of the RH from 0 to 95%. We use the Vogel-Fulcher-Tamman equation to extrapolate our results to colder and warmer temperatures, and based on the extrapolation, the hydrophobic phase is predicted to be glassy (viscosity >1012 Pa s) for temperatures less than 230 K and RHs below 95%, with possible implications for heterogeneous reaction kinetics and cloud formation in the atmosphere. Using a kinetic multilayer model (KM-GAP), we investigated the effect of two phases on the atmospheric lifetime of brown carbon within BBOA, which is a climate-warming agent. We showed that the presence of two phases can increase the lifetime of brown carbon in the planetary boundary layer and polar regions compared to previous modeling studies. Hence, the presence of two phases can lead to an increase in the predicted warming effect of BBOA on the climate.

Learning heterogeneous reaction kinetics from X-ray videos pixel by pixel

Reaction rates at spatially heterogeneous, unstable interfaces are notoriously difficult to quantify, yet are essential in engineering many chemical systems, such as batteries1 and electrocatalysts2. Experimental characterizations of such materials by operando microscopy produce rich image datasets3–6, but data-driven methods to learn physics from these images are still lacking because of the complex coupling of reaction kinetics, surface chemistry and phase separation7. Here we show that heterogeneous reaction kinetics can be learned from in situ scanning transmission X-ray microscopy (STXM) images of carbon-coated lithium iron phosphate (LFP) nanoparticles. Combining a large dataset of STXM images with a thermodynamically consistent electrochemical phase-field model, partial differential equation (PDE)-constrained optimization and uncertainty quantification, we extract the free-energy landscape and reaction kinetics and verify their consistency with theoretical models. We also simultaneously learn the spatial heterogeneity of the reaction rate, which closely matches the carbon-coating thickness profiles obtained through Auger electron microscopy (AEM). Across 180,000 image pixels, the mean discrepancy with the learned model is remarkably small (<7%) and comparable with experimental noise. Our results open the possibility of learning nonequilibrium material properties beyond the reach of traditional experimental methods and offer a new non-destructive technique for characterizing and optimizing heterogeneous reactive surfaces.

(Keynote) Learning Electrochemical Models from Images

As illustrated by the work of Ralph White, traditional methods of electrochemical engineering are based on human intelligence: Mathematical models encoding physical hypotheses are proposed, tested against experimental data and refined by fitting adjustable parameters. Artificial intelligence is beginning to challenge this paradigm, since predictions can be made directly from data without the need for models, but such knowledge is often not transferrable to new situations. This talk will present a hybrid approach of solving PDE-constrained inverse problems to derive new multiscale models, in the context of Li-ion batteries. Examples include learning models of reaction kinetics and surface heterogeneities from x-ray adsorption images of phase separation in lithium iron phosphate nanoparticles [1], x-ray diffraction spectra for fictitious phase separation in nickel-rich oxide porous electrodes [2], and optical videos of lithium plating on graphite particles [3].[1] H. Zhao, H. D. Deng, A. E. Cohen, J. Lim, Y. Li, D. Fraggedakis, B. Jiang, B. D. Storey, W. C. Chueh, R. D. Braatz, and M. Z. Bazant, Learning heterogeneous reaction kinetics from X-ray movies pixel-by-pixel, preprint, https://www.researchsquare.com/article/rs-2320040/v1[2] J. Park, H. Zhao, S. D. Kang, K. Lim, C.-C. Chen, Y.-S. Yu, R. D. Braatz, D. A. Shapiro, J. Hong, M. F. Toney, M. Z. Bazant, and W. C. Chueh, Fictitious Phase Separation in Li Layered Oxides Driven by Electro-Autocatalysis, Nature Materials 20, 991-999 (2021).[3] T. Gao, Y. Han, D. Fraggedakis, S. Das, T. Zhou, C.-N. Yeh, S. Xu, W. C. Chueh, J. Li, M. Z. Bazant, Interplay of lithium intercalation and plating on a single graphite particle, Joule 5, 1-22 (2021).

Hierarchical conductive metal-organic framework films enabling efficient interfacial mass transfer

Heterogeneous reactions associated with porous solid films are ubiquitous and play an important role in both nature and industrial processes. However, due to the no-slip boundary condition in pressure-driven flows, the interfacial mass transfer between the porous solid surface and the environment is largely limited to slow molecular diffusion, which severely hinders the enhancement of heterogeneous reaction kinetics. Herein, we report a hierarchical-structure-accelerated interfacial dynamic strategy to improve interfacial gas transfer on hierarchical conductive metal-organic framework (c-MOF) films. Hierarchical c-MOF films are synthesized via the in-situ transformation of insulating MOF film precursors using π-conjugated ligands and comprise both a nanoporous shell and hollow inner voids. The introduction of hollow structures in the c-MOF films enables an increase of gas permeability, thus enhancing the motion velocity of gas molecules toward the c-MOF film surface, which is more than 8.0-fold higher than that of bulk-type film. The c-MOF film-based chemiresistive sensor exhibits a faster response towards ammonia than other reported chemiresistive ammonia sensors at room temperature and a response speed 10 times faster than that of the bulk-type film.

Quartic autocatalysis on horizontal surfaces with an asymmetric concentration: water-based ternary-hybrid nanofluid carrying titania, copper, and alumina nanoparticles

Understanding the mechanisms and kinetics of homogeneous (i.e. water-based ternary-hybrid nanofluid) and heterogeneous (i.e. catalyst) reactions is capable of creating a more effective distribution of species and quality of fluids at the outlet. Thus, the present study focuses on analyzing the quartic type homogeneous-heterogeneous reactions in ternary-hybrid nanofluid of copper, alumina, and titania nanoparticles with water at the surface of a stationary/moving flat plate. The leading non-linear partial differential equations were transformed into a set of ordinary differential equations using local similarity transformations. The resultant non-linear ordinary differential equations were numerically solved using MATLAB’s built-in BVP4C tool to explore the impact of pertinent factors. From the design side of view, the Tiwari-Das model for nano-liquid was employed to study the flow-thermal behaviour of the nanofluid. The thermal conductivity formulations were taken from experimental works to incorporate the spherical, platelet and cylindrical-shaped nanoparticles. The proposed concept is comparatively analyzed for Blasius and Sakiadis flows. The homogeneous bulk fluid and heterogeneous catalyst concentrations are observed for small and large nanoparticle volume fractions. The investigation shows that the strength of homogeneous and heterogeneous reactions has the opposite effect on the concentration fields. Strong correlations exist between the heterogeneous catalyst’s diffusivity and the homogeneous bulk fluid’s viscosity. Mass diffusion became more prevalent at the surface as the Schmidt number surged. Conversely, the bulk fluid concentration was highest in areas far from the surface.

Multifluid computational fluid dynamics simulation for sawdust gasification inside an industrial scale fluidized bed gasifier

Scarcity of fossil fuels, and to fulfill the energy demand of the increasing population, researchers need to shift their attention to the alternative of fossil fuels. It would be impractical and time-consuming to conduct extensive experiments on Fluidized Bed Gasifiers (FBG) of varying fuel types, sizes, geometries and operating conditions. The objective of the current research work is focused on numerical simulation of hydrodynamics behavior and sawdust gasification in an industrial scale FBG using the Eulerian-Eulerian multifluid model. For the accurate prediction of thermal and hydraulic performance (bed voidage, suspension density, pressure drop, heat transfer, and combustion kinetics), one should incorporate the correct parameters in the Computational Fluid Dynamics (CFD) simulation of a fluidized bed gasifier. It includes the multiphase gasification modeling using the Eulerian-Eulerian approach with an user defined heterogeneous reaction kinetics. The adopted CFD methodology is validated with experimental data and extended for the prediction of gasification characteristics of sawdust by incorporating eight heterogeneous (moisture release, volatile cracking, tar cracking, tar oxidation, char combustion, CO₂ gasification, steam gasification, methanation reaction) and five homogeneous oxidation of CO, CH₄, H₂, forward and backward water gas shift (WGS) reactions. In the result section, composition of gasification products is analyzed, along with the hydrodynamics of sawdust and sand phase, heat transfer between the gas, sand and sawdust, reaction rates of different homogeneous and heterogeneous reactions are being analyzed along the height of the domain.

Evaluation of local oxygen flux produced by photoelectrochemical hydroxide oxidation by scanning electrochemical microscopy

Several in-situ electrochemical approaches have been developed for performing a localized photoelectrochemical investigation of the photoanode. One of the techniques is scanning electrochemical microscopy (SECM), which probes local heterogeneous reaction kinetics and fluxes of generated species. In traditional SECM analysis of photocatalysts, evaluation of the influence of radiation on the rate of studied reaction requires an additional dark background experiment. Here, using SECM and an inverted optical microscope, we demonstrate the determination of O2 flux caused by light-driven photoelectrocatalytic water splitting. Photocatalytic signal and dark background are recorded in a single SECM image. We used an indium tin oxide electrode modified with hematite (α-Fe2O3) by electrodeposition as a model sample. The light-driven flux of oxygen is calculated by analysis of SECM image recorded in substrate generation/tip collection mode. In photoelectrochemistry, the qualitative and quantitative knowledge of oxygen evolution will open new doors for understanding the local effects of dopants and hole scavengers in a straightforward and conventional manner.

Accelerated Sulfur Oxidation by Ozone on Surfaces of Single Optically Trapped Aerosol Particles

The sulfur oxidation in mixed sodium thiosulfate/sucrose/aqueous microdroplets by gaseous ozone is studied in this work via aerosol optical tweezers coupled with Raman spectroscopy, which can simultaneously determine various physicochemical properties and the heterogeneous reaction kinetics of single optically trapped microdroplets, allowing for elucidating their complicated interplay. According to the kinetics measurement results at different relative humidities, ozone concentrations, and stoichiometries of inorganic and organic solutes, this work finds that a high aerosol ionic strength can accelerate the ozone oxidation of thiosulfate at air–water interfaces, while a high aerosol viscosity prolongs the reaction time scales due to diffusion-limited kinetics. The kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) is utilized to investigate the observed heterogeneous kinetics results and to retrieve the surface reaction rate coefficients. The KM-SUB model fit results indicate that the observed kinetics of sulfur oxidation in binary sodium thiosulfate aqueous microdroplets with high ionic strengths is dominated by interfacial reactions, and the fitted surface reaction rate coefficients increase 1 order of magnitude when the droplet ionic strength increases around 40 M. This work further utilizes the kinetics measurements of ozone dependence to discuss the coupling among properties of ozone gas-interface-bulk partitioning and surface reaction rate in the kinetics model. Finally, this work also exploits the surface reaction of thiosulfate with ozone to retrieve the aerosol viscosity of microdroplets containing ternary aqueous sodium thiosulfate and sucrose mixtures, demonstrating the feasibility of pinning down aerosol viscosities via reaction kinetics.

Detailed analysis of the particle residence time distribution in a pressurized drop‐tube reactor

AbstractSolid fuel conversion in a pressurized drop‐tube reactor is studied in detail using a three‐dimensional computational fluid dynamics (CFD) model. The main focus is on analyzing individual particle trajectories and residence times, as these data are crucial for the precise experimental estimation of heterogeneous reaction kinetics. The numerical results were substantiated by radioactive tracer measurements carried out in different operating conditions. The numerical results reveal a complex gas flow that is affected by buoyancy due to a non‐homogeneous temperature distribution, which has a strong affect on the trajectories of particular particle size fractions. In this case, empirical residence time correlations for particles, as commonly used for the evaluation of heterogeneous kinetic measurements, lose their validity since the assumption of a plug flow is no longer valid. It can be shown that if CFD‐assisted data evaluation is used, a significant improvement in the measured heterogeneous reaction kinetics is feasible.

Molecular Tracer Characterization during COVID-19 Pandemic in Shanghai: Changes in the Aerosol Aqueous Environment and Implications for Secondary Organic Aerosol Formation

The COVID-19 lockdown has opened a unique window for investigating aerosol formation and evolution with controlled anthropogenic emissions in urban areas. Here, variations of PM2.5 chemical compositions, gaseous pollutants, meteorological conditions, and secondary organic aerosol (SOA) molecular tracers were monitored during three stages at an urban site (Pudong) and a suburban site (Qingpu) in Shanghai, which were defined as pre-COVID lockdown (PL), during COVID lockdown (DL), and after COVID lockdown (AL) in 2020. Abundances of pollutants during the same periods back in 2019 were also analyzed for a more comprehensive intercomparison and evaluation of the impact of the 2020 COVID-19 lockdown on regional air quality. With the sudden cessation of anthropogenic activities during the lockdown, significant reductions in PM2.5 were observed compared to both PL in 2020 (32% in Pudong and 36% in Qingpu) and the DL period back in 2019 (31% in Pudong and 35% in Qingpu), which was accompanied by the significantly reduced PM2.5 components (29–44% and 14–44% reductions in sulfate, nitrate, ammonium, organic carbon, and elemental carbon for Pudong and Qingpu, respectively). In particular, with the reduced secondary inorganic aerosol (SIA), the time series of SOA molecular tracers also underwent significant reduction that was characteristic to the lockdown. Amid the uncontrolled biogenic emissions and even slightly enhanced atmospheric oxidation capacity during the 2020 DL period, controlling anthropogenic emissions exhibits synergistic effects on the reduction of SIA and SOA, which could be further attributed to the changes in the aerosol aqueous-phase environment, such as aerosol liquid water content (ALWC), ionic strength, sulfate content, and particulate NH4+. Based on thermodynamic modeling, greatly reduced ALWC was observed during 2020 DL, which can prevent the partitioning of oxygenated organics into the condensed phase as well as the aqueous-phase formation of SOA. Higher ionic strength in 2020 DL may have a “salting-out” effect on gas–particle partitioning of oxygenated organics. The reduced SOA during 2020 DL at both sites can generally be reflected by the predicted heterogeneous reaction kinetics (γ) of the isoprene SOA formation pathway. Overall, our study showed a synergistic effect in suppressing SIA and SOA formation upon the reduction of anthropogenic emissions during the COVID-19 lockdown, which shed light on the importance of controlling anthropogenic emissions in regulating secondary aerosol formation in typical urban areas of East China.

Heterogeneous consecutive reaction kinetics of direct oxidation of H2 to H2O2: Effect and regulation of confined mass transfer

Porous catalysts in heterogeneous reactions have played an important role in the modern chemical industry, but it is still challenging to quantitatively describe mass transfer and surface reaction behaviors of reactants in nano-confined space. Direct synthesis of hydrogen peroxide (H2O2) is considered as an attractive alternative to anthraquinone oxidation process, while the confined mass transfer of H2O2 in porous catalysts limits the reactivity. In this work, taking the consecutive reaction of H2O2 synthesis as an example, a quantitative method in modeling the effects of confined mass transfer on the reactivity was studied. More specifically, calorimetry was developed to characterize the confined structures of porous carbon experimentally, the linear nonequilibrium thermodynamics and the statistical mechanics method were further combined. Then, the heterogeneous consecutive reaction kinetics and the Thiele modulus influenced by confined mass transfer were modeled. Consequently, regulation strategies were proposed with the help of theoretical models. The optimized catalyst with biological skeleton carbon support and 0.5 wt% palladium loading shows an excellent catalytic performance. Lastly, for the mesoscience in heterogeneous reaction, the resistance was explored as a quantitative descriptor to compromise in the competition between mass transfer and surface reaction. The mesoscale structures were considered as the dynamic spatiotemporal distribution of substance concentrations, and the resistance minimization multi-scale (RMMS) model was proposed.

Kinetics of two-step catalytic sequence on nanoclusters with limited cluster occupancy

Two-step catalytic sequence was considered for cooperative kinetics of heterogeneous catalytic reactions on nanoclusters with a limited number of adsorbed species due to size restrictions allowing for derivation of the corresponding rate expressions. Dependence of the kinetic parameters on the number of adsorbed species was considered for different models of biographical (intrinsic) and induced non uniform surfaces. Explicit expressions were derived for the rates on evenly nonuniform surfaces exhibiting a linear decrease in the Gibbs energy with site occupancy. An expression for the apparent activation energy was obtained for the two step sequence with both irreversible steps. Simulations were performed for several expressions for the forward reaction rate in a two-step sequence with different number of maximal adsorbed species. The analysis illustrated that the reaction rates in the forward direction pass through maxima even without any competition of both reactants. Strong nonuniformity or lateral interactions make the rate maxima less prominent. Larger clusters, which can accommodate more adsorbed intermediates per cluster, exhibit rather wide maxima in the rates expanding over a broad range of concentrations.The experimental data on resazurin reduction with hydroxylamine over gold clusters bearing 15 to 25 gold atoms were used to illustrate applicability of the developed theoretical approach showing a dramatic switch in kinetic regularities between Au15 and Au18 clusters, which could be attributed to changes in the number of adsorbed species per cluster.

Cooperative catalytic nanokinetics

• Theoretical framework for cooperative kinetics on nanoclusters with several adsorbed molecules. • Development of kinetic equations for common reaction mechanisms. • Analysis of selectivity in consecutive and parallel reactions. • Comparison of theory with experimental data on chemoselective hydrogenation. Cooperative kinetics of heterogeneous catalytic reactions on nanoclusters or in a nanoconfined space is considered for the Eley-Rideal and Langmuir-Hinshelwood types of mechanisms as well as for a reaction mechanism with two kinetically significant steps, by assuming few (minimum two) molecules of different type per a single metal cluster/catalytic ensemble. The treatment accounted for the lateral interactions between adsorbed species by introducing kinetic and adsorption constants dependence on the presence of adsorbed species of the same or another type. Kinetic behavior for the Eley-Rideal type of mechanism with the product adsorption was different from the classical mean –field approximation, resulting in S-shape behavior and cooperativity, while the Langmuir-Hinshelwood mechanism essentially gives the same behavior for the case of two adsorbed species of different type on a two-site ensemble. Differences in the kinetic behavior for the bimolecular mechanism compared to the conventional treatment could be seen for larger ensembles, when at least three species are adsorbed per cluster or located in a nanoconfined space. For parallel reactions of the Eley-Rideal type, it was demonstrated that the cooperative behavior of two adsorbed molecules of a reactant per cluster results in selectivity dependence on conversion for reactions of the same order contrary to the mean field approach. In the case of consecutive reactions with such cooperative behavior, selectivity towards the intermediate product depends on the initial substrate concentration in contrast to the mean field approximation.