Apparent Transfer Coefficient Research Articles

Electrochemical Determination of Cysteine Using a 2-(2,5-Dihydroxy-Phenyl)-Naphtho[1,2-d] Oxazole-5-Sulfonic Acid (DHPNOS) and Cerium Oxide Nanoparticle (CeO2NP) Composite Carbon Paste Electrode (CPE)

An electrochemical sensor for the electrocatalytic oxidation of cysteine was designed based upon a 2-(2,5-dihydroxy-phenyl)-naphtho[1,2-d] oxazole-5-sulfonic acid (DHPNOS) and cerium oxide nanoparticle (CN) modified carbon paste electrode (CPE). An investigation of the redox properties of this modified electrode indicates a reduction in oxidative overvoltage and an intense increase in the analytical current. The pH, modifier, and nanoparticles were optimized to 7, 4% weight percent, and 5% weight percent, respectively. The apparent charge transfer rate constant (ks) and transfer coefficient for electron transfer between modifier and electrode were 2.776 s−1 and 0.47, respectively. Using differential pulse voltammetry, a linear range from 1 to40 µM with a detection limit of 0.33 µM was observed in phosphate buffer (pH 7.0). This work introduces a simple approach for selective determination of L-cysteine in the presence of uric acid. Also, based on chronoamperometry, the diffusion coefficient was determined to be 2.61 × 10−5 cm2 s−1. The effect of interferences on L-cysteine peak current were studied. The repeatability and reproducibility of the designed sensor were also measured with relative standard deviation values of 1.00% and 4.45%, respectively. The constructed sensor was employed for the determination of L-cysteine in acetylcysteine tablets.

Reduction in Apparent Permeability Owing to Surface Precipitation of Solutes by Drying Process and Its Effect on Geological Disposal

Disposal tunnels in geological repositories are ventilated continuously for over 50 years until their closure. Under these conditions, an unsaturated zone of mixed liquid and gas phases forms around the tunnels. Moreover, drying is assumed to progress from the host rock to the tunnels. To understand these drying processes, this study investigated the migration and precipitation of solutes via capillary forces during drying in packed columns using silica sand or glass beads as packed layers and X-ray CT analysis. In addition, the apparent permeability of a column packed with silica sand containing precipitation was examined using a flow experiment. The results indicate that the precipitation and accumulation of solutes were significant near the drying surfaces of the columns. The apparent mass transfer coefficient at a relatively early stage of the drying process indicates that the migration rate of solutes depends strongly on the capillary forces during the drying process. Furthermore, the apparent permeability of the columns with precipitation decreased significantly. These indicate that the precipitation and accumulation of solutes with drying in the groundwater reduce the porosity and permeability, and the advection of groundwater around the repository may be suppressed.

Fibrous catalyst flexibility as a source of mass transfer intensification

The work is devoted to computational fluid dynamics (CFD) simulation of structured cartridges with glass-fiber catalysts (GFCs). The CFD simulation of the model reaction of toluene deep oxidation at 350 °C in the cartridge with corrugated and plain structuring meshes (channel height 5.7 mm, width 8.0 mm, GFC length 44 mm) and sateen-type Pt-based GFC have confirmed the earlier formulated hypothesis, connecting their unusually high mass transfer efficiency with flexibility of GFCs, when the higher apparent transfer coefficient is provided by additional fluid flow in the back-side space of the textile thus leading to the rise of effective external contact surface area. The current study has revealed that relative effective surface area may vary from 1, equivalent to front-sided GFC contact only at low gas speed and up to 1.7 at higher velocities demonstrating that back-side flow may be quite significant even at quite moderate gas velocities (below 0.5 m/sec). The obtained knowledge may open the new approaches for development and optimization of structured GFCs.

Nanosized LiFePO4 Reveals the Electrochemical Kinetics of Electrolytes in Porous Li-Ion Battery Electrodes

Electrified transportation is critical for a decarbonized future, but high cost and range anxiety hamper the adoption of electric vehicles (EV). For low-cost EVs, extreme fast charging of lithium-ion batteries (LIB) is an ideal solution to range anxiety, but this requires significant improvements to rate performance. The measurement of electrochemical kinetics in LIBs is central to understanding and optimizing their rate performance. However, while LIB rate performance is typically limited by the electrolyte, the behaviors of electrolytes during high-rate (dis)charge remain elusive to electrochemical characterization. It is commonly assumed that the effects of ion transport in the electrolyte are negligible in sufficiently thin electrodes (e.g., ≤ 5 μm), allowing measurement of the intrinsic electrochemical kinetics of LIB active materials (AM). On the contrary, we suggest that when the AM is not rate limiting (i.e., with nanosized AMs), electrochemical characterization of LIB electrodes reveals only the kinetics of the electrolyte, making the AM kinetics impossible to measure. In this work, we use LiFePO4 (LFP) as a model system to investigate the fundamental electrochemical kinetics of porous battery electrodes in which the electrolyte is the sole rate limiting component.We first synthesize a nanosized, carbon-coated LFP active material (LFP-C) and fabricate electrodes with both a practical composition (95% AM) and extremely low electronic resistance. Electrochemical impedance spectroscopy shows that charge-transfer resistance is immeasurably small compared to pore resistance, while cyclic voltammetry proves that semi-infinite Li+ diffusion in the electrolyte is the rate limiting process. We develop a novel pseudo-steady-state extrapolation (S3E) technique for quantification of charge-transfer kinetics in porous battery electrodes, which shows that LFP obeys Butler-Volmer kinetics, even at a current density of > 100 mA cm-2. Unexpectedly, we find that the apparent transfer coefficient of LFP-C is ~ 1.5, rather than the commonly assumed value of 0.5. Furthermore, the remarkably low electrode-level exchange current (< 1 mA cm-2) suggests that only a small fraction of particles (~ 1%) are actively reacting, even at extreme rates. These results show that a moving reaction front resulting from rapid interfacial kinetics is the root cause of the widely observed electrode-level reaction heterogeneity of phase-separating AMs. Using LFP three-electrode cells, we conducted rate performance testing on the electrolyte, comparing the conventional LP57 electrolyte to highly conductive 2M LiFSI in acetonitrile (LiFSI-AN). LiFSI-AN achieves up to 70% charge capacity at a 600C rate and 35% discharge capacity at a 3000C rate, exceeding all other reports of LIB rate performance. A novel resistance vs. capacity analysis of the rate performance data enables direct measurement of electrolyte resistance growth, revealing how the complex interaction between a moving reaction front (i.e., increase in conduction path length) and concentration polarization (i.e., decrease in electrolyte conductivity) controls electrolyte rate performance in LFP electrodes.Overall, this work shows that the electrochemical kinetics of nanosized AMs in porous battery electrodes can only be understood by explicitly accounting for the behaviors of the electrolyte. Consequently, the common a priori assumption that the effects of the electrolyte are negligible for electrochemical kinetic measurements in thin electrodes is shown to be, in many cases, fundamentally misleading. Nanosized AMs are, therefore, a powerful tool to investigate the electrochemical kinetics of electrolytes in porous battery electrodes. Particularly, all-LFP three-electrode cells were developed as an ideal platform for rate performance testing of electrolytes, enabling direct measurement of electrolyte resistance growth during high-rate battery operation.

Vanadium-Doped Ti4O7 Porous Transport Layers for Efficient Electrochemical Oxidation of Industrial Wastewater Contaminants

Inexpensive electrode materials and effective cell designs are needed to advance electrochemical technologies for the oxidative treatment of wastewater. Novel vanadium-doped Ti4O7 porous transport layers (PTLs) used in a compact wastewater electrolyzer are developed and characterized and their performance for the electrochemical oxidation of synthetic wastewater is evaluated. An original analytical model predicting performance with the apparent mass transfer coefficient and cell potential is developed. The influence of operating parameters such as volumetric flow, current density, and PTL composition on performance is investigated. Decolorization and chemical oxygen demand (COD) removal of 100 mg L−1 of methyl orange (MO), an azo dye, in 1,500 mgNaCl L−1 is rapid with mass transfer coefficients as great as 377 ± 24 m s−1 for MO at 15 mA cm−2. After 2.5 Ah L−1 at 10 mA cm−2, >99 decolorization and >98% COD removal are achieved with a current efficiency of 19.2% and with specific and volumetric energy consumption of 120 and 84.1 kWh kg−1 for MO and COD, respectively, and 1.34 ± 0.09 and 6.45 ± 0.97 kWh m−3 order−1, respectively. A more energy-efficient electrochemical cell design for industrial wastewater treatment using less expensive high oxidation power (HOP) electrode materials is demonstrated with these results.

Fabrication and Performance of 300-mm Wafer-Scale Silicon Microchannel Cooler

This paper describes the design, fabrication, and thermal performance of 300 mm diameter wafer-scale Si microchannel coolers that demonstrated a junction temperature rise of less than 18°C when dissipating about 14 kW of power. A thermal test wafer, which included hot-spot regions to mimic high-power compute cores, was attached to the wafer-scale microchannel cooler with Pb-Sn solder. Glass manifold layers were attached to the microchannel wafer using a frit material. The average apparent heat transfer coefficient, h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">app</sub> , was about 104,000 W/(K-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

Condensation of NH3/H2O with mass concentrations of 80%-96%: Experimental study in a plate heat exchanger

High concentration NH3/H2O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This paper investigates the condensation of NH3/H2O with NH3 mass concentrations of 80%-96%. The vapor and liquid concentrations are close to equilibrium state, which are different from normal absorbers. The apparent heat transfer coefficients (HTCs) and frictional pressure drop are presented, covering the mass fluxes of 32–86 kgm−2s−1, the averaged vapor qualities of 0.08–0.65 and the saturated pressure of 610 to 780 kPa. Larger mass fluxes noticeably increase the apparent HTCs and frictional pressure drop. At the mass concentrations of 96%, 91% and 88%, higher vapor qualities increase the apparent HTCs for large mass fluxes. The apparent HTCs decrease slightly with vapor qualities for 80% mass concentration. The experimental results are compared with those of pure NH3. The flow patterns of high concentration NH3/H2O are considered as full film flow and partial film flow, which are the same as for NH3. The mass transfer resistance deteriorates the heat transfer especially for partial film flow, which happens at small liquid mass fluxes. The mass transfer resistance has negligible influences on frictional pressure drop.

Reversible Cyclic Voltammetry and Non-Unity Stoichiometry: The Ag/AgBr/Br- Redox Couple.

The voltammetry of electrochemically reversible couples in which a soluble reactant is converted into an insoluble product is investigated computationally via simulation and, in the context of the Ag/AgBr/Br-redox couple, experimentally. The voltammetric waveshape is characterized and, when analyzed via apparent transfer coefficient analysis, shown to give rise to apparent transfer coefficients very considerably in excess of unity, leading to the generic insight for the characterization of electrode reactions involving solution and solid phase reactants.

Predicting the Temperature Evolution during Nanomilling of Drug Suspensions via a Semi-Theoretical Lumped-Parameter Model.

Although temperature can significantly affect the stability and degradation of drug nanosuspensions, temperature evolution during the production of drug nanoparticles via wet stirred media milling, also known as nanomilling, has not been studied extensively. This study aims to establish both descriptive and predictive capabilities of a semi-theoretical lumped parameter model (LPM) for temperature evolution. In the experiments, the mill was operated at various stirrer speeds, bead loadings, and bead sizes, while the temperature evolution at the mill outlet was recorded. The LPM was formulated and fitted to the experimental temperature profiles in the training runs, and its parameters, i.e., the apparent heat generation rate Qgen and the apparent overall heat transfer coefficient times surface area UA, were estimated. For the test runs, these parameters were predicted as a function of the process parameters via a power law (PL) model and machine learning (ML) model. The LPM augmented with the PL and ML models was used to predict the temperature evolution in the test runs. The LPM predictions were also compared with those of an enthalpy balance model (EBM) developed recently. The LPM had a fitting capability with a root-mean-squared error (RMSE) lower than 0.9 °C, and a prediction capability, when augmented with the PL and ML models, with an RMSE lower than 4.1 and 2.1 °C, respectively. Overall, the LPM augmented with the PL model had both good descriptive and predictive capability, whereas the one with the ML model had a comparable predictive capability. Despite being simple, with two parameters and obviating the need for sophisticated numerical techniques for its solution, the semi-theoretical LPM generally predicts the temperature evolution similarly or slightly better than the EBM. Hence, this study has provided a validated, simple model for pharmaceutical engineers to simulate the temperature evolution during the nanomilling process, which will help to set proper process controls for thermally labile drugs.

Comparative studies of the discharge of hydronium ions on zinc, copper and aluminum cathodes

Electrochemical reduction of hydrogen (hydronium ion) was carried out on zinc, aluminum and copper cathodes from acidic aqueous solutions containing sulfuric acid (0.09, 0.18 and 0.36 mol/l) to study the effect of electrolyte acidity, the type of cathodes used and potential values on electrolysis indicators. The studies were carried out on the potentiostat using a three-electrode cell under conditions of intensive electrolyte stirring with a magnetic stirrer. At the initial stage, electrolysis was performed in the following modes: potentiodynamic measurements at a sweep rate of 1 mV/s in the potential range Е = –(700÷850) mV on a copper and aluminum electrode and Е = –(1000÷1150) mV on a zinc electrode. In the indicated potential range, hydronium discharge parameters at each cathode were calculated: Tafel slope, apparent transfer coefficients and exchange currents. Dependences of these parameters on electrolyte acidity were considered. Average values of steady state potentials were obtained, which, similar to the apparent exchange current, significantly depended on the cathode material: –923.1 mV (zinc cathode); +36.1 mV (copper cathode), and –603.7 mV (aluminum cathode) (AgCl/Ag). The effect of surfactants on all the kinetic parameters considered was shown. The order of the reaction with and without surfactant additives was determined. At the next stage, the electrochemical parameters of hydronium discharge on the copper electrode only were compared. It was shown that the electrochemical parameters significantly depend on the cathodic potential range where they are determined, and on the methods used for their calculation. It was noted that the process proceeds in the region of mixed kinetics. As the electrode polarization decreases, the hydrogen discharge mechanism changes, while the proportion of electrochemical kinetics will increase in the region of mixed kinetics. We suppose that the data obtained can also be of practical importance for the zinc electrolysis technology. The data obtained in this research on the electrochemical parameters of hydrogen discharge in a wide range of potentials on cathodes made of different metals as well as on the effect of electrolyte acidity on the behavior of surfactants during electrolysis will expand knowledge about the zinc electrolysis technology.

Modeling the drying kinetics of soybeans under intermittent operation in thin layer

The extraordinary expansion of soybeans production and trade volumes over the past few decades has undoubtedly generated global economic benefits.One factor responsible for the commercial success and extension of soybean shelf life is the drying process, which involves both heat and mass transfer. Alternatively, intermittent drying aims to improve the process in terms of yield, energy use and product quality. Thus, presenting a mathematical model that describes the soybean drying kinetics in intermittent thin-layer operation is a novelty in this study. The first order Runge-Kutta method was used to solve the proposed model, through which it was possible to obtain a detailed knowledge of the heat and mass transfer parameters involved in the drying process. It was possible to properly fit a mathematical model, based on mass and energy balances, which predicted the temperature and moisture profiles of the grain as a function of drying time, as well as the determination of physical parameters, such as global coefficients natural and forced convection heat transfer and the apparent global mass transfer coefficient.

Investigation of Surface Adsorption and Electrode Passivation during Electrochemical Oxidation of Redox-Active Organics

Redox-active organic molecules (ROMs) have gained interest as electroactive materials for redox flow batteries (RFBs). Electrochemical properties of ROMs, i.e., their solubility and redox potential, can be tuned through molecular engineering of the base molecule. Previously, we reported values of the apparent anodic charge transfer coefficient α (values approaching 1) associated with the electrochemical oxidation of a model ROM, 4-hydroxy-TEMPO.1 Also, we have proposed an adsorption-mediated mechanism to explain this unusual value of α, and have presented evidence to support the occurrence of adsorbed intermediates. Here, we extend this investigation to gain further insights using techniques of slow-scan voltammetry and electrode cycling, and apply them to other ROMs. We will discuss results of spectroscopic and micro-gravimetric studies, which provide further insights into the physicochemical properties of the adsorbed surface-passivating films. Ramifications of these to implementation of ROMs in flow batteries will be discussed too.[1] N. A. Shaheen, M. Ijjada, M. B. Vukmirovic, R. Akolkar. J. Electrochem. Soc., 167 (2020) 143505. Acknowledgements: Investigation of the model ROM was supported by Breakthrough Electrolytes for Energy Storage (BEES)—an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0019409. Work completed at Argonne was supported by the Joint Center for Energy Storage Research (JCESR), a U.S. Department of Energy, Energy Innovation Hub, and by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract #DE‐SC0014664. All opinions expressed in this paper are the author’s and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

Radial heat transport in a fixed-bed reactor made of metallic foam pellets: Experiment and particle-resolved computational fluid dynamics

Open-cell metallic foams in the shape of pelletized catalysts are regarded as potential catalyst substrates for the application in fixed-bed reactors. This work reports an experimental study and a detailed Computational Fluid Dynamics (CFD) model to investigate the heat transfer performance of a fixed-bed reactor made of open-cell metallic foam pellets. The steady-state heat transfer behavior of a hot gas flowing through such a packed bed under wall-cooled conditions was examined. The proposed CFD model is in line with the particle-resolved CFD (PRCFD) approach that accounts explicitly for the random packing structure, so that the transport processes in the interstitial regions are fully resolved. The momentum and energy transports inside the highly porous foam particles are considered by closure equations based on the porous-media model. The Rigid Body Dynamics (RBD) method is adopted to create the packing geometry. The axial temperature and the pressure drop obtained by the CFD simulations show good agreement with experimental data. To evaluate the thermal performance, effective heat transport parameters in a packed bed such as the effective radial thermal conductivity and the apparent wall-fluid heat transfer coefficient are determined, with the aid of a 2D pseudo-homogenous plug-flow reactor model. The correlations for heat transport characteristics as a function of particle Reynolds number for the metallic foam packed bed are also presented.

Nitrogen Solubility and Gas Nitriding Kinetics in Fe–Cr–Mo–C Alloy Melts under Pressurized Atmosphere

The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts’ law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s−1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.

The Effect of Temperature on the Cation‐Promoted Electrochemical CO2 Reduction on Gold

AbstractElectrochemical reactions in general, and the CO2 reduction reaction (CO2RR) in particular, are commonly studied at room temperature. However, practical electrolysers may use elevated temperatures. There is currently a lack of fundamental understanding of the effect of temperature on the CO2RR. Here, we performed temperature‐dependent studies on the CO2RR on a relatively simple electrode material, namely gold, to obtain insights into how temperature influences this reaction and the competing hydrogen evolution reaction. A rotating ring disk electrode setup was used to show that the CO2RR activity and selectivity increased with temperature, and to obtain kinetic parameters such as the apparent activation energy and transfer coefficient. The magnitude of the temperature effect and the activation energy was affected by both the cation identity and concentration in the electrolyte. Moreover, the positive effect of temperature on the kinetics of the CO2RR was counteracted by the lower CO2 solubility, making efficient mass transport even more important at higher temperatures.

Theoretical and experimental investigations of time-dependent mass transfer mechanism of fine particulate matter under the sink effect

The mass transfer process of fine particulate matter (PM) under the sink effect is affected by many variables, including purification device performance, environmental conditions, and time. In this paper, the time-dependent mass transfer mechanism of PM was explored while all other variables were to remain the same. Firstly, the spatial and temporal profile characteristics of particulate matter concentrations (PMC) with different size fractions were investigated. Subsequently, the apparent mass transfer coefficient Dapp of PM was calculated based on Fick's second law, and the relevant parameters were determined by laboratory-scale experiments. According to the Dapp, the instantaneous mass transfer coefficient Dins of PM was further discussed and the prediction model of PMC was established. Finally, the stability and applicability of the prediction model were verified by experiments. The results showed that the farther the distance from the purification device, the smaller the particle size, then the smaller the decrease rate of PMC and the stronger the regeneration process. The value of Dapp could reflect the mass transfer process of PM at the macroscopic level. The value of Dins could better characterize the mass transfer process of PM at the level of detail, and it could also be used to predict and evaluate the dynamic mass transfer efficiency of PM. The established PMC prediction model had strong stability and good applicability. The research results of this paper can provide an important reference basis for practical engineering applications.

Interface Dynamic Investigation of Nitrogen Doped 3YSZ (3YSZ:N)

Y2O3 stabilized zirconia (3YSZ, 3 mol%) and nitrogen doped 3YSZ (3YSZ:N) were prepared and characterized by cyclic voltammetry (CV), cathodic polarization and Tafel curve. Through electrochemical methods, the interface kinetic properties of the Pt/YSZ:N system and the reaction process of nitrogen have been studied. At low temperatures, O2− undergo electrochemical reactions. Nitrogen ions show high reactivity above 650 °C. Nitrogen transfer occurred in at least two steps. First step was the loss of electrons from N3− to obtain intermediate product, and second step was the loss of electrons from intermediate product to obtain nitrogen. Both reactions were controlled by diffusion, and apparent transfer coefficient of the reaction was calculated to be 0.39 and 0.27 with exchange current density of 2.59 × 10−3 A·cm−2.

Spectroscopic and Electrochemical Investigation of Adsorbed Nitroxide Radicals

Deep eutectic solvents (DESs) have gained interest as electrolytes for grid-scale energy storage applications such as flow batteries. Here, we report on processes that govern the charge-transfer kinetics involving a redox-active organic (4-hydroxy-TEMPO, HT) species in chloride containing aqueous and DES electrolytes. Using polarization measurements, we show that the apparent anodic charge transfer coefficient α for a one–electron oxidation of HT approaches 1.1 This is a significant deviation from the expected value of α ≈ 0.5 for a symmetric one–electron transfer reaction involving a soluble – soluble redox transition. To explain this unusual value of α, a two–step adsorption – desorption mechanism is proposed, wherein HT undergoes fast charge transfer and adsorption at the electrode surface, followed by slow desorption of the adsorbed oxoammonium cation. Evidence collected using in situ surface enhanced Raman spectroscopy, combined with density functional theory simulations, suggests the presence of the adsorbed oxoammonium on the electrode surface. Additional evidence of adsorption, collected using a novel electrochemical approach, is presented.[1] N. A. Shaheen, M. Ijjada, M. B. Vukmirovic, and R. Akolkar, J. Electrochem. Soc., 167, 143505 (2020).

Analysis and prediction of reaction kinetics using the degree of rate control

“Degree of rate control” (DRC) analysis provides a quantitative approach for analysing the kinetics of multi-step reaction mechanisms that has been widely applied to both heterogeneous and homogeneous catalysis research, as well as electrocatalysis. The DRC of any given transition state or intermediate is defined as a partial derivative such that it approximately equals the fractional increase in net rate to the product of interest per differential decrease in its standard-state free energy for that species (÷RT), holding constant the standard-state free energies of all other transition states and intermediates. Even very complex mechanisms usually have only a few species with non-zero DRCs and are thus the species whose interactions with the catalyst most strongly affect the net rate. These key DRC values thus offer a simple and intuitive route to optimize catalyst materials, especially with the assistance of computational methods like density functional theory (DFT). These high-DRC species are also the species whose energetics must be most accurately measured or calculated to achieve an accurate kinetic model for any reaction mechanism. In simple cases with a single “rate-determining step”, the DRC for its transition state (TS) is + 1. Catalyst-bound intermediates, on the other hand, often have negative DRCs equal to a small integer times their fractional occupancy of catalyst sites. The apparent activation energy equals a weighted average of the standard-state enthalpies (relative to reactants) of all the species (intermediates, transition states and products) in the reaction mechanism, each weighted by its DRC (+RT). It has been shown that the apparent transfer coefficient in electrocatalysis, an inverted form of the Tafel slope, is a weighted average of the number of electrons transferred to generate each intermediate or product species in the mechanism, weighted again by the DRC. Quantitative analysis of kinetic isotope effects (KIEs, or the ratio of net rates for different reactant isotopes) in complex mechanisms has shown that the logarithm of the KIE equals the weighted average over all species in the mechanism of the difference between the two isotopes in their standard-state free energies (÷RT), again weighted by the DRC. The reaction orders with respect to fluid-phase concentrations of reactants, products and intermediates have also been proven to be directly related to DRCs. Thus, there are numerous experimental observables which equate to short linear combinations of DRCs, so that combinations of experimental measurements might provide access to DRC values. Since its invention for transition states in 1994 and its generalization to include intermediates in 2009, the DRC has thus far mainly been calculated for microkinetic models based either entirely on DFT or on DFT with the key energies (i.e., those for high-DRC species) being fine-tuned to match experiments. The relationships summarized above provide new opportunities for using experiments earlier in the development and optimization of microkinetic models that require input from computational catalysis.

Selecting carrier material for efficient biomethanation of industrial biogas-CO2 in a trickle-bed reactor

Biogas upgrading via biomethanation of renewable H2 and the CO2 in biogas poses a potential key technology in the decarbonized energy system. This study uses a methanogenic trickle-bed reactor for upgrading raw biogas from an industrial anaerobic digester to natural gas-grade biomethane through ex situ biomethanation. The carrier material comprises a critical design parameter for trickle-bed reactors by supporting the catalytic methanogenic biofilm and thereby the interfacial area for H2 mass transfer. The present study applied an abiotic mass transfer-based screening of six different carrier types to identify a candidate material that can facilitate efficient H2 mass transfer in a biomethanation trickle-bed reactor. The screening included gas-liquid mass transfer experiments in an air-water system in addition to characterizations of the carriers’ surface area, dynamic liquid hold-up volumes, and external porosities. The abiotic experiments showed that the gas-liquid mass transfer coefficient is highly influenced by material type and cannot be predicted solely from differences in the carriers’ surface area. The abiotic characterizations led to the selection of crushed clay particles, whose application in biomethanation-based upgrading of raw biogas from an industrial anaerobic digester was examined. The clay particles facilitated an apparent volumetric mass transfer coefficient for H2 of 10 min−1, enabling CH4 production rates up to 6.1 L · L−1 · d−1 and more than 99 % H2 conversion over 30 days of continuous operation, where after accumulation of metabolic water in the packed bed impaired H2 mass transfer. Apart from the adverse effect of water accumulation, there was no indication that the maximum H2 mass transfer rate was reached, demonstrating the selected carrier material’s industrial potential for biomethanation-based biogas upgrading.