Improving the Quantitative Description of Reacting Porous Solids: Critical Analysis of the Shrinking Core Model by Comparison to the Generalized Grain Model

The Preparation of an Ultrafine Copper Powder by the Hydrogen Reduction of an Ultrafine Copper Oxide Powder and Reduction Kinetics

Ultrafine copper powders were prepared by the air-jet milling of copper oxide (CuO) powders and a subsequent hydrogen (H2) reduction. After milling, the particle size and grain size of CuO powders decreased, while the specific surface area and structural microstrain increased, thereby improving the reaction activity. In a pure H2 atmosphere, the process of CuO reduction was conducted in one step, and followed a pseudo-first-order kinetics model. The smaller CuO powders after milling exhibited higher reduction rates and lower activation energies compared with those without milling. Based on the unreacted shrinking core model, the reduction of CuO powders via H2 was controlled by the interface reaction at the early stage, whereas the latter was limited by the diffusion of H2 through the solid product layer. Additionally, the scanning electron microscopy (SEM) indicated that copper powders after H2 reduction presented a spherical-like shape, and the sintering and agglomeration between particles occurred after 300 °C, which led to a moderate increase in particle size. The preparing parameters (at 400 °C for 180 min) were preferred to obtain ultrafine copper powders with an average particle size in the range of 5.43-6.72 μm and an oxygen content of less than 0.2 wt.%.

Shedding Light on Solid Sorbents: Evaluation of Supported Potassium Carbonate Particle Size and Its Effect on CO2 Capture from Air

Solid sorbents are essential for developing technologies that directly capture CO₂ from air. In solid sorbents, metal oxides and/or alkali metal carbonates such as potassium carbonate (K₂CO₃) are promising active components owing to their high thermal stability, low cost, and ability to chemisorb the CO₂ present at low concentrations in air. However, this chemisorption process is likely limited by internal diffusion of CO₂ into the bulk of K₂CO₃. Therefore, the size of the K₂CO₃ particles is expected to be an important factor in determining the kinetics of the sorption process during CO₂ capture. To date, the effects of particle size on supported K₂CO₃ sorbents are unknown mainly because particle sizes cannot be unambiguously determined. Here, we show that by using a series of techniques, the size of supported K₂CO₃ particles can be established. We prepared size-tuned carbon-supported K₂CO₃ particles by tuning the K₂CO₃ loading. We further used melting point depression of K₂CO₃ particles to collectively estimate the average K₂CO₃ particle sizes. Using these obtained average particle sizes, we show that the particle size critically affects the efficiency of the sorbent in CO₂ capture from air and directly affects the kinetics of CO₂ sorption as well as the energy input needed for the desorption step. By evaluating the mechanisms involved in the diffusion of CO₂ and H₂O into K₂CO₃ particles, we relate the microscopic characteristics of sorbents to their macroscopic performance, which is of interest for industrial-scale CO₂ capture from air.

Mixed-Combustion Characteristics and Reaction Kinetics of Municipal Sludge and Corn Straw in Micro-Fluidized Bed

With economic development, the output of municipal sludge (MS) continues to increase, and the effective utilization of corn straw (CS) also plays an important role in promoting “carbon neutrality”. The mixed combustion of solid wastes is a very environmentally friendly technology; however, little research has occurred regarding the combustion characteristics and reaction kinetics of MS and CS in a fluidized bed. Therefore, this study used a micro-fluidized bed and process mass spectrometer to evaluate the mixed-combustion characteristics of MS and CS and analyze the effects of the temperature and mixing ratios on the reaction rate. Isothermal kinetics were used to calculate the activation energy, pre-exponential factors, and other kinetic parameters of this reaction. The results showed that with an increasing reaction temperature, the combustion reaction rate of MS and CS under different mixing ratios increased. The reaction rate of mixed combustion of MS and CS was greater than that of MS or CS alone. Compared with the homogeneous model, the shrinking core model is more suitable for analyzing the mixed-combustion behavior of MS and CS. The calculated activation energies of the mixed combustion in different proportions were lower than that of single fuel combustion. When the ratio of MS to CS was 2:8, the activation energy required for the reaction was minimum (28.00 kJ/mol), the pre-exponential factor was 9.06, and the fitting degree was larger than 0.99, which proved the reliability of the results.

A physical-based interpretation of mechanism and kinetics of Cr(VI) reduction in aqueous solution by zero-valent iron nanoparticles

The aim of this paper is to show the results obtained by investigating the reduction of hexavalent Chromium [Cr(VI)] by iron nano-particles in aqueous solution, interpreted in light of the particle-grain model. The diffusional and geometric parameters that govern and describe the reacting system were estimated from the evidences deriving from the characterization and the experiments conducted, allowing assumptions based on physical principles. Such procedure rendered the particle-grain model a valid choice for the interpretation of the results obtained. The model, used in its dimensionless form, was tested according to a preliminary procedure aimed at analyzing the sensitivity of the system, by varying within wide ranges the ratio between the reaction rate, the diffusive mass transfer rate, and the particle-grain radius, to show how reliable its potential application may be. Subsequently, a non-linear regression procedure was used to estimate the two main parameters of the model that affect the reduction process: (i) the diffusion coefficient within the solid layer produced along with the reaction, D^p_c (6.02 E-13 m² s^-1), and (ii) the kinetic constant of the surface reaction, k_c (0.21 m s^-1). The values found for the parameters were perfectly in line with theoretical considerations and experimental evidences.