Catalyst Particles Research Articles

Inhibition of the growth rate of ZnS anode crystal plane: boosting photocatalytic hydrogen production performance

The catalyst's particle size plays a crucial role in enhancing photocatalytic performance. However, it is still unclear to analyse the mechanism by which surfactants affect the particle size of photocatalysts from the perspective of crystal growth. Herein, ZnS nanomaterials with different particle sizes were synthesized using a simple solid-state chemical synthesis method, and the intrinsic influence mechanism between surfactant and catalyst particle size was analyzed from the perspective of crystal growth. It was discovered that the particle size of ZnS kept getting smaller due to changes in polyethylene glycol, cetyltrimethylammonium bromide, and sodium dodecyl sulfate. Analyzing the reasons revealed that the catalyst's particle size was related to the type of surfactant: anionic surfactants could reduce the catalyst size. Correspondingly, the catalyst with a smaller particle size has the optimal performance for photocatalytic hydrogen production. Photocatalysts Z-SDS exhibited outstanding photocatalytic performance, yielding 11.47 mmol/g of hydrogen after a five-hour illumination. Furthermore, the intrinsic influence mechanism between anionic and cationic surfactants, photocatalyst particle size, and catalytic performance was investigated. This investigation provides a new idea for synthesizing small-size and high-performance photocatalysts.

Preparation of high-value carbon nanotubes from real waste plastic towards the negative carbon technology

The preparation of carbon nanotubes (CNTs) from plastics is of great significance for realizing high value utilization of waste and reducing carbon emission. Here, several kinds of real waste plastics were introduced into catalytic pyrolysis, and the process was also optimized. The results showed that the presence of impurities (e.g., adhesive labels) reduced the initial activation energy of the pyrolysis reaction, and the pyrolysis process was extended and could be divided into two stages. During the catalytic process, impurities play a toxic role on the catalyst at higher temperatures and result in the agglomeration of catalyst particles and a decrease in catalytic activity. Less than 10 wt.% carbon fibers were collected from milk cup waste. However, after experimental optimization, the influence of the impurity component was greatly reduced. The toxic effect of organic impurity volatiles on a catalyst was avoided by employing a segmented catalytic pyrolysis process, which led to an increase in solid carbon content of more than 20 % for express package waste. Simultaneously, more uniform and smoother CNTs can be found in the obtained solid carbon. The process of preparing carbon nanotubes with higher yield and better quality is feasible and has important application prospects in the utilization of waste plastics.

Synthesis and characterization of titanium oxynitride catalyst via direct ammonia nitridation of titanium polyacrylate for oxygen reduction reaction

AbstractA titanium oxynitride catalyst for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells was synthesized through the direct ammonia nitridation of titanium complexes. Titanium polyacrylate was employed as the catalyst precursor, and the effect of the calcination temperature between 600 and 1000 °C on the catalyst structure was studied. The catalysts were characterized via X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, cyclic voltammetry, and powder electrical resistivity measurements. The formation of titanium oxynitride particles and deposited carbon was observed for all the samples; however, significant variations in the catalyst structure and catalytic activity were also observed. With an increase in the calcination temperature, nitridation of titanium oxynitride progressed, and the conductivity of the catalyst powder increased. The highest rest potential and ORR current density were achieved with calcination at 800 °C. Importantly, the results suggest that maintaining an optimal nitrogen doping level within the catalyst particles, along with ensuring the formation of electroconductive deposited carbon, is essential for achieving a high ORR current. This work introduces the direct ammonia nitridation of metal complexes as a promising process for designing metal oxynitride catalysts.

Facile Synthesis and Decomposition Kinetics of Energetic Molecular Perovskite DAP-4 Catalyzed with Copper Nanoparticles for Advanced Energetic Systems

AbstractMolecular perovskite DAP-4 (NH4)(C6H12N2)(ClO4)3 can expose high oxidizing capability, with superior decomposition enthalpy compared with ammonium perchlorate (AP). DAP-4 was developed via molecular assembly method. The potential impact of copper nanoparticles (Cu NPs) on DAP-4 thermal decomposition was evaluated. Reactive Cu NPs of 50 nm were integrated into DAP-4 structure via solvent evaporation. Uniform dispersion of Cu NPs into DAP-4 matrix was assessed using elemental mapping. Whereas AP demonstrated decomposition enthalpy of 733 J/g; virgin DAP-4 and Cu/DAP-4 nanocomposite experienced decomposition enthalpy of 3800, and 4150 J/g respectively. Cu NPs offered decrease in DAP-4 main decomposition temperature by 49 oC. Decomposition kinetics was investigated via Kissinger, and Kissinger–Akahira–Sunose (KAS). Cu/DAP-4 demonstrated apparent activation energy of 134.97 ± 3.02 kJ/mol compared with 216.32 ± 4.42 kJ/mol for virgin DAP-4. Cu NPs offered decrease in DAP-4 combustion time from 70 ms to 31 ms, with high intense combustion flame. Copper NPs experienced dramatic change in DAP-4 decomposition mechanism from F1 to F3.Copper NPs could act as efficient reactive catalyst particles; that could be oxidized to CuO with the evolution of 3700 J/g. The evolved CuO nanocatalyst could boost decomposition enthalpy.

Electric-Field-Driven Redistribution of Carriers on Catalyst Particles to Improve Photocatalytic Performance.

The photocatalytic performance is primarily determined by carrier separation efficiency. Applying electric fields to enhance carrier separation efficiency and thereby boost catalytic performance has emerged as a promising approach. However, the carrier behavior on catalyst particles under electric fields was still hardly acknowledged. Herein, using single-molecule fluorescence microscopy with high spatiotemporal resolution, the redistribution behavior of carriers on catalyst particles under an electric field was visualized for the first time, which was closely related to the direction and intensity of the electric field. Single-molecule kinetics of the photocatalytic redox reactions induced by carriers have been further obtained, and the results showed that the external electric field had an obvious regulating role in the conversion process and desorption process in this work, which was attributed to the dynamic redistribution of carriers. The improvement of photocatalytic hydrogen evolution reaction (HER) activity further supports the impact of the external electric field on photocatalysis. This work offers new insight into the microscopic regulation mechanism of photocatalytic activity by electric fields and provides new methods for studying the structure-activity relationship in photocatalytic processes.

Partial upgrading of heavy oils by catalytic aquathermal visbreaking with dispersed iron oxide catalysts

The impact of water and red mud-like Fe oxide catalysts on aquavisbreaking (AVB) of heavy-oils was studied under various circumstances. Vacuum residue (VR) was employed as a good substitute for low-viscous bitumen grades. The reaction products were separated into SARA fractions using open-column liquid chromatography (OCLC), and boiling point groups were classified using thermogravimetric analysis (TGA). In addition, Fourier-transform infrared (FTIR) spectroscopy was used to study the structural features of VR and its derivatives. It was observed that the thermal visbreaking of VR led to a severe coke formation with asphaltene (ASP) to coke selectivity over 130 %, while the addition of water and Fe catalyst could increase the conversion of ASP to liquid oil products while inhibiting coke formation, resulting in improved conversion of ASP. Furthermore, the structural properties of spent catalysts recovered after reactions were analyzed using X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) to identify the active phase, confirming that the Fe precursor is converted to a 20–30 nm Fe2O3 and Fe3O4 during VR AVB. Moreover, the addition of carbon additives to AVB further improved the dispersion of ASP and catalyst particles, contributing to the decrease of the selectivity for ASP conversion to coke by 50 %.

Three-phase CFD-DEM study on the hydrodynamics of a riser system with liquid injection

Riser reactors are frequently applied in the process industry for highly important catalytic processes. In this work, a computational fluid dynamics- discrete element method (CFD-DEM) for a riser with liquid injection was developed. This model treats the gas as a continuous phase whereas the liquid droplets and catalyst particles are treated as discrete elements. The presence of droplets causes particles to be wet, which is taken into account via coverage models of the particles. In addition, the liquid on the particles will change the collision behavior, which is included using an energy balance approach. This CFD-DEM study is conducted simulating a lab-scale pseudo-2D riser. Based on the comparison, the assumption of fully covered particles results in a significant increase in the solids holdup and clustering behavior of the riser which, especially at the low liquid flow rate, results in an over-prediction. The partial coverage simulations show that only half of the particles are covered, which has a major effect on the collisions.

Enhancing carbon dioxide adsorption in a hybrid fixed bed via structuring and thermal management: a numerical study

Systems based on physical sorption are an attractive solution for CO2 capture from flue gases, biogas upgrading or gas storage. Besides the sorbent choice, one of the most important factors related to the design of such systems is proper heat management. Commonly used sorbents typically have low thermal conductivity. Nevertheless, catalyst particles characterized by high conductivity are inherently present in adsorptive (hybrid) reactors. Thus, appropriate structuring of hybrid beds can be used for controlling temperature profiles and improving the bed performance. In this study, the behaviour of a nonadiabatic adsorptive reactor described by a two-dimensional model was analysed for the adsorption step. The effect on the CO2 adsorption performance of different spatial distributions of functionalities in the bed was investigated. The optimality problem for nonuniform radial distribution of sorbent and catalyst in the bed was solved, indicating that such a configuration is a potentially important direction for structuring hybrid beds. Results demonstrate that the optimal configuration of radially distributed functionalities significantly increases the amount of CO2 absorbed under identical boundary and initial conditions for the bed. It appears that precise control of the heat generated and removed from the bed is achievable. Such control could be advantageous for the regeneration phase.

Oxidative Steam Reforming of Methanol over Cu-Based Catalysts

Several Cu and Ni-based catalysts were synthetized over Ce-based supports, either pure or mixed with different amounts of alumina (1:2 and 1:3 mol/mol). Different metal loadings (10–40 wt%) and preparation methods (wet impregnation, co-precipitation, and flame-spray pyrolysis—FSP) were compared for the oxidative steam reforming of methanol. Characterization of the catalysts has been performed, e.g., through XRD, BET, XPS, TPR, SEM, and EDX analyses. All the catalysts have been tested in a bench-scale continuous setup. The hydrogen yield and methanol conversion obtained have been correlated with the operating conditions, metal content, crystallinity of the catalyst particles, total surface area, and with the interaction of the metal with the support. A Cu loading of 20% wt/wt was optimal, while the presence of alumina was not beneficial, decreasing catalyst activity at low temperatures compared with catalysts supported on pure CeO2. Ni-based catalysts were a possible alternative, but the activity towards the methanation reaction at relatively high temperatures decreased inevitably the hydrogen yield. Durability and deactivation tests showed that the best-performing catalyst, 20% wt. Cu/CeO2 prepared through coprecipitation was stable for a long period of time. Full methanol conversion was achieved at 280 °C, and the highest yield of H2 was ca. 80% at 340 °C, higher than the literature data.

Development and performance evaluation of a novel solar mid-and-low temperature receiver/reactor with packed metal foam

Solar-driven hydrogen production from methanol decomposition (MD) is an important method of storing solar energy as clean fuels and improving the solar energy utilization efficiency. Catalyst preparation is one of the key steps, but conventional Cu-ZnO-Al2O3 particle catalysts lead to uneven reactor temperature, reducing the efficient conversion of solar energy into fuels. To alleviate this challenge, a monolithic catalyst utilizing packed foamy copper as a carrier is developed with exceptional thermal and mass transfer properties. The experimental results validate its excellent performance in hydrogen production using MD. The absolute conversion increases by an average of 12.11 % at 250 °C and further by 14.43 % at 270 °C compared to the particle catalyst. Based on these findings, a novel solar reactor is proposed, and a multi-field coupling model is built. The comparative results verify the performance advantage of the developed solar thermochemical receiver/reactor with packed metal foam-based catalyst. It maintains high conversion rates and solar-to-fuel energy efficiency, while alleviating overheating problems caused by traditional particle catalysts. Under the design conditions, the new reactor significantly improves the absolute conversion of methanol and the energy efficiency of solar-to-fuel conversion by 4.57 % and 3.00 %, respectively, resulting in 92.64 % and 67.56 %. This study provides a theoretical basis and technical solution for more stable and efficient use of mid-and-low temperature solar energy.

Investigations of inlet arrangement strategies and particle parameters on thermochemical performance in falling particle solar reactors

The reformation of methane to hydrogen and steam in a falling particle solar reactor (FPSR) is a promising method to use the solar energy, which converts solar energy into chemical energy. FPSR has higher operation temperature, higher pressure bearing capacity and more direct heat energy storage than those of traditional reactor. However, due to the short residence time of the particles, hydrogen production efficiency is reduced. Thus, inlet arrangement strategies and particle catalysts should be carefully designed to improve the thermochemical reaction performance of FPSR. In this contribution, the Eulerian-Eulerian model in CFD program (Fluent 2020 R2) is used to explore the effects of different particle inlet arrangement strategies, non-uniform particle mass flow inlet and particle size on the performance enhancement of methane steam reforming reaction. The results indicate that the two arrangement strategies show different trends in influencing hydrogen production efficiency. Besides, non-uniform particle mass flow rate also has a significant impact on hydrogen production efficiency, with the optimal arrangement conditions increasing the hydrogen production rate by 2.4831 mol/m−3(−|-) s−1. Additionally, smaller particle size demonstrates better hydrogen production efficiency, with a 0.15 mm particle size catalyst showing a 14.02 % improvement in hydrogen production efficiency compared to a 0.75 mm particle size. This contribution can provide a guidance for optimizing the heat and mass transfer performance of the falling particle solar reactor.

Ceria-zirconia solid solution supported platinum catalysts for toluene oxidation: Studying the improved catalytic activity by tungsten addition

The catalytic oxidation of toluene at low temperatures is still an urgent issue to be solved. The key to addressing this issue is the preparation of a high-efficiency catalyst. Herein, ceria-zirconia (CZ) solid solution supported platinum (Pt) catalysts were prepared by citric acid (C)-assisted impregnation method. Surprisingly, the catalytic oxidation activity of the catalyst for toluene combustion significantly improved after introducing W into Pt-C/CZ. In addition, introducing W improved the redox property of the Pt-based catalyst, optimized the distribution of the oxygen vacancies, increased the average particle size of the catalyst particle, modulated the valence state and electronic structure of Pt, elevated the surface lattice oxygen activity, increased the number of surface moderate acidic sites and enhanced toluene adsorption capacity. In this study, the electronic structure of the Pt active site is modulated by introducing a W promoter, providing valuable guidance for the rational design of efficient noble metal catalysts.

Experimental investigation on product and temperature distribution in a continuous flow packed-bed reactor for dodecahydro-N-ethylcarbazole dehydrogenation

The dehydrogenation of dodecahydro-N-ethylcarbazole (H12-NEC) generates enormous gas accompanied by intense heat absorption, affecting heat and mass transfer in porous catalysts. A packed-bed reactor equipped with a temperature measurement device was designed to investigate the continuous flow dehydrogenation of H12-NEC. The effects of weight hourly space velocity (WHSV), reaction temperature, catalyst particle size, and pressure were evaluated. The results reveal that the volume flow rate of hydrogen will increase with the increase of WHSV. And the reduction of gas–liquid ratio and catalyst particle size facilitates heat transfer causing a more uniform axial temperature distribution within the reactor. As the temperature rises, the hydrogen yield increases along with more by-products. 456 K and 2 mm of catalyst particle size are determined to be the optimal reaction conditions in the study, achieving the hydrogen production rate of 9.61 molH2·gPd-1·h-1. With pressure increasing, the hydrogen yield gradually decreases, but the decreased evaporation of H12-NEC results in a smaller temperature difference between the reactor and heating wall. A comparison with the thermodynamic equilibrium model reveals that the effect of pressure on chemical equilibrium may be more significant than the temperature.

Processes of Metal Oxides Catalyst on Conversion of Spent Coffee Grounds into Rich-Synthesis Gas by Gasification

Spent coffee grounds (SCGs), a waste product of the coffee industry, present a significant untapped resource for fuel production. This study aims to optimize the gasification of SCG using various metal catalysts (NiO, MnO2, Al2O3, and Fe2O3) to maximize syngas yield. SCG samples were gasified at different temperatures (800 °C, 900 °C, 1000 °C) and analyzed using Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TG-DTA), and Fourier Transform Infrared (FTIR) spectroscopy to evaluate catalyst performance and reaction mechanisms. The findings indicated that utilizing mixing techniques for physical contact to introduce catalysts led to a uniform distribution of catalyst particles throughout the sample. The decomposition rate of the gasification experiment after adding the catalyst was 24% faster than that of the pure SCGs. In the gasification experiment, the MnO2 catalyst showed the highest CO production, which was 71% higher than that of NiO under the same conditions. At this temperature, MnO2 generated around 171% more CO than at 800 °C, surpassing the yields observed with other catalysts. The study concludes that Mn emerged as the most promising catalyst, significantly improving both CO and CH4 yields. Selecting the appropriate metal catalyst and optimizing operational temperatures are crucial for enhancing the efficiency of SCG gasification.

Exploring the Effect of Ball Milling on the Physicochemical Properties and Oxygen Evolution Reaction Activity of Nickel and Cobalt Oxides

Ball milling is commonly used to reduce catalyst particle size. However, little attention is paid to further changes that ball milling can cause to the rest of the catalysts’ physicochemical properties, which can impact their intrinsic catalytic activity. The effect of ball milling on the physicochemical properties of NiCoO2, NiO, CoO, and NiO:CoO mixtures is reported and correlated with their electrochemical oxygen evolution reaction (OER) activity. It is also shown that particle fragmentation is an inherent consequence of ball milling, but some oxides can also experience a phase transformation. In the case of rocksalt‐structured CoO, it is partially or entirely transformed to spinel‐structured Co3O4. Additionally, NiCo2O4 with a spinel structure can be formed by ball milling NiO and CoO simultaneously (both rocksalt structures), but only in the absence of water. The changes impact the electrochemical activity of the initial oxides. Ball milled NiCoO2 exhibits the highest activity with a mean potential of 1.563 V at 10 mA cm−2, demonstrating the advantage of having Ni and Co in the same structure. Although NiCo2O4 is also a binary oxide, the results indicate that its metal coordination environment makes it intrinsically less active than NiCoO2 for the OER in alkaline media.

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.

Electronic communications between active sites on individual metallic nanoparticles in catalysis

Catalytic activity of metal particles is reported to originate from the appearance of nonmetallic states, but conductive metallic particles, as an electron reservoir, should render electron delivery between reactants more favorably so as to have higher activity. We present that metallic rhodium particle catalysts are highly active in the low-temperature oxidation of carbon monoxide, whereas nonmetallic rhodium clusters or monoatoms on alumina remain catalytically inert. Experimental and theoretical results evidence the presence of electronic communications in between vertex atom active sites of individual metallic particles in the reaction. The electronic communications dramatically lower apparent activation energies via coupling two electrochemical-like half-reactions occurring on different active sites, which enable the metallic particles to show turnover frequencies at least four orders of magnitude higher than the nonmetallic clusters or monoatoms. Similar results are found for other metallic particle catalysts, implying the importance of electronic communications between active sites in heterogeneous catalysis.

Pore Network Modeling of Intraparticle Transport Phenomena Accompanied by Chemical Reactions.

In this work, a 3D pore network model (PNM) is introduced for modeling reaction-diffusion phenomena, with and without coupled heat transfer, in a spherical porous catalyst particle. The particle geometry is generated by packing thousands of microspheres inside a large sphere to represent the 3D geometry, porosity, and tortuosity of a spherical catalyst particle. A pore-network representation is extracted from this geometry, and a PNM for diffusion-reaction and heat conduction is constructed. This newly proposed particle-scale PNM allows for the application of realistic 3D nonuniform boundary conditions on the particle's surface, which is commonly encountered in slender packed-bed reactors. Concentration profiles inside the particle, and effectiveness of the reactions, is analyzed.

Modeling of Catalyst Degradation in Polymer Electrolyte Membrane Fuel Cells Applied to Three‐Dimensional Computational Fluid Dynamics Simulation

ABSTRACTIn a polymer electrolyte membrane (PEM) fuel cell, the following degradation mechanisms are associated with the catalyst particles and their support: carbon support corrosion triggered by carbon and platinum oxidation, platinum dissolution with redeposition, and particle detachment with agglomeration. In this work, an electrochemical model for those degradation effects is presented as well as its coupling with a three‐dimensional computational fluid dynamics PEM fuel cell performance model. The overall model is used to calculate polarization curves and current density distributions of a PEM fuel cell in a fresh and aged state as well as the degradation process during an accelerated stress test with 30 000 voltage cycles. The obtained simulation results are compared to measurements on a three‐serpentine channel PEM fuel cell with an active area of 25 cm2 under various temperatures and humidities. The experimental data are obtained with a segmented test cell using respective degradation protocols and test conditions proposed by the United States Department of Energy. In addition to the temperature and humidity changes, the influence of geometry and material parameters on the degree of degradation and the resulting fuel cell performance is explored in detail.

Simulation analysis of gas-solid two-phase flow and reaction in a novel catalytic cracking reactor

A gas-solid two-phase flow reaction model was developed to evaluate the performance of a novel catalytic cracking reactor in the residue-to-chemicals (RTC) process. Coupled TFM, EMMS drag, and 12-lump kinetic models were employed to simulate the flow reaction process. The reaction temperature and product yields at the reactor exit aligned well with real industrial RTC reactor test results, indicating the reliability and effectiveness of the coupling model. Catalyst particles formed an internal circulation structure in the reactor, increasing the instantaneous catalyst-to-oil ratio. The reactor exhibited low axial and radial temperature gradients with higher reaction temperatures, accelerating the catalytic cracking rate and enhancing the selectivity for high-value products. The effects of the catalyst-to-oil ratio, catalyst inlet temperature, and feedstock mass flow rate on the flow reaction process were optimized. Results showed that both the catalyst-to-oil ratio and catalyst inlet temperature influenced the overall catalyst velocity distribution. Within the limit range, increasing the catalyst-to-oil ratio increased feedstock conversion, gasoline, LPG, dry gas, and coke yields, but decreased diesel yield. Conversely, increasing the feedstock mass flow rate showed opposite trends. Different trends in product yields were observed with varying inlet catalyst temperatures, with optimal product distribution at 973.15 K.