Gold nanoparticles (AuNPs) have received considerable interest owing to their unique properties and applications in catalysis. One of the major challenges for colloidal nanoparticles in catalysis is ...the limited stability and resulting aggregation. Nanoparticle functionalization with ligands or polymers is a common strategy to improve the colloidal stability, which in turn blocks the reactive surface sites and eliminates catalytic activity. Here, we investigate thiolated polyethylene glycol (HS-PEG) as a stabilizing ligand during AuNP catalytic reduction of 4-nitrophenol. We show a direct relationship between the chain length and packing density of HS-PEG with respect to AuNP catalytic activity. High surface coverage of low molecular weight HS-PEG (1 kDa) completely inhibited the catalytic activity of AuNPs. Increasing HS-PEG molecular weight and decreasing surface coverage was found to correlate directly with increasing rate constants and decreasing induction time. Time-resolved UV–vis absorbance spectroscopy of 2-mercaptobenzimidazole (2-MIB) adsorption on AuNPs was used to study the ligand adsorption kinetics and to quantify the free active sites available for catalysis as a function of HS-PEG molecular weight and packing density. HS-PEG packing density and estimation of free active sites, coupled with the kinetics of 2-MBI adsorption onto AuNP ruled out the possibility of an educt diffusion barrier as the main cause of reduced catalytic activity and induction time for HS-PEG functionalized AuNPs (molecular weight ≥1 kDa). Instead, selective blocking of more active sites by adsorbed thiol functionality is attributed to the induction period and reduced catalytic activity. It is also noticed that H– induced desorption/mobility of thiols regenerates the catalytic activity.
Crystallization is an extensively used unit operation for production of solid forms of inorganic and organic compounds, pharmaceuticals, cosmetics, and polymers etc. However, the stochastic nature of ...crystallization makes it difficult to control the product characteristics such as crystal size, size distribution, morphology, polymorphism, and chirality. Use of ultrasound has been shown to greatly influence crystallization and help in gaining control over product characteristics. Ultrasonic waves generate cavitation in the solution resulting in enhanced micromixing, increased mass transfer rates and uniform supersaturation which then lowers induction time, reduces metastable zone width, increases nucleation rates and narrows down particle size and size distribution. This review attempts to present a coherent understanding of the mechanism of sonocrystallization. Further, a newly emerging field of ultrasonic spectroscopy, which is relatively unexplored but holds a tremendous potential for online monitoring of crystallization, has also been discussed.
Ligands are the quintessential synthesis tool in the preparation of colloidal metal catalysts, allowing for the rational design of nanostructured surfaces with high activity and selectivity. These ...same agents can, however, strongly influence the catalytic performance of metal nanostructures in aqueous media. In this regard, the current literature describing the influence of ligands on the model catalytic reaction that sees 4-nitrophenol reduced to 4-aminophenol by borohydride is highly fragmented in that the understanding of reaction rate, induction time, and ligand desorption phenomena is disconnected and, at times, contradictory. Herein, we present a study in which three chemically distinct ligands are applied to bare gold catalysts followed by their exposure to aqueous solutions of relevance to 4-nitrophenol reduction while simultaneously tracking the ligand whereabouts. It is shown that the exposure of ligands to borohydride leads to their near-complete removal from the gold catalyst. This, in turn, gives rise to severe disruptions to the rate of 4-nitrophenol reduction for the scenario where the aqueous reactants are purged of dissolved oxygen and ligand desorption times are slow. In sharp contrast, the reaction rate is little affected when the same reactants contain dissolved oxygen because the resulting induction period provides ample time for the ligands to desorb prior to the onset of the reaction. Moreover, strongly bound ligands are shown to give rise to an induction-time-like feature that is only observable when the reactants are free of dissolved oxygen. Collectively, these findings advocate procedures for catalytic benchmarking that differ from current best practices and underscore the point that a fundamental understanding of 4-nitrophenol catalysis must adopt a holistic approach that accounts for ligand–nanostructure interdependencies.
•Statistically robust data for KHI impact on hydrate induction time probabilities.•KHIs modify induction time probabilities distributions from exponential to Gamma.•Data reveal how KHIs both delay ...formation and reduce crystal growth rate.•Mechanisms by which KHIs affect formation at various subcoolings elucidated.
The formation of gas hydrates is crucial to many technological applications including energy production, energy storage, desalination, and CO2 capture. Kinetic hydrate inhibitor (KHI) chemicals have been used industrially for over 25 years to suppress hydrate formation in key industrial applications. However the mechanisms by which they operate, and specifically whether they delay nucleation or just retard growth, remain open questions. Here induction time probability distributions for methane hydrates were measured at several constant subcoolings as a function of KHI concentration. This allowed the hydrate nucleation and growth rates at each condition to be separately quantified through the observed induction times and initial gas consumption rates, respectively. Adding a KHI produces a Gamma-distribution of induction times, characterized by two parameters: nucleation rate and the average number of events associated with detection. This is qualitatively different to the exponential distributions of induction time probabilities observed in systems without any KHI. These data reveal how KHIs both increase the nucleation work required to form a critical nuclei and increase the effective number of sites where nucleation could occur. By showing unambiguously how KHIs delay hydrate nucleation at low subcoolings, this work opens systematic pathways for developing improved chemicals for either hydrate inhibition or promotion. The approach to inhibitor testing demonstrated here may also help natural gas producers assess the costs and benefits of competing designs for hydrate management.
► Synthesis of silver nanoparticles were using a chemical reduction method. ► Decreasing the induction time of methane hydrate formation in the presence of silver nanoparticles. ► Increasing the ...amount of methane trapped in hydrate crystals formation in the presence of silver nanoparticles.
Using gas hydrates as materials for storage and transportation of natural gas have attracted much attention in recent years. However, there are two barriers in industrializing this new method. Firstly, methane hydrate induction time is relatively high. On the other hand the amount of gas trapped in methane hydrate crystals is too low. In this survey, silver nanoparticles were synthesized using a chemical reduction method and introduced to the hydrate reactor. Experiments were conducted at initial reactor pressures of 4.7MPa and 5.7MPa. At each pressure three independent experiments were performed. According to the results, in the presence of silver nanoparticles, methane hydrate induction time decreased by 85% and 73.9%, and the amount of methane trapped in hydrate crystals increased by 33.7% and 7.4% at the pressures of 4.7MPa and 5.7MPa respectively.
•Effects of boundary layer and bulk crystalliser mixing on crystallisation kinetics are described.•Increased boundary layer mixing (Re) increases nucleation rate which reduces crystal size.•Higher ...bulk agitation (N) reduces induction time and increases crystal growth rate.•High supersaturation levels at increased Re promotes secondary nucleation and agglomeration.•Nucleation and crystal growth kinetics can be decoupled and independently controlled.
While mixing is considered critical in membrane distillation crystallisation, an explicit link between mixing and crystallisation mechanisms has yet to be made. This study therefore used in-line nucleation detection to determine induction time and metastable zone width, as a method to characterise the independent roles of interfacial boundary layer mixing and bulk crystalliser mixing on the kinetics of nucleation and crystal growth in membrane distillation crystallisation. Interfacial boundary layer mixing was investigated by changing Reynolds number (Re, 1300–2050), where an increase in Re also increased flux. This increased supersaturation within the boundary layer, shortened induction time, and was correlated to a higher nucleation rate. The high nucleation rates introduced smaller crystal sizes, which is an effect that can be correlated to classical nucleation theory. Mixing within the crystalliser did not modify the interfacial supersaturation at nucleation (ReN 1562–18741). However, a decrease in induction time was observed when transitioning from lower to higher mixing, which may arise from the improved distribution of the supersaturated feed within the crystalliser. The effect of crystalliser mixing was also evident on crystal growth, where larger crystals were produced at higher crystalliser stirrer speeds due to improved diffusion-controlled growth. This study therefore demonstrates that bulk and interfacial mixing can be collectively used to control crystallisation by decoupling conditions that determine nucleation and crystal growth. Morphological assessment was subsequently undertaken that evidenced how dendritic breeding and wider secondary nucleation mechanisms that dominate crystal growth at high levels of supersaturation can be mitigated through mixing. Membrane crystallisation offers a scalable geometry in which we demonstrate mixing to facilitate good control over crystal growth which is more difficult to realise in existing evaporative crystalliser design.
Based on the problems caused by many oxygen-containing functional groups and poor floatability on the surface of low rank coal, the characteristics of low rank coal were analyzed systematically by ...means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) techniques. The bubble-particle induction time was used to determine the characterization of the bubble-particle attachment, and the bubble-particle attachment of low rank coal modified by soaking the coal samples in an acid or alkaline solution was analyzed. The floatability of the modified coal surface was verified by flotation tests. The results show that the particle size of 0.125–0.074 mm of the coal sample exhibited better bubble-particle attachment characteristics. The small bubble, the larger approach velocity of bubble and the larger bubble deformation were more helpful to enhance the bubble-particle attachment. For an acid solution, the smaller the pH was and the longer the soaking time was, the better the floatability of the coal sample and the higher the combustible material recovery were. The combustible material recovery of low rank coal was increased to 78.79% by soaking the sample in an acid solution of pH = 0 for 180 min. On the contrary there was a best concentration for the alkaline solution.
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•Parallel, stirred experimentation for rapid induction time distribution determination.•Low-cost external bulk imaging-based tracking of the crystallization.•The device applied first ...for temperature and supersaturation dependency determination.•Significant, non-solubility related effect of certain polymer were revealed.•The processing history of polymer solution show significant impact on nucleation rate.
Crystal nucleation is crucial in various fields of science and technology, ranging from materials synthesis to pharmaceutical production. Our research aims to determine the nucleation rates from parallel induction time measurement experiments followed by low-cost external bulk video imaging. Stirred conditions are applied, making our results industry-relevant where the crystallizers are also mixed. The L-Glutamic Acid-water system (L-GA/water) is used as a surrogate. The study is conducted through a series of parallel, small-scale stirred experiments that explore the effects of supersaturation, temperature, and polymer additives on nucleation rates. The methodology involves careful data collection through controlled experimental setup and analysis of induction time distributions that the stochastic nature of nucleation explains the induction time distribution. The discussion section interprets the findings within the context of the research question, highlighting the implications of varying temperature and supersaturation levels as two experimental parameters and polymers on crystal nucleation rates. The specific parameters of supersaturation and temperature dependencies are in the range of those reported in the literature, which validated the developed rapid nucleation rate measurement platform. Stepping outside of these established measurements, analyzing the influence of polymers with high potential in controlling particle size, shape, and polymorphism was touched as well. As an unexpected result, the nucleation rates appeared to depend not only on the chemical structure and amount of the polymer but also on the polymer solution preparation method. The remarkable influence of polymers on nucleation rates was shown that cannot be explained with the solubility altering effect of the polymers alone.
Nucleation rates of carbon dioxide hydrate Lim, Vincent W.S.; Barwood, Mark T.J.; Metaxas, Peter J. ...
Chemical engineering journal (Lausanne, Switzerland : 1996),
09/2022, Letnik:
443
Journal Article
Recenzirano
•CO2 hydrate nucleation rates measured in bulk aqueous phase.•CO2 hydrates nucleate faster than those formed from methane or a gas mixture.•Higher solubility of CO2 in the aqueous phase does not ...significantly affect the nucleation rate.•Lower energy penalty for introducing the CO2 hydrate-water interface yields fast kinetics.
Nucleation rates of carbon dioxide hydrate were extracted via the measurement of induction time distributions in a set of stirred, stainless steel, high-pressure cells. These nucleation rates ranged from (1.71 to 20.6) × 10-4 s−1 at subcoolings between (1.2 and 4.2) K, higher than those measured under the same conditions for methane or structure II hydrates. Formation probability was well-described by the mononuclear mechanism from Classical Nucleation Theory. Analysis under this framework reveals that the comparatively faster nucleation rates observed for CO2 hydrates can be attributed to lower nucleation work rather than guest solubility in the liquid aqueous phase.
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•Liquid-iron contact area had insignificant effects on methane hydrate nucleation.•Corroded particles decreased the induction time of methane hydrate by 80%•Iron promoted nucleation ...was dominated by increasing gas–liquid interface curvature.•Iron surface did not serve as nucleation site for methane hydrate.
Gas hydrate plugging in oil and gas transportation pipelines is a knotty problem for flow assurance practices. The nucleation mechanism of methane hydrate on corroded iron pipeline surface is still unclear. In this work, both smooth and corroded iron particles were packed into a hydrate reactor to investigate the relationship between the induction time of methane hydrate and the liquid–metal contact area. Experimental results indicated an insignificant difference among the particles of varied specific surface areas when submerged in the aqueous phase. However, the induction time was reduced by more than 60% when the particles were located above the gas–liquid interface and further shortened by 80% if the particles were corroded. Molecular dynamics simulations indicated that decreased interlayer spacing between the iron layers would facilitate methane hydrate formation in the initial period. Based on our energy analysis, this was ascribed to an increased curvature of the gas–liquid interface, causing a decrease of energy barrier by 54% for a methane molecule to transfer from the interface into the bulk liquid and subsequently, an increase of methane concentration in solution. Meanwhile, two layers of water molecules were observed to adsorb on the iron surface stably and no hydrate cages formed in such region throughout the simulation time. The microscopic phenomenon agreed well with the experimental results, which highlighted the influence of the gas–liquid–metal three-phase interface other than the liquid–metal contact area upon methane hydrate nucleation. The results shed new insights on predicting the prioritized locations of methane hydrate formation in metallic pipelines to reduce the potential hydrate plugging risk.
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