Kinetics insights into size effects of carbon nanotubes'growth and their supported platinum catalysts for 4,6-dinitroresorcinol hydrogenation

Size effects are a well-documented phenomenon in heterogeneous catalysis,typically attributed to alterations in geometric and electronic properties.In this study,we investigate the influence of catalyst size in the preparation of carbon nanotube(CNT)and the hydrogenation of 4,6-dinitroresorcinol(DNR)using Fe_(2)O_(3)and Pt catalysts,respectively.Various Fe_(2)O_(3)/Al_(2)O_(3)catalysts were synthesized for CNT growth through catalytic chemical vapor deposition.Our findings reveal a significant influence of Fe_(2)O_(3)nanoparticle size on the structure and yield of CNT.Specifically,CNT produced with Fe_(2)O_(3)/Al_(2)O_(3)containing 28%(mass)Fe loading exhibits abundant surface defects,an increased area for metal-particle immobilization,and a high carbon yield.This makes it a promising candidate for DNR hydrogenation.Utilizing this catalyst support,we further investigate the size effects of Pt nanoparticles on DNR hydrogenation.Larger Pt catalysts demonstrate a preference for 4,6-diaminoresorcinol generation at(100)sites,whereas smaller Pt catalysts are more susceptible to electronic properties.The kinetics insights obtained from this study have the potential to pave the way for the development of more efficient catalysts for both CNT synthesis and DNR hydrogenation.

Towards a molecular understanding of the electronic metal-support interaction(EMSI)in heterogeneous catalysis

Tailoring the electronic metal-support interaction(EMSI)has attracted considerable interests as one of the most efficient approaches to improve both the activity and stability of metal catalysts in heterogeneous catalysis.In this viewpoint,we illustrate the methodology and relevant fundamentals on the disentanglement,characterization,and interpretation of EMSI.Under the choice of monometallic catalyst over inert support,a combination of optimal experiment design,multimodal techniques,in situ characterization,with a comprehensive understanding of the underlying measurement protocols is highly desirable for a reliable determination of EMSI.Accordingly,not only the d-band filling but also d-band energy within the EMSI should be taken into consideration for providing general principles to guide the electron-promoting catalytic reaction.

Mechanistic insights into the active intermediates of 2,6-diaminopyridine dinitration

Mechanistic understanding of the active intermediates of 2,6-diaminopyridine(DAP) dinitration in the concentrated nitric-sulfuric acid system is of crucial importance for the selectivity control of target product, i.e., 2,6-diamino-3,5-dinitropyridine(DADNP). The active intermediates determining the product selectivity are theoretically studied. The HSO_(4)^(-)-NO_(2)^(+) complex is proposed as the dominant active nitrating intermediate for the first time, which shows low energy barrier(i.e., 10.19 kcal·mol^(-1),1 kcal = 4.186 k J) for direct dinitration of DAP to DADNP. The formed water during the reaction results in not only the formation of less active SO_(4)^(2-)-NO_(2)^(+) complex, but also the occurance of DAP sulfonation(DAP-SO_(3)H intermediate)to facilitate the formation of mononitration byproduct. Meanwhile, the accompanied thermal effects cause the generation of undesirable pyridine-NHNO_(2) intermediate, which is difficult to be rearranged to yield DADNP, inhibiting the reaction and thus giving low DAP conversion. The insights reported here elucidates the importance of thermal effects elimination and water content control, confirmed experimentally in the batch-and micro-reaction systems.

Polyoxometalates-engineered hydrogen generation rate and durability of Pt/CNT catalysts from ammonia borane

Heterogeneously catalyzed hydrolytic dehydrogenation of ammonia borane is a remarkable structure sensitive reaction. In this work, a strategy by using polyoxometalates(POMs) as the ligands is proposed to engineer the surface and electronic properties of Pt/CNT catalysts toward the enhanced hydrogen generation rate and durability. Three kinds of POMs, i.e., silicotungstic acid(STA), phosphotungstic acid(PTA)and molybdophosphoric acid(PMA), are comparatively studied, among which the STA shows positive effects on the catalytic activity and durability. A catalyst structure-performance relationship is established by a combination of kinetic and isotopic analyses with multiple characterization techniques, such as HAADF-STEM, EDS, Raman spectroscopy and XPS. It is shown that the STA compared to the other two POMs can increase the Pt binding energy and thus promote the reaction. The insights demonstrated here could open a new avenue for boosting the reaction by employing the POMs as the ligands to engineer the catalyst electronic properties.

Active sites and reaction mechanism for N-doped carbocatalysis of phenol removal

Heteroatom-doping of carbocatalysts has been a powerful strategy to remarkably enhance the catalytic performance.Herein,the underlying nature of N promotional effects on peroxymonosulfate(PMS)activation for phenol removal is understood by combining kinetics analysis with multiple techniques.A strategy using mixed acid oxidation of carbon nanotube(CNT)followed by NH3 treatment is employed to yield a series of catalysts with different N-doping contents but similar fraction of sp^(2)-hybridized carbon and defective degree,endowing with a chance to discriminate the dominant N-containing active sites.The multi-sites kinetics analysis suggests the graphitic N-containing sites as the dominant active sites.The mechanism of the surface-bound reactive species is also discriminated as the dominant reaction mechanism.The insights reported here could provide the methodology to fundamentally understand the heteroatom-doping effects of carbocatalysis.

Information Flow Security Models for Cloud Computing

Cloud computing provides services to users through Internet.This open mode not only facilitates the access by users,but also brings potential security risks.In cloud computing,the risk of data leakage exists between users and virtual machines.Whether direct or indirect data leakage,it can be regarded as illegal information flow.Methods,such as access control models can control the information flow,but not the covert information flow.Therefore,it needs to use the noninterference models to detect the existence of illegal information flow in cloud computing architecture.Typical noninterference models are not suitable to certificate information flow in cloud computing architecture.In this paper,we propose several information flow models for cloud architecture.One model is for transitive cloud computing architecture.The others are for intransitive cloud computing architecture.When concurrent access actions execute in the cloud architecture,we want that security domain and security domain do not affect each other,that there is no information flow between security domains.But in fact,there will be more or less indirect information flow between security domains.Our models are concerned with how much information is allowed to flow.For example,in the CIP model,the other domain can learn the sequence of actions.But in the CTA model,the other domain can’t learn the information.Which security model will be used in an architecture depends on the security requirements for that architecture.

Platelet carbon nanofibers as support of Pt-CoO electrocatalyst for superior hydrogen evolution

Exploration of cost-effective Pt/C catalysts has been a significant issue for electrochemical hydrogen evolution reaction(HER) toward sustainable energy conversion and storage.Herein,we report a fabrication strategy by employing platelet carbon nanofibers(p-CNF) as the support to immobilize Pt-CoO HER electrocatalyst using atomic layer deposition method.The edge-rich p-CNF support is found to act as the anchoring sites of Pt nanoparticles and favorably capture electrons from Pt to yield electron-deficient Pt surfaces for the boosted HER.Additionally,the sequential growth of CoO onto the Pt/p-CNF catalyst elaborately constructs the Pt-CoO interface and facilitates the electron transfer from Pt to CoO,which further enhances the HER activity.These advantages endow the fabricated Pt-CoO/p-CNF catalyst with the superior HER activity,e.g.,a very low overpotential of 26 mV at the current density of 10 mA·cm-2 and a mass activity of 4.42 A·mgPt-1at the overpotential of 30 mV,18.8 times higher than that of the commercial20 wt% Pt/C catalyst.The insights reported here could shed light on for the fabrication of cost-effective Pt-based composite HER catalysts.

Coupling non-isothermal trickle-bed reactor with catalyst pellet models to understand the reaction and diffusion in gas oil hydrodesulfurization

In this work,a trickle-bed reactor coupled with catalyst pellet model is employed to understand the effects of the temperature and catalyst pellet structures on the reaction-diffusion behaviors in gas oil hydrodesulfurization(HDS).The non-isothermal reactor model is determined to be reasonable due to non-negligible temperature variation caused by the reaction heat.The reaction rate along the reactor is mainly dominated by the temperature,and the sulfur concentration gradient in the catalyst pellet decreases gradually along the reactor,leading to the increased internal effectiveness factor.For the fixed catalyst bed volume,there exists a compromise between the catalyst reaction rate and effectiveness factor.Under commonly studied catalyst pellet size of 0.8-3 mm and porosity of 0.4-0.8,an optimization of the temperature and catalyst pellet structures is carried out,and the optimized outlet sulfur content decreases to 7.6 wppm better than the commercial level at 0.96 mm of the catalyst pellet size and 0.40 of the catalyst porosity.

Active sites of Pt/CNTs nanocatalysts for aerobic base-free oxidation of glycerol

Understanding the nature of Pt active sites is of great importance for the structure-sensitive base-free oxidation of glycerol. In the present work, the remarkable Pt particle size effects on glycerol conversion and products formation from the oxidation of the primary and the secondary hydroxyl groups are understood by combining the model calculations and DFT calculations, aiming to discriminate the corresponding dominant Pt active sites. The Pt(100) facet is demonstrated to be the dominant active sites for the glycerol conversion and the products formation from the two routes. The insights revealed here could shed new light on fundamental understanding of the Pt particle size effects and then guiding the design and optimization of Pt-catalyzed base-free oxidation of glycerol toward targeted products.

Mechanistic aspects of facet-dependent CH_(4)/C_(2+)selectivity over aχ-Fe_(5)C_(2)Fischer-Tropsch catalyst

Structure-performance relationship is a complex issue in iron-catalyzed Fischer-Tropsch synthesis,and it is not easy to elucidate it by experimental investigations.First-principle calculation is a powerful method for explaining experimental results and guiding catalyst design.In this study,we investigated the reaction mechanisms of CH_(4)formation and C-C coupling on fourχ-Fe_(5)C_(2)surfaces and established the kinetic equations to compare the rates of CH_(4)formation and C_(1)+C_(1)coupling reactions and determine the CH_(4)/C_(2+)selectivity.The results show that the geometry of theχ-Fe_(5)C_(2)surfaces has little effect on the formation rate of CH_(4);however,the C_(1)+C_(1)coupling reactions are significantly affected by the surface geometry.The C_(1)+C_(1)coupling reaction rates on the terraced-like(510)and(021)surfaces are much higher than those on the stepped-like(001)and(100)surfaces.Based on these results,we established a Brùnsted-Evans-Polanyi(BEP)relationship between the effective barrier difference for CH_(4)formation and C_(1)+C_(1)coupling(ΔE_(eff))and the adsorption energy of C+4H(ΔE_(C+4H))onχ-Fe_(5)C_(2)surfaces.ΔE_(C+4H)can be used as a descriptor for CH_(4)/C_(2+)selectivity on different surfaces ofχ-Fe_(5)C_(2).

Explosion limits estimation and process optimization of direct propylene epoxidation with H2 and O2

Direct propylene epoxidation with H2 and O2,an attractive process to produce propylene oxide(PO),has a potential explosion danger due to the coexistence of flammable gases(i.e.,C3 H6 and H2)and oxidizer(i.e.,O2).The unknown explosion limits of the multi-component feed gas mixture make it difficult to optimize the reaction process under safe operation conditions.In this work,a distribution method is proposed and verified to be effective by comparing estimated and experimental explosion limits of more than 200 kinds of flammable gas mixture.Then,it is employed to estimate the explosion limits of the feed gas mixture,some results of which are also validated by the classic Le Chatelier’s Rule and flammable resistance method.Based on the estimated explosion limits,process optimization is carried out using commercially high and inherently safe reactant concentrations to enhance reaction performance.The promising results are directly obtained through the interface called gOPT in gPROMS only by using a simple,easy-constructed and mature packed-bed reactor,such as the PO yield of 13.3%,PO selectivity of 85.1%and outlet PO fraction of 1.8%.These results can be rationalized by indepth analyses and discussion about the effects of the decision variables on the operation safety and reaction performance.The insights revealed here could shed new light on the process development of the PO production based on the estimation of the explosion limits of the multi-component feed gas mixture containing flammable gase s,inert gas and O2,followed by process optimization.

Optimization of catalyst pellet structures and operation conditions for CO methanation

A fundamental understanding of the effects of catalyst pellet structures and operation conditions on catalytic performance is crucial for the reactions limited by diffusion mass transfer. In this work, a numerical investigation has been carried out to understand the effect of catalyst pellet shapes(sphere, cylinder, trilobe and tetralobe) on the reaction-diffusion behaviors of CO methanation. The results reveal that the poly-lobe pellets with larger external specific surface area have shorter diffusion path, and thus result in higher effectiveness factors and CO conversion rates in comparison with the spherical and cylindrical pellets. The effects of operating conditions and pore structures on the trilobular catalyst pellet with high performance are further probed. Though lower temperature can contribute to larger effectiveness factors of pellets, it also brings about lower reaction rates, and pressure has little impact on the effectiveness factors of the pellets. The increase in porosity can reduce the pellet internal diffusion limitations effectively and there exists an optimal porosity for the methanation reaction. Finally, the height of the trilobular pellet is optimized under the given geometric volume, and the results demonstrate that the higher the trilobular catalyst, the better the reaction performance within the allowable mechanical strength range.

Taming Electrons in Pt/C Catalysts to Boost the Mesokinetics of Hydrogen Production

Taming the electron transfer across metal–support interfaces appears to be an attractive yet challenging methodology to boost catalytic properties.Herein,we demonstrate a precise engineering strategy for the carbon surface chemistry of Pt/C catalysts—that is,for the electron-withdrawing/donating oxygencontaining groups on the carbon surface—to fine-tune the electrons of the supported metal nanoparticles.Taking the ammonia borane hydrolysis as an example,a combination of density functional theory(DFT)calculations,advanced characterizations,and kinetics and isotopic analyses reveals quantifiable relationships among the carbon surface chemistry,Pt charge state and binding energy,activation entropy/enthalpy,and resultant catalytic activity.After decoupling the influences of other factors,the Pt charge is unprecedentedly identified as an experimentally measurable descriptor of the Pt active site,contributing to a 15-fold increment in the hydrogen generation rate.Further incorporating the Pt charge with the number of Pt active sites,a mesokinetics model is proposed for the first time that can individually quantify the contributions of the electronic and geometric properties to precisely predict the catalytic performance.Our results demonstrate a potentially groundbreaking methodology to design and manipulate metal–carbon catalysts with desirable properties.

Fabrication of K-promoted iron/carbon nanotubes composite catalysts for the Fischer–Tropsch synthesis of lower olefins

K-promoted iron/carbon nanotubes composite（i.e., Fe K-OX） was prepared by a redox reaction between carbon nanotubes and K2FeO4followed by thermal treatments on a purpose as the Fischer–Tropsch catalyst for the direct conversion of syngas to lower olefins. Its catalytic behaviors were compared with those of the other two Fe-IM and Fe K-IM catalysts prepared by impregnation method followed by thermal treatments. The novel Fe K-OX composite catalyst is found to exhibit higher hydrocarbon selectivity,lower olefins selectivity and chain growth probability as well as better stability. The catalyst structureperformance relationship has been established using multiple techniques including XRD, Raman, TEM and EDS elemental mapping. In addition, effects of additional potassium into the Fe K-OX composite catalyst on the FTO performance were also investigated and discussed. Additional potassium promoters further endow the catalysts with higher yield of lower olefins. These results demonstrated that the introduction method of promoters and iron species plays a crucial role in the design and fabrication of highly active,selective and stable iron-based composite catalysts for the FTO reaction.

Poisoning effect of polyvinyl chloride on the catalytic pyrolysis of mixed plastics over zeolites

Nowadays,the chemical recycling is applied for only 1%of total waste plastics,largely due to contaminants in plastic waste and difficulty in product control.As the major contaminant,polyvinyl chloride(PVC)often forms corrosive hydrogen chloride(HCl)during the chemical recycling,which may cause severe catalyst deactivation and equipment damage.However,the investigation on catalytic pyrolysis(the major route for plastics chemical recycling)of the PVC containing mixed plastics has been rarely reported.Here,catalytic co-pyrolysis of PVC and polyethylene(PE)was studied over an aromatization catalyst,Pt/ZSM-5,since the basic building block aromatics are desired products from plastics chemical recycling.The poisoning effect of PVC vapor on the catalyst stability was explored by collective efforts of thorough product analysis and catalyst characterization.It was found that the HCl evolving from PVC has an autocatalytic effect that promotes the scission of dehydrochlorinated PVC,resulting in the high yield of benzene and acetylene from PVC.On the other hand,the presence of PVC suppressed the aromatics formation from PE,largely due to the poisoning effect of PVC-derived HCl on the Pt/ZSM-5.The deactivation is irreversible as evidenced by the decreased zeolite crystallinity and the loss of strong acid sites that are key to the aromatization,possibly due to the removal of framework Al upon the interaction with HCl.The modification with octadecylphosphonic acid only slightly alleviated the PVC poisoning effect.The insights on the PVC poisoning of zeolite catalysts provided in this work may guide the process design of chemical recycling of PVC containing waste plastics.

Efficient continuous synthesis of 2-hydroxycarbazole and 4-hydroxycarbazole in a millimeter scale photoreactor

2-Hydroxycarbazole and 4-hydroxycarbazole are important chemicals with extensive applications in optoelectronic materials and pharmaceutical field.State of the art yield of 2-hydroxycarbazole is~30%and the reaction time is typically in hours or days.Herein,we developed a green route for the continuous and high-throughput synthesis of 2-hydroxycarbazole and 4-hydroxycarbazole via photochemical intramolecular cyclization of 3–hydroxy-2–chloro-diphenylamine using a self-designed millimeter scale photoreactor,which was designed based on sizing-up and numbering-up strategies for a decent liquid holdup(6.8 m L)and fabricated via femtosecond laser engraving technique.The photochemical synthesis was carried out continuously under the illumination of 365 nm UV-LED with dimethyl sulfoxide as solvent and potassium t-butoxide as catalyst.It was found that under optimized conditions a 2-hydroxycarbazole yield of 31.6%and a 4-hydroxycarbazole yield of 11.1%were obtained with a residence time of 1 min.Compared to semibatch operations,the reaction time was shortened by 1–2 orders of magnitude.As a result,a throughput of 11.3 g/day 2-hydroxycarbazole and 4.0 g/day 4-hydroxycarbazole can be achieved from the photoreactor.It was proposed that the short reaction time and high product yield are resulted from higher photon transfer rates and more uniform photon distribution provided by the millimeter scale photoreactor,which enhances the reaction rates and mitigates overreaction.

Engineering electronic platinum-carbon support interaction to tame carbon monoxide activation

CO oxidation has been studied for more than a century;however,molecular-level understanding of its activation protocol and related intermediates remains elusive.Here,we present a unified mechanistic and kinetic picture of various electronic metal-support interactions within platinum-carbon catalysts via in situ spectroscopic/kinetic analyses and multi-scale simulations.Transient kinetic analysis and molecular dynamics simulations with a reactive force field provided a quantitative description of the competition between the oxygen association and oxygen dissociation mechanisms tuned by the interfacial charge distribution and CO coverage.Steady-state isotopic transient kinetic analysis and density functional theory calculations revealed a simultaneous shift in the rate-determining step(RDS)from O_(2)^(*)dissociation to O^(*)and CO^(*)and O_(2)^(*)and CO^(*)association.A de novo strategy from the interfacial charge distribution to the reaction mechanism,kinetics/thermodynamics of RDS,and,ultimately,catalytic performance was developed to quantitatively map the above CO activation mechanism with an order-of-magnitude increase in reactivity.The proposed catalytic picture and de novo strategy are expected to prompt the development of theories and methodologies for heterogeneous catalysis.

Kinetics and mechanistic insights into the active sites of Au catalysts for selective propylene oxidation

Identification of the catalytically active sites emerges as the prerequisite for an atomic-level comprehensive understanding and further rational design of highly efficient catalysts.Here,we demonstrate a kinetics strategy to identify the active sites of Au catalyst for the disentanglement of geometric and electronic effects on the selective oxidation of propylene to acrolein.Both the Ti-containing titanium-silicalite-1(TS-1)and Ti-free silicalite-1(S-1)were employed as supports to immobilize Au catalysts,which were investigated by a combination of multiple characterization,kinetics analysis,crystal structure modelling.The Au(111)sites are identified as the main active site for acrolein formation,while their electronic effects are highly relevant to the presence or absence of Ti.Moreover,propylene epoxide(PO)formation mainly involves the co-participation of Au and Ti sites,the proximity between Au and Ti sites is found to have less influences on PO formation in a certain distance.In comparison,acrolein is very likely to generate over Au(111)sites via the hydrogen-assisted O_(2) activation to oxygenated species for its oxidizing propylene.The insights gained here could guide the design and preparation of Au catalysts for selective propylene oxidation.

Residence time distribution and heat/mass transfer performance of a millimeter scale butterfly-shaped reactor

A millimeter scale butterfly-shaped reactor was proposed based on sizing-up strategy and fabricated via femtosecond laser engraving. An improvement of mixing performance and residence time distribution was realized by means of contraction and expansion of the reaction channel. The liquid holdup was greatly increased through connection of multiple mixing units. Structure optimization of the reactor was carried out by computational fluid dynamics simulation, from which the effect of reactor internals on mixing and the influence of parallel branching structure on heat transfer were discussed. The UV–vis absorption spectroscopy was used to determine the residence time distribution in the reactor, and characteristic parameters such as skewness and dimensionless variance were obtained. Further, a chained stagnant flow model was proposed to precisely describe the trailing phenomenon caused by fluid stagnation and laminar flow in small scale reactors, which enables a better fit for the experimental results of the asymmetric residence time distribution. In addition, the heat transfer performance of the reactor was investigated, and the overall heat transfer coefficient was 110–600 W m^(-2)K-1in the flow rate range of 10–40 m L/min.

Ultralow specific surface area vermiculite supporting Mn-Ce-Fe mixed oxides as“curling catalysts”for selective catalytic reduction of NO with NH_(3)

In this study,the ultralow specific surface area clay vermiculite(VMT)was selected to be a catalyst support for the NH_(3)-SCR process,and the active components MnCeFeO_(x)loaded on vermiculite was just like curling on ice from the TEM results.The de-NO_(x)performance of Mn-Ce-Fe/VMT exhibited almost complete NO conversion with a gas hourly space velocity(GHSV)of 15,300 h^(-1)at 150℃,which was 25%and 10%higher than that of Mn/VMT and Mn-Ce/VMT,respectively.Ce and Fe co-doping improved the BET surface area,the quantities of active Mn^(4+),the acid sites and NH_(3)adsorption energy of Mn/VMT,all of which contributed to the increase in low-temperature SCR activity.In situ DRIFT measurements suggested that NO_(x)removal over Mn-Ce-Fe/VMT followed both Eley-Rideal(E-R)and Langmuir-Hinshelwood(L-H)mechanisms at 150℃,but the E-R mechanism played a dominant role.Corresponding Mn-Ce-Fe/VMT monolithic catalysts reached 90%NO conversion with a GHSV of 4000 h^(-1).