SIMULATION MODELING ON THE COORDINATION MECHANISM OF ETHYLENE MONOMER ON VARIOUS PREREDUCED Cr（Ⅱ）Ox/SiO2 PHILLIPS POLYETHYLENE MODEL CATALYSTS

As one of the most important catalysts in polyethylene industry,Phillips catalyst(CrO_x/SiO_2)was quite unique for its activation by ethylene monomer without using any activator like alkyl-aluminium or MAO.In this work,the density functional theory(DFT)calculation combined with paired interacting orbitals(PIO)method was applied for the theoretical studies on coordination reaction mechanism between ethylene monomer and two model catalysts namely Cr(Ⅱ)(OH)_2(M1) and silsesquioxane-supported Cr(Ⅱ)(M2)as surfac...

Cobalt porphyrins supported on carbon nanotubes as model catalysts of metal-N_(4)/C sites for oxygen electrocatalysis

Transition-metal based M-N_4/C catalysts are appealing for electrocatalytic oxygen reduction reaction(ORR) and oxygen evolution reaction(OER). Employing model catalysts, which have well-defined molecular structures and coordination environments, to investigate electrocatalytic performance of M-N_4/C sites for ORR and OER is of fundamental significance. Herein, we reported the use of Co tetra(phenyl)porphyrin 1 and Co tetra(pentafluorophenyl)porphyrin 2 as models to probe the role of Co-N_4/C sites for oxygen electrocatalysis. We showed that Co porphyrin 1 is more efficient than its structural analogue 2 for oxygen electrocatalysis in alkaline aqueous solutions, indicating that the electronrich Co-N_4/C site is more favored when noncovalently adsorbed on carbon supports. This work inspires rational design of reaction-oriented catalysts for sustainable energy storage and conversion technologies.

Nanoscale architecture of ceria-based model catalysts: Pt-Co nanostructures on well-ordered CeO_(2)(111) thin films

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt-Co core-shell catalysts. Pt@Co and Co@Pt core-shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields CoCeO2(111) solid solution at low Co coverage(0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt-Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt-Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Unraveling the advantages of Pd/CeO_(2)single-atom catalysts in the NO+CO reaction by model catalysts

Selective catalytic reduction of NO by CO is challenging in environmental catalysis but attractive owing to the advantage of simultaneous elimination of NO and CO.Here,model catalysts consisting of Pd nanoparticles(NPs)and single-atom Pd supported on a CeO_(2)(111)film grown on Cu(111)(denoted as Pd NPs/CeO_(2)and Pd_(1)/CeO_(2),respectively)were successfully prepared and characterized by synchrotron radiation photoemission spectroscopy(SRPES)and infrared reflection absorption spectroscopy(IRAS).The NO+CO adsorption/reaction on the Pd_(1)/CeO_(2)and Pd NPs/CeO_(2)catalysts were carefully investigated using SRPES,temperature-programmed desorption(TPD),and IRAS.It is found that the reaction products on both model catalysts are in good agreement with those on real catalysts,demonstrating the good reliability of using these model catalysts to study the reaction mechanism of the NO+CO reaction.On the Pd NPs/CeO_(2)surface,N_(2)is formed by the combination of atomic N coming from the dissociation of NO on Pd NPs at higher temperatures.N_(2)O formation occurs probably via chemisorbed NO combined with atomic N on the surface.While on the single-atom Pd_(1)/CeO_(2)surface,no N_(2)O is detected.The 100%N_(2)selectivity may stem from the formation of O-N-N-O^(*)intermediate on the surface.Through this study,direct experimental evidence for the reaction mechanisms of the NO+CO reaction is provided,which supports the previous density functional theory(DFT)calculations.

Recent Advances in Mechanistic Understanding of Metal-Free Carbon Thermocatalysis and Electrocatalysis with Model Molecules

Metal-free carbon,as the most representative heterogeneous metal-free catalysts,have received considerable interests in electro-and thermo-catalytic reac-tions due to their impressive performance and sustainability.Over the past decade,well-designed carbon catalysts with tunable structures and heteroatom groups coupled with various characterization techniques have proposed numerous reaction mechanisms.However,active sites,key intermediate species,precise structure-activity relationships and dynamic evolution processes of carbon catalysts are still rife with controversies due to the monotony and limitation of used experimental methods.In this Review,we sum-marize the extensive efforts on model catalysts since the 2000s,particularly in the past decade,to overcome the influences of material and structure limitations in metal-free carbon catalysis.Using both nanomolecule model and bulk model,the real contribution of each alien species,defect and edge configuration to a series of fundamentally important reactions,such as thermocatalytic reactions,electrocatalytic reactions,were systematically studied.Combined with in situ techniques,isotope labeling and size control,the detailed reaction mechanisms,the precise 2D structure-activity relationships and the rate-determining steps were revealed at a molecular level.Furthermore,the outlook of model carbon catalysis has also been proposed in this work.

Metal-N_(4)model single‐atom catalyst with electroneutral quadri‐pyridine macrocyclic ligand for CO_(2)electroreduction

Metal–N–C single‐atom catalysts,mostly prepared from pyrolysis of metalorganic precursors,are widely used in heterogeneous electrocatalysis.Since metal sites with diverse local structures coexist in this type of material and it is challenging to characterize the local structure,a reliable structure–property relationship is difficult to establish.Conjugated macrocyclic complexes adsorbed on carbon support are well‐defined models to mimic the singleatom catalysts.Metal–N_(4)site with four electroneutral pyridine‐type ligands embedded in a graphene layer is the most commonly proposed structure of the active site of single‐atom catalysts,but its molecular counterpart has not been reported.In this work,we synthesized the conjugated macrocyclic complexes with a metal center(Co,Fe,or Ni)coordinated with four electroneutral pyridinic ligands as model catalysts for CO_(2)electroreduction.For comparison,the complexes with anionic quadri‐pyridine macrocyclic ligand were also prepared.The Co complex with the electroneutral ligand expressed a turnover frequency of CO formation more than an order of magnitude higher than that of the Co complex with the anionic ligand.Constrained ab initio molecular dynamics simulations based on the well‐defined structures of the model catalysts indicate that the Co complex with the electroneutral ligand possesses a stronger ability to mediate electron transfer from carbon to CO_(2).

Morphology-Dependent Catalysis of CeO_(2)-Based Nanocrystal Model Catalysts

Comprehensive Summary Recent progress in colloidal synthesis has realized the preparations of uniform nanocrystal(NC)model catalysts with rich and well-controlled morphologies that were employed to explore structure-activity relationships of powder catalysts,similar to single-crystal-based model catalysts under ultrahigh vacuum conditions but can work at the same conditions as powder catalysts without the"materials gap"and"pressure gap".In this perspective,the CeO_(2)-based NC model catalysts with various morphologies are included and their relevant progresses are critically reviewed.The detailed descriptions of morphology-controlled synthesis and characterizations of uniform CeO_(2)NCs.

Combination of a reaction cell and an ultra-high vacuum system for the in situ preparation and characterization of a model catalyst

An in-depth understanding of the structure-activity relationship between the surface structure,chemical composition,adsorption and desorption of molecules,and their reaction activity and selectivity is necessary for the rational design of high-performance catalysts.Herein,we present a method for studying catalytic mechanisms using a combination of in situ reaction cells and surface science techniques.The proposed system consists of four parts:preparation chamber,temperatureprogrammed desorption(TPD)chamber,quick load-lock chamber,and in situ reaction cell.The preparation chamber was equipped with setups based on the surface science techniques used for standard sample preparation and characterization,including an Ar+sputter gun,Auger electron spectrometer,and a low-energy electron diffractometer.After a well-defined model catalyst was prepared,the sample was transferred to a TPD chamber to investigate the adsorption and desorption of the probe molecule,or to the reaction cell,to measure the catalytic activity.A thermal desorption experiment for methanol on a clean Cu(111)surface was conducted to demonstrate the functionality of the preparation and TPD chambers.Moreover,the repeatability of the in situ reaction cell experiment was verified by CO_(2) hydrogenation on the Ni(110)surface.At a reaction pressure of 800 Torr at 673 K,turnover frequencies for the methanation reaction and reverse water-gas shift reaction were 0.15 and 7.55 Ni atom^(-1) s^(-1),respectively.

Surface Active Sites on Co_(3)O_(4) Nanobelt and Nanocube Model Catalysts for CO Oxidation

CO oxidation has been performed on Co_(3)O_(4) nanobelts and nanocubes as model catalysts.The Co_(3)O_(4) nanobelts which have a predominance of exposed{011}planes are more active than Co_(3)O_(4) nanocubes with exposed{001}planes.Temperature programmed reduction of CO shows that Co_(3)O_(4) nanobelts have stronger reducing properties than Co_(3)O_(4) nanocubes.The essence of shape and crystal plane effect is revealed by the fact that turnover frequency of Co3+sites of{011}planes on Co_(3)O_(4) nanobelts is far higher than that of{001}planes on Co_(3)O_(4) nanocubes.

Mechanism and active sites of CO oxidation over single-crystal Au surfaces and a Au/TiO_2(110) model surface

We describe the reaction mechanism and active sites for CO oxidation over a Au/TiO2（110） model surface and Au single‐crystal surfaces, along with the role of H2O, on a molecular scale. At low tem‐perature （＆lt;320 K）, H2O played an essential role in promoting CO oxidation, and the active site for CO oxidation was the perimeter of the interface between the gold nanoparticles and the TiO2 sup‐port （Auδ＋–Oδ––Ti）. We believe that the O–O bond was activated by the formation of OOH, which was produced directly from O2 and H2O at the perimeter of the interface between the gold nanoparticles and the TiO2 support, and consequently OOH reacted with CO to form CO2. This reaction mechanism explains the dependence of the CO2 formation rate on O2 pressure at 300 K. In contrast, at high temperature （＆gt;320 K）, low‐coordinated gold atoms built up on the surface as a result of surface reconstruction due to exposure to CO. The low‐coordinated gold atoms adsorbed O2, which then dissociated and oxidized CO on the metallic gold surface.

Oxidation of formic acid on stepped Au(997) surface

The adsorption and reaction of formic acid （HCOOH） on clean and atomic oxygen‐covered Au（997） surfaces were studied by temperature‐programmed desorption/reaction spectroscopy （TPRS） and X‐ray photoelectron spectroscopy （XPS）. At 105 K, HCOOH molecularly adsorbs on clean Au（997） and interacts more strongly with low‐coordinated Au atoms at （111） step sites than with those at （111） terrace sites. On an atomic oxygen‐covered Au（997） surface, HCOOH reacts with oxygen at‐oms to form HCOO and OH at 105 K. Upon subsequent heating, surface reactions occur among ad‐sorbed HCOO, OH, and atomic oxygen and produce CO2, H2O, and HCOOH between 250 and 400 K. The Au（111） steps bind surface adsorbates more strongly than the Au（111） terraces and exhibit larger barriers for HCOO（a） oxidation reactions. The surface reactions also depend on the relative coverages of co‐existing surface species. Our results elucidate the elementary surface reactions between formic acid and oxygen adatoms on Au surfaces and highlight the effects of the coordina‐tion number of the Au atoms on the Au catalysis.

Reveal the nature of particle size effect for CO_(2) reduction over Pd and Au

Small cluster and periodic surface models with low coverages of intermediates are frequently employed to investigate reaction mechanisms and identify active sites on nanoparticles(NPs)in density functional theory(DFT)studies.However,diverse active sites on NPs cannot be sufficiently represented by these simple models,hampering the in-depth insights into the catalytic behavior of NPs.This paper describes the crucial roles of both model and coverage effect on understanding the nature of active sites for CO_(2)reduction over Au and Pd NPs using DFT calculations.Terrace sites exhibit higher selectivity for CO than edge sites on Au NPs,which is opposite to the results on Au periodic surfaces.This contradiction reveals the computational model effect on clarifying active site properties.For Pd catalysts,the coverage effect is more significant.On bare Pd NPs and periodic surfaces,the selectivity for CO at edge sites is nearly identical to that at terrace sites,whereas edge sites display higher selectivity for CO than terrace sites in the case of high CO coverages.Through considering the more realistic models and the coverage effect,we successfully describe the size effect of Au and Pd NPs on CO selectivity.More importantly,this work reminds us of the necessity of reasonable models in DFT calculations.

Kinetic studies on the dimerization of isobutene with Ni/Al_2O_3 as a catalyst for reactive distillation process

Isooctane is a promising gasoline additive that could be produced by dimerization of isobutene(IB) with subsequent hydrogenation.In this work,the dimerization of IB has been carried out in a batch reactor over a temperature range of 338-383 K in the presence of laboratory prepared Ni/Al_2O_3 as a catalyst and n-pentane as solvent.The influence of various parameters such as temperature,catalyst loading and initial concentration of IB was examined.A Langmuir-Hinshelwood kinetic model of IB dimerization was established and the parameters were estimated on the basis of the measured data.The feasibility of oligomerization of IB based on the reactive distillation was simulated in ASPEN PLUS using the kinetics developed.The simulation results showed that the catalyst of Ni/Al_2O_3 had higher selectivity to diisobutene(DIB) and slightly lower conversion of IB than ion exchange resin in the absence of polar substances.

Estimation of catalytic activity using an unscented Kalman filtering in condensation reaction

The catalytic activity of cation exchange resins will be continuously reduced with its use time in a condensation reaction for bisphenol A(BPA).For online estimation of the catalytic activity,a catalytic deactivation model is studied for a production plant of BPA,state equation and observation equation are proposed based on the axial temperature distribution of the reactor and the acetone concentration at reactor entrance.A hybrid model of state equation is constructed for improving estimation precision.The unknown parameters in observation equation are calculated with sample data.The unscented Kalman filtering algorithm is then used for on-line estimation of the catalytic activity.The simulation results show that this hybrid model has higher estimation accuracy than the mechanism model and the model is effective for production process of BPA.

Unraveling the Dynamic Structural Evolution of Phthalocyanine Catalysts during CO_(2) Electroreduction

Understanding the atomic and electronic changes of active sites promotes the whole new sight into electrochemical carbon dioxide reduction reaction(CO_(2)RR),which provides a feasible strategy to achieve carbon neutrality.Here we employ operando high-energy resolution fluorescence-detected Xray absorption spectroscopy(HERFD-XAS)to track the structural evolution of Ni(II)phthalocyanine(NiPc),considered as the model catalysts with uniform Ni-N_(4)-C_(8) moiety,during the CO_(2)RR.The HERFD-XAS method is in favor of elucidating the interaction of the reactant/catalyst interface from the atomic electronic structure dimension,facilitating the establishment of the catalytic mechanism and the dynamic structure changes.Based on operando measurement,surface sensitive difference spectra(△μ)and spectroscopy simulation,the interfacial interactions between the active sites of NiPc and reactants are monitored and the Ni species gradually reduced by increasing the applied potential is discovered.HERFD-XAS method offers an advanced and powerful tool for elucidating the complex catalytic mechanism in further various systems.

Catalytic process modeling and sensitivity analysis of alkylation of benzene with ethanol over MIL-101(Fe) and MIL-88(Fe)

A solvothermal method was used to synthesize MIL-101(Fe)and MIL-88(Fe),which were used for alkylation of benzene.The synthesized catalysts were characterized by X-ray diffraction,Fourier transform infrared spectroscopy,field emission scanning electron microscope,dynamic light scattering,and BET techniques.Metal-organic frameworks(MOFs)were modeled to investigate the catalytic performance and existence of mass transfer limitations.Calculated effectiveness factors revealed absence of internal a nd external mass transfer.Sensitivity analysis revealed best operating conditions over MIL-101 at 120℃ and 5 bar and over MIL-88 at 142℃ and 9 bar.

Inverse single-site Fe_(1)(OH)_(x)/Pt(111)model catalyst for preferential oxidation of CO in H_(2)

Inverse oxide/metal model systems are frequently used to investigate catalytic structure-function relationships at an atomic level.By means of a novel atomic layer deposition process,growth of single-site Fe_(1)O_(x) on a Pt(111)single crystal surface was achieved,as confirmed by scanning tunneling microscopy(STM).The redox properties of the catalyst were characterized by synchrotron radiation based ambient pressure X-ray photoelectron spectroscopy(AP-XPS).After calcination treatment at 373 K in 1 mbar O_(2).the chemical state of the catalyst was determined as Fe^(3+).Reduction in 1 mbar H_(2) at 373 K demonstrates a facile reduction to Fe2+and complete hydroxylation at significantly lower temperatures than what has been reported for iron oxide nanoparticles.At reaction conditions relevant for preferential oxidation of CO in H_(2)(PROX),the catalyst exhibits a Fe3+state(ferric hydroxide)at 298 K while re-oxidation of iron oxide clusters does not occur under the same condition.CO oxidation proceeds on the single-site Fei(OH)3 through a mechanism including the loss of hydroxyl groups in the temperature range of 373 to 473 K,but no reaction is observed on iron oxide clusters.The results highlight the high flexibility of the single iron atom catalyst in switching oxidation states,not observed for iron oxide nanoparticles under similar reaction conditions,which may indicate a higher intrinsic activity of such single interfacial sites than the conventional metal-oxide interfaces.In summary,our findings of the redox properties on inverse single-site iron oxide model catalyst may provide new insights into applied Fe-Pt catalysis.

Molecular Insights into the Structure-Activity Relationship in Cobalt Porphyrin Catalyzed Oxygen Reduction Reaction

We present a microscopic investigation on the structure-activity relationship of the Co-N4 site in the oxygen reduction reaction(ORR)by electrochemical scanning tunneling microscopy(ECSTM)at the molecular scale.The cobalt porphyrins with various substituents(CoTPPX_(4),X=Cl,H,OCH_(3))that delicately regulate the electronic structure of the active site were investigatedasmodel catalysts.Electrochemical measurements evidenced that the CoTPPCl_(4)exhibits better activity,higher product selectivity for H_(2)O,and lower stability.The CoTPPX_(4)-O_(2)complex with higher contrast can be observed in the STM images and the proportion of the CoTPPCl_(4)-O_(2)is appreciably larger than that of CoTPP-O_(2)and CoTPP(OCH_(3))4-O_(2).Theoretical simulations of the model catalysts and the reaction processes of the ORR reveal the relationship between the electronic structure and the catalytic behavior of the model catalysts.The transformation of the CoTPPX_(4)-O_(2)and CoTPPX_(4)in the electrocatalytic reaction was monitored by in situ ECSTM characterization.The structure-activity relationship clarified by experimental and theoretical investigations in this work should help to guide the rational design and optimization of high-performance catalysts.

Structural Dependence of Competitive Adsorption of Water and Methanol on TiO2 Surfaces

Employing TiO2 anatase （001）-（1 × 4）, futile （110） and futile （011）-（2× 1） single crystal surfaces, we compre- hensively studied the effects of TiO2 surface structures on the competitive adsorption of water and methanol by means of low energy electron diffraction, thermal desorption spectra and X-ray photoelectron spectroscopy. The relative adsorption strengths of chemisorbed methanol and water vary with the TiO2 surface structures and the ad- sorption sites. This leads to TiO2 surface structure-dependent competitive adsorption of water and methanol. The chemisorption of CH3OH on TiO2 anatase （001）-（1 × 4） surface is seldom affected by pre-covered water at low cov- erages but is affected by pre-covered water at high coverages; the chemisorption of CH3OH on TiO2 rutile （110） surface is seldom affected by pre-covered water; and the chemisorption of CH3OH on TiO2 rutile （011）-（2 × 1） sur- face is affected by pre-covered water even at low coverages. These results deepen the fundamental understandings of surface chemistry on TiO2 surfaces.

Preparation of freestanding palladium nanosheets modified with gold nanoparticles at edges

Electronic adjustment is one of the most commonly used strategies to improve the catalytic performance of heterogeneous catalysts. We prepared hexagonal ultrathin Pd nanosheets with edges modified by gold nanoparticles （Au@Pd nanosheets） using galvanic replacement method. By virtue of the electronic interactions between the Pd nanosheets and Au nanoparticles, the Au@Pd nanosheets exhibited excellent catalytic performances in the carbonylation of iodobenzene by carbon monoxide. The novel nanocomposites could be applied as model catalysts to explore electronic effects in catalysis.