Ultra-stable metal nano-catalyst synthesis strategy:a perspective

Xiao-Qing Cao Jun Zhou Song Li Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,School of Materials Science and Engineering,Northeastern University

作者简介：*Jun Zhou,e-mail:hafouniu@163.com,received his Ph.D.in Physical Chemistry from Jilin University in 2015,and then,he entered into the Northeastern University as a postdoctoral fellow in the group of Prof. Gao-Wu Qin.In 2018,he joined the Faculty at the School of Materials Science and Engineering at the Northeastern University.His current research interest focuses on the design of the supported metal catalysts and their application in heterogeneous catalysis.;*Song Li,e-mail:lis@atm.neu.edu.cn;

收稿日期：31 July 2019

基金：financially supported by the National Natural Science Foundation of China (Nos.51601032, 51971059,5197010432);the Fundamental Research Funds for the Central Universities (Nos.N170204015,N180204014);the Provincial Science and Technology Project/Doctor Start Fund (No. 20170520385);

Ultra-stable metal nano-catalyst synthesis strategy:a perspective

Xiao-Qing Cao Jun Zhou Song Li Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,School of Materials Science and Engineering,Northeastern University

Abstract：

Supported metal nanoparticles(NPs) as an important heterogeneous catalyst have been widely applied in various industrial processes.During the catalytic reaction,size of the particles plays an important role in determining their catalytic performance.Generally,the small particles exhibit superior catalytic activity in comparison with the larger particles because of an increase in lowcoordinated metal atoms on the particle surface that work as active sites,such as edges and corner atoms.However,these small NPs are typically unstable and tend to migrate and coalescence to reduce their surface free energy during the real catalytic processes,particularly in high-temperature reactions.Therefore,a means to fabricate stable small metal NP catalysts with excellent sinter-resistant performance is necessary for maintaining their high catalytic activity.In this study,we have summarized recent advances in stabilizing metal NPs from two aspects including thermodynamic and kinetic strategies.The former mainly involve preparing uniform NPs(with an identical size and homogeneous distribution) in order to restrain Ostwald ripening to achieve stability,while the latter primarily involves fixing metal NPs in some special confinement materials(e.g.,zeolites,mesoporous silica and mesoporous carbons),encapsulating NPs using an oxide-coating film(e.g.,forming core-shell structures),or constructing strong metal-support interactions to improve stability.At the end of this review,we highlight our recent work on the preparation of high-stability metal catalysts via a unique interfacial plasma electrolytic oxidation technology,that is,metal NPs are well embedded in a porous MgO layer that has both high thermal stability and excellent catalytic activity.

Keyword：

Nano-catalysts; Thermal stability; Sinterresistant; Synthesis strategy;

Received： 31 July 2019

1 Introduction

Catalysts based on metal nanoparticles (NPs) have attracted great attention because of their excellent catalytic activities.As is known,the metal NP diameter is among the most critical factors in determining their reactivity and specificity for the catalytic reaction [ 1, 2, 3] .Therefore,enormous efforts have been devoted to improving the performance of supported metal catalysts by downsizing NPs [ 3, 4] .In particular,noble metal NPs,as a type of dominant-supported catalyst,show excellent catalytic activity in various reactions due to their special electron structures and small particle sizes that generate a high fraction of surface active atoms.However,because of their low Tammann temperatures and high surface free energy [ 5, 6] ,these small NPs spontaneously tend to sinter and easily deactivate during reactions,particularly under hightemperature conditions.For example,the catalysts used for catalyzing combustion of hydrocarbons [ 7, 8] have to work at a temperature above 600℃,which would cause severe sintering and hence lead to a loss of catalytic activity.Additionally,the regeneration processes of supported metal catalysts are also complex and expensive,typically involving metal leaching,purification and re-deposition [ 9] ,which increase the production cost.Therefore,rationally and effectively stabilizing ultra-small metal NPs on supports against sintering and aggregation are of great significance for heterogeneous catalysis in practical applications.

The ideal sinter-resistant catalyst typically has the following characteristics [ 10] :(1) its integrated components are thermally stable and without structural collapse under hightemperature conditions;(2) its active sites are fully accessible by reactants without substantial mass transfer resistance;(3) its catalytic activity remains unchanged even after treatment at elevated temperatures.Recent theoretical and experimental studies demonstrate that either a thermodynamic approach or a kinetic approach [ 10, 11, 12, 13] can effectively stabilize noble metal NPs on supports.In the thermodynamic approach,NPs of uniform size distribution are essential for improving their stability due to their similar chemical potential that could suppress Ostwald ripening during the catalytic reaction.Kinetic approaches mainly involve the confinement of noble metal NPs via a special material such as a zeolite framework [ 14, 15] or core/shell structure [ 16] to resist aggregation and sintering.Over the past several decades,physical encapsulation methods,as an effective stabilization strategy,have attracted considerable attention because of their attractive features including wide metal applicability,high stability and support persity.Many core/shell heterogeneous catalysts,such as Au@ZrO₂ [ 10] ,Au@SiO₂ [ 17] and Pt@TiO₂ [ 18] ,have been successfully synthesized and exhibit considerable low loss in activity even though they undergo several cycles.In addition,the strong metal-support interaction [ 19] (SMSI)between the active species and support has been regarded as a facile route to form a barrier against NP migration and coalescence to achieve high stability.Importantly,the SMSI effect also causes a positive influence on the catalytic performance due to the electronic and geometric characteristics from the oxide supports [ 20] ,which further motivates the development of new catalysts and catalytic mechanisms.Thus far,various SMSIs have been developed (e.g.,Au/HAP(hydroxyapatite) [ 21] ,Au/Al₂O₃ [ 22] ,Au/TiO₂ [ 23] ,etc.)which not only show enhanced sintering resistance upon calcination,but also the improved activity and selectivity.Therefore,a detailed and systematic description regarding the preparation of stable nano-catalysts is indispensable to better understand catalysis in both fundamental research and practical application.

From this perspective,we will mainly focus on the recent developments regarding the stabilization of NPs.In the first section,two categories based on thermodynamic or kinetic considerations are discussed for enhancing the stability of NPs against aggregation.Then,we introduce a new preparation strategy developed by our group,the interfacial plasma electrolytic oxidation technology,which generates a unique embedded structure to significantly improve the catalytic stability.Finally,we provide our viewpoints about the future development of ultrastable supported metal nano-catalyst synthesis.

2 Thermodynamic approach

The thermodynamic approach aims to reduce the difference in chemical potential and surface energy between the NPs by which the Ostwald ripening can be effectively suppressed.Li et al.studied thermodynamic behavior for supported metal particles in various reaction environments and proposed a general strategy for suppressing the sintering of supported metal NPs via theory calculations [ 5] .They found that particle size and distribution are essential factors for determining their stability and a narrow size and homogeneous distribution suppresses the Ostwald ripening rate due to the NPs similar chemical potential and surface energy.In addition,a support with high surface activation energy can generate the same effect for restraining the Ostwald ripening action.Therefore,for preparation of NPs with identical size and excellent distribution,the choice of an appropriate support is desirable for improving the stability of supported nano-catalysts.

3 Kinetic approach

The dynamical approach mainly takes advantage of an external medium to restrain the growth of NPs in the harsh condition during the catalytic reaction,e.g.,high temperature,high pressure,etc.In this section,three strategies including space confinement,coating structure and strong metal-support interaction are proposed to stabilize the NPs.For the space confinement,porous materials,e.g.,zeolite,porous carbon and porous silica,are used for inhibiting particle migration and aggregation.Regarding the coating means,the core-shell configuration and Al₂O₃ overcoats are developed,in order to prevent particle growth.The last approach is strong metal-support interaction that mainly fabricates some special structures,e.g.,an embedded configuration to maintain the size and structure of NPs in the catalytic surroundings.

3.1 Space confinement strategy

3.1.1 Zeolite support

Zeolite materials have attracted considerable attention as ideal supports for the confinement synthesis of metal NP catalysts due to their well-defined channels,tunable acidity and high stability [ 24] .(Details are summarized in Table 1 [ 25, 26, 27, 28, 29, 30, 31, 32] .) Zeolite materials encapsulating metal NPs into microporous cavities effectively inhibit further NP migration and ripening due to the nano-space confinement effect,leading to improved catalytic stability.Generally,NPs loaded on zeolite supports mainly involve ion exchange [ 33] and wet impregnation [ 34, 35] .However,these methods based on prefabricated zeolite materials cannot efficiently control the size distribution and location of metal species.Partly supported active components on the external surface would occur aggregation and growth during some harsh thermal treatments,which results in severe degradation of catalytic activity.Therefore,the key point of this confinement approach is to develop a facile and controllable method to incorporate NPs into zeolite crystals without damaging their structure.Currently,much effort has been devoted to in situ encapsulating metal NPs during the initial hydrothermal syntheses of zeolites,which mainly involves placing the metal precursors into the microporous framework [ 26, 27] .Unfortunately,the high pH (>12) conditions and complicated hydrothermal circumstances during zeolite crystallization inevitably lead to the precipitation of metal precursors as a form of colloidal complex,thus inhibiting their encapsulation [ 36] .To address these limitations,three approaches have been attempted to construct high-quality supported zeolite catalysts.

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Table 1 Zeolite-encapsulated catalysts prepared using different methods

First,surfactants are used to stabilize the metal precursors.For example,Chen et al. [ 26] successfully encapsulated Pd NPs into mesoporous silicalite-1 nano-crystals(Pd@mnc-S1) using a facile one-pot method (Scheme 1).During this process,polyvinylpyrrolidone (PVP) is used as a stabilizer to maintain the palladium oxide or hydroxide NPs resistance to aggregation.The PVP ligands on the Pd surface not only enhance the stability of the Pd NP but also facilitate the generation of mesopores inside the mesoporous silicalite-1 nano-crystals.Figure 1a shows the morphology of the Pd@mnc-S1.The tiny Pd NPs were highly integrated inside the framework of the silicalite-1nano-crystals.After heating at 550℃for 6 h,the size of the Pd NPs settled inside the silicalite-1 (Pd@mnc-S1)remained unchanged (Fig.1a,c).In addition,Pd NPs on the surface of silicalite-1 (Pd/silicalite-1) were also prepared for comparison.The calcination result shows that the Pd NPs in the contrast sample generate significant aggregation with an increased particle diameter and poor monodispersity (Fig.1b,d).Thus,the comparison confirms the important role of the confinement effect in improving the thermal stability of Pd NPs.

Scheme 1 Encapsulation processes of Pd@mnc-S1 [26]

Fig.1 TEM images of samples calcinated at 550℃:a Pd@mnc-S1 prepared using one-pot method and b compared sample Pd/silicalite-1 in which Pd NPs were prepared on surface of silicalite-1;particle distribution of c Pd@mnc-S1 and d Pd/silicalite-1 before and after calcination [26]

Fig.2 Schematic diagram of metal clusters encapsulated within zeolites (SOD,GIS and ANA) [14]

Second,organic ligands (e.g.,organic amines and NH₃)can be used to stabilize metal cations and prevent their premature precipitation due to complicated hydrothermal circumstances.Recently,Iglesia et al.adopted this strategy to effectively encapsulate small metal NPs into microporous cavities during the synthesis of different zeolite materials (Fig.2) [ 14] .The organic amines and NH₃ligands not only stabilize metal cations against the unexpected precipitation but also act as coordinating agents to isolate the metal precursors during the early stage of framework formation.Notably,the ligand-stabilized metal complexes must be hydrophilic for the Al-rich zeolite template,while hydrophobic organic cations are preferred as stabilizing guests and structure-directing agents for Sirich zeolites.

Third,special metal complex precursors (e.g.,[Pd(NH₂CH₂CH₂NH₂)2]Cl₂) can also be used for the preparation of high-thermal-stability supported zeolite catalysts.Here,well-dispersed and ultrafine Pd clusters encapsulated in the matrix of the silicalite-1 (MFI) zeolite have been synthesized via a direct hydrothermal means using[Pd(NH₂CH₂CH₂NH₂)2]Cl₂ as a precursor [ 27] .The detailed preparation process is shown in Scheme 2.First,Pd complexes and the zeolitic gel interact and mix together.Following the crystallization treatment,Pd precursors are self-assembled into the zeolite framework.Upon calcination and reduction,Pd clusters are uniformly encapsulated into the channel of zeolites-1.Meanwhile,a computational simulation of annealing shows that the tetrapropylammonium cations (TPA⁺) are only in the center of the straight ten-ring channels along the b direction while the[Pd(NH₂CH₂CH₂NH₂)2]²⁺cations are in the intersectional channel.After removing these ligands,the ultra-small Pd NPs settle in the intersectional void spaces between the straight and sinusoidal channels.Thus,it is reasonable to speculate that the interactions between the TPA⁺cations and ethylenediamine ligands play an important role in stabilizing Pd precursors against premature precipitation.Here,the excellent confinement effect of the zeolite matrix endows superior thermal stability and recycling stability of the Pd clusters for formic acid decomposition.

Scheme 2 Schematic diagram of encapsulated Pd metal clusters within the silicalite-1 zeolite [27]

Although the NPs have been successfully encapsulated into the zeolite support using the aforementioned methods,precisely control in particle size inside the zeolite is difficult,especially for an NP diameter between 1.5 and4.0 nm,which is desirable for an industrial catalyst.To solve this problem,Xiao et al. [ 32] demonstrated that the seed-directed growth technology is useful for the immobilization of metal NPs of various sizes within zeolite crystals (Scheme 3).Noble metal NPs are successfully encapsulated using the zeolite seeds that already contain the active species.During the crystallization processes,these active species on the surface of zeolite seeds are further covered and isolated by the aluminosilicate gel without a size limitation and finally an encapsulated metal@zeolite structure with satisfying particle diameters is obtained.This strategy can be applied to stabilize various metals (e.g.,Pt,Pd,Rh,Ag) within a series of zeolites (e.g.,Beta,MOR,silicalite-1),which also exhibit superior sinter resistance for high-temperature reactions including the water-gas shift (WGS) reaction,CO oxidation,oxidative reformation of methane and CO₂ hydrogenation [ 27] .However,the metal NPs deposited on the surface of the zeolite support also undergo significant aggregation and growth during the aforementioned high-temperature reactions.

Scheme 3 Representative diagram of metal@zeolite structures within NPs fixed in inner or on outer surfaces of zeolite crystals [32]

In addition,there are also some other important approaches focusing on the stabilization of tiny metal NPs within zeolite matrix to improve their sintering resistance,for instance,incorporating Pt clusters into high-silica zeolites MCM-22 via the transformation of two-dimensional (2D) zeolite into a three-dimensional (3D) [ 28] structure,stabilizing CoO clusters within silicalite-1 crystals using a steam-assisted method [ 31] ,etc.Even so,simpler and more universal strategies should be put forward to enrich the types of catalysts to meet the increasing demand of industrial applications.

3.1.2 Other me soporous materials

The encapsulation of ultrafine metal NPs into the channel of porous carbons,porous silica,or even carbon nano tubes has also been proven to hinder NPs sintering at high temperatures,indicating an enhanced catalytic stability(Table 2 [ 37, 38, 39, 40] ).

Recently,Wan et al. [ 38] adopted a coordination-assisted synthetic approach,successfully realizing the preparation of highly active and stable gold NP catalysts within ordered mesoporous carbon materials.During this synthesis strategy,silane coupling agent as a coordination complex plays important roles as follows:(1) coordinates with gold species via the thiol group to promote small Au NPs formation;(2) reacts with phenolic resins to benefit the formation of a uniform composite framework;and (3)facilitates the generation of secondary pores in carbon pore walls.Following carbonization,the reduced Au NPs(～9 nm) are entrapped by the ordered mesoporous carbonaceous framework.These monodispersed gold NPs show excellent catalytic performance and can completely convert benzyl alcohol to benzoic acid in water at 90℃under a 1 MPa oxygen condition.Also,excellent stability and reusability were obtained using this effective carbon meso-structure confinement strategy without an obvious loss of activity or metal leaching over five cycles.

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Table 2 Mesoporous materials encapsulated in ultra-stable metal NP catalysts

Additionally,mesoporous silica (mSiO₂) was also introduced to encapsulate ultrafine metal NPs which are on graphene oxide (GO) [ 40] .The preparation process is simple (Scheme 4).First,metal hydroxide (MOH) is loaded on the surface of GO nano-sheets using the depositionprecipitation method.Then,the tetraethyl orthosilicate(TEOS) is directly hydrolyzed and transformed into a mSiO₂ shell covering on the surface of the MOH-GO composite,preventing the aggregation of composites due to the grapheneπ-πstacking interactions.Lastly,the obtained MOH-GO@mSiO₂ nano-sheets are treated under H₂ atmosphere to reduce the graphene oxide (rGO) and MOH and the target M-rGO@mSiO₂ composite material is obtained.Notably,the mesoporous structure of the mSiO₂provides spaces to confine the metal NP growth during the reduction process.Such robust catalysts not only show excellent catalytic activity and high stability in both gas-and solution-phase reactions but can also be fully recovered via high-temperature regeneration.

Scheme 4 Synthesis sketch of M-rGO@mSiO₂ [40]

Confining NPs in nanotubes has also been proven to be an attractive approach to fabricate high-thermal-stability catalysts.By confining active nanoparticles inside the nano tube spaces,the sintering,aggregation,detachment and poisoning can be inhibited effectively [ 41] .Recently,Qin's group designed various confined structures (e.g.,CoO_x/SBA-15 [ 42] and Ni-in-ANTs [ 43] ) using a simple atomic layer deposition (ALD) technology to synthesize highly efficient catalysts.Fortunately,these structures showed excellent recycling stability.Particularly for the Ni-in-ANTs catalyst,Ni NPs were not only confined to Al₂O₃ nano tubes but also embedded in the cavities of the Al₂O₃ interior walls using a template-assisted ALD method.The detailed preparation steps are illustrated in Scheme 5.Owing to these multiple confinement effects,Ni-in-ANTs showed improved activity and stability in cinnamaldehyde and nitrobenzene hydrogenations reactions,without obvious activity lost or metal leaching over four cycles.This general template-as sis ted ALD method provides a new controllable strategy for the stabilization of NPs by fabricating-confined nanoparticles with metal-innanotube structures.

3.2 Coating strategy

3.2.1 Core-shell configurations

Core-shell structures also have important applications in catalysis,whereby the ultra-thin outer shells isolate the active NPs that prevent the tendency for core particles sintering at high temperatures.Table 3 [ 10, 17, 18, 44, 45, 46, 47, 48, 49, 50] shows various core-shell structures fabricated for the stabilization of metal NPs.

The encapsulation of small NPs (<3 nm),highly desirable for catalytic applications,is extremely difficult using conventional sol-gel coating strategies (the StOber process and reverse micelle microemulsion) because of the weak metal/oxide affinity.In this regard,Gao et al. [ 17] developed an un-conventional reverse micelle system that involves employing the gold oxides (Au₂O₃·xH₂O) as precursors to adhere to the silica,followed by a thermal deposition process to successfully realize the coating of ultra-small Au NPs (Scheme 6).The obtained Au@SiO₂single-core/shell NP shows good catalytic performance and stability for the high-temperature CO oxidation reaction.This general synthesis scheme provides a new strategy to fabric ate ultra-sm all metal@oxide core-shell nanostructures.

Scheme 5 Synthesis diagram of Ni-in-ANT and Ni-out-ANT [43]

The stabilization of bimetallic NPs is also essential in the catalysis field,especially for low-temperature-combustion (LTC) engine exhaust purification.Here,sub-5-nm Au-Cu bimetallic NPs encapsulated in SiO₂ were synthesized by Keith et al. [ 46] .In this manner (Scheme 7),HAuCl₄ and CuCl₂ are firstly used as precursors to generate an Au_NCu_MO_X·YH₂O nanocomposite and the obtained sample is covered by SiO₂ in a reverse micelle system.After pretreatment at 400℃using 10%O₂/He and reduction at 350℃using 10%H₂/N₂,the Au-Cu@SiO₂bimetallic catalyst is successfully prepared.The prepared Au-Cu@SiO₂ catalyst exhibits excellent activity for lowtemperature CO oxidation,superior to that of the Au@SiO₂,and demonstrates the positive effect of the copper promoter.Importantly,this special structure endows them with a high catalytic stability that maintains their high performance at 300℃for as long as 116 h.

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Table 3 Core-shell structures encapsulated in ultra-stable metal NP catalysts

Scheme 6 Encapsulation of ultra-small Au NPs via un-conventional reverse micelle system [17]

The surfa ctant-as sis ted colloidal approach is a promising strategy for preparing the supported nano-catalysts due to their controllable NP size,shape and composition.However,the organic capping agents covered on the catalyst surface will inevitably degrade the catalytic performance because they occupy activity sites and block the accessibility of reactants.Although some methods have been developed to remove the organic layer,the aggregation and sintering of naked NPs also occur at elevated temperatures or under harsh reaction conditions.Thus,Dai et al. [ 51] transformed these surface-bound surfactants into carbonaceous layers via heating treatment in an N₂ atmosphere and thus achieved their in situ coating on the NPs(Scheme 8).The generated carbonaceous core/shell structure functioned as robust physical barriers and could effectively slow the coarsening and ripening of noble metals during the heating process.This strategy proposes new design ideas for the stabilization of colloidal catalysts.

Scheme 7 Encapsulation strategy of Au-Cu bimetallic nanocomposite by SiO₂ [46]

Scheme 8 Schematic diagram for construction of carbonaceous coating architectures [51]

Scheme 9 A representative sketch of Pd/Al₂O₃ catalysts with and without ALD Al₂O₃ overcoat for oxidative dehydrogenation of ethane reaction at 675℃ [52]

3.2.2 Al2O3 overcoats

Although core-shell structures largely improve the catalytic stability of the prepared NPs,they are subject to unavoidable partial loss of catalytic activity due to the elevated mass transfer resistance resulting from uncontrollable shell thickness.Stair et al. [ 52] adopted ALD technology to successfully realize over-coating for palladium NPs with 45 layers of alumina by alternately exposing samples in trimethylaluminum and water at200℃.The Al₂O₃ overcoats,synthesized with precise shell thickness control,not only inhibited the coking and sintering of supported metal catalysts,but also maintained their high catalytic activity at high temperatures.Detailed research revealed that the prominent hightemperature sintering resistance was mainly attributed to the selective blocking and stabilization of edge and corner atoms by alumina overcoats,as shown in Scheme 9.Additionally,the pore channels of the alumina overcoat are smaller than the carbon filament diameters (typically 17 nm),leading to an apparent inhibition effect on the formation of carbon filaments.Similarly,bimolecular reactions,which are necessary in the formation of coke,were inhibited due to the size limitation of the microporous channels.Importantly,this unique ALD technology can also be used to stabilize other metal catalysts (e.g.,copper) under liquid-phase reaction conditions [ 53] .

3.3 Strong metal-support interaction

SMSI is also a considerable approach to fabricate ultrastable metal catalysts deriving from the unique electronic or geometric effect between metal particles and oxide supports.SMSI was first presented by Tauster et al. [ 54] during the late 1970s to explain the special phenomenon in which the adsorption of H₂ and CO was severely inhibited on the surface of TiO₂-supported Pt NPs.Since then,SMSI has attracted great attention due to its unique character during catalysis.Some research has shown that the Pt NP surface is covered by a thin layer of metal oxides (e.g.,TiO_2-X) after reduction at 500℃and the electrons transfer from Ti³⁺to metal NPs [ 55, 56] .This phenomenon is closely related to their excellent sintering resistance and enhanced catalytic activities,which reveals the fundamental effect of SMSI.SMSI has been further developed(Table 4 [ 12, 13, 21, 22, 23, 57, 58, 59, 60, 61, 62, 63, 64, 65] ),including support types(scale from reductive metal oxides to metal oxides and to a non-oxide substrate),fabrication approaches (from reduction treatment to oxidation treatment,and even without reduction or oxidation treatment) and interaction forms(from various electron interactions,e.g.,special sites or crystal planes,which can also generate SMSI).Here,we only introduce several special SMSI effects,which are fabricated by some methods that differ from classic synthesis conditions,and show their important role for determining catalytic stability.

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Table 4 Strong metal-support interaction (SMSI) stabilized ultra-stable metal NPs catalysts

3.3.1 Non-oxide substrate

Zhang et al. [ 21] first reported that SMSI could be fabricated between Au NPs and a hydroxyapatite (HAP) support(a non-oxide substrate) under oxidative conditions.As shown in Fig.3,Au NPs are covered by a thin layer of substrate once the calcination temperatures exceed 300℃and the encapsulation degree for Au particles is closely related to the treated temperature.The electron energy loss spectroscopy characterization result shows that the coating layer is composed of P (Fig.4),derived from the HAP support,demonstrating the physical encapsulation of Au NPs by HAP.Also,in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that the adsorption of CO on the surface of Au NPs is sharply inhibited and the electrons spontaneously transfer from the Au NPs to the HAP support (Fig.5),suggesting the existence of electron interactions.In addition,this encapsulation of metal NPs can be completely reversed after treatment under pure H₂ at 500℃,as shown in Fig.3f,which further confirms the SMSI effect between the Au NPs and HAP support.Apparently,the encapsulated Au NPs exhibit excellent sintering resistance,improved selectivity and superior reusability in the liquid-phase reaction.

However,the uncontrolled encapsulation of Au NPs by the HAP support will inevitably reduce the catalytic activity because of the blocking of active sites.To solve this problem,Zhang et al. [ 65] incorporated TiO₂ into the HAP support to regulate the encapsulation degree of the Au NPs to obtain high catalytic performance with both stability and activity.As shown in Fig.6,Au NPs occur in the interfacial region between TiO₂ and HAP.This unique structure makes the Au NPs partially encapsulated and partially exposed to the air,which generates a strong interaction between the Au NPs and the mixed supports that facilitate the stability of the Au particles,and also enhances their catalytic activity due to the partially exposed surface.This work provides a new strategy to improve the catalytic activity and stability simultaneously by controlling the SMSI between NPs and supports.

3.3.2 Non-redox conditions

Classical SMSI is always constructed with redox treatments at high temperatures,which typically cause the sintering of metal NPs before encapsulation.Therefore,researchers have been devoted to fabricating SMSI under more mild conditions without undergoing reduction or oxidation treatment.Recently,Xiao et al.reported the wetchemistry construction of SMSI (wcSMSI) for an Au/TiO₂catalyst at room temperatures in an aqueous solution.The prominent characteristic of this method is the redox interaction between the NPs and support precursors (TiO_x species) [ 23] .Au NPs,acting as a promoter,can accelerate the nucleation and growth of TiO_x species on the surface of the Au NPs,which is favorable for the formation of an oxide-coating structure (Scheme 10).Detailed HRTEM images show that the Au NPs are covered by a thin layer of oxides after treatment using the wet-chemistry method.CO adsorption FTIR spectra show the electron transfer behavior from TiO_x to Au NPs and the corresponding CO adsorption is also suppressed,showing the classical SMSI characteristics.This wet-chemistry SMSI method brings an excellent sintering resistance for catalyst during CO oxidation reaction.Unfortunately,this method cannot be extended to other catalyst systems such as the Pt-FeO_x system due to the weak reducibility of Fe³⁺,which is undesirable for the formation of coating layers.

Fig.3 HRTEM images of Au/HAP samples treated at different temperatures:a Au/H-200,b Au/H-300,c Au/H-400,d Au/H-500,e Au/H-600,f Au/H-500-H₂ [21]

Fig.4 Electron energy loss spectroscopy spectrum of an Au/H-600sample [21]

In addition,SMSI can also be prepared by transforming hydroxide into an oxide support without reduction and oxidation treatment.For example,Xiao et al. [ 58] employed a Mg-Al layered double hydroxide as support to prepare highly sintering-resistant Au catalysts.As shown in Scheme 11,Au NPs are first loaded on the surface of layered double hydroxide (LDH).After calcining treatment in N₂,LDH dehydrates into a layered double oxide (LDO),accompanying with the in situ encapsulation of the Au NPs into the support.The obtained Au/LDO catalyst possesses high thermal stability and excellent sintering resistance.Even after calcination at 600 or 700℃,the mean size of the particles remains at 3.1 or 3.9 nm,respectively,which is obviously smaller than those of NPs prepared using the conventional Au/LDO-IP sample under the same treatment condition.Moreover,the adsorption intensity of CO significantly decreases after calcination in N₂,which suggests a typical SMSI feature and can be assigned to the coverage of Au NPs by the oxides.Notably,excellent catalytic stability is also obtained for CO oxidation and ethanol dehydrogenation.This simple method has been extended to the synthesis of Rh/LDO and Pt/LDO catalysts based on the transformation of hydroxide into oxide.

Fig.5 DRIFT spectra of CO adsorption of different Au/HAP samples [21]

Fig.6 TEM and HRTEM images of a,d Au/TiO₂-800,b,e Au/HAP

3.3.3 Anchoring nanoparticles at special sites or on crystal planes

In addition to the aforementioned SMSI that is mainly based on the overlay structure,some special sites or crystalline planes can also be used to stabilize metal NPs against aggregation and deactivation since adhesion of metal nanoparticle on oxide surface is anisotropic.Li et al. [ 64] reported that thin porous alumina sheets could effectively stabilize Au NPs even undergoing high-temperature calcination.As shown in Fig.7,2-nm Au NPs are highly dispersed on the Al₂O₃ support without aggregation after treatment at 700℃in air.Even calculated at 900℃,the Au NPs (2-4 nm) retain excellent dispersibility on the support surface.HRTEM results show that Au (111) epitaxially grows along with the y-Al₂O₃ (222) in the case of the Au-(Al₂O₃)-700 sample,which could be ascribed to the strong interfacial interaction.In addition,for the Au-(Al₂O₃)-900 sample,Au (002) still corresponds withδ-Al₂O₃ (220) facet that represents the relationship of the epitaxial growth between them and the strong interaction.Besides,the encapsulation action also occurs via a thin alumina layer at 900℃,indicating the SMS I between the Au NPs and Al₂O₃ support.

Scheme 10 Schematic sketch of construction of wcSMSI [23]

Scheme 11 Synthesis and catalytic strategies for constructing a high-temperature-stable Au/LDO catalyst with SMSI [58]

Fig.7 Epitaxial growth of Au along withγ-Al₂O₃ at a 700℃,b 900℃ [64]

Dai et al. [ 22] used theγ-Al₂O₃ nano-sheets to stabilize Pd NPs by designing a sintering-resistant nano-catalyst that is suitable for high-temperature oxidation reactions.Related research has proved that the coordinately unsaturated pentacoordinate Al³⁺ofγ-Al₂O₃ acts as an anchor site to stabilize metal NPs due to SMSI [ 66] .

In addition,Wang et al. [ 67] systematically studied the crystal plane effect on the stability of NPs using anatase(101) and rutile (110) TiO₂ by density functional theory(DFT) calculation.They found that the absorption energies of the Pd₄ cluster on defective anatase (101) and rutile(110) TiO₂ showed an obvious difference (-4.53 and-3.19 eV,respectively),which indicates a stronger interaction between Pd₄ cluster and anatase (101) TiO₂compared to that of the rutile (110) TiO₂.They concluded a possible reason that the anatase (101) had more Ti³⁺species and oxygen defects than those of the rutile (110) TiO₂.Also,they analyze the stability of Rh₄ on the surface of anatase (101) and rutile (110) TiO₂ by the Bader charge scheme [ 68] .The calculation result showed that the Bader valence of the Ti atom on the anatase increased by+0.12e and+0.25e after binding with the Rh atom and thus a charge transfer from Ti to Rh cluster occurred,which endowed the metal cluster electronegative.In contrast,the Bader valence of the Ti atom on rutile decreased by -0.12e and-0.04e after binding with Rh atom that led to an electropositive metal Rh cluster.Therefore,the electronegative Rh cluster on the anatase was more stable than the electropositive Rh on the rutile due to the former'strong interaction with Ti atom.Thus,the crystal plane has a great influence on the stability of metal atoms or clusters that load on the surface of the support.

Scheme 12 Schematic illustration of Au-SA/Def-TiO₂ formation [72]

3.3.4 Support with oxygen defect and doped atom

Oxygen defects in a metal oxide,also known as oxygen vacancies,play an important role in improving catalytic activity and stability [ 69] .Owing to the reserving electron pairs in vacancy,these high electron density centers in a metal oxide typically have a strong attraction for the external metal cluster or single atom that could act as a nucleation site for the metal,generating a strong interaction [ 70] and inhibiting agglomeration [ 71, 72] .The adhesion energy of metal nanoparticles on oxide can thus be tailored by engineering the defects in oxide support.Wang et al. [ 72] constructed a highly stable and reactive Au singleatom (SA) catalyst on TiO₂ by utilizing the oxygen defects on the surface of the TiO₂ (Def-TiO₂) as anchoring sites.As shown in Scheme 12,the oxygen vacancy created on the TiO₂ surface via a reducing approach captured an Au atom to form Au-SA/Def-TiO₂.X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) characterizations indicate that the isolated single Au atom is atomically dispersed on the defect of the TiO₂ and coordinated by Ti atoms.Compared to the Au-SA/Per-TiO₂ (employing perfect TiO₂ as a support),the Au-SA/Def-TiO₂ showed a low conversion temperature (from 230 to 120℃) and high stability for CO oxidation,which could realize the full conversion of CO oxidation without an obvious activity decrease even after600 min of reaction time.DFT calculation results demonstrated that the Au atom on the surface of the defective TiO₂ form a three-center Ti-Au-Ti structure and the binding energy was negative (-1.28 eV for O_2c and-0.69 eV for O_3c) in comparison with that of the Au-SA/Per-TiO₂ (where the Au atom constitutes a Ti-Au-O-Ti structure on the TiO₂ surface and shows neutrality),suggesting that the former possesses better structure stability.

In addition to oxygen defects,a doping atom also has a positive influence on the stability of the supported catalyst.Particularly for metal oxide as a support,the doped atom would weaken the metal-oxygen bond in the metal oxide and hence reduce the energy barrier for the formation of oxygen vacancies and facilitate their creation [ 73, 74, 75, 76, 77, 78] .As previously discussed,the oxygen vacancy can act as the anchoring site to stabilize the metal atom or cluster via bonding effect.Hutchings et al. [ 75] reported the addition of Ti to CeO₂ as a support to deposit gold using a sol-gel methodology and obtained a series of ceria-titania of different doping ratios.The experimental result showed that the Au/Ce_0.2Ti_0.8O₂ exhibited superior catalytic activity and stability,which was mainly ascribed to the large increase in the number of oxygen vacancies that could efficiently stabilize the gold on the support.They also demonstrated that a Ce-based metal oxide might be used as an excellent catalyst support candidate due to its high number of defects that could stabilize the metal species.

4 Interfacial plasma electrolytic oxidation technology

Our group recently focused on the fabrication of ultrastable nano-catalysts [ 79] .Unlike the methods above,a new preparation technology,termed interfacial plasma electrolytic oxidation technology (PEO),has been successfully applied to fabricate a high-quality Au/MgO catalyst with superior stability and activity for catalytic reactions.This method mainly involves a physical-chemical reaction that occurs on the surface of the Mg plate and generates a corresponding porous Mg oxide layer.The detailed synthesis process is illustrated in Scheme 13.First,a noble metal salt as a catalytic precursor (HAuCl₄,PdCl₂,HPtCl₄,etc.) is added into the electrolyte system.Then,an Mg plate and a stainless steel plate are chosen as the anode and cathode,respectively.During the PEO process,noble metal precursors are in situ decomposed and transformed into metal NPs along with their encapsulation into the MgO porous layer.Moreover,the interface temperature of the Mg plate and electrolyte can also be controlled well by two cooling paths (electrolyte cooling and Mg plate solid cooling),which play an important role in determining the particle size and monodispersity.

Scheme 13 Schematic diagram for construction of an Au/MgO catalyst using ultra-low-temperature interfacial plasma electrolytic oxidation technology [79]

Fig.8 Representative pictures of Au/MgO catalyst prepared using PEO:a SEM image (insert being high magnification);b SEM image of cross section;c HAADF-STEM image (insert being an HRTEM image of Au NP in red circle);d HAADF-STEM image of metal-support interface section [79]

The detailed morphology and structural information is shown in Fig.8.The PEO oxide layer surface presents a3D porous structure with a pore size of 0.5-1.0μm(Fig.8a) and thickness of 3-4μm (Fig.8b).Furthermore,high-angle annular dark-field (HAADF) images (Fig.8c)indicate that the obtained Au NPs are uniformly dispersed on the holes of the MgO film with a unique embedding configuration,as illustrated in the insert of Fig.8c.Owing to this unique embedded structure,the obtained Au/MgO catalysts possess excellent thermal stability and sintering resistance.After treatment at 500 or 600℃,the Au/MgO catalysts maintain their original diameter at 5.3 or 4.8 nm,respectively.It is believed that the abundant oxygen vacancies in the MgO overlayer exert a strong adhesion effect on supported Au NPs.Because the oxide layer is firmly attached to metal substrate,the concentration of oxygen vacancies in MgO cannot be annihilated by hightemperature annealing.

The foremost characteristic of this approach is the simultaneous in situ formation of metal NPs and an oxide support that generates a unique embedded structure between them and endows the catalyst outstanding thermal stability and recyclability.Importantly,this general approach has been extended to the preparation of other supported metals,such as Pd/MgO and Ru/MgO,presenting similar performance.We believe that this unique method will bring a new breakthrough for the preparation of ultra-stable heterogeneous catalysts.

5 Summary and outlook

The synthesis of thermally stable catalysts is an important theme that has attracted considerable research interest.Two main paths including thermodynamic and kinetic approaches were performed during the entire developed solution.The thermodynamic approaches focus on the chemical potential and surface energy of the prepared NPs,which is determined by the particles size,distribution and applied support.However,most solutions involve the kinetic approaches including channel confinement,coating and SMSI.In particular,the SMSI effect has a dominant advantage that can not only improve the catalytic stability but also achieve regulation of catalytic activity in comparison with the other kinetic approaches.Consideration SMSI is only suitable for some special metal-support pairs that have strictly limited the application in the field of catalysis,our group developed an SMSI based on universal strategy-interfacial plasma electrolytic oxidation technology for the preparation of ultra-stable supported catalysts,which is appropriate for various noble or transition metal catalysts and hence largely extends the scope of SMSI.

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