Trends and Prospects of Bimetallic Exsolution

Abstract Supported bimetallic nanoparticles used for various chemical transformations appear to be more appealing than their monometallic counterparts, because of their unique properties mainly originating from the synergistic effects between the two different metals. Exsolution, a relatively new preparation method for supported nanoparticles, has earned increasing attention for bimetallic systems in the past decade, not only due to the high stability of the resulting nanoparticles but also for the potential to control key particle properties (size, composition, structure, morphology, etc.). In this review, we summarize the trends and advances on exsolution of bimetallic systems and provide prospects for future studies in this field.


Introduction
Metal nanoparticlesd ispersed on solid oxide supports are increasingly applied in aw ide range of applicationss uch as heterogeneous catalysis,e lectrochemical conversion and photocatalysis. Bimetallic nanoparticles,c omposed of two metal species, often show distinct catalytic properties and stability as compared to individual metals, which is typically ascribed to changes in the electronic and/org eometrics tructures that occur when the metals are combined. [1] Bimetallics are usually prepared by assembly or co-depositionm ethods and varying their particle size, composition (ratio of metals involved), structure (alloys, core-shells, heterostructures, etc.) and morphology,c an lead to aw ide range of functional materials. [2] In spite of this variety, bimetallics prepared through such methods often display limitedl ong-term stability,g enerally due to the weak metal-support interactions. One way to stabilize themi s throught he exsolution method.
Redox exsolution, an alternative to assembly and deposition methods, hast he potentialt op roduce supportedm etal nanoparticles with combined high activity and stability. In this method, active species are substituted in ap erovskite oxide matrix and subsequentlye merge as metal nanoparticles at the surface, normally driven by thermal [3] or electrical reduction. [4] Exsolved nanoparticles are epitaxially grown from the parent oxide and partially socketedi ni t. This results in as trained particle-oxide interface [5] which endows the nanoparticles with enhanced stabilitya gainst sinteringa nd can also enhance resistance to poisoningb yc arbon deposition and sulfur,a sw ell as high activity.
Co-exsolutiono ft wo metals,w hile less studied than single metal exsolution, can lead to attractive structures and func-tionality,i ncludings ynergistic effects between the two co-exsolved metals. [6] There are roughly7 0p aperso nb imetallic exsolution published in the past decade, from which statistical information on the compositions and applications of the exsolved bimetallics is extracted. The Fe-containing compositions are the overwhelming majority followed by Ni-based systems ( Figure 1a). However,o ther bimetallic compositions have also been occasionally reported, as well as alloys containing more than two metals and more complex heterostructures. Interestingly,t here are no bimetallic materials containing two noble metals reported. The use of bimetallic exsolveds ystems spans over all kinds of applicationsi ncluding but not limited to electrochemistry (71 %), catalysis (19 %) and membranes (3 %) (Figure 1b). Among these,e lectrochemistry [6a-c, 7] is the most common application but there are also af ew studies focusing on catalytic applications such as methane conversion, [8] CO oxidation [9] or the water-gas-shift reaction. [10] In spite of these exciting developments, to the best of our knowledge,t here is no review that summarizes the advances on bimetallic exsolution. Here we aim to review the trends that underpin the exsolution of bimetallics ystemsi nt erms of design, tailoring and application and propose future directions for the development of the field.

Mechanismo fA lloy Exsolution
Simultaneous exsolution of two or more reducible metal species from an oxide backbone will, in most instances, lead to the formation of alloy particles as evidenced by modelling studies. [11] Generally,t wo possible exsolutionm echanisms exist, namely the "bulk alloy formation" and "surface alloy formation" as schematically illustrated in Figure 2a and b, respectively. [11a] Exsolutioni sp romoted by the oxygen vacancies introduced by   reduction which are prone to move from the bulk towards the surface, hence it is favourable for exsolvable metals to segregate along with the oxygen vacancies (known asc o-segregation) due to the lower energy required. [11a, b] DFT calculations indicatedt hat the co-segregation of Ni, Co and CoÀNi with an oxygen vacancy are all thermodynamically favourable, due to their negative Gibbs energieso fc o-segregation (À0.39, À0.53 and À0.48 eV,r espectively). However, the Gibbs energiesf or metal aggregation in the bulk (0.02 eV) and at the surface (À0.01 eV) are both much more positive than those of metal co-segregation. Thus, the energetics suggest that alloy formation in the bulk is less favourable than metal co-segregation during the exsolution process andt hus exsolution is more likely to follow the surface alloy formation mechanism due to the lower alloy formation energy at the surface. [11a] This has also been demonstrated experimentally by employing in situ x-ray diffraction where during reduction, and thus exsolution, separated Ni and Co peaks appeared first at al ow temperature while the CoÀNi peak appeareda th ighert emperatures at the expense of the Ni and Co peaks (Figure 2c). [11a] Similar exsolution behaviour has also been observed for CoÀFe alloys using in situ scanning transmission electron microscopy,w here the Co-based nanoparticles and CoÀFe alloys appeared in sequencewiththe increasingt emperature during reduction. [11c] It is worth mentioning that some metals, such as Fe, would probablyn ot exsolvei ft hey are the sole species on the B-site in the perovskite lattice due to their high segregation energy, but it could be possible to exsolvet hem together with a second, substituted, metal (like Ni and Co) with al ower segregation energy.T his is because in mixed cation systems, the Gibbs energy of reduction is af unction of the strength of the metal-oxygen bonds of both substituted metalsa nd hence the energy can be decreased by introducing more reducible ions (Figure 2d). [11c-e] Besides, it is also reported that doping of Co  increases the total energy of the perovskite system and the CoÀFe bond would form more easily than the FeÀFe bond due to the lower formation energy,w hich also accounts for the promoting effects of Co on the Fe exsolution. [11f]

Bimetallic Alloy Nanoparticle Systems
Generally,i np erovskite-based systemst he A-site stoichiometry dictates which types of exsolution can occur:i rreversible or reversible (also called intelligent concept). In the latter,t he original matrix is stoichiometric and thus upon exsolution the thermodynamically stable perovskite segregates into al ess thermodynamically stable mixture of A-site oxide and B-site exsolved components. Thus, these phases could recombined, giving the exsolved nanoparticles the ability to re-dissolve depending on the gas environment, which in turn leads to materials with high durability due to the fact that they can regenerate. This appliesboth for single metals [12] as well as for bimetallic systems. [6d, 7g, h, 10, 11c, e, 13] For example, FeÀCo nanoparticles from La 0.3 Sr 0.7 Cr 0.3 Fe 0.6 Co 0.1 O 3-d were found to completely redissolve into the perovskite when re-oxidized at 800 8Cw hile they remained on the surface as transition-metal oxide at 700 8C. [7g] On the other hand, during exsolution from A-site deficient perovskitesw hicha re thermodynamically metastable, as tabler Asite stoichiometric perovskite forms alongside the exsolved particles which makes re-dissolution less likelya nd thus this exsolution irreversible.M oreover,n anoparticles exsolved from A-site deficient perovskites are partially socketed in the surface and less likely to redissolve into the perovskite lattice under reoxidation leading to coke resistant, highly active materials mainly due to the alignment and socketing betweent he support and the exsolved particles. [3,5] For example, NiÀCo nanoparticles exsolved from La 0.7 Ce 0.1 Co 0.3 Ni 0.1 Ti 0.6 O 3 did not redissolve when treated under oxidizing conditions, but demonstrated coking resistance and matchedn oble metal commercial catalysts in automotive exhaust reaction conditions. [9c] The reversible/intelligente xsolution methoda ccounts for about 15 %o ft he total papers published until today on bimetallic exsolution. Nevertheless both types are reviewed in detail in the following sectiond epending on the chemistry of the exsolved particles.

FeÀNi
ExsolvedF e ÀNi alloys are frequentlyu sed as cathodes for CO 2 or steam electrolysis because such cathodes demonstrate high catalytic activity,g ood stabilityu nder cell operation conditions and resistance to carbon deposition and nanoparticle sintering (Figure 3a), due to the well-known strong anchorage of the exsolved particles on the support (Figure 3b). [7a, b, 14] For example, the interface between the exsolved FeÀNi nanoparticles and the Sr 2 Fe 1. 35 Mo 0.45 Ni 0.2 O 6-d substrate with abundant oxygen vacancies was found to promote the adsorption and activation of CO 2 ,w hich resulted in much better performance for CO 2 reductionr eaction (CO 2 RR) as compared to the cathode based on the pristine perovskite (Figure 3c-f). [14] Similarly,s uch exsolved nanoparticle systems have been provent oh ave en-hanced activity for water splitting since an electrolysis cell with aS r 2 Fe 1.3 Ni 0.2 Mo 0.5 O 6 cathode decorated with FeÀNi exsolved nanoparticles demonstrated about twofold higherc urrent density and half the electrode polarization resistance as compared to the cell using pristine cathode. [15] As imilar system (Sr 2 Fe 1.4 Ni 0.1 Mo 0.5 O 6-d )w as also reported to demonstrate excellent redox cycling stability via self-regeneration of FeÀNi nanoparticles, despite the fact that when re-oxidizedi na ir some FeÀNi nanoparticlesw ere oxidized to (Ni,Fe)O,r emaining on the surfacer ather than reincorporate into the perovskite lattice. [13a] Exsolved FeÀNi alloys have demonstrated excellent catalytic activity,d urability and resistance to coking and sulfur poisoning when used as anode materials for SOFCs. [6a, 7c, d] For instance,S r 2 FeMo 0.65 Ni 0.35 O 6-d was found to partially decompose to am ixed Ruddlesden-Popper (RP) type layeredS r 3 FeMoO 7-d , ap erovskiteS r(FeMo)O 3-d and FeÀNi nanoparticles, and the resulted anode showedh igh electronic conductivity,e xcellent catalytic activity and stabilityu nder both wet H 2 and CH 4 . [16] In addition, exsolvedF e ÀNi nanoparticles were reported to improveh ydrogen dissociative adsorption (rate-limiting step) thus leading to increased catalytic activity. [17] Interestingly,a general thermodynamic model was developed to predict the composition of those exsolved particles based on the approximationt hat the more reducible metal (Ni)c ompletely exsolved while the content of the otherm etal (Fe) in the exsolved alloy is dependent on P O 2 . [17] Doping as mall amount of alkali metals like Na on the A-sites of aS r 2 FeMo 0.65 Ni 0.35 O 6-d perovskite was Adapted with permission. [14] found to facilitate the phase transition into aR Pstructure after reduction,i ncrease the Ni content in the exsolved FeÀNi alloys, and also introduce more surfaceo xygen vacancies ultimately leadingt oh ighera ctivity and enhanced coking resistance when employed in aS OFC anode under H 2 and CH 4 . [18] Lastly, exsolved FeÀNi materials have also demonstrated potential to be used as electrodes for symmetrical SOFCs,a st hey exhibited improved activity for both fuel oxidation and oxygen reduction reaction, good durability and resistance to carbon deposition. [19] Exsolved FeÀNi alloy systemsh ave also been employed as catalytic materials for reactions of methane reforming [8a, b] and CO oxidation. [9a] For the former,t he effect of the A-sitec ations on the materials'a ctivity (LnFe 0.7 Ni 0.3 O 3-d ,L n= La, Pr,S m) was studied and it was found that PrFe 0.7 Ni 0.3 O 3-d composition resulted in the highest activity and stabilityd ue to the optimal composition of exsolved FeÀNi nanoparticles (higher Fe content) and the redox properties of the oxide matrix. [8a] In addition, an increase in the Ni doping level resulted in an increase in the oxygen deficiency of the perovskites due to the reduction of Fe 3 + /Fe 4 + and Ni 2 + speciest ol ower cation valences which in turn enhanced FeÀNi nanoparticle exsolution, resulting in improved activity for CH 4 conversion (20 times over that of the pristine perovskite). [8b] FeÀNi exsolved catalysts have also been prepared through as o-called "topotactice xsolution" method. Here, Fe was introduced as the guest cation (via infiltration or atomic layer deposition) on thes urface of aN i Àcontaining perovskite and the exsolution process was promoted by ion exchange between Fe and Ni driven by the different segregation energies of the two metals. [20,21] Thus, more Ni was draggedt ot he surfacef orming alloyed FeÀNi nanoparticles with high population (Figure 4). This resulted in higher activity for methaned ry reformingw hen compared to that of mono-metallicN ic atalyst prepared via normale xsolution while still maintaining excellent durability. [20,21] For the latter,w hen used in CO oxidation, exsolved FeÀNi nanoparticles from La 0.5 Sr 0.4 Fe 0.1 Ni 0.1 Ti 0.6 O 3 exhibited high activity,g ood long-term stabilityo ver 170 ha nd sulfur tolerance. [9a] Finally,u sing FeÀNi nanoparticles exsolved from Sr 0.9 (Fe 0.81 Ta 0.09 Ni 0.1 )O 3-d in ac atalytic membrane reactor for methanep artial oxidation, resultedi na lmost full CH 4 conversion and nearly 100 %s electivity to CO and H 2 .O xidising reaction conditions can causee xsolved nanoparticles to redissolve into the perovskite lattice (depending on the presence of Asite deficiency) which can prove detrimental fort heir catalytic activity.H ere, the oxygen-permeable membrane could act as an oxygen distributor controlling oxygen partial pressure near the particles hence suppressing their dissolution in the perovskite during oxidation. [8c]

FeÀCo
Exsolution of FeÀCo nanoparticles has also been studied extensively usually as anode materials for SOFCs and similar to their FeÀNi counterparts, they demonstrateaplethora of promisingp roperties such as high activity,s tability,c oking resistancea nd sulfur tolerance. [6b, c, 7e-g] AC o-doped La 0.6 Sr 0.4 FeO 3-d (LSF) anode was found to achieve high powerd ensity and lower polarization resistance in aS OFC cell as compared to its Mn-doped counterpart due to the high catalytic activity of the exsolved FeÀCo particles towards hydrogen oxidationa nd the highero xygen-ion conductivity of the newly formed LaSrFeO 4 -La(Sr)Fe(Co)O 3 (the perovskite phase of the Mn-doped LSF was retained withoutf ormation of other phases after reduction). [22] Similarly,a nS OFC anode material comprising of FeÀCo nanoparticles distributed on RP-type layered Sr 3 FeMoO 7 was prepared by reducing Sr 2 FeMo 2/3 Co 1/3 O 6-d in hydrogen, resulting in greatly improved electrical conductivity and catalytic activity. Thus, the resulting SOFCsa chieved high maximum powerd ensities in H 2 approximately 1.4 times higher than those of SOFCs employing ac onventional Ni-Sm 0.2 Ce 0.8 O 1.9 anode while at the same time also exhibiting high performance andc oking resistance in C 3 H 8 . [23] More interestingly,adouble-layered perovskite (Pr 0.4 Sr 0.6 ) 3 (Fe 0.85 Mo 0.15 ) 2 O 7 decorated with FeÀCo nanoparticles, was used as the anode in ad irect ethane-fuelled proton-con-ductingS OFC;h igh powerd ensity and high ethylene yield with ethylene selectivity of > 91 %w erea chieved together with excellent stabilitya nd coking resistance in ethane. [13b] A Pr 0.4 Sr 0.6 Co 0.2 Fe 0.7 Nb 0.1 O 3-d perovskite wase mployed as semiconductingm ateriali nasingle-layer fuel cell and, despite the lower fuel cell performance, it demonstrated enhanced stability as compared to the conventional lithiated metal oxides. Additionally,e xsolution of FeÀCo alloy nanoparticles enhanced the performance of the starting perovskite, which improved electrode reaction kinetics, facilitated charge separation and ionic conduction. [24]  (c) HAADFs canning TEM with EDS of the FeÀNi alloyed nanoparticles via topotactic exsolution. Adapted with permission. [20] Notably,exsolution of metal particles is sometimesaccompanied by ap hase transition of the perovskite substrate. In this case, the exsolved nanoparticles usually contributet oh igh catalytic activity while the oxide substrate affectst he pathway for electron/ion conduction, hence theya re both important for determining the SOFC anode performance. However,aphase transition of the substrate is not always beneficials ince studies have shown that extreme reducing conditions that promote the exsolution of FeÀCo particles and the phase transition of the perovskite substrates, can also result in decreased conductivity. [25] The exsolved FeÀCo particle systems alsof ind application in electrolysis and various symmetrical cells due to their activity for CO 2 RR [11c] and oxygen evolution reaction(OER). [26] For example, CO 2 electrolysis performance of La 0.4 Sr 0.6 Co 0.2 Fe 0.7 Mo 0.1 O 3-d was enhanced after exsolution of FeÀCo nanoparticles, which seemed to originate from the metal-oxide interface that strengthened CO 2 adsorption and activation.A tt he same time, the materiala lso demonstrated good stability,c oking resistance and redox recyclability. [11c] Additionally,a pproximately 40 times highera ctivity forO ER was demonstrated by an exsolved FeÀCo/LaCo 0.8 Fe 0.2 O 3-d system compared to the pristine perovskite. Operando X-ray adsorption spectroscopy indicated that the enhancement in activity comes from the fact that exsolved nanoparticles self-reconstructedi nto (Co/Fe)O(OH) with unsaturated coordinationo fm etal ions in alkaline media during the OER, which acted as active species. [26] Besides, exsolved FeÀCo materials have also been used as electrodes in symmetrical solid oxide cellsb oth for the oxidation of different kinds of fuels (e.g.,H 2 ,L PG, C 2 H 5 OH and CH 3 OH) as well as for the electrolysis of H 2 Oa nd/or CO 2 ,e xhibitingp romising electrochemical performance and stability. [13c, 27] Last butn ot least,F e ÀCo particles have also been used in chemicall ooping applicationsw here CoFeAlO x was designed as an oxygen carrier material for chemical loopingw ater gas shift reaction, based on the known stability of alumina spinels and the high capacity and reactivity of cobalt ferrites. Here the redox cycle of the reaction causede xsolution and re-dissolution of active FeÀCo particles.I nterestingly,i tw as found that when reduced by mixed CO and CO 2 the exsolved FeÀCo particles remainede mbedded in the support demonstrating high redox stability,w hile under CO, the spinel support was over-reduced to Al 2 O 3 and the metal-support interface structure resembled ad eposited-like manner which led to easy sintering ( Figure 5). [10]

NiÀCo
NiÀCo exsolution studies mostly refer to electrochemical applications, [7h-j] in which these particles demonstrate similar characteristics to those of their Fe based counterparts (Section3 .1 and 3.2). Recently,i tw as reportedt hat an anode material based on NiÀCo alloy nanoparticles exsolved from La 0.52 Sr 0.28 Ti 0.94 Ni 0.03 Co 0.03 O 3-d exhibited high catalytic activity towards NH 3 decomposition and H 2 oxidation due to the abundant active sites and the balanced NH 3 adsorption and N 2 desorptionp rocess ascribed to the synergistic effects of Ni and Co in the alloy.A dditionally the electrode demonstrated good long-term stability duet ot he strongi nteractions between exsolved nanoparticles and the parent oxide. [28] Besides, interesting structural configurations seem to arise from compositions that are co-doped with Co and Ni. It was reported that discrete Co 3 O 4 and NiO nanoparticles could exsolve on the surfaceo f Sr 2 CoMo 1Àx Ni x O 6-d, designedt ob eu sed as as upercapacitor electrode. The exsolved material showed high specific capacitance with dual energy storagem echanisms, namely as urface Faradaic reactiona nd an oxygen intercalation process, which were relatedt ot he presence of the exsolved nanoparticles and the increased availability of oxygen vacancies formed in perovskite, respectively. [29] In catalytic applications,N ia nd Co were also reported to exsolve as alloyedn anoparticlesf rom an A-site deficient perovskite La 0.7 Ce 0.1 Co 0.3 Ni 0.1 Ti 0.6 O 3 and, by tracking individual nanoparticles throughout various chemical transformations, it was demonstrated that these particles have the ability to re-shape into highly activec ubes under reducing conditions. The material achievedh igh catalytic activity matching an oble metal commercial catalyst( Pt/Al 2 O 3 )w hen applied for oxidation of both CO and NO over hundreds of hours of operation. [9c] More recently,aconcept of endogenous-exsolution has emerged where NiÀCo nanoparticles were exsolved both on the surfacea nd in the bulk of the parentp erovskite La 0.7 Ce 0.1 Co 0.3 Ni 0.1 Ti 0.6 O 3-d .T he particles seemt ohave Ni segregated at the core while Co was more homogenously dispersed throughout the nanoparticles. The system demonstrated high oxygen capacity,h igh stability over redox cycling and resistance to deactivation mechanisms like sintering and coking while still exhibiting good surface activity,a ll significant properties for redox cycling applications. [8d]

Other bimetallic systems
Exsolutiono fc opper containing alloys has been reported in electrochemical applications. [6d, 13d, 30] For example, ac omposite (i, j) Schematicillustration of the interface structure during redox cycles for exsolved particles induced by CO + CO 2 and CO reduction, respectively. Adapted with permission. [10] electrode comprising of Ni x Cu 1Àx alloy nanoparticles exsolved in situ on the redox-reversible support of Nb 1. 33 Ti 0.67 O 4 was applied as ac athode for direct CO 2 electrolysis. The ratio of the two componentso ft he alloy appeared to have as ignificant effect on the electrocatalytic activity of the system with Ni 0.75 Cu 0.25 determined as the optimal ratio. [6d] Moreover,aperovskite electrode based on SrFe 0.8 Cu 0.1 Nb 0.1 O 3-d that allowed for reversible exsolution of an FeÀCu alloy was reported to show high conductivities in both oxidising and reducing atmospheres making it ap romising candidate both for SOFCs and SOECs. [13d] Finally,t he emergence of CuÀCo nanoparticles from av anadate Ce 0.8 Sr 0.1 Cu 0.05 Co 0.05 VO 4-d was accompanied by a phase transition of the host into ap erovskite structure Ce 0.8 Sr 0.1 Cu 0.05 Co 0.05 VO 3 .P articles seemed to be Cu-enriched on their surface, probablyd ue to the lower surface free energy of Cu. Such structures weref ound to demonstrate high activity and stabilityu nder hydrocarbon feeds in SOFC anodes; [30a] the catalytic activity was thought to be provided by the Co phase while the suppression of carbon formation was attributed to the Cu-rich surfacel ayer. [30a] Interestingly,n oble metals can be paired with base metals like Ni and Fe mainly,i no rder to boost the catalytic activity of base metals. [6e, 31] PtÀNi alloy (Pt 3 Ni)n anoparticles weree xsolved from La 0.9 Mn 0.9 Pt 0.075 Ni 0.025 O 3-d and the newly exsolved materiale xhibited enhanced activity for oxygen reduction reaction (ORR) comparable to that of the commercial catalystP t/ C, but also higher stability over Pt/C. It was also demonstrated that the enhanced activity was not solely due to the formation of Pt 3 Ni nanoparticles but also their strong interaction with the perovskite substrate. [6e] Additionally,R u ÀFe alloy nanoparticles were reportedt oe xsolve from aS r(Ti 0.3 Fe 0.7 Ru 0.07 )O 3-d in aS OFC anode resulting in lower polarization resistance andh igher maximum power density especiallya tl ow temperatures and hydrogen partial pressures, as compared to the cell using the Ru-free perovskite anode.This was attributedt othe promoting effect of the exsolved RuÀFe nanoparticles on hydrogen adsorptionw hich was the rate limitings tep for the Ru-free anode. [31]

Multielement, Nanostructured Systems
Aside from the traditional bimetallic systems, multi-metal exsolution has been demonstrated, as well as the formation of other nanostructures, such as nano-rods [32] and nano-fibres, [30b] core-shell nanoparticles [33] and even more complex heterostructures. These not only add to the structural diversity of the exsolved systems, but also to multi-metallic systems in general, hence opening more possibilities for tailorable, enhanced catalytic performance.
Simultaneouse xsolution of metals has been shown to be able to lead to particles with ac ore-shell structure.F or instance,e xsolution of Pd and Ni resulted in Pd-NiO core-shell particles whose shell thickness and particle size could be modulated by the initial Ni doping content and reduction temperature, respectively ( Figure 6). The exsolved core-shell particles werer eported to be firmly socketedo nt he surfaceo ft he oxide support resulting in good structuralstability under methane dry reforming reaction conditions. [33] Interestingly,a part from "conventional" co-exsolution, ad ifferent approachh as also been used to prepareb imetallic systems. Exsolution wasu sed in combination with infiltration of a second metal also resulting in bimetallic systems. For instance, Rh was impregnatedo nathree-dimensionally ordered macroporous( 3DOM) Ni-doped perovskite support, and Ni exsolved during reduction formed alloy nanoparticlesw ith the impregnated Rh. The catalyst showedg ood activity and stabilityf or CO 2 methanation owing to the unique 3D porouss tructure, the exsolved NiÀRh alloys and their strong interaction with the support. [34] Moreover,asystemw here Rh was infiltrated on a perovskite with endo-a nd exo-Ni exsolved particles was used in ac hemical looping methane partial oxidation process. Notably,t he infiltrated Rh particles didn ot form any bond with the Ni particlesb ut the modification did result in lowering the temperature at which methane was converted in dynamic temperature programmede xperiments by 200 8Ca dditionally increasinga ctivity by 40 %. [35] Reports have also demonstrated the exsolution of alloys involvingt hree metals,a nd examples include FeÀNiÀRu from LnFe 0.7Àx Ni 0.3 Ru x O 3-d (Ln = La, Pr) [36] and ReÀNiÀFe from La-Ni 0.2 Re x Fe 0.6 O 3 + d -La 3 ReO 8 [37] both being highly active and stable for methaned ry reforming. Spinel oxidesC u 1Àx Ni x Fe 2 O 4 were totally reduced to form as tructure of exsolved metallic nanoparticles on am etal matrix (e.g.,C u-rich CuÀNiÀFe alloys on a Fe-richF e ÀNiÀCu matrix from Cu 0.9 Ni 0.1 Fe 2 O 4 ). [38] The material served as ah ighly active anode for electrochemical H 2 oxidation anddemonstrated much smaller anodic polarization resistance than that of ac onventional NiFe alloy anode, which was ascribed to the unique alloy structure. [38] Another intriguing aspect of exsolution is that it can sometimes causes uch pronouncedc hanges to the host matrixes, that it can generate more complexs tructures than simple exsolved particles. For instance, exsolutiono fB -site metals accompanied by the segregationo fA -site oxides at the surface of perovskites is possible, hence aS rO phase was detected to Figure 6. Exsolution of core-shell particles.( a) HRTEM, (b) HAADF, (c)EDS analysiso fthe exsolved core-shell Pd-NiO particle. (d) Controlling the shell thickness of exsolved particles.A dapted with permission. [33]

Summary and Outlook
Exsolution is ar elatively new research hotspot which can be traced back to 2002. [12a] Exsolution of bimetallic speciesh as gained increased attention even more recently. Herein we have reviewed the, approximately,7 0studies published so far, on bimetallic exsolution and identified research trends in this area. It has been demonstrated that exsolution can endow bimetallicparticles with advantages in the terms of high catalytic activity,i mproved electrochemical properties,p rolonged durability and strong resistance to deactivation in aw ide range of applicationsm ainly including electrochemistry and catalysis. Moreover,a lloying such particles also allows for tuning of the adsorption properties, catalytic activity and stability of the exsolved particles due to the synergistic effects of different metals.H owever,m ost of these studies reported so far are application-oriented whilel ess attention has been paid on the principles of materiald esign. In order to further improve the performance of the exsolvedb imetallic systemsa nd unleash their full potential, some challenging issues should be resolved, including but not limited to the ones identified below.
Firstly,r eduction conditions, initial composition and defects of the parentm aterials can directly affect the ratio of the metals of the exsolved alloys which in turn could determine catalytic activity and selectivity.T aking this into consideration more systematic study is still required of this aspectt oa llow for fine tuning of the chemical nature of the exsolved alloys and indeed to improveu nderstanding of the exsolution process itself.
Secondly,a sc ompared to the exsolved monometallic particles that are usually in the form of simple spheres or ellipsoids, exsolution of different metals offers richer diversity of structures which can bring additional emergent functionalities to materials. However,s tudies on this aspecta re still very limited, and creating, characterisinga nd applying novel structures of exsolved bimetallic materials is highly desirable. A dapted with permission. [39] (b) Fe/MnO x nanoparticles. Adapted with permission. [41] (c) Images from left to right showing the Fe nanorod exsolved from LSF in dry H 2 ,t he FeÀNi alloyedp article formed on Ni-LSF in humidified H 2 ,a nd SrO nanorodg rown from Ni-LSFi nd ry H 2 ,respectively. Adapted with permission. [43] Finally,t he importance of modelling studies should be highlighted, which combined with the experimental observations will strengthent he understanding of the mechanism of bimetallic exsolution ultimately allowing for full control over the design of such materials.