Faraday Discuss. Chem. SOC., 1989, 87, 239-251 Metallic Glasses in Heterogeneous Catalysis Alfons Baiker Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology ( ETH), ETH-Zentrum, CH-8092 Zurich, Switzerland Metallic glasses exhibit some unique properties which make them interesting materials in catalysis. Recently their use as catalyst precursors has been advanced and several efficient catalysts have been prepared by various pretreatments of the metallic glasses. An understanding of the solid-state reactions occurring during the transition of the amorphous precursor to the active catalyst was found to be crucial for successful application of these materials. This paper describes the present knowledge and discusses prin- cipal possibilities and problems involved in the application of metallic glasses in catalysis. The use of metallic glasses in catalysis was first reported at the beginning of this decade.’-3 Early investigations in this field were of a rather phenomenological nature, focusing mainly on the catalytic behaviour with little attention to the surface and bulk characterisa- tion of the materials used.Reviews which cover these activities have been written by S ~ h l o g l , ~ Yoon and Cocke,’ and Shibata and Masumoto.6 It was only recently that researchers started to give intensive consideration to the characterisation of the amor- phous alloys used in the catalytic reaction. These investigations showed that presumably in most applications of metallic glasses in catalysis the surface of the metastable amorphous alloy undergoes chemical and structural changes under reaction conditions.These observations, coupled with the fact that as-quenched alloys exhibit very low surface areas, and consequently low activity, led several investigators to use metallic glasses as catalyst precursors rather than as catalysts. The aim of this paper is to review briefly the knowledge gained and to show where metallic glasses may be of interest in catalysis. Properties and Preparation of Metallic Glasses Metallic glasses’ can be regarded as congealed metallic melts, rigid but devoid of crystalline order. Another name sometimes used is amorphous metal alloys, which underlines the fact that such materials are all alloys and never pure metals. The great variety of metallic glasses reported in the literature’ fall into a few well defined categories: (i) late transition metal + metalloid; (ii) early transition metal + late transition metal or Group IB metal; (iii) alkaline-earth metal +Group IB metal; (iv) early transition metal + alkali metal; and (v) actinide+ early transition metal. For catalysis only metallic glasses of categories (i) and (ii) have been applied so far (table 1).Various factors have been suggested to influence the glass-forming tendency, among which are the thermodynamics (phase diagram), kinetics, metastable phases, stoichiometry, concentration of valence electrons, atomic sizes of constituents and electronegativity. Also a ‘confusion principle’ has been proposed, to the effect that complex mixtures of constituents have greater glass-forming tendency than binary mixtures.Frequently, the search for alloys that readily yield metallic glasses is aided by the fact that eutectic compositions are favoured. 239240 Metallic Glasses in Heterogeneous Catalysis Table 1. Catalytic reactions studied over metallic glasses or catalysts derived therefrom major products alloy ref. C , -C, hydrocarbons C , -C3 hydrocarbons C I -C, hydrocarbons C I -C, hydrocarbons C -C, hydrocarbons methanation methanation methanation C I -C, hydrocarbons C I -C, hydrocarbons C , , C, hydrocarbons methanol synthesis methanol synthesis methanol synthesis methanation acetylene ( + )-apopinene buta-l,3-diene ethene, buta-1,3-diene buta- 1,3-diene buta-1,3-diene buta- 1.3-diene cis-cyclododecene cyclohexene, n-hexene phenylethyne, a-pinene cyclohexene, benzene ethene, propene, isoprene cis-but-2-ene, buta-l,3-diene ethene ethene ethene ethene, isoprene oct-1 -yne, oct-4-yne, phenylacetylene hex-1-ene phenylethyne methyl formate carbon monoxide ethane, cyclopropane 5-aminopentan-1-01 1 3 24 25 26 27 28,29 30 31,32 33 34 35 36 37 38 39 16,17 40 41 42 43 44 2, 14 15 45 46 47,48 49 50 51 18 52 53 54 55 56 57 58 47 59A.Baiker 24 1 Several principal can be used for the production of metallic glasses, including vapour and sputter deposition, plating and melt quenching. Among these methods melt-quenching is the one most widely used. In melt-quenching the melt of the constituents is so rapidly quenched that there is insufficient time for crystallites to nucleate and grow.The common feature of all devices that have been designed to quench molten alloys is that the melt must be converted very rapidly from a jet or stream of droplets into a thin layer in contact with a ‘chill block’ to produce thin foil or ribbon of 10-60 p m thickness. The two most widespread techniques are melt-spinning and melt-extraction. In both, the ‘chill-block’ is a rapidly spinning copper wheel. Melt- spinning produces thin ribbons, melt-extraction makes fine wires. It is important to note that the glassy metals used so far in catalysis have mainly been produced by melt-spinning. Motivation for using Metallic Glasses in Catalysis Metallic glasses possess several properties’”’ which make them interesting materials in catalysis. Ideally, the surface of amorphous materials should be devoid of any long-range ordering of the constituents and exhibit a high density of low-coordination sites and defects.The important role of low-coordination sites in catalysis, such as terrace, step and kink sites, has been demonstrated by Somorjai’ ’ on crystalline materials. Metallic glasses possess high flexibility with regard to fine-tuning of the electronic properties,I2 mainly due to the fact that thermodynamic constraints are less severe in supercooled liquids than in crystalline materials. Metallic glasses are ideally chemically homogeneous and structurally isotropic. Metallic glasses are highly reactive owing to their metastable structure and they undergo solid-state reaction frequently more easily than their crystalline counterparts.This property is of importance in their use as catalyst precursors. Metallic glasses exhibit good conductivity for electricity and heat, making them particularly interesting for application in electrocatalysis. Metallic glasses prepared by melt-quenching exhibit a planar morphology ideal for investigation with modern characterisation tools such as photoelectron spectroscopy and scanning tunnelling microscopy. The manufacture of foils or small ribbons may facilitate the design of new reactor conceptions. Limitations with regard to the application of metallic glasses in catalysis orginate from their metastable structure and the intrinsically low surface area, which corresponds to the geometrical area of as-quenched materials. Catalytic Studies Presently available reports on catalysis over metallic glasses are listed in table 1 .Two different types of investigations can be distinguished depending on the degree of pretreatment of the amorphous metal alloys, namely, catalytic studies on fresh unrecon- structed surfaces of metallic glasses, and studies which were centred on the role of pretreatment and its influence on the catalytic properties of the metallic glasses. Studies on Fresh Unreconstructed Surfaces of Metallic Glasses Relatively little catalytic work has been carried out so far under conditions where the surface of the metal alloys can be regarded as unreconstructed, i.e. where the chemical composition and structure of the surface can be assumed to be in the state characteristic242 Metallic Glasses in Heterogeneous Catalysis of the freshly quenched material.In principle, such investigations can be performed only at temperatures far below the crystallisation temperature of the alloy and require special precautions to eliminate possible contamination of the alloy during its transfer from the site of fabrication to the catalytic reactor. In their initial work, Smith et al.' showed that Pd-Si glasses, made by splat cooling,' are active catalysts for the deuterogenation of cis-cyclododecene at room temperature. Pd-Si glasses produced more trans-isomerisation, more dideutero-saturate and less extensive exchange than crystalline Pd. Later these studies were extended by including Pd-Ge glasses" and comparing the catalytic behaviour of the metallic glasses with their crystalline counterparts.The different selectivities found for the above hydrogenation over amorphous and crystalline Pd systems was attributed to the different surface topography of the metallic glasses and the crystalline alloys. The glassy surfaces were suggested to be free of atomically flat terraces and to be highly populated with protuberances approximating kinks and ledges, as compared with the surface of large crystallites, where the terraces predominate. The kinks and ledges on crystalline surfaces are of discrete dimension^,'^ whereas the glassy surface is assumed to present protuberances with a continuum of coordination numbers. In contrast to the results above, Giessen et a1.15 found no significant differences in the selectivity behaviour between glassy and crystalline phases of Pd,,,Si,,, during hydro- genation reactions of n-hexene, phenylethyne, a-pinene, and cyclododecene.Catalytic selectivities with regard to cis- trans isomerisation, double-bond migration and the stereochemistry of addition were approximately the same regardless of whether a glassy or crystalline catalyst was used. Minor differences were observed in hydrogen-deuterium exchange. The reason for these contrasting results is not clear. More recently, it has been suggested that the hydrogenation of cis-cyclododecene is not very suitable for characterising differences in the structure of these surfaces, since an isomer (trans-cyclododecene) is produced in the reaction which has a rate of hydrogenation different from that of the parent compound.These differences in rates of hydrogenation mask the actual amounts of isomerisation that are occurring and, therefore, conceal the kinds of catalytic sites available on the surface. With this in mind, Smith and c o - ~ o r k e r s ~ ~ ~ " tested the molecule ( + )-apopinene (6,6-dimethyl-1 R,5 R- bicyclo[3.1 . l ] hept-2-ene) as a surface probe to distinguish the relative percentages of terraces, ledges and kinks available on the metallic surface. This probe molecule is more useful than the cis- cyclododecene used earlier because its isomerisation product (-)-apopinene has an identical rate of hydrogenation on a symmetrical surface. The crystallised alloys showed a higher ratio of isomerisation to deuteration than the parent amorphous alloys.I t was suggested that the isomerisation of ( + )-apopinene reflects the total number of ledge, kink and terrace sites, whereas hydrogenation reflects only the number of kinks. Based on these arguments, Smith and co-workers concluded that the surface structure of an amorphous alloy is not two-dimensionally random (flat), but is three-dimensionally random (hilly or rolling). Molnar et aLx studied the selective half-hydrogenation of phenylacetylene, oct- 1-yne and oct-4-yne over Pd-Si and Pd-Ge glassy and crystalline catalysts, and for comparison, over splat-cooled Pd, reduced Pd02 and Pd foil. They found that terminal alkynes comminute Pd structures and expose new active sites. These sites are different on the rapidly cooled catalysts compared to the regularly crystallised catalysts.Although no significant changes were detected in alkyne-hydrogenation selectivities after several hydrogenations, marked changes were revealed by ( + )-apopinene. On the terminal acetylene-treated foils and on the reduced PdO?, the rates of hydrogenation and isomeri- sation of ( + )-apopinene increased, but the ratio of the two rates remained almost the same. In contrast, the splat-cooled catalysts showed a higher rate increase for isomerisa- tion than for hydrogenation. Molnar et al." suggested that the effect of the terminalA. Baiker 243 alkynes is to expose sites of lower coordinative unsaturation, especially ledges and kinked sites by comminution of the Pd structures. These newly exposed sites are assumed not to influence alkyne reactions.It is interesting to note that heat treatment of the amorphous alloys was previously" reported to have a similar effect on the surface structure of the amorphous alloys. Direct insight into the surface structure of metallic glasses has recently been obtained by using scanning tunnelling microscopy (STM). Wiesendanger et ~ 1 . ' ~ performed STM measurements on glassy Rh25Zr75 prepared by melt-spinning. They found that the surface is made up of flat areas, identified as disordered regions, and hill structures, which they attributed to nanocrystals embedded in the amorphous matrix. Similar surface mor- phology was recently found by Walz et al.") for amorphous Feg1Zr9 prepared by melt-spinning. They concluded from their STM investigations that the surfaces of the Fe-Zr samples were at least partially crystallised.Schlogl et aL21 investigated the surface morphology of amorphous Feg1Zr9 alloys using both scanning electron microscopy (SEM) and STM. They also found pronounced anisotropy of the surface structure of the melt-quenched material. The surface was reported to consist of disc-shaped structures several hundred ingstroms in size, with clear valleys separating them. The valleys were attributed to inhomogeneous cooling of microdroplets of the liquid alloy. It could be clearly shown that the smooth surface of the droplets consists of irregular corrugations with an average distance between the maxima of several nanometres and an amplitude of below 1 nm. It has been suggested that the latter structure originates from frozen waves excited on the surface of the formerly liquid microdroplets preserved by the rapid quenching rate of ca.lo6 K s-I. Insight into the electronic structure of amorphous metal alloys surfaces has been gained by studying their interaction with probe molecules using photoelectron spectros- copy (UPS and XPS). Hauert et al.'? studied the chemisorption of CO on Ni-Zr metallic glasses using UPS. Chemisorption of CO was investigated as a function of exposure, temperature and alloy composition. Molecular and/or dissociative CO chemisorption was observed depending on the alloy composition. Molecular chemisorption was pre- dominant on nickel, whereas on zirconium dissociative chemisorption was prevalent. However, Hauert et a1.22 observed that the ratio of molecular to dissociative CO chemisorption was not directly related to the surface 'composition of the Ni-Zr alloys.This phenomenon was attributed to Zr modifying the local electronic structure at the Ni-atom sites in such a way that the chemisorption behaviour of these sites is profoundly different from elemental nickel. More recently, Baiker et al.'3 investigated the adsorption of nitrogen and CO on amorphous Feg1Zr9 using UPS and XPS. The studies showed that both molecular and dissociated nitrogen were present on the surface after exposure to dinitrogen at 79 K. Upon addition of small amounts of hydrogen to the surface the dissociated nitrogen species was desorbed completely, presumably as ammonia. The results of the UPS and XPS studies, including binding energies and line profiles, agreed well with the results of similar investigations carried out on single-crystal surfaces of iron.Similarly, no differences were observed in the CO adsorption normalized to the number of iron sites in the surface (amount adsorbed, kinetics, chemical shift) when comparing CO adsorp- tion on polycrystalline iron and amorphous Fe9,Zr9. This indicated that the local electronic structure of the adsorption sites on the amorphous alloy surface are similar to those of elemental iron. The similarity of the nitrogen adsorption, which is known to be structure-sensitive, may be taken as an indication that both the polycrystalline and the 'amorphous' surfaces exhibit similar microstructures. The similarities in the microstructure of the surfaces may have been produced by the extensive sputter-cleaning of both samples before adsorption experiments.All the experimental investigations discussed above indicate that the surfaces of metallic glasses prepared by melt-quenching are rather inhomogeneous, containing244 Metallic Glasses in Heterogeneous Catalysis disordered regions and crystalline-like regions. These are probably created during the quenching process. Thus the ideal amorphous metal surface, being isotropic and showing uniform short-range ordering, may in reality be difficult to produce, and even more difficult to maintain under conditions where catalytic reactions are performed. This phenomenon imposes severe limitations to all applications of metallic glasses, where a stable disordered surface structure is demanded.The final answer concerning the short-range ordering of amorphous surfaces cannot yet be provided. High-resolution STM of the topography and the local tunnelling barrier height of the amorphous areas could be promising in providing information about short-range order in metallic glass surfaces if the electronic surface structure and the geometric surface structure can be correlated. l Y Studies on Pretreated Metallic Glasses In most catalytic applications of metallic glasses, pretreatment of the as-quenched materials (e.g. in a reducing-gas atmosphere) was found to be crucial to obtain high catalytic activities. Several factors may contribute to this behaviour, the most important being: (i) the surfaces of metallic alloys exposed to air are likely to be covered with a superficial layer of inactive metal oxides; (ii) the surface area of as-quenched materials is very small (usually <0.1 m’g-’) and is therefore easily deactivated in the presence of contaminants. Several different procedures have been applied to improve the catalytic properties of as-quenched materials, including reduction in hydrogen 1,325726~3 1*39743350 or in other reducing-gas atmospheres ( e.g.H2/C0,24,28-30,35*37 H2/CO-,,37338 H2/N253*54) as well as etching in acid solutions ( HCl,33 H N O ~ , ~ ~ ’ ~ ~ . ~ ~ , ~ ~ HF4’,51 ) followed by oxidation and reduction. It seems likely that in most cases where such pretreatments were applied, the original surface structure of the amorphous alloy was altered. Thus comparative studies of the catalytic behaviour of the pretreated amorphous metal alloys and their crystalline counterparts do not generally provide a reasonable basis for answering the question of whether or not the amorphous surface is more active than the corresponding crystalline surface.In any case, it is interesting to note that in most studies [exceptions are reported in ref. (15) and (36)] the amorphous samples were found to exhibit improved or better compared to their crystalline counterparts. The reason for this behaviour is in many cases not clear, since too little effort has been expended on characterising the chemical and structural properties of the amorphous and crystalline alloy surfaces. Several factors, such as degree of ordering and dispersion of the active component, electronic properties, formation of new phases, nucleation and growth of crystalline domains, segregation phenomena and textural properties, will be differently influenced during pretreatment, depending on whether an amorphous or a crystalline alloy is used as starting More recently, highly active catalysts were prepared from glassy metals by selectively oxidising the more electropositive constituent of the material.After reduction, finely dispersed transition-metal particles which are embedded in an amorphous or partially crystalline oxide matrix of the more electropositive constituent were obtained. In principle, this method for the preparation of supported metal catalysts from metal alloys is not new: it has been applied previously by Shamsi and Wallace6’ to crystalline intermetallic alloys.The use of amorphous metal alloys as precursors may offer several advantages, such as higher flexibility in composition, homogeneous distribution of constituents on a molecular scale and higher reactivity. These advantages emerge from the intrinsic properties of the materials outlined above. Next, we shall discuss some crucial factors influencing the structural and chemical properties of catalysts prepared from metallic glasses. catalytic behaviour, i. e. either higher activity 1-3,13,24-26,28.29,3 1,34,38,43,44.50-52,54-57 selectivity,23 13.1 8,25.39,42,43,57A. Baiker 245 Factors influencing Structural and Chemical Properties of Catalysts derived from Metallic Glasses Important properties of metallic glasses influencing the structural and chemical proper- ties of the catalyst derived from them are: (i) chemical composition; (ii) chemical and structural homogeneity; (iii) thermal stability and crystallisation behaviour; (iv) dissol- ution of gases; and (v) segregation phenomena.These factors together with the condi- tions used for the chemical transformation of the precursor are most crucial to obtain catalysts with the desired properties. Chemical Composition The chemical composition influences virtually all properties discussed subsequently and is therefore a controlling factor in the preparation of catalysts from metallic glasses. Note that the flexibility in the composition of metallic glasses is not as large as one would anticipate from the fact that thermodynamical constraints are less stringent for metastable solids.Chemical and Structural Homogeneity Ideally an amorphous metal alloy should be chemically and structurally isotropic. Chemical and structural anisotropies lead to non-uniform propagation of the solid-state reactions occurring during the transformation of the amorphous precursor to the active catalyst. Such inhomogeneities are frequently due to either too slow cooling rate,32 or surface contamination during exposure to air32959 (surface oxide layer). Thermal Stability and Crystallisation Behaviour The thermal stability is a severe limitation if the metallic glass is to be used in the as-quenched state for catalysis; however, that is not necessarily the case if the glassy alloy is used as a catalyst precursor. The thermal stability is mainly influenced by the chemical composition of the metallic glass and the medium to which it is exposed.It has been shown that the crystallisation temperature can be significantly lowered in the presence of a hydrogen a t m o ~ p h e r e ~ ’ ” ~ ~ ~ ~ ” ~ or an adsorbed organic compound.61 Metallic glasses have been found to crystallise by nucleation and growth processes. The driving force is the difference in free energy between the glass and the appropriate crystalline phase(s). Depending on the composition, crystallisation may occur by: ( i ) primary crystallisation, where one crystal phase with a composition different from the amorphous matrix is produced; (ii) polymorphic crystallisation, where one phase with the same composition as the glass is crystallised (occurs only in concentration ranges near the pure elements or compounds) or (iii) eutectic crystallisation, where two crystalline phases grow concomitantly by a discontinuous reaction.Most metallic glasses can crystallise by two or more different reactions. The route by which crystallisation occurs depends not only on the thermodynamic driving force (difference in free energy), but also on the kinetics of the possible routes. In the case of pretreatment of the metallic-glass precursor in reactive gas atmospheres, solid-gas-phase reactions are likely to influence the expected crystallisation behaviour. During rapid solidification, as well as during annealing treatments, surfaces are expected to catalyse nucleation as the crystalline phase replaces a portion of the surface, thus reducing the total energy required for nucleation.An important factor for crystallisa- tion is the oxygen content near the surface. Oxygen may stabilise a number of crystalline phases, thus increasing the driving force for crystallisation. Selective oxidation of one of the components, e.g. the metalloids at the surfaces of metal-metalloid glasses, is likely to result in excessive crystallisation of the metal (e.g. ~ ~ p p e r , ~ ~ * ~ ~ * ~ ~ palladium, 30.38246 Metallic Glasses in Heterogeneous Catalysis and iron44v53 in binary zirconium alloys). Selective oxidation is likely to exhibit the strongest influence on surface crystallisation. Even at temperatures far below any crystallisation event in the bulk glass, primary crystallisation of the transition metal has been observed in metal-metalloid glass.h2 It should also be noted that the crystallisation behaviour of melt-spun ribbons may be different on both ribbon sides.32 Nucleation for primary crystallisation of the transi- tion metals is observed to occur on both sides of the glassy ribbons, while other crystallisation reactions have been observed to prefer usually either the free surface or the contact side of the ribbon.6' This phenomenon may lead to different structural and chemical properties of the two ribbon sides, and consequently also to large anisotropy in the catalysts prepared from such Dissolution of Gases in Metallic Glasses The dissolution of gases is frequently different in metallic glasses than in their crystalline counterparts owing to the marked differences in the structural and electronic properties.As regards the metallic glasses, present knowledge concentrates mainly on the absorption of hydrogen. Since almost all catalysts prepared so far from metallic glass precursors were exposed to a hydrogen-containing atmosphere, either during activation or reaction, understanding the interaction and solid-state reactions induced by hydrogen seems to be crucial. Maeland et aLh3 have shown that the solubility (absorption capacity) of hydrogen in metallic glasses with the general formulae Ti,-,Cu, and Zr,-,Cu, ( x = 0.3-0.7) is larger than in corresponding crystalline alloys. Besides its possible direct influence on the catalytic properties of metallic glasses as a hydrogen source, the absorption of hydrogen generally enhances the formation of metal hydrides, which have been shown to be crucial intermediates in the preparation of catalysts from a r n o r p h o u ~ ~ ~ and crystalline64365 metal alloys.Unfortunately, no similar studies are presently available for the solution of other gases in metallic glasses. Segregation Phenomena The surface composition of metal alloys is often different from that of the bulk. Major driving forces for surface segregation revealed by model calculations" are: different surface free energies of the components and size mismatch in the case of clean surfaces, as well as different heats of chemisorption and reaction of components in the presence of adsorbates. Surface segregation induced by selective oxidation is well known for crystalline and amorphous alloys of the type A-B, where A is an early transition metal or rare-earth metal (e.g.Zr, Ti, lanthanides or actinides), and B a Group VIII (e.g. Ni, Fe, Pd) or Group IB metal (e.g. Cu, Au). Upon exposure of the alloy to oxygen, component A (the more electropositive element) is oxidised and is enriched at the ~ u r f a c e . ~ ~ * ~ ~ - ' ~ As a result of this, phase separation may occur, and the remaining atoms of component B cluster together and precipitate. The phase separation is crucial for the formation of oxide-supported metal particles. Similar segregation phenomena may also occur by adsorption or absorption of hydrogen. However, the enthalpies of hydride formation are much smaller than those of oxide formation.The results on hydrogen-induced surface segregation are rather controversial. The exclusion of oxygen traces in the bulk and surface of the alloys is crucial for proper studies of surface segregation induced by hydrogen. No significant segregation effect has been measured in crystalline Cu30Zr70 and Cu70Zr30,67 LaNi, , 7 ' Mn,Zr, Cr2Zr, V2Zr68 and amorphous Ni26Zr76, C U ~ ~ Z ~ ~ ~ and Fe24Zr76 ,72 whereas strong surface segregation was reported for amorphous Pd-Zr alloys after exposure to a hydrogen atmo~phere.~'Faraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 1. High-resolution electron micrograph and electron diffraction pattern of catalyst ( a ) prepared from amorphnus Pd,,Zr,, . Note the extremely small domain sizes of the phases Pd and ZrO, (baddeleyite), which are also reflected by the partial absence of well defined reflection maxima.Evaluation of the electron diffraction patterns gives evidence for the presence of metallic palladium [ ( 1 11) reflection has highest intensity]. A. Baiker (Facing p. 247)A. Baiker 247 -7.0 h LL v c - -8.0 t 0 \ \ \ -9.0 I I I I I I I I I I I 2.8 3.0 3.2 3.4 3.6 lo3 K/T Fig. 1. Comparison of CO oxidation activities of palladium-on-zirconia catalysts prepared by in situ activation from amorphous Pd,,Zr,, ( a ) and by conventional impregnation of zirconia with a palladium salt ( b ) , respectively. Arrhenius plots of the turnover frequencies are plotted. Conditions were: reactant-gac mixture, 1700ppm CO, and 1700ppm O7 in nitrogen; flow rate, 150 cm' (s.t.p.) min-'; amount of catalyst, ( a ) 0.37 g; ( b ) 1.24 g.Examples illustrating the Potential of Metallic Glasses as Catalyst Precursors There are several examples reported in the literature which demonstrate the potential of metallic as catalyst precursors~'7,10.~5,~~,~~.4'),51.53.54,56.~9.74 For ill us tratio n we may consider the preparation of a palladium-on-zirconia catalyst'" for the oxidation of CO. The catalyst was prepared from amorphous Pd,Zr, by exposure to CO oxidation conditions at 550 K. Under these conditions the amorphous precursor was transformed to a catalyst containing well dispersed palladium particles embedded in a zirconium dioxide matrix. It is interesting to note that the metallic glass precursor was virtually inactive, but the activity developed during the in situ activation, finally reaching a steady state after the transformation was complete.The solid-state reactions occurring in the metallic glass during in situ activation resulted in a large increase in the B.E.T. surface area from 0.02 to 45.5 m2 g-'. The palladium metal surface area of the as-prepared catalyst determined by CO chemisorption was 6.9 m' g-', which corresponds to a palladium dispersion of ca. 6 % . Fig. 1 compares the intrinsic activity of palladium in the catalyst prepared from the metallic glass with the corresponding activity of palladium in a palladium-on-zirconium catalyst prepared by conventional impregnation (incipient wetness) of zirconium dioxide with a palladium salt [( NH4)'PdCl4]. Note the markedly higher turnover frequency measured for the PdlZrO, catalyst prepared from the metallic glass as compared to the conventionally prepared catalyst.The reason for this behaviour is not completely understood so far; however, there is clear indication that the enhanced activity of the palladium in the catalyst prepared from the metallic glass precursor has to be sought in the extremely large interfacial area (metal-metal oxide) of this catalyst. The structural features of the catalyst prepared from the metallic glass are illustrated by the high- resolution electron micrograph shown in plate 1. Note the small intergrown crystalline domains leading to a large interfacial area between palladium and zirconia. This structural feature is characteristic for as-prepared catalysts and is possibly not obtainable by conventional preparation techniques.I t has been found recently that such large248 Metallic Glasses in Heterogeneous Catalysis interfacial areas originating from small intergrown crystals may enhance the formation of solid solutions with hydrogen" and oxygen.75 Photoelectron spectros~opy~~ indicated that the electronic properties of the palladium in the Pd/Zr02 catalyst prepared from the metallic glass were similar to that of a pure palladium foil. This further supports the important role of the interfacial area between palladium and zirconia, particularly in view of the fact that CO oxidation over palladium was found to be structure-insensitive in several investigation^.^^ Conclusions Metallic glasses may be used in the as-quenched state or as catalyst precursors in heterogeneous catalysis.The motivation for using them in the as-quenched state is based on the ability to tailor the electronic properties and the special surface structure which ideally exhibits no long-range ordering of the constituents. The latter property seems, however, partly questionable, at least for surfaces of metallic glasses prepared by melt-spinning, as recent investigations using STM have indicated. Surfaces of metallic glasses already tend to undergo structural relaxation at temperatures far below the crystallisation temperature, and the presence of adsorbed reactants is likely to enhance this relaxation. Thus, in order to make use of the special surface structure of metallic glasses, catalytic reactions have to be performed at low temperatures, which is a severe limitation, in particular in the light of the intrinsically small surface area of these materials.At present, the use of metallic glasses as catalyst precursors appears to be more promising. Several efficient supported-metal catalysts have been prepared by exposing the precursor alloy to an oxygen-containing gas atmosphere at higher temperature. The aim of these pretreatments is to transform the alloy into a supported-metal catalyst by oxidizing the more electropositive component. Catalysts prepared as such have been shown to possess reasonably large B.E.T. surface areas and metal surface areas. A major difference in their structure compared to conventionally prepared supported-metal catalysts is that both the active metal as well as the oxidic support are made up of small disordered and/ or intergrown crystalline particles.As a result of this, the interfacial area between metal particles and support is extremely large, leading to very strong metal-support interactions. Such structures are for most systems difficult to prepare by conventional preparation methods. Thus, the preparation from amorphous alloys offers great potential. The solid-state reactions occurring during pretreatment of the metallic glasses are complex, and the structure of as-prepared catalysts is influenced by several factors determined by the intrinsic properties of the metallic glass and the conditions of pretreatment. Relevant intrinsic properties are: chemical composition, chemical and structural homogeneity, thermal stability and crystallisation behaviour, dissolution of gases and segregational phenomena.Better understanding of all these phenomena is a necessary prerequisite for progress in the use of metallic glasses as catalyst precursors. Although reports on the application of metallic glasses in electrocatalysis are still scarce, these materials seem to offer interesting properties for such applications, par- ticularly since large surface areas are not necessarily required in electrocatalysis. Many metallic glasses exhibit high corrosion resistance, and that coupled with the ability to form homogeneous solid solutions supersaturated with various elements should be beneficial in electrocatalysis. Metallic glasses have been used for electrocatalysis of sodium ~hloride,~' sea water7* and for the hydrogenation of carbon monoxide," the electro-oxidation of methanol8' and the electrocatalytic evolution of hydrogen and oxygen from alkaline so1ution.x'*x3 With regard to the preparation of metallic glasses, it is important that new methods are developed that enable the materials to be fabricated in a form better suited forA. Baiker 249 catalytic purposes.Promising routes include novel chemical technique^.^^-^^ The desired techniques should provide the possibility to deposit the amorphous metal alloys at high dispersion on a support material. Besides these more practical aspects, it should be pointed out that studies performed on metallic glass surfaces are likely to aid us in answering some long-standing funda- mental questions in catalysis.Metallic glasses were suggested to be ideal model systems' for the study of several catalytic problems, among them, the role of bimetals and multimetallics, the role of short-range ordering, the electronic and geometric structure of defects, the influence of' promotors, and surface segregation and clustering. Thanks are due to A. Reller (University of Zurich) for the high-resolution electron micrograph and to H. J. Giintherodt, P. Oelhafen and B. Walz for valuable discussions. Financial support of our work by Lonza AG and the Swiss National Science Foundation is gratefully acknowledged. References 1 H. Komiyama, A. Yokoyama, H. Inoue, T. Masumoto and H. Kimura, Sci Rep. Res. Inst. Tohoku Uniu. Ser. A, 1980, 28, 217. 2 G .V. Smith, W. E. Brower, M. S. Matyaszczyk and T. L. Pettit, in Proc. 7th Int. Congr. Caral., ed. T. Seiyama and K. Tanabe (Elsevier, New York, 1981), vol. A, pp. 355-363. 3 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, J. Catal., 1981, 68, 355. 4 R. Schlogl, in Rapidly Quenched Metals, ed. S. Steeb and H. Warlimont (Elsevier, Amsterdam, 1985), 5 C. Yoon and D. L. Cocke, J. Non-Crysr. Solids, 1986, 79, 217. 6 M. Shibata and T. Masumoto, in Stud. Surf Sci. Catal. (Prep. Catal. I V ) , ed. B. Delmon, P. Grange, 7 F. E. Luborsky, Amorphous Metallic Alloys (Butterworths, London, 1983). 8 C. Suryanarayana, Rapidly Quenched Metals: A Bibliography, 1973- 1979 (IFI/Plenum, New York, 1980). 9 H. J . Giintherodt, in Rapidly Quenched Metals, ed. S. Steeb and H.Warlimont (Elsevier, Amsterdam, VOI. 11, pp. 1723-1727. P. A. Jacobs and G . Poncelet (Elsevier, Amsterdam, 1987), vol. 31, pp. 353-372. 1985) vol. 11, pp. 1591-1598. 10 R. B. Diegle, J. Non-Cryst. Solids, 1984, 61 & 62, 601. 1 1 G. A. Somorjai, Caral. Rev. Sci. Eng., 1978, 18, 173. 12 P. Oelhafen, in Glass?! Metals I I , ed. H. Beck and H. J. Guntherodt (Springer, Berlin, 1983), pp. 283-321. 13 W. E. Brower, M. S. Matyjaszczyk, T. L. Pettit and G. V. Smith Nature (London), 1983, 301, 497. 14 G . A. Somorjai, Adu. Catal., 1977, 26, 1. 15 B. C . Giessen, S. S. Mahmoud, D. A. Forssyth and M. Hediger, in Rapidly Solidified Amorphous and Crysralline Aflop, ed. B. H. Kear, B. C. Giessen and M. Cohen (Elsevier, New York, 1982), pp. 255-258. 16 G. V. Smith, 0. Zahraa, A.Molnar, M. M. Khan, B. Rihter and W. E. Brower, J. Catal., 1983, 83, 238. 17 A. Molnar, G. V. Smith and M. Bartok, J. Catal., 1986, 101, 540. 18 A. Molnar, G. V. Smith and M. Bartok, J. Caral., 1986, 101, 67. 19 R. Wiesendanger, M. Ringger, L. Rosenthaler, H. R. Hidber, P. Oelhafen, H. Rudin and H. J. Giintherodt, Sur$ Sci., 1987, 181, 46. 20 B. Walz, R. Wiesendanger, L. Rosenthaler, H. J. Giintherodt, M. Diiggelin and R. Guggenheim, Mater. Sci. Eng., 1988, 99, 501. 21 R. Schlogl, R. Wiesendanger and A. Baiker, J. Caral., 1987, 108, 452. 22 R. Hauert, P. Oelhafen, R. Schlogl and H. J. Guntherodt, Solid State Commun., 1985, 55, 583. 23 A. Baiker, H . Baris and R. Schlogl, J. Caral., 1987, 108, 467. 24 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H.Kimura, Scr. Metall., 1981, 15, 365. 25 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, ACS Symp. Ser., 1982, 196,237. 26 M. Peuckert and A. Baiker, J. Chem. SOC., Faruday Trans. 1, 1985, 81, 2797. 27 Y. Shimogaki, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, Chem. Lett. ( J p n ) , 1985, 661. 28 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, Chem. Lett. (Jpn), 1983, 195. 29 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, J. Non. Cryst. Solids, 1984, 61 30 M. Shibata, N. Kawata, T. Masumoto and H. Kimura, Chem. Lett. (Jpn), 1985, 1605. 31 G. Kisfaludi, K. Lazar, Z. Schay, 1 . Guczi, C. Fetzer, G. Konczos and A. Lovas, Appl. Sur$ Sci., 1985, 32 G. Kisfaludi, Z. Schay, L. Guczi, G. Konczos, L. Lovas and P. Kovacs, Appl.Swfi Sci., 1987, 28, 1 1 1 . 33 G. Kisf'dudi, Z. Schay and L. Guczi, Appl. Surf Sci., 1987, 29, 367. & 62, 619. 24, 225.250 Metallic Glasses in Heterogeneous Catalysis 34 H. Yamashita, M. Yoshikawa, T. Funabiki a n d S. Yoshida, J. Caral., 1986, 99, 375. 35 M. Shibata, Y. Ohbayashi, N. Kawata, T. Masumoto a n d K. Aoki, J. Catal., 1985, 96, 296. 36 S. J. Bryan, J . R. Jennings, S. J. Kipling, G . Owen, R. M. Lambert and R. M. Nix, Appl. Catal., 1988, 37 D. Gasser a n d A. Baiker, Appl. Catal., 1989, 48, 279. 38 A. Baiker a n d D. Gasser, J. Chem. Soc., Faradaji Trans. 1, 1989, 85, 999. 39 G . Carturan, G. Cocco, E. Baratter, G. Navazio a n d C. Antonione, J. Caral., 1984, 90, 178. 40 S. Yoshida, H. Yamashita, T. Funabiki and T. Yoezawa, J. Chem.Soc., Farada). Trans. 1 , 1984,80, 1435. 41 H. Yamashita, M. Yoshikawa, T. Funabiki a n d S. Yoshida, J . Chem. Soc., Faraday Trans. 1, 1985, 81, 2485. 42 J. C. Bertolini, J. Brissot, T. Le Mogne, H . Montes, Y. Calvayrac a n d J. Bigot, Appl. Surj Sci., 1987, 29, 29. 43 A. Baiker, H. Baris a n d H. J. Guntherodt, J. Chem. Soc., Chem. Commun., 1986, 930. 44 H. Yamashita, M. Yoshikawa, T. Funabiki a n d S. Yoshida S. Yoshida, J. Chem. Soc., Faraday Trans. 45 T. Takahashi, Y. Nishi, N. Otsuji, T. Kai, T. Masumoto a n d H. Kimura, Can. J. Chem. Eng., 1987, 65, 46 S. Yoshida, H. Yamashita, T. Funabiki and T. Yonezawa, J. Chem. Soc., Chem. Commun., 1982, 964. 47 H. Yamashita, M. Yoshikawa, T. Funabiki and S. Yoshida, J. Chem. Soc., Farads), Trans. I , 1986, 82, 1771.48 H. Yamashita, T. Kaminade, T. Funabiki a n d S. Yoshida, J. Muter. Sci. Lett., 1985, 4, 1241. 49 H. Yamashita, M. Yoshikawa, T. Funabiki a n d S. Yoshida, J. Chem. Soc., Faraday Trans. I , 1987, 83, 2883. 50 A. Baiker, H. Baris a n d H. J. Guntherodt, Appl. Catal., 1986, 22, 389. 51 H. Yamashita, M. Yoshikawa, T. Kaminade, T. Funabiki and S. Yoshida, J. Chem. Soc., Faradajl Trans. I , 1986, 82, 707. 52 S. S. Mahmoud, D. A. Forsyth, a n d B. C. Giessen, Marer. Rex Soc. Sjimp. Proc. (1986), vol. 58, pp. 53 A. Baiker, R. Schlogl, E. Armbruster a n d H. J. Guntherodt, J. Catal., 1987, 107, 221. 54 A. Armbruster, A. Baiker, H. J . Guntherodt, R. Schlogl a n d B. Walz, in Stud. Sur-f Sci. Catal. (Prep. Catal. I V ) , ed. B. Delmon, P. Grange P. A. Jacobs a n d G .Poncelet (Elsevier, Amsterdam, 1987), vol. 31, pp. 389-400. 40, 173. I , 1987, 83, 2895. 274. 131-138. 55 R. Lamprecht, PI?. D. Thesis (University of Basel, 1987). 56 A. Baiker, D. Gasser a n d J. Lenzner, J. Chem. Soc,., Chem. Commun., 1987, 1750. 57 H . Yamashita, T. Kaminade, M. Yoshikawa, T. Funabiki a n d S. Yoshida, C , Mol. Chem., 1986, I , 491. 58 G. Kisfaludi, K. Matusek, Z. Schay and L. Guczi, 6th Int. Svmp. Heterogeneous Catal. (Varna, Hungary, 1987). 59 A. Baiker, H. Baris, F. Vanini a n d M. Erbudak, in Proc. 9th int. Congr. Catal. (Cam/-vsis: Theory and Practice), ed. M. J. Phillips and M. Ternan (Chem. Inst. Canada, 19881, vol. 4, pp. 1928-1935. 60 A. Shamsi and W. E. Wallace, Ind. Eng. Chem. Prod. Res. Dev., 1983, 22, 582. 61 W. Kowbel a n d W. E.Brower, J . Catal., 1986. 101, 262. 62 U. Koster, Z. Metallkde., 1984, 75, 691. 63 A. J. Mealand, L. E. Tanner a n d G . G. Libowitz, J . Less-Common Met., 1980, 74, 279. 64 R. M. Nix, T. Rayment, R. M. Lambert, J . R. Jennings and G . Owen, J. Caral., 1987, 106, 216. 65 D. L. Cocke, M. S. Owens, and R. B. Wright Appl. Surf Sci., 1988, 31, 341. 66 A. R. Miedema, Z. Metall., 1978, 69, 455. 67 F. Vanini, S. Buchler, Xin-nan Yu, M. Erbudak, L. Schlapbach and A. Baiker, Surf Sci., 1987, 189/190, 11 17. 68 L. Schlapbach, Nato AS1 Ser. B, 1986, 136, 397. 69 F. Spit, K. Blok, E. Hendriks, G . Winkels, W. Turkenburg, J . W. Drijver a n d S. Radelaar, in Proc. 4th Int. Cor~f.. Rapidlj, Quenched Met., ed. T. Masumoto a n d K. Suzuki ( J p n Inst. Met., Sendai, 19821, pp. 1635-1640. 70 S. Shina, S. Badrinarayanan and A. P. Shina, J. Less-Common Metals, 1986, 125, 1179. 71 L. Schlapbach, A. Seiler, F. Stucki and H. C. Siegmann, J . Less-Common Metals, 1980, 73, 145. 72 S. M. Fries, H. G . Wagner, S. J. Campell, U. Gonser, N. Blaes a n d P. Steiner, J. Phys. F, 1985, 15, 1179. 73 P. Oelhafen, R. Lapka, U. Gubler, J. Krieg, A. DasGupta, H . J. Guntherodt, T. Mizoguchi, C. Hague, J. Kubler a n d S. R. Nagel, in Pro(*. 4th Int. Con/: Rapid!,. Quenched Met., ed. T. Masumoto and K. Suzuki ( J p n Inst. Met., Sendai, 19821, pp. 1259-1265. 74 Y. Shimogaki, H. Komiyama, H. Inoue. T. Masumoto a n d H . Kimura, J. C’hem. Eng. Jpn, 1988,21, 293. 75 A. Baiker, D. Gasser, A. Reller a n d R. Schlogl, in preparation. 76 T. Engel a n d G . Ertl, Adv. Catal., 1979, 28, 1; S. Ladas, H. Poppa a n d M. Boudart, Sut$ Sci., 1981, 77 M. Hara, K. Hashimoto a n d T. Masumoto, J. Appl. Electrochem., 1983, 13, 295. 78 N. Kumagai, A. Kawashima, K. Asami a n d K. Hashimoto, J . Appl. Electrochem., 1986, 16, 565. 79 M. Enyo, T. Yamaguchi, K. Kai and K. Suzuki, Electrochim. Acta, 1983, 28, 1573. 102, 151.A. Baiker 25 1 80 A. Yokoyama, H. Komiyama, H. Inoue, T. Masumoto and H. M. Kimura, Scr. Metall., 1981, 15, 365. 81 A. Kawashima, T. Kanda and K. Hashimoto, Muter. Sci. Eng., 1988, 99, 521. 82 G . Kreysa and B. Hakansson, in Dechema-Monographien (VCH, Weinheim, 1986), vol. 102, pp. 361-373. 83 L. Brossard, R. Schulz and J. Y. Huot, Znt. J. Hydrogen Energy, 1988, 13, 251. 84 J. van Wonterghem, S. Morup, C. 1. W. Koch, W. W. Charles and S. Wells, Nature (London), 1986, 85 J. van Wonterghem, S. Morup, S. W. Charles, S. Wells and J. Villadsen, fhys. Rev. Lett., 1985, 55, 410. 86 J-F. Deng and X-p. Zhang, Appl. Caral., 1988, 37, 339. 322, 622. Paper 8/04941 F; Received 16th December, 1988