journal of the chilean chemical society - chemical reactions at nanometal particles

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Journal of the Chilean Chemical Societyversin On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.3 Concepcin sep. 2005

doi: 10.4067/S0717-97072005000300014

J. Chil. Chem. Soc., 50, N 3 (2005), pgs: 603-612

CHEMICAL REACTIONS AT NANOMETAL PARTICLES

GALO CRDENAS-TRIVIO

Departamento de Polmeros, Facultad de Ciencias Qumicas , Universidad de Concepcin Edmundo Larenas129, Concepcin, Chile [email protected]

SUMMARY

The concept of nanochemistry and the reactions involved are discussed. The work is focused onnanoparticles obtained from colloidal dispersions. The colloidal particles by transmission electronmicroscopy of low and high resolution were analyzed. The high resolution electron miscroscopy (HRTEM)allow us to classify the nanostructure of the metal particles in some polyhedral models: cubooctahedron,truncated octahedron, tetracai decahedron and icosahedron. Some HRTEM of Pd-2 propanol and Ge-2-propanol are analyzed. The Pd atoms exhibit a high crystalline nanoparticles, cubooctahedral arrangement

in the [001] orientation. On the other hand , the Ge atoms are ordered in a pseudohexagonalarrangement. The spacing in the last system are typical of a diamond cubic Ge structure.

The structure of nanometals and also some relevant properties of nanoclusters are discussed.

Some heterogeneous catalysis applications of supported metal clusters are discussed. Selectivehydrogenation of a, b-unsaturated aldehydes are based on Pd, Rh, Ru, Os, Ir, Ni, Co and Pd.. Inparticular the crotonaldehyde hydrogenation in the presence of Pd/SiO2, Pd/Al2O3, PdSn/SiO2 and

PdSn/Al2O3 was measured. The influence of the organic fragments incorporated on the surface of the

particles in the systems mentioned above was studied.

Finally, some remarks of the advantage of this technique in order to obtain nanoclusters are summarized.

INTRODUCTION

The nanochemistry is a new emerging area that links two worlds, one related with molecular bonds andthe chemical engineering of nano or micro sized structures like chemical vapor deposition, lithography orcoating technique.

According to Ozin point of view nanochemistry would be the science of controlled production of materialson the nanometer scale with chemical reactions and explicit principles for the nature of nanosized
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materials(1).

Most of the arguments in nanochemistry are related to secondary valences, geometrical shape andinterphases energy and particles structures. Whitesides establish that this is more a physicochemicalmanage than complexes synthesis (2). We have reported nanoparticle formation in nonaqueous media ofseveral monometallic systems with transition metals (3,4,5,6,7) and recently with lanthanides (8). Thesesystems were prepared by chemical liquid deposition (9,10). The stability of the dispersion depends onthe solvatation effects and the metal used.

CLASIFICATION OF CHEMICAL REACTIONS

The nanoscale particles (atoms, clusters) are strongly related with many disciplines (chemistry, physics,electronics). To achieve this methodology several techniques can be used such as resistive heating,electron beams, arcs, lasers.

A free atoms extremely react ive because it carries a high kinetic energy and orbitals are ready forreactions without steric restrictions. As a consequence, a high temperature particle will usually react atvery low temperature with a substrate of interest. Therefore, temperatures low enough to moderatereaction rates are often desired, and these temperatures are usually in the 50 to 200 C range. The lowtemperature also serve to hold down the vapor pressures for incoming reactants, which is necessary,since almost all reactants of interest must both be allowed to contact the hot source generating thehigh-temperature species.

Usually only species of one, two or three atoms are included, and also can be extended to metal halides,metal oxides or metal sulfides. Practically, these limitations on c overage are not so relevant since most ofthe species that have been studied feasible fall into these categories.

Klabunde (11) has summarized and clasiffied several reactions or process in which atoms are involved.

1. Abstraction processes

Part of the molecule is remove or replace by the reactive specie such as

Ag atoms + CH3 CH2 Br Ag Br + CH2 CH3

2. Electron transfer processes

A transfer of an electron from the reactive specie to the substrate, occurs

Na + + TCNRNa+ TCNR-TCNR = Tetracyano quinone

3. Oxidative addition processes

The oxidative addition or insertion of a reactive specie in a s - bond on the substrate occurs

Pd atoms + C6 F5 Br C6 F5 Pd Br

4.- Simple orbital processes

Either p or s - complexes are formed by mixing the p or nonbonding electrons with the available orbitalsof the metal atom or other species.

Fe atoms + 2C6 H6 ( C6 H6)2 Fe

5.- Substitution processes

Species carried out high temperature substitute a fragment of the substrate.

4 Na + CCl4 CNa4 + 4 Na Cl

6.- Disproportion and ligand transfer

Some groups are attached to the substrate or to an intermediate product. Can be transferred to reactivespecies

2Ni + 2 C6 F5 Br 2C6 F5 Ni Br + Ni Br2

7.- Clusters formation processes

The reactive metal begins to agglomerate generating metal bonds, producing small clusters.


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The nanomaterials exhibit several interesting properties. For example, Canham has suggested thatluminescence in porous Si is caused by quantum confinement (12) and these has been a great deal ofinterest in both porous Li and Si nanocrystals (13,14). There is a general agreement on the mechanism ofluminescence, toward a quantum confinement model for Si nanocrystals (14). Ge has semiconductingproperties similar to those of Si, Ge nanocrystals are also expected to exhibit quantum confinement (15).The quantum size effect shifts the band gap of many semiconductors into the visible range and allowsthe possibility of nanocluster applicat ions in light-emitting diodes.

Another method of increasing the quantum field of emission has been reported (16). It involves theaddition of an impurity to a quantum dot, producing a doped nanocrystalline material. Wang (17) reporteda cluster of mixed semiconductor Znx Mn1-x S being a magnetic semiconductor quantum dot. Particle size

from 3.5 to 7.5 nm were obtained and have shown photoluminescence quantum yields around 18% (18).

Cohen has synthesized semiconductor nanoclusters with controlled size and narrow size distribution usingblock copolymer films prepared by ring-opening metathesis polymerization (19). The presence of apendant group and an electron-transport group can provide, with an electrical access, to nanoclustersfor device applications (20)

Photoluminescence emission spectrum for a polymeric films containing Mn-doped Zn S nanoclusters hasbeen reported (21).

The intensity of emission at 568 nm (Mn emission) was obtained as a function of the excitationwavelength . When the incident radiation decreases below 330 nm, ZnS nanoclusters begin to absorblight, and an increase in the intensity is observed. The excitat ion of ZnS around 330 nm results in theemission at 586 nm. This proves an energy transfer from ZnS to Mn, which indicates that Mn is doped inthe ZnS cluster.

On the other hand , CdS and ZnCd sulfide are useful as photoconductors of visible and infrared radiation(22). These semiconductors are important to make more available photochemical and photovoltaic cells.The most promising photocell designs use their films of photosensitizes interfaced with CdS in which thecrystal latt ice promoters of the two films are molded (23-25).

COLLOIDAL METAL PARTICLES

These bimetallic colloids such Au-Cu (10) system were prepared by chemical liquid deposition (11,12).Figure 1 shows a bimetal atom reactor. The stability of the dispersion depends on the solvation effects,dielectric constant of the solvents, viscosity and the metal used.

Fig.1 Bimetal atom reactor. Simultaneous matalevaporation.

(Reproduced with permission of Elsevier from Colloid& Polym. Sci.)

In our studies of bimetallic systems where the stability of the dispersion is strongly related to thepresence of several electronic properties produce an electronically unstable system, which has beenreported by Henglein (27) for the Pd/Ag system. The instability induces the transfer of atoms from the Pdcluster and their adsorption on the Ag cluster. This overall process is slow and can take place severaldays.

It is observed that colloidal particles grows by the agglomeration of several individual particles and aftera few days flocculation occurs depending on the solvent used.

The monometallic dispersion obtained by the cocondensation method is stabilized mainly by solvationeffect. The presence of oxygen in the polar solvents molecules (acetone or 2-propanol) produce stablecolloids but stability is also induced by the metal involved. Acetone and 2-propanol are excellent solventsfor noble metals such as Ni, Cd, Zn, Pr, Yb and Er (3,4,5,6,7,8).
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We have found that 2-methoxyethanol is the most versatile solvent, it is able to stabilize noble metalsand other very active metals, such as lanthanides (8).

In bimetallic colloids the stability is more ambiguous, due to the presence of several phase that exist inthe system, this produces a thermodynamically system more stable than the homogeneous system.

It is known that the dispersions are thermodynamically unstable and that the equilibrium conditions canbe easily modified, inducing flocculation of dispersed particles. In fact, the dispersion is stable if theparticles have similar physical and chemical properties. A wider size distribution produces less stablesystem, the electric reduction method is known to produce this kind of particles (28). The smallestparticles are strong reducing agents, and are easily oxidized unless they can find another particles to

agglomerate and grows (29,30,31).

Henglein has proposed that the difference in size of the particles produces an "electronically unstable"system, which is observed in the difference on the surface tension and is explained by the difference inthe Fermi levels. This observation has not been probed completely for a nanometallic system, but hasbeen found in Pd/Ag colloids.

The stability of Ni, Cu and Ni-Cu dispersions in organic solvents is summarized in Table I.

Table I Stability of Ni, Cu and Ni-Cu dispersions inorganic solvents. (Reproduced with permission ofElsevier from Colloid & Polym. Sci.)

Micrographs of colloids particles just prepared in 2-methoxyethanol, 2-propanol and acetone show thepresence of a dispersed cluster over the grid. The zones of the low contrast show a continuous surfaceand it is difficult to identify the presence of single particles owing to the resolution of the microscope.

In fig. 2a and b, a bright and dark field are shown. Figure 2a shows zone of low contrast forming a cloudaround the particles and the cluster (see the arrow) several spherical particles with an average size of50 nm. This cloud does not allow to see us particles which can be observed in the dark field ( Fig. 2b)

Fig. 2 Ni-Cu-2-Methoxyethanol colloid electronmicrograph at 20K; 78.4% Ni and 21.6% Cu (a)Bright field (b) Dark field.

The exist bimetallic Ni-Cu particles with low contrast but well defined outline, size and other statistical

parameters. Also particles of pure Ni and Cu in solvents such as 2-methoxy ethanol, 2-propanol andacetone are similar. Figure 3 a shows the electron micrograph.
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Fig. 3 a) Ni-Cu-2-methoxyethanol 50.45%Ni and49.6%Cu; 150K Magnification, b) Ni-Cu-2-methoxyethanol 74.4%Ni and 25.6%Cu; 150KMagnification.

Most of them exhibit a bimodal structure . A positive asymmetry is represented by a third unidimensional

positive (a3 > 0) (32). This positive asymmetry is a consequence of a clustering tendency. However,those histograms with a negative asymmetry over 80% show a symmetric distribution.

The size distribution obtained for Ni particles shows a positive asymmetry and the average size of theparticles is 50,54 nm, but since it shows a positive asymmetry most of the particles are close to themedian 49,4 and 47,9 nm, respectively. Besides, Cu particles show a more symmetric distribution with apositive path and average size of 7.3 nm, much smaller than Ni systems. This asymmetry is aconsequence that particles are growing to produce clusters. The formation of bimetallic particles involvesthis kind of materials. Alloys like colloids consist of a homogeneous mixture of two metals with a colloidaldistribution and colloids with an inner nuclei of one metal, which is covered by a layer of the secondmetal (28). The reduction method is simple when both metals are easy to reduce. Another alternative forthe formation of monometallic and bimetallic colloids is the use of co-condensation technique, which arevery useful in the synthesis of metal colloids in organic solvents method (7,8,9,33).

The stabilities of the Pd/Ag bimetallic particles dispersed in acetone, 2-propanol and 2-methoxyethanolwere lower than those for the single metal clusters in the same solvents.

The electron microscopy studies revealed that the bimetal particles size is close to that of Pd, besidesthe tendency of Ag particles to agglomerate can be decreased by the presence of Pd (4,34). The

stability of bimetal Pd/Ag was acetone> 2-methoxyethanol ~2 propanol. The order of stability has beenobserved in other colloidal dispersions of Pd, Au and Ag (9,35,3).

The electron micrographs of the bimetallic particles showed areas with high contrast , when the majorityof the particles were bigger, most probably to the clustering of small particles on the grid. The TEMimages show zones covered with particles of poor contrast, indicating planar particles, like raft likeparticles (36).These particles can be observed in Ag nanoparticles with a size around 12 nm (fig. 4).

Figure 4. Transmission electron micrograph of Ag-

2-propanol at x 300 K magnification.

Several particles size studies were carried out on a PdAg-acetone dispersion (Pd 57.6% and Ag 42.4%).At the beginning the particles are well dispersed in the grid, 5.2 nm size (fig. 3 a), but on the second daythe agglomeration produced 4.7nm particles (fig. 3b) and after 10 days the clustering increases a particlesize 3.9 nm (fig. 3c) the free particles.
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Figure 5 a/b/c. Colloid growing particles oncopper grid.(Reproduced with permission of Elsevier,Colloid & Polymer Sci.).

In fact, particles growth is due to the agglomeration of several particles at the same time to form biggerclusters.

The diffraction patterns of Pd/Ag in 2-methoxyethanol are shown in fig. 6a and b. The bimetallic particlesbehave as a substitutional solid solution face centered cubic crystals (37). Both metals crystallizesimilarly and atomic radii are similar the difference is less than 15% (rPd= 1.38 A, rAg = 1.44 A), which isa condition for this kind of alloy (38).

Fig. 6a Pd/Ag-2-methoxyethanol electrondiffraction pattern. (a) c omposition: 43.4%Pd and56.4%Ag.

Fig. 6b Pd/Ag-2-methoxyethanol electrondiffraction pattern. (b) composition: 76.6%Pd and27.4% Ag.


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Similar results were obtained with Pd/Ag in 2-propanol, these are bimetallic particles with substitutionsolid solution properties and more than one metallic phase was observed.

STRUCTURE OF NANOMETALS

In order to elucidate cluster structure several techniques are frequently used such as single crystal X-ray, neutron diffraction, multinuclear high resolution nuclear magnetic resonance (NMR) and infraredspectroscopy .

One of the most relevant characteristics is the high symmetry of metal clusters. Metal atoms arearranged in the c luster forming regular polyhedra such as t riangles, tetrahedron, octahedron, etc.

Besides, polyhedra is defined by the metals position in metal clusters are mainly deltahedra, a polyhedrawith all triangular phases. However, another arrangements besides polyhedra with triangular phases aredeltahedric structure such as : square planar, trigonal prismatic or C4vcapped square antiprism

clusters.

Fig.7. Some types of geometries frequently foundin molecular cluster structure and their relationshipto the close-packed arrays found in bulk metals(arachnom nido and tetracapped denominationsrefer to fundamental octahedral geometry).

(Reproduced with permission of Springer-Verlag,Cluster Chemistry, G. Gonzalez, pp 62, 1993)

In a great number of metal atom arrangements Mn in clusters could be considered as representingfragments of any close-packed array of metal atoms, hexagonal close packing or face centered cubicpacked. Then, relatively common cluster geometries such as triangle, tetrahedron, trigonal, square-basepyramid, bipyramid or octahedron, trigonal, square base pyramid, bipyramid or octahedron are fragments

of hexagonal close packing. The following scheme shows some geometries found in molecular clusterstructures and their relationship to close packed was found in bulk metal.

RELEVANT PROPERTIES OF NANOCLUSTERS

More recently, we have reported the synthesis by c hemical liquid deposition of Ge colloidal dispersionusing solvent like 2-propanol, acetone and THF. Strong absorption bands in the UV suggest thatnanoparticles obtained by this procedure exhibit quantum confinement Ge-2-propanol colloid of 3 and 30nm depending on concentration were obtained. (Fig.8aand b).Studies using high resolution transmission(SAED) demonstrate the high crystallinity of the nanoparticles, and it was possible to observe the typicallattice space of a diamond cubic Ge structure.


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Figure 8a. TEM micrograph of Ge-2-propanol

110-3 M colloid. Mean = 3.0 nm 0.7 nm

Figure 8b. TEM micrograph of Ge-2-propanol

110-2 M colloid. Mean = 30 nm 8.1 nm

Quantum size effects have been experimentally observed on several nanocrystalline semiconductors(39,40,41-43). The optical absorption spect rum of a nanocrystalline semiconductor provides an availablemethod for the evaluation of quantum size effects. This quantum size effect is observed as a shifttoward higher energy values for the band edge (a blue shift), as compared to the typical band for thecorresponding macrocrystalline material. The optical properties are highly size dependent, e.g., smallernanoparticles absorb and emit high at higher energies than larger nanoparticles. This kind of effect hasbeen demonstrated for CdS nanoparticles (44). The Ge nanoparticles absorb at 204 nm, this peak slightlyshift to higher energy with time. The reason is that bigger particles flocculate narrowing the sizedistribution. The HRTEM studies results reveal the high cristallinity of the Ge nanoparticles (45).

Figure 9 shows the Ge-2-propanol micrograph in which the atoms are ordered in pseudohexagonalarrangement. The lattice spacing obtained from the picture are d=2.02 A and d=1.75A. These spac ingattributed to [220] and [311] lattice fringes, are typical of a diamond cubic Ge structure. The SAD was

performed in a Jeol 4000 EX operated at 400 kV. The Ge nanoparticles (10 -3 M colloid ) exhibit latticespaces dhkl of a diamond cubic structure. This FTIR shows data in which acetone undergoes attached to

the carbonyl oxygen with loss of an hydrogen on the Ge [100] surface to form an enolic structure. The

loss of the C=O band at 1710 cm -1 and the formation of a new band at 1640 cm-1 corresponding to aC=C bond was observed (46). In our case, there is no loss of the carbonyl band, but there is formation ofGe-C and Ge=C bonds. Further studies needs to be done to make sure about the anchoring fragments in


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the Ge surface.

Figure 9. HRTEM micrograph of a Ge-2-propanol

10-3M nanoparticle.

Teranishi et al synthesized Pd nanoparticles by alcohol reduction of palladium salts in the presence ofPVP as stabilizer (47). Chen et al synthesized alkenethiolate protected Pd nanoparticles by NaBH4

reduction of Pd Cl2 in the presence of thiols (48). Henglein was able to obtain aqueous colloidal palladiumparticles with a narrow size distribution (49). Yonezawa et al. reported the reduction of Pd Cl4

2- by

hydrazine in the presence of cationic alkylisocyanides obtaining a stable aqueous dispersion of cationicpalladium nanoparticles (50).

Our approach has been focused in the characterization of Pd nanoparticles synthesized by CLD. The firstreport on Pd colloids gave particle size of 8 nm very stable in acetone. Now we became interested in thenanostructure of this particles.

We have already reported theobtention of Ag (4), Cu (51), Cd (7), Zn (5), Ni (52), Sn (53), Ga(55),Bi(57) Yb, Er and Pr(56) monometallic dispersions and Ag-Pd (57), Pd-Sn (58), Pd-Sn (59), bimetallicdispersions.

One of the advantages of the technique is that no by products of metal salt reduction are present andpure metal colloids are formed.

In most of the methods reported the particles were formed by the reduction of metal ions in thepresence of stabilizers or heterogeneous supports such as polymers or electrode surface. The differentchemical and physical properties of colloidal metals as a funct ion of their size, partic le size distributionand structurerequires a measurement if these properties should be understood. Small metal particles areknown to present well-defined structural types with cuboctahedral (CO), truncated octahedral (TO),tetrakaidecahedral (TKD), icosahedra, or decahedral morphologies (60-62). Since particles size andstructure are the variables observed as synthesis conditions are changed. It is quite unpredictable thestructure and morphology of small particles, for that reason the use of HRTEM is a powerfull technique todetermine such parameters.

Figure 10, shows polyhedral models of particles that will be used to discuss the particle shapes

Figure 10. Top and side view of Polyhedral models

of nanoparticles.27 (a) Cuboctahedron, CO; (b)Truncated octahedron, TO; (c) Tetracaidecahedron, TKD; and (d) Icosahedron.

(Reproduced with permission of ACS fromLangmuir).

The coagulation of colloid particles can occur for different reasons , being the most important theagglomeration and the subsequent coalescence between particles is due to Brownian motion collisions


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giving place to big aggregates formation. Other reason for colloid coagulation is the chemical modificationof their surface due to the addition of some chemical charged species.

Creighton predict, like in our colloids, a continuous absorption in the visible region that increases untilformation of a band with a maximum around 200 nm (63). This absorption is characteristic of the metallicstate and correspond to superposed interband transition. This behavior indicates an associated quantumsize effect, typical of particles with nanometric size-dimensions. It is know that the nature of metal usedfor the colloid synthesis is an important factor e.g., for noble metals like Au (35) and Pd (64) it is possible

to obtain highly stable colloids in several organic solvents. In the Pd-2-propanol system, the 10-3 M

colloid has an average particles size of 2.2 nm while for the concentrated 10 -2 colloid there is a slightincrease in the particle size to 2.3 nm. Since we are using a 120 kV instrument, only one distribution will

indicate a greater polydispersity in the last case. For instance, a 1.08x10-2 M Pd-2-propanol colloid itwas reported a particle size of 6 nm in low resolution but using HRTEM the average particle size is 2.3nm. In the Pd-acetone colloids the particle size was 8 nm while in higher resolution is 2.7 nm. To improvethe accuracy of the obtained HRTEM images, they were FFT filtered, with low pass radial and latticefilters, and reconstructed (with Crip 1.5f software package, Calidris) to substract the backgroundgenerated by the grid support, and the noise produced during the image acquiring process.

The HRTEM show the high crystallinity of the synthesized nanoparticles since it is possible to reachatomic scale resolution in the structure.

The pronounced truncation of a cube will results in hexagonal faces, producing two kinds of octahedra.In one of them the truncation generates a hexagonal face with different sizes in each edge and willresults in a truncated octahedron (TO) and includes only one or two atoms in each edge of thehexagonal face (fig. 10b). In the other structure with the same number of atoms in each edge of the

hexagonal face (see fig. 10c), and it is correct to refer to it as tetrakai decahedron (TKD). These twostructure are of the same class, moreover the TK shape is more spherical and then it should be morestable. The icosahedrons shape is formed by 20 slightly distorted tetrahedral units (see fig. 10d). Figure11 show two truncated octahedron particles oriented in the [011] zone axis, both particles correspond toPd-2-propanol colloids. The left particles exhibit a 1.6 nm size while the right particle has 3.8 nm size.

Figure 11. HRTEM images of Cuboctahedral (CO)particles in the [011] orientation. The left and rightparticles correspond to Pd-2-propanol. The COshape will be the results of the truncation of acube, producing triangular faces.

METAL CLUSTERS SUPPORTED

The metal catalysts used in the selective hydrogenation ofa, _-unsaturated aldehydes are based on Pd,

Rh, Ru, Os, Ir, Ni, Co and Pd. From all these metals, the latter is not a good catalyst for selective

hydrogenation of the conjugated carbonyl group (65). The selectivity can be improved with the additionof promoters like Sn (66-68) and Ge (69-71), which can increase the formation of unsaturated alcohol.The formations of an alloy with Sn might also help toward select ivity but there is no available literature.Therefore, we considered of interest to prepare a series of PdSn alloy catalysts to determine if theywould cause selectivity change(72).

The results obtained in the crotonaldehyde hydrogenation in the presence of Pd/SiO2, Pd/Al2O3, Pd

Sn/SiO2 and Pd Sn/Al2O3 prepared by the technique "solvated metal atom dispersed" (SMAD) (72). The

technique allows to evaporate simultaneously Pd and solvents (ketones, alcohols or ethers) at 77K. Thecolloids are produced "in situ" and reacted with activated SiO2 or Al2O3 previously introduced in the

reactor bottom . Klabunde (73) have prepared the system Pt-Sn in SiO2 and Al2O3 by SMAD method and

they also found that most of the Sn (84%) was Sn, but Sn2+ and Sn4+ were also present. We study theinfluence of the organic fragment incorporated on the surface of the particles, in the systems Pd/SiO2,

Pd/Al2O3, Pd Sn/SiO2 and Pd Sn/Al2O3 obtained from acetone, 2-propanol and THF. In the followingtable, the sizes obtained from the TEM are summarized. In general, the size of the dispersed particles onAl2O3 are bigger than those supported over SiO2.

Table II. Particle size of Pd Sn supported over SiO2
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and Al2O3

s = standard derivation

It is known that Al2O3 is a support characterized by the strong metal-support interaction modifying the

electric properties of metallic particles (73,74) while SiO2 is a more inert support (75). The bimetal

support either from acetone or 2-propanol showed a narrow particle size distribution, however theparticles dispersed on the supports by THF dispersions present a wider size distribution. Other propertiesto investigate is the crystallinity of this bimetallic particles. The combination between the electrondiffraction and dark field carried out in the reflection [1012] of Pd Sn hexagonal (dhkl = 3.21 A) and

[200] of Pd (dhkl= 1.95 A)

In Figure 12,it can be observed that particles of Pd Sn are hexagonal and particles are rich in Pd. Thepattern of diffraction reveal the presence of SnO (d hkl = 1.60 and 1.15 A). It is necessary to point out

that oxide formation is very low in these systems and they are very stable to oxidation. The catalysts

were tested for crotonaldehyde hydrogenation over Pd/SiO2, Pd/Al2O3 and the bimetallic systems of PdSn are butyraldehyde (main product, 73-89%) crotyl alcohol (12-23%) and a lower percentage ofbutanol. There are some important facts to be considered:

a) The SMAD Pd catalysts show a relatively high selectivity to crotyl alcohol compared with conventionalcatalysts (76-78).

b) The addition of Sn to the SMAD catalyst gave only a slight increase in their selectivity.

c) The SMAD catalysts give less saturated alcohol compared to the conventional catalysts.

Figure 12. Dark field: (a) reflection {1012}, PdSn,hexagonal: (b) reflection {200}, Pd.

FINAL REMARKS

This methodology chemical liquid deposition (CLD) in an alternative to prepare nanoclusters and / or

nanoparticles using main group, transition metals and lanthanides elements. Metal colloids, active solidsand metal and bimetal clusters supported for catalyst can also be obtained.

The main characteristic of this approach is the capacity to get zerovalent metals very active either innon aqueous dispersions or such active solids. The reproducibility is very good if several parameters arecontrolled: vacuum, rate of evaporation, rate of cocondensation and warm up time.

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