Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.3 Concepción sep. 2005
J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 603-612
CHEMICAL REACTIONS AT NANOMETAL PARTICLES
Departamento de Polímeros, Facultad de Ciencias Químicas , Universidad de Concepción Edmundo Larenas 129, Concepción, Chile email@example.com
The concept of nanochemistry and the reactions involved are discussed. The work is focused on nanoparticles obtained from colloidal dispersions. The colloidal particles by transmission electron microscopy 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  orientation. On the other hand , the Ge atoms are ordered in a pseudohexagonal arrangement. 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. Selective hydrogenation of a, b-unsaturated aldehydes are based on Pd, Rh, Ru, Os, Ir, Ni, Co and Pd.. In particular 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.
The nanochemistry is a new emerging area that links two worlds, one related with molecular bonds and the chemical engineering of nano or micro sized structures like chemical vapor deposition, lithography or coating technique.
According to Ozin point of view nanochemistry would be the science of controlled production of materials on the nanometer scale with chemical reactions and explicit principles for the nature of nanosized materials(1).
Most of the arguments in nanochemistry are related to secondary valences, geometrical shape and interphases energy and particles structures. Whitesides establish that this is more a physicochemical manage than complexes synthesis (2). We have reported nanoparticle formation in nonaqueous media of several monometallic systems with transition metals (3,4,5,6,7) and recently with lanthanides (8). These systems were prepared by chemical liquid deposition (9,10). The stability of the dispersion depends on the 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 reactive because it carries a high kinetic energy and orbitals are ready for reactions without steric restrictions. As a consequence, a high temperature particle will usually react at very low temperature with a substrate of interest. Therefore, temperatures low enough to moderate reaction rates are often desired, and these temperatures are usually in the 50 to 200° C range. The low temperature 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 the high-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 coverage are not so relevant since most of the 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 ¾® Ag Br + CH2 CH3
2. Electron transfer processes
A transfer of an electron from the reactive specie to the substrate, occurs
Na + + TCNR ¾®Na+ 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 ¾® 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 orbitals of the metal atom or other species.
Fe atoms + 2C6 H6 ¾® ( C6 H6)2 Fe
5.- Substitution processes
Species carried out high temperature substitute a fragment of the substrate.
4 Na + CCl4 ¾® 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 reactive species
2Ni + 2 C6 F5 Br ¾® 2C6 F5 Ni Br + Ni Br2
7.- Clusters formation processes
The reactive metal begins to agglomerate generating metal bonds, producing small clusters.
The nanomaterials exhibit several interesting properties. For example, Canham has suggested that luminescence in porous Si is caused by quantum confinement (12) and these has been a great deal of interest in both porous Li and Si nanocrystals (13,14). There is a general agreement on the mechanism of luminescence, toward a quantum confinement model for Si nanocrystals (14). Ge has semiconducting properties 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 allows the possibility of nanocluster applications in light-emitting diodes.
Another method of increasing the quantum field of emission has been reported (16). It involves the addition of an impurity to a quantum dot, producing a doped nanocrystalline material. Wang (17) reported a 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 using block copolymer films prepared by ring-opening metathesis polymerization (19). The presence of a pendant group and an electron-transport group can provide, with an electrical access, to nanoclusters for device applications (20)
Photoluminescence emission spectrum for a polymeric films containing Mn-doped Zn S nanoclusters has been reported (21).
The intensity of emission at 568 nm (Mn emission) was obtained as a function of the excitation wavelength . When the incident radiation decreases below 330 nm, ZnS nanoclusters begin to absorb light, and an increase in the intensity is observed. The excitation of ZnS around 330 nm results in the emission at 586 nm. This proves an energy transfer from ZnS to Mn, which indicates that Mn is doped in the 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 the crystal lattice 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.
In our studies of bimetallic systems where the stability of the dispersion is strongly related to the presence of several electronic properties produce an electronically unstable system, which has been reported by Henglein (27) for the Pd/Ag system. The instability induces the transfer of atoms from the Pd cluster and their adsorption on the Ag cluster. This overall process is slow and can take place several days.
It is observed that colloidal particles grows by the agglomeration of several individual particles and after a few days flocculation occurs depending on the solvent used.
The monometallic dispersion obtained by the cocondensation method is stabilized mainly by solvation effect. The presence of oxygen in the polar solvents molecules (acetone or 2-propanol) produce stable colloids but stability is also induced by the metal involved. Acetone and 2-propanol are excellent solvents for noble metals such as Ni, Cd, Zn, Pr, Yb and Er (3,4,5,6,7,8).We have found that 2-methoxyethanol is the most versatile solvent, it is able to stabilize noble metals and 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 in the 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 can be easily modified, inducing flocculation of dispersed particles. In fact, the dispersion is stable if the particles have similar physical and chemical properties. A wider size distribution produces less stable system, the electric reduction method is known to produce this kind of particles (28). The smallest particles 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 in the Fermi levels. This observation has not been probed completely for a nanometallic system, but has been found in Pd/Ag colloids.
The stability of Ni, Cu and Ni-Cu dispersions in organic solvents is summarized in Table I.
Micrographs of colloids particles just prepared in 2-methoxyethanol, 2-propanol and acetone show the presence of a dispersed cluster over the grid. The zones of the low contrast show a continuous surface and 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 cloud around the particles and the cluster (see the arrow) several spherical particles with an average size of 50 nm. This cloud does not allow to see us particles which can be observed in the dark field (Fig. 2b)
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 and acetone are similar. Figure 3 a shows the electron micrograph.
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 the particles is 50,54 nm, but since it shows a positive asymmetry most of the particles are close to the median 49,4 and 47,9 nm, respectively. Besides, Cu particles show a more symmetric distribution with a positive path and average size of 7.3 nm, much smaller than Ni systems. This asymmetry is a consequence that particles are growing to produce clusters. The formation of bimetallic particles involves this kind of materials. Alloys like colloids consist of a homogeneous mixture of two metals with a colloidal distribution and colloids with an inner nuclei of one metal, which is covered by a layer of the second metal (28). The reduction method is simple when both metals are easy to reduce. Another alternative for the formation of monometallic and bimetallic colloids is the use of co-condensation technique, which are very 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-methoxyethanol were 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, besides the 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 been observed 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 majority of the particles were bigger, most probably to the clustering of small particles on the grid. The TEM images show zones covered with particles of poor contrast, indicating planar particles, like raftlike particles (36).These particles can be observed in Ag nanoparticles with a size around 12 nm (fig. 4).
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 day the agglomeration produced 4.7nm particles (fig. 3b) and after 10 days the clustering increases a particle size 3.9 nm (fig. 3c) the free particles.
In fact, particles growth is due to the agglomeration of several particles at the same time to form bigger clusters.
The diffraction patterns of Pd/Ag in 2-methoxyethanol are shown in fig. 6a and b. The bimetallic particles behave as a substitutional solid solution face centered cubic crystals (37). Both metals crystallize similarly and atomic radii are similar the difference is less than 15% (rPd= 1.38 A, rAg = 1.44 A), which is a condition for this kind of alloy (38).
Similar results were obtained with Pd/Ag in 2-propanol, these are bimetallic particles with substitution solid 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 infrared spectroscopy .
One of the most relevant characteristics is the high symmetry of metal clusters. Metal atoms are arranged in the cluster forming regular polyhedra such as triangles, tetrahedron, octahedron, etc. Besides, polyhedra is defined by the metals position in metal clusters are mainly deltahedra, a polyhedra with all triangular phases. However, another arrangements besides polyhedra with triangular phases are deltahedric structure such as : square planar, trigonal prismatic or C4v ¾ capped square ¾ antiprism clusters.
In a great number of metal atom arrangements Mn in clusters could be considered as representing fragments of any close-packed array of metal atoms, hexagonal close packing or face centered cubic packed. Then, relatively common cluster geometries such as triangle, tetrahedron, trigonal, square-base pyramid, 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 cluster structures and their relationship to close packed was found in bulk metal.
RELEVANT PROPERTIES OF NANOCLUSTERS
More recently, we have reported the synthesis by chemical liquid deposition of Ge colloidal dispersion using solvent like 2-propanol, acetone and THF. Strong absorption bands in the UV suggest that nanoparticles obtained by this procedure exhibit quantum confinement Ge-2-propanol colloid of 3 and 30 nm depending on concentration were obtained. (Fig.8a and b). Studies using high resolution transmission (SAED) demonstrate the high crystallinity of the nanoparticles, and it was possible to observe the typical lattice space of a diamond cubic Ge structure.
Quantum size effects have been experimentally observed on several nanocrystalline semiconductors (39,40,41-43). The optical absorption spectrum of a nanocrystalline semiconductor provides an available method for the evaluation of quantum size effects. This quantum size effect is observed as a shift toward higher energy values for the band edge (a blue shift), as compared to the typical band for the corresponding macrocrystalline material. The optical properties are highly size dependent, e.g., smaller nanoparticles absorb and emit high at higher energies than larger nanoparticles. This kind of effect has been demonstrated for CdS nanoparticles (44). The Ge nanoparticles absorb at 204 nm, this peak slightly shift to higher energy with time. The reason is that bigger particles flocculate narrowing the size distribution. 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 pseudohexagonal arrangement. The lattice spacing obtained from the picture are d=2.02 A° and d=1.75A°. These spacing attributed to  and  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 lattice spaces 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  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 a C=C bond was observed (46). In our case, there is no loss of the carbonyl band, but there is formation of Ge-C and Ge=C bonds. Further studies needs to be done to make sure about the anchoring fragments in the Ge surface.
Teranishi et al synthesized Pd nanoparticles by alcohol reduction of palladium salts in the presence of PVP 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 palladium particles with a narrow size distribution (49). Yonezawa et al. reported the reduction of Pd Cl42- by hydrazine in the presence of cationic alkylisocyanides obtaining a stable aqueous dispersion of cationic palladium nanoparticles (50).
Our approach has been focused in the characterization of Pd nanoparticles synthesized by CLD. The first report on Pd colloids gave particle size of 8 nm very stable in acetone. Now we became interested in the nanostructure of this particles.We have already reported the obtention 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), bimetallic dispersions.
One of the advantages of the technique is that no by products of metal salt reduction are present and pure metal colloids are formed.
In most of the methods reported the particles were formed by the reduction of metal ions in the presence of stabilizers or heterogeneous supports such as polymers or electrode surface. The different chemical and physical properties of colloidal metals as a function of their size, particle size distribution and structure requires a measurement if these properties should be understood. Small metal particles are known to present well-defined structural types with cuboctahedral (CO), truncated octahedral (TO), tetrakaidecahedral (TKD), icosahedra, or decahedral morphologies (60-62). Since particles size and structure are the variables observed as synthesis conditions are changed. It is quite unpredictable the structure and morphology of small particles, for that reason the use of HRTEM is a powerfull technique to determine such parameters.
Figure 10, shows polyhedral models of particles that will be used to discuss the particle shapes
The coagulation of colloid particles can occur for different reasons , being the most important the agglomeration and the subsequent coalescence between particles is due to Brownian motion collisions giving place to big aggregates formation. Other reason for colloid coagulation is the chemical modification of 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 until formation of a band with a maximum around 200 nm (63). This absorption is characteristic of the metallic state and correspond to superposed interband transition. This behavior indicates an associated quantum size effect, typical of particles with nanometric size-dimensions. It is know that the nature of metal used for 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 slight increase 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 it was reported a particle size of 6 nm in low resolution but using HRTEM the average particle size is 2.3 nm. In the Pd-acetone colloids the particle size was 8 nm while in higher resolution is 2.7 nm. To improve the accuracy of the obtained HRTEM images, they were FFT filtered, with low pass radial and lattice filters, and reconstructed (with Crip 1.5f software package, Calidris) to substract the background generated 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 reach atomic 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 will results in a truncated octahedron (TO) and includes only one or two atoms in each edge of the hexagonal 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 two structure are of the same class, moreover the TK shape is more spherical and then it should be more stable. The icosahedrons shape is formed by 20 slightly distorted tetrahedral units (see fig. 10d). Figure 11 show two truncated octahedron particles oriented in the  zone axis, both particles correspond to Pd-2-propanol colloids. The left particles exhibit a 1.6 nm size while the right particle has 3.8 nm size.
METAL CLUSTERS SUPPORTED
The metal catalysts used in the selective hydrogenation of a, _-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 addition of 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 selectivity but there is no available literature. Therefore, we considered of interest to prepare a series of PdSn alloy catalysts to determine if they would 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. The colloids 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 the influence 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 following table, the sizes obtained from the TEM are summarized. In general, the size of the dispersed particles on Al2O3 are bigger than those supported over SiO2.
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 the particles dispersed on the supports by THF dispersions present a wider size distribution. Other properties to investigate is the crystallinity of this bimetallic particles. The combination between the electron diffraction and dark field carried out in the reflection  of Pd Sn hexagonal (dhkl = 3.21 A°) and  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°. The pattern 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 Pd Sn are butyraldehyde (main product, 73-89%) crotyl alcohol (12-23%) and a lower percentage of butanol. There are some important facts to be considered:
a) The SMAD Pd catalysts show a relatively high selectivity to crotyl alcohol compared with conventional catalysts (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.
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 solids and metal and bimetal clusters supported for catalyst can also be obtained.
The main characteristic of this approach is the capacity to get zero valent metals very active either in non aqueous dispersions or such active solids. The reproducibility is very good if several parameters are controlled: vacuum, rate of evaporation, rate of cocondensation and warm up time.
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