Metal cluster source

Figure Cl. 1.1. Schematic of a typical laser vaporization supersonic metal cluster source using a pulsed laser and a pulsed helium carrier gas.

The thennal evaporation source was the earliest used to produce metal clusters in the gas phase [H, 12 and 13], mostly for clusters of the alkalis and other low melting point materials. In this technique, a bulk sample is simply... 

The pulsed molecular beam cluster source has produced clusters of virtually every material—we have made clusters of even the most refractory transition metals, of group IIIB and IVB elements, and numerous oxides, carbides, and intermetallic alloys of these elements. 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Previously, intense beams of metal clusters could only be produced for the most volatile metals. The limitation arose from significant materials problems involved in the construction of high temperature ovens. The development of a source that utilizes laser vaporization and subsequent condensation in a rapidly flowing gas eliminated the materials problem and has enabled just about any material to be studi ed(la,8). 

The field of gas-phase transition metal cluster chemistry has expanded rapidly due to the development of the laser vaporization source and the fast flow chemical reactor. The work from the three major laboratories have been reviewed. Many additional laboratories are developing cluster chemistry programs and will soon certainly make significant contributions. 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

The objective of most research in the area of pyrolyzed metal/N/C materials has centered around understanding the nature of the active site for the ORR. Similar to heat-treated macrocycles, there has been a parallel controversy over the nature of the active sites and the role of Fe or Co in these metal-nitrogen-carbon catalysts. Based on the activity attainable from a wide-range of precursors, it seems safe to assume that above a certain temperature, the active site formed is the same regardless of the metal-nitrogen-carbon starting material (macrocycle or otherwise). Initially, some researchers believed that the metal clusters protected by a layer of carbon (which prevented leaching of the metal in the acidic electrolyte) were the source of catalytic... 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

The reactions of some transition metal cluster ions have been described in a review by Parent and Anderson (201). The review covered reactions reported up to 1992 and so the reactions reported here are generally later than 1992. A recent review by Knickelbein (202) discusses the reactions of cation clusters of iron, cobalt, nickel, copper, silver, niobium, and tungsten with small molecules such as H2 and D2. Some of the reactions in Knickelbein s review are included in the following tables of reactions (Tables IV and V). Table IV gives examples of the reactions of transition metal cluster ions and includes the vaporization source, experimental apparatus, the reactants, and the observed product ions. A few examples from these tables will be selected for further discussion. 

Many of the metal cluster anions have been produced by cathode discharge in He doped with Ar and studied in a flow tube reactor. A few clusters have been prepared by laser vaporization using a supersonic cluster beam source and the reac-... 

Support for these proposals comes from several sources first, the disproportionation reactions of mixed metal clusters such as Rh2Co2(CO)j2 or Fe2Ru(CO)12 ... 

Photoinduced deposition of various noble metals onto semiconductor particles has been extensively reported [310-315]. Several factors are controlling this reaction. To control the morphology of metal clusters with desired size and distribution pattern on a given surface area of titania, the most relevant factors are the surfactant, pH, local concentration of cations, and the source of cation [316], In the case of the Ag clusters, the reaction steps proposed include the creation of electron (e )-hole (p+) pairs, the reaction of holes with OH surface species, and the reaction of electrons with adsorbed Ag+ ions ... 

In general, sources of metal cluster fragments, such as Ru3(CO)io(NCMe)2 or Ru3(/i-dppm)(CO)io react readily with 1,3-diynes or poly-ynes initially to give alkyne complexes, which readily undergo further reactions as a result of activation by the cluster core. 

With the development of intense synchrotron X-ray sources, it has become possible to use X-ray absorption spectroscopy to structurally characterize the metal clusters in metalloproteins. A wide variety of systems have been studied in this manner in the last ten years. The present article reviews the nature of X-ray absorption spwtroscopy and the information which can be obtain fix)m its study. The strengths and Kmitations of the technique are discussed with reference to the recent literature. 

The mechanisms of ion formation are usually studied with measurements of mass separated ion energy distributions using a magnetic sector mass spectrometer.144,145 For a liquid metal ion source, both atomic ions and cluster ions of all sizes are emitted. If the total ion current is large, neutral atoms and small droplets may also be emitted. There is little question that most of the atomic ions in a liquid metal ion source at low... 

---