Pt cluster

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

Reductions of [PtClg] " in an atmosphere of CO provide a series of clusters, [Pt3(CO)6] ( = 1-6,10) consisting of stacks of Pt3 triangles in slightly twisted columns Pt-Pt = 266 pm in triangles, 303-309 pm between triangular planes (Fig. 27.12). A feature of these and other Pt clusters is that they mostly have electron counts lower than predicted by the usual electron counting rules. In the series just mentioned for instance, = 1 and n — 2 have electron counts of 44 and 86 whereas 48 and 90 would... 

Figure A.l. Schematic presentation of a catalytic cylindrical Pt cluster interfaced with an O2 -conducting solid electrolyte (YSZ) showing the flux, N, of the promoting species.

The extended fine structure (EXAFS) was used to determine bond distances, coordination number and disorder. The near edge (XANES) was used as an Indication of electronic state. Significant results Include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support. 

Could a change in shape of the small Pt clusters produce sufficiently large changes In the density of states to cause the effect One of us (Horsley) has made preliminary multiple... 

The new results of this study Include a clear demonstration of Pt-0 bonds to the support at low temperature In either He or H2. As the temperature Is raised above 600 K the Pt-0 bonds break and the Pt raft-llke clusters curl up to be more sphere-llke. Concurrently with this bond-breaking and change In shape, electrons flow to the Pt d-band. At temperatures above 600 K there Is a d-electron surplus In the Pt clusters compared to bulk Pt, l.e., they are more noble than Pt. This may be a significant result since practical applications of Pt-contalnlng catalysts are In the temperature range where these changes occur,... 

Similar results could be gained with platinum particles of 35 and 3 nm in diameter. The difference in the relaxation rates corresponds qualitatively with that of 15 and 1.4nm gold particles. Due to the lack of smaller Pt clusters the transition to the molecular state could not be studied. 

As a rule, short nucleation times are the prerequisite for monodisperse particle formation. A recent mechanistic study showed that when Pt(acac)2 is reduced by alkylalu-minium, virtually all the Pt cluster nuclei appear at the same time and have the same size [86]. The nucleation process quickly consumes enough of the metal atoms formed initially to decrease their concentration below the critical threshold. No new metal cluster nuclei are created in the subsequent diffusion-controlled growth stage. 

Controlled decomposition of pre-formed [(COD)Pt(CH3)2] in the presence of triorganoaluminium led to the preparation of the first Pt cluster (size 0.75 + 0.1 nm). The one-shell structure and the metallic state were confirmed by XPS and XANES [352]. 

When exposed to air the organoaluminium shell is transferred to an AI2O3 matrix where the Pt clusters are regularly dispersed (Figure 15). This is a promising pathway for preparing solid catalysts (cf. Section 4.1). 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

Wang YX, Balbuena PB. 2005b. Potential energy surface profile of the oxygen reduction reaction on a Pt cluster Adsorption and decomposition of OOH and H2O2. J Chem Theory Comput 1 935-943. 

Becerra LR, Klug CA, Slichter CP, Sinfelt JH. 1993. NMR-study of diffusion of CO on alumina-supported Pt clusters. J Phys Chem 97 12014-12019. 

Figure 5.13 STM image of Pt clusters electrodeposited onto Au(l 1 1) in 0.1 M H2S04 + 0.1 mM K2PtCl4. E— +0.1 V versus SCE. (Reproduced with permission from Ref. [46].)...

FTIR spectroscopy of CO adsorbed at rt was used to monitor the state of platinum after the different calcination processes, i.e. at 500 and 300°C (Fig. 2A and B, respectively). Besides the broad band of the gaseous probe molecule, IR spectra of CO adsorbed on sample calcined at 500°C (Fig. 2A, a) showed the presence of a large band at ca. 2195 cm"1, accompanied by weak bands at ca. 2170 and 2100 cm 1, due to the presence of both platinum oxide species and Pt° clusters, whose formation is due to platinum autoreduction processes. The presence of large amounts of Pt° particles larger than 30 nm was observed by FIRTEM (data not shown). 

A different approach is the combination of a Pt-carbonyl-cluster with a special dye, Safranine O (Saf 3,7-diamino-2,8-dimethyl-5-phenylphenazinium) in an aqueous/organic two-phase system [48]. The dye is reduced in the organic phase and subsequently, in a type of phase-transfer catalysis, it reduced the cofactor in the aqueous phase. In this example l-LDH is used as a production enzyme, reducing pyruvate to L-lactate (Scheme 43.6). Complete conversion was obtained within 48 h, the mixture containing pyruvate, NAD+ and the Pt-cluster catalyst in a 600 10 1 molar ratio. The TOF for NAD+ was 15 h-1. 

---