Platinum clusters carbon monoxide

Large platinum carbonyl clusters have been investigated as models for the adsorption of carbon monoxide on platinum surfaces and on platinum electrodes. An issue is how large the clusters must be before they adopt the properties of the bulk metal. Teo et al. have investigated the magnetic properties of the clusters [Pt6(CO)12]2+, [Pt9(CO)18]2+, [Pt y(CO)22f+, and... 

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. 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Heiz, U., Sanchez, A., Abbet, S., and Schneider, W.-D., Catalytic oxidation of carbon monoxide on monodispersed platinum clusters Each atom counts. J. Am. Chem. Soc. 121, 3214 (1999). 

CF3H, Methane, trifluoro-cadmium complex, 24 55 mercury complex, 24 52 CF3NOS, Imidosulfurous difluoride, (fluorocarbonyl)-, 24 10 CH2, Methylene ruthenium complex, 25 182 CH2CI4P2, Phosphine, methylenebis-(dichloro)-, 25 121 CH3, Methyl cobalt complexes, 23 170 mercury complexes, 24 143-145 platinum complex, 25 104, lOS CNO, Cyanato silicon complex, 24 99 CN2OS2, l,3k, 2,4-Dithiadiazol-5-one, 25 53 CO, Carbon monoxide chromium complexes, 21 1, 2 23 87 cobalt complex, 25 177 cobalt, iron, osmium, and ruthenium complexes, 21 58-65 cobalt-osmium complexes 25 195-197 cobalt-ruthenium cluster complexes, 25 164... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

The complex [ Pt 2H 2( /i-H X/t-dppm)2] PF6 acts as a catalyst precursor for the water-gas shift reaction at 100°C in methanol. It is most effective at low pressures of carbon monoxide. A possible catalytic cycle is shown in Scheme 9. The catalytic solutions contain various cluster species, however, and a tetra-platinum species [Pt4(/i-CO)2(/i-dppm)3 j -Ph2PCH2P(0)Ph2 ] has been isolated and characterized by X-ray crystallography (109,112). 

The data on the catalyst containing rhenium alone indicate signficant chemisorption of carbon monoxide, but no chemisorption of hydrogen. As expected, the platinum catalyst chemisorbs both carbon monoxide and hydrogen, and the values of CO/M and H/M are nearly equal. The platinum-rhenium catalyst exhibits a value of CO/M about twice as high as the value of H/M. This result approximates what one would expect if hydrogen chemisorbed on only the platinum component of the catalyst. While this chemisorption behavior is consistent with the possibility that the platinum and rhenium are present as two separate entities in the catalyst, they do not rule out the possibility that bimetallic clusters of platinum and rhenium are present. 

During infrared-visible SFG studies of carbon monoxide on Pt(lll) in the 10 -700 Torr pressure range, dramatic changes in the vibrational spectrum of surface species were observed at high CO pressures. The vibrational signatures of the adsorbed species implicate the formation of carbonyl-platinum cluster analogs coadsorbed with an incommensurate CO overlayer. 

An extreme case of chemisorption-induced restructuring of metal surfaces is coirosive chemisorption as observed by SFG. In this circumstance, metal atoms break away from step or kink surface sites and form bonds with several adsorbate molecules. Carbon monoxide can form several carbonyl ligand bonds with platinum atoms leading to the creation of metal-carbonyl species. Thus, metal-metal bonds are broken in favor of forming metal-carbonyl clusters that are more stable at high CO pressures. The SFG vibrational spectra detect the reversible formation of new adsorb carbon monoxide species above 1(X) Torr on Pt(l 11), that appear to be platinum-carbonyl clusters Pt (CO) , with (m/n) > 1 and a CO commensurate overlayer. 

J2.9 Dynamics of the desorption of carbon monoxide from size-selected supported platinum clusters... 

Fig. 25. Reactivity of platinum clusters supported on a MgO fllm as a function of the cluster-size. The reactivity is expressed in the number of carbon dioxide molecules produced either per Ptn cluster, panel (a), or per Pt atom, panel (b), in the cluster-catalyzed oxidation of carbon monoxide. (Adapted from Ref. 36.)...

Red platinum-triangle carbonyl derivatives of the type (R3P) i Pt3 (CO) 3, presumably 84 (M = Pt), may be prepared by carbonylation of K2PtCli in the presence of the tertiary phosphine R3P, carbon monoxide, and hydrazine, or by reaction of the polymeric platinum carbonyl [Pt(CO)2ln (conceivably an uncharacterized platinum cluster) with the tertiary phosphine (33, 34, 53). These trimetallic platinum clusters (R3P)ijPt3-(CO)3 are unstable when subjected to reaction with Ccurbon monoxide to give the tetrametallic derivatives (R3P)ijPtij (CO) 5 (p. 416) the rate of this reaction increases with increasing basicity of the tertiary phosphine R3P. 

Heiz U, Sanchez A, Abbet S, Schneider W-D (1999) Catalytic oxidation of carbon monoxide on monodispersed platinum clusters each atom counts. J Am Chem Soc 121 3214—3217 Henglein A (1989) SmaU-particle research physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 89 1861-1873... 

The terminal hydride in 31 may be removed by one-electron oxidants such as ferrocinium, and under carbon monoxide the reaction affords the symmetrical, cationic complex 32, (Equation (7)). It is possible to replace one of the carbonyl ligands in 32 with other ligands such as ethene. The X-ray structure of this complex, the first platinum cluster to include both a carbonyl and an ethene ligand, is reported the ethene lies in the plane of the platinum triangle with a G-G distance of 1.25(6) A. 

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