Alloy catalysts silica-supported

We illustrate the sensitivity of the C-0 stretching frequency for the bonding configuration with a perhaps somewhat dated but still very instructive study of the adsorption sites of alloy surfaces. Soma-Noto and Sachtler [18] reported an infrared investigation of CO adsorbed on silica-supported Pd-Ag alloys some of their spectra are shown in Fig. 8.5. On pure palladium, CO adsorbs mainly in a twofold position, evidenced by the intense peak around 1980 cm 1, although some CO appears to be present in threefold and linear geometries as well. This is a common feature in adsorption studies on supported catalysts, where particles exhibit a variety of surface... 

High selectivity was also observed on a silica-supported Fe-Cu catalyst prepared by coprecipitation (333 K, 10 atm H2, ethanol)286 and over polymer-protected colloidal Pd-Pt cluster catalysts (303 K, 1 atm H2, ethanol)287,288. In contrast with the above observation, the activity of the bimetallic alloy was 1.4-3 times higher than that of the monometallic Pd cluster reaching the maximum activity at a composition of Pd/Pt = 4 1. 

Careful infrared study of CO chemisorbed by Pd/Si02 catalysts in an SMSI versus a non-SMSI state verified that after HTR, silicon species are distributed in the Pd surface layer. For the catalysts reduced at 300°C (LTR), the B/L intensity ratio (B = bridging CO L = linearly bound CO) is a monotonic function of Pd particle size (Fig. 17). On the other hand, the B/L ratios for Pd/Si02 catalysts that experienced HTR show considerable departure from this universal curve (Fig. 17) (208). Of course, a relatively higher proportion of linearly bound CO for Pd/Si02 catalysts in the SMSI state is believed to follow from the existence of silicon, rather uniformly interdispersed in the metal surface, resembling the case of CO adsorption on silica-supported Pd-Ag alloys (209). 

Praliaud and Martin (77) proposed the formation of Ni-Si and Ni-Cr alloys on silica and chromia supports, respectively, under H2 at sufficiently high temperatures. They suggested that hydrogen spilt over from Ni to the Cr203 carrier and partially reduced it to Cr°, which was then alloyed with Ni as indicated by magnetic measurements. The same technique in conjunction with IR spectroscopy and volumetric adsorption of H2 was applied to partially reduced Ni-on-alumina and Ni-on-zeolite catalysts by Dalmon et al. (78). These supported Ni systems contained Ni° and Ni+. H2 was found to be activated only when the couple Ni°/Ni+ was present according to... 

Table 2 Activation energies (kJ moP ) corresponding to the adsorption, Eai. desorption, E -i, and disproportionation reaetion, 2, of carbon monoxide over silica-supported Pt, Rh, and Pto.5o-Rho.5o alloy catalysts...

The catalytic activation of CO2 and its reaction with C2H4 and H2O was studied over several silica-supported platinum-tin catalysts under different reaction conditions. The lactic acid production is related to the content of the PtSn alloy in the catalyst. 

Catalysts aPtbSn, which were treated under H2 at 673 K before the reaction, have no more Sn or Pt molecular species on surface, and several mono- or bimetallic phases have been identified Pt and PtSn for SPtlSn and 2PtlSn catalysts, PtSn2 and PtSn for lPt2Sn catalyst and PtSn, PtSn2 and Sn for SPtlSn catalyst [10], All samples presented the silica-supported PtSn alloy, but only in IPtlSn sample, which showed the highest value of CH3CH(OH)COOMe production, did this phase not coexist with other metallic phases. 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

The reactions of butan-l-ol (Scheme 3) were explored over silica-supported Pt-Au catalysts (and also Ni-Cu powders). It was confirmed that metals are active in ether formation from higher alcohols, although sensitive to the presence of sodium ions. Alloying decreased the activity of pure Pt for ether formation, ascribed to the diminished number of active ensembles (perhaps containing 4 atoms), although it was not eliminated (unlike Ni-Cu alloys).The percentage formation of C4 hydrocarbons (butane, butene),butanal. 

The formation of filamentous carbon deposits on transition metal catalysts (Fe, Co, Ni) and their alloys have been investigated in some detail over the past two decades.21,38-40 Among them, nickel is the most promising candidate since it forms carbon deposits at temperatures as low as 723-823 K using CH4, C2H6 or CO + H2 feeds. Carbon fibres are usually produced during these reactions. Typical forms of the carbon produced from CH4 decomposition on silica-supported Ni catalysts are shown in Fig. 7.1. The pyrolysis of methane at temperatures somewhat lower than 873 K produces fish-bone type nanofibres.41 The Ni metal particles are present at the tip of each carbon fibre, and catalyse methane decomposition as well as growth... 

Cusumano et al. (128) studied the reaction over Pt on alumina and on silica supports and concluded that the TOF was about the same for both catalysts, which did show quite different atomic rates AR. The later work of Sinfelt et al. (269) on reactions over copper-nickel alloys led also to the suggestion that cyclohexane dehydrogenation over Ni does not require a large ensemble of surface atoms and thus may be structure insensitive on a geometric basis. For ethane hydrogenolysis studied on the same CuNi alloys, it was found that the activity decreased much more rapidly than did the fraction of Ni atoms on the surface of the alloys. This implies that ethane hydrogenolysis requires an ensemble of surface atoms and should show antipathetic structure sensitivity. This reaction will be discussed in connection with Fig. 15 (below). 

Because the hydrogenation of the nitrile group takes place on the Pt sites, the rate enhancement observed upon addition of tin can only be explained by the cooperation of Pt and tin sites on the surface of alloy-type nanocluster and/or the metal support interface. The results obtained on alloy type Sn-Pt/Si02 catalysts strongly resemble results attained on silica supported Ni-Fe catalysts with 75 %... 

Structure of Alloy Type Sn-Pt/Si02 Catalysts used in Low Temperature CO Oxidation. Both Mossbauer and FTIR spectroscopy provided sufficient proof of surface reconstruction during the low temperature CO oxidation. However, the above reconstruction appeared to be reversible as the reversible interconversion of PtSn Sn -I- Pt was demonstrated by both spectroscopic techniques. This reversibility can only be achieved if the segregation described above is within the supported nanoparticle, i.e., when surface reactions involved in CO oxidation do not result in formation of separate Pt and tin-oxide phases on the silica support. 

Microwave calcination of magnesia, alumina, and silica-supported Pd and Pd-Fe catalysts resulted in their having enhanced catalytic activity in test reactions -hydrogenation of benzene and hydrodechlorination of chlorobenzene - compared with conventionally prepared catalysts [14-16]. The greater catalytic activity was attributed to prevention of the formation of a Pd-Fe alloy of low activity, which occurs at the high reduction temperature used in conventional heating. 

The behavior of the CO/H2 synthesis reaction has been studied over silica-supported Ru-Fe catalysts, and an optimum range in the Ru Fe ratio was found to exist in which olefin production was maximized and methane formation was minimized. The catalyst samples were characterized by hydrogen and CO chemisorption, x-ray diffraction measurements, and Mossbauer spectroscopy. Alloy formation was verified at different Ru Fe ratios, and changes in specific activity and selectivity were observed as this ratio varied. Between Ru Fe ratios of 1/2 to 2, 45 mol% of the total hydrocarbon product was C2-C5 olefins while less than 40 moWo was comprised of methane. 

Mossbauer spectroscopy showed that on silica supported catalysts, only Pd-Sn solid solutions or alloys are present, whereas, on alumina supported catalysts, tin is present at... 

Nakagawa Y, Tomishige K (2010) Total hydrogenation of furan derivatives over silica-supported Ni-Pd alloy catalyst. Catal Commun 12 154-156... 

CO adsorption, being an elementary step of SCO, was studied over silica-supported monometallic Rh and Rho.50 + Pto.50 alloy catalysts, under various atmospheres ranging from 25% to 75% The variation... 

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