Noble metal catalysts, features

Noble metal catalysts High yields of synthesis gas were reported for the partial oxidation of methane over nearly all noble metal catalysts (Pd, Rh, Ru, Pt, Ir) [110]. The observed performances (degree of methane conversion and yields of CO and H2) vary with the catalyst and the operating conditions but all reported data share these common features (i) no carbon deposition is observed (Claridge and co-workers [135]... 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

RHE) and coinciding with the peak (see Fig. 8) associated with the Ru(IV)/ Ru(VI) couple beginning at about 1.25 V (RHE) and the second coinciding with the peak usually associated with the Ru(VI)/Ru(VII) couple [209]. Formate was observed to be the product of HCHO oxidation in the first wave, while 4e oxidation of HCHO to carbonate was observed in the second wave. Despite its interesting catalytic features for the electro-oxidation of formaldehyde, Ru02 is not as active as the noble metal catalysts Pt and Pd for this reaction, however. 

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

A representative comparison of the effect of the catalyst bed geometry on methane conversion and product selectivity over a range of methane/air ratios is shown in Fig 4 Unlike typical supported catalysts, where the catalyst is well-dispersed and submicrometer-sized, the noble-metal catalysts in these methane oxidation reactions were basically films with micrometer-sized surface features (Other tests on both extruded cordiente and alumina foam monoliths with lower catalyst loading resulted in similar carbon monoxide production but lower hydrogen yields than those illustrated in the figure, which provided evidence that the reaction is catalyst-dependent and not initiated by the monoliths or gas... 

Fig. 2 shows the conversion vs. temperature curves for H2 oxidation over Iridium catalysts pretreated by hydrogen reduction. Comparing the obtained conversions in H2 oxidation with those in CO oxidation, it was found that the temperatures for 50% conversion of H2 oxidation over Ir catalysts, except for IrA i02-DP, was similar to those in CO oxidation. Over Ir/Al203-DP and Ir/Fe203-DP catalysts, H2 oxidation proceeds at lower temperatures than CO oxidation. This feature is similar to those of other typical noble metal catalysts. It should be noted that CO oxidation over the Ir/Ti02 catalyst takes place at temperatures even below room temperature and at much lower temperature than H2 oxidation. This feature is the same as that of highly dispersed Au catalysts [1-4]. These results indicated that the support effect for the CO oxidation was much larger over Ir catalysts than that over Au catalysts. 

A number of studies dealing with non-noble metal catalysts (NNMCs) have appeared in recent years. " The main advantage of this type of catalyst is their low cost compared to Pt-based systems. In the next sections, relevant features to the ORR and the current status of catalyst development will be discussed. 

Then the reactivity in the CO oxidation was probed under integral conversion conditions to address the suitability of the system for applicative purposes. The full CO conversion attained by the redox system at 150°C, in comparison to the much lower value (ca 38%) of the MlCl-P sample, supports the fact that the redox MnCeOx system features an oxidative performance comparable with that of noble-metal catalysts [1], constituting thus a valuable alternative for environmental applications. 

Previous kinetic investigations dealing with the NO + H2 reaction over supported noble metal-based catalysts showed different kinetic features according to the nature of the support [29,53-58], Initially, this reaction has been described in the absence of oxygen on Rh deposited on silica and alumina by the following mechanism [29],... 

A comparison of various metals as catalysts for the hydrogenolysis of hydrocarbons reveals a wide variation in catalytic activity, even among such closely related metals as the noble metals of group VIII of the periodic table. Striking differences in the distribution of hydrogenolysis products have also been revealed in studies on selected hydrocarbon reactants. These features are emphasized in the following discussion of activity patterns and product distributions in hydrogenolysis. 

Since high current density at the maximum power density and the cost of the noble metals are important parameters for the commercialization of DMECs, H-CNE-supported Pt-Ru alloys maybe classified among the most efficient and cost-effective anode catalysts. It is also worth mentioning that the CNF-supported catalysts feature superior catalytic activity at the high temperatures where the mass transfer of methanol and oxygen is more favorable due to the fibrous network of CNEs. 

One of the characteristic features of the metal-catalysed reaction of acetylene with hydrogen is that, in addition to ethylene and ethane, hydrocarbons containing more than two carbon atoms are frequently observed in appreciable yields. The hydropolymerisation of acetylene over nickel—pumice catalysts was investigated in some detail by Sheridan [169] who found that, between 200 and 250°C, extensive polymerisation to yield predominantly C4 - and C6 -polymers occurred, although small amounts of all polymers up to Cn, where n > 31, were also observed. It was also shown that the polymeric products were aliphatic hydrocarbons, although subsequent studies with nickel—alumina [176] revealed that, whilst the main products were aliphatic hydrocarbons, small amounts of cyclohexene, cyclohexane and aromatic hydrocarbons were also formed. The extent of polymerisation appears to be greater with the first row metals, iron, cobalt, nickel and copper, where up to 60% of the acetylene may polymerise, than with the second and third row noble Group VIII metals. With alumina-supported noble metals, the polymerisation prod-... 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

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