Bronzes catalytically active

In a related system, Feringa and co-workers (104) noted the beneficial effect of copper bronze in the allylic oxidation reaction. The addition of 5 mol% of this addend provides the cyclohexenyl ester in higher selectivity and yield than in the absence of the metal, while allowing the reaction to proceed at 0°C. The role of copper bronze is presumably to effect reduction of Cu(II) to the catalytically active... 

Researchers are looking at ways to reduce the amount of platinum yet retain the catalytic activity. Peter Strasser, a researcher at the University of Houston in Texas, and his colleagues are trying to develop a platinum alloy that will do the job. An alloy such as bronze is a combination of elements, which in the case of bronze are tin and copper. Engineers often use alloys because they offer properties that are superior to those of a single metal, as described in chapter 1. A platinum alloy that acts as an effective catalyst in fuel cell electrodes yet contains less platinum would save a substantial amount of money. 

More recently the transformation of carbon-supported Pt colloids of approximately 1 nm diameter into Pt alloys has been reported, which seems to yield an even better catalyst, since the alloy particles—although coarser than the initial Pt particles—show improved catalytic activity and stability. It is not unlikely that from these alloys by in-situ oxidation transition-metal platinum bronzes like NLPt304 are formed, being the catalysts proper. 

The above discussion demonstrates the multifaceted nature of spillover. The interphase transport of an activated species onto a surface (and sometimes into the bulk) where it is unable to be formed without the activator can induce a variety of changes on, and reactions with, the surface. All the reactions of atomic hydrogen are found to be induced by spillover exchange, bronze formation, reduction, demethoxylation, and catalytic activation. An activated species is able to gain indirect access to the nonsorbing surface. 

The influence of spillover species on an acceptor phase can be in the extreme either subtle or profound. Many of the phenomena associated with hydrogen spillover are as subtle as the influences of type-2 hydrogen on the activity of ZnO (189) or as significant as bulk reduction, bronze formation, or catalytic activation. The effects may be similar to the exposure of a surface to a hydrogen plasma. 

Metals and particularly the noble ones have always attracted attention because of their high catalytic activity. Although economic incentives prompted the examination of nonnoble metal electrocatalysts, few such efforts have proven fruitful. Electrocatalysts that show sufficient activity, but not longevity, include tungsten bronzes and solid organometallic complexes of transition metals, and are discussed in this section. 

The author and Sokolova (193) investigated the catalysis of alcohols on tungsten bronzes, which possess a defective structure. An X-ray structural analysis was also made in this research. In spite of the defective structure, the catalytic activity of W-bronzes proved to be rather low, contrary to the electronic theory of catalysis. According to the latter, however, the catalytic activity decreases as bronzes are being reduced or lithium added, the detectivity decreasing too. From the BET data and the X-ray patterns it follows that the surface of bronzes is practically unaltered on reduction. On the other hand, the low activity... 

The use of a particular catalyst for HOR in alkaline media depends on the operating conditions and cost. For example, non-noble metals such as nickel [37] have been used in H2 technical electrodes for AFCs. It is known that the catalytic activity of Raney Ni (porous Ni doped with Co or alloyed with Fe, Ti or Mo to improve its performance) approaches that of noble metals in these systems. Another catalyst that has been used is nickel boride (Ni2B). Sodium tungsten bronzes [38] and carbon supported Pt and Pt-Pd [39] have also been employed to eatalyze HOR in alkaline solutions. 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

Unfortunately, the activity enhancement between Pt and WC has been shown to be short-lived [127] due to the oxidation of the WC support in the region relevant to the ORR to WO2 at potentials > 0.8 V vs. NHE, followed by the facile oxidation of WO2 to WO3 [126, 127, 136]. In addition, surface oxidation of WC to WO3 is followed by the formation of acid-soluble tungsten bronze species, H W03, which leads to Pt detachment and agglomeration. The agglomeration of Pt is accompanied by a total loss in activity enhancement, returning the activity of Pt to the same level as Pt/C as the catalytic clusters lose their interaction with the surface. The mechanism proposed by Liu for the WC instability and Pt activity loss is shown in Fig. 24.15. 

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

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