Oxide Supported Alloy Catalysts

5 Oxide Supported Alloy Catalysts. - Much of the work in this 

Additions of small proportions of copper to catalysts of each of 

A comparison was made between silica and alumina for supports 102 

The application of EXAFS to the petroleum reforming catalysts with and pt-Re components is more 

Recently there have also been brief reports on bimetallic 107 108 

---

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

Amraig various anode catalysts developed, ft-Ru alloys are generally cmisidered as the best candidates for H2/CO and alcohol oxidation these alloy catalysts show high CO tolerance and acceptable durability under FC operating conditions. Several commercial ft-Ru alloy nanoparticles supported on carbon black have been available for applications in PEMFCs, DMFCs, and DEFCs. Efforts to improve the activity and stability of ft-Ru alloy catalysts continuously are being made. Recently, the nanocapsule method has been successfully employed to synthesize ft-Ru nanoparticles with... 

Recently there has been considerable interest in the reactions of mixed-metal cluster complexes with oxide supports. To a large extent this emanates from the established phenomena within the field of heterogeneous catalysis which show that unusual activities and selectivities can be observed for supported "alloy catalysts [70]. The development of the solution chemistry of heterometallic cluster complexes has reached the stage of providing several series of related complexes which can be used as the basis of systematic studies. Indeed, the complexes Co2Rh2(CO)j 2 Co3Rh(CO)j 2 were reacted with silica and alumina some years ago with the aim of generating new alloy catalysts... 

Figure 4.28 shows an example where STM recognizes the individual metal atoms in an alloy, thus revealing highly important structural information on the atomic level. The technique does not require a vacuum, and can in principle be applied under in situ conditions (even in liquids). Unfortunately, STM only works on well-defined, planar, and conducting surfaces such as metals and semiconductors, and not on oxide-supported catalysts. For the latter surfaces, atomic force microscopy offers better perspectives. 

Xin and co-workers modified the alkaline EG synthesis method by heating the metal hydroxides or oxides colloidal particles in EG or EG/water mixture in the presence of carbon supports, for preparing various metal and alloy nanoclusters supported on carbon [20-24]. It was found that the ratio of water to EG in the reaction media was a key factor influencing the average size and size distribution of metal nanoparticles supported on the carbon supports. As shown in Table 2, in the preparation of multiwalled carbon nanotube-supported Pt catalysts... 

Colmenares L, Wang H, Jusys Z, Jiang L, Yan S, Sun GQ, Behm RJ. 2006. Ethanol oxidation on novel, carbon supported Pt alloy catalysts— Model studies under defined diffusion conditions. Electrochim Acta 52 221-233. 

The use of EM (except in the special case of SEM) demands that the catalyst, whether mono-or multi-phasic, be thin enough to be electron transparent. But, as we show below, this seemingly severe condition by no means restricts its applicability to the study of metals, alloys, oxides, sulfides, halides, carbons, and a wide variety of other materials. Most catalyst powder preparations and supported metallic catalysts, provided that representative thin regions are selected for characterization, are found to be electron transparent and thus amenable to study by EM without the need for further sample preparation. 

Metal oxide sensors (MOS), smart, 22 717 Metal oxide supported catalysts, 5 336-337 coke formation on, 5 267—270 Metal passivation, in industrial water treatment, 26 137 Metal peroxides, 18 410 Metal phosphates, tertiary, 18 840 Metal-phosphorus alloys, 19 59 Metal phthalocyanines, electrochromic materials, 6 572t, 576-577 Metal prefinishing, detersive systems for, 8 413t... 

The most active catalysts for NH3 decomposition are based on Ru, however, cheaper Fe, Co, Ni and alloy systems are also intensely investigated [148]. The impact of the support material is remarkable. In a study by Au et al., Ru/CNTs performed better than all oxide-supported systems, whereas activated carbon resulted in one of the lowest NH3 conversions (Tab. 15.6) [147]. The dispersion of the active component as well as basicity [147] and conductivity [149] of the support are discussed as the relevant factors for high catalytic efficiency. However, the difference between CNT and activated carbon support is still remarkable. Thus it is not surprising that even the residual catalyst material on commercial MWCNTs, which is basically based on Fe and Co, results is a high catalytic performance in NH3 decomposition [150]. 

Of the alternative Ft formulations, the FtMo system has been the most studied in recent years. Work on bulk FtsMo alloys by Grgur, Markovic, and Ross showed similar CO tolerance to FtRu in the presence of H2. This tolerance was correlated with the ability of FtMo to oxidize CO at potentials as low as 0.05 V. However, unlike Ru but similar to Sn, the Mo appeared to oxidize CO just at neighboring Ft sites, with the majority of CO oxidized af potentials typical of pure Ft. The surface Mo atoms were found to be oxidized even at 0.0 V. Therefore, it was postulated that H2O dissociation to form OH was mediated by a Mo(IV)/(Vl) couple. Carbon-supported FtMo catalysts were reported to have better CO tolerance than FtRu in MEA testing up to CO concentrations of 100 ppm.i39... 

The structure of a SPE cell is shown in Fig. 2.3. The basic unit of a SPE electrolyzer is an electrode membrane electrode (EME) structure that consists of the polymer membrane coated on either side with layers (typically several microns thick) of suitable catalyst materials acting as electrodes [43,49,50], with an electrolyzer module consisting of several such cells connected in series. The polymer membrane is highly acidic and hence acid resistant materials must be used in the structure fabrication noble metals like Pt, Ir, Rh, Ru or their oxides or alloys are generally used as electrode materials. Generally Pt and other noble metal alloys are used as cathodes, and Ir, Ir02, Rh, Pt, Rh-Pt, Pt-Ru etc. are used as anodes [43,46]. The EME is pressed from either side by porous, gas permeable plates that provide support to the EME and ensure... 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

The best known heterogeneous catalysts are oxide-supported Ru, Rh, and Ni, and Ru exhibits the highest selectivity. Marked support effects are observed and Ti02 is usually found to be the best support material. Pd on zirconia and Ni on zirconia are particularly effective catalysts when prepared using amorphous Pd-Zr, Ni-Zr, and Ni-containing multicomponent alloys by controlled oxidation-reduction treatment240-242 or generated under reaction conditions.243-245 Stabilized metal nanoparticles of uniform dispersion embedded into the oxide matrix are the... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

There has been interest in using metal cluster compounds as precursors to heterogeneous catalysts on oxide supports.21 It is already known that metal alloys dispersed on supports are heterogeneous catalysts and they are widely used in petroleum refining. 

---