Non-Noble Metal Supported Catalysts

In this body of catalysts, the metal cluster is said to be formed around the carbonyl precursor. According to SEM and TEM imaging, it appears that the carbonyl clusters are on the order of 1 pm in diameter when supported on carbon.192 Analysis with FTIR has shown that the carbonyl is present.189 190 198-200 203 Non-noble metals have also been studied along side the noble-metals in this group of catalysts. Table 4 lists the non-noble metal carbonyl catalysts studied.189-192 198-200 The non-noble metal carbonyl catalysts studied produced mixed results for the ORR activity. 

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

Lin, R., Ding, Y., Gong, L., et al. (2009). Oxidative Bromination of Methane on SUiea-Supported Non-Noble Metal Oxide Catalysts, Appl. Catal. A Gen., 353, pp. 87-92. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

The secret that makes this process work is no surprise, the catalyst. Those that work include some of the noble metals, specifically, platinum or palladium, a rare earth metal like cerium or neodynium (are they rare or what ) on alumina, or a non-noble metal like chromium on a silica-aluminum support. 

Numerous metals have been evaluated as bifunctional catalysts. Those used are noble metals and non-noble or transition metals. Platinum and palladium have the highest catalytic activity. The noble metal content is usually 1% or less, whereas that of non-noble metals is larger 1-30%. The concentration of dispersed metal on supports plays an important role in the activity of the catalyst, e.g. the activity of hydrogenation/dehydrogenation increases then decreases with the concentration of metals. There are some typical reactions of bifunctional catalysts. 

In the case of the ceria supported non-noble metal catalysts, both conventional impregnation techniques (16,32,264,292) and precipitation of the metal precursor onto the ceria support have been used (43). Some P CeO catalysts have also been prepared in the latter way (284,293). Co-precipitation from a mixed solutions containing both... 

TPR and TPO patterns of silica-supported rhodium, iron, and iron-rhodium catalysts are shown in Fig. 11.5 [14]. These catalysts were prepared by pore volume impregnation from aqueous solutions of iron nitrate and rhodium chloride. Note the difference in reduction temperature between the noble metal rhodium and the non-noble metal iron. The bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The TPR patterns of the freshly prepared catalysts consist of two peaks, one coincides with that of the TPR pattern of the fully oxidized catalyst (right panel of Fig. 11.5) and can thus be... 

However, while the commonly used refractory oxide supports, silica and alumina, increase the metal dispersion, they are not inert, especially toward the non-noble metals and less conspicuously also toward the noble metals. The physical and chemical interactions between the active metal, the oxide support and the environment affect the surface properties of the catalyst and consequently influence the shape of crystallites and the particle size distribution. Two sets of experimental observations involving surface phenomena are of interest in the present context Cl) The average size of the... 

Main challenges of present three-way catalysts (TWC) concern the availability of precious metals, lowering of the ignition temperature of the converters and maintenance of a Wgh temperature stability [1]. Development of non-noble-metal alternatives, able to reach the activity levels achieved by usual Pt-Rh-Pd catalysts, is particularly desirable and supported copper catalysts are possible canddates to attain the mentioned requirites. 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

A chemisorption teclmique developed by Koinai et al., based on CO methanation, was successfrilly used to analyze noble metal dispersions of both fresh and vehicle-aged Pt/Rli and Pd/Rli commercial automotive three-way catalysts. The teclmique is relatively rapid (< 2 hours), extremely sensitive, and largely free from complications due to adsorption of CO on non-noble metal components of the washcoat (support, promoters, stabilizers, etc.). Particle sizes of the vehicle-aged catalysts, calculated by applying the spherical particle assumption to the dispersions measured by the CO methanation method, agreed well with particle sizes calculated from x-ray diffraction line-broadening data. These results indicate that the CO methanation teclmique can be applied routinely to obtain fast and accurate measurements of noble metal surface areas in automotive catalysts retrieved from tlie field, even tliose with metal dispersions ca. 2% or less. 

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