Noble metal-based

There are other parallel electrochemical reactions that can occur at the electrodes within the cell, lowering the overall efficiency for CIO formation. Oxygen evolution accounts for about 1—3% loss in the current efficiency on noble metal-based electrodes in the pH range 5.5—6.5. 

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. 

Among the catalysts under investigation, noble metal based systems are the most active in combustion reactions, and specifically PdO based catalysts are best suited for the combustion... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

Smith, G.B., Niklasson, G.A., Svensson, J.S.E.M. and Granqvist, C.G. (1986) Noble-metal-based transparent infrared reflectors experiments and theoretical analyses forverythingoldfilms./oumolqf Applied Physics, 59, 571-581. 

However, some areas of future research need to be highlighted (1) noble metal-based formulations which do not form N20, (2) novel catalyst formulations which decompose/reduce N20 below 300°C, (3) on-board routes to form oxygenated reductants, (4) NTP technologies, (5) maintain catalyst within peak operating temperature window and (6) techniques for storing NO, emissions during cool exhaust conditions followed by re-injection of the stored NO when the catalyst has achieved light-off conditions. There is already an active research on these topics, but a further intensification would be necessary. 

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],... 

Undoubtedly, the most advanced results concerning the development of non-noble metal-based catalysts are related to the use of silver [70-78], Recent investigations showed a significant rate enhancement of the simultaneous conversion of NO and hydrocarbon under lean conditions in the presence of hydrogen. As illustrated in Figure 10.5, such... 

Now, regarding the SCR with hydrocarbons in 02 excess, numerous investigations have shown a low activity below 200°C. However, it was found that H2 can promote the reduction of NO below 200°C on molybdenum and sodium-modified Pt/Si02 and Pt/Al203 catalysts [103]. Such a promotional effect also observed on silver-based catalysts originates extensive investigations in this field and offers new perspectives in the developments of non-noble metal-based catalysts. However, further developments of that variety of catalysts seem to be questionable due to their low sulphur tolerance. 

Table 10.4. Comparison of the catalytic performances of various supported noble metal-based catalysts (reproduced with permission from Ref. [104])...

Presently the effective role of sulphur additive is not well explained because sometimes activation or deactivation phenomena are observed. Such a versatile behaviour is well-illustrated over noble metal-based catalysts particularly when they are dispersed on perovskite supports [111]. The catalytic performances of a prereduced Pt/LaCo03 in H2 at 450°C are illustrated in Figure 10.13a. After preactivation in H2 subsequent bulk and surface characterisation highlighted an extensive reduction of the perovskite... 

Engelmann-Pirez, M., Granger, P. and Leclercq, G. (2005) Investigation of the catalytic performances of supported noble metal based catalysts in the NO + H2 reaction under lean conditions, Catal. Today, 107, 315. 

Dhainaut, F., Pietrzyk, S. and Granger, P. (2007) Kinetics of the NO + H2 reaction over supported noble metal based catalysts Support effect on their adsorption properties, Appl. Catal. B 70, 100. 

Table 1 Non-noble metal-based catalysts employed in the steam reforming of ethanol (SRE) for hydrogen production... 

New generations of noble metal-based catalysts have been developed allowing the researchers to work under milder conditions and to improve yields and selectivity. Asymmetric versions can be envisioned by introduction of chiral ligands and/or substrates and thanks to the mild temperatures and pressures used. 

Identification of unknown crystal structures and determination of phase fields by X-rays can be problematical if the characteristic patterns of the various phases are quite similar, for example in some b.c.c. A2-based ordered phases in noble-metal-based alloys. However, in many cases the characteristic patterns of the phases can be quite different and, even if the exact structure is not known, phase fields can still be well established. Exact determination of phase boundaries is possible using lattice-parameter determination and this is a well-established method for identifying solvus lines for terminal solid solutions. The technique simply requires that the lattice parameter of the phase is measured as a function of composition across the phase boimdary. The lattice parameter varies across the single-phase field but in the two-phase field becomes constant. Figure 4.12 shows such a phase-boundary determination for the HfC(i i) phase where results at various temperatures were used to define the phase boundary as a fimction of temperature (Rudy 1969). As can be seen, the position of is defined exactly and the method can be used to identify phase fields across the whole composition range. 

Figure 9.12 Ni/Al203 (a) and noble metal-based monolith (b) catalysts.

O2/CH4 00, H2O/CH4 (yl, GHSV Figure 9.14 Results of CH4 ATR catalytic activity tests on noble metal-based monolith at lower temperature. 

The electrochemical reduction of nitric oxide in solid-state electrochemical cell is an interesting field surveyed in [95]. The working principle of the cells is the cathodic reduction of NO to nitrogen and oxygen anions. In [95], the properties of various types of solid-state electrochemical cells used for NO reduction are presented and discussed. It is shown that the cathode materials with a high redox capacity and oxygen vacancies are most active for the electrochemical reduction of nitric oxide, whereas noble metal-based electrodes show a much lower selectivity. As an alternative route, the promotion of the reduction with a reductive agent is also considered. 

Noble Metal Catalysts. Noble metal-based catalysts have been widely used in reforming reactions, and are logical choices for ATR. Results of reaction studies generally suggest that these catalysts are comparable in activity to Ni-based catalysts, but they appear to be somewhat more resistant to deactivation. 

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