Precious metal-based catalysts

A further purification step is carried out through the CO preferential oxidation reaction, generally employing precious metal-based catalysts, mostly Pt [10-13]. 

If extra purification is needed to protect and extend the life of nickel-based, pre-forming and steam-reforming catalysts or precious metal-based catalysts, an ultra-purification absorbent may be used. This absorbent can be installed below the ZnO-based, H2S removal absorbent. It removes more H2S and organo-sulphur species so that the feed stream contains very low, parts-per-billion levels of sulphur70. 

Neither the fuel additive nor the catalytic soot combustion coating have the ability to substantially reduce the emission of carbon monoxide or aldehydes. To achieve this, a different, precious metal based, catalytic coating can be applied to the filter, or a precious metal based catalyst can be added in line with the filter system. To date, filtering systems have not reached widespread application, mainly because of the high costs associated with their complexity. 

Despite this effort, no real breakthrough was achieved. One reason for this is that the precious metals have a much higher intrinsic activity, i.e., the activity per gram component, for the simultaneous conversion of CO, HC and NO than base metal catalysts. Furthermore, it is easier to obtain and to keep precious metals in a very finely divided state. Precious metal based catalysts are also much more resistant to sulfur poisoning at temperatures below 750 K than base metal catalysts. 

Figure 40 shows the conversion of CO, HC and NO e at typical automotive catalyst operation conditions for a precious metal based catalyst on a ceramic monolith with an extremely low precious metal loading, and for a precious metal free catalyst in which the same ceramic monolith support was used but with a washcoat consisting of a typical base metal catalyst formulation. The extremely poor con-... 

Finally, deactivation of the catalyst by poisoning elements should be mentioned. Precious metal based catalysts are poisoned by sulfur oxides which mainly originate from the combustion of sulfur-containing fuel constituents, by phosphorus and zinc which mainly originate from some additives in the engine lubricating oil, and by silicium which was sometimes present in some engine seals (Table 21). Also, traces of lead, present in the fuel because of contamination of the fuel supply chain, made an important contribution to the deactivation of the catalyst in the past. 

This paper reports first results of research and development work to achieve nitrogen oxide reduction under lean diesel exhaust gas conditions. Much attention is paid to the influence of operation conditions on catalyst performance. A major part of the paper deals with the influence of the hydrocarbon component, the hydrocarbon concentration and the HC/NO ratio on the activity of a special developed platinum based catalyst. Other aspects discussed are a spectroscopic characterization and a selectivity study. A hypothesis of a "dual-site" reaction mechanism for NOx-reduction in lean diesel exhaust gas precious metal based catalyst is established. Finally, first promising results on the performance of the catalyst system in a vehicle dynamometer test are given. 

Since 1975 catalysts have been fitted to vehicles to control emissions, initially of hydrocarbons and carbon monoxide (oxidation catalysts), and latterly also of oxides of nitrogen (three way catalysts). This contribution will demonstrate the ability of precious metal based catalysts not only to control carbon monoxide, hydrocarbons and nitrogen oxides but also the polynuclear aromatic fraction from both gasoline and diesel fuelled vehicles. The data will include that from both fresh and aged catalyst systems and also those exposed to leaded gasoline. 

This contribution will demonstrate the ability of precious metal based catalysts to control emissions of PAH from both gasoline and diesel fuelled vehicles. The activity of these catalysts, both fresh and aged and including those exposed to leaded gasoline is presented and discussed. 

The most common process for the chemical purification of the hydrogen rich gas is the preferential oxidation (PROX) of carbon monoxide. The preferential oxidation is promoted by precious metal based catalysts. Precious metal catalyst promotes the reaction of hydrogen and oxygen as well. So the main disadvantage of PROX is the side reaction of hydrogen with oxygen to water and heat. Furthermore precious metal based catalysts are expensive. 

Heat-Treated Non-precious-Metal-Based Catalysts for Oxygen Reduction... 

In spite of a very significant progress achieved with heat-treated macrocyclic compounds as ORR catalysts since the early 1970s, the activity and durability of that family of catalysts are stiU insufficient for replacing platinum at the fuel cell cathode and in other applications. Furthermore, the complex structure of macrocyclic compounds makes their synthesis expensive and potentially noncompetitive with precious-metal-based catalysts also from the materials cost point of view. For those reasons, much effort has been invested by the electrocatalysis research community in recent years into finding less expensive and catalytically more active non-precious metal ORR catalysts that would not rely on macrocylic compounds as either catalysts or catalyst precursors. In the past decade, there has been a significant improvement both in the activity and of non-macrocyclic catalysts, expected to be manufactured at a fraction of the cost of their macrocyclic counterparts. In this section, we review the precursors, synthesis routes, and applications of this relatively new family of catalysts. 

Palladium is the precious metal most frequently apphed for methanol steam reforming [176-178]. Despite its higher price compared with the copper-based systems, it is an attractive alternative owing to the potential for higher activity and greater robustness, which are key features for small scale reformers. The combination of palladium and zinc showed superior performance and soon the formation of a palladium-zinc alloy was identified as a critical issue for optimum catalyst performance [179]. Besides palladium/zinc oxide, palladium/ceria/zinc oxide may well be another favourable catalyst formulation [177]. However, precious metal based catalysts have a tendency to show higher carbon monoxide selectivity than copper-zinc oxide catalysts, because it is a primary product of the reforming reaction over precious metals. 

Borup et al. [193] demonstrated that it is possible to heat up an autothermal methanol reformer equipped with a precious metal based catalyst from room temperature, when the O/C ratio of the feed exceeds 1.45. The S/C ratio was set to 1.0 for these investigations. The exothermic oxidation reactions clearly started even at ambient temperature and caused light-off of the reformer. 

Figure 4.14 Dry gas composition of various diesel feedstocks during autothermal reforming over precious metal-based catalyst as measured by Kopasz et al. [260] base feedstock, dodecane ...

However, precious metal based catalysts without an oxygen carrier or additive, such as the rhenium/alumina catalyst as used here for the calculations, are known to have much lower activity compared with catalytic systems, such as platinum/ceria (see Section 4.5.1). 

The current status of precious and non-precious metal-based catalysts employed at the PEMFC cathodes is discussed below. 

A combination of a powdered precious metal-based catalyst, hydrogen peroxide, methanol and water can produce superheated steam in one to two seconds, releasing only CO2 and high-temperature steam for a variety of purposes. 

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