Environmental catalysts catalytic combustion

PGM catalyst technology can also be appHed to the control of emissions from stationary internal combustion engines and gas turbines. Catalysts have been designed to treat carbon monoxide, unbumed hydrocarbons, and nitrogen oxides in the exhaust, which arise as a result of incomplete combustion. To reduce or prevent the formation of NO in the first place, catalytic combustion technology based on platinum or palladium has been developed, which is particularly suitable for appHcation in gas turbines. Environmental legislation enacted in many parts of the world has promoted, and is expected to continue to promote, the use of PGMs in these appHcations. 

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]. 

The emphasis on environmental protection in the last three decades, as industrial and economic growth gave birth to many forms of pollution threatening human health and Earth ecosystems, resulted in the growth of environmental catalysis. So, catalysts ate not only used to promote processes in the production field, but also to reduce the emissions of undesirable or hazardous compounds to the environment. For example, catalytic combustion has been proposed and developed as an effective method for controlling the emissions of hydrocarbons and carbon monoxide. 

Catalytic combustion is an environmentally-driven, materials-limited technology with the potential to lower nitrogen oxide emissions from natural gas fired turbines consistently to levels well below 10 ppm. Catalytic combustion also has the potential to lower flammability at the lean limit and achieve stable combustion under conditions where lean premixed homogeneous combustion is not possible. Materials limitations [1,2] have impeded the development of commercially successful combustion catalysts, because no catalytic materials can tolerate for long the nearly adiabatic temperatures needed for gas turbine engines and most industrial heating applications. 

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

Catalytic combustion of VOC-s is an important technique in environmental pollution control. Total oxidation of methane is a frequently applied test reaction in combustion catalyst development research [Ferri and Fomi 1998, Saiacco et al, 1999] since among VOCs this compound is one of the most difficult to bum off. However, the reaction mechanism is not completely explored yet. 

This chapter reviews recent advances in the use of ceria as a catalyst component in environmentally important catalytic processes. In particular, the oxidation of CO, the oxidation of volatile organic compounds (VOCs), the combustion of methane, and the reaction of NO with CO will be considered. 

Silica-alumina has been used to support the CuO coupled with Ga203, and Sn02 dispersed phases to enhance the catalytic properties of CuO-based catalysts in reactions of environmental importance (hydrocarbon combustion, NO and N2O decomposition and reduction [88]). The acidic properties of such oxide systems were studied from a qualitative (nature of the acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view through the adsorption and desorption of two basic probes (ammonia and pyridine) by coupled volumetric-calorimetric technique and XPS and FT-IR spectroscopy. 

The US Clean Air Act (CAA) of 1970 required that automobile exhaust emissions be regulated to meet new environmental standards. From 1975 all new models were to be fitted with catalytic combustion converters to reduce levels of carbon monoxide and unbumt hydrocarbons in the exhaust. This led to the phase-out of lead additives in gasoline between 1973 and 1996. To compensate for the loss of octane rating of the gasoline more reformate and alkylate needed to be added. Octane catalysts were also developed for FCC units and the aromat-... 

Leanza, R Rossetti, 1 Fabbrini, L Oliva, C Fomi, L. Perovskite catalysts for the catalytic flameless combustion of methane Preparation by flame-hydrolysis and charaeterisation by TPD-TPR-MS and EPR. Appl. Catal, B Environmental, 2000, Volume 28, Issue 1, 55-64. 

Supported noble metals and in particular, palladium, are being widely used for the complete combustion of methane and other alkanes to form CO2 and H2O, environmentally acceptable emission products and extremely low NOx levels [ 1-3], A considerable amount of research effort has been devoted to the process, however, there does not appear to be a consensus with regard to either the mechanism of the reaction or the chemical identity of the active catalytic species [4-8]. This state of affairs is further complicated by the fact that the chemical state of the catalyst is extremely sensitive to the reaction conditions, including time-on-stream and reaction temperature [9-12]. It has also been demonstrated that the nature and form of the support plays a key role in modifying both the activation and deactivation steps encountered with palladium catalyst particles [13-16]. 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

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