Catalytic processes for the control of nitrogen oxides emissions in the presence of oxygen.

The emissions of nitrogen oxides (NOx) from power plant and vehicles are known to cause damages to human health and environmental safety. The introduction of the EURO 6 regulations has imposed new limits on emissions of different types of pollutants present in the flue gases of diesel engines. In particular, the emission limits of autovehicles are different between stoichiometric oxygen-to-fuel ratios and oxygen-lean engine conditions: the latter ones are responsible of large amounts of particulate matter (PM) and nitrogen oxides (NOx), which are very dangerous air pollutants. Conversely, since under stoichiometric conditions PM emissions are quite negligible and NOx can be fruitfully removed by catalytic abatement, the former have been regulated by more stringent legislation. However, emissions limits are becoming stricter and stricter in recent years also for lean engines, although in the presence of oxygen the abatement of NOx results in a much harder task. Also for stationary plant, such as coffee roasters, the NOx emissions will be limited and the new regulations will be enforced from April 2016, and also in these applications the presence of oxygen in the pollutants-containing gas will make any abatement process very difficult. For these reasons, to fulfil the requests of the more stringent regulations, it is important to develop more efficient technologies which result in prevention of pollutants formation, together with improved abatement process for the pollutants emitted in the flue gases. NOx emission levels from autovehicles have been reduced by 66% from Euro 5, requiring the use of NOx after-treatment devices in addition to in-cylinder measures such as cooled EGR (Exhaust gas recirculation). LNTs (Low NOx Trap) have shown good NOx reduction performance and durability while SCR (Selective Catalytic Reduction), while offering also good NOx reduction performance, offers more flexibility for fuel economy and reduction of CO2 emissions. Manufacturers will likely choose the NOx after-treatment technology based on a combination of cost, reliability, fuel economy, and consumer acceptance. In large stationary applications (power plants, chemical industry, etc.), the issue of the NOx removal is often accomplished through the post-treatment of the exhaust gases by means of the selective catalytic reduction (SCR) method. The main advantages got from the exploitation of this technology are its high efficiency and its reliability as well as stability of the catalytic reaction, but the high cost represents a considerable drawback as well as the need to have a proper temperature range of effective performances that obliges to place SCR unit. In addition to these general limits, a further drawback of ammonia SCR process lies in the low acceptability of such a technology in some different contexts, such as the food industry, due to the unpleasant smell and danger of the reactant molecule employed. The extension to vehicle exhausts is very interesting too, provided that a proper catalytic system is developed in ordered to answer to the different requests of mobile NOx sources. The most important difference in the operating conditions can be represented by the range temperature of interest for the process. Hence, SCR devices could be even employed in mobile applications after adequate and calibrated system revision and modification in order to give effectiveness at lower light-off temperature (120 °C), since no solution seems to be viable with non-specific reducers (CO, HCs) mainly in the presence of O2, for instance in diesel engines. In particular, the development of ―Low Temperature‖ NH3-SCR catalysts that work in the range of temperature from 120 to 300 °C is considered an ideal way to control NOx emissions also from stationary sources besides being an unavoidable necessity for vehicles. As a matter of fact, the reduction of the nitrogen oxides by means of ammonia in an oxidizing atmosphere has raised the need of the development of new catalysts, characterized by low cost and capability of ensuring high conversions, even at relatively low temperatures. On the other hand, such a solution appears to be not viable for specific applications such as coffee roasters in food engineering, due to the unacceptance of ammonia, costs and safety reasons. Much more interesting could appear the attempt to prevent pollutant formation by developing intrinsically clean upstream processes. In the present PhD, I focused my attention on the research of lower cost, versatility, environment-friendly nature and, of course, performer catalysts. The work was devoted to synthesize, characterize and testing of MnOx and MnOx-CeO2 catalysts proposed as innovative LT SCR catalysts, and a series of other catalytic systems for the selective oxidation of the organic compound emitted by an industrial roaster of coffee, in order to prevent the NOx formation in such devices. Generally, Mn is used in both reduction and oxidation reactions due to its various types of labile oxygen as well as to the presence of different structures and morphologies. To the best of our knowledge, the optimal oxidation state of the metal has not been assessed for the SCR reaction, despite numerous studies dealing with the role of MnOx and the deactivation of the catalysts. Manganese oxides were investigated as catalysts for low-T SCR. Different MnOx catalysts were prepared by means of different operating conditions and characterized, as well as tested in the SCR process under lab-scale conditions. The goal was thereafter to understand the suitable Mn oxidation state necessary for SCR performances and to study the nature of active sites, which does not seem to be well clarified to date although a great research effort in this field has been made in the last two decades. The contribute of CeO2 with MnOx was investigate so the solution combustion route was chosen for synthesizing MnOx-CeO2 catalysts. The Mn/Ce molar ratio as well as quality and quantity of organic fuel in the reaction were investigated as key parameters and their influence on the structural, microstructural and superficial properties was studied. Moreover, the redox property of MnOx-CeO2 samples and the synergistic effect of Ce4+ and Mnn+ ions were analysed. All these properties were correlated to the catalytic activity in order to optimize the parameters for the best catalytic performance. We have already tried to correlate some chemical properties of the surface with the performances in the NH3-SCR of pure MnOx samples prepared by the SCS technique and in this paper a direct comparison among those results and the evidences obtained for the Mn-Ce catalysts will be performed. On the other hand, and also in connection with a project sponsored by a coffee-maker company, the issue of removal of NOx from roasting effluent was faced by studying catalysts and reactor conditions/configuration to prevention of the formation of nitrogen oxides itself. The attention was given to catalysts consisting of nanoparticles of transition metals, such as copper, nickel, iron and manganese, which replace in part or in full the platinum group catalyst, usally used in industrial context to abate the large mass of VOC emitted in the process. A homogenous dispersion and control the size of the nanoparticles on the substrate was necessary to develop the most effective catalysts and with a higher yield. The total or partial replacement of the platinum group with nanoparticles of transition metal catalysts leads to a considerable reduction in costs and also to the strengthening of sustainability and a secure supply of raw materials for EU producers. The replacement of the catalyst with an oxidative catalyst active and selective at low temperature allows both to reduce VOCs (Volatile Organic Compounds) and the CO is to reduce the nitrogen compounds to molecular nitrogen.