Catalysis for NO_x abatement

Abstract

Research in the field of NO_x abatement has grown significantly in the past two decades. The general trend has been to develop new catalysts with complex materials in order to meet the stringent environmental regulations. This review discusses briefly about the different sources of NO_x and its adverse effect on the ecosystem. The main portion of the review discusses the progress and development of various catalysts for NO_x removal from exhaust by NO decomposition, NO reduction by CO or H₂ or NH₃ or hydrocarbons. The importance of understanding the mechanism of NO decomposition and reduction in presence of metal ion substituted catalysts is emphasized. Some conclusions are made on the various catalytic approaches to NO_x abatement.

Introduction

Environmental catalysis can be defined as technologies using catalysts to reduce the emission of environmentally unacceptable compounds [1], [2]. The exhausts from automobiles and stationary sources such as power plants contain CO, NO_x and hydrocarbons. The conversion of these pollutants to CO₂, N₂ and H₂O using catalysts is a challenge. In the last two decades, significant developments have occurred in this field leading to a better understanding of the catalytic NO_x abatement. A number of reviews regarding various aspects of NO_x abatement have been published, as discussed in the following sections. This review, however, is a compendium of all aspects of NO_x abatement that include NO decomposition, NO reduction by CO, H₂, NO storage and selective catalytic reduction of NO.

The major source of nitrogen oxides is the combustion of fossil fuels such as petroleum in the engines of vehicles or coke in the electrical power plants. The origin of NO_x is generally categorized into mobile and stationary sources. Fig. 1 describes the different sources of NO_x in US and in European countries [3], [4].

NO_x is a generic term for mono-nitrogen oxides namely NO and NO₂, which are produced during combustion at high temperatures. At ambient temperatures, oxygen and nitrogen do not react with each other. However, in an internal combustion engine, high temperatures lead to reactions between nitrogen and oxygen to yield nitrogen oxides. In the presence of excess oxygen, nitric oxide will be converted to nitrogen dioxide.

Bosch and Janssen [5] categorize three types of NO_x formed during the combustion process. NO_x from engine exhaust typically consists of a mixture of 95% NO and 5% NO₂. The first category, thermal NO_x, is formed by the oxidation of N₂ at high temperatures.

$${\text{N}}_2+{\text{O}}_2\iff 2\text{NO,}\mathrm{\Delta }{H}_{298}^{°}=180.6\text{kJ}/\text{mol}$$

This reaction takes place above 1300 K and follows the Zeldovich mechanism of chain reactions involving N^∗ and O^∗ activated atoms:

$${\text{N}}_2+{\text{O}}^{\ast }\to \text{NO}+{\text{N}}^{\ast }$$

$${\text{N}}^{\ast }+{\text{O}}_2\to \text{NO}+{\text{O}}^{\ast }$$

The rate of NO formation is essentially controlled by reaction (2) and increases exponentially with temperature. The Zeldovich mechanism dominates NO formation under most engine conditions [6]. The NO_x emission from the engine can be controlled by lowering the combustion temperature by operating the engine under excess air (fuel-lean) conditions but most these approaches are not very effective [6], though recent approaches based on high temperature air combustion (HiTAC) are effective.

The second category of NO_x is called fuel NO_x and is formed from the oxidation of nitrogen present in fuels such as coal and heavy oils. In contrast to thermal NO_x, fuel, NO_x formation is relatively independent of temperature at normal combustion temperatures [6].

The third category of NO_x is called ‘prompt ${\text{NO}}_x^{\prime }$ (also termed as Fenimore NO) which is formed by the reaction of hydrocarbon fragments with atmospheric nitrogen to yield products such as HCN and H₂CN. These can be subsequently oxidized to NO in the lean zone of the flame. NO can further react with oxygen to NO₂ or N₂O.

$$\text{NO}+1/2{\text{O}}_2\iff {\text{NO}}_2\text{,}\mathrm{\Delta }{H}_{298}^{°}=-113\text{kJ}/\text{mol}$$

$$2\text{NO}\iff {\text{N}}_2\text{O}+1/2{\text{O}}_2\text{,}\mathrm{\Delta }{H}_{298}^{°}=-99\text{kJ}/\text{mol}$$

Prompt NO_x formation is proportional to the number of carbon atoms present per unit volume and is independent of the identity of the parent hydrocarbon. The quantity of HCN formed increases with the concentration of hydrocarbon radicals. Prompt NO_x can be formed in a significant quantity at low-temperature, fuel-rich conditions and where residence times are short.

Another route of formation of NO is via nitrous oxide. In this mechanism, O-atom attacks molecular nitrogen in presence of a third molecule that results in the formation of N₂O. This subsequently reacts with O atom to form NO, N₂O + O → 2 NO with an activation energy of 97 kJ/mol. This reaction route is overlooked because the total NO formed by this reaction is not significant. However, lean conditions suppress Fenimore NO and low temperatures suppress Zeldovich NO. As high pressures promote this reaction, the formation of NO by this route occurs primarily in lean premixed combustion in high pressure gas turbine engines.

The oxides of nitrogen play a major role in the photochemistry of the troposphere and stratosphere. NO_x catalyzed ozone destruction occurs via the following reactions:

$$\text{NO}+{\text{O}}_3\to {\text{NO}}_2+{\text{O}}_2$$

$${\text{NO}}_2+\text{O}\to \text{NO}+{\text{O}}_2$$

These reactions are largely responsible for the ozone decline in middle to high latitudes from spring to fall [7], [8]. Another adverse effect of NO_x is acid rain, which can perturb the ecosystems and can cause biological death of lakes and rivers. Peroxyacetylene nitrates (PAN) can also be formed from nitric oxide and contribute significantly to global photo-oxidation pollution [9].

Some biological studies have shown NO as an essential messenger, which transmits the necessary information to the white blood cells within the bloodstream to destroy tumor cells and to the neurotransmitters to dilate the blood vessels [9], [10]. However, the biologically active NO is a poisonous product of the in vivo enzyme-catalyzed transformation of the amino acid, arginine. NO diffuses through the alveolar-cells and capillary vessels of the lungs and damages the alveolar structures and their functions throughout the lungs provoking both lung infections and respiratory allergies like bronchitis, pneumonia, etc. [11], [12].

Because of the ecological and health hazards due to the presence of NO_x in the environment, regulations have been proposed to control NO_x emissions. There is a wide variation among countries with respect to both the type and the level of regulation employed. The Gothenburg protocol establishes reductions of four main pollutants to reduce acidification, eutrophication and the effect of ozone. Other than Canada and USA, 29 European countries have signed this protocol and these countries have estimated the critical loads themselves. An overview of the different targets [13] is available.

The ongoing emission standards in Europe in Euro IV while Euro V will be effective from September 2009. The latter regulates that the emission will be less than 0.18 g/km for diesel and 0.06 for petrol driven engines, respectively. Since the year 2000, India is adopting Euro I for four-wheeled light-duty and for heavy-duty vehicles. For 2- and 3-wheelers, Bharat Stage II (Euro II) was applicable from April, 2005 and Stage III (Euro III) standards have come in force from April, 2008 [14]. Table 1 shows the emission standards for different Indian vehicles and from new diesel engines used in generator sets.