Low-Temperature Ammonia Oxidation

In a second example, we examine reactions that relate to the Ostwald oxidation process, where NH3 is converted to NO with high selectivity. The reaction is typically run at high temperatures of around 1100 K over Pt/Rh alloy catalysts. The NO that forms is subsequently converted into nitric acid via a series of consecutive reaction steps. At lower temperatures, ammonia reacts to form Ng and NgO instead. The low-temperature conversion of ammonia to N2 would be much more desirable in that it would lower energy costs and, in addition, replace NO, an atmospheric pollutant, with N2, which is environmentally benign. We will describe here the low-temperature catalytic conversion of ammonia to form N2. For a review of high-temperature oxidation, in which coupling with gas-phase radical chemistry plays an important role, we refer to Ref. [50]. 

we compare three different catalytic systems used to carry out the catalytic oxidation of ammonia gas-phase oxidation over transition-metal heterogeneous catalysts, gas-phase oxidation within zeolites and electrocatalytic oxidation of ammonia. 

In this first section, we focus on heterogeneous transition-metal catalysis. Two different reaction paths can be distinguished in the activation of NH3 (see Fig. 6.21). The first distinguishes the direct path through the dissociation of ammonia and subsequent recombination of Nads atoms. The second reaction path involves the reaction of NH3, NO and O to form Ng and H2O (see also Chapter 5, Section 5.6.10). 

Ammonia adsorbed on the transition-metal surface can react to form different NHa, species. Both the presence of adsorbed oxygen and hydroxyl adsorbed on the metal surface can help promote this reaction (see Chapter 3, Section 3.8). 

The activation of ammonia with oxygen to produce NO is endothermic, whereas the activation of ammonia to form N2 in the gas phase is exothermic. The selectivity towards NO will, therefore, increase as the temperature is increased. However, the activation barrier to form adsorbed NO from adsorbed N and O is comparable to that for the recombination of two nitrogen adatoms to form N2. The only data that can be directly compared are those for the (111) surface, where there is a large enough dataset and the methodology applied is similar. These reactions preferentially occim at step edges. NO strongly adsorbs to the surface, so that at low temperatm-es NO likely does not appear as a product. Only N2 and N2O are formed as products at low temperatm-e. As is seen from Table 6.1, N2O formation readily occurs at low temperatm-e due to the recombination of adsorbed NO. N2O is weakly adsorbed and therefore desorbs once it is formed. 

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The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Because acid cbncentratims are favored by low-temperature absorption, several different cooling methods have also been developed, e.g., by external units of the plate, drum, or cascade type by water curtains outside the tower and also by cooling coils strategically located inside the absorption column. In some plants using low-pressure ammonia oxidation, vaporization of the ammonia is used to prechiQ the absorber feedwater and c.Qoling wEiter. ... 

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

The discovery of chemical N2 fixation under ambient conditions is more compatible with a simple, complementary, low temperature and low pressure system, possibly operated electrochemically and driven by a renewable energy resource (qv), such as solar, wind, or water power, or other off-peak electrical power, located near or in irrigation streams. Such systems might produce and apply ammonia continuously, eg, directly in the rice paddy, or store it as an increasingly concentrated ammoniacal solution for later appHcation. In fact, the Birkeland-Eyde process of N2 oxidation in an electric arc has been... 

At low temperatures the SCR reactions dominate and nitrogen oxide conversion increases with increasing temperature. But as temperature increases, the ammonia oxidation reactions become relatively more important. As the temperature increases further, the destmction of ammonia and generation of nitrogen oxides by the oxidation reactions causes overall nitrogen oxide conversion to reach a plateau then decreases with increasing temperatures. Examples are shown in Figure 7 (44). 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

The most popular SCR catalyst formulations are those that were developed in Japan in the late 1970s comprised of base metal oxides such as vanadium pentoxide [1314-62-1J, V20, supported on titanium dioxide [13463-67-7] Ti02 (1). As for low temperature catalysts, NO conversion rises with increasing temperatures to a plateau and then falls as ammonia oxidation begins to dominate the SCR reaction. However, peak conversion occurs in the temperature range between 300 and 450°C, and the fah-off in NO conversion is more gradual than for low temperature catalysis (44). 

Increasing the temperature increases the reaction rate, but decreases the equilibrium (K 500°C = 0.08). According to LeChatlier s principle, the equilibrium is favored at high pressures and at lower temperatures. Much of Haber s research was to find a catalyst that favored the formation of ammonia at a reasonable rate at lower temperatures. Iron oxide promoted with other oxides such as potassium and aluminum oxides is currently used to produce ammonia in good yield at relatively low temperatures. 

The decomposition described in Equation (146) takes place at relatively low temperatures hence, thermal treatment at a relatively slow temperature rate can be sufficient in order to significantly reduce fluorine levels in the final oxides. Nevertheless, treatment at a high temperature rate can lead to another mechanism of ammonium fluoride decomposition yielding gaseous ammonia and molten ammonium hydrofluoride according to the following scheme ... 

It has been reported that titanium supported vanadium catalyst is active for ammonia oxidation at temperatures above 523 K [2,3]. Also, supported vanadium oxides are known to be efficient catalyst for the catalytic reduction of nitrogen oxides (NO ) in the presence of ammonia [4]. This work investigates the nanostructured vanadia/Ti02 for low temperature catalytic remediation of ammonia in air. 

Closed symbols in Fig. 1 show the effect of reaction temperature on ammonia oxidation over CuO by heating with a conventional electric furnace. The reaction started at about 400 K and the conversion of NH3 became 1 at temperatures higher than 500 K. Fig. 1 also indicates that selectivity to N2 was high at low temperatures but it decreased as the temperature increased. Both N2O and NO increased instead of N2, except at 623 K, at which N2O decreases. NO was detected above 583 K, and it monotonously increased by the temperature. High reaction temperature seems to tend deeper oxidation to NOx. Considering that oxidation of N2 to N2O and NO is difficult in the tested temperature range. 

TS-1 is a material that perfectly fits the definition of single-site catalyst discussed in the previous Section. It is an active and selective catalyst in a number of low-temperature oxidation reactions with aqueous H2O2 as the oxidant. Such reactions include phenol hydroxylation [9,17], olefin epoxida-tion [9,10,14,17,40], alkane oxidation [11,17,20], oxidation of ammonia to hydroxylamine [14,17,18], cyclohexanone ammoximation [8,17,18,41], conversion of secondary amines to dialkylhydroxylamines [8,17], and conversion of secondary alcohols to ketones [9,17], (see Fig. 1). Few oxidation reactions with ozone and oxygen as oxidants have been investigated. 

Investigations with the modular multi-channel [28,98] and silicon chip [19, 56-62] micro reactors demonstrate that by exact temperature control the oxidation of ammonia can be run with increased and deliberately steered selectivity. A major application is provided by carrying out former high-temperature reactions in the low-temperature regime. In the case of ammonia oxidation in the chip micro reactor, the yield of the value product NO was actually lower in that regime. In the case of the multi-plate-stack micro reactor, higher yields of the value product NO2 were achieved. 

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