Removal of Nitrogen Oxides

Freshly prepared Pd-Ru and Pd-Ni on a monolithic support converted all the NO (lOOOp.p.m.) with less than 5% ammonia formation in 0.4% O2 (1% CO, 250p.p.m. CaHg) at 753 and 873 K, respectively. As a further example of the metal-support interactions discussed in Section 6, the well known formation of nickel aluminate at high temperatures (in part) caused substantial deactivation of Ni-Pd in real exhaust, whereas Ni-Al204 was a more stable non-interacting support allowing a better performance to be maintained. After lOOh under net reducing conditions there was still a 50% loss of Ru from Pd-Ru catalysts. 

Rhodium-alumina catalysts with only 0.002 wt % Rh had a good activity for converting NO to nitrogen using simulated exhaust but inadequate oxidation activity. The latter situation can be improved by the addition of Pt or Pd, which have a well known ability for CO and hydrocarbon oxidation. However, more ammonia was formed under reducing conditions using R-Rh or Pd-Rh and also the delicate balance of reactions (CO + NO, CO + H2, etc.), which determines NO removal on the lean-side of the stoicheiometric point (i.e., excess of oxygen), was upset. Deposition of Pt 

It also seems that if Pt and Pd are impregnated on the same support, the catalyst poisons almost as fast as palladium, whereas Pt itself appears to retain a residual oxidation activity suggesting that alloy formation occurs with Pd at the surface. However, Pd appeared to resist sintering better in this application and therefore impregnation of the two metals into separate layers of the support was advantageous. For example, both the steady state and light-off performance were improved by impregnating the outer shell with Pt and the inner shell with Pd. 

---

Phthalocyanines are excellent lubricants at temperatures of 149—343°C (191). Combinations with other lubricants, like grease, molybdenum, or tungsten sulfides, have found appHcations in the automotive industry or professional drilling equipment (192—195). Further uses include indicators for iron(Il), molybdenum(V), and uranium(IV) (196) or redox reactions (197), medical appHcations like hemoglobin replacements (198) or sterilisation indicators (199), or uses like in gas filters for the removal of nitrogen oxides from cigarette smoke (200). 

A process development known as NOXSO (DuPont) (165,166) uses sodium to purify power plant combustion flue gas for removal of nitrogen oxide, NO, and sulfur, SO compounds. This technology reHes on sodium metal generated in situ via thermal reduction of sodium compound-coated media contained within a flue-gas purification device, and subsequent flue-gas component reactions with sodium. The process also includes downstream separation and regeneration of spent media for recoating and circulation back to the gas purification device. A full-scale commercial demonstration project was under constmction in 1995. 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

Nitric acid, removal of nitrogen oxides from, 43, 84... 

The presence of PSCs also leads to the removal of nitrogen oxides (NO and NO2) from the gas phase. As long as there are significant amounts of NO2 it will react with chlorine monoxide (CIO) to produce chlorine nitrate (reaction 11). This species subsequently reacts with HQ on PSC surfaces to produce nitric acid (reaction 13), which remains in the condensed phase. Also, nitric acid directly condenses with water to form nitric acid trihydrate particles, hence it is not available to regenerate NO2 by photochemical processes, as it does when it is in the gas phase. 

Bioprocesses for the removal of nitrogen oxides from polluted air are an interesting alternative [58], but current reaction rates are still too low for large-scale applications. Advanced biological processes for the removal of NO from flue gases are based on the catalytic activity of either eukaryotes or prokaryotes, e.g. nitrification, denitrification, the use of microalgae and a combined physicochemical and biological process (BioDeNO ). 

Bogner, W., Kramer, M., Krutzsch, B. et al. (1995) Removal of nitrogen oxides from the exhaust of a lean-tune gasoline engine. Appl. Catal. B Environ., 7, 153. 

Zhao et al. [74] developed a kinetic model to analyze the removal of nitrogen oxides in a pulsed corona discharge in NO/N2 mixtures. They considered reactions of NO and N02 with N and O atoms and with excited N2(A3S) molecules. 

Another computational model for the removal of nitrogen oxides in a pulsed dielectric barrier discharge was developed by Gentile and Kushner [75] for gas mixtures containing N2/02/H20 (85 5 10) and 500ppm NO. The results show that NO concentration decreases relatively fast in time, whereas the densities of the reaction products (HNOz,... 

Mok, Y.S. and Huh, Y.J. (2005) Simultaneous removal of nitrogen oxides and particulate matters from diesel engine exhaust using dielectric barrier discharge and catalysis hybrid system, Plasma Chem. Plasma Process. 25, 625-39. 

Peng, X., Lin, H., Huang, Z. et al. (2006) Effect of catalysis on plasma assisted catalytic removal of nitrogen oxides and soot, Chem. Eng. Technol. 29, 1262-6. 

The present procedure is based on the method published by Fu, Birnbaum and Greenstein. The yields are increased by the very slow addition of an aqueous solution of sodium nitrite to the reaction mixture as well as by a nwdified work-up procedure, i.e, careful removal of nitrogen oxides and the final decomposition of their adducts with carboxylic acids by buffering with sodium carbonate. 

The separatory funnel is removed after the addition is completed to facilitate the removal of nitrogen oxides. 

An increasing demand is expected for the removal of nitrogen oxides from waste gases from stationary and instationary diesel engines for e.g. emergency plants, ships and trucks. Another application is the combined removal of nitrogen oxides and dioxines from waste gases of refuse disposal plants [2.65],... 

Measurements have been carried out for an industrial NOx absorption process [74], The absorption plant (Fig. 9.8) represents a sequence of four units used for the removal of nitrogen oxides from the waste gas of an adipin acid factory. Each unit is separated into two sections, thus, there are altogether eight columns of 2.2 m diameter and 7 m height each, which are connected countercurrently. To enhance the mass transport in the columns, 35 mm INTALOX ceramic saddles with a packed bed height of 3.2 m are used. The plant is operated at atmospheric pressure. 

A gold catalyst with low temperature activity towards carbon monoxide and hydrocarbon oxidation could be suitable to combat cold start-up emission problems and removal of nitrogen oxides from lean-burn gasoline... 

Figure 11. Reaction chamber for removal of nitrogen oxides from power station flue gases [23].

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

An increasing demand is expected for the removal of nitrogen oxides from waste gases from stationary and mobile diesel engines. 

Concern over the role of nitrogen oxides in air pollution. This has stimulated the development of catalysts for the removal of nitrogen oxides from exhaust gases. Nitric oxide is thermodynamically unstable with respect to decomposition to its elements at 25°C and 1 atm ... 

---