Simultaneous oxidation of nitrogen oxides and sulfur dioxide with ozone and hydrogen peroxide

The use of ozone and hydrogen peroxide for the simultaneous oxidation of nitrogen and sulfur oxides was studied in experiments carried out in a stirred cell. It was found that in a gas mixture, containing both nitrogen and sulfur oxides, only the nitrogen oxides are oxidized by ozone. Contrary to earlier results, sulfur dioxide does not disturb the oxidation of nitrogen oxides under dry conditions. The consumption of ozone in the oxidation of nitric oxide was slightly below the stoichiometric level because the ozone was introduced into the reactor in the oxygen flow. When the molar ratio between ozone and nitric oxide was more than 0.4, some of the nitric oxide was oxidized to higher oxides of nitrogen, the final product being a solid mixture of N₂O₅ and (NO)₂S₂O₇. Some nitrosyl sulfuric acid was formed in the aqueous solution of hydrogen peroxide in addition to sulfuric acid under wet conditions. Some white solid was found on the walls of the reactor. This solid is said it the literature to consist of H₂SO₄, HNOSO₄ and (NO)₂S₂O₇.     

CO Oxidation Behavior of Copper and Copper Oxides

Carbon monoxide oxidation activities over Cu, Cu₂O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu₂O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu₂O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu₂O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.     

Simultaneous SO x /NO x removal in a fluidized bed reactor using natural manganese ore

In this experiment, the simultaneous removal of SO₂ and NO from flue gases was investigated through the use of natural manganese ore as a sorbent‐catalyst in a fluidized bed reactor. Selective catalytic reduction behavior was determined as a function of the sulfation degree within the temperature range from 100 °C to 500 °C. The natural manganese ore showed a high activity in the production of nitrogen and water by the reaction of nitric oxide with ammonia and oxygen up to around 200 °C. At higher temperatures, the nitric oxide removal efficiency decreased due to the oxidation of ammonia by oxygen. With the increase of sulfation degree, the temperature at which the maximum selective catalytic reduction of nitric oxide appears gradually increased, however the maximum nitric oxide removal efficiency decreased. Additionally, we investigated the removal efficiency of sulfur dioxide and nitric oxide with reaction time in a batch fluidized bed reactor within a temperature range of 350 °C to 500 °C. As the reaction temperature increased, the adsorption capacity of sulfur dioxide increased, but the nitric oxide removal efficiency decreased. © 2001 Society of Chemical Industry     

Preparation and characterization of nanostructured Ce0.9Cu0.1O2−δ solid solution with high surface area and its application for low temperature CO oxidation

Nanostructured Ce_0.9Cu_0.1O_2−δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N₂. The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H₂-TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H₂-TPR measurement indicated that there were three CuO species in the Ce_0.9Cu_0.1O_2−δ solid solutions: finely dispersed CuO species on the surface of CeO₂, partial Cu²⁺ penetrated into CeO₂ lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.     

Study of the NOχ reaction with reducing gases on Fe/ZrO2 catalyst

The Fe/ZrO₂ catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe²⁺ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NO_χ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures.     

Oxidation of SO2over Supported Metal Oxide Catalysts

A systematic catalytic investigation of the sulfur dioxide oxidation reactivity of several binary (M_xO_y/TiO₂) and ternary (V₂O₅/M_xO_y/TiO₂) supported metal oxide catalysts was conducted. Raman spectroscopy characterization of the supported metal oxide catalysts revealed that the metal oxide components were essentially 100% dispersed as surface metal oxide species. Isolated fourfold coordinated metal oxide surface species are present for most oxides tested at low coverages, whereas at surface coverages approaching monolayer polymerized surface metal oxide species with sixfold coordination are present for some of the oxides. The sulfur dioxide oxidation turnover frequencies (SO₂molecules converted per surface redox site per second) of the binary catalysts were all within an order of magnitude (V₂O₅/TiO₂>Fe₂O₃/TiO₂>Re₂O₇/TiO₂∼CrO₃/TiO₂∼Nb₂O₅/TiO₂>MoO₃/TiO₂∼WO₃/TiO₂). An exception was the K₂O/TiO₂catalyst system, which is inactive for sulfur dioxide oxidation under the chosen reaction conditions. With the exception of K₂O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo, and W) can undergo redox cycles and oxidize sulfur dioxide to sulfur trioxide. The turnover frequency for SO₂oxidation over all of these catalysts is approximately the same at both low and high surface coverages, despite structural differences in the surface metal oxide overlayers. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A comparison of the activities of the ternary catalysts with the corresponding binary catalysts suggests that the surface vanadium oxide and the additive surface oxide redox sites act independently without synergistic interactions: the sum of the individual activities of the binary catalysts quantitatively correspond to the activity of the corresponding ternary catalyst. The V₂O₅/K₂O/TiO₂catalyst showed a dramatic reduction in catalytic activity in comparison to the unpromoted V₂O₅/TiO₂catalyst. The ability of potassium oxide to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic reactivity.     

NH3-Slip Catalysts: Experiments Versus Mechanistic Modelling

The oxidation of ammonia on a Pt/Al₂O₃ coated monolith has been studied under automotive NH₃-slip catalyst conditions. Ammonia conversion as well as the selectivities towards the products N₂, N₂O and NO are well described by a mechanistic model that is based on reaction mechanisms originally developed for NH₃ oxidation in nitric acid production plants.     

NO Adsorption on ZnAl2O4/Al2O3 Powder: A NEXAFS Study at the Nitrogen K Edge

In order to understand the mechanism of the selective catalysis of nitrogen oxide reduction by hydrocarbons on a ZnAl₂O₄/Al₂O₃ catalyst, the NO adsorption step has been studied as a function of the surface state of the catalyst by using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen K edge. The role of oxygen, whose presence is essential for the reaction to occur, is examined. In absence of a preliminary surface oxidation, nitric oxide was found not to be adsorbed on the ZnAl₂O₄/Al₂O₃ surface. After this preliminary treatment, we observed that the nitrogen atom of the NO molecule was linked to a surface oxygen with an adsorption mode parallel or slightly tilted with respect to the catalyst surface. Through these experiments we clearly demonstrate the advantages of soft X-ray experiments in catalysis research even in the case of practical application to real materials.     

NO – oxygen scavenger or reaction intermediate in the decomposition of N2O?

Nitric oxide and nitrogen dioxide were found during the thermal desorption of surface species left on Fe-ferrierites after the decomposition of nitrous oxide. This demonstrates the formation of surface NOx species during N₂O decomposition. Repeated decomposition and subsequent desorption of surface species confirm the active role of surface NOx species. Addition of NO up to a fraction of 0.1 times the amount of N₂O increased the decomposition of nitrous oxide as well as the amount of surface NOx species. The use of nitrous oxide labeled with ¹⁸O demonstrated that the zeolite oxygens participate in the reaction and that the presence of NO enhances this participation.     

Methanol synthesis pathway over Cu/ZrO2 catalysts: a time‐resolved DRIFT 13C‐labelling experiment

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been used to characterize a series of Cu/Ce/Al₂O₃ catalysts. Catalysts were prepared by incipient wetness impregnation using metal nitrate and alkoxide precursors. Catalyst loadings were held constant at 12 wt% CuO and 5.1 wt% CeO₂. Mixed oxide catalysts were prepared by impregnation of cerium first, followed by copper. The information obtained from surface and bulk characterization has been correlated with CO and CH₄ oxidation activity of the catalysts. Cu/Al₂O₃ catalysts prepared using Cu(II) nitrate (CuN) and Cu(II) ethoxide (CuA) precursors consist of a mixture of copper surface phase and crystalline CuO. The CuA catalyst shows higher dispersion, less crystalline CuO phase, and lower oxidation activity for CO and CH₄ than the CuN catalyst. For Cu/Ce/Al₂O₃ catalysts, Ce has little effect on the dispersion and crystallinity of the copper species. However, Cu impregnation decreases the Ce dispersion and increases the amount of crystalline CeO₂ present in the catalysts, particularly in Ce modified alumina prepared using cerium alkoxide precursor (CeA). Cerium addition dramatically increases the CO oxidation activity, however, it has little effect on CH₄ oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of 18O Isotopic Exchange After and During the Interaction of N2O, NO, and NO2 with Fe Ions in Ferrierites

Nitrous oxide deposits its oxygens on Fe-ferrierites at 200–250 °C in contrast with nitric oxide and nitrogen dioxide. This oxygen is readily exchangeable for ¹⁸O₂ at room temperature and the reaction proceeds via a single-step exchange mechanism. All three nitrogen oxides in a mixture with ¹⁸O labeled dioxygen undergo isotopic exchange (IE) at 200–250 °C, N₂O via the same single-step mechanism, while NO and NO₂ react via a multiple-step mechanism. Zeolitic oxygens participate in IE above 250 °C during temperature-programmed desorption of surface species formed in the reaction of nitrogen oxides with ¹⁸O₂.     

Performance of CuO/Al2O3 Catalysts Promoted by SnO2 and ZrO2 in the Selective Oxidation of Carbon Monoxide

The effect of added SnO₂ and ZrO₂ to CuO/Al₂O₃ catalysts was investigated with reference to the oxygen spillover phenomena in the selective oxidation of carbon monoxide. The TPR and TPO analyses indicated that SnO₂ and ZrO₂ addition caused oxygen migration and induced the formation of high concentrations of active oxygen species on the SnO₂ and ZrO₂ surface. The catalytic activities of SnO₂ and ZrO₂ supported CuO/Al₂O₃ catalysts were superior to that for CuO/Al₂O₃ catalysts in the selective oxidation of carbon monoxide. Oxygen, when absorbed to the SnO₂ and ZrO₂ surface can spill over to the CuO phase and easily react with carbon monoxide. Consequently, the addition of SnO₂ and ZrO₂ led to significantly improved activities. This can be attributed to the enhanced migration of oxygen to the catalyst surface.     

Mechanism of electrochemical oxidation of ammonia

The electrochemical oxidation of ammonia has been studied in the most detail in alkaline solution at platinized platinum. Almost all work supports the essence of a mechanism first proposed by Gerischer and Mauerer (1970) [19], in which elemental nitrogen is formed at mildly positive potentials with near quantitative current efficiency through dimerization of partly dehydrogenated ammonia molecules NH_x(ads). The major intermediate, NH₂(ads), is formed at Pt(1 0 0) domains on the metal surface, where it dimerizes to hydrazine, and rapidly oxidizes to N₂. At somewhat more positive potentials, the formation of adsorbed nitrogen atoms poisons the anode, and nitrogen evolution ceases. At potentials where water is oxidized, the Pt anode is modified by a surface oxide; under these conditions, nitrogen evolution is accompanied by nitrogen oxides and oxyanions. Similar mechanisms are most probably followed on other noble metals and their alloys, although there is less experimental information. In the past decade there has been preliminary study of other anode materials, such as Ni/Ni(OH)₂, Ti/IrO₂, and boron-doped diamond, with a view to finding inexpensive and long-lasting anodes for ammonia oxidation, but so far, little is known about the mechanism of oxidation at these materials.     

In situ characterization of interaction of ammonia with carbon surface in oxygen atmosphere

The adsorption and oxidation of ammonia over carbons differing in the chemical structure of surface functional groups have been investigated by FTIR spectroscopy. The reactions of NH₃ with carbons have been studied both in the presence and in the absence of oxygen. As a result of NH₃ chemisorption, in addition to ammonium salts, there are formed surface amide and imide structures. At the higher temperature surface isocyanate species are formed. Thermal stabilities of surface structures, formed as a result of NH₃ chemisorption have been determined by means of FTIR spectroscopy. The activity and selectivity of carbons for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies at 295-623 K. Carbon catalysts are very active for NH₃ oxidation. Nitrogen is generally the predominant product of ammonia oxidation. The selectivity to N₂, N₂O and NO is determined by the surface oxygen coverage and reaction temperature. The data obtained indicate that the N₂ is formed via selective catalytic reduction (SCR) between NH_x surface species and NO formed from NH₄⁺ oxidation. This implies that ammonia is activated in the form of NH₄⁺ species for both SCR and SCO processes.     

Total oxidation of propane over Cu-Mn mixed oxide catalysts prepared by co-precipitation method

The catalytic activity of Cu-Mn mixed oxides with varying Cu/Mn ratios prepared by co-precipitation method was examined for the total oxidation of propane. The nature and phase of the metal oxide species formed were characterized by various methods such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (TPR) as well as BET surface area measurement. The co-precipitation method provides highly interdispersed copper and manganese metallic elements forming Cu-Mn mixed oxide of spinel structure (Cu_1.5 Mn_1.5O₄). Besides the spinel-type Cu-Mn mixed oxide, CuO or Mn₂O₃ phases could be formed depending on the Cu/Mn molar ratio of their precursors. The catalytic activity of Cu-Mn mixed oxide catalyst for propane oxidation was much higher than those of single metal oxides of CuO and Mn₂O₃. The higher catalytic activity likely originates from a synergic effect of spinel-type Cu-Mn mixed oxide and CuO. The easier reducibility and BET surface area seems to be partially responsible for the high activity of Cu-Mn mixed oxide for total oxidation of propane.     

High-temperature oxidation of aluminium in various gases

The oxidation of high-purity aluminium sheet in dry oxygen, moist oxygen, carbon dioxide and carbon monoxide (at total pressure 1.333 × 10³ Nm^?2) was studied in the range 673–923°K, using a vacuum microbalance to follow weight gains. ¹⁴CO₂ and ¹⁴CO were used to elucidate the mechanism of the oxidation in these gases and to estimate the extent of carbon deposition in the oxide layer. The rate of oxidation in moist oxygen was similar to that in dry oxygen, the principle reaction being 2Al + 3H₂O ← Al₂O₃ + 3H₂. It is suggested that there are three steps in the reaction in CO₂, viz. 2Al+3CO₂ ← Al₂O₃ + 3CO, followed by 2Al + 3CO ← Al₂O₃ + 3C, and about 10% of the deposited carbon reacting further by 4Al + 3C ← Al₄C₃. Only the last two reactions are operative in carbon monoxide. The Arrhenius plots show a distinct break in the region 773–823°K for both carbon monoxide and carbon dioxide, but not for dry or moist oxygen. This is tentatively explained by a change in the rate-determining process from diffusion via grain boundaries or cracks in the oxide, to lattice diffusion. It is suggested that carbon may become mobile in the oxide film between 773 and 823°K and may tend to congregate in the grain boundaries and cracks. The oxide film remained protective throughout the duration of the experiments in all the gases.     

Absorption of sulfur dioxide and nitric oxide by some amines and N-cyclohexyl-2-pyrrolidone

In searching for solvents that are able to absorb sulfur dioxide and nitric oxide, solvents triethylenetetramine, diethylenetriamine, N-cyclohexyl-2-pyrrolidone and N, N, N', N'-tetraethylethylenediamine have all been tested. It was found that triethylenetetramine and N, N, N', N'-tetraethylethylenediamine absorb nitric oxide, while this gas is not absorbed by N-cyclohexyl-2-pyrrolidone and diethylenetriamine. Sulfur dioxide, besides being absorbed by triethylenetetramine, is also absorbed by diethylenetriamine and N-cyclohexyl-2-pyrrolidone. The Henry's law constants and activity coefficients at infinite dilution of the gases in the absorbing solvents were determined by gas-liquid chromatography.     

稀土在我国化肥催化剂中的应用

添加稀土氧化物的转化催化剂和甲烷化催化剂是我国化肥催化剂的特色。文中概述了稀土在烃类水蒸气转化催化剂、甲烷化催化剂、中变催化剂、宽变催化剂、氨合成催化剂、二氧化硫氧化催化剂、氨氧化制硝酸催化剂及甲醇催化剂等方面的应用。     

Reduction of nitrogen oxides by ozonization-catalysis hybrid process

Treatment of nitrogen oxides (NO_x) by using a hybrid process consisting of ozonization and catalysis was investigated. The ozonization method may be an alternative for the oxidation of NO to NO₂. It was found that nitric oxide (NO) was easily oxidized to nitrogen dioxide (NO₂) in the ozonization chamber without using any hydrocarbon additive. In a temperature range of 443 to 503 K, the decomposition of ozone into molecular oxygen was not significant, and one mole of ozone approximately reacted with one mole of NO. A kinetic study revealed that the oxidation of NO to NO₂ by ozone was very fast, almost completed in a few tens of milliseconds. When the amount of ozone added was less than stoichiometric ratio with respect to the initial concentration of NO, negligible NO₃ and N₂O₅ were formed. The oxidation of a part of NO to NO₂ in the ozonization chamber enhanced the selective reduction of NO_x to N₂ by a catalyst (V₂O₅/TiO₂), indicating that the mixture of NO and NO₂ reacts faster than NO.