Abstract

Carbon-coated monoliths with different degrees of mesoporosity have been prepared. This has been accomplished by coating cordierite monoliths with a blend of two polymers, viz. Furan resin and polyethylene glycol (PEG), in different proportions. Upon carbonization at 973 K the former yields a carbon coating while the latter pyrolizes generating mesoporosity. Additionally the carbon-coated monoliths were activated with CO₂ to generate microporosity. Vanadium was impregnated in these carbon-coated monoliths by equilibrium adsorption using ammonium metavanadate as precursor and they were tested in the SCR of NO at low temperature. By increasing the amount of PEG, the mesopore volume increases in the range of narrow mesoporosity (2–5 nm). It was found that the more mesopore volume, the more oxygenated surface groups are formed. This turned out to be crucial for the deposition of vanadium in a dispersed fashion and also for the activity in the SCR of NO. On the contrary, the narrow microporosity (<0.7 nm) does not contribute to enhance the dispersion of the catalyst. The kinetic rate constants of the monolithic catalyst prepared are in the range of the most active catalyst reported in the literature for the SCR of NO at low temperature.

Abstract

A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NO_x with ammonia in the presence of excess O₂. These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150 °C at a space velocity of 42,000 h⁻¹. As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N₂ rather than N₂O was found in the product when the temperature was below 150 °C. At higher temperatures, trace amounts of N₂O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH₃ on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N₂ and H₂O. Possible intermediates are proposed and all the intermediates could transform into NH₂NO, which could further react to produce N₂ and H₂O.

Abstract

A series of catalysts of manganese oxide supported on TiO₂ and iron–manganese oxide supported on TiO₂ with different amounts of manganese and iron were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. It was found that the addition of iron oxide not only increased the NO conversion and N₂ selectivity but also increased the resistance to H₂O and SO₂. The Fe–Mn/TiO₂ catalysts yield high activities and high N₂ product selectivity. The N₂O product selectivity increased with the amount of MnO_x as well as temperature. Crystalline phase of MnO_x was present at ≥15% Mn on TiO₂, and the amount increased with Mn content. In addition, SO₂ and H₂O decreased the activities only slightly, while such effect was reversible. The Fe–Mn/TiO₂ with Mn/Fe=1 showed the highest activity. The results showed that this catalyst yielded nearly 100% NO conversion at 120 °C at a space velocity of 15,000 h⁻¹. The effect of oxygen was also studied. Reversible transient behaviors similar to that on other oxide catalysts, including vanadia and chromia, were observed for the Fe–Mn/TiO₂ catalyst.

A series of manganese–cerium oxide catalysts were prepared by different methods and used for low-temperature selective catalytic reduction (SCR) of NO_x with ammonia in the presence of excess O₂. The Mn–Ce–O_x catalysts showed high activities. The experimental results showed that the best Mn–Ce mixed-oxide catalyst yielded nearly 100% NO conversion at 120 °C at a high space velocity of 42,000 h⁻¹. As the manganese content was increased from zero to 30% (i.e., the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.3. The effect of the calcination temperature was also investigated and the optimum calcination temperature was 650 °C. These catalysts are substantially more active than all other catalysts reported in the literature. SO₂ and H₂O (at high concentrations) have slight effects on the SCR activity. From a steady-state kinetics study, it was found that the low-temperature SCR reaction was zero order with respect to NH₃ and first order with respect to NO. Compared with the other catalysts reported for low-temperature SCR of NO with ammonia, based on the first-order rate constants, the MnO_x–CeO₂ catalyst was several times more active than other catalysts reported in the literature. Only N₂ rather than N₂O was found in the product when the temperature was below 150 °C. At higher temperatures, trace amounts of N₂O were detected.

The catalytic properties of the monolith ZrO₂-TiO₂ supported Mn-based catalysts were studied for NH₃-SCR reaction at low temperature. The catalysts were characterized by specific surface area measurements(BET), X-ray diffraction(XRD), oxygen storage capacity(OSC) , temperature programmed reduction(TPR) and X-ray photoelectron spectroscopy(XPS). The results indicate that, compared to the catalysts supported on TiO₂ ,the catalysts supported on ZrO₂-TiO₂ have higher surface area, more steady structure, more OSC, stronger redox properties as well as higher concentration of surface Mn and have good activity at low temperature and excellent stability at high temperature. After Mn-Fe/ZrO₂-TiO₂ is calcined at 700 ℃, the light-off tempe-rature of the catalyst is 182 ℃ and NO conversion is 78% at a high space velocity of 55000 h^-1, and the catalyst is water-resistant. It shows great potential for practical application.

以锆钛复合氧化物为载体, 制备整体式Mn基催化剂并研究其在低温氨选择性催化还原(NH₃-SCR)氮氧化物反应中的性能; 用BET、XRD、储氧量(OSC)、程序升温还原(TPR)和XPS对催化剂进行表征. 实验结果表明, 与以TiO₂为载体的催化剂相比, 以ZrO₂-TiO₂为载体的催化剂具有较大的比表面积、更稳定的晶相结构和较多的储氧量, 并具有较强的低温氧化性能和较高的表面Mn含量, 表现出良好的低温活性和高温稳定性. 在700 ℃焙烧后, Mn-Fe/ZrO₂-TiO₂在高空速55000 h^-1条件下, 仍具有较好的起燃温度(182 ℃)和NO的转化率(78%), 并具有较强的抗水性能, 表现出很好的应用前景.

A series of TiO₂ supported manganese oxide catalysts were prepared by wet-impregnation method for the low temperature selective catalytic reduction (SCR) of NO with ammonia as a reductant. A combination of various physico-chemical techniques such as N₂ physisorption, O₂ chemisorption, TPR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman were used to characterize the chemical environment of these catalysts. O₂ chemisorption and XRD results suggest that Mn exist in a well-dispersed state at below 16.7 wt.% of Mn on TiO₂ anatase (Hombikat), 7.5 wt.% on TiO₂ rutile (Kemira) and P-25 (80% anatase + 20% rutile), and in microcrystalline phase above these loading levels on respective support materials. These results also reveal that Mn interacts very well with pure anatase phase compared to rutile. XPS results of Mn/TiO₂ anatase (Hombikat) catalysts illustrated the presence of MnO₂ as a major phase (peak at 642.0 eV) along with Mn₂O₃ as the minor phase at lower loadings. The presence of Mn₂O₃ disappears at higher loadings. The characterization results indicated that the manganese oxide exists as an isolated species at very low loadings, highly dispersed state probably as two dimensional monolayer species at intermediate loadings, polymeric or microcrystalline form of manganese oxide at higher (above monolayer capacity) loadings was envisaged. The catalytic performance of various amounts of Mn loaded on different TiO₂ supported catalysts for low temperature SCR reaction at catalyst bed temperature 175 °C under power plant conditions using GHSV = 50,000 h⁻¹ was studied. The catalyst with 16.7 wt.% Mn/TiO₂ anatase (Hombikat) was found to be highly active and selective catalyst for this reaction. The Raman studies acted as complimentary tool to XPS in order to characterize the manganese oxides (MnO, Mn₂O₃, Mn₃O₄, MnO₂). Raman data show that there is a strong interaction between the Mn oxides and the support, which is responsible for the impressive catalytic performance in comparison with other systems we investigated.

In this work, the catalytic nature of Mn loaded sulfated zirconia (SZ) catalysts for the selective catalytic reduction (SCR) of NO with methane was investigated by a combination of reactions and characterizations such as FT-IR spectroscopy, H₂-TPR, UV–vis diffuse reflectance spectroscopy (DRS) and NO-TPD. It was found from the results of reactions and FT-IR spectra that the strong Brønsted and Lewis acid sites in the Mn/SZ catalysts were essential for the SCR of NO with methane. The loading of Mn increased the number of strong Lewis acid sites on the surface of SZ catalyst, which is one reason for its promoting effect. On the other hand, FT-IR spectra, H₂-TPR and UV–vis DRS of the catalysts demonstrated that the presence of the SO₄²⁻ species occupied the terminal OH sites on the surface of ZrO₂ support and thereby restrained the formation of more oxidative and nonstoichiometrically dispersed MnO_x (1.5 < x < 2) phase. Such an effect of SO₄²⁻ suppressed the combustion reaction of CH₄ by O₂ and increased the selectivity towards NO reduction. The NO-TPD showed that the loading of Mn increased the adsorption of NO over SZ catalyst, which is another reason for the promoting effect of Mn.

Oxidation of cyclohexene using aqueous hydrogen peroxide (30%, v/v) over iron–cerium oxide (x%Fe/CeO₂; x = 0, 2, 5, 10 and 20 mol%) prepared by co-precipitation was conducted in this study in order to assess the performance of the catalysts of varying composition, and catalytic activity and product selectivity were found to strongly depend on catalyst composition, acidity and reducibility. The effects of varying the catalyst composition, reaction temperature and solvent were investigated. Cyclohexene epoxide and 1,2-cyclohexanediol were found to be the main reaction products, along with small amounts of other oxygenated products. A high cyclohexene conversion rate of 99 mol% and an epoxide selectivity of 98 mol% were observed over 5%Fe/CeO₂ at 100 °C. The catalysts were characterized using various techniques and measures, including BET surface area, XRD, DRS UV–vis, potentiometric titration for acidity measurement, and XPS. XRD showed the formation of iron–cerium oxide solid solutions with cerianite cubic structures for all catalysts except FC4, and the DRS UV–vis results further indicated the formation of solid solutions. Analysis of Raman spectra revealed the presence of aggregate iron oxide species in case of 20%Fe/CeO₂ catalyst. TPR revealed enhancement of the reducibility of the ceria upon Fe doping, and the potentiometric titration results showed an increase in the number of acidic sites with increasing iron content up to 5% and a decrease thereafter. TPR and XPS analyses revealed the presence of highly dispersed Fe species in the 2%Fe/CeO₂ and 5%Fe/CeO₂ samples, whereas for the 10%Fe/CeO₂ and 20%Fe/CeO₂ catalysts, aggregates of Fe species were observed. Catalytic activity was found to be correlated with the composition and acidic properties of the catalyst.

The Fe/SiO₂ catalysts prepared using water-in-oil (w/o) microemulsions exhibited more than two-fold CO conversions as compared to the impregnated catalysts in CO hydrogenation. The catalysts produced the C₂₊ oxygenates with an extremely high selectivity of >40 C mol%, while the impregnated catalysts produced mainly hydrocarbons. The TPR results showed that Fe particles in the catalysts prepared using w/o microemulsion were difficult to reduce and that a considerable amount of FeO remained in the catalyst even after the reduction with H₂ at 873 K for 10 h. The superior activity of the catalysts for the C₂₊ oxygenates synthesis was attributed to the existence of the stable Fe oxide species under the reaction condition.