Low-temperature catalytic reduction of nitrogen oxides with ammonia over supported manganese oxide catalysts

MnO_x supported on γ-Al₂O₃, TiO₂, Y-ZrO₂ and SiO₂ was prepared by an impregnation and a deposition-precipitation method, and their catalytic activities for the low-temperature selective catalytic reduction (SCR) of NO_x with NH₃3 in the presence of excess O₂ were examined. The catalytic activity of the catalysts prepared by a deposition-precipitation method was higher than that of catalysts prepared by an impregnation method. The activity follows in the order: MnO_x/TiO₂ ≈ MnO_x/γ-Al₂O₃ > MnO_x/SiO₂ > MnO_x/Y-ZrO₂. Supported MnO_x catalysts prepared by a deposition-precipitation method appeared to have an amorphous manganese oxide phase and those prepared by an impregnation method exhibited a crystalline MnO₂ phase, respectively. The addition of SO₂ with H₂O in the feeding gas slightly deactivates the SCR activity of MnO_x/TiO₂ catalysts.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

Selective oxidation of H2S to elemental sulfur over chromium oxide catalysts

CrO_x and CrO_x supported on SiO₂ have been found to be active for the selective oxidation of hydrogen sulfide to elemental sulfur. The catalysts show maximum sulfur yield at a stoichiometric ratio of O₂/H₂S, 0.5. Amorphous Cr₂O₃ exhibits higher yield of sulfur and has stronger resistance against water than supported Cr/SiO₂, especially at low temperatures. At high temperatures above 300°C, the sulfur yield over the supported catalyst becomes similar to amorphous Cr₂O₃ because the Claus reaction occurring on the silica support removes SO₂ to increase the sulfur yield. Active sites are the amorphous monochromate species that can be detected as a strong temperature programmed reduction (TPR) peak at 470°C. Catalytic activity can be correlated with the amount of labile lattice oxygen and the strength of Cr–O bonding. The reaction proceeds via the redox mechanism with participation of lattice oxygen.     

Catalytic oxidation of CO over Ag-Co/alumina catalysts

Low-loading silver cobalt catalysts on alumina for the catalytic oxidation of carbon monoxide were synthesized following different preparation methods: the single-step sol-gel method and the impregnation method. A catalyst synthesized by the single-step sol-gel method gave high surface area values. The activities of alumina-supported silver-cobalt catalysts were studied to obtain the effects of the calcination temperature, metal loading, and preparation methods on CO oxidation at low temperature. The catalysts prepared by impregnation metals on the alumina synthesized by the sol-gel method gave the best activity for the CO oxidation. Catalytic activity conversion of (5 wt% Ag-5 wt% Co)/Al 2 O 3 catalysts prepared by the sol-gel-impregnation method reached 100% at 200°C. Increasing silver loading over the catalysts gave rise to more active catalysts. The sol-gel catalysts had poor activity at low temperature.     

The influence of surface structure on the catalytic activity of alumina supported copper oxide catalysts. Oxidation of carbon monoxide and methane

X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electron spin resonance (ESR) have been used to characterize a series of Cu/Al₂O₃ catalysts. The information obtained from surface and bulk characterization has been correlated with CO and CH₄ oxidation activity of the catalysts. For catalysts with Cu/Al atomic ratios 0.051, XPS data indicated that most of the Cu was present as a dispersed surface phase. ESR results showed that the ratio of isolated/interacting copper surface phase decreased with increasing Cu content. For catalysts with Cu/Al atomic ratio 0.077, large CuO crystallites were detected by XRD. XPS results indicated that Cu dispersion decreased with increasing Cu content. The turn over frequency (TOF) for CO oxidation increased with increasing Cu content. This has been attributed to an increase in the amount of crystalline CuO present in the catalysts. The TOF for CH₄ oxidation decreased with increasing Cu content up to Cu/Al = 0.051. This was paralleled by a significant decrease in the relative Cu ESR signal measured for the catalysts. We propose that the isolated Cu surface phase is more active for CH₄ oxidation than the interacting copper surface phase or crystalline CuO. CH₄ oxidation activities were similar for the catalysts with Cu/Al atomic ratio 0.077.     

Kinetics of the catalytic oxidation of carbon monoxide over oxide catalysts

The kinetics of the oxidation of carbon monoxide over five commercially available oxide catalysts have been determined at CO concentrations of 4–15 mol% and initial temperatures up to 170°C. The reaction showed a fractional dependence on the concentration of CO and was inhibited by CO₂. From an analysis of the data a Langmuir-Hinshelwood reaction scheme is proposed.     

Kinetics of the catalytic oxidation of carbon monoxide over oxide catalysts

The kinetics of the oxidation of carbon monoxide over five commercially available oxide catalysts have been determined at CO concentrations of 4–15 mol% and initial temperatures up to 170°C. The reaction showed a fractional dependence on the concentration of CO and was inhibited by CO₂. From an analysis of the data a Langmuir-Hinshelwood reaction scheme is proposed.     

The role of silver species on Ag/Al2O3 catalysts for the selective catalytic oxidation of ammonia to nitrogen

The role of Ag species on Ag/Al₂O₃ catalyst for the selective catalytic oxidation (SCO) of NH₃ to N₂ was studied using 10 wt% Ag/Al₂O₃ catalysts prepared with impregnation, incipient wetness impregnation and sol–gel methods. The catalyst characterization was preformed using N₂ adsorption–desorption, UV/Vis, TEM and XRD. O₂-chemisorption and H₂–O₂ titration were measured to confirm the metal dispersion on the catalyst. The Ag species state and Ag particle size have significant influence on the Ag/Al₂O₃ activity and N₂ selectivity of the SCO of NH₃ at low temperature. Ag⁰ is proposed to be an active species on the H₂ pretreated catalyst at low temperature (<140 °C). It is evident that well-dispersed and small particle Ag⁰ enhances catalytic activity at low temperature, whereas large particle Ag⁰ is relate to a high N₂ selectivity. In contrast, Ag⁺ could also be the active species at temperatures above 140 °C.     

贵金属催化剂催化氧化处理氨氮废水的研究进展

概述了氨氮废水处理的必要性及传统处理技术存在的缺陷,重点综述了贵金属催化剂在氨氮废水催化氧化处理中的应用研究进展,并针对贵金属催化剂在研究应用过程中存在的不足之处,对贵金属催化剂材料的开发及其在工艺应用上的改进等提出了展望。     

Total oxidation of propene and propane over gold–copper oxide on alumina catalysts: Comparison with Pt/Al2O3

Oxidation of propene and propane to CO₂ and H₂O has been studied over Au/Al₂O₃ and two different Au/CuO/Al₂O₃ (4 wt.% Au and 7.4 wt.% Au) catalysts and compared with the catalytic behaviour of Au/Co₃O₄/Al₂O₃ (4.1 wt.% Au) and Pt/Al₂O₃ (4.8 wt.% Pt) catalysts. The various characterization techniques employed (XRD, HRTEM, TPR and DR-UV–vis) revealed the presence of metallic gold, along with a highly dispersed CuO (6 wt.% CuO), or more crystalline CuO phase (12 wt.% CuO).

A higher CuO loading does not significantly influence the catalytic performance of the catalyst in propene oxidation, the gold loading appears to be more important. Moreover, it was found that 7.4Au/CuO/Al₂O₃ is almost as active as Pt/Al₂O₃, whereas Au/Co₃O₄/Al₂O₃ performs less than any of the CuO-containing gold-based catalysts.

The light-off temperature for C₃H₈ oxidation is significantly higher than for C₃H₆. For this reaction the particle size effect appears to prevail over the effect of gold loading. The most active catalysts are 4Au/CuO/Al₂O₃ (gold particles less than 3 nm) and 4Au/Co₃O₄/Al₂O₃ (gold particles less than 5 nm).     

Fischer-Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆

The unpromoted and promoted Fischer-Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer-Emmett-Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al₂O₃ catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.     

Selective catalytic reduction of nitrogen oxide by hydrocarbons over mordenite-type zeolite catalysts

The reduction of NO by hydrocarbons such as C₂H₄, C₂H₆, C₃H₆, and C₃H₈ has been investigated over mordenite-type zeolite catalysts including HM, CuHM, NZA (natural zeolite), and CuNZA prepared by an ion-exchange method in a continuous flow fixed-bed reactor. NO conversion over CuNZA catalyst reaches about 94% with 2000 ppm of C₃H₆ at 500°C. As reductants, alkenes seem to exhibit a higher performance for NO conversion than alkanes regardless of the catalysts. No deterioration of the catalytic activity due to carbonaceous deposits for CuNZA was observed above 400°C even after 30 h of on-stream time, but SO₂ in the feed gas stream causes a severe poisoning of the CuNZA catalyst. The effect of H₂O on NO conversion was significant regardless of the catalysts and the reductants employed in this study. However, CuNZA catalyst shows a unique water tolerance with C₃H₆. The reaction path of NO to N₂ is the most important factor for high performance of this catalytic system. NO is directly reduced by a reaction intermediate, C_n′H_m′(O) formed from hydrocarbon and O₂, N₂O is another reaction intermediate which can be easily removed by C_n′H_m′(O).     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over Bi-V-Sb-O catalysts

We investigated the selective oxidation of hydrogen sulfide to elemental sulfur and ammonium thiosulfate by using Bi₄V_2-xSb_xO_11-y catalysts. The catalysts were prepared by the calcination of a homogeneous mixture of Bi₂O₃, V₂O₅, and Sb₂O₃ obtained by ball-milling adequate amounts of the three oxides. The main phases detected by XRD analysis were Bi₄V₂O₁₁, Bi_1.33V₂O₆, BiSbO₄ and BiVO₄. They showed good H₂S conversion with less than 2% of SO₂ selectivity with a feed composition of H₂S/O₂/NH₃/H₂O/He=5/2.5/5/60/27.5 and GHSV=12,000 h^-1 in the temperature ranges of 220–260 ‡C. The highest H₂S conversion was obtained for x=0.2 in Bi₄V_2-xSb_xO_11-y catalyst. TPR/TPO results showed that this catalyst had the highest amount of oxygen consumption. XPS analysis before and after reaction confirmed the least reduction of vanadium oxide phase for this catalyst during the reaction. It means that the catalyst with x=0.2 had the highest reoxidation capacity among the Bi₄V_2-xSb_xO_11-y catalysts.     

Selective reduction of nitric oxide by methanol and dimethyl ether over promoted alumina catalysts in excess oxygen

Selective catalytic reduction (SCR) activity for NO conversion to N₂ over γ-alumina, vanadia/alumina and molybdena/alumina catalysts has been investigated with methanol (MeOH) and dimethyl ether (DME) as reductants under lean conditions. Molybdena/alumina catalysts showed high efficiency for NO reduction with either reductant, especially at low temperature, which may involve surface formyl produced by oxidative dehydrogenation. Sulphated γ-alumina remains active for NO reduction with MeOH, while sulphated 5 wt.% MoO₃/Al₂O₃ remains active with both MeOH and DME over a broad temperature range.     

硫化铜精矿催化氧化氨浸工艺研究

提出了硫化铜精矿催化氧化氨浸提铜方法。在ＮＨ＋４ 浓度为３００ｇ／Ｌ、氧化剂ＳＮ２２ 浓度为６０ｋｇ／ｔ、催化剂ＡＮ３１ 用量为０－２ｋｇ／ｔ、液固比为５ 的条件下,常温搅拌４ｈ,铜的浸出率可达８０－２５ ％ 。     

Wet air oxidation of p-coumaric acid over promoted ceria catalysts

The catalytic wet air oxidation (WAO) of p-coumaric acid (PCA) has been investigated over Fe- and Zn-promoted ceria catalysts. The catalysts have been prepared by the coprecipitation method and have been characterized by X-ray diffraction (XRD), BET surface area, SEM–EDX and temperature programmed reduction (TPR). The oxidation reaction was carried out in a batch reactor under an air pressure of 2 MPa and in the temperature range 353–403 K. Fe-CeO₂ catalysts, with 20–50 mol% of iron, were found more effective than the unpromoted and Zn-promoted ceria catalysts. On the basis of characterization data, it has been suggested that the higher activity of the Fe-promoted catalysts is related to the modification of the structural and redox properties of the ceria oxide catalyst on addition of iron.     

Catalytic oxidation of toluene over copper and manganese based catalysts: Effect of water vapor

Copper and manganese based catalysts with different supports were prepared by impregnation method for toluene oxidation in the presence of water vapor. Their catalytic activity was tested in the absence and presence of water vapor. The results showed that the activity of catalysts CuMn(y)O_x/γ-Al₂O₃ was higher than that of catalysts CuO_x/γ-Al₂O₃ and MnO_x/γ-Al₂O₃. The presence of water vapor had a negative effect on catalytic activity due to the competition of water molecules with toluene molecules for adsorption on surface active sites. The durability to water vapor followed the order: CuMn(1)O_x/Cordierite > CuMn(1)O_x/TiO₂ > CuMn(1)O_x/γ-Al₂O₃.     

Simultaneous isomerization of n-heptane and saturation of benzene over Pt/Beta catalysts: The influence of zeolite crystal size on product selectivity and sulfur resistance

In this work we show that decreasing the zeolite crystal size has a clear benefit on product selectivity and sulfur resistance of Pt/Beta catalysts during the simultaneous hydroisomerization of n-heptane and hydrogenation of benzene. The higher isomerization selectivity of the catalyst prepared from a nanocrystalline Beta zeolite can be ascribed to a faster diffusion of the iso-C₇ products through the small crystallites preventing cracking reactions to occur, while the improved sulfur resistance can be related to a better dispersion of Pt owing to its higher surface area and mesoporosity.     

超声催化氧化废水中氨氮的实验研究

实验研究了各种条件下超声催化氧化废水中NH3-N的去除效率,并讨论了最优条件组合。实验表明,在超声催化氧化废水中NH3-N时,当pH值为10,温度为60％,反应时间为90min,以Ag2O＋CuO为催化剂的超声催化作用下,NH3-N的去除率可以达到81．5％。     

Partial oxidation of methane and the effect of sulfur on catalytic activity and selectivity

Partial oxidation of methane into syngas was conducted over fresh and sulfided catalysts at a temperature range of 450–750 °C. The temperature dependence of conversion, H₂/CO ratio, and the CO₂ concentration were measured for both fresh and sulfided catalysts. Regardless of metal type, metal loading, support type, and the methods of preparation it appears that all the fresh catalysts were very active and conversions of higher than 70% with H₂/CO ratio of about 2 were observed at 750 °C. Pulse sulfidation appears to be reversible for some of the catalysts but not for all. Under pulse sulfidation conditions, the Rh(0.5%)/Al₂O₃ and NiMg₂O_x-1100 °C (solid solution) catalysts were fully regenerated after reduction with hydrogen. Rh catalyst showed the best overall activity, less carbon deposition, both fresh and when it was exposed to pulses of H₂S. Sulfidation under steady-state conditions, flowing H₂S/Ar mixture over the catalysts, significantly reduce catalyst activity. The catalysts were characterized before and after reaction with H₂S using temperature-programmed oxidation (TPO) and reduction (TPR), X-ray diffraction, and XPS.