The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

The selective catalytic reduction of NO_x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH₄–O₂ reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO_x reduction, while for the NO₂–CH₄–O₂ reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO_x conversion in NO₂–CH₄–O₂ reaction was significantly higher than that in NO–CH₄–O₂ reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO₂]/([NO]+[NO₂]) in the effluent gases in NO–O₂ and NO₂–O₂ reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO_x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO₂, and high activity for NO₂ reduction to N₂ (Pd, Rh); (ii) high activity for NO oxidation to NO₂, and low activity for NO₂ reduction to N₂ (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO_x by methane.     

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH₃–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N₂ adsorption and the H₂–TPR of the catalysts demonstrated that the presence of the SO₄²⁻ species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH₄ by O₂ and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO₄²⁻. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO₂ poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO₂.     

Electrophilic chlorination of methane over superacidic sulfated zirconia

Catalytic chlorination of methane was studied over SO ₄ ^2– /ZrO₂, Pt/SO ₄ ^2– /ZrO₂, and Fe/Mn/SO ₄ ^2– /ZrO₂ solid superacid catalysts. The reactions were carried out in a continuous flow reactor under atmospheric pressure, at temperatures below 240°C, with a gaseous hourly space velocity of 1000 ml/g h and a methane to chlorine ratio of 4 to 1. At 200°C with 30% chlorine converted the selectivity in methyl chloride exceeds 90%. At more elevated temperatures, the selectivity decreases but stays above 80% in methyl chloride at 225°C using the sulfated zirconia catalysts. The selectivity can be enhanced by adding platinum to sulfated zirconia catalysts. An iron and manganese-doped catalyst exhibited excellent selectivities at somewhat lower conversions. Methyl chloride is obtained at 235°C in selectivities greater than 85%. No chloroform or carbon tetrachloride is formed. The electrophilic insertion involves electron-deficient metal-coordinated chlorine into the methane C-H bond.Catalysis by solid superacids, 29. For part 28 see ref. [14].     

NO reduction by CH4 over Pd/Co-sulfated zirconia catalysts

A series of sulfated zirconia supported Pd/Co catalysts was synthesized by the sol–gel method and examined for NO_x reduction by methane. The NO conversion increased up to a Co/S ratio of 0.43, and then decreased at a higher Co loading (Co/S = 0.95). Sulfate content was also essential for obtaining high selectivity to molecular nitrogen. A catalyst loaded with 0.06 wt.% Pd, 2.1 wt.% Co and 2.1 wt.% S (Pd/Co-SZ-2) exhibited remarkable performance under lean conditions and displayed stability in a long-term durability test using a synthetic reaction mixture containing 10% water vapor. This catalyst exhibited the highest sulfur retention most probably as cobalt sulfide. Besides, the catalytic oxidation of NO to NO_y groups was confirmed by FT-IR, in agreement with the general mechanism for the SCR of NO by hydrocarbons. In the absence of oxygen in the feed stream, the catalyst was highly active for NO reduction with methane. IR stretching bands assigned to N₂O and adsorbed nitro groups were identified upon adsorbing NO on Pd/Co-SZ-2. This indicates that under rich conditions disproportionation of NO to N₂O and NO₂ occurs and confirms that the formation of NO₂ species is an essential step for NO reduction by CH₄.     

Role of lanthanide elements on the catalytic behavior of supported Pd catalysts in the reduction of NO with methane

In this study, the role of lanthanide elements (Ce, Gd, La, and Yb) on Pd/TiO₂ catalysts in the catalytic reduction of NO with methane was investigated. Steady-state reaction experiments in the presence of oxygen showed that the addition of lanthanide elements increases the oxygen resistance of the catalyst. The post-reaction XPS characterization results revealed that majority of the Pd sites remained in the zero oxidation state in the presence of Ce or Gd. The effect of SO₂ (145 ppm) and H₂O (0–6.6%) in NO–CH₄–O₂ reaction over supported Pd and Gd–Pd catalysts was also investigated. Over the Gd–Pd catalyst with the presence of SO₂, more than 70% NO conversion was obtained for over 6 h while the Pd only catalyst showed a sharper drop in NO conversion. Over the Gd–Pd catalyst, the presence of H₂O showed no effect on NO conversion activity (>99% conversion) during the 18 h the catalyst was kept on stream. Among the lanthanide elements tested, Gd is the most effective, allowing the use of above stoichiometric oxygen concentration.     

Cobalt catalysts in selective catalytic reduction of NO by methane

Impregnated cobalt-containing catalysts are studied in the selective catalytic reduction of NO by methane. The active component of all the impregnated cobalt-containing catalysts is Co₃O₄. The role of O₂ seems to maintain the surface stoichiometry of Co₃O₄. The main reason of decrease of catalytic activity of samples based on MgO, SiO₂, Al₂O₃ in selective catalytic reduction of NO is due to oxide-oxide interaction promoted by water. Catalysts based on montmorillonite (HMM) are stable in the presence of water. All the carriers can be placed by the capability for oxide–oxide interaction in the following order: SiO₂ ≫ MgO > Al₂O₃ ≫ HMM. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen

Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al₂O₃-PILC (or Al-PILC), ZrO₂-PILC (or Zr-PILC) and TiO₂-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu–Ti-PILC, Cu–Al-PILC, Cu–Zr-PILC, Cu–Al–Laponite, Fe–Ti-PILC, Ce–Ti-PILC, Ce–Ti-PILC, Co–Ti-PILC, Ag–Ti-PILC and Ga–Ti-PILC. Cu–Ti-PILC showed the highest activities at temperatures below 370°C, while Cu–Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N₂O was formed by all of these catalysts. H₂O and SO₂ only slightly deactivated the SCR activity of Cu–Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu–Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce₂O₃ on Cu–Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce₂O₃ decreased the activity of Cu–Ti-PILC due to pore plugging. Cu–Ti-PILC was found to be an excellent catalyst for NO SCR by NH₃, but inactive when CH₄ was used as the reducing agent. Subjecting the Cu–Ti-PILC catalyst to 5% H₂0 and 50 ppm SO₂ at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu²⁺, Cu⁺ and CuO. The TPR temperatures for the Cu–Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity.     

The effect of sodium on the Pd-catalyzed reduction of NO by methane

The kinetics of NO reduction by methane over Pd catalysts supported on 8 mol% yttria-stabilised zirconia (YSZ) has been studied at atmospheric pressure in the 620–770 K temperature range. Langmuir–Hinshelwood type kinetics are found with characteristic rate maxima reflecting competitive adsorption of NO and methane: NO adsorption is much more pronounced than that of methane within the temperature range of this investigation. Pd is an effective catalyst: 100% selectivity towards N₂ can be achieved at 100% conversion of NO over this wide temperature range. Sodium causes strong poisoning of the reaction. The response of the system to variations in NO and methane concentrations, temperature, and sodium loading indicate that this is due to the Na-induced enhancement of NO chemisorption and dissociation relative to methane adsorption, i.e. sodium enhances oxygen poisoning of the catalyst. These results stand in revealing contrast to the strong promotional effect of sodium in the reduction of NO by propene over the same catalysts. The very different response of the two hydrocarbon reductants to Na doping of the Pd catalyst receives a consistent explanation.     

Determination of active palladium species in ZSM-5 zeolite for selective reduction of nitric oxide with methane

The active site in ZSM-5 zeolite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K, and almost all the NO was desorbed at around 673 K, while the palladium species on PdO/SiO₂ hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na⁺ in NaCl solution, indicating that they exist in a cationic state of an isolated Pd²⁺. This method for quantitative analysis of the isolated Pd²⁺ cations is named as ‘NaCl titration’. The amount of the isolated Pd²⁺ cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd²⁺ cation was unchanged after the reaction of NO₂–CH₄, NO₂–CH₄–O₂, or CH₄–O₂ at 673 K, while the adsorbed amount of NO per the Pd²⁺ as determined by NO-TPD decreased after the NO₂–CH₄–O₂ reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO₂–CH₄–O₂ reaction increased with increasing amount of the isolated Pd²⁺ cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO₂ reduction. The palladium species that are active and selective for NO reduction with CH₄ will be proposed.     

MnOx/CeO2 catalysts for the low-temperature selective catalytic reduction of NO with NH3

CeO₂–nanorod support was synthesized by hydrothermal method and different manganese oxides (MnO, MnO_2, and Mn₂O₃) were impregnated over support by the wet-impregnation forming MnO/CeO₂-NR, MnO₂/CeO₂-NRm and Mn₂O₃/CeO₂-NR. The physico-chemical properties of the as-prepared catalysts were analyzed using x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscope–energy-dispersive x-ray spectroscopy (SEM–EDX), hydrogen-temperature-programmed reduction (H₂-TPR), and Raman spectroscopy. These catalysts were further analyzed for NO reduction using NH₃ as a reducing gas in the temperature range of 50 to 450°C. The results confirmed that MnO₂/CeO₂-NR gave the maximum NO conversion (65%) and N₂ selectivity (89%) among all catalysts. Further, MnO₂/CeO₂-NR catalyst was studied for the effect of MnO₂ loading and more than 90% NO conversion and N₂ selectivity were obtained in the temperature range of 250 to 300°C.     

SO2和H2O对Y掺杂Mn/TiO2选择性脱除NO的影响

采用溶胶-凝胶法制备了Y掺杂的TiO₂载体,负载硝酸锰构成了Y掺杂的Mn-Y/TiO₂催化剂。考察了焙烧温度、空速对其催化还原NO性能的影响,并对催化剂的抗SO₂、H₂O毒化性能进行了考察。结果表明,催化剂的最佳焙烧温度为500 ℃,催化剂的活性随空速的降低而升高,XRD分析Y掺杂抑制了锐钛矿晶相的转移,有利于催化剂活性组分的分散,从而提高催化剂的活性。Mn-Y/TiO₂的抗毒化性能优于Mn/TiO₂,在反应温度180 ℃、空速14000 h^－1、氧含量为3%、NO浓度600 mL/L及NH₃/NO为1的条件下,同时通入200 mL/L SO₂和4% H₂O,NO转化率从非掺杂的Mn/TiO₂的48.2%上升到57.6%,Y掺杂提高了催化剂的抗毒化能力;FTIR分析表明催化剂中毒是由于生成了铵的硫酸盐或者锰、钇的硫酸盐。     

NO reduction by CH4 in the presence of excess O2 over Pd/sulfated zirconia catalysts

The selective catalytic reduction (SCR) of NO by methane in the presence of excess oxygen has been studied on a series of Pd catalysts supported on sulfated zirconia (SZ). This support is not as sensitive to structural damage by steaming as the acidic zeolites, such as H-ZSM-5 and H-Mor. In previous studies, it was shown that this type of acidic zeolites are able to stabilize Pd²⁺ ions and promote high SCR activity and selectivity, which are typically not seen in Pd catalysts. In this contribution, it has been demonstrated that SZ is able to promote the NO reduction activity in a similar way to the acidic zeolites, by stabilizing Pd²⁺ ions that is selective for NO reduction. As in the case of acidic zeolites, the stabilization of Pd²⁺ ions can occur through a transfer of Pd species from particle to particle. One of the attractive features of Pd/SZ catalysts is that they are less sensitive to water and SO₂ poisoning than Pd/H-ZSM-5 catalyst and exhibit higher reversibility after removal of water or SO₂.     

Electron spectroscopy of sulfated zirconia, its activity in n-hexane conversion and possible reasons of its deactivation

Sulfated zirconia catalysts were prepared and characterized by X-ray photoelectron spectroscopy taken in the dried state (fresh) and after calcination at 900 K (calc.). A maximum activity was observed as a function of the calcination temperature. The Zr 3d region showed that any Zr hydroxide in the dried catalyst transformed into zirconium oxide upon calcination. The O 1s peak could be fitted by two components corresponding to ZrO₂ and sulfate, respectively. Sulfur was present as sulfate. Both catalysts showed activity in n-hexane conversion (including isomerization) between 300 and 473 K. The activity of the calcined catalyst was much higher. The main products were isopentane and isobutane, along with 2-methyl- and 3-methylpentane. The activity was not stable and only a limited amount of n-hexane transformed before final deactivation. This observation pointed to a limited amount of active sites able to start the reaction. The activity could be fully regenerated by oxygen treatment. Thus, the “oxidative” start of the reaction [ A. Ghenciu, D. Farcasiu, Catal. Lett. 44 (1997) 29] may have also played a role apart from those on strong acid sites. Deactivation may have been due to a partial reduction of sulfate groups rather than to carbon accumulation, as shown also by the minor amounts of S⁴⁺ detected by XPS. Parallel isomerization and splitting of hexane into two C₃ units may occur, followed by the formation of surface C₉ units, the latter being intermediate of larger fragments.     

Searching for the active sites of Co-H-MFI catalyst for the selective catalytic reduction of NO by methane: A FT-IR in situ and operando study

A Co-H-MFI sample has been studied through FT-IR spectroscopy of in situ adsorption and co-adsorption of probe molecules (o-toluonitrile, CO, NO) and has been tested in the CH₄-SCR process under IR operando conditions. The o-toluonitrile (oTN) adsorption and the oTN and NO co-adsorption, show that both Co²⁺ and Co³⁺ species are present on the catalyst surface. Co³⁺ species are located inside the zeolitic channels while Co²⁺ ions are distributed both at the external and at the internal surfaces. The operando study show the activity of Co³⁺ species in the reaction. The existence of three parallel reactions, CH₄-SCR, CH₄ total oxidation and NO to NO₂ oxidation, has been confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH₄-SCR and NO oxidation, respectively. A mechanism for CH₄-SCR has been proposed. Co²⁺ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seem not to play a key role in the SCR process.     

Direct impregnation method for preparing sulfated zirconia supported on mesoporous silica

A new method has been developed to prepare sulfated zirconia (S–ZrO₂) supported on mesoporous silica. With direct exchange of metal containing precursors for the surfactants in the as-synthesized MCM-41 materials, the problem of fill-up of the mesoporous structure was avoided and high sulfur content was achieved. By using this method, the composite of S–ZrO₂/MCM-41 with ZrO₂ content higher than 60 wt.% can be easily obtained without serious blockage of the pore structure of MCM-41. Nevertheless, the pore size and pore volume of the resultant S–ZrO₂/MCM-41 composites were found to vary markedly with the loading of ZrO₂. The strong acidic character of the obtained composites was examined by using them as catalysts in n-butane isomerization. Introduction of other metals such as aluminum as promoter into S–ZrO₂/MCM-41 can be easily conducted by the direct impregnation method.     

A practical scale evaluation of sulfated V2O5/TiO2 catalyst from metatitanic acid for selective catalytic reduction of NO by NH3

The characteristics of sulfated V₂O₅/TiO₂ honeycomb catalyst from metatitanic acid (MTA) were studied in the practical conditions of pilot plant using high dust flue gas from coal fired utility boiler. The effects of reaction temperature, NH₃/NO mole ratio, space velocity and operation time on the reduction of nitric oxide (NO) were mainly investigated for engineering application. The catalyst showed high NO reduction of about 90% at a space velocity of 4000 h⁻¹, NH₃/NO mole ratio of 1.0 and reaction temperature of 300–400 °C. The efficiency of this catalyst remained constant during the present experiment of 2400 h and the erosion by fly ash was lower than that of the commercial catalysts. These results clearly demonstrate the high potential for this catalyst to be applied commercially for the control of NO_x emissions from coal fired utility boiler.     

CoH-FBZ催化剂表征及选择催化CH4还原NO

一种具有FAU和BEA双微孔拓扑结构的分子筛催化剂CoH-FBZ用于富氧条件下CH₄催化还原NO(CH₄-SCR)，应用XRD、FT-IR、DRS-UV-Vis、SEM、AAS、H₂-TPR和NH₃-TPD等技术对催化剂进行表征.结果表明，CoH-FBZ为均匀单一物相，XRD和FT-IR谱图中，FAU和BEA拓扑结构特征峰明显，NH₃-TPD结果表明，双拓扑结构H-FBZ和CoH-FBZ中产生了新的强酸位.CoH-FBZ较CoH-Beta与CoH-Y机械混合样品的催化活性高;CoH-FBZ中FAU与BEA拓扑结构的相对含量FAU/BEA为(0.4∶0.6)～(0.1∶0.9)时，催化剂具有高的CH₄-SCR催化活性.强酸位与Co协同作用有利于提高催化剂的CH₄-SCR催化活性.     

Synthesis and characterization of fly ash supported sulfated zirconia catalyst for benzylation reactions

Synthesis of highly active nano-crystalline, thermally stabilized solid acid catalyst has been reported by loading different weight fractions of sulfated zirconia on chemically activated fly ash through two step sol-gel technique. The catalysts were characterized using powder XRD, FT-IR, N₂-adsorption desorption study, CHNS elemental analysis, SEM-EDAX and their acidity were measured by pyridine adsorbed FTIR. Liquid phase benzylation of benzene and toluene with benzyl chloride was studied as test reaction for catalytic activity of SZF catalysts. A very high conversion of benzene (87%) and toluene (93%) were observed, which is attributed to significant amount of acid site on the catalyst surface. The FTIR study of the pyridine adsorbed samples reflects the presence of Brønsted as well as Lewis acid sites. The catalyst with 12 wt.% zirconia (SZF-12) was regenerated and reused up to four reaction cycles with equal efficiency as in the first run.     

Iron and manganese promoted sulfated zirconia: acidic properties and n-butane isomerization activity

Sulfated zirconia catalysts promoted with Fe and Mn have been synthesized and the acidity characterized by IR spectroscopy of adsorbed pyridine. The catalytic isomerization of n-butane was investigated in a fixed bed reactor operated at low conversion at atmospheric pressure and 250 °C. The main effect of the promoters was to change the ratio Brönsted:Lewis acidity of the samples. The catalytic activity was found to be correlated with the number of strong Brønsted acid sites.