Relative rates of various steps of NO–CH4–O2 reaction catalyzed by Pd/H-ZSM-5

Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH₄–O₂ reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N₂ formation in NO–CH₄–O₂ vs. NO–CD₄–O₂ reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH₄+CD₄)–O₂ reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5.

A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions.

Further, in contrast to Co-ZSM-5, NO_x–CH₄–O₂ reaction was much slower than CH₄–O₂ reaction for Pd/H-ZSM-5; the presence of NO_x retards the reaction of CH₄ over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH₄ is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO₂ strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH₄–O₂ reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5.     

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.     

Methanol formation from methane partial oxidation in CH4–O2–NO gaseous phase at atmospheric pressure

The reaction condition for high yield of methanol in a gaseous reaction between methane and oxygen in the presence of NO at atmospheric pressure was explored. Methane partial oxidation without NO (CH₄–O₂) gave only 1% conversion of methane at 966 K. The addition of NO led to a remarkable increase in methane conversion and to high selectivity to C₁-oxygenates. The conversion of methane attained 10% at 808 K in the presence of NO (0.5%) where the selectivities to methanol and formaldehyde were 22.1 and 24.1%, respectively. Nitromethane and carbon oxides were also observed in the product gas. The amount of nitromethane was almost equal and/or near to that of initial NO. The carbon monoxide produced was several times higher than carbon dioxide. Influences of NO concentration, ratio of methane to oxygen, water vapor, and dilution with helium gas on product distribution were measured. Low concentration of NO (0.35–0.55%) was favorable for methanol formation. High selectivity to methanol was obtained at low value of the ratio of methane to oxygen (2.0–3.0) or low concentration of dilution gas (<16%). The NO₂ added promoted methane partial oxidation and selectivity to methanol. Therefore, it was assured that NO_x promoted the formation of CH₃√ and CH₃O√ in the gas phase reaction for CH₄–O₂–NO.     

Crystal structures of fully dehydrated fully Sr-exchanged zeolite X and of its ammonia sorption complex

The crystal structures of fully dehydrated Sr₄₆–X [Sr₄₆Si₁₀₀Al₉₂O₃₈₄; a=25.214(7) Å] and of its ammonia sorption complex, Sr₄₆–X·102NH₃ [Sr₄₆Si₁₀₀Al₉₂O₃₈₄·102NH₃; a=25.127(7) Å], have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd at 21(1)°C. The Sr₄₆–X crystal was prepared by ion exchange in a flowing stream of aqueous 0.05 M Sr(ClO₄)₂ for 5 days followed by dehydration at 360°C and 2×10⁻⁶ Torr for 2 days. To prepare the ammonia sorption complex, another dehydrated Sr₄₆–X crystal was exposed to 230 Torr of zeolitically dried ammonia gas for 1 h followed by evacuation for 12 h at 21(1)°C and 5×10⁻⁴ Torr. The structures were refined to the final error indices, R₁=0.043 and R_w=0.039 with 466 reflections, and R₁=0.049 and R_w=0.044 with 382 reflections, for which I>3σ(I). In dehydrated Sr₄₆–X, all Sr²⁺ ions are located at two crystallographic sites. 16 Sr²⁺ ions are at the centers of the double six-rings, filling that site (site I, Sr–O=2.592(6) Å). The remaining 30 Sr²⁺ ions are in the supercage (site II); each extends 0.56 Å into the supercage from the plane of its three nearest oxygen atoms (Sr–O=2.469(6) Å). In the structure of Sr₄₆–X·102NH₃, the Sr²⁺ ions are located at three crystallographic sites: 12 are found at site I [Sr–O=2.652(10) Å]; four in the sodalite units (site I′) each coordinated to three framework oxygen atoms at 2.654(9) Å and also to three ammonia molecules at 2.76(8) Å. The remaining 30 Sr²⁺ ions lie at site II. Each extends 1.12 Å into the supercage where it coordinates to three framework oxygen atoms at 2.584(7) Å and also to three ammonia molecules at 2.774(24) Å.     

Synthesis and crystal structure of carbonate cancrinite Na8[AlSiO4]6CO3(H2O)3.4, grown under low-temperature hydrothermal conditions

The synthesis of cancrinite in the system Na₂O–SiO₂–Al₂O₃–Na₂CO₃–H₂O was studied under low-temperature hydrothermal conditions in the 353 K<T<473 K interval. The aim was to reveal the suitable range for the crystallization of pure-phase carbonate cancrinite with the ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)₂ without cocrystallization of sodalite or intermediate disordered phases between cancrinite and sodalite. It was found that cancrinite formation reacts very sensitive on the temperature within the autoclaves whereas the concentration of reactants and the alkalinity of the hydrothermal solution have a much lower influence on the phase formation. Thus the temperature of crystallization of carbonate cancrinite without any by-products should not remain below 473 K. At the lower reaction temperature of 353 K the formation of a disordered intermediate phase between the cancrinite and the sodalite structure has been obtained in every case, independent of the template concentrations and the base. Some problems to detect this in a typical powder product mixture are discussed. Besides the ²⁹Si and ²⁷Al MAS NMR characterization of the products, the crystal structure refinement of pure carbonate cancrinite of ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)_3.4, has been carried out from X-ray powder data using the Rietveld method: P6₃, a=1271.3(1) pm, c=518.6(1) pm, R_WP=0.073, R_F=0.016 for 347 structure factors and 45 variable positional parameters.     

(CH3CH[NH3]CH2NH3)1/2·ZnPO4, a zincophosphate analogue of aluminosilicate zeolite thomsonite

The synthesis and structure of (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, an organically templated zincophosphate (ZnPO) analogue of aluminosilicate zeolite thomsonite (THO), are described. The ZnPO framework is built up from an alternating, vertex-sharing, network of ZnO₄ and PO₄ groups (d_av(Zn–O)=1.944 (8) Å, d_av(P–O)=1.535 (9) Å, θ_av(Zn–O–P)=130.5°) involving distinctive 4=1 secondary building units. The 1,2-diammonium propane cations are highly disordered in the [0 0 1] 8-ring channels. Crystal data: (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, M_r=198.42, orthorhombic, space group Pncn (no. 52), a=14.119 (6) Å, b=14.136 (5) Å, c=12.985 (5) Å, V=2591 (3) ų, Z=10, R(F)=0.057, R_w(F)=0.061 (for a twinned crystal).     

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.     

Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides for methane combustion: Influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture

The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration (λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ containing 30 wt.% of Co₃O₄. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples).

Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g⁻¹ h⁻¹). The catalytic performance of Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ were compared with those of two pure Co₃O₄ oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ as well as to their reducibility. Particular attention was paid to the thermal stability of the Co₃O₄ phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co₃O₄ heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co₃O₄ decomposition into CoO was observed.

Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co₃O₄ and by promoting the reduction at low temperature.     

Synthesis, characterization and catalytic properties of benzyl sulphonic acid functionalized Zr-TMS catalysts

Mesoporous zirconium hydroxide, Zr-TMS (zirconium hydroxide with mesostructured framework; TMS, transition metal oxide mesoporous molecular sieves) catalyst has been prepared through the sol–gel method and functionalized with benzyl sulphonic acid (BSA) using post-synthesis route without destroying the mesoporous structure. The benzyl group anchored Zr-TMS (B-Zr-TMS/≡Zr–O–CH₂–Φ) was achieved by etherification reaction of Zr-TMS with benzyl alcohol at 80 °C using cyclohexane as solvent. Further, B-Zr-TMS was subjected to sulphonation reaction with chlorosulphonic acid (ClSO₃H) at 70 °C using chloroform as solvent to yield BSA-Zr-TMS (≡Zr–O–CH₂–Φ–SO₃H). Maximum sulphonic acid (–SO₃H) loading was optimized with respect to time of functionalization and concentration of ClSO₃H. Functionalization was carried out by loading the maximum amount of benzyl group over Zr-TMS and varying the concentration of –SO₃H. The synthesized materials have been characterized by powder XRD, FT-IR, elemental analysis, N₂ adsorption–desorption and TPD of ammonia. The catalytic activity of the synthesized catalyst has been performed in liquid phase benzoylation of diphenyl ether to 4-phenoxybenzophenone (4-PBP) using benzoyl chloride as benzoylating agent at 160 °C under atmospheric pressure. The same reaction was carried out by sulphated zirconia (SO₄²⁻/ZrO₂) and found very poor activity.     

Synthesis and permeation properties of ion-exchanged ETS-4 tubular membranes

Zeolite membranes, which were composed of ETS-4 with Na cations, were prepared on porous -alumina tubes by hydrothermal synthesis. The membranes, which were formed under optimized conditions, sharply rejected molecules with sizes larger than 0.4 nm. For mixtures of N₂–CO₂, N₂–O₂, N₂–Ar and N₂–CH₄ systems, N₂ permeated faster than the coexisting gas. The N₂/O₂ separation factor for an equimolar mixture was in the range of 2.3–3.5, and the N₂ permeance was in the range of (0.55–2.8)×10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ at permeation temperatures of 283–333 K. Moisture had some effect on the permeation properties for N₂–O₂ mixtures. The separation factor for the N₂/CH₄ system was larger than that of the N₂/O₂ system. When the membrane was ion exchanged with either Li⁺ or Sr²⁺, the separation factors for N₂/O₂ and N₂/CH₄ systems increased, while the permeances decreased.     

Diamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditions

Crystallization of diamond was studied in the CO₂–C, CO₂–H₂O–C, H₂O–C, and CH₄–H₂–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO₂, CO₂–H₂O, H₂O and CH₄–H₂ fluids in experiments with graphite and silver oxalate (Ag₂C₂O₄), oxalic acid dihydrate (H₂C₂O₄·2H₂O), water (H₂O), and anthracene (C₁₄H₁₀), respectively. Diamond nucleation and growth has been found in the CO₂–C, CO₂–H₂O–C, and H₂O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO₂–H₂O–C system. For the CH₄–H₂–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.     

Influence of cycle parameters on periodically operated fluidised bed reactor for CH4 autoreforming

Autothermal reforming of CH₄ has been studied under both periodic and steady state conditions. The investigation was conducted over Co–NiO in a fluidised bed reactor at 873 K and 101.32 kPa. Cycle periods of 1–40 min were used whilst the cycle split, S_ox (with respect to the O₂-rich cycle) was varied from 0.1 to 0.9. Generally, CH₄ oxidation stimulated CO formation, however, steam reforming yielded predominantly CO₂ and H₂. Although O₂-rich cycling (S_ox≥0.5) was detrimental to H₂ formation, H₂O-rich cycling resulted in a 15% improvement in steady state H₂ formation. Theoretical as well as experimental investigations pointed to a resonant frequency of about 6.7 mHz for CH₄ oxidation to produce super steady state H₂ yields. By periodic operation, it is possible to tune H₂/CO ratios over the range 2.5–7 for the same feed composition. Interestingly, S_ox=0.1 yielded the highest ratios, whereas the lowest ratios were attained at S_ox=0.9. Periodic composition cycling introduces a more flexible approach to reactor operation — H₂/CO can be easily modulated by varying the cycle parameters — compared to steady state operation.     

CO, H2 or C3H8 assisted catalytic combustion of methane over supported LaMnO3 monoliths

The effect of the addition of a second fuel such as CO, C₃H₈ or H₂ on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO₃/La-γAl₂O₃ catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C₃H₈ and H₂. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H₂≥C₃H₈>CH₄. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH₄ when pure methane or CO/CH₄ mixtures are used. For H₂/CH₄ and C₃H₈/CH₄ mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH₄ homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures.

Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners.     

VUV/UV/NaClO工艺降解百里香酚协同效应及活性物质贡献

采用VUV/UV/NaClO与UV/NaClO工艺降解百里香酚(Tml)，以协同因子(R)为评价指标，探究了NaClO浓度和pH对Tml去除及协同效应的影响；以硝基苯(NB)和苯甲酸(BA)为探针化合物，确定了不同工艺中HO·和Cl·的稳态浓度及其与Tml的二级反应速率常数；并对比了两种工艺中不同物质对Tml降解的贡献。结果表明，VUV/UV/NaClO与UV/NaClO工艺降解Tml均符合拟一级反应动力学，其一级动力学常数k_VUV/UV/Cl和k_UV/Cl分别为0.0113 s^-1和0.00479 s^-1，且均与NaClO浓度呈正相关；VUV/UV/NaClO和UV/NaClO工艺对Tml的降解具有显著的协同效应，相应的协同因子(R_VUV/UV/Cl、R_UV/Cl)随NaClO的浓度的增加及溶液pH的增大均先增加再降低；当NaClO浓度为0.3 mg·L^-1和pH=7时，R_VUV/UV/Cl和R_UV/Cl达到最大值，分别为1.9和2.1，对应的协同增效为90%和110%。VUV/UV/NaClO和UV/NaClO工艺中HO·的贡献率分别为42.7%和37.6%，Cl·的贡献率分别为42.4%和28.5%。两种工艺中HO·和Cl·均为主要贡献物质。     

Kinetic measurements of CH4 combustion over a 10% PdO/ZrO2 catalyst using an annular flow microreactor

A kinetic study on CH₄ combustion over a PdO/ZrO₂ (10%, w/w) catalyst has been performed in a temperature range between 400 and 550 °C by means of an annular catalytic microreactor.

The role of mass transfer phenomena including diffusion in the catalyst pore, gas–solid diffusion and axial diffusion in the gas phase, has been preliminary addressed by means of mathematical modeling. Simulation results have pointed out the key role of internal diffusion showing that thicknesses of the active catalyst layer as thin as 10–15 μm are required to minimize the impact of mass transfer limitations. The thermal behavior of the reactor has been also addressed by means of catalytic combustion tests with CH₄ and CO–H₂ mixtures as fuels. The results have demonstrated the possibility to obtain nearly isothermal temperature profiles under severe conditions (up to 3% of CH₄) thanks to effective dissipation of reaction heat by radiation from the catalyst outer skin.

Finally the effect of reactants (CH₄ and O₂) and products (H₂O and CO₂) on CH₄ combustion rate has been addressed. The results have shown that both H₂O and CO₂ markedly inhibit the reaction up to 550 °C. The data have been fitted by the following simple power law expression r=k_rP_CH4P_H2O^−0.32P_CO2^−0.25 with an apparent activation energy of 108 kJ/mol.

Evidences have been found and discussed indicating a key role of the support on the extent of such inhibition effects.     

Evaluation of V2O5–WO3–TiO2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V₂O₅–WO₃–TiO₂ system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO₂. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V₂O₅–WO₃–TiO₂ catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration.     

Effect of alternate CH4-reducing/lean combustion treatments on the reactivity of fresh and S-poisoned Pd/CeO2/Al2O3 catalysts

This work investigates the effect of treatments under different CH₄-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al₂O₃/CeO₂ catalysts for methane combustion.

Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH₄-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H₂O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO₂ markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle.

Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO₂-containing atmosphere required very high temperatures (above 750 °C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH₄-reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550–600 °C) due to the reduction of sulphates by CH₄ activated on the surface of Pd metal. A protecting role of CeO₂ on Pd poisoning due either to exposure to SO₂-containing atmosphere or to spill-back of support sulphates species was also evidenced.     

Cu/γ-Al2O3 catalyst for the combustion of methane in a fluidized bed reactor

The applicability of a catalyst based on copper dispersed on γ-Al₂O₃ spheres (1 mm diameter) for fluidized bed catalytic combustion of methane has been assessed. Catalyst properties have been determined by physico-chemical characterization techniques and fixed bed activity tests revealing the presence of a surface CuAl₂O₄ spinel phase, still active and stable in methane combustion after repeated thermal ageing treatments at 800 °C. Methane catalytic combustion experiments have been performed in a 100 mm premixed fluidized bed reactor under lean conditions (0.15–3% inlet methane concentration), showing that complete CH₄ conversion can be attained below 700 °C in a fluidized bed of 1 mm solids with a gas superficial velocity about twice the incipient fluidization velocity.     

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.     

Solid state protonic conductor NH4PO3–(NH4)2Mn(PO3)4 for intermediate temperature fuel cells

A new proton-conductive composite of NH₄PO₃–(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm⁻¹ was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²–10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.