Combustion of CH4 over a PdO/ZrO2 catalyst: an example of kinetic study under severe conditions

A kinetic study on CH₄ combustion over a very active PdO/ZrO₂ catalyst with high Pd loading (10% w/w of Pd) is presented as an example of a demanding problem which requires both the development of appropriate experimental tools and a theoretical insight on surface chemistry.

The use of an annular catalytic reactor as a tool to collect kinetic data under unusually severe conditions (high temperature and CH₄ concentration) is described in comparison with the use of a conventional packed bed reactor. In particular, problems related to the biasing effects of mass, heat and momentum transfer are addressed.

Kinetic data addressing the effects of CH₄, O₂, H₂O and CO₂ concentration in a temperature range from 400 to 550 °C are analysed by means of a purely empirical power law model and of a formal kinetic model based on literature indication assuming methane dissociative adsorption as the rate controlling step.     

High combustion activity of methane induced by reforming gas over Ni/Al2O3 catalysts

During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH₄/H₂O/O₂/Ar = 20/10/20/50 than under CH₄/H₂O/O₂/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH₄/H₂O/O₂/Ar = 20/10/20/50, although it is mainly oxidized to NiAl₂O₄ under CH₄/H₂O/O₂/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH₄/H₂O = 10/10) to the combustion gas (CH₄/O₂ = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity.     

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.     

Regeneration of S-poisoned Pd/Al2O3 catalysts for the combustion of methane

Regeneration of S-poisoned Pd/Al₂O₃ catalysts for the abatement of methane emissions from natural gas vehicles was addressed in this work.

Investigations were devoted to determine the temperature threshold allowing for catalyst reactivation under different CH₄ containing atmospheres. Under lean combustion conditions in the presence of excess O₂, partial regeneration took place only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Short CH₄-reducing, O₂-free pulses led to partial catalyst reactivation already at 550 °C and to practically complete regeneration at 600 °C. Also in this case reactivation was associated with SO₂ release due to the decomposition of stable support sulphates likely promoted by CH₄ activation onto the reduced metallic Pd surface. Rich combustion pulses with CH₄/O₂ = 2 were equally effective to CH₄-reducing pulses in catalyst regeneration.

These results suggest that a regeneration strategy based on periodical natural gas pulses fed to the catalyst by a by-pass line might be efficient in limiting the effects of S-poisoning of palladium catalysts for the abatement of CH₄ emissions from natural gas engine.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst

Nanocomposite Ni/ZrO₂-AN catalyst consisting of comparably sized Ni metal and ZrO₂ nanoparticles is studied in comparison with zirconia- and alumina-supported Ni catalysts (Ni/ZrO₂-CP and commercial Ni/Al₂O₃-C) for steam reforming of methane (SRM) and for combined steam and CO₂ reforming of methane (CSCRM). The reactions are performed under atmospheric pressure with stoichiometric amounts of H₂O and CH₄ or (H₂O + CO₂) and CH₄ at 1073 K. Under a wide range of methane space velocity (gas hourly space velocity of methane GHSV_CH4 = 12,000–96,000 ml/(h g_cat.), the nanocomposite Ni/ZrO₂-AN catalyst always shows higher activity and stability for both SRM and CSCRM reactions. The two supported Ni catalysts (Ni/ZrO₂-CP and Ni/Al₂O₃-C) exhibit fairly stable catalysis under low GHSV_CH4 but they are easily deactivated under high GHSV_CH4 and become completely inactive when they are reacted for ca.100 h at GHSV_CH4 = 48,000 ml/(h g_cat.). The CSCRM reaction is carried out with different H₂O/CO₂ ratios in the reaction feed while keeping the molar ratio (H₂O + CO₂)/CH₄ = 1.0, the results prove that the nanocomposite Ni/ZrO₂-AN catalyst can be highly promising in enabling a catalytic technology for the production of syngas with flexible H₂/CO ratios (ca. H₂/CO = 1.0–3.0) to meet the requirements of various downstream chemical syntheses.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios

A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO₂ in the flue gases from fossil fuel-based power plants without CO₂ separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO₂ reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H₂) with desired H₂/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO₂ reforming of methane. These two advantages have been demonstrated by tri-reforming of CH₄ in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH₄ conversion and about 80% CO₂ conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO₂ conversion in the presence of H₂O and O₂ in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO₂ follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO₂ ≈ Ni/ZrO₂ ≈ Ni/Al₂O₃ > Ni/CeZrO. The higher CO₂ conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO₂ with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO₂ and H₂O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al₂O₃ catalysts for tri-reforming.     

Transient modeling of combined catalytic combustion/CH4 steam reforming

This paper presents an investigation into the complex interactions between catalytic combustion and CH₄ steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H₂ or CH₄ combustion over a supported Pd catalyst and CH₄ steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H₂ combustion over a γ-Al₂O₃-supported Pd catalyst and CH₄ steam reforming over a γ-Al₂O₃-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H₂ combustion. Model results for a reactor with CH₄ combustion indicated that stable reactor operation with reforming fuel conversion to H₂ requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C).     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

SAPO-34 membranes for CO2/CH4 separations: Effect of Si/Al ratio

Silicoaluminophosphate (SAPO) membranes with Si/Al gel ratios from 0.05 to 0.3 were synthesized by in situ crystallization onto porous, tubular stainless steel support. Pure SAPO-34 membranes were obtained when the Si/Al ratio was 0.15 or higher. The adsorbate polarizability correlated with the adsorption capacity on SAPO-34, and the amounts of gases adsorbed were in the order: CO₂ > CH₄ > N₂ > H₂. The Si/Al ratio did not affect the pore volume significantly, but it changed the CO₂ and CH₄ adsorption equilibrium constants. The SAPO-34 membranes effectively separated CO₂ from CH₄ for feed pressures up to 7 MPa. At 295 K, for a pressure drop of 138 kPa and a 50/50 feed, the CO₂/CH₄ selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. At 7 MPa, the CO₂/CH₄ selectivity was 100 and the CO₂ permeance was 4 × 10⁻⁸ mol/(m² · s · Pa) at 295 K. This membrane was also separated CO₂/N₂ (selectivity = 21) and H₂/CH₄ (selectivity = 32) mixtures at 295 K and a pressure drop of 138 kPa. Competitive adsorption and difference in diffusivities are responsible for CO₂/CH₄ and CO₂/N₂ separations, whereas the H₂/CH₄ separation was due to diffusivity differences. For a membrane with Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed, and the CO₂/CH₄ selectivity was lower.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

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.     

Characterization of self-standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of CO2 with H2O

Self-standing porous silica thin films with different pore structures were synthesized by a solvent evaporation method and used as photocatalysts for the photocatalytic reduction of CO₂ with H₂O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH as well as CO and O₂ as minor products. Such thin films having hexagonal pore structure exhibited higher photocatalytic reactivity than the Ti-MCM-41 powder catalyst even with the same pore structure. From FTIR investigations, it was found that these Ti-containing porous silica thin films had different concentrations of surface OH groups and showed different adsorption properties for the H₂O molecules toward the catalyst surface. Furthermore, the concentration of the surface OH groups was found to play a role in the selectivity for the formation of CH₃OH.     

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.     

Reaction and oxygen permeation studies in Sm0.4Ba0.6Fe0.8Co0.2O3 − δ membrane reactor for partial oxidation of methane to syngas

A disk-type Sm_0.4Ba_0.6Co_0.2Fe_0.8O_3 − δ perovskite-type mixed-conducting membrane was applied to a membrane reactor for the partial oxidation of methane to syngas (CO + H₂). The reaction was carried out using Rh (1 wt%)/MgO catalyst by feeding CH₄ diluted with Ar. While CH₄ conversion increased and CO selectivity slightly decreased with increasing temperature, a high level of CH₄ conversion (90%) and a high selectivity to CO (98%) were observed at 1173 K. The oxygen flux was increased under the conditions for the catalytic partial oxidation of CH₄ compared with that measured when Ar was fed to the permeation side. We investigated the reaction pathways in the membrane reactor using different membrane reactor configurations and different kinds of gas. In the membrane reactor without the catalyst, the oxygen flux was not improved even when CH₄ was fed to the permeation side, whereas the oxygen flux was enhanced when CO or H₂ was fed. It is implied that the oxidation of CO and H₂ with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and that CO₂ and H₂O react with CH₄ by reforming reactions to form syngas.     

NO removal by CH4 on Co-NaX-CO and Ag-NaX catalysts in a dual-bed system

NO removal using CH₄ as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co²⁺-, Ag⁺-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co₃O₄, which promotes NO oxidation to NO₂ by excess O₂ at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N₂ by CH₄ in the presence of excess O₂ and H₂O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N₂ in a deNO bed at 523 K and a deNO₂ bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H₂O 10% gas mixture after 160 min.     

A study on the characteristics of the SO2 reduction using coal gas over SnO2-ZrO2 catalysts

SO₂, which is an air pollutant causing acid rain and smog, can be converted into elemental sulfur in direct sulfur recovery process (DSRP). SO₂ reduction was performed over catalyst in DSRP. In this study, SnO₂-ZrO₂ catalysts were prepared by a co-precipitation method, and CO and coal gas, which contains H₂, CO, CO₂ and H₂O, were used as reductants. The reactivity profile of the SO₂ reduction over the catalysts was investigated at the various reaction conditions as follows: reaction temperature of 300–550 °C, space velocity of 5000–30,000 cm³/g_-cat. h, [reductant]/[SO₂] molar ratio of 1.0–4.0 and Sn/Zr molar ratio of SnO₂-ZrO₂ catalysts 0/1, 2/8, 3/5, 5/5, 2/1, 3/1, 4/1 and 1/0. SnO₂-ZrO₂ (Sn/Zr = 2/1) catalyst showed the best performance for the SO₂ reduction in DSRP on the basis of our experimental results. The optimized reaction temperature and space velocity were 325 °C and 10,000 cm³/g_-cat. h, respectively. The optimal molar ratio of [reductant]/[SO₂] varied with the reductants, that is, 2.0 for CO and 2.5 for coal gas. SO₂ conversion of 98% and sulfur yield of 78% were achieved with the coal gas.