Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina supported Pd, Co and Co/Pd catalysts: Part B: On the effect of bimetallic catalyst preparation

The aim of the present work is to study the selective reduction of NO_x from natural gas sources using unburned methane or hydrogen as reducing agents. The results suggest that the NO_x are reduced by H₂ at low temperature, when methane is not activated and at higher temperature the methane is then the main reducing agent. Similar results are obtained for alumina supported palladium and alumina supported cobald-palladium catalysts at low temperature in presence of hydrogen suggesting that the active phase for the reaction NO/H₂ is the palladium. However, at high temperature the higher activity is obtained on bimetallic catalyst. The presence of cobalt enhances the catalytic activity. This result suggests that cobalt and palladium both in cationic form are the active sites when the reducing agent is the methane.     

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.     

The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane

Supported palladium catalysts are very active in the combustion of methane, but still little is known about the kinetic parameters. In this paper a rate expression is presented for an alumina-supported palladium oxide catalyst in the temperature range 180–515°C. Special care was taken to ensure differential conditions during the experiments. In this way, an apparent activation energy of 151±15 kJ/mol was found. The orders in methane, oxygen and water were 1.0±0.1, 0.1±0.1 and −0.8±0.2, respectively. For carbon dioxide a zero order was observed under all conditions. Inhibition by water produced during the reaction was demonstrated to cause non-differential conditions, when a dry feed was used. The rate constant that corrects for this effect could be derived.     

The Influence of Rh Addition on the Catalytic Activity of Cubic Pt Nanocrystals Supported on Alumina for NO/CH4 Reaction

The effect of the Rh addition to the well defined cubic (≈70%) Pt nanocrystals of around 13 nm supported on alumina was investigated for NO/CH₄ reaction. The impact of size and shape of Pt nanoparticles on the catalytic activity were also analyzed by comparing the results with a conventionally prepared catalyst.     

Oxidative dimerisation of methane on supported palladium oxide catalysts

Supported palladium oxide catalysts are able to convert CH₄ to C₂H₆, CO, CO₂, H₂ and H₂O at temperatures 315 °C. Catalysts did not show any support effect when TiO₂, Al₂O₃, ZrO₂, La₂O₃ and MgO were used as supports. With sequential O₂ pulsing the catalyst showed long term activity when used at temperatures below 400 °C. Addition of Pt increased selectivity whereas with Ga it decreased. Results indicate participation of lattice O₂ from catalyst in the reaction pathway.     

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.     

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.     

Al-MCM-41 supported palladium catalyst for methane combustion: Effect of the preparation methodologies

This work aimed at elucidating the beneficial effect of plasma treatment on the catalytic performance of palladium (Pd) catalysts in methane combustion with the ordered mesoporous molecular sieve Al-MCM-41 as the model support. The plasma treated Pd/Al-MCM-41 catalyst exhibited a higher initial activity and a better stability in comparison with the untreated counterpart catalyst. To clarify the plasma effect, the catalysts were characterized by N₂ sorption analysis, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH₃-TPD), pyridine adsorption-infrared spectroscopy (Py-IR), high resolution-transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (CH₄-TPR) experiments. The results obtained confirmed that palladium oxide (PdO) was the active phase. Plasma treatment enhanced the acidity of catalyst and improved the dispersion of PdO particles, which lead to a higher initial activity. The better stability for plasma treated Pd-based catalyst was proved to be closely related to the stronger interaction between palladium oxide and the molecular sieve support. In addition, the sintering of PdO particles over the plasma treated catalyst was not significant during the stability test. These findings may provide useful guidelines for further catalyst design for methane combustion.     

Oscillatory behaviour during the oxidation of methane over palladium metal catalysts

Oscillatory reactions over palladium foil and wire catalysts during the oxidation of methane have been investigated over a wide range of reaction temperatures and argon/methane/oxygen feed gas compositions. Characterisation of the catalyst has also been carried out using scanning electron microscopy (SEM) techniques, which revealed the presence of a porous surface. This suggested that the metal surface has undergone a change since the reaction commenced, and using X-ray powder diffraction (XRD) techniques the palladium phase was shown to be the dominant phase present. Hysteresis phenomena were observed in the activity of the reaction as the temperature was cycled up and down, showing that the metal surface was continually changing throughout the reaction. The activation energies of the reaction during the high reactivity mode, PdO, and low reactivity mode, Pd, were also calculated. Oscillation rates were observed to depend on the dominant surface. Oscillations were frequent when the high reactivity mode was dominant while the activation energy of this mode was found to be low. When the low reactivity mode was dominant, the oscillations were slower and the activation energy was three times larger. The results obtained imply that the behaviour of the palladium surface, switching back and forth from the reduced state to the oxidised state, is responsible for the oscillatory behaviour seen in this system.     

负载型钯催化剂上甲烷催化燃烧的研究进展

概述了负载型钯催化剂对甲烷完全氧化的催化机理,以及钯催化剂在甲烷催化燃烧中的性能特点。     

Carbon-supported cobalt–iron catalysts for ammonia synthesis

The cobalt, iron and Co–Fe catalysts deposited on carbon were prepared, characterised (XRD, H₂ TPD) and studied in ammonia synthesis at 90 bar (H₂:N₂ = 3:1). Partly graphitised carbon material obtained via high temperature treatment (1900 °C) of commercial activated carbon was used as a support for the active metals (10 wt.%) and barium or potassium were used as promoters. XRD studies of unpromoted materials have shown that cobalt (5–20% in Co + Fe) dissolves in the iron phase (alloy formation); the average sizes of crystallites (20–30 nm) are roughly independent of the metal kind (Co, Fe, Co–Fe). The effect of Ba and that of K on the catalyst performance proved to be strongly dependent on the choice of an active phase (Co or Fe or Co–Fe). In the case of Co/C, the promotional effect of barium was extremely large. Furthermore, the Ba–Co/C system was found to be less inhibited by the ammonia product than Ba–Fe/C. At low temperature (400 °C) and at high conversion (8% NH₃ in the gas), the surface-based reaction rate (TOF) for Ba–Co/C is about six times higher than that for Ba–Fe/C.     

Combustion of methane over palladium ZSM-5 and mordenite catalysts

The effect of Pd-loading on Pd-NaZSM-5 and Pd-NaMordenite catalysts prepared by ion exchange was studied for methane combustion with excess oxygen (1% CH₄, 18% O₂, balance N₂) in the temperature range 40–500°C. Fresh and calcined samples (3 h, 450°C) showed methane conversions proportional to Pd-loading on Pd-NaZSM-5 catalysts, while conversions decreased with Pd-loading on calcined Pd-NaMordenite catalysts. TOF (number of methane molecules converted per second per Pd²⁺ ion) for over exchanged Pd-NaZSM5-116 was low as compared to under exchanged Pd-NaZSM5-80 and Pd-NaZSM5-58 samples. Close TOF's were found for the last two samples at 330°C. TOF differences in Pd-NaMordenite catalysts demonstrate the heterogeneity of Pd⁺² sites due to structurally nonidentical locations of cations. TOF's appear to be related to Na/Pd ratios in both catalyst types. Apparent activation energies for Pd-NaZSM-5 materials are higher than those for Pd-NaMordenite catalysts.     

A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn₂O₄, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N₂O decomposition as well as the reduction of N₂O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N₂ and CO₂ was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn³⁺ cations to Mn²⁺, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.     

The effect of mass transfer on the catalytic combustion of benzene and methane over palladium catalysts supported on porous materials

Catalytic combustion of benzene and methane over palladium catalysts supported on FAU and MOR zeolites and MCM-41 and KIT-1 mesoporous materials were studied to illustrate the effect of pore size and shape of supports on their catalytic activities. The palladium catalysts supported on mesoporous materials showed high activity and a steep increase in the conversion of benzene with rising temperature. The low activity of palladium catalysts supported on FAU zeolite was ascribed to mass transfer limitation. However, conversion profiles of methane on palladium catalysts were similar, although their supports were different as zeolites and mesoporous materials. The catalytic behavior of palladium catalysts in the combustion of benzene and methane was explained by the diffusion properties of fuels in the pores of zeolites and mesoporous materials.     

Alumina supported Mo–V–Te–O catalysts for the ammoxidation of propane to acrylonitrile

The effect of Te addition over Mo–V–O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V⁵⁺ species and the formation of V₂O₅ oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V⁵⁺ sites are responsible of propane activation and of the subsequent -H abstraction to form the intermediate propylene. Then, a Mo–V rutile-like structure in which vanadium species are reduced as V⁴⁺, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation.     

Remarkable activity enhancement in the catalytic combustion of methane on supported palladium catalysts

The activity of supported palladium catalysts for the combustion of methane has been determined. It has been observed that the activity increases with time on stream irrespective of whether the catalysts are pre-calcined or pre-reduced. Careful re-reduction experiments have shown that the enhanced activity is maintained. It is concluded that the increase in activity with time on stream is not attributable to slow variations in the chemical state of the palladium particles, but to changes in the morphology of the palladium crystallites under reaction conditions. The possible role of dissolved carbon in stabilising the reconstructed crystallites against reduction is discussed.     

Pillared clay and zirconia-based monolithic catalysts for selective catalytic reduction of nitric oxide by methane

New monolithic catalysts based on zirconia and pillared clays (PILC) have been studied for NO_x removal by CH₄ in the presence of oxygen. A comparative study of the influence of ZrO₂ from various commercial sources for the system Pd–ZrO₂ and the effect of the noble metal chosen for the system NM–PILC was carried out, trying to correlate the catalytic activity with the physico-chemical properties of these catalysts. The obtained results indicate that structure and surface acidity of the support plays an important role on the selectivity to NO_x reduction, although properties such as the surface area or pore volume could also determine the overall activity of the monolithic catalysts.     

Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina-supported Pd, Co and Co/Pd catalysts: Part A. On the effect of palladium precursors and catalyst pre-treatment

The aim of the present work is to study the selective reduction of NO_x from natural gas sources. The unburned methane can be used as reductant. Another reductant such as hydrogen can be created in situ, using a microreformer. The results suggest that the NO_x are reduced by H₂ at low temperature, when methane is not activated and at higher temperature the methane is then the main reductant. However, the catalytic behaviour depends on the metal precursor and the catalyst treatment. The most prominent result is obtained on the palladium catalyst prepared from Pd(NH₃)₄(NO₃)₂ precursor. Comparing the reduction and the calcination step in the course of catalyst preparation, one can conclude that calcination lead to the higher activity in deNO_x, since reduced catalysts are oxidized during the deNO_x process.     

The effect of the Rh–Al, Pt–Al and Pt–Rh–Al surface alloys on NO conversion to N2 on alumina supported Rh, Pt and Pt–Rh catalysts

NO conversion to N₂ in the presence of methane and oxygen over 0.03 at.%Rh/Al₂O₃, 0.51 at.%Pt/Al₂O₃ and 0.34 at.%Pt–0.03 at.%Rh/Al₂O₃ catalysts was investigated.

δ-Alumina and precious metal–aluminum alloy phases were revealed by XRD and HRTEM in the catalysts.

The results of the catalytic activity investigations, with temperature-programmed as well as steady-state methods, showed that NO decomposition occurs at a reasonable rate on the alloy surfaces at temperatures up to 623 K whereas some CH₄ deNO_x takes place on δ-alumina above this temperature. A mechanism for the NO decomposition is proposed herein. It is based on NO adsorption on the precious metal atoms followed by the transfer of electrons from alloy to antibonding π orbitals of NO_(ads.) molecules. The CH₄ deNO_x was shown to occur according to an earlier proposed mechanism, via methane oxidation by NO_2(ads.) to oxygenates and then NO reduction by oxygenates to N₂.