Catalytic Combustion Of Methane On Hexaaluminate And Metallic Monolithic Catalyst

Hexaaluminate powder with different composition was synthesized and metallic monolithic catalysts for methane combustion were prepared by coating the hexaaluminate, acting as the active components, on a FeCrAlloy support.The effect of substitution to the mirror plane ions on the thermal stability and catalytic activity was investigated. Mirror plane cation substituted La1-xAxMnAl11O19, with A= Sr2+,Ca2+,K+; x=0.2, 0.4, 0.8, hexaaluminate catalysts were synthesized by co-precipitation. For the samples with x=0.8 diffraction peaks other than the hexaaluminates structure were observed and the surface areas were decreased. It was also found that the sample Ca0.2La0.8MnAl11O19 possesses the best catalytic activity with T10%=723 K and the highest BET surface areas 26.1 m2/g after calcination at 1373 K. The TEM micrographs revealed that the substitution of La3+ by Ca2+ led to a thinner hexaaluminate planer crystal which improved the sintering resistance and increased the surface area. This substitution increased also the amount of reactive Mn3+ species and the reducibility of the hexaaluminate. The enhanced catalytic activity was attributed to the combined effects of higher specific surface area and Mn3+ content.Mg substitution to the Mn in the spinel blocks was examined. The physico-chemical properties and catalytic activity of the Mg doped samples were investigated and compared with the CeO2 and ZrO2 doped samples. Among the Mg-substituted ones a sample with a composition LaMn0.7Mg0.3Al11O19 has the best activity, with T10%=704 K. XRD and BET analysis suggests that the partial substitution by magnesium suppresses effectively the crystal growth along [001] direction, and retains a larger surface area than the non substituted samples. Although the increase in activity is partly attributable to the high surface area, O2-TPD and H2-TPR results confirm that the substitution affects also the oxygen adsorption property and the oxidation state of Mn ions in the hexaaluminate lattice. The reducibility, the amount of Mn3+ species and the lattice oxygen mobility are also improved. Excess Mg was incorporated into the hexaaluminate with a stoicoimetry LaMn0.7Mg0.3Al11O19. Mixtures of hexaaluminate and MgAl2O4 spinel is obtained after calcination the samples at 1373 K for 5 h. Mg-doping increases further the amount of lattice oxygen and results in a higher catalytic activity than the CeO2 and ZrO2 doped samples. A sample was found very active with T10%=700 K.LaMnAl11O19/FeCrAl monolithic catalysts were prepared by different methods and characterized by XRD, BET, TEM, hydrogen TPR and oxygen TPD. The results show that a sample prepared by ultrasonic co-precipitation possesses the highest BET surface area. Ultrasonic radiation during precipitation leads to an increase of the number of crystal oxygen, improves reducibility of Mn3+ and enhance the catalytic activity. Preparation method of powder samples influence the activity of the monolithic samples. Among the monolithic catalysts the sample prepared by dip-coating, through the powder prepared by ultrasonic co-precipitation as precursor, has the best activity, but the adhesion strength of this sample is poor. The catalyst prepared by spray-pyrolysis of sol possesses the best adhesion strength. The weight loss is lower than 20% during 1 h ultrasonic treatment and than 10% after 10 times of thermal treatment.LaMnAl11O19 and LaMn0.7Mg0.3Al11O19 catalysts were supported onto commercial FeCrAlloy foil, with Al2O3 and La2O3 as a primer. Different active phase/primer combinations were tested. The primers and the active phase were deposited by a dip-coating technique. The effects of primer on the stability and catalytic activity of monolithic catalyst were investigated. The results show that presence of the primer can improve the stability and activity of the monolithic catalyst. With Al2O3 as the primer the catalyst possesses higher adhesion strength, while with La2O3 as primer a higher activity can be obtained. The presence of the primer may reduce the interaction of hexaaluminate and metal support and consequently favor the catalytic activity.