| Because of its high temperature resistance, high thermal conductivity and goodmechanical strength, the use of metal as catalyst support, particularly the FeCrAl alloy,has been widely used with ambitious market prospect. However, the surface activeoxide layer on the metal support is easy to peel off due to the heat exchange shocksand mechanical vibrations in the complicated and harsh environments, resulting in thedeactivation of the catalyst. Therefore, the peeling off of the oxide layer on the metalmonolithic catalyst has been the core problem in the metal monolithic catalyst. Theadhesion properties of the metal monolithic catalyst include the stabilities ofresistance to ultrasonic vibration and thermal shock, which is an important factor indetermining the field of its application. Concentrate on the key problem, this work isto develop a new catalyst with high adhesion stability, as well as to maintaining thehigh activity for methane catalytic combusion.Taking the catalytic combustion of methane as a probe reaction, we used theAl2O3/FeCrAl as both the substrate and aluminum source, used ammonia as theprecipitate/complexant to introduce the high temperature stable lanthanum manganesehexaaluminate (LaMn-hexaaluminate, noted as LHA) in to the Al2O3intermediatelayer (noted as IML), prepared the LHA-Al2O3/FeCrAl composite IML with LHAgrains embedding in the Al2O3layer. The SEM characterization showed that there area large amount of plate-like grains vertically embedding in the Al2O3IML, and thethickness of the composite IML and the embedded plate-like grains is about3μm and50nm, respectively. Combined with the XRD, TEM, EDX and the literature, thisembedded plate-like grains is just the LHA grains, the composite IML is just theLHA-Al2O3/FeCrAl composite IML.On the basis of the obtained LHA-Al2O3composite IML, we modulated theconditions during the preparation process, systematically investigated the influence ofthe preparation conditions on the structure, morphology and performance of thecomposite IML. According to the XRD and SEM characterization, theLHA-Al2O3/FeCrAl sample which was prepared with ammonia asprecipitate/complexant to modulate the pH value of the precursor solution of0.1Mincluding NH4NO3([NH4NO3]/([La3+]+[Mnn+])=6) to pH=8, crystallized at60°C for72h and then calcinated at1100°C for5h, showed complete crystal structure andhad plenty of LHA grains in the IML which is quite rough and porous and beneficialto the adhesion of active LHA layer. The LHA-Al2O3/FeCrAl sample showed a betteradhesion stability whose mass loss remains stable at0.35%and0.13%after40minultrasonic vibration and4thermal cycles, respectively. After coated with active LHAlayer, this catalyst LHA/LHA-Al2O3/FeCrAl showed a good activity in methanecatalytic combustion on1.5vol.%CH4and98.5vol.%air with a weight spacevelocity of38,710ml/g(cat+Al2O3)·h, whose T10=540°C，T90=660°C. Compared withLHA/Al2O3/FeCrAl sample, those LHA grains embedded in the IML bond well withboth the substrate，IML and active LHAlayer like anchors, strengthen the interactionsamong them.This new LHA/LHA-Al2O3/FeCrAl catalyst can not only achieve a betteradhesion stability, but also maintain a good catalytic activity of CH4combustion,compared with LHA/Al2O3/FeCrAl.The author use characterizations such as SEM, ICP to investigate the formation andgrowth of LHA grains in the Al2O3/FeCrAl for pH=7and8samples, and proposedtwo growth mechanisms of the embedding and covering LHA grains. The authorsthink that the embedding LHA precursors are formed by heterogeneous nucleationfrom the adsorbed ions on the pore surface of the Al2O3layer. However, the coveringLHAgrains are only formed under pH=8, as the immersing time is longer than48h,besides the growth of embedding LHA precursors, covering LHA precursors couldalso be formed by depositing the urgently increased Al3+as Al(OH)3gels on the uppersurface ofAl2O3layer, then adsorbing the La3+and Mnn+and recating with them. |
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