Controlled Preparation,Characterization,and Catalytic Performance Of The Supported Pd-Based Bimetallic Nanocatalysts For Methane Oxidation

With the development of industrialization,the application of natural gas has become more and more extensive in various fields.Meanwhile,the pollution caused by methane emissions has become increasingly serious.Catalytic oxidation is one of the most effective pathways to eliminate low concentration methane,in whch the key issue is the availability of effective catalysts.Supported noble metal catalysts exhibit high low-temperature activities and good resistance to poisoning for methane oxidation.However,their low thermal stability and high cost hardly meet the oxidative removal of methane from various sources,which limit their wide applications.Taking into account the electronic and synergistic effects of bimetallic catalysts,it is beneficial for enhancements in activity and stability of the supported noble metal catalysts for catalytic combustion of methane.In addition,the amount of precious metals can be reduced with the use of bimetals.In the thesis,we prepared Pd-GaOx/Al₂O₃ catalysts by calcining the Pd₅Ga₃ intermetallic nanocrystals supported on γ-Al₂O₃.Transmission electron microscopy（TEM）,nitrogen adsorption-desorption（BET）,X-ray photoelectron spectroscopy（XPS）,H₂ temperature-programmed reduction（H₂-TPR）,Fourier transform infrared spectroscopy（FTIR）,and Raman spectroscopy（Raman）were used to characterize physicochemical properties of the catalysts,and methane oxidation was used to evaluate their catalytic activities and stability.The obtained results are as follows:（1）Pd₅Ga₃ and Pd nanocrystals were prepared by the co-reduction method in the presence of oleylamine,and the Pd-GaOx/Al₂O₃ and Pd/Al₂O₃ catalysts were obtained after loading of Pd₅Ga₃ and Pd nanocrystals on γ-Al₂O₃ after calcination.（2）The catalytic activity（T10% = 294 ℃,T50 % = 336 ℃ and T90% = 372 ℃）over Pd-GaOx/Al₂O₃ was much better than that（T10% = 336 ℃,T50% = 379 ℃ and T90% = 418 ℃）over Pd/Al₂O₃.Meanwhile,the Pd-GaOx/Al₂O₃-HT sample(in which the "HT" denotes the sample after hydrothermal treatment at 750 ℃ still showed the outstanding hydrothermal stability with T90% = 410 ℃,whereas the Pd/Al₂O₃-HT sample showed a worse performance（T90% = 507 ℃）.The size of metal nanoparticles in Pd-GaOx/Al₂O₃ did not significantly increase（from 5.3 ± 0.7 to 8.4 ± 2.6 nm）after aging treatment,whereas there was a big increase of Pd nanoparticles in Pd/Al₂O₃（from 5.5 ± 0.6 to 35.8 ± 6.8 nm）.Therefore,the Pd-GaOx/Al₂O₃-HT sample was catalytically stable under the adopted conditions.（3）The exhibited better resistance to H₂ O and CO₂: After the introduction of water vapor and CO₂,the conversion rate of methane over the supported Pd?GaOx catalyst was dropped by 30%,while the activity was restored after the turning off.It can be found that the inactivation caused by the introduction of CO₂ and H₂ O is reversible,while the inactivation caused by SO₂ is irreversible.（4）The supported Pd?GaOx catalyst contained higher valence Pd species and a adsorbed oxygen species content than the supported monometallic Pd catalyst.In addition,the Pd O species in the supported Pd?GaOx bimetallic catalyst exhibited better thermal stability.The excellent catalytic performance of the supported Pd?GaOx bimetallic catalyst was related to its high surface adsorbed oxygen species concentration,stable palladium oxide species,and synergistic action between Pd O and GaOx.