Preparation Of Bnoron Nitride-supported Palladium Catalysts And Their Catalytic Performance For O-xylene Oxidation

Industrialization and urbanization lead to an increase in the emissions of volatile organic compounds（VOCs）.Some of the those like BTX（benzene,toluene,and xylene）result in serious environmental and health hazards.Due to the high activity at low temperatures,the Pd-based catalysts have been considered to be the most efficient catalysts for the oxidation of BTX.However,in the actual volatile organic compounds oxidation system,water vapor are the one of the main products of the catalytic reaction,which in addition to competing the adsorption with VOCs and O₂,also forms the hydroxyl species blocking the active sites on the catalyst surface,which can significantly deactivate the catalyst.Therefore,improving the hydrothermal stability of the palladium-based catalyst is very important.In the present Master Degree thesis,a series of h-BN-supported Pd catalysts（Pd@NC/BN）that were modified by the nitrogen-doped carbon layers were prepared using the pyrolysis method with palladium acetylacetone and 1,10-feriloline as precursor.In the meanwhile,the Pd/BN and Pd/Al₂O₃catalysts were also prepared via the liquid-phase reduction and impregnation routes.Physicochemical properties of the catalysts were characterized by means of various techniques,such as inductively coupled plasma–atomic emission spectroscopy（ICP-AES）,X-ray diffraction（XRD）,nitrogen adsorption–desorption（BET）,transmission electron microscopy（TEM）,high-angle annular dark field–scanning transmission electron microscopy（HAADF-STEM）,X-ray photoelectron spectroscopy（XPS）,laser Raman spectroscopy（Raman）,contact angle measurements,H₂O temperature-programmed desorption（H₂O-TPD）,C₈H₁₀temperature-programmed desorption(C₈H₁₀-TPD),H₂O-C₈H₁₀temperature-programmed desorption(H₂O-C₈H₁₀-TPD),and in situ diffuse reflectance Fourier transform infrared spectroscopy（in situ DRIFTS）.Catalytic activities of theese materials were evaluated for o-xylene oxidation.The obtained main results are as follows:（1）The Pd@NC/BN catalysts were successfully prepared by the pyrolysis method.The TEM and Raman results suggest that the Pd NPs were surrounded by the nitrogen-doped carbon layers,and the average thickness of the nitrogen-doped carbon layers was about 0.30 nm.In the meanwhile,the Pd/BN and Pd/Al₂O₃catalysts were prepared using the liquid-phase reduction and impregnation methods.（2）At a space velocity（SV）of 40,000 m L g^-1h^-1,the Pd@NC/BN-1 and Pd@NC/BN-2 catalysts performed much better than the Pd/Al₂O₃and Pd/BN catalysts.We use the reaction temperatures T_50%and T_90%（corresponding to o-xylene conversion=50 and 90%）to compare activities of the catalysts.The Pd@NC/BN-2catalyst performed the best:the T_50%and T_90%were 170 and 185 ℃,respectively.The T_50%and T_90%over the Pd@NC/BN-1 catalyst were 190 and 200 ℃,respectively,which were much lower than those over the Pd/Al₂O₃and Pd/BN catalysts.The T_50%and T_90%over the Pd/BN catalyst were 205 and 215 ℃,respectively.（3）The Pd@NC/BN catalysts exhibited much better water-resistant performance than the Pd/Al₂O₃and Pd/BN catalysts.The catalytic activity of Pd/Al₂O₃and Pd/BN gradually decreased with the rise in water vapor concentration.When water vapor was cut off,the activity of Pd/Al₂O₃or Pd/BN did not return to its initial activity.With the introduction of 1 vol%water vapor,the conversion of o-xylene over Pd/Al₂O₃first decreased from ca.69 to 44%,then decreased to ca.20%when 10 vol%water vapor was introduced.However,the Pd@NC/BN-1 and Pd@NC/BN-2 catalysts could effectively oxidize o-xylene（with an almost 85%removal efficiency）even in the presence of 10vol%H₂O.（4）The result of the contact angle measurements suggests that the catalysts featuring a structure of nitrogen-doped carbon layers were hydrophobic in nature to a certain degree.The result of the isotope tracing experiments indicates that H₂O was not directly involved in the oxidation of o-xylene but reacted with the adsorbed O₂to generate the new oxygen species（OOH and OH）,and the above new active oxygen species involved in o-xylene oxidation.While the result of in situ DRIFTS reveals that the reactive oxygen species could significantly accelerate the activation and cleavage of C-H bonds in the VOCs molecule,thus facilitating the conversion of key intermediate species（from benzaldehyde to benzoic acid）which played a decisive role in o-xylene oxidation.