On The Preparation Of Highly Efficient Catalysts Using Cold Plasma Treatment

Metal-loaded catalysts have been extensively used in chemical industry. Theoperating efficiency and cost depend on the activity of catalysts. The presentstate-of-art status of technologies for catalyst preparation is far from perfect and theexploring of new preparation method is necessary. In this dissertation, the plasmamethod was systematically investigated with the aim of establishing a new andeffective technique for the preparation of highly efficient catalysts. With regard to theincreasing interest in utilization of solar energy and biomass, the catalysts forphotocatalytic hydrogen generation and glucose oxidation were chose as the modelcatalysts.Pt/TiO₂ was prepared by a plasma-enhanced impregnation method includingimpregnation, glow discharge plasma treatment, calcination and reduction. For thephotocatalytic H2 from water/methanol mixture, such catalysts show significantlyhigh activity. Especially, the plasma-prepared 0.5wt%Pt/TiO₂ exhibits a 2.3 timeshigher activity when compared with the conventional one. The activity ofplasma-prepared catalysts does not change much with the amount of Pt loaded, whilethat of conventional catalysts is greatly dependent on the loading-amount. Catalystcharacterizations show that the plasma treatment produces novel metal clusters. Theseclusters are oxidized with calcinations in air. And a largely distorted metal-supportinterface is produced, inducing an enhanced metal-semiconductor interaction. Thisinteraction improves several properties of the catalysts such as metal dispersion andstability, and optical absorption in near UV region. Moreover, the novel metal-supportinterface greatly facilitates the electron transfer from the semiconductor to metalparticles, leads to remarkably high efficiency.The metal-support interface was designed and controlled to improve the activityof NiO-loaded semiconductor catalysts. In conventional method, semiconductors wereimpregnated with Ni(NO₃)₂, calcined, reduced at 500℃ and re-oxidized at 300℃.The nickel atoms migrate into the crystal of support during the thermal decomposition,and a diffused interfacial region is produced for the resulted catalysts. This is differentfrom the previously designed interface. A plasma treatment was used to replace thethermal calcination to decompose Ni(NO₃)₂. IR image shows that the temperature ofplasma is less than 40℃. Thus the thermal diffusion of nickel atoms is avoided. Withthe sequent reduction-oxidation treatment, a clean metal-support interface is producedwhich is identical to the designed one. In addition, the size and shape of nickelparticles are modified towards larger metal-support interface and larger metal surfacearea. In photocatalysis, the clean metal-support interface is favorable for theseparation of photoinduced charges and transfer of electron from the semiconductor toNiO. The activity for water splitting over NiO/Ta2O5 and NiO/ZrO2 with the cleanmetal-support interface is 1.7 and 1.5 times higher than those with the diffusedinterfacial region, respectively.A novel plasma reduction of metal-loaded catalysts was established. H2PtCl6 isquickly reduced into highly dispersed amorphous metals. For photocatalytic H2generation reaction, such plasma-reduced Pt/TiO2 shows an activity slightly higherthan the hydrogen-reduced sample, much higher than the chemical-reduced andphoto-deposited samples. The metal clusters are crystallized with calcinations in inertatmosphere, with the activity being improved.Pd/Al2O3 was reduced by the plasma method. PdCl2 is reduced into amorphousclusters. For glucose oxidation, this Pd/Al2O3 is as efficient as the hydrogen-reducedone. Thermal calcinations transfer the metal clusters into crystals but decrease theactivity, indicating that amorphous catalysts are more efficient for this reaction.The characteristics of the preparation of highly efficient catalysts using plasmatreatment were studied from the point view of plasma physics and cluster physics. Theplasma behaves similar to dusty plasma in the presence of catalyst powders. Thechemical and physical states of loaded metal are greatly modified by the plasmatreatment. The chemical bonds are elongated and distorted by coulomb repulsions,and then they are quickly split when collided with the energetic species in plasma.Furthermore, they can be reduced by the electrons produced in plasma. A criterion issuggested to determine whether the metal salts can be reduced or not. The ions pair(Mn+/M) with positive standard electrode potential can be reduced and vice verse.Novel amorphous metal (oxide) clusters with lattice defects and vacancies areproduced with the plasma treatment, which may be highly active in catalysis. Whencalcined in air, the defects and vacancies are occupied by oxygen atoms, and novelmetal-support interface is produced. Meanwhile, the clusters are crystallized intocrystals when calcined in inert atmosphere, and the lattice distortion may be healed.