Synthesis and characterization of bimetallic Pt/Ni particles for the application of catalysis

Abstract : The synthesis of PtNi bimetallic particles supported silica catalysts, prepared via non-classical methods using sodium borohydride (NaBH4) as a reducing agent, was studied in detail. The silica supports employed in this work is limited to crystalline silica and mesoporous aluminosilicate (MCM-41). Various preparation techniques as well as reduction parameters were investigated to gain an insight on how these factors influenced the final structure of the PtNi particles on the silica support and their catalytic reactivity towards the hydrogenation of benzene to cyclohexane. It was found that this reduction method enabled total reduction of the metal salts during the preparation stage of the catalysts. Hydrogen consumptions which were detected using H2-TPR analysis were mainly attributed to surface oxidation of the metal phase during storage. Studies on the effect of preparation techniques showed that the surface and catalytic properties of the catalysts are largely affected by the PtNi ratio as well as the method in which the metal salts are introduced onto the support. Catalysts prepared via co-impregnation technique generally exhibited better catalytic reactivity when compared to those prepared via co-precipitation and step-impregnation techniques. Further, catalysts with higher Ni content showed a tendency towards lower reactivity in contrast to those with high Pt content. Several catalysts demonstrated enhanced reactivity when compared to the monometallic Pt catalysts. Investigations showed that the improved reactivity can be attributed to alloying of the Pt and Ni accompanied by surface segregation of Pt. As a means to improve catalytic reactivity, PtNi stabilized oleic acid particles were synthesized prior to incorporation onto a silica support. The intention of this study is to allow better control of the dispersion and alloying between the PtNi particles. Results show that though better dispersed alloys were obtained, very low activity was observed. Nickel surface segregation is likely to be the cause of this due to the presence of oxygen from oleic acid. The effect of several reduction parameters was also investigated to enhance catalytic reactivity. The reduction temperature, NaBH4 concentration and medium in which reduction was carried out were varied. Variations in these parameters affected the particle morphology and dispersion of the PtNi particles. Optimum catalytic reactivity was obtained when small dispersed PtNi particles were formed at 273 K using 0.3 M NaBH4 in a medium of ethanol. Classical methods were also used for the synthesis of PtNi supported catalysts. In this study the PtNi particles were formed using H2 gas as the reducing agent. Several catalysts showed improved reactivity. Investigations show that this is attributed to the anchoring effect of Ni2+ ions which anchors Pt to the support, forming fine dispersed Pt particles available for catalytic reaction. In general, it is obvious that alloyed and non ? alloyed bimetallic particles supported on silica can lead to the enhancement of hydrogenation reactions when compared to the respective monometallic catalysts. However, the PtNi ratios, preparation techniques, environment in which the particles are reduced and support influences the structure of the metallic phase of these catalysts. Therefore it is imperative to gain a thorough understanding on these parameters, in order to synthesize catalysts with desired properties.
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Noor Hana Hanif Abu Bakar. Synthesis and characterization of bimetallic Pt/Ni particles for the application of catalysis. Other. Université Henri Poincaré - Nancy 1, 2010. English. ⟨NNT : 2010NAN10013⟩. ⟨tel-01748212⟩

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