Synthesis and characterization of bimetallic PtNi particles for the application of catalysts
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Date
2010
Authors
Hanif Abu Bakar, Noor Hana
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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, NaB~ 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
NaB~ 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.