An investigation of supported platinum and bimetalic oxidation catalysts
Tiernan, Michael (1996) An investigation of supported platinum and bimetalic oxidation catalysts. PhD thesis, Dublin City University.
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A variety of supported Pt and bimetallic catalysts were investigated for the combustion of iso-butane and attempts were made to associate particular surface characteristics with combustion efficiency. In particular, the effects of Ce addition to Pt/Al203 systems were studied. A number of other metal oxide (Mn, Cr, Zr) additives were investigated and the use of Ce02 and ZrC>2 as alternative supports to AI2O3 was also studied. The effect of high temperature (800°C) aging and reduction (500°C) on the activity of calcined catalysts was determined.
The use of Ce, Cr or Mn additives was found to be potentially beneficial for the iso-butane oxidation activity of Pt/Al2C>3. TPR experiments showed that the addition of Ce, Cr, Mn or Zr to Pt/Al203 resulted in a bimetallic surface interaction which affected surface reducibility. XPS indicated that the nature of Pt-Ce interaction varied depending on the loading of Ce. After impregnation and calcination at 630°C, the presence of Ce, at Ce:Pt>8:1 atomic ratio, generally resulted in poorer oxidation activity relative to Pt/Al203 with higher temperatures required to achieve the same level of i-C4HiQ conversion. However, at lower Ce loadings (Ce:Pt<1:1 atomic ratio), oxidation activity was not so adversely affected and one sample, containing 0.29 wt.% Ce, was found to be noticeably more active than monometallic PtM.1203. Reduction of a 0.5 wt.% Pt/Al203 catalyst resulted in considerable loss of activity. The addition of Ce, Mn, or Cr appeared to help prevent this deactivation. Temperature Programmed Reduction (TPR) profiles indicated that the existence of an interaction between Pt and Ce in Pt-Ce/Al203 samples was associated with improved activity following reduction.
In general, the activity of the Pt catalysts increased after initial testing in the reactant mixture. The most active 0.5 wt.% Pt/Al203 catalysts were obtained after aging in the stoichiometric air : i-C ^ Ijo reaction mixture or in a static air atmosphere. H2 chemisorption measurements indicated the main effect of aging was to increase Pt particle size. The reaction was concluded to be structure sensitive on Pt/Al203 with sintered Pt crystallites being more active than highly dispersed smaller Pt particles. The presence of Ce, Mn, Cr or Zr additives had an adverse effect on the activity of Pt/Al203 after aging in an 0 2 -containing atmosphere.
After impregnation and calcination at 630°C, a 0.5 wt.% Pt/ZrC>2 catalyst was considerably more active than a 0.5 wt.% PI/AI2O3 . Addition of 2.3 wt.% Zr to a 0.5 wt.% Pt/Al203 catalyst had, however, a detrimental effect on catalyst activity. It was concluded that the higher activity of Pt/ZrC>2 relative to Pt/Al203 was due to the presence of larger Pt particles on the former support due to it's considerably lower surface area.
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