Chemistry Reference
In-Depth Information
Other studies involving PtMe
2
COD supercritical deposition to prepare
supported Pt catalysts have looked at gasification of glucose and wheat
straw hydrolysates to form hydrogen rich gas streams.
77,78
The conditions
used were the same as those previously discussed for Erkey and co-
workers preparation of Pt/carbon electrocatalysts.
71
However, in the
range of supports chosen for biomass hydrolysis was extended beyond
carbon, with g-Al
2
O
3
,WO
3
-ZrO
2
and TiO
2
also being used.
78
It was found
that under the same deposition conditions (temperature, pressure, sub-
strate:support ratio and deposition time) the actual Pt loading on the
various supports varied considerably, with the lowest loading on g-Al
2
O
3
and the closest to the target loading was found for activated carbon. The
catalysts in this study were all active, but they were not compared with
those prepared by conventional techniques and this makes critical
analysis of any performance advantages dicult. In another study by
Meryemoglu et al. a comparison of performance with a wet impregnation
catalyst was made, and they found that the supercritical and con-
ventional catalysts had similar activity and selectivity towards H
2
,
although the wet impregnation catalyst was more stable to a chemical
reduction step, which improved the activity of this catalyst.
77
Another study that has used a SC-CO
2
as a solvent to transport metal
salts into mesoporous materials involved the formation of Rh and Rh-Pt
on HMM-1, FSM-16 and standard SiO
2
for use as catalysts for butane
hydrogenolysis.
79
Catalyst preparation involved a multi-step process
outlined in Fig. 11. The first step required a wet impregnation of the
mesoporous supports using a THF, rhodium(II) acetate and platinum(II)
acetylacetonate or rhodium(II) acetate only solution. The THF was then
removed by rotary evaporation and the sample dried under vacuum, prior
to the sample being divided into two portions. One part of the sample
was then placed in a reactor which was heated to 70 1C and charged with
CO
2
to 160 bar and it was then maintained under pressure for 24 h. After
depressurisation the sample was calcined at 400 1C and then reduced at
200 1C under pure H
2
. The other half of the initial impregnated sample
was used a control and calcined and reduced without a supercritical
treatment step.
Carbon monoxide chemisorption analysis on the supported Rh cata-
lysts revealed a significant increase in metal dispersion after SC-CO
2
treatment, for example Rh/FSM-16 dispersion increased from 15% to
71%.
79
This improvement of metal dispersion was confirmed for the
monometallic Rh and bimetallic catalysts by TEM analysis, which also
indicated that SC-CO
2
treatment improved dispersion into the support
micropores. Infrared spectroscopy studies of CO chemisorption on the
bimetallic catalysts indicated that enhanced Rh-Pt alloy formation was
observed after the SC-CO
2
treatment. The butane conversions for the
hydrogenolysis reaction were found to improve for both the mono-
metallic and bimetallic catalysts after SC-CO
2
treatment, although the
improvements were far more substantial with the mesoporous supports.
This observation suggests that the deposition of metal into the pores of
the supports, aided by SC-CO
2
treatment, is beneficial to activity.
Whilst bimetallic catalysts had lower butane conversion relative to the
