Chemistry Reference
In-Depth Information
for the polymerisation of ethylene at 75 bar, 60 1C in hexane. It was found
that the RESS processed catalyst had higher catalytic activity than the
unprocessed material, and this was speculated to be due to the higher
surface area of the former. It was also noted that there was a lower
molecular weight of polymer formed when using the RESS process ma-
terial, which was explained as being due to the exothermic nature of the
reaction enhancing the chain transfer rate.
4.3 Catalysts prepared by reactions in supercritical H
2
O
Supercritical H
2
O is particularly corrosive, nevertheless it offers great po-
tential for the synthesis of catalysts. The preparation of ceria-zirconia
based catalysts has been performed by Darr and co-workers in a simul-
taneous coprecipitation method using SC-H
2
O. The preparation of a
complex 0.5-1.5 wt%Pd/Ce
0.6
Zr
0.3
La
0.05
Y
0.05
O
2-d
/Al
2
O
3
(CZA) catalyst
has been used as a three-way catalyst and also for total oxidation
of toluene.
81,82
These catalysts were prepared by the contact of a
solution containing the relevant metal nitrate salts (Pd precursor was
Pd(NH
3
)
4
Cl
2
) and NaOH with superheated water (240 bar and 450 1C). This
resulted in the direct precipitation of the mixed metal oxide catalyst, un-
like the SAS and RESS preparation techniques where a second calcination
step was required to convert the catalyst precursor to the final catalyst. It
was found that the Ce
0.6
Zr
0.3
La
0.05
Y
0.05
O
2-d
(CZ) component of the material
had the correct elemental concentrations, but that the desired CZ : Al ratio
of 1 : 2 was not quite attained, as a ratio of 1 : 2.5 was produced. XRD an-
alysis showed that the formation of a solid solution of ceria-zirconia with
an average crystallite size of 4 nm and that an AlOOH phase was formed.
These characteristics were similar to a catalyst prepared by a conventional
coprecipitation of the metal nitrate salts with ammonium hydroxide (pH
9.5), although g-Al
2
O
3
was observed as the Al phase in the conventionally
prepared catalyst. HRTEM and STEM analysis of the material prepared
with SC-H
2
O showed exceptional mixing of ceria-zirconia and AlOOH,
whilst the conventional material had distinct ceria-zirconia and alumina
phase segregation. This exceptional mixing seen with the SC-H
2
Oprepar-
ation technique is comparable to that seen with the mixed metal SC-CO
2
SAS preparations and can be explained by the high nucleation rates af-
forded by the use of SCFs. The strong interaction of CZ and Al phases in
the SC-H
2
O preparation resulted in a reduced level of CZ sintering, relative
to the conventionally prepared material, after an accelerated ageing test at
1000 1C. After the ageing process CZ crystallites of 12nm were observed for
the supercritical catalyst, whereas they were 17nm for the conventional
catalyst.
The addition of Pd to the catalysts was investigated in two manners;
the first was a simple wet impregnation onto the preformed SC-H
2
O CZA
and the second was the addition of the Pd salt to the SC-H
2
O copreci-
pitation process. It was found that Pd dispersion was higher in the SC-
H
2
O coprecipitation process, with the Pd being in both Pd
0
and Pd
2
รพ
oxidation states for the SC-H
2
O CZA, whereas it was only present as Pd
0
when added by wet impregnation. For both the three-way catalysts and
total toluene oxidation testing, only the SC-H
2
O CZA samples with the pd
