Thermal characterisation of alumina supported chromium and platinum-chromium catalysts

Article abstract of DOI:10.1007/s10973-005-7006-z

This study presented results on reduction of alumina supported chromium and platinum-chromium catalysts using temperature programmed reduction method (TPR). It has been shown that catalysts after earlier oxidation step but without calcinations one undergo

Full text of DOI:10.1007/s10973-005-7006-z

Journal of Thermal Analysis and Calorimetry, Vol. 85 (2006) 2, 335–337
THERMAL CHARACTERISATION OF ALUMINA SUPPORTED
CHROMIUM AND PLATINUM–CHROMIUM CATALYSTS
Z. Sarbak1* and W. Jóüwiak2
1Laboratory of Adsorption and Catalysis in Environmental Protection, Faculty of Chemistry, Adam Mickiewicz University,
Grunwaldzka 6, 60-780 PoznaÕ, Poland
2Institute of General and Ecological Chemistry, Technical University, ¨eromskiego 116, 90-924 ˜ódü, Poland
This study presented results on reduction of alumina supported chromium and platinum–chromium catalysts using temperature pro-
grammed reduction method (TPR). It has been shown that catalysts after earlier oxidation step but without calcinations one undergo
reduction in lower temperature in comparison to calcined only catalysts. Moreover, addition platinum to Cr/Al2O3 catalysts also
caused decrease of reduction temperature. It has been observed that over the examined catalysts oxidation CO to CO2 and reduction
CO to CH4 occurs. However, on Pt–Cr catalysts both reactions proceed at lower temperature compare to Cr catalysts.
Keywords: Cr/Al2O3, Pt–Cr/Al2O3 catalysts, TPR–H2, TPR–CO
Introduction
idation. The TPR measurements were carrier out on the
apparatus AMI I, with a katharometer as a detector for
the samples reduced by hydrogen (5% H2/95% Ar) or a
quadrupole mass spectrometer made by Dycor for the
samples reduced with CO (5% CO/95% Ar). Two se-
ries of measurements were performed – one for the
samples subjected to calcination at 500°C for 5 hours
and oxidation in the atmosphere of O2 (5% O2/95% Ar)
and the other one for the samples subjected to oxida-
tion only. In the first series of measurements the TPR
curves were recorded for the samples of about 100 mg
at the gas flow rate of 50 cm3 min–1, the temperature in-
crease rate of 20°C min–1 and the time of heating of 30
min. The samples were oxidized at 400 (curve 1), 500
(curve 2) and 700°C (curve 3) and then they were sub-
jected to the temperature programmed reduction with
hydrogen (TPR–H2).
Catalysts containing platinum and chromium species
supported on alumina and other solids, prepared in
various ways, have been studied by several groups
[1–3]. These catalysts become increasingly interest-
ing to many authors in last years. Bimetallic system
have been demonstrated to modify the catalysis of
each component and create new catalytic properties.
As was reported [4] Pt–Cr bimetallic catalysts show
higher activities than Pt or Cr monometallic catalysts.
Our earlier study on the characterisation and struc-
ture of the catalysts Pt–Cr/Al2O3 [5] has proved their
high catalytic activity in the reaction of methane oxida-
tion [6]. A recent study [7] has shown their high cata-
lytic activity in the reaction of simultaneous elimination
of organic compounds of sulphur and nitrogen by
hydrogenolysis of the bonds C–S and C–N. This paper
reports results of the study of these catalysts by the tem-
perature programmed reduction described earlier [8].
In the third series of measurements, the samples
were subjected to oxidation at 500°C then tempera-
ture programmed reduction with carbon monoxide
(TPR–CO) at 700°C, at the gas flow rate 40 cm3 min–1
and the temperature growth rate of 15°C min–1. Dur-
ing oxidation the sample of 200 mg having been
heated to a desired temperature, was annealed at that
temperature for 30 min, while during the TPR–CO
measurements it was done for 10 min.
Experimental
The catalysts were prepared by the method of incipi-
ent wetness using the solution of chromium nitrate,
while platinum was deposited using a solution of
hexachloroplatinic acid according to a procedure de-
scribed earlier [5].
Results and discussion
Prior to the TPR measurements with the use of
hydrogen or carbon monoxide, the catalysts samples
were subjected to calcination and oxidation or only ox-
The TPR measurements were performed for the prep-
arations listed in Table 1.
*
Author for correspondence: sarbak@amu.edu.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

SARBAK, JÓZWIAK
Table 1 Nomenclature, nominal composition and surface
area of catalysts
samples with platinum recorded at 700°C (curve 3c)
shows only one peak, while an analogous profile oxi-
dised for 2Cr/Al2O3 shows two maxima. The maxi-
mum in the TPR profile of the sample 0.3Pt–2Cr/Al2O3
occurs at a temperature intermediate between those of
the two peaks recorded for 2Cr/Al2O3.
Catalysts
Surface area /m2 g–1
Al2O3 (Degussa)
0.3# Pt/Al2O3
2Cr/Al2O3
93
93
92
76
95
77
Figure 2 presents the TPR–H2 profiles of the
samples 20Cr/Al2O3 and 0.3Pt–2Cr/Al2O3. The pro-
file obtained for the sample of 20Cr/Al2O3 showed
peaks at much lower temperatures than those of the
peaks recorded for the samples with 2 mass% Cr,
which means that reduction of the samples with
20 mass% Cr is easier. In all samples presented in
Fig. 2 the consumption of hydrogen was elevated,
which is obvious taking into account a higher content
of chromium. Preliminary calcination of the samples
of 20Cr/Al2O3 (curves 2a, 3a) results in much greater
consumption of hydrogen than when the samples are
subjected to oxidation only (curves 2b and 3b). For
the samples 0.3Pt–2Cr/Al2O3 an increased consump-
tion of hydrogen was observed at lower temperatures
than for the preparations without platinum.
20Cr/Al2O3
0.3Pt–2Cr/Al2O3
0.3Pt–20Cr/Al2O3
The TPR–H2 profiles slightly higher temperature
than for the samples Cr/Al2O3 subjected to oxidation
only obtained revealed peaks with maxima at differ-
ent temperatures. In general, the consumption of hy-
drogen during TPR measurements for the samples
Cr/Al2O3 preliminary calcined and then oxidised was
higher and occurred at a slightly higher temperature
than for samples Cr/Al2O3.
The Cr/Al2O3 samples subjected to calcination at
700°C and oxidation and then reduction with hydro-
gen contain two types of reduced species. According
Fig. 1 TPR–H2 profiles for a, b – 2Cr/Al2O3 and
c – 0.3Pt–2Cr/Al2O3 samples calcined at 1 – 400°C,
2 – 500°C and 3 – 700°C
Figure 1 presents the TPR–H2 profiles for the
samples of 2Cr/Al2O3 calcined at 500°C subsequently
subjected to oxidation at 400, 500 and 700°C
(curves 1a, 2a, 3a) and for the same preparations but
subjected to oxidation without a preliminary calcina-
tion (curves 1b, 2b, 3b). A comparison of the TPR–H2
profiles reveals that the peaks obtained for the samples
subjected to calcination followed by oxidation at 400
and 500°C (curves 1a and 2a) are much greater than the
corresponding ones recorded for the samples oxidised
without preliminary calcination (curves 1b and 2b).
When oxidation was performed at 700°C, the TPR pro-
files obtained for the calcined and oxidised sample and
the only oxidised one are similar with the maxima at
the same temperatures (curves 3a and 3b). The maxima
in the TPR–H2 profiles obtained for the samples of
0.3Pt–2Cr/Al2O3 subjected to oxidation occur at tem-
peratures higher than the maxima obtained for
2Cr/Al2O3 (curves 1c, 2c), when the samples are re-
duced at 400 and 500°C. The TPR–H2 profile for the
Fig. 2 TPR–H2 profiles for a, b – 20Cr/Al2O3 and
c – 0.3Pt–20Cr/Al2O3 samples calcined at 1 – 400°C,
2 – 500°C and 3 – 700°C
336
J. Therm. Anal. Cal., 85, 2006

ALUMINA SUPPORTED CHROMIUM AND PLATINUM–CHROMIUM CATALYSTS
to our earlier study [9] the samples Cr/SiO2 calcined
two different surface groups take part in reduction.
The water formed during the reaction can be involved
in the following reactions:
in high temperatures were found to contain Cr2+ ions
in at least two different coordinatively unsaturated
forms. The type of Cr ions corresponding to the maxi-
mum at a higher temperature does not occur in the
preparation 20Cr/Al2O3 subjected to oxidation only
but it is present in the oxidised 2Cr/Al2O3. The in-
crease in the content of chromium in the preparations
Cr/Al2O3 results in a significant decrease in the reduc-
tion temperature. For the catalysts containing
0.3 mass% platinum the maxima of hydrogen con-
sumption were observed at temperatures lower than
for the catalysts without platinum. Moreover, it was
found that the catalysts Pt–Cr/Al2O3 oxidised in lower
temperatures used greater amounts of hydrogen in the
process of reduction.
CO+H2O→CO2+H2
(2)
(3)
CH4+2H2O→CO2+4H2
The observed decrease in CO consumption in the
range 400–600°C confirms the occurrence of reac-
tions (1) and (2).
Introduction of a small amount of platinum into
the preparation 2Cr/Al2O3 results in a decrease in the
temperatures of formation of CO2 and CH4. As follows
from the above results, the catalyst 0.3Pt–2Cr/Al2O3
can actively catalyse the process of formation of CH4
from CO, and H2 and CH4 oxidation to CO2. This latter
conclusion has been already indicated in our earlier
study [6].
The samples of 2Cr/Al2O3 and 0.3Pt–2Cr/Al2O3
preliminary oxidised at 500°C, were also subjected to
TPR with carbon oxide, the profiles obtained are
shown in Fig. 3. The products of this reduction con-
tained CO and CH4.
Conclusions
As follows from the above discussed results, for the
catalysts subjected to oxidation without the stage of
calcinations, the reduction process occurs at lower
temperatures. It has been shown that an addition of
platinum to Cr/Al2O3 catalysts leads to a decrease in
the reduction temperature.
On the surface of the catalysts studied carbon
monoxide undergoes oxidation to CO2 and reduction
to CH4. An addition of platinum to the catalysts re-
sults in a decrease in the temperature of the reactions
leading to formation of these products.
References
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The presence of these compounds means that
apart from the use of CO for the reduction of Pt and
Cr groups, it is also used for formation of CH4 and ox-
idation to CO2. The latter process starts already at
about 200°C, whereas the reduction of CO to CH4 re-
quires temperatures of about 500°C.
As follows from the curve characterising the for-
mation of CO2, the catalysts studied must have at least
four different types of surface groups responsible for
the oxidation but for formation of CH4, according to
the reaction:
9 Z. Sarbak, Appl. Catal., 124 (1995) 245.
Received: March 3, 2005
Accepted: September 6, 2005
OnlineFirst: December 12, 2005
CO+3H2→CH4+H2O
(1)
DOI: 10.1007/s10973-005-7006-z
J. Therm. Anal. Cal., 85, 2006
337