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activity and necessitate periodic regeneration of the
catalyst. Earlier, we investigated coke deposition on
calcined and hydrogen-prereduced chromia/alumina
catalysts during propane11 and isobutane12 dehydroge-
nation by in situ DRIFT and Raman spectroscopies.
These two complementary methods give information on
different types of carbon-containing deposits. Infrared
spectroscopy can be used to follow the formation of
aliphatic and aromatic hydrocarbon-type species and
oxygen-containing deposits (e.g., carbonates and car-
boxylates) on oxide samples.
13 Raman spectroscopic
measurements may reveal aromatic hydrocarbon spe-
cies14 and graphite-like deposits.
14,15 Our DRIFTS mea-
surements indicated that on calcined chromia/alumina
first carboxylates and aliphatic hydrocarbon deposits
and then with increasing time on stream unsaturated/
aromatic species formed.
11,12 Moreover, the Raman
spectroscopic measurements11 showed formation of graph-
ite-like deposits with longer times on stream. Hydrogen
prereduction decreased the rate of coke deposition but
did not influence the nature of the deposits.
11,12
Isobutane dehydrogenation was investigated by activ-
ity measurements at 580 °C and by in situ DRIFTS
combined with mass spectrometry (MS). The methods
used allowed us to elucidate more reliably the effect of
prereduction on the initial stages of dehydrogenation
and on the deactivation of the catalyst. The activity of
the catalyst could be measured almost continuously, and
because DRIFTS was chosen as the in situ spectroscopic
method, the reduction of Cr6 and the formation of
hydroxyls, oxygen-containing, and hydrocarbon-type
carbonaceous species during reduction and dehydroge-
nation could be followed.
Experimental Section
Samples Used in the Study. Three chromia/
alumina catalysts prepared by the atomic layer deposi-
tion (ALD) technique were used in the study. In the
ALD technique, the precursor of the metal oxide is
deposited on the support from the gas phase through
saturating gas solid reactions.
2 The ç-alumina support
(AKZO 000 1.5E) was crushed, sieved, and calcined
with air at 600 °C for 16 h. The catalysts were prepared
in a flow-type ALD reactor. The chromia precursor,
chromium(III) acetylacetonate, Cr(acac)3 (Riedel-de Hae ¨n,
99%), was vaporized and directed through the support
bed held at 200 °C. After the Cr(acac)3 chemisorption,
excess precursor was flushed from the reactor with
nitrogen, and the acac ligands were removed by air at
520 °C. The chemisorption-ligand removal cycles were
repeated 1, 6, or 12 times to obtain different chromium
loadings, after which the samples were calcined with
air at 600 °C for 4 h. According to earlier analyses,
10,16
the catalysts contained chromium 1.2, 7.5, and 13.5 wt
% (0.7, 2.9, and 8.2 atCr/nm2
support), and Cr6 0.9, 2.4,
and 3.0 wt %. The samples are referred to in the text
according to their chromium contents. No crystalline
Cr2O3 was detected by X-ray diffraction, indicating that
the chromia species were well-dispersed. X-ray photo-
electron spectroscopic (XPS) measurements indicated
that the 13.5CrAl catalyst contained Cr3 and Cr6 after
oxidation and mainly Cr3 after reduction with hydro-
gen, carbon monoxide, or n-butane. The treatments were
done in a reactor connected directly to the XPS system
enabling sample transfer in a vacuum.
For comparison, also the alumina support and a bulk
chromia sample (Cr2O3, Aldrich, 98 ) were investigated.
The chromia sample was calcined with air at 600 °C
for 4 h before use and contained Cr6 0.2 0.1 wt %.