S. Natesakhawat et al. / Journal of Molecular Catalysis A: Chemical 260 (2006) 82–94
93
occupied by Fe2+ and Fe3+ equally, while the oxygen atoms form
a face-centered cubic lattice (fcc) within the spinel. The arrange-
ment of iron and oxygen atoms in magnetite can be written
as Fe(A)3+Fe(B)3+Fe(B)2+(O2−)4, where A and B refer to tetra-
hedral and octahedral sites, respectively. Fast electron hopping
between Fe2+ and Fe3+ located in octahedral sites in magnetite
metallic Cu that forms in the two-step preparation because of
the spacer function of iron oxide. This could explain why the
catalyst prepared by one-step technique has high activity in the
entire reaction temperature range investigated in this study. The
minor Cu species that cannot be reduced may also contribute to
the high activity by providing electronic promotion on magnetite
structure. The next paper in this series will report the synthesis
of catalysts with well-dispersed Cu species in iron oxide matrix
and the electronic function of Cu promoter on the magnetite
structure.
¨
has been confirmed by Mossbauer spectroscopy [35,36] and
isotopic labeling studies [37]. Furthermore, the water-gas shift
reaction over Fe–Cr catalysts was well described by the kinetics
of surface oxygen removal by CO and surface oxygen incor-
poration from H2O, indicating that the redox mechanism is the
primary pathway for the water-gas shift reaction over magnetite
[38,39]. Our reaction results and XPS characterization indicate
that magnetite is the major catalytic phase. In situ DRIFTS
results show that the WGS reaction over Fe-based catalysts
occurs via the redox mechanism. The catalyst surface under-
goes successive oxidation and reduction cycles by H2O and CO
to produce H2 and CO2 respectively, with Fe2+ and Fe3+ occu-
pying the octahedral sites in the magnetite structure constituting
an oxidation-reduction pair (redox).
The role of copper in HTS catalysts for the water-gas shift
reaction is still uncertain. Hutchings and co-workers [15,16]
reported that copper enhanced the WGS activity by modifying
the electronic properties of coprecipitated Fe–Cr catalysts. Their
additional feature was observed with coprecipitated Fe–Cr–Cu
catalyst, suggesting that CuO and Cr2O3 could exist within the
magnetite structure. TPR profile reported by Araujo and Rangel
[18]showedamajorreductionpeakfromreductionofCuspecies
over Fe–Al–Cu catalyst, which was prepared by coprecipitation
of Fe and Al and impregnation of Cu to Fe–Al. They concluded
that copper itself does not have a significant role as an active site,
but rather as a promoter. Andreev et al. [3] suggested that Cu
doped on coprecipitated Fe–Cr catalyst is providing new active
sites and reacting in a similar manner as the Cu species in the
low temperature Cu/ZnO water gas shift catalyst.
In this study, Fe–Al–Cu (two-step) catalyst was prepared by a
coprecipitation–impregnation method by which the catalyst sur-
face could be enriched in copper upon the impregnation step. As
previously discussed, TPR, in situ XRD, and XPS results clearly
indicate that copper is in a metallic form after reduction. This
catalyst showed very high activity at lower temperature. The sin-
tering of metallic Cu could contribute to loss of the WGS activity
when these catalysts are used at higher temperatures. Therefore,
the promotional effect of Cu in these catalysts could be to pro-
vide additional active sites for the water-gas shift reaction. For
the Fe–Al–Cu (one-step) catalyst prepared by a coprecipitation
method, however, XPS and TPR studies indicate that the envi-
ronment of Cu species in the one-step catalyst is significantly
different from that in the two-step catalyst, although, the major
Cu species after reduction is also metallic Cu. It is conceivable
that with one-step preparation, Cu species co-precipitate with
iron oxide to form a solid solution. These Cu species can be
reduced to metallic Cu, which provides additional active sites,
giving a significant boost to the activity of the catalyst, especially
at lower temperatures. At higher temperatures, the sintering of
this metallic Cu species is not very extensive as the surface
4. Summary
Chromium-free iron-based HTS catalysts were prepared
by adding both aluminum and copper using two-step
coprecipitation–impregnation and one-step coprecipitation
methods. The addition of aluminum stabilizes the magnetite
phase by retarding its further reduction to FeO or metallic iron.
Aluminum is a promising chromium replacement to act as a
textural promoter for iron-based water gas shift catalyst. Cop-
per can be used as a structural promoter for high temperature
Fe-based catalyst to enhance the catalytic activity. However, the
promotion of Cu species is affected by preparation methods sig-
nificantly. With two-step preparation, the major portion of Cu
species is on the iron oxide surface, which is reduced to metal-
lic Cu after reduction. The main promotion of the Cu species
in this preparation is to provide additional active sites. With
increasing reaction temperature, the promotional effect of Cu
decays because of sintering of the metallic Cu, the same phe-
nomenon observed in low temperature water gas shift catalysts.
With one-step preparation, the major portion of Cu species co-
precipitates with iron oxide. This Cu species is also reduced to
metallic Cu at reaction conditions to provide additional active
sites. However, this Cu species is not as prone to sintering as the
surface Cu due to the spacer function of the iron oxide. A portion
of Cu species that cannot be reduced may be incorporated into
iron oxide matrix and may serve as an electronic promoter. The
in situ DRIFTS and XPS studies support the conclusion that the
WGS reaction on iron-based catalysts occurs through a redox
process on magnetite phase.
Acknowledgments
The financial contributions from the Ohio Coal Development
Office and the Ohio Department of Development through the
Wright Center of Innovation Program are gratefully appreciated.
References
[1] N.A. Koryabkina, A.A. Phatak, W.F. Ruettinger, R.J. Farrauto, F.H. Ribeiro,
J. Catal. 217 (2003) 233.
[2] G.C. Chinchen, R.H. Logan, M.S. Spenser, Appl. Catal. 12 (1984) 89.
[3] A. Andreev, V. Idakiev, D. Mihajlova, D. Shopov, Appl. Catal. 22 (1986)
385.
[4] J.C. Gonzalez, M.G. Gonzalez, M.A. Laborde, N. Moreno, Appl. Catal. 20
(1986) 3.
[5] G. Doppler, A.X. Trautwein, H.M. Ziethen, E. Ambach, R. Lehnert, M.J.
Sprague, U. Gonser, Appl. Catal. 40 (1988) 119.