Catalysis Today 242 (2015) 168–177
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Catalysis Today
j o ur na l ho me page: www.elsevier.com/locate/cattod
Effect of rhodium on the water–gas shift performance of Fe
2
O
3
/ZrO
2
and CeO
2
/ZrO
2
: Influence of rhodium precursor
Abrar A. Hakeem, Jaikishen Rajendran, Freek Kapteijn, Michiel Makkee
∗
Catalysis Engineering, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, NL 2628 BL Delft,
The Netherlands
a r t i c l e i n f o
Article history:
Received 27 February 2014
Received in revised form 13 May 2014
Accepted 14 May 2014
Available online 24 July 2014
Keywords:
Ceria
Rhodium promotion
Water–gas shift
Methanation
Iron oxide
Rhodium chloride
a b s t r a c t
Iron oxide and ceria are known to have redox properties and their performance as a water–gas shift
(WGS) catalyst is investigated at low H
2
O/CO ratio (∼2) in the temperature range of 623–773 K at 21 bar
with space velocities relevant for industrial applications. The WGS activity of supported iron oxide is
higher than that of supported ceria. The addition of rhodium to supported iron oxide (Rh/Fe
2
O
3
/ZrO
2
)
enhances the WGS activity while for supported ceria (Rh/CeO
2
/ZrO
2
) rhodium enhances the methanation
activity at high pressure (21 bar) WGS conditions. The improved hydrogen production by enhancement
of the WGS activity over Rh/CeO
2
/ZrO
2
due to rhodium is undone by the consumption of hydrogen in the
methane production. Methane is produced over Rh/Fe
2
O
3
/ZrO
2
and Rh/CeO
2
/ZrO
2
catalysts due to the
presence of rhodium.
Rhodium nitrate as precursor results in a better WGS activity performance over Rh/Fe
2
O
3
/ZrO
2
and
a better methanation activity is obtained over Rh/CeO
2
/ZrO
2
in comparison with the use of rhodium
chloride precursor. The presence of residual chlorine suppresses the promotional WGS activity due to
rhodium as well as the methanation activity. In both ex-nitrate and ex-chloride catalysts the smaller
rhodium particles are highly active for methane formation.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The water gas shift (WGS) (1) is a reversible and moderately
exothermic reaction [1].
CO + H
2
O ⇆ CO
2
+ H
2
; H
◦
= -41 kJ mol
-1
(1)
In industry the WGS reaction is carried out in two stages: the
high temperature shift (623–773 K) using Fe
2
O
3
/Cr
2
O
3
/Cu cata-
lyst, reducing the concentration of CO to typically around 2% and
the low temperature shift (473–513 K) using CuO/ZnO/Al
2
O
3
cat-
alyst, reducing the concentration of CO to ppm levels (typically
<3000 ppm) [1,2]. In case of Fe
2
O
3
/Cr
2
O
3
/Cu the redox mechanism
is considered to be operational with Cr as a structural promoter
to prevent sintering of iron oxide [3,4] and Cu as a promoter to
iron oxide [2]. Magnetite (Fe
3
O
4
) [4] is considered as an active
phase of this catalyst in which redox mechanism is dominant by
the presence of Fe
+3
and Fe
+2
. It has been reported that CeO
2
has
redox characteristics due to the presence of Ce
+4
and Ce
+3
under
∗
Corresponding author. Tel.: +31 152781391; fax: +31 152785006.
E-mail address: m.makkee@tudelft.nl (M. Makkee).
net oxidizing and reducing conditions, respectively [5–7]. CeO
2
is
an important component of exhaust catalysts [6], has oxygen stor-
age capacity [8–10], stabilizes the dispersion of noble metals [9],
and promotes noble metals toward WGS [11]. The redox proper-
ties offered by CeO
2
make it a promising catalyst for WGS reaction
similar to that of the iron oxide, which is considered to operate
via a redox mechanism. Ceria supported different noble metals e.g.
Au [12–15], Pt [16,17], (Pt–Cu, Pd–Cu) [18,19], (Pt, Pd, Rh) [11], Rh
[20], (Pt, Rh, Ru, and Pd) [21], (Cu and Ni) [22], (Ni and Re) [23] have
been reported for WGS reaction. Most of these studies focus are on
fuel cell applications in which low temperature WGS is applicable
and the catalyst performance has been tested at atmospheric con-
ditions which is not representative for the industrial application
(20–30 bar) regarding deactivation of the catalyst and side product
formation (especially methane).
In this study we compare the performance of a zirconia sup-
ported ceria and iron oxide under realistic conditions (21 bar)
for high temperature WGS. The aim of catalyst promotion with
rhodium is to enhance the activity of iron oxide and ceria, using
two different rhodium precursors (nitrate and chloride). Further,
the influence of the counter ions in the precursor is illustrated by
the catalyst performance.
http://dx.doi.org/10.1016/j.cattod.2014.05.015
0920-5861/© 2014 Elsevier B.V. All rights reserved.