Resolving the Reaction Mechanism for H2 Formation from High-Temperature Water-Gas Shift by Chromium-Iron Oxide Catalysts

Article abstract of DOI:10.1021/acscatal.6b00659

The reaction mechanism of the high-temperature water-gas shift (HT-WGS) reaction catalyzed by chromium-iron oxide catalysts for H₂ production has been studied for 100 years with two reaction mechanisms proposed: redox and associative (involving surface HCOO?). Direct experimental support for either mechanism, however, is still lacking, which hinders a thorough understanding of catalytic roles of each elements and the rational design of Cr-free catalysts. The current study demonstrates, with temperature-programmed surface reaction (TPSR) spectroscopy (CO-TPSR, CO+H₂O-TPSR, and HCOOH-TPSR), for the first time that the HT-WGS reaction follows the redox mechanism and that the associative mechanism does not take place.

Full text of DOI:10.1021/acscatal.6b00659

Letter
pubs.acs.org/acscatalysis
Resolving the Reaction Mechanism for H2 Formation from High-
Temperature Water−Gas Shift by Chromium−Iron Oxide Catalysts
Minghui Zhu and Israel E. Wachs*
Operando Molecular Spectroscopy & Catalysis Laboratory, Department of Chemical and Biomolecular Engineering, Lehigh
University, Bethlehem, Pennsylvania 18015, United States
S
* Supporting Information
ABSTRACT: The reaction mechanism of the high-temperature water−gas
shift (HT-WGS) reaction catalyzed by chromium−iron oxide catalysts for H2
production has been studied for 100 years with two reaction mechanisms
proposed: redox and associative (involving surface HCOO*). Direct
experimental support for either mechanism, however, is still lacking, which
hinders a thorough understanding of catalytic roles of each elements and the
rational design of Cr-free catalysts. The current study demonstrates, with
temperature-programmed surface reaction (TPSR) spectroscopy (CO-TPSR,
CO+H2O-TPSR, and HCOOH-TPSR), for the first time that the HT-WGS
reaction follows the redox mechanism and that the associative mechanism does
not take place.
KEYWORDS: redox mechanism, formate mechanism, associative mechanism, HT-WGS, iron-based catalysts, hydrogen
he majority of industrial H2 is currently produced by
methane steaming reforming (MSR) followed by the
During the HT-WGS reaction, the equilibrated bulk iron
oxide phase is present as magnetite (Fe3O4), which is prone to
deactivation by thermal sintering. Sintering is inhibited by
addition of chromium oxide (8−14 wt %) to stabilize the
surface area of the magnetite phase.9−12 With increasing
environmental and health concerns about hexavalent chromium
(Cr6+), which is a potent carcinogen, there has been much
interest in developing Cr-free iron oxide HT-WGS catalysts in
recent years.11,13 Rational design of a Cr-free iron oxide HT-
WGS catalyst, however, requires a thorough fundamental
understanding of the reaction mechanism.
The reaction mechanism and kinetics of the HT-WGS
reaction by Cr2O3−Fe2O3 catalysts have been extensively
studied for 100 years, yet no consensus has been
reached.9−11,13,14 The “regenerative” or redox mechanism is
the most accepted reaction mechanism involving alternate
reduction of the catalyst surface oxygen site (O*), with “*”
representing an empty surface site, by gas-phase CO (eq 2) and
oxidation of the reduced catalyst surface empty site by H2O
vapor (eq 3).15−19 The reversible nature of the WGS reaction
also allows reaction steps 2 and 3 to take place from the right to
the left, whereby the catalyst is oxidized by CO2 and reduced by
H2.
T
water−gas shift (WGS) reaction to increase or control the H2/
CO ratio and is employed in numerous applications (ammonia
synthesis (from H2/N2), methanol synthesis (from H2/CO/
CO2), synthetic fuels (from H2/CO), etc.). Ammonia synthesis
alone is responsible for more than 2% of the world’s daily
energy use and produces the synthetic fertilizer required to feed
the world’s growing population.1 Although there is much
interest in developing sustainable H2 production from photo-
catalytic splitting of H2O2−4 and biomass reforming,5,6
production of H2 from fossil fuels (CH4 ≫ hydrocarbons ≫
coal) will be around for quite some time given its established
technology and cost competitiveness. For example, H2 fueling
stations for fuel cell powered automobiles currently being set
up in America and Germany rely on MSR and WGS because of
the availability of abundant and inexpensive natural gas.7
CO + H2O ↔ CO2 + H2 ΔH = −40.6KJ/mol
(1)
The WGS reaction involves carbon monoxide reacting with
steam to produce carbon dioxide and hydrogen and was first
applied by Bosch and Wild in 1914 with a Cr2O3−Fe2O3
catalyst to provide H2 for the synthesis of ammonia.8 Currently,
the WGS reaction is commercially performed in several stages
with different catalysts to optimize the greater CO equilibrium
conversion attained at lower temperatures because the reaction
is exothermic and reversible.9,10 The low-temperature WGS
(LT-WGS) reaction is performed at ∼190−250 °C with a Cu/
ZnO/Al2O3 catalyst, and the high-temperature WGS (HT-
WGS) reaction is performed at ∼350−450 °C with a Cu-
promoted chromium−iron mixed oxide catalyst.
CO + O* ↔ CO2 + *
(2)
H2O + * ↔ H2 + O*
(3)
Received: March 5, 2016
Revised: March 28, 2016
© XXXX American Chemical Society
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Letter
The existing evidence for the redox mechanism is the
observation of the bulk Fe2+ ↔ Fe3+ redox couple with
Mossbauer spectroscopy with bulk Fe2+ oxidized to Fe3+ by
̈
H2O and bulk Fe3+ reduced to Fe2+ by CO.19,20 In situ
gravimetric analysis (GA) demonstrated that the catalyst
oxygen content is dependent on the oxyreduction potential of
the reaction gases (H2/H2O and CO/CO2).18 It was concluded
that the oxygen changes measured with the GA as a function of
the oxyreduction environments correspond to that of surface
oxygen on the catalyst, but GA measures the total weight of the
catalysts and is not able to distinguish between the bulk and
surface oxygen content. The redox mechanism has become
widely accepted because of these reported studies, which,
however, are not able to distinguish between changes taking
place in the bulk lattice and surface of the iron oxide catalysts
because they monitor the entire volume of the catalyst. The
dynamic nature of the iron oxide catalyst bulk phases upon gas
oxyreduction potential further complicates the above con-
clusions.21
Figure 1. MS signals for CO2, H2 and H2O during CO-TPSR.
The alternative mechanism is referred to as the “associative”
mechanism, and it involves surface reaction intermediates
formed by reaction between CO and H2O that subsequently
decompose to CO2 and H2 (eq 4). The most commonly
proposed reaction intermediate is surface formate
(HCOO*).22−25 The associative mechanism has been criticized
mainly by not detecting surface formate species or any other
surface intermediates during the HT-WGS reaction,26,27 which
neglects the possibility of low concentrations and/or transient
formation of surface formate species duirng HT-WGS. Such
complexity has intrigued many computational studies that both
support and refute the associative mechanism.28−30
suggests that the surface oxygen vacancies created by CO
oxidation may be required for activation of the surface
hydroxyls for H2 formation. The independent formation of
CO2 and H2 demonstrates that these two products are not
formed by a common reaction intermediate. Above ∼285 °C,
CO is more extensively oxidized to CO2 with additional oxygen
from the catalyst. Although the CO-TPSR environment is not
the actual WGS reaction condition because of the absence of
H2O, the findings reveal that the redox reaction pathway can
take place during WGS reaction conditions.
CO+H2O-TPSR
■
CO + H2O ↔ (intermediate) ↔ CO2 + H2
(4)
The evolution of H2O, CO, CO2, and H2 during CO+H2O-
TPSR are shown in Figure 2a, and the normalized CO2 and H2
signals are exhibited in Figure 2b. For the normalized spectra,
the MS signals were rescaled to the same maximum and
minimum intensity to better compare their transient behavior.
The slight increase in H2O evolution may be related to water
desorption from the catalyst surface at these low temperatures.
Formation of CO2 initiates at ∼125 °C, but the appearance of
H2 is significantly delayed to ∼240 °C, indicating that the CO2
production between 125 and 240 °C involves CO oxidation by
surface O*. This behavior was already observed above during
CO-TPSR. Even when both CO2 and H2 are simultaneously
formed above 240 °C, the evolution of H2 is retarded relative to
CO2 and the H2/CO2 ratio is less than 1, which finally reaches
1 as equilibrium is achieved at ∼500 °C. The initial delay in H2
formation relative to CO2 evolution has also been previously
observed in constant temperature transient partial pressure
experiments.31,32 The different kinetic responses of CO2 and H2
during CO+H2O-TPSR reveal that these two products are not
generated by a common surface reaction intermediate under-
going the same elementary reaction step.
The focus of the present work is to resolve the reaction
mechanism for H2 production during HT-WGS by the Cr2O3−
Fe2O3 mixed oxide catalyst. We will show how employing
transient kinetic studies, temperature-programmed surface
reaction (TPSR) spectroscopy, allows for the first time to
finally provide solid experimental evidence that demonstrates
the HT-WGS reaction by chromium−iron oxide catalysts only
proceeds via the redox mechanism.
The temperature-programmed surface reaction (TPSR)
studies were carried out using an Altamira Instruments system
(AMI-200) connected to a Dymaxion Dycor mass spectrometer
(DME 200MS). The Cr2O3−Fe2O3 catalyst was prepared by
coprecipitation and consists of 8 wt % Cr2O3 and 92 wt %
Fe2O3. Details of the catalyst preparation synthesis and TPSR
experiment details are given in the Supporting Information.
CO-TPSR
■
The CO-TPSR spectra presented in Figure 1 were collected
from an equilibrated catalyst after a 15 min water vapor
treatment at 110 °C to enhance the surface hydroxyl
concentration. Water does not desorb during the TPSR
experiment reflecting the absence of residual molecular water
on the initial catalyst surface. Evolution of CO2 initiates at
∼135 °C (Tp = 215 °C) and H2 formation initiates at ∼240 °C
(Tp = 285 °C). The CO-TPSR spectra reveal that the
formation of CO2 and H2 occurs at different temperatures. This
indicates that the formation of CO2 proceeds by reaction
between CO and a surface O*, and the formation of H2O
involves reaction of two surface *OH species. TPSR in flowing
He, however, does not produce H2 (see Figure S2). This
HCOOH-TPSR
■
Formic acid (HCOOH) is known to decompose to CO2 and
H2 from HCOO* which is the most proposed reaction
intermediate of associative mechanism.22,33 The evolution of
CO2 and H2 from formic acid decomposition during HCOOH-
TPSR on the equilibrated Cr2O3−Fe2O3 catalyst is presented in
Figure 3. The modest increase in HCOOH evolution at lower
temperatures may be related to formic acid desorption from the
catalyst surface. The production of the CO2 and H2
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Figure 4. Normalized MS signals for CO2 and H2 evolution during
HCOOH-TPSR and CO+H2O-TPSR on equilibrated Cr2O3−Fe2O3
catalyst.
evolution of CO2 and H2 from HCOOH decomposition
initiates at the same temperature and follows the exact same
kinetics as expected for decomposition of a common surface
reaction intermediate (HCOO*), which is the rate-determin-
ing-step.34 In contrast, the production of CO2/(CO+H2O)
begins at a much earlier temperature than H2/(CO+H2O)
formation because of CO oxidation by surface O*. The kinetics
for evolution of CO2/(CO+H2O) and H2/(CO+H2O) above
250 °C are not the same with more CO2 being initially formed
than H2. Furthermore, the kinetics for CO2 and H2 evolution
from CO+H2O-TPSR are also different than found for the
kinetics for CO2 and H2O production from the decomposition
of formic acid. The TPSR findings demonstrate that (i) an
associated mechanism through a common surface intermediate,
especially formic acid or formate, is not supported by the
current findings and (ii) the current findings are only consistent
with a redox or regenerative mechanism.
Figure 2. (a) MS signals for evolution of H2O, CO, CO2, and H2
during CO+H2O-TPSR from the equilibrated Cr2O3−Fe2O3 catalyst
and (b) the normalized CO2 and H2 MS signals (CO: H2O = 1:1).
The new insights suggest the following redox reaction
mechanism for the HT-WGS reaction by chromium−iron
mixed oxide catalysts.
CO + O* ↔ CO2*
(5)
CO2* ↔ CO2 + *
(6)
H2O + * ↔ H2O*
(7)
H2O* + * ↔ OH* + H*
(8)
(9)
OH* + * ↔ O* + H*
H* + H* ↔ H2 + 2*
(10)
The oxidation of CO by surface O* appears rather
straightforward, but isotopic oxygen studies showed rapid
oxygen scrambling that also implicates the presence of surface
carbonates (CO3*) during the HT-WGS.35 The surface
carbonates may just be formed by complexation of the CO2
product with surface O* and not directly involved in the HT-
WGS reaction.36 The details of the elementary steps involved in
water decomposition during HT-WGS are not completely clear
at present because formation of H2 must involve several
reaction steps such as reactions 8−10. The current findings also
Figure 3. Normalized MS signals for HCOOH, CO2, and H2 during
HCOOH-TPSR on equilibrated Cr2O3−Fe2O3 catalyst.
decomposition products initiates at ∼225 °C. The evolution of
CO2 and H2 from HCOOH decomposition follows the exact
same kinetics between 225 and 300 °C as would be expected
for their origin from the same surface reaction intermediate.
The evolution of CO2 and H2 from CO+H2-TPSR and
HCOOH-TPSR are compared in Figure 4. As indicated above,
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Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00659.
■
S
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Catalyst synthesis; in situ Raman spectroscopy details;
temperature-programmed surface reaction (TPSR) spec-
troscopy; Raman spectra of Fe2O3 and Cr2O3−Fe2O3
under dehydrated and reaction conditions (Figure S1);
MS signals during He-TPSR for equilibrated Cr2O3−
Fe2O3 catalyst (Figure S2) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: iew0@lehigh.edu.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from
National Science Foundation Grant CBET- 1511689.
■
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