CO Hydrogenation on Cobalt-Based Catalysts: Tin Poisoning Unravels CO in Hollow Sites as a Main Surface Intermediate

Article abstract of DOI:10.1002/anie.201710301

Site poisoning is a powerful method to unravel the nature of active sites or reaction intermediates. The nature of the intermediates involved in the hydrogenation of CO was unraveled by poisoning alumina-supported cobalt catalysts with various concentrations of tin. The rate of formation of the main reaction products (methane and propylene) was found to be proportional to the concentration of multi-bonded CO, likely located in hollow sites. The specific rate of decomposition of these species was sufficient to account for the formation of the main products. These hollow-CO are proposed to be main reaction intermediates in the hydrogenation of CO under the reaction conditions used here, while linear CO are mostly spectators.

Full text of DOI:10.1002/anie.201710301

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Accepted Article
Title: CO hydrogenation on cobalt-based catalysts: tin poisoning
unravels hollow-CO as a main surface intermediate.
Authors: Frederic Christian Meunier, Anaelle Paredes-Nunez, Davide
Lorito, Laurence Burel, Debora Motta-meira, Giovanni
Agostini, Nolen Guilhaume, and Yves Schuurman
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710301
Angew. Chem. 10.1002/ange.201710301
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CO hydrogenation on cobalt-based catalysts: tin poisoning
unravels hollow-CO as a main surface intermediate.
[
a]
[a]
[a]
[b]
,
[b]
Anaëlle Paredes-Nunez , Davide Lorito , Laurence Burel , Debora Motta-Meira Giovanni Agostini ,
[
a]
[a]
[a]
Nolven Guilhaume , Yves Schuurman and Frederic Meunier*
Abstract: Site poisoning is a powerful method to unravel the nature
of active sites or reaction intermediates. The nature of the
intermediates involved in the hydrogenation of CO was unraveled by
poisoning alumina-supported cobalt catalysts with various
concentrations of tin. The rate of formation of the main reaction
products (i.e. methane and propylene) was found to be proportional
to the concentration of multi-bonded CO, likely located in hollow
sites. The specific rate of decomposition of these species was
sufficient to account for the formation of the main products. These
hollow-CO are proposed to be main reaction intermediates in the
hydrogenation of CO under the reaction conditions used here, while
linear CO are mostly spectators.
bonded CO, which are thought to be located at the particle
edges or steps. This conclusion was reached by noting that the
poisoning of cobalt by chloride affected more the IR band
intensity associated with this bridge/multi-bonded CO than that
of linear (i.e., on-top) CO [11].
In 1980, Fujimoto and his group suggested for the first time
that bridged carbonyls could be active intermediate during CO
hydrogenation over Ru and Rh catalysts [12]. On both catalysts,
bridged CO were hydrogenated (mainly to methane) at a lower
temperature than that needed to hydrogenate linear CO. Several
other groups proposed that bridged CO formed over cobalt-
based catalysts could be involved in the catalytic activity
[
13,14,15,16,17,18,19,20,21,22], yet no quantitative structure-
activity relationship has yet been reported to the best of our
knowledge.
We report herein a quantitative operando diffuse reflectance
FT-IR spectroscopy (DRIFTS) study in which the hydrogenation
Synthetic fuels and base chemicals can be obtained from
both fossil and renewable sources through the catalytic
conversion of synthesis gas (“syngas”, a mixture of carbon
oxides and dihydrogen) [1,2,3,4]. Cobalt-based catalysts are the
most important formulations regarding the conversion of syngas
to synthetic fuels via Fischer-Tropsch synthesis [3]. The nature
of the reaction mechanism of the Fischer–Tropsch synthesis is
still a matter of debate, as well as the nature of the active sites
2 3
of CO is monitored on Sn-modified Co/Al O catalysts under
differential reaction conditions. DRIFTS enables monitoring the
state of the cobalt surface, in particular the various adsorbed CO
species formed under reaction conditions [23,24,25]. Sn was
used as a modifier because it is expected to remain strongly
adsorbed on the cobalt active phase, unlike chlorine used in our
previous study, and does not adsorb CO under our conditions
[
5,6,7,8].
Infrared spectroscopy is
a
powerful technique for
investigating the surface sites and the nature of the reaction
intermediates, especially because CO is both a reactant and a
molecular probe that can be used to characterize metal surfaces
under reaction conditions. IR studies were previously used to
investigate the role of formates, which had been proposed as a
precursor of methane or methanol [9]. Our recent quantitative IR
studies have shown that formates located near to the alumina-
cobalt interface could potentially be the sole precursors of
methanol, while not those of methane and propene, which were
the main reaction products [10]. The present contribution will
now quantitatively investigate the role of adsorbed CO present
over alumina-supported cobalt catalysts.
[
26,27], thereby facilitating DRIFTS spectra interpretation.
The description of the experimental setup and the
preparation of the catalyst samples (Table 1) are detailed in the
experimental section (See Supporting Information).
Table 1. Specific surface area, metal loading and cobalt dispersion of the as-
prepared catalysts.
BET
surface
Area
Metal
Adsorbed
CO
Adsorbed
c
Sn/Co
molar
ratio
Co
Sn
b
disper-
H
2
a
sion
wt.%
wt.%
(m²/g)
%
µmol/g
µmol/g
73
We recently reported that the most active cobalt sites were
possibly those associated with the formation of bridged/multi-
Co
113
107
102
119
15.3
14.3
14.4
14.3
0
7.0
143 ± 10
1
/120
0.24
0.52
1.1
9.8
9.2
7.0
160 ± 10
156 ± 10
103 ± 6
50
33
2
1
/60
1
/30
[
a]
b]
Dr A. Paredes-Nunez, D. Lorito, Dr L. Burel, Dr Y. Schuurman, Dr
F.C. Meunier
Institut de Recherches sur la Catalyse et l’Environnement de Lyon,
Univ Lyon, Université Lyon 1, CNRS, 2, Av. Albert Einstein, 69626,
Villeurbanne, France
a: from TEM analyses
b: from SSITKA experiments, see Fig. S5
c: from chemisorption experiments, see Fig. S6
E-mail: fcm@ircelyon.univ-lyon1.fr
Dr D. Motta-Meira, Dr G. Agostini
[
Typical TEM micrographs show that the cobalt particle size
distribution is narrow and centered for all samples around 9-12
nm (Fig. S1.A to D). Metal (essentially cobalt) dispersions based
on the TEM-derived size distributions are reported in Table 1.
The combined catalytic and DRIFTS experiments were
carried as follows. About 25 mg of sample were placed in the
European Synchrotron Radiation Facility (ESRF), 6 Rue Jules
Horowitz, BP 220, Grenoble, Cedex 9 38043, France
Supporting information for this article is given via a link at the end of
the document.
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DRIFTS cell and reduced in situ at 450°C for 1h. The
temperature was then set at 220°C and the feed (30%CO+ 60%
H
2
/He) was introduced. The reactions were carried out at
atmospheric pressure.
Typical operando DRIFTS spectra recorded over a period
of 18 h are shown in Fig. S2 for the Sn-free catalyst and are
similar to those described in details elsewhere [11,23]. In brief,
-1
the bands at ca. 3000-2800 and below 1650 cm are essentially
related to hydrocarbon waxes and formates/CO adsorbed on
2
the support, respectively. The bands of main interest to this
study are adsorbed on metallic cobalt: on-top (linearly) at 2031
-1
-1
cm and bridged/multi-bonded species below 2000 cm . The
intensity of these bands was essentially constant after 3 h,
except for those associated with waxes that continued growing
with time.
The rates of formation of the main products measured at
the outlet of the DRIFTS cell are shown in Fig. S3. The
selectivity to methane and propene of the samples could not be
determined with any satisfactory precision because of the low
level of CO conversion (well below 5%, inducing a large relative
error on selectivities). Shi et al. [ 28 ], who developed the
synthesis method used here to prepare the Sn-free sample,
reported at 1.0 MPa selectivities to methane and propene of
Figure 1. Operando DRIFTS spectra collected after 23h at 220°C under
30%CO+ 60% H
2
/He over (Red) Co/alumina and the Sn-modified samples
(
Black) Sn/Co = 1/120, (Blue) Sn/Co = 1/60 and (green) Sn/Co = 1/30. Total
-
1
flow was 30 ml min . The signal of gaseous CO was subtracted. The spectra
were normalized with respect to the 2031 cm band.
-
1
The specific concentrations of reversibly adsorbed CO over
each of the samples were determined through isotopic exchange
under reaction conditions at 220°C. Typical normalized
40% and 2%, respectively. The selectivity to methane obtained
1
2
13
here at atmopheric pressure should be larger and methane be
by far the main reaction product. The data reported here were
focused on rates of product formation, which could be
determined directly without using CO conversion by neglecting
the change of moles of gases due to the reaction at these low
levels of conversion.
An induction period of about 3 h was observed during
which the rates of formation decreased rapidly before becoming
steadier (Fig. S3). The Sn-free sample (red symbols) was the
most active catalyst, as the activity markedly decreased with
increasing content of tin. The DRIFTS and activity data are
consistent with an initial surface reconstruction taking place over
ca. 3 h, as suggested earlier [23], while the slower long term
deactivation may be partly related to wax deposition.
exchange plots obtained during the exchange CO- CO are
shown in Fig. S5 and the results are gathered in Table 1. The
number of CO reversibly adsorbed was essentially unchanged
(ca. 150 µmol/g) for the two samples least loaded with Sn as
compared to the case of the Sn-free sample (Table 1). A
significant lower CO adsorption capacity (ca. 103 µmol/g) was
only noted for the most Sn-loaded material.
The effect of Sn on the adsorption of
2
H was also
investigated (Fig. S6) and is discussed in details in the SI
(section Discussion D1).
The plots of the rates of formation of methane and propene
as a function of the concentration of hollow-CO (assumed to be
equal to the product of the concentration of reversibly adsorbed
CO and the molar fraction of hollow-CO) are given in Fig. 2.A.
The contribution of gas-phase CO on the DRIFTS spectra
could be precisely subtracted according to the method reported
elsewhere [29], to facilitate the analysis of adsorbed CO bands.
The signal obtained at pseudo-steady-state (i.e. > 20 h on
stream) of the Sn-modified samples was compared to that of the
Co/alumina (Fig. 1). Band intensities were normalized with
respect to the signal maximum in this region, which turned out to
Linear relationships were obtained, highlighting a direct
correlation between the concentration of hollow-CO and the rate
of formation of the two major reaction products.
The minor offset of the data points at the highest
concentration (Fig. 2.A, red symbols) is likely due to an
underestimation of the true concentration of adsorbed CO from
the SSITKA experiments (Table 1). In fact, the rate plots as a
function the IR band fraction, which is not affected by the lower
precision of the SSITKA data, exhibit much clearer linear
relationships (Fig. 2.B.). The data used to draw Fig. 2.B were
collected from operando experiments carried out entirely on the
DRIFTS cell, while those in Fig. 2.A also relied on
supplementary data obtained from the plug-flow SSITKA reactor,
hence leading to lower precision. Overall the data clearly
indicate that hollow-CO was, at least, a marker of the catalytic
activity, if not a main reaction intermediate.
-1
be located at 2031 cm in all spectra. This position corresponds
to linearly bound CO on metallic cobalt.
Interestingly, the intensity of the bridged/multi-bonded CO
-1
located in the region below 2000 cm decreased gradually with
increasing Sn content. A decomposition of the DRIFTS bands
was carried out to determine the contribution of bridged species
-1
(
Fig. S4). A single band located at ca. 1860 cm was sufficient
to satisfactory represent the DRIFTS signal in this region.
According to Weststrate et al. [30], this band can be assigned to
CO adsorbed on hollow sites (thereafter noted “hollow-CO”).
The molar fractions of linear and hollow-CO could be determined
from the proportion of hollow-CO DRIFTS bands (Fig. S4),
taking into account that the molar absorption coefficient of linear
CO is twice as high as that of hollow-CO [30].
A large number of other samples were tested, which differed
in the nature of the pre-treatment (Table S1). Essentially linear
relationships were obtained when the product rate vs. hollow-CO
fraction of all these samples was plotted (Figure S7), stressing
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the universal nature of the relationship between hollow-CO and
catalytic activity over this type of Co-Sn formulations.
hollow-CO, which are expected to be more strongly bonded than
linear CO, were probably removed in the absence of H through
2
transfer onto sites forming linear species before desorbing as
CO(g).
The decay of the DRIFTS signal of adsorbed CO under H
2
followed first order kinetics for all samples (Fig. S8), suggesting
uniform decomposition in all cases, even when both hollow and
linear CO were present in similar proportions. Such behaviour is
again consistent with a rapid exchange of adsorbed CO between
hollow and top sites at 220°C, decomposition occurring then
through the fastest pathway available: hydrogenation if hollow
sites were available, otherwise through desorption as CO(g) on
top sites. The rate constant of decomposition under H
determined from the slope of best fit lines (Fig. S8).
2
was
Figure 2. Methane (circles) and propene (triangles) production as a function of
the concentration of hollow-CO expressed (A) in µmol/g and (B) as a molar
fraction of adsorbed CO determined by DRIFTS measured over the alumina-
supported cobalt (red symbols) and the three Sn-modified samples: Sn/Co =
Figure 3. Logarithm of the DRIFTS band area of CO(ads) present over (red
symbols) Sn-free Co/alumina and (green symbols) Co/Sn = 30:1 sample at
-
1
220°C under a stream of 20 mL min of (circle) pure H
as a function of time following the removal of CO(g). The feed was composed
of 30% CO + 60% H /He prior to CO removal. The rate constants of CO(ads)
2
and (triangle) pure He
1
/120 (Black), Sn/Co = 1/60 (Blue) and Sn/Co = 1/30 (green). Total flow was
2
-
1
30 ml min .
decomposition were equal to the absolute value of the slope of the
corresponding lines.
Additional experiments were carried out to estimate the initial
decomposition rate of adsorbed CO by removing CO(g) from the
feed. The decomposition rate of adsorbed CO over the Sn-free
The specific rate of decomposition of hollow-CO could thus
be calculated (i.e. rate
= rate constant x hollow-CO
concentration) and compared to the specific rates of methane
and propene formation (Fig. 4). The decomposition rate of
hollow-CO was similar or higher than the sum of methane and
propene formation. These observations would be fully consistent
with hollow-CO being a main surface intermediate in the
formation of these hydrocarbons under the present conditions.
The difference between the two rates could be associated
mostly with the formation of higher hydrocarbons, which were
not measured here, and some desorption as CO(g). The rate
difference of ca. 50% is actually consistent with a selectivity to
methane of ca. 50% expected at atmospheric pressure.
2
sample was ca. 5-fold faster under H than under He (Fig. 3).
This indicates that adsorbed CO was mainly decomposed
through hydrogenation to hydrocarbons rather than desorption to
CO(g) over the most active sample.
In contrast, essentially identical decomposition rates under
2
H and He were obtained for the sample most poisoned with tin,
Co/Sn = 30:1 (Fig. 3). This indicates that adsorbed CO mostly
desorbed as unreacted CO(g) over the least active sample,
which exhibited the largest proportion of linear CO (Fig. 1).
These observations underline that linear CO were unreactive
spectators.
In conclusion, the present quantitative operando study of
syngas reaction over cobalt-based catalysts demonstrates
quantitative relationships between hollow-CO species (i.e. IR
mol fraction and decomposition rates) and the rate of formation
of the main reaction products methane and propene. More work
Interestingly, the initial rate of CO decomposition was almost
identical over the Sn-free sample under He and the most
poisoned sample (whether under He or H ) (Fig. 3). It is likely
2
that at 220°C surface diffusion allowed rapid transfer of
adsorbed CO between hollow-CO and linear CO sites. Thus,
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Entry for the Table of Contents
COMMUNICATION
Text for Table of Contents
Anaëlle Paredes-Nunez, Davide Lorito,
Laurence Burel, Debora Motta-Meira
,
Giovanni Agostini, Nolven Guilhaume,
Yves Schuurman and Frederic Meunier*
Page No. – Page No.
CO hydrogenation on cobalt-based
catalysts: tin poisoning unravels
hollow-CO as a main surface
intermediate.
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