Inverse Bimetallic RuSn Catalyst for Selective Carboxylic Acid Reduction

Article abstract of DOI:10.1021/acscatal.9b02726

Inverse bimetallic catalysts (IBCs), synthesized by sequential deposition of noble and oxophilic metals, offer potential reactivity enhancements to various reactions, including the reduction of carboxylic acids for renewable fuels and chemicals. Here, we demonstrate that an IBC comprising RuSn exhibits high selectivity for propionic acid reduction to 1-propanol, while Ru alone results in cracking. On RuSn, X-ray absorption spectroscopy identified Ru⁰ nanoparticles with a near-surface bimetallic Ru⁰Sn⁰ alloy and small SnO_x domains. Corresponding model surfaces were examined with density functional theory to elucidate the observed selectivity difference. Only selective hydrogenation is predicted to be favorable on SnO_x/Ru, with the SnO_x clusters facilitating C-OH scission and Ru enabling hydrogen activation. Intrinsic barriers along nonselective pathways suggest that the RuSn alloy and SnO_x resist cracking. SnO_x/Ru hydrogenation activity was supported experimentally by inhibiting hydrogenation with phenylphosphonic acid, differentiating the system from fully alloyed RuSn metallic nanoparticles. Overall, this work demonstrates a plausible mechanism for selective reduction of carboxylic acids and proposes a roadmap for rational design of IBCs.

Full text of DOI:10.1021/acscatal.9b02726

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Article
Inverse bimetallic RuSn catalyst for selective carboxylic acid reduction
Vassili Vorotnikov, Todd Eaton, Amy Settle, Kellene Orton, Evan Wegener,
Ce Yang, Jeffrey T. Miller, Gregg T Beckham, and Derek R. Vardon
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02726 • Publication Date (Web): 25 Oct 2019
Downloaded from pubs.acs.org on October 26, 2019
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Inverse bimetallic RuSn catalyst for selective carboxylic acid reduction
1
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3
Vassili Vorotnikov⁼ Todd R. Eaton⁼ , Amy E. Settle , Kellene Orton , Evan C. Wegener , Ce Yang ,
,
2
,3
1
1
Jeffrey T. Miller , Gregg T. Beckham * and Derek R. Vardon **
National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado, USA. Davidson School of Chemical Engineering,
Purdue University, West Lafayette, Indiana, USA. Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne,
1
2
3
Illinois, USA. ⁼authors contributed equally to this work. For correspondence email *gregg.beckham@nrel.gov **derek.vardon@nrel.gov
Abstract
Ru-to-Sn molar ratio of 1:1. We subsequently characterized the
RuSn catalyst and performed density functional theory (DFT)
calculations to elucidate its active species and catalytic
mechanism. Phenylphosphonic acid (PPA) was co-fed to inhibit
Lewis acidic domains on Ru and RuSn, distinguishing catalytic
roles in hydrogenation pathways. Extended X-ray absorption fine
structure (EXAFS) spectroscopy of reduced catalysts was
performed to obtain electronic and structural information.
Mimicking EXAFS results, Ru(0001), Sn /Ru(0001), and
4 4
Sn O /Ru(0001) surfaces were constructed for DFT investigation
and the computed energetics enabled the interpretation of observed
selectivity trends. As a result, we propose a reaction scheme
consistent with DFT calculations, characterization, and observed
reactivity that provides a basis for rational IBC design.
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Inverse bimetallic catalysts (IBCs), synthesized by sequential
deposition of noble and oxophilic metals, offer potential
reactivity enhancements to various reactions, including the
reduction of carboxylic acids for renewable fuels and
chemicals. Here, we demonstrate that an IBC comprising of
RuSn exhibits high selectivity for propionic acid reduction to
1
-propanol, while Ru alone results in cracking. On RuSn, X-
0
0
ray absorption spectroscopy identified Ru nanoparticles with
a near-surface bimetallic Ru Sn alloy and small SnO
domains. Corresponding model surfaces were examined with
density functional theory to elucidate the observed selectivity
difference. Only selective hydrogenation is predicted to be
0
0
x
x x
favorable on SnO /Ru, with the SnO clusters facilitating C-
OH scission and Ru enabling hydrogen activation. Intrinsic
barriers along non-selective pathways suggest that the RuSn
x x
alloy and SnO resist cracking. SnO /Ru hydrogenation
activity was supported experimentally by inhibiting
hydrogenation with phenylphosphonic acid, differentiating the
system from fully-alloyed RuSn metallic nanoparticles.
Overall, this work demonstrates a plausible mechanism for
selective reduction of carboxylic acids and proposes a roadmap
for rational design of IBCs.
Scheme 1. Representations of inverse-prepared catalysts, with inverse
defined as a metal oxide sequentially deposited on top of a noble metal. In
this figure, M represents Ru and M O represents SnO . Alloying of the
1 2 x
metal oxide and noble metal phase can occur to from a distinct M
1 2
M alloy
keywords: propionic acid; 1-propanol; selective hydrogenation;
aqueous phase catalysis; ruthenium; ruthenium-tin alloy; tin oxide
phase, resulting in an inverse bimetallic catalyst.
Results
Introduction
Catalyst activity and selectivity. Batch reactor catalyst screening
Understanding the structure and performance-enhancing
mechanism of industrially-relevant bimetallic catalysts remains a
critical challenge for rational catalyst design, especially in the
context of emerging renewable processes.1, 2 Aqueous phase
reduction of biologically-derived carboxylic acids is one such
identified that the RuSn catalyst, prepared as an IBC with Ru-to-
Sn molar ratio of 1:1, is selective not only in succinic acid, but
1
also propionic acid hydrogenation, performing better than PdRe,
PtSn, or their monometallic counterparts supported on powder
activated carbon (PAC) (see Fig S1-3). While sparse literature is
available on the aqueous phase reduction of propionic acid, a
comparison of RuSn to other catalysts for succinic acid reduction
1
, 3-7
transformation,
as fuel additives,
precursors with potential for lower overall greenhouse gas
capable of producing alcohols that are suitable
8
,
9
10, 11
precursors to acrylonitrile,
or polymer
1
can found in our previous work. The addition of Sn to Ru
1
,
8, 12-15
emissions.
Typical catalyst candidates for selective
markedly reduced the formation of non-selective reduction and
cracking products observed with monometallic Ru, indicative of
over-reduction pathways to propane and cracking to form ethane
and methane (Fig S2). CO was not detected during the analysis of
reduction of carboxylic acids involve combinations of noble (e.g.
Pd, Pt, or Ru) and oxophilic, promoting metals (e.g. Re, Ti, or
1
, 16-25
26
Sn),
but the precise nature of their sites remains unresolved.
Some investigations attributed unique reactivity to the synergy of
gas-phase reaction products, though it may be readily converted to
2
, 16, 18, 20, 21, 27-37
metal and metal oxide functionalities,
analogous to
43, 44
2
CO via the water gas shift reaction over Ru.
CO hydrogenation observed over inverse catalysts, with inverse
2
The high performance of RuSn-PAC led to the testing of RuSn
catalyst defined as the sequential deposition of metal oxide on top
supported on granular activated carbon (GAC) in a trickle bed flow
reactor, as it is necessary to assess a more industrially relevant
pellet-supported catalyst for stability at high conversions and high
yield conditions. The results from the high-conversion time-on-
of a noble metal, or metal-supported metal oxides (Scheme 1,
3
8-40
left).
Other studies point to bimetallic alloys being responsible
1
7, 22-24, 41, 42
for catalyst enhancement.
The possibility of alloying
distinguishes inverse bimetallic catalysts (IBCs) from inverse
catalysts and complicates the ability to interpret the active sites
-
stream run in this reactor are shown in Fig 1. At a WHSV of 0.3 h
1
,
RuSn-GAC demonstrated excellent selectivity for PA
(Scheme 1, right). Accordingly, in-depth characterization and
hydrogenation, achieving 1-PrOH yields of 94 ± 2 mol% at 100%
PA conversion while remaining stable for over 100 hours on
stream.
computational modeling of distinct surface sites are needed to
develop a holistic view of these materials.
In this work, we examined IBCs supported on activated carbon
for selective propionic acid (PA) hydrogenation to 1-propanol (1-
PrOH). We identified the most promising catalyst to be RuSn with
1
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propionic acid
-propanol
product loss
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RuSn-GAC
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0
%
0
%
0
2
4
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8
10
12
14
0
20
40
60
80
100
120
Time (hours)
Fig 2. Change in the overall propionic acid conversion rate (listed in Table
Time on Stream (hours)
Fig 1. Propionic acid hydrogenation in a trickle bed flow reactor with
RuSn-GAC. Conditions: 160°C, 100 bar H at 200 sccm, 1.0 g RuSn-GAC,
-1
-1
1
as a function of time during exposure of the catalysts to a feed of 2 g L
2
-
1
-1
-1
-1
2
PPA, 100 g L propionic acid; Conditions: 160°C, 200 sccm H , 100 bar
propionic acid 25 g L , WHSV 0.3 h .
-1
2
H , propionic acid WHSV 4 h .
To discern RuSn-GAC reactivity, we measured PA
hydrogenation rates and selectivities using low-conversion time-
on-stream (<10%) experiments, with the results provided in
Table 1. The rate of PA hydrogenation was 10 times higher on Ru-
GAC than on RuSn-GAC. Alongside rate changes, chemisorption
experiments showed a 14-fold higher hydrogen uptake on Ru-PAC
compared with RuSn-PAC, suggesting that Sn blocks
adsorption and that the active sites involve Ru . In addition,
negligible CO uptake was previously observed on RuSn-PAC.
While CO is known to poison Ru hydrogenation sites, the high
activity observed over monometallic Ru would suggest that
significant surface sites remain available and active for propionic
acid reduction. However, product selectivities were vastly different
2
with PA mainly converted to light products (CO , methane, ethane,
and propane) with Ru-GAC and to 1-PrOH with RuSn-GAC,
implying that Sn influences the nature of the active site.
In unraveling the role of Sn, PPA was introduced to the mobile
phase during PA hydrogenation over Ru-GAC and RuSn-GAC to
target and inhibit surface metal oxide species. The change in PA
hydrogenation rate upon PPA exposure is shown in Fig 2.
Effectively, no change in the rate was observed on Ru-GAC, but
an 80% reduction in the rate took place on RuSn-GAC.
Interestingly, selectivity to 1-PrOH was maintained on RuSn-GAC
during inhibition, implying that the residual activity is not due to
X-ray absorption spectroscopy. With a previous report pointing
to oxide formation and reactivity testing suggesting oxidic species
1
to be active, further characterization was achieved by employing
X-ray absorption spectroscopy (XAS) to probe the structure of
RuSn IBC. Fresh and spent Ru-PAC and RuSn-PAC catalysts
were examined under ambient and reducing conditions via
controlled-atmosphere EXAFS at both Ru and Sn K edges. X-ray
H
2
0
2
1
absorption near edge structure (XANES) and k -weighted Fourier
transform spectra results, shown in Fig 3 and summarized in Table
4
5
2
, reveal that the spent, air-exposed catalysts contained oxidized
3+
4+
metals Ru and Sn , while the fresh, air-exposed Ru-PAC
0
2
contained some Ru . Treatment of all materials in H at 160°C led
to reduction of Ru to Ru and Sn to Sn (see Table S1, Figs
S5-6 for more details).
The low-intensity of the Ru-Ru peak in the Ru EXAFS of Ru-
PAC (Fig 3C) indicates that Ru formed small nanoparticles; this
was true for fresh and spent Ru-PAC and RuSn-PAC catalysts,
with particle sizes ranging from 2.5 to 4.5 nm, consistent with
previous studies of these materials. There were no significant
differences between fresh and spent catalysts, except for spent
RuSn-PAC, which exhibited a smaller particle size upon treatment
3
+
0
4+
2+
1
2
with H at 160°C. This suggests that the bimetallic catalyst
rearranges under reaction-like conditions. The Ru edge energy in
the XANES region for RuSn-PAC (Fig 3A) was similar to that of
Ru-PAC; however, the shape of the edge was shifted to slightly
lower energy and white line intensity was also slightly lower,
0
Ru sites but rather is related to the reversibility of deactivation
Fig S4). Previous work demonstrated that PPA selectively binds
to Lewis acidic oxides,
of phosphonic acids on SnO
have been shown to exhibit weak Lewis-acidic character and X-ray
photoelectron spectroscopy (XPS) demonstrated evidence of Sn
oxide formation. Thus, the activity inhibition experiments suggest
(
4
6-49
including self-assembled monolayers
50, 51
3 7
similar but not identical to the spectra of a Ru Sn standard (Fig
2
.
Additionally, these catalysts
S5). This suggests that addition of Sn to Ru-PAC led to the
formation of a nanoparticle partially comprised of a RuSn
bimetallic phase. Direct evidence for this formation was difficult to
observe by EXAFS analysis, since Ru and Sn have a similar
number of electrons and scatter similarly. Further, near surface
alloy formation cannot be confirmed from this bulk particle
analysis. The nearly identical edge energy of the Ru-PAC and
RuSn-PAC catalysts suggests that most of the Ru was present as
Ru nanoparticles, implying that the bimetallic RuSn phase may be
present at the nanoparticle surface.
1
that oxidic Sn constitutes part of the RuSn IBC active site.
Table 1. Propionic acid hydrogenation rates on Ru-GAC and RuSn-GAC
catalysts in a flow reactor at 160°C and 100 bar H
PA consumption, conversion <10%, measured via HPLC quantification,
calculated from the difference in the overall rate and 1-PrOH production
a
2
. Measured from total
b
c
d
rate, and determined from H
2
chemisorption on powder catalysts.
a
Individual rates (Selectivity)
d
Overall rate
2
H uptake
Catalyst
-1 -1
1-Propanolb
Lightsc
-1
(
mmol g h )
27.2 (± 0.2)
2.6 (± 0.1)
(μmol g )
Ru-GAC
RuSn-GAC
2.6 (10%)
2.5 (95%)
24.6 (90%)
0.1 (5%)
47.5
3.3
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Table 2. EXAFS results for Ru-PAC and RuSn-PAC catalysts. Unused, as made catalyst; catalyst after 15 h of batch reaction, 100 bar H
2
, 160°C, 25 g L-1 propionic acid.
a
b
1
2
3
4
5
6
7
8
9
XANES energy
Oxidation
state
Ru-O
R (Å)
Ru-Ru
R (Å)
Sn-O
R (Å)
Estimated
size (nm)
Catalyst
Ru-PAC
Condition
Treatment
(keV)
Ru
Sn
n.d.
n.d.
n.d.
n.d.
29.2040
29.0010
29.2040
29.2000
N
N
N
Ru
III
0
III
0
Sn
n.d.
n.d.
n.d.
n.d.
IV
Fresha
Fresha
Spentb
Spentb
Fresha
Fresha
Spentb
Spentb
Air, RT
22.1264
22.1181
22.1280
22.1171
22.2128
22.1171
22.1285
22.1172
3.4
n.d.
5.8
n.d.
5.5
n.d.
5.8
n.d.
2
n.d.
2
n.d.
2
n.d.
2
n.d.
5.3
9.1
n.d.
9.3
n.d.
9.5
n.d.
7.1
2.68
2.65
n.d.
2.65
n.d.
2.66
n.d.
2.65
n.d.
n.d.
n.d.
n.d.
5.8
2.5
6
n.d.
n.d.
n.d.
n.d.
2.04
2.06
2.05
2.05
n.d.
4
n.d.
4
n.d.
4.5
n.d.
2.5
H
2
, 160°C
Air, RT
, 160°C
Air, RT
, 160°C
Air, RT
, 160°C
H
2
III
0
RuSn-
PAC
H
2
II
III
0
IV
II
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
2
1.9
1.2
1.6
.4
.2
0
2+
bimetallic catalysts, namely Sn in the alloyed phase and Sn O
domains.
A
B
1
1
.0
.8
.6
.4
1
0
1.0
0.8
0
Modeling inverse bimetallics. We used periodic DFT calculations
to examine surface-adsorbate interactions over representative
models, chosen to emulate EXAFS results. We modeled Ru with
0.6
0
Ru-PAC, H2 160°C
RuSn-PAC, H2 160°C
RuSn-PAC, air RT
0.4
0
RuSn-PAC, H2 160°C
0.2
2+
0.2
Sn O
1
,
52, 53
0
0
Ru(0001),
replacing surface Ru(0001) with Sn atoms,
a small Sn cluster on Ru(0001) to represent Ru-supported
constructed near-surface Ru Sn alloys by
0.0
0.0
22.10 22.11 22.12 22.13 22.14 22.15 22.16 22.17
Photon Energy (keV)
29.18 29.19 29.20 29.21 29.22 29.23 29.24
Photon Energy (keV)
1, 17, 54, 55
and optimized
3.0
.5
.0
.5
.0
.5
.0
0.8
0.6
0.4
0.2
4
O
4
C
D
RuSn-PAC, H2 160°C
Sn2+O
2
+
2
Sn O domains (Table S2 and Fig S7). The latter model was
chosen to investigate the specific role of the Sn O domains in
facilitating selective carboxylic acid hydrogenation, rather than
study the effects on adjacent Ru sites. DFT-based equilibrium
addressing hydrogen adsorption and shown in
Fig 4, revealed that Sn species inhibit Ru sites, resulting in lower
coverage (see also Fig S8). Hydrogen dissociation barriers also
Ru-PAC, H2 160°C
Ru Foil
Sn-O
Higher shell Sn-O
2+
2
1
Ru-Ru
1
5
6, 57
phase diagrams,
0
0
0
.0
0
0
1
2
3
4
0
1
2
3
4
5
R (Å)
R (Å)
-1
-1
Fig 3. (A) Ru K edge spectra of Ru-PAC and RuSn-PAC treated in H
160°C, and (B) Sn K edge spectra of fresh RuSn-PAC air-exposed and
2
at 160°C compared to Sn O standard. (C) Ru k -weighted
Fourier transformed spectra for Ru foil standard and Ru-PAC treated in H
at 160°C. The low intensity of Ru-Ru scattering for reduced Ru-PAC, in
contrast to the Ru foil, indicates small Ru nanoparticles, and (D) Sn k -
weighted Fourier transformed spectra of RuSn-PAC treated in H
2
at
increased from 0 kJ･mol on Ru(0001) to 11 and 15 kJ･mol on
25% Sn/Ru(0001) and 50% Sn/Ru(0001) (Fig S9). Hydrogen is
predicted to exhibit stronger affinity to SnO/Ru(0001), but the
2
+
2
treated in H
-1
2
associated dissociation barrier was 99 kJ･mol , signifying kinetic
limitations. Consistent with previous reports, we conclude that Ru
2
0
sites are essential for H
2
dissociation while both Sn and oxidic
2
at 160°C
2+
58
2
Sn species contribute to lower H uptake. In the alloy models,
2
+
compared to Sn O standard.
we found that hydrogen interacts preferentially with Ru,
suggesting that Sn solely isolates Ru sites without fundamentally
altering their function. While site isolation was proposed for
RhSn and RuSn catalysts and reflected in PtSn alloy
this alone cannot explain the observed
selectivity changes, requiring mechanistic understanding.
0
Sn K edge spectra (Fig 3B) showed that, after treatment with H
2
4+
at 160°C, most of the Sn in RuSn-PAC was reduced from Sn to
5
9
35
2+
2+
2
Sn , with an edge energy similar to Sn O. The k -weighted
5
4, 55
computational studies,
Fourier transform spectrum (Fig 3D) of RuSn-PAC after treatment
with H
bulk Sn O and Sn O
were quite small.
2
2
at 160°C showed fewer high shell Sn-O-Sn peaks than
2
H coverage (# H/Å )
Ru Sn H
+
4+
2+
4+
2 2
, indicating that Sn O and Sn O domains
0
0.0
0.0
0.0
0.1
0.1
0.1
0.1
0.2
.
0
0
2
5
7
0
2
5
7
0
0
0
5
0
5
0
5
0
5
3
2
1
0
1
From the EXAFS results, we conclude that at 160°C under H
2
,
Ru
6% Sn/Ru
0
the Ru-PAC catalyst consisted of <5 nm Ru . Under the same
conditions, the RuSn-PAC catalyst surfaces consisted of <5 nm
0.18 H/Å2
0.16 H/Å2
0
0
Ru nanoparticles with a surface that was enriched with Sn , as
well as small Sn O domains that were randomly dispersed on the
2+
surface (e.g. both on the activated carbon itself and on RuSn
bimetallic nanoparticles). Observation of Sn oxide supports the
above assertion that RuSn-GAC inhibition was due to PPA binding
to Sn O sites. Previous characterization by chemisorption and
temperature programmed reduction showed negligible hydrogen
and CO uptake with the same RuSn catalyst, in stark contrast to its
monometallic Ru counterpart. XPS was also performed to confirm
the presence of predominantly Ru with mixed Sn oxidation states
(Sn , Sn , Sn ) on the reduced RuSn catalyst, which would
support the presence of alloy Sn/Ru at the near-surface along with
an oxide phase. Collectively, these results would suggest that the
working RuSn catalyst is an inverse bimetallic. However, since the
mechanism of the RuSn bimetallic active surface could not be
deciphered from characterization and reaction testing alone, we
then turned to DFT to provide further insight. Based on these
characterization results, we constructed computational models to
further probe reactivity by examining the main features of inverse
-
3
2
5% Sn/Ru
50% Sn/Ru
2
+
2
1
0.11 H/Å2
0.07 H/Å2
1
0
0
0
2+
4+
-1
-2
1
300 400 500 600 700 800
400 500 600 700 800
T (K)
Fig 4. Phase diagrams showing hydrogen coverage on model surfaces as a
function of temperature and pressure. Yellow rectangles represent the
reaction conditions typical of propionic acid hydrogenation (i.e. 100-200°C
2
). The insets show the equilibrium binding modes at
these conditions, with the top layer represented with spheres.
and 30-130 bar H
3
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1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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6
To gauge the role of Sn in RuSn IBCs, we evaluated PA
reactivity on Ru(0001), 25% Sn/Ru(0001), and Sn /Ru(0001)
Fig S7 A, C, and E), referring to them hereafter as Ru, Sn/Ru and
formation, (ii) C-H formation to propane-1-ol-1-olate followed by
4
O
4
C-OH scission, and (iii) O-H scission to propionate followed by C-
O scission to propionyl and C-H formation. Propanal reduction to
1-PrOH was found to be relatively facile on all model surfaces
irrespective of O-H or C-H formation first.
(
SnO/Ru, respectively. On each model surface, we identified the
likely mechanism by means of intrinsic energetics, i.e. surface-
specific energies and barriers. Reactions were deemed kinetically
accessible on a given surface if the activation energy was below
All three deoxygenation paths (Fig S16) exhibited barriers
-
1
lower than 88 kJ
･mol on Ru, in line with previous studies
-1
52
61
1
10 kJ
･mol , reflecting typical barriers that can be overcome
involving hydrogenation of acetic and propionic acids. On
Sn/Ru, only the propane-1-ol-1-olate deoxygenation path to
propanal (reactions 2A and 2B) was found to be kinetically
accessible, while direct C-OH scission and C-O scission along the
under reaction conditions (T = 160°C and PH2 = 100 bar). The
results of this analysis in Fig 5 highlight the surfaces capable of
promoting each reaction cycle, suggesting that both selective and
non-selective reaction paths are accessible on metallic Ru and
bimetallic Sn/Ru alloys while SnO/Ru is only capable of the
desired, selective PA hydrogenation to 1-PrOH, consistent with
our PPA inhibition experiments (see also Tables S10-12, Figs
S16-26).
0
1
2
3
4
5
6
7
8
9
0
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2
3
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0
-1
propionate path were 64 and 16 kJ ･mol more endothermic with
substantially higher barriers. We did not attempt to discern the
dominant paths beyond kinetic accessibility, since microkinetic
modeling assumptions and extrinsic parameters can alter such
conclusions.52 Instead, we stress the existence of only one low-
barrier path for PA deoxygenation on Sn/Ru, whereas multiple
such paths likely contribute to the overall activity on Ru. The
elevated deoxygenation barriers and fewer accessible paths leading
to 1-PrOH on Sn/Ru suggest that alloying lowers IBC activity, as
observed on RuSn-GAC.
Desired reduction to 1-PrOH. The desired reaction, PA
hydrogenation to 1-PrOH, follows PA deoxygenation to propanal
and its subsequent reduction to 1-PrOH. Analogous to acetic acid
4
1, 52, 60
hydrogenation studies,
we considered three main paths to
propanal: (i) direct C-OH scission to propionyl followed by C-H
Undesired: Decarboxylation
Desired: Reduction to 1-PrOH
H
H
Sn/Ru
Ru
SnO/Ru
2
B
B
2
B
2C
3
H
2
E2
5B
Undesired: Over-reduction
E
5
3
C
E1
5A
H
H
H
H
H
C
3A
1
3B
C2
B
2
1
A
B4
2D
catalyst
surface
+
H
2
catalyst
surface
2
H O +
catalyst
surface
H
C3
3C
Undesired: Decarbonylation
H
H
H
H
H
2
O +
H
H
H
2
A1/D1
1A/4A
1D
A4
3D
C4
H
2
H
H
H
D
C
3
D
4B
2
A
1
2
B
A3
1C
4
Fig 5. Overall mechanism for propionic acid hydrogenation to propanol via direct C-OH scission (pathway 1) and hydrogenation first (pathway 2), propanol
over-hydrogenation to propane (pathway 3), initial steps in propanol decarbonylation (pathway 4) and decarboxylation (pathway 5). The inset images show
-1
catalytic functionalities with accessible barriers (below 110 kJ･mol ) along each catalytic cycle.
4
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(
Table S10). On Ru, all side reactions were found to be accessible.
A: C-H scission
1
1
1
1
75
50
25
00
Propanol easily converts to propane starting with C-H scission
(Fig 5C, Fig S19), consistent with results for ethanol
53
E
A
= 0.33ΔE + 128
decomposition. Similarly, PA decarbonylation (Fig 5D) and
decarboxylation (Fig 5E, Fig S20) had barriers lower than 80 kJ･
mol , aligning with Ru-GAC being non-selective. Sn
incorporation into Ru resulted in higher C-OH, C-O, and C-H
scission barriers, also observed on Sn-modified Pt and Ru surfaces
-1
0
A
E = 0.83ΔE + 68
7
5
0
1
7, 24
for acetic acid hydrogenation.
binding of intermediates and related like-binding fragments,
namely CH , CH , H, OH, and O (Table S13). These surface
species bind weaker on Sn/Ru, causing the final states, which
constitute C-Ru bonded fragments co-adsorbed with H, OH, or O,
to destabilize relative to the initial states. With destabilized final
states, C-H and C-OH activation exhibit greater reaction energies
and higher barriers consistent with the BEP principle, leading to
slower, yet still accessible, reactions on Sn/Ru.
In contrast to either Ru or Sn/Ru, no side reactions were
energetically favored over SnO/Ru. Based on the BEP trend found
in this work (Fig 6, Fig S13), over-reduction to propane was
hindered by C-H scission/formation (reactions 3A and 3D) with
We attributed this to the weaker
5
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2
25
0
-25
-
125 -100 -75 -50 -25
0
25
50
75
B: O-H scission
175
150
125
100
75
Literature Ru
Ru
Sn/Ru
Ru-SnO interface
SnO/Ru
-1
barriers exceeding 120 kJ･mol (Fig S19). Similarly, C-H and C-
C scission barriers along decarbonylation and decarboxylation
E
A
= 0.47ΔE + 85
= 0.60ΔE + 34
-1
exceeded 130 kJ ･ mol , suggesting SnO/Ru is incapable of
cracking. As with the above arguments, simple binding may
explain this behavior. CO binds considerably weaker on SnO/Ru
compared with Ru or Sn/Ru (Table S13), effectively destabilizing
the final state in decarbonylation and increasing barriers associated
with it. To summarize the computational findings, reflected in Fig
5 and Fig S26, Ru is predicted to have low barriers along all paths,
providing an explanation for its high activity but poor selectivity.
Sn/Ru is predicted to exhibit barriers that are higher or comparable
5
0
25
0
E
A
-25
-
125 -100 -75 -50 -25
0
25
50
75
-1
0
ΔE (kJ
･
mol )
to those on Ru, supporting the notion that Sn inhibits and slows
down reactions. The SnO/Ru model showed the greatest contrast
between barriers along selective and non-selective paths,
suggesting this functionality is responsible for enhancing
selectivity of RuSn IBCs.
Fig 6. (A) C-H scission and (B) O-H scission BEP relations based on
literature results for Ru(0001) (grey dots) and values obtained in this work
for reactions at Ru(0001) (blue dots), 25% Sn/Ru(0001) (yellow
diamonds), the interface of SnO cluster and Ru(0001) (green squares), and
SnO/Ru(0001) (orange triangles). The results from this work are ZPE-
corrected.
Discussion
Previous studies of bimetallic catalysts employed for selective
hydrogenation of C=O moieties, be they carboxylic acids, esters,
or aldehydes, have identified two possible active sites: (i) fully
reduced M
metal compositions,
oxidized secondary component.
that conclusions about the active site depend heavily on catalyst
In assessing SnO/Ru, we considered both the SnO cluster on its
own and the reaction at the SnO-Ru interface. On SnO clusters,
only the direct C-OH scission path to propanal was found to be
kinetically accessible (reactions 1A and 1B). Interestingly, the
high-barrier C-H formation steps (reactions 2A and 2C) were
responsible for shutting down the propane-1-ol-1-olate
deoxygenation path, which was accessible on both Ru and Sn/Ru.
We attributed this and lower O-H formation barriers to the
fundamental difference between metallic Ru and oxidic SnO/Ru,
evidenced by significant shifts in Brønsted-Evans-Polanyi (BEP)
relations in Fig 6. In fact, these distinctions may be responsible for
synergistic effects between metallic Ru and oxidic SnO
functionalities. In examining the SnO-Ru interface, we found that
C-H formation can take place on Ru next to SnO (reaction 2A),
resulting in propane-1-ol-1-olate adsorbed on SnO (Fig S21). This
intermediate can then undergo C-OH scission to propanal on SnO
1
M
2
alloys of varying primary (M
1
) and secondary (M
and (ii) M involving an
It is unsurprising
2
)
1
7, 22-24, 41, 42
1
2 x
M O
2
, 16, 18, 20, 21, 27-37
1, 62
22, 37
preparation and its resulting phases,
their characterization,
For instance, in the absence of
information about the oxidation state of the catalyst, binary phase
diagrams prompted the assertion that Ru Sn alloys are responsible
17, 24, 41
and theoretical backing.
3
7
2
2
23
for selective hydrogenation of levulinic and butyric acids.
Similarly, XPS evidence for the alloying of Ru and Sn led Luo et
al. to propose M
Yet, there was no evidence discounting the participation of SnO
species.
presence of Sn and Sn species, suggesting M
responsible for improved selectivity in hydrogenation of fatty
esters
Alcala et al. identified PtSn alloy as the most abundant phase using
Mössbauer spectroscopy and used DFT to show that Sn
incorporation into Pt(111) increases barriers for ethanol
dehydroxylation more so than for acetic acid dehydroxylation,
suggesting
hydrogenation. Yet, the less abundant SnO
despite being observed in the same samples, likely because of
computational limitations. Meanwhile, single crystal studies of
2
4
1 2
M active species for other carboxylic acids.
x
23, 24
In fact, XPS of CoSn and RuSn catalysts revealed the
2
+
4+
1 2 x
M O may be
(
reaction 2B). The subsequent propanal hydrogenation also
3
2-34, 63
1, 24
and carboxylic acids.
For related PtSn catalysts,
benefits from the dual functionality, showing facile O-H formation
on SnO followed by C-H formation on Ru (reactions 1C and 1D,
Fig S22). These results suggest that oxidic domains proximal to
Ru can facilitate carboxylic acid hydrogenation to 1-PrOH as
RuSn IBC active sites.
0
M
1
M
2
active species for selective acetic acid
was not assessed
Assessing selectivity. To better understand selectivity trends, we
assessed 1-PrOH over-reduction to propane and initial steps
leading to PA cracking via decarbonylation and decarboxylation
17
x
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PtSn(111) pointed to M
stemming from observations that Sn incorporation lowers activity
without altering selectivity in crotonaldehyde hydrogenation.
In this work, we aimed to discern the structure-selectivity
relationship by employing an IBC formulation associated with
highly selective and process-relevant RuSn catalysts. We found
that these catalysts exhibit high selectivity of PA to 1-PrOH and
1 2 1 2 x
contain both M M and M M O surfaces. The RuSn near-surface
1
M
2
O
x
as the likely active species,
contribute to selective hydrogenation. Further efforts are needed to
understand the influence of varying concentrations of alloy Sn/Ru
and oxidic SnO/Ru sites that can result when varying the Ru:Sn
ratio, synthesis conditions, and catalyst pretreatment procedures
that were beyond the scope of this work. As highlighted in this
work, careful synthetic control with extensive material
characterization and surface-specific catalytic performance
measurements would be needed to elucidate the relative impact
and co-dependency when varying the amount of surface exposed
SnO/Ru and Sn/Ru alloy.
0
3
6
1
alloy, observed by EXAFS and modeled with DFT, energetically
hinders hydrogenation to light products, largely shutting off the
multiple pathways favored on monometallic Ru, yet still provides
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0
In considering future directions, extrinsic factors such as
sites for H
2
dissociation. Meanwhile, domains of SnO located near
temperature, pressure, and condensed water can also affect catalyst
0
64, 65
Ru were implicated as active sites by both PPA inhibition
experiments and DFT, providing low energy barriers to 1-PrOH
formation while also shutting off undesired cracking pathways.
Thus, both bimetallic arrangements are beneficial in constructing
this highly selective IBC, a conclusion reached only through the
integrated experimental and computational approach herein.
Experimental evidence alone could not rule out either of the
proposed bimetallic sites while DFT relied on characterization to
surface coverage and hence the underlying energetics.
For
instance, microkinetic modeling has the potential to more precisely
5
2
identify the contributions of each type of site, but will require
additional information about coverage effects on binding energies
and intrinsic kinetics. Similarly, reaction conditions (operando)
can provide structural information otherwise unavailable under
reducing conditions (in situ). Accordingly, understanding how
these dynamic effects impact computational and characterization
results, as well as coupling the behavior to reaction kinetics,
remains a worthy pursuit for screening and evaluating promising
IBCs.
inform model surfaces. The resulting active site mechanism is
consistent with models proposed previously,3
3, 34
wherein a fully
reduced primary metal and an oxidized secondary metal work in
0
concert. Our findings are also in line with the proposed Sn
Materials & Methods
3
5, 59
0
poisoning effect,
wherein Sn slows down the reactions
Catalyst synthesis. Catalyst synthesis details have been described
leading to undesired, cracking products. Similar trends were seen
1
previously using a sequential metal deposition procedure. Generally,
1
7, 24
17
54, 55
in DFT calculations for C-O,
PtSn and Ru Sn surfaces.
Based on our findings, we can establish key design criteria for
selective hydrogenation catalysts involving an interplay of M
, and M sites. An optimal catalyst for this type of
reaction would consist of (1) a reduced metal capable of activating
, (2) a secondary metal that can form a bimetallic phase to shut
off non-selective hydrogenation pathways yet still be capable of H
C-C, and C-H
scission on
primary metals were loaded onto the support at approximately 4 wt% and
x
3
7
dried and reduced in H
loaded onto these materials at approximately 4 wt% and dried and reduced
in H for 4h at 450°C. Unless noted otherwise, the catalysts denoted as
metal”-PAC (e.g. Ru-PAC or RuSn-GAC) refer to catalysts on the powder
2
for 4h at 450°C. Secondary metals were then
1
,
2
M
1
M
2
1 2 x
M O
“
support, whereas catalysts denoted as “metal”-GAC refer to catalysts on
the granular support.
H
2
2
Propionic acid hydrogenation. Batch reactor catalyst screening
experiments were performed in a Parr multi-batch reactor system (Parr
Instrument Company). Powder catalyst and reaction solution (20 mL of 25
dissociation, and (3) a Lewis acid oxide vicinal to the reduced
metal to facilitate selective hydrogenation. Optimizing IBCs to
further increase the overall rate or reduce their cost will be crucial
in the adoption of these catalysts. One possible approach involves
computational screening of metallic and oxidic functionalities by
utilizing a set of energetic descriptors, such as atomic binding
energies coupled with BEP relations. Tailored synthesis and
characterization strategies can then be employed to validate their
composition and performance. In this work, the BEP relations
developed for monometallic Ru could be readily applied to
bimetallic alloys because of similarities in the binding of
intermediates, hinting at potential for reducing computational
needs. Meanwhile, metal oxide clusters represent a significant shift
from alloyed materials, suggesting future catalyst formulations can
be inspired by in silico tuning of C-H, O-H, and C-OH bond
activations. Furthermore, these relations may apply beyond
carboxylic acid chemistry. For instance, the increase in C-H
-1
g L aqueous propionic acid) were loaded into the reactors, sealed, and
purged with pressurized helium three times to remove ambient air. The
reactors were then pressurized to 100 bar of H
2
and heated to 160°C. After
1
5 hours at temperature the reactors were quenched in a water bath, cooled
to room temperature, and the solution was filtered and collected for product
analysis.
Trickle bed flow reactor experiments were performed with the
1
equipment and protocol described previously. The reactions were
-1
2
performed at 160°C, 100 bar H , 0.2 mL min liquid feed, at varying
concentrations of aqueous PA. Reactor effluent was collected periodically
for analysis. More detailed descriptions can be found in the SI.
Inhibition of catalysts with phenylphosphonic acid (PPA) was
performed in the flow reactors as described above. Once catalysts had
-1
reached steady state for propionic acid hydrogenation (fed at 0.2 mL min ,
-1
1
00 g L propionic acid) the liquid feed bottle was changed to include PPA
(fed at 0.2 mL min-1, 2 g L-1 PPA, 100 g L-1 propionic acid). Reactor
effluent was collected periodically for analysis.
2
+
formation barriers on Sn O domains supports the trends in
selective crotonaldehyde hydrogenation over PtSn and RuSn,
Liquid reaction products for both batch and flow experiments were
analyzed with an Agilent 1100 series HPLC equipped with a Bio-Rad
Aminex HPX-87H column and cation H+ guard column, operating at 85°C,
a refractive index detector, with dilute sulfuric acid (0.01 N) as the mobile
36, 37, 42
where SnO
x
participation has been suggested.
While their
applicability to other oxides remains to be explored, the
implications for IBCs may be used more broadly to improve the
carbon economy in CO reduction or natural gas processing, where
2
-1
phase at 1.0 mL min . Reactant and product concentrations were measured
using authentic calibration standards prior to each use of the HPLC. The
only compounds detected were propionic acid and 1-propanol, there were
no peaks suggesting formation of any other condensed product (e.g.
ethanol or 2-propanol).
the same elementary steps are involved.
It is important to note that this work focused on discerning the
active sites for a single working catalyst with a specific Ru:Sn
ratio of 1:1, which performed best for the aqueous phase
EXAFS. X-ray absorption spectroscopy (XAS) experiments were
performed at the Materials Research Collaborative Access Team
(MRCAT) and CMC beamlines of the Advanced Photon Source at
Argonne National Laboratory. Powder catalyst samples were loaded as
self-supporting wafers in a 6-sample stainless steel sample holder. For
samples requiring pre-treatment, the sample holder itself was loaded in a
quartz sample tube equipped with gas and thermocouple ports and sealed at
1
hydrogenation of succinic acid and propionic acid. SnO/Ru was
found to be a dominant driver for this chemistry based on PPA
inhibition experiments that demonstrate the need for surface Lewis
acidity (Fig 2, Fig S4) and DFT calculations that show the SnO/Ru
has the greatest impact on selectivity. Still, alloy Sn/Ru may
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both ends by Kapton windows. The samples were heat-treated to 160°C
under flowing H (4% H /He at 100 sccm) in a tube furnace for 30 minutes,
1,2‐Propanediol over Highly Active Ruthenium–Molybdenum Oxide
Catalysts. ChemSusChem 2015, 8, 1170-1178.
2
2
then cooled under flowing He (100 sccm) to room temperature, sealed, and
then placed in the beamline for XAS spectra collection. The XAS spectra
were collected in transmission mode at the Ru (22.1172 keV) and Sn K
edges (29.2001 keV). The XAS data was fit using standard procedures
based on WINXAS software, and k2-weighted Fourier transform data was
used to obtain the EXAFS coordination parameters with least-squares fits
in q- and r-space of the isolated nearest neighbor.
Hydrogen Chemisorption. H chemisorption of Ru-PAC and RuSn-PAC
2
materials was performed on an Autochem II (Micrometrics) using a
temperature-programmed desorption (TPD) method. Prior to analysis
3. Gallezot, P., Conversion of Biomass to Selected Chemical Products.
Chem. Soc. Rev. 2012, 41, 1538-1558.
4. Huber, G. W.; Corma, A., Synergies between Bio- and Oil Refineries
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5. Sauer, M.; Porro, D.; Mattanovich, D.; Branduardi, P., Microbial
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6. Pleissner, D.; Dietz, D.; van Duuren, J. B. J. H.; Wittmann, C.;
Yang, X.; Lin, C. S. K.; Venus, J., Biotechnological Production of Organic
Acids from Renewable Resources. Springer Berlin Heidelberg: Berlin,
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7. Chen, Y.; Nielsen, J., Biobased Organic Acids Production by
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8. Werpy, T.; Holladay, J.; White, J. Top Value Added Chemicals From
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9. Farrell, J., Co-Optimization of Fuels & Engines (Co-Optima)
Initiative: Recent Progress on Light-Duty Boosted Spark-Ignition
Fuels/Engines. ; National Renewable Energy Lab. (NREL), Golden, CO
(United States): 2017; p Medium: ED.
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samples were reduced under flowing H
2
(50 sccm, 10% H
2
in Ar) at 250°C
-1
(2°C min ) for two hours, then cooled to 40°C under inert flow. The
-1
materials were then heated to 450°C (2°C min ) and the desorbed H
detected by TCD.
2
was
Computational Modeling. Periodic DFT calculations were used to
analyze thermodynamics and intrinsic kinetics associated with propionic
acid reactions on Ru(0001), Sn/Ru(0001), and Ru(0001)-supported Sn O
4 4
domains. For selective PA hydrogenation to 1-PrOH, transition states for
each elementary step were computed explicitly using DFT. For non-
selective routes, we used BEP relations for crude estimates, followed by
DFT refinement. The details of our calculations are available in the
Computational Modeling section of the SI.
Supporting Information. catalyst synthesis method, catalyst batch
reactor screening, gas-phase hydrogenation products, catalyst inhibition
experiment, x-ray absorption spectroscopy, computational modeling,
Brønsted-Evans-Polanyi (BEP) relations, reaction mechanism, reaction
energetics, adsorption energetics
1
0. Grasselli, R. K.; Trifirò, F., Acrylonitrile from Biomass: Still Far from
Being a Sustainable Process. Top. Catal. 2016, 59, 1651-1658.
1. Karp, E. M.; Eaton, T. R.; Sànchez i Nogué, V.; Vorotnikov, V.;
1
Biddy, M. J.; Tan, E. C. D.; Brandner, D. G.; Cywar, R. M.; Liu, R.;
Manker, L. P.; Michener, W. E.; Gilhespy, M.; Skoufa, Z.; Watson, M.
J.; Fruchey, O. S.; Vardon, D. R.; Gill, R. T.; Bratis, A. D.; Beckham, G.
T., Renewable Acrylonitrile Production. Science 2017, 358, 1307-1310.
12. Vardon, D. R.; Franden, M. A.; Johnson, C. W.; Karp, E. M.;
Guarnieri, M. T.; Linger, J. G.; Salm, M. J.; Strathmann, T. J.; Beckham,
G. T., Adipic Acid Production from Lignin. Energy Environ. Sci. 2015, 8,
Acknowledgements. We thank the U.S. Department of Energy
Bioenergy Technologies Office for funding this work as part of the
Consortium for Computational Physics and Chemistry, Chemical Catalysis
for Bioenergy Consortium, and Co-Optimization of Fuels & Engines (Co-
Optima) through Contract No. DE-AC36-08GO28308 at the National
Renewable Energy Laboratory. TRE and GTB thank the US Department of
Energy Bioenergy Technologies Office for funding via grant number DE-
FOA-0000996. ECW and JTM acknowledge financial support by the
National Science Foundation under Cooperative Agreement No.
EEC‐1647722. Use of the Advanced Photon Source was supported by the
U.S. Department of Energy, Office of Basic Energy Sciences, under
contract no. DEAC02-06CH11357. MRCAT operations, beamline 10-BM,
are supported by the Department of Energy and the MRCAT member
institutions. The authors are grateful for supercomputer time on Stampede2
provided by the Texas Advanced Computing Center (TACC) under the
National Science Foundation Extreme Science and Engineering Discovery
Grant MCB-09159 to GTB and the NREL Computational Science Center,
which is supported by the DOE Office of Energy Efficiency and
Renewable Energy under Contract no. DE-AC36-08GO28308.
6
1
17-628.
3. Vardon, D. R.; Rorrer, N. A.; Salvachua, D.; Settle, A. E.; Johnson,
C. W.; Menart, M. J.; Cleveland, N. S.; Ciesielski, P. N.; Steirer, K. X.;
Dorgan, J. R.; Beckham, G. T., cis,cis-Muconic Acid: Separation and
Catalysis to Bio-Adipic Acid for Nylon-6,6 Polymerization. Green Chem.
2
016, 18, 3397-3413.
14. Montazeri, M.; Zaimes, G. G.; Khanna, V.; Eckelman, M. J., Meta-
Analysis of Life Cycle Energy and Greenhouse Gas Emissions for Priority
Biobased Chemicals. ACS Sustainable Chem. Eng. 2016, 4, 6443-6454.
1
5. Corona, A.; Biddy, M. J.; Vardon, D. R.; Birkved, M.; Hauschild,
M. Z.; Beckham, G. T., Life Cycle Assessment of Adipic Acid Production
from Lignin. Green Chem. 2018, 20, 3857-3866.
1
6. Rachmady, W.; Vannice, M. A., Acetic Acid Reduction by H2 over
Supported Pt Catalysts: A DRIFTS and TPD/TPR Study. J. Catal. 2002,
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Dumesic, J. A., Experimental and DFT Studies of the Conversion of
Ethanol and Acetic Acid on PtSn-Based Catalysts. J. Phys. Chem. B 2005,
Author Contributions. Conceptualization, V.V., T.R.E., G.T.B.,
D.R.V.; Formal analysis, E.C.W., C.Y., J.T.M.; Funding acquisition,
G.T.B., D.R.V.; Investigation, V.V., T.R.E., A.E.S., E.C.W., K.O.;
Supervision, J.T.M., G.T.B., D.R.V.; Writing – original draft, V.V., T.R.E.;
Writing – review and editing, V.V., T.R.E., G.T.B., D.R.V.
1
09, 2074-2085.
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Nanoparticles and the Support for the Hydrogenation of Functionalized
Carboxylic Acids to Diols on Ru/TiO2. Chem. Commun. 2011, 47, 3613-
3
615.
Conflict of Interest Disclosure. T.R.E., A.E.S., and D.R.V. are
inventors on a patent application submitted by the Department of Energy
on synthesis and use of bimetallic catalysts for selective carboxylic acid
reduction (U.S. non-provisional patent application No. 15/828,658 filed on
December 1, 2017).
19. Khanh Ly, B.; Pham Minh, D.; Pinel, C.; Besson, M.; Tapin, B.;
Epron, F.; Especel, C., Effect of Addition Mode of Re in Bimetallic Pd–
Re/TiO2 Catalysts Upon the Selective Aqueous-Phase Hydrogenation of
Succinic Acid to 1,4-Butanediol. Top Catal 2012, 55, 466-473.
20. Zhang, K.; Zhang, H.; Ma, H.; Ying, W.; Fang, D., Effect of Sn
Addition in Gas Phase Hydrogenation of Acetic Acid on Alumina
Supported PtSn Catalysts. Catal. Lett. 2014, 144, 691-701.
2
1. Xu, G.; Zhang, J.; Wang, S.; Zhao, Y.; Ma, X., A Well Fabricated
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monometallic
C-O scission DeCO
pathways cracking
heterogeneous
alloy
x
DFT
2+
hv
Lights H2
Ru
Sn O
interface propionic acid
t
Ru o
c
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0
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2
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Cc hO em Ci s oH rption Sn0
active site
interface
stability
+
2
2
6
0
0
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C-C
Ru Sn
0 0
diffraction
C-H scission C
H
2
O
2Ru only
Sn4+
Ru Sn
O
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structure
titrant
H
2
ce
reactive surfa
metal oxide
CH4 + C
H
2 6
Ru
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hydrogenatSion
2
n
+
O
alkanes 1-propanol
structure in situ
hv
Ru0
PPA inihibtion
non-selective Reactivity
EXAFS
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