Isolated Single-Atomic Ru Catalyst Bound on a Layered Double Hydroxide for Hydrogenation of CO2 to Formic Acid

Article abstract of DOI:10.1021/acscatal.7b00312

In order to achieve an economical CO₂-mediated hydrogen energy cycle, the development of heterogeneous catalysts for CO₂ hydrogenation to formic acid is an urgent and challenging task. In this study, a stable and well-defined single-site Ru catalyst on the surface of a layered double hydroxide (LDH) in a basic medium is proven to be efficient for selective hydrogenation of CO₂ to formic acid under mild reaction conditions (2.0 MPa, 100°C). The electron-donating ability of triads of basic hydroxyl ligands with a particular location is crucial for an active electron-rich Ru center. There is a strong correlation between catalytic activity and adjustable CO₂ adsorption capacity in the vicinity of the Ru center. Such electronic metal-support interactions and a CO₂ concentration effect result in a significant positive influence on the catalytic activity. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.7b00312

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Letter
Isolated Single-Atomic Ru Catalyst Bound on a Layered
2
Double Hydroxide for Hydrogenation of CO to Formic Acid
Kohsuke Mori, Tomohisa Taga, and Hiromi Yamashita
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00312 • Publication Date (Web): 22 Mar 2017
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ACS Catalysis
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Isolated Single-Atomic Ru Catalyst Bound on a Layered Double Hy-
droxide for Hydrogenation of CO2 to Formic Acid
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Kohsuke Mori*,†, ‡, §, Tomohisa Taga†, and Hiromi Yamashita*,†, §
† Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2ꢀ1 Yamadaoka,
Suita, Osaka 565ꢀ0871, Japan
‡ JST, PRESTO, 4ꢀ1ꢀ8 HonCho, Kawaguchi, Saitama, 332ꢀ0012, Japan
§ Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Kyoto 615ꢀ8520, Japan
KEYWORDS: Singleꢀsite catalyst • Layard double hydroxide • Ruthenium • CO2 hydrogenation • formic acid
ABSTRACT: In order to achieve an economical CO2ꢀmediated hydrogen energy cycle, the development of heterogeneous catalysts for
CO2 hydrogenation to formic acid is an urgent and challenging task. In this study, a stable and wellꢀdefined singleꢀsite Ru catalyst on the
surface of a layered double hydroxide (LDH) in a basic medium is proven to be efficient for selective hydrogenation of CO2 to formic acid
under mild reaction conditions (2.0 MPa, 100°C). The electronꢀdonating ability of triads of basic hydroxyl ligands with a particular locaꢀ
tion is crucial for an active electronꢀrich Ru center. There is a strong correlation between catalytic activity and adjustable CO2 adsorption
capacity in the vicinity of the Ru center. Such electronic metalꢀsupport interactions and a CO2 concentration effect result in a significant
positive influence on the catalytic activity.
The precise design of supported metal nanostructures is a key
challenge in controlling the activity and specificity of heterogeneꢀ
ous catalysts to achieve particular target reactions.1 The downsizꢀ
ing of metal assemblies to clusters, or ultimately isolated singleꢀ
atomic state, with wellꢀdefined metal atoms atomically dispersed
or anchored on supports, offers unrivalled opportunities to maxꢀ
imize atom efficiency.2ꢀ4 In contrast to heterogenization of existꢀ
ing homogeneous organometallic complexes on insoluble matriꢀ
ces, the anchoring of singleꢀsite catalysts directly to metal oxides
has also led to geometric and electronic effects induced by imꢀ
provements in strong metalꢀsupport interactions (SMSI) originatꢀ
ing from particular locations and chemical bonding between metꢀ
als and the associated interfaces.5ꢀ7 Recent remarkable progress in
the development of such isolated singleꢀatomic catalysts has reꢀ
sulted in an exciting research field involving the creation of novel
catalysts and their application in a variety of industrial chemical
reactions.
catalysts, in spite of their practical utility,19ꢀ25 and, to make matꢀ
ters worse, their use frequently requires a high catalyst concentraꢀ
tion, organic solvents, and extremely high pressures.
Here we present an isolated singleꢀatomic Ru catalyst on the
surface of layered double hydroxides (LDHs) for the exploitation
of a heterogeneous CO2 hydrogenation system. The general forꢀ
mula of LDHs are represented as M2+
M
x(OH)2)x+(An‒
3+
1ꢀx
)
x/n·mH2O, where M2+ and M3+ are diꢀ and triꢀvalent metal cations
and Anꢀ represents interlayer anions such as CO3
.
LDHs are
2‒ 26
known to act as efficient base catalysts owing to the sixꢀfold coꢀ
ordinated OH‒ groups with diꢀ and triꢀvalent cations.27 It was
expected, and is herein demonstrated, that an electronꢀrich monoꢀ
nuclear Ru species could be successfully created with the aid of
strong Brønsted OH‒ ligands, which further boost CO2 adsorption
in the vicinity of the active Ru center, ultimately enhancing CO2
hydrogenation even at relatively low pressure.
Various LDHs were prepared by a literature procedure.28
Treatment of a LDH, Mg10Al2(OH)24CO3, with a solution of
RuCl3·nH2O in 0.2 M aqueous NaOH at 50°C for 8 h afforded
Ru/LDH as a gray powder (Ru content: 0.36 wt%, as determined
inductively coupled plasma (ICP) analysis. Other Ru catalysts
were also synthesized by the same method with different supports
(Ru contents: 0.38 ꢀ0.40 wt% by ICP). The reason for treating the
acidified Ru chloride with an alkaline solution is to create an elecꢀ
tronꢀrich ruthenium hydroxide precursor such as [Ru(OH)6]3ꢀ,29 as
well as avoiding dissolution of the basic LDH during the deposiꢀ
tion step. In the UVꢀvis spectrum, a peak assigned to the Ru‒OH
bond was observed at around 400 nm upon addition of NaOHaq to
the aqueous RuCl3·nH2O solution (Fig. S1). The addition of
NaOHaq induced a change in the Ru Kꢀedge XANES spectrum of
the aqueous RuCl3·nH2O solution, indicating a change in the local
coordination environment (Fig. S2A). Fourier transform (FT) of a
k3ꢀweighted Ru Kꢀedge EXAFS spectrum revealed the disappearꢀ
ance of a peak associated with the Ru‒Cl bond, expected to be
around 1.9 Å, upon addition of NaOHaq to the aqueous
RuCl3·nH2O solution (Fig. S2B), coincident with the appearance
The chemical fixation of carbon dioxide (CO2) into syntheticalꢀ
ly beneficial compounds is of great interest in industrial chemistry
because it can be considered an inexpensive, nontoxic, and abunꢀ
dant C1 feedstock.8ꢀ10 The hydrogenation of CO2 to produce forꢀ
mic acid (FA; HCOOH), which is a liquid at room temperature
and contains 4.4 wt% hydrogen, is a promising approach to estabꢀ
lishing FA as a renewable hydrogen storage material, since the
chemically stored H2 in the FA can be liberated in controllable
fashion in the presence of appropriate catalysts even at room temꢀ
perature.11ꢀ14 Thus, the design of novel catalysts for CO2 hydroꢀ
genation to form FA is a crucial task in the realization of economꢀ
ical CO2ꢀmediated hydrogen energy cycles.
Homogeneous transition metal complexes in basic media have
proven to be efficient catalytic systems.15,16 Recent design strateꢀ
gies have been principally focused on the synthesis of electronꢀ
rich metal centers through the use of electronꢀdonating organic
ligands, which exhibit satisfactory reaction rates even under mild
reaction conditions.17,18 However, the development of heterogeꢀ
neous catalysts lags significantly behind that of homogeneous
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of a new scattering peak due to an Ru‒O bond at around 1.6 Å,
demonstrating the formation of ruthenium hydroxide species.
ligand. A highꢀangle annular darkꢀfield scanning transmission
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electron microscopy (HAADFꢀSTEM) image also indicated white
points suggestive of an isolated singleꢀatomic structure (Fig. 1B)
without the significant agglomeration. Conclusively, an isolated
singleꢀatomic Ru species with octahedral coordination geometry
bearing one hydroxyl and two water ligands was grafted onto a
triad of oxygen atoms originating from the basic hydroxyl groups
on the LDH surface (Fig. 1C).
As summarized in Table 1, hydrogenation of CO2 to give forꢀ
mic acid was carried out using various Ru catalysts under several
sets of conditions, and the comparison of catalytic activity was
performed with turnover number (TON) based on Ru. A typical
reaction was performed in a 25 mL stainlessꢀsteel reactor in a
basic aqueous solution containing 1.0 M of NaHCO3 under a total
pressure of 2.0 MPa (H2 : CO2 = 1 : 1) at 100°C for 24 h. Ru/LDH
(Mg2+/Al3+=5) was proven to be an effective heterogeneous cataꢀ
lyst with > 99% selectivity without the formation of methanol,
affording a substantial TON of 461 with a turnover frequency
(TOF) of ca. 19 h–1 (entry 1). No reaction was observed in the
case of the inherent LDH (entry 2). The use of a homogeneous
RuCl3·nH2O solution did not result in efficient catalysis (entry 3).
There was no distinct relationship between the catalytic activity
and SBET; other heterogeneous Ru catalysts, such as Ru/MgO,
Ru/Mg(OH)2, Ru/Al2O3, and Ru/Al(OH)3, were found to be less
active (entries 4–7). Thus, the creation of a singleꢀsite Ru catalyst
on the surface of LDH is indispensable in achieving high catalytic
activity.
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Figure 1. (A) Ru Kꢀedge FTꢀEXAFS spectra, (B) HAADFꢀSTEM
image of Ru/LDH (the circles are drawn around a part of isolated
Ru atoms), and (C) schematic illustration of Ru/LDH (top and
side view).
Table 1. Hydrogenation of CO2 to produce formic acid under
various catalytic conditions.
TON based
on Ru
SBET
entry
catalyst
base
The Brunauer Emmett Teller surface areas (SBET) calculated
from N2 adsorptionꢀdesorption were 81.6 and 84.3 m2·g‒1 for
Ru/LDH and the parent LDH, respectively (Table 1). The XRD
pattern of Ru/LDH did not show any d001 peak shift in comparison
with that of the original LDH (Fig. S3). These facts demonstrated
predominant deposition of Ru species on the surface of LDH,
while the interlayer space was maintained (0.79 nm) without exfoꢀ
liation or segregation. The absence of chlorine in the Ru/LDH was
confirmed by XPS analysis (Fig. S4). The shape and edge posiꢀ
tion in the Ru Kꢀedge XANES spectrum of Ru/LDH (22124.6 eV)
was similar to those of RuCl3·nH2O (22125.2 eV) and RuO2
(22125.9 eV), but differed from that of Ru foil (22118.2 eV) (Fig.
S5), indicating that Ru exist in the +4 oxidation state.30,31 In the
FTꢀEXAFS spectrum of Ru/LDH, there were no peaks due to
metallic Ru‒Ru and contiguous Ru‒O‒Ru bonds, expected to be
around 2.5 and 3.3 Å (Fig. 1A). The absence of a Ru‒Cl bond,
detectable in the spectrum of RuCl3·nH2O at around 2.0 Å, was
also confirmed. The two peaks observed at 2.2 and 2.8 Å may be
assigned to the RuꢀMg or RuꢀAl in the second coordination
spheres,32 which indicate that Ru species is located on the surface
of Brucite layer of the LDH, not within the interlayer space. The
maximum in the envelope of the inverse FT spectrum for Ru/LDH
was located at around k = 7 Å‒1, which was lower than the equivaꢀ
lent result for Ru foil (11 Å‒1), RuO2 (9 Å‒1), and RuCl3 (8 Å‒1)
(Fig. S6). The appearance of maxima in the envelope at smaller k
values suggests a second shell interaction with lighter atoms. The
inverse FT of the main peaks was well fitted by the use of five
longer Ru‒O (2.05 Å) and one short Ru‒O (1.82 Å) shell parameꢀ
ters, as summarized in Table S1 and Fig. S6. In a separate experꢀ
iment, the sample was desorbed under vacuum conditions, and
further subjected to XAFS analysis. The inverse FT of this
Ru/LDH after desorption treatment was well fitted by the use of
three longer Ru‒O (2.04 Å) and one short Ru‒O (1.81 Å) shell
parameters, indicating that Ru center contains two adsorbed water
/m2·gꢀ1
1
2
Ru/LDH (Mg2+/Al3+=5)
LDH (Mg2+/Al3+=5)
RuCl3 solution
Ru/MgO
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
416
ꢀ
81.6
84.3
ꢀ
3
120
239
181
90
4
13.8
30.7
123.0
40.8
73.0
80.1
81.0
ꢀ
5
Ru/Mg(OH)2
6
Ru/Al2O3
7
Ru/Al(OH)3
RuCl/LDH(Mg2+/Al3+=5) NaHCO3
29
8
149
28
9
Rh/LDH (Mg2+/Al3+=5)
Ir/LDH (Mg2+/Al3+=5)
Ru/LDH (Mg2+/Al3+=5)
Ru/LDH (Mg2+/Al3+=5)
Ru/LDH (Mg2+/Al3+=5)
NaHCO3
NaHCO3
NaOH
KOH
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11
12
13
16
698
489
43
ꢀ
ꢀ
ꢀ
In a separate experiment, a reaction with RuCl/LDH that had
been prepared using the RuCl3·nH2O precursor without the addiꢀ
tion of NaOHaq showed moderate activity, indicating the imꢀ
portance of pretreatment (entry 8). The use of Rh/LDH and
Ir/LDH, prepared by the same method as Ru/LDH, gave poor
results (entries 9 and 10). Among the bases examined, NaOH and
KOH were excellent, giving high TONs of 698 and 489 (entries
11 and 12), whose TOFs corresponds to 29 h–1 and 20 h–1, respecꢀ
tively. Notably, the Ru/LDH catalyst under discussion enabled
direct hydrogenation of CO2 even under neutral conditions in the
absence of additional bases (entry 13). The origin of the produced
formic acid was proved by the catalytic reaction using isotopic
13CO2 in D2O solvent (0.5 M NaOH) and the products was identiꢀ
fied by 13C NMR. Deprotonated H13COOH can be observed at
170. 88 ppm as a sole product without the formation of byprodꢀ
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ucts (Figure S7).33 The isotopic 13CO2 reaction provides solid
proof that the produced formic acid is originated from CO2.
It is well accepted that the strong electronꢀdonating ability of
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PNP pincerꢀtype ligands or Nꢀheterocyclic carbenes in Ir(III)
complexes facilitates hydrogenation of CO2.35ꢀ37 Moreover, a
linear correlation between the hydrogenation rate and the elecꢀ
tronꢀdonating ability of bipyridine ligands with various substituꢀ
ents has been reported.38 Upon consideration of these results, it
may be concluded that one possible reason for the high catalytic
activity of Ru/LDH is ascribed to an electronꢀrich singleꢀsite Ru
catalyst with the assistance of a triad of basic hydroxyl ligands.
XAFS analysis proved the formation of an isolated singleꢀatomic
Ru structure for all analogous supports investigated (Fig. S9),
while different coordination environments changed the electronic
state of Ru center via electronic metalꢀsupport interaction
(EMSI),39,40 as determined by XPS analysis (Fig. S10). It should
be noted that the catalytic activity corresponded well with the Ru
3p binding energy; the TON based on Ru increased with decreasꢀ
ing binding energy (Fig. 2A), thus suggesting that special placeꢀ
ment of hydroxyl groups with an ordered arrangement on the
LDH surface is a promising platform for the electronically proꢀ
moted Ru species.
A characteristic feature of the Ru/LDH catalyst is its high activꢀ
ity even under lowꢀpressure conditions. It is noteworthy that the
TON and TOF values achieved by the Ru/LDH system were
comparable or higher than those reported for other heterogeneous
catalyst systems, including Au/Al2O3 (TON and TOF respectively
696 and 29 h–1 at 4 MPa in DMF/NEt3),24, Pd/mpgꢀC3N4 (85 and
3.5 h–1 at 4 MPa in H2O/NEt3),25 and Au/TiO2 (284 and 12 h–1 at
18 MPa in neat NEt3).22 Even under the direct hydrogenation to
produce pure formic acid without bases, the achieved TOF (5.0 x
10‒4 s‒1) is four times higher than that with carbon nanotubeꢀ
graphene supported PdNi alloy (1.2 x 10‒4 s‒1 at 5 MPa).34 Upon
completion of the reaction, separation of the heterogeneous cataꢀ
lyst from the reaction system is straightforward. The recovered
catalyst was recycled at least three times while keeping 90 % of
its original catalytic activity. Ru Kꢀedge XAFS analysis revealed
no structural changes around the isolated singleꢀatomic Ru cataꢀ
lyst in the recovered Ru/LDH (Fig. S8). TEM analysis also
showed that the LDH surfaces remained virtually unchanged even
after the reaction, without the formation of Ru nanocrystallites.
Furthermore, ICP analysis of the filtrate confirmed that the Ru
content was negligible, from which it may be deduced that the
hydrogenation reaction occurs on a solid surface. Such prominent
characteristics may be due to the presence of a structurally robust
mononuclear Ru catalyst on the LDH surface. Because of this, the
Ru/LDH catalyst is a pivotal contribution to the development of
environmentally benign chemical processes.
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A series of LDHs with different M2+/M3+ components and ratiꢀ
os may be easily synthesized, and the catalytic activities of these
materials in CO2 hydrogenation were further investigated. The
optimal ratio among MgꢀAlꢀLDHs investigated was Mg2+/Al3+
=
5 (Fig. 2B). Other combinations with an M2+/M3+ ratio of 5 reꢀ
sulted in low catalytic activity. Interestingly, such catalytic activiꢀ
ty is well correlated with differences in CO2 adsorption originatꢀ
ing from different levels of basicity in the LDHs. Based on metal
atom ordering, two types of hydroxyl site exist on the surface; one
is connected to three M2+ ions, while the other is bound to two
M2+ cations and one M3+.41 Changes in the cation ratio and the
combination used can change the basicity of the LDH, allowing
adjustment.28 It has been reported that basic sites tuned in this
way show a diverse array of pKa values in the range 9.0–16.5.42
High CO2 adsorption property on the hydroxyl site results in a
concentration effect on the surface of LDH support, which boosts
the interaction with active singleꢀsite Ru catalyst and ultimately
influences on the catalytic activity.
Based on the consideration of experimental and theoretical
studies on previously reported reaction mechanisms of metal
complexes,43,44 we propose a plausible catalytic cycle for CO2
hydrogenation over Ru/LDH (Scheme 1). The reaction is initiated
by H2 activation (step 1), in which one basic oxygen ligand assists
the heterolytic dissociation of the adsorbed H2 in the periphery of
the Ru center (step 2). Next, insertion of CO2 into the Ruꢀhydride
species (step 3), followed by the isomerization via the attack of
hydride H species onto C atoms, affording a Ru η1ꢀformate interꢀ
mediate (step 4), which regenerates the initial active species, acꢀ
companied by the formation of formic acid (step 5).
Figure 2. (A) Relationship between the TON for CO2 hydrogenaꢀ
tion based on Ru, and Ru 3p binding energy as determined by
XPS spectroscopy. (B) Relationship between the TON for CO2
hydrogenation based on Ru and CO2 adsorption for various Ruꢀ
supported LDH samples.
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Scheme 1. Possible reaction pathway for CO2 hydrogenation to
produce formic acid with a Ru/LDH catalyst.
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(35)Tanaka, R.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131,
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(38)Himeda, Y.; OnozawaꢀKomatsuzaki, N.; Sugihara, H.; Kasuga, K.
Organometallics 2007, 26, 702ꢀ712.
(39)Hu, P.; Huang, Z.; Amghouz, Z.; Makkee, M.; Xu, F.; Kapteijn, F.;
Dikhtiarenko, A.; Chen, Y.; Gu, X.; Tang, X. Angew. Chem. Int. Ed.
2014, 53, 3418ꢀ3421.
The activation energy (Ea) for CO2 hydrogenation using
Ru/LDH, determined from Arrhenius plots, was 54.3 kJ mol−1, as
shown in Fig. S11. This value is similar to that of
[Cp*Ir(bpy)(OH2)]SO4 (51.4 kJ mol−1), but is different from that
of Au/Al2O3 (74.0 kJ mol−1), which may be indicative of the inꢀ
volvement of a singleꢀsite reaction pathway. The reaction rate (R)
β
of CO2 hydrogenation can be expressed by R = kꢁPH2α·PCO2
,
where k is the rate constant, PH2 and PCO2 represent the partial
pressure of H2 and CO2, and α and β represent the reaction order
of H2 and CO2, respectively. Kinetic investigation determined that
α and β were 1.95 and 1.71 at 100 °C, respectively (Fig. S12).
Based on these facts, it was concluded that the initial Ruꢀhydride
formation is the rateꢀdetermining step, rather than CO2 insertion.
This behavior contrasts with that observed in Au/Al2O3 catalytic
systems, presumably because of the concentration effect of the
adsorbed CO2 in the vicinity of the neighboring Ru center, which
allows the formation of a Ru η1ꢀformate intermediate. The use of
a donating OH ligand is advantageous for this catalytic reaction
because the electronꢀrich Ru species boost not only the insertion
of CO2 into the Ruꢀhydride species, but also the consecutive
isomerization step.44 Such cooperative actions explain the high
activity of Ru/LDH for CO2 hydrogenation.
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In summary, the anchoring of a Ru precursor onto the surface
of LDH in a basic medium successfully generated an isolated
singleꢀatomic Ru catalyst surrounded by OH ligands with strong
basicity. The electronegativity was enhanced by the special locaꢀ
tion of the hydroxyl groups, which have an ordered arrangement
on the LDH surface. The CO2 adsorption capacity in the vicinity
of the Ru center can be tuned by varying the M2+/M3+ components
and ratio in the LDH, for which the maximum was Mg2+/Al3+ = 5.
Even under lowꢀpressure conditions, the Ru/LDH catalyst disꢀ
played significant catalytic activity for selective CO2 hydrogenaꢀ
tion to produce formic acid due to EMSI and CO2 concentration
effects. Further improvement of the catalytic performance under
mild reaction conditions is expected to result in the realization of
an environmentally friendly CO2ꢀmediated reversible hydrogen
storage system as a practical application.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental details, Figures S1ꢀS12 and Table S1 (PDF)
AUTHOR INFORMATION
Corresponding Author
*K. M. mori@mat.eng.osakaꢀu.ac.jp
*H. H. yamashita@mat.eng.osakaꢀu.ac.jp
ACKNOWLEDGMENT
The present work was supported by JSTꢀPRESTO. The part of
this work was supported by GrantsꢀinꢀAid for Scientific Research
(Nos. 26220911, 25289289, and 26630409, 26620194) from the
Japan Society for the Promotion of Science (JSPS) and MEXT.
(40)Campbell, C. T. Nat. Chem 2012, 4, 597ꢀ598.
(41)Cai, H.; Hillier, A. C.; Franklin, K. R.; Nunn, C. C.; Ward, M. D.
Science 1994, 266, 1551ꢀ1555.
REFERENCES
(1) Mori, K.; Yamashita, H. Phys. Chem. Chem. Phys. 2010, 12, 14420ꢀ
14432.
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