Low-temperature hydrogen production from methanol over a ruthenium catalyst in water

Article abstract of DOI:10.1039/d0cy01470b

Traditionally, methanol reforming at a very high temperature (>200 °C) has been explored for hydrogen production. Here, we show that in situ generated ruthenium nanoparticles (ca. 1.5 nm) from an organometallic precursor promote hydrogen production from methanol in water at low temperature (90-130 °C), which leads to a practical and efficient approach for low-temperature hydrogen production from methanol in water. The reactivity of ruthenium nanoparticles is tuned to achieve a high rate of hydrogen gas production from methanol. Notably, the use of a pyridine-2-ol ligand significantly accelerated the hydrogen production rate by 80% to 49 mol H2 per mol Ru per hour at 130 °C. Moreover, the studied ruthenium catalyst exhibits appreciably long-term stability to achieve a turnover number of 762 mol H2 per mol Ru generating 186 L of H2 per gram of Ru. This journal is

Full text of DOI:10.1039/d0cy01470b

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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Low-temperature hydrogen production from methanol over ruthenium
catalyst in water
†
‖
┴
†
3
Mahendra K. Awasthi, Rohit K. Rai, Silke Behrens, and Sanjay K. Singh *
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†Catalysis Group, Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore
‖
453552, M.P., India. KAUST Catalysis Center and Division of Physical Sciences and
Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-
┴
6900, Kingdom of Saudi Arabia. Institut für Katalyseforschung und –technologie (IKFT),
Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, D-76344
Eggenstein-Leopoldshafen, Germany
1
1
0
1
E-mail: sksingh@iiti.ac.in
Abstract
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Traditionally, methanol-reforming at a very high temperature > 200 ℃ has been explored for
hydrogen production. Here, we show that in situ generated ruthenium nanoparticles (ca. 1.5 nm)
from an organometallic precursor promote hydrogen production from methanol in water at low
temperature (90 ℃ ‒ 130 ℃), which leads to a practical and efficient approach for low-temperature
hydrogen production from methanol in water. The reactivity of ruthenium nanoparticles is tuned
to achieve a high rate of hydrogen gas production from methanol. Notable, use of pyridine-2-ol
ligand significantly accelerated the hydrogen production rate by 80% to 49 mol H
2
per mol Ru per
hour at 130 ℃. Moreover, the studied ruthenium catalyst exhibits appreciably long-term stability
to achieve a turn over number of 762 mol H
2
per mol Ru generating 186 L of H per gram of Ru.
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1
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Keywords
Hydrogen, Methanol, Ruthenium, Water, nanoparticles
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Introduction
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Hydrogen is a potential clean energy carrier, and when used in fuel cell it produces only water as
a byproduct. Unfortunately, the presence of hydrogen gas in the earth’s atmosphere is extremely
low (≈ 1 ppm by volume). Therefore, one of the major hurdles in exploring hydrogen economy
with full potential is the safe production and storage of hydrogen gas. Notably, carrying big and
heavy hydrogen cylinders with high pressure has critical safety and economical challenges. On the
other hand, using liquid hydrogen storage materials (such as HCHO, CH OH, HCOOH) in the fuel
3
tank of existing vehicles (using petroleum products) and generate hydrogen on-board to supply to
Fuel Cell is not only a viable concept but is also very economical.1-12 In this context, methanol,
which contains an appreciably high gravimetric content of hydrogen 12.5 wt% is a promising
candidate for on-board and off-board (stationary) large-scale production of hydrogen gas.13-20
Notably, methanol offers several advantages, such as it is an inexpensive liquid, low carbon
content (C1 alcohol), easy to store, and is being produced on large scale from biomass resources
and hydrogen and carbon monoxide, or as industrial byproducts.13-20 Traditionally, hydrogen gas
is being produced from methanol steam reforming process at a high-temperature range (200 ℃ ‒
350 ℃), while catalyst assisted hydrogen production from methanol in water is more energy
efficient as it operates at low temperature (< 190 ℃).
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In principle, hydrogen production from methanol is mildly endothermic, therefore a
suitable catalyst may activate methanol to produce hydrogen gas.14-24 Experimental evidences
revealed that in the presence of a catalyst the dehydrogenation of methanol may follow the three
consecutive pathways: initially, the dehydrogenation of methanol to formaldehyde and hydrogen
(eq. 1), followed by the dehydrogenation of formaldehyde in the presence of water (gem-diol) to
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hydrogen and formic acid (eq. 2), and finally dehydrogenation of formic acid to hydrogen and CO
2
(eq. 3).
-
1
3
4
5
CH
CO + H
HCOOH → CO
3
OH → H
2
CO + H
2
(∆H = 129.8 kJ mol )
(1)
(2)
(3)
-1
H
2
2
O → HCOOH + H
2
(∆H = ‒30.7 kJ mol )
-1
2
+ H (∆H = 31.6 kJ mol )
2
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A wide range of catalysts has been explored to utilize methanol, as a potential liquid
hydrogen storage material, for the production of hydrogen gas at low-temperature with regulated
emission of unwanted CO and methane.21,25 Recently, homogeneous catalysts based on Ru, Ir, Fe,
and Mn have been explored to dehydrogenate methanol in presence of water and produce H
2
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without or very low ppm of CO at the temperature below 100 °C. 14,21,25-30 In particular, ruthenium-
pincer based molecular catalysts exhibited higher activity to produce hydrogen from methanol in
basic condition.25-26 In contrary to the above, heterogeneously catalyzed reforming of methanol to
produce hydrogen and carbon dioxide has been continuously studied and developed, using
31
32
33
different metal-based catalysts such as CuO/ZnO/Al
2
O
3
, Pd/CeO
2
-ZrO
2
, Pt Ni, and Ni-Fe-
3
3
4
Mg alloys, but most of these catalysts require higher temperature over 200 °C and pressure. On
the other hand, industrially viable heterogeneous catalysts for low-temperature hydrogen
production are rarely explored, until recently when Pt/MoC catalyst was explored for hydrogen
production from methanol, but this catalyst worked effectively only at higher temperature (150-
190 ℃) and using an expensive Pt catalyst.35
2
2
2
0
1
2
Notable, the development of catalysts for the selective transformation of methanol to
hydrogen gas (eq. 1) with the generation of formic acid is of critical importance, as it eliminates
the energy-intensive process of CO
2
scrubbing from H
2
and CO mixture obtained from complete
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transformation of methanol (eq. 1 and 2). Furthermore, the byproduct formic acid is also a worthful
product and a potential hydrogen storage material.9 Herein, we synthesized ruthenium
nanoparticles in-situ from an organometallic ruthenium complex and a ligand and utilized it to
achieve efficient catalytic activity to produce hydrogen gas and formate/formic acid from methanol
in the water at the lower temperature (90-130 ℃). The structure of the ruthenium catalyst was
established by TEM, XPS, P-XRD, ICP, TGA and several reaction parameters were evaluated to
achieve high catalytic activity for the low-temperature conversion of methanol to hydrogen gas
and formate over the ruthenium catalyst in water.
9
Results and Discussion
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At an outset, we employed [{(ƞ -benzene)RuCl
2
}
2
] ([Ru]-1) as a pre-catalyst for hydrogen
production from methanol (2:1 molar ratio of CH
3
OH:H
2
O) at 110 °C in the presence of 1.2 equiv.
-1
KOH, where we observed the release of 73 mol H
2
per mol of Ru (initial TOF 9 h ) (Table 1, entry
1). The evolved gas was identified as hydrogen by GC-TCD. Notably, the initial dark brown color
of the reaction solution turned to a black suspension, which was identified as Ru nanoparticles (ca.
18.7 nm) by TEM (Fig. S1). Further, recent studies revealed that pyridine-based ligands can act as
an internal base and play a crucial role in C-H activation reaction.36-37 We, therefore, investigated
the role of 2-hydroxy pyridine (L1) as a promoter in the ruthenium-catalyzed hydrogen production
from methanol. For Ru/L1 catalyst (pre-catalyst [Ru]-1 in the presence of L1), we observed a
significant enhancement of ca. 82% in the initial TOF with the release of 0.66 mol of H
of methanol (TON of 106 mol H per mol of Ru) (Table 1, entry 2 and Fig. 1a). When the reaction
was performed using CH OH:H O molar ratio of 1:1, the intrinsic activity increased further to
yield a TON of 134 mol of H per mol of Ru (0.83 mol of H per mol of methanol) with an
2
per mol
2
3
2
2
2
-1
improved initial TOF of 20 h (Table 1, entry 5 and Fig. 1b). Notably, further increasing the water
4

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content (1:2 molar ratio of CH
3
OH:H
2
O) resulted in lower catalytic activity for H
2
production from
methanol over Ru/L1 catalyst (Table 1, entry 6, and Fig. 1b), as compared to the reaction
performed with high methanol content (CH OH:H O molar ratio of 1:1, 2:1 or 4:1) (Table 1 and
3
2
Fig. 1a). Literature reports on aqueous methanol dehydrogenation outlined the beneficial role of
water in methanol dehydrogenation reaction.11,14 For instance, Grützmacher et al. achieved
dehydrogenation of methanol using CH
3
OH/H O 1:1 molar ratio, and mentioned that in the
2
11
presence of water high gravimetric content of hydrogen can be achieved form methanol. Beller
and other researchers also highlighted the role of water-promoted dehydrogenation of
formaldehyde to formic acid and H
during methanol dehydrogenation reaction.25,38 Notably, Lin
2
1
1
0
1
et al. also reported efficient hydrogen production from methanol over Pt/α-MoC using higher water
O molar of 1:3 and 1:1).35
content (CH
3
OH/H
2
1
2
Table 1. Screening of catalyst to producing hydrogen from methanol a
h
-1
Entry Cat.
n(alc.)/n(H
2
O) T (°C) KOH
n(H
2
)/ n(alc.) n(H
2
)/ n(cat.) TOF (h )
(equiv.)
1b
2
Ru
2:1
2:1
4:1
neat
1:1
1:2
1:1
1:1
110
110
110
110
110
110
110
130
1.2
1.2
1.2
1.2
1.2
1.2
-
0.45
0.66
0.68
0.63
0.83
0.25
0.67
1.42
73
9
Ru/L1
Ru/L1
Ru/L1
Ru/L1
Ru/L1
Ru/L1
Ru/L1
106
109
102
134
40
18
13
14
20
14
19
49
3
4
5
6
7c
8
107
229
1.2
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1
Ru/L1
Ru/L1
1:1
1:1
90
1.2
1.2
1.2
1.2
1.2
0.24
0.50
0.13
0.61
0.69
38
81
21
98
111
5
0d
1e
110
110
110
110
11
12
20
16
1
RuNP/L1 1:1
1
1
2f
Ru/L1
Ru/L1
1:1
1:1
3g
a
Reaction condition: alcohol (16.08 mmol), Ru catalyst (0.625 mol%, n([Ru]-1)/n(L1) 1:2),
b
c
KOH (1.2 equiv.), 10 h, argon, in the absence of ligand L1, reaction with NaOMe in place of
CH OH, with [Ru]-2 and L1 (n([Ru]-2)/n(L1) 1:2), with pre-synthesised Ru nanoparticles and
L1 (1 equiv.), using ethanol as reactant, using n-propanol as reactant and average turnover
d
e
3
f
g
h
frequency (TOF) at 1 h.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Notably, we found that base played a crucial role in generating the active Ru nanoparticles
in the initial hour of the reaction. Moreover, lower activity was observed while using less content
of KOH (0.42 equiv.) (Table S1). Results inferred that compared to KOH, reaction with other bases
t
such as NaOH, K OBu, and K
2
CO
3
exhibited either lower activity or no reaction (Table S2). It is
was released when NaOMe was used as a substrate
worth noting that almost a similar amount of H
2
instead of methanol, in absence of base, suggesting that presumably content of base is crucial for
the deprotonation of methanol (Table1, entry 7, and Fig. S2). Moreover, kinetic isotope effect
(KIE) studies indicated that CD
3
OD is more influential than D O in tuning the reaction rate for the
2
Ru/L1 catalyzed hydrogen production from methanol (Fig. S3 and Table S3). These results
inferred that the activation of methanol C-H bond is presumably the rate-determining step and not
the proton assisted release of hydrogen gas from methanol. Further, 13C NMR of the reaction
aliquot after the completion of the catalytic reaction inferred the presence of formate with traces
of carbonate (Fig. S4a), suggesting that decomposition of formate does not take place. Notably,
performing the reaction with the spent Ru/L1 catalyst resulted in no formate decomposition and
1
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no traces of carbonate was detected in C NMR (Fig. S4b and S5, and Table S4). Furthermore,
the amount of formate formed as a co-product was also quantified to nearly half of the mol of H
2
gas generated during the reaction (Fig. S5). GC-TCD analysis of the evolved gas is in good
agreement with the observation of only purified hydrogen gas (Fig. S6). When reaction
temperature was increased to 130 °C, initial TOF increased by over ten-folds to 49 mol H
Ru per hour as compared to the reaction performed at 90 °C (TOF of 5 mol H per mol of Ru per
hour) (Table 1, entries 5, 8, 9, and Fig. S7). Notably, the Ru/L1 catalyst exhibited the generation
of 1.42 mol H per mol of methanol (~71% conv.) (at 130 °C), which is several-folds higher than
that reported for 0.2%Pt/α-MoC catalyst (0.037 mol H
apparent activation energy for the conversion of methanol to H
2
per mol
2
2
35
2
per mol of methanol at 190 °C). The
1
1
1
0
1
2
2
over Ru/L1 catalyst was estimated
as 18.3 kcal/mol (Fig. S7). Therefore, it is evident from these results that Ru/L1 is active even at
lower reaction temperature of 90 °C – 130 °C.
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Further to detect various species that might form during the dehydrogenation of methanol,
1
13
the reaction mixture at various time intervals (0, 15 and 60 min) was analyzed by H and C NMR,
where no traces of formaldehyde, methanediol, paraformaldehyde and trioxane were detected (Fig.
S5). Consistent with the literature reports, this can be attributed to the faster transformation of
formaldehyde to formic acid and hydrogen in water as compared to methanol dehydrogenation to
formaldehyde (Fig. S5).11,18,38 Furthermore, we performed the catalytic dehydrogenation of
aqueous formaldehyde (37 wt.%) under analogous reaction condition to methanol
dehydrogenation and analyzed the evolved gas by GC-TCD and the reaction aliquots by NMR.
Results inferred that indeed in the presence of base only pure hydrogen and formate was produced
from formaldehyde dehydrogenation as confirmed by GC-TCD and NMR, respectively. Therefore,
these results evidenced that during methanol dehydrogenation under the optimized reaction
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condition, the possible intermediates such as formaldehyde or methanediol formed during the
reaction may also undergo faster transformation to formic acid and H gas, and therefore
2
formaldehyde was not detected in the reaction solution. However, we have detected and isolated
the formic acid generated during the methanol dehydrogenation reaction under the studied reaction
condition (Fig. S5).
3
2
1
00
00
00
(
a)
[
[
Ru]-1 + L1
Ru]-1
2
:1 (CH OH:H O)
3
2
0
0
200
400
600
Time (min)
3
2
1
00
(
b)
1
:0 (CH OH:H O)
3 2
4
2
1
1
:1 (CH OH:H O)
3 2
:1 (CH OH:H O)
00
00
0
3
2
:1 (CH OH:H O)
3
2
:2 (CH OH:H O)
3
2
0
100
200
300
Time (min)
6
7
Fig. 1 Effect of (a) ligand L1 and (b) methanol to water molar ratio on the Ru/L1 catalyzed
hydrogen production from methanol at 110 ℃. (c) Comparative catalytic activity of various
ruthenium catalysts with/without ligands to produce hydrogen from methanol at 110 ℃.
(Reaction condition: methanol (16.08 mmol), Ru catalyst (0.625 mol%, n([Ru]-1)/n(L1) 1:2),
8
9
0
1
2
1
1
1
a
KOH (1.2 equiv.) and methanol to water molar ratio (1:1), argon, methanol to water molar
b
ratio is 2:1, and [Ru]-2 precursor is used).
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Further, to obtain insights into the role of pyridine ligand, catalytic conversion of methanol
to H
2
was also examined using 2-methoxy pyridine (L2) where a significant decline in the initial
TOF (15 mol H
2
per mol Ru per hour) was observed with Ru/L2 catalyst as compared to the TOF
of 20 mol H per mol Ru per hour obtained with Ru/L1 catalyst (Table S5 and Fig. S8). Further,
2
the reaction performed using Ru/pyridine (L3) and Ru/phenol (L4) showed even lower activity
(Fig. 1c, Table S5 and Fig. S8). These observations suggested the possible involvement of the 2-
hydroxy pyridine ligand (L1) in promoting the facile activation of methanol over Ru nanoparticles
to produce hydrogen gas with higher activity. Notably, performing the catalytic reaction with the
larger content of the ligand L1 (Ru/L1 1:4) resulted in lower activity yielding only 46 mol H
mol Ru in 8 h as compared to 62 mol H per mol of Ru for Ru/L1 ratio of 1:2, presumably because
excess ligands may poison the catalyst (Fig. S9). Moreover, using [{(ƞ -p-cymene)RuCl
2
per
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
2
6
2
2
} ] ([Ru]-
2) precursor instead of [Ru]-1 could not improve the catalytic activity (Table 1, entry 10, Fig. 1c
and Fig. S10). Further, performing the catalytic reaction in the presence of pre-synthesized Ru
nanoparticles with L1 resulted in only lower activity, suggesting that in-situ generated Ru
nanoparticles in the presence of L1 in the more suitable condition to generate active Ru
nanoparticle catalysts (Table 1, entry 11, and Fig. 1c). Moreover, to know the nature of real catalyst
for methanol dehydrogenation, we performed a series of poisoning experiments using Whitesides'
2
5
mercury test. Results inferred that the catalytic reaction quenched in the presence of Hg(0) (>300
equiv.) due to the poisoning of the Ru/L1 catalyst by amalgam formation. In contrary to the control
reaction, the addition of Hg(0) (>300 equiv.) at the beginning of the reaction resulted in complete
quenching of the catalytic methanol dehydrogenation, suggesting the heterogeneous nature of the
Ru/L1 catalyst (Fig. S11a). Further, in another experiment, the spent Ru/L1 catalyst was also
stirred with an excess of added Hg(0) (>300 equiv.), before employing it as a catalyst for methanol
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dehydrogenation reaction under the optimized reaction condition, but no gas release was observed
(Fig. S11b). It is worth noting here, that the spent Ru/L1 catalyst was found to be highly active (in
the absence of Hg(0)), supports the heterogeneous nature of the studied catalysts (Fig. S11b).
Moreover, the Ru/L1 catalyst also exhibited appreciable long-term stability, where a total turnover
number (TON) of 762 mol of H
2
per mol of Ru was achieved in a 7-cycle recyclability experiment
generating 186 L of H gas per gram of Ru (Fig. 2). On the other hand, continuing the reaction
2
with the reaction solution (supernatant), obtained after the removal of Ru/L1 catalyst, resulted in
no release of gas even after extending the catalytic reaction for a longer duration (10 h) (Fig. S12).
The literature revealed that reaction performed under reducing condition may result in the
transformation of organometallic Ru precursor to Ru(0) nanoparticles.39-43 Chaudret et al.
established the mechanism and the process for the transformation of organometallic ruthenium
complexes to Ru nanoparticles.42-43 Similarly some other groups have also established the
transformation of organometallic ruthenium complexes to Ru nanoparticles.40 Therefore, in
absence of induction period during the in-situ transformation of [Ru]-1 to Ru nanoparticles can be
1
1
1
1
1
1
1
1
1
1
2
2
2
2
attributed to the highly reducing reaction condition due to the presence of KOH, H gas and 110-
2
130 ℃. Further, ICP-OES analysis of the recovered nanoparticles inferred the presence of ~54
wt.% of Ru, whereas no traces of metal were detected (detection limit of 0.01 ppm) in the
supernatant, evidence the absence of ruthenium species in the reaction mixture also supports the
complete transformation of [Ru]-1 precursor to Ru nanoparticles. Moreover, the liquid portion
(supernatant) was also analyzed by ESI-MS, where any isotopic patterns corresponding to
ruthenium was not observed, further suggesting that the ruthenium complex is converted to
ruthenium nanoparticles during the reaction and no residual ruthenium species is present in the
solution (Fig. S13). Therefore, the above findings evidenced the heterogeneous nature of the
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Ru/L1 catalyst for methanol dehydrogenation. Furthermore, the ethanol and n-propanol were also
investigated under catalytic reaction conditions and the results showed that the studied Ru/L1
catalysts can also efficiently generate hydrogen gas along with value-added products such as acetic
acid (Table 1, entries 12, 13, and Fig. S14). The activity of the catalyst slightly decreased with
subsequent catalytic runs, attributed to the surface oxide coating of the Ru catalyst, as also
confirmed by XPS analysis (Fig . S15). Though the turnover number was relatively low for the
studied Ru/L1 catalyst, it is evident that Ru/L1 catalysts exhibited the generation of higher
equivalents of hydrogen gas per moles of methanol (n(H )/n(MeOH) 1.42) as compared to the
2
earlier reported catalysts, (Table S6). Moreover, in the studied catalytic system, formate/formic
acid is obtained as a worthful byproduct. Advantageously, this resulted in the generation of
hydrogen gas in high purity from methanol. Therefore, the studied Ru/L1 catalytic system could
be a promising candidate for low-temperature bulk hydrogen production from methanol.
1
1
1
0
1
2
1
1
1
1
1
1
1
2
2
2
3
4
5
6
7
8
9
0
1
2
Fig. 2 Long term stability and recyclability experiment for Ru/L1 catalyzed production of H
from methanol at 130 °C.
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To obtain insights into the structural and chemical nature of the in-situ generated Ru/L1
catalyst, we employed several characterization methods. Powder X-ray diffraction of Ru
nanoparticles obtained after the catalytic reaction showed a broad peak at 25-45°, suggesting
4
4
highly dispersed small Ru nanoparticles over the carbon support (Fig. S16). Transition electron
microscopy (TEM) images confirm the existence of a homogenous dispersion of Ru nanoparticles
of particle size ca. 1.5 nm on the carbon support (Fig. 3a and Fig. S17). Furthermore, the dispersion
of in-situ generated ruthenium nanoparticles in the presence of L1 ligand was also estimated as
86%, as calculated from the generalized equations using mean particle size (dTEM) as obtained from
the TEM by considering ~110 particles (Table S7).45-46 In sharp contrast, without ligand, the Ru
nanoparticles have an average particle size of ca. 18.7 nm with poor dispersion (Fig. S1). High
angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images
and energy-dispersive X-ray spectroscopic (EDS) analysis inferred the presence of Ru element
(Fig. 3b-d). Moreover, the EDS line scan also confirmed the even distribution of Ru nanoparticles
over the carbon support (Fig. 3d).
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(
a)
2
2
1
1
5
Average= 1.5 nm
Std. Dev. = 0.3 nm
0
5
0
5
0
1.0
1.5
2.0
2.5
Mean diameter (nm)
1
2
3
Fig. 3 (a) TEM image (inset particle size distribution), (b) EDS point analysis, and (c)
HAADF image and the corresponding (d) EDS line scan analyses for Ru/L1 catalyst.
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8
9
Thermogravimetric analysis of the in-situ generated Ru/L1 catalyst inferred the presence
of large organic content (~30% more) as compared to the Ru nanoparticles obtained in the absence
of the ligand L1 (Fig. S18). Further, the presence of 0.74% nitrogen content observed for Ru/L1
in the elemental analysis, further suggesting the presence of the ligand L1 in the catalyst. Such
behavior is consistent with the stabilization of smaller Ru nanoparticles with 2-hydroxypyridine
ligand (L1). This phenomenon was also revealed earlier where Ru nanoparticles were stabilized
by small ligands.47-48 In addition, the binding energy of Ru/L1 as obtained from X-ray
photoelectron spectroscopy (XPS) experiment for both Ru(3d) and Ru(3p) core levels are assigned
1
1
1
1
0
1
2
3
2
+
to the oxidation state of Ru in the catalyst using CuO as a standard (Cu 3p3/2 933.5 eV in CuO).
In the XPS spectra of Ru/L1 catalyst, peak maxima observed at the binding energy values of 461.7
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2
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eV (Ru 3p3/2) and 279.8 eV (Ru 3d5/2) are assigned to the metallic ruthenium (Fig. 4 and Fig. S16).
Moreover, the low-intensity XPS peak at 399.07 eV corresponding to N 1s for the XPS of Ru/L1
catalyst, manifested the presence of the ligand over the ruthenium nanoparticles (Fig. 4).
(a)
0
(b)
(c)
Ru 3p
3/2
(
461.7 eV)
C=C
284.6 eV)
(
RuO 3p
463.6 eV)
2
3/2
(
281.3 eV)
(
0
Ru 3d
(
RuO 3d
5/2
2
5/2
279.8 eV)
4
71 468 465 462 459 456
Binding energy (eV)
288 286 284 282 280 278
Binding energy (eV)
405
400
395
Binding energy (eV)
4
5
6
Fig. 4 XPS spectra corresponding to the (a) Ru 3p3/2 (b) Ru 3d5/2 and (c) N 1s core levels of
Ru/L1 catalyst.
7
8
9
0
1
2
Notably, XPS of the Ru nanoparticles obtained in the absence of ligand L1 inferred the
higher content of ruthenium oxide (Fig. S15), further suggesting that the presence of ligand L1
over the Ru nanoparticles prevented the facile oxidation of the surface in Ru/L1 catalyst. Hence,
the observed enhanced catalytic performance of the Ru/L1 catalyst can be attributed to the smaller
particle size ruthenium nanoparticles and the ligand L1.
1
1
1
1
3
Experimental
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1
1
1
1
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7
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9
Catalytic hydrogen production from methanol. Typically, an appropriate amount of [{(η6-
benzene)RuCl
2
}
2
] [Ru]-1 (0.05 mmol) (or other precursors) and ligand (0.1 mmol - 0.2 mmol) in
OH)/n(H O) = 1:0 to 1:2) was taken in a 5 mL test tube reaction
methanol-water solution (n(CH
3
2
vessel and was added an appropriate base (1.1 equiv. with respect to methanol). Then the reaction
vessel, equipped with a condenser (–10 °C) and water displacement setup, was de-aerated and
flushed with Ar. Further, the reaction mixture was stirred at a suitable temperature on an oil bath
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(Fig. S19). The amount of gas generated per unit time was quantified by the water displacement
method, and the composition of the released gas was confirmed by GC-TCD. The turnover number
(TON) was calculated by the formula [n(H )/n(catalyst)]. The turnover frequency (TOF) was
2
calculated as TON/time. After the catalytic reaction, the supported ruthenium nanoparticles were
collected by centrifugation and dried in a vacuum oven and weight to ~14 mg of catalysts (as
obtained from 25 mg of [Ru]-1 used in the catalytic reaction) which can be used for the further
catalytic cycles.
8
9
Catalytic hydrogen production from ethanol/n-propanol. Catalytic hydrogen production from
ethanol/n-propanol was performed following the procedure used for methanol, by using ethanol/n-
propanol (16.08 mmol), Ru/L1 catalyst (0.625 mol%, n([Ru]-1)/n(L1) = 1:2) and potassium
hydroxide (1.2 equiv.) in water (1 equiv.). Then the 5 mL test tube reaction vessel, connected
equipped with a condenser (-10 °C) and water displacement setup, was de-aerated and flushed with
Ar. Further, the reaction mixture was stirred at 110 °C on an oil bath. The amount of gas generated
per unit time was quantified by the water displacement method, and the composition of the released
gas was confirmed by GC-TCD. The turnover number (TON) was calculated by the formula
1
1
1
1
1
1
1
0
1
2
3
4
5
6
[n(H
Synthesis of ruthenium nanoparticles. Ru nanoparticles were synthesized by adding the
dropwise of an aqueous solution of NaBH (0.025 g, in 5 mL of water) in an aqueous solution of
RuCl .3H O (0.026 g, 0.1 mmol, in 5 mL of water) and PVP (0.05 g). The content of the flask was
2
)/n(catalyst)]. The turnover frequency (TOF) was calculated as TON/time.
1
1
1
2
2
7
8
9
0
1
4
3
2
sonicated for 10 min to obtain a black suspension of Ru nanoparticles, which were collected by
centrifugation and was washed with distilled water (10 mL x 02).
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0
1
Long term stability and recyclability experiments. Initially, the Ru/L1 catalyst (0.625 mol%,
n([Ru]-1)/n(L1) = 1:2) in methanol (16.08 mmol) and water (1 equiv.) was taken in a 5 mL test
tube reaction vessel, and was added KOH (1.2 equiv.). Then the reaction vessel, connected
equipped with a condenser (–10 °C) and water displacement setup, was de-aerated and flushed
with Ar. Further, the reaction mixture was stirred at 130 °C on an oil bath. The amount of gas
generated per unit time was quantified by the water displacement method. For the subsequent
catalytic run, the reaction mixture was centrifuged to separate the catalyst. Further, the catalyst
was transferred to the reaction vessel, and methanol (16.08 mmol), water (1 equiv.) and KOH (1.2
equiv.) were added to the reaction vessel, and the reaction mixture was stirred at 130 °C on an oil
bath under Argon atmosphere. The release of gas was monitored by water displacement process,
and the composition of the released gas was confirmed by GC-TCD.
1
1
1
1
2
3
Conclusion
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1
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1
1
1
2
2
2
4
5
6
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9
0
1
2
We developed an efficient new Ru/L1 catalyst comprises of ligand capped ruthenium
nanoparticles homogeneously dispersed over the carbon support, in-situ generated from the
ruthenium arene precursor and 2-hydroxypyridine (L1) ligand. The Ru/L1 catalytic system
displayed excellent catalytic activity for CO
of H per mol of methanol) at low temperature (110 – 130 °C). This process also generated formic
acid as a byproduct. The studied catalyst exhibited outstanding long-term stability to generate 186
L of H gas per gram of Ru. This catalytic system may encourage the search of an efficient
2
free hydrogen production from methanol (1.43 mol
2
2
industrially viable process for the low-temperature transformation of methanol to hydrogen gas
and formic acid.
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Acknowledgment
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4
The authors thank SERB, CSIR and IIT Indore for financial support. Instrumentation facility from
SIC (IIT Indore), MRC (MNIT Jaipur), ACMS (IIT Kanpur) and IKFT (KIT) is gratefully
acknowledged. M.K.A. thanks DST New Delhi for his INSPIRE senior research fellowship.
5
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Graphical Abstract
Efficient conversion of methanol to hydrogen gas and formate with appreciably high TOF and
TON achieved over in situ generated ruthenium catalyst in water at low temperature.
2
1