Journal of Catalysis 408 (2022) 316–328
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Facile MOF-derived one-pot synthetic approach toward Ru single atoms,
nanoclusters, and nanoparticles dispersed on CeO2 supports for
enhanced ammonia synthesis
Sanil E. Sivan a,1, Ki Hyuk Kang a,1, Seung Ju Han a, Odongo Francis Ngome Okello b, Si-Young Choi b,
Veeranmaril Sudheeshkumar c, Robert W.J. Scott c, Ho-Jeong Chae a, Sunyoung Park a,⇑, U-Hwang Lee a,⇑
a
Chemical & Process Technology Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
c
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon SK S7N 5C9, Canada
b
a r t i c l e
i n f o
Article history:
Received 6 February 2022
Revised 21 March 2022
Accepted 21 March 2022
Available online 24 March 2022
Keywords:
Metal–organic framework
Ruthenium
Cerium oxide
Nanocluster
Ammonia synthesis
a b s t r a c t
The Haber-Bosch process for NH3 production is one of the largest global energy consumers. Despite
improvements over the past century, developing catalysts with enhanced activities and durabilities
remains challenging. Ru-based catalysts supported on CeO2 are suitable alternatives under mild reaction
conditions. Here, we elucidated the effects of Ru structure on the physicochemical properties and catalytic activities of Ru-CeO2 systems. One-pot syntheses with metal–organic frameworks as sacrificial
templates were used to prepare Ru single-atom, nanocluster, and nanoparticle systems. The Ru nanoclusters and single atoms exhibited superior and more stable performances toward NH3 production, with no
hydrogen poisoning, in relation to the Ru nanoparticles. Kinetic data and electronic energy calculations
demonstrated that the Ru nanoclusters coexisting with Ru single atoms may maximize the NH3 synthetic
activity via the combination of dissociative and associative mechanisms. These results provide guidance
for the rational design and optimization of Ru-based catalysts for use in NH3 synthesis.
Ó 2022 Elsevier Inc. All rights reserved.
1. Introduction
NH3, which is one of the mostly widely produced chemicals
worldwide, is the backbone of the fertilizer industry and is thus
key in sustaining the growing population. Recently, NH3 attracted
increased attention as an ideal energy vector in the hydrogen economy and a futuristic carbon-free fuel [1–7]. Currently, NH3 is
mainly produced via the Haber-Bosch process, using Fe-based catalysts. This process is highly energy intensive owing to its harsh
operating conditions (400–500 °C, 200–300 bar) and integration
with steam reforming to yield H2. Thus, the Haber-Bosch process
accounts for 1–2% of global energy usage and emits 500 million
metric tons of CO2 annually [4–7]. With the increasing demand
for a carbon-free society, recent research focused on developing
catalysts that exhibit higher activities under milder conditions
(<400 °C, <200 bar). Considering that NH3 synthesis is exothermic,
in particular, the relaxation of the thermodynamic limitations by
⇑ Corresponding authors.
E-mail addresses: spark517@krict.re.kr (S. Park), uhwang@krict.re.kr (U-Hwang
Lee).
1
These authors contributed equally.
https://doi.org/10.1016/j.jcat.2022.03.019
0021-9517/Ó 2022 Elsevier Inc. All rights reserved.
decreasing the reaction temperature should enable the use of
lower pressures. This strategy may reduce energy costs and the
carbon footprints of processing facilities by easing reactor material
and compressor capacity requirements, thus enabling on-site NH3
production on a smaller scale.
In this regard, Ru-based catalysts evolved as promising alternatives to Fe-based catalysts owing to their high activities in NH3
synthesis, with a second-generation commercial process (Kellogg
advanced ammonia process) based on graphitized-carbonsupported Ru catalysts developed in 1992 [8–10]. Although Rubased catalysts exhibit enhanced performances under milder conditions, their widespread use is limited by shorter catalyst lifetimes, which are due to a lack of suitable support materials
[11,12]. To overcome this problem, various supports for Ru catalysts have been investigated, including MgO [13], Al2O3 [13,14],
carbon materials [15–17], lanthanide oxides [18–33], perovskites
[34], zeolites [35], boron nitrides [36], electrides [37–41], hydrides
[42–44], oxyhydrides [45,46], oxynitride hydrides [47], and amides
[48–50]. However, for use in industrial processes, metal oxides
remain the most practical support materials for Ru-based catalysts
[33].
Journal of Catalysis 408 (2022) 316–328
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
characterized to investigate the effect of the Ru structure on NH3
synthesis. This study elucidated the advantageous roles of Ru nanoclusters and yielded a structurally optimized Ru-CeO2 system for
use in NH3 synthesis.
Among the metal oxides, CeO2 enhances the activities and stabilities of Ru particles in NH3 synthesis, likely because its intrinsic
redox nature and lattice oxygen vacancies provide strong electrondonating properties [18–20]. Therefore, most studies regarding Ru/
CeO2 systems focused on improving the properties of CeO2 to maximize the metal-support interactions between Ru and CeO2, e.g.,
various synthetic strategies toward CeO2 supports were proposed
[21–27], and the effects of the morphology of CeO2 on the catalytic
performances of Ru particles were investigated [28–31]. Remarkably, interfacial Ru sites based on Ru–O–Ce bonds are effective in
N2 activation, which is the rate-determining step in NH3 synthesis
[18,33]. Moreover, CeO2 supports enable higher catalyst densities
than those observed on conventional carbon supports, which
favors commercial applications in small-scale reactors [18].
Despite research efforts regarding CeO2 modification, including
investigations into the effect of Ru density on the CeO2 surface
[51], the sizes of the Ru structures on the CeO2 surface are not
yet fully optimized NH3 synthesis on Ru-based catalysts displays
structural sensitivity, with B5-type sites comprising five Ru stepedge atoms mainly responsible for N2 activation [52]. To form such
ensemble sites, certain sizes and morphologies of Ru particles are
required, but these requirements depend on the specific support
material, e.g., optimized Ru structures upon BaCeO3 perovskite
and N-doped carbon supports were recently reported [53,54].
Hence, the Ru structures in Ru/CeO2 systems should be designed
considering the proportion of B5 sites and metal-support interactions. If the Ru particles are larger than the optimal size, most Ru
atoms are inactive without the synergistic effect of Ce species,
reducing the usage efficiency of the metal atoms [52]. Although
supported metal single atoms or ensembles were developed for
use in specific reactions, such as the hydrogenation or oxidation
of unsaturated hydrocarbons [55,56], they may exhibit potential
in NH3 synthesis. In general, decreasing metal particle sizes to
the sub-nanometer scale dramatically increases the ratio of
exposed metal atoms and the number of interfacial metal sites
available for metal-support interactions. Recently, several Ru catalysts atomically dispersed on zeolites were proposed for use in NH3
synthesis [35,57]. However, investigations into the effects of such
structures on CeO2 supports for use in NH3 synthesis are ongoing.
Therefore, a strategy for precisely controlling the Ru particle size
from the atomic to the nanoscale level is essential to optimize
Ru/CeO2 systems for use in NH3 synthesis.
Conventional methods of synthesizing Ru/CeO2 systems often
lead to metal particles with a broad size distribution. The scalable,
precise, and reproducible preparations of supported single-atom
and sub-nanocluster catalysts are synthetically challenging. Studies regarding the syntheses of atomically dispersed CeO2supported Ru catalysts, in particular, are limited [58,59]. Moreover,
syntheses involving the impregnation of Ru species on preformed
CeO2 supports show limited success in establishing metalsupport interactions via interfacial Ru–O–Ce bonds, which necessitates the development of novel synthetic protocols. Metal-organic
frameworks (MOFs), which comprise framework structures of
metal ions or clusters linked by multidentate organic ligands, constitute an advanced class of porous hybrid materials. From a catalytic perspective, although the framework structures of
numerous MOFs cannot withstand the harsh reaction conditions
required in most industrial catalytic processes, they may act as
effective sacrificial templates in designing advanced catalytic
materials [60].
In this study, we prepared Ru-CeO2 catalysts for use in NH3 synthesis via a MOF-derived one-pot synthetic method. The sizes of
the Ru particles dispersed on the CeO2 surface were controlled,
with single-atoms, nanoclusters, and nanoparticles produced using
a newly developed Ru-Ce-MOF as a sacrificial template (Fig. 1). The
physicochemical properties of the catalysts were systematically
2. Experimental
2.1. Chemicals
Cerium(III) nitrate hexahydrate (99%), ruthenium(III) acetylacetonate (97%), 1,3,5-benzenetricarboxylic acid (BTC; 95%), and CeO2
powder (99%) were purchased from Sigma-Aldrich (St. Louis, MO,
USA), and N,N-dimethylformamide (DMF; 95%) and ethanol (99%)
were purchased from Samchun Chemical (Seoul, Republic of
Korea). All chemicals were used without further purification.
2.2. Catalyst preparation
Cerium(III) nitrate hexahydrate, ruthenium(III) acetylacetonate,
and BTC (6 mmol) were added to DMF (50 mL). The resultant mixture was stirred at 130 °C for 20 h in an autoclave. After cooling the
autoclave, the separated solid product was washed with DMF and
ethanol and dried for 10 h at 80 °C. The Ru/Ce molar ratio used in
MOF synthesis was adjusted while maintaining a total metal content (Ru + Ce) of 10 mmol. To remove the MOF template completely, the resultant Ru-Ce-BTC MOF powder was calcined at
450 °C for 3 h under air, thereby producing a RuOx-CeO2 composite.
To prepare the Ru-CeO2 catalyst, RuOx-CeO2 was reduced at 450 °C
for 3 h under 10% H2/N2 gas. The Ru contents of the Ru-CeO2 catalysts were 1.4, 2.8, 4.0, and 5.3 wt%, as determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES).
For comparison, the Ce-BTC MOF (Ce(BTC)(H2O) (DMF)1.1) was synthesized using the same procedure without the addition of the Ru
precursor. Subsequent calcination at 450 °C for 3 h yielded CeO2.
For the impregnated Ru/CeO2 catalyst, an ethanolic solution
containing an adequate amount of ruthenium(III) acetylacetonate
was added dropwise to the CeO2 support dispersed in ethanol.
After solvent evaporation, the obtained powder was completely
dried at 100 °C, calcined at 450 °C for 3 h under air, and, finally,
reduced at 450 °C for 3 h under 10% H2/N2 gas.
To minimize air exposure during all ex situ analyses, the prepared samples were stored in a glove box and then transferred to
Ar-filled vials.
2.3. Catalyst characterization
The as-synthesized Ru-Ce-BTC MOFs and the corresponding calcined and reduced materials were characterized using physicochemical analyses. The metal contents of the materials were
analyzed using ICP-AES (iCap 6500 Duo, Thermo Fisher Scientific,
Waltham, MA, USA).
X-ray diffraction (XRD) was conducted using a diffractometer
(D/Max IIIB, Rigaku, Tokyo, Japan) with Ni-filtered CuKa radiation.
The powder XRD patterns of the raw MOF samples were analyzed
in continuous scan mode, whereas step-scan mode (step size:
0.05°) was employed for the calcined and reduced materials. The
unit cell parameters were calculated and refined using the Le Bail
method with the GSAS software (Argonne National Laboratory,
Lemont, IL, USA).
Scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX) were performed at an acceleration voltage
of 10 kV (VEGA-II LSU, Tescan, Brno, Czech Republic).
Thermogravimetric (TG) analysis was conducted under air flow
(30 mL min1) using a thermal analyzer (TGA N-1000, Scinco,
317
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Journal of Catalysis 408 (2022) 316–328
Fig. 1. Synthesis of Ru-CeO2 catalysts. Ru-Ce-BTC, RuOx-CeO2, and Ru-CeO2 represent the raw, calcined, and reduced forms, respectively. Ru-Ce-BTC MOFs were prepared
using a MOF-derived one-pot synthetic method, and the MOF templates were completely removed via calcination to yield RuOx-CeO2 samples, which were subsequently
reduced to produce Ru-CeO2 catalysts.
Seoul, Republic of Korea) at temperatures of 700 °C at a heating
rate of 10 °C min1.
N2 sorption was performed using a sorption analyzer (TriStar
3000, Micromeritics, Norcross, GA, USA). Prior to analysis, each
sample was degassed by heating at 200 °C. The BrunauerEmmett-Teller (BET) surface area was determined using the standard multipoint analysis method (P/P0 = 0.05–0.3), and the pore
volume was evaluated using the single-point method at P/
P0 = 0.995. The pore size distribution was obtained using the
Barrett-Joyner-Halenda method.
H2 temperature-programmed reduction (TPR) studies were conducted using an AutoChem II 2920 adsorption instrument
(Micromeritics). After pretreatment at 200 °C for 2 h under Ar
gas, the sample was heated from 30 to 800 °C (10 °C min1) under
5% H2/Ar flow (30 mL min1).
X-ray absorption spectroscopic data for Ru K-edge X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) analyses were obtained in transmission mode using the HXMA beamline 061D-1 (resolution:
1 104 DE/E, energy range: 5–30 keV) at the Canadian Light
Source (Saskatoon, Canada). The ring current and storage ring electron energy were 250 mA and 2.9 GeV, respectively. A Si(1 1 1) double crystal monochromator was used to select the energy for the Ru
K-edge (22 117 eV), and the higher harmonic was eliminated by
detuning the double crystal. To fit the EXAFS results, the amplitude
reduction factor was fixed at 0.70, based on the Ru foil fitting
results. To extract the fitting parameters, data points in the ranges
of 2.5–11.5 (k-space) and 1.3–3.2 Å were used.
High-angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) and EDX were performed using a
transmission electron microscope (JEM-ARM200F, JEOL, Tokyo,
Japan) with an ASCOR corrector for fifth-order aberrations
(CEOS, Heidelberg, Germany). The convergence angle and probe
diameter of the beam were 27 mrad and 0.7 Å, respectively, under an acceleration voltage of 200 kV. The catalyst
was dispersed in ethanol and transferred to a Quantifoil holey
carbon 200 mesh TEM Au grid. The average particle size was
determined by measuring 500 particles in the TEM image
of each sample.
CO chemisorption studies were performed using an ASAP 2020
Chemi analyzer (Micromeritics). The sample was reduced at 450 °C
for 3 h under 5% H2/Ar flow (30 mL min1) and then maintained at
the same temperature without H2 flow for 1 h prior to cooling to
50 °C. CO molecules were adsorbed by quantitative volumetric
chemisorption.
In situ diffuse reflectance infrared Fourier transform (DRIFT)
measurements upon CO adsorption were performed using a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific) with a PIKE
sample chamber. The catalyst powder was finely ground with
KBr (KBr/catalyst = 7:1, w/w) and placed inside the sample cup
of a DRIFT cell. The catalyst in the DRIFT cell was then reduced at
450 °C for 3 h under 5% H2/Ar flow (30 mL min1), maintained at
the same temperature without H2 flow for 1 h, and cooled to
50 °C. Finally, 2% CO/Ar gas was introduced for 10 min, and the
DRIFT spectrum was recorded under vacuum.
X-ray photon spectroscopy (XPS) was conducted using a Kratos
AXIS NOVA X-ray photoelectron spectrometer (AlKa, 15 kV, 10 mA,
Shimadzu, Kyoto, Japan). Each XPS spectrum was calibrated using
the C 1s signal at 284.5 eV.
Raman spectroscopy was performed using a Raman spectrometer (RAMANforce, Nanophoton, Osaka, Japan) with a 532.04 nm
laser as the excitation source.
318
Journal of Catalysis 408 (2022) 316–328
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
(N2, H2, Ar) were as follows: (N2, H2, Ar) = (6, 30, 24), (10, 30, 20),
(15, 30, 15), and (20, 30, 10) for N2 order (a); (N2, H2, Ar) = (10,
20, 30), (10, 25, 25), (10, 30, 20), and (10, 40, 10) for H2 order (b);
and (N2, H2, Ar) = (10, 30, 0), (15, 45, 0), and (30, 90, 0) for NH3 order
(c).
NH3 decomposition was studied in a fixed-bed quartz reactor
loaded with 0.1 g of catalyst. The calcined catalyst was reduced
at 450 °C under 10% H2/N2 flow (20 mL min1). After cooling to
300 °C, the gas feed was changed to NH3 (100 vol%, 10 mL min1)
1
at a WHSV of 6000 mL g1
. The reaction was conducted over
cat h
the temperature range of 300–450 °C at 1 bar. During the reaction,
the gaseous products were analyzed using a gas chromatograph
(YL6500, YL Instrument, Anyang, Republic of Korea) with a thermal
conductivity detector and Molesieve 13X and Porapak N columns.
Temperature-programmed desorption mass spectrometry
(TPD-MS), with N2 and H2 as reactants in NH3 synthesis, was conducted using an AutoChem II system (Micrometrics). The calcined
sample was heated to 450 °C in a U-shaped quartz cell under He
flow (30 mL min1). The sample was then reduced at 450 °C for
3 h under 10% H2/Ar flow (30 mL min1) and purged at 450 °C
for 1 h under He flow to desorb H atoms on the catalyst surface.
After cooling to 50 °C under He flow, the reduced catalyst was treated with 10% N2/Ar or 10% H2/Ar to saturate the catalytic surface.
Finally, the TPD-MS spectrum was collected by increasing the temperature from 50 to 700 °C at 10 °C min1. Relevant MS signals
were recorded using a gas analyzer (OmniStar GSD 320, Pfeiffer
Vacuum, Aßlar, Germany).
2.4. NH3 synthesis and decomposition
2.5. Computational details
NH3 synthesis was conducted using a fixed-bed Inconel reactor
loaded with 0.05–0.1 g of catalyst. Prior to the reaction, the calcined catalyst was reduced at 450 °C under 75% H2/N2 gas. For
the NH3 syntheses, a stoichiometric feed gas flow (N2/H2 = 1:3,
99.9999 vol%) was introduced at a weight hourly space velocity
1
(WHSV) of 2600–72 000 mL g1
, and the pressure was concat h
trolled using a back-pressure regulator. The reaction was conducted over temperature and pressure ranges of 300–450 °C and
1–80 bar, respectively. Produced NH3 was trapped in a dilute aqueous H2SO4 solution, which was then titrated against KOH (0.03 M),
using phenolphthalein as an indicator, to determine the NH3 mole
fraction in the outlet gas. In addition, the proton conductivity of
this H2SO4 solution was recorded using a conductivity benchtop
meter (Orion Star A212, Thermo Fisher Scientific). To calculate
the kinetic data under steady-state conditions, average reaction
rates, which were well-separated from the equilibrium limits, were
determined. The rate of NH3 synthesis in a plug-flow differential
reactor is calculated using the following equation [61]:
r NH3 ¼
yNH3
V0
1
1 þ yNH3 W cat 22:08
Density functional theory calculations were conducted using
the Vienna ab initio simulation package (University of Vienna,
Vienna, Austria). The generalized gradient approximation-based
van der Waals-Bayesian error estimation exchange–correlation
functional was used [63]. The geometric steps and convergence criteria of the electronic steps were 0.01 eV Å1 and 1 104 eV,
respectively. The bulk structures of Ru (hexagonal close-packed
(hcp)) and CeO2 were relaxed using an 8 8 8 MonkhorstPack grid. The periodic unit cells of Ru (0 0 1) with 64 atomic units
(Ru64) and CeO2 (1 1 1) with 48 formula units (Ce48O96) were optimized using 4 4 1 gamma-centered grids. During surface optimization, the two bottom layers of Ru (0 0 1) and the bottom layer
of CeO2 (1 1 1) were fixed, and the vacuum height was set to 10 Å.
The Ru single-atom structure (Ru1-CeO2) was simulated by substituting a Ce atom in the optimized CeO2 (1 1 1) surface with a Ru
atom. To investigate the effect of Ru clusters, we also simulated
10 (Ru10-CeO2 (1 1 1)) and 22 Ru atoms (Ru22-CeO2 (1 1 1)) with
hcp structures on CeO2 (1 1 1) surfaces. For surface relaxation, the
bottom layer of CeO2 (1 1 1) was fixed, and the vacuum height
was set to 10 Å. The formation (DEint) and free formation (DGint)
energies are calculated using the following equations:
ð1Þ
1
where rNH3 is the average reaction rate (molNH3 g–1
), yNH3 is the
cat h
mole fraction of NH3 at the outlet, V0 is the total flow rate of the
feed gas (L h1, standard temperature and pressure (STP)), Wcat is
the mass of the catalyst (gcat), and 22.08 is the molar volume of
the feed gas (L mol1, STP). The NH3 yield is calculated using the following equation [49]:
Y NH3 ¼
F NH3;outlet
F NH3;theoretical
100
DEint
NxHy ¼ E nN;x ; nH;y Eslab nN EN2 =2 nH EH2 =2
int
DGint
NxHy ¼ DENxHy þ DZPE T DS
ð9Þ
ð10Þ
where E nN;x ; nH;y is the electronic energy of the slab with the
adsorbate and ESlab is the energy of the slab without the adsorbate.
To describe the reaction energies accurately, the electronic energy
of gaseous N2 is corrected based on a previous study [64]. The binding energy of the adsorbate (DGB:E: ) is calculated using the following
equation:
ð2Þ
where YNH3 is the NH3 yield (%), FNH3,outlet is the molar flow rate
(molNH3 h1) of NH3 derived from rNH3, and FNH3,theoretical is the theoretical molar flow rate (molNH3 h1) at complete conversion. The
reaction orders are calculated using the following equations [62]:
DGB:E: ¼ DGadsorbate DGgas
ð11Þ
a
r NH3 ¼ k1 PN2 PbH2 PcNH3
ð3Þ
m ln yNH3 ¼ ln r 0 ln V 0
ð4Þ
c¼1m
ð5Þ
where DGadsorbate and DGgas are the free formation energies of the
adsorbate and gas, respectively. The free energy diagram, along
with the reaction coordinate, was obtained using a normal-mode
analysis of gaseous species under the ideal-gas assumption and
adsorbed species under the harmonic limit [65].
r 0 ¼ k2 PaN2 PbH2
ð6Þ
3. Results and discussion
ð7Þ
3.1. Catalyst characterization
ð8Þ
The raw, calcined, and reduced systems prepared in this study
are denoted XRu-Ce-BTC, XRuOx-CeO2, and XRu-CeO2, respectively,
where X is the Ru content in the Ru-CeO2 catalyst (1.4, 2.8, 4.0, or
5.3 wt%), as determined using ICP-AES (Table 1). The formation of
the Ru-Ce-BTC MOFs is confirmed using powder XRD (Fig. 2a). The
0
ln r0 ¼ ln k2 þ a ln PN2
00
ln r0 ¼ ln k2 þ b ln PH2
where k is the rate constant, P is the partial pressure, r’ is the reaction rate, and a, b, and c are the reaction orders of N2, H2, and NH3,
respectively. The constituent gas flows (mL min1) of the reactants
319
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Journal of Catalysis 408 (2022) 316–328
XRD patterns of the synthesized MOFs with different Ce3+/Ru3+
ratios are similar to that of monometallic tetragonal Ce-BTC with
the general formula Ce(BTC)(H2O)(DMF)1.1 (CCDC ID: 773084),
indicating the formation of the Ce-BTC phase [66]. However, the
XRD peaks of the Ru-containing MOFs are shifted toward higher
2h values owing to the difference in ionic radii between Ru3+
(0.68 Å) and Ce3+ (1.02 Å), indicating that the Ce3+ framework
nodes are partially, isomorphously substituted with Ru3+ ions to
form Ru-Ce-BTC MOFs. For 4.0Ru-Ce-BTC and 5.3Ru-Ce-BTC, the
intensities of the Ce-BTC (1 0 1) peaks decrease significantly,
whereas the full width at half maximum (fwhm) values increase,
suggesting that the crystal growth of Ce-BTC is hindered by the
high Ru contents.
SEM combined with EDX mapping reveal that Ru is uniformly
distributed throughout the nanorod-shaped MOF crystals
(Fig. S1), particularly in 1.4Ru-Ce-BTC and 2.8Ru-Ce-BTC, indicating the formation of mixed-metal MOFs rather than physical mixtures. However, as several Ru-rich domains are observed at high Ru
contents, the formation of other composite structures is possible.
Fig. 2b shows the XRD patterns of CeO2 and the RuOx-CeO2 samples that were calcined at 450 °C under air. According to the TG
analysis, the organic moieties of the MOF template are completely
decomposed at 320 °C (Fig. S2). Therefore, no XRD peaks corresponding to the BTC MOFs are observed for any of the samples.
The XRD patterns of 1.4RuOx-CeO2 and 2.8RuOx-CeO2 are consistent with the cubic fluorite-type structure of CeO2, without peaks
representing RuO2 or metallic Ru, and the CeO2 (1 1 1) peaks are
slightly shifted toward higher 2h angles (Fig. S3a). Moreover, the
lattice parameters of pure CeO2, 1.4RuOx-CeO2, and 2.8RuOx-CeO2
are 5.410108, 5.407345, and 5.405836 Å, respectively. These values
decrease very slightly with increasing Ru doping of the CeO2 lattice, likely due to the contraction of the CeO2 lattice caused by
the smaller ionic radius of Ru4+ (0.62 Å) than that of Ce4+ (0.97
Å). These results suggest the formation of a solid solution phase
(Ce1xRuxO2) with limited Ru content [67-70]. With higher Ru contents, the XRD patterns of 4.0RuOx-CeO2 and 5.3RuOx-CeO2 each
exhibit a RuO2 (1 0 1) peak at 2h 35° owing to phase separation
during calcination.
SEM-EDX mapping further confirms the presence of RuO2 particles in 4.0RuOx-CeO2 and 5.3RuOx-CeO2, as domains with high Ru
concentrations are observed (Fig. S4). In contrast, 1.4RuOx-CeO2
and 2.8RuOx-CeO2 exhibit highly homogeneous distributions of
Ru species without phase separation, as shown using XRD
(Fig. S3a). The solubility of Ru ions in the CeO2 lattice is reportedly
quite limited [67,68], with a maximum Ru content (x) of 0.12 in
Ce1xRuxO2. Therefore, 1.4RuOx-CeO2 and 2.8RuOx-CeO2 consist of
pure solid solutions (Ce0.97Ru0.03O2 and Ce0.94Ru0.06O2, respectively), whereas 4.0RuOx-CeO2 and 5.3RuOx-CeO2 contain Ce1xRuxO2 (0.06 < x < 0.11) phases and separate RuO2 particles.
The textural properties of the RuOx-CeO2 samples were examined using N2 adsorption–desorption (Table 1). Each sample exhibits a type IV adsorption isotherm with an H3-type hysteresis
loop corresponding to a mesoporous structure (Fig. S5a) [71]. The
BET surface areas of 1.4RuOx-CeO2, 2.8RuOx-CeO2, 4.0RuOx-CeO2,
and 5.3RuOx-CeO2 are 92, 103, 79, and 64 m2 g1, respectively
(Table 1). The pore size distributions of all samples exhibit sharp
peaks between 36 and 38 Å (Fig. S5b), with 1.4RuOx-CeO2 also
exhibiting a broad peak at 40–90 Å. The higher surface areas of
1.4RuOx-CeO2 and 2.8RuOx-CeO2 indicate that the Ru species are
finely dispersed, without causing pore blockages and thus restricting reactant access. In contrast, the lower surface areas of 4.0RuOxCeO2 and 5.3RuOx-CeO2 are likely caused by the presence of RuOx
particles on the Ce1xRuxO2 framework, as indicated by XRD
(Fig. 2b) and SEM-EDX (Fig. S4).
Before preparing the Ru-CeO2 catalysts, the reduction behaviors
of CeO2 and the RuOx-CeO2 samples were investigated via TPR
studies (Fig. S6). The profile of CeO2 exhibits a broad peak at
310–500 °C, which is due to the reduction of surface oxygen atoms
bound to Ce4+ species. The reduction peak of ceria surface oxygen
shifts to lower temperatures with increasing Ru content, indicating
that the addition of Ru metal may enhance the reducibility of ceria
via the hydrogen spillover effect [21]. Notably, the profile of
1.4RuOx-CeO2 exhibits a small, broad peak at 136 °C, suggesting
that Ru atoms are finely doped at the single-atom level in the
CeO2 framework without the formation of RuO2 particles. Further,
the weak peak intensity for 1.4RuOx-CeO2 is likely due to the low
Ru content. In contrast, the profiles of the other RuOx-CeO2 samples exhibit sharp peaks representing the reduction of Ru species,
with the maximum hydrogen uptake observed at 110 °C. The
peak representing Ru reduction shifts from 103 to 110 °C as the
Ru content increases, indicating that the reducibilities of the Ru
atoms in the Ce1xRuxO2 solid solutions are higher than those of
the RuO2 particles, likely because the Ru–O–Ce bonds in Ce1xRuxO2 increase the electron density of Ru via strong metal-support
interactions.
Fig. 2c shows the XRD patterns of the Ru-CeO2 catalysts after
reduction at 450 °C. The patterns of 1.4Ru-CeO2 and 2.8Ru-CeO2
only exhibit reflections corresponding to the CeO2 phase, without
peaks related to the Ru metallic phase, indicating that the reduced
Ru species are finely dispersed, with sizes smaller than the detection limit of XRD. In contrast, the patterns of 4.0Ru-CeO2 and
5.3Ru-CeO2 exhibit small Ru (1 0 1) peaks at 2h = 44° corresponding
to metallic Ru particles, which likely originate from the reduction
of RuO2 species (Fig. S3b).
The chemical states and coordination environments of the RuCeO2 catalysts were investigated using XANES and EXAFS spectroscopy. As shown in Fig. 2d, the Ru K-edge XANES spectrum of
1.4Ru-CeO2 exhibits a higher energy absorption edge and stronger
white line intensity, similar to those of the RuO2 reference spectrum, indicating the absence of metallic Ru particles. However, as
the Ru content increases, the XANES spectra of the Ru-CeO2 catalysts exhibit features similar to those of the Ru foil reference spectrum. Fig. 2e shows the Ru K-edge EXAFS spectra of the catalysts in
phase-corrected R-space. The fitted EXAFS spectra and corresponding fitting parameters are shown in Fig. S7 and Table 2, respectively. EXAFS fittings were conducted using only first shell
models of Ru–O and Ru–Ru contributions; thus, the data are only
fitted at distances of up to 3 Å in R-space. Given the lack of overlap between the first Ru–O and Ru–Ru coordination shells and
upper shell data in R-space, the EXAFS parameters extracted from
the first shell fitting are reliable (R factors of 0.02). As the XANES
spectrum of 1.4Ru-CeO2 is similar to that of RuO2, a Ru–O path is
created to fit the data. The peak at 1.99 Å is attributed to the scattering of the first shell of the Ru and O atoms. The estimated coordination number (CN) of Ru–O of 3.6 suggests that four O atoms
are coordinated to each isolated Ru atom. No first-shell Ru–Ru contribution or higher shells are observed at longer distances, indicating that Ru exists as single-atom species, even after reduction. The
lack of higher shell scattering also suggests that Ru atoms are not
located at crystallographically well-defined sites in the CeO2 lattice. At higher Ru contents, Ru–Ru contributions are observed in
the EXAFS spectra, with an hcp Ru model used to fit the Ru–Ru contributions. Unlike that of 1.4Ru-CeO2, the EXAFS spectra of the
other catalysts exhibit two peaks at 1.99 and 2.71 Å owing to the
first shell contributions of Ru–O and Ru–Ru bonds [35]. For
2.8Ru-CeO2, the average CNs of the Ru–O and Ru–Ru contributions
are 3.7 and 1.8, respectively. The lower contribution from Ru–Ru
bonds than that from Ru–O bonds implies the formation of Ru nanoclusters in 2.8Ru-CeO2. Larger average CNs are observed for
4.0Ru-CeO2 and 5.3Ru-CeO2, with CNs of the Ru–Ru contributions
of 3.8 and 5.7, respectively. The EXAFS fitting results suggest that
the Ru content in the Ru-CeO2 catalyst is crucial in controlling
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S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Table 1
Physicochemical properties of the Ru/CeO2 and Ru-CeO2 catalysts.
Catalyst
Ru/CeO2
1.4Ru-CeO2
2.8Ru-CeO2
4.0Ru-CeO2
5.3Ru-CeO2
Ru content
(wt%)a
2.1
1.4
2.8
4.0
5.3
Surface area
(m2 g1)b
12
92
103
79
64
Pore volume
(cm3 g1)c
0.022
0.207
0.222
0.136
0.113
Ru dispersion
(%)d
36.3
149.1
94.3
60.5
35.8
Average metal particle size
(nm)
Intensity ratio
CO chemisorptiond
TEMe
I(D)/I(F2g)f
I(Ru–O–Ce)/I(F2g)g,h
3.6
0.9
1.2
2.0
3.3
3.8
–
1.0
2.7
3.6
–
0.38
0.49
0.45
0.35
–
0.53 (0.58)
0.77 (0.76)
0.75 (0.88)
0.49 (0.95)
a
Determined before reduction through ICP-AES. b Calculated before reduction using the BET equation (Fig. S5). c Total pore volume before reduction at P/P0 = 0.99 (Fig. S5). d
Calculated using CO chemisorption measurements by assuming a stoichiometry of CO/Ruatom = 1. e Determined using STEM (Fig. 3e, S10, and S11). f Calculated using Raman
spectroscopy (Fig. 2f). g I(Ru–O–Ce) = I(Ru–O–Ce at 688.8 cm1) + I(Ru–O–Ce at 969.0 cm1). h The numbers in parentheses are the ratios of the calcined RuOx-CeO2 samples
(Fig. S16).
Fig. 2. XRD patterns of (a) Ru-Ce-BTC MOFs, (b) RuOx-CeO2 samples, and (c) Ru-CeO2 catalysts. (d) XANES profiles, (e) EXAFS and (f) Raman spectra, and (g) low-magnification
HAADF-STEM images of the Ru-CeO2 catalysts. The white circles in (g) indicate the locations of Ru nanoparticles.
nanorods (Fig. 2g). Notably, no Ru particles are observed in
1.4Ru-CeO2 and 2.8Ru-CeO2, whereas 4.0Ru-CeO2 and 5.3Ru-CeO2
contain Ru nanoparticles dispersed on the CeO2 nanorods. According to low-magnification STEM-EDX (Figs. S8 and S9), Ru species
are uniformly dispersed throughout CeO2 in 1.4Ru-CeO2 and
2.8Ru-CeO2, and almost identical Ru contents are obtained using
the Ru structures, while catalysts with low metal loadings are
highly desirable in designing single-atom and nanocluster systems.
The sizes and distributions of the Ru species in the Ru-CeO2 catalysts were investigated using Cs-corrected HAADF-STEM coupled
with EDX mapping. The low-magnification HAADF-STEM images
show that all Ru-CeO2 catalysts form CeO2-based polycrystalline
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S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Journal of Catalysis 408 (2022) 316–328
Table 2
EXAFS curve-fitting results of the Ru-CeO2 catalysts.
Catalyst
Ru–Oa
R (Å)
3.6
3.7
4.6
4.2
1.99
1.99
1.99
1.96
CN
1.4Ru-CeO2
2.8Ru-CeO2
4.0Ru-CeO2
5.3Ru-CeO2
a
Ru–Rua
b
c
R factor
r (Å )
E0 shift (eV)
CN
R (Å)
r (Å )
E0 shift (eV)
0.001
0.003
0.006
0.007
4.7
5.7
1.3
0.3
–
1.8
3.8
5.7
–
2.71
2.69
2.69
–
0.004
0.004
0.004
–
5.7
1.3
0.3
2
2 d
b
c
2
2 d
0.016
0.010
0.018
0.020
Determined via EXAFS curve fitting (Fig. S7). b Coordination number. c Radial distance from the absorber. d Debye-Waller factor.
assigned to the interactions of CO with oxidized Ru species [77].
The band at 1981 cm1 is assigned to bridged monocarbonyl species adsorbed on Ru at the Ru–CeO2 interface [74,75]. For Rudoped CeO2 (Ce0.875Ru0.125O2) materials, wherein Ru is located
within the CeO2 lattice, CO that is linearly bonded on the Ru top
site displays a vibrational frequency of 1972 cm1 [74]. Additionally, geminal CO adsorption on Ru cations located within a ZrO2 lattice (solid solution phase) is energetically unfavorable, and Ru
cations should be located on the ZrO2 surface after catalyst reduction for geminal CO adsorption [73]. Therefore, the monocarbonyl
(1981 cm1) and geminal dicarbonyl (2065 cm1) peaks indicate
that the lattice Ru atoms migrate toward the CeO2 surface during
reduction and form Ru single atoms with cationic coordination in
1.4Ru-CeO2, which confirms the EXAFS results (Fig. 2e). The geminal dicarbonyl band at 2065 cm1 (fwhm 30 cm1) gradually
shifts to lower frequencies and broadens as the Ru content
increases, suggesting the presence of nonuniform Ru sites. The
deconvolution of the spectrum of 5.3Ru-CeO2 (Fig. S14) reveals
new bands at approximately 2053 and 2025 cm1, which are
ascribed to CO that is linearly bonded to the metallic Ru species
[75]. The Ru 3p XPS spectra (Fig. S15) further indicate that the ratio
of metallic Ru particles increases as the Ru content in the Ru-CeO2
catalyst increases.
Raman spectroscopy was performed to investigate the interfacial structures of the Ru-CeO2 catalysts (Fig. 2f). The spectrum of
the CeO2 support exhibits bands corresponding to octahedral local
symmetry (F2g) and oxygen vacancies (D) at 461 and 592 cm1,
respectively [31]. The F2g bands of all Ru-CeO2 catalysts are broadened and red-shifted owing to the addition of Ru species. In addition, Raman bands representing interfacial Ru–O–Ce bonds are
observed at 689 and 969 cm1. I(D)/I(F2g) and I(Ru–O–Ce)/I(F2g)
ratios were calculated based on the peak intensities to compare
the oxygen vacancies and interfacial Ru–O–Ce bonds, respectively
(Table 1). Both ratios decrease in the following order: 2.8RuCeO2 > 4.0Ru-CeO2 > 1.4Ru-CeO2 > 5.3Ru-CeO2. Remarkably, the I
(Ru–O–Ce)/I(F2g) ratios of 1.4Ru-CeO2 and 2.8Ru-CeO2 are similar
to those of the corresponding RuOx-CeO2 samples owing to the
highly dispersed states of the Ru species (Fig. S16). Among the catalysts, 2.8Ru-CeO2 with Ru nanoclusters exhibits the highest density of interfacial Ru–O–Ce bonds, suggesting the presence of
strong metal-support interactions.
TPD-MS was conducted to investigate the dissociative adsorption behaviors of N2 and H2 (Fig. 4b and 4c). For comparison,
deconvoluted TPD peaks in the ranges of 100–250, 250–450,
and >450 °C are characterized as low- (L-T), medium- (M-T), and
high-temperature (H-T) peaks, respectively (Table S1). The N2TPD-MS spectra (Fig. 4b) reveal no desorption peaks for the CeO2
support, indicating that CeO2 is not involved in the dissociative
desorption of N2. Among the catalysts, 1.4Ru-CeO2 exhibits the
highest L-T peak ratio, which suggests that the stable adsorption
of N2 molecules on Ru single-atom sites is unfavorable, likely
because the dissociative adsorption of diatomic molecules stoichiometrically requires metal ensembles containing >1 atom
[56]. The temperatures and areas of the N2 desorption peaks of
ICP-AES and EDX mapping. For 4.0Ru-CeO2 and 5.3Ru-CeO2
(Figs. S10 and S11), Ru nanoparticles with average sizes of 2.7
and 3.6 nm, respectively, are clearly observed. However, as the
average size of the Ru nanoparticles increases, the difference in
the Ru contents obtained using ICP-AES and EDX mapping
increases owing to the nonhomogeneous dispersion of Ru. These
results are consistent with those of XRD and EXAFS.
To observe the Ru single-atom and nanocluster structures in
2.8Ru-CeO2 in detail, high-magnification HAADF-STEM was performed (Fig. 3). No Ru ensembles are observed, apart from defective dark contrast spot-like features in the HAADF-STEM images,
with only CeO2 lattice fringes observed (Fig. 3a and 3b). This is
because there is little contrast between Ru (Z = 44) and Ce
(Z = 58) species, which significantly limits the visualization of Ru
atoms on the CeO2 surface [72]. In addition, the selected area
diffraction pattern is indexed to the fluorite CeO2 structure
(Fig. 3c), which is consistent with the XRD results (Fig. 2c). However, a comparison of the STEM-EDX images of calcined RuOxCeO2 samples (Fig. 3d) and reduced Ru-CeO2 catalysts (Fig. 3e)
reveals the formation of Ru ensembles. Although no RuO2 particles
are observed in the EDX images of 2.8RuOx-CeO2 owing to its solid
solution structure (Fig. 3d), Ru sub-nanoclusters with an average
size of <1 nm are observed on the CeO2 surface in 2.8Ru-CeO2
(Fig. 3e). Therefore, Ru atoms doped into the CeO2 lattice migrate
toward the CeO2 surface during reduction, forming Ru nanoclusters. Moreover, considering the local density of Ru nanoclusters
on the CeO2 surface, Ru single atoms and nanoclusters likely coexist. Although several Ce-deficient defects are observed on the CeO2
surface, likely due to dislocations (Fig. S12), they do not correspond
to the locations of the Ru nanoclusters.
The observations of the Ru single-atom and nanocluster systems are further supported by the Ru dispersion values calculated
based on the CO chemisorption analysis (Table 1 and Fig. S13).
1.4Ru-CeO2, 2.8Ru-CeO2, 4.0Ru-CeO2, and 5.3Ru-CeO2 exhibit Ru
dispersion values of 149.1%, 94.3%, 60.5%, and 35.8%, respectively.
The Ru dispersion value of 1.4Ru-CeO2 is >100% because Ru single
atoms may exist as cations, with geminal CO adsorption (CO/
Ru = 2) predominant over linear CO adsorption (CO/Ru = 1)
[73,74]. For 4.0Ru-CeO2 and 5.3Ru-CeO2, the average Ru particle
sizes calculated based on the CO chemisorption analysis are
slightly smaller than those observed in the TEM images. This is
because the CO adsorption results include contributions from Ru
single atoms, which are difficult to observe via TEM. Considering
that the average Ru particle size of 5.3Ru-CeO2 is similar to the
average pore size (Fig. S5b), most of the Ru nanoparticles are likely
present on the surface of the CeO2 support.
The adsorption properties of the Ru sites in the Ru-CeO2 catalysts were investigated using in situ DRIFT measurements upon
CO adsorption (Fig. 4a). The DRIFT spectrum of 1.4Ru-CeO2 exhibits
three bands at 2120, 2065, and 1981 cm1, with the highfrequency bands at 2120 and 2065 cm1 attributed to weakly
adsorbed tricarbonyl and strongly adsorbed geminal dicarbonyl
species at Run+ sites, respectively [73–77]. Similar band patterns
observed for Ru/CeO2 [75], Ru/MgO [76], and Ru/SiO2 systems are
322
Journal of Catalysis 408 (2022) 316–328
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Fig. 3. (a) Low- and (b) high-magnification HAADF-STEM images and the (c) corresponding fast Fourier transform pattern of 2.8Ru-CeO2. High-magnification HAADF-STEM
images and corresponding EDX mapping of (d) 2.8RuOx-CeO2 and (e) 2.8Ru-CeO2. The white circles in (e) indicate the locations of Ru nanoclusters.
Fig. 4. (a) In situ DRIFT spectra of CO adsorption on Ru-CeO2 catalysts. (b) N2-TPD-MS and (c) H2-TPD-MS spectra of the Ru-CeO2 catalysts and CeO2 support.
323
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Journal of Catalysis 408 (2022) 316–328
Fig. 5. NH3 mole fractions obtained in the temperature range of 350–450 °C at (a) 30, (b) 50, and (c) 80 bar. Rates of NH3 production and NH3 yields over the pressure range of
1
1–80 bar at (d) 400, (e) 425, and (f) 450 °C. All reactions were conducted under a WHSV of 2600 mL g1
.
cat h
Fig. 6. (a) Rates of NH3 formation at 325–400 °C and 10 bar, (b) TOFs at 400 °C and 10 bar, and (c) Arrhenius plots of NH3 syntheses at 325–400 °C and 10 bar under a WHSV of
1
72 000 mL g1
. Reaction orders of (d) N2 and (e) H2 at 400 °C and 10 bar. (f) Longevity study of 2.8Ru-CeO2 for 210 h at 425 °C and 50 bar under WHSVs of 2600 and 18
cat h
1
000 mL g1
.
cat h
Table 3
Activation energies and reaction orders of NH3 syntheses over Ru-CeO2 catalysts.
a
Catalyst
Ru structure
Ea (kJ mol1)a
N2 order (a)b
H2 order (b)b
NH3 order (c)b
1.4Ru-CeO2
2.8Ru-CeO2
5.3Ru-CeO2
Single atom
Single atom, nanocluster
Single atom, nanocluster, nanoparticle
72
64
87
0.62
0.58
1.30
0.27
0.33
–0.35
–0.39
–0.47
–0.64
Determined using Arrhenius plots (Fig. 6c). b Determined via kinetic analyses (Fig. 6d, 6e, and S17).
324
Journal of Catalysis 408 (2022) 316–328
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Table 4
Catalytic activities of Ru/CeO2 catalytic systems in NH3 synthesis.
Catalyst
Ru content
(wt%)
Ru structure
Synthetic method
Temperature
(°C)
Pressure
(bar)
WHSV
1
(mL g1
)
cat h
Rate
(mmolNH3 g1
cat
h1)
Ea
(kJ mol1)
Ref.
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru-CeO2
1.0
1.7
4.0
10.0
2.5
2.5
2.5
3.0
1.0
1.0
3.0
2.8
Nanoparticle
Nanocluster
Nanoparticle
Nanoparticle
Nanoparticle
Nanoparticle
Nanoparticle
Nanoparticle
Nanoparticle
Nanoparticle
Nanocluster, particle
Single atom, nanocluster
Impregnation
Impregnation
Impregnation
Impregnation
Core-shell
Impregnation
Impregnation
Impregnation
Impregnation
Impregnation
Impregnation
MOF-derived composite
315
315
400
430
425
450
450
400
400
400
400
400
1
1
10
100
10
30
30
25
10
10
10
10
3600
–
18 000
70 000
60 000
24 000
24 000
3000
36 000
36 000
36 000
72 000
0.4
1.2
3.8
250
8.5
27
22
4.9
9.2
5.5
22
10.4
–
–
108
104
–
–
–
–
67
72
66
64
18
20
28
29
31
30
21
24
25
26
51
This work
ticles. Among the catalysts, 2.8Ru-CeO2 exhibits the lowest
apparent Ea with the highest TOF in NH3 synthesis (Fig. 6b). This
indicates that 2.8Ru-CeO2, which mainly comprises Ru nanoclusters and single atoms, is the most efficient catalyst in NH3 synthesis in terms of Ru usage and catalytic activity.
The reaction orders of NH3 syntheses over the Ru-CeO2 catalysts
were investigated at 400 °C and 10 bar (Fig. 6d, 6e, and S17); all
kinetic parameters are shown in Table 3. The apparent Ea values
of the Ru-CeO2 catalysts increase with increasing N2 reaction order.
Notably, 5.3Ru-CeO2 exhibits a significantly higher N2 reaction
order and apparent Ea than other catalysts. All N2 reaction orders
of the Ru-CeO2 catalysts are higher than their H2 reaction orders,
indicating that the rate of NH3 synthesis is more sensitive to N2
than H2 over the Ru-CeO2 system. Remarkably, 1.4Ru-CeO2 and
2.8Ru-CeO2 exhibit positive H2 reaction orders, whereas 5.3RuCeO2 exhibits a negative H2 reaction order. This may be due to
hydrogen poisoning caused by the strong adsorption of hydrogen
atoms on the surface of 5.3Ru-CeO2, which renders the reaction
rate independent of the N2 partial pressure [42]. This is consistent
with the H2-TPD-MS spectra (Fig. 4c), which indicate that 5.3RuCeO2 exhibits the highest H-T peak ratio in H2 desorption. In addition, the NH3 reaction order of 5.3Ru-CeO2 is more negative than
those of 1.4Ru-CeO2 and 2.8Ru-CeO2. These kinetic parameters
suggest that the routes of activation of N2 and H2 on Ru nanoclusters and single atoms may differ from those of N2 and H2 on Ru
nanoparticles.
A longevity study of 2.8Ru-CeO2 was performed for 210 h at
425 °C and 50 bar with different WHSVs (Fig. 6f). First, NH3 synthe1
sis was performed for 110 h under a WHSV of 2600 mL g1
, and
cat h
then the reaction was maintained for 100 h under a WHSV of 18
1
000 mL g1
. During the study, the decrease in pressure is negcat h
ligible, and a constant NH3 mole fraction is produced under both
WHSVs. The CO chemisorption results (Fig. S18) reveal that the
Ru dispersion of 2.8Ru-CeO2 decreases slightly from 94.3% to
88.1% after 210 h, demonstrating the high resistance of this catalyst to sintering during NH3 synthesis. This stability is likely due
to the interfacial Ru–O–Ce bonds, which are abundant within
2.8Ru-CeO2. Compared with those of previously reported Ru/
CeO2 catalysts (Table 4), 2.8Ru-CeO2 exhibits a good apparent Ea
and rate of NH3 synthesis. Notably, 2.8Ru-CeO2 contains Ru nanoclusters rather than nanoparticles, despite exhibiting a higher
Ru content than those of most impregnated Ru/CeO2 catalysts,
demonstrating an advantage of the MOF-derived synthesis used
in this study.
Given the high reversibility of NH3 synthesis, we also evaluated
the applicability of the Ru nanoclusters (2.8Ru-CeO2) in NH3
decomposition at 300–450 °C and 1 bar (Fig. S19). The use of
2.8Ru-CeO2 results in 100% NH3 conversion at 450 °C. Notably,
2.8Ru-CeO2 are significantly higher and continue increasing at
higher Ru contents, which indicates that the strength of N atom
adsorption increases with the formation of Ru nanoparticles. The
H2-TPD-MS spectra (Fig. 4c) exhibit similar peak shape and position trends. For all catalysts, the H2 desorption peaks exhibit
broader distributions up to higher temperatures than those of
the N2 desorption peaks, likely due to the hydrogen spillover effect.
3.2. Catalytic activity
NH3 syntheses were performed over the Ru-CeO2 catalysts at
1
350–450 °C and 30–80 bar under a WHSV of 2600 mL g1
cat h
(Fig. 5a–5c). As NH3 synthesis is exothermic, the thermodynamic
equilibrium (dotted) lines decrease with increasing reaction temperature. The NH3 mole fractions formed over all catalysts increase
with increasing reaction temperature and pressure. Except for the
reaction involving 1.4Ru-CeO2, these mole fractions reach thermodynamic equilibrium at 425 °C and then decrease at 450 °C owing
to the reverse decomposition reaction. The effects of reaction pressure on the rate of NH3 production and NH3 yield were investigated
under the same reaction conditions (Fig. 5d–5f). The rates of NH3
production and NH3 yields of the Ru-CeO2 catalysts increase with
increasing reaction pressure and become similar when approaching thermodynamic equilibrium.
Fig. 6a shows the rates of NH3 production of the Ru-CeO2 catalysts at 10 bar over the temperature range of 325–400 °C under a
1
WHSV of 72 000 mL g1
. The NH3 mole fractions and yields
cat h
of all catalysts are <2%, which are well-separated from the equilibrium limits. The highest rate of NH3 production at all temperatures
is observed when the Ru content reaches 2.8 wt% (2.8Ru-CeO2),
and the rate decreases in the following order: 2.8RuCeO2 > 4.0Ru-CeO2 > 5.3Ru-CeO2 > 1.4Ru-CeO2. Fig. 6b shows the
turnover frequencies (TOFs) of NH3 production at 400 °C and
1
10 bar under a WHSV of 72 000 mL g1
. The TOFs are calcucat h
lated based on the total number of Ru atoms and the number of
surface Ru atoms. As the CO/Ru adsorption ratio varies in the range
of 1–2, depending on the Ru content (Fig. 4a), it may be inappropriate to use the number of surface Ru atoms in determining the TOFs
of single-atom and nanocluster systems. The TOFs decrease in the
following order: 2.8Ru-CeO2 > 1.4Ru-CeO2 > 4.0Ru-CeO2 > 5.3RuCeO2, which differs from the trend observed for the rates of NH3
production (Fig. 6a). Fig. 6c shows the apparent activation energies
(Ea) of NH3 formation that are obtained from Arrhenius plots based
on the rates of NH3 production (Fig. 6a). For the Ru-CeO2 catalysts,
the apparent Ea values of NH3 formation vary from 64 to
87 kJ mol1, indicating that the reaction kinetics depend on the
Ru structure. Remarkably, 1.4Ru-CeO2, with Ru single atoms, exhibits a lower apparent Ea than that of 5.3Ru-CeO2 with Ru nanopar325
S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
Journal of Catalysis 408 (2022) 316–328
Fig. 7. Relative energy diagrams of NH3 syntheses at 425 °C and 50 bar based on the dissociative mechanism over (a) Ru (0 0 1) bulk, (b) Ru22-CeO2 (1 1 1), (c) Ru10-CeO2 (1 1 1),
and (d) Ru1-CeO2 (1 1 1). The insets show the structures of representative reaction intermediates, and the initial and final steps of the reaction coordinates are denoted as IS
and FS, respectively. (e) Simplified reaction routes in NH3 synthesis over the 2.8Ru-CeO2 system with Ru nanoclusters and single atoms.
425 °C and 50 bar. As shown in Fig. S20 and Table S3, we modeled
a Ru bulk (0 0 1) surface to represent nanoparticles, Ru nanoclusters (Ru10-CeO2 and Ru22-CeO2) on CeO2 (1 1 1) surfaces, and a
cationic Ru single atom (Ru1-CeO2) on a CeO2 (1 1 1) surface. The
particle sizes of the Ru10 and Ru22 nanoclusters were designed to
be similar to those of the nanoclusters in 2.8Ru-CeO2 (Table 1).
The effect of defects on the CeO2 surface was not considered. In
estimating the energetics of NH3 synthesis, N2 dissociation is
expressed as a two-step reaction (N2_v* ? N2_h* ? 2N*), with
N2_v* and N2_h* indicating vertically and horizontally adsorbed
N2, respectively [78]. On the Ru (0 0 1) surface (Fig. 7a), N2_h*, N*,
2.8Ru-CeO2 exhibits a higher NH3 conversion and faster rate of
decomposition than those of previously reported Ru/CeO2 catalysts
with Ru nanoparticles under similar reaction conditions (Table S2).
These results suggest that Ru nanoclusters supported on CeO2 may
be utilized in NH3 synthesis and decomposition.
3.3. Reaction energetics
Fig. 7 shows the free energy diagrams of NH3 syntheses based
on the dissociative mechanism (N2 (g) + 3/2H2 (g) ? NH3
(g) + N*; initial step ? final step) over Ru-based surfaces at
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S.E. Sivan, Ki Hyuk Kang, Seung Ju Han et al.
associative mechanism of NH3 synthesis was more suitable than
the dissociative mechanism over the Ru single atoms. Among the
prepared Ru-CeO2 catalysts, 2.8Ru-CeO2, with Ru nanoclusters
and single atoms, displayed a superior performance and good stability in NH3 synthesis via the combination of dissociative and
associative routes. The hydrogen spillover effects of the Ru nanoclusters, in particular, promoted the activity of the associative
route over the coexisting Ru single atoms. This study revealed that
the synergistic effect between Ru nanoclusters and single atoms
may maximize NH3 production and the origins of these favorable
effects. Furthermore, the developed synthesis provided advanced
control over the obtained Ru-CeO2 catalysts. Thus, this study
should advance the development of highly active and durable
Ru-based catalysts for use in NH3 synthesis under mild conditions.
and NH* are adsorbed favorably at hollow sites between three Ru
atoms, whereas NH2* is adsorbed at bridge sites between two Ru
atoms. The binding energies (DGB.E.) of N* and H* are –0.72 and
–0.36 eV, respectively. Notably, N* is significantly more stable than
H*, which may be due to the lack of metal-support interactions in
the Ru bulk model. During N* hydrogenation to NH3 (g), NH*
hydrogenation (E5 ? E6, 1.10 eV) requires the most endergonic
state. The Ru22 clusters display stronger adsorption energies for
N2_v* (DGB.E.,Ru22 = –0.32 eV) and N2_h* (DGB.E.,Ru22 = 0.20 eV) compared to those of the other surfaces (Fig. 7b), indicating that a lateral Ru surface favors N2 adsorption. N* and NH* are favorably
adsorbed at lateral hollow sites on the Ru10 and Ru22 clusters
(Fig. 7b and 7c), whereas NH2* is adsorbed at lateral bridge sites
between two Ru atoms. The Ru10 and Ru22 clusters display weaker
adsorption energies for N* (DGB.E.,Ru10 = 0.29 eV and DGB.E.,Ru22 = –
0.15 eV) than those of the Ru (0 0 1) bulk surface (DGB.E.,Ru (0 0 1) = –
0.72 eV). In addition, the Ru clusters mitigate the most endergonic
steps during N* hydrogenation to NH3 (g), with N* hydrogenation
most endergonic (E4 ? E5, 0.14 eV) on the Ru10 cluster and NH*
hydrogenation most endergonic (E6 ? E7, 0.78 eV) on the Ru22
cluster. Among the Ru models, the Ru1-CeO2 (1 1 1) surface displays
the weakest binding energies for N2_v* (0.95 eV), N2_h* (0.95 eV),
N* (1.68 eV), and H* (0.41 eV) (Fig. 7d). This indicates that a Ru
single-atom site incorporated into a CeO2 surface is unfavorable
in NH3 synthesis with respect to the dissociative adsorption of N2
and H2, which is consistent with the experimental pattern of the
adsorption strengths (Fig. 4b). However, according to the kinetic
data (Fig. 6a–6e), the catalytic activities of Ru single atoms
(1.4Ru-CeO2) are significantly higher than those of Ru nanoparticles (5.3Ru-CeO2). This suggests that the dissociative route may
not be a suitable mechanism over Ru single atoms. The associative
mechanism via N2 hydrogenation without directly cleaving the
N N bond is energetically favorable over Ru single atoms
[54,57]. Thus, the NH3 synthetic activity of the Ru single-atom system may originate from this associative mechanism.
On the 2.8Ru-CeO2 surface, NH3 may be produced via the dissociative route over Ru nanoclusters and the associative route over
Ru single atoms (Fig. 7e). Unlike Ru nanoparticles, Ru nanoclusters
and single atoms do not suffer from hydrogen poisoning due to
their relatively weaker adsorption of dissociated hydrogen atoms
(Fig. 4c and 6e). In addition, the hydrogen spillover effects of Ru
nanoclusters likely enhance the activity of the associative route
over the coexisting Ru single atoms (Fig. 4c and S6). Therefore,
based on the catalytic performance and electronic energy calculations, the NH3 synthetic performance of the Ru-CeO2 system may
be maximized by inducing the synergistic effect between Ru nanoclusters and single atoms.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This research was supported by the Korea Research Institute of
Chemical Technology (KRICT), Republic of Korea [project no.
BSK21-401]. RWJS thanks the Natural Sciences and Engineering
Research Council (NSERC) for financial support. X-ray absorption
spectroscopy was performed at the Canadian Light Source, which
is a national research facility at the University of Saskatchewan,
Saskatoon, Canada, which is supported by the Canada Foundation
for Innovation, NSERC, National Research Council, Canadian Institutes of Health Research, Government of Saskatchewan, and
University of Saskatchewan.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2022.03.019.
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