Ni Single Atom Catalysts for CO2 Activation

Article abstract of DOI:10.1021/jacs.8b11729

We report on the activation of CO₂ on Ni single-atom catalysts. These catalysts were synthesized using a solid solution approach by controlled substitution of 1-10 atom % of Mg²⁺ by Ni²⁺ inside the MgO structure. The Ni at

Full text of DOI:10.1021/jacs.8b11729

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Article
Ni single atom catalysts for CO2 activation
Marie-Mathilde Millet, Gerardo Algara-Siller, Sabine Wrabetz, Aliaksei Mazheika, Frank Girgsdies,
Detre Teschner, Friedrich Seitz, Andrey Tarasov, Sergey V. Levchenko, Robert Schloegl, and Elias Frei
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Jan 2019
Downloaded from http://pubs.acs.org on January 14, 2019
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Ni single atom catalysts for CO2
activation
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Marie-Mathilde Millet[a], Gerardo Algara-Siller[a], Sabine Wrabetz[a], Aliaksei Mazheika[b],
Frank Girgsdies[a], Detre Teschner[a,c], Friedrich Seitz[a], Andrey Tarasov[a], Sergey V.
Levchenko[b,d,e], Robert Schlögl[a,c], and Elias Frei*[a]
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[a] M.-M. Millet, Dr.G. Algara-Siller, Dr.S. Wrabetz, Dr.F. Girgsdies, Dr.D.Teschner, Dr.F.Seitz,
Dr.A. Tarasov, Prof.Dr.R. Schlögl, Dr.E. Frei
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin (Germany)
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[b] Dr. Aliaksei Mazheika, Dr. Sergey V. Levchenko
Department of Theory
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin (Germany)
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[c] Dr. Detre Teschner, Prof. Dr. R. Schlögl
Max-Planck-Institut für Chemische Energiekonversion
Abteilung Heterogene Reaktionen
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Stiftstr. 34-36, 45470 Mülheim an der Ruhr (Germany)
[d] Dr. Sergey V. Levchenko
Center for Electrochemical Energy Storage
Skolkovo Institute of Science and Technology
Nobel Str. 3, 143026 Moscow (Russia)
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[e] Dr. Sergey V. Levchenko
Materials Modeling and Development Laboratory
National University of Science and Technology „MISIS“
Leninskii av. 4, 119049 Moscow (Russia)
* E-mail: efrei@fhi-berlin.mpg.de
Keywords: Single atom Catalyst / Ni-Mg solid solution / CO2 activation / NixMg1-xO / Reverse
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water-gas shift / Heterogeneous catalysis
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Abstract
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We report on the activation of CO2 on Ni single atom catalysts. These catalysts were synthesized
using a solid solution approach, by controlled substitution of 1-10 atom-% of Mg2+ by Ni2+ inside the
MgO structure. The Ni atoms are preferentially located on the surface of the MgO, and as predicted
by hybrid-functional calculations, prefer low-coordinated sites. The isolated Ni atoms are active for
CO2 conversion through the reverse water-gas shift (rWGS), but are unable to conduct its further
hydrogenation to CH4 (or MeOH), for which Ni clusters are needed. The CO formation rates correlate
linearly with the concentration of Ni on the surface evidenced by XPS and microcalorimetry. The
calculations show that the substitution of Mg atoms by Ni atoms on the surface of the oxide
structure reduces the strength of the CO2 binding at low-coordinated sites and also promotes H2
dissociation. Astonishingly, the single atom catalysts stayed stable over 100 hours time on stream,
after which no clusters or particle formation could be detected. Upon catalysis, a surface carbonate
adsorbate-layer was formed, of which the decompositions appears to be directly linked to the
aggregation of Ni. This study on atomically dispersed Ni species brings new fundamental
understanding of Ni active sites for reactions involving CO2, and clearly evidences the limits of single
atom catalysis for complex reactions.
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Introduction
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Noble-metals stabilized on metal oxides are one of the most widely used heterogeneous catalysts
also applied in the industry1. The size of the metal particles is a cornerstone for the performances of
such supported catalysts in terms of metal atom efficiency and selectivity, which both tend to
decrease with increasing particle size and broadening size distributions2. Hence there is a need for
sophisticated preparation techniques to control with precision the size of those supported metal
clusters. Furthermore, the diversity and irregularity of the clusters in size and shape strongly hinder
the complete identification of the active site and as such, the understanding of the reaction.
Reducing the complexity of such systems can be achieved by the largest conceivable reduction of
both, the size and the size distribution of the metal particles, by the preparation of “single atom
catalysts” (SAC). The first practical synthesis of single atom catalysts was performed by Zhang’s
group2e, achieving the preparation of isolated Pt atoms on an FeOx support. While the denomination
of SAC is relatively recent2e, the concept of “isolated active site within a solid catalyst” was
introduced much earlier according to John Meurig Thomas3, and became prominent thanks to the
early work of Grasselli et al.4. During the last years many studies were conducted on single atom
catalysts, dealing with supported noble metals like Pt2e, 5, Au2d, 5a, 6, Ir7, Pd8, but rarely using cheaper
and more earth-abundant transition metals.
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The understanding of CO2 activation reactions has become a topic undergoing intense study because
of environmental considerations. Among the transition metals efficient for CO2 activation and
hydrogenation reactions, supported Ni catalysts are prominently represented9. These catalysts have
been extensively studied in methanation10 and dry reforming of methane11 reactions. The size of the
Ni particles in such systems is known to have a large influence on the selectivity of the methanation
reaction12. Larger particles lead to more CH4 selective catalysts, while small particles are more
selective towards CO13. Recently, Vogt et al. provided some new mechanistic insights on the CO2
methanation reaction by correlating the size of the Ni clusters (1-7nm), to the stability of the reaction
intermediates14. Interestingly, they observed even for the smallest Ni clusters (≈1nm) the formation
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of methane. In this study, we go a step further by focusing our interest on the preparation of Ni
single atom catalysts, in particular for the hydrogenation of CO2 to CO or CH4. To our knowledge, such
challenge has not yet been met successfully and only few studies were reported about single Ni atom
catalysts, focusing only on electrocatalytic applications or Ni clusters within a single site concept11c, 15
.
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Wet chemistry based synthesis methods have been selected for this purpose since, unlike physical
deposition synthesis methods like sputtering16, they allow the production of larger amounts of
catalyst, suitable for their full characterization or even practical industrial applications. One main
criterion for the successful synthesis of SAC is to strongly anchor the metal species on the support to
avoid their aggregation1. To overcome this problem, we used the ability of NiO and MgO to form a
solid solution as strategy to prevent the Ni from agglomerating. Both metal oxides crystallize in a face
centered cubic (fcc) structure with almost identical lattice parameters (MgO: 0.4213nm, NiO:
0.41769nm)17. Additionally, MgO is a very suitable support for CO2 activation reactions, since its
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strong basicity enhances the CO2 adsorption capacity 18
.
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In this study, we report for the first time on successful synthesis of Ni SAC in the range of 1-10(at.-%)
in the MgO lattice (NixMg1-xO). The structure and composition of the precursors and catalysts were
thoroughly analyzed with XRD, XPS, XRF, FTIR, STA-MS, microcalorimetry and electron microscopy.
The activity in the rWGS reaction was measured and correlated to the catalyst properties.
Additionnally, DFT hybrid-functional calculations were performed to better elucidate the role of the
Ni atoms with respect to catalytic performance and structural stability. The nature and properties of
the active site will be discussed also in the context of Ni cluster formation as bridge to the Ni particle
based high performance catalysts. Finally, this study addresses the highly important question related
to the use of single atom catalyst as models for supported nanoparticle catalysts, and to their ability
to conduct multi-electron redox reactions without limitations.
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Methods
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3.1 Catalysts preparation
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Materials. Mg(NO3)2*6H2O (99+%, Acros Organics), Ni(NO3)2*6H2O (99+%, Acros Organics), Ammonia
25% (VWR Chemicals, NORMAPUR) and deionized water obtained from laboratory purification
system.
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Synthesis. The NixMg1-x(OH)2 precursors were synthesized by co-precipitation from a Ni and Mg
nitrate solution (Σ 1M) and a 1M Ammonia solution as precipitating agent, in an automated lab
reactor. The equimolar mixture was chosen (as opposed to the stoichiometric one) to avoid the
formation of Mg,Ni(NH3)n. While stirring at 300 r.min-1, both solutions were dosed at a rate of 10
g.min-1 , into deionized water (300ml) at a controlled temperature of 60°C. The resulting solution was
aged for 1hour before cooling down to room temperature. The pH electrode was calibrated at 25°C
with the buffer solutions pH=7 and pH=10 (Carl Roth ±0.02). The pH increased strongly with the first
drops of ammonia (pH=9.3), but then remained constant throughout the whole synthesis and ageing
processes (around pH=8.8). All parameters of the synthesis were recorded and are shown in
supporting information (Figure S1). The slurry was filtrated and the precipitate washed with
deionized water until the conductivity was below 0,5 mS.cm-1. The light green solid was dried at 80°C
for 15 h with a heating ramp of 2°C.min-1 and then calcined in a rotating furnace at 600°C for 3h in a
flow of 100ml.min-1 consisting of 21% O2 in Argon, with a heating ramp of 2°C.min-1.
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3.2 Characterization
XRD. The X-ray diffraction (XRD) measurements were performed in Bragg-Brentano geometry on a
Bruker AXS D8 Advance II theta/theta diffractometer, using Ni filtered Cu K radiation and a position
sensitive energy dispersive LynxEye silicon strip detector. The sample powder was filled into the
recess of a cup-shaped sample holder, the surface of the powder bed being flush with the sample
holder edge (front loading). The powder XRD data were analyzed, and the lattice parameters
calculated using a full pattern fitting, according to the Rietveld method as implemented in the TOPAS
software [TOPAS version 5, copyright 1999-2014 Bruker AXS].
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STA-MS. Simultaneous thermo analysis (TG/DSC) was performed on a Netzsch STA449 Jupiter
thermoanalyzer, using 10-15mg of sample placed into an alumina crucible (85µl) without lid.
Evolution of the gas phase during reaction was monitored with a quadrupole mass spectrometer
(Pfeiffer, QMS200 Omnistar). The thermal decomposition was performed from room temperature to
1000°C, with a heating ramp of 2°C.min-1 in a 21%O2 in Ar atmosphere (100ml.min-1). Experimental
mass loss were calculated from 100 to 1000°C to avoid taking into account the adsorbed surface
species (ex: water).
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The specific CO2 area was calculated using the following formula:
SCO2=ns*S*NA*CSA(CO2 on MgO)
(1)
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Where nS is the amount of CO2 molecules, S the stoichiometric factor, NA the Avogadro number
(6.022140857 × 10 23) and CSA the cross sectional area of a CO2 molecule adsorbed on MgO (22.1 x
10-20 m²)19.
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Catalytic testing. The catalytic measurements were performed in a fixed bed flow reactor. 100mg of
the oxidic compound (100-250 μm particle diameter) were loaded into a 6 mm inner diameter
stainless steel reactor tube. It was first heated in argon to 350°C (1°C.min-1, hold 1h) to get rid of
volatile adsorbed species and then exposed at 200°C to 100ml.min-1 of a Sabatier gas mixture (72 %
H2, 18% CO2, 10% Ar for balance) at 30 bar pressure. Gas analysis was performed online by a gas
chromatograph (Agilent 7890A) equipped with two channels, a thermal conductivity detector was
used to analyze CO2, CO and Argon, while a flame ionization detector was used to analyze MeOH and
CH4 (both of which were combined with different capillary columns: molesieve and plot-Q). All
catalytic testings were performed under differential conditions (< 3% conversion). Testing with pure
silica, at the same conditions revealed negligible formation rates, in the same order of magnitude
than the one of the sample that did not contained any Nickel (pure MgO).
Microcalorimetry. Microcalorimetric experiments were carried out in a Calvet calorimeter (SETARAM
MS70) combined with custom-designed high vacuum and gas dosing apparatus, which enables the
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dosage of probe molecules within a range of 0.02 mmol and to calculate the amount of adsorbed
molecules. The samples were degassed under UHV at 500 °C for 1 h with a heating ramp of 5 °C.min-
1. The pretreatment was conducted in a separate chamber connected to the calorimetric cell. The
final pressure in the degassed cell was approximately 10-7 mbar. The cell was cooled to room
temperature, placed inside the calorimeter, and connected to the microcalorimetric gas-adsorption
system. Subsequently, CO2 was dosed stepwise at 30°C. Pressure, adsorption temperature, and the
heat signals were recorded during all dosing steps.
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XPS. All samples were investigated as pressed pellets. XPS spectra were recorded using non-
monochromatized Al Kα (1486 eV) excitation and a hemispherical analyzer (Phoibos 150, SPECS). The
binding energy (BE) scale was calibrated using standard Au 4f (7/2) and Cu 2p (3/2) procedure.
Charging, in the range of 6-7 eV, was observed with the samples. Binding energy correction for
charging was carried out by adjusting the Mg2p level to 50.3 eV. To calculate the elemental
composition, the theoretical cross sections from Yeh and Lindau20 were used.
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TEM. Samples were characterized using a transmission electron microscope JEOL ARM 200F operated
at 200 kV. Images were taken with scanning TEM (STEM) and high resolution TEM (HRTEM) modes.
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XRF. X-ray fluorescence spectroscopy (XRF) was performed in a Bruker S4 Pioneer X-ray
spectrometer. Sample preparation by melted pellets (100 mg sample with 8. 9g of Li2B4O7 ).
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TPR. Temperature programmed reductions were performed in a fixed bed reactor, in a 5% H2 in
Argon gas mixture, with an heating rate of 5°C.min-1 (100ml.min-1 for 100mg catalyst). The H2
consumption was monitored by a TCD detector. Prior to reduction, the catalysts were heated in
Argon to 500°C (5°C.min-1 for 1h) to get rid of adsorbed surface species. The Ni_10% sample “after
catalysis” was taken out of the reactor and set in contact with air before reduction.
BET. Specific surface areas were determined by N2 physisorption using the BET method21 in a
Quantachrome Autosorb-1 machine. The samples were previously outgassed for 3.5 h at 80 °C.
In situ IR. The Ni_10% sample was pressed into a self supporting disc (40.2mg) and placed in the
sample holder in the center of the furnace of the IR cell. The design of the IR cell was discussed
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elsewhere22. After an activation in Argon at 500°C, the reactive gas was introduced in the cell at a
flow rate of 120.6 ml.min-1 and 10 bar pressure to reproduce the reaction conditions of the plug flow
catalytic testing. The spectra were recorded with a Varian-670 FTIR spectrometer, with a resolution
of 2 cm-1 and an averaging of 128 scans to achieve satisfactory signal-to-noise ratio. The
measurement was performed at 200, 250 and 300°C, after 8h of time on stream.
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CO-adsorption IR. The low temperature IR measurements were carried out in transmission mode
using a Perkin-Elmer 100 FTIR spectrometer equipped with a pre-treatment cell, enabling thermal
treatments in controlled atmosphere. Background spectra were obtained by measuring a spectrum
of the empty cell at -196°C. The Ni_10 self-supporting wafers were pre-treated once in vacuum to
500°C and once in an 72% H2 atmosphere to 300°C (both with a heating rate of 5°C.min-1, and hold at
target temperature for 1h). The IR cell was then evacuated and cooled down to -196°C, for the
progressive adsorption of CO (pCO=0-5mbar).
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3.3 Computational details
All calculations have been performed using hybrid density functional HSE23 with ω = 0.11 and fraction
of exact exchange α = 0.3, denoted − HSE(0.3). The validation of this method is shown in our previous
work24. We have used the FHI-aims code25 which is all-electron with numerical atomic orbitals. The
'tight'25 numerical settings and basis sets were employed. Relativistic effects were treated within
zero-order regular approximation (ZORA)26. Interatomic interaction at middle and large distances was
improved using many-body dispersion correction (MBD)27. The lattice parameter of cubic MgO was
4.217 Å, as obtained from the bulk calculations with HSE(0.3).
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The calculations of adsorption were done using slab-periodic model. The k-grids for slabs were set
with respect to 8x8x8 grid of cubic 8-atomic bulk unit cell. The slab thickness and cell sizes depended
on the surface and considered adsorption sites. We used 5-layer slab with (2x2) unit cell for
calculations of adsorption on (001) terrace. For simulations of (001) steps the 4-layer (2x3) slab with
additional 4-atomic wide semilayer on top was used. The corner position was simulated with 3-layer
(3x3) slab with 16-atomic square island on top. The (110) surface had 8-layers and the unit cell
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depended on the coverage and desired Ni surface concentration. The vacuum-gap between slabs was
about 200 Å. The adsorption of molecules was considered only on one side of the slab, so the dipole
correction was used to prevent unphysical overpolarization. All atoms were allowed to relax with
BFGS algorithm during the search of a minimum on potential energy surface. The procedure was
performed until maximal atomic forces did not exceed 10-2 eV/Å.
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Adsorption energies were calculated as the difference between total energy of adsorbed system and
the sum of total energies of clean surface and gas-phase molecule. We applied the counterpoise
method for correction of superposition error.
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Results and discussion
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ID
Ni% (nominal)
Ni% (XRF)
0
Ni% (XPS)
FHI datbase
#25453
#23134
#25477
#27537
#23659
#23769
MgO
Ni_1
Ni_3
Ni_5
Ni_10
Ni_15
0
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11.2
18.7
1.96
5.66
9.18
15.25
24.5
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Table 1. Chemical constitution of prepared catalysts
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4.1 Structural analysis of phase pure precursors and catalysts
Following a co-precipitation recipe (details see experimental part), a series of NixMg1-xO samples with
varying Ni concentration (0 to 15 at.-%) was synthesized. The samples are named after their Ni
concentration i.e. a sample containing 15 at.-% Ni will be named as Ni_15(OH)2 for the precursor and
Ni_15 for the oxidic compound. The XRD patterns of the precursors are illustrated in Figure S2.
Rietveld refinement confirmed the phase purity of all samples. Since Ni(OH)2 and Mg(OH)2 have the
same crystal structure (brucite type) and close ionic radii, NixMg1-x(OH)2 solid solutions are formed.
The evolution of the lattice parameters a and c as a function of the Ni content is shown Figure S3 and
Table S1. A linear correlation between both lattice parameters and the Ni-loading is observed,
confirming the successful incorporation of Ni in the Mg(OH)2 structure as reported in the literature28.
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All precursors were analyzed using themogravimetry (TG), coupled with simultaneous mass
spectrometry (MS). The results are presented in Figure S4. The pure Mg(OH)2 sample shows one
characteristic endothermic peak with an onset temperature of 275°C. With increasing Ni-content this
characteristic endothermic peak is shifted to lower onset temperatures. The presence of a single
event (as opposed to two distinct ones) further evidences the successful formation of a solid
solution, instead of a physical mixture of Ni(OH)2 and Mg(OH)228b. The theoretical and experimental
weight losses are summarized in Table S3, and less than 1% deviation is observed. The phase pure
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character of the precursors is also a prerequisite for the successful solid solution synthesis in the final
catalysts.
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A)
Measured
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Calculated
Difference
MgO ref.
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2 
B)
Ni_15
Ni_10
Ni_5
Ni_3
Ni_1
MgO
MgO ref.
NiO ref.
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61
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64
65
C)
2 
4.218
4.216
4.214
4.212
4.210
4.208
R²=0.995
0
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NiXRF [%]
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Figure 1. A) Rietveld fit of Ni_10 with the measured (black line) calculated (red line) diffraction
patterns and the corresponding difference curve (grey line) B) Superposition of the 6 XRD patterns,
zoom in the 60-65° region C) Variation of the lattice parameter a versus the Ni content determined by
XRF
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Figure 1A shows a full pattern Rietveld fit of the calcined Ni_10 sample (XRD patterns of all calcined
samples are presented in Figure S5). The fit curve (red line) matches perfectly with the experiment
(black line), also confirmed by the flat difference curve (blue line). Figure 1B shows a zoom of the 60-
65° region of all diffractograms, exemplarily highlighting the shift of the 220 reflection to higher 2θ
values with increasing Ni content. This corresponds to a continuous decrease of lattice parameter a
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from 4.217 to 4.208 Å (Figure 1.C). Such behavior is characteristic of the successful incorporation of
Ni inside the MgO structure17a and a phase pure solid solution sample series. The error bars are
invisible, as they are smaller than the size of the symbols used. All the lattice parameters and their
estimated standard deviations are shown Table S2.
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4.2 Dispersion
To further confirm the atomic distribution of the Ni atoms in the structure, scanning transmission
electron microscopy (STEM) investigations were performed. The high-angle annular dark field
(HAADF) STEM image in Figure 2 illustrates the isolated character of the Ni atoms (brighter spots) as
part of the MgO lattice (Fig. 2B, magnification of 2A, STEM). This was also confirmed by the
comparison of the dark- and bright-field image shown in Figure S6. Representative for all catalysts,
the Ni_10 sample is shown, which has, due to its high Ni loading a high probability of cluster
formation. After extensive analysis of different regions of all the samples, no Ni agglomerates were
observed.
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B)
B)
A)
1 nm
1nm
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Figure 2. A) STEM-HAADF image of Ni_10 before catalytic testing B) Magnification of the selected area.
To guide the eye, some of the brighter spots were circled in red.
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These findings are in line with the TPR profiles of the catalysts (Figure S7A). Since electron
microscopy investigations are not representative of the entire sample, a more integral method was
needed to confirm the dispersion of the Ni species inside the MgO structure. The absence of a low
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temperature reduction event at about 350 °C, indicative for surface agglomerates, and only one
strong signal at 750°C, indicative for Ni atoms embedded in the MgO structure29, underlines the
homogeneous character of the NixMg1-xO solid solution and the absence of NiO agglomerates at the
surface.
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4.3 Elemental composition
The global elemental composition (surface + bulk) was determined using X-ray fluorescence (XRF).
The experimental values show a good agreement with the nominal ones (Table 1). According to a
previous study, the most favorable position for a formal Mg2+ substitutions by Ni2+ ions, in a MgO
structure is on its corners, coordinatively unsaturated24. To investigate the Ni distribution in the
structure, X-ray photoemission spectroscopy (XPS) was applied (Fig. 3A, Ni2p(3/2) core levels, Fig. S9
Mg2p spectra). The more surface sensitive XPS analysis reveals a strong surface enrichment by a
factor of two for Ni in the solid solutions (Fig 3B, Table 1). The obtained spectra of Ni2p (870eV-
850eV) show that only Ni2+ species are present over the whole range of concentration (Fig. 3A).
Contrarily to the study of pure NiO, the presented Ni 2p XPS spectra did not show any peak splitting
expected at 854-858eV. The main Ni 2p peak at 854eV in NiO, which is missing in our spectra, was
identified in the literature as characteristic of Ni with neighboring Ni species30. Its absence in this
study is in line with the high dispersion of the Ni atoms in the MgO structure. A comparison of the
surface composition vs. the global composition (NiXPS as a function of NiXRF), Figure 3B shows that
from Ni_10, the surface composition is not evolving linearly. A surface saturation phenomenon is
indicated for Ni_10 and Ni_15, resulting in a poorer surface Ni concentration (compared to their
global Ni content) when compared to the samples with lower Ni loadings (Ni_1, Ni_3 and Ni_5).
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A)
B)
Ni_15
Ni2p(3/2)
Ni_10
Ni_5
Ni_3
Ni_1
MgO
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0
870
865
860
855
850
845
0
5
10
15
Binding Energy [eV]
NiXRF [%]
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Figure 3. A) Ni2p 3/2 spectra in the 870-845eV region. B) Surface Ni concentration (measured by XPS)
evolution versus the global Ni concentration (measured by XRF)
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As XPS measurements also gain information from the sub-surface layers, another merely surface
sensitive technique has to be applied. Microcalorimetry allows to chemically titrate the surface of the
catalysts with CO2 as probe molecule. The qualitative (differential heat of adsorption) and
quantitative (amount of adsorbed molecules until saturation) information extracted from these
measurements are shown in Figure S10. A linear correlation is obtained when comparing the amount
of CO2 adsorbed, measured by microcalorimetry, and the Ni concentration obtained by XPS (Fig. 5A).
This strongly evidences that the Ni content gained by XPS correlates well with the Ni concentration
on the surface of the catalysts. The linear dependency implies again a saturation effect for the Ni_10
sample (see also Figure S11A, amount of CO2 titrated vs. XRF).
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4.4 Catalytic testing
The catalysts were tested at 30 bar, between 200 and 300°C and in a gas mixture of H2:CO2 of 4:1 (for
details see the experimental part). The H2:CO2 ratio, the low temperatures and the elevated pressure
were chosen in order to favour the formation of CH4 over CO, since this selectivity information serves
as descriptor for multi-electron reactions. In order to study the reactivity of the isolated Ni atoms,
the temperature of 300°C was not exceeded, as testing at higher temperatures (350°C) resulted in
the formation of Ni-clusters of about 10nm (Fig. 4B). Parallel to the Ni cluster formation, the reaction
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rate of the by-product CH4 increased as illustrated in Figure 4A. Details of the EDX mapping are
presented Figure S12. Interestingly, at 350°C MeOH formation was recorded as a rather non-typic
product of Ni catalysts. This might be explained by the intermediate character of the Ni cluster, more
active than an atom but not yet a metallic particle with respect e.g. to its oxophilicity31 or the ability
to activate H2. An ongoing formation of Ni clusters is suppressed as soon as we adjusted the
temperature back to 300 °C. Besides CH4 as reaction product, a much higher reaction rate of CO is
detected due to the already formed Ni clusters.
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1.6
MeOH
CH4
B)
350°C
A)
1.4
CO
1.2
1.0
0.8
0.6
0.4
300°C
0.2
300°C
250°C
200°C
20 nm
0.0
0
2
4
6
8
10 12 14 16 18 20 22 24
Time on stream [hour]
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Figure 4. A) Product formation rate of Ni_10 at 30bar, different temperatures, upper corner left
10
represent the sample after 300°C testing (Figure 7).B) TEM image of the sample after testing at 350°C.
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The Ni_15 sample already showed CH4 formation at 300 °C and a large increase of the CO formation
rate, similar to what is observed for the testing of Ni_10 at 350°C (Figure S13, Figure 4). As the cluster
formation and the development of CH4 as a by-product appear to be directly related, it is very likely
that Ni clusters were already formed at the comparably low temperatures of 300°C in Ni_15 (sample
with the highest Ni concentration). To ensure the reproducibility and to prove that no cluster
formation occurred at 300 °C, all samples were tested following an increasing and then decreasing
temperature profile (Fig. S14). At 300°C, the catalysts were all 100% selective towards CO. During the
testing period of 8h, no deactivation was observed. Figure 5B shows the CO formation rate (rWGS
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reaction) as a function of the Ni content determined by XPS and as evidenced above (see also Fig. 5A)
the Ni concentration on the surface. Since the CO formation rate correlates linearly with the Ni
concentration on the surface, it is very likely that the Ni single atoms are directly involved in the
catalytic cycle. Due to the discrepancy with XRF results (saturation phenomenon, Fig. S11B) the Ni
atoms situated in the bulk do not significantly contribute to the observed reactivity. This
interpretation is in accordance with theory calculations, which show a negligible impact of Ni in sub-
surface region for CO2 and H2 adsorption (Table 2 and details inTable S8) when compared to Ni atoms
on the surface of the oxide. For example, according to theoretical calculations the adsorption
energies i.e. for CO2 on terraces are between 62 and 68 kJ.mol-1 for Ni free and Ni in the first or
second sub-surface layer of MgO (Fig. 6). The small but noticeable increase of CO2 adsorption energy
in the case of MgO (001) surface with Ni in the second atomic layer is explained by additional
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δ-
bonding of CO3 (including the surface oxygen) with Ni d-states. As the Ni is located deeper in
subsurface, the contribution of Ni states to the bonding is decreased.
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A)
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0.372
0.310
0.248
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R²=0.992
0.186
0.108
B)
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0.036
0.000
R²=0.968
0
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NiXPS[%]
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Figure 5. A) Evolution of the amount of CO2 adsorbed (measured by microcalorimetry) versus the
surface nickel concentration (measured by XPS) B) Evolution of the CO formation rate as a function of
the surface nickel concentration (measured by XPS)
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To gain further information about the stability of these catalysts, selected samples (Ni_3 and Ni_10)
were tested over a period of 100 h TOS (Fig. S15A+B). Stable CO formation and the absence of CH4 as
a by-product was observed, which implies indirectly the absence of any cluster formation (as the
cluster formation in our study was always accompanied by the development of CH4 as by-product).
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Eads=-61.8kJ/mol
Eads=-67.5kJ/mol
Eads=-62.7kJ/mol
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Figure 6. Example of the Influence of the adsorption site on the CO2 adsorption energy for terrace sites.
Green represents the Ni which substitutes one of the grey Mg atoms in the first and second sub-surface
layer. Oxygen is shown in red and carbon in black.
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4.5 Characterization after catalysis
After catalysis, the samples were thoroughly characterized (after 40 and 100 h TOS) with STEM, TPR,
XRD and TG-MS. Figure 7A (and 7B as zoom-in) shows the STEM investigation of the Ni_10 catalyst
after 40 h TOS. The analysis did not reveal the presence of any Ni particles or clusters as only single
atoms of Ni in the MgO cubic structure were visible. The stable and selective catalytic performance is
in agreement with the absence of Ni particles. To confirm this with a more integral method on a
larger scale XRD Rietveld analysis was performed after catalysis (100h TOS, Ni_10). Figure 7C shows
the absence of any new phases (hydroxides or carbonates) showing the absence of bulk
transformation processes upon catalysis, again in line with the high stability of the catalysts even at
elevated TOS. Furthermore, the lattice parameter a and the size of the coherent crystalline domains
remained unchanged upon catalysis even after 100h TOS (variations smaller than 3e.s.d.:
abefore=4.21126(7) vs. aafter=4.21104(7); LVol-IBbefore= 9.42(12) vs. LVol-IBafter= 9.10(14)).
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TPR investigations after catalysis (40h TOS, Ni_10), revealed a specific reduction event at 350°C,
which however could not be attributed to the reduction of NiO aggregates29, as illustrated in Figure
S7+S8, but to CO and CH4 formation by hydrogenation of surface species. This confirms again that the
testing at 300°C for extended TOS did not lead to the formation of Ni particles. Furthermore, this
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experiment reveals that carbonate-based species on the surface of the catalysts are present after
reaction.
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B)
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1 nm
1 nm
1 nm
Measured
Calculated
Difference
MgO ref.
C)
SiO2
from catalyst bed
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2 [°]
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Figure 7. A) STEM-HAADF image of Ni_10 after catalysis B) STEM-HAADF image zoom of the indicated
area. To guide the eye, some of the brighter spots were circled in red.C) Rietveld fit of Ni_10 after 100h
time on stream without deactivation, with the measured (black line) calculated (red line) diffraction
patterns and the corresponding difference curve (blue line)
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TG-MS analysis before and after catalysis, revealed a clear surface modification upon testing. The
thermal decomposition experiments showed a large release of CO2 at 300°C, that was not observed
on the fresh catalyst, together with an additional water release, indicating that the in-situ surface
modification corresponds likely to the formation of hydrogenated-carbonate species (Figure S16).
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Considering the amount of CO2 released (measured by integration of the Mass Spectrometry signal
recorded upon thermal decomposition) and a homogeneous distribution of the adsorbates on the
available BET surface area, a coverage of approximately one monolayer is calculated (Table S4). As
expected, XRD was unable to resolve amorphous surface modifications including surface carbonates.
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This over-layer formation appear to be closely linked with the stability of the SAC. While testing at
30bar leads to the stable formation of CO (Fig 5.A and S14), the testing at 1 bar leads to the
formation of CH4 which suggests the formation of Ni clusters and particles at already 300°C, as shown
in Figure S17A and B. We observe that with ongoing TOS the selectivity towards CH4 and the reaction
rates increase, in line with the catalyst behavior at 30 bar and 350°C (Fig. 4, Fig S17B).However,
testing at 1 bar after testing at 30 bar leads to stable production of CO with 100% selectivity. The
absence of CH4 as a by-product consequently evidences the stability of the catalyst without the
formation of any Ni clusters (Figure S18). A reconstruction of the catalysts surface has thus taken
place in-situ at 30 bars. The stability of this surface modification (≤1ML as shown Table S4), formed at
elevated pressures, seems to be directly related to the stability of the Ni atoms, as the agglomeration
of Ni is observed in its absence.
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This is in good agreement with the results of the in-situ IR analysis shown Figure 8 A-C (see Table S5
the assignment of the IR modes). At the surface of the catalysts, were observed a mixture of different
carbonates and hydrogenated carbonates species. With increasing temperature, a progressive
decrease of the carbonate signals (1435,1529cm-1) from the decomposition of the surface over-layer
is observed. This process is accompanied by an increase of the bicarbonate signal (1250cm-1),
formate signals (1342,1606cm-1), and the appearance of CH4 (Fig. 8A, 1305 cm-1; Fig. 8B,3016cm-1).
This findings evidence once again the concerted fashion of a CO2 release due to a progressive
instability of the surface over-layer, the agglomeration of the Ni and the formation of CH4,
respectively. Besides, a pronounced shift in the formate area from 1607 to 1596cm-1 is observed, as
shown in more details Figure 8C. While values around 1600-1606cm-1 were attributed to formates
species on MgO32, values around 1591cm-1 were recently reported for stretching vibration from
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formates species on Ni0 nanoclusters14, further supporting the correlation of cluster formation and
CH4 production. The same shift was observed in the formate C-H area (≈2900cm-1) as seen Figure S19.
The stepwise red-shift of the formate stretching vibration with the formation of Ni clusters might be
interpreted as a weakening of the conjugated C-O/C=O system on Ni clusters or at the metal/metal
oxide interface, consequently visible in its increased catalytic performance (increasing CH4 rates).
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B)
A)
Increasing
Temperature
3200
3100
3000
Wavelength [cm-1]
C)
1650
1625
1600
1700
1600
1500
1400
1300
Wavelength [cm-1]
Wavelength [cm-1]
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Figure 8. IR spectroscopy under reaction conditions, stepwise increase from 200 °C (black), to 250 °C
(dark-red) to 300 °C (red). A) zoom in the 1200-1700cm-1 area. B) zoom on the 2900-3200cm-1 area C)
zoom on the 1550-1650cm-1 area
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4.6 Final discussion
Microcalorimetry. Theoretical calculations (Table 2) predict for the adsorption of CO2 three main
types of sites: low energy sites [terraces -61.8kJ.mol-1], intermediate energy sites [corners -160.2/-
199.7kJ.mol-1] and high energy sites [steps -219.0/- 245.1kJ.mol-1]. Comparing the qualitative analysis
of the catalysts (Figure S10) in terms of their differential heats of adsorption, almost similar values
are obtained (between 105 and 125 kJ.mol-1 for all samples). These energies could be attributed to
the intermediary energy sites (corners), as under a mild vacuum pre-treatment, the high energy sites
(steps) are not accessible (as seen by CO adsorption IR at 77K Figure S20 + Table S6).
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Adsorption Energy (kJ/mol)
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CO2
H2
(dissociative adsorption)
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Adsorption Site
NiMgO
Terraces
-61.8
Corners
-160.2
-199.7
-193.0
-
Steps
-219.0
-245.1
-236.4
-
Terraces
+109.0
+171.7
+171.7
-
Corners
Steps
-98.4
-51.1
-50.2
-
-83.9
-48.2
-47,3
-
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MgO
-61.8
MgO (Ni 1st sublayer)
MgO (Ni 2nd sublayer)
-67,5
-62,7
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Table 2. Calculation of the adsorption energy of CO2, and of the dissociative adsorption of H2 for
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The discrepancy observed between the theoretical values and the experimental ones could be
explained, by the “uniform roughness” of the catalysts surface, the dependency of the Eads on the CO2
coverage33 and by the Ni content (Table S7, S8) that tend to lower the adsorption energies.
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The increase of the CO2 uptake with the Ni concentration indicates that also the amount of available
sites for titration (mainly corners) increased. This is in good agreement with the theoretical
predictions suggesting that the most favorable position for a Ni2+ substitution is at the low
coordinated sites (corners and steps) of the MgO. Further, the incorporation of Ni inside the
structure might cause morphological changes, increasing the amount of corners in the structure, in
order to allow the Ni atoms to reach their thermodynamically most favored position. The roughened
surface rich in corners/steps is directly evidenced by overview TEM images (Fig. S21), which is in
agreement with the presence of Ni on unsaturated sites.
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Catalysis. We observe a linear evolution of the reactivity with the Ni surface concentration, clearly
indicating that the Ni surface sites are involved in the catalytic cycle, while their isolation, and the
absence of agglomeration explains the linear character of the correlation. Under reaction conditions,
a surface adsorbate layer was formed (carbonate-based species). The stability of this reactant
induced restructuring is directly linked with the stability of the catalysts and their high selectivity
towards CO. The involvement of the surface Ni atoms in the catalytic cycle could be related to the
poorer stability of carbonate-based intermediates on Ni sites, in accordance with the calculated
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adsorption energy of CO2 on Ni vs Mg (Table2, S7,S8). Ni would promote the formation of “CO2
vacancies” in this carbonate layer and enable the activation of H2 molecules leading to their catalytic
dissociation. Indeed, to conduct the rWGS reaction, it is crucial to adsorb and dissociate the H2
molecules, a process which is much more favorable on Ni atoms as part of the MgO instead of pure
MgO. That means Ni changes the surface termination and population of corners and steps of the
entire catalysts since coordinatively unsaturated sites are thermodynamically favored.24 Besides this
structural impact, Ni has a direct influence on the reactants, increasing the activation of H2 and
decreasing the stability of activated CO2 accesible for its catalytic conversion.
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Particle formation. While the isolated Ni atoms were active for the activation of CO2, their selectivity
was restricted to CO, and the formation of Ni clusters (even of a few nm) was necessary to form
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higher value-added products such as CH4 or even MeOH. Indeed, the single atoms are only able to
take part in the 2e- redox cycle while, to perform the full hydrogenation of CO2 (the 8e- redox cycle),
Ni clusters are crucial. Furthermore, the product formation rates increased by a factor of three at the
same temperatures after particle formation occurred (Fig. 4).
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Conclusions
Through the synthesis of phase pure precursors and catalysts, we prepared for the first time Ni single
atom catalysts using wet chemistry. Ni concentrations of up to 10 atom-% were achieved and
identified as upper limit for the function of single atom catalyst. The anchoring of the Ni and its
stabilization was achieved by a solid solution approach. A Ni surface enrichment of the solid solutions
is observed, by both physical and chemical analysis methods, in agreement with the prediction made
from first principle calculations24. This enrichment has somehow its restrictions, as samples
containing 10 atom-% Ni or more show a saturation phenomenon. These Ni single atom catalysts are
active for CO2 activation in the reverse-water-gas-shift reaction. A stable performance over time,
which evolved linearly with the amount of Ni on the surface, is observed. We exclude that the
dominant effect of this reactivity is a bulk effect. Further, we show that the main contribution is
originated from the amount of Ni atoms on the surface of the catalysts. During catalysis a carbonate-
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based over-layer was formed and its decomposition appears to be coupled to the Ni agglomeration.
Finally, isolated Ni atoms were not able to hydrogenate CO2 to higher value added products such as
CH4 or MeOH, for which Ni clusters are necessary. The simplification of the size of active sites
towards single atoms seems to have limitations, since the ability to catalyze a multi electron reaction
is attributed to at least Ni clusters. The selectivity change towards CH4 and the progressive increase
of the reaction rates (CO and CH4) for the samples with Ni clusters give insights into the different
reactivity regimes occurring before and after the particle formation. The reactant induced surface
over-layer mechanism for the Ni single atom catalysts is consecutively substituted by a formate
based one, in which the continuously growing Ni cluster/particles decrease the formate stability. This
leads to higher reaction rates and higher CH4 selectivities. Within this NixMg1-xO solid solution study
we bridge for the first time the Ni single atom to Ni cluster regime as example of the limited
applicability of SAC as high performance catalysts. Besides, this fundamental contribution to the
mechanistic understanding in the development of catalytic selectivity and activity serves as new
approach adaptable to other reactions and systems forming solid solutions.
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Supporting Information
Please find additional information on the synthesis protocol, details of all powder X-ray diffraction
measurements, electron microscopy, microcalorimetry, details of the catalytic testing, details of the
thermal analysis, IR-spectroscopy and the theoretical calculations in the SI.
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Acknowledgments
The authors thank J. Plagemann and M. Hashagen for BET analysis, Dr. O. Timpe for XRF analysis, and
J. Allan for TGMS measurements. Marie-Mathilde Millet thanks Unicat for financial support, “FKZ:
EXC 314/1 UNICAT”. Calculations of CO2 and H2 activation at surfaces were supported by the grant
from the Ministry of Education and Science of the Russian Federation (Grant No. 14.Y26.31.0005).
Analysis of the effects of doping on surface chemical properties was supported by the Ministry of
Education and Science of the Russian Federation in the framework of Increase Competitiveness
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Program of NUST ”MISIS” (No. K2-2017-080) implemented by a governmental decree dated 16 March
2013, No.211.
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Graphical abstract
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