Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal-Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

Article abstract of DOI:10.1021/jacs.8b03809

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH₄ → CH₃OH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO₂(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO₂, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.

Full text of DOI:10.1021/jacs.8b03809

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Article
Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal-
Support Interactions and Water-enabled Catalytic Conversion by Site Blocking
Pablo G. Lustemberg, Robert M. Palomino, Ramon A Gutierrez, David C.
Grinter, Mykhailo Vorokhta, Zongyuan Liu, Pedro J Ramirez, Vladimír Matolín, M.
Veronica Ganduglia-Pirovano, Sanjaya D. Senanayake, and Jose A. Rodriguez
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03809 • Publication Date (Web): 28 May 2018
Downloaded from http://pubs.acs.org on May 28, 2018
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Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces:
Metal-Support Interactions and Water-enabled Catalytic
Conversion by Site Blocking
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[a]
[b]
[c]
[d]
Pablo G. Lustemberg, Robert M. Palomino, Ramón A. Gutiérrez, David C. Grinter,
[
e]
[b]
[c]
[e]
Mykhailo Vorokhta, Zongyuan Liu, Pedro J. Ramírez, Vladimír Matolín, M. Verónica
[
f]
[b]
[b]
Ganduglia-Pirovano,* Sanjaya D. Senanayake,* and José A. Rodriguez*
a
Instituto de Fisica Rosario (IFIR), CONICET-UNR, Bv. 27 de Febrero 210bis, S2000EZP Rosario, Santa
Fe, (Argentina)
b
c
d
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973 (United States)
Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A (Venezuela)
Surfaces and Interfaces, Diamond Light Source, Harwell Science and Innovation Campus, Didcot
Oxfordshire OX11 0DE (United Kingdom)
e
Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University,
Prague, (Czech Republic)
f
Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Madrid (Spain)
*
Corresponding authors contact: vgp@icp.csic.es ; ssenanay@bnl.gov ; rodrigez@bnl.gov
Abstract: The transformation of methane into methanol or higher alcohols at moderate
temperature and pressure conditions is of great environmental interest and remains a challenge
despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH
CH OH conversion. This experimental and computational study provides clear evidence that low
Ni loadings on a CeO (111) support can perform a direct catalytic cycle for the generation of
4

3
2
methanol at low temperature using oxygen and water as reactants, with a higher selectivity than
ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission
spectroscopy and density functional theory calculations, we demonstrate that water plays a
crucial role in blocking catalyst sites where methyl species could fully decompose, an essential
2
factor for diminishing the production of CO and CO , and in generating sites on which methoxy
species and ultimately methanol can form. In addition to water-site blocking, one needs the
effects of metal-support interactions to bind and activate methane and water. These findings
should be considered when designing metal/oxide catalysts for converting methane to value-
added chemicals and fuels.
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Introduction
Besides being an excellent source of energy, natural gas is a cheap and abundant source
of carbon for the chemical industry. Methane is the primary component of natural gas and a
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potent greenhouse gas but it is difficult to convert it to upgraded fuels or chemicals due to the
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strength of the CH bonds in the molecule (104 kcal/mol). A synthetic path for a direct CH →
4
CH
3
OH transformation is a major goal in the industrial use of methane since the alcohol is useful
1
as a fuel and a good building block for the generation of many chemical goods. The reaction of
2
molecular oxygen with methane can follow two different reaction paths:
CH
4
4
+ ½ O
2
2
→ CH
3
OH
(1)
(2)
CH
+ ½ O
→ CO + 3H
2
From a thermodynamic viewpoint, both reaction paths are exothermic, but at temperatures higher
2
than 600 K, path (2) is more favourable. Thus, to generate CH
3
OH as a stable product or
at temperatures
intermediate, one must find catalytic materials which can bind and activate CH
4
2
-6
below 500 K.
In recent years, the direct catalytic conversion of methane to methanol has sparked
3
-15
considerable interest.
The direct conversion is performed by the enzyme methane
monooxygenase, but it is very difficult to use this biological system for major industrial-scale
7,10
processes.
4 3
A core of three copper ions is probably responsible for the CH → CH OH
4
transformation in the enzyme. Copper-exchanged zeolites are able to mimic the activity of the
3-6,12
methane monooxygenase.
sequential dosing of O and CH
catalytic CH → CH OH conversion is also possible inside copper-exchanged zeolites or on a
Inside the structure of zeolites, CH
3
OH can be generated by
3
-6
2
4
, and at the end the alcohol is flushed out with H
2
O.
A
12
4
3
1
4
2 x
model inverse oxide/metal catalyst such as CeO /CuO /Cu(111). In this study, we focus our
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attention on the performance of a catalyst with a conventional Ni/CeO configuration that can be
used in technical applications. Using a combination of experiment (ambient pressure X-ray
photoelectron spectroscopy) and theory (density functional calculations), we show that a low-
loaded Ni/CeO (111) system activates CH at 300 K and then, with the help of water, which
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prevents sequential cleavage of CH bonds, performs a direct catalytic cycle for the generation
of CH OH at relatively low temperatures (450 K). These results show the impact that metal-
support interactions have on CH and OH bond breaking and the beneficial effect of water to
diminish the production of CO and CO during methane conversion to methanol. The
3
2
mechanisms and outcomes demonstrated here are valid also for the production of other alcohols.
Methods
Experimental methods. The experiments examining the conversion of methane into
2
methanol over Ni/CeO (111) surfaces were performed in an apparatus which contains an ultra-
high vacuum (UHV) chamber for surface characterization and a batch micro-reactor for catalytic
1
6-18
studies.
The UHV chamber was set up with X-ray photoelectron spectroscopy (XPS), Auger
electron spectroscopy (AES), ion-scattering spectroscopy (ISS), low-energy electron diffraction
16-18
(
LEED), and temperature programmed desorption (TPD).
The Ni/CeO
2
(111) surfaces were
1
6-18
prepared following the methodology described in detail in refs.
In the experiments of CH
4
activation, the sample was transferred in vacuo to the micro-reactor at room temperature then the
hydrocarbon (1 Torr) was introduced. In the studies investigating the activity of the
Ni/CeO
2
(111) catalysts for the conversion of methane to methanol, the samples were exposed to
, 0.5 Torr of O and 0-4 Torr of H O at room temperature and were
a mixture of 1 Torr of CH
4
2
2
2
rapidly heated to a reaction temperature of 450 K. Product yields (methanol, CO or CO ) were
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determined by mass spectroscopy or gas chromatography. In these studies, data were collected
in a systematic way at intervals of 5 min. The amount of molecules generated in the catalytic
studies was normalized by the total reaction time and the active area exposed by the sample. The
kinetic tests were always done in the limit of low conversion (< 10%).
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The ambient-pressure XPS experiments for the Ni/CeO (111) catalysts were performed at
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the Advanced Light Source in Berkeley, CA (beamline 9.3.2). The XPS spectra were recorded
using a VG Scienta R4000 HiPP analyzer. A photon energy of 650 eV was used to excited the O
1s region. The C 1s, Ni 3p, and Ce 4d regions were probed with a photon energy of 490 eV. The
total energy resolution in the photoemission experiments was ~0.2 eV. For the binding energy
calibration, we used the Ce 4d photoemission lines and their 122.8 eV satellite features.
Theoretical models and computational methods. We performed spin-polarized density
1
9
functional theory (DFT) calculations with the DFT+U procedure (an effective onsite Coulomb
interaction parameter, U, was applied to treat the Ce 4f states) and the Perdew, Burke, and
2
0
Ernzerhof (PBE) exchange-correlation functional (GGA). The Vienna ab initio simulation
2
1
22,23
package (VASP) was employed in this study. A value of 4.5 eV
was used for the Hubbard
24,25
U-like term. The projector augmented wave method (PAW)
with a plane-wave cutoff of 415
eV was used. The C (2s, 2p), O (2s, 2p), Ni (3p, 3d, 4s), and Ce (4f, 5s, 5p, 5d, 6s) electrons
were treated as valence states. The supported flat Ni cluster on CeO (111) surface was modeled
by a (33) surface unit cell, with calculated ceria bulk equilibrium lattice constant (CeO : 5.485
4
2
2
Å). The ceria surface was modeled employing a nine atomic layers (three O-Ce-O trilayers) slab
geometry with at least 12 Å vacuum space. A Monkhorst-Pack grid with (331) was used. The
atoms in the bottom trilayer were kept fixed at their optimized bulk-truncated positions during
2 2 4
geometry optimization, whereas the others were allowed to relax. For the O , H O, CH gas-
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phase species, -point calculations were performed using a (121212) Å box. The climbing
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image nudged elastic band method (CI-NEB) was employed in order to locate the TS
structures. For each reaction pathway, NEBs contained nine images. only one imaginary
frequency was found for each of the TS structures reported in this work.
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Results and discussion
Figure 1 displays C 1s XPS spectra acquired after exposing clean Ni(111), CeO
2
(111)
and a CeO (111) surface with ~ 0.15 ML of nickel to 1 Torr of methane at 300 K. Ni(111) is
2
frequently used as a benchmark in experimental and theoretical studies for the activation of
27-29
methane on metal surfaces.
On the Ni(111) surface, the C 1s XPS spectrum points to a
negligible dissociation of methane in agreement with previous studies which show almost no
interaction of the molecule with metal surfaces of pure nickel {(100), (110) and (111)
27-29
terminations} at room temperature.
methane is also negligible. The deposition of small amounts of Ni on a CeO
produces a partial reduction of the oxide surface and adsorbed NiO species (Figure S1 in
On the pristine CeO
2
(111) sample, the adsorption of
2
(111) film at 300 K
x
1
6,17
Supporting Information).
In these systems, individual atoms and tiny clusters (2-10 atoms) of
16-18
nickel are in close contact with the ceria support.
The electronic perturbations induced by
ceria on the nickel have a strong influence on the reactivity of the system towards methane. The
top panel in Figure 1 shows a C 1s XPS spectrum recorded after exposing a CeO (111) surface
containing 0.15 ML of nickel to 1 Torr of CH at room temperature for 5 minutes. Over the
2
4
Ni/CeO
CO groups.
leads to full dissociation: CH
the dissociation from CH
2
(111) surface, methane is activated and decomposes. Clear peaks are seen for CH
x
and
A significant portion of the reacting methane follows a reaction path which
→ CH → CH → CH → C. The calculated reaction profile for
1
8,30
x
4
3
2
4
4 2
up to CH is shown in Figure 2 over a flat Ni /CeO (111) model
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catalysts where Ni atoms are oxidized (Ni ) and the support is partially reduced (cf. Table S1
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and Figure S2), i.e., the computational model matches the essential features of the experimental
model catalysts, and there is a clear similarity of the clusters with fragments of Ni(111) terraces.
Previous theoretical studies have shown that the binding energy of methane on Ni(111) is
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negligible (<0.1 eV) with a large activation barrier (0.81 eV) for dissociation).
In contrast,
methane reacts with the NiO species supported on ceria via the formation of a CH
x
3
fragment and
a H bound to the Ni cluster with a very low activation barrier (0.04 eV). A subsequent
dissociation of the methoxy species produces directly CH and 2 NiH species with a barrier of
0.59 eV, which is surmounted at experimental conditions. Further dissociation produces C
18
adatoms that react with O atoms from the surface to yield CO
x
groups. Less than a monolayer
of methane reacted with the Ni/CeO
2
(111) system at 300 K and there were no significant
changes in the oxidation state of Ni and Ce.
CH4 1 Torr, 300 K
CH
x
+
CH4
0
.15 ML Ni / 1 torr H O
2
CO
CH
x
x
+
CH4
0
.15 ML Ni
Ce 4s
+
CH4
No nickel
294
292
290
288
286
284
282
280
C 1s XPS
CH4 1 Torr, 300 K
H O/Ni(111)
x3
2
clean Ni(111)
x3
280
2
94
292
290
288
286
284
282
Binding Energy (eV)
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Figure 1. C 1s XPS Spectra collected after exposing Ni(111), bottom panel, CeO
2
(111) and Ni/CeO
O/Ni/CeO (111) samples,
the corresponding surfaces were exposed to 1 Torr of water at 300 K before dosing methane.
2
(111), top
panel, to 1 Torr of methane at 300 K for 5 minutes. To generate the H
2
O/Ni(111) and H
2
2
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Figure 2. Energy profile for the CH  CH +H CH+3H reaction on Ni /CeO (111). Energies are referenced to
4
3
4
2
the total energy of CH (g) and the Ni /CeO (111) surface. The structures correspond to the side views of the
4
4
2
3
+
4+
4
molecularly adsorbed CH and dissociated states. Atom color key: Ni (blue), Ce (gray), Ce (white),
surface/subsurface oxygen atoms (red/green), oxygen atoms from chemisorbed species (red), hydrogen (orange), and
carbon atoms (black).
32
Ni(111) interacts poorly with water. After exposing a Ni(111) surface to 1 Torr of water
at 300 K, we found a very small amount (< 0.1 ML) of oxygen-containing species on the surface,
3
2
probably produced by the decomposition of the molecule on defect sites. Pre-dosing with water
did not enhance the interaction of methane with Ni(111), bottom panel in Fig 1. On a
Ni
S3). Experiments of ambient-pressure XPS indicate that H
Ni/CeO (111) surfaces at 300 K when these systems are in equilibrium with a pressure of
gaseous water. However, at higher temperatures (> 400 K), only a small amount of OH groups
4
/CeO
2
(111) substrate, water dissociation, H
2
O → OH + H, is a downhill process (see Figure
2
O and OH species are coadsorbed on
2
3
3
3
3
(< 0.3 monolayer) is bonded to the Ni/CeO
2
(111) surfaces.
2
The ability of Ni/CeO (111) to
3
3
dissociate water can be used to control the reaction selectivity after the activation of CH
4
.
When H
2
O is pre-adsorbed on Ni/CeO
2
(111), the selectivity towards the production of CH
x
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groups increases to 100% (top spectrum in Figure 1). OH groups probably block surface sites
that are highly active for the complete decomposition of methane, formation of Ni(OH) is seen
2
in XPS (Figure S1), and the signals for C and CO
see below, this phenomenon is essential to push the conversion of methane into methanol.
The CH and OH groups coadsorbed on Ni/CeO (111) at 300 K do not interact to produce
x
groups vanish from the C 1s data. As we will
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x
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CH
3
OH. At this temperature, the system cannot overcome the activation barriers associated with
Csurface bonds. At a temperature of 450 K, the
the breaking of HOsurface and H
x
Ni/CeO (111) surface is able to catalyze reactions (1) and (2) but a significant production of
2
methanol was seen only in the presence of water. Figure 3 compares the rates for the reactions on
Ni(111) and Ni/CeO
feed. Ni(111) displays a very low activity for the combustion of methane to CO/CO
not detect the formation of any methanol on this catalyst. No activity for reactions (1) and (2)
was found on plain CeO (111) under the reaction conditions used for the experiments in Figure
. In the case of 0.15 ML of Ni on CeO (111), we found a very good active and stable catalyst
for the combustion of methane with the formation of some methanol. Without water in the
reaction feed, the production of methanol is very small and CO/CO and H are the main reaction
products. The highly active surface sites are not covered by OH species and thus the CH + nO
OH formation. Since both reaction pathways are
2
(111) surfaces as a function of time with a mixture of CH
4
2
and O in the
2
and we did
2
3
2
2
2
4
2
→
CO/CO
2
+ H
2
reaction occurs instead of CH
3
2
possible from a thermodynamic viewpoint, most of the CH OH generated may be decomposed
3
3-6
into CO/CO
2
.
However, in a water rich environment the catalytic production of methanol
O:CH , the selectivity towards
ratio did not improve the selectivity for
increases significantly as shown in Figure 4. At a 4:1 ratio of H
2
4
methanol formation was ~ 35%. A higher H O:CH
2
4
methanol formation and substantially decreased the conversion of methane. OH groups formed
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33
by the dissociation of water on the Ni/CeO
2
(111) surface probably blocked sites necessary for
the bonding and activation of methane. The best selectivity towards methanol production
observed in this study is larger than the best selectivity reported in the literature for ceria- and
34
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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0
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8
9
0
zirconia-based powder catalysts which contain NiO as an active phase.
1.2
1.0
0.8
0.6
Ni/CeO (111)
2
0.4
0
.2
.0
Ni(111)
80
x2
0
0
20
40
60
100
Time (minutes)
0
0
.10
.05
Ni/CeO (111)
2
0.00
0
20
40
60
80
100
Time (minutes)
Figure 3. Production of CO/CO
2
, top panel, and methanol, bottom panel, on Ni(111) and
Ni/CeO (111) catalysts as a function of time. The Ni coverage on ceria was ~ 0.15 ML. The
2
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2
2
2
2
2
2
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3
3
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4
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6
sample was exposed to 1 Torr of CH
detected over the Ni(111) catalyst.
4 2
, and 0.5 Torr of O at 450 K. No methanol production was
0
1
2
3
4
5
6
7
8
9
0
1
2
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0
1
2
3
4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Production of methanol on a Ni/CeO
ceria was ~ 0.15 ML. The sample was exposed to 1 Torr of CH , 0.5 Torr of O and 0, 1 or 4 Torr of H O at 450 K.
2
(111) catalyst as a function of water pressure. The Ni coverage on
4
2
2
The results in Figure 3 point to strong effects of metal-support interactions on the catalytic
process. Formation of methanol was seen only when a nickel-ceria interface existed and the
Ni/CeO
2
(111) catalyst was much more active for the conversion of methane than Ni(111). A plot
(111) as a function of nickel
of the catalytic activity for the production of methanol on Ni/CeO
2
coverage, Figure 5, also reflects the effects of a strong metal-support interaction. For small
coverages of Ni on ceria (< 0.2 ML), results of valence and core-level photoemission show that
3
5
the oxide induces strong electronic perturbations on nickel. These electronic perturbations
decrease at higher Ni coverages and the catalytic activity for the production of CO/CO and
CH OH drops. The catalysts with nickel coverages below 0.3 ML did not show any signs for
2
3
deactivation after performing the reaction for 100 minutes maintaining their activity and
selectivity; they did survive several catalytic cycles at a temperature of 450 K.
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1
Torr CH , 0.5 Torr of O ,
0
0
0
0
0
.8
.6
.4
.2
.0
4
2
4
Torr H O, 450 K
2
Ni/CeO (111)
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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0
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2
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9
0
1
2
3
4
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8
9
0
CO/CO2
CH OH
3
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Ni coverage (ML)
Figure 5. Production of CO/CO
samples were exposed to 1 Torr of CH
2
and CH
3
OH on Ni/CeO2(111) catalysts as a function of nickel coverage. The
and 4 Torr of H O at 450 K.
4
, 0.5 Torr of O
2
2
Using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) we investigated the
surface chemistry associated with the activation and conversion of methane on Ni/CeO (111).
Figure 6 shows C 1s XPS spectra collected while exposing a low-loaded Ni/CeO (111) catalyst
to a mixture of CH /O with and without water. As the pressure of water in the background
increases, there is an increase in the signal for adsorbed CH groups that can be transformed into
methanol with the amount of surface CO present always being small. In the C 1s region we did
not see any signal that could be attributed to adsorbed CH O or CH OH. Thus, at 450 K, the CH
CH O → CH OH conversion probably occurs very fast when water is present in the reaction
2
2
4
2
x
x
3
3
3
→
3
3
mixture. In principle, the oxygen atom present in methanol could be coming from adsorbed O
atoms or OH groups which are present on the surface (Figure S4).
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
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0
1
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9
0
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0
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. C 1s XPS spectra acquired while exposing a CeO
2
(111) surface pre-covered with 0.15 ML of nickel to a
reaction mixture of methane (25 mTorr) and O (13 mTorr) without and with water (0, 25, 50, 100 m Torr). The
2
temperature of the sample was 450 K.
Figure 7 displays Ce 4d and Ni 3p XPS spectra collected together with the experiments
17
2
shown in Figure 6. In Figure 7A, one sees the characteristic line-shape and features for CeO .
We found that the hydrogen produced by the dissociation of pure methane at high temperature
34
could reduce the ceria substrate, but in the presence of oxidant agents (O
2
, H
2
O), a reduction
3
+
process is impossible. This is good because Ce sites present in ceria could decompose the
3
6
formed methanol. In Figure 7B, the Ni 3p features exhibit a line-shape that could be a product
2
+
of the existence of Ni species, such as Ni(OH)
at room temperature (Figure S1) produces Ni(OH)
under a reaction mixture that also contains CH and O
hydroxide decreases. A large fraction of the OH groups seen in AP-XPS (Figure S) is probably
2
and NiO. The dosing of H
as a dominant species, but at 473 K and
, the relative concentration of the nickel
2 2
O to Ni/CeO (111)
2
4
2
4 2
bound to ceria sites. In the catalysts, Ni could dissociate CH , water and O (which, for our
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Ni
4
/CeO
2
(111) theoretical model catalyst, was also considered, see Figs.2, S3 and S5), and ceria
3
+
probably dissociates O and water at Ce sites produced by the Ni deposition. The material was
2
2
active and stable as a catalyst for the production of methanol only when methane, O and water
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
were present in the feed. A feed with only methane and O produced a negligible amount of
2
methanol. For a feed with methane and water, we found a slow reduction of the system and
eventual deactivation by carbon deposition. In the absence of O
2
, the oxidant capacity of water
was not able to balance the reducing power of methane.
2
Figure 7. Ce 4d and Ni 3p XPS spectra acquired while exposing a CeO (111) surface pre-covered with 0.15 ML of
nickel to a reaction mixture of methane (25 mTorr) and O (13 mTorr) without and with water (0, 25, 50, 100 m
Torr). The temperature of the sample was 450 K.
2
To obtain microscopic insight into possible reaction paths and the special role of water into
the behaviour of the low-loaded Ni-CeO
2
(111) system for the production of methanol from
methane and oxygen, we studied the process by using calculations based on a spin-polarized
2
density functional theory (DFT+U) approach. . We modeled the experimental Ni-CeO system
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16
using small flat Ni
4
clusters on a CeO
2
(111) surface. These Ni
4
species partially reduce the
3
+
support, with the formation of two Ce , i.e., the Ni atoms in direct contact with the reducible
ceria support are partially oxidized (cf. Table S1).
0
1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. Energy profile for the CH
2
4 3 4 2
to CH OH reaction over an O/Ni /CeO (111) surface under the presence of
H O. Energies are referenced to the total energy of CH (g), 3H O (g) and the Ni /CeO (111) surface with
chemisorbed oxygen. The side views of the optimized structures are included. Atom color key: Ni (blue), Ce
gray), Ce (white), surface/subsurface lattice oxygen atoms (red/green), oxygen atoms from chemisorbed species
red), hydrogen (orange), and carbon atoms (black). The principal reaction path is depicted in blue, whereas
4
2
4
2
3+
4+
(
(
alternative routes are shown in black, gray, red and light red, as well as light blue.
As mentioned above, molecular oxygen readily dissociates to chemisorbed atomic
species on the ceria-supported Ni
4
species (cf. Figure S5). Thus, we considered a partially
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oxygen covered Ni
4 2 4 3
/CeO (111) system to calculate the CH to CH OH reaction profile (Figure
8
). The molecular binding of CH to the oxidized Ni particles is not that strong (0.14 eV;
4
4
structure II) with an activation barrier for the first hydrogen abstraction of 0.54 eV (TS
1
, Figure
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
). This is somewhat larger than that for the Ni /CeO (111) system without chemisorbed oxygen,
4
2
but is comparable to that for the further decomposition of the methyl species on that system (cf.
Figure 2). After the activation of methane on the O/Ni /CeO (111) surface, methoxy species
could form, but the activation barrier would be large, 1.16 eV (TS , Figure 8). Instead, the
4
2
a1
methyl species would dissociate (CH
3
2
 CH + H) with a smaller barrier of 1.01 eV (TSa2,
Figure 8).
The addition of water, however, changes the reaction path.. The adsorbed water molecule
(structure IV) dissociates by overcoming a relatively low-energy activation barrier (TS = 0.31
eV, Figure 8), producing a surface hydroxyl group (O H) on the ceria support (accompanied by
2
s
3
+
the formation of an additional Ce ), whereas the water OH group recombines with the hydrogen
previously abstracted from methane, forming a new adsorbed water molecule (structure V). Once
more, methoxy species could form, but the activation barrier would also be large, 1.27 eV (TSb1
,
,
Figure 8). Instead, the methyl species would dissociate with a smaller barrier of 0.76 eV (TSb2
Figure 8). Thus, more water is added to the system (structure VI). At this point, there is enough
chemisorbed water to block all Ni sites necessary for methyl species to dissociate relatively
easily (cf. Figure S6) and thus a methoxy species forms on nickel (structure VII) via a 1.01 eV
3
transition state (TS , Figure 8).
Further dissociation of a chemisorbed water species stabilizes the system yielding
chemisorbed H and OH species on the nickel cluster (structure VII). The activation energy for
the formation of CH
3
3
OH from the chemisorbed OCH and H species is calculated to be 1.15 eV
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(
TS
c
, Figure 8), comparable with the methoxy formation barrier. However, if additional water is
adsorbed (structure IX), it could donate an H to the methoxy species and thus form CH OH
3
without barrier (structure X). The desorption of methanol occurs via a 0.69 eV transition state
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
TS , Figure 8) which is accompanied by the adsorption of a OH species at the catalyst site from
5
3
which CH OH has desorbed.
In the present study, we used ambient pressure X-ray photoelectron spectroscopy and
performed DFT+U calculations to elucidate the reaction mechanism for the direct CH4
conversion to CH
3 2
OH at the surface of a low-loaded Ni-CeO catalyst. This system exhibits
features not seen in catalysts able to perform a CH
4
 CH
3
OH transformation such as copper-
6
,12
14
exchanged zeolites
or a model inverse oxide/metal catalyst like CeO
2
/CuO
x
/Cu(111). On
one hand, we show that the reducible ceria support modifies the electronic properties of the
nickel in direct contact with it, changing its oxidation state while becoming oxidized. This, in
turn, results in chemical properties which are very different from those of the corresponding
larger three-dimensional nanoparticles or extended nickel surfaces. These metal-support
interactions play an essential role in the easy cleavage of OH and CH bonds, which is crucial
for the dissociation of H
conversion to CH OH with O
Ni nanoparticles, the presence of Ni adatoms, which are oxidized (Ni ), is not detrimental to the
2
O as well as the activation of CH
4 4
which is needed in the CH
3
2
present in the feed. Note that contrary to large three-dimensional
2
+
1
7,33
activity since OH and CH bonds can be easily cleaved over them.
The metal-support
interactions seen in our catalysts lead to a much better selectivity towards methanol production
34
than that observed in other types of ceria- and zirconia-based catalysts. On the other hand, we
show that water in excess is absolutely necessary for avoiding the complete decomposition of
2
methane on the partially oxidized Ni nanoparticles, which would yield CO and/or CO . The role
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of water is twofold. First, it blocks Ni active sites by dissociating over them, yielding
chemisorbed OH and H species. As a consequence, CH would rather form OCH species with
3
3
existing chemisorbed O species than further decompose. Second, water also helps the formation
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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2
3
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0
1
2
3
4
5
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8
9
0
and desorption of CH OH. Water provides a H atom and a OH species for the
3
NiOCH
3
3 2
+NiHNiOH + Ni + CH OH (g) reaction to occur. On the Ni/CeO surfaces, one
sees a reaction mechanism for the direct CH
4
 CH
3
OH conversion not seen over copper-
6
,12
14
2 x
exchanged zeolites or model inverse oxide/metal catalyst like CeO /CuO /Cu(111).
Conclusion
Metal-support interactions and water site-blocking play a crucial role in the conversion of
methane to methanol on Ni/CeO catalysts. The metal-support interactions make possible the
2
binding and activation of water and methane at low temperature while the water site-blocking
tunes the surface reactivity and enhances the selectivity towards methanol formation. A survey of
the literature shows that water also plays an essential role in the step-wise formation of methanol
3
,6
from methane on copper-exchanged zeolites and also for the direct catalytic conversion over a
1
4
model inverse CeO /CuO /Cu(111) catalyst but the interplay between CH species, O adatoms
2
x
3
and water molecules is different on a Ni/ceria catalyst. Although water-site blocking is
important, one still needs the effects of metal-support interactions to bind and activate methane
and water. We believe that the conceptual ideas presented here can be quite useful for the design
of other conventional metal/oxide catalysts which can achieve the transformation of methane to
methanol.
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Acknowledgements
The work carried out at Brookhaven National Laboratory was supported by the US Department
of Energy (Chemical Sciences Division, DE-SC0012704). S.D.S. is supported by a US
Department of Energy Early Career Award. This research used resources of the Advanced Light
Source (Beamline 9.3.2), which is a DOE Office of Science User Facility under contract no. DE-
AC02-05CH11231. Authors acknowledge contribution of Dr. Ethan Crumlin for assistance with
AP-XPS measurements. M.V.G.P. acknowledges the financial support of the Ministry of
Economy and Competitiveness MINECO-Spain (grant Nr. CTQ2015-78823-R) and P.G.L. that
of the Agencia Nacional de Promoción Científica y Tecnológica-Argentina (grant Nr. PICT-
0
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0
2
016-2750). Computer time provided by the BIFI-ZCAM, RES at the Marenostrum and LaPalma
nodes, SNCAD (Sistema Nacional de Computación de Alto Desempeño, Argentina), and the
DECI resources BEM based in Poland at WCSS and Archer at EPCC with support from the
PRACE aislb, is acknowledged. M.V. thanks the Ministry of Education, Youth and Sports of the
Czech Republic for financial support under project LH15277.
Additional information
Supplementary Information accompanies this paper at xxx, and it includes XPS spectra,
structural Ni /CeO (111) model and Bader charges as well as reaction profiles calculated with
4
2
DFT. Geometries of all the stationary points and transition state structures whose relative
energies are given in the manuscript, along with their calculated absolute energies and number of
imaginary frequencies.
Competing financial interests: The authors declare no competing financial interests.
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