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Unraveling the structure sensitivity in methanol conversion on
CeO2: A DFT+U study
Marçal Capdevila-Cortada,a,* Max García-Melchor,a,b Núria Lópeza,*
(a) Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans, 16, 43007 Tarragona,
Spain; (b) SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator
Laboratory, Menlo Park, CA 94025, USA.

E-mail: mcapdevila@iciq.es; nlopez@iciq.es

Keywords: CeO2; structure sensitivity; methanol oxidation; formaldehyde oxidation;
dehydrogenation; H2 formation; DFT; reaction mechanisms.
Highlights:
- Structure sensitivity correlates with the surface oxygen content in the methanol
decomposition on ceria.
- Methanol selectively converts to formaldehyde on ceria (111) and (110) facets.
- The CeO2(100) facet traps formaldehyde and ultimately enables its oxidation to
produce CO+H2.
- Hydrogen evolution is permitted on (100) due to its ability to stabilize a hydrogen
precursor.
- Formaldehyde can easily exchange its oxygen with the lattice in all facets.

1	
  
	
  

ABSTRACT
Methanol decomposes on oxides, in particular CeO2, producing either formaldehyde or
CO as main products. This reaction presents structure sensitivity to the point that the
major product obtained depends on the facet exposed in the ceria nanostructures. Our
Density Functional Theory (DFT) calculations illustrate how the control of the surface
facet and its inherent stoichiometry determine the sole formation of formaldehyde on
the closed surfaces or the more degraded byproducts on the open facets (CO and
hydrogen). In addition, we found that the regular (100) termination is the only one that
allows hydrogen evolution via a hydride-hydroxyl precursor. The fundamental insights
presented for the differential catalytic reactivity of the different facets agree with the
structure sensitivity found for ceria catalysts in several reactions and provide a better
understanding on the need of shape control in selective processes.

2	
  
	
  

1. Introduction
Cerium oxide, CeO2, is a versatile and economically attractive oxide used in solid
oxide fuel cells [1], biomedicine [2], and catalysis [3]. Either as a catalyst or as a
support, it is widely used in a vast number of oxygen-related reactions such as the
water-gas shift (WGS) [4,5], the preferential oxidation of CO [6,7], SO2 oxidation [8,9],
and the HCl conversion to Cl2 [10]. All these applications are due to the particular redox
character of ceria that permits to cycle between the Ce4+ and Ce3+ states, which
involves oxygen vacancy formation [11] and diffusion [10,12]. The oxygen storage
capacity (OSC) of CeO2 varies with the exposing surface [13], and hence, in
nanoparticles it ultimately depends on their morphology.
Recently, ceria has been also shown to present a high activity and selectivity in
hydrogenation processes [10,14]. More surprisingly, the oxidation and hydrogenation
capacities of different nanostructured CeO2 materials have been found to present a
markedly structure sensitivity [15]. Thus, while hydrogenation takes place preferentially
on (111) facet orientations, oxidation reactions are favored on the more open (110) and
(100) facets exposed in rods and cubes, respectively. Ceria nanoparticles have also
been found to exhibit different activity in soot combustion depending on the exposed
facet [16].
Among oxidation processes, the conversion of methanol to formaldehyde is
considered an ideal test for characterizing the catalytic behavior of metal oxides
[17,18]. The two main byproducts of this reaction are water and hydrogen. Therefore,
the catalytic activity and selectivity of a given material may be a result of the interplay
between its oxidation and hydrogenation abilities.
In industry, methanol is oxidized to produce formaldehyde through the so-called
Formox process [19]. There, methanol and oxygen react at temperatures around 525 –
675 K in the presence of iron oxide mixed with molybdenum or vanadium oxide
according to equations:
CH3OH + ½ O2 → HCHO + H2O

(1)

CH3OH → HCHO + H2

(2)

At higher temperatures, 875 K, silver can also act as catalyst, probably in the form of
a suboxide [20–22]. Hence, the oxidation of methanol to formaldehyde has been
extensively studied on metal oxides, especially on titanium and vanadium oxides [23–
28]. In the case of ceria, most of the investigations have been focused on the (111)
facet [29–33]. On this stoichiometric surface, methanol converts into formaldehyde, at
ca. 550 K, via a dehydrogenation process that involves surface oxygens followed by
3	
  
	
  

the subsequent reduction of two Ce4+ ions per methanol molecule [29]. The presence
of oxygen vacancies was recently reported to have a detrimental effect on the reaction
selectivity as a result of the competition between formaldehyde desorption and its
further oxidation to CO [34]. In the same work, the (100) surface was also shown to
have a reduced selectivity toward formaldehyde. Both HCHO and CO were found to be
the main products on CeO2(100), while on the reduced CeOx(100) almost all methanol
was converted to CO [34]. Temperature-programmed desorption (TPD) experiments on
nanocrystals with different nanoshapes, namely octahedra, cubes and rods, showed
that the latter is the most reactive shape (but non-selective to formaldehyde) one,
evolving CO and H2 at T < 583 K [35]. On the other hand, Zhou and Mullins reported
that formaldehyde does not decompose on stoichiometric CeO2(111), but does evolve
to CO and H2 on the same reduced surface at temperatures above 500 K [36]. Overall,
ceria appears to be very sensitive to the particular choice of active surfaces.
Despite all the above and other theoretical works reported in the literature [37–42],
up to date there is no complete study assessing the selectivity of methanol
dehydrogenation on the most representative ceria facets and proving their different
behavior as experimentally observed. Herein, we present a thorough mechanistic study
on the selective conversion of methanol to formaldehyde and its subsequent
conversion to CO. To account for the particular morphology of the three common
nanoshapes above, we have studied the three lowest index and energy surfaces (111),
(110), and (100) (Fig. 1), as these are the most exposed facets in each nanoshape,
respectively. The desorption of HCHO, CO, and H2 from each surface is also
discussed, and compared to the reported TPD results on the stoichiometric ceria
surfaces [29,33,34]. Overall, this work provides the clues behind the structure
sensitivity in the aforementioned oxidation process.

Fig. 1. Top views of the three lowest index CeO2 surfaces. The outmost oxygen (in red)
and cerium (in green) layers are highlighted, while the rest of atoms are shown dashed.

4	
  
	
  

2. Computational details
All the calculations reported in the present study were performed at the DFT+U level
using the Vienna Ab initio Simulation Package (VASP, version 5.3.3) [43,44]. The
Perdew-Becke-Ernzerhof (PBE) [45] functional was used in combination with an
effective U term, Ueff, of 4.5 eV (this term is defined as the difference between the
Coulomb, U, and the exchange, J, terms) [46], as it has been previously proven to
provide satisfactory results [47–50]. The purpose of adding this Hubbard term U is to
diminish the self-interaction error and to properly localize the Ce 4f states. Projectoraugmented wave (PAW) pseudopotentials [51] were used to describe the core
electrons with a plane-wave cutoff energy of 500 eV for the valence electrons (i.e., 5s,
5p, 4f, 6s for Ce atoms, and 2s, 2p, for O and C atoms).
Ceria presents a fluorite structure, with lattice parameter aexp = 5.411 Å [52]. We
optimized the lattice parameter using a dense Γ-centered 7x7x7 k-point mesh, leading
to the value acalc = 5.497 Å, in good agreement both with the experimental value and
previous computational studies [53]. To maintain the stoichiometry of the CeO2(100)
surface and avoid the dipole moment normal to the surface, a half oxygen monolayer
was transferred from the surface to the bottom of the slab [54]. The (111), (110), and
(100) surfaces were modeled as (2x2) periodically repeated slabs consisting of four,
five, and three O–Ce–O layers, respectively, separated by 15 Å of vacuum space, and
were optimized using a Γ-centered k-point mesh denser than 0.38 Å-1. The layers
allowed to relax in each surface were the five outermost layers in CeO2(111), the three
top-most layers in CeO2(110), and the four outermost layers in CeO2(100), whereas the
rest of atoms were kept fixed to their bulk positions.
Transition states were located by means of the climbing image nudged elastic band
(CI-NEB) method [55]. The nature of all reaction minima and transition states was
confirmed by means of numerical frequency analyses.
3. Results
3.1. Dehydrogenation of methanol to formaldehyde on CeO2.
We investigated the conversion of methanol to formaldehyde on the three lowest
energy surfaces corresponding to the low index planes (111), (110), and (100). Their
surface and relaxation energies are presented in the Supplementary Material (Table
S1).
5	
  
	
  

The reaction consists in the sequence of the following elementary steps:
CH3OH + * → CH3OH*

(R1)

CH3OH* + * → CH3O* + H*

(R2)

CH3O* + * → HCHO* + H*

(R3)

HCHO* → HCHO + *

(R4)

Methanol physisorbs on ceria (R1) and dissociates (R2) giving rise to a chemisorbed
methoxy and a surface OH groups. The methoxy intermediate then leads to
formaldehyde through a C–H bond breaking (R3) with the concomitant hydrogenation
of another surface oxygen. Finally, formaldehyde desorbs from the surface to the gasphase (R4).
Reaction (R2) is controlled by the acid-base properties of the surface and does not
involve any change in the oxidation state of the reactant fragments or the surface metal
atoms. In contrast, reaction (R3) can be seen as the main redox step as two Ce4+
cations are reduced to Ce3+ after the C–H scission and the subsequent formation of a
second surface OH group. The discrimination in terms of acid-base and redox steps
has been recently found to be crucial to tune the catalytic activity of oxides [56].
The calculated energy profiles for the methanol to formaldehyde conversion on the
different CeO2 surfaces are presented in Fig. 2. Methanol adsorption energies range
between -0.5 and -1 eV, whereas the subsequent first deprotonation requires very low
energies, < 0.1 eV, rendering a more stable methoxy intermediate (I1). From this point,
the relative energy barrier for the next step (R3) on all the surfaces is around 1 eV.
More specifically, the (100) surface exhibits the lowest energy barrier (0.89 eV),
whereas the (111) and (110) surfaces have barriers of 1.03 and 1.14 eV, respectively.
In all the cases, the transition state associated to this process (TS2) involves the
reduction of one single Ce4+ to Ce3+, whereas the other unpaired electron resulting from
the C–H scission is delocalized over these C and H atoms and the involved surface
oxygen. On the (111) surface, the C–H scission in I1 results in the physisorbed
formaldehyde as intermediate, as also noted by Kropp and Paier [39]. The next barrier
from the physisorbed to chemisorbed conformation (I2) is lower than 0.05 eV (see
Supplementary Material Fig. S2). Alternatively, on the (110), the C–H bond cleavage
leads to the formation of an intermediate where HCHO is directly chemisorbed on the
hydroxylated surface group. From this species, the following proton transfer to the
adjacent surface oxygen leading to I2 only requires 0.02 eV. Importantly, the energy
barrier for the direct formation of I2 from I1 on the (110) was previously reported [38] as

6	
  
	
  

0.9 eV higher than ours. We found that the direct mechanism from I1 to I2 via the
transition state TS2 is only favored on the (100) surface.
In I2, formaldehyde binds to the surface in a 𝜂! -like configuration, where the C atom
is coordinated to an oxygen from the lattice and the oxygen from HCHO is bound to a
surface cation. This species is rather stable on all the surfaces, with binding energies of
1.23, 0.71, and 2.30 eV for the (111), (110), and (100) facets, respectively. Hence,
formaldehyde desorption from the (111) and (110) surfaces is expected to be
significantly easier compared to (100). That is, the lifetime of HCHO on the (100)
surface may be higher than on the other surfaces, which might eventually lead to
surface poisoning.
Overall, according to the energy diagram in Fig. 2, the most energy-demanding step
in the methanol to formaldehyde conversion on the (110) surface is the C–H bond
breaking, whereas on the (100) and (111) surfaces it corresponds to the formaldehyde
desorption.

Fig. 2. Top panel: energy profile for the MeOH to HCHO conversion on CeO2(111),
CeO2(110), and CeO2(100). Bottom panel: optimized geometries for the intermediates
7	
  
	
  

and transition states. The most relevant distances in the transition states can be found
in Fig. S1. Color code as in Fig.1 with the addition of C (black), O from methanol (dark
red), and H (white).
3.2. Dehydrogenation of formaldehyde to carbon monoxide on CeO2.
The reaction mechanism for the dehydrogenation of formaldehyde to CO on the
clean (111), (110), and (100) surfaces was analyzed assuming the following
elementary reaction steps:
HCHO + * → HCHO*

(R4’)

HCHO* + * → CHO* + H*

(R5)

CHO* + * → CO* + H*

(R6)

CO* → CO + *

(R7)

In this case, the initial step (R4’) is the reverse process of the above (R4) reaction.
After the adsorption of formaldehyde, the reaction then proceeds through two
consecutive H abstractions (R5-R6), which can be again considered as coupled redox
and acid-base steps. Finally, the generated CO desorbs from the surface to the gas
phase. The reaction energy profiles obtained for the three CeO2 surfaces are shown in
Fig. 3. The initial formaldehyde chemisorption (A2) on the (111), (110), and (100)
surfaces is -0.88, -1.25, and -1.84 eV, respectively. Note that the lower binding
energies for the (111) and (100) facets compared to I2 in the methanol
dehydrogenation (Fig. 2) is due to the formation of hydrogen bonds between HCHO
and the surface hydroxyls [57]. The energy difference can be estimated to the
formation of two hydrogen bonds [58]. On (110), A2 binding energy is higher than I2
because the hydroxylated surface presents intrasurface hydrogen bonds (between
Olat–H and another Olat) and formaldehyde adsorbs on the acceptor oxygen disrupting
the H-bond surface network. From A2, the first H abstraction in HCHO is a highly
exothermic process that releases more than -2 eV and results in the chemisorbed
formyl and one surface hydroxyl group (I3). The activation energy for this step requires
only 0.66 eV on the (100) surface, but is very energy demanding on both (111) and
(110), i.e. 1.53 and 1.87 eV, respectively. Similarly to the H stripping in methanol (R3),
in this first transition state (TS3) one Ce4+ is reduced to Ce3+, while the other unpaired
electron is delocalized over the formaldehyde and the reacting surface oxygen. The
second deprotonation step (R6) is endothermic and produces physisorbed CO and two
surface OH groups. The energy barriers for this step on the (111) and (100) surfaces
are 1.60 and 1.64 eV, respectively, and 2.00 eV on the (110). Finally, the CO
8	
  
	
  

desorption from the partially hydroxylated surfaces requires only 0.1 – 0.3 eV. It is
worth noting that CO can also adsorb forming a carbonate structure CO32- with two
surface oxygens. This structure is 3.5 eV more stable than the physisorbed CO [59,60],
although its formation requires two accessible surface oxygens. On hydroxylated
surfaces HCO3- and H2CO3 could be formed, which are 1.8 eV more stable and 1.3 eV
less stable, respectively, than physisorbed CO. Therefore, highly hydroxylated surfaces
are less prone to form carbonates, and their formation (HCO3- and H2CO3) would not
introduce significant alterations to the proposed mechanism.
Alternatively, the oxidation of the chemisorbed formaldehyde on the (100) surface
can also follow the blue dashed path shown in Fig. 3. In this case, the H atom resulting
from the first C–H scission accommodates between two surface Ce4+ cations as a
hydride. Thus, unlike the previous mechanism, this alternative pathway does not
involve the reduction of any surface cerium cations [61]. This first H abstraction is
endothermic by 0.66 eV and requires an activation energy of 0.75 eV. Similarly, the
subsequent C–H bond cleavage is also endothermic by 1.06 eV and has an energy
barrier of 1.82 eV. Although this alternative mechanism is thermodynamically less
favored than the previous one (solid lines in Fig. 3), it might play a role under high
coverage regimes, where all surface oxygens would be poisoned by chemisorbed
formaldehyde. It is worth mentioning that this mechanism is enabled only on the
particularly defective (100) surface, and is unlikely to occur on the other two surfaces
(Fig. S4).
On the whole, Fig. 3 shows that the most energy demanding step for the oxidation
of formaldehyde to CO is the same for the three ceria facets and corresponds to the
cleavage of the second C–H bond (R6). The activation energies for the (R5) and (R6)
steps range between 1.5 – 2 eV, except for (R5) on the (100), which is only 0.66 eV.
On (111) and (110), the energies required to reach TS3 are ca. 0.6 eV higher than the
desorption of formaldehyde from these surfaces.

9	
  
	
  

Fig. 3. Top panel: energy profile for the HCHO conversion to CO upon adsorption on
CeO2(111), CeO2(110), and CeO2(100). The alternative hydride cleavage mechanism
on the latter surface is shown in dashed blue lines. Bottom panel: optimized geometries
for the intermediates and transition states. The most relevant distances in the transition
states can be found in Fig. S3. Color code as in Fig. 2.

10	
  
	
  

3.3. H2 evolution from a hydroxylated CeO2 surface.
Once methanol decomposes to either HCHO or CO, the CeO2 surfaces become
partially hydroxylated, which can eventually lead to hydrogen evolution under certain
reaction conditions. In a previous work, we showed that H2 dissociation on the (111)
surface affords the homolytic product although through an heterolytic path involving the
formation of an ion-pair as precursor [50]. The reaction profile for that reverse process
on the three CeO2 surfaces is presented in Fig. 4. In the first step, one hydrogen of the
two surface hydroxyl groups is transferred to a neighboring cerium atom with the
concomitant oxidation of one Ce3+ to Ce4+. This process is highly endothermic, the
reaction energies being 3.22 eV on (111), 3.61 eV on the (110), and 2.71 eV on the
(100), and the associated barriers 3.55, 4.30, and 3.53 eV, respectively. Interestingly,
the resulting product I5 features a completely different nature depending on the
considered facet, see Fig. 4. While on the (111) and (110) surfaces it can be described
as an ion-pair with a H–H distance of 1.29 and 1.65 Å, respectively, on the (100)
surface this ion-pair is not stable and the proton and the hydride fall apart so that the
latter accommodates between two Ce4+. This is a consequence of the intrinsic oxygen
depletion of the (100) surface. These significant structural differences are also reflected
in substantially different stabilities. More specifically, the ion-pairs formed on the (111)
and (110) surfaces have energies of 0.75 and 0.45 eV above the gas-phase reference,
respectively, whereas the charge separated form on (100) is -0.29 eV more stable.
From these precursors, the activation energy for the following H–H bond formation on
(111) and (110) is very low, 0.1 eV, but it increases to 0.62 eV on the (100).
Thus, hydrogen evolution on the (111) and (110) surfaces is very difficult as H are
always stored as hydroxyls on the regular stoichiometric surfaces. In contrast, as
above mentioned the situation on the (100) surface might be different. In fact, when
methanol is adsorbed on this surface, a large coverage of hydroxyls is likely; therefore,
the hydrogen stripping from A2 can divert towards the production of the intermediate I3
(dashed blue line mechanism), where one of the hydrogens is stored as a hydride. This
situation is equivalent to that presented in structure I5, from which we have shown that
H2 can be evolved with a small energy barrier.
The evolution of hydrogen from the hydroxylated surface might also compete with
the formation of water and its subsequent desorption, thus creating an oxygen
vacancy. This overall process is endothermic on all three surfaces, by 2.22, 2.11, and
2.34 eV on (111), (110), and (100), respectively. Despite being a highly endothermic
process, it is more likely to occur than the hydrogen evolution from the hydroxylated
surface.

11	
  
	
  

Fig. 4. Top panel: energy profile for H2 evolution from partially hydroxylated (0.5 ML)
CeO2(111), CeO2(110), and CeO2(100). Bottom panel: optimized geometries for the
intermediates and transition states. Color code as in Fig. 2.
4. Discussion
In this section we present a detailed comparison between our DFT simulations and
the available experiments in the literature. The theoretical results exposed in the
previous sections are perfectly consistent with the reported TPD experiments on
stoichiometric (111) and (100) CeO2 thin films [29,33,34]. In the case of the (111)
surface, the only desorption products observed are formaldehyde, and methanol.
Methanol desorbs at 220 K (main peak) and 560 K, and formaldehyde at 570 K.
Similarly, TPD experiments on the (100) surface show the same product distribution
but with the additional observation of CO, CO2, H2O, and H2. In this case, methanol
appears at 560 – 580 K, with a large peak of formaldehyde and CO coupled with a
smaller peak assigned to H2 and H2O. Moreover, the reaction on this surface was
reported to occur rapidly and without detecting any formyl or formate intermediates.
12	
  
	
  

The desorption peaks for HCHO, CO, and H2 for both the (111) and (100) facets
proceed prior to any indication of surface reduction (related to CO2 or H2O desorption
peaks).
The above structure sensitivity observed in experiments can be rationalized as
follows on the basis of our theoretical calculations. Even though we showed that the
sequential removal of H from methanol on the (111) and (110) surfaces is energetically
costly, it derives on adsorbed formaldehyde, which can be desorbed more easily than
the further removal of H from it. That is, the adsorption energy of A2 in Fig. 3 is
significantly smaller than the energy required to reach TS3. As a result, methanol
selectively reacts towards formaldehyde on the (111) and (110) surfaces, the latter
being more reactive (lower TS2) than the former. Major differences, however, can be
found in reaction conversions. On the (111) surface, methanol desorption and its
transformation to formaldehyde are competing processes, but the weak interaction
between methanol and the surface results in a large peak in the TPD at low
temperatures. Still, some methanol molecules survive on the surface long enough to
react, possibly pinned by undersurface vacancies [62,63]. This is clearly evidenced in
the TPD experiments where the low temperature peak (methanol desorption) is larger
than the others for the (111) surface under stoichiometric conditions. In contrast, on the
(100) surface the energy demand to reach TS3 is much lower than the adsorption
energy of A2, and therefore, the conversion of formaldehyde to CO is favored
compared to formaldehyde desorption. These results also account for the unique
experimental observation of CO on the most oxidized CeO2(100) surfaces [34]. It was
also shown that CO is more likely to appear on the more defective ceria surfaces. This
can be also explained with the expected increase of the A2 adsorption energy on the
O-vacancy sites, as these may trap HCHO and impedes its desorption thus favoring
CO evolution.
This is in fact the case for the (111) surface, where methanol adsorbs dissociatively
on the defective site with an adsorption energy of -2.12 eV [64]. The subsequent C–H
cleavage to form the chemisorbed formaldehyde demands 1.32 eV, and is an
exothermic process by 0.34 eV. Formaldehyde desorption from this surface requires
1.04 eV, and the energy barrier for the subsequent C–H bond breaking is only 0.58 eV.
Hence, the oxygen deficient (111) traps formaldehyde and enables methanol
conversion to CO, as suggested before and shown in the reported TPD experiments
[34].
As when it comes to hydrogen, experiments show that only CeO2(100) is able to
evolve molecular hydrogen at about 600 K [34]. Indeed, from Fig. 4 it can be observed
that the surface OH groups are too robust to produce H2 from any of the three surfaces.
13	
  
	
  

However, on the (100), the intrinsic lack of surface oxygen makes the situation less
compromised as a both hydrides and protons can be stabilized by the surface Ce and
O atoms. Hence, populating this state is crucial for CeO2 to be able to generate H2.
This also explains why H2 evolution is also observed experimentally on defective
CeO2(111) surfaces [34]. Our results also point out that formaldehyde oxidation on the
(100) surface can take place through a less thermodynamically favored alternative
mechanism and without involving the reduction of surface Ce ions (blue dashed path in
Fig. 3). Upon CO desorption, this reaction path leads to the partially hydroxylated
surface I5, which might enhance H2 evolution on the (100) surface, especially under
high coverage regimes with all the surface oxygens occupied by chemisorbed
formaldehyde. This mechanism, accessible for the intrinsically O-deficient (100)
surface, also explains why reduced (111) surfaces can also produce H2. These
reduced surfaces, can trap hydride atoms and follow a similar mechanism as the one
we proposed for the (100) surface.
Finally, the reported TPD spectrum using isotope labeled ceria and methanol,
Ce18O2(111) and CH316OH, show that an equal amount of HCH16O and HCH18O is
desorbed [29]. The same observation is made upon HCH16O adsorption on reduced
Ce18O1.75(111) [36]. The latter was attributed by the authors to the adsorption on a
vacancy site.
In order to shed light on these experimental findings, we also investigated a reaction
mechanism in which an O atom from the chemisorbed HCHO is exchanged with one
lattice oxygen from the stoichiometric CeO2 surfaces (Fig. 5). On (111) and (110), this
mechanism occurs in an elementary step with an activation energy of 0.40 and 0.17
eV. In contrast, on the (100) termination, the chemisorbed formaldehyde A2 gives rise
to an intermediate (A3) that is 0.07 eV more stable and where the lattice oxygen has
diffused and accommodates on a defective surface site. The energy barrier required to
afford A3 is also very low (0.08 eV). The low energy difference and activation energy
between both A2 and A3 indicate that these two intermediates may be in
thermodynamic equilibrium. Therefore, the low activation energies found for this
process on the different CeO2 surfaces prove that no oxygen vacancies are needed to
desorb HCH18O upon HCH16O adsorption or CH316OH dehydrogenation.

14	
  
	
  

Fig. 5. Left: energy profile for the oxygen exchange between HCHO and a lattice
oxygen from CeO2(111), CeO2(110), and CeO2(100). Right: optimized geometries for
the intermediates and transition states. Color codes as in Fig. 2.
5. Conclusions
A detailed mechanistic study on the reported structure sensitivity of different CeO2
surfaces in the methanol conversion to formaldehyde or syn-gas (CO + H2 mixtures)
has been performed. The reaction proceeds via the activation of the hydroxyl group by
the basic sites on the surface and followed by the subsequent stripping of hydrogen
atoms from the methyl group. Our DFT+U calculations show that while close-packed
surfaces mainly produce formaldehyde, this intermediate is trapped in the O-defective
(100) surface, which finally produces CO and H2. Hydrogen evolution is allowed on the
(100) facet, but not on the closed-packed (111) and (110) terminations. This is due to
the inherent defective sites in the former that stabilize two hydrogen atoms as
hydroxyls and hydrides, which eventually lead to H–H bond formation. Only reduced
(111) and (110) surfaces could present enough vacancy sites to allow a similar
chemistry on the close-packed facets. These active sites in turn extend the lifetime of
adsorbed HCHO enabling its decomposition, and are the ultimate responsible for CO
formation.
The present work sheds light on the structure sensitivity found for the reaction and
that is related to the inherent stoichiometries of the different surfaces. This is more
acute on oxides, in particular reducible ones, as there are more parameters that are
modified when changing the facet orientations. Moreover, shape control is key to obtain
desired selectivities and, thus, methods devoted to a detailed shape selection are
crucial to obtain a more selective and greener chemistry.

15	
  
	
  

Acknowledgements
This research has been supported by the ERC Starting Grant (ERC-2010-StG258406). We acknowledge BSC-RES for providing generous computational resources.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]

Z. Shao, S.M. Haile, Nature. 431 (2004) 170.
A.S. Karakoti, S. Singh, A. Kumar, M. Malinska, S.V.N.T. Kuchibhatla, K. Wozniak, et al., J. Am.
Chem. Soc. 131 (2009) 14144.
A. Trovarelli, Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002.
Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science. 301 (2003) 935.
S. Hilaire, X. Wang, T. Luo, R.. Gorte, J. Wagner, Appl. Catal. A Gen. 215 (2001) 271.
M.M. Schubert, V. Plzak, J. Garche, R.J. Behm, Catal. Letters. 76 (2001) 143.
O. Pozdnyakova, D. Teschner, A. Wootsch, J. Kröhnert, B. Steinhauer, H. Sauer, et al., J. Catal.
237 (2006) 1.
J.S. Yoo, A.A. Bhattacharyya, C.A. Radlowski, Ind. Eng. Chem. Res. 30 (1991) 1444.
J.S. Yoo, A.A. Bhattacharyya, C.A. Radlowski, J.A. Karch, Appl. Catal. B Environ. 1 (1992) 169.
A.P. Amrute, C. Mondelli, M. Moser, G. Novell-Leruth, N. López, D. Rosenthal, et al., J. Catal. 286
(2012) 287.
M. Nolan, S.C. Parker, G.W. Watson, Surf. Sci. 595 (2005) 223.
D.A. Andersson, S.I. Simak, N. V Skorodumova, I.A. Abrikosov, B. Johansson, Proc. Natl. Acad.
Sci. U. S. A. 103 (2006) 3518.
J. Paier, C. Penschke, J. Sauer, Chem. Rev. 113 (2013) 3949.
G. Vilé, B. Bridier, J. Wichert, J. Pérez-Ramírez, Angew. Chem. Int. Ed. 51 (2012) 8620.
G. Vilé, S. Colussi, F. Krumeich, A. Trovarelli, J. Pérez-Ramírez, Angew. Chem. Int. Ed. 53 (2014)
12069.
E. Aneggi, D. Wiater, C. de Leitenburg, J. Llorca, A. Trovarelli, ACS Catal. 4 (2014) 172.
J.M. Tatibouët, Appl. Catal. A Gen. 148 (1997) 213.
M. Badlani, I.E. Wachs, Catal. Letters. 75 (2001) 137.
Http://www.formox.com/. (n.d.).
M. Schmid, A. Reicho, A. Stierle, I. Costina, J. Klikovits, P. Kostelnik, et al., Phys. Rev. Lett. 96
(2006) 146102.
J. Schnadt, A. Michaelides, J. Knudsen, R. Vang, K. Reuter, E. Lægsgaard, et al., Phys. Rev. Lett.
96 (2006) 146101.
I. Karakaya, W.T. Thompson, J. Phase Equilibria. 13 (1992) 137.
J.L. Bronkema, D.C. Leo, A.T. Bell, J. Phys. Chem. C. 111 (2007) 14530.
L.J. Burcham, I.E. Wachs, Catal. Today. 49 (1999) 467.
G. Deo, I.E. Wachs, J. Catal. 146 (1994) 323.
T. Feng, J.M. Vohs, J. Catal. 208 (2002) 301.
Y. Romanyshyn, S. Guimond, H. Kuhlenbeck, S. Kaya, R.P. Blum, H. Niehus, et al., Top. Catal. 50
(2008) 106.
Q. Wang, R.J. Madix, Surf. Sci. 496 (2002) 51.
D.R. Mullins, M.D. Robbins, J. Zhou, Surf. Sci. 600 (2006) 1547.
A. Siokou, R.M. Nix, J. Phys. Chem. B. 103 (1999) 6984.
J.J. Uhlrich, B. Yang, S. Shaikhutdinov, Top. Catal. 57 (2014) 1229.
V. Matolín, J. Libra, M. Škoda, N. Tsud, K.C. Prince, T. Skála, Surf. Sci. 603 (2009) 1087.
R.M. Ferrizz, G.S. Wong, T. Egami, J.M. Vohs, Langmuir. 17 (2001) 2464.
P.M. Albrecht, D.R. Mullins, Langmuir. 29 (2013) 4559.
Z. Wu, M. Li, D.R. Mullins, S.H. Overbury, ACS Catal. 2 (2012) 2224.
J. Zhou, D.R. Mullins, Surf. Sci. 600 (2006) 1540.
D. Mei, N.A. Deskins, M. Dupuis, Q. Ge, J. Phys. Chem. C. 111 (2007) 10514.
D. Mei, N.A. Deskins, M. Dupuis, Q. Ge, J. Phys. Chem. C. 112 (2008) 4257.

16	
  
	
  

[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]

T. Kropp, J. Paier, J. Phys. Chem. C. 118 (2014) 23690.
M. Nolan, J. Chem. Phys. 139 (2013) 184710.
T. Kropp, J. Paier, J. Sauer, J. Am. Chem. Soc. 136 (2014) 14616.
B.-T. Teng, S.-Y. Jiang, Z.-X. Yang, M.-F. Luo, Y.-Z. Lan, Surf. Sci. 604 (2010) 68.
G. Kresse, Phys. Rev. B. 54 (1996) 11169.
G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15.
J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.
S.L. Dudarev, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Phys. Rev. B. 57 (1998) 1505.
S. Fabris, S. de Gironcoli, S. Baroni, G. Vicario, G. Balducci, Phys. Rev. B. 72 (2005) 237102.
M. Ganduglia-Pirovano, J. Da Silva, J. Sauer, Phys. Rev. Lett. 102 (2009) 026101.
R. Farra, M. García-Melchor, M. Eichelbaum, M. Hashagen, W. Frandsen, J. Allan, et al., ACS
Catal. 3 (2013) 2256.
M. García-Melchor, N. López, J. Phys. Chem. C. 118 (2014) 10921.
P.E. Blöchl, Phys. Rev. B. 50 (1994) 17953.
E. Kümmerle, G. Heger, J. Solid State Chem. 147 (1999) 485.
J. Da Silva, M. Ganduglia-Pirovano, J. Sauer, V. Bayer, G. Kresse, Phys. Rev. B. 75 (2007)
045121.
P. Oliver, G. Watson, S. Parker, Phys. Rev. B. 52 (1995) 5323.
G. Henkelman, B.P. Uberuaga, H. Jónsson, J. Chem. Phys. 113 (2000) 9901.
M. Moser, I. Czekaj, N. López, J. Pérez-Ramírez, Angew. Chem. Int. Ed. 53 (2014) 8628.
D. Fernández-Torre, K. Kośmider, J. Carrasco, M.V. Ganduglia-Pirovano, R. Pérez, J. Phys.
Chem. C. 116 (2012) 13584.
G. Revilla-López, N. López, Phys. Chem. Chem. Phys. 16 (2014) 18933.
M. Nolan, G.W. Watson, J. Phys. Chem. B. 110 (2006) 16600.
M. Nolan, S.C. Parker, G.W. Watson, Surf. Sci. 600 (2006) 175.
This fact was confirmed by the obtained spin densities at this surface.
F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, et al., Science. 309 (2005) 752.
G.E. Murgida, M.V. Ganduglia-Pirovano, Phys. Rev. Lett. 110 (2013) 246101.
Calculations on defective ceria(111) were performed using the p(2x2) model slab described in the
Computational details section. Nevertheless, the results are in close agreement with those in ref.
41.

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