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Applied Surface Science 257 (2010) 1175–1180
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Adsorption and dissociation of H2 on the Cu2 O(1 1 1) surface: A density functional
theory study
Riguang Zhang, Baojun Wang ∗ , Lixia Ling, Hongyan Liu, Wei Huang
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history:
Received 21 March 2010
Received in revised form 27 July 2010
Accepted 27 July 2010
Available online 4 August 2010
Keywords:
Hydrogen
Cu2 O(1 1 1)
Adsorption
Dissociation
Density functional theory
a b s t r a c t
Interactions of atomic and molecular hydrogen with perfect and deficient Cu2 O(1 1 1) surfaces have been
investigated by density functional theory. Different kinds of possible modes of H and H2 adsorbed on the
Cu2 O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate
that OSUF , CuCUS and Ovacancy sites are the adsorption active centers for H adsorbed on the Cu2 O(1 1 1)
surface, and for H2 adsorption over perfect surface, CuCUS site is the most advantageous position with the
side-on type of H2 . For H2 adsorption over deficient surface, two adsorption models of H2 , H2 adsorbing
perpendicularly over Ovacancy site and H2 lying flatly over singly-coordinate Cu–Cu short bridge, are typical
of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal
lattice and the other bounded to CuCUS atom, suggesting that the dissociative adsorption of H2 is the main
dissociation pathway of H2 on the Cu2 O(1 1 1) surface. Our calculation result is consistent with that of the
experimental observation. Therefore, Cu2 O(1 1 1) surface with oxygen vacancy exhibits a strong chemical
reactivity towards the dissociation of H2 .
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The formation of dimethyl ether and methanol from syngas conversion has recently attracted more and more attention, because
of its potency for using as an alternative diesel fuel or as a fuel
additive [1,2]. To date, copper is considered as a potential substitute for noble metals because of its low cost and high activity for
CO and H2 [3–9]. However, the active center of copper has been
a matter of debate in the literature over the past two decades
[10–13]. We shall not take part in this debate here. The studies
by Sheffer and King [10], Herman et al. [11] and Zuo et al. [14] have
found that Cu2 O shows the higher catalytic activity in comparison with Cu, and details about the interaction of small molecules
with catalyst surface are important for chemists to understand the
electronic structure of catalyst and the mechanism of the catalytic
processes. Thus, the interaction of CO with Cu2 O has been studied
by many researchers [15–19]. To our knowledge, the interaction of
H2 with Cu2 O surfaces is still lacking. Only literature [14] reported
H2 adsorption on the perfect Cu2 O(1 1 1) surface, but H2 adsorption
on the deficient surfaces has not been mentioned. In fact, experimental solid surfaces are not always perfect and the important
properties of most metal oxides, including surface reactivity, are
closely related to the presence of surface defects. For example, the
∗ Corresponding author at: No. 79 Yingze West Street, Taiyuan 030024, China.
Tel.: +86 351 6018539; fax: +86 351 6041237.
E-mail address: [email protected] (B. Wang).
0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2010.07.095
presence of oxygen vacancies on the NiO(1 0 0) surface leads to an
increase in the adsorption energy of H2 and lowers the energy barrier of the H–H bond cleavage [20]. In contrast, a perfect NiO(1 0 0)
surface exhibits the negligible reactivity towards H2 . And the studies about the adsorption and dissociation of O2 on the deficient
Cu2 O(1 1 1) surface by Sun et al. [21] and the deficient Cu2 O(1 0 0)
surface by Le et al. [22] also show that the surface with oxygen
vacancy exhibits a strong chemical reactivity towards the dissociation of O2 . Meanwhile, experiments about H2 adsorption on the
Cu2 O surface [23] have clearly demonstrated that no H2 uptake is
observed under ultrahigh vacuum conditions, and atomic hydrogen is thought to cap the vacant coordination sites on the surface.
However, the adsorption geometry and behavior of atomic hydrogen and molecular hydrogen on the Cu2 O surface or the role of the
surface defect are less well understood. Therefore, for a detailed
understanding of the surface process, experimental information
is however not always sufficient and accompanying theoretical
calculations can be helpful to clarify some questions. Quantum
chemical methods have become new tools for studying the structure of active surfaces and determining reaction mechanisms. With
recent developments, density functional theory (DFT) is capable of
providing qualitative and quantitative insights into catalyst science
and mechanism about hydrogen dissociation [24–27].
Cu2 O(1 1 1) surface is shown to be the main surface of Cu2 O
[28,29]. In this study, we report a systematic DFT investigation
about the preferred adsorption site of atomic and molecular hydrogen on the perfect and deficient Cu2 O(1 1 1) surfaces. Meanwhile,
the dissociation process of H2 on the Cu2 O(1 1 1) surface is dis-
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R. Zhang et al. / Applied Surface Science 257 (2010) 1175–1180
Fig. 1. The slab model of Cu2 O(1 1 1)-2 × 2. (a) Perfect Cu2 O(1 1 1)-(2 × 2) surface; (b) Oxygen-deficient Cu2 O(1 1 1)-(2 × 2) surface. Orange balls stand for Cu atoms, red balls
stand for O atoms and black ball stands for oxygen vacancy site (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
the article.).
cussed to obtain the interaction mechanisms of H2 with Cu2 O
surface, which may be of interest to researchers attempting to illustrate the catalytic mechanism of the syngas conversion such as
dimethyl ether and methanol synthesis.
2. Computational models and methods
2.1. Surface models
The Cu2 O(1 1 1) surface is modeled using the supercell approach,
where periodic boundary condition is applied to the central supercell so that it is reproduced periodically throughout space. The ideal
and perfect Cu2 O(1 1 1) surface is non-polar including four chemically different types of surface atoms, which are denoted as CuCUS ,
CuCSA , OSUF and OSUB shown in Fig. 1a. CuCUS is the surface copper that is coordinatively unsaturated, i.e., singly-coordinate Cu+
cations. CuCSA is the coordinatively saturated copper atom, i.e.,
doubly coordinate Cu+ . OSUF is the outer-most surface oxygen, i.e.,
threefold-coordinate oxygen anions. And OSUB is the subsurface
oxygen, i.e., fourfold-coordinate oxygen anions.
The removal of the oxygen at the top atomic layer from the
perfect surface results in what is called the oxygen-deficient
Cu2 O(1 1 1) surface [15], as shown in Fig. 1b. Each oxygen vacancy
gives rise to a threefold site of singly-coordinate Cu+ cations (i.e.,
Cu2 , Cu3 and Cu4 atoms as labeled in Fig. 1b).
Our calculations on the perfect (2 × 2) and oxygen-deficient
(2 × 2) surfaces have been done using slab models of six layers. The
vacuum gap is set to 1 nm, at such a distance there is little interaction between the neighboring layers. In all calculations, the copper
and oxygen atoms of the top three layers and hydrogen are allowed
to relax, while those in other layers are fixed.
the Dmol3 program package in Materials Studio 4.0 [34,35] on HP
Proliant DL 380 G5 server system.
The adsorption energy is always regarded as a measure of the
strength of adsorbate–substrate adsorption. The adsorption energies Eads are defined as: Eads = Esub + Eads − Eads/sub , where Eads/sub is
the total energy of adsorbate–substrate system in the equilibrium
state, Esub and Eads are the total energy of fixed substrate and free
adsorbate alone, respectively. With this definition, positive value
of adsorption energy denotes that adsorption is more stable than
the corresponding substrate and free adsorbate.
3. Results and discussion
3.1. Calculations of H2 molecule and bulk Cu2 O
The accuracy of the computational method used in this
study has been tested initially to describe the properties of
H2 molecule in gas phase. The bond length and bond dissociation energy of molecular H2 calculated from our approach are
r(H–H) = 0.0745 nm and EBDE = 442.8 kJ mol−1 , which are in good
agreement with the experimental values of 0.0740 nm [31] and
436.1 kJ mol−1 [36], respectively, as well as to other similar GGA
results [37].
Then, the test is to predict the lattice constant of bulk Cu2 O. The
calculated value for the lattice constant is 0.4430 nm in comparison with the experimental value of 0.4270 nm [38,39]. The largest
deviation of calculation value from the experimental value is only
around 3.74%. Above results obtained in these tests make us confident in pursuing the next step of our investigations, namely the
interaction of H and H2 with Cu2 O surface.
3.2. Adsorption of atomic H on the Cu2 O(1 1 1)-(2 × 2) surface
2.2. Calculation methods
DFT has already been extensively used to study molecular
adsorption on catalyst MoS2 [25], Cu2 O [30], MgO [31], Au [32] and
Cu [33] surfaces. In our study, DFT has been employed to perform
for all calculations using general gradient approximation (GGA)
with the Becke–Lee–Yang–Parr (BLYP) exchange-correlation functional. In the computation, the inner electrons of copper atoms are
kept frozen and replaced by an effective core potential (ECP), other
atoms are treated with an all electron basis set. The double-numeric
basis with polarization functions (DNP) is used for all atoms in the
adsorbed and substrate systems. The k-point sampling scheme of
Monkhorst-Pack grid of 4 × 4 × 1 and Methfessel–Paxton smearing
of 0.005 hartree are employed. All calculations are carried out with
The properties of adsorbed atomic hydrogen are calculated in
our study. Five distinct adsorption sites as presented in Fig. 1a and
b are examined. In the case of atomic H adsorption at CuCUS , CuCSA ,
OSUB and OSUF sites, the perfect surface is employed to calculations,
while in the case of Ovacancy site, the oxygen-deficient surface is
applied to calculations. The adsorption energies and the equilibrium distances between atomic H and surface adsorption sites are
listed in Table 1.
From the calculated adsorption energies listed in Table 1,
the strengths of atomic H adsorption over the five types of
adsorption sites could be assigned in the following order,
H–Ovacancy > Oad –OSUF > H–CuCUS = H–CuCSA > H–OSUB . In the case of
H adsorbed over CuCSA site, initially the atomic hydrogen lies above
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1177
Table 1
Calculated adsorption energies and equilibrium distances for atomic hydrogen adsorption on the Cu2 O(1 1 1)-(2 × 2) surface.
Site
d(H–CuCUS )/nm
CuCUS
CuCSA
OSUF
OSUB
Ovacancy
0.148
0.148
d(H–OSUB )/nm
d(H–OSUF )/nm
d(H–Cu)/nm
q(H)
Eads /kJ mol−1
0.165
0.023
0.023
0.276
0.291
0.422
454.0
454.0
462.4
426.2
553.1
0.098
0.098
Table 2
Properties of H2 adsorbed at different adsorption sites on the perfect Cu2 O(1 1 1)-(2 × 2) surface.
Adsorption mode
r(H–H)/nm
˛/◦
q(H2 )
Eads /kJ mol−1
v(HH)/cm−1
M1
M2
M3
M4
M5
M6
M7
M8
Free H2
0.0788
0.0751
0.0752
0.0748
0.0788
0.0748
0.0752
0.0749
0.0745
25.9
145.4
177.7
161.0
25.9
11.5
177.7
8.0
0.211
−0.006
−0.011
−0.005
0.211
−0.002
−0.011
−0.001
0.000
253.2
242.8
239.6
242.8
253.2
242.4
238.4
242.6
3694.4
3694.4
4431.4
CuCSA site, the configuration is then optimized but the H still bounds
to CuCUS site. In the case of Ovacancy site, the vacancy site is filled
with atomic H. Fig. 2 presents the optimized configuration of atomic
H coordinated at Ovacancy site. The calculated results show that
Ovacancy , OSUF and CuCUS sites are the active centers for H adsorption
on the Cu2 O(1 1 1) surface, and the corresponding Mulliken population analysis also indicates that the charges transfer from atomic
H to Cu2 O(1 1 1) surface.
According to the calculated adsorption energies, we suspect that
there may be three possible products for the decomposition of H2
into two H atoms on the Cu2 O(1 1 1) surface, one is called P1 produced on the perfect surface, i.e., one atomic hydrogen is adsorbed
at CuCUS atom and the other bounds to OSUF atom. The other two
products are called P2 or P3 formed on the deficient Cu2 O(1 1 1) surface, i.e., one atomic hydrogen is almost completely inserted into
the crystal lattice (see Fig. 2) and the other bounds to CuCUS or OSUF
atom. The optimized structures of P1, P2 and P3 are presented in
Fig. 3.
3.3. Adsorption of H2 on the perfect Cu2 O(1 1 1)-(2 × 2) surface
H2 molecule is allowed to approach the perfect Cu2 O(1 1 1)(2 × 2) surface along two adsorption modes of H2 over the four
distinct sites, as shown in Fig. 1a, one is an end-on mode involving
H2 perpendicular to the surface, and the other is a side-on mode
involving H2 parallel to the surface. The geometries used in the calculations are presented in Fig. 4. The adsorption energies, bonding
Fig. 2. Optimized configuration of atomic H coordinated at Ovacancy site. Orange, red
and white atoms stand for Cu, O and H atoms throughout the paper, respectively (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.).
Fig. 3. Optimized geometric structures of P1, P2 and P3.
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R. Zhang et al. / Applied Surface Science 257 (2010) 1175–1180
keep away from the surface, and the corresponding adsorption
energies vary little with adsorption site and orientation, the H–H
bond lengths of adsorbed H2 molecules change little with respect
to free H2 molecule (the calculated H–H bond length for free H2 is
0.0745 nm). The electron transfer from H2 to substrate is 0.211 e
for the M5 mode, and a red-shift of the stretching frequency of the
adsorbed H2 is observed. The trend in vibrational frequency is consistent with that of bond length and Mulliken charges of adsorbed
H2 . Based on above results, CuCUS site is thought to be the most
adsorption site for H2 adsorption on the perfect Cu2 O(1 1 1) surface.
3.4. Adsorption of H2 on the oxygen-deficient Cu2 O(1 1 1)-(2 × 2)
surface
Fig. 4. Adsorption geometries for H2 on the perfect Cu2 O(1 1 1)-(2 × 2) surface.
parameters and stretching frequencies of H2 molecule adsorbed on
the perfect surface are given in Table 2.
According to Table 2, we interestingly find that adsorption of
molecular H2 is less favorable than that of atomic H. For the endon type M1, the optimized M1 structure converts the side-on
type M5, in which two H atoms of H2 molecule are all bound to
the same CuCUS site, the adsorption energy is 253.2 kJ mol−1 , and
the corresponding H–H bond length of adsorbed H2 is elongated
from 0.0745 to 0.0788 nm. In the optimized M2–M4 modes, H2 is
inclined to keep away from the surface. For M5 adsorption mode,
the optimized M5 adsorption mode is similar with the optimized
M1 structure. H2 in optimized IM6–IM8 modes is still inclined to
As above mentioned, the deficient surface exhibits two different kinds of catalytic active sites subjected to the adsorption of
H2 : CuCUS and Ovacancy sites. Owing to the fact that the Cu atoms
nearby the Ovacancy site (i.e., Cu1 , Cu2 , Cu3 and Cu4 atoms remarked
in Fig. 1b) are all singly-coordinated, these Cu atoms belong to
the same kind. Therefore, for the H2 adsorption on the oxygendeficient Cu2 O(1 1 1)-(2 × 2) surface, seven adsorption modes have
been considered: (a) H2 lies flatly over the Cu1 –Cu3 bridge site; (b)
H2 lies flatly over Cu2 –Cu4 bridge site; (c) H2 lies flatly over the
Cu1 –vacancy bridge site; (d) H2 is parallel to Ovacancy site; (e) H2 is
parallel to Cu1 site; (f) H2 is perpendicular to Ovacancy site; (g) H2
is perpendicular to Cu1 site. The adsorption models used in calculation are depicted in Fig. 5. The relevant adsorption energies, H–H
bond distances and Mulliken population analysis of the optimized
adsorption modes are given in Table 3.
According to the results listed in Table 3, the optimized M(b)
structure is very similar to the optimized M(f) mode, which lead
to the structure that one atomic H is almost completely inserted
Fig. 5. Adsorption geometries of H2 adsorbed on the deficient Cu2 O(1 1 1)-(2 × 2) surface.
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R. Zhang et al. / Applied Surface Science 257 (2010) 1175–1180
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Table 3
Properties of H2 adsorbed on the deficient Cu2 O(1 1 1)-(2 × 2) surface with different
adsorption modes.
Adsorption mode
r(H–H)/nm
q(H2 )
Eads /kJ mol−1
v(HH)/cm−1
M(a)
M(b)
M(c)
M(d)
M(e)
M(f)
M(g)
Free H2
0.0748
0.2965
0.0747
0.0750
0.0788
0.2493
0.0788
0.0745
−0.005
0.580
−0.006
−0.017
0.194
0.498
0.194
0.000
314.3
372.3
314.4
314.1
314.7
354.6
314.7
4370.1
4404.0
4360.2
3682.5
3682.5
4431.4
into the crystal lattice and the other bounds to CuCUS , as shown
in Fig. 6(a), this result is consistent with the above result predicted
from atomic H adsorption on the Cu2 O(1 1 1) surfaces, as in Fig. 3(b),
and the rather long distances of H–H bond (0.2965 and 0.2493 nm)
indicate that the H–H bond has been completely broken. Namely,
the dissociation of H2 for M(b) and M(f) modes is spontaneous.
However, for M(a), M(c) and M(d) modes, the optimized structures
show that H2 is inclined to keep away from the surface, and the H–H
bond lengths in the optimized structures are hardly changed with
respect to that calculated for the free H2 molecule (0.0745 nm). In
the case of M(g) mode, the optimized configuration is converted to
the optimized M(e) mode with H2 side-on type on CuCUS atom (see
Fig. 6(b)), and the optimized structure M(g) is very similar to the
structure M(e). The H–H bond is stretched significantly compared
to that in free H2 molecule and that in M(a), M(c) and M(d), such a
configuration can also contribute to the dissociation of H2 . Despite
of the elongation of the H–H bond, the H–H bond has not been broken; the dissociation processes of H2 virtually need to overcome an
energy barrier. The corresponding Mulliken charges and the H–H
stretching vibrational frequencies of adsorbed H2 in the optimized
M(e) and M(g) modes are also listed in Table 3. For deficient surface, H2 molecule still carries positive charges and a red-shift of
the stretching frequency of the adsorbed H2 is observed. These
changes in Mulliken charges and H–H bond stretching frequencies
are consistent with the changes in H–H bond lengths mentioned
above.
Therefore, in the view of the H–H bond lengths and the adsorption energies, M(b) and M(f) are the most favorable modes towards
the dissociation of H2 , which are typical of dissociative adsorption
modes. In other words, no activation energy is required for the dissociation of H2 into two H atoms on the deficient surface, which
means that the dissociative adsorption of H2 is the main dissociation pathway of H2 on the Cu2 O(1 1 1) surface in experiment, and
Cu2 O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H2 molecule. Meanwhile,
Schulz and Cox [23] have investigated the adsorption of H2 on the
Cu2 O surface in detail using LEED, XPS, TDS and UPS, their results
show no H2 uptake is observed under ultrahigh vacuum conditions,
and atomic hydrogen is thought to cap the vacant coordination
sites on the surface. Our calculation results of H2 adsorption on
the deficient Cu2 O(1 1 1) surface are consistent with that of the
experimental observations.
In addition, above calculated results also show that when the
dissociative adsorption of H2 on the deficient surface for M(b) and
M(f) models is occurring as a main dissociation pathway without
any activation energy required, a small quantity of H2 dissociation reactions for M(e) and M(g) modes also occur, which need to
overcome an energy barrier.
3.5. Dissociation of H2 on the deficient Cu2 O(1 1 1)-(2 × 2) surface
To obtain further detailed understanding about a small quantity
of H2 dissociation for M(e) and M(g) modes on the deficient surface,
Fig. 6. Optimized geometric structure of M(b) and M(e) adsorption modes.
what to be investigated first is the dissociation of H2 on the perfect
surface. As the optimized M5 mode is the most stable configuration
for H2 adsorption on the perfect Cu2 O(1 1 1) surface, the optimized
M5 was chosen as the initial reactant, as shown in Fig. 4, which leads
to the product P1 (see Fig. 3(a)). Then, the dissociation of H2 on the
deficient surface is calculated, the optimized M(e) is chosen to be
the initial state, as shown in Fig. 6(b), which results in the product
P2 (see Fig. 6(a)). In order to determine accurate activation energies,
the transition states are searched by means of complete LST/QST,
which has been applied to study the adsorption and dissociation
of NO dimmer [40] and O2 [21] on the Cu2 O(1 1 1) surface, as well
as other adsorption reactions occurred on metal surfaces [41]. The
calculated reaction and activation energies for H2 dissociation on
the perfect and deficient surfaces are shown in Table 4.
From Table 4, it can be clearly seen that the dissociation
process of H2 into P1 needs to overcome a very large energy barrier by 260.8 kJ mol−1 , and this reaction is exothermic slightly by
2.9 kJ mol−1 . The dissociation of H2 leads to P2 with an activation
energy of 81.1 kJ mol−1 , furthermore, this reaction is exothermic
by 40.2 kJ mol−1 . Meanwhile, the bond dissociation energy of free
H2 molecule obtained by experiment is 436.1 kJ mol−1 [36]. Above
reaction and activation energies show that Cu2 O(1 1 1) surface with
oxygen vacancy reduces the activation energy of H–H bond cleavage, and a small quantity of H2 dissociation reaction for M(e) and
M(g) modes on the deficient surface leading to P2 is favorable both
thermodynamically and kinetically in comparison with the dissociation of H2 on the perfect surface.
Table 4
Reaction and activation energies for H2 dissociation reactions on the perfect and
deficient Cu2 O(1 1 1)-(2 × 2) surface.
Reaction
E/kJ mol−1
Ea /kJ mol−1
M5 → P1
M(g) → P2
−2.9
−40.2
260.8
81.1
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R. Zhang et al. / Applied Surface Science 257 (2010) 1175–1180
4. Conclusions
Density functional theory calculations have been carried out to
investigate the adsorption and dissociation of atomic and molecular
hydrogen on the perfect and oxygen-deficient Cu2 O(1 1 1) surface.
The optimized geometries, adsorption energies, vibrational frequency and Mulliken population analysis show that CuCUS , OSUF and
Ovacancy sites are the active centers for H adsorption on the perfect
and deficient Cu2 O(1 1 1) surface, respectively. For H2 adsorption
over perfect surface, H2 adsorbs preferably side-on over CuCUS site,
and the dissociation of H2 needs to overcome an energy barrier
of 260.8 kJ mol−1 . And for H2 adsorption over deficient surface, H2
prefers to dissociatively adsorb perpendicularly over Ovacancy site
and flatly over singly-coordinate Cu–Cu short bridge, which leads
to Ovacancy healing with one H and the other H adsorbed at CuCUS
site. The calculations also mean that the dissociative adsorption
of H2 is the main dissociation pathway of H2 on the Cu2 O(1 1 1)
surface in experiment, in which Cu2 O(1 1 1) surface with oxygen
vacancy induces important changes in the adsorption modes of H2 ,
and exhibits a strong catalytic reactivity towards the dissociation
of H2 . Our calculation is a worthwhile theoretical example for the
interaction of H2 with Cu2 O surface.
Acknowledgements
The authors thank for anonymous reviewers for their helpful
suggestions on the quality improvement of our present paper.
And the authors gratefully acknowledge the financial support
of this study by the National Basic Research Program of China
(2005CB221204) and the National Natural Science Foundation of
China (20906066 and 20976115).
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