A DFT Study of CO Catalytic Oxidation Mechanism on the Defective

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A DFT Study of CO Catalytic Oxidation Mechanism on the Defective CuO (111) Surface 2

Ling-Nan Wu, Zhen-Yu Tian, and Wu Qin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03471 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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The Journal of Physical Chemistry

A DFT Study on CO Catalytic Oxidation Mechanism on the Defective Cu2O (111) Surface Ling-Nan Wua, Zhen-Yu Tiana,b,* Wu Qinc a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China b

c

University of Chinese Academy of Sciences, Beijing100049, China

National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China

Abstract: Understanding the role of surface defects on the catalyst performance is of great significance for a good command of the catalytic mechanism on the real catalyst surface. This work reports the mechanistic study of CO oxidation on the defective Cu2O (111) surface using density functional theory calculations. The effect of surface defects on surface catalytic activity was investigated by creating surface adsorbed O atoms and Cu vacancies on the perfect Cu2O (111) surface. Possible defective surface structures were found and calculated results showed that Cu vacancy on the Cu2O (111) surface could promote CO oxidation in two ways by (I) promoting the reaction between CO and lattice O with the energy barrier of 1.316 eV following Mars-van-Krevelen mechanism; (II) promoting the reaction between CO and adsorbed O atoms below 575 K. For the perfect Cu2O (111) surface, adding adsorbed O to the surface could either cause a strong surface reconstruction with the adsorbed O evolving into a lattice O or stay in an active state. The obtained results could explain the experimental observations that higher O/Cu ratio on Cu2O surface can improve the activity of Cu2O towards CO oxidation. Rate constants were provided according to harmonic transition state theory, which would be helpful for the kinetic modeling of CO catalytic oxidation on the real Cu2O surface.

*

Corresponding author, Tel/Fax: +86-10 8254 3184, E-mail: [email protected]. 1

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1. Introduction The knowledge of CO heterogeneous adsorption and oxidation has gained much attention due to its environmental concern and wide applications. CO is a kind of hazardous pollutants during the combustion of fossil fuels, which is known to harm human health and cause pollution

1-4

. As CO

homogeneous oxidation is hard to proceed at the temperature of interest, it is meaningful to find efficient catalysts with low light-off temperature and a wide range of temperature window

5-7

.

Moreover, Cu2O was found to be a kind of promising catalysts for CO abatement due to its good catalytic activity 8-9, non-toxicity and rich abundance on the earth 8. White et al. 10 synthesized Cu2O nanoparticles supported on silica gel and found that above 99.5% conversion of CO to CO2 could be achieved at temperatures lower than 250 °C for over 12 hours. El Kasmi et al. synthesized thin Cu2O film using pulsed-spray evaporation chemical vapor deposition method and found that Cu2O showed good activity towards CO complete oxidation. The increase of water content in the feedstock could lead to a higher surface oxygen-copper ratio, which was positively related to the catalytic performance of the synthesized Cu2O samples 8. From the previous studies, we can deduce that the surface structure of Cu2O has a significant influence on the surface catalytic behavior, which is an important issue in studying the surface processes. Several studies have been performed to get the atomistic understanding of Cu2O surface properties

10-20

, and a part of them have investigated the effect of Cu2O surface defects on surface

properties. Soon et al.

19

has compared the surface geometries of Cu2O crystal low-index surface

planes. They found that the most stable surface planes of Cu2O crystal were polar surfaces with surface defects rather than nonpolar stoichiometric Cu2O (111) surface planes (O-terminated Cu2O (111) surface 14). Cu2O (111) surface with unsaturated copper site (the CuCUS site) vacancy was found to be an energetically stable polar surface, and it was considered to be one of the most stable surfaces under oxygen-rich condition. Li et al. studied the effect of surface vacancies on the morphology of O-terminated Cu2O (111) surface and they also found that defective surfaces were more stable than 2


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the stoichiometric O-terminated surface

20

. Önsten et al. have studied the atomic structure of Cu2O

(111) using low-energy electron diffraction and scanning tunneling microscopy (STM). They suggested that the absence of undercoordinated copper ions were of great importance for the understanding of Cu2O surface reactivity

21

. They also used high-resolution photoemission

spectroscopy and scanning tunneling microscopy (STM) to investigate the properties of Cu2O (111) surface defects. Their results confirmed the existence of two types of surface vacancies including Cu vacancies and O vacancies, and the Cu vacancies were believed to be the most important defect in H2O and SO2 surface chemistry 22. Several works have attempted to study the CO adsorption and oxidation mechanism on Cu2O surface based on density functional theory (DFT) calculations

15, 17, 23-27

, but most of these studies

used the nonpolar Cu2O (111) surface as the model surface. In such an ideal surface model, the effect of surface defects was not incorporated or it was considered within a limited level. For CO oxidation mechanism on the defective surface, Sun et al. have reported the CO adsorption on oxygen-vacancy Cu2O (111) surface plane 17, and Shen et al. have studied the CO adsorption on pre-adsorbed oxygen atom on the Cu2O (111) surface 24. Information of CO adsorption on the above surface models was considered, but no reaction energy profile was provided, which was of significance to understand the CO oxidation behavior on the Cu2O (111) surface. Furthermore, as the Cu2O (111) surface with unsaturated Cu site vacancy is energetically more stable the nonpolar Cu2O (111) surface

19

, the

Cu2O (111) surface with unsaturated Cu site should also play an important role during the CO oxidation on the Cu2O surface. However, to the best of our knowledge, little is known about the CO oxidation mechanism on the defective surface although such information is needed for the modeling of CO oxidation on the real Cu2O (111) surface. The catalytic properties of defective Cu2O (111) surface deserve in-depth study. To throw light upon the properties of defective Cu2O on CO oxidation, this work mainly focused on the CO oxidation mechanism on the Cu2O (111) surface with O/Cu ratio higher than 3



stoichiometry. Two kinds of defective surface models were established including the Cu2O(111)-CuI surface model where unsaturated Cu vacancies were created on the perfect Cu2O (111) model and the Cu2O(111)+Oad surface model where adsorbed atomic O was placed on the Cu2O(111)-CuI surface and the perfect Cu2O (111) surface. The CO oxidation mechanism on these two surface models was studied. The effect of Cu vacancies and O adatoms on surface activity towards CO oxidation was addressed.

2. Methodology Theoretical calculations were carried out using spin-unrestricted DFT calculations based on DMol3 package (PBE) method

30

28-29

. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof

was used for exchange and correlation potential. Double numerical basis sets plus

polarization function (DNP) 30 was used for atomic basis functions. The DFT Semi-core Pseudopots (DSPP) method was used for pseudopotentials. A nine-atomic layer Cu2O periodic slab was cleaved from an optimized Cu2O crystal. The orbital cutoff was set as 4.0 Å during Cu2O crystal structure and surface geometric optimization, transition state search, and energy calculation. A 3×3×1 Monkhorst–Pack k-point sampling grid was used for Brillouin zone integration and a 3×3×3 k-point sampling grid was chosen for Cu2O crystal geometric optimization. The lattice constant of Cu2O crystal was 4.30 Å after geometric optimization. Nonpolar Cu2O (111) surface model was established first as it was the most stable low-index stoichiometric surface of Cu2O. Then surface Cu vacancies and adsorbed O atoms (O adatoms) were created or added to the perfect Cu2O (111) surface in order to study the effect of surface defects on surface activity. A p(2×2) Cu2O (111) surface superstructure was used to enlarge the surface area, and a 12 Å vacuum layer was placed above the slab to avoid interference from imaging surface slabs. The established surface model of nonpolar Cu2O (111) is shown in Fig. 1, which has four kinds of distinct surface sites including three-fold coordinated O site (OIII site), four-fold coordinated O site (OIV site), singly coordinated Cu site (CuI site) and doubly coordinated Cu site (CuII site), of which CuI site and OIV site are unsaturated sites and they are more 4


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likely to be responsible for the surface activity towards CO oxidation. Adsorption energy ( ) is to characterize the magnitude of interaction between CO and the surface, which is defined as , where is the energy of the total adsorption system after adsorption, is the energy of bare surface before adsorption, and is the energy of adsorbate before adsorption. The basis set superposition error (BSSE) of DNP basis set is smaller than that of 6-311+G(3df,2pd) basis set, and it is

Fig. 1 Structure of nonpolar Cu2O (111) surface (copper atoms are in bronze, and oxygen atoms are in red).

The transition states of CO oxidation elementary reaction steps were searched by the combination of linear synchronous transit (LST) and quadratic synchronous transit (QST) method. Transition state frequencies were calculated and optimized using the eigenvector-following method to guarantee only one imaginary frequency exists. Nudged elastic band (NEB) method was used to scan the minimum energy path (MEP) from initial state to final state through the transition state in order to validate the reaction route. Reaction rate constants of each elementary reaction steps were calculated according to the Harmonic Transition State Theory (HTST) following the equation

32-33

:

∆

, where k is

reaction rate constant, kB is Boltzmann constant, T is temperature, h is Plank constant, ∆ is the Gibbs free energy of activation, R is the gas constant. The transmission coefficient was taken as unity during rate constant calculations. The Gibbs free energy is the sum of electronic energy, zero point energy (ZPE), and thermal contributions from vibrational, rotational and translational degrees of freedom. The enthalpy can be written as ZPE #$% &' ' ( 5



)* . Based on ideal gas approximation, enthalpy can be rewritten as ZPE #$% &' ' ( R . For surface adsorbed species, the rotations and translations were converted into frustrated oscillations and they were considered in frequency calculations. In a similar way, the entropy can be factorized into: , ,#$% , &' ,' ( . The Gibbs energy was calculated by ,, and the ∆ is the Gibbs free energy difference between the transition state and the reactant state.

3. Results and discussion 3.1 Effect of surface Cu vacancies To consider the effect of surface defects on the surface activity, the surface reconstruction after creating a different number of Cu vacancies on the nonpolar Cu2O (111) surface was studied as shown in Fig. 2. When one surface Cu site was removed, it was found that either surface CuI or CuII site removal could yield the same surface structure with one CuI vacancy on the surface, which was in accordance with the conclusion that Cu2O (111)-CuCUS surface was the most stable Cu2O (111) structure with Cu vacancy structure 19. Similarly, creating two to four Cu vacancies on the Cu2O (111) surface would always lead to the disappearance of surface CuI sites. From the current calculation, it could be concluded that unsaturated surface CuI site is more active and less stable than the saturated CuII site, and surface saturated CuII vacancies tend to be fixed by neighboring unsaturated CuI sites if available. The CO oxidation mechanism on the perfect nonpolar Cu2O (111) surface has already been studied in our previous work

25

, and the CuI site was found to play an important role during the

adsorption and subsequent oxidation processes. It is therefore of interest to investigate the surface activity without the presence of CuI. A Cu2O (111) surface model with four surface CuI vacancies

6


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Fig. 2 Cu2O (111) surface reconstruction after creating a different number of Cu vacancies (copper atoms are in bronze, and oxygen atoms are in red, the dashed circles denote the positions of the created Cu vacancies, and the arrows denote the movement of the surface copper sites during surface reconstruction process).

(denoted as Cu2n-4On (111) surface) was established where surface top-layer CuI sites were removed. The highest occupied molecular orbital (HOMO) was analyzed and illustrated in Fig. 3, which could predict the active sites of the surfaces. For the nonpolar Cu2O (111) surface, HOMO is mainly contributed by surface CuI sites, while for the Cu2n-4On (111) surface, both OIII and CuII sites are important contributors to surface HOMO and less HOMO is donated by OIV site, implying that surface CuII and OIII sites are potential surface active sites for following reactions.

Fig. 3 The HOMO analysis of Cu2n-4On (111) surface (copper atoms are in bronze, oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

The Mars-van-Krevelen (MvK) type CO oxidation route was then studied, which was initiated by the reaction between CO and lattice O. The energy profile of CO reaction with a lattice OIII site on 7



the Cu2n-4On (111) surface was illustrated in Fig. 4, and the energy profile of CO reaction with a OIII site on the perfect nonpolar Cu2O (111) surface was also provided for comparison purpose. The adsorption of CO on Cu2n-4On (111) surface releases 0.167 eV energy, which is much lower than that of CO adsorption on the perfect Cu2O (111) surface. This significant decrease of adsorption energy can be explained by the absence of active surface CuI site, which can be strongly bonded to the C atom of CO. Also, the absence of CuI site will change the diffusion path CuI to OIII site, thus affecting the reaction of CO oxidation mechanism on the surface. When the CuI site is available, a CO molecule will first adsorb on the CuI site and then diffuse to react with the OIII site and form CO2. When the CuI site is absent, the CO molecule will first adsorb near the OIII site and then react with the OIII site forming a CO2 molecule. Although the adsorption ability of Cu2n-4On surface towards CO is weakened, its surface OIII site seems to be more active in reacting with the CO molecule, which needs to pass the energy barrier of 1.316 eV and it is lower than that on the perfect Cu2O (111) surface with an energy barrier of 1.629 eV. Besides, the reaction between CO and the OIII site on the defective surface is also an exothermic process releasing 0.603 eV, which is easier than on the perfect surface with the heat adsorption of 0.502 eV. Therefore, from the thermodynamic point of view, the reaction between CO and lattice OIII is also faster on the defective Cu2n-4On (111) surface than on the perfect nonpolar Cu2O (111) surface. After the reaction between the CO molecule and the lattice O, the desorption of CO2 from the surface needs to adsorb 0.141 eV energy and leave an O vacancy on the surface. This O vacancy may cause surface poisoning, and it can be fixed by another O2 molecule. The calculated energy barrier of O2 reaction with O vacancy is only 0.109 eV, which is an exothermic process releasing 1.389 eV energy. Therefore, after introducing the CuI vacancies, the reaction between CO and lattice OIII will be faster with the energy barrier decreasing from 1.629 eV to 1.316 eV, and the formed O vacancy can be readily fixed by gaseous O2 molecules. In addition, a Partial density of states (PDOS) analysis of the surface OIII site on the perfect Cu2O (111) and Cu2n-4On (111) surfaces are further analyzed in Fig. 4b to investigate the effect of 8


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surface Cu vacancies on surface electronic properties. After the removal of surface CuI sites, the electron states of the surface OIII site on the Cu2n-4On (111) surface are closer to the Fermi level compared those on the perfect Cu2O (111) surface, implying that the surface vacancies can improve the surface activity, which is in agreement with the calculated energy barrier values. Therefore, the absence of surface CuI sites will promote the reaction between CO and lattice O forming CO2.

Fig. 4 (a) Energy profile of CO reaction with OIII on the Cu2n-4On (111) surface and the perfect Cu2O (111) surface (The IS, TS , and FS structures of CO reaction with lattice OIII on the defective Cu2n-4On (111) surface are provided, distances of the C atom from CO and the surface OIII site is shown with the energy profile plotted in red, and the reaction energy profile of CO reaction with lattice OIII on nonpolar Cu2O (111) surface is plotted in black); (b) PDOS of the OIII site on the perfect Cu2n-4On (111) and the Cu2O (111)surfaces.

3.2 Effect of surface O adatoms on surface activity for CO oxidation The origin of adsorbed O is first investigated. The surface adsorbed oxygen on the Cu2O surface was previously observed experimentally by Achraf et al. using the X-ray photoelectron spectroscopy characterization 8. Under different preparation conditions, they ratio of surface Cu/O varied a lot, and it was found that the surface activity for CO oxidation increased with the increase of amount of adsorbed O atoms on the surface. Therefore, the adsorbed O could originate from the growth and synthesis process of the Cu2O samples. In addition, the adsorbed O on the surface could also come from O2 dissociation. Based on our calculation scheme, the energy barrier of O2 dissociation into two atomic O adatoms was calculated to be 1.199 eV, which was close to the previously reported value (123.3 kJ/mol, about 1.083 eV by Zhang et al.

34

) on the perfect Cu2O (111) surface. The O2

dissociation is an exothermic process releasing 1.389 eV, so it is also a thermodynamically favorable 9



process. The structure of the formed adsorbed atomic oxygens is similar to the adsorption structure ad1 in Fig. 9. This calculated energy barrier is lower than the energy barriers of CO reaction with lattice O on perfect Cu2O (111) and defective Cu2O (111) surfaces with the values of 1.629 and 1.316 eV respectively, and it is higher than that of CO reaction with the adsorbed O with the structure of ad1 in Fig. 9. Therefore, it can be concluded that the adsorbed oxygen can come from both the catalyst preparation period and O2 dissociation on the surface. To study the effect of surface O adatoms on the surface activity for CO oxidation, possible structures of O adsorption on the Cu2n-4On (111) surface were studied first. Figure 5 gives the stable adsorption geometries of atomic O on the Cu2n-4On (111) surface. Three kinds of stable adsorption configurations were found and they were denoted as Oad1, Oad2, and Oad3. The adsorbed atomic O in structure Oad1 is situated at the bridge site between an OIII site and CuII site with the distance between adsorbed O and OIII site and CuII site to be 1.509 Å and 1.810 Å. The adsorbed O in Oad2 is located in the middle of three CuII sites, the distance between the adsorbed O and the three neighboring CuII is 1.893 Å, 1.895 Å and 1.899 Å. The adsorbed atomic O in structure Oad3 is located among four CuII sites, which is 1.900 Å, 1.983 Å and 2.137 Å away from the top-layer CuII sites and 2.001 Å away from the second-layer CuII site. The adsorption energy of atomic O on the Cu2n-4On (111) surface site 1 to site 3 is -1.195 eV, -2.774 eV, and -2.675 eV, respectively. Therefore, adsorption structure Oad2 is the most stable atomic O adsorption structure on the Cu2n-4On (111) surface, followed by Oad3 and Oad1.

Fig. 5 Stable atomic O adsorption structures on Cu2n-4On (111) surface (the adsorbed atomic O is highlighted in yellow). 10


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The HOMO of the Cu2n-4On (111) surface with different adsorbed O structures is analyzed in Fig. 6 to illustrate their surface electronic properties. It can be seen that the surface HOMO in structure Oad1 is mainly contributed by the adsorbed O and its neighboring CuII site and OIII site, which implies that the active region on the surface is near the adsorbed O site. Similarly, the HOMO in structures Oad2 and Oad3 are also mainly contributed by the adsorbed atomic oxygen and neighboring sites, while less is contributed by the lattice O sites. Thus, we can speculate that the adsorbed O on Cu2n-4On (111) surface is more active than lattice oxygen sites, and they may be helpful for facilitating the CO oxidation on the surface.

Fig. 6 HOMO of Stable Cu2n-4On (111) surface with adsorbed O (copper atoms are in bronze, and oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

The energy profile of these O adatoms reacting with CO molecules was analyzed in Fig. 7. The energy release of CO adsorption on surface structures Oad1, Oad2 and Oad3 is in the range of -0.1 to -0.2 eV. This value is small compared with the CO adsorption energy on surface CuI site of nonpolar perfect Cu2O (111) surface (-1.558 eV). The reactions of CO with the adsorbed O in structures Oad1, Oad2 and Oad3 need to overcome the energy barrier of 0.562 eV, 0.008 eV, and 0.240 eV, respectively. Therefore, the adsorbed O in structure Oad2 is the most active site for CO oxidation, and all of the three kinds of adsorbed O can promote the CO oxidation into CO2 with a significant decrease of the energy barrier. The energy barrier of Oad3 is larger than that in Oad2, while the adsorption energy of CO adsorption of Oad3 is also larger than that of Oad2. The reason can be analyzed in terms of surface deformation induced by second layer CuII sites. For structure Oad2 the average distance between the adsorbed O and three substrate Cu sites before and after CO adsorption 11



is 1.898 Å and 1.896 Å respectively, so the adsorption of CO does not cause strong surface reconstruction. For structure Oad3, the average distance between the adsorbed O and the two substrate Cu sites before and after CO adsorption is 1.826 Å and 1.945 Å respectively, and the adsorbed O in Oad3 moved inward towards the bottom of the surface slab by 1.460 Å along Z direction perpendicular to the surface plane after adsorption. Therefore, it can be deduced that strong substrate deformation induced by the second-layer CuII occurs when CO adsorbs near the adsorbed O in structure Oad3, which results in a larger adsorption energy and a less active adsorbed atomic oxygen. By contrast, there is almost no surface deformation for CO adsorption in structure Oad2, which corresponds to a lower adsorption energy, and the activity of the adsorbed atomic oxygen is almost not affected. Thus, the adsorbed O on the Cu2n-4On (111) surface is beneficial for the catalytic oxidation of CO. A Mulliken charge analysis of the adsorbed oxygen atoms was also performed and analyzed, and the obtained Mulliken charge of the adsorbed atomic oxygens are -0.286, -0.523, -0.464 for Oad1, Oad2, and Oad3 respectively, which is positively (absolute value) correlated to the activity of the adsorbed atomic oxygen on the surface. An increase of the charge of the adsorbed atomic oxygen will decrease the energy barrier of the reaction between adsorbed atomic oxygen and CO forming CO2.

Fig. 7 The energy profile of CO reaction with adsorbed O on Cu2n-4On (111) surface (The adsorbed O is highlighted in yellow and IS, TS and FS structures in reaction route Oad2 is provided).

To investigate the role of Cu vacancies on the activity of adsorbed O towards CO oxidation, the activity of the adsorbed O on the perfect nonpolar Cu2O (111) surface is further discussed. 12


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Adsorption structure and adsorption energy of atomic O on the perfect Cu2O (111) surface are shown in Fig. 8 including the adsorption of atomic O on the surface CuI, CuII, OIII, and OIV top sites. After adsorption, structure ad1 and ad2 give similar stable adsorption geometries, which feature as an adsorbed O located between two CuII sites and one CuI site. The adsorbed O in structure ad3 forms an O-Cu-O linear structure with a surface CuII site and an OIII site, which becomes a three-fold O site after adsorption. In structure ad4, the adsorbed O causes a strong surface reconstruction after adsorption. The adsorbed O is located between three doubly-coordinated Cu sites, and its two neighboring CuI sites undergo strong surface movement after atomic O adsorption. The adsorption energy of atomic O on the perfect Cu2O (111) surface site 1 to site 4 is -4.280 eV, -4.247 eV, -2.037 eV, and -4.978 eV respectively. Therefore, structure ad4 corresponds to the most stable adsorption structure, followed by ad1, ad2, and ad3. The adsorption energy of atomic O on the perfect Cu2O (111) surface is much larger (absolute value) than that on the Cu2n-4On (111) surface, so the interaction between atomic O and the perfect Cu2O (111) surface is stronger than the Cu2n-4On (111) surface.

Fig. 8 Adsorption structures of atomic O on perfect Cu2O (111) surface (copper atoms are in bronze, and oxygen atoms are in red).

The frontier orbital analysis of the adsorbed atomic O is further analyzed as shown in Fig. 9, which predicts the active sites of the surface. The HOMOs of structures ad1 and ad2 are quite similar, and the surface adsorbed O is the main contributor to the surface frontier orbital, which indicates that 13



the surface adsorbed O is likely to be more active than the other sites on the surface to react with the following CO molecule. In a similar way, the adsorbed O in structure ad3 also contributes a lot to the surface HOMO, and it is speculated to be active for surface reactions. However, for adsorption structure ad4, the HOMO is more evenly distributed throughout the whole surface slab rather than distributed on the surface layer as are the cases in ad1 to ad3, so the surface activity of ad4 should be lower than the surface structures of ad1 to ad3. In addition, less HOMO in structure ad4 is contributed by the adsorbed O, and the neighboring three Cu sites are the main contributors the surface HOMO, so the O site in ad4 may not be very active in the following reactions.

Fig. 9 HOMO analysis of adsorbed O atoms on nonpolar Cu2O (111) surface (copper atoms are in bronze, oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

A PDOS analysis of the adsorbed O is then analyzed to investigate the electron states as shown in Fig. 10. It can be seen that the PDOS of the adsorbed O atoms in structures ad1 and ad2 are almost identical, which further proves that the adsorbed O in structures ad1 and ad2 are very similar. The 2s states are located between the range from -7.0 eV to Fermi level (EF) with two strong peaks at around -4.0 eV and near Fermi level. Compared with the lattice O, whose electron distributions are lower than the adsorbed O in structure ad1 and ad2, the PDOS results imply that the adsorbed O in structure ad1 and ad2 should be more active than the lattice O．From the bottom two rows of PDOS in Fig. 10, it can be seen that the PDOS of the adsorbed O of ad4 is very similar to that of lattice O. The 2s orbitals of the adsorbed O and the lattice O are both located near the peak value of -18.5 eV, and the 2p orbitals of them also have a similar distribution from -7.5 eV to Fermi level. It can, therefore, be concluded that the adsorbed O in adsorption structure ad4 has evolved into a lattice O 14


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after surface reconstruction. The PDOS of the O in ad3 has fewer electron states near Fermi level than ad1 and ad2, but its distribution is higher than that of ad4 near Fermi level, so its activity is speculated to be lower than ad1 and higher than ad4.

Fig. 10 PDOS analysis of O adatoms on Cu2O (111) surface (the dashed line represents the Fermi level).

The activities of these adsorbed O in reacting with CO molecules are further investigated by exploring their reaction pathway into CO2 as shown in Fig. 11. CO adsorption near the adsorbed O in adsorption structures ad1 and ad2 releases 0.177 eV. Then the adsorption complex needs to pass the energy barrier of 0.269 eV and release 2.233 eV. Comparatively, the adsorbed O in ad3 is less active, which needs to overcome the energy barrier of 1.148 eV and release 0.275 eV. The adsorbed O in the adsorption structure ad4 has the lowest activity. The energy barrier is as high as 1.732 eV when the adsorbed O reacts with a CO molecule forming CO2. Its energy barrier is even higher than that of the unsaturated surface O site of the perfect Cu2O (111) surface. Therefore, the activity of the surface O adatoms depends on the location where the O adatom is situated. The O adatom situated between two doubly coordinated Cu sites and one singly coordinated Cu site is very active when reacting with the CO molecule. The adsorbed O in adsorption structure ad3 is less active than that in ad1 and 2 but it is still more active than the lattice O site. The activity of the adsorbed O in ad4 is even worse than the lattice O site.

15



Fig. 11 Energy profile of CO reaction with O adatoms on Cu2O (111) surface (the energy profile starts with the structures of nonpolar Cu2O (111) surface after atomic O adsorption including ad1 to ad4, and zero energy has been designated for all the reaction routes at the initial states without illustrating the adsorption energy. Adsorption energies can refer to Fig. 9. “+CO” denotes the CO adsorption step. A lower adsorption energy corresponds to a stronger interaction between CO and the surface. TS denotes transition states.).

The above calculations have considered the surface with one adsorbed atomic oxygen. From the calculated energy barrier and reaction energy, it can be seen that the reaction of O2 dissociation into two adsorbed O is more favorable than the reverse process, i.e. the recombination process of two neighboring adsorbed O atoms is slow compared with the O2 decomposition process. The two adsorbed O atoms from O2 decomposition on the perfect Cu2O (111) surface causes a slight surface reconstruction. The energy profile of CO reaction with one of the two adsorbed O has also been studied, and the calculated energy barrier is 0.146 eV, which is similar to the case of only one adsorbed atomic O on the nonpolar Cu2O (111) surface (with the energy barrier of 0.269 eV). Therefore, two adsorbed oxygen on the perfect Cu2O (111) surface are hard to recombine, and two adsorbed oxygen can cause surface reconstruction, but it does not cause significant change of surface properties.

3.3 Rate constants Rate constants of elementary reaction steps are important for the modeling work of kinetic study. Based on HTST, the reaction rate constants of elementary reaction steps are plotted in Fig. 12. The reaction rate constants in Arrhenius form - . exp /4 are listed in Table 1 with the 16


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Fig. 12 Rate constants of CO oxidation on defective and nonpolar Cu2O (111) surface. Table 1 Rate constants of CO oxidation on defective Cu2O (111) surface in Arrhenius form. Reaction step Oad1+CO Oad2+CO Oad3+CO ad1+O ad3+O ad4+O

A (cm3/mol/s)

n

E (kcal/mol)

7

-0.405

12.543

6

2.018

-0.044

5

2.938

5.771

8

2.009

6.035

10

0.070

25.976

9

1.128

39.320

6.793×10 1.664×10

1.474×10 3.686×10 9.672×10

3.640×10

calculated parameters A, n, and E. At 298.15 K, reaction rates of reaction Oad1+CO, ad3+CO and ad4+CO are negligible, while the reaction rates of ad1+CO, Oad2+CO, and Oad3+CO reaches 1.446×109 s-1mol-1L, 1.728×1011 s-1mol-1L, and 1.386×108 s-1mol-1L respectively. Thus, the adsorbed O on Cu2n-4On (111) surface Oad2 site is more active than other adsorbed atomic O towards CO oxidation, and it is even more than 100 times faster than the adsorbed O on nonpolar Cu2O (111) surface. With the increase of temperature, the rate of the reaction between adsorbed O and CO increases relatively faster than the adsorbed O on Cu2n-4On (111) surface. When the temperature is higher than 575 K, the reaction of adsorbed O and CO is faster on the nonpolar surface than on Cu2n-4On (111) surface. If the temperature exceeds 800 K, the adsorbed O in adsorption structure Oad3 is faster than the adsorbed O in Oad2. Therefore, we can see that the surface activity and surface active site are strongly dependent on temperature, and Cu vacancies have an important promoting effect on surface activity towards CO oxidation especially at the low temperature range 17



below 575 K. If the nonpolar Cu2O (111) surface is available, the adsorbed O in ad1 structure will dominate the surface activity for CO oxidation. If the temperature exceeds 800 K, the adsorbed O in Oad2 structure is the most active site for CO oxidation on Cu2n-4On (111) surface.

4. Conclusions The effect of surface defects on Cu2O surface activity towards CO oxidation was studied based on DFT calculations. The Cu2O (111) surface Cu vacancy was found to occur at the unsaturated Cu site, and Cu2O (111) surface without unsaturated Cu sites could promote the CO oxidation by lowering the energy barrier from 1.629 to 1.316 eV following MvK type surface reaction mechanism. The adsorbed O atoms on both Cu2n-4On (111) surface and perfect Cu2O (111) surface are very active in reacting with CO. The adsorbed O on the polar Cu2n-4On (111) surface can promote the CO oxidation especially below 575 K, and the adsorbed O on the nonpolar Cu2O (111) surface is the most active site for CO oxidation when temperature is higher than 575 K. The adsorbed O in structure ad3 is more active than that in ad2 on Cu2n-4On (111) surface when the temperature is higher than 800 K. The rate constants are provided in Arrhenius form, which may be helpful for future CO modeling work on Cu2O surfaces.

Acknowledgment The authors thank the financial support from the Natural Science Foundation of China (No 51476168/91541102), Ministry of Science and Technology of China (2017YFA0402800), Recruitment Program of Global Youth Experts and the Fundamental Research Funds for the Central Universities (No. 2016YQ07).

References 1. Liu, J.-X.; Filot, I. A. W.; Su, Y.; Zijlstra, B.; Hensen, E. J. M., Optimum Particle Size for Gold-Catalyzed CO Oxidation. J Phys Chem C 2018, 122, 8327-8340. 18


Page 18 of 24

Page 19 of 24 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 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. Xi, Y.; Ren, J.-C., Design of a CO Oxidation Catalyst Based on Two-Dimensional MnO2. J Phys

Chem C 2016, 120, 24302-24306. 3. Song, W.; Su, Y.; Hensen, E. J. M., A DFT Study of CO Oxidation at the Pd–CeO2(110) Interface.

J Phys Chem C 2015, 119, 27505-27511. 4. Chen, S.; Cao, T.; Gao, Y.; Li, D.; Xiong, F.; Huang, W., Probing Surface Structures of CeO2, TiO2, and Cu2O Nanocrystals with CO and CO2 Chemisorption. J Phys Chem C 2016, 120, 21472-21485. 5. Xie, X. W.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. J., Low-Temperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746-749. 6. Qi, J.; Chen, J.; Li, G.; Li, S.; Gao, Y.; Tang, Z., Facile Synthesis of Core–Shell Au@CeO2 Nanocomposites with Remarkably Enhanced Catalytic Activity for CO Oxidation. Energy Environ

Sci 2012, 5, 8937-8941. 7. Widmann, D.; Behm, R. J., Active Oxygen on a Au/TiO2 Catalyst: Formation, Stability, and CO Oxidation Activity. Angew Chem Int Ed Engl 2011, 50, 10241-10245. 8. El Kasmi, A.; Tian, Z.-Y.; Vieker, H.; Beyer, A.; Chafik, T., Innovative CVD Synthesis of Cu2O Catalysts for CO Oxidation. Appl Catal B 2016, 186, 10-18. 9. Yang, Y.; Dong, H.; Wang, Y.; Wang, Y.; Liu, N.; Wang, D.; Zhang, X., A Facile Synthesis for Porous CuO/Cu2O Composites Derived from Mofs and Their Superior Catalytic Performance for CO Oxidation. Inorg Chem Commun 2017, 86, 74-77. 10. White, B.; Yin, M.; Hall, A.; Le, D.; Stolbov, S.; Rahman, T.; Turro, N.; O'Brien, S., Complete CO Oxidation over Cu2O Nanoparticles Supported on Silica Gel. Nano Lett 2006, 6, 2095-2098. 11. Gattinoni, C.; Michaelides, A., Atomistic Details of Oxide Surfaces and Surface Oxidation: The Example of Copper and Its Oxides. Surf Sci Rep 2015, 70, 424-447. 12. Bendavid, L. I.; Carter, E. A., CO2 Adsorption on Cu2O(111): A DFT+U and DFT-D Study. J

Phys Chem C 2013, 117, 26048-26059. 19



13. Casarin, M.; Maccato, C.; Vigato, N.; Vittadini, A., A Theoretical Study of the H2O and H2S Chemisorption on Cu2O(111). Appl Surf Sci 1999, 142, 164-168. 14. Islam, M. M.; Diawara, B.; Maurice, V.; Marcus, P., Bulk and Surface Properties of Cu2O: A First-Principles Investigation. Theochem 2009, 903, 41-48. 15. Le, D.; Stolbov, S.; Rahman, T. S., Reactivity of the Cu2O(100) Surface: Insights from First Principles Calculations. Surf Sci 2009, 603, 1637-1645. 16. Sun, B. Z.; Chen, W. K.; Xu, Y. J., Reaction Mechanism of CO Oxidation on Cu2O(111): A Density Functional Study. J Chem Phys 2010, 133, 154502. 17. Sun, B.-Z.; Chen, W.-K.; Zheng, J.-D.; Lu, C.-H., Roles of Oxygen Vacancy in the Adsorption Properties of CO and NO on Cu2O(111) Surface: Results of a First-Principles Study. Appl Surf Sci

2008, 255, 3141-3148. 18. Yu, X.; Zhang, X.; Tian, X.; Wang, S.; Feng, G., Density Functional Theory Calculations on Oxygen Adsorption on the Cu2O Surfaces. Appl Surf Sci 2015, 324, 53-60. 19. Soon, A.; Todorova, M.; Delley, B.; Stampfl, C., Thermodynamic Stability and Structure of Copper Oxide Surfaces: A First-Principles Investigation. Phys Rev B 2007, 75, 125420. 20. Li, C.; Wang, F.; Li, S. F.; Sun, Q.; Jia, Y., Stability and Electronic Properties of the O-Terminated Cu2O(111) Surfaces: First-Principles Investigation. Phys Lett A 2010, 374, 2994-2998. 21. Onsten, A.; Gothelid, M.; Karlsson, U. O., Atomic Structure of Cu2O(111). Surf Sci 2009, 603, 257-264. 22. Onsten, A.; Weissenrieder, J.; Stoltz, D.; Yu, S.; Gothelid, M.; Karlsson, U. O., Role of Defects in Surface Chemistry on Cu2O(111). J Phys Chem C 2013, 117, 19357-19364. 23. Bredow, T.; Pacchioni, G., Comparative Periodic and Cluster Ab Initio Study on Cu2O(111)/CO.

Surf Sci 1997, 373, 21-32. 24. Shen, Y.; Tian, F. H.; Chen, S.; Ma, Z.; Zhao, L.; Jia, X., Density Functional Theory Study on the Mechanism of CO Sensing on Cu2O (111) Surface: Influence of the Pre-Adsorbed Oxygen Atom. 20


Page 20 of 24

Page 21 of 24 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 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Appl Surf Sci 2014, 288, 452-457. 25. Wu, L.-N.; Tian, Z.-Y.; Qin, W., Mechanism of CO Oxidation on Cu2O (111) Surface: A DFT and Microkinetic Study. Int J Chem Kinet 2018, 50, 507-514. 26. Soon, A.; Söhnel, T.; Idriss, H., Plane-Wave Pseudopotential Density Functional Theory Periodic Slab Calculations of CO Adsorption on Cu2O(111) Surface. Surf Sci 2005, 579, 131-140. 27. Zuo, Z.; Huang, W.; Han, P.; Li, Z., Solvent Effects for CO and H2 Adsorption on Cu2O (111) Surface: A Density Functional Theory Study. Appl Surf Sci 2010, 256, 2357-2362. 28. Delley, B., An All‐Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J Chem Phys 1990, 92, 508-517. 29. Delley, B., From Molecules to Solids with the DMol3 Approach. J Chem Phys 2000, 113, 7756-7764. 30. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple.

Phys Rev Lett 1996, 77, 3865-3868. 31. Sun, B.-Z.; Chen, W.-K.; Xu, Y.-J., Reaction Mechanism of CO Oxidation on Cu2O(111): A Density Functional Study. J Chem Phys 2010, 133, 154502. 32. Vineyard, G. H., Frequency Factors and Isotope Effects in Solid State Rate Processes. J Phys

Chem Solids 1957, 3, 121-127. 33. Piskorz, W.; Zasada, F.; Stelmachowski, P.; Diwald, O.; Kotarba, A.; Sojka, Z., Computational and Experimental Investigations into N2O Decomposition over MgO Nanocrystals from Thorough Molecular Mechanism to Ab Initio Microkinetics. J Phys Chem C 2011, 115, 22451-22460. 34. Zhang, R.; Liu, H.; Zheng, H.; Ling, L.; Li, Z.; Wang, B., Adsorption and Dissociation of O2 on the Cu2O(111) Surface: Thermochemistry, Reaction Barrier. Appl Surf Sci 2011, 257, 4787-4794.

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Table & Figure captions Color figure in electronic versions only

Fig. 1 Structure of nonpolar Cu2O (111) surface (copper atoms are in bronze, and oxygen atoms are in red).

Fig. 2 Cu2O (111) surface reconstruction after creating a different number of Cu vacancies (copper atoms are in bronze, and oxygen atoms are in red, the dashed circles denote the positions of the created Cu vacancies, and the arrows denote the movement of the surface copper sites during surface reconstruction process).

Fig. 3 The HOMO analysis of Cu2n-4On (111) surface (copper atoms are in bronze, oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

Fig. 4 (a) Energy profile of CO reaction with OIII on the Cu2n-4On (111) surface and the perfect Cu2O (111) surface (The IS, TS , and FS structures of CO reaction with lattice OIII on the defective Cu2n-4On (111) surface are provided, distances of the C atom from CO and the surface OIII site is shown with the energy profile plotted in red, and the reaction energy profile of CO reaction with lattice OIII on nonpolar Cu2O (111) surface is plotted in black); (b) PDOS of the OIII site on the perfect Cu2n-4On (111) and the Cu2O (111)surfaces.

Fig. 5 Stable atomic O adsorption structures on Cu2n-4On (111) surface (the adsorbed atomic O is highlighted in yellow).

Fig. 6 HOMO of Stable Cu2n-4On (111) surface with adsorbed O (copper atoms are in bronze, and oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

Fig. 7 The energy profile of CO reaction with adsorbed O on Cu2n-4On (111) surface (The adsorbed O is highlighted in yellow and IS, TS and FS structures in reaction route Oad2 is provided).

Fig. 8 Adsorption structures of atomic O on perfect Cu2O (111) surface (copper atoms are in bronze, 22


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and oxygen atoms are in red).

Fig. 9 HOMO analysis of adsorbed O atoms on nonpolar Cu2O (111) surface (copper atoms are in bronze, oxygen atoms are in red, yellow and blue shaded areas represent the HOMO).

Fig. 10 PDOS analysis of O adatoms on Cu2O (111) surface (the dash line represents the Fermi level).

Fig. 11 Energy profile of CO reaction with O adatoms on Cu2O (111) surface (the energy profile starts with the structures of nonpolar Cu2O (111) surface after atomic O adsorption including ad1 to ad4, and zero energy has been designated for all the reaction routes at the initial states without illustrating the adsorption energy. Adsorption energies can refer to Fig. 9. “+CO” denotes the CO adsorption step. A lower adsorption energy corresponds to a stronger interaction between CO and the surface. TS denotes transition states.).

Fig. 12 Rate constants of CO oxidation on defective and nonpolar Cu2O (111) surface. Table 1 Rate constants of CO oxidation on defective Cu2O (111) surface in Arrhenius form.

23



TOC Graphic

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A DFT Study of CO Catalytic Oxidation Mechanism on the Defective

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