A Density Functional Theory Study of Formaldehyde Catalytic

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A Density Functional Theory Study of Formaldehyde Catalytic Oxidation Mechanism on Au Doped CeO(111) Surface 2

Meizan Jing, Weiyu Song, Lulu Chen, Sicong Ma, Jianlin Deng, Huiling Zheng, Yongfeng Li, Jian Liu, and Zhen Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09276 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

A Density Functional Theory Study of Formaldehyde Catalytic Oxidation Mechanism on Au Doped CeO2(111) Surface

Meizan Jing,† Weiyu Song,†,§ Lulu Chen,† Sicong Ma,† Jianlin Deng,† Huiling Zheng,† Yongfeng Li†, Jian Liu,*,† Zhen Zhao,†, †State

‡

Key Laboratory of Heavy Oil Processing, College of Science, China University

of Petroleum-Beijing, Beijing 102249, P. R. China ‡

Institute of Catalysis for Energy and Environment, Shenyang Normal University,

Shenyang 110034, P. R. China

ABSTRACT Formaldehyde is a harmful and toxic substance. Au-CeO2 catalysts show the excellent formaldehyde catalytic oxidation activity even under ambient temperatures. Here, we present the DFT+U calculations to investigate HCHO oxidation mechanisms and the effects of Au doping and multiply oxygen vacancies. The reaction process of HCHO oxidation mainly consists of the following steps: HCHO adsorption, C-H bond cleavages, CO2 desorption, O2 adsorption, and H2O formation and desorption. The doped Au reduces the energy barriers in C-H bond cleavages on AuCe1-xO2(111) surface comparing with CeO2(111) surface. Au also leads the activation of the surface oxygen species, and then promotes HCHO adsorption and decreases the formation energies of oxygen vacancies. For HCHO adsorption and oxidation reaction on defective surfaces, catalysts with more oxygen vacancies possess higher adsorption 1

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energy and lower activation energy. These results provide deep insights into the effects of Au and multiple oxygen vacancies on HCHO oxidation reactions on Au doped CeO2(111) surface, and reveal the essential reason for the high activity of Au-CeO2 catalysts.

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1. INTRODUCTION Formaldehyde (HCHO), one important volatile organic compound (VOC), is the most common and pernicious indoor gas contaminant with the property of colorless and strongly irritating smells. With the widespread applications of artificial materials and household products, such as paints, adhesives, dyes, decorating materials, permanent-press fabrics, and pressed-wood products and so on1,2, formaldehyde is around us in everywhere. Long-term exposure to the indoor air containing formaldehyde, over 0.08 mg/m3 limit of indoor air in China criterion3, can cause serious health problems2,4. Rich sources and serious hazards need us to make great efforts to eliminate HCHO. Catalytic oxidation5-8 has been considered as the most promising method to convert HCHO from the indoor air at a very low concentration to CO2 and H2O under mild condition. Among some catalysts used in HCHO catalytic oxidation5, oxide-supported noble-metal catalysts (Pt, Pd, Au and Ag) show excellent catalytic activity and thermostability. Despite the outstanding low temperature activity, the high cost and low content limit Pt-based catalyst application. Supported Au catalysts can be a good alternative to Pt catalysts1. In 1989, Haruta et al.9 found that highly dispersed gold nanoparticles supported on transition metal oxides exhibited excellent activity in CO oxidation at low temperature. Since then, gold catalysts have showed their good activity and been applied in many aspects, such as the oxidation of VOCs10,11, the water-gas shift reaction12-16, and the hydrogenation of carbon oxides17. For various supported gold catalysts, CeO2, due to its remarkable redox properties associated with facile conversion of Ce4+ to Ce3+ and strong interaction with metal, is 3



the most widely investigated support. Many experiment researches have showed the excellent performance of Au/CeO2 catalysts on HCHO oxidation18-21. Chen et al.22 found that Au/CeO2 prepared by deposition-precipitation method using urea as precipitating agent (DPU) can afford 100% conversion of HCHO into CO2 at room temperature. They thought the presence of Au3+ and Ce3+ resulted in the generation of active surface oxygen species and hence superior activity. Li et al.23 also reported the enhanced catalytic ability of HCHO oxidation on Au/CeO2. In their opinions, high oxidation states of Au species can provide centers for HCHO chemisorption. Based on the experimental investigations, Au3+ species play the vital role in the outstanding property of Au/CeO 2 for HCHO oxidation. In addition to promoting the activation of surface oxygen and formaldehyde adsorption, the role of Au for HCHO oxidation is still unclear. In some literatures22,24, the possible mechanisms for formaldehyde catalytic oxidation over Au/CeO2 catalyst are proposed. However, some specific reactants and intermediates participated in HCHO oxidation are not clear, e.g. the further oxidation of formate and the formation of CO2 and H2O. Therefore, the exhaustive formaldehyde oxidation pathways on Au/CeO2 catalysts based on DFT calculations are indispensable. So, the first question is to explore the role of Au in catalytic process and the specific mechanism of formaldehyde oxidation. For ceria catalyst, the important role of oxygen vacancy is well acknowledged25. Moreover, the presence of doping metal can decrease the oxygen vacancies formation energy26. However, few DFT works exist to study the influence of oxygen vacancy 4


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concentration on the HCHO oxidation activity, which is a pretty important issue as already observed via some experimental studies. Wang et al.27 found that the catalytic activity of formaldehyde oxidation on Co3O4 was surface oxygen vacancies mediated and showed a direct relationship with the concentration of surface oxygen vacancies. Li23 et al. also reported that the enhanced HCHO oxidation activity of Au/CeO2 lied in the more oxygen vacancies which came from the formation of an AuxCe1-xO2-δ solid solution. Another aim of present study is to investigate the effect of multiple oxygen vacancies on formaldehyde oxidation for Au doped CeO2(111) surface. In view of the above mentioned points, we performed detailed DFT study to explore the reaction pathway of HCHO oxidation based on a model in which one of the surface Ce was substituted by Au (denoted as AuCe1-xO2). The oxygen vacancy models were simulated via the generation of one oxygen vacancy (denoted AuCe1-xO2-y) and two oxygen vacancies (denoted AuCe1-xO2-2y). Meantime, the discussions about effects of Au and multiple oxygen vacancies on HCHO oxidation reactions are also included.

2. COMPUTATIONAL DETAILS All calculations were based on the density functional theory (DFT) and performed with the Vienna ab initio simulation package (VASP) code28,29. The exchange and correlation energy functional was expressed in the GGA-PBE 30. The projector-augmented wave method31,32 was used to describe the interactions between ions and electrons. The valence electrons were solved in the plane-wave basis with a 5



cutoff energy of 400 eV. The convergence criteria for the energy calculation and structure optimization were set to 1.0×10-4 eV and a force tolerance of 0.05 eV/Å, respectively. The Brillouin-zone integration33 was performed using a 1 × 1 × 1 Γ-centered k-point mesh with Gaussian smearing set to 0.05 eV. The larger k-point mesh (3 × 3 × 1), more stringent force convergence threshold (0.02 eV/Å) and higher cut-off energy (500 eV) were used to test the calculation accuracy (Supporting Table S1). Negligible differences justify the accuracy of present computational settings. To accurately describe the localization of Ce 4f electrons, we conducted the DFT+U calculations with a value of Ueff=4.5 eV. The setting of Hubbard-like term (Ueff) follows the approach explored by Fabris et al.34, Cococcioni and De Gironcoli35. The adsorption energy was defined by: Eads = E(adsorbate/surface) − E(adsorbate) − E(surface) Where E(adsorbate/surface) is the total energy of a surface interacting with adsorbate, E(adsorbate) and E(surface) are the energies of the isolated adsorbate and clean surface, respectively. Therefore, a negative value means exothermic adsorption. The more negative the adsorption energy, the stronger the adsorption. The oxygen vacancy formation energy was calculated by: EOv =Esurface-Ov + EO2 -Esurface E2Ov =Esurface-2Ov + EO2 -Esurface-Ov Where Esurface is the energy of perfect surface; Esurface-Ov is the energy of defective surface with an oxygen vacancy; Esurface-2Ov is the energy of defective surface with two oxygen vacancies and EO2 is the energy of a gas-phase O2. 6


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Reaction transition states (TS) were calculated with the climbing image nudged elastic band (CI-NEB) method36,37. Frequency analysis was carried out to ensure that there was only a single imaginary frequency for transition state. The energy barrier (ETS) was defined as the total energy difference between the transition state and initial structure. The charge analysis was carried out by the Bader Charge method 38. We chose the CeO2(111) surface, the most stable surface termination39, which consisted 9 atomic layers (three O-Ce-O trilayers) with a 3×3 unit cell (Figure 1). A set of test calculations on the oxygen vacancy formation and adsorption energy of key intermediates based on the thicker slab models with 4 and 5 O-Ce-O trilayer showed negligible difference (Table S2, S3 and S4). The vacuum space that perpendicular to the surface was set to 12 Å in order to minimize the interaction between distinct slab surfaces. During geometry optimization, the top six atom layers of ceria were relaxed, while bottom three layers were fixed in their bulk positions.

3. RESULTS 3.1 The structure of stoichiometric AuCe1-xO2(111) surface and defective Au doped CeO2(111) surfaces. The optimized structures of stoichiometric AuCe1-xO2(111) surface, defective AuCe1-xO2-y(111) (one oxygen vacancy) surface and defective AuCe1-xO2-2y(111) (two oxygen vacancies) surface are shown in Figure 2. Au atom coordinates to two surface O and two subsurface O atoms, with the Au-O bond distances of 2.12-2.13 Å. The surface of AuCe1-xO2 shows two two-coordinated oxygen atoms (named as O2c, 7



labeled with pink in Figure 2a), which locate on the surface and the sub-surface, respectively. The Bader Charge of the Au is calculated to be +1.27 |e|, suggesting an oxidation state of +340,41. The magnetic moments of cerium do not change, which means no Ce4+ is reduced to Ce3+. The electronic changes due to Au doping bring about the transformation of O2- to O- (delocalized on the uncoordinated O2c). Removal of two uncoordinated oxygen atoms will lead to different reduced AuCe 1-xO2-y surfaces (shown in the Figure 2b and Figure 2c). The oxygen vacancy energy are 0.17 and 0.11 eV, respectively, much lower than that of pure ceria (2.24 eV). Same as that study on pure CeO2, the oxygen vacancy localized preferentially on the subsurface than surface42-44. However, the subsurface vacancy can only be formed after a surface vacancy is generated during reaction45. Accordingly, the AuCe1-xO2-y-surf (Figure 2b) will be chosen to the reaction mechanism study of defective AuCe1-xO2-y. Defective AuCe1-xO2-2y with linearly arranged two surface oxygen vacancies (Figure 2d) is taken to simulate the situation of multiple oxygen vacancies46,47. The removal of the second oxygen will take an endothermic energy of 1.11 eV, lower than same process on ceria (i.e. generation of second oxygen vacancy) without Au doping (2.46 eV). In Au-CeO2 catalysts, CeO2 supported Au nanoparticles catalysts are fairly common in experimental researches22-24. However, the type of Au doped CeO2, as a form represented the strong interaction between gold and ceria, might also exist in experiments21,23. This is confirmed by the large adsorption energy of Au on Ce vacancy, suggesting a strong preference for Au atom to enter into the ceria lattice14. Camellone14 also reported that Au substituted for Ce lattice site was more active than 8


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single Au+ species supported on CeO2 surface about CO oxidation. Given the similarity between CO oxidation and HCHO oxidation, we use the Au doped CeO2. The stability of doped Au single atom on ceria is studied (Figures S1). The high barrier of Au migration suggests a highly stabilized doping state. 3.2 HCHO oxidation reaction mechanism on the stoichiometric AuCe1-xO2(111) surface. The complete reaction pathway for formaldehyde oxidation on the stoichiometric AuCe1-xO2(111) surface is presented in Figure 3, including structures of important intermediates and transition-state, while the magnetic moments of Ce and Bader charge differences of Au in different reaction stages are shown in the Table S5. The whole reaction mechanism can be divided into the following steps: HCHO adsorption (state i-state iii), two-step C-H bond cleavages (state iv-state vii), CO2 desorption (state viii), O2 adsorption and cleavage (state ix-state x(1)), and finally, H 2O formation and desorption (state xi(1)-state i). The possible adsorption configurations and energies of HCHO and O2 are shown in the Figure S2 and Table S6. Chemisorbed HCHO interacts strongly with AuCe1-xO2(111) surface with the high adsorption energy of -1.17 eV, which is stronger than oxygen adsorption (-0.49 eV). So, during the reaction process, HCHO firstly adsorbs on the O2c of catalyst surface to form the dioxy-methylene (state ii). The oxidation state of Au also shows +3 with the Bader Charge of 1.41 |e|. Then, the adsorbed dioxy-methylene spontaneously reverses to a more stable state with an exothermic energy of 0.32 eV (state iii). Next, one of C-H bond dissociates with the H 9



atom transferred to the Au (state iv). The reaction energy of this step is exothermic by 1.14 eV, and no apparent energy barrier is identified in the CI-NEB calculations. The H can be relocated to the adjacently two-coordinated surface O atom (state v) with a small energy barrier of 0.06 eV (state TS1), which releases 2.02 eV. Similarly, for the second C-H bond cleavage, the H atom of adsorbed HCOO- intermediate firstly migrates to the Au atom (state vi) (ETS=0.66 eV, ∆E=0.37 eV), then to the surface O atom of the ceria lattice (state vii) (ETS=0.50 eV, ∆E=-1.32 eV). During the first C-H bond cleavage, the excess electron produced by Au doping exists on the surface oxygen. After H transfers to Au on second C-H bond cleavage, CO2 forms. This step produces four excess electrons. Two excess electrons lead to the changes of Au3+ to Au+ (Bader Charge of 0.43 |e|). The rest of electrons lead to the reduction of O- to O2and formation of Ce3+. The generated CO2 can easily desorb into gas phase with the desorption energy by 0.13 eV (state viii). With the removal of CO2, the catalyst surface remains an oxygen vacancy. Molecular oxygen strongly adsorbs to this vacancy site with Eads=-1.23 eV (state ix). The distance of O-O bond is elongated to 1.42 Å (1.21 Å in gas phase), indicating a peroxide-type O22- species. O2 adsorption also causes the oxidation state of Au backs to +3. Then O-O bond dissociates to two oxygen atoms with the reaction energy of 0.91 eV and the activation barrier of 0.94 eV (TS4(1)). Accompanying with O-O bond cleavage, the excess electron initial localized on Ce atom returns to surface oxygen. One oxygen atom fills in the oxygen vacancy, and the other one attaches to the top site of the Au atom (state x(1)). After that, the two H atoms migrate in succession to oxygen on top of Au atom to form H2O 10


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molecule (state xi(1)-xii(1)) with the reaction energy of 0.28 and 0.10 eV, respectively. An alternative pathway of water formation via the H direct transfer first has also been explored (Figure S3). However, the higher energy barrier suggests the rate of this path should be much lower than that presented in Figure 3 (state xi(1)-state i). It takes 0.20 eV to desorb the generated H2O molecule. Then the whole reaction cycle is completed. CO2 desorption results in the formation of oxygen vacancy, but O2 adsorption and H2O formation contribute to the recovery of catalyst surface. After H2O desorbs from surface, the catalyst returns to the initial state. The potential energy diagram shows the largest reaction energy and energy barrier, which are 0.91 and 0.94 eV, respectively, existing in the process of O-O bond cleavage (state TS4(1)). The O-O bond cleavage becomes the rate control step in the whole reaction. 3.3 HCHO oxidation reaction mechanism on the defective AuCe1-xO2-y(111) surface. With the doping of Au into the ceria lattice, the oxygen vacancy formation energy was dramatically reduced to 0.17 eV (2.24 eV on CeO2(111)). On the surface of AuCe1-xO2-y(111), the excess electrons due to the formation of oxygen vacancy lead to the formation of Ce3+ and Au3+ (Bader Charge of 0.86 |e|). The detailed spin magnetic moments for Ce ion and Bader charge differences of Au ion in overall reaction processes are shown in Table S7. With regard to the adsorption of HCHO and O2 on defective AuCe1-xO2-y(111) surface, the optimized probable adsorption configurations and energies are shown in the Figure S4 and Table S8. Figure 4 shows the most stable configurations of HCHO 11



and O2. For HCHO adsorption, the O atom fills the oxygen vacancy and connects with Ce in lattice. The C atom is close to Au. The distances of O-C and C-Au are 1.28 and 2.38 Å, respectively (Figure 4a). Au also shows +3 valence, no changes with the adsorption of HCHO. HCHO adsorption energy is -0.92 eV, stronger than O2 adsorption (-0.58 eV). The higher adsorption energy of HCHO than O2 suggests that the surface of AuCe1-xO2-y(111) should be preferentially occupied by HCHO instead of O2. For O2 adsorption, the excess electron initially localized on Ce is now present on the O2 molecule, which causes the increase of O-O bond distance. The increased O-O bond length of 1.34 Å indicates a superoxide-type O2- species (Figure 4b). After HCHO adsorbed on AuCe1-xO2-y surface (state ii, Figure 5-1), one of the H atoms in HCHO transfers to the neighboring lattice O with a reaction energy of -1.41 eV (state iii, Figure 5-1). The activation energy in this step is 0.84 eV, which is the largest in whole reaction. So, the first C-H bond cleavage is the rate control step on defective AuCe1-xO2-y catalyst. Now, the surface remains an OH- and a CHO- species. Afterwards, the H of CHO- species transfers to a neighboring lattice O. The reaction energy and active barrier for this step are -1.04 and 0.80 eV, respectively. The two-step C-H bond cleavages lead to an adsorbed CO2- species which connects with Au and two surface hydroxyls (state iv, Figure 5-1). There are no changes of magnetic moment on these reactions. The following reactions undergo different pathways due to the changes of CO2- species. The Path 1 to Path 3 are showed in the Figure 5-2 to Figure 5-4, respectively. Path 1 (Figure 5-2). CO2- can rotate slightly, with a cost of 0.04 eV, to expose 12


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the oxygen vacancy on surface (state v(1)), which can adsorb O2 (Eads=-0.84 eV) (state vi(1)). The excess electron transfers from Ce to adsorbed O2 molecule, resulting in elongation of the O-O bond to 1.34 Å. CO and O2- species can easily react to form CO2 (state vii(1)) with a reaction energy of -3.66 eV. Afterwards, the generated excess electron in this step backs to Ce. It needs the energy of 0.17 eV to desorb CO2 (state viii(1)). After CO2 desorption, the two remaining OH- groups can recombine to form a H2O. Meanwhile, the structure restores to initial one (state ix(1)). However, a relatively large energy (2.45 eV) is needed for the formation and desorption of water. In perfect AuCe1-xO2(111) surface, two H from surface OH- react with the adsorbed active O atom, while there is no adsorbed O atom in defective surface. The formation of water needs to take the three-coordinated lattice oxygen off, which is more difficult than the removal of the adsorbed two-coordinated oxygen. The huge energy indicates that this step is difficult to take place at low temperature. So, it is hard for the catalyst to restore the original structure. Path 2 (Figure 5-3). Starting from state iv, the formed CO2- can also desorb from surface with an endothermic energy of 0.82 eV. The two excess electrons due to CO desorption cause the transfer of Au3+ to Au+ (Bader Charge of 0.64 |e|). Then, one oxygen vacancy and two OH- remain the surface. The oxygen vacancy can attract oxygen molecule, where the excess electron from Ce resulting in the formation of superoxide-type O2- (O-O bond length of 1.34 Å) (state vi(2)). The process is exothermic by 0.64 eV. The adsorbed O2- substance undergoes the O-O bond breakage with the energy barrier of 0.80 eV. One dissociated oxygen atom fills into the oxygen 13



vacancy to form O2-, and the other O- adsorbs on the top of the Au atom (state vii(2)). O-O bond cleavage generates two excess electrons, leading to the formation of Au3+. The oxygen on the top of Au can either reacts with the desorbed CO molecule to form CO2 (Path 2), or accepts the H from adjacent hydroxyl to form H2O (Path 3). The pathway of CO2 formation is pretty easy (state viii(2)), and exothermic with the energy of 3.83 eV. The excess electron first localized on O- returns to Ce. Then, CO2 desorbs from surface with the exothermic of 0.78 eV. The structure after CO2 desorption is the same as the state viii(1) in Path 1. Therefore, it also faces the severe challenge of H2O desorption and catalyst recovery. Path 3 (Figure 5-4). The oxygen adsorbing on gold can also accept the H from neighboring hydroxyl, to produce H2O via a series of elementary reactions, similar with that in the stoichiometric AuCe1-xO2(111) surface. The excess electron always lies in the surface oxygen. One of H from OH- first migrates to the oxygen on the top of Au with the energy of 0.12 eV (state viii(3)). After a slight reorientation (-0.11 eV), the OH- can attract the other H to form H2O (state x(3)) with a reaction energy of 0.14 eV. It takes 0.78 eV to desorb H2O. The structure will recover to the stoichiometric AuCe1-xO2(111) surface. The oxygen vacancy formation energy on perfect AuCe1-xO2 surface is only 0.17 eV, which means that the transformation of the final structure in path 3 (state xi(3) , Figure 5-4) to the initial structure (state i, Figure 5-1) is easy. So, the catalyst also undergoes a complete cycle. 3.4 HCHO oxidation reaction mechanism on the defective AuCe1-xO2-2y(111) surface. On defective AuCe1-xO2-2y(111) surface, the valences of Au and Ce show +1 and 14


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+3, respectively, due to the loss of two oxygen vacancies. The detailed spin magnetic moments of Ce and Bader charge differences of Au in different reaction stages are shown in Table S9. The optimized adsorption configurations and energies of HCHO and O2 on defective AuCe1-xO2-2y(111) surface are displayed on the Figure S5 and Table S10. Among them, the most stable adsorption constructions are shown on the Figure 6. Formaldehyde can adsorb on the defective AuCe1-xO2-2y(111) surface with the O of HCHO filling in oxygen vacancy and C of HCHO coordinating with Au (Figure 6a). HCHO adsorption is exothermic by -1.65 eV. After HCHO adsorption, the valence of Au changes from +1 to +3, as suggested by the Bader Charge changes (from +0.37 |e| to 0.93 |e|). Molecular oxygen can also adsorb on this surface with the energy of -1.52 eV. One of the oxygen atoms fills the oxygen vacancy and the other one is connected with Au atom with the distance of O-Au of 2.04 Å (Figure 6b). The adsorption of O2 also causes the formation of Au3+. The distance of O-O bond is 1.42 Å, so adsorbed O2 molecule forms peroxide-type O22- on defective AuCe1-xO2-2y (111) surface. The co-adsorption of two adsorbates is also examined. After HCHO adsorbs on the defective AuCe1-xO2-2y (111) surface, molecular oxygen adsorbs to the other oxygen vacancy with an exothermic energy of -0.64 eV. The excess electron transfers from Ce3+ to O2 molecule. So, adsorbed O2 is reduced to the superoxide O2- species with O-O bond length of 1.34 Å. Figure 7 shows the complete formaldehyde oxidation pathway. Starting from the adsorbing HCHO and O2 on the defective AuCe1-xO2-2y(111) surface (state iii), the H 15



of adsorbed HCHO transfers to adsorbed O2- species to form the HOO species (state iv). The barrier for this step is 0.73 eV and reaction energy is exothermic by 0.29 eV. This C-H bond cleavage generates two excess electrons. One electron migrates to a surface Ce atom, and the other localizes on the HOO2- intermediate, leading to the elongation of O-O bond to 1.46 Å. Afterwards, the HOO2- species dissociates to form an OH- and lattice O2- (state vi) with the activation barrier and reaction energy are 0.17 eV and -0.43 eV, respectively. Then, the C-H bond of CHO - makes a breakage, accompanying with the transfer of H to adjacent OH- to form H2O (state vii). The barrier for second C-H bond cleavage is only 0.14 eV, and the reaction energy is exothermic by 1.31 eV. It takes 0.51 eV to desorb H2O. CO2- then reacts with adjacent lattice oxygen with the activation barrier of 0.67 eV and an exothermic energy of 2.07 eV (state ix). CO2 desorption generates four excess electrons. Two excess electrons lead to the reduction of Au3+ to Au+, and one electron causes the transfer of O- to O2-. The rest of electron is localized on Ce atoms. After CO2 desorption (0.69 eV), the surface restores to the defective AuCe1-xO2-2y(111) with two oxygen vacancies (state i). The defective AuCe1-xO2-2y catalyst keep a good balance between HCHO, O2 adsorption and oxygen vacancies formation to maintain the catalytic redox cycle.

4. DISCUSSION 4.1 The essential effect of doping metal on CeO2 towards the catalytic activity of formaldehyde oxidation reaction. Pure ceria catalyst shows negligible catalytic activity in HCHO oxidation. 16


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Doping transitional metals in the ceria lattice48 is a general way to improve the activity. It is a key point to identify the role of Au in catalytic process for further optimizing the activity. What about the effect of Au on this reaction? First, Au affects on the adsorption of HCHO. The adsorption energy of HCHO on AuCe1-xO2(111) is -1.17 eV, while the Eads is -0.97 eV on CeO2(111). The promotion in HCHO adsorption is due to the active O2c with the doping of Au22,49,50, which is similar with the doping of Mn48. Low coordinated O2c shows much stronger activity than O3c. The density of states (DOS) clearly shows that the occupied 2p states of O2c atom are closer to Fermi level than the occupied 2p states of O3c atom (Figure 8), explaining the higher activity of O2c and the stronger bonding between O2c and C of HCHO. Second, the effect of Au also can be manifested in the C-H activation step of HCHO oxidation. On the stoichiometric AuCe1-xO2(111) surface, H migrates to Au spontaneously with no barrier, then to the surface O2c with a low barrier of 0.06 eV. The barrier is reduced dramatically when compared with CeO2(111) surface (1.71 eV)51 and Mn doped CeO2 catalyst (1.09 eV)48. Similarly, the dissociation of second C-H bond also undergoes two-step processes, namely to Au then to O. The activation barriers are 0.66 and 0.50 eV, respectively, much lower than that on CeO2(111) (2.28 eV)51. Au assists the H transfer in two-step C-H bond cleavages. The projected density of state (PDOS) for H 1s, Au 5d, Au 6s is shown in Figure 9. The projected DOS of H 1s, Au 5d and Au 6s orbital overlap, indicating the formation of a strong chemical bond between H and Au in two-step C-H bond cleavages. 17



4.2 The effect of oxygen vacancy on formaldehyde oxidation reaction. The doping of Au reduces the oxygen vacancy formation energy greatly50, from 2.24 eV (CeO2(111) surface) to 0.17 eV (AuCe1-xO2(111) surface). The reduction can be due to the function of O2c. As discussed in 4.1, the active O2c species can be taken off easier than O3c. Low oxygen vacancy formation energy allows AuCe1-xO2(111) catalyst to form the oxygen vacancy easily. The effect of oxygen vacancy on formaldehyde oxidation reaction will be discussed via the following aspects. The first aspect is formaldehyde adsorption. The configurations of formaldehyde adsorption are distinct on perfect and defective Au doped CeO2 surfaces. On the stoichiometric AuCe1-xO2 surface, C of HCHO interacts with lattice O2c, and O towards Au (state ii, Figure 3). The active O 2c plays a crucial role in HCHO adsorption. But, on the defective surface, due to the loss of lattice oxygen, oxygen vacancies are eager to obtain electrons and bond with other substances. So, O of formaldehyde fills in the oxygen vacancy and C of HCHO is connected with Au (Figure 4a and 6a). With the increase of oxygen vacancy concentration, the formaldehyde adsorption on defective surface becomes more strongly. The relating adsorption energies (-0.92 eV on AuCe1-xO2-y surface vs. -1.65 eV on AuCe1-xO2-2y surface) and Bader Charge analysis (Table 1) can confirm this point. Second, oxygen vacancy site also affects the formaldehyde oxidation reaction. After HCHO adsorbed on AuCe1-xO2-y surface, two-step C-H cleavages take place with the energy barrier of 0.84 and 0.80 eV, respectively. The energy barriers in the both steps are higher than that on perfect surface. On perfect surface, with the help of Au, H can transfer to Au, 18


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then to the surface oxygen. But on the defective surface, due to the different HCHO adsorption configuration, Au is hard to accept H and assist the H transfer. We analyze the projected density of state of H 1s, C 2p, Au 5d and Au 6s (Figure 10) in two-step C-H bond cleavages. The PDOS show that C 2p orbitals of formaldehyde overlap with Au 5d orbitals. So, C-Au is a strong chemical bond, not H-Au. Therefore, Au on defective surface cannot play the role in assisting the H transfer. This situation can also be seen in AuCe1-xO2-2y surface. The amounts of oxygen vacancies also affect the HCHO oxidation reaction. In consideration of E2ov=1.11 eV, the surface of catalyst will be a lot of oxygen vacancies. On defective AuCe1-xO2-2y surface, the HCHO and O2 occupy an oxygen vacancy respectively and form a co-adsorbed state. In the process of reaction, the adsorbed O2species contributes to the two-step C-H bond cleavages, although Au does not play a role. The energy barriers of two-step C-H bond cleavages are 0.73 and 0.14 eV respectively, which is lower in comparison of the energy barriers on defective AuCe1-xO2-y surface (0.84 and 0.80 eV). So, the adsorbed O2- exhibits the higher activity than lattice oxygen which functions on defective AuCe1-xO2-y surface. The defective AuCe1-xO2-2y surface with two oxygen vacancies is more conducive to formaldehyde oxidation.

5. CONCLUSION The spin-polarized DFT+U calculations are performed to investigate the formaldehyde catalytic oxidation mechanisms on the stoichiometric AuCe 1-xO2(111) 19



surface, defective AuCe1-xO2-y(111) and AuCe1-xO2-2y(111) surfaces. The function of Au species and the effect of multiple oxygen vacancies on the reaction are investigated. On the stoichiometric AuCe 1-xO2(111) surface, HCHO oxidation reactions undergo HCHO adsorption, two-step C-H bond cleavages, CO 2 desorption, O2 adsorption and cleavage, H2O formation and desorption. On the defective AuCe1-xO2-y(111) surface, reaction mechanisms follow the Mars-Van Krevelen-type mechanism involving oxidation of adsorbed HCHO with surface O atoms, which is similar with the perfect AuCe1-xO2(111) surface. Due to the specific property of HCHO and O2 co-adsorption on the defective AuCe1-xO2-2y(111) surface, the processes of HCHO oxidation undergo the Langmuir-Hinshelwood type mechanism. The doping of Au plays an important role in the following aspects: HCHO adsorption, C-H bond dissociation. The active oxygen species form via the Au doping will lead to the facial formations of oxygen vacancies, which influence HCHO adsorption and oxidation reaction. Because of the different HCHO adsorption configurations on perfect and defective Au doped CeO2 surfaces, Au is hard to accept H and assist the H transfer on defective surface. But on the defective AuCe1-xO2-2y(111) surface with two oxygen vacancies, due to the co-adsorption of HCHO and O2, adsorbed O2- species contribute to C-H bond cleavages and decrease the reaction energy barriers. The Au doped CeO2 surfaces with multiple oxygen vacancies are more favorable to HCHO adsorption and oxidation.

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ASSOCIATED CONTENT

Supporting Information Convergence criteria tests (Table S1), the effect of the slab thickness on oxygen vacancy formation and reaction intermediates (Table S2, S3 and S4), the stability of doped Au single atom (Figure S1), the magnetic moments of Ce and Bader charge differences of Au in overall reaction processes (Table S5, S7 and S9), optimized adsorption structures and energies of HCHO and O2 on Au doped CeO2 surfaces (Figure S2, S4, S5 and Table S6, S8, S10), the other reaction pathway of H2O formation without O-O bond cleavage on stoichiometric AuCe1-xO2(111) surface (Figure S3).

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; Tel: +86-10-89732278. Author Contributions §These

authors contributed equally.

ACKNOWLEDGMENTS We acknowledge the financial support from National Natural Science Foundation of China (U1662103, 21503273, 21673290) and the Science Foundation of China University of Petroleum-Beijing (2462015YJRC005).

21



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Figures

Figure 1. The optimized structure of CeO2 (111) (color scheme: light yellow-Ce; red-O; the same below).

29



Figure 2. The structures of (a) stoichiometric AuCe1-xO2(111) surface, (b)(c) defective AuCe1-xO2-y (111) surfaces and (d) defective AuCe1-xO2-2y(111) surface (color scheme: blue-Ce3+; pink-O2c; yellow-Au;).

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Figure 3. The HCHO oxidation reaction pathways on stoichiometric AuCe1-xO2(111) surface including energy data and related structures (color scheme: pink-O; dark gray-C; the same below). 31



Figure 4. The adsorption configurations of (a) HCHO and (b) O2 on the surface of AuCe1-xO2-y(111).

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Figure 5-1. The HCHO oxidation reaction pathway on defective AuCe1-xO2-y(111) surface-the processes of two-step C-H bond cleavages.

33



Figure 5-2. One of HCHO oxidation reaction pathway on defective AuCe1-xO2-y(111) surface after CO2- species remaining on the surface-Path 1.

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35




36


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(a) HCHO adsorption

(b) O2 adsorption

(c) HCHO and O2 co-adsorption Figure 6. The adsorption configurations of (a) HCHO, (b) O2, (c) HCHO and O2 on the AuCe1-xO2-2y(111) surface.

37



Figure 7. The HCHO oxidation reaction pathways on defective AuCe1-xO2-2y(111) surface including energy data and related structures. 38


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Figure 8. Projected density of states for O3c 2p and O2c 2p orbitals on CeO2(111) and AuCe1-xO2(111), respectively. The Fermi level is set at 0 eV.

39



Figure 9. Projected density of states for H 1s and Au (5d, 6s) orbitals in the first C-H bond cleavage (left picture) and second C-H bond cleavage (right picture) on perfect AuCe1-xO2(111) surface. The Fermi level is set at 0 eV.

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Table 1. The Bader Charge and Bader Charge difference of HCHO molecule in gas phase and adsorbing state. HCHO@AuCe1-xO2-y HCHO@AuCe1-xO2-2y

q/|e|

HCHO

q(O)

6.76

6.98

7.02

q(C)

3.37

3.06

3.19

q(H1)

0.94

1.19

1.07

q(H2)

0.93

1.22

1.33

q(HCHO)

12

12.45

12.61

Δq(HCHO)

0

0.45

0.61

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Figure 10. Projected density of states for H 1s, C 2p and Au (5d, 6s) orbitals in the first C-H bond cleavage (left picture) and second C-H bond cleavage (right picture) on defective AuCe1-xO2-y(111) surface. The Fermi level is set at 0 eV.

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TOC Graphic

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A Density Functional Theory Study of Formaldehyde Catalytic

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