Zihao Yao,Jinyan Zhao,Chenxia Zhao,Shengwei Deng,Guilin Zhuang,Xing Zhong,Zhongzhe Wei,Yang Li,Shibin Wang,Jianguo Wang*

Institute of Industrial Catalysis,College of Chemical Engineering,Zhejiang University of Technology,Hangzhou 310032,China

Keywords:Direct synthesis of H2 O2 DFT Supported Pd-catalysts TiO2 -support Carbon-doped Oxygen vacancies

ABSTRACT The choice of support is one of the most significant components in the direct synthesis of H2 O2 .Aiming to improvement of activity and selectivity of H2 O2 on Pd/TiO2 surface,we systematically investigated the important elementary steps on Pd/TiO2 -Vo@C,Pd/TiO2 -Vo,Pd/TiO2 -2Vo,Pd/TiO2 ,and Pd/C using the first-principles calculations.The Bader charge analysis and charge density difference of O2 adsorption elucidate the relationship between the electronic distribution and chemisorption energy.The effective barrier analysis further enables to quantitatively estimate the reactivity of H2 O2 and H2 O.We demonstrate unambiguously that the selectivity of H2 O formation is boosted as the oxygen vacancy concentration raised.Moreover,the introduction of C into a TiO2 with appropriate oxygen vacancies can slightly reduce the effective barrier for H2 O2 formation and increase the effective barrier for H2 O formation leading to a higher activity and selectivity of H2 O2 formation.Our finding suggests that carbon-doped oxygen vacancy TiO2 supported Pd is potential alternative catalyst compared with the Pd/TiO2 .

1.Introduction

Hydrogen peroxide (H2O2) is a vital oxidant with a wide range of applications in industry.The degradation products of H2O2are O2and H2O holding tremendous commercial interest [1,2].Highconcentration of H2O2is widely used in the pulp industry [3],wastewater treatment[4,5],and the synthesis of special chemicals[6,7],while low-concentration of H2O2is extensively applied to disinfections of wound and food.The anthraquinone autooxidation (AO) process [8,9]is the most effective method for the large-scale H2O2production,which accounts for more than 95%of the total global demand [10].However,the AO process is complicated,energy-intensive and involving toxic organic compounds[11].Thus,more benign methods are extremely desired to replace the AO process.The direct synthesis of H2O2on Pd-catalyst derived from H2and O2on Pd-catalyst is a potential alternative method,which has the benefits of lower equipment cost,less energy consumption,and being suitable for small-scale production [12–32].Currently,the industrialization of direct synthesis is still unsolved mainly fall into two aspects:(1)The mixture of H2in O2is facile to trigger an explosion;(2) The direct synthesis on Pd-based catalyst is favored to form H2O,which is strongly related to the decomposition of H2O2[33,34].Therefore,the key to overcome the bottleneck is to increase its selectivity while ensuring safety in the production process.

Supported Pd-catalyst in the direct synthesis of H2O2has extensively investigated during the last hundred years [27,35–43].The support as a crucial component has attracted great attention,mainly due to it determines the exposed crystal planes and the local environment of nanoparticles,which significantly varies the catalytic properties [43–45].Two primary supports (carbon,metal oxide)have been meticulously studied by Hutchings and co-workers[42,46,47].It was found that the order of reactivity in the synthesis of H2O2being C ＞TiO2＞SiO2＞Al2O3＞Fe2O3[42].Followed by the work of Ntainjua et al.,they used CeO2as the support,and its activity for direct synthesis of hydrogen peroxide is Pd/CeO2＞Pd/C ＞Pd/Ti O2＞Pd/Al2O3＞Pd/Fe2O3[48].It strongly demonstrated that the reactivity on metal oxide support Pd catalyst could surpass that on C support Pd catalyst.In addition,recent experimental studies have shown that the size of Pd nanoparticles can be effectively regulated by selecting the support,thereby balancing the catalytic activity and the selectivity[49–51].TiO2,a low-cost metal oxide and reducible,is widely applied in hydrogenation reactions due to its strong interaction with nanoparticles[52–58].Particularly,it was found that the reduced TiO2support (TiO2-Vo) boosting more electrons transfer from support to nanoparticles,while the further decorated TiO2support performs a notable enhancement in the reactions.For example,Chen and co-workers found N-doped TiO2promoted the charge transfer between Pd and the support,resulting in strong metalsupport interaction (MSI),thus improved reaction performance[59].Our recent study of hydrogenation reaction indicates that Pd/TiO2-VOshowed the lowest effective barrier compared with Pd/TiO2and Pd/TiO2-2VO.The introduction of C in Pd/TiO2-VOor Pd/TiO2-2VOled to an increase in the effective barrier and TOF values.We suggested that the proper amount of TiO2-VO,and C is the key to optimizing the Pd electronic properties and balancing the reaction and diffusion[60].So far,to the best of our knowledge,there is still a lack of theoretical understanding of the direct synthesis mechanism on Pd/TiO2-Vo@C.

In this work,we investigated the reaction mechanism over carbon decorated oxygen-deficient TiO2supported Pd catalyst in direct synthesis of H2O2.Our goal was to obtain a general understanding in function of support with oxygen vacancies and carbon,aiming to predict the better catalyst than Pd/TiO2.

The paper is organized as follows.In the next section,we will introduce our models and computational methods.In the results and discussion section,we will present our detailed results of chemisorption of intermediates.Then,we will study the reaction mechanism of direct synthesis of H2O2on the systems of Pd/TiO2-Vo@C,Pd/TiO2-Vo,Pd/TiO2-2Vo,Pd/TiO2and Pd/C.The effective barrier analysis is further carried out to determine the activity and selectivity of H2O2and H2O formation.Finally,some major conclusions will be summarized.

2.Models and Computational Methods

In this work,all the simulations were carried out for the direct synthesis of H2O2within the framework of the generalized gradient approximation with the Perdew-Burke-Ernzerh[61]of functional in the VASP code[62,63].The cutoff energy of plane-wave basis expansion was set to 400 eV.Electronic convergence was set to 10-4eV,and geometries were converged to less than 0.1 eV·nm-1.The PAW was carried out to describe the interaction between coreelectron and valence electron.The structure optimization was calculated using the conjugate-gradient algorithm.A(2×3)anatase TiO2surface unit cell and a(5×5)graphene surface unit cell were constructed with 4 × 4 × 1 Monkhorst–Pack k-point mesh sampling[64].TiO2and C surfaces were built using periodic slabs of four layers.The bottom two layers of atoms were fixed,and the top two layers were relaxed with a 1.5 nm vacuum layer.There were two kinds of oxygen vacancies on the TiO2surface:namely,a two-coordinate oxygen atom and a three-coordinate oxygen atom.The oxygen vacancies were constructed through removing two-coordinate oxygen atoms on the perfect TiO2(1 0 1)anatase surface.Pd13(Ih)was chosen as the active metal to investigate the metal-support interactions between the support and Pd.The transition states(TSs)were searched using the method called a constrained optimization scheme[65–67].The geometries of TSs were converged to less than 0.1 eV·nm-1.The TSs were confirmed by two rules:(1)all forces on atoms vanish;(2)the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom.Vibrational frequency analyses [68]were performed to confirm the integrity of TSs.

3.Results and Discussion

3.1.Pd13 supported on the surfaces of anatase TiO2 (1 0 1) and graphene

Metal-support interaction plays a vital role in the catalytic performance of supported Pd-catalysts,which can directly change the electronic distribution of the Pd and further affecting the chemisorption of intermediates.We designed eight models (Pd/TiO2,Pd/TiO2-Vo@C,Pd/TiO2-Vo,Pd/C,Pd/TiO2-Vo-Vo,Pd/TiO2-2Vo@C,Pd/TiO2-2Vo and Pd/TiO2-2Vo-Vo) for DFT calculations to investigate how support affects the chemisorption energy of intermediates.The optimized surface models are displayed in Fig.1.

Weak binding energy of Pd13was found on the graphene surface(-2.41 eV) compared with that on the TiO2surface (-4.43 eV).In addition,we found that the strong interaction between Pd and TiO2can be reached through increasing the concentration of oxygen vacancies.For instance,the binding energy of Pd13on TiO2-Vo system increased significantly from-4.43 eV to-7.26 eV compared to that on the perfect TiO2support,and it further rose to-9.37 eV with the existent of two oxygen vacancies (TiO2-2Vo).It should be noting that the stronger binding energies of Pd13on TiO2-Vo@C (-7.99 eV) and TiO2-2Vo@C (-10.03) surfaces were observed compared with that on TiO2-Vo (-7.26 eV) and TiO2-2Vo(-9.37 eV)respectively.Thus,it strongly proved that by introducing oxygen vacancies and C,the binding energy of Pd13presented an enhanced trend.Bader charge analysis further unveilings the nature of electron transfer between Pd and support.Positive charge of Pd13was found on TiO2,TiO2-Vo@C,TiO2-Vo and C,while it was a negative charge in other systems (Table 1).The increase in the number of oxygen vacancies made the support more negatively charged,thus donating more electrons to the Pd.Besides,it was interesting to note that Pd13on TiO2-Vo had a positive charge of +0.27 |e|.After carbon doping with oxygen vacancies,the charge of Pd13on TiO2-Vo@C system was +0.31 |e|.Similarly,the TiO2-2Vo had a negative charge of-0.48|e|and doping carbon will reverse the charge to -0.25 |e|.Thus,it indicated that by doping carbon will reverse the effect of oxygen vacancies and regulate the Pd from a negatively charged state to the positively charged state.

3.2.Reaction intermediates relevant to the direct synthesis of H2 O2 on eight models

The active sites on PdAu/CNTs system mainly include three types,namely,top site,bridge site and hollow site,and it was found that O2preferred to adsorb on the bridge site in our previous study[69].However,the effect of an interface becomes highly significant and cannot be ignored for the Pd/TiO2-Vo@C system.As for the adsorption of H2,it was proved easily dissociated into two H atoms [60,70].Thus,we were mainly focused on the activation of O2in this work.There are normally two kinds of states for O2activation:(1)per-oxygen state(2)super-oxygen stateThe shorter bond length of O-O is considered as the super-oxygen stateon the contrary,the larger bond length of that denotes to the per-oxygen statewhich is easily for the direct dissociation of O2.Table 2 listed the most stable O2chemisorption energies and the bond lengths of O－O on the eight surfaces.It showed that the chemisorption energy of O2at the bridge site was stronger than that at the interface in the system of Pd/C.Furthermore,it was interesting to note that O2preferred to adsorb at the interface sites except for Pd/TiO2-Vo,Pd/TiO2-Vo@C and Pd/TiO2-2Vo@C.The chemisorption energy of O2on Pd/TiO2(-2.26 eV) was stronger than that on the surface of Pd/TiO2-Vo (-2.00 eV) for 0.26 eV.It was found that the adsorption ability was enhanced in the systems of Pd/TiO2-Vo-Vo (-2.27 eV) and Pd/TiO2-2Vo (-2.12 eV) compared with the Pd/TiO2-Vo surface.When increasing to three oxygen vacancies of the TiO2support,the chemisorption energy was further enhanced to -3.21 eV (Pd/TiO2-2Vo-Vo).Our results also revealed that the doping carbon was able to weaken the chemisorption energy of O2.For example,the weak chemisorption energy of O2was found in the system of Pd/TiO2-Vo@C(-1.78 eV)and Pd/TiO2-2Vo@C(-1.61 eV).More importantly,the chemisorption energy of O2(-1.05 eV) was weakest on Pd/C in the eight models.As for the other intermediates were similar with the chemisorption of O2,more details were provided in the Supplementary Material.

Fig.1.The configurations of Pd13 on (a) TiO2 ,(b) TiO2 -Vo@C,(c) TiO2 -Vo,(d) C,(e) TiO2 -Vo-Vo,(f) TiO2 -2Vo@C,(g) TiO2 -2Vo and (h) TiO2 -2Vo-Vo.The large yellow dashed square represents the region of Pd13 .The small yellow dashed rectangle represents the oxygen vacancy.The ball in black is C,the ball in gray is Ti,the ball in red is O,and the ball in blue is Pd.(TiO2 -2Vo means that two oxygen vacancies are located below the Pd13 .TiO2 -Vo-Vo means that one of the vacancies is located below the Pd13 ,and the other is placed outside of Pd13 .)

Table 1 The binding energy(eV),charge(|e|)and average Pd-Pd length(nm)on(a)Pd/TiO2 ,(b)Pd/TiO2 -Vo@C,(c)Pd/TiO2 -Vo,(d)Pd/C,(e)Pd/TiO2 -Vo-Vo,(f)Pd/TiO2 -2Vo@C,(g)Pd/TiO2 -2Vo and (h) Pd/TiO2 -2Vo-Vo

3.3.Investigation of O2 adsorption based on electronic distribution analysis

The optimized O2configuration was selected to investigate the electronic distribution between O2and Pd/support.The charge density difference (CDD) and Bader charge analysis on eight models(Table S1 and Fig.2)were carried out to describe the electronic distribution of O2adsorption quantitatively.It was found that the accumulation of charge was mainly localized in the O2and the first neighbor atoms close to O2.The negative charge was localized in the O2indicating the electrons were transferred from the Pd/support.O2obtained electrons from the Pd/TiO2and Pd/TiO2-Vo for 0.89 |e| and 0.86 |e|,respectively.Additionally,compared to the system of Pd/TiO2-Vo,it was worth noting that the charge transfer was remarkable reduced to 0.72|e|on the Pd/TiO2-Vo@C.The least electron of O2(0.64|e|)was localized in the system of Pd/C leading the weakest chemisorption energy.With the formation of two and three oxygen vacancies,the charge of O2in Pd/TiO2-2Vo and Pd/TiO2-2Vo-Vo gradually increased compared with that in Pd/TiO2-Vo,which were 0.88 |e| and 0.95 |e| respectively.

They both lay down to sleep again, and the tailor threw down a stone on the second giant, who sprang up and cried: What s that for? Why did you throw something at me? I didn t throw anything, growled31 the first one

The partial density of analysis (PDOS) involving atomic orbital properties was carried out to investigate the effects of carbondoped oxygen vacancies and oxygen vacancies on the electronicproperties of O2adsorption(Fig.3).There are three kinds of forms for the O2in the gas phase,namely,σ bond orbital,π bond orbital and π*antibond orbital.Comparing the atomic orbital distribution of adsorbed O2and gaseous O2,three orbitals of adsorbed O2had different degrees of offset,which indicated that electron transfer occurs between O2and catalysts.A further detailed comparison of Pd/C and Pd/TiO2series catalysts showed that in the Pd/C system,the π* antibonding orbital of O2shifted to the right of the Fermi level (EF),and Pd(4d),C(2p) And O(2p) formed a relatively large hybrid peak near EF.In the Pd/TiO2series catalysts,compared to O2molecules of gas phase,the π* antibonding orbitals are all shifted to the left of EF,and Ti(3d),Pd(4d),C(2p)and O(2p)are near EF,a highly matched hybrid peak was formed.It indicated that O2had a strong interaction with Pd and Ti atoms in the process of O2adsorption.The π* anti-bonding orbital of O2in the carbon-doped oxygen vacancy system was similar to Pd/C and also shifted to the right of the Fermi level,which leads to the weak chemisorption of O2.According to reports [71],there are two activation methods in the process of O2activation.The first is the electrons of the metal d orbital are filled into the p orbitals of O2,and the other is that the electrons of the O2antibonding orbital have a feedback to the d orbital energy level of metal.O2fed back more electrons to the d orbital of Pd in Pd/C,while only a few electrons in the d orbital of Pd atom filled the π* antibonding orbital of O2,causing it to move in the direction of high energy.For the Pd/TiO2series catalysts,a large number of electrons in the d orbital of Pd atom were filled into the π* antibonding orbital of O2,while only a small number of electrons were fed back from the π*antibonding orbital of O2to the 4d orbital of Pd atom.Therefore,the increase of oxygen vacancies and carbon doping oxygen vacancies can enhance the interaction between Pd and TiO2support.In the case of the same number of oxygen vacancies,the Pd on the carbondoped oxygen vacancy system obviously had stronger binding energy.In addition,the electronic distribution of O2adsorption indicated that the results of CDD and Bader charge analysis were consistent,and both showed that O2had the strong ability to localize more electrons.Moreover,the PDOS analysis of O2adsorption showed that the electron filling of the π* antibonding orbital of O2was mainly derived from the Pd(4d) orbital energy level.

Table 2 The chemisorption energy of O2 (eV),O-O bond length(?)and O2 charge(|e|)on(a)Pd/TiO2 ,(b)Pd/TiO2 -Vo@C,(c)Pd/TiO2 -Vo,(d)Pd/C,(e)Pd/TiO2 -Vo-Vo,(f)Pd/TiO2 -2Vo@C,(g)Pd/TiO2 -2Vo and (h) Pd/TiO2 -2Vo-Vo

Fig.2.(a)The Charge density difference plots of O2 adsorption on(a)Pd/TiO2 ,(b)Pd/TiO2 -Vo@C,(c)Pd/TiO2 -Vo,(d)Pd/C,(e)Pd/TiO2 -Vo-Vo,(f)Pd/TiO2 -2Vo@C,(g)Pd/TiO2 -2Vo and (h) Pd/TiO2 -2Vo-Vo.(i) The trends of O2 chemisorption energy and O2 charge on eight surfaces.

Fig.3.The PDOS plots of (a) Pd/TiO2 (b) Pd/TiO2 -Vo@C (c) Pd/TiO2 -Vo (d) Pd/C (e) Pd/TiO2 -Vo-Vo (f) Pd/TiO2 -2Vo@C (g) Pd/TiO2 -2Vo (h) Pd/TiO2 -2Vo-Vo.

Fig.4.Free energy profiles for the formation of (a) H2 O and (b) H2 O2 on Pd/TiO2-Vo,Pd/TiO2-Vo@C,Pd/TiO2 ,Pd/TiO2 ,Pd/TiO2 -2Vo and Pd/C.(T=300 K).

3.4.Reaction analysis over Pd/TiO2 -Vo@C,Pd/TiO2 -Vo,Pd/TiO2 -2Vo,Pd/TiO2 and Pd/C

3.4.1.The structures of transition states

After investigating the adsorbed configurations of intermediates,we further calculated the elementary steps corresponding to the formation of H2O2and H2O over Pd/TiO2-Vo@C,Pd/TiO2-Vo,Pd/TiO2-2Vo,Pd/TiO2and Pd/C.The selected elementary steps included O2*+*?O*+O*,O2*+H*?OOH*+*,OOH*+H*?H2O2+*+*,OOH*+* ?O*+OH*,O*+H* ?OH*+*,OH*+H* ?H2O+*+*,OH*+OH*?H2O+*+O*.Fig.S1 showed the transition states of the elementary steps for direct synthesis of H2O2and H2O over five selected surfaces.It was interesting to note that direct dissociations of O-O and O-OH occurred at the interface sites for the Pd/TiO2system.One of O atoms sat on the top site of Pd atom and the other located on the top site of Ti atom.For the transition state of O2+H,the O2adsorbed at the interface site,and H sat on the Pd atom nearby.Similarly,for the transition state of OOH+H,the OOH adsorbed at the interface site as well.For the elementary step of OH*+OH*?H2O+*+O*,one of OH adsorbed on the bridge site and the other adsorbed on the top site of Pd atom on the systems of Pd/TiO2-Vo,Pd/TiO2-2Vo and Pd/TiO2.In addition,we found that both OH and OH sat on the top sites of two neighboring Pd atoms for OH*+OH* ?H2O+*+O* on the system of Pd/TiO2-Vo@C.

3.4.2.Effective barrier analysis for the formation of H2O2and H2O

The free energy profiles for the formation of H2O2and H2O are displayed in Fig.4.The details of the barrier and reaction energy of each elementary step are shown in the Supplementary Material.

From the free energy profiles of Fig.4,we can performed effective barrier analysis [60,70,72–79]of H2O2and H2O formation on Pd/TiO2-Vo@C,Pd/TiO2-Vo,Pd/TiO2-2Vo,Pd/TiO2and Pd/C.According to the concept of effective barrier,the activity of product can be written as follow:

TDTS represents the Activity determining transition state.TDI represents the Activity determining intermediate.ΔGris the reaction free energy in the energy profile.

We are now at the position to rationalize general effects of oxygen vacancy and carbon (Fig.5).

Fig.5.Histogram of the effective barrier for the formation of H2 O2 and H2 O on Pd/TiO2 -Vo@C,Pd/TiO2 -Vo,Pd/TiO2 -2Vo,Pd/TiO2 ,Pd/C.The bar in red is the effective barrier of H2 O formation.The bar in blue is the effective barrier of H2 O2 formation.

First,it was found that the effective barrier of H2O2and H2O formation on Pd/TiO2was much higher than other systems,which indicated lower reaction rate on the formation of H2O2and H2O.Besides,the similar effective barrier showed the comparable selectivity on the formation of H2O2and H2O.As the concentration of oxygen vacancy increased,effective barrier of H2O formation dropped dramatically leading to higher reaction rate of H2O compared with that of H2O2.This phenomenon further illustrated that the selectivity of H2O formation was boosted as the oxygen vacancy concentration raised.Second,we found that the introduction of C into a TiO2with appropriate oxygen vacancies can slightly reduce the effective barrier for H2O2formation and increase the effective barrier for H2O formation.For instance,it was found that effective barrier of H2O2formation declined from 1.47 eV to 1.36 eV,while the effective barrier of H2O formation increased from 1.24 eV to 1.38 eV.Therefore,the activity and selectivity of H2O2formation can be improved to a certain extent.Third,Pd/C was still the best catalysis with the lowest effective barrier of H2O2formation in the five system,followed by the system of Pd/TiO2-Vo@C.In addition,the effective barrier of H2O formation is much higher than H2O2for 0.12 eV,leading to better selectivity of H2O2.p

4.Conclusions

In this work we carried out extensive DFT calculations to investigate the direct synthesis of H2O2on the Pd supported TiO2modified by oxygen vacancy and its carbon substitution.The following major conclusions can be reached:

(1) The stronger binding energy of palladium clusters can be achieved through constructing the carbon decorated oxygen-deficient TiO2support and oxygen-deficient TiO2support.

(2) Compared with the Pd/TiO2and Pd/TiO2-Vo,the chemisorption energy of O2is significantly weakened in the system of Pd/TiO2-Vo@C.Electronic distribution analysis further demonstrates that the less electron is transferred from Pd/TiO2-Vo@C to O2compared with that from Pd/TiO2-Vo.

(3) Effective barrier analysis indicates that the selectivity of H2O formation was boosted as the oxygen vacancy concentration raised.The introduction of C into a TiO2with appropriate oxygen vacancies can slightly reduce the effective barrier for H2O2formation and increase the effective barrier for H2O formation leading to a higher activity and selectivity of H2O2formation.Carbon-doped oxygen vacancy TiO2supported Pd is potential alternative catalyst compared with the Pd/TiO2.

(4) Pd/C is still the best catalysis with the lowest effective barrier of H2O2formation in the five system (Pd/TiO2-Vo@C,Pd/TiO2-Vo,Pd/TiO2-2Vo,Pd/TiO2,Pd/C),followed by the system of Pd/TiO2-Vo@C.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC-21625604,91934302,and 22008211) and Zhejiang Innovation Team(2017R5203).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.016.