Jixun Zhng,Pengfei Tin,Aiho Xu,Like Ouyng,Zixu Yng,?,Jing Xu,,?

a State Key Laboratory of Chemical Engineering,School of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China

b Key Laboratory of Pressure Systems and Safety,Ministry of Education,East China University of Science and Technology,Shanghai 200237,China

c Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology,School of Chemistry and Chemical Engineering,Guangxi University,Nanning 530004,China

d College of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:H2O2 synthesis Bimetallic catalysts Palladium particle size Pd0/Pd2+ ratio Structure-performance relationship

ABSTRACT Direct synthesis of H2O2 from H2 and O2 via heterogeneous catalysis is an environmentally friendly and atomically economic alternative to the traditional anthraquinone oxidation (AO) process.Optimizing the electronic and geometric structures of the active metals to break the current limitations of hydrogenation rate and H2O2 selectivity is a promising and challenging topic.In this study,a series of Pd-Au bimetallic catalysts supported on TiO2 with a metal loading of 3.0 wt% and a constant Pd/Au molar ratio(Pd:Au=2:1) were prepared.The catalysts were reduced in H2 at different temperatures (473,573 and 673 K),and their catalytic activity for the direct H2O2 synthesis were evaluated at 283 K and 0.1 MPa.H2 reduced Pd-Au catalysts exhibited superior performance in direct H2O2 synthesis.The maximum H2O2 selectivity of 87.7% and H2O2 yield of 3116.4 mmol h-1 gPd-1 were achieved over the Pd2.0Au1.0-573 catalyst with a H2 conversion of 12.8%.The tailored local chemical environment caused by H2 reduction creates a balanced ratio of Pd0 and PdOx sites,thus improving the selectivity towards H2O2.This work developed an effective strategy for fabrication of highly active and stable Pd-based H2O2 synthesis catalysts with high H2O2 yield.

Hydrogen peroxide (H2O2),one of the 100 most important chemicals in the world,has been recognized as an efficient and green oxidant because of its numerous advantages such as mild reaction conditions,fast reaction speed,high selectivity and pollution-free.As a result,hydrogen peroxide is heavily used as a feedstock or additive (e.g.,oxidant [1,2],bleaching agent [3],disinfectant [4]) in a wide range of manufacturing sectors,such as chemicals [5],papermaking,textile [6],medicine [7],electronics,and environmental protection [8].

The anthraquinone oxidation (AO) has maintained its position as the most mature hydrogen peroxide synthesis technology since 1840s [9,10],and still accounts for more than 95% of the world’s H2O2production today.Despite the high H2O2yield per cycle,the AO process also suffers from the complicated process,side reactions that lead to a considerable consumption of alkyl anthraquinone,rapid deactivation of hydrogenation catalyst and high separation cost [11].Furthermore,this process is only profitable on a large-scale production (>4×104t/a) [12,13].Given the existing issues in AO process,the direct synthesis of hydrogen peroxide from H2and O2over heterogeneous catalysts offers an attractive and green alternative process,which was first introduced in 1914[14].However,the process has not been commercialized yet due to the strong explosive potential of the mixture of H2and O2at high pressures,as well as the low productivity caused by the side reactions,such as hydrogenation and degradation of H2O2in the process.

The high activity and selectivity of Pd-based catalysts for direct synthesis of H2O2is well known and has received great attention for decades [15].Recent studies have highlighted the enhanced H2O2selectivity by alloying Pd with a secondary metal such as Au.The latter is often attributed to the geometrical effect,which gives emphasis on the advantage of Pd monomers surrounded by Au atoms towards H2O2formation,while contiguous Pd ensembles are more favorable for H2O2hydrogenation [16,17].As H2O2synthesis over Pd-based bimetallic catalysts is a surface-catalyzed and structure-sensitive reaction,modification of catalyst surface compositionviasurface pretreatment may alter the chemical environment of active sites and the product selectivity.

For selective H2oxidation on Pd catalysts,there are discrepancies regarding the active phases since the copresence of metal Pd(Pd0) and Pd oxide (PdOx) in the reaction system.For example,Gudarzietal.reported that PdO(II) species showed a higher selectivity towards H2O2than Pd0in both mono Pd [18] and bimetallic Pd-Au [19] catalysts.The lower H2O2selectivity on Pd0was attributed to the reabsorption and the subsequent degradation of H2O2on Pd0surface according to Choudhary [20–25].Contrarily,some studies reported higher conversion and selectivity toward H2O2on partially reduced PdOxspecies [26–28].Recently,N-heterocyclic carbenes (NHC) was adopted as electronic modifiers of Pd,and the optimized catalyst with an appropriate surface Pd0/Pd2+ratio yielded twice as much H2O2formation rate as the unmodified counterpart [29].

In Pd catalyzed H2O2synthesis reaction,the chemical composition (e.g.,Pd0/Pd2+molar ratio) or particle size of the Pd species can be finely tuned during the calcination of the precursor and the pretreatment prior to reaction [30,31].Calcination under prolonged duration or elevated temperature was found to result in an increase in the fraction of Pd2+to formation of a surface oxide layer [32],and hence a decreased H2O2formation rate [33].But on the other hand,the increasing calcination temperature will undoubtedly be accompanied by a growing average crystallite size[34,35] and an affected H2O2selectivity.More recently,Hanget al.reported a Pd/C catalyst calcined at 523 K showing a promising H2O2productivity of 4443 mmol h-1gPd-1with 94.5% selectivity [36].The enhanced catalytic performance was attributed to the formation of small and highly oxidized Pd NPs (1.4–2.5 nm).Similarly,Pritchard observed that a mild heat treatment at 575 K was capable of forming an optimal Pd0/Pd2+ratio without causing a significant growth of particle size [37].Simonetal.successfully encapsulated small Pd-rich NPs in the amorphous SnOxlayer through an oxidation-reduction-oxidation (O-R-O) heat treatment cycle,which prevents the sequential hydrogenation and degradation of H2O2[38].

In our previous work,we have known that small Pd NPs (particle size of 1.4–2.5 nm) and exposure of active sites at Pd0/PdO interfaces are important parameters enhancing H2O2selectivity over Pd catalysts [39,40].We also demonstrated that alloying Pd with appropriate amount of Au effectively reduced the quantity of contiguous Pd ensembles,thus inhibiting the degradation of H2O2viahydrogenation pathway.The highest H2O2selectivity of 48.1% was achieved with an optimal Pd/Au ratio of 2.0 and total metal content of 3.0 wt% [41].On the basis of our previous studies on direct synthesis of H2O2over Pd-based catalysts (Table S1 in Supporting information),this study demonstrated that catalyst calcination and pretreatment protocol had great impacts on the morphology and chemical composition of Pd NPs,and ultimately on catalytic performance with regard to selectivity and productivity of H2O2.

The four catalyst samples were prepared and tested for their capability to catalyze the direct synthesis of H2O2under atmospheric pressure.A prior blank experiment was used to exclude the catalytic activity from the support.Detailed catalyst preparation,characterization and testing methods are available in Supporting information.Since the formation of H2O2always occurs with its hydrogenation/decomposition reactions,the H2O2productivity is reported as its net formation rate [42].Table 1 shows that H2O2selectivity of unreduced catalyst remains 71.8%,but increases abruptly to 87.7% after reduction under 573 K.A further increase of reduction temperature to 673 K leads to a slight decrease in H2O2selectivity.It is also noticed that catalysts reduced at mild temperatures (473 K and 573 K) exhibit higher H2conversion than the unreduced catalyst.Higher temperature reduction tends to trigger the sintering of the metal NPs,which may cause the reduction in the number of active sites as well as the activity loss.This is verified by the abrupt decrease in H2conversion when the reduction temperature increases to 673 K.A similar trend was observed for the H2O2productivity as well.The highest H2O2productivity was obtained with Pd2.0Au1.0-473 (3116.4 mmol h-1gPd-1),which also exhibited the highest H2conversion (17.7%).By evaluating the dispersion of metal atoms exposed to reactants based on a hemispheric model,we also calculated the turnover frequency (TOF,h-1),which showed a similar trend with the productivities.The TOFs of the reduced catalysts were generally higher than that of the unreduced one,with a highest value of 1103.3 h-1occurring on the catalysts reduced at the lowest temperature (473 K).And then,it showed a slight decrease after the first additional 100 K,but when it came to another same magnitude of temperature increase,it responded with a rapid decline of more than 30%.

Table 1Performance of Pd2.0Au1.0/TiO2 catalysts with different heat treatment temperature.a

The impacts of H2reduction pretreatment on decomposition/hydrogenation side reactions of H2O2were also studied.Fig.S1 (Supporting information) shows that the H2O2concentration barely changes within 60 min under pure N2flow,indicating no activity for H2O2decomposition,which agrees with our previous studies [41].When switching the gas feed from N2to 15H2/85N2,a significant drop in H2O2concentration was observed for all catalysts.Higher reduction temperature results in a faster H2O2hydrogenation rate,which matches the growth of the average particle size measured by TEM (Fig.1) which will be discussed in the next section.The enhanced hydrogenation activity on large Pd NPs is due to the fact that the cleavage of O-O bond is favored on the extended Pd0ensembles.To sum up,the hydrogenation side reaction is the primary cause affecting the selectivity and net productivity of H2O2during direct H2O2synthesis from H2and O2.

Fig.1.TEM images and Pd particle sizes of the fresh catalysts.(a) Pd2.0Au1.0-0,(b) Pd2.0Au1.0-473,(c) Pd2.0Au1.0-573,(d) Pd2.0Au1.0-673.

The discrepancies in catalytic performance seen for Pd-Au catalysts with the same nominal composition but different reduction procedures encourage us to examine the surface/bulk structure and chemical composition of the catalysts for better understanding of the structure-performance relationship.

Fig.1 shows the TEM images of the Pd-Au bimetallic catalysts,as well as the corresponding particle size distribution of the Pd particles.It can be seen that the TiO2particles exhibit the same morphology after reduction pretreatment under different temperatures with an average particle size of 25 nm,suggesting that reduction treatment does not lead to obvious agglomeration of TiO2support.Pd NPs with different dimensions,ranging from small clusters of several atoms (<1 nm) to large Pd-Au particles (>10 nm),were clearly visualized on the images.Reduction pretreatment results in an abrupt increase in average Pd particle size from 2.8±0.7 nm to 3.7±0.8 nm.Increasing reduction temperature from 473 K to 673 K causes a moderate growth on the average particle size from 3.7±0.8 nm to 4.2±0.7 nm,equivalent to a growth rate ofca.0.2–0.3 nm per 100 K.Analysis of the TEM images and particle size distribution indicate that reduction treatment under mild temepratures causes the agglomeration of metal NPs to some extent.

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to analyze the micromorphology and the elemental distribution of Au and Pd (Fig.2).The distributions of both Pd and Au are almost overlapped with that of Ti and O,confirming the homogeneity of the supported catalyst in the form of Pd-Au bimetallic particles.In the same figure,it can also be seen that the spatial distribution of aggregated forms of Au is highly consistent with that of large Pd NPs with a size of 10–15 nm,suggesting the presence of highly-enriched large particles in addition to the other highly-dispersed ones.The energydispersive X-ray spectroscopy (EDS) elemental mapping images confirmed a uniform distribution of Pd and Au in the bimetallic particles and the tight contact between the two metals.The hybrid support was also identified by the elemental mapping of Ti and O,as shown in Figs.2b and c,respectively.The XRD patterns(Fig.3a) of the both unreduced and reduced Pd-Au bimetallic catalysts reveal both characteristics of rutile (25.3°,37.8° and 48.0°)and anatase phases (27.4°,36.1° and 54.3°,Degussa P25) with identical 2θpositions,manifesting that the crystal structure of the TiO2support is intact during reduction pretreatment [43].Pd shows a weak signal belonging to Pd (111) at 2θ=40.1° in Pd2.0Au1.0-673[44],whereas the diffraction peaks of Au are absent,presumably owing to the low metal loading and high metal dispersion.Such cases are not rare in palladium-based catalysts prepared by similar impregnation method [31].Nevertheless,all catalysts are consisted of mainly small metal particles,in good agreement with the TEM results.

Fig.2.Microstructure analysis of Pd2.0Au1.0/TiO2 catalysts.(a) STEM-HAADF grayscale image of Pd2.0Au1.0/TiO2,(b-f) STEM-EDS maps of Pd2.0Au1.0/TiO2.Elements shown in blue,yellow,green and red refer to Ti,O,Pd and Au,respectively.

Fig.3.(a) XRD patterns,(b) CO-DRIFTS,(c) fitting Pd 3d XPS spectra,and (d) Au 4f XPS spectra of Pd2.0Au1.0/TiO2 catalysts with different heat treatment temperature: (A)Pd2.0Au1.0-0,(B) Pd2.0Au1.0-473,(C) Pd2.0Au1.0-573,(D) Pd2.0Au1.0-673.

Fig.4.(a) O2-,(b) H2-TPD profiles obtained for different catalysts in an Ar flow with a ramping rate of 10 K/min.(A) Pd2.0Au1.0-0,(B) Pd2.0Au1.0-473,(C) Pd2.0Au1.0-573,(D)Pd2.0Au1.0-673.

The surface characteristics of Pd-Au bimetallic catalysts after reduction treatment was studied using CO-DRIFTS,and the observed spectra were displayed in Fig.3b.CO absorption bands located～1800–2100 cm-1were observed in all catalysts,which were attributed to CO adsorbed on metallic Pd atoms with different bonds(e.g.,linear,bridged,and hollow).No CO adsorption bands on Au(ca.2100 cm-1) was observed [45].The peak at 2084 cm-1(peak 1) associated with the linearly bonded CO (Pd-CO) on Pd atoms of low coordination,e.g.,corner or edge sites [46] is visible for all reduced catalysts,and this peak splits into two small peaks located at 2104 and 2078 cm-1for the unreduced catalyst [47].The three successive peaks at 1985,1936,and 1870 cm-1(peak 2) were assigned to chemisorbed CO on bridged-and multi-folded hollow Pd0sites [48].

More intriguingly,the bands related to CO adsorbed on these Pd0sites grow sharper upon increasing reduction temperature.The peak area ratio of the COlinearbandsvs.CObridged+COhollowbands was decreased regularly from 0.059 to 0.025 (Table S2 in Supporting information),indicating that higher reduction pretreatment results in the exposure of more flat sites with continuous Pd ensembles.These features also shifted slightly to lower wavenumbers with increasing reduction temperatures,and eventually ended at 1976,1890 and 1859 cm-1in the case of Pd2.0Au1.0-673.These redshifts may be attributed to a stronger electronic back-donation from the d orbital of Pd metal to the 2πmolecular orbitals of CO molecules,indicating a remarkable enhancement in the d orbital electron density from the increase in Pd0[49,50].Moreover,a small band around 1607 cm-1on Pd2.0Au1.0-0 corresponding to surface carbonate species was absent in the spectra of the reduced catalysts presumably because the carbonate species were removed after reduction treatment.

In Pd-catalyzed direct H2O2synthesis,Pd0is considered as responsible for H2dissociation while PdO is suggested to activate O2associatively.A balanced ratio of Pd0/Pd2+is necessary for high H2O2yield.Herein,the chemical states of the supported Pd NPs were studiedviaXPS survey of Pd 3d spectra,and the results were shown in Fig.3c and Table 2.The curve-fitting of Pd 3d spectra show Pd0(335.2 and 340.5 eV) and Pd2+(336.8 and 342.1 eV)components for all catalyst samples [51,52].The molar ratio of Pd0/Pd2+increases steadily along with the rising reduction temperature,from 0.1 in the unreduced catalyst to 1.6 in Pd2.0Au1.0-673.The increased surface molar ratio of Pd0/Pd2+after H2reduction treatment provides more sites for H2dissociation,thus resulting in a faster hydrogenation rate,as observed in Pd2.0Au1.0-473[53,54].However,excessive Pd0would lead to a decrease in the PdAu-PdAuOxinterface sites as well as an increase in metallic Pd sites,which is more feasible for the breakage of O-O bond in O2,as evidenced by theoretical calculation (Fig.S2 in Supporting information).The rising reduction temperature also leads to an adverse aggregation of both surface Pd and Au atoms,resulting in the gradual destruction of the high dispersion,which may dominate the downward catalyst performance,manifested as an even lower H2O2productivity than the untreated catalysts when the reduction temperature reaches 673 K.

Table 2The quantified XPS data of the surface Pd atoms.

For XPS Au 4f (Fig.3d),the Au 4f5/2and Au 4f7/2BE peaks appeared at 87.2 eV and 83.5 eV,respectively,lower than those of naturally occurring metallic Au (BE 87.7 and 84.0 eV [55]) by 0.5 eV.This can be partly ascribed to the quantum size effect,which implies that the nano-sized Au particles have more surface atoms than bulk Au,resulting in a shift of the d-band towards Fermi level[56],as well as the electron transfer from Au0to both Pd0[57] and TiO2[58].Upon increasing reduction temperature,the Au 4f peaks are further negatively shifted.It implies that high temperature reduction results in an increase in the electron density of Au.

To get more insights on the effects of surface chemical composition on the adsorption behaviors of the Pd catalysts,O2-and H2-TPD studies were carried out and the results were shown in Figs.4a and b,respectively.Two desorption peaks at 365 K and 447 K were detected for all catalysts,mainly attributed to the adsorption of O2molecules on oxygen vacancies of surface TiO2and Pd particles [59],respectively.The strong O2desorption peaks observed in the high temperature region of 850–950 K can be attributed tothe decomposition of surface palladium oxide [60].Upon rising reduction temperature,the molecular O2peaks on Pd particles gradually shifted to lower temperature with increased peak intensities.On the other hand,the high temperature O2desorption peak broadens and even flatten out in the Pd2.0Au1.0-673 sample,suggesting an increasing ratio of Pd0/Pd2+,which is in line with the XPS results.More importantly,it is known that the activated O2molecules are important for the outcome of the reaction,which means excessively strong adsorption may lead to oxygen dissociation while weak adsorption may lead to oxygen desorption from the catalyst surface.The 573 K reduced one seems to have the optimal adsorption strength,thus turning out with the highest H2O2selectivity.

For H2-TPD,all catalysts showed similar desorption profiles with two major desorption peaks,denoted as Hα(350–500 K) and Hβ(620–670 K).The Hαpeaks corresponding to the chemisorbed H2species desorption on metallic Pd sites contained two overlapped peaks.The peak centered atca.378 K was allocated to H2derived fromβ-hydride decomposition [61,62];the peak centered atca.431 K can be attributed to the desorption of H2on isolated Pd0sites that have no interactions with Au [61,63].The Hβpeaks were probably due to the H2desorption from highly dispersed Pd atoms in Pd-Au particles.Increasing reduction temperature results in a decrease of Hαdesorption amount.Such behavior implies a loss of metallic Pd sites which may be ascribed to the particle growth upon H2reduction under elevated temperatures.

A hydrogenation reaction pathway study in H2O2synthesis on Pd catalysts through isotopic experiments evidenced an associative adsorption pathway for O2molecules [53],and the formation of OOH intermediate from the dissociated H atoms and activated O2molecules was considered as the key step for H2O2synthesis [64].Reaction pathway that leads to the dissociation of O2molecules tends to end up with H2O as the main product.But the O-O bond cleavage,which was considered to be adverse for H2O2formation,was significantly suppressed on Pd monomers surrounded by less active Au atoms [41,65].Our previous study has already unraveled a Pd–PdO–TiO2interface where the Pd0atoms were most exposed on the upper layers of particles,and PdO species were present at the interface of Pd NPs and TiO2[40].In this work,we adopted the modeled PuAu-PdAuOx interface as the active sites for H2O2synthesis,where the upper metallic Pd species take responsibility for H2dissociated activation and the oxidized Pd species for O2nondissociated activation.The reducible support TiO2was selected to better ensure the coexistence of metallic and oxidized metal components on the catalyst surface and the highly dispersed Au serves to stabilize the enrichment of metal Pd at the top of the particles.Single metallic or oxidized Pd species will lead to a dissatisfactory H2O2productivity.Because the initial structure of the interface between PdAu and PdAuOxis very much dictated by the activation conditions,so as the particle size,the final catalytic performance will be completely different.These insights on the critical role of PdAu–PdAuOxinterface in H2O2synthesis inspired us to further refine the geometric and chemical properties of the interface for optimal H2O2selectivity and yield by reduction pretreatment.

The reduction pretreatment on Pd-Au bimetallic catalysts obviously outperforms the unreduced one in terms of both H2conversion and H2O2selectivity.The low H2conversion of the Pd2.0Au1.0-0 sample can be attributed to a lack of Pd0sites for H2dissociation,and the reduction of PdOxby H2leads to a rapid accumulation of H2O at the initial stage of the reaction,which undoubtedly compromises the average H2O2selectivity.However,increasing reduction temperature not only leads to more Pd0for H2dissociation,but also to a growth in particle size caused by varying degrees of metal sintering,as observed in the TEM results.And the latter explains the dropping H2conversion from 17.7% to 8.1% with the increasing reduction temperature,along with the H2O2productivity.

As mentioned above,H2O2synthesis was primarily occurred on the PdAu–PdAuOxinterface (Fig.5a),the structure of which was affected by the reduction conditions.The surface Pd0/Pd2+ratio of the original Pd2.0Au1.0/TiO2increases from 0.1 to 0.3 (reduced at 473 K) and 0.9 (reduced at 573 K).The optimal Pd0/Pd2+ratio in the Pd2.0Au1.0-573 demonstrates the highest H2O2selectivity due to the downward shift of the PdAu and PdAuOxinterface demarcation line (Fig.5b) to form the maximum active sites towards H2O2synthesis.Nonetheless,a further increase in reduction temperature brings an opposite effect.We owe this to a combined effect: (I) The decreased PdAu–PdAuOxinterface in step with the continues increased Pd0/Pd2+ratio,directly reduced the number of active sites;(Ⅱ) The high rate of parallel side reaction caused by the uncontrollable particle size growth,for O2prefers to be dissociated on continuous Pd0sites;(Ⅲ) The excessive tandem side reactions,as the hydrogenation tests have shown that contiguous Pd ensembles are more active for H2O2hydrogenation than small Pd clusters [66].

Fig.5.(a) The proposed reaction mechanism of H2O2 synthesis on Pd2.0Au1.0/TiO2 catalysts.(b) The possible change of structural model after reduction treatment.

Based on the discussion above,it can be summarized that the hydrogenation activity and H2O2selectivity depends on the balance of the sites for hydrogenation and associative O2activation,and it is highly sensitive to the reduction conditions (e.g.,temperature).It should be also noted that the reduction temperature should be finely tuned to avoid the activity loss caused by the sintering of metal NPs.

In conclusion,we successfully demonstrated an engineering control on the chemical states and geometry of PdAu–PdAuOxinterface in a bimetallic Pd-Au/TiO2catalyst through a facile reduction pretreatment method,and eventually identified 573 K as the optimum reduction temperature.The optimal Pd0/Pd2+ratio at about 0.9 observed in Pd2.0Au1.0-573 showed the highest H2O2selectivity of 87.7% compared with the unmodified one (71.80%),which is able to locate the PdAu–PdAuOxinterface closest to the largest diameter on the catalyst surface,thereby increasing the number of the exposed number of active sites despite particle agglomeration.The insights on valence and morphology effect on catalytic activity provides a feasible approach to improve the direct synthesis of H2O2viathe modification of the Pd active surface through pre-activation treatment.

Declaration of competing interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest associated with the work submitted.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos.91934302 and 22178110),the Dean/Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (No.2020K001)and the Project of the Department of Science and Technology of Sichuan Province (No.2023YFQ0086).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2023.108446.