Role of Hydroxyl Groups in Low Temperature CO Catalytic Oxidation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Role of Hydroxyl Groups in Low Temperature CO Catalytic Oxidation over ZnSiO(OH) Nanowires Supported Gold Nanoparticles 4

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Jingjing Li, Baolin Zhu, Guichang Wang, Zunfeng Liu, Weiping Huang, and Shou-min Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08209 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry

Role of Hydroxyl Groups in Low Temperature CO Catalytic Oxidation over Zn4Si2O7(OH)2 Nanowires Supported Gold Nanoparticles Jing-Jing Li1,2, Bao-Lin Zhu3, *, Gui-Chang Wang2, *, Zun-Feng Liu1, Wei-Ping Huang2, Shou-Min Zhang2, * 1.State Key Laboratory of Medicinal Chemical Biology, College of Pharmacy, Nankai University, Tianjin, 300071, China 2.College of Chemistry, The Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), and Tianjin Key Lab of Metaland Molecule-based Material Chemistry, Nankai University, Tianjin, 300071, China 3.National Demonstration Center for Experimental Chemistry Education (Nankai University), Tianjin, 300071, China

*Corresponding authors: Bao-Lin Zhu; E-mail: [email protected] Gui-Chang Wang; E-mail: [email protected] Shou-Min Zhang; E-mail: [email protected] 1

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Abstract: Zn4Si2O7(OH)2 nanowires (ZSO NWs) with abundant hydroxyl groups can be easily fabricated by hydrothermal method. Gold nanoparticles (Au NPs) supported on Zn4Si2O7(OH)2 catalysts were prepared by deposition-precipitation method and exhibited superior catalytic activity for CO oxidation at low temperatures. XPS and in situ DRIFT spectroscopy suggested that the metal-support interactions (MSIs) and abundant hydroxyl groups existed in the Zn4Si2O7(OH)2 were beneficial to CO oxidation. The influences of gold contents and calcination temperature on their catalytic performance were also investigated. Obtained results demonstrated that the 0.8%Au-Zn4Si2O7(OH)2 catalysts pretreated at 300 C exhibited excellent CO catalytic activity and long-term stability. Based on experiment results, a possible mechanism was illustrated in atomic-scale by density functional theory (DFT) analyze to reveal the role of hydroxyl groups in CO oxidation for the Au-Zn4Si2O7(OH)2 catalyst.

1 Introduction Catalytic oxidation of carbon monoxide is one of the most important reactions in heterogeneous catalysis for its potential applications, such as automobile exhaust purification, protective mask, CO gas sensors, air cleaning, etc.Among numerous catalyst formulations that have been investigated, the supported gold nanoparticles (Au NPs) catalysts have attracted intensive research due to their exceptional catalytic activity at low temperatures.3 Compared with other active components, gold can exhibit good catalytic performance at low temperatures even at much lower contents.4-11 The catalytic performance of gold catalysts essentially relies on the size of the constituent Au NPs. Nevertheless, there are other factors that can inherently 2


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determine the catalytic activity, such as the gold state, the synthesis method and the nature of support. It is generally accepted that irreducible oxides (IROs) (e.g., SiO2, Al2O3, MgO, and ZnO)12-15 supported Au NPs are difficult in activating oxygen and often exhibit high catalytic activity for CO oxidation only at high temperatures in comparison to the reducible metal oxides (RMOs) (e.g., TiO2, CeO2, and Fe2O3 ).16-18 Therefore, the states and particle size of Au NPs, the structure and morphology of the IROs, and the interaction between Au NPs and IROs supports play vital role in achieving high catalytic performance for CO oxidation. IROs are often as inert supports in CO oxidation for their hardly reducible. So, searching for a highly active catalyst supported on IROs in catalytic oxidation of CO is still a crucial issue. Silicate nanomaterials, which combine NPs’ high chemical activity and microscale structures, abundant natural resources, special physiochemical properties and high thermal stability, are appealing materials as adsorbents in water treatment and catalyst fields.19 Among the different silicate materials, zinc silicate is a low-cost material and have attracted considerable attention in the catalysis fields. Most of the reported zinc silicate nanomaterials are composed of hemimorphite (Zn4Si2O7(OH)2·H2O) or willemite units. Hemimorphite with urchin-like and nanowires structure was intensively investigated for their high crystalline.20 Moreover, it has large lattice spacing, which endow it a higher affinity to adsorb heavy metal ions.21 In the hemimorphite, two zinc-oxygen or silicon-oxygen tetrahedrons linked with each other by oxygen bridges to form 4-, 6-, and 8-membered rings, with water 3



molecules existing in the resulting pores. Drying treatment can remove the free water, and dehydrated hemimorphite Zn4Si2O7(OH)2 (ZSO) support with pore structure can be obtained. After a simple hydrothermal and drying treatment, ZSO nanowires (NWs) with an orthorhombic system, which can be considered as composites of zinc and silica oxides could be obtained.21 Qian et al. modified the surface of silica with ZnO and found the MSIs was enhanced.22 They also found that the size of Au NPs could be well controlled by tuning the morphology of ZnO. It is well known that MSIs play an important and decisive role for the catalytic performance of supported metal catalysts. It not only strongly affects the catalytic performance, but also, to a great extent, determines the NPs’ stability as a durable catalyst.23 Consequently, the structure and the morphology of the supports play a major role in the gold catalysts’ performance.24 Wang et al. found that the monoclinic steps and the tetragonal surfaces of ZrO2 were closely related to the Au NPs size by DFT analyze, suggesting that the shape effect cause the increase of oxygen defects and the adsorption of gold atom.25 Tang et al. found the surface rich hydroxyl groups in SiO2-AlOOH (SA) composite nanosheets supporting CuO catalysts were beneficial for CO oxidation.26 ZSO is belong to orthorhombic system, and has a large numbers of hydroxyl groups. The hydroxyl groups are active in CO oxidation or photocatalytic oxidation reaction.27 It seems that high activity can be anticipated on the ZSO supported Au catalysts. However, there are few reports about zinc silicate that be used as catalyst support for CO oxidation. 4


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Here, Au-ZSO catalysts were prepared by deposition-precipitation method. The catalyst exhibited superior catalytic activity for CO oxidation. To explore the structural effect of the ZSO support in CO oxidation, the physicochemical properties of the Au-ZSO catalysts were investigated by diverse characterizations. Obtained results indicated that the prepared catalysts exhibited excellent catalytic activity and stability in CO oxidation due to its unique structure and strong interaction. The DFT calculation results reveal a mechanism that the surface hydroxyl groups can activate oxygen for low-temperature CO oxidation. 2. Experimental All the reagents were analytical grade and used without any further purification. 2.1. Catalyst Preparation 2.1.1. Preparation of ZSO and ZnO The ZSO NWs was prepared by hydrothermal method.21 The obtained precipitates were centrifuged and washed with distilled water for several times to remove the residual ions. After the precipitates were dried in oven at 80 C for 12 h, the ZSO was obtained and was used as support without further treatment. ZnO support was prepared according to the previous report.14 2.1.2. Preparation of Au-ZSO catalysts and Au-ZnO catalyst Au-ZSO catalyst: 0.4 g ZSO powder was stirred in 150 mL distilled water. A certain amount of HAuCl4 solution (0.01 mol/L) was added to the slurry and stirred for 0.5 h. A certain amount of urea solution was added drop-wise to adjust pH = 7.0, and the mixture was refluxed at 90 C for 4 h. Then the precipitate was collected by centrifugation and washed with distilled water to remove the chloride. After the powder was dried at 80 C, the samples were calcined at 200 C, 300 C and 400 C 5



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for 2 h, respectively. Au-ZnO catalyst: According to the same fabrication process, Au-ZnO catalysts was prepared for comparison. 2.2 Characterizations: The crystalline structure of the product was characterized by X-ray powder diffraction (XRD, Rigaku D/max-2200). The morphology was observed by scanning electron microscopy techniques (SEM, JSM-7500) and transmission electron microscope (TEM, Philips FEI T20ST). UV-vis DRS was recorded on a UV-Vis-NIR spectrophotometer (JASCO Corp V-570). The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe). Brunauer-Emmett-Teller (BET) specific surface area was measured by Nitrogen adsorption at liquid N2 temperature on a micromeritics apparatus (ASAP 2020/Tristar 3000). The in-situ IR on CO absorption was recorded by a TP-5080 fully automatic multi-purpose absorption instrument

connected with

temperature-programmed reduction (H2-TPR) profiles

IR

system.

The H2

were recorded on a

quantachrome chemBET pulsar TPR. Temperature-programmed desorption (TPD) of CO was carried out using a quantachrome chemBET pulsar TPD. 2.3 CO oxidation test: The catalytic activity was measured in a fixed-bed stainless steel tubular reactor using 200 mg catalyst powder. The reaction gas contains 10% CO balanced with air at a total flow rate of 36.3 mL/min. The operation temperature was controlled with a thermo-couple. The effluent gases were analyzed online by a GC-508A gas chromatography with a thermal conductivity detector. The activity of catalyst was evaluated by the CO conversion according to the following equation, CO conversion (%) ＝𝐶

𝐶(𝐶𝑂2 )

(𝐶𝑂) +𝐶(𝐶𝑂2 )

×100%

where 𝐶(𝐶𝑂) and 𝐶(𝐶𝑂2) represent the outlet CO and CO2 concentration, 6


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respectively. The activity was expressed by the degree of CO conversion. 3. Results and discussion 3.1 Characterization of Au-ZSO catalysts XRD. X-ray diffraction analysis was performed to investigate the crystalline phase of the support and the prepared catalysts. In the case of Figure 1, the XRD patterns of the ZSO support (a) and Au-ZSO with different loadings (b) could be indexed to the orthorhombic ZSO (JCPDS:19-1480). After Au NPs were supported on the ZSO, the structure did not show any significant difference. The XRD patterns of 0.8%Au-ZSO catalysts without calcination (a) and calcined at different temperatures (b: 200 C, c: 300 C, d: 400 C) are shown in Figure1B. The peak intensity increased with the increasing calcination temperatures, which should be attributed to the improvement of crystallinity and the increasement of crystalline size of ZSO. The XRD peak of 1%Au-ZnO shows hexagonal structure of ZnO (JCPDS: 36-1451), as shown in Figure 1C. Small peaks appear at 38 , 44.4  are the diffraction of gold (JCPDS: 4-0784). Compared with Figure 1C, the peaks assigned to diffractions of gold (111) and (200) at 38 , 44.4  were not apparently visible in Figure 1A and B. This was probably due to the small particle size and highly dispersity of Au NPs on the ZSO support. Also, the gold peaks might be overlaid by the peaks of ZSO. SEM. The scanning electron microscopy (SEM) is used to study the morphology of zinc silicate, the chemical composition and distribution of the catalyst. The ZSO and 0.8%Au-ZSO catalyst calcined at 300 C showed one-dimension (1D) morphohology, as shown in Figure 2A and Figure 2B. The length of the nanowires was in macro-scale and they were intertwined with each other with an average diameter of about 60.0 nm. When the Au NPs were supported on the ZSO, and calcined at 300 C, 7



the 1D morphology almost maintained. But, the surface was roughness and was covered with several fragments. Element mapping and energy dispersive spectrum (EDS) analysis were employed to determine chemical composition and content of the Au-ZSO catalysts. The elemental mappings and EDS analysis verified the presence of Zn, Si, O, and Au, consistent with the formation of Au-ZSO catalyst, as shown in Figure 2C. Besides, Au NPs exhibited a low content and were highly dispersed on the ZSO support, as shown by the gold mapping. TEM. The TEM images on the morphology and structure of 0.8%Au-ZSO calcined at 300 C under different magnifications are shown in Figure 3. The length of the nanowires is several micrometers, and the nanowires are interweaved together to form bundle-like structures, as shown in Figure 3A. The catalysts retained their nanowires morphologies after the Au NPs supported on the ZSO, and many black particles were observed on the ZSO NWs (Figure 3B). Under higher magnification, highly dispersed black nanoparticles with particle size less than 5 nm can be obviously observed on the surface of the ZSO NWs, and the whole surface of the nanowires structure is rough (Figure 3C). The microstructure of the deposited nanoparticles was further verified by HR-TEM images in Figure 3D. The lattice spacing in the area marked by dashed lines of the supported nanoparticles was measured to be about 0.235 nm, well assigned to the (111) plan of metallic gold, which indicated that Au NPs (less than 5 nm) were highly dispersed on the ZSO support. It has been reported that the Au NPs with small size is critical for high CO catalytic activity.28 Therefore, the formed Au NPs seems to be favorable for the high CO catalytic activity. 8


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Calcination temperatures have obvious influence on the particle size of Au NPs. The corresponding size distributions of Au NPs for the Au-ZSO catalysts calcined at 300 C and 400C were showed in Figure 4. When the catalysts were calcined at 300 C, it was obviously that the Au NPs were highly dispersed on the support, and the average diameters of Au NPs were 4.41 nm. It can be seen from the corresponding particle size distributions that the number of Au NPs less than 4 nm accounts for about 50%. Au NPs less than 5 nm could effectively chemisorb CO and catalyze CO oxidation at room temperature, which can enhance the CO catalytic performance. When the calcination temperature was 400 C, the nanowires morphology destroyed to a certain extent and Au NPs agglomerated together with an average diameter of 5.76 nm. Therefore, the calcination temperature had a great difference in Au NPs size. From the above results, it can be concluded that the Au-ZSO catalysts calcined at 300 C exhibit higher catalytic activity than that calcined at 400 C. UV-vis DRS. The UV-vis spectra of the ZSO support (a) and the 0.8% Au-ZSO catalysts calcined at different temperatures (b: 200 C, c: 300 C, d: 400 C) are shown in Figure 5. Compared with the support, a new adsorption peak at about 560 nm appeared in the Au-ZSO catalysts, which was the surface plasmon resonance (SPR) of metallic Au NPs.29 The absorption peaks were weak and broad, which indicated that the size of Au NPs was small, corresponding well to the HRTEM images. Moreover, the absorption bands were red-shifted slightly with the increasing of the calcination temperatures, indicating that the particle size of Au NPs increased in a small range. 9



BET. The N2 adsorption-desorption isotherms and pore size distributions of ZSO and 0.8%Au-ZSO catalysts calcined at 300 C was exhibited in Figure 6. The isotherms of the Au-ZSO shows a type IV nitrogen adsorption-desorption isotherm, implying the existence of mesoscale pores in the structure. The support displays a H1-type hysteresis loop, indicating the presence of nearly cylindrical channels produced by the stacking of the particles.30 The specific surface area (SSA) of the ZSO is 18.4827 m2/g. After Au NPs supported on the ZSO, the SSA is 22.1293 m2/g. The pore-size distribute of the ZSO support (inset of Figure 6) mainly located in 10 nm, indicating that the pore channels are in the mesoporous region. After deposition of Au NPs, the pore mainly disappears, which might be related to the calcination treatment and Au NPs might fill up the pore channel.31 XPS. The X-ray photo-electron spectra (XPS) were employed to elucidate the surface elemental composition and the oxidation states of the catalyst. Figure 7A depicts the survey XPS spectrum, indicating the presence of Zn, O, Si and Au elements in the catalysts. The residual carbon resulted from the XPS itself. Figure 7B depicts the O 1s spectrum. The O 1s peaks at 530.3 eV is attributed to Zn-O bond.14 The peak at 531.7 eV are attributed to Si-O bonds in Si2O76- and OH- in Zn4Si2O7(OH)2 support.32 The binding energy of Zn 2p was 1022.3 and 1045.3 eV, respectively, indicating the existence of Zn2+ in ZSO support (Figure 7C).31 The Si 2p peaks at 102.1 eV and 103 eV are shown in Figure 7D. The peak at 102.3 eV and 101.0 eV indicated the coexistence of two different states of Si bonds. The peak at 101.0 eV is attributed to Si-O bonds in the SiOx, while that at 102.3 eV is attributed to the Si-O bonds within 10


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the Si2O76-.32 The binding energy of Zn 3p was close to the binding energy of Au 4f5/2, and the intensity of the Zn 3p peak was higher than Au 4f5/2, resulting in a broader and larger peak of Au 4f5/2 than Au 4f7/2, as shown in Figure 8. By XPS peak fit software, the binding energy of Au 4f7/2 and 4f5/2 was 83.3 and 87.2 eV, respectively, demonstrating the existence of Au0.33 The peak at 85.9 and 88.8 eV indicated the existence of oxidized gold (Au3+).34 H2-TPR: To gather more information on the reducibility of different chemical species and synergistic eﬀect between gold and the support in the catalysts, the characteristic reducibility of the samples was determined by H2-TPR. Figure 9 displays the reduction profiles of ZSO and Au-ZSO calcined at 300 C. The reduction peak of the ZSO support at about 307 C might be the reduction of oxygen species of the support or the oxygen species absorbed on the surface of ZSO.35 For gold catalysts, there were no obvious peaks assigned to the reduction of gold cations, which would be located below 200 C, indicating the low content of oxidized gold species.36 A shift of reduction peak to lower temperature (249 C) was observed on Au-ZSO by comparison with the ZSO support, which was consistent with the study in literatures that noble metal strongly facilitated the reduction of support on account of the interaction between them.37 Besides, the TPR profiles of gold catalysts showed a new reduction peak (513 C) compared with corresponding supports, which could be attributed to the reduction of oxygen species around the nanosized gold particles.38 The Au-ZSO could produce more active sites, which were favorable for low temperature CO oxidation. So, the lower reduction peak temperature and more active 11



sites accounted for the excellent CO catalytic activity of Au-ZSO catalysts. The TPR profiles of gold catalysts differ from those of the corresponding bare supports depending on the synergistic effect between Au NPs and ZSO. CO-TPD: In order to understand the behavior of Au-ZSO catalysts for CO adsorption and activation, the temperature-programmed desorption (TPD) of CO was further investigated. The desorption profiles of the samples were similar, indicating that all adsorbed CO was transformed into CO2. The adsorbed species were desorbed with raising the temperature, as shown in Figure 10. The ZSO support has two desorption profiles (44 and 462 C), while the Au-ZSO has three TPD profiles, indicating different types of CO adsorption sites on the sample surface. The relatively high intensity peaks of the Au-ZSO demonstrated a higher intake of CO than the ZSO support. It is well known that the CO TPD profiles shows no low-temperature CO desorption peaks on irreduction oxides such as Al2O3, ZnO, SiO2, and etc., further demonstrating the existence of the oxygen species. The desorption peak at about 44 C for the samples may be assigned to the desorption of CO2 at low temperature, while the desorption peak at around 500 C might be the decomposition of formate or carbonate-like species on the supports.39 The CO2 peak at 591 C for the Au-ZSO catalysts might relate to the reaction of CO with surface hydroxyl groups associated with the gold species to form carbonate-like species, which would be thermally decomposed to gaseous CO2.40 DRIFT spectrum: To verify the valence of Au species and the reaction pathway of CO adsorption, a spectral zone of 2600-1400 cm-1 was investigated by DRIFT 12


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spectrum measurements. Figure 11 shows in situ DRIFT spectra of CO adsorption on the Au-ZSO catalysts. The catalysts performed at an adsorption temperature of 50 C. The samples were first purged in He for 1 h to remove surface impurities at the evaluated temperature, followed by CO adsorption until the samples reached the steady state. The adsorption peak appeared at about 2171 cm-1 can be ascribed to CO linearly adsorbed on the Au3+ sites,41 agreeing well with the above XPS results, or CO species bound to Zn2+ cations at the ZSO surface.42 The new peak at 2117 cm-1 agree well with those reported for CO chemisorbed on the top of low-coordinated Au atoms.43 The additional peak around 2060 cm-1 can be attributed to CO linearly adsorbed on the surface of very small gold particles.41 Furthermore, two peaks located at about 1630 and 1567 cm-1 resulting from the interaction of CO with surface hydroxyl groups might be associated with the carbonate like surface species on Au-ZSO catalysts, indicating the chemisorption capacity of surface hydroxyl groups for gaseous CO.44 It can be clearly observed that the peaks intensity of the CO absorption on the active sites enhanced with the increase of the absorption time from 5 min to 20 min. To further research the reaction mechanism and prove the produce of CO2, the IR spectra in the zone of 3800-3400 cm-1 under the same condition was shown in Figure 11 B. The zone at higher wave-number displays bands corresponding mainly to hydroxyl species.45 The intensity of the peak at about 3538 cm-1 enhanced with the increase of CO concentration, which confirmed the OH stretching vibration interaction of CO with hydroxyl groups on the ZSO support, i.e. CO+OH→COOH.

13



When the catalysts reached the CO adsorption equilibrium, the temperature was increased from 50 to 400 C to elucidate the physical desorption as a temperature function, as shown in Figure 11 C, D. The peak at 2060 cm-1 disappeared gradually with the increase of the desorption temperature, which implied that CO was rather weakly bound to small Au NPs. Besides, the peaks at 2171 and 2117 cm-1 did not disappeared, which might indicate that the Au-CO and Au3+-CO or Zn2+-CO had a strong interaction. Interestingly, when the desorption temperature was 400 C, new peaks at about 2360 and 2341 cm-1 appeared. It might be the produce of CO2 gaseous resulting from the decomposition of carbonate-like species,46 accompanied by the decrease of the peaks at 1630, 1567 and 3538 cm-1. The possible reaction might be the following: COOH→CO2+H. In conclusion, the hydroxyl groups can react with CO, and generates carbonate intermediates, which is favorable for CO oxidation. 3.2 Catalytic performance of Au-ZSO catalysts The catalytic activities of Au-ZSO catalysts for CO oxidation were monitored. In Figure 12A, all the samples were calcined at 300 C for 2 h. It can be clearly seen that the pure ZSO support (curve e) showed no catalytic activity, while the Au-ZSO catalysts showed high catalytic activities for CO oxidation. The catalytic activity of CO oxidation enhanced with the increase of gold loadings for the Au-ZSO catalysts. When the nominal loadings of gold were 1.0% and 0.8%, the temperature of CO full conversion (T100%) of Au-ZSO catalysts were 25 C and 60C, respectively. To investigate the influence of support and the MSIs, 1.0%Au-ZnO (c) catalyst’ activity was also monitored. T100% of 1.0%Au-ZnO catalysts was 90 C, which is higher than 14


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the T100% of 1.0%Au-ZSO catalysts. It was known that the catalytic activity was influenced by many factors. The nanowires morphology should play an important role in catalytic activity for the Au-ZSO catalysts. It can be observed that Au NPs are highly dispersed on the ZSO support, and about 50% of the particle size is less than 4 nm in HR-TEM. So, the morphology of nanowires could greatly affect the Au NPs size, and the interaction between gold and the support. Apparently, the metal-support interactions (MSIs) plays an essential role in the gold-based catalysts for the excellent catalytic activity. In our catalytic system, the electronic interactions between gold and the support may lead to the abundant low-coordination Au atoms. They can chemisorb and catalyze CO oxidation effectively, and the Au3+ observed by XPS or in DRIFT spectra could enhance the catalytic activity to some extent for the Au-ZSO catalyst.47 Furthermore, the hydroxyls at the interfaces played a key role in assisting gold to activate oxygen, and affecting the CO catalytic activity,48 which could be confirmed by the DRIFT spectra. The Au+ can bond to hydroxyl to form Au-OH, then the CO adsorbed on metal gold insert into the Au-OH bond to form a hydroxy carbonyl, as in the following: CO + Au+–OH- Au+–COOH-H–Au0–CO2, which could enhance the catalytic activity.49, 50 Besides, the hydroxyl groups at the ZSO surface might react with CO directly, and promote the CO oxidation. The presence of surface OH groups associated with Au is critical for the formation of COOH species, and could extend the lifetime of the catalyst and prevent deactivation by carbonate accumulation.51 Therefore, abundant hydroxides facilitate the CO oxidation, and excellent catalytic 15



performance was obtained on the Au-ZSO catalyst. Figure 12B depicts the catalytic activity of 0.8%Au-ZSO catalysts calcined at different temperatures (a: 80 C, b: 200 C, c: 300 C, d: 400 C). When the calcination temperatures ranged between 80 C300 C, the catalytic activity improved gradually. T100% of the catalyst calcined at 300 C was 60 C, while the T100% of the catalysts calcined at 200 C and 80 C were 70 C and 100 C, respectively. The different catalytic activities rose from the different interactions between the support and the Au NPs. The interaction enhanced with the increase of the calcination temperatures from 80 to 300 C. However, the high-temperature treatment might lead to the increased mobility and growth of Au NPs. As shown in Figure 4, the average particle size of Au NPs increased from 4.41 nm to 5.76 nm, resulting in the decrease of the catalytic activity (T100%  110 C). In summary, the catalysts calcined at 300 C exhibits the best CO catalytic activity. It is well known that the catalyst with high catalytic activity at low and high temperature can be applied in air purifies and automotive exhaust gas application, respectively. So, the stabilities of the Au-based catalysts at low and high temperatures were monitored.52, 53 In Figure 12C, the T100% of 0.8%Au-ZSO catalysts keep stable at 60 C for 10 h. Similarly, the catalysts exhibit excellent stability at 220 C with a lifetime test for 10 h (Figure 12D). The long-term stability of the catalysts shows the potential practical utility. Calculation methods and model To further understand the effect of OH species on the CO oxidation over Au(111), 16


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a theoretical study was performed. This study uses the Vienna ab initio simulation package (VASP) to perform the periodic, self-consistent density functional theory (DFT) calculations.54, 55 The generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) was used to calculate the electronic structures.56 The inner cores were described by the project-augment wave (PAW) scheme.57, 58 The electronic wave functions were expanded in a plane wave basis where the kinetic cut-off energy was 400 eV. In this work, all of the calculations are applied by spin polarization. Au(111) surface was modeled by periodically repeated slabs. Each supercell consists of four layers of metal with the top two layers allowed to relax. A vacuum region of 15Å was used. The calculations were performed on p(3×3) surface unit cells. The calculated optimized Au lattice constant was 4.18 Å. The Brillouin-zone integrations were performed using a 3×3×1 Monkhorst-Pack grid which was tested to be a reasonable k-points grid.59 The climbing image nudged elastic band (CI-NEB) method was used to calculate the reaction barriers.60 The adsorption energy Eads was calculated as, Eads = E(adsorbate + surface) – E(free adsorbate) – E(free surface). The more negative the Eads, the stronger the adsorption. The activation barrier (Ea) and the reaction energy were calculated using the following equations, Ea  ETS  EIS , E  EFS  EIS , where EIS, ETS, and EFS represent the energies of initial state (IS), transition state (TS), and final state (FS), respectively. Considering the formed CO2 is physiosorbed on the Au, the dispersion force 17



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would be important, therefore the van der Waals interactions correction was taken into account in our study. In general, the method based on the optPBE-vdW exchange-correction functional can give the magnitude that is close to the experiment results,

while the DFT-D2(D3) method usually overestimates

the VDW

interaction.61-63 Thus, the optPBE-vdW functional was employed in the present work. The possible reaction mechanisms of CO oxidation on Au(111) and OH/Au(111) systems were explored in the following section. Figure 13 and 14 displays the configurations of key species like TSs for possible elemental steps. a) Au(111) On the pure Au(111), two reaction paths for CO oxidation were considered: one is the molecular oxygen mechanism, namely CO+O2OOCOCO2+O, and the other one is the atomic oxygen mechanism, that is, CO+O2CO+2OCO2+O. CO+O2OOCOCO2+O For the molecular mechanism, the first step is the formation of OOCO, and it was found that it is a non-activated and downhill process with the reaction energy of -1.27 eV. After the formation of OOCO, it then decomposes to produce CO2. This step requires a barrier of 1.34 eV (TS1), and it is exothermic by 2.12 eV. At TS1, the bond length of C-O is 1.55 Å. CO+O2CO+2OCO2+O For the dissociation of O2 goes through TS2 with an activation energy of 1.41 eV, and the O-O bond length is 1.89 Å at TS3. For the formation of CO2 via CO+OCO2, the barrier is much smaller, 0.33 eV, and the distance between C and O at TS3 is 2.15 Å. The formed CO2 desorbs to the gas phase with the energy of 0.25 eV. Comparing these two different reaction mechanisms, one can find that the 18


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molecular mechanism is more active because the barrier of the rate-limiting step is lower (1.34 vs 1.41 eV). Due to the high barrier of CO oxidation on Au for both atomic and molecular oxygen mechanism (>1.3 eV), the probability that CO oxidation occurs at room temperature is very low. Moreover, we have also studied the mechanism of CO+O2CO3 (Carbonate)CO2+O, which needed a rather high barrier of 1.96 eV and was thus ignored in the present work. b) OH/Au(111) For the OH/Au(111) surface, the reaction mechanism can be assumed to be CO+OHCOOH(Carboxylate)CO2+H, i.e, via a carboxylate type intermediate. The formation of COOH is a barrierless process, and the associated reaction energy is -1.36 eV. The formed COOH then dissociates to form CO2 via TS4 (d/C-O/O-H=1.25/1.36 Å), which needs to overcome a barrier of 0.77 eV and is exothermic by 0.52 eV. After the formation of CO2, it would be released from surface to gas phase, and the corresponding energy is 0.28 eV. At the same time CO2 is formed, the other produced H atom reacts with molecular O2 to form HO2 by the path of H+O2HO2. This step has a barrier of 0.31 eV via TS5 (dO-H= 2.08 Å), and it is exothermic by 1.17 eV. Then the formed HO2 can either dissociate to form OH and H (HO2OH+H), or react with H to form H2O2 (HO2+HH2O2). It was found that the latter path is energetic favorable while the former one is not (1.46 eV vs 0.26 eV). The corresponding reaction energy of latter path is -1.51 eV, and the distance between O and H is 1.83 Å at TS6. After the formation of H2O2, it then decomposes to form OH through the cleavage of O-O bond. This step is exothermic by 0.43 eV, and the 19



energy barrier is 0.84 eV via TS7 (dO-O=1.86 Å). The formed OH can react with CO again to complete the catalytic cycle. So the reaction for the CO oxidation on OH/Au(111) can be expressed as .00( 1.36) .77( 0.52) .31( 1.17) CO  OH  O2  H 0  COOH  O2  H 0  CO2  H  O2  H 0  CO2  HO2  H .26( 1.51) .84( 0.42) 0  CO2  H 2 O2 0  CO2  2OH

Here the data in parentheses means reaction energy. As mentioned above, for the CO oxidation process, the controlling step is the formation of OH species by the decomposition of H2O2 with an energy barrier of 0.84 eV. Compared to that on pure Au(111), the energy barrier is significantly reduced from 1.34 eV to 0.84 eV, which leads to a much higher activity in the presence of OH specie. Mechanism: The DFT calculation results reveal that hydroxyl groups play a vital role in the CO oxidation on Au-ZSO catalysts. Based on the above results, a main pathway was proposed to explain the high CO catalytic activity at low temperature of Au-ZSO, as illustrated in Figure 15. CO adsorbed on Au0 is attacked by a hydroxyl group that absorbed on the Au3+ giving rise to the formation of COOH intermediate. The COOH then decomposes to CO2 and H as in the following: Au3+-OH- +CO  CO-Au3+-OH-Au3+-COOH-H-Au0-CO2. The formed H can readily react with O2 to form HO2.The HO2 further reacts with H to produce H2O2, and then dissociates to two OH, completing the reaction cycle. Also, the hydroxyl groups existed in ZSO catalysts might react with CO directly, and enhanced the CO catalytic activity. Therefore, the presence of surface hydroxyl groups associated with Au is critical for the formation of COOH species, and result in the high activity of Au-ZSO catalysts. 20


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Conclusion The Au-ZSO catalysts with nanowire morphology and rich hydroxyl groups were successfully fabricated via deposition-precipitation method. The catalysts exhibited high catalytic activity and thermal stability in CO oxidation. The strong metal-support interactions, which related to the hydroxyl groups, played a vital role in the high catalytic activity in CO oxidation. The hydroxyl groups can react with CO, and promote the CO oxidation. Also, the Au3+ can bond to hydroxyl in the Au/ZSO, and react with CO molecules adsorbed on metallic gold or other active species to form intermediate COOH. The DFT analyze further verified the positive effect of hydroxyl groups on CO catalytic performance. These research results proved the practicability of ZSO, which can be employed as a promising catalyst support for the applications in other catalytic fields.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21271110, 21373120, 21301098, 21271107) and MOE Innovation Team of China (IRT 13022). Reference (1) Choi, Y.; Stenger, H.G. Kinetics, Simulation and Insights for CO Selective Oxidation in Fuel Cell Applications. J. Power Sources 2004, 129, 246254. (2) Liu, W.; Flytzanistephanopoulos, M. Total Oxidation of Carbon Monoxide and Methane over Transition Metal Fluorite Oxide Composite Catalysts: I. Catalyst Composition and Activity. J. Catal. 1995, 153, 304316. 21



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Figure Captions Figure 1. XRD patterns of (A) Au-ZSO with different gold loadings, (B) 0.8% Au-ZSO calcined at different temperatures, (C) 1% Au-ZnO calcined at 300 C.

Figure 2. SEM images of (A) ZSO, (B) 0.8% Au-ZSO catalysts calcined at 300 C, and the elemental mapping, (C) EDS analysis of the 0.8% Au-ZSO catalysts.

Figure 3. TEM (A, B) and HRTEM (C, D) images of the 0.8% Au-ZSO catalysts calcined at 300 C.

Figure 4. HRTEM images of the 0.8% Au-ZSO catalysts calcined at (A) 300 C, (B) 400 C and the corresponding size distributions of gold nanoparticles.

Figure 5. UV-vis spectra of the ZSO and 0.8% Au-ZSO calcined at different temperatures.

Figure 6. The N2 adsorption-desorption isotherm of the ZSO support and the 0.8% Au-ZSO calcined at 300 C (the inset is the pore-size distribution).

Figure 7. (A) Survey XPS spectrum of 0.8% Au-ZSO catalyst calcined at 300 C and high-resolution spectrum for (B) O 1s, (C) Zn 2p, (D) Si 2p.

31



Figure 8. Au 4f spectrum of 0.8% Au-ZSO catalyst calcined at 300 C.

Figure 9. H2-TPR profiles of the ZSO and the 0.8%Au-ZSO calcined at 300 C.

Figure 10. CO-TPD profiles of the ZSO support and the Au-ZSO calcined at 300 C

Figure 11. (A, B) DRIFT spectra of with different CO adsorption time and (C, D) DRIFT spectra of the 0.8%Au-ZSO calcined at 300 C after exposing to CO at 50 C and subsequently heating to the indicated temperatures.

Figure 12. CO conversion of the ZSO and Au-ZSO with (A) different gold loadings, (B) different calcination temperatures, and the stability testing at (C) 60 C and (D) 220 C.

Figure 13. Snapshots of CO oxidation on pure Au(111) catalysts.

Figure 14. Snapshots of CO oxidation on OH-modified Au(111) catalysts.

Figure 15 Schematic illustration of the main reaction mechanism for Au-ZSO catalysts with enhanced catalytic activity for CO oxidation.

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Figure 8

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Figure 14

Figure 15

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TOC Graphic

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Role of Hydroxyl Groups in Low Temperature CO Catalytic Oxidation

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