Zr Ratios on the Physicochemical Properties and

Sep 5, 2018 - A series of CuO/Ti–Zr–O composites was prepared and applied for CO oxidation. Then, they were further characterized by X-ray diffrac...
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Study of Different Ti/Zr Ratios on the Physicochemical Properties and Catalytic Activities for CuO/Ti−Zr−O Composites Junning Qian,⊥,† Qun Hu,⊥,† Xueyan Hou,† Fangting Qian,§ Lihui Dong,*,†,‡ and Bin Li*,†,‡ †

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/13/18. For personal use only.

Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P. R. China ‡ Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing University, Nanjing 210093, P. R. China § School of Economics and Management, Anhui Agricultural University, Hefei 230000, P. R. China ABSTRACT: A series of CuO/Ti−Zr−O composites was prepared and applied for CO oxidation. Then, they were further characterized by X-ray diffraction, Brunauer−Emmett−Teller surface area, UV−vis diffuse reflectance measurement, O2 temperature-programmed desorption, H2 temperature-programmed reduction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy. Several points were made as follows: (1) This series of 5CuXTZ composites could still maintain the anatase TiO2 type after the doping of ZrO2 and CuO. (2) CuO species kept a scattered state on Ti−Zr−O support, forming the active Cu+ species and oxygen vacancies. (3) This 5CuXTZ catalyst (Ti:Zr = 20:1) displayed the best reductive property because of a higher number of Cu+ species and better reductive properties. (4) The CO oxidation over this 5CuXTZ catalyst followed the L−H mechanism. CuO/TiO2 and CuO/ZrO2 catalysts in CO oxidation.16−18 However, the key factors in improving these catalytic activities are still worth investigating. Particularly, the effect of different ratios of Ti/Zr on the enhanced catalytic performance remains to be studied. Though metallic Cu-based catalysts have been widely studied,32−35 knowledge on characteristics of catalytic activities is still incomplete. Qin et al.36 thought that Cu2+ was more active than other copper species in the CuO−CeO2 system. Agudo et al. found that Cu+/Cu0 species were active in the oxidation of CO by studying the CuO/Al2O3 composites.37 On one hand, catalytic behavior has a close relation with the dispersion of various phases. Active component usually keeps a dispersed state on the support, existing as a molecular monolayer when the loading amount is low. These dispersed parts come together to produce a close-packed monolayer, and the remaining ones generate a crystalline phase when the loading amount is high. On the other hand, catalytic properties of CuO are highly affected by different carriers. Thus, the active state of Cu species in the CuO/TiO2−ZrO2 system over CO oxidation is still worth exploring. In this work, our focus is primarily on following points: (1) explore the correlation between structure and catalytic

1. INTRODUCTION Nowadays, CO gases receive more attention due to the importance of CO sensing, safety of human beings, air cleaning, and so on.1,2 Furthermore, CO reaction is also harmful in some industrial processes (methanol production and water−gas shift reaction).3−6 Therefore, it is meaningful to solve these problems caused by CO gases, and catalytic oxidation is a valid method.7 Noble metal-based composites are well-known as useful catalysts in CO oxidation.8−10 But, the biggest disadvantage of these catalysts is the expensive price. Thus, related studies about free noble metal catalysts in CO oxidation attract a wide interest.11−13 This research mainly includes following aspects: CuO/CeO2,14,15 CuO/TiO2,16,17 CuO/ZrO2,18 CuO/Fe2O3,19 CuO/Al2O3,20,21 etc. In all catalysts, CuO/TiO2 system shows better catalytic behaviors compared to MoOx/TiO2, FeOx/TiO2, and CoOx/TiO2.22−24 Lately, studies on the TiO2−ZrO2 mixed oxides have attracted a wide interest because this system contains a larger Brunauer−Emmett−Teller specific surface area.25−29 Further, ZrO2 has an excellent redox nature and is easy to use to form pores and interact with active components. Huang et al.30 prepared a CuO/Ti0.6Zr0.4O2 catalyst by the surfactant-assisted method and found that this catalyst performed 100% CO conversion at 140 °C. Gong et al.31 synthesized CuMn−TZ composites by prehydrolysis of alkoxide and discovered these catalysts were very active in CO oxidation, and CO conversion reached 100% at 180 °C. Previous research found that CuO/ TiO2−ZrO2 composites showed behaviors better than those of © XXXX American Chemical Society

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June 15, 2018 September 5, 2018 September 5, 2018 September 5, 2018 DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research performance of CuO/TiO2−ZrO2 catalysts; (2) investigate the chemical state of Cu species on the surface of catalysts; and (3) surmise the reaction mechanism over Cu-based catalysts in CO oxidation.

that, the sample was interacted with a stream of O2−N2 and/or CO-N2 for 1 h. By heating the adsorbed species and recording the spectra at different temperatures, the corresponding reaction study was obtained from room temperature to 300 °C at 5 °C·min−1. 2.3. Activity Measurement for CO Oxidation. Catalytic performance was tested in a fixed-bed reactor, which worked under a certain stream of 20.8% O2, 1.6% CO, and 77.6% N2 with 30,000 mLg−1·h−1. Each experiment needed about 50 mg of sample. The sample then was disposed in N2 stream for 1 h before introducing the reaction gases. The conversion of CO was measured from room temperature to the temperature of 100% CO conversion using a GC-2010 gas chromatograph. Then, the catalytic activity was expressed with CO conversion.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. TiO2−ZrO2 supports with different Ti/Zr molar ratios (3:1, 5:1, 7:1, 12:1, 15:1, 20:1, and 30:1) were synthesized by coprecipitation. First, a quantity of TiCl4 and ZrOCl2·8H2O was dissolved in HCl(aq). Afterward, the mixture was dropped into NH3(aq) until the pH was 10. Next, the obtained mixture was kept stirring for 3 h and aged for 24 h. After that, this mixture was washed until the pH was 7. Then, the solid was dried at 110 °C and calcined in a muffle furnace at 500 °C for 4 h. The pure TiO2 support was prepared in a similar way. Furthermore, 5% Cu(Ac)2 was impregnated on these obtained supports. Subsequently, the solid was kept mixing for 0.5 h and dried at 90 °C. Finally, the preliminary products were dried for 12 h at 110 °C, and then these products were calcined at 500 °C for 4 h. They were named as 5CuXTZ (X = 3, 5, 7, 12, 15, 20, and 30) and 5CuT. 2.2. Catalyst Characterization. 2.2.1. X-ray Diffraction (XRD) Measurement. XRD data were collected according to the D/MAX-RB X-ray diffractometer (Rigaku, Japan), which possessed the radiation of Cu Kα (λ = 0.15418 nm), working under 40 kV and 40 mA with the scanning rate of 8° min−1 and the range between 10° and 80°. 2.2.2. UV−Vis Diffuse Reflectance Measurement (DRS) Measurement. Information on composites was gained using a TU-1901 spectrophotometer (Shimadzu), applying a reference material of BaSO4 and a scanning range between 200 and 800 nm. 2.2.3. BET Measurement. To obtain the information on BET specific surface area and pore distribution, these samples were tested on the Micrometrics TriStar II 3020 analyzer at 77 K. 2.2.4. Temperature-Programmed Reduction (TPR) Measurement. Chemisorption analyzer (Finetec Instruments) was applied to test the reductive property. The sample was first treated in purified N2. Then, it was treated with a H2−Ar mixture for 0.5 h; after that, it was heated from room temperature to 800 °C. 2.2.5. Temperature-Programmed Desorption (TPD) Measurement. O2-TPD was implemented using a chemisorption analyzer (Finetec Instruments). About 100 mg of sample was treated in highly purified He at 200 °C for 1 h. In addition, oxygen adsorption was conducted under the O2−He mixture at 200 °C for 0.5 h. After total cooling, the system was purged in He (60 mL·min−1) for 1 h. Then, the temperature was raised to 700 °C at a rate of 10 °C·min−1. 2.2.6. X-ray Photoelectron Spectroscopy (XPS) Measurement. XPS was carried out with a PHI 5000 Versa Probe machine with Al Kα radiation at 1486.6 eV. Then, the sample was outgassed in a UHV room. 2.2.7. High-Resolution Transmission Electron Microscopy (HRTEM) Images and Energy-Dispersive X-ray Spectroscopy Mapping. HRTEM and EDX mapping images were collected using a Tecnai G2 20 instrument from American Company, which works under 200 kV. 2.2.8. In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). In situ DRIFTS information was obtained using a Nicolet IS50FT-IR spectrometer. First, the sample was treated with purified N2 for 1 h at 300 °C. After

3. EXPERIMENTAL RESULTS 3.1. XRD and UV−vis DRS Results. Figure 1 displays the XRD spectra of 5CuXTZ catalysts. It can be seen that these

Figure 1. XRD patterns of these catalysts.

peaks all belong to anatase-type TiO2. This means that ZrO2 is mainly in the amorphous state.38 In addition, these peaks move to low diffraction after the doping of ZrO2 compared to 5CuT sample. This reveals that Zr4+ has completely entered into the lattice of Ti4+, as the ionic radius of Zr4+ (0.072 nm) is larger than that of Ti4+ (0.061 nm).39 Therefore, it can be summarized that Zr4+ has entirely entered into the lattice of TiO2, forming the Ti4+−O−Zr4+ band but still keeping in anatase-like structure through the substitution effect.40,41 Moreover, the absence of crystalline CuO reveals that the CuO species maintains a dispersed state on the surface.42 As depicted in Figure 2, these UV−vis absorption bands move to higher wavelengths compared with those of TiO2. It signifies that the doping of CuO has a great impact on the optical absorption edge.43 Moreover, compared to 5CuT, the red-shift adsorption of 5CuXTZ samples indicates that Zr4+ has entered into the lattice of TiO2,44 which is in agreement with XRD results. Furthermore, a wide absorption band (490− 800 nm) is observed for 5CuXTZ samples. This absorption band is ascribed to Cu+, proving the presence of Cu+.45 Meanwhile, it can be observed that the intensities of absorption of 5CuT and 5CuXTZ samples are higher than those of TiO2. This means that support structure can affect the copper species coordination environment, presenting broader and weaker d-d bands in the visible region.46 3.2. N2 Physisorption Analysis. Figure 3 presents the N2 adsorption/desorption equilibrium curves and pore size B

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Table 1. Textural Properties of 5CuT and 5CuXTZ Samples samples

BET specific surface area (m2·g−1)

average pore diameter (nm)

5Cu3TZ 5Cu5TZ 5Cu7TZ 5Cu12TZ 5Cu15TZ 5Cu20TZ 5Cu30TZ 5CuT

160 139 127 123 121 107 99 77

5.4 8.5 8.8 11.2 11.4 12.6 12.8 11.5

Figure 2. UV−vis DRS spectra of catalysts.

distribution profiles. As shown in Figure 3 (a), all samples display the IV-type isotherm which contains a hysteresis loops of H2 model, indicating the existence of mesopore in these samples.47−49 Deeply, it can be seen in Figure 3 (b) that all the pore sizes are around 5.4−12.8 nm, and detailed information are collected in Table 1. As seen from the table, the size of pore diameter increases with the increase of TiO2 content. In addition, BET specific surface areas of 5CuXTZ samples increase with the doping of ZrO2 content and decrease with the increase of TiO2 content. The maximum BET specific surface area can even reach 160 m2·g−1 (5Cu3TZ). It reveals that the doping of Zr4+ can greatly improve the BET specific surface area. 3.3. H2-TPR Results. H2-TPR characterization is taken to evaluate the reductive properties of these samples. As shown in Figure 4, two peaks in each sample are ascribed to the changing processes of α (Cu2+ → Cu+) and β (Cu+ → Cu0), respectively.50,51 It can be seen that the temperatures of α and β peaks decrease at first and then increase with the increase of TiO2 content, and 5Cu20TZ sample possesses the lowest temperatures for both α (163.4 °C) and β (178.8 °C) peaks, which proves that this 5Cu20TZ sample has the best reductive properties. Detailed changes are attributed to the following reasons: (1) Usually, the larger BET specific surface area is beneficial for the reduction of surface CuO. Furthermore, Ti4+ can combine with Zr4+ to generate Zr−O−Ti band, enhancing

Figure 4. TPR profiles of 5CuXTZ catalysts.

the effect of delocalization and enrichment between Ti4+ and Zr4+ through the introduction of ZrO2.52 Additionally, the electron from Zr−O bond to Ti−O bond, and the charge redistribution comes to form a newly mixed Zr−O−Ti band. Besides, the actual H2 consumption is calculated in Table 2. According to Table 2, the actual H2 consumption for 5CuXTZ samples are larger than that for 5CuT, which implies that the introduction of ZrO2 enhances the overall reductive properties of the 5CuXTZ catalysts. (2) Just as previously reported,53,54 the most active facet of anatase TiO2 is the (001) plane. Figure 5 displays the changes of different structures. It can be seen from Figure 5a that the corresponding octahedral configuration

Figure 3. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves of these samples. C

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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286, and 500 °C except for 5CuT and 5Cu3TZ. For 5CuT, these three peaks are centered at 120, 220, and 400 °C, while the three peaks are centered at 120, 350, and 650 °C for 5Cu3TZ. The unstable physical oxygen species are responsible for the formation of the first peak.56 The second peak arises from the O2− species generated by O2 in surface oxygen vacancies.57 These two peaks are usually related to the oxidation process.58 The third peak is concerned with lattice oxygen species.57 Moreover, this third peak area is highly increased, while ZrO2 dopes into the lattice of TiO2, which correspondingly causes the formation of Ti−O−Zr bond.46 The temperatures of oxygen desorption are higher than those of pure TiO2 when Zr4+ dopes into the TiO2 lattice. It not only enhances the interaction between support and copper species but also increases the temperature of oxygen desorption.59 3.5. Catalytic Performance. Figure 7 shows the catalytic behaviors of a series of 5CuXZT catalysts in CO oxidation.

Table 2. Peaks areas of the H2-TPR profiles and the H2 consumption (mmol·g−1) of the peak area samples

α (left)

β (right)

total

actual H2 consumption (mmol·g−1)

5CuT 5Cu3TZ 5Cu5TZ 5Cu7TZ 5Cu12TZ 5Cu15TZ 5Cu20TZ 5Cu30TZ

4793.8 4993.7 5117.0 6031.4 5555.6 5315.1 5732.7 7582.3

4909.1 5708.0 5358.4 3848.3 4388.6 5473.1 4837.3 4016.0

9702.9 10 701.7 10 475.4 9879.7 9944.2 10 788.2 10 570.0 11 598.3

0.5570 0.6144 0.6014 0.5672 0.5709 0.6193 0.6068 0.6658

Figure 5. Tentative model of the surface-dispersed copper oxide species formed on the (001) plane of (a) the anatase TiO2 surface and (b) the ZrO2-doped anatase TiO2 surface.

is generated when Cu2+ dopes into the octahedral vacancy. Moreover, various lengths of M−O (M = Ti, Zr) bonds can be formed because of the effect of different radii and charge densities. Thus, the octahedral configuration of Cu2+ is changed (Figure 5b), staying in an unstable state. As a result, this structure will be much easier for the generation of Cu+. 3.4. O2-TPD Results. O2-TPD results are shown in Figure 6. The surface O species goes through the following conversion process: O2(ad) → O2−(ad) → O−(ad) → O2−(ad/lattice).55 Physically adsorbed O2(ad) is usually removed at the beginning of the experiment. Adsorbed O2−(ad) and O−(ad) species are not stable; O2−(ad/lattice) species are assigned to oxygen in the lattices, which are not easy to desorb from surface. As seen from Figure 6, most samples show three typical peaks at 188,

Obviously, the amount of ZrO2 has an important effect on the performance of catalysts. The CO conversion is very low (∼30%), below 75 °C for these catalysts. However, it starts to increase rapidly with the further increase of temperature. Besides, the CO conversion first increases but then decreases as the amount of ZrO2 decreases (Figure 8). This means that a

Figure 6. O2-TPD profiles for these catalysts.

Figure 8. Temperature of 90% CO conversion over 5CuXTZ and 5CuT catalysts.

Figure 7. CO conversion over 5CuXTZ and 5CuT catalysts.

D

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. XPS spectra (a) Ti 2p; (b) Zr 3d; (c) Cu 2p; (d) Cu LMM; and (e) O 1s of 5CuTZ, 5Cu20T, and 5Cu30TZ samples.

Table 3. Surface (XPS) Compositions of 5CuTZ, 5Cu20T, and 5Cu30TZ Samples samples

Ti 2p

Zr 3d

Cu 2p

O 1s

Ti/Zr

O‴/(O″ + O′ + O‴) %

Cu+/(Cu2+ + Cu+) %

5CuT 5Cu20TZ 5Cu30TZ

17.21 17.59 15.75

1.29 0.73

3.84 2.91 2.79

53.01 52.18 52.23

13.64 21.58

59.72 79.06 73.88

16.03 36.10 21.93

centers at 933.7 and 932.2 eV are ascribed to copper species, and the positions between 937.5 and 947.5 eV belong to Cu2+ (Figure 9c). As reported elsewhere,61 lower valence copper species for Cu0 and Cu+ are located at 932.0 eV. The further Cu−O−Ti (Zr) band is caused by Cu+, which causes the formation of oxygen vacancy. The spectrum of Cu LMM is presented to further explore the state of copper. As seen in Figure 9d, 568.5 and 570.6 eV belong to Cu2+ and Cu+, accordingly.62 Furthermore, it is notable from Figure 9e that there are up to three peaks in O 1s spectra. Peaks at 531.2 eV (O″) and 529.5 eV (O‴) are in these three samples while 532.7 eV (O′) peak only appears in 5Cu30TZ sample. O″ and O‴ are ascribed to these adsorbed O and lattice O species, and O′ belongs to this water molecular on the surface. Further, this

proper doping of ZrO2 favors the improvement of catalytic behavior. Furthermore, the 5Cu20TZ presents the best catalytic activity, whose CO conversion reaches 100% at 120 °C. This excellent catalytic performance is related to the best reductive property among all catalysts.46 3.6. XPS Analysis. To investigate the influence of composition on catalytic properties, 5CuTZ, 5Cu20T, and 5Cu30TZ samples were characterized by XPS. The results of Ti 2p, Zr 3d, O 1s, Cu 2p, and Cu LMM are shown in Figure 9. As seen in Figure 9a, the peaks of 458.2 and 464.0 eV are associated with Ti 2p3/2 and Ti 2p1/2, implying that Ti basically forms as Ti4+.30 Further, the binding energies located at 181.4 and 183.8 eV correspond to Zr 3d5/2 and Zr 3d3/2 (Figure 9b), which means that Zr exists chiefly in +4.60 Additionally, the E

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. (a and f) HRTEM images and (b−e) EDX mapping diagrams of the 5Cu20TZ sample.

Figure 11. IR spectra following an exposure of 5Cu20TZ catalysts to (a) CO and (b) CO + O2 streams at different temperatures.

In addition, detailed information on element analysis is summarized in Table 3. The Ti/Zr ratios on the surface are lower than theoretical Ti/Zr ratios, suggesting that the catalyst surface is enriched with Zr4+. Furthermore, 5Cu20TZ sample shows the lowest Ti/Zr ratio (13.64:1). Beyond that, it can be found that 5Cu20TZ sample also contains the largest amount

5Cu20TZ catalyst exhibits the highest amount of lattice oxygen (Table 3). According to the literature,51 the ability to store O2 for catalyst is quite relevant to the amount of lattice oxygen oxidation, which is beneficial for the enhanced catalytic properties. This result further proves the excellent catalytic performance of 5Cu20TZ catalyst. F

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research of Cu+. The low Ti/Zr ratio and large Cu+ amount imply that there are more oxygen vacancies in this sample, which is helpful for the enhanced catalytic performance.63 3.7. Structural Characterization (HRTEM + EDX Mapping). HRTEM images and EDX mapping diagrams were characterized to study the effect of microstructure on enhanced catalytic properties for 5Cu20TZ sample (Figure 10). Figure 10a displays the nanostructure of this material, and these elements Cu, Ti, Zr, and O on the catalyst surface correspond to Figures 10b−d, which agrees with EDX mapping diagram of Figure 10a. The results also prove that Cu, Ti, and Zr elements exist on the catalyst surface. In addition, these Figures 10a−e show a similar morphology, revealing that these elements are evenly dispersed in 5Cu20TZ sample. Furthermore, specific distribution of various crystal surfaces are shown in Figure 10f. It exhibits two ZrO2 crystal surfaces (121) and (111), the lattice parameters of which are 0.204 and 0.286 nm. Further, there is a crystal surface of d = 0.188 nm, and it is assigned to the TiO2 (200). Moreover, CuO expresses an existence of (−112), which hass a 0.196 nm distance. The results show that this d-spacing with ZrO2 is bigger than that with {(PDF-ICDD86-1449)}, but this dspacing with TiO2 is smaller than that with {(PDF-ICDD711166)}, which reveals that the interaction between ZrO2 and TiO2 exists, and these results cause the changes of lattices.51 However, the d-spacing of CuO does not show a clear distinction with {(PDF-ICDD80-1916)}, which means that the doping CuO almost did not enter the Ti−Zr−O structure. 3.8. CO and/or O2 Interaction over 5Cu20TZ Catalyst. The in situ DRIFTS spectra of CO oxidation were measured to further investigate the adsorption on 5Cu20TZ surface. As shown in Figure 11a, the peak centered at 2108 cm−1 from 25 °C is the Cu+−CO species.64 This Cu+−CO band implies the existence of Cu+ on the surface, which is also consistent with XPS results. In addition, this band increases first but then decreases when temperature increases. Furthermore, it is greatly enhanced at 75 °C, while this peak is decreased with continuously increasing temperature. These changes mean that Cu2+ species on the surface are first reduced to Cu+ species and then further reduced to Cu0 species with the continuously increasing temperature, which also agrees with the TPR results. Afterward, the coadsorption of CO and O2 over 5Cu20TZ catalyst was carried out to study the mechanism of CO oxidation. As seen in Figure 11b, the peak for Cu+−CO species appears at 2106 cm−1 from room temperature, whereas this peak becomes weaker than the corresponding peak in single CO adsorption. It indicates that O2 species preferentially occupies the vacancies on the surface. As a result, the CO molecules cannot first adsorb on the surface. Further, this peak intensity increases at first, gradually decreases, and eventually disappears at 200 °C with the slow increase of temperature. This also reflects the same change with single CO oxidation (Cu2+ → Cu+ → Cu0). 3.9. CO Oxidation Mechanism with 5Cu20TZ. According to the FT-IR results, a probable reaction process is raised to explain the CO oxidation (Figure 12). First, Cu+ is formed according to the electron transfer from Ti4+ and Zr4+ to Cu2+ at 25 °C because of the bigger electronegativity of Cu. As discussed above, the O2− species in oxygen vacancy are formed by the preferentially adsorbed O2 on the catalyst surface, and these O2− species further partially inhibit the adsorption of CO molecules at low temperature, which belongs to the L−H mechanism.65 Thus, a few CO2 species are formed by CO

Figure 12. Possible reaction mechanism for CO oxidation over 5Cu20TZ catalyst.

oxidation below 75 °C, which is corresponding to the catalytic performance. On the other hand, this Cu+ species seems to have a great effect on the reaction. As shown in the FT-IR results, the amount of Cu+ is gradually increasing when the temperature is less than 75 °C. However, it starts to decrease when the temperature continues to increase. Combined with catalytic properties, it can be concluded that copper species mainly keep in +1 state before the CO oxidation is completed. In addition, there are more CO molecules and Cu+−CO species gradually appearing on the surface. Then, more CO2 species are generated by the reaction of more adsorbed CO molecules and O2− species.

4. CONCLUSIONS In summary, different ZrO2 amounts have an apparent influence on the catalytic performance. This work reports the correlation of different Ti/Zr ratios and catalytic behavior of a series of CuO/Ti−Zr−O catalysts. Here are the conclusions: (1) This series of 5CuXTZ composites maintain the type of anatase TiO2 after the doping of ZrO2 and CuO. (2) The lattice of TiO2 is changed after the doping of ZrO2, making the generation of Cu+ easier. (3) It is usually thought that the catalyst with larger specific surface area and pore diameter will possess better catalystic properties. However, 5Cu20TZ shows the best catalytic performance in this Cu-based composite system. This means that specific surface area and pore diameter are not the determining factors of catalytic activity, as it is also affected by other factors (such as the amount of active species and reductive ability). (4) The 5Cu20TZ catalyst displays the best reductive property, as it not only possesses a lot of Cu+ species on the surface but also owns better reductive properties, which correspondingly contributes to the best catalytic performance. (5) The CO oxidation over 5CuXTZ catalyst follows the L−H mechanism.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.) *E-mail: [email protected] (L.H.D.); Tel.: +86 07713233718; Fax: +86 0771-3233718. ORCID

Lihui Dong: 0000-0002-1184-7317 Bin Li: 0000-0002-4080-3268 G

DOI: 10.1021/acs.iecr.8b02674 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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J.Q. and Q.H. contributed equally to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This study was funded by National Natural Science Foundation of China (Grants 21507014, 21663006, and 21763003) and the Program for Science and Technology Development Plan of Nanning (Grant 20163146).

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