Oxygen Vacancy Promoting Dimethyl Carbonate Synthesis from CO2

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Oxygen Vacancy Promoting Dimethyl Carbonate Synthesis from CO2 and Methanol over Zr-doped CeO2 Nanorods Bin Liu, Congming Li, Guoqiang Zhang, Xuesi Yao, Steven S. C. Chuang, and Zhong Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00415 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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Oxygen Vacancy Promoting Dimethyl Carbonate Synthesis from CO2 and Methanol over Zr-doped CeO2 Nanorods Bin Liu†, Congming Li, †,* Guoqiang Zhang, † Xuesi Yao, ‡ Steven S. C. Chuang, ‡Zhong Li†,* †

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China ‡ Department of Polymer Science, the University of Akron, 170 University Avenue, Ohio 44325, United States

Corresponding author: Congming Li; Zhong Li Email: [email protected]; [email protected]

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ABSTRACT: The synthesis of dimethyl carbonate (DMC) from CO2 and methanol by Zr-doped CeO2 nanorods with different ratios of Zr/Ce has been studied at 6.8 MPa and 140 °C. The catalysts were characterized extensively by TEM, XRD, N2 adsorption, Raman spectroscopy, UV-Vis spectroscopy, XPS, CO2-TPD, and in situ FTIR techniques. Doping of Zr atoms into the ceria lattice produced a fluorite-like solid solution, promoting the formation of oxygen vacancy sites. Zr-doped CeO2 nanorods exhibited a significantly more oxygen vacancy sites than pure CeO2 nanorods. Zr0.1Ce nanorods which exhibited DMC synthesis activity also possess the highest concentration of oxygen vacancy sites. In situ FTIR studies further revealed that CO2 can adsorb on the oxygen vacancy to form bidentate carbonate and as intermediate to participate in the reaction. This study presents a strategy to design a high-efficiency CeO2-based catalysts by controlling the concentration of the surface oxygen vacancies. KEYWORDS: Zr-doped CeO2 nanorods; Oxygen vacancy; CO2 adsorption; Dimethyl carbonate; bidentate carbonate

1. INTRODUCTION Dimethyl carbonate (DMC) is considered as an environmentally benign chemical in the emerging area of ‘green chemistry’.1 DMC molecule contains a number of organic functional groups: methoxy, carbonyl, and methyl groups, which can be used as a potential building block in the methylation and carbonylation reactions for replacing toxic precursors such as dimethyl sulfate (DMS) and phosgene.2 DMC has also used as a fuel additive because of its high octane number (105) and oxygen content (53%).2 The direct synthesis of DMC from methanol/CO2 has been gaining its importance because of the abundance of CO2, its environmental benign nature, and high atom efficiency of the synthesis process. Catalysts such as Cu-Fe bimetal catalyst, ionic liquid, zirconia, CeO2, and CeO2-ZrO2 have been studied for the direct synthesis of DMC from methanol and CO2.3-7 Among these reported catalysts, ceria-based catalysts showed promising selectivity and activity.8-11 However, the activation of CO2 is still a great challenge because CO2 is a fully oxidized, thermodynamically stable and chemically inert molecule.12 It is well known that the nature of catalytic sites for CO2 adsorption is an important factor affecting the efficiency of CO2 activation. The surface oxygen vacancy sites of a catalyst could promote the adsorption and activation of CO2.13 Oxygen vacancies have been proposed to serve as the active sites which could promote the CO2 conversion in the CO2 methanation.14-16 It has also been reported that oxygen vacancies were acid active centers active in CO2 hydrogenation and stabilize the thermodynamically unstable metal oxides.17-18 A number of studies found that the surface oxygen vacancies could provide a means of stabilizing the end products, by filling one oxygen atom of the CO2 molecule to the oxygen vacancy.19 In our previous research,20 we also found the surface oxygen vacancies favored CO2 adsorption and improved the catalytic performance for the synthesis of DMC from CO2 and methanol. The morphology of CeO2 catalyst has strong influence on the exposed crystal


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planes and the concentration of their surface oxygen vacancies. The latter has been shown to govern a number of catalytic reactions.21-24 CeO2 has been successfully synthesized with different morphologies: nanorods, nanocubes, and nano-octahedra. Among these different morphologies: the surface of the nanorods has the highest concentration of oxygen vacancy sites because of their enclosed (110) and (100) crystal planes.25 Wu and co-workers have studied the activity of CeO2 nano-crystal with well-defined surface planes for CO oxidation reaction, which followed the sequence of: rods > cubes > octahedra.26 Wang et al. studied the morphology effects of CeO2 on the direct synthesis of DMC with the 2-Cyano pyridine as dehydration agent and they found that CeO2 nanorods exhibited higher activity than nanocubes and nano-octahedra because of their high density of defect sites, and acid-base sites.27 Compared to pure CeO2, doping the transition metal oxides such as Y2O3, ZrO2, and TiO2 into the CeO2 could vary the surface acid-base property and oxygen vacancies.5, 28-29 Chen et al. demonstrated that the CeO2-CuO nanorods exhibited strikingly high catalytic activity for CO oxidation, which was ascribed to the introduction of CuO species into CeO2 nanorods generating oxygen vacancies.30 Liu et al. reported that ZrO2-doped CeO2 nanorods possessed high activities for the selective oxidation of styrene to styrene oxide compared to pure CeO2 nanorods because of the presence of high concentration of oxygen vacancies on the doped CeO2 nanorods.31 According to these previous studies, oxygen vacancies could play a significant role in the direct synthesis of DMC on CeO2-based catalysts.32-33 However, the effect of the surface oxygen vacancy on this reaction is poorly understood. In this work, the CeO2 nanorods with various Zr content were prepared and their activity in DMC synthesis from CO2 and methanol was investigated. It was found that the surface oxygen vacancy concentration and CO2 adsorption varied with Zr content, which are crucial to determine the catalytic activity of direct synthesis DMC from methanol and CO2.

2. EXPERIMENTAL SECTION 2.1. Materials Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99.9%, AR) and Zirconium nitrate (Zr(NO3)4·5H2O, 99.0%, AR) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China), sodium hydroxide (NaOH, 99%, AR) was obtained from Feng Chuan Chemical Reagent Co, Ltd (Tian jin, China). All reagents were used as received without further purification. 2.2. Catalyst preparation The nanorods were synthesized by hydrothermal process based on the previous report.34 Typically (shown in Scheme 1), Zr(NO3)4·5H2O and Ce(NO3)3·6H2O with different mole ratios were dissolved in 30 mL deionized water under vigorous stirring until completely dissolved. Simultaneously, 84.0 g of NaOH was dissolved in 210 mL deionized water. Then, the two solutions were mixed together and kept stirring for 30 min. The obtained mixed slurry was transferred into a 300 mL stainless steel


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autoclave and kept at 100 °C for 24 h. After the autoclave was cooler down to room temperature, the product were washed with deionized water until the pH reaches to 7 and washed several times with ethanol. The resulting product was dried at 80 °C overnight, and then calcined at 600 °C for 5 h in a muffle furnace to obtain ZrxCe nanorods, where x was the mole ratio of Zr to Ce in the catalysts. The Zr/Ce mole ratio was controlled in the range of 0 to 0.3 in order to keep the morphology of nanorods. For comparison, the pure ZrO2 were synthesized by the same process. To highlight the important role of hydrothermal synthesis in preparation of nanorod catalyst, we have also prepared CeO2 particles for XPS and catalyst activity studies (SI. Activity and Characterization of CeO2 particles prepared by co-precipitation)

Scheme 1. Schematic illustration of the synthesis process of the Zr-doped CeO2 nanorods. 2.3. Material Characterization The morphology of the obtained powder of CeO2, ZrxCe nanorods and ZrO2 was characterized by JEOL JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Samples for TEM analyses were prepared by dispersing the powdered products in ethanol, and then deposited on a carbon film coated on a copper grid. The powder X-ray diffraction patterns of the investigated catalysts were obtained from an X-ray diffractometer equipped with Cu Kα radiation source (in the 2θ range 10º-85º with a scanning speed of 8º/min). Specific surface areas measurement was carried out by a Beishide 3H-2000PS2 surface analyzer using nitrogen adsorption at a liquid-nitrogen temperature (77 K). Before the test, the sample was degassed in situ at 250 °C for 4 h. The surface areas of the catalysts were estimated by the Brunauer-Emmett-Teller (BET) method, and the pore size distributions were calculated from the analysis of desorption branch of the isotherm via the conventional the Barrett-Joyner-Halenda (BJH) model. The contents of Ce and Zr were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Before to analysis, 2 ml of conc. HF was used to dissolve 80 mg of the samples, followed by adding 2 ml of 30 wt% H2O2, and then the solution was diluted to 1000 ml with de-ionized water. The elemental analysis was performed. Raman spectra were recorded by a Renishaw in ViainVia micro laser Raman spectrometer (U.K.), Ar+ laser (514.5 nm wavelength), with an output power of 4 mW. The UV-Vis spectra of the catalyst samples were obtained by a Perkin-Elmer Lambda 900 UV-vis/NIR spectrophotometer. X-ray photoelectron spectroscopy (XPS) data were obtained by an ESCALab220i-XL electron spectrometer (VG, UK) using 300 W Al Ka radiation. The


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samples were compressed into a pellet of 2 mm thickness and then mounted on a sample holder by utilizing double-sided adhesive tape for XPS analysis. The sample holder was then placed into a fast entry air load-lock chamber without exposure to air and evacuated under vacuum ( Zr0.2Ce > Zr0.3Ce >CeO2. Furthermore, the Ce3+/Cetotal ratio of ZrxCe catalysts is higher than the pure CeO2 sample. Oxygen vacancy associated exposed Ce3+ ions on CeO2 are potentially potent surface sites in the reaction of DMC synthesis from CO2 and methanol. These results implied that the addition of Zr promoted the formation of Ce3+ on the surface of the CeO2 catalyst.47 The transformation of Ce4+ (0.97 Å) in CeO2 to a larger Ce3+ (1.10 Å) could compensate lattice contraction induced by the slightly smaller radius of Zr4+ (0.84 Å).48-49 It has been proved that the existence of Ce3+ could generate the oxygen vacancies on the catalyst surface.50 Thus, the concentration of Ce3+ increased with the concentration of oxygen vacancies. The Zr0.1Ce catalyst has the highest Ce3+/Cetotal ratio than other ZrxCe catalysts, hence, it has the highest concentration oxygen vacancies.


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Table 2 shows the concentration of Ce3+ and oxygen vacancy increased with the increasing of Zr content to a level, then decreased with further increase in Zr content. This decrease may be explained by the aggregation of Zr on the surface of the catalysts.31 Table 2 also lists the surface and bulk Zr/Ce of the catalysts, determined by XPS and ICP. The surface Zr/Ce atomic ratios are slightly higher than the bulk ratio, further indicating aggregation of Zr on the surface of the catalysts. Aggregation of Zr on the surface of Zr0.3Ce catalysts is also evidenced by its low surface area as compared with those of Zr0.05Ce, Zr0.1Ce, and Zr0.2Ce. The O 1s spectra of the catalysts in Figure 4b showed three peaks: (i) lattice oxygen (OL) in the CeO2 lattice at ～529.0 eV ), (ii) the O component associated with the O2- ions in surface oxygen vacancies (OV) at ～530.5 eV and (iii) chemisorbed oxygen species (OC) at ～532.5 eV.51 The concentration of surface oxygen vacancies of the catalysts can be estimated by the integrated peak areas using the following equation

C[O ]% = V

AO × 100 % + AO + AO V

AO

L

V

C

A୓ై : Photoelectron peaks areas of lattice oxygen; A୓౒ : photoelectron peaks areas of surface oxygen vacancies; A ୓ి : Photoelectron peaks areas of chemisorbed oxygen

Table 2 shows the total concentration of OV is 7.1%, 9.4%, 10.4%, 8.8%, 7.7%, 6.0%, corresponding to pure CeO2, Zr0.05Ce, Zr0.1Ce, Zr0.2Ce, Zr0.3Ce, ZrO2, respectively. The concentrations of OV agree well with the results obtained from the Ce 3d spectra, which was indicated that surface oxygen vacancies were enhanced by doped the Zr into CeO2 nanorods. Doping Zr into CeO2 lattice promoted the formation of surface oxygen defects/vacancies.31 The Zr 3d XPS spectra of ZrxCe nanorods and ZrO2 are shown in Figure S3 in the supporting information. ZrO2 and ZrxCe catalysts exhibit two Zr 3d peaks at 181.8 eV and 184.2 eV that could be related to the Zr 3d3/2 and 3d5/2 the spin-orbit splitting. The binding energy of Zr 3d in the ZrxCe catalysts (182.1eV, 184.5eV) was higher than that in ZrO2.52 These results further confirmed that Zr4+ entered into the CeO2 lattice and formed the solid solutions, as shown by the results of XRD and TEM.49, 52


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3.5. TPD analysis of the catalysts CO2 adsorption

ZrO2

(mmol/g) 0.46

Intensity (a.u.)

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Zr0.3Ce 0.72

Zr0.2Ce

0.79

Zr0.1Ce

0.96

Zr0.05Ce

0.89 0.59

CeO2 100

200

300 400 Temperature (°C)

500

600

Figure 5. CO2-TPD profile of CeO2, ZrxCe nanorods and ZrO2. It has been reported that the surface oxygen vacancy acts as Lewis acid sites and plays a key role in the adsorption of CO2.13 Thus, we have further studies the nature of catalyst surface by CO2-TPD. The results were shown in Figure 5. The amount of CO2 desorbed from the catalysts are calculated from the TPD peak area and listed in Figure 5. The CO2 desorption peaks appeared in the range of 50-200, 200-400, and 400-600 °C which were varied with the Zr/Ce ratios. The amount of CO2 desorbed from these sites are calculated and displayed in Table S3. In figure 5, we can see the intensity of the peak below 200 °C increases and then decreased with higher Zr content. The total amount of adsorbed CO2 of the ZrxCe nanorods was higher than those of CeO2 and ZrO2 catalysts. The Zr0.1Ce gave the highest adsorption capacity of CO2 at 0.96 mmol/g-1 among these catalysts. The amount of adsorbed CO2 on these catalysts showed a volcano-shaped curve with respect to Zr content and followed the order: Zr0.1Ce > Zr0.05Ce > Zr0.2Ce > Zr0.3Ce > CeO2 > ZrO2. It is noted that the order of CO2 adsorption ability of the catalysts is consistent with the variations in the concentration of surface oxygen vacancies determined by XPS in Table 2. In Table S3, it can be noted that the weak and moderate adsorption sites of the ZrxCe catalysts increased with rise of the Zr content and decreased with higher Zr content. The Zr0.1Ce nanorods possess the largest amount of weak and moderate adsorption sites. The surface acid properties of the catalysts were investigated by NH3-TPD and the results were exhibited in Figure S5. The amount of NH3 of all the catalysts are


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calculated from the TPD peak area and also displayed in Figure S5. It is observed from FigureS5 that the CeO2, ZrxCe nanorods and ZrO2 catalyst contained the same types of acidic sites: the desorption peaks in the range of 50-200 °C, 200-400 °C, 400-600 °C can be assigned to weak, moderate and strong adsorption, respectively. The amount of NH3 desorbed from these sites are calculated and summarized in Table S3. It is noted that the acid properties of the catalysts were strongly affected after doping Zr into CeO2. The Zr0.1Ce has the highest acidic sites density compared to other samples and the adsorption quantity of NH3 is 0.54 mmol/g-1, which indicated that Zr0.1Ce sample has the largest number of acidic sites. The amount of adsorption NH3 for all catalysts showed a volcano-shaped curve with respect to Zr content and followed the order: Zr0.1Ce > Zr0.05Ce > Zr0.2Ce > CeO2 > Zr0.3Ce > ZrO2. It has to be emphasized that the total acidity of ZrxCe nanorods catalysts was higher than pure CeO2 and ZrO2 catalysts (except Zr0.3Ce), which may be ascribed to synergic effect between ZrO2 and CeO2. At the same time, the findings also showed that the acidity of Zr0.3Ce nanorods was lower than the acidity of CeO2 nanorods. It may be due to the BET surface and pore volume of Zr0.3Ce nanorods (59 m2/g) are lower than pure CeO2 nanorods (74 m2/g) and exposed less acidic sites. Similar to CO2-TPD results, for all ZrxCe catalysts, weak and moderate acidic sites increased first and then decreased with higher Zr content. It can be found that the Zr0.1Ce nanorods possess the largest amount of acidic sites. By doping oxides, the surface acid-base sites of CeO2 will be varied and will bring about the performance differences, the similar results were obtained by the previous reports.53-54

16

1.0

12

0.8 0.6

8

0.4 4 0.2

Methanol conversion (%)

3.6. Catalytic activity

DMC yield (mmolDMC/gcat)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 CeO2

Zr0.05Ce Zr0.1Ce Zr0.2Ce Zr0.3Ce

ZrO2

0.0

Figure 6. Catalytic performance of the CeO2, ZrxCe nanorods and ZrO2 catalysts. Methanol conversion = 2nDMC/nCH3OH×100%. Error bars represent one standard deviation (n=5)


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Figure 6 shows the catalyst performance over the catalysts. The ZrxCe nanorods gave higher activity than CeO2 and ZrO2. The yield reached a maximum value of 14.2 mmol/g at Zr/Ce = 0.1, and then decreased with further increasing Zr/Ce ratios. The activity of Zr0.1Ce is 1.3 and 6.7 times higher than that of the pure CeO2 and ZrO2, respectively. The activity of the catalysts decreased in the order: Zr0.1Ce > Zr0.05Ce > Zr0.2Ce > Zr0.3Ce > CeO2 > ZrO2, which followed the trend of the surface oxygen vacancy concentrations and the capacity of CO2 adsorption. Meanwhile, it can be observed that the methanol conversion reached a maximum value of 0.65% at Zr/Ce = 0.1, and then decreased with further increasing Zr/Ce ratios. In figure S6, the yields increased from 7.2 to 12.1 mmol/g for CeO2 and 10.2 to 14.9 mmol/g for Zr0.1Ce with increasing the reaction time. It is also noted that further increasing the reaction time has not exhibited obvious increase of the yields for the Zr0.1Ce catalyst. These results indicated that the reaction may reach a plateau state after 6 h reaction time for Zr0.1Ce catalyst because of accumulation of water. According to the reaction equation of the DMC synthesis from carbon dioxide and methanol, the thermodynamic information about this reaction were calculated by ௵ ௵ the thermodynamic data of various substances.55 The ߂‫ܩ‬ଶଽ଼ ௄ and ߂‫ܪ‬ଶଽ଼ ௄ values are 26.21 kJ/mol and -27.90 kJ/mol, respectively. The △G of this reaction in our reaction conditions is about 32.95 kJ/mol. It can be seen that the Gibbs free energy of this reaction at ambient temperature is higher than 0, which indicated that this reaction does not occur spontaneously at 413K. However, we can see from the estimated result ௵ the △G (413 k, 6.8 MPa) is smaller than ߂‫ܩ‬ସଵଷ ௄ , which indicate that increase the pressure of the reaction is in favor of the formation of the DMC. We also found that ௵ ௵ △G (413 k, 6.8 MPa) is still larger than ߂‫ܩ‬ଶଽ଼ ௄ , this is due to the ߂‫ܪ‬ସଵଷ ௄ < 0, hence the higher temperature is not favor of the formation of the DMC. The stability is one of the most important concern of the heterogeneous catalyst. The reusability of the catalysts in the DMC synthesis was tested with six reaction cycles under the identical reaction as mentioned in the experimental section. The results are shown in Figure S7 in the support information. The activity from these catalysts decreased slightly after six cycles. However, the activity of Zr0.1Ce remained the highest among all samples. The activities of the catalysts were reduced by 21, 24, 22, 26, 19 and 68% after six cycle times corresponding to CeO2, Zr0.05Ce, Zr0.1Ce, Zr0.2Ce, Zr0.3Ce, ZrO2, respectively. 3.7. Characterization of the used catalyst To determine the deactivation mechanism, we compared the TEM, XRD, and XPS results of the fresh and used Zr0.1Ce nanorods. The TEM results and the XRD patterns of the fresh Zr0.1Ce and used Zr0.1Ce nanorods are shown in Figure S8 in the support information. The TEM images of the morphologies of Zr0.1Ce nanorods showed that the fresh and used Zr0.1Ce have the same morphologies after six cycles. However, the used Zr0.1Ce nanorods had a mean diameter of ～10 nm and an average length of ～46 nm, it appears that the used Zr0.1Ce nanorods become a little wider and longer than fresh Zr0.1Ce nanorods (～9 nm and ～40 nm). After six cycles, the


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used Zr0.1Ce catalysts still display the characteristic diﬀraction peaks corresponding to the cubic ﬂuorite structure, which is fully consistent with the TEM results.

Figure 7. XPS spectra of the fresh and used Zr0.1Ce catalyst. Figure 7(a), (b) compare the deconvoluted Ce 3d and O 1s XPS spectra of fresh and used Zr0.1Ce. The relative abundances of the Ce3+ and Ov of the samples, listed in Figure 7, showed that the content of Ce3+ and the surface oxygen vacancy decreased from 21.5% to 17.4%, and 10.4% to 7.1%, respectively. This decrease could be attributed to partial filling of surface oxygen vacancy with the O atom of adsorbed CO2.56 A number of DFT studies have indicated that the surface oxygen vacancy is thermodynamically unstable and highly reactive, experimental studies also showed that the surface oxygen vacancy can interact with O from adsorbed CO2.57-58 3.8. The effect of the surface vacancy concentrations on the CO2 adsorption and activity of catalysts

15

1.08

12

0.96 Zr0.1Ce Zr0.05Ce

0.84

9

Zr0.2Ce 0.72

Zr0.3Ce

0.60

CeO2

0.48

ZrO2 6

7 8 9 10 Surface oxygen vacancy concentration (%)

6

3

DMC yield (mmolDMC/gcat)

The adsorption of CO2 (mmolCO2/gcat)

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0 11

Figure 8. The relationship between catalyst activity, CO2 adsorption amount and the concentration of the surface oxygen vacancy


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The surface oxygen vacancy on the catalyst surface could serve as Lewis acid sites that promote CO2 adsorption.13, 33 Thus, we further investigated the relationship between the surface oxygen vacancy, CO2 adsorption capacity, and catalyst activity in in Figure 8. The adsorption capacity of CO2 of the catalysts increased with the surface oxygen vacancy concentrations, indicating that surface oxygen vacancies promote CO2 adsorption. Oxygen vacancies can be associated with Ce3+. XRD, Raman and XPS results in Figure 2, 3, and 4 showed that Zr-doping led to the formation of Ce3+ in the fluorite structure of CeO2. The Lewis acid nature of oxygen vacancy allowed it to interact with nonbonding electrons in the O atom of CO2 through Lewis acid-base interactions.13 A number of theoretical and experimental studies suggest that the surface oxygen vacancy allows inserting one of the O atoms of CO2 into the oxygen vacancy, forming a bent CO2- intermediate.59-60 Marco Fronzi et al.60 suggested that the higher energy needed to form an oxygen vacancy reflecting higher reactivity of a reduced nanocluster toward CO2. Our results showed ZrxCe nanorods with increasing surface oxygen vacancy concentration exhibited a propensity to increase CO2 adsorption. Figure 8 also showed that catalytic activity of ZrxCe catalysts increased with the surface oxygen vacancy concentration. Among these catalysts, the Zr0.1Ce nanorods which exhibited the highest activity among the investigated samples possessed the highest concentration of surface oxygen vacancy. Oxygen vacancies promote the direct DMC synthesis from CO2/methanol. Based on the previous reports,61-62 CO2 could be adsorbed and activated on the oxygen vacancy sites over the defect surface of metal oxide. In DFT studies, the C=O bond cleavage of CO2 is achieved by using one of the O atoms to fill the oxygen vacancy with a small barrier, which is easier than that on the metal surface.60 Wu et al.63 studied the CeO2 nanocrystals with different morphology (rod, cube, octahedral) and found CeO2 nanorods bind CO2 strongly because the surface of CeO2 nanorods are more defective. In CO2 methanation,15, 61 some authors proposed that the surface oxygen vacancy played an important role in the reduction of CO2 to CO and proved that the surface oxygen vacancy catalyzed the rate-determining step with a much lower activation temperature. According to the previous reports and the obtained results in this work, doped Zr into CeO2 lattice resulted in more oxygen vacancies on CeO2 surface serving as the active sites, which can promote the interaction between CO2 and ZrxCe nanorods.


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856

1018

1205 1288

Bidentate Bicarbonate

1686

a

1585

1612

3.9. In situ FTIR study

Bidentate Carbonate

Ionic Bicarbonate

Absorbance

CeO2

Zr0.1Ce

Zr0.3Ce

4000

3000

1600

1200

800

1106 1052

Wavenumber (cm-1)

1156

b Terminal

Bridged

Terminal

CeO2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Zr0.1Ce

Zr0.3Ce

4000

3000

1600

1200 -1

Wavenumber (cm )


800

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Figure 9. DRIFTS IR spectra of adsorbed species on CeO2 and ZrxCe nanorods: (a) adsorbed CO2 and (b) adsorbed methanol.

1106

Terminal

1052

Monomethyl carbonate

Absorbance

1193

1288

1345

1460

1595

a

Bridged

50 s 10 s 3s 0s

4000

3000

1600

1200

800

-1

Wavenumber (cm )

Bridged

1016

Monomethyl carbonate

1052

1193

1345

1460

1595

Terminal

1106

b

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 s

Triple-bonded

10 s 3s 0s 4000

3000

1600

1200

800

-1

Wavenumber (cm ) Figure 10. DRIFTS spectra of adsorbed species on Zr0.1Ce nanorods: (a) Interaction


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of CO2 with adsorbed methoxy and (b) adsorbed DMC. We have further employed in situ FTIR to characterize the structure of adsorbed species and the nature of the active sites. Figure 9a showed that CO2 adsorption on CeO2 produced carbonate and bicarbonate.64-65 Zr-doping decreased the IR intensity of bidentate carbonate. Bidentate carbonate and bidentate bicarbonate can be assigned to CO2 adsorbed on the surface oxygen vacancy sites. CO2 may adsorb on the surface oxygen vacancy site through inserting one O atoms in the vacancy site on the surface of the catalysts. The C atoms bind the O atom of the catalyst surface to form the carbonate. A number DFT studies have demonstrated that the surface oxygen vacancy assists CO2 activation and adsorption.57, 59, 66 This proposition is further supported by our XPS and CO2-TPD results. Figure 9b showed that the adsorption of methanol on CeO2 produced terminal methoxy (1106 cm-1) and bridged methoxy (1052 cm-1) on Ce4+. Zr-doping produced a methoxy (1156 cm-1) on Zr4+. The formation of these positive methoxy bands is accompanied with an increase in the H2O band in the 3300 to 3400 cm-1 region and the negative OH band at 3674 cm-1. This observation has also been reported by former studies on the interaction of alcohol with the surface hydroxyl groups of metal oxides.67-68 This interaction allowed the transfer of the H atom from methanol’s OH to a coordinately unsaturated O2- center on the catalyst surface, producing bridging OH group. The H in methanol’s OH could also react the surface OH group to produced surface water (Figure S9).69 Accordingly, the whole process can be summarized in Scheme S1. Because of its high activity, we have further studied the interaction of CO2 with adsorbed methoxy species on Zr0.1Ce, shown in Figure 10a. Exposure of adsorbed methoxy species to CO2 resulted in a rapid decrease in terminal methoxy at 1106 cm-1 and the formation of monomethyl carbonate. The IR spectra of monomethyl carbonate were further verified by the adsorption of DMC on the catalysts in Figure 10b. Monomethyl carbonate has been considered to be as the reaction intermediate of direct synthesis DMC from CO2 and methanol. Adsorption of DMC on Zr0.1Ce produced not only monomethyl carbonate species but also adsorbed methoxy species. Adsorbed DMC was not detected because low pressure condition used favor its conversion to monomethyl carbonate.69

3.10. A proposed mechanism for DMC formation over the surfaces of ZrxCe nanorods Scheme 2 illustrates the proposed mechanism for the DMC synthesis which begins with the activation of adsorbed CO2 by an oxygen vacancy site through a Lewis acid-base interaction. It should be noted that the linear OH in the vicinity of oxygen vacancy site is a hypothetic species which will be further investigated by in situ IR study. DFT studies have suggested that the oxygen vacancy site could locate between Ce and Zr atoms.70 Zr0.3Ce produced high intensity of terminal methoxy, as shown in Figure 9(b) for the insertion into adsorbed methanol. The abundance of terminal methoxy could attribute to the high specific activity (i.e., catalyst activity on


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the basis of oxygen vacancy sites) of Zr0.3Ce when compared with that of Zr0.1Ce. Thus, the DMC activity is governed by abundance of adsorbed active species and surface oxygen vacancy sites. The surface oxygen vacancy promoted CO2 adsorption and activation of a number of CO2-related reactions.15, 59, 71 The XPS and CO2-TPD results show that doping Zr into the CeO2 nanorods promotes the generation of the surface oxygen vacancy. The oxygen vacancy sites allow CO2 to adsorb as bidentate carbonate as evidenced by the presence of IR bands at 1018 and 1288 cm-1 (Figure 9a). At a neighboring acidic site, methanol may adsorb as methyl and methoxy species, respectively. A number of studies have suggested that the existence of acid-base pair sites play a key role in the direct synthesis of DMC.3, 69, 72 Activated CO2 could react with the terminal CH3O-M/CH3-M bond in metal oxides to produce methyl carbonate species, as evidenced by a decrease in terminal CH3O-M at 1106 cm-1 and an increase in methyl carbonate in Figure 10 (a). Subsequent reaction of methyl carbonate with CH3 from adsorbed methoxy should lead to DMC with regeneration of oxygen vacancy. According to the references,12, 72 elementary mechanistic steps for the formation of DMC from CO2 and methanol were proposed by the Langmuir-Hinshelwood (LH) and the Eley-Rideal (ER) mechanism. Some studies revealed that the CO2 and methanol adsorbed on the surface of the CeO2 nanorods in two separate steps, which are consistent with the Langmuir-Hinshelwood mechanism, meanwhile, the adsorption and the activation of CO2 is the rate-determining step.53, 73 To determine the dynamic behavior of CO2 in our reaction need further work in future, such as the study of the initial formation rate of DMC with different pressure of CO2 and the effect of the amount of the methanol on the DMC yield. In the present work, based on the XPS, CO2-TPD and FTIR results, it can be found that CO2 and methanol interact with the surface oxygen vacancy and the surface hydroxyl, respectively, this is consistent with the LH mechanism. Moreover, there existed a linear relationship between the DMC yield of the catalysts and the adsorption capacity of the CO2. Hence, we considered that the steps of the adsorption and activation of the CO2 may be as rate-determining steps of the direct synthesis of the DMC from CO2 and methanol. Alternative proposed mechanisms for DMC formation also involved the surface oxygen vacancy over the Ce0.1Ti0.9O2 and H3PW12O40/Ce0.1Ti0.9O2 catalysts.32-33 The surface oxygen vacancies could act as Lewis acid sites to interact with the O atom of CO2. Methanol could adsorb at adjacent oxygen vacancy to produce the intermediate which further reaction adsorbed CO2 to from DMC and then regenerate oxygen vacancy It is important to note that methyl carbonate which has been produced from methanol and CO2 can be also produced from DMC, as shown in Figure 10b. This observation further confirms the micro-reversibility of the steps involved in the proposed mechanism in Scheme 2.


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Scheme 2. The proposed mechanism of DMC synthesis on the ZrxCe nanorods.

4. CONCLUSION In summary, a series of Zr-doped CeO2 nanorods were synthesized via a hydro-thermal method and the effect of the doping content of Zr on the lattice structure, microstructure, especially the amount of oxygen vacancy as well as the catalytic activity of DMC synthesis from CO2 and methanol were studied in detail. Zr0.1Ce nanorods which gave the highest amount of oxygen vacancies exhibited the highest activity for DMC synthesis. The DMC synthesis activities were found to be correlated with the concentration of the surface oxygen vacancy. FTIR results suggest that bidentate carbonate and terminal methoxy species are actively involved in DMC synthesis. The results of this study provide an insight into the effect of Zr-doping on DMC synthesis on CeO2 nanorods catalysts and provide a technical basis for devising a new strategy to design high-efficiency CeO2-based catalysts.

■ ASSOCIATED CONTENT Supporting Information Supporting figures and tables related to the additional UV-Vis spectra, XPS, NH3-TPD, TEM, XRD data. The authors declare no competing financial interest

■ AUTHOR INFORMATION Corresponding Authors ﹡E-mail: [email protected]


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﹡E-mail: [email protected] ORCID Li Zhong: 0000-0001-6087-6854 Li Congming: 0000-0002-0707-8826 ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (U1510203).

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Oxygen Vacancy Promoting Dimethyl Carbonate Synthesis from CO2

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