CO2-assisted oxidative dehydrogenation of ethane to ethylene over the ZnO-ZrO2 catalyst

LIAO Duo-hua ，YANG Liang ，SONG Geng-zhe ，MA Xue-dong ，LI Shuang

（College of Chemical Engineering, Northwest University, Xi’an 710069, China）

Abstract: The ZnO-ZrO2 catalyst was prepared by the deposition-precipitation method using ZrO2 as the carrier obtained from calcining commercial zirconium hydroxide (Zr(OH)4). And the catalytic performance was evaluated at 873 K in CO2-assisted ethane oxidative dehydrogenation reaction (CO2-ODHE). The physical-chemical properties and morphology were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectra, High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectra (XPS), CO2 temperature-programmed desorption (CO2-TPD).The results show that ZnO were doped into the surface lattice of ZrO2 on the 5%ZnO-ZrO2 catalyst, generating highly dispersed ZnO species and oxygen-deficient regions on catalyst surface. 5%ZnO-ZrO2 catalyst could selectively breaking C-H bond instead of C-C bond, delivering excellent catalytic performance. 210 μmol/(gcat·min) of C2H4 formation rate could compare favorably with the data reported on noble metal and transition metal carbides. Additionally, the possible mechanism is discussed.

Key words: ethylene；ethane oxidative dehydrogenation；CO2-assisted；ZnO-ZrO2 catalyst

Ethylene, as an important organic raw material, is an important precursor for the preparation of highvalue-added fine chemical products such as polyethylene, polyvinyl chloride, and ethylenepropylene rubber. Ethylene production is one of the important symbols to measure the development level of a country's petrochemical industry[1,2]. By 2022, global ethylene production has been 202 million tons, which will increase to 263 million tons by 2030, with an estimated average annual growth rate of 4%[3]. At present, most of the industrial production of ethylene comes from the high-temperature steam cracking process of ethane, which is an energy-intensive process. Its shortcomings, such as high energy consumption, carbon deposition, and frequent equipment shutdown, hinder the continuous and efficient production of ethylene[4]. Compared with hightemperature steam cracking of ethane, the ethane oxidative dehydrogenation to ethylene (ODHE) has been proposed as one of the most potential alternative processes to produce ethylene. Because ODHE reaction typically was operated at relatively low temperature and could dramatically reduce carbon deposition[5,6].

However, it is still a challenge to prevent the overoxidation of desired product ethylene to carbon oxides(COx) in ODHE reaction[7]. Therefore, oxidative dehydrogenation of ethane using greenhouse gas CO2(CO2-ODHE) as a mild oxidant is a more atomeconomical route to produce ethylene[8]. With increasing emissions of CO2in recent years, converting CO2via pollutant-to-fuel processes is industrially attractive[9-11]. The CO2-ODHE reaction is a complex system in which a variety of reactions take place simultaneously, including ethane oxidative dehydrogenation reaction (equation (1)), direct dehydrogenation of ethane (equation (2)), reverse water gas shift (RWGS) reaction (equation (3)), reverse-Boundouard reaction (equation (4)), hydrogenolysis reaction of ethane (equation (5)), and dry reforming reaction of ethane (equation (6))[12,13]. CO2can effectively remove the carbon deposition on the catalyst surface through the reverse Boundouard reaction, promoting the stability of catalyst. In addition,RWGS reaction will drive the main reaction and reduce other side reactions[14].

Therefore, the CO2-ODHE reaction takes the advantages of low carbon deposition rate, high ethylene selectivity, and reasonable utilization of CO2into high value-added chemicals[15]. However, the complexity of CO2-ODHE reaction as discussed above limits the ethylene yield[16]. Rational design of catalysts is the key to break through this difficulty, especially the selective tailoring of C-C and C-H bonds.

Heterogeneous catalysts using noble metals were frequently reported due to their extraordinary catalytic activity in breaking C-H bond. The Pt-Ce bimetallic catalyst was found to catalyze the CO2-ODHE reaction with a high ethane conversion (40%) and ethylene selectivity (80%) at 700 °C and a space velocity of 15000 h-1[17]. However, the high cost of raw materials suppresses the extensive application and industrial development of noble metal catalysts. Transition metal oxides have been widely used as active phases (Zn, Cr,Ni, etc.) or supports (TiZrOx, Al2O3, MCM-41, etc.) in the oxidative dehydrogenation of light alkanes[13,18].Among them, ZnO-based catalysts showed the ability to break C-H bonds of alkanes. The low-coordination Zn2+dispersed on the Zn/TiZrOxcatalyst was proposed to be the active site in propane oxidative dehydrogenation with CO2[19]. Liu et al.[20]found that the highly dispersed ZnO species on Zn/NaS50 catalyst resulted in 64% ethylene selectivity and 69% ethane conversion at 650 °C and a space velocity of 1800 h-1.And the coupling mechanism was proposed on Zn/NaS50 catalyst, and the introduction of CO2can effectively accelerate the desorption of by-product H,promoting the reaction equilibrium to positive direction. Due to the low cost and redox properties as well as thermal/chemical stability of ZrO2, surfacemodified ZrO2using Cr, Fe, Y, etc. were reported in oxidative dehydrogenation reactions. For example,Bugrova et al.[21]found that the highly dispersed Cr6+species on Cr/ZrO2surface were the active sites for CO2-ODHE reaction. And the typical MvK mechanism are proposed, the basic sites (e.g., Oxygen vacancies)on ZrO2improve the activation of CO2to supply lattice oxygen, ensuring the highly effective redox cycle of Cr6+/Cr3+. Interestingly, Li et al.[22]found that on the NiFe/ZrO2catalyst, the coupling mechanism is favored,and abundant oxygen vacancies on monoclinic ZrO2increase the reactivity of RWGS reaction, giving an enhanced ethylene yield. At 600 °C and a space velocity of 9000 h-1, 11% ethane conversion and 70% ethylene selectivity were obtained. Therefore, the mechanism of CO2-ODHE reaction varied on different catalysts, and the controversy still exists.

In this paper, ZnO-ZrO2catalyst was prepared by the deposition-precipitation method using commercial zirconium hydroxide as the carrier. And the catalytic performance was evaluated at 873 K in CO2-ODHE reaction. The morphology and physical-chemical properties were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM), Raman spectra, transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), and CO2temperature-programmed desorption (CO2-TPD). Also,the high-performance active sites and possible reaction mechanism were discussed.

1 Experimental

1.1 Experimental raw materials and reagents

Zinc acetate (C4H6O4Zn·2H2O, Kermel, AR,99.9%) was used as the precursors of ZnO and ZnOZrO2, respectively. Commercial zirconium hydroxide(Zr(OH)4, Kermel, AR, 99.9%) was employed as the carrier of catalyst. Also, ammonium hydroxide solution(NH3·H2O, Damao, AR, 99.9%) was served as a precipitating agent. All of the materials were used as received without any further purification.

1.2 Catalyst preparation

ZrO2was obtained by the calcined commercial zirconium hydroxide at 600 °C for 5 h. ZnO-ZrO2catalyst was prepared by the deposition-precipitation method. Firstly, 0.1127 g zinc acetate hydrate was dissolved in 50.0 mL deionized water and stirred until a clear solution was formed. Subsequently, 1.0 g commercial zirconium hydroxide was added to zinccontaining solution at 65 °C, and vigorous stirring continued for 20 min to form a homogeneous suspension. And then, diluted ammonia water was gradually added dropwise and stirred vigorously until the pH value reached 9.0 ± 0.1. After the complete precipitation, the product was filtered and washed with deionized water of 300 mL, dried at 100 °C for 6 h, and then calcined at 600 °C for 5 h with a heating rate of 5 °C/min. The obtained sample was named as 5%ZnO-ZrO2. The preparation process of pure ZnO was the same as the above, but the commercial zirconium hydroxide was not added in the preparation process.

1.3 Catalyst characterization

The phase composition of the series catalyst was investigated by X-ray diffraction (XRD). The XRD patterns were recorded on a Smart LAB SE diffractometer using a Ni-filtered CuKα radiation source (λ= 0.15418 nm) with a scanning performed from 10° to 80° at a rate of 10(°)/min. Scanning electron microscopy (SEM) images and elemental mapping were taken by an SU8220 microscope. Raman spectra were acquired from a LabRAM HR Horiba Scientific Raman spectrometer with a 532 nm-1laser.High-resolution transmission electron microscopy(HRTEM) images were obtained by a FEI TF20 microscope operated at 200 kV. X-ray photoelectron spectra (XPS) analyses were conducted on a Thermo Scientific K-Alpha with AlKα (hν= 1486.6 eV). The C 1speak was fixed at 284.8 eV as a reference. CO2desorption profiles (CO2-TPD) of all samples were obtained using a gas-phase flow reactor with an outlet flowing to a TCD. One certain sample (100.0 mg) was loaded into a quartz tube reactor and plugged at both ends with appropriate quartz wool. In the pretreated stage, the sample was heated to 873 K at a heating rate of 10 K/min and held for 1 h in a He flow of 50 mL/min.And then, the reactor was cooled to 323 K. The pretreated sample was exposed to the flowing of 10%CO2/He (50 mL/min) and held for 90 min to adsorb CO2. After the probe gas flowing, 100 mL/min He flow was passed over the catalyst surface for 120 min to remove the physical adsorption of CO2. Subsequently,the sample was heated to 923 K at a temperature ramp rate of 15 K/min and the effluent gas was monitored using the TCD.

1.4 Catalyst evaluation

For each catalyst, 300 mg sample was loaded in a quartz tube and activated at 873 K under a N2flow(30 mL/min) for 1 h in fixed bed reactor. After the removal of adsorbed surface species, the reactants(C2H6/CO2/N2= 1∶1∶2, total flow = 60 mL/min,GHSV = 12000 mL/(gcat·h)) were introduced into the reactor. All the feed components and products were analyzed using an online GC (2060) equipped with TCD.

C2H6conversion (%), C2H4selectivity (%), C2H4yield (%), C2H4formation rate (STY, μmol/(gcat·min)),and CO formation rate (STY, μmol/(gcat·min)) were defined based on the carbon balance:

whereXwith ‘inlet’ and ‘outlet’ stand for the molar flow of gas component (mol/min) at the inlet or outlet,respectively.Mcatalystrepresents the weight (g) of used catalyst in catalytic evaluation process.

1.5 EDH experiment

The experiment of ethane dehydrogenation without CO2(EDH) was also conduct. For each catalyst, 300 mg sample was loaded in a quartz tube and activated at 873 K under a N2flow (30 mL/min) for 1 h in fixed bed reactor. After the removal of adsorbed surface species, the reactants (C2H6/N2= 1∶3, total flow = 60 mL/min, GHSV = 12000 mL/(gcat·h)) were introduced into the reactor. All the feed components and products were analyzed using an online GC (2060)equipped with TCD. The reaction data was acquired using the same method as mentioned above.

2 Results and discussion

2.1 Catalyst characterization

2.1.1 XRD analysis

In order to understand the phase composition of the catalysts, the XRD patterns of ZrO2, ZnO, and 5%ZnO-ZrO2catalyst are shown in Figure 1(a). The strong peaks at 30.4°, 35.2°, 50.2°, 59.8° of tetragonal phase (i.e.,T-ZrO2) were observed on bare ZrO2and 5%ZnO-ZrO2catalysts[23]. As for pure ZnO, the sharp peaks at 32°, 34°, 37°, 47°, 56°, 62.9°, 67.9° were assigned to the hexagonal wurtzite structure of ZnO(i.e.,h-ZnO)[24]. With the addition of ZnO, the diffraction peak intensity ofT-ZrO2on 5%ZnO-ZrO2catalyst declined, indicating a smaller grain size ofTZrO2[25]. Interestingly, no diffraction peak ofh-ZnO was detected on 5%ZnO-ZrO2catalyst, suggesting that the ZnO may be highly dispersed on the catalyst, or doped into the lattice of ZrO2. Figure 1(b) exhibits the partially enlarged XRD patterns of ZrO2and 5%ZnOZrO2catalysts. It is worth noting that the diffraction peak corresponded to (101) lattice plane at 2θ= 30.4°ofT-ZrO2shifted towards the higher angle after the addition of ZnO, illustrating that the Zn cations may be doped into the lattice ofT-ZrO2on 5%ZnO-ZrO2catalyst[26].

Figure 1 (a) XRD patterns of ZrO2, ZnO, and 5%ZnO-ZrO2 catalysts; (b) partially enlarged XRD patterns of ZrO2 and 5%ZnO-ZrO2 catalysts

2.1.2 Raman spectra analysis

To further understand the surface composition and structure, Raman spectra of ZrO2, ZnO, and 5%ZnOZrO2catalysts are illustrated in Figure 2. The Raman bands at 149, 269, 312, 465, and 643 cm-1on ZrO2and 5%ZnO-ZrO2were attributed to the tetragonal phase of ZrO2[27], which is consistent with XRD analysis. The strong band at 331, 381, 432 cm-1on pure ZnO was assigned toh-ZnO[28]. With the addition of ZnO, the bands at ～312 cm-1ofT-ZrO2shifted to high wavenumber, and this phenomenon could be explained by the shorted Zr-O bonds in ZrO2[29], further indicating that the Zn cations were doped into the lattice ofTZrO2on 5%ZnO-ZrO2. Additionally, the broad bands in the region of 550-620 cm-1related to oxygen vacancy were observed on ZrO2and 5%ZnO-ZrO2[30], and the higher intensity on 5%ZnO-ZrO2indicates higher oxygen-deficient region compared to bare ZrO2.

Figure 2 Raman spectra of ZnO, ZrO2 and 5%ZnO-ZrO2 catalysts

2.1.3 SEM and HRTEM analysis

Figure 3(a) and Figure 3(b) show the SEM images of ZrO2and 5%ZnO-ZrO2catalysts, respectively. The SEM images clearly demonstrate that both of them were nanoparticle aggregates, in which ZrO2showed larger nanoparticle aggregates (100-400 nm), while 5%ZnO-ZrO2exhibited uniform and smaller nanoparticle aggregates (50-100 nm). It confirms that the addition of ZnO leads to the smaller grain size of 5%ZnO-ZrO2, which is consistent with XRD analysis.In order to further study the distribution of ZnO species on the 5%ZnO-ZrO2catalyst, the elemental mappings of catalyst were conducted, as shown in Figure 3(c) and Figure 3(d). The Zn element on the surface of catalyst were highly dispersed, which further indicates that the ZnO was incorporated into the lattice of ZrO2on the 5%ZnO-ZrO2catalyst.

Figure 3 SEM images of ZrO2 (a) and 5%ZnO-ZrO2 (b); Elemental mapping of 5%ZnO-ZrO2 ((c), (d))

The HRTEM images of ZrO2and 5%ZnO-ZrO2are displayed in Figure 4. The aggregations of round nanoparticles are observed on both samples. The particle size of 5%ZnO-ZrO2centered at 14 nm, while that of bare ZrO2centered at 22 nm. This is consistent with the XRD and SEM results. In addition, the highresolution photograph of 5%ZnO-ZrO2shows the (101)crystal face ofT-ZrO2, while no crystal face belonging to ZnO are observed. More importantly, in the lattice fringe of (101) crystal plane, the loss of clarity and parallelism (blue circle in Figure 4(d)) are observed on 5%ZnO-ZrO2catalyst, further illustrating the presence of Zn cations and oxygen-deficient region on the surface of 5%ZnO-ZrO2catalyst[31].

Figure 4 HRTEM images of ZrO2 ((a), (b)) and 5%ZnO-ZrO2 ((c), (d))

2.1.4 XPS analysis

The surface chemical states of ZrO2, ZnO and 5%ZnO-ZrO2catalysts were determined by XPS.Figure 5 shows the Zr 3d, Zn 2pand O 1sXPS spectra of all the samples, and the binding energies of Zr 3d5/2,Zr 3d3/2, O 1s, Zn 2p1/2and Zn 2p3/2are shown in Table 1. As shown in Figure 5(a), the binding energies of Zr 3d5/2and Zr 3d3/2of pure ZrO2are 182.40 and 184.80 eV with a separation of 2.4 eV.This indicates that the Zr cation has a valence of+ 4[32]. With the addition of ZnO, the Zr 3d5/2peak of the 5%ZnO-ZrO2catalyst shifted to a lower binding energy, which may be attributed to the formation of oxygen-deficient regions on the catalyst surface,distributing its partial charge to adjacent Zr sites[19,33].Figure 5(b) shows the XPS spectrum of Zn 2p, which renders the Zn 2p3/2and Zn 2p1/2spectra of pure ZnO at 1021.26 and 1044.37 eV, respectively. The spinorbit splitting of the Zn 2ppeak is 23.11 eV for this catalyst, which is characteristic of ZnO[34]. Thus, the present state of Zn species should be Zn2+. However,on the 5%ZnO-ZrO2catalyst, the Zn 2p3/2peak shifted to the direction of high binding energy, which proves the presence of strong interaction between ZnO and ZrO2[35]. It may be attributed to that the ZnO was doped into the lattice ofT-ZrO2on 5%ZnO-ZrO2catalyst. This further explains the high dispersion of ZnO species on 5%ZnO-ZrO2catalyst. The strong interaction hinders the migration of ZnO on the surface of catalyst and then promotes its high dispersion state. Notably, the addition of ZnO on ZrO2may influence the distribution of surface oxygen species. As shown in Figure 5(c), two peaks of the O 1sspectrum in 530.1-530.2 and 531.9-532.0 eV could be ascribed to the lattice oxygen (OI) and the oxygen-deficient region (OII) caused by oxygen defects on catalyst surface[36]. The concentration of oxygen-deficient regions on the catalyst surface was estimated by the ratio of peak area, as shown in Table 1. With the introduction of ZnO, the related content of surface oxygen-deficient regions increased from 20% to 30%, which clearly demonstrates that the introduction of ZnO to the ZrO2surface enhanced oxygen-deficient regions.

Table 1 Binding energy of Zr 3d, Zn 2p and O 1s spectra and surface oxygen-deficient region (OII) concentration on ZrO2, ZnO and 5%ZnO-ZrO2 catalysts

Figure 5 XPS spectra of the ZrO2, ZnO, and 5%ZnO-ZrO2 catalysts: Zr 3d (a); Zn 2p (b); O 1s (c)

2.1.5 CO2-TPD analysis

The effect of ZnO doping on the surface basic sites and CO2adsorption capacity on ZrO2and 5%ZnOZrO2catalysts were investigated by CO2-TPD profiles,which can be seen in Figure 6. ZrO2and 5%ZnO-ZrO2catalysts show similar CO2desorption trends. The lowtemperature desorption peak at 70-240 °C was attributed to the adsorption of CO2on the weak basic sites such as surface hydroxyl groups on ZrO2[33]. The amount of CO2desorption decreased with the doping of ZnO. The strong interaction between ZnO and ZrO2on 5%ZnO-ZrO2led to the formation of unstable surface hydroxyl groups which were easily removed[37]. The high-temperature desorption peak at 395-600 °C was due to the adsorption of CO2on strong basic sites such as oxygen-deficient regions and lattice oxygen over catalyst[38,39]. With the incorporation of ZnO, the concentration of oxygen-deficient regions dramatically increased on 5%ZnO-ZrO2, which is consistent with XPS results. Therefore, it’s proposed that the addition of ZnO is an alternative way to improve the surface nature of ZrO2, especially the oxygen-deficient regions.In particular, the oxygen-deficient regions on the catalyst have been considered as the main active sites for the adsorption and activation of CO2, and increasing the content of oxygen-deficient regions can significantly improve the CO2adsorption capacity of the catalyst system[40]. Additionally, the oxygendeficient regions in metal oxides can effectively promote the activation and conversion of CO2in RWGS reactions[41]. These imply that the 5%ZnO-ZrO2catalyst proposes a powerful ability to adsorb and activate CO2, which ultimately promotes the activity of RWGS reaction.

Figure 6 CO2-TPD profiles of ZrO2 and 5%ZnO-ZrO2 catalysts

2.2 Catalytic performance evaluation

Figure 7(a) demonstrates the ethane conversion for the CO2-ODHE reaction over ZrO2and 5%ZnOZrO2catalysts. It can be seen that the ZrO2proposes only 3.2% ethane conversion under the reaction condition of 873 K, indicating that pure ZrO2possesses almost no ethane dehydrogenation activity. After the addition of ZnO, the ethane conversion over 5%ZnOZrO2can reach 11.2%, approximately 5 times higher than that of bare ZrO2. It illustrates that the highly dispersed ZnO species could be identified as the main active sites for the oxidative dehydrogenation of ethane. In addition, the excellent ethane conversion stability can be observed over 5%ZnO-ZrO2catalyst for the time-on-stream reaction experiment of 20 h, which may be related to that the highly dispersed active sites were more inclined to adsorb and activate the C-H bond. Because the electrically heterogeneous highlydispersed metal oxide might be difficult to adsorb homogeneous C-C bonds of ethane[42]. This effectively inhibits the formation of carbon precursors and promotes the efficient selective production of ethylene.Figure 7(b) shows the product yield, selectivity, and CO2conversion over the ZrO2and 5%ZnO-ZrO2catalysts. As expected, 5%ZnO-ZrO2achieves an excellent ethylene selectivity of 84%, slightly lower than that of ZrO2, and the main by-product is methane.This may be attributed to the significant increase in ethane conversion, and the excessive production of byproduct H would result in the hydrogenolysis reaction of ethane. However, it is worth noting that 5%ZnOZrO2still obtains an ethylene yield of 9.4%,approximately 3 times higher than that of bare ZrO2.This indicates that highly dispersed ZnO species can not only efficiently activate and convert ethane, but also exhibit the ability to selectively cleave C-H bond.This may be related to the ability of the catalyst to adsorb and activate CO2. Consistent with the results in Raman, XPS and CO2-TPD analysis, the CO2conversion (8%) of 5%ZnO-ZrO2is higher than that of pure ZrO2, indicating the enhanced oxygen-deficient regions on 5%ZnO-ZrO2improve its ability to adsorb and activate CO2.

Figure 7 (a) Ethane conversion of ZrO2 and 5%ZnO-ZrO2 catalysts; (b) Products yield, selectivity and CO2 conversion of ZrO2 and 5%ZnO-ZrO2 catalystsReaction conditions: for all catalyst, test in a flow of C2H6/CO2/N2 = 1∶1∶2, the total flow of 60 mL/min, reaction temperature of 600 °C and atmospheric pressure, GHSV = 12000 mL/(gcat·h)

In order to investigate the reaction mechanism of CO2-ODHE reaction on 5%ZnO-ZrO2catalyst, the catalytic performance of 5%ZnO-ZrO2catalyst in CO2-ODHE and EDH reaction was evaluated. As shown in Table 2, the formation rate of C2H4on 5%ZnO-ZrO2was higher than that of CO in CO2-ODHE reaction,which suggested that the equilibrium of direction dehydrogenation reaction in the presence of CO2moved positively due to RWGS reaction, leading to an increased ethylene formation rate[21]. Additionally, the C2H6conversion and C2H4yield of EDH reaction were much lower than those of CO2-ODHE reaction over the 5%ZnO-ZrO2, further confirming that CO2enhanced the C2H4formation rate through RWGS reaction. Also,the coupling reaction mechanisms have been proposed on ZnO-based catalyst[20,43,44]. Therefore, we can conclude that the reaction coupling mechanism is involved on the ZnO-ZrO2catalyst. Firstly, ethane was activated on the surface of ZnO-ZrO2catalyst, giving C2H4and by-product H; secondly, CO2was adsorbed and activated on the oxygen-deficient regions, and then reacted with by-product H through RWGS reaction,making the whole reaction move forward, as shown in Figure 8.

Table 2 Details of catalytic performance over the ZrO2 and 5%ZnO-ZrO2 catalysts in CO2-ODHE and EDH reaction

Figure 8 Reaction mechanism of CO2-ODHE reaction on ZnO-ZrO2 catalyst

To further confirm the excellent catalytic performance of the 5%ZnO-ZrO2catalyst, its catalytic performance was compared with that of the reported catalysts, as shown in Table 3. However, it is not reasonable to directly compare ethane conversion and ethylene selectivity due to the different reaction conditions used by the researchers. Therefore, we compared the catalysts reported by different works using the rate of ethylene formation (STY) as a benchmark. The 5%ZnO-ZrO2catalyst illustrates a higher ethylene formation rate than the previously reported NiFe/CeO2, Cr/Ce-MCM-41, Zn2.92/NaS50,etc., and is even close to the current cutting-edge 1%Fe/Mo2C catalyst. This result clearly shows that the 5%ZnO-ZrO2catalyst proposed in this work has excellent catalytic performance in CO2-ODHE reaction.

Table 3 Catalytic performance of various catalysts evaluated in CO2-ODHE reaction

3 Conclusions

In this paper, 5%ZnO-ZrO2catalyst was prepared and tested in CO2-ODHE reaction, the main conclusion are as follows:

The prepared 5%ZnO-ZrO2delivered 84% C2H4selectivity and 11% C2H6conversion at 873 K and a space velocity of 12000 h-1. 210 μmol/(gcat·min) of C2H4formation rate was observed, reaching the data obtained on noble metal or carbides.

The doping of ZnO to the surface of ZrO2is main reason in achieving excellent catalytic performance.Highly dispersed ZnO species and oxygen-deficient regions were found on the 5%ZnO-ZrO2catalyst surface, the former could selectively breaking C-H bond instead of the C-C bond, and the latter could enhance the adsorption and activation of CO2.