Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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How CO2 Chemisorption States Affect Hydrogenation Activity Bin Yang,† Lijun Wang,‡ Zile Hua,§ and Limin Guo*,† †
School of Environmental Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China School of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing 312000, China § State Key Laboratory of High Performance Ceramic and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ‡
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S Supporting Information *
ABSTRACT: Carbon dioxide (CO2) activation and then hydrogenation is an alternative route to achieve the efficient control of CO2 emission. For activation, the neutral CO2 molecule is first chemisorbed as carboxylate (CO2δ−) or carbonate (CO32−) on catalysts, which is an important step. Up to now, the relationship elucidation between CO2 chemisorption states and hydrogenation activity still lacks of direct experimental evidence. Herein, the model Zn−Cr mixed oxide catalysts were created to fulfill the goal. The chemisorption states and hydrogenation mechanism were carefully identified by in situ DRIFTS experiments. Furthermore, additional characterizations demonstrated that CO2 chemisorption states depended on surface oxygen concentration and species. Combined with the results of CO2 hydrogenation experiments, the conclusion that chemisorption state as carboxylate was beneficial for higher hydrogenation activity was concluded.
C
ZnO.1,7,12 Hence the connection between CO2 chemisorption states and hydrogenation activity still lacks direct experimental evidence. Herein, we synthesized and characterized Zn−Cr mixed-oxide as model catalyst to fulfill the goal. According to above introduction, CO2 σ-bond activation by chemisorption on metal ions and π-bond activation by chemisorption on oxygen ions are associated with the oxygen on the uppermost layer of oxide-support catalysts.9 The key for this performance is whether the surface oxygen participate in CO2 activation or not. This assumption that oxide-support interface tuning to affect CO2 activation therefore can be realized by surface oxygen concentration variation through calcination treatments at reductive or oxidative atmosphere. The model catalysts presented in this study is hinted by ZnO− Cr2O3 mixed-oxide which has been widely employed in the past for methanol synthesis from syngas (CO2, CO, and H2) at high temperature and high pressure.13 By calcination of Zn−Cr layered double hydroxides (LDH) precursor at oxidative or reductive atmosphere, the oxygen-rich or -deficiency Zn−Cr mixed-oxide catalysts were prepared. Further characterizations indicated this oxide-support interface tuning affected not only the chemisorption of CO2, but also the hydrogenation activity of CO2 dramatically. This work provides direct experimental evidence for the relationship of CO2 activation and catalytic hydrogenation activity in oxide-support catalysts.
arbon dioxide (CO2) is known as a crucial greenhouse gas that has a severe impact on climate change and global temperature rise.1−3 The reduction of CO2 emission by storage and conversion has attracted much attention for decades.4 The bond length and dissociation energy of CO2 are 116.3 pm and 1072 kJ mol−1, respectively, which suggests the high stability of CO2 molecules.5 Thus, it is crucial to activate CO2 first by an electron transfer with associated energy of about 0.6 eV,56 which can be realized by the interaction between heterogeneous surface and CO2. There are two chemisorption species of CO2, which relate to the formation of carbonate-like species. One is carbonate (CO32−) which describes a charge transfer from surface oxygen to the approaching CO2 and a molecule bending to form a [O−CO2]2− complexes.1,7,8 The other one is carboxylate (CO2δ−) that is similar to [M−CO2δ−] complex but the adsorption sites differ substantially from metal ions to oxygen ions, which is ascribed to the polar surface terminated by metal ions other than oxygen ions.1,6,9 These CO 2 chemisorption species indicate two different activated states: σ-bond activation and π-bond activation, and the appropriate adsorption polar sites are important.10 The oxide-support interaction can be further described as metal−metal or metal− oxygen−metal interaction in mixed-metal oxides.11 It should be noted that surface heterogeneity of metal oxides is hard to accurately describe due to a wide range of active sites and morphological features, such as steps, edges, polar facet, and so on. Recently works have revealed CeOx/Cu which activated CO2 as CO2δ‑ showed far faster CH3OH production rate than Cu/ZnO, on which CO2 was chemisorbed as CO32−.1 But the different hydrogenation pathways and support metals cannot be discarded in the comparison between CeOx/Cu and Cu/ © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
March 8, 2019 May 22, 2019 May 27, 2019 May 27, 2019 DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Structural characterizations of ZnCrOx-H2 and ZnCrOx-O2. (a) X-ray diffraction, (b) Raman spectra, (c) and (d) High resolution transmission electron microscope.
further understanding of the overall morphology and crystalline state. ZnCrOx-O2 nanoparticles ensemble (SI Figure S2-b4) showed a polycrystalline-type diffraction pattern corresponding to the ZnO and ZnCr2O4 phase. And the discrete spots indicated the existence of ZnCr2O4 with well crystallinity and large crystal size, and the diffraction rings can be ascribed to the many ZnO tiny crystallites. However, these diffraction spots were indistinct in ZnCrOx-H2 (SI Figure S2a4) and replaced by several diffraction rings, which corresponded to the ZnO diffraction and was consistent with the results of XRD and Raman results (Figure 1a and b). It was noted that spinel structure formation could be chromate (ZnCrO4) decomposition at high temperatures and ZnCrO4 was generated by oxidation of Cr(III) in the presence of ZnO and O2.14,19,20 Hence, oxidative atmosphere during calcination was one of the necessary factors in the formation of chromate and Zn−Cr spinel. The calcined atmosphere changed from oxidative to reductive atmosphere may affect the spinel formation. As XPS results of Cr 2p (Figure 2a) showed, such valence state variation was accompanied by the obvious appearance of Cr(VI) located at 579.7 eV in ZnCrOxO2.13 The binding energy of Cr(III) in ZnCrOx-H2(577.2 eV) was also slightly higher than ZnCrOx-O2(576.4 eV), which may also imply the different electron excited ability.14 Meanwhile, the binding energy of 1021.9 eV (1021.5 eV) and 1045 eV localized in Zn 2p3/2 and Zn 2p1/2 (SI Figure S3) consisted of Zn2+. Previous XPS experiments also demonstrated Cr(III)−Cr(VI) (formed as CrO42−) transformation was much easily completed in oxidative atmosphere and this transformation was inhibited in reductive atmosphere, which was important to form spinel structure and create different surface oxygen concentrations.16,21
The catalysts, which were denoted as ZnCrOx-O2 or ZnCrOx-H2, were prepared by calcination of Zn−Cr LDH precursor at 673 K within oxidative or reductive atmosphere, respectively. Once calcined, the basal reflections of LDH (Supporting Information (SI), Figure S1) disappeared in an Xray diffraction pattern, while the reflections of ZnO and spinel structure of ZnCr2O4 appeared as shown in Figure 1a.13,14 ZnCrOx-O2 consisted of both ZnO and ZnCr2O4. As displayed in Figure 1d, there was no clear boundary of oxide and support in ZnCrOx-O2 due to the state of Zn existence as ZnO and nonstoichiometry ZnCr2O4 spinel, in which the Zn atoms occupied the octahedral sites.13 This ideal spinel structure also could be verified by the characteristic Raman active modes in Figure 1b, among which the spectra of A1g, Eg, and F2g could be assigned to the symmetric Cr−O stretching vibration originated from CrO6 groups, symmetric Zn−O vibration and Cr−O stretching vibration in ZnCr2O4, respectively.15 Normal ZnCr2O4 spinel structure formed a cubic close-packed lattice oxygen surrounded by octahedral and tetrahedral sites, which indicated oxygen-rich of the catalyst.13,16,17 In contrast, ZnCrOx-H2 (Figure 1a) only showed distinctly hexagonal wurtzite phase of ZnO, and oxide-support may be a defective spinel, which was poor-crystallization and further verified by HRTEM image (Figure 1c) and Raman spectra (Figure 1b). The interplanar spacing of 0.24 and 0.48 nm belonged to ZnO facet (101) and ZnCr2O4 facet (111), respectively.18 Both lattice fringes can be found in ZnCrOxO2(Figure 1d), whereas only ZnO (101) existed in ZnCrOxH2(Figure 1c). The absence of characteristic spinel Raman spectra of ZnCrOx-H2 (Figure 1b) also verified the inhibition of spinel formation. More TEM observation images and diffraction patterns were enclosed as seen in SI Figure S2 for B
DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. XPS and temperature-programmed profiles of ZnCrOx-H2 and ZnCrOx-O2. (a) XPS of Cr 2p, (b) XPS of O 1s, (c) H2-TPR, and (d) O2TPD.
calcination for catalyst preparation under reductive atmosphere played a key role on surface oxygen distribution. Theoretical simulation revealed that Zn−Cr spinel surface was terminated by (100) facet with Cr and O-rich structure that was thermodynamically stable configuration.13,16 Oxygen-rich (100) facet of ZnCrOx-O2 not only contained more oxygen, but also facilitated [O] reduction by the strong interactions between ZnO and ZnCr2O4 comparing with ZnCrOx-H2. Nevertheless, the dominant polar facet of ZnO and poorcrystallization oxide support in ZnCrOx-H2 had more oxygen vacancies. The oxygen species were further characterized by O2temperature-programmed desorption (O2-TPD) and the results were showed in Figure 2d. Both ZnCrOx-H2 and ZnCrOx-O2 had a great potential to adsorb O2 in spite of different oxygen types. The peak at about 520 K can be assigned to chemically adsorbed oxygen molecular (O2−) since desorption the physically adsorbed oxygen O2 usually happened below 400 K.24,25 Whereas, ZnCrOx-O2 presented an additional peak at 757 K that contributed to desorption of chemically adsorbed oxygen atom O −(ad).25 [O]-rich terminated polar facet (100) of spinel was more electron rich than those of nonpolar facets, which was electron transfer from metal ions to oxygen ions at the interface.16 The slightly lower binding energy of Cr 2p in ZnCrOx-O2 than ZnCrOxH2(Figure 2a) also implied the higher electrons excited ability. This unique active oxygen atom-O−(ad) in ZnCrOx-O2
Both catalysts were also characterized by temperatureprogrammed reduction in hydrogen (H2-TPR) to investigate the surface oxygen concentration. As displayed in Figure 2c, there was one main reducible peak for both catalysts due to the reduction of lattice oxygen atoms from the dominant polar facet. The same onset reduction temperature at approximate 490 K showed similar reducibility of both catalysts. Quantitative analysis of H2-TPR profiles suggested different interactions between the oxide-support interface and the amount of oxygen on the surface.2 It cost 0.8 mmol g−1 H2 in ZnCrOx-O2, and the amount was twice than that in ZnCrOx-H2(0.4 mmol g−1). This may be the reason for different temperature positions of reduction peaks, which were 590 K for ZnCrOx-O2 and 580 K for ZnCrOx-H2 (Figure 2C). This oxygen concentration difference was fairly compatible with XPS O 1s analysis results shown in Figure 2b, which showed the surface oxygen defects increased from 16.0% in ZnCrOx-O2 to 42.0% in ZnCrOx-H2.22 The band at about 532 eV was attributed to oxygen atoms in the vicinity of oxygen vacancy and 530 eV was belong to M−O lattice oxygen atoms in fully coordinated.22 Notably, both catalysts were prereduced before CO2 hydrogenation experiments, which also can diminish surface oxygen.23 In SI Table S1, the amount of oxygen defects of ZnCrOx-O2 showed monotonously and insignificant increase to the highest value 20.3% from 373 to 673 K at H2/Ar atmosphere, which was still far lower than the corresponding value (42.0%) in ZnCrOx-H2. Thus, the C
DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. In situ DRIFTS of (a) ZnCrOx-H2 and (b) ZnCrOx-O2. Both catalysts were prereduced by 20% H2/N2 at 623 K for an hour. The spectra were collected after the exposure of catalysts to CO2 or CO2+H2 mixture at the certain pressures and temperatures.
which may be O− precisely, through carbon atom leaving an alignment of the O−O axis parallel to the surface. The complexes were similar to π-bond activation. Along with the pressure increased and H2 added, unidentate carbonate transformed to bidentate carbonate according to the peak intensity change between 1670 and 1545 cm−1, and no other changes were observed. Further investigation of temperature effect may propose CO2 hydrogenation mechanism of both catalysts. Once the catalysts in CO2+H2 mixtures are annealed to 613 K, formate (HCOO−) and methoxy (H3CO−) species appeared in both catalysts. In detail, the peak at 1326 cm−1 was assigned to symmetric OCO stretches, whereas 1577 and 1591 cm−1 were the asymmetric OCO stretches and 2872 cm−1 was for CH stretch modes of HCOO−.28 Meanwhile, the peak at 1055 cm−1 was assigned to the CO stretch mode of terminal methoxide species.27,29 These species were generated by coordinated CO32− or CO2δ− hydrogenation which was only formed under the reaction conditions. It was suggested that the reaction pathways of both catalysts were based on the formate pathway due to the presence of formate and methoxy intermediate species. Recently, experimental and theoretical studies showed CO2 hydrogenation followed either RWGS or formate reaction, which involved dissociation of carbon dioxide to carbon monoxide or directly hydrogenate to formate, respectively.10,27 Moreover, the mechanism of formate reaction was that CO2 hydrogenation occurred from hydrogen and CO2 via formate intermediate over the catalyst surface. It showed the same intermediates according to the results of in situ DRIFTS of both catalysts under reaction conditions in this research, which suggested the same hydrogenation pathways that CO2 hydrogenation followed the formate reaction in spite of different CO2 chemsisorbed species. CO2 hydrogenation reduction experiments were implemented after catalysts activation in 20%H2/Ar at 623 K. General knowledge is that the strong metal-support interaction (SMSI) occurs when oxide-support catalysts reduced by H2 at different temperature and can further affect the activity. Nevertheless, the prerequisite was the reducible supports. According to the results of experiments (SI Figure S6), the hydrogenation activity of ZnCrOx-O2 did not have an obvious relationship with the reduction temperature, which indicated SMSI interaction had no obvious effects on the interaction
verified the electron transfer from metal ions to oxygen and stronger oxygen-activity ability than ZnCrOx-H2. Generally, the coordination of oxygen at the (100) surface of the spinel was known to strongly affect the adsorption properties of the catalyst surface.16 It was possible that over ZnCrOx-O2, CO2 adsorbed on these O− type easily formed CO32− as anticipated and CO2 may be directly adsorbed on metal ions of ZnCrOxH2 due to more oxygen vacancies and weak oxygen-activity ability. The assumption was further confirmed by the later characterization of in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). In situ DRIFTS was used to investigate the interaction of CO2 or CO2/H2 mixtures with catalyst surfaces at a certain pressure and temperature. As Figure 3a showed, the adsorption of CO2 on ZnCrOx-H2 surface generated strongly bound carboxylate CO 2 δ ‑ due to the presence of 1268 cm−1(symmetric stretching vibration OCO) and 1678 cm−1(antisymmetric stretching vibration OCO) peaks after exposure of ZnCrOx-H2 to 10 Torr CO2 at 300 K.1 This indicated ZnOx-(oxygen-poor)-support interface activated CO2 by transfer an electron through metal ions. According to the surface selection rules, the presence of antisymmetric stretching vibration suggested an adsorption geometry with an alignment of the O−O axis nonparallel to the surface.7 This meant the bent CO2δ‑ molecule adsorbed with the metal ions via an oxygen atom similarity to the σ activation instead of carbon atom which similarly existed in CeOx/Cu system.1,26 Once CO2 pressure was up to 102 Torr, new peaks appeared at 1419 and 1218 cm−1, which belonged to noncoordinated carbonate (CO32−).1 Both carboxylate and noncoordinated carbonate species showed no other change after adding H2 at this temperature. These operations were also carried out on ZnCrOx-O2 and the results are shown in Figure 3b. Compared with ZnCrOxH2, there were four totally different peaks located at 1670, 1545, 1364, and 1065 cm−1, which belonged to carbonate (CO32−).27 However, it differed from the noncoordinated carbonate in ZnCrOx-H2, the 1670 and 1065 cm−1 peaks were assigned to bidentate carbonate (CO32−) and the other two attributed to unidentate carbonate (CO32−), which were displayed by diagrammatic sketches.27 This activation state was due to charge transfer from surface oxygen to CO2 and bent further. The CO2 molecule adsorbed to surface oxygen, D
DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Kinetics and hydrogenation activities results. (a) Arrhenius plot for CO2 hydrogenation on ZnCrOx-O2 and ZnCrOx-H2, (b) CO2 hydrogenation activities at different GHSV. Reaction condition: H2/CO2 = 3/1, 4 MPa,613 K.
about +0.33 eV.31,32 Moreover, the apparent activation energy for CO2 conversion also decreased from 105.42 kJ mol−1 for ZnCrOx-O2 to 85.38 kJ mol−1 for ZnCrOx-H2(Figure 4A), which further demonstrated that ZnCrOx-H2 was more active for CO2 hydrogenation than ZnCrOx-O2. Overall, the relationship between CO2 chemisorption states and hydrongenation activity was investigated based on Zn−Cr mixed-oxide catalysts with different surface properties. CO2 chemisorption state was related to oxygen concentration and species of catalysts. And the different CO2 chemisorption states on ZnCrOx-H2 and ZnCrOx-O2 were well identified by in situ DRIFTS. Oxygen-deficient ZnCrOx-H2 which absorbed CO2 as CO2δ− showed higher hydrogenation activity compared with oxygen-rich ZnCrOx-O2 on which CO2 was chemisorbed as CO32−. This research showed the direct experimental evidence on the relationship between CO2 chemisorption states and hydrogenation activity, which may provide a new understanding of CO2 hydrogenation mechanism and a strategy for catalyst design for CO2 catalytic hydrogenation.
between oxide and support within ZnCrO x -O 2 . CO 2 conversion activities of both catalysts were compared in a fixed-bed flow reactor at 4 MPa and 613 K varying with gas hourly space velocity (GHSV), and the products of hydrogenation were CO and CH3OH, which was further shown in SI Figures S7 and S8. According to the results of Figure 4b, the catalyst-area-based activities of ZnCrOx-H2 were, overall, much higher than those of ZnCrOx-O2 at all tested GHSV ranges. To make a more objective comparison, the external diffusion should be eliminated, and all active sites should take full advantage by GHSV adjusting. As GHSV increased (Figure 4b), the activity of both samples were enhanced gradually and kept steady above 22 000 mL g−1 h−1. At this optimized condition, the activity of ZnCrOx-H2 was about 15 × 10−5 molCO2 m−2 s−1, which was three times than that of ZnCrOxO2 (about 5 × 10−5 molCO2m−2 s−1). The products selectivity of both catalysts showed no such diversity and just slightly changed along with the GHSV increased. This activity enhancement (Figure 4b) can not be simplily attributed to the reducing capacity of catalysts, which was no different revealed by H2-TPR (Figure 2c). Although the specific surface area and surface alkali showed some difference (SI Figure S5 and Table S1), the promoting effect on activity should not be so substantial. Generally, it is also known that oxygen vacancy on the surface cannot enhance CO2 hydrogenation activity due to the quenching effect by CO2.30 This phenomenon s corresponded to the ZnCrOx-O2−HX samples. Though the amount of oxygen defects was tuned by reduction temperature (SI Table S1), the catalytic activity was insignificantly affected (SI Figure S6). Meanwhile, CO2 adsorption characterizations of ZnCrOx-O2−HX in SI Figure S9 was practically identical. In other words, CO2 adsorption state was little affected by reduction temperature in ZnCrO2− O2 catalysts. Thus, the CO2 activation on surface may play the vital function on final hydrogenation activity. Notably, CO2δ− species in ZnCrOx-H2 still can be found in reaction condition at 613 K due to the presence of 1288 cm−1, and unidentate CO32− species still existed in ZnCrOx-O2 besides the reaction intermediates peaks (Figure 3b). The formation of formate species via CO2δ− hydrogenation was energetically favorable, which involved reaction of dissociated H and bending CO2δ−. Whereas, CO32− to formate should undergo C−O bond breaking and C−H bond formation which was endothermic by
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01316. Detailed procedures for characterizations and catalytic activity measurements, catalytic activities, XRD patterns, SEM, and TEM images (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lijun Wang: 0000-0002-8392-2195 Limin Guo: 0000-0002-6834-2758 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was financially supported by the Huazhong University of Science and Technology and National Natural Science Foundation of China (U1510107, 21776297, 21878116). The authors thank the Analysis and Testing E
DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.9b01316 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
