Oxidative Dehydrogenation of Propane to Propylene in the Presence

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Oxidative Dehydrogenation of Propane to Propylene in the Presence of HCl Catalyzed by CeO2 and NiO-Modified CeO2 Nanocrystals Quanhua Xie, Huamin Zhang, Jincan Kang, Jun Cheng, Qinghong Zhang, and Ye Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00650 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Catalysis

Oxidative Dehydrogenation of Propane to Propylene in the Presence of HCl Catalyzed by CeO2 and NiOModified CeO2 Nanocrystals

Quanhua Xie†, Huamin Zhang†, Jincan Kang, Jun Cheng, Qinghong Zhang*, and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. *CORRESPONDING AUTHOR: E-mail: [email protected] and [email protected]; Phone: +86-592-2186156; Fax: +86-592-2183047 †

These authors contributed equally.

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ABSTRACT: The oxidative dehydrogenation of propane is an attractive reaction for propylene production, but the over-oxidation leads to low propylene selectivity at considerable propane conversions. Here, we report the oxidative dehydrogenation of propane by oxygen in the presence of hydrogen chloride. CeO2 was found to be an efficient catalyst for the conversion of propane to propylene by (O2 + HCl). The reaction was structure sensitive and the catalytic behavior depended on the exposed facet of CeO2 nanocrystals. The nanorod exposing {110} and {100} facets showed the highest activity, whereas the nanocube enclosed by {100} facets was the most selective for propylene formation. The modification of CeO2 nanorods by NiO increased both propane conversion and propylene selectivity. A propylene selectivity of 80% was achieved at propane conversion of 69% over an 8 wt% NiO−CeO2 catalyst at 773 K, offering a single-pass propylene yield of 55%. No significant catalyst deactivation was observed in 100 h of reaction. HCl played a pivotal role in the selective formation of propylene and more than 95% of HCl could be recovered after the reaction. The structure-property correlation indicates that the surface oxygen vacancy and the surface chloride coverage are two crucial factors determining the activity and selectivity. The mechanistic studies suggest that the peroxide species (O22-) formed by adsorption of O2 on surface oxygen vacancies may activate chloride, generating a radical-like active chlorine species. The active chlorine species accounts for the activation of C-H bond of propane, forming propylene as a major product.

KEYWORDS: propane, oxidative dehydrogenation, propylene, hydrogen chloride, cerium oxide, nickel oxide, oxygen vacancy, surface chloride coverage


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1. INTRODUCTION Propylene is used for production of many commodity chemicals such as polypropylene, acrylonitrile, acrylic acid and propylene oxide, and is one of the most important building blocks in the chemical industry. Currently, propylene is primarily produced as a co-product of ethylene via high-temperature steam cracking of petroleum-derived naphtha. Recent emergence and use of abundant shale gas resources, in particular ethane, as the feedstock to steam cracker would increase the gap between the production of ethylene and propylene. This has motivated the development of “on-purpose” propylene-production technologies such as the dehydrogenation of propane, which produces exclusively propylene of high purity suitable for polymer synthesis instead of a mixture of lower olefins.1 Many recent studies have been devoted to the non-oxidative dehydrogenation of C3H8 to C3H6.1−4 The CO2-assisted dehydrogenation of alkanes has also been reported.5 The technology for the non-oxidative dehydrogenation of C3H8 using CrOx/Al2O3 or Pt−Sn/Al2O3 catalyst is relatively mature.1 However, this reaction is a thermodynamically restricted reaction at moderate temperatures (equilibrium C3H8 conversion = ~20% at 773 K). The need of a high reaction temperature and the endothermic nature make this reaction less energy efficient. Moreover, the easy coke deposition requires a frequent catalyst regeneration by combustion, which not only increases process cost but also causes CO2 formation. On the other hand, the oxidative dehydrogenation of C3H8 is exothermic, and it is estimated that the energy savings reach ~45% by using the oxidative dehydrogenation to replace the non-oxidative process.6 The presence of O2 may also avoid the coke deposition. However, the over-oxidation to CO2 particularly at high C3H8 conversions is a serious problem that limits the development of catalysts for the oxidative dehydrogenation of C3H8. C3H6 is more reactive than C3H8 because of the lower bond


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dissociation energy of the weakest C-H bond in C3H6 (361 kJ mol-1 versus 397 kJ mol-1 in C3H8)7 and the presence of C=C bond. To increase the selectivity of the target product, which is more reactive than the substrate, is a great challenge for many selective reactions.8-14 A number of catalysts have been reported for the oxidative dehydrogenation of C3H8 by O2.10,11 Supported vanadium oxide or vanadium-containing composite oxide catalysts show relatively high activity.10,11,14,15 However, C3H6 selectivity drops quickly with an increase in C3H8 conversion, and the single-pass C3H6 yield is usually lower than 20%.11 Only a few papers have claimed high single-pass C3H6 yield (> 20%).11,16-19 Besides the conventional transition metal oxide-based catalysts, novel metal-free carbon materials and boron nitrides as well as metal-organic frameworks have also demonstrated excellent performances for the oxidative dehydrogenation of C3H8.20-23 Despite some recent encouraging progress, high C3H6 selectivity (>70%) is still difficult to achieve at a high C3H8 conversion (> 30%). The use of halogen (X2) as an oxidant for the functionalization of lower alkanes can avoid the formation of CO2 and has attracted much attention in recent years.13,14,24-32 For example, the conversion of C3H8 using Br2 in the presence of I2 could produce a mixture of C3H6, C3H7Br and C3H7I with excellent single-pass yields.29 The conversion of C3H8 with Cl2 over a Ru/TiO2 catalyst provided C3H6 with good selectivity.31 Although high single-pass (C3H6 + C3H7X) yields could be obtained, the regeneration of X2 from HX via the Deacon reaction or via electrolysis is required.24 The integration of the functionalization of lower alkanes and the regeneration of X2 in one step via oxidative halogenation (eq. 1) enables process intensification and represents a promising route for selective conversion of lower alkanes. CnH2n+2 + HX + 1/2O2 → CnH2n+1X + H2O

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The oxybromination of CH4 to CH3Br could proceed over supported Ru and Rh,33,34 FePO4,35 CeO2,36 and (VO)2P2O737 catalysts. CeO236 and LaOCl38,39 catalyzed the oxychlorination of CH4 to CH3Cl, whereas CO was selectively formed during the oxychlorination of CH4 over (VO)2P2O7 catalyst.40 Many factors may control the product selectivity in the oxychlorination and oxybromination of CH4 over different types of catalysts.41 Although many studies have been devoted to the oxidative halogenation of CH4, only a few papers have focused on the conversion of C2H6 or C3H8 by this strategy.37,42-44 Pérez-Ramírez and co-workers once mentioned that C2H5Br was formed as the dominant product in the oxybromination of C3H8 at 550-700 K over (VO)2P2O7 catalyst and the increase in the reaction temperature led to coke formation.37 They recently reported that EuOCl could catalyze the oxychlorination of C2H6 or C3H8 to C2H4 or C3H6, and the yield of C3H6 reached 40%.43 A TiC–SiC catalyst was also efficient for the upgrading of natural gas via oxyhalogenation, in particular for the conversion of C2H6 to C2H4.44 It is noteworthy that the catalytic transformation of C3H8 or C2H6 is quite different from that of CH4 because of the different types of products and the difference in the reactivity of these lower alkanes, although the activation of C-H bond is usually a common initial step.7-11 Moreover, the oxidative dehydrogenation of C3H8 to C3H6 is generally more difficult than that of C2H6 to C2H4. The yield of C3H6 obtained from the oxidative dehydrogenation of C3H8 is typically lower than that of C2H4 from C2H6 because of the presence of reactive allylic hydrogen atoms in C3H6, which lead to consecutive oxidation of C3H6 more easily.7,11 Under this background, it would be attractive to develop an efficient catalytic system for the oxidative functionalization of C3H8 by O2 in the presence of HCl, which is much cheaper and more plentiful than other halogen-containing compounds. This paper reports our recent finding that CeO2 catalyzes the direct conversion of C3H8 to C3H6 by (O2 + HCl) with high single-pass


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yields. Only marginal organic halides (selectivity 99.999 %) and 80% He (> 99.999%) at 823 K for 1 h. After the reactor was cooled down to room temperature, the reactant gas mixture composed of C3H8 (> 99.9%), HCl (≥ 99.999%), and O2 (≥ 99.999%) was introduced into the reactor. He (≥ 99.999%) and N2 (≥ 99.999%) were also added in the reactant gas mixture. He was used to adjust the partial pressures of reactants, while N2 was employed as an internal standard for the calculation of C3H8 conversion. The gas feeding and gas flow rate were controlled by mass-flow controllers (Brooks Instrument). The reaction was started by raising the temperature to the desired reaction temperature (typically 773 K). We confirmed that the catalytic reaction under our conditions was free of heat-transfer and mass-transfer limitations.


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The products were analyzed by two online gas chromatographs (Shanghai Haixin GC-950 and GC-930), which were equipped with two thermal conductivity detectors (TCDs) and one flame ionization detector (FID). A molecular sieve-5A column was used for the separation of CH4, O2 and N2, while a Porapak Q column was used for the separation of CO, CH4, CO2, C2H4, C2H6, C3H6 and C3H8. A DB-624 capillary column in connection with the FID was used to separate and analyze CH4, C2H4, C2H6, C3H6, C3H8 and halogenohydrocarbons. The conversions of O2 and C3H8 were calculated based on the concentrations of O2 and C3H8 in the inlet and outlet of gas flow with N2 as an internal standard using the following equations: O2 conversion (%) = ([O2 in] – [O2 out])/[O2 in] × 100%

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C3H8 conversion (%) = ([C3H8 in] – [C3H8 out])/[C3H8 in] × 100%

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The selectivity of each carbon-containing product was evaluated on a molar carbon basis. As examples, the selectivities of C3H6, CO, and CO2 were calculated by the following equations: C3H6 selectivity (%) = [C3H6 out]/([C3H8 in] – [C3H8 out]) × 100%

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CO selectivity (%) = [CO out]/(3×[C3H8 in] − 3×[C3H8 out])× 100%

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CO2 selectivity (%) = [CO2 out]/(3×[C3H8 in] − 3×[C3H8 out]) × 100%

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The catalytic result after 3 h of reaction was used for discussion unless otherwise stated. Carbon balance was evaluated and was typically better than 95%. The dehydrochlorination of 1-C3H7Cl (> 99%) was carried out with the same fixed-bed flow reactor. After the pretreatment of catalyst in O2-He mixed gas flow, the reaction was started by introducing 1-C3H7Cl (flow rate of liquid, 0.30 cm3 h-1) in a mixed N2 and He gas flow (48 ml min-1). The catalytic performance after 3 h was typically used for discussion. For the analyses of Cl2 and HCl in the products, the gas at the reactor outlet was passed through two serial impingers, equipped with a porous frit immersed into an aqueous KI solution.


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The collected Cl2 and HCl during a fixed time were measured for several typical catalysts. Cl2 was quantified from the amount of I2 formed by titration with an aqueous solution of Na2S2O3 (0.05 M).47 Cl− in the solution was quantified by the Mohr’s method.48

2.4. DFT Calculations. The DFT calculation was performed with CP2K Code.49 The exchange-correlation functional was treated by the Perdew–Burke–Ernzerhof (PBE) within generalized gradient approximation (GGA).50 Analytic Goedecker-Teter-Hutter (GTH) pseudopotentials were employed to represent the core electrons with 1, 4, 6, 7 and 12 valence electrons for H, C, O, Cl and Ce, respectively. DZVP basis set was applied and the cut-off energy was set to 500 Ry. In order to diminish the self-interaction error and to localize the Ce 4f states properly, Hubbard term U was added (7.0 eV here), as reported in literature.51 For structural optimizations, all atoms were allowed to relax and convergence criterion for wave function optimization was set by a maximum electronic gradient of 3.0E-7 a.u. with an energy difference tolerance between self-consistent field (SCF) cycles of 1.0E-13 a.u. The experimental lattice parameter, i.e., a = 5.411 Å (ICSD #157419),52 was applied in our optimization. To model the surface of CeO2, the well-known slab approach with periodic boundary conditions was adopted. Following the DFT studies on CeO2 in literature,51,53 our calculation was performed on the most stable {111} surface of CeO2. A stoichiometric 2×4 supercell slab model containing 4 O−Ce−O trilayers was established for CeO2 {111} with the vacuum scale setting to 20 Å between repeated slabs along z direction and its volume is 13.3 ×15.3 ×30.9 Å3. The transition state (TS) of the reaction was determined using a constrained optimization scheme, and vibrational analysis was used to confirm the transition state, which corresponded to true saddle point with only one imaginary frequency on the potential energy surface.


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The formation energy of O-vacancy (∆Evac) of CeO2 (or Ni-modified CeO2) was calculated by eq. 7: ∆Evac = [E(reduced slab) + 1/2E(O2)] − E(slab)

(7)

Here, E(reduced slab), E(O2), and E(lab) represent the total energy of CeO2 (or Ni-modified CeO2) containing an O-vacancy, the energy of a gas phase O2, and the total energy of stoichiometric CeO2 (Ni modified CeO2) slab, respectively.

3. RESULTS AND DISCUSSION 3.1. Catalytic Behaviors of Metal Oxides for Conversion of C3H8 by (O2 + HCl). Table 1 compares the catalytic behaviors of various metal oxides for the conversion of C3H8 by (O2 + HCl). Most of these metal oxides have been reported as catalysts for the oxidative halogenation of methane.33-40 Without a catalyst, no conversion of C3H8 was observed under the current reaction conditions. RuO2, a well-known catalyst for the Deacon reaction, i.e., HCl oxidation,54 showed the highest O2 conversion and also a higher C3H8 conversion, but the major product was CO2, suggesting that the reactivity of the oxygen species on RuO2 was too high. C3H6 was formed as the major product from C3H8 conversion over other metal oxide catalysts displayed in Table 1. This is different from the conversion of CH4 by (O2 + HCl), where CH3Cl was formed as the major selective oxidation product.36 The formation of halogenated hydrocarbons was also observed, mainly including chloropropane (C3H7Cl) and chloropropylene (C3H5Cl), but their selectivities were low. We speculate that C3H6 may be formed either via C3H7Cl intermediate (eq. 8) or via direct conversion of C3H8 by (O2 + HCl). C3H8 + 1/2O2 + HCl→ C3H7Cl + H2O → C3H6 + HCl + H2O


(8)

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The transition metal oxides such as Fe2O3, CuO and NiO exhibited higher C3H6 selectivities but their activities were lower. Moreover, these metal oxides were not stable during the reaction. For example, C3H8 conversion and C3H6 yield decreased rapidly in the initial 3 h and both dropped to < 2% after 10 h of reaction over Fe2O3 (Supporting Information Figure S1). This is probably because of the change of metal oxides into lower melting-point metal chlorides, which moved out from the hot zone of catalyst bed during the reaction (Supporting Information Figure S1). VOPO4 showed a medium C3H8 conversion and medium C3H6 selectivity. The selectivity of CO was higher over VOPO4, similar to the observation for the oxychlorination of CH4 over this catalyst.40 We also examined the catalytic performances of rare earth metal oxides, which were stable for the oxychlorination of CH4.36 Among the rare earth metal oxides examined, CeO2 demonstrated the highest C3H6 yield in the conversion of C3H8. In addition to C3H6, CO and CO2 were formed as the major by-products. The total selectivity of organic chlorides (including C3H7Cl and C3H5Cl) over CeO2 was 2.8%. It is noteworthy that CeO2 also catalyzes the oxidation of HCl to Cl2 by O2, i.e., the Deacon reaction, although its activity is lower than RuO2.55 CeO2 is unique for the high stability in the Deacon reaction even at high temperatures.56 Our result shows that CeO2 is much more stable than Fe2O3 (Supporting Information Figure S1), and thus is a promising candidate for the conversion of C3H8 to C3H6 by (O2 + HCl).

3.2. Effect of Morphology and Exposed Facet of CeO2. Many recent studies have pointed out that the catalytic behaviors of CeO2 depend on its morphology or the exposed facets.57-59 Our previous study suggests that the exposed facet of CeO2 affects its catalytic performance for the oxidative halogenation of methane.36 However, the nature of such a facet-effect is not clear. Here, we investigate the structure-sensitive feature of CeO2 for the conversion of propane by (O2 + HCl)


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and study the nature behind the phenomenon in the present work to gain insights into the key catalyst or site requirement for the conversion of C3H8 and the selective formation of C3H6. TEM measurements for CeO2 nanocrystals used in this work showed relatively uniform morphologies of nanorods, nanocubes, nano-octahedra, and nanoparticles (Supporting Information Figure S2). HRTEM revealed that the nanorod exposed both {110} and {100} facets, while the nanocube was enclosed by {100} facets. The nano-octahedron and nanoparticle predominantly exposed {111} facets. These results are in agreement with many previous studies.45,46,57-59 Furthermore, the morphology of CeO2 nanocrystals kept essentially unchanged after the reaction, although the nanorod became shorter and wider (Supporting Information Figure S3). We compared the catalytic properties of CeO2 nanocrystals with different morphologies for the conversion of C3H8 by (O2 + HCl). Since the specific surface areas of CeO2 nanocrystals with different morphologies are different, we have evaluated the rates of C3H8 conversion and C3H6 formation per CeO2 surface area, expressed as r(C3H8) and r(C3H6), for a better comparison (Table 2). The r(C3H8) and r(C3H6) were calculated from the slopes of straight lines, which were obtained by plotting C3H8 conversion and C3H6 yield versus the total surface area of each CeO2 nanocrystal by changing catalyst amount (Supporting Information Figure S4). The result summarized in Table 2 shows that the CeO2 nanorod exhibits the highest r(C3H8) and r(C3H6). The r(C3H8) decreased in the order of nanorod > nanocube > nano-octahedron > nanoparticle. The r(C3H6) changed in the same order except for nano-octahedron and nanoparticle. Essentially, the r(C3H8) and r(C3H6) for nano-octahedra were similar to those for nanoparticles. These observations suggest that the {110} facet shows slightly higher activity than the {100} facet, which is significantly more active than the {111} facet.


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For selective oxidation reactions including the oxidative dehydrogenation of lower alkanes, the selectivity of the target product usually decreases with increasing the substrate conversion.8-13 We observed a similar tendency for the CeO2-catalyzed conversion of C3H8 by (O2 + HCl) (Figure 1A). Upon increasing C3H8 conversion, the selectivity of C3H6, the major product, as well as that of organic chlorides (denoted as CnHmCl) decreased, while those of over-oxidation products, i.e., COx (CO + CO2), increased (Figure 1B). The comparison of product selectivity at the same C3H8 conversion reveals that the CeO2 nanocube is the most selective toward C3H6 formation and the selectivity of C3H6 decreases in the following sequence: nanocube > nanorod > nano-octahedron ≈ nanoparticle. Thus, the {100} facet is the most selective for C3H6 formation, followed by the {110} and {111} facets. It becomes clear that not only the activity of C3H8 conversion but also the selectivity are facet-dependent, demonstrating that the CeO2catalyzed conversion of C3H8 by (O2 + HCl) is a structure-sensitive reaction. We performed studies to understand the nature of the observed facet effect. The concentration of oxygen vacancies (expressed as O-vacancies hereafter) has been proposed to be a key descriptor for catalytic properties of CeO2 in many oxidation reactions.58,59 To clarify the role of O-vacancies in our reaction, we have measured the concentration of O-vacancies for CeO2 nanocrystals with different morphologies by Raman spectroscopy. Figure 2 shows Raman spectra obtained using laser wavelengths of 514 and 325 nm as excitation sources. Typically, the UV Raman (λex = 325 nm) is more surface sensitive than the visible Raman (λex = 514 nm).60-62 Our visible Raman spectra show a strong Raman band at 462 cm-1 and weak Raman bands at 260, 596, and 1176 cm−1. The Raman band at 462 cm−1 corresponds to the F2g mode (Ce−O−Ce vibration) of CeO2 fluorite phase, while the bands at 260, 596, and 1176 cm-1 arise from the second-order transverse acoustic (2TA), the defect-induced (D), and the second-order


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longitudinal optical (2LO) modes, respectively.63 These Raman bands were also observed in the UV Raman spectra except for that at 260 cm-1, but the relative intensities changed significantly (Figure 2B). Because of the multiphonon excitation by resonance Raman effect, the 2LO band at 1179 cm-1 became much stronger.63 The D band at 592 cm-1 also became intense as compared with the F2g band at 462 cm−1 due to the resonance Raman effect. In general, the D band at 592 cm−1 is associated with the O-vacancy sites, and thus the ratio of intensities of D and F2g bands (expressed as ID/IF2g) evaluated from UV Raman spectra can be used to estimate the relative concentration of surface O-vacancies.60-63 The ID/IF2g values evaluated for CeO2 nanocrystals are displayed in Table 2. The ID/IF2g depends on the exposed facets and decreases in the order of nanorod > nanocube > nano-octahedraon ≈ nanoparticle. This indicates that the concentration of surface O-vacancies decreases in the order of {110} > {100} > {111} and this trend is the same as that for the rate of C3H8 conversion (Table 2). Therefore, the concentration of O-vacancies on CeO2 nanocrystals may play a key role in C3H8 conversion. Since CeO2 is usually used for complete oxidation catalysis due to its strong oxidation ability,64,65 it is of interest to understand the key factors controlling C3H6 selectivity. We have confirmed that CO2 is the major product in the conversion of C3H8 by O2 over CeO2 in the absence of HCl. Thus, HCl plays a key role in the selective formation of C3H6. We speculate that the ability of CeO2 surfaces for HCl chemisorption or the surface coverage of Cl− may determine C3H6 selectivity. We measured the coverage of chloride species on surfaces of CeO2 nanocrystals by XPS after the chemisorption of HCl at 773 K. The XPS spectra confirmed the presence of Cl species with binding energies of Cl 2p3/2 at 198.5 eV and Cl 2p1/2 at 200.1 eV (Supporting Information Figure S5), which could be ascribed to Cl− anions. These XPS peaks for Cl− anions could not be observed before HCl chemisorption. The intensity of XPS peaks for the


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chemisorbed Cl− anion depended on the morphology of CeO2 or the exposed facet. The quantitative result revealed that the coverage of Cl− (expressed with Cl/Ce molar ratio) at surface region decreased in the order of {100} > {110} > {111} (Table 2). This trend agrees well with that for C3H6 selectivity (Figure 1), supporting our speculation that the capability of catalyst surfaces for HCl chemisorption is a key factor controlling C3H6 selectivity.

3.3. Effect of CeO2 Modification. CeO2 with fluorite structure can form solid solutions with a large variety of metal oxides,64,66 which may tune the O-vacancy formation energy,67 and thus promote the catalytic activity. We investigated the effect of modification of CeO2 nanorods, which exhibited the highest rates of C3H8 conversion and C3H6 formation, by various metal oxides. The addition of V2O5 or MoO3, which is usually employed as a catalyst component for the oxidative dehydrogenation of lower alkanes,8,11,14 into CeO2 decreased C3H8 conversion although C3H6 selectivity was improved (Supporting Information Table S1). On the other hand, the modification of CeO2 with lower-valent dopants except for Co3O4 increased C3H8 conversion. We speculate that the possible incorporation of lower-valent cations into CeO2 lattice to replace Ce4+ ions may create more O-vacancy sites,64-66 thus enhancing the activity. The C3H6 selectivity was also enhanced by some lower-valent modifiers. Among all the modifiers examined, NiO was the most efficient to promote the yield of C3H6 (Supporting Information Table S1). Both C3H8 conversion and C3H6 selectivity were significantly enhanced by modification of CeO2 nanorods with NiO. The catalytic behaviors of NiO-modified CeO2 nanorods depended on NiO content. Both O2 and C3H8 conversions increased after doping of NiO up to 5 wt%, but a higher NiO content was unbeneficial to the activity (Figure 3A). The C3H6 selectivity increased gradually from 55% to


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72% with an increase in NiO content to 8 wt% (Figure 3B). The selectivities of CO and CO2 decreased at the same time. The selectivity of CnHmCl kept at 8 wt%. Several other groups also reported the formation of Ni−Ce−O solid solution by incorporation of Ni2+ cations in CeO2 lattice,67-73 which could create more O-vacancy sites and was beneficial to oxidative dehydrogenation of hydrocarbons,68,69 NO reduction,70-72 and methane oxidation73. Visible and UV Raman spectroscopic studies were performed to gain further insights into the structure of the NiO−CeO2 catalysts. The visible Raman spectra show that the band at 460 cm-1 ascribed to F2g mode of CeO2 crystalline structure becomes broader with an increase in NiO content up to 8 wt% (Figure 6A). This also suggests that some Ce4+ cations in CeO2 lattice have been substituted by Ni2+.70 At the same time, the shoulder band at 550-600 cm-1 became stronger, indicating the increase in the concentration of O-vacancies. The UV Raman spectra also showed the change in the relative intensity of F2g and D bands at 462 and 592 cm-1 (Figure 6B). The ID/IF2g values estimated from UV Raman spectra suggest that the concentration of surface Ovacancies increases with an increase in NiO content up to 8 wt% (Table 3). The coverage of Cl− species on the surfaces of NiO−CeO2 catalysts after chemisorption of HCl was evaluated by XPS. The XPS peaks of Cl 2p3/2 and Cl 2p1/2 at 198.5 eV and 200.1 eV ascribed to Cl− species were observed for all these catalysts (Supporting Information Figure S8). The intensity of the Cl 2p peaks increased significantly after the modification of CeO2 nanorods


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by NiO. The quantification confirmed that the Cl/Ce molar ratio at surface region increased with NiO content up to 8 wt% (Table 3). A further increase in NiO content from 8.0 to 20 wt% only slightly increased the Cl/Ce molar ratio. This trend is essentially in agreement with that for the change of C3H6 selectivity with NiO content (Figure 3B), further indicating that the surface coverage of Cl− is a key factor in controlling C3H6 selectivity. We have characterized the 8 wt% NiO−CeO2 catalyst after 100 h of reaction. The content of Ni in the catalyst measured by the ICP-OES technique decreased from 8.0 to 7.1 wt% after 100 h of reaction. Only XRD peaks belonging to fluorite CeO2 were observed (Supporting Information Figure S9), probably suggesting that the NiO particles without incorporation into the CeO2 lattice leached out during the long-term reaction in the gas flow containing HCl, whereas the Ni species in the Ce−Ni−O solid solution was stable. The result that the XRD peaks became narrower indicates an increase in particle size after the reaction. The average sizes of crystallites estimated by the Scherrer equation were 8.3 and 21 nm for the fresh and the used catalysts, respectively. The specific surface area evaluated by N2 physisorption decreased from 40 to 21 m2 g-1 after the reaction. The rod morphology could be confirmed after the reaction from TEM image (Supporting Information Figure S9b), but the rod became much shorter and wider. In spite of this change, the HRTEM revealed that the catalyst after the reaction still mainly exposed {110} and {100} facets (Supporting Information Figure S9c). These may result in the stable performance of the 8 wt% NiO−CeO2 catalyst.

3.4. Role of HCl. To understand the role of HCl more deeply, we have investigated the effect of partial pressure of HCl, expressed as P(HCl), on catalytic performances for the oxidative conversion of C3H8. In the absence of HCl, CO2 was formed with a selectivity of 93% over CeO2


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(Figure 7A), confirming that CeO2 is a complete oxidation catalyst without HCl. O2 was almost completely consumed in the formation of CO2 and C3H8 conversion was 17%. The presence of HCl decreased the conversion of O2 but increased that of C3H8. The addition of HCl even with a low P(HCl) could significantly suppress the formation of CO2 and enhance the formation of C3H6. CO was also formed with a selectivity of 15-20% in the presence of HCl. The catalytic behavior of the 8 wt% NiO−CeO2 catalyst was somewhat different from that of CeO2 in the absence of HCl. Instead of CO2, CH4 became the major product for the (C3H8 + O2) reaction over the 8 wt% NiO−CeO2 catalyst (Figure 7B). The NiO−CeO2 catalyst had been reported for the oxidative dehydrogenation of C3H8 by O2 in a previous study, but high C3H6 selectivity could only be achieved at low temperatures and low C3H8 conversions.69 The increase in reaction temperature led to the formation of non-reoxidizable Ni0 particles probably due to the complete consumption of O2 at higher temperatures. Our XRD measurements confirmed the reduction of NiO into metallic Ni0 after the reaction in the absence of HCl (Supporting Information Figure S10). The metallic Ni0 particles may be responsible for the cracking reaction at high C3H8 conversions, forming CH4 and C2H4, and the later may be further oxidized into CO and CO2.69 The presence of HCl completely changed the reaction pattern and the product distribution became similar to that for CeO2. The increase in P(HCl) to > 2.5 kPa suppressed the conversion of O2, and thus could keep Ni species in oxidized state. The conversion of C3H8 and the selectivity of C3H6 increased. A C3H6 selectivity of 80% was obtained at C3H8 conversion of 69% at P(HCl) of 25 kPa, providing a single-pass C3H6 yield of 55%. These results clearly point out that HCl suppresses the reactivity of oxygen species on CeO2 or NiO−CeO2 catalyst for overoxidation and induces new active species for the selective conversion of C3H8 to C3H6.


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We further investigated the oxidation of HCl by O2 in the absence of C3H8, i.e., the Deacon reaction (eq. 9), over CeO2 nanorods and the 8 wt% NiO−CeO2 catalyst under reaction conditions similar to those employed for the conversion of C3H8. 2HCl + 1/2O2 → Cl2 + H2O

(9)

The conversions of HCl and O2, and the formation of Cl2 were observed (Table 4). CeO2 could catalyze the Deacon reaction. Our result showed that the NiO−CeO2 catalyst was more active for the conversion of HCl and the formation of Cl2. Thus, the modification of CeO2 by NiO promotes the Deacon reaction, probably through facilitating the activation of HCl, which is believed to be the most energy-demanding step for the Deacon reaction.55 We further measured the formation of Cl2 in the conversion of C3H8 by (O2 + HCl). The rate of Cl2 formation in the presence of C3H8 became negligibly smaller as compared to that in the absence of C3H8 over both CeO2 nanorods and the NiO−CeO2 catalyst (Table 4). This allows us to propose that the active chlorine species, which is generated during the activation of HCl on catalyst surfaces under O2, is used for the conversion of C3H8. We collected HCl in the product gas stream and measured the amount of Cl− collected in 3 h of C3H8 conversion by (O2 + HCl) over CeO2 nanorods and the 8 wt% NiO−CeO2 catalyst. Our result demonstrated that 96-97% HCl could be recovered after the reaction over both catalysts (Table 4), and the remaining small fraction of HCl ( 773 K). The comparison of catalytic performances of CeO2 and NiO−CeO2 catalysts at different temperatures demonstrates once again that the modification of CeO2 by NiO not only enhances C3H8 conversion but also increases C3H6 selectivity by suppressing the formation of CO and CO2. The dependence of catalytic behaviors on the pseudo contact time, which is expressed as the ratio of catalyst weight to total flow rate (W/F), at 773 K, is displayed in Figure 9. Over both CeO2 and the 8 wt% NiO−CeO2 catalysts, C3H8 conversion increased with the contact time almost linearly at shorter contact times and became saturated at higher ones because of the depletion of O2. At a short contact time of 0.063 s g cm-3, where the C3H8 conversion was < 6%, the C3H6 selectivities were 66% and 81% over CeO2 and NiO−CeO2 catalysts, respectively. This suggests that C3H6 is formed as the major primary product from C3H8 over both catalysts at 773 K. The increase in the contact time decreased the selectivity of C3H6 and increased those of CO and CO2. However, it is worth mentioning that the decrease in C3H6 selectivity with an increase in the contact time or the C3H8 conversion in the present system is milder than that in most of the


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catalytic systems reported for the oxidative dehydrogenation of C3H8.11 C3H6 selectivities of 57% and 70% could still be obtained at C3H8 conversions of 50% and 68% over CeO2 and NiO−CeO2, respectively. Figure 9 also suggests that CO and CO2 are probably formed via two paths. A small part of CO and CO2 is formed directly from C3H8, and the other part of CO and CO2 comes from the consecutive oxidation of C3H6. The selectivity of organic halides was quite low in the whole range of contact times in Figure 9, suggesting that organic halides may be decomposed to C3H6 very quickly at 773 K. To clarify whether organic chlorides are intermediates for C3H6 formation, we have performed the conversion of C3H7Cl on our catalysts. The results show that C3H7Cl is very reactive over both

CeO2

and

NiO−CeO2

catalysts

(Supporting

Information

Figure

S11).

The

dehydrochlorination of C3H7Cl initiated from ~425 K over CeO2, which was much lower than the starting temperature (~648 K) for the conversion of C3H8 by (O2 + HCl) (Figure 8). The 8 wt% NiO–CeO2 catalyst exhibited a higher activity for the dehydrochlorination of C3H7Cl than CeO2. C3H7Cl can be transformed stoichiometrically into C3H6 (yield > 96%) at 773 K over both CeO2 and the 8 wt% NiO−CeO2 catalysts. These results suggest that the dehydrochlorination of C3H7Cl proceeds much faster than the conversion of C3H8. This may result in a very low selectivity of C3H7Cl at high temperatures even if it is formed as a reaction intermediate in the conversion of C3H8. Actually, we observed the formation of organic chlorides over our catalysts particularly at lower temperatures (Figure 8). Therefore, organic chlorides may be formed as intermediates in the conversion of C3H8 to C3H6 by (O2 + HCl) over CeO2-based catalysts.

3.6. Structure-Property Relationships and Reaction Mechanism. Our studies on CeO2 nanocrystals with different morphologies or different exposed facets suggest that that the higher


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concentration of surface O-vacancies results in higher C3H8 conversion rate (Table 2). The modification of CeO2 nanorods by NiO with a proper content also increases the concentration of O-vacancies probably due to the incorporation of a part of Ni2+ into CeO2 lattice, and thus further increases the C3H8 conversion activity (Table 3 and Figure 3). To understand more deeply the formation of oxygen vacancies on our catalyst surfaces, we have calculated the formation energy of O-vacancies using the DFT method. The result shows that the formation energy of O-vacancies (∆Evac) decreases in the sequence of CeO2 {111} > CeO2 {100} > CeO2 {110} (Supporting Information Figure S12). This trend is in agreement with that reported in literature.58,59 Furthermore, our DFT calculations reveal that the addition of Ni2+ into any of the three facets of CeO2 can decrease the formation energy of O-vacancies. These calculation results can interpret the difference in the concentration of O-vacancies on CeO2 nanocrystals with different morphologies and the NiO−CeO2 catalysts. Our experimental results for CeO2 nanocrystals with different morphologies and NiO−CeO2 catalysts further demonstrate that the coverage of Cl− on catalyst surfaces is a key factor controlling the product selectivity. The coverage of Cl− (Table 2) and the C3H6 selectivity (Figure 1) on CeO2 nanocubes enclosed by {100} facets are both the highest among CeO2 nanocrystals with different morphologies. The modification of CeO2 nanorods by NiO further enhances the coverage of Cl− on catalyst surfaces (Table 3), leading to a further increase in C3H6 selectivity (Figure 3). It is reasonable to consider that the increase in Cl− coverage on catalyst surfaces may suppress the reactivity of lattice oxygen or adsorbed oxygen species, which may be responsible for over-oxidation on CeO2. A few previous studies have pointed out that the modification of metal oxide (such as MgO or Dy2O3-doped MgO) surfaces by alkali chlorides can suppress the reactivity of oxygen species, and thus increase the selectivity for the oxidative dehydrogenation


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product.12,74-76 However, the gradual loss of Cl− from the catalyst may occur under O2 gas flow at reaction temperatures (> 773 K). We also performed the effect of catalyst pretreatment by HCl and found that the pretreatment of CeO2 nanorods or the 8 wt% NiO−CeO2 catalyst by HCl could enhance the formation of C3H6 (Supporting Information Figure S13). However, such an enhancement could not be maintained in the absence of gaseous HCl because of the loss of Cl− during the reaction. Our studies have further suggested that the presence of HCl not only suppresses the reactivity of oxygen species for complete oxidation on CeO2 but also induces new active species for the selective conversion of C3H8 to C3H6 (Figure 7). Previous studies for chloride-modified metal oxide catalysts implied the generation of chlorine radicals in the presence of O2, which might be responsible for the oxidative chlorination of CH4 and the oxidative dehydrogenation of lower alkanes.12 Actually, we observed the considerable formation of Cl2 without C3H8, and the presence of C3H8 significantly inhibited the formation of Cl2, forming C3H6 as the major product (Table 4). Therefore, we speculate that an active chlorine species, possibly a Cl•-like species, is generated from HCl, which accounts for the selective conversion of C3H8 to C3H6 on our CeO2based catalysts. It is reasonable to consider that an oxidative species on CeO2 surfaces may oxidize the adsorbed Cl− into the Cl•-like species. In a previous work, we proposed that the redox of Ce4+/Ce3+ might be responsible for the oxidation of adsorbed Cl− species.36 However, the redox potential of Ce4+/Ce3+ [1.72 V versus SHE (the standard hydrogen electrode)77] is remarkably lower than that of Cl•/Cl− (2.41 V versus SHE78), suggesting that Ce4+ is unable to oxidize Cl− to Cl•. This has urged us to consider other possibilities. It is known that adsorbed oxygen species, in particular peroxide species (O22-), can be formed on CeO2 surfaces with abundant O-vacancies under O2 atmosphere.63 The adsorbed oxygen species may also work as


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the oxidative species. The redox potential of O22-/O2- (2.42 V versus SHE77) is slightly higher than that of Cl•/Cl−. Therefore, we speculate that it is not the surface Ce4+ but the adsorbed O22species accounts for the oxidation of Cl− to the active chlorine species. We characterized the adsorbed oxygen species using Raman spectroscopy. Raman band belonging to the O22- species on CeO2 surfaces typically appears at 830-835 cm-1.63 We clearly observed a Raman band at 831 cm-1 for CeO2 nanorods and nanocubes after pretreatment at 673 K in He for 1 h followed by adsorption of O2 (Figure 10A). This Raman band for CeO2 nanorods was stronger than that for nanocubes, while the band for CeO2 nano-octahedra or CeO2 nanoparticles was too weak to be observed. We further found that the Raman band at 831 cm-1 became significantly stronger after the modification of CeO2 nanorods with NiO (Figure 10B). The change in the intensity of this Raman band shows a similar trend to that in the concentration of O-vacancies (Tables 2 and 3), indicating that the O22- species may result from the adsorption of O2 on O-vacancy sites. Furthermore, a similar trend in the change of catalytic activity for the conversion of C3H8 has been observed (Table 2 and Figure 3). Therefore, the O-vacancy may control the catalytic activity by providing O22- species for the activation of HCl. Based on these results and discussion, we propose a reaction mechanism in Figure 11 for the selective conversion of C3H8 to C3H6 by (O2 + HCl) over CeO2-based catalysts. In brief, O2 is adsorbed on the O-vacancy site on CeO2 surfaces, forming O22- species. Our DFT calculations reveal that the formation of O22- species on CeO2 surfaces is an energy-favored step (Figure 12 and Supporting Information Figure S14). Then, HCl is oxidized by the O22- species to form Cl•like species. Cl2 would be formed through the coupling of two Cl•-like species in the absence of C3H8. The DFT calculation suggests that C3H8 can be activated by Cl• radical, forming C3H7•


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intermediate with an energy barrier of 0.91 eV (Figure 12 and Supporting Information Figure S14). CeO2-based catalysts also catalyzed the Deacon reaction in the absence of C3H8 (Table 4). In the presence of C3H8, the formation of Cl2 became negligible. A previous study reported that the desorption of Cl2 from CeO2 was endothermic (1.42 eV).56 Our calculation has shown that the energy barrier for the activation of C3H8 by surface Cl• species is 0.91 eV. We suggest that the surface Cl•-like species prefers to react with C3H8 rather than to couple and desorb into gas phase in the presence of C3H8. Therefore, we propose that the surface reaction between Cl•-like species and C3H8 is the main path for the formation of C3H6. However, we cannot completely exclude the gas-phase chain radical reaction path for C3H6 formation. In other words, Cl• species formed by dissociation of Cl2 released into gas phase might also participate in the activation of C3H8. Possibly, such a gas-phase reaction might occur as a minor reaction path particularly at high reaction temperatures. The C3H7• intermediate may be transformed into C3H6 via a direct path by removing another H atom with an energy barrier of 1.0 eV (Figure 12a). The indirect path via C3H7Cl can also produce C3H6 (Figure 12b). In this path, C3H7Cl is first formed by coupling C3H7• and Cl• radicals without energy barrier, followed by dehydrochlorination. The dehydrochlorination of C3H7Cl is a Lewis acid-base-catalyzed reaction. The DFT calculation reveals that the defective CeO2 surface with O-vacancies can efficiently catalyze the heterolytic cleavage of C−Cl bond and the β-H transfer from C3H7+ to the basic site of lattice O2- (Supporting Information Figure S15). Our experimental results also indicate that both paths may occur under our reaction conditions. At relatively lower temperatures (≤ 650 K), the formation of C3H7Cl has been observed with considerable selectivity (Figure 8), whereas C3H6 is formed as a major primary


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product at a relatively higher temperature (773 K, Figure 9). The experiment using C3H7Cl as a reactant also confirms that it can be converted to C3H6 almost quantitatively at 773 K over our catalysts (Supporting Information Figure S11). HCl is desorbed from the surface and the Ovacancy is regenerated in the final two steps.

4. Conclusions We developed a catalytic route for selective conversion of C3H8 to C3H6 by (O2 + HCl) with CeO2-based catalysts. CeO2 mainly catalyzed the complete oxidation of C3H8 to CO2 in the absence of HCl and the presence of HCl was essential for the selective formation of C3H6. HCl not only induced the selective formation of C3H6 but also enhanced the conversion of C3H8. Both the activity and selectivity depended on the exposed facet of CeO2 nanocrystals, and thus the reaction was structure sensitive. CeO2 nanorods exposing {110} and {100} facets were the most active for C3H8 conversion, while CeO2 nanocubes enclosed by {100} facets were the most selective for C3H6 formation. The modification of CeO2 nanorods with NiO further increased both C3H8 conversion and C3H6 selectivity. We achieved a C3H6 selectivity of 80% at C3H8 conversion of 69% over an 8 wt% NiO−CeO2 catalyst at 773 K. The catalyst was stable in 100 h. The structure-property correlation suggests that the concentration of surface O-vacancies and the surface coverage of Cl− are two crucial factors that determine the activity and selectivity. We propose that oxygen is adsorbed on the O-vacancy site on CeO2 surfaces, forming O22- species and the O22- species oxidizes Cl− into Cl•-like species, which accounts for the activation and selective conversion of C3H8 to C3H6.

ASSOCIATED CONTENT


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Supporting Information Details for the syntheses of CeO2 nanocrystals with different morphologies, Comparison of stability of Fe2O3 and CeO2, Catalytic properties and characterizations of CeO2 nanocrystals with different morphologies, Catalytic properties of CeO2 nanorods modified with transition metal oxides, Catalytic properties and characterizations of NiO−CeO2 catalysts with different NiO contents, Dehydrochlorination of C3H7Cl over CeO2 and 8 wt% NiO−CeO2 catalysts, Calculation of formation energy of oxygen vacancies, Conversion of C3H8 over catalysts with HCl prechemisorbed, Configurations of key intermediates in DFT calculations, Vibration frequencies for transition states.

AUHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Nos. 21373170, 21433008 and 91545203) and the National Basic Research Program of China (No. 2013CB933102).

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24 McFarland, E. Unconventional Chemistry for Unconventional Natural Gas. Science 2012, 338, 340-342. 25 Olah, G. A.; Gupta, B.; Farina, M.; Felberg, J. D.; Ip, W. M.; Husain, A.; Karpeles, R.; Lammertsma, K.; Melhotra, A. K.; Trivedi, N. J. Selective Monohalogenation of Methane over Supported Acid or Platinum Metal Catalysts and Hydrolysis of Methyl Halides over γAlumina-Supported Metal Oxide/Hydroxide Catalysts. A Feasible Path for the Oxidative Conversion of Methane into Methyl Alcohol/Dimethyl Ether. J. Am. Chem. Soc. 1985, 107, 7097-7105. 26 Zhou, X. P.; Yilmaz, A.; Yilmaz, G. A.; Lorkovic, I. M.; Laverman, L. E.; Weiss, M.; Sherman, J. H.; McFarland, E. W.; Stucky, G. D.; Ford, P. C. An Integrated Process for Partial Oxidation of Alkanes. Chem. Commun. 2003, 9, 2294-2295. 27 Lorkovic, I. M.; Yilmaz, A.; Yilmaz, G. A.; Zhou, X. P.; Laverman, L. E.; Sun, S. L.; Schaefer, D. J.; Weiss, M.; Noy, M. L.; Cutler, C. I.; Sherman, J. H.; McFarland, E. W.; Stucky, G. D.; Ford, P. C. A Novel Integrated Process for the Functionalization of Methane and Ethane: Bromine as Mediator. Catal. Today 2004, 98, 317-322. 28 Ding, K.; Zhang, A.; Stucky, G. D. Iodine Catalyzed Propane Oxidative Dehydrogenation Using Dibromomethane as an Oxidant. ACS Catal. 2012, 2, 1049-1056. 29 Ding, K.; Metiu, H.; Stucky, G. D. Interplay Between Bromine and Iodine in Oxidative Dehydrogenation. ChemCatChem 2013, 5, 1906-1910. 30 Ding, K.; Metiu, H.; Stucky, G. D. The Selective High-Yield Conversion of Methane Using Iodine-Catalyzed Methane Bromination. ACS Catal. 2013, 3, 474-477. 31 Testova, N. V.; Shalygin, A. S.; Kaichev, V. V.; Glazneva, T. S.; Paukshtis, E. A.; Parmon, V. N. Oxidative Dehydrogenation of Propane by Molecular Chlorine. Appl. Catal. A Gen.


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2015, 505, 441-446. 32 Upham, D. C.; Gordon, M. J.; Metiu. H.; McFarland, E. W. Halogen-Mediated Oxidative Dehydrogenation of Propane Using Iodine or Molten Lithium Iodide. Catal. Lett. 2016, 146, 744-754. 33 Wang, K. X.; Xu, H. F.; Li, W. S.; Zhou, X. P. Acetic Acid Synthesis from Methane by NonSynthesis Gas Process. J. Mol. Catal. A 2005, 225, 65-69. 34 Liu, Z.; Huang, L.; Li, W. S.; Yang, F.; Au, C. T.; Zhou, X. P. Higher Hydrocarbons from Methane Condensation Mediated by HBr. J. Mol. Catal. A 2007, 273, 14-20. 35 Lin, R.; Ding, Y.; Gong, L.; Dong, W.; Wang, J.; Zhang, T. Efficient and Stable SilicaSupported Iron Phosphate Catalysts for Oxidative Bromination of Methane. J. Catal. 2010, 272, 65-73. 36 He, J.; Xu, T.; Wang, Z.; Zhang, Q.; Deng, W.; Wang, Y. Transformation of Methane to Propylene: A Two-Step Reaction Route Catalyzed by Modified CeO2 Nanocrystals and Zeolites. Angew. Chem. Int. Ed. 2012, 51, 2438-2442. 37 Paunović, V.; Zichittella, G.; Moser, M.; Amrute, A. P.; Pérez-Ramírez, J. Catalyst Design for Natural-Gas Upgrading through Oxybromination Chemistry. Nat. Chem. 2016, 8, 803809. 38 Podkolzin, S. G.; Stangland, E. E.; Jones, M. E.; Peringer, E.; Lercher, J. A. Methyl Chloride Production from Methane over Lanthanum-Based Catalysts. J. Am. Chem. Soc. 2007, 129, 2569-2576. 39 Peringer, E.; Salzinger, M.; Hutt, M.; Lemonidou, A. A.; Lercher, J. A. Modified Lanthanum Catalysts for Oxidative Chlorination of Methane. Top. Catal. 2009, 52, 1220-1231. 40 Paunović, V.; Zichittella, G.; Verel, R.; Amrute, A. P.; Pérez-Ramírez, J. Selective


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Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate. Angew. Chem. Int. Ed. 2016, 55, 15619-15623. 41 Zichittella, G.; Paunović, V.; Amrute, A. P.; Pérez-Ramírez, J. Catalytic Oxychlorination versus Oxybromination for Methane Functionalization. ACS Catal. 2017, 7, 1805-1817. 42 Yu, F.; Wu, X.; Zhang, Q.; Wang, Y. Oxidative Dehydrogenation of Ethane to Ethylene in the Presence of HCl over CeO2-Based Catalysts. Chin. J. Catal. 2014, 35, 1260-1266. 43 Zichittella, G.; Aellen, N.; Paunović, V.; Amrute, A. P.; Pérez-Ramírez, J. Olefins from Natural Gas by Oxychlorination. Angew. Chem. Int. Ed. 2017, 56, 13670-13674. 44 Zichittella, G.; Puértolas, B.; Siol, S.; Paunović, V.; Mitchell, S.; Pérez-Ramírez, J. An Activated TiC-SiC Composite for Natural Gas Upgrading via Catalytic Oxyhalogenation. ChemCatChem 2018, 10, 1282-1290. 45 Mai, H.; Sun, L.; Zhang, Y.; Si, R.; Feng, W.; Zhang, H.; Liu, H.; Yan, C. Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109, 24380-24385. 46 Yan, L.; Yu, R.; Chen, J.; Xing, X. Template-Free Hydrothermal Synthesis of CeO2 Nanooctahedrons and Nanorods: Investigation of the Morphology Evolution. Cryst. Growth Des. 2008, 8, 1474-1477. 47 Amrute, A. P.; Mondelli, C.; Hevia, M. A. G.; Pérez-Ramírez, J. Temporal Analysis of Products Study of HCl Oxidation on Copper- and Ruthenium-Based Catalysts. J. Phys. Chem. C 2011, 115, 1056-1063. 48 Belcher, R.; Macdonald, A.M.G.; Parry, E. On Mohr’s Method for the Determination of Chlorides. Anal. Chim. Acta 1957, 16, 524-529. 49 VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J.


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QUICKSTEP: Fast and Accurate Density Functional Calculations Using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103-128. 50 Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 51 Wang, Y. G.; Mei, D.; Li, J.; Rousseau, R. DFT+U Study on the Localized Electronic States and Their Potential Role During H2O Dissociation and CO Oxidation Processes on CeO2(111) Surface. J. Phys. Chem. C 2013, 117, 23082-23089. 52 Lee, S. W.; Kim, D.; Won, H. J.; Chung, W. Y. Electrical Conductivity and Defect Structure of CeO2-ZnO System. Electron. Mater. Lett. 2006, 2, 53-58. 53 Wang, Y.; Mei, D.; Glezakou, V.; Li, J.; Rousseau, R. Dynamic Formation of Single-Atom Catalytic Active Sites on Ceria-Supported Gold Nanoparticles. Nat. Commun. 2015, 6, 65116518. 54 Pérez-Ramírez, J.; Mondelli, C.; Schmidt, T.; Schlüter, O. F. K.; Wolf, A.; Mleczko, L.; Dreier, T. Sustainable Chlorine Recycling via Catalysed HCl Oxidation: from Fundamentals to Implementation. Energy Environ. Sci. 2011, 4, 4786-4799. 55 Amrute, A. P.; Mondelli, C.; Hevia, M. A. G.; Pérez-Ramírez, J. Mechanism-Performance Relationships of Metal Oxides in Catalyzed HCl Oxidation. ACS Catal. 2011, 1, 583-590. 56 Amrute, A. P.; Mondelli, C.; Moser, M.; Novell-Leruth, G.; López, N.; Rosenthal, D.; Farra, R.; Schuster, M. E.; Teschner, D.; Schmidt, T.; Pérez-Ramírez, J. Performance, Structure, and Mechanism of CeO2 in HCl Oxidation to Cl2. J. Catal. 2012, 286, 287-297. 57 Li, Y.; Shen, W. Morphology-Dependent Nanocatalysts: Rod-shaped Oxides. Chem. Soc. Rev. 2014, 43, 1543-1574. 58 Huang, W.; Gao, Y. Morphology-Dependent Surface Chemistry and Catalysis of CeO2


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Nanocrystals. Catal. Sci. Technol. 2014, 4, 3772-3784. 59 Capdevila-Cortada, M.; Vilé, G.; Teschner, D.; Pérez-Ramírez, J.; López, N. Reactivity Descriptors for Ceria in Catalysis. Appl. Catal., B: Environ. 2016, 197, 299-312. 60 Luo, M.; Yan, Z.; Jin, L.; He, M. Raman Spectroscopic Study on the Structure in the Surface and the Bulk Shell of CexPr1-xO2-δ Mixed Oxides. J. Phys. Chem. B 2006, 110, 13068-13071. 61 Li, M.; Feng, Z.; Xiong, G.; Ying, P.; Xin, Q.; Li, C. Phase Transformation in the Surface Region of Zirconia Detected by UV Raman Spectroscopy. J. Phys. Chem. B 2001, 105, 81078111. 62 Chang, S.; Li, M.; Hua, Q.; Zhang, L.; Ma, Y.; Ye, B.; Huang, W. Shape-Dependent Interplay between Oxygen Vacancies and Ag-CeO2 Interaction in Ag/CeO2 Catalysts and Their Influence on the Catalytic Activity. J. Catal. 2012, 293, 195-204. 63 Wu, Z.; Li, M.; Howe, J.; Meyer III, H. M.; Overbury, S. H. Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595-16606. 64 Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Fundamentals and Catalytic Applications of CeO2-Based Materials. Chem. Rev. 2016, 116, 5987-6041. 65 Hu, Z.; Liu, X.; Meng, D.; Guo, Y.; Guo, Y.; Lu, G. Effect of Ceria Crystal Plane on the Physicochemical and Catalytic Properties of Pd/Ceria for CO and Propane Oxidation. ACS Catal. 2016, 6, 2265-2279. 66 Paier, J.; Penschke, C.; Sauer, J. Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment. Chem. Rev. 2013, 113, 3949-3985. 67 Wang, X.; Shen, M.; Wang, J.; Fabris, S. Enhanced Oxygen Buffering by Substitutional and Interstitial Ni Point Defects in Ceria: A First-Principles DFT+U Study. J. Phys. Chem. C


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2010, 114, 10221-10228. 68 Xu, J.; Xue, B.; Liu, Y. M.; Li, Y. X.; Cao, Y.; Fan, K. N. Mesostructured Ni-Doped Ceria as an Efficient Catalyst for Styrene Synthesis by Oxidative Dehydrogenation of Ethylbenzene. Appl. Catal. A 2011, 405, 142-148. 69 Jalowiecki-Duhamel, L.; Ponchel, A.; Lamonier, C.; D’Huysser, A.; Barbaux, Y. Relationship between Structure of CeNiXOY Mixed Oxides and Catalytic Properties in Oxidative Dehydrogenation of Propane. Langmuir 2001, 17, 1511-1517. 70 Tang, K.; Liu, W.; Li, J.; Guo, J.; Zhang, J.; Wang, S.; Niu, S.; Yang, Y. The Effect of Exposed Facets of Ceria to the Nickel Species in Nickel-Ceria Catalysts and Their Performance in a NO+CO Reaction. ACS Appl. Mater. Interfaces 2015, 7, 26839-26849. 71 Maitarad, P.; Han, J.; Zhang, D.; Shi, L.; Namuangruk, S.; Rungrotmongkol, T. StructureActivity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies. J. Phys. Chem. C 2014, 118, 9612-9620. 72 Wang, Y.; Zhu, A.; Zhang, Y.; Au, C. T.; Yang, X.; Shi, C. Catalytic Reduction of NO by CO over NiO/CeO2 Catalyst in Stoichiometric NO/CO and NO/CO/O2 Reaction. Appl. Catal. B 2008, 81, 141-149. 73 Shan, W.; Luo, M.; Ying, P.; Shen, W.; Li, C. Reduction Property and Catalytic Activity of Ce1-xNixO2 Mixed Oxide Catalysts for CH4 Oxidation. Appl. Catal. A 2003, 246, 1-9. 74 Wang, D.; Rosynek, M. P.; Lunsford, J. H. The Effect of Chloride Ions on a Li+−MgO Catalyst for the Oxidative Dehydrogenation of Ethane. J. Catal. 1995, 151, 155-167. 75 Kumar, C. P.; Gaab, S.; Müller, T. E.; Lercher, J. A. Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts. Top. Catal. 2008, 50, 156167.


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76 Gärtner, C. A.; van Veen, A. C.; Lercher, J. A. Oxidative Dehydrogenation of Ethane on Dynamically Rearranging Supported Chloride Catalysts. J. Am. Chem. Soc. 2014, 136, 12691-12701. 77 Lide, D. R.; Haynes, W. M. eds. CRC Handbook of Chemistry and Physics 2016-2017: a Ready-Reference Book of Chemical and Physical Data. Chemical Rubber Publishing Company, 2016, 5, 78-84. 78 Cheng, J.; Liu, X.; VandeVondele, J.; Sulpizi, M.; Sprik, M. Redox Potentials and Acidity Constants from Density Functional Theory Based Molecular Dynamics. Acc. Chem. Res. 2014, 47, 3522-3529.


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Table 1. Catalytic Performances of Metal Oxides for the Conversion of C3H8 by (O2 + HCl)ab conversion (%)

selectivity (%)

C3H6

catalyst

a

C3H8

O2

C3H6

C3H7Cl

C3H5Cl

C2H6

CO

CO2

yield (%)

blank

0

0

-

-

-

-

-

-

0

RuO2

25

93

2.4

0.2

0.1

0.07

28

70

0.6

Fe2O3

8.1

4.0

94

3.0

0.4

0.9

1.0

0.5

7.6

CuO

4.4

4.2

84

4.4

1.5

6.1

2.6

0.8

3.7

NiO

2.2

1.0

74

0.8

4.2

3.3

3.5

12

1.6

VOPO4

16

23

57

0.2

0.3

4.7

28

8.8

9.1

La2O3

3.1

2.8

72

0.3

7.0

5.0

5.0

11

2.3

CeO2

29

44

61

1.1

1.7

2.5

17

16

18

Eu2O3

10

13

75

1.5

0.6

4.6

8.3

9.1

7.5

Reaction conditions: catalyst, 0.10 g; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; F =

48 mL min-1; T = 773 K; time on stream, 3 h. b The carbon balance for each catalyst was better than 95%.


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ACS Catalysis

Table 2. Rates of C3H8 Conversion and C3H6 Formation as Well as Some Physical Properties for CeO2 Nanocrystals with Different Morphologiesa

CeO2 morphology

a

specific

reaction rate

surface areab

(mmol m-2 h-1)

exposed

ID/IF2gc

Cl/Ced

facet (m2 g-1)

r(C3H8)

r(C3H6)

nanorod

{110}+{100}

42

1.8

1.1

1.1

0.38

nanocube

{100}

21

1.4

0.90

0.98

0.44

nanooctahedron

{111}

17

0.68

0.40

0.67

0.24

nanoparticle

{111}

38

0.56

0.47

0.52

0.21

Reaction conditions: P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; F = 48 mL min-1; T

= 773 K; time on stream, 3 h. b Surface area of catalyst after reaction. cValues obtained from UV Raman measurements. d Surface Cl/Ce molar ratio estimated from XPS after adsorption of HCl at 773 K for 1 h.


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Table 3. The ID/IF2g Values Obtained from UV Raman and the Surface Cl/Ce Molar Ratios Estimated from XPS for NiO−CeO2 Catalysts with Different NiO Contents NiO content ID/IF2g

Cl/Cea

0

1.10

0.38

2

1.36

0.45

8

1.74

0.62

20

1.25

0.78

(wt%)

a

Surface Cl/Ce molar ratios estimated from XPS after adsorption of HCl at 773 K for 1 h.


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ACS Catalysis

Table 4. HCl Conversion and Cl2 Formation Rate in the Deacon Reaction and the C3H8 Conversion by (O2 + HCl) over CeO2 and 8 wt% NiO−CeO2 Catalystsa conversion (%) catalyst

selectivity (%)

r(Cl2)

reaction C3H8

HCl

O2

C3H6

CO

CO2

(mmol g-1 h-1)

CeO2 rod

(HCl + O2)

−

14

3.5

−

−

−

8.8

NiO−CeO2

(HCl + O2)

−

20

6.3

−

−

−

12

CeO2 rod

(HCl + O2 + C3H8)

38

2.6b

78

55

19

20

0.0050

NiO−CeO2

(HCl + O2 + C3H8)

52

3.4b

68

72

7.6

15

0.0062

a

Reaction condition: catalyst, 0.10 g; P(HCl) = 10 kPa; P(O2) = 18 kPa; P(C3H8) = 0 or 18 kPa;

F = 48 mL min-1; T = 773 K; time on stream, 3 h.

b

The conversion of HCl was mainly

contributed to the formation of organic chlorides. 96-97% HCl was recovered in the outlet gas from the conversion of C3H8 by (O2 + HCl).


ACS Catalysis 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

Figure 1. Catalytic performances of CeO2 nanocrystals with different morphologies. (A) C3H6 selectivity versus C3H8 conversion. (B) Selectivities of other products including organic chlorides (CnHmCl) and COx (CO and CO2) versus C3H8 conversion. Reaction conditions: catalyst 0.005-0.10 g; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; F = 48 mL min-1; T = 773 K; time on stream, 3 h.


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ACS Catalysis

Figure 2. Raman spectra for CeO2 nanocrystals with different morphologies. (A) Visible Raman spectra. (B) UV Raman spectra.


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Figure 3. Effect of NiO content on catalytic performances of NiO−CeO2 catalysts for the conversion of C3H8 by (O2 + HCl). (A) Conversion and yield. (B) Selectivity. Reaction conditions: catalyst 0.10 g; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; F = 48 mL min-1; T = 773 K; time on stream, 3 h.


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ACS Catalysis

Figure 4. Long-term stability of the 8 wt% NiO−CeO2 catalyst for the conversion of C3H8 by (O2 + HCl). Reaction conditions: catalyst 0.15 g; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; F = 48 mL min-1; T = 773 K.


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Figure 5. XRD patterns and HRTEM micrographs for NiO−CeO2 catalysts with different NiO contents. (A) XRD patterns. (B) HRTEM micrographs: (a) CeO2, (b) 2 wt% NiO−CeO2, (c) 8 wt% NiO−CeO2, (d) 20 wt% NiO−CeO2.


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ACS Catalysis

Figure 6. Raman spectra for the NiO−CeO2 catalysts with different NiO contents. (A) Visible Raman spectra. (B) UV Raman spectra.


ACS Catalysis 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

Figure 7. Effect of partial pressure HCl on catalytic performances for the conversion of C3H8 by (O2 + HCl). (A) CeO2. (B) 8 wt% NiO−CeO2. Reaction conditions: catalyst 0.10 g, F(total) = 48 mL min-1; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; time on stream, 3 h.


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ACS Catalysis

Figure 8. Effect of temperature on catalytic performances for the conversion of C3H8 by (O2 + HCl). (A) CeO2. (B) 8 wt% NiO−CeO2. Reaction conditions: catalyst 0.10 g, F(total) = 48 mL min-1; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; time on stream, 3 h.


ACS Catalysis 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

Figure 9 Effect of contact time on catalytic performances for the conversion of C3H8 by (O2 + HCl). (A) CeO2. (B) 8 wt% NiO−CeO2. Reaction conditions: catalyst 0-0.40 g; T = 773 K; F(total) = 48 mL min-1; P(C3H8) = 18 kPa; P(O2) = 18 kPa; P(HCl) = 10 kPa; time on stream, 3 h.


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ACS Catalysis

Figure 10. Raman spectra (λex = 514 nm) for CeO2 catalysts under O2 atmosphere. (A) CeO2 nanocrystals with different morphologies. (B) NiO−CeO2 catalysts.


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Figure 11. Proposed reaction mechanism for the oxidative dehydrogenation of C3H8 to C3H6 by O2 in the presence of HCl over CeO2-based catalysts.


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ACS Catalysis

Figure 12. Calculated energy profiles for key reaction intermediates for the activation of O2, HCl and C3H8 and the conversion of C3H8 to C3H6 on CeO2 surfaces. (a) Direct path for C3H7• to C3H6, (b) C3H7• to C3H6 via C3H7Cl. The configurations of key intermediates were displayed in Figure S14.


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79x129mm (300 x 300 DPI)


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ACS Catalysis

79x109mm (300 x 300 DPI)


ACS Catalysis 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

79x59mm (300 x 300 DPI)


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ACS Catalysis

142x255mm (300 x 300 DPI)


ACS Catalysis 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

79x129mm (300 x 300 DPI)


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ACS Catalysis

79x129mm (300 x 300 DPI)


ACS Catalysis 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

79x129mm (300 x 300 DPI)


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ACS Catalysis

79x129mm (300 x 300 DPI)


ACS Catalysis 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

79x129mm (300 x 300 DPI)


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ACS Catalysis

88x65mm (300 x 300 DPI)


ACS Catalysis 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

94x63mm (300 x 300 DPI)


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ACS Catalysis

47x26mm (300 x 300 DPI)


Oxidative Dehydrogenation of Propane to Propylene in the Presence

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