Elucidating structure-performance correlations in gas-phase selective ethanol oxidation and CO oxidation over metal-doped γ-MnO2

Chinese Journal of Catalysis 41 (2020) 1298–1310

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/chnjc

Article

Elucidating structure-performance correlations in gas-phase selective ethanol oxidation and CO oxidation over metal-doped γ-MnO2 Panpan Wang, Jiahao Duan, Jie Wang, Fuming Mei, Peng Liu * Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China

A R T I C L E

I N F O

Article history: Received 3 December 2019 Accepted 25 December 2019 Published 5 August 2020 Keywords: MnO2 Metal doping Ethanol oxidation Acetaldehyde Catalytic CO oxidation

A B S T R A C T

Despite of considerable efforts on the MnO2-based catalytic combustion, the different structural and component requirements of MnO2 for gas-phase selective oxidation and complete oxidation largely remain unknown. By comparing four types of MnO2 with different crystal structures (α, β, γ and δ), γ-MnO2 was found to be the most efficient catalyst for both aerobic selective oxidation of ethanol and CO oxidation. The structural effect of γ-MnO2 was further investigated by doping metal ions into the framework and by comparing the catalytic performance in the gas-phase aerobic oxidation of CO and ethanol. Among ten M-γ-MnO2 catalysts, Zn-γ-MnO2 showed the lowest temperature (160 °C) for achieving 90% CO conversion. The CO oxidation activity of the M-γ-MnO2 catalysts was found to be more relevant to the surface acidity-basicity than the reducibility. In contrast, surface reducibility has been demonstrated to be more crucial in the gas-phase ethanol oxidation. Cu-γ-MnO2 with higher reducibility and more oxygen vacancies of Mn2+/Mn3+ species exhibited higher catalytic activity in the selective ethanol oxidation. Cu-γ-MnO2 achieved the highest acetaldehyde yield (75%) and space-time-yield (5.4 g gcat–1 h–1) at 200 °C, which are even comparable to the results obtained by the state-of-the-art silver and gold-containing catalysts. Characterization results and kinetic studies further suggest that the CO oxidation follows the lattice oxygen-based Mars-van Krevelen mechanism, whereas both surface lattice oxygen and adsorbed oxygen species involve in the ethanol activation. © 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction Manganese dioxides (MnO2) have been widely used as heterogeneous catalysts for aerobic oxidations because of their high availability, tunable crystal structures, mixed Mn valence and high mobility of lattice oxygen [1,2]. Diverse MnO2 materials are formed by octahedral MnO6 units shared with corners or edges, which lead to various tunnel and layered structures (such as α-, β-, γ- and δ-MnO2, etc.) [3]. For gas-phase oxida-

tions, the crystal structure of MnO2 is suggested to play a crucial role in determining the catalytic performance, with the optimal crystal form varying by the different oxidations [4–7]. However, most studies for the gas-phase oxidation with crystalline MnO2 are focusing on the catalytic combustion of CO [4], formaldehyde [5,8] and aromatics [7]; the reports on the aerobic selective oxidation are scarce. Therefore, it is highly desired to develop efficient and practical MnO2 catalysts for gas-phase selective oxidation.

* Corresponding author. Tel: +86-27-87543032; Fax: +86-27-87543632; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (21673088, 21972050). DOI: 10.1016/S1872-2067(20)63551-3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 8, August 2020

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

Recently, gas-phase aerobic oxidation of ethanol to acetaldehyde (AC) has attracted wide attention, because this process is increasingly competitive to the traditional AC production by ethylene-based Wacker oxidation [9]. Although supported noble metal catalysts can achieve moderate to high AC yield [10–13], noble-metal-free oxide catalysts are more promising for practical application given their lower cost and easier regeneration [14]. MnO2 catalysts have been used for gas-phase aerobic oxidation of ethanol [15–22]. However, most of reports focused on the catalytic combustion of ethanol with excessive oxygen [17–22]. Only silver-doped α-MnO2 and the supported catalysts derived thereof were effective for the selective ethanol oxidation [16,23]. We have found that the unreducible metal-doped α-MnO2 (M-OMS-2), especially Na-OMS-2, could act as stable and efficient catalysts for the selective ethanol oxidation, achieving a comparable AC yield (~66% at 200 °C) to the Ag-α-MnO2 (~71% at 230 °C) [24]. However, besides the α-MnO2, other crystalline MnO2 materials have not yet been studied in the selective ethanol oxidation. Moreover, it is still unknown whether the phase structure effect of MnO2 on the catalytic selective oxidation is different from that on the catalytic complete oxidation. Undoubtedly, clarifying these points will provide rational guidance for designing improved MnO2 catalysts for selective oxidation. In the present work, four types (α, β, γ and δ) of MnO2 catalysts were initially prepared and evaluated in the gas-phase aerobic oxidation of ethanol and CO with one equivalent of O2. Interestingly, γ-MnO2 achieved the highest AC yield (~65%) and CO conversion (~100%) at 200 °C, which are apparently higher than the results obtained by the other three MnO2 catalysts. Because the catalytic oxidation capability of MnO2 can be modulated by metal doping [3,15,24–27], there are potential opportunities to further improve the catalytic activity of γ-MnO2 by framework modification with metal dopants. Catalytic CO oxidation can be employed as a complete oxidation to compare with the selective ethanol oxidation. To the best of our knowledge, there are no reports on the aerobic selective oxidation of ethanol or CO oxidation by using metal-doped γ-MnO2 materials [28]. In this sense, it would be interesting to clarify the difference in the structural and component requirements of γ-MnO2 for the selective ethanol oxidation and CO oxidation. Therefore, various M-γ-MnO2 (M = Cu2+, Zn2+, Mg2+, Co2+, Ni2+, Ca2+, Al3+, Fe3+, La3+) catalysts were prepared by a one-pot hydrothermal method and evaluated in the two oxidations. The results strongly evidenced that Zn-γ-MnO2 with the highest molar ratio of surface base/acid sites obtained the best activity in the CO oxidation, whereas Cu-γ-MnO2 with the highest surface reducibility achieved the highest AC yield (~75% at 200 °C) in the selective ethanol oxidation. Cu-γ-MnO2 can show higher activity than previously reported noble-metal-containing catalysts at lower temperature. Their kinetic behavior were also studied. 2. Experimental 2.1. Materials

1299

Various salts (KMnO4, MnSO4·H2O, (NH4)2S2O8, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Mg(NO3)2·6H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Ca(NO3)2·4H2O, Al(NO3)3·9H2O, Fe(NO3)3·9H2O, La(NO3)3·6H2O) with > 99% purity, acetaldehyde (99%) and ethanol (> 99.8%) were provided by Sinopharm Chemical Reagent Co., Ltd. CH3CH2OD (99 atom% D) was purchased from Sigma-Aldrich. 2.2. Preparation of catalysts α-MnO2 was prepared by oxidation of MnSO4·H2O with KMnO4 under refluxing, as describing in our previous paper [24]. β-MnO2 and γ-MnO2 were prepared by the hydrothermal reaction of MnSO4·H2O (3.38 g) and (NH4)2S2O8 (4.56 g) in deionized water (100 mL), which was transferred into an autoclave (200 mL), then heated at 140 °C for 12 h and at 90 °C for 24 h, respectively [27]. δ-MnO2 was synthesized from the oxidation of MnSO4·H2O (0.22 g) by KMnO4 (1.2 g) in deionized water (60 mL), which was hydrothermally heated at 220 °C for 24 h [4]. The resulting solid was filtered, washed and dried at 110 °C overnight. All metal-doped M-γ-MnO2 (M = Cu2+, Zn2+, Mg2+, Co2+, Ni2+, Ca2+, Al3+, Fe3+, La3+) catalysts were synthesized by using the similar procedure for the undoped γ-MnO2. The metal ion dopant with M/(Mn+M) ratio of 10 mol% was initially added with MnSO4·H2O to form a homogeneous mixture, followed by the above procedure. All catalysts were calcined at 300 °C in air before use. 2.3. Characterization of catalysts XRD patterns were collected on an Empyrean apparatus with Cu Kα radiation (40 mA and 40 kV). The metal ratio of M-γ-MnO2 was evaluated by X-ray fluorescence spectrometry (XRF-1800). The surface areas were analyzed by the BET method using a Micromeritics ASAP 2420-4MP apparatus. SEM micrographs were taken using a FEI Nova NanoSEM 450 microscope. TEM micrographs were collected on a FEI Tecnai G2 F30 electron microscope at an acceleration voltage of 300 kV. XPS analysis was performed with an AXIS-ULTRA DLD-600W spectrometer with Al Kα irradiation. The binding energies were adjusted by setting the C 1s peak of adventitious carbon at 284.5 eV. The surface acidity and basicity of M-γ-MnO2 catalysts were studied with the temperature-programmed desorption (TPD) of NH3 and CO2 on a Micrometrics AutoChem 2920II instrument, respectively. Typically, 50 mg sample was loaded in a quartz tube and heated in Ar at 350 °C for 2 h to diminish the release of O2 during TPD. After the sample was cooled to 100 °C, 0.3% NH3 in Ar or 10% CO2 in Ar with a flow rate of 50 mL/min were passed through the sample for 0.5 h. Then pure Ar was used to flush the sample at 100 °C for 1 h, which can remove the physically adsorbed NH3 or CO2. After that, the sample was heated from 100 to 400 °C with a ramp rate of 10 °C/min and a flow rate of 20 mL/min Ar. The NH3 or CO2 evolution was monitored by a thermal conductivity detector (TCD). The amount of desorbed NH3 or CO2 was calculated by using the peak areas and a calibration coefficient. H2-TPR experiments were con-

1300

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

ducted on the same instrument as TPD. After the sample (20 mg) was pretreated at 300 °C in Ar for 1 h and cooled to room temperature, it was treated with 10% H2 in Ar at a flow rate of 10 mL/min and heated to 600 °C at a ramp rate of 10 °C/min. The H2 consumptions were calculated by the peak area which was recorded by TCD. 2.4. Catalytic reaction Gas-phase aerobic oxidation of ethanol was performed on a fixed-bed reactor at atmospheric pressure. The detailed procedure is similar to our previous report [24]. Briefly, 60 mg catalyst (80–100 mesh) mixed with 600 mg α-Al2O3 (60–80 mesh) was pretreated with O2 at 300 °C for 1 h. Then the preheated gas mixture with 2.5 mL/min of ethanol in the gas phase and a molar ratio of ethanol/O2/N2 = 1/1/38 was passed by the catalyst bed with the gas-hourly-space-velocity (GHSV) of about 100000 mL gcat–1 h–1. GC-FID and GC-TCD were simultaneously employed to analyze the reaction products online. In all cases, the carbon balances closed at 100 ± 2%. Other catalyst particle sizes and weights were checked to exclude the presence of diffusion and mass transfer limitations. Blank experiments were also performed to exclude the activity contribution by the empty quartz reactor and α-Al2O3. Catalytic CO oxidation was carried out in a fixed-bed reactor with stainless steel tube (i.d. = 8 mm) under atmospheric pressure. Typically, 0.1 g M-γ-MnO2 (80–100 mesh) diluted with 0.5 g quartz sand (60–80 mesh) were loaded into the reactor. After the catalyst was pretreated with dry air at 300 °C for 1 h and cooled to the room temperature, a mixture of 1 vol% CO and 1 vol% O2 in N2 was introduced as the reactants at a flow rate of 50 mL/min. The resulting GHSV is about 30000 mL gcat–1 h–1. The reaction temperature was monitored by a thermal couple inside the catalyst bed and raised from room temperature to 250 °C by steps of 10–30 °C. The reactor was kept at each temperature for 15 min to obtain experimental data at steady state. The composition of the outlet gas was analyzed by GC (Fuli GC 9070) with a TDX-01 packed column. The carbon balance was near 100% in all cases. Similarly, control and blank experiments were performed to exclude the presence of diffusional limitations and background activity, respectively. 3. Results and discussion 3.1. Preparation and characterization of catalysts The XRD patterns of α-MnO2, β-MnO2, γ-MnO2 and δ-MnO2 (Fig. S1) are in good agreement with those of cryptomelane-type (JCPDS 42–1348), pyrolusite-type (JCPDS 24–0735), nsutite-type (JCPDS 14–0644), and birnessite-type (JCPDS 80–1098) MnO2, respectively. The average crystallite sizes and surface areas of the four types of MnO2 are listed in Table 1. The layered δ-MnO2 has the smallest crystallite size (~15.1 nm) and the highest surface area (97 m2/g), which is consistent with previous reports [5,27]. As shown in Fig. 1, the SEM images of α-, β- and γ-MnO2 show nanorod-like morphologies with widths of 10–200 nm and lengths of 200–2000 nm, whilst the

Table 1 Structural and textural properties of various MnO2 samples. Sample

ABET (m2/g)

Crystallite M/(Mn+M) b size a (mol%) (nm)

Crystal radius c (nm)

α-MnO2 62 25.2 — — β-MnO2 19 23.1 — — γ-MnO2 55 19.3 — — δ-MnO2 97 15.1 — — 57 22.8 0.9 0.087 Cu-γ-MnO2 Zn-γ-MnO2 64 21.4 0.7 0.088 Mg-γ-MnO2 51 19.7 0.8 0.086 Co-γ-MnO2 71 14.9 0.8 0.079 (L)/0.089 (H)d Ni-γ-MnO2 58 21.1 1.0 0.083 Ca-γ-MnO2 47 18.8 0.3 0.114 59 17.7 3.1 0.068 Al-γ-MnO2 Fe-γ-MnO2 125 13.8 9.7 0.069 (L)/0.079 (H)d La-γ-MnO2 56 20.6 0.3 0.117 a Estimated by Scherrer equation using the strongest XRD peak. b Calculated by the XRF results. c Shannon crystal radius of the doping metal with six-coordination [29]. d For low (L) and high (H) spin species.

δ-MnO2 shows a distinct nanosheet-like shape. Since various metal-doped M-γ-MnO2 (M = Cu2+, Zn2+, Mg2+, 2+ Co , Ni2+, Ca2+, Al3+, Fe3+, La3+) catalysts were prepared by the same “one-pot” hydrothermal method as the undoped γ-MnO2, the urchin-like nanosphere morphology was largely retained (see Fig. 1 and Fig. S2), with different lengths and widths of the nanorods. The average nanorod size of Fe-γ-MnO2 is below one-fifth that of the γ-MnO2, resulting in that the surface area of the former (125 m2/g) is much higher than the latter (55 m2/g). The surface areas of the other M-γ-MnO2 catalysts decreased from 71 m2/g for Co-γ-MnO2 to 47 m2/g for Ca-γ-MnO2. As seen in Table 1, all the resulting M/(Mn + M) ratios were lower than the theoretical 10 mol%, indicating that the dopant metal ions incapable of entering the framework of γ-MnO2 were removed by washing with water. It is known that the substituting degree of the octahedral MnO2 framework depends on the coordination and the crystal radius (CR) of dopant ions [29,30]. Only six-coordinated metal ions with similar CR sizes to those of Mn3+ (0.072 nm for low spin species, 0.078 for high spin species) and Mn4+ (0.067 nm) can be readily incorporated in the octahedral MnO2 framework [27]. In this study, various metal dopants with different CR sizes (Table 1) allow six-coordination, thus different degree of substitution could be expected. Evidently, doping with Fe3+, whose CR sizes (0.069 nm for low spin species and 0.079 nm for high spin species) are more close to the Mn3+/Mn4+ species, resulted in the highest Fe/(Mn + Fe) molar ratio (9.7 mol%). In contrast, doping La3+ rendered the lowest La/(Mn + La) ratio (0.3 mol%), as La3+ has the largest CR size (0.117 nm). The XRD patterns of all M-γ-MnO2 are shown in Fig. 2. Clearly, the nsutite structure was basically retained after metal doping, but some peaks for the doped M-γ-MnO2 shifted to lower angles. The Ca-γ-MnO2 with low doping content (0.3 mol%) showed similar 2θ shift as the Fe-γ-MnO2, suggesting that the presence of dopant in both the mixture solution and

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

1301

Fig. 1. SEM images of four types (α, β, γ and δ) MnO2 and representative M-γ-MnO2 samples.

(160)

(300)

(131) (230)

(031)

(120)

the final framework may render partial loss of nsutite structure symmetry of γ-MnO2 and some lattice expansion [31,32]. Furthermore, as shown in Table 1, the crystallize sizes of M-γ-MnO2 estimated from the Scherrer equation using the strongest (131) peak (2θ = 37.1o) varied by the metal dopants, with Cu-γ-MnO2 showing the biggest average crystallize size (22.8 nm). The Fe-γ-MnO2 showed the smallest crystallize size (13.8 nm), which is in good agreement with its highest surface area and the smallest nanorod observed by SEM. The reducibility of various M-γ-MnO2 samples was evaluated by H2-TPR (Fig. 3A). There are two main peaks between

Zn Cu Ni

Intensity (A.U.)

Co Fe La Al Ca Mg  -MnO2

20

30

40 2 Theta (degree)

50

Fig. 2. XRD patterns of various M-γ-MnO2 samples.

60

250–500 °C attributed to reduction of (i) MnO2 to Mn3O4 (via Mn2O3) and (ii) Mn3O4 to MnO [2,33], with the initial reduction starting from 125–200 °C. Evidently, except for La-γ-MnO2, all M-γ-MnO2 samples showed lower temperature for the first reduction peak than the undoped γ-MnO2, which may be ascribed to the weakening effect of the metal dopants (other than La3+) on Mn-O bonds [26,32]. These results point to the improved reducibility of M-γ-MnO2 materials after metal doping. Especially for Cu-γ-MnO2, the first reduction peak was shifted to 285 °C with the initial reduction starting as low as 125 °C, which can be due to the surface reduction of adsorbed oxygen species to form oxygen vacancies without framework degradation. This result suggests that doping with Cu2+ can greatly facilitate the reduction of γ-MnO2. On the contrary, the reduction of γ-MnO2 is markedly retarded by doping with La3+. As shown in Table 2, the measured H2 consumptions for all M-γ-MnO2 samples (7.7–10.7 mmol/g) are lower than the theoretical value (11.5 mmol/g) for the complete reduction of MnO2 to MnO, pointing to the mixed Mn valence in the framework. The NH3-TPD and CO2-TPD profiles of various M-γ-MnO2 are shown in Fig. 3B and 3C, respectively. The corresponding amount of surface acid/base sites is listed in Table 2. Clearly, the NH3/CO2-TPD profiles of the M-γ-MnO2 samples varied with the metal dopants. The peak at lower temperatures (~130 °C) could be ascribed to desorption at surface Brönsted acid sites (Mn-OH and M-OH groups) for NH3-TPD and surface weak base sites (such as O2‒, OH‒) species for CO2-TPD. The higher temperature peaks in the NH3-TPD and CO2-TPD curves are attributed to the stronger Lewis acid sites derived from the defective metal atoms [32,34] and the stronger base sites derived from lattice oxygen species (such as metal-O2‒ pairs and defective O2‒ anions) [35], respectively. Compared with the undoped γ-MnO2, doping with Fe3+, La3+, Co2+ and Al3+ resulted in a considerable increase of surface acidity and basicity, especially for stronger acid/base sites. Fe-γ-MnO2 with the highest specific

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

A 285

H2 consumption (A.U.)

125

H2-TPR

B

NH3-TPD

133

C

CO2-TPD

130

Zn

Zn

Zn

Cu

Cu

Cu

Ni

Ni

Ni

Fe La Al

Co

Intensity (A.U.)

Co

Fe La Al

Intensity (A.U.)

1302

Co Fe La Al Ca

Ca

Ca

Mg

Mg

Mg

 -MnO2

 -MnO2

 -MnO2

100 150 200 250 300 350o 400 450 500 Temperature ( C)

100

150

200 250 o Temperature ( C)

300

100

150

200 250 o Temperature ( C)

300

Fig. 3. H2-TPR (A), NH3-TPD (B) and CO2-TPD (C) profiles of various M-γ-MnO2 samples. Table 2 Chemical and catalytic properties of various MnO2 catalysts. STY g T90 d Conv. e Selec. e TOF f (°C) (%) (%) (h1) (h1) 225 58 99.3 α-MnO2 8.9 43 396 4.6 2.2 275 52 99.6 β-MnO2 9.5 16 138 3.9 2.0 185 65 99.4 γ-MnO2 10.7 49 472 4.8 2.4 190 45 49.3 δ-MnO2 8.5 95 906 3.4 0.8 170 75 99.6 Cu-γ-MnO2 9.5 43 495 5.6 2.8 160 68 96.0 9.8 40 529 5.1 2.5 Zn-γ-MnO2 185 62 99.3 Mg-γ-MnO2 9.6 43 422 4.6 2.3 205 70 99.4 Co-γ-MnO2 9.7 75 632 5.2 2.6 200 72 99.5 Ni-γ-MnO2 9.2 45 391 5.4 2.7 175 71 99.5 Ca-γ-MnO2 10.1 50 548 5.3 2.7 195 60 99.2 Al-γ-MnO2 8.3 69 634 4.5 2.2 180 56 93.8 7.7 87 875 4.2 2.0 Fe-γ-MnO2 240 49 99.1 3.6 La-γ-MnO2 10.3 80 643 1.8 a Estimated by H2-TPR. b By NH3-TPD. c By CO2-TPD. d T90 stands for the temperature at which CO conversion reaches 90%. e Ethanol conversion and acetaldehyde selectivity at 200 °C. f Turnover frequency (TOF) based on ethanol conversion, and given in molethanol molMn‒1 h‒1. g Space-time-yield (STY) of acetaldehyde in gAC gcat‒1 h‒1. Sample

H2 uptake a (mmol/g)

Acidity b (μmol/g)

Basicity c (μmol/g)

surface area possessed the highest surface acidity (87 μmol/g) and basicity (875 μmol/g). For all samples, the surface basic sites are more than the acidic sites by about an order of magnitude. This suggests that the basic oxygen species are predominant on the surface [24], which leads to the alkaline nature of the M-γ-MnO2 materials. 3.2. Effect of crystal structure of MnO2 on the catalytic performance We initially investigated the influence of crystal structure of MnO2 on the catalytic performance for the aerobic ethanol oxidation and CO oxidation with one equivalent of O2 (Fig. 4). Interestingly, the activity order in ethanol oxidation varied with reaction temperature, but even so, γ-MnO2 exhibited the highest catalytic activity in the range of 125225 °C. As listed in Table 2, the turnover frequency (TOF) and space-time-yield (STY) at 200 °C increased in the order of δ-MnO2 < β-MnO2 <

α-MnO2 < γ-MnO2, and the AC selectivity of various catalysts maintained > 99% except for δ-MnO2, which yielded CO2 (selectivity ~40%) and ethyl acetate (selectivity ~10%) as by-products. Above 200 °C, CO2 became the main by-product for all catalysts. This activity order is totally different from previous report on the catalytic ethanol combustion [36], in which the optimal catalyst was α-MnO2 followed by δ-, γ- and β-MnO2. This confirms that there are different structure effects of MnO2 on the aerobic selective oxidation and complete oxidation of ethanol. Furthermore, a different activity order was also observed for CO oxidation, increasing with β-MnO2 < α-MnO2 < δ-MnO2 ~ γ-MnO2. Clearly, γ-MnO2 and β-MnO2 showed the lowest (185 °C) and highest (~275 °C) temperature for reaching 90% CO conversion, respectively. The different activity orders in the selective ethanol oxidation and CO oxidation further point to the different structural requirements of MnO2 in the two oxidations. It is noteworthy that α- and δ-MnO2 were reported to show higher activity than γ- and β-MnO2 in the pre-

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

MnO2

80

MnO2 MnO2

60 40 20

B100 AC selectivity (%)

MnO2

80 60 40 20 0 100 125 150 175 o200 225 Temperature ( C)

C100

D H2 consumption (A.U.)

0 100 125 150 175 o200 225 Temperature ( C)

CO conversion (%)

Ethanol conversion (%)

A100

80 60 40 20 0

50

100 150 200o 250 Temperature ( C)

1303

150 MnO2 MnO2 MnO2 MnO2

100

200 300 400 o Temperature ( C)

500

Fig. 4. Ethanol conversion (A), acetaldehyde selectivity (B) and CO conversion (C) as a function of reaction temperature, and H2-TPR profiles (D) of MnO2 catalysts with four crystal structures.

vious report on MnO2-catalyzed CO oxidation [4], which may be due to the different catalyst preparation, pretreatment and reaction conditions. Undoubtedly, γ-MnO2 showed the highest activity not only in the selective ethanol oxidation but also in the CO oxidation in this study. Given the moderate surface area of γ-MnO2, it could be deduced that the surface area was not the main factor determining the catalytic activity of MnO2 catalysts. According to the H2-TPR results of the four catalysts (Fig. 4D), β-MnO2 and δ-MnO2 showed lower temperature for the first reduction peak (~300 °C), but γ-MnO2 exhibited the lowest starting reduction temperature (~150 °C). These results imply that the catalytic activity in ethanol oxidation depends not on the bulk reducibility but on the surface reducibility of MnO2. The higher surface reducibility can render more oxygen vacancies for O2 activation at lower temperature and benefit the oxidations [24,37]. Therefore, the drastic activity increase observed by β-MnO2 above 175 °C could be primarily due to the formation of more surface oxygen vacancies during ethanol oxidation. Although δ-MnO2 also showed lower starting reduction temperature (~175 °C), its selectivity to CO2 dramatically increased above 175 °C. This may be ascribed to the presence of stronger surface acid sites in δ-MnO2 (Table 2). Evidently, the sample with the highest surface acidity and basicity (δ-MnO2) showed much lower activity than that with moderate surface acidity and basicity (γ-MnO2) in the ethanol oxidation, whereas the former showed comparable activity to the latter in the CO oxidation. These results imply that both the surface reducibility and the surface acid-base property can influence the catalytic perfor-

mance of MnO2 in the oxidation of ethanol and CO. Since the surface acid-base and redox properties of the optimal γ-MnO2 catalyst can be tuned by metal-doping, there are great opportunity to further improve the catalytic performance and to clarify the different structural and component requirements of M-γ-MnO2 for the selective ethanol oxidation and CO oxidation, which would provide an interesting example for the comparative study on these two oxidations using the same catalyst series [38]. 3.3. Effect of metal doping in γ-MnO2 on the catalytic performance 3.3.1. CO oxidation The CO conversions as a function of temperature over various M-γ-MnO2 catalysts are shown in Fig. 5A and 5B. Almost all catalysts achieved ~100% CO conversion below 250 °C. Obviously, all M-γ-MnO2 (except for La-γ-MnO2) showed higher activity than the undoped γ-MnO2 below 150 °C, plausibly due to the presence of more surface defect sites after metal doping as suggested by the H2-TPR results (Fig. 3A). To better compare the activity, T90 was used to identify the temperature at which CO conversion reaches 90% over various catalysts. As listed in Table 2, the T90 value decreased in the order of metal dopant La > Co > Ni > Al > γ ~ Mg > Fe > Ca > Cu > Zn, with Zn-γ-MnO2 and La-γ-MnO2 showing the lowest (160 °C) and highest (240 °C) T90, respectively. Fig. 5C and 5D show the Arrhenius plots and the estimated apparent activation energy (Ea) between room temperature and 100 °C. The listed Ea values (13.726.0

1304

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

A

100

B -MnO2

100

-MnO2

80

Fe--MnO2

Ca--MnO2 Al--MnO2

CO conversion (%)

CO conversion (%)

Mg--MnO2

La--MnO2

60

Zn--MnO2

40 20 0

Co--MnO2

80

Ni--MnO2 Cu--MnO2

60 40 20

50

100 150 o Temperature ( C)

200

0

250

C

50

100 150 o Temperature ( C)

200

250

D -1

-1

ln(k)

ln(k)

-2 -MnO2 (Ea = 19.7 kJ/mol)

-3

-2 -MnO2 (Ea = 19.7 kJ/mol)

Mg--MnO2 (Ea = 19.0 kJ/mol)

Fe--MnO2 (Ea = 16.3 kJ/mol)

Ca--MnO2 (Ea = 15.4 kJ/mol)

Co--MnO2 (Ea = 24.5 kJ/mol)

Al--MnO2 (Ea = 20.3 kJ/mol)

Ni--MnO2 (Ea = 23.1 kJ/mol)

La--MnO2 (Ea = 26.0 kJ/mol)

-4

Zn--MnO2 (Ea = 13.7 kJ/mol)

2.7

2.8

2.9 3.0 -1 1000/T (K )

-3

3.1

3.2

Cu--MnO2 (Ea = 14.5 kJ/mol)

2.7

2.8

2.9 3.0 -1 1000/T (K )

3.1

3.2

Fig. 5. CO conversion (A, B) as a function of the reaction temperature, and Arrhenius plots (C, D) for CO oxidation over various M-γ-MnO2 catalysts.

kJ/mol) are in good agreement with the previously reported Ea values (1285 kJ/mol) for CO oxidation over metal doped α-MnO2 catalysts [3]. Interestingly, the decreasing order of the Ea value is very similar to that of the T90, with Zn-γ-MnO2 showing the lowest Ea and La-γ-MnO2 showing the highest Ea. Therefore, among various M-γ-MnO2, Zn-γ-MnO2 is the most efficient catalyst for CO oxidation and enables O2 and CO activation at lower temperatures. To establish the structure-performance relationship in the M-γ-MnO2-catalyzed CO oxidation, we attempted to relate the catalytic activity with the structural and chemical properties of the catalysts. However, the optimal Zn-γ-MnO2 is not the catalyst with the highest surface area, reducibility, acidity or basicity, implying that the CO oxidation activity is not determined by one factor but influenced by the factors comprehensively. As listed in Table 2, the Zn-γ-MnO2 showed the lowest surface acid sites (40 μmmol/g), a moderate surface base sites (529 μmmol/g) but the highest molar ratio of surface base/acid sites (13.2). In contrast, the La-γ-MnO2 showed the lowest surface base/acid sites ratio (8.0). In an attempt to correlate the ratio of surface base/acid sites with the catalyst activity, we compared the orders of the ratio of surface base/acid sites and the Ea of CO oxidation (Fig. 6). Interestingly, the Ea values decrease with increasing the molar ratio of surface base/acid sites, suggesting that the less acid sites and more base sites in the M-γ-MnO2 would benefit the CO oxidation. Doping with Zn2+

can significantly decrease the surface acid sites and increase the base sites in the γ-MnO2, and thus may facilitate the CO adsorption and activation. 3.3.2. Ethanol oxidation The catalytic performances of various M-γ-MnO2 catalysts in the gas-phase ethanol oxidation as the function of temperature are shown in Fig. 7. Obviously, all M-γ-MnO2 catalysts showed high AC selectivity (up to 99%) below 200 °C and a dramatic selectivity decrease above 200 °C with CO2 as the predominant by-product. It is evident that both reducible metals (such as Cu2+, Ni2+, Co2+) and unreducible metals (such as Ca2+, Zn2+) doped M-γ-MnO2 catalysts can render higher activity than the undoped γ-MnO2. As shown in Table 2, the ethanol conversion at 200 °C increased in the order of metal dopant La < Fe < Al < Mg < γ < Zn < Co < Ca < Ni < Cu, with Cu-γ-MnO2 and La-γ-MnO2 achieving the highest (75%) and lowest (49%) conversion, respectively. The maximum AC yield at 200 °C was achieved by Cu-γ-MnO2 (~75%), accompanied by the highest TOF (5.6 molethanol molMn‒1 h‒1) and STY (2.8 gAC gcat‒1 h‒1). The STY value of Cu-γ-MnO2 was compared with that of the previously reported catalysts for the gas-phase selective ethanol oxidation (see Table S1). Evidently, the Cu-γ-MnO2 shows higher STY of AC than the previously reported Na-OMS-2 (2.5 gAC gcat‒1 h‒1 at 200 °C) [24] and the noble-metal-containing Ag-α-MnO2 (2.5 gAC gcat‒1 h‒1 at 230 °C) [16] and Au/MgCuCr2O4 (2.8 gAC gcat‒1

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

La--MnO2 Co--MnO2 Ni--MnO2 Al--MnO2 -MnO2

Mg--MnO2

20

Fe--MnO2 Ca--MnO2 Cu--MnO2 Zn--MnO2

15

10 12 Mole ratio of surface base/acid sites

14

Fig. 6. Correlation between Ea of CO oxidation and molar ratio of surface base/acid sites of various M-γ-MnO2 catalysts.

h‒1 at 250 °C) [12], confirming the enhancement effect of metal doping in γ-MnO2 on gas-phase selective ethanol oxidation. As expected, the Ea values (40.096.7 kJ/mol), evaluated from the Arrhenius plots in the range of 100150 °C (Fig. 7E and 7F), are in inverse proportion to the catalytic activity of M-γ-MnO2. The Cu-γ-MnO2 showed the lowest Ea (40.0 kJ/mol), which is much lower than the undoped γ-MnO2 (58.8 kJ/mol) and the Na-OMS-2 (63.7 kJ/mol) [24], suggesting that ethanol and oxygen are more prone to activation on Cu-γ-MnO2 at lower temperatures.

80 60

Al--MnO2 La--MnO2

80 60

150 175 200 o Temperature ( C)

225

80

-4

Mg--MnO2 Ca--MnO2

60

Al--MnO2 La--MnO2 Zn--MnO2

100

125

150 175 200 o Temperature ( C)

Co--MnO2 Ni--MnO2 Cu--MnO2

20

-MnO2 (Ea = 58.8 kJ/mol)

-6

Mg--MnO2 (Ea = 60.5 kJ/mol)

-7

Ca--MnO2 (Ea = 48.1 kJ/mol)

-9

Al--MnO2 (Ea = 63.5 kJ/mol) La--MnO2 (Ea = 96.7 kJ/mol) Zn--MnO2 (Ea = 56.2 kJ/mol)

2.4

225

2.5 2.6 -1 1000/T (K )

2.7

F

100

Fe--MnO2

-5

-8

D

-MnO2

40

0 100

-MnO2

40 125

-2 -3

Ca--MnO2

20

100

Ethanol conversion (%)

Mg--MnO2

40

B

E

100

Zn--MnO2

0 100

The TEM images of the Zn-γ-MnO2 catalyst before and after use in the CO oxidation are shown in Fig. S3. Clearly, the nanorod-like morphology remained unchanged. The recycled catalyst displayed the lattice fringes of (120) planes and (131) planes with the d spacing distances of 0.39 nm and 0.24 nm, respectively, which are in good agreement with the XRD results for the fresh Zn-γ-MnO2 sample. It is worth noting that there are many defect sites at the surface and the edge of the

C

-MnO2

AC selectivity (%)

Ethanol conversion (%)

100

AC selectivity (%)

A

3.4. Stability of M-γ-MnO2 catalysts

ln(k)

8

The activity order of M-γ-MnO2 catalysts in ethanol oxidation is clearly different from that in CO oxidation, confirming that MnO2-catalyzed different oxidations have different structure-performance relationships. Although the Fe-γ-MnO2 possesses the highest surface area, acidity and basicity among various M-γ-MnO2 catalysts, it shows even lower activity than the undoped γ-MnO2. Given that the Cu-γ-MnO2 shows the highest reducibility and activity, whereas the La-γ-MnO2 shows the lowest reducibility and activity, it is reasonable to propose that the surface reducibility is more important than the other properties of M-γ-MnO2 catalysts in the gas-phase selective oxidation of ethanol. As shown in Fig. 3A, the surface reduction of Cu-γ-MnO2 started as low as 125 °C, implying that more surface adsorbed oxygen species or oxygen vacancies are present in the catalyst. It is known that the higher MnO2 reducibility and more surface oxygen vacancies would benefit the O2 and ethanol activations [22,37]. Accordingly, the enhanced catalytic performance after metal (Cu2+, Ni2+, Ca2+, Co2+, Zn2+) doping could be mainly attributed to the increased surface oxygen vacancies in the doped γ-MnO2.

-2 -MnO2

80

-3

Fe--MnO2 Co--MnO2

ln(k)

Ea of CO oxidation (kJ/mol)

25

1305

Ni--MnO2

60

Cu--MnO2

-4 -5 -6

40

-7

-MnO2 (Ea = 58.8 kJ/mol)

Fe--MnO2 (Ea = 65.1 kJ/mol) Co--MnO2 (Ea = 51.8 kJ/mol) Ni--MnO2 (Ea = 45.1 kJ/mol) Cu--MnO2 (Ea = 40.0 kJ/mol)

2.5 2.6 2.7 -1 1000/T (K ) Fig. 7. Ethanol conversion (A, B) and acetaldehyde selectivity (C, D) as a function of the reaction temperature, and Arrhenius plots (E, F) for ethanol oxidation over various M-γ-MnO2 catalysts. 125

150 175 200 o Temperature ( C)

225

100

125

150 175 200 o Temperature ( C)

225

2.4

1306

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

B O 1s

Fresh Zn-MnO2 Spent MnO2

529.2 530.7

Intensity (A.U.)

Spent Zn-MnO2

Spent Zn-MnO2 Fresh Zn-MnO2

Spent MnO2

Fresh MnO2

Fresh MnO2

655

536

650 645 640 Binding Energy (eV)

C Mn 2p

Intensity (A.U.)

641.6 642.9 640.3 644.3

640.3 641.6 642.9 644.3

Spent Cu-MnO2 Fresh Cu-MnO2 Spent -MnO2 Fresh MnO2

655

650 645 640 Binding Energy (eV)

534 532 530 528 Binding Energy (eV)

D O 1s

529.3 532.8

Intensity (A.U.)

Intensity (A.U.)

A Mn 2p

531.0

Spent Cu-MnO2 Fresh Cu-MnO2 Spent MnO2

Fresh MnO2

536

534 532 530 528 Binding Energy (eV)

Fig. 8. XPS spectra of the representative M-γ-MnO2 catalysts before and after CO oxidation (A,B) and ethanol oxidation (C,D). (A,C) Mn 2p; (B,D) O 1s.

nanorods, which would be the active sites for the CO oxidation. To clarify the redox property of M-γ-MnO2 in the CO oxidation, the oxidation states of Mn and O in the fresh and spent Zn-γ-MnO2 and γ-MnO2 catalysts were studied by XPS (Fig. 8A and 8B). The Mn 2p3/2 spectra can be deconvoluted into four peaks locating at ca. 640.3, 641.6, 642.9 and 644.3 eV, which can be ascribed to Mn2+, Mn3+, Mn4+ and a satellite, respectively [24,39,40]. Due to the partial decomposition of MnO2 to Mn2O3 and Mn3O4 upon calcination at 300 °C, the fraction of Mn2+ species (Mn2+/Mn ratio) in the fresh samples reached about 10%. After the CO oxidation, the fractions of surface Mn2+ (~ 10%), Mn3+ (~51%) and Mn4+ (~25%) species remained almost unchanged. In line with this, the fractions of surface lattice oxygen (~529.2 eV) and adsorbed oxygen (~530.7 eV) species [33,41] kept ~ 69% and ~ 30% before and after the reaction, respectively. However, the surface O/Mn ratio increased notably from 1.65 to 1.96 for γ-MnO2 and from 2.07 to 2.37 for Zn-γ-MnO2, pointing to the enhanced ratio of surface base/acid sites in the recycled catalysts. The higher surface O/Mn ratio for the fresh Zn-γ-MnO2 is consistent with the NH3/CO2-TPD results and can account for its much higher activity than the undoped γ-MnO2 at lower temperatures (< 200 °C). Based on the above characterization and activity results, we can conclude that surface acidity-basicity is more important than the reducibility of M-γ-MnO2 in the CO oxidation. The M-γ-MnO2 catalysts can maintain their defect-enriched structure without framework

degradation during CO oxidation, and the reaction may follow the Mars-van Krevelen (MVK) mechanism starting from CO activation on surface lattice oxygen with the consumed lattice oxygen being replenished by O2 rapidly [3,42]. Continuous gas-phase oxidation of ethanol was carried out at 200 °C to evaluate the stability of M-γ-MnO2 catalysts (Fig. 9). To our delight, the four representative catalysts can keep stable without significant decrease in activity and selectivity for 30 h on-stream. The AC selectivity was maintained over 99% for all catalysts. The ethanol conversion of γ-MnO2 and Cu-γ-MnO2 decreased slightly from 65% to 64% and from 75% to 74%, respectively. These results suggest that Cu-γ-MnO2 can serve as a stable and efficient catalyst for the gas-phase selective oxidation of ethanol. The TEM images of the fresh and recycled Cu-γ-MnO2 catalysts (see Fig. S3) point to the unchanged nanorod-like morphology with clear lattice fringes of (120) planes and (131) planes, but many defects occur at the surface and edge. The XPS spectra of the fresh and spent Cu-γ-MnO2 and γ-MnO2 catalysts (Fig. 8C and 8D) verify that the surface Mn2+ fraction significantly increased from 10% to 44% for the spent γ-MnO2 and from 17% to 49% for the spent Cu-γ-MnO2. Moreover, the fractions of Mn3+ and Mn4+ species decreased somewhat, pointing to the partial reduction of surface Mn3+ and Mn4+ species to Mn2+ during ethanol oxidation. In line with this, the surface lattice oxygen (~529.3 eV) fraction decreased from 70% to 64% for the spent γ-MnO2 and from 68% to 65% for the

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

Ethanol conversion

40

 -MnO2

Cu--MnO2 La--MnO2

20

Zn--MnO2 AC selectivity

0 0

5

10 15 20 Time on stream (h)

25

30

Fig. 9. Ethanol conversion and acetaldehyde selectivity as a function of time on-stream using representative M-γ-MnO2 catalysts at 200 °C.

spent Cu-γ-MnO2, whilst the fraction of surface oxygen vacancies (~531.0 eV) increased from 24% to 28% for the spent γ-MnO2 and from 25% to 30% for the spent Cu-γ-MnO2. The more Mn2+ species and oxygen vacancies observed in the spent Cu-γ-MnO2 is in good agreement with its higher surface reducibility. Moreover, these observations confirm that increasing the defective Mn2+ sites and oxygen vacancies in the γ-MnO2 framework could enhance the catalytic oxidation activity plausibly by promoting the reactivity and mobility of active oxygen species [24,37]. 3.5. Reaction kinetics for selective ethanol oxidation over Cu-γ-MnO2 To further understand the reaction kinetics and mechanism for the selective ethanol oxidation over M-γ-MnO2, the influence of reaction conditions including reactant concentration

B

100

1.0

80

0.8

80

0.8

60

0.6

60

0.6

-1

AC selectivity

40

o

125 C o 200 C

20 0

Ethanol conversion

0

2 4 6 8 10 O2 concentration (vol%)

0.4 0.2 12 0.0

AC selectivity

40

o

125 C o 200 C

20 0

C

Ethanol conversion

1

0.4 0.2

2 3 4 5 0.0 Ethanol concentration (vol%)

6

100

-1

1.0

Reaction rate (mmol h ) Conv. & Selec.(%)

100

Conv. & Selec. (%)

A

-1 -1

60

80 4 60 40 Ethanol conversion AC selectivity

20 0

Space time yield (gAC gcat h )

Conv. & Selec. (%)

80

and GHSV on the catalytic performance of Cu-γ-MnO2 was studied. Fig. 10A shows the effect of O2 concentration at 2.5 vol% ethanol concentration and a GHSV of 100000 mL gcat‒1 h‒1. Evidently, Cu-γ-MnO2 achieved 47% ethanol conversion at 200 °C in the absence of O2, which is much higher than the previously reported Na-OMS-2 (< 5% at 200 °C) [24], pointing to the higher reactivity and mobility of the lattice oxygen in the former case. The reaction rate and AC selectivity were almost not influenced by O2 concentrations (1.2512.5 vol%) when O2 was fed at 125 °C. These results suggest that the number of surface oxygen vacancies for O2 activation keeps about the same level at lower reaction temperature [43]. When more than 2.5 vol% O2 was fed at 200 °C, the ethanol conversion increased steadily, but the AC selectivity decreased drastically. This can be explained by the presence of more Mn2+ defect sites and oxygen vacancies at 200 °C, which would enhance the reactivity of surface oxygen species and lead to complete oxidation of ethanol to CO2. The effect of ethanol concentration on the catalytic performance of Cu-γ-MnO2 at 2.5 vol% O2 concentration and a GHSV of 100000 mL gcat‒1 h‒1 is shown in Fig. 10B. Clearly, the ethanol conversion decreased steadily from 18% to 6% at 125 °C and from 90% to 52% at 200 °C with the ethanol concentration increasing from 1 vol% to 5 vol%, whilst the AC selectivity kept stable (up to 99%). The reaction rates were nearly independent of ethanol concentration at 125 °C. This kinetic behavior suggest that the active sites at the catalyst surface can be almost saturated with ethanol-derived intermediates (e.g., C2H5O-) at lower ethanol concentration (~2 vol%). Fig. 10C shows the effect of GHSV on the catalytic behavior of the Cu-γ-MnO2 catalyst at 200 °C. The GHSV was changed by the amount of catalyst, while keeping ethanol (2.5 vol%) and O2 (2.5 vol%) and the total flow rate (100 mL h‒1). With the GHSV increasing from 50000 to 300000 mL gcat‒1 h‒1, the ethanol conversion decreased from 92% to 48%. Interestingly, the STY increased with GHSV and achieved up to 5.4 gAC gcat‒1 h‒1, which is much higher than that obtained by Na-OMS-2 (4.7 gAC gcat‒1 h‒1) under similar conditions [24] and is unequivocally the highest value for the gas-phase selective ethanol oxidation over noble-metal-free catalysts at 200 °C [14,44] . It needs to point out that further optimization of the reaction conditions can im-

Reaction rate (mmol h ) Conv. & Selec. (%)

100

1307

50

100 150 200 250 300 3 -1 -1 GHSV ( 10 mL gcat h )

Fig. 10. Effect of O2 concentration (A), ethanol concentration (B) and GHSV (C) on the catalytic performance of Cu-γ-MnO2.

2

0

1308

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

prove the STY value to higher level. As we proposed previously [12,24,45], the initial steps of ethanol oxidation over reducible metal oxides may involve both the ethanol activation on surface lattice oxygen and the O2 activation on the reduced metal (e.g. Cu+, Mn2+/3+)-derived oxygen vacancies. Superoxide-type species (O2‒) is thought to serve as the activated oxygen species, which can dissociate the hydroxyl group of ethanol by a endothermic process [46,47]. To verify the possible OH activation by O2‒ species, ethanol molecule deuterated at the hydroxyl group (C2H5OD) was employed to probe the kinetic relevance of the elementary step involving OH bond cleavage to form ethoxide during ethanol oxidation on Cu-γ-MnO2. The ratio of oxidative dehydrogenation rates for undeuterated and deuterated ethanol (KIE) for C2H5OD at 125 °C was measured to be 1.58, which confirms that the OH bond activation is a kinetically relevant step for ethanol oxidation over Cu-γ-MnO2. This result is opposite to the literatures on aerobic oxidation of ethanol over VOx/Al2O3 [43] and MoO2/TiO2 [48], in which the OH bond cleavage is not kinetically relevant but quasi-equilibrated. The normal KIE value obtained by Cu-γ-MnO2 is comparable to that observed by Na-OMS-2-catalyzed ethanol oxidation [24]. Therefore, the reaction process of ethanol oxidation over Cu-γ-MnO2 not only follows the MVK mechanism [19] but also involves the O2‒-mediated OH and α-CH activations. Finally, we attribute the higher activity of Cu-γ-MnO2 than Na-OMS-2 and other metal doped γ-MnO2 catalysts in the gas-phase selective ethanol oxidation to the higher reducibility, which results in more surface Mn2+/Mn3+ defects and oxygen vacancies thus benefit the activation of oxygen and ethanol. 4. Conclusions

Zn-γ-MnO2 shows the highest catalytic activity, which has been associated with its highest molar ratio of surface base/acid sites. The stable oxidation state of surface Mn and oxygen species suggests that CO oxidation over M-γ-MnO2 follows the lattice oxygen-based Mars-van Krevelen mechanism. In contrast, Cu-γ-MnO2 shows the best catalytic activity and high stability in selective ethanol oxidation, which have been attributed to its highest surface reducibility and the presence of more Mn2+/Mn3+ defects and oxygen vacancies for O2 and ethanol activation at lower temperature. The different structure-performance relationships for M-γ-MnO2-catalyzed selective ethanol oxidation and CO oxidation would provide useful guidelines for the design of improved MnO2-based catalysts for selective gas-phase oxidation. Acknowledgments The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology, and the Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education) for use of the facilities. References [1] S. L. Suib, Acc. Chem. Res., 2008, 41, 479487. [2] E. Hayashi, Y. Yamaguchi, K. Kamata, N. Tsunoda, Y. Kumagai, F.

Oba, M. Hara, J. Am. Chem. Soc., 2019, 141, 890900. [3] L. Pahalagedara, D. A. Kriz, N. Wasalathanthri, C. Weerakkody, Y.

[4] [5]

In summary, we have shown that γ-MnO2 is the most efficient catalyst for gas-phase selective ethanol oxidation and CO oxidation among four types (α, β, γ and δ) of MnO2 catalysts, and metal doping has a strong influence on the surface properties and catalytic performance of M-γ-MnO2 catalysts. The doped M-γ-MnO2 can be readily synthesized by a “one-pot” hydrothermal method. The resulting catalysts show different activity orders in CO and ethanol oxidations. For CO oxidation,

[6] [7] [8] [9] [10]

Meng, S. Dissanayake, M. Pahalagedara, Z. Luo, S. L. Suib, P. Nandi, R. J. Meyer, Appl. Catal. B, 2017, 204, 411420. S. Liang, F. Teng, G. Bulgan, R. Zong, Y. Zhu, J. Phys. Chem. C, 2008, 112, 53075315. J. Zhang, Y. Li, L. Wang, C, Zhang, H. He, Catal. Sci. Technol., 2015, 5, 23052313. B. Zhao, R. Ran, X. Wu, D. Weng, Appl. Catal. A, 2016, 514, 2434. X. Weng, Y. Long, W. Wang, M. Shao, Z. Wu, Chin. J. Catal., 2019, 40, 638646. S. Guan, W. Li, J. Ma, Y. Lei, Y. Zhu, Q. Huang, X. Dou, J. Ind. Eng. Chem., 2018, 66, 126140. C. Angelici, B. M. Weckhuysen, P. C. A. Bruijnincx, ChemSusChem, 2013, 6, 15951614. N. Zheng, G. D. Stucky, J. Am. Chem. Soc., 2006, 128, 1427814280.

Graphical Abstract Chin. J. Catal., 2020, 41: 1298–1310

doi: 10.1016/S1872-2067(20)63551-3

Elucidating structure-performance correlations in gas-phase selective ethanol oxidation and CO oxidation over metal-doped γ-MnO2 Panpan Wang, Jiahao Duan, Jie Wang, Fuming Mei, Peng Liu * Huazhong University of Science and Technology The different structural and component requirements of MnO2 for gas-phase selective ethanol oxidation and CO oxidation were clarified by using metal-doped M-γ-MnO2 catalysts, with surface reducibility and acid-base property as the crucial factor, respectively.

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310 F. Garcés, S. L. Suib, Adv. Funct. Mater., 2011, 21, 312323.

[11] O. A. Simakova, V. I. Sobolev, K. Y. Koltunov, B. Campo, A.-R. Leino, [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

K. Kordás, D. Y. Murzin, ChemCatChem, 2010, 2, 15351538. P. Liu, E. J. M. Hensen, J. Am. Chem. Soc., 2013, 135, 1403214035. J. Mielby, J. O. Abildstrøm, F. Wang, T. Kasama, C. Weidenthaler, S. Kegnӕs, Angew. Chem. Int. Ed., 2014, 53, 1251312516. T. Takei, N. Iguchi, M. Haruta, Catal. Surv. Asia, 2011, 15, 8088. H. Zhou, J. Y. Wang, X. Chen, C.-L. O’Young, S. L. Suib, Microporous Mesoporous Mater., 1998, 21, 315324. J. Chen, X. Tang, J. Liu, E. Zhan, J. Li, X. Huang, W. Shen, Chem. Mater., 2007, 19, 42924299. L. Lamaita, M. A. Peluso, J. E. Sambeth, H. J. Thomas, Appl. Catal. B, 2005, 61, 114119. V. P. Santos, M. F. R. Pereira, J. J. M. Órfão, J. L. Figueiredo, Appl. Catal. B, 2010, 99, 353363. J. Li, R. Wang, J. Hao, J. Phys. Chem. C, 2010, 114, 1054410550. H. Li, G. Qi, Tana, X. Zhang, X. Huang, W. Li, W. Shen, Appl. Catal. B, 2011, 103, 5461. B. Bai, J. Li, J. Hao, Appl. Catal. B, 2015, 164, 241250. B. Bai, Q. Qiao, Y. Li, Y. Peng, J. Li, Chin. J. Catal., 2018, 39, 630638. V. V. Dutov, G. V. Mamontov, V. I. Sobolev, O. V. Vodyankina, Catal. Today, 2016, 278, 164173. P. Liu, J. Duan, Q. Ye, F. Mei, Z. Shu, H. Chen, J. Catal., 2018, 367, 115125. W. Y. Hernández, M. A. Centeno, S. Ivanova, P. Eloy, E. M. Gaigneaux, J. A. Odriozola, Appl. Catal. B, 2012, 123124, 2735. J. Yang, H. Zhou, L. Wang, Y. Zhang, C. Chen, H. Hu, G. Li, Y. Zhang, Y. Ma, J. Zhang, ChemCatChem, 2017, 9, 11631167. T. Uematsu, Y. Miyamoto, Y. Ogasawara, K. Suzuki, K. Yamaguchi, N. Mizuno, Catal. Sci. Technol., 2016, 6, 222233. X. Li, J. Ma, L. Yang, G. He, C. Zhang, R. Zhang, H. He, Environ. Sci. Technol., 2018, 52, 1268512696. R. D. Shannon, Acta Crystallogr. A, 1976, 32, 751767. C. K. King’ondu, N. Opembe, C. Chen, K. Ngala, H. Huang, A. Iyer, H.

1309

[31] V. P. Santos, O. S. G. P. Soares, J. J. W. Bakker, M. F. R. Pereira, J. J. M.

[32] [33] [34] [35] [36] [37] [38] [39]

[40] [41] [42] [43] [44] [45] [46] [47] [48]

Órfão, J. Gascon, F. Kapteijn, J. L. Figueiredo, J. Catal., 2012, 293, 165174. S. Ming, P. Wang, P. Liu, J. Duan, F. Mei, L. Pang, Z. Chen, T. Li, Chem. Eng. J., 2020, 379, 122287. J. Nie, H. Liu, J. Catal., 2014, 316, 5766. R. Kumar, S. Sithambaram, S. L. Suib, J. Catal., 2009, 262, 304313. P. Liu, M. Derchi, E. J. M. Hensen, Appl. Catal. B, 2014, 144, 135143. J. Zhang, J.-H. Zhang, C.-B. Zhang, H. He, Acta Phys.-Chim. Sin., 2015, 31, 353359. J. Hou, Y. Li, L. Liu, L. Ren, X. Zhao, J. Mater. Chem. A, 2013, 1, 67366741. L. Wang, J. Zhang, Y. Zhu, S. Xu, C. Wang, C. Bian, X. Meng, F.-S. Xiao, ACS Catal., 2017, 7, 74617465. D. Gu, J.-C. Tseng, C. Weidenthaler, H.-J. Bongard, B. Spliethoff, W. Schmidt, F. Soulimani, B. M. Weckhuysen, F. Schüth, J. Am. Chem. Soc., 2016, 138, 95729580. M. Sun, W. Li, B. Zhang, G. Cheng, B. Lan, F. Ye, Y. Zheng, X. Cheng, L. Yu, Chem. Eng. J., 2018, 331, 626635. M. Luo, Y. Cheng, X. Peng, W. Pan, Chem. Eng. J., 2019, 369, 758765. W. Gac, Appl. Catal. B, 2007, 75, 107117. B. Kilos, A. T. Bell, E. Iglesia, J. Phys. Chem. C, 2009, 113, 28302836. G. Zhao, S. Fan, X. Pan, P. Chen, Y. Liu, Y. Lu, ChemSusChem, 2017, 10, 13801384. P. Liu, T. Li, H. Chen, E. J. M. Hensen, J. Catal., 2017, 347, 4556. Y.-C. Son, V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. Int. Ed., 2001, 40, 42804283. W. Song, P. Liu, E. J. M. Hensen, Catal. Sci. Technol., 2014, 4, 29973003. W. Zhang, S. T. Oyama, J. Phys. Chem., 1996, 100, 1075910767.

金属掺杂γ-MnO2在乙醇气相选择氧化和CO氧化中的构效关系 王盼盼, 段嘉豪, 王

杰, 梅付名, 刘

鹏*

华中科技大学化学与化工学院, 能量转换与存储材料化学教育部重点实验室, 材料化学与服役失效湖北省重点实验室, 湖北武汉430074

摘要: 二氧化锰(MnO2)因具有制备简单、Mn4+/Mn3+/Mn2+混合的金属价态、可调变的晶型结构(如α, β, γ, δ等)、丰富的表面 氧缺陷和活泼的晶格氧等特点, 在多相催化领域被广泛用作需氧氧化催化剂. MnO2催化气相氧化主要集中在催化燃烧或 完全氧化, 催化性能最佳的MnO2晶型因反应底物不同会有所差异. 由于其氧化催化活性较高、选择性难于控制, MnO2这类 结构丰富、价廉易得的催化剂在气相选择氧化制高附加值化学品中面临着机遇和挑战. 乙醇气相选择氧化制乙醛符合绿 色和可持续性化学工业的发展需求, 被认为是替代传统高污染、高成本的乙烯瓦克氧化工艺的最佳选择. MnO2在催化乙醇 燃烧中应用广泛, 不同晶型MnO2中α-MnO2的活性最高. 我们已经报道了非变价金属离子掺杂α-MnO2 (M-OMS-2), 特别是 具有最强表面碱性和可还原性的Na-OMS-2在乙醇气相选择氧化中获得了高达66%的乙醛产率和与贵金属催化剂相接近 的催化效率. 然而, 不同晶型MnO2对乙醇气相选择氧化催化性能的影响, 以及选择氧化和完全氧化所需的MnO2晶型和结 构性质是否一致尚不清楚. 这些问题的阐明无疑能够指导更高性能MnO2选择氧化催化剂的设计合成. 本文首先制备了四种晶型(α, β, γ, δ)MnO2, 并将其应用于气相乙醇选择氧化和CO氧化, 惊喜地发现γ-MnO2在这两类反 应中均表现出最高的催化活性, 这与之前报道的乙醇催化燃烧和CO氧化结果明显不同, 证明了MnO2的晶型结构和反应条 件对其催化氧化性能有重要影响. 为了深入研究γ-MnO2在乙醇气相选择氧化和CO氧化中的构效关系, 通过一锅水热法制 备了金属掺杂的M-γ-MnO2 (M = Cu2+, Zn2+, Mg2+, Co2+, Ni2+, Ca2+, Al3+, Fe3+, La3+). 采用多种表征手段证明了金属掺杂能够 有效地调变γ-MnO2的结构组成、表面酸碱性和可还原性. 制得的M-γ-MnO2催化剂在CO和乙醇氧化中显示出不同的活性顺 序, 发现CO氧化中催化剂的酸碱性更加重要. 具有最高表面碱性/酸性位点摩尔比的Zn-γ-MnO2在CO氧化中活性最佳, 其

1310

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310

表面锰物种和氧物种的氧化态在反应前后基本不变, 表明M-γ-MnO2上的CO氧化遵循基于晶格氧的Mars-van Krevelen机 理. 与之不同的是, 催化剂的可还原性是影响乙醇气相选择氧化性能的更主要因素. 具有最高表面可还原性的Cu-γ-MnO2 于200 °C获得了最高75%的乙醛产率和较好的催化稳定性, 这归因于其在较低温度下存在更多的Mn2+/Mn3+缺陷位和氧空 位, 这有利于O2和乙醇的低温活化. 进一步的动力学研究表明, O2和乙醇浓度对反应速率影响不大, 而空速的增大能够大 幅度提高乙醛的时空收率. 同位素效应证明醇羟基断裂仍然是反应的速控步骤, 因此表面晶格氧和吸附氧物种均参与了 乙醇活化. 关键词: 二氧化锰; 金属掺杂; 乙醇氧化; 乙醛; CO催化氧化 收稿日期: 2019-12-03. 接受日期: 2019-12-25. 出版日期: 2020-08-05. *通讯联系人. 电话: (027)87543032; 传真: (027)87543632; 电子信箱: [email protected] 基金来源: 国家自然科学基金(21673088, 21972050). 本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).