ZrO2 for higher alcohols synthesis from syngas

Catalysis Today xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Mixed oxides of La-Ga-O modified Co/ZrO2 for higher alcohols synthesis from syngas ⁎

Na Kanga,b, Qilei Yanga,b, Kang Ana,b, Shuangshuang Lia,b, Lihong Zhanga,b, , Yuan Liua,b, a b

⁎

Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite-type oxide Higher alcohols synthesis Ethanol Cobalt Gallium

In this work, a series of catalysts of LaCo1-xGaxO3 with perovskite phase supported on ZrO2 was prepared by combining the citrate complexing method with incipient impregnation. The catalysts of LaCoO3/ZrO2 and Co3O4-Ga2O3/ZrO2 were similarly prepared and used for comparison. The catalysts were characterized by using BET, XRD, H2-TPD, H2-TPR, CO-TPD, TG, XPS and TEM techniques. Reducing LaCo1-xGaxO3/ZrO2 resulted in forming mixed oxides of La-Ga-O modified Co/ZrO2, which was used as the catalyst for higher alcohols synthesis from syngas. The prepared catalyst exhibited very high selectivity to higher alcohols especially to ethanol, high resistance to sintering, high resistance to carbon deposition and good activity, which are attributed to Ga-doping made Co in positive charge, the confinement of mixed oxides of La-Ga-O for Co nanoparticles, the restriction of Ga-doping for carbon formation and the high dispersion of Co nanoparticles, respectively.

1. Introduction Higher alcohols synthesis (HAS) from syngas has been attracted great attention owing to the following reasons. (1) Higher alcohols (C2+OH) can be used as high quality of power fuel or fuel additives, which can reduce harmful substance emission and possess superior octane value and good explosion-proof anti-seismic performance. (2) Higher alcohols can be converted into value-added chemicals (e.g.: lower olefins, etc.) through the further chemical process [1–4]. The catalysts reported for HAS include four categories. Rh-based catalysts show high selectivity to C2+ oxygenates, especially ethanol, while its high price restricts extensive application [4–6]. The modified methanol synthesis catalysts mainly produce methanol rather than higher alcohols [7]. Mo-based catalysts (mainly including Mo2C and Mo2S) exhibit very good resistance to sulfur poisoning, but it is operated under strict reaction conditions [8]. The last kind of modified Fisher-Tropsch (FT) catalysts, mainly Cu-Fe and Cu-Co based catalysts, suffer from deactivation due to phase separation and sintering of the active metallic nanoparticles (NPs) [9,10]. For all of the four kinds of catalysts, to elevate the selectivity to C2+OH and suppress the formation of hydrocarbons and methane are the key challenges. Therefore, the technology for HAS is still in developing and effort on its commercialization is on the way. HAS is an extremely complicated reaction, taking Cu-Co based catalysts as the example. CO is associatively adsorbed on metallic

⁎

copper, and dissociative adsorption of CO is on the sites of metallic cobalt [4,11]. Coupling of the two hydrogenated CO would generate C2+OH [12]. The hydrogenated products of CO on metallic cobalt could interact to form eCH3, eCH2CH3, eCH2CH2CH3, etc., which could interact with associatively adsorbed CO on copper to generate C2H5OH, C3H7OH, C4H9OH, etc., respectively. On mono copper and mono cobalt NPs, methanol and hydrocarbons would be produced, respectively. The problem is that the existence of mono copper and mono cobalt can hardly be avoided. To favor the synergy between copper and cobalt, previously perovskite-type oxides (PTOs) were used as the precursors for preparing bimetallic Cu-Co catalysts [12,13]. PTO is generally written as ABO3, where the metallic ions at A and/or B sites can be partially substituted by other ions, resulting in complex oxides with various compositions. For nano crystallines of LaCo1-xCuxO3 loaded on ZrO2, the ions of copper and cobalt are uniformly mixed at the atomic level and confined in a crystalline of the PTO. Therefore, reducing LaCo1-xCuxO3 would result in bimetallic Cu-Co NPs with copper and cobalt in close contact, which was loaded on La2O3-doped ZrO2. Thus prepared catalysts showed much higher selectivity to C2+OH as compared with the regularly impregnated Cu-Co catalysts [12]. Recently, Ning et al. [14] reported that Ga-doped Co catalyst supported on alumina is highly selective for HAS, which was prepared by using a layered double hydroxides as the precursor. They claimed [15] that the Ga atoms contiguous distribution Co atoms contributed to

Corresponding authors at: Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China. E-mail addresses: [email protected] (L. Zhang), [email protected] (Y. Liu).

https://doi.org/10.1016/j.cattod.2018.01.034 Received 22 November 2017; Received in revised form 17 January 2018; Accepted 27 January 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Kang, N., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.01.034

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

amount in 30%LCG-Z. For the H2 temperature programmed desorption (H2-TPD) process, 100 mg of catalyst was reduced in a micro-reactor with pure hydrogen at 800 °C (xLCG-Z) and 700 °C (19%LC-Z and CG-Z) for 2 h. As for the used catalysts, the reduction temperature and time are 400 °C and 0.5 h, respectively, in order to reduce the partially oxidized cobalt as the catalysts were exposed in air. After the reduction, hydrogen was switched to N2 and outgassed for 30 min to remove excess hydrogen, then cooled down to room temperature (25 °C) for re-adsorption of H2 1 h. Finally, the sample was kept in 30 mL min−1 N2 stream with a temperature ramp of 10 °C min−1 from room temperature to 800 °C to perform the TPD. The dispersion of Co was calculated based on the volume of the chemisorbed H2 using the following simplified equation [19]:

isolating Co centers to associative CO adsorption as well as Co and Ga in the well-defined CoGa particles, which is beneficial to Ga donate electrons to neighboring Co sites, being responsible for CO dissociative adsorption. In another work, Gao et al. [16] studied Ga-modified Co supported on active carbon, which was prepared according to general co-impregnation method. This catalyst showed good catalytic performance for HAS. The modifying effect of gallium oxide is ascribed to the fact that Ga2O3 made part of cobalt in the state of Co2+, where Co2+ work likes the metallic copper. No any other reports on Ga-modified Co catalysts for HAS can be found, except the above papers. Based on the background above, in this work, Ga-modified Co catalyst prepared by using PTOs as the precursors was investigated. Several supports and the influence of Co/Ga ratio in Table S1 (in the Supplementary material) were investigated. The results indicated that LaCo0.65Ga0.35O3/ZrO2 showed the best catalytic performance for HAS. Thus, in this work, the catalytic performance for HAS, the structure and the effect of Ga were studied for LaCo0.65Ga0.35O3/ZrO2.

D(%) =

2 × Vad × M × SF × 100 m × P × Vm × dr

(1)

Where Vad (mL) is the volume of chemisorbed H2 at standard temperature and pressure (STP) conditions measured in the TPD procedure; m stands for the weight of the sample (g); M is the molecular weight of Co (58.9 g mol−1); P represents the weight fraction of Co in the catalysts; SF is molar ratio Co/H in the chemisorption, which deems as 1, and Vm is the molar volume of H2 (22414 mL mol−1) at STP; and dr is the reduction degree of cobalt calculated based on H2-TPR. The CO temperature programmed desorption (CO-TPD) experiments were performed. Firstly, the reduced catalyst of 0.1 g was flushed with He at 400 °C for 2 h with a flow rate of 20 mL/min. Secondly, the sample was cooled to 50 °C and chemisorption from 5% CO/He gas mixture for 60 min. After CO adsorption, the sample was swept with a He flow (20 mL/min) until the TCD signal was stable. CO-TPD runs were conducted in the 40–700 °C. Bright field scanning transmission electron microscopy (BF-STEM), the corresponding STEM-EDS element analysis and line scanning were obtained on a JEOL JEM-2100F field-emission transmission electron microscope. Samples were finely ground to fine particles in an agate mortar and then dispersed into ethanol ultrasonically. Finally, the welldispersed samples were deposited onto copper grids covered by holey carbon film. Thermal analysis was carried out by DTG-50/50H thermogravimetric analyzer to acquire Thermogravimetric analysis (TG) data. During TG tests, 20 mg of sample was used and heated from room temperature to 800 °C with a ramping rate of 10 °C min−1 in air atmosphere. X-ray photoelectron spectroscopy (XPS) tests were performed on ESCALAB 250Xi photoelectron spectrometer using Al Kα (hγ = 1253.6 eV) radiation. The samples were carefully mounted on a sample holder by means of a double-sided adhesive tape. The sample loading was performed in argon flow in order to avoid the contact of the sample with the air. The binding energy (BE) of C 1s (284.8 eV) was used to correct all XPS spectra.

2. Experimental 2.1. Catalyst preparation ZrO2 support was prepared according to precipitation method as reported in the literature [17]. Solution of 0.5 M ZrO(NO3)2·xH2O in water and ammonia solution were dropped into a beaker, kept the pH between 10 to 11 under vigorous stirring at 70 °C for 3 h. Then, the hydrous zirconia was aged for 24 h at room temperature. After filtering, the precipitate was dried by freeze-drying, calcined in air at 650 °C for 3 h at a heating rate of 2 °C min−1. LaCo0.65Ga0.35O3/ZrO2 was prepared by combining the incipient wetness impregnation method with citrate complexing method, mentioned in our previous works [12,18]. At first, lanthanum, cobalt and gallium nitrate, citric acid and glycol at a molar ratio of La/Co/Ga/ citric acid/glycol = 1/0.65/0.35/2.4/0.48 were dissolved in deionized water. Then, the obtained aqueous solution was incipient impregnated on ZrO2 support, staying overnight, the resulting sample was dried at 80 and 120 °C for 6 and 12 h, respectively. The dried sample was calcined at 350 and 650 °C for 2 h and 5 h, respectively, at a heating rate of 2 °C min−1 for the two temperature increasing steps. The prepared catalysts were named as xLCG-Z, where the “x” stands for loading percentage of LaCo0.65Ga0.35O3 in the calcined catalysts. For comparison, LaCoO3/ZrO2, LaGaO3/ZrO2, Co3O4-Ga2O3/ZrO2 and pure LaCo0.65Ga0.35O3 were prepared similarly, and were named as LC-Z, LG-Z, GC-Z and LCG, respectively. For comparison, the content of cobalt in 19%LC-Z is the same as that in 30%LCG-Z, the content of gallium in 11%LG-Z is the same as that in 30%LCG-Z, and the content of cobalt and gallium in CG-Z are accordance with the 30%LCG-Z. 2.2. Catalyst characterization The specific surface areas of the catalysts were measured by N2 adsorption on the basis of the BET method on Tristar 3000 Micromeritics instrument. The samples were pretreated in vacuum at 300 °C for 4 h before starting the experiment. Meanwhile, pore size distributions and pore volumes were derived from the BJH method using the adsorption branch of the isotherms. X-ray diffraction (XRD) tests were performed on a Bruker D8-Focus X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The spectra were collected between 20 and 75° of 2θ at a scanning speed of 8° min−1. Temperature programmed reduction (TPR) tests were carried out on a fixed bed micro-reactor. 50 mg of the catalyst was used, which was heated from room temperature to 880 °C at a heating rate of 10 °C min−1 in the presence of 5% H2/Ar at a flow rate of 30 mL min−1. As for LCG, 16 mg catalyst was used, which is close to the PTO loading

2.3. Catalytic performance test The catalytic performance tests for HAS from syngas were carried out on a stainless-steel continuous flow fixed-bed micro-reactor combined with GC (gas chromatography) system. The xLCG-Z and 11%LG-Z were reduced at 800 °C as well as 19%LC-Z and CG-Z were reduced at 700 °C with pure hydrogen for 3 h at a heating rate of 5 °C min−1 with H2 flow rate of 20 mL min−1. Then, 400 mg of catalyst in 40–60 mesh was diluted with 400 mg quartz sand with the same size and then loaded into a micro-reactor, along with the pressure increased to 4 MPa by feeding the syngas mixture of H2/CO/N2 = 8/4/1 from the pressurized manifold via individual mass flow controller. The gas hourly space velocity (GHSV) was set at 3900 mL (gcat h)−1. At the first temperature (290 °C), the reaction time was kept for 50 h, the period at other temperatures was maintained for 12 h to collect data from GC. 2

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

as compared with that over 30%LCG-Z. Thus, it can be concluded that gallium adding can improve the selectivity to alcohols and to C2+OH obviously. Over CG-Z catalyst, methanol is much high in the total alcohols, which may be ascribed to Ga2O3. Ga2O3 is active for CO2 hydrogenation to generate methanol [22,23]. At the same time, the selectivity to CO2 is much higher compared with other catalysts, which is likely attributed to the fact that Ga2O3 is also active for water-gas-shift reaction as stated in [24]. From Table 1, it is seen that the selectivity to hydrocarbons is high over CG-Z, which is mainly due to no PTO formed in the catalyst. Taking LaCoO3 as the example, ions of La3+ and Co3+ are confined in the crystalline of the PTO. Therefore, after reduction Co NPs are in close contact with La2O3. However, for CG-Z catalyst, after reduction, many of Co NPs would be away from Ga2O3. The mono Co NPs would generate hydrocarbons, leading to the high selectivity to hydrocarbons [14]. The no formation of PTO for CG-Z also resulted in its low activity. Taking LaCoO3 as the example too, after reduction Co NPs are in close contact with La2O3, which confined Co NPs from migration. So Co NPs are highly dispersed. However, for CG-Z after reduction, many of Co NPs are away from Ga2O3. Thus there is no the confinement, resulting in lower dispersion of Co NPs and hence lower activity. The lower dispersion of Co NPs in CG-Z was confirmed by TPD results. LG-Z is not active for syngas conversion, CO conversion could not be detected at 300 °C.

The GC was equipped with two packed columns. One is TDX-01 packed column (3 m) connected to the TCD detector to analyze CO, H2, CH4 and CO2, the other is Porapak Q column (3 m) connected to FID detector to detect hydrocarbons and the condensed liquid products. CO conversion (XCO), the product selectivity (Si) and the mass fraction of a certain alcohol in the total alcohols (Wi) were calculated according to the following equations:

X CO =

COin − COout × 100% COin

Si =

nCi × 100% ∑ nCi

Wi =

mi × 100% ∑ mi

Where COin and COout are the moles of CO in the feed-gases and offgases, respectively; n and Ci represent the number of carbon atoms in a molecule of the products and moles of a carbon-containing product, respectively; mi is the weight of a certain alcohol in the products of total alcohols. 3. Results and discussions 3.1. Characterizations 3.1.1. Catalytic performance The results of the catalytic performance tests are listed in Table 1. It is known that metallic Co0 is the active species for dissociating CO, which would lead to the formation of methane and other hydrocarbons via hydrogenation [14]. Therefore the CO conversion and hydrocarbon selectivity increase with the increase of LaCo0.65Ga0.35O3 content, accompanying with decreasing the selectivity to alcohols. Meanwhile, the article [20] pointed out that La2O3 as additive may restrain water gas shift reaction (WGSR). Maybe, La-Ga-O mixed oxides could also restrain WGSR. The two reasons led to the low CO2 selectivity over LCG-Z catalysts. Considering CO conversion, selectivity to total alcohols and distribution of the alcohols, 30%LCG-Z showed the best catalytic performance among the catalysts of xLCG-Z. It can be seen from Table 1 that in the higher alcohols, the selectivity to ethanol and C4+OH is high while to C3+OH is much less, which is likely due to the support of ZrO2 as pointed out in [21]. The coordinative unsaturated Zr4+-O2− pairs favors the formation of C4+OH especially iso-synthesis. 19%LC-Z, where the loading amount of cobalt is the same as in 30% LCG-Z, exhibited higher CO conversion than 30%LCG-Z while the selectivity to alcohol and to C2+OH are obviously lower at 300 °C. In order to eliminate CO conversion influence, to compare the results at lower CO conversion of 10.6% at 290 °C, when the selectivity to alcohols is 48% over 19%LC-Z, which is lower than 52.1% of the selectivity to alcohols with CO conversion of 17% at 300 °C over 30%LCG-Z. Besides, the selectivity to C2+OH is also obviously lower over 19%LC-Z

3.1.2. Stability of 30%LCG-Z The results of the stability test for 30%LCG-Z are shown in Fig. 1. In the reaction period of 200 h, the CO conversion maintained around 17%, the selectivity to total alcohols kept in 51%-52% and C2+OH selectivity maintained at 76%, showing excellent stability. Table 2 summarizes some results of excellent Co-based and Fe-based catalysts for HAS from syngas. It can be seen that 30%LCG-Z catalyst is excellent among catalysts. Compared with Co-Ga/AC [16], 30%LCG-Z showed obviously higher selectivity to C2+OH at higher CO conversion under nearly same space velocity condition. The stability of Co-Ga/AC was not shown in the paper [16]. CoGa/ZnO/Al2O3, the best catalyst prepared by using layer double hydroxide as the catalyst precursor, showed better selectivity for HAS, which maintained stable running for 100 h at low space velocity of 2000 h−1 and 260 °C [14]. Compared with CoGa/ZnO/Al2O3 reported in Ref. [14], 30%LCG-Z catalyst has two advantages: one is higher ethanol selectivity: ethanol in the total alcohols is 52.8% over 30%LCG-Z and 30.8% over CoGa/ZnO/Al2O3; the other is that the stability of 30%LCG-Z seems better: the stability tests on 30%LCG-Z was investigated at higher temperature (300 vs. 260 °C) and longer period (200 vs. 100 h). 3.2. Characterizations 3.2.1. Physical properties and XRD results The N2 adsorption-desorption isotherms and the corresponding pore

Table 1 Catalytic performance for HAS from syngas. Samples

25%LCG-Z 30%LCG-Z 35%LCG-Z 19%LC-Z CG-Z

T (°C)

300 300 300 290 300 300 310

XCO (%)

10.2 17.0 19.3 10.6 22.0 5.0 10.8

SCO2 (%)

1.5 2.5 2.9 2.3 4.7 5.0 10.6

SRH (%)

SCOH (%)

44.3 45.4 51.6 49.7 53.3 44.0 53.7

54.2 52.1 45.5 48.0 42.0 51.0 35.7

Reaction conditions: P = 4 MPa, H2/CO/N2 = 8/4/1, GHSV = 3900 mL (gcat h)−1.

3

Alcohols distribution (wt.%) C1

C2

C3

C4

25.7 23.9 26.6 31.5 33.3 66.7 80.3

51.8 52.8 49.2 47.0 44.1 29.6 16.2

0.8 1.5 1.7 1.2 1.2 0.0 0.0

21.7 21.8 22.5 20.3 21.4 3.7 3.5

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

Fig. 1. (a) CO conversion and selectivity to alcohols, hydrocarbons and CO2 and (b) distribution of alcohols over 30%LCG-Z vs. reaction time at 300 °C, 4 MPa, GHSV of 3900 mL (gcat h)−1 and H2/CO/N2 = 8/4/1.

highly dispersed. Considering the molar ratio of La/Ga, La2O3 should exist in the reduced products of LaCo0.65Ga0.35O3, while no diffraction peaks of La2O3 could be detected, suggesting that La2O3 is highly dispersed. The similar situation was reported by Sun [31], who found that if the peak of La2O3 in the XRD patterns could be seen, the loading amount of La2O3 on ZrO2 would exceed 14 wt.%. Similarly, for reduced 19%LC-Z and reduced CG-Z, Co NPs and La2O3/Ga2O3 would be highly dispersed. The used catalysts show many similar XRD patterns (Fig. 2c) as compared with the reduced catalysts, even after reaction 200 h, much weak diffraction peaks of metallic Co and no diffraction peaks corresponding to La2O3/Ga2O3 were shown, indicating that the resistant ability to sintering is very good. Also, the peaks corresponding to La4Ga2O9 and LaGaO3 can be seen, similar to the reduced catalysts. Meanwhile, the grain size of La4Ga2O9, LaGaO3 and ZrO2 are close for the reduced and the corresponding used catalysts of xLCG-Z (the sizes are listed in Table 3), indicating that La4Ga2O9, LaGaO3 and ZrO2 are stable in the reaction process. However, there is an exception as comparing the XRD patterns of the reduced and the corresponding used catalysts. That is to say, for used 19%LC-Z, the diffraction peak of Co2C can be seen while Co2C can't be detected for xLCG-Z and CG-Z. It is known that La2O3 could facilitate the formation of Co2C during the reaction of HAS [32,33]. The XRD results suggest that the element of Ga can suppress the formation of Co2C.

size distribution of ZrO2, xLCG-Z, CG-Z and 19%LC-Z are shown in Fig. S1 (in the Supplementary information). On the basis of the IUPAC classification, IV-type adsorption isotherms are revealed by all of the samples, indicating the existence of mesopores. The catalysts have an approximate H4 hysteresis loop at relative pressures (p/p0) of 0.8–1.0, suggesting that the catalysts have relatively narrow pore size distribution [18,29]. The BET surface area, average pore diameter and the VBJH are listed in Table 3. It can be learnt that those are lowered a little as loading the PTOs on the support of ZrO2. The XRD results of the fresh, reduced and used catalysts are shown in Fig. 2. For all of the catalysts, the XRD results indicate that ZrO2 existing form is the monoclinic phase. In the XRD patterns of the fresh catalysts xLCG-Z in Fig. 2a, the diffraction peaks at 2θ≈33° are attributed to perovskite phase. From the enlarged pattern of the insert, it is seen that the diffraction peaks of perovskite for xLCG-Z shifted a little to lower 2θ value, as compared with LaCoO3 in LC-Z, which is caused by the difference of ionic radius of Ga3+ (0.062 nm) with Co3+ (0.0545 nm) [30]. Meanwhile, no any impurity phases of Ga2O3, Co3O4 and La2O3 could be detected for xLCGZ. Along with the loading capacity of LaCo0.65Ga0.35O3 increase, the diffraction intensity of LaCo0.65Ga0.35O3 increases. Those suggest that LaCo0.65Ga0.35O3 with perovskite phase was successfully loaded on the ZrO2 support. For fresh CG-Z, only ZrO2 phase was detected and no peaks corresponding to Ga2O3 and Co3O4 could be seen, which may be due to the high dispersion and comparatively low amount of Ga2O3 and Co3O4 loading. The reduced catalysts from the XRD results are shown in Fig. 2b. For xLCG-Z catalysts, the perovskite structure was completely destroyed, accompanying with the appearance of the diffraction peaks of La4Ga2O9 (PDF#53-1108) and LaGaO3 (PDF#70-2784). Much weak diffraction peaks corresponding to metallic cobalt (PDF#15-0806) can be seen in the enlarged XRD patterns of the insert, indicating that Co NPs are

3.2.2. H2-TPR TPR results of the calcined catalysts are presented in Fig. 3. For the pure support of ZrO2 and LG-Z catalysts, no reduction peaks were observed, showing they are stable. For 19%LC-Z, the reduction peaks at the temperature range of around 300–450 °C and 450–680 °C are corresponding to Co3+ to Co2+ and Co2+ to Co0, respectively, which have been well studied in these literatures [12,34].

Table 2 The catalytic performance over representative excellent catalysts for HAS from syngas reported in literatures. Catalyst

T (°C)

H2/CO

P (MPa)

CHSV (h−1)

XCOa (%)

SROHb (%)

C2+OH in alcohols (%)

Cu-Co/ZrO2-La2O3-CeO2 [13] Cu-Co/La2O3-SiO2 [25] Cu–Co/Al2O3 [26] Cu-Fe-Mg [27] Co4P/SiO2 [28] CoGa/ZnO-Al2O3 [14] Co-Ga/AC [16] 30%LCG-Z (This work)

270 330 250 280 280 260 220 300

2 2 2 2 2 2 2 2

4 3 2 3 5 3 3 4

6000 3900 1800 3900 5000 2000 4000 3900

11.7 32.1 23.2 29.4 5.3 43.5 13.1 17

51.6 39.5 23.3 32.9 44.8 59.0 30.3 52.1

71.0 66.1 79.3 66.4 77.0 92.8 80.8 76.1

a b

CO conversion. ROH selectivity.

4

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

Table 3 Physical properties of ZrO2 and the catalysts. Sample

SBET (m2 g−1)

Pore size (nm)

VBJH (cm3 g−1)

DaZrO2 (nm)

DaLaGaO3 (nm)

DaLa4Ga2O9 (nm)

Co Dispersion (%)

ZrO2 19%LC-Z 25%LCG-Z 30%LCG-Z 35%LCG-Z CG-Z

53.4 48.0 43.4 38.9 36.8 37.5

21.4 21.0 20.3 20.1 19.9 21.0

0.29 0.27 0.23 0.20 0.18 0.20

13.5 11.2 11.5 12.4 12.6b 12.4c 12.9d 14.2 11.5

– – – 26.7b 27.0c 27.6d 28.7b 29.7c –

– – – 31.7b 32.9c 34.7d 37.5b 38.6c –

– 11.0f 7.9g 9.9f 9.4g 10.5f 10.1g 9.6h 9.1f 8.5g 8.7f 5.9g

e

a

Crystalline sizes calculated from XRD results with Scherrer formula. Grain sizes derived from XRD results for the catalyst after reduction, after reaction for 86 h and after 200 h stability test, respectively. e Derived from H2-TPD results. f,g,h Co dispersion derived from H2-TPD results for the catalysts after reduction, after reaction for 86 h and after 200 h stability test, respectively. b,c,d

xLCG-Z suggests that LaCo0.65Ga0.35O3 with perovskite phase was formed and supported on ZrO2 support. For CG-Z, the two reduction peaks are obviously higher than the corresponding reduction peaks of cobalt ions in Co3O4/ZrO2 reported in the literature [35]. The reported reduction peaks for cobalt ions in Co3O4/ZrO2 are generally around 300 and 354 °C for Co3+ to Co2+ and Co2+ to Co0, respectively. This information supports the above viewpoint that Ga-doping could suppress the reduction of cobalt ions.

For xLCG-Z, the shape of TPR profiles is similar to that of 19%LC-Z, while the two reduction peaks moved to higher temperature, suggesting that adding gallium ions into LaCoO3 could suppress the reduction of the cobalt ions. By the way, the area of the high-temperature peak is about two times of the area of the corresponding low-temperature peaks, supporting the two steps reduction of cobalt ions. The temperature of the reduction peaks increase with the increase of LaCo0.65Ga0.35O3 content in xLCG-Z, and the peak temperatures for LCG (non-supported LaCo0.65Ga0.35O3) are obviously higher than xLCG-Z catalysts, which should be ascribed to the crystalline size increase of the PTO. The crystalline size of LaCo0.65Ga0.35O3 increase with the increase of its loading amount and the non-supported LaCo0.65Ga0.35O3 has the largest crystalline. The comparison of the TPR profiles of LC-Z with

3.2.3. STEM Representative STEM images and line scannings of 30%LCG-Z after reduction are presented in Fig. 4A. It can be seen that the Co, La and Ga elements showed the same variation from the red line corresponding to

Fig. 2. XRD patterns of (a) the fresh catalysts, (b) the catalysts after reduction and (c) the catalysts used for 86 h and 200 h (♣): LaCo0.65Ga0.35O3 (Ω): LaCoO3 (●): ZrO2 (♥): Co (♦): La4Ga2O9 (♠): LaGaO3 (&): Co2C.

5

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

where Co2C works like metallic copper [32,33,41]. For CG-Z used for 86 h, the BE of Co 2p3/2 is 779.0 eV, which is higher than 778.5 eV of pure metallic Co as well as the BE of Ga 3d5/2 (20.3 eV) is a little lower than 20.5 eV for pure Ga2O3 [42], suggesting that Co NPs and Ga2O3 are in interaction and Co donate electrons to Ga2O3. For 30%LCG-Z after reaction for 200 h, the BEs of 834.7 and 838.4 eV for La 3d5/2 are much close to 834.7 and 838.3 eV for LaGaO3, obviously higher than 834.3 and 837.5 eV for La2O3. Meanwhile, the BEs of Ga 3d5/2 are 16.5 and 19.8 eV, close to 17.3 and 19.4 eV for LaGaO3. The XRD results indicate that LaGaO3 and La4Ga2O9 were formed, and there is residue La in the form of La2O3. However, XPS results do not show the existence of La2O3, indicating that the residue La was possible in strong interaction with LaGaO3/La4Ga2O9. The BE of Co 2p3/2 is 778.8 eV, belonging to metal cobalt, which is higher than that of the pure metallic cobalt, suggesting that the interaction between Co NPs and LaGaO3/La4Ga2O9 made metallic Co in positive charge. Anyway, from XPS results, it can be concluded that Ga-adding makes metallic Co in the positive charge of Coδ+. The surface compositions of used 19%LC-Z and CG-Z, and 30%LCGZ after stability test of 200 h are listed in Table 4. For all of the used catalysts, the practical atomic ratios of Co are lower than corresponding theoretical values, which is likely attributed to the fact that the metallic Co is in the shape of the nanoparticle, and the bottom of a Co NP contribute less to the surface composition. On the contrast, the surface ratios of La and Ga for 19%LC-Z and CG-Z are obviously higher than that of the corresponding theoretical values, which should be due to the reason that La2O2CO3 and Ga2O3 could be “wetted” on the surface of the ZrO2 support. The reason is that ZrO2 support and La2O2CO3 and Ga2O3 are oxides, hence those inclined to interact. For 30%LCG-Z, the surface ratio of La is higher than that of the theoretical, while the surface ratio of Ga is close to the theoretical. It seems that LaGaO3 and La4Ga2O9 are not so well “wetted” on ZrO2.

Fig. 3. TPR profiles of catalysts.

scanning route, indicating that the three elements are in close contact. The EDS mapping images (Fig. S2A in the Supplementary information) exhibited that La, Co and Ga are uniformly distributed. After reaction for 200 h, the EDS mapping (Fig. S2B) and line scannings (Fig. 4B) indicate that Co, La and Ga elements are still uniform distributed and in close contact, respectively. Meanwhile, XRD results show that La combined with Ga formed LaGaO3 and La4Ga2O9 and all the elements of La, Ga and Co are confined and originated from a nano crystalline of LaCo0.65Ga0.35O3. Therefore, after reduction, Co NPs would be supported on LaGaO3-La4Ga2O9. LaCo0.65Ga0.35O3 was supported on ZrO2, thus, the catalyst after reduction and in the reaction process was in the state of Co/LaGaO3-La4Ga2O9-ZrO2.

3.2.4. XPS The binding energies (BEs) of Co 2p3/2, La 3d5/2 and Ga 3d5/2 and Zr 3d5/2 for the used catalysts of 19%LC-Z and CG-Z and for 30%LCG-Z after stability for 200 h obtained from XPS results are summarized in Table 4, and the XPS spectra are shown in Fig. S3. For 19% LC-Z after reaction for 86 h, the BEs of La 3d5/2 is 835.3 and 839.1 eV (Table 4), which are close to 835.5 and 839.4 eV for La2O2CO3 [36], respectively, larger than the corresponding 834.3 and 837.5 eV for La2O3 [37], suggesting lanthanum is in the state of La2O2CO3. The BEs of Co is 778.6 and 781.8 eV, the former is close to the BE of pure metallic Co of 778.5 eV [38,39] and the latter is a little lower than 782.1 eV for Co2p3/2 in Co2C [40], suggesting that there are two kinds of cobalt, one is metallic Co and the other is Co2C. XRD results also showed the existence of Co and Co2C in used 19%LC-Z. It is known that metallic Co and Co2C in synergy can generate C2+OH,

3.2.5. TG To learn about the carbon deposition resistance of the catalysts, TG analysis was adopted, seen in Fig. 5. The lines were verified by using the corresponding reduced catalyst as the baseline. For 30%LCG-Z catalyst reacted for 200 h and 86 h, the amounts of coke deposited are 2.1 wt.% and 1 wt.%, respectively. The coke was combusted at the temperature range of 250–400 °C, which is assigned to the hydrocarbons with large molecule from FT reaction [43]. As for CGZ after reaction for 86 h, the amount of the heavy hydrocarbons deposited is 1.1 wt.%. For 19%LC-Z, the amount of carbon deposited reaches 3.6 wt.%, much higher than the 30%LCG-Z. The deposited carbon was mainly combusted at the temperature range of 250–400 °C, belonging to large molecule from FT reaction. Besides, small amount of the deposited Fig. 4. STEM images and EDS scanning lines of chemical elements for 30%LCG-Z (A) after reduction and (B) after 200 h stability test.

6

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

Table 4 Binding energy (BE) and surface composition of 19%LC-Z, 30%LCG-Z and CG-Z from XPS results. Sample

BEs (eV)

Atomic ratio (%)

Co 2p3/2

Ga 3d5/2

La 3d

–

30%LCG-stability

778.6 781.8 778.8

CG-Z-used

779

835.3 839.1 834.7 838.4 –

19% LC-Z-used

19.8 16.5 20.3

5/2

Zr 3d5/2

Co/(La + Co + Ga)

La/(La + Co + Ga)

Ga/(La + Co + Ga)

181.8

31.0 (50.0)

69 (50.0)

–

181.9

14.4 (32.5)

68.8 (50.0)

16.9 (17.5)

182.1

24.2 (65.0)

–

75.8 (35.0)

Data in the parentheses are theoretical values.

100–300 °C and 350–600 °C. The desorptions at the low temperature are assigned to weakly adsorbed CO, and the desorptions at the high temperature are attributed to strongly adsorbed CO. Tejuca et al. [47] reported that the low-temperature desorption for Co/La2O3 derived from reducing LaCoO3 is ascribed to desorption of CO on Co2+ sites and CO species on metallic Co sites is at the high temperature for CO-TPD. For 30%LCG-Z, compared with 19%LC-Z, the high-temperature peak shifted to lower temperature, indicating that the adsorption intensity decreased and therefore CeO dissociation was restricted to some degree. As stated in papers [28,48], the higher the desorption temperature of CO, the more favorable the hydrocarbon generation will be for CO hydrogenation. Therefore, addition gallium is beneficial to decrease hydrocarbon generation. 3.3. Discussion Fig. 5. TG curves for the used catalysts.

By comparing the catalytic performance of LCG-Z and LC-Z, it can be concluded that adding Ga increased the selectivity to C2+OH markedly, which is attributed to the fact that the interaction between Co NPs and La-Ga-O made Co in partial positive charge (Coδ+) and Coδ+ is favor of generating C2+OH [28]. The XRD, TEM and H2-TPR results indicate that after reduction LaCo0.65Ga0.35O3 was completely destroyed, transformed into Co NPs supported La-Ga-O mixed oxides and the mixed oxides spread on ZrO2 support. The XPS results above suggest the interaction between Co NPs and LaGaO3/La4Ga2O9 (the mixed oxides) made metallic Co in positive charge. Compared with metallic Co, CO adsorbed on Coδ+ is weaker, which prefer to interact with other alkyls to form C2+OH [28,49] while the rate for hydrogenation of the adsorbed CO to generate methane would be lowered from CO-TPD. Co dispersions were measured by using H2-TPD, and the results are listed in Table 3. After reaction, the Co dispersion decreased only a little for 30%LCG-Z from 10.5% of reduced catalyst to 10.1% of the catalyst reacted for 86 h and 9.6% for reacted for 200 h, and the grain size of LaGaO3, La4Ga2O9 and ZrO2 also grew up only a little, suggesting that 30%LCG-Z possesses excellent resistance to sintering. The excellent resistance to sintering of Co NPs is ascribed to the confinement of the mixed oxides of La-Ga-O. Perovskite as the precursor is beneficial for Co NPs being in close contact with LaGaO3/ La4Ga2O9, owing to the mixed oxides La-Ga-O originated from the perovskite, which can be proved by the EDS line scanning profiles in Fig. 4. The close contact favors the interaction between Co NPs and the mixed oxides, which could fix the Co NPs from migration. For CG-Z, perovskite structure was not formed, which indicates Co NPs were not always accompanied with gallium oxide, resulting in lower resistance to sintering. For 19%LC-Z, PTO of LaCoO3 was formed, meaning that Co NPs were in close contact with La2O3. However, after reaction for 86 h for 19%LC-Z, the XRD results show that Co NPs grew a little, indicating the confinement of La2O3 for Co NPs is inferior to La-Ga-O mixed oxides. From TG results, 30%LCG-Z showed the best resistance to carbon deposition, as compared with 19%LC-Z and CG-Z catalysts, which should be attributed to the following reasons. Firstly, it is known that

carbon was combusted over 500 °C, which is attributed to graphitized carbon [44]. The TG results show that over the Ga-containing catalysts of 30%LCG-Z and CG-Z, only coke of large molecule from FT reaction was deposited, while on 19%LC-Z, there is containing a few graphitized carbon besides the coke. It is known that La2O3 as an additive can eliminate the carbon deposited via the reaction of La2O3 + CO2 → La2O2CO3 and then C + La2O2CO3 → 2CO + La2O3 [45,46]. From XPS results, La2O2CO3 can be observed for the used 19%LC-Z catalysts. However in 30%LCG-Z and CG-Z, La2O2CO3 was not detected. Therefore the Ga-containing catalysts exhibited much better resistance to carbon deposition, compared with 19%LC-Z, suggesting that Ga-adding can restrict carbon deposition effectively. 3.2.6. CO-TPD Fig. 6 displays the TPD spectra of adsorbed CO on 19%LC-Z and 30%LCG-Z catalysts, where two desorption peaks of CO appear in

Fig. 6. CO-TPD patterns of 19% LC-Z and 30% LCG-Z.

7

Catalysis Today xxx (xxxx) xxx–xxx

N. Kang et al.

oxygen vacancies can help to eliminate the deposited carbon [50]. Meanwhile, LaGaO3 and La4Ga2O9 [51–53] could generate oxygen vacancies, which can activate oxygen-containing compounds to react with the carbon deposited, thus the carbon would be eliminated. Secondly, Ga-adding can restrict the formation of Co2C, as indicated by the results of XPS and XRD. It is known that graphite carbon is formed via Co2C by migrating carbon species from Co2C onto the surface [54,55]. Lastly, Ga-adding made metallic Co in positive charge (Coδ+) as indicated by XPS results, and Coδ+ is not favored for carbon chain growth for FT synthesis [28,49], thus the formation of the coke of high molecules from FT was suppressed. Two kinds of reaction mechanism for CO hydrogenation to higher alcohols over Co-based catalysts have been proposed in literatures. (1) For the catalysts promoted by alkali metals, it is accepted that the addition of alkali metals elevate the surface basicity, leading to the formation of a lot of basic hydroxy groups [56]. The alcohols are generated via inserting surface hydroxy groups into the metal-carbon bonds, which is an accepted mechanism for La2O3 promoted cobalt catalysts to yield alcohols [41,57]. (2) Synergistic catalyzing over Co2CeCo [32,33,41] or CoeCo2+ [58,59] could form higher alcohols. Co2C and Co2+ species played an important role in CO insertion, which is beneficial to C2+OH generation. For 30%LCG-Z catalyst, there are La2O3 promoter, metallic Co and Co2+. The XPS results (Fig. S3) indicate that there are Co2+ species besides positively charged metal Co. Therefore, it can’t be determined whether higher alcohols were formed via reaction pass way of (1) or (2) stated in the above paragraph. This is a much hard work, which can’t be clarified in this paper. Anyway, whatever higher alcohols were formed via (1) or (2), letting metallic Co in positive charge would improve the selectivity to higher alcohols, as discussed in above.

[3] R. Xu, S. Zhang, C. Stewart, E. Durham, M.R. Eden, C.B. Roberts, AIChE J. 60 (2014) 1786–1796. [4] H.T. Luk, C. Mondelli, D.C. Ferre, J.A. Stewart, J. Perez-Ramirez, Chem. Soc. Rev. 46 (2017) 1358–1426. [5] Z. Fan, W. Chen, X. Pan, X. Bao, Catal. Today 147 (2009) 86–93. [6] V. Abdelsayed, D. Shekhawat, J.A. Poston Jr., J.J. Spivey, Catal. Today 207 (2013) 65–73. [7] E. Heracleous, E.T. Liakakou, A.A. Lappas, A.A. Lemonidou, Appl. Catal. A Gen. 455 (2013) 145–154. [8] S. Zaman, K.J. Smith, Catal. Rev. 54 (2012) 41–132. [9] K. Xiao, Z. Bao, X. Qi, X. Wang, L. Zhong, K. Fang, M. Lin, Y. Sun, J. Mol. Catal. A Chem. 378 (2013) 319–325. [10] Y. Yang, X. Qi, X. Wang, D. Lv, F. Yu, L. Zhong, H. Wang, Y. Sun, Catal. Today 270 (2016) 101–107. [11] K. Xiao, Z. Bao, X. Qi, X. Wang, L. Zhong, K. Fang, M. Lin, Y. Sun, Chin. J. Catal. 34 (2013) 116–129. [12] G. Liu, Y. Geng, D. Pan, Y. Zhang, T. Niu, Y. Liu, Fuel Process. Technol. 128 (2014) 289–296. [13] Z. Song, X. Shi, H. Ning, G. Liu, H. Zhong, Y. Liu, Appl. Surf. Sci. 405 (2017) 1–12. [14] X. Ning, Z. An, J. He, J. Catal. 340 (2016) 236–247. [15] Z. An, X. Ning, J. He, J. Catal. 356 (2017) 157–164. [16] S. Gao, X. Li, Y. Li, H. Yu, F. Zhang, Y. Sun, H. Fang, X. Zhang, X. Liang, Y. Yuan, Fuel (2017) Unpublished results. [17] N. Guillén-Hurtado, A. Bueno-López, A. García-García, J. Mater. Sci. 47 (2011) 3204–3213. [18] J. Si, G. Liu, J. Liu, L. Zhao, S. Li, Y. Guan, Y. Liu, RSC Adv. 6 (2016) 12699–12707. [19] J. Liu, C. Li, F. Wang, S. He, H. Chen, Y. Zhao, M. Wei, D.G. Evans, X. Duan, Catal. Sci. Technol. 3 (2013) 2627. [20] Y. Sun, Q. Sun, J. Liu, Z. Zhang, Petrochem. Technol. 43 (2014) 887–891. [21] D. He, Y. Ding, H. Luo, C. Li, J. Mol. Catal. A Chem. 208 (2004) 267–271. [22] S. Collins, M. Baltanas, A. Bonivardi, J. Catal. 226 (2004) 410–421. [23] S. Natesakhawat, P.R. Ohodnicki, B.H. Howard, J.W. Lekse, J.P. Baltrus, C. Matranga, Top. Catal. 56 (2013) 1752–1763. [24] W. Jochum, S. Penner, R. Kramer, K. Föttinger, G. Rupprechter, B. Klötzer, J. Catal. 256 (2008) 278–286. [25] G. Liu, T. Niu, D. Pan, F. Liu, Y. Liu, Appl. Catal. A Gen. 483 (2014) 10–18. [26] J. Wang, P.A. Chernavskii, A.Y. Khodakov, Y. Wang, J. Catal. 286 (2012) 51–61. [27] A. Cao, Q. Yang, Y. Wei, L. Zhang, Y. Liu, Int. J. Hydrogen Energy 42 (2017) 17425–17434. [28] X. Song, Y. Ding, W. Chen, W. Dong, Y. Pei, J. Zang, L. Yan, Y. Lu, Energy Fuels 26 (2012) 6559–6566. [29] Y. Jiao, J. Zhang, Y. Du, D. Sun, J. Wang, Y. Chen, J. Lu, Int. J. Hydrogen Energy 41 (2016) 10473–10482. [30] N. Tien-Thao, H. Alamdari, M.H. Zahedi-Niaki, S. Kaliaguine, Appl. Catal. A Gen. 311 (2006) 204–212. [31] H. Sun, Y. Ding, J. Duan, Q. Zhang, Z. Wang, H. Lou, X. Zheng, Bioresour. Technol. 101 (2010) 953–958. [32] V.M. Lebarbier, D. Mei, D.H. Kim, A. Andersen, J.L. Male, J.E. Holladay, R. Rousseau, Y. Wang, J. Phys. Chem. C 115 (2011) 17440–17451. [33] G. Jiao, Y. Ding, H. Zhu, X. Li, J. Li, R. Lin, W. Dong, L. Gong, Y. Pei, Y. Lu, Appl. Catal. A Gen. 364 (2009) 137–142. [34] F.S. Toniolo, R.N.S.H. Magalhães, C.A.C. Perez, M. Schmal, Appl. Catal. B Environ. 117–118 (2012) 156–166. [35] Z. Pu, Y. Liu, H. Zhou, W. Huang, Y. Zheng, X. Li, Appl. Surf. Sci. 422 (2017) 85–93. [36] W. Gu, J. Liu, M. Hu, F. Wang, Y. Song, ACS Appl. Mater. Interfaces 7 (2015) 26914–26922. [37] H. Zhan, F. Li, P. Gao, N. Zhao, F. Xiao, W. Wei, L. Zhong, Y. Sun, J. Power Sources 251 (2014) 113–121. [38] A.B. Mandale, S. Badrinarayanan, S.K. Date, A.P.B. Sinha, J. Electron. Spectrosc. Relat. Phenom. 33 (1984) 61–72. [39] I. Alstrup, I. Chorkendorff, R. Candia, B.S. Clausen, H. Topsøe, J. Catal. 77 (1982) 397–409. [40] T. Majima, E. Kono, S. Ogo, Y. Sekine, Appl. Catal. A Gen. 523 (2016) 92–96. [41] Z. Wang, J.J. Spivey, Appl. Catal. A Gen. 507 (2015) 75–81. [42] Y. Mizokawa, H. Iwasaki, R. Nishitani, S. Nakamura, J. Electron. Spectrosc. Relat. Phenom. 14 (1978) 129–141. [43] Z. Wang, C. Wang, S. Chen, Y. Liu, Int. J. Hydrogen Energy 39 (2014) 5644–5652. [44] S.M. de Lima, A.M. da Silva, L.O.O. da Costa, U.M. Graham, G. Jacobs, B.H. Davis, L.V. Mattos, F.B. Noronha, J. Catal. 268 (2009) 268–281. [45] X. Verykios, Int. J. Hydrogen Energy 28 (2003) 1045–1063. [46] V.A. Tsipouriari, X.E. Verykios, J. Catal. 187 (1999) 85–94. [47] L.G. Tejuca, A.T. Bell, J.L.G. Fierro, M.A. Peña, Appl. Surf. Sci. 31 (1988) 301–316. [48] Y. Pei, Y. Ding, J. Zang, X. Song, W. Dong, H. Zhu, T. Wang, W. Chen, Chin. J. Catal. 33 (2012) 808–812 Chinese version. [49] M.L. Smith, N. Kumar, J.J. Spivey, J. Phys. Chem. C 116 (2012) 7931–7939. [50] F. Liu, L. Zhao, H. Wang, X. Bai, Y. Liu, Int. J. Hydrogen Energy 39 (2014) 10454–10466. [51] J. Kapała, J. Alloys Compd. 373 (2004) 179–182. [52] I. Stijepovic, R. Djenadic, V.V. Srdic, M. Winterer, J. Eur. Ceram. Soc. 35 (2015) 3545–3552. [53] M. Zinkevich, S. Geupel, H. Nitsche, M. Ahrens, F. Aldinger, J. Phase Equilib. Diff. 25 (2004) 437–447. [54] J. Pola, M. Urbanová, D. Pokorná, J. Šubrt, S. Bakardjieva, P. Bezdička, Z. Bastl, J. Photochem. Photobiol. A Chem. 210 (2010) 153–161. [55] Z. Wang, Z. Yusop, P. Ghosh, Y. Hayashi, M. Tanemura, Appl. Surf. Sci. 257 (2011) 3168–3173. [56] T. Ishida, T. Yanagihara, X.H. Liu, H. Ohashi, A. Hamasaki, T. Honma, H. Oji, T. Yokoyama, M. Tokunaga, Appl. Catal. A Gen. 458 (2013) 145–154. [57] V.M. Lebarbier, D. Mei, D.H. Kim, A. Andersen, J.L. Male, J.E. Holladay, R. Rousseau, Y. Wang, J. Phys. Chem. C 115 (2011) 17440–17451. [58] J.E. Baker, R. Burch, S.J. Hibble, P.K. Loader, Appl. Catal. 65 (1990) 281–292. [59] M. Blanchard, H. Derule, P. Canesson, Catal. Lett. 2 (1989) 319–322.

4. Conclusion LaCo0.65Ga0.35O3/ZrO2 was prepared by combining the citrate complexing method with incipient impregnation. After reduction, the catalyst was transformed to Co NPs/LaGaO3-La4Ga2O9 supported on ZrO2, which showed excellent catalytic performance for higher alcohols synthesis from syngas, including very high selectivity to higher alcohols especially ethanol, very good resistance to sintering and to carbon deposition, and excellent activity. The high selectivity to higher alcohols was attributed to that Ga-doping made Co in positive charge which weakened the adsorption strength of CO on the surface of Co NPs, lowering the selectivity to hydrocarbons; the very good resistance of Co NPs was owing to the confinement of the mixed oxides of La-Ga-O for Co NPs and that the mixed oxides were in close contact with Co NPs; the high resistance to carbon deposition was attributed to the fact that Gadoping restricted the formation of Co2C and carbon chain growth for FT synthesis which would generate graphite carbon and coke respectively, and the carbon eliminating effect of oxygen vacancies on LaGaO3/ La4Ga2O9. Acknowledgement The financial support of this work by National Natural Science Foundation of China (NSFC) (Nos. 21576192 and 21776214) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2018.01.034. References [1] P. Forzatti, E. Tronconi, I. Pasquon, Catal. Rev. 33 (1991) 109–168. [2] V.R. Surisetty, A.K. Dalai, J. Kozinski, Appl. Catal. A Gen. 404 (2011) 1–11.

8