K-Modulated Co Nanoparticles Trapped in La-GaO as Superior Catalysts for Higher Alcohols Synthesis from Syngas

Owing to the outstanding catalytic performance for higher alcohol synthesis, Ga-Co catalysts have attracted much attention. In view of their unsatisfactory stability and alcohol selectivity, herein, K-modulated Co nanoparticles trapped in La-Ga-O catalysts were prepared by the reduction of La1−xKxCo0.65Ga0.35O3 perovskite precursor. Benefiting from the atomic dispersion of all the elements in the precursor, during the reduction of La1−xKxCo0.65Ga0.35O3, Co nanoparticles could be confined into the K-modified La-Ga-O composite oxides, and the confinement of La-Ga-O could improve the anti-sintering performance of Co nanoparticles. In addition, the addition of K modulated parts of La-Ga-O into La2O3, which ameliorated the anti-carbon deposition performance. Finally, the addition of K increased the dispersion of cobalt and provided more electron donors to metallic Co, resulting in a high activity and superior selectivity to higher alcohols. Benefiting from the above characteristics, the catalyst possesses excellent activity, good selectivity, and superior stability.


Introduction
Due to their sufficient combustion and release of less harmful substances during combustion, higher alcohols with 2-6 carbon atoms are regarded as a kind of clean energy [1].In addition, due to the high octane number, higher alcohols can also be used as a high-quality fuel additive.After separation, a series of basic chemicals with very high economic value, such as ethanol, propanol, and butanol, can be obtained [2][3][4].Currently, ethanol is mainly produced by fermentation and ethylene hydration, while other alcohols are refined from petroleum.Obviously, in the long run, the above synthesis routes for higher alcohol would be restricted by increasingly depleted petroleum and food [5].Recently, the synthesis of higher alcohols from syngas has attracted much attention, while this process is usually restricted by the low selectivity to higher alcohols and the poor stability of the catalyst.
Nowadays, four kinds of catalysts for higher alcohol synthesis (HAS) from syngas have been reported.Among them, the Rh-based catalysts show good activity and superior selectivity to ethanol, while the high price of Rh limits its industrial applications [6,7].The harsh reaction conditions usually restrict large-scale applications for Mo-based catalysts [8,9].For modified methanol synthesis catalysts, the main product is still methanol [10,11].Fortunately, modified Fischer-Tropsch catalysts, mainly the modified Co and modified Fe catalysts, exhibit good activity and high selectivity for HAS at milder reaction conditions.However, the modified Fe catalysts are more beneficial to the water gas shift reaction (WGSR), generating lots of CO 2 ; and the typical Cu modified Co catalysts usually show poor stability because of the phase separation of cobalt and copper [12][13][14].Therefore, it has become an important issue for researchers to explore new catalysts for HAS from syngas with better catalytic performance.
Recently, Ga-modified Co catalysts were reported and showed excellent catalytic performance for higher alcohol synthesis [15][16][17].He et al. prepared a series of Co-Ga catalysts by using Co-Ga-LDHs (layered double hydroxides) and found that Ga was beneficial to the non-dissociative adsorption of CO [15,16].Gao et al. reported that gallium oxide can reduce the reduction degree of CoO and generate some Co 2+ in the reduced catalysts, which act as non-dissociative CO adsorption sites for HAS, resulting in the high selectivity to alcohols for the Ga-Co/AC catalyst [17].While the stability of Co-Ga catalysts should be further improved.
Here, considering the good activity and high selectivity on Co-Ga catalysts, K doped Co-Ga catalysts are explored by the reduction of La 1−x K x Co 0.65 Ga 0.35 O 3 .The results show that the addition of K modulates the composition of La-Ga-O, enhances the dispersion of Co, and adjusts the electronic structure of Co, and as a result the catalysts possess excellent catalytic performance.Typically, an outstanding selectivity of 43.6% to the higher alcohols, and a stable catalytic performance during the 200 h reaction can be obtained.

X-ray Powder Diffraction (XRPD)
The X-ray powder diffraction (XRPD) patterns of the three catalysts for LCG (LaCo 0.65 Ga 0.35 O 3 ) and LKCG-x (La 1−x K x Co 0.65 Ga 0.35 O 3 , x = 0.1 and 0.2, where x is the K content in perovskite) (see 3.2 catalysis synthesis) are shown in Figure 1a.The diffraction peaks at 2θ = 23.2, 33.3, 40.6, 47.4 and 58.8 • are attributed to the characteristic diffraction peaks of perovskite-type oxide (PTO).For LKCG-0.1 catalyst, with the addition of K ions into LCG, the perovskite diffraction peaks move to lower 2θ values (seen from the illustration of Figure 1a), for that the ion radius of K + (0.155 nm) is larger than that of La 3+ (0.136 nm) [18].The existence of perovskite structure after calcination is beneficial to the interaction and the even dispersion of all the elements.
For the LKCG-0.2, a new Co 3 O 4 diffraction peak in Figure 1a can be seen.Since the amount of K entering the perovskite is limited, when the K doping amount is more than 0.1, part of potassium cannot incorporate into the perovskite structure and cover the surface of the catalyst in the form of oxide [18].The presence of K 2 O disrupted the dispersion of elements in the catalyst precursor, resulting in the formation of Co Meanwhile, Ga-containing oxides among the three samples cannot be detected, indicating that Ga entered into the structure of perovskite.The uniformly dispersed Co and Ga ions in the LKCG-0.1 catalyst are advantageous for the synergism between them, favoring the catalytic performance.
The XRPD profiles of three catalysts reduced at 750 • C (see 3.4 Catalysts' Performance) are presented in Figure 1b.As for LCG, the perovskite structure disappears and transfers to Co and La 4 Ga 2 O 9 .As for the reduction of LKCG-0.1, phases of La 2 O 3 , Co, LaGaO 3 , and a small amount of La 4 Ga 2 O 9 can be observed.The existence of the characteristic diffraction peak of LaGaO 3 and La 2 O 3 indicated that the adding of K weakened the interaction between lanthanum and gallium.In other words, the addition of K in the perovskite modulated the composition of La-Ga-O, resulting in the change of La-Ga-O from La 4 Ga 2 O 9 to LaGaO 3 and La 2 O 3 .
Based on the above discussion in Figure 1a, parts of K cannot be doped into perovskite, resulting co-exist of LaCo y Ga 1−y O 3 and La 1−z K z Co 1−m Ga m O 3 in LKCG-0.2.During reduction,  Figure 1c shows the XRPD profiles of the three catalysts after reaction and LKCG-0.For the LCG catalyst after reaction, it should be noted that the catalyst was still Co/La 4 Ga 2 O 9 , which is the same as that after reduction.La 2 O 2 CO 3 and La 2 O 3 cannot be detected, indicating the above reaction of eliminating carbon deposition may be hard to occur due to a strong interaction existing between lanthanum and gallium.
It should be noted that no Co 2 C was observed in all the used samples, suggesting that the existence of gallium can prevent the formation of Co 2 C and stabilize the catalyst composition in the process of reaction.This is in accordance with the literature, which illustrated that the existence of gallium could improve the catalyst's stability [21].

Temperature-Programmed Reduction (TPR)
Figure 2 and Table 1 illustrates the temperature-programmed reduction (TPR) and the hydrogen consumptions values of LCG and LKCG-x (x = 0.1 and 0.2) catalysts.Seen from the Figure 2, all the three catalysts contain two major hydrogen-consuming peaks, one at 400-500 • C and the other at 600-800 • C. According to the literature, the H 2 -consuming peak of LaCoO 3 at 500 • C can be attributed to Co 3+ → Co 2+ , while that above 500 • C to Co 2+ → Co 0 [22][23][24][25].Herein, seen from Table 1, the area ratio of the low temperature peak to the high temperature peak (T L /T H ) is 1/2.Therefore, we believe that the peak of 400-500 • C in all the three catalysts is classified to Co 3+ → Co 2+ , and the peak of 600-800 • C can be assigned to Co 2+ → Co 0 .At the same time, the similar theoretical and experimental H 2 -consumption also confirmed the attribution of the above reduction peak.
words, the addition of K in the perovskite modulated the composition of La-Ga-O, resulting in the change of La-Ga-O from La4Ga2O9 to LaGaO3 and La2O3.
Based on the above discussion in Figure 1a, parts of K cannot be doped into perovskite, resulting co-exist of LaCoyGa1-yO3 and La1-zKzCo1-mGamO3 in LKCG-0.2.During reduction, La1-zKzCo1-mGamO3 would be reduced into LaGaO3 while LaCoyGa1-yO3 to La4Ga2O9, as a result, the LKCG-0.2 are reduced to Co/LaGaO3-La4Ga2O9, as can be seen in Figure 1b.
Figure 1c shows the XRPD profiles of the three catalysts after reaction and LKCG-0.1 after 200 h reaction.After reaction, part of La2O3 transferred to LaCO3OH and La2O2CO3, for that La2O3 and CO2 can react to generate La2O2CO3, and the further reaction between La2O2CO3, H2O and CO2 can generate LaCO3OH [19,20].Since XRPD in this work was carried out ex situ, the catalysts containing La2O2CO3 could readily absorb H2O and CO2 in air, and then LaCO3OH formed.The co-existence of La2O3 and La2O2CO3 in the catalysts after reaction illustrated the feasibility of reaction of CO2 + La2O3 → La2O2CO3  → 2CO + La2O3, which can help the catalysts eliminate carbon.For the LCG catalyst after reaction, it should be noted that the catalyst was still Co/La4Ga2O9, which is the same as that after reduction.La2O2CO3 and La2O3 cannot be detected, indicating the above reaction of eliminating carbon deposition may be hard to occur due to a strong interaction existing between lanthanum and gallium.
It should be noted that no Co2C was observed in all the used samples, suggesting that the existence of gallium can prevent the formation of Co2C and stabilize the catalyst composition in the process of reaction.This is in accordance with the literature, which illustrated that the existence of gallium could improve the catalyst's stability [21].

Temperature-Programmed Reduction (TPR)
Figure 2 and Table 1 illustrates the temperature-programmed reduction (TPR) and the hydrogen consumptions values of LCG and LKCG-x (x = 0.1 and 0.2) catalysts.Seen from the Figure 2, all the three catalysts contain two major hydrogen-consuming peaks, one at 400-500 °C and the other at 600-800 °C .According to the literature, the H2-consuming peak of LaCoO3 at 500 °C can be attributed to Co 3+ →Co 2+ , while that above 500 °C to Co 2+ →Co 0 [22][23][24][25].Herein, seen from Table 1, the area ratio of the low temperature peak to the high temperature peak (TL/TH) is 1/2.Therefore, we believe that the peak of 400-500 °C in all the three catalysts is classified to Co 3+ →Co 2+ , and the peak of 600-800 °C can be assigned to Co 2+ →Co 0 .At the same time, the similar theoretical and experimental H2-consumption also confirmed the attribution of the above reduction peak.Compared to the TPR results of LaCoO3 in the literature, the H2-consuming peak of Co 2+ → Co 0 migrates to higher temperatures in the LCG catalyst, suggesting that the existence of Ga in LaCoO3 will restrain the conversion of Co 2+ to Co 0 and a strong effect between Co and Ga exists for LCG [16].Compared to the TPR results of LaCoO 3 in the literature, the H 2 -consuming peak of Co 2+ → Co 0 migrates to higher temperatures in the LCG catalyst, suggesting that the existence of Ga in LaCoO 3 will restrain the conversion of Co 2+ to Co 0 and a strong effect between Co and Ga exists for LCG [16].
Seen from Figure 2, with the addition of K ions in LCG, the H 2 -consuming peaks at around 750 • C moved to lower temperature, illustrating the addition of K promotes the reduction of Co 2+ → Co 0 .While for LKCG-0.2, a shoulder peak around 620 • C appeared.According to XRPD, the catalyst contains Co 3 O 4 .Therefore, this small shoulder can be attributed to the reduction of Co 2+ → Co 0 in Co 3 O 4 .The presence of the small shoulder also confirms the XRPD results.For the other samples, no shoulders can be observed, which indicates that all the Co ions have entered into the crystal lattice of the perovskite.

N 2 Adsorption and Desorption Curves
Figure 3 and Table 2 show the N 2 physical adsorption curves and the physical properties of the investigated catalysts.Seen from the Figure 3, the curves are typical type II isotherms accompanying with a H 3 type hysteresis loop, which indicates that mesoporous exits in the catalysts.The presence of mesoporous can also be seen from the pore size distribution curves.
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 14 Seen from Figure 2, with the addition of K ions in LCG, the H2-consuming peaks at around 750 °C moved to lower temperature, illustrating the addition of K promotes the reduction of Co 2+ → Co 0 .While for LKCG-0.2, a shoulder peak around 620 °C appeared.According to XRPD, the catalyst contains Co3O4.Therefore, this small shoulder can be attributed to the reduction of Co 2+ →Co 0 in Co3O4.The presence of the small shoulder also confirms the XRPD results.For the other samples, no shoulders can be observed, which indicates that all the Co ions have entered into the crystal lattice of the perovskite.

N2 Adsorption and Desorption Curves
Figure 3 and Table 2 show the N2 physical adsorption curves and the physical properties of the investigated catalysts.Seen from the Figure 3, the curves are typical type II isotherms accompanying with a H3 type hysteresis loop, which indicates that mesoporous exits in the catalysts.The presence of mesoporous can also be seen from the pore size distribution curves.For LKCG-x (x = 0.1 and 0.2) catalysts, with the addition of K, the hysteresis loop increases and moves to lower P/P 0 , illustrating the investigated catalysts have bigger BET surface area.The larger surface area results in the higher dispersion of metal cobalt nanoparticles (see in Table 2).

X-ray Photoelectron Spectroscopy (XPS)
Figure 4 and Table 3 summarized the binding energies (BEs) and the X-ray photoelectron spectroscopy (XPS) profiles of La 3d, Co 2p, and Ga 3d for the reduced LCG and LKCG-0.1 catalysts.All XPS profiles showed almost similar peak patterns except the different value of binding energies of each element.The binding energy of Co in the both samples is similar to that of metal cobalt, illustrating that Co exists in the form of Co 0 in the catalyst, which is the same as the XRPD results.
Catalysts 2018, 8, x FOR PEER REVIEW 6 of 14 For LKCG-x (x = 0.1 and 0.2) catalysts, with the addition of K, the hysteresis loop increases and moves to lower P/P0, illustrating the investigated catalysts have bigger BET surface area.The larger surface area results in the higher dispersion of metal cobalt nanoparticles (see in Table 2).

X-ray Photoelectron Spectroscopy (XPS)
Figure 4 and Table 3 summarized the binding energies (BEs) and the X-ray photoelectron spectroscopy (XPS) profiles of La 3d, Co 2p, and Ga 3d for the reduced LCG and LKCG-0.1 catalysts.All XPS profiles showed almost similar peak patterns except the different value of binding energies of each element.The binding energy of Co in the both samples is similar to that of metal cobalt, illustrating that Co exists in the form of Co 0 in the catalyst, which is the same as the XRPD results.According to the reported literature, the binding energies of La 3d5/2 are 834.4 and 837.8 eV for pure La2O3, 834.7 eV and 838.1 eV for LaGaO3 [26][27][28].Herein, the binding energies of La 3d5/2 for LCG are 835.2eV and 838.6 eV, which is larger than that of pure La2O3 and LaGaO3.The binding energies of Ga 3d5/2 are 17.6 and 19.7 eV, which is also a little larger than that of LaGaO3 at 17.4 and 19.4 eV [28].The binding energies of Co 2p3/2 is 778.3 eV, which is less than 778.5 eV for metal cobalt [29].The higher binding energies of La 3d5/2 and Ga 3d5/2, and the lower binding energies of Co 2p3/2 illustrate that an interaction among La, Ga, and Co existed.At the same time, La and Ga could donate elector to Co.
Compared to the reduced LCG catalyst, it was found that the binding energy of La in LKCG-0.1 decreased, suggesting that the doping of K modulated the interaction between La and Ga, which agrees with the above XRPD results.In addition, the binding energy of Co is lower, which means that the K could donate electron to Co.The enhanced electron for Co is beneficial for the selectivity to higher alcohols.

Transmission Electron Microscopy (TEM)
Figure 5a-h shows the transmission electron microscopy (TEM) images, the line scans profiles, the energy dispersive spectrometer (EDS) mapping scans image, and the elements distribution of the reduced LKCG-0.1 catalyst.In Figure 5a, 5-11 nm Co nanoparticles are uniform dispersed in the reduced LKCG-0.1 catalyst even after 750 °C high temperature reduction.In Figure 5b, the lattice spacing of [112] and [011] planes for La2O3, [200] and [111] planes for metal Co, [220] planes for La4Ga2O9 and LaGaO3 can be clearly seen.That is to say, the composition of the reduced LKCG-0.1 catalyst is Co/La2O3-La4Ga2O9-LaGaO3, which is consistent with the XRPD.In addition, as seen from Figure 5b, the metal cobalt nanoparticles are encircled and located between La-Ga-O oxides.And this confinement effect result in the highly dispersion of Co nanoparticles in Figure 5a.According to the reported literature, the binding energies of La 3d 5/2 are 834.4 and 837.8 eV for pure La 2 O 3 , 834.7 eV and 838.1 eV for LaGaO 3 [26][27][28].Herein, the binding energies of La 3d 5/2 for LCG are 835.2eV and 838.6 eV, which is larger than that of pure La 2 O 3 and LaGaO 3 .The binding energies of Ga 3d 5/2 are 17.6 and 19.7 eV, which is also a little larger than that of LaGaO 3 at 17.4 and 19.4 eV [28].The binding energies of Co 2p 3/2 is 778.3 eV, which is less than 778.5 eV for metal cobalt [29].The higher binding energies of La 3d 5/2 and Ga 3d 5/2 , and the lower binding energies of Co 2p 3/2 illustrate that an interaction among La, Ga, and Co existed.At the same time, La and Ga could donate elector to Co.
Compared to the reduced LCG catalyst, it was found that the binding energy of La in LKCG-0.1 decreased, suggesting that the doping of K modulated the interaction between La and Ga, which agrees with the above XRPD results.In addition, the binding energy of Co is lower, which means that the K could donate electron to Co.The enhanced electron for Co is beneficial for the selectivity to higher alcohols.

Transmission Electron Microscopy (TEM)
Figure 5a-h shows the transmission electron microscopy (TEM) images, the line scans profiles, the energy dispersive spectrometer (EDS) mapping scans image, and the elements distribution of the reduced LKCG-0.1 catalyst.In Figure 5a, 5-11 nm Co nanoparticles are uniform dispersed in the reduced LKCG-0.1 catalyst even after 750 • C high temperature reduction.In Figure 5b, the lattice spacing of [112] and [011] planes for La 2 O 3 , [200] and [111] planes for metal Co, [220] planes for La 4 Ga 2 O 9 and LaGaO 3 can be clearly seen.That is to say, the composition of the reduced LKCG-0.1 catalyst is Co/La 2 O 3 -La 4 Ga 2 O 9 -LaGaO 3 , which is consistent with the XRPD.In addition, as seen from Figure 5b, the metal cobalt nanoparticles are encircled and located between La-Ga-O oxides.And this confinement effect result in the highly dispersion of Co nanoparticles in Figure 5a. Figure 5c-h exhibits the line scans profiles and the corresponding element distribution of the reduced LKCG-0.1 catalyst.The red lines represent the scanning routes in Figure 5c.Seen from the illustration in Figure 5c, the La and Ga have the same change trend, indicating the formation of La-Ga-O.In addition, there is no La and Ga where Co appears, illustrating metal cobalt nanoparticles are highly dispersed and located between the La-Ga-O oxides, which is in accordance with the XRPD and Figure 5b.
Figure 6a-h displays the TEM images, the line scanning profiles, the EDS mapping image, and the corresponding elements distribution of LKCG-0.1 after 200 h stability tests.As can be seen from Figure 6a, Co is still located between La-Ga-O oxide and the average crystal size of the Co nanoparticles is 9.5 nm, indicating that the sintering of the catalyst is not obvious, which is consistent with the results in Table 2. Seen from the Figure 6b, the lattice spacing of 0.205 and 0.  Figure 5c-h exhibits the line scans profiles and the corresponding element distribution of the reduced LKCG-0.1 catalyst.The red lines represent the scanning routes in Figure 5c.Seen from the illustration in Figure 5c, the La and Ga have the same change trend, indicating the formation of La-Ga-O.In addition, there is no La and Ga where Co appears, illustrating metal cobalt nanoparticles are highly dispersed and located between the La-Ga-O oxides, which is in accordance with the XRPD and Figure 5b.
Figure 6a-h displays the TEM images, the line scanning profiles, the EDS mapping image, and the corresponding elements distribution of LKCG-0.1 after 200 h stability tests.As can be seen from Figure 6a, Co is still located between La-Ga-O oxide and the average crystal size of the Co nanoparticles is 9.5 nm, indicating that the sintering of the catalyst is not obvious, which is consistent with the results in Table 2. Seen from the Figure 6b

CO Hydrogenation Performance
Table 4 lists the catalytic performance of all the investigated catalysts.The carbon monoxide hydrogenation performance of the different molar ratio of Co/Ga for LaCo y Ga 1−y O 3 catalyst were explored in our lab, and the results revealed the optimum molar ratio is 0.65/0.35[30].Therefore, the molar ratio of Co/Ga of all the investigated samples was fixed at 0.65/0.35.

CO Hydrogenation Performance
Table 4 lists the catalytic performance of all the investigated catalysts.The carbon monoxide hydrogenation performance of the different molar ratio of Co/Ga for LaCoyGa1-yO3 catalyst were explored in our lab, and the results revealed the optimum molar ratio is 0.65/0.35[30].Therefore, the molar ratio of Co/Ga of all the investigated samples was fixed at 0.65/0.35.As for LCG, seen from the above TEM and XRPD results, the composition after reduction is the same with that after reaction, and all are Co/La4Ga2O9.In addition, the smaller BET surface area for LCG makes the Co nanoparticle severely sintered and unevenly dispersed (see in Table 2), thus   C, GHSV = 6000 mL g cat −1 •h −1 .a X co is CO conversion.b S co 2 is the selectivity to CO 2 .c S ROH is the selectivity to the total alcohols.i in C i is number of carbon atoms for the carbon-contained products.C 4+ represents the carbon-contained products with 4 or more carbon atoms.
As for LCG, seen from the above TEM and XRPD results, the composition after reduction is the same with that after reaction, and all are Co/La 4 Ga 2 O 9 .In addition, the smaller BET surface area for LCG makes the Co nanoparticle severely sintered and unevenly dispersed (see in Table 2), thus resulting in a poor activity.At the same time, the larger Co particle sizes and the strong effects between Ga and La in the catalyst are also detrimental to the generation of the Co-Ga interfaces.The interfaces of Co and Ga were usually considered to be the active sites for HAS [16], while metal cobalt was the active sites of hydrocarbon generation [14].Therefore, the LCG catalyst has the highest hydrocarbon selectivity among all the samples.However, as for LKCG-0.1 catalyst, the main composition after reduction is Co/K 2 O-La 2 O 3 -LaGaO 3 and the main composition after reaction is Co/K 2 O-La 2 O 3 -La 2 O 2 CO 3 -LaGaO 3 (seen from the XRPD results).It is known from Table 4 that the catalyst with the optimal catalytic performance is LKCG-0.1.Since they have a larger specific surface area, cobalt nanoparticles are highly dispersed on the catalyst surface, which can be seen in Figure 5 and Table 2.In addition, since all the elements located in the lattice of perovskite, smaller size and uniformly dispersed cobalt nanoparticles are also conducive to generating more Co-Ga interfaces.In the process of reaction, cobalt exists in the form of Co 0 .The close contact of cobalt with the Co-Ga interface at the atomic level is beneficial to the synergistic effect of the catalyst, and thus the LKCG-0.1 catalyst exhibits the best catalytic performance.In addition, the electron donating effect of K can promote the increase of the selectivity of higher alcohols [31][32][33][34].
For the LKCG-0.2catalyst, with the increasing of K content, the catalytic activity decreased, for that the addition of K can make part of Co outside the perovskite structure, resulting in a non-uniform dispersion of Co and Ga.A relatively lower activity and selectivity is observed in Table 4.
Other catalysts with outstanding performance reported in the literature are revealed in Table 5 [17,[35][36][37][38].By comparison, the activity of LKCG-0.1 catalyst in this work is not the optimal, but it may be one of the good catalysts in general considering relatively lower reaction temperatures and higher alcohol selectivity for HAS. a Molar ratio.b The ethanol's mass fraction in all alcohols.c The mass fraction of the higher alcohols in all alcohols.
Figure 7 presents the carbon monoxide hydrogenation performance for 200-h stability tests of LKCG-0.1 catalyst.Seen from Figure 7, the alcohol's selectivity and CO conversion still maintained stability, which are remained at 19.8% and 41.8%, and the higher alcohols in all alcohols stabilized at 72.8%.The outstanding stability can be owned to the uniform dispersion of the active sites, stable catalyst structure, good sintering resistance, and more Co-Ga interfaces.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 14 resulting in a poor activity.At the same time, the larger Co particle sizes and the strong effects between Ga and La in the catalyst are also detrimental to the generation of the Co-Ga interfaces.The interfaces of Co and Ga were usually considered to be the active sites for HAS [16], while metal cobalt was the active sites of hydrocarbon generation [14].Therefore, the LCG catalyst has the highest hydrocarbon selectivity among all the samples.However, as for LKCG-0.1 catalyst, the main composition after reduction is Co/K2O-La2O3-LaGaO3 and the main composition after reaction is Co/K2O-La2O3-La2O2CO3-LaGaO3 (seen from the XRPD results).It is known from Table 4 that the catalyst with the optimal catalytic performance is LKCG-0.1.Since they have a larger specific surface area, cobalt nanoparticles are highly dispersed on the catalyst surface, which can be seen in Figure 5 and Table 2.In addition, since all the elements located in the lattice of perovskite, smaller size and uniformly dispersed cobalt nanoparticles are also conducive to generating more Co-Ga interfaces.In the process of reaction, cobalt exists in the form of Co 0 .The close contact of cobalt with the Co-Ga interface at the atomic level is beneficial to the synergistic effect of the catalyst, and thus the LKCG-0.1 catalyst exhibits the best catalytic performance.In addition, the electron donating effect of K can promote the increase of the selectivity of higher alcohols [31][32][33][34].
For the LKCG-0.2catalyst, with the increasing of K content, the catalytic activity decreased, for that the addition of K can make part of Co outside the perovskite structure, resulting in a non-uniform dispersion of Co and Ga.A relatively lower activity and selectivity is observed in Table 4.
Other catalysts with outstanding performance reported in the literature are revealed in Table 5 [17,[35][36][37][38].By comparison, the activity of LKCG-0.1 catalyst in this work is not the optimal, but it may be one of the good catalysts in general considering relatively lower reaction temperatures and higher alcohol selectivity for HAS.  Figure 7 presents the carbon monoxide hydrogenation performance for 200-h stability tests of LKCG-0.1 catalyst.Seen from Figure 7, the alcohol's selectivity and CO conversion still maintained stability, which are remained at 19.8% and 41.8%, and the higher alcohols in all alcohols stabilized at

Thermo-Gravimetry (TG)
Figure 8 exhibited the TG curves of the reduced and used LCG and LKCG-x (x = 0.1 and 0.2) catalysts and the corresponding differential thermal gravity (DTG) curves of the used catalysts.In Figure 8, the weight of the three reduced samples increases in the temperature range of 200-320 • C, which is attributed to the oxidation of metal cobalt nanoparticles on the surface of the catalysts.Therefore, the TG profiles of the catalysts after reduction was severed as a datum to explore the carbon deposition amount of the catalysts after reaction.In Figure 8b, two exothermic peaks can be seen for all the samples.The peak located at 300-600 • C can be attributed to amorphous carbon; while the other peak at 600-800 • C to the graphitized carbon [39].Seen from Figure 8b, the incorporation of K can significantly reduce the total amount and formation rate of amorphous carbon, and for that K can modulate the composition of the catalysts and produce amount of La 2 O 3 , which is beneficial to the coke elimination.
72.8%.The outstanding stability can be owned to the uniform dispersion of the active sites, stable catalyst structure, good sintering resistance, and more Co-Ga interfaces.

Thermo-Gravimetry (TG)
Figure 8 exhibited the TG curves of the reduced and used LCG and LKCG-x (x = 0.1 and 0.2) catalysts and the corresponding differential thermal gravity (DTG) curves of the used catalysts.In Figure 8, the weight of the three reduced samples increases in the temperature range of 200-320 °C , which is attributed to the oxidation of metal cobalt nanoparticles on the surface of the catalysts.Therefore, the TG profiles of the catalysts after reduction was severed as a datum to explore the carbon deposition amount of the catalysts after reaction.In Figure 8b, two exothermic peaks can be seen for all the samples.The peak located at 300-600 °C can be attributed to amorphous carbon; while the other peak at 600-800 °C to the graphitized carbon [39].Seen from Figure 8b, the incorporation of K can significantly reduce the total amount and formation rate of amorphous carbon, and for that K can modulate the composition of the catalysts and produce amount of La2O3, which is beneficial to the coke elimination.
Seen from the Figure 8, the carbon deposition amounts of LKCG-0.2,LKCG-0.1 and LCG after reaction are 3.9%, 5.3% and 10.7%, respectively; in other words, the addition of K significantly relieves the formation of carbon deposition.Seen from the Figure 8, the carbon deposition amounts of LKCG-0.2,LKCG-0.1 and LCG after reaction are 3.9%, 5.3% and 10.7%, respectively; in other words, the addition of K significantly relieves the formation of carbon deposition.
For LKCG-x (x = 0.1 and 0.2) catalysts, the containing carbon content of the two catalysts is almost similar.Compared to LCG catalyst, the adding of K leads to a decrease of carbon deposition.According to the above XRPD and XPS results for the reduced LKCG-x catalysts, the doping of K can modulate the composition of La-Ga-O, generating more La 2 O 3 , which has a better carbon-depleting effect.
Herein, the formula of CO 2 + La 2 O 3 → La 2 O 2 CO 3 C → 2CO + La 2 O 3 is used to illustrate the process of removing carbon deposits during the reaction, indicating that more La 2 O 3 favors the anti-carbon effect of catalysts [19,[40][41][42].In addition, seen from the used XRPD pattern for LKCG-x catalysts, both La 2 O 3 and La 2 O 2 CO 3 can also be detected, explaining that the above mechanism is correct.Therefore, the K-doped catalyst exhibits the best anti-carbon deposition performance.
For the LCG catalyst, seen from the used XRPD result, no La 2 O 3 and La 2 O 2 CO 3 can be detected.Meanwhile, there is a strong effects between La and Ga, which is not conducive to eliminating carbon deposited, and thus the carbon content of the LCG catalyst is largest.

Material
Lanthanum (III) nitrate hexahydrate, citrate acid, cobalt (II) nitrate hexahydrate, potassium nitrate, and glycol were bought from Shanghai Aladdin.Gallium (III) nitrate nonahydrate was purchased from Beijing HWRK Chem.All of above materials were used without further purification.

Catalysts' Synthesis
The citrate complexation method was used to prepare the K-doping catalysts [43].Firstly, a solution with La, K, Co, Ga nitrates and citric acid, glycol are mixed by using deionized water, in which the molar ratio of lanthanum:potassium:cobalt:gallium:citric acid:glycol is 1−x:x:0.65:0.35:2.4:0.48.Secondly, the above prepared mixed solution was continuously stirred to gel at 80 • C, and then dried overnight at 120 • C. Finally, the powder catalysts were calcined at 350 • C for 2 h and 700 • C for 5 h (with a heating speed of 2 • C min −1 ), respectively.The obtained La 1−x K x Co 0.65 Ga 0.35 O 3 were labeled as LKCG-x (x = 0.1 or 0.2, which is the content of K in perovskite).
For comparison, LaCo 0.65 Ga 0.35 O 3 without K doping was prepared according to the above method, and the sample was labeled as LCG.

Catalysts' Characterization
XRPD patterns were performed at the speed of 8 • C min −1 between 20 • and 80 • (2θ).TPR were performed between 30 • and 900 • C in 5% H 2 /Ar (50 mL min −1 ) at the heating rate 10 • C min −1 .TEM images, the EDS line scans and the corresponding element mapping scans analysis were performed to observe the component structure of the catalysts.The N 2 physical adsorption curve were tested to calculate the BET surface areas, the pore size distributions, and the pore volumes of the catalysts.XPS profiles were used to analyze the binding energy of elements, and the binding energy (BE) of C 1s was 284.6 eV.TG was performed between 30 • C and 900 • C in air at the heating speed of 10 • C min −1 .H 2 -TPD was used to analyze the dispersion degree and size of metal cobalt.The formula of d (Co 0 ) = 96/D was used to calculate the size of metal cobalt, and the dispersion of metal cobalt was conducted by using the method reported in literatures [44,45].

Catalysts' Performance
The CO hydrogenation performance of catalysts were tested in a fixed-bed reactor; 0.4 g samples and 0.4 g quartz sand in 40-60 mesh were mixed and used for the HAS reaction.Temperature, pressure and gas hourly space velocity were set to 290 • C, 4 MPa and 6000 mL•g cat −1 •h −1 , respectively.
The molar ratio of H 2 /CO is 2/1, and the internal standard gas of reaction was N 2 .Before reaction, the catalysts precursors were reduced at 750 • C and remained 3 h in pure H 2 atmosphere.A TCD detector was used for analyzing H 2 , N 2 , CO, CO 2 , and CH 4 ; the hydrocarbons and alcohols were detected by

3 O 4 .
It is worth noting that a part of LaCo y Ga 1−y O 3 and La 1−z K z Co 1−m Ga m O 3 should also exist accompanied by the formation of Co 3 O 4 .

a
4) c (9.9) d 7.0 b (8.4) c (9.7) d LKCG-Calculated from X-ray diffraction results with the Scherrer equation.b The reduced catalysts.c The catalysts after reaction.d The 200 h stability test.e The degree of dispersion.f The crystal size calculated by hydrogen temperature-programmed desorption (H2-TPD).

a
Calculated from X-ray diffraction results with the Scherrer equation.b The reduced catalysts.c The catalysts after reaction.d The 200 h stability test.e The degree of dispersion.f The crystal size calculated by hydrogen temperature-programmed desorption (H 2 -TPD).
Figure5c-h exhibits the line scans profiles and the corresponding element distribution of the reduced LKCG-0.1 catalyst.The red lines represent the scanning routes in Figure5c.Seen from the illustration in Figure5c, the La and Ga have the same change trend, indicating the formation of La-Ga-O.In addition, there is no La and Ga where Co appears, illustrating metal cobalt nanoparticles are highly dispersed and located between the La-Ga-O oxides, which is in accordance with the XRPD and Figure5b.Figure6a-h displays the TEM images, the line scanning profiles, the EDS mapping image, and the corresponding elements distribution of LKCG-0.1 after 200 h stability tests.As can be seen from Figure6a, Co is still located between La-Ga-O oxide and the average crystal size of the Co nanoparticles is 9.5 nm, indicating that the sintering of the catalyst is not obvious, which is consistent with the results in Table2.Seen from the Figure6b, the lattice spacing of 0.205 and 0.177 nm are assigned to parameters of the [111] and [200] planes of Co; the lattice spacing of 0.228 nm, 0.276 nm, 0.306 nm and 0.294 nm corresponds to the [012], [200], [023], and [103] planes for La2O3, LaGaO3, La4Ga2O9 and La2O2CO3, respectively.That is to say, the component of the LKCG-0.1 catalyst after 200 hours reaction is Co/La2O3-La4Ga2O9-LaGaO3-La2O2CO3, which is in accordance with the XRPD results.
Figure5c-h exhibits the line scans profiles and the corresponding element distribution of the reduced LKCG-0.1 catalyst.The red lines represent the scanning routes in Figure5c.Seen from the illustration in Figure5c, the La and Ga have the same change trend, indicating the formation of La-Ga-O.In addition, there is no La and Ga where Co appears, illustrating metal cobalt nanoparticles are highly dispersed and located between the La-Ga-O oxides, which is in accordance with the XRPD and Figure5b.Figure6a-h displays the TEM images, the line scanning profiles, the EDS mapping image, and the corresponding elements distribution of LKCG-0.1 after 200 h stability tests.As can be seen from Figure6a, Co is still located between La-Ga-O oxide and the average crystal size of the Co nanoparticles is 9.5 nm, indicating that the sintering of the catalyst is not obvious, which is consistent with the results in Table2.Seen from the Figure6b, the lattice spacing of 0.205 and 0.177 nm are assigned to parameters of the [111] and [200] planes of Co; the lattice spacing of 0.228 nm, 0.276 nm, 0.306 nm and 0.294 nm corresponds to the [012], [200], [023], and [103] planes for La 2 O 3 , LaGaO 3 , La 4 Ga 2 O 9 and La 2 O 2 CO 3 , respectively.That is to say, the component of the LKCG-0.1 catalyst after 200 h reaction is Co/La 2 O 3 -La 4 Ga 2 O 9 -LaGaO 3 -La 2 O 2 CO 3 , which is in accordance with the XRPD results.

Figure 8 .
Figure 8.(a) Thermo-gravimetric (TG) curves of the reduced and used catalysts and (b) differential thermal gravity (DTG) curves of the used catalysts.

ForFigure 8 .
Figure 8.(a) Thermo-gravimetric (TG) curves of the reduced and used catalysts and (b) differential thermal gravity (DTG) curves of the used catalysts.
1 after 200 h reaction.After reaction, part of La 2 O 3 transferred to LaCO 3 OH and La 2 O 2 CO 3 , for that La 2 O 3 and CO 2 can react to generate La 2 O 2 CO 3 , and the further reaction between La 2 O 2 CO 3 , H 2 O and CO 2 can generate LaCO 3 OH [19,20].Since XRPD in this work was carried out ex situ, the catalysts containing La 2 O 2 CO 3 could readily absorb H 2 O and CO 2 in air, and then LaCO 3 OH formed.The co-existence of La 2 O 3 and La 2 O 2 CO 3 in the catalysts after reaction illustrated the feasibility of reaction of CO 2 + La 2 O 3 → La 2 O 2 CO 3 C → 2CO + La 2 O 3 , which can help the catalysts eliminate carbon.

Table 1 .
Theoretical and experimental H 2 consumption value of the LKCG-x (x = 0.1 and 0.2) and LCG catalysts.Calculated from the TPR results.b The unit of H 2 consumption value is mmol H 2 /50 mg.T L and T H represent low and high temperature, respectively. a

Table 1 .
Theoretical and experimental H2 consumption value of the LKCG-x (x = 0.1 and 0.2) and LCG catalysts.
a Calculated from the TPR results.b The unit of H2 consumption value is mmol H2/50 mg.TL and TH represent low and high temperature, respectively.

Table 2 .
The physical properties of all the investigated catalysts.

Table 2 .
The physical properties of all the investigated catalysts.

Table 3 .
The binding energies of the reduced LCG and LKCG-0.1 catalysts.

Table 3 .
The binding energies of the reduced LCG and LKCG-0.1 catalysts.

Table 4 .
CO hydrogenation performance of the investigated catalysts.
Reaction conditions: P = 4 MPa, H2/CO/N2 = 8/4/1, T = 290 °C, GHSV = 6000 mL gcat −1 •h −1 .a Xco is CO conversion.b Sco2 is the selectivity to CO2. c SROH is the selectivity to the total alcohols.i in Ci is number of carbon atoms for the carbon-contained products.C4+ represents the carbon-contained products with 4 or more carbon atoms.

Table 4 .
CO hydrogenation performance of the investigated catalysts.

Table 5 .
Performance of CO hydrogenation reported in the literature.

Table 5 .
Performance of CO hydrogenation reported in the literature.The ethanol's mass fraction in all alcohols.c The mass fraction of the higher alcohols in all alcohols.
a Molar ratio.b