**1. Introduction**

Higher alcohols are attracting considerable attentions owing to their broad applications, such as fuels, fuel additives, and feedstock for the production of various chemicals and polymers [1–3]. With increasing concerns for environmental pollution and depletion of non-renewable petroleum resources, there is a growing interest in the direct synthesis of oxygenates, especially higher alcohols synthesis via syngas derived from coal, natural gas, or biomass [4]. Generally, the catalysts suitable for higher alcohols synthesis can be divided into the following classes: (I) Rh-based catalysts [5,6], (II) the modified Fischer–Tropsch catalysts [7,8], (III) Mo-based catalysts [9], and (IV) the modified Cu-based catalysts for methanol synthesis [10–14]. Non-noble Cu-based catalysts, due to their

comparable high activity, are regarded as one kind of the most promising candidates for higher alcohols synthesis [4,13,15].

With respect to Cu-based catalysts, remarkably important advances have been made and reported owing to the simple preparation method and full utilization of active components [2,4,10–18]. It has been well documented that interaction between metal oxide and the support significantly improved the dispersion of the active species [19,20], stabilized active species [12], and promoted the generation of new inter-phases [16,17,21,22], thereby strongly influencing the catalytic performance [19–29]. Lemonidou et al. [19] compared the catalytic activities of the three Ni-Mo catalysts supported by activated carbon (AC), Al2O3, and ZrO2, respectively. They revealed that the activity was closely related to the dispersion of the active phase on support surface, and AC support with a higher surface area was helpful for the exposure of more active Ni-O-Mo sites. Y. Khodakov et al. [20] found that Cu-Co supported on Al2O3, due to relatively high metal dispersion and formation of copper cobalt bimetallic species, exhibited much higher alcohol selectivity than that supported on other materials. Wang et al. [21] studied the Al2O3-supported Cu-Co bimetallic catalysts for CO hydrogenation and revealed that the employment of Al2O<sup>3</sup> can significantly increase the interaction between cobalt and copper particles compared with unsupported catalysts, thereby improving the selectivity of the catalysts to higher alcohols. Lee et al. [23] investigated the effect of supports (ZnO, MgO, and Al2O3) on the activity of Cu-Co catalysts for the hydrogenation reaction of CO and suggested that the high surface area and strong interaction between active centers and support played a vital role in improving alcohol formation. By comparing the Cu-Zn catalysts with and without γ-Al2O3, Choi [30] et al. pointed out that the selectivity of higher alcohols and CO conversion over a Cu-Zn catalyst supported on γ-Al2O<sup>3</sup> were higher than 1.8 and 2.7 times that of a Cu-Zn catalyst without γ-Al2O3, respectively. They further found that a refractory CuAl2O4, formed via the thermal reaction of CuO and Al3+ , was able to enhance the long-term stability by increasing the resistance to sintering of the catalyst. Sun et al. [31] studied methanol synthesis from CO<sup>2</sup> hydrogenation over micro-spherical SiO<sup>2</sup> support Cu/ZnO catalysts and found that the catalytic activity was enhanced as a result of the small Cu particle size and uniform metal dispersion. Co-Cu bimetallic catalysts with SiO<sup>2</sup> support have been thoroughly investigated for higher alcohols synthesis from syngas by Han et al [32]. It suggested that CoCu bimetallic particles covered by Cu atoms were responsible for alcohols synthesis. Ma et al. [22,33,34] reported that the improvement of Cu dispersion was mainly ascribed to the generation of copper phyllosilicate (Cu2SiO5(OH)2) caused by enhanced metal-support interactions, which was quite vital for the high activity and stability in the ethanol synthesis. The above literature clearly showed that Al2O<sup>3</sup> and SiO<sup>2</sup> are good support candidates to prepare the catalyst with good performance in the synthesis of higher alcohols.

Our group has also spent considerable effort to study the Al2O<sup>3</sup> and SiO<sup>2</sup> supported Cu-based catalysts for the higher alcohols synthesis from syngas [12,16,17,35]. In our latest work, we found that the interaction between Cu and Al2O<sup>3</sup> support on K-Cu/Al2O<sup>3</sup> catalysts could be effectively tuned by changing the calcination temperature, which led to the different distribution of CuO, CuAl2O4, and CuAlO<sup>2</sup> on the catalysts and strongly affected the reaction behaviors in the direct synthesis of ethanol from syngas [16,17]. For the Cu catalyst with SiO<sup>2</sup> support, the correlation of catalyst structure evolution and ethanol selectivity during the reaction process was systematically discussed [35]. Although we had somewhat understood the relation between copper species and performance of the supported Cu-based catalysts, the direct comparison of Al2O<sup>3</sup> and SiO<sup>2</sup> supported Cu catalysts and the effects of supports, treated at similar conditions, on the direct synthesis of higher alcohols from CO hydrogenation had not been sufficiently discussed.

Therefore, this work was mainly to clarify the reason of difference in reaction behaviors over the Cu catalysts supported on Al2O<sup>3</sup> and SiO<sup>2</sup> for CO hydrogenation into higher alcohols. Considering that alkali addition strongly affected the selectivity towards higher alcohols [13,15,36–41], herein, the present study also put forth effort to explore the effects of potassium addition on the structure and performance of Al2O<sup>3</sup> and SiO<sup>2</sup> supported Cu catalysts. Moreover, the physicochemical

properties of the prepared catalysts were characterized via various techniques, including X-ray diffraction (XRD), N<sup>2</sup> absorption-desorption, H2-temperature-programmed reduction (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and in situ Fourier-transform infrared spectroscopy (FTIR), and the characterization results were discussed alongside with the catalytic data in detail.

#### **2. Materials and Methods**

#### *2.1. Materials*

Analytical-grade chemicals, including Cu(NO3)2·× 3H2O, NaOH, AlCl3·× 6H2O, and K2CO3, were purchased from the Beijing Chemical Co. Ltd. (Beijing, China) and used directly without further purification. The employed SiO<sup>2</sup> was purchased from Aladdin Industrial Co. Ltd. (Los Angeles, US). The γ-Al2O3, as a support was synthesized using a hydrothermal route, which was similar to the procedure described by Yang et al. [42]. Typically, the ammonia solution (28% NH3), AlCl3·× 6H2O solution, and NaOH solution were mixed under hydrothermal treatment. Then, the mixture was dried and calcined to obtain the γ-Al2O3.
