Effect of FSP-inserted Cu on Physicochemical Properties of Cu/Al2O3 Catalyst

The copper inserted on Cu/Al2O3 catalysts with various Cu loading (10-40 wt%) were synthesized via flame spray pyrolysis (FSP). These catalysts were characterized using X-ray diffraction (XRD), N2 physisorption, temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy (XANES). The XRD results confirmed the formation of copper aluminate spinel (CuAl2O4) on the FSPinserted Cu catalyst. The CuO crystallite size of the Cu/Al2O3 catalysts was increased with increasing Cu loading during the flame spray pyrolysis step. The incorporation of copper and aluminum precursors during the flame spray pyrolysis step can inhibit the growth of Al2O3 particles resulting in higher BET surface area and smaller particle size than pure Al2O3 support. The data from TPR and XANES results can predict the ratio of CuO and CuAl2O4 in the FSP-made support. Less than 20 wt% loading of the FSP-inserted Cu showed high concentration of CuAl2O4 phase in the FSP-made material. The composition of CuO and CuAl2O4 phase can be controlled by varying Cu loading in flame spray pyrolysis step. This is a promising alternative way to synthesize the desired catalyst. An example was the catalytic testing of the selective hydrogenolysis of glycerol. The presence of both CuO and CuAl2O4 phases in the Cu/Al2O3 catalyst enhanced the catalytic activity and promoted the selectivity to acetol product. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Nowadays, the copper-based materials are widely used in several applications. These are electrical conductors, ceramic-reinforced composites and catalysts. In the field of catalyst, the copper-based catalysts play important roles in many commercial catalytic processes high thermal stability and high surface for active metal loading. An example is the use of this catalyst for glycerol hydrogenation that shows two steps to produce propylene glycol. This reaction required both acidic and metallic sites for converting glycerol to acetol and producing propylene glycol from acetol, respectively [4,5]. In general, the catalytic property mostly depends on condition of process and nature of catalyst. A morphology of the catalyst particle, an interaction with the support and an atomic structure on the catalyst surface affect significantly the catalytic performance. Many forms of Cu-containing species, e.g. metals, oxides, alloys and spinels, can be valid as the active sites. Many observations have suggested that the different form of Cu species on the catalyst surface related to the catalytic activity. It was reported that the participation of Cu 0 , Cu + , and Cu 2+ surface species enhanced hydrogenation of dienes and methanol synthesis [6−8], while only Cu 0 surface was active for CO hydrogenation [9]. On the other hand, the copper aluminate species (CuAl2O4) had promotional effect in glycerol hydrogenolysis, methanol stream reforming, benzyl alcohol oxidation and photocatalytic degradation of organic pollutants [10−14].
The catalysts containing desired forms of Cu species are obtained by a wide variety of preparation techniques such as impregnation, sol-gel method, and co-precipitation. Recently, the copper supported on alumina was prepared by a novel chemical reduction method. It was found that the Cu metal was the main species of Cu/Al2O3 catalyst and the Cu2O was slightly produced by oxidation of Cu metal on the surface [15]. Shim et al. [16] synthesized CeO2 promoted Cu/-Al2O3 catalyst by coimpregnation and sequential impregnation method. They reported that the catalyst preparation techniques affected the transformation of bulk Cu species to cluster Cu species, and the catalytic performance for high temperature water gas shift reaction. Morales-Leal and coworkers [17] suggested that the CuAl2O4 species was apparent on the catalyst surface when the Cu/Al2O3 catalyst was calcined at temperature beyond 500 °C. The presence of CuAl2O4 showed high catalytic activity for methanol-ethanol oxidation. Wolosiak-Hnat et al. [18] mentioned that the CuAl2O4 phase gave good catalytic activity in glycerol hydrogenolysis to 1,2-propanediol. From the above information, the synthesis of suitable Cu forms on the catalyst surface benefits the development of catalyst field. One of popular synthesis methods is the flame spray pyrolysis (FSP). The simultaneous metal oxides and spinels can be prepared by this method in a single step. The phase composition of both was adjusted by various parameters such as precursor concentration, precursor feed rate and precursor composition [19,20]. Granados et al. [21] found that the addition of cobalt and aluminum in liquid precursor resulted in the formation of cobalt aluminate spinel (CoAl2O4) and cobalt oxides (CoO) phases. Kim et al. [22] noted that the Ce-Al-O system with 5-10 mol% CeOx concentration appeared both cerium oxide (CeO2) and cerium magnetoplumbite spinel structure (CeAl11O18).
The metal aluminate spinel has been synthesized by several techniques, e.g. coprecipitation, so-gel method, solid state fusion method and microwave irradiation method [23−26]. However, the aluminate spinel was generally obtained at high calcination temperature about 1000 °C for several days in order that the spinel phase was completely formed. This was difficult to scaling up of synthesis and economic perspective. The flame spray pyrolysis (FSP) was an established commercial process to prepare metal-oxide spinel in a single step [27,28]. The FSP products with low contaminated compounds were made at high temperature (up to 3000 K) [29]. Moreover, the characteristic of these products can be controlled by fuel composition, precursor feed rate, precursor concentration and type of precursor etc. [30,31]. In this work, the Cu/Al2O3 catalysts were synthesized by two sequential methods. The FSP-made material was prepared using mixed copper and aluminum precursors and then the copper was again impregnated on this material. The Cu content in the FSP-made material was varied to obtain the different phase composition. The Cu/Al2O3 catalysts were characterized by N2 physisorption, inductively coupled plasma (ICP), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) and temperature programmed reduction (TPR).

Material Synthesis
A series of FSP-made material were prepared by varying the Cu content in a range of 0-40 wt%. The flame spray pyrolysis reactor is shown in Figure 1. The starting precursors were copper (II) acetylacetonate (Sigma-Aldrich, 97 wt%) and aluminum-tri-sec-butoxide (Sigma-Aldrich, 98 wt%). These precursors were diluted with xylene to 0.5 M solution. This liquid solution was fed with 5 ml/min into the center capillary of nozzle using a syringe pump and dispersed into spray by 5 L/min of oxygen. The pressure drop at the nozzle was held constant at 1.5 bar. A supporting premixed methane and oxygen with 1.5 and 3.2 L/min of gas flow rates, respectively, ignited and stabilized the flame. A sinter metal ring surrounding the flame was supplied by an additional 5 L/min of sheath oxygen flow. The powder product was collected on a glass microfiber filter (Diameter 150 mm, Whatman) with the aid of a vacuum pump. This product was addressed as the xCu-Al2O3 support. The x symbol meant the weight percentage of Cu based on the support material. All of the xCu-Al2O3 supports were impregnated by the Cu. The impregnated Cu loading was 10 wt% based on the whole material. A copper (II) nitrate trihydrate (Sigma-Aldrich, 98 wt%) was dissolved in deionized water and mixed with the xCu-Al2O3 support. The slurry was stirred at 70 °C for 6 h to obtain the paste. This paste was dried overnight at 110 °C and subsequently calcined at 400 °C for 3 h in airflow. The obtained material was addressed as the Cu/xCu-Al2O3 catalyst.

Material Characterization
The pore characteristics and BET surface area of the catalyst were evaluated by the N2 physisorption using BET DORP mini II. The catalyst was pretreated in a helium gas flow of 50 ml/min at 220 °C for 3 h to remove moisture in the solid particle. Analysis was measured at −196 °C with N2 gas. The specific surface areas were determined by the BET multipoint method with relative pressure of 0.001−0.20. The average particle size was calculated using DBET = 6000/(×SBET), where DBET was the particle diameter (nm),  was the powder density (g/cm 3 ), and SBET was specific surface area (m 2 /g). The XRD patterns were performed on X-Ray Diffractometer Bruker AXS Model D8 Discovere using Cu-K radiation ( = 1.5406 Å). The diffractograms were recorded from 10 to 80° 2θ using a scanning rate of 2°/min, scan speed of 0.3 sec/step with operating voltage and current of 40 kV and 40 mA, respectively. The crystallite size was estimated from line broadening according to the Scherrer equation as follows: crystallite size = K/Bcosθ, where K = 0.9 (crystallite-shape factor),  = 1.5418 Å for Cu K (wavelength of X-ray) and B was Xray diffraction broadening. The bulk reduction behavior of each catalyst was studied by H2 temperature programmed reduction using a Micromeritics Pulse Chemisorb 29100 instrument. A 0.1 g of catalyst samples in a quartz tubular reactor was initially heated under 30 ml/min of nitrogen flow at a rate of 10 °C/min to 150 °C and then held at this temperature for 1 h. After cooled down to room temperature by nitrogen, the reducing gas (10% H2 in N2) was switched on at 30 ml/min, and the temperature was raised at a rate of 10 °C/min until it reached 800 °C. The XANES spectra Cu K-edge was performed on the beamline BL8 at Synchrotron Light Research  Institute (SLRI), Thailand [32]. The XANES data were measured in transmission mode and then analyzed by the Athena -program [33].

N2 Adsorption/desorption Isotherms
The N2 adsorption/desorption isotherms and pore size distribution of the xCu-Al2O3 supports and the Cu/xCu-Al2O3 catalysts are shown in Figures 2 and 3

Bulletin of Chemical Reaction Engineering & Catalysis, 15 (3), 2020, 645
Copyright © 2020, BCREC, ISSN 1978-2993 hysteresis loop that were characteristic of aggregates of plate-like particles or slit-shaped pores. Figure 3(a) also confirms that the pores with size less than 40 nm was abundant on the 10Cu-Al2O3 and 20Cu-Al2O3 supports. The pore distribution of xCu-Al2O3 supports exhibited the multimodal pore size. After the impregnation of 10 wt% Cu on the xCu-Al2O3 supports, the amount of multilayer adsorbed N2 was clearly decreased but the pore characteristics were similar. This was due to plugging the pores by impregnated Cu particles [34], which was consistent with loss of the pores in size range less than 40 nm as shown in Figure 3(b). The BET surface area and the pore volume of all the materials are shown in Table  1. The average particle size was also calculated from the BET data. The specific surface area was significantly increased when 10-20 wt% Cu was introduced during the flame spray pyrolysis step. It was indicated that the Cu dopant disturbed the formation of Al2O3 by inhibiting the growth of Al2O3 particles. This was in agreement with the catalyst synthesis by doping some metals during the flame spray pyrolysis step [35−37]. However, a large amount of Cu showed low BET surface area because the Cu particles aggregated themselves to form larger particles.

X-Ray Diffraction
The xCu-Al2O3 samples were characterized by X-ray diffraction technique as shown in  Table 1. Figure  5 shows the XRD pattern of the Cu/xCu-Al2O3 samples. The Al2O3-supported Cu catalyst exhibited the diffraction peaks of only CuO and γ-Al2O3. The CuAl2O4 phase was not apparent in this case. It was implied that the impregnated Cu did not react with Al2O3 to form the CuAl2O4 phase. The sharp peaks assigned as the CuO phase were certainly observed on all the Cu/xCu-Al2O3 catalysts. This indicated that the amount of CuO particles was increased by the impregnation of Cu on the FSP-made catalysts.

H2 Temperature Programmed Reduction
The reduction behavior of the xCu-Al2O3 supports is shown in Figure 6. Two reduction regions were proposed by deconvolution. The first region addressed as the α zone was at low temperature below 300 C. This attributed to the overlap of two reduction steps of CuO to Cu2O and then Cu metal. These CuO species were well dispersed and weakly interacted with the alumina matrix [42,43]. This was consistent with a general reduction by H2 of the bulk CuO in a temperature range of 200−450 C [44]. When increasing copper loading, the hydrogen consumption in  region was increased. This related to a large amount of copper oxides on the catalyst surface. Moreover, the peaks in  region were shifted to higher temperature with increasing copper loading. It was implied that the particle sizes of copper oxides became larger consistent with the CuO crystallite size calculated from XRD results as shown in Table 1. The copper oxide particles with large sizes was more difficultly reduced than those with small sizes [34,45].
The second reduction region assigned as the β zone was occurred at high temperature above 300 C. The peaks were rather broad indicating the reduction of CuAl2O4 spinel structure. This structure is confirmed by the XRD results in Figure 4. In general, the bulk CuAl2O4 species was initially reduced at 400 C until completely at 900 C because of the strong interaction between metal and support [46]. The degree of reduction (DOR) calculated from the area under TPR profiles is presented in Table 2. This value was based on the theoretical loading of Cu in each catalyst. It was found that at 10 and 20 wt% Cu loading of the support, the H2 consumption ratio of  and  regions was almost in the same level. This implied the same behavior of formation of segregated CuO and CuAl2O4 during the flame spray pyrolysis step for low Cu loading. It was also remarked that the 10Cu-Al2O3 showed higher DOR than the 20Cu-Al2O3. An increase of Cu loading from 20 to 40 wt% resulted an increase of the H2 consumption ratio of α and β region. It was indicated that the segregated   CuO was easily formed and its agglomeration was inevitable. Much more excess of Cu loading exhibited strong interaction of large CuO particles and alumina. Figure 7 presents the TPR profiles of the Cu/xCu-Al2O3 catalysts. Without loading Cu in the flame spray pyrolysis step, the Cu/Al2O3 catalyst showed two reduction peaks at 240 and 290 C. This attributed to two reduction steps of dispersed CuO species to Cu2O and then metallic copper. The TPR pattern was changed when the Cu was impregnated on the xCu-Al2O3 supports. The reduction of CuO in the α region was completed at higher temperature. Several  peaks in a wide temperature range indicated the CuO with different particle sizes dispersed on the catalyst surface.
However, it was proposed that the reduction behavior of CuAl2O4 species in the  region was still similar. An addition of impregnated Cu in each support showed high DOR in  region. Before the catalytic testing, the Cu-based catalyst was generally reduced in the temperature range of 300−400 C under H2 flow to obtain metal active sites. Figure 8 demonstrates the comparative TPR profiles of the Cu/40Cu-Al2O3 catalyst before and after reduction. The fresh catalyst was reduced at 300 and 400 C under H2 ambient flow for 3 h. The peak in  region observed for the fresh catalyst was disappeared after reduction at both temperatures. It was implied that the CuO species can be reduced to metallic Cu particles on the catalyst surface. On the other hand, the presence of broad peaks in  region for three catalysts indicated a difficult reduction of CuAl2O4 species and therefore before the reaction, the catalyst surface contained both dispersed Cu metals and CuAl2O4 species. Based on the CuAl2O4 species in the reduced catalyst at 300 C, there was about 75% remaining CuAl2O4 in that at 400 C as shown in Table 2.    intensity of the pre-edge peak of the xCu-Al2O3 samples decreased with increasing copper loading as shown Figure 9(b) because of a decrease of Cu 2+ ions in tetrahedral site. The CuAl2O4 was a spinel structure with Cu 2+ ions in tetrahedral site and Al 3+ ions in octahedral site. In addition, the Cu K-edge of high copper loading was closer with the CuO standard, where a XANES spectra around 8987 eV was observed. Meanwhile, the addition of 10 wt% Cu on the FSP-made supports showed nearly the CuO standard, revealing the Cu 2+ oxidation state as shown in Figure 10.

X-ray Absorption Near Edge Structure
The XANES linear combination fit was used for investigating the composition of Cu species in the xCu-Al2O3 samples as presented in Table  3. The formation of CuAl2O4 was preferred at 20 wt% loading of the FSP-inserted Cu. The composition ratio of copper oxide and copper aluminate was again shown in Figure 11. This ratio was dramatically increased when more than 20 wt% Cu was loaded by the flame spray pyrolysis. From the TPR results, the catalytic reduction in the α and β regions was speculated to the presence of copper oxide and copper aluminate, respectively. The DOR in each region was converted to be the weight of each copper species and then the mass ratio of copper oxide and copper aluminate was calculated. The relationship of this ratio and the Cu loading had a same tendency with the XANES results. It was indicated that the postulation from the TPR data was reliable.
A low mass ratio of CuO/CuAl2O4 composition obtained by the 10 and 20 wt% Cu loading related to a large content of copper aluminate. The mass ratio of CuO/CuAl2O4 was increased with increasing copper loading during flame spray pyrolysis. These results speculated that the interaction between CuO species and Al2O3 support became weaken with increasing the Cu loading. This could be explained by the growth of copper oxide particle via sintering process during flame spray pyrolysis process. A high copper loading could lower surface contact between copper oxide and alumina and therefore collision between copper oxides were easily occurred at high temperature. As shown in Figure 12, when the particles are rapidly cooled, a low

The Catalytic Testing by Selective Hydrogenolysis of Glycerol
The catalytic property of FSP-made Cu catalysts was evaluated by glycerol hydrogenolysis reaction in a batch reactor. The benchmark was 30 wt% Cu impregnated on the FSP-made Al2O3 support addressed as the 30Cu/Al2O3. The BET surface area of this catalyst was 19 m 2 /g. For comparison, total Cu loading of the selected catalysts was fixed as 30 wt%. These catalysts were 30Cu-Al2O3 and Cu/20Cu-Al2O3. The catalytic activity and selectivity for selective hydrogenolysis of glycerol over three catalysts are shown in Figure 13. Comparing to the 30Cu/Al2O3, both 30Cu-Al2O3 and Cu/20Cu-Al2O3 catalysts displayed higher glycerol conversion and acetal selectivity. It was also remarked that both catalysts with FSP-inserted Cu showed the same trend of product distribution. It was reported that the catalytic pathway in glycerol hydrogenolysis was occurred through glycerol dehydration to acetol on acidic sites, acetol hydrogenation to 1,2-propnaediol on metallic copper, and then decomposition of 1,2-propanediol to propanal [50−52]. From this result, the presence of CuAl2O4 spinel on the catalyst surface can promote the acetol production and inhibit the 1,2propanediol decomposition. Therefore, the catalyst modification by the FSP-inserted Cu was promising alternative way to enhance the desired reaction.

Conclusion
The insertion of Cu in the Cu based catalysts by flame spray pyrolysis was studied. Incorporation of Cu and Al precursors during the flame spray pyrolysis can produce both CuO and CuAl2O4 phases while the addition of impregnated Cu on the FSP-made Al2O3 showed only CuO phase. The formation of CuAl2O4 was occurred by the flame spray pyrolysis. The FSP-inserted Cu inhibited the alumina growth as a consequence of high specific surface area. The specific surface area was decreased while the crystallite size of Cu was increased with increasing the FSPinserted Cu loading. A large amount of FSPinserted Cu showed a weaken interaction between copper oxide with alumina support. The XANES result indicated that a large amount of CuAl2O4 was formed at 10-20 wt% loading of the FSP-inserted Cu. For the application of catalyst field, the presence of both CuO and CuAl2O4 particles on the catalyst surface showed higher activity for glycerol hydrogenolysis and promoted the formation of acetol product. The composition of CuO and CuAl2O4 phases on the catalyst surface can be adjusted by varying the FSP-inserted Cu loading and therefore the flame spray pyrolysis was promising technique to synthesize the Cu based catalyst for the desired reaction.

Acknowledgment
The authors would like to thank Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University and Graduate School for financial support. In addition, the