Keywords

Zr-promoter; Zr-Al₂O₃; Water-in-oil; Microemulsion method; Cobalt catalyst; Co₃O₄ reducibility; CO hydrogenation

Introduction

In the search for highly active and selective catalysts for the CO hydrogenation reaction, all elements in metallic form from group VIII are able to chemisorb and dissociate CO and H₂. However, only Ru, Co, Fe, and Ni are considered for use in commercial applications [1]. Fischer-Tropsch synthesis (FTS) is an exothermic reaction between H₂ and CO producing water and a wide variety of hydrocarbons in gas, liquid, and solid state used as fuels and chemicals [2,3]. Supported cobalt based catalysts have been used for FTS due to their higher activity, high selectivity to linear hydrocarbons, and low activity for water-gas shift (WGS) reaction [4,5]. The activity and selectivity of cobalt catalysts are dependent on metal dispersion and reduction degree, support and promoter. The interaction of cobalt and alumina is high and promoters have been incorporated in order to avoid metal-support interactions [6,7].

Zirconium seems to increase the performance of Co/Al₂O₃ catalysts [7-14]. Some authors attribute its promotion effect to the increase of active intermediates (-CH₂-) which causes an enhancement of the catalyst activity and the selectivity to long-chain hydrocarbons [15]. Other authors found that Zr enhances the cobalt reducibility and consequently the catalyst activity [13,16,17]. On the other hand, as important as the choice of the promoter is the choice of the synthesis method. Considering the relevance of the synthesis procedure for obtaining promoted alumina supports, the microemulsion preparation method is a promising strategy. It allows the synthesis of highly homogeneous materials with controlled structural properties and particle sizes [18]. This methodology has been employed for preparation of metal nanoparticles, metal oxides and mixed metal oxides for catalytic and electrochemical processes [19,20]. The synthesis of inorganic nanoparticles in water-in-oil microemulsions is driven by microscopic micelles that act as nanoreactors where the nanoparticle synthesis occurs. A water-in-oil (W/O) microemulsion is a transparent or translucent solution which is optically isotropic and thermodynamically stable. It is made up of droplets of water surrounded by a continuous oil phase, where the interfacial tension between oil and water is overcome by the use of surfactants [18,21].

The synthesis of a variety of binary metal oxides has been studied as catalysts supports for several catalytic reactions and has shown good results due to a high homogeneity and intimate binary metal interaction. In addition, small size particles maximize the surface area exposed to the reactant, allowing more reactions to occur [22,23]

Based on the presented literature, the presence of Zr in alumina increases the CO hydrogenation reaction; however, the method of Zr incorporation into Al₂O₃ has not been fully investigated. In addition, binary oxide nanoparticles used as supports have given good results in different application [24]. To the best of our knowledge, no Zr-Al nanoparticles co-precipitated by microemulsion method have previously been prepared and studied. Therefore the synthesis of Zr-Al oxide nanoparticles is an adequate candidate for preparing cobalt catalyst supports. The aim of this work is to synthetize as well as to study the characteristics of co-precipitated Zr-Al nanoparticles. At the same time understand how affect this material compared with similar ones on the cobalt deposition and further application as catalyst for CO hydrogenation reaction.

Experimental

Catalyst preparation

The co-precipitation of Zr-Al nanoparticles was accomplished by mixing two water-in-oil microemulsion solutions (microemulsion 1 (ME1) and microemulsion 2 (ME2)) (Table 1). ME1 contained Zr and Al precursors while ME2 contained the precipitating agent NH₄OH. ME2 was added to ME1 dropwise under continuous stirring at 30°C until pH 9 was reached. The solution was kept at constant conditions for 12 h to complete the reaction. The final solution was destabilized with acetone and the solid product was separated by centrifugation and washed with acetone and water. The product was freeze-dried in order to avoid particle agglomeration. Afterwards, the product was calcined in air for 6 h at 550°C (heating rate 10°C/min). The obtained material was labeled as Zr-Al₂O₃ (ME).

Table 1: Selected composition of the microemulsion systems.

For comparison, Al₂O₃ and Zr/Al₂O₃ were also prepared. A commercial pseudo-boehmite Al₂O₃ (Versal 250) was dried at 120°C for 5 h and calcined at 550°C for 6 h. Afterwards, an aqueous solution of ZrO(NO₃)2 was prepared and added to the treated alumina by incipient wetness impregnation (molar ratio Al:Zr=8). The material was thermally treated in the same way as Zr-Al₂O₃(ME). The obtained material was labeled as Zr-Al₂O₃(IM).

The carriers (Zr-Al₂O₃ (ME), Zr-Al₂O₃(IM) and Al₂O₃) were dried at 120°C for 5 h prior to the 12 wt% of cobalt deposition. An aqueous solution of Co(NO₃)2·6H₂O with volume equivalent to the pore volume of each support was added to the supports dropwise. The carrier pore volume was determined by N₂-adsorption technique. After metal impregnation, the materials were dried for 6 h at 120°C and calcined in air at 350°C for 10 h (heating rate: 1°C/ min). The final catalysts were labeled as Co/Zr-Al₂O₃(ME), Co/Zr- Al₂O₃(IM) and Co/Al₂O₃.

Catalyst characterization

X-ray diffraction (XRD) of the fresh samples was performed on a Siemens D5000 X-ray diffractometer with Cu Kα radiation (40 kV, 30 mA). The measurements were recorded from 10° to 90° in the 2θ range using a step size of 0.020° and a step time of 12s for all the samples. The phases were identified by the Eva software (version 13.0.0.2, 2007). Crystallite sizes of Co₃O₄ were calculated using the Scherrer equation and assuming spherical particles [25]. The Co° crystallite size was estimated from Co₃O₄ using the formula d(Co°)=0.75·d(Co₃O₄) [26,27]. The analyses were performed in a pressure interval between 20 and 510 mmHg. Chemisorption isotherms were extrapolated at zero pressure in order to determine the adsorption of hydrogen [28]. The stoichiometry assumption was that two cobalt atom per molecule of hydrogen. The average particle size of Co°, was estimated according to d(Co°)H=96 D · DOR, assuming spherical shape [29,30].

Brunauer–Emmet–Teller (BET) surface area and porosity data was collected with a Micromeritics ASAP 2000/2020 unit. 0.2 g of the samples was outgassed at 250°C overnight prior to analysis. The data was recorded by N₂ adsorption at liquid nitrogen temperature at relative pressures between 0.06 and 0.2.

The reducibility of the catalysts was investigated by hydrogen temperature-programmed reduction (H₂-TPR) [31]. The calcined catalysts (0.15 g) were studied in a Micromeritics Autochem 2910 at a flow of 5 vol% H₂ in Ar in a range of temperatures from 30°C to 930°C (heating rate: 10°C/min). The H₂ consumption was monitored during the study by the difference in thermal conductivity between the inlet and outlet gases. The degree of reduction (DOR, %) was calculated using H₂-TPR of the in situ reduced catalysts. 0.15 g of the fresh catalyst was reduced at 350°C (1°C/min) for 16 h in flowing H₂, then flushed with helium gas for 30 min. Afterwards, the helium was changed to 5 vol % H₂ in Ar and the temperature was increased from 350 to 930°C (10°C/min) and the H₂ consumption was monitored. The TCD was calibrated with Ag2O as standard. The DOR was calculated assuming that unreduced cobalt after the reduction pre-treatment was in the form of Co2+ according to:

where A_TCD is the integration of the TCD signal, normalized per mass catalyst; AW_Co is the atomic weight of Co (58.9 g/mol), f is a calibration factor correlating the area of the TCD signal and the H₂ consumed; X_Co is the cobalt loading (12% Co).

The cobalt dispersion (D, %) and the cobalt crystallite size (d(Co°), nm), was calculated by hydrogen static chemisorption on the reduced catalysts. The measurements were performed on a Micromeritics ASAP 202°C unit at 35°C, after reducing about 0.15 g of the fresh catalysts under the same conditions as in TPR analysis (H₂ flow at 350°C for 16 h (heating rate: 1°C/min)).

The morphology of the supports and final catalysts was studied by high resolution-scanning electron microscopy (HR-SEM) using an XHR-SEM Magellan 400 instrument supplied by the FEI Company. The samples were investigated using a low accelerating voltage and no conductive coating.

Transmission electron microscopy (TEM) analysis was performed using a Philips CM300UT-FEG electron microscope with a point resolution of 0.17 nm, information limit of 0.1 nm, which was operated at 200 kV, in which images were acquired with a TVIPS CCD camera. The samples were prepared by immersing a Quantifoil R copper micro grid in a fresh catalyst dispersed in ethanol.

Catalytic testing

CO hydrogenation was tested at operating conditions similar to Fischer-Tropsch synthesis. Experiments were performed in a stainless-steel fixed-bed reactor (i.d. 9 mm) at process conditions: 210°C, 20 bar, molar H₂/CO ratio=2.1. A mixture of 1 g of catalyst with a pellet size between 53–90 μm was diluted and mixed with 5 g of SiC (for an even temperature profile) and thereafter placed in the reactor [28,32]. Prior to the reaction the catalyst was activated by reducing it in situ with hydrogen at 350°C for 16 h at atmospheric pressure. After activation, the reactor was cooled down to 180°C and then flushed with He before increasing the pressure to 20 bar. The catalysts were tested during two periods, first at a syngas flow of 100 cm³/min (NTP) and in the second period the gas flow was adjusted in order to obtain a CO conversion of 30% [7,32-34]. The heavy hydrocarbons and most of the water were condensed in two traps kept at 120°C and room-temperature, respectively. The product gases leaving the traps were depressurized and analyzed on-line with a gas chromatograph (GC) Agilent 6890 equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). H₂, N₂, CO, CH₄, and CO₂ were separated by a Carbosieve II packed column and analyzed on the TCD. The percentage of CO conversion was calculated by:

C1–C6 products were separated by an alumina-plot column and quantified on the FID detector, from which it was possible to determine the C₅₊ selectivity (S_C5+). The CO₂-free S_C5+ (i.e., S_C5+ if excluding CO₂ from the C-atom balance) is defined as follows [28,32]:

Results and Discussion

Synthesis approach

Zr-Al co-precipitated in water-in-oil microemulsion: Several microemulsions were prepared in order to define the composition and temperature of the water/surfactant/oil (W/S/O) system at which the microemulsion was formed and stable. The selected weight ratio in percentages was 7.9/26.4/65.7 (Table 1). The Zr- Al₂O₃ precursor was formed by collision and coalescence of water droplets between microemulsions 1 and 2 (ME1 and ME2). Oxo-hydroxo complexes of zirconium and aluminum were produced when the base came in contact with the metal initiating the nucleation and formation of the first particles inside the water droplets [18,35]. The simultaneous co-precipitation of the precursors inside the water droplets favored in this way a good dispersion of Zr in alumina, and uniform growth of the particles. In addition, the EDX spectra of the material showed Al/Zr/Co atomic ratios (Table 2) similar to the added metals. These results show that both Zr and Al precipitated during the synthesis and no loss of metal was detected.

Table 2: Representative EDX elemental analysis of the supports and the catalysts.

Wetness impregnation: The commercial alumina used as support has a pseudo ϒ-Al₂O₃ porous structure. This framework allows the deposition of zirconium first and after cobalt oxides inside the pores and on the surface of the alumina. During the calcination step, decomposition of the precursors and reactions between the Zr, Co and ϒ-Al₂O₃ might occur.

Characterization of the materials

X-Ray diffractograms: The X-ray diffractograms of the carriers and Co-catalysts are illustrated in Figure 1. The Zr‑Al₂O₃(ME) support presents a low crystalline of ϒ-Al₂O₃ phase. In addition, no Zr species were detected which might be attributed to the applied synthesis method. One explanation could be that Zr is encapsulated in the alumina matrix and consequently Zr oxide species crystal formation was inhibited [36,37]. In contrast, the Zr- Al₂O₃(IM) material presents characteristic peaks for ϒ-Al₂O₃ and a band at 2θ=32° assigned to a metastable ZrO₂ with orthorhombic structure (Figure 1).

Figure 1: X-ray diffractograms of the carriers (left) calcined at 500°C for 6 h and cobalt-catalysts (right) calcined at 350°C for 10 h. 2θ (°).

After Co deposition, the XRD patterns (Figure 1 right) showed that highly crystalline Co₃O₄ species were formed in all the catalysts with similar crystallite sizes of approximately 11 nm (Figure 1 and Table 3). Thus, it can be concluded that the presence of Zr does not affect the Co₃O₄ particle size.

Table 3: Physicochemical characterization of the supports and catalysts (^aDetermined from a single point of adsorption at P/P0=0.998; ^bEstimated by BJH formalism (adsorption branch); ^cAverage crystallite size of Co₃O₄ estimated from Scherrer equation; ^dAccording with: d(CoO)= 0.75·d(Co₃O₄); ^eAccording to: d(CoO)H= 96·DOR; ^fMetal dispersion, after reduction at 350°C for 16 h in H₂; ⁱDegree of reduction (DOR) from TPR of reduced catalysts).

N₂ physisorption: The catalyst porosity is presented in Figure 2. The isotherms correspond to type IV [38]. The hysteresis loop for Co/Zr-Al₂O₃(ME), corresponds to type H₂(b) [38], associated with complex pore networks consisting of pores with ill-defined shapes in the mesopore range. Materials with textural porosity formed by voids between particles can be associate with type H₂(b). In addition, this material showed narrow pore size distribution. Co/ Zr-Al₂O₃(IM) and Al₂O₃ supports showed type H3 hysteresis loop [38], correspondent to materials with non-rigid aggregates and wide pore size distribution like amorphous alumina. The carrier isotherms were similar and therefore not included in Figure 2.

Figure 2: The N₂ adsorption-desorption isotherms and pore size distribution curves for the Co-catalysts.

The surface area for all materials was between 190 and 248 m²/g (Table 3). Incorporation of Co and/or Zr phases on alumina leads to a decrease in BET surface area and pore volume (Table 3), due to partial pore blockage of the deposited oxides inside the pores [39,40]. The Co₃O₄ particle size was smaller than the Al₂O₃ and Zr-Al₂O₃(IM) pore sizes, therefore Co₃O₄ deposition is favored inside the pores. On the other hand, Zr-Al₂O₃(ME) had smaller pore diameter size than the Co₃O₄ particles (Table 3), which leads to the conclusion that some of the Co₃O₄ was deposited on the carrier surface.

Scanning and transmission electron microscopy: The Zr- Al₂O₃(ME) and Co/Zr-Al₂O₃(ME) morphology (Figure 3) showed non-agglomerated uniform particle size distribution. However, Co/Al₂O₃, Zr-Al₂O₃(IM) and Co/Zr-Al₂O₃(IM) showed heterogeneous spherical agglomerations of smaller particles of 120, 80 and 80 μm respectively. These agglomerations are attributed to Zr and/ or Co deposition. Zr-Al₂O₃(ME) does not agglomerate after cobalt deposition (Figure 3). Based on these findings, it is considered that Zr prevents particle agglomeration, especially when Zr is highly dispersed.

Figure 3: SEM pictures for the carriers and the co-catalysts.

For a better understanding of the species and morphology in the promoted-catalysts, TEM pictures were taken (Figure 4). Co/Zr-Al₂O₃(ME) shows a homogeneous material formed by agglomerated nanoparticles. Co/Zr-Al₂O₃(ME) shows carrier particle sizes between 4-7 nm and Co₃O₄ cubic crystals (Figure 4a). STEM-EDX mapping results show a homogeneous distribution of Zr on Co/Zr-Al₂O₃(ME) (Figure 4a). This picture demonstrates how the ME technique can be applied for the synthesis of highly disperse oxide promoter on a carrier. The Zr dispersion on alumina in Co/Zr-Al₂O₃(IM) (Figure 4b) was lower, forming Zr-rich islands on the Al₂O₃ surface. Furthermore, the cobalt deposition seems to be better in the ME material than in the Zr-impregnated material.

Figure 4: Representative STEM-EDX elemental mapping for a) Co/Zr-Al₂O₃(ME) and b) Co/Zr-Al₂O₃(IM).

H₂-Temperature programmed reduction: A typical TPR profile for all the catalysts is shown in Figure 5. In general, the first two peaks correspond to the reduction of Co₃O₄ + H₂ → 3CoO + H₂O [41] and 3CoO + 3H₂ → 3Co⁰ + 3H₂O [42]. The peak around 700°C is attributed to Co₃AlO₆ (Co₃O₄-AlO₂) and/or CoO-Al₂O₃, and the peak at 900°C corresponds to CoAl₂O₄ [43-45].

Figure 5: TPR profile for the fresh catalyst (left) and after activation at 350°C for 16 h (right).

Co₃O₄ in Co/Zr-Al₂O₃(ME) was harder to reduce, as a consequence the reduction temperature was shifted towards higher temperatures compared to the other catalysts. The lack of crystallinity in the ME carrier favored the cobalt-aluminate formation and also its reduction temperature.

The TPR for Co/Zr-Al₂O₃(IM) presents similar peaks as for Co/Al₂O₃ with the difference that the reduction temperature was lower by about 50°C. In addition an extra H₂ uptake is seen at 608°C which can correspond to the partial reduction of Zr. The amount of cobalt aluminate species was decreased compared with Co/Al₂O₃, attributed to the presence of Zr. CoAl₂O₄ (spinel) was not detected by the XRD technique since its diffractogram peaks overlaps with the Co₃O₄ peaks.

Additionally, TPR experiments (Figure 5 right) were performed after the catalyst activation in order to identify the unreduced cobalt amount and consequently the degree of reduction (DOR) (i.e., from 350 to 930°C in H₂). Co₃O₄ in Co/Zr-Al₂O₃(IM) is completely reduced after catalyst activation with a DOR of 70%, while the DOR for Co/Al₂O₃ and for Zr-Al₂O₃(ME) is 30 % and 11%, respectively (Table 3 and Figure 5). Thereafter, it can be concluded that the presence of Zr in islands as is the case of Co/ Zr-Al₂O₃(IM) decrease the cobalt-alumina interactions, favoring in this way a more metallic formation which is required for a CO hydrogenation reaction.

Interesting to note in all the catalysts (Figure 5 right) is that the unreduced cobalt species (peaks around 700 and 900°C) shifted the reduction temperature to higher temperatures compared with the first TPR analysis (Figure 5 left). The explanation given is that during catalysts activation, the remaining unreduced-cobalt in the form of Co° interacts with water (produced by the metal reduction) to form Co-aluminate which is reduced at a higher temperature [15].

Table 3 presents the dispersion of metallic cobalt Co° calculated by H₂ chemisorption (see experimental part). The results show that Co° dispersion is quiet similar in Co/Zr-Al₂O₃(IM) and Co/Zr- Al₂O₃(ME). This results compared with Co/Al₂O₃ are higher, so it is concluded that Zr favors the dispersion of cobalt in alumina support. In addition, the measured Co° particle size by this technique and by TEM is higher than the calculated from the Scherrer equation, from which it can be concluded that during catalyst activation the metallic particles are sintered. This effect is higher in Co/Zr-Al₂O₃(ME) and one of the explanations might be due to the textural porosity and the lack of structural porosity which makes the cobalt-sintering easier.

Catalytic test

Comparing CO conversions for all the catalysts after 25 h of syngas stream (H₂:CO=2.1) (Table 3), the catalyst activity decreases in the following order: Co/Zr-Al₂O₃(IM)>Co/Al₂O₃>Co/Zr-Al₂O₃(ME). The results are related to: the higher the DOR (degree of reduction of Co) the higher the CO conversion in period one (Table 3).

In all the cases, the selectivity is affected by the CO conversion; the higher the CO conversion, the higher the C₅₊ selectivity (S_C5+) and as a consequence the selectivity to CH₄ and C2-C4 are decreased. The selectivities to CH₄ and C2-C4 were higher for the ME catalyst during both periods; this might be attributed to two facts: low Co⁰ formation in the Co/Zr-Al₂O₃ (ME) catalyst and the small pore size of the carrier, around 4 nm, leading to internal mass transfer limitations favoring the faster H₂ diffusion due to its smaller size compared to the CO molecule which diffuses more slowly. This led to higher H₂/CO ratios within the catalyst particles than at the pellet surface.

Conclusion

For the first time, Zr-Al oxides nanoparticles were synthetized by the water-in-oil microemulsion method. The material presented a high Zr dispersion in alumina and it was highly homogeneous with uniform particle sizes, narrow pore size distribution and high surface area. This material was used as cobalt support and compared with similar material prepared by Zr impregnated on commercial alumina. The presence of ZrO₂-islands on alumina favored the dispersion and degree of reduction of cobalt, while the high Zr dispersion in the Zr-Al₂O₃ (ME) material hindered ZrO₂ crystallization. This produced producing a more amorphous material, leading to a higher degree of CoAl₂O₄ formation and therefore increased selectivity to methane and short-chain hydrocarbons C₂-C₄. The catalytic activity and SC₅₊ is favored by the Co/Zr-Al₂O₃ (IM) catalyst. These results are attributed to the catalyst porosity and higher Co⁰ availability on the surface. However, even if the cobalt on Zr-Al₂O₃ nanoparticles catalysts (prepared by water-in-oil microemulsion) are not the best for use in CO hydrogenation reaction where a high C₅₊ selectivity is desired; the material has very good properties to be considered for other applications: such as base material in three-way-catalysts; or as catalyst support for other catalytic reaction like hydrodesulphurization, or to stabilize alumina phases when it is used at high temperatures.

Acknowledgments

Swedish International Development Cooperation Agency (SIDA), Nanoandes network and European project COST Action: CM1101 for financial support. Special thanks to Dr. Emanuela Negro (Deft University of Technology), Edgar Cardenas (Luleå University) and Cesar Leyva Porras (CIMAV, S.C.) for measurements of TEM, SEM and HRTEM/STEM.

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