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TiO2 catalysts
Accepted Manuscript Title: Direct conversion of CO2 to long-chain hydrocarbon fuels over K–promoted CoCu/TiO2 catalysts Authors: Zhibiao Shi, Haiyan Yang, Peng Gao, Xiaopeng Li, Liangshu Zhong, Hui Wang, Hongjiang Liu, Wei Wei, Yuhan Sun PII: DOI: Reference:
S0920-5861(17)30668-5 https://doi.org/10.1016/j.cattod.2017.09.053 CATTOD 11058
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
31-5-2017 12-8-2017 25-9-2017
Please cite this article as: Zhibiao Shi, Haiyan Yang, Peng Gao, Xiaopeng Li, Liangshu Zhong, Hui Wang, Hongjiang Liu, Wei Wei, Yuhan Sun, Direct conversion of CO2 to long-chain hydrocarbon fuels over K–promoted CoCu/TiO2 catalysts, Catalysis Today https://doi.org/10.1016/j.cattod.2017.09.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Direct conversion of CO2 to long-chain hydrocarbon fuels over K–promoted CoCu/TiO2 catalysts
Zhibiao Shi a,b, Haiyan Yang a, Peng Gao a,*, Xiaopeng Li a, Liangshu Zhong a, Hui Wang a,*, Hongjiang Liu b, Wei Wei a,c, Yuhan Sun a,c
a
CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, No.99 Haike Road, Zhangjiang Hi–Tech Park, Shanghai 201210, China
b
Department of chemistry, College of Sciences, Shanghai University, Shanghai 200444, China
c
School of physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
* Corresponding author. Tel: +86–021–20608002, Fax: +86–021–20608066. E-mail: [email protected] (P. Gao), [email protected] (H. Wang)
Highlights
CO2 was directly hydrogenated to long-chain hydrocarbons over CoCu/TiO2 catalysts.
1
Adding K on CoCu/TiO2 sample suppressed H2 adsorption but enhanced CO2 adsorption.
The added K promoted formation of C5+ hydrocarbons and suppressed CH4 formation.
The catalyst with addition of suitable amount of K exhibited the best performance.
K–promoted CoCu/TiO2 catalysts afforded a substantial stability.
Abstract: A series of TiO2 supported Co–Cu catalysts with the weight percent of potassium oxides ranged from 0 to 3.5 wt.% were synthesized. This work investigates the influence of potassium promoter on CO2 hydrogenation to long-chain (C5+) hydrocarbons. The introduction of suitable amount of K into the CoCu/TiO2 catalyst remarkably promoted the formation of C5+ hydrocarbons and suppressed methane formation. The temperature-program desorption measurements demonstrate that the addition of K increases the chemisorption of CO2, whereas H2 adsorption is decreased, which enhanced production of liquid fuels. However, these effects were not obvious with the addition of excess amount of K due to the slight change of surface K content. A maximum C5+ yield with CO2 conversion of 13% and C5+ selectivity of 35.1 C-mol% is obtained over the CoCu/TiO2 catalyst with 2.5 wt.% of potassium promoter loading, which also exhibits a substantial stability. 2
Keywords: CO2 hydrogenation; long-chain hydrocarbons; Fischer-Tropsch synthesis; Co–Cu catalysts; K promoter
3
1 Introduction In the past two centuries, the utilization of coal, fossil fuels and natural gas has allowed an unprecedented era of prosperity and advancement for human development. However, with the burning of carbon-rich fossil fuels, the concentration of carbon dioxide in the atmosphere has raised remarkably, which results in the global temperature increasing and climate change due to the “greenhouse effect” [1, 2]. Therefore, mitigating the CO2 emissions and converting CO2 into value-added chemicals have attracted much attention. In addition, most research to date, not surprisingly, is focusing on the CO2 hydrogenation to various C1 feedstock (e.g., CH4, CH3OH, CO, HCOOH) [3-7], while few studies are focused on the liquid fuels (C5+ hydrocarbons) due to the extreme inertness of CO2 and a high C–C coupling barrier[8]. Production of hydrocarbons from CO2 hydrogenation includes two routes, direct and indirect route: one promising route is the direct production of hydrocarbons, including both alkanes and olefins, which combined the reduction of CO 2 to CO via reverse water-gas shift (RWGS) reaction (equation (1)) and subsequent hydrogenation of CO to hydrocarbons via Fischer-Tropsch synthesis (FTS, equation (2)) [9]. The indirect route is often performed by using different reactors with syngas (a mixture of CO and H2) and/or methanol intermediate formation [10, 11]. However, as compared with the indirect route, the direct route would be more economic and energy-efficient. CO2 + H2 → CO + H2O,
△rH300 oC = +38 KJ mol–1
(1)
CO + 2H2 → –(CH2)n– + H2O,
△rH300 oC = –166 KJ mol–1
(2)
Thermodynamically, as it is a slightly endothermic reaction, the conversion of CO2 by RWGS is limited at the low temperature. Therefore, most researchers studied the catalysts for CO2 hydrogenation to hydrocarbons at high temperature (300~400 oC) 4
[12-14]. So far, among the two industrially applied FTS catalysts (Fe and Co), Fe is usually selected for FTS starting from CO2 because Co performs as a methanation catalyst rather than acting as an FTS catalyst at high temperature [15-18]. In contrast, reports on the CO2 hydrogenation to hydrocarbons or alcohols by cobalt-based catalysts are scarce. In traditional FTS at low temperature (<250 oC), cobalt-based catalysts are preferred for their high activity, high yields of long-chain hydrocarbons, high mechanical strength and high stability compared to iron-based catalysts and lower price than noble metals such as Ru-based catalysts [19, 20]. Recently, Co-based catalysts have exhibited a promising catalytic performance for CO2 hydrogenation to light hydrocarbons and C2+ alcohols [21-23]. Besides, other metals (such as Cu, Pd, Pt and Ru) should be introduced to enhance CO production, because the Co was not active in WGS and RWGS reactions. Copper-based catalysts, the most popularly studied catalytic systems for the WGS reaction, have also been applied to the RWGS reaction [24]. Therefore, the Co–Cu bimetallic catalyst can be used as an efficient catalyst for CO2 hydrogenation to hydrocarbons. During CO2-based FTS process, the degree of hydrogenation of surface-adsorbed intermediates is higher due to the slower adsorption rate of CO2 compared with CO hydrogenation, leading to much easier formation of CH4 with a decrease in chain growth. Therefore, currently there remains a key challenge associated with increasing reactivity for chain growth and suppressing methane formation. Akin et al. found that products of CO2 hydrogenation contain about 70 C-mol% of methane over Co/Al2O3 catalyst [25]. Alkali metals like Na and K have been extensively studied as promoter of iron-based for CO2 hydrogenation [13, 17, 26, 27]. It has been shown that they suppressed the formation of CH4, increased the chain growth probability and enhanced the production of olefins. Furthermore, its effects on the product selectivity 5
have been found to be strongly dependent on its concentration. For FTS, the support can significantly influence the morphology, adsorption, and activity/selectivity properties of the active phase. Some researchers found the TiO2 supported cobalt-based catalyst possessed higher reducibility and catalytic activity for CO hydrogenation compared with other typical oxide support, such as Al2O3, SiO2 and MgO [28, 29]. In this work, a series of potassium promoted CoCu/TiO2 catalysts with varied weight
percentage
of
potassium
were
prepared
by
the
homogeneous
deposition-precipitation (DP) technique followed by incipient wetness impregnation, and tested for CO2 hydrogenation to long-chain hydrocarbons. The DP method is widely used to the prepare highly loaded and highly dispersed oxide supported metal catalysts and the obtained metal particles do not easily sinter due to their strong interaction with the support [30-33]. The aim of this work is to investigate the effect of the potassium promoter on the properties of the obtained CoCu/TiO2 catalysts and catalytic performance for the direct conversion of CO2 into liquid fuels. 2 Experimental 2.1 Catalyst preparation The supported cobalt copper catalysts were prepared by the DP method described as follows [34]. In a typical synthesis, 10.18 g Co(NO3)2·6H2O and 8.45 g Cu(NO3)2·3H2O were dissolved in 400 mL of deionized water and 190 mL ammonia aqueous (Aladdin, AR) was then dropped into it. The mixed solution was stirred for 30 minutes to form a cobalt/copper ammonia complex solution and 10 g supports (TiO2 P25 Evonik Industry, 80% anatase and 20% rutile) were added into above solution under vigorous stirring for another 30 minutes, all of which were operated at 6
room temperature. Then, the solution was kept at 90 oC to evaporate ammonia with N2 bubbling and deposited the cobalt and copper species on the support. When the pH value of the solution decreased to ~7, the evaporation process stopped. Then, the black products were separated and washed several times with deionized water. The obtained catalyst was dried overnight at 100 oC and calcined in air at 350 oC for 4 h. And the catalysts promoted with K were prepared by a consecutive incipient-wetness impregnation step using the solution of potassium nitrate. The weight percent of potassium promoter in six catalysts was designed to be 0, 1.5, 2.0, 2.5, 3.0 and 3.5 wt.%, respectively (Table 1). And the promoted samples were further dried overnight at 100 oC. 2.2 Catalyst characterization 2.2.1 N2 adsorption The surface areas of the catalysts were measured by a standard BET procedure using N2 adsorption at –196 oC on a Micromeritics ASAP 2420 apparatus. The samples were degassed under vacuum for 10 h at 200 oC prior to adsorption. The surface areas of supports and catalysts were determined by the BET method. The total pore volume (TPV) was calculated from the amount of vapor adsorbed at a relative pressure (p/p0) close to unity, where p and p0 are the measured and equilibrium pressures, respectively. Pore size distribution curves were established from the desorption branches of the isotherms using the BJH model. 2.2.2 XRD characterization X–ray powder diffraction (XRD) experiments were conducted using Rigatku Ultima 4 X–ray diffractometer utilizing Cu Kα radiation (40 kV, 40 mA) in the range of 5°–90° and scanning step length of 0.0667°. The in situ XRD patterns were 7
measured during catalyst reduction in hydrogen (5% H2 in Ar). The full width at half maximum (FWHM) of Cu (111) and Co (111) was used to calculate the mean kλ
crystallite size using the Scherrer equation (D = β cosθ) at 43.2o and 44.2o, respectively, where the k value was 0.89 and λ was 0.15418 nm. 2.2.3 TEM characterization The morphology of the samples was observed by d a Tecnai G220 high-resolution transmission electron microscope (TEM) operated at 200 kV. Samples were prepared by directly depositing the powders on a lacey carbon film supported on a Cu grid. Prior to the analyses, samples were exposed to air at ambient conditions after the reaction. 2.2.4 XPS characterization X–ray photoelectron spectroscopy (XPS) measurements were studied on a Kratos AXIS ULTRA DLD spectrometer equipped with Al Kα radiation (150 W, hv = 1486.6 eV) under ultrahigh vacuum (10-7 bar). The binding energies were calibrated internally by adventitious carbon deposit C (1s) with Eb = 284.8 eV. 2.2.5 ICP–OES analysis The chemical compositions of calcined catalysts were analyzed by an inductively coupled plasma-optical emission spectroscopy (ICP–OES) using Thermo iCAP 6300. 2.2.6 H2–TPR characterization The reducibility of the catalysts was studied by temperature-programmed reduction (TPR). The TPR was carried out in a U–tube quartz reactor using a Micromeritics ChemiSorb 2920 with a thermal conductivity detector (TCD). Typically, the sample (50 mg) was first pretreated with Ar gas flow at 120 oC for 2 h to remove physically 8
adsorbed water and cooled down to 50 oC. Then the sample was reduced in 5 vol.% H2/Ar stream (40 mL·min–1) from room temperature to 600 oC at a heating rate of 10 oC·min–1. 2.2.7 CO2–TPD characterization CO2 temperature-program desorption (CO2–TPD) measurements were conducted to investigate the basicity of the catalysts on the same apparatus as that employed for the H2–TPR test. 100 mg of the sample was in situ reduced with pure H2 (30 mL·min–1) at 350 oC for 90 minutes. After cooling to 50 oC, the sample was flushed with Ar (40 mL·min–1) for 30 minutes at 50 oC, after which the sample was exposed to pure CO2 (30 mL·min–1) for 1 h and then flushed with Ar flow (40 mL·min–1) to remove all physically adsorbed molecules. Desorption was then performed by increasing the temperature from 50 to 750 oC at a rate of 5 oC·min–1 under a flow of Ar. The CO2 desorption profiles were detected by a MS. 2.2.8 H2–TPD characterization H2 temperature-programmed desorption (H2–TPD) were performed in the same apparatus as TPR. The catalyst was first reduced at 350 oC for 2 h in a flow of 5 vol.% H2/Ar. After cooling down to ambient temperature, the catalyst was saturated with pure H2 at 50 oC for 1h and then flushed with Ar to remove the physically adsorbed molecules. Afterward, the TPD experiment was started with a ramping rate of 5 C·min–1 in the temperature range of 50–750 oC under Ar atmosphere. The change of
o
hydrogen signal was monitored by a TCD detector. 2.3 Catalytic performance tests Carbon dioxide hydrogenation reaction was carried out in a fixed bed stainless tubular reactor (dint. = 10 mm). Prior to reaction, 1.5 g of catalyst diluted with 2.0 g 9
quartz sand (both in 40–60 mesh) was reduced at 350 oC for 8 h in a pure hydrogen flow (80 mL·min–1) at the pressure of 0.5 MPa. After reduction, the sample was cooled to 50 oC and then the reactant gas mixture H2/CO2/N2 (73/24/3 vol.%, N2 used as an internal standard) was fed into the reactor. Subsequently the pressure was increased gradually to 5.0 MPa and the temperature was increased to the reaction temperature with a ramp of 1 oC·min–1. The steady–state activity measurements were taken after at least 24 h on the stream. The tail gas, after passing through a cold trap (ice-water bath), was analyzed on–line by gas chromatographs (GC). All products were analyzed with two online gas chromatographs (GC). H2, N2, CO, CH4 and CO2 were analyzed through a TDX carbon molecular sieve column with a thermal conductivity detector (TCD) using Ar as the carrier gas. Hydrocarbons were analyzed through a modified alumina packed column with Ar as the carrier gas and a hydrogen flame ionization detector (FID). Moreover, the hydrocarbon distribution was calculated based on the total carbon moles with a unit of C–mol% on all tested catalysts. CO2 conversion was calculated according to equation (3): CO2 conversion (%) =
CO2 in −CO2 out CO2 in
× 100%
(3)
where CO2 in and CO2 out represent the molar fraction of CO2 at the inlet and outlet, respectively. CO selectivity was calculated by equation (4): CO selectivity (C-mol%) =
COout CO2 in − CO2 out
× 100%
(4)
The distribution of different hydrocarbon in total hydrocarbons was given as equation (5): Ci hydrocarbon distribution (C–mol%) =
Mole of C𝑖 hydrocarbon ×𝑖 ∑𝑛 𝑖=1 Mole of C𝑖 hydrocarbon ×𝑖
× 100% (5)
10
The denominator in the equation represents the total hydrocarbons. 3 Results and discussion 3.1 Textural properties N2 adsorption–desorption isotherms and pore distributions of the Co–Cu-based catalysts with different potassium promoter loading are shown in Fig. 1 and their textural properties are listed in Table 1. As can be seen from Fig.1, TiO 2 supported Co–Cu-based catalysts exhibit the same type of N2–sorption isotherm with a type–H (I) hysteresis loop at relative pressures (p/p0) of 0.8–1.0 and a very broad pore distribution in the range of 5–65 nm with a maximum at ~31 nm. It implies that those samples have more regular cylindrical channel. The BET surface area of catalysts decreased with the addition of promoter K probably due to the partial coverage of the surface by potassium [35], and it changed slightly with increasing K content.
3.2 Structure and morphology The XRD patterns of the calcined, reduced and spent catalysts are shown in Fig. 2a–c, respectively. For the calcined sample (Fig. 2a), the diffraction peaks at 35.5°, 38.7°, 48.6°, 58.2°, 61.4° and 68.0° are assigned to the phase of CuO (JCPDS card No.80–1916), and the diffraction peaks at 2θ value of 31.3°, 36.8° and 65.2° are related to the phase of Co3O4 (JCPDS card No.74–2120). No diffraction peaks associated with K can be detected in all samples probably due to low concentration and good dispersion. Otherwise, the average crystallite sizes of CuO and Co3O4 were calculated from the strongest diffraction peaks by Scherrer equation at 38.7° and 36.8°, and the values for all the catalysts are around 23 and 16 nm, respectively. For the reduced catalysts (350 oC, H2 for 8 h), the diffraction pattern shows three peaks at 11
43.3°, 50.4° and 74.1°, corresponding to (1 1 1), (2 0 0) and (2 2 0) of metallic Cu0 (JCPDS card No.85-1326), respectively (Fig. 2b). The diffraction peak intensity of the calcined and reduced catalysts changed only slightly with increasing K content, indicating that the potassium promoter has no obvious effect on the crystallite sizes. The XRD patterns of the spent catalysts (250 oC, 5.0 MPa, H2/CO2 = 3.0 and time on stream 100 h), the intensity of the characteristic peaks assigned to metallic Cu0 and Co0 increased slightly compared with the correspondingly reduced samples (Fig. 2c).
The calcined, reduced and used CoCu/TiO2 and 2.5K–CoCu/TiO2 catalysts were also characterized by TEM and HRTEM. As shown in the TEM images of calcined CoCu/TiO2 (Fig. S1a in the supporting information) and 2.5K–CoCu/TiO2 (Fig. 3a), Co3O4 and CuO nanoparticles were highly dispersed and the particle size changed slightly with the introduction of K, which suggested that the impregnation process nearly had no significant effect on the dispersion of nanoparticles. No significant sintering was observed after reduction and reaction (Fig. S1b, c and Fig. 3b, c), indicating that the TiO2 support and DP method can enhance the dispersion and stabilization of the active components. In addition, metal Co and Cu are formed when 2.5K–CoCu/TiO2 is reduced in H2 prior to reaction (Fig. 3b-2), and metal phase can be also observed after reaction (Fig. 3c-2). These are consistent with XRD results. In order to investigate the surface evolution of the catalysts, XPS was carried out, and the original spectra were given in Fig. S2 and S3 and the results were listed in Table 2 and 3. From the tables we can see that the content of potassium on the surface of calcined catalysts increased with the increase of K loading. However, the surface K content decreased slightly when the promoter loading was beyond 2.5 wt.%. 12
Compared with the results detected by ICP method (Table 1), the Co and K content and Co/Cu ratio on the surface are higher than those on the bulk, while the surface Cu content is lower, indicating that the surface is enriched in Co and K, whereas the surface content of Cu is found reduced. For the spent catalysts without K-modified, the surface content of cobalt and copper increased, while the Co/Cu ratio did not change much comparing with the calcined sample (Table 3). However, for the K-modified catalyst, the content of cobalt and potassium increased significantly after reaction, while the copper changed slightly, which resulted in remarkable increase of surface Co/Cu ratio. Therefore, the introduction of potassium inhibited the precipitation of copper on the surface during the reduction and reaction process. The binding energies (BEs) of Co 2p3/2, Cu 2p3/2, Ti 2p3/2 and K 2p3/2 for calcined and spent catalysts are also listed in Table 2 and 3. For calcined samples, the binding energies of Co 2p3/2, Cu 2p3/2 and Ti 2p3/2 increased gradually with the K content increasing, while the binding energy of K 2p3/2 unchanged almost, which suggested that the interaction among them was enhanced. The reference Co and Cu metal have the binding energy of the Co 2p3/2 and Cu 2p3/2 bands at 778.2 eV and 932.7 eV, respectively. For used catalysts, the binding energy for Co 2p3/2 and Cu 2p3/2 are located at around 780.5 eV and 933.9 eV, respectively, and the weak satellite peaks can also be observed from the spectra of Co 2p3/2 and Cu 2p3/2 (Fig. S3). They revealed the presence of oxidation states of both cobalt and copper species, indicating metal Co and Cu may be partly oxidized during reaction. Moreover, it was very difficult to prevent and diminish the phase transformation that probably occurred 13
during exposure to air for the ex situ XPS measurement. At the same time, the Co 2p3/2 and Cu 2p3/2 shifted to lower binding energy with the potassium content increasing for spent samples, indicating an electronic modification of Co and Cu with K and the interaction of cobalt and copper was weakened [36]. For the K-promoted catalyst the peaks at around 293.0 and 295.5 eV are definitely related to presence of K+ ions (Fig. S2 and S3). 3.3 Reduction behaviors To explore the phase evolution of representative catalyst, 2.5K–CoCu/TiO2, in situ XRD was exhibited in flowing 5% H2/Ar under different reaction temperature. For the sample after treatment in hydrogen at lower temperatures (<150 oC), the XRD patterns displayed characteristic peaks ascribed to TiO2, CuO and Co3O4 (Fig. 4). When the temperature increased to 200 oC, the patterns showed characteristic peaks of metallic Cu [37]. In addition, the characteristic peak at 43.8° was detected with the reduction temperature increasing to 300 oC, which was assigned to metallic Co. Moreover, the crystallite sizes of Cu and Co calculated by Scherrer equation increased from 15.2 to 30.7 nm and 8.3 to 17.4 nm when the reduction temperature enhanced from 250 to 450 oC, respectively.
The reduction behaviors of the promoted Co–Cu-based catalysts are characterized by H2–TPR. As is shown in Fig.5, all the H2–TPR profiles displayed a broad reduction peak consisting of several overlapping peaks which were attributed to several reduction steps. In order to analyze the results of TPR measurements, the profiles were deconvoluted into four Gaussian peaks, which are denoted as α, β, γ and δ peak, respectively. Since TiO2 is not reduced within the experimental region, the four peaks 14
are ascribed to the reduction of CuO and Co3O4. According to in situ XRD results, the lower temperature peaks are attributed to the reduction of highly dispersed CuO (α peak) and the reduction of bulk CuO (β peak) [38], while the higher temperature peaks are attributed to the reductions of Co3O4 to CoO (γ peak) and CoO to metallic Co (δ peak), respectively [39]. It can be seen that the reduction maximum shifted towards much higher temperature when the potassium promoter was added into the catalysts [40], and increased gradually with the K loading increasing. Combined with XPS results, it can be concluded that the metal support interaction was enhanced with increasing potassium content, and to some degree this would decrease the reducibility of catalysts [35]. In fact, the changes of the total amount of hydrogen consumption and the sum of H2/Cu and H2/Co ratios were pretty small with increasing K content (Table S2). Typically, the reduction degree will be lower when using the same reduction procedure.
3.4 Surface adsorption behaviors The information about the surface basicity could be obtained from the CO2–TPD profiles for CoCu/TiO2 catalysts with different K promoter loading shown in Fig. 6. As can be seen, a high and broad peak at around 110 oC was present in all of the TPD profiles, corresponding to the desorption of CO2 weakly adsorbed in bulk phase. For the K–free sample and low K–loading samples (1.5 wt.% K and 2.0 wt.% K), no other peaks were observed at higher temperature. With the K loading increasing, high temperature peaks at 440 oC were observed, which originated from the desorption of CO2 that interacted strongly with the surface basic sites. These results indicated that potassium played a critical role in improving the surface basicity. In addition, the total 15
CO2 uptake amount increased significantly with the increasing of promoter content. However, when the potassium promoter content further increased from 2.5 to 3.5 wt.%, the CO2 uptake amount changed slightly (Table 4). As mentioned above, the surface K content for the calcined and spent catalysts changed slightly with further increase of promoter content. Rather, the surface basicity depends not only on the amount of potassium content but also on how well it is distributed over the catalysts’ surface.
Fig. 7 shows the H2–TPD profiles of the samples. It was found from Fig.7 that there was an intense peak at about 150 oC for all the samples. This peak is corresponding to their combinative desorption of atomic hydrogen on the surface of metallic Cu or Co sites [41, 42]. The addition of K decreased the amounts of H2 desorption (Table 4) and slightly shifted the peaks towards lower temperature. A similar result was obtained by Zhang et al. [43]. They reported that the CO2 chemisorption was enhanced while H2 chemisorption was weakened on the iron based catalyst surface with the addition of K. In addition, with the amount of potassium increasing, it donates more electrons to metal species, resulting in the decrease of electrons from adsorption hydrogen.
3.5 Catalytic performance
The catalytic performances of CO2 selective hydrogenation to higher hydrocarbons over CoCu/TiO2 catalysts with different potassium promoter content were investigated under given reaction conditions to study the effect of the promoter of potassium. To clear the reaction mechanism, Cu/TiO2, 2.5K–Cu/TiO2, Co/TiO2 and 16
2.5K–Co/TiO2 catalysts were also evaluated in the same reaction condition and the
results are listed in Table S3. The products are mainly CO and hydrocarbons. The selectivity of oxy–compound is not listed here, because it was less than 0.5 C-mol% when the reaction temperature was higher than 230 oC (Table S1). Cobalt catalysts are widely used in FT application using syngas for its higher chain growth probabilities [9]. However, when switching the feed gas from syngas to a gas mixture containing CO2 and H2, the products were mainly methane and CO selectivity was only 0.13 C-mol% over the Co/TiO2 catalyst (Table S3). With the addition of K, the methanation activation of Co/TiO2 catalyst decreased significantly and C5+ selectivity increased. For Cu/TiO2, the CO selectivity was 73.0 C-mol% and nearly no C2+ hydrocarbons were formed. The activity of Cu/TiO2 decreased and CO selectivity increased to 98.9 C-mol% with the introduction of K. In addition, the CO2 conversion over cobalt-based catalysts was much higher than that over copper-based catalysts. Comparing the △H of equation (1) and (6), we deduced that the methanation reaction on cobalt phase was easier than the RWGS reaction on copper phase. Therefore, during CO2 hydrogenation over CoCu/TiO2 catalysts with different K content, CO2 is initially reduced to CO by H2 via RWGS on Cu sites, followed by a subsequent hydrogenation of CO to hydrocarbons via FTS on Co sites.
17
As can be seen, for the pure CoCu/TiO2 catalyst, the main product was also CH4, and the selectivity of CH4 was up to 89.5 C-mol%, thus it performed as a methanation catalyst rather than FT catalyst (equation (6) below) [44, 45]. With the introduction of K, it can be seen clearly that methane formation was suppressed and C5+ selectivity increased significantly compared with the K–free catalyst. Meanwhile, the C5+ selectivity increased gradually with increasing potassium content, and reached a maximum (35.1 C-mol%) when the potassium promoter content is 2.5 wt.%. And the selectivity of CH4 and C5+ changed slightly with further increasing potassium promoter content from 2.5 to 3.5 wt.% (Fig. 8). At the same time, however, CO2 conversion decreased and CO selectivity increased gradually. The C5+ yield increased with increasing K content until it reached a maximum for 2.5K–CoCu/TiO2 and then decreased. Those results indicated that an appropriate amount of potassium can improve the catalytic performance for CO2 hydrogenation to C5+ hydrocarbons within the potassium promoter content lower than 2.5 wt.%, while beyond this content the promotive effect is no longer obvious. Moreover, the increase of K content also increased the C2–C4 olefin/paraffin ratio in CO2 hydrogenation over TiO2 supported Co–Cu catalysts (Table 5). As mentioned from CO2/H2–TPD characterizations, the addition of K increased the chemisorptions of CO2, whereas H2 adsorption was decreased. The H/C ratio around active surface decreased resulting in the capacity of CO2 and CO hydrogenation decreased at the same time. In traditional FT reaction, the potassium promoter can increase the chain growth probability (α), the product shift to
18
long chain hydrocarbons [46]. In our case, α increased with increasing potassium content and a maximum of 0.63 was obtained over 2.5K–CoCu/TiO2, and then the value changed slightly with further increase of potassium content (Table 5). Considering this result, these trends for CO2 conversion and hydrocarbons distributions can be easily understood. CO2 + 4 H2 → CH4 + 2 H2O,
△rH300 oC = –177 KJ·mol–1
(6)
The effect of temperature has been assessed by running experiments at 230, 250 and 270 oC (Fig. 9). By increasing the reaction temperature, the CO2 conversion was enhanced strongly. For example, the conversion of CO2 increased from 7.1% to 22.2% and the selectivity to CO decreased from 38.5 to 22.0 C-mol% for 2.5K–CoCu/TiO2 catalyst (Fig. 9 b). As mentioned above, the process of CO2 hydrogenation proceeds via a two-step reaction mechanism. In the RWGS reaction, CO2 converted to CO, which is an intermediate product in a consecutive reaction and the CO converted to hydrocarbons through FT reaction. When the reaction temperature increased, the rate of FT reaction improved greatly[47]. The CO formed from RWGS reaction can convert into hydrocarbons quickly, and thus the CO selectivity decreased and the production of total hydrocarbons enhanced with temperature increasing. The selectivity for CH4 increased while C5+ selectivity decreased gradually with increasing temperature (Fig. 9). The formation of methane is favorable at relatively high temperature and the chain-growth probabilities decreased with increasing reaction temperature according to the Anderson–Schulz–Flory model [48, 49]. Therefore, the 19
high reaction temperature was not favor the formation of long-chain hydrocarbons. The long-term stability and activity of catalysts are vital for hydrocarbons synthesis from CO2 hydrogenation. The catalytic performance as a function of reaction time for 2.5K–CoCu/TiO2 catalyst was displayed in Fig. 10. The CO2 conversion and C5+ selectivity dropped slightly during the initial 24 h on stream. However, the CO2 conversion and C5+ selectivity remained stable around 12% and 34 C-mol%, respectively, after a time-on-stream of 200 h, indicating potential for industrial application. 4 Conclusions A series of potassium promoted CoCu/TiO2 catalysts with different K content were prepared and used for the CO2 hydrogenation to long-chain hydrocarbons. The potassium promoter had no obvious effect on the textural properties and crystal structures, while decreased the reducibility of CoCu/TiO2 catalysts. In addition, the introduction of K increased the chemisorptions of CO2, as seen by the increased desorption amount of CO2 with increasing K content. However, the amount of H2 adsorption decreased with the increase of K loading. The CO2/H2 adsorption behaviors changed slightly with further increase of K content, which was related to the slight change of surface K content. For the pure CoCu/TiO2 catalyst, the main product was CH4, and its selectivity was up to 89.5 C-mol%. With the introduction of K, methane formation was suppressed and C5+ selectivity increased significantly with increasing potassium content. At the same time, CO2 conversion decreased and CO selectivity increased gradually. Therefore, a maximum C5+ yield with CO2 conversion of 13% and C5+ selectivity of 35.1 C-mol% was obtained over the CoCu/TiO2 catalyst with 2.5 wt.% 20
of potassium promoter loading, which also afforded a considerable stable catalytic performance, indicating promising potential for industrial application.
Acknowledgements This work was financially supported by the “Frontier Science” program of Shell Global Solutions International B. V. (PT65197), the National Natural Science Foundation of China (21503260, 214032080), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA02040602), and Shanghai Municipal Science and Technology Commission, China (16DZ1206900). We thank Dr Sander van Bavel, Dr Carl Mesters, Dr Alexander van der Made, and Dr Tim Nisbet from Shell for helpful discussions.
21
References [1] G.A. Olah, A. Goeppert, G.K.S. Prakash, J. Org. Chem. 74 (2009) 487-498. [2] R.M. Cuellar-Franca, A. Azapagic, J. CO2 Util. 9 (2015) 82-102. [3] S. Scire, C. Crisafulli, R. Maggiore, S. Minico, S. Galvagno, Catal. Lett. 51 (1998) 41-45. [4] C.S. Chen, J.H. Lin, J.H. You, C.R. Chen, J. Am. Chem. Soc. 128 (2006) 15950-15951. [5] S. Moret, P.J. Dyson, G. Laurenczy, Nat. Commun. 5 (2014) 4017. [6] F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C.F. Elkjaer, J.S. Hummelshoj, S. Dahl, I. Chorkendorff, J.K. Norskov, Nat. Chem. 6 (2014) 320-324. [7] S. Xiao, Y.F. Zhang, P. Gao, L.S. Zhong, X.P. Li, Z.Z. Zhang, H. Wang, W. Wei, Y.H. Sun, Catal. Today 281 (2017) 327-336. [8] P. Gao, S.G. Li, X.N. Bu, S.S. Dang, Z.Y. Liu, H. Wang, L.S. Zhong, M.H. Qiu, C.G. Yang, J. Cai, W. Wei, Y.H. Sun, Nat Chem, doi:10.1038/nchem.2794. [9] R.W. Dorner, D.R. Hardy, F.W. Williams, H.D. Willauer, Energ. Environ. Sci. 3 (2010) 884-890. [10] S. Abello, D. Montane, Chemsuschem 4 (2011) 1538-1556. [11] N. Meiri, R. Radus, M. Herskowitz, J. CO2 Util. 17 (2017) 284-289. [12] U. Rodemerck, M. Holena, E. Wagner, Q. Smejkal, A. Barkschat, M. Baerns, ChemCatChem 5 (2013) 1948-1955. [13] J. Wei, J. Sun, Z.Y. Wen, C.Y. Fang, Q.J. Ge, H.Y. Xu, Catal. Sci. Technol. 6 (2016) 4786-4793. [14] S. Hu, M. Liu, F.S. Ding, C.S. Song, G.L. Zhang, X.W. Guo, J. CO2 Util. 15 (2016) 89-95. [15] R. Satthawong, N. Koizumi, C.S. Song, P. Prasassarakich, Top. Catal. 57 (2014) 588-594. [16] P. Kangvansura, L.M. Chew, W. Saengsui, P. Santawaja, Y. Poo-arporn, M. Muhler, H. Schulz, A. Worayingyong, Catal. Today 275 (2016) 59-65. [17] J.L. Zhang, S.P. Lu, X.J. Su, S.B. Fan, Q.X. Ma, T.S. Zhao, J. CO2 Util. 12 (2015) 22
95-100. [18] M. Albrecht, U. Rodemerck, M. Schneider, M. Bröring, D. Baabe, E.V. Kondratenko, Appl. Catal. B 204 (2017) 119-126. [19] B. Zeng, B. Hou, L.T. Jia, J.G. Wang, C.B. Chen, D.B. Li, Y.H. Sun, Catal. Sci. Technol. 3 (2013) 3250-3255. [20] R. Xie, H. Wang, P. Gao, L. Xia, Z. Zhang, T. Zhao, Y. Sun, Appl. Catal. A 492 (2015) 93-99. [21] M.K. Gnanamani, G. Jacobs, R.A. Keogh, W.D. Shafer, D.E. Sparks, S.D. Hopps, G.A. Thomas, B.H. Davis, Appl. Catal. A 499 (2015) 39-46. [22] R.E. Owen, J.P. O'Byrne, D. Mattia, P. Plucinski, S.I. Pascu, M.D. Jones, Chem. Commun. 49 (2013) 11683-11685. [23] M.K. Gnanamani, G. Jacobs, H.H. Hamdeh, W.D. Shafer, F. Liu, S.D. Hopps, G.A. Thomas, B.H. Davis, ACS Catal. 6 (2016) 913-927. [24] W. Wang, S.P. Wang, X.B. Ma, J.L. Gong, Chem. Soc. Rev. 40 (2011) 3703-3727. [25] A.N. Akin, M. Ataman, A.E. Aksoylu, Z.I. Onsan, React. Kinet. Catal. Lett. 76 (2002) 265-270. [26] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B 200 (2017) 530-542. [27] M. Rafati, L.J. Wang, A. Shahbazi,J. CO2 Util. 12 (2015) 34-42. [28] R.C. Reuel, C.H. Bartholomew, J. Catal. 85 (1984) 78-88. [29] P. Munnik, P.E. de Jongh, K.P. de Jong, Chem. Rev. 115 (2015) 6687-6718. [30] Z.Q. Wang, Z.N. Xu, S.Y. Peng, M.J. Zhang, G. Lu, Q.S. Chen, Y.M. Chen, G.C. Guo, ACS Catal. 5 (2015) 4255-4259. [31] I.U. Din, M.S. Shaharun, D. Subbarao, A. Naeem, F. Hussain, Catal. Today 259 (2016) 303-311. [32] P. Burattin, M. Che, C. Louis, J. Phys. Chem. B, 103 (1999) 6171-6178. [33] C. Zhang, H.Y. Yang, P. Gao, H. Zhu, L.S. Zhong, H. Wang, W. Wei, Y.H. Sun, J. CO2 Util. 17 (2017) 263-272. [34] J. Lin, X. Zhao, Y. Cui, H. Zhang, D. Liao, Chem. Commun. 48 (2012) 23
1177-1179. [35] M. Khobragade, S. Majhi, K.K. Pant, Appl. Energy 94 (2012) 385-394. [36] S. Ali, N.A.M. Zabidi, D. Subbarao, AIP Conference Proceedings 1621 (2014) 29-33. [37] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B 200 (2017) 530-542. [38] X. Guo, D. Mao, G. Lu, S. Wang, G. Wu, J. Catal. 271 (2010) 178-185. [39] J. Wang, P.A. Chernavskii, Y. Wang, A.Y. Khodakov, Fuel 103 (2013) 1111-1122. [40] C.M. Balonek, A.H. Lillebø, S. Rane, E. Rytter, L.D. Schmidt, A. Holmen, Catal. Lett. 138 (2010) 8-13. [41] Y. Soong, V.U.S. Rao, R.J. Gormley, B. Zhong, Appl. Catal. 78 (1991) 97-108. [42] A. Infantes-Molina, E. Gralberg, J.A. Cecilia, E. Finocchio, E. Rodríguez -Castellón, Catal. Sci. Technol. 5 (2015) 3403-3415. [43] H. Zhang, H. Ma, H. Zhang, W. Ying, D. Fang, Catal. Lett. 142 (2011) 131-137. [44] D.R.H. Robert, W. Dorner, F. W. Williams, B. H. Davis, H.D. Willauer, Energy Fuels 23 (2009) 4190–4195. [45] Y. Zhang, G. Jacobs, D.E. Sparks, M.E. Dry, B.H. Davis, Catal. Today 71 (2002) 411-418. [46] J. Gaube, H.F. Klein, Appl. Catal. A 350 (2008) 126-132. [47] M.E. Dry, Appl. Catal. A 138 (1996) 319-344. [48] M.J. Choi, J.S. Kim,H.K. Kim, S.B.Lee, Y. Kang, K.W. Lee, Korean J. Chem. Eng. 18 (2001) 646-651. [49] G.P. Van der Laan, A.A.C.M. Beenackers, Catal. Rev. Sci. Eng. 41 (1999) 255-318.
Figures
24
180
0.5 0.4 Pore volume (cm3/g)
Quantity adsorbed (cm3/g)
160 140 120 100
0.3 0.2 0.1
80
0.0
60
CoCu/TiO2
1.5K-CoCu/TiO2
40
2.0K-CoCu/TiO2 3.0K-CoCu/TiO2
2.5K-CoCu/TiO2 3.5K-CoCu/TiO2
0
20
10
20 30 40 50 Pore diameter (nm)
60
70
0 0.0
0.2
0.4 0.6 0.8 Relative pressure (p/p0)
1.0
Fig. 1 N2 adsorption–desorption isotherm and pore distribution (inset) of CoCu/TiO2 catalysts with different K promoter loading.
TiO2 Co3O4 CuO
a)
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
20
30
40
50 2(o)
60
70
80
90
25
b)
TiO2 *Cu Co
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
c)
20
30
40 50 2(o)
60
70
80
90
TiO2 *Cu Co
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
20
30
40
50 2(o)
60
70
80
90
Fig. 2 XRD patterns of (a) calcined, (b) reduced and (c) spent catalysts. (▽) TiO2, () Co3O4, () CuO, (*) Metallic Cu, (♣) Metallic Co.
26
Fig. 3 TEM and HRTEM images of calcined (a-1, a-2), reduced (b-1, b-2) and spent (c-1, c-2) 2.5K–CoCu/TiO2 model catalyst.
TiO2 Co3O4 CuO *Cu Co dCu (nm) dCo (nm)
450 oC
30.7 26.6 22.1 16.9 15.2
Intensity (a.u.)
400 oC 350 oC
300 oC
10
20
30
40
250 oC 200 oC 150 oC 100 oC
50 2(o)
60
17.4 16.8 13.1 8.3
30 oC
70
80
90
Fig. 4 In situ XRD pattern of 2.5K–CoCu/TiO2 taken during reduction in 5% H2/Ar.
27
β α
γ
δ
3.5K-CoCu/TiO2
TCD Signal (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2
CoCu/TiO2
100
200
300 400 o Temperature ( C)
500
600
Fig. 5 H2–TPR profiles of various catalysts after calcination with different K loading.
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
100
200
300 400 500 Temperature (oC)
600
700
Fig. 6 CO2–TPD profiles of reduced catalysts with different K loading.
28
H2 desorption (a.u.)
3.5K-CoCu/TiO2 3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
0
100
200
300 400 o 500 Temperature ( C)
600
700
CH4
100
C2-C4
30
C5+ CO2 Conv
80 20 60 40
10
Conversion (%)
Hydrocarbons distribution (C-mol%)
Fig. 7 H2–TPD profiles of catalysts after reduction with different K promoter loading.
20 0
0 0
1.5
2.0 2.5 K Content (wt.%)
3.0
3.5
Fig. 8 Catalytic performance for CO2 hydrogenation with different K promoter loading under reaction conditions shown in Table 5.
29
Hydrocarbon distribution (C-mol%)
CH4
C2-C4
35 30
80 25 60
20
40
15 10
20 5 0
o
o
230 C
50
CH4
250 C C2-C4
o
0
270 C
C5+ CO Sel CO2 Con.
50 40
40 30
30
20
20
10
10 0
o
230 C
o
250 C
o
0
Conversion (%) and selectivity (C-mol%)
b)
Hydrocarbon distribution (C-mol%)
C5+ CO Sel CO2 Conv
Conversion (%) and selectivity (C-mol%)
a) 100
270 C
30
Hydrocarbon distribution (C-mol%)
C2-C4
CH4
60
C5+ CO Sel CO2 Conv
50
40
40 30 30 20 20 10
10
0
o
230 C
o
0
o
250 C
Conversion (%) and selectivity (C-mol%)
c) 50
270 C
Fig. 9 Effect of reaction temperature on the CO2 hydrogenation performance over (a) CoCu/TiO2, (b) 2.5K–CoCu/TiO2, (c) 3.5K–CoCu/TiO2 catalysts under reaction conditions shown in Table 5.
Hydrocarbon distribution (C-mol%)
60 C5+
50 30 CO sel
40
20
30 20 CO2 conv
10
10 0
0 0
40
80 120 Time on stream (h)
160
Conversion (%) and selectivity (C-mol%)
40
200
Fig. 10 Catalytic performance of 2.5K–CoCu/TiO2 catalyst as a function of reaction time. Reaction conditions: P = 5.0 MPa, T = 250 oC, GHSV = 3000 mL·g–1·h–1,
31
H2/CO2 /N2 = 73/24/3. Table 1 Textural properties of CoCu/TiO2 catalysts with different K promoter loading. Metal content (mol%) a
K2O
Co/Cu
content
mol
(wt.%)
ratio
SBET Sample
VBJH
Dpore
(m2·g–1) (cm3·g–1)
(nm)
Co
Cu
Ti
K
CoCu/TiO2
17.2
18.8
64.0
0.0
0.0
0.91
36.9
0.25
17.3
1.5K–CoCu/TiO2
17.3
18.3
62.0
2.4
1.4
0.94
30.2
0.23
32.3
2.0K–CoCu/TiO2
17.1
18.5
61.0
3.4
2.0
0.93
31.3
0.23
33.3
2.5K–CoCu/TiO2
17.0
18.7
60.0
4.2
2.5
0.91
31.1
0.22
30.9
3.0K–CoCu/TiO2
17.1
18.0
59.9
5.0
2.9
0.95
30.2
0.21
29.7
3.5K–CoCu/TiO2
16.8
17.8
59.7
5.8
3.4
0.94
29.0
0.24
32.4
a
Determined by ICP–OES analysis.
Table 2 XPS results for the CoCu/TiO2 catalysts with different K content after calcination. Relative surface concentration of metal (at.%)
Co 2p3/2
Cu 2p3/2
Ti 2p3/2
K 2p3/2
Co
Cu
Ti
K
Co/Cu atomic ratio
CoCu/TiO2
779.6
933.1
458.2
--
24.7
15.0
60.3
--
1.65
1.5K–CoCu/TiO2
779.7
933.1
458.4
292.9
18.7
13.5
64.2
3.6
1.38
2.0K–CoCu/TiO2
779.6
933.2
458.3
292.7
18.6
13.2
64.0
4.3
1.41
2.5K–CoCu/TiO2
779.8
933.5
458.4
292.9
19.4
13.4
60.6
6.6
1.45
3.0K–CoCu/TiO2
780.0
933.8
458.5
292.9
20.0
12.8
62.1
5.2
1.57
Binding energy (eV) Sample
32
3.5K–CoCu/TiO2
780.0
933.5
458.5
292.8
19.1
12.4
64.3
4.2
1.53
Table 3 XPS results for the CoCu/TiO2 catalysts with different K content after reaction. Relative surface concentration of metal (at.%)
Co 2p3/2
Cu 2p3/2
Ti 2p3/2
K 2p3/2
Co
Cu
Ti
K
Co/Cu atomic ratio
CoCu/TiO2
780.9
934.1
458.4
--
33.9
20.4
45.7
--
1.67
1.5K–CoCu/TiO2
781.0
934.2
458.6
293.0
30.3
15.7
43.5
10.5
1.94
2.0K–CoCu/TiO2
780.5
933.8
458.2
292.6
31.8
16.4
39.7
12.1
1.94
2.5K–CoCu/TiO2
780.1
933.6
458.1
292.6
30.5
16.1
37.9
15.5
1.90
3.0K–CoCu/TiO2
780.2
933.7
458.1
292.6
30.0
15.5
38.5
16.0
1.93
3.5K–CoCu/TiO2
780.1
933.5
458.0
292.6
28.9
15.0
38.7
17.3
1.92
Binding energy (eV) Sample
Table 4 CO2/H2–TPD results of the reduced CoCu/TiO2 catalysts with different K promoter loading. Total CO2
Total H2
uptake amounta (a.u.)
uptake amountb (a.u.)
CoCu/TiO2
33.0
100.0
1.5K–CoCu/TiO2
59.6
74.7
2.0K–CoCu/TiO2
65.3
69.9
2.5K–CoCu/TiO2
96.2
62.8
3.0K–CoCu/TiO2
95.4
62.0
Sample
33
3.5K–CoCu/TiO2 a
100.0
59.7
The largest total CO2 uptake amount of the 3.5K–CoCu/TiO2 catalyst is defined as
100, and the values of the other catalysts are relative to this catalyst. b
The largest total H2 uptake amount of the CoCu/TiO2 catalyst is defined as 100, and the values of
the other catalysts are relative to this catalyst.
Table 5 Catalytic performance for CO2 hydrogenation with different K promoter loading.a
CO2 Conv (%)
CO Sel (C-mol%)
CoCu/TiO2
23.1
1.5K–CoCu/TiO2
Sample
Hydrocarbon distribution (C-mol%)
Chain growth Olefin/Paraffin C5+Yield probability (C2~C4) (C-mol%)b α
CH4
C2~C4
C5+
1.3
89.5
5.6
4.9
0.34
0.01
1.1
21.2
4.7
62.0
24.3
13.8
0.47
0.02
2.8
2.0K–CoCu/TiO2
13.8
19.7
45.4
33.3
21.3
0.52
0.14
2.4
2.5K–CoCu/TiO2
13.0
35.1
34.1
30.8
35.1
0.63
0.33
3.0
3.0K–CoCu/TiO2
12.8
35.9
34.6
34.6
30.8
0.60
0.46
2.5
3.5K–CoCu/TiO2
11.9
45.9
35.0
33.9
31.1
0.61
0.41
2.0
a
Reaction conditions: P = 5.0 MPa, T = 250 oC, GHSV = 3000 mL·g–1·h–1, H2/CO2
/N2 = 73/24/3. b Yield C5+ = 𝐶𝐶𝑂2 × 𝑆𝑡𝑜𝑡𝑎𝑙 ℎ𝑦𝑑𝑟𝑜𝑐𝑎𝑟𝑏𝑜𝑛 × 𝑆𝐶5+ .
34
S0920-5861(17)30668-5 https://doi.org/10.1016/j.cattod.2017.09.053 CATTOD 11058
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
31-5-2017 12-8-2017 25-9-2017
Please cite this article as: Zhibiao Shi, Haiyan Yang, Peng Gao, Xiaopeng Li, Liangshu Zhong, Hui Wang, Hongjiang Liu, Wei Wei, Yuhan Sun, Direct conversion of CO2 to long-chain hydrocarbon fuels over K–promoted CoCu/TiO2 catalysts, Catalysis Today https://doi.org/10.1016/j.cattod.2017.09.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Direct conversion of CO2 to long-chain hydrocarbon fuels over K–promoted CoCu/TiO2 catalysts
Zhibiao Shi a,b, Haiyan Yang a, Peng Gao a,*, Xiaopeng Li a, Liangshu Zhong a, Hui Wang a,*, Hongjiang Liu b, Wei Wei a,c, Yuhan Sun a,c
a
CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, No.99 Haike Road, Zhangjiang Hi–Tech Park, Shanghai 201210, China
b
Department of chemistry, College of Sciences, Shanghai University, Shanghai 200444, China
c
School of physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
* Corresponding author. Tel: +86–021–20608002, Fax: +86–021–20608066. E-mail: [email protected] (P. Gao), [email protected] (H. Wang)
Highlights
CO2 was directly hydrogenated to long-chain hydrocarbons over CoCu/TiO2 catalysts.
1
Adding K on CoCu/TiO2 sample suppressed H2 adsorption but enhanced CO2 adsorption.
The added K promoted formation of C5+ hydrocarbons and suppressed CH4 formation.
The catalyst with addition of suitable amount of K exhibited the best performance.
K–promoted CoCu/TiO2 catalysts afforded a substantial stability.
Abstract: A series of TiO2 supported Co–Cu catalysts with the weight percent of potassium oxides ranged from 0 to 3.5 wt.% were synthesized. This work investigates the influence of potassium promoter on CO2 hydrogenation to long-chain (C5+) hydrocarbons. The introduction of suitable amount of K into the CoCu/TiO2 catalyst remarkably promoted the formation of C5+ hydrocarbons and suppressed methane formation. The temperature-program desorption measurements demonstrate that the addition of K increases the chemisorption of CO2, whereas H2 adsorption is decreased, which enhanced production of liquid fuels. However, these effects were not obvious with the addition of excess amount of K due to the slight change of surface K content. A maximum C5+ yield with CO2 conversion of 13% and C5+ selectivity of 35.1 C-mol% is obtained over the CoCu/TiO2 catalyst with 2.5 wt.% of potassium promoter loading, which also exhibits a substantial stability. 2
Keywords: CO2 hydrogenation; long-chain hydrocarbons; Fischer-Tropsch synthesis; Co–Cu catalysts; K promoter
3
1 Introduction In the past two centuries, the utilization of coal, fossil fuels and natural gas has allowed an unprecedented era of prosperity and advancement for human development. However, with the burning of carbon-rich fossil fuels, the concentration of carbon dioxide in the atmosphere has raised remarkably, which results in the global temperature increasing and climate change due to the “greenhouse effect” [1, 2]. Therefore, mitigating the CO2 emissions and converting CO2 into value-added chemicals have attracted much attention. In addition, most research to date, not surprisingly, is focusing on the CO2 hydrogenation to various C1 feedstock (e.g., CH4, CH3OH, CO, HCOOH) [3-7], while few studies are focused on the liquid fuels (C5+ hydrocarbons) due to the extreme inertness of CO2 and a high C–C coupling barrier[8]. Production of hydrocarbons from CO2 hydrogenation includes two routes, direct and indirect route: one promising route is the direct production of hydrocarbons, including both alkanes and olefins, which combined the reduction of CO 2 to CO via reverse water-gas shift (RWGS) reaction (equation (1)) and subsequent hydrogenation of CO to hydrocarbons via Fischer-Tropsch synthesis (FTS, equation (2)) [9]. The indirect route is often performed by using different reactors with syngas (a mixture of CO and H2) and/or methanol intermediate formation [10, 11]. However, as compared with the indirect route, the direct route would be more economic and energy-efficient. CO2 + H2 → CO + H2O,
△rH300 oC = +38 KJ mol–1
(1)
CO + 2H2 → –(CH2)n– + H2O,
△rH300 oC = –166 KJ mol–1
(2)
Thermodynamically, as it is a slightly endothermic reaction, the conversion of CO2 by RWGS is limited at the low temperature. Therefore, most researchers studied the catalysts for CO2 hydrogenation to hydrocarbons at high temperature (300~400 oC) 4
[12-14]. So far, among the two industrially applied FTS catalysts (Fe and Co), Fe is usually selected for FTS starting from CO2 because Co performs as a methanation catalyst rather than acting as an FTS catalyst at high temperature [15-18]. In contrast, reports on the CO2 hydrogenation to hydrocarbons or alcohols by cobalt-based catalysts are scarce. In traditional FTS at low temperature (<250 oC), cobalt-based catalysts are preferred for their high activity, high yields of long-chain hydrocarbons, high mechanical strength and high stability compared to iron-based catalysts and lower price than noble metals such as Ru-based catalysts [19, 20]. Recently, Co-based catalysts have exhibited a promising catalytic performance for CO2 hydrogenation to light hydrocarbons and C2+ alcohols [21-23]. Besides, other metals (such as Cu, Pd, Pt and Ru) should be introduced to enhance CO production, because the Co was not active in WGS and RWGS reactions. Copper-based catalysts, the most popularly studied catalytic systems for the WGS reaction, have also been applied to the RWGS reaction [24]. Therefore, the Co–Cu bimetallic catalyst can be used as an efficient catalyst for CO2 hydrogenation to hydrocarbons. During CO2-based FTS process, the degree of hydrogenation of surface-adsorbed intermediates is higher due to the slower adsorption rate of CO2 compared with CO hydrogenation, leading to much easier formation of CH4 with a decrease in chain growth. Therefore, currently there remains a key challenge associated with increasing reactivity for chain growth and suppressing methane formation. Akin et al. found that products of CO2 hydrogenation contain about 70 C-mol% of methane over Co/Al2O3 catalyst [25]. Alkali metals like Na and K have been extensively studied as promoter of iron-based for CO2 hydrogenation [13, 17, 26, 27]. It has been shown that they suppressed the formation of CH4, increased the chain growth probability and enhanced the production of olefins. Furthermore, its effects on the product selectivity 5
have been found to be strongly dependent on its concentration. For FTS, the support can significantly influence the morphology, adsorption, and activity/selectivity properties of the active phase. Some researchers found the TiO2 supported cobalt-based catalyst possessed higher reducibility and catalytic activity for CO hydrogenation compared with other typical oxide support, such as Al2O3, SiO2 and MgO [28, 29]. In this work, a series of potassium promoted CoCu/TiO2 catalysts with varied weight
percentage
of
potassium
were
prepared
by
the
homogeneous
deposition-precipitation (DP) technique followed by incipient wetness impregnation, and tested for CO2 hydrogenation to long-chain hydrocarbons. The DP method is widely used to the prepare highly loaded and highly dispersed oxide supported metal catalysts and the obtained metal particles do not easily sinter due to their strong interaction with the support [30-33]. The aim of this work is to investigate the effect of the potassium promoter on the properties of the obtained CoCu/TiO2 catalysts and catalytic performance for the direct conversion of CO2 into liquid fuels. 2 Experimental 2.1 Catalyst preparation The supported cobalt copper catalysts were prepared by the DP method described as follows [34]. In a typical synthesis, 10.18 g Co(NO3)2·6H2O and 8.45 g Cu(NO3)2·3H2O were dissolved in 400 mL of deionized water and 190 mL ammonia aqueous (Aladdin, AR) was then dropped into it. The mixed solution was stirred for 30 minutes to form a cobalt/copper ammonia complex solution and 10 g supports (TiO2 P25 Evonik Industry, 80% anatase and 20% rutile) were added into above solution under vigorous stirring for another 30 minutes, all of which were operated at 6
room temperature. Then, the solution was kept at 90 oC to evaporate ammonia with N2 bubbling and deposited the cobalt and copper species on the support. When the pH value of the solution decreased to ~7, the evaporation process stopped. Then, the black products were separated and washed several times with deionized water. The obtained catalyst was dried overnight at 100 oC and calcined in air at 350 oC for 4 h. And the catalysts promoted with K were prepared by a consecutive incipient-wetness impregnation step using the solution of potassium nitrate. The weight percent of potassium promoter in six catalysts was designed to be 0, 1.5, 2.0, 2.5, 3.0 and 3.5 wt.%, respectively (Table 1). And the promoted samples were further dried overnight at 100 oC. 2.2 Catalyst characterization 2.2.1 N2 adsorption The surface areas of the catalysts were measured by a standard BET procedure using N2 adsorption at –196 oC on a Micromeritics ASAP 2420 apparatus. The samples were degassed under vacuum for 10 h at 200 oC prior to adsorption. The surface areas of supports and catalysts were determined by the BET method. The total pore volume (TPV) was calculated from the amount of vapor adsorbed at a relative pressure (p/p0) close to unity, where p and p0 are the measured and equilibrium pressures, respectively. Pore size distribution curves were established from the desorption branches of the isotherms using the BJH model. 2.2.2 XRD characterization X–ray powder diffraction (XRD) experiments were conducted using Rigatku Ultima 4 X–ray diffractometer utilizing Cu Kα radiation (40 kV, 40 mA) in the range of 5°–90° and scanning step length of 0.0667°. The in situ XRD patterns were 7
measured during catalyst reduction in hydrogen (5% H2 in Ar). The full width at half maximum (FWHM) of Cu (111) and Co (111) was used to calculate the mean kλ
crystallite size using the Scherrer equation (D = β cosθ) at 43.2o and 44.2o, respectively, where the k value was 0.89 and λ was 0.15418 nm. 2.2.3 TEM characterization The morphology of the samples was observed by d a Tecnai G220 high-resolution transmission electron microscope (TEM) operated at 200 kV. Samples were prepared by directly depositing the powders on a lacey carbon film supported on a Cu grid. Prior to the analyses, samples were exposed to air at ambient conditions after the reaction. 2.2.4 XPS characterization X–ray photoelectron spectroscopy (XPS) measurements were studied on a Kratos AXIS ULTRA DLD spectrometer equipped with Al Kα radiation (150 W, hv = 1486.6 eV) under ultrahigh vacuum (10-7 bar). The binding energies were calibrated internally by adventitious carbon deposit C (1s) with Eb = 284.8 eV. 2.2.5 ICP–OES analysis The chemical compositions of calcined catalysts were analyzed by an inductively coupled plasma-optical emission spectroscopy (ICP–OES) using Thermo iCAP 6300. 2.2.6 H2–TPR characterization The reducibility of the catalysts was studied by temperature-programmed reduction (TPR). The TPR was carried out in a U–tube quartz reactor using a Micromeritics ChemiSorb 2920 with a thermal conductivity detector (TCD). Typically, the sample (50 mg) was first pretreated with Ar gas flow at 120 oC for 2 h to remove physically 8
adsorbed water and cooled down to 50 oC. Then the sample was reduced in 5 vol.% H2/Ar stream (40 mL·min–1) from room temperature to 600 oC at a heating rate of 10 oC·min–1. 2.2.7 CO2–TPD characterization CO2 temperature-program desorption (CO2–TPD) measurements were conducted to investigate the basicity of the catalysts on the same apparatus as that employed for the H2–TPR test. 100 mg of the sample was in situ reduced with pure H2 (30 mL·min–1) at 350 oC for 90 minutes. After cooling to 50 oC, the sample was flushed with Ar (40 mL·min–1) for 30 minutes at 50 oC, after which the sample was exposed to pure CO2 (30 mL·min–1) for 1 h and then flushed with Ar flow (40 mL·min–1) to remove all physically adsorbed molecules. Desorption was then performed by increasing the temperature from 50 to 750 oC at a rate of 5 oC·min–1 under a flow of Ar. The CO2 desorption profiles were detected by a MS. 2.2.8 H2–TPD characterization H2 temperature-programmed desorption (H2–TPD) were performed in the same apparatus as TPR. The catalyst was first reduced at 350 oC for 2 h in a flow of 5 vol.% H2/Ar. After cooling down to ambient temperature, the catalyst was saturated with pure H2 at 50 oC for 1h and then flushed with Ar to remove the physically adsorbed molecules. Afterward, the TPD experiment was started with a ramping rate of 5 C·min–1 in the temperature range of 50–750 oC under Ar atmosphere. The change of
o
hydrogen signal was monitored by a TCD detector. 2.3 Catalytic performance tests Carbon dioxide hydrogenation reaction was carried out in a fixed bed stainless tubular reactor (dint. = 10 mm). Prior to reaction, 1.5 g of catalyst diluted with 2.0 g 9
quartz sand (both in 40–60 mesh) was reduced at 350 oC for 8 h in a pure hydrogen flow (80 mL·min–1) at the pressure of 0.5 MPa. After reduction, the sample was cooled to 50 oC and then the reactant gas mixture H2/CO2/N2 (73/24/3 vol.%, N2 used as an internal standard) was fed into the reactor. Subsequently the pressure was increased gradually to 5.0 MPa and the temperature was increased to the reaction temperature with a ramp of 1 oC·min–1. The steady–state activity measurements were taken after at least 24 h on the stream. The tail gas, after passing through a cold trap (ice-water bath), was analyzed on–line by gas chromatographs (GC). All products were analyzed with two online gas chromatographs (GC). H2, N2, CO, CH4 and CO2 were analyzed through a TDX carbon molecular sieve column with a thermal conductivity detector (TCD) using Ar as the carrier gas. Hydrocarbons were analyzed through a modified alumina packed column with Ar as the carrier gas and a hydrogen flame ionization detector (FID). Moreover, the hydrocarbon distribution was calculated based on the total carbon moles with a unit of C–mol% on all tested catalysts. CO2 conversion was calculated according to equation (3): CO2 conversion (%) =
CO2 in −CO2 out CO2 in
× 100%
(3)
where CO2 in and CO2 out represent the molar fraction of CO2 at the inlet and outlet, respectively. CO selectivity was calculated by equation (4): CO selectivity (C-mol%) =
COout CO2 in − CO2 out
× 100%
(4)
The distribution of different hydrocarbon in total hydrocarbons was given as equation (5): Ci hydrocarbon distribution (C–mol%) =
Mole of C𝑖 hydrocarbon ×𝑖 ∑𝑛 𝑖=1 Mole of C𝑖 hydrocarbon ×𝑖
× 100% (5)
10
The denominator in the equation represents the total hydrocarbons. 3 Results and discussion 3.1 Textural properties N2 adsorption–desorption isotherms and pore distributions of the Co–Cu-based catalysts with different potassium promoter loading are shown in Fig. 1 and their textural properties are listed in Table 1. As can be seen from Fig.1, TiO 2 supported Co–Cu-based catalysts exhibit the same type of N2–sorption isotherm with a type–H (I) hysteresis loop at relative pressures (p/p0) of 0.8–1.0 and a very broad pore distribution in the range of 5–65 nm with a maximum at ~31 nm. It implies that those samples have more regular cylindrical channel. The BET surface area of catalysts decreased with the addition of promoter K probably due to the partial coverage of the surface by potassium [35], and it changed slightly with increasing K content.
3.2 Structure and morphology The XRD patterns of the calcined, reduced and spent catalysts are shown in Fig. 2a–c, respectively. For the calcined sample (Fig. 2a), the diffraction peaks at 35.5°, 38.7°, 48.6°, 58.2°, 61.4° and 68.0° are assigned to the phase of CuO (JCPDS card No.80–1916), and the diffraction peaks at 2θ value of 31.3°, 36.8° and 65.2° are related to the phase of Co3O4 (JCPDS card No.74–2120). No diffraction peaks associated with K can be detected in all samples probably due to low concentration and good dispersion. Otherwise, the average crystallite sizes of CuO and Co3O4 were calculated from the strongest diffraction peaks by Scherrer equation at 38.7° and 36.8°, and the values for all the catalysts are around 23 and 16 nm, respectively. For the reduced catalysts (350 oC, H2 for 8 h), the diffraction pattern shows three peaks at 11
43.3°, 50.4° and 74.1°, corresponding to (1 1 1), (2 0 0) and (2 2 0) of metallic Cu0 (JCPDS card No.85-1326), respectively (Fig. 2b). The diffraction peak intensity of the calcined and reduced catalysts changed only slightly with increasing K content, indicating that the potassium promoter has no obvious effect on the crystallite sizes. The XRD patterns of the spent catalysts (250 oC, 5.0 MPa, H2/CO2 = 3.0 and time on stream 100 h), the intensity of the characteristic peaks assigned to metallic Cu0 and Co0 increased slightly compared with the correspondingly reduced samples (Fig. 2c).
The calcined, reduced and used CoCu/TiO2 and 2.5K–CoCu/TiO2 catalysts were also characterized by TEM and HRTEM. As shown in the TEM images of calcined CoCu/TiO2 (Fig. S1a in the supporting information) and 2.5K–CoCu/TiO2 (Fig. 3a), Co3O4 and CuO nanoparticles were highly dispersed and the particle size changed slightly with the introduction of K, which suggested that the impregnation process nearly had no significant effect on the dispersion of nanoparticles. No significant sintering was observed after reduction and reaction (Fig. S1b, c and Fig. 3b, c), indicating that the TiO2 support and DP method can enhance the dispersion and stabilization of the active components. In addition, metal Co and Cu are formed when 2.5K–CoCu/TiO2 is reduced in H2 prior to reaction (Fig. 3b-2), and metal phase can be also observed after reaction (Fig. 3c-2). These are consistent with XRD results. In order to investigate the surface evolution of the catalysts, XPS was carried out, and the original spectra were given in Fig. S2 and S3 and the results were listed in Table 2 and 3. From the tables we can see that the content of potassium on the surface of calcined catalysts increased with the increase of K loading. However, the surface K content decreased slightly when the promoter loading was beyond 2.5 wt.%. 12
Compared with the results detected by ICP method (Table 1), the Co and K content and Co/Cu ratio on the surface are higher than those on the bulk, while the surface Cu content is lower, indicating that the surface is enriched in Co and K, whereas the surface content of Cu is found reduced. For the spent catalysts without K-modified, the surface content of cobalt and copper increased, while the Co/Cu ratio did not change much comparing with the calcined sample (Table 3). However, for the K-modified catalyst, the content of cobalt and potassium increased significantly after reaction, while the copper changed slightly, which resulted in remarkable increase of surface Co/Cu ratio. Therefore, the introduction of potassium inhibited the precipitation of copper on the surface during the reduction and reaction process. The binding energies (BEs) of Co 2p3/2, Cu 2p3/2, Ti 2p3/2 and K 2p3/2 for calcined and spent catalysts are also listed in Table 2 and 3. For calcined samples, the binding energies of Co 2p3/2, Cu 2p3/2 and Ti 2p3/2 increased gradually with the K content increasing, while the binding energy of K 2p3/2 unchanged almost, which suggested that the interaction among them was enhanced. The reference Co and Cu metal have the binding energy of the Co 2p3/2 and Cu 2p3/2 bands at 778.2 eV and 932.7 eV, respectively. For used catalysts, the binding energy for Co 2p3/2 and Cu 2p3/2 are located at around 780.5 eV and 933.9 eV, respectively, and the weak satellite peaks can also be observed from the spectra of Co 2p3/2 and Cu 2p3/2 (Fig. S3). They revealed the presence of oxidation states of both cobalt and copper species, indicating metal Co and Cu may be partly oxidized during reaction. Moreover, it was very difficult to prevent and diminish the phase transformation that probably occurred 13
during exposure to air for the ex situ XPS measurement. At the same time, the Co 2p3/2 and Cu 2p3/2 shifted to lower binding energy with the potassium content increasing for spent samples, indicating an electronic modification of Co and Cu with K and the interaction of cobalt and copper was weakened [36]. For the K-promoted catalyst the peaks at around 293.0 and 295.5 eV are definitely related to presence of K+ ions (Fig. S2 and S3). 3.3 Reduction behaviors To explore the phase evolution of representative catalyst, 2.5K–CoCu/TiO2, in situ XRD was exhibited in flowing 5% H2/Ar under different reaction temperature. For the sample after treatment in hydrogen at lower temperatures (<150 oC), the XRD patterns displayed characteristic peaks ascribed to TiO2, CuO and Co3O4 (Fig. 4). When the temperature increased to 200 oC, the patterns showed characteristic peaks of metallic Cu [37]. In addition, the characteristic peak at 43.8° was detected with the reduction temperature increasing to 300 oC, which was assigned to metallic Co. Moreover, the crystallite sizes of Cu and Co calculated by Scherrer equation increased from 15.2 to 30.7 nm and 8.3 to 17.4 nm when the reduction temperature enhanced from 250 to 450 oC, respectively.
The reduction behaviors of the promoted Co–Cu-based catalysts are characterized by H2–TPR. As is shown in Fig.5, all the H2–TPR profiles displayed a broad reduction peak consisting of several overlapping peaks which were attributed to several reduction steps. In order to analyze the results of TPR measurements, the profiles were deconvoluted into four Gaussian peaks, which are denoted as α, β, γ and δ peak, respectively. Since TiO2 is not reduced within the experimental region, the four peaks 14
are ascribed to the reduction of CuO and Co3O4. According to in situ XRD results, the lower temperature peaks are attributed to the reduction of highly dispersed CuO (α peak) and the reduction of bulk CuO (β peak) [38], while the higher temperature peaks are attributed to the reductions of Co3O4 to CoO (γ peak) and CoO to metallic Co (δ peak), respectively [39]. It can be seen that the reduction maximum shifted towards much higher temperature when the potassium promoter was added into the catalysts [40], and increased gradually with the K loading increasing. Combined with XPS results, it can be concluded that the metal support interaction was enhanced with increasing potassium content, and to some degree this would decrease the reducibility of catalysts [35]. In fact, the changes of the total amount of hydrogen consumption and the sum of H2/Cu and H2/Co ratios were pretty small with increasing K content (Table S2). Typically, the reduction degree will be lower when using the same reduction procedure.
3.4 Surface adsorption behaviors The information about the surface basicity could be obtained from the CO2–TPD profiles for CoCu/TiO2 catalysts with different K promoter loading shown in Fig. 6. As can be seen, a high and broad peak at around 110 oC was present in all of the TPD profiles, corresponding to the desorption of CO2 weakly adsorbed in bulk phase. For the K–free sample and low K–loading samples (1.5 wt.% K and 2.0 wt.% K), no other peaks were observed at higher temperature. With the K loading increasing, high temperature peaks at 440 oC were observed, which originated from the desorption of CO2 that interacted strongly with the surface basic sites. These results indicated that potassium played a critical role in improving the surface basicity. In addition, the total 15
CO2 uptake amount increased significantly with the increasing of promoter content. However, when the potassium promoter content further increased from 2.5 to 3.5 wt.%, the CO2 uptake amount changed slightly (Table 4). As mentioned above, the surface K content for the calcined and spent catalysts changed slightly with further increase of promoter content. Rather, the surface basicity depends not only on the amount of potassium content but also on how well it is distributed over the catalysts’ surface.
Fig. 7 shows the H2–TPD profiles of the samples. It was found from Fig.7 that there was an intense peak at about 150 oC for all the samples. This peak is corresponding to their combinative desorption of atomic hydrogen on the surface of metallic Cu or Co sites [41, 42]. The addition of K decreased the amounts of H2 desorption (Table 4) and slightly shifted the peaks towards lower temperature. A similar result was obtained by Zhang et al. [43]. They reported that the CO2 chemisorption was enhanced while H2 chemisorption was weakened on the iron based catalyst surface with the addition of K. In addition, with the amount of potassium increasing, it donates more electrons to metal species, resulting in the decrease of electrons from adsorption hydrogen.
3.5 Catalytic performance
The catalytic performances of CO2 selective hydrogenation to higher hydrocarbons over CoCu/TiO2 catalysts with different potassium promoter content were investigated under given reaction conditions to study the effect of the promoter of potassium. To clear the reaction mechanism, Cu/TiO2, 2.5K–Cu/TiO2, Co/TiO2 and 16
2.5K–Co/TiO2 catalysts were also evaluated in the same reaction condition and the
results are listed in Table S3. The products are mainly CO and hydrocarbons. The selectivity of oxy–compound is not listed here, because it was less than 0.5 C-mol% when the reaction temperature was higher than 230 oC (Table S1). Cobalt catalysts are widely used in FT application using syngas for its higher chain growth probabilities [9]. However, when switching the feed gas from syngas to a gas mixture containing CO2 and H2, the products were mainly methane and CO selectivity was only 0.13 C-mol% over the Co/TiO2 catalyst (Table S3). With the addition of K, the methanation activation of Co/TiO2 catalyst decreased significantly and C5+ selectivity increased. For Cu/TiO2, the CO selectivity was 73.0 C-mol% and nearly no C2+ hydrocarbons were formed. The activity of Cu/TiO2 decreased and CO selectivity increased to 98.9 C-mol% with the introduction of K. In addition, the CO2 conversion over cobalt-based catalysts was much higher than that over copper-based catalysts. Comparing the △H of equation (1) and (6), we deduced that the methanation reaction on cobalt phase was easier than the RWGS reaction on copper phase. Therefore, during CO2 hydrogenation over CoCu/TiO2 catalysts with different K content, CO2 is initially reduced to CO by H2 via RWGS on Cu sites, followed by a subsequent hydrogenation of CO to hydrocarbons via FTS on Co sites.
17
As can be seen, for the pure CoCu/TiO2 catalyst, the main product was also CH4, and the selectivity of CH4 was up to 89.5 C-mol%, thus it performed as a methanation catalyst rather than FT catalyst (equation (6) below) [44, 45]. With the introduction of K, it can be seen clearly that methane formation was suppressed and C5+ selectivity increased significantly compared with the K–free catalyst. Meanwhile, the C5+ selectivity increased gradually with increasing potassium content, and reached a maximum (35.1 C-mol%) when the potassium promoter content is 2.5 wt.%. And the selectivity of CH4 and C5+ changed slightly with further increasing potassium promoter content from 2.5 to 3.5 wt.% (Fig. 8). At the same time, however, CO2 conversion decreased and CO selectivity increased gradually. The C5+ yield increased with increasing K content until it reached a maximum for 2.5K–CoCu/TiO2 and then decreased. Those results indicated that an appropriate amount of potassium can improve the catalytic performance for CO2 hydrogenation to C5+ hydrocarbons within the potassium promoter content lower than 2.5 wt.%, while beyond this content the promotive effect is no longer obvious. Moreover, the increase of K content also increased the C2–C4 olefin/paraffin ratio in CO2 hydrogenation over TiO2 supported Co–Cu catalysts (Table 5). As mentioned from CO2/H2–TPD characterizations, the addition of K increased the chemisorptions of CO2, whereas H2 adsorption was decreased. The H/C ratio around active surface decreased resulting in the capacity of CO2 and CO hydrogenation decreased at the same time. In traditional FT reaction, the potassium promoter can increase the chain growth probability (α), the product shift to
18
long chain hydrocarbons [46]. In our case, α increased with increasing potassium content and a maximum of 0.63 was obtained over 2.5K–CoCu/TiO2, and then the value changed slightly with further increase of potassium content (Table 5). Considering this result, these trends for CO2 conversion and hydrocarbons distributions can be easily understood. CO2 + 4 H2 → CH4 + 2 H2O,
△rH300 oC = –177 KJ·mol–1
(6)
The effect of temperature has been assessed by running experiments at 230, 250 and 270 oC (Fig. 9). By increasing the reaction temperature, the CO2 conversion was enhanced strongly. For example, the conversion of CO2 increased from 7.1% to 22.2% and the selectivity to CO decreased from 38.5 to 22.0 C-mol% for 2.5K–CoCu/TiO2 catalyst (Fig. 9 b). As mentioned above, the process of CO2 hydrogenation proceeds via a two-step reaction mechanism. In the RWGS reaction, CO2 converted to CO, which is an intermediate product in a consecutive reaction and the CO converted to hydrocarbons through FT reaction. When the reaction temperature increased, the rate of FT reaction improved greatly[47]. The CO formed from RWGS reaction can convert into hydrocarbons quickly, and thus the CO selectivity decreased and the production of total hydrocarbons enhanced with temperature increasing. The selectivity for CH4 increased while C5+ selectivity decreased gradually with increasing temperature (Fig. 9). The formation of methane is favorable at relatively high temperature and the chain-growth probabilities decreased with increasing reaction temperature according to the Anderson–Schulz–Flory model [48, 49]. Therefore, the 19
high reaction temperature was not favor the formation of long-chain hydrocarbons. The long-term stability and activity of catalysts are vital for hydrocarbons synthesis from CO2 hydrogenation. The catalytic performance as a function of reaction time for 2.5K–CoCu/TiO2 catalyst was displayed in Fig. 10. The CO2 conversion and C5+ selectivity dropped slightly during the initial 24 h on stream. However, the CO2 conversion and C5+ selectivity remained stable around 12% and 34 C-mol%, respectively, after a time-on-stream of 200 h, indicating potential for industrial application. 4 Conclusions A series of potassium promoted CoCu/TiO2 catalysts with different K content were prepared and used for the CO2 hydrogenation to long-chain hydrocarbons. The potassium promoter had no obvious effect on the textural properties and crystal structures, while decreased the reducibility of CoCu/TiO2 catalysts. In addition, the introduction of K increased the chemisorptions of CO2, as seen by the increased desorption amount of CO2 with increasing K content. However, the amount of H2 adsorption decreased with the increase of K loading. The CO2/H2 adsorption behaviors changed slightly with further increase of K content, which was related to the slight change of surface K content. For the pure CoCu/TiO2 catalyst, the main product was CH4, and its selectivity was up to 89.5 C-mol%. With the introduction of K, methane formation was suppressed and C5+ selectivity increased significantly with increasing potassium content. At the same time, CO2 conversion decreased and CO selectivity increased gradually. Therefore, a maximum C5+ yield with CO2 conversion of 13% and C5+ selectivity of 35.1 C-mol% was obtained over the CoCu/TiO2 catalyst with 2.5 wt.% 20
of potassium promoter loading, which also afforded a considerable stable catalytic performance, indicating promising potential for industrial application.
Acknowledgements This work was financially supported by the “Frontier Science” program of Shell Global Solutions International B. V. (PT65197), the National Natural Science Foundation of China (21503260, 214032080), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA02040602), and Shanghai Municipal Science and Technology Commission, China (16DZ1206900). We thank Dr Sander van Bavel, Dr Carl Mesters, Dr Alexander van der Made, and Dr Tim Nisbet from Shell for helpful discussions.
21
References [1] G.A. Olah, A. Goeppert, G.K.S. Prakash, J. Org. Chem. 74 (2009) 487-498. [2] R.M. Cuellar-Franca, A. Azapagic, J. CO2 Util. 9 (2015) 82-102. [3] S. Scire, C. Crisafulli, R. Maggiore, S. Minico, S. Galvagno, Catal. Lett. 51 (1998) 41-45. [4] C.S. Chen, J.H. Lin, J.H. You, C.R. Chen, J. Am. Chem. Soc. 128 (2006) 15950-15951. [5] S. Moret, P.J. Dyson, G. Laurenczy, Nat. Commun. 5 (2014) 4017. [6] F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C.F. Elkjaer, J.S. Hummelshoj, S. Dahl, I. Chorkendorff, J.K. Norskov, Nat. Chem. 6 (2014) 320-324. [7] S. Xiao, Y.F. Zhang, P. Gao, L.S. Zhong, X.P. Li, Z.Z. Zhang, H. Wang, W. Wei, Y.H. Sun, Catal. Today 281 (2017) 327-336. [8] P. Gao, S.G. Li, X.N. Bu, S.S. Dang, Z.Y. Liu, H. Wang, L.S. Zhong, M.H. Qiu, C.G. Yang, J. Cai, W. Wei, Y.H. Sun, Nat Chem, doi:10.1038/nchem.2794. [9] R.W. Dorner, D.R. Hardy, F.W. Williams, H.D. Willauer, Energ. Environ. Sci. 3 (2010) 884-890. [10] S. Abello, D. Montane, Chemsuschem 4 (2011) 1538-1556. [11] N. Meiri, R. Radus, M. Herskowitz, J. CO2 Util. 17 (2017) 284-289. [12] U. Rodemerck, M. Holena, E. Wagner, Q. Smejkal, A. Barkschat, M. Baerns, ChemCatChem 5 (2013) 1948-1955. [13] J. Wei, J. Sun, Z.Y. Wen, C.Y. Fang, Q.J. Ge, H.Y. Xu, Catal. Sci. Technol. 6 (2016) 4786-4793. [14] S. Hu, M. Liu, F.S. Ding, C.S. Song, G.L. Zhang, X.W. Guo, J. CO2 Util. 15 (2016) 89-95. [15] R. Satthawong, N. Koizumi, C.S. Song, P. Prasassarakich, Top. Catal. 57 (2014) 588-594. [16] P. Kangvansura, L.M. Chew, W. Saengsui, P. Santawaja, Y. Poo-arporn, M. Muhler, H. Schulz, A. Worayingyong, Catal. Today 275 (2016) 59-65. [17] J.L. Zhang, S.P. Lu, X.J. Su, S.B. Fan, Q.X. Ma, T.S. Zhao, J. CO2 Util. 12 (2015) 22
95-100. [18] M. Albrecht, U. Rodemerck, M. Schneider, M. Bröring, D. Baabe, E.V. Kondratenko, Appl. Catal. B 204 (2017) 119-126. [19] B. Zeng, B. Hou, L.T. Jia, J.G. Wang, C.B. Chen, D.B. Li, Y.H. Sun, Catal. Sci. Technol. 3 (2013) 3250-3255. [20] R. Xie, H. Wang, P. Gao, L. Xia, Z. Zhang, T. Zhao, Y. Sun, Appl. Catal. A 492 (2015) 93-99. [21] M.K. Gnanamani, G. Jacobs, R.A. Keogh, W.D. Shafer, D.E. Sparks, S.D. Hopps, G.A. Thomas, B.H. Davis, Appl. Catal. A 499 (2015) 39-46. [22] R.E. Owen, J.P. O'Byrne, D. Mattia, P. Plucinski, S.I. Pascu, M.D. Jones, Chem. Commun. 49 (2013) 11683-11685. [23] M.K. Gnanamani, G. Jacobs, H.H. Hamdeh, W.D. Shafer, F. Liu, S.D. Hopps, G.A. Thomas, B.H. Davis, ACS Catal. 6 (2016) 913-927. [24] W. Wang, S.P. Wang, X.B. Ma, J.L. Gong, Chem. Soc. Rev. 40 (2011) 3703-3727. [25] A.N. Akin, M. Ataman, A.E. Aksoylu, Z.I. Onsan, React. Kinet. Catal. Lett. 76 (2002) 265-270. [26] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B 200 (2017) 530-542. [27] M. Rafati, L.J. Wang, A. Shahbazi,J. CO2 Util. 12 (2015) 34-42. [28] R.C. Reuel, C.H. Bartholomew, J. Catal. 85 (1984) 78-88. [29] P. Munnik, P.E. de Jongh, K.P. de Jong, Chem. Rev. 115 (2015) 6687-6718. [30] Z.Q. Wang, Z.N. Xu, S.Y. Peng, M.J. Zhang, G. Lu, Q.S. Chen, Y.M. Chen, G.C. Guo, ACS Catal. 5 (2015) 4255-4259. [31] I.U. Din, M.S. Shaharun, D. Subbarao, A. Naeem, F. Hussain, Catal. Today 259 (2016) 303-311. [32] P. Burattin, M. Che, C. Louis, J. Phys. Chem. B, 103 (1999) 6171-6178. [33] C. Zhang, H.Y. Yang, P. Gao, H. Zhu, L.S. Zhong, H. Wang, W. Wei, Y.H. Sun, J. CO2 Util. 17 (2017) 263-272. [34] J. Lin, X. Zhao, Y. Cui, H. Zhang, D. Liao, Chem. Commun. 48 (2012) 23
1177-1179. [35] M. Khobragade, S. Majhi, K.K. Pant, Appl. Energy 94 (2012) 385-394. [36] S. Ali, N.A.M. Zabidi, D. Subbarao, AIP Conference Proceedings 1621 (2014) 29-33. [37] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B 200 (2017) 530-542. [38] X. Guo, D. Mao, G. Lu, S. Wang, G. Wu, J. Catal. 271 (2010) 178-185. [39] J. Wang, P.A. Chernavskii, Y. Wang, A.Y. Khodakov, Fuel 103 (2013) 1111-1122. [40] C.M. Balonek, A.H. Lillebø, S. Rane, E. Rytter, L.D. Schmidt, A. Holmen, Catal. Lett. 138 (2010) 8-13. [41] Y. Soong, V.U.S. Rao, R.J. Gormley, B. Zhong, Appl. Catal. 78 (1991) 97-108. [42] A. Infantes-Molina, E. Gralberg, J.A. Cecilia, E. Finocchio, E. Rodríguez -Castellón, Catal. Sci. Technol. 5 (2015) 3403-3415. [43] H. Zhang, H. Ma, H. Zhang, W. Ying, D. Fang, Catal. Lett. 142 (2011) 131-137. [44] D.R.H. Robert, W. Dorner, F. W. Williams, B. H. Davis, H.D. Willauer, Energy Fuels 23 (2009) 4190–4195. [45] Y. Zhang, G. Jacobs, D.E. Sparks, M.E. Dry, B.H. Davis, Catal. Today 71 (2002) 411-418. [46] J. Gaube, H.F. Klein, Appl. Catal. A 350 (2008) 126-132. [47] M.E. Dry, Appl. Catal. A 138 (1996) 319-344. [48] M.J. Choi, J.S. Kim,H.K. Kim, S.B.Lee, Y. Kang, K.W. Lee, Korean J. Chem. Eng. 18 (2001) 646-651. [49] G.P. Van der Laan, A.A.C.M. Beenackers, Catal. Rev. Sci. Eng. 41 (1999) 255-318.
Figures
24
180
0.5 0.4 Pore volume (cm3/g)
Quantity adsorbed (cm3/g)
160 140 120 100
0.3 0.2 0.1
80
0.0
60
CoCu/TiO2
1.5K-CoCu/TiO2
40
2.0K-CoCu/TiO2 3.0K-CoCu/TiO2
2.5K-CoCu/TiO2 3.5K-CoCu/TiO2
0
20
10
20 30 40 50 Pore diameter (nm)
60
70
0 0.0
0.2
0.4 0.6 0.8 Relative pressure (p/p0)
1.0
Fig. 1 N2 adsorption–desorption isotherm and pore distribution (inset) of CoCu/TiO2 catalysts with different K promoter loading.
TiO2 Co3O4 CuO
a)
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
20
30
40
50 2(o)
60
70
80
90
25
b)
TiO2 *Cu Co
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
c)
20
30
40 50 2(o)
60
70
80
90
TiO2 *Cu Co
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
10
20
30
40
50 2(o)
60
70
80
90
Fig. 2 XRD patterns of (a) calcined, (b) reduced and (c) spent catalysts. (▽) TiO2, () Co3O4, () CuO, (*) Metallic Cu, (♣) Metallic Co.
26
Fig. 3 TEM and HRTEM images of calcined (a-1, a-2), reduced (b-1, b-2) and spent (c-1, c-2) 2.5K–CoCu/TiO2 model catalyst.
TiO2 Co3O4 CuO *Cu Co dCu (nm) dCo (nm)
450 oC
30.7 26.6 22.1 16.9 15.2
Intensity (a.u.)
400 oC 350 oC
300 oC
10
20
30
40
250 oC 200 oC 150 oC 100 oC
50 2(o)
60
17.4 16.8 13.1 8.3
30 oC
70
80
90
Fig. 4 In situ XRD pattern of 2.5K–CoCu/TiO2 taken during reduction in 5% H2/Ar.
27
β α
γ
δ
3.5K-CoCu/TiO2
TCD Signal (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2
CoCu/TiO2
100
200
300 400 o Temperature ( C)
500
600
Fig. 5 H2–TPR profiles of various catalysts after calcination with different K loading.
3.5K-CoCu/TiO2
Intensity (a.u.)
3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
100
200
300 400 500 Temperature (oC)
600
700
Fig. 6 CO2–TPD profiles of reduced catalysts with different K loading.
28
H2 desorption (a.u.)
3.5K-CoCu/TiO2 3.0K-CoCu/TiO2 2.5K-CoCu/TiO2 2.0K-CoCu/TiO2 1.5K-CoCu/TiO2 CoCu/TiO2
0
100
200
300 400 o 500 Temperature ( C)
600
700
CH4
100
C2-C4
30
C5+ CO2 Conv
80 20 60 40
10
Conversion (%)
Hydrocarbons distribution (C-mol%)
Fig. 7 H2–TPD profiles of catalysts after reduction with different K promoter loading.
20 0
0 0
1.5
2.0 2.5 K Content (wt.%)
3.0
3.5
Fig. 8 Catalytic performance for CO2 hydrogenation with different K promoter loading under reaction conditions shown in Table 5.
29
Hydrocarbon distribution (C-mol%)
CH4
C2-C4
35 30
80 25 60
20
40
15 10
20 5 0
o
o
230 C
50
CH4
250 C C2-C4
o
0
270 C
C5+ CO Sel CO2 Con.
50 40
40 30
30
20
20
10
10 0
o
230 C
o
250 C
o
0
Conversion (%) and selectivity (C-mol%)
b)
Hydrocarbon distribution (C-mol%)
C5+ CO Sel CO2 Conv
Conversion (%) and selectivity (C-mol%)
a) 100
270 C
30
Hydrocarbon distribution (C-mol%)
C2-C4
CH4
60
C5+ CO Sel CO2 Conv
50
40
40 30 30 20 20 10
10
0
o
230 C
o
0
o
250 C
Conversion (%) and selectivity (C-mol%)
c) 50
270 C
Fig. 9 Effect of reaction temperature on the CO2 hydrogenation performance over (a) CoCu/TiO2, (b) 2.5K–CoCu/TiO2, (c) 3.5K–CoCu/TiO2 catalysts under reaction conditions shown in Table 5.
Hydrocarbon distribution (C-mol%)
60 C5+
50 30 CO sel
40
20
30 20 CO2 conv
10
10 0
0 0
40
80 120 Time on stream (h)
160
Conversion (%) and selectivity (C-mol%)
40
200
Fig. 10 Catalytic performance of 2.5K–CoCu/TiO2 catalyst as a function of reaction time. Reaction conditions: P = 5.0 MPa, T = 250 oC, GHSV = 3000 mL·g–1·h–1,
31
H2/CO2 /N2 = 73/24/3. Table 1 Textural properties of CoCu/TiO2 catalysts with different K promoter loading. Metal content (mol%) a
K2O
Co/Cu
content
mol
(wt.%)
ratio
SBET Sample
VBJH
Dpore
(m2·g–1) (cm3·g–1)
(nm)
Co
Cu
Ti
K
CoCu/TiO2
17.2
18.8
64.0
0.0
0.0
0.91
36.9
0.25
17.3
1.5K–CoCu/TiO2
17.3
18.3
62.0
2.4
1.4
0.94
30.2
0.23
32.3
2.0K–CoCu/TiO2
17.1
18.5
61.0
3.4
2.0
0.93
31.3
0.23
33.3
2.5K–CoCu/TiO2
17.0
18.7
60.0
4.2
2.5
0.91
31.1
0.22
30.9
3.0K–CoCu/TiO2
17.1
18.0
59.9
5.0
2.9
0.95
30.2
0.21
29.7
3.5K–CoCu/TiO2
16.8
17.8
59.7
5.8
3.4
0.94
29.0
0.24
32.4
a
Determined by ICP–OES analysis.
Table 2 XPS results for the CoCu/TiO2 catalysts with different K content after calcination. Relative surface concentration of metal (at.%)
Co 2p3/2
Cu 2p3/2
Ti 2p3/2
K 2p3/2
Co
Cu
Ti
K
Co/Cu atomic ratio
CoCu/TiO2
779.6
933.1
458.2
--
24.7
15.0
60.3
--
1.65
1.5K–CoCu/TiO2
779.7
933.1
458.4
292.9
18.7
13.5
64.2
3.6
1.38
2.0K–CoCu/TiO2
779.6
933.2
458.3
292.7
18.6
13.2
64.0
4.3
1.41
2.5K–CoCu/TiO2
779.8
933.5
458.4
292.9
19.4
13.4
60.6
6.6
1.45
3.0K–CoCu/TiO2
780.0
933.8
458.5
292.9
20.0
12.8
62.1
5.2
1.57
Binding energy (eV) Sample
32
3.5K–CoCu/TiO2
780.0
933.5
458.5
292.8
19.1
12.4
64.3
4.2
1.53
Table 3 XPS results for the CoCu/TiO2 catalysts with different K content after reaction. Relative surface concentration of metal (at.%)
Co 2p3/2
Cu 2p3/2
Ti 2p3/2
K 2p3/2
Co
Cu
Ti
K
Co/Cu atomic ratio
CoCu/TiO2
780.9
934.1
458.4
--
33.9
20.4
45.7
--
1.67
1.5K–CoCu/TiO2
781.0
934.2
458.6
293.0
30.3
15.7
43.5
10.5
1.94
2.0K–CoCu/TiO2
780.5
933.8
458.2
292.6
31.8
16.4
39.7
12.1
1.94
2.5K–CoCu/TiO2
780.1
933.6
458.1
292.6
30.5
16.1
37.9
15.5
1.90
3.0K–CoCu/TiO2
780.2
933.7
458.1
292.6
30.0
15.5
38.5
16.0
1.93
3.5K–CoCu/TiO2
780.1
933.5
458.0
292.6
28.9
15.0
38.7
17.3
1.92
Binding energy (eV) Sample
Table 4 CO2/H2–TPD results of the reduced CoCu/TiO2 catalysts with different K promoter loading. Total CO2
Total H2
uptake amounta (a.u.)
uptake amountb (a.u.)
CoCu/TiO2
33.0
100.0
1.5K–CoCu/TiO2
59.6
74.7
2.0K–CoCu/TiO2
65.3
69.9
2.5K–CoCu/TiO2
96.2
62.8
3.0K–CoCu/TiO2
95.4
62.0
Sample
33
3.5K–CoCu/TiO2 a
100.0
59.7
The largest total CO2 uptake amount of the 3.5K–CoCu/TiO2 catalyst is defined as
100, and the values of the other catalysts are relative to this catalyst. b
The largest total H2 uptake amount of the CoCu/TiO2 catalyst is defined as 100, and the values of
the other catalysts are relative to this catalyst.
Table 5 Catalytic performance for CO2 hydrogenation with different K promoter loading.a
CO2 Conv (%)
CO Sel (C-mol%)
CoCu/TiO2
23.1
1.5K–CoCu/TiO2
Sample
Hydrocarbon distribution (C-mol%)
Chain growth Olefin/Paraffin C5+Yield probability (C2~C4) (C-mol%)b α
CH4
C2~C4
C5+
1.3
89.5
5.6
4.9
0.34
0.01
1.1
21.2
4.7
62.0
24.3
13.8
0.47
0.02
2.8
2.0K–CoCu/TiO2
13.8
19.7
45.4
33.3
21.3
0.52
0.14
2.4
2.5K–CoCu/TiO2
13.0
35.1
34.1
30.8
35.1
0.63
0.33
3.0
3.0K–CoCu/TiO2
12.8
35.9
34.6
34.6
30.8
0.60
0.46
2.5
3.5K–CoCu/TiO2
11.9
45.9
35.0
33.9
31.1
0.61
0.41
2.0
a
Reaction conditions: P = 5.0 MPa, T = 250 oC, GHSV = 3000 mL·g–1·h–1, H2/CO2
/N2 = 73/24/3. b Yield C5+ = 𝐶𝐶𝑂2 × 𝑆𝑡𝑜𝑡𝑎𝑙 ℎ𝑦𝑑𝑟𝑜𝑐𝑎𝑟𝑏𝑜𝑛 × 𝑆𝐶5+ .
34