Article pubs.acs.org/EF
Synthesis and Characterization of Silica-Supported Cobalt Phosphide Catalysts for CO Hydrogenation Xiangen Song,†,§ Yunjie Ding,*,†,‡ Weimiao Chen,† Wenda Dong,†,§ Yanpeng Pei,†,§ Juan Zang,†,§ Li Yan,† and Yuan Lu† †
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, People’s Republic of China ‡ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, People’s Republic of China § Graduate School, Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ABSTRACT: A series of silica-supported cobalt phosphides with different metal/P molar ratios were synthesized by means of a temperature-programmed reduction (TPR) method. Their catalytic performances of CO hydrogenation were tested in a fixedbed reactor under conditions of 553 K, 5.0 MPa, and H2/CO = 2 (molar). The catalytic activity of the phosphides ranked in the following order: Co2P/SiO2 < Co4P/SiO2 < Co8P/SiO2 (x denotes the molar ratio of metal to P). It is interesting to find that C2+ oxygenates were the main product over the CoxP catalysts. Evidence from X-ray photoelectron spectroscopy (XPS) and highresolution transmission electron microscopy (HRTEM) measurements proved that the Coδ+ species might be the active sites for the formation of higher oxygenates during CO hydrogenation.
1. INTRODUCTION With rapid economic development and intensive improvement of living standards worldwide, more and more energy is needed. As a nonrenewable resource, the storage capacity of petroleum is being reducing day by day. Increasing concern about an energy crisis has driven the efforts to find substitute energies. The conversion of syngas (CO + H2) derived from coal, natural gas, and renewable biomasses to oxygenates (especially ethanol) may be a route to relax the crisis in the near future.1 Recently, ethanol production from syngas has drawn growing attention worldwide. Conventionally, ethanol is produced in large scale primarily via the fermentation of sugars derived from corn or sugar cane, or via the hydration of petroleum-based ethylene. Catalysts for syngas conversion to oxygenated hydrocarbons can be broadly grouped into four categories: (1) Rh-based catalysts;2−7 (2) alkali-doped Cu-based catalysts;8 (3) modified Fischer−Tropsch (FT) synthesis catalysts;9−11 and (4) Mobased catalysts.12−14 Rh-based catalysts have the highest ethanol selectivity among the various syngas conversion catalysts investigated so far.2−7 However, because of the cost and supply issues involved with Rh, it would be difficult for this type of catalyst to be commercialized in large scale. Therefore, there is significant interest in developing less-expensive syngas conversion catalysts with high alcohol selectivity. Several researchers have reported that Mo carbides15,16 and cobalt carbides10,17 have catalytic properties similar to certain precious metal catalysts in syngas conversions. Furthermore, transition-metal phosphides, such as Fe2 P, Co2 P, and Ni2P,18−25 when supported on SiO2 and Al2O3, were recently evaluated for hydrodesulfurization (HDS), hydrodeoxygenation (HDO), and hydrodenitrogenation (HDN) reactions and exhibited relatively satisfactory results. The structures of the metal-rich phosphides, based on trigonal prisms, can easily © 2012 American Chemical Society
accommodate the relatively large P atoms, thus leading to a better exposure of the surface metal atoms to the fluid-phase reactants.18 In addition, supported phosphides can be prepared easily via the temperature-programmed reduction (TPR) of phosphate precursors. Oyama18 has predicted that the catalytic ability of metal phosphides would remain a substantial area and they may also have good activities in hydrogen transfer reactions, such as synthesis gas conversion to hydrocarbons and oxygenates, and so on. Zaman and Smith26−28 have reported some CO hydrogenation results over K promoted MoP/SiO2 catalysts, and showed that the product distribution over K promoted MoP/SiO2 catalysts comprised mainly C2+ oxygenates, which was distinct from that obtained on other Mo based catalysts. Enlightened by the catalytic results obtained from Mo phosphides26−28 and cobalt carbides10 for syngas conversion, we have studied the catalytic properties of cobalt phosphides for CO hydrogenation. In the present work, CoxP/SiO2 catalysts with different metal/P molar ratios were prepared and characterized using TPR, CO−temperature-programmed desorption (CO-TPD), X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) techniques. The performances of these catalysts for syngas conversion have also been investigated in detail.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. The catalysts with different metal/ P molar ratios were prepared by a temperature-programmed reduction (TPR) method from supported phosphate precursors prepared by Received: August 16, 2012 Revised: October 17, 2012 Published: October 18, 2012 6559
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sand inside the reactor, and the space above the catalyst was also filled with quartz sand, which acted as a preheater for the syngas. The effluent passes through a condenser filled with cold and deionized (DI) water to collect the liquid products. The outlet gases CO, H2, CH4, CO2, and C1−C5 hydrocarbons were online analyzed by an Agilent 3000A Micro GC with four packed columns of molecule sieve, Plot Q, Al2O3 and OV-1, and a TCD device as detectors. The organic liquid products and the aqueous products were analyzed off-line on a Varian Model 3800 GC gas chromatograph with a HP-FFAP capillary column and a flame ionization detection (FID) device, after being carefully separated. The CO conversion and the product selectivity were calculated according to the following equations:
incipient impregnation of the silica support. The silica support was calcined at 773 K for 3 h prior to use. To avoid the formation of an insoluble precipitate of the metal phosphate upon mixing the aqueous solutions of metal nitrate and ammonium phosphate, a two-step impregnation was performed to prepare the supported metal phosphate precursors. First, the silica support was impregnated with an aqueous solution of (NH4)2HPO4. After drying at 393 K for 8 h, the support containing phosphorus was further impregnated with the solution of metal nitrate. The impregnated supports were then dried at 393 K for 8 h and calcined at 773 K for 4 h. Second, the precursors were reduced to phosphides by heating from room temperature to 623 K at a ramping rate of 5 K/min and from 623 K to 973 K at a ramping rate of 1 K/min. The samples were held at 973 K for 2 h in flowing H2 with a flow rate of 350 mL/min per gram of precursor. The prepared catalysts and the spent catalysts were cooled to room temperature in a H2 flow and then passivated in a 1 vol % O2/Ar flow with a flow rate of 100 mL/min for 4 h. The catalysts with different M/P molar ratios are denoted by MxP/SiO2 (where x = M/P molar ratio). The metal weight loading of all catalysts was 10%, while the P loading varied. 2.2. Characterization of Catalysts. TPR experiments were performed on a Micromeritics Autochem 2910 apparatus. Eighty milligrams (80 mg) of the precursor was placed in a quartz reactor and reduced by a 10% H2/Ar gas mixture at a flow rate of 50 mL/min. The temperature was ramped at 10 K/min to the final temperature and the hydrogen consumption was recorded using a thermal conductivity detection (TCD) device. The CO uptake was measured using the same apparatus as H2-TPR. A 100-mg sample was loaded in the reactor and reduced in a H2 flow with a flow rate of 60 mL/min at 500 °C for 2 h. Afterward, the sample was flushed with a He flow with a flow rate of 80 mL/min at 500 °C for 2 h. Subsequently, the sample was cooled to 50 °C. 5% CO-He was injected into the He carrier gas with a flow rate of 50 mL/min, and the CO uptake was measured using a TCD device. CO pulses were repeatedly injected until the effluent areas of consecutive pulses were constant. The total dynamic CO uptake was then calculated. After CO adsorption, the sample was swept with a He flow (20 mL/min) until the TCD signal was stable. CO-TPD was performed in the He flow (20 mL/min) at a heating rate of 10 °C/min with a quadrupole mass spectrometer (QMS, Balzers OmniStar 300) as a detector to monitor the desorbed species. MS signals at m/z = 2 (H2), 16 (CH4), 18 (H2O), 28 (CO), 44 (CO2), and 12 (C) were continuously recorded. The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface areas. Pore size distributions were determined from the desorption branches of the isotherms, according to the Barrett−Joyner−Halenda (BJH) method. Powder X-ray diffraction (XRD) measurements were recorded with a PANalytical X’Pert-Pro powder X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm), operating at 40 kV and 150 mA. HRTEM analyses were performed with a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV. Before the HRTEM experiments, the samples were ultrasonically dispersed in ethanol and then several drops of the solution were deposited onto copper grids. X-ray photoelectron spectroscopy (XPS) was carried out using a VANTAGE-ESI instrument with Mg Kα radiation (1253.6 eV). Before the measurements, the fresh and spent catalysts were both reduced under static conditions by passing a H2 flow for 1 h at 723 K. After the reduction, the samples were degassed in high vacuum for 2 h at 723 K, then cooled to room temperature. Binding energies were determined using C 1s (284.6 V) as the reference. 2.3. Reactivity Studies. The passivated catalysts were reduced in situ at 723 K for 1 h with a H2 flow rate of 100 mL/min and at a ramping rate of 5 K/min starting from room temperature before the reaction was conducted. After cooling to 553 K, the sweeping gas was switched to syngas (H2/CO = 2), and the reaction was carried out at 553 K, 5.0 MPa, and GHSV = 5000 h−1. Supported pure metal catalyst of cobalt was also tested for comparison. The volume of catalyst was 1 mL for each test. The reaction system consisted of a small fixed-bed tubular reactor made of 316L stainless steel with a length of 300 mm and an inner diameter of 9.0 mm, together with an external heating system. The catalyst particles were placed on a packed bed of quartz
conversion (%) = selectivity (%) =
∑ niMi × 100 MCO niMi × 100 ∑ niMi
where ni is the number of carbon atoms in product i, Mi the total mole number of product i during a certain reaction time, and MCO the total mole number of carbon monoxide (CO) in the feed during a certain reaction time.
3. RESULTS AND DISCUSSION 3.1. H2-TPR. In order to shed light on the interaction between the metal component and phosphorus, H2-TPR was carried out from room temperature to 1200 K. The H2-TPR profiles of the supported cobalt oxide and cobalt phosphates are shown in Figure 1. The first peak on Co/SiO2 sample centered
Figure 1. Temperature-programmed reduction (TPR) profiles of supported cobalt phosphate samples.
at ∼500 K was ascribed to the reduction of Co3O4 to CoO.29 Moreover, the higher-temperature peak occurring at ∼661 K was attributable to the reduction CoO to Co.29 For the precursors of CoxP/SiO2 sample, the reduction peaks of all species overlapped to form a big peak. The fast heating rate and the strong interaction originated from intimate contact of the cobalt and the phosphorus species might cause the reduction peaks difficult to separate from each other. In fact, the amount of P was not sufficient for all the Co to form Co2P crystal phases in the cases of the precursors with lower P contents. Consequently, excessive cobalt species would transform into metallic Co in advance during reduction, which would, in turn, assist in the reduction of the other components. Therefore, the main reduction peak shifted from 890 K to 860 K as the Co/P ratio increased from 2 to 8. This also indicated the strong interaction between the cobalt and phosphorus species. The 6560
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particle sizes of cobalt species over Co2P/SiO2 and Co8P/SiO2 catalysts are given in Table1. The Co2P and Co particles size Table 1. Surface Areas and CO Chemisorptions over Fresh SiO2-Supported Phosphides particle sizea catalyst SiO2 Co2P/ SiO2 Co4P/ SiO2 Co8P/ SiO2 a
BET surface area (m2/g)
average pore diameter (nm)
CO uptake (μmol/gcatl)
180 126
19.9 19.6
0 10
139
18.4
26
16.8
18.6
141
18.1
48
16.3
20.4
Co
Co2P 19.9
The particle sizes were calculated from the peaks of XRD patterns.
were ∼16 nm and ∼19 nm, respectively, on the CoxP/SiO2 catalysts. This indicated that the particle size of the cobalt species may be determined by the Co loading, because the metal weight loading of all catalysts was 10%. Therefore, the effect derived from different particle sizes during the reduction process might be weak and could be ruled out. 3.2. Texture Properties and Chemisorption of CO. N2 adsorption was used to measure the surface areas, pore diameters, and pore volumes of the catalysts. CO chemisorption capacities were also measured. The results are listed in Table 1. In comparison to the support, the prepared fresh catalysts possessed lower specific areas and smaller average pore diameters, presumably because of pore blockage by the phosphides. The surface areas of the catalysts became smaller as the P content increased. Experimental CO uptake was small for pure fresh phosphide catalysts with a metal/P molar ratio of 2 in all cases. The enhanced CO uptake with the increase of the metal/P molar ratio can be attributed to the adsorption on the active metallic sites. In many cases, the CO chemisorptions on the cobalt phosphides catalysts were low.24,30,31 Wang et al.24 reported that an excess of surface P or possibly oxygen derived from the unreduced precursor or passive catalysts blocked the active sites, thus giving rise to a lower CO uptake and hindering the chemisorptions of CO. According to Burns et al,31 the decrease in CO uptake could be due to the incorporation of phosphorus. They found the values of chemisorption to rank in the following order: Co > Co2P > CoP. This ordering is probably due to an inhibition of the formation of bridgebonded CO species. In the present work, the CO uptake also ranked in the following order: Co8P/SiO2 > Co4P/SiO2 > Co2P/SiO2. In addition, because of the formation of phosphide, the number of Co atoms that are exposed on the surface would decrease. This may also lead to a decrease in the number of adsorption sites.18 Thus, we believed that the introduction of P species led to the lower CO uptake of cobalt phosphides. 3.3. CO-TPD. Figure 2a displays the TPD spectra of adsorbed CO on CoxP/SiO2 catalysts. CO-TPD is a powerful tool that can be used to explain the strength of the CO-metal bond, and the peak position of CO desorption was related to the types of active sites.32,33 In general, when the temperature of CO desorption increases, it indicates that CO is adsorbed on the small-sized metal particles or high coordination sites, where CO was adsorbed strongly. In addition, the mass spectrum of the corresponding products during the CO-TPD process over Co4P/SiO2 catalyst is also shown in Figure 2b. Two desorption peaks of CO appear in the profile of Figure 2a. The small peak
Figure 2. (a) CO-TPD spectra on CoxP/SiO2 catalysts. (b) Mass spectrum of during the CO-TPD process over Co4P/SiO2 catalyst.
occurring at low temperature (362 K) was aroused from weakly adsorbed CO that may be located at the Co2+ site,32 while the next one, centered from 748 K to 914 K, may be caused by the CO desorption from reduced Co particles.33 Figure 2b shows that the mass spectra signal of CO2 was detected during CO desorption at high temperature. This implied that the disproportionation reaction of CO (2CO → CO2 + C) took place during CO-TPD at ∼780 K. With the increase of Co/P molar ratio, the high-temperature desorption peak shifted noticeably from 748 K to 914 K. This seemed indicate that the introduction of P obviously changed the electronic proportion and weakened the CO adsorption strength on cobalt metal. The weakening of interaction between CO and cobalt metal may favor the insertion of CO and enhance the selectivity of oxygenate in the products. 3.4. XRD Characterization. The diffraction patterns of the fresh and spent CoxP/SiO2 catalysts are shown in Figure 3. The four major peaks for cobalt phosphide aligned well with the standard Co2P pattern (International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF) No. 6-0595), suggesting that the major phase in the Co2P/SiO2 sample before and after CO hydrogenation was Co2P. The patterns for the fresh and spent CoxP/SiO2 catalysts (x = 4 and 8) exhibited peaks for both metallic Co and Co2P. The metallic Co and Co2P phases formed on the fresh catalyst remained unchanged after the CO hydrogenation reaction. This suggested that the metallic Co and Co2P phases were both stable under the 6561
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Figure 3. XRD patterns of CoxP/SiO2 catalysts before and after reaction.
reaction conditions. A metallic Co particle size of 16.8 nm and a Co2P particle size of 18.6 nm could be calculated from the peaks of the fresh Co4P/SiO2 catalyst. 3.5. TEM Characterization. Elemental analyses of HRTEM for the fresh Co4P/SiO2 catalyst are shown in Figure 4. The component of the dark domain in the dark-field scanning tunneling electron microscopy (STEM) image was silica alone, which was opposite that in the bright-field transmission electron microscopy (TEM) image in Figure 4a (the Cu element came from the copper grids). The white domain in Figure 4b contained both elemental Co and P. This indicated that Co and P elements were dispersed homogeneously on certain domains of the support, rather than scattered on the support independently and with no interactions between each other. Low-resolution and highresolution TEM images of the Co4P/SiO2 catalyst before reaction are also shown in Figure 5. The low-resolution image revealed spherically shaped particles with a uniform size distribution on the support. The particle size derived from the TEM image was ∼15 nm, which was consistent with that calculated from its XRD pattern. The measured spacing between lattice fringes of 2.04 and 2.21 Å in Figure 5b are consistent with the d-spacing of the (111) crystallographic plane of metallic Co (ICDD PDF No. 01-089-4307) and the (112) crystallographic plane of Co2P (ICDD PDF No. 03-0652380), respectively. This observation corresponded perfectly to the XRD pattern of the Co4P/SiO2 catalyst, in which metallic Co and Co2P crystal phases coexisted on the sample. 3.6. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) analyses were performed in order to analyze the surface chemical nature of the catalyst particles. Figure 6 displays the XPS spectra in Co (2p) and P (2p) regions for the Co4P/SiO2 catalyst before and after reaction. On the XPS spectrum of the fresh Co4P/SiO2 catalyst, two peaks were apparent in the Co (P1/2) region at 794.2 eV and the Co (2P3/2) region at 779.2 eV, along with a spin−orbit splitting energy of 15.0 eV. The peaks at 794.2 and 779.2 eV were assigned to reduced Co species,21,22 and the P (2P) peaks at 130.6 and 126.6 eV corresponded to P3+ in the HPO3H− and elemental phosphorus, respectively.21,22,25 In the case of the spent Co4P/SiO2 catalyst, the weak peak at 782.6 eV was the Co2+ species, as assigned in the literature.22 Two Co (2P) peaks at ∼793.7 eV and ∼778.8 eV were ascribed to reduced Co, and
Figure 4. Elemental analyses of HRTEM for the fresh Co4P/SiO2 catalyst.
the peak at 126.8 eV was attributed to the reduced P species.22,23 The existence of reduced P indicates the formation of phosphide, which is in accordance with the result of the XRD patterns. The binding energy of 132.6 eV indicated the presence of P5+ in the PO43−, which might be a counterion of Co2+, presumably due to the presence of Co3(PO4)2.22 The binding energies of Co in the fresh and spent catalysts were higher than those of metallic Co (2P1/2: 793 eV, 2P3/2: 778 eV),21 indicating that the Co species in the catalysts possessed a partial positive charge (δ+). Furthermore, the P (2p) binding energy of 126.6−126.8 eV was a considerable shift from that of elemental phosphorus (130.2 eV),23 suggesting that the P had a partial negative charge (δ−). The binding energy of reduced Co and phosphorus shifted by ∼0.9 eV and ∼3.3 eV, respectively, suggesting a considerable magnitude of charge transfer from Co to P in the catalysts. The binding energy shift of P (2p) in our work is bigger than that reported by Burns et al.21,31 This may be attributed to the composition of the phosphides. The binding energy shift was induced by the charge transfer from Co to P species. Therefore, in the Co4P/SiO2 catalyst, the accepted charge amount for each P atom from Co may be larger than that in Co2P and CoP catalysts. The oxidized P and 6562
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3.7. Catalytic Performance and Discussions. From a kinetic of point of view, the effect of cobalt phosphide formation on altering the reaction rates should be reflected by the change of activation energy (Ea) or the pre-exponential factor (k0) in the Arrhenius equation of r = k0 exp[−Ea/(RT)]. But, before we calculated the Arrhenius parameters, necessary experimental work must be done to eliminate the influence of internal mass transport and external diffusion. The catalytic performances of the catalysts supported on the silica with different particle sizes and with different volumes are presented separately, in Tables 2 and 3, respectively. It could be observed Table 2. Catalytic Performance of the Catalysts Supported on Silica with Different Particle Sizes Selectivity (%)a silica particle size 10−20 mesh
20−40 mesh
40−60 mesh
catalystb Co4P/ SiO2 Co/ SiO2 Co4P/ SiO2 Co/ SiO2 Co4P/ SiO2 Co/ SiO2
CO conv (%)
CH4
6.7
38.8
16.6
23.1
20.3
7.0
C2+ oxyd
CO2
5.1
34.7
4.8
74.1
1.0
2.6
2.0
37.8
16.8
6.1
34.7
4.6
23.9
21.3
73.1
1.0
1.6
3.0
7.0
37.8
16.8
6.1
34.7
4.6
22.9
21.7
73.9
1.0
1.2
2.2
C2+Hc MeOH
a Molar selectivity = carbon efficiency = nici/∑nici. bReaction conditions: T = 563 K, P = 5.0 MPa, H2:CO = 2:1, and GHSV = 5000 h−1. The metal loading of all catalysts was 10 wt %, and the phosphorus loading was different for different metal/P molar ratios. c Hydrocarbons with two or more carbons. dOxygenates with 2−5 carbons, such as ethanol, propanal, propanol, butanol, butyraldehyde, and ethyl acetate.
from Table 2 that the variation of reaction rate with the change of catalyst particles size was almost negligible when the particle size reduced to the range of 10−60 mesh. The catalytic result did not show any obvious difference with the increase in catalyst volume; this could rule out the effect of external diffusion. The kinetic parameters, based on the data in Table 4, are shown in Table 5. From Table 5, the decrease in Ea indicated the promotion of the introduction of P, while the decrease in k0 suggested a compensation effect, which may be caused by the decrease in reaction active sites. Table 6 presents the detailed reaction data of CO hydrogenation over different phosphide catalysts. The supported pure cobalt metal catalyst mainly produced hydrocarbons. It can be found that CO conversion and hydrocarbon selectivity increased as the metal/P molar ratio increased, while the selectivity of oxygenates had an opposite trend. The formation of phosphide led the total number of Co atoms exposed on the outer surface decrease.18 Therefore, although the incorporation reduced the Ea value, the decrease in active sites dominated the reaction rate. Moreover, the turnover frequency (TOF) decreased as the introduction of P. Detailed selectivities of the products and their variation trends with different metal/P molar ratios are shown in Figure 7. It was interesting to note that the selectivity of C2+ oxygenates over the Co2P/SiO2 catalyst was as high as 45%,
Figure 5. (a) Low-resolution and (b) high-resolution TEM images of the fresh Co4P/SiO2 catalyst.
Figure 6. XPS spectra of the Co4P/SiO2 catalyst: (a) fresh catalyst and (b) spent catalyst.
Co species might arise from an incomplete reduction of superficial passivation layers before the XPS measurement. 6563
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chain growth ability of reduced metallic Co, which did not form the Co2P phase but was dispersed homogeneously and coexisted with Co2P in the catalyst, was restricted to some extent, because of the influence of the P species. The HRTEM image in Figure 5b shows that the distance between the Co particles and the Co2P species was only ∼0.5 nm, which was just the size of a Co cluster composed of several Co atoms. The intimate contact between the Co particles and the Co2P species should have an impact on the electronic state of the cobalt particles. The overlapped TPR profiles in Figure1 supported this speculation of intimate contacts between the Co particle and the Co2P species. The XPS characterization results have also verified this assumption. The cobalt species existed mainly as a Coδ+ state on the Co4P/SiO2 catalyst surface, because of the result of a synergistic effect from Co and Co2P. Based on the above observations, it could be proposed that the Coδ+ species were the active sites for the formation of C2+ oxygenates in our catalysts. In the four catalysts studied in the present work, the Co4P/ SiO2 sample showed the best combination of activity and selectivity toward C2+ oxygenates. Although further work must be done to improve the activity of this supported CoxP catalyst, its product distribution was quite distinct from other reported Co-based and Mo-based catalysts. Several Co-based and Mobased catalysts have been widely investigated for CO hydrogenation to produce mixed alcohols,10,14,35,36 and Table 7 summarizes some reported results, with comparison to the Co4P/SiO2 catalyst in this work. It is obvious that the cobalt phosphide catalysts can produce a good deal of oxygenates, even in the absence of alkali-metal oxides as a promoter. The selectivity of C2+ oxygenates of our catalyst has a certain competitive advantage, compared with those of CoCu, CoMoK, and RhMoK catalysts under similar reaction conditions. It is well-known that the reaction of higher oxygenate synthesis involves several elementary steps, including CO dissociation, undissociative CO activation, CO insertion, and reaction intermediate hydrogenation, so that the catalyst surface must comprise a variety of active sites.1,3−9 There are different viewpoints about the reaction active sites for the formation of higher alcohols over the Co−Cu catalysts. Some researchers believe that Co atoms provide the alkyl group, while Cu atoms neighboring to the Co atoms are responsible for chain termination via the addition of CO into an alkyl group locating on the boundaries of Co and Cu atoms.8,35,37 However, other investigators proposed that Co−Cu alloys favor the formation of Co2C, which can activate CO undissociatively, and this step is essential for the synthesis of oxygenates.11,37 Furthermore, some researchers have demonstrated that the growth of oxygenate chains can occur on Co2C via C−C coupling between CO and CHx species, and CO dissociation is favored on Co sites instead of Co2C sites, based on both theoretical calculation17 and experimental results.10 They claimed that the Co2C species played an important role in CO insertion, because of their noble-metal-like characteristics, and an increasing Co2C/Co ratio in the catalysts seemed to be the
Table 3. Catalytic Performance of the Catalysts Supported on Silica with Different Volumes of Catalyst Selectivity (%)a volume of catalyst (mL) 1
catalyst Co4P/ SiO2 Co/ SiO2 Co4P/ SiO2 Co/ SiO2
2
Co4P/ SiO2 Co/ SiO2
4
b
CO Conv (%)
CH4
C2+Hc
MeOH
C2+ oxyd
CO2
6.9
37.1
18.2
6.8
33.5
4.4
23.1
21.7
72.9
1.2
1.4
2.8
7.0
37.8
16.8
6.1
34.7
4.6
23.9
21.3
73.1
1.0
1.6
3.0
6.8
37.8
17.8
6.1
33.7
4.6
23.5
21.3
73.1
1.0
1.6
3.0
a Molar selectivity = carbon efficiency = nici/∑nici. bReaction conditions: T = 563 K, P = 5.0 MPa, H2:CO = 2:1, and GHSV = 5000 h−1. The metal loading of all catalysts was 10 wt %, and the phosphorus loading was different for different metal/P molar ratios. c Hydrocarbons with two or more carbons. dOxygenates with 2−5 carbons, such as ethanol, propanal, propanol, butanol, butyraldehyde, and ethyl acetate.
Table 4. Catalytic Performance of the Catalysts at Different Reaction Temperaturesa CO Conv (%) T = 543 K 1.8 4.0 6.4 13.1
T = 553 K Co2P/SiO2 2.3 Co4P/SiO2 5.3 Co8P/SiO2 8.7 Co/SiO2 17.5
T = 563 K 3.2 7.0 11.6 23.9
a
Reaction conditions: P = 5.0 MPa, H2:CO = 2:1, and GHSV = 5000 h−1. The metal loading of all catalysts was 10 wt %, and the phosphorus loading was different for different metal/P molar ratios.
which was very close to selectivities over Rh-based catalysts.4,5 With the increase of the Co/P ratio, the selectivity of C2+ oxygenates decreased, but it still could reach 37.7%, which almost corresponds to that of the unpromoted Rh-based catalyst.6 Thus, it is envisaged that cobalt phosphide catalysts are promising potential substitutes of Rh catalysts in the field of C2+ oxygenates synthesis from syngas. Methane still was the main component in the hydrocarbon products, although XRD and TEM characterizations showed that there existed quite a quantity of metallic Co particles with a particle size range that was suitable for Fischer−Tropsch (F-T) synthesis to produce long-chain hydrocarbons.34 This observation suggested that the
Table 5. Apparent Kinetic Parameters and Correlation Coefficients (R2) for the Catalysts
Ea (kJ/mol) K0 (μmol(g of Co)−1 s−1) R2
Co2P/SiO2
Co4P/SiO2
Co8P/SiO2
Co/SiO2
67.9 7.71 × 1011 0.989
70.4 2.43 × 1012 0.997
75.3 1.07 × 1013 0.985
80.2 6.4 × 1013 0.991
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Table 6. Detailed Results of CO Hydrogenation over CoxP/SiO2 Catalysts Selectivity (%)a catalyst
b
Co2P/SiO2 Co4P/SiO2 Co8P/SiO2 Co/SiO2
c
−1
d
CO Conv (%)
TOF (s )
CH4
C2+H
MeOH
EtOH
other C2+ oxye
CO2
carbon balance (%)
2.3 5.3 8.7 17.5
0.08 0.09 0.11 0.12
26.7 33.8 44.2 19.3
10.0 15.8 23.4 74.1
13.3 7.1 4.5 1.0
9.5 13.2 9.4 2.0
35.2 24.5 13.2 1.6
5.3 5.6 5.3 2.0
99.9 99.7 99.8 99.9
a Molar selectivity = carbon efficiency = nici/∑nici. bReaction conditions: T = 553 K, P = 5.0 MPa, H2:CO = 2:1, and GHSV = 5000 h−1. The metal loading of all catalysts was 10 wt %, and the phosphorus loading was different for different metal/P molar ratios. cTOF = turnover frequency; this parameter was based on CO conversion and CO chemisorption. dHydrocarbons with two or more carbons. eOxygenates with 2−5 carbons except ethanol, such as propanal, propanol, butanol, butyraldehyde, and ethyl acetate.
was longer than that over the phosphide catalyst, we could infer from the distribution of the oxygenate product that the effect in the phosphide was stronger than that in the carbide. By analogy, we might propose that the Co2P/Co species is playing a similar role as the Co2C/Co in oxygenate synthesis, that is, metallic Co provided alkyl groups and Co2P was responsible for the insertion of CO to terminate chain growth and to form oxygenates. Based on the discussion above, we proposed a reaction network scheme in Figure 8. The chain growth was via the insertion of adsorbed CHx into another adsorbed CxHy. CO insertion means the termination of chain growth.
Figure 7. Product distributions over CoxP/SiO2 catalysts. (Reaction conditions: T = 553 K, P = 5.0 MPa, H2:CO = 2:1, and GHSV = 5000 h−1.)
Table 7. Reaction Data on Several Co-Based and Mo-Based Catalyst for Syngas Conversion
reference temperature, K GHSV, h−1 H2:CO ratio pressure, MPa CO conversion, % selectivity, C at.% hydrocarbons + CO2 methanol total C2+ oxygenate
CoCuLa
CoMoK
MoRhK
CoLaZr
Co4P/ SiO2
35 548 4000 2.0 6.89 15.7
36 603 4800 2.0 5.0 14.3
10 495 1500 2.0 3.0 21.4
14 573 2000 1.0 8.3 27.5
this paper 553 5000 2.0 5.0 5.3
59.5
72.6
61.1
76.2
55.2
15.5 25.0
11.1 16.3
3.0 35.9
9.6 14.2
7.1 37.7
Figure 8. Possible reaction network scheme on the CoxP/SiO2 catalysts.
4. CONCLUSIONS CO hydrogenation activities of a series of cobalt phosphide catalysts have been investigated and correlated with the metal/ P molar ratios of the catalysts. The CoxP/SiO2 catalysts showed a high selectivity of 45% toward C2+ oxygenates, which was a very different product distribution, compared to other Cobased catalysts or conventional mixed-alcohol catalysts. This may be attributed to the change in electronic and adoption properties of the metallic cobalt caused by the introduction of the P atoms, and Coδ+ is proposed to the active sites for the formation of higher oxygenates.
most likely approach to enhance selectivity toward alcohols. XPS characterization has showed that Coδ+ in cobalt carbide was the predominant Co species.11 The CoO species was also reported as active sites for CO insertion, while metallic Co was the active sites for CO dissociation that led to chain growth.38,39 Moreover, the high concentration of surface Coδ+ species has been suggested to increase the probability of chain termination to form oxygenate products.12 It was worth to note that, from the XRD pattern of the Co4P/SiO2 sample, there existed two Co species, namely, metallic and phosphide, and the XPS result showed that the Coδ+ (0 < δ < 2) was the main chemical state on the surface. The electronic properties of cobalt in carbides and phosphides were influenced by the C and P atoms, respectively. On reasoning that the alcohol chain length over the carbide catalyst
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (+86)-411-84379143. Fax:(+86)-411-84379143. E-mail:
[email protected]. Funding
This work was financially supported by the National Natural Science Foundation of China (No. 20903090). Notes
The authors declare no competing financial interest. 6565
dx.doi.org/10.1021/ef301391f | Energy Fuels 2012, 26, 6559−6566
Energy & Fuels
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Article
(38) Blanchard, M.; Derule, H.; Canesson, P. Catal. Lett. 1989, 2 (5), 319−322. (39) Matsuzaki, T.; Hanaoka, T.; Takeuchi, K.; Arakawa, H.; Sugi, Y.; Wei, K. M.; Dong, T. L.; Reinikainen, M. Catal. Today 1997, 36 (3), 311−324.
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