CO2 Hydrogenation to Hydrocarbons over Iron-based Catalyst: Effects

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CO2 Hydrogenation to Hydrocarbons over Iron-Based Catalyst: Effects of Physico-Chemical Properties of Al2O3 Supports Fanshu Ding, Anfeng Zhang, Min Liu, Yi Zuo, Keyan Li, Xinwen Guo, and Chunshan Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5031166 • Publication Date (Web): 21 Oct 2014 Downloaded from http://pubs.acs.org on October 25, 2014

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Industrial & Engineering Chemistry Research

CO2 Hydrogenation to Hydrocarbons over Iron-Based Catalyst: Effects of PhysicoChemical Properties of Al2O3 Supports Fanshu Ding,†Anfeng Zhang,† Min Liu,† Yi Zuo,†Keyan Li,†XinwenGuo,*,†and Chunshan Song **,†,‡ †

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Centre for Energy Research,

School of Chemical Engineering, Dalian University of Technology, Dalian, P. R. China ‡

Department of Energy and Mineral Engineering, EMS Energy Institute, PSU-DUT Joint

Centre for Energy Research, Pennsylvania State University, University Park, PA16802, USA Keywords: CO2 hydrogenation; Dispersion; Hydroxyls; Alumina; PZC.

ABSTRACT: The effects of surface hydroxyl groups and SiO2 content of alumina supports for FeK/Al2O3 catalysts on their activity and product selectivity for CO2 hydrogenation to hydrocarbons reaction were investigated. A series of iron-based catalysts supported on six commercial Al2O3supports were prepared and tested for hydrocarbons production in a fixed-bed reactor. Surface acidic-basic hydroxyls distribution of the supports were evaluated by FT-IR and point of zero charge (PZC) measurement. It was found that the PZC of Al2O3 supports strongly affectsthe dispersion as well as particle size of the Fe-based catalysts. Increasing PZC value of Al2O3, the iron 1 ACS Paragon Plus Environment


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dispersion increased and particle size decreased. The highest CO2 conversion (54.4%) and C5+ hydrocarbons selectivity (31.1%) were achieved when the PZC was 8.0. CO2 conversion

and

the

long-chain

products

selectivity

decreased

when

PZC

declined.Moreover, SiO2-doping of alumina strongly affected the surface acidic-basic hydroxyls distribution of the supports, while a desired SiO2 content improved the catalyst activity by adjusting CO adsorption and the reduction behavior of the iron-based catalysts.

1. Introduction

CO2 turns out to be attractive for making value-added chemicals because CO2 is an abundant, non-toxic and renewable carbon source.1, 2 The utilization of CO2 for chemicals production not only contributes to alleviate greenhouse effect which is caused by the increasing CO2emissions, but also provides a potential approach for developing sustainable energy technology.3, 4 Hydrogenation of CO2 over iron-based catalysts may beregarded as a modified Fischer-Tropschsynthesis (FTS) reaction using CO2 and H2 as feed, where CO2 is first reduced to CO via the Reverse Water Gas Shift (RWGS) reaction.5, 6 Potassium is a significant promoter in CO2 hydrogenation for enhancing the CO2 adsorption and olefins selectivity, leading to a significant increase in CO2conversion.7Structure promoters or support are also preferred because of the suppression of active phase agglomeration and 2 ACS Paragon Plus Environment

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improvement in mechanical properties of the catalysts.High surface area Al2O3 has been widely used as a support material for metal and metal oxide catalysts.8Investigation of various modified iron-based catalysts indicatedAl2O3 to be the best support for CO2 hydrogenation, compared with SiO2 and TiO2.9 Coating the Fe-K/Al2O3 catalyst with a certain amount of SiO2 increased both CO2 conversion and also higher hydrocarbon selectivity.10,

11

Qu et al. synthesized a series of Al2O3 materials with different pore

properties by using surfactant template such as CTAB, SDS, P123 and blank templates; it was found that the surfactant templates affect textural properties, and the adsorption properties were strongly influenced by the pore texture of the Al2O3.12 Hydrothermal treatment in the range of 100-350 oC also leads to changes inγ-Al2O3pore structure with specific surface area of 200 to 70 m2/g.13 Compared to most other metal oxide support materials, Al2O3 surfaces exhibit various surface Al-OH groups which cause high dispersion.14According to the model that Peri. et al. proposed, there are five types of Al-OH groups over Al2O3 surface, the different IR vOH wavenumber can be attributed to differing numbers of surrounding surface oxide sites.15 Knözinger and Ratnasamy discussed the assignment of the Al-OH IR bands thoroughly, these bands were assigned to the vOH of either bridging group bound to tetrahedrally and octahedrally coordinated Al atoms or terminal group bound to octahedrally coordinated Al atom, as OH-µ1-AlIV (100), OH-µ1-AlIV (110), OH-µ2-AlVI (110) and OH-µ3-AlVI (110), respectively, moreover, the bands assigned to the more basic Al-OH appearedat 3 ACS Paragon Plus Environment


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high wavenumbers, increasing acidity of Al-OH groups would shift the IR bands to lower wavenumbers.8 Although a great deal of approaches have been reported to improve catalyst activity and C2+ hydrocarbon selectivity over the Al2O3 supported Fe catalysts, little is known about how the surface Al-OH groups affects the catalyst performance. In the present work, a series of Fe-based catalysts supported on various commercial aluminasupports were prepared and tested for CO2 hydrogenation to hydrocarbons. Catalyst activity and product selectivity were evaluated in a fixed-bed reactor. The supports and catalysts were characterized by OH-IR, fast titration, N2-adsorption, XRD,H2-TPR, and CO-TPD.

2. Experimental

2.1 Catalyst Preparation

Six γ-Al2O3supports which are denoted as A1 to A6 were prepared by the calcination of different commercial boehmite (AlO(OH)·nH2O) in air at 540

o

C for 4 h. The

corresponding pore size, pore volume, BET surface area and SiO2 content are listed in Table 1. The catalysts were prepared by impregnation method,Fe(NO3)3·9H2O and KNO3 were used as precursor, the corresponding content of Fe and K were 15wt% and 10wt% respectively.The fresh catalyst was obtained after drying at 120 oC for 12 h, calcined in air at 540 oC for 4 h, the heating rate was 2 oC/min. 4 ACS Paragon Plus Environment

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2.2 Catalyst Characterization

The OH-IR spectra were recorded with an EQUINOX55 (Bruker) FT-IR spectrometer by means of the KBr pellet technique. Prior to the measurements, the samples were heated to 300 oC under vacuum (~1×10-5Torr) for 1 h. The spectra of all samples were presented by subtracting the background spectrum. Surface acid-base properties of the Al2O3supports were estimated by the potentiometric titration method.16, 17 About 200 mg samples was equilibrated for 40 min in 30 mL of 0.01 M NaNO3 solution with continuous magnetic stirring at 25 ℃, followed by the addition of 2 mL of 0.01 M HNO3. The suspension was agitated for 30 min, the pH value of the solution was recorded as the initial pH with pH meter. The suspension was then titrated with 0.2 mL 0.01 M NaOH, the pH of the suspension was recorded after every 2 min. The surface charge density σo (µC/m2) was calculated using the formula:

 C A − CB + OH −  −  H +       σo =  F mS where CA (mol/dm3) and CB (mol/dm3) are the concentrations of acid and base added, [OH-] and [H+] are the concentrations of OH- and H+ ions measured from the pH of the suspension, m is the mass of samples (g), and S (m2/g) is the specific surface area of the samples,F (C/mol) is the Faraday constant.

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N2 isotherms were measured using a Quantachrome AUTO-SORB sorption analyzer at 77 K. Prior to the measurements, the samples were degassed invaccumat 300 oC for 8 h. The Brunauer-Emmauer-Teller (BET) method was used to calculate the specific surface area. The Barrett-Joyner-Halenda (BJH) method was applied to evaluate the pore size distribution.The total pore volume was obtained from the amount of vapor adsorbed at a relative pressure (P/P0) close to unity, where P and P0 are the measured and equilibrium pressure respectively. X-ray diffraction (XRD) patterns of the supports and catalysts were recorded on a RigakuSmartLab diffractometer with Cu Kα radiation (λ=1.5406 Å) source. The spectra were recorded from 2Ɵ=5°to 80° with a step size of 0.02°. The crystallite phases were identified by comparing the diffraction patterns with the data of the Joint Committee on Power Diffraction Standards (JCPDS). H2-temperature programmed reduction (H2-TPR) were measured with ChemBET Pulsar TPR/TPD equipment (Quantachrome, USA) to analyze the reducibility of the calcined catalysts. About 100 mg sample was placed in a quartz tube reaction in the interior of a controlled oven.To remove moisture and other contaminants, the sample was flushed with He at 200 ℃ for 1 h prior to the reduction, then cooled down to room temperature. The sample was reduced with a gas mixture containing 5% H2/Ar at a flow rate of 30 mL/min and heating rate at 10 oC/min up to 900 oC. For removal of released water formed during the reduction process, a cooling trap was placed between the sample 6 ACS Paragon Plus Environment

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and the TCD detector, the temperature and TCD signals were continuously recorded during the H2-TPR analyze. CO-temperature programmed desorption (CO-TPD) were conducted in the same equipment as TPR. The sample (~200 mg) was reduced with 5%H2/95%Ar (30 mL/min) at a temperature of 400 oC for 2 h, then flushed at 400 oC with He for 50 min. After pre-reduction, the sample was cooled to 30 oC, and CO flow was continued for 30 min at 30 oC, purged with He for 30 min to remove weakly adsorbed species. The temperature and TCD signals were continuously recorded during the CO-TPD analyze while the temperature was increased from 30 to 500 oC at a rate of 10 oC/min. Iron dispersion of the Al2O3 supported catalyst was measured by CO titration at 30oC using ChemBET Pulsar TPR/TPD equipment. The sample (~200 mg) was reduced in situ with H2 at 400 oC for 2 h, then flushed at 400oC with He for 50 min. After pre-reduction, the sample was cooled to 30 oC and CO chemisorption was carried out.A Fe: CO stoichiometry is assumed to be 2:1.18-23 After CO titration, the sample was re-oxidized at 400 oC by 5% O2/N2 to determine the reduction extent of iron-based catalyst. Iron dispersion (D%) was calculated according to the equation as described in the literature.20-23 D% =

1.117 X Wf



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where X (µmol/g) is the total uptake of CO, W is the weight percentage of iron in the Al2O3 supported catalyst, and f is the fraction of iron reduction extent determined from O2 uptake. Average particle size (dp (nm)) was calculated from D% assuming spherical metal crystallite of uniform diameter. dp =

96 D%

2.3 Activity Tests

CO2 hydrogenation to hydrocarbons reaction was carried out in a pressurized fixed-bed flow reactor (inner diameter 8 mm), 1.0 g catalyst (10-20 mesh) was loaded for each test. The catalyst was pre- reduced with H2 at 400 oC overnight prior to the reaction. After the reduction, the feed gas was switched to the mixture of CO2and H2with H2/CO2 molar ratio of 3.0 under the reaction conditions of P=3.0 MPa, T= 400 oC, space velocity of 1800 mL/(gcat·h). Products were analyzed by anon-lineFULIGC97gas chromatograph. CO, CO2 and CH4 were analyzed on a carbon molecular sieve column with TCD,and CH4 and C2-C8 hydrocarbons (C2+) were analyzed byFID with a HayeSep Q column. Chromatograms of FID and TCD were correlated through CH4, and product selectivity was obtained based on carbon.


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3. Results and discussion

3.1 Physico-chemical properties of supports and catalysts

Fig. 1 shows infrared spectra in the hydroxylνOH region for different Al2O3 supports; all the Al2O3samples were initially outgassed at 300 oC for 1 h to remove moisture adsorbed. In the IR spectra of the six Al2O3supports, four bands were observed around 3760, 3720, 3670 and 3570 cm-1. According to the report of Knözinger, more basic hydroxyls appear at high wavenumbers, from this point of view alumina has rather more basic hydroxyls and less strong acidic hydroxyls, indicating a more basic surface.8 For the A2 to A4 samples, vibrations at lower frequencies became stronger, showing a more acidic surface. Doping of silica into alumina could affect the hydroxyls distribution, and the bonds of strong acid hydroxyls of A5 and A6 in Fig. 1 became even stronger than the other four samples.From Table 1, it can be seen that the silica content of support samples A5 and A6 is much higher than the other four supports. Al2O3surface existed OH groups that protonate or deprotonate, depending on the acidity or basicity of the solution; the pH is termed as PZC when the net electric chargeis zero. In this context, PZC value is a useful measure to evaluate the balance of the amounts of acidic and basic hydroxyls on Al2O3 supports.The influence of solution pH on surface charge density σ0 (µC/cm2) of various Al2O3materials is summarized in Fig. 2, the PZC value of different Al2O3 can be seen in Table 1. For all tested samples, σ0 is positive 9 ACS Paragon Plus Environment


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at lower pH, decreases with increasing pH value, become negative at higherpH. Dependence of σ0 on pH could be illustrate by the protonation/de-protonation of surface Al-OH groups of Al2O3. Generally speaking, PZC of Al2O3 is round 7~8 and that of SiO2 is round 2~3. The PZC value in Fig. 2 and Table 1 show that doping of alumina with silica cause a decrease of PZC, the values of A5 and A6 are 7.48 and 7.36, respectively. It should be mentioned that the exact PZC values not only depend on the chemical composition of the oxides, but also on the method by which the oxide was prepared. Hydrothermal treatment of γ-Al2O3 could increase the quantity of surface bridging OH groups as well as the concentration of Lewis acid sites, thus alter the metal-support interaction.24 A1 to A4 had relatively low silica content, and PZC value decreased from A1 to A4, indicating increasing acidic hydroxyls which could be confirmed by the spectroscopic data in OH-IR measurement. Fig. 3presents the N2 adsorption-desorption isotherms and pore size distribution of the Al2O3 supports and the corresponding iron-based catalysts. A sharp increase of N2 uptake was exhibited at P/P0=0.7-0.95 for the A3-A6 samples, these four samples exhibited typical IV isotherm in IUPAC classification, the H3-type hysteresis loop suggested their slit shaped pores and broader pore size distribution. An IV isotherms with H1 hysteresis loop were observed on the isotherms of A1 and A2. As can be seen from Fig. 3 and Table 1, compared with other alumina samples, A2 sample exhibited the smallest pore volume (0.37 cm3/g) and the narrowest pore size distribution. The largest surface areas and the 10 ACS Paragon Plus Environment

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broadest pore diameter distribution were observed on the A5 and A6 samples, in which SiO2 content was more than 3%.The differences of pore properties on these six aluminasupports were probably due to the differences in preparation methods and feed materials during the production of the commercial Al2O3supports. The differences in surface both Al-OH groups and pore structures resulted in the variations in adsorption behavior of the metal precursor and thus the differences in dispersion as well as particle size of the iron catalysts.

3.2 Crystallite structure of supports and catalysts

The crystallite structure of the supports and catalystswas determined using X-ray diffraction. Fig. 4 (a) shows the XRD patterns of the six Al2O3 supports. All supports exhibited basically the same XRD patterns, the broad diffraction peaks at 2Ɵ=32oand 37.5o are correspondedto Fe2O3, peaks at 46.0o and 67.0o are the characteristic peaks corresponding to the (4 0 0) and (4 4 0) reflections of γ-Al2O3phase.25 XRD patterns of the fresh FeK/Al2O3 catalysts are showed in Fig. 4 (b). For A3~A6 catalysts, the XRD patterns showed the characteristic peaks of hematite (α-Fe2O3) and Al2O3, while catalysts supported with A1 and A2 showed only the peaks of Al2O3. The disappeared hematite diffraction peaks might be due to the highly dispersed Fe2O3 particles onto Al2O3, and the



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particle size is outside thesize range of the Fe2O3 nanoparticles capable of diffracting X-rays.25

3.3 CO chemisorption and reduction behaviourof catalysts

CO-TPD in Fig. 5 was used to analyze CO adsorption behavior on the reduced catalysts, the main CO desorption peaks are located in the temperature range 300-500oC for all the six samples. For the A1-A4 samples with low silica content, desorption peak of CO shifted to higher temperature when the PZC value of supports decreased, indicating that basic hydroxyls of Al2O3 favored the adsorption of CO. For the high silica samples A5 and A6, desorption peak shifted to lower temperature with increasing silica content, along with an increase of peak area, probably due to Fe-SiO2 interaction.According to the work of Suo et al., associative CO desorbs on Fe single-crystal planes below 200 oC, anddesorption temperature of dissociative CO desorbs are at about 500 oC, desorption peaks at 300-650oC could be assigned to the recombination of surface-adsorbed C species and Oatoms.26We also found in our previous work that adding a suitable content of SiO2 into the FeK/Al2O3 could increase the CO adsorption, possibly by changing the metal-support interactions between iron and alumina.11

Determination of iron particle size for the reduced catalyst was performed using CO-titration measurement. Table 2 shows the results of CO-titration and re-oxidation of 12 ACS Paragon Plus Environment

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alumina-supported catalysts. The calculated particle size from CO chemisorption and reduction degree show that the size of Fe2O3 estimated from XRD analyze does not differ much from the reduced iron-based catalyst. The highest dispersion of iron was observed on the A1 supported catalyst, the corresponding Fe diameter is 2.9 nm, which could explain the diminished iron oxides diffraction peaks in XRD measurement. Particle size is found to be between 2.9 nm (A1) to 5.2 nm (A6) from this calculation, the iron particle size increases with decreasing PZC value of the alumina support, indicating surface hydroxyl groups of Al2O3 supports affect the metal-support interaction and thus the iron dispersion, while Al2O3 with a great amount of basic Al-OH favored the dispersion of iron oxides. Fig. 6 showed the reduction behavior of Fe-based catalyst using H2-TPR characterization. As shown in the figure, surface hydroxyls and PZC as well as pore structure and of Al2O3supports affect the reduction behavior of the iron-based catalysts, which is in agreement with iron dispersion and particle size. Large metal oxides had rather low reduction temperature and the decreasing iron dispersion resulted in the decline of reduction temperature, the reduction temperature of A1 supported catalyst was 522 oC, when the iron dispersion decreased from 32.9% to 25.2% and particle size increased from 2.9 to 3.8 nm in the A4 supported catalyst, the reduction temperature dropped to 509 oC.SiO2 supported catalysts had weaker metal-support interaction



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compared with Al2O3, and reduction temperature of catalysts supported A5 and A6 decreased to 499 and 491 oC, in which SiO2 contents in supports were more than 3%.

3.4 Catalytic Hydrogenation of CO2 to Hydrocarbons

Catalytic activity and product selectivity of the Fe catalysts are on six differentAl2O3 supports are shown in Fig.7. CO2 conversion decreased from 54.4 to 40.5% on the A1 to A4 supported catalysts, while the corresponding C5+ hydrocarbons selectivity dropped from 31.1 to 11.4%. High activity for the A1 supported catalyst may be related to the high metal dispersion, leading to a high density of active sites.It is likely that iron transformed into iron carbide during the CO2 hydrogenation reaction, and smaller iron carbide particles exhibit higher activity. A5 and A6 supported catalysts contained 3.1% and 4.5% silica, respectively, and CO2 conversion first increased to 54.4% on the A5 supported catalyst, but on the A6 supported catalyst, CO2 conversion decreased to 45.8%. In our previous work, we found that a small amount of SiO2 could decrease the interaction between iron and aluminum; however, when SiO2 content was higher than a certain level, the catalysts did not have enough active sites for the absorption of reactant.11 In the present work, 3% SiO2 could be a proper promoter to enhance the adsorption of CO, which could be seen in CO-TPD profiles in Fig. 5, the desorption temperature decreased. However, when SiO2 content increased continuously, the 14 ACS Paragon Plus Environment

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increasing active sites for CO adsorption resulted in a lack of CO2 and H2 adsorption capacity, and in this situation, the CO2 conversion decreased sharply.

4. Conclusions

This work investigated the effects of physico-chemical properties of alumina supports on the supported iron catalysts for CO2 hydrogenation to hydrocarbons. Hydroxyl distribution over the Al2O3support surface and its effect on acid-base properties of supports, iron dispersion and catalytic performance of the FeK/Al2O3 catalysts were studied. Basic hydroxyls favoured the dispersion of iron particles and the adsorption of intermediate product CO. High dispersion also resulted in smaller particle size of iron, which probably contributed to the formation of smaller iron carbide particles that exhibit higher activity. In CO2 hydrogenation to hydrocarbons reaction, Fe catalyst supported on Al2O3 with more basic hydroxyls exhibited high catalyst activity and long-chain product selectivity. Doping of silica into alumina favoured the catalysts reduction and increased CO uptake on the reduced catalysts from H2-TPR and CO-TPD. A large amount of silica inhibits the adsorption of reactants and a suitable doping level is around 3%.

ASSOCIATED CONTENT Supporting Information 15 ACS Paragon Plus Environment


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Description of (1) X-ray diffraction patterns of Fe-K/Al2O3 catalysts after reaction; (2) TPD profiles of H2 and CO2; (3) FT-IR spectra of Al2O3 supports and fresh catalysts; (4) TEM images of the fresh catalysts. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Fax: +86 0411 84986134; E-mail address: [email protected].

**

Fax: +1 814 863 4466; E-mail address: [email protected].

REFERENCES (1) Song, C. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal. Today 2006, 115, 2. (2) Ma, X.; Wang, X.; Song, C. “Molecular Basket” Sorbents for Separation of CO2 and H2S from Various Gas Streams. J. Am. Chem. Soc.2009, 131, 5777. (3) Saeidi, S.; Amin, N. A. S.; Rahimpour, M. R. Hydrogenation of CO2 to value-added products—A review and potential future developments. J. CO2 Util. 2014, 5, 66.


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(4) Perathoner, S.; Centi, G. CO2 Recycling: A Key Strategy to Introduce Green Energy in the Chemical Production Chain. ChemSusChem 2014, 7, 1274. (5) Riedel, T.; Schaub, G.; Jun, K.-W.; Lee, K.-W. Kinetics of CO2 Hydrogenation on a K-Promoted Fe Catalyst. Ind. Eng. Chem. Res. 2001, 40, 1355. (6) Kang, S.-H.; Bae, J. W.; Cheon, J.-Y.; Lee, Y.-J.; Ha, K.-S.; Jun, K.-W.; Lee, D.-H.; Kim, B.-W. Catalytic performance on iron-based Fischer–Tropsch catalyst in fixed-bed and bubbling fluidized-bed reactor. Appl. Catal. B: Environ.2011, 103, 169. (7) Dorner, R. W.; Hardy, D. R.; Williams, F. W.; Willauer, H. D. K and Mn doped iron-based CO2 hydrogenation catalysts: Detection of KAlH4 as part of the catalyst's active phase. Appl. Catal. A: Gen.2010, 373, 112. (8) Knözinger, H.; Ratnasamy, P. Catalytic Aluminas: Surface Models and Characterization of Surface Sites. Catal. Rev.1978, 17, 31. (9) Riedel, T.; Claeys, M.; Schulz, H.; Schaub, G.; Nam, S. S.; Jun, K. W.; Choi, M. J.; Kishan, G.; Lee, K. W. Comparative study of Fischer-Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts. Appl. Catal., A: Gen.1999, 186, 201. (10)Ding, F.; Zheng, B.; Song, C.; Guo, X. Modification of Fe-K/Al2O3 catalysts with TEOS for carbon dioxide hydrogenation into hydrocarbons. Prepr. Symp. - Am. Chem. Soc., Div. Fuel Chem. 2012, 57, 444.



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(11) Ding, F.; Zhang, A.; Liu, M.; Guo, X.; Song, C. Effect of SiO2-coating of FeK/Al2O3 catalysts on their activity and selectivity for CO2 hydrogenation to hydrocarbons. RSC Adv.2014, 4, 8930. (12) Chen, D.; Qu, Z.; Sun, Y.; Wang, Y. Adsorption–desorption behavior of gaseous formaldehyde on different porous Al2O3 materials. Colloids and Surfaces A: Physicochem. Eng. Aspects2014, 441, 433. (13) Chertov, V. M.; Zelentsov, V. I.; Lyashkevich, B. N.

Production of finely divided

boehmite power. J. Appl. Chem. Ussr.1982, 55, 2120. (14)Hardcastle; D., F.; Wachs; E., I. Raman spectroscopy of chromium oxide supported on Al2O3, TiO2 and SiO2: a comparative study.J. Mol. Catal. 1988, 46, 173 (15)Peri, J. B. A Model for the Surface of γ-Alumina1. J. Phys. Chem. 1965, 69, 220. (16)Farooq, M.; Ramli, A.; Subbarao, D. Physiochemical Properties of γ-Al2O3–MgO and γ-Al2O3–CeO2 Composite Oxides. J. Chem. Eng. Data 2011, 57, 26-32. (17) Mahmood, T.; Saddique, M. T.; Naeem, A.; Westerhoff, P.; Mustafa, S.; Alum, A. Comparison of Different Methods for the Point of Zero Charge Determination of NiO. Ind. Eng. Chem. Res.2011, 50, 10017. (18) Jung, H.; Vannice, M.A.; Mulay, L. N.; Stanfield, R. M.; Gelgass, W. N. The characterization of carbon-supported iron catalysts: Chemisorption, magnetization, and mössbauer spectroscopy. J. Catal.1982, 76, 208.


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(19) Jones, V.; Neubauer, L.; Bartholomew, C. Effect of crystallite size and support on the CO hydrogenation activity/selectivity properties of Fe/carbon. J. Phys. Chem. 1986, 90, 4832. (20) Lohitharn, N.; Goodwin Jr, J. G. Impact of Cr, Mn and Zr addition on Fe Fischer-Tropsch synthesis catalysis: Investigation at the active site level using SSITKA. J. Catal.2008, 257, 142. (21) Lohitharn, N; Goodwin Jr, J. G. Effect of K promotion of Fe and FeMn Fischer-Tropsch synthesis catalysts: Analysis at the site level using SSITKA. J. Catal.2008, 260, 7-16. (22) Xiong, H.; Moyo, M.; Motchelaho, M.; Jewell, L.; Coville, N. Fischer-Tropsch synthesis over model iron catalysts supported on carbon sphere: The effect of iron precursor, support pretreatment, catalyst preparation method and promoters. Appl. Catal. A: Gen.2010, 388, 168. (23) Park, J.-Y.; Lee, Y.-J.; Khanna, P. K.; Jun, K.-W.; Bae, J. W.; Kim, Y. H. Alumina-supported iron oxide nanoparticles as Fischer–Tropsch catalysts: Effect of particle size of iron oxide. J. Mol. Catal. A: Chem.2010, 323, 84. (24)Mironenko, R. M.; Belskaya, O. B.; Talsi, V. P.; Gulyaeva, T. I.; Kazakov, M. O.; Nizovskii, A. I.; Kalinkin, A. V.; Bukhtiyarov, V. I.; Lavrenov, A. V.; Likholobov, V. A. Effect of γ-Al2O3 hydrothermal treatment on the formation and properties of platinum sites in Pt/γ-Al2O3 catalysts. Appl. Catal. A: Gen.2014, 469, 472. 19 ACS Paragon Plus Environment


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Table 1 Textural properties of the supports (A1-A6) and catalysts (C1-C6).

Samples

BET surface area (m2/g)

Pore Volume (cm3/g)

Average pore size (nm)

SiO2 content (wt%) a

PZC

A1/C1

222/145

0.54/0.29

9.7/7.9

0.07/-

8.00/-

A2/C2

189/100

0.37/0.21

7.8/7.9

0.16/-

7.77/-

A3/C3

247/205

1.03/0.86 17.4/12.8

0.02/-

7.63/-

A4/C4

257/141

0.85/0.54 18.1/17.7

0.08/-

7.62/-

b

Type and manufacture of commercial boehmitematerials SB, Sasol, Germany BD-DF-LS, Zi Bo Bai Da Chemical Co., Ltd., China BD-09-LSi, Zi Bo Bai Da Chemical Co., Ltd., China LSI-MP,Liaoning-HaitaiSci-Tech Development Co., Ltd., China HSI-MP,

A5/C5

324/159

1.15/0.40

12.8/7.8

3.11/-

7.48/-

Liaoning-HaitaiSci-Tech Development Co., Ltd., China

A6/C6 a

325/189

1.02/0.56

12.4/9.7

4.46/-

7.36/-

BD-09-HSi,Zi Bo Bai Da Chemical Co., Ltd., China

Weight percentage of SiO2 as determined by ICP.

b

PZC was determined using fast titration method.



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Fig. 1. Infrared spectra in the region of hydroxyl stretching vibrations for different Al2O3 supports.


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Fig. 2.Surface charge density (σo) of different Al2O3 supports vs pH at room temperature.



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Fig. 3.N2 adsorption-desorption isotherms and pore size distribution curves for six types of Al2O3 supports and FeK/Al2O3 catalysts. 24 ACS Paragon Plus Environment

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Fig. 4.XRD patterns of the supports and catalysts. (a) Al2O3; (b) FeK/Al2O3 as prepared;



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Fig. 5. CO-TPD of the Al2O3 supported catalysts.


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Table 2. Iron dispersion and particle size as determined by CO chemisorption and re-oxidation of the Al2O3 supported catalysts after reduction at 400oC.

(µmol/g)

Reduction degree (%)c

Corrected dispersion (%)d

Corrected Fe diameter (nm)e

8.7

659

33.5

32.9

2.9

11.8

8.1

751

38.2

31.0

3.1

10.2

10.3

9.3

714

36.3

28.4

3.4

A4

7.5

7.7

12.5

596

30.3

25.2

3.8

A5

6.7

6.8

14.1

707

36.0

18.9

5.1

A6

6.2

6.3

15.2

678

34.5

18.3

5.2

CO uptake

Uncorrected Fe diameter (nm)b

O2 uptake

(µmol/g)

Uncorrected dispersion (%)a

A1

10.8

11.0

A2

11.7

A3

Supports

a

Dispersion (D)=surface Fe0 atom/total Fe atom × 100, assumed stoichiometric adsorption ratio of CO/Fe =1/2. b

Fe particle size calculated from CO chemisorption using d(Fe)=96/D

c

Calculated from O2 uptake.

d

Corrected dispersion (D)=surface Fe0 atom/total reduced Fe0 atom × 100=surface Fe0 atom/(total Fe atom × reduced fraction ) × 100 e

Corrected Fe diameter =uncorrected Fe diameter × reduced Fe fraction.



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Fig. 6.H2-TPR profiles of Al2O3 supported catalysts.


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Fig. 7. CO2 conversion and product selectivity of catalysts supported with different Al2O3 materials.


CO2 Hydrogenation to Hydrocarbons over Iron-based Catalyst: Effects

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