Direct Transformation of Carbon Dioxide to Value-Added

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Kinetics, Catalysis, and Reaction Engineering

Direct Transformation of Carbon Dioxide to Value-added Hydrocarbons by Physical Mixtures of Fe5C2 and K-modified Al2O3 Junhui Liu, Anfeng Zhang, Xiao Jiang, Min Liu, Jie Zhu, Chunshan Song, and Xinwen Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02017 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Industrial & Engineering Chemistry Research

Direct Transformation of Carbon Dioxide to Value-added Hydrocarbons by Physical Mixtures of Fe5C2 and K-modified Al2O3 Junhui Liua, Anfeng Zhanga, Xiao Jiangb, Min Liua, Jie Zhua, Chunshan Songa,c*, Xinwen Guoa* a

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, P. R. China.

b

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.

c

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering and Chemical Engineering, The Pennsylvania State University, University Park, PA, 16802, USA

*

Corresponding authors. E-mail: [email protected] (X.W. Guo); [email protected] (C.S. Song)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Conversion of CO2 by hydrogenation into higher hydrocarbons has attracted much attention due to the continuous increasing concentration of CO2 in the atmosphere. Fe5C2 was used as the active site and mixed with K-modified supports for CO2 hydrogenation to synthesize value-added hydrocarbons. A series of supports were tested and the alkaline Al2O3 was the best choice for light olefins and C5+ hydrocarbons. The transformation of product selectivity at time on stream demonstrated that potassium migrated into Fe5C2 during reaction. The selectivity of C2+ is 69.2 % containing 35.8 % C2-C4= and 29.1 % C5+ value-added hydrocarbons with the 31.5 % CO2 conversion (H2/CO2=3). When the ratio of H2 and CO2 was switched to 4, CO2 conversion improved to 40.9 %, and the C2+ selectivity to 73.5 %, containing 37.3 % C2-C4= and 31.1 % C5+ value-added hydrocarbons. The mixture catalysts can be directly used without any reduction before reaction, reducing the consumption H2 and simplifying the operation. KEYWORDS: CO2 hydrogenation, Value-added hydrocarbons, No reduction, Fe5C2


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1 Introduction

The continued growth of anthropogenic emissions of CO2 caused the climate change. Carbon capture and storage (CCS) and carbon capture and utilization (CCU) have been proposed as the possible technologies to mitigate climate change.1-5 Transformation of CO2 to valuable chemicals can not only mitigate CO2 emissions, but also reduce consumption of fossil resources.6-8 For CO2 conversion, most research paid attention to C1 products, such as CH4, CO, and methanol.9-15 CO2 molecule is very stable and the C-C coupling is more difficult, therefore, few studies focus on the C2+ hydrogenations synthesis directly from CO2. Hydrocarbons in the fuel range are more important for transportation, and light olefins are key building blocks in the chemical industry.16, 17 Iron-based catalysts were applied to directly synthesize hydrocarbons, over which the selectivity of C2-C4= and C5+ hydrocarbons are both high.18,

19

Bifunctional catalysts were used in the

methanol-mediated approach, and zeolite is responsible for high selectivity of C2-C4= (e.g.,SAPO-34) or C5+ hydrogenations (e.g., HZSM-5).20 For iron-based catalysts, potassium was usually added into catalysts as promoter to optimize the product distribution.21, 22 The catalysts need to be reduced in hydrogenation to make the iron oxides transform to metallic Fe before reaction, then it is easier to form the iron carbides during reaction, which is responsible for chain growth.19 Metal oxides, such as Al2O3, ZrO2 and TiO2, are good supports for CO2 hydrogenation reaction.9, 21, 23, 24 However, the strong metal-support interactions between metal and metal oxide support could inhibit the reduction and the carbonization of active iron phase during 3 ACS Paragon Plus Environment


reaction.25-27 Thus long time and a large quantity of H2 are needed to reduce catalysts before reaction. Fe species were mixed with MOFs, which were used as supports, for synthesis of hydrocarbons from CO2 hydrogenation, and the supports have a significant impact on the activity and selectivity of Fe catalysts.28 Fe5C2 is considered as the active phase of iron-based catalysts in Fischer-Tropsch synthesis (FTS).29, 30 In the present work, pure Fe5C2 was synthesized and physical mixtures of Fe5C2 and K-modified supports were applied as the active catalysts to catalytic hydrogenation of CO2. The Fe5C2-10K/a-Al2O3 displays excellent desired valuable hydrocarbons selectivity, containing 37.3 % C2-C4= and 31.1 % C5+ hydrocarbons. More importantly, the application of these catalysts is free of H2 reduction, thereof possessing great potential in practice in terms of cost-efficiency and environmental-friendliness. It is anticipated that this unique catalyst will attract much attention and give a prospective direction for catalyst preparation in conversion of CO2.

2. Experimental section

2.1 Preparation of catalysts

2.1.1 Preparation of Fe5C2 Iron oxalate dihydrate was synthesized by a solvothermal method.31 8 mmol FeSO4·7H2O was dissolved in the mixture of 16 mL ethylene glycol and 64 mL water. Then, 8 mmol H2C2O4 was dissolved in the ethylene glycol and water solution with the same proportion. The two solutions were slowly mixed under stirring to obtain a 4 ACS Paragon Plus Environment

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transparent mixture. The mixture was tightly sealed and hydrothermally treated at 100 °C for 24 h. The product was centrifuged and washed with deionized water and ethanol several times and dried at 80 °C overnight. Fe5C2 was synthesized by thermally treating iron oxalate dihydrate under CO flow. 1g iron oxalate hydrate powders were transferred into a quartz boat in a horizonal tubular furnace, and activated to the iron carbide phase at 350 °C (ramp= 0.45 °C min-1) under a 10 % CO/90 % (v:v) N2 with the flow rate at 100 mL min-1 for 4 h.

2.1.2 Preparation of Fe2O3 The Fe2O3 was synthesized by precipitation method.32 Na2CO3 aqueous solution (2M) was dropped into Fe(NO3)3·9H2O aqueous solution (1M) until pH=9.0 under continuously stirring condition. After aged at 80 °C for 5 h, the liquid was separated by centrifugalization, and washed with a great amount of deionized water. After drying at 60 °C overnight, the powder was calcinated at 400 °C for 4 h.

2.1.3 Preparation of supports

Activated carbon (AC), TiO2, ZrO2, and Al2O3 were commercial samples (Aladdin Chemicals). SBA-15 was prepared by using the method of Zhao Group.33 Boron-containing ZSM-5 (B-ZSM-5) was synthesized by using the reported method of our group.34

2.1.4 Preparation of potassium modified supports

Potassium modified supports were prepared by the incipient wetness impregnation 5 ACS Paragon Plus Environment


(IWI) method using aqueous solution of KNO3. The samples were obtained after drying at 100 °C overnight, followed by calcinations in air (in N2 for AC) at 500 °C for 4 h. The potassium modified supports were denoted as xK/support, in which x represents the mass percent of potassium.

2.2 Catalyst characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab(9) diffractometer using a Cu Kα radiation (λ= 1.5406 Å) source at a step size of 0.02°. Scanning electron microscopy (SEM) images were obtained on a Hitachi SU8220 instrument with an acceleration voltage of 5 kV. CO2 temperature-programmed desorption (CO2-TPD) measurements were conducted on the ChemBETPulsar TPR/TPD equipment (Quantachrome, USA). Prior to adsorption, about 100 mg sample was mounted and flushed with Ar (ca. 30 mL min−1) at 400 °C for 1 h. After cooling to 50 °C, the sample was exposed to pure CO2 (30 mL min–1) for 1 h and then flushed with Ar flow (30 mL min–1) for 1 h to remove all physically adsorbed molecules. The TPD program was initiated by heating up to 500 °C with a rate of 10 °C min−1.

2.3 Catalytic tests

CO2 hydrogenation was conducted in a pressurized fixed-bed flow reactor with 8 mm inner diameter at 3 MPa. Typically, 0.133 g Fe5C2 or Fe2O3and 0.867 g potassium modified supports were used for each test. No H2 reduction was performed before reaction. The feed gas, consisting of CO2 and H2 (CO2/H2=1/3), was introduced to the 6 ACS Paragon Plus Environment

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reactor with the GHSV= 3600 ml g-1 h-1. The products were analyzed by using an on-line gas chromatograph (FULI GC 97). CO, CO2, and CH4 were analyzed on a carbon molecular sieve column with TCD, while CH4 and C2-C8 hydrocarbons (C2+) were analyzed by FID with a HayeSep Q column. Chromatograms of FID and TCD were correlated through methane, and product selectivity was obtained based on carbon balance.

3. Results and discussion

We synthesized pure Fe5C2 and α-Fe2O3 as the active iron species. Figure 1 shows the XRD patterns of Fe5C2 and α-Fe2O3. The peaks at 39.7°, 40.8°, 42.9°, 43.7°, 44.7°, 45.9°, and 58.3° are assigned to Fe5C2 (JCPDS no.36-1248), while the peaks at 24.1°, 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4°, and 64.0° are corresponded to α-Fe2O3 (JCPDS no.33-0664). Fe5C2 or α-Fe2O3 was mixed with K-modified alkaline Al2O3 (denoted as a-Al2O3) for CO2 hydrogenation reaction. For all catalysts, no H2 reduction was performed before reaction. Table 1 presents the catalytic performance of the two catalysts. CO2 conversion over Fe5C2-10K/a-Al2O3 was 44.8 % at 400 °C and 31.5 % at 320 °C. In contrast, the selectivity of C2+ was 69.3 % at 320 °C, which was higher than that at 400 °C (53.7 %). On the other hand, for Fe2O3-10K/a-Al2O3, the conversion of CO2 was 25.1 % at 400 °C, which was far lower than that over Fe5C2-10K/a-Al2O3. Moreover, CO, with the selectivity of 99.6 %, was dominant in the products, and the production of C2+ hydrocarbons was absent. At lower temperature, namely 320 °C, Fe2O3-10K/a-Al2O3 exhibited negligible catalytic 7 ACS Paragon Plus Environment


property, while only reverse water-gas shift (RWGS) occurred with CO as the only product. Clearly, the striking contrast in the activity performance between Fe5C2 and Fe2O3 catalysts demonstrates that the mixtures of Fe5C2 and K-modified a-Al2O3 enables effective catalytic CO2 conversion to long-chain hydrocarbons through C-C coupling, while it cannot be achieved in the case of Fe2O3 as active iron species in the absence of reduction. The catalytic performance of reduced Fe2O3-10K/a-Al2O3 was also measured and the results were exhibited in Table 1. The 24.1 % of CO2 conversion was obtained, which is far lower than that over Fe5C2-10K/a-Al2O3. The C5+ hydrocarbons selectivity was higher, while the C2-C4= selectivity was lower. Fe5C2-10K/a-Al2O3 could achieve the similar catalytic performance even though without pre-reduction. In the following step, activated carbon (AC), SBA-15, TiO2, B-ZSM-5, ZrO2, neutral Al2O3 (n-Al2O3), and alkaline Al2O3 (a-Al2O3) were selectively chosen as support materials for screening tests, and SEM images are presented in Figures S1 and S2(c). Alkali metals like Na and K are favorable promoters that have significant influence on catalytic performance for Fe-based FTS and CO2 hydrogenation catalysts.35-37 As reported, alkali metals benefit the formation of iron carbide, improve both CO and CO2 adsorption, suppress the formation of CH4, and increase the chain growth probability.21, 38 In the present work, K (10 wt%) was introduced into support materials by impregnation method, followed by mixing with Fe5C2 by grinding. Figure 2 shows the catalytic performance of catalysts by incorporating different support materials. The main product over Fe5C2-10K/AC, Fe5C2-10K/TiO2, and 8 ACS Paragon Plus Environment

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Fe5C2-10K/ZrO2 was CO, as well as low selectivity toward C2+ hydrocarbons. In spite of lower CO selectivity, Fe5C2-10K/SBA-15 and Fe5C2-10K/B-ZSM-5 exhibited a large proportion of CH4, but lower C2+ hydrocarbons (e.g., < 20 %). When the Al2O3 were used as supports, CO2 conversion was improved to 41.5 % and 42.6 % on n-Al2O3- and a-Al2O3-supported catalysts, respectively. Interestingly, the production of undesired CO and CH4 was markedly suppressed. In striking contrast, the C2+ selectivity was significantly increased to 45.4 % and 50.3 %, respectively, wherein the selectivity of value-added C2-C4= and C5+ was more advanced than other catalysts. CO2-TPD profiles (Figure 3) illustrate that the 10K/TiO2 and 10K/ZrO2 show negligible adsorptive capability towards CO2 adsorption, while 10K/n-Al2O3 and 10K/a-Al2O3 have the strongest affinity with CO2 adsorption. Such differentiation might provide insight into the role of Al2O3 in the observed advanced activity performance. Recently, our group reported that surface acidic-basic hydroxyl distribution on Al2O3 strongly affects the product distribution, and the alkaline-doped Al2O3 catalysts exhibited the best catalytic performance for CO2 hydrogenation to hydrocarbons.23 Therefore, a-Al2O3 was chosen as support for Fe5C2 exclusively in the following activity tests. Integration manner of the Fe5C2 and 10K/a-Al2O3 was also investigated, and the catalytic behavior is depicted in Figure 4. Types A-C exhibited relatively lower CH4 selectivity in comparison to duel-bed packed catalysts. Type A presented more advanced C2-C4= selectivity (25.7 %), while type C performed better in terms of C5+ selectivity (26.0 %). However, the latter stacking manner displayed highest total 9 ACS Paragon Plus Environment


selectivity of valuable C2-C4= and C5+, as well as the highest CO2 conversion of 44.8 %. In contrast, the CH4 selectivity of type D and E was above 40 %. Besides, the selectivity of C2-C4= and C5+ over these two type catalysts was much lower than that over type A-C. Figure 5 shows the evolution of the catalyst performance with time-on-stream (TOS). CO2 conversion over type A-C was practically stable during11 h TOS, whereas the product selectivity had an obvious change. Interestingly, the desired value-added products, namely C2-C4= and C5+, over type A and B had a distinct variation with TOS, except the C5+ selectivity over type C. Specifically, the selectivity of C5+ over type B gradually increased from 13.9 to 21.8 %, while 16.0 to 19.6 % for the type A. The selectivity of C2-C4= over type A and C increased 6 %, whereas the C2-C4= selectivity over type B increased from 12.5 to 21.0 %. On the contrary, the C2-C40 selectivity over type B decreased from 20.9 to 6.3 %, while type A and C barely showed any obvious alteration. The packing-dependent behavior of Fe5C2 and 10K/a-Al2O3 demonstrates that the interparticle contact of the two components is crucial for CO2 conversion, and governs the product distribution. The intimate contact of the two components is beneficial for high valuable hydrocarbons. As a matter of fact, the evolution of product selectivity results from the migration of K with TOS, which will be discussed later. As reported, K is crucial for the product distribution in CO2 hydrogenation, as it enables the adjustment of adsorption property on the surface.21 Therefore, the migration of K to active Fe benefited for the olefins formation and the enhancement 10 ACS Paragon Plus Environment

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of the C-C chain growth. As a result, the desired value-added products (C2-C4= and C5+) further increased. The content of doped K was varied in a wide range (0-15 wt%), and resulting catalytic performance is presented in Figure 6. CO2 conversion over Fe5C2-a-Al2O3 in the absence of K was 28.8 %, while it was significantly improved to 46.9 % when 5 wt% of K was added. However, a further addition of K led to a slight decrease in CO2 conversion. The addition of K resulted in a drastic reduction of CH4 selectivity from 33.3 % over Fe5C2-a-Al2O3 to 17.7 % over Fe5C2-15K/a-Al2O3. On the other hand, CO selectivity presented an envelope-shape trend with the increase of K content, and reached the minimum over Fe5C2-5K/a-Al2O3. As reported, the alkali metals favored CO2 adsorption and suppressed hydrogenation of the formed alkenes on the surface of the catalysts.18, 32 Such K-dependent behavior can also be reflected in the present work, as the C2-C4= selectivity increased from 3.4 to 25.4 %, while the C2-C40 selectivity decreased from 17.9 to 4.8 %. Among all, Fe5C2-10K/a-Al2O3 had the highest selectivity of C5+ and C2+. The effect of temperature has been clarified by evaluating the catalytic performance of Fe5C2-10K/a-Al2O3 (type C) at 320, 350, and 400 °C, respectively. As presented in Figure 7, CO2 conversion decreased from 44.8 to 31.5 % by reducing the temperature from 400 to 320 °C. The selectivity of CO and CH4 decreased from 23.2 to 18.6 % and from 23.1 to 12.1 %, respectively. Notably, the light olefins increased from 26.0 to 34.4 % when the temperature was decreased from 400 to 350 °C, however, that increase became slow by a further reduction of temperature to 320 °C, as the increment of C2-C4 olefins was only 1.4 %. Meanwhile, the selectivity of C2-C4 11 ACS Paragon Plus Environment


paraffins slightly decreased, thereof leading to an enhancement of the ratio of olefin/paraffin (O/P) from 5.4 (400 °C) to 8.2 and 8.3 (350 °C and 320 °C, respectively). Interestingly, the C5+ selectivity was increased only 0.6 % when the temperature decreased from 400 to 350 °C, while the selectivity of C5+ at 320 °C was improved 6.2 % and maximized at 29.1 %. Evidently, lower temperatures are beneficial for higher selectivity of C2-C4= and C5+. The effect of the H2/CO2 ratio was also investigated by altering the H2/CO2 ratio from 2 to 4, as depicted in Figure 8. With the increase of H2/CO2 ratio, CO2 conversion exhibited an improvement from 24.4 % to 40.9 %. CO selectivity was decreased from 23.3 to 12.4 %, whereas CH4 presented a slight increase from 9.3 to 14.0 %. It is worth noting that the selectivity of both C2-C4 paraffins and olefins showed increasing trend with the increased H2/CO2 ratio, wherein the improvement of the latter, from 33.6 to 37.3 %. Not surprisingly, the increase of H2/CO2 ratio resulted in a slight drop of O/P ratio from 8.7 to 7.4， due to the higher H/C ratio on the catalyst surface, which caused the faster secondary olefins reactions. Generally, the selectivity of valuable products, C2+, was maximized at H2/CO2=4 with an optimal value of 73.5 C-mol%, whereas that of C5+ hardly altered with the H2/CO2 ratios. On the other hand, the gradually increased H2 partial pressure led to a negative impact on the chain-growth probability (α), the values of which decreased from 0.71 to 0.67 (Figure S3). The stability test was also conducted. Figure 9 shows the variation of catalytic performance of Fe5C2-10K/a-Al2O3 with TOS in different integration manners. The 12 ACS Paragon Plus Environment

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selectivity of hydrocarbons over Fe5C2-10K/a-Al2O3 (type A) at 400 °C showed a distinct improvement during the first 5 h on stream, and then was practically in the following 30 h on stream. CO2 conversion was initially stable during the first 12 h on stream, however, a slight decrease was evidenced with the TOS. At TOS=34 h, the CO2 conversion was decreased with respect to the initial collected value. Apparently, the loss of activity was not significant. On the other hand, the hydrocarbons selectivity and CO2 conversion over Fe5C2-10K/a-Al2O3 (type C) at 320 °C remained relatively stable at high level during the tested 30 TOS. Clearly, these composite catalysts, comprising of Fe5C2 and alkali-treated Al2O3, exhibited good stability within the tested TOS. The XRD pattern of used Fe5C2-10K/a-Al2O3 was shown in Figure S4. The Fe3O4 phase was observed, which was from the oxidation of Fe5C2 during the reaction.

3. Conclusions

In conclusion, pure Fe5C2 was synthesized and mixed with K-modified to catalyze hydrogenation of CO2. Compared with most iron-based catalysts, the mixture catalysts exhibited superior catalytic activity without reduction before reaction. Alkaline Al2O3 is the best support for high selectivity of value-added hydrocarbons among various supports. The intimate contact between Fe5C2 and K/a-Al2O3 is important. CO2 conversion over Fe5C2-10K/a-Al2O3 is as high as 40.9 %, with the 68.4 % desired valuable hydrocarbons. Supporting Information 13 ACS Paragon Plus Environment


SEM images of AC, SBA-15, TiO2, ZrO2, B-ZSM-5, n-Al2O3; SEM images of Fe5C2, a-Al2O3, images of Fe5C2 and 10K/a-Al2O3 mixture granules; ASF plots of the Fe5C2-10K/a-Al2O3 (type C). Reaction conditions, H2/CO2 =2, H2/CO2 =3, H2/CO2 =4, 320 °C, 3.0 MPa, 3600 ml g-1 h-1. XRD pattern of used Fe5C2-10K/a-Al2O3 at 400 °C.

Author Information a

Corresponding author. Fax: +86 411 84986134

b

Corresponding author at: EMS Energy Institute, PSU-DUT Joint Center for Energy

Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, United States. Fax: +1 814 865 3573. E-mail addresses: [email protected] (X. Guo), [email protected] (C. Song)

Acknowledgments

This work was financially supported in part by the National Key Research and Development Program of China (2016YFB0600902-5).

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Figure 1 XRD patterns of Fe5C2 and Fe2O3.



Figure 2 CO2 conversion and product selectivity over Fe5C2-based catalysts supported on different support materials. Reaction conditions, 400 °C, 3.0 MPa, 3600 ml g-1 h-1, H2/CO2 = 3.


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Figure 3 CO2-TPD of potassium modified supports.



Figure 4 Effect of integration manner on catalytic behaviors of the composite catalysts containing Fe5C2 and 10K/a-Al2O3. (A) Mixing of powder of two components; (B) Mixing of granules of two components; (C) Stacking of mixture granules; and (D, E) Dual bed with different component on the top. Reaction conditions, 400 °C, 3.0 MPa, 3600 ml g-1 h-1, H2/CO2 = 3.


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Figure 5 CO2 conversion and product selectivity over catalysts with different integration mannerat time on stream (TOS).Reaction conditions, 400 °C, 3.0 MPa, 3600 ml g-1h-1, H2/CO2 = 3.



Figure 6 Effect of K content on catalytic performance over Fe5C2-(x)K/a-Al2O3 (type C). Reaction conditions, 400°C, 3.0 MPa, 3600 ml g-1 h-1, H2/CO2 = 3.


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Figure 7 Effect of reaction temperature on catalytic performance over Fe5C2-10K/a-Al2O3 (type C). Reaction conditions, 3.0 MPa, 3600 ml g-1 h-1, H2/CO2=3.



Figure 8 Effect of the H2/CO2 ratio on catalytic performance over Fe5C2-10K/a-Al2O3 (type C). Reaction conditions, 320 °C, 3.0 MPa, 3600 ml g-1 h-1.


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Figure 9 Catalytic performance over (a) Fe5C2-10K/a-Al2O3 (Type A, 400 °C), (b) Fe5C2-10K/a-Al2O3 (Type C, 320 °C) with TOS. Reaction conditions, 3.0 MPa, 3600 ml g-1 h-1, H2/CO2 = 3.



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Table 1 CO2 hydrogenation performance of catalystsa CO2 conv.

(°C)

(%)

CO

CH4

C5+

C2-C4=

C2-C40

Fe5C2-10K/a-Al2O3

400

44.8

23.2

23.1

26.0

22.9

4.8

Fe2O3-10K/a-Al2O3

400

25.1

99.6

0.4

0

0

0

Fe5C2-10K/a-Al2O3

320

31.5

18.6

12.1

29.1

35.8

4.4

Fe2O3-10K/a-Al2O3

320

2.0

100

0

0

0

0

320

24.1

23.4

7.5

40.4

25.3

3.4

Fe2O3-10K/a-Al2O3 (reduced) a

Product sel. (%)

Temp.

Catalyst

Reaction conditions:1.0 g catalyst, 3 MPa, H2/CO2 = 3, 3600 ml h–1 gcat-1.


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Table of Contents Graphic


Direct Transformation of Carbon Dioxide to Value-Added

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