Reverse water-gas shift reaction over co-precipitated

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 3 6 8 2 e3 6 8 9

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Reverse wateregas shift reaction over co-precipitated CoeCeO2 catalysts: Effect of Co content on selectivity and carbon formation Luhui Wang a,*, Hui Liu b, Ying Chen a, Shuqing Yang a a

Department of Chemical Engineering, School of Petrochemical Technology and Energy Engineering, Zhejiang Ocean University, Zhoushan, 316022, PR China b School of Food and Pharmaceutical, Zhejiang Ocean University, Zhoushan, 316022, PR China

article info

abstract

Article history:

The CoeCeO2 catalysts with different cobalt contents were prepared via co-precipitation

Received 30 March 2016

method and used for reverse wateregas shift (RWGS: CO2 þ H2 / CO þ H2O) reaction.

Received in revised form

Characterizations of the catalysts were conducted by XRD, TPR, TPO, TEM and SEM. The

4 July 2016

results show that highly dispersed Co3O4 interacted with CeO2 is formed in 2% CoeCeO2

Accepted 6 July 2016

catalyst, and the catalyst shows excellent RWGS catalytic performance in terms of activity,

Available online 26 July 2016

selectivity and low carbon deposition. However, in the catalysts with high Co loading amount (5%), bulk Co3O4 with larger particle size is formed, which leads to obviously

Keywords:

increased carbon deposition and by-product CH4 production during the reaction. These

Reverse wateregas shift

results suggest that highly dispersed Co, reduced from highly dispersed Co3O4 on CeO2,

CoeCeO2

should be the key active component for RWGS reaction; while bulk Co with large particle

Cobalt content

size should be the key active component for methanation and carbon deposition.

Catalyst

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Carbon deposition

Introduction CO2 conversion has attracted extensive attention around the world due to global warming associated with positive carbon accumulation. The hydrogenation of CO2 with renewable hydrogen has been proposed to utilize CO2 as a raw material to produce more useful chemicals, such CO [1,2], CH4 [3e5], and methanol [6,7]. The reverse wateregas shift (RWGS) reaction, i.e., CO2 þ H2 / CO þ H2O, has been recognized as one of the most promising processes for CO2 utilization [8]. RWGS reaction can convert CO2 to more valuable CO, which is the main raw material of FischereTropsch synthesis. Through

RWGS reaction and F-T synthesis, the greenhouse gas CO2 can be converted to light olefins and liquid hydrocarbons [9,10]. The RWGS reaction is endothermic, and higher temperatures can result in higher equilibrium conversion of CO2. The water formed in the RWGS reaction must be removed, because water can deactivate the catalytic activity of subsequent reactions, such as methanol synthesis or FischereTropsch reactions. The high-temperature operation of the RWGS reaction makes the cascade utilization of thermal energy more difficult [11]. Considering the conversion and energy consumption, an intermediate-temperature process is more desirable for the RWGS reaction [12].

* Corresponding author. Fax: þ86 580 2551439. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.048 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.


Copper [13e15], noble metal [16e18], nickel [19e22] and cobalt-based [23] catalysts have been studied for RWGS reaction. Due to poor thermal stability, copper catalysts deactivate easily, and addition of iron can promote its stability [15]. For noble metal, nickel and cobalt-based catalysts, one crucial issue is methanation: CO2 þ 4H2 / CH4 þ 2H2O and CO þ 3H2 / CH4 þ H2O [19,21]. Kim et al. [17] reported that Pt/ TiO2 catalysts showed excellent activity and selectivity for the RWGS reaction, and found that the CO selectivity was dependent on the reducible Ti site on the catalyst surface. Lu et al. [19,20] reported that, a single (isolated) Ni particle in NiO/ SBA-15 catalysts catalyzed the RWGS reaction, giving a high selectivity of 100% for reducing CO2 to CO, while the paired NieNi particles catalyzed CO2 methanation. Recently, we found that the 10% CoeCeO2 catalyst is very active for RWGS reaction. However, because Co is also very active in the COx methanation [24,25], large amount of methane was produced over the 10% CoeCeO2 catalyst [23]. To the best of our knowledge, there has been no report on Co-CeO2 catalyst which is active and selective for RWGS reaction. In the present work, co-precipitated CoeCeO2 catalysts with different Co contents were tested for RWGS reaction and characterized by XRD, TPR, TPO, TEM and SEM. The 2% CoeCeO2 catalyst was founded to be active and selective for RWGS reaction. The relationship between the structure and the catalytic properties was studied.

Experimental Catalyst preparation The CoeCeO2 catalysts were prepared by co-precipitation (CP) method. The appropriate amounts of Co(NO3)2$6H2O and Ce(NO3)3$6H2O were dissolved in deionized water to make 0.10 M mixed solutions. NaOH and Na2CO3 were also dissolved in deionized water with a 1:1 M ratio to make a 0.20 M mixed solution. The hydrolysis of the metal salts to hydroxides and carbonates were achieved by adding the two solutions dropwise into deionized water while stirring and at a pH value between 9.9 and 10.1. The precipitates were aged overnight at room temperature. Finally, the catalysts were calcined at 600 C for 4 h in the air after centrifuging, washing and drying. The catalysts were denoted as X CoeCeO2, where X is the mass ratio of Co/(Co þ Ce) (X ¼ 1%, 2%, 5%, 10%). The pure ceria was prepared by precipitation method under the same preparation conditions as those used for the CoeCeO2 catalyst.

Catalyst characterization N2 adsorptionedesorption isotherms were measured at 196 C in an physisorption analyzer (AutosorbeiQ, Quantachrome). Pore size distributions were calculated using the BJH model. The specific surface areas were calculated by the BrunauereeEmmetteeTeller (BET) equation, and the pore size distributions and average pore diameters were determined from desorption branch of isotherms using the BarretteeJoynereeHalenda (BJH) model.

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Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a DX-2700 X-Ray diffraction equipment with Cu radiation operated at 40 kV and 30 mA. The scan was processed at a rate of 1.2 /min, and with a step of 0.02 . The average CeO2 crystalline size (D) was calculated from XRD patterns by Scherer equation. The lattice parameter of cubic Fluorite phase was calculated by Rietveld refinement analysis with Jade 6.5 software. Transmission electron microscopy (TEM) measurements were conducted on a Tecnai TEM instrument (Tecnai G2 F20, FEI). The samples for TEM measurement were suspended in ethanol and supported onto a holey carbon film on a Cu grid. The scanning electron microscopy (SEM) image of the used catalyst was performed via a Hitachi S4800 scanning electron microscope. Temperature-programmed reduction (TPR) experiment was carried out on a fabricated TPR apparatus equipped with a thermal conductivity detector (TCD) for analysis. The TPR experiment was performed under a flow of 20 mL/min 5% H2/ Ar mixture over 20 mg of catalyst using a heating rate of 10 C/ min. Prior to TPR, the catalysts were treated under a 5% O2/He mixture at 400 C for 30 min in order to yield clean surfaces. To quantify the amount of carbon deposited on the used catalysts, temperature-programmed oxidation (TPO) experiments were performed after the activity test. For the TPO experiments, a stream of 5% O2 in He (50 mL/min) was passed over the sample while the temperature was increased from room temperature to 800 C at 10 C/min. The effluent gas was analyzed using a mass spectrometer (HPR-20, Hiden Analytical Ltd.), and mass 44 (CO2) were monitored. The CO2 signal was calibrated so that the TPO peak area could be converted to the amount of carbon deposited.

Evaluation of catalytic performance The RWGS reaction was performed in a packed-bed quartz reactor (I.D. ¼ 8 mm) operated under atmospheric pressure within the temperature range of 400 Ce600 C. For the RWGS reaction, 20 mg catalyst (60e100 mesh) was used by feeding a stream of H2/CO2 at 100 mL/min with a 1:1 feed ratio, and the GHSV was 300 000 mL/(g h). 50 mg inert SiO2 (60e100 mesh) was used to dilute the catalyst. The catalysts were reduced insitu at a rate of 30 C/min up to 450 C for 1 h in a flow of a 20% H2/N2 mixture (50 mL/min) prior to the reaction. The catalysts were kept for 30 min at each reaction temperature before analysis of the product. The long-term stability test was performed at 600 C, using 10 mg catalyst (60e100 mesh) with a high GHSV of 600 000 mL/(g h). The feed and product gas streams were analyzed using an online gas chromatograph equipped with a packed column (TDX-01), a methanator and a flame ionization detector (FID).

Results and discussion Catalyst characterization The specific surface area (SBET) of the CeO2 and CoeCeO2 catalysts are listed in Table 1. As presented in Table 1, the SBET values were increased with the doping of Cobalt. For the 1%

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Table 1 e Textural properties of the CeO2 and CoeCeO2 catalysts. Catalysts CeO2 1% CoeCeO2 2% CoeCeO2 5% CoeCeO2 10% CoeCeO2

SBET (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

28.0 34.3 33.5 39.5 47.2

0.102 0.124 0.065 0.089 0.110

3.70 3.70 3.69 6.80 7.43

CoeCeO2 and 2% CoeCeO2 catalysts, the SBET values were very similar. For the 5% CoeCeO2 and 10% CoeCeO2 catalysts, the SBET values were obviously increased to 39.5 m2/g and 47.2 m2/ g. The nitrogen adsorption/desorption isotherms of the CeO2 and CoeCeO2 catalysts are shown in Fig. 1(A). All samples exhibit typical IV shape isotherms, with the P/P0 position of the inflection point corresponding to a diameter in the mesoporous range. From the BJH pore size distributions shown in Fig. 1(B), it can be seen that all samples possessed mesopore size distributions, and pore diameter of the catalysts with high cobalt content was larger. XRD patterns for CoeCeO2 catalysts with different Co contents are shown in Fig. 2. For the 1% CoeCeO2, only the fluorite oxide-type structure was identified in these samples

Fig. 1 e N2 adsorption/desorption isotherms (A) and pore size distributions (B) of CeO2 and CoeCeO2 catalysts.

and no obvious diffraction peaks of crystalline CoO or Co3O4 were observed in the XRD patterns. It suggests that most cobalt species may be in form of CoxCeyO2 solid solution or highly dispersed cobalt oxides in 1% CoeCeO2 catalyst [26,27]. For the 2% CoeCeO2, a very weak diffraction peak of crystalline Co3O4 can be observed as shown in Fig. 2(B). The result indicates that highly dispersed Co3O4 is formed in 2% CoeCeO2 catalysts. At higher metal content (5% Co), an obvious diffraction peak of crystalline Co3O4 can be identified by XRD in Fig. 2(B), suggesting that bulk Co3O4 with large particle size is present in these catalysts. To further investigate the structure of the CoeCeO2 samples, the average crystalline sizes and lattice constants of ceria were calculated and shown in Fig. 3. As shown in Fig. 3, their mean crystallite size of ceria decreases with increasing Co content, while the lattice constants of ceria remains almost unchanged. The decrease in the size of ceria suggests that the addition of Co inhibits the crystal growth of ceria, in agreement with previous result reported by Wang et al. [27]. The doping effect, i.e. substitution of a larger Ce4þ ion (0.101 nm) by a smaller Co ions (Co2þ: 0.075 nm; Co3þ: 0.061 nm) to form the CoxCeyO2 solid solution, contracts the lattice constant of

Fig. 2 e XRD patterns of wide range (A) and narrow range (B) for CeO2 and CoeCeO2 catalysts.


Fig. 3 e CeO2 mean crystallite size (solid) and lattice parameter (open) of CoeCeO2 catalysts as a function of Co content.

ceria [27]. The unchanged lattice constants of ceria in Fig. 3 suggested that the CoxCeyO2 solid solution should be limited in the CoeCeO2 catalysts. The XRD and SBET results clearly indicate that cobalt ions incorporated into CeO2 crystal lattice,

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or cobalt species dispersed on CeO2 particle surface, can inhibit the increase of CeO2 particle size during calcination process. In addition, it should be noted that our results of the unchanged lattice constants of CeO2 is different with the previous report by Wang et al. [27]. They reported that the lattice parameters have been observed to experience a decrease followed by an increase due to the influence of the maximum solubility limit of cobalt oxides in the CeO2. The cause of the difference with the previous report was likely due to the different preparation methods of CoeCeO2 catalysts. The morphological properties of the CoeCeO2 catalysts, with Co content of 2%, 5% and 10%, were investigated by TEM as shown in Fig. 4. From Fig. 4(A), it is clear that the particle sizes of the 2% CoeCeO2 catalyst are 8e20 nm. High-resolution TEM image of the 2% CoeCeO2 catalyst (Fig. 4(B)) revealed that the Co3O4 particles were less than 5 nm. For 5% CoeCeO2 and 10% CoeCeO2 catalysts (Fig. 4(C,D)), the Co3O4 particles were larger than 10 nm. The results suggested Co3O4 particle size were obviously increased with high Co content, which are consistent with XRD results. Fig. 5 shows the TPR profiles of CeO2 and CoeCeO2 catalysts. In pure CeO2, the high temperature d1 peak, results from

Fig. 4 e TEM images for 2% CoeCeO2 (A, B), 5% CoeCeO2 (C), and 10% CoeCeO2 catalysts (D).

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Fig. 5 e H2-TPR patterns of CeO2 and CoeCeO2 catalysts.

the removal of the bulk oxygen of CeO2, while the weak d2 peak results from the removal of the surface oxygen of CeO2. When Co content is greater than or equal to 2%, CoeCeO2 exhibits four hydrogen consumption peaks (a, b, g and d). According to the literature [28], the reduction of Co3O4 supported on CeO2 is a stepwise reduction process (Co3þ / Co2þ / Co). The a peak locates at temperature lower than 300 C is ascribed to the reduction of Co3þ to Co2þ. The b peak resulted from the reduction of Co2þ that weakly interacts with CeO2 to Co, which maybe locate on out surface of CoeCeO2 catalyst; while the g peak resulted from the reduction of Co2þ interacting with CeO2 to Co, which maybe locate in the pores of CoeCeO2 catalyst. The result indicates that Co species exist as the form of Co3O4 which strongly or weakly interacted with CeO2 when the Co content is greater than or equal to 2%. In the case of 1% CoeCeO2 catalyst, only g and d peaks exist. The g peak is ascribed to the reduction of Co2þ interacting with CeO2 to Co. The result suggests that Co species maybe exist as the form of CoeCeeO solid solution or CoO which strongly interacts with CeO2. This result is consistent with those reported by other groups, which show that CoO or CoxCeyO2 solid solutions is prevalent in the CoOx/CeO2 materials with lower cobalt content, whereas Co3O4 is prevalent in the materials with high Co/Ce atomic ratios [26,29].

The activity of 2% CoeCeO2 catalyst is obviously higher than pure CeO2 and 1% CoeCeO2 catalyst, without obviously CH4 formation at reaction temperatures higher than 450 C. The catalysts with high Co content (5% Co), is more active than the 2% CoeCeO2 catalyst. However the high Co content (5% Co) gives progressively high CH4 selectivity, which results in a lower CO selectivity, suggesting that more Co (5% Co) makes contribution to improving catalytic activity for the methanation reaction. From Fig. 6, it can be seen that 2% CoeCeO2 shows the best catalytic performance in terms of activity and selectivity for RWGS reaction among the catalysts tested. More Co (5% Co) resulted in large amount of methane production, and is not suitable for the RWGS reaction. The result suggests that the nature of the active sites for the methanation may not be the same for the RWGS reaction. Carbon deposition on Co-based catalysts is a severe problem in some reactions, such as FischereTropsch synthesis [30] and CO2 reforming of CH4 [31,32]. To examine the possible carbon deposition on the catalysts during RWGS reaction, post-reaction characterization experiments were performed. Fig. 7 shows the results of TPO experiments performed over the used catalysts. As the temperature was raised, significant levels of CO2 formation were observed over the 5% CoeCeO2 and 10% CoeCeO2 catalysts, while CO2 signal was very weak over the 1% CoeCeO2 and 2% CoeCeO2 catalysts.

Catalytic performance The catalytic activity and selectivity performance of the CoeCeO2 catalysts with different Co contents for RWGS reaction are shown in Fig. 6. It is obvious that the Co content can effectively affect the catalytic performance of CoeCeO2 catalysts. As shown in Fig. 6(A), the activity increases with Co content increasing from 1 to 10%. On CoeCeO2 catalysts, methane was the only by-product, while on pure CeO2 and 1% CoeCeO2 catalyst, there was no methane produced. Pure CeO2 is almost inactive below 500 C, and the activity of 1% CoeCeO2 catalyst is also very low.

Fig. 6 e CO2 conversion (A) and selectivity (B) for CeO2 and CoeCeO2 catalysts.


Fig. 7 e CO2 (m/z ¼ 44) traces during TPO over used CoeCeO2 catalysts.

Fig. 8 e The amount of deposited carbon on used CoeCeO2 catalysts.

The amount of deposited carbon, calculated according to TPO peak area, is shown in Fig. 8. It can be seen that the amount of carbon deposition over 5% CoeCeO2 catalyst is about 12 times greater than that over the 2% CoeCeO2 catalyst. The results indicate that large amounts of carbon were

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deposited over the 5% CoeCeO2 and 10% CoeCeO2 catalysts surface during reaction. Fig. 9 shows the SEM images of the used 2% CoeCeO2 and 5% CoeCeO2 catalyst. Small amounts of carbon nanofibers were observed over the used 2% CoeCeO2 catalyst, while large amounts of carbon nanofibers over the used 5% CoeCeO2 catalyst. The results indicate the deposited carbon was mostly in the form of carbon nanofiber. CH4 and CO, which can be present during the RWGS reaction conditions, were reported to be potential reactants with Co metal to produce CNTs or CNFs materials [33,34]. Co/Al2O3 catalysts, with cobalt particle size of about 15e20 nm, were active in the formation of CNFs during CH4 decomposition (CH4 / C þ 2H2) at 500 C [33]. Co/Al2O3 catalysts were also active in CO disproportionation (2CO / C þ CO2) to produce CNTs, especially in the presence of H2 [34]. Therefore, it was reasonable to deduce that CNFs could be produced from CH4 and/or CO over CoeCeO2 catalysts during the RWGS reaction. It should be note that it was also possible to produce CNFs directly form CO2 hydrogenation, and the pathway was independent of the reaction products CO and CH4, as reported by Chen et al. [35,36] over Ni-based catalysts. Additional work is necessary to gain an understanding of the mechanisms of carbon deposition over CoeCeO2 catalysts during the RWGS reaction. In order to investigate the 2% CoeCeO2 catalyst stability, a long-term reaction test was performed at 600 C with a high GHSV of 600 000 mL/(g h). The results were expressed as the CO2 conversion versus time on stream in Fig. 10. No CH4 can be detectable in this reaction conditions. The 2% CoeCeO2 catalyst showed a very high catalytic activity at the initial stage of the reaction, but after 5 h on stream, the CO2 conversion decreased from 24.8% to 16.1%. Then the CO2 conversion was slowly increasing with time on stream. After 50 h on stream, the catalyst achieved 20.5% stable CO2 conversion. Inset of Fig. 7 shows the result of TPO experiment performed over the catalysts after 60 h on stream. No obvious CO2 signal in TPO experiment, indicated that no deposited carbon was formed on the catalyst. The result indicates the 2% CoeCeO2 catalyst is a good candidate for the long-term RWGS reaction. The reason for catalyst deactivation at initial stage was not clear, which maybe due to the sinter of Co particles or the oxidation of Co particles by water produced through the reversed water gas shift reaction [37], and it needs further research.

Fig. 9 e The SEM images of the used catalysts: (A) 2%CoeCeO2, (B) 5%CoeCeO2.

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Sciences in the Most Important Subjects of Zhejiang (No. 20130201).

references

Fig. 10 e Long-term stability test of 2% CoeCeO2 catalyst (reaction conditions: 10 mg of catalyst, 600 C, CO2/ H2 ¼ 1:1, GHSV ¼ 600 000 mL/(g h)), and CO2 (m/z ¼ 44) trace during TPO over 2% CoeCeO2 catalyst after 60 h reaction (inset).

Based on the structure features, it is reasonable to propose that the differences in catalytic performance are related to the different form of Co species. In the 1% CoeCeO2 catalysts, most Co species exist in the form of CoxCeyO2 solid solution or CoO particles strongly interacted with CeO2, which are almost inactive for both RWGS and methanation reaction. On the contrary, most Co species existed in the form of highly dispersed Co3O4 on CeO2 in the 2% CoeCeO2 catalysts. The high selectivity of 2% CoeCeO2 catalysts should be contributed to highly dispersed Co reduced from highly dispersed Co3O4 on CeO2. However, more Co (5% Co) results in the formation of bulk Co3O4 with large particle size, which leads to improving CH4 production and large amount of deposited carbon. These results demonstrate that highly dispersed Co, which is reduced from highly dispersed Co3O4 on CeO2, should be the key active component for RWGS; while bulk Co with large particle size should be the key active component for methanation and carbon deposition.

Conclusions The CoeCeO2 catalysts with different Co contents were prepared by a co-precipitation method and used for RWGS reaction. The 2% CoeCeO2 catalyst showed excellent catalytic performance in terms of activity, selectivity for RWGS reaction. The high selectivity of 2% CoeCeO2 catalyst is related to highly dispersed Co3O4 on CeO2. Increased Co content results in larger Co particle size, leading to increased carbon deposition and by-product CH4 production.

Acknowledgments This work was supported by the Natural Science Foundation of China (21406206) and the Open Foundation from Marine

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