Selective CO Methanation over Ru Catalysts Supported on ...

CHINESE JOURNAL OF CHEMICAL PHYSICS

VOLUME 25, NUMBER 4

AUGUST 27, 2012

ARTICLE

Selective CO Methanation over Ru Catalysts Supported on Nanostructured TiO2 with Different Crystalline Phases and Morphology Gui-ying Wang, Yu-xian Gao, Wen-dong Wang ∗ , Wei-xin Huang CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China (Dated: Received on May 28, 2012; Accepted on June 4, 2012)

Nanostructured titanium dioxides were synthesized via various post-treatments of titanate nanofibers obtained from titanium precursors by hydrothermal reactions. The microstructures of TiO2 and supported Ru/TiO2 catalysts were characterized with X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, and nitrogen adsorption isotherms. The phase structure, particle size, morphology, and specific surface area were determined. The supported Ru catalysts were applied for the selective methanation of CO in a hydrogen-rich stream. The results indicated that the Ru catalyst supported on rutile and TiO2 -B exhibited higher catalytic performance than the counterpart supported on anatase, which suggested the distinct interaction between Ru nanoparticles and TiO2 resulting from different crystalline phases and morphology. Key words: Selective CO methanation, Ru catalyst, Titanium dioxide, Microstructure

consumption and CO production [2, 7].

I. INTRODUCTION

CO + 3H2 = CH4 + H2 O CO2 + 4H2 = CH4 + 2H2 O CO2 + H2 = CO + H2 O

For the envisioned hydrogen energy economy, the hydrogen production is one of the crucial technologies for the application of the polymer electrolyte membrane fuel cell (PEMFC), which is considered as one of the most promising hydrogen-based energy systems owing to its high energy efficiency and environment-friendly characteristics [1]. The most common hydrogen production method is the fuel processing of hydrocarbons, mainly consisting of the reforming and water gas shift reactions, which may also produce appreciable amounts of carbon monoxide [2]. This remaining CO should be removed to a trace level below the poisoning limit of the Pt-based anode of PEMFC.

(1) (2) (3)

The activities of Ru-based catalysts are strongly dependent on the kind of support material, the metal loading and dispersion, and the reaction conditions. Titanium dioxides have been one of the preferred catalyst supports for the selective CO methanation [8−10]. In addition to the effect of the nature of supports with respect to different oxides, it is demonstrated that the different crystalline phase of supports, such as alumina [11], also has influence on the properties of the Ru/Al2 O3 catalysts and thus the catalytic performance of selective CO removal in a H2 -rich stream. However, the effect of different crystalline phase of TiO2 supports is still not clear on these reactions. It has been recently developed that the TiO2 with different crystalline phase can be prepared by controllable phase transition via simple wet-chemical reactions from the titanate precursors obtained by hydrothermal treatments [12−15]. In the present study, we investigate the structure properties of Ru catalysts supported on TiO2 with different crystalline phase and morphology, and the influence on the catalytic performance for the selective CO methanation in a hydrogen-rich stream was also studied.

The Ru-based catalyst has been proven active for the representative catalytic approaches to remove CO in a H2 -rich gas stream, the selective CO methanation and the preferential CO oxidation (PROX) [3−6]. Both approaches have their advantages and challenges. The selective CO methanation (Eq.(1)) is less system complexity compared with PROX in that no addition of oxygen/air is required into the hydrogen-rich stream; however, careful control of the methanation reactor conditions is needed in order to avoid CO2 methanation (Eq.(2)) and the reverse water-gas shift (RWGS) reaction (Eq.(3)), which may result in the undesirable H2

II. EXPERIMENTS ∗ Author

to whom correspondence should be addressed. E-mail: [email protected], Tel.: +86-551-3603683, FAX: +86-5513601592

DOI:10.1088/1674-0068/25/04/475-480

The hydrogen titanate (H-titanate) was prepared via hydrothermal reactions of different titanium precursors 475

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Gui-ying Wang et al.

100 nm

100 nm

(b)

(a)

100 nm

100 nm (c)

(d)

FIG. 1 TEM images of (a) H-titanate fibers and the products after different post-treatments: (b) calcination at 450 ◦ C, hydrothermal reaction in (c) 0.05 mol/L and (d) 2.65 mol/L HNO3 solution.

in concentrated NaOH solution and the following neutralization of the resultant sodium titanate in a diluted acid solution [12−14]. Typically, 4.01 g of TiOSO4 was dissolved into 30 mL of water to form a clear solution, which was mixed with 50 mL of 15 mol/L NaOH solution. The mixture of white suspension was then placed into a 100 mL Teflon-lined autoclave with an 80% filling factor and kept at 200 ◦ C for 48 h. The precipitate after hydrothermal reaction was isolated by filtration and washed repeatedly with deionized water until the pH value was close to 7. The resultant titanate was subsequently neutralized using 0.1 mol/L HCl solution and washed again with deionized water until the neutral pH value and the negative Ag ions test. The obtained Htitanate nanofibers were separated and dried at 80 ◦ C, and then subjected to various post-treatments, including calcination at 450 ◦ C, hydrothermal reaction for 48 h in 0.05 and 2.65 mol/L HNO3 solution, respectively, to produce titanium dioxides with different crystalline phases and morphology. The supported Ru catalysts were prepared by the impregnation of RuCl3 solution over different TiO2 supports with the nominal metal loading at 4%, which proved an optimized metal loading in our preliminary test.

2100F instrument equipped with an energy-dispersive X-ray (EDX) detector. The catalytic performance was examined with a fixbed reactor using a mixture of 50%H2 and 1%CO or 0.7%CO+15% CO2 balanced with N2 at a total flow rate of 100 mL/min. The catalyst (100 mg, 0.15−0.25 mm) was loaded into a vertical tubular quartz reactor (4 mm i.d.) and kept in the isothermal center part of the reactor. The catalyst was activated in situ at 350 ◦ C for 2 h under H2 flow prior to each test. The reactants and products were analyzed by gas chromatographs (Shimadzu GC-14C and Fuli 9750T) with thermal conductivity detectors. The conversion of CO (XCO ) and CO2 (XCO2 ) was calculated as follows:

The phase composition was analyzed by powder Xray diffraction (XRD) performed with a Philips X’Pert Pro Super diffractometer (Cu Kα λ=0.15406 nm). BET surface area was determined from N2 adsorption isotherm with a Coulter 3100 system. The morphology and microstructure of the samples was examined with transmission electron microscopy (TEM) using a JEM-

Figure 1 shows TEM images of the hydrogen titanate precursor and the derived titanium dioxides. The Htitanate fibers (Fig.1(a)) obtained by hydrothermal reaction of TiOSO4 with concentrated NaOH solution at 200 ◦ C represent the layered hydrogen titanates (Fig.2(a)) [12, 14−16], which may undergo various posttreatments to produce TiO2 with different crystalline

DOI:10.1088/1674-0068/25/04/475-480

XCO = XCO2 =

in out FCO − FCO in FCO

(4)

out in − FCO FCO 2 2 in FCO2

(5)

III. RESULTS AND DISCUSSION



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TABLE I Characteristics for different TiO2 and supported Ru catalysts. Catalyst Ru/TiO2 (B) Ru/TiO2 (A) Ru/TiO2 (R) a

Support TiO2 -B nanofiber Anatase TiO2 nanorod Rutile TiO2 nanoflower

TiO2 38.9 88.1 54.3

SBET /(m2 /g) Ru/TiO2 a 34.1 64.9 31.8

dRu /nm 2.4 2.0 3.0

BET surface area of Ru catalysts supported on different TiO2 after being activated at 350 ◦ C.

catalysts originated from H-titanate precursor [12, 15], in contrast with the high-temperature annealing that may result in the transition from anatase to rutile with the rather poor surface area and nonporous texture [17].

FIG. 2 XRD patterns of (a) H-titanate fiber and the products after different post-treatments: (b) calcination at 450 ◦ C, hydrothermal reaction in (c) 0.05 mol/L and (d) 2.65 mol/L HNO3 solution.

phases and morphology. Upon heating at 450 ◦ C, the layered hydrogen titanates may be converted to the titanium dioxide polymorph TiO2 -B [14, 15] retaining the fibril morphology as indicated in Fig.1(b) and Fig.2(b). The post-treatment of hydrothermal reaction in a diluted 0.05 mol/L HNO3 solution makes the H-titanate transform into anatase TiO2 (Fig.2(c)), while rutile TiO2 (Fig.2(d)) is obtained via the treatment in a concentrated 2.65 mol/L HNO3 solution. It is noticed that the morphology of anatase nanorod (Fig.1(c)) and rutile nanoflower (Fig.1(d)) products are completely different from that of the parent H-titanate nanofibers after the treatment in the acid solution, which may suggest an essential structure reorganization during the hydrothermal reaction. These results are well consistent with the phase transition between nanostructured titanates and titanium dioxides as previously reported [12, 14]. The BET surface areas of TiO2 with different crystalline phases and morphology are listed in Table I. Since TiO2 -B yielded upon heating at 450 ◦ C, its surface area was lower than those of anatase and rutile TiO2 through hydrothermal treatments. The surface areas of supported Ru catalysts over different TiO2 decreased to various extents after being activated by reduction at 350 ◦ C. It is noticeable that the controllable phase transition under relatively milder conditions can produce comparable surface areas for the titania and supported DOI:10.1088/1674-0068/25/04/475-480

Figure 3 (a), (b), and (c) show TEM images of the Ru catalysts supported on TiO2 with different crystalline phases and morphology after reduction. It is observed that the morphology of the TiO2 supports can be well preserved after loading Ru particles. The small Ru dots are highly dispersed on the TiO2 supports with relatively homogeneous distribution. The Ru particle size distribution, obtained from more than 50 nanoparticles, is compared in Fig.3 (d), (e), and (f) along with the corresponding TEM images. The mean particles size (dRu ) is accordingly calculated and also listed in Table I. It is noted that Ru/TiO2 (B) features a rather even dispersion of Ru particles along the wall of TiO2 (B) nanofibers. The sizes of Ru particles are distributed in the range of 1.5 nm to 3.3 nm (Fig.3(d)) and the mean particle size is calculated to be about 2.4 nm. Similar range of particle sizes is evident for Ru/TiO2 (A) but a higher distribution of the smaller particles is observed on TiO2 anatase support, which may bring on a less dRu value of 2.0 nm. In the case of Ru/TiO2 (R), the Ru particle size distribution on TiO2 rutile support is found to be in a little wider range of about 2.0 nm to 4.8 nm, thus the slightly increased mean particle size of 3.0 nm. Given the same preparation process for supported Ru catalysts involving impregnation and activation, the difference in the Ru particle size distribution may suggest the different interaction between Ru and TiO2 supports, which could originate from the diverse nature of surface structure related to the TiO2 supports with different crystalline phases and morphology. EDX analyses were carried out to verify the composition of the supported Ru catalysts. Typically, the EDX spectrum of TiO2 -B nanofibers supported Ru catalyst (shown in Fig.4) confirms the presence of Ti, O and Ru besides the signals of Cu grid. However, the XRD patterns of the supported Ru catalysts (Fig.5), identical to those of the corresponding TiO2 supports (Fig.2), are in the absence of any diffraction peak related to either metallic Ru or ruthenium oxides. This result is in agreement with the presence of small Ru nanoparticles on the all Ru/TiO2 catalysts with the mean particles size less than 3.0 nm that may be beyond the detectable c °2012 Chinese Physical Society

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(b)

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20 nm


20 nm

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FIG. 3 TEM images of (a) Ru/TiO2 (B), (b) Ru/TiO2 (A) and (c) Ru/TiO2 (R), and the corresponding Ru particle size distribution in (d), (e) and (f), respectively.

FIG. 4 Typical EDX analysis of Ru/TiO2 (B).

FIG. 5 XRD patterns of (a) Ru/TiO2 (B), (b) Ru/TiO2 (A), and (c) Ru/TiO2 (R).

limitation of XRD, which is also perceived in the case of Ru supported on a commercial TiO2 support [18] as well as ceria, alumina, and carbon nanotubes [19, 20]. Figure 6 shows the results of CO methanation over Ru catalysts supported on TiO2 with different crystalline phases and morphology in the hydrogen-rich stream without the presence of CO2 . It is observed that the conversion of CO increases with increasing reaction temperature for all the catalysts. Under the experimental conditions employed, Ru/TiO2 (R) and Ru/TiO2 (B) catalysts are apparently more active than Ru/TiO2 (A). Ru/TiO2 (R) catalyst can achieve almost complete CO conversion around 220 ◦ C. For Ru/TiO2 (B) catalyst, the conversion is a bit lower than that of Ru/TiO2 (R) at each temperature and reaches 100% at 230 ◦ C. The conversion curve of Ru/TiO2 (A) catalyst shifts notably

toward higher temperature, where the reaction is initiated at about 200 ◦ C with 100% conversion at 300 ◦ C. The complete conversion of CO can maintain in the test temperature range up to 400 ◦ C for all the catalysts. This characteristic for solo-CO methanation is analogous to those reported Ru catalysts on different supports [7, 8, 21, 22]. The catalytic performances for the selective methanation of CO in the hydrogen-rich stream containing CO/CO2 mixture are compared in Fig.7 for the Ru catalysts supported on different TiO2 . It is noticed that the evolution of CO conversion for selective methanation differs from that for solo-CO methanation. In the presence of 15% CO2 in the reaction gas, all the catalysts feature a maximum value of CO conversions to various extents. The slight increase in CO conversion

DOI:10.1088/1674-0068/25/04/475-480



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FIG. 6 CO conversion for Ru/TiO2 (B) (square), Ru/TiO2 (A) (triangle) and Ru/TiO2 (R) (circle) catalysts. Reaction conditions: 1%CO, 50%H2 , and N2 balanced; total flow rate of 100 mL/min, catalyst mass of 100 mg.

FIG. 7 Conversion of CO (solid symbols) and CO2 (open symbols) for Ru/TiO2 (B) (square), Ru/TiO2 (A) (triangle) and Ru/TiO2 (R) (circle) catalysts. Reaction conditions: 0.7%CO, 15%CO2 , 50%H2 , and N2 balanced; total flow rate of 100 mL/min, catalyst mass of 100 mg.

before the maximum is due to the less CO concentration [23] in the reaction gas mixture for selective CO methanation compared to those for solo-CO methanation. At the same time, the CO2 conversion becomes apparent after the maximum of CO conversion, which is attributed to the CO2 methanation and the adverse rise of the CO concentration from the effect of RWGS reaction at higher temperature. It is observed that Ru/TiO2 (R) is the most active catalyst, exhibiting the premier CO conversion initially with increasing temperature. The complete CO conversion can be achieved at 220 ◦ C and maintained till 240 ◦ C, at which the obvious CO2 conversion of 5% appears. Further increasing temperature may cause slightly decreasing CO conversion and progressively increasing CO2 conversion. The catalytic performance for CO conversion of Ru/TiO2 (B) is comparable to that of Ru/TiO2 (R) with a less temperature window of complete CO conversion between 220−230 ◦ C, but the CO2 conversion is boosted to more than 30% at 240 ◦ C. However, only a maximum CO conversion of about 90% is reached at 240 ◦ C in the case of Ru/TiO2 (A) catalyst. Further increasing temperature results in a rapid decrease of CO conversion, which indicates that the CO production rate via the RWGS reaction increase more promptly than the CO consumption via the methanation reaction, though CO2 methanation over Ru/TiO2 (A) is not as predominant as that over the other two catalysts at higher temperature. It has been demonstrated that CO hydrogenation is a structure-sensitive reaction dependent on the size and oxidation states of Ru particles [24, 25]. With respect to the effect of Ru particles size on selective CO methanation, the results from open literature are still debatable. Some results suggested that the small Ru particles size exhibited positive effect on the catalytic activity [8, 10, 26], but others indicated that the specific activity increased with increasing Ru size [9, 27, 28]. This apparent difference may be understandable by taking account of the varied Ru loading and the interaction between

metal and different supports, which was essentially related to different mechanisms involving dissociative or associative adsorption of carbon oxides and thus responsible for the diverse activity [23, 28−30]. In the present investigation, it is more reasonable to ascribe the observed difference in activity for the selective CO methanation to the distinct interaction between Ru nanoparticles and TiO2 supports with different crystalline phases and morphology, though it seems that the smaller Ru particles are more counterproductive for the activity.

DOI:10.1088/1674-0068/25/04/475-480

IV. CONCLUSION

Based on the titanate nanofibers obtained from titanium precursors by hydrothermal reactions, the different nanostructured titanium dioxides, including anatase, rutile and TiO2 -B, have been prepared via various post-treatments by a second hydrothermal reaction in diluted or concentrated acid solution, or direct calcination, respectively. The highly dispersed Ru nanoparticles with relatively homogeneous distribution are identified for the Ru catalysts supported on the TiO2 with different crystalline phases and morphology. It is revealed that the catalysts supported on rutile and TiO2 -B are more active than that supported on anatase during the solo- and selective CO methanation reaction. It seems that the observed difference in catalytic performance apparently increases with Ru particles size, which is essentially originated from the distinct interaction between Ru nanoparticles and TiO2 supports with different crystalline phases and morphology. The optimized catalyst of 4%Ru supported on rutile can achieve the preferentially complete CO methanation while avoiding or minimizing CO2 methanation and RWGS reaction, which proves a promising one for the selective CO removal in H2 -rich stream. c °2012 Chinese Physical Society

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V. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.20703042), the National Basic Research Program of China (No.2010 CB923300), USTC-NSRL Association Funding (No.KY 2060030009), and Fundamental Research Funds for the Central Universities (No.WK2060030010).

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