CO selective methanation in hydrogen-rich gas mixtures over carbon nanotube supported Ru-based catal

Available online at www.sciencedirect.comJOURNAL.OFScienceDirectNATURAL GASCHEMISTRYEI SEVIERJourmal of Natural Gas Chemistry 21(2012)445- 451www.elsevier.com/locat/jngcCO selective methanation in hydrogen-rich gas mixturesover carbon nanotube supported Ru-based catalystsJun Xiong，Xinfa Dong*，Lingling LiSchool of Chemistry and Chemical Engineering. South China University of Technology, Guangzhou 510640, Guangdong, China[ Manuscript reeived October 23, 2011; revised January 16, 2012 ]Abstractof CO by CO selective methanation from H2-rich gas stream conducted in a fixed-bed quartz tubular reactor at ambient pressure. It was foundthat the metal promoter, reduction temperature and metal loading afcted the catalytic properties significantly. The most excellent performancewas presented by 30 wr% Ru Zr/CNTS catalyst reduced at 350。C. Since it decreased CO concentration to below 10 ppm from 12000 ppm byCO sective methanation at the temperature range of 180 -240 。C, and kept Co selectivity higher than 859% at the temperature below 200 °C.Characterization using XRD, TEM, H2-TPR and XPS suggests that Zr modification of Ru/CNTs results in the weakening of the interactionbetween Ru and CNTs, a higher Ru dispersion and the oxidization of surface Ru. Amorphous and high dispersed Ru particles with small sizewere obtained for 30 wt% Ru-Zr/CNTs catalyst reduced at 350 °C, leading to excellent catalytic performance in co selective methanation.Key wordscO selective methanation; Ru-based/CNTs catalyst; hydrogen-rich gases; CNTs; hydrogen energy1. Introductionof the membrane at high temperature is often needed throughthis process [5]. Many investigators also focus on the prefer-The proton-exchange membrane fuel cell (PEMFC)ential oxidation of CO (PROX) for this purpose [6-8]. How-which relies on hydrogen as fuel is considered to play an im-ever, this approach requires the addition of oxygen (or air) inportant role in the future energy generation for mobile appli-the bydrogen-rich gas stream, which may result in a high con-cations due to its high power density and zero emission, andversion of hydrogen into water [9]. Recently, Co selectivefor stationary applications due to the generation of electricitymethanation has been proved to be an effective strategy, be-and heat [1]. At present, most hydrogen is produced throughcause it does not require any oxygen (or air) addition since thesteam reforming, partial oxidation or auto thernal reformingnecessary reactants (CO and H2) are already present[1 - 3,10].(a combination of steam reforming and partial oxidation) ofCO methanation, shown in Equation (1) is highly exother-methane or methanol. About 1- 2 vol% of Co which can poi- mic; At the same time, the undesirable reaction ofCO2 metha-son the Pt anodes of PEMFC and decrease the efficiency ofnation in Equation (2) is also highly exothermic:PEMFC significantly is inevitably co produced in these re-CO+ 3H2←- + CH4 +H2O△H = -206 kJ/mol(1actions, since most PEMFC today cannot tolerate Co higherthan 10- 100 ppm [2,3]. Thus, thorough removal of CO inhydrogen-rich gas streams has been required to decrease theCO2 +4H2←+ CH4+ 2H2O NH= -165 kJ/mol (2concentration of CO. When CO removal is applied on boardfor transport applications powered by PEMFC, such as vehi-The amount of hydrogen consumed during CO selectivecles, it should be performed at temperatures as low as possi-methanation is negligible, due to low Co content. The chal-ble in order to make the volume smaller [4]. One of the COlenge is the simultaneous methanation of CO2, which con-deep removal methods is selective diffusion of H2 through thesumes 4 mol H2 per mol CO2, amplified by high CO2 contentmembrane. The high pressure differential between both sidesin hydrogen-rich gas mixtures. Therefore, in order for this中国煤化工MYHCNM HG●Crreponding author. Tel: +86-20-87110659; E-mail: cexfdong@scu.edu.cnCopyrightO2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.do:10.1016/51003-99311)60389-7446Jun Xiong et al./ Jourmal of Natural Gas Chemistry Vol. 21 No. 42012method to be effective, low hydrogen consumption throughThe CNTs were purified and functionalized by refluxing inCO selective methanation is required. Thus, selectivity of the5 M aqueous HNO3 solution for 2 h, and then the acid-treatedcatalyst in Co methanation versus CO2 methanation (Sco) isCNTS were diluted with water, filtered, washed with excessa very important parameter. It can be defined as Equation (3):deionized water, and dried at 120。C overnight for further use.Sco=- rCO3)Ru-based/CNTs catalysts were prepared through an im-rco+rCo2pregnation method reported by Chetty et al. [14]. The re-where, rco is the rate of CO reduction, and rco2 is the rate ofquired amount of RuCl3 and metal promoter were dissolvedCO2 reduction.in ethanol-water (3 : 1) mixture, respectively. ZrOC2, FeCl3,Furthermore, a second undesirable reaction is the reverse-CoCl2, La(NO3)3, Ca(NO3)2 were used as metal promoterwater-gas shift (RWGS) reaction which is favored at high tem-sources. While stiring, the purified and functionalized CNTsperatures and can shift CO2 to CO (Equation 4):were added, followed by drying at 373 K for 5 h. The atomicratio of Ru to metal promoter was 2:5. Unless otherwiseCO2+H2←+CO+H2O OH = 41.1 kJ/molspecified, the metal loading in the catalysts was controlled tobe 30 wt%.In order to minimize the forward reaction of Equation (4),the reaction temperature must be kept as low as possible. Con-2.2. Catalyst characterizationsequently, it is important to develop high selective CO metha-nation catalysts characterized by high activity at sufficientlyThe microstructure and morphology of the catalysts werelow temperatures. Among the catalysts, Ru-based catalystscharacterized by a transmission electron microscope (FEI Tec-for selective methanation of CO have exhibited excellent per-nai 12) at an accelerating voltage of 100 kV. The powder sam-formances. Takenaka et al. [4] reported that Ru/TiO2 canple was dispersed in ethanol and kept in an ultrasonic bathdecrease a concentration of Co from 0.5% to 20 ppm in the .for 2 h, then the sample was deposited onto a carbon coveredgases formed by methane steam reforming with a significantlyCu supporting grid and dried at 25 °C. X-ray diffractionlow conversion of CO2 into methane at 493 K. Dagle et al.(XRD) patterns were obtained on a rotation anode X-ray[2] employed a Ru/Al2O3 catalyst for CO selective methana-diffractometer (D8 Advance, German) equipped with graphitetion. It was found that over 3 wt% Ru metal loading catalystwith a crystallite size of 34.2 nm, the concentration of CO inmonochromatized Cu Ka radiation. The working voltageoutput was less than 100 ppm in a temperature range of 240and working current of the diffractometer were kept at 40 kVto 280 °C, while hydrogen consumption was maintained lessand 40 mA, respectively. X-ray photoelectron spectra (XPS)were achieved with an AXIS UItra DLD spectrometer (Kratosthan 10%. .Carbon nanotubes (CNTs) are suitable support for manyAnal.), using the radiation of Al Ka line as the excitationkinds of metal catalysts owing to their intrinsic properties suchsource. Spectra were corrected using C (18) peak of graphiteas high surface area, unique electronic properties and chem-at 284.6eV.ical inertness, thermal stability and high mechanical strengthH2 temperature-programmed reduction (H2 TPR) exper-[11-13]. To develop new functional material and improveiments were carried out in a U-type tubular quartz reactor,the catalytic performance of Ru-based catalyst, CNTs are in-where 50 mg sample (40- 60 mesh) was loaded in the ther-troduced as catalyst support to synthesize well dispersed Ru-mostatic zone. The reduction was conducted in a flow ofbased/CNTs. Our strategy for the development of new excel-H2/Ar mixture (volume ratio of 10: 90) with a flow rate oflent carbon nanotube supported Ru-based catalysts will focus50 mL/min and a heating rate of 5。C/min. The consumptionon the outlet concentration and selectivity of CO, due to theof H2 was detected using a TCD detector.activity and selectivity for the methanation of Co are crucialin the purification of hydrogen-rich gas mixtures by means of2..3. Catalytic performance testingCO selective methanation method. In this study, the effects ofmetal promoter, reduction temperature and metal loading onthe catalytic performances of CNTS supported Ru-based cat-fixed-bed quartz tubular reactor at ambient pressure. A K-typealysts were explored. The intrinsic factors that influence thethermocouple was utilized to measure the temperature of theperformance of Ru/CNTs were also investigated by means ofsystem. About 250 mg powder catalysts (40- -60 mesh particleXRD, TEM, XPS and H2 -TPR techniques.size) diluted with 1 g quartz sand (20-40 mesh) was packed inreactor. Prior to the reaction, the catalysts were reduced with2. Experimental20% hydrogen/nitrogen flow at various temperatures rangingfrom 350 tq 550°C fnp?h Tha ronntion was initiated by in-2.1. Catalyst preparationtroducing n中国煤化工1%), CO (~1.2 vol%)and CO2 (|YHCNMHGowrateof50mL/minThe multi-wall carbon nanotubes (O.D. <8 nm), producedinto the reactor. During the reaction, a part of effluent gasby exploiting chemical vapor deposition (CVD) processes,through the catalyst bed was sampled out and analyzed by GCwere purchased from Chengdu Organic Chemicals Co. Ltd.(Agilent 4890D) with TCD and FID detectors.Joumal of Natural Gas Chemitry Vol. 21 No.4 20124473. Results and discussion■Ru3.1. Catalysts characterization●MCNTs▲1-ZrO23.1.1. XRD analysisFigure 1 shows the X-ray diffraction patterms of variousCNTs supported catalysts reduced at 350 °C. The diffractionpeak at 26.20 in all spectra can be well indexed as the (002)reflection of graphite [15]. The peaks ascribable to tetragonalZrO2 (t-ZrO2), La2O3, CaClOH, Co were found in ZrOCl2,350 (CoCl2, La(NO3)3, Ca(NO3)2-modified samples, respectively[16-19]. The peaks characterizing FeCl3 in Ru-Fe/CNTs506C7(sample were absent, due to the freedom of XRD spectra201(0 )from FeCl3 crystal [20]. From XRD patterns of Ru/CNTsFigure 2. XRD patterns of Ru-Zr/CNTS reduced at dfferent temperaturesreduced at 350 °C, the broad diffraction peak of amorphousRu was clearly recognizable, as indicated by the characteris-tic peak at 440 [21,22]. It was seen that the amorphous Ru3.1.2. XPS analysisdiffraction peak weaken greatly after modification with metalpromoter. The weakest peak of amorphous Ru was showed byRu 3p3/2 region was chosen for the XPS analysis. TheRu-Zr/CNTs, suggesting a highest dispersion of Ru particle inspectra from as-prepared catalysts can be deconvoluted intoall samples.three peaks of different intensities located around 461.4, 462.7nd 463.9eV, as shown in Figure 3. The lower bindingenergies (BE) at 461.4 and 462.7 eV were assigned to Ru0●-Zr0， ◆La,Oand Ru'+ species, respectively, whereas the component withCaClOH★CoBE 463.9eV matched the character of Ru4+ [15]. It wasfound that most of the Ru was in the metallic state, and verysmall amounts of Ru'+ were detectable from the spectrum ofRu/CNTs catalyst. In contrast, XPS spectrum obtained from461.4eV 462.8eV 463.9eV(270是1t-十20/(° )N|Figure 1. XRD patterms of CNTs supported catalysts reduced at 350°C.(1) Ru/CNTs, (2) Ru-Zr/CNTs, (3) Ru-Fe/CNTs, (4) Ru-La/CNTs, (5) Ru-Ca/CNTs, (6) Ru-Co/CNTsThe effect of reduction temperature on the diffractionstructure of Ru-Zr/CNTs catalyst was also explored, as shownin Figure 2. It was found that Ru particles were present in(1a typical amorphous form for the catalyst reduced at 350°C.However, two crystallite diffractional peaks at 44.10 and 42.40corresponding to metallic Ru appeared when the catalyst was中国煤化工reduced at above 450°C [23,24], indicating the crystallizationof Ru particles. The variation in the intensity of Ru and t-ZrO2MHC NMHG4 466 468 470crytallite diffractional peaks clearly revealed that the crystal-Binding energy (eV)lization degree of Ru and t-ZrO2 increased significantly as theFigure 3. X-ray photelectron spectra of CNTs supported catalysts. (1)reduction temperature increased from 450 to 550 °C [25,26].Ru/CNTs, (2) Ru-Zr/CNTsJoumal of Natural Gas Chemistry Vol. 21 No. 42012449decreased co concentration to 510 ppm at 280 °C. The cata-dispersion was showed, which might be caused by the exis-lysts, Ru-La/CNTs, Ru- Fe/CNTs and Ru-Ca/CNTs, were con- .tence of appropriate interaction between Ru species and ZrO2siderably more active, reaching much lower CO levels (16-in Ru-Zr/CNTs catalyst [31]. The active metal particles of42 ppm) at temperatures between 220 and 260 °C. It was ob-Ru-Zr/CNTS catalyst were very small, with a narrow particleviously found that Ru-Zr/CNTs catalyst showed the best activ-distribution ranging from 4 to 8 nm, as showed in Figure 5.ity, with a wide temperature window between approximatelyGenerally, the small size and high dispersion of active parti-180 and 240°C where the CO level was less than 10 ppm.cles lead to a good catalytic activity of catalyst [12]. Thus, theThe optimal reaction temperature decreased about 40°C af-significant promoting effects on the catalytic activity of Ru-ter Zr modification. This suggests that a great improvementZr/CNTs in CO selective methanation may mainly attribute toof catalytic activity can be obtained after Zr modification onZr modification.Ru/CNTS catalyst.3.2.2. Effects of reduction temperature10000 (It was reported that amorphous Ru-based catalystsshowed excellent catalytic properties in CO selective metha-1000nation due to their unique isotropic structural and chemicalproperties [41,42]. This result was also found in our work.- Ru-Fe/CNTsAs shown in Figure 7, the best catalytic activity was showedRu-La/CNis- Ru-Ca/CNTsSby amorphous Ru-Zr/CNTs catalyst reduced at 350 °C with- Ru/CNTsa widest temperature window where CO level was less than10 ppm between 180- 240 °C, keeping CO seletivity higherthan 85% before 200 °C. By increasing the reduction temper-ature to 450 °C, the catalyst sample began to transfer from theamorphous state to its crystallized state (Figure 2). A rela-tively narrow reaction temperature window between 180 and220 °C was exhibited and CO selectivity decreased dramati-101520250300cally. For the catalyst reduced at 550 °C with a relatively highTemperature (C)crystallization degree of Ru particles, the optimal reactionFigure 6. Outlet concentration of CO as a function of reaction temper-atures in Cselective methanation over different CNTs supported cat-alysts. Operating conditions: P= 1bar, total flow rate = 50 mL/min(CO/CO2/H2 = 1.2/20.0/78.8), catalyst weight = 250 mg10000十350CThe catalytic activity improvement of Ru-La/CNTs andRu-Zr/CNTS may be related to the addition of ZrO2 andLa2O3 which are widely used as electronic modifier, respec-色100tively [18,38]. Electron transfer has been proposed in the caseof Ru catalysts supported on different kinds of metal oxides[39]. The adsorptive and catalytic behavior of Ru is corre-lated to the electronegativity of the support through electron-donating effect [27]. Ishihara et al. [39] reported that the ac-tivity of Co methanation, which depended on the electronega-900tivity of the oxide supports, can be enhanced by Ru on supportoxides with high electronegativity. ZrO2 has a higher elec-80_。_450个tronegativity than La2O3 [40]. Thus, a better promoting effecton the catalytic activity was exhibited by the catalyst modified60 F8with Zr in Co selective methanation.XPS results indicate that the oxidization of surface Ru4Cparticles was occured after Zr modification. The oxidizedcharge transfer to the active Ru particles would weaken C=Obond of the adsorbed CO molecules and strengthen the in-teraction between C=O bond and the catalyst surface for Ru-中国煤化工240 260 280 30based catalyst [37]. Therefore, it is expected that Ru-Zr/CNTsC)surface would be more reactive for CO methanation as com-YHCNM HGpared with Ru/CNTs surface. Ru particles became more ac-Figure 7.action temperatures in CO selective methanation over Ru-Zr/CNTs reducedtive after Zr modification, due to significant weakening of theat various temperatures. Operating conditions: P= 1 bar, total flow rate =interaction between Ru and CNTs. Additinally, a higher Ru50 mL/min (CO/CO2/H2 = 1.2/20.0/78.8), catalyst weight = 250 mg450Jun Xiong et al./ Jourmal of Natural Gas Chemistry VoL. 21 No. 42012temperature increased to 200 °C and the reaction tempera-reduced to a relatively low value [1,5]. CO selectivity de-ture window where CO level was less than 10 ppm was thecreased rapidly from 100% to 41% at 200°C, when Ru-Zrmost narrow, ranging only from 200- 220 °C. Thus, the cat-loading increased from 5 wt% to 20 wt%. CO selectivities ofalytic performance of Ru-Zr/CNTs catalysts deteriorated sig-30 wt% and 50 wt% Ru-Zr/CNTs were almost the same andnificantly with the increase of reduction temperature, and Ru-higher than that of 20 wt% Ru-Zx/CNTs significantly. Appar-Zr/CNTs catalyst with amorphous form exhibited more excel-ently, choice of metal loading is extremely important in properlent catalytic performance in Co selective methanation thanbalancing of activity with selectivity. Therefore, 30 wt% Ru-the corresponding catalyst with crystallized form.Zr loading appeared to be optimal in consideration of the out-let concentration and selectivity of CO under the conditions3.2.3. Effect of metal loadingstudied.To study the effects of metal loading on catalyst per-4. Conclusionsformance, a series of Ru-Zr/CNTs catalysts with differentRu-Zr loadings were evaluated. As shown in Figure 8,Series of Ru-based catalysts supported on carbon nan-Co outlet concentration was depleted to a minimum levelotube are prepared by impregnation method. The effects(<10 ppm) rapidly for all samples, before CO levels beganof metal promoter, reduction temperature and metal loadingto increase. The optimal reaction temperature decreased sig-n the catalytic performance of Ru-based catalysts in COnificantly from 220 to 180。C, when Ru-Zr loading increasedselective methanation are investigated. Zr modification offrom 5 wt% to 20 wt%. This was well consistent with previousRu/CNTs results in the weakening of interaction between Rufindings that an increase in activity of catalyst can be causedand CNTs, a higher Ru dispersion and the oxidization of sur-by increasing active metal loading [2,12]. A maximum activ-face Ru. A great improvement of catalytic activity is ob-ity in CO selective methanation was showed over 30 wt% Ru-tained for Ru/CNTs catalyst modified with Zr. 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