Two step copper impregnated zinc oxide microball synthesis for the reduction of activation energy of methanol steam reformation Supamas Danwittayakul a,b , Joydeep Dutta b,c,⇑ a National Metal and Materials Technology Center, 114 Thailand Science Park, Klong Nueng, Klong Luang, Pathumthani 12120, Thailand b Center of Excellence in Nanotechnology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand c Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 17, Al-Khodh 123, Oman highlights Cu/ZnO microballs catalysts can be used to enhance the MSR activities. Urea addition resulting to reduction of activation energies for MSR. Microball SSA was found to increase with an increase of Cu loading. Hydrogen yield was found to be dependent on activation energy. graphical abstract article info Article history: Received 22 January 2013 Received in revised form 1 March 2013 Accepted 6 March 2013 Available online 13 March 2013 Keywords: Cu–ZnO microballs Methanol steam reforming Activation energy Hydrogen production abstract Cu/ZnO microball catalysts were prepared by a two-step process, where ZnO nanorods supports were first grown hydrothermally followed by the impregnation of copper nanoparticles. Catalytic activities for methanol steam reforming by using Cu/ZnO microball were found to increase with higher copper con- tent. Addition of urea during the metal impregnation process was found to enhance the methanol steam reforming catalytic activity attributed to the larger surface area of the catalyst. Activation energies of syn- thesized catalyst and CuZnAl commercial catalyst were calculated from the Arrhenius plots of the rate of reaction and were found to affect hydrogen yield. The lowest activation energy of 4.74 kJ mol 1 was achieved for the optimized catalyst which was half of the activation energy of commercial catalysts. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen is a renewable and clean energy and considered to be one of the promising energy source for the future. Methanol (CH 3 OH) is the smallest biomass molecule that has the highest po- tential to be used for hydrogen production because it has a high H/ C ratio (high energy density), no sulfur and require low reforming temperatures [1–3]. Methanol steam reforming (MSR) reaction exhibits high yield of hydrogen gas since three moles of hydrogen can be potentially obtained from 1 molecule of methanol as shown in Eq. (1). However, being an endothermic reaction this requires a minimum activation energy (E a ) to activate the reaction. CH 3 OH þ H 2 O $ CO 2 þ 3H 2 ðDH ¼þ49:4 kJ=molÞ ð1Þ Heterogeneous catalysts have been used for accelerating methanol reforming process [4–6]. Copper–Zinc oxide (Cu/ZnO) composites have been used as catalysts for MSR due to very high selectivity to the reaction [7–11]. Zinc oxide has an inherent ability to crack methanol and incorporation of copper nanoparticles on zinc oxide support enhances catalytic activity due to synergistic catalytic activation of the bimetallic system [12,13]. Latest catalyst development efforts have been focused on the reduction of activation energy in order to reduce the 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.029 ⇑ Corresponding author at: Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 17, Al-Khodh 123, Oman. Tel.: +968 2414 3266; fax: +968 2441 3532. E-mail address: [email protected](J. Dutta). Chemical Engineering Journal 223 (2013) 304–308 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Transcript
Chemical Engineering Journal 223 (2013) 304–308
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier .com/locate /ce j
Two step copper impregnated zinc oxide microball synthesisfor the reduction of activation energy of methanol steam reformation
1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.03.029
⇑ Corresponding author at: Chair in Nanotechnology, Water Research Center,Sultan Qaboos University, P.O. Box 17, Al-Khodh 123, Oman. Tel.: +968 2414 3266;fax: +968 2441 3532.
Supamas Danwittayakul a,b, Joydeep Dutta b,c,⇑a National Metal and Materials Technology Center, 114 Thailand Science Park, Klong Nueng, Klong Luang, Pathumthani 12120, Thailandb Center of Excellence in Nanotechnology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailandc Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 17, Al-Khodh 123, Oman
h i g h l i g h t s
� Cu/ZnO microballs catalysts can beused to enhance the MSR activities.� Urea addition resulting to reduction
of activation energies for MSR.� Microball SSA was found to increase
with an increase of Cu loading.� Hydrogen yield was found to be
dependent on activation energy.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:Received 22 January 2013Received in revised form 1 March 2013Accepted 6 March 2013Available online 13 March 2013
Keywords:Cu–ZnO microballsMethanol steam reformingActivation energyHydrogen production
a b s t r a c t
Cu/ZnO microball catalysts were prepared by a two-step process, where ZnO nanorods supports werefirst grown hydrothermally followed by the impregnation of copper nanoparticles. Catalytic activitiesfor methanol steam reforming by using Cu/ZnO microball were found to increase with higher copper con-tent. Addition of urea during the metal impregnation process was found to enhance the methanol steamreforming catalytic activity attributed to the larger surface area of the catalyst. Activation energies of syn-thesized catalyst and CuZnAl commercial catalyst were calculated from the Arrhenius plots of the rate ofreaction and were found to affect hydrogen yield. The lowest activation energy of 4.74 kJ mol�1 wasachieved for the optimized catalyst which was half of the activation energy of commercial catalysts.
� 2013 Elsevier B.V. All rights reserved.
1. Introduction can be potentially obtained from 1 molecule of methanol as shown
Hydrogen is a renewable and clean energy and considered to beone of the promising energy source for the future. Methanol(CH3OH) is the smallest biomass molecule that has the highest po-tential to be used for hydrogen production because it has a high H/C ratio (high energy density), no sulfur and require low reformingtemperatures [1–3]. Methanol steam reforming (MSR) reactionexhibits high yield of hydrogen gas since three moles of hydrogen
in Eq. (1). However, being an endothermic reaction this requires aminimum activation energy (Ea) to activate the reaction.
CH3OHþH2O $ CO2 þ 3H2 ðDH� ¼ þ49:4 kJ=molÞ ð1Þ
Heterogeneous catalysts have been used for acceleratingmethanol reforming process [4–6]. Copper–Zinc oxide (Cu/ZnO)composites have been used as catalysts for MSR due to veryhigh selectivity to the reaction [7–11]. Zinc oxide has aninherent ability to crack methanol and incorporation of coppernanoparticles on zinc oxide support enhances catalytic activitydue to synergistic catalytic activation of the bimetallic system[12,13]. Latest catalyst development efforts have been focusedon the reduction of activation energy in order to reduce the
S. Danwittayakul, J. Dutta / Chemical Engineering Journal 223 (2013) 304–308 305
operating temperature for the reforming process. Increasedactive surface area of such heterogeneous catalyst can both en-hance its catalytic activity and reduce the working temperature.Fernández et al. reported that addition of urea can control rateof precipitation of metal hydroxide powders which Wang et al.attributed to arise due to better dispersion of metal catalysts[14,15].
In this work, we have synthesized Cu/ZnO microball catalystsusing a simple two-step process including the growth of ZnOnanorods on a catalyst support (cordierite) followed by the wetimpregnation of copper nanoparticles [16]. Urea was added intoCu/ZnO microball catalyst and the catalytic activities werecompared. Copper–zinc–aluminum (CuZnAl) catalysts sourcedcommercially from SÜd Chemie were used for comparison of ourresults. Activation energy of each catalyst for methanol steamreforming was determined from the rate of hydrogen formationfrom Arrhenius plots and correlated to the net hydrogen yield ofthe steam reforming processes.
2. Experimental
2.1. Growth of ZnO nanorods
ZnO nanorods were grown following our earlier reports using amodified method suggested by Sugunan et al. [17] that composedof seeding a substrate with ZnO nanocrystals followed by acontrolled growth process [18,19]. Seeding process was modifiedby growing zinc nanoparticles directly on the ceramic substratesby dipping cordierite ceramic substrates into a 2 mM solution ofzinc acetate in ethanol followed by heating the substrate to350 �C in the ambient until the solvent was completely evaporated.The seeded substrates were then annealed at 350 �C for 5 h in theatmosphere. ZnO nanorods were grown in a sealed chemical bathfollowing a process described in detail elsewhere [20]. The seededsubstrates were then immersed in an equimolar solution (10 mM)
Fig. 1. Scanning electron micrographs of catalysts, (a) ZnO nanorod on cordierite substrtime), (b) 05Cu/ZnO microball catalyst (prepared by impregnation method using 10 mL o(prepared by impregnation method using 10 mL of 0.75 M Cu(NO3)2), inset is a high ma
of zinc nitrate and hexamine and heated 95 �C for up to 10 h. In thechemical bath, the growth solution was refreshed every 5 h toensure ready availability of zinc ions and hydroxyl ions for thenanorod growth [21]. The substrates after with ZnO nanorods werewashed rigorously with deionized water and then annealed at350 �C for 1 h in the ambient prior to further use.
2.2. Preparation of Cu/ZnO microball catalysts
ZnO nanorods grown on cordierite substrates were impreg-nated into copper nitrate (Cu(NO3)2) solutions. Briefly 10 mL of0.1 M, 0.5 M and 0.75 M of Cu(NO3)2 solutions were first preparedin deionized water into which the cordierite substrates with ZnOnanorods were immersed. The samples were indexed with respectto the copper nitrate concentrations used for the nanoparticlegrowth, i.e. 0.1 M, 0.5 M and 0.75 M of Cu2+ solutions as 01Cu/ZnO, 05Cu/ZnO and 075Cu/ZnO, respectively. Impregnationprocess was conducted at 95 �C until the solvent was completelyevaporated. These samples were then calcined at 300 �C for 3 hprior to further use. In a separate experiment, urea was added inthe Cu(NO3)2 solutions with Cu:urea molar ratio of 5:1. The cata-lysts with urea addition were prepared using 0.1 M, 0.5 M and0.75 M Cu(NO3)2 solutions and designated respectively as 01Cu/ZnOU, 05Cu/ZnOU and 075Cu/ZnOU.
2.3. Catalyst characterization
Microstructure of each catalyst was investigated using fieldemission scanning electron microscope (FESEM; JEOL, JSM-6301)working at 20 kV and the specific surface area (henceforth calledS.S.A.) were ensured by gas adsorption technique (BET; Quanta-chrome, Autosorb-1C) first by outgassing at 300 �C for 5 h followedby nitrogen gas adsorption at 77 K.
Copper and zinc contents were determined using inductivelycoupled plasma-optical emission spectrometer (ICP-OES: Horiba,
ate grown by hydrothermal route (10 mM of growth solution with 10 h of growthf 0.5 M Cu(NO3)2) and (c) low magnification image of 075Cu/ZnO microball catalystgnification image.
Table 1Composition and specific surface area of prepared catalysts.
S.S.A. = Specific surface area.a Commercial catalyst from SÜd Chemie Co., Ltd.
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Activa). First, catalyst samples were weighed and heated at 95 �Cwhile soaking in strong sulfuric acid for 3 h to allow the releaseof metals and metal oxides from the substrates. Zinc oxide, copperand copper (I) oxide are readily dissolved in sulfuric acid whilecopper (II) oxide forms copper sulfate prior to its dissolution inwater. Adjusted final volume of released metal ion solutions weremeasured and the copper and zinc contents were determined byusing ICP-OES.
2.4. Steam reforming of methanol
The experiments for studying the steam reforming of methanolwere carried out at atmospheric pressure in a packed tube placed inan electrically heated tubular reactor. Reforming process was con-ducted at temperatures of 200–350 �C. 0.5 g of catalysts wereground in a mortar and packed in the reactor for separate experi-ments. Prior to methanol steam reforming studies, catalysts wereactivated by flowing 5% H2 in argon at a flow rate of 60 mL/minfor 1 h at 300 �C [22]. A ratio of water to methanol of 0.8 mol wasused for all the reforming experiments. Gas products werecollected and analyzed by a gas chromatograph (GC, Buck Scientific)attached with a thermal conductivity detector (TCD). Hydrogenselectivity and methanol conversion were calculated by using Eqs.(2) and (3). Rates of reaction were calculated from rate of hydrogenproduction whereas all the other side reactions were neglected. Theactivation energies were evaluated form the slope of Arrheniusplots between natural log of rate of reaction against 1/T. Stabilitytest over 8 h were determined at 250 �C upon using 05Cu/ZnO.
Methanol conversion ð%Þ ¼ moles of methanol consumedmoles of methanol fed
� �
� 100 ð2Þ
Fig. 2. Methanol steam reforming activity of Cu/ZnO microball catalysts compared to CuZ(b) methanol conversion.
H2 selectivityð%Þ ¼ moles of H2 produced� 0:5moles of methanol consumed
� �� 100 ð3Þ
3. Results and discussion
ZnO nanorods were grown on ceramic substrates as shown inFig. 1a. On an average, 2.7 lm long and 120 nm wide ZnO nanorodsformed upon carrying out hydrothermal synthesis for 10 h whenusing 10 mM of growth solution. The ZnO microball structurewas then formed using ZnO nanorods as Zn2+ precursor. An advan-tage of using ZnO nanorods as a Zn2+ source is the ease to controlthe available Zn2+ ions in the impregnation solution by slightlyadjusting pH of the solution as the rate of ZnO dissolution dependson pH [21]. For the transformation of the ZnO nanorods into Cu/ZnO nanoballs, catalysts with different copper loading (01Cu/ZnO, 05Cu/ZnO and 075Cu/ZnO) were prepared and representativemicrographs are shown in Fig. 1b and c. Cu/ZnO binary oxideformation occurs due to the dissolution and subsequent co-precip-itation reactions, wherein Cu2+ ions available in the precursorsolution react with Zn2+ ions supplied by the ZnO nanorods [23].Table 1 summarizes the specific surface area of Cu–ZnO microballcatalysts for various copper loading (Table 1). For high concentra-tions of Cu2+ solution during the impregnation step (more acidicpH), more Zn2+ ions are released from ZnO nanorods in the solutionleading to more uniformly distributed Cu/Zn nanoparticles, result-ing in higher S.S.A. (Table 1). In addition, the presence of urea in theprecursor solution resulted in the formation of larger S.S.A.catalysts that can be attributed to the hydrolysis reaction of ureagradually generating hydroxyl ions leading to the homogeneousnucleation resulting in highly uniform Cu/Zn nanocatalysts[14,15,24,25].
Catalytic activity of hydrogen selectivity by methanol steamreforming reaction using bare ZnO nanorod was found to be 11%(at 350 �C) while more than 60% methanol could be converted evenat low temperatures (�200 �C). Catalytic activity of Cu/ZnO ismuch higher than pure ZnO because of the synergistic combinationof the bimetallic oxides [26,27]. It was observed that hydrogenselectivity increased with an increase in Cu content in the catalysts(Fig. 2a and Table 1) [28]. The highest hydrogen selectivity of 43%was found for 075C/10Zn samples at 350 �C. Theses samples alsoshowed acceptable hydrogen selectivity of �33% at temperaturesas low as 200 �C. Methanol conversion using Cu/ZnO microballwas found to be very high especially for catalysts with higher cop-per content (05Cu/ZnO and 075Cu/ZnO), exhibiting more than 90%conversion in the range of 200–350 �C temperature (Fig. 2b). Cata-lytic performance of 075Cu/ZnO catalyst showed better hydrogen
nAl commercial catalyst, H2O:CH3OH molar ratio = 0.8, (a) hydrogen selectivity and
Fig. 3. Stability test of steam reformation of methanol upon using 05Cu/ZnOmicroball catalyst over 250 �C for 8 h.
Fig. 5. Arrhenius plots for the methanol steam reforming upon using Cu/ZnO basedcatalysts. The reaction rate is calculated in correspondence of the followingoperating conditions: H2O:CH3OH molar ratio = 0.8, temperature range 200–350 �C.
Fig. 6. Plot of hydrogen yield against activation energy for methanol steamreforming.
Fig. 4. Scanning electron micrograph of 05Cu/ZnO microball catalyst after refor-mation process at 300 �C for 8 h.
S. Danwittayakul, J. Dutta / Chemical Engineering Journal 223 (2013) 304–308 307
selectivity and methanol conversion rate than the CuZnAl commer-cial catalyst (Fig. 2 and Table 1). Stability test upon using 05Cu/ZnOwas determined over 8 h at 250 �C and found that 05Cu/ZnO cata-lyst was stable over the observation duration as shown in Fig. 3with more than 35% hydrogen selectivity and above 86% methanolconversion. However, we found that higher reformation tempera-ture was not suitable for the catalysts with high copper contentsince sintering effect lead to a reduction of hydrogen yield asshown in Fig. 4. In addition, Cu/ZnO microball with urea additionwas found to be very selective to MSR showing 47% hydrogen
Table 2Catalytic activity for steam methanol reforming.
Methanol conversion and hydrogen yield data obtained from SRM at 250 �C.
selectivity (075Cu/ZnOU at 350 �C) while >99% methanol conver-sion could be achieved which is attributed to the better dispersionof the copper nanoparticles in the zinc oxide matrix [14,29].
Fig. 4 shows the Arrhenius plots of Cu/ZnO microball catalystswhere we observe that the activation energies decreased withincreasing copper loading. This corresponds well with the factthat high Cu/Zn ratio catalysts are very selective to MSR and canbe used at lower operating temperatures [30–32]. The lowest acti-vation energy upon using 075Cu/ZnO was found to be22.32 kJ mol�1 while activation energy for CuZnAl catalyst was8.92 kJ mol�1 (Table 2). It should be noted here that the activationenergy of bare ZnO nanorods for MSR was found to be almost80 kJ mol�1. Activation energy of modified Cu/ZnO microball cata-lyst (075Cu/ZnOU) by adding urea was found to be 4.74 kJ mol�1
which clearly shows that an addition of urea enhances the cata-lytic activity. Activation energies for respective hydrogen yieldsobtained from SRM are plotted in Fig. 5 showing a quasi-linearrelationship of the activation energies with the hydrogen yield(see Fig. 6).
4. Conclusions
Cu/ZnO microball catalysts were successfully prepared by atwo-step method including the growth of ZnO nanorods on cordi-erite substrates followed by the in situ copper nanoparticleimpregnation. Catalytic activity of Cu/ZnO microball catalysts de-pend upon copper loading and an increase in activity was observedwith increasing copper content in the catalyst. 4552 lmol g�1 h�1
308 S. Danwittayakul, J. Dutta / Chemical Engineering Journal 223 (2013) 304–308
of hydrogen yield was obtained upon using 075Cu/ZnO samples.An addition of urea was found to enhance the MSR catalyticactivity (5385 lmol g�1 h�1 of hydrogen yield upon using 075Cu/ZnOU) due to better dispersion of the metal catalysts and largersurface area in the zinc oxide matrix. Activation energies of eachcatalyst were calculated from the Arrhenius plots which werefound to influence the hydrogen yields linearly. The lowest activa-tion energy for SRM was found to be 4.74 kJ mol�1 upon using075Cu/ZnOU which was half the activation energy required usingCuZnAl commercial catalyst (8.92 kJ mol�1).
Acknowledgements
The authors would like to acknowledge partial financial supportfrom the Center of Excellence in Nanotechnology at the AsianInstitute of Technology and the National Metal and Materials Tech-nology Center (MTEC) and National Nanotechnology Center(NANOTEC) both belonging to the National Science & TechnologyDevelopment Agency (NSTDA), Thailand. JD is thankful to GrantFunding from the Research Council of the Sultanate of Oman underGrant Agreement No. (RCP SQU 011 001).
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