EFFECTS OF PREPARATION OF Cu/Zn OVER Al O CATALYSTS … · 2017. 8. 20. · Cu-Zn based catalysts were prepared with urea (CZU) and without urea (CZ) followed by impregnation over

Suranaree J. Sci. Technol. 16(2):103-112

1 School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology,Thammasat University, P.O. Box 22, Pathumthani, Thailand 12121. E-mail: [email protected]: +66-2-986-9009 ext 2305

2 National Metal and Materials Technology Center, National Science and Technology Development Agency,Klong-1, Klong-luang, Pathumthani, Thailand 12120

* Corresponding author

EFFECTS OF PREPARATION OF Cu/Zn OVER Al2O3CATALYSTS FOR HYDROGEN PRODUCTION FROMMETHANOL REFORMING

Suparoek Henpraserttae1,2 and Pisanu Toochinda1*

Received: Sept 4, 2008; Revised: Jan 27, 2009; Accepted: Jan 30, 2009

Abstract

A novel catalyst for hydrogen production from the catalytic methanol reforming process could play animportant role in hydrogen production to be used as a feed for a fuel cell. This study focuses on thepreparation methods of active Cu/Zn based catalysts with and without urea by incipient wetnessimpregnations to lower the metal loading and catalyst cost. The catalytic methanol reforming wasstudied in a fixed-bed reactor under mild conditions and a reaction temperature range of 453 and 523K in order to lower energy costs. The activity in the hydrogen production of the impregnated catalystsand a commercial catalyst was analyzed by gas chromatography and compared in terms of hydrogenproduction yield. The catalysts were also characterized by XRD and SEM in order to identify thephysical and chemical properties of the catalysts. The data show that the activity in hydrogen productionfrom the catalysts with urea is higher than that of the catalysts without urea. The impregnated catalystscould exhibit activity at as low a temperature as 453 K which indicates the possibility of lowering thereaction temperature for the methanol reforming process. The Cu/Zn catalysts prepared by incipientwetness impregnation over Al

2O

3 with urea could exhibit high activity about 28% H

2 yield with

efficiency about 97% compared with the commercial catalyst. The impregnated catalysts could bealternative catalysts for hydrogen production from methanol reforming with a lower cost of the catalystcompared with the co-precipitation method used in a commercial one.

Keywords: Methanol reforming, Hydrogen production, Cu/Zn based catalysts, Incipient wetness impregnations

Introduction

The growing world population has had a largeeffect on the demand for energy. To date, fossilfuels have been used as the main energy sources

to support such demand. Unfortunately, fossilfuel reserves are expected to last no longer than50-100 years. The shortage of energy is thus

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104 Hydrogen Production from Methanol Reforming

becoming an important problem for mankind,prompting worldwide searches for alternativeenergy (Shafiee and Topal, 2007).

One interesting alternative energy isenergy from hydrogen feeding to a fuel cellcreating electricity. A fuel cell unit generateselectrical energy from an electrochemicalreaction of hydrogen with oxygen, yielding anenvironmentally benign by product, water(Zhang et al., 2008; Kulikovsky, 2008;Sholklapper et al., 2008). The hydrogen fuel cellis claimed as an alternative energy resource, withhigh efficiency compared with conventionalinternal combustion engines, to replace fossilfuel (Edwards et al., 1998; Bowers et al., 2007).

Hydrogen could be produced from thereforming reaction of hydrocarbons such asmethane, ethanol, methanol, etc (Alessandraand Elisabete, 2006; Papavasiliou et al., 2007;Campos-Skrobot et al., 2008). The methanereforming reaction requires a huge amount ofenergy, 973-1273 K (Zhou et al., 2008; Xuet al., 2008) while the ethanol has been used invarious applications such as gasohol and foodproduction. Methane and ethanol have beenwidely used as energy resources, so there is noneed to spend significant amount of energy toconvert methane and ethanol to be hydrogen.One of the attractive choices for the hydrogenproduction from the reforming reaction ismethanol. Methanol is considered as low cost,as having a low boiling point, low reformingtemperature, high energy density, high H/Cratio and as being easily stored. This studyfocuses on methanol steam reforming reactionto produce hydrogen for a fuel cell. The reactionis shown as follows (Patel and Pant, 2007;Shishido et al., 2007; Basile et al., 2008).Steam reforming:

Hydrogen could be produced from areaction of methanol and water. This reactionprovides the highest hydrogen production amongother various reforming reactions. Unfortunately,

the process still requires a huge amount ofenergy in order to produce a high yield ofhydrogen. This is a major drawback of hydrogenproduction from the reforming reaction. TheCu-Zn based catalysts require a high operationtemperature of 523-573 K to produce theeffective hydrogen yield (Shishido et al., 2004;Yao et al., 2006; Patel and Pant, 2007).Ironically, in order to operate a hydrogen fuelcell, the hydrogen could be obtained from theprocess with a high cost of energy expenditure.Most of the Cu-Zn based catalysts andcommercial catalysts were prepared by usingCu and Zn contents as high as 60-90 weightpercent by the co-precipitation method toproduce the high yield of hydrogen (Patel andPant, 2007; Shishido et al., 2007). Literatureshave been reported on the use of urea inprecipitation of Cu/Zn-based catalysts couldexhibit higher activity of methanol reformingthan those of the catalysts prepared byprecipitation without using urea. The additionof urea allows the formation of precipitates tohave better homogeneity of metal cationdistribution in the solution and could yield highsurface area precipitates which composed bysmall pieces of metals stick together inspherical shape (Murcia-Mascarós et al., 2001;Shishido et al., 2004; Turco et al., 2004;Shishido et al., 2007).

The incipient wetness impregnation overporous materials have been widely used formany catalyst preparations due to the ease ofmethod and small amount of active metalcontent relative to other catalyst preparationmethods which could lower the cost ofcatalyst production (Patel and Pant, 2006). Theimpregnation of Cu and Zn with urea on thesupport such as alumina might be able toexhibit a high activity of hydrogen productionwithout using a high metal loading on thecatalyst.

The objective of this work is to studythe performance of various metal contentsimpregnated on Al2O3 for the methanolreforming reaction. This study also focus onthe Cu-Zn based catalyst with and without ureaimpregnated on Al2O3 to study the catalyticperformance of the methanol reforming

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105Suranaree J. Sci. Technol. Vol. 16 No. 2; April - June 2009

reaction. The performances of the catalysts arecompared with the performance of a commercialone. The results of this study could lead to abetter understanding of the role of catalystpreparation in order to develop novel catalystsfor hydrogen production fromthe methanolreforming process.

Experimental

Catalyst Preparation

Cu-Zn based catalysts were prepared withurea (CZU) and without urea (CZ) followed byimpregnation over Al2O3. The catalyst preparedwith urea (CZU) was synthesized by impregnatingthe aqueous solution of Cu(NO3)2•3H2O(99%, Fluka) and Zn(NO3)2•6H2O (99%,Fluka) with the addition of urea (99%, CarloErba) ver Al2O3 (98%, Riedel-de Ha n). Thecatalyst prepared without urea was synthesizedby impregnating the aqueous solution ofCu(NO3)2•3H2O and Zn(NO3)2•6H2O overAl2O3. The impregnated samples were driedin air at 373 K for 12 h and then calcined inair at 573 K for 3 h. The commercial catalystused for comparison was Cu/Zn/Al2O3 fromS d-Chemie AG (M nchen, Germany). Thecompositions of the catalysts synthesized andtheir abbreviations are shown in Table 1.

Methanol Reforming Reaction in TubularReactor

The methanol steam reforming reaction

for the hydrogen production was studied on theprepared catalysts in a stainless steel tubularreactor which has an inside diameter of 1 cm.Two grams of catalyst were packed betweenquartz wool in the tubular reactor. The catalystwas reduced with 50 ml/min of 10% H2 : N2

balanced at 453 K for 1 h prior to the methanolreforming reaction test. The reactor was flushedwith 50 ml/min N2 flow at 453 K for 30 min toget rid of adsorbed hydrogen from thereduction process. The mixture of methanoland water solution was loaded into a saturatorwhich was heated at 333 K. The compositionsof methanol and water in the liquid phaseare 0.125:0.875 molar ratio. The compositionof solution was calculated by the AspenPlus simulation program (Aspen TechnologyInc., Burlington, Mass., USA) to ensure thatthe composition of the mixture in the vaporphase is a 1:1 molar ratio at the outlet from thesaturator. The vapor of the mixture in thesaturator was carried at 20 ml/min N2 into thereactor at 453 K and 523 K in a continuoussystem. Therefore, the space time of the packedbed reactor in a continuous mode was about6 second. The product was collected and theproduction of hydrogen was determined via gaschromatography (GC).

Analysis Section

A Perkin Elmer (Waltham, Mass., USA)Autosystem XL gas chromatograph withPorapak Q column (Supleco, Bellefonte, PA,USA) coupled with a thermal conductivity

Table 1. The compositions of Cu-Zn catalysts for methanol reforming

Catalyst Composition Urea(Cu/Zn/Al2O3) (%wt) (mole/moleCu)

10CZ 5 / 5 / 90 0

10CZU1 5 / 5 / 90 1

10CZU2 5 / 5 / 90 2

20CZ 10 / 10 / 80 0

20CZU1 10 / 10 / 80 1

20CZU2 10 / 10 / 80 2

Commercial 40 / 30 / 30 0

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detector (TCD) was used to determine theamount of hydrogen production. The GCwas affixed to a computer for automaticdetermination of peak areas which could beconverted to concentration.

The surfaces of the catalysts wereinspected by a scanning electron microscope(SEM, JEOL JSM-5410, Jeol Inc., Tokyo,Japan). The elemental composition of thecatalyst surface was determined by energydispersive spectrometry (EDS, Oxford) thatwas equipped with SEM.

The catalysts were characterized by theX-ray diffraction technique (XRD, JEOL JDX-3530, Jeol Inc., Tokyo, Japan) using Cu Kα1

radiation, 30° - 41° 2-theta, 0.04° step size, 0.5sec step time. JADE software (Jade SoftwareCorporation Ltd., Christchurch, New Zealand)was used to determine the crystallize size ofthe CuO and identify phases of the catalystswith the database of an X-ray diffractogram fromthe International Centre for Diffraction Data

(Newtown Square, PA, USA).

Results and Discussion

Activity of the Catalysts for MethanolReforming

Hydrogen production from the methanolreforming reaction over various catalysts wasinvestigated and the results are reported inTable 2. Percent H2 yields were calculatedusing the following equation;

Table 2 shows that the higher metalloading on the catalysts leads to a higheractivity of the catalyst which is indicated by the20 wt% metal loading catalysts (20CZ, 20CZU1,20CZU2) exhibiting an activity higher than those

Table 2. Hydrogen production from methanol reforming over various catalysts at 453 Kand 523K

Cu-Zn-Al2O3 Catalysts Temperature% H2 yield

(Cu/Zn/Al2O3) (%wt) (K)

10CZ (5/5/90) < 10

10CZU1 (5/5/90) < 10

10CZU2 (5/5/90) 19

20CZ (10/10/80) 453 13

20CZU1 (10/10/80) 17

20CZU2 (10/10/80) 21

Commercial (40/30/30) 25

10CZ (5/5/90) 13

10CZU1 (5/5/90) 22

10CZU2 (5/5/90) 26

20CZ (10/10/80) 523 20

20CZU1 (10/10/80) 25

20CZU2 (10/10/80) 28

Commercial (40/30/30) 29

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of the 10 wt% metal loading catalysts (10CZ,10CZU1, 10CZU2) at both 453 and 523 K. Thedata also show that the amount of urea affectsthe catalytic activity of the hydrogen productionat both 10% and 20% metal loading which isindicated by the results of the higher CZU2 ureacatalysts, which exhibit higher activity thanthat of the lower CZU1 urea catalysts at 453 and523 K. Among all the catalysts in this study, thecommercial catalyst exhibits the highesthydrogen yield but the Cu and Zn loading onthe commercial catalyst (Cu/Zn/Al2O3 : 40/30/30 wt%) is much higher than that of theimpregnated catalysts in this study. The metalloading of the commercial catalyst is about4 times higher than that of the 20CZU1and 20CZU2 but the hydrogen yield of thecommercial one is about 1.5 and 1.2 timesrespectively, at 453 K and only about 1.2 and1.04 times respectively, at 523 K. From theresults, even though the impregnated catalystshave much a lower metal loading, they stillcan exhibit a comparable hydrogen yield to thecommercial catalyst. The results also indicate

that the higher metal loading on the impregnatedcatalysts with urea could exhibit a high activityand be able to compete with the commercialone.

The lower metal loading and simplepreparation method of the impregnated catalystsin this study could provide another choice ofcatalyst for methanol reforming with a lowercost of catalyst production compared with theconventional catalysts. This study also showsthat the impregnated catalysts in this study couldexhibit activity of hydrogen production at as lowa temperature as 453 K which indicates thepossibility of lowering the reaction temperaturefor the methanol reforming process.

Catalyst Characterization

The CZ-catalyst, CZU-catalyst, andcommercial catalyst were characterized byscanning electron microscope with energydispersive spectrometer (SEM-EDS) and X-raydiffractometer (XRD).

Figures 1 and 2 shows the SEM-EDSimages of the CZ-catalyst and CZU-catalyst,

Figure 1. SEM image and EDS profiles of the impregnated Cu-Zn without urea over Al2O3

catalyst for methanol reforming

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Figure 2. SEM image and EDS profiles of the impregnated Cu-Zn with urea over Al2O3

catalyst for methanol reforming

Figure 3. SEM image and EDS profiles of the commercial catalyst for methanol reforming

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respectively. The images of these two typesof catalysts show that the Cu and Zn depositedtogether in small clusters on the Al2O3 surface.The EDS profiles indicate that the smallclusters are composed of Cu and Zn located onthe support which is Al. The SEM images alsoshow that the sizes of Cu-Zn clusters on thecatalysts with urea, CZU (H ≈ 0.5-1.5 µm), aremuch smaller than those of the catalysts withouturea, CZ (H ≈ 2-4 µm). The addition of ureacould improve the homogeneity of the Cu andZn composition in the impregnated solution andallow the formation of smaller metal cluster size.The smaller metal clusters with the same metalloading could provide more surface area ofthe active sites for the reaction. The highernumbers of active sites on the catalysts with urea,CZU, exhibit a higher activity of hydrogenproduction than that of the catalysts without urea,CZ, in both 10wt% and 20wt% metal loadings.

Figure 3 shows the SEM-EDS image ofthe commercial catalyst. The image shows thatthe metals are mixed well throughout the entirecatalyst without forming any separated metalclusters. The EDS profile shows that thecatalyst surface is composed of Cu, Zn, and Alspecies.

Figure 4 shows the SEM images ofcatalysts with and without urea at the same metalloading (20 wt%). The SEM images of 20CZ(Figure 4(a)) shows the agglomeration of a largeCu-Zn cluster (≈ 5-10 µm) over Al2O3. Thedistribution of metal clusters is not consistentthroughout the surface of Al2O3. The SEMimages of 20CZU1 (Figure 4(b)) shows smallerCu-Zn clusters (≈ 2-6 µm) and betterdispersion of Cu-Zn clusters over Al2O3 thanthat of 20CZ. The SEM images of 20CZU2(Figure 4(c)) clearly show the smallest Cu-Znclusters (≈ 0.2-1.0 µm) among the catalysts andalso the best distribution of Cu-Zn clustersthroughout the surface of Al2O3. The imagesimply better dispersion of Cu-Zn clusters andmore active sites on Al2O3 of the 20CZU2catalyst over other impregnated Cu-Zn catalysts.The SEM results also support the highesthydrogen yield from 20CZU2 among otherimpregnated Cu-Zn catalysts in this study, whichcould be explained by the smallest metal

clusters and the best metal dispersion.Figure 5 shows the X-ray diffractogram

of the impregnated catalysts with and withouturea and the commercial catalyst. The peak at2θ equal to 31.8, 34, and 36.2 degrees indicateszinc oxide (Zincite, JCPDS No.: 36-1451). Thecopper oxide (Tenorite, JCPDS No.: 45-0937)peaks were observed at 2θ equal to 35.8 and38.8 degrees. The diffractogram of the catalystswith and without urea exhibit a similar patternto that of the commercial catalyst which ensuresthe same phases of Cu-Zn active sites on thesecatalysts. Table 3 shows the estimated crystallizesize of CuO on the catalysts. Table 3 also showsthat the higher moles of urea used in thepreparation process yields the smaller crystallizesize of CuO on the catalysts. The smallercrystallize size of CuO with the same metalloading could provide more active sites of CuOfor the methanol reforming reaction whichsupports the results from the activity of thecatalysts for the hydrogen production.

Conclusions

The study shows that a higher metal loadingon the catalyst leads to a higher activity of thecatalyst which results in a higher hydrogen yieldfor the 20 wt% metal loading catalysts thanfor the 10 wt% metal loading catalysts. Theimpregnated Cu-Zn catalysts on Al2O3 with ureaexhibit a higher activity than those of thecatalysts without urea.

The commercial catalyst has the highesthydrogen yield but also contains much higherCu and Zn contents than the impregnatedcatalysts with and without urea. The hydrogenyield of the commercial catalyst is only about1.2 and 1.04 times higher while the metalloading is about 4 times higher than those ofthe 20CZU1 and 20CZU2 catalysts at 523 K.Although, the impregnated catalysts have amuch lower metal loading, they still can give acomparable hydrogen yield. The SEM-EDSimages show that the Cu-Zn is deposited as acluster over alumina. The SEM images also showthat the amount of urea used in the preparationprocess plays an important role in the sizeof metal clusters and the dispersion of metal

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Figure 5. X-ray diffractogram of the impregnated catalysts and the commercial catalystfor methanol reforming

Figure 4. SEM images of the impregnated Cu-Zn with and without urea over Al2O3 catalystfor methanol reforming, (a) 20CZ, (b) 20CZU1 and (c) 20CZU2

(a)

(b)

(c)

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clusters over Al2O3. The good dispersion of thesmall metal clusters over Al2O3 results in a higheractivity of hydrogen production. The crystallizesize of CuO determined via XRD could confirmthat the size of the CuO crystal depends onthe amount of urea used in the preparationprocess. The study also shows that the methanolreforming reaction could be carried out at as lowa temperature as 453 K which could save a lotof energy expenditure.

The production cost of the catalysts couldbe lower due to the ease of the preparationprocess and lower metal loading comparedwith the commercial catalyst. The proper metalloading of Cu-Zn with urea impregnated on thehigh surface area support could enhance thereaction activity and lower the operationtemperature. The results of this study could leadto the development of the novel catalyst forhydrogen production from the methanolreforming process.

Acknowledgments

The authors gratefully acknowledge Dr. SumittraCharojrochkul and Dr. Pimpa Limthongkul fromthe National Metal and Materials TechnologyCenter for providing the experimental facility.We also thank Dr. Paul Jerus for the commercialcatalyst and Thammasat University ResearchFund for financial support.

Sample Estimated crystallize size of CuO a

(nm)

10CZ 28

10CZU1 21

10CZU2 14

20CZ 29

20CZU1 22

20CZU2 14

Commercial 10

a determined by XRD using peak at 2-theta equal to 38.8°

Table 3. Estimated crystallize size of CuO

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EFFECTS OF PREPARATION OF Cu/Zn OVER Al O CATALYSTS … · 2017. 8. 20. · Cu-Zn based catalysts were prepared with urea (CZU) and without urea (CZ) followed by impregnation over