Journal of Natural Gas Chemistry 18(2009) 436–440

Monolithic macroporous catalysts — a new route forminiaturization of water-gas shift reactor

Hao Liang, Yuan Zhang, Yuan Liu∗Department of Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

[ Received April 2, 2009; Revised May 13, 2009; Available online November 9, 2009 ]

AbstractMonolithic macroporous Pt/CeO2/Al2O3 catalysts were prepared using concentrated emulsions synthesis route, and the obtained sampleswere characterized with SEM, TG, TEM, XRD and TPR techniques. These monolithic catalysts were applied to water gas shift (WGS)reaction in reformed gases. The SEM and TEM results indicated that the monoliths possessed macroporosity, and that the platinum particleshomogeneously dispersed on the supports with the particle size in the range of 1−2 nm. The reducibility of the catalysts was characterizedby TPR method, and it was shown that the monolithic PtOx/CeO2/Al2O3 exhibited the similar reducibility property to that of the particlePtOx/CeO2 reported in literatures. The CO conversion over the monolithic catalysts is higher than that over micro-reactor catalysts for WGSreaction in the reformed gases conditions, indicating that the monolithic macroporous catalysts is a potential new route for miniaturization ofWGS reactor.

Key wordsmacroporous; monolith; miniaturization; water gas shift; platinum

1. Introduction

Water-gas shift (WGS) reaction, as a key step for the purehydrogen production from hydrocarbons, has recently beenattracting growing interest rapidly due to fuel cell power sys-tem development, which is considered as a potential energysource [1,2]. Through WGS reaction, about 10–16 vol% COin the reformed gases can be reduced to below 1vol% and ex-cess H2 can be produced [3,4]. For effective production of H2,a fuel cell system needs to be installed in space-restricted ar-eas [5]. The key challenge for fuel cell oriented hydrogen pro-duction is the miniaturization of the hydrogen production re-actors. WGS reactor occupies almost two thirds of the volumein the hydrogen production system from hydrocarbons [6], sothe miniaturization of WGS reactor is of great significance.Lots of highly active catalysts have been reported, such as no-ble metal catalysts [3,7], Au catalysts [8,9] and base metalcatalysts [10]. Among these catalysts, Pt/CeO2 is a promisingcandidate for WGS reaction, as it is highly active and non-pyrophoric [11,12].

For the miniaturization ofWGS reactor, studies have beenmainly concentrated on general monolith catalysts and mi-crochannel reactor up to now. Microchannel reactor can re-

duce the size of conventional chemical reactor without lower-ing the throughput [12]. It was reported that forWGS reactionof fuel processor, compared with the conventional processingreactor, microchannel reactor can reduce the reactor size ofone or two orders of magnitude [13]. Fu et al. [11] studied thegeneral monolith (with channel size in about 1 mm) catalystsfor water gas shift reaction and suggested that a monolith-based design not only gave the required mechanical strengthbut also led to a better Pt utilization and thus smaller reactorvolumes, compared with a fixed bed configuration catalysts.However, the two types of catalysts mentioned above are stilltoo bulky to meet the application requirements.

The channel sizes of the general monolith and microchan-nel reactor are in the range of millimeter level and severalhundred microns (μm), respectively. The pore size of macro-porous materials is in the range from several tens of nm toseveral tens of μm. Hence we try to realize miniaturizationof hydrogen production system by preparing catalysts withmonolithic macroporous characters.

Based on considerations above, in this work, WGS wasselected as the example reaction, and Pt/CeO2 was selectedas the WGS catalysts. The possibility of the new route forminiaturization of the reactor was studied.

∗ Corresponding author. Tel: 022-87401675; E-mail: yuanliu@tju.edu.cnThis work is financial supported by the Ministry of Sciences and Technology of China (863 programs, No 2006AA05Z115 and 2007AA05Z104), and the

National Natural Science Foundation of China (No. 20976121)

Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(08)60138-3

Journal of Natural Gas Chemistry Vol. 18 No. 4 2009 437

2. Experimental

2.1. Preparation of monolithic polystyrene (PS) template

The polystyrene foams were obtained by polymerizationof styrene in highly concentrated water-in-oil (W/O) emul-sions. Both of the monomer, styrene (Tianjin Reagent Co,China) and the crosslink agent, divinylbenzene (DVB, AlfaAesar, UK) were washed with 0.2 mol·L−1 NaOH and thenwith deionized water. Styrene (2.0 g), DVB (0.5 g), AIBN(20 mg, Tianjin Reagent Co, China) and Span 80 (90 mg,Tianjin Reagent Co, China) were introduced into a flask toform a homogeneous phase. Then water was added drop-wisely into the homogeneous phase with a syringe at roomtemperature under stirring, until the volume fraction of thewater reached 0.82. Thus the highly concentrated water-in-oil (W/O) emulsion was generated. The emulsion was thenput into sealed glass molds with the temperature holding at60 ◦C for 24 h, during which polymerization of styrene wasproceeding. The wet polystyrene monoliths were firstly re-moved from the molds by carefully breaking the glass con-tainers, then washed with deionized water and ethanol to re-move surfactant species. At last, they were dried at 60 ◦C for24 h and polystyrene monolithic templates were obtained.

2.2. Preparation of macroporous monolithic alumina

Macro-porous alumina monoliths were prepared by im-bibing macro-porous monolithic polystyrene foams with alu-mina sols. The sols were prepared as follows. 4 g of pseudo-boehmite was put into 60 ml deionized water under stirring.After 1 h aging, 1 M HNO3 was dropped into the system witha ratio of n(Al3+) : n(H+) = 1 : 0.03. A translucent alu-mina sol was obtained by aging it for 12 h under stirring atroom temperature. The alumina sol was imbibed into thepolystyrene template pores under modest vacuum, and thethus coated templates were dried at 60 ◦C for 12 h. The coat-ing and drying procedures were repeated for several times.The coated templates were calcined in air at 600 ◦C for 4 hto obtain macroporous alumina monoliths. The heating ratewas 1 ◦C·min−1. Then the prepared macroporous aluminamonolith was calcined in air from 600 ◦C to 1300 ◦C and holdat 1300 ◦C for 2 h. The mechanical strength of the calcinedmacroporous alumina monoliths is good enough to be usedas supports. The calcined macroporous monolith of Al2O3was impregnated with alumina sols one more time in orderto increase the surface area of the macroporous Al2O3. Themonoliths after coating with alumina sols were dried at 80 ◦Cfor 8 h, and then calcined at 550 ◦C for 2 h.

2.3. Preparation of macroporous monolithic CeO2/Al2O3

Macroporous monolithic CeO2/Al2O3 was prepared byimpregnating the alumina monoliths with an aqueous solutionof Ce(NO3)3 for 12 h. After impregnation, the samples were

dried in air for 8 h, and then at 80 ◦C for 8 h. The sampleswere calcined in air at 550 ◦C for 3 h.

2.4. Preparation of macroporous monolithic Pt/CeO2/Al2O3

Macroporous monolithic Pt/CeO2/Al2O3 catalysts wereprepared by impregnating CeO2/Al2O3 monoliths with anaqueous solution of H2PtCl6 for 12 h. After impregnation, thesamples were dried in air for 8 h, and then at 80 ◦C for 8 h.The samples were calcined in air at 500 ◦C for 3 h. It is ab-breviated to Pt/Ce/Al in which contents of Pt and CeO2 were0.77wt% and 20wt%, respectively.

2.5. Characterization

Thermo gravimetric analysis was carried out on a Perkin-Elmer Pyris TG instrument at a dry-air atmosphere with aheating rate of 10 ◦C·min−1.

Photographs of the monoliths were taken by using anOlympus l7000 digital camera.

Scanning electron microscopy (SEM) tests were per-formed on a Philips XL-30ESEM microscope with Au-sputtered specimen operated at 15 kV to observe the macro-porous structures of the samples.

Transmission electron microscopy (TEM) pictures wereobtained on a Technai G2 F20 microscope operated at 200 kV.The well dispersed samples were deposited on a Cu grid cov-ered by a holey carbon film for measurements.

X-ray diffraction (XRD) patterns of the samples wererecorded on a Philip X’pert Pro diffractometer. Cobalt Kα

radiation (λ = 0.178901nm) was used with a power setting of40 kV and 40 mA.

Temperature programmed reduction (TPR) experimentswere carried out in a quartz reactor with a reduction gas mix-tures of 5vol%H2-Ar and at a heating rate of 10 ◦C·min−1.25 mg of samples were used and were pretreated in He at400 ◦C for 1 h in each run. TCD was used as the detector.

The mechanical strength of the samples were measuredusing Shimadzu DSS-25T universal testing machine fittedwith flat plates closing with a circular head at a speed of0.5 mm·min−1.

2.6. Catalytic performance test

Catalytic performance tests were carried out in a fixedtubular reactor operating at atmospheric pressure. In each run,a piece of monolith was put into a high temperature-resistantsilicone rube, and the silicone tube was connected with twoquartz tube at its both sides. The reaction temperature wasmonitored by a K-type thermocouple placed in the mono-lithic catalysts and controlled by a temperature controller.The reaction mixtures consisted of 3vol%CO, 8vol%CO2,24vol%H2, 24vol%H2O in N2. The mass space velocity was50000 ml·g−1·h−1. The reactant and product mixture wereanalyzed with a SP-2100 gas chromatograph (GC) equippedwith a thermal conductivity detector.

438 Hao Liang et al./ Journal of Natural Gas Chemistry Vol. 18 No. 4 2009

3. Results and discussion

Photo pictures of the macroporous monoliths are shownin Figure 1. Figure 1(a) shows the photograph of monolithicPS template with a cylindrical shape. Figure 1(b) shows thecorresponding monolithic Al2O3 smaller than PS template,mainly due to occurrence of shrinkage during calcination pro-cess. The corresponding monolithic CeO2/Al2O3, as shownin Figure 1(c), takes on yellowish color, which is due to theloading of CeO2. The shape of monolithic Pt/CeO2/Al2O3 isshown in Figure 1(d), and its black ash color comes from theexistence of Pt species.

Figure 1. The photographs of monoliths. (a) PS template, (b) Al2O3, (c)CeO2/Al2O3, (d) Pt/Ce/Al

Figure 2 is the TG curve of dried PS monolith after im-bibing alumina sols. The weight loss before 120 ◦C is due tothe evaporation of solvents such as ethanol and water. Theweight loss between 120∼270 ◦C is caused by the evapora-tion of structural water. There is a sharp weight loss in thetemperature range of 270∼380 ◦C, which is ascribed to thecombustion of PS template. The weight loss at the tempera-ture higher than 380 ◦C is attributed to the phase transforma-tion of γ-AlOOH to γ-Al2O3. As the temperature is higherthan 550 ◦C, no weight loss could be detected. The monolithAl2O3 in this work was calcined at 1300 ◦C. The mechanicalstrength of the monoliths is 3.08 MPa, which is good enoughfor catalyst support application.

The SEM images of the template and the monolithicoxides are shown in Figure 3. Figure 3(a) is a representative

Figure 2. TG curve of macroporous Al2O3

picture of polystyrene foam monolith. It is shown that themacropores are in spherical shape, in size of 5−50 μm. Thespherical macropores are interconnected via windows withthe diameters approximately 1−10 μm. The macroporeswere occupied by water during polymerization. After dry-ing, the water vaporized, leaving interconnected macropores.The microscopic appearance of macro-porous alumina mono-lith is shown in Figure 3(b), and macropores and pore win-dows become large, because of calcination at 550 ◦C. Fig-ure 3(c) shows the images of macroporous Pt/CeO2/Al2O3.The macroporous structure becomes larger than that of alu-mina monolith due to the calcination process of 1300 ◦C. TheCeO2 particles locate on the walls of macroporous Al2O3,which are undetectable in SEM, while can be seen from XRDpatterns (shown in Figure 4).

Figure 3. SEM images of monolith materials. (a) PS template, (b) Al2O3-550, (c) Pt/CeO2/Al2O3-1300

The preparationmethod of this work is flexible for prepar-ing macroporous monolith, and the diameter of the intercon-nected macropores can be controlled and adjusted by varyingamount of styrene, water or divinylbenzene.

Journal of Natural Gas Chemistry Vol. 18 No. 4 2009 439

Figure 4 presents the XRD patterns of macroporousmonolithic Al2O3 and Pt/CeO2/Al2O3. The sharp diffractionpeaks are corresponding to α-Al2O3. The characteristic peaksof CeO2 were marked in Figure 4(2). The average particlesize of CeO2 is 23.7 nm calculated by Scherrer calculator from(111) crystal plane. The diffraction peaks of ceria are muchweaker than that of the alumina, while the content of ceria inthe samples is 20% in weight. Thus it is speculated that partof ceria is highly dispersed on the alumina support which cannot be detected by the XRD.

Figure 4. XRD patterns of macroporous monolithic materials. (1) Al2O3-1300, (2) Pt/CeO2/Al2O3-1300

The platinum species on monolithic CeO2/Al2O3 cata-lysts can be seen from Figure 5. The Pt crystallites are blackparticles, homogeneously dispersed on support, with a narrowcrystallite size distribution of 1−2 nm, which are undetectablein XRD patterns.

Figure 5. TEM images showing the nano-crystallinity of monolithic catalyst

Figure 6 shows the TPR profile of macroporous mono-lithic Pt/CeO2/Al2O3. The reduction property of the sample isdirectly reflected by TPR profile. It is clearly shown that theexistences of four reduction peaks in the temperature rangefrom room temperature to 800 ◦C. The reduction peak of α

around 50 ◦C is ascribed to the reduction of PtOx species [14].This reduction peak appears in a broad shape, which shouldbe due to the hydrogen spillover. Both β1 and β2 peaks in thetemperature range of 200∼550 ◦C are attributed to the sur-face oxygen of CeO2 [15]. It is assumed that CeO2 particlessituate at two kinds of surroundings, one is interacted withplatinum species, the other is in interaction with alumina; β1is owing to the reduction of the former kind of ceria, and β2is the reduction of the latter. It is known that platinum canpromote the reduction of ceria. γ peak centered at 700 ◦C isdue to the reduction of bulk of CeO2. The TPR behavior ofPt/CeO2/Al2O3 is much like that of particle Pt/CeO2 catalystsas reported in literatures [14,15], indicating that the platinumspecies are supported on ceria other than on alumina.

Figure 6. TPR profile of monolithic Pt/CeO2/Al2O3

Micro-structured reactors appear as a highly promisingtechnology for miniaturization of hydrogen production sys-tem [14]. CO conversions over the catalyst in this workand a typical micro-channel reactor [16] are compared andlisted in Table 1. The catalyst of the micro-channel reactor isPt/CeO2/Al2O3 too. In both cases, the reaction mixtures arecomposed of simulated reformed gases and tested under thesame space velocity of 50000 ml·g−1cat ·h−1. The volume of thecatalyst (including its support) in the micro-structured reac-tor is 2.5 cm3, and the volume of the monolithic macroporouscatalyst is 0.13 cm3. The amount of CO converted on per vol-ume of catalysts in one minute over the two kinds of catalyticreactors is listed in Table 1.

As can be seen from Table 1, the amount of CO con-verted over per volume catalyst in one minute over monolithicmacroporous catalyst at the reaction temperature of 180 ◦Cis 1.18 ml, while no CO is converted over the micro-channelreactor. The amount of CO converted increases with the in-crease of reaction temperature. At 300 ◦C, the amount ofCO converted over the monolithic macroporous catalyst is6.02 ml, while it is 0.76 ml over the micro-channel reactor.The amount of CO converted over per volume catalyst in oneminute is almost one order of magnitude higher in the case ofthe monolithic macroporous catalyst than that of in the case of

440 Hao Liang et al./ Journal of Natural Gas Chemistry Vol. 18 No. 4 2009

the micro-channel reactor. This result suggests that the mono-lithic macroporous catalysts provides a potential route for theminiaturization of hydrogen production system.

Table 1. Comparison of monolithic macro-porous catalyst withmicro-channel reactor catalyst for CO conversion

CaCO(ml)Sample180 ◦C 210 ◦C 240 ◦C 270 ◦C 300 ◦C

Mono Ptb 1.18 1.62 2.26 3.21 6.02Micro Ptc 0 0.08 0.18 0.36 0.76a Cco is the amount of CO reacted on per volume of catalyst in oneminute calculated from Cco = (VCO×Xco)/Vcat, VCO is the flow rateof CO (ml·min−1), Xco is the CO conversion (%), Vcat is the volumeof the catalyst (ml);b Mono Pt is the abbreviation of macroporous monolithic Pt/Ce-Al/Al;c Micro Pt is the abbreviation of Pt/CeO2/Al2O3

4. Conclusions

Preparing catalysts with monolithic macroporous struc-ture is a potential route for the miniaturization of water gasshift reactor, and is likely a way for the miniaturization ofother hydrogen production reactors. In the monolithic macro-porous Pt/CeO2/Al2O3 catalysts, the macro-pore size was inthe range of 5−50 μm, platinum crystallites were in size of1∼2 nm and homogeneously dispersed on ceria surface. Themonolithic macro-porous Pt/CeO2/Al2O3 catalysts exhibitedvery good catalytic performance for water gas shift reaction.

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