Effects of preparation methods on the performance of Cu-Mo-Fe-Ox
in the hydrogen production from water
Si Chen1, Wei Chu1∗, Xu Liu1, Dongge Tong2∗1. College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China;
2. College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, Sichuan, China[Manuscript received February 25, 2011; revised April 11, 2011 ]
AbstractTwo Cu-Mo-Fe-Ox samples, which can store and supply pure hydrogen through repeated redox reaction (Fe3O4+4H2↔3Fe+4H2O), wereprepared by co-precipitation (FCM-C) and impregnation (FCM-I) methods, respectively, and the performance of hydrogen production fromwater were investigated. Compared with the impregnated sample, the co-precipitation sample presented better catalytic activity. The sampleswere characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy(XPS), Fourier transform infrared spectroscopy (FT-IR) and temperature-programmed reduction (H2-TPR) techniques. XRD, FE-SEM andXPS results suggest that the FCM-C sample has smaller particle size and higher dispersion of iron oxide than that of FCM-I sample. Inaddition, FT-IR and H2-TPR analyses indicate that the weak interaction among metal oxides in FCM-C sample may induce facile reduction ofactive metal and superior property of hydrogen production by decomposing water in succession.
Key wordscatalytic activity; hydrogen production; redox; water decomposition
1. Introduction
The global boosted environmental consciousness and theenergy crises have pressurized many countries to develop al-ternative energy resources in the past decades [1−3]. Hydro-gen owns favorable properties such as high energy densityvalues per mass, lightweight, great abundance without toxiceffects and environmental pollutants [4], and so on. There-fore, hydrogen is considered to be one of the classic substi-tutes for energy resources [5]. However, it is short of a safe,efficient and low-cost storage system, which is crucial to var-ious stationary and mobile applications [6] and subsequentlybecomes the main difficulty in its wide application.
Recently, Takenaka et al. [6−9] proposed a simple, safeand environmentally benign technology for the storage, trans-port and supply of H2 to polymer electrolyte fuel cell (PEFC)vehicles. The technology can obtain pure hydrogen via sim-ple redox reactions between Fe3O4 (initial state Fe2O3) andFe: Fe3O4+4H2→3Fe+4H2O (Step 1, chemical storage of H2)and 3Fe+4H2O→Fe3O4+4H2 (Step 2, H2 formed by decom-posing water) [5,10]. Meanwhile, the decomposition temper-
ature of water to produce H2 was below 300 ◦C. Soon af-ter, a lot of works were done. These works were mainly fo-cused on the modification of iron oxide by adding metal ox-ides [7], monometal, bimetal and even precious metals [8].Different preparation methods caused diverse roles of exoticmetal cation in FeOx, mainly in enhancing the rate of wa-ter decomposition or preventing the sintering of iron oxide.However, works in these areas are still scarce. In this study,the samples were prepared by different methods to resolve theabove problems.
2. Experimental
2.1. Preparation of samples
The sample prepared by co-precipitation method was de-noted as FCM-C. In a typical experiment, Fe(NO3)3·9H2O,Cu(NO3)2·3H2O and (NH4)6Mo7O24·4H2O (all purities of99%) were dissolved in deionized water. The amounts of Cuand Mo in the mixture were 3 and 8 mol% of total metal
∗ Corresponding author. Tel: +86-28-85403836; Fax: +86-28-85461108; E-mail: [email protected]; [email protected] work was supported by the National Basic Research Program of China (973 Program, 2011CB201202) of Ministry of Science and Technology of
554 Si Chen et al./ Journal of Natural Gas Chemistry Vol. 20 No. 5 2011
cations, respectively. The mixture was co-precipitated withammonia under constant magnetic stirring. The precipitatewas washed five times by deionized water, and then driedat 80 ◦C for 12 h. The dried sample was calcined at 300 ◦Cfor 4 h and subsequently at 500 ◦C for 8 h in air. The ironoxide sample without adding Cu and Mo was denoted asFe2O3-none-C.
During the preparation of sample via impregnationmethod, Fe2O3 powder (Fe2O3-none) was obtained by cal-cinning Fe(NO3)3·9H2O (purity of 99%) at 500 ◦C. Itwas directly impregnated in the aqueous solution containingCu(NO3)2·3H2O and (NH4)6Mo7O24·4H2O. The proportionof metal cations in the following process was the same as thatof co-precipitation. And the as-prepared sample was denotedas FCM-I.
2.2. Activity evaluation of the samples in the redox reaction
The performance of H2 production by the as-preparedsamples from decomposing H2O was evaluated by a gas flowsystem with a fixed bed. 50 mg sample was placed in themiddle of a quartz tubular reactor with 6 mm inner diameter.The total pressure of the gas flow system was maintained at0.1 MPa. The temperature of the fixed bed was monitored bya thermocouple.
First, hydrogen (50 mL·min−1) was introduced into thereactor at 100 ◦C. The temperature was raised from 100 ◦C to800 ◦C at a rate of 5 ◦C·min−1, and then the temperature washeld constant until no consumption of hydrogen was detectedby the on-line gas chromatograph.
Second, nitrogen was used to remove the remnant hydro-gen in the reactor and the temperature of the reactor cooledto room temperature naturally. Then, the water vapor was in-troduced with nitrogen (50 mL·min−1) into the reactor. Thetemperature increased from 100 ◦C to 600 ◦C at a rate of4 ◦C·min−1. The temperature was kept at 600 ◦C until noformed hydrogen was detected by the on-line gas chromatog-raphy [8].
2.3. Sample characterizations
The X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on a Philips X’pert MPDdiffractometer equipped with Cu Kα (40 kV, 40 mA) radi-
ation in the range of 20o−80o. The morphologies of thesamples were observed by a field-emission scanning electronmicroscope (FE-SEM; Inspect F, FEI Corporation). X-rayphotoelectron spectra (XPS) experiments were performedon the XSAM 800 spectrometer with an Al anode for Kα
(1486.6 eV) radiation. Temperature-programmed reduction(H2-TPR) was carried out in a quartz reactor under atmo-spheric pressure. Briefly, the as-prepared sample (50 mg) wasloaded in the middle of the quartz reactor tube, and the samplewas flushed with 5% H2/N2 gas (30 mL·min−1) at 100 ◦C for1 h. After the system was stabilized, the temperature of thereactor was increased from 100 ◦C to 800 ◦C with a rate of10 ◦C·min−1 by a temperature-programmed controller. Theamount of hydrogen consumption was analyzed by a ther-mal conductivity detector (TCD) and recorded by a computerwork-station. FT-IR spectra of the samples were recorded ona Bruker Fourier transform infrared spectrophotometer with awave number range of 4000−400 cm−1.
3. Results and discussion
3.1. Performance of hydrogen production
To observe the effects of preparation methods on the re-oxidation of the samples, the capacity of forming H2 for sam-ples in repeated redox cycles was investigated in our study.Studies indicated that vehicle powered by hydrogen fuel cellsneeded 5 kg hydrogen to run 500 km. The rate of reactionshould be higher than 143 μmol·min−1·g−1 if the speed of ve-hicle was 180 km·h−1. As shown in Table 1, three parameters,such as the temperature (defined the H2 production tempera-ture) of H2 formed (Step 2) at the rate of 143 μmol·min−1·g−1,H2 formed interval and the amount of H2 formed, were usedto evaluate the performance of the samples during cycling.
From Table 1, we could see that the H2 production tem-perature for FCM-C was lower than that of FCM-I during cy-cling. Meanwhile, the average amount of H2 formed in 4 cy-cles for FCM-C (3.84 wt%) was larger than that of FCM-I(2.42 wt%). It was possibly attributed to the longer intervalof H2 formed and the nearly equal rate of H2 formed at peaktemperature for FCM-C in each cycle (Table 1). However, itshould be pointed out that the average hydrogen storage ca-pacity for FCM-C was lower than its theoretical value, whichcould be ascribed to the conditions of the un-optimized addi-tional amount of Cu and Mo.
Table 1. Cycle performance of H2 production from water with FCM-I and FCM-C
Samples Cycles H2 production temperature (◦C) H2 production interval (min) Volume of H2 production (mL) Ratio of H2 to Fe2O3FCM-I 1 456 86 9.94 2.85
Journal of Natural Gas Chemistry Vol. 20 No. 5 2011 555
The kinetic curves for the samples in the first reoxidationare shown in Figure 1. It could be seen that the rate of H2production for FCM-C was higher than that of FCM-I. It alsoindicated that FCM-C was more active than FCM-I in the re-action of water decomposition.
Figure 1. Variations of H2 production rate versus temperature in reoxidationfor FCM-I and FCM-C in the first cycle
3.2. Crystal phase analysis of catalysts
XRD patterns of FCM-C and FCM-I samples are pre-sented in Figure 2. For both samples, no peaks of any otherphases or impurities were detected fromXRD patterns, exceptfor the crystalline phase of Fe2O3 (JCPDS 33-0664), indicat-ing that the formed copper and molybdenum particles wereamorphous [5,11,12,13]. Broad peaks in the XRD pattern ofFCM-C could be observed, which indicated that the crystallitesize of Fe2O3 in FCM-C was smaller than that in FCM-I.
Figure 2. XRD patterns of the FCM-I (1) and FCM-C (2) samples
3.3. XPS characterization of the samples
Table 2 shows the binding energies of iron, molybde-num and copper on the fresh catalysts surface. For FCM-C
and FCM-I samples, the binding energies of Fe 2p1/2 were724.7 and 724.6 eV, respectively, indicating the presence ofFe3+ species in both of these two catalysts. In comparisonwith FCM-I catalyst, the Fe 2p1/2 binding energy of FCM-Cslightly shifted to high energy region (0.1 eV) and the peakof Cu 2p1/2 shifted to higher binding energy by 3 eV, indi-cating that the synergic interaction between iron and copperoxides in FCM-C was improved. The iron content on the cat-alyst surface was calculated by semi-quantitative method. Re-sults showed that the ratio of Fe/(Fe+Mo+Cu) on the surfaceof FCM-C was about two times higher than that of FCM-Icatalyst, suggesting that most Fe2O3 particles were well dis-persed on the surface of the FCM-C catalyst.
Table 2. XPS characterization of the samples
Fe/(Fe+Mo+Cu) ratio Banding energy (eV)Catalystson catalyst surface Fe 2p1/2 Mo 3d3/2 Cu 2p1/2
TPR profiles of the samples are shown in Figure 3. ForFe2O3-none sample, three consumption peaks were observedat 515, 614 and above 800 ◦C. They were ascribed to the re-duction of Fe2O3→ Fe3O4→ FeO→ Fe [14,15]. For FCM-Isample, the first peak at 440 ◦C, which was lower than thatof Fe2O3-none sample, could be ascribed to the reduction ofCuO to Cu0 and Fe2O3 to Fe3O4. It could be concluded thatthe addition of copper significantly improved the reducibilityof iron oxide [16,17]. The second peak was possibly relatedto the reduction of Fe2(MoO4)3 to Fe and 2FeO·MoO2 to Mometals at above 800 ◦C [18]. The broader shoulder peak ofFCM-I at 612 ◦C in comparison with that of FCM-C indicatedthat the addition of Mo into Fe2O3 might inhibit the reductionof Fe2O3 to Fe metal. Al-Shihry et al. [18] also reported thatMo6+ ions could split Fe–O bonds whenMo was incorporatedinto iron oxide, which made iron more ionic and consequentlyless reducible.
Figure 3. H2-TPR profiles of the samples. (1) Fe2O3-none, (2) FCM-I, (3)Fe2O3-none-C, (4) FCM-C
556 Si Chen et al./ Journal of Natural Gas Chemistry Vol. 20 No. 5 2011
For Fe2O3-none-C sample, the peaks at 396 ◦C and648 ◦C were attributed to the reduction of Fe2O3 to Fe3O4and Fe2O3 to Fe via FeO [17], respectively. Compared withFe2O3-none, the lower shift of peak temperatures and theincreased peak areas suggested that the iron oxide speciesin Fe2O3-none-C were easily reduced by H2 and consumedmore H2.
For FCM-C sample, the peak appeared at 318 ◦C, whichcould be ascribed to the reduction of CuO to Cu0 and Fe2O3 toFe3O4 [17,19]. In comparison with FCM-I, the first reductionpeak dramatically decreased about 122 ◦C, and the peak areawas much bigger for FCM-C sample. It could be attributed tothe interaction between Fe and Cu, smaller metal oxides clus-ter and higher dispersed surface iron oxide particles in FCM-C. Meanwhile, the broad peak for Fe3O4 reduction slightlyshifted toward lower temperature, further indicating that the
existence of interaction among iron oxides caused easy reduc-tion of the metal oxides in FCM-C. These results were consis-tent with the XRD and XPS results. Thus, the facile reductionof iron oxide in FCM-C sample accounted for its higher hy-drogen production.
3.5. FE-SEM observation
SEM images of FCM-I and FCM-C samples are shown inFigure 4. It could be clearly identified those the grain sizes ofFCM-C were smaller than those of FCM-I, indicating that co-precipitation made the sample escape sintering of crystalliteduring high temperature calcination, and thus caused an im-provement of active components dispersion. It was generallyconsistent with the XRD results, in which the crystallite sizeof FCM-C was smaller than that of FCM-I.
Figure 4. SEM images of the samples FCM-I (a) and FCM-C (b)
3.6. FT-IR spectra
Figure 5 exhibits the FT-IR spectra in the range of1000−400 cm−1 for the samples. The extended distinct bandsat around 554 cm−1 and 475 cm−1 were assigned to vibra-tional modes of Fe–O bonds in Fe2O3 [18]. Compared withFe2O3-none, two new peaks in the spectra of FCM-I appearedat 964 cm−1 and 845 cm−1 which were ascribed to Fe-O-Movibrations in Fe2(MoO4)3 [18,20,21].
For FCM-C sample, a broad band with a shoulder peakwas observed in the range of 800−1000 cm−1 which couldbe ascribed to the tetrahedral Mo-O unit in Fe2(MoO4)3 [22].Furthermore, the peaks at 750−400 cm−1 were wider thanthose of Fe2O3-none-C, which could be attributed to the in-troduction of Cu element into the sample [11,23].
Undoubtedly, there were obvious differences of the FT-IRspectra between FCM-I and FCM-C samples. Xu et al. [11]reported that with the decrease of the crystallite size, the sur-face atoms and bulk atoms were in different chemical envi-ronment, which could cause the weakening or even disappear-ance of infrared absorption frequency. Therefore, the weakerbands in the spectrum of FCM-C indicated the weakened vi-brations in these bonds, and further suggested the weakly-
interacted metals species in the FCM-C sample. This resultsupported the XPS and TPR results, in which the reductionpeak at low temperature was attributed to the reduction ofweakly-interacted metals species.
Figure 5. FT-IR spectra of different samples. (1) Fe2O3-none, (2) FCM-I,(3) Fe2O3-none-C, (4) FCM-C
Journal of Natural Gas Chemistry Vol. 20 No. 5 2011 557
4. Conclusions
In this work, we synthesized Cu-Mo-Fe-Ox samples viaco-precipitation and impregnation methods, respectively. Theperformance of hydrogen production by decomposing waterwith the two samples is investigated. Compared with FCM-I,FCM-C sample shows better performance. It is possibly at-tributed to that FCM-C sample exhibits highly dispersed ac-tive sites and shows synergistic effect among the differentmetal oxides.
AcknowledgementsThis work was supported by the National Basic Research Pro-
gram of China (973 Program, 2011CB201202) of Ministry of Sci-ence and Technology of China (MOST). The authors gratefully ac-knowledge the TEM Group of Analytical Testing Center of SichuanUniversity. The useful discussions with Wang Ning, Deng Jie andZheng Jian are highly appreciated.
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