Effect of the transition metal oxide supports on hydrogen production from bio-ethanol reforming

Catalysis Today 194 (2012) 2–8

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Effect of the transition metal oxide supports on hydrogen production from bio-ethanol reforming Zhong He a , Min Yang a , Xianqin Wang a,∗ , Zhen Zhao b , Aijun Duan b a b

Chemical, Biological and Pharmaceutical Engineering Department, New Jersey Institute of Technology, 323 MLK Blvd., Newark, NJ 07102-1982, USA State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China

a r t i c l e

i n f o

Article history: Received 22 February 2012 Received in revised form 24 April 2012 Accepted 4 May 2012 Available online 14 June 2012 Keywords: Active sites Transition metal oxides Pt Oxygen vacancy Synergistic effect

a b s t r a c t We studied the effect of transition metal oxides on hydrogen production from bio-alcohols on a series of Pt catalysts supported on activated carbon, ZrO2 , TiO2 and CeO2 nanoparticles. These catalysts were synthesized with incipient wetness impregnation method and were characterized with CO pulse chemisorption, TPR, TPD, TPO, and synchrotron-based X-ray diffraction and X-ray absorption techniques. The activity of the catalysts for bio-alcohol reforming with steam/carbon ratio of 3/2 was tested using a fixed bed flow reactor system. Transition metal oxide supports affect the hydrogen production activity and selectivity significantly. The results indicate that the active sites for hydrogen production from reforming bio-alcohols are the sites where there exists strong synergistic interaction between Pt and oxygen vacancies in transition metal oxide nanoparticles. This interaction affects the electronic and structural properties of Pt and transition metal oxide nanoparticles. The higher activity and better stability with less coke formation are seen over the catalyst that has stronger Pt and oxygen vacancy interaction and weaker surface acidity. This work provides us insight to prepare a better catalytic system for bio-alcohol reforming process. Published by Elsevier B.V.

1. Introduction Currently, over 95% H2 is produced from steam reforming of natural gas or coal-gasification [1]. These processes require operations at high temperatures. Moreover, both processes are based on fossil fuel sources, contributing to net emission of CO2 [1]. In order to minimize the environmental impact, hydrogen produced from biomass resource has been a critical issue to the Department of Energy (DOE) in USA and abroad [2]. Considered as an effective means for renewable hydrogen production, steam reforming (SR) of biomass resources, has been studied as an on-board hydrogen feed for fuel cells [3,4]. Since ethanol can be produced in large quantities from biomass and offers several advantages related to natural availability, storage and safety in handling [5]. In fact, bio-alcohols are considered as a promising energy resource and are attracting increased attentions [6–10]. Different processes have been used to produce hydrogen from ethanol, including partial oxidation [10–14], oxidative steam reforming [6,15–20] and steam reforming [9,21–29]. Steam reforming of ethanol (SRE) is advantageous for its high efficiency among the processes. Oxides [8,28], nickel-based [7,25,27,29–32], cobaltbased [9,22,24,26] and noble-metal-based [12,13,21,24,33–35]

∗ Corresponding author. Tel.: +1 973 596 5707; fax: +1 973 596 8436. E-mail address: [email protected] (X. Wang). 0920-5861/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.cattod.2012.05.004

catalysts have been studied for steam reforming of ethanol. Nickelbased catalysts have been more extensively studied because of their high activity and low cost [22]. On the other hand, supported Pt catalysts showed high selectivity to H2 [36,37]. In the aqueous reforming of ethylene glycol, the rate of reforming decreased in the order of Pt, Ni > Ru > Rh, Pd > Ir, while hydrogen selectivity showed to follow Pd > Pt > Ni > Ru > Rh [2]. Overall, Pt was identified as the most promising catalyst and thus it was selected as the active metal in this study. Transition metal oxides have been chosen for various catalytic processes because varying oxidation states is possible in these oxides, making the oxidation state potentially a tunable parameter to control the selectivity [38]. The local oxidation state is expected to be affected by the defect sites, and will determine the chemisorption properties of the catalysts [39]. For example, Dumesic et al. investigated aqueous-phase reforming of 10 wt% ethylene glycol at temperatures of 483 and 498 K over Pt-black and Pt supported catalysts, and found the rate of H2 production decreased in the order of TiO2 > Al2 O3 , carbon, Ptblack > SiO2 –Al2 O3 , ZrO2 > CeO2 , ZnO, SiO2 [2]. CeO2 has been used both as catalyst support and promoter to improve the dispersion of active metal and promote the interaction between the support and the metal [40]. When Pt was supported on ceria, the highest ethanol conversions (+90%) and hydrogen selectivity (30%) were achieved [41]. Low coordination defect sites on transition metals, such as steps and kinks, as well as open “rough” crystal faces that make low

Z. He et al. / Catalysis Today 194 (2012) 2–8

coordination metal sites available have been uniquely active for breaking H H, C H, C C, C O, O O and N N bonds [42]. Oxidemetal interfaces provide highly active sites for reactions of C H and C O bonds. It has been demonstrated that in highly dispersed metal catalysts, the supporting oxide stabilizes the metal particles, but may also promote individual reaction steps. In particular, many reactions are believed to proceed, preferentially or exclusively, at the metal–oxide interface which contains sites with distances between metal atoms and support in the range of atomic dimensions [42]. The unique properties of metal–support borderline sites were first emphasized by Schwab [43] who also wrote a review on that topic [44]. Several examples from the independent experiments have also been summarized by Solymosi [45]. Boudart [46] termed the case of a support, modifying the electron density of small metal clusters by charge transfer or polarization, the ‘Schwab effect of the second kind. Ponec [47–49] considered the possible ways of additives affecting the catalytic behavior of supported metal particles by adding extra charge. These charge anisotropies cannot propagate through the metal as a consequence of a very effective screening by the metal electrons. Calculations by Smith et al. [50] showed that this screening will be, indeed, very effective: Creating an electron hole can hardly affect even the nearest neighbor. A similar situation has been found for the reverse (support) effect [51]. All the information above focused one important fact: the defect sites (holes, vacancies, kinks, steps, etc.) in the catalysts play critical roles in the catalyst activity and selectivity. In this work, we investigated the effect of transition metal oxides on hydrogen production from bio-alcohols on Pt nanoparticles. This work provided us insight to prepare a better catalytic system for bio-alcohol reforming process. 2. Experimental 2.1. Catalyst preparation Pt noble metal and CeO2 , ZrO2 , TiO2 and amorphous C were chosen as model systems. Activated carbon (Strem), CeO2 (Strem) and TiO2 (Degussa P25) were used as received. ZrO2 nanoparticles were synthesized with the recipes from Takahashi and Liang’s work [52,53]. Oxide-supported and carbon-supported Pt catalysts were prepared by incipient wetness impregnation technique. Aqueous solution of tetraamineplatinum(II) nitrate (Pt(NH3 )4 (NO3 )2 , Strem 99%) was added dropwise onto the supports until they were fully wet. Then the samples were dried in air at 100 ◦ C for 12 h (heating ramp of 2 ◦ C/min), followed by calcination at 260 ◦ C for 3 h (heating ramp of 2 ◦ C/min). For all samples, the amount of Pt was 1 wt%. The samples were refereed as Pt/CeO2 , Pt/TiO2 , Pt/ZrO2 and Pt/C. 2.2. Characterization CO chemisorption and BET surface area: The synthesized materials were characterized by CO chemisorption. Irreversible CO uptake measurements are used to titrate the surface metal atoms and to provide an estimate of the active sites on the catalysts. CO chemisorptions were measured in an AutoChem 2920 II (Micromeritics) equipped with a thermal conductivity detector (TCD) to measure CO consumption. 100 mg of sample was generally loaded in a quartz U-tube, and catalysts were pretreated in H2 at 623 K for 1 h, and then cooled to room temperature. To minimize or remove the contribution from the ceria support, CO chemisorptions were carried out at 273 K by soaking quartz tube in an ice bath. The carrier gas is helium at a rate of 50 ml/min. BET surface area measurement: BET surface area measurements are carried out on H2 reduced fresh samples. Adsorption at liquid nitrogen temperature was performed using a 30% N2 /He stream, and the desorption

3

area obtained after rapid heating was compared with the area of a calibrated volume. Temperature programmed reduction (TPR): Temperatureprogrammed reduction (TPR) was used to reduce PtOx species and activate the catalyst species. TPR was carried out in the same apparatus described previously for CO chemisorption measurements. The thermal conductivity detector was used to measure H2 consumption during TPR experiment. An ice water trap was used to remove moisture from the TPR effluent stream at 273 K before the TCD. 40–60 mg of sample was generally loaded in a quartz U-tube. The TPR was performed using 30 ml/min of 10 vol.% H2 /Ar from room temperature to 623 K at a heating rate of 10 K/min monitoring the thermal conductive detector (TCD) signals after removing any residual oxygen in a line by flowing He at room temperature for 10 min. Temperature programmed desorption (TPD): Ethanol TPD was used to investigate the surface activity and the types of active sites on the surface. NH3 TPD was to probe the surface acidity of the catalysts. Ethanol or NH3 vapor was vaporized and dosed to the reduced samples at room temperature till the sample was saturated. After the saturated catalyst was flushed in helium at the room temperature for half an hour, it was heated to 1073 K at a rate of 10 K/min in the helium (50 sccm). The desorbed species were detected using an on-line mass spectrometer (QMS 200, Stanford Research Systems). In case of NH3 , the thermal conductivity detector (TCD) was applied to measure NH3 consumption and an on-line mass spectrometer was used (QMS 200, Stanford Research Systems) to detect NH3 (e/m = 16, 17) and H2 O (e/m = 18) in exhaust gas lines. X-ray diffraction: X-ray diffraction (XRD) data was collected at beam line X7B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), Upton, NY. A MAR345 detector was used to record full X-ray patterns [54] and the powder rings were integrated with the FIT2D code [55]. The FIT2D parameters for the integration of the data were obtained with a standard LaB6 crystal compound. X-ray absorption near edge spectroscopy (XANES): XANES measurements (XAS) at the Pt L3-edge and Ce L3-edge were performed at the beam line X-18A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), Upton, NY. The storage ring was operated with the electron energy of 3 GeV; the ring current was in the range 180–300 mA. Spectrum of a Pt foil or ceria was collected simultaneously to provide energy calibration for each sample. Oxygen storage capacity (OSC): OSC measurements were conducted in an AutoChem 2920 II (Micromeritics). The samples were reduced in a flow of 10 vol.% H2 /Ar at 623 K for 1 h, then cooled down to 573 K, and held at this temperature with pulse of 10 vol.% O2 /He mixture till the sample was saturated during OSC analysis. Oxygen consumption was calculated based on the signals from the TCD detector. 2.3. Catalytic tests and products analysis Steam reforming of ethanol (SRE) was performed in a fixed-bed reactor with an inner diameter of 9.4 mm at the temperature of 573 K and the atmosphere pressure. 0.4 g catalyst was diluted by 24 g inert (SiC) to improve the reaction heat transfer, and loaded to the reactor. Before the reaction, the catalyst was firstly treated in hydrogen flow at 623 K following the same protocol as described for temperature programmed reduction session. After the pretreatment of the catalysts, a liquid feed of water premixed with ethanol (water/ethanol molar ratio = 3:1) at the rate of 0.1 ml/min was introduced by a liquid pump (Series I Metering HPLC Pump from Scientific Systems, Inc) to the reaction system. 60 sccm of N2 was used as the carrier gas. The liquid was mixed with N2 and then preheated to 473 K before going through the tubular reactor. The 1st

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Z. He et al. / Catalysis Today 194 (2012) 2–8

PtOx/CeO 2

a

PtOx/ZrO2

b

H2 (2amu)

H2 ((2 amu)) 427.18

300.66 174.14

232.05 164.05 H2O (18amu) (18amu)

CeO2

H2 (2amu)

100

200

300

400

500

o

Temperature p ( C))

H2O (18 amu)

204.81

ZrO2

H2O (18amu)

164.75

0

120.47

Inte ensity (a.u.)

Intensity (au.u)

252.05

H2 (2 amu)

H2O ((18 amu))

116.87

0

100

200

300

400

500

o

Temperatur p e ( C))

Fig. 1. Temperature programmed reduction profiles of (a) PtOx/CeO2 , CeO2 , (b) PtOx/ZrO2 , ZrO2 .

liquid sample was collected after the reaction was carried out for one and half hours. The liquids were drained every 1 h and then analyzed by a gas chromatograph (Agilent 7890A) with FID detector and a HP-5 capillary column (length 30 m, i.d. 0.32 mm, film 0.25 ␮m) through auto sampling injections, while gas was sampled every half an hour and analyzed by an online GC (Agilent 7890A) with a thermal conductivity detector (TCD) and a HP-MOLESIEVE capillary column (length 30 m, i.d. 0.32 mm, film 25 ␮m) through an automatic injection valve. The GC was controlled by Chemstation Rev. B.04.02. The main liquid products were identified and calibrated using their pure chemicals. 2.4. Temperature-programmed oxidation Temperature-programmed oxidation (TPO) was used to probe the amount of coke formation and their species. TPO was carried out after the catalyst was tested for the ethanol steam reforming reaction for 5 h. After the catalyst was tested for the reforming reaction, the liquid reactant was shut off and the carrier gas was left on only. The temperature was cooled down to room temperature. The system was purged for half hour to one hour till there was no carbonaceous or other species seen in mass spectrometer. Then, the helium carrier gas was switched to 10 vol.% O2 /He at 50 sccm, and the catalyst was heated to 1073 K at a rate of 10 K/min. CO2 and other possible product species were analyzed using a quadrupole mass spectrometer (QMS 200, Stanford Research Systems) and recorded by the on-line computer. 3. Results and discussion 3.1. Characterization results TPR experiments were conducted and the H2 and H2 O concentration profiles were presented in Fig. 1. No H2 consumption peak is observed on both ZrO2 and CeO2 , showing that neither CeO2 nor ZrO2 is reducible till 500 ◦ C. H2 O production peaks from both supports are from the water physically absorbed in CeO2 and ZrO2 . Comparing the pure supports and the catalyst samples, first H2 O peaks on both PtOx/CeO2 (164 ◦ C) and PtOx/ZrO2 (120 ◦ C) are from

the supports, and there are no hydrogen consumption peaks for the first water production signals. H2 consumption peaks on PtOx/CeO2 and PtOx/ZrO2 appeared at the temperatures 20–30 ◦ C lower than the second water peaks, indicating the water was from reduction of PtOx. Comparing H2 consumption peaks on PtOx/CeO2 (232 ◦ C) and on PtOx/ZrO2 (172 ◦ C), we can see that PtOx on CeO2 is more stable and requires a higher temperature to be reduced. This implied that PtOx interacts more strongly with CeO2 , compared with ZrO2 . Moreover, the H2 areas of PtOx/ZrO2 and PtOx/CeO2 are 1.31 × 10−4 and 2.85 × 10−4 respectively, showing that there is more reducible species on PtOx/CeO2 than on PtOx/ZrO2 . No H2 consumption is observed on PtOx/TiO2 and PtOx/C (data are not shown in the figure), showing that there is no much reducible species on both catalysts. It is well known that PtOx itself undergoes an autoreduction process during the calcination process. Thus, we believe that the interaction of PtOx between different supports followed the sequence of PtOx/CeO2 > PtOx/ZrO2 > PtOx/TiO2 ∼ PtOx/C. TPR results suggest that the support can influence the properties of the catalyst systems significantly. Table 1 summarizes the BET N2 physisorption and CO pulse chemisorption results, including surface area, mean particle size of the supports, metal dispersion and active particle size. CO uptakes are relative values because CO can adsorb on CeO2 , especially when metals are introduced [56]. In order to remove support contribution, we measured the CO uptakes on Pt/CeO2 at different temperatures from 298 K to 263 K. We found that the results from 273 K to 263 K are basically the same. Thus, we used this 273 K for the CO uptake measurements for all the samples. Ceria and zirconia are nano-scale, with CeO2 of 13 nm and ZrO2 of 11 nm. The particle size of TiO2 is 28 nm and is relatively large and it contains about 30% rutile titania. Amorphous activated carbon has the highest surface area of 637.6 m2 /g. However, the metal dispersions on all the supports are very similar, around 40 ± 5%. The Pt particle sizes calculated from the dispersions are around 2–3 nm for all the samples. The results are confirmed with XRD data. Fig. 2 shows the XRD patterns of the samples by converting all 2 angles into the values ˚ As shown, the corresponding to the Cu k␣1 radiation (␭ = 1.54 A). distinct diffraction patterns of the supports are observed in all the samples. Noticeably, Pt particles are too small to be detected with

Z. He et al. / Catalysis Today 194 (2012) 2–8

5

Table 1 BET surface area, mean particle size of supports, metal dispersion and Pt particle size. Sample

Surface area SBET (m2 /g)

Pt/CeO2 Pt/ZrO2 Pt/TiO2 Pt/C

64.7 99.7 54.5 637.6

± ± ± ±

1.0 6.3 4.1 67.0

Mean particle size* of supports DBET (nm)

Metal dispersion (%)

12.9 ± 0.2 10.8 ± 0.7 28.3 ± 2.1 –

44.5 38.5 35.7 39.9

XRD technique, even with synchrotron resource. This approves that Pt crystallite sizes are smaller than 2 nm, which are too small to be detected with XRD technique. Fig. 3 presents XANES spectra of the samples before the reduction. Treatment of the raw X-ray absorption data, including the alignment, background removal and normalization, was performed with Athena package [57]. X-ray absorption near-edge structure (XANES) was used to study the Pt electronic property change (Fig. 3) [58,59]. The intensity of the absorption edge, known as white line [60], is connected to electron transitions from the atomic 2p3/2 state to the 5d3/2 and 5d5/2 levels. An intense white line is observed if the absorbing atom has a large number of d-electron vacancies. Thus, the white line can be correlated to the oxidation state of platinum as the electron vacancies in the 5d levels reflect the oxidation state of the metal [61]. The intensity of the white line clearly indicated the oxidation state difference in Pt on the different supports. It follows the order of Pt/CeO2 > Pt/ZrO2 > Pt/C. This order implied that there are more oxidized Pt species on ceria, compared with other supports. This result is consistent with the reduction temperatures

Pt/C

Intensity

Pt/ZrO2

Pt/C O2 Pt/CeO

a

10

12

14

16

18

20

22

2.5 2.9 3.2 2.8

± ± ± ±

0.04 0.05 1.5 0.3

CeO2 C O2 L CeO2 S Pt/CeO2 L

2.0

1.5

24

2 Theta

Norm (E)

8

Pt particle size (nm)

0.9 2.1 2.3 3.5

and hydrogen consumption peak areas, implying that there exists a much stronger interaction between Pt and CeO2 , which prevents the auto-reduction of PtOx species. In the other hand, the Ce L3 edge XANES spectra in Fig. 4a show that there is a large amount of Ce3+ in Pt/CeO2 samples. XRD data in Fig. 2 shows that ceria is in cubic fcc structure, not Ce2 O3 which has a hexagonal hP5 structure [62–65]. The spent catalyst was also characterized, ceria fcc structure is maintained after reaction (Fig. 4b). But Ce L3 edge XANES spectra (Fig. 4a) clearly show the existence of Ce3+ . Due to the charge balance, two Ce3+ formations would be associated with one oxygen vacancy formation. In other words, Ce3+ and O vacancy are in the lattice of ceria fcc structures, which would introduce the stress and lattice parameter change in ceria fcc structure and result in the shift and asymmetric change in diffraction peaks [62–65]. Comparing pure ceria and Pt/CeO2 sample in Fig. 4a, Ce3+ intensity in Pt/CeO2 is much higher than that in pure ceria, indicating that Pt on ceria promotes Ce3+ formation, thus leads to the formation of oxygen vacancies. It is logical that the oxygen vacancy sites are intimately next to Pt atoms. Thus, a synergistic interaction (Pt ↔ Ovac) is clearly revealed in these samples [62–65]. Our oxygen storage capacity measurement not only confirmed the existence of oxygen vacancy, but estimated the amount of oxygen vacancy in the samples. The oxygen vacancy amounts in each sample varied significantly. We measured the OSC for all the catalysts, and the results are shown in Table 2. The OSC analysis demonstrates a high density of oxygen vacancies in CeO2 -based catalysts. Pt/CeO2 has the highest oxygen consumption during OSC

Pt/TiO2 6

± ± ± ±

1.0

0.5 Fig. 2. XRD patterns of the freshly reduced catalysts.

0.0 5700

5720

2.0 Pt foil f il Pt/C Pt/ZrO2

1.5

5760

5780

b Pt/CeO2 after ethanol reaction

Pt/CeO2

Pt/CeO2 before reduction

Intensity / a.u.

Norm (E)

5740

Energy / ev

1.0

Pt/CeO2 after reduction

0.5

0.0

5 11550

11560

11570

11580

11590

11600

10

15

20

25

2 Theta/degree

Energy /ev Fig. 3. Pt L3-edge XANES spectra of freshly prepared catalysts.

Fig. 4. (a) Ce L3-edge XANES spectra of freshly reduced Pt/CeO2 and CeO2 references. (b) XRD patterns of Pt/CeO2 samples.

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Z. He et al. / Catalysis Today 194 (2012) 2–8

Table 2 Oxygen storage capacity of the freshly reduced catalysts. Catalysts

Loading (g)

Oxygen storage capacity (␮mol O2 /g)

Oxygen storage capacity (␮mol O/g)

Pt/C Pt/TiO2 Pt/ZrO2 Pt/CeO2

0.1028 0.1035 0.1026 0.1032

0 17.3 11.4 252.5

0 34.6 22.8 505.0

analysis (505 ␮mol O/g), while Pt/ZrO2 and Pt/TiO2 have a much lower oxygen storage capacities, 22.8 and 34.6 ␮mol O/g respectively. No oxygen storage capacity is observed on Pt/C catalyst. The result again confirms the above results, Pt facilitates the oxygen vacancy formation in the transition metal oxide, but the Pt–support interaction strength varies between Pt and different supports: the better interaction between Pt and support, the higher oxygen storage capacity is obtained. 3.2. NH3 -TPD, ethanol-TPD

Pt/CeO2

Gas pressure (a.u.)

1.5x10 -6

Pt/CeO2 1.0x10 -6

Pt/TiO2 Pt/ZrO2

Pt/C

0.0 0

200

300

400 o

200

400

600

500

800

o

Temperature / C Fig. 6. H2 production curves from ethanol desorbed TPD on CeO2 and Pt/CeO2 .

Pt/C Pt/CeO2 Pt/TiO2 P /Z O2 Pt/ZrO2

100 100

CeO2

5.0x10 -7

H2 Production from Ethanol TPD

NH3 -TPD methods are capable of providing qualitative information of the total acid strength. The strength of acid sites corresponds to the temperature of the NH3 desorption peaks. Generally, the acid sites are classified into the weak (≤200 ◦ C), moderate (200–350 ◦ C) and strong (≥350 ◦ C) acid sites [66]. Fig. 5 shows the NH3 -TPD profiles of the catalysts in the temperature range from 50 to 500 ◦ C. Except that the Pt/C exhibits nearly a neutral property since there is only a weak peak observed at the temperature above 300 ◦ C, each of the metal oxide supported samples has one big and clear NH3 desorption peak. But the acid strength follows the order of Pt/CeO2 < Pt/ZrO2 < Pt/TiO2 . The small peak on Pt/C indicates NH3 mainly interacts with the catalyst supports instead of Pt. Due to the strong interaction between Pt and ceria support, the interaction between NH3 and ceria is possibly weakened. Thus the acid strength is observed to be weaker, compared with Pt/ZrO2 and Pt/TiO2 . Ethanol-TPD experiments are used to investigate the hydrogen production temperatures and reaction mechanism study. To highlight the hydrogen production activity and production temperature, the hydrogen production curves from ethanol TPD are presented. The curves here are not normalized by the dispersion and weight. Fig. 6 presents the hydrogen evolution curves from the ethanol TPD profile of CeO2 and Pt/CeO2 . Two distinct hydrogen production peaks are observed over both samples. Over Pt/CeO2 sample, the first H2 peak at low temperature (156 ◦ C) was suggested by de Lima et al. [67] as the decomposition product of adsorbed ethanol on the catalyst surface, which was further confirmed [68]. The second H2 peak at high temperature (376 ◦ C) is also from the

ethanol decomposition, and the H2 shoulder at 645 ◦ C could be attributed to the decomposition of the strongly bonded intermediate species. Compared with Pt/CeO2 sample, the hydrogen was produced at much high temperatures. Two main evolution peaks appeared at 300 ◦ C and 550 ◦ C. The hydrogen signal from CeO2 is less intensive compared with Pt/CeO2 . This result demonstrated that transition metal oxide nanoparticle alone is not a good catalyst for the reaction system. For comparison purpose, H2 evolution curves from all samples are presented in Fig. 7. Activity from Pt/C is negligible compared with that from other metal oxide supported catalysts, indicating that Pt alone is not a good catalyst either for the bio-alcohol reforming. Thus, neither Pt nor support alone can be the very good catalysts for the bio-alcohol reforming process. We observe that the hydrogen production temperatures are in the order of Pt/CeO2 < Pt/ZrO2 < Pt/TiO2 < Pt/C, which follows the exactly the same order of the reduction temperature decrease. The results implied that the hydrogen production temperature is closely related to the reduction temperature, thus the interaction between Pt and transition metal oxide supports. The detailed ethanol TPD

200

300

400

500

600

700

800

o

Temperature ( C)

Temperature ( C) Fig. 5. NH3 evolution curves from NH3 -TPD experiments.

Fig. 7. Hydrogen production curves from ethanol TPD experiments over Pt/C, Pt/CeO2 , Pt/TiO2 and Pt/ZrO2 .

Z. He et al. / Catalysis Today 194 (2012) 2–8

100

Eth anol conversion (%)

Pt/CeO2

80 Pt/ZrO2

60

Pt/TiO Pt/Ti O2

40

Pt/C

20

1

2

3

4

5

Time (hours) Fig. 8. Ethanol conversion as a function of time over the catalysts.

results from other supports and catalyst samples are not included in this paper and are to be published together with ethylene glycol work in a paper for the reaction mechanism study [69]. 3.3. Catalytic activity and stability Based on the ethanol TPD results, we chose 350 ◦ C to investigate the activity and stability of the catalysts in a fixed bed continuous flowing reactor. The catalysts were tested at 350 ◦ C under atmosphere pressure for 5 h with a steam/carbon ratio of 3/2. The conversion of the ethanol as a function of time obtained on the catalysts is shown in Fig. 8. Obviously, Pt/CeO2 has the highest ethanol conversion, while Pt/C and/or Pt/TiO2 shows the lowest ethanol conversion among the systems tested. The activity order follows the sequence of Pt/CeO2 > Pt/ZrO2 > Pt/TiO2 ∼ Pt/C. The sequence is nearly exactly the same as the order of the reduction temperature, or the Pt–support interaction, or the oxygen vacancy amounts in the supports. Excluding Pt/C, the higher activity is observed with the catalysts with weaker acid sites. After the activity tests, we conducted TPO measurements for all the samples and the CO2 evolution curves are shown in Fig. 9. No obvious CO2 evolution peak is observed over the Pt/CeO2 catalyst indicating that there is no stable coke formation on this catalyst surface after the reaction with the different water/ethanol ratios in the feed. However, both Pt/ZrO2 and Pt/TiO2 samples have big and distinct CO2 evolution peaks. The peak areas follow the opposite

CO2 pressure (torr)

Pt/TiO2

7

trend of the activity order. According to the literature [70], the deposited cokes may have different morphologies and reactivity, depending on the specific reaction conditions and structures of the catalyst. The two CO2 desorption peaks could assigned to active carbon species which were different from graphite-like carbon species. However, the desorption temperature suggested that the activity of these two carbon species were also different. Coke buildup might be one of the reasons causing the lower activity in Pt/ZrO2 and Pt/TiO2 catalysts compared with that in Pt/CeO2 catalyst. TPO experiment was not carried out on Pt/C since C support can be oxidized during TPO and would contribute to CO2 evolution signal. The coke amount in these series of samples is consistent with the coke formation in the conventional methane reforming process [71]. It is known that the strong acid sites promote the dehydration of ethanol and promote ethylene formation. Ethylene species are the precursor for the coke formation on the catalysts [72]. Overall, the results in this work demonstrate that there exists a synergistic interaction between Pt and transition metal oxide supports. This interaction plays an important role in the bio-alcohol reforming. Neither Pt nor transition metal oxide alone can be a good catalyst for the reforming process. The activity of ethanol conversion is closely related to the strength of the synergistic interaction. A better activity with less coke formation is achieved with the Pt/CeO2 that has stronger Pt–support interaction and weak acid sites. 4. Conclusion Results from different transition metal oxide nanoparticles indicated that the hydrogen production temperature and amount are closely related to the interaction strength between Pt and transition metal oxide nanoparticles. Based on the results, the active sites for hydrogen production from reforming alcohols are the sites where there exists strong synergistic interaction between Pt and oxygen vacancies. This interaction affects the electronic and structural properties of Pt and transition metal oxide nanoparticles. The stronger Pt–metal oxide nanoparticle interaction with weaker surface acidity leads to the higher H2 productivity and less coke formation. Acknowledgements This work was supported by Dr. Wang’s NJIT startup package. The XRD and XAFS work were performed at X7B and X18A beamlines in the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). We would like to thank Jonathan Hanson, José A. Rodriguez, Laura Barrio-Pliego, Gong Zhou, Michael Estrella, Syed Khalid, Nebojsa Marinkovic, and Qi Wang for the assistance in XRD and EXAFS data collection. The authors thank George Barnes and Shawn Yetman from the chemical, biological and pharmaceutical engineering at NJIT for their constant help during our lab setup. References

Pt/ZrO2 Pt/CeO2

100

200

300

400

500

600

700

o

Temperature ( C) Fig. 9. CO2 evolution curves from temperature-programmed oxidation experiments after the ethanol steam reforming activity tests.

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