Hydrogen production by autothermal reforming of

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 5 1 1 4 e5 1 2 4

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Hydrogen production by autothermal reforming of dodecane over strontium titanate based perovskite catalysts K. Hbaieb a,b,*, K.K.A. Rashid c, F. Kooli d a

Science and Technology Unit, First Floor, Room G-08, Saudi Arabia Mechanical Department, College of Engineering, Taibah University, P.O. Box 344, Al-Madinah Al-Munawwara, Saudi Arabia c Sud-Chemie India Ltd, Cochin, India d Department of Chemistry, Taibah University, P.O. Box 344, Al-Madinah Al-Munawwara, Saudi Arabia b

article info

abstract

Article history:

Yttrium strontium titanate (YST) based perovskites with different substitutions of Ti by Ni,

Received 9 September 2016

Co and Ru have been investigated for the autothermal reforming (ATR) of dodecane to

Received in revised form

produce hydrogen. The performance of these catalysts have been benchmarked against

14 November 2016

that of the Ni-Al2O3 commercial catalyst under varying temperature, steam to carbon and

Accepted 18 November 2016

oxygen to carbon ratios. Even though high liquid hourly space velocity (LHSV) has been

Available online 13 January 2017

used, all metal doped YST catalysts had shown relatively high hydrogen yield and performed as well as the commercial catalyst with consistently little lower performance for

Keywords:

the cobalt based catalyst. Though the undoped yttrium strontium titanate performed well,

Hydrogen

its resistance to carbon formation was relatively lower than that of all metal doped YST

Fuel reforming

catalysts. All catalysts were further tested for ATR activity under highly severe conditions,

Catalysis

at a gas hourly space velocity (GHSV) of 160,000 1/h and liquid hourly space velocity (LHSV)

Perovskite

of 18, using a fuel mixture of dodecane and xylene simulating sulfur-free jet fuel. The results showed stable performance for all catalysts with the activities following the order Ru-YST > Co-YST > Ni-YST > Ni-Al2O3. Ru-YST catalyst has shown the highest resistance to carbon formation and sintering. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen economy has been an ambition for many researchers for the last several decades. Many advantages are assured including sustainability, greenhouse gases mitigation and green environment. However, many setbacks have been

encountered in establishing such green economy. In particular, the problem of hydrogen storage, supply and transportation is unsettled. As an alternative, the development of an on-board reformer for the existing logistic commercial fuel stands out to be a valid intermediate solution. An effective onboard reformer requires an exceptional reforming catalyst.

* Corresponding author. Mechanical Department, College of Engineering, Taibah University, P.O. Box 344, Al-Madinah Al-Munawwara, Saudi Arabia. E-mail address: [email protected] (K. Hbaieb). http://dx.doi.org/10.1016/j.ijhydene.2016.11.127 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.


Traditional nickel based catalysts for steam e hydrocarbon reforming are susceptible to deactivation through nickel crystallite growth and carbon formation [1e3]. Morales-Cano et al. [4] claimed that incorporating the noble metals rhodium and iridium improved the resistance to sintering of Ni-Al2O3 catalyst, but there was no benefit when incorporating ruthenium noble metal. The mobility of nickel into the face centered cubic (FCC) lattice of rhodium and iridium caused the formation of NieRh and NieIr alloy particles whereas diffusion into ruthenium lattice structure is not energetically favorable [4]. Li et al. gave a comprehensive review on the enhancement of the resistance to oxidation, hot spot formation, and coke deposition of the nickel based catalysts under methane reforming when modified by incorporating small amounts of noble metals [5]. Nickel coupling with transitional metals has also been investigated. For example, the bimetallic NieCo and NieCu on different supports have shown enhancement in activity and resistance to carbon formation [6e13]. Other strategies for improving the coke resistance involved the addition of elements like potassium and gold to block the step sites on nickel catalysts responsible for graphite formation [14]. Doping Ni-Al2O3 catalyst with other carbon formation resistant elements has also shown some positive impact [15e18]. Nevertheless, the susceptibility of Ni-Al2O3 catalyst to carbon deposition remains considerable. As such the search for alternative catalysts is warranted. Recently much attention was drawn to perovskite based catalysts with the general formula ABO3. The key advantage of the perovskite catalyst is that the active catalytic elements are incorporated in the B-site. Thus, unlike in metal supported catalysts, the active phase is atomically dispersed. Active metals inserted on the B-site of the perovskite lattice structure have been shown to undergo reduction during catalytic reaction while maintaining good dispersion within the catalyst structure [19]. Liu and Krumpelt [19] used temperature programmed reduction (TPR) experiment to observe that ruthenium, when doped at the B site of a lanthanum strontium chromite calcined at 800 C, reduced from an oxidized to zero valence state at the surface. Such observation was absent when the calcination took place at 1200 C. That is, the lower the calcination temperature the more effective is the reduction of precious metals at the surface. The growth of reduced metals at the catalyst surface has a positive impact on the activity of the catalyst as more active sites would be available for the gas phase reaction. Valderrama et al. [20] characterized reduced La1x SrxCoO3 under TEM and observed well dispersed cobalt particles of size of the order 10e15 nm at the surface of the perovskite. Therefore, perovskite with active metals embedded at the B-site could act as precursor of well dispersed metal catalysts. The potential of inhibiting carbon formation due to the well dispersion of active metals in perovskite has been reported [20e22]. Mota et al. [21] explored the use of LaCo1xRuxO3 perovskite to produce hydrogen from diesel under oxidative reforming conditions. The catalyst, with high degree of ruthenium loading, showed lowest tendency towards carbonaceous deposition. Valderrama et al. [20] observed little formation of carbon nanotubes on the surface of La1x SrxCoO3 after 30 h of methane dry reforming at 800 C. The

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nanosize Co particles and their well dispersion at the surface are believed to be the reason for the high activity and resistance to carbon formation. Erri et al. [22] have shown that manipulation of the perovskite structure can be tailored towards inhibiting carbon formation. For example, doping LaFe1xNixO3 with ceria at the A-site at 40% significantly suppressed carbon formation. This has been attributed to improved oxygen ion conductivity that favored gasification reaction at catalyst surface. This observation was subsequent to an autothermal reforming of Jet-fuel surrogates at 775 C for 12 h at 130,000 1/h GHSV, Steam/Carbon (S/C) ¼ 3 and Oxygen/ Carbon (O/C) ¼ 0.364. Even though many perovskites have been successfully used for hydrocarbon reforming, the quest for new catalysts with potentially better performance is needed. Subject to similar conditions to reforming, strontium titanate based perovskites have shown good performance as anode for direct carbon solid oxide fuel cell [23e27]. This material is considered a good electrical conductor at operating temperatures and more interestingly showed high ionic conductivity after heat treatment under reductive atmosphere at low temperatures. Fergus [25] reported that such ionic conductivity can be further enhanced by replacing strontium with various lanthanides or yttrium at the A-site. The highest peak in conductivity among these materials is for Y0.08Sr0.88TiO3 (YST) [26]. The A-site deficiency in the latter material is intended to create oxygen vacancies to enhance ion conductivity. Such process has to be carefully controlled as high A-site deficiency may result in phase impurities. Ma et al. [27] recommended 2e6% A-site deficiency in YST to maintain phase purity yet benefit from the enhancement in ionic conductivity. Perovskite may constitute a platform for very small well dispersed active sites. That is, the metal elements initially incorporated on the B site of the perovskite structure may pop up to the surface and grow into tiny well dispersed particles when subject to reducing environment. Alternatively, perovskite may also replace the traditional oxide supports due to the excellent oxygen ionic conductivity providing lattice oxygen to the reforming reaction. Here the active metals are not incorporated into the B site of the perovskite structure but just loaded onto the perovskite surface (by impregnation). Sekine et al. [28e32] extensively studied the reforming performance of Ni and Co loaded onto perovskite support. When replacing the traditional Al2O3 support with the perovskite LaAlO3, they noticed that the usual formation of inactive carbon species onto Ni/Al2O3 was not detected on Ni/LaAlO3 when subject to methane steam formation [28]. Further partial substitution of La with Sr at 30% showed improvement in catalytic activity [29,30]. When reforming under toluene, Ni/La0.7Sr0.3O3 showed high activity and good resistance to carbon formation provided high surface area is obtained by lowering calcination temperature [31]. The decomposition of the fuel is promoted by the supported Ni metal, while the oxidation of the intermediates is facilitated by the lattice oxygen of the perovskite support and steam feeding [32]. The presence of perovskite lattice oxygen in/on the support structure helped reducing the coke deposition and improving catalyst performance. The aim of this paper is to explore the use of YST as reforming catalyst for dodecane to produce hydrogen.

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Dodecane is chosen as it closely mimics some of the specific properties of diesel [33]. Xylene is separately added to simulate the effect of aromatics on the catalyst performance. A-site deficiency at 4% is applied throughout. The effect of doping active metals at the B-site on the catalyst activity and characteristics is discussed.

Experimental Material preparation Hydrocarbon steam reforming catalyst of Sud-Chemie India Ltd, C11-9-02 (16 6 8 mm ring shaped), containing 15% NiO (12% Ni) and rest alumina, was used as a commercial reference sample for the purpose of comparative evaluation. The modified (solegel) Pechini method was used to synthesize the perovskite YST materials [34,35]. The starting precursors were titanium isopropoxide and metal nitrates. Titanium isopropoxide was dissolved in ethylene glycol in 1:12 M ratio. The solution was diluted with ethanol and heated at ~70e80 C. Citric acid was added slowly while stirring until a clear solution was obtained. Aqueous metal nitrate solution was subsequently added before pH was adjusted to 6e7. The resulting solution was left overnight for thorough mixing and solvent evaporation. After long time stirring, a gel is obtained. The gel was dried at elevated temperatures. Slow heating up to 400 C was conducted with dwells at various intermediate temperatures to avoid instability manifested by abnormal gel swelling. The dried gel was crushed into fine powder and homogenously mixed. The organic substances were removed upon heating at 600 C for one hour. The obtained powder was further mixed and brought into a furnace for a temperatureprogrammed calcination up to 800 C. The final product was yttrium strontium titanate doped with nickel, cobalt or ruthenium at 10%, Y0.08Sr0.88Ti0.9M0.1O3 (YSTM where M ¼ R for ruthenium, N for nickel, C for cobalt).

Characterization The catalyst crystallinity and phase structure were analyzed by X-ray diffractions (XRD), using Bruker D8 Advance diffractometer, equipped with Ni-filtered Cu-Ka radiation. The patterns were recorded in the 2q range of 10 e 90 . The specific surface area was measured by N2 adsorption isotherms, using a Micromeritics ASAP2020. The samples were degassed at 120 C overnight prior to measurements. Particle morphology and size distribution were investigated using Transmission electron microscope (TEM) micrographs obtained from JEOL model JEM1400. The thermogravimetric analyses (TGA) were conducted using a TA instrument, model SDT Q600. Raman spectra were recorded using Bruker 400 spectrometer.

Testing The activity experiments were conducted using a benchtop customized microreactor supplied by Hi-Tech (India). The reactor consists of 4 gas lines equipped with gas flow meters and 2/3 way valves. Water and fuel were introduced through 2 liquid lines by 2 HPLC pumps. All of the gas and liquid lines led

to an oven encompassing a reactor furnace. The oven may be adiabatically heated to a temperature up to 200 C. Once reactants entered the oven, they passed through spring mixers before and after they mixed together. Due to design constraint (space limitation within the oven) no separate pre-heater was provided. Instead pre-heating was conducted using the first zone of a 3-zone reactor furnace. Even though this is inconvenient and pre-heating may not be well controlled, compactness is commonly a necessity for an on-board reformer. The catalyst diluted with SiC particles was mounted in the middle of the tube. Between the mixing pre-heating zone and the catalyst, space was kept vacant to ensure fast access of reactants to catalyst after pre-heating and thus avoiding pre-combustion of fuel. Inside the reactor tube a hollow thermowell was inserted enclosing thermocouples placed right on top and bottom of the catalyst zone to ensure uniform temperature distribution within the catalyst zone. A gas liquid separator with chiller side connections was attached downstream for water and heavy fuel products condensation. The gases were either vented out or directed to a gas chromatograph (Agilent 7890A Series GC) equipped with thermal conductivity and flame ionization detectors.

Results and discussion Catalyst activity Evaluation at different operating temperatures Autothermal reforming activity tests of catalysts were carried out using the heavy liquid hydrocarbon fuel, dodecane. Dodecane is chosen as it closely simulates the physical and chemical properties of diesel or jet fuel. High liquid hourly space velocity of 4 was chosen in the hope of distinguishing the best performing catalyst from the less performing ones. The effect of reaction temperatures on the activity of the different catalysts under autothermal conditions were studied for steam to carbon ratio (S/C) ¼ 3 and oxygen to carbon ratio (O/C) ¼ 0.31. Every experiment is run for 6 h which allowed taking nine injections at the gas chromatograph. Nitrogen is used as a carrier gas. Product gas concentrations at dry basis and free of nitrogen (not shown) are nearly constant throughout which indicates that the catalysts are stable for the time period tested. Hydrogen yield expressed as hydrogen moles to fuel moles ratio at different reaction temperatures and for the different catalysts is shown in Fig. 1. The results are surprisingly similar irrespective of the reaction temperatures and the catalyst type. However, YSTN and YSTR are consistently slightly better than YSTC and do sometimes outperform Ni-Al2O3 catalyst although by slight amount. The undoped YST catalyst has performed unexpectedly well and showed only slightly lower activity than the commercial and doped YST catalysts. Earlier studies showed that yttrium, alone or with alkaline earth metals like Ca or Sr, is good for the low temperature steam reforming and auto-thermal reforming of alcohols [36].

Effect of varying steam to carbon and oxygen to carbon ratio The effect of varying the oxygen to carbon ratio on the catalytic activity of the YSTM (M ¼ Co, Ni, Ru) catalysts has been

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25

30 Ni-Al2O3

YST

YSTN

YSTR

YSTC

20

20

H2 Yield

H2 Yield [H2Mol/FuelMol]

25

15 10

15 Ni-Al O YSTC YSTN YSTR

10

5 5

0 700

750

800

900

Temperature (oC)

0 1.0

Fig. 1 e H2 yields for all catalysts under autothermal reforming of dodecane under varying temperatures.

1.5

2.0

2.5

3.

Steam to Carbon Ratio

Fig. 3 e Hydrogen yields of Ni-Al2O3 and YST catalysts after tests under varying S/C ratios. studied at an invariable steam to carbon ratio of S/C ¼ 3. The reaction temperature was consistently set at 750 C, while conditions of steam reforming (O/C ¼ 0), autothermal reforming (O/C ¼ 0.31) and oxidative reforming (O/C ¼ 0.5) have been applied sequentially, each for 6 h. Fig. 2 shows the effect of varying O/C ratio on the activity of the 3 different YSTM catalysts and the commercial Ni-Al2O3 catalyst. All catalysts increased in activity with decreasing O/C ratio. The highest activity was under steam reforming. Fig. 2 shows that the activity of YSTC is lower than that of all other catalysts. Even though ruthenium is considered catalytically very active, nickel based catalyst is performing as well as ruthenium based catalyst and sometimes even better. The effect of varying steam to carbon (S/C) ratio on the activity of the catalysts is shown in Fig. 3. The tests were run under autothermal reforming with O/C ¼ 0.31. The activity increased with increasing S/C ratio. YSTC catalyst has lowest performance. The differences in the activity of the catalysts were not large. The ruthenium doped catalyst performed well under S/C ¼ 2 and only slight increase is observed when

increasing S/C to 3. In contrary, increasing S/C to 3 for the other catalysts clearly resulted in higher activity.

Effect of testing at high LHSV in the presence of aromatics The difference in performance of the various catalysts, as shown in Fig. 1, is quite small and could just be a usual manifestation of experimental errors. As such we cannot conclusively distinguish the top performing catalyst from the rest. Anticipating that increasing the LHSV would expedite the deactivation conditions and expose the least performing catalyst, LHSV was sharply increased from 4 to 18. Moreover xylene was added to dodecane at 23 volume % as a representative aromatic component. The latter fuel composition is the recommended blend of a JP-8 surrogate by a SERDP development project [37e39]. Several ATR experiments at 750 C were conducted using 0.1 g of catalyst for at least 20 h. The same ATR experimental conditions described for the previous tests were employed with the exception that

35 25

30 20

20

H2 Yield

H2 Yield

25

15

10

Ni-Al O YSTC YSTN YSTR

10

15

Ni-Al O YSTR YSTN YSTC

5

5

0

0

0.0

0.1

0.2

0.3

0.4

0.

Oxygen to Carbon Ratio

Fig. 2 e Hydrogen yields of Ni-Al2O3 and YST catalysts after tests under varying O/C ratios.

0

5

10

15

20

time (hours)

Fig. 4 e Hydrogen yields of Ni-Al2O3 and YST catalysts after autothermal runs under dodecane þ xylene fuel mixture.


LHSV was set very high. It is startling that all catalysts showed excellent stability and high activity as shown in Fig. 4. YSTR clearly outperformed all other catalysts. Unexpectedly, Ni-Al2O3 showed least performance. The performance of YSTC is higher than YSTN and lower than YSTR. The order of the performance of the catalysts is: YSTR > YSTC > YSTN > Ni-Al2O3. It was shown that YSTC has lowest performance under changing temperature and varying S/C and O/C ratios. It was pointed out that the drop in activity of YSTC as compared to other catalysts is only slight and may not be representative. Extra severity due to the increase of LHSV to 18 in the presence of aromatics might have been the reason for the differences in the observed trends in performance. According to many studies reported elsewhere cobalt has better activity than nickel which agrees well with our observations under extremely severe operating conditions [40e45].

Characterization of catalysts Transmission electron microscopy (TEM) Fresh catalysts were characterized for morphology, specific surface area and phase structure and purity before activity testing. The specific area of all catalysts was measured to be in the range of 10e20 m2/g. Fig. 5 is TEM micrograph of the inhouse prepared ruthenium doped YST catalyst, YSTR. As shown, particles are nano in size with a wide size distribution. Particles of 20 nm diameter or smaller are shown.

X-ray diffraction (XRD) The crystalline structure of the catalysts was determined by applying the X-Ray Diffraction (XRD) technique. XRD patterns of the commercial Ni-Al2O3 and the prepared YSTM catalysts are given in Fig. 6. The Scherrer equation was employed to determine the particle size of the different phases based on their most intense diffraction peaks. The diffraction pattern was identified by referencing to those of known structures in the Joint Committee of Powder Diffraction Standards (JCPDS) database. The observed peaks for the Ni-Al2O3 catalyst at 2q around 43.3 and 62.7 could be attributed to the presence of NiO. The

Fig. 5 e TEM micrograph of a typical YSTM powder.

Ni-Al2O3

Intensity (a.u.)

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YSTR YSTC

YSTN YST 20

30

40

50

60

70

80

2 theta

Fig. 6 e XRD pattern of a fresh Ni-Al2O3 and YST(M) catalysts.

peak observed at 28 could be either due to the presence of NiO or q alumina. Intensities of peaks expected at 2q around 19 and 46 , attributed to crystalline nickel aluminate, are much lesser compared to that of NiO thus indicating chances of more NiO being well dispersed on the surface of the catalyst. Other characteristic peaks, for nickel aluminate spinel, at 2q around 19 and 59.6 are too tiny and therefore nickel aluminate spinel formation is much less on the catalyst. The peak observed at 31.4 could be attributed to the presence of a-alumina. Characteristic peaks for a-alumina, nickel aluminate and NiO are overlapped at 2q around 37 [46]. Yttrium doped strontium titanate, YST (Fig. 6) showed the characteristic XRD peak at 2q of 32.4 . YSTN and YSTC exhibited similar sharp intense peak at the same 2q value. This peak was found to be shifted to 32.55 for YSTR. Such shift towards higher angle cannot be correlated with the variations in ionic radii of doped elements as reported elsewhere [47]. Crystallite size was found to increase more or less at the same level upon doping YST with Co/Ni/Ru. All YSTs showed XRD peaks at 2q values around 23, 32, 40, 46, 52, 58, 68 and 77 , well in agreement with the patterns reported in literature for strontium titanate [48,49]. Further the small peak at 25 for doped YST supports the presence of free Sr or Ti compounds especially for YSTC. The phase crystallinity of all spent catalysts, after ATR tests under varying temperatures, was maintained as shown in Fig. 7. The peak observed in XRD of YSTN catalyst at 2q ¼ 25.75 could be attributed to the presence of carbon for which the crystallite size was found to be 83.5 A. No carbon peak was observed with other YST catalysts (may be too low to get detected in XRD). The peak of carbon in YSTN is extended over a broad range and therefore could be considered as mostly amorphous. Amorphous carbon can be easily removed upon regeneration of the used catalysts. Table 1 gives the crystallite sizes of fresh and used catalysts. The crystallite size A. Such a low crystallite size is of NiO in Ni-Al2O3 is only ~100 favorable for dispersion, activity and further for the carbon removal and regeneration of the catalyst, but more vulnerable to sintering and grain coarsening. Unlike YST catalysts that

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Table 2 e XRD crystalline size/growth after S/C and O/C variation tests.

Intensity (a.u.)

XRD Peak max (2q) YSTM/NiO/Ni peak Crystallite size (nm) YSTM/NiO/Ni (fresh) Crystallite size (nm) YSTM/NiO/Ni (Used) Crystal growth (%) Crystallite size (nm) carbon (after ATR)

YSTC YSTR YSTN

YSTN

YSTR

YSTC

Ni-Al2O3

32.32

32.35

43.2

32.35

27.26

27.03

28.8

10.48

27.01

34.57

30.87

12.06

7.2 5.2

15.1 6.3

e 28 No peak at 2q ~26

YST Ni-Al2O3 30

40

50

60

70

80

2 theta

Table 3 e XRD crystalline size/growth after activity tests under dodecane þ xylene mixture. XRD

Fig. 7 e XRD of used catalysts after autothermal runs under varying temperatures.

showed good resistance to sintering, Ni-Al2O3 catalyst underwent considerable crystal growth which is detrimental for catalyst stability and durability. The temperature rise up to 900 C might have a major contribution to such abnormal growth; thus it is recommended to maintain temperature of operation as low as possible. The XRD of the used catalysts (not shown) tested under varying S/C and O/C ratios showed the same crystal structure as that of the fresh catalysts. The crystallite size of carbon calculated from XRD for the peak at 2q of 25.75 was found to be 63 and 52 A for YSTR and Ni-Al2O3, respectively, while no peaks for carbon were observed for the other catalysts. All catalysts showed resistance to crystal growth with the highest growth of 28% recorded for YSTR (see Table 2). The XRD of the used catalysts tested under dodecaneexylene mixture (not shown) showed stable crystal structure throughout. The crystallite size of carbon calculated from XRD for the peak at 2q of 25.75 was found to be 38.9 A for Ni-Al2O3. No carbon peak was observed for all YST catalysts. Table 3 again shows large crystal growth for NiAl2O3 catalyst and excellent resistance to sintering for all YST catalysts.

Peak max (2q) YSTM/NiO/Ni peak (fresh) Crystallite size (nm) YSTM/NiO/Ni (fresh) Peak max (2q) YSTM/NiO/Ni peak (after test with xylene) Crystallite size (nm) YSTM/NiO/Ni(after test with xylene) Crystal growth (%) Crystallite size (nm) carbon (after test with xylene)

Ni-Al2O3

YSTN

YSTC

YSTR

43.26

32.38

32.4

32.55

10.48

27.26

28.8

27.03

51.5

31.72

32.43

32.04

18.43

29.76

34.58

30.83

76 3.89

9.2 20 14 No peak detected at 2q ~26

Intensity (a.u.)

20

Ni-Al2O3

YSTR YSTC YSTN

Raman spectra The laser Raman spectra for fresh catalysts, Ni-Al2O3 and doped YSTs are presented in Fig. 8 in the range 50e3500 cm1. The Raman band observed at 560 cm1 could be attributed to poorly crystalline or amorphous NiO phase. Similar behavior is reported earlier for the supported nickel catalysts [50].

0

1000

2000

3000 -1

wavenumber (cm )

Fig. 8 e Raman spectra of fresh Ni-Al2O3 and YST(M) catalysts.

Table 1 e XRD crystalline size/growth after temperature variation activity tests. XRD Peak max (2q) YSTM/NiO/Ni peak Crystallite size (nm) YSTM/NiO/Ni (fresh) Crystallite size (nm) YSTM/NiO/Ni (Used) Crystal growth (%) Crystallite size (nm) carbon (after ATR)

YST

YSTN

32.18 23.36 22.15 e No peak at 2q ~26

32.43 27.26 28 2.7 8.35

YSTR

YSTC

32.61 32.37 27.03 28.8 32.03 37.57 31 30.4 No peak detected at 2q ~26

Ni-Al2O3 51.84 10.48 18.45 76

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Strontium titanate is expected to have Raman bands in the second order active mode at 81 cm1, 250e350 cm1 region, 630e727 cm1 region, 1038 cm1, 1325 cm1 and 1618 cm1 [51]. The bands observed at 81, 770 and 1570 are prominent. Raman shift at 1315 is visible as a broad tiny peak only in the case of YSTN and it is not evident in the case of YSTR and YSTC. The most dominant band at 770 in all YSTs is assigned to the transverse optic mode TO2 þ TO4. Some minor deviation from standard strontium titanate spectrum may be due to some structural distortion upon metal doping. However such a distortion is not evident from XRD studies. In laser Raman spectroscopy of the spent catalysts, two intense peaks are of interest: The D band corresponding to a wavelength of 1350 cm1 associated with structure disorder of carbon and the G band at 1580 cm1 associated with carbon atom vibrations in their hexagonal sheets. Moreover, the intensity ratio of both bands (R ¼ ID/IG) reflects the graphitization degree of the deposited carbon. Fig. 9 shows the Raman spectra of the spent catalysts tested for ATR under varying temperatures accompanied by those of fresh catalysts for

comparison. The D and G peaks for Ni-Al2O3 catalyst are very small, which is an indication for the low carbon formation. For both YSTN and YSTR, no D peaks were observed and the G peaks were small and broad. A tiny G peak was also observed for the fresh catalysts. Thus even though the carbon deposited is mostly of graphitic type, only low percentage should be present as suggested by TGA (Fig. 12). For both YST and YSTC, broad G and D peaks are apparent. The G peaks are shouldered at lower frequencies and smaller than the D peaks which imply that the graphitization degree of formed carbon is low [52]. The ID/IG ratios are 1.5 and 1 for YST and YSTC, respectively. The laser Raman spectra of spent catalysts after testing at various S/C and O/C showed no peaks for Ni-Al2O3 catalyst in the range of 1200 cm1e1800 cm1 (Fig. 10) potentially because no carbon is deposited on the catalyst. No peaks are observed for YSTR that in turn reflects high resistance to carbon formation. The Raman spectra for both YSTN and YSTC showed D and G peaks that are quite broad especially for YSTN. The graphitization degree is more prominent the lower is the ID/IG ratio. The latter is estimated to be 0.8 for YSTC and 0.94 for YSTN, an indication for sizeable disorder in the deposited carbon. The graphitization extent of the deposited carbon is not manifested in a peak on XRD pattern but well caught by the Raman spectra. It is well proven that Laser Raman microprobe spectroscopy is a powerful technique for identifying carbon deposits on catalyst surfaces [53,54]. Raman spectra of spent catalysts after testing under dodecaneexylene mixture are shown in Fig. 11. YSTR and YSTN showed very broad and small D and G peaks. Ni-Al2O3 showed peaks at 1310 cm1 and 1570 cm1. The ratio ID/IG is estimated to be 1.05, 1.44 and 1.03 for Ni-Al2O3, YSTN and YSTR, respectively. Even though YSTN underwent larger carbon formation (see Fig. 14 for TGA) as compared to Ni-Al2O3 and YSTR, its ID/IG ratio is largest with the implication that graphitization degree of the deposited carbon is low. The YSTC catalyst, unlike the other catalysts, showed relatively sharp peak at 1560 cm1 and unobservable tiny peak at 1320 cm1. This reflects high graphitization degree of the deposited carbon.

Intensity (a.u.)

YSTC

Ni-Al2O3

YSTN

YSTR

1200

Fig. 9 e Raman spectra of (a) fresh catalyst and (b) used catalysts after autothermal runs under varying temperatures.

1300

1400

1500

1600

1700

1800

wavenumber (cm-1)

Fig. 10 e Raman spectra of used catalysts after tests under varying O/C and S/C ratios.


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Intensity (a.u.)

YSTC

YSTN

YSTR

Ni-Al O

1200

1300

1400

1500

1600

1700

1800

wavenumber (cm-1)

Fig. 11 e Raman spectra of used catalysts after tests under dodecane þ xylene fuel mixture.

Thermogravimetric analysis TGA is used to investigate the extent of carbon deposition on the catalyst. Such analysis measures the weight loss of the catalyst while increasing temperature. Weight loss due to moisture is usually observed at temperatures below 200 C, although the presence of chemisorbed or coordinated water molecules may persist at higher temperatures. As the spent catalyst is mostly in oxide form and carbon is organic, the measured weight loss at higher temperature could be attributed to carbon burn out. It usually occurs at two temperature ranges: the first is the low temperature range from ~300 to 600 C where amorphous carbon is removed. This type of carbon is not critical as it can be easily removed during catalyst regeneration. A more critical type is the graphitic carbon that is removed at high temperature (>600 C) [55]. Such crystalline carbon persists at the catalyst surface during regeneration at moderate conditions, particularly at temperatures at which the desired structure and texture is well maintained. Thermograms of the fresh catalysts (Fig. 12a) indicate considerable weight loss for the Ni-Al2O3 catalyst: 1.7% at 215 C and 3.2% between 215 and 700 C. This could be mainly due to the presence of coordinated, physisorbed and chemisorbed water molecules which will get completely dehydrated only at higher temperatures [56,57]. Commercial NiAl2O3 catalyst having alumina around 85% is more prone to rehydration and hence initial removal of moisture is recommended before reduction and further reaction. A small contribution to this weight loss could probably be attributed to the removal of small amounts of impurities which is being observed as tiny G and D bands in the Raman spectrum of fresh Ni-Al2O3 catalyst (Fig. 9). YST and metal doped catalysts had shown weight loss less than 1% each in the temperature region ambient to 200 C and 200e700 C. Prominent peaks for amorphous or graphitic carbon are not noticed in the Raman spectra of the unused catalysts and hence the observed weight loss could be mainly attributed to the presence of physisorbed and chemisorbed water molecules. However tiny broad G band peaks indicate the presence of very small

Fig. 12 e TGA results of (a) fresh and (b) used catalysts after autothermal runs under varying temperature.

amounts of impurities. These peaks are much lower than those observed for the spent catalysts. It is difficult to make a comparison for the thermogravimetric curves between fresh and used catalysts as the severity of thermal, hydrothermal and reduction/reaction conditions experienced by these two are entirely different. Thermo-gravimetric analysis (TGA) curves of spent catalysts tested for ATR under varying temperatures showed initial drop at low temperature for all catalysts (Fig. 12b). This drop is attributed to moisture evaporation and rapid oxidation of tiny carbonaceous species [58,59]. All catalysts except YSTR underwent weight gain above 300 C due to oxidation of metallic species. The oxidation is more substantial for NiAl2O3 catalyst. Even though YST suffered carbon deposition, the amount is less than 2%. YST has shown respectable activity for ATR at different temperatures. However, it was not considered for the more severe tests, not only because it suffered carbon deposition but also because better performances are seen upon doping with different active metals. TGA characterization of the spent catalysts after activity tests under varying S/C and O/C showed excellent resistance

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to carbon formation for Ni-Al2O3 catalyst, as depicted in Fig. 13. All catalysts except for YSTR underwent weight gains. The initial weight drop at low temperature in all samples is due to the presence of moisture. Only small weight loss is observed for all YST catalysts up to 600 C. Weight gain is observed for Ni-Al2O3. Weight loss for all YST catalysts in the high temperature region (>600 C) was found to be within 2%. Ni-Al2O3 is most resistant to carbon formation followed by YSTR, YSTN and lastly by YSTC. TGA results of spent catalysts after testing under dodecaneexylene mixture are shown in Fig. 14. Again weight gain is observed clearly for Ni-Al2O3 and much lesser for YSTN. This weight gain is preceded by weight loss due to moisture evaporation. There is hardly any carbon burn out up to 600 C. Beyond this temperature observable weight loss is recorded for YSTN and YSTC. Ni-Al2O3 also suffered sizable carbon formation. YSTR is most resistant for carbon formation, clearly better than Ni-Al2O3 catalyst. The severity of the

104

mass (%)

102

Ni-Al O

100

98

YSTR

96

experimental conditions adversely affected the resistance of Ni-Al2O3 catalyst to both sintering and carbon formation, whereas YSTR maintained excellent carbon formation resistance, underwent almost no sintering and showed best activity. The large weight gain observed in TGA experiment for NiAl2O3 catalyst is due to the presence of well dispersed nickel metal particles at the catalyst support. It seems that the formation of the reduced metals at the surface of the catalyst with high dispersion helped reducing carbon formation. Prolonged pretreatment under reducing environment is therefore important for the perovskite catalysts prior to reforming to ensure sufficient active phase on the surface prior to the reforming reaction. The key observation of this study is the possibility of YST ceramic catalysts to reach high activity. This activity exceeded sometimes that of the Ni-Al2O3 catalyst even at very high rate of fuel reforming reaction. Even though this was accompanied by some carbon formation the deactivation rate is vanishing small for all catalysts for the time tested (it is constant over 20 h of testing period). It is worth noting that ruthenium doped YST outperformed Ni-Al2O3 catalyst in all aspects, namely non-vulnerability to sintering, resistance to carbon formation and high activity. As the main drive for using ceramic catalysts is their excellent stability under both oxidative and reducing environment, the atomistic dispersion of the active metals and the high sulfur tolerance reported particularly for YST based materials, these catalysts could be proposed as promising potential catalysts for diesel reforming. Sulfur tolerance study is currently undergoing and will be reported in a future paper.

YSTN YSTC

94 0

200

400

600

800

tempearture (oC)

Fig. 13 e TGA results of used catalysts after tests under varying O/C and S/C ratios.

101 Ni-Al2O3 100 YSTR

mass (%)

99

98

97 YSTN

Conclusions YST catalysts doped with different active metals are tested for dodecane reforming under varying temperatures, S/C and O/C ratios and benchmarked against Ni-Al2O3 catalysts. The effect of adding the aromatic xylene is also separately studied under much severe conditions. In general the ceramic catalysts showed very high activity sometimes overpassing that of NiAl2O3 catalyst. Carbon formation observed after tests should not be alarming as the reaction conditions were very severe and the deactivation rate is vanishing small. Ruthenium doped YST (and to a lesser degree YSTN) has the benefit of both excellent activity and high resistance to carbon formation and sintering. Further efforts should be directed toward reducing the level of doping, introducing coke resistant promoters, growing reduced metals on the YST surface as well as studying the sulfur tolerance.

96 YSTC 95

Acknowledgements

94 0

200

400

600

temperature (oC)

Fig. 14 e TGA results of used catalysts after tests under dodecane þ xylene fuel mixture.

800

This project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH)dKing Abdulaziz City for Science and Technologydthe Kingdom of Saudi Arabia, award number 09-ENE807-05.


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