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Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol
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 x x x ( 2 0 1 6 ) 1 e1 3
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Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol M. Surendar, T.V. Sagar, G. Raveendra, M. Ashwani Kumar, N. Lingaiah, K.S. Rama Rao, P.S. Sai Prasad* I & PC Division, CSIR-IICT, Hyderabad 500 607, India
article info
abstract
Article history:
Au, Ag, Cu, Pt doped cobalt based perovskites (LaCo0.99X0.01O3) were prepared by co-
Received 10 July 2015
precipitation method and investigated for their activity in glycerol steam reforming re-
Received in revised form
action in the temperature range of 400e700 C and at atmospheric pressure. These cat-
3 December 2015
alysts were characterized by ICP- OES, BET surface area, X-ray diffraction (XRD), Fourier
Accepted 5 December 2015
transform infrared spectroscopy (FT-IR), Hydrogen chemisorption, Temperature pro-
Available online xxx
grammed reduction (TPR), UVeVis diffuse reflectance spectroscopy (UVeVis DRS), Scanning electron microscopy (SEM) techniques and CHN analysis. The physico-chemical
Keywords:
properties were compared for the fresh, reduced and spent catalysts. The Pt-doped
Lanthanum cobaltate perovskite
catalyst displayed the best hydrogen yield (78.4%) at 96.0% conversion of glycerol. The
Pt doping
superior performance of the catalyst was explained in terms of the dispersion of the
Glycerol steam reforming
cobalt metal. The catalyst was also found to be stable during the time on stream analysis
Hydrogen yield
due to low carbon formation. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The ever increasing global demand for energy on one hand and the rapid depletion of fossil fuel sources on the other have necessitated the quest for alternative fuels which are clean and renewable [1]. Among the renewable energy carriers hydrogen has received much attention because of its high calorific value (1300 kJ/kg) and zero carbon emission during its usage. Hydrogen is also a desirable fuel for fuel cells in the electricity generation sector. Under these circumstances, hydrogen production from renewable sources has acquired enormous importance [2]. Bioglycerol, obtained as a major by-
product from the biodiesel industry, is considered to be an excellent renewable feedstock. The rapid growth of biodiesel industry in recent years is expected to result in a glut of crude glycerol in the market. The current methodologies for the purification of crude glycerol are not economic, necessitating the search for better options for its direct use [3]. One of the promising ways to utilize this impure glycerol is to produce hydrogen. Currently, over 50% of the world's hydrogen demand is met by steam reforming of hydrocarbons [4]. Among the several possible technologies for the production of hydrogen, steam reforming has acquired paramount importance because of the
* Corresponding author. E-mail address: [email protected] (P.S. Sai Prasad). http://dx.doi.org/10.1016/j.ijhydene.2015.12.075 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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high selectivity for hydrogen (7 mol H2/mol glycerol) [5]. Though, glycerol steam reforming (GSR) is endothermic and requires relatively high temperatures compared to the exothermic nature of other reactions like WGS and FTsynthesis the higher yield of hydrogen compensates for the energy input [6e10]. The general equation for glycerol steam reforming is given below.
low coke formation. Chemisorption studies indicated further scope for reducing the Co particle size and thereby enhancing the hydrogen yield. It is expected that metals like Pt are responsible for maintaining part of the Co phase at its reduced/metallic state. In the present work LaCoO3 perovskite was modified with the introduction of small amount of dopant like Pt, Cu, Ag or Au and the change in hydrogen yield was investigated.
C3 H8 O3 þ 3H2 O/7H2 þ 3CO2 Steam reforming ðiÞ The following reactions also occur under the reaction conditions. C3 H8 O3 /4H2 þ 3CO Direct decomposition
ðiiÞ
CO þ H2 O/H2 þ CO2 WGS reaction ðiiiÞ CO þ 2H2 /CH4 Methanation ðivÞ 2CO/CO2 þ C Boudouard reaction ðvÞ In the case of GSR, the glycerol molecule first undergoes dehydrogenation, followed by decarbonylation and WGS reaction. Depending on the nature of the catalytic site the CO may participate in reactions like methanation and Boudouard reaction. Many innovations have taken place on glycerol reforming for which Group VIII metals are found to be highly active. The application of Ni as catalyst is very common [11e14]. However, the major problems associated with Ni catalyst are: metal sintering, unwanted metal-support interaction and high carbon formation. These drawbacks limit the industrial scale application of Ni [15]. Several attempts have been made to overcome these problems by modifying the crystal structure of the metal, adopting different preparation methods and treatment procedures during catalyst synthesis [16e18]. In recent times, perovskite-type oxides have received considerable attention for the steam reforming process due to their rigid crystal structure [19e23]. In the ABO3 type perovskites, A is usually an alkali or alkaline earth metal and B is a transition metal. Partial substitution of either A or B with promoters like (Fe, Ca, Mn and Sr) results in new material formation possessing superior catalytic activity. Improvements in the redox and structural properties are also reported [24]. The perovskites with well defined structures, after reduction with hydrogen before the reforming reaction, produce well dispersed active metals [25e28]. The CeC bond scission activity of Co is comparable with that of Ni [29]. However, only limited studies are reported on this catalyst, particularly on the Co based perovskite catalysts. In a recent study on the LaeCeeCo mixed oxide derived Co catalysts we observed the collapse of the perovskite structure after reduction [30]. However, a highly dispersed Co-metal catalyst has emerged exhibiting the best catalytic activity at 700 C with complete conversion of glycerol and a 68.0% hydrogen yield. The active cobalt area constituted a major part of the BET surface area of the reduced catalysts, and this was correlated with the turnover frequency (TOF). The formation of a highly dispersed catalyst also helped to achieve
Experimental Catalyst preparation Perovskites with the general formula LaCo0.99X0.01O3 (X ¼ Au, Ag, Cu and Pt) were prepared by the coprecipitation method. The chemicals used in the preparation of the catalysts were: La(NO3)3.6H2O, H2PtCl6.6H2O, AgNO3, HAuCl4$3H2O supplied by M/s. Sigma Aldrich Chemicals, USA and Co(NO3)2.6H2O, Cu(NO3)3.3H2O and 25 vol.% NH4OH solution procured from M/s. SD Fine Chem, India. Typically, the LaCo0.99Ag0.01O3 was prepared by mixing required amounts of aqueous La(NO3)3.6H2O, Co(NO3)2.6H2O and AgNO3 solutions with continuous stirring for 2 h, at 80 C on a hot plate. This solution was cooled and then the precipitation carried out using 5 vol % aqueous ammonia until the pH of the solution reached 10. The mixture was then aged for 24 h at room temperature and filtered. The precipitate thus obtained was washed to remove the excess NH4OH. The sample was dried for 48 h at 100 C and finely ground into powder and subjected to the calcination at 700 C for 6 h in N2 flow. Similar procedure was followed for the preparation of LaCoO3, LaCo0.99Au0.01O3, LaCo0.99Cu0.01O3 and LaCo 0.99Pt0.01O3.
Characterization studies Chemical analysis of the samples was carried out by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian 725 ES instrument. The concentrations of the components obtained in parts per million were converted into atomic concentrations using the atomic weights of the metals. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Miniflex X-ray diffractometer (M/s. Rigaku Corporation, Japan) using Ni filtered Cu Ka radiation (l ¼ 1.5406 A) with a scan speed of 2 min1 and a scan range of 10e80 at 30 kV and 15 mA. The average crystallite size of the fresh, reduced and spent catalysts was calculated using the Scherrer formula. TPR studies were performed using a home-made apparatus. Catalyst sample (50 mg), taken in a quartz reactor, was reduced with 10% H2eAr gas mixture flowing at a flow rate of 30 ml min1 and heated at 5 C min1 up to 900 C. Before the TPR run, the catalysts were pretreated in argon at 300 C for 2 h. The hydrogen consumption was monitored using thermal conductivity detector of a gas chromatograph (Varian-8301). BET surface areas were determined by N2 adsorption on a SMART SORB 92/93 instrument (M/s. SMART Instruments, India). Prior to BET
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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measurement, the samples were dried at 150 C for 2 h. UVeVis DRS spectra of the catalyst samples were recorded in the UVeVisible region of 200e800 nm with a split width of 1.5 nm and scan speed of 400 nm per minute with M/s. GBC Cintra 10e UVeVisible spectrometer. Pellets were made by taking 15 mg of the catalyst sample and dry KBr, ground thoroughly for uniform mixing before making the pellet. The spectra were recorded at room temperature. FT-IR spectra were obtained on a Perkin Elmer (M/s. Spectrum GX, USA) instrument, using KBr pellet method. H2 Chemisorptions studies were performed using an Autosorb iQ Unit (M/s Quantachrome, USA). Catalyst sample (100 mg), taken in a quartz reactor, was first reduced in H2 gas at a flow rate of 60 ml min1 and a heating rate of 10 C min1 up to 650 C. They were then cooled in He flow to room temperature. Hydrogen gas was then introduced in pulses and the adsorption uptake was analyzed using a TCD. The coke content of the used catalysts was determined in CHNS analyzer (M/s. ElementaV, Germany).
Catalytic activity test Steam reforming of glycerol was performed in a fixed bed reactor. About 1 g of the catalyst was loaded in the middle of the reactor, suspended between two quartz wool plugs. Prior to reaction, the catalyst was reduced in pure hydrogen (100 mL/min) for 6 h at 700 C. Then, hydrogen was swept away with N2 and the reactor bed temperature was brought to the required level. An aqueous solution of 30 wt. % glycerol was introduced into the preheater kept above the reactor, and passed at a rate of 0.08 mL/min using a HPLC pump (M/s. Lab Alliance). After reaching the steady state, over a period of 1 h, the product mixture coming out of the reactor was condensed at 0 C. The liquid products were separated in a gaseliquid separator and the gaseous products analyzed online, using a TCD equipped gas chromatograph (M/s. Agilent, 7820A) equipped with a carboseive packed column for quantifying H2, CO, CH4 and CO2. The liquid products (methanol, ethanol, 1-propanol, 1,2- and 1,3-propanediols) and unreacted glycerol were analyzed gas chromatically using an Innowax capillary column of an FID attached chromatograph. The data acquired were used to determine the performance of the catalyst using the following equations [31,32].
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Results and discussion ICP-OES & X-ray diffraction The elemental composition of the catalysts was determined by the ICP-OES technique. The data presented in Table 1 indicate the closeness of these values with the theoretical values. The XRD patterns of the fresh doped LaCoO3 catalysts are shown in Fig. 1(A). The patterns indicate the formation of LaCoO3 perovskite as proved by the appearance of peaks at 2q values of 23.3, 30.3 32.9 33.3, 38.1, 40.7, 42.6, 44.4, 47.5, 58.8, 68.9, 69.9 and 79.2 [PCPDF-84-0848]. Shifting of diffraction peaks of the doublet at 2q values of 32.9 and 33.3 to the lower side is observed for the doped catalysts indicating the incorporation of the dopants into the perovskite unit [33]. In the case of Pt-containing sample the doublet appears as a single broad peak reflecting decrease in crystallinity [34]. This confirms the influence of the additional metal in changing the crystallinity of the perovskite. XRD patterns of reduced catalysts are displayed in Fig. 1(B). The structure of the perovskite is sensitive to the reduction temperature. It is reported that reduction treatment at 600 C results in the collapse of the perovskite structure with a consequent formation of La2O3 [JCPDS 71-1144] and metallic Co [35]. In the present catalysts also the absence of diffraction lines relating to the perovskite or cobalt oxides (Co3O4/CoO) is clearly observed indicating the transformation of the perovskite structure, presumably forming Co/La2O3. The presence of La(OH)3 is ascribed to the hydration of La2O3 due to exposure of the sample to humidified air, prior the XRD analysis [36e38]. It is worth mentioning that all the doped samples displayed a different metallic Co [39e42] than the unprompted (LaCoO3) perovskite catalyst.
Temperature programmed reduction The TPR profiles of fresh LaCo0.99X0.01O3 catalysts are presented in Fig. 2(A). The reduction patterns of the LaCoO3 catalysts are explained by a two-step mechanism in the literature [43]. In some cases the primary (low temperature) signal is interpreted as due to the reduction of Co3þ and Co2þ and the secondary (high temperature) signal due to the reduction of Co2þ to metallic Co0 [43,44]. Low temperature: LaCoO3 þ 1/2H2 / LaCoO2.5 þ 1/2H2O
H2 Produced experimentally H2 yield ¼ 100% H2 Calculated theoretically High temperature: LaCoO2.5 þ H2 / 1/2 La2O3 þ Co0 þ H2O H2 selectivity ¼
molecules of H2 produced experimentally 1 100%; C atoms in gas products R
R is the H2/CO2 reforming ratio 7/3 for glycerol. Carbon conversion to vapor ¼
Selectivity of productðiÞ ¼
C in the gas products 100% C fed into reactor
i Produced experimentally 100%; C atoms in gas products
where i is CO, CH4 and CO2
Conventionally, the two-step reduction is confirmed when the amount of H2 consumed in the second step is much higher than that of the first step. In the present case, unpromoted LaCoO3 clearly displays this behavior. However, the promoted catalysts differ in this aspect. Further, there is a considerable shift in the primary reduction signal towards the lower temperature region. It is reported that the nature and amount of promoter influences the shifting of primary signal towards the low temperature region [45]. In the present case, the shift is more pronounced in case of Pt-doped
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Table 1 e Elemental analysis and BET surface area values of catalysts. Catalyst (LaCo0.99X0.01O3) X ¼ Pt X ¼ Cu X ¼ Ag X ¼ Au LaCoO3
BET surface area (m2/g)
Atomic ratio (mol.) La
Co
Dopant
Fresh
Reduced
0.999 1.000 0.966 0.999 1.000
0.980 0.971 1.002 1.034 1.001
0.0079 0.0089 0.0092 0.0080 0.0079
20.1 16.5 13.9 10.0 08.9
58.8 50.3 45.7 30.4 25.1
catalyst than that of the Cu-doped followed by Ag and Audoped catalysts. The intensity of the primary signal is higher than that of the secondary signal in the TPR patterns of promoted catalysts. This can be explained by the simultaneous reduction of the dopant oxide along with the Co3þ to Co2þ reduction. The enhanced intensity of peak maximum appearing in the low temperature region in the case of Ptdoped catalyst can be due to hydrogen spillover [46]. The multiple temperature maxima appearing in the low temperature region in the pattern of the Cu containing sample can be due to the existence of copper in different oxidation states. The extent of shifting of Tmax in the TPR patterns of supported Co catalysts is reported to depend on the nature and concentration of dopant [47]. The BET surface area values (Table 1) show increasing trend as we move from Au to Pt doping, corroborating well with the XRD results.
UVevis diffuse reflectance spectroscopy The chemical environment and coordination geometry of the metal ions can be understood from their UVeVis DRS spectra. These spectra of the doped samples (fresh) are shown in Fig. 3(A). Each pattern presents two broad bands, one in the 300e400 nm region and the other in the 600e800 nm. Literature reveals that the Co3O4 spinnel exhibits peaks due to Co3þ ions in the octahedral geometry appearing at 380 and 700 nm [48] along with the tetrahedrally coordinated Co2þ ions at 540, 580 and 640 nm [49]. In the present spectra of the promoted catalysts, such combination peaks are not clearly visible indicating that the cobalt oxide is completely involved in the perovskite (LaCoO3) formation. However, in the case of Ptdoped catalyst a large difference in the intensities and also the blue and red shifts are seen in the bands at low and high regions. This clearly suggests that the chemical environment of the cobalt metal ions in the Pt containing sample is completely different from the remaining catalysts. The Co3þ species are more predominant than the Co2þ species. This information is well supported by the TPR results where the shifting of Tmax to lower temperature evidences the easy reduction of Co3þ ions. In the UV-DRS spectra of the reduced samples shown in Fig. 3(B), the above characteristic peaks are conspicuously absent indicating the formation of metallic Co, as a consequence of reduction.
Scanning electron microscopy The SEM pictures of the catalysts are displayed in Fig. 4. Agglomeration of the particles is visible in the case of Ag-
doped sample, whereas bulky nature can be identified in Audoped sample. The agglomeration of particles is much less in the case of Cu and Pt doped samples.
Fourier transform infrared spectroscopy The FT-IR spectra of LaCo0.99X0.01O3 catalysts (fresh) are shown in Fig. 5(A). The characteristic stretching and bending vibrations of the LaCoO3 are observed between 450 and 750 cm1. The IR band found in the range of 586e592 cm1 is the characteristic stretching vibration of the metaleoxygen bond of the CoO6 octahedral unit present in LaCoO3 perovskite [50]. The two additional bands observed at 1380 and 1471 cm1 are due to the bending vibration frequencies of (NO3)2 group. The IR bands in the 2800e3600 cm1 region can be ascribed to the adsorbed H2O molecules. In the spectra of the reduced samples shown in Fig. 5(B), the band observed at 638 cm1 is due to the LaeO vibration [50], the bands at 1641.2, 1485.5, and 1380.1 cm1 are assignable to the LaCoO3 crystal lattice vibrations as reported earlier [51] and the sharp band at 3612.3 cm1 could be due to the atmospheric moisture absorbed during the handling of the samples.
Reforming activity All the catalysts have displayed complete conversion of glycerol in the studied range of reaction temperature: 400e700 C. Table 3 indicates the distribution of products in liquid and gas phases. It is observed that at lower temperatures the amount of liquid product is more than the gaseous product. Compared to the doped catalysts, LaCoO3 has produced higher amount of liquid products. The compositions of liquid collected at different reaction temperatures are shown in Table 4. Lower alcohols like MeeOH, EteOH and PreOH are the main products. Small amounts of propanediols are also present. The formation of liquid products on the catalysts decreased in the order: LaCoO3>LaCo0.99Au0.01O3> LaCo0.99Ag0.01O3> LaCo0.99Cu0.01O3> LaCo0.99Pt0.01O3. An interesting observation is that for the formation of gaseous products, the order is reverse. A considerable difference exists in the extent of carbon conversion to gaseous products. The distribution depends on the nature of the doped element (Au, Ag, Cu and Pt). The hydrogen yield followed the order: LaCoO3 < LaCo0.99Au0.01O3 < LaCo0.99Ag0.01O3 < LaCo0.99Cu0.01O3 < LaCo0.99Pt0.01O3. Cu and Pt-doped samples exhibit superior functionality. This can be explained on the basis of easy reduction of cobalt oxide at lower temperature, as revealed by the H2-TPR analysis. The UV-DRS results also have indicated high concentration of Co3þ
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 1 e XRD patterns of LaCo0.99X0.01O3 catalysts; (A) Fresh (B) Reduced (C) Spent catalysts.
in the octahedral position in Cu and Pt promoted samples. The presence of Co3þ in the octahedral position seems to be responsible for the CeC scission in the glycerol leading to 96% of carbon conversion into gas phase. Thus, it appears that Co3þ species in octahedral environment are easily reducible (TPR results).
The reactivity of the catalysts is further correlated with the hydrogen chemisorption data (Table 2). The particle size of Co in Cu and Pt doped samples is obtained as 2.4 and 1.8 nm, respectively. Au and Ag doping has generated cobalt particles larger than those in Cu and Pt-doped catalysts. The rate of production of H2 per cobalt atom per second can be regarded
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 2 e H2-TPR profiles of LaCo0.99X0.01O3 catalysts; (A) Fresh (B) Spent catalysts.
as the quasi-turn over frequency (QTOF). Turn over frequency (TOF), on the other hand, is a measure of rate of H2 production per active cobalt site per second. Thus, for calculating QTOF, the amount of cobalt present in the catalyst is sufficient. For TOF calculation one has to find out the number of surface cobalt atoms. In the present case H2-chemisorption technique was used to find out the number of surface/active cobalt atoms, the dispersion, metal area and particle size (Table 2). Fig. 6 shows the effect of cobalt particle size on the inverse QTOF value which yields a straight line with an intercept on Yaxis. From this graph, it is clear that a cobalt particle size of 1 nm gives a 1/QTOF value of ~0.12. If all the cobalt atoms are dispersed at atomic level (with no bulk cobalt species), then the inverse of Y-axis intercept, can be taken as the rate of H2 by one cobalt site i.e., TOF. Thus the TOF derived from this graph is equal to ~10 (1/0.1). In other words, at atomic level
cobalt dispersion, each cobalt atom is able to yield 10 hydrogen molecules in 1 s during the reforming reaction. In order to achieve a high TOF towards the production of hydrogen by cobalt catalysts one has to concentrate on a suitable catalyst preparation methodology or doping the promoter more efficiently or both. The present manuscript deals with the effect of promoters on achieving the smaller cobalt particle size and thereby maximizing the H2 yield. In this aspect, Pt appears to be an efficient dopant in yielding the smallest cobalt particle size, i.e., 1.8 nm (Table 2) and in turn, achieving higher TOF value i.e., 2.7 (Fig. 7). A comparison of results derived from Figs. 6 and 7 shows that there is still a chance to improve TOF by decreasing the cobalt particle size. Since Fig. 7 shows an exponential decrease of TOF against cobalt particle size a small decrease in cobalt particle size would be sufficient to get higher TOF. This
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 3 e UVeVis DRS spectra of LaCo0.99X0.01O3 catalysts; (A) fresh (B) reduced (C) spent catalysts.
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 4 e SEM images of the LaCo0.99X0.01O3 catalysts.
work thus shows that there is still a scope for further enhancement in TOF. Recently, using LaeCeeCo mixed oxides with different LaO2eCeO2 ratios we have reported that the TOF is influenced by the cobalt particle size. The optimum results i.e. 68% hydrogen yield was achieved for cobalt particles having size 4e5 nm with TOF of 1.12 s1 [30]. In the present investigation the turn over frequency value is improved by more than two fold (2.54 s1) due to the presence of Pt and as a result the hydrogen yield reaches a maximum value of 78.4%. The stability of the catalysts was tested during the 10 h time-on-stream analysis and the results are presented in Fig. 8. The hydrogen yield of the Au doped perovskite is found to be decreasing suddenly after 4 h, while the Ag doped sample showed a steady decrease. On the other hand, both the Cu and Pt-doped catalysts showed no deactivation even after 10 h. The CHN analysis was carried out on the used catalysts (collected after 10 h on stream) in order to estimate the extent of carbon deposited on the samples during the reforming reaction. The results obtained (Table 2) clearly indicate a considerable difference in the amount of carbon formation. Cu and Pt-doped catalysts show much lower carbon accumulation than that of Ag and Au-doped catalysts. The higher amount of the carbon formed over the Au and Ag doped catalysts reveals that these catalysts are less efficient than the others.
Characterization of the spent catalysts The XRD patterns of the spent LaCo0.99X0.01O3 catalysts are presented in Fig. 1(C). The major peaks indicate the presence
of La2O3 and metallic cobalt. The diffraction lines observed at 25.86 and 30.44 can be assigned to the hexagonal (L) phase of La2O3 [JCPDS -74-2430, 42], while the peaks at 27.54, 39.98 and 54.76 represent the La2O3 in the cubic (L1) phase [JCPDS-220369, 51]. The peaks centered at 33.72, 38.18 and 47.34 in all the catalysts represent the unreduced perovskite (P) LaCoO3 [JCPDS-75-0279]. The peaks at 44.4, 51.88, 56.90 and 75.98 reveal the presence of metallic Co in the samples [52e56]. The metallic particles are not re-oxidized to cobalt oxides even when exposed to atmosphere. This is evidenced by the absence of the diffraction patterns related to cobalt oxides in all the samples. The active metal area varies inversely with the Co dispersion, as expected. Increase in Co dispersion due to the promotional effect of metals is in accordance with the literature report [47], particularly in the case of Pt [59]. The UV-DRS patterns of spent (LaCo0.99X0.01O3) catalysts are displayed in Fig. 3(C). In the case of Au catalyst the absorbance of peaks with maxima at 540, 580 and 670 nm confirm the presence of the cobalt in 2 þ oxidation state. On the other hand, no such peaks are identified in the rest of the studied samples explaining the presence of the finely dispersed metallic cobalt on the La2O3. It can also be clearly understood that except Au, all the other dopants (Ag, Cu and Pt) improved the reducibility of cobalt oxide and stabilized the Co metal in zero oxidation state, which is essential for the reforming activity. The H2-TPR profiles of spent catalysts are displayed in Fig. 2 (B). The analysis was mainly focused on understanding the nature of the carbon deposited on the catalysts during the reaction. It was reported previously that the free carbon (without hydrogen) yields a positive peak in the
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 5 e FT - IR spectra of the LaCo0.99X0.01O3 catalysts; (A) fresh (B) reduced catalysts.
range of 400 C and a negative peak for the carbonaceous moiety (CHx; x ¼ 2e3) in the temperature range 700 C [5658]. In the present investigation too, similar peaks are identified at relatively higher temperatures than the literature value indicating the formation of the free carbon as
well as the CHx moieties. The shift in the maxima of both the positive and negative peaks might be attributed to the stronger interaction of carbon with cobalt/support. The decrease in the intensity of the negative peak over the unpromoted catalyst compared to that over the promoted
Table 2 e Chemisorption and carbon formation data of LaCo 0.99X 0.01O3 catalysts. Catalyst LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3 LaCoO3
H2 uptake (mmole/g)
Active metal surface area (m2/g)
Average particle size (nm)
Metal dispersion (%)
Rate of carbon formation (m mol C) (mol carbon converted1)
465.9 446.4 394.4 311.5 305.2
37.8 35.6 31.2 24.9 22.0
1.8 2.4 5.1 8.1 8.6
47.6 43.5 19.4 12.3 11.1
4.2 4.5 6.6 11.9 19.9
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Table 3 e Composition of the reaction products formed at various temperatures. Catalysts
Percentage of products (wt. %) 400 C
LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3 LaCoO3
500 C
600 C
700 C
Liquid
Gas
Liquid
Gas
Liquid
Gas
Liquid
Gas
67.2 65.6 76.1 79.4 81.5
32.8 34.4 23.9 20.6 18.5
14.4 17.2 51.1 67.8 71.0
85.6 82.8 48.9 32.2 29.0
13.4 4.3 25.7 50.2 55.2
86.6 95.7 74.3 49.8 44.8
3.9 6.5 10.6 49.6 54.6
96.1 93.5 89.4 50.4 45.4
Table 4 e Product distribution during the evaluation of the LaCo0.99Pt0.01O3 catalyst. Composition of liquid product Reaction temperature ( C) (water-free) (wt. %) 400 500 600 700 MeeOH EteOH PreOH 1,2-PD 1,3-PD Total products
33.6 20.2 6.7 3.4 3.4 67.2
7.2 4.3 1.4 0.7 0.7 14.4
6.7 4.0 1.3 0.7 0.7 13.4
2.0 1.2 0.4 0.2 0.2 3.9
PD ¼ propanediol.
Fig. 7 e TOF vs. Co Average crystal size of the LaCo0.99X0.01O3 catalysts.
Fig. 6 e (1/QTOF) vs. Co Average crystal size of the LaCo0.99X0.01O3 catalysts.
catalysts (x ¼ Au, Ag, Cu, Pt) indicates the formation of partial hydrogenation products of the CO/CO2 liberated during the reaction or the dehydration products of the glycerol. From this it is clear that only the Pt and Cu promoted samples are efficient in promoting the WGS reaction which is necessary for improving the H2 yield. However, all the catalysts have suffered from coke formation as a result of CO disproportination (Boudouard) reaction. The early deactivation of the un-promoted as well as the Au promoted
Fig. 8 e Comparison of hydrogen yield at 700 C. catalysts might be attributed to high deposition of the free carbon and the CHx moiety. These results are in good agreement with the carbon analysis which indicates that the amount of the carbon is the maximum for the LaCoO3
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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 x x x ( 2 0 1 6 ) 1 e1 3
Table 5 e Crystal size data from XRD (Scherrer equation) analysis. Catalysts
Average particle size, (nm) Fresh (perovskite)
Co metal After reduction Spent
LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3
12.2 28.4 39.8 62.5
e 3.9 5.7 9.3
e 4.1 17.2 39.3
and Au promoted samples. The carbon formation follows the order: LaCoO3> Au > Ag > Cu ~ Pt. These results were in good agreement with the CHN analysis and the H2 e TPR of the spent catalysts. From this it is clearly understood that Pt sites are predominantly responsible for maintaining part of the Co phase at its reduced/metallic state than supplying hydrogen for producing the partially hydrogenated products. The crystallite size data obtained from XRD analysis are presented in Table 5. Broadly, it appears that reduction has decreased the crystallite size, even though such phenomenon could not be emphasized in Pt-doped catalyst due to its amorphous nature. Whereas Cu-doped catalyst retained its crystallite size after the reaction, Ag and Au-doped catalysts showed considerable increase in crystallite size. Thus, Ag and Au-doped catalysts appear to be more susceptible to deactivation due to sintering than Pt and Cu-doped catalysts which explains the continuous decrease in hydrogen yield (Fig. 8).
Conclusions LaCo0.99Pt0.01O3 and LaCo0.99Cu0.01O3 exhibit higher hydrogen yield than LaCo0.99Au0.01O3 and LaCo0.99Ag0.01O3 catalysts. In addition, the Pt-doped catalyst shows close to 96% carbon conversion to gas. Cu and Pt are found to be efficient dopants for producing smaller cobalt metal particles with higher metal dispersion during the reduction process compared to Ag and Au dopants. A significant amount of carbon is formed during the time stream analysis on LaCo0.99Au0.01O3 and LaCo0.99Ag0.01O3, blocking the active sites responsible for the reforming reaction leading to faster deactivation. Cu and Pt incorporation in LaCoO3 appears to promote WGS reaction leading to enhancement in hydrogen yield.
Acknowledgment The authors thank Council of scientific industrial research & Department of Science and Technology (DST), New Delhi, India.
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Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol M. Surendar, T.V. Sagar, G. Raveendra, M. Ashwani Kumar, N. Lingaiah, K.S. Rama Rao, P.S. Sai Prasad* I & PC Division, CSIR-IICT, Hyderabad 500 607, India
article info
abstract
Article history:
Au, Ag, Cu, Pt doped cobalt based perovskites (LaCo0.99X0.01O3) were prepared by co-
Received 10 July 2015
precipitation method and investigated for their activity in glycerol steam reforming re-
Received in revised form
action in the temperature range of 400e700 C and at atmospheric pressure. These cat-
3 December 2015
alysts were characterized by ICP- OES, BET surface area, X-ray diffraction (XRD), Fourier
Accepted 5 December 2015
transform infrared spectroscopy (FT-IR), Hydrogen chemisorption, Temperature pro-
Available online xxx
grammed reduction (TPR), UVeVis diffuse reflectance spectroscopy (UVeVis DRS), Scanning electron microscopy (SEM) techniques and CHN analysis. The physico-chemical
Keywords:
properties were compared for the fresh, reduced and spent catalysts. The Pt-doped
Lanthanum cobaltate perovskite
catalyst displayed the best hydrogen yield (78.4%) at 96.0% conversion of glycerol. The
Pt doping
superior performance of the catalyst was explained in terms of the dispersion of the
Glycerol steam reforming
cobalt metal. The catalyst was also found to be stable during the time on stream analysis
Hydrogen yield
due to low carbon formation. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The ever increasing global demand for energy on one hand and the rapid depletion of fossil fuel sources on the other have necessitated the quest for alternative fuels which are clean and renewable [1]. Among the renewable energy carriers hydrogen has received much attention because of its high calorific value (1300 kJ/kg) and zero carbon emission during its usage. Hydrogen is also a desirable fuel for fuel cells in the electricity generation sector. Under these circumstances, hydrogen production from renewable sources has acquired enormous importance [2]. Bioglycerol, obtained as a major by-
product from the biodiesel industry, is considered to be an excellent renewable feedstock. The rapid growth of biodiesel industry in recent years is expected to result in a glut of crude glycerol in the market. The current methodologies for the purification of crude glycerol are not economic, necessitating the search for better options for its direct use [3]. One of the promising ways to utilize this impure glycerol is to produce hydrogen. Currently, over 50% of the world's hydrogen demand is met by steam reforming of hydrocarbons [4]. Among the several possible technologies for the production of hydrogen, steam reforming has acquired paramount importance because of the
* Corresponding author. E-mail address: [email protected] (P.S. Sai Prasad). http://dx.doi.org/10.1016/j.ijhydene.2015.12.075 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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high selectivity for hydrogen (7 mol H2/mol glycerol) [5]. Though, glycerol steam reforming (GSR) is endothermic and requires relatively high temperatures compared to the exothermic nature of other reactions like WGS and FTsynthesis the higher yield of hydrogen compensates for the energy input [6e10]. The general equation for glycerol steam reforming is given below.
low coke formation. Chemisorption studies indicated further scope for reducing the Co particle size and thereby enhancing the hydrogen yield. It is expected that metals like Pt are responsible for maintaining part of the Co phase at its reduced/metallic state. In the present work LaCoO3 perovskite was modified with the introduction of small amount of dopant like Pt, Cu, Ag or Au and the change in hydrogen yield was investigated.
C3 H8 O3 þ 3H2 O/7H2 þ 3CO2 Steam reforming ðiÞ The following reactions also occur under the reaction conditions. C3 H8 O3 /4H2 þ 3CO Direct decomposition
ðiiÞ
CO þ H2 O/H2 þ CO2 WGS reaction ðiiiÞ CO þ 2H2 /CH4 Methanation ðivÞ 2CO/CO2 þ C Boudouard reaction ðvÞ In the case of GSR, the glycerol molecule first undergoes dehydrogenation, followed by decarbonylation and WGS reaction. Depending on the nature of the catalytic site the CO may participate in reactions like methanation and Boudouard reaction. Many innovations have taken place on glycerol reforming for which Group VIII metals are found to be highly active. The application of Ni as catalyst is very common [11e14]. However, the major problems associated with Ni catalyst are: metal sintering, unwanted metal-support interaction and high carbon formation. These drawbacks limit the industrial scale application of Ni [15]. Several attempts have been made to overcome these problems by modifying the crystal structure of the metal, adopting different preparation methods and treatment procedures during catalyst synthesis [16e18]. In recent times, perovskite-type oxides have received considerable attention for the steam reforming process due to their rigid crystal structure [19e23]. In the ABO3 type perovskites, A is usually an alkali or alkaline earth metal and B is a transition metal. Partial substitution of either A or B with promoters like (Fe, Ca, Mn and Sr) results in new material formation possessing superior catalytic activity. Improvements in the redox and structural properties are also reported [24]. The perovskites with well defined structures, after reduction with hydrogen before the reforming reaction, produce well dispersed active metals [25e28]. The CeC bond scission activity of Co is comparable with that of Ni [29]. However, only limited studies are reported on this catalyst, particularly on the Co based perovskite catalysts. In a recent study on the LaeCeeCo mixed oxide derived Co catalysts we observed the collapse of the perovskite structure after reduction [30]. However, a highly dispersed Co-metal catalyst has emerged exhibiting the best catalytic activity at 700 C with complete conversion of glycerol and a 68.0% hydrogen yield. The active cobalt area constituted a major part of the BET surface area of the reduced catalysts, and this was correlated with the turnover frequency (TOF). The formation of a highly dispersed catalyst also helped to achieve
Experimental Catalyst preparation Perovskites with the general formula LaCo0.99X0.01O3 (X ¼ Au, Ag, Cu and Pt) were prepared by the coprecipitation method. The chemicals used in the preparation of the catalysts were: La(NO3)3.6H2O, H2PtCl6.6H2O, AgNO3, HAuCl4$3H2O supplied by M/s. Sigma Aldrich Chemicals, USA and Co(NO3)2.6H2O, Cu(NO3)3.3H2O and 25 vol.% NH4OH solution procured from M/s. SD Fine Chem, India. Typically, the LaCo0.99Ag0.01O3 was prepared by mixing required amounts of aqueous La(NO3)3.6H2O, Co(NO3)2.6H2O and AgNO3 solutions with continuous stirring for 2 h, at 80 C on a hot plate. This solution was cooled and then the precipitation carried out using 5 vol % aqueous ammonia until the pH of the solution reached 10. The mixture was then aged for 24 h at room temperature and filtered. The precipitate thus obtained was washed to remove the excess NH4OH. The sample was dried for 48 h at 100 C and finely ground into powder and subjected to the calcination at 700 C for 6 h in N2 flow. Similar procedure was followed for the preparation of LaCoO3, LaCo0.99Au0.01O3, LaCo0.99Cu0.01O3 and LaCo 0.99Pt0.01O3.
Characterization studies Chemical analysis of the samples was carried out by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian 725 ES instrument. The concentrations of the components obtained in parts per million were converted into atomic concentrations using the atomic weights of the metals. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Miniflex X-ray diffractometer (M/s. Rigaku Corporation, Japan) using Ni filtered Cu Ka radiation (l ¼ 1.5406 A) with a scan speed of 2 min1 and a scan range of 10e80 at 30 kV and 15 mA. The average crystallite size of the fresh, reduced and spent catalysts was calculated using the Scherrer formula. TPR studies were performed using a home-made apparatus. Catalyst sample (50 mg), taken in a quartz reactor, was reduced with 10% H2eAr gas mixture flowing at a flow rate of 30 ml min1 and heated at 5 C min1 up to 900 C. Before the TPR run, the catalysts were pretreated in argon at 300 C for 2 h. The hydrogen consumption was monitored using thermal conductivity detector of a gas chromatograph (Varian-8301). BET surface areas were determined by N2 adsorption on a SMART SORB 92/93 instrument (M/s. SMART Instruments, India). Prior to BET
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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 x x x ( 2 0 1 6 ) 1 e1 3
measurement, the samples were dried at 150 C for 2 h. UVeVis DRS spectra of the catalyst samples were recorded in the UVeVisible region of 200e800 nm with a split width of 1.5 nm and scan speed of 400 nm per minute with M/s. GBC Cintra 10e UVeVisible spectrometer. Pellets were made by taking 15 mg of the catalyst sample and dry KBr, ground thoroughly for uniform mixing before making the pellet. The spectra were recorded at room temperature. FT-IR spectra were obtained on a Perkin Elmer (M/s. Spectrum GX, USA) instrument, using KBr pellet method. H2 Chemisorptions studies were performed using an Autosorb iQ Unit (M/s Quantachrome, USA). Catalyst sample (100 mg), taken in a quartz reactor, was first reduced in H2 gas at a flow rate of 60 ml min1 and a heating rate of 10 C min1 up to 650 C. They were then cooled in He flow to room temperature. Hydrogen gas was then introduced in pulses and the adsorption uptake was analyzed using a TCD. The coke content of the used catalysts was determined in CHNS analyzer (M/s. ElementaV, Germany).
Catalytic activity test Steam reforming of glycerol was performed in a fixed bed reactor. About 1 g of the catalyst was loaded in the middle of the reactor, suspended between two quartz wool plugs. Prior to reaction, the catalyst was reduced in pure hydrogen (100 mL/min) for 6 h at 700 C. Then, hydrogen was swept away with N2 and the reactor bed temperature was brought to the required level. An aqueous solution of 30 wt. % glycerol was introduced into the preheater kept above the reactor, and passed at a rate of 0.08 mL/min using a HPLC pump (M/s. Lab Alliance). After reaching the steady state, over a period of 1 h, the product mixture coming out of the reactor was condensed at 0 C. The liquid products were separated in a gaseliquid separator and the gaseous products analyzed online, using a TCD equipped gas chromatograph (M/s. Agilent, 7820A) equipped with a carboseive packed column for quantifying H2, CO, CH4 and CO2. The liquid products (methanol, ethanol, 1-propanol, 1,2- and 1,3-propanediols) and unreacted glycerol were analyzed gas chromatically using an Innowax capillary column of an FID attached chromatograph. The data acquired were used to determine the performance of the catalyst using the following equations [31,32].
3
Results and discussion ICP-OES & X-ray diffraction The elemental composition of the catalysts was determined by the ICP-OES technique. The data presented in Table 1 indicate the closeness of these values with the theoretical values. The XRD patterns of the fresh doped LaCoO3 catalysts are shown in Fig. 1(A). The patterns indicate the formation of LaCoO3 perovskite as proved by the appearance of peaks at 2q values of 23.3, 30.3 32.9 33.3, 38.1, 40.7, 42.6, 44.4, 47.5, 58.8, 68.9, 69.9 and 79.2 [PCPDF-84-0848]. Shifting of diffraction peaks of the doublet at 2q values of 32.9 and 33.3 to the lower side is observed for the doped catalysts indicating the incorporation of the dopants into the perovskite unit [33]. In the case of Pt-containing sample the doublet appears as a single broad peak reflecting decrease in crystallinity [34]. This confirms the influence of the additional metal in changing the crystallinity of the perovskite. XRD patterns of reduced catalysts are displayed in Fig. 1(B). The structure of the perovskite is sensitive to the reduction temperature. It is reported that reduction treatment at 600 C results in the collapse of the perovskite structure with a consequent formation of La2O3 [JCPDS 71-1144] and metallic Co [35]. In the present catalysts also the absence of diffraction lines relating to the perovskite or cobalt oxides (Co3O4/CoO) is clearly observed indicating the transformation of the perovskite structure, presumably forming Co/La2O3. The presence of La(OH)3 is ascribed to the hydration of La2O3 due to exposure of the sample to humidified air, prior the XRD analysis [36e38]. It is worth mentioning that all the doped samples displayed a different metallic Co [39e42] than the unprompted (LaCoO3) perovskite catalyst.
Temperature programmed reduction The TPR profiles of fresh LaCo0.99X0.01O3 catalysts are presented in Fig. 2(A). The reduction patterns of the LaCoO3 catalysts are explained by a two-step mechanism in the literature [43]. In some cases the primary (low temperature) signal is interpreted as due to the reduction of Co3þ and Co2þ and the secondary (high temperature) signal due to the reduction of Co2þ to metallic Co0 [43,44]. Low temperature: LaCoO3 þ 1/2H2 / LaCoO2.5 þ 1/2H2O
H2 Produced experimentally H2 yield ¼ 100% H2 Calculated theoretically High temperature: LaCoO2.5 þ H2 / 1/2 La2O3 þ Co0 þ H2O H2 selectivity ¼
molecules of H2 produced experimentally 1 100%; C atoms in gas products R
R is the H2/CO2 reforming ratio 7/3 for glycerol. Carbon conversion to vapor ¼
Selectivity of productðiÞ ¼
C in the gas products 100% C fed into reactor
i Produced experimentally 100%; C atoms in gas products
where i is CO, CH4 and CO2
Conventionally, the two-step reduction is confirmed when the amount of H2 consumed in the second step is much higher than that of the first step. In the present case, unpromoted LaCoO3 clearly displays this behavior. However, the promoted catalysts differ in this aspect. Further, there is a considerable shift in the primary reduction signal towards the lower temperature region. It is reported that the nature and amount of promoter influences the shifting of primary signal towards the low temperature region [45]. In the present case, the shift is more pronounced in case of Pt-doped
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Table 1 e Elemental analysis and BET surface area values of catalysts. Catalyst (LaCo0.99X0.01O3) X ¼ Pt X ¼ Cu X ¼ Ag X ¼ Au LaCoO3
BET surface area (m2/g)
Atomic ratio (mol.) La
Co
Dopant
Fresh
Reduced
0.999 1.000 0.966 0.999 1.000
0.980 0.971 1.002 1.034 1.001
0.0079 0.0089 0.0092 0.0080 0.0079
20.1 16.5 13.9 10.0 08.9
58.8 50.3 45.7 30.4 25.1
catalyst than that of the Cu-doped followed by Ag and Audoped catalysts. The intensity of the primary signal is higher than that of the secondary signal in the TPR patterns of promoted catalysts. This can be explained by the simultaneous reduction of the dopant oxide along with the Co3þ to Co2þ reduction. The enhanced intensity of peak maximum appearing in the low temperature region in the case of Ptdoped catalyst can be due to hydrogen spillover [46]. The multiple temperature maxima appearing in the low temperature region in the pattern of the Cu containing sample can be due to the existence of copper in different oxidation states. The extent of shifting of Tmax in the TPR patterns of supported Co catalysts is reported to depend on the nature and concentration of dopant [47]. The BET surface area values (Table 1) show increasing trend as we move from Au to Pt doping, corroborating well with the XRD results.
UVevis diffuse reflectance spectroscopy The chemical environment and coordination geometry of the metal ions can be understood from their UVeVis DRS spectra. These spectra of the doped samples (fresh) are shown in Fig. 3(A). Each pattern presents two broad bands, one in the 300e400 nm region and the other in the 600e800 nm. Literature reveals that the Co3O4 spinnel exhibits peaks due to Co3þ ions in the octahedral geometry appearing at 380 and 700 nm [48] along with the tetrahedrally coordinated Co2þ ions at 540, 580 and 640 nm [49]. In the present spectra of the promoted catalysts, such combination peaks are not clearly visible indicating that the cobalt oxide is completely involved in the perovskite (LaCoO3) formation. However, in the case of Ptdoped catalyst a large difference in the intensities and also the blue and red shifts are seen in the bands at low and high regions. This clearly suggests that the chemical environment of the cobalt metal ions in the Pt containing sample is completely different from the remaining catalysts. The Co3þ species are more predominant than the Co2þ species. This information is well supported by the TPR results where the shifting of Tmax to lower temperature evidences the easy reduction of Co3þ ions. In the UV-DRS spectra of the reduced samples shown in Fig. 3(B), the above characteristic peaks are conspicuously absent indicating the formation of metallic Co, as a consequence of reduction.
Scanning electron microscopy The SEM pictures of the catalysts are displayed in Fig. 4. Agglomeration of the particles is visible in the case of Ag-
doped sample, whereas bulky nature can be identified in Audoped sample. The agglomeration of particles is much less in the case of Cu and Pt doped samples.
Fourier transform infrared spectroscopy The FT-IR spectra of LaCo0.99X0.01O3 catalysts (fresh) are shown in Fig. 5(A). The characteristic stretching and bending vibrations of the LaCoO3 are observed between 450 and 750 cm1. The IR band found in the range of 586e592 cm1 is the characteristic stretching vibration of the metaleoxygen bond of the CoO6 octahedral unit present in LaCoO3 perovskite [50]. The two additional bands observed at 1380 and 1471 cm1 are due to the bending vibration frequencies of (NO3)2 group. The IR bands in the 2800e3600 cm1 region can be ascribed to the adsorbed H2O molecules. In the spectra of the reduced samples shown in Fig. 5(B), the band observed at 638 cm1 is due to the LaeO vibration [50], the bands at 1641.2, 1485.5, and 1380.1 cm1 are assignable to the LaCoO3 crystal lattice vibrations as reported earlier [51] and the sharp band at 3612.3 cm1 could be due to the atmospheric moisture absorbed during the handling of the samples.
Reforming activity All the catalysts have displayed complete conversion of glycerol in the studied range of reaction temperature: 400e700 C. Table 3 indicates the distribution of products in liquid and gas phases. It is observed that at lower temperatures the amount of liquid product is more than the gaseous product. Compared to the doped catalysts, LaCoO3 has produced higher amount of liquid products. The compositions of liquid collected at different reaction temperatures are shown in Table 4. Lower alcohols like MeeOH, EteOH and PreOH are the main products. Small amounts of propanediols are also present. The formation of liquid products on the catalysts decreased in the order: LaCoO3>LaCo0.99Au0.01O3> LaCo0.99Ag0.01O3> LaCo0.99Cu0.01O3> LaCo0.99Pt0.01O3. An interesting observation is that for the formation of gaseous products, the order is reverse. A considerable difference exists in the extent of carbon conversion to gaseous products. The distribution depends on the nature of the doped element (Au, Ag, Cu and Pt). The hydrogen yield followed the order: LaCoO3 < LaCo0.99Au0.01O3 < LaCo0.99Ag0.01O3 < LaCo0.99Cu0.01O3 < LaCo0.99Pt0.01O3. Cu and Pt-doped samples exhibit superior functionality. This can be explained on the basis of easy reduction of cobalt oxide at lower temperature, as revealed by the H2-TPR analysis. The UV-DRS results also have indicated high concentration of Co3þ
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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 x x x ( 2 0 1 6 ) 1 e1 3
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Fig. 1 e XRD patterns of LaCo0.99X0.01O3 catalysts; (A) Fresh (B) Reduced (C) Spent catalysts.
in the octahedral position in Cu and Pt promoted samples. The presence of Co3þ in the octahedral position seems to be responsible for the CeC scission in the glycerol leading to 96% of carbon conversion into gas phase. Thus, it appears that Co3þ species in octahedral environment are easily reducible (TPR results).
The reactivity of the catalysts is further correlated with the hydrogen chemisorption data (Table 2). The particle size of Co in Cu and Pt doped samples is obtained as 2.4 and 1.8 nm, respectively. Au and Ag doping has generated cobalt particles larger than those in Cu and Pt-doped catalysts. The rate of production of H2 per cobalt atom per second can be regarded
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Fig. 2 e H2-TPR profiles of LaCo0.99X0.01O3 catalysts; (A) Fresh (B) Spent catalysts.
as the quasi-turn over frequency (QTOF). Turn over frequency (TOF), on the other hand, is a measure of rate of H2 production per active cobalt site per second. Thus, for calculating QTOF, the amount of cobalt present in the catalyst is sufficient. For TOF calculation one has to find out the number of surface cobalt atoms. In the present case H2-chemisorption technique was used to find out the number of surface/active cobalt atoms, the dispersion, metal area and particle size (Table 2). Fig. 6 shows the effect of cobalt particle size on the inverse QTOF value which yields a straight line with an intercept on Yaxis. From this graph, it is clear that a cobalt particle size of 1 nm gives a 1/QTOF value of ~0.12. If all the cobalt atoms are dispersed at atomic level (with no bulk cobalt species), then the inverse of Y-axis intercept, can be taken as the rate of H2 by one cobalt site i.e., TOF. Thus the TOF derived from this graph is equal to ~10 (1/0.1). In other words, at atomic level
cobalt dispersion, each cobalt atom is able to yield 10 hydrogen molecules in 1 s during the reforming reaction. In order to achieve a high TOF towards the production of hydrogen by cobalt catalysts one has to concentrate on a suitable catalyst preparation methodology or doping the promoter more efficiently or both. The present manuscript deals with the effect of promoters on achieving the smaller cobalt particle size and thereby maximizing the H2 yield. In this aspect, Pt appears to be an efficient dopant in yielding the smallest cobalt particle size, i.e., 1.8 nm (Table 2) and in turn, achieving higher TOF value i.e., 2.7 (Fig. 7). A comparison of results derived from Figs. 6 and 7 shows that there is still a chance to improve TOF by decreasing the cobalt particle size. Since Fig. 7 shows an exponential decrease of TOF against cobalt particle size a small decrease in cobalt particle size would be sufficient to get higher TOF. This
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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 x x x ( 2 0 1 6 ) 1 e1 3
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Fig. 3 e UVeVis DRS spectra of LaCo0.99X0.01O3 catalysts; (A) fresh (B) reduced (C) spent catalysts.
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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 x x x ( 2 0 1 6 ) 1 e1 3
Fig. 4 e SEM images of the LaCo0.99X0.01O3 catalysts.
work thus shows that there is still a scope for further enhancement in TOF. Recently, using LaeCeeCo mixed oxides with different LaO2eCeO2 ratios we have reported that the TOF is influenced by the cobalt particle size. The optimum results i.e. 68% hydrogen yield was achieved for cobalt particles having size 4e5 nm with TOF of 1.12 s1 [30]. In the present investigation the turn over frequency value is improved by more than two fold (2.54 s1) due to the presence of Pt and as a result the hydrogen yield reaches a maximum value of 78.4%. The stability of the catalysts was tested during the 10 h time-on-stream analysis and the results are presented in Fig. 8. The hydrogen yield of the Au doped perovskite is found to be decreasing suddenly after 4 h, while the Ag doped sample showed a steady decrease. On the other hand, both the Cu and Pt-doped catalysts showed no deactivation even after 10 h. The CHN analysis was carried out on the used catalysts (collected after 10 h on stream) in order to estimate the extent of carbon deposited on the samples during the reforming reaction. The results obtained (Table 2) clearly indicate a considerable difference in the amount of carbon formation. Cu and Pt-doped catalysts show much lower carbon accumulation than that of Ag and Au-doped catalysts. The higher amount of the carbon formed over the Au and Ag doped catalysts reveals that these catalysts are less efficient than the others.
Characterization of the spent catalysts The XRD patterns of the spent LaCo0.99X0.01O3 catalysts are presented in Fig. 1(C). The major peaks indicate the presence
of La2O3 and metallic cobalt. The diffraction lines observed at 25.86 and 30.44 can be assigned to the hexagonal (L) phase of La2O3 [JCPDS -74-2430, 42], while the peaks at 27.54, 39.98 and 54.76 represent the La2O3 in the cubic (L1) phase [JCPDS-220369, 51]. The peaks centered at 33.72, 38.18 and 47.34 in all the catalysts represent the unreduced perovskite (P) LaCoO3 [JCPDS-75-0279]. The peaks at 44.4, 51.88, 56.90 and 75.98 reveal the presence of metallic Co in the samples [52e56]. The metallic particles are not re-oxidized to cobalt oxides even when exposed to atmosphere. This is evidenced by the absence of the diffraction patterns related to cobalt oxides in all the samples. The active metal area varies inversely with the Co dispersion, as expected. Increase in Co dispersion due to the promotional effect of metals is in accordance with the literature report [47], particularly in the case of Pt [59]. The UV-DRS patterns of spent (LaCo0.99X0.01O3) catalysts are displayed in Fig. 3(C). In the case of Au catalyst the absorbance of peaks with maxima at 540, 580 and 670 nm confirm the presence of the cobalt in 2 þ oxidation state. On the other hand, no such peaks are identified in the rest of the studied samples explaining the presence of the finely dispersed metallic cobalt on the La2O3. It can also be clearly understood that except Au, all the other dopants (Ag, Cu and Pt) improved the reducibility of cobalt oxide and stabilized the Co metal in zero oxidation state, which is essential for the reforming activity. The H2-TPR profiles of spent catalysts are displayed in Fig. 2 (B). The analysis was mainly focused on understanding the nature of the carbon deposited on the catalysts during the reaction. It was reported previously that the free carbon (without hydrogen) yields a positive peak in the
Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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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 x x x ( 2 0 1 6 ) 1 e1 3
Fig. 5 e FT - IR spectra of the LaCo0.99X0.01O3 catalysts; (A) fresh (B) reduced catalysts.
range of 400 C and a negative peak for the carbonaceous moiety (CHx; x ¼ 2e3) in the temperature range 700 C [5658]. In the present investigation too, similar peaks are identified at relatively higher temperatures than the literature value indicating the formation of the free carbon as
well as the CHx moieties. The shift in the maxima of both the positive and negative peaks might be attributed to the stronger interaction of carbon with cobalt/support. The decrease in the intensity of the negative peak over the unpromoted catalyst compared to that over the promoted
Table 2 e Chemisorption and carbon formation data of LaCo 0.99X 0.01O3 catalysts. Catalyst LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3 LaCoO3
H2 uptake (mmole/g)
Active metal surface area (m2/g)
Average particle size (nm)
Metal dispersion (%)
Rate of carbon formation (m mol C) (mol carbon converted1)
465.9 446.4 394.4 311.5 305.2
37.8 35.6 31.2 24.9 22.0
1.8 2.4 5.1 8.1 8.6
47.6 43.5 19.4 12.3 11.1
4.2 4.5 6.6 11.9 19.9
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Table 3 e Composition of the reaction products formed at various temperatures. Catalysts
Percentage of products (wt. %) 400 C
LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3 LaCoO3
500 C
600 C
700 C
Liquid
Gas
Liquid
Gas
Liquid
Gas
Liquid
Gas
67.2 65.6 76.1 79.4 81.5
32.8 34.4 23.9 20.6 18.5
14.4 17.2 51.1 67.8 71.0
85.6 82.8 48.9 32.2 29.0
13.4 4.3 25.7 50.2 55.2
86.6 95.7 74.3 49.8 44.8
3.9 6.5 10.6 49.6 54.6
96.1 93.5 89.4 50.4 45.4
Table 4 e Product distribution during the evaluation of the LaCo0.99Pt0.01O3 catalyst. Composition of liquid product Reaction temperature ( C) (water-free) (wt. %) 400 500 600 700 MeeOH EteOH PreOH 1,2-PD 1,3-PD Total products
33.6 20.2 6.7 3.4 3.4 67.2
7.2 4.3 1.4 0.7 0.7 14.4
6.7 4.0 1.3 0.7 0.7 13.4
2.0 1.2 0.4 0.2 0.2 3.9
PD ¼ propanediol.
Fig. 7 e TOF vs. Co Average crystal size of the LaCo0.99X0.01O3 catalysts.
Fig. 6 e (1/QTOF) vs. Co Average crystal size of the LaCo0.99X0.01O3 catalysts.
catalysts (x ¼ Au, Ag, Cu, Pt) indicates the formation of partial hydrogenation products of the CO/CO2 liberated during the reaction or the dehydration products of the glycerol. From this it is clear that only the Pt and Cu promoted samples are efficient in promoting the WGS reaction which is necessary for improving the H2 yield. However, all the catalysts have suffered from coke formation as a result of CO disproportination (Boudouard) reaction. The early deactivation of the un-promoted as well as the Au promoted
Fig. 8 e Comparison of hydrogen yield at 700 C. catalysts might be attributed to high deposition of the free carbon and the CHx moiety. These results are in good agreement with the carbon analysis which indicates that the amount of the carbon is the maximum for the LaCoO3
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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 x x x ( 2 0 1 6 ) 1 e1 3
Table 5 e Crystal size data from XRD (Scherrer equation) analysis. Catalysts
Average particle size, (nm) Fresh (perovskite)
Co metal After reduction Spent
LaCo0.99Pt0.01O3 LaCo0.99Cu0.01O3 LaCo0.99Ag0.01O3 LaCo0.99Au0.01O3
12.2 28.4 39.8 62.5
e 3.9 5.7 9.3
e 4.1 17.2 39.3
and Au promoted samples. The carbon formation follows the order: LaCoO3> Au > Ag > Cu ~ Pt. These results were in good agreement with the CHN analysis and the H2 e TPR of the spent catalysts. From this it is clearly understood that Pt sites are predominantly responsible for maintaining part of the Co phase at its reduced/metallic state than supplying hydrogen for producing the partially hydrogenated products. The crystallite size data obtained from XRD analysis are presented in Table 5. Broadly, it appears that reduction has decreased the crystallite size, even though such phenomenon could not be emphasized in Pt-doped catalyst due to its amorphous nature. Whereas Cu-doped catalyst retained its crystallite size after the reaction, Ag and Au-doped catalysts showed considerable increase in crystallite size. Thus, Ag and Au-doped catalysts appear to be more susceptible to deactivation due to sintering than Pt and Cu-doped catalysts which explains the continuous decrease in hydrogen yield (Fig. 8).
Conclusions LaCo0.99Pt0.01O3 and LaCo0.99Cu0.01O3 exhibit higher hydrogen yield than LaCo0.99Au0.01O3 and LaCo0.99Ag0.01O3 catalysts. In addition, the Pt-doped catalyst shows close to 96% carbon conversion to gas. Cu and Pt are found to be efficient dopants for producing smaller cobalt metal particles with higher metal dispersion during the reduction process compared to Ag and Au dopants. A significant amount of carbon is formed during the time stream analysis on LaCo0.99Au0.01O3 and LaCo0.99Ag0.01O3, blocking the active sites responsible for the reforming reaction leading to faster deactivation. Cu and Pt incorporation in LaCoO3 appears to promote WGS reaction leading to enhancement in hydrogen yield.
Acknowledgment The authors thank Council of scientific industrial research & Department of Science and Technology (DST), New Delhi, India.
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Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075
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Please cite this article in press as: Surendar M, et al., Pt doped LaCoO3 perovskite: A precursor for a highly efficient catalyst for hydrogen production from glycerol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.075