Glycerol steam reforming over noble metal nanocatalysts

Accepted Manuscript Title: Glycerol steam reforming over noble metal nanocatalysts Authors: Alireza Zarei Senseni, Mehran Rezaei, Fereshteh Meshkani PII: DOI: Reference:

S0263-8762(17)30309-X http://dx.doi.org/doi:10.1016/j.cherd.2017.05.020 CHERD 2692

To appear in: Received date: Revised date: Accepted date:

26-2-2017 5-5-2017 22-5-2017

Please cite this article as: Zarei Senseni, Alireza, Rezaei, Mehran, Meshkani, Fereshteh, Glycerol steam reforming over noble metal nanocatalysts.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2017.05.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Glycerol steam reforming over noble metal nanocatalysts Alireza Zarei Senseni a, Mehran Rezaei a, *, Fereshteh Meshkani a

a

Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department,

Faculty of Engineering, University of Kashan, Kashan, Iran



Corresponding Author: Tel.: +98 315 5912469; E-mail address: [email protected]

Graphical abstract

1

highlights  Noble metal nanocatalysts were investigated in glycerol steam reforming. 

The alumina-stabilized magnesia support was prepared by co-precipitation technique.



The crystallite sizes of samples were in the range of 1.28 - 4.2 nm.



Rh catalyst showed the best catalytic performance in glycerol steam reforming.

Abstract The precious metal nanocatalysts (Rh, Ru, Pt, Ir) prepared over promoted Al2O3 with MgO were evaluated in steam reforming of glycerol for the production of hydrogen. Different characterization techniques (TEM, BET, TPR, TPO and H2S chemisorption) were employed to determine the physicochemical properties of the prepared samples. The chemisorption of H2S showed the crystallite sizes of the active metals were in the range of 1.28 - 4.2 nm. The catalytic results indicated that among the prepared catalysts, Rh and Pt possessed the highest activity in C3H8O3 steam reforming and Rh catalyst showed the highest stability with time on stream.

Keywords: C3H8O3; Steam reforming; Noble metal; Hydrogen

Nomenclature XRD

X-ray diffraction

BET

Brunauer-Emmett-Teller surface area analysis

TPO

Temperature programmed oxidation

TPR

Temperature programmed reduction

TEM

Transmission electron microscopy 2

SEM

Scanning electron microscopy

1. Introduction The fossil fuels combustion caused air pollution and global warming over recent years. In recent years the researches on the renewable feedstock have been raised to meet the energy requirements of the world(Surendar et al., 2015). Hydrogen can be used in fuel cells for the generation of clean power. Various methods for the production of hydrogen have been used based on the different types of feedstocks such as CH4, C2H5OH, C3H8O3, C2H6O2, etc (de Rezende et al., 2015). Some technologies have been proposed in the literature to produce hydrogen from glycerol such as: steam, dry and autothermal reforming, catalytic partial oxidation and gasification under H2O supercritical conditions (Kamonsuangkasem et al., 2013; Wess et al., 2015). C3H8O3 has several advantages among other renewable sources for the production of hydrogen such as renewable, getatable, environmental friendly and and it is used in different chemical and food industries. The high quantity of glycerol can be produced in biodiesel production via transesterification reaction of fatty acids. In this reaction, one mole of C3H8O3 can be produced from 3 moles of the synthesized methyl ester, which is equal to about 10 % of the weight of the product (GallegosSuárez et al., 2015; Sanchez and Comelli, 2014). One of the interesting methods for the use of the glycerol is reforming of C3H8O3 with steam for the generation of H2 with high purity. In this catalytic reaction, based on the Eq. 1, one mole of C3H8O3 produces 7 moles of hydrogen (Wang et al., 2013). (1) Which may be derived from the combination of Eqs. (2) and (3) (Senseni et al., 2016b): 3

(Glycerol decomposition) (2) and (Water gas shift reaction) (3) In addition, the influences of the operating conditions in glycerol steam reforming based on the thermodynamic calculations were reported (Dieuzeide and Amadeo, 2010; Kamonsuangkasem et al., 2013; Silva et al., 2015b). Over the recent years extensive efforts have been made in order to develop the heterogeneous catalysts for glycerol steam reforming process (Silva et al., 2015a). Precious metals show high catalytic activity and low carbon formation in this reaction (Pompeo et al., 2011). However, the use of these metals is limited due to their high cost. The nickel catalysts can be considered as the substitution of precious metals because of their lower costs. However, they suffer from the deactivation due to nickel sintering and coke formation (Jiménez-González et al., 2015). The results showed that the addition of precious metals in small content can improve the catalytic performance of the Ni catalysts in glycerol stream reforming (Araque et al., 2013; Huang et al., 2013). In recent years, there are only a few research studies on the catalytic performance of the Ni, Ir, Rh, Ru, Co, Pt, Ce and Pd based catalysts in glycerol steam reforming (Iriondo et al., 2009; Wu et al., 2013). The other reported results revealed that the nickel catalysts prepared over different carriers (Al2O3, CeO2 and ZrO2) possessed the H2 selectivity in the following order: Ni/ZrO2 > Ni/Al2O3 ≈ Ni/CeO2 (Manfro et al., 2013). Roger et al.(Araque et al., 2013) studied the effect of the Ce/Zr ratio on the catalytic characteristics of nickel catalyst in glycerol steam reforming. Dieuzeide et al. (Dieuzeide et al., 4

2016) investigated the influence of the MgO as a promoter for nickel catalyst. It was found that MgO played an important role in improving the dispersion of nickel and catalytic activity. The increase of the Mg content from 1.50 to 2.75%, improved the dispersion of Ni and increased the resistance of the catalyst against carbon formation. The effect of CeO2 on the catalytic performance of Ni catalysts was studied by Profeti et al. (Profeti et al., 2009). They concluded that the modification of the catalyst by Pd, Ru, Ir, and Pt increased the glycerol conversion and declined the formation of coke. Iriondo et al. investigated the catalytic activity of the Ni, Pt and Ni-Pt catalysts and also the influence of the La promoter in the glycerol steam reforming (Iriondo et al., 2009). The Ni-Pd and Ni catalysts supported on Al2O3-ZrO2, Al2O3-ZrO2-La2O3 were prepared and evaluated in the glycerol steam reforming by Yurdakul et al. (Yurdakul et al., 2016). They found that the most promising catalyst was Ni-Pd/Al2O3-ZrO2 and the maximum hydrogen yield for this catalyst was 74% at 800 °C. Rh/Al2O3 catalyst also showed higher glycerol conversion and catalytic stability compared to Ni supported catalysts. In this reaction, the catalytic activity is affected by the amount of deposited carbon produced by the thermal decomposition of glycerol (Chiodo et al., 2010). In this paper, the catalytic performance of noble metal catalysts supported on modified alumina with magnesium oxide was studied in the catalytic steam reforming of glycerol and the influences of various factors such as reaction temperature, feed ratio, and gas hour space velocity (GHSV) were investigated.

2. Experimental 2.1. Catalyst preparation 5

The samples were synthesized by the described method in our previous published paper (Nematollahi et al., 2011). In this method, Pt(NH3)4(NO3)2, Rh(NH3)6(NO3)3, IrCl3, and Ru(NO)(NO3) were used as active metal precursors. The catalysts with 1 wt.% of active metals were prepared by wet impregnation method. For this purpose, the alumina stabilized with MgO (Mg/Al molar ratio = 7, calcined at 950 °C) was impregnated with aqueous solutions of metal precursors with desired concentration to obtain 1 wt.% active metal in the final catalyst. The impregnated supports were then dried and calcined at 450 ºC.

2.2. Characterization The surface area was measured by a Tristar 3000 (Micromeritics) instrument. The H2S chemisorption was used to determine the dispersion, crystallite size and the surface area of the active metals. The details of the analysis were reported in (Nematollahi et al., 2011; RostrupNielsen, 1984). Temperature programmed reduction and oxidation (TPR and TPO) were performed using a ChemBET-3000 TPR/TPD (Quantachrome) instrument. The morphology of the samples was studied using scanning and transmission electron microscopes (SEM, Vega@Tescan and TEM, JEM-2100 UHR).

2.3. Catalytic tests The catalytic tests were done in a quartz microreactor (id: 7 mm, od: 10 mm and length: 600 mm) in the temperature range of 300 - 600 °C. 100 mg of the sieved particles (35 - 60 µm ) was placed inside the reactor and the catalyst was reduced under a pure hydrogen stream (30 ml/min) 6

at 600 ºC for 5 h. A mixture of C3H8O3 and H2O with the desired ratio was injected into a vaporizer and the mixture of N2 and Ar with a molar ratio of 1 (15 ml/min) was used as internal standard and carrier. The product composition was determined using a gas chromatograph (Varian 3400, TCD and Carboxen 1000). The following equations were applied in order to calculate the C3H8O3 conversion and the product selectivity (Senseni et al., 2016b; Wang et al., 2013).

In these equations, ndry,out was the total molar flow of gaseous products at the outlet of the reactor. Also, mole fraction, molar flow (mol.min-1) of gaseous products at the outlet stream of the reactor and CO2, CO, and CH4 selectivity are shown by n, y and i, respectively. 3. Results and discussion The textural properties of the catalysts are reported in Table 1 (Nematollahi et al., 2011). As can be seen, Rh and Ru samples exhibited higher BET area compared to other catalysts. In addition, the BET areas of the catalysts were higher than that observed for the catalyst support, which is related to change in the distribution of pores of the catalyst carrier after impregnation (Nematollahi et al., 2011). The prepared catalysts possessed smaller pore size compared to the catalyst support. The chemisorption of H2S indicated the crystal size of the active metals in the range of 1.28 - 4.2 nm. It is seen that the sizes of the active metal crystals are smaller than the 7

pore size of the carrier. This leads to the entering of the active metals into the pores of the carrier and altering the distribution of pore size (Khajenoori et al., 2013; Nematollahi et al., 2011). The results reported in Table 1 showed that the Rh sample exhibited the smallest crystal size and the highest Rh surface area due to the highest amount of adsorbed sulfur, while Pt catalyst displayed the biggest crystal size and the lowest value of adsorbed sulfur. The TEM images of the calcined samples are shown in Figure 1. The TEM analysis shows a high dispersion of active precious metals on the carrier. The sizes of the active metals were smaller than 5 nm, which were close to the crystal sizes determined by the H2S chemisorption analysis. The TPR analyses of the catalysts are shown in Figure 2. The Rh, Ir and Ru samples were reduced at the lowest temperatures, whereas the Pt catalyst was reduced at the highest temperature due to the strongest interaction with the carrier. The reduction peaks observed at 160 and 275 ºC in Ru and Ir catalysts, confirmed that the most content of the Ru and Ir can be reduced at low temperature. In other samples several reduction peaks were observed in TPR profile, indicating the several active metal species with different interactions with catalyst carrier (Khajenoori et al., 2013; Nematollahi et al., 2011). The catalytic activity of the samples is shown in Figure 3a. The results indicated that the catalytic activity varied in the following order: Rh> Pt > Ru>Ir. Among the prepared catalysts, the Rh and Pt exhibited the highest glycerol conversion in the temperature range of 300 - 600 ºC, while the lowest activity was seen for the Ir catalyst. The results revealed that the increase in reaction temperature improved the reactant conversion, due to the endothermic characteristic of the steam reforming of C3H8O3 (Adhikari et al., 2007; Dhanala et al., 2013, 2015; Senseni et al., 2016b). It is known that the formation of methane and other hydrocarbons such as ethylene are possible at temperatures below 600 ˚C (Sadanandam et 8

al., 2014). The CH4 and CO selectivities decreased with the increase of reaction temperature, as a result of steam reforming reaction of CH4 and water gas shift reaction, respectively. In addition, the selectivities of H2 and CO2 increased due to the occurrence of glycerol decomposition, water gas shift reaction, carbon gasification and methane steam reforming. The trend of change of selectivity of all components is shown in Figures 3b – e. This trend was adapted to thermodynamic analysis (Adhikari et al., 2007; Arandiyan et al., 2012; Dhanala et al., 2013, 2015; Dieuzeide and Amadeo, 2010; Kamonsuangkasem et al., 2013; Sadanandam et al., 2012; Senseni et al., 2016a; Senseni et al., 2016b; Silva et al., 2015b; Wang et al., 2013). Rh catalyst has the lowest CH4 production than the other catalysts. According to the results, the following trend was observed for H2 selectivity in all temperatures Rh > Ir > Ru > Pt Rh as the active metal is able to improve the desirable reactions such as CH4 steam reforming and carbon gasification and breakages of C-C and C-H bonds, which favor the H2 production (Araque et al., 2013). According to the results of H2 selectivity and glycerol conversion obtained in this study, the Rh catalyst showed the most promising catalytic performance in glycerol stream reforming. To study the stability of the catalysts, the glycerol conversion was monitored during 5h time on stream at 600 ºC and the results are shown in Figure 4. Rh showed the highest stability among the prepared catalysts and the Ru catalyst exhibited a dramatic decrease in glycerol conversion with time on stream, which could be due to carbon accumulation on the catalyst surface. The influence of H2O:C3H8O3 molar ratio on the catalytic characteristics of the Rh catalyst is shown in Figure 5. It is seen that the C3H8O3 conversion and H2 and CO2 selectivities increased 9

by the increase of H2O:C3H8O3 from 3 : 1 to 9 : 1, whereas CO and CH4 selectivities decreased due to the progress of water gas shift reaction and steam reforming of methane (Bobadilla et al., 2012; Iriondo et al., 2012; Manfro et al., 2013). The effects of GHSV on the glycerol conversion and the components selectivity are shown in Figure 6. As seen, by increasing GHSV, the contact time between the catalyst surface and reactants decreased and led to a decline in C3H8O3 conversion. In addition, the H2 and CO2 selectivities decreased by increasing the GHSV, whereas CO and CH4 selectivities increased. The reasons for this change could be related to the reverse water gas shift reaction and the coke formation on the catalyst surface (Benito et al., 2005; Senseni et al., 2016a; Senseni et al., 2016b; Sepehri et al., 2016). The long-term stability of the Rh/MgAl2O4 is shown in Figure 7. As indicated from the results, the conversion remained almost stable for 20 h time on stream. Figures 8a shows the TPO analysis of the spent catalysts. The Rh/MgAl2O4 catalyst exhibited the oxidation peaks at about 380 ºC and 480 ºC, which are related to the formation of amorphous and graphitic carbon, respectively. The corresponding peaks for Ru, Ir and Pt catalysts at 420 ºC and 450 ºC are attributed to the graphitic carbon. Figure 8b shows the TPO profiles of the spent Rh catalyst under various H2O:C3H8O3 ratios and stability test.As can be seen, one peak was observed in TPO curve of the catalysts after the reaction. Increasing in H2O:C3H8O3 ratio decreased the peak area, indicating the decrease in the content of deposited carbon. The SEM image of the spent Rh/MgAl2O4 catalyst in Figure 9 showed a high resistance of this catalyst against carbon formation. As can be seen, no observable carbon was seen on this catalyst. 10

4. Conclusions Glycerol steam reforming was studied over precious metal catalysts. According to the BET results the Rh and Ru catalysts displayed the highest specific surface among the other prepared catalysts (Ir and Pt). The following trends were observed for the glycerol conversion and H2 selectivity at all temperatures: Glycerol conversion: Rh> Pt > Ru > Ir and H2 selectivity: Rh > Ir > Ru > Pt The Rh catalyst was selected as the optimum catalyst and exhibited high catalytic performance under different feed ratios and feed flow rates. According to the result of the catalytic performance, the optimum performance was achieved under the reaction temperature of 600 °C, GHSV of 35000 mL.g-1.h-1 and feed ratio of 9. The Rh catalyst showed glycerol conversion and H2 selectivity of about 100% at a reaction temperature of 600 ºC. By the increase of GHSV from 35000 to 70000 ml. h-1 g-1, the glycerol conversion declined from 100% to 84%. The Rh catalyst also exhibited a high stability for 20 h under time on steam.

Acknowledgements The authors are grateful to University of Kashan for supporting this work by Grant No. 158426/137.

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Fig. 1: TEM analysis of the reduced catalysts, (a) Rh, (b) Ru, (c) Pt and (d) Ir

17

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Fig. 2: TPR profiles of the noble metal catalysts, Rh (1), Ru (2), Ir(3) and Pt (4) (Nematollahi et al., 2011).

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Fig. 3. a) Glycerol conversion, b) H2 selectivity, c) CO selectivity, d) CO2 selectivity and e) CH4 selectivity, GHSV=35000 mL.g-1.h-1 and H2O:C3H8O3=9

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21

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Fig. 4. Stability of the catalysts at 600 °C, GHSV= 35000 mL.g-1.h-1and H2O:C3H8O3=9

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Fig. 5. Effect of H2O:C3H3O3 ratio on the catalytic performance of Rh/MgAl2O4 catalyst at T = 600 °C, GHSV=35000 mL.g-1.h-1

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Fig. 6: Effect of GHSV on the catalytic performance of Rh/MgAl2O4 catalyst at 600 ºC and H2O:C3H8O3 = 9

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Fig. 7. Long-term stability of Rh/MgAl2O4 catalyst at 600 °C, GHSV = 35000 mL.g-1.h-1 and H2O:C3H8O3 = 9

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Fig. 8. TPO analysis of the spent catalysts (a) Noble metal catalysts at T = 300 – 600 ˚C, H2O/C3H8O3 = 9 and GHSV = 35000 mL g

-1

h -1: and (b) Rh/MgAl2O4 catalyst at different

H2O/C3H8O3 ratios at T = 600 ˚C and GHSV = 35000 mL. g -1 .h-1

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Fig. 9. SEM image of spent Rh/MgAl2O4 after 20 h time on stream at T=650˚C, H2O:C3H8O3 = 9:1 and GHSV = 35000 mL.g-1.h-1

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Table 1. Structural properties of the reduced catalysts (Khajenoori et al., 2013; Nematollahi et al., 2011)

Catalyst

Pt Rh Ru Ir Support

BET area (m2.g-1)

85.56 159 120.84 95.33 36.60

Pore

Pore

Sulfur

Metal

volume

diameter

capacity

area

(cm3.g-1)

(nm)

(wt%)

(m2/g)

0.238 0.276 0.253 0.262 0.142

10.66 6.95 7.53 8.95 14.33

295 1650 700 625 _

0.67 3.75 1.59 1.40 _

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Metal crystallite size (nm) 4.24 1.28 3.96 1.86 _