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Procedia Engineering 51 ( 2013 ) 461 – 466

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of Institute of Technology, Nirma University, Ahmedabad.doi: 10.1016/j.proeng.2013.01.065

Chemical, Civil and Mechanical Engineering Tracks of 3rd Nirma University International Conference(NUiCONE 2012)

Production of synthesis gas by carbon dioxide reforming of methane over nickel based and perovskite catalysts

Sandeep K. Chawla, Milka George, Femina Patel*, Sanjay Patel

Chemical Engineering Department, Institute of Technology, Nirma University, Ahmedabad, 382481 India

Abstract

CO2 reforming of methane shows significant environmental and economic benefits by consuming two major greenhouse gases, carbon dioxide (CO2) and methane (CH4) to produce synthesis gas (CO+H2). It offers advantages such as the production of syngas with a lower H2 /CO ratio and it obviates a water vaporization step to produce steam, an energy consumer process and eliminate CH4 and CO2. However, during process significant catalyst deactivation due to carbon formation takes place. Therefore, it is of great interest to develop catalysts which pose high stability without severe deactivation. This paper is mainly focused on the development of base metal like nickel-based catalyst by impregnation and perovksites by sol gel (SG) and coprecipitation (CP) method for CO2 reforming of CH4. The catalysts were characterized by the XRD and BET. The Nickel catalyst prepared by impregnation method using support -Al2O3 and different types of promoters to improve activity, stability and selectivity in order to reduce coke formation and to achieve long-term operation. Nickel catalysts promoted by the ZrO2 shows higher dispersion of the metal particle on the surface of the support than the unpromoted catalysts. It has been found that the ZrO2, CeO2, K2O and MgO promoted 10 % Ni/ -Al2O3 catalysts exhibited good activity, stability and long-term operation as compared to the unpromoted catalysts. It reduces the deactivation rate. The use of perovskite like oxides ABO3 has increased recently for carbon dioxide reforming of methane. Conversion of CH4 and CO2 were found in the order: LaNiO3(CP)> 10%Ni/ ZrO2- -Al2O3 > LaNiO3(SG) >10%Ni/ K2O - -Al2O3 > 10%Ni/ MgO - -Al2O3 > LaCoO3(CP) > LaCoO3(SG) > 10%Ni/ CeO2 - -Al2O3 > 10%Ni/ -Al2O3 > 5%Ni/ -Al2O3 > 20%Ni/ -Al2O3. LaNiO3 (CP) gave high activity and stability due to further reduction of B-site cations which remain distributed in the structure and form well dispersed and stable metal particle catalysts which improved the stability and activity.

Keywords: CO2 reforming; perovskites; syngas; sol gel

1. Introduction

The demand of syngas in industrial fields has increased due to its potential in many applications such as Fischer-Tropsch process in which syngas is converted into a series of liquid hydrocarbon fuels or its direct conversion as a reactant to dimethyl ether or methanol for petrochemical industries. Syngas can be produced in many ways such as through steam reforming (SR), auto thermal reforming of methane (ARM), dry CO2

reforming of methane (DRM) and partial oxidation of methane (POM). Each of the process described above shows different advantages and limitations such as H2/CO ratio, energy efficiency, catalytic activity and stability. Recently the development of CO2

reforming of methane has been investigated to produce syngas [1-3].

CO2 reforming can be economically advantageous over the other syngas production ways depending on the required H2 to CO ratio. For example, if the ratio (H2 to CO) 0-0.5 is needed, CO2 reforming is preferred. For a ratio of 0.5-0.9, CO2reforming competes with the partial oxidation of bitumen. The major problem in CO2 reforming is the carbon deposition on the surface of the catalyst [1-5].

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462 Sandeep K. Chawla et. al / Procedia Engineering 51 ( 2013 ) 461 – 466

CH4 + CO2 2CO + 2H2 ( H 298K = 247 kJ/mol) (1) Moreover, the rapid catalyst deactivation due to coke formation is a serious concern especially for nickel metal catalyst. The use of perovskite material as the catalyst could potentially overcome this problem due to its well-defined structures which produces highly-dispersed metallic particles to promote high activity which suppresses coke formation and enhances catalytic stability [7-14]. The general chemical formula for perovskite compounds is ABX3 or ABO3 , where 'A' and 'B' are two cations of very different sizes and X or O is an anion that bonds to both. The 'A' atoms are larger than the 'B' atoms. A ion can be rare earth, alkaline earth, alkali and other large ions such as Pb+2, Bi+3 that fits in to the dodecahedral site of the framework. The B ion can be 3d, 4d and 5d transition metal ions which occupy the octahedral sites (surrounded by six oxygen atoms in octahedral coordination). Perovskite compounds can also tolerate significant partial substitution (A and/or B with metals (A', B' correspondingly) of different oxidation states) and non-stoichiometric while still maintaining the perovskite structure. Metal ions having different valence can replace both A and B ions. This may generate a non-integral number of oxygen atoms [9-14].

2. Experimental 2.1. Catalysts preparation Preparation of perovskites based catalyst - LaBO3 (B=Ni, Co) Sol Gel Method (SG)

Metal nitrates of La(NO3)3.6H2O, Co(NO3)2.6H2O and Ni(NO3)2.6H2O along with citric acid were dissolved in water at an equivalent ratio of 1:1 (metal cations:citric acid). Ethylene glycol and citric acid were used to make the gel. The Ethylene Glycol and citric acid were added drop-wise to the nitrate solution and they were stirred for 15 min. The resulting solutions were heated to 80 0C to form a viscous gel finally yield a solid precursor upon slow solvent evaporation at that temperature for several hours. This gel was dried in an oven at 110 0C overnight and after thorough grinding of the resulting powder, it was finally calcined under air at 750 oC for 5 hr in order to achieve the corresponding perovskite structure in the samples.

Co-Precipitation Method (CP)

The co-precipitation samples were produced by precipitation of the nitrate precursors. A certain amount of nitrate salts containing the following cations La+3, Co+2 and Ni+2 were dissolved in distilled water. The aqueous 1 M Na2Co3 solution was added drop-wise to the nitrate solution under vigorous stirring until pH 10 was attained at room temperature. The precipitate was allowed to age for 1 hr at room temperature. The obtained precipitate was filtered and washed with distilled water until a pH of 7 was reached. The compound was then dried at 110 0C overnight in hot air oven and crushed to a fine powder. Fine powder was calcined in muffle furnace at 750 0C under air atmosphere for 5 hr. Preparation of Ni based catalyst supported by -Al2O3 Impregnation

Alumina supported nickel catalyst with different promoters were prepared by impregnation. The desired concentration of nitrate solution was prepared for the given mass of alumina pellets and stirred well. The excess water from the slurry was removed by evaporation. Then impregnated pellets dried at 110 0C overnight in oven followed by calcinations at 500 0C in the presence of air. The solutions of nickel nitrate, cerium nitrate, potassium nitrate, zirconium nitrate, magnesium nitrate were made by dissolving them in sufficient quantity of distilled water. Alumina pellets were crushed to a size of approximately 1 mm before it was used for impregnation. The crushed particles were allowed to soak for 4 h under vigorous stirring in the solution to ensure uniformity of deposition. The excess water was removed by evaporation and impregnated wet pellets were dried overnight at 110 0C followed by calcinations in presence of air at 500 0C for 4 hr.


2.2. Catalyst characterization

The crystalline phases identification was done by X-ray diffraction using PW1774 Spinner Diffractometer system XPERT-MPD operated at 40 kV and 30 mA with Ni-filtered Cu K radiation ( = 1.5406 Å). Diffractograms were recorded with step scans from 20 to 990 in 2 angle and 1 s for each 0.050 step. Phase detection was made by comparison with JCPDS data bank. Specific surface areas were obtained from adsorption/desorption isotherms of N2 at -196 0C determine using Micromeritics ASAP 2020 instrument. Samples were degassed at 300 0C under vacuum (10-3 Pa) until complete removal of humidity for 3 h prior to adsorption/desorption experiments. The specific surface area was determined from the linear part of the BET curve. The pore size distribution was calculated from the desorption branch of N2 adsorption/desorption isotherms using the Barrett–Joyner–Halenda (BJH) formula. Pore volume and average diameter were also obtained from the pore size distribution curves using the software. 2.3. Catalyst activity test

Catalytic activities were carried out in a fixed bed of catalyst particles (ca. 0.75 g) mixture previously added with 3 g of SiO2 (0.5–1.5 mm granulate) in order to reduce the specific pressure drop across the reactor between two ceramic blanket wool and inserted into stainless steel fixed bed reactor (I.D. 1.805 cm, O.D. 1.905 cm and L.50 cm). The reactor was placed in a tubular PID-regulated oven and the temperature was monitored with a K type thermocouple positioned in correspondence to the catalyst bed. The catalyst was first reduced with H2 mixed with N2 (H2:N2=10:90 by v/v) at room temperature to 350 0C with ramp of 5 0C/min for 2 hr. After reduction, feed gas (CO2 :CH4 :N2 = 1:1: 1) with the total flow rate 504 ml/min was passed through the rector. Reaction temperature was raised from 200 to 800 0C and product stream was analyzed by gas-chromatography (GC 2010 Model) using an Shin Carbon ST 100/120 micropacked column and a μTCD detector.

3. Results and discussion

The XRD patterns of 10 %Ni/CeO2- -Al2O3 (fig. 1 (a)) shows intense diffraction lines of NiO (2u=440,640,760) and -Al2O3 (2u=470,680). In this catalyst, two phases of -Al2O3 and three crystalline phase of nickel were detected. The overlap of -Al2O3 and nickel oxide form composite layer of NiO - -Al2O3 which is an amorphous phase. The smaller amount of Ni interact with alumina and form nickel aluminate (NiAl2O4) composite layer which is an amorphous phase or a crystalline phase with crystallite sizes smaller than the detection limit of XRD.

(a) (b)

Fig.1. XRD patterns of (a) 10Ni /CeO2- -Al2O3 (b) LaNiO3 (Sol-Gel)

The XRD pattern of LaNiO3 (Sol-Gel) shows intense diffraction lines of LaNiO3 (2 =32.75, 47.28, 58.62). In fig. 1 (b) the catalysts calcined at 750 ˚C exhibit an intensive peak of LaNiO3 and a weak peak of LaNiO3 It is shown that, for LaNiO3 prepared by Co-precipitation method, the pure Rhombohedral perovskite-type structure is formed with other phase like La(OH)3.

The specific surface area (BET surface area), pore size and pore volume of the samples are listed in Table 1.


Table 1. BET surface area, porev volume , pore diameter of different Catalysts

Sr. no.

Catalyst BET Surface Area (m2/g )

Pore diameter (nm)

Pore volume (cm³/g)

1. LaCoO3– Sol-gel 12.0632 17.0473 0.051411

2. LaNiO3– Co-precipitation 12.1 12.623 0.038

3. 10%Ni/ -Al2O3 137 0.218 6.385

4. 10%Ni/CeO2- -Al2O3 106.8 0.206 7.740

Conversion of CH4 and CO2 were found in the order: LaNiO3(CP)> 10%Ni/ ZrO2- -Al2O3 > LaNiO3(SG) >10%Ni/ K2O

- -Al2O3 > 10%Ni/ MgO - -Al2O3 > LaCoO3(CP) > LaCoO3(SG) > 10%Ni/ CeO2 - -Al2O3 > 10%Ni/ -Al2O3 > 5%Ni/ -Al2O3 > 20%Ni/ -Al2O3

Conversion of CH4 and CO2 over LaNiO3 catalyst were slightly higher than those on the LaCoO3 catalyst prepared by the same preparation method (either CP or SG) at the same temperature. This may be attributed to the large number of active sites available on pre-reduced LaNiO3 catalysts compared to pre-reduced LaCoO3. This may be also due to the low reducibility of the Co-based perovskites under pre-treated conditions. Co-based perovskite is more stable under reducing atmosphere than Ni- based perovskite. This may be the reason for the low reforming activity over Co-based catalysts. LaNiO3 catalyst prepared by the Co precipitation method gives higher conversion when compared to LaNiO3 catalyst prepared by the Sol gel method.

Table.2. Activity of different catalyst for CO2 reforming with CH4

Catalysts % Conversion of CH4 % H2 Yield

% CO Yield

H2/CO ratio

Stability Ratio (C8/C1) Ratio

5%Ni/ -Al2O3 51.53 10.5 21.7 0.48 -

10%Ni/ -Al2O3 68.08 45.1 86.5 0.52 -

20%Ni/ -Al2O3 34.23 9.4 21.0 0.44 -

10%Ni/CeO2- -Al2O3 71.08 65.4 88.9 0.73 0.81

10%Ni/ZrO2- -Al2O3 77.92 68.9 74.6 0.92 0.48

10%Ni/2% K2O- -Al2O3 75.09 76.2 83.0 0.91 0.68

10%Ni/2%MgO- -Al2O3 74.78 74.6 82.5 0.90 0.69

LaCoO3 - SG 72.00 25.0 37.0 0.65 0.73

LaNiO3- SG 77.00 47.0 60.0 0.85 0.71

LaCoO3- CP 74.00 42.0 49.0 0.74 0.80

LaNiO3- CP 86.00 63.0 66.0 0.90 0.67

These results are much similar to: CO2 Reforming of CH4 over Nickel and Cobalt Catalysts Prepared from La-Based Perovskite Precursors by Jianjun Guo et al [1] and structural features and performance of LaNi1-x RhxO3 system for the dry reforming of methane by M.E. Rivas et al [13].


Fig. 2. Conversion of CH4 as function of temperature over different catalysts

Comparison for Time on stream stability test of catalysts for CO2 reforming with CH4

The stability improvement can be estimated by the ratios between the CH4 conversion after 8 hr and CH4 conversion after 1 hr of time -on- stream (as shown in the fig. 3. The stability was better over the CeO2 promoted catalyst compared to other promoted Ni based catalysts. The observed stability of these Ni catalysts during 8 hrs on-stream indicates formation of small metal particles evenly distributed over the support.

In the reaction on pre-reduced LaNiO3 catalysts, the conversion of CH4 and CO2 increased with the temperature but the rate of coke formation over pre- reduced LaNiO3 catalyst was high that the CH4 conversion decreased over 22% over 8 hrs. This may be attributed to the large number of active sites available on pre-reduced LaNiO3 catalysts compared to pre-reduced LaCoO3 catalysts. So, the effect of preventing Ni from agglomerating by La2O3 varies and this maybe the main cause of the large amount of carbon deposited on LaNiO3 catalysts. The stability was better over the pre-reduced LaCoO3-Coprecipitation catalyst where the CH4 conversion decreased over 16% over 8 hrs. The catalyst obtained from activated LaCoO3 -Co precipitation precursor displayed good stability. The observed stability of these Co catalysts during 8 hrs on-stream indicates formation of small metal particles evenly distributed over the support. High stability may be also linked to the formation of La2O2CO3 phase. The carbon species formed on the Co-sites are favourably removed by the oxygen species originating from La2O2O3 giving rise to active and stable performance due to the existence of synergetic sites consisting of Co and La elements. As observed H2/CO ratios are slightly lowers than 1 corroborating the occurrence of RWGS reaction by which some of the produced H2 reacts with CO2 to yield CO and H2O.

Fig. 3. Time on stream stability test of catalysts for CO2 reforming with CH4

4. Conclusion

Nickel-based catalyst synthesized by impregnation and perovksites catalyst by sol gel and co precipitation method for CO2 reforming of CH4. The results of XRD, BET characterization revealed that the strong interaction between nickel and promoter (ZrO2) over support which prevents metal particles from aggregating and increase the resistance to coking. The


LaNiO3 and 10%Ni/ZrO2- -Al2O3 catalyst displayed high catalytic performance for CO2/ CH4 reforming at 800 °C because the small size of nickel particles maintained enough active sites on the surface of catalyst. Conversion of CH4 and CO2 were found in the order: LaNiO3(CP)> 10%Ni/ ZrO2- -Al2O3 > LaNiO3(SG) >10%Ni/ K2O - -Al2O3 > 10%Ni/ MgO - -Al2O3 > LaCoO3(CP) > LaCoO3(SG) > 10%Ni/ CeO2 - -Al2O3 > 10%Ni/ -Al2O3 > 5%Ni/ -Al2O3 > 20%Ni/ -Al2O3. The stability was better over the pre-reduced LaCoO3-CP and 10%Ni/CeO2- -Al2O3 catalyst. This may be also due to the low reducibility of the Co-based perovskites. The carbon species formed on the Co-sites and Ni sites are favourably removed by the oxygen species originating from La2O2CO3 and CeO2 giving rise to active and stable performance due to the existence of synergetic sites consisting of Co and La, Ni and Ce elements. References

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