Dry reforming of methane over LaNi1−yByO3±δ (B = Mg, Co) perovskites used as catalyst precursor

doi:10.1016/j.apcata.2007.10.010

Applied Catalysis A: General

Volume 334, Issues 1–2, 1 January 2008, Pages 251-258

https://doi.org/10.1016/j.apcata.2007.10.010 Get rights and content

Abstract

Perovskites LaNiO₃, LaNi_1−xMg_xO_3−δ and LaNi_1−xCo_xO_3−δ were synthesized by auto combustion method. TPR analysis reveled that Mg or Co substituted perovskites were more difficult to reduce. The perovskites were evaluated as catalyst precursors in the dry reforming of methane. Catalysts obtained by reduction of LaNiO₃ and LaNi_1−xMg_xO_3−δ perovskite had the highest catalytic activity for CO₂ reforming of CH₄ at 700 °C using drastic reaction conditions (10 mg of catalyst, a mixture of CH₄/CO₂ without dilution gas). Methane and carbon dioxide conversions were 57% and 67%, respectively, with a H₂/CO ratio equal to 0.47.

The presence of cobalt leads to a decrease of the catalytic activity. This decreasing of activity may be attributed to the Co–Ni alloy formation. Computational calculations revealed that Ni atom cleaves the C–H atom while Co is not able to activate the CH₄ molecule. The interaction energy of CH₄ with the Ni and CO atom was 18 kcal/mol and 0.7 kcal/mol, respectively.The catalysts were characterized by TPR, TEM and in situ XRD.

Graphical abstract

Perovskites LaNi_1−xB_xO_3−δ (B = Mg,Co) were evaluated as catalyst precursors in the dry reforming of methane. LaNi_1−xMg_xO_3−δ perovskite had the highest catalytic activity at 700 °C using drastic reaction conditions without dilution gas. CH₄ and CO₂ conversions were 57% and 67%, respectively, with a H₂/CO ratio equal to 0.47. The partial substitution of Ni by Co leads to a decrease of the catalytic activity.

Introduction

Methane reforming using carbon dioxide as oxidant agent (CH₄ + CO₂ → 2CO + 2H₂) to produce synthesis gas (CO + H₂) has received considerable attention in recent years. This process is particularly important to be taken into consideration when a gas field contains a significant amount of methane and carbon dioxide, both gases being undesirable greenhouse gases. A low H₂/CO ratio is preferentially used for the production of liquid hydrocarbons in the Fischer–Tropsch synthesis as well as for the production of formaldehyde, polycarbonates or methanol [1], [2].

Due to the high endothermicity of the reaction, the process can be used with different purposes such as: a system for solar energy transfer to chemical energy, energy storage in the form of CO and H₂ and also in chemical energy transmission systems (CETS) [3].

The two main drawbacks of the process are: (1) the requirement of temperatures as high as 800 °C required to obtain high conversions and (2) the catalyst deactivation due to carbon deposition [4], [5]. The principal reasons for coke formation are methane decomposition (Reaction (1)) and the Boudouard reaction (Reaction (2)). The first reaction is favored at high temperatures and low pressures, whereas the second one is favored at low temperatures and high pressures.CH₄ ⇔ 2H₂ (Methane decompostition)2CO ⇔ C + CO₂ (Boudouard reaction)

The methane reforming reaction has been investigated over supported catalysts using noble metals [6], [7], [8], [9], [10] and transition metals [11], [12], [13]. The reported order of activity is the following: Rh, Ru > Ir > Ni, Pt, Pd > Co > Fe, Cu [14] being the novel metals less sensitive to deactivation by carbon deposition.

Industrially the metal of choice is nickel due to its inherent availability, high activity in the methane reforming reaction, interesting redox properties and relatively low cost. However, it is difficult to prevent sintering of nickel and deposition of carbon at high temperatures. Nevertheless it has been shown that a high dispersion of the metal species over the support can reduce coke formation [15].

Shiozaki et al. [16] showed that a metal oxide with a well-defined structure can be a source of small metal particles. Hayakawa et al. used CaTi_1−xNi_xO₃ as catalyst precursor for the partial oxidation of methane to syngas [17], highly dispersed Ni metals were formed in situ during the reaction resulting in a high activity and stability.

In this direction, efforts have been carried out to synthesize Ni catalysts resistant to carbon deposition by introducing the metal in the perovskite type oxides (ABO₃). These are well-defined structures which can produce very small particles after reduction treatment, well-dispersed at the surface of a basic support [18], [19]. This particle size provides favorable conditions to avoid carbon formation [20] and increases the activity and stability of the catalyst [21].

Thermodynamic calculations of graphitic carbon deposition suggest that carbon formation could be avoided by working at high temperatures (e.g., ±1000 °C) as well as by the presence of excess of CO₂, H₂O or O₂ [22], [23]. Also, coke formation can be suppressed by addition of alkaline earth metals with high basicity [24].

From an industry standpoint, it is desirable to operate the dry methane reforming process at a relatively lower temperature and with a CO₂/CH₄ ratio close to unity without using gas of dilution; nevertheless this process has not been implemented industrially.

In previous papers we have shown that LaNiO_3−δ and La₂NiO₄ type perovskites used as catalyst precursor lead to a very active and stable catalysts for the CO₂ reforming of CH₄ [19], [21].

In this work magnesium and cobalt modified Ni perovskite type oxides, of the general formula LaNi_1−xB_xO₃, were synthesized by the auto combustion method and used as catalyst precursors in the dry reforming of methane, using a gas feed of CH₄/CO₂ = 50/50 mL/min without dilution gas with the purpose of simulating desirable industrial conditions. The effect of adding cobalt or nickel on the catalytic activity and stability was studied.

Section snippets

Catalysts precursor preparation

The precursor perovskites LaNi_1−xB_xO_3−δ (B = Mg, Co) were prepared by the self-combustion method [25], [26]. In the synthesis, La(NO₃)₃·6H₂O (Rhodia), Ni(NO₃)₂·6H₂O (Aldrich), Mg(NO₃)₂·6H₂O (Aldrich), Co(NO₃)₂·6H₂O (Merck) and glycine (Merck) were used.

Glycine (H₂NCH₂CO₂H) used as ignition promoter was added to an aqueous solution of metal nitrates with appropriated stoichiometry, in order to get a NO₃⁻/NH₂ = 1 ratio. The resulting solution was slowly evaporated at about 100 °C until a vitreous

Crystalline structure

The X-ray diffraction patterns obtained for LaNiO_3−δ show that after calcination at 700 °C only the presence of the LaNiO_3−δ perovskite rhombohedral phase (JCPDF Card No. 88-0633) was observed. After the reduction treatment under hydrogen at 700 °C, the LaNiO_3−δ perovskite structure was completely destroyed being Ni° and La₂O₃ observed by X-ray.

The XRD patterns of the perovskites type oxide LaNi_1−xCo_xO_3−δ and LaNi_1−xMg_xO_3−δ after calcination, after reduction and after reaction are shown in Fig. 1

Conclusions

LaNiO₃, LaNi_1−xMg_xO_3−δ (x = 0.02, 0.06 and 0.1) and LaNi_1−xCo_xO_3−δ (x = 0.1, 0.3, 0.5, 0.8 and 1) oxides were synthesized and evaluated as catalyst precursors in the dry reforming of methane. TPR analysis reveled that Mg or Co substituted perovskites were more difficult to reduce. The ease of reduction of the cobalt series was: LaNiO₃ > LaNi_0.9Co_0.1O₃ > LaNi_0.7Co_0.3O₃ > LaNi_0.5Co_0.5O₃ > LaNi_0.2Co_0.8O₃ > LaCoO_3. The ease of reduction of the LaNi_x−1Mg_xO₃ series was: LaNiO₃ > LaNi_0.98Mg_0.02O₃ > LaNi_0.94Mg_0.06O₃.

Acknowledgements

The authors are grateful to the PICS program: “Valorization of natural gas and Fischer–Tropsch synthesis” for the financial support given. F. Mondragon and G. Sierra Gallego acknowledge to the University of Antioquia for the financial support of the Sostenibilidad Program and to Colciencias for the support of the project 1115-06-17639. G. Sierra thanks Colciencias and the University of Antioquia for the Ph.D. scholarship.

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Dry reforming of methane over LaNi1−yByO3±δ (B = Mg, Co) perovskites used as catalyst precursor

Abstract

Graphical abstract

Introduction

Section snippets

Catalysts precursor preparation

Crystalline structure

Conclusions

Acknowledgements

Fuel Processing Techn.

J. Catal

J. Catal.

J. Catal.

Catal. Today

J. Catal.

J. Catal.

Catal. Today

Appl. Catal. A

Stud. Surf. Sci. Catal.

Appl. Catal. A

Stud. Surf. Sci. Catal.

J. Catal.

Catalysis Today

J. Catal.

J. Catal.

Appl. Catal.

Mater. Lett.

Catal. Today

J. Catal.

Appl. Catal. A: Gen.

J. Catal.

Dry reforming of methane over LaNi_1−yB_yO_3±δ (B = Mg, Co) perovskites used as catalyst precursor