Catalytic partial oxidation of methane to synthesis gas over Ni–CeO2

doi:10.1016/S0926-860X(00)00728-6

Applied Catalysis A: General

Volume 208, Issues 1–2, 14 February 2001, Pages 403-417

https://doi.org/10.1016/S0926-860X(00)00728-6 Get rights and content

Abstract

The partial oxidation of methane to syngas was studied in this work over Ni-containing ceria catalysts with nickel content of 5, 10 and 20 at.% at atmospheric pressure. All catalysts, in the as prepared state, showed similar activity and CO selectivity at T≥550°C. Catalyst pre-reduction was not required. Reaction mixtures were dilute, containing 3 mol% CH₄ and 1.5 mol% O₂. Methane conversion and CO selectivity approached their respective thermodynamic equilibrium values above 550°C. The H₂/CO ratio was equal to 2 at T>600°C. In the range 0.54–0.04 g s/cm³ (STP), contact time effects were absent in partial oxidation of methane over the 5 at.% Ni–Ce(La)O_x catalyst. The phase composition, nickel dispersion and carbon deposition on the catalysts were investigated by various characterization techniques, including XRD, STEM/EDS, XPS and TPO analyses. The 5 at.% Ni–Ce(La)O_x catalyst, comprising highly dispersed nickel oxide in ceria, showed excellent resistance to carbon deposition and stable performance during 100 h-on-stream at 650°C. On the other hand, high-content (>10 at.%) nickel in ceria, comprising both dispersed nickel and bulk nickel oxide particles, was unstable even after a much shorter time-on-stream; carbon deposition was clearly the cause of this performance instability.

Introduction

Natural gas is the cleanest fossil fuel and the most desirable feedstock for chemicals production. Steam reforming of natural gas is widely used to produce synthesis gas for various chemicals. The catalytic partial oxidation of methane (POM) to synthesis gas: $CH_{4} + 12 O_{2} → CO +2 H_{2} (Δ H=−22.2 kJ / mol, 1000 K)$ has been under intense study [1], [2], [3], [4], [5], [6], [7], [8], [9] as a potential alternative to the highly endothermic steam reforming process. Adoption of POM would result in energy savings. The stoichiometry of reaction (1) with a product molar ratio H₂/CO=2, is suitable for Fisher–Tropsch and methanol synthesis. Of course, in conjunction with the water–gas-shift reaction, POM may be used to produce H₂ for fuel cell applications.

The first-row transition metals (Ni, Co and Fe) [2], [3], [4], [5] and the noble metals (Ru, Rh, Pd, Pt, Ir) [1], [6], [7], [8], [9] have been reported as active catalysts for the partial oxidation of methane. Several problems, including the pyrophoric nature and deactivation of these catalysts remain to be solved. The Ni-based catalyst is the most studied one for POM due to its low cost. However, a rapid deactivation due to carbon deposition or metal loss at high temperature has been reported for nickel catalysts. Carbon deposition mainly comes from methane decomposition: CH₄→C_s+2H₂; and CO disproportionation: 2CO→C_s+CO₂, where C_s refers to surface carbon. The former dominates at high temperature, while the latter is a low-temperature pathway to carbon. Claridge et al. [10] showed that methane decomposition is the principal route for carbon formation over a supported nickel catalyst at the typical methane partial oxidation temperature of 1050 K. Both ‘whisker’ and ‘encapsulated’ forms of carbon were present on a catalyst with a high Ni loading. Recent studies have focused on developing a highly active and stable catalyst for partial oxidation. Different additives were studied for the Ni–Al₂O₃ system [11], [12], [13], [14]. Mixed metal oxides, NiO–MgO solid solutions [15], [16], [17], Ni–BaTiO₃ [18], Ni–Mg–Cr–La–O [19] and Ca_0.8Sr_0.2Ti_1.0Ni_0.2 [20] mixed oxides, were reported to be highly active and selective catalysts at high space velocity (10⁵–10⁶ ml/g h) and high temperature (>700°C) with improved carbon resistance.

Ceria, a stable fluorite-type oxide, has been studied for various reactions utilizing its redox properties, which can be further enhanced in the presence of a metal or metal oxide [21], [22], [23], [24], [25], [26], [27]. Ceria-based materials, such as CuO–CeO₂, have been mostly examined as active catalysts for total oxidation, such as CO oxidation [25], [28], [29], [30], [31] and CH₄ combustion [24], [29], [30], [32]. Recently, Otsuka et al. [33], [34], [35] showed that ceria is able to directly convert methane to syngas with H₂/CO=2 at temperatures higher than 600°C. Ceria has also been examined as a promoter of both the activity and selectivity of supported Ni or Pt catalysts for partial oxidation of methane [11], [12] or CO₂ reforming of methane [36], [37]. Ceria-supported Ni with high Ni-loading (13 wt.%) was reported by Tang et al. [17] to be an active catalyst for POM at T=750°C. However, this catalyst rapidly deactivates due to carbon deposition.

In this paper, we report on the activity/selectivity and stability of Ni-ceria catalysts, with Ni content ranging from 5 to 20 at.% (corresponding to 2.5–10 wt.%), for the partial oxidation of CH₄ to syngas in the medium-high temperature range 550–700°C at atmospheric pressure. Parametric studies included the effect of contact time and catalyst pre-reduction. Carbon deposition was checked by temperature-programmed oxidation, on-line NDIR–CO₂ analysis, and post-reaction surface analysis of the catalysts. Selected samples were characterized by XRD, XPS and STEM/EDS.

Section snippets

Experimental

Bulk Ni–Ce(La)O_x catalysts were synthesized by the urea coprecipitation/gelation method using metal nitrates and urea [30], [38]. This method provides well-dispersed and homogeneous mixed metal oxides. Supported Ni/Ce(La)O_x catalysts were prepared by impregnation of Ce(La)O_x, itself prepared by the urea coprecipitation/gelation method, with a solution of nickel nitrate of appropriate concentration, corresponding in volume to the total pore volume of the support (incipient wetness). For the

Catalyst composition and activity

In this work, all catalysts were doped with ∼4 at.% lanthanum. La dopant was used to achieve high surface area and nanocrystalline ceria [24], which is denoted as Ce(La)O_x throughout the paper. Table 1 lists the BET surface area of various catalyst compositions, and particle size determined by XRD. Also, Table 1 shows the effect of La addition on the lattice parameter of ceria. Oxide solid solution was formed in La-doped CeO₂ with a dopant level of 4–20 at.% [39]. For the 5 at.% Ni–Ce(La)O_x, only

Conclusions

Ni-containing ceria, with nickel content in the range of 5–20 at.% (2.5–10 wt.%), is a highly active and selective catalyst for partial oxidation of methane to syngas at temperatures higher than 550°C. However, only the 5 at.% Ni–Ce(La)O_x material with high nickel dispersion in ceria showed excellent resistance to carbon deposition and, thus, had a high stability under reaction conditions. In contrast, carbon deposition occurred on high nickel-containing ceria, which comprised both dispersed

Acknowledgements

We wish to acknowledge the assistance of Dr. Anthony Garratt-Reed and Ms. Elisabeth Shaw of the Center for Materials Science and Engineering at the Massachusetts Institute of Technology, with the STEM/EDX and XPS analysis, respectively. We also thank the reviewers of the paper.

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