Catalytic partial oxidation of methane to synthesis gas over Ni–CeO2
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: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→Cs+2H2; and CO disproportionation: 2CO→Cs+CO2, where Cs 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 Ca0.8Sr0.2Ti1.0Ni0.2 [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)Ox 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)Ox catalysts were prepared by impregnation of Ce(La)Ox, 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)Ox 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)Ox, 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)Ox 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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