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Rh, Ni, and Ca substituted pyrochlore catalysts for dry reforming of methane

Paper ID Volume ID Publish Year Pages File Format Full-Text
41414 45888 2011 10 PDF Available
Title
Rh, Ni, and Ca substituted pyrochlore catalysts for dry reforming of methane
Abstract

Dry reforming of methane (DRM) has been widely investigated, with most studies showing rapid deactivation due to carbon deposition. This suggests a need to develop catalysts that limit carbon formation while avoiding structural changes at the elevated temperatures typical of this reaction. Here, we report CO2 reforming of methane on four pyrochlore catalysts. First, rhodium was partially substituted for Zr in lanthanum zirconate (La2Zr2O7) to give La2Zr2−xRhxO7−δ (LRZ, x = 0.112, 2 wt% Rh) pyrochlore. A second pyrochlore catalyst was synthesized in which Ca was further substituted into the La-site to give La1.95Ca0.05Zr2−xRhxO7−δ (LCRZ, x = 0.055, 1 wt% Rh). This was done to compare the effect of Ca substitution on the La site along with Rh substitution on Zr site. A third catalyst was synthesized where Ni was substituted in the Zr-site to give La2Zr2−xNixO7−δ (LNZ, x = 0.112, 1 wt% Ni). A fourth catalyst, containing no catalytically active metal, La1.97Sr0.03Zr2O7−δ (LSZ), was synthesized to provide a direct comparison to the substituted pyrochlores. Effects of substitution and kinetic measurements were examined for dry reforming of methane in a fixed-bed reactor. XRD results prior to reaction showed that all the pyrochlore catalysts had a cubic unit-cell lattice. Results from XPS suggested that there were two oxidic phases (RhO2 and Rh2O3) in Rh-substituted LRZ and LCRZ and two phases (NiO and Ni2O3) present in Ni substituted LNZ. TPR of the catalysts confirmed the presence of two reducible Rh species in LCRZ and LRZ, and four reducible Ni species in LNZ. Textural measurements revealed that among the active catalysts, LCRZ had the highest BET surface area (10.0 m2/g) and pore volume (0.10 cm3/g). Temperature programmed surface reaction (TPSR) tests indicated different light-off curves for different catalysts, with LCRZ being the most active by this measure. Steady state tests at 750 °C using an equimolar reactant feed for 450 min showed that the Ni-based pyrochlore (LNZ) deactivated rapidly. LCRZ and LRZ showed similar activity, however, LCRZ showed lower carbon built-up. XRD of the spent catalysts showed that the pyrochlore structure was unchanged during reaction for all catalysts. Carbon deposited on catalyst surface during reaction was characterized by TPO. The Ni-based pyrochlore showed higher carbon deposition (1.4 g/gcat.) than either LCRZ (0.26 g/gcat.) or LRZ (0.44 g/gcat.). These results suggested that Rh substituted into the pyrochlore was more active and selective for synthesis gas compared to a directly comparable atomic loading of Ni. In addition, the replacement of Ca2+ for La3+ may provide improved oxygen mobility of the catalyst (through the introduction of lattice oxygen defects) resulting in the oxidation of carbonaceous species deposited on the active sites on the catalyst during the reaction. Substitution of metals into the crystal lattice might have also lowered the bond energy of La–O and Zr–O lattice bonds resulting in the release of oxygen from the lattice, which probably oxidized surface carbon thus slowing down carbon accumulation.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (216 K)Download as PowerPoint slideHighlights► Rh-, Ni-, and Ca-substituted La2Zr2O7 pyrochlore catalysts were synthesized. ► High catalytic activity of Rh, and Ca-substituted pyrochlores with time-on-stream was observed. ► Rapid and continuous deactivation of Ni-substituted catalyst was observed. ► Partial substitution of a metal in the lattice creates Oxygen vacancies.

Keywords
Pyrochlores; CO2 reforming of methane; Ni catalyst; Rh catalyst
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Rh, Ni, and Ca substituted pyrochlore catalysts for dry reforming of methane
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Publisher
Database: Elsevier - ScienceDirect
Journal: Applied Catalysis A: General - Volume 403, Issues 1–2, 22 August 2011, Pages 142–151
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us