Perovskite-like catalysts for the catalytic flameless combustion of methane

Abstract

QUESTIONNAIREThe catalytic flameless combustion (CFC) of methane outperforms conventional flame combustion because of lower emission of pollutants (HC, CO and NO x ) and high thermal efficiency. The catalysts traditionally used for CFC are mainly based on supported noble metals, such as Pd and Pt, which ensure high activity, however accompanied by some drawbacks, due to high cost and poor thermal and chemical stability. La-based catalysts with ABO 3 perovskite-like structure have been proposed as a valid alternative for the present application. Indeed, perovskite catalysts combine low cost, thermochemical stability at high operating temperature and satisfactory catalytic activity. In spite of this, there are still some open questions, especially regarding their resistance to sulphur poisoning when used for the CFC of methane. Indeed, the poisoning mechanism of perovskite-like catalysts has not been completely understood and only a few papers deal with this very important topic.In general, the partial substitution of the perovskitic lattice metal ions is one of the important means to modulate catalytic activity. In LaBO 3 perovskites, the influence of dopants such as A' 2+ and A' 4+ has been widely investigated in the literature.* Corresponding author: fax +39-02-50314300; e-mail ilenia.rossetti@unimi.it 2In the recent past, attention has been devoted to catalyst nanostructuring, aiming at obtaining high surface area and defectivity, which should enhance oxygen mobility through the lattice and ensure good catalytic activity. To this purpose, we set up and optimised a flame-pyrolysis (FP) apparatus for the onestep synthesis of perovskitic mixed oxides. It is based on a specially designed burner fed with oxygen and an organic solution of the precursors, the solvent acting as fuel for the flame. FP-synthesized perovskites, showed good phase purity, along with nanometer-size particles and hence very high surface area (sometimes in excess of 100 m 2 /g). In addition, the high temperature of the flame in principle should also ensure thermal stability.In the present work we prepared a set of samples by FP, namely La 1-x A x MeO 3 , with A' = Ce, Sr, Ag Me=Mn, Co, x=0, 0.1, 0.2, either pure or doped with noble metals (Pt, Pd or Ag). In addition silver containing samples were synthesized by a traditional sol-gel (SG) method. The catalysts were tested for the CFC of methane, monitoring the activity by online mass spectrometry. Also the resistance to sulphur poisoning was investigated, using tetrahydrothiophene as poisoning agent.