Effect of sulphur poisoning on perovskite catalysts prepared by flame-pyrolysis

Abstract

ABO3 perovskite-like catalysts are known to be sensitive to sulphur-containing compounds. Possible solutions to increase resistance to sulphur are represented by either catalyst bed protection with basic guards or catalyst doping with different transition or noble metals. In the present work La(1-x)A'xCoO3, La(1-x)A'xMnO3 and La(1-x)A'xFeO3, with A' = Ce, Sr and x=0, 0.1, 0.2, either pure or doped with noble metals (0.5 wt% Pt or Pd), were prepared in nano-powder form by flame pyrolysis. All the catalysts were tested for the catalytic flameless combustion of methane, monitoring the activity by on-line mass spectrometry. The catalysts were then progressively deactivated in operando with a new procedure, consisting of repeated injection of some doses of tetrahydrothiophene (THT), usually employed as odorant in the natural gas grid, with continuous analysis of the transient response of the catalyst. The activity tests were then repeated on the poisoned catalyst. Different regenerative treatments were also tried, either in oxidising or reducing atmosphere.Among the unsubstituted samples, higher activity and better resistance to poisoning have been observed in general with manganites with respect to the corresponding formulations containing Co or Fe at the B-site. The worst catalyst showed LaFeO3, from both the points of view of activity and of resistance to sulphur * Corresponding author: fax +39-02-50314300, e-mail ilenia.rossetti@unimi.it 2 poisoning. La0,9Sr0,1MnO3 showed, the best results, exhibiting very high activity and good resistance even after the addition of up to 8.4 mg of THT per g of catalyst.Interesting results were attained also by adding Sr to Co-based perovskites. Sr showed a first action by forcing Mn or Co in their highest oxidation state, but, in addition, it could also act as a sulphur guard, likely forming stable sulphates due to its basicity. Among noble metals, Pt doping proved beneficial in improving the activity of both the fresh and the poisoned catalyst.Keywords: Methane, catalytic combustion; Sulphur poisoning; Perovskite-like catalysts. -INTRODUCTIONThe catalytic flameless combustion (CFC) of methane outperforms conventional flame combustion because of lower emission of pollutants (HC, CO and NOx) 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 ABO3 perovskite-like structure [1][2][3][4] have been proposed as a valid alternative for the present application. Indeed, perovskite catalysts combine low cost, thermo-chemical stability at high operating temperature and satisfactory catalytic activity [5][6][7][8]. 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 a...

La–Ag–Co perovskites for the catalytic flameless combustion of methane

“…Indeed, no decrease of conversion has been observed when doping similar samples with Sr [16]. With the present Ag-doped samples the main advantage is the improved initial activity of some of them, which leads to satisfactory results even after more or less severe poisoning.…”

“…It has also been reported [12] that some of the formed species are thermally unstable and react with oxygen (SO4 -) or desorb at high temperature. Once formed, sulphur species may depress suprafacial activity [16] and bring about a less efficient intrafacial reactivity, due to a decreasing concentration of available surface sites for oxygen release from the bulk. Their subsequent restoration through a Mars van Krevelen mechanism is inhibited too, for the same reasons.…”

“…450°C. This temperature has been chosen because most samples exhibit the highest reaction rate around this value and the conversion curve is very steep, so the effect of poisoning can be magnified [16].…”

“…Its protective action is necessarily time-limited by its consumption. In a previous investigation [16] intra-framework Sr doping proved an interesting mean to improve catalyst resistance to poisoning, as well as the addition of noble metals, such as Pd or Pt. However, at least the former method showed beneficial for LaMnO3 based catalysts, whereas LaCoO3 showed decreasing activity upon Sr addition, besides being intrinsically less resistant to S poisoning.…”

Oxygen transport in nanostructured lanthanum manganites

“…The samples were prepared by flame pyrolysis [19][20][21][22] and basically characterised, as extensively described in [23]. Sample compositions are reported in Table 1.…”