Perovskite-like catalysts for the catalytic flameless combustion of methane

Modified LaCoO3 and LaMnO3 were investigated as catalysts for low tvemperature flameless combustion of methane. Modifications were carried out by the substitution part of La for Sr2+ and Ce4+, by the addition of 0.5% of Pt or Pd and by the substitution with Ag, which have limited solubility in the perovskite structure and may exist as intraframework Ag+ and extraframework metallic silver. Catalysts were synthesized by flame pyrolysis, which lead to a significant increase of both surface area and thermal resistance in comparison with the catalysts prepared by traditional sol-gel method. Samples were mainly characterized by XRD, BET and TPR techniques. Catalytic activity for the flameless combustion of methane was investigated by means of bench scale continuous apparatus, equipped with a quadrupolar mass spectrometer. In addition the resistance of every catalysts against sulphur poisoning was tested by using tetrahydrothiophene (THT) as poisoning agent. In most cases modification of perovskites led to an activity improvement, which was much more evident in the case of silver substitution. All the FP-prepared catalysts showed full methane conversion below 600\ub0C, with CO2 and H2O as the sole detected products. Sr-substitution and addition of noble metals increased resistance to sulphur poisoning, while silver was not effective from this point of view, its main advantage being a substantial increase of the initial activity, which lead to satisfactory performance even after poisoning

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

ABO(3) 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(x)'CoO(3), La((1-x))A(x)'MnO(3) and La((1-x))A(x)'FeO(3), 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 LaFeO(3), from both the points of view of activity and of resistance to sulphur poisoning. La(0.9)Sr(0.1)MnO(3) showed, the best results, exhibiting very high activity and good resistance even after the addition of up to 8.4 mg of THT/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

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

Ag represents an interesting dopant for the highly active LaCoO3 perovskites used for the catalytic flameless combustion (CFC) of methane, due to its ability to adsorb and activate oxygen and to the possibility of incorporation into the framework as Ag+ or Ag2+, with formation of oxygen vacancies. In the present work we compared the catalytic activity and resistance to sulphur poisoning of a series of LaCoO3, x%Ag/LaCoO3, La1-xAgxCoO3 samples (nominal composition), the latter two notations indicating post-synthesis Ag loading or direct incorporation during the synthesis, respectively. The samples were prepared by flame pyrolysis (FP) and by the sot-gel (SG) method, leading to different particle size and possibly to different incorporation degree of the dopant, quantified by Rietveld refinement of XRD patterns. Higher activity was observed, in general, with fresh catalysts synthesised by FP. The SG samples demonstrated a slightly better resistance to sulphur poisoning when considering the conversion decrease between the fresh and the poisoned samples, due to lower surface exposure. However, interesting data have been obtained with some of the Ag-doped poisoned FP samples, performing even better than the fresh SG-prepared ones. Ag addition led to a complex change of activity and resistance to poisoning. The activity of FP-prepared samples doped with a small amount of Ag (e.g. 5 mol%) was indeed lower than that of the undoped LaCoO3. By contrast, a further increase of Ag concentration led to increasing catalytic activity, mainly when big extra framework Ag particles were present. By contrast, for SG samples a low Ag amount was beneficial for activity, due to an increased reducibility of Co3+

Effective Ag doping and resistance to sulphur poisoning of La-Mn perovskites for the catalytic flameless combustion of methane

Perovskite-like structured catalysts showed satisfactory activity for the low temperature flameless combustion of methane. This process reduces the emission of CO, NOx and unburnt hydrocarbons. Partial metal ion substitution in the composition of perovskites may improve catalytic activity and it can modulate their resistance to sulfur poisoning. Silver is an interesting dopant due to its limited solubility in the perovskite structure and suitable activity both in extra-and intra-framework positions. The amount of lattice silver tightly depends on the preparation procedure. Samples with nominal composition La1-xAgxMnO3+/-delta with x = 0; 0.05; 0.10 were prepared by flame spray pyrolysis (FP) and by the so-called sol-gel citrate method (SG). Temperature-programmed analysis, X-ray powder diffraction (XRPD) and Electron Paramagnetic Resonance (EPR) spectroscopy were used as the main characterisation tools. Almost all of the catalysts were very active for the flameless combustion of methane. The activity of the FP-prepared catalysts was always higher than that of the SG-prepared ones with identical nominal composition. Furthermore, partial substitution of Ag for La led to substantially higher activity both for SG- and FP-prepared catalysts and the catalytic activity increased with increasing Ag substitution. The residual activity after poisoning with tetrahydrothiophene, a common odoriser used in the natural gas grid, is also presented, together with the transient response of the samples upon poisoning

NANOSTRUCTURED MIXED OXIDES CATALYSTS: RELATIONSHIPS BETWEEN PHYSICAL-CHEMICAL AND CATALYTIC PROPERTIES

Perovskitic catalysts showed satisfactory activity for the low temperature flameless combustion of methane. This process allows to decrease the emission of CO, NOx and partially unburnt hydrocarbons. Partial substitution in the composition of perovskites may improve catalytic activity and it can modulate their resistance to sulphur poisoning. Samples with nominal composition La1-xA\u2019xBO3, where A\u2019= Sr, Ce, Ag, B=Co, Mn with x = 0; 0.05; 0.10, 0.20 and x%NB/LaBO3, where NB=Pt, Pd, Ag were prepared by flame spray pyrolysis (FP) and by the so-called sol-gel citrate method (SG). Temperature-programmed analysis, X-ray powder diffraction (XRD) and Electron Paramagnetic Resonance (EPR) spectroscopy were used as main characterisation tools. Almost all of the catalysts showed very active for the flameless combustion of methane. The activity of the FP-prepared catalysts was always higher than that of the SG-prepared ones with identical nominal composition. Partial substitution didn\u2019t lead to a significant change of activity, though an evident result was achieved in case of Sr-substituted manganites. Also activity may be increased by the addition of a small amount of Pt. Ag addition led to a complex change of activity and resistance to poisoning for Co-based perovskites. The activity of samples doped with a small amount of Ag (e.g. 5 mol%) was indeed even lower than that of the \u201cpure\u201d LaCoO3. By contrast, a further increase of Ag concentration led to increasing catalytic activity, mainly when extraframework Ag was present. Partial substitution of Ag for La in Mn-based perovskites led to increasing activity both for SG- and FP-prepared catalysts and the catalytic activity increased with increasing Ag substitution. From the point of view of resistance against poisoning Sr and noble metals may be good protectors since they react more promptly with sulphur. Silver is not as effective from the point of view of protection against poisoning. The main advantage is the improved initial activity of some catalysts, which leads to satisfactory results even after more or less severe poisoning. A brief investigation of different poisoning agents (nitrogen-containing compounds) was also made. The transient response of the catalysts during the injection of poisons was also analyzed

Effect of nitrogen-containing impurities on the activity of perovskitic catalysts for the catalytic combustion of methane

LaMnO3, either pure or doped with 10 mol % Sr, has been prepared by flame pyrolysis in nanostructured form. Such catalysts have been tested for the catalytic flameless combustion of methane, achieving very high catalytic activity. The resistance toward poisoning by some model N-containing impurities has been checked in order to assess the possibility of operating the flameless catalytic combustion with biogas, possibly contaminated by S- or N-based compounds. This would be a significant improvement from the environmental point of view because the application of catalytic combustion to gas turbines would couple improved energy conversion efficiency and negligible noxious emissions, while the use of biogas would open the way to energy production from a renewable source by means of very efficient technologies. A different behavior has been observed for the two catalysts; namely, the undoped sample was more or less heavily poisoned, whereas the Sr-doped sample showed slightly increasing activity upon dosage of N-containing compounds. A possible reaction mechanism has been suggested, based on the initial oxidation of the organic backbone, with the formation of NO. The latter may adsorb more or less strongly depending on the availability of surface oxygen vacancies (i.e., depending on doping). Decomposition of NO may leave additional activated oxygen species on the surface, available for low-temperature methane oxidation and so improving the catalytic performance