Perovskite type manganese oxides are widely used as catalysts in environmental chemistry that deal with the conversion of carbon monoxide and hydrocarbons. These oxide catalysts possess both high thermal stability and activity and are useful in the combustion of fuels in automobiles and in power plants. However, the major obstacles to the successful application of these materials in a large scale are both their low resistance to sulphur poisoning and also their scarce BET surface area which is often linked to the catalytic activity.

In the present study, a study on the synthesis of the precursor of La_1-xAg_xMnO_3+d (x = 0; 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factors under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 210°C, P = 22 atm) followed by a thermal treatment at 600°C for 10 h.

Structural and physico-chemical properties of the catalysts were investigated using X ray diffraction, BET-sorption, temperature programmed reduction-, and desorption-mass spectrometry, (TPR-MS and TPD-MS) and X-ray photoelectron spectroscopy (XPS). Permittivity measurements of the obtained powders in the range 0.5-3 GHz have been performed using an Agilent 85070E dielectric probe kit, varying the compaction degree.

While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation procedure. The influence of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in catalytic combustion of methane in the presence and in the absence H₂S has been studied.

The MWhyd prepared catalysts were found to exhibit a much higher resistance to sulphur poisoning than those prepared using CHhyd preparative procedure. MWhyd La_1-xAg_xMnO_3+d was the most active catalyst in methane combustion. These results were considered to be due to the nature, amount and mobility of the oxygen species of the solids.

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