Influence of Cr3+ and Fe3+ cations on the structure of alumina–aluminium phosphate solids and their catalytic activity for N2O decomposition

Abstract

Phosphate salts of the metals Al, Fe and Cr supported on alumina were prepared by co-precipitation with NH₃ from solutions containing calculated amounts of the metal nitrates and phosphoric acid. The atomic ratio of the elements participating in the solids was Al : P : M = 100 : x : 20 where M = Al, Fe, Cr and x= 0, 4.5, 9, 18, 36, 72 and 144. The parent compounds Al₁₀₀P_xAl₂₀ are formed by an endothermic decomposition of NH₄NO₃, which exists in the dried precipitant, to NH₃ and HNO₃. The resulting solids for x= 0 contain just γ-Al₂O₃, for x= 144 AlPO₄ plus an amorphous phase, while for intermediate values of x a totally amorphous phase is formed. Substitution of 20 % Al by Fe alters slightly the route of decomposition, the endo-effect noticed above is followed by a small exothermic one, meaning probably the decomposition of part of the resulting nitric acid through anion breakdown to N₂, H₂O and O₂. The structure of the resulting solids corresponds to α-M₂O₃ for x= 0 and MPO₄ for x= 144, while for intermediate values amorphous materials are formed. An addition of Cr instead of Fe alters completely the route of decomposition of NH₄NO₃ to N₂, H₂O and O₂ for reasons similar to those noticed for Fe. For x= 0 the resulting solids contain α-M₂O₃ oxides alone, for x= 144 they are totally amorphous and for intermediate values of x an amorphous phase plus crystalline Cr₂O₃ is formed. The behaviour of these systems for a simple redox reaction, namely N₂O decomposition, shows a continuous drop of activity with phosphorous content. An analysis of the results according to the kinetic theory of poisoning indicates an order of deactivation between one and two, depending on the catalyst and the temperature. A mechanism which may explain this result is proposed. Thiele modulus and effectiveness factors which have been calculated show that for AlPAl an AlPCr species internal diffusion is the rate-determining step and the reaction occurs mainly on the external surface. For the AlPFe catalysts the rate of diffusion is almost equal to that of reaction.