Recently, supported iron oxide catalysts have received much attention because their potentiality for many applications in environmental catalysis (N2O decomposition and reduction) and in fine chemical industry for reactions requiring strong Lewis acid sites (e.g., Friedel-Crafts, isomerisations, etc.). [1-2]. Because nanostructured materials have unique physical and chemical properties completely different from those of bulk state, it is expected that catalytic systems consisting of nanosized iron oxide should have high performances. A series of catalysts prepared by dispersing iron oxide on supports of different nature and acidity have been studied in their surface and catalytic properties. Silica (S), with its neutral nature, zirconia (Z), possessing oxidizing, reducing, acid and basic properties, and mixed oxides of silica-zirconia (SZ) with different ZrO2 content (5, 15, 30 wt.%) were used as supports for the iron phase deposited by an equilibrium-adsorption method (Fe/S, Fe/Z, Fe/SZ-5, Fe/SZ-15 and Fe/SZ-30). Among the different techniques of thermal analysis used to characterize solids, temperature programmed reduction (TPR) found great success in catalysis. In this analysis, a reducing gas mixture flows through the sample while the sample temperature is linearly increased at fixed rate. Quantitative results in terms of maximum temperature of reduction (Tmax) and amount of reducible gas consumed are obtained from position and integration of the experimental peaks, respectively. In this work, the prepared Fe-catalysts have been analyzed by TPR (from 50°C to 1000°C at constant rate of 8°C/min) using H2/Ar (5% v/v) as reducing mixture, to identify the supported iron phases and their interactions with the different supports. TPR profiles of the samples are characterized by two well defined and narrow peaks assignable to the two step reduction: 1) Fe2O3 Fe3O4 FeO (around 400°C) and 2) FeO Fe(0) (750-850°C). The temperature of the second-step-peak increased with the zirconia content in the support, probably due to strong interaction of the iron phase with the support. A third peak at very high temperature (around 1000°C) appeared, indicative of the presence of isolated Fe3+ species (Figure 1) [3]. The shape of TPR profile of all samples is typical of the presence of iron oxide aggregates of nanosized dimensions. A computational procedure has been applied to the experimental data for a deep investigation of the reduction kinetics of the samples. Moreover, relations have been individuated between the catalytic activity in a test reaction (-pinene-oxide isomerisation to -campholenic-aldheyde) and the nature of the iron species on the different supports.
Kinetics of reduction of supported nanoparticles of iron oxides by temperature programmed reduction (tpr) analysis / P. Carniti, A. Gervasini, C. Messi. ((Intervento presentato al 28. convegno Congresso AICAT-GICAT tenutosi a Milano nel 2006.
Kinetics of reduction of supported nanoparticles of iron oxides by temperature programmed reduction (tpr) analysis
P. Carniti;A. Gervasini;C. Messi
2006
Abstract
Recently, supported iron oxide catalysts have received much attention because their potentiality for many applications in environmental catalysis (N2O decomposition and reduction) and in fine chemical industry for reactions requiring strong Lewis acid sites (e.g., Friedel-Crafts, isomerisations, etc.). [1-2]. Because nanostructured materials have unique physical and chemical properties completely different from those of bulk state, it is expected that catalytic systems consisting of nanosized iron oxide should have high performances. A series of catalysts prepared by dispersing iron oxide on supports of different nature and acidity have been studied in their surface and catalytic properties. Silica (S), with its neutral nature, zirconia (Z), possessing oxidizing, reducing, acid and basic properties, and mixed oxides of silica-zirconia (SZ) with different ZrO2 content (5, 15, 30 wt.%) were used as supports for the iron phase deposited by an equilibrium-adsorption method (Fe/S, Fe/Z, Fe/SZ-5, Fe/SZ-15 and Fe/SZ-30). Among the different techniques of thermal analysis used to characterize solids, temperature programmed reduction (TPR) found great success in catalysis. In this analysis, a reducing gas mixture flows through the sample while the sample temperature is linearly increased at fixed rate. Quantitative results in terms of maximum temperature of reduction (Tmax) and amount of reducible gas consumed are obtained from position and integration of the experimental peaks, respectively. In this work, the prepared Fe-catalysts have been analyzed by TPR (from 50°C to 1000°C at constant rate of 8°C/min) using H2/Ar (5% v/v) as reducing mixture, to identify the supported iron phases and their interactions with the different supports. TPR profiles of the samples are characterized by two well defined and narrow peaks assignable to the two step reduction: 1) Fe2O3 Fe3O4 FeO (around 400°C) and 2) FeO Fe(0) (750-850°C). The temperature of the second-step-peak increased with the zirconia content in the support, probably due to strong interaction of the iron phase with the support. A third peak at very high temperature (around 1000°C) appeared, indicative of the presence of isolated Fe3+ species (Figure 1) [3]. The shape of TPR profile of all samples is typical of the presence of iron oxide aggregates of nanosized dimensions. A computational procedure has been applied to the experimental data for a deep investigation of the reduction kinetics of the samples. Moreover, relations have been individuated between the catalytic activity in a test reaction (-pinene-oxide isomerisation to -campholenic-aldheyde) and the nature of the iron species on the different supports.
Pubblicazioni consigliate
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
