Abstract
The thermal stability of Ru–Re NPs on γ-alumina support was studied in hydrogen at 800 °C and in air at 250–400 °C. The catalysts were synthesized using Cl-free and Cl-containing Ru precursors and NH4ReO4. Very high sintering resistance of Ru–Re NPs was found in hydrogen atmosphere and independent of Ru precursors and Re loading, the size of them was below 2–3 nm. In air, metal segregation occurred at 250 °C, leading to formation of RuO2 and highly dispersed ReOx species. Ruthenium agglomeration was hindered at higher Re loading and in presence of residual Cl species. Propane oxidation rate was higher with the Ru(N)–Re catalysts than with Ru(N) and that containing Cl species. The Ru(N)–Re (3:1) catalyst exhibited the highest activity and the lowest activation energy (91.6 kJ mol−1) what is in contrast to Ru(Cl)–Re (3:1) which had the lowest activity and the highest activation energy (119.3 kJ mol−1). Thus, the synergy effect was not observed in Cl-containing catalysts.Graphic
Highlights
The thermal stability of supported metal nanoparticles (NPs) at high temperatures is a very important issue determining their applications as catalysts in the various industrial processes
The following conclusions can be drawn based on the results reported in this study: An increase of the heating temperature in hydrogen from 500 to 800 °C of the bimetallic Ru–Re/γ-Al2O3 catalysts leads to some lowering of the metal dispersion
Ru–Re bimetallic NPs were unstable and at a moderate temperature of 250 °C segregation of metals is visible. This process leads to RuO2 agglomeration, which was lower in the Ru–Re catalysts containing a higher amount of Re and in the presence of residual Cl species, and to the transformation of the Re phase into a highly dispersed ReOx species
Summary
The thermal stability of supported metal nanoparticles (NPs) at high temperatures is a very important issue determining their applications as catalysts in the various industrial processes. Numerous literature data indicate that particle sintering is one of the most critical mechanisms causing catalyst. Thermally-induced deactivation of the supported metalbased catalysts results from loss of catalytic surface area caused by the growth of the metal crystallite size, as well as from reducing of support area due to support collapse and of catalytic surface area due to pore collapse on crystallites of the active metal phase. The sintering process of metal species can be described by the two main mechanisms, the Ostwald ripening or particle migration and coalescence [3,4,5]. Many other factors affect the sintering process, for example, metal particle size, the loading or metal dispersion, the interaction between the metal and support, or the atmosphere and time in which the catalyst is heated [6]. The presence of residue chloride on the surface of the metal catalysts (mainly from chloride-based precursors) may cause severe sintering of the metal particles
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