Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Abstract

noble metals are widely used as valuable catalysts in deNOx and combustion technology. However, like most noble metal-reducible oxide combinations they are subjected to strong metal-support interaction upon high temperature reduction, resulting in changing catalytic properties [1] and eventually deactivation. An investigation of these phenomena and of structureactivity correlations is greatly facilitated by model systems with planar thin supports. In the present work, well-facetted Pt particles, obtained by epitaxial growth on (001) NaCl surfaces, were supported by thin films of crystalline ceria. These model catalysts are well suited for characterisation by electron microscopy and provide a defined reference state for studies of structural changes during catalyst treatment and reaction. Catalyst changes due to calcination and reduction have been studied after treating the samples in oxygen (673K, 1h), and after hydrogen exposure at increasing temperatures from 673K to 1073K for 1h, either in a flow system or in a circulating batch reactor. The morphology and composition were followed by (HR)TEM, electron diffraction and EELS. In their initial state, most Pt particles are truncated half octahedra with (001) faces perpendicular to the electron beam, exhibiting square or rectangular shapes at lower magnification [2]. They are surrounded by grains of CeO , partly in epitaxial correlation to the metal. In contrast, after a 2 reductive treatment at 723 K, Pt particles embedded in ceria recrystallise into a cubic or plateletlike structure, thereby losing in part their original azimuthal orientation. Some particles exhibit round edges but have increased in size, obviously due to agglomeration. At this reduction stage the bulk stoichiometry of the ceria support is still unchanged. With rising reduction temperature the average particle size increases but the platelet or cube like structure persists. On Pt/silica and Pt/alumina a similar transformation of the Pt particles was observed to occur at higher temperature and possibly connected with alloy formation [3]. These phase transformation are reversible by low-temperature oxidation and can be repeated by subsequent reduction. At present, HREM, image simulations and EELS measurements are carried out in order determine the origin of this effect. Further heating of the ceria-based Pt catalyst in hydrogen up to 1073K results in clustering of the Pt particles on large flat areas of a sintered support of partly reduced ceria, exhibiting large lattice periodicities between 0.8 and 1.2 nm. EELS measurements confirm the formation of a chemically stable Ce (III) oxide. These structural changes are connected to changes of the catalytic properties [4], as will be exemplified and discussed for the CO hydrogenation. This work was supported by the Austrian Science Fund (Project S 8105)[1] A. Trovarelli, Catal. Rev

Structure–activity correlations in thin film model catalysts: CO hydrogenation on Rh/VOx

“…This is consistent with a model where V penetrates the Rh-V interface and forms a shell of a comparatively V-richer alloy before a further raise of the reduction temperature will equilibrate the V concentration inside the Rh particles [19]. (A similar behaviour was observed in corresponding experiments on ceria-and silica-supported Pt and Rh particles [26], [27], [28] and [29].) Finally, no signs of alloying are detected upon reduction of catalyst II-0.3 up to 673 K, in contrast to the findings on the Rh/V 2 O 3 catalysts of type I discussed above.…”

“…As outlined above, the use of well-defined thin film model systems is an adequate tool for this kind of studies, because thin film metal-oxide systems with Pt [26], [27], [28] and [29] and Rh [29] particles are prone to enter an alloy state upon reduction at comparatively low temperatures (> 573 K). This was confirmed in two previous studies on thin films containing Rh particles supported by pure vanadia [19] and [20], where different V-Rh alloys could be identified under different reduction conditions.…”

Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen

“…Here also further reduction leads to different particle habits [9], but also to changes of composition and structure of the support, as will be discussed separately [10].…”

Pd–Al interaction at elevated temperatures: a TEM and SAED study

“…In close correlation to experiments performed previously on Pd, Pt and Rh particles supported on reducible and non-reducible oxides [24][25][26] we also focused on the question if a metal-support interaction can also be triggered by reduction at elevated temperatures and if there are substantial differences to simple annealing treatments. Thus, the Pd/Al 2 O 3 films were subjected to reductive treatments between 523 and 873 K in H 2 for 1 h each.…”

“…As we had already outlined previously, partial reduction of ''non-reducible'' oxides (e.g. SiO 2 or Al 2 O 3 ) becomes feasible in thin film model catalysts at temperatures T ‡ 773 K thereby mediating the formation of Pt-rich alloy compounds [24,25]. The first stages of alloy formation are believed to occur via a reduction of the oxide species and the subsequent migration of the is not yet pronounced.…”