No abstract

Using colloidally synthesized nanoparticles for the preparation of supported catalysts offers several advantages (e.g. precise control of particle size and morphology) when compared to traditional preparation techniques. Although such nanoparticles have already been very successfully used for catalytic applications in the liquid phase, applications in heterogeneous gas phase catalysis are still scarce. One aspect, usually considered as a problem, is organic stabilizers typically employed during the nanoparticle synthesis since they or their decomposition products are supposed to block catalytically active sites on the nanoparticle surface. Thus, in many studies so far, the removal of the organic ligands prior to use in gas phase catalysis has been proposed. In this perspective article, however, we will discuss a number of benefits such ligand shells may have for heterogeneous gas phase catalysis, including the protection against chemical modification, prevention of sintering and tuning of SMSI effects.

Nanostructured praseodymium oxides were successfully prepared via four different methods: two traditional methods (calcination of praseodymium nitrate and sol-gel method with propylene oxide) and two more sophisticated, modern techniques (citrate method and modified Pechini method). Powder X-ray diffraction revealed that all synthesis methods led to praseodymium oxide Pr 6 O 11 with cubic fluorite-like structure. The temperature necessary for the formation of the crystalline oxide phase, however, was dependent on the method and synthesis parameters. The size of the nanocrystalline domains was in the range of some 10 nm in all cases. The catalytic properties of the nanostructured oxides were studied choosing CO oxidation as a first test reaction. According to infrared spectroscopy, the surface of all samples was covered with monodentate carbonate species after the synthesis. After exposure to CO, two types of bidentate carbonates were observed on the oxide surface, and under the feed of both CO and O 2 , carbon dioxide was observed by IR spectroscopy as product in the gas phase at temperatures from 300 °C on. The activity with respect to CO oxidation was further investigated in a catalytic test reactor. The maximum conversion of CO was reached at ∼550 °C, and it was ∼95-96% independent of the synthesis method. At moderate temperatures (∼350-500 °C), the activities of the catalysts prepared in the present work were dependent on the synthesis method and synthesis parameters, only to a small extent, but all of them were more active than commercial Pr 6 O 11 . The differences between the various samples prepared in this study can be explained by an influence of the synthesis on the oxygen ion mobility. Mechanistically, the results of our work suggest that CO oxidation occurs through the adsorption of CO as a bidentate carbonate, which is then transformed into a monodentate carbonate finally desorbing as CO 2 .

Catalysis by noble metals is still extremely important for the industrial production of chemicals, [1] in catalytic converters, [2] and in new areas of energy generation (e.g. fuel cells [3] ) and storage (e.g. batteries [4] ). Since the supplies of these metals are limited and their prices continuously increase, the search for new classes of catalytic materials, such as cheap and abundant oxides, [5] and significant improvement of the performance of existing metal catalyst systems is indispensable.An attractive option in this context is the use of the support as a co-catalyst; in other words, shifting the main task of the support from ensuring sufficient particle dispersion to participating in the catalytic cycle. Particularly interesting in this respect are so-called strong metal-support interactions (SMSI) which have been the subject of a number of catalytic studies since their discovery in the 1970s.[6] Many different transition-metal oxides, such as TiO 2 , [7] CeO 2 , [8] and WO 3 [9] are known to show a SMSI effect which can strongly influence the electronic structure of metal catalysts and even lead to thin oxide layers covering the metal nanoparticles.[10] While in some cases (e.g. toluene hydrogenation on Pt/TiO 2 [11] ) the SMSI effect was found to decrease the overall catalytic activity (by diminishing the number of active sites through encapsulation of the metal particles by the oxide), in other cases it can considerably improve activity (e.g. CO oxidation on Pt/TiO 2 [12] ). Such synergism can for instance result from the fact that the oxide provides special adsorption sites at the perimeter of the particles, [11] induces a different particle morphology, [7] or delivers active species.[12] Since CO oxidation on Pt, for instance, suffers from the fact that CO is strongly bound to Pt and thus poisons the surface so that O 2 cannot adsorb at low temperatures, a supply of oxygen by the support should improve the performance drastically. Indeed, this type of SMSI effect was recently observed: [13] The interaction of Pt with FeO, which delivers oxygen at the border between the two constituents, was exploited in the development of a very efficient catalyst for PROX (preferential oxidation of CO in the presence of hydrogen) which already works at room temperature. Besides the possible contribution of subsurface Fe, [14] the active site was discussed to be a highly reduced FeO layer on Pt, while hydrogen is necessary to replenish these reduced sites. [13] Although the potential of SMSI is clear and industrial companies are already benefitting from such catalysts, rational concepts to generate and tune SMSI are still largely missing in the literature. One possible strategy is the use of supported preformed colloidal nanoparticles instead of metal precursors which are assembled into particles only in subsequent steps on the surface. Here, spacers between the catalytic particle and the support in the form of organic ligands can be employed which can possibly mediate the interaction between the metal and the sup...

In contrast to conventional methods, colloidally prepared heterogeneous supported metal catalysts are excellent systems to study the catalytic properties as a function of metal loading, monodispersity, particle shape, or the type of support without changing the other parameters, as will be demonstrated herein. Colloidal, ligand‐capped Pt nanoparticles deposited on oxide supports are investigated for CO adsorption and oxidation. Dodecylamine and different alkanethiols are used as ligands. IR spectroscopic experiments reveal that small molecules, such as CO, can pass through the ligand shell and can adsorb on the particle surface, even if the ligand shell is not removed by a special pretreatment. The ability to penetrate the shell was found to depend on the type of ligand used which renders ligand‐capped nanoparticles potentially interesting for reaction and selectivity control. In the case of CO oxidation, high activity is detected only at temperatures at which a partial loss of ligands has already occurred, resulting in a rather similar catalytic behavior independent on the type of ligand. However, there are no indications for poisoning of the catalysts by decomposition of the ligand shell. Simple purification procedures of the Pt nanoparticles are sufficient to avoid further poisoning effects. Depositing nanoparticles with the same size in different amounts on a support enabled a detailed study of the influence of metal loading on the activity. The activity per gram metal increases with the metal loading. Local autothermal heating is responsible for this effect, which is also detected for a reference system consisting of Pt nanoparticles prepared without a ligand shell.