Cluster Catalysis with Lattice Oxygen: Tracing Oxygen Transport from a Magnetite (001) Support onto Small Pt Clusters

Oxidation catalysis on reducible oxide-supported small metal clusters often involves lattice oxygen. The present work aims at differentiating whether the reaction takes place at the cluster/support interface or on the cluster. To that purpose, we trace the path of lattice oxygen from Fe3O4(001) onto small Pt clusters during the CO oxidation. While oxygen vacancies form on many other supports, magnetite maintains its surface stoichiometry upon reduction thanks to high cation mobility. To investigate whether size-dependent oxygen affinities play a role, we study two specific cluster sizes, Pt-5 and Pt-19. By separating different reaction steps in our experiment, migrating lattice oxygen can be accumulated on the clusters. Temperature-programmed desorption (TPD) and sophisticated pulsed valve experiments indicate that CO oxidation occurs with this highly reactive oxygen on the Pt clusters. Scanning tunneling microscopy (STM) shows a decrease in the apparent height of the clusters, which density functional theory (DFT) explains as a restructuring following lattice oxygen reverse spillover.

Synthesis of monodispersed model catalysts using softlanding cluster deposition

Laurent Klinger

In nanocatalysis, clusters deposited on solid, well-defined surfaces play an important role. For the detection of size effects it is, however, important to prepare samples consisting of deposited clusters of a single size, as their chemical properties change with the exact number of atoms in the cluster. In this paper, the experimental tools are presented to prepare such model systems. The existence of monodispersed clusters is confirmed by various experimental findings. First, the carbonyl formation of deposited Ni-n clusters shows no change in the nuclearity when comparing the size of the deposited clusters with one of the formed carbonyls. Second, scanning tunneling microscopy (STM) studies show that fragmentation of Si-n clusters upon deposition can be excluded. In addition, the adsorption behavior of CO on deposited Pd atoms points to the existence of single atoms on the surface. Furthermore, CO oxidation results on Au-n clusters confirm the existence of monodispersed clusters trapped on well-defined adsorption sites. Finally, we use Monte-Carlo simulations to define the range of clusters and defect densities, for which monodispersed clusters can be expected.

De Gruyter2002

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

Nitrous oxide is the third most important gas contributing to global warming and its concentration in the atmosphere is still on the rise. Nitric acid production represents the largest source of N2O in the chemical industry, with a total annual emission of about 400 kt N2O. An efficient way to reduce the N2O is the catalytic decomposition into N2 and O2. ZSM-5 zeolites containing iron are known to be effective in this reaction involving Fe(II)-extraframework species as active sites. But the nature of these sites and the reaction network is still under debate. Often the N2O is in the exhaust together with other gases like NOx, COx, oxygen, and water vapor, which may influence the catalyst efficiency towards N2O decomposition. The effect of these gases on the reaction mechanism is still not understood yet. The aim of this work is to study the decomposition mechanism of N2O over isomorphously substituted Fe-ZSM-5 (0.02-0.55% Fe) using transient response methods. Kinetic studies under steady-state conditions give overall information about the reaction, whereas transient experiments lead to information on the individual steps and the possibility to suggest a reaction model. Transient response methods are applied in vacuum using a Temporal Analysis of Products (TAP) setup and at ambient pressure using a Micromeritics AutoChem 2910 analyzer. The influence of different gases such as NO, H2O and NO2 on the decomposition of N2O over Fe-ZSM-5 is investigated to gain insight into their effect on this reaction. The TAP multifunctional reactor system is used to perform transient pulse experiments for micro-kinetic analysis of complex heterogeneous catalytic reactions. This setup allows to dose precisely reactants and to monitor product formation with submillisecond time resolution implying the absence of external mass or heat transfer. A reproducible low hydroxylation level of the catalyst can be attained in vacuum. The results from the transient responses with the Micromeritics analyzer are obtained under conditions more relevant to "operando". The mechanism of N2O decomposition over Fe containing ZSM-5 was studied by transient response methods at ambient pressure with the Micromeritics analyzer and under vacuum with the TAP setup. Both approaches showed that the mechanism strongly depends on the reaction temperature, but in all cases involves atomic oxygen loading from N2O with N2 evolution as the first reaction step. The surface oxygen, (O)Fe, formed from N2O possesses very high reactivity and oxidizes CO to CO2 already at 373 K. A complete saturation of the active sites can be reached by the catalyst exposure to N2O at low temperatures (T < 600 K in vacuum, T < 523 K at ambient pressure). Under these conditions oxygen is stored and does not desorb. On that basis, the concentration of sites active for the surface atomic oxygen loading from N2O is determined by pulse experiments using the TAP setup. The concentration of active sites is found to be the same as in a flow experiment with the Micromeritics analyzer. The mechanism of surface atomic oxygen recombination and desorption as O2 depends on the temperature. At temperatures higher than 773 K, the mechanism probably involves direct (O)Fe recombination. The investigation of the response profiles upon N2O pulses at 803 K evidences the O2 formation is the rate-determining step since the oxygen desorption is found to be slow compared to N2O and N2. Furthermore, a fit of the oxygen desorption curve corresponded to a first order dependence on the oxygen surface coverage. In the range of 523 to 773 K, the O2 formation is facilitated by NO, which is formed on the catalyst surface from N2O. The addition of NO to the gas phase confirmed the accelerating effect of NO on the oxygen desorption. NO and (O)Fe are adsorbed on adjacent, but different sites and are not competing for the same ones. Adsorbed NO accommodates oxygen from N2O forming higher oxidized nitrogen oxide species NO2/NO3 which decompose back to NOads and molecular oxygen. Thus, the recombination of surface oxygen loaded from N2O is accelerated by a NO/NOx redox cycle whereas adsorbed NOx acts as co-catalyst. Adsorbed NO is oxidized by N2O easily as shown also by temperature-programmed desorption (TPD) experiments. Therefore, the interaction of gaseous NO2 with Fe-ZSM-5 at 523 K in vacuum and at ambient pressure is investigated and demonstrates the formation of two different NOx surface species accompanied by the evolution of gaseous NO. These surface species block the sites for oxygen loading from N2O whereas the amount of reversible adsorbed N2O stays constant compared to the standard pre-treated catalyst. The formation of reactive surface oxygen from NO2 could neither be evidenced in TPD nor with CO oxidation experiments. Adsorbed water strongly influences the N2O decomposition inhibiting it completely, at least at temperatures ≤ 673 K. Water pulses at 523 and 593 K result in partial desorption of the loaded atomic surface oxygen as O2, indicating the competition of water for the same adsorption sites. The residual adsorbed oxygen desorbs during TPD experiments at 849 K instead of 715 K for the dry catalyst and it can not oxidize CO to CO2. It means that the active sites are transformed into an inactive form in the presence of water. The oxygen which stays on the catalyst seems to participate in the Fe(II) oxidation to the hydroxylated Fe(III) species. These Fe(III) species are inactive in N2O decomposition and could be activated again by high temperature treatment in He or in vacuum. During TAP experiments only small amounts of water are dosed and the water desorption is facilitated due to vacuum. Therefore, the effect of water could be shown to be reversible at temperatures > 700 K. At 803 K oxygen desorbs significantly faster with increasing water amount suggesting a water assisted O2 desorption. Simultaneously, the catalyst activity decreases to about a half of the initial activity of the dry zeolite. To conclude, from the TAP reactor data obtained under vacuum conditions and the results from the transient flow experiments at ambient pressure it follows that the mechanism of the N2O decomposition over Fe-containing ZSM-5 depends strongly on the reaction temperature and on the presence of NO, H2O and NO2. In general, below 523 K only surface atomic oxygen loading from N2O takes place with simultaneous evolution of N2 in the gas phase. The O2 formation proceeds via direct (O)Fe recombination at temperatures above 773 K. In the range of 523 to 773 K, the recombination of the atomic oxygen involves NOx adsorbed on the catalyst surface.