Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt-n/TiO2(110)-(1x1)

We determined the CO oxidation rates for size-selected Pt-n (n is an element of {3,7,10}) clusters deposited onto TiO2(110). In addition, we investigated the cluster morphologies and their mean sizes before and after the reaction. While the clusters are fairly stable upon annealing in ultrahigh vacuum up to 600 K, increasing the temperature while adsorbing either one of the two reactants leads to ripening already from 430 K on. This coarsening is even more pronounced when both reactants are dosed simultaneously, i.e., running the CO oxidation reaction. Since the ripening depends on the size initially deposited, there is nevertheless a size effect; the catalytic activity decreases monotonically with increasing initial cluster size.

Effect of the TiO2 Reduction State on the Catalytic CO Oxidation on Deposited Size-Selected Pt Clusters

Simon Bonanni, Harald Brune, Wolfgang Harbich

The catalytic activity of deposited Pt-7 clusters has been studied as a function of the reduction state of the TiO2(110)-(1 x 1) support for the CO oxidation reaction. While a slightly reduced support gives rise to a high catalytic activity of the adparticles, a strongly reduced one quenches the CO oxidation. This quenching is due to thermally activated diffusion of Ti3+ interstitials from the bulk to the surface where they deplete the oxygen adsorbed onto the clusters by the formation of TiOx (x similar or equal to 2) structures. This reaction is more rapid than the CO oxidation. The present results are of general relevance to heterogeneous catalysis on on TiO2-supported metal clusters and for reactions involving oxygen as intermediate.

2012

Transition metal clusters on h-BN/Rh(111)

Hamed Achour

The main aim of this dissertation is to study the catalytic aspects of very small transition metal clusters supported on h-BN/Rh(111). The catalytic reactions of interest were ammonia synthesis and CO oxidation on mass selected and soft landed iron and platinum clusters, respectively. A special focus was devoted to the investigation of the stability of these very small clusters on h-BN before and after the reaction. It was found that Pt7 clusters soft landed at room temperature with an energy of 1.2 eV per atom and annealed to 700 K exhibit Smoluchowski ripening. Above this temperature, these clusters undergo partial intercalation between the h-BN monolayer and the Rh(111). The intercalation initiates at 900 K and becomes more pronounced when the clusters are annealed under gas reaction conditions. A high catalytic activity was observed when the Pt catalyst remains supported on h-BN. In this case, the reaction starts at 480 K and follows Langmuir-Hinshelwood mechanism. However, the catalytic activity strongly reduces when the Pt clusters undergo partial intercalation. Nevertheless, in this case the reaction starts at only 380 K, revealing a reduction by 100 K in the Pt poisoning as a result of substrate effect and charge redistribution. In a second trail, the cluster-h-BN interaction was studied through h-BN irradiation with Pt clusters, at room temperature, within an energy window of 30-416 eV/atom. The results show that even irradiation at 30 eV/atom can lead to entire non thermal intercalation of the clusters. The energetic Pt clusters were found to be site selective as they settle only under the h-BN wires, in contrast to soft landed Pt7 clusters that were found to settle at the side edge of the h-BN depressions. After being exposed to irradiated Pt clusters, at an energy above 100 eV/atom, the h-BN layer starts to display visual cavity-type defects. These defects, induced by collision, disappeared after annealing to 600 K under gas reaction leaving behind an h-BN with no visual defects. CO temperature desorption spectroscopy (TDS) indicates that the intercalated Pt is inactive towards CO oxidation due to h-BN screening. Subsequently, we were able to produce a catalyst system made of soft-landed Pt/h-BN/intercalated Pt/Rh(111) using a combination between soft and energetic Pt deposition with which a reduction by 100 K in CO poisoning was obtained. Finally, ammonia synthesis was conducted on soft landed Fe clusters, supported on h-BN/Rh(111) and deposited at 100 K under Ultra High Vacuum (UHV). Scanning tunneling microscopy (STM) displays two imaging states of the as deposited clusters; a ring state surrounding h-BN depression which disappears above 300 K and a dot like structure. The Fe clusters are found to grow by Ostwald ripening after annealing to 600 K and above this temperature, they endure partial intercalation. It was found that, in the presence of iron, the nitrogen reaction gas experienced an exchange with the nitrogen species forming the h-BN layer. TPR (Temperature Programmed Reduction) of N

_2

with hydrogen revealed that nitrogen reduction occurs at 620 K under high vacuum following the Haber-Bosch method. The detection of reaction intermediates NH and NH

_2

together with desorbed atomic nitrogen, during ammonia synthesis, confirms that NH

_3

formation involves the stepwise hydrogenation of adsorbed nitrogen.

EPFL2017

Production d'hydrogène dans un réacteur microstructuré

Pierre Reuse

The aim of this work was to operate the steam reforming of methanol and the total oxidation of methanol in a microstructured two-passage reactor. The steam reforming enables the production of hydrogen at relatively low temperatures (250 °C) with only a small amount of carbon monoxide (~ 1%). After a cleaning step (CO preferential oxidation, for example), this hydrogen would be a good feed for a fuel cell to produce electricity for mobile applications. Steam reforming is an endothermic reaction. The energy needed to run this reaction is produced by the total oxidation, a strongly exothermic reaction. To allow an efficient heat transfer between these two reactions, a microstructured reactor was used. These structures, with dimensions in the micron range, are widely known to have far better heat transfer capacity than traditional heat exchangers. Furthermore, these structures have some unique features with respect to classical reactors. A narrow residence time distribution, close to that of a plug flow reactor, can be obtained, keeping selectivity high for complex reactions. Microstructured reactors also have small residence time, allowing very quick variations in the feed and experimental conditions. In chapter 4, the concepts relating to the channel coatings will be presented. A Cu/Zn/Al catalyst provided by Süd-Chemie was transformed into a suspension of nanoparticles in isopropanol by using an attrition grinding apparatus. A thin layer (~5 µm) of catalyst was formed on the microchannel walls. Several tests were performed to characterize the adhesion of this layer. The influence of the layer on the residence time distribution was also investigated; it was possible to maintain Bodenstein numbers as high as 50. Chapter 5 is focused on the steam reforming of methanol, the reaction that generates hydrogen. A kinetic analysis in differential and integral modes established that the rate of the reaction is inhibited by hydrogen, the main product of the reaction; the partial order is - 0.2. The influence of water on the rate of reaction is negligible (partial order of 0.1), and methanol has a positive partial order of 0.7. A kinetic constant of 5.57 · 10-6 mol0.4 · (m3)0.6 · gcat-1 · s-1 at 200 °C and an activation energy of 73.9 kJ · mol-1 complete the description of the kinetics of the steam reforming. Throughout the comparisons, we showed that the catalyst coated in the microchannels has an initial rate for the steam reforming 30% higher than the original catalyst in fixed bed. We can therefore conclude that the catalyst modification procedure does not diminish the catalyst activity. This was also shown by comparing our results with results available in the scientific literature for similar catalysts. CO2 selectivity remains high and constant then decreases as 90% conversion is approached. Working with an excess of water with respect to methanol contributes to keeping the selectivity high. A molar ratio of 1.2 is a good compromise between selectivity and increased energy costs due to the vaporization of the additional water. It's also beneficial to work at conversions below 90% to be in the high selectivity domain and to diminish the inhibiting influence of hydrogen. Finally, we showed that this catalyst deactivates over 100 hours at a temperature of 300 °C. A kinetic model for the deactivation at temperatures between 300 °C and 330 °C was established. In chapter 6 the total oxidation of methanol was studied. A cobalt based catalyst was developed to allow the synchronization of the two reactions temperatures. The minimum temperature for complete conversion for the total oxidation reaction must clearly not exceed an acceptable working temperature for the steam reforming catalyst (i.e. a temperature that results in an acceptable catalyst lifetime). A kinetic analysis determined partial orders for methanol and oxygen as 0.67, the kinetic constant at 230 °C as 5.45 · 10-3 mmol-0.33 · m4 · (gcat · s)-1 and the activation energy as 130 kJ · mol-1. The copper based catalyst used for the steam reforming of methanol was also active for the total oxidation of methanol. Unfortunately the minimal temperature to ensure a total conversion was not compatible with the working temperature of the steam reforming. In chapter 7, the results concerning the coupling of the two previously studied reactions are presented. The numerous fittings and cables necessary for the operation of the reactor didn't allow the reactor to be run in an autothermal mode. Therefore constant, not regulated, heating was used to compensate the thermal losses. By coupling the two reactions and by studying the dynamic response time of the reactor (temperature and conversion of the steam reforming reaction) at the time of stopping and starting the oxidation reaction, we've seen that the temperature increase was around +10 °C within 60 seconds and that the conversion for the steam reforming of the methanol fitted well with the temperature increase. This reactor also has good dynamics and responds quickly to feed variations. When the coupling mode was changed from co-current to counter-current we noticed a significant increase in conversion. Furthermore, selectivity in counter-current mode remained high, even for conversion approaching 100%.

EPFL2003