van J Joop Grondelle

An in situ cell for transmission EXAFS measurements on catalytic samples

An in situ cell suitable for transmission EXAFS measurements on catalytic samples is described. The cell can be used for catalyst pretreatments in various atmospheres (including H2, H2S, O2 and CO) in a temperature range upto 700 K. The sample is heated by conducting heat from an external heater to the sample. During measurement the samples can be cooled down to 77 K by conducting heat from the sample to an external liquid nitrogen container. During the pretreatment and the measurement a waterflow through the body of the cell keeps certain crucial parts from overheating or icing up. To avoid radiation leaks in powdery samples these samples are pressed in a selfsupporting wafer and held in a disk‐shaped sampleholder. Tests by various catalytic groups have proven the suitability of the design.

A temperature programmed reduction study of Pt on Al2O3 and TiO2

The reducibility of platinum on γ-Al2O3- and TiO2 was studied with the aid of temperature programmed reduction. The reduction peak temperature was found to be dependent on the temperature of the primary oxidation after impregnation and drying. The higher the oxidation temperature the lower the TPR peak temperature and the higher the H2 consumption. During the oxidation small PtO2 particles were formed which were more easily reduced than the original isolated Pt2+ ions. For Pt/Al2O3 no decomposition of PtO2 was observed up to 750 K, while bulk PtO2 decomposed around 600 K. This demonstrated that there is a substantial interaction between PtO2 and Al2O3. For PtO2 supported on TiO2 and SiO2 this interaction is much weaker and on these supports PtO2 decomposed at lower temperatures than on Al2O3. Reduction of passivated catalysts, with H/Pt < 0.8 in the metallic state, took place even at 223 K. After passivation these catalysts consist of a metal core surrounded by a metal oxide skin. Due to the presence of the metallic core, H2 can be dissociatively chemisorbed at low temperatures and induce reduction of the oxide layer. The implications of this for the O2-H2 titration method are discussed. Reduction of Pt/TiO2 led to metal assisted reduction of the support. Below 500 K only a small part of the support is (reversibly) reduced in the near vicinity of the metal particles. Above 500 K further metal assisted reduction of the TiO2 support takes place, probably promoted by increased ion mobility.

N2O decomposition over Fe/ZSM-5: effect of high-temperature calcination and steaming

N2O decomposition over Fe/ZSM-5 made by the sublimation method was investigated. Further calcination or steaming treatment (700°C) increased the decomposition rate of N2O. FTIR showed that such treatments lead to the disappearance of Brønsted acid sites. The various catalysts have different apparent activation energies, confirming the presence of different active Fe species. The observation that after high-temperature calcination a higher activity is obtained while no dealumination occurs indicates a mechanism where neutral Fe oxide clusters occluded in the zeolite micropores react at high temperature with the zeolite protons to form [FeO]+-like species.

Hydrogenation of carbon monoxide over rhodium/silica catalysts promoted with molybdenum oxide and thorium oxide

The promotion of silica-supported rhodium catalysts in the hydrogenation of carbon monoxide by molybdenum oxide and thorium oxide has been examined. Temperature programmed reduction studies indicated the formation of rhodium molybdates, while no evidence was found for the formation of such mixed oxides in the thorium oxide-promoted catalysts. Hydrogen and carbon monoxide chemisorption were suppressed by the presence of molybdenum oxide, pointing to a coverage of the rhodium particles by this promoter oxide. The catalysts with MO: Rh ratios exceeding one even exhibited an almost complete suppression of the rhodium chemisorption capacity. In the thorium oxide-promoted catalysts the chemisorption of hydrogen and carbon monoxide were not suppressed. Infrared spectroscopy of adsorbed carbon monoxide showed that molybdenum oxide completely suppressed bridge-bonded and linearly bonded carbon monoxide, as well as the gemdicarbonyl species. Thorium oxide addition resulted in a minor decrease of the linearly bonded carbon monoxide, while the bridge-bonded carbon monoxide was suppressed to a greater extent. The IR spectra of the thorium oxide-promoted catalysts also exhibited a broad absorption band between 1300 and 1750 cm-‘, which is thought to be due to carbon monoxide bonded with the carbon atom to the metal and with the oxygen atom to the promoter ion. Carbon monoxide hydrogenation was greatly enhanced by the presence of both molybdenum oxide and thorium oxide. Thorium oxide-promoted catalysts had a high selectivity to C,-oxygenates, while the molybdenum oxide-promoted catalysts exhibited a high methanol selectivity. Ethylene addition to a working catalyst showed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of oxygenates, was not enhanced by molybdenum oxide, nor by thorium oxide. The ethylene addition experiments indicated that the role of the promoter is to enhance carbon monoxide dissociation. The results can be understood by assuming that side-bonded carbon monoxide, with its weakened C-O bond, is responsible for the higher carbon monoxide dissociation activity.

The morphology of rhodium supported on TiO2 and Al2O3 as studied by temperature-programmed reduction-oxidation and transmission electron microscopy

Supported RhAl2O3 and RhTiO2 catalysts with varying metal loadings were investigated by chemisorption and temperature-programmed reduction and oxidation. Hydrogen chemisorption showed that all the Rh on Al2O3 was well dispersed (HRh > 1 for loadings below 5 wt% and H>Rh > 0.5 up to 20 wt%), while the dispersion on TiO2 was much lower. TPR/TPO showed that this was due to the growth of two different kinds of RhRh2O3 particles on TiO2; one kind was easily reduced/oxidized, with a high dispersion, and the other kind was harder to reduce/oxidize, with a lower dispersion. TEM showed that the first kind of Rh2O3 consisted of flat, raftlike particles and the second kind of spherical particles.

Hydrogenation of carbon monoxide over vanadium oxide-promoted rhodium catalysts

The effect of vanadium oxide as support and promoter on supported rhodium catalysts on the CO hydrogenation has been investigated at 0.15 and 4.0 MPa. Rh/V,O, reduced at 723 K has a good selectivity toward oxygenated products, especially C,-oxygenates, but has a low activity added as a promoter to catalysts consisting and stability. Vanadium oxide of rhodium supported on silica and alumina showed a remarkable effect on the activity of these systems. For the silica-supported systems the activity increased by a factor of 40, the deactivation of these catalysts was low (2 % h-l) and the oxo-selectivity was very high (70 %I. Although the vanadium oxide blocks part of the active metal surface, as became evident from a suppressed chemisorption capacity, it also enhances the -rate of CO dissociation in those locations where reaction is still possible, The enhancement prevails over the blocking in the case of silica- and alumina-supported vanadium oxide-promoted catalysts, while blocking dominates for the vanadium oxide-supported catalyst after high temperature reduction. Experiments in which ethylene was added to a working catalyst, provided indications that the main promoter action of the vanadium oxide is to increase the CO dissociation, thereby increasing the activity of the catalyst. For the alumina-supported catalysts, most of the vanadium oxide is scavenged by the support and only at a high V/Rh ratio, the activity of the Rh/Al 0 catalyst is increased. The addition of vanadium oxide to the alumina-support

Preparation and characterization of vanadium oxide promoted rhodium catalysts

d3catalysts caused a suppression of the formation of ethers, covers the acidic The vanadium oxide probably

NH3 oxidation to nitrogen and water at low temperatures using supported transition metal catalysts

Abstract The location of the promoter element in rhodium on alumina and silica catalysts promoted by vanadium oxide has been studied by various techniques. Our results prove that an intimate contact between the active component, rhodium, and the promoter, vanadium oxide, is present in most catalysts studied. For the silica-supported systems, temperature programmed reduction and diffuse reflectance infrared spectroscopy pointed to the formation of a mixed oxide (RhVO 4 ) during calcination. Reduction of this oxide phase resulted in a vanadium oxide layer on top of the metal particle, as could be concluded from carbon monoxide chemisorption experiments. CO chemisorption was suppressed in the Rh/V 2 O 3 /SiO 2 catalysts, while transmission electron microscopy showed that the rhodium particle size was not influenced by the addition of vanadium oxide. This indicates that the suppression of CO chemisorption is not due to a decrease of metal particle size, but due to covering of the metal particle. Infrared spectroscopy showed that the amount of linearly bonded and bridge-bonded CO was almost completely suppressed, while the amount of gem-dicarbonyl species remained unaffected. No suppression of hydrogen chemisorption was observed. From this and TPD experiments it could be concluded that hydrogen adsorption occurs both on the exposed Rh atoms, as well as on the vanadium oxide patches partly covering the surface Rh atoms, pointing to the formation of hydrogen bronzes. Temperature programmed reduction experiments showed that RhVO4 was not formed in Rh/V 2 O 3 /Al 2 O 3 during calcination. For V/Rh 2 O 3 and V 2 O 3 particles exist separately on the support, due to the strong interaction between V 2 O 5 and Al 2 O 3 . Only for catalysts with a V/Rh value around 7.0 (near-monolayer of vanadium oxide on alumina), oxidation at 898 K resulted in the formation of RhVO 4 . For these catalysts, Rh2O3 might be positioned on top of the vanadium oxide layer after calcination. Almost all adsorbed CO was present in the form of the gem-dicarbonyl species and only a minor suppression of CO adsorption was observed.

Reduction and oxidation of Rh/Al2O3 and Rh/TiO2 catalysts as studied by temperature-programmed reduction and oxidation

Abstract The ability of several alumina-supported metal catalysts and transition-metal ion-exchanged zeolite Y catalysts to oxidize ammonia to nitrogen and water at low temperatures (between 200 and 350°C) was tested both at high and low ammonia concentrations. Copper-containing zeolite Y catalysts were comparable in activity and were more selective than noble-metal containing zeolite catalysts of similar metal loading. Cu/zeolite-Y catalysts were superior to copper/molybdenum and vanadium/alumina catalysts. Postsynthesis treatment of Cu/zeolite-Y with NaOH increased the activity for ammonia oxidation; dispersion and size of the supported copper-oxide particles were very important parameters. Co-fed steam dramatically increased the deactivation on all catalysts, especially at lower temperatures.

Preparation and characterization of very highly dispersed iridium on Al2O3 and SiO2

Abstract Careful preparation of Rh/Al 2 O 3 catalysts leads to ultradisperse systems (H/Rh > 1.0). Temperature-programmed reduction (TPR) shows that these catalysts are almost completely oxidized during passivation. Identical preparation of Rh/TiO 2 catalysts leads to less disperse systems (H/Rh = 0.3), which exhibit two reduction peaks in TPR. These peaks are due to the reduction of small, well-dispersed Rh 2 O 3 particles and of large, bulk-like Rh 2 O 3 particles. In all cases reduction of Rh 2 O 3 is complete above 450 K. TiO 2 is partly reduced by a metal-catalysed process above 500 K.