Hilde Johnsen Venvik

Using an annular structured reactor the oxidation of CO and H ₂ over silver was studied at temperatures employed during the industrial partial oxidation of methanol to formaldehyde. Silver tubes were systematically exposed to different atmospheres and imaged at regular intervals using scanning electron microscopy to gain insight into both reaction activity and catalyst restructuring. The reactant feed has a profound effect on restructuring, and a “dynamic steady-state morphology” is identified for each gas mixture. The exposed catalysts show clear signs of stepped surface faceting, grain growth and formation of small pinholes (< 1 μm). H ₂ oxidation conditions promote the formation of numerous angular surface cavities (> 3 μm). Addition of steam to the feed, however, inhibits the formation of stepped facets. Silver is active towards both CO and H ₂ oxidation, and co-feeding both reactants has a synergistic effect. Addition of steam leaves the H ₂ oxidation activity unaffected but completely inhibits the formation of CO ₂ via CO oxidation. The results clearly show that the effects of H ₂ O formation are very different from those of H ₂ O addition and that CO is not a precursor towards CO ₂ in the silver-catalysed methanol to formaldehyde system, in which steam is usually co-fed. The systematic approach taken here allows the effects of temperature and gas composition to be decoupled, and the results can be used in future studies to ascribe restructuring features to various reactants.

Dynamic restructuring of Ag catalysts was visualised in real time using in situ X-ray ptychography. Formation of pores and cavities was observed upon heating under various gas environments, allowing quantitative assessment of material redistribution.

Platinum supported on ceria and zirconia was prepared through different preparation methods: Coprecipitation (CP), spray drying (SD), and flame spray pyrolysis (FSP). The catalysts were characterized by XRD, TPR, N₂ adsorption, and H₂ chemisorption, and the water–gas shift activity in the range 190–310 °C and initial stability at 300–310 °C were tested. Although the spray-dried Pt/CeO₂/ZrO₂ catalyst shows the highest initial activity, it deactivates rapidly at 300 °C and levels out at similar activity as the coprecipitated Pt/CeO₂ and Pt/CeO₂/ZrO₂ within a few hours. Flame spray pyrolysis appears to be a promising preparation method concerning the stability of catalysts, although the initial activity is rather poor. High activity is related to high Pt dispersion, low reduction temperature, and small support particles. The support particle size is also much affected by the preparation method.

The co-adsorption of hydrogen and carbon monoxide on Pd₃Ag(111) alloy surfaces has been studied as a model system for Pd-Ag alloys in membrane and catalysis applications using periodic density functional theory calculations (PW91-GGA). We explored the effects of Pd–Ag surface composition, since segregation of silver towards and away from the surface has been suggested to explain the experimentally observed changes in H₂activation, CO inhibition and reactivity. We found that CO pre-adsorbed on the surface weakens the adsorption of H on Pd₃Ag(111) alloy surfaces irrespective of whether the surface termination corresponds to the bulk Pd₃Ag composition, or is purely Pd-terminated. A higher coverage of H with CO present is obtained for the Pd-terminated surface; this surface also exhibits a larger range of chemical potentials for co-adsorbed hydrogen and CO. The barrier for H₂activation increases with increasing CO coverage, but the surface composition has the largest impact on H₂activation at intermediate CO coverage. The results imply that Pd-based membranes with typically ~ 23 wt% Ag are less prone to CO poisoning if the surface becomes Pd-terminated.