Water and Hydroxyl Reactivity on Flat and Stepped Cobalt Surfaces

Abstract

Hydroxyl adsorbates generally appear as transient species during water formation from adsorbed oxygen and hydrogen atoms on a metal surface, a reaction that is part of the catalytic cycle in various important surface-catalyzed reactions such as Fischer−Tropsch synthesis. In the present work, temperature-programmed desorption and in situ synchrotron XPS were used to study water adsorption and OH reactivity on a flat and a stepped cobalt single crystal surface. Water adsorbs intact on the flat Co(0001) surface and desorbs around 160 K. Electrons induce dissociation of water and produce OH species at low temperature. Hydroxyl species can also be formed by the reaction between O ad and H 2 O, but only for high initial oxygen coverage while low coverage O ad appears largely unreactive. Reactive hydrogen species (H atoms) produced by a hot tungsten filament hydrogenate adsorbed oxygen atoms at low temperature already and both OH ad and H 2 O are formed. In all cases, hydroxyl adsorbates react around 190 K to form water via 2 OH ad → H 2 O (g) + O ad associated with an activation barrier of 40−50 kJ mol −1 . Water readily dissociates on the step sites exposed by vicinal . A part of the OH ad species recombine to form water and oxygen between 200 and 300 K, while decomposition of OH ad into O ad and H ad dominates above 370 K. For catalysis, the high reactivity of step sites for water dissociation and the high stability of OH ad at these sites implies that O removal from these sites may be difficult and may limit the overall rate of Fischer−Tropsch synthesis on cobalt catalysts.

Water Formation Kinetics on Co(0001) at Low and Near-Ambient Hydrogen Pressures in the Context of Fischer–Tropsch Synthesis

“…The reference spectrum for a mixed CO ad −O ad layer was obtained by dosing CO at 300 K onto the O-covered sample, while the reference spectrum for OH ad was obtained by electron-induced dissociation of H 2 O ad , described in more detail elsewhere. 23,24 A cold trap held at liquid nitrogen temperature was used for the hydrogen gas line to eliminate water. For the NAP-XPS experiments at HIPPIE a Pall GLPSIPVMM4 filter was used in addition to the cold trap.…”

“…Binding energies are reported relative to the Fermi edge. The reference spectrum for a mixed CO ad –O ad layer was obtained by dosing CO at 300 K onto the O-covered sample, while the reference spectrum for OH ad was obtained by electron-induced dissociation of H 2 O ad , described in more detail elsewhere. , …”

Wetting of a Stepped Platinum (211) Surface

“…Platinum is an effective redox catalyst for the formation of water in electrochemical fuel cells, stimulating considerable interest in how it interacts with water, OH, and other intermediate species. , As a result, a number of studies have investigated how water binds on the close-packed Pt surface, revealing details of how the film nucleates, , the complex mix of pentamer, hexamer, and heptamer rings that stabilizes the first layer of water, − and how the structure of this layer influences the growth and crystallization of multilayer films. − The importance of surface steps as nucleation sites was identified in the earliest STM studies, with water forming narrow “quasi-one-dimensional chains” less than 10 Å wide on the top edge of Pt steps, with more extended 2D islands nucleating on the lower terrace . Steps were also found to stabilize water on other metal surfaces, − and this, along with the importance of low coordinate sites for practical catalysts, − has spurred experiments to examine how step sites influence water adsorption on Pt. − The general conclusion is that low coordinate metal steps enhance the binding energy of water and stabilize adsorption, but there is less clarity about the precise structures formed on different surfaces or indeed the amount of water present.…”