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

Abstract

Understanding the kinetics of oxygen removal from catalytically active metal surfaces by hydrogen is important for several catalytic reactions such as Fischer–Tropsch synthesis, methanation of CO or CO2, and the reverse-water–gas-shift reaction. Motivated by FTS, a Co(0001) single crystal model catalyst was used to study the kinetics of oxygen removal through reaction with hydrogen. Kinetic studies in the 10–7–10–4 mbar H2 pressure regime show that water formation is first order in the surface hydrogen concentration while the order in oxygen concentration changes from one at low oxygen coverage to zero at high oxygen coverage. In situ XPS shows that the hydrogen surface concentration saturates around 10–1 mbar and on this basis the typical temperature of 450 K needed for water formation in this pressure regime can be considered as typical for high pressures as well. The absence of OH buildup during the reaction points to O + H as the rate-limiting step, with a barrier of ∼120 kJ mol–1. Such a high barrier shows that slow removal of adsorbed oxygen from the surface of reactive metal catalysts such as cobalt may be rate-limiting for the overall reaction.

Water and Hydroxyl Reactivity on Flat and Stepped Cobalt Surfaces

“…This is similar to the situation on Ni(111) reported by Pache et al, and we attribute it to intact water coordinated to, and stabilized by, O ad . The situation changes above 0.25 ML, the coverage regime where islands of another high density structure of chemisorbed oxygen form alongside the p(2 × 2) islands . From this point on, we find the growth of a fourth H 2 O peak at 185 K at the expense of the 175 K peak.…”

“…The initial H 2 O coverage prior to electron irradiation was around 2.5 ML in this particular experiment, and the H 2 O multilayers produce a large, broad peak around 534 eV in the O 1s spectrum . This does not stop electrons from penetrating to the interface and produce both OH ad and O ad at the H 2 O/Co interface, as evident from the presence of a peak at 530.9 attributed to OH ad , and at 529.4 eV due to atomic oxygen. ,, Figure (b) provides the concentrations of O ad , H 2 O ad and OH ad during heating of a H 2 O layer after exposure to 100 eV electrons derived from the series of O 1s spectra shown as a heat map in Figure (c). The water multilayers desorb below 160 K, but the O 1s spectrum at 176 K still shows a substantial amount of H 2 O at a temperature where water had already left the surface in the experiment without electron bombardment.…”

“…The TPD spectrum after irradiation shows desorption of H 2 with a peak area that corresponds to the decomposition of 0.2 ML H 2 O into atomic oxygen and hydrogen atoms. Water dissociation was confirmed by the LEED pattern, which was recorded after heating to 500 K and shows a p(2 × 2) pattern typical for atomic oxygen at a concentration of 0.2 ML. , Irradiation has a significant impact on the water desorption signal. Quantitative comparison with the water desorption without irradiation shows that the amount of desorbing water decreases by 50%, which is attributed to a combination of decomposition and electron-induced desorption.…”

“…Temperature-programmed desorption and reaction experiments were performed using the setup described previously in refs. ,, The “shielded” quadrupole mass spectrometer (QMS) used to measure the water formation rate is located inside a separately pumped and moveable compartment with a 5 mm aperture that connects to the main chamber. Desorption signals and reaction products from other parts of the sample holder are largely suppressed when the sample is placed at 2 mm from the aperture.…”

“…The removal of adsorbed oxygen atoms by reaction with hydrogen to form water is a crucial step in Fischer–Tropsch synthesis (FTS) on cobalt catalysts and also in several other important catalytic reactions. Our recent studies , indicated that the formation of OH ad species is the rate-limiting step in the reaction of adsorbed oxygen and hydrogen on Co(0001), with a significant activation barrier of around 120–130 kJ mol –1 . Through in situ measurements at both low and near-ambient H 2 pressures, we found that the concentration of OH ad species during water formation on Co(0001) remains below the detection limit, which points to a high reactivity of hydroxyl species.…”