No abstract

By means of scanning tunneling microscopy (STM) measurements we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O2) and hydrogen (H2), respectively. The FeO islands respond very dynamically towards oxygen (O2), with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O2 dissociation and O adatom incorporation. An STM movie obtained during oxidation reveales how further O2 molecules can dissociate after the consumption of all initially existing CUF sites though the formation of new CUF sites. In contrast, we found that H2 molecules only dissociate when vibrationally excited through the ion gauge, and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H2 dissociation. Our STM results reveal how excess O adatoms are incorporated and released in O2 and H2, and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.

Using a multi-technical approach, we studied the oxidation of anatase TiO 2 (101)-supported vanadium (V) clusters at room temperature. We found by ex situ XPS that the highest oxidation state is +4 at sub-monolayer coverage regardless of the O 2 pressure, and STM studies revealed that the initial oxidation proceeds through oxygen-induced disintegration of V clusters into monomeric VO 2 species. By contrast, for ~2 monolayer V coverage, a partial oxidation to V 5+ is achieved. By in situ APXPS measurements, we found that V can be maintained in the V 5+ oxidation state irrespective of the coverage; however, in the sub-monolayer range, an O 2 pressure of at least ~1 × 10-5 mbar is needed. Our results suggest an enhanced reducibility of V in direct contact with the TiO 2 support compared to V in the 2 nd layer, which is in line with the observed optimum V 2 O 5 loading in catalytic applications just slightly below a full monolayer.

The adsorption of ammonia on anatase TiO is of fundamental importance for several catalytic applications of TiO and for probing acid-base interactions. Utilizing high-resolution scanning tunneling microscopy (STM), synchrotron X-ray photoelectron spectroscopy, temperature-programmed desorption (TPD), and density functional theory (DFT), we identify the adsorption mode and quantify the adsorption strength on the anatase TiO(101) surface. It was found that ammonia adsorbs non-dissociatively as NH on regular five-fold coordinated titanium surface sites (5f-Ti) with an estimated exothermic adsorption energy of 1.2 eV for an isolated ammonia molecule. For higher adsorbate coverages, the adsorption energy progressively shifts to smaller values, due to repulsive intermolecular interactions. The repulsive adsorbate-adsorbate interactions are quantified using DFT and autocorrelation analysis of STM images, which both showed a repulsive energy of ∼50 meV for nearest neighbor sites and a lowering in binding energy for an ammonia molecule in a full monolayer of 0.28 eV, which is in agreement with TPD spectra.

Note: This article is part of the JCP Special Topic on Oxide Chemistry and Catalysis.