A multi-technique study of CO2 adsorption on Fe3O4 magnetite

Abstract

The adsorption of CO 2 on the Fe 3 O 4 (001)-( √ 2 × √ 2)R45 • surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO 2 binds most strongly at defects related to Fe 2+ , including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages, CO 2 adsorbs at fivefold-coordinated Fe 3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (• unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO 2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO 2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD mea-

Local Structure and Coordination Define Adsorption in a Model Ir1/Fe3O4 Single‐Atom Catalyst

“…When the sample is annealed at 960 K, the Ir 4f signal shifts to 60.8 eV, consistent with the formation of metallic Ir nanoparticles (Ir in a bulk metal environment has a 4f 5/2 peak at 60.7 eV). It should be noted that the sample temperatures in the XPS/TPD setup were measured by a thermocouple spot‐welded directly on the sample mount, and thus are more reliable than those measured in the STM chamber, where the thermocouple is placed further away from the sample plate on the annealing stage. Additionally, the sample mount was getting loose during STM experiments, therefore we estimate the annealing temperatures to be up to 100 K lower than indicated by the thermocouple readout in the STM chamber.…”

“…To study how the different coordination environments affect the reactivity of the model catalyst we performed TPD experiments using isotopically labelled 13 CO as a probe molecule (Figure B). First, 0.3 ML Ir was deposited on the freshly prepared Fe 3 O 4 (001) surface, and the model catalyst with two‐fold Ir exposed to 1 ML 13 CO using a calibrated molecular beam source at 100 K. The sample was then heated with a temperature ramp of 1 K s −1 and the desorbing species monitored by a mass spectrometer. We have previously shown that CO interacts weakly with the as‐prepared Fe 3 O 4 (001) surface, desorbing in two peaks below 120 K. The addition of the two‐fold‐coordinated Ir adatoms leads to a new desorption feature at about 610 K, well above the temperature where CO desorbs from metallic Ir surfaces (500–560 K) .…”

Electron transfer between anatase TiO 2 and an O 2 molecule directly observed by atomic force microscopy

“…The amount of dosed gases was calibrated according to the method described in ref. 58. One monolayer is defined as 1 molecule per surface 5-coordinated titanium atom and corresponds to an exposure of 1.6 Langmuir (L; 1 L = 1.33×10 −4 Pa×s).…”

Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface

“…The LEED, XPS, and UPS data were acquired in a vacuum system (base pressure <5 × 10 –11 mbar) using a SPECS Phoibos 150 energy analyzer, a SPECS FOCUS 500 monochromatized X-ray source (Al Kα anode), a SPECS UVS10/35 source with both He I and He II discharge, and a commercial LEED setup. Full details of this vacuum system are described in ref (34). Finally, nc-AFM data were taken in a two-vessel UHV setup (preparation chamber <10 –10 mbar, analysis chamber <10 –11 mbar) based on a commercial Omicron LT-STM equipped with a commercial Omicron q-Plus LT head and tuning-fork-based AFM sensors ( k = 1900 N/m, f 0 = 30500 Hz, Q ≈ 20 000).…”