Impact of ambient oxygen on the surface structure of α-Cr2O3(0001)

Surface x-ray diffraction has been employed to quantitatively assess the surface structure of α-Cr2O3(0001) as a function of oxygen partial pressure at room temperature. In ultrahigh vacuum, the surface is found to exhibit a partially occupied double layer of chromium atoms. At an oxygen partial pressure of 1×10−2 mbar, the surface is determined to be terminated by chromyl species (CrO), clearly demonstrating that the presence of oxygen can significantly influence the structure of α-Cr2O3(0001)

Planar model system of the Phillips (Cr/SiO₂) catalyst based on a well-defined thin silicate film

The Phillips catalyst (Cr/SiO2) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years. However, the atomic structure of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ~1007 cm-1 typical for Cr=O vibrations. In addition, CO titration experiments suggested also the presence of "naked" Cr, which transforms into mono- and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions

Methods for Expanding the Diversity in the Response of Metal Oxide Based Gas Sensors

As all aspects of life become more automated and interconnected, sensors will be needed in various applications. In particular, gas sensors will find widespread use, in e.g. indoor air quality monitoring and breath analysis. Versus other detection methods, semiconducting metal oxide (SMOX) based sensors are more compact, sensitive, robust and inexpensive. Their major drawback is their inherent lack of selectivity. This limitation could be addressed by using arrays of SMOX materials with complementary sensing behavior. Today as a result of the historical development, despite the decades of research, most commercially available sensors are still based on SnO2. The work here examines three different options for creating complementary sensors: using a different n-type base metal oxide (WO3), noble surface loading and the creation of metal-oxide-metal-oxide mixtures. Based on a literature review, WO3 appeared promising and here its complementarity was verified. It was identified that the sensing behavior of WO3 is robust against changes in synthesis. Using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, it was possible to identify why the resistance of WO3 increases with humidity. From this finding it became apparent why the response to oxidizing gases strongly decreases in the presence of atmospheric humidity. In order to tune the sensing behavior, surface loading with metal oxides is commonly used. Although two mechanisms, chemical sensitization and Fermi level pinning, were already suggested in the 1980s, experimental evidence was limited. Here, the effect of rhodium, palladium and platinum loading on WO3 was examined. Using operando DRIFT spectroscopy, in situ transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) it was shown that the Fermi level pinning mechanism dominates. As a result, Rh-loading reduces the complementarity of WO3 and SnO2 based sensors. Finally, sensors based on SnO2 and Cr2O3 mixtures were examined. Reports of gas sensors based on combinations of metal oxides, in particular mixtures of n- and p-type materials, are common in literature. These mixed materials are usually created using sophisticated and expensive methods, like the electrospinning of nanofibers. Here sensors based on nanofibers were compared to those based on randomly dispersed particles. By breaking apart the nanofibers using soft mechanical grinding, it was possible, for the first time, to clearly separate the effects of the secondary structure from the coupling between the materials. It was identified that the junctions between the materials are largely responsible for the changed sensing. Furthermore, it was shown that by varying the ratio of the metal oxides, the sensor response can be tuned, i.e. shows a p- or n- type response, and in some cases no response. In total it has been shown that other n-type materials should be considered for integration into arrays with SnO2. It has been found that the applicability of noble metal oxide surface loadings to increase the complementarity of materials is limited. It has been shown that metal-oxide-metal-oxide mixtures can be used to tune the sensing behavior and that the mechanical mixing of materials is a sufficient preparation method to attain the desired results