Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy

Lauritsen JV, Foster AS, Olesen GH, et al. Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy. Nanotechnology. 2006;17(14):3436-3441.Atomic force microscopy in the non-contact mode (nc-AFM) can provide atom-resolved images of the surface of, in principle, any material independent of its conductivity. Due to the complex mechanisms involved in the contrast formation in nc-AFM imaging, it is, however, far from trivial to identify individual surface atoms or adsorbates from AFM images. In this work, we successfully demonstrate how to extract detailed information about defects and the chemical identity of adsorbates on a metal oxide surface from nc-AFM images. We make use of the observation that the apex of the AFM tip can be altered to expose either a positive or negative tip termination. The complementary set of images recorded with the two tip terminations unambiguously define the ionic sub-lattices and reveal the exact positions of oxygen vacancies and hydroxyl (OH) defects on a TiO2 surface. Chemical specificity is extracted by comparing the characteristic contrast patterns of the defects with results from comprehensive AFM simulations. Our methodology of analysis is generally applicable and may be pivotal for uncovering surface defects and adsorbates on other transition metal oxides designed for heterogeneous catalysis, photo-electrolysis or biocompatibility

Combined (Q)EXAFS/XRD: Technique and applications

The advantages of the combined (Q)EXAFS/XRD technique are discussed with focus on recent studies where the technique has given additional insight into catalyst properties. XRD and also more recently EXAFS have become routine techniques in catalysis research. However, by use of a combination of the two methods both the long and the short range ordered structures can be probed enabling a much better description of many catalysts. Of vital importance is also the possibility to perform the studies in situ, revealing structural information directly related to the active state of the catalyst. Time dependent changes of both the crystalline and the amorphous phases in catalysts can be followed by using a technique which combines EXAFS in the quick scanning mode (QEXAFS) and XRD using a position sensitive detector

In-situ studies of catalysts by XAFS and Mössbauer spectroscopy

The unique advantages of XAFS and Mössbauer spectioscopy applied to studies of catalysts are discussed. These advantages include the importantin situ capability, as well as the possibility to provide information on X-ray amorphous materials. The successful use of the two techniques is demonstrated by examples of two important technical catalyst systems, hydrotreating and ammonia synthesis catalysts, representing supported and unsupported catalysts

X-Ray absorption studies of the Ni environment in Ni-Mo-S

The local environment of the Ni atoms in the Ni-Mo-S phase has been elucidated by Ni K-edge EXAFS and XANES measurements of carbon-supported sulfided Ni-Mo catalysts. The results show that the Ni atoms have a low sulfur coordination number (less than six). This is contrary to recent reports in which an octahedral-like sulfur coordination have been suggested. It is proposed that the Ni atoms in Ni-Mo-S are located at the(1010)(1010) edges of the MoS$_2$ structure in square and tetragonal pyramidal type sites. These two types of sites can easily interconvert during sulfur addition/extraction and may as such be involved in the catalytic cycle