High-resolution transmission electron microscopy reactions

However, in more recent times science has made rapid strides in this direction. It is now possible to use EXAFS in situ during a catalytic reaction to examine the average coordination of metal atoms in the small particles which often exist in precious metal catalysts [2]. High resolution transmission electron microscopy has evolved to the level of atomic resolution, but can only be used ex-situ, or in situ with moderate pressures when special cells are fitted [3]. 

The formation of covalently PEG-grafted SWCNT hybrid material was achieved by the reaction of SWCNT-COC1 with hydroxyl-terminated PEG in various solvents. Two different self-assembling morphologies, depending on the quality of the solvent, were discovered by high-resolution transmission electron microscopy (HR-TEM) [141]. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

In this study, a powerful combination of high resolution transmission electron microscopy (HRTEM), mass spectrometry (MS) and Quick EXAFS (QEXAFS) is used to study the reactions of [Pt2+(NH3)4](N03 )2 impregnated on Si02 during different pretreatment processes. MS is used to monitor which gases are produced during the pretreatment. QEXAFS is used to study the local structure of the Pt complex during the pretreatment. The timescale of... 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures in aqueous electrolytes at 274—333 K. We focus on the study of both the structure sensitivity of the reactions as gleaned from studies of the bulk (bi) metallic surfaces and the resultant crystallite size effect expected or observed when the catalyst is of nanoscale dimension. Physical characterization of the nanoparticles by high-resolution transmission electron microscopy (HRTEM) techniques is shown to be an essential tool for these studies. Comparison with well-characterized model surfaces have revealed only a few nanoparticle anomalies, although the number of bimetallics... 

High resolution transmission electron microscopy examinations were performed on catalyst particles that have treated at temperatures in excess of 725°C in the presence of CO/H2. Most of the bimetallic particles had short filaments associated with them, indicating that under these conditions the catalytic activity towards the growth of this form of carbon was relatively low. It was significant, however, to find that the particle surfaces on which gas phase adsorption and decomposition reactions occurred had remained completely free of any accumulated carbon residues. This observation is consistent with the argument that the... 

High resolution transmission electron microscopy (TEM) (Jeol lOOCX) was employed to determine the size of the metal particles on the surface of the catalyst support, and the composition of individual metal particles was ascertained (for thin sections cut with an ultramicrotome) using a field-emission scaiming transmission electron microscope (STEM) (VG HB 501) (at 1.5 mm resolution) and an energy dispersive X-ray (EDX) analyser. The metal loading of catalysts was determined by ICP-AES (Spectro D), following dissolution in concentrated hydrochloric and sulphuric acids. Direct analysis of aqueous samples taken from the reaction medium, using the same analytical technique, allowed the corrosion of metallic components from the catalyst surface to be studied. 

The growth mechanism appears to be the same, irrespective of the hydrocarbon. However, the resulting morphology depends not only on the metal and particle size but also on the type of hydrocarbon and the reaction temperature. The importance of step sites on the catalyst surface for the nudeation of carbon was recently confirmed by in situ investigations employing high resolution transmission electron microscopy the results indicate the segregation of carbon when the formation of whiskers takes place at specific sites on the nickel surface. The addition of potassium to the nickel catalyst leads to an increase in the induction period (t ) for CH4 decomposition, which... 

The sizes of nanoparticles can be measured directly by the means such as HRTEM (High Resolution Transmission Electron Microscopy), SAXS (Small Angle X-ray Scattering). The traditional methods for measuring the amount of sm-face metal atoms are built on the foundations of chemisorptions (see the details in Chapter 7). Total amount of surface metal atoms measured by this method is related to the amount of gas molecules adsorbed on it. This value is not always equal to total amounts of catal dic active sites for some reactions. 

---