Electron microscopy, small metal particle

We first review the factors affecting catalyst structures, sintering of small metal particles and ceramic substrates and describe the unique contributions of electron microscopy. 

Experimental and theoretical developments in small metal-particle catalysis using electron microscopy... 

There are many ways in which small metal particles can be created and examined (Section 3.2). When the gold particles are supported, the first step is to determine their mean size and size distribution for this there is no real substitute for transmission electron microscopy (TEM). The various energetic and electronic properties then need to be examined, and the bases of the available experimental techniques will be briefly rehearsed in Section 3.3. Of particular interest is the point at which the change from metallic to nonmetallic behaviour occurs as size is decreased, because this corresponds very roughly to the point at which catalytic activity (at least for oxidation of carbon monoxide) starts to rise dramatically. Relevant experimental results and theoretical speculations are reviewed in Section 3.4. 

The characteristics of reforming catalysts make them one of the most frequent cases where EXAFS is used. The very small metallic particles cannot be detected in transmission electron microscopy the long range order required for XRD analysis is absent, and the low metal contents make XPS analysis difficult. The observation of XANES structures at Pi and Re thresholds can be used to determine the electronic state. Some examples of reference compounds are shown in Figure 11.10. In particular a sharp peak (called a white line for historical reasons) is visible at the edge with an intensity related to the oxidisation state of the element. This peak in the absorption coefficient is a consequence of the existence of the empty electron states close to zero binding energy. 

It is not surprising therefore that the optical properties of small metal particles have received a considerable interest worldwide. Their large range of applications goes from surface sensitive spectroscopic analysis to catalysis and even photonics with microwave polarizers [9-15]. These developments have sparked a renewed interest in the optical characterization of metallic particle suspensions, often routinely carried out by transmission electron microscopy (TEM) and UV-visible photo-absorption spectroscopy. The recent observation of large SP enhancements of the non linear optical response from these particles, initially for third order processes and more recently for second order processes has also initiated a particular attention for non linear optical phenomena [16-18]. Furthermore, the paradox that second order processes should vanish at first order for perfectly spherical particles whereas experimentally large intensities were collected for supposedly near-spherical particle suspensions had to be resolved. It is the purpose of tire present review to describe the current picture on the problem. 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

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]. 

To avoid sintering, the nanoparticles are isolated from each other by a small loading. For a support of circa lOOm g" , and metal loading of about 1 wt%, with ruthenium particles of about 2 nm in diameter one obtains a statistical repartition of each particle every 30 nm. This means that such metallic particles of ruthenium particles are at a random distance of about 30 nm from each other. Electron microscopy indicates that this is frequently the case. 

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