Dynamic imaging of structural changes in silver catalysts by environmental scanning electron microscopy.

Polycrystalline silver catalysts are used extensively for the partial oxidation of methanol to formaldehyde, which is then primarily incorporated in the synthesis process for adhesives and resins. In order to maximize formaldehyde production it is essential to gain a comprehensive understanding of the complex microstructural changes which occur in the catalyst during reaction conditions. However, conventional electron microscopic techniques are incapable of imaging catalysts at high temperatures and in the presence of a gaseous atmosphere. Therefore, an environmental scanning electron microscope (ESEM) has been used to image polycrystalline silver catalysts during simulated industrial conditions. The most dramatic effect of heating various catalysts to 700 degrees C in the ESEM chamber was the formation of "pinholes" in the silver surface. These "pinholes" occur at specific temperatures and are inherently associated with the catalytic process, resulting from near-surface explosions caused by subsurface hydroxyl recombination. Of particular interest was the nature and location of the holes, which preferentially occur in the vicinity of surface defects such as platelets and edge structures. To the best of our knowledge, this study represents the first time that the progress of a catalytic reaction has been observed under in situ conditions by scanning electron microscopy.

Monitoring water pathways through mungbean pods using backscattered electron microscopy.

Weather damage by rainfall is a major constraint to production of high quality mungbean seed. Breeding mungbeans for weathering resistance is assisted by an understanding of the pod wall structure and how this structure can affect water absorption. This paper describes a simple microscopic technique for determining the transport route of salt solutions from the external pod surface through the pod wall tissues and into the pod lumen. Different mungbean cultivars were selected based on differences in field performance. Mungbean pods were immersed in concentrated salt solutions (lanthanum nitrate, caesium chloride and sodium chloride) for different time periods, embedded in resin blocks and polished prior to backscatter imaging in a field emission scanning electron microscope. The salts precipitated between the pod wall tissues and through timed experiments clearly demonstrated the passage of salt solution through the pod wall tissues and into the pod lumen. The sale molecules penetrated the outer epidermis and parenchyma but were unable to penetrate the dense sclerenchymatous layer beneath. The salt solution entered the lumen of the pod via the small gap between the suture cap and the dense tissue sheath. Although this technique may not provide a true determination of fresh water absorption through mungbean pods it does demonstrate a simple means of identifying more resistant pod structures suitable for use in achieving genetic improvement.

Effects of four different processing techniques on the microstructure of potatoes: comparison with fresh samples in the ESEM.

Four common scanning electron microscope (SEM) processing techniques involving freeze-substitution and chemical fixation were compared with fresh unprocessed samples imaged in an environmental scanning electron microscope (ESEM) using small pieces of potato tubers as test specimens. Potato tubers were chosen for this investigation because of their high moisture content and, consequently, the common need for extensive processing for conventional, high vacuum SEM imaging. ESEM results showed that the fresh unprocessed specimens were essentially unaltered, showing clear potato cell structure, morphology, and cell content. However, processed samples showed strong differences to the fresh samples: freeze-substituted specimens showed fine networks of material stretching across the surface of cells. These structures may represent fibrillar material or may be artifact caused during processing. Chemical fixation almost entirely destroyed the microstructure of these potato samples.

Imaging fluid/solid interactions in hydrocarbon reservoir rocks.

The environmental scanning electron microscope (ESEM) has been used to image liquid hydrocarbons in sandstones and oil shales. Additionally, the fluid sensitivity of selected clay minerals in hydrocarbon reservoirs was assessed via three case studies: HCl acid sensitivity of authigenic chlorite in sandstone reservoirs, freshwater sensitivity of authigenic illite/smectite in sandstone reservoirs, and bleach sensitivity of a volcanic reservoir containing abundant secondary chlorite/corrensite. The results showed the suitability of using ESEM for imaging liquid hydrocarbon films in hydrocarbon reservoirs and the importance of simulating in situ fluid-rock interactions for hydrocarbon production programmes. In each case, results of the ESEM studies greatly enhanced prediction of reservoir/borehole reactions and, in some cases, contradicted conventional wisdom regarding the outcome of potential engineering solutions.