Particle size, measurement sample preparation

Kinetic parameters from selected publication on CO2 reactivity are shown in Table 1. A wide span of apparent activation energies is published for biomass chars, 80.3 kJ/mol -318 kJ/mol and for coal chars, 79 kJ/mol -359.5 kJ/mol Some of the discrepancy can be explained as due to different experimental procedures (such as sample load, particle size and sample preparation) and the application of different analysing equipment. Concerning the latter one, the potential role of systematic errors in temperature measurements among various thermobalances is evident. Variations might also be explained by different extraction procedures, lack of accuracy caused by the approximations used in the different computational methods and the kinetic compensation effect. 

Einally, europium oxide nanorods have been prepared by the sonication of an aqueous solution of europium nitrate in the presence of ammonia [85]. The particle sizes measured from transmission electron micrographs and HRSEM are about 50 X 500 nm (W x L). Sonication of an aqueous solution of europium nitrate in the presence of ammonia results in the precipitation of europium hydroxide The as-prepared material is europium hydroxide, as confirmed by TGA, DSC, XPS, and Mossbauer spectroscopy measurements, as well as by PXRD of the as-prepared sample assisted by microwave irradiation... 

These have the potential of eliminating a number of problems dealing not only with sample handling and preparation, but also with ultimate versus agglomerate size issues, and open the door to a whole new way of addressing particle size measurements. 

After well-defined times the circulating pump was stopped and various volumes of the gel were taken from the reactor. The gel was washed with water and ammonium nitrate solution to minimise the potassium contents and dried over night at 333 K. The resulting powder was divided into portions using a laboratory-sampler (0.2 g for the particle size measurements and 0.5 g for the surface area measurements) Some of the prepared precursors were calcined for 14 h increasing the temperature by 0.5 K per minute up to a temperature of 543 K. A fraction between 45 pm and 180 pm of these samples was reduced with H2/Ar (5/95 Vol.%) at a maximum temperature of 523 K for 21 h. The reaction rate for the methanol synthesis with a synthesis gas of CO/CO2/H2 = 49/2/49 Vol % was measured in a tubular-reactor at 5 MPa and 553 K at a GHSV of 12000 h , following the reactions (2a and 2b) ... 

As with any physical measurement, sample preparation is of utmost importance. This is particularly true for particle size determinations. If the measurement is performed in suspension, dispersing aids may have to be added and ultrasound may have to be apphed to ensure homogeneous dispersion. This may disrupt aggregates, which can be desired or undesired. In any case, it is highly advisable to observe the effects of sample preparation by additional methods such as microscopy. Guidelines for method development, instrument calibration, and acceptance criteria can be found in regulatory documents. ... 

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

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