Ex Situ Characterizations

Department of Chemistry, Fudan University, Shanghai, P.R. China 

Heterogeneous Catalysis at Nanoscale for Energy Applications, First Edition. Edited by Franklin (Feng) Tao, William F. Schneider, and Prashant V. Kamat. 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

Because there are more ex situ characterization techniques than this chapter could cover, here we only describe briefly the principles of some selected ex situ techniques. These characterization tools could provide average or collective chemical information, local chemical information with spatial resolution at micrometer and submicrometer ranges, and local structural information at nanometer and atomic scales for heterogeneous catalysts. Examples are selected that demonstrate the performances of these techniques, with a special focus on their applications in characterizing nanocatalysts for energy production and energy conversion. 

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We have recently modified U7) one of the several radiochemical methods (U5) which have been used for surface electrochemistry investigations in order to characterize adsorption on well-defined, single crystal electrodes. Below, we will describe the technique and identify some challenging issues which we will be able to address. The proposed method is sensitive to a few percent of a monolayer at smooth surfaces, is nondestructive and simple to use. The radiochemical measurements can be made with all compounds which can be labelled with reasonably long-lived, preferably g- emitting radioisotopes. We believe this technique will fulfill the quantitative function in in situ surface analysis as Auger spectroscopy currently does in vacuum, ex situ characterization of electrodes. 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

HSAPO-34 during methanol-to-olefin catalysis ex situ characterization after cryogenic grinding. Catal. Lett., 76, 89-94. 

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are recognized as powerful and versatile tools for the characterization of supported metal catalysts, because real-space images of catalysts with spatial resolution down to 0.1 nm can be recorded and combined with high-spatially resolved spectroscopic information. However, TEM has been used mainly for ex situ characterization, for example, of catalysts after gas treatments. 

The results of transient experiments by Ballarini et al. (152) showed that the active surface of equilibrated catalysts is different, depending on the reaction conditions and the P/V ratio of the catalyst. At low temperature (320 °C), an active surface forms that is selective and probably is more like VOPO4 than (VO)2P207. However, as the temperature is increased to 380 °C, this material becomes less selective. The active phase formed at temperatures >380 °C was found to be less active than the low-temperature phase but has increased selectivity at this temperature. At these temperatures, the active site is found to imdergo hydrolysis and oxidation, and Ballarani et al. proposed that the active surface is a VO / pol)q5hosphoric acid mixture. The authors speculated that the evolution of different phases at different temperatures (which is also dependent on very minor changes in the P/V ratio) could be the cause of the existence of markedly different surfaces observed in both in situ and ex situ characterizations of the active catalyst. 

A significant research effort has been devoted to the characterization of Pd surface oxides [17, 63]. In situ studies under non-UHV conditions are most relevant, because surface oxides may only be present under reaction conditions (and not accessible by post-reaction (ex situ) characterization). A stepwise oxidation of Pd(lll) was reported [64-66], starting from the well-known (2x2) chemisorbed oxygen adlayer, followed by the formation of a two-dimensional Pd O surface oxide, which eventually transforms to a PdO bulk oxide. At high temperature, PdO decomposes with the concurrent dissolution of oxygen atoms into the bulk. In situ synchrotron HP-XPS [64, 65] indicated that a two-dimensional Pd O surface oxide was formed on Pd(lll) at 0.4 mbar O and >470 K, which transformed to PdO at temperatures >660 K. PdO was found to decompose at temperatures >720 K. 

Spent catalyst samples were recovered and submitted to ex-situ characterization analyses following the experiments. 

Solid-liquid inter face structure—investigation under rigorously controlled conditions of solid-liquid interfaces, with both in situ and ex situ characterization of components, is nearing feasibility. Improved understanding of the extended structure of both fluid and solid phases in the vicinity of the interface would be a major advance in the scientific level of this field. 

The initi findings of the Tg depression in PLLA were ex situ characterization by thermal analysis. In the remainder of this section the use of advanced characterization methods is illustrated to provide additional insight into the novel behavior. The methods used to address the polymer were those appropriate to provide in situ, real-time information on morphology via SALS, and on segmental dynamics via DRS. The rationale for the use of these tools is that the dipolar loss peak in the frequency domain (from DRS) is closely related to the glass transition of a given polymer system — hence, under isothermal conditions, a change in the loss peak frequency is... 

In both cases ex-situ characterization experiments were carried out to probe the resulting structure, using techniques like density measurements and Wide Angle X-ray Diffraction (WAXD) patterns. The cooling mechanism and the temperature distribution across the sample thickness were analysed. Results show that the final structure is determined only by the imposed thermal history and pressure. 

Santasalo-Aamio A, Hietala S, Rauhala T, Kallio T (2011) In and ex situ characterization of an anion-exchange membrane for alkaline direct methanol fuel cell (ADMFC). J Power... 

Even more valuable than the ex situ characterization of carbon materials with the aforementioned methods, is their in situ characterization during operation or at least under conditions very similar to their application. However, not all measurements can be applied in situ, as for instance in XPS, the mean free wavelength of the electrons in not-vacuum conditions is too low and in TEM ultrahigh vacuum (UHV) is required to detect the electrons. Consequently, electron probe techniques relying on UHV are not (or only with immense effort) applicable in situ [26]. Novak et al [27] give an overview of advanced in situ characterization applied to carbonaceous materials for Li-ion batteries. 

Small amounts of Ru or Ir were sputter-deposited on Pt-NSTF substrate to determine their stability and OER activity in a fuel cell environment. Ex situ characterization of as-grown material was first performed in order to characterize the morphology and surface state of each OER catalyst. Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) were employed to complete this task. Two different OER catalyst loadings were studied, 2 and 10 pg/cm, in order to explore the impact of layer thickness on the catalyst morphology and composition. 

SOME EXAMPLES ON EX SITU CHARACTERIZATION OF NANOCATALYSTS FOR ENERGY APPLICATIONS... 

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