Operando analysis

Hoekman et al. [40] studied CO2 methanation reaction over Haldor Topspe commercially available methanation catalysts consisting of Ni and NiO on an alumina substrate with total nickel loading of 20-25% and an operating temperature range of 190-450 C in an extruded ring-shaped catalyst. Approximately 60% conversion of CO2 was observed at r= 300-350°C and stoichiometric CO2/H2 ratio. Aldana et al. [41] found that Ni over ceria-zirconia (prepared by sol-gel synthesis) shows an initial COj activity of almost 80%, with a CH4 selectivity of 97.3%, decreasing down to 84.7% after 90 hours of reaction. By IR operando analysis, they found that for Ni-ceria-zirconia catalysts the main mechanism for CO2 methanation does not require CO as reaction intermediate and the mechanism is based on CO2 adsorption on weak basic sites of the support. 

The weak interaction of X-rays with matter causes quasi none-destractiveness in comparison to other probes such as ions and electrons and thus allows for in situ or operando analysis. Nonetheless, radiation induced damage can become an issue especially for experiments which have a long exposure, or high radiation dose per area (focused beam) and can be non-neglectable. 

However, in electron detection tools, only ex situ characterization technique was possible, i.e.. Auger electron spectroscopy (AES) or EELS in electrochanisiry [74]. Now, with the emergence of AP-XPS, another opportunity is opened up in the fields of electrochemistry, providing in sim quantitative information on chemical and electronic states of surface/interface under operating condition, i.e., operando analysis [75, 76]. 

Groothaert et al., using operando UV-vis spectroscopy combined with online GC analysis [176] and operando X-ray absorption fine structure (XAFS) [177], presented the first experimental evidence for the formation of the bis( x-oxo)dicopper core in Cu-ZSM-5 and for its key role of intermediate in the sustained high activity of Cu-ZSM-5 in the direct decomposition of NO into N2 and 02. In particular, monitoring the catalytic conversion of NO and N20 above 673 K, they found that the bis( x-oxo)dicopper core is formed by the O abstraction of the intermediate N20 (Figure 4.14). Subsequently,... 

Most of the techniques discussed above are typically used ex situ for catalyst characterization before and after reaction. This is normally the easiest way to carry out the experiments, and is often sufficient to acquire the required information. However, it is known that the reaction environment plays an important role in determining the structure and properties of working catalysts. Consequently, it is desirable to also try to perform catalytic studies under realistic conditions, either in situ [113,114,157, 191-193] or in the so-called operando mode, with simultaneous kinetics measurements [194-196], In addition, advances in high-throughput (also known as combinatorial) catalysis call for the fast and simultaneous analysis of a large number of catalytic samples [197,198], This represents a new direction for further research. 

It is imperative to monitor the details of chemical reactions at the molecular level, which operando data can contribute to immensely through the use of spectroscopy. Spectroscopic techniques have been used in the past mostly to characterize fresh or used catalysts, obtaining structural information relating to the bulk and surface of the solids. In addition, on-line gas analysis of... 

Significant attention has once again been devoted to the in-situ or operando EPR characterisation of the supported VOx catalysts under working conditions.99 100 The term operando spectroscopy has been recently introduced115116 to distinguish it from in-situ spectroscopy which is commonly used in a broader sense not necessarily implying the simultaneous product analysis and spectroscopic characterisation of the catalysts under working conditions. Bruckner et... 

Kubacka A, Martinez-Arias A, Fernandez-Garcia M, Newton MA. Dynamic operando observation of 10 wt% Pd-based TWCs simultaneous XAS/DRIFTS/mass spectrometry analysis of the effects of Ceo.5Zro.5O2 loading on structure, reactivity and performance. Catal Today. 2009 145 288. 

Because the terms "in situ" and "operando" are used inconsistently to represent investigations of catalysts in reactive atmospheres (Banares, 2005 Weckhuysen, 2003), the latter is avoided here, and use of the former is minimized. Relevant in the present context of structural analysis are investigations of catalysts under dynamic conditions representing those of catalytic application. 

The operando methodology couples electrical and spectroscopic techniques and aims at the correlation of the sensor activity with the spectroscopic data obtained under the same conditions on the same sample (Fig. 5). In an ideal case, one would obtain four types of information (i) gas-phase changes (and reaction products) from the on-line gas analysis, (ii) species adsorbed on the surface, (iii) changes in the oxide surface and lattice, and (iv) sensor activity. However, in practice, all these items information are seldom obtained. 

Figure 9.8 Size effect in CO oxidation by Au-Ti02 (1 1 0) observed under operando conditions. Black dots - laboratory-based results blue dots - catalytic testing results under operando conditions obtained simultaneously with GISAXS analysis, (a) Reaction...

The FT-IR analysis showed that SiH4-modified catalysts generated PE at a rate about seven times faster than that of the standard Phillips catalyst. Both catalysts were exposed to ethene (at low pressure) for eight minutes at room temperature, during which time operando FT-IR was collected. It was... 

Daniel, C., Clarte, M., Provendier, H., etal. (2009). Spatially resolved operando infrared analysis of a microstructured catalytic surface for CO oxidation over Pt based catalysts, C. R. Chimie, 12, pp. 647-653. 

In situ EPR and UV-vis studies of oxidation catalysts can be divided in two main categories 1) monitoring of catalysts at elevated temperature under flowing gases including true reaction conditions, in some cases in connection to product analysis devices such as gas chromatography (GC) or mass spectrometry (MS) (known as operando spectroscopy) 2) measurement of catalysts at ambient or lower temperature after pre-treatment in certain gas atmospheres and/or adsorption of probe molecules without intermediate contact to the ambient atmosphere. In addition to... 

In situ micro analysis on Li ion batteries has been reviewed recently focusing on electron microscopy and touching also other microscopic probes as well [8]. Electron microprobes offer spatial resolution of a few nano meters and less. However in situ and operando capabilities are limited due to the low depth of information and the restriction of the probe environment. 

---