Operando conditions

Spectroscopic techniques, carried out in in situ and operando conditions, obviously represent powerful tools for the description of the reactions and the catalysts in running conditions. In fact, the exigency of the scientist to look at the chemical process at a molecular level cannot only address the traditional kinetics modelling, where the reactor itself behaves as a black box. The use of spectroscopy allows monitoring the catalytic material under duty, directly revealing species and transformations, which can then support the hypothesis made for mathematical calculations applied to a kinetic model [1],... 

The main inconvenient of this methodology is that the results cannot be considered stricto sensu as obtained in operando conditions, because the system was perturbed from the steady state to reveal hidden species. It could be even hypothesized that such compounds are uniquely due to the particular test conditions and not to the real reaction pathway. A method to discard such kind of criticism is to maintain the chemical steady state of the reaction, while introducing a perturbation via a sudden exchange of one... 

Montanari el al., for example, studied a Co—H-MFI sample through FT-IR spectroscopy of in situ adsorption and coadsorption of probe molecules [o-toluonitrile (oTN), CO and NO] and CH4-SCR process tests under IR operando conditions. The oTN adsorption and the oTN and NO coadsorption showed that both Co2+ and Co3+ species are present on the catalyst surface. Co3+ species are located inside the zeolitic channels while Co2+ ions are distributed both at the external and at the internal surfaces. The operando study showed the activity of Co3+ sites in the reaction. The existence of three parallel reactions, CH4-SCR, CH4 total oxidation and NO to NOz oxidation, was also confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH4-SCR and NO oxidation, respectively. A mechanism for CH4-SCR has been proposed. On the contrary, Co2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seemed not to play a key role in the SCR process [173],... 

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...

Figure 9.9 Correlation of the catalytic activity observed under operando conditions with geometric features using a truncated octahedron-shaped gold nanoparticle model (a) in (b) black dots/curve represent...

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

The other technique we use under operando conditions is infrared spectroscopy, which has also been used in catalysis research for more than 50 years. The new surname as an operando technique makes it sound more glamorous, but it is really an additional qualifier for in situ IR, which has been around for decades. In fact, the editor of this book, John Regalbuto, did his Ph.D. work using in situ and operando... 

Transmission infrared spectra of pressed disks ( 14 mg) of Pt/SiOj were collected in situ and under operando conditions in an IR reactor-cell placed in an FTIR spectrometer (Mattson, Galaxy 6020) at a resolution of 2 cm" and 30 scans/spec-trum. The IR cell is equipped with NaCl windows, and it has connections for inlet and outlet flows, and thermocouples connected to a temperature controller to monitor and control its temperature. The spectra were obtained in absorbance mode after subtraction of the background spectrum of the catalyst s disk under He atmosphere at the corresponding temperature. The samples were pretreated at various conditions prior to study CO adsorption and reaction. During CO oxidation experiments, 10% O2 was added to the 1% CO-He feed. In most experiments, the heating rate was l°C/min with a total flow of 120 cc/min unless indicated otherwise. 

The EXAFS results show that calcined catalysts are fully oxidized when calcined below 400°C, but higher calcination temperatures lead to formation of metallic Pt. The amount of metallic Pt increases with increasing calcination temperature. Hydrogen reduction leads to fully metallic particles. As expected, the dispersion decreases as the calcination temperature increases. The calcined and oxidized catalysts do not adsorb CO at room temperature, indicating no exposed metallic Pt. Nonetheless, the results shown previously indicate that both calcined and oxidized samples can oxidize CO at temperatures above 100°C. To analyze this apparent contradiction in more detail, several samples were studied using in situ FTIR under reaction (operando) conditions. 

The EXAPS results were obtained in situ but not under operando conditions, and thus it can only give information about the initial state of the surface. The most important result obtained from these experiments is that no Pt-S signal fit the results in the SOj poisoned catalysts. This suggest that, in this case, S is bonded to O species adsorbed on Pt, which agrees well with the conclusion that S is inhibiting oxygen dissociative adsorption instead of CO adsorption. 

Notwithstanding this limitation a considerable number of research areas have greatly benefited from a combination of XAFS and X-ray scattering. In material science one has seen beautiful results in for instance the field of catalysis, where one can now carry out experiments in operando conditions, and the study of the structural development of nano-technological materials. Also the usefulness has been proven in the case where the sample material is inhomogeneous and scarce like for instance in archaeological artefacts. 

SOFC anodes Ni-gadolininm doped ceria (Ni/GDC) are studied for sulfur poisoning under operando conditions by Nurk et al. [62], The molecular structure of sulfur species formed on the anodes in the temperature range 250-550 °C is studied by XANES (K shell). With Fl2 fuel containing 5 ppm H2S, several sulfur species in different oxidation states (6-1-, 4- -, 0, —2) are detected the species could either relate to -S04 or SO3 (g), -S03 or SO2 (g), S2 (g) or surface-adsorbed S atoms, and Ni or Ce snlfides. These results do not agree with thermodynamic phase calculations, in particnlar the formation of sulfate species is not expected at the highest temperatures this can be related to the difference between eqnilibrium conditions and the steady-state conditions in the fuel ceU that are determined by kinetic-controlled processes. 

Indeed, it seems obvious that under reaction conditions, when the reactants adsorb onto the surface, and transform into intermediates and products, that they finally desorb rapidly hence the concentration at the surface of most or all of these species is expected to be very low. In particular, this is expected to be true for species involved in successive steps down to the slowest step. Indeed, the detectability of the active adsorbed species (the surface intermediates) is rare, if even possible, in operando conditions (i.e. at relatively high temperatures) with usual... 

IR measurements. These labile adsorbed species may be detected using unusual measurements such as femtosecond laser excitation followed by nanosecond time resolved FTIR measurements. On the contrary, heavy adsorbed species that are not involved in the main reaction concentrate at the catalyst surface and can be easily detected. It is possible to forecast that, if a species is determined to be highly concentrated and stable in reaction conditions, it is either the true catalyst (e.g. sometimes carbonaceous materials deposited at the surface during the induction period), or a poison (e.g. coke that is a main cause of deactivation of acidic catalysts), or finally, a spectator species. In any case, the information obtained in operando conditions is always interesting and sometimes useful for revealing the reaction mechanisms. 

Hereinafter we will review data concenung oxidation reactions performed in ovn lab using this approach and, when possible, we will compare these data with those arising from other studies, including studies performed in operando conditions. 

In PROX reaction conditions only a resistant band, although complex, in the region of Cu+ monocarbonyls, is observed. However, the intensity of this band correlates with the selective PROX activity. This suggested to the authors that CO and H2 oxidation would occur on the same copper sites and that strong adsorption of CO prevents adsorption and oxidation of H2 on the catalyst. According to our interpretation, however, the possibility of CO oxidation occurring on Cu +,which cannot be revealed in operando conditions, should also be considered. 

Bulk X-ray diffiraction, thus averaging the X-ray difiiraction signal over a large amount of sample ( mm ), ex situ or under operando conditions can be considered a standard technique using laboratory based or synchrotron based sources. Countless findings rely on the precise description of the atomic order obtained from Laue diffraction patterns to describe structure, phases, reaction pathways, strain, disorder etc. 

Robert et al. have illustrated the beneficial effect of combination of spatially resolved XRF with XRD for active battery electrodes under in operando conditions, by detecting and correlating the distribution of Cu and Mn in layered oxysulfide Sr2Mn02Cu3 5S3 particles [65]. 

Singer A, Ulvestad A, Cho H et al (2014) Noequilibrium stmctural dynamics of nanoparticles in LiNil/2Mn3/204 cathode under operando conditions. Nano Lett, ASAP... 

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