In situ XAS

The study described above provides fundamental accounts of enzyme behavior, albeit following chemical treatment, and at 77 K. Thus, although the findings are significant, it is unclear if the mechanism can be extrapolated to room temperature and physiologically relevant electrochemical conditions, such as those in a BFC. Thus, the primary benefit of the XAS studies described herein is the ability to investigate laccase in situ, under realistic electrochemical and BFC conditions, and correlated to various electrochemical and spectroscopic methods. 

The in situ XAS analyses deseribed herein were performed using X-ray absorption near-edge structure (XANES) analysis and extended X-ray absorption fine strueture (EXAFS) analysis and analyzed in conjunction with FEFF8.0 modeling to investigate changes to the active site as a function of potential and presenee of O2. Fundamental electrochemical methods sueh as cyclic voltammetry (CV), chronoamperometry, and ORR polarization measurements were then compared to illustrate the electrocatalytic activity of laccase. Finally, an in-depth analysis of the A //FFFF8 results lead to a new proposed mechanism for the laccase-catalyzed ORR. 

FIGURE 15.2 Drawings of the three osmium-hased redox polymers employed in this study. 

TABLE 15.1 Summary of Redox Polymers Described in This Chapter 

The redox event observed from CVs shows peak separation, A p 59 mV, that indicates an Os redox couple that is reversible with = 1 electrons transferred, as 

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J J. R. Johnson, S. Mukerjee, G. D. Adzic, J. J. Reilly, J. McBreen, In situ XAS studies on AB2 type metal hydride alloys for battery ap-... 

Mukeijee S, McBreen J. 1989. Effect of particle size on the electrocatalysis by carbon-supported Pt electrocatalysts an in situ XAS investigation. J Electroanal Chem 448 163-171. 

A number of designs of transmission in situ XAS cells have been published for the study of bound catalyst electrodes.These cells all utilize a thin-layer geometry to minimize the contribution to the absorbance by electrolyte solution. The cell design reported by McBreen and co-workers shown in Figure 9 uses three layers of filter paper soaked in the electrolyte as a separator, or later a Nafion membrane between the working electrode and a Grafoil counter electrode. Bubbles in the electrolyte, that would result in noise in the XAS data, are... 

Collection of in situ XAS data using a single cell fuel cell avoids problems associated with bubble formation found in liquid electrolytes as well as questions regarding the influence of adsorption of ions from the supporting electrolyte. However, the in situ study of membrane electrode assemblies (MEAs) in a fuel cell environment using transmission... 

Ru provides sites for water activation as well as having an electronic effect on the Pt atoms, such that CO is less strongly adsorbed. In situ XAS measurements have been used to determine the structure of PtRu catalysts, to assess the magnitude of any electronic effect that alloy formation may have on the Pt component of the catalyst, and to provide evidence in support of the bifunctional mechanism. 

Figure 30. Correlation of the oxygen reduction performance (log igoo mv) of Pt and Pt alloy electrocatalysts in a PEM fuel cell with Pt—Pt bond distance (filled circles) and the d band vacancy per atom (open circles) obtained from in situ XAS at the Pt L3 and L2 edges.(Reproduced with permission from ref 34. Copyright 1995 The Electrochemical Society, Inc.)...

Surface analytical methods — X-ray absorption spectroscopy, XAS — Figure. Electrochemical cell for in situ XAS measurements in reflection, set up with a gracing incident X-ray beam, beam shaping slit, ionization chambers for the intensity measurement of incoming (ii) and reflected beam (I2) and beam stop for the direct nonreflected beam [vii]... 

The use of synchrotron-based in-situ x-ray absorption spectroscopy (XAS) for the study of catalysis, both heterogeneous and electrocatalysis, has matured over the last decade with simultaneous efforts in the United Sates, European Union, and Japan. An excellent review of these efforts has recently been published [27]. Application of in-situ XAS for investigating battery and fuel-cell materials has recently been reviewed [28,29], and an extensive database of literature on its application to catalysis can be accessed electronically [30]. Detailed aspects on the application of the technique and methodology used for data analysis have recently been published [31]. 

In the context of supported electrocatalysts typically used in the current state-of-the-art PEMFCs, the in-situ XAS spectroscopy has three important functions. 

Recently in-situ XAS spectra have shown that alloying of Pt with base transition elements such as Co and Cr enable changes in the electronic properties of Pt that in turn shift the onset of Pt-OH formation to higher potentials. This has been shown to enable a lowering of ORR overpotential losses by approximately 50mVs. In this endeavor, however, the surface nature of the cluster is very important and has to be predominantly Pt with the inner core as the alloy. Hence, the methodologies of preparation of these metal clusters are of prime importance. 

There are lots of questions still to be answered in the area of chemical speciation, and these can only be addressed by using the most appropriate analytical speciation methods. In future studies, this will involve the combined use of complementary in situ (XAS) and ex situ (GC-ICP-MS, HPLC-ICP-MS, and ESTMS/MS) methods. 

Ferrocene was incorporated into the templating micelles of synthetic siliceous MCM-41, and found to affect the morphology and structure of MCM-41. Ferrocene was oxidized to tetrahedrally coordinated oxide isolatedly grafted on the pore wall after calcination at 600°C in oxygen, and transformed to oxide nanoparticles in MCM-41 by further heating at 800°C under vacuum, characterized by TEM, in-situ XAS and EPR measurements. 

Figure 5. Temperature programmed reduction of 12CoRe/y-Al203 using the in situ XAS cell. Temperature ramping is indicated in the figure (A = C03O4, = CoO, T = cobalt metal,... = temperature).

Comparative in situ XAS of Ru, Pt, and Pd/Al203 catalysts under SCCO2 indicated that rathenium is oxidized during reaction [147], although the resulting RuO, may be less active than the initial metal. The accumulation of carbonaceous residues and slow removal of CO (a by-product of undesired decarbonylation) may play a role in differentiating the activity of Pd versus Pt systems. In situ and ex situ XAS have also... 

The active site responsible for the aerobic oxidation of alcohols over Pd/AljO, catalysts has long been debated [96-lOOj. Many reports claim that the active site for this catalyst material is the metallic palladium based on electrochemical studies of these catalysts [100, 101]. On the contrary, there are reports that claim that palladium oxide is the active site for the oxidation reaction and the metalhc palladium has a lesser catalytic activity [96,97). In this section, we present examples on how in situ XAS combined with other analytical techniques such as ATR-IR, DRIFTS, and mass spectroscopic methods have been used to study the nature of the actual active site for the supported palladium catalysts for the selective aerobic oxidation of benzylic alcohols. Initially, we present examples that claim that palladium in its metallic state is the active site for this selective aerobic oxidation, followed by some recent examples where researchers have reported that ojddic palladium is the active site for this reaction. Examples where in situ spectroscopic methods have been utilized to arrive at the conclusion are presented here. For this purpose, a spectroscopic reaction cell, acting as a continuous flow reactor, has been equipped with X-ray transparent windows and then charged with the catalyst material. A liquid pump is used to feed the reactants and solvent mixture into the reaction cell, which can be heated by an oven. The reaction was monitored by a transmission flow-through IR cell. A detailed description of the experimental setup and procedure can be found elsewhere [100]. Figure 12.10 shows the obtained XAS results as well as the online product analysis by FTIR for a Pd/AljOj catalyst during the aerobic oxidation of benzyl alcohol. 

Until now, we have discussed the examples where metallic palladium has been reported to be the active site for the selective aerobic oxidation reaction using supported palladium catalyst. However, it is interesting to note that recently Lee and coworkers [97, 103-105] have utilized in situ XAS technique, in combination with other spectroscopic techniques, to study the active site of supported palladium catalysts for the selective aerobic oxidation reactions and found that oxidic palladium... 

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-... 

From investigations of the active phase of a bulk metallic copper catalyst under reaction conditions of the partial methanol oxidation by means of in situ XAS at the oxygen K-edge and copper L2-,L3-edges it was concluded that the partial oxidation of methanol to formaldehyde is catalysed by a copper plus oxygen phase where oxygen atoms probe defects of tiie copper lattice, which represent the catalytically active sites [3, 4, 6j. 

In situ XAS-measurements of Cu at the oxygen K-edge under conditions of the partial methanol oxidation revealed that a copper oxide different from CU2O is responsible for the partial oxidation of methanol [3, 4, 6]. A surface species of CU2O is reacting to COx and is consumed by the methanol reaction. Variation of the abundance of this copper oxide may therefore cause the oscillations of the CO2 production. 

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