Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale

In situ Investigations of Structure-Activity Relationships of a Cu/ZrO2Cu/ZrO_2 Catalyst for the Steam Reforming of Methanol

Structure–activity relationships of a nanostructured Cu/ZrO2 catalyst for the steam reforming of methanol (MSR) were investigated under reaction conditions by in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) combined with on-line mass spectrometry (MS). Temperature-programmed activation by reduction in hydrogen or by reduction in a mixture of methanol and water (feed) was studied by time-resolved Cu K edge XANES and TG/DSC/MS measurements. Small and disordered CuO particles were identified as the main copper phase present in the precursors. After extended time on stream and treatment at 673 K in hydrogen, no significant sintering of the copper particles or deactivation of the reduced Cu/ZrO2 catalysts was detected, indicating a superior stability of the material. The initially low steam-reforming activity of the Cu/ZrO2 catalyst after reduction in hydrogen could be significantly increased by a temporary addition of oxygen to the feed. This increased activity after oxidative treatment is correlated with an increasing amount of oxygen in the copper particles. 63Cu NMR studies detected only a minor degree of microstrain in the active copper phase of the Cu/ZrO2 catalyst. The decreased reducibility of CuO/ZrO2, the low degree of microstrain, and the correlation between the amount of oxygen remaining in the copper particles and the catalytic activity indicate a different metal support interaction compared with Cu/ZnO catalysts

Phosphorus-31 MAS NMR investigations of pyridine stabilized dithiomonometaphosphoryl halides

Phosphorus-31 magic-angle spinning NMR spectroscopy was chosen in order to study a series of N-donor stabilized dithiomonometaphosphoryl halides (F, Cl, and Br) and to extract structural and bonding information from their spectra. Additionally, an unusual line splitting was observed in the 31P MAS NMR spectra of pyridine dithiomonometaphosphoryl chloride (Py→PS2Cl) and the analogous bromide (Py→PS2Br). It is shown that this effect is due to a residual dipolar coupling between phosphorus and chlorine or bromine, respectively. This work is to our knowledge the first observation of this effect between P and Br in 31P MAS NMR spectra

Triorganophosphine-dithiomonometaphosphoryl halides

The title compounds, ethyldiphenylphosphine- dithiomonometaphosphoryl chloride, EtPh2P --> PS2Cl, C14H15ClP2S2, (I), and tris-n-propylphosphine- dithiomonometaphosphoryl chloride and bromide, (Pr3P)-Pr-n --> PS2Cl, C9H21ClP2S2, (II), and nPr(3)P --> PS2Br, C9H21BrP2S2, (III), respectively, are the first phosphine-stabilized dithiomonometaphosphoryl halides to be structurally characterized. In the tris-n-propylphosphine derivatives, the central P --> P donor-acceptor bond becomes longer in the order bromo P bond. These structural features agree with the observed 31 P NMR data. In (II) and (III), the central P-P bond coincides with the crystallographic threefold axis, entailing site- occupational disorder for the S2Y group

Triorganophosphine-dithiomonometaphosphoryl halides

The title compounds, ethyldiphenylphosphine- dithiomonometaphosphoryl chloride, EtPh2P --> PS2Cl, C14H15ClP2S2, (I), and tris-n-propylphosphine- dithiomonometaphosphoryl chloride and bromide, (Pr3P)-Pr-n --> PS2Cl, C9H21ClP2S2, (II), and nPr(3)P --> PS2Br, C9H21BrP2S2, (III), respectively, are the first phosphine-stabilized dithiomonometaphosphoryl halides to be structurally characterized. In the tris-n-propylphosphine derivatives, the central P --> P donor-acceptor bond becomes longer in the order bromo < chloro < fluoro. Substitution of the tris-n- propylphosphine group in (II) by the more bulky ethyldiphenylphosphine group also leads to a longer P --> P bond. These structural features agree with the observed 31 P NMR data. In (II) and (III), the central P-P bond coincides with the crystallographic threefold axis, entailing site- occupational disorder for the S2Y group

U₂Ru₂Sn: a new Kondo insulator?

We present magnetic susceptibility, specific heat, electrical resistivity, magneto resistance, Hall coefficient, thermal conductivity, thermoelectric power, and Sn-119 NMR data for U2Ru2Sn. Clear evidence is found for the formation of a gap in the electronic density of states at the Fermi level. Together with the disappearance of the local magnetic moments at low temperatures and the observation of enhanced effective masses this suggests to classify U(2)Ru(2)sn as a Kondo insulator. (C) 2002 Elsevier Science B.V. All rights reserved