On the complexity of the water-gas shift reaction mechanism over a Pt/CeO2 catalyst: Effect of the temperature on the reactivity of formate surface species studied by operando DRIFT during isotopic transient at chemical steady-state

The present report investigates the role of formate species as potential reaction intermediates for the WGS reaction (CO + H2O -> CO2 + H-2) over a Pt-CeO2 catalyst. A combination of operando techniques, i.e., in situ diffuse reflectance FT-IR (DRIFT) spectroscopy and mass spectrometry (MS) during steady-state isotopic transient kinetic analysis (SSITKA), was used to relate the exchange of the reaction product CO2 to that of surface formate species. The data presented here suggest that a switchover from a non-formate to a formate-based mechanism could take place over a very narrow temperature range (as low as 60 K) over our Pt-CeO2 catalyst. This observation clearly stresses the need to avoid extrapolating conclusions to the case of results obtained under even slightly different experimental conditions. The occurrence of a low-temperature mechanism, possibly redox or Mars van Krevelen-like, that deactivates above 473 K because of ceria over-reduction is suggested as a possible explanation for the switchover, similarly to the case of the CO-NO reaction over Cu, I'd and Rh-CeZrOx (see Kaspar and co-workers [1-3]). (c) 2006 Elsevier B.V. All rights reserved.

A comparative study on the reactivity of electrogenerated bromine with cyclohexene in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide

The reactivity of electrogenerated bromine with cyclohexene has been studied on a platinum microelectrode by linear sweep and cyclic voltammetry in both the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, and the conventional aprotic solvent, acetonitrile. Variation in the voltammetric response was observed in the two solvents, indicating that the bromination reaction proceeded via separate mechanisms. To identify the different products, electrolysis was conducted on the preparative scale and NMR spectroscopy confirmed that while bromination of the organic substrate in the ionic liquid yields trans-1,2-dibromocyclohexane, in acetonitrile, trans-1-(N-acetylamino)-2-bromocyclohexane is instead obtained as the major product. The reaction mechanism for bromination in acetonitrile has been modeled using digital simulation.

Reactivity of the 4d transition metals toward N hydrogenation and NH dissociation: A DFT-based HSAB analysis

The density functional theory (DFT) based hard-soft acid-base (HSAB) reactivity indices, including the electrophilicity index, have been successfully applied to many areas of molecular chemistry. In this work we test the applicability of such an approach to fundamental surface chemistry. We have considered, as prototypical surface reactions, both the hydrogenation of atomic nitrogen and the dissociative adsorption of the NH molecular radical. By use of a DFT methodology, the minimum energy reaction pathways, and corresponding reaction barriers, of the above reactions over Zr(001), Nb(110), Mo(110), Tc(001), Ru(001), Rh(111), and Pd(111) have been determined. By consideration of the chemical potential and chemical hardness of the surface metal atoms, and the principle of electronegativity equalization, it is found that the charge transferred to the NH radical during the process of dissociative adsorption correlates very well with that determined by Mulliken population analysis. Furthermore, it is found that the stability of the NH/surface transition state complex relates directly to this charge transfer and that the trend in transition state stability predicted by a HSAB; treatment correlates very strongly with that determined by DFT calculations. With regards to N hydrogenation, we find that during the course of the reaction, H loses cohesion to the surface, as it must migrate from a 3-fold hollow site to either a bridge or top site, to react with N. Partial density of states (PDOS) and Mulliken population analysis reveal that this loss of bonding is accompanied by charge transfer from H to the surface metal atoms. Moreover, by simple modeling, we show that the reaction barriers are directly proportional to this mandatory charge transfer. Indeed, it is found that the reaction barriers correlate very well with the electrophilicity index of the metal atoms.

A density functional theory study of the surface relaxation and reactivity of Cu2O(100)

Density functional theory has been used to investigate the surface relaxation of Cu2O(100) and the adsorption of NO. The calculations indicate the formation of surface copper dimers on relaxation coupled with a large contraction of the spacing between the first and second layers. Local density of states for atoms in the top three layers shows that the third layer copper atoms have the greatest change in bonding character. Adsorption energies have been calculated for the N-down and O-down adsorption of NO on the Cu2O(100) surface. These indicate that N-down adsorption is favoured and that in this case NO-lattice oxygen interactions dominate the adsorbate structure. (C) 2000 Elsevier Science B.V. All rights reserved.

The effects of the specific adsorption of anion on the reactivity of the Ru(0001) surface towards CO adsorption and oxidation: in situ FTIRS studies

The dynamics of adsorption and oxidation of CO on Ru(0001) electrode in sulfuric acid solution have been studied using in situ FTIR spectroscopy under potential control and at open circuit, the latter at 20 and 55 degrees C. The in situ IR data show clearly that the bisulfate anion adsorbs on the Ru(0001) surface over the potential range from -200 mV to 350 mV (vs. Ag/AgCl) at 20 degrees C in the absence and presence of adsorbed CO; however, increasing the temperature to 55 degrees C and/ or increasing the concentration of dissolved O-2 reduces the bisulfate adsorption. The formation of surface (hydro-) oxide at higher potentials replaces the bisulfate adsorbates. Both linear (COL) and three-fold hollow bonded CO (COH) adsorbates were produced following CO adsorption at Ru(0001) in H2SO4, as was observed in our previous studies in HClO4. However, the amount of adsorbed CO observed in H2SO4 was ca. 10% less than that in HClO4; in addition, the COL and COH frequencies were higher in H2SO4, and the onset potential for COads oxidation 25 mV lower. These new results are interpreted in terms of a model in which the adsorbed bisulfate weakens the CO adlayer, allowing the active Ru oxide layer to form at lower potentials. Significantly different results were observed at open circuit in H2SO4 compared both to the data under potential control and to our earlier data in HClO4, and these observations were rationalized in terms of the adsorbed HSO4- anions (pre-adsorbed at -200 mV) inhibiting the oxidation of the surface at open circuit (after stepping from the initial potential of -200 mV), as the latter was no longer driven by the imposed electrochemical potential but via chemical oxidation by trace dissolved O-2. Results from experiments at open circuit at 55 degrees C and using oxygen-saturated H2SO4 supported this model. The difference in Ru surface chemistry between imposed electrochemical control and chemical control has potential implications with respect to fuel cell electrocatalysis.

Reactivity of ammonium chloride/mercuric chloride mixtures with monel containers. The new compounds (NH4)(2)(NH3)(x)[Ni(NH3)(2)Cl-4] and (NH4)(5)Cl-2[CuCl2][CuCl4]

Ammonium chloride/mercuric chloride mixtures (molar ratio 2: 1) react at 350degreesC with Monel (Cu68Ni32) to yield (NH4)NiCl3 and mercury and copper amalgam, respectively. With larger amounts of (NH4)Cl in the reaction mixture, dark green (NH4)(2)(NH3)(x)[Ni(NH3)(2)Cl-4] (x approximate to 0.77) (1) is also formed as a main product. Light blue crystals of the mixed-valent copper(I,II) chloride (NH4)(5)Cl-5[CuCl2][CuCl4] (2) were obtained as a minor byproduct from a 4:1 reaction mixture. The crystal structures were determined from single crystal X-ray data; (1): tetragonal, I4/mmm, a = 770.9(1), e = 794.2(2) pm, 190 reflections, R-1 = 0.0263; (2): tetragonal, I4/mcm, a = 874.8(1), c = 2329.2(3) pm, 451 reflections, R-1 = 0.0736. In (1) Ni2+ resides in trans-[Ni(NH3)(2)Cl-4](2-) octahedra, and in (2) copper(l) is linearly two-coordinated in ECUC121- and copper(II) resides in a flattened tetrahedron [CuCl4](2-) with a tetrahedricity of 89%. (C) 2001 Elsevier Science.