Activité chimique

Résumé

En chimie physique, et plus particulièrement en thermodynamique, l'activité chimique, ou activité, d'une espèce chimique exprime l'écart entre les propriétés de cette espèce pure ou dans un mélange réel et les propriétés de cette même espèce dans un état standard à la même température. La notion d'activité chimique est surtout employée pour les phases liquide et solide. Elle permet notamment le calcul des équilibres de phases et des équilibres chimiques. De nombreux modèles de coefficients d'activité ont été développés pour les phases liquides, avec comme état standard les liquides purs. Les grandeurs d'excès, calculées à partir des coefficients d'activité, expriment l'écart entre les grandeurs extensives d'un mélange réel et celles d'une solution idéale. Historique À la suite de l'apparition de l'équation d'état de van der Waals en 1873, il devenait possible de calculer les équilibres de phases gaz-liquide. En effet, cette équation novatrice pour son époque permettait

Source officielle

https://fr.wikipedia.org/wiki/Activit%C3%A9_chimique

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Concepts associés (33)

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Cours associés (12)

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Séances de cours associées (23)

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This thesis aims to find, for the first time, a direct relation between the size and morphology of small metallic nanostructures (gold in this case) supported on a metal-oxide surface to their catalytic activity. In this perspective, three main topics have been treated during this thesis. The first part concerns the design and realization of a Scanning Tunneling Microscope (STM) supposed to work over a wide temperature range (4K < T ≤ 300K). This new device replaces an existing solution. The coarse approach of the scanning tip towards the sample is realized by an axial motor based on a sapphire prism gliding on shear piezos in the stick&slip mode. The new STM is very rigid moving the resonance frequencies to higher values with respect to the existing solution. Operation of the motor down to T = 8K has been proven, however topographic imaging has been performed only in the temperature range 77K < T ≤ 300K. The second part of this work focuses on a study of the evolution of the morphology of gold nanoparticles on a TiO2(110) surface. Size-selected clusters Aun+ (n = 5, 7) are deposited at a well defined kinetic energy on the surface held at room temperature. Subsequent annealing of the sample has been performed stepwise. After each temperature increase, the morphology has been determined by STM. The evolution as a function of surface temperature has been studied for two different surface reconstructions, TiO2(110)-(1×1) and TiO2(110)-(2×1). The deposition process leads only to small fragmentation and the morphology is stable up to T = 400K. Further increase of the surface temperature leads to sintering of the particles by Ostwald ripening as shown by an exponential decrease of the island density with temperature. The main topic of this thesis, the correlation between morphology and catalytic activity, is described in the last part of this manuscript. For the first time we are able to relate the onset of CO2 production from CO and O2 to a clear change in the morphology. The catalytic activity of the particles strongly depends on their size and dimensionality. The relative activity per particle has been determined and we find a clear maximum for clusters containing 60 atoms and are 3 to 4 monolayers high. These results are discussed in contrast of literature data on the same but also on different metal-oxide surfaces.

EPFL2009

Chargement

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3)

Rainer Beck, Harmina Vejayan

Reflection–absorption infrared spectroscopy (RAIRS) is widely used to identify molecular adsorbates on metals during surface chemical reactions, but the interpretation of RAIRS data can be difficult with experiment alone. Here, we reveal from first-principles calculations the origin of the contrasting RAIRS spectra of methyl adsorbed on Pt(111) and Ni(111). We find that the dynamic dipole associated with the symmetric C–H stretch vibration of CH3 along surface normal is significant on Pt(111) but negligibly small on Ni(111), explaining the strong IR activity in the former and the absence of any RAIRS peaks in the latter. This difference is correlated to different charge transfer patterns between metals and the adsorbate, which are determined by the different preferred adsorption sites of methyl on the two surfaces. This work highlights the need of electronic structure calculations in interpreting RAIRS spectra of adsorbates on metal surfaces.

2021

Chargement

Biomimetic catalysis with immobilized transition metal complexes

Alexander Schiller

The tetradentate chelate ligand tris[(1-vinylimidazol-2-yl)methyl]amine (10) was synthesized in five steps from commercially available starting materials. Upon reaction with MCl2 (M = Zn, Cu, Ni, Co) in the presence of NH4PF6, the complexes [M(10)Cl]PF6 (M = Zn (11), Cu (12), Co (14)) and [Ni(10)Cl]2(PF6)2 (13) were obtained. The structure of all complexes was determined by single crystal X-ray crystallography. Immobilization of 11 and 12 was achieved by copolymerization with ethyleneglycol dimethacrylate (EGDMA) to obtain P11-Zn and P12-Cu. Immobilized versions of 13 and 14 have been generated by cleavage the metal from P12-Cu and subsequent assembly of MCl2 (M = Ni (P12-Ni), Co (P12-Co)). The supported complexes P11-Zn, P12-Cu, P12-Ni, and P12-Co were found to be efficient catalysts for the hydrolysis of bis(ρ-nitrophenyl)phosphate (BNPP) at 50 °C. At pH 9.5, the heterogeneous catalyst P12-Cu was 56 times more active than the homogeneous catalyst 12. Partitioning effects, which increase the local concentration of BNPP in the polymer, are shown to contribute to the enhanced activity of the immobilized catalyst. The tridentate N,N',N''-chelate ligand tris(1-vinylimidazol-2-yl)phosphine (20) was synthesized from commercially available starting materials. Three immobilized Cu(II) complexes were generated by the following: (a) homopolymerization of 20 and subsequent metalation with CuCl2; (b) copolymerization of 20 with EGDMA and subsequent metalation with CuCl2; or (c) molecular imprinting with the organometallic Mo-complex Mo(η3-C4H7)(CO)2(20) (28) and EGDMA and subsequent replacement of Mo(II) by Cu(II). All three polymeric Cu complexes were found to very efficiently promote the hydrolysis of activated phosphoesters with the relative activity being dependent on the nature of the polymer and the substrate. The complex Fe2O(CH3COO)(10)23 (42) was synthesized and structurally investigated by single crystal X-ray crystallography. After copolymerization with EGDMA and subsequent replacement of Fe(III) by Cu(II), Zn(II) or Mn(III), hydrolysis and oxidation reactions were investigated. A higher activity compared to the corresponding monometallic immobilized complexes was observed. This increase in activity is most likely caused by cooperative effects.

EPFL2006

Chargement

Biomimetic catalysis with immobilized transition metal complexes

Alexander Schiller

EPFL2006

EPFL2009