Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules

https://ris.utwente.nl/ws/files/6651605/Lercher96infra-red.pdf

Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules

Infrared−visible sum frequency generation (SFG) is a surface-specific vibrational spectroscopy that can operate in a pressure range from ultrahigh vacuum (uhv) to atmospheric pressures. SFG is therefore one of the few surface science techniques that permits atomic scale monitoring of surface species during catalytic reactions at high pressures (around 1 atm) and high temperatures. Using single-crystal surfaces of transition metals, platinum and rhodium, reaction rates can be simultaneously determined by gas chromatography, and correlations between the concentration of adsorbates under reaction conditions and the observed turnover numbers can help to elucidate the reaction mechanism. To bridge the gap to traditional surface science experiments, SFG is also employed under uhv or low pressures. The technique has been successfully applied to the adsorption and oxidation of CO, hydrocarbon conversion such as ethylene hydrogenation and cyclohexene hydrogenation and dehydrogenation on Pt(111). The experiments de...

Molecular Studies of Catalytic Reactions on Crystal Surfaces at High Pressures and High Temperatures by Infrared−Visible Sum Frequency Generation (SFG) Surface Vibrational Spectroscopy

Aerosol mass size distributions of 41 polycyclic aromatic hydrocarbons (PAHs) were measured during 20 different 12-h periods in urban Chicago and over Lake Michigan during July 1994 and January 1995. Geometric mean aerodynamic equivalent diameters (GMDs) range from 0.72 to 2.39 μm for particulate matter and from 0.33 to 9.85 μm for individual PAHs. GMDs of the less volatile PAHs are larger in the urban atmosphere than over the water during the summer. Geometric standard deviations of the particle size distributions, however, are larger at the urban location for many PAHs, indicating a broader mass size distributions. GMDs of unsubstituted PAHs (except perylene) are well correlated with their log subcooled liquid vapor pressures ( , Pa), following the form:  GMD = mg log + bg . Values for mg and bg range from 0.03 to 0.88 and from 0.83 to 8.80, respectively. Higher molecular weight PAHs are sorbed to the finest sized aerosols, but more volatile PAHs are associated with larger particles. The slope (mg) and ...

Aerosol size distributions of polycyclic aromatic hydrocarbons in urban and over-water atmospheres

Diesel vehicles are a major source of fine, atmospheric particulate matter in urban environments. The influences diesel particles exert on solar radiation, on atmospheric chemistry, and on humans depend crucially on the size and chemical character of these particles. In this work, size-resolved diesel particle chemistry has been examined by collecting particles directly from a diesel car exhaust with a low-pressure impactor. The impactor samples have been weighed and analyzed chemically to construct continuous size distributions for selected compounds present in the particulate phase. Submicron diesel-particle mass size distributions displayed three log-normal modes that were centered at 0.09, 0.2, and 0.7−1 μm of particle aerodynamic diameter (EAD) and that had average geometric standard deviations of 1.34, 1.61, and 1.34, respectively. The lowest two modes had approximately the same particulate mass, whereas over 80% of the number of particles were estimated to be found in the mode around 0.1 μm. The th...

Characterization of the Particulate Phase in the Exhaust from a Diesel Car

Etude par spectroscopie infrarouge des vibrations ν(CH) de composés cycliques: Dioxanne-1,4, dithianne-1,4, oxathianne-1,4 et cyclohexane

IR Spectroscopic evidence, obtained with deuterated dimethyl ether, methanol, ethanol, and 2-trichloro-1-ethanol, suggested that the CH stretching frequencies increase when alcohols or ethers are coordinated, and decrease when dissociative adsorption occurs. The adsorption of isobutanol on alumina, reported by Knoezinger and Stubner as coordinative adsorption, was therefore probably dissociative adsorption. Knoezinger agrees.

Adsorption of alcohols on alumina. 1. Gravimetric and infrared spectroscopic investigation. Comment

Les sondes les plus souvent employees pour l'etude infrarouge de l'acidite de Lewis des sites superficiels sont la pyridine, l'ammoniac et les nitrites. Dans ce travail, nous avons cherche a utiliser l'ether dimethylique qui est une base faible. Nous montrons que l'adsorption de CD3 OCD2 H sur l'alumine, l'oxyde de zinc ou la silice conduit a la formation d'especes coordinees (Al2 O3 , ZnO) ou liees par liaison hydrogene (SiO2 ). Les vibrations v(CH) sont dans tous les cas sensibles a la complexation. La comparaison des resultats avec ceux obtenus par l'utilisation des autres sondes montre que les glissements Δν(CH) observes peuvent etre relies a la force des sites acides de Lewis superficiels. CD3 OCD2 H est une sonde tres sensible qui a, de plus, l'avantage de ne former que des especes chimisorbees coordinees permettant ainsi l'emploi de la gravimetrie pour le denombrement des sites acides forts.

CD3OCD2H : une nouvelle molécule-sonde pour l'étude infrarouge de l'acidité de Lewis des sites d'adsorption des oxydes metalliques: I – Généralités

L'etude infrarouge de l'adsorption de quantites successives de CD 3 OCD 2 H sur des echantillons d’alumine DEGUSSA C actives a differentes temperatures montre la formation de trois types d'especes : des especes fortement coordinees, des especes coordinees re- versiblement et des especes liees par liaison pontale hydrogene. Les especes irreversiblement coordinees se decomposent a T > 470 K pour donner des groupements methoxy superficiels. Par gravimetrie (mesures faites a 370 K), nous montrons que les sites acides forts impliques dans la formation des especes irreversiblement coordinees n'apparaissent que lorsque la temperature d'activation de l'alumine est superieure a 400 K. Leur nombre augmente avec la temperature d'activation, atteignant une limite voisine de 0,66 site nm –2 .L'utilisation de CD 3 OCD 2 H permet de montrer l’heterogeneite des sites acides superficiels. Il apparait de plus que le pouvoir donneur de proton des groupements hydroxyle de l'alumine est plus faible que celui des groupements OH restant apres activation de la silice a 870 K.

CD3OCD2H : une nouvelle molécule-sonde pour l'étude infrarouge de l'acidité de Lewis des sites d'adsorption des oxydes metalliques - II – Adsorption sur alumine

Jack Caillod

Papers

Etude par spectroscopie infrarouge des vibrations ν(CH) de composés cycliques: Dioxanne-1,4, dithianne-1,4, oxathianne-1,4 et cyclohexane

Adsorption of alcohols on alumina. 1. Gravimetric and infrared spectroscopic investigation. Comment

CD3OCD2H : une nouvelle molécule-sonde pour l'étude infrarouge de l'acidité de Lewis des sites d'adsorption des oxydes metalliques: I – Généralités

CD3OCD2H : une nouvelle molécule-sonde pour l'étude infrarouge de l'acidité de Lewis des sites d'adsorption des oxydes metalliques - II – Adsorption sur alumine

Etude des vibrations ν(CH) du triméthyl-2,4,6 trioxane