Surface Atomic Ratio Research Articles

Determination of framework and non-framework aluminium in HY dealuminated zeolites by X-ray photoelectron spectroscopy

The surface atomic ratio of framework and non-framework aluminium in a deep bed HY zeolite and in an HY zeolite dealuminated with SiCl4 has been determined by X-ray photelectron spectroscopic measurements.

Application of EXAFS in Catalysis. Structure of Bimetallic Cluster Catalysts

Abstract In recent years the utility of extended x-ray absorption fine structure (EXAFS) as a probe for the study of catalysts has been clearly demonstrated [1–13]. Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. With such catalysts it is difficult or impossible to obtain structural information by conventional x-ray diffraction methods [14].

Sulfur deactivation of nickel methanation catalysts

Sulfur deactivation of supported Ni in CO hydrogenation was studied in an all-quartz internalrecycle reactor with a feed containing 4% CO in H 2. Thirteen ppb H 2S reduced the steady-state methanation activity of Ni γ- Al 2 O 3 about 200-fold at 661 K; 100 ppb H 2S reduced the activity 5000-fold. A dual site Langmuir-Hinshelwood rate expression predicts both the CO partial pressure dependence and the S poisoning. Poisoning and chemisorption data indicate formation of a stable two-dimensional, surface sulfide with a S: Ni surface atom ratio of 1: 2 for 13 ppb H 2S in H 2 at 661 K. The surface sulfide has a free energy of formation of at least −26 kcal/mole which is 15 kcal/mole more stable than bulk Ni 2S 3. Sulfur poisoning is due to geometric effects, i.e., site blockage, rather than electronic effects since the activation energy for methanation over S-poisoned Ni was the same as that over unpoisoned Ni, 24 kcal/mole.

Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Pt–Ir clusters

The extended x-ray absorption fine structures (EXAFS) associated with the LIII absorption edges of platinum and iridium were investigated for several catalysts consisting of metal clusters of these elements dispersed on silica or alumina. Previous x-ray diffraction studies on silica supported platinum–iridium catalysts containing 5%–20% by weight of platinum plus iridium indicated that these elements were present in the form of bimetallic clusters with sizes in the approximate range of 25–50 Å. The present EXAFS investigation has provided more detailed structural information on such clusters and has extended the scope of the research to include more highly dispersed platinum–iridium catalysts which cannot be investigated satisfactorily by x-ray diffraction. In this way, it has been possible to extend the structural evidence for platinum–iridium bimetallic clusters to the case in which the ratio of surface atoms to total atoms approaches unity.

Hydrogenolysis and hydrogenation of hydrocarbons on supported Rh–Ir bimetallic catalysts

A study of the characteristics and catalytic behaviour of silica-supported Rh–Ir catalysts has been carried out. Catalysts spanning the spectrum of possible metal compositions were used. Both chemisorption and X.p.s. results suggest a modest Rh enrichment of the surface relative to bulk composition. Hydrogenation activity was characterized by cyclopropane hydrogenation and pure Rh shows activity orders-of-magnitude greater than that of pure Ir. The hydrogenation turnover frequency (normalized by assuming a hydrogen-to-metal surface atom ratio of one) shows a slight maximum on the Rh-rich catalysts which may reflect an electronic effect or be the result of selective chemisorption. The hydrogenolysis reactions are most rapid on Rh and the turnover frequencies decrease in a non-linear manner from Rh to Ir. Even though the absolute rates are different, both hydrogenation and hydrogenolysis reactions have reactivity patterns qualitatively similar to the Ni–Cu system. The results for both systems can be interpreted on the basis of different numbers of atoms in the ensembles which constitute the site, larger ensembles being required for the hydrogenolysis reaction. The relative rate of H–D exchange of methane and the pattern of the rates as a function of catalyst composition indicate that methane desorption is not the rate-limiting step in ethane hydrogenolysis.

Auger spectroscopy of osmium, and the segregation of platinum to the osmium surface

The surface of osmium, in the form of a film electrodeposited on a platinum backing, has been examined by Auger spectroscopy. Allocation of peaks in the spectrum to Auger transitions is made by comparison with the published spectra of neighbouring elements. When the film was heated platinum was found to have diffused through it and segregated to the osmium surface. The platinum to osmium surface atomic ratios could be derived from the spectra, with some plausible assumptions, and, when this ratio was plotted as a function of temperature, the region of maximum rate of segregation of platinum was found to be 650–750°C. Not all the platinum could be removed by ion bombardment, indicating a certain solid solubility of platinum in osmium. The solubility ratio was found to be about double that deduced from other work, suggesting that osmium is sputtered more readily than platinum from their solid solution.

Correlation of the wetting behaviour of sodium on nimonic PE16 with the surface O/Cr ratio determined by Auger spectroscopy

The contact angle of liquid sodium on Nimonic PE16 was measured by the sessile drop technique, as a function of temperature of the substrate. Prior surface treatments given to the PE16 included mechanical polishing, chemical polishing, and oxidation at 700°C; the effect of subsequent argon-ion bombardment was also studied. Surface analyses by Auger spectroscopy were performed on duplicate specimens after each of the above treatments, and again after ion bombardment. Although on any of the surfaces at any stage there were between six and nine elements present, the only obvious correlation was that between the low-temperature (160°C) contact angle and the oxygen/chromium atomic ratio at the surface. Up to about 260°C the sodium was apparently inert and non-wetting but beyond that temperature the contact angle dropped rapidly until at about 500°C irreversible wetting occurred. At the same time the correlation with the oxygen/chromium surface atomic ratio disappeared.

The change of conductivity of erbium films caused by the sorption of hydrogen and oxygen

Abstract Films of erbium were evaporated onto the inside wall of a pyrex glass reaction vessel at 1O−9 torr. Their mean thickness (200–600 A) was deduced from their mass and geometrical area. Estimates of their surface areas were made from the physical adsorption of krypton at 78 °K (BET method) giving a mean specific surface area of 71 m−2 g−1. The number of surface sites was calculated from a relationship given by Brennan et al.1). The sorption of hydrogen and oxygen was studied separately, by measuring the changes in the electrical resistance of the films as a function of the amount of pure gas admitted to the reaction vessel in measured doses ∼ 1018 atoms per dose. The experiments were repeated at 295 °K, 200 °K, 130–140 °K for both gases (measurements at 78 °K were not reproducible, probably because of a magnetic phase change). Hydrogen at 295 °K [as reported2)] caused an initial increase ΔR in the original resistance R to a maximum ΔR R ∼ 20 %, which was followed by a decrease to ΔR R = 0 and then to ΔR R A similar pattern of resistance changes was observed at 200°K and 130–140°K with smaller changes of ΔR R , i.e. 16 % and 8 % respectively, but no significant differance in surface (θ) and bulk atomic ratios at which the maxima occurred, e.g. ΔR R was a maximum for 1.6⩽θ⩽1.9.