Gar B. Hoflund

Comparison of the performance characteristics of Pt/SnOx and Au/MnOx catalysts for low-temperature CO oxidation

Abstract Platinized tin oxide (Pt/SnO x ) and gold supported on manganese oxide (Au/MnO x ) catalysts have been shown to be good CO oxidation catalysts at low temperatures (30–100°C). The performance of these catalysts has been compared by reacting mixtures of CO and OZ and He under similar conditions. The Au/MnO x catalyst is superior to Pt/SnO x catalysts with regard to both activity and decay characteristics under the conditions examined. As expected, Au/MnO x catalysts exhibit greater activity as the reaction gas mixture becomes more oxidizing.

Au/MnOx catalytic performance characteristics for low-temperature carbon monoxide oxidation

Manganese oxide-supported gold (Au/MnOx) catalysts have been prepared and tested for low-temperature (< 100°C) carbon monoxide oxidation in stoichiometric mixtures of carbon monoxide and oxygen containing no carbon dioxide in the feed gas. Even with no pretreatment these catalysts are superior to the best, pretreated platinized tin oxide (Pt/SnOx) catalysts under the conditions tested. The very small decay observed for Au/MnOx catalysts is mostly due to carbon dioxide retention. The optimum gold content has been determined to be 10 at.-% of the manganese content, and a lithium promotor results in improved catalytic behavior over K- or Na-promoted Au/MnOx catalysts for the conditions examined in this study.

Computer‐interfaced digital pulse counting circuit

An electronic circuit is presented which can be used in digital pulse counting applications such as ESCA, Auger spectroscopy, UPS, etc. It consists of a quartz crystal clock and integrated circuits, and can be interfaced easily to a laboratory computer through a parallel board. A modification allows this circuit to be used as a counter with a time delay, thus making it useful in applications involving time‐of‐flight mass spectrometry.

In situ oxygen atom erosion study of a polyhedral oligomeric silsesquioxane-polyurethane copolymer

The surface of a polyhedral oligomeric silsesquioxane-polyurethane copolymer has been characterized in situ using X-ray photoelectron spectroscopy before and after exposure to incremental fluences of oxygen atoms produced by a hyperthermal oxygen atom source. The data indicate that the atomic oxygen initially attacks the cyclopentyl groups that surround the polyhedral oligomeric silsesquioxane cage most likely resulting in the formation and desorption of CO and/or CO2 and H2O from the surface. The carbon concentration in the near-surface region is reduced from 72.5 at.% for the as-entered surface to 37.8 at.% following 63 h of O-atom exposure at a flux of 2.0 × 1013 O atom/cm2-s. The oxygen and silicon concentrations are increased with incremental exposures to the O-atom flux. The oxygen concentration increases from 18.5 at.% for the as-entered sample to 32.6 at.% following the 63-h exposure, and the silicon concentration increases from 8.1 to 11.1 at.% after 63 h. The data reveal the formation of a silica layer on the surface, which serves as a protective barrier preventing further degradation of the polymer underneath with increased exposure to the O-atom flux.

Evidence of alloy formation during reduction of platinized tin oxide surfaces

Ion scattering spectroscopy, Auger electron spectroscopy, and electron spectroscopy for chemical analysis have been used to examine a platinized tin oxide catalyst surface before, during, and after reduction by annealing under vacuum at 250 to 450 C. These techniques were then used to examine the reduced surface after a room-temperature, low-pressure oxygen exposure. The spectral results and the behavior of the reduced surface toward oxygen exposure both indicate that a Pt/Sn alloy is produced during reduction.

Oxygen transport through high-purity, large-grain Ag

The permeability of oxygen through pure, large-grain Ag membrames has been found to be linear and repeatable over the 400-800 C range, but only at a magnitude that is a factor of 3.2 smaller than ascribed by prior research. AES data indicate the pertinence of grain-boundary considerations, due to the virtual undetectability of intragranular oxygen. Vacuum-desorption of oxygen-saturated Ag is found to occur at the critical temperature of 630 C; this is consistent with the increased mobility of oxygen atoms in the higher temperature regime.

Surface characterization study of AgF and AgF2 powders using XPS and ISS

Abstract XPS and ISS data have been collected from pressed AgF and AgF2 powder samples obtained from AESAR. The samples were sputter-cleaned in vacuum for 10 min with 1 keV He+ to reduce the amounts of surface contaminants present. Negative shifts in XPS BEs and increased peak widths are observed with increasing silver oxidation state for the Ag 3d peaks according to the XPS data. This negative BE shift with increasing oxidation state is opposite to that expected based on simple charge-transfer arguments. Significant negative KE shifts between the Auger features are obtained from Ag metal, AgF, and AgF2 similar to those observed for Ag metal, Ag2O and AgO. The XPS F 1s and F 2s peaks indicate the presence of multiple chemical states of fluorine including fluoro-hydrocarbon and/or-alcohol-like species in addition to fluorides. As prepared AgF and AgF2 samples were found to have F Ag ratios of 0.60 and 1.75, respectively. Since the stoichiometries of AgF and AgF2 predict F Ag ratios of 1.00 and 2.00, respectively, the excess Ag is present in chemical forms other than the silver-halide. Both samples contain mixed chemical states of Ag0, Ag+ and Ag2+. The primary contaminants consist of O and C. Multiple chemical states of O are present on the outermost and near-surface regions of the as-prepared samples including atomically adsorbed oxygen, carbonates, absorbed water, hydroxyl groups and various oxygen-containing carbon compounds. The XPS C 1s peak shapes are quite complex with features due to the presence of several different carbon species including carbide, hydrocarbons, alcohols, carbonate, CHF and CFO(OH)-like species.

In Situ Erosion Study of Kapton ® Using Novel Hyperthermal Oxygen Atom Source

A novel hyperthermal oxygen atom source has been used to perform in situ erosion of Kapton ® surfaces at room temperature, and these surfaces have been examined using x-ray photoelectron spectroscopy before and afterexposureto different e uencesofoxygen atoms and then afterexposureto air. Thedata indicatethat theinitial attack site is the carbonyl portion of the Kapton by reaction with atomic oxygen to form carbon dioxide, which desorbs. The oxygen-to-carbon-atom ratio decreases from 0.23 to 0.11 during a 24-h exposure to a hyperthermal oxygen-atom e ux of about 1 :4 £ 10 14 atoms/cm 2 -s. Following the 24-h oxygen-atom exposure, the sample was exposed to air for 3 h. The oxygen, nitrogen, and carbon concentrations return to values similar to those obtained before the oxygen-atom exposure due to reaction with molecular oxygen in the air. Previous data from space and ground simulations indicate an increase in the surface oxygen content with exposure to atomic oxygen and then air before analysis. The results obtained demonstrate that it is necessary to examine the chemical effects of oxygen-atom degradation of Kapton without air exposure before surface characterization.

TPD and ESD studies of the interaction of hydrogen and oxygen on polycrystalline zirconium

Temperature programmed desorption (TPD) and electron stimulated desorption (ESD) have been used to study the interaction of hydrogen and oxygen on a polycrystalline zirconium surface. ESD and TPD features due to hydrogen bonding directly both to oxygen and to Zr are identified. Deuterium adsorption studies reveal that the presence of an oxide layer retards hydrogen adsorption but that the oxygen attracts bulk hydrogen forming a hydroxylated surface oxide layer.

A surface study of the oxidation of polycrystalline tin

The oxidation of a polycrystalline tin foil was examined using Auger electron spectroscopy (AES), x‐ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), ion scattering spectroscopy (ISS), and electron‐stimulated desorption (ESD). An oxidation state which does not correspond to SnO or SnO2 has been identified using XPS and retarding mode AES at low oxygen exposures (≤3000 L). It has been suggested in previous studies that oxygen penetrates beneath the outermost layers during the early stages of oxidation of certain metals such as tin. This suggestion has been verified in this study using ISS and ESD. Very small amounts of oxygen are detected with ISS on a sputter‐cleaned Sn surface, and this O signal does not increase with oxygen exposure even though XPS and AES show a significant oxygen uptake. Additionally, no O+signal is detected with ESD whereas large ESD O+ signals are obtained from surfaces of bulk tin oxides. The oxygen concentration is uniform across the sample surface at these ex...