David R. Schryer

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.

Effects of pretreatment conditions on a Pt/SnO2 catalyst for the oxidation of CO in C02 lasers

CO oxidation catalysts are important for long-life closed-cycle operation of CO{sub 2} lasers. The electrical discharges frequently used to excite such lasers decompose some CO{sub 2} to CO and O{sub 2} but many of their applications, including remote sensing from space vehicles, prohibit addition of makeup gas or removal of decomposition products because of volume and weight constraints. These lasers thus represent a new and important application for low-temperature CO oxidation catalysts, since for space applications, no catalyst heating is allowed to minimize power consumption. The most promising catalysts whose performance has been verified by actual laser operation consist of Pt and/or Pd on tin(IV) oxide. This paper presents results of a study of various pretreatment techniques on the activity of a commercially available Pt/SnO{sub 2} catalyst. Pretreatment with the reducing gases, CO and H{sub 2}, produces approximately equal steady-state CO{sub 2} yields which are significantly higher than those for the other pretreatment gases, although the steady-state is more rapidly attained with the H{sub 2} pretreatment. Pretreatment with O{sub 2} in He results in only slightly greater CO{sub 2} yields than pretreatment with He alone.

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.

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.

The enhanced oxidation of SO2 by NO2 on carbon particulates

Abstract The oxidation of SO 2 on carbon particles in dry air and in air at 65% relative humidity (RH) was found to be greatly enhanced by the presence of gaseous NO 2 . Exposures of 20–80 ppm SO 2 + 10 ppm NO 2 on 1 mg samples of commercial carbon black were found to produce both sorption and desorption coverages (weight retained after desorption into N 2 ) of over one order of magnitude greater than for corresponding SO 2 exposures. Significant agglomeration and wetting were observed to occur progressively during exposures at 65% RH and samples, even after 150 h exposure, rarely reached steady-state weight gain. The wetting may have regenerated fresh reactive carbon surface. Sorptions conducted in nitrogen atmospheres, rather than in air, appeared to produce slightly higher sorptions and weight retentions for equivalent exposure concentrations and times, indicating that NO 2 served as the oxidizer and that molecular oxygen, or some trace constituents in air, may have weakly inhibited the oxidation by NO 2 . Wet chemical analysis of the desorbed phase indicated that sulfate, presumably H 2 SO 4 , accounted for over half of the retained weight. Measurements of pH from water-quenched samples indicated a highly acidic surface phase and suggested the oxidation process could process in an acidic environment.

The oxidation of SO2 on carbon particles in the presence of O3, NO2 and N2O

Abstract The oxidation of SO 2 on carbon particles at 65 % relative humidity (RH) by O 3 , NO 2 and N 2 O was investigated gravimetrically and compared with oxidation by air. Approximately 1 mg samples of carbon black were exposed to continuously flowing mixtures of SO 2 , SO 2 + O 3 , SO 2 + NO 2 , and SO 2 + N 2 O in air (or in N 2 ). Both O 3 and NO 2 in the 0.07–10ppmv range with 20–40ppmv SO 2 were found to produce sorption and desorption coverages significantly higher than those for corresponding SO 2 in air exposures. N 2 O was determined to be much less effective as an oxidant than O 3 or NO 2 . Identical concentrations of O 3 or NO 2 were found to produce equivalent conversions of SO 2 to sulfate on carbon for equal exposure times. Wet chemical analysis of the residue following desorption indicated that sulfate generally accounted for well over half of the retained weight. Exposure at low concentrations (0.10 ppmv SO 2 + 0.02 ppmv O 3 or NO 2 ), however, appeared to produce little, if any, enhancement in SO 2 transformation when compared with equivalent SO 2 in air. Weight retentions for these runs were very small, however, and measurement errors of up to 25% would be anticipated.

Surface characterization study of the reduction of an air-exposed Pt3Sn alloy. IV

Abstract Ion scattering spectroscopy (ISS), angle-resolved Auger electron spectroscopy, and electron spectroscopy for chemical analysis (ESCA or XPS) have been used to examine the reduction (300 °C under 1 Torr of H2 for 1 h) of an air-exposed, polycrystalline Pt3Sn alloy surface. Initially, the surface was covered with a thick tin oxide layer. The reduction resulted in a loss of oxygen particularly from the near-surface region, migration of Pt to the surface from a tin-depleted Pt region lying beneath the tin oxide layer, and reduction of tin oxide to metallic tin which alloys with the Pt which migrated to the near-surface region. ISS shows that the outermost atomic layer of the reduced surface contains a large amount of Pt which probably occupies vacancies left by oxygen.

INFLUENCE OF PROMOTERS ON THE PERFORMANCE OF AU/MNOX AND PT/SNOX/SIO2 LOW-TEMPERATURE CO OXIDATION CATALYSTS

The influence of promoters on Pt/SnOx/SiO2 and Au/MnOx low-temperature CO oxidation catalysts has been investigated under stoichiometric reaction conditions with no CO2 added to the feed gas. The performance of Pt/SnOx/SiO2 catalysts is improved significantly by the addition of 1 wt.% Fe but reduced by the addition of 5 wt.%Fe, 1 wt.% Sb, 5 wt.% Sb, 1 wt.% As, 5 wt.%As and 1.8 wt.% P. The performance of Au/MnOx is improved significantly by the addition of 1 at.% Ce but reduced by the addition of 1 at.% Co. For the catalysts and conditions examined, the Au/MnOx catalysts are superior to the Pt/SnOx/SiO2 catalysts with respect to both activity and decay characteristics.

Influence of an Fe promoter on silica-supported Pt/SnOx catalysts used for low-temperature CO oxidation

Silica-supported Pt/SnOx catalysts used for low-temperature CO oxidation have been prepared without and with an Fe promoter. Reaction studies demonstrate that the addition of the Fe promoter results in higher catalytic activity in the presence of 8 at% CO2 and a lower decay rate. Ion scattering spectroscopy (ISS) has been used to examine the outermost atomic layers of the promoted and nonpromoted catalysts before and after activation by a reductive pretreatment. The nonpromoted catalyst exhibits agglomeration of the platinized tin oxide film exposing the catalytically inactive silica support. This agglomeration does not occur when Fe is present, and a large catalytically active surface area is maintained during the reduction.

A characterization study of a hydroxylated polycrystalline tin oxide surface

In this study Auger electron spectroscopy, electron spectroscopy for chemical analysis (ESCA) and electron-stimulated desorption (ESD) have been used to examine a polycrystalline tin oxide surface before and after annealing in vacuum at 500 °C. Features due to surface hydroxyl groups are present in both the ESCA and ESD spectra, and ESD shows that several chemical states of hydrogen are present. Annealing at 500 °C causes a large reduction in the surface hydrogen concentration but not complete removal.