Donald D. Beck

The dissociative adsorption of hydrogen sulfide over nanophase titanium dioxide

A variety of TiO{sub 2} materials, including a nanophase TiO{sub 2} powder, were evaluated for their ability to dissociatively adsorb H{sub 2}S in a H{sub 2} environment. A temperature programmed desorption technique was used to determine the rate of sulfide accumulation on the surface of the samples as a measurement of initial activity. The initial activity for the gas condensation-produced nanophase TiO{sub 2} with its rutile structure was found to be greater than that for other samples of TiO{sub 2} tested. When normalized for surface area, the initial specific activities of the rutile samples studied for the dissociative adsorption of H{sub 2}S were similar in magnitude, but significantly higher than those of the anatase TiO{sub 2} samples investigated. Thus, the improvement in the activity is attributed mainly to the ability of the nanophase synthesis method to produce high surface area rutile TiO{sub 2}. When evaluated using x-ray photoelectron spectroscopy, the nanophase TiO{sub 2} was found to be significantly deficient in oxygen. Annealing this material in oxygen decreased the number of anion vacancies and lowered the activity. Thus, we conclude that oxygen vacancies also contribute to the H{sub 2}S dissociative adsorption activity.

Impact of sulfur on the performance of vehicle-aged palladium monoliths

Abstract A commercially prepared catalyst which uses Pd technology and which had been vehicle-aged for 56 000 miles was characterized for three-way catalytic performance in a laboratory reactor under conditions simulating the operation of the catalyst in a vehicle. The tests were conducted with the concentration of SO2 in the simulated exhaust feedstream set at 1 ppm, 5 ppm and 30 ppm (corresponding to 15, 75 and 450 ppm in fuel, respectively) to evaluate the magnitude and reversibility of the effect of sulfur on HC, CO and NOx activity. Tests were conducted using a propene/propane mixture as a surrogate for the hydrocarbon mixture in exhaust. The lightoff and warmed-up activities, particularly for HC and NOx, were found to be deteriorated when SO2 was added to the simulated exhaust feedstream, in agreement with previous work on the impact of sulfur on three-way catalysts conducted in this laboratory. In addition, the impact on lightoff and warmed-up activity of NOx and HC was found to be generally non-linear with sulfur concentration. The impact of sulfur on activity was found to be partially non-reversible at temperatures below 650°C, but the original activity could be restored when the catalyst was operated at temperatures above 650°C. Consistent with this result, the impact of sulfur on activity was found to be minimal for operation above 700°C.

Impact of sulfur on model palladium-only catalysts under simulated three-way operation

Abstract Three catalysts consisting of palladium supported on Al2O3, 5% CeO2/Al2O3, and 8.6% La2O3/Al2O3 were characterized for three-way catalytic performance using a laboratory reactor designed to simulate the operation of the catalyst in a vehicle. Different concentrations of SO2 were used in the feedstream to evaluate the magnitude and reversibility of the impact of sulfur on three-way activity. Both lightoff and warmed-up activities of all of the catalyst formulations were significantly decreased when sulfur was present in a cycled, stoichiometric feedstream. The magnitude of the impact on activity increased with increasing sulfur content. The lightoff activity also decreased with increasing sulfur content, following a similar trend as with the warmed-up activity. For all three catalyst formulations, the increase in the lightoff temperature due to the addition of 30 ppm SO2 to the feedstream was about the same as the increase in the lightoff temperature due to the effect of relatively severe thermal aging. Although the addition of base metal oxides to the catalyst resulted in an apparent improvement in activity compared to Pd/Al2O3 alone, the improvement attributable to cerium was completely canceled when SO2 was added to the feedstream, while the improvement attributable to lanthanum was partially canceled when SO2 was added. During isothermal tests at 500°C, the aged catalysts recovered only part of their lost activity when sulfur was removed from the feedstream. Exposure to a rich or cycled stoichiometric environment at or above 700°C was required to obtain the original activity of these aged catalysts.

A study of thermal aging of Pt/Al2O3 using temperature-programmed desorption spectroscopy

The metal dispersion of 1.0% PtAl2O3 was characterized using temperature-programmed desorption (TPD) of D2 in an ultrahigh vacuum apparatus. On the nonaged catalyst desorption of D2 gives rise to two peaks, one which can be identified as weak desorption from the Al2O3 support surface and the other as D2 desorption from the supported Pt particle surfaces. The PtAl2O3 sample was thermally aged in 5% O2N2 or in 5% H2N2 at a selected aging temperature for several hours. At intervals as small as 1 min, the Pt dispersion was obtained by D2 TPD. For aging temperatures of 700, 800, and 900 °C, the Pt dispersion decreased rapidly within the first few minutes of treatment and then more gradually afterward, consistent with the change in the dominant sintering mechanism from particle migration to interparticle transport. For sintering temperatures of 700, 800, and 900 °C, the D2 TPD peak shape changed as a function of aging time; this change suggests that faceting of the Pt particles occurs during sintering. The data also indicate that aging in an oxygen environment does not redisperse PtAl2O3, contrary to other reports. For all aging temperatures, sintering was less severe in the H2 environment than in the O2 environment. The aging data from both environments were fitted to a simple kinetic sintering model.

Axial characterization of catalytic activity in close-coupled lightoff and underfloor catalytic converters

Abstract A exhaust system consisting of a close-coupled Pd technology 32 in3 lightoff converter and Pt/Rh technology 170 in3 underfloor converter was vehicle-aged for 56000 miles on a vehicle equipped with a 3.8 l engine. Following this aging, the converters were taken off the vehicle and cut into 1″ thick sections along their axis and characterized for lightoff and warmed-up activity using a laboratory reactor to simulate vehicle exhaust. Each section was also analyzed for the quantity of oil additive poisons (phosphorus and zinc) deposited. Following this initial characterization, the phosphorus and zinc deposits were removed, and the sections were characterized again for lightoff and warmed-up activity. This procedure was used to qualitatively determine the relative contribution of oil additive poisoning and thermal sintering to the total activity deterioration as a function of axial position in the catalyst monoliths. Analysis of the lightoff converter as taken from the vehicle showed a dramatic axial gradient in the lean and stoichiometric lightoff and warmed-up (600°C) performance for HC, CO and NOx, with most of the deterioration having taken place in the forward-most 1″ section of the converter, which was consistent with the gradient in the deposition of phosphorus (P) and zinc (Zn) in this converter. Comparison of these data sets with those obtained after removal of the P and Zn poisons indicates that most of the total deterioration of lean HC and CO activity can be attributed to P and Zn poisoning of the forwardmost 1″ section. When tested under stoichiometric conditions, most of the deterioration of HC activity is attributable to P and Zn poisoning, while most of the deterioration of CO and NOx activity is attributable to thermal deterioration. A similar activity and poison deposition gradient was detected in the underfloor converter, but to a smaller extent.

Axial characterization of oxygen storage capacity in close-coupled lightoff and underfloor catalytic converters and impact of sulfur

Abstract The oxygen storage capacity of a 56,000 mile aged warmup and underfloor converter system was characterized as a function of axial location along the converters and compared with fresh samples having the same formulation. Measurements of oxygen storage were made using a titration technique and at conditions expected to be commonly encountered during OBD-II diagnosis of catalyst performance. Vehicle aging resulted in a dramatic loss of oxygen storage in the warmup converter presumably due to the severe thermal sintering, but the significant amount of phosphorus (P) and zinc (Zn) poison accumulation on this converter was found to impact oxygen storage minimally. This is in contrast to the measured impact of P and Zn deposition on warmed-up hydrocarbon conversion, which was found to be significant relative to the impact of thermal sintering. The underfloor converter was found to have retained nearly all of its original oxygen storage after vehicle aging, consistent with operation of this converter at moderate temperatures which do no result in severe thermal sintering of the noble metals and the ceria. The impact of sulfur on the oxygen storage of both warmup and underfloor converter sections was dramatic. Sections in the forward part of the warmup converter and in the front brick of the underfloor converter had relatively modest oxygen storage capacity which was almost completely blocked as the sulfur concentration reached 75–150 ppm (equivalent in gasoline). Other sections such as the rear of the warmup converter and the rear monolith of the underfloor converter had more oxygen storage capacity, which was significantly decreased as the sulfur concentration reached 150 ppm equivalent in fuel, and was approached complete loss near 500 ppm sulfur equivalent in fuel.

Surface enrichment of Pt10Rh90(111): I. Annealing in vacuum and low pressure environments

Abstract The variation in the surface composition of a model catalyst surface, a Pt 10 Rh 90 (111) single crystal, has been studied as a function of temperature and gas phase composition using ion scattering spectroscopy (ISS) with Ne 20 , Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). As the single crystal is heated in vacuum, the surface composition of the first layer of this alloy becomes enriched in Pt beginning at 600°C, reaching a surface composition of 30% Pt at 1000°C, in general agreement with previously reported findings. The equilibrated surface compositions measured by ISS and AES were very similar. Depth profiling shows the Pt-enriched layer is of atomic thickness on the surface. A comparison of the ISS and AES data suggests that the sub-surface layer is enriched in Rh, in agreement with the oscillatory behavior in the composition of near-surface layers reported by other researchers. In low pressure reducing conditions (1 × 10 −6 Torr H 2 ), Pt enrichment measured by ISS on the surface approaches 40%. In low pressure oxidizing conditions (1 × 10 −6 Torr O 2 ), the surface layer equilibrates near, but above the bulk composition in Pt concentration, but no oxidation of the metal was observed.

THE EFFECT OF SULFUR ON THREE-WAY CATALYSTS

Abstract Both pelleted and monolithic catalysts have been tested in a laboratory reactor under conditions designed to simulate the operation of the catalyst in a vehicle. The warmed-up and light-off performances of both types of catalyst were deteriorated by the sulfur. The monolithic catalyst rapidly recovered all of the lost activity when sulfur was removed from the feed gas, but the pelleted catalyst only recovered a portion of the lost activity. Examination of the effect of sulfur on Pt, Pd, and Rh indicated that both Pt and Rh rapidly recovered all of its lost activity when sulfur was removed from the feed, but Pd did not. Additionally, a pelleted Pt/Rh catalyst which did not contain any Ce also rapidly recovered all lost activity when sulfur was removed from the feed, but a similar catalyst which contained Ce did not. Hence, the slow recovery from sulfur poisoning is related both to the the high Ce surface area seen in pelleted catalysts and to the presence of Pd. The impact of the sulfur decreased when the cycling frequency was increased and the cycling amplitude decreased, indicating that sulfur will have less of an effect on cars with better A/F control. This result was confirmed in tests which used engine exhaust.

Impact of sulfur on three-way catalysts: Comparison of commercially produced Pd and Pt-Rh monoliths

Abstract Commercially-prepared Pt-Rh and Pd monolith catalysts were thermally aged then characterized for catalytic performance using a laboratory reactor to evaluate the magnitude and reversibility of the impact of sulfur on three-way activity. The SO 2 concentration in the feedstream was varied from 0 ppm to 30 ppm, which was comparable to sulfur levels in gasoline ranging from 0 to 450 ppm. Tests were first conducted using propylene and repeated using propane to represent the hydrocarbon mixture in exhaust. Of the two catalysts, Pd showed better propylene lightoff activity while Pt-Rh showed better propane lightoff activity, regardless of the sulfur content. For each catalyst, increasing the sulfur concentration from 0 ppm SO 2 to 30 ppm SO 2 of sulfur resulted in a lightoff temperature increase by 40 to 60°C. Under warmed-up conditions, the loss of activity for HC, CO and NOx due to the presence of sulfur was greater under slightly rich conditions than under lean conditions for both Pd and Pt-Rh, while the magnitude of the impact on NOx and particularly on HC activity under warmed-up stoichiometric conditions was significant and much greater for Pd than for Pt-Rh catalyst. Using propylene, the effect of SO 2 on the activity of the Pd catalyst was partly reversible, while the effect on Pt-Rh was completely reversible. Using propane, the effect of sulfur on the activity of both catalysts was larger than for propylene. The resulting decrease in activity due to the presence of sulfur was partly reversible on Pt-Rh, but the poisoning of the Pd catalyst was mostly irreversible. Part of the irreversible poisoning effect is attributed to a direct interaction or reaction between SO 2 and Pd, while the other part is attributed to the promotional effect of SO 2 in hydrocarbon coking of the catalyst when alkane hydrocarbons are present.

Surface enrichment of Pt10Rh90(111). II: Exposure to high temperature environments at 760 Torr

The variation in the surface composition of a model catalyst surface, a Pt10Rh90(111) single crystal, has been studied after exposure to oxidizing and reducing gas phase environments at 760 Torr at temperatures typical of automotive catalytic converter operation using ion scattering spectroscopy (ISS) with Ne20, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Under reducing conditions, the surface composition is enriched in Pt, consisting of 30% Pt in the surface layer. The Pt enrichment occurs only on the top one-two surface layers. This condition is identical to that obtained after annealing in vacuum at high temperature (1000°C) followed by cooling. Under oxidizing conditions, the surface layer quickly becomes enriched in Rh compared to the bulk, and is accompanied by oxidation of Rh to Rh2O3, as indicated by AES and XPS. The Rh enrichment in the oxide form extends several layers below the surface (5–6 nm), but the Rh oxide only accounts for part of the oxygen incorporated in the sample during the high temperature treatment in oxidizing conditions. This suggests a significant portion of the oxygen occupies interstitial sites in the alloy. Temperature programmed desorption in vacuum results in the observance of an intense oxygen desorption peak around 700°C, accounting for both the decomposition of surface Rh2O3 and removal of interstitial oxygen. At this point, the surface is still enriched in Rh, but is converted to a Pt-enriched surface by 950°C.