Craig L. DiMaggio

CO adsorption site occupation on Pt(335): a quantitative investigation using TPD and EELS

Abstract In a study of CO on Pt(335) at various coverages using TPD and EELS we observe bridge-bonded CO. Bridge-bonded CO on Pt(335) is inconsistent with existing models of CO adsorption on highly stepped Pt surfaces. Previous studies of CO on Pt(335) and on the similar Pt(112) surface identified two atop CO species: edge atop and terrace atop. We observe two additional species: edge bridge and terrace bridge. Edge atop and edge bridge CO both adsorb before CO adsorbs at terrace sites. Activation energies are estimated for CO desorption from edge and terrace sites. A model that explains our data, based on a quantitative analysis of the TPD and EELS spectra, is presented. Possible overlayer structures are proposed at selected coverages. At saturation, all edge sites are occupied with atop CO, half of the terrace sites are occupied, and on the terrace the atop-to-bridge ratio is 2:1. At low coverage the overlayer reaches equilibrium within a few minutes at 100 K. At high coverage equilibrium is not reached in tens of minutes at 100 K, but annealing 10 s at 280 K does allow the overlayer to reach equilibrium.

Adsorption and dissociation of oxygen on Pt(335)

Abstract Oxygen adsorption and dissociation on Pt(335) have been studied with high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption. Five vibrational peaks are found: from atomic oxygen on the terraces at 470 cm −1 and on the steps at 560 cm −1 , from molecular oxygen on the terraces at 860 cm −1 and on the steps at 830 cm −1 , and, under limited conditions, from a distinct molecular step species at 700 cm −1 . Four desorption features correspond to the first four vibrational peaks: terrace O 2 at 125 K, step O 2 at 210 K, terrace O at 750 K and step O at 850 K. Our analysis of the HREELS intensities suggests that the step species are bound on the exposed step edges, not in the trough as suggested by previous studies. Dissociation occurs almost exclusively at the steps: At saturation coverage about 95% of the step O 2 dissociates before desorbing, while 95% of the terrace O 2 desorbs directly. Dissociation of the edge O 2 occurs in part directly and in part through the metastable 700 cm −1 state. Direct dissociation produces step O and terrace O with equal probabilities, while the 700 cm −1 state produces predominantly step O. Equilibration between step and terrace O occurs only above 400 K.

The adsorption of nitrogen dioxide on the Ag(110) surface and formation of a surface nitrate

The adsorption of NO2 on the Ag(110) surface has been characterized by temperature programmed reaction spectroscopy and high resolution electron energy loss spectroscopy. At 95 K the NO2 is dimerized to N2O4 in a multilayer and a distinct molecular layer, both of which desorb below 200 K. The first adsorption layer contains chemisorbed NO2 and NO3, the latter formed by a reaction between NO2 and its decomposition products. Part of the NO2 is molecularly chemisorbed via the oxygen atoms with a proposed symmetric bidentate geometry, desorbing at 270 K. Nitrogen dioxide also undergoes partial dissociation to nitrogen and oxygen adatoms. An NO3 species is formed by the reaction of NO2 with the oxygen adatoms produced from the partial dissociation of NO2. The NO3 is attached to the surface via one oxygen atom and has C2v symmetry; it decomposes below 500 K. The geometry of both the chemisorbed NO2 and NO3 have analogues among inorganic metal complexes.

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.

Identification of molecular and subsurface oxygen on stepped Rh(711)

Abstract The adsorption of oxygen on Rh(711) was studied over a wide range of oxygen coverages using temperature programmed desorption, high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Oxygen adsorption on Rh(711), a Rh(100) surface with four atom terraces and a (111) step, differs significantly from the behavior observed on the flat Rh(100) surface. Unlike Rh(100), the Rh(711) surface shows a strong O-O stretch at 1079 cm −1 , considerable amounts of subsurface oxygen, and a LEED pattern which indicates a surface reconstruction. Rh(711) adsorbs oxygen dissociatively at 100 K and at low coverages, gives a single HREELS Rh-O mode which varies from about 410 to 435 cm −1 with increasing coverage, and a sharp c(2 × 2) LEED pattern. Desorption from this state grows in with the peak temperature shifting from 1250 to 1135 K characteristic of second-order desorption kinetics. We conclude that oxygen atoms adsorb initially on the terraces, rather than the steps, on this surface. At higher coverages, a new first-order or quasi-zero-order peak appears at 835 K at the same time that desorption from a molecular state is observed in two peaks at 125 and 150 K. On the fully saturated surface, the O-O stretch is observed at 1079 cm −1 which we assign to a superoxo molecular oxygen species. At high oxygen coverages, complicated LEED patterns are observed that suggest a reconstruction of the clean Rh(711) surface structure. O/Rh AES peak intensity ratios show that oxygen goes subsurface at these high coverages and HREELS shows two atomic oxygen stretches at 413 and 534 cm −1 , assigned to oxygen in four- and three-fold sites, respectively. The results show that the four-atom wide Rh(100) terraces are different from flat Rh(100) single crystals with respect to: atomic oxygen ordered structures, facile subsurface oxygen diffusion, and HREELS detection of molecular oxygen.

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.

REACTIONS OF N AND NO ON PT(335)

As part of a study of species important in automotive exhaust chemistry, the reactivity of atomic N and NO on Pt(335) at low temperature has been studied. The atomic N was produced by dissociating adsorbed NO with a 76 eV electron beam. Cross sections for electron-stimulated desorption and dissociation are estimated for NO on terrace and step sites. Terrace NO is at least five times more likely to desorb than to dissociate. Step NO has a lower desorption cross section than terrace NO, but probably a higher dissociation cross section. Temperature-programmed desorption was used to monitor desorption, dissociation, and the formation of N2 and N2O from adsorbed N and NO. Five distinct desorption states of N2 formed by NO dissociation are identified. The dominant N2 peak (435 K) comes from electron-dissociated step NO; its desorption temperature is higher than the N2 peaks from electron-dissociated terrace NO. Coadsorbed N and NO react to form N2O even below 100 K, with an activation barrier of ∼6 kcal/mol. On...

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.

HCO interactions on the terraces and step edges of a stepped Pt surface

Abstract We have studied coadsorbed CO and H on the stepped Pt(335) surface with electron energy loss spectroscopy and temperature-programmed desorption. Radically different coadsorption behavior is found on the step edges and the terraces. Along the edge coadsorbed H strongly affects COs vibrational spectrum, shifting CO from atop to bridge sites and enhancing the cross section of bridge CO. On the terrace coadsorbed H has virtually no effect on COs spectrum, even at very high H coverage. Both of these behaviors differ from the interactions observed on Pt(111). Possible explanations are discussed.