Final Report of a CRADA Between Pacific Northwest National Laboratory and the Ford Motor Company (CRADA No. PNNL/265): ?Deactivation Mechanisms of Base Metal/Zeolite Urea Selective Catalytic Reduction Materials, and Development of Zeolite-Based Hydrocarbon Adsorber Materials?

Reducing NOx emissions and particulate matter (PM) are primary concerns for diesel vehicles required to meet current LEV II and future LEV III emission standards which require 90+% NOx conversion. Currently, urea SCR as the NOx reductant and a Catalyzed Diesel Particulate Filter (CDPF) are being used for emission control system components by Ford Motor Company for 2010 and beyond diesel vehicles. Because the use of this technology for vehicle applications is new, the relative lack of experience makes it especially challenging to satisfy durability requirements. Of particular concern is being able to realistically simulate actual field aging of the catalyst systems under laboratory conditions. This is necessary both as a rapid assessment tool for verifying improved performance and certifiability of new catalyst formulations, and to develop a good understanding of deactivation mechanisms that can be used to develop improved catalyst materials. In addition to NOx and PM, the hydrocarbon (HC) emission standards are expected to become much more stringent during the next few years. Meanwhile, the engine-out HC emissions are expected to increase and/or be more difficult to remove. Since HC can be removed only when the catalyst becomes warm enough for its oxidation, three-way catalyst (TWC) and diesel oxidation catalyst (DOC) formulations often contain proprietary zeolite materials to hold the HC produced during the cold start period until the catalyst reaches its operating temperature (e.g., >200°C). Unfortunately, much of trapped HC tends to be released before the catalyst reaches the operating temperature. Among materials effective for trapping HC during the catalyst warm-up period, siliceous zeolites are commonly used because of their high surface area and high stability under typical operating conditions. However, there has been little research on the physical properties of these materials related to the adsorption and release of various hydrocarbon species found in the engine exhaust. For these reasons, automakers and engine manufacturers have difficulty improving their catalytic converters for meeting the stringent HC emission standards. In this collaborative program, scientists and engineers in the Institute for Integrated Catalysis at Pacific Northwest National Laboratory and at Ford Motor Company have investigated laboratory- and engine-aged SCR catalysts, containing mainly base metal zeolites. These studies are leading to a better understanding of various aging factors that impact the long-term performance of SCR catalysts and improve the correlation between laboratory and engine aging, saving experimental time and cost. We have also studied materials effective for the temporary storage of HC species during the cold-start period. In particular, we have examined the adsorption and desorption of various HC species produced during the combustion with different fuels (e.g., gasoline, E85, diesel) over potential HC adsorber materials, and measured the kinetic parameters to update Ford’s HC adsorption model. Since this CRADA has now been completed, in this final report we will provide brief summaries of most of the work carried out on this CRADA over the last several years

Mechanism of CO Oxidation on Pd/CeO2(100): The Unique Surface-Structure of CeO2(100) and the Role of Peroxide

Understanding the atomic mechanism of low-temperature CO oxidation on a heterogeneous catalyst is challenging. We performed density functional theory (DFT) calculations to identify the surface structure and reaction mechanism responsible for low-temperature CO oxidation on Pd/CeO2(100) surfaces. DFT calculations reveal the formation of a unique zigzag chain structure by the oxygen and Ce atoms of the topmost surface of CeO2(100) with Pd atoms located between the zigzag chains. O(2)adsorbed on such Pd atoms is stable in the presence of CO but plays a very important role in lowering the activation barrier for low-temperature CO oxidation by forming a square-planar PdO(4)structure and facilitating further O(2)adsorption.In-situRaman spectroscopy studies confirm the adsorbed oxygen species to be peroxides. The calculated activation barrier for CO oxidation, based on the mechanism suggested by these unique structures and peroxides, is 31.2 kJ/mol, in excellent agreement with our experimental results

Ethanol dehydration on gamma-Al2O3: Effects of partial pressure and temperature

Ethanol dehydration was investigated using platelet gamma-Al2O3 over a wide range of reaction temperature (180-300 degrees C) and ethanol partial pressure (0.5-2 kPa) by X-ray diffraction, ethanol Temperature programmed desorption and reactions. The turnover frequencies for commercial and platelet gamma-Al2O3 were almost identical (1.2-1.3 x 10(-2) ethanol/site s) when normalized to the number of ethoxide quantified by ethanol TPD. The desorption barrier of ethoxide was 183.6 kj/mol, similar to the activation barrier of ethylene formation. These results demonstrate that ethoxide is a key intermediate rather than molecular ethanol, possibly suggesting an El mechanism for ethylene formation, consistent with recent spectroscopic studies. Detailed kinetic measurements demonstrate the nature of the species on alumina surface varied with reaction temperature. At low temperature (180 degrees C), the ethanol dimer, one of which would be the ethoxide, saturated the surface, leading to the inhibition of ethylene formation and constant ether formation rates with ethanol pressure. At high temperature (260 degrees C), the ethanol monomer became dominant, consistent with the constant ethylene formation rates and increased ether formation rates with ethanol pressure. The apparent activation energies also changed with reaction temperature and ethanol partial pressure. Especially, the inhibition by ethanol dimer clearly contributed the increased apparent activation barrier at 180 degrees C

Origin of Higher CO Oxidation Activity of Pt/Rutile than That of Pt/ Anatase

Herein, we show that the weak interaction of CO with Pt/TiO2 under the CO oxidation condition is the origin of higher CO oxidation activity of Pt/rutile than that of Pt/anatase. The results of CO temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectros-copy (DRIFTS) indicate that the onset temperatures of CO desorption on freshly prepared Pt/rutile and Pt/anatase are the same. However, the CO-TPD curves of Pt/rutile after the reaction test show that the desorption temperature of CO shifts to a lower temperature, while that for Pt/anatase does not change. The in situ pulse reaction using DRIFTS reveals that CO on Pt/rutile reacted with oxygen faster than CO on Pt/anatase. IR spectra with peak deconvolution of adsorbed CO on Pt/rutile exhibit that CO adsorbed on the terrace sites of Pt clusters on rutile (2089 cm-1) reacts readily with O2. These results indicate that the higher low-temperature activity of Pt/rutile is related to its weaker interaction with CO compared with Pt/anatase under the reaction conditions. Our findings deepen the fundamental understanding of metal-support interaction and CO oxidation on Pt/TiO2 catalysts

Morphology change and phase transformation of alumina related to defect sites and its use in catalyst preparation

The surface properties of support have a large influence on the catalyst properties and the catalyst properties are usually interpreted based on the characterization results of the support surface. However, in many cases, especially when gamma-Al2O3 is used as the support, the alumina surface can be changed during catalyst preparation. Here, the significant changes in the physical and chemical properties of gamma-Al2O3 were observed during catalyst preparation by using incipient wetness impregnation, which follows the general protocols of catalyst production. These changes were confirmed with transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. To investigate the origin of such changes from hydrolysis, low-crystalline gamma-Al2O3 (A600) was prepared, which has abundant defect sites (Al-V) on its surface and bulk. Through Al-27 magic angle spinning nuclear magnetic resonance (MAS-NMR) and XRD studies, it could be demonstrated that defect sites (Al-V) play a crucial role in the hydrolysis of alumina. Further, the hydrolysis of the alumina surface could be controlled by altering the solvent properties like changing the volume ratio of ethanol. These results suggest that the change in the support's surface during the catalyst preparation should be considered carefully when correlating the characteristics of the alumina surface with the characteristics of an alumina-based catalyst

Effect of number and properties of specific sites on alumina surfaces for Pt-Al2O3 catalysts

In this work, how the number and properties of specific sites on alumina surfaces affect the specific interaction between Pt and alumina was investigated by using X-ray diffraction, ethanol temperature programmed desorption, diffuse reflectance infrared Fourier transform spectroscopy, H2 chemisorption, scanning transmission electron microscopy and benzene hydrogenation reaction. Here, we chose two sets of model aluminas having different number of sites with the identical properties and different properties of sites with the same number based on ethanol TPD. The H2 chemisorption results for the model aluminas show that H/Pt are all similar for low Pt loadings, but significantly different for high Pt loadings. For 1 wt% Pt/Al2O3, the number of specific sites on all the aluminas was sufficient to disperse all the Pt, leading to only highly dispersed Pt clusters (???1 nm). However, at 10 wt% Pt/Al2O3, the number of Pt atoms is greater than that of the specific sites on the alumina surface, resulting in a bimodal distribution of large agglomerated Pt (>10 nm) and highly dispersed Pt clusters (<3 nm) revealed by XRD and TEM. Overall, the results clearly demonstrated that Pt shows higher dispersion with increasing number of sites and interaction strength, because the Pt atoms can interact with specific sites on alumina in greater numbers and more strongly. However, these Pt dispersion changes do not represent the gradual change in Pt cluster sizes, but the relative population change of small (<3 nm) and large agglomerated Pt clusters (>10 nm) under bimodal distribution. The number of large agglomerated Pt clusters decreased with increasing number of sites and interaction strength. This fundamental understanding provides an important perspective for designing Al2O3-based supported catalysts

Using a Surface-Sensitive Chemical Probe and a Bulk Structure Technique to Monitor the gamma- to theta-Al2O3 Phase Transformation

In this work, we investigated the phase transformation of gamma-Al2O3 to theta-Al2O3 by ethanol TPD and XRD. Ethanol TPD showed remarkable sensitivity toward the surface structures of the aluminas studied. Maximum desorption rates for the primary product of ethanol adsorption, ethylene, were observed at 225, 245, and 320 degrees C over gamma-, theta-, and alpha-Al2O3, respectively. Ethanol TPD over a gamma-Al2O3 sample calcined at 800 degrees C clearly shows that the surface of the resulting material possesses theta-alumina characteristics, even though only the gamma-alumina phase was detected by XRD. These results strongly suggest that the gamma-to-theta phase transformation of alumina initiates at oxide particle surfaces. The results obtained are also consistent with our previous finding that the presence of penta-coordinated Al3+ sites, formed on the (100) facets of the alumina surface, is strongly correlated with the thermal stability of gamma-aluminaopen141

Modification of the acid/base properties of gamma-Al2O3 by oxide additives: An ethanol TPD investigation

The electronic properties of oxide-modified gamma-Al2O3 surfaces were investigated by using ethanol TPD. Ethanol TPD showed remarkable sensitivity toward the surface structures and electronic properties of the aluminas modified by various transition metal oxides. Maximum desorption rates for the primary product of ethanol adsorption, ethylene, were observed at 225 degrees C on non-modified gamma-Al2O3. Desorption temperature of ethanol over a gamma-Al2O3 samples with different amounts of BaO linearly increased with increasing loading. On the contrary, ethanol desorption temperature on Pt modified gamma-Al2O3 after calcined at 500 degrees C linearly decreased with increasing Pt loading. These results clearly suggested that the acid/base properties of the gamma-Al2O3 surface can be strongly affected by ad-atoms. For confirming these arguments, we performed ethanol TPD experiments on various oxide modified gamma-Al2O3 and normalized the maximum desorption temperatures based on the same number of oxide dopants. These normalized ethanol desorption temperatures linearly correlate with the electronegativity of the metal atom in the oxide. This linear relationship clearly demonstrates that the acidic properties of alumina surfaces can be systematically changed by ad-atoms.close

Critical role of (100) facets on ??-Al2O3 for ethanol dehydration: Combined efforts of morphology-controlled synthesis and TEM study

In this work, the effect of crystal facets on the catalytic behavior of ??-Al2O3 was investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed desorption of ethanol, solid-state 27Al NMR, infrared spectroscopy, and ethanol dehydration reaction. A series of platelet ??-Al2O3 were synthesized, in which the relative ratio of (100) facets had been systematically increased. Ethylene formation increased with increasing (100) facets, clearly demonstrating the critical role of these facets as active sites for ethanol dehydration on ??-Al2O3. This systematic approach is helpful for a better understanding of facet-dependent catalytic properties of ??-Al2O3 that arise from the interaction between the supported metal and the crystal facets