Formation of carbonaceous deposits on Pd-based hydrodechlorination catalysts: Vibrational spectroscopy investigations over Pd/Al2O3 and Pd/SOMS

The widespread utilization and commercialization of hydrodechlorination (HDC) over Pd-based catalysts as a remediation technique has been impeded because of catalyst deactivation problems such as formation of carbonaceous deposits under the reductive environment of HDC. In this study, we investigated the use of a novel animated material, swellable organically-modified silica (SOMS), as a catalyst scaffold for HDC of trichloroethylene (TCE) to develop a catalytic system resistant to carbon formation. The state of aggregation of adsorbed TCE on Pd/SOMS was characterized. It was found that the unique nature of SOMS scaffold caused condensation of adsorbents in the SOMS matrix. This is of particular importance considering the fact that the increase of local concentration of reactants due to condensation may enhance the kinetics of catalytic reactions. To determine the resistance to the formation of carbonaceous materials under reaction conditions, in-situ vibrational spectroscopy experiments (diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and laser Raman spectroscopy) were undertaken over Pd-incorporated SOMS in the absence and presence of water vapor in the reactant stream. The commonly used HDC catalyst Pd/Al2O3 was also studied for comparison purposes. Formation of carbonaceous deposits of different nature were observed over Pd/Al2O3 whereas no detectable carbon formation was observed over Pd/SOMS. It was confirmed that surface hydroxyl groups which are in basic character act as coking agents. The carbon formation resistant behavior of Pd/SOMS is closely related to the nature and low concentration of surface hydroxyl groups

Swellable Organically Modified Silica (SOMS): A Novel Support for Aqueous Phase Hydrodechlorination of Trichloroethylene

This study focuses on the use of swellable organically modified silica (SOMS) as a catalyst support for aqueous-phase hydrodechlorination of trichloroethylene. SOMS is extremely hydrophobic and swells volumetrically when contacted with organics. These properties should help overcome the disadvantages associated with the commercial catalyst (Pd/Al2O3), such as deactivation by anions

Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions

Swellable-organically modified silica (SOMS) has been demonstrated to be an efficient catalyst scaffold for catalytic treatment of water contaminated with trichloroethylene (TCE). In this study, deactivation characteristics of Pd-incorporated SOMS for aqueous-phase hydrodechlorination (HDC) of TCE were investigated. Pd/SOMS catalysts were exposed to highly-concentrated chloride solutions (up to 1 M NaCl or 0.01 M HCl) to examine the deactivation resistant behavior of Pd/SOMS. The commonly used HDC catalyst Pd/Al2O3 was also studied for comparison purposes. Pd/SOMS and Pd/Al2O3 in their pristine and treated states were tested for aqueous-phase HDC of TCE and characterized by several techniques including N2 physisorption, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. The aqueous-phase treatments had a pronounced adverse effect on the textural properties of Pd/Al2O3, although the effect was independent of the type of the chloride precursor, NaCl or HCl. Treating Pd/Al2O3 with chloride-containing solutions lowered the catalytic activity due to formation of Pd-Cl complexes and active metal leaching. The leached Pd obtained from the treatment solution was shown to be inactive for aqueous-phase HDC of TCE. While Pd/Al2O3 underwent severe deactivation due to the chloride treatments, Pd/SOMS exhibited resistance to chloride deactivation and metal leaching. The chloride treatments did not impact the textural properties of Pd/SOMS. The achieved deactivation resistance was attributed to the novel characteristics of the SOMS support

Aqueous-Phase Hydrodechlorination of Trichloroethylene over Pd-Based Swellable Organically Modified Silica: Catalyst Deactivation Due to Sulfur Species

One of the problems of catalytic water treatment systems is that sulfur-containing species present in contaminated water have a detrimental effect on the catalytic performance because of strong interactions of sulfur species with active metal sites. In order to address these problems, our research has focused on developing a poison-resistant catalytic system by using a novel material, namely, swellable organically modified silica (SOMS), as a catalyst scaffold. Our previous investigations demonstrated that the developed system was resistant to chloride poisoning, active metal leaching, and carbon deposition under reaction conditions. This study examines the sulfur tolerance of the developed catalytic system for hydrodechlorination (HDC) of trichloroethylene (TCE) by subjecting Pd-incorporated samples to different sulfur species, including sulfates (SO42–), bisulfides (HS–), and hydrogen sulfide (H2S). The pristine and sulfur-treated catalysts were then tested for aqueous- and gas-phase HDC of TCE and characterized by several techniques, including N2 physisorption, X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS), and temperature-programmed reaction (TPrxn) with H2. The investigations were also performed on Pd/Al2O3, a commercially used HDC catalyst, to have a basis for comparison. The activity and characterization results revealed that Pd/Al2O3 underwent deactivation due to exposure to sulfur-containing compounds. Pd/SOMS, however, exhibited better resistance to aqueous sulfates, bisulfides, and gas-phase H2S. In addition, the removal of sulfur species from completely poisoned catalysts was found to be more facile in Pd/SOMS than Pd/Al2O3. The tolerance of Pd/SOMS to sulfur poisoning was attributed to stem from the novel characteristics of SOMS, such as swelling ability and extreme hydrophobicity

Wash-coat development for Lean-burn Engine-exhaust Aftertreatment: A Novel Way of Incorporation of Binder to Pd Supported on Sulfated Zirconia

As the environmental regulations are getting more stringent, it is of great importance to develop a catalytic aftertreatment system for natural gas-fired lean-burn engines to reduce the emissions of nitrogen oxides (NOx), unburned hydrocarbons (CH4, C2H6, and C3H8) and carbon monoxide efficiently. We have developed a dual-catalyst aftertreatment system which utilizes the hydrocarbons present in the exhaust streams of lean-burn engines for reducing NOxemissions. The dual-catalyst aftertreatment system is a physical mixture composed of a reduction catalyst, palladium supported on sulfated zirconia (Pd/SZ), and an oxidation catalyst, cobalt supported on ceria. Such a catalytic system which utilizes methane in the exhaust stream offers several advantages considering that the emissions of air-pollutant greenhouse gases are controlled in a single unit without a need of injecting and handling an external reducing agent such as ammonia [1-3]. Development of a catalytically active washcoat for monolith cores is essential for the practical use of this dual-catalyst system. However, if the washcoat adheres poorly to the monolith core, the aftertreatment unit will suffer from the irreversible loss of the catalytically active phase. In order to improve the adhesive properties, conventionally, binders are used in a catalyst slurry. In this study, we have aimed to improve the adhesive properties at the molecular level. For this purpose, alumina, as a binder, was incorporated in situ to the sol-gel medium of Pd/SZ prior to the gelation during synthesis. Samples prepared by this novel approach have shown superior differences in terms of catalytic performance than the samples prepared by the conventional method. Addition of binder before gelation to the sol-gel medium also resulted in changes of textural and structural properties of binder-free samples, and adhesive properties of the washcoat as shown by N2 physisorption, in-situ X-ray diffraction (XRD) calcination, 27Al-NMR, laser RAMAN and infrared spectroscopy, scanning electron microscopy (SEM) and ultrasonication, Water and sulfur tolerance as well as hydrothermal stability of the binder-incorporated catalyst will be presented as well