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Exhaust gas aftertreatment systems play a key role in controlling transportation greenhouse gas emissions. Modern aftertreatment systems, often based on Pd metal supported on aluminum oxide, provide high catalytic activity but are vulnerable to sulfur poisoning due to formation of inactive sulfate species. This paper focuses on regeneration of Pd-based catalyst via the decomposition of alumina-supported aluminum and palladium sulfates existing both individually and in combination. Decomposition experiments were carried out under hydrogen (10% H2/Ar), helium (He), low oxygen (0.1% O2/He), and excess oxygen (10% O2/He). The structure and composition of the model catalysts were examined before and after the decomposition reactions via powder X-ray diffraction and elemental sulfur analysis. The study revealed that individual alumina-supported aluminum sulfate decomposed at a higher temperature compared to individual alumina-supported palladium sulfate. The simultaneous presence of aluminum and palladium sulfates on the alumina support decreased their decomposition temperatures and led to a higher amount of metallic palladium than in the corresponding case of individual supported palladium sulfate. From a fundamental point of view, the lowest decomposition temperature was achieved in the presence of hydrogen gas, which is the optimal decomposition atmosphere among the studied conditions. In summary, aluminum sulfate has a two-fold role in the regeneration of a catalyst—it decreases the Pd sulfate decomposition temperature and hinders re-oxidation of less-active metallic palladium to active palladium oxide.

In the present research, an industrially aged Pt/Al 2 O 3 catalyst was used as a basis for the study on the sulfur deactivation and the development of more resistant catalytic materials. The catalytic activities of both industrially and laboratory-aged materials in DMDS oxidation were studied in addition to characterization by XRD, XPS, FESEM, TEM and N 2 adsorption. The industrial ageing induced a phase change from γ-Al 2 O 3 towards θ-Al 2 O 3 , formation of aluminum sulfates and an increase in Pt particle size as well as a change in the oxidation state of Pt to a higher state. These changes caused an increase of 30 °C in the light-off temperature for DMDS oxidation. Accelerated ageing in the presence of SO 2 and H 2 O vapor at 400 °C for 5 h decreased the activity of the Pt/Al 2 O 3 at the same level than for the industrially aged catalyst even though smaller sulfur content and no sintering of γ-Al 2 O 3 were observed. Pt sintering (10–20 nm) in both cases was observed. The XPS results confirmed the formation of new sulfate phases and the interaction between sulfur and the active phase as well as the support of the catalyst undergone accelerated ageing. After the accelerated ageing of copper-based catalysts, the 0.3Pt10Cu/Al 2 O 3 ǀ 0.8 SiO 2 ǀ 0.2 catalyst showed an interesting resistance towards sulfur deactivation, as it was expected. [ABSTRACT FROM AUTHOR]

La 2 O 3 /γ–Al 2 O 3 , NiO/γ–Al 2 O 3 and NiO/La 2 O 3 /γ–Al 2 O 3 samples have been prepared by conventional impregnation, using silica-free γ -Al 2 O 3 support. The materials have been characterized, as such or after reaction, with XRD, skeletal IR, UV–vis–NIR, XPS and FESEM techniques. The catalytic activity has been evaluated in ethanol decomposition through temperature programmed surface reaction (TPSR); and in ethanol steam reforming (ESR); and in mixed ethanol and phenol steam reforming (EPSR as a model reaction for biomass tar steam reforming) in a continuous flow reactor. Ni on alumina exists as a surface Ni x Al 2 O 3+ x spinel, evident by XRD, skeletal IR and vis spectroscopy measurements. La disperses on alumina in a disordered state. In the ternary system, XPS reveals significant Ni–La interactions. The addition of some lanthanum further increases the activity of Ni/Al 2 O 3 for ESR and EPSR. Fresh unreduced catalysts are conditioned in the feed at temperatures above 973 K. Conditioned catalysts give rise to full conversion of reactants in ESR and EPSR at 873 K and higher temperatures, but are severely deactivated by sulfur. The sudden start of the steam reforming activity at 873 K likely corresponds to the temperature onset for the activation of water by metallic nickel. [ABSTRACT FROM AUTHOR]

Practical copper (Cu)-based catalysts for the water-gas shift (WGS) reaction was long believed to expose a large proportion of Cu(110) planes. In this work, as an important first step toward addressing sulfur poisoning of these catalysts, the detailed mechanism for the splitting of hydrogen sulfide (H2S) on the open Cu(110) facet has been investigated in the framework of periodic, self-consistent density functional theory (DFT-GGA). The microkinetic model based on the first-principles calculations has also been developed to quantitatively evaluate the two considered decomposition routes for yielding surface atomic sulfur (S*): (1) H2S → H2S* → SH* → S* and (2) 2H2S → 2H2S* → 2SH* → S* + H2S* → S* + H2S. The first pathway proceeding through unimolecular SH* dissociation was identified to be feasible, whereas the second pathway involving bimolecular SH* disproportionation made no contribution to S* formation. The molecular adsorption of H2S is the slowest elementary step of its full decomposition, being related with the large entropy term of the gas-phase reactant under realistic reaction conditions. A comparison of thermodynamic and kinetic reactivity between the substrate and the close-packed Cu(111) surface further shows that a loosely packed facet can promote the S* formation from H2S on Cu, thus revealing that the reaction process is structure sensitive. The present DFT and microkinetic modeling results provide a reasonably complete picture for the chemistry of H2S on the Cu(110) surface, which is a necessary basis for the design of new sulfur-tolerant WGS catalysts. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: The steam reforming of 85% pure ethanol, 15% gasoline (E85) with and without sulfur was studied over a bimetallic precious metal (Rh/Pt) catalyst deposited on a ceramic monolith. Tests performed at low space velocities (22,000h−1) confirmed that the catalyst could achieve 100% ethanol and gasoline conversion to equilibrium concentrations of H2, CO, CO2 and CH4 with no signs of deactivation for at least 110h reforming a sulfur-free E85 fuel. In the presence of 5ppm sulfur the catalyst maintained 100% ethanol and 100% gasoline conversion for approximately 22h before rapid deactivation resulted in ethanol conversion values below 21%. TPO analysis established large carbon deposits had formed on the catalyst surface demonstrating that sulfur promoted carbon formation. Following such extensive deactivation full activity was recovered after treating the catalyst with air; however subsequent deactivation occurred more rapidly indicating that some amount of permanent damage had occurred. A process with preemptive regeneration via air treatment was studied and it was found to extend the period of stable activity. [Copyright &y& Elsevier]

• First investigation of the impact of H 2 S on chemical looping hydrogen production. • Strong temperature dependency of hydrogen sulfide chemisorption. • ICP-EOS analysis confirmed the chemisorption of sulfur on oxygen carrier material. • Significant sulfur contamination of product hydrogen in the oxidation phase. • Fulfillment of the overall sulfur mass balance within a mean deviation of 3.7%. Chemical looping with iron-based oxygen carriers enables the production of hydrogen from various fossil and biogenic primary energy sources. In applications with real producer gases, such as biogas or gasified biomass, hydrogen sulfide represents one of the most challenging contaminants. The impact of H 2 S on the reactivity of a Fe 2 O 3 /Al 2 O 3 oxygen carrier material in chemical looping hydrogen production was investigated in the present work. First, potential sulfur deactivation mechanisms are discussed in detail on the basis of thermodynamic data. Afterwards, an experimental study in a fixed-bed reactor system gave experimental evidence on the fate of sulfur in chemical looping hydrogen systems. The chemisorption of hydrogen sulfide (H 2 S) was identified as the main cause for the accumulative adsorption of H 2 S in the reduction phase and was confirmed by ex-situ ICP-EOS analysis. In the subsequent steam oxidation step, significant quantities of H 2 S were released resulting in an undesirable contamination of the hydrogen product gas. The reason was found as weakened sulfur bonds through increasing reactor temperatures caused by the exothermic oxidation reactions. In additional air oxidation steps no further contaminants as sulfur dioxide were identified. A profound interpretation was achieved through the fulfillment of the overall sulfur mass balance within a mean deviation of 3.7%. Quantitative investigations showed that the hydrogen consumption decreased by 12% throughout the reduction phase in the event of 100 ppm H 2 S in the feed gas. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we investigated the catalytic activity of 2% Pd/γ–Al2O3 and 2% Pd–1% Sn/γ–Al2O3 for CH4 oxidation in lean conditions in the presence and in the absence of SO2 in the reaction feed. The catalysts were studied by the Pd3d 5/2 electron binding energy values determined by XPS analysis. Sulfates formation and/or tin addition to Pd/Al2O3 resulted in an increase of the Pd3d 5/2 electron binding energy. Results showed a direct relation between Pd activity for CH4 oxidation and the degree of oxidation of Pd species. [Copyright &y& Elsevier]

Pt/Al2O3 and Pt/BaO/Al2O3 catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR and XPS studies of sulfur removal from Pt/Al2O3 and Pt/BaO/Al2O3 catalysts indicate that the sulfur removal from Al2O3 surface precedes reductive decomposition of BaSO4 (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface atomic ratio Ba:S = 1 (~450o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing conditions. [ABSTRACT FROM AUTHOR]

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO x storage sites are regenerated when H2 is used in combination with CO2 compared to H2 only. [ABSTRACT FROM AUTHOR]

Abstract: Flow reactor experiments were performed to study the effect of SO2 +O2 and SO2 +H2 exposures on the NO x storage performance of Pt/BaO/Al2O3 and BaO/Al2O3 catalysts. In addition, Pt/SiO2 samples were used to study the interaction between Pt and SO2 under the two exposure conditions. For BaO/Al2O3 the two SO2 exposure conditions caused similar deactivation of the NO x storage capacity, whereas for Pt/BaO/Al2O3 the decline in the NO x storage capacity was faster during the SO2 +H2 exposure than for SO2 +O2 exposure. The presence of Pt enhances the adsorption of SO2 for both SO2 exposure conditions. Quantitative analysis of sulfur showed that exposure to SO2 +O2 caused higher accumulation of sulfur in Pt/BaO/Al2O3 samples in comparison with SO2 +H2 exposure. Thus there is no correlation between the total amount of adsorbed sulfur in the samples and the reduction of the NO x storage capacity, probably because of the adsorption of sulfur on sites not necessarily important for the NO x storage process. [Copyright &y& Elsevier]

Abstract: The selective catalytic reduction (SCR) of NO with C3H8 was studied over unsulfated and pre-sulfated 1% Pt/γ-Al2O3, and 1% Pt-2% Sn/γ-Al2O3 catalysts. Carbon residues deposited were evaluated on all samples, after 12h of C3H8-NO-O2-SO2 reaction. X-ray photoelectron spectra (XPS) were recorded over unsulfated and pre-sulfated fresh samples. Pre-sulfated 1% Pt-2% Sn/γ-Al2O3 catalyst showed a high resistance to deactivation by sulfur during C3H8-NO-O2-SO2 reactions. Results are explained on basis of the effect of tin addition to 1% Pt/γ-Al2O3 that results in (i) lowering the sulfates generated over the surface of the catalyst during sulfation; and (ii) preventing Pt particles from sintering during sulfation process. We also investigated the effect of Sn addition to 1% Pt/γ-Al2O3 catalyst, and the effect of pre-sulfation on 1% Pt/γ-Al2O3 and 1% Pt-2% Sn/γ-Al2O3, on carbon depositions over the catalysts. Results showed a strong diminution of carbon depositions on sulfated 1% Pt-2% Sn/γ-Al2O3 due to Pt-Sn surface interactions. The facts that tin addition to a Pt/γ-Al2O3 catalyst, prevents Pt particles from sintering during high temperature sulfation and that sulfating a 1% Pt-2% Sn/γ-Al2O3 catalyst prevents carbon residues deposition over the catalyst, suggest that this catalyst could resist high temperature reactions during automotive exhaust control without deactivation. [Copyright &y& Elsevier]