Characterization of SO\u3csub\u3e2\u3c/sub\u3e-Poisoned Ceria-Zirconia Mixed Oxides

CeO2, ZrO2, and a series of CexZr1-xO2 catalysts with 1 wt% Pd were exposed to fixed exposures of SO2 under oxidizing environments and then characterized by FTIR, pulse-reactor studies with CO and O2, and temperature-programmed desorption (TPD). For exposures above 473 K, sulfates were formed on all of the materials; however, the results are consistent with the formation of bulk sulfates on CeO2 and only surface sulfates on ZrO2. For the mixed oxides, the quantity of sulfates formed at 673 K increased linearly with the Ce content. In TPD, the sulfates on ZrO2 were stable to higher temperatures than those formed on CeO2, which decomposed in a well-defined peak between 900 and 1050 K. The sulfates on both oxides were reduced by CO above 750 K. Even though XRD patterns for the mixed oxide were significantly different from that of the physical mixture, the TPD and pulse-reactor results were similar to what would be expected for physical mixtures of CeO2 and ZrO2, suggesting that sulfate species are associated with individual metal cations. Finally, pulse-reactor studies with CO and O2 at 873 K show that the sulfates can be reversibly reduced and oxidized on both CeO2 and ZrO2, so that sulfur poisoning gives rise to an apparent increase in oxygen storage, demonstrating that this method is not acceptable for measurement of this quantity

A Mechanistic Study of Sulfur Poisoning of the Water-Gas-Shift Reaction Over Pd/Ceria

The effect of sulfur on the water-gas-shift (WGS) activity of Pd/ceria catalysts has been studied using steady-state rate measurements, pulse-reactor studies, and FTIR. After exposing Pd/ceria to SO2 at 673 K in an oxidizing environment, the WGS rates dropped to a value close to that observed on Pd/alumina. Both pulse-reactor and FTIR measurements showed that cerium sulfates can be readily reduced by CO and re-oxidized by O2 at 723 K; however, unlike reduced ceria, the Ce2O2S formed by reduction of the sulfates cannot be re-oxidized by H2O or CO2. The implications of these measurements for understanding oxygen-storage capacity (OSC) of three-way catalysts are discussed

An investigation of NO\u3csub\u3ex \u3c/sub\u3estorage on Pt–BaO–Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

A series of samples containing 5-wt% or 20-wt% BaO on γ-Al2O3 with different loadings of Pt were prepared and examined for their NO2 adsorption properties using temperature programmed desorption (TPD), temperature programmed reduction (TPR), and x-ray diffraction (XRD). For calcination at 873 K or above, BaO/Al2O3 formed BaAl2O4. While carbonates were found to be unstable on the aluminate phase, NO2 reacted with the aluminate to form bulk Ba(NO3)2 and Al2O3, even at room temperature. With BaO/Al2O3, reaction to form the nitrate required slightly higher temperatures because of the need to displace CO2; however, pulsing NO2 over pure Ba(CO3) showed rapid reaction to form CO2 and NO in the gas phase, along with Ba(NO3)2, at 673 K. The decomposition temperature for Ba(NO3)2 shifted by more than 100 degrees when TPD was carried out in vacuum rather than in a carrier gas, showing that re-equilibration with the gas phase is important in the decomposition process. The addition of Pt had a minimal effect on the thermal stability of the nitrates but was essential for the reduction of the nitrate in H2. Since a relatively small amount of Pt was sufficient to cause the complete reduction of the Ba(NO3)2 phase at temperatures below 400 K, it appears that the nitrates must be extremely mobile within the Ba-containing phase. Finally, trapping studies of NO2 at 573 K, with or without 10% CO2 in the gas phase, showed no measurable difference between BaO/Al2O3 and BaAl2O4, with or without CO2

Deactivation of the Water-Gas-Shift Activity of Pd/Ceria by Mo

The effect of surface Mo on the water-gas-shift (WGS) activity of Pd/ceria was studied. A series of 1-wt% Pd catalysts, with varying Mo content, were prepared from supports obtained by aqueous impregnation of (NH4)2MoO4 onto ceria. Rates were found to decrease linearly with Mo coverage up to 1.8 Mo/nm2 and were 10% of that on Pd/ceria after the addition of this amount of Mo. TPD studies with 2-propanol on the Mo-containing ceria demonstrate a relationship between the loss of WGS activity and ceria sites that decompose the alcohol to propene and water. FTIR measurements suggest that Mo ions exchange with surface hydroxyls on ceria and that carbonates are not formed on ceria surfaces that have 1.8 Mo/nm2. The results from CO-O2 pulse measurements suggest that the Mo-containing surface is much harder to reduce than pure ceria. Raman spectra of the Mo-containing ceria show features associated with molybdena only for Mo coverages greater than 1.8 Mo/nm2. The implications of these results for understanding WGS activity on Pd/ceria are discussed

An Examination of Sulfur Poisoning on Pd/Ceria Catalysts

The species formed by exposure of a Pd/ceria catalyst to SO2 under various conditions have been studied using temperature-programmed desorption (TPD) and FTIR. For adsorption of SO2 between 298 and 473 K, a molecular SO2 species adsorbs on the surface, possibly as a surface sulfite; and this species converts to a sulfate above 473 K. Exposure of Pd/ceria to SO2 at temperatures above 473 K in the presence of O2 results in the formation of bulk sulfates. These sulfates decompose to form SO2 and O2 upon TPD in He, with O2 and SO2 peaks at 1023 K assigned to Ce+4 sulfates and peaks at 1123 K assigned to Ce+3 sulfates. When H2 is present in the TPD carrier gas, the sulfates are reduced and a significant fraction of the sulfur is removed as H2S, with the rest remaining as Ce2O2S. When CO is present in the TPD carrier gas, all of the sulfates are reduced to Ce2O2S, with the simultaneous formation of CO2. The formation of CO2 from the reduction of the sulfate occurs in the same temperature range as CO2 production from reduction of Pd/ceria, except that more CO2 is formed from the sulfur-poisoned catalyst. The implications of these results for understanding oxygen storage capacity (OSC) in automotive, three-way catalysts is discussed

Ultra-Thin CeO\u3csub\u3e2\u3c/sub\u3e Overlayer on YSZ Studied by X-Ray Surface Scattering

Transition metal catalysts such as Rh/Pt used in a three-way automotive catalytic converter have to perform reduction and oxidation functions at the same time. This can be accomplished only in a specific range of oxygen pressure and temperature. In order to maintain a constant partial pressure of oxygen in the vicinity of catalysts mixtures of ceria and zirconia are used. Ceria is an essential component due to its capability of storing oxygen under oxidizing and releasing oxygen under reducing conditions. However, this function deteriorates with time and eventually a catalytic converter stops working properly. It is not well understood why this particular mixture of oxides can achieve the role as a oxygen buffer and why its lifetime is limited. In order to address this issue and to understand the structural interplay at the ceria/zirconia interface, we studied the atomic structure of ultra-thin ceria layers deposited on single crystals of (001) oriented Y-stabilized zirconia (YSZ), in situ, during annealing in air using the synchrotron x-ray surface diffraction technique

Recreation on Federal Lands

This report discusses recreation in federal lands that covers approximately 653 million acres of federally-owned land in the United States. This preservation/use dichotomy, while varying among agencies, is a focal point for debate over recreation on federal lands

Nano-Socketed Nickel Particles with Enhanced Coking Resistance Grown \u3cem\u3ein situ\u3c/em\u3e by Redox Exsolution

Metal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic spplications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal-oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimenstion for tailoring particle-substrate interactions in the context of increasing interest for emergent interfactial phenomena