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Chemistry of NO2 on CeO2 and MgO: Experimental and theoretical studies on the formation of NO3 (2000)

Rodriguez, José A. ; Jirsak, Tomas

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 112 (2000), S. 9929-9939

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: In environmental catalysis the destruction or removal of nitrogen oxides (DeNOx process) is receiving a lot of attention. Synchrotron-based x-ray absorption near-edge spectroscopy, high-resolution photoemission, and first-principles density-functional calculations (DFT-GGA) were used to study the interaction of nitrogen dioxide with CeO2 and MgO. The only product of the reaction of NO2 with pure CeO2 at 300 K is adsorbed nitrate. The NO3 is a thermally stable species which mostly decomposes at temperatures between 450 and 600 K. For the adsorption of NO2 on partially reduced ceria (CeO2−x), there is full decomposition of the adsorbate and a mixture of N, NO, and NO3 coexists on the surface of the oxide at room temperature. Ce3+ cations can assist in the transformation of NO and NO2 in DeNOx operations. Adsorbed NO3 (main product) and NO2 are detected after exposing MgO to NO2 gas. A partial NO2,ads→NO3,ads transformation is observed on MgO(100) from 150 to 300 K. DFT-GGA calculations show strong bonding interactions for NO2 on Mg sites of this surface, and dicoordination via O, O is more favorable energetically than monocoordination via N. The NO2,ads species disappears from magnesium oxide at temperatures below 600 K, whereas part of the NO3,ads is stable up to temperatures near 800 K. MgO can be very useful as a sorbent for trapping NO2. A general trend is found after comparing the chemical behavior of NO2 on different types of oxides (CeO2, MgO, TiO2, Fe2O3, CuO, ZnO). On all these systems, the main product after adsorbing NO2 at 300 K is nitrate with minor amounts of chemisorbed NO2 and no signs of full decomposition of the adsorbate. This trend and the results of DFT-GGA calculations indicate that NO2 is very efficient for the nitration (i.e., formation of NO3 as a ligand) of metal centers that are missing O neighbors in oxide surfaces. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.481629

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Interaction of sulfur with Pt(111) and Sn/Pt(111): Effects of coverage and metal–metal bonding on reactivity toward sulfur (2000)

Rodriguez, José A. ; Hrbek, Jan ; Kuhn, Mark ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 113 (2000), S. 11284-11292

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: In the chemical and petrochemical industries, Pt-based catalysts are very sensitive to sulfur poisoning. Synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy (TDS), and first-principles density-functional slab calculations were used to study the adsorption of sulfur on Pt(111) and a p(2×2)-Sn/Pt(111) surface alloy. Our results show important variations in the nature of the bonding of sulfur to Pt(111) depending on the coverage of the adsorbate. For small coverages, θS〈0.3 ML, atomic sulfur is the most stable species. The adsorbate is bonded to hollow sites, has a large adsorption energy (〉75 kcal/mol), and desorbs as S. The Pt–S bonds are mainly covalent but sulfur induces a significant decrease in the density of Pt 5d states near the Fermi level. When the sulfur coverage increases on the surface, θS〉0.4 ML, there is a substantial weakening in the Pt↔S interactions with a change in the adsorption site and a tendency to form S–S bonds. Desorption of S2 is now observed in TDS and the S2p core levels shift to higher binding energy. At coverages near a full monolayer, S2 is the most stable species on the surface and its adsorption energy is ∼45 kcal/mol. Similar trends are observed for the adsorption of sulfur on a p(2×2)-Sn/Pt(111) surface alloy, but the adsorbate↔substrate interactions are weaker than on Pt(111). The formation of Pt–Sn bonds reduces the reactivity of Pt toward sulfur. Electronic effects associated with bimetallic bonding can be useful for controlling or preventing sulfur poisoning. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1327249

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Reaction of S2 and H2S with Sn/Pt(111) surface alloys: Effects of metal–metal bonding on reactivity towards sulfur (1998)

Rodriguez, José A. ; Chaturvedi, Sanjay ; Jirsak, Tomas ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 109 (1998), S. 4052-4062

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a ((square root of 3)×(square root of 3))R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. At 100–300 K, S2 chemisorbs and reacts on polycrystalline tin to form metal sulfides. The reactivity of pure tin toward sulfur is large even at a temperature as low as 100 K. In contrast, tin atoms in contact with Pt(111) interact weakly with S2 or H2S. Tin does not prevent the bonding of S to Pt in a ((square root of 3)×(square root of 3))R30°-Sn/Pt(111) surface alloy, but the alloy is less reactive toward H2S than polycrystalline Sn or pure Pt(111). At room temperature, S2 and H2S adsorb dissociatively on Pt sites of ((square root of 3)×(square root of 3))R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to ((square root of 3)×(square root of 3))R30°-Sn/Pt(111), one sees the formation of only a chemisorbed layer of sulfur (i.e., no sulfides of tin or platinum are formed). The Pt–Sn bond is complex, involving a Sn(5s,5p)→Pt(6s,6p) charge transfer and a Pt(5d)→Pt(6s,6p) rehybridization that localize electrons in the region between the metal centers. These phenomena reduce the electron donor ability of Pt and Sn, and the metals are not able to respond in an effective way to the presence of species that are strong electron acceptors like S2, HS, and S. The redistribution of charge produces surfaces that have a remarkable low reactivity toward sulfur. When compared to other admetals (Cu, Zn, Ag, Au), tin is the best choice as a site blocker that can enhance the tolerance of Pt reforming catalysts to sulfur poisoning. The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that are less sensitive to the presence of S-containing molecules.© 1998 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.477005

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Reaction of S2 and SO2 with Pd/Rh(111) surfaces: Effects of metal–metal bonding on sulfur poisoning (1999)

Rodriguez, José A. ; Jirsak, Tomas ; Chaturvedi, Sanjay

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 110 (1999), S. 3138-3147

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The surface chemistry of S2 and SO2 on Rh(111), Pd/Rh(111) and polycrystalline Pd has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. Pd adatoms lead to an increase in the rate of adsorption of S2 on Rh(111), but they are less reactive than atoms of pure metallic palladium: Rh(111)〈Pd/Rh(111)〈Pd. The adsorption of sulfur induces a large reduction in the density of states (DOS) near the Fermi level of Pd/Rh(111) surfaces. The decrease in the DOS is smaller than in S/Pd(111) but bigger than in S/Rh(111). The chemistry of SO2 on Rh(111), Pd/Rh(111), and Pd is rich. At 100 K, SO2 adsorbs molecularly on these systems. Above 200 K, the adsorbed SO2 decomposes (SO2,a→Sa+2Oa) or transforms into SO3/SO4 species. The molecular SOx species disappear upon annealing to 450 K and only atomic S and O remain on the surfaces. A Pd monolayer supported on Rh(111) is not very active for the dissociation of SO2. In this respect, the Pd1.0/Rh(111) system is less chemically active than pure Pd or Rh(111). The electronic perturbations associated with the Pd–Rh bonds reduce the electron donor ability of Pd, weakening the interactions between the Pd 4d orbitals and the lowest unoccupied molecular orbitals of S2 and SO2. The behavior of the S2/Pd/Rh(111) and SO2/Pd/Rh(111) systems shows that bimetallic bonding can reduce the reactivity of Pd towards sulfur-containing molecules. A very large drop in reactivity can be expected when Pd is bonded to s,p or early transition metals. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.477910

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Reaction of H2S with MgO(100) and Cu/MgO(100) surfaces: Band-gap size and chemical reactivity (1999)

Rodriguez, José A. ; Jirsak, Tomas ; Chaturvedi, Sanjay

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 111 (1999), S. 8077-8087

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The interaction of H2S, SH, and S with MgO(100) and Cu/MgO(100) surfaces has been investigated using synchrotron-based high resolution photoemission and density functional calculations. Metallic magnesium reacts vigorously with H2S fully decomposing the molecule at temperatures below 200 K. In contrast, the Mg atoms in MgO exhibit a moderate reactivity. At 80 K, most of the H2S molecules (∼80%) chemisorb intact on a MgO(100) surface. Annealing to 200 K induces cleavage of S–H bonds leaving similar amounts of H2S and SH on the surface. The complete disappearance of H2S is observed at 300 K, and the dominant species on the oxide is SH which is coadsorbed with a small amount (∼10%) of atomic S. The adsorbed SH fully decomposes upon heating to 400 K producing S adatoms that are stable on the surface at temperatures well above 500 K. The results of density functional calculations indicate that the bonding interactions of SH and S with pentacoordinated Mg sites of a flat MgO(100) surface are strong, but the bonding of the H2S molecule is relatively weak. Defect sites probably play an important role in the dissociation of H2S. Cu adatoms facilitate the decomposition of H2S on MgO(100) by providing electronic states that are very efficient for interactions with the frontier orbitals of the molecule. The rate of H2S decomposition on MgO is substantially lower than those found on Cr3O4, Cr2O3, ZnO, and Cu2O. For these systems, the smaller the band-gap in the oxide, the bigger its reactivity towards H2S. Theoretical calculations indicate that this trend reflects the effects of band–orbital mixing. The electrostatic interactions between the dipole of H2S and the ionic field generated by the charges in an oxide play only a secondary role in the adsorption process. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.480141

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Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies (1999)

Rodriguez, José A. ; Jirsak, Tomas ; Freitag, Andrea ; [et al.]

Springer

Catalysis letters 62 (1999), S. 113-119

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ISSN: 1572-879X

Keywords: cerium oxides ; SO2 ; desulfurization ; copper ; X‐ray absorption spectroscopy ; photoemission and XPS

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology

Notes: Abstract CeO2 and Cu/CeO2 are effective catalysts/sorbents for the removal or destruction of SO2. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO2 with pure and reduced CeO2 powders, ceria films (CeO2, CeO2−x, Ce2O3+x) and model Cu/CeO2 catalysts. The results of XANES and photoemission provide evidence that SO4 was formed upon the adsorption of SO2 on pure powders or films of CeO2 at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO2 and some SO3 evolving into gas phase. At 670 K, there was still a significant amount of SO4 present on the CeO2 substrates. The introduction of O vacancies in the CeO2 powders or films favored the formation of SO3 instead of SO4. Ceria was able to fully dissociate SO2 to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO2 new active sites for the destruction of SO2 were created improving the catalytic activity of the system. The surface chemistry of SO2 on the Cu‐promoted CeO2 was much richer than on pure CeO2. The behavior of ceria in several catalytic processes (oxidation of SO2 by O2, reduction of SO2 by CO, automobile exhaust converters) is discussed in light of these results.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1019007308054

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