Tomas Jirsak

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

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.

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

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 i...

Role of Au-C interactions on the catalytic activity of au nanoparticles supported on TiC(001) toward molecular oxygen dissociation.

High-resolution photoemission and density functional calculations on realistic slab surface models were used to study the interaction and subsequent dissociation of O(2) with Au nanoparticles supported on TiC(001). The photoemission results indicate that at 150 K O(2) adsorbs molecularly on the supported gold nanoparticles, and upon heating to temperatures above 200 K the O(2) --> 2O reaction takes place with migration of atomic oxygen to the TiC(001) substrate. The addition of Au to TiC(001) substantially enhances the rate of O(2) dissociation at room temperature. The reactivity of Au nanoparticles supported on TiC(001) toward O(2) dissociation is much larger than that of similar nanoparticles supported either on TiO(2)(110) or MgO(001) surfaces, where the cleavage of O-O bonds is very difficult. Density functional calculations carried out on large supercells show that the contact of Au with TiC(001) is essential for charge polarization and an enhancement in the chemical activity of Au. Small two-dimensional particles which expose Au atoms in contact with TiC(001) are the most reactive. While O(2) prefers binding to Au sites, the O atoms interact more strongly with the TiC(001) surface. The oxygen species active during the low-temperature (<200 K) oxidation of carbon monoxide on Au/TiC(001) is chemisorbed O(2). Once atomic O binds to TiC(001), the chemisorption bond is so strong that temperatures well above 400 K are necessary to remove the O adatoms from the TiC(001) substrate by direct reaction with CO. The high reactivity of Au/TiC(001) toward O(2) at low-temperature opens the route for the transformation of alcohols and amines on the supported Au nanoparticles.

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

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 bon...

Fundamental studies of desulfurization processes: reaction of methanethiol on ZnO and Cs/ZnO

The reaction of methanethiol on ZnO and Cs promoted ZnO surfaces has been studied with synchrotron based photoemission and thermal desorption spectroscopy. On ZnO, methanethiol undergoes selective reaction to produce carbon monoxide (37–58%), methane (23–38%), formaldehyde (12–15%), ethane (1–11%), and a mixture of ethylene and acetylene (3–13%). At low temperatures (<100 K), methanethiol reacts to yield thiolate intermediate bound to Zn2+ cations. The thiolate is stable to 500 K. Above this temperature, C–S bond cleavage occurs to yield methyl intermediate and atomic S. Carbon is removed from the surface as gaseous products above 500 K, and atomic sulfur remains bound to the zinc sites of the surface. Submonolayer amounts of cesium do not have a significant promotional effect on C–S bond cleavage, whereas Cs multilayers are found to significantly lower the activation barrier for C–S bond cleavage. This study illustrates the chemistry associated with the desulfurization of thiols on a catalytically relevant oxide surface.

The interaction of oxygen with TiC(001): photoemission and first-principles studies.

High-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of oxygen with a TiC(001) surface. Atomic oxygen is present on the TiC(001) substrate after small doses of O(2) at room temperature. A big positive shift (1.5-1.8 eV) was detected for the C 1s core level. These photoemission studies suggest the existence of strong O<-->C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of O. Ti and C atoms are involved in the adsorption and dissociation of the O(2) molecule. In general, the bond between O and the TiC(001) surface contains a large degree of ionic character. The carbide-->O charge transfer is substantial even at high coverages (>0.5 ML) of oxygen. At 500 K and large doses of O(2), oxidation of the carbide surface occurs with the removal of C and formation of titanium oxides. There is an activation barrier for the exchange of Ti-C and Ti-O bonds which is overcome only by the formation of C-C or C-O bonds on the surface. The mechanism for the removal of a C atom as CO gas involves a minimum of two O adatoms, and three O adatoms are required for the formation of CO(2) gas. Due to the high stability of TiC, an O adatom alone cannot induce the generation of a C vacancy in a flat TiC(001) surface.

Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S-O bonds

Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilitates the cleavage of the S–O bonds. In this article, we explore three different approaches (formation of O vacancies, promotion with alkali metals, and doping with transition metals) that lead to the activation of SO2 and S–O bond breaking on MgO and oxides in general. Basic principles for a rational design of catalysts with a high efficiency for the destruction of SO2 are presented.Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilita...

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

The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a (∛×∛)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 (∛×∛)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 (∛×∛)R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to (∛×∛)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) rehybri...

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

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 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 TD...

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

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...