A comparative study of the thermal stability and reactions of CH2, CH3 and C2H5 species on the Pd(100) surface

Adsorbed CH₂, CH₂ and C₂H₅ moieties were produced on Pd(100) at 90 K by photoinduced dissociation of the corresponding iodo compounds, and their thermal reactions were established.This laboratory is a part of the Center of Catalysis, Surface and Material Science at the University of Szeged.     

HREELS study of photo-induced formation of CO2 anion radical on Rh(111) surface

High-resolution electron loss spectroscopy revealed, probably for the first time, that the illumination of adsorbed CO₂ on K-promoted Rh(111) induces or enhances formation of the CO₂ radical.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Adsorption and Reactions of CH2I2 on Clean and Oxygen-Modified Ag(111): a RAIRS and TPD Study

The surface chemistry of CH₂I₂ on Ag(111) in the presence and absence of pre-adsorbed O, produced by NO₂ adsorption at elevated temperature, has been examined using temperature-programmed desorption and reflection absorption infrared spectroscopy. There is good evidence for the formation of adsorbed methylene, CH₂(a), that reacts with another CH₂(a) to form and desorb ethylene, C₂H₄(g), in a reaction-limited process. Increasing the surface coverage of CH₂I₂ hinders both the dissociation and recombination processes indicated by the upward temperature shift in the formation of C₂H₄. Co-adsorbed O atoms strengthen the bonding of CH₂I₂ to the surface; the increased thermal stability is up to 60 K. The formation of C₂H₄ decreases with increasing amounts of pre-adsorbed O; the main reaction product is CH₂O produced in a reaction-limited process. CH₂O forms either on the chemisorbed or on the oxide phase with desorption peak temperatures of 225 and 270 K, respectively. The formation of gas-phase carbon dioxide suggests that a formate intermediate is involved in a secondary reaction pathway.     

Catalytic reaction of CH4 with CO2 over alumina-supported Pt metals

The dissociation of CH₄ and CO₂, as well as the reaction between CH₄ and CO₂ at 723–823 K have been studied over alumina supported Pt metals. In the high temperature interaction of CH₄ with catalyst surface small amounts of C₂H₆ were detected. In the reaction of CH₄+CO₂, CO and H₂ were produced with different ratios. The specific activities of the catalysts decreased in the order: Ru, Pd, Rh, Pt and Ir, which agreed with their activity order towards the dissociation of CO₂.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Hysteresis and oscillations in the selectivity during the NO-H2 reaction over Rh(533)

The NO-H₂ reaction over Rh(533) shows oscillatory behaviour at H₂-rich mixtures in the 10^–6 mbar pressure regime around 470 K. The selectivity changes periodically in time: the rate of N₂ formation is out of phase with the NH₃ and H₂O formation rates. Accumulation of atomic N plays a central role in the oscillating behaviour. A comparison will be made with the NO-H₂ reaction over Pt(100).     

Formation and reactions of CH3 species over Mo2C/Mo(111) surface

The reaction pathways of adsorbed CH₃ on the Mo₂C/Mo(111) surface were investigated by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). CH₃ fragments were produced by the dissociation of the corresponding iodo-compound. CH₃I adsorbs molecularly on Mo₂C at 90 K and dissociates at and above 140 K. The main products of the reaction of adsorbed CH₃ are hydrogen, methane and ethylene. The coupling into ethane was not observed. The results are discussed in relevance to the conversion of methane into benzene on Mo₂C deposited on ZSM-5. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photocatalytic reaction of H2O+CO2 over pure and doped Rh/TiO2

In the photocatalytic reduction of carbon dioxide to formic acid, formaldehyde and methanol in aqueous suspensions of TiO₂ and Rh/TiO₂, the effects of doping the TiO₂ with W⁶⁺ were investigated.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH3CH2NH3)PbI3

A new nanocrystalline sensitizer with the chemical formula (CH₃CH₂NH₃)PbI₃ is synthesized by reacting ethylammonium iodide with lead iodide, and its crystal structure and photovoltaic property are investigated. X-ray diffraction analysis confirms orthorhombic crystal phase with a = 8.7419(2) Å, b = 8.14745(10) Å, and c = 30.3096(6) Å, which can be described as 2 H perovskite structure. Ultraviolet photoelectron spectroscopy and UV-visible spectroscopy determine the valence band position at 5.6 eV versus vacuum and the optical bandgap of ca. 2.2 eV. A spin coating of the CH₃CH₂NH₃I and PbI₂ mixed solution on a TiO₂ film yields ca. 1.8-nm-diameter (CH₃CH₂NH₃)PbI₃ dots on the TiO₂ surface. The (CH₃CH₂NH₃)PbI₃-sensitized solar cell with iodide-based redox electrolyte demonstrates the conversion efficiency of 2.4% under AM 1.5 G one sun (100 mW/cm²) illumination.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Effects of potassium on the chemisorption of CO2 and CO on the Mo2C/Mo(100) surface

The interaction of CO₂ with K-promoted Mo₂C/Mo(100) has been studied with high-resolution electron energy loss spectroscopy, work function measurements and temperature-programmed desorption. Pre-adsorbed potassium dramatically affects the adsorption behavior of CO₂ on the Mo₂C/Mo(100) surface. It increases the rate of adsorption, the binding energy of CO₂ and it induces the dissociation of CO₂ through the formation of negatively charged CO₂. Potassium adatoms also promote the dissociation of adsorbed CO over Mo₂C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bimetallic catalysts as dissipative structures: stationary concentration patterns in the O2 + H2 reaction on a composite Rh(110)/Pt surface

The O₂ + H₂ reaction has been studied under low pressure conditions (10^-5 mbar) employing a microstructured Rh(110)/Pt surface as catalyst. Photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM) were used as spatially resolving in situ methods. Under reaction conditions stationary concentration patterns (Turing‐like structures) of the adsorbates develop inside the Pt domains which are associated with a compositional change of the metallic substrate. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of titania overlayers on C2H4/CO/H2 reactions over a Rh foil

The effects of submonolayer deposits of titania on the activity and selectivity of a Rh foil catalyst for C₂H₄/CO/H₂ reactions have been investigated. Reactions were carried out at 1 atm total pressure and at temperature of 488 K and 523 K. The addition of titania to the catalyst enhances the total rate of C₃-oxygenate formation. This rate enhancement is due entirely to an increase in the rate of 1-propanol formation, which reaches a maximum at a TiO _x . coverage of 0.2 ML. The rate of propanal formation, by contrast, is not enhanced. The rates of formation of methane, ethane, and C₃-hydrocarbons also exhibit rate maxima at a TiO _x . coverage of 0.2 ML. The rates of formation of C₄- and C₅-hydrocarbons, on the other hand, are suppressed by titania addition. The higher rate of 1-propanol production in the presence of titania is attributed to an interaction between Ti³⁺ ions at the edge of TiO _x . islands and the carbonyl bond of adsorbed C₃-oxygenated species. Such interactions are envisioned to facilitate hydrogenation of the carbonyl bond.     

TPD study of the reaction of CH3CH2SH and (CH3CH2)2S2 on ZnO(0001) and ZnO

Superior activation of on 1 wt% Pt/TiO₂ catalysts for the oxidation of CO was attained by loading a large amount of Fe-oxide (100 wt%) and TiO₂. In situ IR spectra of CO proved that the structural transformation is brought about on the Pt-sites by loading of Fe-oxide, where predominant Pt-sites giving linear CO change to highly reactive bridge CO Pt-sites. In contrast, no transformation of the linear CO sites to the bridge CO sites takes place by loading of TiO₂ but the environment of Pt-sites for linear CO is changed.     

Pressure swing adsorption separation of H2S/CO2/CH4 gas mixtures with molecular sieves 4A, 5A,and 13X

Pressure swing adsorption experiments were carried out for the separation of equimolar mixtures of carbon dioxide and methane containing small amounts of hydrogen sulfide, utilizing 4A, 5A, and 13X molecular sieves. High-purity methane of zero or nearly zero hydrogen sulfide concentration was produced in the adsorption stage with 13X and 5A sieves, at high product recovery rates; high-purity carbon dioxide was obtained with the same sieves in the desorption stage. Zeolite 4A was found capable of raising considerably the hydrogen sulfide concentration in the accumulated desorption product (vs. the adsorption feed) at high recovery rates too. Adsorption selectivity values derived from the experimental results for all three gas pairs were in line with some theoretical predictions and experimental data of the literature.     

Effective Coke Removal in Methane to Benzene (MTB) Reaction on Mo/HZSM-5 Catalyst by H2 and H2O Co-addition to Methane

The catalytic dehydro-aromatization reaction over Mo/HZSM-5 catalyst was drastically stabilized by the co-addition of 5.4% H₂ and 1.8% H₂O to methane feed at 750 °C, 0.3 MPa and methane space velocity of 3000 mL g⁻¹ h⁻¹, suppressing the coke formation effectively, compared with single hydrogen or steam addition.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

The intermolecular interaction studies in the complexes of isothiocyanic acid (HNCS) and its derivatives with H2S: a computational study

The computational investigations are carried out on the complexes of isothiocyanic acid (HNCS) and its derivatives with H₂S through MP2/aug-cc-PVTZ//MP2/aug-cc-PVDZ level. Five, four, three and four structures are located on the potential energy surface of the HNCS?H₂S, HSCN? H₂S, HCNS? H₂S and HSNC?H₂S heterodimers, respectively. The calculated results reveal that the most stable heterodimers among all heterodimers obtained for the HNCS?H₂S, HSCN?H₂S, HCNS?H₂S and HSNC?H₂S systems belong to HSCN?H₂S system. Therefore, HSCN?H₂S system has a key role in the atmosphere.