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.     

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.     

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.     

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.     

Trapping of CH3O formed from CO + H2

Methoxy formed on Al₂O₃ from¹³CO and H₂ coadsorption on Ni/Al₂O₃ was trapped by C₂H₅OH adsorption and temperature-programmed reaction (TPR). The presence of excess C₂H₅OH significantly increases the rate of¹³CH₃OH and (¹³CH₃)₂O formation. The¹³CH₃OH forms by the reaction of C₂H₅OH with¹³CH₃O on Al₂O₃. In the absence of C₂H₅OH,TPR following¹³CO and H₂ coadsorption did not produce significant amounts of¹³CH₃OHor(¹³CH₃)₂O.     

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.     

Marked Role of Carbon Monoxide in the Formation of Benzene During CH4-CO Reaction Over Rh/SiO2 Catalysts

Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H₂-TPR), carbon monoxide (CO-TPR) or methane (CH₄-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH₄-CO reaction over SiO₂-supported Rh catalysts. The steady-state surface for the CH₄-CO reaction was different from that of the CH₄ decomposition reaction. The existence of benzene-like adsorbed species as building blocks was demonstrated on the CH₄-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH₄ decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH₄ decomposition surface, which was not the case from the CH₄-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH₄-CO reaction.     

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.     

Catalytic Conversion of N2O to N2 over Metal-Based Catalysts in the Presence of Hydrocarbons and Oxygen

The catalytic conversion of N₂O to N₂ in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al₂O₃, SiO₂, TiO₂, ZrO₂ and calcined hydrotalcite MgAl₂(OH)₈·H₂O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on -alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH₄, C₃H₆, C₃H₈) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N₂O to N₂ was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO₂ and H₂O in the feed. A shift of the N₂O conversion versus temperature curve to higher temperatures was observed when SO₂ and H₂O were added, separately or simultaneously, to the feed. The inhibition caused by SO₂ was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H₂O and N₂O on the same active sites.     

Role of support in reforming of CH4 with CO2 over Rh catalysts

The reforming of CH₄ with CO₂ over supported Rh catalysts has been studied over a range of temperatures (550–1000 K). A significant effect of the support on the catalytic activity was observed, where the order was Rh/Al₂O₃>Rh/TiO₂>Rh/SiO₂. The catalytic activity of Rh/SiO₂ was promoted markedly by physical mixing of Rh/SiO₂ with metal oxides such as Al₂O₃, TiO₂, and MgO, indicating a synergetic effect. The role of the metal oxides used as the support and the physical mixture may be ascribed to the promotion in dissociation of CO₂ on the surface of Rh, since the CH₄ + CO₂ reaction is first order in the pressure of CO₂, suggesting that CO₂ dissociation is the rate-determining step. The possible model of the synergetic effect was proposed.     

CO hydrogenation over a RhVO4/SiO2 catalyst after H2 reduction

The hydrogenation of CO over a RhVO₄/SiO₂ catalyst has been investigated after H₂ reduction at 773 K. A strong metal–oxide interaction (SMOI) induced by the decomposition of RhVO₄ in H₂ enhanced not only the selectivity to C₂ oxygenates but also the CO conversion drastically, compared with an unpromoted Rh/SiO₂ catalyst. The selectivity of the RhVO₄/SiO₂ catalyst was similar to those of conventional V₂O₅‐promoted Rh/SiO₂ catalysts (V₂O₅–Rh/SiO₂), but the CO dissociation activity (and TOF) was much higher than for V₂O₅–Rh/SiO₂, and hence the yield of C₂ oxygenates was increased. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption of CH3 and its reactions with CO2 over TiO2

The adsorption, decomposition of CH₃ and its reactions with CO₂ were followed by means of Fourier transform infrared spectroscopy combined with mass spectrometry. Methyl radicals were produced by the pyrolysis of azomethane. Absorption bands, observed at room temperature adsorption, were attributed to adsorbed CH₃ and CH₃O species. The decomposition of adsorbed CH₃ in vacuum started above 400 K and was accelerated by CO₂. In the study of the interaction of methane with titania, activated in different ways, we found no convincing spectroscopic evidence for the activation of methane at 300 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Generation of alkyl fragments on Rh/SiO2 and VO x -promoted Rh/SiO2

The adsorption of ethylene on Rh/SiO₂ and Rh-VO _x /SiO₂ has been studied by transmission FTIR spectroscopy. For C₂H₄ chemisorption at 298 K on Rh/SiO₂ molecularly adsorbed C₂H₄ and ethylidyne species are identified. It has been found that ethylidyne can be hydrogenated toward surface ethyl species in a reversible reaction at room temperature. On Rh-VO _x /SiO₂ the fragmentation of C₂H₄ into methylene species has been observed during C₂H₄ adsorption at 298 K.On leave from Zentralinstitut für Physikalische Chemie, Rudower Chaussee 5, O-1199 Berlin, Germany.     

Electrochemical promotion of the CO2 hydrogenation reaction using thin Rh, Pt and Cu films in a monolithic reactor at atmospheric pressure

A monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au plate cells was used to investigate the hydrogenation of CO₂ at atmospheric pressure and temperatures 220–380 °C. The Rh/YSZ/Pt cells lead to CO and CH₄ formation and the open-circuit selectivity to CH₄ is less than 5%. Both positive and negative applied potentials enhance significantly the total hydrogenation rate but the selectivity to CH₄ remains below 12%. The Cu/TiO₂/YSZ/Au cells produce CO, CH₄ and C₂H₄ with selectivities to CH₄ and C₂H₄ up to 80% and 2%. Both positive and negative applied potential significantly enhance the hydrogenation rate and the selectivity to C₂H₄. It was found that the addition of small (0.5 kPa) amounts of CH₃OH in the feed has a pronounced promotional effect on the reaction rate and selectivity of the Cu/TiO₂/YSZ/Au cells. The selective reduction of CO₂ to CH₄ starts at 220 °C (vs 320 °C in absence of CH₃OH) with near 100% CH₄ selectivity at open-circuit and under polarization conditions at temperatures 220–380 °C. The results show the possibility of direct CO₂ conversion to useful products in a MEPR via electrochemical promotion at atmospheric pressure.     

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.     

C2H2 gas sensor based on Ni-doped ZnO electrospun nanofibers

Pure and Ni-doped ZnO nanofibers were synthesized using the electrospinning method. The morphology, crystal structure and optical properties of the nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy, respectively. It is found that Ni doping does not change the morphology and crystal structures of the nanofibers, and the ultraviolet emissions of ZnO nanofibers present red shift with increasing Ni doping concentration. C₂H₂ sensing properties of the sensors based on the nanofibers were investigated. The results show that the C₂H₂ sensing properties of ZnO nanofibers are effectively improved by Ni doping, and 5 at% Ni-doped ZnO nanofibers exhibit a maximum sensitivity to C₂H₂ gas.     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.     

Improvement of H2/CH4 Separation Performance of PES Hollow Fiber Membranes by Addition of MWCNTs into Polymeric Matrix

Carboxylated multiwalled carbon nanotubes (MWCNTs) were added to polyethersulfone hollow fiber membranes to improve their H₂/CH₄ separation properties. The addition of MWCNTs up to 1 wt% increased macrovoids formation in cross-section, while in 2 wt% loading, decreased due to increase in dope viscosity. The best gas separation performance for the mixed-matrix hollow fiber membranes was achieved at 1 wt% MWCNTs loading with hydrogen permeance of 69 GPU and H₂/CH₄ selectivity of 44.1 at 5 bar(g). Tensile test results showed that incorporation of MWCNTs into the polymeric matrix affected the mechanical properties of the fabricated membranes.     

Method of estimating absolute concentrations of C<Subscript>2</Subscript>H<Subscript>5</Subscript> and H radicals in hydrocarbon diffusion flames

A method for estimating absolute concentrations of C₂H₅ and H radicals in hydrocarbon diffusion flames is proposed and substantiated. Concentration profiles of C₂H₅ and H on the flame axis are obtained. In the method proposed, the concentration of C₂H₅ is determined from the equality of two quantities — the rate of loss of n-butane by diffusion and the rate of its formation by recombination of two C₂H₅ radicals. The concentration of H radicals is determined from the relation between the ratio C₂H₅/H and experimental profiles of C₂H₄, C₂H₆, and O₂. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 13–20, November–December, 2007.