Applied Catalysis A, General (v.202, #2)
Preface by P Grange (155).
Contribution of spillover effects to heterogeneous catalysis by G.M Pajonk (157-169).
This paper, which is dedicated to Prof. Bernard Delmon for his outstanding contributions to heterogeneous catalysis, will only encompass his remarkable findings in the field of spillover phenomena. In a certain way, this paper is not a new additional review about spillover itself, i.e. its formation, but an attempt to highlight the many parts spillover can play in catalysis in general and the implications of Bernard Delmon and his various teams in particular. Of course much will be described about the remote control concept but not exclusively. On the eve of the 5th International Conference on Spillover (5th ICSP) to be held in Madrid in September 2001, it is of particular interest to try getting a good picture of how the spillover-baby has grown up since its experimental discovery in the 1960s.
Keywords: Spillover; Heterogenous catalysis; Remote control;
Hydrogen spillover through the gas phase by E Baumgarten; L Maschke (171-177).
Hydrogen spillover through the gas phase was investigated, using a reactor, with a separate activation compartment, containing the catalyst (metal/alumina), and a reaction compartment with graphite or activated carbon, both separated by a frit. The only product, methane, was analyzed, using GC. In presence of catalyst the formation rate of methane was clearly above the limit of detection in contrast to measurements without catalyst. Different structure of the metal (Pt, Ni, Rh, and Pt–Ni), obtained under helium or hydrogen lead to great differences in the reaction rates.
Keywords: Spillover through the gasphase; Hydrogen spillover; Spillover and structure of metal crystallites;
Influence of spilt-over hydrogen on the electrical properties of H-ZSM-5 by M.E Franke; U Simon; F Roessner; U Roland (179-182).
In this work the effect of H2 on the electrical properties of H-ZSM-5 and Pt/H-ZSM-5 has been studied applying impedance spectroscopy in order to clarify the nature of migrating H* species (charged or uncharged, atomic or molecular) in the spillover process. The observed drop of the resistive part of a charge carrier relaxation process is attributed to the increase of mobile charges in the zeolite. This can be explained in terms of the dissociation of hydrogen on the Pt nanoparticles, the subsequent diffusion onto the zeolitic support and the formation of H+ as migrating species.
Keywords: Spill-over hydrogen; Zeolites; Pt/H-ZSM-5; Electrical properties; Impedance spectroscopy;
Mechanistic aspects of the dry reforming of methane over ruthenium catalysts by P Ferreira-Aparicio; I Rodrı́guez-Ramos; J.A Anderson; A Guerrero-Ruiz (183-196).
Carbon dioxide reforming of methane has been studied over two ruthenium catalysts supported on silica and on γ-alumina. Catalytic activity measurements, infrared spectroscopic analysis and isotopic tracing experiments applied to the study of the surface hydroxyl groups of the supports have allowed different reaction mechanisms to be proposed on the bases of the detected surface species, their mobility, stability and reactivity. Activation of both reactants takes place on the ruthenium surface for Ru/SiO2 catalyst. The accumulation of carbon adspecies formed from methane decomposition on the metallic particles finally impedes carbon dioxide dissociation and induces rapid deactivation of this catalyst. The alumina support provides an alternate route for CO2 activation by producing formate intermediates on its surface that subsequently decompose releasing CO. This bifunctional mechanism, in which the hydroxyl groups of the support play a key role, induces greater stability on the Ru/Al2O3 catalyst by significantly decreasing the rate of carbon deposition on the metal. The proposed reaction pathway requires continuous surface mobility of species from the metal to the support and vice versa.
Keywords: Ruthenium; Carbon dioxide; Methane dry reforming; FTIR;
Hydrogenation of formate species formed by CO chemisorption on a zirconia aerogel in the presence of platinum by Heiko Kalies; Nicolas Pinto; Gérard Marcel Pajonk; Daniel Bianchi (197-205).
The adsorption of CO (10% CO/He) on pure ZrO2 aerogel and on a 0.5% Pt/ZrO2 solid is studied at 638 K using in situ FTIR. It is shown that the two solids lead to the formation of the same adsorbed species: formate and carbonate species, in comparable superficial concentration. On ZrO2, the formate species are not hydrogenated in presence of pure hydrogen at 544 K. On Pt/ZrO2, they are hydrogenated at 544 K into adsorbed methoxy groups by hydrogen spilled over species. The methoxy groups are hydrogenated into methane by a process which probably involves a reverse spillover.
Keywords: Chemisorption; Carbon monoxide; Spill over; Hydrogenation; Platinum; ZrO2;
Kinetic study of hydrogen adsorption on sulfated zirconia-supported platinum by N Satoh; J.-i Hayashi; H Hattori (207-213).
Rate of hydrogen adsorption on sulfated zirconia-supported platinum (Pt/SO4 2−–ZrO2) was measured in the temperature range 323–523 K to elucidate the slowest step involved in the hydrogen adsorption and the corresponding energy barrier which the hydrogen species should cross over. At a nearly constant hydrogen pressure (50 Torr (6.7 kPa)), the hydrogen uptake continued more than 24 h, and the uptake in 24 h increased with an increase in the adsorption temperature. Above 473 K, the hydrogen uptake far exceeded the H/Pt ratio of unity, indicating that the hydrogen adsorption involves hydrogen spillover. Without Pt, the hydrogen uptake was scarcely appreciable.The hydrogen adsorption rate followed the Fick’s equation for surface diffusion. Activation energy for the diffusion constant was obtained as 84 kJ/mol. The rate could not be expressed by the rate equation derived on the assumption that the rate controlling step is the spillover step.
Keywords: Sulfated zirconia; Hydrogen adsorption; Spillover; Surface diffusion;
Oxygen spillover effects induced by Rh-modification on the low-temperature oxidation of CO over Cu-incorporated zeolite A studied by the forced oscillating reaction method by Tomoyuki Inui; Yukiyoshi Ono; Yoichi Takagi; Jin-Bae Kim (215-222).
In order to develop a CO oxidation catalyst exhibiting at a low-temperature range, an Rh-modified Cu-incorporated zeolite A catalyst was investigated.Supported 8.6 wt.% Cu catalysts were prepared by different procedures. One (Cu/SiO2) was prepared by ordinary incipient impregnation method using SiO2 as the support. Another (Cu–A) was prepared by incorporation of Cu into the framework of zeolite A by using Cu nitrate in the mixed solution for crystallization. Only 1 wt.% Rh to Cu was added to Cu–A.The redox properties were measured by temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) and it was found that Cu in Cu–A has much higher dispersion than in Cu/SiO2, and by Rh addition not only reduction of oxidized Cu–A but also reduced Cu–A enhanced reduction and oxidation, respectively. The combustion reaction of CO was conducted by adopting the forced oscillating reaction method and it confirmed marked acceleration in CO combustion by the Rh addition, and a low-temperature CO oxidation at a temperature range of 100±50°C could be achieved. The reason of the results is ascribed to the oxygen spillover effect induced by the existence of Rh.
Keywords: CO oxidation; Cu-incorporated zeolite A; Catalysts; Rh-modification; TPR; TPO;
The use of solid electrolyte potentiometry in the study of oxygen transfer in oxidic catalysts by M Estenfelder; H.-G Lintz (223-229).
The cooperation of single phases in a multicomponent oxidic catalyst has been investigated by combined kinetic and potentiometric (SEP) measurements. Carbon monoxide and an aldehyde have been oxidized on CuMoO4, Mo–V–O x and a mixture of both solids. Synergetic effects in the mixed catalyst can be explained by the flow of oxygen from one single oxide to the other. Its driving force, the difference of the oxygen activity between both phases in the mixture, is determined by SEP. The direction of the flow is dependent on the reaction catalyzed and is inversed in the examples studied.
Keywords: Solid electrolyte potentiometry; Multicomponent oxidic catalyst; Oxidation reaction; Oxygen transfer; Synergism;
Oxygen surface mobility and isotopic exchange on oxides: role of the nature and the structure of metal particles by C. Descorme; D. Duprez (231-241).
Oxygen surface mobility on oxides-supported metals has been studied for many years in our laboratory. The first step of the process is the activation of the oxygen molecule on the metal particle. Measurements of the kinetics of the adsorption–desorption of oxygen on metal surfaces were possible by the way of isotopic exchange studies. The influence of the nature of the metal, of the accessibility of the metal, of the oxide support are reviewed.
Keywords: Oxide-supported metals; Oxygen activation; Oxygen mobility; 18 O/16 O isotopic exchange;
Effect of potassium doping on the structural and catalytic properties of V/Ti-oxide in selective toluene oxidation by Dmitri A Bulushev; Lioubov Kiwi-Minsker; Vladimir I Zaikovskii; Olga B Lapina; Alexei A Ivanov; Sergei I Reshetnikov; Albert Renken (243-250).
Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VO x , increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O−, O2 −), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O2−), responsible for the partial oxidation.The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and 51V NMR. Potassium addition introduces a disorder in the crystalline structure of bulk V2O5 particles resulting in better spreading of V2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde.
Keywords: V/Ti-oxide catalysts; Doping by potassium; Toluene partial oxidation; Transient response technique; FT-Raman spectroscopy; HRTEM; 51V NMR;
Novel Re–Sb–O catalysts for the selective oxidation of isobutane and isobutylene by Haichao Liu; Eric M Gaigneaux; Hideo Imoto; Takafumi Shido; Yasuhiro Iwasawa (251-264).
It was found that three crystalline Re–Sb–O compounds, SbOReO4·2H2O, Sb4Re2O13 and SbRe2O6, were active for the selective catalytic oxidation of isobutane (773 K) and isobutylene (623–723 K). Among the three compounds, SbRe2O6 exhibited the highest catalytic activity for the selective oxidation of isobutane with a high selectivity of 83% to methacrolein (MAL)+isobutylene at the steady-state conversion of 7%. The Re–Sb–O catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and in situ laser Raman microscopy (LRM). SbRe2O6 decomposed to Re2O7 and Sb4Re2O13 partly at the surface under isobutylene oxidation at 673 K, while Sb4Re2O13 was stable under the reaction conditions. Under isobutane oxidation conditions at 773 K, the three Re–Sb–O compounds underwent a progressive decomposition to Sb4Re2O13, Re2O7 and Sb2O4. The catalytic behavior and structural characterization of the new catalysts allowed us to refer the reactivity of the catalysts to surface Re2O7 species benefiting from a cooperation with Sb4Re2O13.
Keywords: Re–Sb oxide catalysts; Selective oxidation; Isobutane; Isobutylene; Methacrolein; Lattice oxygen; XRD; Raman; SEM;
Dynamic phenomena and catalytic reactivities of oxide surfaces by E.M Gaigneaux; H.M Abdel Dayem; E Godard; P Ruiz (265-283).
Oxide catalysts undergo numerous dynamic phenomena at their surfaces under the conditions used for selective oxidation of hydrocarbons. This dynamic behavior influences catalytic performances and should thus be taken carefully into account in the understanding of the exact state of catalysts during the reaction and of phenomena like deactivations, synergetic effects, etc. This contribution reports examples in which dynamic behaviors of oxide surfaces under oxidation conditions dramatically dictate performances. It is concluded that the crucial key to maximize performances is to stabilize catalytic surfaces in slightly reduced suboxidic states. Such states allow catalytic oxidation cycles to proceed with high frequencies and favor reconstruction of catalysts and creation of active sites. Contrary, dynamic behaviors leading catalysts to more reduced or more oxidized states result in lower performances. Next to the tuning of O2 and hydrocarbon partial pressures in the feed, adjustments of the stoichiometries and uses of the remote control mechanism and spillover oxygen appear as efficient approaches to master the dynamism of catalysts ‘at work’ and thus to optimize their performances. Finally, implications and perspectives of the understanding of dynamic phenomena at the surface of oxides are discussed.
Keywords: Mo trioxide; Bi molybdate; Selective oxidation; Dehydration; Catalyst ‘at work’;
Patent report (285-292).