Temperature programmed desorption of oxygen from Pd films interfaced with Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZrO<Subscript>2</Subscript>

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y₂O₃–stabilized–ZrO₂ (YSZ), an O²⁻ conductor, under high-vacuum conditions and temperatures between 50 and 250 °C. Oxygen was adsorbed both via the gas phase and electrochemically, as O²⁻, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300 °C (state β₁) and 340–500 °C (state β₂). The creation of the low temperature peak is favored at high exposure times (exposure >1 kL) and low adsorption temperatures (T_ads < 200 °C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450 °C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O²⁻ pumping to the Pd catalyst film show two peaks (at 350 and 430 °C) corresponding to spillover O_ads and according to the reaction:

Effect of chloride on sintering of Pt/Al2O3 catalysts

Reduced Pt/Al₂O₃ catalysts with different chloride contents were treated at different temperatures under oxygen flow. TPR and TPD studies of oxidized species show that at low Cl/Pt atomic ratio (1) PtO₂ is formed at low temperature (400–500 K) and is totally decomposed (900 K) yielding reduced metallic Pt and inducing metal sintering. At high Cl/Pt atomic ratio (6) formation of stable (up to 1000 K) platinum oxichloride avoids metal sintering.     

The activity and selectivity of oxygen atoms adsorbed on a Ag/α-Al2O3 catalyst in ethene epoxidation

The bonding of the oxygen species held on a Ag/-Al₂O₃ catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/-Al₂O₃ catalyst was produced by the thermal decomposition of a Ag oxalate/-Al₂O₃ precursor. Oxygen desorbs from this Ag/-Al₂O₃ catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol^–1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol^–1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/-Al₂O₃ catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO₂ evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO₂ shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.     

High oxygen ion conduction in sintered oxides of the Bi2O3-Er2O3 system

The phase diagram of the Bi₂O₃-Er₂O₃ system was investigated. A monophasic f c c structure was stabilized for samples containing 17.5–45.5 mol% Er₂O₃. Above and below this concentration range polyphasic regions appear. The f c c phase showed high oxygen ion conduction. The ionic transference number is equal to one for specimens containing 30 mol% Er₂O₃ or less, while an electronic component is introduced at low temperatures for specimens containing 40–60 mol% Er₂O₃. Between 673 K and 873 K a maximum in the conductivity was found at 20 mol% Er₂O₃. (Bi₂O₃)_0.8.(Er₂O₃)_0.20 is found to be the best oxygen ion conductor so far known. The conductivity at 773 K and 973 K is 2.3 ^–1m^–1 and 37 ^–1 m^–1 respectively. These values are 2–3 times higher than the best oxygen ion conductor reported for substituted Bi₂O₃ systems and 50–100 times higher than those of stabilized zirconia (ZrO₂)_0.915(Y₂O₃)_0.085 at corresponding temperatures.     

Supported and sulfided Pb-Mo oxides: Active and stable hydrotreatment catalysts

A newly developed PbMo/Al₂O₃ HDS catalyst shows activity and stability that are comparable to or better than the traditional CoMo/Al₂O₃. Activity is optimum when the atomic ratio PbMo is 16. At that ratio, the generated surface phase(s) display maximum degree of Mo coordinative unsaturation, as measured by low temperature oxygen chemisorption.     

The Temperature-Programmed Desorption of Oxygen from an Alumina-Supported Silver Catalyst

The associative desorption kinetics of O₂ from a 15 wt% Ag/-Al₂O₃ atalyst were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments. Saturation with chemisorbed atomic oxygen (O*) was achieved by dosing O₂ for 1 h at 523 K and at atmospheric pressure followed by cooling in O₂ to room temperature. The TPD spectra showed almost symmetric O₂ peaks centred above 500 K, indicating associative desorption of O₂ from Ag metal surface sites. By varying the heating rates from 2 to 20 K min^-1, the O₂ TPD peak maxima were found to shift from 508 to 542 K, respectively. A microkinetic analysis of these TPD traces yielded an activation energy for desorption of 149±2 kJ mol^-1 and a corresponding pre-exponential factor of 2×10¹²±1×10¹² s^-1 in good agreement with the kinetic parameters reported for O₂ desorption under UHV conditions from Ag(111) and Ag(110) single crystal surfaces.     

Hydrogen adsorption on clean and oxygen covered Pt(111)

The dissociative adsorption of hydrogen on oxygen covered Pt(111) has been investigated using molecular beam techniques. The D₂-sticking probability has been measured as a function of oxygen coverage (0 < _O < 0.25 ML) and angle of incidence for two incident energies, 14 and 63 meV. In addition, the order of the oxygen layer has been measured using thermal He-scattering. The measurements show clear evidence for the existence of two distinct adsorption processes both on the oxygen covered and on the clean Pt(111) surface, i.e. in the limit were _O approaches zero: an activated process which depends on the total oxygen coverage and a non activated process which is sensitive only to the amount of locally ordered oxygen. The non activated process can be explained in terms of a mechanism involving a short living precursor state. The picture for the activated process is less clear. The dependence of this process on the incident energy seems strong evidence for a mechanism involving a barrier to dissociation directly upon impact, whereas the dependence on the oxygen coverage supports previously reported experiments which seem to be only compatible with a precursor mechanism.     

Bismuth-Modified Vanadyl Pyrophosphate Catalysts

A 1 bismuth-doped VPO catalyst was prepared by refluxing Bi(NO₃)₃ and VOPO₄ 2H₂O in isobutanol. The incorporation of Bi into the VPO lattice lowered the overall V oxidation state from 4.24 to 4.08. It also lowered both the peak maximum temperature for the desorption of oxygen from the lattice from 1001K (undoped) to 964K with a shoulder at 912K and the peak maxima for H₂ temperature-programed reduction from 863, 1011 and 1143K (undoped) to 798, 906 and 1151K. The total oxygen desorbed from the Bi-doped catalyst was only one-fourth that of the undoped catalyst, while the amount of oxygen removed by TPR was roughly the same for both catalysts. These results suggest that in anaerobic oxidation, the Bi-doped catalyst will have roughly the same activity as in undoped catalyst in C₄ hydrocarbon oxidation but will have a higher selectivity to products such as olefins and maleic anhydride.     

The Preferential Oxidation of CO: Selective Combinatorial Activity and Infrared Studies

From EXAFS (extended X-ray absorption fine structure) analysis, gold was found to have mainly oxygen in its nearest coordination shell in the fresh Au/-Al₂O₃ catalyst prepared by AuCl₃ impregnation and vacuum drying at room temperature. After thermal treatment under helium, chlorine appeared within the nearest neighbors of gold and more chlorine showed up as the treatment temperature was increased from 323 to 473K. No reduced Au species was observed up to 473K under He. However, the gold became reduced during CO oxidation at 373K and above. The precursor AuCl₃ was found to deposit on -Al₂O₃ via bonding to surface hydroxyl groups. This catalyst showed nearly 100% CO conversion at 573K, but a very low activity at 373 K under the conditions used in this study. Neither the residual chlorine nor the extent of reduction can explain the low activity at lower temperatures.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

Direct conversion of methane to higher hydrocarbons via an oxygen free,low-temperature route

The direct conversion of methane to higher hydrocarbons over a silica-supported Ru catalyst has been investigated via an oxygen free, two-step route. The reaction consists of decomposition of methane over a 3% silica-supp orted Ru catalyst at temperatures between 400 and 800 K to produce surface carbonaceous species followed by rehydrogenation of these species to higher hydrocarbons at of 368 K. It was found that the Ru/SiO₂ catalyst exhibits a trend similar to that for single-crystal Ru catalysts. However, the temperature at which a maximum in ethane selectivity occurs shifts toward a higher temperature. It was also found that the ethane yield can be optimized by changing the surface carbon coverage. Under optimum conditions a net ethane yield of about 13–15% has been realized. For this two-step reaction sequence, only a few reaction cycles could be operated without intermediate high temperature rehydrogenation and without significant loss in ethane yield. This is attributed to large amounts of inactive carbon that could not be hydrogenated at 368 K. Higher methane partial pressures were found to be desirable for this reaction. The activity of the catalyst could also be maintained at total pressures up to 10 atm.     

Isokinetic effects in ethane hydrogenolysis and their relation to the mechanism of the reaction

Literature data on the kinetics of ethane hydrogenolysis over metal catalysts are used to indicate the existence of isokinetic temperatures, . For platinum = 725 K, for Ni is probably in the same temperature range. For Fe and Co on the other hand is found at much lower values, about 330 K for Fe and 320 K for Co. From an analysis of the model of selective energy transfer, these values support the proposition of Sinfelt that the elements of high atomic number in one and the same transition metal series favour the splitting of the carbon-carbon bond as rate determining step, whereas those elements that have lower atomic numbers cause the splitting of the metal-carbon bond to be the rate determining step.     

High oxide ion conduction in sintered Bi2O3 containing SrO,CaO or La2O3

Ionic conduction in sintered oxides of the system Bi₂O₃-SrO was investigated by measuring the conductivity and ion transference number under various conditions. The ion transference numbers were measured by an oxygen concentration cell employing the specimen as the electrolyte.It was found that the solid solution containing 2040 mole% SrO which had a rhombohedral structure was an almost pure oxide ion conductor under a relatively high partial pressure of oxygen, and that the conductivity was several times higher than that of stabilized zirconias at the same temperatures up to 800°C. Oxide ion conduction was confirmed also by quantitative determination of generated O₂ from the anode of the oxygen concentration cell during discharge.The sintered specimens of the systems Bi₂O₃-CaO and Bi₂O₃-La₂O₃ were found also to be oxide ion conductors, and the ion transference numbers were greater than 0.9.     

CO Oxidation on Gold Surfaces Studied on the Atomic Scale

The interaction of small gold crystal tips with oxygen gas and CO/O₂ gas mixtures was studied by means of field ion microscopy (FIM). High-resolution FIM-images of clean tips were obtained with hydrogen and neon as imaging gas. At temperatures between 300 and 450 K the exposure of a clean Au sample to O₂ gas at 100–1000 mbar, in the absence of an electric field, led to oxygen chemisorption and formation of a surface oxide. The presence of an electric field of 12–15 V/nm was found to enhance the oxidation process. Exposure to CO gas at 300 K led to the removal of the surface oxide. This was associated with the occurrence of a wave front which started in the apex centre and extended to the outskirts of the tip sample. The build-up of the surface oxide and its titration by carbon monoxide was completely reversible. Our results strongly suggest that pure gold crystals are active catalysts for the CO oxidation at 300 K.     

Reduction and oxidation kinetics of Mn3O4/Mg-ZrO2 oxygen carrier particles for chemical-looping combustion

The kinetics of reduction with methane and oxidation with oxygen of Mn₃O₄ supported on Mg-ZrO₂ prepared by freeze granulation has been investigated. The reactivity experiments were performed in a thermogravimetric analyzer (TGA) using different reacting gas concentrations and temperatures in the range of 1073-1223 K. The oxygen carrier particles showed high reactivity during both reduction and oxidation at all investigated temperatures. An empirical reaction model, which assumes a linear relation between time and conversion, was used to determine the kinetic parameters for reduction and oxidation, with chemical reaction being the main resistance to the reaction. The order of reaction found was 1 with respect to CH₄ and 0.65 with respect to O₂. The activation energy for the reduction reaction was 119 and for the oxidation reaction. The reactivity data and kinetic parameters were used to estimate the solid inventory in the air and fuel reactor of a CLC system. The optimum solid inventory obtained was at a value of ΔX_s=0.4. At these conditions, the recirculation rate of oxygen carrier between air and fuel reactor was per MW of fuel, which could be accomplished in an industrial reactor. The high reactivity of the Mn₃O₄/Mg-ZrO₂ with both methane and oxygen showed that this is a very promising oxygen carrier for CLC.     

Surface oxygen species and their reactivities in the mild oxidation of ethylene on cerium oxide studied by FT-IR spectroscopy

Surface oxidation of ethylene on cerium oxide was studied at mild temperatures (293–473K) by usingin situ FT-IR spectroscopy, and isotopic technique. Ethylene oxidation took place even at around 330 K on a well outgassed cerium oxide independent of the presence of gaseous O₂. At mild temperatures, the surface adsorbed product was mainly formate species. Adsorbed Superoxide (O ₂ ^– ) species was definitely observed at temperatures up to 373 K when gaseous oxygen was present. However isotopic experiment confirmed that the Superoxide was not the active form of oxygen due to the mild oxidation of ethylene. The principal oxygen species participating in the mild oxidation of ethylene was surface lattice oxygen which is supposed to be in O^–-like form created by an outgassing at high temperatures. The mild oxidation of ethylene could be also initiated by surface peroxide (O ₂ ^2– ) and O^– species which were formed via the adsorption of O₂ on a partially reduced cerium oxide.     

An Investigation of the Reduction and Reoxidation of Isolated Vanadate Sites Supported on MCM-48

The reduction and subsequent reoxidation of isolated vanadate species supported on silica was investigated using temperature-programmed reduction and oxidation, along with in-situ XANES and Raman spectroscopy. Approximately 70–80% of the vanadium was reduced to V³⁺ after reduction in H₂ at temperatures up to 923 K. Upon reduction, the vanadyl oxygen was removed and the three remaining V–O bonds are lengthened by 0.2 Å. The vanadate species are rapidly reoxidized when exposed to O₂, with the amount of oxygen uptake matching well with the amount removed during reduction. In-situ Raman spectroscopy during reoxidation in ¹⁸O₂ showed that significant scrambling occurs between gas phase oxygen and surface oxygen species during the reoxidation of the vanadate species.     

The activation of O2 and the oxidative dehydrogenation of C2H6 over SmOF catalyst

The activation of O₂ over SmOF was studied by in situ laser Raman spectrometry and temperature programmed desorption (TPD). When the hydrogen- and helium-treated (1 h for each gas at 973 K) SmOF sample was cooled to 303 K in oxygen, Raman bands which correspond to the existence of O ₂ ^2– , O ₂ ^n– (2 >n > 1), O ₂ ^– and O ₂ ^- (1 > > 0) species were observed. From 303 to 973 K, there was no O₂ desorption but the Raman bands observed at 303 K reduced in intensity and vanished completely at 973 K, even though the sample was under an atmosphere of oxygen. We suggest that as the sample temperature increased, dioxygen species were converted to mono-oxygen species such as O^– which were undetectable by Raman spectrometry. O₂ desorption occurred above 973 K, giving a TPD-peak at 1095 K. When C₂ H₆ was pulsed over the sample pretreated with oxygen and helium at 973 K, C₂H₄ selectivity was 91.8%. We conclude that the mono-oxygen species is responsible for the oxidative dehydrogenation of ethane to ethene.     

Synergistic Catalysis of Carbon Dioxide Hydrogenation into Methanol by Yttria-Doped Ceria/γ-Alumina-Supported Copper Oxide Catalysts: Effect of Support and Dopant

Methanol synthesis from carbon dioxide hydrogenation was studied over ceria/-alumina- and yttria-doped ceria (YDC)/-alumina-supported copper oxide catalysts to seek insight into the catalysis at metal–support interfaces. It was found that, in comparison with Cu/-Al₂O₃, the Cu/CeO₂/-Al₂O₃ and Cu/YDC/-Al₂O₃ catalysts exhibited substantial enhancement in activity and selectivity toward methanol formation. The extent of enhancement was augmented by increased ceria loading on -alumina and with increased yttria doping into ceria. The enhancement is inferred to result from the synergistic effect between copper oxide and surface oxygen vacancies of ceria.