Jonathan C. Otamiri

Ammoxidation of toluene by YBa2Cu3O6+x and copper oxides: Activity and XPS studies

The catalytic activities of YBa2Cu3O6+x and copper oxides have been studied in the ammoxidation of toluene. At low oxygen pressures, over YBa2Cu3O6+x selective ammoxidation to benzonitrile occurs, whereas at high pressures, only total combustion of toluene to carbon oxides is observed. X-ray photoelectron spectroscopy (XPS) analysis of YBa2Cu3O6+x catalysts indicates predominance of Cu(I) states under partial ammoxidation conditions, while under total oxidation conditions, Cu(II) states are predominant. Comparison with the catalytic performance of copper oxide catalysts shows that differences do exist. The catalysis by YBa2Cu3O6+x is greatly influenced by the oxygen content of the bulk material despite its surface composition.

Kinetics and mechanisms in the ammoxidation of toluene over a V2O5 catalyst. Part 2: Non-selective reactions

In the ammoxidation of toluene over a V2O5 catalyst, the variation of initial rates with partial pressures of oxygen, toluene and ammonia was determined for formation of carbon oxides. Dependencies obtained were analyzed and expressed in terms of rate equations. It was found that CO and CO2 are initial products formed at different sites, none of which are involved in selective reactions. The mechanisms derived for the formation of the two oxides have many features in common. In each mechanism there are two parallel routes originating from the same active site, which is suggested to be an ensemble exposing vanadium ions and electrophilic oxygen species. One of the routes proceeds without participation of ammonia, while in the other route ammonia is adsorbed. In both routes, the rate-determining step can be considered to be one of the steps in a stepwise reoxidation process. The rates for formation of carbon oxides decrease strongly with increasing partial pressure of ammonia, which is due to a combined effect of introduction of a new reaction pathway and competitive adsorption between oxygen and ammonia.

Surface and bulk composition of YBa2Cu3O6+x compounds studied by XPS

Abstract YBa2Cu3O6+x compounds prepared by a formic acid route where characterized by XPS. The samples were investigated both after scraping in air and in vacuum to reveal the surface composition after partial degradation during short time air exposure and to gather data on the pure bulk state. The surface layer is characterized by the XPS data of O 1s = 530.8–531.5 eV, Y3d5/2 = 157.5 eV, Ba3d 5 2 = 779.9 eV , Ba4d 5 2 = 89.4 eV and Cu2p 3 2 = 934.4 eV with a strong satellite structure at 942.5 eV. The data indicate the presence of barium hydroxide and carbonate, copper(II) oxide, the green phase and some yttrium compound. The bulk state is characterized by XPS data of O 1s = 528.8 eV, Y3d 5 2 = 156.2 eV , Ba3d 5 2 = 778.6 eV , Ba4d 5 2 = 88.3 eV and Cu2p 3 2 = 932.6 eV without a satellite and a similar copper state as for the surface layer. There exist single states of yttrium and barium in the bulk, both copper(I) and copper(II) valence states and for oxygen possibly several components closely spaced accounting for the structurally different oxide ions.

Kinetics of carbon monoxide oxidation over YBaCuO perovskites

Abstract The kinetics of carbon monoxide oxidation over YBa 2 Cu 3 O 6+x and PrBa 2 Cu 3 O 6+x catalysts was investigated in the temperature range 160 to 200°C and a single rate expression for all catalysts was derived. A reaction mechanism consistent with the kinetics is proposed: it involves the formation of carbon dioxide via two routes. In these, carbon monoxide reacts with either mobile lattice oxygen or with adsorbed oxygen species to give carbon dioxide. Oxygen-deficient samples are more active than oxygen-rich samples due to higher concentrations of oxygen surface vacancies and defects. The presence of water significantly diminishes activity reversibly, probably as a result of competitive adsorption with carbon monoxide.

Ammoxidation of toluene over Y-Ba-Cu-Co-O perovskites

Ammoxidation of toluene over the perovskites YBa2Cu3O6.1, YBa2Cu2CoO6.7 and YBaCuCoO4.9 was investigated at 400 °C. At low partial pressures of O2 benzonitrile was selectively formed, while CO2 was the main product at high pressures of O2. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O2 pressures, Cu-sites are active for nitrile formation, while Co-sites give CO2. At high O2 pressures, the activity for CO2 of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies.

Oxidation of carbon monoxide over cobalt and aluminium substituted YBaCuO perovskites

CO oxidation over YBa2Cu3−δMeδO6+x, where Me is Co or Al, has been studied. The activity for CO oxidation decreases with increasing degree of substitution in the B site, in the case of substitution both with a more active element (Co) and with an inactive element (Al). The active site is believed to consist of -Cu-()-Cu-, where () can be an oxygen species O or an oxygen vacancy []. Dilution of this site with less active-Cu-()-Me-results in a lower activity. Nitrogen-annealed samples show a considerably higher activity than oxygen-annealed samples. Oxidation-reduction experiments suggest that bulk influences such as oxygen diffusion and filling of vacancies also affect activity.

Preparation of YBa2Cu3O6 + x by a Formic Acid Method

Synthesis of YBa2Cu3O6+x from formic acid solutions of stoichiometric amounts of the initial materials has been investigated in the temperature range 750–950°C. The reaction is strongly influenced by the concentration of formic acid. Dilute solutions favor synthesis of purer samples under moderate conditions of temperature and sintering time. At lower temperature the same effect can be achieved if sintering time is longer, while at higher temperature, multiple grinding and heating at very short intervals are necessary to avoid formation of Y2BaCuO5. The observed effect of HCOOH concentration is explained by the different mechanisms involved in the decomposition of the formates obtained.

YBa2Cu3O6 - A Selective Ammoxidation Catalyst

YBa 2 Cu 3 O 6+x , 0≤x1≤, was used as a catalyst for the oxidation of toluene in presence of oxygen and ammonia. The partial pressures of reactants were varied. It was observed that when x is above zero, the material is active for total combustion. The activity is highly dependent on the value of x. After reductive treatment of sample having x>0 in reactant stream without molecular oxygen producing YBa 2 Cu 3 O 6 , the partial pressure of oxygen was increased from low to high. At low oxygen pressure, the material was active and selective for formation of benzonitrile. A dramatic transition from selective to non-selective region was observed to occur at a distinct pressure, which according to X-ray diffraction analysis is due to incorporation of oxygen species into the lattice. Under selective conditions, YBa 2 Cu 3 O 6 is more active for nitrile formation in comparison with vanadium oxide catalysts. Catalytic behaviours are discussed considering possible surface structures.