Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O₂, N₂O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O₂, N₂O and NO) and NH₃. In contrast to adsorbed oxygen species formed from NO, those from O₂ and N₂O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O₂, N₂O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O₂ and N₂O rather than from NO. These species may be of bi-atomic nature.     

Influence of electronic properties of Na2O/CaO catalysts on their catalytic characteristics for the oxidative coupling of methane

For Na₂O/CaO catalysts of different sodium content the adsorption of oxygen and their electrical properties were studied by transient experiments and measurements of contact potential differences (CPD) as well as electrical conductivity. CPD results show a change of the mechanism of oxygen activation with increasing sodium concentration due to changing the type of ionic conductivity from cationic to anionic. Anion vacancies are formed by incorporation of sodium into the CaO lattice. As CPDs show, the cation conductivity promotes an accumulation of oxygen species on the catalyst surface resulting in a decrease of C₂ product selectivity for the catalyzed oxidative coupling of methane. The anion conductivity favors a dissociation of molecular adsorbed oxygen and a subsequent incorporation into the oxide lattice, hereby, decreasing its concentration on the catalyst surface which favors in term selective formation of ethane and ethylene. This revised version was published online in July 2006 with corrections to the Cover Date.     

New catalytic materials for the high-temperature synthesis of hydrocyanic acid from methane and ammonia by high-throughput approach

For converting methane and ammonia to hydrocyanic acid, catalysts were prepared and tested in a 48-parallel channel fixed-bed reactor unit operating at temperatures up to 1373 K. The catalysts were synthesized with a robot applying a genetic algorithm as the design tool. New and improved catalyst compositions were discovered by using a total of seven generations each consisting of 92 potential catalysts. Thereby, the catalyst support turned out as an important input variable. Furthermore, platinum, which is well known as a catalytic material was confirmed. Moreover, improvements in HCN yield were achieved by addition of promoters like Ir, Au, Ni, Mo, Zn and Re. Multi-way analysis of variance and regression trees were applied to establish correlations between HCN yield and catalyst composition (support and metal additives). The obtained results are considered as the base for future even more efficient screening experiments.     

Effect of Electronic Properties of Catalysts for the Oxidative Coupling of Methane on Their Selectivity and Activity

The oxidative coupling of methane (OCM) to higher hydrocarbons may eventually become an interesting alternative for the chemical utilization of natural gas. Extensive studies have been conducted since the works of Keller and Bhasin [l] and of Hinsen and Baerns [2].