Search

Abstract: The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary and secondary () interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of , both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage. [Copyright &y& Elsevier]

Abstract: A temporal analysis of products (TAP) reactor was used to study the mechanism of ammonia oxidation at high temperature (1073 K) over Fe2O3, Cr2O3, and CeO2. The results were compared with those obtained over the industrially applied Pt95–Rh5 alloy. Analysis of characteristic times of the transient responses of N2 and NO made it possible to gain insight into the sequence of their formation. A common feature of both metal oxide and noble metal catalysts is that NO is a primary product of NH3 oxidation, whereas N2 results mainly from secondary transformations of NO. The amount of N2O formed over the oxides was minimal. Multipulse NH3 experiments in the absence of gas-phase O2 give unequivocal evidence of the participation of surface lattice oxygen of the metal oxides in the reaction of NH3 to NO. The degree of reduction of the oxide surface determines the product distribution but does not alter the catalytic activity for NH3 conversion. Greater reduction favors the reaction channel to N2 over that to NO. The desired reaction follows a Mars–van Krevelen-type scheme involving the participation of lattice oxygen in the NH3 conversion to NO and regeneration of the so-formed vacancies by gas-phase O2 and bulk lattice oxygen. Replenishment of surface vacancies by the diffusion of bulk oxygen occurs to a significant extent over Fe2O3. However, when NH3 and O2 are fed together, dissociative adsorption of gas-phase oxygen can be considered the main mechanism for vacancy regeneration. [Copyright &y& Elsevier]

Abstract: Transient isotopic studies in the Temporal Analysis of Products (TAP) reactor evidenced a clear mechanistic regularity between methane and propane for N2O reduction over Fe-silicalite in the temperature range of 623–773K. The lifetime of the oxygen species generated by N2O decomposition on extraframework iron sites was found to be crucial for alkane oxidation. When N2O and CH4 or C3H8 were pulsed together over Fe-silicalite, oxygen species deposited on iron sites effectively activated the hydrocarbon. However, these oxygen species become inactive for alkane oxidation if the pulses of N2O and the alkane were separated by more than 0.1s. Transient studies have indicated that C3H8 is more efficient for N2O reduction than CH4, as expected from the facilitated activation of the Con increasing the carbon number in alkanes. The performance derived from the TAP reactor agrees well with experiments under flow conditions at ambient pressure. [Copyright &y& Elsevier]

Abstract: The mechanism of the reaction of N2O and 13CO over Fe-silicalite was investigated with the use of the temporal analysis of products (TAP) reactor and compared with that of the reaction of N2O and C3H8 previously reported (Appl. Catal. A 267 (2004) 181). Upon direct N2O decomposition at 523–573 K, Fe-silicalite stored ca. 1018 atoms of oxygen per gram, with a ratio of 1 O atom per each 30–60 Fe atoms in the sample. Only a small fraction of the deposited oxygen was reactive for CO oxidation. Pump-probe experiments at different time delays (0–2 s) between the pulses of nitrous oxide and the reducing agent indicated the markedly different mechanisms of the N2O–13CO and N2O–C3H8 reactions in the temperature range of 623–673 K. Fe-silicalite is active for propane oxidation in the presence of short-lived oxygen species, that are produced when N2O and C3H8 are pulsed simultaneously. Time delays between the N2O and C3H8 pulses greater than 0.1 s are sufficient to transform these active oxygen species for hydrocarbon conversion into inactive ones. In contrast, the oxidation of CO by N2O does not depend on the lifetime of the oxygen species in the range of time delays investigated. The mechanisms of the N2O-mediated 13CO and C3H8 oxidations differ as a consequence of the different interactions of the two reducing agents with iron species in the zeolite. Pulse experiments support the occurrence of the scavenging mechanism with both propane and carbon monoxide. In this mechanism, short-lived oxygen deposited by N2O is efficiently eliminated by the reductant. Distinctive to propane, the strikingly high affinity of carbon monoxide for isolated Fe3+ ions in the zeolite gives rise to an additional pathway for N2O reduction in the presence of chemisorbed CO species. These particular Fe3+–CO species were identified by in situ UV/vis and EPR spectroscopies (J. Catal. 223 (2004) 13). [Copyright &y& Elsevier]

Abstract: The oxidation of HCl to Cl2 (Deacon process) was studied over RuO2-based catalysts in bulk and supported forms. RuO2 (7wt.%) was incorporated onto different TiO2 carriers (rutile, anatase, and P25) by deposition–precipitation of Ru(OH)2 followed by calcination. RuO2/TiO2-rutile exhibits the highest activity at ambient pressure in a continuous flow fixed-bed reactor. Insights into the mechanism of this experimentally demanding reaction on RuO2 and the influence of the TiO2-rutile carrier were gained by studies in the Temporal Analysis of Products (TAP) reactor at 623K. HCl multipulsing in the absence of O2 leads to chlorination of the catalysts with no detectable gas-phase Cl2 production. The chlorine uptake of RuO2 amounts to 0.16mmolCl , which enables to estimate that ca. 75% of the total surface Ru atoms (coordinatively unsaturated and bridge sites) are chlorinated, forming a RuO2− x Cl x oxychloride. RuO2 is also chlorinated by Cl2, but to a lesser extent compared to HCl. The chlorine uptake of fresh RuO2/TiO2-rutile per gram of sample was four times higher than that of fresh RuO2, namely due to chlorination of the high surface area carrier. Upon equilibration at ambient pressure under Deacon conditions, the TAP-derived chlorine uptake in the supported catalyst experienced a 6-fold decrease with respect to the fresh sample due to permanent chlorination. Pulsing of a mixture of O2 and HCl over the catalysts revealed that both gas-phase Cl2 evolution and H2O desorption are impeded processes. Pump–probe experiments of O2 and HCl evidenced the tight dependence of the net Cl2 production on both the oxygen and chlorine coverage. In addition, a limited contribution of lattice surface species could be detected by the production of small amounts of Cl2 at very low coverages. The equilibration procedure (ambient or TAP conditions) influences the reaction mechanism and the net Cl2 production, giving account of the reaction complexity due to the highly dynamic state of the catalyst surface. The Deacon process on RuO2− x Cl x -based catalysts can be mostly described by a Langmuir–Hinshelwood scheme with a minor 2D Mars–van Krevelen contribution. [Copyright &y& Elsevier]