Transient Product Analysis Research Articles

Enhanced performance of boron nitride catalysts with induction period for the oxidative dehydrogenation of ethane to ethylene

Hexagonal boron nitride (h-BN) has recently aroused continuous interest in the oxidative dehydrogenation (ODH) of light alkanes. Here we found that commercial h-BN exhibits an induction period during the ODH of ethane to ethylene. It can achieve better performance only on activated BN after the first cycle run of ODH or treatment of reactants together. The conversion of ethane was 36% at a selectivity of 79% on the activated BN at 570 °C, which nearly tripled that on the fresh BN (12% conversion with 84% selectivity). An excellent productivity of 110 μmol gcat−1 s−1 was obtained, which was one order of magnitude higher than that on most previous catalysts. Various methods of characterization such as infrared spectroscopy, O2 adsorption microcalorimetry, and transient analysis of products by mass spectroscopy indicated the formation of B–OH species on BN after the induction period. There was a linear relationship between the reaction rate of ethane and the quantity of OH species. It is suggested that B–OH species can facilitate the adsorption of O2 and then be transformed to B–O species, which would react with ethane to produce ethylene with the assistance of O2, thus resulting in a higher reaction rate and a lower activation energy.

Catalytic Abatement of Nitrous Oxide Coupled with Functionalization of C1–C3 Alkanes

AbstractMechanistic aspects of selective reduction of N2O by C1–C3 alkanes resulting in N2, olefins, and hydrogen over CaO‐based catalysts were studied by combining catalytic tests at ambient pressure with transient analysis of products in high vacuum with microsecond time resolution. In the frame of this reaction, oxygen species formed from N2O participate in selective (olefin production) and nonselective (COx formation) heterogeneous interactions. The coverage by these species and the interplay of exothermic heterogeneous (N2O decomposition and oxidative alkane conversions) and endothermic homogeneous (thermal nonoxidative dehydrogenation and cracking of hydrocarbons) reaction pathways determine the overall olefin selectivity.

Dehydrogenation of propane with selective hydrogen combustion: A mechanistic study by transient analysis of products

Abstract The role of lattice and adsorbed oxygen species in propane dehydrogenation in a perovskite hollow fiber membrane reactor containing a Pt–Sn dehydrogenation catalyst was elucidated by transient analysis of products with a sub-millisecond time resolution. Propane is mainly dehydrogenated non-oxidatively to propene and hydrogen over the catalyst, while lattice oxygen of the perovskite oxidizes preferentially hydrogen to water. For achieving high propene selectivity at high propane conversions, the formation of gas phase O 2 on the shell side of the membrane reactor should be avoided. Otherwise, oxygen species adsorbed over the Pt–Sn catalyst participate in non-selective C 3 H 8 /C 3 H 6 transformations to C 2 H 4 and CO x .

Challenges in macroscopic measurement of diffusion in zeolites

The paper presents a brief historical overview of the field of the measurement of diffusion in zeolites. The focus will be on macroscopic measurements, i.e. when diffusion through entire crystals is studied. While this may appear to be a simple task, we will see that there are difficulties in obtaining reliable results. Two case studies, the volumetric/piezometric experiment and the Transient Analysis of Products (TAP) apparatus are discussed and ways to improve either the experiments or the way in which the results are analyzed are presented. The author’s perspective into what are the current challenges in the field concludes the paper.

Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.

Rapid lightoff of syngas production from methane: A transient product analysis

AbstractSteady‐state production of syngas (CO and H2) can be attained within 10 s from room‐temperature mixtures of methane and air fed to a short‐contact‐time reactor by initially operating at combustion stoichiometry (CH4/O2 = 0.5) and then quickly switching to syngas stoichiometry (CH4/O2 = 2.0). The methane/air mixture is first ignited, forming a premixed flame upstream of the catalyst that heats the Rh‐impregnated α‐alumina foam monolith to catalytic lightoff (T > 500°C) in a few seconds. The methane/oxygen ratio is then increased to partial oxidation stoichiometry, which extinguishes the flame and effects immediate autothermal syngas production. Transient species profiles are measured with a rapid‐response mass spectrometer (response time constant ≅ 0.5 s), and catalyst temperature is measured with a thermocouple at the catalyst back face. Because the monolith thermal response time (∼ 1 s) is several orders of magnitude larger than the reaction timescales (∼ 10−12 to 10−3 s), chemistry and flow should be mathematically decoupled from local transient variations in catalyst temperature. Using this assumption, a transient temperature profile is combined with detailed surface chemistry for methane on Rh in a numerical plug‐flow model. This approach accurately reproduces the transient species profiles measured during experimental lightoff for short combustion time experiments and lends insight into how the monolith temperature develops with time. The combined experimental and numerical efforts supply useful information on the transient reactor behavior for various combustion times and identify a combustion time to avoid undershoot or overshoot in catalyst temperature and minimize start‐up time. © 2004 American Institute of Chemical Engineers AIChE J, 51: 247–260, 2005