Oscillations in the reaction rate of nitric oxide reduction by ammonia over polycrystalline platinum foil catalysts

Abstract

Self-sustained oscillations were obtained in the NO + NH3 reaction a t atmospheric pressure with reactant partial pressures of 133-600 Pa (1-4.5 Torr), in the temperature range of 603-673 K. The effects of reaction parameters, temperature, partial pressure, and reactant gas velocity were studied. The onset temperature of the oscillations (603 K) was slightly dependent on the partial pressure of nitric oxide in the feed gas. Near this temperature the oscillations were uncontrolled (chaotic), while increasing the temperature resulted in periodic oscillations in the reaction rates. The oscillation phenomena were studied in both isothermic and adiabatic modes. The oscillations, when initiated by a temperature increase, start up only after an induction period during which the rates of NO consumption and N 2 formation sharply increase as opposed to the slow enhancement of the rate of N 2 0 formation. T h e two reaction branches found a t the high-and low-temperature regimes in the batch mode' have product distributions which are similar to those found at the extremes of the amplitudes of rate oscillations.

Transient studies on the effect of oxygen on the high-temperature NO reduction by NH3 over Pt–Rh gauze

“…It was concluded that secondary reactions involving NO and NH 3 are the main cause for the occurrence of loss reactions in ammonia burners, yielding the undesired N 2 O and N 2 products. The mechanism and kinetics of the NO reduction by NH 3 in the absence of O 2 have been investigated over platinum catalysts of different nature, including single crystals (Pt(1 0 0) [4,5] or stepped Pt 12(1 1 l) Â (l 1 1) [6]), polycrystalline Pt wire [7,8] and foils [9][10][11], as well as supported Pt catalysts [12][13][14], at different molar feed NH 3 /NO ratios (1)(2)(3)(4)(5)(6)(7)(8)(9), pressures (0.1-1000 Pa), and temperatures (473-873 K), using batch and flow reactors, as well as surface science techniques in ultra-high vacuum (UHV). N 2 (mainly) and N 2 O were found as the reaction products, with a typical increase in the N 2 /N 2 O ratio upon increasing the temperature and/or the feed NH 3 concentration [7].…”

A kinetic model of standard selective catalytic reduction of NO by NH3 on two sites

“…In general, due to the correlations related to spatial constraints, these probabilities are not reduced to the average coverages of a site. To articulate the difference in the treatments of the reaction kinetics on single sites or pairs of sites and in an adsorbed overlayer, it is of interest to mention that under certain conditions the reaction (1) exhibits kinetic oscillations on polycrystalline Pt [14][15][16]. Such oscillations are usually associated with non-linear dependence of the rates of reaction steps on adsorbate coverages.…”

Copper based catalysts for the selective ammonia oxidation into nitrogen and water vapour—Recent trends and open challenges

“…93,173,202,203] suggesting the subsequent dehydrogenation of ammonia: -NH3  -NH2  -NH  -N, until the formation of Nads species, which interact with Oads to produce NO [e.g. [204][205][206][207] and NO2 [30], while the formation of N2 occurs by interaction of two Nads species [138] over noble metals. Recently, Cui et al [138] proposed a synergistic catalytic effect between RuO2 and CuO during ammonia oxidation suggesting that coordinatively unsaturated Ru atoms (cus), present in RuO2 can easily adsorb and activated NH3 molecules and oxygen atoms.…”

“…Therefore, a significant population of NH3-x species on their surface could be expected [30]. Such species were reported to react with NO to form N2 and/or N2O [205,206,208,209]. Experiments of Burch and Southward [37] emphasized that under rich conditions (NH3:O2 = 1:1), CuO was partially reduced by ammonia to produce a reservoir of NH3-x species.…”