Pulse Reaction Studies of Transient Nature of Adsorbates during NO-CO Reaction over Rh/SiO

Abstract

The transient nature of adsorbates for the reaction of NO with CO over a 4 wt % Rh/Si02 catalyst has been studied by in situ infrared spectroscopy combined with pulse transient techniques. Dynamic behavior of infrared-observable Rh-NO-, Rh-NO+, Rh+(CO)2, and linear and bridged CO reveals that dissociation of Rh-NO-oxidizes Rho to R h ' ; R h ' chemisorbs CO and NO as Rh+(C0)2 and Rh-NO', respectively; and Rh+(CO)* can either undergo reductive agglomeration or react with adsorbed oxygen to produce C02 at 473-523 K. At 573 K, Rh is in the reduced state where linear CO and bridged CO react with the oxygen produced from dissociation of Rh-NO-to produce C02. The catalyst in the reduced surface state is more active for the formation of N20 and C02 products, and the response of N20 formation leads that of COZ formation at temperatures below the light-off temperature. The rapid C02 response during the pulse reaction studies indicates that the reaction can quickly reach steady state at temperatures above 473 K when the reaction conditions are altered.

Effects of W6+ Doping of TiO2 on the Reactivity of Supported Rh toward NO: Transient FTIR and Mass Spectroscopy Studies

“…The observed differences in the transient gas phase responses of NO, N 2 and N 2 O obtained at 250 and 450 °C, following the switch He → 1% NO/He (Figures and ), over Rh supported on TiO 2 and W 6+ -doped TiO 2 carriers can qualitatively be discussed with respect to the following sequence of elementary steps, which are based on results either obtained in this work or which have been reported in the literature. ,, Here, (S) is a site on the Rh surface. Assuming that the rate-determining step is that of NO dissociation (eq 6), as suggested by many investigators in the case of Rh catalysts, ,,, the higher initial yield of N 2 observed over the Rh/TiO 2 (W 6+ ) as compared to the Rh/TiO 2 catalyst can be attributed to either the higher initial surface coverage of adsorbed NO, or to the larger value of the rate constant, k 2 , associated with this step, or to both. FTIR results obtained after 20 min of reaction with 1% NO/He mixture at 450 °C, followed by Ar treatment at 450 °C, revealed that the Rh−NO + species (the only kind of adsorbed NO species existing, 1920 cm -1 ) disappear very fast in Ar flow at 450 °C over the Rh/TiO 2 catalyst, while they persist even after 10 min of Ar purge in the case of Rh/TiO 2 (W 6+ ) catalyst.…”

Applying Dynamic and Synchronous DRIFTS/EXAFS to the Structural Reactive Behaviour of Dilute (≤1 wt%) Supported Rh/Al2O3 Catalysts using Quick and Energy Dispersive EXAFS

“…This band is associable with the formation of Rh(NCO) [40][41][42] species and rapidly disappears after the switch back to oxidising conditions, in tandem with the other carbonyls species, and concomitant with a burst of CO 2 production.…”

In Situ Infrared Study of Catalytic Decomposition of NO

“…A number of the NO-CO reaction studies on Rh/SiO 2 catalysts have revealed that the reaction pathway for N 2 and CO 2 formation below light-off temperature is different from those above light-off [13,17]. Tables 2A and 2B list both reaction pathways.…”

“…Under such conditions, CO adsorbs on Rh + sites as Rh + (CO) 2 and NO adsorbs as Rh-NO -; the surface of supported Rh catalyst contains both Rh 0 and Rh + sites [13,17]. Increase in reaction temperature allows NO dissociation to occur.…”