Strong promotional effects of Li, K, Rb and Cs on the Pt-catalysed reduction of NO by propene

Abstract

The catalytic activity and selectivity of Pt dispersed on ␥-Al 2 O 3 for the reduction of NO by propene is promoted extremely strongly by Li, K, Rb and Cs alkalis in a wide temperature range ca. 450-800 K. Remarkable and unprecedented effects on both activity and selectivity are found. The best promotion effects are achieved by Rb-promotion, for which rate increases as high as 420-, 280-and 25-fold are obtained for the formation rates of N 2 , CO 2 and N 2 O, respectively, in comparison with the performance of un-promoted (alkali-free) Pt/␥-Al 2 O 3 . From the other hand, the selectivity towards N 2 is improved from ∼20% over the alkali-free unpromoted Pt catalyst, to >90% over the optimally alkali-promoted catalyst. These effects are understandable in terms of the effect of alkali promoter on the relative adsorption strengths of reactant species. Alkali promotes the adsorption, and consequently the dissociation, of electronegative adsorbates (NO) and inhibits the adsorption of electropositive adsorbates (propene), on a catalyst surface predominantly covered by propene and its fragments.

The NO x reduction mechanism by H2 under near isothermal conditions over Pt–Ba/Al2O3 Lean NO x Trap systems

“…As a matter of facts, it has been reported that the addition of Na or Ba to supported Pt catalysts results in increase in NO x reduction activity in the absence of oxygen [3,10,11]. According to the mechanism previously suggested (reactions (1)-(3)), it may be argued that the presence of the alkaline or earth-alkaline metal ion weakens the N-O bond, leading to more easy dissociation and subsequent reduction.…”

Preparation and NO reduction property of apatite-type ALa₉Si₆O₂₆ (A = Li, Na, K) supported Pt catalyst

“…Konsolakis et al showed that the NO reduction on Pt/£-Al 2 O 3 catalyst was promoted by addition of Li (0.164.7 wt %), K (0.98.8 wt %), Rb (1.915.5 wt %) and Cs (3.024.0 wt %) for C 3 H 6 NO reaction. 14) In this case, NO conversion and N 2 selectivity were increased by addition of alkali metals. It seems that the catalytic activity of PGM catalyst for NO reduction is promoted by basicity of the catalyst.…”

Novel electropositively promoted monometallic (Pt-only) catalytic converters for automotive pollution control

“…alkalies, can drastically enhance both the de-NO x activity and N 2 -selectivity of both Pt and Pd. Hydrocarbons and CO oxidation reactions were also significantly enhanced by the same way [3][4][5][6].…”

“…The origin of the promotional effects of Na on TWC catalytic chemistry is not the object of the present study; the way of action of Na modifier has been extensively investigated in previous detailed kinetics and spectroscopic studies where the origins of the promoting phenomena have been well rationalized [3][4][5][6]. The main object of this paper is to record the performance of these materials in more realistic conditions and in the form that they are used in practical applications.…”