An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

Abstract

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H 2 over supported Pt-, Rh-and Pd-based catalysts. Different activity sequences have been established for the NO+H 2 reaction . It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H 2 reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steadystate rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al 2 O 3 during the NO+H 2 reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N 2 O) irrespective of the nature of the reducing agent (CO or H 2 ). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N 2 via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H 2 +O 2 reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.

Rh1−x Pd x Nanoparticle Composition Dependence in CO Oxidation by NO

“…No activation energies are presented for the 100% Pd nanocrystals due to extremely low turnover frequencies for all products in these reaction conditions. These results compare relatively well to previous studies on Rh and Pd nanoparticle catalysts, such as studies by Granger et al, who found higher turnover rates and activation energies on Rh/Al 2 O 3 catalysts as compared to Pd/Al 2 O 3 catalysts [16]. No synergism was observed in these conditions with the Rh 1-x Pd x nanocrystals.…”

“…Rhodium is acknowledged to be the best catalyst for this reaction, with intense efforts devoted to understanding its behavior [3][4][5][6][7][8][9][10][11][12][13]. However, in recent years much research has also been aimed at understanding the role of Pd in the reaction [1][2][3][14][15][16][17][18][19] because of the relatively high cost of rhodium catalysts, palladium's improved resistance to sintering, and because new improvements in gasoline purification have helped reduce the significance of previous problems with Pd site poisoning by sulfur and lead [15]. Pd has been found to have lower activity than Rh for the reaction of NO with CO [16] and has also been shown to suffer from deactivation [14].…”

Rhodium Nanoparticle Shape Dependence in the Reduction of NO by CO

“…The reaction proceeds much faster on Rh than on Pd or Pt [12] and is known to be structure sensitive on Rh single crystals [13]. The products of the reaction are CO 2 at all temperatures, N 2 O primarily (*75%) at the lower (\320°C) temperatures investigated in this study, and N 2 primarily at higher temperatures ([320°C) [12].…”

“…The reaction proceeds much faster on Rh than on Pd or Pt [12] and is known to be structure sensitive on Rh single crystals [13]. The products of the reaction are CO 2 at all temperatures, N 2 O primarily (*75%) at the lower (\320°C) temperatures investigated in this study, and N 2 primarily at higher temperatures ([320°C) [12]. Although many mechanisms have been proposed for this reaction [14][15][16][17][18][19][20], it remains an open area of investigation [21,22] due to its enormous complexity.…”

Pd-W and Pd-Mo catalysts for NO decomposition and NO/CO reduction reactions

“…Studies have shown that Pd can catalyze the reduction of NO to N 2 , in addition to its activity for hydrocarbon and CO oxidation. [5][6][7][8][9][10][11] However, in the presence of hydrocarbons, the NO reducing capacity of Pd is lower than that reported for Rh-based catalysts. 8,12 In order to overcome this problem while employing relatively inexpensive metals, bimetallic Pd-Mo and Pd-W catalysts have been proposed.…”