Kinetics of the NO + H2 reaction over supported noble metal based catalysts: Support effect on their adsorption properties
Introduction
Future implementations of restrictive standard regulations on NOx emissions may concern a wide variety of industrial plants and also mobile sources due to the fact that NOx are partially reduced into nitrous oxide (N2O) during the cold start engine. Regarding stationary sources, the selective reduction of nitric oxides with ammonia as reducing agent is currently the most powerful technology in spite of the high toxicity of ammonia. However, actual catalytic processes could be less efficient for NOx abatement from low and high temperature exhaust gases, which would imply additional developments in the performances of the conventional vanadia–titania based catalysts. Alternatively, NH3 could be replaced by hydrogen in the case of low temperature sources because previous investigations showed that supported noble metals can be profitably used for such an application [1], [2], [3], [4], particularly when noble metals are dispersed on reducible supports such as perovskites [3], [5]. By way of illustration, Pd/LaCoO3 exhibits promising catalytic properties in comparison with a reference Pd/Al2O3 catalyst under lean conditions, in the presence of water [5]. A better resistance towards deactivation phenomena in the presence of SO2 is also observed [6]. The involvement of the redox properties of LaCoO3 could originate such a difference in the catalytic behaviour. As the matter of fact, the nature of interactions between noble metals and the support can be greatly influenced by the activation thermal treatment under hydrogen [1], [5]. It was earlier found that a pre-activation in H2 at 450 °C leads to an extensive reduction of LaCoO3, such a process being catalysed by the presence of metallic palladium particles. The peculiar interactions with the reduced support may strongly enhance the adsorption properties of palladium.
The kinetics of the NO + H2 reaction on supported Pd catalysts has been investigated. This reaction has been considered as a probe reaction either for the elucidation of reaction mechanisms or for the quantitative evaluation of the adsorptive properties of palladium particularly after deposition on LaCoO3.
Section snippets
Catalyst preparation and characterisation
The preparation procedure of LaCoO3 was described elsewhere [7]. A so-called sol–gel method involving a citrate route [8] was implemented using aqueous solutions of La(NO3)3·6H2O (Fluka) and Co(NO3)2·6H2O (Fluka) with a molar citric acid/(La + Co) ratio of 1. LaCoO3 obtained after evaporation, drying overnight at 120 °C, and subsequent calcinations in flowing air at 600 °C, exhibited a specific surface area of 20 m2 g−1. The characteristic rhombohedral structure of LaCoO3 was evidenced from X-ray
Influence of the pre-activation thermal treatment on the physicochemical properties of LaCoO3 before and after Pd addition
Fig. 1 shows two ranges of hydrogen consumption on LaCoO3 and Pd/LaCoO3 which underline a two-step reduction process. According to the calculation of the atomic H/Co ratios the low temperature range corresponds to the reduction of Co3+ into Co2+ species while above 450 °C an extensive reduction of LaCoO3 operates. The values of the overall atomic H/Co ratio close to 3, accounting for the margin of error, indicate a complete reduction of Co3+ into metallic cobalt particles. However, it is
Discussion
Up to now, only few publications report extensive investigations over noble metals on the kinetics of the reduction of NO by hydrogen in comparison with those dealing with CO or hydrocarbons. As a matter of fact controversial statements arise in the literature with the involvement or not of a hydrogen-assisted dissociation step of NO or different elementary steps for the formation of N2O and N2 which could obey either Langmuir–Hinshelwood or Eley–Rideal mechanisms [14], [15]. The deposition of
Conclusion
This study reports a kinetic investigation of the NO + H2 reaction on supported palladium catalysts. This reaction has been considered as a probe for the adsorption properties of palladium and their modifications depending on the nature of the support material. It was found that a classical Langmuir–Hinshelwood mechanism where the dissociation of NO is assisted by chemisorbed H atoms likely occurs on Pd/Al2O3 on the other hand, the kinetic behaviour of Pd supported on LaCoO3 differs which could
Acknowledgments
The authors would like to thank the Region Nord-Pas-de-Calais and the CNRS for a grant (F. Dhainaut).
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