An infrared and XAS study on NO adsorption on Pd/zeolite under complex gas feed

Dongxia  Liu, Johnson Matthey

Reducing nitrogen oxides from diesel engines is an essential requirement to meet the emissions legislation standards. Recent studies have shown that, alongside selective catalytic reduction and lean NOx traps, the passive NOx adsorbers are an efficient way of controlling the NOx emissions [1]. Palladium zeolites have proved to be one of the most remarkable candidates for this application. They are able to adsorb NO at temperatures below 200o C with a high storage capacity and trapping efficiency [2]. Various characterization techniques have already identified Pd2+ ions at the exchange sites as the storage site for the NO, under simplified gas feeds. Still of great interest is the recognition of any changes in the storage sites in the presence of other gases. This would provide insight to “in-situ” behavior of these materials during engine operation. The present study offers a combined infrared and XAS approach to shed light on the NO storage mechanism with Pd/zeolites.

Materials and Methods
The samples employed in the present study were 0.5 wt% Pd on various zeolites. Chabazite (CHA) and Beta (BEA) zeolite were chosen as representative of small and large pore structures respectively. They were compared to alumina as support. The samples were prepared by incipient wetness impregnation.

In order to probe the type of the adsorbed surface species as well as the nature of the Pd active sites during the storage and the release cycles, the properties of these materials were initially evaluated in a simplified gas mixture. Stepwise extra gases were added, leading towards a synthetic gas mixture that simulated cold start exhaust emissions from typical diesel vehicles (NO; CO; O2; H2O). Thus influence of each gas feed component was monitored during storage at 100°C, and during release between 100 and 400°C. Similar conditions were used for the infrared and the XAS measurements. Infrared spectra were performed in house using a Perkin Elmer Frontier IR spectrometer and a Harrick diffuse reflectance cell. X-ray absorption spectra at the Pd K-edge (24350 eV) were measured on the SuperXAS beamline at the Swiss Light Source (PSI, Villigen, Switzerland) [3] using a Si(111) double-crystal monochromator and gridded ionisation chambers before and after sample and reference foil. Data were analysed using Demeter Athena and Artemis software [4].

Results and Discussion
Previously Pd2+ in the zeolite exchange sites was identified as the main adsorption site for NO in a gas feed consisting of NO and O2 only. Part of NO is also adsorbed on the Brønsted acid sites in the absence of water [2]. With Pd on Al2O3 support NO is mostly stored as nitrites and nitrates. “In-situ” infrared spectroscopy has shown that when water is added to the feed preferential adsorption to the Brønsted acid sites occurs. A change in the coordination sphere of Pd is also observed, with the NO being adsorbed on a Pd2+ site which also binds hydroxyl or water molecule (Figure1.). Exchanged Pd2+ ions remain the main site for NO uptake even in the presence of CO and H2O, with a CO and NO being coordinated on the same Pd site. The infrared assignments and the possible coordination configuration around the Pd sites have been supported by molecular modeling simulations.

Storage features were also evident by XAS. In the XANES region, NO adsorption resulted in a shift in the edge position to lower energies as a result of increased electronegativity of palladium. In the EXAFS region, a “beating” effect was present due to of closely-spaced Pd-N and Pd-O distances. Features in both regions during NO adsorption were used to confirm the infrared findings and provide the extra advantage of quantification of the stored NO. For example, in an NO and O2 gas mix, Pd-chabazite was shown to coordinate to an average of one NO molecule during storage. With the addition of water, storage was found to be lower, and in the presence of CO, NO storage was greatly enhanced as shown by other techniques.

The results revealed a unique characteristic of Pd zeolites: the Pd2+ exchanged cations are able to maintain their location and store NO without chemical transformation under complex gas feeds. This has provided a useful insight into the functional performance of this class of materials for NOx storage and help establish the structural requirements for this critical emission control process.

1. Rajaram, R. R,, Chen, H. Y., Liu, D., Passive NOX adsorber. U.S. patent 14/563,340, December 8, 2014.
2. Hai-Ying Chen, H.Y, Collier, J.E., Liu, D., Mantarosie, L., Durán-Martín, D., Novák, V., Rajaram, R. R., Thompsett, D., Catal. Lett. 146, 1706 (2016).
3. Abdala, P.A., Safanova, O. V., Wiker, G., Beek, W., Emerich, H., A. van Bokhoven, J; Sá, J., Szlachetko, J., Nachtegaal, M., Chimia 66(9), 699,(2012).
4. Ravel, B., and Newville, M., J. Synchrotron Radiat. 12, 537 (2005).

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