(560fl) A Temporal Analysis of Products (TAP) Study of Passive NOx Adsorption (PNA) on 1% Pd/SSZ-13

Authors: 
Menon, U. - Presenter, University of Houston
Thirumalai, H., University of Houston
Rahman, B. M., University of Houston
Gupta, A., University of Houston
Harold, M., University of Houston
Grabow, L. C., University of Houston
Most engine exhaust after-treatment catalytic processes such as selective catalytic reduction (SCR) are effective at temperatures above 200°C. Temperatures below 200°C are considered to be the cold start period when the majority of NOx and hydrocarbons slip through the after-treatment system. To meet strict NOx emission standards, trapping NOx upstream of the SCR catalyst during the cold start period and releasing the NOx at higher temperatures to enable its reduction is a potential way to lower NOx emissions further. A material with demonstrated capability for this purpose is Pd-exchanged zeolites, such as ZSM-5, BEA, and SSZ-13. While the PNA concept has been demonstrated in a group of recent studies, the nature and role of the various active sites determining their performance and NOx absorption chemistry is inadequately understood despite intensive research efforts.1 In this investigation, we characterize the active sites of a model passive NOx adsorber (PNA), Pd/SSZ-13 using Temporal Analysis of Products (TAP). The data are used to further develop a working mechanism comprising reaction steps that describe the NOx trapping and release over a range of temperatures. The TAP data are interpreted with the help of help of flow reactor and computational methods.

The NOx trapping and release properties on the 1% Pd/SSZ-13 were tested using feeds with different compositions. The experimental results showed the potential of this material to be an effective PNA at low temperatures. Literature shows multiple Pd species co-existing in the catalyst: (i) atomically dispersed or ‘naked’ Pd in the cationic sites in the pores of the zeolites, and (ii) PdOx particles on the external surfaces. Forms of active Pd sites present in the catalyst are referred to as PdII, PdI, PdII-OH and PdIIO.

The 1% Pd/SSZ-13 catalyst was prepared using an incipient wetness impregnation method. Powder catalysts of 250-400 µm were used for experiments. A generation-1 TAP reactor2 was used for conducting transient experiments. Controlled pulse experiments were carried out over 5 mg of the catalyst at a defined state to unravel the reaction mechanism. The products from the reaction were measured by a high resolution 100C UTI mass spectrometer. The catalyst was pretreated with 1500 pulses (1015 molecules/pulse) of O2/Ar (1:1) mixture at 500°C before each experiment. Experiments were conducted at temperatures between 100°C and 450°C with mixtures of isotopic and non-isotopic NO, with and without O2 and CO under dry and wet conditions to study the participation of different Pd species and the effects of H2O, CO on NOx uptake and release. The interpretation of experimental data was supported through periodic, spin-polarized density functional theory (DFT) calculations using the Vienna ab-initio simulation package (VASP) with the BEEF-vdW exchange correlation functional. Different active site motifs of Pd were modeled in bulk unit cells of SSZ-13 with one or two framework Al atoms.

A DFT investigation on the stability of the above-mentioned sites and the NO adsorption energies on them suggest that NO binds strongly on PdI sites, the formation of which can be explained through the two step site modification mechanism as in Eq (1).

2 Z [PdIIOH] → Z [PdI – O –PdI] Z + H2O → 2 ZPdI (1)

H2-temperature programmed reduction was performed in the TAP reactor and the H2 consumption at temperatures below 300°C is indicative of the amount of cationic Pd present in the catalyst. It was estimated to be ~85% of the total Pd present in the catalyst. A 16O2 pretreated catalyst pulsed with a 15N18O and 16O2 resulting in the production of 15N16O18O and 15N16O2 is consistent with the formation of the Pd+ sites from two adjacent Pd2+OH sites:

15N18O + 2 PdII -16OH ↔ 15N16O18O + 2 PdI + H216O (2)

15N18O + PdII 16O ↔ 15N16O18O + PdII (3)

The two step modification mechanism for the formation of Pd+ sites was confirmed by additional isotopic experiments with C18O on 16O2 pretreated catalyst by monitoring the production of C16O18O.

The presence of 18O2 and N2 in the product outlet provide evidence for possible NO dissociation. A more detailed insight into the reaction mechanism is provided by the observation of oxygen scrambling between the feed 18O and the catalytic 16O.

15N18O + ZPdII ↔ 15N18O- ZPdII (4)

215N18O- ZPdII ↔ 15N2 + 2 ZPdII-18O (5)

2 ZPdII-18O ↔ 2 ZPdII + 18O2 (6)

PdII-18O + PdII-16O ↔ 2PdII + 18O16O (7)

The effects of O2, H2O and CO in the feed were also studied for PNA performance of Pd/SSZ-13. The substantial reduction of NOx storage in the presence of H2O was primarily attributed to the loss of NO uptake on Brønsted acid sites, which are easily poisoned by H2O. NO uptake on PdI is only mildly impeded by H2O, as suggested by DFT results. Considering that NOx uptake over Pd zeolites is enhanced in the presence of CO,3 our results suggest that the oxidation or reduction potential of the gas phase alters the relative availability of active storage sites. In particular, the strongly exothermic nature of CO oxidation renders the reduction from PdIIOH to PdI thermodynamically favorable.

This study aims to identify the various active sites for passive NOx adsorption using Pd/SSZ-13 catalyst, which is the most promising material to date. An atomic-level understanding of the ad-/desorption and reaction steps on these active sites will greatly accelerate the design and development of improved PNA materials.

References

1 Y. Zheng, L. Kovarik. M. H. Engelhard, Y. Wang, F. Gao and J. Szanyi (2017). The Journal of Physical Chemistry

2 J. Gleaves, G. Yanblonsky, X. Zheng, R. Fushimi, P. L. Mills (2010). Journal of Molecular Catalysis A: Chemical

3 A. Vu., J. Y. Luo, J. H. Li; W. S. Epling (2017); Catalysis Letters