Article pubs.acs.org/JPCC
Low-Temperature Pd/Zeolite Passive NOx Adsorbers: Structure, Performance, and Adsorption Chemistry Yang Zheng,† Libor Kovarik, Mark H. Engelhard, Yilin Wang, Yong Wang, Feng Gao,* and János Szanyi* Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Pd/zeolite passive NOx adsorber (PNA) materials were prepared with solution ion-exchange between NH4/zeolites (Beta, ZSM-5, and SSZ-13) and PdCl2 solutions. The nature of Pd (dispersion, distribution, and oxidation states) in these materials was characterized with Na+ ion exchange, TEM imaging, CO titration with FTIR, and in situ XPS. The NOx trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites and PdO2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO2 clusters. NOx trapping and release are not simple chemisorption and desorption events but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO2 clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO adsorption sites. In the presence of H2O, the as-formed NO2 desorbs immediately. In the presence of CO in the feed, metallic Pd, “naked” Pd2+, and Pd+ sites are responsible for NO adsorption. For Pd adsorption sites with the same oxidation states but in different zeolite frameworks, NO binding energies are not expected to vary greatly. However, NO release temperatures do vary substantially with different zeolite structures. This indicates that NO transport within these materials plays an important role in determining release temperatures. Finally, some rational design principles for efficient PNA materials are suggested.
1. INTRODUCTION High fuel efficiency combustion modes in new-generation internal combustion engines cause a considerable decrease in exhaust temperatures. The lower exhaust temperature poses a great challenge for current lean NOx (NO + NO2) control technologies to meet the strict NOx emission standards that mandate a significant reduction in cold-start NOx emission.1 Leading NOx abatement techniques, i.e., three-way catalysis (TWC), lean NOx trap (LNT), and selective catalytic reduction (SCR), all have light-off temperatures typically above 200 °C.2−4 The decreasing exhaust temperature extends the time required for the inlet exhausts to warm up the catalytic convertors. As a result, a majority of the tailpipe NOx emission is emitted during the cold-start period of the vehicles.5 For SCR in particular, since this technology relies on on-board NH3 generation via urea decomposition at temperatures above ∼180 °C, noncatalytic obstacles, e.g., urea deposition and clogging, can also occur during cold starts. A promising approach to address cold-start NOx emission is to employ a passive NOx adsorber (PNA) material upstream of the main catalytic convertor (TWC or SCR).6−8 Efficient PNA is designed to adsorb NOx, preferably NO, during the cold-start period. The stored NOx should then be readily released once © XXXX American Chemical Society
the TWC/SCR system becomes operational. In contrast to the LNT catalyst that only adsorbs NO2 and requires periodical chemical reduction via rich purging to release stored NOx,9 the PNA thermally releases NOx under continuous lean conditions at higher temperatures, e.g., 200−350 °C, where the catalytic convertor functions efficiently. The elimination of the need for the rich purging significantly enhances fuel economy and engine durability and simplifies electronic controlling. Ceria/ alumina-supported Pd/Pt and zeolite-supported Pd are two typical PNA formulations. For the former formulation, the use of ceria/alumina instead of alkaline earth oxide (e.g., BaO) used for LNT enables low-temperature NOx storage as nitrites, negating the need to oxidize NO to NO2.7,8 This process is similar to the nitrite route for NOx adsorption on the LNT reported by Lietti et al.10 Nitrites decompose readily at temperatures above 200 °C. However, the susceptibility to sulfur poisoning, especially for ceria-based materials, limits their practical application. Received: May 5, 2017 Revised: July 8, 2017 Published: July 11, 2017 A
DOI: 10.1021/acs.jpcc.7b04312 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
(ramping rate 2 °C/min). The actual Pd loading of each sample was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis conducted at Galbraith Laboratories (Knoxville, TN), and the results are shown in Table 1. Chlorine was undetectable in the calcined samples. In
Zeolite-supported Pd catalysts for low-temperature NO adsorption have recently attracted great interest, due to their superb low-temperature NO trapping efficiency and storage capacity and good resistance to sulfur and hydrocarbon (HC) poisoning. Chen et al.6 very recently compared Pd-supported Beta, ZSM-5, and SSZ-13 PNA materials. The highly dispersed Pd cations at the exchange sites in the zeolite framework were claimed to be responsible for NO adsorption. Zeolite structure and chemical states of Pd were found to play critical roles in NO storage and release, with NO desorption temperatures increasing in the order of Beta, ZSM-5, and SSZ-13. Vu et al. investigated the effect of CO on Pd/Beta PNA and concluded that CO can improve the amount of NOx stored at low temperatures and can also induce higher-temperature desorption.11 Note that zeolites themselves can also function as HC traps, thus reducing both cold-start HC and NOx slips.12,13 Zeolite-supported Pd catalysts have been extensively studied for hydrocarbon SCR in the presence of excess O2, where only atomically dispersed Pd was found to be selective.14−17 It has been acknowledged that the nature of Pd species in zeolites is a function of acid strength, Al distribution, zeolite structure, Pd loading, pretreatment, and reaction conditions.15,17−20 Ogura et al.19 used NaCl titration (i.e., ion exchange between Na+ and isolated Pd cations) to quantify the amount of isolated Pd cations as a function of Pd/Al ratio and found that the predominance of isolated Pd cations in ZSM-5 can only be achieved when the Pd/Al ratio is very low (
