Low-Temperature Pd/Zeolite Passive NO_x Adsorbers: Structure, Performance, and Adsorption Chemistry

Abstract

Pd/zeolite passive NO x adsorber (PNA) materials were prepared with solution ion-exchange between NH 4 /zeolites (Beta, and PdCl 2 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 NO x 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 PdO 2 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 PdO 2 clusters. NO x 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 PdO 2 clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO adsorption sites. In the presence of H 2 O, the as-formed NO 2 desorbs immediately. In the presence of CO in the feed, metallic Pd, "naked" Pd 2+ , 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.

Achieving Atomic Dispersion of Highly Loaded Transition Metals in Small‐Pore Zeolite SSZ‐13: High‐Capacity and High‐Efficiency Low‐Temperature CO and Passive NOx Adsorbers

“…[11,50] In summary,w ep rovide ad irect, simple,a nd scalable route to highly loaded ionic Pd and Pt in asmall-pore siliceous (3 < Si/Al < 12) zeolite.This route utilizes only wet chemistry and does not require the use of expensive organometallic precursors or organic solvents.The key is to use the NH 4 -form of the zeolite and the modified IWI method, not the conventional ion exchange.F urthermore,w er econcile the contradictory literature data which the FTIR characterization for high loaded Pt and Pd species in ZSM-5 and other zeolites Angewandte Chemie Zuschriften always indicates the presence of significant amounts of metallic nanoparticles (not well-dispersed Pd or Pt) because: 1) H-forms of zeolite are often used for ion exchange,2)Si/Al ratios > 10 are not able to disperse metals as individual atoms because of the decreased hydrophilicity of the zeolite micropores 3) ForP t, the calcination temperature should not exceed 350 8 8Cb ecause of the instability of ionic Pt 2+ above this temperature.T his new insight led to the synthesis of Pd/ SSZ-13 with up to 2wt% of atomically dispersed Pd for immediate industrial application as CO and passive NO x adsorbers.T hese materials are able to abate 180 mmole g À1 NO x and simultaneously CO during cold-start of the vehicle while maintaining atomic dispersion. We achieved complete utilization of each Pd atom, surpassing the best performance of Pd/zeolite adsorbers reported in both patent [38,39] and open [35][36][37][40][41][42][43][44][45] literature.…”

“…Calcination at 400 8 8C, however, resulted in the formation of metallic Pt nanoparticles on the external surface of the zeolite crystals ( Figure S15): ionic Pt in SSZ-13 is unstable at temperatures > 400 8 8C, even when O 2 is present in the gas-phase and thermal treatment leads to the formation of metallic Pt particles.F TIR spectra of adsorbed CO on the 400 8 8Ccalcined 1wt% Pt/SSZ-13 showed ap rominent feature approximately 2090 cm À1 ,c onfirming the formation of metallic Pt clusters ( Figure S16). We achieved complete utilization of each Pd atom, surpassing the best performance of Pd/zeolite adsorbers reported in both patent [38,39] and open [35][36][37][40][41][42][43][44][45] literature. We achieved complete utilization of each Pd atom, surpassing the best performance of Pd/zeolite adsorbers reported in both patent [38,39] and open [35][36][37][40][41][42][43][44][45] literature.…”

“…[35][36][37][38][39][40][41][42][43][44][45][46] This is the major source of NO x pollution in the world and it affects public health and atmospheric chemistry.W eh ave shown that Pd-loaded zeolites were able to store NO x at low temperatures as NO + and Pd I/II -NO nitrosyl complexes. [35][36][37][38][39][40][41][42][43][44][45][46] This is the major source of NO x pollution in the world and it affects public health and atmospheric chemistry.W eh ave shown that Pd-loaded zeolites were able to store NO x at low temperatures as NO + and Pd I/II -NO nitrosyl complexes.…”

“…Most NO x emissions from internal combustion engines occur during cold-start when the SCR catalyst is still inactive. [35][36][37][38][39][40][41][42][43][44][45][46] This is the major source of NO x pollution in the world and it affects public health and atmospheric chemistry.W eh ave shown that Pd-loaded zeolites were able to store NO x at low temperatures as NO + and Pd I/II -NO nitrosyl complexes. However,i nt he engine exhaust gas mixture H 2 Oi sa lways present in significant amounts:H 2 Ocompetes for Pd sites and blocks NO adsorption.…”

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

“…However, based on the provided evidence, this Pd/SSZ-13 system selectively forms non-classical Pd +2 (CO)2 fragments and not Pd +3 (CO)2 as previously proposed in the literature. 11,12,24,28 The non-classical nature of this complex is further indirectly corroborated by the fact that its infrared signature does not depend on zeolite type to any significant extent and presents a Pd +2 (CO)2 fragment stabilized as ion pairs (with the Al-associated negative oxygen atoms of zeolite acting as a negative part of the ion pair, whereas Pd +2 and Pd +2 (CO)2 are the positive +2-charged part of the ion pair) in a small (~0.85 nm) SSZ-13 cage ( Fig. S10).…”