Effects of Pd particle size and water pressure on the structural transformation of Pd nanoparticles to mononuclear Pd(II) cations in CHA zeolites
Trevor Lardinois, Purdue University
Trevor M. Lardinois,1 Harrison H. Lippie,1 Vamakshi Yadav,2 Christina W. Li,2 Rajamani Gounder1,*
- Charles D. Davidson School of Chemical Engineering, Purdue University, IN
- Department of Chemistry, Purdue University, IN
As-exchanged, mononuclear [Pd(NH3)4]2+ complexes supported in CHA zeolites agglomerate to particles during thermal treatments in air (>598 K) before converting to exchanged Pd2+ cations,1 which are the purported passive NOx adsorption site.2 This agglomeration process results in a broad distribution of Pd particle sizes, which precludes a clear understanding of the effects of particle size on the structural transformation of Pd particles to exchanged Pd2+ cations under high temperature air treatments (>598 K) with and without water. Here, a series of Pd-CHA zeolites (Si/Al = 12) were synthesized via colloidal nanoparticle deposition procedures to contain similar bulk Pd content (0.7‒1.3 wt%) but varied Pd particle sizes (2, 7, 14 nm).3 Samples were treated in flowing air to 598‒973 K before characterizing the amounts of agglomerated PdO particles and exchanged Pd2+ with H2 temperature programmed reduction (5 kPa H2 in balance Ar, 10 K min-1, 203‒573 K).1 At a fixed air treatment temperature, the amount of mononuclear Pd2+ cations formed were consistently higher when Pd nanoparticles were initially smaller. The largest Pd clusters studied (14 nm) consistently showed residual metallic Pd (Pd0/Pdtot = 0.2‒0.5) after high temperature (598‒973 K) air treatments, suggesting that Pd oxidation was kinetically limited. Additionally, adding water (2‒6 kPa) to the air stream during thermal treatments (598‒973 K) decreased the amounts of exchanged Pd2+ relative to the air-only treatments for all particle sizes, contrasting proposals that water facilitates Pd mobility and thus increases exchanged Pd2+ content. The conversion of Pd nanoparticles to exchanged Pd2+ cations as a function of time was measured with (2 kPa) and without water in balance air (598‒673 K) and showed that co-feeding water did not influence the apparent rate of redispersion. Together, these data show that water does not influence the rate of Pd structural interconversion but does influence the thermodynamics.
(1) Lardinois, T. M.; Bates, J. S.; Lippie, H. H.; Russell, C. K.; Miller, J. T.; Meyer, H. M.; Unocic, K. A.; Prikhodko, V.; Wei, X.; Lambert, C. K.; Getsoian, A. B.; Gounder, R. Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites. Chem. Mater. 2021.
(2) Khivantsev, K.; Jaegers, N. R.; Kovarik, L.; Hanson, J. C.; Tao, F. (Feng); Tang, Y.; Zhang, X.; Koleva, I. Z.; Aleksandrov, H. A.; Vayssilov, G. N.; Wang, Y.; Gao, F.; Szanyi, J. 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. Angew. Chemie 2018, 130 (51), 16914–16919.
(3) Goodman, E. D.; Johnston-Peck, A. C.; Dietze, E. M.; Wrasman, C. J.; Hoffman, A. S.; Abild-Pedersen, F.; Bare, S. R.; Plessow, P. N.; Cargnello, M. Catalyst Deactivation via Decomposition into Single Atoms and the Role of Metal Loading. Nat. Catal. 2019, 2 (9), 748–755.