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

Abstract

We provide the first observation and characterization of super-electrophilic metal cations on a solid support. For Pd/SSZ-13 the results of our combined experimental (FTIR, XPS, HAADF-STEM) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd+3 and Pd+4, are in fact present as super electrophilic Pd+2 species (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution we re-assign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligandfree) super-electrophilic Pd+2 in SSZ-13, and their complexes with CO and NO. With CO, non-classical, highly positive [Pd(CO)2]2+ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Non-classical carbonyl complexes also form with Pt+2 and Ag+ in SSZ-13. The Pd+2(CO)2 complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd+2 complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd+2-carbonyl-olefin [Pd(CO)(C2H4)] and -nitrosyl-olefin [Pd(NO)(C2H4)] complexes. Overall, we show that zeolite framework can stabilize super electrophilic metal (Pd) cations, and show the new chemistry of Pd/SSZ13 system with implications for adsorption and catalysis. Introduction: Zeolite-supported transition metals represent an important class of catalysts utilized extensively in the industry: from petroleum refining to emissions control.1-6 Zeolites are highly crystalline aluminosilicates with well-defined binding sites inside microporous channels/cages of various sizes.7 We recently demonstrated the preparation of atomically dispersed Pd (and Pt) in a small-pore zeolite SSZ-13 (Si/Al ratio ~6) with loadings up to 2 wt% by a simple method (modified incipient wetness impregnation)8. Previous studies suggested that harsh hydrothermal aging is required to atomically disperse Pd ions in SSZ-13 and other zeolites.57,58 In those studies, however, the as-prepared materials initially contained a significant amount of PdO nanoparticles. Although, aging was suggested to improve the metal dispersion, it does not lead to complete dispersion of Pd because of simultaneous loss of Pd and zeolite dealumination under such harsh conditions. We have also shown that Pd/SSZ-13 with atomically dispersed Pd was capable of simultaneously adsorbing NO and CO from vehicle exhaust streams with unrivaled efficiency under practical conditions (i.e., in the presence of water in the stream) at low temperatures.8-10 The importance of their adsorption capacity lies in the fact that the current state-of-the-art selective catalytic reduction (SCR) materials61 cannot effectively convert NOx to N2 under practical conditions at temperatures lower than 200 0C. To circumvent this problem, Pd/SSZ-13 can be used as a low temperature passive NOx adsorber (PNA). In this process NOx is adsorbed at low temperature during cold start/vehicle idle and released at temperatures above 300 0C,8 when downstream SCR catalyst composed primarily of Cu/SSZ-13 (previously characterized extensively using various spectroscopic and DFT methods)61-63 becomes active. Their unique performance for PNA applications, coupled with their excellent hydrothermal stability puts Pd/SSZ-13 at the forefront for low-temperature emission control applications. Furthermore, the high loading of atomically dispersed Pd serves as a perfect example to investigate and understand the behavior of transition metal ions in microporous materials. tThe high uniformity of atomically dispersed Pd ions in the micropores of this zeolite allows us to investigate the fundamentals of NOx uptake and release, and the oxidation state of the metal. Moreover, the ability to synthesize M/SSZ-13 materials with uniform, atomic metal dispersion opens a new avenue to investigate other transition metal ion exchanges zeolites, e.g., Pt and Ag. The oxidation state of Pd in zeolites has been considered a settled matter for the last three decades. For example, Bell and co-workers11 concluded that Pd/ZSM-5 with 0.44 wt% loading could be prepared by ion-exchange of oxidized Pd. The IR spectra of adsorbed CO on this sample displayed numerous bands besides the C-O stretching features of Pd0CO species: Pd+2-CO, Pd+1-CO and even bands attributed to Pd+3(CO)2 were observed. Subsequently, Hadjiivanov and co-workers12 concluded that adsorption of CO at 100 K produces Pd3+(CO)2 complexes (unselectively) as well as a range of Pd+2(CO)x, Pd+1-CO and Pd0-CO complexes upon warming from 100 to 298 K. These studies suggested that Pd is initially present as either Pd+4 or Pd+3 ions that are reduced to Pd0 metal upon CO adsorption. However, as mentioned earlier, strategies employed in most previous works to produce atomically monodisperse Pd species were not successful and, as a result, both Pd ions and polynuclear PdO moieties co-existed in the micropores accompanied by larger PdO particles on the external surface. Surprisingly, only one density functional theory (DFT) study has been employed for Pd/zeolite systems, reported by us earlier.8 Prior to our studies on Pd(Pt)/SSZ-13, high loadings of atomically dispersed metals at elevated levels (> 1%) had not been attained. For example, Moliner et al. demonstrated a route to introduce 0.23 wt% atomically dispersed Pt in SSZ-13 with Si/Al ratio 7 to 9.13 Here we report the results of a combined spectroscopy/microscopy/computation study on a 1 % Pd loaded SSZ-13 zeolite in which the metal ions are present in atomic dispersion. Here we identify super-electrophilic metal ions in a solid support with the help of FTIR spectroscopy (coupled with DFT calculations) and, quasi in-situ XPS spectroscopy. This latter technique showed an unprecedented 2.3 eV shift of the binding energy (BE) of Pd(II) ion upon dehydration compared to the atomically dispersed partially hydrated ion. This unusually high shift allows us to quantify super-electrophilicity quantitatively : shifts higher than 2 eV upon dehydration of a partially hydrated, atomically dispersed metal cation in zeolite would mean it forms a super electrophilic M cation. This finding can be extended to other metals in zeolites (vide infra), and efforts are underway in our group to highlight the unusual catalytic chemistry of such unique super electrophilic as well as highly electrophilic metal fragments.64 For example, such fragments were recently shown to be exceptionally active for homolytic activation of the C-H bond of ethylene (22 kJ/mole stronger than methane C-H bond at RT) under ambient conditions.64 We also highlight that zeolitic confined micropore architecture is imperative for the formation of such super electrophilic species. that do not form selectively (on regular solid, nonmicroporous/non-zeolitic supports. For example, the high-lying IR bands for the complexes of CO with super electrophilic transition metal ions (stable.g., Pd(II)(CO)2 and Pt(II)(CO)2 complexes) form in zeolites with high yields, but not on any other commonl solid supports like alumina, zirconia, silica etc. The interaction of these super-electrophilic Pd cations with CO results in the formation of non-classical carbonyl complexes. Similar complexes are also observed with Pt2+ and Ag+ ions in SSZ-13. Experimental: Na-SSZ-13 with Si/Al = 6 and ion-exchanged twice with 2 M NH4NO3 aqueous solution at 80 °C for 3 hours yielding the ammonium forms of SSZ-13. NH4SSZ-13 was subsequently dried under ambient conditions and then at 80 oC. Samples with 0.1 and 1 wt% Pd, 1 wt% Pt and 3 wt% Ag loadings were prepared by modified ion exchange (for Pd and Pt) with 10 wt% Pd(NH3)4(NO3)2 solution (Sigma-Aldrich 99.99%) with NH4-SSZ-13, and platinum(II) tetraamine nitrate solution, and regular ion exchange for AgNO3 (99.99%) solution with H-SSZ-13 (produced by decomposition of NH4-SSZ-13 in air at 550 °C). More specifically, minimum amount of the Pd(II) or Pt(II) precursor solution was added to zeolite in the amount approximately equivalent to the total pore volume of the zeolite. The thick paste was mixed and stirred vigorously for 30 minutes, followed by calcination in air at 650 °C for 5 h (ramping rate 2 °C/min) in case of Pd and 350°C in case of Pt. H-forms of zeolites could be used as well with identical results: in that case, Pd and Pt tetramine salts were dissolved in the minimum amount of dilute ammonium hydroxide solution (pH=11.5), mixed with zeolite to form thick paste (mixed vigorously), followed by drying and calcination in air at 650 °C for Pd and 350 °C for Pt. In the case of Ag, 1 g of H-SSZ-13 was dispersed in water and stirred with ~20 ml of 0.1 M silver nitrate solution for 3 hours; then the sample was purified by 5 successive centrifugationredispersion cycles and dried at 80 oC overnight. To avoid silver auto-reduction under high vacuum in the FTIR cell, the sample was heated to 180 °C to remove residual water from Ag/SSZ-13 as quickly as possible and then immediately cooled down prior to IR measurements. The in situ static transmission IR experiments were conducted in a home-built cell housed in the sample compartment of a Bruker Vertex 80 spectrometer, equipped with an MCT detector and operated at 4 cm-1 resolution. The powder sample was pressed onto a tungsten mesh which, in turn, was mounted onto a copper heating assembly attached to a ceramic feedthrough. The sample could be resistively heated, and the sample temperature was monitored by a thermocouple spot welded onto the top center of the W grid. The cold finger on the glass bulb containing CO was cooled with liquid nitrogen to eliminate any contamination originating from metal carbonyls, while NO was cleaned with multiple freeze–pump–thaw cycles. Prior to spectrum collection, a background with the activated (annealed, reduced or oxidized) sample in the IR beam was collected. Each spectrum reported is obtained by averaging 256 scans. HAADF-