Substrate–Support Interactions Mediate Hydrogenation of Phenolic Compounds by Pd/CeO2 Nanorods

Ceria-supported palladium (Pd/CeO2) has spawned significant attention in recent years due to its ability to catalyze selective hydrogenation of phenolic compounds to cyclohexanones and cyclohexanols at a mild temperature and pressure. However, the mechanistic basis by which ceria enhances catalytic conversion is still unclear. Here, we use the increase in the 13C transverse relaxation rate upon the addition of nanoparticles (NPs) (13C ΔR2) to investigate the adsorption of phenolic compounds on the surface of the Pd/CeO2 catalyst by solution NMR. We show that hydroxyphenols adsorb on the support more efficiently than underivatized phenol and methoxyphenols and that phenol derivatives with an oxygen atom at position 2 (i.e., 2-hydroxyphenol and 2-methoxyphenol) form very stable interactions with the Pd site of Pd/CeO2. An analysis of the kinetics of hydrogenation revealed that catalytic conversion is linearly correlated with the ability of the substrate to form interactions with the CeO2 support and is inhibited by the formation of stable substrate–Pd adducts. Our data suggest that CeO2–substrate interactions mediate phenol hydrogenation more efficiently than Pd–substrate interactions and explain the exceptional catalytic performance reported for Pd/CeO2

Mechanistic Insight into Nanoparticle Surface Adsorption by Solution NMR Spectroscopy in an Aqueous Gel

Engineering nanoparticle (NP) functions at the molecular level requires a detailed understanding of the dynamic processes occurring at the NP surface. Herein we show that a combination of dark‐state exchange saturation transfer (DEST) and relaxation dispersion (RD) NMR experiments on gel‐stabilized NP samples enables the accurate determination of the kinetics and thermodynamics of adsorption. We used the former approach to describe the interaction of cholic acid (CA) and phenol (PhOH) with ceria NPs with a diameter of approximately 200 nm. Whereas CA formed weak interactions with the NPs, PhOH was tightly bound to the NP surface. Interestingly, we found that the adsorption of PhOH proceeds via an intermediate, weakly bound state in which the small molecule has residual degrees of rotational diffusion. We believe the use of aqueous gels for stabilizing NP samples will increase the applicability of solution NMR methods to the characterization of nanomaterials

Transfer hydrogenation over sodium-modified ceria: Enrichment of redox sites active for alcohol dehydrogenation

Ceria (CeO2) and sodium-modified ceria (Ce-Na) were prepared through combustion synthesis. Palladium was deposited onto the supports (Pd/CeO2 and Pd/Ce-Na) and their activity for the aqueous-phase transfer hydrogenation of phenol using 2-propanol under liquid flow conditions was studied. Pd/Ce-Na showed a marked increase (6×) in transfer hydrogenation activity over Pd/CeO2. Material characterization indicated that water-stable sodium species were not doped into the ceria lattice, but rather existed as subsurface carbonates. Modification of ceria by sodium provided more adsorption and redox active sites (i.e. defects) for 2-propanol dehydrogenation. This effect was an intrinsic property of the Ce-Na support and independent of Pd. The redox sites active for 2-propanol dehydrogenation were thermodynamically equivalent on both supports/catalysts. At high phenol concentrations, the reaction was limited by 2-propanol adsorption. Thus, the difference in catalytic activity was attributed to the different numbers of 2-propanol adsorption and redox active sites on each catalyst

‘Surface Contrast’ NMR reveals non‐innocent role of support in Pd/CeO2 catalyzed phenol hydrogenation

Ceria (CeO 2 )‐supported metals are widely used as catalysts because of their exceptional redox properties. Here, we use surface contrast NMR methods to investigate the hydrogenation of phenol by Pd supported on ceria nanoparticles. We show that the rigid and planar binding of phenol to Pd is mediated by a weak and highly mobile association of the small molecule to ceria. Interestingly, while addition of phosphate to the mixture does not perturb the adsorption of phenol on Pd, it destabilizes its interaction with ceria and proportionally decreases the rate of catalytic conversion. Our data provide strong experimental evidence that weak interactions between adsorbate and ceria are catalytically competent, and explain the exceptional performance of Pd/CeO 2 for reductive conversions under mild reaction conditions

Probing O-H Bonding Through Proton Detected 1H-17O Double Resonance Solid-State NMR Spectroscopy

The ubiquity of oxygen in organic, inorganic, and biological systems has stimulated the application and development of 17O solid-state NMR spectroscopy as a probe of molecular structure and dynamics. Unfortunately, 17O solid-state NMR experiments are often hindered by the combination of broad NMR signals and low sensitivity. Here, it is demonstrated that fast MAS and proton detection with the D-RINEPT pulse sequence can be generally applied to enhance the sensitivity and resolution of 17O solid-state NMR experiments. Complete 2D 17O→1H D-RINEPT correlation NMR spectra were typically obtained in fewer than 10 hours from less than 10 milligrams of material, with low to moderate 17O enrichment (less than 20%). 2D 1H-17O correlation solid-state NMR spectra allow overlapping oxygen sites to be resolved on the basis of proton chemical shifts or by varying the mixing time used for 1H-17O magnetization transfer. In addition, J-resolved or separated local field (SLF) blocks can be incorporated into the D-RINEPT pulse sequence to allow direct measurement of one-bond 1H-17O scalar coupling constants (1JOH) or 1H-17O dipolar couplings (DOH), respectively; the latter of which can be used to infer 1H-17O bond lengths. 1JOH and DOH calculated from planewave density functional theory (DFT) show very good agreement with experimental values. Therefore, the 2D 1H-17O correlation experiments, 1H-17O scalar and dipolar couplings, and planewave DFT calculations provide a method to precisely determine proton positions relative to oxygen atoms. This capability opens new opportunities to probe interactions between oxygen and hydrogen in a variety of chemical systems

Catalytic properties of intermetallic platinum-tin nanoparticles with non-stoichiometric compositions

Intermetallic compounds are unique catalyst platforms for mechanistic studies and industrial applications, because of their ordered structures in comparison to random alloys. Despite the intrinsically defined stoichiometry of intermetallic compounds, compositional deviations can still occur in intermetallic catalysts. The location of the extra metal atoms could differ the catalytic properties of intermetallic compounds with non-stoichiometric composition if those metal atoms end up on/near the surface. In this study, we synthesized PtSn intermetallic compounds with accurate stoichiometry and slightly Pt-/Sn-rich compositions. We used furfural hydrogenation and acetylene semi-hydrogenation as probe reactions to investigate the surface structures of PtSn intermetallic catalysts after reduction at different temperatures. Even though the intermetallic PtSn is the major bulk phase among non-stoichiometric compositions, the intermetallic PtSn surface can only be observed under the high-temperature reduction in Sn-rich PtSn intermetallic nanoparticles (iNPs), while the Pt-rich PtSn iNPs show Pt-rich-surfaces regardless of reduction temperatures. Four structural models were constructed based on the comprehensive surface and bulk characterizations. This work extends the understanding of intermetallic catalysts with non-stoichiometric compositions to tailor the intermetallic surface structures for catalysis

Controlling activity of palladium-catalyzed reactions via supported materials

This dissertation investigates how two metal oxide supports (mesoporous silica and ceria) influence the activity of Pd catalyzed liquid-phase reactions. The first chapter provides a general introduction to Pd catalysts as well as a review of previous attempts to control and modify Pd properties for various catalytic applications. Chapter 2 describes a method to create ligand-free size-controlled Pd nanoparticles by confining them in the pores of mesoporous silica nanoparticles (MSN). Surfactants and pore-expanding agents were used to fine-tune the size of pores between 2 and 5 nm. These pores successfully controlled the size of Pd nanoparticles between 2 and 4 nm as evidenced by STEM analysis and H2 pulse chemisorption. Because of the increased number of coordinatively unsaturated sites and higher back donation capacity, the Pd particles with the smallest size had the highest catalytic activity for a Suzuki-Miyaura cross-coupling reaction and the hydrogenation of phenol in the aqueous phase. In Chapter 3, palladium supported on high surface ceria (Pd/CeO2) demonstrated to be an efficient catalyst for hydrodehalogenation (HDH) of halophenols under mild conditions. The catalyst's high HDH activity can be attributed to the formation of electron-rich phenoxide species during dissociative adsorption of the halophenol onto the ceria support. Much lower rates are obtained when the reaction proceeds directly on Pd nanoparticle without dissociative adsorption onto the support. The mechanism also involves oxidative addition of C-halogen bond into Pd followed by reductive elimination to give phenol and hydrogen halide. Various spectroscopy techniques provided evidence supporting the proposed mechanism. Comparison of HDH reactivity of different halophenols results in high activities for Cl- and Br-, moderate for F- and poor for I-. The reason for this trend is the rate determining reductive elimination of halide from the catalyst surface. For iodophenols the strong chemisorption of the halide blocks active sites and results in catalyst poisoning. However, addition of pyridine to the reaction mixture, promotes reductive elimination of the halide in the form of pyridinium iodide, which ultimately restores catalyst turnover. Chapter 4 explores Pd/CeO2 as a catalyst for the transfer hydrodehalogenation (THD) of halophenols under mild conditions (65 °C) using isopropanol as hydrogen source. In contrast to HDH, the conversion was most efficient for fluorophenol, and reactivity decreased with halogen size suggesting that oxidative addition, which is the rate limiting step in HDH at 35 °C is not limiting under our THD conditions. THD kinetics, NMR, and temperature programmed surface reaction (TPSR) experiments indicated that oxidative addition of C-X and isopropanol oxidation compete for the same active sites on Pd. The isopropanol-derived hydride is adsorbed onto Pd sites and then used for hydrogenation of the Pd-chemisorbed halophenol. In the case of THD, the H abstraction is the rate-determining step. Chapter 5 relates the kinetics of Pd/CeO2 catalyzed phenol hydrogenation to the binding dynamics of the substrate onto the catalytic material. To enable the characterization of the mode, kinetics and equilibria of binding of the substrate onto the catalyst, we synthesized ceria nanocubes with narrow size distribution and uniform surface termination (100). Incorporation of Pd onto the support resulted in Pd nanoparticles of size ~1.5 nm. The catalytic hydrogenation of phenol appears to involve the initial binding of the substrate to the support characterized by anisotropic motions of the weakly adsorbed molecule. This event is followed by migration to the metallic sites where the substrate has decreased mobility, likely involving flat binding on the Pd surface. Disruption of the anisotropic binding to the support by the addition of phosphate interferents leads to a proportional decrease in reaction rates indicating that substrate-support interactions are critical to catalytic activity