‘Surface Contrast’ NMR Reveals Non‐innocent Role of Support in Pd/CeO₂ Catalyzed Phenol Hydrogenation

Abstract

Ceria (CeO2)‐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/CeO2 for reductive conversions under mild reaction conditions.

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

“…The ΔR 2 data measured for PhOH in the presence of the CeO 2 nanorods show the same overall trend observed previously for PhOH adsorption on CeO 2 nanocubes, 7 with the para position displaying larger ΔR 2 than the ortho and meta positions ( Figure 3). This pattern indicates that binding modes (i) and (iii) coexist on the surface of the NP.…”

“…In addition, our data revealed that methoxy groups are not capable of effectively hydrogen-bonding to ceria and that PhOH derivatives with an O atom at position 2 form very stable adducts with the Pd sites of Pd/CeO 2 .A comparison of the NMR data with the rate of catalytic hydrogenation measured for the phenolic compounds in the presence of Pd/CeO 2 revealed that % conversion is linearly correlated with the amount of adsorbed small molecule but is inhibited by strong interactions between the substrate and the Pd sites. These data agree with our recent study indicating that the weak interactions between substrate and ceria are competent for catalysis,7 and suggest that stable substrate-Pd adducts may compete with H 2 adsorption and activation and/or reduce catalytic conversion by reducing the rate of product release from the catalyst.Finally, we have shown that13 C ΔR 2 data provide important insight into sorption equilibria. Although they do not return the comprehensive kinetic, thermodynamic, and structural description of NP surface adsorption obtained by a rigorous analysis of a complete set of SC NMR experiments,7 13 C ΔR 2 measurements are less time-consuming and easier to analyze as they do not require extensive modelling and data fitting procedures.…”

“…Conversely, deviation of the C-H bond from the axis of rotation will introduce additional modulation of the local magnetic fields that will reduce the 13 C transverse relaxation and will result in 13 C R 2 minima (corresponding to the R 2 measured for the free state) when θ corresponds to the magic angles (54.74° and 125.26°). 7,[20][21] Therefore, for an NMR sample of PhOH and Pd/CeO 2 in which binding modes (i)-(iii) coexist, the observed 13 C ΔR 2 can be modelled using the formula , , where R iso is a scaling factor that depends on the amount of PhOH in binding modes (i) and (ii) and R ani is a scaling factor that depends on the amount of PhOH in binding mode (iii) (Figure 2e).…”

“…This condition results in all C-H bonds to reorient with the same rotational correlation time as the NP (τ NP ) and experience similar13 C ΔR 2 upon binding (note that similar conclusions can be drawn for PhOH bound at the interface between Pd and CeO 2 , which would be indistinguishable from binding mode (iii) in our experiments). Finally, the presence of a single rotatable bond in binding mode (iii) generates a dependency of the 13 C R 2 from the angle θ formed by the C-H bond vector and the axis of rotation that can be modelled by a , spherical harmonic(Figure 2d) 7,[20][21]. Indeed, a C-H bond aligned with the axis of rotation (the para C-H bond in the case of PhOH bound to an oxygen vacancy on CeO 2 ,Figure 2c,d) will appear as rigidly attached to the NP (i.e.…”

“…C R 2 NMR data was collected at 25 °C on a Bruker 800 MHz spectrometer equipped with triple resonance z-gradient cryoprobe 13. C R 2 was measured using a double INEPT-based CPMG sequence with proton decoupling during the 13 C relaxation period 7. The 13 C carrier frequency was set at the center of the aromatic region of each molecule.…”