Reduction of PdO/Al₂O₃ in Liquid Cyclohexane Followed In Situ by ATR-IR, High-Energy XRD, and XAS

Abstract

Supported Pd-based catalysts used in heterogeneously catalyzed liquid phase processes often undergo an activation step through reduction of the oxide phase or of the oxidic passivation layer prior to reaction. An oxidized Pd/Al2O3 catalyst was reduced by using H2-saturated cyclohexane at 70 °C, and reduction was followed in real time by attenuated total reflection infrared spectroscopy (ATR-IR), high-energy X-ray diffraction (XRD), and quick scanning X-ray absorption spectroscopy (XAS) to obtain the perspective from different materials properties. We show that dynamic Pd speciation changes can be monitored during reduction in the liquid phase. XRD and XAS indicated that the isothermal PdO reduction passes through an intermediate Pd0 phase toward the formation of a well-defined Pd hydride phase. Based on the different sensitivities of the two techniques, XRD suggested that the process ends with a dominant Pd hydride phase, while XAS revealed an equal proportion of Pd0 and Pd hydride. The time-resolved XAS data suggested that the isothermal reduction of PdO in cyclohexane to Pd0 occurs through a nucleation mechanism and that Pd0 is rapidly converted into the hydride phase according to a contracting sphere model. The identification and the presence of a mixed Pd0/PdH x phase appears crucial in view of the relevance of these species in for instance hydrogenation reactions.

Preparation, Quantification, and Reaction of Pd Hydrides on Pd/Al2O3 in Liquid Environment

“…At 30 °C, the fraction of PdO decreased below 10% within 150 s in the reducing flow (Figure S11) and was replaced by the spectrum of hydrides through a short period (ca. 60 s) of Pd 0 as already observed before …”

“…60 s) of Pd 0 as already observed before. 22 After reduction at 30 °C, the catalyst was heated to 160 °C to enable observation of desorption of the hydrides by XAS, similar to TPR-1 in Figure 2. Finally, the gas flow was switched to He to obtain the reference spectrum of the bare Pd nanoparticles and the sample was cooled to 30 °C.…”

“…In situ experiments were carried out in a dedicated setup (Figure S1) using a flow cell described previously for gas-phase experiments that was adapted for liquid-phase experiments under pressure and was fitted with two 200 μm thick Teflon windows . This cell was validated for transient experiments in liquid phase (Figures S2–S4).…”

Electrocatalytic hydrogenation of acetonitrile to ethylamine in acid

“…At open circuit potential (OCP) and +0.1 V, weak vibration bands located at ~787 and 1015 cm −1 , which can be assigned to the vibration mode of δ (C-C≡N) and r (CH 3 ) in MeCN, respectively, were observed 31 . In addition, the vibration band at 2098 cm −1 originates from the emergence of Pd–H stretching mode 32 , 33 . Along with increasing potential (more negative than +0.1 V), the vibration intensities of δ (C-C≡N), CH 3 and Pd–H are gradually strengthened, suggesting the gradually increased surface coverage of MeCN and H on the surface of Pd/C catalyst.…”

Mechanocatalytic Hydrogenolysis of the Lignin Model Dimer Benzyl Phenyl Ether over Supported Palladium Catalysts

“…62 The spectra were dominated by metallic Pd (2.5 Å in R space), but a contribution of PdO (1.5 Å in R space) was clearly visible. 63,64 The spectra of the samples milled in hydrogen showed a decreased contribution of PdO, while the primary peak for Pd−Pd bonds at 2.5 Å increased in intensity for Pd/Al 2 O 3 and slightly shifted to a higher bond distance for both catalysts from 2.54 to 2.57 Å. The intensity shifts indicated that hydrogen milling created a sufficiently reducing environment to reduce any oxidized Pd in situ; for Pd 04 /SiO 2 , the Pd present in the fresh sample had a lower PdO concentration, which is why a significant increase in the peak intensity -was not observed, but a slight hydride shift still occurred.…”