Revealing the Active Sites of Pd Nanocrystals for Propyne Semihydrogenation: From Theory to Experiment

Abstract

The development of highly selective catalysts has been remarkably relying on the understanding of catalytic active sites. Pd-catalyzed semihydrogenation of propyne has been a focus of research with industrial applications toward the production of polymer-grade propylene. In this work, combining density functional theory (DFT) calculations and experimental observations, we propose that, different from the existing debates where the formation of palladium carbide (Pd–C) species or specific facets of Pd nanoparticles are critical, the apexes of Pd (111) octahedrons are the active sites for highly selective propyne semihydrogenation. The propylene selectivity on Pd octahedrons can be ascribed to site-selective propyne adsorption on the apexes prior to reactions and subsequent difficult to access intermediate states toward overhydrogenation. To reveal the active sites of Pd, propyne semihydrogenation was performed on shaped-Pd nanoparticles with designed exposed facets: e.g., (111) and (100) facets. Of practical importance, the propyne conversion and propylene selectivity exceed ∼94% and ∼96% on Pd octahedrons, respectively, at low temperature (35 °C) and atmospheric pressure. In addition, more control experiments have been performed to verify the effects of apexes of Pd octahedrons on propylene selectivity. It is shown that the propylene selectivity decreases to ∼50% when the apexes of Pd octahedrons are gradually removed. The experimental observations have further confirmed that the apexes of Pd octahedrons can be used as the active sites for propyne semihydrogenation, which is in good agreement with the results from theoretical calculations. This work may not only reveal the active sites of Pd nanoparticles for selective semihydrogenation of propyne but also open an avenue for designing highly active and selective catalysts in the chemical industry.

Revealing the Correlation between Catalytic Selectivity and the Local Coordination Environment of Pt Single Atom

“…To further investigate the correlation of Pt−O coordination number and catalyst selectivity, Pt/FeO x H y SACs were used to catalyze PSH in a fix-bed reactor with an inner diameter of 10 mm. 48 As shown in Figure 3a, all Pt/FeO x H y SACs exhibited similar activities. When the coordination number of Pt−O was increased from ∼3.43 to ∼5.04, the TOF value slightly increased from 9.2 × 10 5 to 9.4 × 10 5 h −1 , whereas the propylene selectivity significantly increased from 65.4% to 94.1%.…”

“…FeOOH was gradually converted into Fe 2 O 3 with the increased treatment temperature, leading to the weakening peak intensity in the DRIFTS spectra (Figure 3c). When C 3 H 4 and H 2 were introduced, the characteristic peaks of C 3 H 4 at 2100−2200 and 3275−3390 cm −1 were observed (Figure 3d), which corresponded to the stretching vibration of CC and C−H associated with CC, respectively, 48 The shoulder peak at ∼3015 cm −1 was indexed as the stretching vibration unsaturated C−H associated with CC, indicating that propylene was formed (Figure 3d,e). More importantly, the intensity of the propylene peak strongly varied in the DRIFTS spectra of different catalysts, suggesting the propylene selectivities of Pt/FeO x H y SACs were different (Figure 3e).…”

“…For 0.5%Pt-FeO x H y and 1%Pt-FeO x H y catalysts, the weak peak at 3015 cm −1 but intense peaks at 2900−2985 cm −1 suggested that most of propyne was converted into propane. 48 Moreover, the spent catalysts were characterized by XRD and XPS measurement. Compared to the XRD patterns of the fresh SACs, no obvious changes in the XRD patterns of the spent SACs suggest the excellent stability of catalyst under the indicated conditions (Figure S10).…”

“…To further investigate the correlation of Pt–O coordination number and catalyst selectivity, Pt/FeO x H y SACs were used to catalyze PSH in a fix-bed reactor with an inner diameter of 10 mm . As shown in Figure a, all Pt/FeO x H y SACs exhibited similar activities.…”

“…The gradual increase in the propylene peak intensity at 3015 cm –1 in the DRIFTS spectra implied that the increase in treatment temperature favored PSH. For 0.5%Pt-FeO x H y and 1%Pt-FeO x H y catalysts, the weak peak at 3015 cm –1 but intense peaks at 2900–2985 cm –1 suggested that most of propyne was converted into propane . Moreover, the spent catalysts were characterized by XRD and XPS measurement.…”