Dual doping effects (site blockage and electronic promotion) imposed by adatoms on Pd nanocrystals for catalytic hydrogen production (original) (raw)
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Applied Catalysis B: Environmental, 2018
The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ diluted solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average A C C E P T E D M A N U S C R I P T particle size 4.5 nm) during both CO oxidation (in H2-free atmosphere) and preferential oxidation of CO in H2 excess (PROX) by operando X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H2 is around 30% at 150 °C and reaches 100% at 200 °C, whereas in the presence of H2 the conversion reaches a maximum of 15% (at 250 °C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 °C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 °C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts.
Physical Chemistry Chemical Physics, 2012
Using single-crystalline Fe 3 O 4 (111) films grown over Pt(111) in UHV as a model-support, we have characterized the nucleation behaviour and chemical properties of Pd particles grown over the film using different deposition techniques with scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Comparison of Pd/Fe 3 O 4 samples created via Pd evaporation under UHV conditions and those resulting from the solution deposition of Pd-hydroxo complexes reveals that changes in the interfacial functionalization of such samples (i.e. roughening and hydroxylation) govern the differences in Pd nucleation behavior observed over pristine oxides relative to those exposed to alkaline solutions. Furthermore, it appears that other differences in the nature of the Pd precursor state (i.e. gas-phase Pd in UHV vs. [Pd(OH) 2 ] n aqueous complexes) play a negligible role in Pd nucleation and growth behaviour at elevated temperatures in UHV, suggesting facile decomposition of the Pd complexes deposited from the liquid phase. Applying temperature programmed desorption and infrared spectroscopy to probe the CO chemisorption properties of such samples after reduction in different reagents (CO, H 2) shows the formation of bimetallic PdFe alloys following reduction in H 2 , but monometallic Pd particles after CO reduction.
The Journal of Physical Chemistry C, 2010
Using first principles calculations, we examine how the ensemble effect on the performance of bimetallic catalysts is affected by the change of surface electronic structure associated with their geometric parameters. We look at H 2 O 2 formation from H 2 and O 2 based on three different Pd monomer systems including AuPd adlayers with a Pd monomer each on Pd(111) [AuPd M /Pd(111)] and Au(111) [AuPd M /Au(111)] and a 55atom cluster with Au 41 Pd shell and Pd 13 core [Au 41 Pd@Pd 13 ]. Our calculations show that H 2 O 2 selectivity tends to be significantly deteriorated in the Au 41 Pd@Pd 13 and AuPd M /Au(111) cases, as compared to the AuPd M /Pd(111) case. This is largely due to enhancement of the activity of corresponding surface Pd and its Au neighbors, while isolated Pd surface sites surrounded by less active Au are responsible for the H 2 O 2 formation by suppressing O-O cleavage. This study highlights that ensemble contributions in multimetallic nanocatalysts can be a strong function of their geometric conditions, particularly local strain and effective atomic coordination number at the surface, that are directly related to surface electronic states.
Journal of the American Chemical Society, 2011
Supported nanoparticulate Pd is the focus of research for application in a number of important areas. These include biomass conversion, coupling (e.g., Mizoroki-Heck and Suzuki-Miyaura, among others), and selective oxidation reactions for fine or high-value chemical(s) production, water gas shift, methane oxidation, and autoexhaust catalysis for pollution abatement. 1 Central to all these applications is the unique behavior of the noble metal in oxidation steps and/or processes. As such, a clear and fundamental comprehension of the nanoscale redox properties of Pd that underpin all these catalytic conversions is highly sought after.
Applied Catalysis A: General, 2012
A Pd catalyst supported on a layered double hydroxide (LDH or hydrotalcite-like material) has been synthesized in a one step reaction using a precipitation-reduction method in which hydrolysis of hexamethylenetetramine was used to both precipitate the LDH by virtue of the resulting increase in pH and provide formaldehyde which reduces the Pd 2+ precursor to Pd 0 . The resulting Pd nanoparticles were mostly tetrahedral in shape, and were highly dispersed on the surface of the LDH. After introducing a capping agent during the synthesis, the morphology of the Pd particles changed to truncated octahedral, although a similar high degree of dispersion was obtained. Compared with a conventional impregnated catalyst composed of pseudo-spherical Pd particles, catalysts prepared by the new method showed both higher activity and selectivity in the hydrogenation of acetylene. The enhanced activity is due to the specific morphology of the Pd particles, which results in their higher dispersion, while the higher selectivity is attributed to the Pd-C phase formed in the catalyst during the reaction. Furthermore, compared with the truncated octahedral Pd particles enclosed by and facets, the tetrahedral particles with only (1 1 1) facets exposed showed higher ethylene selectivity, suggesting that the Pd (1 1 1) facet is the preferred facet in the selective hydrogenation of acetylene to ethylene. (D. Li).
Two sets of bimetallic Pd-Pt (Pd: 1.0; Pt: 0.25-1.0%, w/w) and Pd-Au (Pd: 1.0; Au: 0.25-1.0%, w/w) catalysts have been used, with no added promoter, in the catalytic direct synthesis (CDS) of hydrogen peroxide from its elements at 2 • C with a CO 2 /O 2 /H 2 mixture (72/25.5/2.5%, respectively). The catalysts were supported on the commercial macroreticular ion-exchange resin Lewatit K2621 and were obtained from the reduction with H 2 of ion-exchanged cationic precursors at 5 bar and at 60 • C. The addition of Pt or Au to Pd produced an increase of the initial overall catalytic activity in comparison with monometallic Pd with both the second metals, but with Pt the increase was much higher than with Au. Moreover, the addition of 0.25% (w/w) Pt, or more, invariably made all the Pd-Pt catalysts less selective with respect to Pd alone. In the case of Au, by contrast, the addition of 0.25% w/w produced an increase, albeit small, of the selectivity. As the result, the most active and productive Pd-Pt catalyst was 1Pd025PtK2621 with 1891 mol (H 2) mol −1 (Pd+Pt) h −1 initially consumed, 1875 mol (H 2 O 2) mol −1 (Pd+Pt) h −1 initially produced, a 45% selectivity towards H 2 O 2 at 50% conversion of H 2. In the case of the Pd-Au bimetallic catalysts, 1Pd025AuK2621 was the best one, with 1184 mol (H 2) mol −1 (Pd+Pt) h −1 initially consumed, 739 mol (H 2 O 2) mol −1 (Pd+Pt) h −1 initially produced, a 55% selectivity towards H 2 O 2 at 50% conversion of H 2. Although the characterization of the Pd-Pt and Pd-Au catalysts with TEM showed that the morphology of the nanostructured metal phases in the Pd-Pt and Pd-Au catalysts was very different from each family to the other, no clear correlation between the size of the nanoparticles and their distribution and the catalytic performance was apparent. These catalysts were also generally different, especially the Pd-Au ones, from previously reported related materials obtained from the same support and the same precursor, but with a different reducing agent (formaldehyde).
Solar Energy, 2020
The bimetallic core-shell nanostructures of galvanic metals have gained considerable scientific interest in improving the TiO 2 photocatalysis under solar radiations over the monometallic analogues. In the present research work, Pd@Au core-shell supported TiO 2 nanostructures were synthesized via galvanic replacement reaction and were examined for their catalytic/ photocatalytic hydrogenation. Three different types of bimetallic Pd@Au nanostructure were synthesized by varying Pd:Au weight ratio i.e. (1:1), (1:2) and (1:3). DLS measurements revealed that with increasing Au weight ratio, the hydrodynamic size increases from 126 to 157 nm. The optical studies showed a considerable blue shift in the absorption band of Au nanoparticles from 529 to 518 nm in the case of Pd@Au (1:1). The coexistence of absorption characteristic of Pd and Au suggests the successful synthesis of bimetallic nanostructure. STEM and EDS mapping further confirmed the formation of Pd@Au nanostructure with inner Pd core and outer Au shell. Bimetallic Pd@Au nanocatalyst displayed superior activity and selectivity towards hydrogenation of cinnamaldehyde in comparison to monometallic analogues. However, when Pd@Au nanostructures were impregnated on the surface of TiO 2, a significant improvement in the hydrogenation reaction was observed under solar radiations relative to catalytic conditions. The photocatalytic performance of Pd@Au-TiO 2 was found to be varied as a function of shell thickness and the optimized APT-2 (Pd 1 @Au 2-TiO 2) photocatalyst exhibited higher rate constant (2.3 × 10 −1 h −1) for cinnamaldehyde hydrogenation. Hence, the plasmonic Pd@Au-TiO 2 hetero-junction could be a promising greener photocatalyst for selective hydrogenation of unsaturated carbonyls for large scale industrial applications.
ACS Catalysis, 2013
Incorporating small amounts of Pd into supported Au catalysts has been shown to have beneficial effects on selective hydrogenation reactions, particularly 1,3butadiene hydrogenation and the hydrogenation of nitroaromatics, especially p-chloronitrobenzene. Appropriate Pd incorporation enhances hydrogenation activity while maintaining the desirable high selectivity of supported Au catalysts. To better understand this phenomenon, a series of aluminaand titania-supported Au and dilute Pd−Au catalysts were prepared via urea deposition−precipitation. The catalysts were studied with infrared spectroscopy of CO adsorption, CO oxidation catalysis, and cyclohexene hydrogenation catalysis with the goal of understanding how Pd affects the catalytic properties of Au. CO adsorption experiments indicated a substantial amount of surface Pd when the catalyst was under CO. Adsorption experiments at various CO pressures were used to determine CO coverage; application of the Temkin adsorbate interaction model allowed for the determination of adsorption enthalpy metrics for CO adsorption on Au. These experiments showed that Pd induces an electronic effect on Au, affecting both the nascent adsorption enthalpy (ΔH 0 ) and the change in enthalpy with increasing coverage. This electronic modification had little effect on CO oxidation catalysis. Michaelis−Menten kinetics parameters showed essentially the same oxygen reactivity on all the catalysts; the primary differences were in the number of active sites. The bimetallic catalysts were poor cyclohexene hydrogenation catalysts, indicating that there is relatively little exposed Pd when the catalyst is under hydrogen. The results, which are discussed in the context of the literature, indicate that a combination of surface composition and Pd-induced electronic effects on Au appear to increase hydrogen chemisorption and hydrogenation activity while largely maintaining the selectivities associated with catalysis by Au.