Dual doping effects (site blockage and electronic promotion) imposed by adatoms on Pd nanocrystals for catalytic hydrogen production (original) (raw)
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).
Synthesis of Supported Pd0 Nanoparticles from a Single-Site Pd2+ Surface Complex by Alkene Reduction
Chemistry of Materials, 2018
A surface metal-organic complex, (-AlO x )Pd(acac) (acac = acetylacetonate), is prepared by chemically grafting the precursor Pd(acac) 2 onto γ-Al 2 O 3 in toluene at 25 °C. The resulting surface complex is characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and dynamic nuclear polarization surface-enhanced solid-state nuclear magnetic resonance spectroscopy (DNP SENS). This surface complex is a precursor in the direct synthesis of size-controlled Pd nanoparticles under mild reductive conditions and in the absence of additional stabilizers or pretreatments. Indeed, upon exposure to gaseous ethylene or liquid 1-octene at 25 °C, the Pd 2+ species is reduced to form Pd 0 nanoparticles with a mean diameter of 4.3 ± 0.6 nm, as determined by scanning transmission electron microscopy (STEM). These nanoparticles are catalytically relevant using the aerobic 1-phenylethanol oxidation as a probe reaction, with rates comparable to a conventional Pd/Al 2 O 3 catalyst but without an induction period. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction mass spectrometry (TPR-MS) reveal that the surface complex reduction with ethylene coproduces H 2 , acetylene, and 1,3-butadiene. This process reasonably proceeds via an olefin activation/ coordination/insertion pathway, followed by β-hydride elimination to generate free Pd 0 . The well-defined nature of the singlesite supported Pd 2+ precursor provides direct mechanistic insights into this unusual and likely general reductive process.
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.
Hydrodechlorination catalysis of Pd-on-Au nanoparticles varies with particle size
Journal of Catalysis, 2013
Trichloroethene (TCE), a common carcinogen and groundwater contaminant in industrialized nations, can be catalytically degraded by Au nanoparticles partially coated with Pd (“Pd-on-Au NPs”). In this work, we synthesized Pd-on-Au NPs using 3, 7, and 10 nm Au NPs with Pd surface coverages between 0–150% and studied how particle size and composition influenced their TCE hydrodechlorination (HDC) activity. We observed volcano-shape dependence on both Au particle size and Pd surface coverage, with 7 nm Au NPs with Pd coverages of 60–70% having maximum activity. Using extended X-ray absorption fine-structure spectroscopy, we found a strong correlation between catalytic activity and the presence of 2-D Pd ensembles (as small as 2–3 atoms). Aberration-corrected scanning transmission electron microscopy further confirmed the presence of Pd ensembles. The Pd dispersion and oxidation state generally changed from isolated, metallic Pd atoms to metallic 2-D Pd ensembles of varying sizes, and to partially oxidized 3-D Pd ensembles, as Pd surface coverage increased. These changes occurred at different surface coverages for different Au particle sizes. These findings highlight the importance of controlling particle size and surface coverage in bimetallic catalysts.
Oxidation and reduction of Pd(100) and aerosol-deposited Pd nanoparticles
Physical Review B, 2011
Using in situ high-pressure x-ray photoelectron spectroscopy, we have followed the oxidation and the reduction of Pd model catalysts in oxygen and CO pressures in the millibar range. The study includes a Pd(100) single crystal as well as SiO x -supported Pd nanoparticles of 15 or 35 nm diameter, respectively. We demonstrate that nanoparticles also form ultrathin surface oxides prior to the onset of the bulk PdO. The Pd nanoparticles are observed to bulk oxidize at sample temperatures 40 degrees lower than the single-crystal surface. In the Pd 3d 5/2 and the O 1s spectrum, we identify a component corresponding to undercoordinated atoms at the surface of the PdO oxide. The experimentally observed PdO core-level shift is supported by density functional theory calculations. In a CO atmosphere, the Pd 3d 5/2 component corresponding to undercoordinated PdO atoms is shifted by + 0.55 eV with respect to PdO bulk, demonstrating that CO molecules preferably adsorb at these sites. CO coordinated to Pd atoms in the metallic and the oxidized phases can also be distinguished in the C 1s spectrum. The initial reduction by CO is similar for the single-crystal and the nanoparticle samples, but after the complete removal of the oxide we detect a significant deviation between the two systems, namely that the nanoparticles incorporate carbon to form a Pd carbide. Our results indicate that CO can dissociate on the nanoparticle samples, whereas no such behavior is observed for the Pd(100) single crystal. These results demonstrate the similarities, as well as the important differences, between the single crystals used as model systems for catalysis and nm-sized particles on oxide supports.
ACS Catalysis, 2021
Hydrogen peroxide production by direct synthesis (H 2 + O 2 → H 2 O 2) is a promising alternative to the commercialized indirect process involving sequential hydro genation and oxidation of anthraquinones. Metal dopants are known to enhance the performance of Pd based catalysts in this reaction by increasing H 2 O 2 rates and selectivity. Recently, binary and ternary Pd based alloys with Pb have been proposed as catalysts by theoretical studies, but these compositions lack experimental proof. Herein, shape selective Pd 3 Pb nanocrystals were created to produce catalysts where the active and doping metal are colocalized to a fine extent. This strategy enables us to study the effects of both Pb doping and nanocrystal shape on the catalytic performance in direct H 2 O 2 synthesis. In order to achieve these goals, we developed a procedure for the shape controlled synthesis of Pb doped nanocrystals with phase pure, intermetallic Pd 3 Pb composition. By a change of the ligands, uniform Pd 3 Pb nanocrystals with cubic, cuboctahedral, and spherical shapes as well as flowerlike aggregates were obtained, which were supported on acid treated TiO 2. We show that the catalytic efficiency in direct H 2 O 2 synthesis not only is influenced by the nanocrystal composition but also depends on the particle shape. Pd 3 Pb cubes, predominately terminated by their (200) facets, outperformed not only the monometallic Pd reference catalyst but also Pd 3 Pb nanocrystals with other shapes. Further DFT calculations and surface studies indicated not only the electronic modification of Pd surface atoms with a higher barrier for O 2 dissociation on Pd 3 Pb but also a lack of larger Pd ensembles in Pd 3 Pb cubes which are known to cleave O−O bonds and form water.
2018
The electrochemical hydrogen evolution reaction (HER) is an emerging route for producing clean hydrogen. Recently, Ru-based catalysts have exhibited good activity towards the HER, making them attractive substitutes for Pt. However, the HER performance of Ru-based catalysts still cannot reach the same level as that of commercial Pt/C in acid solution. The atomic layer deposition (ALD) technique has proved to be an effective route for the preparation of noble metal catalysts on substrates. Herein, we successfully deposited RuOx catalysts on octahedral and cubic Pd seeds through an ALD process. Interestingly, we found that RuOx was selectively deposited on Pd particles when Pd/nitrogen-doped carbon nanotubes (NCNTs) were used as the substrate. Importantly, the as-prepared Pd@RuOx/NCNT showed comparable activity with state-of-the-art commercial Pt/C catalysts for the HER. The performance exceeds all of the current Ru-based nanocatalysts for the HER in acid solution. The X-ray absorption...
Applied Catalysis B: Environmental, 2018
A widespread approach to modulate the performances of heterogeneous catalysts is the use of bimetallic nanoparticles (NPs) as the active phase. However, studying the relationship between the NPs structure and catalytic properties requires well-defined systems, having uniform composition, size and nanostructure, which cannot be achieved by traditional methods (e.g. impregnation). Here, we developed wet-chemistry synthetic routes to prepare PdePt NPs or Pt-core@Pd-shell NPs of small size and well-controlled composition and structure, protected by mercaptoundecanoic acid (MUA) moieties. The pristine NPs were tested for H 2 production by NH 3 BH 3 hydrolysis, in order to systematically investigate the effect of composition and of synthetic route on the activity of the systems. Depending on the preparation method, two distinct trends of activity were observed, rationalized in terms of the extent of surface functionalization by MUA. The MUA protective layer was found to effectively stabilize the NPs dispersion while maintaining high activity in certain cases (Pt-rich NPs), and was demonstrated to be essential for catalyst recycling. In order to further study structure-activity relationships of PdePt NPs after ligand removal, nanostructured PdePt@CeO 2-based catalysts were prepared by self-assembly route. Regardless of the starting NPs structure (alloy or core-shell), similar water gas shift reaction performances were observed, due to the structural rearrangements occurring upon oxidation and reduction thermal treatments, which led to the formation of Pt-rich core@PdePt-shell under reducing conditions.
Atomic {Pdn+-X} States at Nanointerfaces: Implications in Energy-Related Catalysis
Energies
Palladium is among the most versatile noble-metal atoms that, when dispersed on solid supports, can be stabilized in 0, +1, +2, +3 redox states. Moreover, despite its noble-metal character, Pd shows a considerable degree of chemical reactivity. In Pd Nanoparticles (NPs), atomic {Pdn+-X} states, where n = 0, 1, 2, 3, and X = atom or hydride, can play key roles in catalytic processes. Pd-oxygen moieties can be stabilized at nanointerfaces of Pd in contact with metal-oxides. These {Pdn+-X}s can be either isolated Pd atoms dispersed on the support, or, more interestingly, atomic states of Pd occurring on the Pd NPs. The present review focuses on the role of such {Pdn+-X} states in catalytic processes related to energy storage or energy conversion, with specific focus on photocatalysis, H2 production reaction (HRR), oxygen reduction reaction (ORR), and water-splitting. Synthesis of atomic {Pdn+-X} states and their detection methodology is among the current challenges. Herein, the chemist...
Journal of Catalysis, 2007
Model catalytic studies under ambient conditions require materials with well-defined structures for the establishment of clear relationships between structural and catalytic properties. In the present work, such a system was prepared by controlled decomposition of organometallic Pd precursors on a MgO support. The resulting Pd nanoparticles of highly defined morphology were characterized by high-resolution transmission electron microscopy and investigated with respect to their interaction with CO, O 2 , and H 2 by IR spectroscopy (DRIFTS). Temperature-dependent CO adsorption experiments revealed the available adsorption sites on the Pd particles and indicated CO disproportionation on the Pd particles. We report in particular on the interplay between metal and support in these processes, which was analyzed by a thorough comparison of Pd/MgO with the pure MgO support. Investigations of the interaction with CO and H 2 suggest that Pd facilitates the formation of formates on MgO via dissociation of hydrogen on the Pd particles. Furthermore, CO oxidation was studied as a function of the CO/O 2 ratio in the temperature range 100-150 • C.
Shape-Dependence of Pd Nanocrystal Carburization during Acetylene Hydrogenation
The Journal of Physical Chemistry C, 2015
This interdisciplinary work combines the use of shape-and size-defined Pd nanocrystals (cubes of 10 and 18 nm, and octahedra of 37 nm) with in situ techniques and DFT calculations to unravel the dynamic phenomena with respect to Pd reconstruction taking place during acetylene hydrogenation. Notably, it was found that the reacting Pd surface evolved at a different pace depending on the shape of the Pd nanocrystals, due to their specific propensity to form carbides under reaction conditions. Indeed, Pd cubes (Pd(100)) reacted with acetylene to form a PdC 0.13 phase at a rate roughly 6-fold higher than that of octahedra (Pd(111)), resulting in nanocrystals with different degrees of carburization. DFT calculations revealed changes in the electronic and geometric properties of the Pd nanocrystals imposed by the progressive addition of carbon in its lattice.