Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts (original) (raw)
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Journal of Physical Chemistry C, 2007
The surface structure of Pd-Ag alloy and its alteration in the presence of atomic hydrogen have been studied using density functional calculations on Pd 1-x Ag x (111) (x ≈ 0.2) models. In the absence of an adsorbate, silver atoms are found to segregate on the surface, in line with previous experimental observations under vacuum conditions. At equilibrium, the surface is predicted to expose mainly Ag atoms. Isolated Pd atoms incorporated in this Ag-rich layer appear to be slightly preferred over the Pd 2 dimers. Increasing the coverage of adsorbed H atoms on the Pd-Ag substrate gradually suppresses surface segregation of silver, such that migration of all surface Ag atoms into the subsurface region becomes favorable at a H coverage of ∼0.25 ML. For the latter structures, with solely Pd atoms in the surface layer and Ag atoms in the subsurface layer, the propensity of H to be accommodated in interstitial sites below the surface layer essentially vanishes: subsurface H atoms are predicted to be energetically driven to escape to the surface without an activation barrier. These results might have strong implications in understanding the promoting role of Ag in selective hydrogenation reactions over Pd-Ag catalysts. The presented adsorbate-induced resegregation in bimetallic systems is a general concept applicable to a broad variety of catalytic systems and advanced materials.
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.
Journal of Catalysis, 2009
Previous works have shown that palladium overlayers (Pd/Ni, Pd/Cu) are more active than pure Pd surfaces for alkene hydrogenation. These results have been ascribed to the specific nanostructure of the alloy surfaces. Here, we compare Pd(100), Pd(110) and Pd 8 Ni 92 (110) single-crystal surfaces toward 1,3butadiene hydrogenation and hydrogen absorption, using a gas-phase static reactor. We show that the lower rate of butene formation on clean Pd surfaces can in fact be explained by the initial fast diffusion of hydrogen into the Pd crystal (conversely, hydrogen dissolution in Pd-Ni is negligible). However, the activity of Pd becomes higher at steady state, i.e. after several reaction cycles, due to the increase of the near-surface H concentration. Unlike the butane formation rate, the partial hydrogenation rate appears poorly affected by the Pd surface structure. These results suggest that, when hydrogen supply is ratedetermining, hydrogen absorption effects can be more critical than structural effects for Pd-catalyzed hydrogenations.
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.
The surface structure of the catalyst is a key factor to affect its catalytic performance toward the targeted reaction. In this work, aiming at revealing the surface structure influences of Pd-based alloy catalysts on the catalytic performance of C 2 H 2 selective hydrogenation, four kinds of surface structures of Pd-based alloy catalysts, including the core−shell Pd nL @M (M = Cu and Ag), the core−shell Pd nL @Pd x M y , the uniform alloy Pd 1 Cu 3 and Pd 1 Ag 1 , and the subsurface structure Pd 1L-M sub are engineered, and the corresponding catalytic performance is fully examined using DFT calculations. Our results reveal that the catalytic performance of C 2 H 2 selective hydrogenation is closely related to the surface structures of Pd-based alloy catalysts; among them, the
Chemical communications (Cambridge, England), 2015
Three distinctive doping effects to modify the electronic and geometric properties of Pd nanocrystals for HCOOH decomposition to H2/CO2 are presented: Bi atoms take preferable residence on higher index sites, which leads to a reduction in HCOOH dehydration; Te atoms dwell favourably on terrace sites, which reduces the rate of dehydrogenation; Ag atoms, without site specificity, induce strong electronic effects to promote the activity on the dwindling number of surface Pd sites at high coverage.
Theory meets experiment: Electrocatalysis of hydrogen oxidation/evolution at Pd–Au nanostructures
Catalysis Today, 2011
The oxidation and evolution reactions of hydrogen are investigated at nanostructures of palladium-gold combining experimental and theoretical approaches. The extraordinary reactivity of submonolayers of Pd on Au(1 1 1) has been explained by a direct correlation with the changes in the electronic properties and geometrical arrangements. The application of the electrocatalysis theory, which goes beyond the qualitative approach of the d band centers, allows explaining quantitatively the experimental finding.
ChemCatChem, 2012
This paper reviews recent experimental and theoretical results on palladium-based catalysts in selective hydrogenation of alkynes obtained by a number of collaborating working groups in a joint multimethod and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X in the sub-surface of Pd metal was observed by in situ spectroscopy for X = H, C under hydrogenation conditions. Under certain conditions (low H 2 partial pressure) alkyne fragmentation leads to formation of a Pd-C surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface considerably increases the selectivity of alkyne semi-hydrogenation over Pd-based catalysts by decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that Pd-C hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high leading to rather unselective hydrogenation over in situ formed bulk Pd-H. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X = Ga. As a great advantage, such Pd x Ga y catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd 3 Ga 7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga 2 O 3 upon heating of Pd/Ga 2 O 3 in hydrogen was shown to lead to formation of Pd-Ga intermetallic compounds at moderate temperatures. In this way, Pd 5 Ga 2 and Pd 2 Ga are accessible in form of supported nanoparticles, in thin film models and realistic powder samples, respectively.
Applied Catalysis A: General, 2016
The thermal effects and activity of silica and alumina supported bimetallic Pd-Au catalysts (of various Pd/Au ratio) in the exothermic H 2 and O 2 recombination reaction have been investigated in view of their potential use in the industrial passive autocatalytic recombiners (PAR). The catalysts were prepared by the colloid-based reverse "water-in-oil" microemulsion method which provided metal particles of size in a very narrow range (4-7 nm). In both SiO 2 and Al 2 O 3 − series catalysts the Pd-Au particles aggregated to some extent, especially strongly in alumina-series samples. The H 2 + O 2 reaction has been monitored using Microscal gas-flow through microcalorimeter at temperature of 22 • C and atmospheric pressure. The observed pattern of changes in both the heat evolution and the conversion of hydrogen seem to reflect the effect of water and/or other oxygen-containing surface species (like OH) on the activity/deactivation of catalysts. The nature of support and the composition of metal particles (Pd/Au ratio) played a role. Deactivation of alumina supported catalysts was stronger than silica supported counterparts. Among all studied catalysts, the best behavior was offered by low Au content-containing Pd-Au-0.1/SiO 2 (Pd 90 Au 10) catalyst. Its almost stable activity during the catalytic run may be attributed to relatively weak interactions with water molecules and/or other oxygen-containing species (like OH), intermediates formed in the hydrogen oxidation. It may be supposed that electronic modification of palladium sites by gold assisted by the surface composition of Pd-Au particles reflecting in "surface arrangement of Pd and Au-atoms" are decisive. This experimental observation seems to correlated with the DFT calculation indicating that besides the number of Au atoms, their location with respect to the Pd, e.g "surface arrangement of Au" is more important for the energy/strength of interaction with water molecules.