Pd Ag Catalysts Research Articles

Silicate fiberglass catalysts: From science to technology

Efficient methods for the confining of highly dispersed Pd and Pt particles 1–1.5 nm in size in the subsurface layers of silicate glass fibers at depths of 10–20 nm are developed. Fiberglass catalysts reveal high activity in the selective hydrogenation of acetylene and the deep oxidation of hydrocarbons despite their low metal contents (0.01–0.02 wt %). The monometallic Pd/FG catalyst thus exhibits higher C2H2 hydrogenation selectivity at high acetylene conversion (99%+) in comparison to the commercial bimetallic Pd–Ag catalyst, and the activity of the Pt/FG catalyst in the deep oxidation of organic and chlororganic compounds, including dioxins, is found to be higher than that of the commercial AP-56 (Pt/Al2O3) catalyst. The efficiency of Pt/FG has successfully been demonstrated in the removal volatile organic compounds (VOC) from Monomer plant of PJSC Nizhnekamskneftekhim for more than four years of continuous operation.

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Pd–Ag alloy catalysts with very dilute amounts of Pd were synthesized. EXAFS results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H2 dissociation. In addition, for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein molecule for facile hydrogenation of the aldehyde functionality.

Fabrication of a PdAg mesocrystal catalyst for the partial hydrogenation of acetylene

Mesocrystalline PdAg nanoalloys have been assembled from PdAg nanoparticle units via an oriented attachment pathway and subsequently immobilized on hydrotalcite—a support that offers the advantages of tunable basicity and acidity, as well as high adsorption capacity. Pd and Ag atoms were homogeneously distributed throughout the sea urchin-like mesocrystals. The supported PdAg mesocrystals showed a significant enhancement in catalytic performance in the partial hydrogenation of acetylene when compared with the conventional PdAg nanoparticles, with a fourfold increase in turnover frequency. The high activity can be ascribed to the presence of grain boundaries in the mesocrystalline PdAg catalyst, which facilitate H2 activation/dissociation. In addition, the increase in the selectivity toward ethene over supported PdAg mesocrystals catalyst may be ascribed to the potential ordered facets, which originated from the oriented attachment of building units.

Role of PdAg interface in Pd-Ag/SiO2 bimetallic catalysts in low-temperature oxidation of carbon monoxide

An approach to the development of bimetallic nanoparticles with the separated but interacting phases of silver and palladium was proposed to effectively separate the active sites of silver, which possess high activity in activation of molecular oxygen, and the sites of palladium-containing phases, which are responsible for CO adsorption. The structure of PdAg particles was determined using X-ray diffraction analysis, electronic diffuse reflectance spectroscopy, and temperature-programmed reduction. It was found that the introduction of silver led to easier reduction of palladium from oxidized states in an atmosphere of CO. The activity of the bimetallic PdAg catalysts in the oxidation of CO increased, as compared with that of the Pd/SiO2 catalyst. The oxidized bimetallic catalyst, in which palladium occurred in an oxide state, was even more effective in the oxidation of CO; this can be related to the better cooperation of the active sites of silver and palladium oxide, which are responsible for the activation and conversion of CO and oxygen at the Ag-PdO interfacial boundary.

Room‐Temperature Hydrogenation of Citral Catalyzed by Palladium–Silver Nanocrystals Supported on SnO 2

We have developed two strategies to optimize Pd catalysts. On one hand, Ag was introduced into Pd and then they were applied to the selective hydrogenation of citral under mild conditions. The addition of metallic Ag could tune the selectivity of Pd and made it suitable for the selective hydrogenation of the conjugated double bond. The selectivity of citronellal increased from 0 (for Pd/C and Pd0.7Ag0.3/C) to 96 % (for Pd0.4Ag0.6/C) as the Ag content increased. On the other hand, the addition of SnO2 made the Pd catalysts more inclined to activate the C=O bond and gave better performance for the hydrogenation of the conjugated double bond compared with the corresponding Pd–Ag catalysts. The selectivity of citronellal increased from 0 (for Pd0.7Ag0.3/C) to 76 % (for Pd0.7Ag0.3–SnO2/C was used) after the addition of SnO2. On the basis of these results, we developed a catalyst (Pd0.6Ag0.4–SnO2/C) with the best catalytic performance for the selective hydrogenation of citral (the conversion of citral reached 99 %, and the selectivity was up to 96 %).

Bimetallic Supported Pd-Ag Catalysts

Series of Pd-Ag/SiO2, Pd/CeO2 and Pd-Ag/CeO2 catalysts with different Pd/Ag ration were synthesized and investigated. It was shown that simultaneous reduction of Pd and Ag took place for Pd-Ag/SiO2 and Pd Ag/CeO2 catalysts during TPR-H2. That may be associated with formation of bimetallic Pd-Ag particles. The high temperature of Pd reduction was observed for Pd/CeO2 and Pd-Ag/CeO2 catalysts. That was associated with strong interaction of PdO with CeO2 or formation of mixed PdxCeO2-δ phase.

Partial hydrogenation of acetylene over a NiTi-layered double hydroxide supported PdAg catalyst

Supported PdAg/NiTi-LDH with rich defective sites was synthesized and exhibited high activity and selectivity in the partial hydrogenation of acetylene.

Comparative study of carbon-supported Pd and PdAg catalysts synthesised by the polyol process and reverse micelles methods

Carbon-supported Pd and PdAg nanoparticles were synthesised by the polyol (P) and reverse micelles (RM) methods for use in a comparative study as catalysts for ethanol electro-oxidation in alkaline media. The electrocatalysts were physically and chemically characterised by transmission electron microscopy, X-ray diffraction and thermogravimetric analysis. Electrochemical characterisation was performed by cyclic voltammetry in the absence and presence of ethanol (0.1, 1, 3 M ethanol), chronoamperometry and electrochemical impedance spectroscopy. The electrochemical measurements showed that the catalysts obtained by the RM method presented excellent dispersion and small crystal size, resulting in materials with high electro-active area. However, the PdAg/C catalysts prepared by the polyol process presented better catalytic activity and stability during the electro-oxidation reaction. In addition to the excellent dispersion and low agglomeration promoted by the polyol method, catalytic enhancement can also be associated with the synergic effect between Pd and Ag elements.

Cu/Al2O3 catalysts modified with Pd for selective acetylene hydrogenation

A Cu/alumina catalyst has been modified by addition of various quantities of Pd. Characterisation suggests formation of a bimetallic with the surface dominated by Cu. Optimisation of the Cu:Pd ratio (50:1) resulted in a catalyst which combined the properties of Cu and Pd (high ethylene selectivity and activity at low temperature). Using a 3:1 H2:acetylene feed, >99% acetylene conversion and >70% ethylene selectivity was attained at 373K, representing a considerable reduction in the temperature necessary to achieve equivalent activity/selectivity over monometallic Cu. Increasing the H2:acetylene ratio to 10 resulted in >99% acetylene conversion at 363K as well as enhanced ethylene selectivity (>80%). Enhanced activity of Cu at low temperature is attributed to H2 dissociation on Pd with spillover onto Cu sites where the reaction takes place. These bimetallic CuPd catalysts offer sufficient activity at low temperature and could represent a replacement to the current industrial PdAg catalysts.

A novel TiO2 nanofiber supported PdAg catalyst for methanol electro-oxidation

A novel PdAg/TiO2 nanofiber electrocatalyst for methanol oxidation was fabricated by using TiO2 nanofiber as support. The structural and electrochemical properties of the PdAg/TiO2 nanofiber catalyst were characterized by XRD (X-ray diffraction), TEM (the transmission electron microscopy) and electrochemical analysis. The experiment results showed that the large surface area and porous structure of TiO2 nanofiber were preferable to the dispersion of nanosized PdAg alloy on the surface of TiO2 nanofiber support. The electrocatalytic activity for methanol oxidation of the PdAg/TiO2 nanofiber catalyst has shown promising electrocatalytic and anti-poisoning capabilities. The methanol oxidation mechanism of the PdAg/TiO2 nanofiber catalyst was mainly resulted from the photocatalytic function of the composite TiO2 material that had high catalytic activity without UV light illumination. The PdAg/TiO2 catalyst might become a promising candidate for direct methanol fuel cell.

Performance, structure and mechanism of Pd–Ag alloy catalyst for selective oxidation of glycerol to dihydroxyacetone

The mechanism of the oxidation of glycerol to dihydroxyacetone over Pd–Ag catalysts was discussed. Characterization results suggest that the metal composition of the surface of the crystalline Pd–Ag alloy particles is almost the same as the bulk composition. The synergetic effects of Pd and Ag appear on the oxidation of the secondary OH group of vic-diols. The reaction order with respect to glycerol concentration over Pd–Ag/C was zero, suggesting the strong interaction between glycerol and the catalyst surface. The reaction order with respect to O2 pressure was 0.4, suggesting that the rate-determining step is the reaction involving oxygen species. These reaction trends and characterization results support the mechanism where the terminal OH group of glycerol is adsorbed on the Ag site and the neighboring secondary OH group (CH–OH) is attacked by the oxygen species dissociatively adsorbed on the Pd site.

Bimetallic catalysts for the hydrogenation of aromatic nitro compounds

The catalytic activity of bimetallic Pd-Ni, Pd-Cu, and Pd-Ag catalysts in the reaction of nitrobenzene hydrogenation was studied. It was found that the rate of hydrogenation on the bimetallic catalysts was higher than that on palladium metal (palladium black), and it depended on catalyst preparation pro cedures. The catalytic activity of the bimetallic systems increased in the order Pd-Cu< Pd-Ag ≪ Pd-Ni.

Selective oxidation of glycerol to dihydroxyacetone over a Pd–Ag catalyst

Glycerol oxidation with molecular oxygen was conducted using Pd-based catalysts under neutral conditions. Pd–Ag/C showed higher selectivity to dihydroxyacetone (DHA) and higher activity than Pd/C. The DHA yield reached 44% at 52% glycerol conversion over Pd–Ag/C (Ag/Pd = 1). The Pd–Ag alloy phase is responsible for the high activity and DHA selectivity.

Ethanol electro-oxidation activity of Nb-doped-TiO2 supported PdAg catalysts in alkaline media

Various mesoporous Nb-doped titania materials were synthesized by a hydrothermal method and examined as catalyst support for PdAg alloy particles for alkaline direct ethanol fuel cell (ADEFC). X-ray diffraction, nitrogen adsorption/desorption, transmission electron microscopy (TEM), cyclic voltammetry (CV) and chronoamperometry (CA) were used to investigate the properties of the supports and catalysts. 700°C was found as the optimal activation temperature for Nb-doped TiO2 due to the high electronic conductivity achievement and mesoporosity conservation. X-ray diffraction (XRD) revealed that NbxTi1−xO2 had a structure of mixed anatase and rutile phases. PdAg/NbxTi1−xO2 exhibited excellent performance for the electrooxidation of ethanol and higher durability in alkaline solution compared with PdAg/C and PdAg/commercial TiO2. The results open a new way to develop durable and effective electrocatalysts for ADEFC.

Performance of Ni-added Pd-Ag/Al2O3 catalysts in the selective hydrogenation of acetylene

Effects of Ni addition on the performance of Pd-Ag/Al2O3 catalysts in the selective hydrogenation of acetylene were investigated. Ni-added Pd-Ag catalysts showed higher conversions than Ni-free Pd-Ag catalyst under hydrogen-deficient reaction conditions, hydrogen/acetylene <2.0, due to the spillover of hydrogen from reduced Ni to Pd and the suppression of hydrogen penetration into the Pd bulk phase, which enriched the Pd surface with hydrogen. Ethylene selectivity was also increased by Ni addition because the amounts of surface hydrogen originating from the Pd bulk phase, which was responsible for the full hydrogenation of ethylene to ethane, were decreased due to the presence of Ni at the sub-surface of Pd-Ag particles. Added Ni also modified the geometric nature of the Pd surface by blocking large ensembles of Pd into isolated ones, which eventually improved ethylene selectivity.

Divergent catalytic behaviors of Pt and Pd films in the cathode of a sandwiched type membrane reactor

Relative electrochemical and catalytic hydrogenation responses to NO3− reduction by the Ag modified Pt, Pd and Pt–Pd cathodes, deposited on H+ conducting Nafion membrane, have been studied. The electrochemical investigations noticed the maximum reactivity to show by an Ag–Pt electrode exhibiting a reduction current of 24.26mAcm−2 at −1.26V vs. Ag/AgCl reference electrode. Conversely, the catalytic hydrogenation reaction exhibited maximum nitrate reduction rate by an Ag–Pd catalyst having a rate constant of 25.1×10−3min−1. While both electrochemical reactions and catalytic hydrogenation reactions were combined in the electrolyte reactor, the highest reduction efficiency was found to show by an Ag–Pt–Pd cathode assembly, where the reduction current was 20.12mAcm−2 and the rate constant was 84.3×10−3min−1 under controlled current electrolysis condition. In this paper, reasons of such divergent catalytic behaviors have been discussed.

Preparation and characterization of Pd xAg y/C electrocatalysts for ethanol electrooxidation reaction in alkaline media

Carbon-supported bimetallic PdAg catalysts with Pd/Ag atomic ratios varying from 4/1 to 1/2 were prepared by an impregnation–reduction method. The impregnated black mixture was treated in H 2/N 2 atmosphere at a temperature varying from 180 to 500 °C. The obtained Pd x Ag y /C catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV) and chronoamperometry (CA). XRD results show that the lattice constant of Pd is dilated, suggesting the formation of PdAg alloy. The lattice constant of Pd for the Pd x Ag y /C-500 (reduced at 500 °C by H 2) increases linearly and the average metal particle size decreases slightly from 6.8 to 5.1 nm with increasing Ag fractions from 20% to 67% in the PdAg composition. For Pd x Ag y /C catalysts with a certain specific Pd/Ag atomic ratio, e.g., Pd 2Ag 1/C, the dilated lattice constant of Pd is independent of the reducing temperature, indicating the alloy degree for the Pd 2Ag 1/C- t catalysts is comparable. The average metal particle size for the Pd 2Ag 1/C- t catalysts increases from 3.4 to 5.2 nm with H 2 reduction temperature increasing from 180 to 500 °C. The potentiodynamic measurements on ethanol electrooxidation reaction (EOR) show that the catalytic activities for the Pd x Ag y /C- t catalysts toward the EOR are improved by alloying Pd with Ag. At typical potential of a working fuel cell, e.g., −0.4 V vs. Hg/HgO, the EOR current density presents a volcano shape as a function of the Ag fractions in PdAg with the maximum occurs at the Pd/Ag atomic ratios between 2/1 and 3/1. The CA tests show that the Pd x Ag y /C-500 catalysts perform high stability than that of Pd/C-500. The improved EOR activity for the Pd x Ag y /C- t catalysts, compared with whether Pd/C or Ag/C catalyst, may possibly be attributed to the formation of PdAg alloy and the fitted particle size.

Pd-Ag Catalyst Supported on Nb-Doped Titania for Alkaline Direct Ethanol Fuel Cell

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Complex intermetallic compounds as selective hydrogenation catalysts – A case study for the (1 0 0) surface of Al 13Co 4

Recently, a novel concept for the design of selective and stable catalysts for the hydrogenation of alkynes has been announced – the isolation of the active sites on the surface of a complex intermetallic compound. The basic idea is that isolated active sites enable only a reduced number of possible adsorption geometries for the reactants, leading to a narrower range of possible reaction products. However, so far an atomistic scenario for the complex multi-step hydrogenation process catalyzed by complex intermetallic compounds has not yet been developed. Here, we present detailed ab initio density-functional simulations of the surface structure and of the selective hydrogenation of acetylene to ethylene on the (1 0 0) surface of the compound Al 13Co 4. In agreement with recent STM investigations, our calculations show that the surface is highly corrugated with well-separated Co surface atoms in the center of CoAl 5 pentagons. The activation energies for the rate-controlling steps are found to be comparable or even lower than those calculated for conventional Pd or Pd–Ag catalysts, and the desorption energy of ethylene is lower than the barrier to further hydrogenation to ethyl. This confirms that Al 13Co 4 is an efficient as well as highly selective catalyst. Our results demonstrate that the active sites are not the Co atoms alone, but pentagonal CoAl 5 clusters forming zig-zag chains on the surface separated by wide troughs. The high activity of the Al atoms in these clusters is promoted by the strong, partially covalent bonding between Al and Co atoms, as well as by their low coordination in a surface with a very complex topology.

Selective Hydrogenations with AgPd Catalysts Prepared by Galvanic Exchange Reactions

Treading on eggshells: AgPd nanoparticles were prepared by galvanic exchange reactions and used as hydrogenation catalysts for allylic alcohols. The results show that thin Pd shells on Ag cores leads to improved selectivity of hydrogenation reactions versus isomerization reactions. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine that nanoparticles were more accurately described as cluster-in-cluster structures with Pd clusters on a central Ag core.