Pd Ag Catalysts Research Articles

In Situ Surface Structures of PdAg Catalyst and Their Influence on Acetylene Semihydrogenation Revealed by Machine Learning and Experiment.

PdAg alloy is an industrial catalyst for acetylene-selective hydrogenation in excess ethene. While significant efforts have been devoted to increase the selectivity, there has been little progress in the catalyst performance at low temperatures. Here by combining a machine-learning atomic simulation and catalysis experiment, we clarify the surface status of PdAg alloy catalyst under the reaction conditions and screen out a rutile-TiO2 supported Pd1Ag3 catalyst with high performance: i.e., 85% selectivity at >96% acetylene conversion over a 100 h period in an experiment. The machine-learning global potential energy surface exploration determines the Pd-Ag-H bulk and surface phase diagrams under the reaction conditions, which reveals two key bulk compositions, Pd1Ag1 (R3̅m) and Pd1Ag3 (Pm3̅m), and quantifies the surface structures with varied Pd:Ag ratios under the reaction conditions. We show that the catalyst activity is controlled by the PdAg patterns on the (111) surface that are variable under reaction conditions, but the selectivity is largely determined by the amount of Pd exposure on the (100) surface. These insights provide the fundamental basis for the rational design of a better catalyst via three measures: (i) controlling the Pd:Ag ratio at 1:3, (ii) reducing the nanoparticle size to limit PdAg local patterns, (iii) searching for active supports to terminate the (100) facets.

Kinetics of liquid-phase diphenylacetylene hydrogenation on “single-atom alloy” Pd-Ag catalyst: Experimental study and kinetic analysis

• Pd 1 Ag 3 SAA catalyst demonstrates extremely high selectivity in diphenylethene (ca. 98 %). • Selectivity of Pd 1 Ag 3 does not depend on P(H 2 ) and DPA concentration. • Proposed kinetic model is capable to describe the experimental dependencies. Catalytic performance of Pd 1 Ag 3 /Al 2 O 3 single-atom alloy (SAA) catalyst was studied in liquid-phase hydrogenation of diphenylacetylene (DPA) and compared with the performance of the reference monometallic Pd/Al 2 O 3 . Formation of SAA structure in Pd 1 Ag 3 /Al 2 O 3 was confirmed by FTIR CO technique. It was found that Pd 1 Ag 3 /Al 2 O 3 single-atom alloy catalyst exhibits excellent selectivity in diphenylethene (stilbene) (ca. 98 %), which remains constant over a wide range of DPA conversions (0–95%), while over Pd/Al 2 O 3 selectivity decreases steadily with the increase in DPA conversion. It is remarkable that the selectivity of Pd 1 Ag 3 /Al 2 O 3 depends neither on hydrogen pressure (5–15 bar), nor on DPA concentration (0.0262 – 0.159 mol/l). In contrast, over the reference Pd/Al 2 O 3 selectivity tends to decrease with increasing P(H 2 ) and decreasing DPA concentration. The proposed reaction network comprises hydrogenation of DPA in a parallel formation of cis and trans -stilbene with following hydrogenation to bibenzyl and direct formation of the latter from the initial DPA. The calculations clearly shows the capability of the kinetic model to describe the experimental dependencies for Pd 1 Ag 3 single-atom catalyst in an excellent way with the degree of explanation equal to ca. 99 %.

Understanding composition-dependent catalytic performance of PdAg for the hydrogenation of 1,3-butadiene to 1-butene

The composition-dependent adsorption and catalytic properties of PdAg bimetallic surfaces for the partial hydrogenation of 1,3-butadiene to 1-butene were studied by density functional theory. With the increase of Ag content, the activity and selectivity for the partial hydrogenation of 1,3-butadiene to 1-butene were enhanced with the decrease in binding strength of adsorbates. The increase of surface Ag content in PdAg catalysts can thus enhance the catalytic performance of the hydrogenation of 1,3-butadiene.

Enhanced Oxygen Reduction Activity on Silver-Palladium Alloyed Thin Film Electrocatalysts in Alkaline Media

The oxygen reduction reaction (ORR) requires a large overpotential, which is a factor limiting the large-scale deployment of clean energy technologies such as hydrogen fuel cells (H2 FCs), direct methanol FCs, and metal-air batteries. Designing strategies to enhance catalyst activity, stability, and selectivity for the ORR is one method towards enhancing H2 FCs technology. Many predicted active catalysts are not stable in acidic media, resulting in limited catalyst options for standard proton exchange membrane H2 FCs. Some theoretically active, yet acid-unstable, catalysts, such as Ag, can be stabilized in alkaline media making anion exchange membrane (AEM) fuel cells an attractive alternative. Ag-rich catalysts are specifically ideal because of their low cost and Ag’s high abundance relative to other catalysts materials such as Pt. We have previously shown that Cu atoms in Ag-Cu alloys can be modified by Ag to produce an enhanced activity for the ORR at high Cu atomic loadings. Better understanding the nature of intrinsic enhancements for the ORR on alloys will help in the development of new ORR alloy electrocatalysts.Besides, Ag-Cu alloys, several other Ag-bimetallics (including Ag-Pd, Ag-Mn, Ag-Fe, and Ag-Co) have been predicted to be good catalysts for the ORR. Alloying could lead to activity enhancements via changes in electronic structure, strain, and/or localized geometry. The activity of Ag-based catalysts for the ORR needs to be enhanced. One strategy to do this is to alloy Ag, which binds oxygen adsorbates too weakly, to a metal such as Pd, which binds oxygen adsorbates too strongly and is also very active. In this work, we use a thin film model system to show intrinsic activity enhancements for the ORR on Ag-Pd catalysts, resulting in activities higher than those of both parent materials even after reducing the Pd content by half.Ag-Pd thin films of different compositions are prepared using electron beam physical vapor co-deposition. X-ray diffraction indicate alloying across the full compositional range tested and atomic force microscopy shows that the films are highly flat (roughness factor < 1.02). Electrochemical testing in 0.1 M KOH shows significant activity improvements for the Ag-Pd films. The films with +40 at% Pd have activities, at 0.9 V vs RHE, greater than the linear combination of pure Ag and Pd activities. On a Pd content basis, the thin films with +50 at% Pd exhibit higher kinetic current densities, at 0.9 V vs RHE, than both pure Ag and Pd. Ag10Pd90 is the most enhanced thin film, both on a surface area and Pd loading bases, demonstrating a 5-fold activity enhancement relative pure Pd. By correlating density functional theory with experimental activity and physical characterization measurements, we hypothesize that Ag is modifying Pd’s surface electronic structure to adsorb OH more weakly via ligand and strain effects. These enhanced Ag-Pd ORR catalysts could facilitate the large-scale implementation of efficient clean energy electrochemical systems such as fuel cells and metal-air batteries.

Additive-Free Aqueous Phase Synthesis of Formic Acid by Direct CO2 Hydrogenation over a PdAg Catalyst on a Hydrophilic N-Doped Polymer–Silica Composite Support with High CO2 Affinity

A series of N-doped polymer containing silica (NPS) materials were synthesized via condensation reactions of 3-aminopropyltriethoxysilane (APT) and various aldehydes. These materials were assessed ...

Study on the reaction mechanism of acetylene selective hydrogenation catalysts Pd-Ag/Al2O3

Taking active component Pd-Ag alloy and catalyst carrier Al2O3 as the research object, the catalytic mechanism of acetylene hydrogenation reaction was explored by quantum mechanics theory and Monte Carlo method. In this paper, the activation energy on the catalyst surface, H2 dissociation process, acetylene and ethylene hydrogenation reaction were calculated. From the thermodynamic point of view, the increase of the co-catalyst Ag content enhances the desorption of ethylene on the catalyst surface, reduces the probability of ethane formation, and thus enhances the selectivity of acetylene hydrogenation. Through further process simulation studies, the quantum chemistry method and Monte Carlo method can be used to obtain the mechanism and optimal conditions for the selective catalytic hydrogenation of acetylene, with saving a lot of manpower and material resources.

Understanding the influence of the composition of the Ag[sbnd]Pd catalysts on the selective formic acid decomposition and subsequent levulinic acid hydrogenation

Formic acid is obtained in equimolar amount with levulinic acid during the hydrolysis of cellulose and thus can be used as a sustainable hydrogen source in the direct levulinic acid hydrogenation towards gamma-valerolactone (biofuel additive). AgPd catalysts prepared by various methods and containing different Ag:Pd ratio were investigated in this context. By combining activity tests, characterization of the main physicochemical properties of the catalysts and DFT study of formic acid decomposition, the key factors responsible for the activity of AgPd catalysts in both the formic acid decomposition and the subsequent hydrogenation of levulinic acid were specified. Pd is shown to be active, but prone to poisoning by CO, while the CO poisoning remains limited on diluted AgPd alloy with strong intermetallic interaction, where its adsorption is very weak thanks to the isolation of Pd atoms. Therefore, the catalyst containing 4%Ag-1%Pd/AlOOH showed the highest selectivity in formic acid decomposition as well as the highest activity in levulinic acid hydrogenation (34% conversion in 5 h at 190 °C).

(Ag)Pd-Fe3O4 Nanocomposites as Novel Catalysts for Methane Partial Oxidation at Low Temperature.

Nanostructured composite materials based on noble mono-(Pd) or bi-metallic (Ag/Pd) particles supported on mixed iron oxides (II/III) with bulk magnetite structure (Fe3O4) have been developed in order to assess their potential for heterogeneous catalysis applications in methane partial oxidation. Advancing the direct transformation of methane into value-added chemicals is consensually accepted as the key to ensuring sustainable development in the forthcoming future. On the one hand, nanosized Fe3O4 particles with spherical morphology were synthesized by an aqueous-based reflux method employing different Fe (II)/Fe (III) molar ratios (2 or 4) and reflux temperatures (80, 95 or 110 °C). The solids obtained from a Fe (II)/Fe (III) nominal molar ratio of 4 showed higher specific surface areas which were also found to increase on lowering the reflux temperature. The starting 80 m2 g−1 was enhanced up to 140 m2 g−1 for the resulting optimized Fe3O4-based solid consisting of nanoparticles with a 15 nm average diameter. On the other hand, Pd or Pd-Ag were incorporated post-synthesis, by impregnation on the highest surface Fe3O4 nanostructured substrate, using 1–3 wt.% metal load range and maintaining a constant Pd:Ag ratio of 8:2 in the bimetallic sample. The prepared nanocomposite materials were investigated by different physicochemical techniques, such as X-ray diffraction, thermogravimetry (TG) in air or H2, as well as several compositions and structural aspects using field emission scanning and scanning transmission electron microscopy techniques coupled to energy-dispersive X-ray spectroscopy (EDS). Finally, the catalytic results from a preliminary reactivity study confirmed the potential of magnetite-supported (Ag)Pd catalysts for CH4 partial oxidation into formaldehyde, with low reaction rates, methane conversion starting at 200 °C, far below temperatures reported in the literature up to now; and very high selectivity to formaldehyde, above 95%, for Fe3O4 samples with 3 wt.% metal, either Pd or Pd-Ag.

Liquid-phase acetylene hydrogenation over Ag-modified Pd/Sibunit catalysts: Effect of Pd to Ag molar ratio

A series of bimetallic Pd–Ag catalysts supported on the carbon material Sibunit was synthesized for selective liquid-phase acetylene hydrogenation. Using XRD and EXAFS it was shown that palladium and silver in the 0.5% Pd–Ag/Sibunit can form bimetallic PdxAg(1−x) nanoparticles with the composition depending on the Pd:Ag ratio. The active component of Pd–Ag(1:0.25)/Sibunit, Pd–Ag(1:1)/Sibunit and Pd–Ag(1:4)/Sibunit is represented by the particles of Pd0.74Ag0.26, Pd0.60Ag0.40 and Pd0.41Ag0.59 nanoalloys, respectively. An increase of the quantity of silver decreases the catalyst activity. The XPS study revealed that this phenomenon is caused by a decrease in the surface concentration of palladium (geometrical effect) and an increase in the degree of palladium electronic modification with silver (electronic effect). The latter exerts a beneficial effect on the reaction selectivity: as the Pd:Ag molar ratio is raised to 1:2, selectivity to ethylene increases to 81% (X > 97%), which exceeds 2.7-fold the selectivity of Pd/Sibunit.

Porous organic polymer supported PdAg bimetallic catalyst for the hydrodeoxygenation of lignin-derived species

A nitrogen-riched porous organic polymer named as BA-M was synthesized by co-polymerization of benzene and aniline monomers with dimethoxymethane as cross-linker through a simple Friedel-Crafts alkylation reaction. A series of the porous organic polymer supported bimetallic PdAg nanoparticles catalysts were developed for the selective hydrodeoxygenation of lignin-derived species including vanillin, isoeugenol and so on. The remarkable catalytic performance with 100% conversion of vanillin and 100% selectivity to p-creosol were achieved over the Pd9Ag1/BA-M catalyst at mild condition (low hydrogen pressure and temperature). The Pd9Ag1/BA-M catalyst also showed excellent recyclability with the catalytic activity and selectivity being well preserved after six catalytic recycles. The interaction between metal nanoparticles and porous organic polymer support, together with the nitrogen species in porous organic polymer support were primarily responsible for the excellent catalytic performance of bimetallic PdAg catalysts.

Hydrogen Production from Formic Acid Attained by Bimetallic Heterogeneous PdAg Catalytic Systems

The production of H2 from the so-called Liquid Organic Hydrogen Carriers (LOHC) has recently received great focus as an auspicious option to conventional hydrogen storage technologies. Among them, formic acid, the simplest carboxylic acid, has recently emerged as one of the most promising candidates. Catalysts based on Pd nanoparticles are the most fruitfully investigated, and, more specifically, excellent results have been achieved with bimetallic PdAg-based catalytic systems. The enhancement displayed by PdAg catalysts as compared to the monometallic counterpart is ascribed to several effects, such as the formation of electron-rich Pd species or the increased resistance against CO-poisoning. Aside from the features of the metal active phases, the properties of the selected support also play an important role in determining the final catalytic performance. Among them, the use of carbon materials has resulted in great interest by virtue of their outstanding properties and versatility. In the present review, some of the most representative investigations dealing with the design of high-performance PdAg bimetallic heterogeneous catalysts are summarised, paying attention to the impact of the features of the support in the final ability of the catalysts towards the production of H2 from formic acid.

PdAg bimetallic electrocatalyst for highly selective reduction of CO2 with low COOH* formation energy and facile CO desorption

For electrocatalytic reduction of CO2 to CO, the stabilization of intermediate COOH* and the desorption of CO* are two key steps. Pd can easily stabilize COOH*, whereas the strong CO* binding to Pd surface results in severe poisoning, thus lowering catalytic activity and stability for CO2 reduction. On Ag surface, CO* desorbs readily, while COOH* requires a relatively high formation energy, leading to a high overpotential. In light of the above issues, we successfully designed the PdAg bimetallic catalyst to circumvent the drawbacks of sole Pd and Ag. The PdAg catalyst with Ag-terminated surface not only shows a much lower overpotential (-0.55 V with CO current density of 1 mA/cm2) than Ag (−0.76 V), but also delivers a CO/H2 ratio 18 times as high as that for Pd at the potential of -0.75 V vs. RHE. The issue of CO poisoning is significantly alleviated on Ag-terminated PdAg surface, with the stability well retained after 4 h electrolysis at -0.75 V vs. RHE. Density functional theory (DFT) calculations reveal that the Ag-terminated PdAg surface features a lowered formation energy for COOH* and weakened adsorption for CO*, which both contribute to the enhanced performance for CO2 reduction.

Volcano curves for homologous series reactions: Oxidation of small alkanes

Volcano curves are typically developed and used for predicting new catalysts for a single reaction and one substrate. The concept of using the volcano curve to predict catalysts for an entire homologous series of reactions has been unexplored. Herein the catalytic activity of seven monometallic catalysts (Pt, Pd, Rh, Ag, Ni, Cu, and Co/Al2O3) and three Ag-Pd/Al2O3 bimetallic catalysts is evaluated in the total oxidation of small alkanes (methane, ethane, propane, and isobutane) in the 280–400 °C temperature range under fuel lean and rich conditions. We show that hysteresis in activity, with three distinct kinetic regimes, is a common phenomenon of alkane oxidation over all catalysts studied when varying the oxygen concentration, and the size of the hysteresis loop depends on the oxophilicity of the catalyst and the reducing ability of the hydrocarbon. Expectedly, the concept of the universality of the volcano curve for a homologous series is valid but only when a suitable surrogate substrate is chosen. Hydrocarbons with two or more carbon atoms can serve as surrogates of the alkane homologous series, whereas methane is not. Interestingly and consistent with the hysteresis, the feed composition controls the catalyst oxidation state and potentially impacts the optimal catalyst descriptors used to determine new catalysts. The predicted 1:3 Ag-Pd catalyst is indeed superior to single metals for the homologous series under fuel lean conditions for ethane and larger alkanes. It is inferior to Pt under fuel rich conditions and better than Pd and Pt for methane rich conditions. A method for qualitative inference of the catalyst structure, based on the volcano curve and the oxidation state of the catalyst, is proposed.

Electrochemical Ammonia Synthesis Using a BZCYYb4411 Proton Conductor

The Haber Bosch process is the dominant route to synthesizing ammonia. Ammonia is an important component in fertilizers, which are widely used across the agriculture community. Dependence on the Haber Bosch process is detrimental because: (1) this process operates at high temperatures (~450°C)1 and pressures (150-300 bar) and accounts for 2% of the worlds energy supply2, (2) contributes 1% of all CO2 emissions, and (3) is typically centralized which limits access in developing worlds. Electrochemical ammonia production has the potential to greatly reduce society’s carbon footprint and energy consumption, as well as increase ammonia production in developing worlds and rural communities. This type of ammonia synthesis method has been shown to operate with air and water sources as opposed to highly pure nitrogen and hydrogen reactants used in commercial plants. Solid state electrochemical systems are able to operate at elevated temperatures (300-600 °C) unlike ambient processes (low temperature fuel cells) and thus have the potential for higher yields. Solid state ceramic proton conductors show high ionic conductivities (1E-2 Scm-1 at 600°C)3 and are stable in this intermediate temperature range. Currently, the investigation of intermediate temperature ion conductors in solid state ammonia synthesis is nascent but growing in interest. Herein, we describe the synthesis and processing of a highly conductive proton conductor BaZr0.4Ce0.4Y0.1Yb0.1O3- δ (BZCYYb4411) (Fig. 1a,b) is characterized and implemented in a solid state fuel cell arrangement. BZCYYb4411 is synthesized using a solid-state reaction route and Ag-Pd catalyst is used to make a symmetric cell. In this one chamber reactor employing 3% H2/N2, the faradaic efficiency and ammonia production rate is explored at different temperatures and operating voltages to elucidate the optimal conditions for ammonia synthesis. (1) Kyriakou, V.; Skodra, A.; Stoukides, M.; Vasileiou, E.; Garagounis, I. Electrochemical Synthesis of Ammonia in Solid Electrolyte Cells. Front. Energy Res. 2014, 2 (January), 1–10. (2) Martín, A. J.; Shinagawa, T.; Pérez-Ramírez, J. Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf. Chem 2019, 5 (2), 263–283. (3) Choi, S.; Kucharczyk, C. J.; Liang, Y.; Zhang, X.; Takeuchi, I.; Ji, H. Il; Haile, S. M. Exceptional Power Density and Stability at Intermediate Temperatures in Protonic Ceramic Fuel Cells. Nat. Energy 2018, 3 (3), 202–210. Figure 1

110th Anniversary: Kinetics and X-ray Absorption Spectroscopy in Methane Total Oxidation over Alumina-Supported Pt, Pd, and Ag–Pd Catalysts

The kinetics of methane oxidation over Pt/Al2O3, Pd/Al2O3, and Ag–Pd/Al2O3 is investigated as a function of the O2/CH4 ratio. We show that as the oxygen fraction increases from low values, three distinct kinetic regimes exist that are nearly first order, negative order, and zero order with respect to the oxygen concentration. At O2/CH4 < 3, Pd and Ag–Pd are superior catalysts, whereas at higher oxygen concentrations, Pt is superior. Hysteresis with bistability is demonstrated over all three catalysts with two distinct activity regimes at identical feed compositions: higher activity upon increasing oxygen concentration from low values and lower activity upon decreasing oxygen concentration from high values. In situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) over the Ag–Pd catalyst suggest that the bistability stems from different oxidation states of the catalyst as well as a core–shell structure in the reduced state.

Hydrodechlorination of 1,2-dichloroethane on supported AgPd catalysts

AgPd catalysts have been synthesized on SiO2, TiO2, and carbon supports using controlled surface reactions to form well defined active site structures. These catalysts were studied for hydrodechlorination of 1,2-dichloroethane to form ethane, ethylene, and chloroethane. The formation of bimetallic AgPd structures is most effective for AgPd/TiO2, followed by AgPd/C and then AgPd/SiO2 catalysts. FTIR spectra of adsorbed CO suggest that Pd is well dispersed on the Ag nanoparticles for the TiO2-supported catalyst, while more contiguous Pd sites are present on the SiO2-supported catalyst. Ethylene selectivities of up to 97% and 82% are achieved on AgPd/TiO2 and AgPd/C catalysts, respectively. The AgPd/SiO2 catalyst is 91% selective to ethane. These results indicate that the well dispersed Pd achieved at low Pd loadings on TiO2 and carbon is selective to ethylene, while the contiguous Pd species formed on SiO2 are selective to ethane. Furthermore, increased reactivity is correlated to high ethylene selectivity.

Model Bimetallic Pd–Ag/HOPG Catalysts: An XPS and STM Study

A series of model bimetallic Pd–Ag catalysts with different ratios of metals and size distributions of supported particles was prepared by successive metal deposition onto a modified surface of highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structural and electronic properties, as well as the morphology of nanoparticles at all stages of catalyst preparation. The use of the method made it possible to obtain bimetallic particles with various structures: alloyed, core–shell, and those consisting of individual monometallic particles. The XPS method with synchrotron radiation was used to determine the patterns in the formation of alloyed bimetallic particles, their chemical composition, structure, and the ranges of thermal stability under conditions of ultrahigh vacuum.

Adsorption-Induced Segregation as a Method for the Target-Oriented Modification of the Surface of a Bimetallic Pd–Ag Catalyst

The effect of palladium segregation was studied which resulted from the effect of CO and O2 on the surface structure and catalytic characteristics of the Pd–Ag2/Al2O3 catalyst. The IR-spectroscopic study of adsorbed CO showed that Pd1 centers isolated from each other by silver atoms predominated on the surface of reduced Pd–Ag2/Al2O3, as evidenced by the almost complete absence of absorption bands typical for the multicentred CO adsorption. In the course of catalyst treatment with CO and O2, the intensity of absorption bands characteristic of the multicenter CO adsorption considerably increased due to the transformation of a portion of monatomic Pd1 centers into multiatomic Pdn ones as a result of the surface segregation of Pd. In this case, a substantial increase in the catalyst activity in the liquid-phase hydrogenation of diphenylacetylene was observed. It was established that, after treatment with CO, the catalyst selectivity for the formation of a target olefin (stilbene) remained almost constant, whereas the treatment with O2 led to a decrease in the selectivity because of more considerable surface modification.

Ozone-assisted catalysis of CO: In situ Fourier transform IR evidence of the cooperative effect of a bimetallic Ag-Pd catalyst

The room-temperature oxidation of CO using ozone-assisted catalysis (OAC) over monometallic or bimetallic catalysts supported on γ-Al2O3 is presented. Within the tested particle size range, ozone decomposition was insensitive to the size of the Ag particles (6.6–11.9 nm) at weight hourly space velocities of up to 360 L·g−1·h−1. Among the tested monometallic catalysts (Ag Pd, Fe, Mn, and Cu), Ag showed the highest activity for CO oxidation. Bimetallic catalysts (Ag-M or Pd-M where M = Cu, La, Ru, and Fe) were also considered by adding a second metal or perovskite oxide (LaFeO3) to Ag. A cooperative effect was observed with the bimetallic Ag-Pd catalyst for the OAC of CO oxidation at room temperature, whereas the other bimetallic catalysts showed slightly lower performance compared to the monometallic catalyst. A high ozone utilization efficiency of 0.94 was achieved with the bimetallic Ag-Pd/γ-Al2O3 catalyst. High-angle annular dark-field scanning tunneling electron microscopy (HAADF) and energy dispersive X-ray (EDX) spectroscopy measurements confirmed the proximity of the two components, which is essential for their interaction. The in situ FTIR measurements revealed that the cooperative effect in the bimetallic Ag-Pd catalyst involved modification of CO adsorption and the suppression of product accumulation. Two typical IR absorption bands of linear- (2090 cm−1) and bridge-CO (1918 cm−1) on Pd disappeared in the presence of Ag nanoparticles. Thus, the presence of Ag in contact with Pd inhibited the accumulation of carbonates, which led to enhanced catalytic performance. Adsorption site-dependent CO oxidation on Pd was also confirmed by the in situ Fourier transform IR measurement.

Surface Engineering of a Supported PdAg Catalyst for Hydrogenation of CO2 to Formic Acid: Elucidating the Active Pd Atoms in Alloy Nanoparticles.

The hydrogenation of carbon dioxide (CO2) to formic acid (FA; HCOOH), a renewable hydrogen storage material, is a promising means of realizing an economical CO2-mediated hydrogen energy cycle. The development of reliable heterogeneous catalysts is an urgent yet challenging task associated with such systems, although precise catalytic site design protocols are still lacking. In the present study, we demonstrate that PdAg alloy nanoparticles (NPs) supported on TiO2 promote the efficient selective hydrogenation of CO2 to give FA even under mild reaction conditions (2.0 MPa, 100 °C). Specimens made using surface engineering with atomic precision reveal a strong correlation between increased catalytic activity and decreased electron density of active Pd atoms resulting from a synergistic effect of alloying with Ag atoms. The isolated and electronically promoted surface-exposed Pd atoms in Pd@Ag alloy NPs exhibit a maximum turnover number of 14 839 based on the quantity of surface Pd atoms, which represents a more than 10-fold increase compared to the activity of monometallic Pd/TiO2. Kinetic and density functional theory (DFT) calculations show that the attack on the C atom in HCO3- by a dissociated H atom over an active Pd site is the rate-determining step during this reaction, and this step is boosted by PdAg alloy NPs having a low Pd/Ag ratio.