Pd-Cu Catalysts Research Articles

Revealing mechanisms for the formation of acetone and propanal in the acetoxylation of propylene on PdCu catalysts combined DFT with kMC analysis

1,4-butanediol has attracted much attention and Lyondell method is a competitive production technology due to its wide range of feedstock sources, in which the acetoxylation of propylene to allyl acetate on PdCu catalysts is an essential step. By combining Density Functional Theory and kinetic Monte Carlo simulations, the formation mechanisms and pathways of byproducts acetone and propanal in the acetoxylation of propylene to allyl acetate on PdCu catalysts were systematically investigated. Most species tend to be adsorbed on Pd atoms, which is consistent with that Pd atoms are main active sites. Acetone is obtained by the hydrogenation of CH3COCH2*, which comes from the oxidation and then dehydrogenation of propylene. Propanal is produced by the hydrogenation of CH3CHCHO*, which can be formed by the dehydrogenation and then oxidation of propylene or the oxidation and then dehydrogenation of propylene. Compared with acetone, propanal is more difficult to form and its formation reactions have higher Gibbs activation energy. This work provides theoretical guidance for the better modification of catalysts and the reduction of byproducts acetone and propanal on microscopic and mesoscopic scales.

A self-reactivated PdCu catalyst for aldehyde electro-oxidation with anodic hydrogen production.

The low-potential aldehyde oxidation reaction can occur at low potential (~0 VRHE) and release H2 at the anode, enabling hydrogen production with less than one-tenth of the energy consumption required for water splitting. Nevertheless, the activity and stability of Cu catalysts remain inadequate due to the oxidative deactivation of Cu-based materials. Herein, we elucidate the deactivation and reactivation cycle of Cu electrocatalyst and develop a self-reactivating PdCu catalyst that exhibits significantly enhanced stability. Initially, in-situ Raman spectroscopy confirm the cycle involved in electrochemical oxidation and non-electrochemical reduction. Subsequently, in-situ Raman spectroscopy and X-ray absorption fine structure reveal that the Pd component accelerates the rate of the non-electrochemical reduction, thereby enhancing the stability of the Cu-based electrocatalyst. Finally, a bipolar hydrogen production device is assembled utilizing the PdCu electrocatalyst, which can deliver a current of 400 mA cm-2 at 0.42 V and operate continuously for 120 h. This work offers guidance to enhance the stability of the Cu-based electrocatalyst in a bipolar hydrogen production system.

Pd catalysts in the mild reductive depolymerization of Soda lignin: Support and Cu addition effects

This study pioneers the exploration of the structure-performance relationship of 5 wt% Pd and PdCu catalysts on 3 supports (γ-Al2O3, SiO2, and activated carbon (AC)) in the mild reductive depolymerization (MRD) of Soda lignin. While all catalysts achieve similar final weight average molecular weight (Mw) reduction (≈ 85 %), Pd/AC and PdCu/AC show significant differentiation from solvolysis already after 2 h and 3 h, compared to 6 h and 20 h for Pd and PdCu on oxide supports. This study also highlights the importance of hydrogenation of C=C bonds in monomer side-chains for maximizing monomer yields. Particularly PdCu/SiO2 is characterized by the lowest monomer yields (7 wt%) due to its low hydrogenation activity, while catalysts like Pd(Cu)/Al2O3, Pd/SiO2, and Pd/AC, with more expressed hydrogenation, achieve a total monomer yield of up to 14 wt%. Additional findings confirm that on γ-Al2O3, the co-impregnation of Cu and Pd leads to the formation of a chemical bond between Pd2+ and Cu1+, resulting in a new mixed metal oxide Pd-Cu–O phase. On SiO2, Pd2+ and Cu1+ do not form a chemical bond, however, the presence of Cu1+ in close proximity to Pd2+ significantly alters the selectivity of the PdCu/SiO2 catalyst due to synergetic effects. PdCu/AC maintains similar selectivity to Pd/AC, except for reduced hydrogenation over time, due to the lack of interaction between Pd and Cu on the AC surface. This research also delves into the MRD mechanism of Soda lignin over Pd and PdCu catalysts, focusing on the β-O-4 linkage cleavage and the subtle differences in reaction pathways depending on the catalyst. These findings bridge gaps in understanding the MRD of actual lignin feedstocks, offering valuable insights for catalyst development and process optimization.

Increasing electrochemical carbon dioxide reduction to methane via a novel copper-based conductive metal organic framework

The development of a new system for the electrochemical carbon dioxide reduction reaction (ECO2RR) to methane (CH4) is challenging, and novel conductive metal organic frameworks (c-MOFs) for efficient ECO2RR to CH4 are critical to this system. Here, we report a novel c-MOF, copper-pyromellitic dianhydride-2-methylbenzimidazole (Cu-PD-2-MBI), in which the introduction of electron-withdrawing 2-methylbenzimidazole (2-MBI) into the copper-pyromellitic dianhydride (Cu-PD) interlayer elevated the valence of copper (Cu) ions, which improved the ECO2RR performance of Cu-PD-2-MBI. Cu-PD-2-MBI was tested in a flow cell, and the Faradaic efficiency of CH4 reached 73.7 %, with a corresponding partial current density of −428.3 mA·cm−2 at −1.3 V, which was higher than those of most reported Cu-based catalysts. Further exploration via theoretical calculations indicated that the intercalated 2-MBI in Cu-PD-2-MBI induced a shift in the d-band center in the Cu sites from −2.63 to −1.86 eV and reduced the formation energy of the *COOH and *CHO intermediates in the process of generating CH4 compared with those of the reference Cu-PD catalyst.

Hydrogenation of Furfural to Furfuryl Alcohol over PdCu/zirconium Phosphonate with Metal and Lewis Acid Sites

AbstractHydrogenation of furfural to furfuryl alcohol under H2 and N2 atmosphere was studied over zirconium phosphonate (ZrPO)‐supported PdCu (PdCu/ZrPO) catalysts. The direct hydrogenation and transfer hydrogenation activities of PdCu metal active sites and the transfer hydrogenation activity of Zr4+−O2− Lewis acid‐basic sites were investigated, respectively. The size and the distribution of Cu particles could be adjusted by altering the P/Zr molar ratio. Bimetallic PdCu could enhance the acidity of the PdCu/ZrPO catalysts, and improve the ratio of Lewis/Brønsted acid, which led to an increase in the transfer hydrogenation activity of Lewis acid‐basic pairs. The electronic effect between Pd and Cu promoted furfural to absorb onto electron‐deficient Cu through η1(O)‐aldehyde configuration, which contributed to the direct hydrogenation and the transfer hydrogenation at metal active sites. Especially, the adsorption and dissociation of H2 in electron‐rich Pd were promoted, which significantly improved the direct hydrogenation activity.

Synthesis of new 4, 5-disubstituted-6-methyl-2-(methylthio) pyrimidines via C-C coupling reactions

A convenient synthetic protocol for diverse 4, 5-disubstituted-6-methyl-2-(methylthio)pyrimidines was successfully developed by Sonogashira reactions. In the presence of Pd-Cu catalysts, one-pot, multi-step reaction of amines, terminal alkynes, and 4-chloro-5-iodo-6-methyl-2-(methylthio)pyrimidine in DMF at 80°C resulted in 4, 5-disubstituted-6-methyl-2-(methylthio)pyrimidine derivatives in moderate to good yields.

Kaolin and zinc oxide supported PdCu catalysts as a superior catalyst in methanol oxidation reaction

Electrocatalysts, which are important in increasing the efficiency of fuel cells, still have disadvantages such as low stability and high cost. To overcome these disadvantages, research on the synthesis of low-cost nanoparticle (NP) catalysts has been carried out. In this study, a new approach to develop a low-cost, high electrocatalytic activity and more stable catalyst was realized. In this context; kaolin ((Al2O3(SiO2)2(H2O)2)) and zinc oxide (ZnO) supported palladium-copper (PdCl2–CuCl2) nanostructures were synthesized by chemical reduction technique. The particle sizes of the synthesized PdCu@kaolin and PdCu@ZnO NPs were found to be 12.02 nm and 23.02 nm, respectively, according to the Debye Scherrer formula. The PdCu@kaolin and PdCu@ZnO NPs obtained within the scope of the study were also used in the anodic oxidation of methanol (CH₃OH), a one-carbon organic compound. According to the results obtained, the current density for methanol oxidation peaks of PdCu@kaolin and PdCu@ZnO NPs were found to be 27.77 mA cm−2 and 41.52 mA cm−2, respectively. It was observed that the ZnO-supported nanoparticles provided 1.49 times higher electrocatalytic activity compared to the other one. The 500-cycle results showed that the ZnO-supported catalyst exhibited a continuously increasing current density. The tests also proved that the catalysts are diffusion-controlled systems and have high stability. Within the scope of the study, ZnO and Kaolin support not only decreased the cost of the catalyst but also significantly affected the electrocatalytic activity, offering better results than the commercial Pd/C catalyst. Cost-effective ceramic structures such as kaolin and ZnO were crucial to show high stability and electroactive behavior close to the literature. The current study has performed an important contribution to the literature on the use of ceramic materials in electrochemistry.

Electrons redistribution of palladium-copper nanoclusters boosting the direct oxidation of methane to methanol

The direct oxidation of methane to methanol (DOMM) at mild conditions is an enormous attractive yet highly challenging, which is known as the “Holy Grail” reaction. In this work, we prepare the PdxCuy NCs supported on mesoporous sulfur-carbon materials and investigate their catalytic DOMM performance. The Pd14Cu1 NCs exhibits the excellent DOMM performance with the productivity of 34.3 mmol g−1 h−1 and the selectivity of 92.3 % at 150 °C when H2/O2 as active oxygen species, which was obviously superior to other PdCu catalysts. The enhanced performance of Pd14Cu1 NCs in DOMM is attributed to the non-crystalline atomic arrangement of tiny cluster structures and the synergistic electronic effect between Pd and Cu atoms, which can reduce the desorption temperature for methane, improve the in-situ generation of H2O2, and provide more active oxygen. The time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that a radical-involved reaction pathway on Pd14Cu1 NC. This work provides the understanding for synergistic effect dual metallic and helps to design highly efficient and selective catalysts for the direct oxidation of CH4.

Assessment of sulfuric acid as pH control agent in catalytic nitrate reduction in drinking waters

H2SO4 was thoroughly assessed as an alternative to CO2 as pH controller, as it allows for to a much precise dosage control. Catalytic nitrate reduction tests using a Pd-Cu catalyst supported on a carbon black were performed using both Milli-Q grade and real natural drinking waters spiked with NO3– in batch reactors. Conversion and selectivity results were equivalent to those obtained for CO2 use, obtaining total NO3– conversion and final NH4+ selectivity around 30 % for a pH range of 5.5–6 using a 0.05 M H2SO4 solution. Good stability of the catalyst in natural drinking water with the higher salt content studied was observed through reaction cycles, despite the presence of SO42- in the reaction medium, with total NO3– conversion after four reaction cycles and final NH4+ concentration of less than 16 mg/L were obtained, making H2SO4 a promising alternative to CO2 as pH controller in the catalytic NO3– reduction.

Selectivities of Stepped Cu-M (M = Pt, Ni, Pd, Zn, Ag, Au) Bimetallic Surface Environment for C1 and C2 Pathways.

Copper (Cu) emerges as a highly efficient and cheap catalytic agent for the electrochemical reduction of carbon dioxide (CO2RR), promising a sustainable route toward carbon neutrality. Despite its utility, the Cu catalyst exhibits limitations in terms of product selectivity, highlighting the need for the development of a superior catalyst design. Herein, we present a density functional theory (DFT) investigation into the selectivities of Cu-M (M = Pt, Ni, Pd, Zn, Ag, Au) bimetallic catalysts (BMCs) for the carbon dioxide reduction reaction (CO2RR). The interaction between the metals of Cu-M makes the surface electrons reconstruct so that the d-band center shifts to the Fermi level. In terms of CO2 activation, the Cu-Ni catalyst exhibits superior performance. Additionally, the Cu-Pd catalyst favors the formation of *COH along the reaction pathway, favoring the generation of CH4. Conversely, the Cu-Ni catalyst preferentially produces *CHO, thereby favoring the production of CH3OH. For the Cu-Ag catalyst, the reaction intermediates along the C2 pathway are *CO-*CHO and *COH-*CHO. The Cu-Ni catalyst follows a reaction path that proceeds via *CO-*CO → *CO-*COH → *COH-CHO. On the other hand, the Cu-Pt catalyst exhibits a reaction sequence of *CO-*CO → *CO-*CHO → *OCH-*OCH. This study provides guiding significance for the design of Cu-based bimetallic catalysts aimed at improving the selectivities and efficiency of the CO2RR process.

Green synthesis of carbon coated macrostructured catalysts: Optimized methodology and enhanced catalytic performance

A novel green and efficient synthesis methodology for carbon-coated macrostructured catalysts was assessed. Sodium alginate was used as a dispersant to promote a suitable dispersion of the modified carbon nanotubes (CNT) to be incorporated on a structured support. The coating conditions were optimized to synthesize active and stable carbon-coated macrostructured catalysts. Achieving a total dispersant decomposition was one of the key steps to guarantee the coating stability, 550 °C being the optimal temperature. To assess the catalytic activity for these catalysts, the carbon-coated structures were tested as main catalysts for oxalic acid degradation through ozonation, and as support for bimetallic Pd-Cu catalysts, applied in NO3− conversion. The results achieved during ozonation allowed to outperform the ones obtained with macrostructured catalysts synthesized by more established techniques (such as chemical vapor deposition) with oxalic acid degradation, in a continuous system, of around 75%. As for the bimetallic samples, the species proved to be quite dispersed on the catalyst support, allowing a NO3− conversion of around 20% with a corresponding N2 selectivity of 55%, outperforming, again, the same type of catalyst synthesized with “less green” surfactants. This newly developed methodology is an important step for the sustainability of the structured catalyst design area.

Cooperation of Different Active Sites to Promote CO2 Electroreduction to Multi-carbon Products at Ampere-Level.

Electroreduction of CO2 to C2+ products provides a promising strategy for reaching the goal of carbon neutrality. However, achieving high selectivity of C2+ products at high current density remains a challenge. In this work, we designed and prepared a multi-sites catalyst, in which Pd was atomically dispersed in Cu (Pd-Cu). It was found that the Pd-Cu catalyst had excellent performance for producing C2+ products from CO2 electroreduction. The Faradaic efficiency (FE) of C2+ products could be maintained at approximately 80.8 %, even at a high current density of 0.8 A cm-2 for at least 20 hours. In addition, the FE of C2+ products was above 70 % at 1.4 A cm-2. Experiments and density functional theory (DFT) calculations revealed that the catalyst had three distinct catalytic sites. These three active sites allowed for efficient conversion of CO2, water dissociation, and CO conversion, ultimately leading to high yields of C2+ products.

Exploring the synergistic effect of palladium nanoparticles and highly dispersed transition metals on carbon nitride/super-activated carbon composites for boosting electrocatalytic activity

In the present work, multifunctional electrocatalysts formed by palladium nanoparticles (Pd NPs) loaded on Fe or Cu-containing composite supports, based on carbon nitride (C3N4) and super-activated carbon with a high porosity development (SBET 3180 m2/g, VDR 1.57 cm3/g, and VT 1.65 cm3/g), were synthesised. The presence of Fe or Cu sites favoured the formation of Pd NPs with small average particle size and a very narrow size distribution, which agreed with Density Functional Theory (DFT) calculations showing that the interaction of Pd clusters with C3N4 flakes is weaker than with Cu- or Fe-C3N4 sites. The electroactivity was also dependent on the composition and, as suggested by preliminary DFT calculations, the Pd-Cu catalyst showed lower overpotential for hydrogen evolution reaction (HER) while bifunctional oxygen reduction reaction/ oxygen evolution reaction (ORR/OER) behaviour was superior in Pd-Fe sample. The Pd-Fe electrocatalyst was studied in a zinc-air battery (ZAB) for 10 h, showing a performance similar to a commercial Pt/C + RuO2 catalyst with a high content of precious metal. This study demonstrates the synergistic effect between Pd species and transition metals and shows that transition metals anchored on C3N4-based composite materials promote the electroactivity of Pd NPs in HER, ORR and OER due to the interaction between both species.

PdCu Alloy Catalyst for Inhibition‐free, Low‐temperature CO Oxidation

AbstractDesigning robust catalysts for low‐temperature oxidation is pertinent to the development of advanced combustion engines to meet increasingly stringent emissions limitations. Oxidation of CO, hydrocarbon, and NO pollutants over platinum‐group catalysts suffer from strong inhibition due to their competitive adsorption, while coinage metals are generally slow at activating O2. Through computational screening, we discovered a PdCu alloy catalyst that completely oxidizes CO below 150 °C without inhibition by NO, propylene or water. This is attributed primarily to geometric effects and the presence of CO bound to Pd sites within the Cu‐rich surface of the PdCu alloy. We demonstrate that the novel PdCu catalyst can be used in tandem with a PtPd catalyst to achieve sequential, inhibition‐free, complete oxidation of CO in a two‐bed system, while also achieving 50 % NO conversion below 120 °C. Moreover, neither water nor propylene adversely affect the low temperature CO oxidation activity.

Experimental and computational study of the catalytic activity of Pd and PdCu nanoparticle catalysts and clusters supported on reduced graphene oxide and graphene acid for the suzuki cross-coupling reaction

The catalytic activity of Pd and PdCu nanoparticle catalysts and clusters supported on reduced graphene oxide (RGO) and graphene acid (GA) toward the Suzuki cross-coupling reaction between bromobenzene and phenylboronic acid is investigated experimentally and computationally. The experimental results indicate that the bimetallic PdCu nanoparticle catalysts supported on RGO or GA outperform the supported Pd catalysts and that the PdCu/RGO catalyst exhibits superior activity, reaching a full conversion to the biphenyl product in only 15 min under ambient conditions. The experimental results show that both the Pd and PdCu catalysts have higher activities when supported on RGO than when supported on GA. The computational results using density functional theory show that the RGO provides relatively better support for the C-C coupling reaction than GA. Investigations based on the charge analysis have further revealed that the RGO is a better charge donor and acceptor than GA, facilitating both the reaction's oxidative addition and reductive elimination steps by reducing the respective barrier heights. Doping a Cu atom near the Pd reaction site is shown to enhance this effect further. The relative reactivity trends of RGO- and GA-supported Pd and PdCu clusters obtained from the DFT calculations show exact agreement with the experimental results.

Spectroscopic determination of metal redox and segregation effects during CO and CO/NO oxidation over silica-supported Pd and PdCu catalysts

We report the effects of alloying Cu into silica supported Pd nanoparticles on the catalytic activity, surface composition, and particle structure during CO oxidation Pd is highly active for CO oxidation but is inhibited by relevant competitive reagents (e.g. NO) at low temperatures (< 150 °C). By alloying Cu into Pd nanoparticles, NO inhibition is suppressed without any CO oxidation activity loss. Infrared spectroscopy illustrates the formation of surface nitrosyl (NO-Pd) is eliminated in the PdCu alloy catalyst, preventing CO oxidation inhibition by NO. X-ray absorption spectroscopy studies show that Pd in the monometallic catalyst forms a surface oxide after light-off. Conversely, in the PdCu catalyst, Pd remains metallic during reaction, independent of temperature, whereas the Cu oxidizes when the catalyst becomes active, after light-off. Furthermore, DRIFTS studies show that the Cu segregates to the surface as the catalyst becomes active for diesel oxidation, presumably becoming the O2 adsorption site.

A Graphene Oxide‐Supported PdCu Catalyst for Enhanced Electrochemical Synthesis of Ammonia

AbstractThe conventional Haber‐Bosch method for the ammonia synthesis process requires high temperature and pressure. Electrochemical synthesis of ammonia, an emerging ammonia synthesis technology, is a promising approach for sustainable ammonia production that is energy‐efficient and free of greenhouse gas emissions. The design and development of high‐performance catalysts are the keys to promoting the sustainable ammonia production process. This work synthesized a PdCu alloy catalyst loaded on a graphene oxide carrier through a simple liquid phase reduction method. Which greatly enhanced the catalytic performance with an ammonia yield of 1.62 mg h−1cm−2 and Faradaic efficiency of 38.2 % under the nitrate reduction ammonia synthesis (NO3RR) reaction at an overpotential of −0.4 V. For the nitrogen reduction ammonia (NRR), the ammonia yield was 20.83 μg h−1 cm−2 with a Faradaic efficiency of 3.8 %. This study may provide a new idea for material design and promote ammonia synthesis development under ambient conditions.

Synthesis of Vinyl Acetate Monomer Over PdCu Alloys: The Role of Surface Oxygenation in the Reaction Path

AbstractThis work uses sol–gel and sonochemical methods to prepare bimetallic PdCu catalysts supported on modified ZrO2 with Ti and Al. The catalysts are tested for vinyl acetate synthesis from ethylene, acetic acid, and oxygen, varying the reaction temperature for two catalysts prepared, which show higher activity. Catalysts characterization results show bimetallic species and distort alloys with a dispersed distribution of active metal onto the support. The in situ reaction by DRIFT‐MS identifies the surface formation of main intermediates like palladium acetate monomers and mono and bidentate intermediates, associated to vinyl acetate formation, like vinyl hydrogenated species over PdCu. Finally, this behavior will be attributed to the bimetallic distortions of PdCu samples, provided by interaction effects between PdCu and supports, indicating a more exposition of PdCu species suitable for the reaction, according to high‐resolution transmission microscopy electron microscopy results for PdCu/ZrTi sample. Thus, this sample exhibits a minimal formation of sub‐products and catalytic stability for 18 hh. These results evidence the participation of hydroxyls and oxygen vacancies of the catalyst in the catalytic reaction measured in situ, monitoring the products formed at the reactor outlet. Finally, it is proposed a reaction pathway as a function of reaction conditions.

Influence of bicarbonate, other anions and carbon dioxide in the activity of Pd-Cu catalysts for nitrate reduction in drinking water

Synthetic and commercial drinking waters with different composition were studied as reaction media to study the influence of salts in NO3- catalytic reduction using a Pd-Cu catalyst supported on a carbon black. As a general trend, a decrease in NO3- conversion and an increase in NH4+ selectivity were observed for high HCO3- concentration media in mixed salts waters. Literature has commonly ascribed HCO3- effect to competitive adsorption with NO3-. However, in the current work, the mechanism for effect HCO3- is reconsidered basis on HCO2- formation during NO3- catalytic reduction, here reported for the first time. HCO2- formation indicates that hydrogenation of HCO3- occurs in addition to adsorption. Likewise, decomposition of HCO2- on the catalysts surface releases hydrogen leading to increased spill-over and relevant hydrogenation of NO3- to NH4+. The presence of SO42-, Cl- reduces NH4+ selectivity due to competition for active sites and lower HCO2- generation. Furthermore, it was observed that the use of CO2 as buffer also contribute to the hydrogenation of NO3- to NH4+ through HCO2- route.

Pd and Pd-Cu supported on different carbon materials and immobilized as flow-through catalytic membranes for the chemical reduction of NO3-, NO2- and BrO3- in drinking water treatment

Powdered catalysts are commonly used in lab-scale tests for the catalytic reduction of oxoanions in drinking water, but their powder nature limits their application at full scale. In this work, Pd and Pd-Cu catalysts (5% wt.) supported on carbon materials with different structural properties, in powder form, were used to prepare catalytic membranes that were tested in a reactor with flow-through configuration (FTCMR) to study their performance in the reduction of NO3-, NO2- and BrO3-. Pd catalytic membranes showed high activity in the reduction of NO2-, being the selectivity to NH4+ lower than 2% at 80% NO2- conversion in all cases. In BrO3- reduction, they exhibited a wide range of conversions being the catalyst supported on materials with high conductivity the most active ones, which may be ascribed to the charge distribution at the metal-carbon interface. NO3- reduction using Pd-Cu catalytic membranes showed that catalysts supported on materials with small nanoparticle size and low electrical conductivity exhibited higher selectivity to NH4+. FTCMR led to a good control of H2 transfer and availability in the active sites, facilitating the tuning of H2 availability conditions to preserve the activity, while maintaining/diminishing selectivity to NH4+. In simultaneous oxoanions reduction tests, NO3- reduction was inhibited by Br species, probably by affection of the Pd-Cu redox cycle. This fact could be crucial to the future development of drinking water treatment processes, as conditions the order of the disinfection and NO3- reduction steps.