Carbon-supported Palladium Catalysts Research Articles

Structure sensitivity in the formic acid-driven catalytic transfer hydrogenation of maleic acid to succinic acid over Pd/C catalysts

A series of carbon-supported palladium catalysts (Pd/C) with average particle size in the range from 3 to 7 nm was prepared by varying the Pd loading. These materials and the influence of their physicochemical properties in the catalytic transfer hydrogenation (CTH) of maleic acid (MAc) to succinic acid (SAc) using formic acid (FAc) as H2 donor were evaluated. The chemical, textural, and surface properties were studied by a number of characterization techniques (ICP-OES, XRD, TEM, and XPS). It was found that the intrinsic rate of SAc formation per surface Pd atom (turn-over frequency, TOF Pdsur) is structure-sensitive. The TOF Pdsur rate increases with the particle size following a volcano-type curve, reaching a maximum for an average particle size of ca. 6 nm. Assuming a cuboctahedral shape with a cubic close-packed structure, an approximation frequently adopted for Pd particles, it was found that, for the series of catalysts, neither the calculated TOF of Pd surface atoms at low coordination sites nor at high coordination are constant. This suggests that no geometric effect is responsible for the structure-sensitivity. Among the different Pd species present in the catalysts (i.e., metallic Pd (Pd0), palladium carbide (PdCx), and oxidized Pd), it was observed that the relative number of surface Pd0 sites also follows a volcano-type curve, which indicates that Pd0 sites are necessarily involved in the most active centers. For particles with an average size larger than 4 nm, the TOF rate of surface Pd0 sites was constant and in the range of 0.12 s−1. For smaller sizes, a more accurate determination of their Pd0 surface concentration is required to obtain reliable surface Pd0 TOF rates.

Additively Manufactured Silicone Polymer Composite with High Hydrogen Getter Content and Hydrogen Absorption Capacity.

Hydrogen getters consisting of 1,4-bis[phenylethynyl] benzene (DEB) and a carbon-supported palladium catalyst (Pd/C) have been used to mitigate the accumulation of unwanted hydrogen gas in a sealed system. Here, we report the formulation of a composite resin consisting of silicone polymer plus DEB-Pd/C as an active getter material and the additive manufacturing of silicone getter composites with a high getter content (up to 50 wt %). NMR and DSC studies suggest no reaction between the silicone polymer resin and DEB even at elevated curing temperatures (75 °C). Getter composites with varying amounts of getter and filler were formulated, and their rheological properties were studied. The two composite resins with good printability parameters and different getter contents were chosen to make 3D-printed samples. The hydrogen absorption capacity of these samples was studied at a low hydrogen pressure of 750 mTorr of pure hydrogen. The getter composite with 50 wt% of getter showed normalized DEB conversion of 83%, with the hydrogen adsorption capacity of 100.2 mL of H2 per gram of polymer getter composite.

Efficient room-temperature hydrogenation of nitroaromatic compounds to primary amines using nitrogen-doped carbon-supported palladium catalysts

We report the preparation of novel Pd/CN catalysts, which can efficiently catalyze the hydrogenation of nitroaromatic compounds to produce primary amines under very mild conditions.

Enhanced stability of nitrogen-doped carbon-supported palladium catalyst for oxidative carbonylation of phenol

Enhancing the stability of supported noble metal catalysts emerges is a major challenge in both science and industry. Herein, a heterogeneous Pd catalyst (Pd/NCF) was prepared by supporting Pd ultrafine metal nanoparticles (NPs) on nitrogen-doped carbon; synthesized by using F127 as a stabilizer, as well as chitosan as a carbon and nitrogen source. The Pd/NCF catalyst was efficient and recyclable for oxidative carbonylation of phenol to diphenyl carbonate, exhibiting higher stability than Pd/NC prepared without F127 addition. The hydrogen bond between chitosan (CTS) and F127 was enhanced by F127, which anchored the N in the free amino group, increasing the N content of the carbon material and ensuring that the support could provide sufficient N sites for the deposition of Pd NPs. This process helped to improve metal dispersion. The increased metal-support interaction, which limits the leaching and coarsening of Pd NPs, improves the stability of the Pd/NCF catalyst. Furthermore, density functional theory calculations indicated that pyridine N stabilized the Pd2+ species, significantly inhibiting the loss of Pd2+ in Pd/NCF during the reaction process. This work provides a promising avenue towards enhancing the stability of nitrogen-doped carbon-supported metal catalysts.

Selective Hydrogenation of Benzylidenethiazolidinedione Exocyclic Alkene with Anti-poisoning Nitrogen-Doped Carbon Supported Palladium Catalysts

Selective Hydrogenation of Benzylidenethiazolidinedione Exocyclic Alkene with Anti-poisoning Nitrogen-Doped Carbon Supported Palladium Catalysts

A layer-like FAU zeolite with palladium particles embedded in situ as catalyst for hydrogenation of 1,4-bis(phenylethynyl) benzene

The active carbon supported palladium catalyst (Pd/C) is commonly used for hydrogenation of 1,4-bis(phenylethynyl) benzene (DEB), to avoid its disadvantage of easy combustion, a nonflammable Pd-containing zeolite was developed as a novel catalyst for the DEB hydrogenation. In the present work, this Pd-containing zeolite catalyst was synthesized via a one-pot hydrothermal technique, and its structure and catalytic performance were investigated. The results show that the support of this prepared Pd-containing zeolite catalyst is a layer-like FAU zeolite with a hierarchical pore structure, and the Pd particles is embedded uniformly inside the layer-like FAU zeolite with an average size of 2.02 ± 0.41 nm. The results also show that the catalytic performance of this prepared Pd-containing zeolite catalyst (Pd@FAU) is much better than that of the traditional Pd/C catalyst with the same content of Pd particles, the DEB-Pd@FAU mixture can reach its maximal hydrogen uptake capacity of 257.41 ml/g in 20 h, and the DEB-Pd/C mixture can only reach its maximum of 223.81 ml/g in 45 h. These differences are mainly caused by that the Pd particles in the prepared Pd@FAU catalyst has a smaller size, a better dispersion and a more metallic Pd species than those in the Pd/C catalyst. All these results indicate that the Pd@FAU catalyst would be a promising substitute for the traditional Pd/C catalyst for hydrogenation of DEB.

Electrochemical Metal Recycling: Recovery of Palladium from Solution and In Situ Fabrication of Palladium-Carbon Catalysts via Impact Electrochemistry.

Recycling of critical materials, regeneration of waste, and responsible catalyst manufacture have been repeatedly documented as essential for a sustainable future with respect to the environment and energy production. Electrochemical methods have become increasingly recognized as capable of achieving these goals, and “impact” electrochemistry, with the advantages associated with dynamic nanoelectrodes, has recently emerged as a prime candidate for the recovery of metals from solution. In this report, the nanoimpact technique is used to generate carbon-supported palladium catalysts from low-concentration palladium(II) chloride solutions (i.e., a waste stream mimic) as a proof of concept. Subsequently, the catalytic properties of this material in both synthesis (Suzuki coupling reaction) and electrocatalysis (hydrogen evolution) are demonstrated. Transient reductive impact signals are shown and analyzed at potentials negative of +0.4 V (vs SCE) corresponding to the onset of palladium deposition in traditional voltammetry. Direct evidence of Pd modification was obtained through characterization by environmental scanning electron microscopy/energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy, and thermogravimetric analysis of impacted particles. This showed the formation of deposits of Pd0 partially covering the 50 nm carbon black particles with approximately 14% Pd (wt %) under the conditions used. This material was then used to demonstrate the conversion of iodobenzene into its biphenyl product (confirmed through nuclear magnetic resonance) and the successful production of hydrogen as an electrocatalyst under acidic conditions (under cyclic voltammetry).

Homologation of Aryl Aldehydes Using Nitromethane as a C1 Source Enabled by Nitrogen-Doped Carbon-Supported Palladium Catalysts

Homologation of aryl aldehydes provides useful synthetic intermediates, but it requires multistep reactions and generates significant amounts of waste. We considered such reactions using nitromethane as a C1 source through nitroolefin formation, partial hydrogenation to oximes, and hydration of oximes; however, the control of selectivity in the second reaction is challenging. To achieve this pathway, nitrogen-doped carbon-incarcerated palladium nanoparticle catalysts were developed for selective hydrogenation. We found that the presence of nitrogen dopants effectively tuned the catalytic activity to show almost perfect selectivity, high activity, and reusability under ambient pressure. We then performed a three-step homologation reaction in both batch and flow systems with only one purification step. The sequential continuous-flow system worked efficiently for more than 2 days to afford the product in a high yield.

Ambient photothermal catalytic CO oxidation over a carbon-supported palladium catalyst

The XC-72R supported Pd catalysts exhibit low-temperature catalytic activity for CO oxidation under light irradiation. The CO conversion can reach 100% with the light irradiation of 900 mW·cm-2 at a catalyst temperature of 100 °C. Pd/C catalysts with different Pd loading have similar performance enhancement under light irradiation. The activation energy of 1% Pd/C catalyst remains unchanged implying that CO oxidation belongs to photothermal catalytic oxidation. The enhanced performance could be ascribed to ultrahigh surface temperature (275 °C) induced by the light irradiation of 800 mW·cm-2 and detected by an IR camera. That is higher than the catalyst temperature (187 °C) measured by the thermocouple. Carbon generates a higher temperature than Pd NP, causing heat transfers from carbon to Pd NP. In contrast, Pd NP produces a higher temperature than Al2O3 for Pd/Al2O3 catalyst, and heat is transferred from Pd to Al2O3. These demonstrate that the enhanced performance contributes to the photothermal effect of carbon.

Palladium catalyzed liquid phase oxidation of glycerol under alkaline conditions - Kinetic analysis and modelling

Glycerol is a major by-product of bio-diesel production and hence its utilization has been an area of high interest in the scientific community. Oxidizing glycerol to high-value products can improve the economics of bio-diesel production. In the present work, we study glycerol oxidation in alkaline medium using an activated carbon supported Palladium catalyst. Kinetic studies have been performed in a batch reactor to study the effect of different parameters such as NaOH concentration, temperature, and pressure. Our results point to product inhibition by adsorbing reactant and product species. We also observe a relative insensitivity of product yields at a given conversion, to the temperature. Our observations further suggest that higher NaOH concentration gives better C3 selectivity while higher oxygen pressure results in lower C3 selectivity. Based on our own studies and the literature available, the most likely kinetic pathway and a kinetic model have been proposed. Our results suggest the possibility that C–C cleavage occurs directly from the primary intermediate aldehydic species, as a result of which, carbon chain scission products (including CO2) form from very early stages of the reaction. Various strategies such as pooling temperature data to estimate rate ratios prior to full parameter estimation, use of statistical significance tests to reduce model parameters etc have been used to arrive at a minimalistic model still capable of explaining all features of the oxidation. Confidence bounds on the model parameters have been estimated.

Hydrogenation aging mechanisms of a porous composite with polyethylene as matrix and 1,4-bis(phenylethynyl)benzene as hydrogen getter

Although 1,4-bis(phenylthynyl)benzene (DEB) blended with a carbon supported palladium catalyst (Pd/C) is one of the most widely used irreversible hydrogen getter in a closed environment, it is hard to form and easy to burn. To overcome these deficiencies of this powdery DEB-Pd/C hydrogen getter, a DEB-Pd/C/LDPE porous hydrogen absorbing composite has been developed in our group. In the present work, hydrogen absorption aging mechanisms of this porous hydrogen absorbing composite samples are investigated by using a accelerated aging test method, their hydrogen absorption performances are measured by hydrogen absorption experiments, and their aging mechanisms are studied by X-ray diffraction, attenuated total reflection-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and field emission transmission electron microscope. The results show that both the hydrogen absorption rate and the hydrogen absorption capability of each aging composite sample decrease with the increasing of storage time. The results also show that the decrease of its hydrogen absorption rate is mainly attributed to both the surface migration of its DEB and the surface oxidation of its Pd catalyst, and the decrease of its hydrogen absorption capacity is mainly attributed to the surface migration of its DEB.

Hydrogenation of 4-Propylphenol over Carbon-supported Palladium Catalyst without External Hydrogen: Effect of Carbon Support and Palladium Loading

The ring hydrogenation of 4-propylphenol in aqueous ethanol solution was studied over graphite- and activated carbon-supported palladium catalysts (Pd/G and Pd/C) with 0.1–5 wt % of palladium loadi...

A Combined Spectroscopic and Electrochemical Approach to Investigate the Selectivity Determining Factors for CO2 Electroreduction on Pd-Nanoparticles

The electrochemical conversion of carbon dioxide into added-value products keeps on receiving increased interest as a means to valorize CO2 while diminishing its anthropogenic emissions. A broad spectrum of CO2-electroreduction products (e.g., CH4, C2H4) has been reported, but only a few are expected to be economically competitive with regards to industrially established chemical production pathways [1]. Interestingly, Pd-based nanoparticle catalysts are particularly well suited for the production of two of such products, since they feature the unique ability to selectively produce formate and carbon monoxide (CO) at low vs. high overpotentials, respectively [2]. This switching of the selectivity with the applied potential has been linked to a difference in the nature of the Pd catalyst and corresponding reaction mechanism. Specifically, at potentials below − 0.4 VRHE at which CO is the main reduction product [2,3], the reaction is believed to proceed on a fully CO-covered Pd-surface [4]. At higher potentials between − 0.4 and 0 VRHE, though, formate is predominantly produced [2,3] through a hypothesized CO2-hydrogenation mechanism on palladium hydride (PdHx) [5].Chiefly, the design of better-performing catalysts requires a better understanding of the relation between the precise stoichiometry of this hydride phase (i.e., the value of x in PdHx) and the applied potential, reaction duration and corresponding coverage of surface adsorbates (e.g., CO).To determine the relation between the above-described parameters, in this study we used a commercial carbon-supported palladium catalyst (40 % Pd/C) that displayed the above-described switchable selectivity between formate and CO in online gas chromatography measurements in a newly designed parallel plate cell (Figure 1). Specifically, very high selectivities of up to 90 % for formate were found at very low overpotentials (− 0.1 to − 0.3 VRHE), while at high overpotentials (− 0.7 to − 0.9 VRHE) CO is the main product with faradaic efficiencies of over 60 %, and only low amounts of formate are found. The interplay between surface adsorbates and palladium hydride formation was then investigated by quantifying the catalyst’s time and potential-dependent hydride content using chronoamperometry complemented with operando X-ray absorption spectroscopy (XAS) in our group's spectroelectrochemical flow cell [6]. Here, we investigated the influence of CO2 presence in the electrolyte acquiring XAS over 10 minutes of potential hold in both N2- and CO2-saturated potassium bicarbonate.As an example of our results, Figure 2 shows the X-ray absorption near edge structure (XANES) spectra after ten minutes of holding potential at − 0.1 VRHE. While we observed full hydride formation in N2-saturated KHCO3, the palladium in the CO2 saturated electrolyte remained in full metallic state, suggesting that in the presence of carbon dioxide surface adsorbates prevent the formation of PdHx. Additionally, we were able to monitor changes of the palladium hydride stoichiometry in the seconds- to minutes-timescales, thereby enabling us to investigate the hydride formation speeds with an unprecedented time resolution. Finally, the strongly adsorbed surface species formed during CO2 electroreduction were identified and their surface coverage quantified using a novel combined rotating disk electrode (RDE) cyclic voltammetry and in situ external reflection Fourier-transform infrared spectroscopy (FTIR) setup. This allowed us to determine that in the CO2 saturated bicarbonate electrolyte the Pd-surface is covered by a full monolayer of surface-adsorbed CO that prevents hydrogen adsorption and subsequently palladium hydride formation, as unveiled by our XAS results.In conclusion, we have combined electrochemical with spectroscopic methods providing us with the means to improve our understanding of the palladium hydride stoichiometry and adsorbed surface species under CO2 electroreduction conditions. Thus, this contribution will provide novel insight into the relation among the time-dependent bulk and surface composition of Pd-nanoparticles and their link to their CO2-reduction selectivity.Literature:[1] J. Durst et al., Chimia, 2015, 69, 769. [2] M. Rahaman et al., ChemSusChem, 2017, 10, 1733. [3] D. Gao et al., NanoResearch, 2017, 10, 2181. [4] R. Kortlever et al., Catalysis Today, 2015, 244, 58. [5] X. Min et al., J. Am. Chem. Soc., 2015, 137, 4701. [6] T. Binninger et al., J. Elec. Soc., 2016, 163, H906. Figure 1

Geometric, electronic, and synergistic effect in the sulfonated carbon-supported Pd catalysts for the direct synthesis of hydrogen peroxide

Although carbon-supported palladium catalysts have been generally used for the direct synthesis of H2O2 (DSHP), the low H2O2 selectivity remains a challenge. In this work, it was systematically studied how the sulfonated carbon surface affects the physicochemical properties of Pd during catalyst preparation, and thereby affects the catalytic activity of Pd/C for the DSHP reaction. Nano-sized Pd particles (about 2–3 nm) with a high Pd2+/Pd0 ratio resulted in a high H2O2 selectivity. The sulfonated carbon material acted as an efficient solid acid support and enhanced the H2O2 selectivity. Fine-tuning the Pd properties on the sulfonated carbon support significantly improved the H2O2 selectivity by as much as 87 % in the absence of caustic promoters, which clearly demonstrates that the synergistic combination of the active metal properties and that of the carbon support are extremely important to realize a highly active and selective carbon-supported palladium catalyst for the DSHP reaction.

Gas-phase hydrodeoxygenation of bio-oil model compound over nitrogen-doped carbon-supported palladium catalyst

The hydrodeoxygenation (HDO) of bio-oil has attracted considerable attention but is extremely challenging due to low yield and selectivity. In this study, we report the gas-phase hydro-deoxygenation of guaiacol, a typical model compound of lignin-derived bio-oil, into aromatic hydrocarbons with benzene yield up to 85.1% over a nitrogen-doped carbon-supported palladium (Pd/NC) catalyst under atmospheric pressure. The guaiacol HDO products are detected by an on-line VUV photoionization time-of-flight mass spectrometry (TOF-MS) in real-time. Stability evaluation based on time-on-stream profiles of typical markers (benzene and phenol) shows that the benzene selectivity and stability are kept unchanged during 6 h time-on-stream. The mechanism of guaiacol HDO over Pd/NC is also discussed. We infer that the enhanced stability of Pd/NC catalyst is attributed to the prevention of Pd particles' leaching and aggregation by nitrogen-doped carbon materials and the high activity is due to the redistribution of electron density caused by the interaction of nitrogen functionalities and Pd particles. More importantly, efficient deoxygenation of lignin model compound while preserving its aromatic nature is feasible and successfully achieved with Pd/NC catalyst, which is extremely advantageous for the concept of selective conversion of lignin into value-added aromatic chemicals.

Hydrogenation of chlorate ions by commercial carbon supported palladium catalysts\u2014a comparative study

The chlorate elimination potential of three commercial activated carbon supported 10 wt% palladium catalysts (Cat-I, Cat-II and Cat-III) have been compared in heterogeneous catalytic hydrogenation. The physical–chemical properties of the catalysts were characterized by using high-resolution transmission electron microscopy, X-ray diffractometry, Fourier-transform infrared spectroscopy and ζ potential measurements. Chlorate reduction tests have been carried out by applying the same procedure and conditions in each case. The studied catalysts were active, but Cat-I and Cat-III showed higher activity, and eliminated 93% and 91% of chlorate, respectively. Reuse tests have also been carried out to compare the catalysts. Although Cat-I and Cat-III were shown almost equally high activity in the first cycle, the reuse tests showed that Cat-III could have a better applicability.

Polymer Framework with Continuous Pores for Hydrogen Getters: Molding and a Boost in Getter Rate

The hydrogen getter consisting of 1,4-bis­[phenylethynyl]­benzene (DEB) and a carbon-supported palladium catalyst (Pd/C) is limited by its powder forms in practical application. Development of the molded DEB-Pd/C is thus highly demanded. To solve the contradiction between molding and hydrogen-getter rate, herein, we prepared a porous polymer matrix composite for DEB-Pd/C by a facile one-phase removal method of their cocontinuous polymer matrices. Such a porous composite exhibits excellent hydrogen-getter activities. More interestingly, under low hydrogen partial pressure, the porous composite had a faster getter rate than that of powder getters. The significantly enhanced activity is attributed to the confinement effects of the continuous pore structure of the polymer frameworks, which further indicates its promise as a highly active and flexible supporting framework for hydrogen absorption or other gas–solid reactions.

Unleash electron transfer in C-H functionalization by mesoporous carbon-supported palladium interstitial catalysts.

The functionalization of otherwise unreactive C–H bonds adds a new dimension to synthetic chemistry, yielding useful molecules for a range of applications. Arylation has emerged as an increasingly viable strategy for functionalization of heteroarenes which constitute an important class of structural moieties for organic materials. However, direct bisarylation of heteroarenes to enable aryl-heteroaryl-aryl bond formation remains a formidable challenge, due to the strong coordination between heteroatom of N or S and transitional metals. Here we report Pd interstitial nanocatalysts supported on ordered mesoporous carbon as catalysts for a direct and highly efficient bisarylation method for five-membered heteroarenes that allows for green and mild reaction conditions. Notably, in the absence of any base, ligands and phase transfer agents, high activity (turn-over frequency, TOF, up to 107 h−1) and selectivity (>99%) for the 2,5-bisarylation of five-membered heteroarenes are achieved in water. A combination of characterization reveals that the remarkable catalytic reactivity here is attributable to the parallel adsorption of heteroarene over Pd clusters, which breaks the barrier to electron transfer in traditional homogenous catalysis and creates dual electrophilic sites for aryl radicals and adsorbate at C2 and C5 positions. The d-band filling at Pd sites shows a linear relationship with activation entropy and catalytic activity. The ordered mesopores facilitate the absence of a mass transfer effect. These findings suggest alternative synthesis pathways for the design, synthesis and understanding of a large number of organic chemicals by ordered mesoporous carbon supported palladium catalysts.

Insights into the Synergistic Effect in Pd Immobilized to MOF-Derived Co-CoOx@N-Doped Carbon for Efficient Selective Hydrogenolysis of 5-Hydroxylmethylfurfural

Co-CoOx-containing N-doped porous carbon-supported palladium catalysts were prepared via a ZIF precursor calcination and a simple impregnation route. The material was employed for the hydrogenolysis of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF). The Co-CoOx-containing N-doped porous carbon provides a high dispersion and anchoring effect on active metal as well as abundant surface defects with strong adsorption ability to the oxygenated groups and enhanced electron transfer capability, which could promote the activation and H2 spillover and the cascade reaction of HMF hydrogenolysis to DMF. As-obtained palladium-based nanocatalysts exhibited excellent catalytic hydrogenolysis performance, and there is no apparent change in activity in six runs. The reasonable synergism between surface defects and active metallic species is the primary reason for such excellent catalytic effects. Most importantly, the strategy proposed enables us to adjust the physicochemical properties of the catalyst surface to design new bifunctional catalysts with significantly improved performance.

Rectifying the chemisorption – XRD discrepancy of carbon supported Pd: Residual chloride and/or carbon decoration

A conspicuous discrepancy is prevalent in the characterization of carbon supported palladium catalysts: particle size estimated by XRD and electron microscopy agree, while the chemisorption of hydrogen is substantially suppressed with respect to the small size measured by XRD/STEM. In this work, a systematic study of carbon materials, pretreatments, and characterization has isolated the contributions of carbon decoration and residual chloride, both of which can cause a chemisorption shortfall. The degree of decoration decreases with graphitization of the carbon supports due to stronger CC interaction, whereas increased density of oxygen functional groups on the surface increases decoration, due to enhanced Pd-C interactions. The discrepancy can be rectified by employing a low temperature burnoff of the decorating carbon layers prior to chemisorption, and by using metal precursors without chloride ligands or counterions.