Recent Progress in Pd-Based Nanocatalysts for Selective Hydrogenation

Highly efficient catalytic transfer hydrogenation for the conversion of nitrobenzene to aniline over PdO/TiO2: The key role of in situ switching from PdO to Pd

The reduction of nitrobenzene to aniline is very important for both pollution control and chemical synthesis. Nevertheless, difficulties still remain in developing a catalytic system having high efficiency and selectivity for the production of aniline. Herein, it was found that PdO nanoparticles highly dispersed on TiO2 support (PdO/TiO2) functioned as a highly efficient catalyst for the reduction of nitrobenzene in the presence of NaBH4. Under favorable conditions, 95% of the added nitrobenzene (1 mmol/L) was reduced within 1 min with an ultra-low apparent activation energy of 10.8 kJ/mol by using 0.5%PdO/TiO2 as catalysts and 2 mmol/L of NaBH4 as reductants, and the selectivity to aniline even reached up to 98%. The active hydrogen species were perceived as dominant species during the hydrogenation of nitrobenzene by the results of isotope labeling experiments and ESR spectroscopic. A mechanism was proposed as follows: PdO activates the nitro groups and leads to in-situ generation of Pd, and the generated Pd acts as the reduction sites to produce active hydrogen species. In this catalytic system, nitrobenzene prefers to be adsorbed on the PdO nanoparticles of the PdO/TiO2 composite. Subsequently, the addition of NaBH4 results in in-situ generation of a Pd/PdO/TiO2 composite from the PdO/TiO2 composite, and the Pd nanoclusters would activate NaBH4 to generate active hydrogen species to attack the adsorbed nitro groups. This work will open up a new approach for the catalytic transfer hydrogenation of nitrobenzene to aniline in green chemistry.

Modifying the Substrate-Dependent Pd/Fe2O3 Catalyst-Support Synergism with ZnO Atomic Layer Deposition

Low-loading Pd supported on Fe2O3 nanoparticles was synthesized. A common nanocatalyst system with previously reported synergistic enhancement of reactivity that is attributed to the electronic interactions between Pd and the Fe2O3 support. Fe2O3-selective precoalescence overcoating with ZnO atomic layer deposition (ALD), using Zn(CH2CH3)2 and H2O as precursors, dampens competitive hydrogenation reactivity at Fe2O3-based sites. The result is enhanced efficiency at the low-loading but high reactivity Pd sites. While this increases catalyst efficiency toward most aqueous redox reactions tested, it suppresses reactivity toward polyaromatic core substrates. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) show minimal electronic impacts for the ZnO overcoat on the Pd particles, implying a predominantly physical site blocking effect as the reason for the modified reactivity. This serves as a proof-of-concept of not only stabilizing supported nanocatalysts but also altering reactivity with ultrathin ALD overcoats. The results point to a facile ALD route for selective enhancement of reactivity for low-loading Pd-based supported nanocatalysts.

Tannic Acid-Decorated Bimetallic Copper-Gold Nanoparticles with High Catalytic Activity for the Degradation of 4-Nitrophenol and Rhodamine B

In this study, tannic acid (TA) was applied as a stabilizing agent for synthesizing bimetallic copper-gold (CuAu) nanoparticles. Cu(NO3)2 and NaAuCl4 were used as the sources of copper and gold ions, respectively, and NaBH4 was employed as a reducing agent. The prepared TA-CuAu nanoparticles were extensively characterized via ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction, and zeta potential analyses. To evaluate their catalytic activity, the TA-CuAu nanoparticles and NaBH4 were applied in the degradation of 4-nitrophenol (4-NP) and rhodamine B (RB) individually and in a mixture. The individual degradation of 4-NP and RB was completed within 10 min, and the apparent rate constants were calculated as 0.3046 and 0.2628 min-1, respectively, emphasizing the efficient catalytic activity of the TA-CuAu nanoparticles. Additionally, controlled experiments were performed for the degradation of 4-NP and RB in the absence of catalysts or NaBH4 to investigate the kinetic feasibility of the catalytic reactions. A mixture of 4-NP and RB was successfully degraded within 10 min using the TA-CuAu nanoparticles as catalysts. Furthermore, the reuse of the catalysts after five successive cycles demonstrates an outstanding performance with no significant loss in the catalytic activity. Finally, the successful treatment of the tap and lake water samples spiked with 4-NP and RB using the TA-CuAu nanoparticles further confirmed their application potential as catalysts in environmental water remediation.

Hydrogenation of cyclohexene over single-atom Pt or Pd incorporated porous ceria nanoparticles under solvent-free conditions

In order to maximize the utilization of noble metals in catalysis, single atom of palladium (Pd) and platinum (Pt) were incorporated individually in the framework of porous ceria (CeO2) by using a one-step flash combustion method. Samples with different Pd and Pt loading (0.5, 1, 2.5, and 5 wt%) were prepared and examined by using different analysis techniques such as XRD, ICP, N2 sorption measurements, SEM, HR-TEM, and XPS. The characterization data confirms the formation of zero-state single-atom Pt and Pd (with possible formation of Pd nanoparticles with a size less than 5 nm) incorporated onto the three-dimensional porous ceria structure. The catalytic activity of the synthesized materials was studied in the cyclohexene reduction to cyclohexane at 393 K and 3 atm of pure hydrogen (H2) gas as a model reaction. The obtained results demonstrated that the conversion percentage of cyclohexene is increasing with Pd or Pt loading. The best cyclohexene conversion, 21% and 29%, was achieved over the sample that contains 5 wt% of Pt and Pd, respectively. The collected catalytic data fit the zero-order reaction model, and the rate constant of each catalyst was determined. The catalytic experiments of the most-performed catalysts were repeated five times and the obtained loss in activity was insignificant.

Advances and Challenges in Development of Transition Metal Catalysts for Heterogeneous Hydrogenation of Organic Compounds

Actual problems of development of catalysts for hydrogenation of heterocyclic compounds by hydrogen are summarized and discussed. The scope of review covers composites of nanoparticles of platinum group metals and 3d metals for heterogeneous catalytic processes. Such problems include increase of catalyst activity, which is important for reduction of precious metals content; development of new catalytic systems which do not contain metals of platinum group or contain cheaper analogues of Pd; control of factors which make influence on the selectivity of the catalysts; achievement of high reproducibility of the catalyst's performance and quality control of the catalysts. Own results of the authors are also summarized and described. The catalysts were prepared by decomposition of Pd0 and Ni0 complexes, pyrolysis of Ni2+ and Co2+ complexes deposited on aerosil and reduction of Ni2+ in pores of porous support in situ. The developed catalysts were used for hydrogenation of multigram batches of heterocyclic compounds.

Mesoporous α-Al2O3-supported PdCu bimetallic nanoparticle catalyst for the selective semi-hydrogenation of alkynes

The selective hydrogenation of alkynes to alkenes is widely applied in the chemical industry; nevertheless, achieving highly selective hydrogenation with high catalytic activity is considerably challenging. Herein, ultrafine PdCu bimetallic nanoparticles encapsulated by high-surface-area mesoporous α-Al2O3 were prepared by high-temperature calcination-reduction using a porous organic framework (POF) as the template. As-obtained PdCu@α-Al2O3 exhibited a high selectivity of 95% for the semi-hydrogenation of phenylacetylene as a probe reaction under mild reaction conditions. The separation of continuous Pd atoms and modification of the Pd electronic state by Cu atoms suppressed β-hydride formation and alkene adsorption, contributing to high selectivity for the catalytic hydrogenation of alkynes. The catalytic activity was maintained after 7 cycles due to the strong interaction between the PdCu bimetallic nanoparticles and α-Al2O3 as well as the encapsulation effect of mesoporous α-Al2O3. Thus, the current work provides a facile strategy for fabricating high-surface-area mesoporous α-Al2O3-supported catalysts for industrial catalysis applications.

The surface chemistry of norbornadiene and norbornene on Pd(111) and Pd(100): a comparative DFT study

CONTEXT

The interaction of norbornadiene (NBD) and norbornene (NBE) with the palladium (111) and (100) surfaces have been investigated using density functional theory (DFT). Five configurations of adsorbed NBD may be formed on Pd(111): endo-tetra-σ, endo-di-σ,π, endo-di-π, exo-di-σ, and exo-π. The NBE molecule adsorbed on Pd(111) may exist in 4 configurations: endo-di-σ, endo-π, exo-di-σ, and exo-π. On Pd(100), a smaller number adsorption configurations of NBD and NBE are formed, since the double bonds of these molecules in the endo-orientation are bound only in a di-σ mode. The adsorption energy of NBD and NBE molecules on Pd(100) is noticeably higher compared to Pd(111), which is due to the surface geometry of Pd(100). The most stable configurations on both Pd facets are endo-tetra-σ for NBD and exo-di-σ for NBE. However, due to smaller adsorption area of the exo-di-σ configuration on Pd(111), a larger number of NBD molecules may adsorbed on the same surface area. Energetically favorable endo-tetra-σ (NBD) and exo-di-σ (NBE) configurations are very mobile on Pd(111). On Pd(100), only NBE molecules can migrate, while NBD migration is hindered due to the high activation barrier.

METHODS

All DFT calculations were performed using the Perdew-Burke-Ernzerhof density functional (PBE) with the relativistic SBK effective core potential and TZ2P basis set in the PRIRODA program.

In Situ DRIFTS Analysis during Hydrogenation of 1-Pentyne and Olefin Purification with Ag Nanoparticles

The catalytic performance of nanoparticles (NPs) of Ag anchored on different supports was evaluated during the selective hydrogenation of 1-pentyne and the purification of a mixture of 1-pentene/1-pentyne (70/30 vol %). The catalysts were identified: Ag/Al (Ag supported on ɣ-Al2 O3 ), Ag/Al-Mg (Ag supported on ɣ-Al2 O3 modified with Mg), Ag/Ca (Ag supported on CaCO3 ) and Ag/RX3 (Ag supported on activated carbon-type: RX3). In addition, in situ DRIFTS analysis of 1-pentyne adsorption on each support, catalyst, and 1-pentyne hydrogenation were investigated. The results showed that the synthesized catalysts were active and very selective (≥85 %) for obtaining the desired product (1-pentene). Different adsorbed species (-C≡C- and -C=C-) were observed on the supports and catalysts surface using in situ DRIFT analysis, which can be correlated to the activity and high selectivity reached. The role of the supports and electronic properties over Ag improve the H2 dissociative chemisorption during the hydrogenation reactions; promoting the selectivity and the high catalytic performance. Ag/Al and Ag/Al-Mg were the most active catalysts. This was due to the synergism between the active Ag/Ag+ species and the supports (electronic effects). The results show that Ag/Al and Ag/Al-Mg catalysts have favorable properties and are promising for the alkyne hydrogenation and olefin purification reactions.

Supraparticles for naked-eye H2 indication and monitoring: Improving performance by variation of the catalyst nanoparticles

The recent transition to H₂-based energy storage demands reliable H₂ sensors that allow for easy, fast, and reliable detection of leaks. Conventional H₂ detectors are based on the changes of physical properties of H₂ probes induced by subsurface H-atoms to a material such as electrical conductivity. Herein, we report on highly reactive gasochromic H₂ detectors based on the adsorption of H₂ on the material surface. We prepared supraparticles (SPs) containing different types of noble metal nanoparticles (NPs), silica NPs, and the dye resazurin by spray-drying and tested their performance for H₂ detection. The material undergoes a distinct color change due to the hydrogenation of the purple resazurin to pink resorufin and, finally, colorless hydroresorufin. The stepwise transition is fast and visible to the naked eye. To further improve the performance of the sensor, we tested the reactivity of SPs with different catalytically active NPs by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the choice of the NP catalyst has a pronounced effect on the response of the H₂ indicator. In addition, we demonstrate that the performance depends on the size of the NPs. These effects are attributed to the availability of reactive H-atoms on the NP surface. Among the materials studied, Pt-containing SPs gave the best results for H₂ detection.

Single-Atom Alloy Pd1Ag10/CeO2–ZrO2 as a Promising Catalyst for Selective Alkyne Hydrogenation

The effect of support on the performance of Pd1Ag10/Al2O3 and Pd1Ag10/CeO2–ZrO2 catalysts in the selective hydrogenation of diphenylacetylene (DPA) was studied. Characterization of the catalyst by DRIFTS-CO and HRTEM revealed the formation of a PdAg single-atom alloy (SAA) structure on the surface of PdAg nanoparticles, with Pd1 sites isolated by Ag atoms. It was found that the use of CeO2–ZrO2 as a carrier makes it possible to increase the activity of the Pd1Ag10 catalyst by a factor of three without loss of selectivity compared to the reference Pd1Ag10/Al2O3. According to the HRTEM data, this catalytic behavior can be explained by an increase in the dispersion of Pd1Ag10/CeO2–ZrO2 compared to its Pd1Ag10/Al2O3 counterpart. As evidenced by DRIFTS-CO data, the high selectivity of the Pd1Ag10/CeO2–ZrO2 sample presumably stems from the stability of the structure of isolated Pd1 sites on the surface of SAA Pd1Ag10/CeO2–ZrO2.

Electron-Poor Phosphines Enable the Selective Semihydrogenation Reaction of Alkynes with Pd on Carbon Catalysts

An alternative to the Lindlar catalyst for the semihydrogenation reaction of alkynes to alkenes is of high interest. Here we show that palladium on carbon (Pd/C), i.e., a widely available supported Pd catalyst, is converted from an unselective to a chemoselective catalyst during the semihydrogenation reaction of alkynes, after the addition of catalytic amounts of commercially available electron-poor phosphines. The catalytic activity is ≤7 times greater, and the selectivity is comparable to that of the industrial benchmark Lindlar catalyst.

Acceleration of the semi-hydrogenation of alkynes over an N-doped porous carbon sphere-confined ultrafine PdCu bimetallic nanoparticle catalyst

Selective hydrogenation of alkynes to obtain alkenes is a key reaction in petrochemical and fine chemical industries. However, the development of stable and highly selective catalysts with uniformly dispersed active sites is still immensely challenging for the semi-hydrogenation of alkynes. In this study, N-doped porous carbon nanospheres (NPCNs) were synthesized by the nanoemulsion self-assembly and subsequently carbonization method. Ultrafine PdCu bimetallic nanoparticles (NPs) were uniformly dispersed and immobilized on NPCNs. The obtained PdCu/NPCNs catalyst exhibited an open framework and abundant active sites originating from ultrafine PdCu NPs. In the semi-hydrogenation of alkynes, the PdCu/NPCNs catalyst exhibited a remarkable performance and stability, outperforming most of the classical catalysts. The excellent performance was related to the introduction of a secondary metal Cu, which can regulate the electronic state of Pd active sites to further enhance the hydrogenation activity and selectivity. Hence, the facile approach reported herein may be useful for constructing highly dispersed bimetallic NP-based catalysts for selective hydrogenation of alkynes in the petrochemical industry.

Highly Active Bimetallic Single-Atom Alloy PdAg Catalysts on Cerium-Containing Supports in the Hydrogenation of Alkynes to Alkenes

Abstract

A study of a series of single-atom-alloy catalysts Pd1Ag3/Al2O3, Pd1Ag3/CeO2–Al2O3, and Pd1Ag3/CeO2–ZrO2 in the selective hydrogenation of diphenylacetylene (DPA) showed a significant (five-fold) increase in activity for the PdAg3/CeO2–ZrO2 sample in comparison with that of Pd1Ag3/Al2O3. It was especially noted that the increase in activity was not accompanied by a decrease in the selectivity for the target product. This catalytic behavior can be explained by two factors: (1) a more than twofold increase in the dispersity of the PdAg3/CeO2–ZrO2 catalyst and (2) a change in the electronic state of the nanoparticles, as determined from the results of an IR-spectroscopic study of adsorbed CO. The retention of the high selectivity of the synthesized catalysts indicated the stability of the structure of Pd1 monoatomic sites in the catalysts prepared by deposition on Ce-containing supports, which was also confirmed by the IR spectroscopy of adsorbed CO. The experimental results indicate that Ce-containing supports are promising for the synthesis of catalysts for the selective hydrogenation of substituted alkynes.

Hybrid Pd-Nanoparticles within Polymeric Network in Selective Hydrogenation of Alkynols: Influence of Support Porosity

This work is addressing the selective hydrogenation of alkynols over hybrid catalysts containing Pd-nanoparticles, within newly synthesized hyper-cross-linked polystyrenes (HPS). Alkynols containing C₅, C₁₀, and C₂₀ with a terminal triple bond, which are structural analogues or direct semi-products of fragrant substances and fat-soluble vitamins, have been studied. Selective hydrogenation was carried out in a batch mode (ambient hydrogen pressure, at 90 °C, in toluene solvent), using hybrid Pd catalysts with low metal content (less than 0.2 wt.%). The microporous and mesoporous HPS were both synthesized and used as supports in order to address the influence of porosity. Synthesized catalysts were shown to be active and selective: in the case of C₅, hydrogenation selectivity to the target product was more than 95%, at close to complete alkynol conversion. Mesoporous catalysts have shown some advantages in hydrogenation of long-chain alkynols.

Robust palladium catalysts on nickel foam for highly efficient hydrogenations

The combination of a polydopamine interface, solvothermal seeding of Pd(OAc)2, and ALD Al2O3 overcoat enables the formation of evenly-coated, ultralow Pd loading Ni foam monolith materials.