Photodeposited Pd Nanoparticles with Disordered Structure for Phenylacetylene Semihydrogenation

Renaissance in Alkyne Semihydrogenation: Mechanism, Selectivity, Functional Group Tolerance, and Applications in Organic Synthesis

Alkenes constitute a significant class of chemical compounds with applications in the bulk, pharmaceutical, or perfume industry. Among the known methods of olefin production, semihydrogenation of the C-C triple bond seems to be the most straightforward one. Nonetheless, the success of this reaction requires full control over diastereoselectivity, eradication of a parasitic process of over-reduction or migration of the C-C double bond formed, and achieving satisfactory functional-group compatibility. The review demonstrates developments in the field of alkyne semihydrogenation over the period 2010-2022, with selected papers published in 2023 and 2024, emphasizing solutions to the above-mentioned limitations. We discuss mechanistic aspects of this transformation, including those related to unconventional systems. The review includes examples of applications of alkyne semihydrogenation in organic synthesis, confirming the considerable utility of this process. Finally, strategies to enhance catalyst selectivity are summarized. For the reader's convenience, we provided a graphical guidebook to catalytic systems, illustrating the efficiency of the particular method.

Ambient-condition acetylene hydrogenation to ethylene over WS2-confined atomic Pd sites

Ambient-condition acetylene hydrogenation to ethylene (AC-AHE) is a promising process for ethylene production with minimal additional energy input, yet remains a great challenge due to the difficulty in the coactivation of acetylene and H2 at room temperature. Herein, we report a highly efficient AC-AHE process over robust sulfur-confined atomic Pd species on tungsten sulfide surface. The catalyst exhibits over 99% acetylene conversion with a high ethylene selectivity of 70% at 25 oC, and a record space-time yield of ethylene of 1123 molC2H4 molPd-1 h-1 under ambient conditions, which is nearly four times that of the typical Pd1Ag3/Al2O3 catalyst, and exhibiting superior stability of over 500 h. We demonstrate that the confinement of Pd-S coordination induces positively-charged atomic Pdδ+, which not only facilitates C2H2 hydrogenation but also promotes C2H4 desorption, thereby enabling a high conversion of C2H2 to C2H4 at room temperature while suppressing over-hydrogenation to C2H6.

Machine Learning for Quantitative Structural Information from Infrared Spectra: The Case of Palladium Hydride

Infrared spectroscopy (IR) is a widely used technique enabling to identify specific functional groups in the molecule of interest based on their characteristic vibrational modes or the presence of a specific adsorption site based on the characteristic vibrational mode of an adsorbed probe molecule. The interpretation of an IR spectrum is generally carried out within a fingerprint paradigm by comparing the observed spectral features with the features of known references or theoretical calculations. This work demonstrates a method for extracting quantitative structural information beyond this approach by application of machine learning (ML) algorithms. Taking palladium hydride formation as an example, Pd-H pressure-composition isotherms are reconstructed using IR data collected in situ in diffuse reflectance using CO molecule as a probe. To the best of the knowledge, this is the first example of the determination of continuous structural descriptors (such as interatomic distance and stoichiometric coefficient) from the fine structure of vibrational spectra, which opens new possibilities of using IR spectra for structural analysis.

Excellent Pd-Loaded Magnetic Nanocatalyst on Multicarboxyl and Boronic Acid Biligands

An effective palladium nanocatalyst (Fe3O4@SiO2-FPBA-DTPA-Pd) was proposed and prepared, which was immobilized on magnetic silica with ethylenediamine pentaacetic acid and formylphenylboronic acid as biligands. A series of characterizations showed that Fe3O4@SiO2-FPBA-DTPA-Pd was 5-15 nm and contained 1.44 mmol/g Pd2+/Pd0. It was stable below 232.7 °C, and its saturation magnetization value was 21.17 emu/g which was easily recycled by a magnet. Its catalytic ability was evaluated through 7 Suzuki reactions and 15 Heck reactions. Results showed that the yields of 14 reactions catalyzed by Fe3O4@SiO2-FPBA-DTPA-Pd were more than 90%, while were better than those of the other two immobilized Pd catalysts on a single diethyltriamine pentaacetic acid (DTPA) group or boronic acid group. Moreover, Fe3O4@SiO2-FPBA-DTPA-Pd showed good reusability in both Suzuki and Heck reactions. In two model Suzuki and Heck reactions, after seven cycles, its yields were still above 95% without significant loss, which exceeded those of many reported catalysts; therefore, it has great potential in future large-scale industrial production.

Triple Play of Band Gap, Interband, and Plasmonic Excitations for Enhanced Catalytic Activity in Pd/HxMoO3 Nanoparticles in the Visible Region

Plasmonic photocatalysis has been limited by the high cost and scalability of plasmonic materials, such as Ag and Au. By focusing on earth-abundant photocatalyst/plasmonic materials (HxMoO3) and Pd as a catalyst, we addressed these challenges by developing a solventless mechanochemical synthesis of Pd/HxMoO3 and optimizing photocatalytic activities in the visible range. We investigated the effect of HxMoO3 band gap excitation (at 427 nm), Pd interband transitions (at 427 nm), and HxMoO3 localized surface plasmon resonance (LSPR) excitation (at 640 nm) over photocatalytic activities toward the hydrogen evolution and phenylacetylene hydrogenation as model reactions. Although both excitation wavelengths led to comparable photoenhancements, a 110% increase was achieved under dual excitation conditions (427 + 640 nm). This was assigned to a synergistic effect of optical excitations that optimized the generation of energetic electrons at the catalytic sites. These results are important for the development of visible-light photocatalysts based on earth-abundant components.

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.

Photocatalytic Semi-Hydrogenation of Alkynes: A Game of Kinetics, Selectivity and Critical Timing

The semi-hydrogenation reaction of alkynes is important in the fine chemicals and pharmaceutical industries, and it is thus important to find catalytic processes that will drive the reaction efficiently and at a low cost. The real challenge is to drive the alkyne-to-alkene reaction while avoiding over-hydrogenation to the saturated alkane moiety. The problem is more difficult when dealing with aromatic substitution at the alkyne center. Simple photocatalysts based on Palladium tend to proceed to the alkane, and stopping at the alkene with good selectivity requires very precise timing with basically no timing tolerance. We report here that the goal of high conversion with high selectivity could be achieved with TiO2-supported copper (Cu@TiO2), although with slower kinetics than for Pd@TiO2. A novel bimetallic catalyst, namely, CuPd@TiO2 (0.8% Cu and 0.05% Pd), with methanol as the hydrogen source could improve the kinetics by 50% with respect to Cu@TiO2, while achieving selectivities over 95% and with exceptional timing tolerance. Further, the low Palladium content minimizes its use, as Palladium is regarded as an element at risk of depletion.

Moderating the interaction among Pd, CeO2, and Al2O3 for improved three-way catalysts

The Pd distribution and the CeO₂-Al₂O₃ combination are among the decisive factors for the performance of commercial three-way catalysts. Generally, the sufficient doping of Pd into ceria-based oxides and the intimate interaction between CeO₂ and Al₂O₃ could both benefit the three-way catalytic reactions. However, in the present work, the moderate doping of Pd into CeO₂ and less intimate CeO₂-Al₂O₃ interaction were found to be responsible for the much higher catalytic activity (the decrease in T₅₀ was 52, 119, or 55 °C for C₃H₆, CO, or NO) in PdCe/Al₂O₃-CP than PdCe/Al₂O₃-Imp, for which the Pd and Ce species were co-loaded onto Al₂O₃ through the co-precipitation or impregnation method, respectively. It was intriguing to find that the co-precipitated PdCeO_x in PdCe/Al₂O₃-CP showed less sufficient doping of Pd into CeO₂ than the co-impregnated PdCeO_x in PdCe/Al₂O₃-Imp; as a result, both a higher fraction of highly active metallic Pd and a higher Pd dispersion were realized in PdCe/Al₂O₃-CP. Moreover, due to the less intimate CeO₂-Al₂O₃ interaction, specifically the less severe penetration of the Pd and Ce species into Al₂O₃, PdCe/Al₂O₃-CP showed higher Pd dispersion, specific surface area, pore volume and size than PdCe/Al₂O₃-Imp. The presence of more abundant reactive Pd⁰, and the higher accessibility of the active Pd and CeO₂ sites, together with improved redox properties and enriched oxygen vacancies contributed much to the enhanced three-way catalytic activity of PdCe/Al₂O₃-CP. Additionally, simultaneously optimizing the Pd distribution and the CeO₂-Al₂O₃ combination in a single step, as reported in this work, is also highly desirable in industry.

Pd Nanoparticles Supported on N-Doped TiO2 Nanosheets: Crystal Facets, Defective Sites, and Metal-Support Interactions Boost Reforming of Formaldehyde Solution for Hydrogen Production

To produce H₂ from formaldehyde (HCHO), dehydrogenation offers an alternative approach to future hydrogen-based energy sources, but the unsatisfactory efficiency hinders its practical application. Here, ultrafine Pd nanoparticle (NP) decorated N-doped TiO₂ nanosheets exposed with (001) facet catalysts (denoted as Pd/TiO_2-x) have been prepared and exhibit superior H₂ production performance from alkaline HCHO aqueous solution. Under our current conditions, the Pd/TiO_2-x catalyst with a Pd loading of 1 wt % exhibits a H₂ production rate of 183.77 mL/min/g, which is 1.75 and 3.66 times that of Pd/TiO₂ and Pd NPs, respectively. Based on the results of Fourier transform infrared spectroscopy (FTIR), Raman, and liquid-phase electron paramagnetic resonance (EPR) spin-trapping experiments, the excellent H₂ generation of Pd/TiO_2-x can be attributed to the synergistic contribution among the reactive crystal facets, defective sites, and metal-support interactions in boosting the breakage of C-H bonds in HCHO, dissociation of H₂O, and ultimately the formation of H₂. This work is expected to provide a paradigm of an efficient catalyst to produce H₂ from HCHO/H₂O solution.

Grafting nanometer metal/oxide interface towards enhanced low-temperature acetylene semi-hydrogenation

Metal/oxide interface is of fundamental significance to heterogeneous catalysis because the seemingly “inert” oxide support can modulate the morphology, atomic and electronic structures of the metal catalyst through the interface. The interfacial effects are well studied over a bulk oxide support but remain elusive for nanometer-sized systems like clusters, arising from the challenges associated with chemical synthesis and structural elucidation of such hybrid clusters. We hereby demonstrate the essential catalytic roles of a nanometer metal/oxide interface constructed by a hybrid Pd/Bi₂O₃ cluster ensemble, which is fabricated by a facile stepwise photochemical method. The Pd/Bi₂O₃ cluster, of which the hybrid structure is elucidated by combined electron microscopy and microanalysis, features a small Pd-Pd coordination number and more importantly a Pd-Bi spatial correlation ascribed to the heterografting between Pd and Bi terminated Bi₂O₃ clusters. The intra-cluster electron transfer towards Pd across the as-formed nanometer metal/oxide interface significantly weakens the ethylene adsorption without compromising the hydrogen activation. As a result, a 91% selectivity of ethylene and 90% conversion of acetylene can be achieved in a front-end hydrogenation process with a temperature as low as 44 °C.

Metal/oxide interface is of fundamental significance to heterogeneous catalysis. Here, the authors construct a nanometer Pd/Bi₂O₃ interface by grafting Pd clusters onto Bi₂O₃ clusters and demonstrate its essential roles in the low-temperature semi-hydrogenation of acetylene.

Zero-Valent Palladium Single-Atoms Catalysts Confined in Black Phosphorus for Efficient Semi-Hydrogenation

Single-atom catalysts (SACs) represent a new frontier in heterogeneous catalysis due to their remarkable catalytic properties and maximized atomic utilization. However, single atoms often bond to the support with polarized electron density and thus exhibit a high valence state, limiting their catalytic scopes in many chemical transformations. Here, it is demonstrated that 2D black phosphorus (BP) acts as giant phosphorus (P) ligand to confine a high density of single atoms (e.g., Pd₁ , Pt₁ ) via atomic layer deposition. Unlike other 2D materials, BP with relatively low electronegativity and buckled structure favors the strong confinement of robust zero-valent palladium SACs in the vacancy site. Metallic Pd₁ /BP SAC shows a highly selective semi-hydrogenation of phenylacetylene toward styrene, distinct from metallic Pd nanoparticles that facilitate the formation of fully hydrogenated products. Density functional theory calculations reveal that Pd atom forms covalent-like bonding with adjacent P atoms, wherein H atoms tend to adsorb, aiding the dissociative adsorption of H₂ . Zero-valent Pd in the confined space favors a larger energy gain for the synthesis of partially hydrogenated product over the fully hydrogenated one. This work provides a new route toward the synthesis of zero-valent SACs on BP for organic transformations.

Atomically dispersed Pd-Ru dual sites in an amorphous matrix towards efficient phenylacetylene semi-hydrogenation

Optimizing the active sites to balance the conversion and selectivity of the target reaction has long been a challenging quest in developing noble metal-based catalysts. By dispersing Pd and Ru in an amorphous zirconium hydrogen phosphate matrix cross-linked by ionic inorganic oligomers, highly diluted noble metal (<0.2 mol%) can be utilized as dual single-atom sites in oxides for the semi-hydrogenation of phenylacetylene with optimized conversion and selectivity (both >90%) to styrene. In situ DRIFT-IR results suggested the fast generation of surface hydroxyl groups during the catalytic reaction, indicating the high efficiency of the single-atom sites to dissociate bound H2. This work provides an easily scaled-up method for the production of cost-effective single-atom catalysts extendable to various oxide matrices.

Cu2O hydrides promote the selective semihydrogenation of alkynes on Pd–Cu2O/TiO2 under mild conditions

High activity, chemoselectivity and stereoselectivity were unveiled by the synergy of the p–n heterojunctions with Pd⁰ in a novel Pd–Cu₂O/TiO₂ catalyst for spillover semihydrogenations.

Photodegradation of Phenolic Compounds from Water in the Presence of a Pd-Containing Exhausted Adsorbent

A closed-cycle technology regarding the use of an exhausted Pd-based adsorbent as a photocatalyst in the degradation process of phenol is presented. Pd (II) represents a precious metal of great economic importance. Its obtained from natural sources become more difficult to achieve. Therefore, also considering the regulations of the “circular economy,” its recovery from secondary sources turn out to be a stringent issue in the last years. Pd(II) ions are removed from aqueous solution through adsorption onto Florisil (an inorganic solid support—magnesium silicate) impregnated with Cyphos IL 101 (trihexyl tetradecyl phosphonium chloride). It was observed that the presence of the ionic liquid (IL) in the adsorbent structure doubles the adsorption efficiency of the studied materials. The newly obtained Pd-based photocatalyst was exhaustively characterized and was used in the degradation process of phenol from aqueous solutions. The phenol degradation process was studied in terms of the nature of the photocatalyst used, time of photodegradation and solid: liquid ratio. It was observed that both the presence of IL and Pd lead to an increase in the efficiency of the phenol degradation process. The new Pd-based photocatalyst could be efficiently used in more cycles of phenol photodegradation processes. When is used as a photocatalyst the Florisil impregnated with IL and loaded with 2 mg/g of Pd, a degree of mineralization of 93.75% is obtained after 180 min of irradiation of a phenol solution having a concentration of 20 mg/L and using a solid:liquid ratio = 1:1.

Pd nanoparticle growth monitored by DRIFT spectroscopy of adsorbed CO

Synchrotron-based X-ray absorption spectroscopy and scattering are known in situ probes of metal nanoparticles (NPs). A limited number of laboratory techniques allow post-synthesis diagnostics of the active metal surface area. This work demonstrates the high potential of infrared spectroscopy as an in situ laboratory probe for the growth of metal NPs on a substrate. We introduce a small fraction of CO molecules into the reaction mixture as a probe to monitor the reduction kinetics of the Pd2+ precursor on ceria in hydrogen.

Transfer hydrogenation of alkynes into alkenes by ammonia borane over Pd-MOF catalysts

Ammonia borane with both hydridic and protic hydrogens in its structure acted as an efficient transfer hydrogenation agent for selective transformation of alkynes into alkenes in non-protic solvents.

Ultra-Small Pd Nanoparticles on Ceria as an Advanced Catalyst for CO Oxidation

In this study, we demonstrate the preparation and characterization of small palladium nanoparticles (Pd NPs) on modified ceria support (Pd/CeO2) using wet impregnation and further reduction in an H2/Ar flow. The obtained particles had a good dispersion, but their small size made it difficult to analyze them by conventional techniques such as transmission electron microscopy (TEM) and X-ray powder diffraction (XRPD). The material demonstrated a high catalytic activity in the CO oxidation reaction: the 100% of CO conversion was achieved at ~50 °C, whereas for most of the cited literature, such a high conversion usually was observed near 100 °C or higher for Pd NPs. Diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy in combination with CO probe molecules was used to investigate the size and morphology of NPs and the ceria support. On the basis of the area ratio under the peaks attributed to bridged (B) and linear (L) carbonyls, high-dispersion Pd NPs was corroborated. Obtained results were in good agreement with data of X-ray absorption near edge structure analysis (XANES) and CO chemisorption measurements.

Microwave assisted hydrogenation of olefins by Pd NPs@polystyrene resin using a gas addition kit: a robust and sustainable protocol

Polystyrene (PS) resin bead supported palladium nanoparticles (Pd NPs@PS resin) were prepared and their catalytic activity for the hydrogenation of olefins was investigated under microwave heating.