Synergy of Contact between ZnO Surface Planes and PdZn Nanostructures: Morphology and Chemical Property Effects in the Intermetallic Sites for Selective 1,3-Butadiene Hydrogenation

Circumventing the activity-selectivity trade-off via the confinement effect from induced potential barriers on the Pd nanoparticle surface

The request for both high catalytic selectivity and high catalytic activity is rather challenging, particularly for catalysis systems with the primary and side reactions having comparable energy barriers. Here in this study, we simultaneously optimized the selectivity and activity for acetylene semi-hydrogenation by rationally and continuously varying the doping ratio of Zn atoms on the surface of Pd particles in Pd/ZnO catalysts. In the reaction temperature range of 40-200 °C, the conversion of acetylene was close to ∼100%, and the selectivity for ethylene exceeded 90% (the highest ethylene selectivity, ∼98%). Experimental characterization and density functional theory calculations revealed that the Zn promoter could alter the catalyst's potential energy surface, resulting in a "confinement" effect, which effectively improves the selectivity yet without significantly impairing the catalytic activity. The mismatched impacts on activity and selectivity resulting from continuous and controllable alteration in the catalyst structure provide a promising parameter space within which the two aspects could both be optimized.

Host-induced alteration of the neighbors of single platinum atoms enables selective and stable hydrogenation of butadiene

Tuning the coordination neighbors of the metal center is emerging as an elegant approach to manipulating the performance of supported single-atom catalysts in heterogeneous catalysis. Herein, atomically dispersed Pt species with different coordination neighbors hosted on nitrogen-doped carbon (NC) and graphitic carbon nitride (C₃N₄) are constructed through an impregnation-activation approach. Advanced characterization techniques including X-ray electron microscopy, X-ray absorption spectroscopy, and high angle annular dark-field scanning transmission electron microscopy reveal the different nature of active sites induced by the hosts: i.e., the Pt-N_x configuration in NC but both Pt-N and Pt-O coordinations in C₃N₄. H₂-D₂ exchange experiments and electron microscopy further evidence that Pt/NC exhibits a high propensity for H₂ splitting and high thermal stability of the Pt species against agglomeration, whereas Pt/C₃N₄ cannot dissociate H₂ and the Pt atoms easily aggregate in the reductive stream. Consequently, when applied in the selective hydrogenation of 1,3-butadiene, Pt/NC exhibits higher selectivity to butenes and excellent stability, but Pt/C₃N₄ behaves as a nanoparticle analogue favoring deep hydrogenation. The superior selectivity patterns of the single Pt atoms over Pt nanoparticles are rationalized by the inversed adsorption strength between the H₂ and 1,3-butadiene molecules at different metal sites, which is substantiated by the kinetic studies.

Tuning the electronic property of Pd nanoparticles by encapsulation within ZIF-67 shells towards enhanced performance in 1,3-butadiene hydrogenation

The low olefin selectivity of Pd-based catalysts is a long-term challenge for the selective hydrogenation of 1,3-butadiene.

Construction of Pd-Zn dual sites to enhance the performance for ethanol electro-oxidation reaction

Rational design and synthesis of superior electrocatalysts for ethanol oxidation is crucial to practical applications of direct ethanol fuel cells. Here, we report that the construction of Pd-Zn dual sites with well exposure and uniformity can significantly improve the efficiency of ethanol electro-oxidation. Through synthetic method control, Pd-Zn dual sites on intermetallic PdZn nanoparticles, Pd-Pd sites on Pd nanoparticles and individual Pd sites are respectively obtained on the same N-doped carbon coated ZnO support. Compared with Pd-Pd sites and individual Pd sites, Pd-Zn dual sites display much higher activity for ethanol electro-oxidation, exceeding that of commercial Pd/C by a factor of ~24. Further computational studies disclose that Pd-Zn dual sites promote the adsorption of ethanol and hydroxide ion to optimize the electro-oxidation pathway with dramatically reduced energy barriers, leading to the superior activity. This work provides valuable clues for developing high-performance ethanol electro-oxidation catalysts for fuel cells.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O₂/Ar and 10 vol % H₂/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au³⁺ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au³⁺ possesses the optimal activity with 1.8 × 10^-2 s^-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au³⁺ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au³⁺ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H₂ via constructing frustrated Lewis pairs with O.

Fabrication of PdZn alloy catalysts supported on ZnFe composite oxide for CO2 hydrogenation to methanol

The conversion of CO₂ to methanol is of great significance for providing a means of CO₂ fixation and the development of future fuels. Supported Pd catalysts have been demonstrated to be active for CO₂ hydrogenation to methanol and PdZn alloy plays a key role in this reaction. Therefore, using ZnO-enriched support to increase the amount of nanometric PdZn alloy particles on the surface is an effective strategy to develop ideal catalysts. Herein, we fabricated a PdZn alloy catalyst supported on ZnO-enriched ZnFe₂O₄ spinel for efficient CO₂ hydrogenation to methanol. The amount of formed PdZn alloy and catalyst structure influenced by ZnO concentration on ZnFe₂O₄ were explored to obtain the best Pd-Z₁FO catalyst, which achieves a methanol space-time yield (STY) of 593 gkg_cat^-1h^-1 (12 gg_Pd^-1h^-1) with CO₂ conversion of 14% under reaction conditions of 290 °C, 4.5 MPa and 21600 mLg^-1h^-1. Furthermore, the amount of exposed PdZn alloy sites were measured by using CO-pulse chemisorption and we find a linearity between methanol production rate and PdZn alloy sites.

Multifunctional ionic porous frameworks for CO2 conversion and combating microbes

Porous organic frameworks (POFs) with a heteroatom rich ionic backbone have emerged as advanced materials for catalysis, molecular separation, and antimicrobial applications. The loading of metal ions further enhances Lewis acidity, augmenting the activity associated with such frameworks. Metal-loaded ionic POFs, however, often suffer from physicochemical instability, thereby limiting their scope for diverse applications. Herein, we report the fabrication of triaminoguanidinium-based ionic POFs through Schiff base condensation in a cost-effective and scalable manner. The resultant N-rich ionic frameworks facilitate selective CO2 uptake and afford high metal (Zn(ii): 47.2%) loading capacity. Owing to the ionic guanidinium core and ZnO infused mesoporous frameworks, Zn/POFs showed pronounced catalytic activity in the cycloaddition of CO2 and epoxides into cyclic organic carbonates under solvent-free conditions with high catalyst recyclability. The synergistic effect of infused ZnO and cationic triaminoguanidinium frameworks in Zn/POFs led to robust antibacterial (Gram-positive, Staphylococcus aureus and Gram-negative, Escherichia coli) and antiviral activity targeting HIV-1 and VSV-G enveloped lentiviral particles. We thus present triaminoguanidinium-based POFs and Zn/POFs as a new class of multifunctional materials for environmental remediation and biomedical applications.

Zn-Promoted Selective Gas-Phase Hydrogenation of Tertiary and Secondary C4 Alkynols over Supported Pd

We have investigated the gas-phase (P = 1 atm; T = 373 K) hydrogenation of (tertiary alkynol) 2-methyl-3-butyn-2-ol (MBY) and (secondary) 3-butyn-2-ol (BY) over a series of carbon (C), non-reducible (Al₂O₃ and MgO), and reducible (CeO₂ and ZnO) supported monometallic [Pd (0.6-1.2% wt) and Zn (1% wt)] and bimetallic Pd-Zn (Pd:Zn mol ratio = 95:5, 70:30, and 30:70) catalysts synthesized by deposition-precipitation and colloidal deposition. The catalysts have been characterized by H₂ chemisorption, hydrogen temperature-programmed desorption (H₂-TPD), specific surface area (SSA), X-ray photoelectron spectroscopy (XPS), and transmission (TEM) and scanning transmission electron microscopy (STEM) analyses. Reaction over these catalysts generated the target alkenol [2-methyl-3-buten-2-ol (MBE) and 3-buten-2-ol (BE)] through partial hydrogenation and alkanol [2-methyl-butan-2-ol (MBA) and 2-butanol (BA)]/ketone [2-butanone (BONE)] as a result of full hydrogenation and double-bond migration. The catalysts exhibit a similar Pd nanoparticle size (2.7 ± 0.3 nm) but a modified electronic character (based on XPS). Hydrogenation activity is linked to surface hydrogen (from H₂ chemisorption and H₂-TPD). An increase in H₂:alkynol (from 1 → 10) results in enhanced alkynol consumption with a greater rate in the transformation of MBY (vs BY); H₂:alkynol had negligible effect on product distribution. Reaction selectivity is insensitive to the Pd site electron density with a similar response (S_MBE = 65 ± 9% and S_BE = 70 ± 8%) over Pd^δ- (on Al₂O₃ and MgO) and Pd^δ+ (on C and CeO₂). A Pd/ZnO catalyst delivered enhanced alkenol selectivity (S_MBE = 90% and S_BE = 96%) attributed to PdZn alloy phase formation (proved by XRD and XPS) but low activity, ascribed to metal encapsulation. A two-fold increase in the consumption rate was recorded for Pd-Zn/Al₂O₃ (30:70) versus Pd/ZnO with a similar alloy content (32 ± 4% from XPS), ascribed to a contribution due to spillover hydrogen (from H₂-TPD) where high alkenol selectivity was maintained.

The role of hydrogen coverage and location in 1,3-butadiene hydrogenation over Pt/SiO2

The coverage and location of H atoms are two key aspects for understanding the behavior of small Pt particles towards butadiene hydrogenation.

Pd–Au bimetallic catalysts supported on ZnO for selective 1,3-butadiene hydrogenation

The effect of the ZnO morphology on the properties of Pd–Au bimetallic catalysts has been discussed.

An investigation of the photovoltaic parameters of ZnS grown on ZnO(101̄1)

The selective growth of ZnS on ZnO (zinc nitrate versus acetate precursors) affects the photovoltaic parameters when the material is used as a photoanode in solar cells.

A flexible cell for in situ combined XAS-DRIFTS-MS experiments

A new cell for in situ combined X-ray absorption, diffuse reflectance IR Fourier transform and mass spectroscopies (XAS-DRIFTS-MS) is presented. The cell stands out among others for its achievements and flexibility. It is possible to perform XAS measurements in transmission or fluorescence modes, and the cell is compatible with external devices like UV-light and Raman probes. It includes different sample holders compatible with the different XAS detection modes, different sample forms (free powder or self-supporting pellet) and different sample loading/total absorption. Additionally, it has a small dead volume and can operate over a wide range of temperature (up to 600°C) and pressure (up to 5 bar). Three research examples will be shown to illustrate the versatility of the cell. This cell covers a wider range of applications than any other cell currently known for this type of study.

XAS/DRIFTS/MS spectroscopy for time-resolved operando investigations at high temperature

The combination of complementary techniques in the characterization of catalysts under working conditions is a very powerful tool for an accurate and in-depth comprehension of the system investigated. In particular, X-ray absorption spectroscopy (XAS) coupled with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) is a powerful combination since XAS characterizes the main elements of the catalytic system (selecting the absorption edge) and DRIFTS monitors surface adsorbates while MS enables product identification and quantification. In the present manuscript, a new reactor cell and an experimental setup optimized to perform time-resolved experiments on heterogeneous catalysts under working conditions are reported. A key feature of this setup is the possibility to work at high temperature and pressure, with a small cell dead volume. To demonstrate these capabilities, performance tests with and without X-rays are performed. The effective temperature at the sample surface, the speed to purge the gas volume inside the cell and catalytic activity have been evaluated to demonstrate the reliability and usefulness of the cell. The setup capability of combining XAS, DRIFTS and MS spectroscopies is demonstrated in a time-resolved experiment, following the reduction of NO by Rh nanoparticles supported on alumina.

Internally Supported Metal-Oxide Nanocatalyst for Hydrogenation of Nitroaromatics

The uncalcined but highly dispersive oxide-supported metal catalyst for liquid phase reactions may suffer from the agglomeration of metal nanoparticles and the drop of metal catalyst in solution, which will decrease the activity and shorten their life in catalysis. Here, a one-pot successive polyol reaction was developed to prepare M-E _xO _y colloidal particles as heterogeneous nanocatalysts, which merge the controlled synthesis of metal catalysts and oxide supports, the in situ loading of catalyst, and even the mesopore amplification into a highly integrated process. Unlike the traditional surface-deposited catalysts, the noble metal nanoparticles even with a large amount of loading are internally dispersed in the mesoporous oxide particles, which show higher activity and stability in the hydrogenation of nitroaromatics compared to the isolated nanocatalysts or surface-deposited catalysts. The improved activity and stability comes from the physical confinement of metal nanoparticles and good mass transportation of substrate/product within the support particles. This work proposed a novel method to prepare highly dispersed metal catalysts, which could be potentially useful to heterogeneous catalytic reactions with high-throughput and long-life demands.

Regio- and Chemoselective Hydrogenation of Dienes to Monoenes Governed by a Well-Structured Bimetallic Surface

Unprecedented surface chemistry, governed by specific atomic arrangements and the steric effect of ordered alloys, is reported. Rh-based ordered alloys supported on SiO₂ (Rh_xM_y/SiO₂, M = Bi, Cu, Fe, Ga, In, Pb, Sn, and Zn) were prepared and tested as catalysts for selective hydrogenation of trans-1,4-hexadiene to trans-2-hexene. RhBi/SiO₂ exhibited excellent regioselectivity for the terminal C═C bond and chemoselective hydrogenation to the monoene, not to the overhydrogenated alkane, resulting in a high trans-2-hexene yield. Various asymmetric dienes, including terpenoids, were converted into the corresponding inner monoenes in high yields. This is the first example of a regio- and chemoselective hydrogenation of dienes using heterogeneous catalysts. Kinetic studies and density functional theory calculations revealed the origin of the high selectivity: (1) one-dimensionally aligned Rh arrays geometrically limit hydrogen diffusion and attack to alkenyl carbons from one direction and (2) adsorption of the inner C═C moiety to Rh is inhibited by steric repulsion from the large Bi atoms. The combination of these effects preferentially hydrogenates the terminal C═C bond and prevents overhydrogenation to the alkane.

Hierarchical α-Ni(OH)2 Composed of Ultrathin Nanosheets with Controlled Interlayer Distances and Their Enhanced Catalytic Performance

Hierarchical α-Ni(OH)₂ assembled of ultrathin nanosheets with the intercalation of diatomic alcohol molecules were synthesized via a facile one-step solvothermal process. The assembly structure avoided the agglomeration of ultrathin nanosheets while retaining their atomic-scale thickness and high surface area. The intercalation of the diatomic alcohol molecules into the transition-metal layers provided larger interlayer spacing and more exposed active sites, which guaranteed the high activity of the α-Ni(OH)₂. The as-obtained hierarchical α-Ni(OH)₂ exhibited excellent catalytic performance in the reduction of p-nitrophenol, with a maximum reaction rate constant (k) of 6.23 × 10^-3 s^-1 and a super high activity factor K (K = k/m) of 216.69 s^-1 g^-1. The layer spacing played the most important role in the reaction, and the catalytic efficiency increased greatly with the increase of the layer spacing of the α-Ni(OH)₂. This design concept and synthetic method can also be extended to the production of a wide variety of hierarchical catalysts for other reactions.