Catalytic activity of Pd octahedrons/SiO 2 for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Integrated Optimization of Crystal Facets and Nanoscale Spatial Confinement toward the Boosted Catalytic Performance of Pd Nanocrystals

Tuning the surface chemical property and the local environment of nanocrystals is crucial for realizing a high catalytic performance in various reactions. Herein, we aim to elucidate the structure sensitivity of Pd facets on the surface catalytic hydrogenation reaction and to identify what role the nanoconfinement effect plays in the catalytic properties of Pd nanocrystal catalysts. By controlling the coating structures of mesoporous silica (mSiO2) on Pd nanocrystals with different exposed facets that include {100}, {111}, and {hk0}, we present a series of Pd@mSiO2 nanoreactors in core-shell and yolk-shell structures and the discovery of a partial-coated structure, which can provide different types of nanoconfinement, and we propose a seed size-dominated growth mechanism. We demonstrate that a superior activity was exhibited in Pd nanocrystals enclosed by the {hk0} facet as compared to the Pd{100} and Pd{111} facets, and substantially enhanced efficiency and stability were achieved in Pd@mSiO2 particles with yolk-shell structures, indicating a crucial superiority of optimizing the configuration of crystal facets and nanoconfinement. Our study provides an efficient strategy to rationally design and optimize nanocatalysts for promoting catalytic performance.

Heterogeneous Trimetallic Nanoparticles as Catalysts

The development and application of trimetallic nanoparticles continues to accelerate rapidly as a result of advances in materials design, synthetic control, and reaction characterization. Following the technological successes of multicomponent materials in automotive exhausts and photovoltaics, synergistic effects are now accessible through the careful preparation of multielement particles, presenting exciting opportunities in the field of catalysis. In this review, we explore the methods currently used in the design, synthesis, analysis, and application of trimetallic nanoparticles across both the experimental and computational realms and provide a critical perspective on the emergent field of trimetallic nanocatalysts. Trimetallic nanoparticles are typically supported on high-surface-area metal oxides for catalytic applications, synthesized via preparative conditions that are comparable to those applied for mono- and bimetallic nanoparticles. However, controlled elemental segregation and subsequent characterization remain challenging because of the heterogeneous nature of the systems. The multielement composition exhibits beneficial synergy for important oxidation, dehydrogenation, and hydrogenation reactions; in some cases, this is realized through higher selectivity, while activity improvements are also observed. However, challenges related to identifying and harnessing influential characteristics for maximum productivity remain. Computation provides support for the experimental endeavors, for example in electrocatalysis, and a clear need is identified for the marriage of simulation, with respect to both combinatorial element screening and optimal reaction design, to experiment in order to maximize productivity from this nascent field. Clear challenges remain with respect to identifying, making, and applying trimetallic catalysts efficiently, but the foundations are now visible, and the outlook is strong for this exciting chemical field.

Synthesis and performance of PdMulti@HCS catalysts with Pd nanoparticles partially embedded in the inner wall of hollow carbon spheres for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

PdMulti@HCS catalysts ensure the maximum exposure of Pd active sites and optimal transfer and diffusion ability for H2O2 synthesis.

Direct synthesis of H2O2 over Pd–M@HCS (M = Sn, Fe, Co, or Ni): effects of non-noble metal M on the electronic state and particle size of Pd

Direct synthesis of H2O2 in a yolk–shell structure assisted by M (M = Fe,Co,Ni,Sn) metal doping.

Electrochemically Modeling a Nonelectrochemical System: Hydrogen Peroxide Direct Synthesis on Palladium Catalysts

Nonelectrochemical hydrogen peroxide direct synthesis (HPDS) under ambient conditions is an environmentally benign and energy-efficient process that produces a green oxidizer, yet the reaction mechanism of HPDS is still controversial. Inspired by the recently suggested heterolytic mechanism that involves electron and proton transfer at Pd catalysts, we propose a new electrochemical density functional theory (DFT) model that combines the Butler-Volmer equation and constant-potential DFT with hybrid explicit-implicit solvent treatment. Application of this model to Pd surfaces showed that the heterolytic mechanism has a lower barrier for the protonation steps for H₂O₂ production than for the nonelectrochemical hydrogenation steps, leading to advantageous kinetics for H₂O₂ production over H₂O production, while the conventionally accepted Langmuir-Hinshelwood mechanism fails to explain the experimental kinetics. This work resolves the unanswered discrepancies between previous experimental and DFT results, and we expect that these results will readily help the systematic development of improved catalysts for HPDS.

Atomistic Insights into H2O2 Direct Synthesis of Ni-Pt Nanoparticle Catalysts under Water Solvents by Reactive Molecular Dynamics Simulations

In computational catalysis, density-functional theory (DFT) calculations are usually utilized, although they suffer from high computational costs. Thus, it would be challenging to explicitly predict the catalytic properties of nanoparticles (NPs) at the nanoscale under solvents. Using molecular dynamics (MD) simulations with a reactive force field (ReaxFF), we investigated the catalytic performance of Ni-Pt NPs for the direct synthesis of hydrogen peroxide (H₂O₂), in which water solvents were explicitly considered along with the effects of the sizes (1.5, 2.0, 3.0, and 3.5 nm) and compositions (Ni₉₀Pt₁₀, Ni₈₀Pt₂₀, and Ni₅₀Pt₅₀) of the NPs. Among the Ni-Pt NPs, 3.0 nm NPs show the highest activity and selectivity for the direct synthesis of H₂O₂, revealing that the catalytic performance is not well correlated with the surface areas of NPs. The superior catalytic performance results from the high H₂ dissociation and low O₂ dissociation properties, which are correlated with the numbers of NiNiPt-fcc and NiNi-bridge sites on the surface of Ni-Pt NPs, respectively. The ReaxFF-MD simulations propose the optimum composition (Ni₈₀Pt₂₀) of 3.0 nm Ni-Pt NPs, which is also explained by the numbers of NiNiPt-fcc and NiNi-bridge sites. Furthermore, from the ReaxFF-MD simulations, the direct synthesis of H₂O₂ for the Ni-Pt NPs can be achieved not only with the Langmuir-Hinshelwood mechanism, which has been conventionally considered, but also with the water-induced mechanism, which is unlikely to occur on pure Pd and Pd-based alloy catalysts; these results are supported by DFT calculations. These results reveal that the ReaxFF-MD method provides significant information for predicting the catalytic properties of NPs, which could be difficult to provide with DFT calculations; thus, it can be a useful framework for the design of nanocatalysts through complementation with a DFT method.

Facet Engineering of Pd Nanocrystals for Enhancing Photocatalytic Hydrogenation: Modulation of the Schottky Barrier Height and Enrichment of Surface Reactants

Metal cocatalyst loading is one of the most widely explored strategies in promoting photocatalytic solar energy conversion. Engineering surface-active facets of metal cocatalyst and exploring how they modulate the reactivity is crucial for the further development of advanced photocatalysts. In this work, through controlled hybridization of two-dimensional (2D) TiO₂ nanosheets with well-designed Pd nanocube (Pd NC) with exposed {100} facet and Pd nano-octahedron (NO) with exposed {111} facet, we unravel the distinct crystal facet effect of Pd cocatalyst in promoting the selective hydrogenation of nitroarenes to amines of TiO₂ photocatalyst. The activity tests show that the Pd NO with {111} facet is a more efficient cocatalyst than the Pd NC with exposed {100} facet. The prepared TiO₂-Pd NO composite displays a 900% enhancement of photocatalytic hydrogenation rate in comparison with bare TiO₂, while the TiO₂-Pd NC sample only shows a 200% photoactivity enhancement. Microscopic mechanism study discloses that the distinctive photoactivity improvement of Pd NO is ascribed to the concurrent modulation of the Schottky barrier height and enrichment of surface reactants: (i) the Pd NO with a lower Fermi level could result in steeper band bending of TiO₂ (i.e., higher Schottky barrier) than the Pd NC, which is more efficient in boosting interfacial separation and inhibiting the recombination of photoexcited charge pairs; and (ii) the {111} facet of Pd has higher nitroarenes adsorption ability and especially stronger hydrogen enrichment capability, thus accelerating the surface hydrogenation process and contributing to a higher reaction rate. This work emphasizes the rational facet control of cocatalysts for enhancing the photocatalytic hydrogenation performance.

The direct synthesis of hydrogen peroxide from H2 and O2 using Pd-Ni/TiO2 catalysts

The direct synthesis of hydrogen peroxide (H2O2) from molecular H2 and O2 offers an attractive, decentralized alternative to production compared to the current means of production, the anthraquinone process. Herein we evaluate the performance of a 0.5%Pd-4.5%Ni/TiO2 catalyst in batch and flow reactor systems using water as a solvent at ambient temperature. These reaction conditions are considered challenging for the synthesis of high H2O2 concentrations, with the use of sub-ambient temperatures and alcohol co-solvents typical. Catalytic activity was observed to be stable to prolonged use in multiple batch experiments or in a flow system, with selectivities towards H2O2 of 97% and 85%, respectively. This study was carried out in the absence of halide or acid additives that are typically used to inhibit sequential H2O2 degradation reactions showing that this Pd-Ni catalyst has the potential to produce H2O2 selectively. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Palladium-Based Bimetallic Nanocrystal Catalysts for the Direct Synthesis of Hydrogen Peroxide

The direct synthesis of H2 O2 from H2 and O2 is a strongly desired reaction for green processes and a promising alternative to the commercialized anthraquinone process. The design of efficient catalysts with high activity and H2 O2 selectivity is highly desirable and yet challenging. Metal dopants enhance the performance of the active phase by increasing reaction rates, stability, and/or selectivity. The identification of efficient dopants relies mostly on catalysts prepared with a random and non-uniform deposition of active and promoter phases. To study the promotional effects of metal doping on Pd catalysts, we employ colloidal, bimetallic nanocrystals (NCs) to produce catalysts in which the active and doping metals are colocalized to a fine extent. In the absence of any acid and halide promotors, PdSn and PdGa NCs supported on acid-pretreated TiO2 (PdSn/s-TiO2 , PdGa/s-TiO2 ) were highly efficient and outperformed the monometallic Pd catalyst (Pd/s-TiO2 ), whereas in the presence of an acid promotor, the overall H2 O2 productivity was also further enhanced for the Ni-, Ga-, In-, and Sn-doped catalysts with respect to Pd/s-TiO2 .

Effect of polyvinylpyrrolidone (PVP) on palladium catalysts for direct synthesis of hydrogen peroxide from hydrogen and oxygen

Addition of polyvinylpyrrolidone to Pd/SiO₂ catalyst improved H₂O₂ selectivity by adjusting electronic state of palladium active species.

Moving Frontiers in Transition Metal Catalysis: Synthesis, Characterization and Modeling

Nanosized transition metal particles are important materials in catalysis with a key role not only in academic research but also in many processes with industrial and societal relevance. Although small improvements in catalytic properties can lead to significant economic and environmental impacts, it is only now that knowledge-based design of such materials is emerging, partly because the understanding of catalytic mechanisms on nanoparticle surfaces is increasingly improving. A knowledge-based design requires bottom-up synthesis of well-defined model catalysts, an understanding of the catalytic nanomaterials "at work" (operando), and both a detailed understanding and a prediction by theoretical methods. This article reports on progress in colloidal synthesis of transition metal nanoparticles for preparation of model catalysts to close the materials gap between the discoveries of fundamental surface science and industrial application. The transition metal particles, however, often undergo extensive transformations when applied to the catalytic process and much progress has recently been achieved operando characterization techniques under relevant reaction conditions. They allow better understanding of size/structure-activity correlations in these systems. Moreover, the growth of computing power and the improvement of theoretical methods uncover mechanisms on nanoparticles and have recently predicted highly active particles for CO/CO₂ hydrogenation or direct H₂ O₂ synthesis.

Looking for the “Dream Catalyst” for Hydrogen Peroxide Production from Hydrogen and Oxygen

The reaction between hydrogen and oxygen is in principle the simplest method to form hydrogen peroxide, but it is still a “dream process”, thus needing a “dream catalyst”. The aim of this review is to analyze critically the different heterogeneous catalysts used for the direct synthesis of H2O2 trying to determine the features that the ideal or “dream catalyst” should possess. This analysis will refer specifically to the following points: (i) the choice of the metal; (ii) the metal promoters used to improve the activity and/or the selectivity; (iii) the role of different supports and their acidic properties; (iv) the addition of halide promoters to inhibit undesired side reactions; (v) the addition of other promoters; (vi) the effects of particle morphology; and (vii) the effects of different synthetic methods on catalyst morphology and performance.

Boosting the Characterization of Heterogeneous Catalysts for H2O2 Direct Synthesis by Infrared Spectroscopy

Infrared (IR) spectroscopy is among the most powerful spectroscopic techniques available for the morphological and physico-chemical characterization of catalytic systems, since it provides information on (i) the surface sites at an atomic level, (ii) the nature and structure of the surface or adsorbed species, as well as (iii) the strength of the chemical bonds and (iv) the reaction mechanism. In this review, an overview of the main contributions that have been determined, starting from IR absorption spectroscopy studies of catalytic systems for H2O2 direct synthesis, is given. Which kind of information can be extracted from IR data? IR spectroscopy detects the vibrational transitions induced in a material by interaction with an electromagnetic field in the IR range. To be IR active, a change in the dipole moment of the species must occur, according to well-defined selection rules. The discussion will be focused on the advancing research in the use of probe molecules to identify (and possibly, quantify) specific catalytic sites. The experiments that will be presented and discussed have been carried out mainly in the mid-IR frequency range, between approximately 700 and 4000 cm−1, in which most of the molecular vibrations absorb light. Some challenging possibilities of utilizing IR spectroscopy for future characterization have also been envisaged.

Direct and Selective Synthesis of Hydrogen Peroxide over Palladium-Tellurium Catalysts at Ambient Pressure

Highly selective hydrogen peroxide (H₂ O₂ ) synthesis directly from H₂ and O₂ is a strongly desired reaction for green processes. Herein a highly efficient palladium-tellurium (Pd-Te/TiO₂ ) catalyst with a selectivity of nearly 100 % toward H₂ O₂ under mild conditions (283 K, 0.1 MPa, and a semi-batch continuous flow reactor) is reported. The size of Pd particles was remarkably reduced from 2.1 nm to 1.4 nm with the addition of Te. The Te-modified Pd surface could significantly weaken the dissociative activation of O₂ , leading to the non-dissociative hydrogenation of O₂ . Density functional theory calculations illuminated the critical role of Te in the selective hydrogenation of O₂ , in that the active sites composed of Pd and Te could significantly restrain side reactions. This work has made significant progress on the development of high-selectivity catalysts for the direct synthesis of H₂ O₂ at ambient pressure.