Model Heterogeneous Catalysts Research Articles

Chemical activation of atom-precise Pd3 nanoclusters on γ-Al2O3 supports for transfer hydrogenation reactions.

Deposition of atom-precise nanoclusters onto solid supports is a promising route to synthesize model heterogeneous catalysts. However, to enhance nanocluster-support interactions, activation of the nanoclusters by removal of surface ligands is necessary. Thermal treatment to remove surface ligands from supported metal nanoclusters can yield highly active heterogeneous catalysts, however, the high temperatures employed can lead to poor control over the final size and speciation of the nanoclusters. As an alternative to high-temperature thermal treatments, chemical activation of [Pd3(μ-Cl)(μ-PPh2)2(PPh3)3]+ (Pd3) nanoclusters on γ-Al2O3 supports under mild reaction conditions has been demonstrated in this work. Hydride-based reducing agents such as NaBH4, LiBH4, and LiAlH4 have been examined for the activation of the Pd3 nanoclusters. The structural evolution and speciation of the nanoclusters after activation have been monitored using a combination of XAS, XPS, STEM-EDX mapping, and solid-state NMR techniques. The results indicate that treatment with borohydride reducing agents successfully removed surface phosphine and chloride ligands, and the extent of size growth of the nanoclusters during activation is directly correlated with the amount of borohydride used. Borate side products remain on the γ-Al2O3 surface after activation; moreover, exposure to high amounts of NaBH4 resulted in the incorporation of B atoms inside the lattice of the activated Pd nanoclusters. LiAlH4 treatment, on the other hand, led to no significant size growth of the nanoclusters and resulted in a mixture of Pd single-atom sites and activated nanoclusters on the γ-Al2O3 surface. Finally, the catalytic potential of the activated nanoclusters has been tested in the transfer hydrogenation of trans-cinnamaldehyde, using sodium formate/formic acid as the hydrogen donor. The catalytic results showed that smaller Pd nanoclusters are much more selective for hydrogenating trans-cinnamaldehyde to hydrocinnamaldehyde, but overall have lower activity compared to larger Pd nanoparticles. Overall, this study showcases chemical activation routes as an alternative to traditional thermal activation routes for activating supported nanoclusters by offering improved speciation and size control.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters.

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Grafting of Group‐10 Organometallic Complexes on Silicas: Differences and Similarities, Surprises and Rationale

AbstractSurface organometallic chemistry (SOMC) represents a unique synthetic platform for the preparation of model heterogeneous catalysts resembling those broadly applied in industry. SOMC techniques usually rely on the grafting of tailored molecular precursors onto the surface OH groups of oxide supports. The development of such precursors and the understanding of their reactivity with the supports are therefore crucial for the development of well‐defined surface species. While a large number of organometallic precursors of early transition metals are known, only few examples of group‐10 metal complexes are reported, in spite of the great interest for heterogeneous catalysts based on the Pt‐group elements. Herein, we report the reactivity of a family of group‐10 (Ni, Pd and Pt) alkyl complexes, towards partially dehydroxylated SiO2 yielding well‐defined supported species. We studied the effect of the metal, ligand, and support on the grafting mechanism of such precursors through a combined experimental and computational approach. Ultimately, we showed that at least two grafting pathways are possible for these compounds, namely the protonolysis of the M‐alkyl bond by surface OH groups and the opening of strained siloxane bridges: the proportion of the two depending on the nature of the metal and its ancillary ligand.

Active Sites in a Heterogeneous Organometallic Catalyst for the Polymerization of Ethylene.

Heterogeneous derivatives of catalysts discovered by Ziegler and Natta are important for the industrial production of polyolefin plastics. However, the interaction between precatalysts, alkylaluminum activators, and oxide supports to form catalytically active materials is poorly understood. This is in contrast to homogeneous or model heterogeneous catalysts that contain resolved molecular structures that relate to activity and selectivity in polymerization reactions. This study describes the reactivity of triisobutylaluminum with high surface area aluminum oxide and a zirconocene precatalyst. Triisobutylaluminum reacts with the zirconocene precatalyst to form hydrides and passivates −OH sites on the alumina surface. The combination of passivated alumina and zirconium hydrides formed in this mixture generates ion pairs that polymerize ethylene.

Mechanistic insights into carbon-carbon coupling on NiAu and PdAu single-atom alloys.

Carbon-carbon coupling is an important step in many catalytic reactions, and performing sp3-sp3 carbon-carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C-I cleavage and selective sp3-sp3 carbon-carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp3-sp3 carbon-carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants.

Regulating Heterogeneous Catalysis of Gold Nanoparticles with Polymer Mechanochemistry.

Polymer mechanochemistry has emerged as a unique approach to regulate homogeneous catalysis in chemical transformations. The utilization of polymer mechanochemistry to regulate heterogeneous catalysis, however, still remains to be investigated. In this study, using polymer-grafted gold nanoparticles as the model heterogeneous catalysts, we show that polymer chains can be mechanically ruptured from the surface of gold nanoparticles, and thus, the catalytic activity of gold nanoparticles can be accelerated under sonication. The mechanical activation of polymer-grafted gold nanoparticles only occurs when the grafted polymer chains exceed a threshold molecular weight. This mechanical behavior is similar to those mechanophore-linked polymers. More importantly, further characterizations reveal that the Au-Au bonds instead of the Au-S bonds are broken at the heterointerfaces of polymer chains and gold nanoparticles. Our study unveils an unprecedented characteristic of polymer-grafted metallic nanoparticles in response to external mechanical stress.

Development of Metal Oxides Heterostructure for Highly Selective CO2 Reduction to Methanol.

Carbon dioxide conversion into valuable chemicals using solar light considered an ideal goal in renewable energy studies. For the purpose of effective photocatalytic materials, a large number of strategies have been studied. Coupling of semiconductors is considered one of the best approaches to design a good photocatalyst for producing selective solar fuels. In this study, we fabricate ZnO/ Co(OH)2 model heterogeneous catalysts for CO2 reduction under visible light irradiation. Firstly, Three ZnO samples with different morphologies and surface area were fabricated by varying preparation methods. It is found that ZnO nanoflakes with a high ratio of {0002} facets possess a higher conversion rate yielding Methanol, Acetaldehyde, and Acetone as the major products of photocatalytic CO2 reduction in a continuous flow gas-phase reaction condition. The obtained ZnO nanoflakes were then treated to form the heterostructure of ZnO/Co(OH)2. When the mass ratio between hexagonal-phased ZnO and Co(OH)2 is varied, the composite catalyst shows selective methanol formation as a major product compared to pristine ZnO phases. The synthesized samples were characterized by means of a multi techniques approach involving powder X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray analysis, transmission electron microscopy, Photoluminescence, UV-Visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and surface area measurements. In conclusion, the results can not only provide the importance indication about the influence of {0002} facets of ZnO, but also demonstrated a strategy for the selective formation of hydrocarbons over the heterostructure semiconductors of ZnO/Co(OH)2 nanocomposites.

Colloidal nanocrystals for heterogeneous catalysis

Abstract Catalytic materials are an essential component of the chemical industry. They find applications in everything from fine chemical manufacturing to greenhouse gas mitigation. They are indispensable for developing a sustainable future. Their development has been continuous, from early trial and error efforts to the first fundamental insights gained through surface science, to modern in-situ characterization and computational predictions. The accumulation of knowledge on the working principles of catalytic surfaces allowed designing and producing better systems with improved performance. Even though tremendous progress has been made thanks to surface science techniques, these studies are usually performed under ultra-high vacuum and are therefore limited in their applicability to more relevant industrial conditions. The control over size, shape and composition in colloidal nanocrystals makes them formidable precursors for model heterogeneous catalysts. These model systems enable linking the insights from surface science studies via in-situ and operando studies to realistic catalytic reaction conditions. In this review, colloidal nanocrystals are presented as powerful building blocks for catalytic materials in the quest for fundamental understanding. A review of the principal methods to produce colloidal nanocrystals with a high level of control is reported, complemented by procedures for how to prepare active catalysts from these particles. Examples and guidelines for the catalytic applications of these materials revolve around the three guiding objectives in catalysis science: activity, selectivity and stability. This work will be limited to examples of this colloidal approach in the areas of thermal, electro- and photocatalysis. The exposed approaches can be used and extended to many other areas of catalysis science, thus providing a new avenue to explore fundamentals and applications of catalytic materials.

Models for heterogeneous catalysts: studies at the atomic level

A systematic approach to model heterogeneous catalyst material and characterization at the atomic level is presented. Two examples are used to illustrate the concepts derived from those studies. They document the problems arising on the way to create such model systems and they also indicate possible solutions. The first example is connected with activation of CO2 at the rim of electron rich MgO (thin film-supported Au islands), and the second example aims at the creation of a model system for the Phillips catalyst for ethylene polymerization and, in particular, the creation of a hydroxylated silica support.

Ultrafine Alloy Nanoparticles Converted from 2D Intercalated Coordination Polymers for Catalytic Application

Supported multimetallic alloy nanoparticles (NPs) have shown great potential for applications owing to combined functions of constituent metals, and more remarkably, enhanced physicochemical properties and even novel synergistic effects that are not possessed by their parent metals. Nevertheless, synthesizing this kind of nanocomposites has been a long‐standing challenge using conventional wet chemistry. Here, this study reports an efficient, versatile strategy for the preparation of multimetallic alloy NPs supported by layered double hydroxides (LDH) and/or layered double oxides (LDO). In this approach, different metal precursors are intercalated stepwise into the gallery space of LDH. Along with the coordination reaction between the metal precursors, 2D cyanide bridged coordination polymers (CP) are formed in the confined space. Afterward, supported multimetallic alloy NPs can be obtained via either liquid‐phase reduction or thermal autoreduction. Due to the homogeneous mixing of metals in the 2D CP, ultrafine alloy NPs can be obtained with high particulate uniformity and compositional tailorability. A large series of supported binary alloy NPs (FePd, FePt, CoPd, CoPt, NiPd, NiPt, and PtPd) and ternary alloy NPs (FePdPt, FeNiPt, FeCoPt, and NiCoPt) are successfully synthesized with this approach. The resulting supported multimetallic alloy NPs present great potential in numerous applications. To demonstrate their workability, one class of LDH/NiPd nanocomposite is explored as a model heterogeneous catalyst with respect to the carbon–carbon cross‐coupling reactions (Suzuki–Miyaura, Heck, and Sonogashira cross‐coupling reactions).

ChemInform Abstract: Colloidal Metal Nanocatalysts: Synthesis, Characterization, and Catalytic Applications

AbstractReview: 143 refs.

Colloidal Metal Nanocatalysts: Synthesis, Characterization, and Catalytic Applications

Metal nanoparticles are key materials in heterogeneous catalysis due to their high catalytic activity and selectivity to the desired product. Accordingly, they are playing a pivotal role in most heterogeneous catalytic reactions that are steeply growing with the development of a colloidal synthetic protocol that enables fine control of size, shape, morphology and composition of metal nanoparticles at an atomic level. These colloidal metal nanoparticles can be dispersed on a rigid support such as mesoporous silica, metal oxide and zeolite, which utilizes metal nanoparticles as model heterogeneous catalysts in industrially important processes involving hydrogenation/dehydrogenation, isomerization and cracking. In this review article, we highlight the recent progress on general colloidal synthetic routes with technological advances in characterization tools that enable the atomic-scale observation of metal nanoparticles. Structure-dependent contributions on the control of product selectivity and turnover rate are also discussed by combining advanced ex situ and in situ surface characterization tools that can monitor the structural change of metal nanocatalysts as well as the evolution of reaction intermediates under the reaction conditions.

ChemInform Abstract: Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis

AbstractReview: 107 refs.

Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis

AbstractA nanoparticle with well‐defined surfaces, prepared through colloidal chemistry, enables it to be studied as a model heterogeneous catalyst. The colloidal synthetic approach provides versatile tools to control the size and shape of nanoparticles. Traditional nucleation and growth mechanisms have been utilized to understand how nanoparticles can be uniformly synthesized and unprecedented shapes can be controlled. Now, the size of metal particles can be controlled to cluster regimes by using dendrimers. By using seeds and foreign atoms, specific synthetic environments such as seeded growth and crystal overgrowth can be induced to generate various shaped mono‐ or bi‐metallic, core/shell, or branched nanostructures. For green chemistry, catalysis in 21st century is aiming for 100 % selectivity to produce only one desired product at high turnover rates. Recent studies on nanoparticle catalysts clearly demonstrate size and shape dependent selectivity in many catalytic reactions. By combining in situ surface characterization techniques, real‐time monitoring of nanoparticles can be performed under reaction environments, thus identifying several molecular factors affecting catalytic activity and selectivity.

ChemInform Abstract: Model Studies in Catalysis

AbstractReview: 71 refs.

Model Studies in Catalysis

We review our approach to model heterogeneous catalysts on the basis of four examples. It is shown that Pd nanoparticles in hydrogenation incorporate hydrogen which turns out to be crucial for the actual hydrogenation step. The second example correlates the structure of vanadia monolayer catalysts with its reactivity in methanol oxidation. Further examples refer to ultrathin oxide film catalysts where the oxide metal interface controls either the charge state of Au particles grown on the film (example 3), and, in the fourth example, the oxide film itself exhibits remarkable CO-oxidation activity which can be traced to a reactive intermediate structure of the ultrathin film.

Preparation and Adsorption Properties of Pd Nanoparticles Supported on TiO2Nanotubes

Here we present preparation methods for a series of novel model heterogeneous catalysts consisting of Pd nanoparticles dispersed on self-organized TiO2 nanotube arrays (TiNTs). For a comprehensive comparative study of TiNTs’ surface adsorption properties, different chemical and physical methods were employed for the deposition of the Pd nanoparticles: (i) e-beam deposition in high vacuum (HV) and (ii) in ultrahigh vacuum (UHV), (iii) impregnation, and (iv) metal precipitation. Following preparation, the Pd/TiNTs were exposed to several O2 (oxidative) and CO (reductive) cleaning cycles at 500 K in UHV. After each cleaning cycle, the CO adsorption properties were systematically probed with time-resolved infrared reflection−adsorption spectroscopy to monitor the surface state of the catalyst. It is shown that clean and well-defined Pd/TiNT catalysts can be obtained, which are well suited for future adsorption and reactivity studies.

Construction of heterogeneous Ni catalysts from supports and colloidal nanoparticles – A challenging puzzle

Well-defined nanoparticles obtained by modern wet chemical synthesis methods can be very useful building blocks for construction of model heterogeneous catalysts. Here we show the challenges involved in the construction of heterogeneous Ni catalysts from supports and colloidal nanoparticles. Surprisingly, “1 + 1” is not equal to “the best catalyst”, following this concept. The thought-provoking lack of catalytic activity of supported Ni NPs in the hydrogenation of cyclohexene and the low activity of these materials in the steam reforming of ethanol were analyzed in details by TEM, XPS, DRIFTS and CO adsorption experiments. The factors responsible for the low catalytic activity of Ni/SiO 2 system were proposed. The removal of the ligands, used in the synthesis of the colloidal Ni nanoparticles, in the post-steps of catalyst preparation is cumbersome. In fact, low temperature treatments are not able to efficiently expose the Ni surface whereas high-temperature treatments lead to particle sintering. In this systematic study several aspects of catalyst preparation through impregnation of colloidal nanoparticles are addressed.

Transition Metal-Catalyzed Dissociation of Phosphine−Gallane Adducts: Isolation of Mechanistic Model Complexes and Heterogeneous Catalyst Poisoning Studies

Attempts to induce the catalytic dehydrocoupling of the phosphine-gallane adduct Cy2PH.GaH3 (Cy=cyclohexyl) (1) by treatment with ca. 5 mol% of either the Rh(I) complex [{Rh(mu-Cl)(1,5-cod)}2] (cod=cyclooctadiene) or the Rh(0) species Rh/Al2O3 and [Oct4N]Cl-stabilized colloidal Rh led to catalytic P-Ga bond cleavage to generate the phosphine, H2, and Ga metal. Interestingly, subsequent treatment of the reaction mixtures with Me2NH.BH3 failed to lead to the formation of [Me2N-BH2]2 via Rh-catalyzed dehydrocoupling, which suggested that catalyst deactivation was taking place. Poisoning studies involving the treatment of the active Rh(0) catalyst with Cy2PH, PMe3, or GaH3.OEt2 showed that deactivation indeed occurred as the dehydrocoupling of Me2NH.BH3 either dramatically decreased in rate or did not take place at all. The X-ray photoelectron spectroscopy analysis of colloidal Rh(0) that had been treated with Cy2PH and PMe3 confirmed the presence of phosphorus on the catalyst surface in each case, consistent with catalyst poisoning via phosphine ligation. A mechanism for the Rh-catalyzed P-Ga bond cleavage reaction of 1 and Me3P.GaH3 (2) is proposed and involves the initial reaction of Ga-H bonds with the Rh colloid surface, which weakens and ultimately breaks the P-Ga bond. The reasonable nature of this mechanism is supported by a model reaction between the zerovalent group 9 complex Co2(CO)8 and 2 which afforded Me3P.Ga[Co(CO)4]3 (3). Consistent with the elongated and thus weakened P-Ga bond in 3, solutions of this species in Et2O subsequently form the known complex [(Me3P)Co(CO)3]2 (4) and Ga metal after 4 h at 25 degrees C.

Surface science of single-site heterogeneous olefin polymerization catalysts

This article reviews the surface science of the heterogeneous olefin polymerization catalysts. The specific focus is on how to prepare and characterize stereochemically specific heterogeneous model catalysts for the Ziegler-Natta polymerization. Under clean, ultra-high vacuum conditions, low-energy electron irradiation during the chemical vapor deposition of model Ziegler-Natta catalysts can be used to create a "single-site" catalyst film with a surface structure that produces only isotactic polypropylene. The polymerization activities of the ultra-high vacuum-prepared model heterogeneous catalysts compare well with those of conventional Ziegler-Natta catalysts. X-ray photoelectron spectroscopic analyses identify the oxidation states of the Ti ions at the active sites. Temperature-programmed desorption distinguishes the binding strength of a probe molecule to the active sites that produce polypropylenes having different tacticities. These findings demonstrate that a surface science approach to the preparation and characterization of model heterogeneous catalysts can improve the catalyst design and provide fundamental understanding of the single-site olefin polymerization process.