Acidic Properties and Structure-Activity Correlations of Solid Acid Catalysts Revealed by Solid-State NMR Spectroscopy.

Surface Coordination Decouples Hydrogenation Catalysis on Supported Metal Catalysts

Precise modulation of local interatomic interactions affecting the electronic structure is an important method to control the catalytic activity and reaction pathways. In this study, we focused on the hydrogenation reaction of naphthalene and employed density functional theory calculations to investigate the specific influence of electronic effects triggered by the coregulation of Ni and sulfur edge engineering on the hydrogenation performance of Ni-doped MoS2 at different edge sulfur coverages (Ni-MoS2-X-θs). Our findings reveal that the interaction between Ni and S in the catalyst matrix material modifies the local electronic structure surrounding the sulfur atoms in the active site. Notably, Ni doping facilitates significant electron transfer, altering the charge and the electronic states at the catalyst edge. This, in turn, affects the adsorption capacity and reactivity of the catalyst, thereby reducing the energy barrier of the hydrogenation reaction. Furthermore, we paid particular attention to the modulation of catalytic activity and reaction pathways under the Eley-Rideal (E-R) mechanism. Interestingly, the sulfur coverage exhibited a nonlinear relationship with the adsorption and activation properties of the probe molecule. Typically, changes in the Mo edge sulfur coverage probability of Ni-MoS2-X-θs have a greater impact on the activation properties. Through comprehensive studies, we demonstrated that both compositional and structural factors must be considered when tailoring the catalytic performance. Importantly, adjusting the ratio of marginal sulfur atoms to metal atoms to 1:1 can effectively enhance the catalytic activity. This study provides valuable insights into the electronic effects regulating the hydrogenation reaction activity of MoS2-based catalysts. It also opens the way for the rational design of novel hydrogenation catalysts, offering a new strategy for optimizing the catalytic performance.

Review: 40 refs.

Electrocatalytic CO ₂ Reduction over Bimetallic Bi-Based Catalysts: A Review

ConspectusMetal heterogeneous catalysis is the workhorse of the chemical industry, driving the conversion of reactants to desirable products. Traditional design approaches for metal catalysts rely on trial-and-error tests and take a lot of time to identify promising catalytic active species from the large candidate space. Over the decades, much focus has been placed on identifying the factors affecting the active sites, which, in turn, guides the design and preparation of more active, selective, and stable catalysts. In the context of theoretical design method for catalysts, the concept of the energy descriptor strategy provides correlations between the adsorption energy of key intermediates and catalytic reactivity. Such energy descriptors for catalytic reactivity can be used to predict the activity of candidate catalysts and understand trends among different catalysts.However, a more efficient descriptor strategy is still attractive and needed that avoids density functional theory calculation on the adsorption energy of each candidate and possesses the guidance power for the rational design of microstructural characteristics of catalytic active species. In this regard, bridging the gap between the electronic/atomic-level descriptions of the microscopic properties of the catalytic active species and the macroscopic catalytic performance of the desirable reaction, that is, the microscopic-to-macroscopic relationship, remains intriguing yet challenging, toward which progress leads to revolutionizing catalyst design.In this Account, we propose a structural descriptor strategy that for the first time maps the quantitative relationship between microstructural features and catalytic performances for metal catalysts, as well as its application in the high-throughput screening and rational design of catalytic active species. We begin with the analysis of the microstructural characteristics of the reaction center and its coordination environment and extract key feature parameters to build a mathematical expression of the structural descriptor. Next, through regression fitting, a mathematical correlation is built between the structural descriptor and the energetics involved with the reaction pathway. Finally, substituting the above statistical correlations into the rate equation derived from microkinetic model offers the structural descriptor-based prediction model for metal catalysts. The use of easily accessible structural descriptors has proven to be a powerful method to advance and accelerate the discovery and design of metal catalysts, including atomically dispersed metal catalysts, metal alloy catalysts, and metal cluster catalysts. Overall, the structural descriptor strategy not only demonstrates much potential to elucidate the quantitative interplay between microstructural features of catalytic active species and intrinsic catalytic reactivity but also provides a new approach in kinetics analysis to rationalize metal catalyst design. We conclude with an outlook for further constructing a universal structural descriptor and accelerating predictions on catalytic performance of metal catalysts by leveraging material databases and machine learning.

Catalytic hydrogenation reactions play crucial roles in fine chemical production and pharmaceutical synthesis. Compared to direct hydrogenation of organic compounds with pressurized hydrogen gas, catalytic transfer hydrogenation reactions using inexpensive and readily accessible small molecules as hydrogen-donors has been considered as a more versatile, scalable, and sustainable pathway toward enhanced chemoselectivity under mild reaction conditions. Noble metal nanoparticles, especially those within the sub-5 nm size regime, can efficiently catalyze the dehydrogenation of a series of hydrogen-storing molecules, such as ammonia borane, hydrazine, formaldehyde, formic acid, and isopropanol, to produce surface-adsorbed hydrogen species that actively hydrogenate a variety of organic substrate molecules. The transfer hydrogenation/hydrogenolysis reactions are mechanistically complex, and may occur selectively along multiple distinct pathways, exhibiting kinetic features signifying the Langmuir–Hinshelwood, Eley–Rideal, autocatalysis, and reversible reaction mechanisms, respectively, depending on the compositions of the metal catalysts and the chemical nature of the hydrogen donors. In this talk, I will share with the audience some new insights concerning the detailed mechanisms of transfer hydrogenation reactions over metal nanocatalyst surfaces. We employ surface-enhanced Raman scattering (SERS) as an in situ fingerprinting spectroscopic tool to precisely resolve the detailed structural evolution of molecular adsorbates during catalytic reactions, based upon which the key intermediates along different reaction pathways are unambiguously identified. The results of deliberately designed in situ SERS measurements show that the chemoselective transfer hydrogenation of nitrophenyl isocyanide by ammonia borane may proceed on noble metal nanocatalyst surfaces selectively through either a unimolecular or a bimolecular pathway, depending on how the nitrophenyl isocyanide adsorbates interact with the metal nanoparticle surfaces. The experimental observations are corroborated by density functional theory calculations, which shed light on the underlying relationships between catalyst-adsorbate interactions and reaction pathway selection. We have further demonstrated that the photoexcited plasmonic hot carriers in the metal nanoparticles can be effectively harnessed to fine-regulate the activation energy barriers associated with the rate-limiting steps for the transfer hydrogenation of nitrophenyl isocyanide on Pd nanocatalyst surfaces when ammonium formate serves as the hydrogen donor. Our results clearly demonstrate the feasibility of using plasmonic hot carriers to kinetically modulate catalytic molecule-transforming processes.

In-situ/operando techniques to identify active sites for thermochemical conversion of CO2 over heterogeneous catalysts

Zeolites are important heterogeneous catalysts widely used in the modern chemical and petrochemical industries. Metal-containing zeolites show distinct performance in the catalytic processes such as fluid catalytic cracking, activation and conversion of light alkanes, methanol-to-aromatics conversion, biomass transformation, and so on. The metal speciation, distribution, and interactions on zeolites have enormous impact on their property and catalytic performance. Significant efforts have been devoted to the synthesis of more active and selective zeolites by engineering the metal active sites. However, the nature of metal species and their role in the reactions are still poorly understood, which makes it difficult to establish the structure-activity relationship toward the rational design and application of zeolites. For example, synergic active sites are often present on the metal-containing zeolites, but their structure, property and quantification still remain to be resolved. Solid-state NMR is a powerful tool for the characterization of heterogeneous catalysts and catalytic reactions by providing information about both molecular structure and dynamics. The heterogeneity and low concentration of the metal sites on zeolites usually leads to a great challenge for their characterization. In this Account, we will describe our effort to study the metal active sites, host-guest interactions, and reaction intermediates by using solid-state NMR spectroscopy, with the aim to highlight recent advances in solid-state NMR techniques for probing the structure and property of metal-containing zeolites as well as the relevant reaction mechanisms. Using sensitivity-enhanced NMR methods such as 67Zn, 71Ga, and 119Sn, NMR enables the identification of metal speciation on zeolites. The synergic active sites constituted by metal species (as Lewis acid sites) and acidic protons (as Brønsted acid sites) on zeolites that amount to only a small fraction of the whole system can be directly probed and quantified with advanced 1H-67Zn or 1H-71Ga double-resonance solid-state NMR. We developed NMR methods to study the host-guest interactions in zeolites by observing the spatial interaction/proximity between aluminum sites (associated with Brønsted or Lewis acid sites) in zeolite host and carbon atoms in organic molecule guest formed during catalytic reaction, which leads to the formation of supramolecular reaction centers in the methanol-to-olefins reaction. The mechanisms underlying the catalytic reactions on metal-modified zeolite are revealed by the identification of key reaction intermediates with in situ 13C MAS NMR spectroscopy. Our discussion based on the representative examples shows how the metal species serving as active sites significantly affect the property and activity of zeolites and related reaction pathways. The structural information obtained by the state-of-the-art solid-state NMR techniques provides new insights into the structure-activity relationship of zeolites in heterogeneous catalysis, which should be beneficial for rational design of highly efficient zeolite catalysts.

Catalytic oxidation of formaldehyde by MnOx-decorated anatase TiO2{001}: Investigation of the effect of calcination temperature on catalytic activity and reaction mechanism

A quantum chemical study on the mechanism of chiral N-oxides-catalyzed Strecker reaction

Elucidating molecular mechanisms of two-dimensional chemical reactions

We discuss in this article methodologies and strategies for rational design of solid acid catalysts. In particular we will demonstrate how a new computational technique, de novo molecular design, is effective in suggesting new template molecules which can be utilised in the synthesis of target solids. A new microporous aluminophosphate-based catalyst, DAF-4, was prepared which is effective for converting methanol to light olefins. Other methodologies which may prove effective in designing new materials are also discussed.

Heterogeneous catalysts, often consisting of metal nanoparticles supported on high-surface-area oxide solids, are common in industrial chemical reactions. Researchers have increasingly recognized the importance of oxides in heterogeneous catalysts: that they are not just a support to help the dispersion of supported metal nanoparticles, but rather interact with supported metal nanoparticles and affect the catalysis. The critical role of oxides in catalytic reactions can become very prominent when oxides cover metal surfaces forming the inverse catalysts. The source of the catalytic activity in homogeneous catalysts and metalloenzymes is often coordinatively unsaturated (CUS) transition metal (TM) cations, which can undergo facile electron transfer and promote catalytic reactions. Organic ligands and proteins confine these CUS cations, making them highly active and stable. In heterogeneous catalysis, however, confining these highly active CUS centers on an inorganic solid so that they are robust enough to endure the reaction environment while staying flexible enough to perform their catalysis remains a challenge. In this Account, we describe a strategy to confine the active CUS centers on the solid surface at the interface between a TM oxide (TMO) and a noble metal (NM). Among metals, NMs have high electron negativity and low oxygen affinity. This means that TM cations of the oxide bind strongly to NM atoms at the interface, forming oxygen-terminated-bilayer TMO nanostructures. The resulting CUS sites at the edges of the TMO nanostructure are highly active for catalytic oxidation reactions. Meanwhile, the strong interactions between TMOs and NMs prevent further oxidation of the bilayer TMO phases, which would otherwise result in the saturation of oxygen coordination and the deactivation of the CUS cations. We report that we can also tune the oxide-metal interactions to modulate the bonding of reactants with CUS centers, optimizing their catalytic performance. We review our recent progress on oxide-on-metal inverse catalysts, mainly the TMO-on-Pt (TM = Fe, Co, and Ni) systems and discuss the interface-confinement effect, an important factor in the behavior of these catalytic systems. We have studied both model catalyst systems and real supported nanocatalysts. Surface science studies and density functional theory calculations in model systems illustrate the importance of the oxide-metal interfaces in the creation and stabilization of surface active centers, and reveal the reaction mechanism at these active sites. In real catalysts, we describe facile preparation processes for fabricating the oxide-on-metal nanostructures. We have demonstrated excellent performance of the inverse catalysts in oxidation reactions such as CO oxidation. We believe that the interface confinement effect can be employed to design highly efficient novel catalysts and that the inverse oxide-on-metal catalysts may find wide applications in heterogeneous catalysis.

A promising redox-mediated bromate-ion (BrO3−) based system was investigated through research combined with mathematical modeling, analytical study, and full-cell test to understand the characteristic effect of electrochemical reaction coupled with chemical reaction, called catalytic regenerative reaction (EC’), on the cell behaviors. From the full-cell based discharge test, a unique unsteady behavior induced by the catalytic regenerative reaction was found for the first time, and to understand the underlying physics and nature of the system, theoretical study was conducted through mathematical modeling and analytical method. Three reaction mechanisms (E, EC, and EC’) were compared in their characteristic reaction behaviors to understand the nature of each reaction mechanism, and the results were analyzed with respect to reaction time and species concentration at the electrode surface. Furthermore, the effect of actual condition of bromate system (i.e. non-unity stoichiometry and limited amount of bromate) was analyzed at unsteady condition for the first time to understand its characteristic behavior in the actual battery condition. It was found that cell voltage and operation time were increased substantially by the catalytic regenerative reaction, and inclusion of non-unity stoichiometric coefficients induced a significant change in the concentration profiles for reactant and product species, leading to operation time about 4.2 times longer than the unity stoichiometric EC’ reaction. As a final step, the effect of catalytic reaction was analyzed in analytical method to define the parameters to control the catalytic reaction, and especially, the relative effect of electrochemical reaction and coupled chemical reaction rates on the autocatalytic reaction was compared through a zone diagram where conditions for pure diffusion controlled and pure kinetic controlled cases were defined.

Selective regulation of chemical reactions is crucial in chemistry. Oxygen, as a key reagent in ubiquitous oxidative chemistry, exhibits great potential in regulating molecular assemblies, and more importantly, chemical reactions in molecular systems supported by metal surfaces. However, the unique catalytic performance and reaction mechanisms of oxygen species remain elusive, which are essential for understanding reaction selection and regulation. In this study, by a combination of scanning tunneling microscopy (STM) imaging/manipulations and density functional theory (DFT) calculations, we showed that the on-surface reaction pathways of terminal alkynes could be steered from C-C coupling to C-H activation with high selectivity by introducing O2 into the molecular system. The catalytic performance and reaction mechanisms of oxygen species were explored in the C-H activation processes, and both molecular O2 and atomic O could efficiently steer the reaction pathways. These results would provide a fundamental understanding of interfacial catalytic reaction processes.