Different Types Of Active Sites Research Articles

Synthesis of Rhenium-Doped Copper Twin Boundary for High-Turnover-Frequency Electrochemical Nitrogen Reduction.

The precise design and synthesis of active sites to improve catalyst's performance has emerged as a promising tactic for electrochemistry. However, it is challenging to combine different types of active sites and manipulate them simultaneously at atomic resolution. Here, we present a strategy to synthesize Re atom-doped Cu twin boundaries (TBs), through pulsed electrodeposition and boundary segregation. The Re-doped Cu TBs demonstrate a highly efficient nitrogen reduction reaction (NRR) performance. Re-doped Cu TBs showed a turnover frequency of ∼5889 s-1, ∼800 times higher than the pure Cu TB active centers (∼7 s-1). In addition to the "acceptance-donation" activation of N2 molecules, theoretical calculations also reveal that the Re-Re dimer on TB can boost the NRR and impede the hydrogen evolution reaction synchronously, rendering Re-doped Cu TB catalysts with high NRR activity and selectivity.

Competitive double bond isomerization and competitive esterification of C8 to C18 linear 1-alkenes on amberlyst®15 catalyst: Calculation of relative adsorption coefficients in two concurrent reactions

Relative initial reaction rates for both double bond isomerization and direct esterification with heptanoic acid were determined for the series of linear 1-alkenes from 1-octene to 1-octadecene. In competitive reactions between two 1-alkenes on Amberlyst®15 catalyst at 90 °C, rates for both double bond isomerization and esterification decreased with increasing carbon chain length. Data suggested each 1-alkene had two different adsorption coefficients on the catalyst, one for an isomerization site and one for a site for the addition reaction. Results were consistent with previous data suggesting that Amberlyst®15 catalyst has two different types of active sites.

Dynamics of chemical reactions on single nanocatalysts with heterogeneous active sites.

Modern chemical science and industries critically depend on the application of various catalytic methods. However, the underlying molecular mechanisms of these processes still remain not fully understood. Recent experimental advances that produced highly-efficient nanoparticle catalysts allowed researchers to obtain more quantitative descriptions, opening the way to clarify the microscopic picture of catalysis. Stimulated by these developments, we present a minimal theoretical model that investigates the effect of heterogeneity in catalytic processes at the single-particle level. Using a discrete-state stochastic framework that accounts for the most relevant chemical transitions, we explicitly evaluated the dynamics of chemical reactions on single heterogeneous nanocatalysts with different types of active sites. It is found that the degree of stochastic noise in nanoparticle catalytic systems depends on several factors that include the heterogeneity of catalytic efficiencies of active sites and distinctions between chemical mechanisms on different active sites. The proposed theoretical approach provides a single-molecule view of heterogeneous catalysis and also suggests possible quantitative routes to clarify some important molecular details of nanocatalysts.

Aging effect on the structure formation of active sites in single-atomic catalysts and their electrochemical properties for oxygen reduction reaction

FeNC catalysts have attracted attention because of their superior oxygen reduction reaction (ORR) performance in alkaline media. However, there is a conflict between the active sites of FeNC catalysts, namely single-atomic Fe–Nx sites and dual active sites consisting of atomic sites/iron carbide nanoparticles. Herein, we synthesized catalysts with different types of active sites by controlling the facile aging process. The main active sites varied from single active sites to dual active sites when the aging time and Fe content was decreased and increased. A catalyst with 0.6 wt% Fe that is aged for 24 h (FeNC-24–0.6) has dual active sites, whereas a catalyst with 0.9 wt% Fe that is aged for 48 h (FeNC-48–0.9) consists mainly of single active sites. The catalysts showed outstanding ORR activity, exceeding the half-wave potential of the commercial 20 wt% Pt/C catalysts by 11 mV. Interestingly, the FeNC-48–0.9 catalyst exhibited a rare negative shift compared to the FeNC-24–0.6 catalyst in the durability tests. Therefore, it is believed that increasing the number of single-atomic Fe–Nx sites is an effective approach to enhance the ORR performance of FeNC catalyst in alkaline media.

Observation of H2 Evolution and Electrolyte Diffusion on MoS2 Monolayer by In Situ Liquid-Phase Transmission Electron Microscopy.

Unit-cell-thick MoS2 is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its tunable catalytic activity, which is determined based on the energetics and molecular interactions of different types of HER active sites. Kinetic responses of MoS2 active sites, including the reaction onset, diffusion of the electrolyte and H2 bubbles, and continuation of these processes, are important factors affecting the catalytic activity of MoS2 . Investigating these factors requires a direct real-time analysis of the HER occurring on spatially independent active sites. Herein, the H2 evolution and electrolyte diffusion on the surface of MoS2 are observed in real time by in situ electrochemical liquid-phase transmission electron microscopy (LPTEM). Time-dependent LPTEM observations reveal that different types of active sites are sequentially activated under the same conditions. Furthermore, the electrolyte flow to these sites is influenced by the reduction potential and site geometry, which affects the bubble detachment and overall HER activity of MoS2 .

Superior Iodine Uptake Capacity Enabled by an Open Metal-Sulfide Framework Composed of Three Types of Active Sites

Superior Iodine Uptake Capacity Enabled by an Open Metal-Sulfide Framework Composed of Three Types of Active Sites

Syngas Production by Dry Methane Reforming over Alumina‐Supported Noble Metals and Kinetic Studies

AbstractDry reforming of methane (DRM) provides valuable syngas from two greenhouse gases, CH4 and CO2. Three alumina‐supported noble metals, Ru, Pt, and Pd, were used as catalysts for the DRM process. Using CO2/CH4 mixtures in the range of 0.5–2 mol mol−1, catalyst performance was assessed in an integral fixed‐bed reactor between 773 and 1073 K. All catalysts were stable for at least 10 h. The DRM kinetics was studied in a differential reactor in the range of 848–898 K, using the power law model. The apparent activation energy values for the consumption of CH4 (86, 89, and 95 kJ mol−1) and CO2 (84, 86, and 91 kJ mol−1) over Ru/Al2O3, Pt/Al2O3, and Pd/Al2O3 as catalysts were found. Several kinetic models of the Langmuir‐Hinshelwood type were considered. A pathway with slow surface reaction between CH4 and CO2 chemisorbed on different types of active sites was found to be most suited to describe the kinetic data.

Heterogeneous catalytic direct amide bond formation

The synthesis of amides by the direct reaction of carboxylic acids with amines is an industrially relevant reaction in fine chemistry and drug synthesis, as well as a fundamental process for the construction of proteins in living organisms. To promote this process, the use of heterogeneous catalysts is preferred over their homogeneous counterparts, due to their advantages related to simple isolation, recovery and recycling. In addition, peptide bond formation over solid surfaces is interesting not only from the point of view of industrial organic synthesis but also from their implications in the understanding of prebiotic chemistry. For all this, we have highlighted some of the most relevant reported heterogeneous catalysts for the direct amide bond formation, focusing not only on the physicochemical properties of the solid catalyst –such as porosity and acidity- and its catalytic performance but also on the current understanding of the reaction mechanism. Different types of active sites incorporated at the surface (and bulk) of the solid are reviewed here: inorganic, organic and hybrid organic-inorganic. Besides the nature of the active sites, their interplay with the support in which they are embedded is thoroughly described, where surface area and pore size are key properties of the solid catalyst.

Bio-Templating: An Emerging Synthetic Technique for Catalysts. A Review

In the last few years, researchers have focused their attention on the synthesis of new catalyst structures based on or inspired by nature. Biotemplating involves the transfer of biological structures to inorganic materials through artificial mineralization processes. This approach offers the main advantage of allowing morphological control of the product, as a template with the desired morphology can be pre-determined, as long as it is found in nature. This way, natural evolution through millions of years can provide us with new synthetic pathways to develop some novel functional materials with advantageous properties, such as sophistication, miniaturization, hybridization, hierarchical organization, resistance, and adaptability to the required need. The field of application of these materials is very wide, covering nanomedicine, energy capture and storage, sensors, biocompatible materials, adsorbents, and catalysis. In the latter case, bio-inspired materials can be applied as catalysts requiring different types of active sites (i.e., redox, acidic, basic sites, or a combination of them) to a wide range of processes, including conventional thermal catalysis, photocatalysis, or electrocatalysis, among others. This review aims to cover current experimental studies in the field of biotemplating materials synthesis and their characterization, focusing on their application in heterogeneous catalysis.

Stabilizing Fe-N-C Catalysts as Model for Oxygen Reduction Reaction.

The highly efficient energy conversion of the polymer‐electrolyte‐membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt‐based catalysts, non‐noble‐metal ORR catalysts are developed, among which transition metal–heteroatoms–carbon (TM–H–C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two‐electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe–N–C material with the relatively simple constitution and structure, is selected as a model catalyst for TM–H–C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe–N–C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM–H–C materials for advanced ORR catalysis.

Porous Silica-Based Organic-Inorganic Hybrid Catalysts: A Review

Hybrid organic-inorganic catalysts have been extensively investigated by several research groups in the last decades, as they allow combining the structural robust-ness of inorganic solids with the versatility of organic chemistry. Within the field of hybrid catalysts, synthetic strategies based on silica are among the most exploitable, due to the convenience of sol-gel chemistry, to the array of silyl-derivative precursors that can be synthesized and to the number of post-synthetic functionalization strategies available, amongst others. This review proposes to highlight these advantages, firstly describing the most common synthetic tools and the chemistry behind sol-gel syntheses of hybrid catalysts, then presenting exemplificative studies involving mono- and multi-functional silica-based hybrid catalysts featuring different types of active sites (acid, base, redox). Materials obtained through different approaches are described and their properties, as well as their catalytic performances, are compared. The general scope of this review is to gather useful information for those approaching the synthesis of organic-inorganic hybrid materials, while providing an overview on the state-of-the art in the synthesis of such materials and highlighting their capacities.

Formation of admixed phase during microwave assisted Cu ion exchange in mordenite

Emerging technologies aimed to tune properties of microporous materials, including zeolitic catalysts, involve microwave processing that accelerates chemical reactions and often increases efficiency of target materials. Here we report on the results of our comprehensive study of copper-exchanged mordenites obtained from sodium mordenite with Si/Al = 6.5 and CuSO4 solution using both conventional and microwave assisted ion-exchange procedures. The current study confirms that microwave irradiation not only enhance the ion exchange but also is accompanied by a chemical reaction resulting in formation of an antlerite admixed phase, which explains the over-exchange of copper ions. Fourier transform infrared studies evidence epitaxial growth of antlerite on the surface of mordenite particles. Annealing at 450 °C leads to the transformation of antlerite into mesoporous CuO. Altogether it suggests that the resulting composite material obtained by the microwave assisted ion-exchange procedure can be considered as a promising catalyst with several different types of active sites.

Silica Aerogels/Xerogels Modified with Nitrogen-Containing Groups for Heavy Metal Adsorption.

Heavy metals are common inorganic pollutants found in the environment that have to be removed from wastewaters and drinking waters. In this work, silica-derived aerogels and xerogels were modified via a co-precursor method to obtain functional adsorbents for metal cations. A total of six formulations based upon four different functional precursors were prepared. The materials′ structural characterization revealed a decreased porosity and surface area on modified samples, more prominent in xerogel counterparts. Preliminary tests were conducted, and the prepared samples were also compared to activated carbon. Three samples were selected for in-depth studies. Isotherm studies revealed that the pre-selected samples remove well copper, lead, cadmium and nickel, and with similar types of interactions, following a Langmuir trend. The adsorption kinetics starts very fast and either equilibrium is reached quickly or slowly, in a two-stage process attributed to the existence of different types of active sites. Based on the previous tests, the best sample, prepared by mixing different functional co-precursors, was selected and its behavior was studied under different temperatures. For this material, the adsorption performance at 20 °C is dependent on the cation, ranging from 56 mg·g−1 for copper to 172 mg·g−1 for lead.

Recent progress on functional mesoporous materials as catalysts in organic synthesis

The heterogenization of homogeneous catalysts has attracted great attention owing to their increasing environmental and economic considerations. Mesoporous materials are a kind of attractive candidates due to their unique structural features, such as high surface areas, uniform and tunable pore sizes and shapes, large pore volumes, and easy to be functionalized. Therefore, mesoporous materials have been used as catalysts or their supports for homogeneous catalysis in the past decades. This review concerns the very recent development in this area, including the catalyst design, the types of active species, and the reaction catalyst by this kind of catalysts. Mesoporous catalysts can be classified as supported catalyst and bulk one. The supported catalyst includes active site in the pore wall framework, the active site on the pore wall surface, and the active site encapsulated in the mesopores. The bulk one can be directly used as catalyst, which includes metal oxides and carbon nitrides. Next, four different types of active sites, nanoparticles, highly distributed metals, anchored organic catalysts, and immobilized enzymes, are summarized in this review. Then five types of organic reactions are summarized. With the development of electrochemistry in organic synthesis in recent years, a brief outlook is also discussed.

Experimental reactivity descriptors of M-N-C catalysts for the oxygen reduction reaction

Pyrolyzed non-precious metal catalysts (NPMCs) are promising materials to replace platinum-based catalysts in the cathode of the fuel cells. These catalysts present high catalytic activity both in alkaline and acid media for the oxygen reduction reaction (ORR). These catalysts are essentially heterogeneous as they can present different types of active sites. MNx structures have been proposed as the most active for the ORR, similar to those of the MN4 structures of metal porphyrins and phthalocyanines. Several parameters have been proposed as reactivity descriptors to correlate the structure of these materials with their catalytic activity, such as the amount of MNx and of pyridinic nitrogens in the graphitic structure. In this study, we have explored the metal center redox potential of the catalyst as an overall reactivity descriptor. We have investigated this descriptor for pyrolyzed and intact catalysts for the ORR in acid and basic media. We have found that for all catalysts tested, there is a linear correlation between the redox potential of the catalyst and the catalytic activity expressed as (log i)E). The activity increases as the redox potential becomes more positive. The correlation gives a straight line of slope close to +0.12 V/decade which agrees with the theoretical slope proposed in a previous publication assuming the adsorbed M − O2 follows a Langmuir isotherm and that the redox potential is directly linked to the M − O2 binding energy. The Tafel plots present two slopes, at low and high overpotentials. Based on these results, we proposed two different mechanisms. The low Tafel slopes of −60 mV appear at potentials where the surface concentration of M(II) active sites is potential dependent (close to the onset potential). At higher overpotentials the surface coverage of M(II) becomes constant and the slope changes to −0.120 V/decade.

Catalytic oxidation of Hg0 with O2 induced by synergistic coupling of CeO2 and MoO3

Based on Volcano plotting, the controlled combination of weak and strong bond strengths of a bimetallic catalyst has the potential to maximize the catalytic effect via the formation of intermediate bond strength between the reactant and the two different types of active sites. In this study, a rational design approach was adopted to couple MoO3 and CeO2 to maximize the catalytic oxidation of Hg0 using oxygen as the oxidizing agent. It is found that CeO2 displayed a relatively strong bond strength with Hg0 while MoO3 has relatively weak bond strength with Hg0; the pre-doping of MoO3 results in the transformation of CeO2 from clusters to the form with additional exposed CeO2 (111) surface; the CeO2 and MoO3 show synergistic effect on the formation of Brønsted acid sites. Moreover, the results show that there is an overlap between the Hg0 desorption region of MoO3 and the Hg0 adsorption region of CeO2 (with adjusted optimum bond strength with Hg0), which contributes to the catalytic reaction of Hg0 by O2. Therefore, this study reveals that the synergistic effects of the coupling of CeO2 and MoO3 induced the reaction between Hg0 and O2, which is otherwise difficult.

A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation

The active sites on the methane dehydroaromatization (MDA) catalyst Mo/HZSM-5 are very hard to characterize, because they are present in various geometries and sizes and only form under reaction conditions with methane at 700 °C. To address these issues an experimental strategy is presented that enables distinguishing different active sites for MDA present on Mo/HZSM-5 and helps determining the Mo charge, nuclearity and chemical composition. This approach combines a CO pretreatment to separate the active Mo site formation from coke formation, quasi-in situ spectroscopic observations using DNP, 13C NMR, CO IR and theory. This allows the discrimination between three different types of active sites. Distinct spectroscopic features were observed corresponding to two types of mono- or dimeric Mo (oxy-)carbide sites as well as a third site assigned to Mo2C nanoparticles on the outer surface of the zeolite. Their formal Mo oxidation state was found to be between 4+ and 6+. Dynamic nuclear polarization (DNP) measurements of samples carburized in CO as well as in CH4 confirm the assignment and also show that accumulated aromatic carbon covers the bigger Mo nanoparticles on the outer surface of the zeolite, causing deactivation. It was previously observed that after an initial period where no desired products are formed yet, benzene starts slowly forming until reaching its maximum productivity. Direct observation of the active site with 13C NMR confirmed that Mo-sites do not transform further once benzene starts forming, meaning that they are fully activated during the period where no desired products are observed yet. Therefore the slow increase of the benzene formation rate cannot be attributed to a further transformation of Mo sites.

Evidence for product-specific active sites on oxide-derived Cu catalysts for electrochemical CO2 reduction

Carbon dioxide electroreduction in aqueous media using Cu catalysts can generate many different C2 and C3 products, which leads to the question whether all products are generated from the same types of active sites or if product-specific active sites are responsible for certain products. Here, by reducing mixtures of 13CO and 12CO2, we show that oxide-derived Cu catalysts have three different types of active sites for C–C coupled products, one that produces ethanol and acetate, another that produces ethylene and yet another that produces 1-propanol. In contrast, we do not find evidence of product-specific sites on polycrystalline Cu and oriented (100) and (111) Cu surfaces. Analysis of the isotopic composition of the products leads to the prediction that the adsorption energy of *COOH (the product of the first step of CO2 reduction) may be a descriptor for the product selectivity of a given active site. These new insights should enable highly selective catalysts to be developed.

Constructing A Rational Kinetic Model of the Selective Propane Oxidation Over A Mixed Metal Oxide Catalyst

This research presents a kinetic investigation of the selective oxidation of propane to acrylic acid over a MoVTeNb oxide (M1 phase) catalyst. The paper contains both an overview of the related literature, and original results with a focus on kinetic aspects. Two types of kinetic experiments were performed in a plug flow reactor, observing (i) steady-state conditions (partial pressure variations) and (ii) the catalyst evolution as a function of time-on-stream. For this, the catalyst was treated in reducing atmosphere, before re-oxidising it. These observations in long term behaviour were used to distinguish different catalytic routes, namely for the formation of propene, acetic acid, acrylic acid, carbon monoxide and carbon dioxide. A partial carbon balance was introduced, which is a ‘kinetic fingerprint’, that distinguishes one type of active site from another. Furthermore, an ‘active site’ was found to consist of one or more ‘active centres’. A rational mechanism was developed based on the theory of graphs and includes two time scales belonging to (i) the catalytic cycle and (ii) the catalyst evolution. Several different types of active sites exist, at least as many, as kinetically independent product molecules are formed over a catalyst surface.

Progress in the design of zeolite catalysts for biomass conversion into biofuels and bio-based chemicals

ABSTRACTThis article reviews the recent advances in the development of zeolite catalysts for biomass valorization processes to produce both biofuels and/or bio-based chemicals, which is an emerging and fast expanding field. The work deals with different types of feedstocks, including vegetable oils, lignocellulose and sugars, as well as with a number of relevant intermediates and platform molecules. Transformation of biomass into valuable products is hindered by a number of factors, mainly related to its complex composition, as biomass typically consists of bulky molecules with high oxygen content. Accordingly, biomass processing usually requires the combination of multiple steps and severe conditions, hence concepts like atom efficiency, product selectivity, and catalyst deactivation become of special relevance. A great progress has been achieved in the past years engineering the properties of zeolites for being adapted to the challenges associated to biomass valorization. The possibility of tailoring the main physicochemical properties of zeolites has become now a reality, being the major reason that explains the success achieved by this class of materials in a growing variety of biomass conversion pathways, as those described in this work: catalytic cracking and pyrolysis, hydrotreatments, with special relevance for hydrodeoxygenation processes, as well as in a high number of condensation, isomerization, and dehydration reactions. Thus, the development of hierarchical zeolites, exhibiting enhanced accessibility, and the possibility of introducing and combining in a controlled way different types of active sites (Brønsted and Lewis acid centers, basic sites, and metal phases) are the main basis of the excellent performance of zeolites in numerous biomass conversion routes.