Role Of Active Centers Research Articles

Role of Active Centers in Predicting the Catalyst Turnover: A Theoretical Study.

Water oxidation catalysis has garnered significant attention due to its potential for sustainable energy conversion. Among molecular catalysts, [Fe(OTf)2 (Me2Pytacn)] complexes have exhibited notable turning-over rates. Although various [M(OTf)2 (Me2Pytacn)] complexes (M = Mn, Co, Ni) have been synthesized, however, the role of active centres has not been thoroughly investigated. In this study, we apply our newly developed catalytic models, efficiency conceptualization model (ECM) and the maximum kinetic efficiency MaxKinEff framework to assess the role of the active centres in its catalytic performance. Our computational analysis identifies cobalt-based [Co(OTf)2 (Me2Pytacn)] as a superior alternative for water oxidation reactions. Notably, cobalt catalysts exhibit a longer lifespan (~ 44 days) and higher turnover numbers (TON), with computed values of (ECM) = 3.30 h-1, (ECM) = 3456, (MaxKinEff) = 12.43 h-1, and (MaxKinEff) = 3616. These findings suggest that cobalt could play a pivotal role in improving the efficiency of molecular catalysts for water oxidation.

Quasi-one-dimensional Mo chains for efficient hydrogen evolution reaction

Structural modulation of catalytic nanostructures and fundamental understanding of their active sites at the atomic scale are important for predicting and improving the catalytic properties of nanostructures. Here, we prepared quasi-one-dimensional (1D) metal molybdenum (Mo) chains confined in atom-thick molybdenum disulfide (MoS2), referred henceforth as Mo/MoS2 nanosheets, and evaluated their hydrogen evolution reaction (HER) properties. The experiment and theoretical calculations show that the quasi-1D Mo chain with unsaturated coordination exhibits high HER activity. The unsaturated Mo sites in the chains increase the carrier density and facilitate the diffusion of hydrogen along the chains, mimicking an atomic scale reactor which leads to an experimentally observed enhanced catalytic performance. Within the framework of Volmer-Tafel model, the calculated kinetic barrier for H2 evolution is only 0.48 eV for Mo/MoS2, which is significantly lower than that for the Pt (111) surface (∼0.8 eV). In particular, Mo/MoS2 nanosheets supported on reduced graphene oxide (Mo/MoS2/RGO) outperformed commercial Pt on glassy carbon (Pt/C) in the practically meaningful high-current region (>140 mA cm−2) in 0.5 M H2SO4 solution and (15 mA cm−2) in 1.0 M NaOH solution, demonstrating that the Mo/MoS2/RGO could potentially replace Pt catalysts in practical HER systems. Additionally, this study provides crucial insights into the role of active centers in catalysis through a model-structure-performance relationship, thus pointing the way to a commercially viable technology for HER.

Hydriding properties of the nanocomposite 85 wt.% Mg–15 wt.% Mg2Ni0.8Co0.2 obtained by ball milling

The hydrogen sorption properties of the nanocomposite 85 wt.% Mg–15 wt.% Mg2Ni0.8Co0.2 obtained by mechanical alloying in inert atmosphere were investigated. Absorption measurements were performed under a hydrogen pressure P = 1 MPa at temperartures ranging from 373 to 573 K, while desorption studies proceeded at P = 0.15 MPa and temperatures of 573 and 553 K. The addition of the intermetallic compound Mg2Ni0.8Co00.2 was shown to improve the hydriding kinetics of magnesium. The composite exhibited a high hydrogen capacity which did not decrease even after a large number of absorption–desorption cycles. Comparison of the hydriding kinetics of the intermetallic compounds Mg2Ni and Mg2Ni0.8Co0.2 indicated facilitation of the process by the presence of cobalt in the alloy. Magnetic measurement data on Mg2Ni0.8Co0.2 showed formation of superparamagnetic precipitations of nickel and cobalt playing the role of active centres for dissociative chemisorption of hydrogen. The behaviour of the composite was explained by the catalytic effect of the intermetallic Mg2Ni0.8Co00.2, the existence of Ni and Co clusters on the surface and the process of mechanical alloying.

Surface Effect on Branching Chain Reactions In Filtration Combustion of Gases

The existence of lean and rich concentration limits of the low-velocity regime of flame propagation was shown experimentally under filtration of a combustible gas mixture in narrow tubes. The values of the limits depend on the inner diameter of the tube. Experiments with an inhibitor affecting the concentration limits of combustion-wave propagation in tubes in a low-velocity regime showed that branching chain reactions occur in the filtration combustion wave and the concentrations of active centers thus exceed the equilibrium values. In the low-velocity regime, the surface of the narrow quartz tube slightly decreases the role of active centers in combustion-wave propagation.

Methyl iodide reactions on the surface of mechanically pre-activated platinum(II) salt in the heterogeneous system: K 2PtCl 4 powder–MeI vapor

Mechanically pre-activated K 2PtCl 4 salt consumes methyl iodide producing methyl chloride at room temperature. The reaction mechanism includes the following steps sequence: oxidative addition of methyl iodide to platinum(II) complexes with intermediate formation of methyl platinum(IV) complexes and further decomposition of the latter in the course of innersphere reductive elimination yielding methyl chloride. The first step of the reaction proceeds owing to the assistance of active centers regenerated in the course of each event of MeI into MeCl transformation taking part in the chain halogen substitution process. It could be assumed that the role of active centers is played by coordinatively unsaturated platinum(II) complexes located on the surface. These species bearing a positive efficient charge can render electrophilic assistance for the nucleophilic substitution. The chain termination can be caused by recombination of coordinatively unsaturated platinum(II) complexes and interstitial chloride ions forming an inactive K 2PtCl 4 complex.

Role of Active Center and Lysine Binding Sites of Plasmin in Plasmin-Induced Platelet Activation and Disaggregation

Previous studies have shown that plasmin can activate platelets and can also disperse platelet aggregates by degradation of fibrinogen bound to platelets. In this study, the role of the active center and the lysine binding sites (LBS) of human plasmin in activating platelets and in dispersing platelet aggregates is investigated using aprotinin and the tripeptide Val-Phe-Lys-CH2Cl to inhibit the active center and using epsilon-aminocaproic acid (EACA) to specifically block the LBS. Our results show that the catalytic activity of plasmin is indispensable both for activating platelets and for dispersing platelet aggregates. Binding of plasmin to platelets through the LBS enhances its activating potential, since both EACA (1 mM) and Lys-plasminogen (molar ratio of plasminogen:plasmin at 2:1 to 4:1) inhibit plasmin-induced platelet activation, whereas Glu-plasminogen at a molar ratio of 15:1 had no effect. Furthermore, plasmin which lacks the LBS (miniplasmin), is about 3 fold less effective in activating platelets. However, plasmin binding through the LBS is not absolutely required to disperse platelet aggregates, since EACA at 30 mmol/l was unable to prevent disaggregation by plasmin (half disaggregation time: 40 min in the presence of EACA against 27 min in its absence). It also appeared that fibrinogen receptors on activated platelets are resistant to plasmin degradation, and that disaggregation of plasmin-induced platelet aggregates was much slower than the degradation of fibrinogen by plasmin.

Role of active centres in the process of 2D nucleation and growth on perfect crystal surfaces

The role of “active centres” on a perfect crystal face in the growth of the face is estimated. The transient for multinuclear, multilayer growth is calculated, using a model, including “active centres” and compared with the transient for homogeneous nucleation.

Role of active centres in the process of 2D nucleation and growth on perfect crystal surfaces

The role of “active centres” on a perfect crystal face in the growth of the face is estimated. The transient for multinuclear, multilayer growth is calculated, using a model, including “active centres” and compared with the transient for homogeneous nucleation.

The role of active centers in the kinetics of new phase formation

A general equation for the time dependence of the number of nuclei is derived in the case of a nucleation process limited by the finite number of active centers and by the existence of zones around the growing nuclei forbidden for nucleation. The particular case of nucleation onto substrate from the vapour phase is considered. A method for determinating the number of active centers and the forbidden zone spreading rate constant with the help of an experimentally-found nucleation rate, a saturation nucleus density and a saturation time is developed.