Catalyst Active Sites Research Articles

Exploring Ti active sites in Ziegler-Natta catalysts through realistic-scale computer simulations with universal neural network potential

We investigate the interaction dynamics between TiCl4 molecules and a MgCl2 (110) cluster to understand the complexities of Ziegler-Natta catalysts through molecular dynamics simulations. Our study reveals significant distortion of Mg atoms at the edge of cluster, influencing TiCl4 adsorption behavior, cluster stability, and reactivity. Various TiCl4 configurations on the MgCl2 surface, such as octahedral and bridging forms, illustrate the versatility of surface in accommodating different molecular arrangements, affecting catalytic performance. We explore diverse adsorption sites influenced by surface topology and electronic properties, revealing a range of adsorption energies from 15.7 to 29.2 kcal/mol. This indicates a complex interplay of molecular forces, significantly impacting catalytic efficiency at distorted sites. Nudged Elastic Band calculations reveal varied activation energies for ethylene insertion, suggesting faster reaction rates at certain sites and highlighting highly favorable dynamics at Corradini type Ti active sites. All these findings offer new directions for enhancing catalyst performances.

Pd octahedra nanocubes mediated photo-fenton catalytic performance for sustainable degradation of methylene blue

This research utilized a typical solvothermal approach, by employing Na2PdCl4 as the synthetic precursor, L-ascorbic acid (AA) as the reductant agent, and polyvinylpyrrolidone (PVP) as the practical surfactant, palladium (Pd) octahedra was finally effectively synthesized by the hydrothermal and solvothermal methods. Subsequently, characterizations of Pd octahedra were conducted by using TEM, XPS, XRD, and other analytical methods. Whereafter, Pd octahedra displayed a consistent shape and size, featuring a distinctive tip-shaped structure, which conferred upon them outstanding optical absorption properties within the near-infrared (NIR) region. Consequently, under NIR laser exposure, Pd octahedra could elevate local reaction temperature via in-situ photothermal conversion, thereby stimulating the formation of active sites of Fenton catalysts. The Photo-Fenton catalytic activity of Pd octahedra was investigated by using methylene blue (MB) as a model pollutant. The degradation efficiency of MB reached 94.8 % within 35 min under 808 nm laser irradiation, and the removal rate of MB by Pd octahedra could still reach more than 90 % after five cycles of use. All the results manifest that Pd octahedra exhibit excellent purification performance for MB dyestuff wastewater.

Praseodymium-Doped Cr2O3 Prepared by In Situ Pyrolysis of MIL-101(Cr) for Highly Efficient Catalytic Oxidation of 1,2-Dichloroethane.

The development of economical catalysts that exhibit both high activity and durability for chlorinated volatile organic compounds (CVOCs) elimination remains a challenge. The oxidizing and acidic sites play a crucial role in the oxidation process of CVOCs; herein, praseodymium (Pr) was introduced into CrOx catalysts via in situ pyrolysis of MIL-101(Cr). With the decomposition of the ligand, a mixed micro-mesoporous structure was formed within the M-Cr catalyst, thereby reducing the contact resistance between catalyst active sites and the 1,2-dichloroethane molecule. Moreover, the synergistic interaction between chromium and praseodymium facilitates Oβ species and acidic sites, significantly enhancing the low-temperature catalytic performance and durability of the M-PrCr catalyst for 1,2-dichloroethane (1,2-DCE) oxidation. The M-30PrCr catalyst possess enhanced active oxygen sites and acid sites, thereby exhibiting the highest catalytic activity and stability. This study may provide a novel and promising strategy for practical applications in the elimination of 1,2-DCE.

Microwave initiated synthesis of carbon nanosphere from plastic and its application as anode electrocatalyst support in direct methanol fuel cell

The term plastisphere was coined to describe the human-made plastic environment, addressing the challenges of plastic waste. This study presents a rapid method using microwave susceptor catalysts (Co/Mo/Biochar, Ni/Mo/Biochar, Fe/Mo/Biochar) for the catalytic degradation of polystyrene into valuable carbon in microwave synthesizer. Using SEM, we found that active metal nanoparticles are uniformly distributed and the sizes of the active porous sites varied depending upon the catalyst used where the porous active sites of catalyst Co/Mo/biochar, Ni/Mo/biochar and Fe/Mo/biochar ranges between 300 nm and 700 nm, 2.21 μm and 4.05 μm, and 5 μm to larger pore size respectively. The microwave initiated catalytic synthesis process transforms uniform-sized polystyrene pellets into carbon nanospheres within 90 s. From XRD characterization, the broadened peaks at (002) plane resulted from the graphitic flakes' waving structure, confirming that the carbon nanospheres formed are graphitic in nature which is in agreement with results obtained from Raman spectroscopy. This study also explores polystyrene as an energy feedstock, utilizing Co/Mo/Biochar synthesized carbon nanospheres (CNS) which is mesoporous in nature with BET average surface area about 1428.3 m2/g as anode electro catalyst support for methanol oxidation in direct methanol fuel cells. The Pt Ru (1:1) supported on CNS catalyst exhibited higher electro catalytic activity compared to commercial PtRu/C, as indicated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS).

Enhanced tetracycline hydrochloride degradation at near neutral conditions with FeOx modified Bi4TaO8Cl catalyst: Coupling the photocatalytic and Fenton oxidation

The efficient regeneration of Fe2+ active sites in heterogeneous catalysts is of great significance for the photo-Fenton progress. Herein, amorphous FeOx as one efficient cocatalyst was anchored on the Bi4TaO8Cl nanosheets to boost the activation of H2O2 and the separation of photogenerated carriers. The surface-exposed FeOx can produce Fe2+ active sites by extracting the photogenerated electrons from Bi4TaO8Cl. As a result, the redox cycle of Fe3+/Fe2+ was built and the carriers were separated to boost the continuous photo-Fenton degradation. As a result, H2O2 are continuously activated through the cycle of Fe3+/Fe2+. The coupling of photocatalysis and the Fenton reaction enhances the TC-H degradation performance and suppress the loss-activation of iron sites. The optimized FeOx/Bi4TaO8Cl sample exhibited the highest degradation ratio of about 83.5 % in 20 min, which was nearly 4 times that of the pure Bi4TaO8Cl. In addition, the better performance of FeOx/Bi4TaO8Cl catalyst under near-neutral conditions also sheds new insights about the design of efficient photo-Fenton catalysts that can work under mild conditions.

Experimental study on Finding stable catalytic methane decomposition for hydrogen production

This study attempts to find active and stable catalysts for H2 production from catalytic methane decomposition (CMD). Experimental results show that metal-based (Ni, Fe, and Ni–Fe alloy) and carbon-based (biochar and activated carbon) catalysts are gradually deactivated due to carbon deposition during the CMD reaction. Although the CMD performance can be enhanced using bimetallic catalysts supported by either metal oxide (Al2O3) or activated carbon, deactivation due to carbon deposition is still inevitable. It was also found the carbon-based carbon catalyst has lower activity as compared with the metal-based catalysts. When using commercially available nickel foams with pore per inch (PPI) higher than 90 as the catalysts, stable H2 production can be obtained. The reasons for obtaining the stable CMD are attributed to high surface area and catalyst active sites, better heat transfer characteristics, and enhanced Ni dispersion of Ni. For the reaction temperature of 900 °C and the nickel foam with a PPI of 110, the stable methane conversion of 90% can be obtained without COx production.

Defect engineering on constructing surface active sites in catalysts for environment and energy applications

Defect engineering on constructing surface active sites in catalysts for environment and energy applications

Spatial confinement and erbium-mediated electronic modulation for enhanced 4-nitrophenol destruction through peroxymonosulfate activation

The development of efficient strategies to modulate the electron structure of active sites in heterogeneous catalysts is crucial but remains challenging for enhancing peroxomonosulfate (PMS) activation to degrade organic contaminants. In this study, Co-Er-N-doped carbon black materials are successfully synthesized by pyrolyzing the CB@Er/Co-complex precursor. Er-doping does not enhance the defect level or graphitization degree of the carbon matrix, but rather regulates the electronic structure of Co sites, thereby generating abundant metallic Co species for activating PMS and yield increased reactive oxygen species towards 4-nitrophenol (4-NP) degradation. The optimal Er/Co@NCB-0.6 catalyst exhibits a removal efficiency of 97.9 % in 20 min. Furthermore, this catalyst exhibits broad degradation capabilities against various organic contaminants. Due to the spatial confinement effect from carbon layers, the encapsulated Co nanoparticles in the Er/Co@NCB-0.6 catalyst demonstrate excellent stability and reusability for the reaction. Radical quenching and electron paramagnetic resonance results indicate that the Er/Co@NCB-0.6/PMS system involves radical and non-radical pathways, and 1O2 and SO4•− play essential role in degrading 4-NP. The pollutant is degraded into seventeen intermediates via two plausible pathways. This study provides a valuable rare metal-mediated strategy to regulate the electronic structure of Co-based catalytic materials towards activating PMS in removing persistent organic pollutants.

Active sites of the cobalt catalysts controlled by surface silanols of silicalite-1 in CO2 hydrogenation to ethanol

The soaring emissions of CO2 have resulted in an extreme climate crisis worldwide. One of the promising solutions is to hydrogenate the CO2 into ethanol. However, a significant challenge lies in the low activity or ethanol selectivity of the catalysts, hindering its industrialization. For the Co-based catalysts, regulation of the active cobalt valance state is crucial for boosting ethanol synthesis by CO hydrogenation. In this work, silicalite-1 (S-1) was used as support to control cobalt sites, which was synthesized by a novel grinding synthesis method. The S-1 can form Si-O-Co chemical bonds with cobalt particles by grafting cobalt atoms onto isolated silanols located on the external surface of S-1. The strong metal-support interactions originating from the Si-O-Co bonds stabilized the coexistence of Co0 and CoO (CoOx sites), preventing complete reduction to Co0 during reduction process. The S-1-supported catalysts with CoOx sites showed superior catalytic performance compared to other catalysts with only Co0 sites. The S-1-supported catalysts with Co loading of 5 wt% exhibited an ethanol selectivity of 27 % at 250 °C and 2 MPa, corresponding to a space–time yield (STY) of 0.83 mmolethanol·gcat−1·h−1. In situ DRIFTS and DFT calculation results demonstrated that Co0 sites facilitated the formation of CHx* species, whereas CoO sites favored the HCOO* species. The CHx* species on Co0 sites tended to be further hydrogenated to CH4, whereas the CHx* species on CoOx sites could be coupled with the HCOO* species, inhibiting the formation of CH4 and enhancing ethanol selectivity. Our work provides an approach to prepare highly efficient catalysts with controlled cobalt sites and deepens an insight into the roles of Co0 and CoO sites in CO2 hydrogenation to ethanol.

Eugenol Hydrodeoxygenation Over Mixed Mo-W Carbides.

The modification of molybdenum carbide catalysts by another transition metal has raised an increasing research interest due to the significant improvement of catalyst activity in hydrodeoxygenation of lignin derivatives. At par with the commonly used Co and Ni that add a strong hydrogenation functionality, it was found that the addition of the more oxophilic W restricts ring hydrogenation while allowing the deoxygenation of oxygenated compounds and thus yielding higher selectivity toward the formation of non-oxygenated aromatic compounds. The coexistence of Mo2C with W2C along with metallic W altered the electronic properties of Mo2C which resulted in an increase of catalyst active site density and facilitated further total eugenol deoxygenation. Propyl-benzene selectivity of up to 83 % was reached at close to 100 % eugenol conversion. These findings will allow a better overview of the effect of different metal phases of mixed carbides on the catalyst performance and raise the prospect of optimizing catalyst design for a hydrodeoxygenation processing of lignin depolymerization products.

To overcome the unconventional polymerization behavior of single-site symmetrical metallocene catalyst in the presence of borate

The β-H transfer of a propagating chain shows an unconventional polymerization behavior with single-site catalysts. In this work, we seek to address these challenges in the presence of different alkylaluminum cocatalysts. Adjusting the amount and type of cocatalyst in the polymerization process can affect molecular weight distribution (MWD) and chain ends formed via β-H transfer. The cocatalyst's elevated triethylelaluminum (TEA) by more than 50 % resulted in a drop in isoprene (IP) concentration, Mw, and activity, while pure TEA exhibited a smaller [C*]/[Zr] % than both pure Triisobutylaluminum (TIBA) and TIBA/TEA mixture. These results indicate that the type of alkylaluminum and its varying ratio affect the activation of metallocene catalysts. Secondly, this study also includes chain termination and transfer reactions and how comonomers participate in overcoming these issues. For this complicated process, we additionally performed the quench-labeling metal-polymer bonds technique (quantitative experiments (catalyst's active site)) leading to building a kinetic model that plays a significant role in mechanistic studies. The quantitative of active sites results indicate that the lower propagation rate constant (kp) was increased by nearly 20 % with TEA (25 %), and continuously increased with the TEA amount, meaning that TEA is a noticeable essential component in the cocatalyst for higher kp. The higher kpE and kpIP with TIBA might be associated with faster propagation and lower chain transfer reactions to Al−iBu and active centers and faster re-initiation of Al−iBu. The active center and 3,4 connections of IP are increased with increased TEA, meaning that the active site based on Al−Et bonds is facilitating more coordinated-to-IP through 3,4 connections. Finally, to clarify the impact of cocatalysts on MWD, a detailed theoretical splitting of the corresponding GPC was conducted. However, compared to PE MWD, TEA decreases the MWD of the polymers and becomes narrow. It is evident that introducing IP as a comonomer and TEA as a combined cocatalyst significantly reduces the chain transfer reaction that occurs during PE.

Metal (Ca, Cu, Zn)-modified defect-rich Bi nanosheets for electrocatalytic reduction of CO2 to HCOOH at high current density

Bismuth-based materials have emerged as promising electrocatalysts for the electrochemical reduction of carbon dioxide (eCO2RR) to HCOOH. However, it remains challenges to meet the practical application requirements and identify the active sites of bismuth-based catalysts in eCO2RR. Herein, bismuthate (MBi2O4, M = Ca, Cu, Zn) derived nanosheet-like electrocatalysts with metal-modified edge defects are developed. During electrochemical processes, MBi2O4 pre-catalyst undergoes surface reconstruction to expose more active Bi0 sites and to form abundant metal-modified edge defects, which facilitate the modulation of charge state of adjacent Bi sites and improve the adsorption of *OCHO intermediate, thereby enhancing the selectivity of eCO2RR towards HCOOH. The Ca-modified defect-rich Bi catalyst exhibits a maximum HCOOH Faradaic efficiency of 94.3 % at an industrially relevant current density of 400 mA cm−2, together with high stability/reproducibility of 55 h. This work proposes a new strategy for construction of Bi-based catalysts and explores its structural reconstruction mechanism during the electrocatalytic process, opening a new window for design of eCO2RR catalysts.

Supported LaCoO3 perovskite-like oxides for N2O decomposition: The key role of the support nature and LaCoO3 content

In this work a detailed comparison of the N2O decomposition catalytic performance of LaCoO3 perovskite-like mixed oxides supported over different commercial materials was conducted. Zirconia-, silica- and γ-alumina-based supports were studied. Zirconia-based supports turned out to be the most applicable because of the formation of the pure LaCoO3 perovskite phase on their surface. Among these supports, ZrO2-La revealed to possess the best activity owing to the highest Co2+/Co3+ (0.82) and Oads./OL (0.67) ratios on its surface. The raw of activities was as follows: LaCoO3(20 %)/ZrO2-La > LaCoO3(20 %)/ZrO2 > LaCoO3(20 %)/ZrO2-W. When the LaCoO3 content in LaCoO3/ZrO2-La was 20 wt%, the maximum of activity was observed – 94.78 mmol(N2O)/[g(cat.)·h] at 450 °C. This fact was in accordance with the highest Co2+/Co3+ (0.82) and Oads./OL (0.67) ratios at the 20 wt% LaCoO3 content. Coordinatively unsaturated Co2+, forming Lewis acid-base pairs, were proposed to be the active sites of the zirconia-based LaCoO3 catalysts in the N2O decomposition.

Enhancing Surface Hydroxyl Group Modulation on Carbon Nitride Boosts the Effectiveness of Photodynamic Treatment for Brain Glioma.

The effectiveness of photodynamic therapy (PDT) in treating brain gliomas is limited by the solubility of photosensitizers and the production of reactive oxygen species (ROS), both of which are influenced by the concentration of photosensitizers and catalyst active sites. In this study, we developed a controllable surface hydroxyl concentration for the photosensitizer CN11 to address its poor water solubility issue and enhance PDT efficacy in tumor treatment. Compared to pure g-C3N4 (CN), CN11 exhibited 4.6 times higher hydrogen peroxide production under visible light, increased incidence of the n → π* electron transition, and provided more available reaction sites for cytotoxic ROS generation. These findings resulted in a 2.43-fold increase in photodynamic treatment efficacy against brain glioma cells. Furthermore, in vivo experiments conducted on mice demonstrated that CN11 could be excreted through normal cell metabolism with low cytotoxicity and high biosafety, effectively achieving complete eradication of tumor cells.

Transforming crystal structures of cobalt molybdate to generate electron-rich sites for electrochemical detection of Pb(II)

BackgroundMost of the investigations on distinct crystal structures of catalysts are individually focused on the difference of surface functional groups or adsorption properties, but rarely explore the changes of active sites to affect the electrocatalytic performance. Catalysts with diverse crystal structures had been applied to modified electrodes in different electrocatalytic reactions. However, there is currently a lack of an essential understanding for the role of real active sites in catalysts with crystalline structures in electroanalysis, which is crucial for designing highly sensitive sensing interfaces. ResultsHerein, cobalt molybdate with divergent crystal structures (α-CoMoO4 and β-CoMoO4) were synthesized by adjusting the calcination temperature, indicating that α-CoMoO4 (800 °C) (60.00 μA μM−1) had the highest catalytic ability than β-CoMoO4 (700 °C) (38.68 μA μM−1) and α-CoMoO4 (900 °C) (29.55 μA μM−1) for the catalysis of Pb(II). It was proved that the proportion of Co(II) and Mo(IV) as electron-rich sites in α-CoMoO4 (800 °C) were higher than β-CoMoO4 (700 °C) and α-CoMoO4 (900 °C), possessing more electrons to participate in the valence cycles of Co(II)/Co(III) and Mo(IV)/Mo(VI) to boost the catalytic reduction of Pb(II). Specifically, Co(II) transferred a part of electrons to Mo(VI), promoting the formation of Mo(IV). Co(II) and Mo(IV), as the electron-rich sites, providing electrons to Pb(II), further accelerating the conversion of Pb(II) into Pb(0). SignificanceIn the process of detecting Pb(II), the CoMoO4 structures under different temperatures have distinct content of electron-rich sites Co(II) and Mo(IV). α-CoMoO4 (800 °C), with the highest content are benefited to detect Pb(II). This work is conducive to understanding the effect of the changes of active sites resulting from crystal transformation on the electrocatalytic performance, and provides a way to construct sensitive electrochemical interfaces of distinct active sites.

Deciphering the Role of Native Defects in Dopant‐Mediated Defect Engineering of Carbon Electrocatalysts

AbstractDoping–dedoping chemistry lays the cornerstone for converting heteroatom dopants into intrinsic defects as the emerging active sites of carbon catalysts, but the defect content is yet hindered by inadequate doping efficiencies. Comprehending crucial factors behind the doping of pristine carbon and their correlation to the catalytic properties of dedoped carbon is thus of high significance. Here, the overlooked impact of native defects in pristine carbon on dopant‐mediated defect engineering of carbon catalysts is explicitly unveiled. Intact fullerene (C60), C60‐derived carbon, and carbon black in distinct pentagon/edge defect states are employed as respective precursors to undergo a nitrogen doping–dedoping treatment. Theoretical and experimental evidence consistently indicates that native pentagons change the preferred N doping site from the edge to the basal plane, leading to a substantially higher doping level. Importantly, in addition to pentagons from the removal of zigzag‐edged pyridinic N, N dopants in in‐plane pentagons are more easily dedoped than those in hexagons, generating even more pentagons in a new pentagon–heptagon–pentagon structure as oxygen reduction active sites. The optimized defect‐rich carbon gives an outstanding half‐wave potential of 0.834 V (0.846 V for Pt/C) via the four‐electron pathway, excellent long‐term durability, and prospective applicability in zinc–air batteries.

New insights into the pH-dependent removal of sulfamethoxazole in peracetic acid activation systems: From mechanistic exploration to practical application potentials

Peracetic acid (PAA) as emerging oxidant in advanced oxidation processes (AOPs) has attracted widespread attention in purifying water pollution. In this research, the removal of target contaminant (sulfamethoxazole, SMX) was investigated through PAA activation by a facile catalyst (Co@C), and the active sites of catalyst were identified as sp3-C, Oads, and Co0 by correlation analysis. Especially, different pH adjustment strategies were designed, including System A (adjusting pH after adding PAA) and System B (adjusting pH before adding PAA), to investigate the impact of oxidant acidity and alkalinity on solution microenvironment as well as effect and mechanism of pollutant removal. The results showed that HO· and CH3C(O)OO· dominated in System A, while Co(IV)O2+ was also observed in System B. Both systems showed optimal SMX degradation (98 %). However, System A exhibited excellent water quality tolerance (efficiency > 78 %), superior sustained catalyst activation (efficiency > 80 % in 40 h), less ion leaching (41 μg L−1), and lower products toxicity. Moreover, the pH of solution after reaction in System B was intensely acidic, requiring costly pH adjustments for discharge. This study unveils the strategy of adjusting pH after adding PAA is preferable for water purification, enriching the emerging research of PAA-based AOPs for the remediation of environments.

Theoretical evidence on pyridinic nitrogen in N, S-coordinated Co single atom catalyst as dominant active site promoting H2 cleavage, H diffusion, and hydrogenation activity

The limitation of isolated metal active sites in single atom catalysts (SAC) hinder their application in hydrogenation reactions, necessitating the creation of additional active site beyond the metal site. This study introduces pyridinic N and S atoms into the N/C sheet, optimally adjusting the local environment of the Co site. This adjustment synergistically enhances H2 activation, H diffusion, and promotes the hydrogenation of nitrobenzene into aniline. The sterically hindered Co acidic site and N basic site, known as frustrated Lewis pairs, in defect-containing Co-N3SN-def1 model cooperatively facilitate the highly efficient cleavage of the H2 molecule with a barrier of only 0.37 eV. The findings suggest that the pyridinic N site in the Co-N3SN-def1 model acts as a reservoir for *H, significantly contributing to the enhanced activity for the hydrogenation of nitrobenzene. The mechanism unveiled in this study offers valuable insights for designing efficient heterogeneous catalysts for target hydrogenation reactions.

Synergistic enhancement of wearable biosensor through Pt single-atom catalyst for sweat analysis

Real-time monitoring of biological markers in sweat is a valuable tool for health assessment. In this study, we have developed an innovative wearable biosensor for precise analysis of glucose in sweat during physical activities. The sensor is based on a single-atom catalyst of platinum (Pt) uniformly dispersed on tricobalt tetroxide (Co3O4) nanorods and reduced graphene oxide (rGO), featuring a unique three-dimensional nanostructure and excellent glucose electrocatalytic performance with a wide detection range of 1–800 μM. Additionally, density functional theory calculations have revealed the synergetic role of Pt active sites in the Pt single-atom catalyst (Co3O4/rGO/Pt) in glucose adsorption and electron transfer, thereby enhancing sensor performance. To enable application in wearable devices, we designed an S-shaped microfluidic chip and a point-of-care testing (POCT) device, both of which were validated for effectiveness through actual use by volunteers. This research provides valuable insights and innovative approaches for analyzing sweat glucose using wearable devices, contributing to the advancement of personalized healthcare.

Ethanolysis of enzymatic hydrolysis lignin with Ni catalysts on different supports: The roles of catalytic sites

Catalytic lignin solvolysis (CLS) holds promise for efficient lignin utilization, yielding small molecules with minimal or no char formation. However, the role of different active sites of catalyst in CLS are rarely discussed. Here, Ni catalysts on different supports, i.e., SiO2, Al2O3, MgO, and ZrO2, were prepared with the aim of manipulating the relative importance of metal and acid-base functionalities to investigate the role of different active sites in the enzymatic hydrolysis of lignin (EHL) ethanolysis. The main ether linkage, i.e., β-O-4, in EHL is cleaved without the participation of a catalyst. Metal sites suppress repolymerization through hydrogenating active intermediates, while acid and base sites facilitate the conversion of phenolic monomers into complex alkylated and etherified products and also promote repolymerization reactions. Among the catalysts, Ni/SiO2 demonstrated the highest hydrogenation activity and yielded the most monomers (24.7 wt %) at 280 °C for 6 h under 2 MPa H2 in ethanol. These findings shed light on the catalytic mechanisms in CLS, offering valuable insights for future catalyst design.