Catalyst Active Sites Research Articles

Role of Cr-doped NiO in reduction under a low concentration of H2 and CO

Nickel-chromium alloys are often utilized in high-resistance and heating metamaterials because of their passivity; however, their reducibility behavior has never been reported. Herein, Ni-Cr reduction has been conducted in H2 and CO atmospheres. Nickel oxide (NiO) catalysts were impregnated with chromium and subsequently subjected to temperature-programmed reduction (TPR) at low concentrations of CO and H2 to determine their reduction performance. The TPR profile fitting using a Gaussian function indicated the Cr-NiO reduction transformation of Cr6+→Cr3+ at temperatures below ∼300 °C for both reductant gasses. Furthermore, the addition of Cr shifted the reduction peaks to a lower temperature owing to the interaction of Cr and Ni atoms, leading to increased reducibility. The characterizations of the Cr-NiO catalyst using X-ray diffraction, Brunauer–Emmet–Teller surface area, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) analyses demonstrated the formation of NiCrxOy alloy with increasing surface area and porosity. The kinetic analysis using the Kissinger method demonstrated the reducibility of the catalysts with a low Ea value after the addition of Cr, where an Ea value of 68.0 kJ mol−1 (NiO) decreased to 49.3 kJ mol−1 (Cr-NiO) in the H2 atmosphere and an Ea value of 95.8 kJ mol−1 (NiO) decreased to 91.9 kJ mol−1 (Cr-NiO) in the CO atmosphere. The synergistic effects between Cr and Ni contributed to the increase in catalyst active sites by the transformation of the nickel oxidation state, which demonstrated the formation of oxygen vacancies and alloys, as proven via XPS analysis.

Exploring the mechanism of electron transfer-mediated peroxymonosulfate activation over Cu-based catalyst for the selective decomposition of bisphenol A

Effective degradation of aquatic contaminants highly depends on the adsorption of peroxymonosulfate (PMS) on active sites of catalysts. However, the influence of PMS adsorption state on PMS activation remains unclear. In this study, we investigated the catalytic performance of a Cu-NC catalyst, composed of dispersed Cu atoms on a nitrogen-doped carbon skeleton, for the activation of PMS in decomposing bisphenol A (BPA). The Cu-NC-4 catalyst exhibited remarkable selectivity and efficiency, achieving a kinetic rate of 0.997 min−1 with over 99.9% BPA removal in just 3 min. Through various experiments and characterizations, we determined that the degradation process was primarily facilitated by electron transfer from BPA to Cu-NC. Furthermore, the dosing sequence of PMS and pollutants was found to be crucial in the system. Density functional theory (DFT) simulation revealed that PMS with bridging adsorption on the Cu site exhibited more favorable adsorption energy, leading to enhanced electron transfer and more efficient and selective destruction of electron donors. Based on these findings, we propose a mechanism in which the electron donor plays a crucial role in PMS activation, while the nearby Cu sites optimize the adsorption configuration of PMS, thereby boosting electron transfer. These two factors synergistically contribute to the efficient removal of pollutants.

Enhanced Degradation of Methyl Orange and Trichloroethylene with PNIPAm-PMMA-Fe/Pd-Functionalized Hollow Fiber Membranes.

Trichloroethylene (TCE) is a prominent groundwater pollutant due to its stability, widespread contamination, and negative health effects upon human exposure; thus, an immense need exists for enhanced environmental remediation techniques. Temperature-responsive domains and catalyst incorporation in membrane domains bring significant advantages for toxic organic decontamination. In this study, hollow fiber membranes (HFMs) were functionalized with stimuli-responsive poly-N-isopropylacrylamide (PNIPAm), poly-methyl methacrylate (PMMA), and catalytic zero-valent iron/palladium (Fe/Pd) for heightened reductive degradation of such pollutants, utilizing methyl orange (MO) as a model compound. By utilizing PNIPAm's transition from hydrophilic to hydrophobic expression above the LCST of 32 °C, increased pollutant diffusion and adsorption to the catalyst active sites were achieved. PNIPAm-PMMA hydrogels exhibited 11.5× and 10.8× higher equilibrium adsorption values for MO and TCE, respectively, when transitioning from 23 °C to 40 °C. With dip-coated PNIPAm-PMMA-functionalized HFMs (weight gain: ~15%) containing Fe/Pd nanoparticles (dp~34.8 nm), surface area-normalized rate constants for batch degradation were determined, resulting in a 30% and 420% increase in degradation efficiency above 32 °C for MO and TCE, respectively, due to enhanced sorption on the hydrophobic PNIPAm domain. Overall, with functionalized membranes containing superior surface area-to-volume ratios and enhanced sorption sites, efficient treatment of high-volume contaminated water can be achieved.

Phosphorus-Doped PdSn Nanocatalyst with Abundant Defective Atoms for Enhanced Methanol Oxidation.

The design of the nanostructure of palladium-based nanocatalysts is considered to be a very effective way to improve the performance of nanocatalysts. Recent studies have shown that multiphase nanostructures can increase the active sites of palladium catalysts, thus effectively improving the catalytic efficiency of palladium atoms. However, it is difficult to regulate the phase structure of Pd nanocatalysts to form a compound phase structure. In this work, PdSnP nanocatalysts with different compositions were synthesized by fine-regulating the doping amount of phosphorus atoms. The results show that the doping of phosphorus atoms not only changes the composition of PdSn nanocatalysts but also changes the microstructure, forming amorphous and crystalline multiphase structures. This multiphase nanostructure contains abundant interfacial defects, which effectively promotes the electrocatalytic oxidation efficiency of Pd atoms in small-molecule alcohols. Compared with the undoped PdSn nanocatalyst (480 mA mgPd-1 and 2.28 mA cm-2) and the commercial Pd/C catalyst (397 mA mgPd-1 and 1.15 mA cm-2), the mass (1746 mA mgPd-1) and specific activities (8.56 mA cm-2) of PdSn0.38P0.05 nanocatalysts in the methanol oxidation reaction were increased by 3.6 and 3.8 times and 4.4 and 7.4 times, respectively. This study provides a new synthesis strategy for the design and synthesis of efficient palladium-based nanocatalysts for the oxidation of small-molecule alcohols.

Facile gas-steamed synthesis strategy of N, F co-doped defective porous carbon for enhanced oxygen-reduction performance in microbial fuel cells

The metal-free carbon-based catalyst with low cost and high oxygen reduction reaction (ORR) activity is urgently desired to satisfy the demands of microbial fuel cells (MFCs). However, it is still a great challenge to develop a facile and feasible strategy to construct efficient active sites of heteroatom doping for carbon-based electrocatalyst. Herein, we report a strategy based on an ammonium fluoride (NH4F) gas-steamed metal-organic frameworks (MOFs) to heighten structural defects and density of N, F active sites of metal-free catalyst. Oxygen temperature-programmed deposition and density functional theory results confirm that the NH4F gas-steamed process greatly enhances the adsorption affinity of O2 and oxygen intermediates on the catalysts. The resulted N and F co-doped porous carbon cage (FNC-15) demonstrates outstanding ORR catalytic activity and long-term stability in alkaline and neutral electrolytes. This work proposes a facile and efficient in situ gas-steamed strategy to develop metal-free cathode catalysts with superior performance.

Deconvoluting the roles of polyolefin branching and unsaturation on depolymerization reactions over acid catalysts

Developing a circular economy for plastics is a societal priority with a range of stakeholders engaged in developing pathways to mechanical and chemical recycling. Acid catalysis has been proposed as an attractive strategy to chemically recycle or upcycle plastic waste. In principle, the energetics that govern polyolefin decomposition reactions over Brønsted acid sites should be the same ones known for alkane cracking. However, the nature of polymers as feedstock imposes new challenges for these well-known processes, e.g., strong mass transfer limitations. To better understand the role of polymer structure and diffusion on cracking rates, the effect of tertiary carbons, chain unsaturation, and catalyst active sites distribution were studied. Our findings confirm that higher concentrations of tertiary carbons will govern decomposition reactions, regardless of diffusion limitations. Also, we unveil that LLDPE reactions mostly take place on the external surface of ZSM-5 crystals while HDPE conversion takes place within the zeolite pores.

Bimetallic Cu-Fe catalysts on MXene for synergistically electrocatalytic conversion of nitrate to ammonia

NO3− is a common water pollutant that can serve as a potential nitrogen source for electrocatalytic NH3 production. However, an efficient and complete removal of low NO3− concentrations remains a challenge. Fe1Cu2@MXene bimetallic catalysts were constructed on two-dimensional Ti3C2Tx MXene carriers via a simple solution-based synthetic method and used for the electrocatalytic reduction of NO3−. The combination of the rich functional groups, high electronic conductivity on the MXene surface, and the synergistic effect between the Cu and Fe sites enabled the composite to effectively catalyse NH3 synthesis, with a 98% conversion of NO3− in 8 h and a selectivity for NH3 of up to 99.6%. In addition, Fe1Cu2@MXene showed excellent environmental and cyclic stability at various pH values and temperatures over multiple (14) cycles. Semiconductor analysis techniques and electrochemical impedance spectroscopy confirmed that the synergistic effect provided by the dual active sites of the bimetallic catalyst enabled fast electron transport. This study provides new insights into the synergistic promotion of NO3− reduction reactions using bimetals.

Modulable Cu(0)/Cu(I)/Cu(II) sites of Cu/ C catalysts derived from MOF for highly selective CO2 electroreduction to hydrocarbons

Electrocatalytic CO2 reduction has been perceived as sustainable to realize the closed-loop carbon cycle and obtain high-energy-dense fuels and chemicals. Nevertheless, the efficient CO2 reduction into specific hydrocarbons is still highly challenging. In particular, the effects of Cu(0)/Cu(I)/Cu(II) active sites in copper-based catalysts remain elusive. Hence, this work investigated the modulable valence states of Cu(0)/Cu(I)/Cu(II) active sites derived from MOF Cu(btc) under different calcination conditions to enhance CO2RR performance. As evidenced by characterizations, the Cu/C-T-air was more inclined to produce C2H4, which may be associated with Cu(I)/Cu(II) coexistence and abundant oxygen vacancy. Cu/C-450-air showed excellent CO2RR properties with 34.8% Faradaic efficiency of C2H4 at 1.1 V vs. RHE after a continuous measurement of 12 h. Meanwhile, the Cu/C-T-H2 was inclined to produce C2H4, while Cu/C-T-N2 tended to produce CH4, which may be associated with smaller grain size and thinner carbon layer than the latter to exposure to more active sites and rapid electron transport. Moreover, the mechanism of electrocatalytic CO2 reduction of Cu/C-T-gas was conjectured. We furnished rational thinking to design Cu/C catalysts with tunable active sites to bolster CO2RR.

Methane Activation by [OsC3]+: Implications for Catalyst Design.

Gas-phase reactions of [OsC3]+ with methane at ambient temperature have been studied by using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. The comparison of [OsC3]+ with the product clusters revealed significant changes in cluster reactivity. In particular, with different ligands, the cluster may produce multiple products or, alternatively, just a single product. Theoretical calculations reveal the influence of electronic features such as molecular polarity index, charge and spin distribution, and HOMO-LUMO gap on the reactivity of the Os complexes. Fundamentally, it is the polarity of the clusters that leads to the cluster reactivity in the methane activation. Furthermore, reducing the local polarity of the catalyst active site may be one means of reducing the number of byproducts in the reaction.

B atom dopant-manipulate electronic structure of CuIn nanoalloy delivering wide potential activity over electrochemical CO2RR

The accuracy and efficiency tuning the local electronic structure of catalyst active sites are pre-requisites for achieving high selectivity CO2 reduction reaction on a wide potential window, whereas remain a great challenge. Here, a B-doped CuIn alloy catalyst with tunable electronic structure for the highly effective electrochemical conversion of CO2 to CO has been exploited. The obtained B-doped CuIn alloy performs an optimal CO Faraday efficiency of 99% at –0.6 V (vs. the reversible hydrogen electrode) and particularly keeps outstanding CO Faraday efficiency (> 90%) over a wide cathodic electrochemical window (400 mV). Density functional theory theoretical calculation manifests that the enhanced performance is primarily ascribed to the electron-capturing ability of high valence state B atom, which optimizes the local electronic structure of adjacent metal active sites and adjusts the binding energy between catalyst and intermediates. A foundation of designing advanced electrochemical CO2 reduction reaction catalysts can be served by the insights gained though this research.

Chemical Recycling Processes of Waste Polyethylene Terephthalate Using Solid Catalysts.

Polyethylene terephthalate (PET) is a non-degradable single-use plastic and a major component of plastic waste in landfills. Chemical recycling is one of the most widely adopted methods to transform post-consumer PET into PET's building block chemicals. Non-catalytic depolymerization of PET is very slow and requires high temperatures and/or pressures. Recent advancements in the field of material science and catalysis have delivered several innovative strategies to promote PET depolymerization under mild reaction conditions. Particularly, heterogeneous catalysts assisted depolymerization of post-consumer PET to monomers and other value-added chemicals is the most industrially compatible method. This review includes current progresses on the heterogeneously catalyzed chemical recycling of PET. It describes four key pathways for PET depolymerization including, glycolysis, pyrolysis, alcoholysis, and reductive depolymerization. The catalyst function, active sites and structure-activity correlations are briefly outlined in each section. An outlook for future development is also presented.

Auto-selective reaction mechanism on Al-substituted ZnFe2O4 spinel electrode and sustainable water oxidation by oxygen vacancy transition

This study aims to optimize the oxygen evolution reaction (OER) by maximizing OH– adsorption on the mixed spinel ZFAO surface, in which Al3+ is substituted for Fe3+ in the ZnFe2O4 (ZFO) spinel structure composed of inexpensive transition metals. The substituted Al3+ ions simultaneously occupy the tetrahedral and octahedral sites of Fe in the ZFO spinel lattice. As the amount of substituted Al3+ increases, the amount of OH– adsorbed on the crystal surface increases significantly, and the OER activity is maximized at the ZFA0.75O/NF electrode, in which 0.75 M of Al is substituted. The overpotential is greatly reduced to 270 mV at 10 mA cm−2, and the Tafel slope is very low at 79 mA dec−1. These results were constant even after 10 d, which means that the charge transfer reaction proceeds rapidly between the ZFAO spinel catalyst and aqueous solution during the OER without side reactions. In particular, this study confirms that the main OER active sites of the ZFAO catalyst are the Al-tetrahedral and Fe-octahedral sites, and oxygen through the lattice vacancies rapidly transfers, maintaining a stable spinel structure. This study also reveals that the OER selectively pursues two mechanisms: an adsorbate evolution mechanism (AEM) on the Al-tetrahedral site and a lattice oxygen participation mechanism (LOPM) on the Fe-octahedral site. This study contributes to making the catalyst cost-effective, which has been an obstacle to the commercialization of water electrolysis technology, and securing its long-term durability, thereby opening the door to the mass production of clean hydrogen energy.

Unexpected promotional effects of HCl over CeO2-based catalysts for NOx reduction against alkali poisoning

In the flue gas of stationary NOx emission source, HCl was deemed to be one of the poisons of NOx reduction catalysts since it can react with ammonia to form NH4Cl and adhere to oxide surface, reducing the redox property and blocking the active sites of catalysts for selective catalytic reduction of NOx with NH3 (NH3-SCR). Herein, unexpected promotional effects of HCl over CeO2-based catalysts for NOx reduction against alkali poisoning have been unraveled. The modification of gaseous HCl on the surface of CeO2 constructed Cl-Ce-O-Ce-OH and Cl-Ce-O-Ce3+ sites, providing additional Brønsted and Lewis acid sites to facilitate ammonia adsorption. It is worth noting that HCl can not only directly neutralize alkali metal to reduce alkalinity, but also increase the acidity to offset the encroachment of acid sites by alkali metal poisoning. The lower NO adsorption energy and weaker oxidation property of Cl-Ce-O-Ce3+ sites also prevented the accumulation of inert nitrate on the surface of alkali metal poisoned catalyst. This work unveils multi-promotion mechanisms for the enhanced catalytic activity and anti-alkali poisoning of gaseous HCl modified cerium-based NOx reduction catalysts, providing a new thinking based on pollutant interaction for further development of highly efficient NOx reduction catalysts in multipollutant flue gas.

Selectivity of CO2 reduction reaction to CO on the graphitic edge active sites of Fe-single-atom and dual-atom catalysts: A combined DFT and microkinetic modeling

We study the carbon dioxide reduction reaction (CO2RR) activity and selectivity of Fe single-atom catalyst (Fe-SAC) and Fe dual-atom catalyst (Fe-DAC) active sites at the interior of graphene and the edges of graphitic nanopore by using a combination of DFT calculations and microkinetic simulations. The trend of limiting potentials for CO2RR to produce CO can be described by using either the adsorption energy of COOH, CO, or their combination. CO2RR process with reasonable reaction rates can be achieved only on the active site configurations with weak tendencies toward CO poisoning. The efficiency of CO2RR on a catalyst depends on its ability to suppress the parasitic hydrogen evolution reaction (HER), which is directly related to the behavior of H adsorption on the catalyst’s active site. We find that the edges of the graphitic nanopore can act as potential adsorption sites for an H atom, and in some cases, the edge site can bind the H atom much stronger than the main Fe site. The linear scaling between CO and H adsorptions is broken if this condition is met. This condition also allows some edge active site configurations to have their CO2RR limiting potential lower than the HER process favoring CO production over H2 production.

Nanoscale Ni-NiO-ZnO Heterojunctions for Switchable Dehydrogenation and Hydrogenation through Modulation of Active Sites.

Catalysts consisting of metal-metal hydroxide/oxide interfaces are highly in demand for advanced catalytic applications as their multicomponent active sites will enable different reactions to occur in close proximity through synergistic cooperation when a single component fails to promote it. To address this, herein we disclosed a simple, scalable, and affordable method for synthesizing catalysts consisting of nanoscale nickel-nickel oxide-zinc oxide (Ni-NiO-ZnO) heterojunctions by a combination of complexation and pyrolytic reduction. The modulation of active sites of catalysts was achieved by varying the reaction conditions of pyrolysis, controlling the growth, and inhibiting the interlayer interaction and Ostwald ripening through the efficient use of coordinated acetate and amide moieties of Zn-Ni materials (ZN-O), produced by the reaction between hydrazine hydrate and Zn-Ni-acetate complexes. We found that the coordinated organic moieties are crucial for forming heterojunctions and their superior catalytic activity. We analyzed two antagonistic reactions to evaluate the performance of the catalysts and found that while the heterostructure of Ni-NiO-ZnO and their cooperative synergy were crucial for managing the effectiveness and selectivity of the catalyst for dehydrogenation of aryl alkanes/alkenes, they failed to enhance the hydrogenation of nitro arenes. The hydrogenation reaction was influenced by the shape, surface properties, and interaction of the hydroxide and oxide of both zinc and nickel, particularly accessible Ni(0). The catalysts showed functional group tolerance, multiple reusabilities, broad substrate applicability, and good activity for both reactions.

Kinetics insights into the active sites of Au catalysts for the oxidative esterification of methacrolein to methyl methacrylate

The direct oxidative esterification of methacrolein (MAL) has drawn considerable attention as a promising route to produce methyl methacrylate (MMA). Here, we report a kinetics strategy to identify the active sites of Au catalyst for its disentanglement of geometric and electronic effects on this reaction. A series of Al2O3 and CeO2 supported Au catalysts are prepared by urea deposition–precipitation method, whose particle sizes are tailored by changing the vacuum-drying temperature. Their catalytic activity is correlated with Au particle size based on a crystal structure modelling, in which the Au edge site is identified as the main active site. The reduction in the valence electron population of Au 5d states facilitates the adsorption of MAL, thus the positive core-level shift of Au 4f levels for each support gives rise to a higher production of MMA. This kinetics strategy could be extended to the design of other metal catalysts for this reaction.

How water molecules occupying the active site of a single-atom catalyst affect the electrochemical reduction of carbon dioxide

How water molecules occupying the active site of a single-atom catalyst affect the electrochemical reduction of carbon dioxide

Tuning the Site-to-Site Interaction of Heteronuclear Diatom Catalysts MoTM/C2N (TM = 3d Transition Metal) for Electrochemical Ammonia Synthesis.

Ammonia (NH3) synthesis is one of the most important catalytic reactions in energy and chemical fertilizer production, which is of great significance to the sustainable development of society and the economy. The electrochemical nitrogen reduction reaction (eNRR), especially when driven by renewable energy, is generally regarded as an energy-efficient and sustainable process to synthesize NH3 in ambient conditions. However, the performance of the electrocatalyst is far below expectations, with the lack of a high-efficiency catalyst being the main obstacle. Herein, by means of comprehensive spin-polarized density functional theory (DFT) computations, the catalytic performance of MoTM/C2N (TM = 3d transition metal) for use in eNRR was systematically evaluated. Among the results, MoFe/C2N can be considered the most promising catalyst due to its having the lowest limiting potential (-0.26 V) and high selectivity in the context of eNRR. Compared with its homonuclear counterparts, MoMo/C2N and FeFe/C2N, MoFe/C2N can balance the first protonation step and the sixth protonation step synergistically, showing outstanding activity regarding eNRR. Our work not only opens a new door to advancing sustainable NH3 production by tailoring the active sites of heteronuclear diatom catalysts but also promotes the design and production of novel low-cost and efficient nanocatalysts.

Non-radical dominated degradation of chloroquine phosphate via Fe-based O-doped polymeric carbon nitride activated peroxymonosulfate: Performance and mechanism

The utilization of peroxymonosulfate (PMS)-based advanced oxidation process as a proficient approach for treating wastewater has garnered significant attention. However, the activation of PMS and the degradation of contaminants are highly dependent on or severely limited by the design and structured active sites of catalysts. Herein, an Fe-based O-doped polymeric carbon nitride catalyst, namely Fe-ONLH, was synthesized to activate PMS for the efficient removal of model pollutant chloroquine phosphate (CQP) via a non-radical dominant pathway with singlet oxygen (1O2). The rapid degradation of CQP in the developed Fe-ONLH/PMS system was achieved by the optimum degradation conditions predicted by Response Surface Method (RSM) and the catalyst dosage was considered to have the highest relative importance among the variables. Particularly, the ability of Fe and oxygen co-dopants to generate non-radical active species is mainly attributed to the various active sites on Fe-ONLH (e.g., Fe-O, graphite N, CO and NC-N2), which also exhibited advantages in adaptability of wide pH range, catalyst reusability, stability in complicated ionic environmental or water matrixes. Besides, probable intermediates and decomposition pathways of CQP attacked by non-radicals and radicals were analyzed. The current study provides new insights of Fe-base heterogeneous catalysts for PMS activation and non-radical pathway degradation of pollutants during wastewater treatment.

Fulvic acid enhanced peroxymonosulfate activation over Co–Fe binary metals for efficient degradation of emerging bisphenols

Bisphenol F (BPF) and bisphenol S (BPS) are emerging bisphenols, which have become the main substitutes for bisphenol A (BPA) in industrial production and are also considered as new environmental pollution challenges. Thus, the necessity for an effective approach to remove BPF and BPS is essential. In this study, fulvic acid (FA) was used to modify Co–Fe binary metals (CFO) for peroxymonosulfate (PMS) activation. The characterization results demonstrated that CFO changed significantly in morphology after compounding with FA, with smaller particle size and 5.6 times larger specific surface area, greatly increasing the active sites of catalyst; Moreover, humic acid-like compounds increased the surface functional groups of CFO, especially phenolic hydroxyl, which could effectively prolong the PMS activation. The concentration of all reactive species, such as SO4•−, •OH, O2•−, and 1O2 increased in FA@CFO/PMS system. As a result, the degradation efficiency of CFO for both BPF and BPS was significantly improved after compounding FA, which also had a wide range of pH applications. The degradation pathways of both BPF and BPS were proposed based on liquid chromatography-mass spectrometry (LC-MS) analysis and the density functional theory (DFT) calculations. Our findings are expected to provide new strategies and methods for remediation of environmental pollution caused by emerging bisphenols.