Catalyst Regeneration Research Articles

Development of Processes and Catalysts for Biomass to Hydrocarbons at Moderate Conditions: A Comprehensive Review.

This comprehensive review explores recent catalyst advancements for the hydrodeoxygenation (HDO) of aromatic oxygenates derived from lignin, with a specific focus on the selective production of valuable aromatics under moderate reaction conditions. It addresses critical challenges in bio-crude oil upgrading, encompassing issues related to catalyst deactivation from coking, methods to mitigate deactivation, and techniques for catalyst regeneration. The study investigates various oxygenates found in bio-crude oil, such as phenol, guaiacol, anisole, and catechol, elucidating their conversion pathways during HDO. The review emphasizes the paramount importance of selectively generating arenes by directly cleaving C-O bonds while avoiding unwanted ring hydrogenation pathways. A comparative analysis of different bio-crude oil upgrading processes underscores the need to enhance biofuel quality for practical applications. Additionally, the review focuses on catalyst design for HDO. It compares six major catalyst categories, including metal sulfides, transition metals, metal phosphides, nitrides, carbides, and oxides, to provide insights for efficient bio-crude oil upgrading toward sustainable and eco-friendly energy alternatives.

Thermo-Catalytic Decomposition Comparisons: Carbon Catalyst Structure, Hydrocarbon Feed and Regeneration

Thermo-catalytic decomposition (TCD) activity and stability depend upon the initial carbon catalyst structure. However, further transitions in the carbon structure depend on the carbon material (structure and composition) originating from the TCD process. In this article, reaction data are presented that illustrates the time-dependent TCD activity as TCD-formed carbon contributes and then dominates conversion. A variety of initial carbon catalysts are compared, including sugar char, a conductive carbon black (AkzoNobel Ketjenblack), a rubber-grade carbon black (Cabot R250), and its graphitized analogue as formed and partially oxidized. Regeneration of carbon catalysts by partial oxidation is evaluated using nascent carbon black as a model, coupled with subsequent comparative TCD performance relative to the nascent, non-oxidized carbon black. Activation energies for TCD with nascent and oxidized carbons are evaluated by a leading-edge analysis method applied to TCD. Given the correlation between nanostructure and active sites, two additional carbons, engine soots, are evaluated for regeneration and dependence upon nanostructure. Active sites are quantified by oxygen chemisorption, followed by X-ray photoelectron spectroscopy (XPS). The structure of carbon catalysts is assessed pre- and post-TCD by high-resolution transmission electron microscopy (HRTEM). Last, energy dispersive X-ray analysis mapping (EDS) is carried out for its potential to visualize oxygen chemisorption.

Reaction microkinetic model of xylose dehydration to furfural over beta zeolite catalyst

AbstractIn recent decades, there has been a growing interest in bio-refineries as a crucial element in transitioning to a low-carbon economy. One specific aspect of this interest is the conversion of carbohydrates into separate platform chemicals, such as furfural (FUR), which play a significant functional role in various daily life processes. This research paper focuses on investigating the use of a H-beta catalyst with SiO2/Al2O3 = 28 for producing furfural from xylose in water. Various conditions, such as temperature and initial solution concentration, are studied to determine their effect on FUR yield. The highest FUR yield (40 mol.%) is obtained when FUR is the only product species. We also report that about 90% yield from reaction with fresh catalyst can be achieved after catalyst regeneration. The activation energies for the reaction on the catalyst surface are found to be in the range of 38–75 kJ/mol. A mathematical kinetic model with three irreversible steps is derived to estimate the reaction sequence at 160, 180, and 200 °C. The model takes into account mechanisms such as adsorption, desorption, and transport (internal or external). Our results suggest that the H-beta catalyst shows high activity toward FUR yield and could be a promising alternative for mass-scale production of the latter.

The effect of dopants (Fe, Al) on the low-temperature activity and SO2 tolerance in solvothermally synthesized MnOx NH3-SCR catalysts

Fe-doped MnOx catalyst prepared by a preferred solvothermal method displayed noticeably better low-temperature (LT) NH3-SCR performance and water stability than an analogously prepared Al-doped MnOx catalyst. The SCR activity of both catalysts decreased markedly when exposed to SO2, but the resultant MnFeOx-S catalyst retained higher LT activity than MnAlOx-S and recovered significantly more of its original activity after thermal regeneration (400 °C). Comprehensive characterization confirmed that the deactivation of the catalysts was governed by formation of stable metal sulfates, which only decomposed to a minor extent upon thermal treatment although Al doping lowered the thermal stability of the adsorbed sulfur species. Additionally, Fe doping was found to facilitate electron transfer between Mn and Fe ions and weaken the interaction between active sites and deposited sulfates during the heating procedure, which promoted re-oxidation of Mn2+ to catalytically active Mn3+/Mn4+. Altogether, the altered redox properties resulted in improved LT SCR performance, enhanced water stability, higher SO2 tolerance and superior regeneration of the MnFeOx catalyst.

Regeneration of PbO-deactivated Mn-Ce/Ti-bearing blast furnace slag SCR catalyst: Washing and Mn-Ce reloading

The NH3-SCR of NOx from sintering flue gas can be hindered by PbO, deactivating catalysts and making its regeneration a significant challenge. To address this issue, a novel MnOx-CeO2/Ti-bearing blast furnace slag catalyst was developed and its Pb poisoning mechanism was previously elucidated. In this work, regeneration of Pb-poisoned catalyst was systematically evaluated, including washing with different agents and replenishing MnOx-CeO2 components. The optimal conditions for catalyst cleaning were 1.5 mol/L H2SO4 pickling at 30 °C for 2 h. The doped SO42− could interact with catalyst ingredients, generating sulfate substances that partly transformed into various active sulfur-bearing species. These species enriched the catalyst surface and led to the formation of new S-OH/S = O chemical bonds. Loading MnOx-CeO2 further improved the catalyst's redox capacity by facilitating electronic conversions and increasing chemisorbed oxygen Oα content. The regeneration process also enhanced catalyst surface acidity by generating new N = O and N-OH bonds (N = Mn, Ce), intensifying the adsorption and activation of gaseous NH3. The present work offers new insights into the regeneration of MnOx-CeO2/Ti-bearing blast furnace slag catalyst, particularly for their application in flue gas streams with high Pb content.

Rapid synthesis of in situ nitrogen-doped ZnO nanoparticles for visible-light-driven photocatalytic removal of emerging contaminants

The effective functionalization of photocatalysts suitable for removing emerging contaminants poses a significant challenge. This study proposes a novel and rapid method for synthesizing in situ nitrogen-doped ZnO nanoparticles to address these limitations. Using a combustion synthesis scheme, we modified the urea-to-sucrose ratio and oxide precursors to characterize the catalysts systematically. The resulting material exhibited a remarkable 40% increase in visible-light-driven photocatalytic degradation of tetracycline hydrochloride (TC-HCl) over undoped ZnO. A maximum photocatalytic efficiency of 74.7% (60 min) was achieved by incorporating 50% urea in the fuel, which provided N–ZnO with a size of 120 nm, a surface area of 26 m2/g, mesoporosity, and a bandgap of 2.89 eV. With the rapid synthesis, this work surpassed the values reported in the literature. Moreover, this study presents a pathway for catalyst regeneration, underscoring the significance of the method and its potential in photocatalysis.

Regeneration Strategies for Ruthenium-Poisoned ORR Catalysts in Reformate PEM Fuel Cells

PEM fuel cells operating on reformate gases suffer from Ru crossover and its re-deposition in the cathode catalyst layer. Therefore, strategies for regeneration of Ru-poisoned ORR catalysts are needed. Here, we developed a regeneration protocol and tested this for commercial PtRu2/Vulcan, which represents a worst case scenario of Ru-poisoned ORR catalysts due to the high Ru content and good intermixture with Pt. Our protocol consists of two chronoamperometric cycles, namely at 1.4 VRHE for 5 minutes followed by 1.6 VRHE for 100 s. The Ru deletion as a function of the number of regeneration cycles was in-situ monitored by electrochemical CO stripping method. The mass and specific ORR activities of the recovered PtRu2/Vulcan increase by 6-times and 4.4-times, respectively. Very remarkably, our regeneration protocol aims to trigger the Ru dissolution by forming the RuO4 during the OER without a negative impact on the catalytic ORR properties of Pt.

Regeneration of a sulfur-poisoned selective catalytic reduction catalyst at ambient conditions

Despite the high sulfur resistance of vanadia-titania catalysts with physically mixed zeolite in the selective catalytic reduction of NOx (SCR), their regeneration is still challenging because it typically requires a large amount of energy or hazardous chemicals. This paper proposes an innovative regeneration strategy for prolonging the lifetime of the sulfur-poisoned catalyst. Our spectroscopic results revealed that the adsorbed water in the catalyst dissolves the deactivating material, ammonium bisulfate (ABS). We developed a regeneration protocol based on this finding, which was facilitated by dissolution of ABS in water adsorbed on the catalyst during the exposure to humid air at ambient temperature. The superior hydrophilic nature of zeolite resulted in a different water content between two physically mixed domains, promoting ABS migration from the vanadia active sites to zeolite and thus recovering the catalytic activity. This strategy is beneficial for the development of energy-efficient and environmentally friendly SCR systems.

In-situ oxidation of Sn catalysts for long-term electrochemical CO2 reduction to formate

Electrochemical carbon dioxide (CO2) reduction (ECR) into valuable products such as formate is a compelling approach to managing excessive CO2 emissions. Tin (Sn)-based catalysts have been used for selective CO2 reduction to formate. However, they are often unstable for long-term ECR operation, especially at high reaction rates. In this work, we report highly selective Sn-based catalysts for CO2 reduction to formate at high current densities. The catalysts were prepared by depositing a layer of Sn nanoparticles on the surface of sputtered Ag on a polymer membrane. The catalysts exhibited over 90% Faradaic efficiency toward formate in the current density range of 50–300 mA/cm2 and were stable for over 20 h at a current density of 100 mA/cm2. Characterization of catalysts after ECR reaction reveals significant changes in catalyst morphology during ECR. To extend ECR operation time, we developed an in-situ catalyst regeneration strategy in which the catalysts are periodically oxidized during the reduction reaction. Using this approach, the operation time of Sn-catalysts was extended to more than 150 h with a selectivity over 90%, at the current density of 100 mA/cm2.

Ni-Co catalyst-assisted carbon cycling for CH4-CO2 reforming

Carbon cycling, instead of conventional oxygen cycling in chemical looping, overcomes the limitations of oxygen storage capacity of oxygen carriers and enables the reaction to proceed under relatively mild conditions. In this study, we designed a high-performance carbon-storage catalyst (Ni-Co/Al2O3) for chemical looping conversion of CH4 and CO2 to H2 and CO. The physico-chemical characterizations suggested that the Co alloyed with Ni could result in larger active metal particles and simultaneously benefit the bending of carbon deposits toward the periphery of the active sites, avoiding the quick deactivation of catalysts due to the encapsulation of the active sites by carbon deposits and generating a higher carbon capacity (23.59 vs. 14.08 mol/kg). In addition, the alloying of Co with Ni could weaken the graphitization of carbon deposits, thus facilitating the carbon elimination and catalyst regeneration in the CO2 conversion step, strongly improving the cycling stability during long-term testing.

Carbon nitride with a tailored electronic structure toward peroxymonosulfate activation: A direct electron transfer mechanism for organic pollutant degradation

The persulfate-based nonradical reaction process has gained increasing attention in removing organic contaminants, and the process dramatically needs affordable, robust, high-performance heterogeneous catalysts. Herein, dimethyl formamide (DMF) incorporated graphitic carbon nitride (DCN) was synthesized. The molecular engineering of the heptazine unit in DCN improves the electron mobility, strengthens the built-in electric field, and provides abundant active sites, thus enabling DCN to efficiently activate PMS for organic degradation, with 100 % removal of BPA within 12 min. The investigation of the reactive species and electrochemical properties confirmed the direct electron transfer in the PMS activation for organic degradation over DCN. More importantly, DCN can be used as a membrane catalyst in a flowing catalytic unit, enabling continuous water purification and catalyst regeneration. This work provides novel guidance for metal-free catalyst design and profound insights into the persulfate-based heterogeneous catalytic oxidation process for water decontamination technology.

N-H Bond Activation Catalyzed by an Anderson-Type Polyoxometalate-Based Compound: Key Role of Transition-Metal Heteroatom.

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis to achieve N-H bond activation. Herein, the mechanism of the Anderson-type POM-based catalyst [FeIIIMoVI6O18{(OCH2)3CNH2}2]3- ([TrisFeIIIMoVI6O18]3-, Tris = {(OCH2)3CNH2}2) for the N-H bond activation of hydrazine (PhHNNHPh) was investigated by density functional theory calculations. The results reveal that [TrisFeIIIMoVI6O18]3- as the active species is responsible for the continuous abstraction of two electrons and two protons of PhHNNHPh via a proton-coupled electron transfer pathway, resulting in the activation of two N-H bonds in PhHNNHPh and thus the product PhNNPh. H2O2 acts as an oxidant to regulate catalyst regeneration. Based on the proposed catalytic mechanism, the key role of the heteroatom FeIII in [TrisFeIIIMoVI6O18]3- was disclosed. The d-orbital of FeIII in [TrisFeIIIMoVI6O18]3- acts as an electron receptor to promote the electron transfer (ET) in the rate-determining step (RDS) of the catalytic cycle. The substitution of the heteroatom FeIII of [TrisFeIIIMoVI6O18]3- with CoIII, RuIII, or MnIII is expected to improve the catalytic activity for several reasons: (i) the unoccupied molecular orbitals of POM-based compounds containing CoIII or RuIII are low, which is beneficial for the ET of RDS; (ii) For N-H bond activation catalyzed by the MnIII-containing POM-based compound, the transition state of RDS is stable because the d-orbital of its active site is half-filled, which results in a low free-energy barrier.

Ruthenium-Catalyzed Selective α-Alkylation of β-Naphthols using Primary Alcohols: Elucidating the Influence of Base and Water.

Functionalized arenes and arenols have diverse applications in chemical synthesis and material chemistry. Selective functionalization of arenols is a topic of prime interest. In particular, direct alkylation of arenols using alcohols is a challenging task. In this report, a ruthenium pincer catalyzed direct α-alkylation of β-naphthol using primary alcohols as alkylating reagents is reported. Notably, aryl and heteroaryl methanols and linear and branched aliphatic alcohols underwent selective alkylation reactions, in which water is the only byproduct. Notably, catalytically derived α-alkyl-β-naphthol products displayed high absorbance, emissive properties, and quantum yields (up to 93.2 %). Dearomative bromination on α-alkyl-β-naphthol is demonstrated as a synthetic application. Mechanistic studies indicate that the reaction involves an aldehyde intermediate. DFT studies support this finding and further reveal that a stoichiometric amount of base is required to make the aldol condensation as well as elementary steps required for regeneration of catalytically active species. In situ-generated water molecule from the aldol condensation reaction plays an important role in the regeneration of an active catalyst.

Non-thermal plasma for catalyst regeneration: A review

An alternative technology to the thermal regeneration of coked catalysts is non-thermal plasma (NTP). The main advantage of plasma is that it allows activation of the molecules at low temperature and atmospheric pressure. The species formed under oxygen plasma are able to oxidize coke from various catalysts: zeolite, metal supported over oxide. In the present review catalysts regeneration are discussed with a strong focus on the effect of the nature and location of coke into the porosity of catalyst. Two families of catalysts that differ in pore size have been considered (i) zeolites possessing well-defined, nanometer sized pores (<1 nm) and (ii) metal/support with mesopores (2–50 nm) and macropores (>50 nm). Moreover, the interest of NTP assisted by temperature for the regeneration of a coked material is also shown. The review ends with some concluding remarks and outlook in the field of catalyst regeneration.

Catalytic ozone oxidation of m-cresol by spirulina biochar-modified perovskite: Catalyst deactivation and in situ regeneration

In this study, a LaFe0.26Mn0.74O3-δ perovskite catalyst modified using spirulina biochar (LFMO-xBiochar) was used to compare the effects of water flushing on the ozonation performance on a pilot scale. The results showed that timely water flushing operation could effectively extend the service life of LFMO-xBiochar and maintain the chemical oxygen demand (COD) removal rate above 60% for nearly 40 h during the reaction. Density functional theory (DFT) calculations showed that the protonated Fe, Mn, and oxygen vacancies on the catalyst surface were the main active sites. Combined with SEM/EDS mapping, BET, FTIR and XPS measurements, the results showed that organic pollutants and deposits hindered the ozone decomposition at the active sites on the catalyst surface, leading to its final deactivation. However, timely water flushing could effectively remove the organic pollutants and inorganic salts adsorbed onto the catalyst surface during the initial deactivation stage. Additionally, water flushing loosened or removed surface deposits, re-exposing the active sites and restoring some pores, thereby promoting the catalyst regeneration and prolonging its service life. This study provides new insights into the deactivation and regeneration of ozonation catalysts, which can support practical industrial applications of this process.

Erratum to: Deactivation and Regeneration of a Zeolite-Containing Cobalt Catalyst in a Fischer–Tropsch Synthesis Reactor

Erratum to: Deactivation and Regeneration of a Zeolite-Containing Cobalt Catalyst in a Fischer–Tropsch Synthesis Reactor

Ion Beam as an External and Dynamic Metal Reservoir to Induce Nanoparticle Exsolution in Oxides

A central theme in solid oxide fuel cells (SOFC) and electrolyzers (SOEC) is to design stable and catalytically active solid/gas interfaces toward desired reactions. A recent advance in this regard is to synthesize nanoparticles decorated electrodes in a process termed “exsolution”. Exsolution generates stable and catalytically active metal nanoparticles via phase precipitation out of a host oxide. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This strong metal-oxide interaction makes the exsolved nanoparticles more resistant against particle agglomeration as compared to the infiltrated ones. In addition, the exsolved particles also open up the possibility of regeneration of catalysts. To date, the concept of exsolution has been successfully applied to a number of applications including SOFC/SOEC, ceramic membrane reactors, chemical looping combustion, and (electro)catalysis.As the exsolution process starts with a solid-solution that contains the to-be-exsolved transition metal ions, this method is often limited by the intrinsic solubility limit of the host oxide. Here, we propose that ion beam irradiation to be a unique tool to promote exsolution as it can introduce defects and dopants into the host oxides and at concentrations higher than thermodynamic limits. Moreover, due to the surface sputtering effect, ion beam irradiation can modulate the surface morphology, creating novel nanostructures.To demonstrate this approach, we chose thin-film perovskite SrTi0.65Fe0.35O3 (STF) and fluorite Ce0.5Zr0.5O2 (CZO) as model systems, both of which are promising materials in SOFC and SOEC. We modulated metal exsolution in both materials with in-situ 10-150 keV Ni irradiation at 800 °C in vacuum. Ni irradiation on STF controllably changed the exsolved particle composition from unitary Fe to bimetallic Fe-Ni. Moreover, it also reduced the particle size down to sub-2 nm, which outperforms other tuning methods thus far in the literature. As a result, the irradiation-modified STF demonstrated superior catalytic activity toward room-temperature oxygen evolution reactions (OER) than the conventional thermally exsolved STF. Regarding CZO, Ni irradiation also decorated the surface with well-dispersed nanoparticles, leading to enhanced high-temperature H2O splitting kinetics. The effective size and composition control over exsolution highlights the utility of metal ion beams in promoting and tailoring exsolved nanocatalysts for a broad range of applications in clean energy and fuel conversion. Figure 1

Nitrogen–Fluorine co-doped TiO2/SiO2 nanoparticles for the photocatalytic degradation of acrylonitrile: Deactivation and regeneration

In this study, we investigated the deactivation kinetics and mechanism of N–F–TiO2/SiO2 nanopowder as a model photocatalyst for the purpose of facilitating the photocatalytic degradation of acrylonitrile (AN) in aqueous environment. Prior research has already displayed the proficient degradation of AN through the utilization of N–F–TiO2/SiO2 catalysts, revealing a degradation efficiency of 81.2% within a span of 6 min at an initial AN concentration of 10 mg/L. Multiple variables including the initial AN concentration, illumination intensity, and initial pH value were extensively analyzed during the degradation process. The kinetics of photocatalytic degradation of AN, facilitated by the N–F–TiO2/SiO2 photocatalyst, were modeled by fitting the pseudo first-order reaction kinetics to each individual factor. Furthermore, the adverse effect of catalyst poisoning during the photocatalytic breakdown of AN using the N–F–TiO2/SiO2 photocatalyst was analyzed through a range of different techniques including SEM, XPS, BET, XRD, TG, and NH3-TPD. The incorporation of findings from these diverse techniques revealed that, the primary factors contributing to the photocatalyst's poisoning were as follows: (i) During the degradation process, the build-up of intermediate molecules on active sites hindered their functionality, leading to a decrease in the efficiency of the photocatalytic reaction, (ii) Carbonaceous deposits formed when the catalyst's pore structure was obstructed by pollutants or intermediate products that had not undergone timely photocatalytic breakdown and (iii) The persistent erosion of active sites due to hydraulic forces resulted in inadequate performance of the N–F–TiO2/SiO2 photocatalyst in aqueous solutions. A comprehensive analysis of the deactivation kinetics was conducted, deciding in the formulation of a detailed poisoning mechanism for the N–F–TiO2/SiO2 photocatalyst. Additionally, we explored the catalysts regeneration, involving thermal treatment, ultrasonic irradiation, and catalyst reloading. This study not only advances our insight into the waning performance of catalysts in aqueous media but also establishes a conceptual framework for extrapolating analogous deactivation dynamics in other catalysts, grounded in precedent experimental knowledge. This research contributes to the development of a deactivation model for catalysts in the aqueous environment, based on existing experimental research, providing a theoretical framework for understanding the deactivation process of photocatalysts.

Conversion of primary alcohol to ester in presence of Ruthenium(II)-PNP pincer complex and comparison with isoelectronic (PNP)Os and (PNP)Rh+ complexes: A computational study

Mechanistic pathway of (PNP)Ru {PNP = bis[(2-dimethylphosphino)-ethyl]amine} mediated synthesis of an ester (2-methoxyethyl-2-methoxyacetate) from two equivalent of a primary alcohol (3-oxabutanol) is investigated using density functional theory. A ten-stage catalytic cycle is proposed. The key steps of this process is broadly described as the oxidation of first equivalent of primary alcohol into aldehyde with the elimination of H2 molecule, coupling of the aldehyde with second equivalent of alcohol to form an ester followed by the regeneration of the pincer catalyst with the liberation of second H2 molecule. Two isoelectronic pincer complexes (PNP)Os and (PNP)Rh+ are also considered to shed light on the variation of the energetics. Prior to H2 elimination, a η2-bonded metal-dihydride complex is obtained for (PNP)Ru and (PNP)Rh+, whereas pure Os-H bonds are observed in case of (PNP)Os. Theoretical catalytic efficiency and TOF values infer that (PNP)Ru is the best amongst the three pincer complexes considered here.

Regeneration of Co-based bead type catalyst in ammonia borane hydrolysis for hydrogen generation: Artificial neural networks and response surface methodology

The usage of ammonia borane (NH3BH3, AB) as a hydrogen storage medium and the release of hydrogen from its structure via hydrolysis has gained attention as for the hydrogen energy systems. Cobalt (Co)-based catalysts are the one of the activist material that can effectively catalyze the hydrogen production at current applications. However, Co-based catalysts have gradually lost their initial activities in long-term reactions due to by-product deposition on their surface. To address this issue, a catalyst regeneration strategy based on solvent washing to re-activation of Co-based catalysts with help of new perspective is presented in this study. Regeneration parameters as solution properties (pH:3–7), temperature (15–75 °C), and duration (10–50 min) have been modeled by both Response Surface Methodology (RSM) and Artificial Neural Network (ANN) techniques with different levels to optimize the process conditions for the regeneration process and to regain the high initial catalytic activity. The results have showed that solvent washing at optimization conditions was the effective method for restoring the activity of the catalyst. The findings indicate that time and temperature have a significant impact on the regeneration of the Co-based catalyst. The characterization techniques such as XRD, FTIR, HR-TEM, SEM/EDS, and XPS were performed to illuminate and correlate the effect of regeneration on the internal and external properties of fresh, used and regenerated catalysts. The optimum regeneration conditions, the pH of 6.5, the temperature of 44 °C, and the duration of 42 min under the optimal conditions identified by RSM, provided the highest hydrogen production (3.23 l min−1.g−1), showed much closer results to ANN technique predicted value (3.45 l min−1.g−1). It is shown that ANN has a relatively higher prediction accuracy and optimization ability compared to RSM for hydrogen production rate.