Surface Poisoning Research Articles

Catalytic hydrodehalogenation of iodinated contrast agents in drinking water: A kinetic study

The occurrence of iodinated contrast media (ICM) in water bodies represents a significant threat for public health due to their strong biochemical stability and recalcitrance to conventional water treatment systems. In this work, the feasibility of catalytic hydrodehalogenation (HDH) for the degradation of a representative group of ICMs (iohexol (IHX), iopromide (IPR), iopamidol (IPM) and diatrizoic acid (DIA)) is investigated. Complete degradation and deiodination of the isolated ICMs (1.6·10−2 mmol L−1) was achieved in less than 30 min in the presence of 0.25 g L−1 of different precious metal catalysts (Pd/Al2O3, Rh/Al2O3 and Pt/Al2O3) under ambient conditions, closing iodine balance by >95 %. Based on the identified reaction intermediates and final products, reaction mechanisms were accordingly proposed. A sequential reaction pathway was proposed for DIA removal, while reaction mechanisms encompassing both sequential and concerted reaction steps were proposed for the non-ionic ICMs (IHX, IPR, and IPM). Subsequent kinetic models, which successfully described experimental data, were developed from the elucidated reaction mechanisms, considering all species involved in the reaction. The versatility of the process was finally demonstrated in the treatment of real aqueous matrices (surface water and tap water) spiked with the ICMs mixture, except in the case of Pt/Al2O3 which suffered deactivation due to surface poisoning by halides. For this catalyst, kinetic constants ranged from 0.31 to 0.22 min−1 in deionized water, and from 0.04 to 0.08 min−1 in surface water as the reaction matrix. Among the tested catalysts, Pd/Al2O3 showed the highest activity and stability for the removal of ICMs.

Physical insight into the enhanced urea electrooxidation using Ni and Fe-based LDH, LDO, and hydroxides under different dissolved gas saturation conditions in electrolyte

The production of green hydrogen is one of the most demanding and challenging for modern technology. A promising and an effective approach is electrocatalytic anodic urea oxidation reaction to generate hydrogen at cathode under alkaline electrolysis condition. However, it remains crucial for the development of active and stable electrocatalysts for efficient urea oxidation. In this study, we employed the hydrothermal synthesis route for NiFe LDH, NiFe LDO, and Ni(OH)2. The superior performance of NiFe LDH compared to other catalysts is observed due to easy charge transfer facilitated by Fe ions. Moreover, Fe incorporation prevents surface poisoning of the electrocatalyst, resulting in increased activity and stability for electrocatalytic urea oxidation reaction. The influence of different electrolyte environments on the performance of urea-based electrolyzers for sustainable hydrogen production and management of urea-rich wastewater has been measured for the first time by varying oxygen saturation and nitrogen purging conditions. In cyclic voltammetry studies, O2-saturated and purging conditions outperformed N2 gas saturation or purging. The findings of this study provide valuable insight into the design of practical and environmentally friendly urea electrooxidation systems.

Selective electroreduction of CO2 to C2+ products on cobalt decorated copper catalysts

Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) to multi-carbon (C2+) products is often plagued by low selectivity because the adsorption energies of different reaction intermediates are in a linear scaling relationship. Development of Cu-based bimetallic catalysts has been considered as an attractive strategy to address this issue; however, conventional bimetallic catalysts often avoid metals with strong CO adsorption energies to prevent surface poisoning. Herein, we demonstrated that limiting the amount of Co in CuCo bimetallic catalysts can enhance C2+ product selectivity. Specifically, we synthesized a series of CuCox catalysts with trace amounts of Co (0.07-1.8 at%) decorated on the surface of Cu nanowires using a simple dip coating method. Our results revealed a volcano-shaped correlation between Co loading and C2+ selectivity, with the CuCo0.4% catalyst exhibiting a 2-fold increase in C2+ selectivity compared to the Cu nanowire sample. In situ Raman and Infrared spectroscopies suggested that an optimal amount of Co could stabilize the Cu oxide/hydroxide species under the CO2RR condition and promote the adsorption of CO, thus enhancing the C2+ selectivity. This work expands the potential for developing Cu-based bimetallic catalysts for CO2RR.

Titanium oxynitride thin films by the reactive sputtering process with an independent pulsing of O2 and N2 gases

Titanium oxynitride thin films are deposited by DC reactive magnetron sputtering. A pure titanium target is sputtered in a reactive atmosphere composed of argon, oxygen and nitrogen gases. The oxygen mass flow rate as well as that of the nitrogen gas are both pulsed during the deposition time using an independent and rectangular signal for each reactive gas. A constant pulsing period T = 45 s is applied for both reactive gases and a delay time δ of 34 s between N2 and O2 injection times is set for all depositions. Oxygen and nitrogen duty cycles are systematically and independently changed from 0 to 100 % of their pulsing period. From real time measurements of the Ti target potential and total sputtering pressure, it is shown that the reactive process alternates between oxidized, nitrided and elemental sputtering modes as a function of the oxygen and nitrogen injection times. The full poisoning of the Ti target surface by oxygen and/or nitrogen can be avoided for some given ranges of O2 and N2 duty cycles. Deposition rates of titanium oxynitride films are substantially enhanced and can be adjusted between that of pure Ti and TiN films with a gradual transition of their electrical conductivity and optical transmittance in the visible range. These results support that titanium oxynitride compounds exhibiting absorbent to transparent behaviors can be precisely sputter-deposited by means of a two reactive gases pulsing process.

Plume mode instability enhanced by emitter surface poisoning in hollow cathode

The unstable plume mode of hollow cathodes should be avoided in practical applications because it severely degrades the overall cathode lifetime. In this study, we investigate the spot-plume transition and plasma stability characteristics of an unused segmented lanthanum hexaboride emitter. The expansion of the unstable plume mode region is observed during a discharge experiment. Subsequently, the segmented emitter is retrieved, the inner surface of the emitter is observed, and the work function on the surface is measured at room temperature. The emitter surface exhibits color variations with oxygen and carbon detection. The downstream edge shows the original purple color and almost no degradation in the work function. The high temperature in this region promotes the desorption of carbon and oxygen. In the spot mode, this region mainly contributes to thermionic electron emission; therefore, the discharge voltage in the spot mode does not change during the discharge experiment. Carbon or carbide is detected in the middle of the axial direction on the emitter surface, where the surface temperature is not sufficiently high to desorb carbon during discharge. Based on the surface analysis results, the dominant substance in the region where carbon is detected was lanthanum carbide. An increase in the work function is indicated in the region, which appears to increase the plasma instability. According to previous studies, an increase in the work function results in a rise in the potential in the emitter, and an increase in the electron temperature in the outside plume region induces the plasma instability. Further investigation is needed to understand the mechanism connecting the rise in the work function and the rise in the electron temperature in the plume region.

Unraveling the impact of temperature on the reaction kinetics of the electro-oxidation of methanol on Pt(1 0 0)

Methanol is one of the key molecules in the challenge towards a sustainable future, particularly as a renewable hydrogen carrier fuel and as a low-carbon and net carbon-neutral liquid chemical. For most applications, it is imperative to understand the impact of temperature on the methanol electro-oxidation reaction (MEOR). In this study, the influence of the temperature on the kinetics of the MEOR and the parallel reaction pathways is assessed by investigating responses in both conventional and oscillatory regimes using a single-crystal Pt(100) electrode. Our findings demonstrate that chronoamperometric measurements under steady-state conditions provide more reliable values for apparent activation energies compared to transient conditions. Furthermore, a temperature-dependent shift in the dominance of specific oxidation pathways is observed, analogous to a kinetic and thermodynamic control mechanism, preventing the complete poisoning of the electrode surface. Specifically, oxidation pathways leading to the formation of reaction byproducts predominate at lower temperatures, while the oxidation pathway via COad becomes dominant at temperatures exceeding 30 °C. Moreover, our research shows that, at shorter times, temperature changes minimally affect the mean potential required to sustain the applied current during the oscillations in a galvanostatic experiment, which is closely linked with the voltaic efficiency. However, over longer periods, when mass transport phenomena become significant and mixed-mode oscillations occur, elevated temperatures increase the mean potential, resulting in reduced voltaic efficiency. Therefore, to facilitate the complete conversion of methanol to CO2 without increasing the mean potential for current maintenance, it is essential not only to increase the temperature but also to improve the mass transport conditions to mitigate the mixed-mode oscillations, despite their lower minima reached during oscillation. This idea challenges the conventional assumption that a lower minimum potential implies a lower mean potential during oscillations. This advancement propels our understanding to a more sophisticated level, providing valuable insights to guide the materials design to increase the conversion efficiency and optimize the operating temperature of devices crucial to energy conversion.

Light‐off Performance of Three‐Way Catalysts Before and After Accelerated Aging Test with an Engine‐Dynamometer

AbstractThis study demonstrates the reaction behavior during the purification of model automotive exhaust gases over Pd catalysts before and after thermal degradation. In particular, to investigate the relationship between the Pd state and the reaction behavior of Pd/Al2O3 and Pd/CeO2−ZrO2 (CZ), operando X‐ray absorption spectroscopy measurements were performed during purifying exhaust gases over real and model catalysts mimicking the degradation of Pd particles and CZ supports after accelerated aging tests. The NO reduction activity of the aggregated Pd metal species was as high as that of the highly dispersed Pd species, but hydrocarbon (HC) poisoning was significantly enhanced by the aggregation of Pd metal particles caused by thermal aging. The existence of a three‐phase boundary (TPB) between the CZ, the Pd particles, and the gas phase strongly affected the catalytic activity at low temperatures, and the presence of a sufficient TPB facilitated the combustion of unburned HCs owing to the oxygen storage performance of CZ. Thus, the TPB reduced the poisoning of the precious metal surface by HC species at low temperatures. Therefore, the findings of this study will facilitate the development of next‐generation gas purification catalysts with high activity and durability.

General impurity gas blanket effect mechanism and elimination strategies for hydrogen storage materials

Hydrogen energy has attracted considerable attention as the promising alternative to fossil energy. However, pollutants in H2 inevitably affect the hydrogenation kinetics of hydrogen storage materials (HSMs). In this work, the general physical blanket effect mechanism of impurity gases (He/Ar/CH4/N2/CO2/NO2) on the hydrogenation kinetics is extracted and confirmed to be a single physical process unrelated to HSMs for the first time. Specifically, the selective absorption of H2 by HSMs induces the local enrichment of impurity gases and impede H2 diffusion physically. For example, ZrCo can achieve its theoretical capacity (2.0 wt% H2) in 1.2 bar pure H2 within 40 s, while only 0.46 wt% H2 could be absorbed in 1.2 bar 3 mol% CH4 + 97 mol% H2 mixed gas even after 0.4 h. Such blanket effect may interlace with chemical surface poisoning effect and bring variability on hydrogenation kinetics. Notably, among the investigated impurity gases, He/Ar/N2/CH4 are absolute inactive gases which cannot impede hydrogenation, thus excluding the chemical surface poisoning effect originated from active gases. This allows the precise extraction of intrinsic physical blanket effect quantitatively, instructing comprehensive strategies to promote the fast hydrogenation saturation for HSMs. In summary, optimizing engineering parameters including dynamical hydrogenation operation provides an effective approach to remarkably promote hydrogen absorption kinetics of HSMs.

Theoretical study on the surface poisoning of high-entropy alloys during hydrogen storage cycles: the effect of metal elements and phases.

High-entropy alloys offer promising hydrogen storage properties and design versatility but suffer from compromised capacity and stability in practical industrial applications owing to surface poisoning caused by trace impurities or unexpected contact with air. Theoretical simulations provide a rapid and efficient platform for estimating anti-poisoning performance, particularly concerning alloys versus metal elements in various phases. This work explores the surface poisoning behavior of two typical high entropy materials: BCC-phase V35Ti30Cr25Fe10 and Laves-phase ZrTiVNiCrFe, along with pure metals V, Ti, Cr, and Fe as well as single AB2 (A = Zr, Ti, B = V, Ni, Cr, and Fe) compounds, at various phase stages during hydrogen storage cycles using density functional theory (DFT) simulations. Results show that surfaces of V35Ti30Cr25Fe10 and ZrTiVNiCrFe with a hydrogen uptake of 100% can facilitate O2 adsorption over dissociation, especially when O2 adsorbs on Fe sites, and formation of hydroxyl. The O2 poisoning behavior of high-entropy alloys was roughly estimated using the molar ratio weighted sum of constituent components, with the maximum deviation of 15.92% between predicted values and calculated values. This study sheds light on anti-poisoning mechanisms and aids in designing resilient high-entropy alloys.

Doping Graphene By Physisorption to Influence Electrochemical Activity and Probing Local Electrochemistry on 10 Micron Spots

Graphene as an electrochemical electrode for energy devices and catalytic activity is a catch 22 situation. Pristine graphene is highly conducting with exquisite chemical inertness to avoid poisoning of the electrode surface. However, this inertness leads to poor chemisorption which is generally considered a prerequisite for electron transfer during redox. Thus, pristine graphene is a poor electrochemical electrode. The challenge is being addressed by distorting the perfect sp2 structure by interfacing with other 2D materials to form (complex) heterostructures, and/or doping other atoms or punching holes (i.e., vacancies). However, chemical stability, i.e., inertness, becomes vulnerable at the defects. Here we propose a concept of “doping” where the graphene becomes electrochemically active, and the pristineness is maintained. The dopant is physiosorbed to the graphene. The outcome will be a redox active graphene electrode. DFT simulations are conducted to explain the structure.The electrochemical behavior will be studied by a special opto-electrochemical method called the Scanning electrometer for Electrical double-layer (SEED). SEED quantitively measures local electrochemistry on a 10-micron spot to avoid edge effect and (potential) cracks in graphene. Figure 1

Electrochemical Valorization of Glycerol: Catalyst Development and Product Analysis

The growing interest in biofuels as a substitute for traditional fossil fuels has raised environmental concerns about the extensive production of their by-products. Among these by-product, Glycerol, which accounts for over 10% of the total by-products, resulted in the production of nearly 4 billion liters in 2020. (1) Glycerol is a polyol organic molecule, viscous, and water-soluble liquid that if not disposed of properly, it can have detrimental impacts on the environment, including soil and water pollution, ultimately contributing to an increase in the carbon footprint. Hence, addressing this growing concern has prompted a surge of interest in developing sustainable and economically viable techniques for converting glycerol into value-added products.The electrochemical glycerol oxidation reaction (GOR) is a cost-effective and reliable method to enable circular economy practices by selectively producing highly value-added products such as dihydroxyacetone, glyceric acid, and glycolic acid from glycerol. However, it requires the development of active, selective, and stable electrocatalysts to steer GOR at low overpotentials. To date, catalysts based on platinum group metals (PGMs) have outperformed other GOR catalysts in terms of activity. However, in addition to the high cost of fabrication, these catalysts are susceptible to surface poisoning by glycerol intermediates, thereby impeding their commercialization due to the absence of long-term stability. (2) As a result, developing electrocatalysts based on earth-abundant element has become the focal point in GOR catalyst research.Here in, we highlight the latest advancements in the development of low-PGM content GOR catalysts. Our discussion focuses on the strategies for understanding how the type and concentration of PGM can influence selectivity, with a particular emphasis on the significance of GOR overpotentials. We also aim to highlight the inconsistencies in GOR product analyses, specifically regarding the use of H-NMR analysis, with an ultimate goal of advancing accurate GOR product quantification. References Nomanbhay S, Hussein R, Ong MY. Sustainability of biodiesel production in Malaysia by production of bio-oil from crude glycerol using microwave pyrolysis: a review. Green Chemistry Letters and Reviews. 2018;11(2):135-57. Li T, Harrington DA. An Overview of Glycerol Electrooxidation Mechanisms on Pt, Pd and Au. ChemSusChem. 2021;14(6):1472-95.

Ultrathin covalent organic overlayers on metal nanocrystals for highly selective plasmonic photocatalysis

Metal nanoparticle-organic interfaces are common but remain elusive for controlling reactions due to the complex interactions of randomly formed ligand-layers. This paper presents an approach for enhancing the selectivity of catalytic reactions by constructing a skin-like few-nanometre ultrathin crystalline porous covalent organic overlayer on a plasmonic nanoparticle surface. This organic overlayer features a highly ordered layout of pore openings that facilitates molecule entry without any surface poisoning effects and simultaneously endows favourable electronic effects to control molecular adsorption–desorption. Conformal organic overlayers are synthesised through the plasmonic oxidative activation and intermolecular covalent crosslinking of molecular units. We develop a light-operated multicomponent interfaced plasmonic catalytic platform comprising Pd-modified gold nanoparticles inside hollow silica to achieve the highly efficient and selective semihydrogenation of alkynes. This approach demonstrates a way to control molecular adsorption behaviours on metal surfaces, breaking the linear scaling relationship and simultaneously enhancing activity and selectivity.

Dual function of H2O on interfacial intermediate conversion and surface poisoning regulation in simultaneous photodegradation of NO and toluene

Co-existing air pollutants, especially NOx and VOCs, will generate secondary photochemical pollution under light irradiation. However, simultaneous elimination of multi-pollutants has long been a challenge. Photocatalysis could turn the reaction pathway between pollutants to convert them into harmless products, which is a promising technology for multi-pollutant control. Here we achieved synergistic photocatalytic degradation of NO and C7H8 on InOOH photocatalyst, and the performance can be adjusted by H2O through affecting the interaction between surface species and catalyst. In situ DRIFTS and GC-MS revealed that the improved efficiency originated from the fast conversion of C–N coupling intermediates led by additional H2O. Surface characterizations and DFT simulation determined that accumulated nitrates will compete with the adsorption of NO and C7H8, resulting in a decline in efficiency in the later stage. Although improved efficiency would bring more nitrates, as H2O has comparable adsorption to nitrate at the same site, high humidity can mitigate the deactivation. The photocatalyst can be also simply regenerated by water washing. This work reveals the complex interaction in the multi-pollutant system and provides guidelines for precisely regulating the synergistic removal of NOx and VOCs.

Reductive Transformation of O-, N-, S-Containing Aromatic Compounds under Hydrogen Transfer Conditions: Effect of the Process on the Ni-Based Catalyst.

The influence of the reaction medium on the surface structure and properties of a Ni-based catalyst used for the reductive transformations of O-, N-, and S-containing aromatic substrates under hydrogen transfer conditions has been studied. The catalysts were characterized by XRD, XPS, and IR spectroscopy and TEM methods before and after the reductive reaction. It has been shown that the conversion of 1-benzothiophene causes irreversible poisoning of the catalyst surface with the formation of the Ni2S3 phase, whereas the conversion of naphthalene, 1-benzofuran, and indole does not cause any phase change of the catalyst at 250 °C. However, after the indole conversion, the catalyst surface remains enriched with N-containing compounds, which are evenly distributed over the surface.

A platinum ensemble catalyst for room-temperature removal of formaldehyde in the air

Minimal use of noble metals is ideal in developing catalytic systems against carcinogenic formaldehyde (FA) in air. Although single-atom catalysts (SACs) have been proposed to maximize atomic utilization, metals dispersed to the single-atom limit are less durable in redox environments. A highly dispersed platinum (Pt) ensemble (Ptn) on titanium dioxide (TiO2) was synthesized and validated to achieve 100% conversion of 100 ppm FA in dry air at room temperature (RT) at a gas hourly space velocity of 47,771 h−1. In contrast, Pt SAC (Pt1/TiO2) and a reference Pt nanoparticle catalyst (PtNP/TiO2) exhibited much lower performances. The turnover frequencies (TOFs) of Ptn/TiO2, Pt1/TiO2, and PtNP/TiO2 for the RT FA oxidation reaction were 0.03, 0.01, and 0.005 s−1, respectively. The critical role of the surface lattice oxygen (Olatt) in the overall reaction was supported by the prominence of the Mars van Krevelen kinetics in FA oxidation by the Ptn catalyst. The performance of the PtNP catalyst matched with Ptn only when the Pt loading in the former was raised to 2 wt%. Hence, the Pt dose can be reduced by one-fourth through the ensemble form dispersed at the sub-nanometer scale. The density functional theory simulation also distinguished the roles of different Pt catalysts. The Ptn sites could serve as an oxygen reservoir (effective dissociation of molecular oxygen) to promote proximate reactions (between the adsorbed –CHO and surface Olatt species). Conversely, Pt1 is a single site that restricts proximate reactions with vulnerability to surface poisoning.

Effective Suppression of LSCF Air Electrode Degradation by Air Cleaning

The performance degradation of air electrodes in solid oxide cells is strongly affected by impurities such as sulfur dioxide. Air filters are applied in most fuel cell systems to avoid poisoning and subsequent degradation of fuel cell stacks [1, 2]. Filtering solutions designed for the high air purity demands of proton-exchange membrane fuel cells (PEMFC) are commercially available.This study evaluates the applicability of such a filter for the LSCF air electrode in an electrolyte-supported SOC. Three different air qualities were applied in single-cell tests to distinguish the degradation mechanism caused by the ambient air impurities from the intrinsic aging phenomena of the cell. The utilized gases are (i) compressed air, (ii) filtered compressed air, and (iii) synthetic air. The compressed air (i) consists of ambient air supplied by a conventional compressor with a downstream particle filtering and oil removal system. In (ii), an additional filter layer was applied between the mass flow controller and the cell housing to filter the compressed air. The filter consisted of specially modified activated carbons designed to remove harmful gases and was operated at ambient temperature and pressure. The active area of the filter was 0.95 cm², corresponding to an air velocity of 0.088 m s-1 at an airflow rate of 500 sccm. The synthetic air is a mixture of 21% oxygen (99.95% O2, the main impurities are H2O and Ar) and 79% nitrogen (99.999% N2) provided by the Air Liquide company. Symmetrical cells based on a 3YSZ substrate coated with GDC buffer layers and LSCF electrode applied on both sides with an active surface area of 1 cm2 were applied in this study.Figure 1 shows the polarization resistance of the LSCF electrode operated in different types of air. Similar to previous reports [3-5], a significant degradation rate is recorded within the first 300 hours of operation. Filtering the air reduces the degradation rate significantly. The performance degradation of the cell with filtered air is comparable to that of the cell with synthetic air, demonstrating the capability of the filter to adsorb the gas impurities. Fig. 1 polarization resistance of the LSCF-cathode of 3 symmetrical cells operated at 750 °C with unfiltered and filtered compressed air and synthetic air, respectively.To better understand the degradation mechanism in the LSCF air electrode, we analyzed the impedance spectra measured during aging tests using the distribution of relaxation times (DRT) and then a complex nonlinear least squares (CNLS) fit. This contribution discusses differences in the temporal development of air electrode impedance and degradation mechanisms related to surface poisoning and subsequent changes in LSCF bulk.Keywords: LSCF air electrode, Impedance spectra, Degradation mechanisms, Air filtration.[1] M. Harenbrock, A. Korn, A. Weber, E. Hallbauer, in, SAE Technical Paper, 2020.[2] M. Harenbrock, E. Hallbauer, T. Heilmann, M. Hanselmann, ATZheavy duty, 14 (2021) 34-37.[3] C. Endler, A. Leonide, A. Weber, F. Tietz, E. Ivers-Tiffée, Journal of the Electrochemical Society, 157 (2010) B292-B298.[4] C. Endler-Schuck, A. Leonide, A. Weber, S. Uhlenbruck, F. Tietz, E. Ivers-Tiffée, Journal of Power Sources, 196 (2011) 7257-7262.[5] C. Endler-Schuck, J. Joos, C. Niedrig, A. Weber, E. Ivers-Tiffée, Solid State Ionics, 269 (2015) 67-79. Figure 1

Solvation of furfural at metal-water interfaces: Implications for aqueous phase hydrogenation reactions.

Metal-water interfaces are central to understanding aqueous-phase heterogeneous catalytic processes. However, the explicit modeling of the interface is still challenging as it necessitates extensive sampling of the interfaces' degrees of freedom. Herein, we use abinitio molecular dynamics (AIMD) simulations to study the adsorption of furfural, a platform biomass chemical on several catalytically relevant metal-water interfaces (Pt, Rh, Pd, Cu, and Au) at low coverages. We find that furfural adsorption is destabilized on all the metal-water interfaces compared to the metal-gas interfaces considered in this work. This destabilization is a result of the energetic penalty associated with the displacement of water molecules near the surface upon adsorption of furfural, further evidenced by a linear correlation between solvation energy and the change in surface water coverage. To predict solvation energies without the need for computationally expensive AIMD simulations, we demonstrate OH binding energy as a good descriptor to estimate the solvation energies of furfural. Using microkinetic modeling, we further explain the origin of the activity for furfural hydrogenation on intrinsically strong-binding metals under aqueous conditions, i.e., the endothermic solvation energies for furfural adsorption prevent surface poisoning. Our work sheds light on the development of active aqueous-phase catalytic systems via rationally tuning the solvation energies of reaction intermediates.

Band-gap engineering of zirconia by nitrogen doping in reactive HiPIMS: a step forward in developing innovative technologies for photocatalysts synthesis.

In the global context of climate change and carbon neutrality, this work proposes a strategy to improve the light absorption of photocatalytic water-splitting materials into the visible spectrum by anion doping. In this framework, reactive high power impulse magnetron sputtering (HiPIMS) of a pure Zr target in Ar/N2/O2 gas mixture was used for the deposition of crystalline zirconium oxynitride (ZrO2-xNx) thin films with variable nitrogen doping concentration and energy band-gap. The nitrogen content into these films was controlled by the discharge pulsing frequency, which controls the target surface poisoning and peak discharge current. The role of the nitrogen doping on the optical, structural, and photocatalytic properties of ZrO2-xNx films was investigated. UV-Vis-NIR spectroscopy was employed to investigate the optical properties and to assess the energy band-gap. Surface chemical analysis was performed using X-ray photoelectron spectroscopy, while structural analysis was carried out by X-ray diffraction. The increase in the pulse repetition frequency determined a build-up in the nitrogen content of the deposited ZrO2-xNx thin films from ∼10 to ∼25at.%. This leads to a narrowing of the optical band-gap energy from 3.43 to 2.20eV and endorses efficient absorption of visible light. Owing to its narrow bandgap, ZrO2-xNx thin films obtained by reactive HiPIMS can be used as visible light-driven photocatalyst. For the selected processing conditions (pulsing configuration and gas composition), it was found that reactive HiPIMS can suppress the hysteresis effect for a wide range of frequencies, leading to a stable deposition process with a smooth transition from compound to metal-sputtering mode.

Catalytic dechlorination of 1,2-DCA in nano Cu0-borohydride system: effects of Cu0/Cun+ ratio, surface poisoning, and regeneration of Cu0 sites

Aqueous-phase catalyzed reduction of organic contaminants via zerovalent copper nanoparticles (nCu0), coupled with borohydride (hydrogen donor), has shown promising results. So far, the research on nCu0 as a remedial treatment has focused mainly on contaminant removal efficiencies and degradation mechanisms. Our study has examined the effects of Cu0/Cun+ ratio, surface poisoning (presence of chloride, sulfides, humic acid (HA)), and regeneration of Cu0 sites on catalytic dechlorination of aqueous-phase 1,2-dichloroethane (1,2-DCA) via nCu0-borohydride. Scanning electron microscopy confirmed the nano size and quasi-spherical shape of nCu0 particles. X-ray diffraction confirmed the presence of Cu0 and Cu2O and x-ray photoelectron spectroscopy also provided the Cu0/Cun+ ratios. Reactivity experiments showed that nCu0 was incapable of utilizing H2 from borohydride left over during nCu0 synthesis and, hence, additional borohydride was essential for 1,2-DCA dechlorination. Washing the nCu0 particles improved their Cu0/Cun+ ratio (1.27) and 92% 1,2-DCA was removed in 7 h with kobs = 0.345 h−1 as compared to only 44% by unwashed nCu0 (0.158 h−1) with Cu0/Cun+ ratio of 0.59, in the presence of borohydride. The presence of chloride (1000–2000 mg L−1), sulfides (0.4–4 mg L−1), and HA (10–30 mg L−1) suppressed 1,2-DCA dechlorination; which was improved by additional borohydride probably via regeneration of Cu0 sites. Coating the particles decreased their catalytic dechlorination efficiency. 85–90% of the removed 1,2-DCA was recovered as chloride. Chloroethane and ethane were main dechlorination products indicating hydrogenolysis as the major pathway. Our results imply that synthesis parameters and groundwater solutes control nCu0 catalytic activity by altering its physico-chemical properties. Thus, these factors should be considered to develop an efficient remedial design for practical applications of nCu0-borohydride.

Durable N-doped carbon electrocatalysts derived from NH3-activated coffee waste for the oxygen reduction reaction

Polymer electrolyte membrane fuel cells typically use expensive electrocatalysts comprising Pt-group metals (PGMs), particularly for the oxygen reduction reaction (ORR). However, PGMs can undergo surface poisoning resulting in severe performance deactivation. Therefore, metal-free electrocatalysts have been investigated as alternatives. In this study, N-doped carbon electrocatalysts for the ORR were derived from coffee waste (CW), which originates from carbonaceous natural resources. The electrocatalysts by CW recycling and simple thermal activation under NH3 improve the electrocatalytic activity toward ORR and exhibit excellent durability in 40,000 cycles of accelerated stability tests. Although further investigation is required to optimize the electrochemical performance and gain deeper insights into the mechanistic roles of active sites, this simple strategy can promote cost-effective waste recycling and valorization for carbon neutrality.