Catalytic Splitting Research Articles

Investigating the Role of Mixed Oxides for Oxygen Evolution Reaction on Iridium Oxide Electrocatalysts

Ongoing concerns over anthropogenic emissions, fossil fuel sustainability, and related geopolitical aspects point to an increasing need for cost-effective renewable fuel sources. A promising solution is the generation of hydrogen via the catalytic splitting of water, which can be achieved with the aid of proton exchange membrane (PEM) electrolyzers. Notably, the anode material in current commercially available PEM electrolyzers is typically iridium oxide. Despite its effectiveness in facilitating the oxygen evolution reaction (OER), the use of iridium oxide poses challenges due to the scarcity and high cost associated with this precious metal. As the quest for sustainable energy intensifies, ongoing research explores innovative approaches, such as the incorporation of alternative materials and catalysts, to enhance efficiency while addressing the economic and environmental concerns associated with iridium oxide. Studies have shown that incorporating mixed oxides and switching the supports for iridium oxide catalysts help lower the iridium loading while enhancing the OER performance.1,2 In this study, we investigate a series of different sputtered, mixed iridium oxides (Ta, Ti, Ir). We use a rotating disk electrode system to benchmark the catalytic activity of different loadings of mixed oxides. To measure the intrinsic activity of each catalytic active site, we use a home-built operando UV-Vis system to quantify the populations of surface species as well as track the kinetics of different redox transitions during OER. We incorporate nanoscale secondary ion mass spectrometry to probe the migration of different mixed oxide species before and after electrochemical cycling and use kelvin probe force microscopy to estimate the conductivity through the different films. In this study, we show the importance of cooperative effects between neighboring iridium oxide species and the coordination of iridium that are responsible for enhanced OER performance. These results indicate that doping modulates coverage dependent interactions at the surface, which affects both the binding energetics and populations of rate limiting intermediates. We believe this study will serve as a steppingstone for future designs of more cost-efficient PEM electrolyzers.(1) Oh, Hyung-Suk, et al. "Electrochemical catalyst–support effects and their stabilizing role for IrO x nanoparticle catalysts during the oxygen evolution reaction." Journal of the American Chemical Society 138.38 (2016): 12552-12563.(2) Zheng, Ya-Rong, et al. "Monitoring oxygen production on mass-selected iridium–tantalum oxide electrocatalysts." Nature Energy 7.1 (2022): 55-64.

Catalytic performance of metal chloride for dehydrochlorination of trichloroethane

Catalytic 1,1,2-TCE splitting was an endothermic step targeted to production of VDC, cis-1,2-DCE or trans-1,2-DCE. Nevertheless, how to optimize active sites on carriers to improve activity was a key challenge. In this study, the catalytic splitting experiments of 1,1,2-TCE with metal chloride as catalysts were carried out at 260℃, 101 kPa. The results showed that CsCl with 3% molar fraction had the best catalytic effect. The characterization results of CO2-TPD and NH3-TPD indicated that the alkaline sites were beneficial for the selective generation of VDC by 1,1,2-TCE, within the range of 71 to 79%. Designing effective catalysts would make it possible to replace the saponification reaction with catalytic splitting in industry.

Surface species structure of deactivated catalyst for dehydrochlorination of trichloroethane

ABSTRACT Catalytic splitting method was expected to replace saponification method in industry to produce vinylidene chloride from trichloroethane, but catalyst deactivation had become the main factor hindering its industrial application. There were many speculations based on theory about the reasons for catalyst deactivation, but there was barely any analysis of the surface substance composition of deactivated catalysts. A systematic and scientific study of the composition of deactivating substances on the surface of deactivated catalysts would contribute to further research on the causes of deactivation. Several characterizations and computing methods were combined to analyze the structure of the deactivation species on the catalyst surface. The total number of rings calculated for the extracted species was approximately 11. The number of cycloalkanes and aromatic rings was 5 and 7, respectively, and the carbon fraction of cycloalkanes was 0.38. The ratio of unsubstituted hydrogen to carbon atoms was 0.42. The results show that the molecular formula of the deactivation species on the catalyst surface was C37.72H31.51Cl15.85, which was a multi-cyclic mixed component mainly composed of aromatic hydrocarbons and aliphatic hydrocarbons.

Cobalt-doped CsPbBr3 perovskite quantum dots for photoelectrocatalytic hydrogen production via efficient charge transport

Photoelectrochemical cells (PECs) based on all-inorganic perovskite quantum dots (QDs) have great potential for the catalytic splitting of water using sunlight due to their good photovoltaic properties and inexpensive manufacturing process. However, their development is severely constrained by their own large carrier transport barriers and poor water stability. Herein, we report the synthesis of cobalt doped CsPbBr3 perovskite quantum dots (Co-doped CsPbBr3 QDs) by hot-injection method and their preparation into FTO/c-TiO2/m-TiO2/Co-CsPbBr3 QDs photoanode devices, and further encapsulation of the devices using low-temperature conductive carbon paste (LTCC). The partial replacement of Pb2+ sites by Co2+ in Co-doped CsPbBr3 QDs with appropriate concentrations enhances the binding energy of QDs, forms impurity energy levels, reduces carrier transport barriers, improves carrier lifetime and carrier separation and transport rate, so as to improve optoelectronics purpose of catalytic activity. Ultimately, the Co-CsPbBr3 QDs photoanode produced 1.944 mA cm−2 at 1.23 VRHE under simulated sunlight (AM 1.5, 100 mW cm−2) and achieved a stable photocurrent density of more than 1 mA cm−2 for up to 5500 s in aqueous KOH solution. The development of effective all-inorganic perovskite quantum dots sheds light on the realization of catalysts for PEC water splitting.

Study of hydrogen absorption in a novel three-dimensional graphene structure: Towards hydrogen storage applications

The use of a novel three-dimensional graphene structure allows circumventing the limitations of the two-dimensional nature of graphene and its application in hydrogen absorption. Here we investigate hydrogen-bonding on monolayer graphene conformally grown via the epitaxial growth method on the (0001) face of a porousified 4H-SiC wafer. Hydrogen absorption is studied via Thermal Desorption Spectroscopy (TDS), exposing the samples to either atomic (D) or molecular (D2) deuterium. The graphene growth temperature, hydrogen exposure temperature, and the morphology of the structure are investigated and related to their effect on hydrogen absorption. The three-dimensional graphene structures chemically bind atomic deuterium when exposed to D2. This is the first report of such an event in unfunctionalized graphene-based materials and implies the presence of a catalytic splitting mechanism. It is further shown that the three-dimensional dendritic structure of the porous material temporarily retains the desorbed molecules and causes delayed emission. The capability of chemisorbing atoms after a catalytic splitting of hydrogen, coupled to its large surface-to-volume ratio, make these structures a promising substrate for hydrogen storage devices.

(Invited) Electrocatalysts of Complex Oxides for Water Splitting

Low temperature electrolysis plays a critical role in our effort for energy generation with net zero or negative carbon impacts. This process produces green hydrogen through catalytic splitting of water and requires the development of highly active and, perhaps more importantly, highly stable electrocatalysts for oxygen evolution reaction (OER). This presentation will focus on examining the structure-property relationship of complex oxides that are made of AxByOz, where A and B can be a single metal cation or mixed cations, respectively. Depending the compositions, perovskite, pyrochlore, and Ruddlesden-Popper (RP) phase solids all have been found to be highly active for OER in either acid or base conditions. Noticeably, structurally defective solid catalysts such as those observed in defect perovskite (J. Am. Chem. Soc., 2014, 136, 14646-14649) often possess high activity. I will review some of the latest advancements in understanding the mechanisms of OER and discuss the new results on regulating the cation sites and oxygen defect chemistry for enhancing the bond-stability of key catalytic constituents and surface structures for OER performance.

(Invited) Memristive Devices based on Two-Dimensional Materials

Two-dimensional layered materials are emerging as candidates for neuromorphic switches. So far, several different phenomena have been proposed as device switching mechanisms. These include the lateral movement of ions, either in van der Waals gaps [1] or grain boundary mediated [2], phase changes in 2D materials [3], defect mediated resistive switching [4], or vertical ion movement and subsequent modulation of electronic barriers [5].In this talk, vertically aligned molybdenum disulfide (MoS2) will be discussed as a memristive material that allows ionic transport along its van der Waals gaps. These ions contribute to the electron transport properties across the vertical device structures [6]. In addition, the motion of ions can be used to tune energy barriers at MoS2/semiconductor Schottky junctions [5]. We have identified hydroxyl (OH-) ions that likely originate from catalytic splitting of water molecules in the devices [7]. In addition, vacancies are present in the 2D films that manifest as absorbers for infrared radiation, i.e. wavelengths beyond what is expected from a semiconductor with a bandgap of approximately 1.2 eV [8], [9].AcknowledgementsThe authors acknowledge funding by the European Commission in the Horizon 2020 grant agreements 881603 (Graphene Flagship) and 829035 (QUEFORMAL), the German BMBF grant NEUROTEC (16ES1134) and the DFG projects MOSTFLEX (407080863) and ULTIMOS2 (412113712).

K-doped CeO2-ZrO2 for CO2 thermochemical catalytic splitting.

Green syngas production is a sustainable energy-development goal. Thermochemical H2O/CO2 splitting is a very promising sustainable technology allowing the production of H2 and CO with only oxygen as the by-product. CeO2–ZrO2 systems are well known thermochemical splitting catalysts, since they combine stability at high temperature with rapid kinetics and redox cyclability. However, redox performances of these materials must be improved to allow their use in large scale plants. K-doped systems show good redox properties and repeatable performances. In this work, we studied the effect of potassium content on the performances of ceria–zirconia for CO2 splitting. A kinetic model was developed to get insight into the nature of the catalytic sites. Fitting results confirmed the hypothesis about the existence of two types of redox sites in the investigated catalytic systems and their role at different K contents. Moreover, the model was used to predict the influence of key parameters, such as the process conditions.

(Invited) Transistors, Memristors and Optoelectronics Based on Two-Dimensional Molybdenum Disulfide

Molybdenum disulfide (MoS2) is a semiconducting transition metal dichalcogenide material. This two-dimensional (2D) layered material has a band gap ranging from 1.3 eV in bulk form (indirect) to 1.88 eV as a monolayer (direct). The material has been suggested as a potential candidate for applications in nanoelectronics, optoelectronics and neuromorphic computing [1]–[4]. In this talk, the principles of scalable growth through thermal conversion of metals and metal-organic vapor phase growth will be discussed [5], [6]. These materials are then integrated into the silicon technology platform to demonstrate device applications.Conventional field effect transistors with MoS2 channels show promising characteristics. In addition, ion-based plasticity promises to enable memristive functionality on MoS2 devices. In particular, vertically aligned MoS2 layers allow the tuning of energy barriers at MoS2/semiconductor Schottky junctions through mobile ions [7], [8]. These ions (OH-) likely originate from catalytic splitting of water molecules.Spectral response measurements of MoS2 based devices provide insight into the band structure and beyond, as photodetectors based on MoS2/silicon/graphene and MoS2/amorphous silicon heterostructures show peculiar responsivity in the infrared regime [9]–[11]. Acknowledgements The authors acknowledge funding through the European Union’s Horizon 2020 research and innovation programme under grant agreements 785219 (Graphene Flagship) and 829035 (QUEFORMAL), the German BMBF grant NEUROTEC (16ES1134) and the DFG projects MOSTFLEX (407080863) and ULTIMOS2 (412113712).

Nonvolatile Resistive Switching in Nanocrystalline Molybdenum Disulfide with Ion‐Based Plasticity

AbstractNon‐volatile resistive switching is demonstrated in memristors with nanocrystalline molybdenum disulfide (MoS2) as the active material. The vertical heterostructures consist of silicon (Si), vertically aligned MoS2, and chrome/gold metal electrodes. Electrical characterizations reveal a bipolar and forming‐free switching process with stable retention for at least 2500 s. Controlled experiments carried out in ambient and vacuum conditions suggest that the observed resistive switching is based on hydroxyl ions (OH−). These originate from catalytic splitting of adsorbed water molecules by MoS2. Experimental results in combination with analytical simulations further suggest that electric field driven movement of the mobile OH− ions along the vertical MoS2 layers influences the energy barrier at the Si/MoS2 interface. The scalable and semiconductor production compatible device fabrication process used in this work offers the opportunity to integrate such memristors into existing Si technology for future neuromorphic applications. The observed ion‐based plasticity may be exploited in ionic‐electronic devices based on transition metal dichalcogenides and other 2D materials for memristive applications.

A class of two-dimensional SiAs monolayers with novel electronic and optical properties from ab initio investigations

Exploring new 2D monolayer materials with intrinsic novel properties is of great significance from both fundamental and practical points of view. Using first-principles calculations based on density functional theory, we propose three stable hexagonal SiAs monolayers with the excellent thermal and dynamic stabilities, whose crystal structures are similar to the monolayer GaSe, named $ \alpha$ -, $ \beta$ - and $ \gamma$ -SiAs, respectively. Our results show that $ \alpha$ - and $ \beta$ -SiAs monolayers are wide-band-gap semiconductors with indirect band gaps, which have great potential applications in photoelectronic devices, especially used for catalytic splitting water. Further, the effective masses of electron transport in monolayers $ \alpha$ - and $ \beta$ -SiAs exhibit anisotropic characteristics. Intriguingly, $ \gamma$ -SiAs is a direct band gap semiconductor with a band gap of 1.13eV as bulk silicon, which results in a significant absorption in the visible-light region. More importantly, the effective masses in the $ \gamma$ -SiAs monolayer are only a little larger than 0.20 m0, indicating that it may have high carrier mobility. Our findings provide three promising candidates for low-dimensional nanodevices with excellent performances in the future.

Adsorption of Phosgene on Titanium Doped Graphene Nanoribbons: A DFT Study

In this study, we have studied the interaction of Phosgene (COCl2) on two configurations of Titanium doped Graphene Nanoribbons (GNR)- Armchair GNR and ZigZag GNR. We analyze the changes in the electronic structure of the substrate before and after the adsorption of Phosgene, with the help of Density Functional Theory (DFT) calculations. The Titanium doping was assumed to be substitutional. The Phosgene gas molecule was introduced in multiple configurations to account for the randomness of the incoming gas molecule on the substrate. Our results indicate that the Titanium atom causes a catalytic splitting of the C-Cl bond, reducing the toxic gas into less harmful by-products. Carbon mono-oxide gas is released after the adsorption, while the Chlorine atoms attach to the substrate. The Oxygen atom in the gas can partially shield the C-Cl bond and form a physical barrier to the splitting. However, this effect was seen only in a small minority of the cases we studied. This study was further extended to two other molecules – one with an S-Cl bond (SOCl2 -Thionyl Chloride) and one with an O-Cl bond (POCl3) to determine whether the catalytic splitting observed in Phosgene is a consequence of the interaction of Ti with the C-Cl bond, or the interaction of Ti with the electron cloud of the Cl atom. Our results demonstrate that Ti-doped graphene nanoribbons have great potential to be used as a platform for toxic gas treatment as well as sensing.

Fundamental Basis for Distinctive Sensing of H2 in Humid Environment

Accurate sensing of trace amount of hydrogen without interference from environment is highly desirable in timely detection of possible leakage when using hydrogen as a clean energy source. Humidity in air is usually considered as the main challenge to the performance of room temperature hydrogen sensors. In this work, we have demonstrated distinctive sensing of H2 in humid environment at room temperature, using (002) oriented TiO2 thin film as active sensing material. Through first‐principle calculations in the framework of density functional theory, we have clarified the interaction between H2O and rutile TiO2 (002) surface. It was found that H2O can be easily split into –H and –OH with a negligible activation energy below 0.07 eV on the TiO2 (002) surface. Catalytic splitting of water has rather little effect on the height of Schottky barrier between Pt and the TiO2 (002) surface, thus resulting in ignorable impact on hydrogen sensing property. The (002) oriented TiO2 thin film hydrogen sensor could be utilized in various humid environment at room temperature.

Helium nanodroplets doped with copper and water

Copper nanoparticles are promising, low-cost candidates for the catalytic splitting of water and production of hydrogen gas. The present gas-phase study, based on the synthesis of copper-water complexes in ultracold helium nanodroplets followed by electron ionization, attempts to find evidence for dissociative water adsorption and H2 formation. Mass spectra show that H2O–Cu complexes containing dozens of copper and water molecules can be formed in the helium droplets. However, ions that would signal the production and escape of H2, such as (H2O)n−2(OH)2Cum+ or the isobaric (H2O)n−1OCum+, could not be detected. We do observe an interesting anomaly though: While the abundance of stoichiometric (H2O)nCum+ ions generally exceeds that of protonated or dehydrogenated ions, the trend is reversed for (H2O)OHCu2+ and (H2O)2OHCu2+; these ions are more abundant than (H2O)2Cu2+ and (H2O)3Cu2+, respectively. Moreover, (H2O)2OHCu2+ is much more abundant than other ions in the (H2O)n−1OHCu2+ series. A byproduct of our experiment is the observation of enhanced stability of He6Cu+, He12Cu+, He24Cu+, and He2Cu2+.Graphical abstract

Mathematical Modeling of Ion Transport and Water Dissociation at the Ion-Exchange Membrane/Solution Interface in Intense Current Regimes

At current densities exceeding the limiting current density, H+ and OH− ions are generated at the interface of the ion-exchange membrane with a depleted solution as a result of the dissociation of water molecules. At present, it is generally accepted that water splitting occurs in a thin (a few nanometers) layer inside the membrane, this reaction being catalytic in nature. The mathematical model of ion transport in the diffusion layer near the membrane surface has been constructed and numerically studied under conditions when dissociation and recombination processes involving water molecules and H+ and OH− ions occur simultaneously. It has been shown that in overlimiting current regimes under very high voltages, intense noncatalytic dissociation of water molecules in the extended space charge region of the depleted solution can occur irrespective of the catalytic splitting of water. Since this region has macroscopic dimensions, the rate of noncatalytic water dissociation is comparable with the rate of the corresponding catalytic process. The obtained results significantly supplement modern concepts of the mechanism of generation of H+ and OH− ions in membrane systems, showing that this process can proceed not only in accordance with the conventional mechanism with the catalytic participation of functional groups and/or other compounds, but also via the noncatalytic mechanism that has not been taken into account to the present.

Immobilization of pyridinium nitrate and its application for the catalytic converting of β-pinene oxide

In this paper, differences among the catalytic effects of individual ammonium nitrates and influences of the catalysts used either in homogeneous and, moreover, pyridinium nitrate in heterogeneous forms of the catalytic converting of β-pinene oxide to perillic alcohol were studied. A special attention was paid to the heterogeneously catalyzed reaction where the representative of the used catalysts, pyridinium nitrate, was immobilized on various supports—mesoporous molecular sieves (MCM-41, SBA-15), silica and smectites (hectorite, montmorillonite K-10 and bentonite). Based on the obtained results, the individual influences on the course of the catalytic splitting, namely the properties and modifications of supporting materials were discussed. The best results using the catalyst immobilized on hectorite were achieved. The maximum relative concentration was 64 % for perillic alcohol and 17 % for perillic alcohol isomers 17 %.

Low Energy CO2-free Carbon Hydrate Splitting

Energy economics increasingly develop awareness of Efficiencies. From extraction through transformation, storage, distribution, etc. to the use of energy efficiency is topic. Some of it is addressed by going local in feedstock and distributed in generation, also opening new opportunities for energy use of biomass, usually limited to logistic constraints.Hydrogen recently has taken a prominent position in contributing towards flexibility of Renewable Energy generation and could potentially save a part of the 80% crude oil energy loss for transformation into gasoline for the transportation sector.Splitting organic waste and biomass into Hydrogen and Carbon in a bio-refinery, flexible product outputs could be enabled, as demonstrated at the European Centre for Renewable Energies in Guessing, Austria. By applying our thermo catalytic splitting of Carbon Hydrate gases at 55% of the energy needs of SMR only, no scarce water needs to be used nor CO2 is generated.Bio refineries could uplift energy efficiency of biomass use, from today's limitation to electricity and heat to flexible synthetic fuels or electricity on demand, potentially providing economic back-up solutions for volatile renewable energy generation. A model for a 200.000 capita MSW installation will be introduced.

Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces

Water is the renewable, bulk chemical that nature uses to enable carbohydrate production from carbon dioxide. The dream goal of energy research is to transpose this incredibly efficient process and make an artificial device whereby the catalytic splitting of water is finalized to give a continuous production of oxygen and hydrogen. Success in this task would guarantee the generation of hydrogen as a carbon-free fuel to satisfy our energy demands at no environmental cost. Here we show that very efficient and stable nanostructured, oxygen-evolving anodes are obtained by the assembly of an oxygen-evolving polyoxometalate cluster (a totally inorganic ruthenium catalyst) with a conducting bed of multiwalled carbon nanotubes. Our bioinspired electrode addresses the one major challenge of artificial photosynthesis, namely efficient water oxidation, which brings us closer to being able to power the planet with carbon-free fuels.

Decreased emission of nitrous oxide from delivery wards—case study in Sweden

The very potent greenhouse gas nitrous oxide (N2O) is widely used as a mild anaesthetic for mothers in delivery work in Sweden. As a part of the Stockholm County Council environmental program it was decided in 2002 that the emissions should be drastically reduced. Different ways were theoretically evaluated, and catalytic splitting to nitrogen and oxygen gas (N2 and O2) was chosen for a demonstration installation. A Japanese commercial unit for treatment of mixed anaesthetic gases (Anesclean® from Showa Denko K.K.) was thoroughly modified and installed at the Karolinska University Hospital at Huddinge in Stockholm in 2004. The destruction of N2O was optimised and studied for 2 years. Data from both collection and destruction are given in the article. Of the collected N2O more than 95% was split to N2 and O2 in the very stable system. The overall emission decrease was mainly dependent on the share that could be collected in the specific exhaustion system as compared to the normal room ventilation. Life cycle assessment (LCA) and life cycle costing (LCC) were used to evaluate the actual environmental value and economical cost for the process. Important factors are pointed out.

The dissociation kinetics of H2S over an alumina supported Co-Mo sulphide catalyst

In this study, a high surface area 4Co∶6Mo∶100γ-Al2O3 sulphide prepared using precipitation from homogeneous solution (PFHS) has been used for the catalytic splitting of hydrogen sulphide into H2 and elemental sulphur. The activity of this new formulation was significantly better than previously reported recipes. Kinetic data collected over a wide range of H2S partial pressures between 883 and 983 K revealed that, although the decomposition followed a first-order law, a mechanism involving H2S adsorption on co-ordinative unsaturation sites of the Co-Mo sulphide catalyst gave a Langmuir-Hinshelwood rate expression that yielded satisfactory model parameters. In particular, the scission of the surface H-S bond appeared to be the rate determining step.