Development Of Catalyst Materials Research Articles

(Invited) High Productivity Electrocatalytic Water Splitting Using Diverse Water Sources

Water electrolysis is a commercially established process. However, significant cost reductions are desired for the large scale applications anticipated in the future. Electrolysis is typically performed under extreme pH conditions due to the high reaction rate of the electrocatalyst and low solution resistance. On the other hand, energy conversion using electricity generated from renewable energy requires a water splitting electrocatalyst that is versatile, safe, and durable. Recent water splitting research has focused on the development of catalyst materials and elucidation of their efficient action under non-extreme pH conditions. However, there are several problems associated with water splitting in the mild pH range. The first problem is the lack of protons and hydroxide ions at mild pH, which leads to large concentration overpotentials. To solve these problems, concentrated buffered electrolytes are used to suppress local pH shifts and to compensate for the steady-state proton or hydroxyide ion. To maximize the buffering effect, concentrated buffer electrolyte solutions should be applied near the pKa values where protonated and deprotonated species coexist. A second problem at near-neutral pH levels is the inherently lower conductivity of the electrolyte solution compared to strongly acidic or strongly alkaline solutions. The ohmic potential (iR) drop between the anode and cathode is critical to the cell voltage of water splitting devices, especially at the high current densities that are industrially important. The conductivity at pH 7.0 of phosphate buffer can be maximized by varying the molar ratio and selecting the appropriate counter cation (K among Li, Na, K, and Cs). The choice of conditions essentially reduces viscosity and maximizes solubility, which strongly affects the conductivity of the solution. The conductivity of saturated K phosphate reaches a maximum of ~20 S m−1, but this value is still not as high as the alkaline one (~130 S m−1). Viscosity and solution resistance can be adjusted by mixing different electrolytes and fine-tuning the position of pKa within the target pH range. In water containing Mg2+ and Ca2+, precipitation of these ionic salts can be a problem. Keeping the pH between 8 and 10 prevents such precipitation. We are also attempting to intentionally use chloride ions, which have the highest concentration anion in seawater. In this way, solution resistance is further improved. Thus, it is emphasized that electrolyte engineering is an important technology for cost-effective water electrolysis by finding new reaction environments to efficiently electrolyze water under more moderate conditions.

Why copper catalyzes electrochemical reduction of nitrate to ammonia.

Electrochemical reduction of nitrate (NO3RR) has drawn significant attention in the scientific community as an attractive route for ammonia synthesis as well as alleviating environmental concerns for nitrate pollution. To improve the efficiency of this process, the development of catalyst materials that exhibit high activity and selectivity is of paramount importance. Copper and copper-based catalysts have been widely investigated as potential catalyst materials for this reaction both computationally and experimentally. However, less attention has been paid to understanding the reasons behind such high activity and selectivity. Herein, we use Density Functional Theory (DFT) to identify reactivity descriptors guiding the identification of active catalysts for the NO3RR, establish trends in activity, and explain why copper is the most active and selective transition metal for the NO3RR to ammonia among ten different transition metals, namely Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Ru, and Co. Furthermore, we assess NO3RR selectivity by taking into account the competition between the NO3RR and the hydrogen evolution reaction. Finally, we propose various approaches for developing highly active catalyst materials for the NO3RR.

Facile one pot synthesis of nitrogen doped reduced graphene oxide supported Co3O4 nanoparticles as bifunctional catalysts for the reduction of 4-nitrophenol and NaBH4 hydrolysis

The development of catalyst materials with significant potential and multifunctional applicability is essential for sustainable energy and environmental applications. The present work reports the preparation of nitrogen doped reduced graphene oxide supported Co3O4 nanoparticles (NGCO) via a simple and cost effective one pot synthesis method, at a relatively low temperature. The prepared NGCO composites were employed for the first time as active bifunctional catalysts in the reduction of 4-nitrophenol (4NP) and NaBH4 hydrolysis. The NGCO catalyst exhibited excellent catalytic activity and stability for the 4NP conversion with a mass normalized rate constant of 97.8 min−1g−1 at 303 K. The concentration (0.7 mM) and volume (20 mL) of the 4NP solution used in this study was higher than most of the works in the literature and that signifies the novelty of the prepared NGCO catalyst materials. The NGCO catalysts also showed potential activity and durability in liberating hydrogen from NaBH4, via hydrolysis, with a maximum specific hydrogen generation rate of 2090.6 mL min−1 gCo−1. The kinetics of the catalytic NaBH4 hydrolysis reaction was investigated using zero order and first order kinetic models. The experimental data was well-described with the zero-order kinetic model. The improved catalytic activity of the NGCO composites relative to the unsupported Co3O4 nanoparticles is due to the excellent dispersibility of the active sites and the synergetic action of both Co3O4 and N doped graphene support in NGCO.

Photoelectrolysis of TiO2 is highly localized and the selectivity is affected by the light

On the way to sustainable prosperity for future generations, photoelectrochemistry is becoming a key area for energy conversion and the environmentally friendly generation of chemical resources because it combines the advantages of electrochemical and photochemical processes. Highly active catalyst materials with excellent selectivity towards the desired reaction are certainly required for efficient processes. Understanding the underlying processes, including detailed in situ information, facilitates the design and development of catalyst materials. In this work, we utilize the scanning photoelectrochemical microscopy for the spatially resolved in situ investigation of the electrochemical and photoelectrochemical evolution of hydrogen, oxygen, reactive oxygen species, and chlorine for energy conversion. Herein, we demonstrate that the activity and the selectivity of the TiO2 photoelectrocatalyst are highly localized despite their apparently uniform composition based on regular morphological characterization. Furthermore, the results suggest that the illumination dramatically changes the selectivity in electrolysis reactions, which is demonstrated on the competing oxygen- and chlorine- evolution reaction. Consequently, this leads to the critical implication that the activity and selectivity should not be considered uniform per se, especially under the illumination. This implies that light might be used for controlling the selectivity in electrolysis reactions. Such discovery shall dramatically impact the optimization of photoelectrocatalysts in general.

(Invited) From Atomic-Scale Understanding to Design of Advanced Electrocatalyst Materials

Increasing concerns regarding the environmental impact of power generation and the reduction of carbon emissions have become a major motivation for the use of sustainable electrical power. In this perspective, renewable energy sources are expected to take up an important part of electric energy produced in the near future. Since renewable sources such as wind and solar are intermittent in nature, their increased installation will result in either excessive or insufficient power generation at times. A large variety of concepts for using this surplus power is currently under development.Acidic water electrolysis is an attractive environmentally-safe solution for the conversion and storage of excess electricity into hydrogen and oxygen. The large scale application of water electrolysis systems is currently hindered by low efficiency and instability of catalysts during long term operation [1]. This is especially crucial for the anodic oxygen evolution reaction (OER) which catalyzed by Iridium based alloys and oxides making the issues related to efficiency of catalyst utilization and durability even more critical. The development of catalyst materials with superior performance demands a deep understanding of the OER itself as well as electrocatalysts degradation mechanisms. Although a large number of possible OER mechanisms have been proposed [2], the topic remains a subject of intensive debates across the scientific literature, since the intermediates and their possible interrelations are in most cases unknown. Additional challenges in mechanistic studies are related to the changes in the composition of the catalyst and its degradation triggered by OER. Though the existence of experimental correlation between the structure, activity of OER catalyst and its dissolution was numerously reported and discussed [2,3], reports addressing all these aspects in one study are rare. This indicates that further research efforts are required to reveal reaction intermediates and products by means, for instance, of mass spectrometry at the atomic scale by atom probe tomography, that complement spectroscopic insight from synchrotron-based experiments, ideally in operando conditions.In this presentation, the main fundamental limitations that hinder the current understanding of the reactions mechanisms occurring in water electrolysis will be discussed. It will be stressed that only the combination of insights enables the establishing of reliable structure-function relationships in electrocatalyst materials. We present our approach to investigate complex processes in electrocatalysis with a combination of independent methods like online electrochemical mass spectrometry, atom probe tomography and synchrotron-based spectroscopy [4,5]. In particular, we discuss approaches to resolving the reaction mechanism on constantly evolving surfaces; present our recent results on the interplay between the nature of the active sites in Ir-based oxides and their stability towards the OER and establish structure–function relationships between the reactivity and the atomic-scale surface structure [5]. Finally, using the new insights gained in our recent studies [4-6], we discuss potential design strategy for Ir-based OER catalysts with high catalytic activity and long-term stable performance.[1] S. Cherevko et al., Catalysis Today 2016, 262, 170-180[2] T. Reier, H.N. Nong, D. Teschner, R. Schlögl, P. Strasser, Adv. Energy Mater. 2017, 7(1), 1601275.[3] O. Kasian, J.P. Grote, S. Geiger, S. Cherevko, K.J.J. Mayrhofer, Angew. Chem. Int. Ed. 2018, 57, 2488 –2491.[4] K. Schweinar, R.L. Nicholls, C.R. Rajamathi, P. Zeller, M. Amati, L. Gregoratti, D. Raabe, M. Greiner, B. Gault, O. Kasian, J. Mater. Chem. A 2020, 8(1), 388-400.[5] O. Kasian, S. Geiger, T. Li, J-P. Grote, K. Schweinar, S. Zhang, C. Scheu, D. Raabe, S. Cherevko, B. Gault, K. Mayrhofer, Energy Environ. Sci. 2019, 12(12), 3548-3555.[6] K. Schweinar, I. Mouton, B. Gault, O. Kasian, J. Phys. Chem. Lett. 2020, submitted.

Deciphering the Oxygen Reduction Reaction on Platinum: A Theoretical Framework

The conversion of oxygen into water is crucial to the operation of polymer electrolyte fuel cells and other emerging electrochemical energy technologies. Chemisorbed oxygen species play a central role in this reaction, since they necessarily act as intermediates but also as site-blockers. Any attempt to decipher the oxygen reduction reaction must relate the formation of oxygen intermediates to basic electronic and electrostatic properties of the catalyst surface and, on the other hand, link it to effective parameters of the electrode activity. An approach that accomplishes this feat will be of great utility in guiding design and development of catalyst materials and developing predictive models of electrode operation. Here, we present a theoretical framework for the multiple interrelated surface phenomena and processes involved. It correlates the formation of chemisorbed oxygen intermediates, metal surface charging phenomena, ion density and potential distribution in electrolyte, field-dependent ordering of interfacial water molecules, and effective kinetic parameters of the ORR. Parameterized with density functional theory results and rotating disk electrode data, the model sheds light on the double-edged roles of oxygen intermediates; it produces as output the Tafel slope and exchange current density as continuous functions of electrode potential and presents a new perspective on the volcano relation the ORR. The optimal oxide coverage is a result of two oppositely-headed trends upon increasing coverage by oxygen intermediates, viz, the intuitive site-blocking effect and the increasing protophilicity of the surface.

Design and development of catalyst materials for the production of fuels and chemicals in a sustainable manner

Design and development of catalyst materials for the production of fuels and chemicals in a sustainable manner

(Invited) Electrochemical Reduction of CO2 to Value-Added Chemicals: Status and Remaining Challenges

Significant reduction in atmospheric CO2 emissions as well as in the development of alternative energy sources will be critical to curb the unwanted effects of climate change (e.g., global warming, erratic weather patterns) and to reduce our dependence on fossil fuels. A variety of strategies needs to be pursued to curb the increase in atmospheric CO2 level. One potential strategy that can be employed to overcome this challenge is the electrochemical reduction of CO2 into useful chemicals such as formic acid, carbon monoxide, hydrocarbons, or alcohols. To drive thus process in a cost effective manner otherwise-wasted renewable energy that is produced in excess of grid demand can be used. A key challenge is the development of catalyst materials that are active and selective for the electrochemical reduction of CO2, i.e., catalysts that can reduce CO2 at low overpotentials (and thus high energetic efficiency), produce desired products at high current densities over long periods of time, and do so selectively while suppressing unwanted reactions. Most present efforts to re-energize spent CO2 via electrochemical methods struggle to achieve simultaneously high energetic efficiency, high product selectivity, and high throughput. This presentation will provide an overview of progress by us and others in (i) developing suitable cathode catalysts for CO2 reduction (primarily to CO) as well as suitable anode catalysts (typically for the oxygen evolution reaction) needed for a CO2 electrolysis cell; (ii) optimizing operation conditions for these catalysts (pH, electrolyte composition, …); and (iii) developing gas diffusion electrodes suitable for a CO2 electrolysis cells. A projection for the potential for further improvement of catalysts, electrodes, operation conditions, and electrolysis cell designs will be made. Also, a gross margin model to estimate economic feasibility, as well as the results of a techno-economic analysis of a process that converts the CO2 of a 0.5 MW power plant to fuel via electroreduction of CO2 to CO, followed by Fischer-Tropsch will be presented.

Hierarchical Cr2O3@OPC composites with octahedral shape for rechargeable nonaqueous lithium-oxygen batteries

The development of catalyst materials is the most significant issue that hinders the practical applications of Li-O2 batteries. Herein we show the design and synthesis of the hierarchical chromic oxide-octahedral porous carbon (Cr2O3@OPC) composites catalyst with octahedral shape that derived from Cr-based metal-organic frameworks (MIL-101(Cr)) precursor. When applied as cathode catalysts in rechargeable Li-O2 batteries, the electrode with Cr2O3@OPC composites catalyst exhibits a low charge and discharge over-potential, high discharge capacity and excellent cycling stability. What's more, the electrode with Cr2O3@OPC composite shows a discharge capacity up to ∼4800 mAh g(catalyst + carbon)−1 at a current density of 0.1 mA cm−2, and exhibits a very stable discharge voltage plateau of 2.7 V and a charge voltage plateau of ∼3.9 V. With the addition of Cr2O3@OPC composite, the Li-O2 batteries can obtain good cycle performance over 50 cycles at a fixed capacity of 800 mAh g(catalyst + carbon)−1. These results indicating that the Cr2O3@OPC composite derived from MIL-101(Cr) would be a promising catalyst for Li-O2 batteries.

Development of catalyst materials being effective for microwave sterilization

Recently, airborne virus infections have emerged as some of the most challenging medical problems. To prevent the threat of infection, the processes of sterilization have been studied widely. Microwave sterilization has many advantages in comparison with conventional methods. It is able to raise the temperature of a material in a short time and selectively heat the material. This results in the reduction of usage and the rapid completion of sterilization. We developed a novel microwave sterilization system that can raise the temperature in quite a short time using a lower microwave power (100 W). Filters made with Kao wool (Al2O3) were coated with TiO2 (anatase) by sol–gel method and used to trap microorganisms. In addition, these filters were coated with Pt or Ag by impregnated method. We also prepared a Tyranno-fiber textile filter and a honeycomb SiC filter. Two microorganisms, Bacillus subtilis ATCC 9372 and Bacillus stearothermophilus ATCC 7953, were used in this experiment, where either microorganism was loaded onto a filter. After irradiation, filters loaded with B. subtilis and B. stearothermophilus were incubated for 48 h in a TSB medium (Bactoe Tryptic Soy Broth, Becton, Dickinson and company sparks, MD) at 37 and 56 °C, respectively. B. subtilis and B. stearothermophilus loaded on to Ag-impregnated filters were sterilized in 30 and 15 s, respectively. The Tyranno-fiber textile filter and the honeycomb SiC also filter showed effective microwave sterilization. These results showed that this system could sterilize B. subtilis and B. stearothermophilus in quite a short time and that microwave absorbable materials are effective as microwave sterilization filters.

Developments of electron microscopy methods in the study of catalysts

Recent developments in electron microscopy methods for in-situ probing of catalysis at the atomic level, ultrahigh resolution surface tools, energy filtered imaging for elucidating the structure and chemistry of nanoparticles and electron tomography for three-dimensional visualization of nanocatalysts are transforming heterogeneous catalysis research. In this short review, recent experimental and theoretical developments of electron microscopy methods for the study of gas–solid catalyst reactions, liquid–catalyst reactions, polymerization in the solution and solid phase, oxides and nanocatalysts are presented. They demonstrate the powerful role of electron microscopy in the design and development of catalyst materials and technologies.

Multisolute Metal Ion Sorption at the γ-Al2O3/Water Interface

Many environmental problems associated with raw material production and past waste management practices require reliable quantitative prediction of the fate and transport of metal ion contaminants in surface and groundwaters. Sorption reactions at the mineral/water interface typically reduce solute mobility and often control the fate and transport of metal ions. Therefore, determination of specific sorption mechanisms has been a primary focus of research aimed at understanding metal ion transport, mineral growth and the development of catalyst materials. Typically, reactions at the mineral/water interface have been studied with a combined macroscopic/microscopic approach. Sorption experiments conducted in batch reactors describe specific ion sorption to oxide and clay materials as a function of pH, background electrolyte, surface coverage and competing adions. Select samples are then studied using various spectroscopy and microscopy methods including XAS, IR, XPS, AFM, SEM and TEM to determine the structure of sorbed species relative to macroscopic observations. Spectroscopic investigations have focused primarily on single solute systems. XAS data has been used to differentiate between innersphere and outersphere complexes and reveal the formation of polymeric species at high surface coverages. The variability in structure of many transition metal ion surface complexes as a function of surface coverage indicates that competing adions may have an impact on surface complexation and speciation. The goal of this study is to determine the effect of various adions including Co 2+, Fe 2+, SeO~and SeO 2on the sorption mechanism of cobalt at different surface coverages and to evaluate the types of surface complexes formed in multisolute systems using XAS techniques. Materials and methods. Batch equilibrium studies were conducted using y-AI203 (Buehler Alumina), 82 g/m 2, and metal basis grade nitrate or chloride salts C0(NO3)2, FeC12, Na2SeO3, Na2SeO4 and NaNO3 (Johnson Matthey Co.). Metal stock solutions University of Maine, Orono, ME, USA