Catalytic tests to assess the performance of mixed perovskite-type oxides (La0.9Ca0.1FeO3-δ, La0.6Ca0.4FeO3-δ, Nd0.9Ca0.1FeO3-δ, Nd0.6Ca0.4FeO3-δ, Nd0.6Ca0.4Fe0.9Co0.1O3-δ, ...Nd0.6Ca0.4Fe0.97Ni0.03O3-δ, and LSF) with respect to CO oxidation are presented as well as characterization of the materials by XRD and SEM. Perovskites are a highly versatile class of materials due to their flexible composition and their ability to incorporate dopants easily. CO oxidation is a widely used “probe reaction” for heterogeneous catalysts. In this study, it is demonstrated how tuning the composition of the catalyst material (choice of A-site cation, A-site and B-site doping) greatly influences the activity. Changing the A-site cation to Nd3+ or increasing the concentration of Ca2+ as A-site dopant improves the performance of the catalyst. Additional B-site doping (e.g., Co) affects the performance as well—in the case of Co-doping by shifting ignition temperature to lower temperatures. Thus, perovskites offer an interesting approach to intelligent catalyst design and tuning the specific properties towards desired applications.
Abstract
Over the last decade, exsolution has emerged as a powerful new method for decorating oxide supports with uniformly dispersed nanoparticles for energy and catalytic applications. Due to their ...exceptional anchorage, resilience to various degradation mechanisms, as well as numerous ways in which they can be produced, transformed and applied, exsolved nanoparticles have set new standards for nanoparticles in terms of activity, durability and functionality. In conjunction with multifunctional supports such as perovskite oxides, exsolution becomes a powerful platform for the design of advanced energy materials. In the following sections, we review the current status of the exsolution approach, seeking to facilitate transfer of ideas between different fields of application. We also explore future directions of research, particularly noting the multi-scale development required to take the concept forward, from fundamentals through operando studies to pilot scale demonstrations.
In heterogeneous catalysis, surfaces decorated with uniformly dispersed, catalytically-active (nano)particles are a key requirement for excellent performance. Beside standard catalyst preparation ...routines—with limitations in controlling catalyst surface structure (i.e., particle size distribution or dispersion)—we present here a novel time efficient route to precisely tailor catalyst surface morphology and composition of perovskites. Perovskite-type oxides of nominal composition ABO3 with transition metal cations on the B-site can exsolve the B-site transition metal upon controlled reduction. In this exsolution process, the transition metal emerges from the oxide lattice and migrates to the surface where it forms catalytically active nanoparticles. Doping the B-site with reducible and catalytically highly active elements, offers the opportunity of tailoring properties of exsolution catalysts. Here, we present the synthesis of two novel perovskite catalysts Nd0.6Ca0.4FeO3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ with characterisation by (in situ) XRD, SEM/TEM and XPS, supported by theory (DFT+U). Fe nanoparticle formation was observed for Nd0.6Ca0.4FeO3-δ. In comparison, B site cobalt doping leads, already at lower reduction temperatures, to formation of finely dispersed Co nanoparticles on the surface. These novel perovskite-type catalysts are highly promising for applications in chemical energy conversion. First measurements revealed that exsolved Co nanoparticles significantly improve the catalytic activity for CO2 activation via reverse water gas shift reaction.
Perovskite-type oxides are highly flexible materials that show properties that are beneficial for application in reverse water-gas shift processes (rWGS). Due to their stable nature, the ability to ...incorporate catalytically active dopants in their lattice structure, and the corresponding feature of nanoparticle exsolution, they are promising candidates for a materials design approach. On an industrial level, the rWGS has proven to be an excellent choice for the efficient utilisation of CO2 as an abundant and renewable carbon source, reflected by the current research on novel and improved catalyst materials. In the current study, a correlation between rWGS reaction environments (CO2 to H2 ratios and temperature), surface morphology, and catalytic activity of three perovskite catalysts (Nd0.6Ca0.4Fe0.9Co0.1O3-δ, Nd0.6Ca0.4Fe0.97Co0.03O3-δ, and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ) is investigated, combining catalytic measurements with SEM and NAP-XPS. The materials were found to react dynamically to the conditions showing both activation due to in situ nanoparticle exsolution and deactivation via CaCO3 formation. This phenomenon could be influenced by choice of material and conditions: less reductive conditions (larger CO2 to H2 or lower temperature) lead to smaller exsolved particles and reduced carbonate formation. However, the B-site doping was also important; only with 10% Co-doping, a predominant activation could be achieved.
High temperature water-gas shift (HT-WGS) is an industrially highly relevant reaction. Moreover, climate change and the resulting necessary search for sustainable energy sources are making WGS and ...reverse-WGS catalytic key reactions for synthetic fuel production. Hence, extensive research has been done to develop improved or novel catalysts. An extremely promising material class for novel highly active HT-WGS catalysts with superior thermal stability are perovskite-type oxides. With their large compositional flexibility, they enable new options for rational catalyst design. Particularly, both cation sites (A and B in ABO3) can be doped with promoters or catalytically active elements. Additionally, B-site dopants are able to migrate to the surface under reducing conditions (a process called exsolution), forming catalytically active nanoparticles and creating an interface that can strongly boost catalytic performance. In this study, we varied A-site composition and B-site doping (Ni, Co), thus comparing six novel perovskites and testing them for their HT-WGS activity: La0.9Ca0.1FeO3-δ, La0.6Ca0.4FeO3-δ, Nd0.9Ca0.1FeO3-δ, Nd0.6Ca0.4FeO3-δ, Nd0.6Ca0.4Fe0.9Ni0.1O3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ. Cobalt and Nickel doping resulted in the highest activity observed in our study, highlighting that doped perovskites are promising novel HT-WGS catalysts. The effect of the compositional variations is discussed considering the kinetics of the two partial reactions of WGS-CO oxidation and water splitting.
For an in-depth characterization of catalytic materials and their properties, spectroscopic in-situ (operando) investigations are indispensable. With the rapid development of advanced commercial ...spectroscopic equipment, it is possible to combine complementary methods in a single system. This allows for simultaneously gaining insights into surface and bulk properties of functional oxides, such as defect chemistry, catalytic characteristics, electronic structure, etc., enabling a direct correlation of structure and reactivity of catalyst materials, thus facilitating effective catalyst development. Here, we present a novel sample-stage, which was specifically developed to pave the way to a lab–based combination of near ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy with simultaneous catalytic operando measurements. This setup is designed to probe different (model) systems under conditions close to real heterogeneous catalysis, with a focus on solid oxide electrochemical cells. In a proof of concept experiment using an electrochemical model cell with the doped perovskite Nd0.6Ca0.4Fe0.9Co0.1O3-δ as working electrode, the precise control of the surface chemistry that is possible with this setup is demonstrated. The exsolution behavior of the material was studied, showing that at a lower temperature (500 °C) with lower reducing potential of the gas phase, only cobalt was exsolved, forming metallic particles on the surface of the perovskite-type oxide. Only when the temperature was increased to 600 °C and a cathodic potential was applied (−250 mV) Fe also started to be released from the perovskite lattice.
Understanding the behaviour of active catalyst sites at the atomic level is crucial for optimizing catalytic performance. Here, the evolution of Pt and Cu dopants in Au
clusters on CeO
supports is ...investigated in the water-gas shift (WGS) reaction, using operando XAFS and DRIFTS. Different behaviour is observed for the Cu and Pt dopants during the pretreatment and reaction. The Cu migrates and builds clusters on the support, whereas the Pt creates single-atom active sites on the surface of the cluster, leading to better performance. Doping with both metals induces strong interactions and pretreatment and reaction conditions lead to the growth of the Au clusters, thereby affecting their catalytic behaviour. This highlights importance of understanding the behaviour of atoms at different stages of catalyst evolution. These insights into the atomic dynamics at the different stages are crucial for the precise optimisation of catalysts, which ultimately enables improved catalytic performance.
Single crystals of the title compound, Na(H
2
AsO
4
), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution. The crystal structure of Na(H
2
AsO
4
) is ...isotypic with the phosphate analogue and the asymmetric unit consists of two sodium cations and two tetrahedral H
2
AsO
4
−
anions. Each of the sodium cations is surrounded by six O atoms of five H
2
AsO
4
−
groups, defining distorted octahedral coordination spheres. In the extended structure, the sodium cations and dihydrogen arsenate anions are arranged in the form of layers lying parallel to (010). Strong hydrogen bonds range of O...O distances 2.500 (3)–2.643 (3) Å between adjacent H
2
AsO
4
−
anions are observed within and perpendicular to the layers. The isotypic structure of Na(H
2
PO
4
) is comparatively discussed.
Perovskite-type oxides have gained significant attention in the scientific community due to their unique properties and potential applications. Their ability to exsolve reducible B-site cations (e.g. ...Co, Ni, Cu) combined with their flexibility regarding A-site and B-site composition allows for the tailoring of novel catalytic materials. This study focuses on B-site doped perovskite-type oxides with a general formula of Nd0.6Ca0.4Fe1-xCuxO3 and Pr0.6Ca0.4Fe1-xCuxO3 (x = 0.0, 0.03, 0.05, 0.10) for potential use as a catalyst for Methanol Steam Reforming via the exsolution of catalytically active Cu nanoparticles. The atomic and electronic structure, morphology, and exsolution behaviour of these materials were investigated experimentally and with density functional theory, with a specific emphasis on the impact of B-site doping with varying Cu content as well as choice of A-site element. Both parameters influenced the crystal structure, surface area, and morphology of the materials. The exsolution behaviour of the materials was observed using in-situ XRD at DESY beamline P02.1 at PETRA III, with nanoparticles forming after reductive treatments on the host oxide surface. The quantity and size of the nanoparticles were found to be adjustable by selecting the A-site ion, doping content at the B-site, and the choice of reducing agent. Materials with higher Cu content on the B-site exhibited facilitated exsolution. Furthermore, exsolution was promoted with Nd as the A-site element compared to Pr. In conclusion, the controlled exsolution of Cu nanoparticles introduces Cu-doped perovskite-type oxides as promising candidates for developing novel catalytic systems. The findings underscore the importance of fine-tuning the oxide composition (A-site element, amount of B-site dopant) to achieve tailored exsolution of nanoparticles, which is crucial for rational material design. By leveraging this knowledge, catalysts with finely tuned properties can be created for specific applications and operational environments.
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•Nanoparticle exsolution on Nd and Pr perovskites verified by synchrotron XRD.•Cu-doping found to improve exsolution.•Number and size of nanoparticles adjustable by catalyst design.•DFT confirms influence of A- and B-site doping.
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