In situ time-resolved XAFS study on the structural transformation and phase separation of Pt3Sn and PtSn alloy nanoparticles on carbon in the oxidation process

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Application of Time-Resolved Synchrotron X-ray Absorption Spectroscopy in an Energy Conversion Reaction

The rational design of high-efficiency catalysts is hindered by the knowledge of active sites, which always experience dynamic transformations within different time scales. In this regard, tracking these time-dependent processes is essential to building the correlation between the active site and catalytic performance. Achieving this goal requires powerful characterization techniques to overcome the obstacle induced by the time mismatch. By virtue of the local structure sensitivity, synchrotron X-ray absorption spectroscopy (XAS) comprising step-scanning XAS, quick-scanning XAS, and energy-dispersive XAS has been widely applied to record structural evolution events. In this Perspective, we highlight the substantial accomplishments achieved by these time-resolved XAS techniques. Their principles, advantages, and limitations involved in monitoring energy-involving electrocatalysis were also introduced. Meanwhile, the key challenges that we are encountering and the further directions of time-resolved XAS are also provided. We sincerely hope that these insights could offer a reliable guideline for other researchers to design more efficient in situ experiments.

Differences in the Electrochemical Performance of Pt-Based Catalysts Used for Polymer Electrolyte Membrane Fuel Cells in Liquid Half- and Full-Cells

A substantial amount of research effort has been directed toward the development of Pt-based catalysts with higher performance and durability than conventional polycrystalline Pt nanoparticles to achieve high-power and innovative energy conversion systems. Currently, attention has been paid toward expanding the electrochemically active surface area (ECSA) of catalysts and increase their intrinsic activity in the oxygen reduction reaction (ORR). However, despite innumerable efforts having been carried out to explore this possibility, most of these achievements have focused on the rotating disk electrode (RDE) in half-cells, and relatively few results have been adaptable to membrane electrode assemblies (MEAs) in full-cells, which is the actual operating condition of fuel cells. Thus, it is uncertain whether these advanced catalysts can be used as a substitute in practical fuel cell applications, and an improvement in the catalytic performance in real-life fuel cells is still necessary. Therefore, from a more practical and industrial point of view, the goal of this review is to compare the ORR catalyst performance and durability in half- and full-cells, providing a differentiated approach to the durability concerns in half- and full-cells, and share new perspectives for strategic designs used to induce additional performance in full-cell devices.

Pt3Sn nanoparticles enriched with SnO2/Pt3Sn interfaces for highly efficient alcohol electrooxidation

Pt₃Sn nanoparticles (NPs) enriched with Pt₃Sn/ultra-small SnO₂ interfaces (Pt₃Sn@u-SnO₂/NG) were synthesized through a thermal treatment of Pt₂Sn/NG in a H₂ atmosphere, followed by annealing under H₂ and air conditions. The unique structure of Pt₃Sn NPs enriched with Pt₃Sn/SnO₂ interfaces was observed on the Pt₃Sn@u-SnO₂/NG catalyst based on HRTEM. The optimized Pt₃Sn@u-SnO₂/NG catalyst achieves high catalytic activity with an ethanol oxidation reaction (EOR) activity of 366 mA mg_Pt ^-1 and a methanol oxidation reaction (MOR) activity of 503 mA mg_Pt ^-1 at the potential of 0.7 V, which are eight-fold and five-fold higher than those for the commercial Pt/C catalyst (44 and 99 mA mg_Pt ^-1, respectively). The Pt₃Sn@u-SnO₂/NG catalyst is found to be 3 times more stable and have higher CO tolerance than Pt/C. The outstanding performance of the Pt₃Sn@u-SnO₂/NG catalyst should be ascribed to the synergetic effect induced by the unique structure of Pt₃Sn NPs enriched with Pt₃Sn/SnO₂ interfaces. The synergetic effect between Pt₃Sn NPs and ultra-small SnO₂ increases the performance for alcohol oxidation because the Sn in both Pt₃Sn and SnO₂ favors the removal of CO_ads on the nearby Pt by providing OH_ads species at low potentials. The present work suggests that the Pt₃Sn@u-SnO₂ is indeed a unique kind of efficient electrocatalyst for alcohol electrooxidation.

Contrasting the EXAFS obtained under air and H2 environments to reveal details of the surface structure of Pt-Sn nanoparticles

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ∼0.09 Å shorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.

Metal-Support Interactions and C1 Chemistry: Transforming Pt-CeO2 into a Highly Active and Stable Catalyst for the Conversion of Carbon Dioxide and Methane

There is an ongoing search for materials which can accomplish the activation of two dangerous greenhouse gases like carbon dioxide and methane. In the area of C1 chemistry, the reaction between CO₂ and CH₄ to produce syngas (CO/H₂), known as methane dry reforming (MDR), is attracting a lot of interest due to its green nature. On Pt(111), high temperatures must be used to activate the reactants, leading to a substantial deposition of carbon which makes this metal surface useless for the MDR process. In this study, we show that strong metal–support interactions present in Pt/CeO₂(111) and Pt/CeO₂ powders lead to systems which can bind CO₂ and CH₄ well at room temperature and are excellent and stable catalysts for the MDR process at moderate temperature (500 °C). The behavior of these systems was studied using a combination of in situ/operando methods (AP-XPS, XRD, and XAFS) which pointed to an active Pt-CeO_2-x interface. In this interface, the oxide is far from being a passive spectator. It modifies the chemical properties of Pt, facilitating improved methane dissociation, and is directly involved in the adsorption and dissociation of CO₂ making the MDR catalytic cycle possible. A comparison of the benefits gained by the use of an effective metal-oxide interface and those obtained by plain bimetallic bonding indicates that the former is much more important when optimizing the C1 chemistry associated with CO₂ and CH₄ conversion. The presence of elements with a different chemical nature at the metal-oxide interface opens the possibility for truly cooperative interactions in the activation of C–O and C–H bonds.

Catalytic conversion of ethane to valuable products through non-oxidative dehydrogenation and dehydroaromatization

Chemical utilization of ethane to produce valuable chemicals has become especially attractive since the expanded utilization of shale gas in the United States and associated petroleum gas in the Middle East. Catalytic conversion to ethylene and aromatic hydrocarbons through non-oxidative dehydrogenation and dehydroaromatization of ethane (EDH and EDA) are potentially beneficial technologies because of their high selectivity to products. The former represents an attractive alternative to conventional thermal cracking of ethane. The latter can produce valuable aromatic hydrocarbons from a cheap feedstock. Nevertheless, further progress in catalytic science and technology is indispensable to implement these processes beneficially. This review summarizes progress that has been achieved with non-oxidative EDH and EDA in terms of the nature of active sites and reaction mechanisms. Briefly, platinum-, chromium- and gallium-based catalysts have been introduced mainly for EDH, including effects of carbon dioxide co-feeding. Efforts to use EDA have emphasized zinc-modified MFI zeolite catalysts. Finally, some avenues for development of catalytic science and technology for ethane conversion are summarized.

In situ, operando studies on the size and structure of supported Pt catalysts under supercritical conditions by simultaneous synchrotron-based X-ray techniques

Investigation of the size and structure of supported Pt catalysts under supercritical conditions leads to a fundamentally new level of understanding of nanoscale materials under extreme conditions.

The importance of direct reduction in the synthesis of highly active Pt–Sn/SBA-15 for n-butane dehydrogenation

Supported Pt–Sn bimetallic catalysts directly reduced by H₂ are highly active for the dehydrogenation of n-butane, while the catalysts calcined in air, followed by H₂ reduction are totally inactive.

Control of crystallographic phases and surface characterization of intermetallic platinum tin nanoparticles

Colloidal synthesis and characterization of intermetallic tin-rich platinum–tin nanoparticles with detailed surface characterization.

Kinetics of Lifetime Changes in Bimetallic Nanocatalysts Revealed by Quick X-ray Absorption Spectroscopy

Alloyed metal nanocatalysts are of environmental and economic importance in a plethora of chemical technologies. During the catalyst lifetime, supported alloy nanoparticles undergo dynamic changes which are well-recognized but still poorly understood. High-temperature O₂ -H₂ redox cycling was applied to mimic the lifetime changes in model Pt₁₃ In₉ nanocatalysts, while monitoring the induced changes by in situ quick X-ray absorption spectroscopy with one-second resolution. The different reaction steps involved in repeated Pt₁₃ In₉ segregation-alloying are identified and kinetically characterized at the single-cycle level. Over longer time scales, sintering phenomena are substantiated and the intraparticle structure is revealed throughout the catalyst lifetime. The in situ time-resolved observation of the dynamic habits of alloyed nanoparticles and their kinetic description can impact catalysis and other fields involving (bi)metallic nanoalloys.

Effects of heat treatment atmosphere on the structure and activity of Pt3Sn nanoparticle electrocatalysts: a characterisation case study

In this work, a variation in heat treatment atmosphere approach is taken to provide a series of related PtSn catalysts with the same nominal composition of Pt₃Sn, but with different surface compositions.

Insight into the structure evolution and the associated catalytic behavior of highly dispersed Pt and PtSn catalysts supported on La2O2CO3 nanorods

This work reports the composition-dependent structure evolution and the associated catalytic behavior of PtSn catalysts supported on La₂O₂CO₃ nanorods.

Dehydrogenation of propane over PtSn/SBA-15 catalysts: effect of the amount of metal loading and state

The state and distribution of Pt and Sn change with different metal amounts, resulting in different catalytic performances on PtSn/SBA-15.