The experimental evidence of the role of surface restructuring during catalytic reactions

Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O₂, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O₂ enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis

Some fundamental concepts of catalysis are not fully explained but are of paramount importance for the development of improved catalysts. An example is the concept of structure insensitive reactions, where surface-normalized activity does not change with catalyst metal particle size. Here we explore this concept and its relation to surface reconstruction on a set of silica-supported Ni metal nanoparticles (mean particle sizes 1-6 nm) by spectroscopically discerning a structure sensitive (CO₂ hydrogenation) from a structure insensitive (ethene hydrogenation) reaction. Using state-of-the-art techniques, inter alia in-situ STEM, and quick-X-ray absorption spectroscopy with sub-second time resolution, we have observed particle-size-dependent effects like restructuring which increases with increasing particle size, and faster restructuring for larger particle sizes during ethene hydrogenation while for CO₂ no such restructuring effects were observed. Furthermore, a degree of restructuring is irreversible, and we also show that the rate of carbon diffusion on, and into nanoparticles increases with particle size. We finally show that these particle size-dependent effects induced by ethene hydrogenation, can make a structure sensitive reaction (CO₂ hydrogenation), structure insensitive. We thus postulate that structure insensitive reactions are actually apparently structure insensitive, which changes our fundamental understanding of the empirical observation of structure insensitivity.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Preparation of nanostructured and nanosheets of MoS2 oxide using oxidation method

Molybdenum disulfide (MoS₂), a two-dimensional transition metal has a 2D layered structure and has recently attracted attention due to its novel catalytic properties. In this study, MoS₂ has been successfully intercalated using chemical and physical intercalation techniques, while enhancing its surface properties. The final intercalated MoS₂ is of many interests because of its low-dimensional and potential properties in in-situ catalysis. In this research, we report different methods to intercalate the layers of MoS₂ successfully using acid-treatment, ultrasonication, oxidation and thermal shocking. The other goal of this study is to form SO bonds mainly because of expected enhanced in-situ catalytic operations. The intercalated MoS₂ is further characterized using analyses such as Fourier Transform Infrared Spectroscopy (FTIR), Raman, Contact Angle, X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Microanalysis (EDAX), Transmission electron microscopy (TEM), and BET.

Some insights in the sonochemical preparation of cobalt nano-particles

The preparation of cobalt nano-particles from a solution of Co(CO)(3)(NO) in n-decane under ultrasonication with a frequency of 20 kHz yielded cobalt particles of a size of ca. 5 nm. The presence of either silica or oleic acid in the solution reduced the particle size to ca. 3 and 2 nm, respectively. The resulting particle size is independent of the ultrasonication time, initial Co(CO)(3)(NO) concentration, ultrasound intensity and solution temperature. It is postulated that bubble collapse generates multiple nucleation sites resulting in the formation of cobalt particles with a rather uniform particle size distribution.

Probing the interface in vapor-deposited bimetallic Pd-Au and Pt-Au films by CO adsorption from the liquid phase

Bimetallic Pd-Au and Pt-Au and monometallic Pd, Pt, and Au films were prepared by physical vapor deposition. The resulting surfaces were characterized by means of XPS, AFM, and CO adsorption from the liquid phase (CH2Cl2) monitored by ATR-IR spectroscopy. CO adsorption combined with ATR-IR proved to be a very sensitive method for probing the degree of interdiffusion occurring at the interfaces whose properties were altered by variation of the Pd and Pt film thickness from 0.2 to 2 nm. Because no CO adsorption was observed on Au, the evaporation of Pt-group metals on Au allowed us to study the effect of dilution on the adsorption properties of the surfaces. At equivalent Pd film thickness, the evaporation of Au reduced the amount of adsorbed CO and caused the formation of 2-fold bridging CO, which was almost absent in monometallic surfaces. Additionally, the average particle size on Pd-Au surfaces was smaller than that on monometallic Pd surfaces. The results indicate that a Pd/Au diffuse interface is formed that affects the Pd particle size even more drastically than the simple decrease in Pd film thickness in monometallic surfaces. Pt-Au surfaces were less sensitive to CO adsorption, indicating that the two metals do not mix to a significant extent. The difference in the interfacial behavior of Pd and Pt in the bimetallic gold films is traced to the largely different Pd-Au and Pt-Au miscibility gaps.

DFT Studies of Pt/Au Bimetallic Clusters and Their Interactions with the CO Molecule

Density functional theory (DFT) calculations were performed to study Pt/Au clusters of different size, structure, and composition as well as their interactions with a CO molecule. Among the Pt/Au isomers studied here, the planar structure is the most stable structure in many Pt compositions, although three-dimensional structures become more stable with increasing Pt composition. Furthermore, structures with the Pt atoms surrounded by Au atoms are more stable among homotops. However, these conclusions will be altered if ligands are attached to the Pt/Au bimetallic clusters, as evidenced from the results of CO adsorption. When both Au and Pt sites are exposed, CO adsorption at the Pt site is stronger. If only a Au site is available for CO adsorption, the strongest adsorption occurs at approximately 25% Pt composition, which may correlate with the experimentally observed reactivity of the core-shell structured Pt/Au nanoparticles.