Structure-sensitive Reaction Research Articles

Size-dependent effects of Cu° nanoparticles on electronic properties and ethanol dehydrogenation catalysis via Cu⁺-O-Cu⁺ species

Copper (Cu) nanoparticles, widely utilized as catalysts in industrial applications, exhibit intriguing behavior in structure-sensitive reactions like ethanol dehydrogenation. Contrary to expectations, the catalytic activity does not consistently increase as the size of Cu nanoparticles decreases. In Cu catalysts, the particle size significantly effects the Cu0/Cu+ ratio on the nanoparticle surface. Decreasing the size of Cu nanoparticles promotes their oxidation, leading to the formation of Cu+ and O2− species. This alteration subsequently influences the structural and electronic properties of the catalytic Cu sites. Through Cu nanoparticle calculations, the presence of an O ad-atom was identified to modify the electron density at the Cu site, thereby altering the ethanol adsorption energy and impacting the overall catalytic activity. The increase in O coverage, due to size effects, resulted in a notable decrease in the heat of adsorption on Cu nanoparticles (by approximately 20 kJ mol− 1). The observed high ΔEads indicates that more ethanol molecules could reach the transition state for the reaction, subsequently augmenting turnover rates (TOF) and reducing the apparent activation energy (Eaap) concerning Cu particle size.

Structure Sensitive Reaction Kinetics of Chiral Molecules on Intrinsically Chiral Surfaces.

Enantiospecific heterogeneous catalysis utilizes chiral surfaces to resolve enantiomers via structure sensitive surface chemistry. The catalyst design challenge is the identification of chiral surface structures that maximize enantiospecificity. Herein, we develop data driven models for the enantiospecificity of tartaric acid reactions on chiral Cu(hkl)R&S surfaces. Measurements of enantiospecific rate constants were obtained by using curved Cu(hkl)R&S surfaces that enable kinetic measurements on hundreds of chiral surface orientations. One model uses feature vectors derived from generalized coordination numbers to capture the local structure around Cu atoms exposed by the Cu(hkl)R&S surfaces. The second model introduces the use of chiral cubic harmonic functions to capture the symmetry constraints of the face-centered cubic Cu structure. The model using 58 generalized coordination numbers has a fitting error similar to that of the model using only 5 cubic harmonic functions. The two models predict maxima in the enantiospecificity on surfaces with very similar surface orientations. The models developed in this work are applicable for any enantiospecific reaction happening on any chiral material with a cubic lattice structure, opening the way to understanding the surface structure sensitivity of the enantiospecific reaction kinetics.

Facet sensitivity of iron carbides in Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) is a structure-sensitive reaction of which performance is strongly related to the active phase, particle size, and exposed facets. Compared with the full-pledged investigation on the active phase and particle size, the facet effect has been limited to theoretical studies or single-crystal surfaces, lacking experimental reports of practical catalysts, especially for Fe-based catalysts. Herein, we demonstrate the facet sensitivity of iron carbides in FTS. As the prerequisite, {202} and {112} facets of χ-Fe5C2 are fabricated as the outer shell through the conformal reconstruction of Fe3O4 nanocubes and octahedra, as the inner cores, respectively. During FTS, the activity and stability are highly sensitive to the exposed facet of iron carbides, whereas the facet sensitivity is not prominent for the chain growth. According to mechanistic studies, {202} χ-Fe5C2 surfaces follow hydrogen-assisted CO dissociation which lowers the activation energy compared with the direct CO dissociation over {112} surfaces, affording the high FTS activity.

P-modified Pt/Al2O3 catalysts for selective propane dehydrogenation

The rigorous operating conditions required for propane dehydrogenation result in undesired side reactions on Pt catalysts. Herein, we present the successful suppression of structure-sensitive side reactions by modifying Pt/Al2O3 catalysts with P. The characterization of the P-modified Pt/Al2O3 catalysts revealed the coexistence of metallic Pt and Pt–P alloys on the surfaces of the Pt nanoparticles. At a reaction temperature of 600 °C, the Pt catalyst modified with an optimal amount of P exhibited a notably higher propylene selectivity (95%) than that of the unmodified Pt/Al2O3 catalyst (typically ∼83%), without any loss of activity. A detailed investigation revealed that the geometric effect resulting from the formation of the Pt–P alloy, which reduced the size of the Pt ensembles, was mainly responsible for the enhanced performance of P-modified Pt/Al2O3. This study offers vital insights into the formation of noble metal–P alloys on nanoparticle surfaces and their advantageous effects on heterogeneous catalysis.

Selective reduction of NO by CO using copper-aluminum oxides from hydrotalcite-type precursors: Study of chemical species and thermal stability

The objective of this work was to analyze the influence of Al on copper catalysts, regarding thermal stability and performance during NO reduction by CO. Two copper-based catalysts were prepared by precipitation and subsequent calcination at 600 °C, with and without aluminum in their composition (CuAl-HT-c and Cu-p). The precursor CuAl-HT was synthesized to obtain a hydrotalcite structure. The catalysts obtained after calcination of the precursors were characterized by N2 physisorption, TPR, N2O titration, in situ XRD and in situ XANES. They showed good catalytic performance, however with Cu-p being the most active but with larger formation of N2O (important greenhouse gas). In this catalyst, the interaction between the adsorbed NO molecules is facilitated, probably due to the higher concentration of Cu+.Regarding the aged catalysts, Cu-p was deactivated due to sintering, which is attributed to the lack of aluminum. CuAl-HT-c-900 had its performance improved during aging, either because the reduction of NO by CO is a structure-sensitive reaction for this catalyst (and the increase in particle size after aging is related to larger activity), or because of the presence of copper aluminate, which can also improve activity and selectivity.

Effect of Pt nanoparticle size on resistance to chloride-induced inhibition for hydrodechlorination of trichloroethylene in water

Catalysts used for hydrodechlorination (HDC) of trichloroethylene (TCE) undergo partial or complete loss of catalytic activity due to inhibition by inevitable reaction product HCl. Changing the size of Pt NPs could be used as a strategy to prevent inhibition of HDC catalysts if the structure sensitivity of the reaction was known. This study aims to obtain structure-sensitivity of aqueous-phase HDC of TCE and investigate the effect of Pt NPs’ size on resistance to chloride inhibition. The research encompassed the synthesis of Pt NPs, characterization studies, and catalytic activity measurements at 30 °C & 1 atm H2 in batch or discontinuous pulse mode. Characterization results showed that targeted Pt NPs were successfully synthesized with average particle sizes of 3.0 nm, 5.8 nm, 8.9 nm, and 60.9 nm. Catalytic activity experiments revealed that initial turnover frequency values exhibited a decrease with increasing particle size of Pt NPs, indicating that aqueous-phase HDC of TCE is a structure sensitive reaction. The fastest kinetics and the highest resistance to HCl inhibition were observed over the smallest Pt NPs employed in this study. This information can serve as a rational synthesis parameter for designing catalysts that could mitigate chloride inhibition caused by the reaction product HCl.

Toward High-Energy-Density Fuels for Direct Liquid Organic Hydrogen Carrier Fuel Cells: Electrooxidation of 1-Cyclohexylethanol.

Electrochemically active liquid organic hydrogen carriers (EC-LOHCs) can be used directly in fuel cells; so far, however, they have rather low hydrogen storage capacities. In this work, we study the electrooxidation of a potential EC-LOHC with increased energy density, 1-cyclohexylethanol, which consists of two storage functionalities (a secondary alcohol and a cyclohexyl group). We investigated the product spectrum on low-index Pt single-crystal surfaces in an acidic environment by combining cyclic voltammetry, chronoamperometry, and in situ infrared spectroscopy, supported by density functional theory. We show that the electrooxidation of 1-cyclohexylethanol is a highly structure-sensitive reaction with activities Pt(111) ≫ Pt(100) > Pt(110). Most importantly, we demonstrate that 1-cyclohexylethanol can be directly converted to acetophenone, which desorbs from the electrode surface. However, decomposition products are formed, which lead to poisoning. If the latter side reactions could be suppressed, the electrooxidation of 1-cyclohexylethanol would enable the development of EC-LOHCs with greatly increased hydrogen storage capacities.

The relationship between structure and performance of Mo-ZrO2 catalysts in CO2 hydrogenation

CO₂ hydrogenation is a structure-sensitive reaction, producing a diverse range of products based on the structure of catalysts. In this study, we present a catalyst Mo-ZrO2 for CO2 hydrogenation and observed the products vary among CO, CH4, and CH3OH as Mo changes in its loading and local environment. Kinetic studies reveal that increasing Mo loading amplifies hydrogenation ability of the catalyst while simultaneously reducing CO2 adsorption on ZrO2. Specifically, Mo loadings under 10 wt% result in Mo integration into ZrO2 support, forming a Mo-O-Zr structure. Conversely, a higher Mo loading leads to the formation of Zr(MoO4)2 clusters on ZrO2 support. The Mo5⁺-Ov-Zr structure, formed in Mo-doped ZrO2 during reduction pretreatment, emerges as an active site for methanol synthesis. In contrast, metallic Mo formed from the reduction of Zr(MoO4)2 clusters loaded on ZrO2 is responsible for methane formation. The evolution of the surface structure from Mo-O-Zr structure to Zr(MoO4)2 clusters on ZrO2 as Mo loading changes, resulting in alterations in the number of active sites for methanol synthesis and methanation, consequently leading to variations in product distribution.

Phyllosilicate-derived Ni catalysts with small nanoparticle size and strong metal-support interaction for efficient and robust decomposition of ammonia

Ammonia decomposition is a structure-sensitive reaction, so the difference in structure of similar catalysts may have a great impact on the catalytic performance of ammonia decomposition reaction. However, it is not clear which structural properties can play a role in ammonia decomposition reaction and the degree of influence on catalytic performance. To explore this question, ammonia evaporation-hydrothermal (AEH), impregnation (IM), and evaporation-induced self-assembly (EISA) methods were used to synthesize Ni/SiO2 catalysts to obtain carbon-free hydrogen from catalyzing NH3 decomposition reaction. Among the three, the Ni/SiO2 catalyst synthesized via ammonia evaporation-hydrothermal method is the smallest in terms of Ni nanoparticles (∼3.0 nm) and the strongest Ni-SiO2 interaction. For ammonia decomposition, it is the highest in activity and thermal stability. The NH3 conversion at 650 °C and 30 000 mL gcat−1h−1 (GHSV) over Ni/SiO2-AEH was close to 90 % and remained stable in an evaluation period of 60 h.

A disordered oxide as an active phase during CO catalytic oxidation on Ru(0001)

The identification of the active phase of a catalyst under operando conditions is probably the most relevant task in basic catalysis research. In this respect, the Ru(0001) surface during CO oxidation is a traditional studied model, for both its importance in applied catalysis and for its peculiar behavior. In this work, we study the CO catalytic oxidation reaction on flat and defective Ru(0001) surfaces, concluding that the highest activity is achieved in oxidizing conditions when a disordered and thin Ru oxide is formed at the surface. Also, we find that in defective surfaces, with a high density of surface steps, this reaction is enhanced, related to the facile sub-surface incorporation of atomic oxygen and to the easier formation of the oxide. Thus, the CO catalytic oxidation on Ru(0001) is a structure-sensitive reaction, being active on the surface of the oxide formed in reaction conditions.

Automated descriptor selection, volcano curve generation, and active site determination using the DescMAP software

The material space for catalyst discovery is expansive. Volcano curves are traditionally employed to provide physical insights into optimal catalyst characteristics for new material selection. Their generation lies on a single descriptor picked using expert knowledge. Here we present DescMAP, a Python-based software, to automate the selection of descriptors, the generation of volcano maps, and the identification of active sites for structure-sensitive reactions. We consider traditional energy-based and geometric descriptors for structure-sensitive reactions. DescMAP is integrated with the Virtual Kinetic Laboratory (VLab) to provide multiple functionalities. It inputs spreadsheets or template files for flexibility and outputs interactive graphs for post-processing. We demonstrate its features using the non-oxidative dehydrogenation of ethane to ethylene over (111) closed-packed surfaces and the methane total oxidation over various Pt facets. It can be easily applied to other complex chemistries and achieves quick screening of potential catalysts. Program summaryProgram title: DescMAPCPC Library link to program files:https://doi.org/10.17632/g399b3xyfy.1Developer's repository link:https://github.com/VlachosGroup/DescriptorMapCode Ocean Capsule:https://codeocean.com/capsule/7598436Licensing provisions: MIT licenseProgramming language: PythonExternal routines: pMuTT, NumPy, Pandas, Matplotlib, Plotly, Scikit-learn, ScipyNature of problem: Screening potential catalysts via microkinetic modeling and generating volcano curves is time-consuming. An automated tool to accelerate this process is lacking.Solution method: Python package with descriptor selection that automates descriptor-based microkinetic modeling. Interactive volcano curves generated for post-analysis.

Dynamics of Pd Subsurface Hydride Formation and Their Impact on the Selectivity Control for Selective Butadiene Hydrogenation Reaction.

Structure-sensitive catalyzed reactions can be influenced by a number of parameters. So far, it has been established that the formation of Pd-C species is responsible for the behavior of Pd nanoparticles employed as catalysts in a butadiene partial hydrogenation reaction. In this study, we introduce some experimental evidence indicating that subsurface Pd hydride species are governing the reactivity of this reaction. In particular, we detect that the extent of formation/decomposition of PdHx species is very sensitive to the Pd nanoparticle aggregate dimensions, and this finally controls the selectivity in this process. The main and direct methodology applied to determine this reaction mechanism step is time-resolved high-energy X-ray diffraction (HEXRD).

Adsorption and activation of CO on perfect and defective h-Fe7C3 surfaces for Fischer-Tropsch synthesis

As a structure-sensitive reaction, one important issue in Fischer–Tropsch synthesis (FTS) reaction is to clarify the relationship between the iron catalyst composition and its effects on activity and selectivity. In this work, the adsorption and activation of CO on both perfect and defective h-Fe7C3 surfaces were investigated theoretically. The results indicate that the direct CO dissociation pathway is preferred with the effective energy barriers less than 1.50 eV on both perfect (101) and (11¯1) surfaces, which play a dominant role in FTS reaction. Instead, the effective energy barriers of the preferred HCO pathway on the perfect (11¯1¯) and (11¯0) surfaces are higher than 2.40 eV. When defects occur on the surfaces, CO is inclined to adsorb on resemble B5 site and direct CO bond dissociation with barriers around 0.8 eV is more favorable than hydrogenation routes for all the four defective surfaces. Thus it can be concluded that the presence of B5-like site can facilitate to form a multiple coordinated state and lead to a stretched CO bond. Specifically, we have theoretically revealed that on B5-like site, the stronger interaction between CO and h-Fe7C3 surface with more electrons transformation, the higher activation of CO bond facilitated to CO direct dissociation.

Natural attapulgite supported nano-Ni catalysts for the efficient reductive amination of biomass-derived aldehydes and ketones

The efficient synthesis of useful primary amines via reductive amination of biomass-based aldehydes and ketones over earth-abundant base metal catalysts is an attractive biomass value-adding technology yet facing lots of challenges. Herein, natural attapulgite (ATP) was applied as support for the fabrication of active Ni catalysts with different Ni loadings (5–30 ​wt%) by the deposition-precipitation method. The Ni/ATP-550 catalyst with 10–15 ​wt% Ni loadings was found to present the highest catalytic performance for the synthesis of valuable 5-amino-1-pentanol (5-AP) via reductive amination of biofurfural-derived 2-hydroxytetrahydropyran among a variety of commonly used oxide supports loaded Ni catalysts, as well as ATP supported nickel catalysts with other loadings, achieving 5-AP yield up to 94%. The intrinsic activity of the Ni/ATP catalysts was found to depend strongly on the Ni0 crystallite size and Ni0 fraction of the catalysts, which generally increased with increasing Ni0 crystallite size and fraction, owing probably to the hydrogenation of imine intermediate is a structure-sensitive reaction. The efficient 10Ni/ATP-550 catalyst also exhibited good activity and stability in the reductive amination of several other biomass-derived aldehydes and ketones to their corresponding primary amines with good to excellent yields (81%–99%). This work provided a clean and efficient natural ATP-supported non-noble metal nickel-based catalytic system for the reductive amination of aldehydes and ketones to synthesize high-value-added primary amines, which could be a promising candidate for the industrial production of amines.

Rate Equations of Structure-Sensitive Catalytic Reactions with Arbitrary Kinetics

A general mathematical framework for the quantitative description of the cluster size dependence in heterogeneous catalytic reactions has been developed based on an analysis of the Gibbs energy of elementary reactions. The methodology was illustrated for a generic linear sequence of elementary reactions with three steps, a multi-step mechanism of ethanol oxidation comprising linear, nonlinear and quasi-equilibria steps and a network of parallel reactions in transformations of furfural.

The Necessity for Multiscale In Situ Characterization of Tailored Electrocatalyst Nanoparticle Stability

Tailored nanoparticles have opened up several exciting avenues to boost the activity and selectivity of structure-sensitive electrocatalytic reactions, such as the electrochemical carbon dioxide (CO2) reduction (eCO2RR). Colloidal chemistry provides the perfect toolbox to synthesize electrocatalyst nanoparticles on demand with atomic precision, in order to control and steer the electrocatalytic reactions of interest to the desired products. Not only does colloidal chemistry offer a means to prepare nanoparticles with well-defined sizes and shapes, it also allows easy deposition on any desired substrate (e.g., porous substrates) due to the solution processability. But what you see after synthesis with ex situ characterization techniques is not always what you get during the electrocatalytic reaction. Like any other electrocatalyst material, colloidal nanoparticles are prone to restructuring, and hence, the reaction output is altered due to destabilization of the electrocatalyst. This destabilization of the electrocatalyst nanoparticles is currently one of the major bottlenecks for the widespread implementation of electrocatalysts in the chemical industry. This calls for the necessary development and application of in situ characterization techniques to probe the morphology and composition of the tailored electrocatalyst nanoparticles over multiple length scales, in order to rationally design the next generation of stable electrocatalyst nanoparticles. Through detailed spatiotemporal in situ characterization, we can take full advantage of the possibilities that colloidal chemistry offers for electrocatalyst preparation with superior activity, selectivity, and stability. In this perspective, the necessity for in situ characterization of electrocatalyst nanoparticle stability is highlighted. For this purpose, first the progress of colloidal nanoparticles for electrocatalytic conversion reactions is briefly discussed, after which the focus shifts toward in situ characterization of the (in)stability of the tailored nanoparticles during the reaction of interest, ideally under industrially relevant conditions. This perspective shows that in situ characterization of electrocatalyst deactivation requires a multiscale approach, and that without combined in situ characterization we will remain blind to several aspects that are known to influence electrocatalyst performance.

Controlled-Release Mechanism Regulates Rhodium Migration and Size Redistribution Boosting Catalytic Methane Conversion

The migration of Rh atoms under a gas/reactive environment has a great impact on the dynamic restructuring and size redistribution of Rh catalysts in a variety of structure-sensitive catalytic reactions. To date, regulating the size distribution of active Rh species via controlled atomic migration remains challenging. Here, we show a controlled-release mechanism to regulate Rh atom migration through two-dimensional (2D) zeolite nanosheets, enabling quasi-continuous size redistribution of active Rh species from a single atom to nanoparticles with reversibility. Utilizing state-of-the-art in situ characterizations, a reversible aggregation/redispersion of Rh catalysts in/out of the 2D zeolite was directly observed under H2 or CO environment. The interplay between gas environment and support confinement was disclosed that substantially restrained the dynamic migration of Rh species and enabled a quasi-continuous control of Rh size distribution over a wide temperature window and size range. The catalytic testing for mild oxidation of methane demonstrated a volcano correlation of methanol activity with increasing particle size. The sub-nanometer Rh clusters with an average diameter of 0.9 nm exhibited the highest methanol activity of 39.7 molCH3OOH·molRh–1·h–1 with remarkable selectivity as high as 73.2%, far beyond that of single atom species and larger particles. Density functional theory calculations and electron paramagnetic resonance spectroscopy further revealed that this size dependency is related to the preferential formation of •OH radicals on Rh clusters with different sizes, which acts as a key initiator for methane activation. Our results thus provide a practical approach to synthesize size-specific metal catalysts via controlled atomic migration.

Dynamics of Co/Co2C redox cycle and their catalytic consequences in Fischer–Tropsch synthesis on cobalt–manganese catalysts

Fischer–Tropsch synthesis over CoMn oxides is a surface-catalyzed structure-sensitive reaction. Active site changes greatly alter product distribution. The dynamics and essence of the changes in Co species during the reaction have seldom been explored. Herein, multiple characterizations were applied to confirm that two types of active sites, i.e., Co and Co2C, coexist on CoMnOx under the reaction conditions. Apart from the direct participation of carbon species in the reaction, the steady-state isotopic transient kinetic analysis also suggested the mutual transformation of carbon species in Co2C. The ratio of the two paths altered with the temperature, thus leading to Co2C-rich and Co-rich surfaces at intermediate and high temperatures, respectively. Kinetic calculations and microkinetic modeling showed that Co2C-rich surfaces facilitated CO adsorption and suppressed hydrogen adsorption. This gave high carbon-chain growth activity. The Co-rich surfaces showed high vacancy and hydrogen coverages, lowering the CO activation and CH4 formation energies.

Interfacial Auδ--OV-Zr3+ structure promoted C[sbnd]H bond activation for oxidative esterification of methacrolein to Methyl methacrylate

Constructing abundant metal-support interfacial active sites is an effective strategy to boost catalytic performance toward interfacial electronic structure-sensitive reactions, such as oxidative esterification of oxygenates. Herein, we successfully promote the generation of the interfacial active sites via tuning the crystal phases of Au/ZrO2 catalysts. Multi-characterizations (such as in-situ FT-IR spectra and reaction kinetics) and density functional theory (DFT) calculations demonstrated that the interfacial Auδ--OV-Zr3+ structure with the synergistic effect exhibits strong electrons transfer from tetragonal-ZrO2 to Au atom. Specifically, the OV-Zr3+ could facilitate the adsorption of methacrolein substrate, and the electron-rich Auδ- species could promote the CH bond activation of the CH2C(CH3)CHOOCH3 intermediate. Therefore, the interface-rich Au/t-ZrO2 catalyst achieved nearly 90 % yield of MMA and excellent catalyst stability without the incorporation of second promoter. The outcome of this work could provide some insights into developing an efficient Au-based catalyst in the oxidative esterification system.

Structure Sensitivity of Ammonia Synthesis on Cobalt: Effect of the Cobalt Particle Size on the Activity of Promoted Cobalt Catalysts Supported on Carbon

This work presents a size effect, i.e., catalyst surface activity, as a function of active phase particle size in a cobalt catalyst for ammonia synthesis. A series of cobalt catalysts supported on carbon and doped with barium was prepared, characterized (TEM, XRPD, and H2 chemisorption), and tested in ammonia synthesis (9.0 MPa, 400 °C, H2/N2 = 3, 8.5 mol% of NH3). The active phase particle size was varied from 3 to 45 nm by changing the metal loading in the range of 4.9–67.7 wt%. The dependence of the reaction rate expressed as TOF on the active phase particle size revealed an optimal size of cobalt particles (20–30 nm), ensuring the highest activity of the cobalt catalyst in the ammonia synthesis reaction. This indicated that the ammonia synthesis reaction on cobalt is a structure-sensitive reaction. The observed effect may be attributed to changes in the crystalline structure, i.e., the appearance of the hcp Co phase for the particles with a diameter of 20–30 nm.