Controlling Selectivity in Unsaturated Aldehyde Hydrogenation Using Single-Site Alloy Catalysts

Tuning the Hydrogenation Selectivity of an Unsaturated Aldehyde via Single-Atom Alloy Catalysts

Selective hydrogenation of α,β-unsaturated aldehydes to produce unsaturated alcohols remains a challenge in catalysis. Here, we explore, on the basis of first-principles simulations, single-atom alloy (SAA) catalysts on copper as a class of catalytic materials to enhance the selectivity for C═O bond hydrogenation in unsaturated aldehydes by controlling the binding strength of the C═C and C═O bonds. We show that on SAA of early transition metals such as Ti, Zr, and Hf, the C═O binding mode of acrolein is favored but the strong binding renders subsequent hydrogenation and desorption impossible. On SAA of late-transition metals, on the other hand, the C═C binding mode is favored and C═C bond hydrogenation follows, resulting in the production of undesired saturated aldehydes. Mid-transition metals (Cr and Mn) in Cu(111) appear as the optimal systems, since they favor acrolein adsorption via the C═O bond but with a moderate binding strength, compatible with catalysis. Additionally, acrolein migration from the C═O to the C═C binding mode, which would open the low energy path for C═C bond hydrogenation, is prevented by a large barrier for this process. SAA of Cr in Cu appears as an optimal candidate, and kinetic simulations show that the selectivity for propenol formation is controlled by preventing the acrolein migration from the more stable C═O to the less stable C═C binding mode and subsequent H-migration and by the formation of the O-H bond from the monohydrogenated intermediate. Dilute alloy catalysts therefore enable tuning the binding strength of intermediates and transition states, opening control of catalytic activity and selectivity.

Designing Efficient Single-Atom Alloy Catalysts for Selective C═O Hydrogenation: A First-Principles, Active Learning and Microkinetic Study

Selective hydrogenation of α,β-unsaturated aldehydes into unsaturated alcohols is a process in high demand in organic synthesis, pharmaceuticals, and food production. This process requires the precise hydrogenation of C═O bonds, a challenge that requires a tailored catalyst. Single-atom alloys (SAAs), where individual atoms of one metal are distributed in a host metal matrix, offer a potential solution to this challenge. Nevertheless, identifying the appropriate SAA capable of targeted adsorption and the efficient activation of C═O bonds remains a substantial hurdle. In this work, we synergistically combine density functional theory (DFT) calculations, active learning, and microkinetic simulations to design SAAs for the efficient and selective hydrogenation of α,β-unsaturated aldehydes. We first comprehensively assessed the potential of 66 SAAs across 264 surfaces (including (100), (110), (111), and (320) crystal planes), to gauge their potential in activating C═C and C═O bonds. Our assessment unveiled the excellent selectivity of the Ti1Au SAA in activating C═O bonds. Moreover, our detailed DFT calculations further demonstrated the high catalytic activity of Ti1Au(320) and Ti1Au(111) surfaces with a low activation energy barrier of only 0.60 eV. Subsequently, we conducted microkinetic simulations on the selective hydrogenation process of crotonaldehyde, by selecting Ti1Au (320) and (111) surfaces as the catalysts and demonstrated that they exhibited a remarkable selectivity and nearly 100% conversion toward crotyl alcohol in the temperature range from 373 to 553 K. The present study not only reveals novel SAAs for targeted hydrogenation of α,β-unsaturated aldehydes but also establishes a promising path toward efficient design of selective hydrogenation catalysts more broadly.

Contrasting Capability of Single Atom Palladium for Thermocatalytic versus Electrocatalytic Nitrate Reduction Reaction

The occurrence of high concentrations of nitrate in various water resources is a significant environmental and human health threat, demanding effective removal technologies. Single atom alloys (SAAs) have emerged as a promising bimetallic material architecture in various thermocatalytic and electrocatalytic schemes including nitrate reduction reaction (NRR). This study suggests that there exists a stark contrast between thermocatalytic (T-NRR) and electrocatalytic (E-NRR) pathways that resulted in dramatic differences in SAA performances. Among Pd/Cu nanoalloys with varying Pd-Cu ratios from 1:100 to 100:1, Pd/Cu(1:100) SAA exhibited the greatest activity (TOFPd = 2 min-1) and highest N2 selectivity (94%) for E-NRR, while the same SAA performed poorly for T-NRR as compared to other nanoalloy counterparts. DFT calculations demonstrate that the improved performance and N2 selectivity of Pd/Cu(1:100) in E-NRR compared to T-NRR originate from the higher stability of NO3* in electrocatalysis and a lower N2 formation barrier than NH due to localized pH effects and the ability to extract protons from water. This study establishes the performance and mechanistic differences of SAA and nanoalloys for T-NRR versus E-NRR.

Crotonaldehyde Adsorption on Cu-Pt Surface Alloys: A Quantum Mechanics Study

The adsorption of crotonaldehyde on Cu-Pt alloy surfaces was characterized by density functional theory (DFT). Two surfaces were considered: Cu2Pt/Cu(111) and Cu3Pt/Cu(111). It was determined that the presence of Pt on the surface, even when isolated as single atoms fully surrounded by Cu, provides additional stability for the adsorbates, increasing the magnitude of the adsorption energy by as much as 40 kJ/mol. The preferred bonding on both surfaces is via multiple coordination, with the most stable configuration being a cis arrangement with di-σ bonding of the C=O bond across a Cu–Cu bridge and an additional π bonding to a Pt atom. The fact that Pt significantly affects the adsorption of unsaturated aldehydes such as crotonaldehyde explains why the kinetics of their hydrogenation using single-atom alloy (SAA) catalysts vary with alloy composition, as we previously reported, and brings into question the simple model in which the role of Pt is only to promote the dissociation of H2.

In situ identification of surface sites in Cu-Pt bimetallic catalysts: Gas-induced metal segregation

The effect of gases on the surface composition of Cu-Pt bimetallic catalysts has been tested by in situ infrared (IR) and x-ray absorption spectroscopies. Diffusion of Pt atoms within the Cu-Pt nanoparticles was observed both in vacuum and under gaseous atmospheres. Vacuum IR spectra of CO adsorbed on CuPt_x/SBA-15 catalysts (x = 0-∞) at 125 K showed no bonding on Pt regardless of Pt content, but reversible Pt segregation to the surface was seen with the high-Pt-content (x ≥ 0.2) samples upon heating to 225 K. In situ IR spectra in CO atmospheres also highlighted the reversible segregation of Pt to the surface and its diffusion back into the bulk when cycling the temperature from 295 to 495 K and back, most evidently for diluted single-atom alloy catalysts (x ≤ 0.01). Similar behavior was possibly observed under H₂ using small amounts of CO as a probe molecule. In situ x-ray absorption near-edge structure data obtained for CuPt_0.2/SBA-15 under both CO and He pointed to the metallic nature of the Pt atoms irrespective of gas or temperature, but analysis of the extended x-ray absorption fine structure identified a change in coordination environment around the Pt atoms, from a (Pt-Cu):(Pt-Pt) coordination number ratio of ∼6:6 at or below 445 K to 8:4 at 495 K. The main conclusion is that Cu-Pt bimetallic catalysts are dynamic, with the composition of their surfaces being dependent on temperature in gaseous environments.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Applications of Single Atom Catalysts for Environmental Management

With the rapid development of industrialization, human beings have caused many negative effects on the environment that have endangered the survival and development of human beings, such as the greenhouse effect, water pollution, energy depletion, etc [...].

Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts

The dissociation of H₂ is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H₂-D₂ exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H₂-D₂ exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H₂ is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H₂-D₂ exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H₂ pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H₂-D₂ exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Tuning Ligand-Coordinated Single Metal Atoms on TiO2 and their Dynamic Response during Hydrogenation Catalysis

Ligand-coordinated supported catalysts (LCSCs) are of growing interest for heterogeneous single-atom catalysis. Here, the effect of the choice of organic ligand on the activity and stability of TiO₂ -supported single-atom Pt-ligand catalysts was investigated for ethylene hydrogenation. The activity of these catalysts showed a significant dependence on the choice of ligand and also correlated with coordination number for Pt-ligand and Pt-Cl^- . Of the three ligands examined in this study, the one with the lowest Pt coordination number, 1,10-phenanthroline-5,6-dione (PDO), showed the lowest reaction temperature and highest reaction rate, likely due to those metal sites being more accessible to reactant adsorption. In-situ X-ray absorption spectroscopy (XAS) experiments showed that the activity also correlated with good heterolytic dissociation of hydrogen, which was supported by OH/OD exchange experiments and was the rate-determining step of the hydrogenation reaction. In these in-situ XAS experiments up to 190 °C, the supported Pt-ligand catalyst showed excellent stability against structural and chemical change. Instead of Pt, the PDO ligand could be coordinated with Ir on TiO₂ to form Ir LCSCs that showed slow activation by loss of Ir-Cl bonds, then excellent stability in the hydrogenation of ethylene. These results provide the chance to engineer ligand-coordinated supported catalysts at the single-atom catalyst level by the choice of ligand and enable new applications at relatively high temperature.

A new family of copper-based MXenes

In this work, we demonstrate, through first-principles calculations, the existence of a new family of copper-based MXenes. These add up new structures to the previously reported universe and span the interest of such 2D materials for applications in heterogeneous catalysis, ion-based batteries, sensors, biomedical applications, and so on. First, we propose the MXene-like structures: Cu₂N, Cu₂C, and Cu₂O. Phonon spectra calculations confirmed their dynamical stability by showing just positive frequencies all through the 2D Brillouin zone. The new MXenes family displays metallic characteristics, mainly induced by the Cu-3d orbitals. Bader charge analysis and charge density differences depict bonds with ionic character in which Cu is positively charged, and the non-metal atom gets an anionic character. Also, we investigate the functionalization of the proposed structures with Cl, F, O, and OH groups. Results show that the H3 site is the most favorable for functionalization. In all cases, the non-magnetic nature and metallic properties of the pristine MXenes remain. Our results lay the foundations for the experimental realization of a new MXenes family.

Copper-based single-atom alloys for heterogeneous catalysis

Heterogeneous catalysts, as crucial industrial commodities, play an important role in industrial production, especially in energy catalysis. Traditional noble metal catalysts cannot meet the increasing demand. Therefore, the exploration of cost-effective catalysts with high activity and selectivity is important to promote chemical production. Single-atom alloy (SAA) catalysts reduce the use of precious metals compared with traditional catalysts. The unique structure of SAAs, extremely high atom utilization and high catalytic selectivity give them a prominent position in heterogeneous catalysis. SAAs are widely used in selective hydrogenation/dehydrogenation, carbon dioxide reduction reaction (CO2RR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitric oxide reduction reaction (NORR). Here, the applications and research progress of copper-based single-atom alloys in the various catalytic reactions mentioned above are mainly introduced, and the factors (such as synthesis method, composition content, etc.) affecting the catalytic performance are analyzed using a combination of various characterization and testing methods.

Adsorption of crotonaldehyde on metal surfaces: Cu vs Pt

The thermal chemistry of crotonaldehyde on the surface of a polished polycrystalline copper disk was characterized by temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) and contrasted with previous data obtained on a Pt(111) single crystal substrate. A clear difference in the adsorption mode was identified between the two surfaces, highlighted by the prevalence of RAIRS peaks for the C=C bond on Cu vs for C=O on Pt. Adsorption was also determined to be much weaker on Cu vs Pt, with an adsorption energy on the former ranging from -50 kJ/mol to -65 kJ/mol depending on the surface coverage. The experimental data were complemented by extensive quantum mechanics calculations using density functional theory (DFT) to determine the most stable adsorption configurations on both metals. It was established that crotonaldehyde adsorption on Cu occurs via the oxygen atom in the carbonyl group, in a mono-coordinated fashion, whereas on Pt multi-coordination is preferred, centered around the C=C bond. The contrasting surface adsorption modes seen on these two metals are discussed in terms of the possible relevance to selectivity in single-atom alloy hydrogenation catalysis.

Atomically dispersed Ru in Pt3Sn intermetallic alloy as an efficient methanol oxidation electrocatalyst

We successfully fabricate a novel concave nanostructure that is composed of atomically dispersed Ru atoms in Pt₃Sn nanoconcaves (Ru-Pt₃Sn NCs), which shows enhanced performance in methanol electroxidation compared to commercial Pt/C. This could be ascribed to the stable intermetallic structure and active surface structure, as well as the synergy among Pt, Sn and Ru.

Hydrogenation of pentenal over supported Pt nanoparticles: influence of Lewis-acid sites in the conversion pathway

The superb TOF and high selectivity of Pt/CeAl are associated with the surface properties (e.g. medium Lewis acidic site). The unsaturated Ce4+/Al3+ cations pairs act as the acid sites and electron acceptors to polarize the CO bonds.

Aldehyde trapping by self-propagating atom-exchange reactions on a gallium nitride monolayer: role of the molecule complexity

The design of novel gas sensors based on two-dimensional systems has grown rapidly in the last few years due to the remarkable reactivity of their surfaces.

Pd catalysts supported on dual-pore monolithic silica beads for chemoselective hydrogenation under batch and flow reaction conditions

Two different types of palladium catalysts supported on dual-pore monolithic silica beads [5% Pd/SM and 0.25% Pd/SM(sc)] for chemoselective hydrogenation were developed.