On the Real Nature of Rh Single‐Atom Catalysts Dispersed on the ZrO 2 Surface

Modeling Single-Atom Catalysis

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

Interfacing single-atom catalysis with continuous-flow organic electrosynthesis

The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. "Single-atom catalysis" has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc.) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc.). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.

Insight into the dual action mechanism of 3V-PPh3 polymers as carriers and ligands in the Rh/3V-PPh3 heterogeneous catalytic hydroformylation of ethylene to propionaldehyde

An experimentally confirmed porous vinyl-functionalized PPh₃ (3V-PPh₃) polymer-supported Rh-based catalyst exhibits the significant advantages of high activity, high stability, and easy separation in the synthesis of propionaldehyde, which fundamentally solves the problem of Rh precious-metal loss. In this paper, the microscopic mechanism and electronic structure characteristics of two kinds of cross-linked 3V-PPh₃ polymer-supported Rh-based catalyst were studied by means of quantum chemistry (QC). With 3V-PPh₃ as the carrier, stable adsorption configurations of Rh and 3V-PPh₃ were investigated, and the results showed that Rh and P had the strongest effects, while the vinyl group enhanced the adsorption strength of Rh. Moreover, it was found that a high concentration of exposed P was beneficial to the dispersion of Rh. With 3V-PPh₃ as the ligand, the properties of the HRh(CO)(P-frame)₃ complex were investigated, and the results of structure analysis indicated that there were strong interactions between Rh and P, which contributed more to the non-loss of Rh. Among the four different configurations, the Rh-P coplanar configuration of cross-linking mode 2 had the highest Rh-P bond energy. The results of AIM analysis suggested that the Rh-P and Rh-C(CO) bonds involve closed-shell (donor-acceptor) interactions. The Mulliken charge and molecular electrostatic potential results revealed that the Rh activity of the Rh and P non-coplanar configuration was higher in the two cross-linking methods. Hopefully, this work will clarify the structure-activity relationship between 3V-PPh₃ polymer and Rh, and provide theoretical guidance for the design and development of high-efficiency heterogeneous catalysts for the hydroformylation of ethylene to propionaldehyde.

Vibrational Properties of CO Adsorbed on Au Single Atom Catalysts on TiO2(101), ZrO2(101), CeO2(111), and LaFeO3(001) Surfaces: A DFT Study

The nature and local environment of Au single atoms supported and stabilized on four different oxides is studied by means of DFT + U calculations using CO as probe molecule and its stretching frequency, ωe, as a fingerprint of the site where the Au atom is bound. Four oxides are considered, anatase TiO2, tetragonal ZrO2, cubic CeO2, and a perovskite LaFeO3. In this latter case a recently reported experimental study has detected a stretching mode for CO adsorbed on Au1/LaFeO3 of 2215 cm−1, with a large blue shift, ∆ω(CO) = 72 cm−1 with respect to free CO. In order to identify the Au adsorption site that can give rise to this large blue-shift we have considered five cases: (a) Au replacing a lattice cation, (Au)subM; (b) Au replacing a lattice O anion, (Au)subO; (c) Au adsorbed on the surface, (Au)ads; (d) Au bound to an extra O atom on the surface, (AuO)ads, or (e) Au bound to two extra O atoms on the surface, (AuO2)ads. It turns out that the correct reproduction of ∆ω for CO adsorbed on positively charged gold, Auδ+, is challenging for DFT. Therefore, we have performed a comparative study of Auδ+-CO molecular compounds for which ωe(CO) is known experimentally using various kinds of DFT functionals and accurate CCSD and CCSD(T) quantum chemistry methods. Also based on this comparison we propose a tentative assignment for the observed frequency of CO adsorbed on Au1/LaFeO3 single atom catalyst.

Graphic Abstract

Oxygen-deficient metal oxides supported nano-intermetallic InNi3C0.5 toward efficient CO2 hydrogenation to methanol

Direct CO₂ hydrogenation to methanol using renewable energy-generated hydrogen is attracting intensive attention, but qualifying catalysts represents a grand challenge. Pure-/multi-metallic systems used for this task usually have low catalytic activity. Here, we tailored a highly active and selective InNi₃C_0.5/ZrO₂ catalyst by tuning the performance-relevant electronic metal-support interaction (EMSI), which is tightly linked with the ZrO₂ type-dependent oxygen deficiency. Highly oxygen-deficient monoclinic-ZrO₂ support imparts high electron density to InNi₃C_0.5 because of the considerably enhanced EMSI, thereby enabling InNi₃C_0.5/monoclinic-ZrO₂ with an intrinsic activity three or two times as high as that of InNi₃C_0.5/amorphous-ZrO₂ or InNi₃C_0.5/tetragonal-ZrO₂ The EMSI-governed catalysis observed in the InNi₃C_0.5/ZrO₂ system is extendable to other oxygen-deficient metal oxides, in particular InNi₃C_0.5/Fe₃O₄, achieving 25.7% CO₂ conversion with 90.2% methanol selectivity at 325°C, 6.0 MPa, 36,000 ml g_cat ^-1 hour^-1, and H₂/CO₂ = 10:1. This affordable catalyst is stable for at least 500 hours and is also highly resistant to sulfur poisoning.

Reactivity of Single Transition Metal Atoms on a Hydroxylated Amorphous Silica Surface: A Periodic Conceptual DFT Investigation

The drive to develop maximal atom-efficient catalysts coupled to the continuous striving for more sustainable reactions has led to an ever-increasing interest in single-atom catalysis. Based on a periodic conceptual density functional theory (cDFT) approach, fundamental insights into the reactivity and adsorption of single late transition metal atoms supported on a fully hydroxylated amorphous silica surface have been acquired. In particular, this investigation revealed that the influence of van der Waals dispersion forces is especially significant for a silver (98 %) or gold (78 %) atom, whereas the oxophilicity of the Group 8-10 transition metals plays a major role in the interaction strength of these atoms on the irreducible SiO₂ support. The adsorption energies for the less-electronegative row 4 elements (Fe, Co, Ni) ranged from -1.40 to -1.92 eV, whereas for the heavier row 5 and 6 metals, with the exception of Pd, these values are between -2.20 and -2.92 eV. The deviating behavior of Pd can be attributed to a fully filled d-shell and, hence, the absence of the hybridization effects. Through a systematic analysis of cDFT descriptors determined by using three different theoretical schemes, the Fermi weighted density of states approach was identified as the most suitable for describing the reactivity of the studied systems. The main advantage of this scheme is the fact that it is not influenced by fictitious Coulomb interactions between successive, charged reciprocal cells. Moreover, the contribution of the energy levels to the reactivity is simultaneously scaled based on their position relative to the Fermi level. Finally, the obtained Fermi weighted density of states reactivity trends show a good agreement with the chemical characteristics of the investigated metal atoms as well as the experimental data.