Adsorption and activation of CO2 on Zrn (n = 2-7) clusters

Cu-Based Materials for Enhanced C2+ Product Selectivity in Photo-/Electro-Catalytic CO2 Reduction: Challenges and Prospects

Carbon dioxide conversion into valuable products using photocatalysis and electrocatalysis is an effective approach to mitigate global environmental issues and the energy shortages. Among the materials utilized for catalytic reduction of CO2, Cu-based materials are highly advantageous owing to their widespread availability, cost-effectiveness, and environmental sustainability. Furthermore, Cu-based materials demonstrate interesting abilities in the adsorption and activation of carbon dioxide, allowing the formation of C2+ compounds through C-C coupling process. Herein, the basic principles of photocatalytic CO2 reduction reactions (PCO2RR) and electrocatalytic CO2 reduction reaction (ECO2RR) and the pathways for the generation C2+ products are introduced. This review categorizes Cu-based materials into different groups including Cu metal, Cu oxides, Cu alloys, and Cu SACs, Cu heterojunctions based on their catalytic applications. The relationship between the Cu surfaces and their efficiency in both PCO2RR and ECO2RR is emphasized. Through a review of recent studies on PCO2RR and ECO2RR using Cu-based catalysts, the focus is on understanding the underlying reasons for the enhanced selectivity toward C2+ products. Finally, the opportunities and challenges associated with Cu-based materials in the CO2 catalytic reduction applications are presented, along with research directions that can guide for the design of highly active and selective Cu-based materials for CO2 reduction processes in the future.

Growth pattern and electronic and magnetic properties of Cr-doped silver clusters

Growth pattern and electronic and magnetic properties of Agn Cr (n = 1-16) clusters have been investigated via density functional theory (DFT) combined with CALYPSO structure search method. The optimized geometry shows that the growth of the global minimum structures of Agn Cr clusters have obvious rule. when n > 12, silver atoms grow around an icosahedron which is almost unchanged in each structure. Analyses of electronic properties indicate that the doped Cr atom can only enhance the stability of larger silver clusters. Optical absorption and photoelectron spectra of Agn Cr isomers have been predicted and can be used for their structural identification. The icosahedral Ag12 Cr cluster with large energy level gap can be seen as a superatom. The adsorption capacity of Cr atom in Agn Cr cluster to CO is much higher than that of free Cr atom. The intensity of IR and Ramam spectra can be dramatically enhanced when CO is absorbed on Agn Cr cluster that Cr atom is encapsulated by Ag atoms. Moreover, the red shift of IR and Raman spectra of CO adsorbed on these clusters is also very small compared to free CO. Magnetism calculations show that the magnetic moment of Agn Cr clusters decreases linearly from n = 6 to 12 and increases linearly from n = 12 to 16. The total magnetic moment of Agn Cr cluster is mainly localized on the Cr atom. The change of magnetic moment of Cr atom is related to the charge transfer between Cr and Ag atoms.

Ab initio investigation of the role of the d-states on the adsorption and activation properties of CO2 on 3d, 4d, and 5d transition-metal clusters

We report a theoretical investigation of the adsorption and activation properties of CO2 on eight-atom 3 d, 4 d, and 5 d transition-metal (TM) clusters based on density functional theory calculations. From our results and analyses, in the lowest energy configurations, CO2 binds via a chemisorption mechanism on Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt (adsorption energy from −0.49 eV on Pt up to −1.40 eV on Os), where CO2 breaks its linearity and adopts an angular configuration due to the charge transfer from the clusters toward the C atom in the adsorbed CO2. In contrast, it binds via physisorption on Cu, Ag, and Au and maintains its linearity due to a negligible charge transfer toward CO2 and has a small adsorption energy (from −0.17 eV on Cu up to −0.18 eV on Ag). There is an energetic preference for twofold bridge TM sites, which favors binding of C with two TM atoms, which enhances the charge transfer ten times than on the top TM sites (onefold). We identified that the strength of the CO2–TM8 interaction increases when the energy values of the highest occupied molecular orbital (HOMO) of the TM8 are closer to the energy values of the lowest unoccupied molecular orbital of CO2, which contributes to maximize the charge transfer toward the molecule. Beyond the energy position of the HOMO states, the delocalization of 5 d orbitals plays an important role in the adsorption strength in TM, especially for the iron group, e.g., the adsorption energies are −1.08 eV (Fe, 3 d), −1.19 eV (Ru, 4 d), and −1.40 eV (Os, 5 d).

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

Adsorption and Activation of CO2 on Small-Sized Cu-Zr Bimetallic Clusters

Adsorption and activation of CO₂ is a key step in any chemical reaction, which aims to convert it to other useful chemicals. Therefore, it is important to understand the factors that drive the activation process and also search for materials that promote the process. We employ the density functional theory to explore the possibility of using small-sized bimetallic Cu-Zr clusters, Cu_4-nZr_n, with n = 1-3 for the above-mentioned key step. Our results suggest that after adsorption, a CO₂ molecule preferably resides on Zr atoms or at the bridge and triangular faces formed by Zr atoms in bimetallic Cu-Zr clusters accompanied with its high degree of activation. Importantly, maximum activation occurs when CO₂ is adsorbed on the CuZr₃ cluster. Interestingly, we find that the adsorption energy of CO₂ can be tuned by varying the extent of the Zr atom in Cu-Zr clusters. We rationalize the high adsorption of CO₂ with the increase in the number of Zr atoms using the d-band center model and the concept of chemical hardness. The strong chemisorption and high activation of CO₂ are ascribed to charge migration between Cu-Zr clusters and the CO₂ molecule. We find an additional band in the infrared vibrational spectra of CO₂ chemisorbed on all of the clusters, which is absent in the case of free CO₂. We also observe that the energy barriers for the direct dissociation of the CO₂ molecule to CO and O decrease significantly on bimetallic Cu-Zr clusters as compared to that on pure Cu₄. In particular, the barrier heights are considerably small for Cu₃Zr and CuZr₃ clusters. This study demonstrates that Cu₃Zr and CuZr₃ clusters may serve as good candidates for activation and dissociation of the CO₂ molecule.

Role of metcar on the adsorption and activation of carbon dioxide: a DFT study

Metallocarbohedrenes or metcars belong to one of the classes of stable nanoclusters having a specific stoichiometry. In spite of the available theoretical and experimental studies, the structure of pristine Ti8C12 metcar is still uncertain. We study the geometric structure of a titanium metcar, Ti8C12, together with its electronic properties and chemical activity towards adsorption and activation of CO2 molecule by means of density functional theory. Our results suggest that the CO2 molecule is strongly adsorbed and undergoes a significantly high degree of activation onto the Ti8C12 metcar. The migration of charge from titanium metcar to CO2 molecule attributes the high degree of activation of this molecule. In the infrared vibrational spectra for CO2 molecule adsorbed onto Ti8C12, we find a new signal which is absent in the corresponding spectra for gaseous CO2. In addition to adsorption energy, we also estimate the energy barrier for the dissociation of CO2 molecule to CO and O fragments on a Ti8C12 cluster. As a whole, this work reveals the ground state geometry of Ti8C12 metcar and highlights the role of this metcar in CO2 adsorption and activation, which are the key steps in designing potential catalysts for CO2 capture and its conversion to industrially valuable chemicals.