CO2 adsorption and activation over medium sized Cun (n=7, 13 and 19) clusters: A density functional study

Calculations of the effect of catalyst size and structure on the electrocatalytic reduction of CO2 on Cu nanoclusters

The structure and catalytic properties of Cu nanoclusters of sizes between 55 and 147 atoms were examined to understand if small Cu clusters could provide enhancement over traditional catalysts for the electrocatalysis of CO2 to CO and carbon-based fuels, such as CH4 and CH3OH, compared to bulk Cu surfaces and large Cu nanoparticles. Clusters studied included Cu55, Cu78, Cu101, Cu124, and Cu147, the structures of which were determined using global optimisation. The majority of Cu clusters examined were icosahedral, including the perfect closed-shell, partial-shell, elongated and distorted icosahedral clusters. Free energy diagrams for the reduction of CO2 showed the potential required for the formation of CO is notably smaller for all cluster sizes considered, relative to Cu(111). Less variation is observed for the limiting potential for the formation of CH4 and CH3OH. However, it was found that clusters that are either a distorted motif or contain vacancy defects yielded the best activity and provide an interesting synthesis target for future experiments.

Subnanometric Ru clusters with upshifted D band center improve performance for alkaline hydrogen evolution reaction

Subnanometric metal clusters usually have unique electronic structures and may display electrocatalytic performance distinctive from single atoms (SAs) and larger nanoparticles (NPs). However, the electrocatalytic performance of clusters, especially the size-activity relationship at the sub-nanoscale, is largely unexplored. Here, we synthesize a series of Ru nanocrystals from single atoms, subnanometric clusters to larger nanoparticles, aiming at investigating the size-dependent activity of hydrogen evolution in alkaline media. It is found that the d band center of Ru downshifts in a nearly linear relationship with the increase of diameter, and the subnanometric Ru clusters with d band center closer to Femi level display a stronger water dissociation ability and thus superior hydrogen evolution activity than SAs and larger nanoparticles. Benefiting from the high metal utilization and strong water dissociation ability, the Ru clusters manifest an ultrahigh turnover frequency of 43.3 s^-1 at the overpotential of 100 mV, 36.1-fold larger than the commercial Pt/C.

The 2D or 3D morphology of sub-nanometer Cu5 and Cu8 clusters changes the mechanism of CO oxidation

The mechanism of the CO oxidation reaction catalysed by planar Cu₅, three dimensional (3D) Cu₅, and 3D Cu₈ clusters is theoretically investigated at the B3PW91/Def2TZVP level. All three clusters are able to catalyse the reaction with similar activation energies for the rate determining step, about 16-18 kcal mol^-1, but with remarkable differences in the reaction mechanism depending on cluster morphology. Thus, for 3D Cu₅ and Cu₈ clusters, O₂ dissociation is the first step of the mechanism, followed by two consecutive CO + O reaction steps, the second one being rate determining. In contrast, on planar Cu₅ the reaction starts with the formation of an OOCO intermediate in what constitutes the rate determining step. The O-O bond is broken in a second step, releasing the first CO₂ and leaving one bi-coordinately adsorbed O atom which reacts with CO following an Eley-Rideal mechanism with a low activation energy, in contrast to the higher barriers obtained for this step on 3D clusters.

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.

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

The first step in the conversion of CO2 to useful chemicals involves the adsorption of this molecule on a catalyst accompanied with its high degree of activation. In this paper, we explore the efficacy of small sized zirconium clusters, Zrn (n = 2-7), in the adsorption and activation of the CO2 molecule by using the density functional theory based ab initio method. The results of our calculations provide compelling evidence for the chemisorption and very high degree of activation of CO2 with the elongation of the C-O bond in the range of 1.27-1.42 Å compared to 1.16 Å for free CO2 and the deformation of the O-C-O bond angle from linear to 115-136°. This activation takes place through a charge migration from the Zrn cluster to the CO2 molecule resulting in the formation of CO2δ- species. To assess the catalytic potential of Zr-clusters for CO2 conversion, we also analyse the reaction pathways and the transition barrier heights for the dissociation of CO2 (CO2 → CO + O) on all the Zrn clusters. Our results for the dissociation of CO2 to CO and O fragments reveal that the transition barrier is small for all the Zrn clusters except for Zr2 and Zr4 and it attains a minimum value of 0.11 eV for an isomer of the Zr6 cluster. The present work clearly demonstrates that small-sized monometallic Zr-clusters are highly efficient in activating and dissociating a CO2 molecule adsorbed on these clusters.

Cluster size effects on the adsorption of CO, O, and CO2and the dissociation of CO2on two-dimensional Cux(x1, 3, and 7) clusters supported on Cu(111) surface: a density functional theory study

In this study, we performed density functional theory based calculations to determine the effect of the size of Cux(x= 1 (adatom), 3 (trimer), 7 (heptamer)) clusters supported on Cu(111) toward the adsorption of CO, O, and CO2, and the dissociation of CO2. CO adsorbs with comparable adsorption energies on the different cluster systems, which are influenced by the reactivity of the Cu atoms in the cluster and the interaction of CO with the Cu atoms in the terrace. The O atom, on the other hand, will always favor to adsorb on hollow sites and is more stable on the hollow sites of smaller clusters. CO2dissociates with lower activation energy on the cluster region than on flat Cu(111). We obtained the lowest activation energy on Cu3due to its more reactive Cu atoms than the Cu7case and due to the possibility of O to adsorb on the cluster region, which is not observed in the Cu1case. The presented results will provide insights on future studies on supported cluster systems and their possible use as catalysts for CO2-related reactions.

Ab initio investigation of quantum size effects on the adsorption of CO2, CO, H2O, and H2 on transition-metal particles

Adsorption is a crucial preliminary step for the conversion of CO2 into higher-value chemicals, nonetheless, the atomistic understanding of how substrate particle size affects this step is still incomplete. In this study, we employed density functional theory to investigate the effects of particle size on the adsorption of model molecules involved in the CO2 transformations (CO2, CO, H2O and H2) on Con, Nin and Cun particles with different sizes (n = 13, 55, 147) and on the respective close-packed surfaces. We found significant size-dependence of the adsorption properties for physisorbed (linear) and chemisorbed (bent) CO2 on the substrates and distinct (symmetric or asymmetric) stretching of the C-O bonds, which can play a crucial role to understand the CO2 dissociation pathways. For CO and H2, some properties showed small oscillations, due to size effects that induced alternation of the adsorption site preference for different particle sizes; for H2O, the adsorption properties were almost independent of particle size. The presence of low-coordinated adsorption sites resulted in a trend for stronger adsorption and greater charge transfer for smaller clusters. Fixing the size-independent factors (e.g., type of metal), our results show that CO2 adsorption on transition-metal clusters is significantly affected by particle size, suggesting that substrate particle size could be a key factor to understand and control the catalytic transformations of CO2.

Electrochemical behaviour of naked sub-nanometre sized copper clusters and effect of CO2

In size-controlled naked Cu₅ and Cu₂₀ nanoclusters the latter show anodic redox processes occurring at much lower potential with respect to Cu₅, but the latter coordinate effectively CO₂ and allow to reduce CO₂ under cathodic conditions at lower overpotential.

Carbon dioxide activation and transformation to HCOOH on metal clusters (M = Ni, Pd, Pt, Cu, Ag & Au) anchored on a polyaniline conducting polymer surface – an evaluation study by hybrid density functional theory

Computational electrocatalytic reduction of CO₂ to HCOOH was achieved on different metal-anchored polyaniline using density functional theory. Cu was found to perform better than other metals at an applied potential −0.58 V through the H*COO pathway.

Highly effective catalysis of the double-icosahedral Ru(19) cluster for dinitrogen dissociation - a first-principles investigation

The N2 bond cleavage is the rate-limiting step in the synthesis of ammonia, and ruthenium is a catalyst well known for this reaction. The double-icosahedral (D5h) Ru19 cluster is famous as an active catalyst, and has a remarkable stability towards the adsorption of H2, N2 and CO. Using first-principles calculations, we have investigated the adsorption and dissociation of dinitrogen on a double-icosahedral Ru19 cluster. Our results show that the hollow site in the rhombus region (BHB site) of the Ru19 cluster possesses the greatest catalytic activity to dissociate N2, with the reaction barrier of 0.89 eV and an exothermicity of -1.45 eV. Multiple coadsorption of N2 on the cluster (i.e. coadsorption of 2N2 and 3N2 on a single Ru19 cluster) causes the barrier to dissociate N2 to be less on a BHB site than for adsorption of a single N2. To understand the catalytic properties of a Ru19 cluster towards N2 bond cleavage, we calculated the electron population, vibrational wavenumbers and local densities of states; the results are explicable.