Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

CO2 hydrogenation to formic acid on Pd-Cu nanoclusters: a DFT study

Carbon dioxide (CO₂) hydrogenation to formic acid is a promising method for the conversion of CO₂ to useful organic products. The interaction of CO₂ with hydrogen (H₂) on Pd_mCu_n (m + n = 4, 8 and 13) clusters to form formic acid (HCOOH) has been explored using density functional theoretical (DFT) calculations. Pd₂Cu₂, Pd₄Cu₄ and 13-atom Pd₁₂Cu clusters are found to be the most stable among all of the Pd_mCu_n (m + n = 4, 8 and 13) clusters with binding energies of -1.75, -2.16 and -2.40 eV per atom, respectively. CO₂ molecules get adsorbed on the Pd₂Cu₂, Pd₄Cu₄ and Pd₁₂Cu clusters in an inverted V-shaped way with adsorption energies of -0.91, -0.96 and -0.44 eV, respectively. The hydrogenation of CO₂ to form formate goes through a unidentate structure that rapidly transforms into the bidentate structure. To determine the transition state structures and minimum energy paths (MEPs) for CO₂ hydrogenation to formic acid, the climbing image nudge elastic band (CI-NEB) method has been adopted. The activation barriers for the formation of formic acid from formate on Pd₂Cu₂ and Pd₄Cu₄ are calculated to be 0.79 and 0.68 eV, respectively whereas that on the Pd₁₂Cu cluster is 1.77 eV. The enthalpy for the overall process of CO₂ hydrogenation to formic acid on the Pd₂Cu₂, Pd₄Cu₄ and Pd₁₂Cu clusters are found to be 0.83, 0.48 and 0.63 eV, respectively. Analysis of the density of states (DOS) spectra show that the 4d orbital of Pd, the 3d orbital of Cu, and the 2p orbitals of C and O atoms are involved in the bonding between CO₂ molecules and the Pd₂Cu₂ clusters. The CO₂ adsorption on the Pd_mCu_n (m + n = 4 and 8) clusters has also been explained in terms of the charge density distribution analysis.

Edge engineering on layered WS2 toward the electrocatalytic reduction of CO2: a first principles study

Transition-metal dichalcogenides (TMDCs) have been modified to show excellent electrocatalytic performance for the CO₂ reduction reaction (CO₂RR). However, little research has been reported on the edge modification of WS₂ and its electrocatalytic CO₂RR. In this work, the edge structure of WS₂ with W atoms exposed in the top layer was established by density functional theory calculations. Through using WS₂-xTM-y (x = 1, 2 or 3; y = 1 or 2; TM = Zn, Fe, Co or Ni) models by doping TM atoms on the top layer of WS₂, the effects of dopant species, doping concentration and adsorption sites on their electrocatalytic activity were investigated. Among the models, the active site for the CO₂RR is the W atoms. The doping of TM atoms would affect the bond strength between W and S atoms. After the doping of TM atoms in WS₂-2TM-1 ones, the electrical conduction of S atoms and the underlying W atoms can greatly be improved. Thus the catalytic activities can be significantly increased, in which the WS₂-2Zn-1 model shows the best catalytic activity. The limiting potential (U_L) of the CO₂RR to CO on the WS₂-2Zn-1 model is -0.51 V and the Gibbs energy change (ΔG) for the adsorption of intermediates on the WS₂-2Zn-1 model is ΔG(COOH*) = -0.37 and ΔG(CO*) = -0.51 eV, respectively. Solvation correction showed that WS₂-2Zn-1 could maintain good catalytic performance in a wide range of pH values. The present results may provide a theoretical basis for the design and synthesis of novel electrocatalysts with high performance for the CO₂RR.

The role of surface oxidation and Fe-Ni synergy in Fe-Ni-S catalysts for CO2 hydrogenation

Increasing carbon dioxide (CO₂) emissions, resulting in climate change, have driven the motivation to achieve the effective and sustainable conversion of CO₂ into useful chemicals and fuels. Taking inspiration from biological processes, synthetic iron-nickel-sulfides have been proposed as suitable catalysts for the hydrogenation of CO₂. In order to experimentally validate this hypothesis, here we report violarite (Fe,Ni)₃S₄ as a cheap and economically viable catalyst for the hydrogenation of CO₂ into formate under mild, alkaline conditions at 125 °C and 20 bar (CO₂ : H₂ = 1 : 1). Calcination of violarite at 200 °C resulted in excellent catalytic activity, far superior to that of Fe-only and Ni-only sulfides. We further report first principles simulations of the CO₂ conversion on the partially oxidised (001) and (111) surfaces of stoichiometric violarite (FeNi₂S₄) and polydymite (Ni₃S₄) to rationalise the experimentally observed trends. We have obtained the thermodynamic and kinetic profiles for the reaction of carbon dioxide (CO₂) and water (H₂O) on the catalyst surfaces via substitution and dissociation mechanisms. We report that the partially oxidised (111) surface of FeNi₂S₄ is the best catalyst in the series and that the dissociation mechanism is the most favourable. Our study reveals that the partial oxidation of the FeNi₂S₄ surface, as well as the synergy of the Fe and Ni ions, are important in the catalytic activity of the material for the effective hydrogenation of CO₂ to formate.

Recyclabl Metal (Ni, Fe) Cluster Designed Catalyst for Cellulose Pyrolysis to Upgrade Bio-Oil

A new recyclable catalyst for pyrolysis has been developed by combining calculations and experimental methods. In order to understand the properties of the new cluster designed catalysts, cellulose (a major component of plants) as a biomass model compound was pyrolyzed and catalyzed with different cluster designed catalysts. The NiaFeb (2 ≤ a + b ≤ 6) catalyst clusters structures were calculated by using Gaussian and Materials Studio software to determine the relationships between catalyst structure and bio-oil components, which is essential to design cluster designed catalysts that can improve bio-oil quality. GC-MS analysis of the bio-oil was used to measure the effects on the different catalyst interactions with cellulose. It was found that the NiFe cluster designed catalysts can increase the yield of bio-oil from 35.8% ± 0.9% to 41.1% ± 0.6% and change the bio-oil composition without substantially increasing the water content, while substantially decreasing the sugar concentration from 40.1% ± 1.3% to 27.5% ± 0.9% and also producing a small amount of hydrocarbon compounds. The catalyst with a high Ni ratio also had high Gibbs free energy, ΔG, likely also influencing the decrease of sugar and acid while increasing the ketone concentrations. These results indicate the theoretical calculations can enhance the design next-generation cluster designed catalysts to improve bio-oil composition based upon experiments.

Homogeneous and Heterogeneous Catalysis Impact on Pyrolyzed Cellulose to Produce Bio-Oil

Effectively utilizing catalytic pyrolysis to upgrade bio-oil products prepared from biomass has many potential benefits for the environment. In this paper, cellulose (a major component of plants and a biomass model compound) is pyrolyzed and catalyzed with different catalysts: Ni2Fe3, ZSM-5, and Ni2Fe3/ZSM-5. Two different pyrolysis processes are investigated to compare homogeneous and heterogeneous catalysis influence on the products. The results indicate that the Ni2Fe3 cluster catalyst shows the best activity as a homogeneous catalysis. It can also be recycled repeatedly, increases the yield of bio-oil, and improves the quality of the bio-oil by decreasing the sugar concentration. Furthermore, it also catalyzes the formation of a small amount of hydrocarbon compounds. In the case of Ni2Fe3/ZSM-5 catalyst, it shows a lower yield of bio-oil but also decreases the sugar concentration significantly. Ni2Fe3, not only can it be used as homogeneous catalysis mixed with cellulose but also shows catalytic activity as a supported catalyst on ZSM-5, with higher catalytic activity than ZSM-5. These results indicate that the Ni2Fe3 catalyst has significant activity for potential use in industry to produce high quality bio-oil from biomass.

Surface strategies for catalytic CO2 reduction: from two-dimensional materials to nanoclusters to single atoms

This work constructively reviewed and predicted the surface strategies for catalytic CO₂ reduction with 2D material, nanocluster and single-atom catalysts