Effect of N‑Containing Functional Groups on CO₂ Adsorption of Carbonaceous Materials: A Density Functional Theory Approach

The amount of anthropogenic CO₂ emission keeps increasing worldwide, and it urges the development of efficient CO₂ capture technologies. Among various CO₂ capture methods, adsorption is receiving more interest, and carbonaceous materials are considered good CO₂ adsorbents. There have been many studies of N-containing carbon materials that have enhanced surface interaction with CO₂; however, various N-containing functional groups existing in the carbon surface have not been investigated in detail. In this study, first-principle calculations were conducted for carbon models having various N-functional groups to distinguish N-containing heterogeneity and understand carbon surface chemistry for CO₂ adsorption. Among N-functional groups tested, the highest adsorption energies of −0.224 and −0.218 eV were observed in pyridone and pyridine groups, respectively. Structural parameters including bond angle and length revealed an exceptional hydrogen-bonding interaction between CO₂ and pyridone group. Charge accumulation on CO₂ during interaction with pyridine-functionalized surface was confirmed by Bader charge analysis. Also, the peak shift of CO₂ near Fermi level in the DOS calculation and the presence of HOMO on pyridinic-N in the frontier orbital calculation determined that the interaction of pyridinic-N is weak Lewis acid–base interaction by charge transfer. Furthermore, adsorption energies of N₂ were calculated and compared to those of CO₂ to find its selective adsorption ability. Our results suggest that pyridone and pyridine groups are most effective for enhancing the interaction with CO₂ and have potential for selective CO₂ adsorption

Rhodium–Tin Binary NanoparticleA Strategy to Develop an Alternative Electrocatalyst for Oxygen Reduction

A Rh–Sn nanoparticle is achieved by combinatorial approaches for application as an active and stable electrocatalyst in the oxygen reduction reaction. Both metallic Rh and metallic Sn exhibit activities too low to be utilized for electrocatalytic reduction of oxygen. However, a clean and active Rh surface can be activated by incorporation of Sn into a Rh nanoparticle through the combined effects of lateral repulsion, bifunctional mechanism, and electronic modification. The corrosion-resistant property of Rh contributes to the construction of a stable catalyst that can be used under harsh fuel cell conditions. Based on both theoretical and experimental research, Rh–Sn nanoparticle designs with inexpensive materials can be a potential alternative catalyst in terms of the economic feasibility of commercialization and its facile and simple surfactant-free microwave-assisted synthesis

Effects of Carbohydrates on the Hydrodeoxygenation of Lignin-Derived Phenolic Compounds

Simulating lignocellulose-derived pyrolysis oil, the effects of carbohydrate derivatives on the hydrodeoxygenation of lignin-derived phenolic compounds, guaiacol in this study, were observed using supported ruthenium catalysts. Among several carbohydrate derivatives which possibly exist in the pyrolysis oil, the addition of furfural and 5-hydroxy­methylfurfural (5-HMF) significantly decreased the conversion of guaiacol, with density functional theory (DFT) calculations indicating that guaiacol competes with furfural and 5-HMF to adsorb onto the ruthenium nanoparticle surface, thus suppressing the hydrogenation of guaiacol

Role of Heteronuclear Interactions in Selective H₂ Formation from HCOOH Decomposition on Bimetallic Pd/M (M = Late Transition FCC Metal) Catalysts

In this study, by using spin-polarized density functional theory calculations, we have elucidated the role of heteronuclear interactions in determining the selective H₂ formation from HCOOH decomposition on bimetallic Pd_shell/M_core (M = late transition FCC metal (Rh, Pt, Ir, Cu, Au, Ag)) catalysts. We found that the catalysis of HCOOH decomposition strongly depends on the variation of surface charge polarization (ligand effect) and lattice distance (strain effect), which are caused by the heteronuclear interactions between surface Pd and core M atoms. In particular, the contraction of surface Pd–Pd bond distance and the increase in electron density in surface Pd atoms in comparison to the pure Pd case are responsible for the enhancement of the selectivity to H₂ formation via HCOOH decomposition. Our calculations also unraveled that the d band center location and the density of states for the d band (particularly d_<i>z</i>², d_<i>yz</i>, and d_<i>xz</i>) near the Fermi level are the important indicators that explain the impact of strain and ligand effects in catalysis, respectively. That is, the surface lattice contraction (expansion) leads to the downshift (upshift) of d band centers in comparison to the pure Pd case, while the electronic charge increase (decrease) in surface Pd atoms results in the depletion (augmentation) of the density of states for d_<i>z</i>², d_<i>yz</i>, and d_<i>xz</i> orbitals. Our study highlights the importance of properly tailoring the surface lattice distance (d band center) and surface charge polarization (the density of states for d_<i>z</i>², d_<i>yz</i>, and d_<i>xz</i> orbitals near the Fermi level) by tuning the heteronuclear interactions in bimetallic Pd_shell/M_core catalysts for enhancing the catalysis of HCOOH decomposition toward H₂ production, as well as other chemical reactions

Importance of Ligand Effect in Selective Hydrogen Formation via Formic Acid Decomposition on the Bimetallic Pd/Ag Catalyst from First-Principles

The critical role of the Ag–Pd ligand effect (which is tuned by changing the number of Pd atomic layers) in determining the dehydrogenation and dehydration of HCOOH on the bimetallic Pd/Ag catalysts was elucidated by using the spin-polarized density functional theory (DFT) calculations. Our calculations suggest that the selectivity to H₂ production from HCOOH on the bimetallic Pd/Ag catalysts strongly depends on the Pd atomic layer thickness at near surface. In particular, the thinnest Pd monolayer in the Pd/Ag system is responsible for enhancing the selectivity of HCOOH decomposition toward H₂ production by reducing the surface binding strength of specific intermediates such as HCOO and HCO. The dominant Ag–Pd ligand effect by the substantial charge donation to the Pd surface from the subsurface Ag [which significantly reduce the density of state (particularly, <i>d</i>_<i>z</i>²_{–<i>r</i>²} orbital) near the Fermi level] proves to be a key factor for the selective hydrogen production from HCOOH decomposition, whereas the expansive (tensile) strain imposed by the underlying Ag substrate plays a minor role. This work hints on the importance of properly engineering the surface activity of the Ag–Pd core–shell catalysts by the interplay between ligand and strain effects

Impact of d‑Band Occupancy and Lattice Contraction on Selective Hydrogen Production from Formic Acid in the Bimetallic Pd₃M (M = Early Transition 3d Metals) Catalysts

Catalysts that are highly selective and active for H₂ production from HCOOH decomposition are indispensable to realize HCOOH-based hydrogen storage and distribution. In this study, we identify two effective routes to promoting the Pd catalyst for selective H₂ production from HCOOH by investigating the effects of early transition metals (Sc, Ti, V, and Cr) incorporated into the Pd core using density functional theory calculations. First, the asymmetric modification of the Pd surface electronic structure (d_<i>z</i>² vs d_<i>yz</i> + d_<i>zx</i>) can be an effective route to accelerating the H₂ production rate. Significant charge transfer from the subsurface Sc atom to the surface Pd atom and subsequent extremely low level of d band occupancy (<0.1) around the Sc atoms are identified as a key factor in deriving the asymmetric modification of the Pd surface electronic structure. Second, in-plane lattice contraction of the Pd surface can be an effective route to suppressing the CO production. Compressive strain of the Pd surface is maximized as a result of alloying with V and induces subsequent changes in adsorption site preference of the key intermediates for the CO production path, resulting in a significant increase in the activation energy barrier for the CO production path. The unraveled atomic-scale factors underlying the promotion of the Pd surface catalytic properties provide useful insights into the efforts to overcome limitations of current catalyst technologies in making the HCOOH-based H₂ storage and distribution economically feasible

Reversible Surface Segregation of Pt in a Pt₃Au/C Catalyst and Its Effect on the Oxygen Reduction Reaction

Reversible surface segregation of Pt in Pt₃Au/C catalysts was accomplished through a heat treatment under a CO or Ar atmosphere, which resulted in surface Pt segregation and reversed segregation, respectively. The Pt-segregated Pt₃Au/C exhibited a significantly improved oxygen reduction reaction (ORR) activity (227 mA/mg_metal) compared to that of commercial Pt/C (59 mA/mg_metal). For the Pt-segregated Pt₃Au/C, the increased OH-repulsive properties were validated by a CO bulk oxidation analysis and also by density functional theory (DFT) calculations. Interestingly, the DFT calculations revealed that the binding energy for Pt-segregated Pt₃Au (111) surfaces was 0.1 eV lower than that for Pt (111) surfaces, which has been previously reported to exhibit the optimum OH binding energy for the ORR. Therefore, the reversible surface segregation is expected to provide a practical way to control the surface states of Pt–Au bimetallic catalysts to enhance ORR activity. In addition, the Pt-segregated Pt₃Au/C showed excellent electrochemical stability, as evidenced by its high-performance retention (96.4%) after 10 000 potential cycles, in comparison to that of Pt/C (55.3%)

Electrochemical Synthesis of NH₃ at Low Temperature and Atmospheric Pressure Using a γ‑Fe₂O₃ Catalyst

The electrochemical synthesis of NH₃ by the nitrogen reduction reaction (NRR) at low temperature (<65 °C) and atmospheric pressure using nanosized γ-Fe₂O₃ electrocatalysts were demonstrated. The activity and selectivity of the catalyst was investigated both in a 0.1 M KOH electrolyte and when incorporated into an anion-exchange membrane electrode assembly (MEA). In a half-reaction experiment conducted in a KOH electrolyte, the γ-Fe₂O₃ electrode presented a faradaic efficiency of 1.9% and a weight-normalized activity of 12.5 nmol h^–1 mg^–1 at 0.0 V_RHE. However, the selectivity toward N₂ reduction decreased at more negative potentials owing to the competing proton reduction reaction. When the γ-Fe₂O₃ nanoparticles were coated onto porous carbon paper to form an electrode for a MEA, their weight-normalized activity for N₂ reduction was found to increase dramatically to 55.9 nmol h^–1 mg^–1. However, the weight- and area-normalized N₂ reduction activities of γ-Fe₂O₃ decreased progressively from 35.9 to 14.8 nmol h^–1 mg^–1 and from 0.105 to 0.043 nmol h^–1 cm^–2_act, respectively, during a 25 h MEA durability test. In summary, a study of the fundamental behavior and catalytic activity of γ-Fe₂O₃ nanoparticles in the electrochemical synthesis of NH₃ under low temperature and pressure is presented

Influence of Cation Substitutions Based on ABO₃ Perovskite Materials, Sr_1–<i>x</i>Y_<i>x</i>Ti_1–<i>y</i>Ru_<i>y</i>O_3−δ, on Ammonia Dehydrogenation

In order to screen potential catalytic materials for synthesis and decomposition of ammonia, a series of ABO₃ perovskite materials, Sr_1–<i>x</i>Y_<i>x</i>Ti_1–<i>y</i>Ru_<i>y</i>O_3−δ (<i>x</i> = 0, 0.08, and 0.16; <i>y</i> = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) were synthesized and tested for ammonia dehydrogenation. The influence of A or B site substitution on the catalytic ammonia dehydrogenation activity was determined by varying the quantity of either A or B site cation, producing <b>Sr</b>_{<b>1</b>–<b><i>x</i></b>}<b>Y</b>_{<b><i>x</i></b>}Ti_0.92Ru_0.08O_3−δ and Sr_0.92Y_0.08<b>Ti</b>_{<b>1</b>–<i><b>y</b></i>}<b>Ru</b>_{<b><i>y</i></b>}O_3−δ, respectively. Characterizations of the as-synthesized materials using different analytical techniques indicated that a new perovskite phase of SrRuO₃ was produced upon addition of large amounts of Ru (≥12 mol %), and the surface Ru⁰ species were formed simultaneously to ultimately yield <b>Ru</b>_{<b><i>z</i></b>}(surface)/Sr_0.92Y_0.08<b>Ti</b>_{<b>1</b>–<b><i>y</i></b>}<b>Ru</b>_{<i><b>y</b></i>–<b><i>z</i></b>}O_3−δ and/or <b>Ru</b>_{<b><i>z</i></b>–<b><i>w</i></b>}(surface)/Sr_<i>w</i>Ru_<i>w</i>O₃/Sr_{0.92–<i>w</i>}Y_0.08<b>Ti</b>_{<b>1</b>–<b><i>y</i></b>}<b>Ru</b>_{<b><i>y</i></b>–<b><i>z</i></b>}O_3−δ. The newly generated surface Ru⁰ species at the perovskite surfaces accelerated ammonia dehydrogenation under different conditions, and Sr_0.84Y_0.16Ti_0.92Ru_0.08O_3−δ exhibited a NH₃ conversion of ca. 96% at 500 °C with a gas hourly space velocity (GHSV) of 10 000 mL g_cat^–1 h^–1. In addition, Sr_0.84Y_0.16Ti_0.92Ru_0.08O_3−δ further proved to be highly active and stable toward ammonia decomposition at different reaction temperatures and GHSVs for >275 h