A DFT Study of CO Oxidation at the Pd–CeO₂(110) Interface

Ceria-supported Pd is one of the main components in modern three-way catalysts in automotive applications to facilite CO oxidation. The exact form in which Pd displays its high activity remains not well understood. We present a DFT+<i>U</i> study of CO oxidation for single Pd atoms located on or in the ceria surface as well as a Pd_<i>n</i> nanorod model on the CeO₂(110) surface. The oxidation of Pd to the 2+ state by ceria weakens the Pd–CO bond for the single Pd models and, in this way, facilitates CO₂ formation. After CO oxidation by O of the ceria surface, Pd relocates to a position below the surface for the Pd-doped model; in this state, CO adsorption is not possible anymore. With Pd on the surface, O₂ will adsorb and dissociate leading to PdO, which can be easily reduced to Pd. The reactivity of the Pd nanorod is low because of the strong bonds of the metallic Pd phase with CO and the O atom derived from O₂ dissociation. These findings show that highly dispersed Pd is the most likely candidate for CO oxidation in the Pd–CeO₂ system

Computational Design of a CeO₂‑Supported Pd-Based Bimetallic Nanorod for CO Oxidation

Engineering a bimetallic system with complementary chemical properties can be an effective way of tuning catalytic activity. In this work, CO oxidation on CeO₂(111)-supported Pd-based bimetallic nanorods was investigated using density functional theory calculations corrected by on-site Coulomb interactions. We studied a series of CeO₂(111)-supported Pd-based bimetallic nanorods (Pd–X, where X = Ag, Au, Cu, Pt, Rh, Ru) and found that Pd–Ag/CeO₂ and Pd–Cu/CeO₂ are the two systems where the binding sites of CO and O₂ are distinct; that is, in these two systems, CO and O₂ do not compete for the same binding sites. An analysis of the CO oxidation mechanisms suggests that the Pd–Ag/CeO₂ system is more effective for catalyzing CO oxidation as compared to Pd–Cu/CeO₂ because both CeO₂ lattice oxygen atoms and adsorbed oxygen molecules at Ag sites can oxidize CO with low energy barriers. Both the Pd–Ag and Pd–CeO₂ interfaces in Pd–Ag/CeO₂ were found to play important roles in CO oxidation. The Pd–Ag interface, which combines the different chemical nature of the two metals, not only separates the binding sites of CO and O₂ but also opens up active reaction pathways for CO oxidation. The strong metal–support interaction at the Pd–CeO₂ interface facilitates CO oxidation by the Mars–van Krevelen mechanism. Our study provides theoretical guidance for designing highly active metal/oxide catalysts for CO oxidation

Formation of a Rhodium Surface Oxide Film in Rh_<i>n</i>/CeO₂(111) Relevant for Catalytic CO Oxidation: A Computational Study

The structure of small Rh_<i>n</i> (<i>n</i> = 1–10) clusters and corresponding Rh-oxide (Rh_<i>n</i>O_<i>m</i>) clusters (<i>n</i> = 1–4; <i>m</i> = 1–9) supported on a stoichiometric CeO₂(111) surface has been investigated using density functional theory corrected for on-site Coulombic interactions (DFT+U) with the goal to identify a realistic model for Rh/CeO₂-based CO oxidation catalysts. Rh_<i>n</i> clusters on ceria prefer to adopt a three-dimensional morphology. The adsorption of oxygen leads to the reconstruction of such clusters into a two-dimensional Rh-oxide film. The stability of Rh_<i>n</i>O_<i>m</i> species is determined by evaluating the reaction energy for the stepwise oxidation of Rh_<i>n</i>, which is to be compared with data for the experimental fresh catalysts. It is found that with increasing cluster size the surface oxide phase becomes increasingly stable against the isolated RhO₃ form under oxidative conditions. The Helmholtz free energy change for Rh_<i>n</i>O_<i>m</i> clusters with varying <i>m</i> was determined for the reduction by CO and oxidation by O₂. In this way, it was found that Rh-oxide species are more stable than the corresponding pure Rh clusters when supported on CeO₂(111). This suggests that the active site for CO oxidation is a Rh surface-oxide

Mechanistic Study of Selective Catalytic Reduction of NO_<i>x</i> with NH₃ over Mn-TiO₂: A Combination of Experimental and DFT Study

Mn-TiO₂ oxide catalyst has been studied intensively for selective catalytic reduction (SCR) of NO with NH₃ due to its extraordinarily good low-temperature performance. However, the mechanism of SCR on Mn-TiO₂ still remains unclear, especially with regard to the decomposition pathway of the NH₂NO intermediate and the reason for the decreasing N₂ selectivity with the increasing of temperature. In this work, we attempt to provide a molecular level understanding of these questions via a combination of DFT and experimental study. A complete catalytic cycle of the SCR reaction was proposed based on a model in which Mn is doped into the TiO₂(101) surface by quantum-chemical DFT+U calculations. In situ DRIFTS experiments were performed to provide evidence to the important intermediates as proposed in the reaction mechanism. The doping Mn enhances NH₃ adsorption and activation due to its lower conduction band. NH₂NO can decompose into N₂ and H₂O fast via a concerted H migration step. The decreasing selectivity with rising temperature can be explained by the deep oxidation of NH₃. This study provides atomic-scale insights into the catalytic cycle and the important role of doping Mn in NH₃–SCR reaction on Mn-TiO₂ catalysts, which is of significance for the design of high activity low-temperature SCR catalysts

Nature of Cu Species in Cu–SAPO-18 Catalyst for NH₃–SCR: Combination of Experiments and DFT Calculations

A series of Cu–SAPO-18 catalysts with various Cu loadings were prepared and their catalytic activities were tested for the selective catalytic reduction of NO with NH₃. The catalysts were characterized by means of XRD, N₂ adsorption–desorption, TEM, XPS, UV–vis DRS, H₂-TPR, NH₃-TPD and EPR. Isolated Cu²⁺ ions are confirmed to be the catalytic active sites. Cu-4.42 catalyst exhibits high NO conversion (>80%) at the lowest temperature of 200 °C among all catalysts. It can be attributed to the maximum amount of isolated Cu²⁺ ions in Cu-4.42 catalyst. DFT calculations show that the isolated Cu ions are located in the pear shaped cavity and exhibit a preference for the neighboring of 6R planes of Cu–SAPO-18. NH₃–SCR mechanism over Cu–SAPO-18 catalyst is elucidated by a combination of in situ DRIFTS technique and DFT calculations, in which the dissociation of NH₃ and the oxidation of NO are shown to be key steps in the reaction

Selective Propylene Oxidation to Acrolein by Gold Dispersed on MgCuCr₂O₄ Spinel

Gold nanoparticles supported on a MgCuCr₂O₄ spinel catalyze the aerobic oxidation of propylene to acrolein. At 200 °C, the selectivity is 83% at a propylene conversion of 1.6%. At temperatures above 220 °C, propylene combustion dominates. The good performance of Au/MgCuCr₂O₄ in selective propylene oxidation is due to the synergy between metallic Au and surface Cu⁺ sites. Kinetic experiments (H₂ addition, N₂O replacing O₂) show that the reaction involves molecular oxygen. DFT calculations help to identify the reaction mechanism that leads to acrolein. Propylene adsorbs on a single Au atom. The adsorption of propylene via its π-bond on gold is very strong and can lead to the dissociation of the involved Au atom from the initial Au cluster. This is, however, not essential to the reaction mechanism. The oxidation of propylene to acrolein involves the oxidation of an allylic C–H bond in adsorbed propylene by adsorbed O₂. It results in OOH formation. The resulting CH₂–CH–CH₂ intermediate coordinates to the Au atom and a support O atom. A second C–H oxidation step by a surface O atom yields adsorbed acrolein and an OH group. The hydrogen atom of the OH group recombines with OOH to form water and a lattice O atom. The desorption of acrolein is the most difficult step in the reaction mechanism. It results in a surface oxygen vacancy in which O₂ can adsorb. The role of Cu in the support surface is to lower the desorption energy of acrolein

Interface Engineering of Hollow CoO/Co₄S₃@CoO/Co₄S₃ Heterojunction for Highly Stable and Efficient Electrocatalytic Overall Water Splitting

The key to improve the performance of electrochemically water splitting and simplify the entire system is to develop a dual-functional catalyst, which can be applied to catalyze the process of HER and OER. Therefore, we synthesized a novel hollow CoO/Co4S3@CoO/Co4S3 heterojunction with a core–shell structure as an excellent dual-functional catalyst. This sample is composed of an outer hollow CoO/Co4S3 cubic thin shell and an inner hollow CoO/Co4S3 sphere, and it can provide abundant catalytic active sites while effectively promoting the flow of reactants, products, and electrolytes. Meanwhile, the O–Co–S bond in the heterojunction interface can promote both the CoO active site in OER and theCo4S3 active site in HER. Therefore, the overpotential of the hollow CoO/Co4S3@CoO/Co4S3 is only 190 mV (OER) and 81 mV (HER), respectively, at the current density of 10 mA cm–2. Moreover, the hollow CoO/Co4S3@CoO/Co4S3 showed the outstanding electrochemical stability in 60 h. In addition, in the two-electrode system assembled from the hollow CoO/Co4S3@CoO/Co4S3, only the potential of 1.48 V can achieve the current density of 10 mA cm–2. Impressively, the commercial solar panel is sufficient to drive the two-electrode electrolyzer consisting of hollow CoO/Co4S3@CoO/Co4S3. This finding offers a promising nonprecious metal-based catalyst that can be applied to catalyze the electrochemical overall water splitting

Density Functional Theory Study of the Formaldehyde Catalytic Oxidation Mechanism on a Au-Doped CeO₂(111) Surface

Formaldehyde is a harmful and toxic substance. Au-CeO₂ catalysts show the excellent formaldehyde catalytic oxidation activity even under ambient temperatures. Here, we present the DFT+<i>U</i> calculations to investigate HCHO oxidation mechanisms and the effects of Au doping and multiple oxygen vacancies. The reaction process of HCHO oxidation mainly consists of the following steps: HCHO adsorption, C–H bond cleavages, CO₂ desorption, O₂ adsorption, and H₂O formation and desorption. The doped Au reduces the energy barriers in C–H bond cleavages on the AuCe_1–<i>x</i>O₂(111) surface compared with the CeO₂(111) surface. Au also leads to the activation of the surface oxygen species and then promotes HCHO adsorption and decreases the formation energies of oxygen vacancies. For HCHO adsorption and oxidation reaction on defective surfaces, catalysts with more oxygen vacancies possess higher adsorption energy and lower activation energy. These results provide deep insights into the effects of Au and multiple oxygen vacancies on HCHO oxidation reactions on the Au-doped CeO₂(111) surface and reveal the essential reason for the high activity of Au-CeO₂ catalysts

Interface Engineering of Hollow CoO/Co₄S₃@CoO/Co₄S₃ Heterojunction for Highly Stable and Efficient Electrocatalytic Overall Water Splitting

The key to improve the performance of electrochemically water splitting and simplify the entire system is to develop a dual-functional catalyst, which can be applied to catalyze the process of HER and OER. Therefore, we synthesized a novel hollow CoO/Co4S3@CoO/Co4S3 heterojunction with a core–shell structure as an excellent dual-functional catalyst. This sample is composed of an outer hollow CoO/Co4S3 cubic thin shell and an inner hollow CoO/Co4S3 sphere, and it can provide abundant catalytic active sites while effectively promoting the flow of reactants, products, and electrolytes. Meanwhile, the O–Co–S bond in the heterojunction interface can promote both the CoO active site in OER and theCo4S3 active site in HER. Therefore, the overpotential of the hollow CoO/Co4S3@CoO/Co4S3 is only 190 mV (OER) and 81 mV (HER), respectively, at the current density of 10 mA cm–2. Moreover, the hollow CoO/Co4S3@CoO/Co4S3 showed the outstanding electrochemical stability in 60 h. In addition, in the two-electrode system assembled from the hollow CoO/Co4S3@CoO/Co4S3, only the potential of 1.48 V can achieve the current density of 10 mA cm–2. Impressively, the commercial solar panel is sufficient to drive the two-electrode electrolyzer consisting of hollow CoO/Co4S3@CoO/Co4S3. This finding offers a promising nonprecious metal-based catalyst that can be applied to catalyze the electrochemical overall water splitting

Interface Engineering of Hollow CoO/Co₄S₃@CoO/Co₄S₃ Heterojunction for Highly Stable and Efficient Electrocatalytic Overall Water Splitting

The key to improve the performance of electrochemically water splitting and simplify the entire system is to develop a dual-functional catalyst, which can be applied to catalyze the process of HER and OER. Therefore, we synthesized a novel hollow CoO/Co4S3@CoO/Co4S3 heterojunction with a core–shell structure as an excellent dual-functional catalyst. This sample is composed of an outer hollow CoO/Co4S3 cubic thin shell and an inner hollow CoO/Co4S3 sphere, and it can provide abundant catalytic active sites while effectively promoting the flow of reactants, products, and electrolytes. Meanwhile, the O–Co–S bond in the heterojunction interface can promote both the CoO active site in OER and theCo4S3 active site in HER. Therefore, the overpotential of the hollow CoO/Co4S3@CoO/Co4S3 is only 190 mV (OER) and 81 mV (HER), respectively, at the current density of 10 mA cm–2. Moreover, the hollow CoO/Co4S3@CoO/Co4S3 showed the outstanding electrochemical stability in 60 h. In addition, in the two-electrode system assembled from the hollow CoO/Co4S3@CoO/Co4S3, only the potential of 1.48 V can achieve the current density of 10 mA cm–2. Impressively, the commercial solar panel is sufficient to drive the two-electrode electrolyzer consisting of hollow CoO/Co4S3@CoO/Co4S3. This finding offers a promising nonprecious metal-based catalyst that can be applied to catalyze the electrochemical overall water splitting