Transition metal doping of Pd(1 1 1) for the NO + CO reaction

A microkinetic study of CO2 hydrogenation to methanol on Pd1-Cu(111) and Pd1-Ag(111) catalysts: a DFT analysis

The thermochemical conversion of CO2 into methanol, a process known for its selectivity, often encounters a significant obstacle: the reverse water gas reaction. This problem emerges due to the demanding high temperatures and pressures, causing instability in catalytic performance. Recent endeavours have focused on innovatively designing catalysts capable of withstanding such conditions. Given the costliness of experimental approaches, a theoretical framework has emerged as a promising avenue for addressing the challenges in methanol production. It has been reported that transition metals, especially Pd, provide ideal binding sites for CO2 molecules and hydrogen atoms, facilitating their interactions and subsequent conversion to methanol. In the geometric single-atom form, their surface enables precise control over the reaction pathways and enhances the selectivity towards methanol. In our study, we employed density functional theory (DFT) to explore the conversion of CO2 to CH3OH on Pd1-Cu(111) and Pd1-Ag(111) single-atom alloy (SAA) catalysts. Our investigation involved mapping out the complex reaction pathways of CO2 hydrogenation to CH3OH using microkinetic reaction modelling and mechanisms. We examined three distinct pathways: the COOH* formation pathway, the HCOO* formation pathway, and the dissociation of CO2* to CO* pathway. This comprehensive analysis encompassed the determination of adsorption energies for all reactants, transition states, and resultant products. Additionally, we investigated the thermodynamic and kinetic profiles of individual reaction steps. Our findings emphasised the essential role of the Pd single atom in enhancing the activation of CO2, highlighting the key mechanism underlying this catalytic process. The favoured route for methanol generation on the Pd1-Ag(111) single-atom alloy (SAA) surface unfolds as follows: CO2* progresses through a series of transformations, transitioning successively into HCOO*, HCOOH*, H2COOH*, CH2O*, and CH2OH*, terminating in the formation of CH3OH*, due to lower activation energies and higher rate constants.

Boosted Light Alkane Deep Oxidation via Metal Bond Length Modulation-Induced C-C Bond Preferential Activation

Light alkanes (LAs), typical VOCs existing in both stationary and mobile sources, pose significant environmental concerns. Although noble metal catalysts demonstrate strong C-H bond activation, their effectiveness in degrading LAs is hindered by inherent challenges, including poor chemical stability and water resistance. Here, from a new perspective, we propose a feasible strategy that adjusting the metal bond lengths within Pd clusters through partial substitution of smaller radius 3d transition metals (3dTMs) to prioritize the activation of low-energy C-C bonds within LAs. Benefiting from this, PdCo/CeO2 exhibits exceptional catalytic performance in propane degradation due to their high capacity for C-C cleavage stemming from the shorter Pd-Co length (2.51 Å) and lower coordination number (1.73), boosting the activation of α-H and β-H of propane simultaneously and accelerating the mobility of postactivated oxygen species to prevent Pd center deep oxidation. The presence of 3dTMs on Pd clusters improves the redox and charge transfer ability of catalysts, resulting in an amplified generation of oxygen vacancies and facilitating the adsorption and activation of reactants. Mechanistic studies and DFT calculations suggest that the substitution of 3dTMs significantly accelerate C-C bond cleavage within C3 intermediates to generate the subsequent C2 and C1 intermediates, suppressing the generation of harmful byproducts.

A comprehensive investigation of the plasmonic-photocatalytic properties of gold nanoparticles for CO2 conversion to chemicals

Understanding the interactions between plasmonic gold (Au) nanoparticles and the adsorbate is essential for photocatalytic and plasmonic applications. However, it is often challenging to identify a specific reaction mechanism in the ground state and to explore the optical properties in the excited states because of the complicated pathways of carriers. In this study, photocatalytic reduction of carbon dioxide (CO₂) to C1 products (for example, CO and CH₄) on the Au(111) nanoparticle (NP) surface was studied based on reaction pathway analysis, adsorbate reactivity, and its ability to stabilize or deactivate the surface. The calculated reaction Gibbs free energies and activation barriers revealed that the first step in CO reduction via a direct hydrogen transfer mechanism on Au(111) is the formation of formyl (*CHO) instead of hydroxymethylidyne (*COH). Furthermore, the size enhanced and symmetry sensitive optical responses of cuboctahedral Au(111) NPs on localized surface plasmon resonance (LSPR) were investigated by using time-dependent DFT (TDDFT) calculations. Although near field enhancement around cuboctahedral Au(111) NPs is only weakly dependent on the morphology of NPs, it was observed that corner sites stabilize *C-species to drive the CO₂ reduction to CO. The density of active surface states interacting with the adsorbate states near the Fermi level gradually decreases from the (111) on-top site toward the corner site of the Au(111) NP-CO system, which strongly affects the molecule's binding on catalytic sites and, in particular, electronic excitation. Finally, the spatial distribution of the charge oscillations was determined as a guide for the fabrication of Au NPs with an optimal LSPR response.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper–Mixed Oxide Interfaces

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu_xMn_1–xO₃ (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu_yPd_x/perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N₂ selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N₂O at low temperatures, enhancing the selectivity toward N₂. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal–mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

Single atom alloys vs. phase separated alloys in Cu, Ag, and Au atoms with Ni(111) and Ni, Pd, and Pt atoms with Cu(111): a theoretical exploration

A single-atom alloy (SAA) consisting of an abundant metal host and a precious metal guest is a promising catalyst to reduce the cost without a loss of activity. DFT calculations of Ni- and Cu-based alloys nX/M(111) (X = Cu, Ag, or Au for M = Ni; X = Ni, Pd, or Pt for M = Cu; n = 1-4) reveal that a phase-separated alloy (PSA) is produced by Cu atoms with Ni(111) but an SAA is produced by Au atoms with Ni(111) and Pd and Pt atoms with Cu(111). In the Ni(111)-based Ag alloy and Cu(111)-based Ni alloy, the relative stabilities of the SAA and PSA depend on coverages of Ag on Ni(111) and Ni on Cu(111). The interaction energy (E_int) between the X_n cluster and M(111) host is larger than that between one X atom and the M(111) host, because the X_n cluster forms more bonding interactions with the M(111) host than does one X atom. When going from one X atom to the X₄ cluster, the E_int values of Au and Pt clusters respectively with Ni(111) and Cu(111) increase to a lesser extent than those of Cu and Ni clusters respectively with Ni(111) and Cu(111). Consequently, Au and Pt atoms tend to form SAAs respectively with Ni(111) and Cu(111) hosts compared to Cu and Ni atoms. This trend in the E_int value is determined by the valence orbital energies of the X atom and the X_n cluster. Cu atoms in nCu/Ni(111) have a slightly positive charge but Ag atoms in nAg/Ni(111), Au atoms in nAu/Ni(111), and Ni, Pd, and Pt atoms in nX/Cu(111) (X = Ni, Pd, or Pt) have a negative charge. The negative charge increases in the order Ag < Au in nX/Ni(111) and Ni < Pd < Pt in nX/Cu(111). The Fermi level decreases in energy in the order nCu/Ni(111) ≥ Ni(111) > nAg/Ni(111) > nAu/Ni(111) and Cu(111) ≥ nNi/Cu(111) > nPd/Cu(111) > nPt/Cu(111). The d valence band center decreases in energy in almost the same order. The CO adsorption energy decreases in the order Ni(111) ∼ nCu/Ni(111) > nAg/Ni(111) ∼ nAu/Ni(111) and Cu(111) > nNi/Cu(111) > nPd/Cu(111) > nPt/Cu(111). These properties are explained based on the electronic structures.

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

Pt supported on oxygen storage materials (CeO2 and CeO2–ZrO2) as effective three-way catalysts.

Improved Pd/CeO2 Catalysts for Low-Temperature NO Reduction: Activation of CeO2 Lattice Oxygen by Fe Doping

Developing better three-way catalysts with improved low-temperature performance is essential for cold start emission control. Density functional theory in combination with microkinetics simulations is used to predict reactivity of CO/NO/H2 mixtures on a small Pd cluster on CeO2(111). At low temperatures, N2O formation occurs via a N2O2 dimer over metallic Pd3. Part of the N2O intermediate product re-oxidizes Pd, limiting NO conversion and requiring rich conditions to obtain high N2 selectivity. High N2 selectivity at elevated temperatures is due to N2O decomposition on oxygen vacancies. Doping CeO2 by Fe is predicted to lead to more oxygen vacancies and a higher N2 selectivity, which is validated by the lower onset of N2 formation for a Pd catalyst supported on Fe-doped CeO2 prepared by flame spray pyrolysis. Activating ceria surface oxygen by transition metal doping is a promising strategy to improve the performance of three-way catalysts.

The catalytic decomposition of nitrous oxide and the NO + CO reaction over Ni/Cu dilute and single atom alloy surfaces: first-principles microkinetic modelling

Density functional theory calculations and microkinetic modelling reveal that well-engineered Ni/Cu dilute alloys are promising for the catalytic reduction of NO by CO.

Linear Activation Energy-Reaction Energy Relations for LaBO3 (B = Mn, Fe, Co, Ni) Supported Single-Atom Platinum Group Metal Catalysts for CO Oxidation

Single-atom catalysts are at the center of attention of the heterogeneous catalysis community because they exhibit unique electronic structures distinct from nanoparticulate forms, resulting in very different catalytic performance combined with increased usage of often costly transition metals. Proper selection of a support that can stably keep the metal in a high dispersion is crucial. Here, we employ spin-polarized density functional theory and microkinetics simulations to identify optimum LaBO₃ (B = Mn, Fe, Co, Ni) supported catalysts dispersing platinum group metals as atoms on their surface. We identify a strong correlation between the CO adsorption energy and the d-band center of the doped metal atom. These CO adsorption strength differences are explained in terms of the electronic structure. In general, Pd-doped surfaces exhibit substantially lower activation barriers for CO₂ formation than the Rh- and Pt-doped surfaces. Strong Brønsted-Evans-Polanyi correlations are found for CO oxidation on these single-atom catalysts, providing a tool to predict promising compositions. Microkinetics simulations show that Pd-doped LaCoO₃ is the most active catalyst for low-temperature CO oxidation. Moderate CO adsorption strength and low reaction barriers explain the high activity of this composition. Our approach provides guidelines for the design of highly active and cost-effective perovskite supported single-atom catalysts.

Design of Pd-based pseudo-binary alloy catalysts for highly active and selective NO reduction

The development of Pd-based alloy catalysts for highly active and selective reduction of NO by CO was investigated. A survey of Pd-based bimetallic catalysts (PdM/Al2O3: M = Cu, In, Pb, Sn, and Zn) revealed that the PdIn/Al2O3 catalyst displayed excellent N2 selectivity even at low temperatures (100% at 200 °C). The catalytic activity of PdIn was further improved by substituting a part of In with Cu, where a Pd(In1-x Cu x ) pseudo-binary alloy structure was formed. The optimized catalyst, namely, Pd(In0.33Cu0.67)/Al2O3, facilitated the complete conversion of NO to N2 (100% yield) even at 200 °C and higher, which has never been achieved using metallic catalysts. The formation of the pseudo-binary alloy structure was confirmed by the combination of HAADF-STEM-EDS, EXAFS, and CO-FT-IR analyses. A detailed mechanistic study based on kinetic analysis, operando XAFS, and DFT calculations revealed the roles of In and Cu in the significant enhancement of catalytic performance: (1) N2O adsorption and decomposition (N2O → N2 + O) were drastically enhanced by In, thus resulting in high N2 selectivity; (2) CO oxidation was promoted by In, thus leading to enhanced low-temperature activity; and (3) Cu substitution improved NO adsorption and dissociation (NO → N + O), thus resulting in the promotion of high-temperature activity.

Improved NO–CO reactivity of highly dispersed Pt particles on CeO2 nanorod catalysts prepared by atomic layer deposition

Highly dispersed platinum (Pt) nanoparticles are deposited on CeO₂ nanorods via atomic layer deposition (ALD) to improve the catalytic activity towards the NO–CO reaction.