Brønsted–Evans–Polanyi relation for CO oxidation on metal oxides following the Mars–van Krevelen mechanism

Oxygen Vacancy Activates the Second-Nearest-Neighbor Lattice Oxygen for Oxidation Reaction

Oxygen vacancies on the oxide surface are regarded to play critical roles in catalytic oxidation reactions because they can alter the electronic and geometric properties of oxide catalysts. However, the effects of the oxygen vacancy on the CO oxidation activity of the surrounding lattice oxygen have remained elusive. In this work, using high-pressure scanning tunneling microscopy we identify that oxygen vacancy can activate surface lattice oxygen on the Mn3O4 thin layer. It is found that CO reacts with the lattice oxygen located at the second-nearest-neighbor position to the original oxygen vacancies more easily than that at the closest position and at the defect-free surface. This can be ascribed to the lower formation energy of the oxygen vacancies. Our study provides atomic-level insights into the promoting effect of oxygen vacancies on catalytic oxidation reactions.

Implementing the Blowers-Masel Approximation to Scale Activation Energy Based on Reaction Enthalpy in Mean-Field Microkinetic Modeling for Catalytic Methane Partial Oxidation

Mean-field microkinetic modeling is a powerful tool for catalyst design and the simulation of catalytic processes. The reaction enthalpies in a microkinetic model often need to be adjusted when changing species' binding energies to model different catalysts, when performing thermodynamic sensitivity analyses, and when fitting experimental data. When altering reaction enthalpies, the activation energies should also be reasonably altered to ensure realistic reaction rates. The Blowers-Masel approximation (BMA) relates the reaction barrier to the reaction enthalpy. Unlike the Brønsted-Evans-Polani relationship, the BMA requires less data because only one parameter, the intrinsic activation energy, needs to be determined. We validate this application of BMA relations to model surface reactions by comparing against density functional theory data taken from the literature. By incorporating the BMA rate description into the open-source Cantera software, we enable a new workflow, demonstrated herein, allowing rapid screening of catalysts using linear scaling relationships and BMA kinetics within the process simulation software. For demonstration purposes, a catalyst screening for catalytic methane partial oxidation on 81 hypothetical metals is conducted. We compared the results with and without BMA-corrected rates. The heat maps of various descriptors (e.g., CH4 conversion, syngas yield) show that using BMA rates instead of Arrhenius rates (with constant activation energies) changes which metals are most active. Heat maps of sensitivity analyses can help identify which reactions or species are the most influential in shaping the descriptor map patterns. Our findings indicate that while using BMA-adjusted rates did not markedly affect the most sensitive reactions, it did change the most influential species.

Reactivity of Lattice Oxygen in Ti-Site-Substituted SrTiO3 Perovskite Catalysts

An environmental catalyst in which a transition metal (Mn, Fe, or Co) was substituted into the Ti site of the host material, SrTiO₃, was synthesized, and the reactivity of lattice oxygen was evaluated. For CO oxidation, Mn- and Co-doped SrTiO₃ catalysts, which provided high thermal stabilities, exhibited higher activities than Pt/Al₂O₃ catalysts despite their low surface areas. Temperature-programmed reduction experiments using X-ray absorption fine structure (XAFS) measurements showed that the lattice oxygen of Co-doped catalyst was released at the lowest temperature. Isotopic experiments with CO and ¹⁸O₂ revealed that the lattice oxygen was involved in CO oxidation on Fe- and Co-doped catalysts; that is, CO oxidation on these catalysts proceeded via the Mars-van Krevelen mechanism. On the other hand, for Mn-doped catalyst, the contribution of lattice oxygen to CO oxidation was relatively negligible, indicating that the reaction proceeded according to the Langmuir-Hinshelwood mechanism. This paper clearly demonstrates that the catalytic mechanism can be adjusted by substituting transition metals into SrTiO₃.

Porous unsupported CuO nanoplates for efficient photothermal CO oxidation

It is a significant issue for environmental protection and industrial production to eliminate CO, a gas harmful to life and some important reaction sites. Real environmental conditions require catalytic CO oxidation to occur at relatively low temperature. Nowadays, photothermal catalysis has been exploited as a new way to achieve CO elimination, different from thermal catalysis. CuO, as cheap and abundant substitute for precious metals, is considered to have potential in photothermal catalysis. Oxygen vacancies (O_V) and lattice oxygen (O_L) activity are considered extremely crucial for oxide catalysts in CO oxidation, according to Mars-van Krevelen mechanism. Herein, porous CuO nanoplates with adjustable O_Vand O_Lactivity were prepared by a facile method via controlling the morphology and phase composition of precursors. The light-off temperature (50% conversion) of the best sample obtained under the optimal conditions was ∼110 °C and an almost complete conversion was reached at ∼150 °C. It also achieved nearly 70% conversion under 6 standard Suns (6 kW cm^-2irradiation) and could work in infrared radiation (IR) regions, which could be attributed to the photo-induced thermal effect and activation effect. The simple synthesis and characterization provide a good example for the future photothermal catalysis.

Deep reaction network exploration at a heterogeneous catalytic interface

Characterizing the reaction energies and barriers of reaction networks is central to catalyst development. However, heterogeneous catalytic surfaces pose several unique challenges to automatic reaction network characterization, including large sizes and open-ended reactant sets, that make ad hoc network construction the current state-of-the-art. Here, we show how automated network exploration algorithms can be adapted to the constraints of heterogeneous systems using ethylene oligomerization on silica-supported single-site Ga3+ as a model system. Using only graph-based rules for exploring the network and elementary constraints based on activation energy and size for identifying network terminations, a comprehensive reaction network is generated and validated against standard methods. The algorithm (re)discovers the Ga-alkyl-centered Cossee-Arlman mechanism that is hypothesized to drive major product formation while also predicting several new pathways for producing alkanes and coke precursors. These results demonstrate that automated reaction exploration algorithms are rapidly maturing towards general purpose capability for exploratory catalytic applications.

Active sites and deactivation of room temperature CO oxidation on Co3O4catalysts: combined experimental and computational investigations

Co₃O₄is a well-known low temperature CO oxidation catalyst, but it often suffers from deactivation. We have thus examined room temperature (RT) CO oxidation on Co₃O₄catalysts by operando DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO₂. Catalysts pretreated in oxygen at 400 °C are most active, with the initial interaction of CO and Co₃O₄being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high RT activity then levels-off, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co₃O₄surface that upon reaction onset loses its most active oxygen. This specific active oxygen is not reestablished by gas phase O₂during the RT reaction. When the reaction temperature is increased to 150 °C, full conversion can be maintained for 100 h, and even after cooling back to RT. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational modeling using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at RT was identified, which may block a vacancy site from further reaction unless CO is removed at higher temperature. The interaction between oxygen vacancies was found to be small, so that in the active state several lattice oxygen species are available for reaction in parallel.

Effects of Subsurface Oxide on Cu1/CeO2 Single-Atom Catalysts for CO Oxidation: A Theoretical Investigation

Supported atomic dispersion metals are of great interest, and the interfacial effect between isolated metal atoms and supports is crucial in heterogeneous catalysis. Herein, the behavior of single-atom Cu catalysts dispersed on CeO₂ (100), (110), and (111) surfaces has been studied by DFT + U calculations. The interactions between ceria crystal planes and isolated Cu atoms together with their corresponding catalytic activities for CO oxidation are investigated. The CeO₂ (100) and (111) surfaces can stabilize active Cu⁺ species, while Cu exists as Cu²⁺ on the (110) surface. Cu⁺ is certified as the most active site for CO adsorption, which can promote the formation of the reaction intermediates and reduce reaction energy barriers. For the CeO₂ (100) surface, the interaction between CO and Cu is weak and the CO adsorbate is more likely to activate the subsurface oxygen. The catalytic performance is closely related to the binding strength of CO to the active Cu single atoms on the different subsurfaces. These results bring a significant insight into the rational design of single metal atoms on ceria and other reducible oxides.

Influence of the Support on Propene Oxidation over Gold Catalysts

The epoxidation of propene without forming a substantial amount of byproducts is one of the holy grails of catalysis. Supported Cu, Ag and Au catalysts are studied for this reaction and the activity of the supported metals is generally well understood. On the contrary, limited information is available on the influence of the support on the epoxide selectivity. The reaction of propene with equal amounts of hydrogen and oxygen was tested over gold nanoparticles deposited onto CeO2, TiO2, WO3, γ-Al2O3, SiO2, TiO2-SiO2 and titanosilicate-1. Several metal oxide supports caused further conversion of the synthesized propene oxide. Strongly acidic supports, such as WO3 and titanosilicate-1, catalyzed the isomerization of propene oxide towards propanal and acetone. Key factors for achieving high PO selectivity are having inert or neutralized surface sites, a low specific surface and/or a low density of surface -OH groups. This work provides insights and practical guidelines to which metal oxide support properties lead to which products in the reaction of propene in the presence of oxygen and hydrogen over supported gold catalysts.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Efficient and stable low-temperature CO oxidation over Pt/In–SnO2 composite triggered by abundant oxygen vacancies and adsorption sites

In ion doping can greatly improve the active oxygen migration ability in the Pt/In–SnO₂ catalyst, which is beneficial to CO oxidation at low temperature.