Low-Temperature CO Oxidation Catalyzed by Free Palladium Clusters: Similarities and Differences to Pd Surfaces and Supported Particles

Synergism of Iron and Platinum Species for Low-Temperature CO Oxidation: From Two-Dimensional Surface to Nanoparticle and Single-Atom Catalysts

CO oxidation is one of the most studied reactions in heterogeneous catalysis. It is present in air cleaning and automotive emission control. It also participates in the removal of CO from streams of hydrogen used in fuel cells. Because of the competitive adsorption of CO and O₂ over active sites, the use of Pt-based catalysts for low-temperature CO oxidation remains a challenge. Recently, great progress has been made with catalysts containing Pt-Fe species because of the contribution of Fe species to O₂ activation. The structure-activity relationship and reaction mechanisms have been investigated with various Pt-Fe catalysts. In this Perspective, we give a summary of the recent advances of low-temperature CO oxidation over Pt-Fe catalysts with a focus on the synergistic effect of Pt and Fe species in the CO and O₂ activation of catalytic reactions. Future prospects for the preparation of highly effective Pt-Fe catalysts are also proposed.

Catalytic Non-Oxidative Coupling of Methane on Ta8O2

Mass-selected Ta₈O₂⁺ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H₂, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

Degradation of Phenol Using Peroxymonosulfate Activated by a High Efficiency and Stable CoMgAl-LDH Catalyst

In this study, we report on an active and stable CoMgAl layered double hydrotalcite (LDH) catalyst for phenol degradation by heterogeneous activation of peroxymonosulfate (PMS). The CoMgAl-LDH catalyst was synthesized by hydrothermal method. The PMS/CoMgAl-LDH system overcomes the drawbacks of traditional Fenton processes. Various effects, e.g., scavengers, chloride ion, catalyst dosage, PMS concentration, temperature, and pH, were also inspected to evaluate the system. The results indicated that the PMS/CoMgAl-LDH system had extremely high efficiency for phenol degradation; 0.1 mM phenol could be completely degraded by 0.3 g/L catalyst and 3 mM PMS within 60 min at 30 °C. The CoMgAl-LDH catalyst appeared to possess outstanding reusability and stability. After four rounds of recycling, nearly 100% of the phenol was removed within 80 min by the PMS/CoMgAl-LDH system, with only 0.05 mg/L Co2+ leaching. A sulfate radical was the main oxidation species in the PMS/Co-LDH system. The degradation rate of phenol was influenced by temperature, and the activation energy was 65.19 kJ/mol. These advantages proved the PMS/CoMgAl-LDH system is an effective strategy for the treatment of organic contaminants.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.

Identification of active sites in CO oxidation over a Pd/Al2O3 catalyst

The active sites of Pd/Al₂O₃ catalysts for CO oxidations were identified by investigating the dependence of CO oxidation activities on the surface structure and morphology of Pd nanoparticles.

In Situ Formation of Isolated Bimetallic PtCe Sites of Single-Dispersed Pt on CeO2 for Low-Temperature CO Oxidation

Identification of the chemical states of catalytic sites is critical for an atomic-level understanding of catalytic mechanisms. Herein, hydrogen thermal pretreatment of the Pt single atoms on porous nanorods of CeO₂ (Pt₁/ PN-CeO₂) induced the formation of isolated bimetallic PtCe sites as a new type of active center for CO oxidation. The evolutions of Pt₁/ PN-CeO₂ catalysts during the hydrogen pretreatment and CO oxidation were examined by various in situ techniques including infrared, ambient-pressure X-ray photoelectron and X-ray absorption spectroscopy. The experimental results demonstrate that these bimetallic sites can be partially preserved or reoxidized into Pt-O-Ce with a low coordination number with oxygen under realistic conditions, leading to the appropriate CO adsorption and activating the efficient activity of Pt₁/ PN-CeO₂ for CO oxidation at low temperature.

Identification of a flattened Pd–Ce oxide cluster as a highly efficient catalyst for low-temperature CO oxidation

An improved deposition method was employed to prepare a Pd/CeO₂ catalyst, which exhibited highly efficient activity in low-temperature CO oxidation (LTO).

Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si2M with M = H, Li, Na, Cu, and Ag

High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si2M with M = H, Li, Na, Cu, and Ag toward the first step of methanol activation reaction. Standard heats of formation of these small triatomic Si clusters were determined. Potential-energy profiles were constructed using the coupled-cluster theory with extrapolation to complete basis set CCSD(T)/CBS, and CCSD(T)/aug-cc-pVTZ-PP for Si2Cu and Si2Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si2M possess low-spin states and mainly stem from an M-O connection in preference to Si-O interaction, except for the Si2H case. In two competitive pathways including O-H and C-H bond breakings, the cleavage of the O-H bond in the presence of all clusters studied becomes predominant. Of the mixed clusters Si2M considered, the dissociation pathways of both O-H and C-H bonds with Si2Li turns out to have the lowest energy barriers. The most remarkable finding is the absence of the overall energy barrier for the O-H cleavage with the assistance of Si2Li. The breaking of O-H and C-H bonds with the assistance of Si2H, Si2Li, and Si2Na is kinetically preferred with respect to the Si2Cu and Si2Ag cases, apart from the case of Si2Na for O-H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu3, Pt3, and PtAu2, the energy barriers for the O-H bond activation in the presence of small Si species, especially Si2H and Si2Li, are found to be lower. Consequently, these small mixed silicon clusters can be regarded as promising alternatives for the expensive metal-based catalysts currently used for methanol activation particularly and other dehydrogenation processes of organic compounds. The present study also suggests a further extensive search for other doped silicon clusters as efficient and more realistic gas-phase catalysts for important dehydrogenation processes in such a way that they can be experimentally prepared and implemented.

CO oxidation by the atomic oxygen on silver clusters: structurally dependent mechanisms generating free or chemically bonded CO2

Oxidation of CO by the atomic oxygen on Ag_nO⁻ (n = 1–8) forms free or chemically bonded CO₂.

Co-Adsorption of O2 and CO on Au2- : Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2 O2 (CO)n ]- (n=2-6)

The co-adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂ O₂ (CO)_n ]^- (n=2-6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂ O₂ (CO)_n ]^- (n=2-6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co-adsorbed on Au₂^- with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂ O₂ (CO)]^- core ion is observed around ν˜ =2032-2042 cm^-1 , which is about 200 cm^-1 higher than that in [Au₂ (CO)₂ ]^- . This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co-adsorbed O₂ and CO on Au₂^- may have relevance to the elementary reaction steps on real gold catalysts.

The complex behavior of the Pd7 cluster supported on TiO2(110) during CO oxidation: adsorbate-driven promoting effect

The CO saturation-driven 3D to 2D structural transformation of the Pd₇ cluster supported on TiO₂(110) promotes CO oxidation activity via bifunctional synergy of interfacial sites.

Release of Oxygen from Palladium Oxide Cluster Ions by Heat

Palladium oxide cluster ions, PdnOm(+), were prepared in the gas phase using laser ablation of a palladium rod in the presence of oxygen. The cluster ions were heated to 1000 K downstream from the cluster source (post heating), and the abundance of PdnOm(+) (n = 2-7) was examined using mass spectrometry. Temperature-programmed desorption experiments revealed that an oxygen molecule is released from oxygen-rich PdnOm(+), forming oxygen-deficient PdnOm-2(+). It was found that Pd6O4(+) was thermally stable up to 1000 K. The activation energy for oxygen molecule desorption has been obtained and compared with previous results by Lang et al.