Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO2 catalysts

CO2 Methanation over Nickel Catalysts: Support Effects Investigated through Specific Activity and Operando IR Spectroscopy Measurements

Renewed interest in CO2 methanation is due to its role within the framework of the Power-to-Methane processes. While the use of nickel-based catalysts for CO2 methanation is well stablished, the support is being subjected to thorough research due to its complex effects. The objective of this work was the study of the influence of the support with a series of catalysts supported on alumina, ceria, ceria–zirconia, and titania. Catalysts’ performance has been kinetically and spectroscopically evaluated over a wide range of temperatures (150–500 °C). The main results have shown remarkable differences among the catalysts as concerns Ni dispersion, metallic precursor reducibility, basic properties, and catalytic activity. Operando infrared spectroscopy measurements have evidenced the presence of almost the same type of adsorbed species during the course of the reaction, but with different relative intensities. The results indicate that using as support of Ni a reducible metal oxide that is capable of developing the basicity associated with medium-strength basic sites and a suitable balance between metallic sites and centers linked to the support leads to high CO2 methanation activity. In addition, the results obtained by operando FTIR spectroscopy suggest that CO2 methanation follows the formate pathway over the catalysts under consideration.

Interactions of water and short-chain alcohols with CoFe2O4(001) surfaces at low coverages

Iron and cobalt-based oxides crystallizing in the spinel structure are efficient and affordable catalysts for the oxidation of organics, yet, the detailed understanding of their surface structure and reactivity is limited. To fill this gap, we have investigated the (001) surfaces of cobalt ferrite, CoFe₂O₄, with the A- and B-layer terminations using density functional theory (DFT/PBE0) and an embedded cluster model. We have considered the five-fold coordinated Co^2+/3+ (O_h), two-fold coordinated Fe²⁺ (T_d), and an oxygen vacancy, as active sites for the adsorption of water and short-chain alcohols: methanol, ethanol, and 2-propanol, in the low coverage regime. The adsorbates dissociate upon adsorption on the Fe sites whereas the adsorption is mainly molecular on Co. At oxygen vacancies, the adsorbates always dissociate, fill the vacancy and form (partially) hydroxylated surfaces. The computed vibrational spectra for the most stable configurations are compared with results from diffuse reflectance infrared Fourier transform spectroscopy.

Understanding methanol dissociative adsorption and oxidation on amorphous oxide films

Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction. We here use ambient...

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species

The study provides deep insight into the origin of photocatalytic deactivation of Nb₂O₅ after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO₂/Nb₂O₅ heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb₂O₅ and CeO₂) and as-prepared CeO₂/Nb₂O₅ heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO₂/Nb₂O₅ heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb₂O₅ to CeO₂. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

The effect of the calcium dopant on the activity and selectivity of gold catalysts supported on SBA-15 and Nb-containing SBA-15 in methanol oxidation

Gold catalysts based on SBA-15, NbSBA-15 (Nb introduced in one pot synthesis) and Nb₂O₅/SBA-15 (prepared by impregnation of SBA-15) were doped with calcium species introduced before Au loading and were tested in gas-phase methanol oxidation.

Complete Degradation of Gaseous Methanol over Pt/FeOx Catalysts by Normal Temperature Catalytic Ozonation

Normal temperature catalytic ozonation (NTCO) is a promising yet challenging method for the removal of volatile organic compounds (VOCs) because of limited activity of the catalysts at ambient temperature. Here, we report a series of Pt/FeO_x catalysts prepared by the co-precipitation method for NTCO of gaseous methanol. All samples were found to be active and among them, the Pt/FeO_x-400 (calcined at 400 °C) catalyst with a Pt cluster loading of 0.2% exhibited the highest activity, able to completely convert methanol into CO₂ and H₂O at 30 °C. Extensive experimental research suggested that the superior catalytic activity could be attributed to the highly dispersed Pt clusters and an appropriate molar ratio of Pt⁰/Pt²⁺. Furthermore, electron paramagnetic resonance and density functional theory computational studies revealed the mechanism that the Pt/FeO_x-400 catalyst could activate O₃ and water effectively to produce hydroxyl radicals responsible for the catalytic oxidation of methanol. The findings of this work may foster the development of technologies for normal temperature abatement of VOCs with low energy consumption.

Surface chemistry of methanol on different ZnO surfaces studied by vibrational spectroscopy

The adsorption and reactions of CH₃OH on nonpolar mixed-terminated ZnO(101[combining macron]0), polar O-ZnO(0001[combining macron]) and Zn-ZnO(0001) surfaces have been studied systematically using high-resolution electron energy loss spectroscopy (HREELS) in conjunction with temperature programmed desorption (TPD). For all three ZnO surfaces, exposure to methanol at room temperature leads to (partially) dissociative adsorption resulting in the formation of hydroxyl and methoxy species. Upon heating to higher temperatures, the dissociated and intact methanol species on ZnO(101[combining macron]0) predominantly undergo molecular desorption releasing CH₃OH at 370 and 440 K. The Zn-O dimer vacancies are responsible for the decomposition of a small fraction of methanol yielding H₂, CH₂O and CO at 540 and 565 K. The interaction of methanol with polar O-ZnO and Zn-ZnO surfaces is dominated by thermal decomposition of CH₃OH to produce CH₂O, H₂, CO, CO₂ and H₂O at elevated temperatures. The high chemical reactivity of both polar surfaces is related to the high abundance of different types of surface defects formed via massive restructuring. Importantly, the reconstructed Zn-ZnO surface exhibits high selectivity for hydrogen production at 520 K, which was not observed for the polar O-ZnO surface. The HREELS data revealed that this low-temperature hydrogen evolution on Zn-ZnO results from methoxy oxidation to a formate species occurring at O-terminated step-edge sites.

Structural and electronic properties of bulk and low-index surfaces of zincblende PtC

Transition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Continuous flow gas phase photoreforming of methanol at elevated reaction temperatures sensitised by Pt/TiO2

Gas phase photoreforming of methanol using a Pt/TiO₂ photocatalyst has been performed under flow conditions at elevated temperatures.

On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO2

Partial insight into the reaction mechanism of the photooxidation of 2-propanol over Au/TiO₂. 2-Propanol is activated by hole capture and converted to acetone requiring the presence of O₂ as an electron acceptor. The deposited Au nanoparticles are assumed to facilitate the electron transfer from the TiO₂ conduction band to adsorbed O₂.

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study

This work is a mechanistic study of total and partial methanol photooxidation using operando FTIR coupled to gas phase analysis techniques (gas-IR and MS).

Gas-phase oxidation of ethanol over Au/TiO2 catalysts to probe metal–support interactions

Ethanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal, and the role of metal–support interactions.

Heterogeneous Metal Catalysts for Oxidation Reactions

Oxidation reactions may be considered as the heart of chemical synthesis. However, the indiscriminate uses of harsh and corrosive chemicals in this endeavor are threating to the ecosystems, public health, and terrestrial, aquatic, and aerial flora and fauna. Heterogeneous catalysts with various supports are brought to the spotlight because of their excellent capabilities to accelerate the rate of chemical reactions with low cost. They also minimize the use of chemicals in industries and thus are friendly and green to the environment. However, heterogeneous oxidation catalysis are not comprehensively presented in literature. In this short review, we clearly depicted the current state of catalytic oxidation reactions in chemical industries with specific emphasis on heterogeneous catalysts. We outlined here both the synthesis and applications of important oxidation catalysts. We believe it would serve as a reference guide for the selection of oxidation catalysts for both industries and academics.