Evalyn Mae C. Alayon

Generating Highly Active Partially Oxidized Platinum during Oxidation of Carbon Monoxide over Pt/Al2O3: In Situ, Time-Resolved, and High-Energy-Resolution X-Ray Absorption Spectroscopy

High activity is generated by sudden formation of disordered oxidic platinum over a platinum catalyst supported on alumina (see picture). High temperature and low concentration of carbon monoxide are required to generate high activity.

Dynamic Structure Changes of a Heterogeneous Catalyst within a Reactor: Oscillations in CO Oxidation over a Supported Platinum Catalyst

Kinetic oscillations in the oxidation of CO occur because of local changes in the catalyst structure inside the reactor. Activity loss within an oscillation originates from partial reduction of the active surface, which occurs at distinct positions within the catalyst bed. The original activity is regained partly by re-oxidation of the catalyst, during which a short-lived phase is formed.

Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper-containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper-exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di- or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.

Methane to methanol over copper mordenite: yield improvement through multiple cycles and different synthesis techniques

Copper mordenite was used for the conversion of methane to methanol in a cyclic operation. Repeated cycling was possible using catalysts having different loadings prepared via aqueous ion exchange using copper(II) acetate (Cu-MORA) and solid state ion exchange using copper(I) chloride (Cu-MORS). For Cu-MORA, the yield increased by at least 30% on the second cycle and remained constant afterwards. Linear combination fitting of the XANES identified a similar increase in the fraction of CuI formed upon reacting with methane on the second cycle. For Cu-MORS residual chlorine initially hindered the production of methanol. Successive cycles removed chlorine and yielded significantly more methanol per copper than Cu-MORA. Over successive cycles of Cu-MORS, the fraction of the CuII species that was reduced to CuI upon reacting with methane correlated well with the amount of methanol produced. Although commonly done, analyzing only one reaction cycle is not representative of the long-term performance of methane to methanol over copper mordenite. Copper species equilibrate with cycling.

Catalytic conversion of methane to methanol using Cu-zeolites.

The conversion of methane to value-added liquid chemicals is a promising answer to the imminent demand for fuels and chemical synthesis materials in the advent of a dwindling petroleum supply. Current technology requires high energy input for the synthesis gas production, and is characterized by low overall selectivity, which calls for alternative reaction routes. The limitation to achieve high selectivity is the high C-H bond strength of methane. High-temperature reaction systems favor gas-phase radical reactions and total oxidation. This suggests that the catalysts for methane activation should be active at low temperatures. The enzymatic-inspired metal-exchanged zeolite systems apparently fulfill this need, however, methanol yield is low and a catalytic process cannot yet be established. Homogeneous and heterogeneous catalytic systems have been described which stabilize the intermediate formed after the first C-H activation. The understanding of the reaction mechanism and the determination of the active metal sites are important for formulating strategies for the upgrade of methane conversion catalytic technologies.

In situ XAS probes partially oxidized platinum generating high activity for CO oxidation

In situ x-ray absorption spectroscopy identified partially oxidized platinum as the active phase for generating high activity during carbon monoxide oxidation over Pt/Al2O3. CO covers and poisons metallic platinum, which results in a low catalytic activity. The conversion in CO abruptly increased during ignition, switching from low activity to high activity, accompanied by oxidation as observed in the increased intensity of the XANES. EXAFS analysis indicated breaking of Pt-Pt bonds and the appearance of a Pt-O scatterer. The partially oxidized catalyst had a lower oxygen coordination number and shorter Pt-O bond length than bulk PtO2, suggesting that a strongly defected platinum oxide was formed with a coordination possibly lower than octahedral. From infrared data, ignition was accompanied by the abrupt disappearance of CO adsorbed on metallic particles. The reverse changes happen with the sudden decrease in CO conversion during extinction: partially oxidized catalyst becomes reduced and covered with CO.