Untersuchungen zur Reaktivität von Vanadiumoxidfilmen auf Au(111)

Im Rahmen dieser Arbeit wurde die Bildung von Methoxygruppen auf als Modellkatalysatoren dienenden V2O3(0001)- und V2O5(001)-Oberflächen untersucht. Dieses ist ein Zwischenschritt einer industriell bedeutsamen Reaktion, der oxidativen Dehydrierung von Methanol zu Formaldehyd.Unter typischen UHV-Bedingungen wird die V2O3(0001)-Oberfläche von Vanadylgruppen und die V2O5(001)-Oberfläche von Doppelreihen aus Vanadylgruppen terminiert. Die Sauerstoffatome dieser Vanadylgruppen können durch Elektronenstrahlen entfernt werden. Dabei besteht die Möglichkeit, den Grad der Oberflächenreduktion durch die Elektronenstrahldosis zu variieren. Vanadylterminierte V2O3(0001)-Oberflächen sind unreaktiv im Hinblick auf die partielle Oxidation von Methanol. Die höchste Reaktivität weist eine nur schwach reduzierte Oberfläche auf. Details der Methanoladsorption sind mittels STM untersucht worden. Dabei wurde herausgefunden, dass n elektronenstrahl-induzierte Defekte zu 2n Methoxygruppen führen. Diese zusätzlichen Adsorptionsplätze werden durch desorbierendes Wasser, welches aus miteinander kombinierenden Hydroxygruppen entsteht, gebildet.Defektfreie V2O5(001)-Oberflächen sind ebenso inaktiv bezüglich der Formaldehydproduktion. Die höchste Reaktivität wurde wiederum für eine nur schwach reduzierte Oberfläche gefunden. Durch die gleichzeitige Desorption von rekombiniertem Methanol und Wasser sind bei Raumtemperatur nicht alle Defekte belegt, woraus sich eine geringere Menge Formaldehyd, verglichen mit V2O3,ergibt. Dosiert man eine große Menge Methanol bei Raumtemperatur, ist dieses nicht der Fall. Methoxygruppen werden auf der Oberfläche erst stabilisiert, wenn der Wasserstoff in Form von Wasser desorbiert ist. Daraus resultiert eine Zeitabhängigkeit der Methoxybedeckung, die durch kinetische Modelle gut reproduziert werden kann.The formation of methoxy groups, which is an intermediate step in the oxidative dehydrogenation of methanol to formaldehyde has been investigated on the surface of two model catalysts, V2O3(0001) and V2O5(001).Under typical UHV-conditions a layer of vanadyl groups terminates the V2O3 films whereas the V2O5 films are terminated by double rows of vanadyl groups. The oxygen atoms of the vanadyl groups can be removed by electron irradiation. The applied dose controls the degree of reduction. Vanadyl terminated V2O3(0001) surfaces were found to be unreactive towards the partial oxidation of methanol. The highest reactivity is observed for a partial vanadium termination. Details of the surface reaction were investigated with STM. It was found that n electron induced surface defects lead to the formation of 2n methoxy groups. The additional adsorption sites were created by water desorption, which is formed by the combination of hydroxy groups. Defect free V2O5(001) films were also shown to be inactive for formalde-hyde production. The highest reactivity is found for a partially reduced surface. But here the simultaneous formation of methanol and water via recombination is a likely process. This leads to a decrease of the surface methoxy coverage. At room temperature not all reactive defects are covered which gives a relatively low formaldehyde yield. Dosing a relatively large dose of methanol at room temperature leads to an almost full coverage of the defects and to a higher formaldehyde yield. The way to stabilize methoxy up to the formaldehyde formation temperature is removal of hydrogen out of the system by water formation. This competes with methanol formation for hydrogen atoms. These processes lead to a certain time dependence of the methoxy concentration, which could be well reproduced with kinetic modelling

Well Ordered Molybdenum Oxide Layers on Au 111 Preparation and Properties

MoO₃ layers on Au(111) were prepared via oxidation of molybdenum at elevated temperature in an atmosphere of 50 mbar of O₂. Three different types of oxide structures were identified. Up to monolayer oxide coverage a structure with a c(4 × 2) unit cell relative to the Au(111) unit cell forms. This structure was previously identified as being similar to a monolayer of α-MoO₃.(1) At larger coverages of up to two layers an oxide with a 11.6 Å × 5 Å rectangular unit cell appears. Further increase in the coverage leads to the occurrence of crystallites of regular α-MoO3 with a very small density of defects. These crystallites grow with the (010) plane parallel to the substrate surface and with random azimuthal orientation leading to rings in the LEED pattern. With increasing layer thickness the crystallites start to coalesce until finally a closed film forms. These layers sublimate at temperatures between about 670 and 770 K with the MoO₃ aggregates sublimating at lower temperature than the bilayer and monolayer films

Partial oxidation of methanol on well ordered V2O5 001 Au 111 thin films

The partial oxidation of methanol to formaldehyde on well-ordered thin V2O5(001)films supported on Au(111) was studied. Temperature-programmed desorption shows that bulk-terminated surfaces are not reactive, whereas reduced surfaces produce formaldehyde. Formaldehyde desorption occurs between 400 K and 550 K, without evidence for reaction products other than formaldehyde and water. Scanning tunnelling microscopy shows that methanol forms methoxy groups on vanadyl oxygen vacancies. If methanol is adsorbed at low temperature, the available adsorption sites are only partly covered with methoxy groups after warming up to room temperature, whereas prolonged methanol dosing at room temperature leads to full coverage. In order to explain these findings we present a model that essentially comprises recombination of methoxy and hydrogen to methanol in competition with the reaction of two surface hydroxyl groups to form water

Methanol Adsorption on V2O3 0001

Well ordered V2O3(0001) layers may be grown on Au(111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V2O3(0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V2O3(0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol → methoxy + H reaction