Methanol adsorption and desorption are important on many oxide surfaces, for instance, as probe molecules of surface active sites. In this study, atomic layer deposited (ALD) tungsten oxide on silica was used as oxide sample and dimethyl ethylene (DME) synthesis was observed by operando FTIR-mass spectroscopy (IR-MS). Methanol adlayer, which contains both methanol and methoxy, was formed by exposing the sample with methanol at low partial pressure (~1 torr) with carrier inert gas at ~ 1 bar. The stability of the adlayer molecules was observed under two conditions: a) exposed to pure argon purging at different temperatures up to 300 °C overnight; b) with constant methanol partial pressure (1 torr) in ambient flow while DME synthesis processed. The kinetics study here requires isotopic deuterated methanol exchange(CD3OH/CH3OH). Operando IR results shows under condition a), the majority of the adlayer, both methanol ad methoxy species, remained on the surface overnight after the highest temperature treatment. Methanol stability is even stronger than methoxy. Temperature dependent IR results show an isosbestic point between the surface water and Methanol species which is direct evidence of water replacement by methanol adsorption. These results further confirms other earlier studies reported. However, under condition b), both replacements transient of surface methanol and methoxy species by deuterated flow are by far much faster. As expected, only CD3/CH3 group exchange was observed upon isotope switch in IR and MS results. Measured isotope displacement time constant for methoxy was ~700 s and that for methanol was ~1700 s. These surface species transients are compared with simultaneous MS data of DME gas products which comes out with the similar scale of the surface species. These preliminary results indicate that the methanol and methoxy are by far more active (&gt;100X) on a crowdie surface with gas phase methanol exposing comparing a surface with slightly lower coverage and no methanol exposure. This is a good example of reactivity strong dependence on the surface coverage.

How stable are methanol species on oxide? A SSITKA study

251st National Meeting (2016)

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