Within the context of lignin valorisation, selective heterogeneous catalysis plays a major role. However it demands more and more insights on how lignin-based aromatic oxygenates interact and react on typical metallic surfaces. To tackle this complex problem we performed a combined approach blending periodic DFT study (optPBE functional) and UHV experiments (XPS and TPD) on a simple model, namely anisole on Pt(111). Although structurally quite simple, the decomposition of this ether can give different products (such as phenol, benzene, hydrogen and carbon monoxide) depending on which C—O bond is cleaved. Guided by a continual exchange between the theoretical and the experimental teams, we finally build up a good picture of what the TPD experiment of anisole on Pt(111) can be.  At 110 K large amounts of anisole adsorb on Pt(111) forming multilayers. When the sample is heated the second layer and part of the first layer desorb until its coverage reaches nearly 0.7 ML (determined by XPS). At this point, anisole desorption energy (DFT: 166 kJ/mol) is much higher than the first dehydrogenation barrier (DFT: 108 kJ/mol) suggesting that hydrogen should be produced when anisole decomposition starts. Because of the low pressure, hydrogen desorbs and a cascade of reaction brings the system to two quite stable fragments namely CH and PhO. Since there is most likely almost no hydrogen on the surface in UHV conditions, phenol cannot be produced. Moreover PhO cannot deoxygenate on the surface without involving other intermediates (DFT: 205 kJ/mol uphill). Nonetheless CH can abstract the oxygen atom from PhO to afford phenyl and CHO that can hydrogen phenyl to benzene, the main desorbing decomposition product. That fundamental science surface study turns out to be interesting for catalysis as well since it appears that the ratio CH/H seems to be controlling the selectivity of benzene over phenol

Anisole decomposition in UHV conditions: When DFT and experiments play ping-pong on a Pt(111) surface

251st National Meeting (2016)

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