Constrained Chemical Dynamics of CO Dissociation/Hydrogenation on Rh Surfaces. Issue 28 (30th April 2018)

• Record Type: Journal Article
• Authors: Kraus, Peter; Frank, Irmgard
• Is Part Of: Chemistry. Volume 24:Issue 28(2018)
• Page Start: 7188
• Page End: 7199
• Publication Date: 2018-04-30
• Subjects: ab initio calculations -- density functional calculations -- microkinetic modeling -- molecular dynamics -- rhodium -- syngas; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201705867
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Among noble metal catalysts, rhodium (Rh) is unique in its ability to perform a one‐step synthesis of ethanol from syngas. The first steps following the adsorption of syngas on Rh surfaces are assumed to be responsible for the conversion of CO and the selectivity effects between C1, C2, and oxygenated species. In the current work, constrained ab initio molecular dynamics are applied to investigate the kinetics of CO dissociation and hydrogenation over flat and stepped Rh surfaces. The obtained barriers for the Rh(111) surface are in good agreement with the literature data. On the stepped Rh(211) surface, a large site‐dependent variation in barrier height is shown, with the upper terrace exhibiting behavior comparable to the Rh(111) surface, whereas the barriers over the lower terrace site are generally significantly lower. The rate constants are calculated using transition state theory for both surfaces, and are applied successfully in a microkinetic model, confirming the predicted impact on CO conversion and CH4 /C1 ‐oxygenate/C2 H n selectivity. In addition to the high‐accuracy energetics and rate constants reported for CO dissociation/hydrogenation and the presentation of an updated microkinetic mechanism for Rh catalysts, the applicability of constrained molecular dynamics for reaction barrier calculation is confirmed, and sensitive pathways affecting the selectivity between formaldehyde/methanol over Rh catalysts are highlighted. Abstract : CO activation over Rh surfaces : The first steps in the synthesis of ethanol from syngas over Rh surfaces are investigated using constrained ab initio molecular dynamics. The results over two Rh surfaces (see picture) compare well with available density functional theory data. The derived rate constants are applied in a microkinetic model to confirm the impact of CO dissociation and hydrogenation on the selectivity between C1 and C2 products.