Combined Theoretical and Experimental Study of CO Adsorption

Combined Theoretical and Experimental Study of CO Adsorption and
Oxidation over Platinum
G.T. Kasun Kalhara Gunasooriya*, Mark Saeys
European Research Institute of Catalysis
http://www.lct.UGent.be
E-mail: Kasun.Gunasooriya@UGent.be
1. Molecular-scale hypotheses and concepts about elementary
steps, active sites, RDS,…
“Playing”
2. Modeling to evaluate hypotheses
“Insight”
3. Kinetic model or improved catalyst (activity, stability)
“Validate”
4. Macro-scale experimental validation (intrinsic kinetics)
Introduction
CO oxidation: Schwab effect
CO adsorption and oxidation over Pt catalysts has been studied extensively,
both experimentally and theoretically.
Many studies have shown that most conventional DFT functionals
overestimate the CO adsorption energy. This feature has been coined “the
CO Puzzle”. We demonstrate that recent vdW-DF functional provide an
accurate description of CO adsorption on Pt(111).
The activity and selectivity of supported metal clusters can in principle be
manipulated by tuning their electronic properties using the support, known as
the “Schwab Effect”. To demonstrate this effect, CO oxidation was
performed over 1-2 nm Pt clusters supported on a series of TiO2 thin films
with an order of magnitude variation in carrier concentration.
• Band Engineering controls the free electron concentration and the Fermi
level (Efermi) in the TiO2 support
• This tunes charge transfer to supported 1 nm Pt clusters (Schwab effect)
CO adsorption on Pt(111)
Experimental Results
Low coverage CO adsorption
• CO oxidation rate measured for Pt clusters on a range of TiO2 films in a
low pressure batch reactor at 350 °C
• Excess CO: rate (kCO) increases by 70% with carrier conc.
• Excess O2: rate (kO2) decreased by 30% with carrier conc.
1/9-top
−144 /−95
1/9-bridge
−141/−91
1/9-hollow
−136 /−85
1/3-top
−138/−90
Average CO adsorption energy / Gibbs free adsorption energy at 300 K, 1 bar (kJ/mol)
High coverage CO adsorption
kmolecular ,O 2 PO 2
KCO PCO
Free electron concentration
TOF ≈ k
Free electron concentration
DFT calculations: charge injection
Petrova et al. 2T+4B
−120/−69
Stability diagram for CO adsorption on Pt(111)
CO coverage gradually increases
up to 1/3 ML
Then, several phases are found
with increasing coverage: c(4x2)4CO, (√3×5)rect -6CO, (√3×3)rect 4CO
Correct site preference and
accurate adsorption energy
Correct high coverage structures
with bridge/top balance
Natural Bond Orbitals (NBO) analysis
- Charge injection increases 2π* occupancy
CO stretch
- Charge injection increases of C-Pt Pauli repulsion
CO adsorption energy decreases by charge injection
Surface charge per Pt atom
First demonstration of a controllable Schwab effect
anti-bonding Pt-C occupancy
Avery et al. 4T+2B
−129/−78
CO adsorption energy
Persson et al. 4T+2B
−131/−80
Chua, Gunasooriya, Saeys, Seebauer, J. Catal., 311 (2014) 306; Schwab, Koller, JACS, 90 (1968) 3078
Conclusions
- revPBE-vdW functional accurately describes CO adsorption on Pt(111) for a wide range of coverages and structures. CO Puzzle Solved!
- Controllable Schwab effect for CO oxidation over Pt/TiO2. Charge injection decreases CO adsorption energy.
Acknowledgments: Prof. Edmund G. Seebauer, University of Illinois – Urbana Champaign , Dr. Y. P. Gavin Chua, Institute of Chemical Engineering
Sciences (A*STAR-ICES), Shell Global Solutions, Odysseus Type I grant from the Research Foundation - Flanders (FWO Vlaanderen), NUS
08/06/2015 – 19/06/2015, EMAT Workshop on Transmission Electron Microscopy, Antwerp