[1] CO and NO-induced disintegration of Rh, Pd, and Pt nanoparticles on TiO 2 (110): A first principles study Bryan Goldsmith, 1 Evan Sanderson, 2 Runhai Ouyang, 3 Wei-Xue Li 3 1. Department of Chemical Engineering, University of California, Santa Barbara, CA, 93106-5080 2. Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106-9510 3. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Introduction Free energy of reactant- induced NP disintegration Computational Methodology Adatom complex formation and particle energetics adatom/support support bulk formation E E E E = − − NP disintegration in the presence of reactants is common e.g. Rh/TiO 2 , Rh/Al 2 O 3 , Pd/Fe 2 O 3 , Pt/SiO 2 , Ir/Al 2 O 3 , Ni/TiO 2 , Ru/TiO 2 Most stable adatom positions on TiO 2 (110), corresponding to formation energies of 2.88, 2.01 and 3.12 eV for Rh, Pd, and Pt, respectively. If ΔG disintegration is negative, then the NP should be expected to disintegrate under equilibrium conditions. Disintegration of supported nanoparticles (NPs) in the presence of reactants can lead to catalyst deactivation or be exploited to redisperse sintered catalysts. Nanoparticle disintegration can cause catalyst deactivation Rh/SiO 2 Disintegration Most active size Density Functional Theory modeling using Vienna Ab-initio Simulation Package Projector Augmented Wave method RPBE Functional + Spin polarization Plane wave kinetic energy cutoff = 400 eV Forces converged to 0.03 eV/Å via conjugate gradient Vacuum layer thickness of 15 Å Bottom two tri-layers are fixed in positions. (4x2) Rutile TiO 2 (110) Periodic model 0 0 0 3 ( , , ) (, ) x f x B p G RT p E n Tp k TLn TS R p µ Ω ∆ = − − + − me disintegration γ Formation energy of adatom complex Standard gas phase chemical potential Configurational entropy of complexes Nanoparticle energy 0 G ∆ < disintegration Disintegration can be modeled by the Gibbs Free Energy An exothermic adatom complex formation energy and a small particle radius will promote NP disintegration. Increasing the reactant species chemical potential, by raising pressure or lowering temperature, will also enhance disintegration. We address the following questions: Between supported Rh, Pd and Pt catalysts, which one is more susceptible to the disintegration. Among NO and CO, which one is more efficient for the redispersion of the low surface area catalyst. How sensitive do these results depend on the particle size, temperature and partial pressure. Density functional theory allows the rapid calculation of the effects of reactant partial pressure (p), temperature (T), NP radius (R), as well as particle morphology on the stability of supported NPs toward disintegration into adatom complexes. [1] Rh(CO) 2 and Rh(NO) 2 have more favorable formation energies than Rh(CO) and Rh(NO) Formation energy = 0.75 eV -1.35 eV -0.02 eV -1.58 eV C O Rh N Rh(CO) 2 Rh(NO) 2 Rh(CO) Rh(NO) Rh Pt Pd NO binding on (111) facet NO chemical potential (eV) Acknowledgments References Pt, Pd, Rh NP/TiO 2 (110) disintegration Conclusions The interaction between CO and NO with the Rh adatom is greater than for the Pd and Pt adatoms. The Rh/TiO 2 (110) NPs are less resistant to CO and NO-induced disintegration than the Pd/TiO 2 (110) and Pt/TiO 2 (110) NPs NO is a more efficient reactant for particle redispersion than CO Influence of CO and NO adsorption on Pt, Pd, or Rh nanoparticle surface energetics � Reactant environment and particle size effects on supported nanoparticle energy Rh NPs are more susceptible to CO and NO-induced disintegration compared to both Pd and Pt NPs, and NO is a greater promoter of NP disintegration than CO. The facile disintegration of Rh NPs is due to the large and exothermic formation energy of the Rh adatom complexes. These findings provide insights for how to either prevent or promote reactant-induced NP disintegration. [1] McClure, S. M.; Lundwall, M. J.; Goodman, D. W. Proc. Natl. Acad. Sci. 2011, 108, 931 [2] Ouyang, R.; Liu, J.-X.; Li, W.-X. J. Am. Chem. Soc. 2012, 135, 1760 The interaction of CO and NO with Rh adatom is greater than for Pd and Pt adatoms Formation energy Rh Pd Pt Metal(CO) 0.75 0.26 0.09 Metal(CO) 2 -1.35 -0.54 -0.69 Metal(NO) -0.02 -0.05 -0.10 Metal(NO) 2 -1.58 -0.67 -0.68 Energies are in eV. Exothermic formation energy promotes particle disintegration CO and NO bind strongest to Rh metal compared to Pd and Pt metals 111 111 (, ) [ (, )] (, ) me i i i me i me TP f TP TP γ γ γ γ γ ∆ = +∆ ≈ + ∑ Rh Pt Pd CO binding on (111) facet CO chemical potential (eV) Reduction in surface energy (eV/Å 2 ) Temperature ↓, Pressure ↑, Coverage ↑ Reduction in surface energy (eV/Å 2 ) NP E ∆ LESS STABLE MORE STABLE (111) NP 3 γ ΔE = R Ω NP E ∆ contours (Ω is molar volume) In the presence of CO In the presence of NO eV eV This work was funded by the NSFC (21173210, 21225315), the 973 Program (2013CB834603), and the NSF-OISE 0530268. B.R.G. acknowledges the PIRE-ECCI program for a graduate fellowship. E.D.S. thanks IRES-ECCI for an undergraduate summer research fellowship. These trends, which are manifested by the interplay between coverage dependent adsorption energies, reactant-induced surface stabilization, and NP size, give insight into the variability of particle stability over a broad range of reaction conditions. diameter = 20 Å In agreement with experiment, Rh(CO) 2 predicted to form but not Rh(CO) Rh(CO) 2 Exp. Detected not detected Pd and Pt not predicted to form adatom complexes In the presence of CO CO chemical potential (eV) Gibbs free energy of disintegration (eV) Exp. Suggested In agreement with experimental suggestions, Rh(NO) 2 predicted to form Pd and Pt not predicted to form adatom complexes (Under typical conditions) In the presence of NO Gibbs free energy of disintegration (eV) NO chemical potential (eV) diameter = 20 Å Include gas phase disintegration Consider adatom translation Particle size-dependent binding energies Future work Reduction in surface energy due to reactant binding At a specified gas phase chemical potential, the adatom complex that is most likely to form has the most negative disintegration free energy. Pd(CO) 2 (g)