Catalytic oxidations: finding the optimum composition of AuPd coreshell nanoparticle catalysts

Transcription

1 Brazilian ChemComm Symposium Chemistry and Sustainable Energy 5 th November 2012, São Paulo, Brazil Catalytic oxidations: finding the optimum composition of AuPd coreshell nanoparticle catalysts Prof. Dr. Liane M. Rossi Laboratory of Nanomaterials and Catalysis Instituto de Química Universidade de São Paulo Av. Prof. Lineu Prestes 748 São Paulo , SP Brasil lrossi@iq.usp.br

2 Catalytic oxidations are widely used in the manufacture of bulk petrochemicals, but are not a commonplace in the fine chemicals and pharmaceutical industry, and at the organic laboratory level: Stoichiometric Oxidations are very far from being ideal from the green point of view! hazardous or toxic chemicals volatile organic solvents large amounts of toxic wastes Oxidizing reagent Residue KMnO 4 Mn 2+ /MnO 2 K 2 CrO 4 Cr 3+ CH 3 COOOH t-buooh CH 3 COOH t-buoh ClO - Cl - H 2 O 2 O 2 H 2 O H 2 O

3 Green oxidizing agents, O 2 and H 2 O 2, do not readily react in a selective way with organic substrates, unless a catalyst is present. Catalytic Oxidations Control the reactivity of oxygen species to obtain valuable organic oxygenates and avoid overoxidation. R-CH 2 -OH R-CHO R-COOH CO 2 + H 2 O Discriminate functional groups in the same molecule. Oxidations in fine chemicals is generally more difficult, however, owing to the multifunctional nature of the molecules of interest.

4 Development of metal nanoparticle catalyst *Gold was discovered as an active catalyst in the late 80s after the seminal contribution of Haruta and Hutching.

5 Metal nanoparticle catalyst Soluble NPs high control on particle size, size distribution and surface chemistry Supported NPs Future Targets Control on particle size, size distribution and uniform dispersion of NPs on solid supports - dial up the active sites Prevent metal leaching Improve metal recovery Understand the role of stabilizers on metal NP catalysis

6 Supported metal NPs Immobilization of pre-formed metal NPs Metal salt impregnation and reduction method Catalyst Support M x+ M x+ M x+ M x+ M x+ M x+ M x+ Poor control on metal dispersion Particles size and sized distribution

7 Supported metal NPs Ligand-assisted method Nanoscale, 2012, 4, Catalyst Support M x+ M x+ M x+ M x+ M x+ M M x+ x+ R= NH 2 R= NH NH 2 None Si(OR) 3 = NH 2, en, COOH, SH, PR 2,... Tune NPs size with uniform dispersion of NPs on supports Inorganic Chemistry, 2009, 48, 4640.

8 Supported metal NPs Ligand-assisted method Nanoscale, 2012, 4, Catalyst Support M x+ M x+ M x+ M x+ M x+ M M x+ x+ Si(OR) 3 = NH 2, en, COOH, SH, PR 2,... Improve metal recovery using magnetic support Applied Catalysis. A, General, 2008, 338, 52.

9 Supported metal NPs Ligand-assisted method Nanoscale, 2012, 4, Catalyst Support M x+ M x+ M x+ M x+ M x+ M M x+ x+ Si(OR) 3 = NH 2, en, COOH, SH, PR 2,... Low metal leaching ChemCatChem, 2012, 4, 698.

10 Supported metal NPs Ligand-assisted method: metal support interaction Chemistry A European Journal, 2011, 17, 4626.

11 t Supported metal NPs Ligand-assisted method: metal support interaction X-ray absorption fine structure spectroscopy studies Au 3+ Au 3+ Au 3+ Au (a) (b) (c) Au L 3 -edge SiO 2 -Au (d) Au σ+ H 2 N H 2 N Au σ+ NH 2 NH 2 NH 2 O O Si O Si O O O O Si O O O O Si O Au σ+ NH 2 NH 2 Au σ+ SiO 2 -NH 2 -Au 3+ NH (e) Energy / ev (a) Au(CH 3 COO) 3 (b) SiO 2 -Au 3+ (c) HAuCl 4 (d) SiO 2 -NH 2 -Au (e) Au foil Chemistry A European Journal, 2011, 17, 4626.

12 Selected examples of magnetically recoverable catalysts Rh NPs PtNPs Ir NPs Ru NPs Applied Catalysis. A, General, 2008, 338, 52. Catalysis Communications, 2009, 10, ChemCatChem, 2012, 4, 698. Applied Catalysis. A, General, 2009, 360, 177. Applied Catalysis. B, Environmental, 2009, 90, 688.

13 Selected examples of magnetically recoverable catalysts NiNPs PdNPs AuNPs ACS Catalysis, 2012, 2, 925. Inorganic Chemistry., 2009, 48, Appl. Catal. B, Environ., 2010, 100, 42. Journal of Catalysis, 2010, 276, 382. Chemistry A European Journal, 2011, 17, Green Chemistry, 2010, 12, 144. Green Chemistry, 2009, 11, 1366.

14 Conversion (%) Supported Au NP catalysts AuNPs Oxidation of benzyl alcohol K 2 CO 3 = high selectivity and conversion rates, but low catalyst stability In the search for a more stable catalysts, we first chose to adhere to the literature by adding Pd to our supported gold catalyst 100 Selectivity =96% Chemistry A European Journal, 2011, 17, Green Chemistry, 2010, 12, 144. Green Chemistry, 2009, 11, K 2 CO 3 KOH Et 3 N absence of base

15 Considerations Open question Supported Au NP catalysts AuNPs... AuPdNPs Bimetallic NPs = metallic domain distributions: alloys (AB) or core-shell (A@B or B@A) NPs How much Pd should be added to activate Au NPs? Chemistry A European Journal, 2011, 17, Green Chemistry, 2010, 12, 144. Green Chemistry, 2009, 11, AuPd alloy NPs have received special attention in catalytic applications. However, the surface of an AuPd alloy NP differs from its corresponding bulk concentration Core-shell NPs can be obtained by the reduction of palladium over pre-formed gold NPs, and vice versa

16 Supported AuPd NP catalysts Oxidation of benzyl alcohol Chemistry A European Journal, 2011, 17, 4626.

17 Supported AuPd NP catalysts Catalytic performance of the catalysts in the oxidation reaction with benzyl alcohol. The amount of Au is fixed (3.4 mol), while the amount of Pd varies from 0 to 40 mol % (i.e., 0 to 1.4 mol). Reaction conditions: 1 ml (10 mmol) benzyl alcohol, 75 mg catalyst (3.4 µmolau), 0 to 1.4 µmol Pd(OAc) 2, 6 bar O 2, 2.5 h, 100 C.

18 Supported AuPd NP catalysts morphologically structured Au-rich core and a Pd-rich shell TEM and HAADF-STEM image of a Au:Pd = 10:1 supported catalyst particle and the respective Au and Pd maps. The particle compositional distribution is observed in the line scans, measured from the regions delimited by the lines indicated in both maps. The hemispherical shape observed in the Au line scan contrasts with the flat distribution measured for Pd, which shows its concentration at the particle shell.

19 Supported AuPd NP catalysts morphologically structured Au-rich core and a Pd-rich shell (a) BF-STEM image of a supported catalyst particle Au:Pd = 5:2 (b) HAADF-STEM image of the supported catalyst particle and the respective Au and Pd maps. The particle compositional distribution is observed in the line scans, measured from the regions delimited by the lines indicated in both maps. The hemispherical shape observed in the Au line scan contrasts with the flat distribution measured for Pd, which shows its concentration at the particle shell.

20 Supported AuPd NP catalysts morphologically structured Au-rich core and a Pd-rich shell Catalytic performance of the Au@Pd catalysts in the oxidation reaction with benzyl alcohol. The amount of Au is fixed (3.4 mol), while the amount of Pd varies from 0 to 40 mol % (i.e., 0 to 1.4 mol). Reaction conditions: 1 ml (10 mmol) benzyl alcohol, 75 mg catalyst (3.4 µmolau), 0 to 1.4 µmol Pd(OAc) 2, 6 bar O 2, 2.5 h, 100 C.

21 Supported AuPd NP catalysts Full-shell cluster model: The most active catalyst, with 89.9% Au and 9.1% Pd, is very close to the nominal composition for the complete coverage of Au cores ( nm) with one atomic layer of Pd.

22 Activity Final Remarks Hypothesis based on morphological and catalytic studies: the deposition of one atomic layer of Pd on Au resulted in a Au core-pd-rich shell catalyst of maximum activity Pd added

23 Final Remarks The Au:Pd molar ratio needed to form a monolayer of Pd might change as a function of Au core size. Consequently, one can expect the maximum activity to occur at differing AuPd compositions when using Au core size or size distribution other than the one we used in our study. ~3 nm AuNP ~45% surface atoms ~40 mol% Pd for monolayer ~10 nm AuNP ~16% surface atoms ~15 mol% Pd for monolayer ~20 nm AuNP ~8% surface atoms ~8 mol% Pd for monolayer Experiments in progress!

24 Final Remarks Au core Pd-rich shell: the distribution of metal domains and the Au:Pd ratio are both important for the synergistic effect observed. high selectivity of Au high activity of Pd High activity and selectivity meeting the increasing demand for environmentally friendly chemical processes

25 ACKNOWLEDGMENTS Group Tiago Artur da Silva - PhD Fernanda Parra da Silva -PhD Lucas L. R. Vono - PhD Marco Aurélio S. Garcia - PhD Natália J. S. Costa Post Doc Jean-Claudio Costa Post Doc Leonardo Gomes Santos Undergrad Bruna Julio - Undergrad Rafael L. Oliveira Inna M. Nangoi Marcos J. Jacinto Fernando B. Effenberger Collaboration Pedro K. Kiyohara (IF/USP) Renato F. Jardim (IF/USP) Richard Landers (Unicamp) Daniela Zanchet (Unicamp) Érico Teixeira-Neto (IQ-USP) Elena Goussevskaia (UFMG) Paulo A. Z. Suarez (UnB) Joel C. Rubim (UnB) Karine Philippot (LCC/CNRS, Toulouse, France)