Recent activities in TP C6:

Transcription

1 Recent activities in TP C6: Adsorption, diffusion, and reaction at MoO 3 and V 2 O 5 substrate K. Hermann, M. Gruber, and X. Shi Theory Department, Fritz-Haber-Institut, Berlin Sfb 546 Workshop, Schmöckwitz, Feb. 16, 2010

2 TP C6 Introduction Personnel - Matteo Cavalleri (Postdoc), NEXAFS Wiley (phys. stat. solidi), since March Xuerong Shi (PhD student), adsorption, reactions Key State Lab. of Coal Conversion, Taiyuan (PR China), since Jan Mathis Gruber (IMPRS PhD student), adsorption, reaction, NEXAFS (learning), until Murat Mesta (PhD student), NEXAFS (starting), relativistic effects, until NN (Sfb 546 PostDoc), NEXAFS, searching since Jan Any suggestions? Please tell me quickly, time is running!

3 TP C6 Introduction Subjects considered - Adsorption, diffusion, substitution, vacancies, spectroscopy - Extended substrates : V 2 O 5, MoO 3, MoS 2, Mo 2 C - Adsorbates : H, NH x, CO, NO, H 2 O, O, S - Small particles : V x O y, Mo x O y, (NEXAFS) Publications Refereed journals : 5 apprd., 1 in print, 1 submt., 1 in prep. - Proceedings : 1 apprd.

4 Specific subjects Discrimination between V x O y particles at SBA-15 SiO 2 using O 1s NEXAFS (collaboration with TP B2) - most recent: confirmation of theoretical spectra [1], improved resolution in experiment [2] [1] M. Cavalleri et al., J. Catal. 262, 215 (2009). [2] M. Häveckeret al., Phys. Stat. Solidi (b) 246, 1459 (2009).

5 O 1s NEXAFS for V x O y / SiO 2, Introduction Structure / type of reactive VO x species at catalyst support, monomeric vs. non-monomeric VO x Model system (B2) : - small VO x particles on SBA-15 SiO 2 support, - determine structural details of VO x Analysis of different oxygen species inside VO x and at support interface Discrimination of V=O, V-O-V, V-O-Si, Si-O-Si bonds in NEXAFS spectra

6 O 1s NEXAFS for V x O y / SiO 2, Theoretical Details Model clusters - from vibrational studies (A4) [1,2] - hydrogen termination at periphery - clusters VSi 7 O 13 H 7, V 2 Si 6 O 14 H 6, Electronic structure - DFT, GGA (RPBE) functional, StoBe - Slater s TP method for O 1s excitations - atom resolved NEXAFS spectra [1] N. Magg et al., J. Catal. 226, 88 (2004) [2] J. Döbler, M. Pritzsche, and J. Sauer, in preparation.

7 O 1s NEXAFS for V x O y / SiO 2, Exp. NEXAFS Spectra [1] NEXAFS measurements (B2 [1]) - (a) clean SBA-15 - (b) SBA-15 + V x O y (2.7% V weight loading) Excitation regions OLD - V 2p 3d - O 1s O 2p V 3d broad asymmetric oxygen peak at about ev [1] M. Häveckeret al., Phys. Stat. Solidi (b) 246, 1459 (2009).

8 O 1s NEXAFS for V x O y / SiO 2, Theory vs. Experiment [1, 2] Comparison with experiment [1] - all V x Si y O z H w clusters considered, total NEXAFS spectra [2] - higher non-monomers, umbrella type V x O y contribute to peak asymmetry OLD non-monomeric V x O y must exist at SBA-15 surface [1] M. Häveckeret al., Phys. Stat. Solidi (b) 246, 1459 (2009). [2] M. Cavalleri et al., J. Catal. 262, 215 (2009).

9 O 1s NEXAFS for V x O y / SiO 2, Theory vs. Experiment [1, 2] Comparison with experiment [1] - all V x Si y O z H w clusters considered, total NEXAFS spectra [2] - higher non-monomers, umbrella type V x O y contribute to peak asymmetry NEW non-monomeric V x O y must exist at SBA-15 surface [1] M. Häveckeret al., Phys. Stat. Solidi (b) 246, 1459 (2009). [2] M. Cavalleri et al., J. Catal. 262, 215 (2009).

10 Specific subjects NH x (de)hydrogenation at V 2 O 5 (010) - NH 4 formation at OH groups (Bronstedt sites), NH 4 diffusion - NH x binding at oxygen vacancies (Lewis sites) - H binding, diffusion, - surface OH + OH reaction to form H 2 O + O, H 2 O desorption Selective Catalytic reduction (SCR) of NO x with NH 3 - reaction schemes with OH groups (Bronstedt sites) - reaction schemes near oxygen vacancies (Lewis sites)

11 NH x at V 2 O 5 (010), Introduction Use of vanadia based catalysts in ammoxidation reactions example: selective catalytic reduction (SCR) of NO x with NH 3 4 NH NO + O 2 4 N H 2 O 4 NH NO 2 + O 2 3 N H 2 O NH x reactions (dehydrogenation, hydrogenation ) NH x NH x-1 + H ads, x = 1-3 NH x + H ads NH x+1 Adsorption / desorption of NH x, N, H Diffusion of NH x, N, H at surface

12 NH x at V 2 O 5 (010), Theoretical Details Substrate clusters for V 2 O 5 (010) - 1- / 2-layer sections, start from exp. geometry - hydrogen termination at periphery (embedding) - clusters V 12 O 40 H 20, V 14 O 42 H 14, V 14 O 46 H 22, - adsorbates added (H, NH x, NO) - O removed from O(1-3) sites Electronic structure - DFT, GGA (RPBE) functional, StoBe - equilibrium geometries - transition states, barriers (NEB)

13 NH x at V 2 O 5 (010), Adsorption sites, Energetics Adsorption sites Adsorption energies E ads from total energy differences [ev] O(1) O(2) O(3) V H N NH NH NH V NH NH 3 bound quite weakly NH 4, H bound quite strongly

14 NH x at V 2 O 5 (010), Adsorption sites, Energetics Adsorption (vacancy) sites NH 3, NH 4 Adsorption energies E ads [ev] from total energy differences vacancy clusters O(1) vac O(2) vac O(3) vac s-o(1) vac H N NH NH NH s-o(1) vac NH Binding stronger near O vacancy NH 3 bound near sub-surface s-o(1) vac

15 V 2 O 5 (010), sub-surface oxygen vacancy Vacancy exchange reaction path O(2) vac s-o(1) vac Diffusion process oxygen : s-o(1) O(2) O vacancies : O(2) vac s-o(1) vac s-o(1) vac more stable than O(2) vac small diffusion barrier, 0.1 ev importance for reactions?

16 NH x at V 2 O 5 (010), Hydrogen diffusion Reactant diffusion at surface important for reaction, example hydrogen Diffusion paths, barriers from nudged elastic band (NEB) calculations H hops between O sites, OH diffusion O(2) V O(1) O(3 ) O(3)

17 NH x at V 2 O 5 (010), Hydrogen diffusion Reactant diffusion at surface important for reaction, example hydrogen Diffusion paths, barriers from nudged elastic band (NEB) calculations H hops between O sites, OH diffusion Low barriers : O(1) / O(1) : 0.2 / 0.4 ev O(2) / O(2) : 0.8 ev ( Barriers in ev )

18 NH x at V 2 O 5 (010), Hydrogen diffusion Reactant diffusion at surface important for reaction, example hydrogen Diffusion paths, barriers from nudged elastic band (NEB) calculations H hops between O sites, OH diffusion H diffusion O(1) / O(1) ( Barriers in ev ) animation Mobile oxygen facilitates O-H bond breaking / making

19 NH x at V 2 O 5 (010), Hydrogen reaction Hydrogen diffusion at surface may lead to adjacent surface OH Reaction of surface OH : V=OH + V=OH V-H 2 O + V=O Surface H 2 O formed (barrier = 0.3 ev) Surface H 2 O weakly bound (0.4 ev) easy desorption oxygen vacancy filled by gas phase O 2

20 NH x at V 2 O 5 (010), Hydrogen reaction Hydrogen diffusion at surface may lead to adjacent surface OH Reaction of surface OH : V=OH + V=OH V-H 2 O + V=O Surface H 2 O formed (barrier = 0.3 ev) Surface H 2 O weakly bound (0.4 ev) oxygen vacancy filled by gas phase O 2 easy desorption animation Δ = 0.3 ev Mobile oxygen facilitates reaction

21 SCR of NO x with NH 3 at V 2 O 5 (010) Use of vanadia based catalysts in ammoxidation reactions Selective Catalytic Reduction (SCR) of NO x with NH 3 4 NH NO + O 2 4 N H 2 O 4 NH NO 2 + O 2 3 N H 2 O Reaction schemes near OH groups (Bronstedt sites) [1] Reaction schemes near oxygen vacancies (Lewis sites) [1] Initial testing steps, conclusive results Schmöckwitz 2011 [1] S. Soyer, A. Uzun, S. Senkan, and I. Onal, Catal. Today 118 (2006) 268.

22 SCR of NO x with NH 3 at V 2 O 5 (010) 4 NH NO 2 + O 2 3 N H 2 O Scheme 1: NH 3 adsorbs near OH group, NH 4+ formation [1] : V=OH + NH 3 + NO V=O-NH 4+ + NO 2: V=O-NH 4+ + NO V=O-NH 4+ -NO 3: V=O-NH 4+ -NO V=OH-NH 2 NOH 4: V=OH-NH 2 NOH 2 * V=OH-NH 2 NO 5: 2 * V=OH-NH 2 NO V=OH + V=OH + N 2 + H 2 O OH-H surface OH (Brønsted sites) OH-NH 3 [1] S. Soyer, A. Uzun, S. Senkan, and I. Onal, Catal. Today 118 (2006) 268.

23 SCR of NO x with NH 3 at V 2 O 5 (010) 4 NH NO 2 + O 2 3 N H 2 O Scheme 2: NH 3 adsorbs near O(1) vacancy [1] (= reduced metal center, Lewis site) 1a: V + NH 3 +NO V-NH 3 + NO 2a: V-NH 3 + NO V-NH 3 -NO 3a: V-NH 3 -NO V=O-NH 2 NH 2a 1a + 1b 2b 1b: V + NH 3 + NO V-NO + NH 3 2b: V-NO + NH 3 V-NO-NH 3 3b: V-NO-NH 3 V=O-NH 2 NH 4: V=O-NH 2 NH V + V=OH + N 2 + H 2 O 3a 4 3b + O(1) vac -NH 3 O(1) vac -NO O vacancies (Lewis sites) [1] S. Soyer, A. Uzun, S. Senkan, and I. Onal, Catal. Today 118 (2006) 268.

24 Other subjects MoO 3 (010) [1] ( MoS 2 oxidation ) - sulfidation vs. sulfur adsorption - importance of hydrogen Molecular vs. dissociative CO and NO adsorption at Mo 2 C(0001) [2] ) - preferred adsorption, sites and energetics Private discussions [1] X. Shi, J. Wang und K. Hermann, J. Phys. Chem. C, (2010), in print. [2] X. Shi, J. Wang und K. Hermann, J. Phys. Chem. C, (2010), submitted.

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