Interaction of carbon monoxide with Cu nanoclusters grown on alumina surface

Transcription

1 Interaction of carbon monoxide with Cu nanoclusters grown on alumina surface J.A. Olmos-Asar, E. Vesselli, A. Baldereschi, Maria Peressi Department of Physics, University of Trieste and CNR-IOM 100 Congresso Nazionale SIF - Pisa, September 22-26, 2014

2 Outline Motivations and state-of-the-art Self-seeded nucleation of Cu nanoclusters on Al 2 O 3 /Ni 3 Al(111) CO chemistry on supported Cu nanoclusters Conclusions

3 Why CO? Why supported nanoclusters? Carbon monoxide: important reactant in many industrial catalytic processes. Dissociation is a fundamental step (e.g. in Boudouard reaction: 2CO CO2+C ). => basis research on CO adsorption and dissociation on different catalysts using surface science techniques + quantum-mechanical calculations Metal nanoclusters or nanoparticles (NPs): important in heterogeneous catalysis; for applicative purposes, grown on a support (useful in controlling NPs size, preventing sintering at high T, selectivity,...) => the synthesis of ordered arrays of well-defined equally sized NPs is the goal of many efforts

4 A good template: Ultrathin films of alumina onto Ni3Al(111) substrate Oxide ultrathin films: recently proposed as promising substrates for growing highly ordered nanostructure arrays through self-organization of adatoms. Alumina (Al2O3) on Ni3Al(111) shows a superstructure with two different appearances according to the voltage bias: LT (T=23 K) - STM images unit cell of the alumina superstructure network structure U bias = 3.2 V dot structure U bias = 2.0 V 278 Å 278 Å Degen et al., Surf. Sci. 2005; Moors et al., Appl. Surf. Sci. 2008

5 Outline Motivations and state-of-the-art Self-seeded nucleation of Cu nanoclusters on Al 2 O 3 /Ni 3 Al(111) CO chemistry on supported Cu nanoclusters Conclusions

6 ofabdoi: micas, erio 2, non a (Tri-, Italy priate, like for example epitaxial graphene, showing periodic variations in the potential energy surface for ad-atoms. Oxide ultrathin films have been recently proposed as alternative, promising substrates. In particular, it has been found that the oxidation of clean Nix Al single crystal ter3 Therefore, DFT suggests that the dot site is the p minations under ultra high vacuum (UHV) conditions gives adsorption site also for Cu. rise to highly ordered, ultrathinofnonni3al(111) In the experiments chemical vapour depositi stoichiometric Al2O3 extremely homogeneous the Cuonto atoms impinging uniforml stoichiometric alumina films onto Thealumina one 22,23 formed superstructure with unit cell domly on the surface havecell a much larger probabilit Ni3p Al(111) is arranged into a superstructure whose unit p acting with the outermost alumina layer rather tha is ( 67 67)R12 with respect to the supporting alloy surately and directly with the hole: infact, the unit cell A good template: Ultrathin films of alumina onto Ni Al(111) substrate with respect to the substrate 3-fold and 6-fold symmetry sites ultrathin layer of Al2O3 Ni3Al(111) substrate { { Schmid et al., PRL 2007; Vesselli et al., PRL 2010 of the Ni3 Al(111) surface is 2.45 A while the one of structure is 41.5 A, yielding a coverage of holes of monolayers metallic alloy surface. I Therefore, DFTreferred suggests thatto thethe dot site is the preferential adsorption site also Cu. tial therefore toforinvestigate the Cu interaction with In the experiments of chemical vapour deposition (CVD) tions of the22,23 surface, in impinging terms of adsorption onto alumina the Cu atoms uniformly and ran- and dif domly 1 on we the surface have a much larger probability Table summarise the results of ofcuinteradsorptio acting with the outermost alumina layer rather than immedisites region ofhole: theinfact, hole,theindicated in Figure 2 atelyin andthe directly with the unit cell parameter of the Ni3 Al(111) surface is 2.45 A while the one of the oxide dot surfac representative also of other sites of the structure is 41.5 A, yielding a coverage of holes of terized by referred a similar In most oxi monolayers to the local metalliccoordination. alloy surface. It is essential therefore to investigate the Cu interaction with other porsites the strength of adsorption energy is about 0.5 tions of the surface, in terms of adsorption and diffusion. In any case 1 ev. Comparing Table 1 wesmaller summarisethan the results of Cu adsorption in the someresults o in the region of thethat, hole, indicated in Figure 2b: theythe are netwo wesites can conclude after the dot, representative also of other sites of the oxide surface, characsecond adsorption maybe possi terized bystable a similarsite localfor coordination. In most(and oxide surface sites the strength of adsorption energy is about 0.5 ev, and in ation) of Cu on alumina/ni3 Al(111), followed by t any case smaller than 1 ev. Comparing the results of Table 1,

7 Technical details DFT calculations: GGA-PBE pseudopotentials reduced models (~150 atoms + vacuum in all three spatial directions) Experiments: Ni3Al(111) treated under UHV by sputtering & annealing recipes alumina obtained by thermal oxidation at 1000 K in 10-7 mbar O2 Cu deposition by CVD XPS spectra SuperESCA beamline of ELETTRA Synchrotron

8 Single Cu atom adsorption & diffusion no interaction among the Table 1 Adsorption energies and equilibrium height topmost O atoms) of Cu in the dot, network, and oxide surface shown in Fig. 2a and b Site Single atom energies in the most favored adsorption sites. In other sites, Eads < 1 ev. oxide surface Therefore, DFT suggests that the dot site is the preferential adsorption site also for Cu. In the experiments of chemical vapour deposition (CVD) onto alumina 22,23 the Cu atoms impinging uniformly and randomly on the surface have a much larger probability of interacting with the outermost alumina layer rather than immediately and directly with the hole: infact, the unit cell parameter of the Ni3 Al(111) surface is 2.45 A while the one of the dot structure is 41.5 A, yielding a coverage of holes of monolayers referred to the metallic alloy surface. It is essential therefore to investigate the Cu interaction with other portions of the surface, in terms of adsorption and diffusion. In Table 1 we summarise the results of Cu adsorption in some sites in the region of the hole, indicated in Figure 2b: they are representative also of other sites of the oxide surface, characterized by a similar local coordination. In most oxide surface sites the strength of adsorption energy is about 0.5 ev, and in any case smaller than 1 ev. Comparing the results of Table 1, we can conclude that, after the dot, the network is the second stable site for adsorption (and maybe possible nucleation) of Cu on alumina/ni3 Al(111), followed by the adsorption on-top of some surface O ions (Eads on T1 = 0.97 ev). In general, it has been reported that for several transition metals the adsorption energy in the hole is typically 2 ev larger than on the oxide surface 20. Cu follows therefore this general trend, although the difference between the adsorption energy inside and outside the hole is smaller (0.79 ev considering NB and 1.15 ev considering T1 ). It is also interesting to notice that an adsorption energy of 0.99 ev is reported for Cu ontop of an O atom of MgO films 35. E ads (ev) h (Å) Site D!2.12!3.42 H2! H3 NT the! B1 NB B(eV)2 h (A ) is NHor hole )!0.98 e adsorption energy the of dot a (D, Site most E 1.91 (ev) favored. h (A ) Site E Concerning energetics, sites are clearly the Kinetics H 0.54! L T1 onofthe two Cu adsorption nucleation sites, not opposing to the nucleation process Cuprimary clusters at dot theandholesnd (energy barriers for H network. The adsorption of Cu in the dot site (D) is inn B deedh quite strong, with energy of overcome 2.12 ev when the Cuat atomcvdn conditions) diffusion are < 0.5 ev, small enough to be easily! L B is in the most stable position at the bottom of the hole, slightly bs Fig. 1 Up: top view of the structural model of the Al2 O3 /Ni3 Al(111), with the sketch of the periodically repeated unit cell. Directions of the primitive vectors of the underlying alloy are also indicated (a 2 nm x 2 nm portion is shown in inset). Light blue spots highlight the dot structure and small triangles mark the network. The white circle and square identify the reduced models considered for calculations. Bottom: side view of one unit cell. Atomic coordinates are those obtained by Schmidt et al. 20. Red: O, green: Al, blue: Ni ads ads 2 T 3 B 1 H displaced offcenter. For comparison, the adsorption energy for a Rh atom in the same site, reported as its preferential nucleation site, is 3.5 ev 31. The network site, reported as the most stable adsorption position for other metals, is a triangular structure formed by three O ions (Figures 1 and 2a), therefore three different adsorption configurations have to be considered: on-top of one O atom (NT ), in between two O atoms (NB ) and in the middle T1 H L1 L !Most ECu of the Cu (1) atoms can eventually reach the dot sites and!!!!!! Table 1 Adsorption energies and equilibrium heights (from the closest topmost O atoms) of Cu in the dot, network, and other sites of the oxide surface shown in Figure 2a,b nucleate there, even taking into account their low concentration hole system, ESubs is the Ea In the experiments of chemical vapour d Single atom diffusion: A question arising for a

9 Cu self-seeding in the hole Strong adhesion of Cu to supporting metallic alloy => guarantees the stability of the nucleation centers, offering stiff anchoring also for multi-atomic seeds and further growth of 3D Cu NPs. multi-atomic seeds

10 Seed-anchored NPs growth Self-seeded nucleation of ordered patterns of Cu NPs proceed... Calculations here up to Cu15; exp. from CVD: minimum estimated size: Cu30

11 Outline Motivations and state-of-the-art Self-seeded nucleation of Cu nanoclusters on Al 2 O 3 /Ni 3 Al(111) CO chemistry on supported Cu nanoclusters Conclusions

12 XPS on clusters C 1s core level spectra measured in situ : (i) upon exposure to CO at LN2, (ii) at saturation, (iii) upon heating upon heating, CO desorbs, and the intensity related to the atomic C species increases up to a plateau, which is reached when the CO coverage drops (cluster size: 30 atoms) (cluster size: 100 atoms) (cluster size: 200 atoms) (iii) CO adsorption & dissociation are influenced by CO gas pressure (i) and temperature (iii) (ii) (i) during uptake, CO dissociates and C adsorbs on clusters Binding Energy (ev)

13 XPS on clusters: cluster size C 1s core level spectra measured in situ : (i) upon exposure to CO at LN2, (ii) at saturation, (iii) upon heating, for different cluster sizes (cluster size: 30 atoms) (cluster size: 100 atoms) (cluster size: 200 atoms) (iii) (ii) (i) Look at C/CO and / CO: cluster size does matter!

14 XPS on clusters: CO background mbar Look at C/CO ratio: CO background does matter!

15 XPS on clusters: cluster size and T C/CO signal after exposure of the Cu clusters to CO at LN 2 temperature (filled markers) and at RT upon annealing (empty circles) in CO background: temperature & cluster size & CO background do matter in CO dissociation

16 Hints from atomistic simulations: CO dissociation & Boudouard reaction (2CO CO2+C) Eley- Rideal Langmuir- Hinshelwood CO dissociation: highly endothermic Boudouard reaction: also endothermic

17 Hints from atomistic simulations: CO dissociation & Boudouard reaction Eley- Rideal Langmuir- Hinshelwood C incorporated CO dissociation: highly endothermic Boudouard reaction: also endothermic

18 Effect of the catalyst finite size Finite size (clusters instead of single crystal surfaces) helps CO dissociation and coadsorption

19 Effect of the reactant coverage CO coverage can reach the supra-monolayer limit in clusters! High reactant coverage improves dramatically CO dissociation and makes Boudouard reaction exothermic!

20 ... and everything together

21 ... and everything together DFT projected density of states on the C atom and surrounding Cu atoms (average) after CO dissociation

22 Conclusions - holes of the dot structure of ultra-thin Al2O3 films on Ni 3 Al(111): preferential seeding sites for Cu, offering strong adhesion to the metal and stiff anchoring for further growth of 3D Cu clusters - thermodynamics & kinetics contribute to make thin alumina films@ni3al(111) a good template for the growth of highly ordered Cu nanoclusters arrays - finite size of Cu nanoclusters AND high reactant coverage favour the CO dissociation - on Cu nanoclusters, supramonolayer reactant coverage can be reached! - support makes nanoclusters more malleable - J.A. Olmos-Asar, MP et al., in press on PCCP (2014) - J.A. Olmos-Asar, MP et al., submitted

23 Thanks to: Jimena A. Olmos-Asar, Enrico Monachino, Carlo Dri, Angelo Peronio, Cristina Africh, Paolo Lacovig, Giovanni Comelli, Alfonso Baldereschi, and Erik Vesselli and for financial support to: General Directorate for the Country Promotion Executive Program with Argentina Congresso Nazionale SIF - Pisa, September 22-26, 2014