M. Rutigliano a, D. Santoro a, and M. Balat-Pichelin b

Transcription

1 M. Rutigliano a, D. Santoro a, and M. Balat-Pichelin b a CNR IMIP, Istituto di Metodologie Inorganiche e dei Plasmi, Via G. Amendola 122/D, Bari, Italy b Laboratoire Procédés, Matériaux et Energie Solaire, PROMES-CNRS, 7 rue du four solaire, Font-Romeu Odeillo, France maria.rutigliano@cnr.it

2 Outline Introduction Experiment Setup Evaluation method for recombination coefficient Results Simulations Molecular Dynamics (MD)calculations Results Comparison experimental vs theoretical for recombination coefficient Summary

3 Introduction The realization of nuclear fusion reactors like ITER has revealed the need of new data on hydrogen recombination on several kinds of materials at high temperature level. In the literature, one can find only data at low temperature (around 300 K) for silica, stainless steel, several pure metals and carbon. For a same material, data are widely scattered depending on the gas temperature and gas pressure and on the surface temperature of the material. According to the literature review, it seems necessary to perform new measurement of the recombination coefficient of atomic hydrogen at higher temperature levels.

4 Experimental Setup MESOX set-up (Moyen d Essai Solaire d OXydation) at the focus of the 6 kw Odeillo solar furnace 1 - concentrator 2 - shutter 3 - mass flowmeter 4, 8 and 9- mirrors 5 - quartz reactor 6 - waveguide 7 - optical pyrometer 10 - magnetron 11 - support bracket 12 - monochromator 13 - CCD 14 - computer 15 - vacuum pump 16 - control valve 17 - pressure gauge marianne.balat@promes.cnrs.fr

5 Experimental evaluation of recombination coefficient γ H Actinometry, Optical Emission Spectroscopy : the intensity ratio profile I H /I H2 of the two lines, Hβ at nm and H2 at nm was used for the determination of γ H as these two lines can be acquired simultaneously. Resolution of the diffusion equation at steady state : mean free path of H atoms <<< reactor radius some hypotheses: convective transfer negligible, radial gradient negligible, the stability of the ratio I H /I H2 in the reactor (without sample) allows neglecting the recombination in volume and on the reactor wall Finally : γ H = 4.D H,H 2 V.L γ H =H rec /H tot I H I H2 I H x= L I H2 x=0 T s 1 T L

6 Tungsten sample BEFORE The chemical composition of the W sample is: W 99.98%, Mo 0.004%, Fe 0.002%, P 0.002% and other components (0.012%) AFTER Heat treatment in H2 plasma at 2143 K XRD pattern

7 Experimental Results for the recombination of hydrogen atoms Process activation energy 14 kj/mol

8 Recombination of atoms on surfaces H 2 H H LANGMUIR-HINSHELWOOD H H 2 H ELEY-RIDEAL H H 2 H HOT ATOM

9 Semiclassical Molecular Dynamics calculations The dynamical simulation of surface processes is worked out through three main steps: building up of a 3D lattice crystal and phonon dynamics determination determination via DFT calculations of Potential Energy Surface(PES)where the reaction takes place Trajectory Propagation Semiclassical collisional method G. D. Billing 2000 Dynamics of Molecule Surface Interactions John Wiley & Sons, New-York

10 Recombination Probability: site and surface temperature effect 3F SITE T 3F T SITE 6.0e Recombination Probability 4.0e-3 2.0e-3 Recombination Probability e Kinetic Energy (ev) Kinetic Energy (ev) T S =1000K T S =700K

11 Factors Influencing the surface temperature effect The effect of surface temperature on the reaction dynamics is not wholly predictable due to the influence of different chemico-physical factors at both microscopic and macroscopic levels. The main factors include: mobility of adsorbed species surface coverage surface structural modifications reaction mechanism energy exchange mechanism between the chemisorbed species and the substrate

12 Probability for other surface processes T S =1000K H ad *W(1110) + H gas products 3F SITE T SITE 1.00 H ad + H gas 1.00 H ad + H ad Probability H ad + H ad Probability H ad + H gas 0.00 [H2] ad 0.00 [H2] ad Kinetic Energy (ev) Kinetic Energy (ev)

13 Recombination coefficient: theoretical versus experimental values (γ H ) th 3F T T S =T gas =1000K T S =T gas =700K T S =1000K T gas = 800K T S = 700K T gas =800K T S (K) 700 (γ H ) exp In the experiment the gas temperature was assumed to be around 700 K for similar surface temperature and 800 K for surface temperatures up to 1350 K. M. R., D. Santoro, M. Balat-Pichelin, Surface Science 628 (2014)

14 Comments: theoretical versus experimental ❶ In MD simulations, we considered a perfect (110) crystal while in the sample used in the experiment, there are other crystalline planes, the considered surface plane can have some defects and the sample contains, some impurities. ❷ The adsorption density considered in the calculations is different from that of the real surfaces in which different sites can be occupied by hydrogen atoms at the same time altering the reaction dynamics. This latter circumstance could lead the activation of Langmuir Hinshelwood mechanism. This issue requires further investigation. ❸ In the experiment it was observed that recombination acts for approximately 3 mm above the surface. This distance is too large to ascribe the decrease in the flux intensities just to the surface processes. In fact, surface processes act for distances of few Å from the surface, while for larger distances gas-phase atomic recombination can be effective.

15 Recombination Reaction Energetics 3F SITE E kin (ev) E ph E vib (%) E rot (%) E tr (%) 4.0 < < < T SITE Total Energy Fraction E rot E vib E tr 0.00 E ph Kinetic Energy (ev)

16 Vibrational Distribution of H 2 formed molecules 3F SITE 0.3 E kin =5 ev 0.15 E kin =6 ev T SITE v=0 v=1 v=2 v=3 v=4 v=5 v= v=7 v=8 v=9 v=10 v=11 v=12 P(v) P(v) P(v) P(v) Vibrational Number Vibrational Number Kinetic Energy (ev) Kinetic Energy(eV)

17 Summary The recombination coefficient of hydrogen atoms on W(110) has been evaluated by using a method based on the actinometry technique. The experimental results obtained for the recombination coefficient of H atoms on W were presented in the high temperature range and have shown that this material is very catalytic with values going from at 700 K up to 0.59 at 1350 K. MD simulations on the two most active sites on W(110) surface for the surface temperatures of 700 and 1000 K and a gas temperature of 700, 800 and 1000 K have been presented. A quite good agreement has been found between the experimental and the theoretical γ H. The energy transferred to the surface during recombination reaction is negligible. H2 molecule are formed in excited vibrational levels.

18 Acknoldegments This research was supported by : CNRS, France (ref. EDC 25071) and CNR, Italy (Bilateral Project n ) through the funding project: HYDROGEN ATOMS RECOMBINATION ON TUNGSTEN FOR ITER - EXPERIMENTS AND MOLECULAR DYNAMICS SIMULATION