Dr. Jinasena W. Hewage

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Molecular adsorption of H2 on small silver- copper bimetallic nanoparticles A search for novel Hydrogen storage materials

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“Molecular adsorption of H2 on small silver- copper bimetallic nanoparticles A search for novel Hydrogen storage materials” Dr. Jinasena Hewage University of Ruhuna Sri Lanka

Molecular adsorption of H2 on small silver- copper bimetallic nanoparticles A search for novel Hydrogen storage materials :

Molecular adsorption of H 2 on small silver- copper bimetallic nanoparticles A search for novel Hydrogen storage materials Dr. Jinasena Hewage Department of Chemistry University of Ruhuna Matara , Sri Lanka G.S.M. Perera 1 , T. P. Kodippily 2 , J. W. Hewage 1* , and K. M. Nalin de Silva 3 1 Department of Chemistry, University of Ruhuna , Matara , Sri Lanka, 2 Department of Chemistry, University of Colombo, Sri Lanka, 3 Sri Lanka Institute of Nanotechnology, Nanotechnology and Science Park, Homagama , Sri Lanka. * jwhewage@gmail.com

Why we need cleaner burning fuel ?:

Why we need cleaner burning fuel ? Since 1750, atmospheric levels of carbon dioxide have increased from 275 ppm to 392 ppm in 2013, with continued annual increases of 2-3 ppm . Future energy needs call for abundant, low-carbon technologies to reduce the environmental effects of fossil fuel consumption. Also transportation currently relies heavily on either gasoline or diesel fuel and the increased demand for oil supplies may cause energy crises. In addition, air pollution is a major environmental concern for the whole world. The transportation sector and electric power plants which generate greenhouse gas ( e.g CO 2 ) are two major contributors to environmental quality issues . It is clear that this current situation requires an alternative clean form of energy source be developed in order to reduce dependence on fossil energy

Why Hydrogen gas as a fuel ? :

Why Hydrogen gas as a fuel ? Its use creates neither air pollution nor greenhouse-gas emissions . It is the most abundant element on Earth, but less than 1% is present as molecular hydrogen gas H 2 . The overwhelming majority is chemically bound as H 2 O in water and some are bound to liquid or gaseous hydrocarbons. The clean way to produce hydrogen from water is to use sunlight in combination with photovoltaic cells and water electrolysis The chemical energy per mass of Hydrogen is (142 MJ kg –1 ) It is at least three times larger than that of other chemical fuels (for example, the equivalent value for liquid hydrocarbons is 47 MJ kg –1 ). Once produced, Hydrogen is a clean synthetic fuel: when burnt with oxygen, the only exhaust gas is water vapor Note: However, its low volumetric energy density has only a quarter of the energy of gasoline (8 MJ/liter for liquid hydrogen versus 32 MJ/liter for gasoline).

Energy content (per Kg) for some common combustibles vs H2:

Energy content (per Kg) for some common combustibles vs H 2

Ways of storing/using Hydrogen as a fuel..:

Ways of storing/using Hydrogen as a fuel.. There are essentially two ways to run a road vehicle on hydrogen In the first, hydrogen in an internal combustion engine is burnt rapidly with oxygen from air. The efficiency of the transformation from chemical to mechanical through thermal energy is limited by the Carnot efficiency It is slightly higher for hydrogen–air mixtures (around 25%) than for petrol–air mixtures. When a clean mixture is used, the exhaust gas contains nothing but water vapor In the second, hydrogen is ‘burnt’ electrochemically with oxygen from air in a fuel cell It produces electricity (and heat) and drives an electric engine The efficiency of the direct process of electron transfer from oxygen to hydrogen is not limited by the Carnot efficiency it can reach 50–60%, twice as much as the thermal process

Volume of 4kg of Hydrogen compacted in different ways..:

Volume of 4kg of Hydrogen compacted in different ways..

Recent advancements..:

Recent advancements.. The current methods of storing hydrogen as compressed gas or in the liquid form do not meet the industrial requirements because the energy densities are much lower than that in gasoline . Moreover, there are issues of safety and cost involved in compressing hydrogen under high pressure or liquefying it at cryogenic temperatures. Although storage of hydrogen in porous solid-state materials and metal hydrides offers an alternative, there are no current solid-state storage materials that meet the industry requirements Metal Hydrides and solids with large surface area (e.g . Carbon Nanotubes) also have been experimented with extensively for Hydrogen storing, often coming up with contradicting results.

Use of Ag/Cu small nanoclusters as means for H2 adsorbent.. :

Use of Ag/Cu small nanoclusters as means for H 2 adsorbent.. We investigated the use of silver-copper bimetallic nanoclusters [ Ag m Ni n n+m ≤ 8]as the clean storage materials for hydrogen storage. Heterogeneous clusters of silver and copper of size 2-8 were designed in all permutations. Density functional approach was utilized in the Gaussian 9 Quantum Mechanical package for modeling and calculation Selection of level of theory was based on literature and the previous work on small silver, copper and gold clusters

Method and Basis set..:

Method and Basis set.. B3LYP and LANL2DZ basis set were used in geometry optimization of the Cu/Ag mixed nanoclusters LANL2DZ is considered reliable for d-block metals with large number of electrons The optimized geometries of the Cu/Ag mixed nanoclusters were compared with literature and was found out to be in good accuracy. The next step was the introduction of molecular H 2 to the medium and using the mixed basis set. cc- pVQZ basis set for H 2 and LANL2DZ for clusters were used

Optimized geometries of some of the clusters..:

Optimized geometries of some of the clusters.. Clusters of 2 atoms Clusters of 3 atoms Clusters of 4 atoms Clusters of 5 atoms Results and Discussion

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Clusters of 7 atoms Clusters of 8 atoms Clusters of 6 atoms

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Average Binding E nergy (ABE ) : ABE= [{ mE (Cu)+ nE (Ag) - E ( Cu m Ag n )}/ ( m + n )] Adiabatic I onization P otential (AIP) : AIP = E ( Cu m Ag n ) + - E ( Cu m Ag n ) Adiabatic Electron A ffinity (VEA) : AEA = E ( Cu m Ag n ) - E ( Cu m Ag n ) - C hemical hardness ( η ) : η = (AIP - AEA)/ 2 ABE of a cluster is a parameter to measure its thermodynamic stability. Higher binding energy indicates higher thermal stability Higher AIP suggests higher chemical stability. Lower AEA suggests higher chemical stability Higher HLG and η also suggests higher chemical stability

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Cluster ABE ( eV /atom ) HLG ( eV ) VIP ( eV ) VEA( eV ) η ( eV ) Magnetic moment ( μB )   Cu 0.000 1.954 7.826 0.812 3.507 1.000   Ag 0.000 1.776 7.752 1.088 3.332 1.000     Cu 2 1.008 3.254 7.930 0.673 3.628 0.000   CuAg 0.892 3.082 7.830 0.827 3.502 0.000   Ag 2 0.776 2.906 7.726 0.985 3.371 0.000     Cu 3 1.005 1.395 5.742 1.912 1.915 1.000   Cu 2 Ag 0.926 1.336 5.794 2.057 1.868 1.000   CuAg 2 0.860 1.284 5.878 2.154 1.862 1.000   Ag 3 0.740 1.262 5.795 2.221 1.787 1.000     Cu 4 1.306 1.905 6.588 1.335 2.627 0.000   Cu 3 Ag 1.244 1.924 6.582 1.357 2.613 0.000   Cu 2 Ag 2 1.181 1.955 6.579 1.366 2.606 0.000   CuAg 3 1.066 1.760 6.498 1.510 2.494 0.000   Ag 4 0.953 1.612 6.423 1.615 2.404 0.000  

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Cluster ABE ( eV /atom) HLG (eV) AIP ( eV ) AEA( eV ) η(eV) Magnetic moment ( μB )   Cu 5 1.412 1.341 5.986 1.749 2.119 1.000   Cu 4 Ag 1.353 1.316 6.019 1.819 2.100 1.000   Cu 3 Ag 2 1.295 1.288 6.045 1.890 2.078 1.000   Cu 2 Ag 3 1.217 1.241 5.984 1.947 2.019 1.000   CuAg 4 1.139 1.195 5.918 1.991 1.963 1.000   Ag 5 1.041 1.179 5.818 2.002 1.908 1.000     Cu 6 1.592 3.244 7.106 1.011 3.047 0.000   Cu 5 Ag 1.543 3.136 7.116 1.099 3.009 0.000   Cu 4 Ag 2 1.493 3.059 7.035 1.178 2.928 0.000   Cu 3 Ag 3 1.443 3.010 7.035 1.255 2.890 0.000   Cu 2 Ag 4 1.359 2.979 6.993 1.273 2.860 0.000   CuAg 5 1.279 2.981 6.676 1.213 2.731 0.000   Ag 6 1.199 3.000 6.950 1.274 2.838 0.000  

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Cluster ABE ( eV /atom) HLG (eV) AIP ( eV ) AEA( eV ) η(eV) Magnetic moment (μB) Cu 7 1.625 1.273 5.886 1.826 2.030 1.000   Cu 6 Ag 1.566 1.270 5.821 1.796 2.013 1.000   Cu 5 Ag 2 1.505 1.266 5.758 1.760 1.999 1.000   Cu 4 Ag 3 1.444 1.258 5.691 1.727 1.982 1.000   Cu 3 Ag 4 1.382 1.254 5.628 1.684 1.972 1.000   Cu 2 Ag 5 1.319 1.247 5.564 1.640 1.962 1.000   CuAg 6 1.246 1.212 5.671 1.800 1.935 1.000   Ag 7 1.166 1.174 5.760 1.978 1.891 1.000     Cu 8 1.723 2.696 6.407 1.242 2.583 0.000   Cu 7 Ag 1.671 2.639 6.383 1.292 2.545 0.000   Cu 6 Ag 2 1.644 3.161 6.465 1.082 2.692 0.000   Cu 5 Ag 3 1.600 3.129 6.783 1.125 2.829 0.000   Cu 4 Ag 4 1.555 3.099 6.542 1.168 2.687 0.000   Cu 3 Ag 5 1.481 3.103 6.439 1.172 2.633 0.000   Cu 2 Ag 6 1.407 3.092 6.315 1.165 2.575 0.000   CuAg 7 1.318 2.493 6.190 1.392 2.399 0.000   Ag 8 1.251 2.558 6.452 1.410 2.521 0.000  

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Series consist of odd number of atoms (m + n = 3, 5, 7) => contain an unpaired electrons => show comparatively lower AIP => show higher AEA => show lower chemical hardness ( η) => show lower HLG values also Show obvious odd-even oscillation These properties indicates that even-numbered clusters are more stable than that of adjacent odd-numbered clusters. All clusters prefer the lowest possible spin multiplicity that is 1 for even-numbered clusters : Magnetic moment 0 μ B . 2 two for odd-numbered clusters: Magnetic moment 1 μ B .

Adsorbtion of H2 onto the optimized structures of the Nanoclusters..:

Adsorbtion of H 2 onto the optimized structures of the Nanoclusters .. As described previously, the suitable theory and mixed basis set was used in the computation Every possible adsorption site was tested for each individual cluster to determine the optimum position for the H 2 molecule Adsorption should be physisorption due to many reasons The maximum and minimum operating pressure in a storage tank is 3 and 100 bar, respectively According to the protocol introduced by DOE-USA, hydrogen adsorption energy should be around -18 kJ mol -1 and well-above the -5 kJ mol -1 at ambient temperature

Structures of some optimized H2 adsorbed nanoclusters.. :

Structures of some optimized H2 adsorbed nanoclusters ..

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Cluster ABE ( kJ mol -1 ) E ads ( kJ mol -1 ) HLG ( kJ mol -1 ) AIP ( kJ mol -1 ) AEA ( kJ mol -1 ) η ( kJ mol -1 ) Cu 2 171.719 -30.965 374.901 775.331 33.840 370.745 CuAg 165.737 -29.416 359.332 765.283 66.815 349.234 Ag 2 153.614 -3.193 299.128 743.530 97.431 323.050 Cu 3 161.450 -54.920 137.132 550.722 149.670 200.526 Cu 2 Ag 155.937 -50.110 131.330 553.058 164.953 194.053 CuAg 2 150.181 -40.525 125.842 555.664 174.991 190.337 Ag 3 136.309 -5.908 123.794 541.537 212.111 164.713 Cu 4 171.996 -66.602 274.842 641.283 66.835 287.224 Cu 3 Ag 167.412 -63.046 274.107 639.959 69.943 285.008 Cu 2 Ag 2 162.745 -59.047 273.135 638.971 73.495 282.738 CuAg 3 154.791 -56.022 245.961 632.311 93.399 269.456 Ag 4 141.418 -19.306 213.903 616.402 143.786 236.308

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Cluster ABE ( kJ mol -1 ) E ads ( kJ mol -1 ) HLG ( kJ mol -1 ) AIP ( kJ mol -1 ) AEA ( kJ mol -1 ) η ( kJ mol -1 ) Cu 5 166.977 -26.403 131.855 550.039 153.548 198.245 Cu 4 Ag 163.142 -27.609 126.630 554.599 163.116 195.742 Cu 3 Ag2 158.786 -25.392 125.711 558.310 168.455 194.927 Cu 2 Ag 3 153.348 -24.665 122.272 551.860 180.522 185.669 CuAg 4 145.672 -8.548 115.366 556.395 194.010 181.193 Ag 5 138.397 -5.125 113.949 562.405 195.509 183.448 Cu 6 174.950 -16.713 312.466 602.970 102.555 250.207 Cu 5 Ag 171.309 -15.889 308.449 606.835 92.272 257.281 Cu 4 Ag 2 167.651 -15.469 304.353 652.955 93.445 279.755 Cu 3 Ag 3 163.683 -12.886 289.939 618.791 124.288 247.252 Cu 2 Ag 4 157.344 -10.405 287.234 607.866 125.607 241.129 CuAg 5 153.089 -22.700 290.464 624.996 104.118 260.439 Ag 6 145.181 -5.706 288.967 669.331 125.412 271.960

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Cluster ABE ( kJ mol -1 ) E ads ( kJ mol -1 ) HLG ( kJ mol -1 ) AIP ( kJ mol -1 ) AEA ( kJ mol -1 ) η ( kJ mol -1 ) Cu 7 175.722 -22.205 122.770 554.298 168.540 192.879 Cu 6 Ag 171.263 -22.514 122.560 549.420 180.500 184.460 Cu 5 Ag 2 166.727 -22.364 122.350 544.430 160.738 191.846 Cu 4 Ag 3 161.734 -19.186 121.195 537.134 175.710 180.712 Cu 3 Ag 4 157.106 -19.292 120.854 532.192 174.671 178.761 Cu 2 Ag 5 150.832 -5.417 113.975 478.437 187.494 145.471 CuAg 6 145.588 -7.450 118.438 494.543 196.577 148.983 Ag 7 138.957 -1.796 113.266 552.856 205.830 173.513 Cu 8 181.840 -26.914 292.591 608.574 90.889 258.843 Cu 7 Ag 178.622 -34.577 287.733 627.188 94.304 266.442 Cu 6 Ag 2 175.011 -19.739 279.620 605.597 102.273 251.662 Cu 5 Ag 3 171.438 -18.136 274.816 622.954 107.814 257.570 Cu 4 Ag 4 167.780 -15.733 270.063 603.933 112.957 245.488 Cu 3 Ag 5 161.716 -12.734 271.980 599.432 111.928 243.752 Cu 2 Ag 6 155.808 -10.407 273.267 576.050 110.474 232.788 CuAg 7 149.871 -20.189 272.400 614.548 120.170 247.189 Ag 8 142.872 -1.724 246.749 597.884 137.372 230.256

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Conclusion and future work Neat silver and copper particles are out of two extremes (-6 kJ mol -1 to -18 kJ mol -1 ) Mixed systems provides an excellent physisorption range as a storage material Clusters size of six and eight are shown to be better options Cu 5 Ag 1 , Cu 4 Ag 2 , Cu 3 Ag 3 , Cu 2 Ag 5 / Cu 5 Ag 3 , Cu 4 Ag 4 , Cu 3 Ag 5 , Cu 2 Ag 6 Temperature and pressure dependence of adsorptions ?? Adsorption density, ρ m (wt%) or ρ v (kgm -3 ) ?? Optimization of operational temperature and pressure with functionalizations

References:

References   R. D. Cortight , R.R. Davada , J.A. Dumesic , Nature 418 (2002) 964. Alper , Science 299 (2003) 1686. L. Schlapbach , A. Züttel , Nature 414 (2001) 353. S. X. Tao, Notten P. H. L. Notten , Santen R. A. V. Santen and Jansen A. P.J. Jensen, Phys. Rev. B 83 (2011) 195403. L. Kyuho , Y.H. Kim Y H, Y.Y. Sun, Phys. Rev. Lett.104 (2010) 36101. C. Liu, Y.Y. Fan Y, Science 286 (1999)1127. P. C. Aeberhard , K. Refson , Phys. Rev. B 83 (2011) 174102 L. Zbigniew , Phys. Rev. B 81(2010) 144108

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