1. Density Functional Theory Studies of Candidate Carbon Capture Materials OMS-2 and Cu-BTC Eric Cockayne NIST

2. Thanks to Eric B. Nelson, Boise State University Lan Li, Boise State University Winnie Wong-Ng, NIST Laura Espinal, NIST Andrew Allen, NIST

3. Motivation CCS (Carbon Capture and Sequestration) Current Technologies Step 1- Gas Production Step 2- CO 2 Capture Step 3- CO 2 Transportation Step 4- CO 2 Injection Amine Scrubbing Increase Costs to Plants by ~30% Increase Electricity Costs by 60-80% 3

4. New Materials and Designs Needed Low Energy Costs Introduction into Existing Technology CO 2 removal using solid sorbents Sorbents may be recycled by either a temperature or a pressure cycle The Need for New Materials [3] 4 Nanoporous solids show great promise. Espinal et al., MRS Bulletin 37, 431 (2012).

5. Nanoporous materials predicted to have lower energy costs in a carbon capture and removal cycle than liquid amines Lin et al., Nature Mater. 11, 633 (2012).

6. Nanoporous materials : Many are already known Many more hypothetical structures Possibilities for both designing new nanoporous materials and tuning the properties of existing ones. Geometry Size, shape and dimensionality of pores and tunnels Chemistry Ionic substitution to control sorbate-framework interaction Change ligands in metal-organic frameworks to achieve the above goals

7. Outline Can we use density functional theory to guide the design of nanoporous materials for carbon capture?  -MnO 2 : can we control the hysteresis of carbon-dioxide sorption? Cu-BTC: can we solve the problem that water absorption reduces the CO 2 uptake?

8. Advice to a DFT novice studying nanoporous solids Use PBEsol pseudopotentials “Goldilocks” between LDA & PBE GGA Use a Hubbard U parameter for magnetic ions Fit U by fitting bandgap of known simpler system. Set up antiferromagnetic structure if possible Include van der Waals forces at an empirical level, e.g. Grimme’s formulation Fully ab-initio vdW computationally expensive If studying H 2 O sorption, use meta-GGA, (uses 2 nd derivative information) Accurate hybrid functionals computationally expensive

9. MnO 2 : Many allotropes  -MnO 2 (a) most stable  -MnO 2 (c) a.k.a. OMS-2 (octahedral molecular sieve) 2x2 pores Cations in tunnels K (cryptomelane); Ba (hollandite); Na, Mg, Ca, Cu, Fe, Al (etc.) Other MnO 2 OMS structures (b;d) Different mxn pore sizes Geometry and chemistry Can be changed.  - MnO 2 : Hysteresis control?

10. Experiment :  - MnO 2 only stabilized in presence of additional species such as K +. Above calculations of most stable location of K + in the two compounds For  - MnO 2, the tunnels are too small to easily accommodate K + For  - MnO 2, the tunnels easily accommodate K +.

11. DFT Calculations show that  - K x MnO 2 is stabilized for x > 0.002, consistent with experiments. Cockayne and Li, Chem. Phys. Lett. 544, 53 (2012).

12. (a) Experimental magnetic state of  - MnO 2. Experimental volume and bandgap are reproduced for U = 2.8 eV and J = 1.2 eV. (b) Predicted ground state magnetic structure o f  - MnO 2. (Antiferro-) Magnetism of MnO 2

13. Experimental Observations N 2 and CO 2 adsorption and desorption isotherms at T = 303 K using 15 min equilibration time for OMS-2. Solid and open symbols represent adsorption and desorption points, respectively. Sorption Hysteresis: a path to adsorption of gas molecules by porous host differs from that of desorption. The width of the hysteresis loop is time- and pressure- dependent. Scanning pressure curves using 5 min dwell time at 303 K, The dotted line represents a common adsorption curve while the colored solid lines are desorption curves after reaching different maximum pressures on the adsorption branch. Espinal et al., JACS, 134, 7944, 2012 Espinal et al., J AC A 134, 7944 (2012).  - MnO 2 : Hysteresis control? Critical pressure of 7 bar before hysteresis occurs

14. Possible CO 2 sorption mechanisms by OMS-2. a. Perspective view of a single tunnel of OMS-2 (front view) showing the cation inside the tunnel: For clarity, translucid yellow walls are shown to highlight the location of the octahedrally coordinated Mn b-g, Schematic representation of the cross-sectional side view of the OMS-2 tunnel showing a possible mechanisms of CO 2 sorption as a function of pressure and time. “Gatekeeper” model “Ratchet” model

15. Gatekeeper model: single CO 2 diffusion barrier

16. Gatekeeper model: two CO 2 per 0.3 nm repeat distance reduces diffusion barrier Li et al., Chem. Phys. Lett. 580, 120 (2013).

17. Ratchet model P < 7 bar P > 7 bar P >> 7 bar Decreasing P

18. Engineering hysteresis by controlling cations Replace K+ with another species that  -MnO 2 accommodates Computationally, we tested: Ba 2+ (effect of cation charge) and Na + (effect of cation size)

19. Energy Barriers in  -MnO 2 CO 2 sorption models Critical pressure for hysteresis: highest for Ba 2+ ; lowest critical pressure is model-dependent Recent experiments indicate critical pressure for hysteresis is higher in Ba 2+ doped  -MnO 2 ! Gatekeeper Model Ratchet Model K + 0.13 eV 0.37 eV Na + 0.87 eV 0.04 eV Ba 2+ 1.02 eV 0.96 eV

20. Cu-BTC (a.k.a. HKUST-1): Metal-organic framework material 1.3 nm, 1.1 nm and 0.7 nm pores connected by square and triangular windows. Exposed Cu 2+ ions face into large pores

21. Cu-BTC and other MOF materials: Large CO 2 uptake. Liu et al., Langmuir 26, 14301 (2010)

22. Cu-BTC and other nanporous materials: H 2 O sorption kills CO 2 uptake Can’t use for post-combustion CO 2 capture Liu et al., Langmuir 26, 14301 (2010)

23. Past computational work: One H 2 O per Cu 2+ Water oxygen (O W ) bonds with exposed Cu 2+ inside the large pore Present study: Stability analysis shows that the H 2 O molecules want to “flop” to one side or the other

24. Cu-BTC: Comparative X-ray powder diffraction results (Wong-Ng et al., in press) dry Highly hydrated (2.3 H 2 O per Cu 2+ ) 3 distinct partially-occupied O W Positions, only one next to Cu 2+ Water absorption experiments: as Many as 6.5 H 2 O per Cu 2+

25. Fitting experimental O W positions with realistic arrangements of H 2 O Principle: O W -O W separations should optimally be around 0.29 nm (separation of O W in hydrogen-bonded H 2 O molecules) Two possible arrangements of the O W shown above: Model 28 and Model 30. Model 30: 6 rings of 5 O W ~28 H 2 O per large pore seen experimentally

26. Similarity beween arrangements of O W and arrangements of C in fullerenes: Inspired a third model: Model 42, based on fullerene on right

27. DFT-relaxed H 2 O arrangements Model 28 gets ripped apart Models 28 and 42 show some H bonds to framework (shown in red) 30 28 12 42

28. Comparative binding energetics of (H 2 O) 28 clusters in large (lp) and medium pores (mp) of Cu-BTC Intracluster vdW “chemical” total -5.29 -3.06 -7.91 -16.15 -13.89 -2.19 +1.30 -14.78 lp mp Experiment: all O W sites are in large pores

29. Can we design a MOF where H 2 O uptake doesn’t hinder CO 2 uptake? 0.54 nm If the Cu-Cu separation was just a 0.05 nm larger, then the O W -O W would be less favorable for the structure to “ice up” Experimental O w -O w pair distribution functions for ice Geiger et al., J. Phys. Chem. C 118, 10989 (2014).

30. Conclusions Density functional theory calculations used as a tool for design of nanoporous carbon capture materials  - MnO 2 : Cation (i.e. chemical) changes predicted to change the hysteresis behavior Predictions being verified experimentally. Cu-BTC: Water forms large stable hydrogen bonded clusters, particularly in large pores Changing metal-metal distance should help