Density Functional Theory Studies of Candidate Carbon Capture Materials OMS-2 and Cu-BTC Eric Cockayne NIST Thanks to Eric B. Nelson, Boise State University Lan Li, Boise State University Winnie Wong-Ng, NIST Laura Espinal, NIST Andrew Allen, NIST
Motivation CCS (Carbon Capture and Sequestration) Step 1- Gas Production Step 2- CO2 Capture Step 3- CO2 Transportation Step 4- CO2 Injection Current Technologies Amine Scrubbing
Increase Costs to Plants by ~30% Increase Electricity Costs by 6080% 3 The Need for New Materials CO2 removal using solid sorbents Sorbents may be recycled
by either a temperatu re or a pressure cycle New Materials and Designs Needed Low Energy Costs Introduction into Existing Technology Espinal et al., MRS Bulletin 37, 431 (2012).
Nanoporous solids show great promise. [3] 4 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). 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 Outline Can we use density functional theory to
guide the design of nanoporous materials for carbon capture? a-MnO2: can we control the hysteresis of carbon-dioxide sorption? Cu-BTC: can we solve the problem that water absorption reduces the CO2 uptake? 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. Grimmes formulation Fully ab-initio vdW computationally expensive
If studying H2O sorption, use meta-GGA, nd a-MnO2: Hysteresis control? MnO2: Many allotropes b-MnO2 (a) most stable a-MnO2 (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 MnO2 OMS structures (b;d) Different mxn pore sizes Geometry and chemistry Can be changed. Experiment: a-MnO2 only stabilized in presence of additional species such as K+. Above calculations of most stable location of K+ in the two compounds For b-MnO2, the tunnels are too small to easily accommodate K+
For a-MnO2, the tunnels easily accommodate K+. DFT Calculations show that a-KxMnO2 is stabilized for x > 0.002, consistent with experiments. Cockayne and Li, Chem. Phys. Lett. 544, 53 (2012). (Antiferro-) Magnetism of MnO2 (a) Experimental magnetic state of b-MnO2. Experimental volume and bandgap are reproduced for U = 2.8 eV and J = 1.2 eV. (b) Predicted ground state magnetic structure of a-MnO2. a-MnO2: Hysteresis control?
Experimental Observations N2 and CO2 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 pressuredependent. 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 Critical pressure of 7 bar before hysteresis occurs Espinal et al., J AC A 134, 7944 (2012). Ratchet model
Gatekeeper model Possible CO2 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 CO2 sorption as a function of pressure and time.
Gatekeeper model: single CO2 diffusion barrier Gatekeeper model: two CO2 per 0.3 nm repeat distance reduces diffusion barrier Li et al., Chem. Phys. Lett. 580, 120 (2013). Ratchet model P < 7 bar
P > 7 bar P >> 7 bar Decreasing P Engineering hysteresis by controlling cations Replace K+ with another species that a-MnO2 accommodates Computationally, we tested: Ba2+ (effect of cation
charge) and Na+ (effect of cation size) Energy Barriers in a-MnO2 CO2 sorption models Gatekeeper Model Ratchet Model K+ 0.13 eV
0.37 eV Na+ 0.87 eV 0.04 eV Ba2+ 1.02 eV
0.96 eV Critical pressure for hysteresis: highest for Ba 2+; lowest critical pressure is model-dependent Recent experiments indicate critical pressure for hysteresis is higher in Ba2+ doped a-MnO2 ! 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 Cu2+ ions face into large pores Cu-BTC and other MOF materials: Large CO2 uptake. Liu et al., Langmuir 26, 14301 (2010) Cu-BTC and other nanporous materials: H2O sorption kills CO2 uptake Cant use for post-combustion CO2 capture Liu et al., Langmuir 26, 14301 (2010)
Past computational work: One H2O per Cu2+ Water oxygen (OW) bonds with exposed Cu2+ inside the large pore Present study: Stability analysis shows that the H2O molecules want to flop to one side or the other Cu-BTC: Comparative X-ray powder diffraction results
(Wong-Ng et al., in press) dry Highly hydrated (2.3 H2O per Cu2+) 3 distinct partially-occupied OW Positions, only one next to Cu2+ Water absorption experiments: as Many as 6.5 H2O per Cu2+ Fitting experimental OW positions with realistic arrangements of H2O
Principle: OW-OW separations should optimally be around 0.29 nm (separation of OW in hydrogen-bonded H2O molecules) Two possible arrangements of the OW shown above: Model 28 and Model 30. Model 30: 6 rings of 5 OW ~28 H2O per large pore seen experimentally Similarity beween arrangements of OW and arrangements of C in fullerenes: Inspired a third model: Model 42, based on fullerene on right
12 DFT-relaxed H2O arrangements 28 Model 28 gets ripped apart 30 42
Models 28 and 42 show some H bonds to framework (shown in red) Comparative binding energetics of (H2O)28 clusters in large (lp) and medium pores (mp) of Cu-BTC Intracluster vdW lp mp
-5.29 -13.89 -3.06 chemical -7.91 total
-16.15 -2.19 +1.30 Experiment: all OW sites are in large pores -14.78 Can we design a MOF where H2O uptake doesnt hinder CO2 uptake?
0.54 nm Experimental Ow-Ow pair distribution functions for ice Geiger et al., J. Phys. Chem. C 118, 10989 (2014). If the Cu-Cu separation was just a 0.05 nm larger, then the OW-OW would be less favorable for the structure to ice up
Conclusions Density functional theory calculations used as a tool for design of nanoporous carbon capture
materials a-MnO2 : 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
