Density Functional Theory Studies of - PowerPoint Presentation

Slide1

Density Functional Theory Studies of

Candidate Carbon Capture Materials OMS-2 and Cu-BTC

Eric

Cockayne

NIST

Slide2

Thanks toEric B. Nelson, Boise State University

Lan

Li, Boise State University

Winnie Wong-Ng, NIST

Laura

Espinal

, NIST

Andrew Allen, NIST

Slide3

Motivation

CCS (Carbon Capture

and Sequestration

)

Current Technologies

Step 1- Gas Production

Step

2- CO

2

Capture

Step 3- CO2 TransportationStep 4- CO2 Injection

Amine Scrubbing Increase Costs to Plants by ~30%Increase Electricity Costs by 60-80%

3

Slide4

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]4Nanoporous

solids show great promise

.

Espinal

et al., MRS Bulletin 37, 431 (2012).

Slide5

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).

Slide6

Nanoporous materials

:

Many are already known

Many more hypothetical structures

P

ossibilities for both designing new

nanoporous

materials and tuning the properties of existing ones.

GeometrySize, shape and dimensionality of pores and tunnels

ChemistryIonic substitution to control

sorbate-framework interactionChange ligands in metal-organic frameworks to achieve the above goals

Slide7

Outline

Can we use density functional theory to guide the design of

nanoporous

materials for carbon capture?

a

-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?

Slide8

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 possibleInclude van der Waals forces at an empirical level, e.g. Grimme’s

formulationFully ab-initio vdW computationally expensive

If studying H2O sorption, use meta-GGA, (uses 2nd derivative information)Accurate hybrid functionals computationally expensive

Slide9

MnO

2

:

Many allotropes

b

-MnO

2

(a)

most stable

a

-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 chemistryCan be changed.

a

-

MnO

2

: Hysteresis control?

Slide10

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

+

.

Slide11

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).

Slide12

(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

.

(

A

ntiferro

-) Magnetism of MnO

2

Slide13

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).a-MnO2: Hysteresis control?

Critical pressure of

7 bar before hysteresis

occurs

Slide14

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 CO2 sorption as a function of pressure and time.

“Gatekeeper” model“Ratchet” model

Slide15

Gatekeeper model: single CO

2

diffusion barrier

Slide16

Gatekeeper model: two CO

2

per 0.3 nm repeat

distance reduces diffusion barrier

Li et al., Chem. Phys.

Lett

. 580, 120 (2013).

Slide17

Ratchet model

P < 7 bar

P > 7 bar

P >> 7 bar

Decreasing P

Slide18

Engineering hysteresis by controlling

cations

Replace K+ with another

species that

a

-MnO

2accommodates

Computationally, we tested:Ba2+ (effect of

cation charge)and Na+ (effect of cation size)

Slide19

Energy Barriers in a

-MnO

2

CO

2

sorption models

Critical pressure for hysteresis: highest for Ba

2+;lowest critical pressure is model-dependentRecent

experiments indicate critical pressure for hysteresisis higher in Ba

2+ doped a-MnO2 !

Gatekeeper Model Ratchet ModelK+ 0.13 eV 0.37 eVNa+ 0.87 eV 0.04 eVBa2+ 1.02 eV 0.96 eV

Slide20

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

Slide21

Cu-BTC and other MOF materials:Large CO

2

uptake.

Liu et al., Langmuir 26, 14301 (2010)

Slide22

Cu-BTC and other

nanporous

materials:

H

2

O sorption kills CO

2

uptakeCan’t use for post-combustion CO

2 capture

Liu et al., Langmuir 26, 14301 (2010)

Slide23

Past computational work:

One H

2

O per Cu

2+

Water oxygen (O

W

) bonds

with exposed Cu

2+ inside

the large porePresent study:Stability analysis shows thatthe H2O molecules want to“flop” to one side or the other

Slide24

Cu-BTC: Comparative X-ray powder diffraction results

(Wong-Ng et al., in press)

dry

H

ighly hydrated (2.3 H

2

O per Cu

2+

)

3 distinct partially-occupied OW Positions, only one next to Cu2+Water absorption experiments: asMany as 6.5 H2O per Cu2+

Slide25

Fitting experimental O

W

positions with

realistic arrangements of H

2

O

Principle: O

W

-OW separations should optimally be around 0.29 nm(separation of O

W 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

Slide26

Similarity

beween

arrangements of O

W

a

nd arrangements of C in fullerenes:

Inspired a third model: Model 42, based on fullerene on right

Slide27

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

Slide28

Comparative

binding

energetics

of (H

2

O)

28

clusters in

large (lp) and medium pores (mp

) of Cu-BTCIntracluster

vdW “chemical” total-5.29 -3.06 -7.91 -16.15-13.89 -2.19 +1.30 -14.78

lpmpExperiment: all OW sites are in large pores

Slide29

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-OW would be less favorable for the structure to “ice up”Experimental Ow-Ow

pair distribution functionsfor iceGeiger et al., J. Phys. Chem.C 118, 10989 (2014).

Slide30

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