Chemisorption
Pulse titration
- Homework
- Silver (2.2 wt% on silica)
- Surface reaction: 2Agsurf + 02 -» 2 AgOsurf
@T = 170 °C (no bulk oxidation!)
- msample = °-5021 8
vpuise = 0.00925 cm3 02@RT@latm Max TCD signal = 0.022
- D = ?
Heterogeneous catalysis (C9981)
Lecture 6 Catalysts characterization -
continuation
styskalik@chemi.muni.cz styskalik.sci.muni.cz
Catalyst characterization
• Outline
- Solid state NMR techniques
- Hydrophilicity vs. Hydrophobicity meas
• Water sorption
• Dynamic water sorption
• Inverse gas chromatography
• Microcallorimetry
• Quartz crystal microbalance
- In situ and operando techniques
Solid state NMR techniques
• Magic angle spinning (MAS)
• Multiple quantum experiments (MQ)
• Dynamic nuclear polarization (DNP)
Solid state NMR techniques
- Spinning averages dipolar interaction, chemical shift anisotropy, and quadrupolar interaction
- "Residues" of these interactions are observed spinning sidebands
- Static and double rotation NMR possible
A
Magic angle spinning (MAS)
- 54.74 °
Solid state NMR techniques
31
polymeric product, gel
molecular condensation products
precursor, OP(OSiMe3)3
2h, 100 °C, SSNMR
oligomeric condensation products
168 h, r.t.
48 h, r.t.
■10
■20
■30
-40
■50
-60
Chemical shift [ppm]
Solid state NMR techniques
13CCPMAS
CH3C00
SiHexP \ I
CgH^ +CH3COO
Me3SiO
C
a>
a> >
a>
Me3SiO + MeSi03
MeSiP
CH3COO
CH3COO
SiP
*
CHoCOO
yx_A.
Me3Sid CH3COO
T"
0
250
200
150 100 Chemical shift [ppm]
50
Solid state NMR techniques
Dynamic nuclear polarization (DNP)
^ n
Solid state NMR techniques
• Dynamic nuclear polarization (DNP)
200 150 100 50 0
13C Chemical Shift (ppm)
Solid state NMR techniques
Solid state NMR techniques
i-■-1-■-1-■-1-■-1-■-1
150 100 50 0 -50 -100
Chemical shift (ppm)
Solid state NMR techniques
Static NMR (=no spinning)
27AI
triAl styskalik, 11 kHz, RTt spin echo (L1 =2) 90CT(Siq)=3.9usat200W 90CT(sot)=1.3usat200W September 2020
—I—i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—
300 200 100 0 -100
i—i—i—i—|—i—i—i—i—|—i—r - 200 [ppm]
Solid state NMR techniques
Multiple quantum MAS (MQMAS)
2D 27AI TQM AS
□
□
Row :    .56.92 ppm [189 of 512] -1-r^—\-1-^-1-r-1-1-f
\- E n. a.
100
50
i—I—r 0
o
o 00
o o
~~I-1—r-1-1—
50      F2 [ppm]
Hydrophilicity vs. Hydrophobicity
• Why?
- Remember: Adsorption/desorption step in cata as important as cata rxn itself (@Lecture 1)
- Moreover: strong efforts nowadays put forward bio-sources instead of fossil fuels
- Compare
• Oil - long hydrocarbon chains (hydrophobic)
• Wood - cellulose, lignin = sugar based materials = a lot of oxygen, OH groups (hydrophilic)
Hydrophilicity vs. Hydrophobicity
• Possibilities:
— Water sorption
— Dynamic vapor sorption/Quartz crystal microbalance
— Inverse gas chromatography
— Microcallorimetry
Hydrophilicity vs. Hydrophobicity
• Problems:
- What is a measure of hydrophobicity?
• A material can have high/low affinity to both water and organic molecules (i.e. if a sample is hydrophilic, it does not necessarily mean it is hydrophobic!)
- % of pore volume filled with water at certain p/p0
- Hydrophobic index (water/toluene competitive sorption)
- Heat of adsorption, heat of immersion
a a a
— Chemisorption: Do chemisorbed molecules account for hydrophobicity/philicity?
Hydrophilicity vs. Hydrophobicity
• Water sorption
- Similar to N2 physisorption
— Known volume and pressure in the cell, known mass
of the sample
Hydrophilic materials			Hydrophobic materials		
V	r-		V	III/	
( V	f M		( V	1 p/p0 V	
i V	> p/p0 /		( V	) p/p0 VII	
( V	/		[	1 p>p„	
o p/po					
Fig. 1. Adsorption isotherms classified according to IUPAC: type I: very hydrophilic material, type II: hydrophilic material, type III: hydrophobic/ low hydrophilic material with weak sorbent-water interactions, type IV: hydrophilic material, type V: hydrophobic/low hydrophilic material with weak sorbent-water interactions, type VI: hydrophilic material with multiple sorbent-water interactions and stepwise sorption, type VII: very hydrophobic material.
Hydrophilicity vs. Hydrophobicity
Water sorption
Very strong interaction water-adsorbent
Strongly hydrophilic
Water clustering = water-water interaction (liquefaction, condensation) stronger than water-adsorbent
Strongly hydrophobic
Very weak interaction water-adsorbent
& - P. Ngf S~ Mint oval M icroporo us and M esop o ro us M a te rials 114 (2008) 1-26
Hydrophilicity vs. Hydrophobicity
Water sorption
- Plotting In p against 1/7at constant adsorption uptake gives a straight line with a slope equal to
Hiso/R
- Hiso = isosteric heat of adsorption
- Isotherms at multiple temperatures needed!
1 Phys. Chem. C201Z, 116, 13316-13323
Hydrophilicity vs. Hydrophobicity
Water sorption
Hydrophobic material = very low
Hiso = negligible interaction water-adsorbent
40 H
c
.O
a.
2
id
0
1 »H
- Hiso = isosteric heat of adsorption
Hydrophilic material = very high Hiso = strong interaction water-adsorbent
30-1 ►
Hiso more or less constant = water-water interaction (condensation)
■  H-ZSM5 (15)
• NH4-ZSM5(15)
* H-ZSM5 (140)
t NH4ZSM5 (140)
4  Silicalite (OH) Silicalite (F")
~T~
2
1
3
T
4
-r
5
Q (mmol/g)
Hydrophilicity vs. Hydrophobicity
• Dynamic vapor sorption/Quartz crystal microbalance
- You deposit your material on an accurate microbalance
- You expose it to vapors of different gases/liquids (water, alcohols, hydrocarbons,...)
— You follow the uptake by changes of mass
— If we can control/follow pressure, then isotherms can be obtained similar to a classic physisorption
Hydrophilicity vs. Hydrophobicity
• Inverse gas chromatography
— You pack column (~ 50 cm) with the material you want to test (~ 0.5 g)
-You inject series of gases/liquids (e.g. methane, ethane,...hexane, heptane; methanol, ethanol,...; benzene, toluene, xylene...)
— You follow retention time (you directly see „affinity" of your material to selected liquids)
— Models (math) can give surface energy,...
Hydrophilicity vs. Hydr
• Immersion microcalorimetry
- Evacuated sample sealed in a bulb with brittle end
— Bulb immersed in a testing liquid, sealed
— Bulb broken (rod pushed down)
- Liquid gets into the bulb, adsorbs, heat of immersion released and measured
Fig. 1. Set-up for immersion calorimctry.
Hydrophilicity vs. Hydrophobicity
• Immersion microcalorimetry
— Ti-MCM-41, pure inorganic vs. increasing degree of surface silylation (increasing carbon content)
C content (Wt.%) Time (in hours)
Fig, 7. Evolution of the areal enthalpy of immersion (mj/m2) in methanol and 2- Fig. & Catalytic activity of Ti-MCM-41 materials with different silylation degrees
metbyl-2-propane I as a function of carbon content for the different si ly la ted Ti- inthecydohexene epoxidation with TBHP (and 0.5 wtK of catalyst) at 33 3 K during
MCM-41 samples. ^   {ajCyclohexene Conversion (KMoL); (b) Epoxide Selectivity (^Mol,),
J. Silvestre-Albero et al. / Applied Catalysis A: General 507 (2015) 14-25
In situ and operando techniques
In situ = online analysis of a working catalyst
Operando = online analysis of a working catalyst at relevant conditions (p, T, WHSV)
Technique	Information	Suitable to operando charac.
IR spectroscopy (FTIR/DRIFTS/ATR/IRAS)	Adsorbate nature, adsorption site	✓
Raman spectroscopy (SERS/TERS)	Solid structure, adsorbate nature	
XAS (EXAFS/XANES/QXAS)	Local environment, oxidation state	✓
SAXS (GISAXS)	NP size and morphology	✓
XRD (PXRD/SXRD/HEXRD)	Crystal phase and dimension	✓
XPS(NAP-XPS)	Chemical composition, oxidation state	Low pressure, large volume
TEM (Environmental TEM) [In situ TEM)	Atomic structure, chemical distribution	Low pressure, large volume Low catalyst amount
SPM (STM, AFM...)	Surface structure	Planar model catalysts
In situ and operando techniques
We already know these techniques, let's look at the examples!
- Low energy electron diffraction (LEED, gives similar results to x-ray diffraction = analysis of crystal structures)
- X-ray absorption near edge structure (XANES)
- Diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS)
In situ and operando techniques
• Example: LEED - low energy electron diffraction
• Ethylbenzene dehydrogenation to styrene
- 600 °C, 1 atm, 10-fold excess water vapor
- Over Fe203 epitaxially grown on Pt(lll)
• Flow reactor located in a high pressure cell
- Heated by lasers
- GC-MS analysis of catalytic products
- LEED analysis enabled by gate valve (high pressure/ultra high vacuum) and sapphire window
W. Weiss, R. SchlQgl / An integrated surface science approach towards, metal oxide catalysis
In situ and operando techniques
• Example: LEED - low energy electron diffraction
PEEM chamber STM chamber
Before catalytic reaction
After catalytic reaction
Initial period, no catalytic activity
f 4
§3
3
a-Fe3O,(0001)
hydrogen
xO.01
ethyl benzene
heater on (870 K)
I
Working catalyst
styrent
defect density
- high
......medium
low
low
medium high
high medium
Low
10
20 30
Time / min
40
50
In situ and operando techniques
• Example: LEED - low energy electron diffraction
Loss of catalytic activity
In situ and operando techniques
• Example: XANES - x-ray absorption near edge structure
• Methanol oxidation to formaldehyde over Cu
- 25-450 °C, 1 mbar
- Cu in the form of polycrystalline foil
2 CH3OH + 02 -> 2 CH20 + 2 H20
In situ and operando techniques
• Example: XANES - x-ray absorption near edge structure
2 CHaOH + O, -> 2 CH-,0 + 2 H.,0
Catalytic Activity
0.4 d.6 a.a
Row ratio O 21 ChfeOH
n situ and operando techniques
• Example: XANES - x-ray absorption near edge structure
10 h
co
(U
S 6
(Ľ
S 4
-2
O K-edge
Tempe rati
531.6
Flow ratio 02/CH3OH
670 K
Ö70 K
570 K
300 K
Reference Cli20
........i i i i
less active
very active
2 oxide and 1 suboxide species
520        530        540        550        560 570
Photon Energy / eV
In situ and operando techniques
Example: XANES - x-ray absorption near edge
structure
~ 720 K	• Positive correlation between
670 K t       |-a-1 T, / , / X 620 K	catalytic activity and Cu suboxide
570K   T /	species
	•  Explanation/idea/description of
f           520 K	active species: Subsurface oxygen
	cover by a strained layer of copper
/                 485 K	atoms
1—4H / 470 K /	•  Results confirmed by near ambient
/h-#H 420 K i                           i                           i i	pressure XPS (NAP XPS)
0.4 0£ 0.8
Integrated intensity (norm, u)
In situ and operando techniques
• Example: DRIFTS - diffuse reflectance infrared Fourier transform spectroscopy
• Coupling of ethanol and acetaldehyde to 1,3-butadiene overTa doped zeolite
- 300 °C, 1 atm
- Well dispersed (virtually isolated) Ta sites 2 C2H5OH + CH3CHO -> C4H6 + 2 H20
DOh KM021/acscatal.6bQ1642 ACÍ Carol 2016, 6, 6323-6332
In situ and operando techniques
• Example: DRIFTS - diffuse reflectance infrared Fourier transform spectroscopy
Scheme 2. Schematic Overview of the DRIFTS-MS Setup"
Time Time Time
Scheme 2. Schematic Overview of the DRIFTS-MS Setup"
Time Time Time
*TBy using two syringe pumps and heated coils that enter a two-position—four-way valve, the gas-phase composition can be modulated between substrates A and B. Depending on the chosen position, either flow A or B flows through the DRIFTS accessory followed by the online mass spectrometer. In addition, two three-way valves allow the substrates to bypass the DRIFTS accessory to ensure steady gas-phase concentrations prior to measurements.
In situ and operando techniques
• Example: DRIFTS
EtOH
AA
AOL
CA
COH
OH |-H2
Table L Chosen m/z Signals" and Some 1R Vibrational Frequencies'* for the Most Important Intermediates
Molecule	m/z	C-H stretch [cm-1]	c=o stretch	c=c stretch [cm-1]	other vibrations [cm-1]
EtOH	46	2988, 2970, 2900			106S
AA	II	2820, 2725, 2700	1750		
BD	54	3108, 3090, 3045		1605, 1588	
CA	70	2935, 2820, 2730	1722, 1710	1640	
COH	57	3025, 2937, 2880		1676	1450, 1440
In situ and operando techniques
• Example: DRIFTS - diffuse reflectance infrared Fourier transform spectroscopy