S1

Supporting Information (SI):

Understanding Sulfur Poisoning of Bimetallic Pd-Pt

Methane Oxidation Catalysts and their Regeneration

Patrick Lotta, Mario Ecka, Dmitry E. Doronkina,b, Anna Ziminab, Steffen Tischerb, Radian

Popescuc, Stéphanie Belind, Valérie Brioisd, Maria Casapua, Jan-Dierk Grunwaldta,b,*, Olaf

Deutschmanna,b,*

a Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of

Technology (KIT), Engesserstr. 20, 76131 Karlsruhe, Germany

b Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology

(KIT), Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

c Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT),

Kaiserstr. 12, 76131 Karlsruhe, Germany

d Synchrotron SOLEIL, L’Orme des Merisiers - St. Aubin, BP 48, 91192 Gif-sur-Yvette Cedex,

France

* Jan-Dierk Grunwaldt: grunwaldt@kit.edu

* Olaf Deutschmann: deutschmann@kit.edu

S2

Sequence of the test procedures

Figure S1. Reaction protocol for catalytic tests with lab bench aging experiments with SO2.

Selected tests were repeated with a fresh sample, however, a CH4-TPR in 3200 ppm CH4 and N2

was performed as the final step instead of the TPD.

Figure S2. Reaction protocol for the lab bench regeneration experiments after ageing with SO2.

S3

Additional material characterization data

Table S1. Results of the elemental analysis (by inductively coupled plasma optical emission

spectrometry, ICP-OES) of the 2.0 wt.%Pd – 0.4 wt.% Pt catalysts supported on Al2O3 and CZ.

Catalyst Pd-Pt/Al2O3 Pd-Pt/CZ

Pd content [wt.%] 1.99 1.92

Pt content [wt.%] 0.41 0.38

Figure S3. HAADF STEM image and EDXS maps of palladium (Pd-Lα1 line, blue), platinum

(Pt-Lα1 line, red) and sulfur (S-Kα1 line, lavender) of Pd-Pt/Al2O3 after 15 h of SO2-aging at

500°C.

S4

Figure S4. Chemical composition (the figure shows the sulfur amount in at.%) of Pd-Pt

nanoparticles supported on Al2O3 as a function of their diameter D. Particle size is derived from

HAADF-STEM images, chemical composition is derived from EDXS analysis.

Table S2. Maxima of S K-edge whiteline based on spectra with first derivative (cf. Figure S5).

Sample Pd/-Pt/Al2O3

(SO2-poisoned)

Pd-Pt/CZ

(SO2-poisoned)

PdSO4 · 2 H2O PdSO4

Maximum position

first derivative [eV]

2480.6 2480.0 Pre-edge:

2477.7

Whiteline:

2480.9

Pre-edge:

2477.7

Whiteline:

2480.9

Sample PdS Al2(SO4)3 Ce(SO)4 Zr(SO)4

Maximum position

first derivative [eV]

2469.4 2480.9 2480.2 2480.4

0 2 4 6 8 10 12 14 16 18 20

0

5

10

15

20

S [

at.

%]

D [nm]

S5

Figure S5. Normalized first derivative of S K-edge spectra.

2471 2478 2485 2492

-6

-4

-2

0

2

4

6

8

Norm

. d

eriv

ati

ve

[a.u

.]

E [eV]

Pd-Pt/Al2O

3 (400°C)

Pd-Pt/Al2O

3 (450°C)

Pd-Pt/Al2O

3 (500°C)

PdSO4 · 2 H

2O

PdSO4

(a)

2471 2478 2485 2492

-4

-2

0

2

4

6(b)

Norm

. d

eriv

ati

ve

[a.u

.]

E [eV]

Pd-Pt/Al2O

3 (400°C)

Pd-Pt/Al2O

3 (450°C)

Pd-Pt/Al2O

3 (500°C)

Al2(SO

4)

3

2471 2478 2485 2492

-4

-2

0

2

4

6(c)

Norm

. d

eriv

ati

ve

[a.u

.]

E [eV]

Pd-Pt/CZ (400°C)

Pd-Pt/CZ (450°C)

Pd-Pt/CZ (500°C)

Ce(SO4)

2

2471 2478 2485 2492

-6

-4

-2

0

2

4

6(d)

Norm

. d

eriv

ati

ve

[a.u

.]

E [eV]

Pd-Pt/CZ (400°C)

Pd-Pt/CZ (450°C)

Pd-Pt/CZ (500°C)

Zr(SO4)

2

2471 2478 2485 2492

-6

-4

-2

0

2

4

6

8(e)

Norm

. d

eriv

ati

ve

[a.u

.]

E [eV]

Pd-Pt/CZ (400°C)

Pd-Pt/CZ (450°C)

Pd-Pt/CZ (500°C)

PdSO4 · 2 H

2O

PdSO4

S6

Figure S6. DRIFTS spectra (difference reflectance spectra [aged catalyst]-[catalyst as prepared])

obtained for (a) Pd-Pt/Al2O3 and (b) Pt-Pt/CZ after aging the catalyst in 3200 ppm CH4, 10% O2,

12% H2O, 5 ppm SO2 in N2 (GHSV = 80 000 h-1) at 400°C, 450°C or 500°C for 15 h.

1600 1400 1200 1000 800

(a)R

efle

cta

nce

Wavenumber [cm-1

]

aluminum

surface sulfates

bulk aluminum

sulfates

aluminum

surface sulfites

400°C

450°C

500°C

1600 1400 1200 1000 800

(b)

Ref

lect

an

ce

Wavenumber [cm-1

]

500°C

450°C

400°Csurface sulfates

bulk sulfates

S7

Additional catalytic and TPD/TPR data from lab bench tests

Figure S7. Light-off (dark color) and light-out (light color) curves for Pd-Pt/Al2O3 (a) and Pd-

Pt/CZ (b) before (black/grey) and after aging (dark/light blue) at 400°C, 450°C and 500°C; gas

composition during activity test: 3200 ppm CH4, 10% O2, 12% H2O in N2. GHSV = 80 000 h-1.

250 300 350 400 450 500

0

20

40

60

80

100

250 300 350 400 450 500 250 300 350 400 450 500 550

(a)

CH

4 c

on

ver

sio

n [

%]

Temperature [°C]

Pd-Pt/Al2O

3

aged at 400°C

Pd-Pt/Al2O

3

aged at 450°C

Pd-Pt/Al2O

3

aged at 500°C

Temperature [°C] Temperature [°C]

250 300 350 400 450 500

0

20

40

60

80

100

250 300 350 400 450 500 250 300 350 400 450 500 550

(b)

CH

4 c

on

ver

sio

n [

%]

Temperature [°C]

Pd-Pt/CZ

aged at 400°C

Pd-Pt/CZ

aged at 450°C

Pd-Pt/CZ

aged at 500°C

Temperature [°C] Temperature [°C]

S8

Figure S8. Comparison of Temperature-Programmed-Desorption of SO2 (S-TPD) in N2 for SO2-

poisoned Pd-Pt catalysts supported on CZ and CeO2 (poisoning temperature 450°C).

TPD after aging at 450°C

Pd-Pt/CeO2

Pd-Pt/CZ

300 400 500 600 700 800 900225

0

20

40

60

80

100

120

140

SO

2 c

on

cen

tra

tio

n [

pp

m]

T [°C]

S9

Additional data from (in situ) XAS experiments

Figure S9. (a) XANES spectra of Pt-references used for linear combination fitting (LCF) and a

Pd-Pt/Al2O3 sample poisoned with SO2 at 450°C. (b) Corresponding Fourier-transformed

k2-weighted EXAFS spectra (k-range 2.5 - 14 Å-1, uncorrected for the phase shift).

Figure S10. Evolution of Pt-species during a Temperature-Programmed-Desorption of SO2 (S-

TPD) in He (GM1, Table 1 in the main manuscript) of a Pd-Pt/Al2O3 sample after 15 h of SO2

poisoning at 450°C.

11540 11560 11580 11600 11620

0.0

0.5

1.0

1.5

2.0

(a)

No

rm.

ab

sorp

tio

n [

a.u

.]

E [eV]

Pd-Pt/Al2O

3

PtO2

Pt foil

0 2 4 6 8

0.0

0.5

1.0

1.5

2.0

FT

k2-w

eig

hte

d E

XA

FS

[Å

-3]

R [Å]

Pd-Pt/Al2O

3

PtO2

Pt foil

(b)

11550 11575 11600

0.0

0.5

1.0

1.5

2.0

2.5

(a)

Pt L3-edge during TPDNo

rm.

ab

sorp

tio

n [

a.u

.]

E [eV]

PtO2

Pt foilT

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

1.0

Pt L3-edge during TPD

(b)

PtO2

Pt

Fra

ctio

n o

f re

fere

nce

s

T [°C]

S10

Figure S11. In situ XANES at the Pt L3-edge and evolution of Pt-species during a Temperature-

Programmed-Reduction (CH4-TPR) in 3200 ppm in He (GM3, Table 1 in the main manuscript)

of a Pd-Pt/Al2O3 sample after 15 h of SO2 poisoning at 450°C.

11550 11575 11600

0.0

0.5

1.0

1.5

2.0

2.5

PtO2

Pt foil

(a)N

orm

. ab

sorp

tion

[a.u

.]

E [eV]

T

Pt L3-edge during TPR

0 100 200 300 400 500 600

0.0

0.2

0.4

0.6

0.8

1.0

(b)

Pt L3-edge during TPR

Fra

ctio

n o

f re

fere

nce

T [°C]

PtO2

Pt

S11

Figure S12. In situ XANES spectra at the Pd K-edge of aged samples (SO2 poisoning at 450°C)

during a Temperature-Programmed-Desorption of SO2 (TPD, GM1, Table 1), during heating in a

reaction mixture (RM, GM2, Table 1) and during a Temperature-Programmed-Reduction (CH4-

TPR, GM3, Table 1).

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(a)

Pd-Pt/Al2O

3

No

rm.

ab

sorp

tio

n [

a.u

.]

E [eV]

T

Pd K-edge during TPD

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(b)

No

rm.

ab

sorp

tio

n [

a.u

.]

E [eV]

T

Pd-Pt/CZ

Pd K-edge during TPD

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(c)

No

rm.

ab

sorp

tio

n [

a.u

.]

E [eV]

T

Pd-Pt/Al2O

3

Pd K-edge during TPO

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(d)

Norm

. ab

sorp

tion

[a.u

.]

E [eV]

T

Pd-Pt/CZ

Pd K-edge during TPO

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(e)

Norm

. ab

sorp

tion

[a.u

.]

E [eV]

Pd-Pt/Al2O

3

Pd K-edge during TPR

T

24320 24340 24360 24380 24400 24420

0.0

0.5

1.0

1.5

(f)

Norm

. ab

sorp

tion

[a.u

.]

E [eV]

T

Pd-Pt/CZ

Pd K-edge during TPR

S12

Figure S13. Exemplary fits at 20°C and 750°C for in situ XANES spectra at the Pd K-edge of

aged samples (SO2 poisoning at 450°C) during a Temperature-Programmed-Desorption of SO2

(TPD, GM1, Table 1). The low- and high-temperature fits represent the two observed types of Pd

species in the TPD experiment: surface-poisoned PdO and dominating reduced Pd nanoparticles.

A misfit that originates from nanoparticle sintering on the alumina support can be seen for the

spectrum of Pd-Pt/Al2O3 at 750°C (Figure S13c). On CZ (Figure S13d) no sintering was

observed during the TPD, which we attribute to strong noble metal support interactions.

S13

S14

Figure S14. Exemplary fits at 20°C, ~300°C, ~600°C and 750°C for in situ XANES spectra at

the Pd K-edge of aged samples (SO2 poisoning at 450°C) during a Temperature-Programmed-

Desorption of SO2 (CH4-TPR, GM3, Table 1). The fits represent four different temperature

regions in which the relevant catalyst states were found. The misfit in Figure S14h probably

originates from alloy formation of Pd with the support, e.g. CeO2. The spectrum clearly shows

metallic features, however, it is shifted to lower energies, which points to higher electron density

on Pd. As oxidation or sulfidation would rather decrease the electron density and would change

the XANES shape, and hydride formation is unlikely at high temperature (750°C), alloy

formation remains as the only reasonable explanation.

S15

Figure S15. Evolution of Pd-species of aged (a) Pd-Pt/Al2O3 and (b) Pd-Pt/CZ samples (sulfur

ageing at 450°C) during Temperature-Programmed-Reduction (CH4-TPR) in 3200 ppm, 1.5%

H2O in He.

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

1.0

Fra

ctio

n o

f re

fere

nce

s(a)

T [°C]

PdO

PdSO4

Pd

PdS

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

1.0

Fra

ctio

n o

f re

fere

nce

s

PdSO4

PdO

Pd

PdS

(b)

T [°C]

S16

Thermodynamic analysis

Figure S16. Pd-PdO-PdSO4 phase diagram for bulk materials in a gas mixture of 10% O2 in N2.

The partial pressure of SOx is the predominant factor governing the presence of PdSO4.

The phase diagram for bulk materials displayed in Figure S16 was calculated with the

DETCHEM software package using the DETCHEMEQUIL tool [1]. Thermodynamic data were

taken from literature [2-4]. Please note that the thermodynamic constants of small noble metal

particles on a support, as subject to our study, may differ from crystalline bulk species. Hence,

the phase diagram can only provide a rough estimation of the actual phase stability for our Pd-

based methane oxidation catalysts.

[1] O. Deutschmann, S. Tischer, S. Kleditzsch, V. Janardhanan, C. Correa, D. Chatterjee, N. Mladenov, H. D. Minh,

H. Karadeniz, M. Hettel, V. Menon, A. Banerjee. DETCHEM Software package, 2.8 ed., www.detchem.com,

Karlsruhe 2019.

[2] T. Dahmen, P. Rittner, S. Böger-Seidl, R. Gruehn, Thermal Behavior of Sulfates 14. Thermal Behavior of PdSO4

· 2 H2O and PdSO4 · 0.75 H2O and Structure of M-PdSO4, J. Alloy Compd., 216 (1994) 11-19.

[3] C. Mallika, O.M. Sreedharan, J.B. Gnanamoorthy, Determination of the standard free energy of formation of

PdO(s) from the solid oxide electrolyte E.M.F., J. Less-Common Met., 95 (1983) 213-220.

[4] A. Burcat, Thermochemical species in polynomial form, https://burcat.technion.ac.il/dir/, Budapest 2020.

S17

Supplementary decomposition study

To evaluate the stability of reference bulk sulfates (commercial compounds), we performed

thermogravimetric analysis (TGA). Approximately 30 mg sample were heated with 5 K/min in

N2 (TPD) or 5% CH4 in N2 (TPR) using a STA 409 (Netzsch) thermal gravimetric analysis

(TGA) unit. Analysis of the evolved gas species with a mass spectrometer (OmniStar GSD 320)

for selected tests revealed, that the first mass loss is mainly due to dehydration of the water-

containing materials. At elevated temperatures we observed SO2 release due to sulfate

decomposition.

Moreover, catalyst samples of Pd-Pt/Al2O3 and Pd-Pt/CZ, which were poisoned with SO2 at

450°C, were investigated in a Netzsch STA 449 F3 Jupiter while analyzing the evolved gases

with a mass spectrometer.

S18

Figure S17. Weight loss of bulk sulfates with increasing temperature (heating rate 5 K/min).

TPR in 5% CH4 and balance He, TPD in pure N2.

0 300 600 900

0

20

40

60

80

100

TPD

TPR

Al2(SO

4)

3 18 H

2O

(a)M

ass

[%

]

T [°C]0 300 600 900

0

20

40

60

80

100

TPD

TPR

(b)

Ce(SO4)

2 4 H

2O

Mass

[%

]

T [°C]

0 300 600 900

0

20

40

60

80

100

TPD

TPR

Zr(SO4)

2 4 H

2O

(c)

Ma

ss [

%]

T [°C]0 300 600 900

0

20

40

60

80

100

TPD

TPR

(d)

Y2(SO

4)

3 8 H

2O

Mass

[%

]

T [°C]

0 300 600 900

0

20

40

60

80

100La

2(SO

4)

3

(e)

Mass

[%

]

T [°C]

TPD

TPR

S19

Figure S18. TG-MS experiment with SO2-poisoned Pd-Pt/Al2O3 (a) and Pd-Pt/CZ (b) catalysts

during a TPD in Ar.

Figure S19. TG-MS experiment with SO2-poisoned Pd-Pt/Al2O3 (a) and Pd-Pt/CZ (b) catalysts

during a CH4-TPR in 3200 ppm CH4 in N2.