www.sciencemag.org/content/353/6295/150/suppl/DC1

Supplementary Materials for

Thermally stable single-atom platinum-on-ceria catalysts via atom trapping

John Jones, Haifeng Xiong, Andrew T. DeLaRiva, Eric J. Peterson, Hien Pham, Sivakumar R. Challa, Gongshin Qi, Se Oh, Michelle H. Wiebenga, Xavier Isidro Pereira Hernández, Yong Wang, Abhaya K. Datye*

*Corresponding author. Email: datye@unm.edu

Published 8 July 2016, Science 353, 150 (2016) DOI: 10.1126/science.aaf8800

This PDF file includes: Materials and Methods

Figs. S1 to S14

Tables S1 to S3

References

2

Materials and Methods

Catalyst preparation

Nano faceted CeO2 crystals

Synthesis of different shapes of CeO2 was performed using methods described in the

literature (18). The precursor used was Ce(NO3)3 6H2O (99.999%; Sigma-Aldrich). To

prepare CeO2 polyhedra, the Ce(NO3)3 6H2O was heated in air at 350 ˚C for 2 h,

producing a yellow powder. This was ground in a mortar and pestle to obtain a powder

suitable for catalyst preparation. CeO2 rods and cubes were obtained via hydrothermal

synthesis of a mixture of Ce(NO3)3 6H2O, NaOH, and H2O in a Teflon liner sealed tightly

in a stainless steel Parr autoclave. This mixture was subjected to a hydrothermal

treatment for 24 h at 100 ˚C for the rods and 180 ˚C for the cubes. The products were

then dried in air at 80 ˚C for 12 h and finally ground in a mortar and pestle for uniformity.

Pt/La-Al2O3 and Pt/CeO2 catalysts

Pt/La-Al2O3 and Pt/CeO2 catalysts (1 wt. %Pt, nominal) were prepared by incipient

wetness impregnation. Briefly, the appropriate amount of chloroplatinic acid (Sigma

Aldrich, 8 wt.%) was added drop-wise to the La-Al2O3 (MI 386, 4 wt.% La2O3, W R

Grace) or to the CeO2, respectively. The mixture was dried at 80 oC for 12 h. Then, the

Pt/La-Al2O3 was calcined at 350 oC for 6 h in flowing air. This catalyst was termed the

as-prepared catalyst, an image of which is shown in Figure 1A in the manuscript, and

used for preparing the physically-mixed samples, as described in the next section. The

samples of Pt/CeO2 were directly calcined at 800 oC for 10 h in flowing air to simulate

the accelerated aging that the other catalysts were subjected to.

Physically-mixed samples

The Pt/La-Al2O3 and nanoshaped CeO2 powders were mixed at a weight ratio of Pt-La-

Al2O3:CeO2 of 2:1. The mixture was ground in a mortar and pestle for 15 min and then

aged at 800 oC for 10 h in flowing air.

Pt/Al2O3 catalyst

The 1wt.%Pt/Al2O3 catalyst used for the DRIFTS study (Fig. 4D) was prepared using

the incipient wetness impregnation method. The Pt precursor used was Pt(NH3)4(NO3)2

from Sigma Aldrich and the Al2O3 support used was Catalox SBA 200 (γ-Al2O3) from

SASOL. After impregnating the support with the precursor solution, the catalyst was

dried at 150°C for 16h and calcined at 500°C for 2h, both steps in air.

Catalyst characterization

X-ray diffraction (XRD) data was collected using a Rigaku Smart Lab diffractometer

employing Cu Kα radiation and a Rigaku D/teX position-sensitive detector with a

nickel filter. Scans were performed from 20˚ to 90˚ with a scan rate of 6.2 degrees/min.

The data was analyzed by whole pattern fitting using the software Jade© obtained from

Materials Data Inc. For the physically-mixed samples, the broad -alumina peaks were

treated as an amorphous component of the diffraction pattern, and the CeO2 and Pt were

modeled using their respective crystal structures. Because the samples were composed of

3

a 2:1 (by weight) mixture of alumina:ceria, it was possible to consider the CeO2 as an

internal standard, fixing the weight fraction of CeO2 to be 0.333, and allowing for a

quantitative determination of the weight fraction of crystalline Pt (i.e., the Pt visible via

XRD).

The surface area of the samples was measured using a Micromeritics Gemini 2360

surface area analyzer according to the multi-point Brunauer Emmett Teller (BET) method

with N2 adsorption at –196 ˚C.

Electron Microscopy Transmission electron microscopy was performed using a JEOL

2010F microscope. The powders were deposited on holey carbon support films after

being dispersed in ethanol. The high resolution transmission electron microscopy was

carried out using a JEOL 2010F 200kV transmission electron microscope (resolution of

0.2 nm). For atomic resolution imaging, we used a JEOL JEM ARM200CF 200 kV

aberration-corrected (AC) transmission electron microscope (resolution of 0.08 nm) at

the University of Illinois at Chicago. The single Pt atoms on CeO2 can be clearly seen in

the AC-STEM dark field images. Scanning electron microscopy was performed on a

Hitachi S-5200 SEM (resolution ~ 2 nm at 1 kV). The samples for this were prepared on

a double sided carbon tape mounted on an aluminum boat.

CO, H2, and O2 Pulse chemisorption experiments were performed on a Micromeritics

Autochem 2920 using a TCD detector. The gases used for each experiment were

10%CO/He, 10%H2/Ar and 10%O2/He, respectively. For CO and O2 the carrier gas was

He, while for H2 the carrier gas was Ar. The procedure to run the pulse chemisorption

experiments was as follows: The 1 wt.%Pt/CeO2 catalyst was heated to 450°C under

helium at a heating rate of 10°C/min. Once at 450°C, the catalyst was oxidized by

flowing 10%O2/He (40ml/min) for 30 min. Residual oxygen was removed by purging

with helium for 30min (40ml/min) at the same temperature. Temperature was decreased

to 40°C and 20 pulses of the gas (CO, H2, or O2) were injected into the carrier gas to be

flowed into the reactor. The volume of each pulse was 0.5 ml. The time for injection was

3 min, followed by another 3 min wait step. This allows the removal of physisorbed

species from the catalyst.

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

DRIFTS was used to investigate the adsorption and desorption behavior of CO

molecules on Pt species of the catalysts during CO oxidation reaction. The infrared

spectrometer used was a Tensor 27 from Bruker, coupled with a Praying Mantis™

Diffuse Reflection accessory from Harrick. The products generated during the CO

oxidation reaction were monitored by a ThermoStar GSD 320 T Quadrupole Mass

Spectrometer (QMS) from Pfeiffer Vacuum, using a Secondary Electron Multiplier. The

procedure for DRIFTS during the CO oxidation reaction was as follows: the 1

wt.%Pt/CeO2 catalyst was heated to 450°C under helium at a heating rate of 10 °C/min.

When the temperature reached 450°C, the catalyst was oxidized by flowing 10%O2/He

(40 ml/min) for 30 min. After that, residual oxygen was removed by purging with helium

for 30 min (40 ml/min) at the same temperature. Then, the temperature was decreased to

the reaction temperature(s) and a background spectrum was taken at each temperature.

After taking the background spectrum at the reaction temperature, 15 ml/min of

10%CO/He and 10 ml/min of 10%O2/He were flowed (this equals to a 33% excess

4

oxygen). This represents the start of the reaction. In order to generate the IR spectra for

Figure 4, the reaction was run at 125 °C for 30 min. One spectrum was taken every

minute. At the end of the 30 min, the flow of CO was stopped and the remaining CO

adsorbed on the surface was allowed to react for 10 min with the flowing O2. Once again,

one spectrum was taken every minute. The spectra and backgrounds taken had a

resolution of 4 cm-1

and 128 scans were averaged for each spectrum and background.

CO oxidation measurements

CO oxidation experiments were performed in a U-shaped stainless steel reactor, and

the gases (CO, O2, and CO2) were analyzed using a Varian CP-4900 Micro-GC utilizing a

TCD detector. The samples of as-prepared and aged 1 wt. % Pt/La-Al2O3 catalysts were

tested in the reactor using 20 mg of catalyst, while the physically mixed samples were

tested using 30 mg of catalyst in order to keep the same amount of Pt for each

experiment. The samples were packed into a stainless steel U-tube reactor that was

mounted inside an insulated oven. The powder samples were packed between quartz

wool with a thermocouple placed touching the sample inside the reactor. CO oxidation

conditions used were 1.5ml/min CO, 1ml/min O2, and 75ml/min He (~2%CO), with a

ramp rate of 2 °C/min, and sampling performed every 3 min.

Evaporation-rate estimates for Pt

At 800 C, the pressure of PtO2 in equilibrium with Pt metal is 1.6 10-3

Pa (6).

Based on a model for metal evaporation developed by Langmuir in 1913 (31), we would

expect a rate of evaporation of 0.13 nm/s from a flat surface. This provides a lower limit

on the time it would take for a 5-nm particle to sublime at this temperature: it would be

only 50 milliseconds for a Pt particle having the stated vapor pressure of PtO2. Since the

formation of PtO2 occurs in the presence of oxygen, we find that Pt sublimes in our aging

studies. Because a stagnant gas film surrounds the particles we would expect the rate of

sublimation to slow down due to the diffusion resistance. The effect of such stagnant

film on the volatilization rates of thin cylindrical wires of tungsten at ~2500 C was

reported by Fonda (32). It was found that the rates of metal loss decreased by factor of

20–75 when compared to the rates observed in vacuum. The evaporation rates were

shown to depend on two quantities: the constituents of the vapor phase (higher rates

under lighter gases such as nitrogen,) and the characteristic dimension of the material

evaporating (higher rates from thinner wires.) In the absence of reliable data on transport

properties for Pt evaporating through PtO2 or data on the gas film properties, we have

assumed an attenuation in evaporation rates similar to those reported by Fonda (32) to

come up with an estimate for the rates of volatilization for Pt nanoparticles. We also

accounted for the effect of temperature on diffusivity, which is proportional to T1.5

.

Taking into consideration these two effects (stagnant film and high temperature) we

should expect an ~80-fold decrease in evaporation rates, or ~4s lifetime of a 5-nm Pt

particle. The rate of oxidation of Pt to form volatile PtO2 is rapid compared to the rates of

emission based on the work by Fryburg and Petrus and by Jehn (33-35). In other

experiments by Johns et al. (4), we found that ca. 5-nm particles were lost within 15s at

5

650 oC in air due to Ostwald ripening. Hence, the estimates for emission to the vapor

phase presented here seem quite reasonable.

Weight-loading measurement by scanning electron microscopy-energy dispersive

spectroscopy (SEM-EDS)

The weight loading of the Pt/La-Al2O3 was measured in an SEM fitted with Oxford

Aztec EDS system. Five scans on different areas were collected on the as-prepared Pt/La-

Al2O3 and aged Pt/La-Al2O3 samples (Table S1).

Weight loading measured by X-ray diffraction (XRD)

A control sample was prepared to assess the use of XRD to quantify the weight

loading of Pt in the catalyst samples. This sample consisted of a 2:1 weight ratio of 1 wt.

% Pt on La-Al2O3 (that was first aged at 800 oC for 10 h – as shown in Figure 1 in the

manuscript) and as-prepared polyhedral CeO2 mixed together. Using CeO2 as an internal

standard against which to measure the Pt peak, a Rietveld refinement was performed and

we derived a loading of 0.8 wt.% Pt, which matches the loading found by SEM-EDS

(Table S1).

It is evident from the two patterns in Figure S5 that the ceria causes an attenuation of

the Pt peak, which is why the Reitveld refinement approach is needed, using ceria as an

internal standard, to determine the Pt content. This method of quantification will only

work when the Pt is present in the form of large particles. If the Pt was highly dispersed,

the XRD reflection would be broad and the accuracy of quantitation would suffer. This,

however, is not a problem here because we have a bimodal distribution, Pt on ceria is

atomically dispersed, Pt on alumina grows into large crystalline particles.

Weight loading measurement and distributions of Pt on La-Al2O3 and CeO2 by scanning

transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS)

A STEM fitted with Oxford Aztec EDS was used to measure the weight loading of

Pt on separate CeO2 and La-Al2O3 particles to determine whether Pt has remained on La-

Al2O3 after aging. The average weight loading of Pt in the aged physically-mixed

sample, as determined by EDS, was 0.8 + 0.3 wt%. Figure S4 shows three images where

EDS was acquired on separate CeO2 and La-Al2O3 particles. Pt was detected on the

CeO2 particles, whereas no Pt was detected on the La-Al2O3 particles. Not only does

EDS match what we observed in Figure S3d, where very small Pt nanoparticles are

present on CeO2, it also verifies that all of the Pt has migrated from La-Al2O3 to CeO2.

Furthermore, no large Pt particles were observed in the aged physically-mixed sample, in

comparison to the aged 1 wt% Pt/La-Al2O3 without CeO2 (Figure 1B).

6

Fig. S1 HAADF STEM images and BET surface areas of CeO2 a) polyhedra, b) rods, c)

cubes, and HR-TEM images of d) polyhedra, e) rods, f) cubes (19). Note the

characteristic internal voids seen in the polyhedral and rods, but not in the cubes which

have atomically smooth surfaces.

7

Fig. S2 Bright-field AC-STEM images (a and c) and the corresponding dark-field

HAADF AC-STEM images (b and d) of CeO2 rods showing that the surfaces are clean

and devoid of any structures similar to those seen on the Pt containing ceria. These show

clear evidence of (111) facets and internal voids.

8

Fig. S3 Representative STEM/TEM images of physical mixtures of the Pt/La-Al2O3 and

polyhedral ceria after aging at 800 oC for 10 h in air: (a and b) low magnification; (c and

d) high magnification, showing that Pt migrated from Al2O3 to ceria. These images

obtained on a JEOL 2010F do not have the spatial resolution required to see isolated

single atoms of Pt, which are seen most clearly with the AC-STEM images shown in

Figures 3 and 4 in the manuscript. However, the Pt scatters electrons more strongly than

the lower atomic number ceria, and hence the Pt is visible only as a diffuse feature in

these images, as indicated by arrows (our microscope resolution is ~0.2 nm). The size of

the atomically dispersed Pt cannot be accurately determined from the images in this

figure.

9

Fig. S4 Representative STEM images and EDX results of physically-mixed fresh Pt/La-

Al2O3 and polyhedral ceria after aging at 800 oC for 10 h in air (a-c). These images were

obtained on a JEOL 2010F electron microscope, and EDS was acquired on separate La-

Al2O3 and CeO2 particles. Pt is detected on CeO2, whereas no Pt is detected on La-

Al2O3, indicating that all of the Pt migrated from La-Al2O3 to CeO2.

0 wt% Pt on La-Al2O3

1.8 wt% Pt on CeO2

a

0 wt% Pt on La-Al2O3 2.9 wt% Pt on CeO2

1.6wt% Pt on CeO2

b

0 wt% Pt on La-Al2O3

2.3 wt% Pt on CeO2

c

10

Fig. S5 Reference sample used to quantify the amount of Pt visible to XRD (Pt in

crystalline form). The inset shows a Reitveld refinement fit to the Pt(111) peak. By

considering the ceria added to this sample after aging as an internal standard we can

quantify the amount of Pt relative to ceria. Quantification of the Pt/La-alumina sample

without the added ceria (lower pattern) is not possible due to the broad reflections from

poorly crystallized alumina phase.

11

Fig. S6 CO conversion as a function of 1/T and Arrhenius plot for TOF for CO oxidation

on the Pt/Al2O3 samples and the physically-mixed samples of Pt/La-Al2O3 and ceria after

aging at 800 oC for 10h and a week.

0.00

0.01

0.10

1.00

1.9 2 2.1 2.2 2.3 2.4 2.5 2.6

TOF

(s-1

)

1000/T

Arrhenius Plot AGED 1 week Mixed Polyhedra

AGED Mixed Polyhedra

AGED Mixed Rods

AGED Mixed Cubes

12

Fig. S7 AC-STEM images of 1 wt.%Pt/ceria cubes after aging at 800 oC for 10 h in air,

showing the absence of atomically dispersed Pt. Instead, the cubes sample showed large

Pt particles, as seen under SEM in Fig. S9.

13

Fig. S8 XRD patterns of 1 wt% Pt on CeO2 with different nanoshapes after aging for 10 h

at 800 oC in flowing air. A Pt peak is seen only on the ceria-cube sample; all ceria

reflections are indicated with asterisks. There is evidence of K 1-K 2 peak splitting for

many of the ceria reflections for the cube sample, and the peak widths at these reflections

are smaller in the profile for the cube sample than for the widths of peaks in the other

nanoshaped-ceria samples. Both of these observations from the XRD profiles indicate

that the ceria crystallites are relatively large in the cube sample.

14

Fig. S9 SEM images of 1 wt% Pt on CeO2 after aging for 10 h at 800 oC: (a) 1 wt% Pt on

CeO2 cubes; b) 1 wt% Pt on CeO2 polyhedra; c) 1 wt% Pt on CeO2 rods. Scale bars are

500 nm. The ceria-cubes sample undergoes significant sintering as evident from this

image.

15

Fig. S10 Light-off curves and Arrhenius plots for CO oxidation on the Pt/CeO2 samples

with different ceria shapes.

16

Fig. S11 STEM image of Pt/CeO2 rod after aging at 800 oC for 10 h in air, captured using

the JEOL 2010F microscope.

17

18

Fig. S12 (A) DRIFTS of the 1wt.% Pt/CeO2 during CO oxidation after oxidative

treatment in 10% O2 at 450°C. After 30 min of CO oxidation, the CO flow was stopped

and spectra recorded at 125oC, 250

oC and 350

oC at the end of 10 min. while the oxygen

flow continued. There is significant drop in the peak intensity at 2095 cm-1

that is

assigned to isolated ionic Pt sites on the ceria. The three runs at different temperatures

show that the catalyst is stable after reaction and the Pt species remain atomically

dispersed. (B) QMS data for the CO oxidation reaction performed using DRIFTS. TOP:

The reaction was conducted at 350°C. At 2000 s, 10%CO/He and 10%O2/He started to

flow at a rate of 15 ml/min and 10 ml/min, respectively. At around 4000 s, the flow of

CO was stopped to allow species adsorbed on the surface to react with O2. At around

4500 s, the flow of CO was restarted using the same conditions. Around 5000 s, the flow

of CO and O2 was stopped and CO species adsorbed on the surface were allowed to

desorb in flowing He. The rate of reaction is very high at this temperature, as seen from

the CO2 signal. BOTTOM: At around 6000s, 10%CO/He and 10%O2/He started to flow

at a rate of 15 ml/min and 10 ml/min, respectively. The catalyst was at 125°C and there

was no CO2 formation. At around 8500 s, the flow of CO was stopped to allow for

species adsorbed on the surface to react with O2. Close to 9500 s the flow of CO was

restarted using the same flow concentrations and conditions, while at the same time the

temperature was raised to 250 °C. The reaction was run at 250 °C between close to 10000

s and 12000 s. The temperature was raised to 300 °C and decreased back to 250 °C

between 12000 s and around 12750 s. The CO2 level reached the same level as before the

temperature rise, attesting to the stability of the catalyst. At around 13000 s, the flow of

CO was stopped to allow for species adsorbed on the surface to react with O2. The CO2

concentration dropped first as the gas phase CO was depleted, and then the adsorbed CO

reacted, leading to a discontinuity in the CO2 curve. Around 13750 s, the flow of CO was

restarted using the same conditions. At 14000 s, the flow of CO and O2 was stopped and

species adsorbed on the surface were allowed to desorb under He.

19

Fig. S13 (A) DRIFTS of the 1wt.% Pt/CeO2 during CO oxidation after oxidative

treatment in 10% O2 at 450°C. After 30 min of CO oxidation at 125 oC, the CO flow was

stopped and spectra recorded at 0, 2 and 10 minutes while the oxygen flow continued.

There is negligible drop in the symmetrical feature at 2095 cm-1

that is assigned to

isolated ionic Pt sites on the ceria. (B) QMS data of the 1wt.% Pt/CeO2 during the

DRIFTS spectral acquisition for CO oxidation as shown in (A). No CO2 is detected,

indicating that the catalyst is not active at this temperature of 125 oC.

20

Fig. S14 (A) DRIFTS of the 1wt.% Pt/Al2O3 during CO oxidation after oxidative

treatment in 10% O2 at 450 °C. After 30 min of CO oxidation at 125oC, the CO flow was

stopped and spectra recorded at 0, 5 and 10 min. while the oxygen flow continued. There

is significant drop in the peak intensity at 2064 cm-1

that is assigned to CO adsorbed on

metallic Pt nanoparticles on the Al2O3. A residual feature at 2100 cm-1

persists,

corresponding to CO adsorbed on oxidized Pt species (B) QMS data of the 1wt.%

Pt/Al2O3 during the DRIFTS spectral acquisition for CO oxidation as shown in (A). CO2

is detected in the initial transient, but no CO2 is detected after 750s, indicating that the

steady state catalyst activity is very low at the reaction temperature of 125 oC.

21

Table S1. Pt weight loading measured by SEM-EDS in different areas.

Spectrum Pt wt.% on as-

prepared La-Al2O3

(fresh)

Pt wt.% on La-Al2O3

Aged at 800 ˚C

1 1.2 0.9

2 1.2 0.8

3 1.3 0.8

4 1.0 1.0

5 1.0 0.5

Average 1.14 0.8

22

Table S2. Characterization and catalytic performance of Pt/Al2O3 catalysts and

physically-mixed samples (2:1) after aging in air at 800 oC in flowing air (except as noted

below) Column label A B C D E F

Sample Crystalline Pt

by XRD

(wt%)

Dispersed Pt

(wt%)

Micromoles of

dispersed Pt per gram

catalyst

% CO

conversion

225 °C

TOF

s-1

EA

kJ/mol

Pt/Al2O3 fresh 1.1 (via SEM

EDS)

NA 46 (via chemisorption) 12.9 0.16 85

Pt/Al2O3 aged 10 h 0.81 (via XRD

& confirmed

by SEM-EDS)

NA NA 0.8 - 120

Pt/Al2O3+Cube Ceria aged

10 h

0.60 0.5 25.6 3.9 0.08 70

Pt/Al2O3+Rod Ceria aged

10 h

0.34 0.76 39.0 7.3 0.10 57

Pt/Al2O3+Polyhedra Ceria

aged 10 h

0.16 0.94 48.2 6.9 0.08 64

Pt/Al2O3+Polydedra Ceria

aged 1 week

ND 1.1 56.4 15.3 0.15 56

1 wt%Pt/CeO2 Polyhedra

aged 10 h

ND 1.0 51.3 15.4 0.17 55

Pt(100) from Berlowitz et

al. (25)

NA NA NA NA 0.12 54

A. Obtained by XRD from Reitveld refinement; ND = not detected; NA = not available

B. Dispersed Pt = 1.1 wt% (amount on Pt/Al2O3 aged via EDX) – crystalline Pt (column A)

C. Column C = Column B 104 / (195 g/mol)

Flow rate: 1.5ml/min CO = 66.9 µmol/min CO = 1.115 mol/s of CO

D. CO conversion (%) for the samples at 225 oC

E. Turnover Frequency (TOF) for the dispersed Pt phase: Average 0.12±0.04 s-1

TOF is calculated by using the following formula

23

Table S3. Surface areas and Pt atom coverage data on the Pt/CeO2-polyhedral catalyst

Catalyst Surface area (m2/g)

Pt coverage (atom·nm-2) Before aging After aging

Pt/CeO2-polyhedra 110.6 29.5 1.1

Polyhedral ceria 88.7 13.4 -

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