International Forum for Green Asia 2020

Catalytic properties of Pd/CeO2 and Pd-Cu/CeO2
Saki Shigenobu,1 Hajime Hojo,2 Hisahiro Einaga2
1

Interdisciplinary Graduate School of Engineering, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan2

Department of Material Sciences, Faculty of Engineering Sciences, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, JapanAbstract
The effect of adding Cu to Pd catalysts (Pd/CeO2, Pd/TiO2, Pd/Al2O3) on their catalyticproperties was investigated. The low-temperature oxidation activity of Pd/CeO2 was superior
to that of Pd/TiO2 and Pd/Al2O3, and the Cu addition enhanced the CO oxidation activity in all
temperature ranges. The structural changes of Pd/CeO2 before and after sulfur poisoning wereinvestigated by in situ XAFS. The sulfidation of the CeO2 support was promoted by the Pd
deposition.
1. Introduction
Regulations on factory and vehicle
emissions are becoming increasingly
stringent worldwide to achieve a sustainable
society. Pd catalysts are a key material topurify these exhaust gases. However, there isa strong need to reduce the amount of Pd usedbecause Pd is scarce and expensive.
Furthermore, catalyst poisoning by sulfur
content (e.g., SO2 and H2S) is an important
issue in the industrial exhaust gas purification
process. Because the catalytic performance of
Pd catalysts after sulfur poisoning is seriouslyaffected, therefore, it is necessary to developa Pd catalyst with high activity and resistanceto sulfur poisoning.
One way to improve the activity of thenoble metal catalyst is to add transition
metals. Our previous studies revealed that theco-deposition of transitional metals enhances
the activity of TiO2-supported noble metals
for CO oxidation.[1] In this study, we focusedon Cu, which has high redox properties andattempted to improve the catalytic activity of
Pd catalysts by the addition of Cu.
Understanding
thesulfur
poisoningmechanism is essential for designing
catalysts that are highly resistant to sulfur
poisoning. However, the process has not beenclarified. in situ X-ray absorption finestructure (XAFS) measurements caninvestigate the local structure of catalyst

under reaction condition. Therefore, in situ
XAFS measurements under SO2 gas flow
were carried out to investigate the local
structure change to elucidate the sulfur
poisoning process.
2. Experimental
2.1. Catalyst preparation
Pd catalysts (Pd/TiO2, Pd/Al2O3,Pd/CeO2) and Pd-Cu co-deposition catalysts(Pd-Cu/TiO2, Pd-Cu/Al2O3, Pd-Cu/CeO2)
were prepared by impregnation methods. For
the preparation of catalysts, Pd(NO3)2・4NH3
and (CH3COO)2Cu・H2O were dissolved intowater. The solution was added dropwise tothe powders of catalyst support andevaporated to dryness at 60℃. The catalyst
samples were dried, calcined, and thenreduced in H2 flow. The Pd loading was set to
1 wt %.
2.2. Catalysts characterisations
Catalytic CO oxidation was carried out
with a fixed bed flow reactor. Catalyst
samples were loaded in a U-shaped glass
flow reactor, which was placed in a furnace
for catalyst heating. Reaction gases (CO0.5 %, O2 0.25 %, He-balance) were fed tothe reactor with a flow rate of 100, 300
mL/min. The concentrations of CO2 and CO
were determined using GC-TCD. Prior to thereaction, the catalyst was heated at 300 ℃ in
a He flow for 1 h. Reaction temperature was23

International Forum for Green Asia 2020
set in the range of 90 to 250℃.
XAFS spectra were obtained using an insitu XAFS measurement cell at Kyushu
University beamline (SAGA-LS BL-06). Thecatalysts were contacted with SO2 containinggas (500 ppm of SO2) at 500℃ and thencooled down to room temperature for spectra
measurements under the flow of He. Data
reduction was performed with the software
Athena and Artemis.

4. Conclusions
Pd/CeO2 showed high CO oxidationactivity at low temperature, and the oxidation
activity of Pd-Cu/CeO2 was enhanced in all
temperature ranges. FTIR studies suggested
that the addition of Cu enhances CO
desorption on Pd by CO oxidation reaction.
Therefore, the activity of CO oxidation wasimproved at low temperature. in situ XAFS
analsys showed that the support CeO2 ofPd/CeO2 was more reduced by SO2 than thatof CeO2 which did not supported Pd. The
support CeO2 reduced by SO2 were notreoxidized by O2.

3. Results and discussion
Figure 1 shows the effect of Cu additionon the CO oxidation activity of the Pd
catalyst. The light-off temperature ofPd/Al2O3 and Pd/TiO2 was 150°C and 120°C,respectively. The CO oxidation activity ofPd/CeO2 was higher than that of Pd/Al2O3and Pd/TiO2 in the low-temperature region(<180°C). The addition of Cu on Pd/CeO2
enhanced the activity in both the low andhigh-temperature
ranges.
Pd-Cu/CeO2
showed the highest CO oxidation activity.
FTIR spectra of Pd/CeO2 showed the peak oflinearly coordinated CO on Pd around andbridge coordinated CO around. Both peaks oflinearly coordinated CO and bridge
coordinated CO on Pd-Cu/CeO2 decreased
significantly in the presence of CO oxidation
reaction gas. This behavior is due to thedesorption of CO on Pd by the CO oxidation
reaction. Therefore, the addition of Cu to thePd/CeO2 promoted oxidative desorption ofCO which had been adsorbed on Pd sites at
low temperature.
Figure 2 shows the Ce L3-edge XANES
spectra of CeO2 and Pd/CeO2 in the presenceof SO2. CeO2 and Pd/CeO2 showed a peak
attributed to Ce4+. The reduction of Ce4+ toCe3+ occurred by SO2. The change of peak toCe3+ under SO2 flow was remarkable for
Pd/CeO2. This shows the reduction of thesupport CeO2 by SO2 was promoted by the Pddeposition. When the reaction gas waschanged to O2 at 500℃, the XANES spectra
did not change. This indicates that sulfurized
CeO2 support was not reoxidized by O2. Inaddition, the peak of sulfide did not detect in
Pd L3-edge XANES spectra. This means nosulfide was formed on Pd by SO2Figure 1. CO oxidation activity of Pd catalysts
and Pd-Cu catalysts

Figure 2. Ce L3-edge XANES spectra of CeO2
and Pd/CeO2 under the flow of 500 ppm SO2
Measurement timeCeO2：20,40,60,80 min
Pd/CeO2：20,40,60,80,120,240 min

Reference[1] H. Einaga, R. Shiranita, H. Saito, and S.
Hata, Chem. Lett., 44, 1786, 2015.
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