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Plasma activated HP and OP samples exhibited much higher activities than C500. HP sample possessed the merits of both good
stability and high activity. Its HCHO conversion reached 69% quickly and kept at around 74% for 9 h. OP sample suffered an
activation process before reaching stable. It obtained an initial conversion of 59%, then underwent a 4 h activation process to
reach 100% conversion and finally maintained at over 96% for 6 h. The HCHO conversions exhibited the same tendency with time
on stream for each sample in the whole temperature range of 120-150 C, as shown in Fig. S1 (Supporting information). The
activation process for OP sample could be accelerated as temperature lifted. That was over 10 h at reaction temperature of 120
C and could be shortened to half hour with temperature increasing to 150 C.
For HP, OP and C500 catalysts, the BET surface area exhibited no obvious changes, as shown in Fig. S2 (Supporting
information). The diffraction patterns of Co3O4 are observed from X-ray diffraction patterns (XRD), as shown in Fig. S3 (Supporting
information). This indicates the Co3O4 could be formed after both plasma activation and calcination. Those peaks intensities of HP
and OP samples are much weaker than C500, implying poor crystallinity of plasma activated samples. However, no diffraction
peaks corresponding to Ag species could be distinguished for all samples. Ag and Co could be easily distinguished at the catalyst
surface from HRTEM images for all catalysts, as shown in Fig. S4 (Supporting information).
Fig. S5 (Supporting information) displays the Raman spectra of Ag-Co/CeO2 catalysts. For fresh sample, only two peaks are
observed. The strong bond at 461 cm1 corresponds to the fluorite F2g mode of ceria. The peak at 1043 cm1 is due to the
presence of nitrate ions [27]. This suggests the existence of silver and cobalt nitrates after impregnation since no further
calcination or reduction process was conducted. The nitrate peak disappeared completely for HP and C500 samples. While it still
existed along with a declination of intensity for OP sample, indicating the incomplete decomposition of nitrate species. After both
plasma activation and calcination, four new peaks occurred. The peaks at 483, 524 and 689 cm1 correspond to the Eg, F2g and A1g
symmetries of Co3O4, respectively [11,28]. This implies the formation of Co3O4, which is consistent with XRD result. The band at
557 cm1 could be attributed to the formation of oxygen vacancy after plasma activation and calcination [29, 30].
Fig. S6 (Supporting information) shows the IR spectra of CeO2 and Ag-Co/CeO2 catalysts. Four intense bands in the 1000-
1
1800 cm region are observed on ceria surface. The bands at 1065 and 1340 cm1 could be ascribed to the unidentate carbonate
1
species, and the band at 1535 cm is due to carboxylate species [31,32]. The band centered at 1630 cm1 could be assigned to the
deformation vibration of water. The broad absorbance at 2700-3700 cm1 is attributed to the stretching vibration of OH groups
and adsorbed water. After supporting Ag and Co nitrates, the bands in 1000-1600 cm1 region are due to the superposition of
carbonate, carboxylate and nitrate species. The bands at 1325 and 1455 cm1 could be ascribed to the asymmetric stretch of
nitrate species, and the band at 1040 cm1 is the symmetric stretch of nitrate species [33]. This result indicates the existence of
silver and cobalt nitrates over fresh catalyst. After oxygen plasma treatment, only trace of nitrate species could be observed. The
species distribution over HP sample was the same as ceria and nitrate species completely disappeared. This suggests both silver
and cobalt nitrates species could be decomposed quickly and efficiently by plasma activation in a very short time. And hydrogen
plasma is more efficient for nitrate decomposition than oxygen. After 500 C calcination, all the surface species entirely
disappeared and nothing could be observed, suggesting the complete decomposition of nitrates and removal of carbonate,
carboxylate and water species during high temperature process.
Fig. S7 (Supporting information) illustrates the H2-TPR profiles of Ag-Co/CeO2 catalysts. For fresh Ag-Co/CeO2 catalyst, two
peaks at 177 and 280 C are observed. The peak at 177 C is due to the consumption of hydrogen and the formation of NO2, corresponding
to the reduction and decomposition of silver and cobalt nitrates. This could be proved by mass spectra result as shown in Fig. S7b. An
intensive peak of nitrogen dioxide at 177 C formed along with the consumption of hydrogen at the same time, indicating the existence of
both reduction and decomposition processes in hydrogen atmosphere. While only hydrogen consumption was observed and no nitrogen
dioxide formed at 280 C. This suggests both silver and cobalt nitrates could be consumed entirely at 177 C, and the peak at 280 C is
only due to the reduction of cobalt oxide. For plasma activated HP and OP samples, a main reduction peak at 273 C belonging to the
reduction of Co3O4 is observed. Two small peaks at 165 and 179 C ascribed to the reduction of remained nitrates are observed over OP
sample. This implies the incomplete decomposition of nitrate exists. For C500, only one reduction peak of Co3O4 is observed, suggesting
the complete decomposition of nitrates under high temperature calcination. Moreover, its position moves to a higher temperature of 387 C
than others. This could be ascribed to its higher crystallinity degree of Co3O4, which could be proved by XRD results. In addition, a
reduction peak at high temperature of 754 C ascribed to reduction of bulk ceria is observed for all samples.