7.7 Unique Properties of Silver Cations …
187
three equivalent Ag atoms or ions interacted with a hydrogen nucleus to generate
the Ag–hydride species, Ag 3 –H. The formation of this Ag–hydride species indicated
that the heterolytic dissociation of hydrogen molecules proceeds over cationic silver
clusters, i.e., Ag
+
3 , as shown in the reactions below.
2 Ag + + H 2
2 Ag o + 2 H +
(7.43)
2 Ag o + Ag +
Ag 3
+
(7.44)
Ag 3
+
+ H 2
Ag 3 –H + H +
(7.45)
The reactions (7.43) and (7.45) show that acidic H
+ (O–H groups) is generated
not only by the reduction of Ag
+ cations with hydrogen, but also by the heterolytic
dissociation of the H–H bond in H 2 molecules on Ag
+
3 cationic clusters. The formation
of a triangular Ag
+
3 cationic cluster had previously been proposed as the result of
treatment of Ag-Y with hydrogen at low-temperature [58] and γ-irradiation of Ag-A
[69].
Baba et al. also found that reactions (7.43), (7.44), and (7.45) proceeded reversibly,
as shown in Fig. 7.2. They elucidated the reversibility of Ag 3 –H and acidic proton
formation by measuring
1 H MAS NMR spectra of Ag-A. When the reduced sample
was evacuated at 313 K for 2 h, the intensities of the signals associated with the acidic
protons and Ag 3 –H were reduced to 13% of their intensities in the spectrum of the
sample before evacuation (Fig. 7.2b). However, re-exposing the sample to hydrogen
(40 kPa) at 313 K for 30 min (second hydrogen reduction) caused the signals at
4.0 ppm and −1.8 ppm to reappear, as shown in Fig. 7.2c. The intensities of these
signals returned to almost their original levels, which are shown in Fig. 7.2a. Here,
the hydrogen consumption in the second hydrogen reduction was almost equal to
that in the first hydrogen reduction in Fig. 7.2a.
The results shown in Fig. 7.2 clearly indicate that reactions (7.43), (7.44), and
(7.45) are reversible when the degree of Ag
+ cation reduction in the Ag-A zeolite is
low. However, the extent of reversibility decreased at Ag
+ cation reduction degrees
higher than ~30%.
Furthermore, they reported the reversibility of heterolytic dissociation on Ag-Y
zeolites, as shown in Fig. 7.3 [4, 64]. When Ag-Y (46% Ag
+ exchange) was placed in
contact with H 2 at 373 K for 12 min, the degree of Ag
+ cation reduction was 21%. In
this first hydrogen reduction, three peaks were observed in the presence of hydrogen
at 4.7, 3.9, and −0.1 ppm, as shown in Fig. 7.3a. The peaks at 4.7 and 3.9 ppm were
attributed to the acidic O–H groups formed in the sodalite cage and the supercage
of the Y zeolite, respectively, because these chemical shift values were in agreement
with those previously observed for H-Y [70]. The peak at −0.1 ppm was assigned
187
three equivalent Ag atoms or ions interacted with a hydrogen nucleus to generate
the Ag–hydride species, Ag 3 –H. The formation of this Ag–hydride species indicated
that the heterolytic dissociation of hydrogen molecules proceeds over cationic silver
clusters, i.e., Ag
+
3 , as shown in the reactions below.
2 Ag + + H 2
2 Ag o + 2 H +
(7.43)
2 Ag o + Ag +
Ag 3
+
(7.44)
Ag 3
+
+ H 2
Ag 3 –H + H +
(7.45)
The reactions (7.43) and (7.45) show that acidic H
+ (O–H groups) is generated
not only by the reduction of Ag
+ cations with hydrogen, but also by the heterolytic
dissociation of the H–H bond in H 2 molecules on Ag
+
3 cationic clusters. The formation
of a triangular Ag
+
3 cationic cluster had previously been proposed as the result of
treatment of Ag-Y with hydrogen at low-temperature [58] and γ-irradiation of Ag-A
[69].
Baba et al. also found that reactions (7.43), (7.44), and (7.45) proceeded reversibly,
as shown in Fig. 7.2. They elucidated the reversibility of Ag 3 –H and acidic proton
formation by measuring
1 H MAS NMR spectra of Ag-A. When the reduced sample
was evacuated at 313 K for 2 h, the intensities of the signals associated with the acidic
protons and Ag 3 –H were reduced to 13% of their intensities in the spectrum of the
sample before evacuation (Fig. 7.2b). However, re-exposing the sample to hydrogen
(40 kPa) at 313 K for 30 min (second hydrogen reduction) caused the signals at
4.0 ppm and −1.8 ppm to reappear, as shown in Fig. 7.2c. The intensities of these
signals returned to almost their original levels, which are shown in Fig. 7.2a. Here,
the hydrogen consumption in the second hydrogen reduction was almost equal to
that in the first hydrogen reduction in Fig. 7.2a.
The results shown in Fig. 7.2 clearly indicate that reactions (7.43), (7.44), and
(7.45) are reversible when the degree of Ag
+ cation reduction in the Ag-A zeolite is
low. However, the extent of reversibility decreased at Ag
+ cation reduction degrees
higher than ~30%.
Furthermore, they reported the reversibility of heterolytic dissociation on Ag-Y
zeolites, as shown in Fig. 7.3 [4, 64]. When Ag-Y (46% Ag
+ exchange) was placed in
contact with H 2 at 373 K for 12 min, the degree of Ag
+ cation reduction was 21%. In
this first hydrogen reduction, three peaks were observed in the presence of hydrogen
at 4.7, 3.9, and −0.1 ppm, as shown in Fig. 7.3a. The peaks at 4.7 and 3.9 ppm were
attributed to the acidic O–H groups formed in the sodalite cage and the supercage
of the Y zeolite, respectively, because these chemical shift values were in agreement
with those previously observed for H-Y [70]. The peak at −0.1 ppm was assigned