2.4. UV-vis DRS Analysis
Figure 4a, b show UV-vis DRS diagrams of pure m-BiVO
4 and Ag/Pd/m-BiVO
4 with different mass ratios and with different loading amounts. From the diagram, it can be seen that the absorption edges of all test samples are approximately 500 nm [
15], indicating that m-BiVO
4 absorbs in the visible region;
Figure 4a shows that among the catalysts with different mass ratios, Ag/Pd/m-BiVO
4 (1 wt%, 2:1) has the strongest capacity to absorb visible light. As seen from the UV-vis DRS diagram of catalysts with different loading amounts in
Figure 4b, Ag/Pd/m-BiVO
4 (2 wt%, 2:1) strongly absorbed visible light, which may be related to the synergy between the carrier and the precious metal.
Figure 4c shows the band gap energy (E
g) spectra of m-BiVO
4 and Ag/Pd/m-BiVO
4 (2 wt%, 2:1). The E
g of these samples can be calculated based on the UV-vis absorption spectra data by using the equation αhν = A(hν-E
g)
n/2, where α, h, ν, A and Eg represent the absorption coefficient, Planck’s constant, the frequency, a constant and the optical band gap energy, respectively [
11]. The value of n depends on the type of optical transition of the semiconductor. As a direct transition semiconductor, m-BiVO
4 has an n value of 1. The Eg values of the two catalysts are approximately 2.40 eV and 2.17 eV, respectively. When Ag and Pd are loaded on the surface of m-BiVO
4, the band gap of the prepared composite becomes narrower, thus increasing the visible light response of the prepared sample [
12,
32].
2.5. XPS Analysis
Figure 5b–f show the XPS analysis of Bi 4f, V 2p, O 1s, Pd 3d and Ag 3d. The binding energy and element composition of Ag/Pd/m-BiVO
4 (2 wt%, 2:1) nanocomposites were studied. As shown in
Figure 5a, the results show that the main components of the prepared sample are Bi, V, O, Pd and Ag.
Figure 5b shows the 4f high-resolution XPS spectrum of Bi. It can be seen from the spectrum that the characteristic peaks at 158.5 eV and 163.8 eV correspond to the binding energies of Bi 4f
7/2 and Bi 4f
5/2, respectively.
Figure 5c shows a high-resolution XPS spectrum of V 2p, in which the main peaks at 516.0 eV and 523.5 eV correspond to V 2p
3/2 and V 2p
1/2.
Figure 5d shows a high-resolution XPS spectrum of O 1s, which can be divided into three peaks of 529.2 eV, 531.5 eV and 533.1 eV, with the main peak at 529.2 eV corresponding to the lattice oxygen of Bi
2O
22+, the main peak at 531.5 eV corresponding to the acidic oxygen of VO
42- and the 533.1 eV peak corresponding to hydroxyl groups on the sample surface or H
2O adsorbed on the surface [
21].
Figure 5e shows a 3d high-resolution XPS spectrum of Pd, in which the main peaks at 334.5 eV and 339.9 eV are attributed to Pd 3d
5/2 and Pd 3d
3/2, which correspond to the zero valence binding energy of the metal state of Pd.
Figure 5f shows two characteristic peaks at 367.2 eV and 373.2 eV are attributed to Ag 3d
5/2 and Ag 3d
3/2, which correspond to the zero valence binding energy of the metal state of Ag [
30,
33]. The experimental results show that Ag and Pd are successfully loaded on the surface of m-BiVO
4 in the form of metals.
Figure S1 shows the XPS analysis of Bi 4f, V 2p, O 1s, and Pd 3d. The binding energy and element composition of Pd/m-BiVO
4 (2 wt%) nanocomposites were studied. As shown in
Figure S1a, the results show that the main components of the prepared sample are Bi, V, O, and Pd.
Figure S1b shows the 4f high-resolution XPS spectrum of Bi. It can be seen from the spectrum that the characteristic peaks at 158.6 eV and 163.9 eV correspond to the binding energies of Bi 4f
7/2 and Bi 4f
5/2, respectively.
Figure S1c shows a high-resolution XPS spectrum of V 2p, in which the main peaks at 516.1 eV and 523.7 eV correspond to V 2p
3/2 and V 2p
1/2.
Figure S1d shows a high-resolution XPS spectrum of O 1s, which can be divided into three peaks of 529.3 eV, 531.5 eV and 533.1 eV, with the main peak at 529.3 eV corresponding to the lattice oxygen of Bi
2O
22+, the main peak at 531.5 eV corresponding to the acidic oxygen of VO
42- and the 533.1 eV peak corresponding to hydroxyl groups on the sample surface or H
2O adsorbed on the surface [
21].
Figure S1e shows a 3d high-resolution XPS spectrum of Pd, in which the main peaks at 334.7 eV and 334.1 eV are attributed to Pd 3d
5/2 and Pd 3d
3/2, which correspond to the zero valence binding energy of the metal state of Pd [
30,
33]. The experimental results show that Pd is successfully loaded on the surface of m-BiVO
4 in the form of metals. The comparison between the XPS results of Pd/m-BiVO
4 (2 wt%) and the XPS results of Ag/Pd/m-BiVO
4 (2 wt%, 2:1) in
Figure 5 shows that the binding energies of the elements of monometallic material Pd/m-BiVO
4 (2 wt%) and bimetallic material Ag/Pd/m-BiVO
4 (2 wt%, 2:1) have slight change, which also proves that bimetallic material Ag/Pd/m-BiVO
4 (2 wt%, 2:1) has been successfully prepared.
2.8. Catalytic Activity
The reaction formula for the selective oxidation of benzyl alcohol to benzaldehyde is shown as follows:
The activities of the catalysts were tested in a photochemical reaction box. The selective oxidation of benzyl alcohol into benzaldehyde was carried out continuously for 12 h, including light reaction and corresponding dark reaction experiments. The conversion rate and selectivity of the reactants and products were analyzed by GC. All the experiments are performed twice, and the average values are calculated.
Table 1 shows the effects of eight solvents with different polarities on the selective oxidation of benzyl alcohol. The reactions were carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrates included 50 mg of Ag/Pd/m-BiVO
4 (1 wt%, 1:1) catalyst, 1.0 mmol of KOH, 2 mmol of benzyl alcohol and 6 mL of solvent.
From the data in
Table 1, it can be seen that the conversion rate and selectivity of the photoreactions are much higher than those of the dark reactions. With a decrease in the polarity of the solvent, the conversion rate of benzyl alcohol increased first and then decreased. When the polarity of the solvents was reduced to less than 0.2, the selectivity of benzaldehyde was reduced to less than 90%. When toluene was used as the solvent, the best conversion rate of benzyl alcohol was 70.7%, and the selectivity of benzaldehyde was over 99%.
Table 2 shows the effects of different types and amounts of alkali on the selective oxidation of benzyl alcohol. The reactions were carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrate included 50 mg of catalyst Ag/Pd/m-BiVO
4 (1 wt%, 1:1), 2 mmol of benzyl alcohol, 6 mL of toluene and a certain amount of base.
The experiment explored the influence of seven kinds of base with different strengths and amounts on the reactions. The conversion rate with a strong base was significantly higher than that with a weak base, and the conversion rate of the light reactions was much higher than that of the dark reactions. In this experiment, the base can neutralize part of H+ in the reaction environment, and the base can promote oxidation of hydroxyl hydrogen to aldehyde, thus promoting the progress of photocatalytic reaction. When NaOH was selected as the reaction substrate, the conversion rate was the best, and the amount of NaOH used was optimized. The results showed that with increasing alkali content, the conversion first increased and then decreased. When the amount of NaOH added was 1.0 mmol, the conversion rate reached 75.9%, which may be due to the presence of excessive alkali occupying the active sites of the catalyst itself, thus affecting the reaction. The choice of base is based on specific experiments, including the choice of photocatalyst and the type of reactant as well as the specific environment of the reaction.
Table 3 shows the effects of different catalysts on the selective oxidation of benzyl alcohol. The reaction was carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrates included 50 mg of catalysts, 1.0 mmol of NaOH, 2 mmol of benzyl alcohol and 6 mL of toluene.
The conversion rate of m-BiVO
4 loaded with noble metal was higher than that of pure m-BiVO
4 under visible light irradiation, and the conversion rate of Ag and Pd bimetal loaded was higher than that of the catalysts loaded with single metal in
Table 3. The conversion rate of benzyl alcohol on Ag/Pd/m-BiVO
4 (2 wt%, 2:1) was the best (89.9%). Both the loading ratio and the total loading amount of noble metals affect the conversion rate of the reaction, which may be determined by the interaction of metals and the relationship between the loading amount of noble metals and the specific surface area of the carrier m-BiVO
4.
Table 4 shows the effects of different catalyst dosages on the oxidation of benzyl alcohol. The reactions were carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrates included 1.0 mmol of NaOH, 2 mmol of benzyl alcohol, 6 mL of toluene and different amounts of Ag/Pd/m-BiVO
4 (2 wt%, 2:1) catalyst.
When the amount of catalyst added was 50 mg, the conversion rate was the best in
Table 4. When the amount of catalyst used is small, it cannot generate enough photogenic charge carriers and cannot provide enough specific surface area, resulting in the lack of active substances and active sites involved in the reaction. When the amount of catalyst used is large, a large amount of photocatalyst is suspended in the reaction liquid, which scatters the incident light, thus affecting the reaction [
27].
Table 5 shows the effects of different illumination times on the selective oxidation of benzyl alcohol. The reaction was carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrates included 50 mg of Ag/Pd/m-BiVO
4 (2 wt%, 2:1) catalyst, 1.0 mmol of NaOH, 2 mmol of benzyl alcohol and 6 mL of toluene. With increasing illumination time, the conversion rate of benzyl alcohol increased. With an illumination time of more than 12 h, the conversion rate of benzyl alcohol increased slightly. Therefore, 12 h is the best illumination time.
To study the selective oxidation of different alcohol derivatives by Ag/Pd/m-BiVO
4 (2 wt%, 2:1), 13 kinds of alcohol derivatives were selected. The reaction was carried out under a light source with an intensity of 3.0×10
−2 W/cm
2. The reaction substrates included 50 mg of Ag/Pd/m-BiVO
4 (2 wt%, 2:1) catalyst, 1.0 mmol of NaOH, 2 mmol of alcohol derivatives and 6 mL of toluene. The results are shown in
Table 6.
The catalyst has a poor catalytic effect on saturated aromatic alcohols or fatty alcohols with branched chain C, which may be because the hydroxyl hydrogen of such alcohols is not easy to separate. In addition, due to the large steric resistance of secondary aromatic alcohols or fatty alcohols, the catalytic effect of this catalyst on such alcohols is also not good. If an electron-donating group is attached to the para-position of a benzyl alcohol, the catalytic activity of the catalyst is not good, but if an electron-withdrawing group is attached to the para-position, the catalyst has good catalytic activity, which may be because the electron-donating group does not facilitate separation of the hydroxyl H.
In short, the Ag/Pd/m-BiVO4 (2 wt%, 2:1) nanocomposite has strong catalytic activity for a class of aromatic alcohols with electron-withdrawing groups during the selective oxidation of alcohol derivatives to corresponding aldehydes and has poor catalytic activity for saturated fatty alcohols and secondary alcohols.