Understanding selective enhancement by silver during photocatalytic oxidation

Abstract

The superiority of silver deposited titania particles over bare titania particles for the photocatalytic oxidation of selected organic compounds is explained: the presence of silver mainly enhances the photocatalytic oxidation of organic compounds that are predominantly oxidised by holes, while it has only an insignificant effect on those organic compounds that require hydroxyl radicals for their mineralisation.

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The quantum efficiencies of the photocatalytic mineralisation of organic pollutants on commercial bare titania is very low (<1%) owing to the rapid recombination of photogenerated electrons and holes created within semiconductor particles.¹ There have been extensive studies to improve the TiO₂ photocatalytic activity including deposition of noble metals (e.g. Ag or Pt) on the TiO₂ surface.^2–12 However, the results obtained are not universal. Metallisation of TiO₂ with silver was reported to significantly increase the photocatalytic degradation/oxidation rates of oxalic acid,² sucrose,³ methyl orange,⁴ 2-chlorophenol,⁵ and phenol, ranging from 50 to 500%. The degradation/oxidation of 4-chlorophenol,⁶ 1,4-diclorobenzene,⁷ and various dyes,^8–10 however, was only mildly enhanced (less than 30%) in the presence of silver. Insignificant effects were reported for malic acid¹¹ and salicylic acid.³

The reactor configuration, experimental conditions (including type of titania or silver precursor), organic concentrations, and dissolved oxygen levels in photocatalyst suspensions could play a role in controlling the performance of Ag/TiO₂. For example, silver deposits increased the mineralisation rate of 2-propanol when rutile TiO₂ was used whereas it had no effect for anatase TiO₂.^13,14 Dobosz and Sobczynski¹⁵ and Liu et al.¹⁶ both reported silver deposits improved the efficiency of anatase up to 300% for the photocatalytic oxidation of phenol^15,16 at silver loadings of 0.5 and 1 wt%, respectively. On the contrary, Vamathevan et al.¹⁷ observed a detrimental effect at a silver loading of 2.5 wt% for the same reaction. Lee et al.⁷ also reported that Ag/TiO₂ prepared from AgF showed a better performance for the mineralisation of 2,4-dichlorophenol compared to that prepared from AgNO₃.

Regardless of the conflicting conclusions in the literature, the question of the role and selectivity of silver in enhancing the photocatalytic degradation and/or mineralisation of organic compounds by TiO₂ remains unresolved. In order to gain an in depth understanding at a molecular level on the role of Ag, the photocatalytic mineralisation of 16 organic compounds was investigated. Degussa P25 was used as the source of TiO₂ and a 0.1 M AgNO₃ solution was used as the precursor for preparing 2 atom% Ag/TiO₂, which has been reported as the optimum loading of silver when prepared by the photodeposition method.³ In all experiments, the catalyst loading and the concentration of organic compounds were kept constant at values of 1 g l⁻¹ and 2000 µg carbon, respectively. Suspension pH was adjusted to 3.0 ± 0.5 using perchloric acid.

The selected organic compounds were divided into groups, namely the saccharides (sucrose, glucose and fructose), carboxylic acids (maleic acid, oxalic acid, dichloroacetic acid, formic acid, citric acid, malonic acid, acetic acid, and iso-butyric acid), alcoholic (methanol and hexan-1-ol) and aromatic organics (salicylic acid, phenol and 2,4-dichlorophenol), and photocatalytic mineralisation rates of these groups to carbon dioxide and water were assessed. Fig. 1 shows the mineralisation rates (at 50% conversion to CO₂) for TiO₂ and Ag/TiO₂.

Fig. 1 Comparison of the mineralisation rates of 16 organic compounds by bare TiO₂ and 2 atom% Ag/TiO₂.

From Fig. 1 the silver deposits did not enhance the mineralisation of organic compounds possessing alcoholic or aromatic structures. Alternatively, silver provided an enhancing effect for the saccharide group, increasing the glucose, fructose and sucrose mineralisation rates by factors of 7.2, 8.0 and 5.7, respectively. Similarly, the mineralisation of carboxylic acids, including oxalic acid and formic acid, were enhanced 3.7 and 2.9 times by the presence of silver deposits. Silver only marginally enhanced the mineralisation rates of acetic acid and dichloroacetic acid, and had no significant effect on the mineralisation of iso-butyric acid.

Initial examination of the results shows that positive effects of silver on the photocatalytic oxidation by TiO₂ can be related to the number of C–H, C–O and C=O bonds in the molecular structure of each organic compound. From Table 1, apart from acetic acid, mineralisation is enhanced by silver deposits if the number of total C–O and C=O bonds is comparable or greater than the number of C–H bond in the molecule. It can be seen that the rates of mineralisation of sucrose, maleic acid, oxalic acid, formic acid, malonic acid, acetic acid and dichloroacetic acid were enhanced by factors of 5.7, 4, 3.7, 2.9, 2.2, 1.8, 1.3 and 1.2, respectively. No enhancement or detrimental effects were observed for iso-butyric acid, the aromatics phenol, salicylic acid, and 2,4-dichlorophenol, or alcohols methanol, hexan-1-ol, where the number of C–H bond in each molecule outnumbered the sum of C–O and C=O bonds.

Table 1 Numbers and ratios of C–H, C–O and C=O bonds in organic compounds and photocatalytic enhancement provided by Ag/TiO₂

Organic compounds	Number of C–O & C=O bonds (1)	Number of C–H bonds (2)	Ratio of (1)/(2)	Relative rate enhancement
Sucrose	14	14	1.00	5.7
Glucose	6	7	0.86	7.2
Fructose	7	7	1.00	8.0
Maleic acid	4	2	2.00	4.0
Oxalic acid	4	0	4.00	3.7
Formic acid	2	1	2.00	2.9
Citric acid	7	4	1.75	2.2
Malonic acid	4	2	2.00	1.8
Acetic acid	2	3	0.67	1.3
Dichloroacetic acid	2	1	2.00	1.2
Iso-butyric acid	2	7	0.29	Nil
Phenol	1	5	0.20	Nil
Salicylic acid	3	4	0.75	Nil
Methanol	1	3	0.33	Nil
Hexan-1-ol	1	13	0.08	Nil
2,4-dichlorophenol	1	3	0.33	Nil

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Sucrose, glucose and fructose do not appear to match the observed rule as their total number of C–O and C=O bonds are equal to or less than the number of C–H bonds but significant positive effects of silver were seen. However, these “large” compounds that have high numbers of C–O and C=O bonds are degraded to short chain carboxylic acid intermediates. Intermediates of sucrose, glucose and fructose have been reported mainly to be aliphatic carboxylic acids,^18,19 such as arabonic acid (6 C–O and C=O bonds, and 5 C–H bonds) and gluconic acid (7 C–O and C=O bonds, and 6 C–H bonds). The molecular structures of these intermediates agree with the observed correlation.

Carraway et al.²⁰ reported there were different oxidising entities responsible for breaking different bonds in a molecule. Photocatalytic oxidation may proceed via H-atom abstraction by (surface adsorbed or diffused) hydroxyl radicals or direct electron transfer to the holes in the valence band of TiO₂.²⁰ Mills and Hoffmann²¹ reported selected halogenated aromatic compounds to be photocatalytically oxidised by hydroxyl radicals whereas Mao et al.²² observed carboxylic acids such as trichloroacetic acid and oxalic acid (which contain no C–H bond in their structure) were oxidised primarily by valence band holes on TiO₂via a photo-Kolbe process. Mao et al.'s findings agree with the results obtained by Enriquez et al.²³ who reported the degradation of dichloroacetic acid can only be initiated by direct interaction with TiO₂. Based on these reports and results from the present study, the enhancement observed in the Ag/TiO₂ system for the photooxidation of sucrose, glucose and fructose, and some carboxylic acids is thought to be attributed to attack by the photogenerated holes. Silver deposits generate a greater number of holes by acting as electron sinks and assisting their transfer to species in solution (e.g. dissolved oxygen). This reduces the possibility of electron/hole recombination. Moreover, the presence of silver does not necessarily generate greater numbers of hydroxyl radical as production of these radicals can be limited by the number of hydroxyl groups present on the surface. Hence, silver can only improve the mineralisation of organic compounds which are likely to be oxidised by holes. For this reason, as the degradation of C–H bonds and aromatics rings requires the participation of hydroxyl radicals,^24,25 the mineralisation of organic compounds, which contain many C–H bonds and/or aromatic rings, will not be enhanced by the presence of silver.

In order for photogenerated holes to be capable of direct oxidation, the organic compounds must be adsorbed on the TiO₂ surface. Subsequently, the enhancement provided by silver is possibly related to the adsorption of carboxylic functional groups on the TiO₂ surface. That is, silver does not promote adsorption of these groups on the TiO₂ surface rather than these groups need to be adsorbed for the beneficial effects of silver to be provoked. Franch et al.²⁶ reported that at pH lower than the point of zero charge (pzc) of TiO₂, oxalic, fumaric and maleic acids adsorbed very well on TiO₂ and oxidised rapidly to carbon dioxide. However, at pH higher than the pzc of TiO₂, the rate of oxalic acid oxidation decreased noticeably and both fumaric and maleic acids were only oxidised to formic acid. This suggests the carboxylic functional groups, which are well-adsorbed on the TiO₂ surface at low pH,²⁶ can be efficiently oxidised by holes via a photo-Kolbe process. Therefore, under low pH conditions the mineralisation of short chain carboxylic acids such as formic and oxalic acid will be converted into CO₂ much faster by the surplus of holes in the Ag/TiO₂ system. For longer chain carboxylic acids with high number of C–H bonds such as iso-butyric acid or aromatic carboxylic acids such as salicylic acid, after the initial decarboxylation, hydroxyl radicals are required to degrade the C–H containing or phenolic intermediates. Hence, the presence of silver will not produce any significant enhancements of the mineralisation process.

In conclusion, we found that the enhancing effect of silver deposits on TiO₂ can initially be predicted by the molecular structure of the substrate to be oxidised. The fewer C–H bonds and/or more C=O and C–O bonds a molecule possesses, the more probable the enhancement of mineralisation in the presence of silver. Ongoing work into characterising Ag/TiO₂ and identifying intermediates is being conducted to draw a clearer picture of the photocatalytic reactions. Greater discussion on adsorption and reaction mechanisms and exceptions to the rule will be provided in a future correspondence.

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