Impact of H₂O₂ on the Lactic and Formic Acid Degradation in Presence of TiO₂ Rutile and Anatase Phases under UV and Visible Light

Abstract

The degradation rates of formic acid and lactic acid in the presence and absence of H₂O₂ were studied, utilizing several TiO₂ catalysts: PC105 (100% anatase), MPT 625 (100% rutile), and P25 (80% anatase/20% rutile), and the results were discussed with regards to the current literature. The impact of hydrogen peroxide on the photocatalytic efficiency of eleven TiO₂ samples was then determined, using commercial anatase structures (PC105, PC500, UV100), commercial mixed anatase/rutile (P25 and P90), and six rutile (two commercial samples: MPT 625 and C-R160, and four home-made rutile samples were synthesized by TiCl₄ hydrolysis). The effect of catalyst surface area and TiO₂ phase on the degradation rate of lactic acid (LA) and the decomposition of H₂O₂ was studied and discussed in regard to the active species generated. The intermediate products formed in the absence and presence of H₂O₂ were also an important factor in the comparison. Finally, the efficiency of the degradation of LA and formic acid (FA) in the presence of rutile and H₂O₂ was determined under visible light, and their reactivity was compared. The intermediate products formed in the degradation of LA were identified and quantified and compared to those obtained under UV (Ultra-Violet).

1. Introduction

Several publications [1,2,3,4,5,6,7,8,9,10,11,12,13,14] have mentioned the impact that H₂O₂ addition has on the degradation of different organic compounds in the presence of TiO₂. Most studies performed using P25 TiO₂ found that H₂O₂ has a favorable impact. It was explained by the elevated hydroxyl radical production, either due to hydrogen peroxide’s reaction with conduction band electrons, or to indirect formation via O₂°⁻, which is generated by the reduction of water and is able to avoid electron–hole recombination (Equations (1) and (2)).

H₂O₂ + e⁻ → OH° + OH⁻

(1)

H₂O₂ + O₂⁻° → OH° + OH⁻ + O₂

(2)

However, some researchers have also shown an unfavorable effect of H₂O₂ on TiO₂ [2,3] which is explained by the competition between H₂O₂ and a pollutant for the adsorption sites. H₂O₂ is also proposed to be in competition with photoproduced holes, since H₂O₂ competes with the reaction of water (Equation (3)), which limits the formation of OH radicals (Equations (3) and (4)):

H₂O₂ + h⁺ → H⁺ + HO₂^°

(3)

H₂O + h⁺ → H⁺ + OH^°

(4)

To our knowledge, the impact of anatase and rutile phase on the UV degradation of organic molecules in the presence of H₂O₂ has not been well studied, and the interpretation of the results is still under debate [1,15]. H₂O₂ seems to always have a higher impact in the presence of rutile TiO₂ under UV irradiation. While differences in the nature of peroxo-complex formation on the surface of anatase and rutile phases was suggested by Ohno et al. [1], Tang et al. [15] reported that the difference could be attributed to a heterogeneous reaction on the particle surface of the photocatalyst for anatase TiO₂, and a reaction in solution for rutile TiO₂.

Contrary to the latter hypothesis, it is well known that in the presence of H₂O₂ a complex is formed on the surface of TiO₂, allowing for H₂O₂ decomposition. Moreover, several publications report the decomposition of this peroxo-complex on rutile and anatase phase in the absence of pollutants and show that the decomposition of H₂O₂ is favored on rutile phase. These results are in agreement with the work of Hirakawa et al. [16] and Zhang et al. [17]. Both publications provide evidence of an increase in OH° radical formation by the addition of hydrogen peroxide on rutile and rutile-containing TiO₂, and a decrease in O₂⁻° formation. For anatase TiO₂, a decrease in OH° radical formation and an increased formation of O₂⁻° were observed. However, to our knowledge, no connections have been made in previous works on the impact that pollutants have on these results.

Several other works were also published on the possibility of using visible light to degrade pollutants in the presence of TiO₂ and H₂O₂, and have mentioned the activation of the peroxo-complex formed on the surface of TiO₂ [15,18,19,20,21]. However, no comparison was made between the impact of H₂O₂ on the efficiency of TiO₂ under UV and visible light.

Many other issues remain subject to debate: is the favorable or unfavorable impact of H₂O₂ dependent on the surface area of the catalyst? Is there a correlation between the disappearance rate of organic compounds and the disappearance rate of H₂O₂? What is the impact of H₂O₂ on the formation of intermediate products? What is the efficiency of H₂O₂/rutile under visible light in comparison to UV? Is the mechanism similar under UV and visible activation?

The objective of our work is to try to answer to some of these questions using two pollutants: formic acid and lactic acid. We also plan to utilize several rutile, anatase and mixed phase photocatalysts with varying surface areas under UV irradiation. Finally, we plan to draw a comparison between the H₂O₂/rutile efficiency under UV and visible light using formic acid and lactic acid.

2. Results and Discussion

2.1. Comparison of the Impact of H₂O₂ on Formic and Lactic Acid Photocatalytic Degradation in Presence of P25, PC105 and C-R100

Before studying the impact of H₂O₂ on formic acid and lactic acid degradation, a control experiment was carried out with H₂O₂ in the dark. No oxidation of formic acid and lactic acid was observed within a 2 h period of darkness at room temperature, indicating that the possible degradation of hydrogen peroxide in the absence of light can be ruled out for all experiments. Similarly, control experiments using UV irradiation in the absence of a photocatalyst (while maintaining constant pollutant and H₂O₂ concentrations) confirm the lack of photolysis.

The impact of H₂O₂ on lactic acid and formic acid was then determined in the presence of three commercial TiO₂ samples: PC105, C-R100 and P25—a commonly used reference in photocatalysis. The disappearance rate of these two organic compounds was represented as a function of time in the presence and absence of H₂O₂ (Figure 1).

Regardless of which organic compound was used, the same behavior was observed: a significant improvement of the degradation rate in the presence of rutile phase TiO₂ (Figure 1a,c,d,f), and no impact or a slightly negative impact in the presence of pure anatase phase.

Few works have studied the impact of the anatase and rutile phase on the degradation of organic molecules in the presence of H₂O₂, and the interpretation of their results is still subject to debate [1,15]. In all cases, the authors found that H₂O₂ had a greater impact in the presence of rutile TiO₂ and under UV irradiation, which agrees with our results. While Ohno et al. [1] suggested that the difference is due to the nature of the H₂O₂ complex formed, Tang et al. [15] suggested that photodegradation catalyzed by rutile TiO₂ occurs mainly in the solution, but takes place on the surface of the photocatalyst when anatase TiO₂ is present.

Further explanations have been published using experiments performed in the absence of pollutant [16,17,22], which show a difference in the nature of Reactive Oxygen Species (ROS) generation on the two TiO₂ phases in the presence of H₂O₂. The authors found that when H₂O₂ is utilized, hydroxyl radicals (OH°) are primarily generated on rutile TiO₂, while in the presence of anatase TiO₂, hydroperoxide radical (HO₂°) (a much fewer active species) formation predominates [16,17]. These differences are explained by the formation of alternate H₂O₂ complexes on TiO₂: Ti–η²-peroxide on the surface of rutile, and Ti–μ-peroxide on the surface anatase [17]. Although Density Functional Theory (DFT) analyses indicate that for both phases the more favorable structure of the Ti–peroxo-complex is Ti–O–O–H [23,24].

The differences in the reduction and oxidation properties of these two phases were suggested by our previous results [22] and evaluated by the determination of conduction band energies [25]. It was found that the Conduction Band (CB) edge of rutile TiO₂ is localized at a lower potential than the CB of anatase, inducing a stronger reducing potential for rutile TiO₂. On the surface of rutile TiO₂, a direct reduction of H₂O₂ or indirect reaction between O₂°⁻ and H₂O₂ (Equations (1) and (2)) would likely occur. While on anatase TiO₂, due to its stronger oxidizing potential, H₂O₂ is primarily oxidized into HO₂°⁻ (Equation (3)) in competition with water (Equation (4)).

According to these various studies, the greater impact of H₂O₂ on rutile phase TiO₂ can be explained by a higher production of OH° due to differences in the oxydo-reduction properties of rutile and anatase.

2.2. Impact of H₂O₂ on the Photocatalytic Degradation of Lactic in Presence of Different TiO₂ Rutile, TiO₂ Anatase and Mixture of These Both Phases

The impact of hydrogen peroxide on lactic acid (LA) degradation was determined in the presence of two commercial rutile, three commercial anatase, and two mixed phase TiO₂ samples. Experiments were also conducted using four home-made rutile samples. The LA degradation rates obtained in the presence and absence of H₂O₂ are reported in Figure 2a.

As formerly observed and discussed in the previous paragraph, a substantially positive impact on rutile TiO₂ (C-R160 is an exception) was observed, while no impact or a slightly negative impact was observed for the pure anatase phase, shown in Figure 2b by representing the ratio of the LA degradation rate with and without H₂O₂.

In addition to differences between the rutile and anatase phases, some differences are also observed within the same phase. H₂O₂ had a significantly positive impact on our two home-made TiO₂ rutile samples, which were both calcined after 2 and 48 h of hydrolysis. Improvement factors of 10 and 18 were found, respectively. Moreover, the lactic acid degradation rates on these two catalysts in the presence of H₂O₂ are about 1.4 times higher than that of TiO₂ P25, an international reference in the photocatalytic field.

The lack of a positive impact on LA degradation in the presence of C-R160 and its low efficiency are attributed to the presence of surface impurities observed by the release of organic acid in water, and by the presence of about 5% Si. On the one hand, the presence of SiO₂ on TiO₂ surface modifies the adsorption properties of LA due to the ZPC (zero point charge) of silica which is about 2 and favors the recombination of (e⁻, h⁺) pairs, on the other hand, the organic impurities present in C-R160 are degraded in competition with LA degradation.

The slightly improved efficiency of MPT-625 (C-R100) towards lactic acid degradation in the presence of H₂O₂ could be partly attributed to the photo-Fenton reaction caused by the presence of iron in the structure, which was observed by Inductively Coupled Plasma (ICP) and X-ray photoelectron spectrometry (XPS).

Considering our home-made rutile TiO₂ samples, the most efficient catalysts are those which have been calcined at 300 °C. A likely cause is the improvement in crystallinity which favors the formation of OH° radicals. It can also be attributed to a decrease in pore volume. After calcination, the volume of the pores are 2.2 and 1.7 10⁻² cm³g⁻¹ for HM-R2c and HMr-R48c, respectively, while prior to calcination, the pore volumes were 0.7 and 0.9 10⁻² cm³g⁻¹ [18].

Moreover, it is apparent that the larger the surface area, the less lactic acid is degraded in the presence of H₂O₂ (Figure 3). Similar behavior was also observed for the pure anatase phase (Figure 3). While the impact is significant on the rutile phase, in the presence of pure anatase or anatase mixed with 20% rutile TiO_2, an increase in the surface area has much less of an impact.

Considering lactic acid adsorption, the pollutant degradation rate in the presence of H₂O₂ tends to decrease with increasing adsorption; however, this decrease seems to depend on the TiO₂ phase type (Figure 4). This behavior can be explained by an enhancement in the formation of H₂O₂ complexes on the surface of TiO₂ leading to a greater formation of reactive oxygen species (ROS). These results agree with the impact that H₂O₂ degradation has on LA degradation (Figure 5).

Regardless of which TiO₂ phase is used, the degradation of LA in the presence of H₂O₂ is directly correlated to the decomposition of H₂O₂ as observed in Figure 5. This clearly indicates that the degradation of lactic acid is due to the activation of the complex formed between TiO₂ and H₂O₂.

While our results clearly show that LA degradation is correlated to the decomposition of an H₂O₂ complex formed on the surface of TiO₂, the negative impact of the surface area on the LA degradation rate is difficult to understand. Indeed, in the absence of H₂O₂, increasing the surface area of TiO₂ improves pollutant degradation [22,26]. In combination with our results showing the decomposition of H₂O₂ increasing as a function of the surface area, LA degradation should also increase with surface area. However, this is not the case. Moreover, we observe that depending on the nature of the TiO₂ phase, the impact of the surface area differs. A more negative impact was found for rutile TiO₂, whereas the surface area has much less of an impact on anatase TiO₂.

This behavior could be explained by an increase in the deactivation of reactive oxygen species on the surface of TiO₂ with increasing surface area. The varying degree of deactivation on anatase and rutile phases is potentially due to the different active species generated due to the more important amount of H₂O₂ adsorbed on important surface area. Active species initially formed could react with the more important amount of H₂O₂ adsorbed on more important surface area.

On rutile TiO₂, OH° generated by the decomposition of peroxo-complexes can react with H₂O₂ forming HO₂°, a less active species in comparison to OH° (Equation (5)):

OH° + H₂O₂ → HO₂° + H₂O

(5)

While HO₂°, which is generated in the anatase phase, can react with H₂O₂ to form OH°, this limits the negative impact of the surface area (Equation (6)):

HO₂° + H₂O₂ → OH° + H₂O + O₂

(6)

However, it is just a hypothesis which should be verified by determining the active species generated using a high surface area of TiO₂ compared to the small surface area of TiO₂.

2.3. Impact of H₂O₂ on the Chemical Pathways of Lactic Acid Photocatalytic Degradation

Two initial chemical pathways can occur during the degradation of lactic acid: either decarboxylation or dehydrogenation, giving ethanol and pyruvic acid, respectively. Unfortunately, ethanol cannot be detected due to the sensitivity of our analyses, but also due to its reactivity towards hydroxyl radicals. Ethanol behaves as a scavenger of hydroxyl radicals [27]. The pathways were evaluated based on the detection of acetic acid and pyruvic acid.

Regardless of which TiO₂ sample is used, the main product detected in the aqueous phase and in the presence of H₂O₂ is acetic acid (Figure 6b). In the absence of H₂O₂, acetic acid is not initially present for anatase and anatase-containing samples, but is observed in the majority of rutile samples (Figure 6a).

In the presence of H₂O₂, the acetic acid yield is lower for anatase than for samples containing rutile. These results agree with the formation of HO₂°, which is less active than OH°. It is also interesting to note that pyruvic acid formed from the initial dehydrogenation of OH groups appears after about 60% LA conversion for all of the catalysts, except for the two commercial rutile samples where it is detected before 60% (Figure 7a). Its detection is attributed to the partial decomposition of H₂O₂, allowing for the direct adsorption of LA and immediate photocatalytic degradation. In fact, more than half of the H₂O₂ is degraded on a majority of the catalysts, and about 30% is degraded for P25 and P90 samples (Figure 7b).

In the presence of H₂O₂, the absence of pyruvic acid at low LA conversion and the high formation of acetic acid supports the hypothesis that lactic acid undergoes rapid decarboxylation upon the generation of OH° radicals.

2.4. Visible Photocatalytic Efficiency of TiO₂ Rutile and P25 in Presence of H₂O₂

Previously, our results showed that the photocatalytic degradation of LA and formic acid (FA) upon irradiation and in the presence of H₂O₂ and TiO₂ is correlated to the decomposition of H₂O₂. As the H₂O₂ peroxo-complex also absorbs light in the visible region, we tested the degradation of FA and LA using a rutile TiO₂ sample and visible irradiation.

Prior to investigating the impact of H₂O₂ on the photocatalytic degradation of the pollutant under visible light, a control experiment was carried out in the absence of H₂O₂. It is clear that degradation is non-negligible in the presence of rutile TiO₂ (Figure 8). These results are consistent with the visible absorption of rutile TiO₂ (absorbance drops off past 413 nm).

In the presence of H₂O₂, a significant enhancement in the degradation of FA and LA was observed, which was found to correlate directly with the decomposition of H₂O₂ (Figure 8a,b). Moreover, the disappearance rates of FA and LA are the same (6.7 µmol/L/min) with an H₂O₂ disappearance rate of about 28 µmol/L/min. In the case of FA, decarboxylation can occur, whereas with LA, both dehydrogenation and decarboxylation can occur [28]. To track their formation, we studied the intermediate products generated from LA under visible light irradiation. As shown in Figure 9a, in the presence of H₂O₂, only acetic acid was detected using visible light irradiation. This indicates that only decarboxylation occurs, which explains the comparable degradation rates for LA and FA.

In the absence of H₂O₂ and under visible light exposure, the first step is likely to be dehydrogenation to form pyruvic acid (Figure 9b). A similar behavior has been observed in the presence of rutile under UV irradiation [28].

Comparing to the efficiency observed under UV sources, the photocatalytic efficiency under visible irradiation is lower, about seven times smaller than the photocatalytic efficiency observed under UV irradiation at 365 nm. This difference is due to lower absorption of the H₂O₂ complex formed on the surface of the catalyst above 400 nm compared to its absorption under UV but also to the number of photons emitted by these two irradiation sources, about 4.6·10¹⁵ photons/s/cm² under visible light and 7.3 photons/s/cm² under UV light. Concerning the degradation mechanism under UV and visible light, in both cases, it is due to the decomposition of the H₂O₂ complex confirmed by the similar ratio of 4–5 observed between the degradation of organic pollutant and H₂O₂ decomposition and to the main formation of acetic acid under these two irradiation sources.

The efficiency of TiO₂ under visible light in the presence of H₂O₂ has already been mentioned by several authors studying different molecules (terebutylazine [15], Linuron [18], Prometryn [19], salicylic acid [20] and decene [21]). Li et al. [20] propose a mechanism involving photoinduced electron transfer from the surface complexes of Ti(IV)–OOH. Ohno et al [21] considered the possibility of a photochemical reaction involving TiO₂⁻ peroxide. Tang et al. [15] suggests that in the presence of rutile TiO₂ the reaction occurs in the solution, while in the presence of anatase phase, heterogeneous reactions occur. Rao et al [18] also showed that the demethoxylation and demethylation of linuron are the main reactions, and dechlorination and hydroxylation are only minor reactions.

In our conditions, we showed that decarboxylation is the major reaction pathway, favored over dehydrogenation.

3. Materials and Methods

3.1. Chemicals

Formic acid (99%) and lactic acid (80%), for the photocatalytic degradation tests, were supplied, respectively, by Acros Organics (Geel, Belgium) and Sigma Aldrich Chemie S.A.R.L. (Saint-Quentin Fallavier, France). Pyruvic acid (100%) and acetic acid (99.7%), intermediate products formed in the degradation of lactic acid, were supplied by Sigma Aldrich Chemie S.A.R.L. H₂O₂ (50%) was purchased from Acros Organics. Ultrapure water (18 MΩ·cm⁻¹) was used throughout all of the experiments.

3.2. Catalysts

Eleven titanium dioxide samples were used: two commercial TiO₂ samples composed of 80% anatase and 20% rutile (TiO₂ P-25 and TiO₂ P-90) from Evonik, Essen, Nordrhein-Westfalen, two commercial anatase structures (PC105 and PC-500) from Millennium Chemicals (Hunt-Valley, MD, USA), and one (Hombikat UV100) purchased from Sachtleben Chemie GmbH (Duisburg, Germany), two commercial rutile structures (MPT-625 or C-R100, and C-R160) from Ishihara Sangyo Kaisha Ldt (Osaka, Japan) and Nanostructured, respectively, and four home-made TiO₂ rutile catalysts synthesized from TiCl₄ and hydrolyzed for either 2 h or 48 h. The non-calcined samples were named HM-R2 and HM-R48, and the calcined samples were named HM-R2c and HM-R48c. A detailed description of these photocatalysts was given in our previous work [28].

The characterizations of these catalysts are given in

Table 1. Structure, surface area, Iso Electric Point (IEP) and crystallite size of each photocatalyst.

Photo-Catalysts	Structure	SBET (m2·g−1)	Crystallite Size (nm)	IEP	Reference
P25	80% Anatase 20% Rutile	50	21 − 30	7 − 6.4	[29,30]
P90	80% Anatase 20% Rutile	90	14	7− 6.6	[29,30]
PC 105	100% Anatase	88	15–25	4.7 +/− 0.5	[28]
PC 500	100% Anatase	340	5–10	6.2	[29]
UV 100	100% Anatase	300	<10	5.3	[30]
C-R100 (MPT 625)	100% Rutile	103	13	5.4 +/− 0.5	[28]
C-R160	100% Rutile	160	8–10	5.1 +/− 0.5	[28]
HM-R2	100% Rutile	173	7.5	4.3	[28]
HM-R2c	100% Rutile	112	9.9	3.5	[28]
HM-R48	100% Rutile	117	10.5	4.4	[28]
HM-R48c	100% Rutile	92	12.8	3.6	[28]

.

3.3. Photocatalytic Experiments and Analytical Procedures

The photocatalytic experiments were conducted using an aqueous solution of about 1000 µM lactic acid (LA) or formic acid (FA) and 5000 µM H₂O₂. The reactions were carried out in Pyrex photoreactors. For all degradation experiments, 1 g·L⁻¹ of photocatalyst was used. A PLL18W Philips lamp and an HPK 125 W mercury lamp (Koninklijke Philips N.V., Amsterdam, The Netherlands) with a filter cutting all wavelengths above 340 nm were used for the LA and FA experiments, respectively. In all cases, the UV irradiation was centered at 365 nm. A white LED emitting from 400 to 880 nm was used for visible irradiation. Prior to UV irradiation, the catalyst suspensions were allowed to reach adsorption equilibrium by stirring in the dark for 1 h. Samples of the reaction solution were taken periodically for several hours during UV or visible irradiation, filtered on a DURAPORE 0.45 µm hydrophilic membrane (Merck Millipore, Burlington, MA, USA) to remove the photocatalyst, and used for further analysis.

3.4. Analytical Procedure

H₂O₂ was complexed with an acidic solution of TiCl₄. Then, the H₂O₂ content of each sample was monitored at 410 nm by performing UV–vis spectroscopy in order to detect the yellow complex which forms under acidic conditions in the presence of Ti⁴⁺ ions [31,32].

The samples were analyzed with a Shimadzu (Shimadzu Corporation, Kyoto, Japan) High Performance Liquid Chromatograph (HPLC) equipped with a Coregel-87H3 column (300 mm × 7.8 mm—Concise Separations) thermo-stated at 30 °C. A H₂SO₄ (5 × 10⁻³ mol·L⁻¹) mobile phase was used at a flow rate of 0.7 mL·min⁻¹. A diode array detector was used and set at 210 nm.

4. Conclusions

When studying the impact of H₂O₂, we found that utilizing it in combination with rutile TiO₂ greatly favors the oxidation of FA and LA. No additional improvement was observed in the presence of anatase TiO₂ under the same conditions. The higher the amount of rutile in the TiO₂ photocatalyst, the more significant the improvement.

By using 11 commercial and home-made TiO₂ samples to study the degradation of LA, we also showed that the improvement depends on the surface area of TiO₂. Opposing our results observed in the absence of H₂O₂, an increase in the surface area is harmful to the degradation. However, regardless of which TiO₂ sample is used, the degradation of LA is always correlated to the decomposition of H₂O₂.

The larger the surface area, the less lactic acid is degraded in the presence of H₂O₂. While the impact is significant in the rutile phase, in the presence of pure anatase or anatase mixed with 20% rutile TiO₂, an increase in surface area has much less of an impact. This behavior could be explained by an increase in the deactivation of ROS on the surface of TiO₂ with increasing surface area. The varying degrees of deactivation on anatase and rutile phases are potentially due to the different active species generated. On rutile TiO₂, OH° is generated by the decomposition of peroxo-complexes. It can then react with H₂O₂ forming HO₂°, a less active species in comparison to OH°. While HO₂°, which is generated in the anatase phase, can react with H₂O₂ to form OH°, this limits the negative impact of the surface area.

Our work also highlights a modification in the chemical pathways of LA in the presence of H₂O₂. Regardless of which TiO₂ sample is used, the formation of acetic acid was favored suggesting a promotion of the decarboxylation reaction over the dehydrogenation reaction.

Our investigation into the performance of H₂O₂ on rutile TiO₂ exposed to visible irradiation indicates that, for both UV and visible light, the mechanism of H₂O₂/TiO₂ light-driven photocatalysis is ascribed to the chemisorption of H₂O₂ on the surface of TiO₂ and the subsequent formation of a yellow surface complex, which is decomposed into ROSs. This mechanism is confirmed by the existence of a strong correlation between the decomposition of H₂O₂ and degradation of LA/FA under visible and UV irradiation. Moreover, we highlight that the photocatalytic degradation rates of FA and LA under visible light are identical, indicating that the decarboxylation reaction is the main pathway in the degradation of LA under visible light. This is also in agreement with our observation of acetic acid formation alone, whereas in the absence of H₂O₂, the first step is likely dehydrogenation.

These studies show that visible light, and consequently, solar light, can be efficiently used for removing some organic pollutants by photocatalysis in the presence of H₂O₂, and also by utilizing rutile TiO₂. It could potentially also be applicable for valorization reactions. In the future, it will be insightful to investigate the degradation of different families of molecules and their chemical pathways under visible and/or UV irradiation.