Influence of Gold on Hydrotalcite-like Compound Catalysts for Toluene and CO Total Oxidation

X6Al2HT500 hydrotalcites, where X represents Mg, Fe, Cu or Zn were synthetized and investigated before and after gold deposition for toluene and CO total oxidation reactions. The samples have been characterized by specific areas, XRD measurements and Temperature Programmed Reduction. Concerning the toluene total oxidation, the best activity was obtained with Au/Cu6Al2HT500 catalyst with T50 at 260 °C. However, catalytic behavior of Au/X6Al2HT500 sample in both reactions depends mainly on the nature of the support.


Introduction
Volatiles Organic Compounds (VOCs) often cause breathing problems, many of them are carcinogen and they participate in the formation of tropospheric ozone.These VOCs are mainly issued from exhaust of industrial and transport activities.Due to the harm caused to the human environment, the VOCs' emissions are strictly regulated.Among many technologies available for VOCs control, the catalytic oxidation of these pollutants to carbon dioxide and water has been recognized as one of the most promising technologies [1,2].Complete catalytic oxidation with noble metal supported on solids is an effective way of VOCs removal at low temperature [3][4][5].Moreover, during the past decade, many studies have established that nanosized Au-on-reducible support has a remarkable catalytic activity for many oxidation reactions [3][4][5][6][7][8][9].In a previous work, we have shown that the gold catalyst supported on Ce-Ti mixed oxide prepared by deposition-precipitation method provides a small gold particle size and high catalytic activity in propene oxidation [10,11].However, in the literature [12], few works have discussed the gold deposition on hydrotalcite for VOCs total oxidation.Thus, an interesting way to obtain mixed oxide catalysts is the use of hydrotalcite precursors.Hydrotalcite-like compounds (HT), a class of layered double hydroxides, consist of positively charged metal hydroxide layers separated from each other by anions and water molecules.The general formula of HT compounds is [M II  1 x− where M II and M III are divalent and trivalent metal ions and A n− is an interlayer n-valent anion.Typically, the x value in hydrotalcites is in the range from 0.20 to 0.33 [13,14].The layers contain metal cations of at least two different oxidation states [13,14].Indeed, after calcination treatment, mixed oxides are formed and possess unique properties like high specific surface area and porosity, good thermal stability, good mixed oxides homogeneity basic properties and high metal dispersion [15][16][17][18][19].The partial or total substitution of Mg 2+ and Al 3+ is possible by a divalent or trivalent cation in HT structure.It seems interesting to replace the bivalent Mg 2+ by another efficient element in order to achieve improved toluene and carbon monoxide oxidation.
The copper oxide catalysts are known to be active for the combustion reaction [20][21][22][23].In fact, CuO/Al 2 O 3 has shown excellent performance for the total oxidation of CO [20], propene and toluene [21,22].The iron oxide catalysts are known to be an active solid and support for the CO oxidation [23].These oxides are recognized to be a good gold support for the VOC oxidation, in particular for the methanol, propanol and toluene oxidation [24].The gold supported on the zinc oxide was already reported as a good catalyst for the benzene, toluene and xylene oxidation [25].This solid was reported as having high activity for these reactions compared to the Au/MgO and Au/Al 2 O 3 .
The aim of the present study is to investigate the effect of gold deposition on X 6 Al 2 HT hydrotalcites (where X represents Mg, Fe, Cu or Zn) for toluene and carbon monoxide total oxidation reactions.The gold-based catalysts and the supports were characterized by powder X-Ray Diffraction (XRD), BET measurements, UV-visible diffuse reflectance (DR-UV-vis) spectroscopy and hydrogen temperature programmed reduction (H 2 -TPR).The gold-based compounds and the mixed oxide derived from HT are tested in catalytic total oxidation of toluene and CO.

Characterizations of Gold Catalysts and Supports
Table 1 presents the BET specific surface areas of the samples before and after gold deposition.The gold content of the catalysts is mentioned too.After gold deposition, a low decrease of the specific area is observed for all solids.The gold losses in case of Fe 6 Al 2 HT500 were explained because the pH of the solution was higher than IEP of iron oxide in order to deposit gold in the form of Au(OH) 3 .In this case, the support is likely to be negative, which leads it to adsorb only the gold cation form and leads to an increase in the losses of gold [3,8].In order to investigate the structure of the samples, XRD patterns were performed (Figure 1).First of all, it could be seen that for the four mixed oxides prepared by the hydrotalcite route, several oxide structures were revealed.Thus, for the compound with magnesium (Mg 6 Al 2 HT500) or copper (Cu 6 Al 2 HT500), a phase of metal oxide is observed: MgO periclase (JCPDS-ICDD 45-0946) and CuO Tenorite phase (JCPDS-ICDD 48-1548), respectively.Concerning the compound containing iron (and Fe 6 Al 2 HT500), the hematite phase (Fe 2 O 3 (JCPDS-ICDD 48-1548)) and magnetite (Fe 3 O 4 (JCPDS-ICDD 48-1548)) are observed in the solid, while for the zinc solids (Zn 6 Al 2 HT500), two phases are observed: Zincite phase (ZnO (JCPDS-ICDD 36-1451)) and Gahnite phase (ZnAl 2 O 4 (JCPDS-ICDD 05-0669)).After gold loading, it could be observed that gold deposition affected the oxide crystallinity only for the Au/Fe 6 Al 2 HT500.Indeed, for this compound, the Fe 3 O 4 phase was not observed and a decrease of intensity of the others patterns was revealed.An interaction between gold and iron species seems to have occurred.However, concerning the others mixed oxide, same oxide structure of the support has been observed with an increase of the X-ray patterns intensity.For the gold based solids, it is notable that the X-ray diffraction patterns of metallic gold (JCPDS-ICDD 04-0784) at 2θ = 38.2°,44.4°, 64.5° and 77.5° are very intense in the case of Au/Mg 6 Al 2 HT500 while catalysts with copper (Au/Cu 6 Al 2 HT500) and zinc (Au/Zn 6 Al 2 HT500) have broad and weak peaks.As regards the compound with iron, no diffraction peak for gold metallic is visible which could be due to this low content in the sample.The difference in the intensity gold diffraction peak between Au/Cu 6 Al 2 HT500, Au/Zn 6 Al 2 HT500 and Au/Mg 6 Al 2 HT500 could be explained by the size of the gold crystallites.An estimation of gold crystallite size was only performed in the case of Au/Mg 6 Al 2 HT500 using the Scherrer Equation with the diffraction peak at 2θ = 38.2°.This gold crystallite size was estimated at 15.3 nm.Indeed, for Au/Cu 6 Al 2 HT500 and Au/Zn 6 Al 2 HT500 the low intensity of X ray diffraction patterns and the overlapping with other signal from several phases did not allow an evaluation of the gold crystallite size.The low intensity of metallic gold X-ray patterns could indicate a high dispersion of gold nanoparticle.This information revealed that the gold dispersion is better in these catalysts.
In order to identify the nature of the gold species, the solids were analyzed by UV-Vis spectrophotometry in diffuse reflectance.The different spectra were presented in Figure 2. The UV-Vis spectra show new bands concerning the gold nanoparticles.Concerning the sample Au/Fe 6 Al 2 HT500 beside the support contribution, no clear change in the spectra was observed.However, for Au/Mg 6 Al 2 HT500 and Au/Zn 6 Al 2 HT500 three absorption bands are detected at 260, 330 and 560 nm.These bands could correspond respectively to: (i) Au + cations, (ii) (Au) n δ+ small clusters and (iii) plasmon resonance of Au nanometallic particles [10,26,27].Indeed, in gold metallic nanoparticles, the plasmon absorption arises from the collective oscillations of the free conduction band electrons that are induced by the incident electromagnetic radiation.This last band was observed on Au/Zn 6 Al 2 HT500 at 530 nm.Concerning the band at 330 nm, as in the case of Au/Mg 6 Al 2 HT500, this one could be attributed to (Au) n δ+ small clusters present on the solid.In the case of Au/Cu 6 Al 2 HT500, the broad band at around 620 nm could correspond to atoms of edge which are in strong interaction with the support [27].

Catalytic Activity for Toluene and CO Oxidation
A reactivity characterization of the solids has been done for two total catalytic oxidation reactions (toluene and CO). Figure 3 represents the conversion of toluene as a function of the temperature in the presence of catalytic support hydrotalcite (X 6 Al 2 HT500 where X represents Mg 2+ , Fe 2+ , Cu 2+ or Zn 2+ ) and in the presence of gold-based catalysts (Au/X 6 Al 2 HT500).When the conversion is complete, H 2 O and CO 2 are the only products observed.However, at the beginning of toluene conversion for all samples, a few ppm of benzene are detected.The T 50 temperatures (temperature for 50% toluene conversion) are reported into the Deposition of gold has a beneficial effect on the catalytic activity.Moreover, it could be noticed that the reactivity depends mainly on the nature of the mixed oxides support.The best reactivity is obtained for Au/Cu 6 Al 2 HT500 (T 50 = 260 °C).The solids were tested also for the CO total oxidation.The CO total oxidation on gold catalyst could be contributed to a better knowledge of the gold dispersion on the support.The relationship between catalyst activity and gold metallic particle size has been shown by several authors [28][29][30].Figure 4 showed the conversion of carbon monoxide as a function of the temperature in presence of mixed oxide and gold based catalysts.According to the T 50 temperatures (temperature for 50% conversion of carbon monoxide), this catalytic order is established: Au/Fe 6 Al 2 HT500 > Au/Cu 6 Al 2 HT500 > Au/Zn 6 Al 2 HT500 > Cu 6 Al 2 HT500 > Fe 6 Al 2 HT500 >> Zn 6 Al 2 HT500 > Au/Mg 6 Al 2 HT500 > Mg 6 Al 2 HT500.These results show that the gold catalysts are active for the CO total oxidation.The use of gold has a beneficial effect on the activity and depends on the composition of supports.However, good interaction between gold and a support containing copper (Au/Cu 6 Al 2 HT500), iron (Au/Fe 6 Al 2 HT500) and zinc (Au/Zn 6 Al 2 HT500) was also evidenced.It could be noted that the sample showing the lower dispersion (Au/Mg 6 Al 2 HT500) by XRD patterns, is less active in the CO total oxidation reaction.Moreover, the solids which have not presented a gold diffraction peak in XRD, showed a greater conversion at room temperature.This information is in accordance with the relationship between the dispersion of gold nanoparticles and catalytic activity for the CO oxidation [28][29][30].
For a better understanding of the difference in catalytic reactivity between these solids and the redox properties of the best solids, an H 2 -TPR experiment study was carried out for the solids containing copper and iron.
The results of H 2 -TPR experiments are reported on the Figure 5 and on the Table 2. Concerning the solid with iron (Fe 6 Al 2 HT500), three reduction steps are detected, the first at 354 °C corresponding to the reduction of Fe 2 O 3 into Fe 3 O 4 while the second step (at 610 °C) concurs to the reduction of Fe 3 O 4 into FeO and the last reduction step (at 780 °C) is attributed to the transformation of FeO into Fe 0 metallic [31].When gold is added to the support, these three reduction peaks are observed in addition to another at 133 °C, corresponding to the reduction of gold species.Concerning the solid with copper (Cu 6 Al 2 HT500), three reduction steps are detected.The first (at 201 °C) corresponds to the reduction of highly dispersed CuO species into Cu 2 O; the second (at 250 °C) is assigned to the reduction of bulk CuO species into Cu 2 O; the last peak corresponds to the reduction of Cu 2 O species into metallic copper [32,33].The adding of gold on the support allows the shifting of the reductions to lower temperatures.This is possible with the increasing of oxygen species mobility on the surface of solids.Temperature (°C) Table 2 resumes the H 2 -TPR experimental and calculated consumption of gold-based catalysts samples.For calculations of hydrogen consumption during H 2 -TPR measurement for the gold-based catalysts, if we consider the hypothesis that gold is only in the ionic form Au 3+ , we could evaluate a maximal H 2 consumption from gold species.Thus, knowing the gold content, and taking into account the gold reduction reaction of Au 3+ into Au°, we could evaluate a theoretical H 2 consumption by gold.By subtracting the theoretical consumption of gold from experimental consumption, it could be observed that the H 2 consumption of the solids increase when gold is deposited.Indeed, it is known that gold species permit the facilitation of the reduction of oxygen species from the support surface on the border with gold [4,5,10,11].Thus, in our case, the presence of gold promotes the reduction of iron or copper species at lower temperature and enhances the redox properties of the mixed oxide support prepared by hydrotalcite precursors.The results of the toluene total oxidation could be correlated with the results of the H 2 -TPR.In fact, the solids with the reduction at low temperature (Au/Cu 6 Al 2 HT500 and Cu 6 Al 2 HT500) present the best catalytic activity for the toluene oxidation.For the Toluene oxidation, the important point is the reducibility of the support.Indeed, the importance of the size of the gold nanoparticle is less than in the case of the CO total oxidation.Thus, the gold species enhanced the redox properties of the catalysts which are beneficial for the toluene total oxidation.A comparison between Au/Cu 6 Al 2 HT500 (most active sample) and some results reported in other works using gold catalysts and toluene catalytic oxidation were shown in the Table 3 The activity results (T 50 ) obtained by the Au/Cu 6 Al 2 HT500 close in comparison with best solids from literature [10,[34][35][36].Despite the difference between operating conditions, the results of the catalysts were very attractive; indeed, the T 50 obtained for Au/Cu 6 Al 2 HT500 was better than the other solids.

Catalysts Preparation and Characterization
Hydrotalcite (HT) like compounds with XAl molar ratio 6:2 (denoted as X 6 Al 2 HT where X correspond to Mg 2+ , Fe 2+ , Cu 2+ or Zn 2+ ) were prepared by coprecipitation.An aqueous solution containing appropriate amounts of nitrates elements (Mg, Fe, Cu, Zn and Al) was added, under stirring, dropwise into Na 2 CO 3 solution.During the synthesis the temperature and pH were maintained respectively at 60 °C and 10 by addition of NaOH solution.The solution was dried at 60 °C during 18 hours.The resulting suspension was filtered off and washed several times with hot deionized water (50 °C) and dried at 60 °C during 48 h.The thermal treatment was performed under flow of air (4 L h −1 −1 °C.min−1 -4 h at 500 °C).The catalysts are named X 6 Al 2 HT500 where X corresponds to Mg 2+ , Fe 2+ , Cu 2+ or Zn 2+ .
Gold-based catalysts were also prepared with the Deposition Precipitation method using sodium hydroxide as precipitating agent.One gram of the hydrotalcite support was added to an aqueous solution at 80 °C containing the suitable amount of gold to obtain 4 weight% in the solid.The pH of HAuCl 4 solution was adjusted to 8 by addition of NaOH (0.1 mol L −1 ) drop by drop under stirring during 4 h.The suspension was filtered, washed several times with hot water in order to eliminate Na + and Cl − ions.The catalyst was then dried in the oven at 80 °C followed by a thermal treatment under air at 400 °C (1 °C min −1 ) during 4 h.The code names of these catalysts are, respectively: Au/X 6 Al 2 HT500 (where X represents Mg, Fe, Cu or Zn).

Catalysts Characterization
To determine the gold content of samples, chemical analysis of Au was performed by inductively coupled plasma atomic emission spectroscopy at the CNRS Centre of Chemical Analysis (Vernaison, France).
The specific surface areas of solids were determined by the BET method using a QSurf M1 apparatus (Thermoelectron), and the gas adsorbed at −196 °C was pure nitrogen.
X-ray diffraction (XRD) measurements were carried out on a Bruker AXS D8 Advance diffractometer equipped with a copper anode (λ = 1.5406Å) and a LynxEye detector.The scattering intensities were measured over an angular range of 20° ≤ 2θ ≤ 80° for all the samples with a step-size of ∆(2θ) = 0.02 and a count time of 4 s per step.The diffraction patterns have been indexed by comparison with the JCPDS files.Crystallite size of gold nanoparticles was calculated (with a graphics based profile analysis program "TOPAS from Bruker AXS") from the line broadening of the Au diffraction line using the Scherrer equation.
UV-vis diffuse reflectance (UV-vis-DR) spectra were recorded with a Cary 5000 Scan (Varian) spectrophotometer in the spectral range of 200-800 nm.
The temperature programmed reduction (H 2 -TPR) experiments were carried out in an Altamira AMI-200 apparatus.The TPR profiles were obtained by passing a 5% H 2 /Ar flow (30 mL min −1 ) through 30 mg of samples heated at 5 °C min −1 from ambient temperature to 900 °C.The hydrogen concentration in the effluent was continuously monitored by a thermoconductivity detector (TCD).

Catalytic Tests
The activity for the toluene total oxidation of the catalysts (100 mg) was measured in a continuous flow system on a fixed bed reactor at atmospheric pressure.Before each test, the catalyst was reactivated in flowing air (2 L h −1 ) at 400 °C for 4 h.The flow of the reactant gases (100 mL min −1 with 1000 ppm of C 7 H 8 and balance with air) was adjusted by a Calibrage CALPC-5 apparatus constituted of a saturator and mass flow controllers.After reaching a stable flow, reactants passed through the catalyst bed and the temperature was increased from room temperature to 400 °C (1 °C.min−1 ).The feed and the reactor outflow gases were analyzed on line by a micro-gas chromatograph (Agilent technologies 490 Micro gas chromatography).
CO oxidation reaction was conducted at atmospheric pressure in a quartz flow microreactor containing 100 mg of catalyst in a fixed bed, using a series of mass flow controllers with diluted gases.The catalytic tests were made using a gas mixture containing 1000 ppm CO and 10% O 2 (He as eluant gas) with a flow of 100 mL min −1 , in the temperature range 20-300 °C with a ramp rate 1 °C min −1 .
ADEV 4400 IR CO-CO 2 infrared analyzers were used to perform the analysis both CO and CO 2 .The catalysts performance was assessed in terms of T 50 temperature, defined as the temperature, when 50% conversion was obtained.

Conclusions
In this study, X 6 Al 2 HT compounds with several divalent cations are prepared by the coprecipitation method.The samples with gold (Au/X 6 Al 2 HT500) are synthesized by the deposition-precipitation method.The TPR profiles, XRD patterns and UV-Vis spectra confirm the presence of metallic gold nanoparticles.The use of gold has a beneficial effect on the catalytic activity for the toluene and CO oxidation.This activity also depends on the composition of the support, particularly the divalent cation.A correlation between catalytic properties and reducibility is observed.

Figure 3 .
Figure 3. Toluene conversion on mixed oxide supports and gold catalysts vs. reaction temperature.

Figure 4 .
Figure 4. Carbon monoxide conversion on mixed oxide supports and gold catalysts vs. reaction temperature.

Table 1 .
BET surface area and gold content of the solids.

Table 2 .
H 2 -TPR consumption and conversion temperature (T 50 ) of 50% toluene and CO of catalysts.

Table 3 .
Results observed for catalytic toluene oxidation with gold-based catalysts.