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Catalysts 2017, 7(6), 175; https://doi.org/10.3390/catal7060175

Article
Carbon-Modified Mesoporous Anatase/TiO2(B) Whisker for Enhanced Activity in Direct Synthesis of Hydrogen Peroxide by Palladium
1
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
2
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
*
Authors to whom correspondence should be addressed.
Academic Editor: Morris D. Argyle
Received: 16 March 2017 / Accepted: 22 May 2017 / Published: 2 June 2017

Abstract

:
The regulation of the interaction between H2O2 and its catalysts is a promising route to achieve high productivity and selectivity towards H2O2. Herein, mesoporous anatase/TiO2(B) whisker (mb-TiO2) modified with heterogeneous carbon was prepared as the support of Pd-based catalysts for the direct synthesis of H2O2. The morphology and structure of the catalyst were investigated by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Brunner-Emmet-Teller measurements, and X-ray photoelectron spectroscopy. The interaction between H2O2 and the support was studied by isothermal calorimeter. The carbon heterogeneous modification can weaken the interaction between H2O2 and the support, then accelerate the desorption of H2O2 and reduce the re-adsorption of H2O2 in the reaction medium. Meanwhile, the synergistic effects between TiO2 and Pd nanoparticles are not influenced by the heterogeneous carbon distribution. The catalyst exhibits better performance for the synthesis of H2O2 compared with the corresponding unmodified catalyst; the productivity of H2O2 increases more than 40%, which can be ascribed to the decrease of further H2O2 conversion under the weakened interaction.
Keywords:
direct synthesis of H2O2; mesoporous anatase/TiO2(B) whisker; carbon modification; palladium; H2O2 desorption; isothermal microcalorimetry

1. Introduction

As a green and excellent oxidant, hydrogen peroxide (H2O2) has been widely applied in the pulp/paper industry, water purification, and chemical synthesis [1]. With the increasing demand for H2O2 in the international market, the direct synthesis of H2O2 from hydrogen and oxygen is believed to be a promising route because of its remarkable advantages [2,3,4,5,6,7], such as its lower levels of environmental pollution and lower cost. However, three inevitable side reactions in the direct synthesis process, including the combustion of H2 and the hydrogenation/decomposition of H2O2, reduce the productivity and selectivity towards H2O2, which is the major drawback of this process [2,4,8].
The surface property of the support has been proved to have an important effect on the performance of H2O2 synthesis. For example, the hydrophilic/hydrophobic moderation of the support is an efficient approach to improve the catalytic activity. As early as the 1970s, Pd catalysts supported on more hydrophobic materials were prepared to improve the productivity of H2O2 [9]. Hu et al. came to similar conclusions; they speculated further that more hydrophilic support might increase the re-adsorption of H2O2, resulting in the further conversion of H2O2 and the lower selectivity of H2O2 synthesis [8]. In addition, the crystal phase of the support surface can also lead to the enhancement of catalytic performance by the suppression of side reactions. TiO2 is a promising candidate support for H2O2 synthesis and has been widely studied [10,11,12]. According to our previous work with assistance from molecular dynamic simulations [13], it has been indicated that introducing the proper amount of TiO2(B) phase in the catalyst surface can accelerate the desorption of H2O2 from the catalyst due to the weak interaction between H2O2 and TiO2(B), and consequently improve the productivity of H2O2 over Pd-TiO2(B)/anatase catalyst.
In fact, the interaction between H2O2 and the support is the origin of the support effect according to the analysis mentioned above, and the selectivity and productivity of H2O2 is thus influenced by the moderation of the interaction between H2O2 and the support. Carbon-modified TiO2 has been proved to weaken the adsorption of polar molecules and accelerate their transfer [14]. Meanwhile, the carbon modification method is facile, inexpensive, and minimally changes the structure of supports. Herein, we fabricated a heterogeneous carbon-modified catalyst for the direct synthesis of H2O2 on the basis of mesoporous anatase/TiO2(B) whisker (mb-TiO2). mb-TiO2 whisker with excellent performance in various catalytic systems has been successfully prepared in our previous work [15,16,17,18]. Carbon heterogeneous surface modification can regulate the adsorption of H2O2 on the catalyst surface without changing the strong synergy between active sites and TiO2. The experimental results showed that the Pd-supported, carbon-modified mb-TiO2 (C-mb-TiO2) catalyst exhibited higher productivity and selectivity towards H2O2 compared with the unmodified Pd/mb-TiO2 catalyst. X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and isothermal microcalorimeter (IMC) characterization techniques were applied to analyze the structure–performance relationship.

2. Results and Discussion

2.1. Structural Characterization of the Catalysts

Figure 1 shows the XRD and Raman patterns of mb-TiO2, C-mb-TiO2, and the corresponding Pd-supported catalysts. Five distinct diffraction peaks are assigned to anatase phase of TiO2 (JCPDF 21-1272) [19]. Meanwhile, the weak observed peaks at 2θ of 14.3°, 28.7° and 43.6° correspond to (001), (002) and (003) of TiO2(B) phase (JCPDF 35-0088) [20]. Evidently, no extra diffraction peaks of carbon can be found in C-mb-TiO2, which implies that either the amount of carbon is too little or the generated carbon is amorphous [21]. In addition, the diffraction peaks for Pd are indistinguishable, which may be caused by the low content or good dispersion, as previously reported [22]. The bicrystalline structure was also confirmed by Raman spectra. Peaks at 234 cm−1, 431 cm−1 and 469 cm−1 are all indexed to the TiO2(B) phase [13]. The specific crystal structure has been explored in our previous work [15]. As shown in Figure 1b, two Raman bands located at 1300–1450 cm−1 (D-band) and 1500–1650 cm−1 (G-band) are only observed in C-mb-TiO2 rather than mb-TiO2, which verifies the existence of carbon species on the surface of C-mb-TiO2 [8]. The TG analysis shows that the content of carbon species of C-mb-TiO2 is less than 1 wt % (seen in Figure S2), and only about one third of mb-TiO2 surface is covered by the introduced carbon species according to our previous calculation [14].
N2 adsorption-desorption isotherms were measured at 77 K to characterize the physical and chemical properties of supports and corresponding catalysts, as shown in Figure 2. The typical type IV isotherms according to IUPAC classification are observed for all samples, indicating the presence of a mesoporous structure [17]. The corresponding pore size distribution curves (seen in Figure 2) show a narrow distribution ranging from 5 to 9 nm with a maximum around 7 nm for all samples. Details about the BET surface area and average pore volume/size are summarized in Table 1. It can be seen that the surface area and pore volume of C-mb-TiO2 are nearly identical to that of mb-TiO2, indicating that almost nothing has changed in the initial structure of mb-TiO2 after the carbon modification. Both the average pore size and the pore volume decrease after loading with Pd, which can be ascribed to the adhesion of metallic Pd nanoparticles inside the pores.
To obtain more information about the surface morphology, the samples were characterized by SEM and HRTEM. Figure S1 shows the SEM images of C-mb-TiO2, which is fibrous whisker with a uniform width of 300–400 nm, and the morphology shows little change from the mb-TiO2. The structure of Pd-supported C-mb-TiO2 was further studied by HRTEM and ED mapping. Well-crystallized TiO2(B) can be observed in Figure 3a; the nanocrystal with lattice fringes of 0.38 nm is assigned to d003 of TiO2(B) facet, and the lattice fringe spacing of 0.35 nm corresponds to d101 of anatase [15], with the majority being the anatase phase. Furthermore, we find that the evident anatase phase is covered by a thin area without crystal lattice fringes (see the two yellow dot lines in Figure 3b), which can be ascribed to the generated amorphous carbon species [21], and is consistent with the results of the Raman analysis. Figure 3c shows that the Pd particles are closed to the carbon and anatase phases, but it is difficult to obtain the accurate positioning of Pd from HRTEM images. The loading of Pd is confirmed by elemental mapping, as shown in Figure 3d. The blue regions represent the presence of Pd on the surface of TiO2 in the TEM maps. The relatively low levels of carbon cannot be distinguished due to the external introduction of the carbon source during the testing process. The average particle size of Pd on C-mb-TiO2 (11.2 nm) is very close to that on mb-TiO2 (10.5 nm), indicating that the size effect of the Pd particles can be ignored.
Figure 4 shows the Pd3d core-level spectra of the fresh catalysts. The binding energies at 336.6 and 341.9 eV are ascribed to Pd2+3d5/2 and Pd2+3d3/2, respectively [23]. The binding energy position of Pd0 and Pd2+ is unchanged between the two catalysts. The presence of Pd2+ in the fresh reduced sample is caused by TiO2-assisted partial oxidation of Pd0 when exposed to air. As shown in Table S1, the ratio of Pd2+/Pd0 of the two catalysts is very similar, indicating that the property of active sites is not affected by the carbon heterogeneous modification [11,13]. According to the literature focusing on carbon riveted supported catalysts, the electronic structure of the supported metal would be altered obviously when the generated carbon species are in intimate contact with metallic particles [24,25]. Therefore, it can be deduced that most of the Pd particles deposit onto the TiO2 phase.

2.2. The Adsorption Properties of H2O2 on the Supports

Isothermal calorimeter was used to investigate the adsorption properties of H2O2 on the supports. The adsorption thermodynamic data was calculated from the calorimetric profile recorded through in situ monitoring; the actual amount of adsorbed H2O2 was confirmed by the colorimetric method. Finally, their ratio (the adsorption energy of H2O2) can be used to represent the interaction strength between H2O2 and the support. The detailed process is described in Electronic Supplementary Information (ESI). Figure 5 shows the data of the adsorption properties of H2O2 on the support. It can be seen that the adsorption heat of H2O2 on C-mb-TiO2 is obviously lower than that on mb-TiO2, meaning that the adsorption of H2O2 on C-mb-TiO2 is weaker. The value of adsorption heat of H2O2 indicates a reversible physical adsorption. In other words, the generated H2O2 is more easily desorbed from the C-mb-TiO2 surface and H2O2 in the reaction medium is more difficult to re-adsorb on the support.

2.3. The Catalytic Activity of the Catalysts

The direct synthesis of H2O2 from H2 and O2 over Pd/mb-TiO2 and Pd/C-mb-TiO2 catalysts were investigated at 283 K and ambient pressure. As shown in Figure 6a, the conversion of H2 shows little variation over time, while the selectivity and productivity of H2O2 decrease along with the time for both catalysts. The Pd/C-mb-TiO2 catalyst exhibits better activity for H2O2 synthesis. The conversion of H2 over Pd/C-mb-TiO2 is approximately 35% higher than that over Pd/mb-TiO2. The Pd/mb-TiO2 catalyst shows H2O2 productivity of 1933 and 1717 mmol/gPd/h after 0.5 h and 1h, respectively, while the corresponding productivity on Pd/C-mb-TiO2 is 2848 and 2411 mmol/gPd/h with an improvement of more than 40%. Meanwhile, the H2O2 selectivity over Pd/C-mb-TiO2 is also promoted by 9% when compared with that over Pd/mb-TiO2. The analysis above indicates that the catalyst performance is significantly improved due to the carbon modification. Notably, the catalytic performance of both the Pd/mb-TiO2 and Pd/C-mb-TiO2 are superior to the Pd/P25 catalyst [4,8,11], which is widely investigated for the direct synthesis of H2O2.
To further unveil the effect of the support, we also performed the decomposition and hydrogenation of H2O2 under N2 and N2/H2 atmospheres, respectively, which are the two main side reactions in the direct synthesis of H2O2. As illustrated in Figure 6b, neither of the catalysts showed activity for the decomposition of H2O2 in pure N2 flow. When the H2/N2 mixture was fed into the reactor, the H2O2 was catalytically hydrogenated to form H2O, demonstrating that the hydrogenation of H2O2 is mainly responsible for the productivity and selectivity in this system. Similar results have also been observed for Pd/P25 [26]. Furthermore, it is observed that Pd/C-mb-TiO2 exhibits much lower hydrogenation rate than Pd/mb-TiO2. Willock et al. suggested that the production of H2O2 takes place at the interface between the particle and TiO2 [27]. Therefore, it is reasonable for us to assume that the properties of both active sites and supports have great impact on the catalytic synthesis of H2O2. Additionally, in our previous work, we confirmed that the accelerated desorption of H2O2 from the support can decrease the hydrogenation rate of H2O2 and then improve the selectivity towards H2O2 over the corresponding catalyst [13]. As mentioned above, the presence of carbon species can significantly weaken the interaction between H2O2 and the catalyst under the premise of similar property of active sites. Therefore, it can be concluded that the weaker interaction can reduce the concentration of H2O2 on the surface of the catalyst and then decrease the hydrogenation rate of H2O2. In addition, the improvement of H2O2 desorption can provide more reactive sites for H2O2 formation. As a result, the enhanced selectivity of H2O2 and the conversion of H2 improve the productivity of H2O2.

3. Materials and Methods

3.1. Synthesis of Carbon-Modified TiO2 Whisker

First, the mesoporous and bicrystalline TiO2 whisker was prepared according to our previous report [15]. Briefly, K2Ti2O5 was suspended in 0.1 M H2SO4 solution with vigorous stirring to form H2Ti2O5. The suspension was filtered and washed, following which it was dried at 353 K under vacuum and then calcinated at 723 K for 2 h in muffle to obtain mesoporous and bicrystalline TiO2 whisker. The final catalyst was denoted as mb-TiO2.
The carbon-modified mb-TiO2 was prepared as follows [14]. 1 g of mb-TiO2 was dispersed in 20 mL of dichloromethane containing 0.377 g of benzenephosphonous acid. After stirring for 24 h, the precipitation was filtered and washed with water/acetone (50/50 by volume) solution, and then dried in an oven at 373 K. The dried sample was treated at 673 K in Ar gas flow for 4 h and denoted as C-mb-TiO2.
3 wt % Pd-supported catalysts were prepared by incipient wetness impregnation. A certain amount of PdCl42− solution was slowly dropped into the support under stirring. The paste was left in natural environment for 4 h, following which it was dried at 383 K overnight. Prior to the reaction, all the catalysts were reduced at 573 K in H2 (40 mL·min−1) for 1 h. For comparison, commercial titania (P25, Degussa Co., Frankfurt am Main, Germany) was loaded with Pd using the same method.

3.2. Catalyst Characterization

X-ray diffraction (XRD) Spectrogram was collected on a D8 Advance X-ray diffractometer operating at a 40 kV with a current of 100 mA with Cu-Kα radiation. The data was collected in the 2θ range from 5° to 60° at a rate of 0.2 s step−1. Raman spectroscopy (Horiba HR 800) with a multichannel air-cooled CCD camera detector was employed to determine the surface crystal phases of supports using a 514.5 nm He-Cd laser beam. Nitrogen adsorption/desorption at −196 °C were achieved by using the TriStarII 3020M machine (Micromeritics Instrument Co., Atlanta, GA, USA). Surface area was calculated by the BET method; pore volume was determined by N2 adsorption at a relative pressure of 0.99. The Pd content of the catalysts was confirmed by ICP-AES (PerkinElmer Co., Waltham, MA, USA). SEM for the morphology of catalyst samples was taken by an America Quanta200 environmental scanning electron microscope. The information of dispersion and size of Pd nanoparticles were obtained by high-resolution transmission electron microscopy (HRTEM) performed on a Japan JEM-2100 at 200 kV. XPS was recorded by an AXIS UltraDLD spectrometer with the monochromatic Al Ka X-ray source. The interaction between H2O2 and the support was characterized on a TAM air Calorimeter from TA Instruments.

3.3. Catalytic Reaction

The direct synthesis of H2O2 was performed under atmosphere pressure in a glass tri-phase reactor. The reagent gases (H2/O2/N2 = 9:36:15) with a total flow rate of 60 mL/min were premixed in a steel mixer, and then introduced into the reactor via a frit fixed at the bottom. The temperature of 60 mL slurry containing 50 mg catalyst and 0.38 mL H2SO4 was controlled at 10 °C through the water circulated jacket. Vigorous stirring (950 rpm) was adopted to minimize mass transfer resistance. The outlet of the reactor was linked to a gas chromatography equipped with a 5A molecular sieve packed column and a thermal conductivity detector (SP-6890, Rainbow Chemical Instrument Co., Jinan, China), which helped to continuously analyze the conversion of H2. The concentration of H2O2 in the medium was determined by the colorimetric method after the complexation with a TiOSO4/H2SO4 reagent using a UV-vis spectrophotometer (UV-2802S, Unico (Shanghai) Instrument Co., Shanghai, China) at a wavelength of 400 nm. The selectivity of H2O2 ( S H 2 O 2 ) was calculated from the following formula:
S H 2 O 2 = rate   of   H 2 O 2   formation   ( mol / min ) rate   of   H 2   conversion   ( mol / min ) × 100 %
The reaction of H2O2 hydrogenation and decomposition was also implemented. The process was similar to that described above except for the initial composition of reaction medium. H2O2 was added to ethanol solution with the concentration of 0.25 wt %, then the decomposition and hydrogenation of H2O2 were tested under a flow of pure N2 (60 mL/min) and a flow of H2/N2 (9:51 mL/min), respectively. The same tests using bare supports or the absence of catalysts were also performed under the conditions mentioned above.

4. Conclusions

In this work, a simple heterogeneous carbon modification was applied to introduce carbon species on the surface of mb-TiO2. Pristine mb-TiO2 and carbon-modified mb-TiO2 were used as the supports of Pd-based catalysts for the direct synthesis of H2O2. The interaction between H2O2 and the supports was characterized by isothermal microcalorimetry. It was demonstrated that the carbon-decorated TiO2 exhibited weaker interaction with H2O2 than that achieved with the pristine TiO2. Therefore, the carbon-modified catalyst can effectively accelerate the desorption of H2O2 from the catalyst surface and reduce the re-adsorption of H2O2 in the reaction medium, which benefits the decrease of side reaction and the improvement of H2O2 productivity. Meanwhile, the properties of metallic active sites are not influenced by the carbon modification. As a result, the Pd/C-mb-TiO2 shows better performance for the direct synthesis of H2O2 compared with Pd/mb-TiO2. More than 40% increase in the productivity of H2O2 was achieved, and the selectivity of H2O2 improved by 9%. In addition, the catalytic performance of both Pd/mb-TiO2 and Pd/C-mb-TiO2 are superior to Pd/P25, which is widely investigated for the direct synthesis of H2O2. Hence, heterogeneous carbon modification represents a promising approach to tune the properties of the catalysts, as demonstrated here with TiO2-supported Pd catalyst for H2O2 synthesis.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/7/6/175/s1, Figure S1: SEM images of (a,b) mb-TiO2 and (c,d) C-mb-TiO2, Figure S2: TG and DSC curves of C-mb-TiO2, Figure S3: The scheme of reaction process, Table S1: Quantified XPS data for the surface Pd atoms, Table S2: Calorimetric measurement of H2O2 adsorption, Table S3: The performance of Pd catalysts for H2O2 synthesis, decomposition and hydrogenation.

Acknowledgments

This work was supported by the National Key Basic Research Program of China (2013CB733505, 2013CB733501), the National Natural Science Foundation of China (Nos. 91334202, 21406118), the Natural Science Foundation of Jiangsu Province of China (BK2012421, BK20160993), the Postdoctoral Science Foundation of China (2016M591837), the Research Fund for the Doctoral Program of Higher Education of China (20123221120015), and the Project for Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Author Contributions

Rui Tu, Jun Li and Xiaohua Lu designed the framework of experiments; Rui Tu and Shuying Chen performed all the experimental work; Rui Tu analyzed the experimental data and wrote the paper; Licheng Li, Suoying Zhang and Jun Li participated in the modification of the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) XRD pattern and (b) Raman spectra of the samples. (□ anatase, ● TiO2(B)).
Figure 1. (a) XRD pattern and (b) Raman spectra of the samples. (□ anatase, ● TiO2(B)).
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Figure 2. N2 adsorption-desorption isotherms and the pore-size distribution of the samples.
Figure 2. N2 adsorption-desorption isotherms and the pore-size distribution of the samples.
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Figure 3. TEM images of the samples: (ae) Pd/C-mb-TiO2 and (f) Pd/mb-TiO2; (d) Elemental Pd and Ti maps of Pd/C-mb-TiO2.
Figure 3. TEM images of the samples: (ae) Pd/C-mb-TiO2 and (f) Pd/mb-TiO2; (d) Elemental Pd and Ti maps of Pd/C-mb-TiO2.
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Figure 4. XPS Pd3d spectroscopy of (a) Pd/mb-TiO2 and (b) Pd/C-mb-TiO2.
Figure 4. XPS Pd3d spectroscopy of (a) Pd/mb-TiO2 and (b) Pd/C-mb-TiO2.
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Figure 5. Adsorption heat of H2O2 in the studied TiO2 samples: (a) mb-TiO2 and (b) C-mb-TiO2.
Figure 5. Adsorption heat of H2O2 in the studied TiO2 samples: (a) mb-TiO2 and (b) C-mb-TiO2.
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Figure 6. (a) H2 conversion, H2O2 selectivity and productivity, and (b) decomposition and hydrogenation of H2O2 over Pd/mb-TiO2 and Pd/C-mb-TiO2. (Blue color indicates Pd/C-mb-TiO2, purple color indicates Pd/mb-TiO2. Filled triangles: H2 conversion. Open circles: H2O2 selectivity. Histogram: H2O2 productivity).
Figure 6. (a) H2 conversion, H2O2 selectivity and productivity, and (b) decomposition and hydrogenation of H2O2 over Pd/mb-TiO2 and Pd/C-mb-TiO2. (Blue color indicates Pd/C-mb-TiO2, purple color indicates Pd/mb-TiO2. Filled triangles: H2 conversion. Open circles: H2O2 selectivity. Histogram: H2O2 productivity).
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Table 1. Structural properties of the supports and catalysts.
Table 1. Structural properties of the supports and catalysts.
SampleSurface Area (m2·g−1)Pore Volume (cm3·g−1)Average Pore Size (nm)
mb-TiO282.20.2210.5
C-mb-TiO281.80.2210.4
Pd/mb-TiO2780.2010.2
Pd/C-mb-TiO2790.1910.1

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