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Article

Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2

by
Bin Zhao
1,
Xiaoqiang Zhang
2,
Jingbo Mao
3,
Yanli Wang
1,
Guanghui Zhang
1,
Zongchao Conrad Zhang
2,* and
Xinwen Guo
1,*
1
State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
2
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
3
College of Environmental and Chemical Engineering, Dalian University, Dalian 116622, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(4), 699; https://doi.org/10.3390/catal13040699
Submission received: 3 March 2023 / Revised: 30 March 2023 / Accepted: 2 April 2023 / Published: 4 April 2023
(This article belongs to the Special Issue Heterogeneous Catalysis for Selective Hydrogenation)
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Abstract

:
TiO2-supported catalysts have been widely used for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about the effect of their different crystalline surfaces on their activity during the hydrodeoxygenation process. In this work, Au supported on anatase TiO2, mainly exposing 101 or 001 facets, was investigated for the hydrodeoxygenation (HDO) of guaiacol. At 300 °C, the strong interaction between the Au and TiO2-101 surface resulted in the facile reduction of the TiO2-101 surface with concomitant formation of oxygen vacancies, as shown by the H2-TPR and H2-TPD profiles. Meanwhile, the formation of Auδ−, as determined by CO-DRIFT spectra and in situ XPS, was found to promote the demethylation of guaiacol producing methane. However, this strong interaction was absent on the Au/TiO2-001 catalyst since TiO2-001 was relatively difficult to be reduced compared with TiO2-101. The Au on TiO2-001 just served as the active site for the dissociation of hydrogen without the formation of Auδ−. The hydrogen atoms spilled over to the surface of TiO2-001 to form a small amount of oxygen vacancies, which resulted in lower activity than that over Au/TiO2-101. The catalytic activity of the Au/TiO2 catalyst for hydrodeoxygenation will be controlled by tuning the crystal plane of the TiO2 support.

1. Introduction

Strong metal-support interactions (SMSI) have been recognized as a common phenomenon in reducible oxide-supported noble metals, and have developed as one of the most important concepts in heterogeneous catalysis [1,2,3,4]. Tauster et al. [4] first reported that group VIII metals supported on TiO2 lost their chemisorption capacity for H2 and CO after high-temperature reduction. SMSI effects were manifested by the encapsulation of the metal particles [5] and stabilization by supports [6], and by charge transfer between the metal particles and supports [7]. More specifically, Campbell et al. [8] proposed the concept of electronic metal–support interaction (EMSI) via charge transfer between the metal and the support in supported metal catalysts. The SMSI effect can be practically exploited for developing a catalyst by a design strategy considering the chemical bonding and charge transfer at the interface between metal nanoparticles and partially reduced support products to tune the surface electronic and chemical properties of the metal particles [8].
The reducible oxide, TiO2, usually shows a strong EMSI effect with metal nanoparticles, which can effectively tune the electronic structure of the support metal catalysts, showing better catalytic performance in many catalytic reactions [9,10]. Zhang et al. [11] reported that the different oxidation states of the catalytically active metal sites correlate with different EMSI between the Pt and TiO2 supports. Highly oxidized Pt clusters demonstrated higher catalytic activity and thermal stability compared with less oxidized Pt clusters. Fu et al. [5] demonstrated that the electron transfer from TiO2 to Pt nanoparticles due to the EMSI effect led to the formation of negatively charged Pt. It favored oxygen activation and phenol oxidation. Although TiO2 is widely reported to tune the electronic properties of the supported metal particles due to EMSI, the origins of the charge transfer on TiO2 supported metal catalysts remain poorly understood. SMSI between metal and TiO2 in the hydrodeoxygenation reaction has been widely studied [12,13], but EMSI between them in the hydrodeoxygenation reaction has been less reported.
In our group, TiO2-supported metal catalysts have been extensively studied due to their excellent catalytic activity and selectivity to phenolics for the HDO of guaiacol [14,15,16,17]. A previous study demonstrated Ag supported on reducible TiO2 exhibited superior activity and selectivity to aromatics for the HDO of guaiacol compared with Ag/SiO2 or Ag/Al2O3. This was attributed to the fact that hydrogen dissociated on the surface of Ag, and then spilled over to and reduced the anatase TiO2 surface, creating more oxygen vacancies on the Ag/TiO2-A. These oxygen vacancies were the active sites for the hydrodeoxygenation of guaiacol determined by a linear relationship between the hydrogen consumption for forming oxygen vacancies and the yield of phenols [16]. The evolution of multiple spillover hydrogen species has recently been identified with increasing pre-treatment temperature in H2 [18]. The oxygen vacancies were found to be filled by hydride species on Ag/TiO2-A with a very low Ag loading. On Ni/TiO2-A catalysts, Niδ− atoms were found to partially fill in the oxygen vacancies [15]. Mao et al. [17] reported that pure TiO2-A or Au nanoparticles showed only marginal activity for guaiacol hydrodeoxygenation. However, the Au nanoparticles supported on the surface of TiO2-A exhibited excellent activity and selectivity to phenolics. The results showed that the easy reduction of TiO2-A by spillover hydrogen dissociated from the Au nanoparticles facilitated the formation of oxygen vacancies as active sites for HDO of guaiacol.
Intriguingly, little EMSI effect was observed on TiO2-R (rutile)-supported metal catalysts, which consequently exhibited poor catalytic activity in the hydrodeoxygenation of guaiacol. TiO2-A has different crystal faces, while the catalytic performance of supported TiO2-A catalysts in some reactions is dramatically affected by these faces [19,20,21]. However, several studies have shown an EMSI effect of crystal faces in TiO2-A on the hydrodeoxygenation of guaiacol. Conceivably, not all the crystal faces in TiO2-A may contribute equally to the EMSI effect and the catalytic performance. Understanding the differences in the role of the crystal faces in TiO2-A in the supported catalysts should enable the design of high-performance catalysts for biomass applications. In this work, we report the crystal plane effect of TiO2-A on the hydrodeoxygenation of guaiacol over Au/TiO2 catalysts. We synthesized two Au/TiO2 catalysts exposing different crystal planes of TiO2 (Au/TiO2-101 and Au/TiO2-001). The aim of this work was to study the catalytic performance of Au supported on the different crystal planes of TiO2 for the HDO of guaiacol, and, further, to understand the fundamental differences between Au/TiO2-101 and Au/TiO2-001 catalysts.

2. Results

2.1. Structure Characteristic of TiO2 and Au/TiO2

The crystal structure and morphology of the synthesized TiO2 and Au/TiO2 catalysts were investigated by XRD, TEM, high-resolution TEM (HRTEM), and BET characterizations. Table 1 summarizes the physical properties of TiO2 and Au/TiO2. The BET surface areas of the TiO2 were both around 110 m2·g−1. The actual Au contents in the catalysts were close to 1 wt%. The surface areas of the Au catalysts were slightly lower than those of the corresponding supports, This may be have been due to the surface of TiO2 being occupied by the loaded gold [22].
Figure 1 displays the XRD patterns and Raman spectra of the TiO2 and Au/TiO2 catalysts exposing different TiO2 crystal planes. The XRD patterns (Figure 1A) of the as-synthesized catalysts showed the typical diffraction patterns of anatase TiO2 (JCPDS card no. 21-1272). Obviously, the XRD patterns of TiO2-101 were the same as the purity anatase TiO2; however, the intensity of the (004) diffraction peaks of TiO2-001 was lower and its full width at half-maximum was broadened compared with that of TiO2-101, indicating that the thickness of the TiO2 in the 001 direction decreased. In addition, the intensity of the (200) diffraction increased and its full width at half-maximum became narrowed, revealing that the side length of the TiO2 in the 100 direction increased. The changes in the diffraction peaks of (004) and (200) indicated the formation of TiO2-001 nanosheets [23]. The anions used in the synthesis were completely removed by washing with NaOH aqueous solution, as shown in XPS (Figure S1, in the Supplementary Materials). No XRD diffraction peak of Au was observed for all the Au/TiO2 catalysts, indicating the gold of the small nanoparticles was highly dispersed on the surface of TiO2.
The Raman spectra (Figure 1B) of the catalysts showed similar bands at 144, 398, 515, and 638 cm−1, which were Raman-active vibrational modes of anatase TiO2, consistent with the XRD results. The vibrational modes of the Eg peaks at 144 cm−1 and 638 cm−1 were mainly caused by symmetric stretching vibration of O-Ti-O in TiO2, the B1g peak at 398 cm−1 was caused by the symmetric bending vibration of O-Ti-O, and the A1g peak at 515 cm−1 was caused by the antisymmetric bending vibration of O-Ti-O [23]. Compared with TiO2-101, the distinct intensities of the Eg peaks in TiO2-001 at 144 and 638 cm−1 were lower, while those of the B1g peak at 394 cm−1 and the A1g peak at 514 cm−1 were higher. This change was consistent with previous reports that the number of the symmetric stretching vibration modes (Eg) of O-Ti-O on exposed 001 facets in TiO2 is less than the exposed 101 facets, whereas that of the symmetric bending vibration (A1g) and the antisymmetric bending vibration (B1g) of O-Ti-O is dominant on 001 facets [24].
Figure 2 presents the TEM images of TiO2-101, Au/TiO2-101, TiO2-001 and Au/TiO2-001 and HRTEM images of Au/TiO2-101 and Au/TiO2-001. The morphologies of these nanocrystals were quite uniform. The TiO2-101 (Figure 2A and Figure S2A, in the Supplementary Materials) nanocrystals possessed a typically octahedral structure, and the size of the nanocrystals was between 30 and 40 nm. The TiO2-001 (Figure 2B and Figure S2B (in the Supplementary Materials)) was composed of uniform square nanosheets, and the size distribution was 40–60 nm. The lattice fringes resolved in the HRTEM images (Figure S2, in the Supplementary Materials) were about 0.35 nm and 0.24 nm, respectively, which matched with the 101 and 001 crystal planes of anatase TiO2. These results agree with previously reported results [20,25]. Based on the Wulff construction (Figures S3 and S4, in the Supplementary Materials), we calculated the percentage of each crystal plane in these samples (Table S1, in the Supplementary Materials). The proportion of the 101 or 001 facets was above 80%, indicating that these TiO2 crystal planes can be used as model supports. The 1 wt% Au/TiO2 catalysts were prepared by a classic deposition-precipitation method (DP). HRTEM (Figure 2E,F) was used to determine the shape and location of Au nanocrystals. The morphology of Au loaded on the different TiO2 crystal planes of TiO2 and the Au nanoparticle size distributions of the Au/TiO2-101 and Au/TiO2-001 catalysts are presented in Figure 2C,D. The overall morphology of the Au/TiO2 samples was comparable to the TiO2 samples. Tiny particles of Au nanocrystals were well-dispersed on the Au/TiO2-101 and Au/TiO2-001 catalysts, as highlighted in the dashed yellow box (Figure 2C,D). The HRTEM images (Figure 2E,F) showed the morphology of the Au nanoparticles and lattice fringes with d = 0.23 nm, consistent with Au (111) [26]. The average particle sizes of the Au nanoparticles was 3.1 ± 0.5 nm for the Au/TiO2-101 sample (insert in Figure 2C) and 3.6 ± 0.5 nm for the Au/TiO2-001 sample (insert in Figure 2D).

2.2. Chemical Properties of TiO2 and Au/TiO2

Figure 3 shows the H2-TPR profiles of TiO2-101, TiO2-001, Au/TiO2-101 and Au/TiO2-001. Lv et al. [27] reported that the H2 molecule desorption temperature on Au surfaces was 276 K for H2-TPD. However, there was no peak at this temperature, as shown in Figure 4. This indicated that there was no H desorption on Au nanoparticles in this case. The pure TiO2 supports were barely reducible in the low temperature region, while a significant reduction was observed in the high temperature (HT) region (a peak above 500 °C). The peak c was ascribed to the reduction of TiO2 in the presence of Au. The interaction between Au and TiO2 promoted the reduction of TiO2. For the two Au/TiO2 catalysts, not only did the high temperature peak shift to low temperature, but a low temperature reduction peak (peak b at about 403 °C) also appeared. The peak b was ascribed to the reduction of Ti4+ on the surface of TiO2 [28], suggesting that the Au interacted with TiO2 and consequently decreased the reduction temperature of TiO2. However, a new low temperature reduction peak (peak c at about 221 °C) appeared in the Au/TiO2-101 catalyst; this peak could be ascribed to the reduction of the surface of TiO2-101 activated by gold species, forming oxygen vacancies [29]. Shapovalov et al. [30] reported that the bond energy of oxygen on the TiO2 surface can be weakened in the presence of Au species, implying that, compared with the surface of TiO2-001, the O atoms on the surface of TiO2-101 are easier to be removed in the presence of gold species. Moreover, the reducibility of the Au/TiO2 catalysts below the catalytically relevant temperature of 300 °C followed the order of Au/TiO2-101 > Au/TiO2-001, implying a reducibility influence of the exposed crystal plane of the TiO2 support.
Figure 4 shows the H2-TPD profiles of TiO2-101, TiO2-001, Au/TiO2-101 and Au/TiO2-001. Little hydrogen was desorbed from the pure TiO2 with different crystal planes. Two desorption peaks can be observed for all the Au/TiO2 catalysts. The high temperature (about >400 °C) desorption peaks were assigned to spillover hydrogen associated with the TiO2 [31]. Compared with Au/TiO2-001, although the temperature of hydrogen desorption of Au/TiO2-101 was higher, the amount of hydrogen desorption was much larger than that of Au/TiO2-001. Moreover, a relatively low temperature (<200 °C) desorption peak was detected on the Au/TiO2-101 catalyst; this was attributed to the desorption of spillover H adsorbed on the oxygen vacancy of the TiO2 surface [15]. This phenomenon has been reported in previous studies indicating that the spillover hydrogen promoted the reduction of titania on the surface of TiO2 [16,32,33]. As seen in Figure 4, the temperature of hydrogen desorption of Au/TiO2-101 was significantly lower than for Au/TiO2-001, suggesting a less energetically demanding desorption of H2 on the Au/TiO2-101 catalyst [34,35]. Hydrogen adsorbed on the oxygen vacancy of the Au/TiO2-101 surface was more easily desorbed than that on Au/TiO2-001. On the other hand, the area of desorbed hydrogen on the Au/TiO2-101 catalyst was much greater than on the Au/TiO2-001 catalyst, providing further evidence of the reducibility of TiO2-101 and the strong interaction of Au with this support.
Figure 5 shows the in situ DRIFTS of adsorbed CO on TiO2-101, TiO2-001 and two Au/TiO2 catalysts. As shown in Figure 5 A and B, the CO adsorption peaks at 2173 and 2120 cm−1 on the surface of fresh TiO2 were attributed to physically adsorbed CO. After hydrogen pretreatment, an apparent CO absorption peak located at 2183 cm−1 on TiO2-101 and TiO2-001 was observed. This peak can be associated with CO adsorbed on the coordinatively unsaturated Ti4+ site (Ti4+-CO) [36,37,38]. Ulrike Diebold [39] reported that reduced TiO2 can easily form oxygen vacancies on the surface. The formation of oxygen vacancies occurs at approximately 300 °C or above in a hydrogen atmosphere [40]. Therefore, the unsaturated Ti4+ site (Ti4+-CO) was attributed to CO adsorbed on the oxygen vacancy. Obviously, the spectral intensity of TiO2-101 was much stronger than TiO2-001, indicating that TiO2-101 was more easily reduced [41,42]. For Au/TiO2-001, another new CO absorption peak appeared at 2109 cm−1 when Au was supported on the surface of TiO2-001, which was assigned to the linear adsorption of CO on metallic Au0 sites based on previous investigations [37,43,44]. Nevertheless, another apparent CO absorption signal was found at 2102 cm−1 for Au/TiO2-101 compared with the v(CO) characteristics of Au/TiO2-001 at 2109 cm−1. The red shift of CO adsorption on Au/TiO2-101 indicated that a partial negative state of gold species (Auδ−) was formed on the surface of Au/TiO2-101 due to the strong metal–support interaction [45]. Moreover, the intensity of CO at 2102 cm−1 was higher than that at 2109 cm−1 on Au/TiO2-001, indicating that the Auδ− on the Au/TiO2-101 surface showed superior CO adsorption ability.
In order to study the various chemical electronic states of Au, Ti and O species, X-ray photoelectron spectroscopy (XPS) was performed after in situ reduction at 300 °C as a result of the interaction of Au with titania (Figure 6). As shown in Figure 6A, the peaks of Au 4f7/2 were centered at 83.3 eV on the Au/TiO2-101 and Au/TiO2-001, which were very close to the binding energy of metallic Au [43,46]. On the surface of Au/TiO2-101, another weak signal of 82.3 eV at the 83.3 eV shoulder could be assigned to the presence of electron-rich gold species (Auδ−) [47,48]. The Auδ− species was found in the DRIFTS of adsorbed CO (Figure 5C). However, this species was not found on the surface of Au/TiO2-001.
The Ti 2p XPS spectra are shown in Figure 6B. Two binding energies of the Ti 2P3/2 spectrum at 458.5 and 458.9 eV for Au/TiO2-101 were observed, assigned to the Ti4+ and the more electron-deficient Ti(4+δ)+ species, respectively, according to a previous assignment [49,50,51]. Radnik et al. [52] reported that the electrons from the carriers were transferred to the gold nanoparticles and accompanied by the formation of negatively charged gold (Auδ−) after hydrogen pretreatment. In this case, the discovery of the Ti(4+δ)+ species can most probably be attributed to electron transfer from the TiO2 surface to Au, resulting in the loss of electrons of the Ti species on the TiO2 surface, accompanied by the formation of the Auδ− species on the TiO2 surface. The Auδ− species was found in the XPS of the Au 4f spectra (Figure 6A). Compared with Au/TiO2-101, the Auδ- and Ti(4+δ)+ species were also found in the XPS spectrum of the Au/TiO2-001 sample. However, the Au/TiO2-001 surface was dominated by Ti4+, while the Ti(4+δ)+ spectral signals were very weak, implying again that the TiO2-001 surface was difficult to reduce. The DRIFTS of adsorbed CO (Figure 5) and H2-TPR (Figure 3) also illustrated this phenomenon. The absence of a Ti3+ signal in the XPS may have been due to the oxygen vacancies in the filled Auδ− species or hydride. Taking all the XPS results together, the Auδ− species and the Ti(4+δ)+/O(2−δ)− species were found in the pre-reduced Au/TiO2-101 catalyst; however, only Au0 species and Ti4+/O2− species were discovered in the pre-reduced Au/TiO2-001 catalyst. This suggests that electrons bound to oxygen and titanium ions shifted toward Au atoms occupying the oxygen vacancies. Our group’s previous studies [14] demonstrated that adsorbed H on the surface of TiO2-A formed TiO-H species and Ti-H species for pre-reduced Ni/TiO2-A.
In addition, XPS spectra (Figure S5, in the Supplementary Materials) in the O 1s region for these two samples were fitted to two peaks at 529.7 and 531.8 eV, attributed to the lattice oxygen (O2−) and adsorbed surface hydroxyl group (OH−), respectively [46,53,54,55,56]. After in situ reduction (Figure 6C), a new binding energy at 530.3 eV was discovered, which was 0.6 eV higher than the O2− species (529.7 eV). Senna et al. [51] reported that the bonding energy of O 1s was shifted to higher energy by co-milling of TiO2 with poly(tetrafluoroethylene) (PTFE) powder; this shift was caused by the transfer of electrons to adjacent oxygen vacancies. A similar shift was found for Ti species. As shown in the Figure 6C, for Au/TiO2-101, the binding energy of a large number O2− species (529.7 eV) transferred to higher energy O(2−δ)− (530.3 eV); simultaneously, the binding energy of many of the Ti4+ species at 458.5 eV shifted to Ti(4+δ)+ at 458.9 eV. However, for Au/TiO2-001, only a small quantity of O2− species shifted to O(2−δ)−. This suggested that it was easier to form the oxygen vacancies on the Au/TiO2-101 surface than on the Au/TiO2-001. Combining the in situ XPS of all the elements of Au, Ti and O, it was found that electrons bound to oxygen and titanium ions shifted toward Au atoms, suggesting that the formation of oxygen vacancies was accompanied by the formation of negatively charged Au sites, reflecting the apparent electron metal–support interactions (EMSI) [57].

2.3. Catalytic Performance

TiO2-101, TiO2-001 and two Au/TiO2 catalysts were evaluated by guaiacol HDO to study the effects of TiO2-101 and TiO2-001 planes on catalytic performance. The catalyst was pre-reduced at 300 °C before the catalytic reaction took place; however, the morphology of the catalyst did not change before and after the pre-treatment (Figure S6, in the Supplementary Materials). The guaiacol conversion and product distribution are presented in Table 2. First, only phenol was produced as the major product and the C8+ product and coke were obviously formed on TiO2-101 and TiO2-001 catalysts for guaiacol hydrodeoxygenation at 300 °C. In our previous work [17], we reported that hydrodeoxygenation of guaiacol over Au/AC exhibited marginal activity. The guaiacol conversion was very low on both the two pristine supports. This observation was consistent with our earlier report that TiO2-A showed low hydrodeoxygenation activity [17]. Second, when Au was supported on TiO2-101 and TiO2-001, the guaiacol conversion was much higher over the Au-supported catalysts than that over the TiO2-101 and TiO2-001 catalysts. The conversion of guaiacol followed the order: Au/TiO2-101 (39.4%) > Au/TiO2-001 (19.7%) > TiO2-101 (5.8%) ≈ TiO2-001 (6.4%). The results demonstrated that the reaction behaviors showed distinct crystal plane dependency. For guaiacol conversion on Au/TiO2 catalysts, phenol was the primary aromatic product; methylphenol, xylenol, tricresol and catechol were products in small amounts; and cyclohexanol was a byproduct from phenol hydrogenation. The selectivity to phenol followed the order: Au/TiO2-101 (60.2%) > Au/TiO2-001 (49.1%) > TiO2-101 (35.8%) ≈ TiO2-101 (34.8%). Moreover, the selectivity of phenols increased with decreasing selectivity of the C8+ and coke.
The guaiacol molecule contains three types of C-O bonds: CArO-CH3, CAr-OCH3, and CAr-OH, with bond dissociation energies of 262–276, 409–421 and 466 kJ/mol, respectively [58]. It was discovered that the energy barrier of the CarylO-CH3 bond scission was the lowest, corresponding closely to Au/TiO2-101 in which the selectivity of methane was much higher than that of methanol (Table 2). Demethylation via cleavage of the CarylO-CH3 bond was preferential in terms of bond strength compared to demethoxylation by Caryl-OCH3 bond scission. For instance, sulfide catalysts usually lead to the hydrogenolysis of guaiacol through demethylation forming catechol [59]. In the previous work on Au/TiO2-A [17], only methanol was formed as the C1 product during HDO of guaiacol. In contrast, methane was the main C1 product over a Au/TiO2-101 catalyst (Table 2). The fact of methanol being formed at a selectivity of 90.6%, together with the small amount of methane in the C1 product over the Au/TiO2-001 catalyst, confirmed the different reaction pathway of guaiacol hydrodeoxygenation over the Au/TiO2-101 catalyst.
In summary, demethylation was much faster than demethoxylation for guaiacol hydrodeoxygenation over the Au/TiO2-101 catalyst, in contrast to guaiacol hydrodeoxygenation over the Au/TiO2-001 catalyst, which was possibly due the different properties of Au. The detailed mechanism of guaiacol conversion is discussed below.

2.4. Pathways of Au/TiO2 Catalyst

The different catalytic pathways of the Au/TiO2-101 and Au/TiO2-001 catalysts in guaiacol HDO are closely related to the different interactions between Au and TiO2 with different crystal planes. There are two possible pathways in guaiacol conversion (Scheme 1). No anisole was detected during the HDO process, thus excluding the pathway of guaiacol HDO to form anisole. Methanol was detected as a major C1 by-product (90.6%) over Au/TiO2-001 (Table 2), indicating that phenol can be obtained directly by the hydrodemethoxylation of guaiacol through path 2. Path 2 is a dominant mechanism in oxygen vacancy [16]. Methylated phenols (cresols, xylenols and trimethylphenols) were also produced. The results confirm the findings of Mao et al. regarding guaiacol HDO on an Au/TiO2-A catalyst [17].
On the Au/TiO2-101 catalyst, catechol and CH4 were all detected. CH4 was the dominant C1 by-product (Table 2), suggesting catechol may form through the hydrodemethylation of guaiacol through path 1. The direct hydrodemethylation process of guaiacol occurred mainly on the Auδ− species. Catechol was further rapidly hydrogenated to phenol, because the rate of catechol HDO is much faster than that of guaiacol [17]. In addition, a small amount of methanol was also detected, implying that the direct demethoxylation of guaiacol may proceed through path 2.

2.5. Stability of Au/TiO2 Catalyst

The stability of the Au/TiO2-101 catalyst was investigated by guaiacol hydrogenation testing using the catalyst in three consecutive tests. The HDO conversion of guaiacol and the product selectivity are shown in Figure 7. At the end of the reaction, the catalyst was filtered and recovered without further treatment. It can be seen that there was an insignificant decrease for the guaiacol conversion and product yields in the recycling tests, possibly caused by small losses of catalysts during operation. This suggests that the Au/TiO2-101 catalyst showed good catalytic stability for the HDO conversion of guaiacol under the investigated conditions.

3. Discussion

As the guaiacol hydrodeoxygenation took place at 300 °C, hydrogen was preferentially dissociated on the Au/TiO2-101 catalyst under this condition, as shown by H2-TPD (Figure 4), then spillover occurred to reduce the surface of TiO2-101 (Figure 3), forming oxygen vacancies. Meanwhile, an EMSI effect and favorable electron transfer from the support to gold caused the formation of negatively charged gold (Auδ−). The DRIFT spectroscopy and in situ XPS demonstrated the presence of the Auδ− species. Compared with Au/TiO2-101, the Auδ− species were not detected on the surface of Au/TiO2-001. In situ DRIFT spectroscopy demonstrated that the spectral intensity of the Ti4+-CO absorption signal was evidently much higher than for Au/TiO2-001, indicating a higher concentration of oxygen vacancies on the Au/TiO2-101 surface than on the Au/TiO2-001 surface. In situ XPS also demonstrated the binding energy of a large number of O2− species (529.7 eV) transferred to higher energy O(2−δ)− (530.3 eV) for Au/TiO2-101, in clear contrast to only a small amount of O2− species shifted to O(2−δ)− for Au/TiO2-001. This evidence also confirmed the stronger EMSI effect on the Au/TiO2-101 surface than on the Au/TiO2-001 surface. Wang et al. [60] reported that electron-rich Ru species inhibited the ring hydrogenation reaction and the defect sites on TiO2 promoted the deoxygenation reaction for guaiacol HDO. Zhang et al. [14] demonstrated that Niδ− species of Ni/TiO2-A selectively catalyzed the hydrogenolysis of C–O bonds of guaiacol and that oxygen vacancies broke the ether-oxygen bond of guaiacol. On the Au/TiO2-101 catalyst, the presence of large amounts of CH4 (Table 2) suggests possible C-O hydrodemethylation in the guaiacol conversion, which could take place on the Auδ− site. The deoxygenation of catechol follows as a kinetically facile step [15] However, on the Au/TiO2-001 catalyst, the presence of large amounts of methanol (Table 2) indicates that the guaiacol hydrodeoxygenation reaction occurred mainly at the oxygen vacancy. Moreover, the higher concentration of oxygen vacancies on the Au/TiO2-101 surface is beneficial to improved catalytic activity for guaiacol hydrodeoxygenation compared to the Au/TiO2-001 surface.

4. Materials and Methods

4.1. Synthesis of TiO2-101

4.1.1. Synthesis of Precursor

For the preparation of the Ti(OH)4 precursor [20], 6.6 mL TiCl4 was added dropwise to 20 mL HCl solution (0.43 mol/L) in an ice bath. After stirring for 30 min, the solution obtained was added dropwise into 50 mL NH3·H2O solution (5.5 wt%) under stirring at room temperature. Afterward, the PH was adjusted to 6–7 with 4 wt% NH3·H2O solution to obtain a white suspension. Then the mixture was stirred continuously for 2 h. The suspension was centrifugated and then the precipitate obtained was washed using water and ethanol until the residual Cl− was completely removed. Finally, the precipitate was centrifuged and dried at 70 °C for 12 h to obtain the Ti(OH)4 precursor.

4.1.2. Synthesis of TiO2-101

TiO2-101 was prepared according to a previously reported method [20]. A quantity of 0.4 g NH4Cl was added to a mixture of 30 mL of water and 30 mL of isopropyl alcohol under stirring at room temperature. Next, 4.0 g Ti(OH)4 precursor was dispersed into the above mixed solution under stirring for 20 min, and then subject to ultrasound for 10 min. The obtained suspension was then transferred into a 100 mL Teflon-lined autoclave and reacted for 24 h at 180 °C. On completion, the suspension was centrifuged and then the precipitate obtained was washed using 0.1 M NaOH aqueous solution until the residual Cl− was completely removed. The precipitate was then washed with ultrapure water and ethanol until neutral. Finally, the precipitate was centrifuged and dried at 80 °C for 12 h to obtain TiO2-101 nanocrystals.

4.2. Synthesis of TiO2-001

TiO2-001 was prepared by the hydrothermal method [61]. A quantity of 4 mL HF solution (40 wt%) was added into 25 mL Ti(OBu)4 (TBOT) under stirring for 30 min at room temperature. The mixed solution was then transferred into a 100 mL Teflon-lined autoclave and heated for 24 h at 180 °C. After reaction, the hydrothermal product was centrifuged and washed with 0.1 M NaOH aqueous solution until the residual Cl- was completely removed. The precipitate was then washed with ultrapure water and ethanol to neutrality. Finally, the white powder was centrifuged and dried at 80 °C for 12 h to obtain TiO2-001 nanocrystals.

4.3. Preparation of Au/TiO2 Catalysts

Au/TiO2 catalysts were prepared by the deposition−precipitation method [62]. A quantity of 1 g TiO2 powders were dispersed in 50 mL water under stirring for 15 min at room temperature. (NH4)2CO3 solution (25 mL, 1 M) was added dropwise into the above suspension under stirring for 15 min to obtain the TiO2 suspension. Then, the mixed solution of 4.2 mL HAuCl4‧4H2O solution (24.8 mM) and 25 mL water was added dropwise into the TiO2 suspension. The system was subsequently stirred at room temperature for 1 h; the as-formed slurry was centrifuged and washed with ultrapure water and ethanol. Finally, the precipitate was dried in vacuum for 12 h at 60 °C, and calcined in air for 2 h at 300 °C.

4.4. Catalyst Characterization

X-ray diffraction (XRD) measurements were performed using a PANalytical X’Pert Powder X-ray diffractometer (Tokyo, Japan) equipped with a graphite monochromator and Cu Kα radiation. The diffraction data was scanned 2θ from 5° to 80° and collected using a PIXcel 1D detector (Tokyo, Japan).
UV resonance Raman spectroscopy (UV-Raman) with the exciting line of 325 nm was performed using a home-made triple-stage UV-Raman spectrograph with a spectral resolution of 2 cm−1 (Jobin Yvon, Paris, France).
Transmission electron microscopy (TEM) images of all samples were taken on a JEM-2100 instrument (FEI Corp. Portland, OR, USA) with an acceleration voltage of 200 kV and high-resolution TEM (HRTEM) images were obtained using a Tecnai F30 HRTEM instrument (FEI Corp. Portland, OR, USA) operated at 300 kV. The sample was dispersed in ethanol and sonicated for 5 min. The suspension was then mounted in a Cu TEM grid. The distributions of Au particle size were established from the measurements of 100 particles in the TEM images.
The surface areas (SBET) of the sample were determined by the Brunauer–Emmett–Teller method using N2 adsorption at 77.3 K with a Micromeritics ASAP 2020 physical adsorption analyzer (Norcross, GA, USA). The samples were degassed at 280 °C for 5 h prior to the measurements.
Elemental analysis of samples was conducted using a PerkinElmer Optima 8000 inductively coupled plasma-optical emission spectrometer (ICP-OES) (Shanghai, China).
Temperature-programmed reduction in hydrogen (H2-TPR) experiments were performed using a Micromeritics AutoChem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA). A 100 mg sample was placed in a tubular quartz reactor and heated from room temperature to 750 °C under 10% H2/Ar at a heating rate of 10 °C/min. H2 consumption was measured using a thermal conductivity detector (TCD) (Micromeritics, Norcross, GA, USA).
Temperature programmed desorption in hydrogen (H2-TPD) was carried out on a Micromeritics AutoChem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA). A 300 mg sample was placed in a tubular quartz reactor, and reduced at 300 °C for 1 h, then cooled to −40 °C for 30 min in 10% H2/Ar. Next, the sample was purged in Ar for 1 h. H2 desorption was performed by heating to 800 °C (10 °C min−1) in Ar.
In situ DRIFT spectra of CO adsorption on the samples were collected using a Thermo Scientific Nicolet iS50 (Waltham, MA, USA) equipped with an MCT detector and the spectra were obtained as the average of 64 scans at a resolution of 4 cm−1. Prior to testing, the sample was pressed into the in situ reaction cell. For the fresh sample, the high purity He was introduced for 3 min to collect the background spectrum. Then, the gas mixture of 5% CO/He was passed into a reaction cell at 30 °C for 3 min; then the DRIFT spectrum was recorded after the chemisorption of CO. For the pre-treatment, the sample was heated in 10% H2/Ar at 10 °C/min to 300 °C for 1 h. After treatment, the sample was purged in He and cooled to 30 °C to collect the background spectrum. Subsequently, the sample was then exposed to 5% CO/He at 30 °C for 3 min and the DRIFT spectrum was recorded after the chemisorption of CO.

4.5. Catalyst Activity Measurements

Guaiacol hydrodeoxygenation was performed in a sealed 50 mL stainless-steel batch reactor. Prior to each reaction, the catalyst was pre-reduced at 300 °C in 10% H2/Ar for 1 h. A quantity of 1.3 g guaiacol and 25 mL decane were added into the reactor and then 0.20 g catalyst was dispersed into the mixed solution. The reactor was purged with N2 5 times to remove the air and filled with 3 MPa hydrogen. Further, the reactor was heated to 300 °C and held for 4 h under magnetic stirring at 700 rpm. On completion, the reactor was cooled to ambient temperature. A quantity of 25 mL ethanol absolute as solvent and 0.2394 g n-tetradecane as internal standard were added into the reactor. The liquid reactant mixture was identified by GC-MS and qualitatively analyzed by gas chromatography (Agilent 7890A) (Agilent, Santa Clara, CA, USA) with an HP-5 column (30 m × 0.32 mm × 0.25 µm) and a flame ionization detector (FID) (Agilent, Santa Clara, CA, USA).
The conversion of guaiacol (Xguaiacol%) and the product yield (Yproduct%) were calculated according to the following equations:
X g u a i a c o l = n c o n s u m e d   g u a i a i c o l n i n i t i a l   g u a i a i c o l × 100 %
Y p r o d u c t = n p h e n o l o c   c o m p o u n d n i n i t i a l   g u a i a i c o l × 100 %

5. Conclusions

In summary, in this work, we synthesized Au supported on TiO2-101 and TiO2-001 and found that crystal planes of TiO2 had a strong effect on guaiacol HDO. The Au/TiO2-101 catalyst produced a much higher guaiacol conversion than that over the Au/TiO2-001 catalyst. The H2-TPR and H2-TPD profiles showed easier reduction of the TiO-101 surface resulted in more oxygen vacancies. Moreover, in situ XPS and DRIFTS of adsorbed CO demonstrated the formation of electron-rich Au species (Auδ−) due to electron transfer between Au and TiO2-101 after hydrogen treatment. In contrast, this phenomenon was not found on the Au/TiO2-001 catalyst. The high activity of the Au/TiO2-101 catalyst was attributed to the Auδ− site at the surface due to the strong electronic metal-support interaction. The Auδ− site was found to promote the demethylation of guaiacol by producing methane. However, the Au on TiO2-001 dissociated the hydrogen, and the spillover H to the surface of TiO2-001 resulted preferentially in the generation of small amounts of oxygen vacancies, which served as active sites for the direct hydrodemethoxylation of guaiacol. This work provides an effective strategy to improve the HDO activity of Au/TiO2 catalysts by tuning the crystal plane of the support.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040699/s1, Figure S1: XPS spectrum of Cl 2p and F 1s in the Au/TiO2-101(A) and Au/TiO2-001(B); Figure S2: HRTEM images of TiO2-101(A), TiO2-001(B); Table S1: The percentages of different crystal planes in the as-prepared TiO2 nanocrystals were based on geometric calculation; Figure S3: Geometric model of TiO2-101 nanocrystals; Figure S4: Geometric model of TiO2-001 nanocrystals; Figure S5: XPS spectrum of Au 4f (A), Ti 2p (B) and O 1s (C) in Au/TiO2 catalysts before in-situ reduction; Figure S6: TEM images of Au/TiO2 catalyst before and after pre-treatment. Au/TiO2-101-before (A), Au/TiO2-101-after (B), Au/TiO2-001-before (C) and Au/TiO2-001-after (D).

Author Contributions

Conceptualization, B.Z., Z.C.Z. and X.G.; methodology, B.Z. and X.Z.; validation, B.Z.; formal analysis, B.Z., X.Z. and J.M.; investigation, B.Z.; resources, B.Z.; data curation, B.Z.; writing—original draft preparation, B.Z.; writing—review and editing, B.Z., X.Z., Y.W., G.Z., Z.C.Z. and X.G; project administration, X.G.; All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Liaoning Revitalization Talent Program (XLYC2008032), Fundamental Research Funds for the Central Universities (DUT22LAB602).

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns (A) and Raman spectra (B) of TiO2-101, Au/TiO2-101, TiO2-001 and Au/TiO2-001.
Figure 1. XRD patterns (A) and Raman spectra (B) of TiO2-101, Au/TiO2-101, TiO2-001 and Au/TiO2-001.
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Figure 2. TEM images of TiO2-101 (A), TiO2-001 (B), Au/TiO2-101 (C), Au/TiO2-001 (D) and HRTEM images of Au/TiO2-101 (E) Au/TiO2-001 (F).
Figure 2. TEM images of TiO2-101 (A), TiO2-001 (B), Au/TiO2-101 (C), Au/TiO2-001 (D) and HRTEM images of Au/TiO2-101 (E) Au/TiO2-001 (F).
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Figure 3. H2-TPR profiles of TiO2 and Au/TiO2 catalysts with different TiO2 crystal planes. (a) the high temperature peak (above 500 °C), (b) the medium temperature peak (about 403 °C), (c) the low temperature peak (about 221 °C).
Figure 3. H2-TPR profiles of TiO2 and Au/TiO2 catalysts with different TiO2 crystal planes. (a) the high temperature peak (above 500 °C), (b) the medium temperature peak (about 403 °C), (c) the low temperature peak (about 221 °C).
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Figure 4. H2-TPD profiles of TiO2 and Au/TiO2 catalysts with different TiO2 crystal planes.
Figure 4. H2-TPD profiles of TiO2 and Au/TiO2 catalysts with different TiO2 crystal planes.
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Figure 5. In situ DRIFTS of CO adsorption of TiO2-101 (A), TiO2-001 (B) and Au/TiO2 catalysts with different TiO2 crystal planes (C).
Figure 5. In situ DRIFTS of CO adsorption of TiO2-101 (A), TiO2-001 (B) and Au/TiO2 catalysts with different TiO2 crystal planes (C).
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Figure 6. In situ XPS of Au 4f (A), Ti 2p (B) and O 1s (C).
Figure 6. In situ XPS of Au 4f (A), Ti 2p (B) and O 1s (C).
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Scheme 1. Possible pathways in guaiacol hydrodeoxygenation.
Scheme 1. Possible pathways in guaiacol hydrodeoxygenation.
Catalysts 13 00699 sch001
Catalysts 13 00699 g007 550
Figure 7. Recycling tests of Au/TiO2-101. The recycled catalyst was directly used in the next recycling experiments without any treatment.
Figure 7. Recycling tests of Au/TiO2-101. The recycled catalyst was directly used in the next recycling experiments without any treatment.
Catalysts 13 00699 g007
Table
Table 1. Physical properties of TiO2 and Au/TiO2.
Table 1. Physical properties of TiO2 and Au/TiO2.
SamplesSBET (m2·g−1)Au Content (wt%) a
TiO2-101118-
TiO2-001107-
Au/TiO2-1011090.91
Au/TiO2-0011011.04
a Actual metal loading determined by ICP-AES.
Table
Table 2. Conversion and product selectivity for guaiacol hydrodeoxygenation over the various TiO2 and Au/TiO2 catalysts a.
Table 2. Conversion and product selectivity for guaiacol hydrodeoxygenation over the various TiO2 and Au/TiO2 catalysts a.
Sample bConv. (%)C6-Ring Sel. (%)C1 Sel. (%)
PhenolCatecholMethylated PhenolsCyclohexanolOthers cCH4CH3OH
TiO2-1015.850.100049.4-100
TiO2-0016.451.600048.4-100
Au/TiO2-10139.460.26.023.46.53.980.219.8
Au/TiO2-00119.749.19.427.78.55.69.490.6
a Reaction conditions: guaiacol 1.30 g, catalyst 0.20 g, decane 25 mL, 3 MPa, 700 rpm, 300 °C, 4 h. b All the catalysts were pre-treated in 10% H2/Ar at 300 °C for 1 h before reaction. c “Others” in the product yields refer to mostly C8+ and coke formed during the reaction testing.
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Zhao, B.; Zhang, X.; Mao, J.; Wang, Y.; Zhang, G.; Zhang, Z.C.; Guo, X. Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2. Catalysts 2023, 13, 699. https://doi.org/10.3390/catal13040699

AMA Style

Zhao B, Zhang X, Mao J, Wang Y, Zhang G, Zhang ZC, Guo X. Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2. Catalysts. 2023; 13(4):699. https://doi.org/10.3390/catal13040699

Chicago/Turabian Style

Zhao, Bin, Xiaoqiang Zhang, Jingbo Mao, Yanli Wang, Guanghui Zhang, Zongchao Conrad Zhang, and Xinwen Guo. 2023. "Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2" Catalysts 13, no. 4: 699. https://doi.org/10.3390/catal13040699

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