Investigation of the Performances of TiO₂ and Pd@TiO₂ in Photocatalytic Hydrogen Evolution and Hydrogenation of Acetylenic Compounds for Application in Photocatalytic Transfer Hydrogenation

Abstract

The development of effective bifunctional catalysts demonstrating high performance in both photocatalytic hydrogen evolution and selective hydrogenation of unsaturated compounds is of great interest for photocatalytic transfer hydrogenation. In this work, TiO₂ and Pd@TiO₂ catalysts were studied in two separate processes: photocatalytic H₂ evolution and conventional hydrogenation reactions. Photocatalytic properties of titanium dioxide synthesized by a simple precipitation method were compared with those of commercial ones. Commercial anatase with a lower agglomeration degree showed better activity in H₂ evolution. Further modification of the commercial anatase with Pd resulted in increasing its activity, achieving an H₂ evolution rate of 760 μmol/h g_cat. The Pd catalysts supported on different TiO₂ samples were tested in hydrogenation of acetylenic compounds. The activity of the Pd@TiO₂ catalysts was found to be dependent on the photocatalytic properties of TiO₂ supports. XPS studies of Pd catalysts indicated that commercial anatase with better photocatalytic properties provided easier reduction of Pd²⁺ to active Pd⁰ particles. The Pd catalyst supported on commercial anatase demonstrated the highest activity in the hydrogenation process. The W_C≡C rate achieved 2.6 × 10⁻⁶, 9.0 × 10⁻⁶ and 35.7 × 10⁻⁶ mol/s for hydrogenation of 2-hexyne-1-ol, 5-hexyne-1-ol and 2-hexyne, respectively. The selectivity of the catalyst to target olefinic compounds was 94–96%. In addition, the hydrogenation rate was found to be significantly affected by reaction conditions such as hydrogen concentration and solvent composition. The W_C≡C rate decreased linearly with decreasing hydrogen concentration in a H₂:He gas mixture (30–100 vol%). Performing the reaction in 0.10 M NaOH ethanolic solution resulted in increasing the W_C≡C rate and selectivity of the process. The Pd catalyst was reused in an alkali medium (NaOH in ethanol) for 35 runs without significant degradation in its catalytic activity. Thus, the results obtained in this work can be useful in photocatalytic transfer hydrogenation.

1. Introduction

Hydrogenation is one of the most important chemical processes widely used in the petrochemical, fine chemical, and pharmaceutical industries. It is estimated that 25% of chemical transformations include at least one hydrogenation step [1]. In petrochemistry, selective hydrogenation is the direct route to eliminate the impurities of alkynes and dienes from alkene-containing hydrocarbons for downstream polymerization [1,2,3]. In fine organic synthesis, selective hydrogenation of acetylene compounds is the main way to obtain valuable chemical products such as biologically active compounds and pharmaceuticals [4,5,6,7].

Pd nanoparticles immobilized on support materials are the most promising catalysts for selective hydrogenation of unsaturated C≡C bonds due to their superior ability in dihydrogen dissociation and the effect of supports on their properties [4,5,8]. It is known that the support material affects the dispersion of the active phase and prevents particle aggregation, and therefore can significantly improve the catalytic properties of metal catalysts [9,10].

Various materials such as silica, carbon, alumina, titanium dioxide, etc. are used as catalytic supports [11]. Among them, titanium dioxide has proven excellent support for Pd hydrogenation catalyst due to strong metal–support interaction [12,13,14,15,16,17]. Titanium dioxide due to its excellent optical and electronic properties is also well known as an efficient photocatalyst widely used in various redox reactions, such as degradation of organic compounds and production of hydrogen [18,19,20]. In addition, a modification of titanium dioxide with palladium was shown to be an effective tool to improve its photocatalytic properties for further use in the H₂ evolution process [21,22].

Therefore, Pd/TiO₂ photocatalysts have attracted attention to be used in photocatalytic transfer hydrogenation, which is a promising alternative to conventional hydrogenation technology due to the advantages of being eco-friendly [23,24]. Lavorato et al. [25] have used TiO₂ and Pd/TiO₂ in the photocatalytic transfer hydrogenation of acetophenone and butanone. The Pd/TiO₂ showed higher activity in both reactions compared with that of commercial TiO₂. However, despite the high conversion of acetophenone (96.2%), the yield of phenyl ethanol on Pd/TiO₂ was lower than on TiO₂. In contrast, the reduction of butanone to butanol with Pd/TiO₂ gave a yield of 15.6%which was six times more than that of bare TiO₂. Imamura K. et al. [26] have reported that Pd loaded on TiO₂ was the most effective photocatalyst for transfer hydrogenation of alkenes compared with similar catalysts containing Au, Ag, Pt, Cu, Rh, and Ru. After 30 min, photoirradiation of various aromatic or aliphatic alkenes in methanolic suspensions of 0.1 wt% Pd–TiO₂, the formation of corresponding alkanes with a yield of 92–99% was observed. In [27], a regio- and stereoselective photocatalytic hydrogenolysis of various allylic alcohols to unsaturated hydrocarbons by using Pd^II/TiO₂(P₂₅) was demonstrated. The hydrogenolysis proceeds at room temperature under light irradiation without stoichiometric generation of salt waste, allowing a step-economical synthesis of (S)-(+)-lavandulol by avoiding otherwise necessary protection/deprotection steps. Lv S. et al. [28] studied photocatalytic transfer hydrogenation of furfural using the TiO₂-based photocatalysts with alcohols as both the solvent and hydrogen donor. It was found that ultralow loading metals (Pd, Pt) on TiO₂, together with adding a small amount of water in the system, led to a great increase in the selectivity of the process to furfuryl alcohol.

Thus, over the last decade, some progress in the field of photocatalytic transfer hydrogenation has been made. However, the development of effective bifunctional catalysts, providing high performance in both photocatalytic hydrogen evolution and selective hydrogenation of unsaturated compounds remains a challenge. One of the possible options to overcome this problem is to study the competitive relationship between hydrogenation and hydrogen evolution, and its dependence on the structure of photocatalysts [24].

Therefore, in this work, TiO₂ and Pd/TiO₂ catalysts were studied in two separate processes to evaluate the relationship between the photocatalytic properties of TiO₂ photocatalysts in H₂ production and the behavior of Pd/TiO₂ catalysts in the conventional hydrogenation process. Namely, different titanium dioxide powders were studied in photocatalytic water splitting into H₂ and then used for the preparation of Pd/TiO₂ catalysts, which were studied in hydrogenation of 2-hexyne and hexynols with molecular hydrogen. TiO₂ and Pd/TiO₂ catalysts were characterized using a complex of physical and chemical methods. Additionally, the effect of agglomeration of TiO₂ particles and modification of TiO₂ agglomerates with Pd on their photocatalytic activity in H₂ evolution was discussed. The effect of reaction conditions on the kinetics of 5-hexyn-1-ol hydrogenation over the most active Pd@TiO₂ catalyst was also evaluated. The results obtained can be useful in photocatalytic transfer hydrogenation as it involves both photocatalytic H₂ evolution and hydrogenation reactions.

2. Results and Discussion

2.1. Characterization of TiO₂ Samples and Pd Catalysts Based on Them

Titanium dioxide (TiO₂SA) synthesized by a simple precipitation method was characterized by X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) methods. The results obtained were compared with those of commercial anatase (TiO₂CA) and a commercial mixture of rutile and anatase (TiO₂CRA).

Figure 1 shows the XRD patterns of the samples based on titanium dioxide. Characteristic peaks at 29.3°, 44.2°, 56.4°, 63.6°, 64.8°, 74.4°, 82.1°, 83.8° and 90.1° corresponding to the (101), (112), (200), (105), (211), (204), (116), (220), and (215) crystal planes of TiO₂ in the anatase phase (JCPDS card № 21-1272) [21] were observed in XRD patterns of the TiO₂SA, TiO₂CA and TiO₂CRA (Figure 1a–c). At the same time, the XRD pattern of TiO₂CRA (Figure 1c) exhibited additional characteristic peaks at 31.8°, 42.0° and 48.0° corresponding to the (110), (101) and (111) of TiO₂ in the rutile phase (JCPDS card № 21-1276) [21] Processing of the XRD pattern using “Crystallographica Search-Match” software (version. 2.1.1.1; Oxford Cryosystems, Oxford, UK) showed that the content of the rutile phase in TiO₂CRA was about 10%wt. The mean crystallite sizes estimated from the FWHM of the anatase (101) line by the Scherrer equation are 8, 16, and 15 nm for TiO₂SA, TiO₂CA, and TiO₂CRA, respectively.

The values of mean crystallite sizes were used to calculate the specific surface area (S_XRD) of the samples by the modified Equation (1), proposed in [29]:

$$S_{XRD}={\displaystyle \frac{6000}{d\times 3.9}}$$

(1)

where, 6000—conversion factor, d—mean crystallite sizes of TiO₂ in nm, and 3.9—density of TiO₂ in g/cm³.

The calculated S_XRD values were found to be higher when compared with values of the specific surface area (S_BET) determined using the BET method (

Table 1. Results of the study of TiO₂ samples using XRD and BET methods.

Samples	Crystalline Size (XRD), nm	Surface Area, m2/g		Surface Blocked (calc.), %
		SXRD	SBET
TiO2SA	8	192	98	50
TiO2CRA	15	103	61	40
TiO2CA	16	96	66	30
Pd@TiO2CA	16	96	67	30

). Such a difference in surface area size can be explained by the blocking surface of TiO₂ particles due to their agglomeration. The agglomeration of TiO₂ has been evaluated by determining the percentage of blocked surface by adopting the formula (S_XRD − S_BET) · 100%/S_XRD, where S_XRD is the surface area calculated from crystal size measured with the Scherrer formula, whereas S_BET is the surface area determined with the BET method.

The results presented in Table 1 show that TiO₂ samples demonstrate different degrees of surface blocking. The calculated values of surface blockage were 30%, 40%, and 50% for TiO₂CA, TiO₂CRA and TiO₂SA, respectively. This suggests that TiO₂SA possessed a higher degree of agglomeration of TiO₂ particles compared with those of TiO₂CRA and TiO₂CA. At the same time, TiO₂SA showed the highest BET surface area, which can be explained by the smaller crystalline size of TiO₂SA compared with TiO₂CRA and TiO₂CA.

Pd@TiO₂ hydrogenation catalysts were prepared by deposition of [PdCl₄]²⁻ ions on different TiO₂ samples using sodium hydroxide as a precipitant. For simplicity, from now on we refer to the Pd catalysts supported on commercial anatase (TiO₂CA), commercial mixture of rutile and anatase (TiO₂CRA), and synthesized anatase (TiO₂SA) as Pd@TiO₂CA, Pd@TiO₂CRA, and Pd@TiO₂SA, respectively.

Table 2. Results of assessing the degree of deposition of palladium ions on TiO₂ support materials.

Catalyst	Amount of Pd in Mother Liquor, ×10−5 Mol		The Degree of Deposition, %	Pd Content in a Catalyst, %wt.
	before Deposition	after Deposition		PEC	EDX *
Pd@TiO2SA	9.5	0.4	96	1.0	1.0
Pd@TiO2CA	9.5	0.1	99	1.0	1.0
Pd@TiO2CRA	9.5	0.3	97	1.0	1.0

* Results of EDX elemental analysis of untreated Pd catalysts (before their reduction with hydrogen) presented in the Supplementary Materials (Figures S1–S3).

shows the results of assessing the degree of palladium ion deposition on different TiO₂ supports. According to the photoelectric colorimetric (PEC) analysis, the addition of NaOH to a suspension containing [PdCl₄]^2– and TiO₂ promotes almost complete (96–99%) deposition of palladium ions onto a TiO₂ support material. The palladium content in the catalysts, calculated based on the PEC data, was about 1%wt., which was confirmed by energy dispersive X-ray elemental (EDX) analysis (Table 2).

In order to assess the state of palladium during the hydrogenation process, the catalysts were treated with molecular hydrogen in the reactor at 40 °C and then studied using X-ray photoelectron spectroscopy (XPS). Figure 2 shows Pd 3d and Ti 2p regions of the XPS spectra of the reduced Pd@TiO₂ catalysts. Deconvolution of Pd 3d signals showed the different oxidation states of Pd existing on the surface of TiO₂ supports (Figure 2a,b). The peaks at ~335 and ~340 eV correspond to the orbits of metallic Pd 3d5/2 and Pd 3d3/2, respectively. For Pd²⁺, the binding energy peaks at ~337 and ~342 eV belong to the Pd 3d5/2 and Pd 3d3/2 orbits [30]. The Pd peak at ~337 eV was positively shifted compared with the reference value reported for PdO (336.3 eV) [31]. Comparison of the Pd 3d region of XPS spectra of the catalysts showed that the metallic Pd was the dominant species (~70%) in Pd@TiO₂CA, while in Pd@TiO₂SA, the proportion of palladium in the zero-valence state was about 50%. This can be explained by the fact that different titanium dioxides possess a different effect on the Pd⁰/PdO ratio in supported catalysts due to the metal–support interaction [32].

The two strong peaks from the Pd@TiO₂ catalysts at around 464 eV and 458 eV can be attributed to Ti 2p1/2 and Ti 2p3/2, respectively (Figure 3c,d). The Ti2p3/2 and Ti2p1/2 peaks are separated by 5.7 eV, which is in agreement with the energy observed for anatase TiO₂ [33]. The Ti2p3/2 peak positions were also close to the reference value (458.7 eV) reported for Ti⁴⁺ in anatase [34].

Figure 2. Pd 3d (a,b) and Ti 2p (c,d) XPS spectra of Pd@TiO₂CA (a,c) and Pd@TiO₂SA (b,d). The reference values of binding energies for Pd⁰, PdO [31] and Ti⁴⁺ in anatase [34] were presented in spectra as dashed lines.

Figure 2. Pd 3d (a,b) and Ti 2p (c,d) XPS spectra of Pd@TiO₂CA (a,c) and Pd@TiO₂SA (b,d). The reference values of binding energies for Pd⁰, PdO [31] and Ti⁴⁺ in anatase [34] were presented in spectra as dashed lines.

Figure 3. HAADF-STEM and EDX elemental mapping images of Ti and Pd (a,b) and corresponding Pd particle size distribution histograms (c,d) from the Pd@TiO₂CA (a,c) and Pd@TiO₂SA (b,d) catalysts.

Figure 3. HAADF-STEM and EDX elemental mapping images of Ti and Pd (a,b) and corresponding Pd particle size distribution histograms (c,d) from the Pd@TiO₂CA (a,c) and Pd@TiO₂SA (b,d) catalysts.

In order to determine the size of Pd particles and their distribution on the surface of TiO₂ supports, the catalysts were studied using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Figure 3 shows the HAADF-STEM and EDX elemental mapping images of Ti and Pd from the reduced Pd@TiO₂ catalysts (Figure 2a,b). According to the results obtained, the catalysts represent the aggregates of TiO₂ particles, uniformly coated with smaller Pd nanoparticles (the brightest spots in the HAADF-STEM images) of 4–5 nm in size (Figure 2c,d). At the same time the catalysts differ in size of TiO₂ particles and their agglomeration degree. The Pd@TiO₂CA catalyst is composed of 10–20 nm oval TiO₂ particles, forming an aggregate of 100–120 nm in size. In contrast, Pd@TiO₂SA is composed of smaller spherical TiO₂ particles (7–12 nm), forming a larger aggregate (400–500 nm) with poorly traced boundaries. It should be noted that HAADF-STEM data were consistent with the results of XRD and BET studies, confirming the size and agglomeration of TiO₂ particles. In addition, Pd particles in Pd@TiO₂CA were clearly seen, while in some sites of the Pd@TiO₂SA catalyst, the boundaries between Pd species (Pd and PdO) and TiO₂ particles were less visible (Figure S4). This can be explained by difference in morphology of the catalysts (TiO₂ particles in Pd@TiO₂SA were smaller than those of Pd@TiO₂CA). The higher content of PdO in the Pd@TiO₂SA catalyst compared with that of Pd@TiO₂CA (Figure 2a,b) could also have affected the visibility of the Pd species in HAADF-STEM images of the catalysts.

The XRD pattern of the Pd@TiO₂CA catalyst (Figure 1d) was nearly the same as starting TiO₂CA (Figure 1b). Diffraction peaks related to the palladium species (Pd or PdO) were not detected in the XRD pattern of the Pd catalyst, which can be explained by low Pd loading (Table 2), small particles sizes and their good distribution on the support surface (Figure 3) [35].

2.2. Photocatalytic H₂ Production

Before conducting photocatalytic H₂ evolution experiments, the optical properties of different titanium dioxides and Pd@TiO₂CA were evaluated using the Ultraviolet–Visible (UV-vis) diffuse reflectance spectroscopy method (Figure 4).

The UV-vis absorption spectra presented in Figure 4a shows that TiO₂CA, TiO₂SA, and TiO₂CRA samples possess slightly different optical properties, affecting their absorption of light. Decorating TiO₂CA with Pd shifts the wavelength towards the longer wavelengths, changing its optical properties.

The Kubelka–Munk function (denoted by Equation (2)) of the Tauc plot method was used to calculate the band gaps of the photocatalysts [36].

(hυF(Rα)^{^}(1/n) = A × (hυ − E_g),

(2)

where, hυ: photon energy (h: Planck constant, υ: vibration frequency), α: absorption coefficient, E_g: direct band gap, A: constant, n = ½, R: reflectance, and F(R): proportional to the absorption coefficient (α).

Figure 4b displays the plots of the Kubelka–Munk function with the axis of (f (R)hυ)² versus energy (hυ). The band gap is determined by extending the tangent line until it intersects the hυ axis. The band gap values were 2.95, 2.98, 3.00, and 3.11 eV for Pd@TiO₂CA, TiO₂SA, TiO₂CRA, and TiO₂CA, respectively. The presence of Pd species in Pd@TiO₂CA resulted in a decrease in the band gap energy in comparison with bare TiO₂CA. This phenomenon is ascribed to the characteristic absorption of surface excitation caused by the plasmon effect of the metal, which leads to enhanced light absorption towards the visible region [21]. The band gap values for TiO₂SA (2.98 eV) and TiO₂CRA (3.00 eV) were lower than those of TiO₂CA (3.11 eV). Since low band gap value is one of the most important indicators of high photocatalytic performance, the activity of the samples should have been decreased in the following order: Pd@TiO₂CA > TiO₂SA > TiO₂CRA > TiO₂CA. Testing of TiO₂-based photocatalysts in the H₂ evolution reaction showed that the total amount of hydrogen produced within 2 h was 317, 860, 1023, and 1236 μmol/g_cat for TiO₂SA, TiO₂CRA, TiO₂CA, and Pd@TiO₂CA, respectively (Figure 5). Such contradiction in the results is probably due to different agglomeration of TiO₂ particles. Pellegrino F. et al. have reported that the agglomerate/aggregate size reduction clearly decreases the scattering in the visible region and possibly increases the absorption in the UV region, directly affecting the photocatalytic activity of titanium dioxide [37]. According to XRD and BET studies’ data (Table 1), TiO₂CA showed a lower degree of agglomeration of TiO₂ particles, and, therefore, it was more effective compared with TiO₂SA and TiO₂CRA. Meanwhile, the modification of TiO₂CA with palladium did not significantly affect the surface area of titanium dioxide (Table 1), and as expected, the Pd@TiO₂CA photocatalyst showed higher activity than bare TiO₂CA. The total amount of hydrogen produced on Pd@TiO₂CA was 1236 μmol/g_cat within two hours. The photocatalytic H₂ evolution rate (PCHE) achieved up to 760 μmol/h g_cat, within the first hour and it decreased to 476 μmol/h g_cat during the second hour (Table S1) due to the decreasing concentration of the sacrificial reagent in the electrolyte, which is comparable with that of other photocatalysts from the literature (

Table 3. A comparison of photocatalytic properties of Pd@TiO₂CA with other known photocatalysts in the H₂ evolution process.

Photocatalysts	Amount of the Photocatalyst, mg	Sacrificial Agent	Light Intensity	PCHE Rate (μmol/h gcat)	Ref.
5%Pd–ZnS	50	Na2S/Na2SO3	100 mW cm−2	940	[38]
NiSe/MnO2–CdS	100	H2O	100 mW cm−2	455	[39]
RGO-Cd0.60Zn0.40S-0.5%Mo	70	Na2S/Na2SO3	Solar Simulator (1000 Wm−2)	1543	[40]
Pd@TiO2CA	70	Na2S/Na2SO3	Solar Simulator (1000 Wm−2)	760 *	This work

* PCHE rate for Pd@TiO₂CA presented in the Supplementary Materials (Table S1).

).

2.3. Catalytic Activity of Pd@TiO₂ Catalysts in the Hydrogenation Process

The Pd@TiO₂ catalysts were tested in the hydrogenation process under mild conditions (0.1 MPa, 40 °C). The hydrogenation of 5-hexyn-1-ol, 2-hexyn-1-ol, and 2-hexyne were chosen as the model reactions. Due to the good solubility of the substrates in ethanol, it was used as a solvent. The plausible pathways of the reactions are illustrated in Figure 6. Hydrogenation of the acetylenic compounds is a consecutive process. In the first step, the triple C–C bond of an acetylenic compound is reduced to C–C double one, forming its olefinic derivative (Figure 6, reactions 1 and 1′) which is then hydrogenated to a saturated alcohol or hydrocarbon (Figure 6, reactions 2 and 2′). However, direct formation of a saturated compound is possible (Figure 6, reactions 3). It should be noted that during the hydrogenation of 2-hexyn-1-ol and 2-hexyne (Figure 6b,c), the process is accompanied by the formation of both cis- and trans-isomers of an olefinic compound as intermediate products. In this case, a transformation of the cis-olefinic compound to it trans-isomer is also possible (Figure 6b,c, reactions 4).

The activity of the catalysts was estimated by measuring the H₂ uptake against time. Figure 7a shows the variation in H₂ uptake versus time during the hydrogenation of 2-hexyn-1-ol. The Pd@TiO₂CA and Pd@TiO₂CRA demonstrated higher catalytic activity than that of Pd@TiO₂SA. The semi-hydrogenation point (50 mL) was reached after 14, 16 and 21 min for Pd@TiO₂CRA, Pd@TiO₂CA and Pd@TiO₂SA, respectively. The 2-hexyn-1-ol hydrogenation rate, calculated from the hydrogen uptake data, is presented in Figure 7b. In all cases, the reaction rate (W) increased in the first minute and remained constant until a semi-hydrogenation point. Then, the reaction rate increased and after passing a maximum, it sharply decreased.

According to the chromatographic analysis, cis-2-hexen-1-ol is accumulated in the reaction medium on Pd@TiO₂CA in the initial period, and then is reduced to 1-hexanol (Figure 7c), confirming occurrence paths 1 and 2 in Figure 6b, respectively. Accumulation of cis-2-hexen-1-ol was accompanied by the formation of small amounts of trans-2-hexen-1-ol and 1-hexanol, and their yield at the semi-hydrogenation point was 3% and 1%, respectively. At the same time, the yield of cis-2-hexen-1-ol was 93%. Formation of trans-2-hexen-1-ol and 1-hexanol in the initial period of reaction (before reaching the semi-hydrogenation point) confirms occurrence paths 1′ and 3 in Figure 6b. After passing the semi-hydrogenation point (16 min), a small amount of cis-2-hexen-1-ol accumulated was also transformed to trans-2-hexen-1-ol (path 4 in Figure 6b), which was eventually reduced to 1-hexanol (path 2′ in Figure 6b). The composition of the reaction mixture changed similarly on the rest Pd@TiO₂ catalysts (Figure S5). The curves of the dependence of selectivity on conversion (Figure 7d) shows that Pd@TiO₂CA catalyst possessed a slightly higher selectivity to cis-2-hexen-1-ol compared with that of Pd@TiO₂SA and Pd@TiO₂CRA.

The hydrogenation rate and selectivity of the catalysts were calculated from the hydrogen uptake and chromatographic analysis data, respectively (Figure 7 and Figures S5–S9). A comparison of the catalytic properties of the catalysts during the hydrogenation of 5-hexyn-1-ol, 2-hexyn-1-ol and 2-hexyne is presented in

Table 4. A comparison of catalytic properties of Pd@TiO₂ catalysts in hydrogenation of acetylenic compounds.

Catalyst	W × 10−6, mol/s		WC≡C:WC=C	Selectivity *, %	Conversion **, %
	C≡C	C=C
2-hexyn-1-ol
Pd@TiO2SA	2.2	5.5	1:2.5	95	86
Pd@TiO2CRA	3.1	7.7	1:2.5	94	80
Pd@TiO2CA	2.6	6.1	1:2.3	96	97
2-hexyne
Pd@TiO2SA	27.0	5.2	5.2:1	92	61
Pd@TiO2CRA	29.5	8.1	3.6:1	88	62
Pd@TiO2CA	35.7	10.0	3.6:1	96	92
5-hexyn-1-ol
Pd@TiO2SA	6.0	13.8	1:2.3	96	77
Pd@TiO2CRA	6.1	13.6	1:2.2	92	76
Pd@TiO2CA	9.0	12.7	1:1.4	94	90

*—Selectivity to cis-2-hexen-1-ol, cis-2-hexene, and 5-hexen-1-ol for hydrogenation of 2-hexyn-1-ol, 2-hexyne and 5-hexyn-1-ol, respectively. **—The value of the substrate conversion used for the calculation of selectivity of the catalysts. Reaction conditions: 50 mg of catalyst, 0.25 mL of a hexynol or 1.0 mL of 2-hexyne, 25 mL of ethanol, at 40 °C and 0.1 MPa.

.

Accumulation of cis-2-hexen-1-ol on Pd@TiO₂SA occurred selectively (95%) at a rate (W_C≡C) of 2.2 × 10⁻⁶ mol/s, and then the rate increased to 5.5 × 10⁻⁶ mol/s, corresponding to the hydrogenation of double C–C bond (W_C=C). The W_C≡C to W_C=C rates ratio was 1:2.5. The Pd@TiO₂CA and Pd@TiO₂CRA catalysts showed higher catalytic activity and had near the same W_C≡C to W_C=C rates ratio. However, the selectivity of the Pd@TiO₂CRA catalyst was lower than that of Pd@TiO₂CA.

The hydrogenation of 2-hexyne on Pd@TiO₂ catalysts occurred much faster compared with that of 2-hexyn-1-ol. In addition, the hydrogenation rate of the triple C-C bond was higher than that of the double C-C one. This is probably due to the fact that after passing the semi-hydrogenation point, the cis-2-hexene accumulated was preferably converted to its trans-isomer reaching 50–55% in yield (isomerization of cis-2-hexene occurred much faster than the hydrogenation of its double C-C bond), which then reduced to hexane with a lower hydrogenation rate (Figure S9) [41]. It should be noted that the change in composition of the reaction mixture during hydrogenation of 2-hexyne (Figure S9) was near the same as in the case of 2-hexyn-1-ol (Figure 7c and Figure S5). Thus, the occurrence of all reactions presented in Figure 6c was also confirmed. A comparison of the catalytic properties of the catalysts shows that Pd@TiO₂CA possesses a higher catalytic activity and selectivity to cis-2-hexene compared with those of Pd@TiO₂SA and Pd@TiO₂CRA.

The hydrogenation of 5-hexyn-1-ol occurred faster in comparison with the hydrogenation of 2-hexyn-1-ol. This is probably due to the effect of the position of the -OH alcohol group on the electronic density of the C-C triple bond. The hydrogenation rates of triple and double C–C bonds in the presence of Pd@TiO₂SA achieved 6.0 × 10⁻⁶ and 13.8 × 10⁻⁶ mol/s, respectively. The hydrogenation of 5-hexyn-1-ol on Pd@TiO₂CRA occurred at the same W_C≡C and W_C=C rates. The composition of the reaction mixture on the catalysts also changed in nearly the same way (Figure S7). The hydrogenation of 5-hexyn-1-ol to 5-hexen-1-ol (path 1 in Figure 6a) was accompanied with the formation of a small amount of 1-hexanol (path 3 in Figure 6a). Then, the 5-hexen-1-ol accumulated was reduced to 1-hexanol (path 2 in in Figure 6a). However, the Pd@TiO₂CRA catalyst demonstrated lower selectivity to 5-hexen-1-ol (92%) compared with that of Pd@TiO₂SA (96%) and Pd@TiO₂CA (94%). The W_C≡C rate on Pd@TiO₂CA was 1.5-fold higher than that of the rest catalysts, while the W_C=C rate almost remained unchanged, which led to changing the W_C≡C to W_C=C rates ratio to 1:1.4.

A comparison of catalytic properties of Pd@TiO₂ catalysts in hydrogenation of different acetylenic compounds allowed revelation of the following facts: Pd@TiO₂CRA demonstrated the lowest selectivity to target products; Pd@TiO₂SA showed the lowest activity; and Pd@TiO₂CA was found to be the most optimal due to its higher activity and selectivity compared with Pd@TiO₂SA and Pd@TiO₂CRA, respectively. This correlates with photocatalytic studies data for different TiO₂, according to which TiO₂CA demonstrated the highest activity in H₂ evolution experiments, while TiO₂SA was found to be the least effective. The effect of support material on the properties of Pd particles was also confirmed by XPS studies data. Pd@TiO₂CA showed higher content of metallic Pd compared with Pd@TiO₂SA. In addition, it should be noted that a modification of the TiO₂CA with Pd led to an increase its photocatalytic activity in H₂ evolution experiments, making such Pd@TiO₂ catalysts good candidates for photocatalytic transfer hydrogenation. However, H₂ evolution and hydrogenation experiments were carried out at different conditions.

It well known that varying the operation parameters can affect the hydrogenation rate [42]. Therefore, a series of experiments were performed to investigate the kinetics of 5-hexyn-1-ol hydrogenation over Pd@TiO₂CA. The hydrogenation of 5-hexyn-1-ol was chosen due to it better solubility in water and ability to be hydrogenated more easily compared with 2-hexyne and 2-hexyn-1ol, respectively. The reaction parameters such as catalyst dosage (25–150 mg), hydrogen concentration in the H₂:He gas mixture (30–100 vol%), 5-hexyn-1-ol amount (0.25–1.00 mL), temperature (20–40 °C) and solvent (ethanol, water, 0.10 M NaOH aqueous solution, 0.35 M Na₂S and 0.25 M Na₂SO₃ aqueous solution and 0.10 M NaOH in ethanol) were varied (Figure 8). Figure 8a shows that the W_C≡C reaction rate is proportional to the amount of catalyst in the range of 25–100 mg. A further increase in catalyst amount (150 mg) did not affect the rate of reaction. This suggests that measurements under the experimental conditions studied (50 mg) are within the kinetic regime. A variation in the substrate amount, in the range of 0.25–1.00 mL (2.3–9.1 mmol), did not significantly affect the rate (Figure 8b), and the reaction seemed to be of zero order to 5-hexyn-1ol under the reaction conditions studied. Decreasing the reaction temperature from 40 to 20 °C led to a decrease in the triple C-C bond hydrogenation rate (W_C≡C), while W_C=C decreased up to 8.5 × 10⁻⁶ mol/s, passing through the maximum (W_C=C = 16.5 × 10⁻⁶ mol/s) at 30 °C (Figure 8c). The triple C-C bond hydrogenation rate decreased linearly with decreasing H₂ concentration in the direction of the origin of the coordinates (Figure 8d).

Hydrogenation of 5-hexyn-1-ol in water occurred 1.4 times slower than in ethanol without changing the W_C≡C to W_C=C rates ratio and selectivity to 5-hexen-1-ol. A similar W_C≡C hydrogenation rate was observed when 0.10 M NaOH aqueous solution was used as a solvent. However, the hydrogenation rate of the double C-C bond slightly decreased, changing the W_C≡C to W_C=C rates ratio from 1:2.8 to 1:2.5, which was also accompanied with an increase in selectivity up to 96%. This effect was more noticeable when the reaction was carried out in a 0.10 M NaOH solution in ethanol. The hydrogenation rate of the triple C-C bond increased up to 7.7 × 10⁻⁶ mol/s, while W_C=C decreased up to 11.3 × 10⁻⁶ mol/s. The W_C≡C to W_C=C rates ratio was 1:1.5, and the selectivity achieved was 97% (

Table 5. Catalytic properties of Pd@TiO₂CA catalysts in hydrogenation of 5-hexyn-1-ol in different solvents.

Solvent	W × 10−6, mol/s		WC≡C:WC=C	Selectivity, %	Conversion *, %
	C≡C	C=C
Ethanol	5.7	16.5	1:2.9	92	81
Water	4.1	11.6	1:2.8	93	79
NaOH in water	4.0	9.9	1:2.5	96	77
NaOH in ethanol	7.7	11.3	1:1.5	97	66

*—The value of the substrate conversion used for calculation of selectivity of the catalysts. Reaction conditions: 50 mg of catalyst, 0.25 mL of 5-hexyn-1-ol, 25 mL of solvent, at 30 °C and 0.1 MPa.

). Pd@TiO₂CA was not active in the aqueous solution, containing Na₂S and Na₂SO₃, which was probably due to the poisoning of the catalyst by the sulfur species [43].

As a 0.10 M NaOH solution in ethanol was found to be a good solvent for hydrogenation of 5-hexyn-1-ol, the recyclability of the Pd@TiO₂CA catalyst was studied in an alkali medium. The catalyst was reused for 35 runs without significant degradation in the catalytic activity (Figure 9). Moreover, after the 1st run, the activity of the catalyst increased, and the W_C≡C and W_C=C reaction rates were found to be up to 18 × 10⁻⁶ and 29 × 10⁻⁶ mol/s for the next consecutive runs. Such behavior can be explained by the effect of alkali on the charge of the TiO₂ support [44], which can provide more complete Pd reduction [32] during first run, and eventually an increase in activity of the catalyst (Pd⁰ is more active than PdO) for the next runs [45].

Thus, the results of experiments on varying the reaction conditions showed that the 5-hexyn-1-ol hydrogenation rate depends on the catalyst dosage, temperature, H₂ concentration and composition of the solvent. Among them, the effect of H₂ concentration and composition of the solvent should be separately highlighted. Decreasing the hydrogen concentration in the H₂:He gas mixture had a more significant effect on the performance of the process than the change in the catalyst amount and temperature. This suggests that performance of such bifunctional catalysts in photocatalytic transfer hydrogenation may be significantly limited by their activity in the H₂ evolution process. The composition of the solvent can also be crucial. A Na₂S/Na₂SO₃ alkaline aqueous solution cannot be used as a sacrificial agent in photocatalytic transfer hydrogenation due to its ability to suppress the hydrogenation process. On the contrary, performing the hydrogenation in an alkaline ethanol solution positively affected the selectivity of the process. In addition, the catalyst can be reused in an alkaline ethanol solution for at least 35 runs without significant degradation in the catalytic activity. A comparison of H₂ evolution (Table S1) and hydrogenation (Table 4) experiments’ data for the Pd@TiO₂CA catalyst showed that the hydrogenation process occurred much faster compared with H₂ evolution. This suggests that the photocatalytic transfer hydrogenation process can be significantly accelerated by the development of effective Pd-containing photocatalysts, providing high rates of H₂ production (comparable with those of hydrogenation process) in the presence of organic sacrificial agent.

3. Materials and Methods

3.1. Chemicals and Materials

TiCl₃ (10–15% aqueous solution, Sigma-Aldrich), NaOH (pure grade), TiO₂ (anatase, 99.7%, Sigma Aldrich, St. Louis, MI, USA), TiO₂ (mixture of rutile and anatase, 99.5%, Sigma Aldrich), ammonia solution (25% NH₃ in H₂O, analytical grade), PdCl₂ (59–60% Pd, Sigma Aldrich), KCl (pure grade), and ethanol (96.3%, pure grade) was used without additional purification. The purity of 2-hexyne (99%, Sigma Aldrich), 2-hexyn-1-ol (97%, Sigma Aldrich), and 5-hexyn-1-ol (96%, Sigma Aldrich) was confirmed by chromatography.

3.2. Synthesis of Titanium Dioxide (TiO₂SA)

Titanium dioxide (anatase) was synthesized by the chemical precipitation method. A 100 mL of 10–15% TiCl₃ aqueous solution was mixed with 400 mL of ethyl alcohol using a magnetic stirrer for 30 min. Fifty-six mL of a 25% NH₄OH solution was added dropwise to the resulting ethanol solution of TiCl₃ under magnetic stirring (pH reached 8.6). Then, the mixture was kept under stirring for 4 h until the color of the precipitate changed from violet to white (transition of Ti³⁺ to Ti⁴⁺). The resulting white precipitate was separated from the solution, washed several times with water and alcohol, dried by evaporation at a temperature of 90 °C, and then calcined at 350 °C for 2 h.

3.3. Preparation of Palladium Catalysts

To obtain supported Pd catalysts, commercial anatase (TiO₂CA), a commercial mixture of rutile and anatase (TiO₂CRA), and synthesized anatase (TiO₂SA) were used as support materials. The catalysts were prepared by a precipitation method according to the procedure described in [46]. In more detail, a potassium (II) tetrachloropalladate (K₂PdCl₄) precursor solution was prepared by crushing 168.4 mg of palladium (II) chloride and 155.7 mg of KCl in an agate mortar. The obtained K₂PdCl₄ was dissolved in 50 mL of distilled water at 70 °C. Five mL of 1.9 × 10⁻² M water solution of K₂PdCl₄ was added dropwise to the aqueous suspension of the support material (1.0 g in 20.0 mL of water) and stirred for 2 h. Then, 2.0 mL of 0.25 M NaOH aqueous solution was added dropwise to the mixture and stirred for 1 h. Further, the resulting catalyst was removed from the supernatant by centrifugation, washed several times with DI water until there was a neutral reaction of the wash water, and dried in air. The completeness of Pd immobilization on support materials was assessed by the residual concentration of palladium ions in the supernatant after the deposition process. The supernatant solution was neutralized with 0.25 M HCl in an amount equivalent to the added NaOH and analyzed to determine the concentration of [PdCl₄]²⁻ ions. The palladium concentration in the supernatant solutions was determined on an SF-2000 UV-Vis spectrophotometer (OKB Spectr, Saint Petersburg, Russia) using calibration curves at a wavelength of λ = 430 nm. The error of measurements is lower than 5%.

3.4. Characterization of the Composites and Catalysts

Powder X-ray diffraction (XRD) patterns were obtained with a DRON-4-0.7 X-ray diffractometer (Bourevestnik, Saint Petersburg, Russia) using cobalt-monochromatized Co Kα radiation (λ = 0.179 nm). The error of measurements is ±0.015 degree. Measurement of a specific surface area (BET) was carried out by the low-temperature N₂ adsorption-desorption technique using Accusorb equipment (Micromeritics, Norcross, GA, USA). The error of measurements is lower than 5%. Ultraviolet-visible diffuse reflectance spectra were recorded by a T92+ UV-Vis spectrophotometer (PG instruments, Wibtoft, UK). The spectra were measured against BaSO₄ at a wavelength range of 240–700 nm. The obtained diffuse reflectance data were converted to absorbance spectra using the Kubelka–Munk function. Band gap energies (Eg) were evaluated by Tauc plots of [F(R∞) E]^1/2 versus photon energy E, where R∞ = R_Sample/R_BaSO4. The elemental analysis was carried out using a JSM-6610LV (Jeol, Tokyo, Japan) scanning electron microscope with an EDX detector. Standard deviation determined for Pd content (Figures S1–S3) was less than 0.17%wt. X-ray photoelectron spectra (XPS) of catalysts were recorded on an ESCALAB 250Xi X-ray and Ultraviolet Photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with AlKα radiation (photon energy 1486.6 eV). Spectra were recorded in the constant pass energy mode at 50 eV for a survey spectrum and 20 eV for an element core level spectrum, using an XPS spot size of 650 μm. The total energy resolution of the experiment was about 0.3 eV. Investigations were carried out at room temperature in an ultrahigh vacuum of the order of 1 × 10⁻⁹ mbar. An ion-electronic charge compensation system was used to neutralize the sample charge. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrographs were obtained on a Zeiss Libra 200FE transmission electron microscope (Carl Zeiss, Oberkochen, Germany) with an accelerating voltage of 100 kV.

3.5. Photocatalytic H₂ Evolution Experiments

Photocatalytic hydrogen production reactions took place in a homemade quartz reactor. An Asahi Spectra MAX-303 Xenon device was used as a light source during the photocatalytic hydrogen production measurements. The light source was adjusted to 1000 W/m² via a light meter (HD 2302.0) in a manner that the source lighted an area of 3.5 × 3.5 cm of the base of the reactor in which the photocatalyst was fully located. Before the photocatalytic hydrogen production tests, TiO₂SA, TiO₂CRA, TiO₂CA, and Pd@TiO₂CA photocatalyst particles (70 mg) were suspended in aqueous solution of a 0.35 M Na₂S and 0.25 M Na₂SO₃ mixture (pH = 13) of sacrificial reagent and allowed to settle down for 15 h. Just after the light irradiation, H₂ generation took place and the qualitative and quantitative characterization of the produced H₂ was performed by connecting the photocatalytic reactor directly to Agilent 6890 GC gas chromatograph (Agilent, Santa Clara, CA, USA).

3.6. Hydrogenation of Acetylenic Compounds with Molecular H₂

The catalysts were studied during the hydrogenation of acetylenic compounds (2-hexyn, 2-hexyn-1-ol, 5-hexyn-1-ol) according to the procedure described in [42]. The process was carried out in a homemade thermostatically controlled long-necked glass flask reactor in an ethanol solution at 40 °C and atmospheric pressure of hydrogen, with intensive stirring (600–700 rounds per minute). A catalyst was pre-treated with hydrogen directly in the reactor at 40 °C (30 min) for its activation (reduction of Pd to a zero-valent state), and then, 0.25 mL (2.23 mmol) of a substrate was introduced in the reaction medium (50 mg of a catalyst in 25 mL of ethanol) to perform the reaction. The substrate amount was taken based on the uptake of 100 mL of hydrogen. For hydrogenation of 2-hexyne, the amount of the substrate was 1.0 mL. The reaction rate was calculated as the hydrogen consumption per unit of time. The amount of hydrogen uptake was determined by measuring the H₂ volume in a gas storage burette connected to the reactor. The reaction products were analyzed by gas chromatography on a Chromos GC-1000 chromatograph (Chromos, Russia) with a flame ionization detector using a BP21 (FFAP) capillary column with a polar phase (PEG-modified with nitroterephthalate) of 50 m in length and a 0.32 mm inside diameter. The selectivity of the catalyst was calculated as the ratio of the target product to the sum of all reaction products at a fixed conversion. For the hydrogenation of 5-hexyn-1-ol on Pd@TiO₂CA, a variation in operating parameters such as the catalyst loading (25–150 mg), amount of substrate (0.25–1.00 mL), temperature (20–40 °C), H₂ concentration in a hydrogen-helium gas mixture (30–100%vol.) and composition of solvent (ethanol, water, 0.1 M NaOH aqueous solution, 0.35 M Na₂S and 0.25 M Na₂SO₃ aqueous solution and 0.1 M NaOH in ethanol) was carried out.

4. Conclusions

In this work, different titanium dioxide powders were studied in photocatalytic hydrogen evolution from water and then used for the preparation of Pd@TiO₂ catalysts followed by their study in the hydrogenation of acetylenic compounds (2-hexyne, 2-hexyn-1-ol, and 5-hexyn-1-ol) with molecular hydrogen. For the hydrogenation of 5-hexyn-1-ol over the most active Pd@TiO₂CA catalyst, a variation in reaction parameters such as catalyst dosage, hydrogen concentration in the H₂:He gas mixture, amount of the substrate, temperature, and composition of solvent was also carried out. In addition, Pd@TiO₂CA was studied in the photocatalytic hydrogen evolution process. The main aim was to gain insights into the influence of various factors on photocatalytic H₂ evolution and conventional hydrogenation of acetylenic compounds for use in photocatalytic transfer hydrogenation. The results of this study revealed the following noticeable facts. The TiO₂CA photocatalyst with a lower agglomeration degree showed better activity in H₂ evolution than TiO₂CRA and TiO₂SA (showed the lowest activity). Further modification of TiO₂CA with Pd resulted in increasing its activity and achieving an H₂ evolution rate of 760 μmol/h g_cat, which is comparable to that of other known photocatalysts from the literature. The Pd@TiO₂CA catalyst was found to be more active in the hydrogenation of various acetylenic compounds compared with Pd@TiO₂CRA and Pd@TiO₂SA. TiO₂CA support in the Pd@TiO₂CA catalyst provided easier H₂-assisted reduction of Pd²⁺ species into Pd⁰ active particles than TiO₂SA in Pd@TiO₂SA. The Pd@TiO₂CA and Pd@TiO₂SA catalysts were found to be different in morphology. The Pd@TiO₂CA catalyst is composed of 10–20 nm oval TiO₂ particles, forming an aggregate of 100–120 nm in size. Meanwhile, Pd@TiO₂SA is composed of smaller spherical TiO₂ particles (7–12 nm), forming a larger aggregate (400–500 nm) with poorly traced boundaries. A variation in reaction parameters such as hydrogen concentration and composition of a solvent during hydrogenation of 5-hexyn-1-ol over the most active Pd@TiO₂CA affected the hydrogenation rate more significantly than that of other ones (catalyst dosage, amount of the substrate, and temperature). The hydrogenation rate decreased linearly with the decreasing hydrogen concentration in the H₂:He gas mixture (30–100 vol%) in the direction of the origin of the coordinates. The Pd@TiO₂CA catalyst was not active when the Na₂S/Na₂SO₃ alkaline aqueous solution was used as a solvent due to the sulfur species’ ability to suppress the hydrogenation process. In contrast, performing hydrogenation in 0.1 M NaOH ethanolic solution positively affected the selectivity of the process. In addition, the catalyst was reused in alkaline ethanol solution for 35 runs without significant degradation in catalytic activity. Moreover, the activity of the catalyst increased in two times after the 1st run. A comparison of performance of Pd@TiO₂CA in two separate processes showed that the hydrogenation of acetylenic compounds occurred much faster than photocatalytic H₂ evolution from water. Based on the mentioned facts, the following findings and recommendations have been formulated: (a) The activity of TiO₂ in the photocatalytic H₂ evolution reaction was found to be affected by the agglomeration degree of TiO₂ particles and their modification with Pd. Therefore, there are possibilities to regulate the photocatalytic properties of TiO₂-based materials. (b) The state of Pd in a Pd@TiO₂ catalyst can be affected by the photocatalytic properties of the TiO₂ support. Therefore, using TiO₂-based materials with excellent photocatalytic properties as supports for Pd particles may result in the formation of effective Pd hydrogenation catalysts. (c) The Na₂S/Na₂SO₃ alkaline aqueous solution was found to be a bad solvent for the hydrogenation process over the Pd@TiO₂CA catalyst. Therefore, it cannot be used as a sacrificial agent in photocatalytic transfer hydrogenation over Pd@TiO₂ catalysts. (d) Sodium hydroxide solution (0.1 M) in ethanol was found to be a good solvent for hydrogenation processes. Therefore, performing the photocatalytic transfer hydrogenation in alkaline ethanol solution as both solvent and sacrificial reagent may positively affect the activity, selectivity and stability of Pd@TiO₂ catalysts. (e) The hydrogenation of acetylenic compounds occurred much faster than photocatalytic H₂ evolution. Therefore, the progress in photocatalytic transfer hydrogenation can be achieved after the development of new photocatalysts, possessing high performance in H₂ evolution reaction.