Arzoo
Chauhan‡
a,
Rajat
Ghalta‡
a,
Rajaram
Bal
b and
Rajendra
Srivastava
*a
aCatalysis Research Laboratory, Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab-140001, India. E-mail: rajendra@iitrpr.ac.in; Tel: +91-1881-232064
bNanocatalysis Area Conversion and Catalysis Division, CSIR-Indian Institute of Petroleum, Dehradun-248005, India
First published on 3rd May 2023
The selective hydrogenation of α–β unsaturated carbonyl compounds requires a catalyst with a suitable combination of the support and active sites to activate a specific functional group. In this work, ZnO and g-C3N4(CN) nanocomposite (ZnO/CN) supported Ru catalysts were synthesized for thermal and photochemical selective hydrogenation of cinnamaldehyde (CAL). Under thermal conditions, formic acid (FA) was employed as a hydrogenating agent in water, and 85% cinnamyl alcohol (COL) selectivity was achieved with nearly complete CAL conversion. The acidity of ZnO activated the CO group of CAL, the basicity of CN facilitated the adsorption of FA, and the decorated Ru assisted the FA to H2 formation leading to the selective production of COL. A FT-IR study confirmed the effective adsorption of CAL through C
O, yielding the selective formation of COL. In contrast, under photochemical conditions, hydrocinnamaldehyde (HCAL) was the selective hydrogenation product that was formed due to the efficient migration of charge carriers at the interface of the Z-scheme heterojunction of CN and ZnO. The synergistic effects at the interface were crucial for the charge transfer mechanism, enhancing the charge carriers' lifetime and enabling excellent charge separation under photocatalytic conditions. Detailed characterization and control reactions were performed to establish the structure–activity relationship and to conclude the plausible reaction mechanism under both conditions.
A low-cost Ru-based catalyst has shown promising activity for the selective hydrogenation of α–β unsaturated aldehydes to α–β unsaturated alcohols.8–10 Research has been conducted to increase the catalytic efficiency of Ru by modulating the nature of the support, hydrogenating agents, and introducing a second metal with Ru, solvents, and reaction conditions.11–13 The introduction of a second metal is witnessed to improve the efficiency but inevitably adds to the cost.14 Good metal–support interaction improves the metal dispersion, stability, and selectivity, owing to more efficient catalysis. N-containing carbon supports such as C3N4 (CN) exhibit considerable basicity due to high N content, which holds the metal nanoparticles, preventing the deactivation of the catalyst.15,16 The incorporation of metal oxides assists in establishing the interaction between the carbonyl group of the substrate and the catalyst surface through effective adsorption of CO, resulting in selective hydrogenation due to the Lewis acidic character of metal oxides.17 Primarily, molecular hydrogen is used as a hydrogenating agent for α–β unsaturated compounds.18–23 However, the process becomes more challenging when H2 is substituted by indirect hydrogenating agents such as formic acid, as maintaining alcohol selectivity is difficult.24 We have recently developed metal-free and metal-based catalysts for utilizing formic acid as an H2 and formylation source to form selective products.25–28 In addition to the catalyst design, the solvent governs the activity and product selectivity. Water is preferable in developing sustainable catalytic protocols. Due to its dipolar nature, water is effective in hydrogenating C
O over C
C in unsaturated substrates.29,30 Moreover, the water molecules undergo a hydrogen exchange with the formic acid and, thus, accelerate the reaction kinetics.31
Besides thermocatalytic exploration, cinnamaldehyde reduction can also be explored under photocatalytic conditions with different light sources.32 Generally, COL is detected as the primary product, and very few reports suggest the formation of HCAL as a major product under photocatalytic conditions.33 Most of the photocatalytic processes are based on noble-metal catalysts. A bimetallic catalyst composed of Au and Pd decorated on an acidic support offered good selectivity for HCAL (91%).34 A fully hydrogenated product, hydrocinnamyl alcohol (HCOL), was observed as the final product over a transition metal catalyst.35,36 Thus, designing an economic catalyst without compromising the HCAL selectivity needs to be developed.
Herein, a Ru decorated ZnO/g-C3N4 catalyst is synthesized for the selective hydrogenation of cinnamaldehyde under thermocatalytic and photocatalytic conditions. Under thermal conditions, COL was obtained with 85% selectivity in water with formic acid as a hydrogen source. To the best of our search, it will be the first report wherein formic acid has been utilized as a hydrogen donor for the selective hydrogenation of cinnamaldehyde in water. Interestingly, ∼100% selective production of HCAL was achieved under photocatalytic conditions (visible light). The interface formation between g-C3N4 (CN) and ZnO was key in the charge transfer mechanism, enhancing the charge carriers' lifetime and resulting in excellent charge separation under photocatalytic conditions. Two different products by switching thermocatalytic to photocatalytic conditions make this study exciting and would attract significant attention from researchers in diverse areas.
All the ZnO/CN nanocomposites have a distinctive peak at 27.4°, attributed to the (002) plane of graphitic carbon of CN (Fig. 1a).39 Decrease in the intensity of (002) was observed with increasing ZnO content (Fig. 1a). It indicates the successful formation of the ZnO/CN composites where (002) plane intensity varies with the amount of CN in nanocomposites. Fig. 1b shows the XRD patterns of the Ru-decorated ZnO/CN composites. For better presentation, a plot keeping the intensity at the same scale on the Y-axis for all the catalysts is provided (Fig. S2a†), indicating no change in the intensity of CN with increasing Ru content. No peaks corresponding to Ru NPs were observed. For an accurate determination of the existence of Ru NPs, a 10% Ru/SS catalyst was prepared by loading 10 weight % of Ru NPs on silica spheres. The material was reduced employing the same reduction conditions as were used for the preparation of Ru-decorated ZnO(1.5)/CN catalysts. The XRD pattern of 10% Ru/SS shows intense peaks ascribed to different planes of the Ru NPs (Fig. S2b†) corresponding to the JCPDS card number 6-663. Comparing the XRD patterns of 10% Ru/SS and Ru-decorated ZnO(1.5)/CN, no peak corresponding to Ru NPs was observed in Ru-decorated ZnO(1.5)/CN catalysts. No diffraction peak corresponding to Ru NPs in the catalysts synthesized in this study is attributed to lower % Ru loading, small size, and high dispersion of Ru NPs.
The surface morphology of the catalysts was examined by scanning electron microscopy (SEM), and an irregular morphology was observed in the SEM image of ZnO at low magnification. On higher magnification, a mixture of rod and filament-like ZnO was observed with rods in excess, as suggested by the XRD analysis (Fig. S3a and b†).38 The SEM images of ZnO/CN composites are provided in Fig. S3c–e,† and the SEM images of 1% Ru@ZnO(1.5)/NC and 3% Ru@ZnO(1.5)/NC are provided in Fig. S3f and g.† The FESEM images of the ZnO(1.5)/CN composite show rods of ZnO distended outwards from the sheets of CN, which proposes the 3-dimensional growth of ZnO over the sheet-like morphology of CN, confirming the stacked interlinked network at the interface of the two materials (Fig. 3c and d). The HR-TEM images of 1% Ru@ZnO(1.5)/NC confirm the ZnO and CN phases. ZnO nanorods are visibly protruding outwards from the nanosheets of CN, forming an interface between the two materials (Fig. 3e and f). The (002) plane of hexagonal wurtzite ZnO was identified as the source of lattice fringes with a d-spacing of 0.261 nm (Fig. 3g).40 In Fig. 2h, the distant lattice fringes, at 0.200 and 0.253 nm, are associated with the (101) plane of metallic Ru NPs and (011) of ZnO, which suggests the successful Ru(0) incorporation in the 1% Ru@ZnO(1.5)/NC catalyst.41,42 Similar trends are observed for 3% Ru@ZnO(1.5)/NC (Fig. S3h–j†). The atomic % and metal dispersion of 1% Ru@ZnO(1.5)/NC and 3% Ru@ZnO(1.5)/NC catalysts were analyzed by EDS and elemental mapping (Fig. S4 and S5†). The elemental mapping demonstrates that Ru is homogeneously dispersed on the ZnO/CN nanocomposite.
X-ray photoelectron spectroscopy analysis was conducted for the 1% Ru@ZnO(1.5)/CN and 3% Ru@ZnO(1.5)/CN to examine the chemical states of the elements. The catalyst comprises Ru, Zn, C, N, and O, specified by the surface survey scan of 1% Ru@ZnO(1.5)/CN (Fig. 3a). The Zn XPS spectrum is composed of two peaks, at binding energies of 1044.8 and 1021.7 eV, assigned to the Zn 2p1/2 and Zn 2p3/2 peaks of Zn2+, respectively.43 The O 1s exhibits four peaks at binding energies 533.0, 531.4, 531.7, and 530.4 eV associated with Ru–O, defects present in the framework, O–H groups attached to the CN sheets, and the lattice oxygen (Zn–O), respectively.44 N 1s was deconvoluted into three peaks, 400.8 eV (NHx, amine functional groups), 399.8 (NC3, tertiary nitrogen), and 398.5 eV (pyridinic N).45 The dominance of the pyridinic N is in accordance with the urea-derived pristine CN. The high-resolution spectrum for C 1s is composed of three peaks. The peaks at 287.9, 284.8, and 284.1 are ascribed to sp2 hybridized N–C–N carbon, (C)3–N bonded carbon, and C–C bonded carbon.46 The peak at 284.47 is due to the overlapping of Ru 3d and C 1s and the high intensity is due to the merger of Ru and C signals.45 Furthermore, the greater peak intensity of C 1s in 3% Ru@ZnO(1.5)/CN (Fig. S6e†) than that of 1% Ru@ZnO(1.5)/CN confirms this observation. Fig. 3e shows the deconvoluted peaks for Ru 3d and C 1s. For the sake of clarity and better understanding, Ru 3p was considered for the further confirmation of chemical states (Fig. 3f), which also consist of an intense peak at 475.2 for Zn LMM47 (refer to surface scan, Fig. 3a) due to which Ru 3p3/2 and Ru 3p1/2 were deconvoluted separately (Fig. 3g and h). Fig. 3g shows two deconvoluted peaks of Ru 3p3/2 at 461.9 and 464.4 eV corresponding to Ru(0) and Ru(+4). Similarly, Fig. 3h shows peaks at 483.6 and 486.3 eV for Ru 3p1/2 corresponding to Ru(0) and Ru(+4), respectively.48,49 The XPS analysis plots of 3% Ru@ZnO(1.5)/CN are provided in Fig. S6 in the ESI.† Similar results were obtained except for more intensified peaks corresponding to Ru due to the higher % loading. The atomic % of the elements based on XPS is provided in Table S1.† Bulk elemental analysis (using microwave plasma-atomic emission spectroscopy) was conducted to determine the Ru NP concentration, which was estimated to be 0.94% and 2.72% for 1% Ru@ZnO(1.5)/CN and 3% Ru@ZnO(1.5)/CN, respectively.
![]() | ||
Fig. 3 (a) XPS surface survey of 1% Ru@ZnO(1.5)/CN, (b) Zn 2p, (c) O 1s, (d) N 1s, (e) Ru 3d and C 1s, (f) Ru 3p and Zn LMM, (g) Ru 3p3/2, and (h) Ru 3p1/2. |
The BET analysis was conducted for all the synthesized catalysts to compare the effect of composite formation and metal loading on the surface properties. The adsorption isotherms for all the ZnO/CN composites and Ru decorated composite show a type II isotherm with H3 hysteresis loops, indicating the formation of interparticle mesopores in the nanocomposites (Fig. S7†).50 Moreover, with the introduction of ZnO into the CN, the surface area of the composite increased from 45 to 65 m2 g−1 (Table S2†), which could be due to the thermal exfoliation and fragmentation of CN sheets, as indicated by SEM images. The metal loading marginally increased the surface area because Ru NP sites provided additional sites for N2-adsorption.51,52 Table S2† summarises all synthesized catalysts' surface area and total pore volume.
For calculating the compositions of the ZnO/CN nanocomposites, the zinc source (Zn(OAc)2)·2H2O was subjected to TGA. 1/5 mass of ZnO was obtained from Zn(OAc)2·2H2O (Fig. S8a†). A sharp weight loss near 300 °C was observed in the TGA of Zn(OAc)2·2H2O. Above 300 °C, Zn(OAc)2·2H2O got converted into ZnO, and no significant weight loss was observed above 400 °C. Therefore, 400 °C was the best calcination temperature for preparing ZnO. The ZnO showed good stability in the thermogram with ∼2% weight loss up to 800 °C (Fig. S8b†). In contrast, CN showed 100% weight loss below 700 °C (Fig. S8a†). The composites show different % of weight loss up to 700 °C, which depends on the composition of the nanocomposite (Fig. S8b†). Furthermore, all the catalysts were thermally stable up to 400 °C, which signifies that catalysts are not prone to decomposition at the reaction temperature adopted in this study.
CO2-TPD was conducted to estimate the basicity of the catalysts imparted by CN predominantly. The basicity is helpful for the adsorption and activation of FA. A peak associated with the moderate basic sites in the temperature range of 400 to 500 °C was observed in all the ZnO(X)/CN catalysts. The peak intensity was directly proportional to the CN content in the catalyst (Fig. S9a†). Similarly, NH3-TPD was undertaken to evaluate the acidic sites due to ZnO, which is required for the adsorption and activation of the CO group of CAL. The acidity increased with the increasing amount of ZnO in the composite catalysts. ZnO(1)/CN had the lowest acidity, whereas ZnO(2)/CN had the highest value of acidity (Fig. S9b†).
A photoluminescence (PL) experiment was performed to evaluate the charge separation efficiency (Fig. 4b). CN exhibited a broad and most intense emission covering the visible region with a peak maximum of 480 nm. ZnO showed a lower intense emission spectrum than CN. The heterojunction of CN and ZnO separates the charge carriers more efficiently; therefore, less intense emission spectra were observed.55 Moreover, the Ru-decorated nanocomposite showed relatively lower emission with a broader peak in the visible region. The Ru NPs, with their higher work function and low Fermi level, effectively captured the photogenerated electrons from the semiconductor, resulting in efficient separation of charge carriers. This process leads to a reduction in the intensity of the photoluminescence (PL) emission peak. The Ru NPs also increased the lifetime of charge carriers by reducing their recombination rate, potentially leading to enhanced photocatalytic efficiency. The luminescence decay characteristics of unmodified CN, ZnO, and their composite were examined with the Time-Correlated Single Photon Counting (TCSPC) technique. The decay curves were fitted using a mathematical equation that accounts for the contribution of discrete emissive species.45 The pre-exponential factors and the time of excited-state luminescence decay were determined for each component. The average decay time, 〈τ〉, was calculated using
, and the fractional contribution for each decay was obtained using
.52 A three-exponential fit was used to model the decay curve. The result showed that the nanocomposite had a higher average decay lifetime of 3.2 ns than the individual components, and the introduction of Ru NPs further raised the decay lifetime to 4.11 ns (Fig. 4c). The TCSPC analysis confirmed that the nanocomposite had a better decay lifetime than pristine CN and ZnO, and introducing Ru NPs enhanced the charge retention capability by accommodating photogenerated electrons. The details of the investigation and calculated parameters are provided in Table S4.†
The transient current response of all photocatalysts was recorded with an on–off photocurrent experiment (i–t) (Fig. 4d). The semiconductor material exhibited an increased current response upon irradiation and promptly returned to its initial value upon turning off the light source. The analysis of the current response was repeated for multiple test cycles, and a consistent response was observed in each cycle, indicating stable performance. CN showed the least response among all catalysts because of easy charge recombination. ZnO possessed a moderately higher response for the test cycle evidencing the slow recombination. The CN/ZnO heterojunction showed a relatively higher response than bare CN and ZnO because, in the heterojunction, the electrons and holes were separated in different parts of the junction, enhancing its catalytic activity. The Ru NP decorated CN/ZnO catalysts exhibited a higher photocurrent response than the bare ZnO(1.5)/CN. The rise in the Ru content in the heterojunction resulted in an increase in the current response because Ru NPs aided in the effective migration of charge carriers.57 The heterojunction catalyst with 3% Ru@ZnO(1.5)/CN showed the highest current response, which can be attributed to its higher conductivity. The relationship between carrier concentration (N) and electrical conductivity (σ) can be expressed using the equation σ = Neμ, where “e” is the charge and μ denotes carrier mobility.45 Therefore, in 3% Ru@ZnO(1.5)/CN, a significantly higher charge carrier generation and migration was observed during the light illumination. In conclusion, Ru NPs promoted the mobility of the produced charge carriers and improved the catalyst's performance.
EIS was performed for all the photocatalysts under light illumination, and the resulting Nyquist plots (Fig. 4e) showed semicircles, the size of which correlates directly with the charge transfer rate.59 The diameter of the semicircle reflects the interfacial charge transfer resistance (Rct) present at the electrode–electrolyte interface. A reduction in the diameter of the semicircle signifies a decline in interfacial charge transfer resistance, thereby indicating improved charge transfer for the migration of charge carriers. The Rct values were determined for CN, ZnO, ZnO/CN, and Ru-decorated ZnO/CN. The largest semicircle diameter (Rct) was obtained for ZnO, whereas CN has a comparably smaller arc diameter, indicating better charge migration. The heterojunction showed a lower semicircle arc compared to ZnO and CN. The introduction of Ru NPs facilitated the charge carriers' migration, resulting in a reduction of the arc diameter with an increase in the Ru content. Among the various photocatalysts tested, 3% Ru@ZnO(1.5)/CN demonstrated the smallest Rct value, indicating its superior photoactivity. The Ru NPs expedited the proficient migration of charge carriers by stimulating the charge transfer mechanism.
The complete band structures of CN, ZnO, and ZnO(1.5)/CN were explored using valence band X-ray photoelectron spectroscopy (VB-XPS), ultraviolet photoelectron spectroscopy (UPS), and diffuse reflectance UV-visible (DRUV-vis) spectroscopy. The VB-XPS spectra of CN and ZnO (Fig. 5a and c) revealed that their valence band maxima (VBM) were positioned at 1.71 eV and 2.30 eV, respectively, with respect to their Fermi levels (Ef).63 The Fermi level of both CN and ZnO was linked to their work function Φ, which is the energy difference between the Fermi level and vacuum. The UPS spectra (Fig. 5b and d) provided further insight into the work function Φ values for CN and ZnO, which were calculated to be 4.51 eV and 5.1 eV, respectively, using the relation Φ = 21.22 − ESE cutoff + Ecutoff.45,64 By combining the data of VB-XPS, DRUV-vis, and UPS spectra, the calculated conduction band (CB) and valence band (VB) values of CN were 6.22 eV and 3.40 eV, respectively, with respect to vacuum.64 The CB and VB values of ZnO were 7.40 eV and 4.27 eV, respectively (Fig. 5e). The band edge values are expressed in Evac and can be changed to ENHE through the addition of −4.44 ± 0.02 eV.65 The band edges of CN and ZnO (Fig. 5e) were observed to align well with the band structure estimated from the Mott–Schottky plots, with a slight shift.
![]() | ||
Fig. 5 (a) VB-XPS spectrum of CN, (b) UPS spectrum of CN, (c) VB-XPS spectrum of ZnO, (d) UPS spectrum of ZnO, and (e) band structures of photocatalysts of this study. |
The Fermi level is the maximum energy level at which an electron can exist. When CN and ZnO are combined in a composite, the Fermi level of CN (4.51 eV vs. vacuum) declined, and ZnO (5.1 eV vs. vacuum) rose till it attained a new equilibrium at 4.63 eV vs. vacuum observed in the UPS spectrum of ZnO(1.5)/CN (Fig. S12a†). Therefore, the nanocomposite had a newly formed Fermi level at 4.63 eV vs. vacuum (Fig. 5e). It is thoroughly described in the literature that Ru has a Fermi level of 4.7 eV vs. vacuum, which is lower than the calculated Fermi level of the nanocomposite (Fig. 5e).66–68 Therefore, the decoration of Ru NPs in the ZnO(1.5)/CN nanocomposite leads to the transfer of electrons over Ru NPs with marginal rearrangement in the Fermi level (discussed later). The UPS spectrum of 3% Ru@ ZnO(1.5)/CN showed that the material had a Fermi level at 4.66 eV vs. vacuum (Fig. S12b†), which lies between the Ru and ZnO(1.5)/CN nanocomposite Fermi levels (Fig. 5e).
EN | Catalyst | Conversion (%) | Selectivity (%) | TOF (min−1) | ||
---|---|---|---|---|---|---|
CAL | COL | HCAL | HCOL | |||
a Reaction conditions: CAL (0.5 mmol), FA (4.5 mmol), catalyst (20 mg), water (3 mL), temperature (140 °C), and time (6 h). b Average of three measurements presented as an integer. c Reaction conducted with 10 bar H2 instead of FA. d TOF is calculated considering the whole mass of the catalyst. e TOF is calculated considering the Ru content. | ||||||
1 | None | 2 | 0 | 100 | 0 | — |
2 | CN | 6 | 0 | 100 | 0 | 1.4 |
3 | ZnO | 15 | 100 | 0 | 0 | 18.3 |
4 | ZnO(1)/CN | 20 | 60 | 40 | 0 | 4.5 |
5 | ZnO(1.5)/CN | 18 | 65 | 35 | 0 | 3.9 |
6 | ZnO(2)/CN | 16 | 67 | 33 | 0 | 0.7 |
7 | 1% Ru@ZnO | 40 | 89 | 0 | 11 | 8.1 e(1015) |
8 | 1% Ru@CN | 100 | 0 | 0 | 100 | 23 e(2538) |
9 | 0.5% Ru@ZnO(1.5)/CN | 61 | 73 | 0 | 27 | 13.4 e(3096) |
10 | 1% Ru@ZnO(1.5)/CN | 96 | 85 | 0 | 15 | 40 e(2436) |
11 | 1.5% Ru@ZnO(1.5)/CN | 100 | 77 | 0 | 23 | 42 e(1685) |
12 | 1% Ru@ZnO(1.5)/CN | 30 | 100 | 0 | 0 | 12.6 e(761) |
The involvement of water in the hydrogen-exchange pathway with formic acid is already documented, which suggests that it is a suitable solvent for the selective formation of COL.25,31 Furthermore, water also assists in the effective dissociation of FA by the exchange mechanism, already proposed in our previous report.25 Two control reactions for the formic acid-mediated hydrogenation were conducted to confirm the exchange pathway between water and formic acid: first using H2O as a solvent and the other based on isotope labelling in which D2O was used instead of H2O. Both reactions were conducted under optimized conditions. The reaction mixtures were then subjected to GC-MS analysis. The mass spectra of the products formed in both cases are provided in Fig. S14.† In H2O, cinnamyl alcohol (C9H10O) was the established product with a molecular mass of 134 g mol−1, consistent with the analyzed mass spectrum (B). Three noticeable fragments, in this case, are 105, 115, and 134 (highlighted in yellow), with the base peak 92. However, in the D2O mediated reaction, deuterated cinnamyl alcohol (C9H9DO) with a molecular mass of 135 (A) was formed instead of cinnamyl alcohol (C9H10O). An increment of 1 was observed in the three fragments, which appeared at 106, 116, and 135 with the same base peak of 92. It confirms that an exchange reaction occurred between formic acid and H2O, which facilitated formic acid to dissociate into the H2 and CO2 (confirmed by GC analysis, Fig. S13†). When D2O was used, the exchange of “D” from D2O occurred with HCOOH and formed HCOOD, which decomposed to HD and CO2. The generated HD reacted with cinnamaldehyde to produce deuterated cinnamyl alcohol (C9H9DO). Hence, the molecular mass of deuterated cinnamyl alcohol increased by “1” when the aldehydic group of cinnamaldehyde was hydrogenated in the presence of formic acid and D2O. A GC-MS chromatogram of the reaction mixture recovered after 3 h in thermal hydrogenation of CAL shows the formation of COL and HCOL (Fig. S15†). 1H NMR of COL in the reaction mixture of thermal hydrogenation of CAL is provided in Fig. S16.†
All the parameters influencing the catalytic activity were varied to optimize the reaction conditions. Initially, the temperature was varied from 120 °C to 160 °C. Only 18% CAL conversion with 90% COL selectivity was obtained at 120 °C. On conducting the reaction at an elevated temperature (160 °C), almost 100% conversion was achieved, but instead of COL, HCOL was selectively obtained. So, the temperature (140 °C) was the optimum temperature, yielding 85% COL with 96% conversion (Fig. 6a). The role of FA concentration was also explored (Fig. 6b). 4.5 mmol of FA was sufficient for achieving the highest yield of COL (85%), whereas further increasing the FA, the COL selectivity was reduced. Similarly, the reaction time and catalyst amount were optimized. The best COL yield was achieved in 6 h with 20 mg catalyst (Fig. 6c and d). To our knowledge, this is the first report where FA is employed as a hydrogen source for the selective reduction of CAL to COL over a heterogeneous catalyst. Therefore, providing comparative activity data for this catalytic protocol is difficult.
The kinetics of the reaction was investigated by conducting experiments at 140 °C for different durations. The CAL conversion was assessed for various order equations, and a first-order rate equation provided a linear graph. The rate constant (k) at 140 °C was 4.33 × 10−5 s−1, deduced from the slope of the linear plot. The experiment was conducted at different temperatures (130–150 °C), resulting in an increased reaction rate and rate constant with a temperature rise. The rate constant values were 4.33 × 10−5 s−1 at 130 °C and 7.26 × 10−5 s−1 at 150 °C. The Arrhenius equation was used to compute the Ea, and a linear plot of ln[k] vs. 1/T was obtained with an R2 value of 0.99 (Fig. 7b). The resulting straight-line plot ascertained the Ea as 82.44 K J mol−1.
![]() | ||
Scheme 3 Schematic presentation for the thermo-catalytic plausible reaction mechanism for FA-assisted CAL hydrogenation over the 1% Ru@ZnO(1.5)/CN catalyst. |
EN | Catalyst | Conversion (%) | Selectivity (%) | AQY |
---|---|---|---|---|
a Reaction conditions: substrate (0.5 mmol), catalyst amount (20 mg), IPA (5 mL), 150 W LED, room temperature, H2 (2 bar), and time (5 h). b Average of three measurements is presented as an integer. c Without H2. d Dark. e Sunlight (4 h reaction time and 10 am–2 pm). | ||||
1 | ZnO | 2 | >99 | 13.9 × 10−3 |
2 | CN | 3 | >99 | 18.8 × 10−3 |
3 | ZnO(1.5)/CN | 7 | >99 | 43.5 × 10−3 |
4 | 1% Ru@CN | 13 | >99 | 80.8 × 10−3 |
5 | 1% Ru@ZnO | 10 | >99 | 68.4 × 10−3 |
2 | 1% Ru@ZnO(1.5)/CN | 18 | >99 | 111.1 × 10−3 |
3 | 2% Ru@ZnO(1.5)/CN | 32 | >99 | 194.3 × 10−3 |
4 | 3% Ru@ZnO(1.5)/CN | 96 | >99 | 580.2 × 10−3 |
5 | 4% Ru@ZnO(1.5)/CN | 99 | >99 | 595.8 × 10−3 |
6c | 3% Ru@ZnO(1.5)/CN | 12 | >95 | 72.5 × 10−3 |
7d | 3% Ru@ZnO(1.5)/CN | — | — | 6 × 10−3 |
8e | 3% Ru@ZnO(1.5)/CN | 99 | >99 | 931.4 × 10−3 |
The spectral response of 3% Ru@ZnO(1.5)/CN was analyzed by photocatalytic reactions with violet, blue, green, red, and white 9 W LED bulbs under the optimized reaction conditions. The emission peak maxima for violet, blue, green, and red LEDs were observed in the visible region. The effect of the spectral response on the photocatalytic CAL conversion was studied, and the results demonstrate that the catalyst's activity followed the order: violet > blue > white > green > red, which correlates with the catalyst's light absorption properties (Fig. S20†).
The photocatalytic reduction of CAL was carried out at room temperature, and only the first-order rate equation resulted in a linear graph for CAL conversion (Fig. 7c). The rate constant was calculated to be 6.25 × 10−5 s−1 from the straight-line plot for first-order kinetics under photocatalytic conditions. Since the reaction is mediated by the photocatalyst, the kinetics depend on the light source's power. Thus, ln [CAL] vs. time plots were obtained for reactions at different light intensities. The kinetic profiles revealed that the rate constant decreased from 6.25 × 10−5 to 1.23 × 10−5 s−1 as the light intensity was reduced from 1210 to 270 W m−2. A linear relationship between the rate constant and light intensity with an R2 value of 0.98 was detected (Fig. 7d), indicating that the reaction kinetics were directly proportional to light intensity.
To scrutinize the ability of the photocatalyst to form OH radicals, a terephthalic acid test was executed as a control experiment, utilizing PL spectroscopy. The generation of OH necessitates an oxidation potential of 2.8 vs. NHE. However, the VB position of CN is less negative than this oxidation potential, resulting in its incapacity to produce the OH radical. Nonetheless, the VB of ZnO is more positive than the oxidation potential of OH radical formation; therefore, ZnO can proficiently produce OH radicals, as validated by the terephthalic acid test (Fig. S21b†). When a heterojunction is formed, two possibilities exist: oxidation can occur at the VB of CN (type II heterojunction) or oxidation can occur at the VB of ZnO (Z-scheme heterojunction). The terephthalic acid test evinced that the heterojunction effectively generated OH radicals and manifested robust PL spectrum emission, evincing that oxidation in the heterojunction occurred at the VB of ZnO. It verifies that the heterojunction is a Z-scheme heterojunction (Fig. 7b). Electrons from the CB of ZnO and holes from the VB of CN move to the interface and recombine (Fig. 8(iii)). The photogenerated electron perseveres in the CB of CN and is transported to the Ru NPs owing to the low Fermi level of Ru (Fig. 8(iv)).
Control experiments were carried out to elucidate the role of charge carriers in photocatalytic reduction using the Ru@ZnO/CN heterojunction. Initially, CAL reduction was executed in the presence of electron scavengers, CCl4, and formic acid (Fig. S22a†). A marked drop in CAL conversion was noted in both scenarios, implying the significance of electrons in photocatalytic reduction. Moreover, IPA served as a hole scavenger, solely offering photogenerated electrons for reduction. To scrutinize the impact of isopropyl alcohol (IPA) on the photocatalytic reduction process, a control experiment was carried out by substituting IPA with acetonitrile (ACN), which proved to be ineffective (Fig. S22b†). To further amplify the effectiveness of the photocatalytic reduction reaction, a strategic approach involving the addition of external hole scavengers to the ACN solvent was employed. In one control experiment, triethanolamine (TEA) was added as an external additive to scavenge holes, resulting in a reduction of CAL. The CAL conversion was markedly improved with increasing amounts of TEA, from 100 μL to 200 μL (Fig. S22b†). In this instance, the reduction of CAL is solely attributed to the presence of H2, in the absence of IPA. Therefore, the overall photocatalytic reduction process is mainly driven by molecular hydrogen, with IPA's minimal involvement in the form of transfer hydrogenation. Nonetheless, IPA plays a critical role in quenching the photogenerated holes and enhancing the availability of photogenerated electrons, thus enabling Ru NPs' efficient photocatalytic reduction. Another control experiment involved adding IPA to the ACN solvent, which made the catalyst activity in a mixture of these two solvents, thus demonstrating IPA's role as a hole scavenger (Fig. S22c†). As the amount of IPA was increased, CAL conversion also increased. Consequently, effective quenching of photogenerated holes is an indispensable prerequisite for a successful photocatalytic reduction process, as it enables the availability of electrons, ultimately enhancing the production of HCAL.
The Ru nanoparticles (NPs) in the Ru@ZnO/CN heterojunction accept and trap the electrons produced by ZnO/CN under illumination, thereby promoting the desorption of hydrogen molecules (as discussed subsequently). To test this hypothesis, a non-photocatalytic material comprising Ru NPs supported on SBA-15 was utilized. A reaction using 3% Ru@SBA-15 under 150 W LED irradiation at room temperature resulted in negligible CAL conversion. The absence of electrons on the Ru NPs at room temperature prevented the desorption of H2 molecules. Thus, the photocatalyst absorbs light and generates electron–hole pairs, with IPA serving as a hole scavenger and the electrons accumulating on the Ru NP sites enabling successful desorption of hydrogen and reducing the double bond of CAL.
The photoactive catalyst (3% Ru@ZnO(1.5)/CN) induces the separation of charges within the ZnO/CN composite upon exposure to light. The resulting photogenerated electrons originating from the conduction band of ZnO and holes from the valence band of CN are recombined at the interface of the two materials. Subsequently, the electrons from the conduction band of CN are transferred to the surface of Ru NPs due to their higher work function and lower Fermi energy relative to the ZnO/CN composite. Furthermore, the externally supplied H2 adsorbed on the surface of the Ru NPs. The surface electrons on the Ru NPs efficiently dissociate and desorb the adsorbed H2, thereby promoting the reduction of the CC bond of CAL adsorbed on the catalyst surface, ultimately resulting in the selective production of HCAL (Fig. 8c). Additionally, the holes present in the valence band of ZnO participate in the oxidation of isopropyl alcohol (IPA), leading to the formation of acetone (Fig. 8c).
Overall, under thermal conditions, the role of ZnO is to adsorb and activate the –CO group of cinnamaldehyde. CN adsorbs formic acid due to the basic nature of the CN support. The role of Ru is to assist the effective dissociation of HCOOH into H2 + CO2 and dissociatively adsorb the generated H2. The independent roles of all the counterparts of the catalyst were determined by various control experiments. The catalytic activity of ZnO, CN, and Ru was independently evaluated, and the following results were obtained. ZnO afforded 17% CAL conversion and 100% COL selectivity. The pronounced selectivity for COL is attributed to the electropositive nature of Zn2+, which assisted in the adsorption and activating of the carbonyl group. CN alone offered only 6% conversion, but combining ZnO/CN elevated the catalytic activity. This observation suggests that formic acid activation was required along with CAL activation. CN being a basic support provided adsorption sites to FA, which were further enhanced by Ru NPs. Ru NPs dissociate FA into H2 + CO2 following the decarboxylation route (also confirmed by GC analysis), and the H2 generated was dissociatively adsorbed on Ru NPs. The dissociatively adsorbed –H hydrogenated the cinnamaldehyde. Moreover, CN also assisted the dispersion of the Ru NPs by providing a surface. Hence, it can be inferred that an optimum balance of Ru NPs, ZnO, and CN was essential to acquire efficient catalytic performance.
Under photocatalytic conditions, ZnO plays the role of an oxidative photocatalyst, whereas CN plays the role of a reductive photocatalyst because of their respective band edge positions. Combining both materials, a Z-scheme heterojunction photocatalyst was prepared (proved through control experiments) with enhanced charge separation compared to the individual components. Furthermore, due to the lower Fermi level of Ru, Ru accepted electrons generated by the ZnO/CN Z-scheme heterojunction and helped in the efficient dissociation of adsorbed H2 over Ru NPs. Hence, Ru assisted in photo-generated electron-mediated dissociation of H2 for the efficient hydrogenation of CAL.
Footnotes |
† Electronic supplementary information (ESI) available: XRD patterns, XPS spectra of the 3% Ru@ZnO/CN catalyst, SEM images of all the materials, TEM images of the 3% Ru@ZnO/CN catalyst, N2-adsorption/desorption isotherms, EDS and elemental mapping, TGA profiles of composites, CO2 & NH3 temperature-programmed desorption profiles, Tauc plots of all synthesized photocatalysts, LSV spectra in the dark and light, UPS spectra of 3% CN(1.5)/ZnO and Ru@CN(1.5)/ZnO, catalytic activity data of Ni and Co decorated catalysts, GC chromatogram of H2 + CO2, GC-Mass spectra recorded to establish the deuterium exchange, 1H NMR spectra of the crude reaction mixture showing the formation of COL and HCAL, temperature-dependent FT-IR spectra of CAL adsorbed on the catalyst, comparative absorption spectra of NBT and fluorescence spectra of THA solution and characterization of the spent catalyst under thermal and photochemical conditions. See DOI: https://doi.org/10.1039/d3ta02000b |
‡ Equal contribution. |
This journal is © The Royal Society of Chemistry 2023 |