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Article

The Contradicting Influences of Silica and Titania Supports on the Properties of Au0 Nanoparticles as Catalysts for Reductions by Borohydride

by
Gifty Sara Rolly
1,
Alina Sermiagin
1,
Krishnamoorthy Sathiyan
1,
Dan Meyerstein
1,2,* and
Tomer Zidki
1,*
1
Department of Chemical Sciences, The Radical Research and Material Research Centers, Ariel University, Ariel 4077625, Israel
2
Department of Chemistry, Ben-Gurion University, Beer-Sheva 8410501, Israel
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(9), 606; https://doi.org/10.3390/catal14090606
Submission received: 17 July 2024 / Revised: 2 September 2024 / Accepted: 6 September 2024 / Published: 9 September 2024
(This article belongs to the Special Issue Novel Nanocatalysts for Sustainable and Green Chemistry)
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Abstract

:
This study investigates the significant impact of metal–support interactions on catalytic reaction mechanisms at the interface of oxide-supported metal nanoparticles. The distinct and contrasting effects of SiO2 and TiO2 supports on reaction dynamics using NaBD4 were studied and focused on the relative yields of [HD]/[H2] and [D2]/[H2]. The findings show a consistent increase in HD yields with rising [BD4−] concentrations. Notably, the sequence of HD yield enhancement follows the order of TiO2-Au0-NPs < Au0-NPs < SiO2-Au0-NPs. Conversely, the rate of H2 evolution during BH4- hydrolysis exhibits an inverse trend, with TiO2-Au0-NPs outperforming the others, followed by Au0-NPs and SiO2-Au0-NPs, demonstrating the opposing effects exerted by the TiO2 and SiO2 supports on the catalytic processes. Further, the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) confirms the catalytic mechanism, with TiO2-Au0-NPs demonstrating superior activity. The catalytic activity observed aligns with the order of TiO2-Au0-NPs > Au0-NPs > SiO2-Au0-NPs, suggesting that SiO2 donates electrons to Au0-NPs, while TiO2 withdraws them. It is of interest to note that two very different processes, that clearly proceed via different mechanisms, are affected similarly by the supports. This study reveals that the choice of support material influences catalytic activity, impacting overall yield and efficiency. These findings underscore the importance of selecting appropriate support materials for tailored catalytic outcomes.

1. Introduction

Due to their unique optical, electronic, and catalytic properties, noble metal nanoparticles (M0-NPs) are considered an intriguing class of compounds for environmental applications [1]. The direct application of metal nanoparticles is inconvenient due to their high tendency to agglomerate. Thus, metal NPs are often supported onto organic and inorganic supports to prevent agglomeration and particle growth [2]. Organic supports, such as polymers or carbon materials, are frequently used for applications such as catalysis, sensors, and drug delivery due to their biocompatibility, ease of functionalization, and tuneable properties [3]. Inorganic oxide supports such as silica, alumina, and titania are often used for catalytic applications due to their high surface area, mechanical stability, and thermal resistance [4,5,6,7,8]. Inorganic oxides can also be functionalized with various surface groups, such as hydroxyl and amino groups, which can enhance the reactivity of the metal particles and improve their catalytic performance. Moreover, using support materials to anchor or immobilize metal nanoparticles can facilitate their separation and recycling from suspensions by filtration or centrifugation [9].
Metal particles supported on the surface of inorganic oxides have shown strong metal–support interactions [10,11,12,13]. Metal oxide supports play essential roles in catalytic reactions through synergistic interactions with the deposited M0-NPs, generating new interface phenomena such as electron transfer or shape rearrangement that can influence the activity of the catalysts [14,15,16]. Most studies on metal–support interactions were performed in a vacuum or a gas-solid interface [17,18]. The study of metal–support interactions in a liquid phase is relevant to many practical applications, such as catalysis and biomedicine [19]. Some studies have shown that the properties of M0-NPs and their interaction with the support materials can be significantly different in a liquid environment compared to a vacuum or gas-solid interface [20].
Studies suggesting that metal oxides affect the catalytic and physical properties of metal NPs were often associated with the reducibility of the central metal in the metal-oxide supports [21,22]. However, such effects were also reported for irreducible oxides like nano-silica supports. Silver, gold, and platinum NPs supported on silica NPs are poorer catalysts for the hydrogen evolution reaction (HER) initiated by C(CH3)2OH· radicals than the unsupported M0-NPs catalysts [23,24]. An earlier study showed that nano-silica, as the support of Ag0-NPs, similarly affects their chemical properties above and below the point of zero charge (PZC) of the silica support [25]. However, the silica support does not affect the reaction between CH3· and Ag0-NPs [26]. Another study indicated some electron transfer from the SiO2 to the silver that increases the negative charge on the Ag0-NPs [27].
Catalytic processes involving borohydride as the reducing agent were chosen for this study as borohydride is known to reduce all oxide/hydroxide covers formed on M0-nanoparticles that might be formed during the catalytic process [28]. It is commonly assumed that the M0-NPs catalyze the borohydride hydrolysis reaction (BHR) via hydride transfer from the BH4− to the M0-NPs [29,30,31,32,33,34,35]:
n · B H 4 − + M 0 - N P + 3 n · H 2 O → n · B O H 3 + M 0 - N P − H 4 n 4 n − N + 3 n · H +
Reaction (1) is then followed by H2 evolution via either the Heyrovsky or Tafel mechanisms [34]. Alternatively, the (M0-NP)-Hm− may be oxidized in the presence of an oxidizing agent [28,36,37,38]. However, recent DFT studies indicate that at least on Ag0 [39] and Au0 [40], the BH4– hydrolysis proceeds via different mechanisms. For Ag0, the proposed mechanism is [39]:
B H 4 −   * + H 2 O   * → B H 3 ( O H ) −   * + 2 H   *
B H 3 ( O H ) −   * + H 2 O   * → B H 2 ( O H ) 2   * + H 2   *
B H 2 ( O H ) −   * + H 2 O   * → B H ( O H ) 2   * + H +   * H 2 O   * + e −   *
B H ( O H ) 2   * + H   * + H 2 O   * + e −   * → B H ( O H ) 3   * + H 2   *
B H ( O H ) 3 −   * + H 2 O   * → B ( O H ) 4 −   * + H 2   *
According to this mechanism, at most, three adsorbed H atoms are formed during the catalytic BH4− hydrolysis.
The experimental results [34] for the catalytic H2 evolution on Ag0 agree with reactions (2)–(6). However, the catalytic H2 evolution results on Au0 indicate a different mechanism, Reactions (7)–(10) [34,40].
Catalysts 14 00606 i001
(where * indicates species adsorbed on the surface). Reaction (7) is followed by [40]:
B H 3 ( O H )   −   * + H 2 O   * → B H 2 ( O H ) 2 −   * + 2 H     *
The latter reaction occurs when no *H atoms are present; in their presence, the reaction occurring is:
B H 3 ( O H )   −   * + H 2 O   * → B H 2 ( O H ) 2 −   * + H 2   *
This reaction is followed by [40]:
B H 2 ( O H ) 2 −   * → B H   * ( O H ) 2 + H + e −   *   *
The *BH(OH)2 thus formed is stable and desorbs from the Au0-NPs into the solution [40]. Furthermore, not all the adsorbed hydrogen atoms on Au0-NPs form H2; these conclusions were experimentally supported [40]. According to this mechanism, only three H. atoms are adsorbed on the Au0-NPs, and the rest of the H2 is formed without the involvement of adsorbed hydrogen atoms or hydrides. Furthermore, when H. atoms are adsorbed on Au, Reaction (8) is replaced by Reaction (9) [40]. Hence, H2 evolution and substrate reduction mechanisms must be reconsidered.
Previously, the BD4– hydrolysis in the presence of silica-supported silver nanoparticles (SiO2-Ag0-NPs) was studied [27]. The results indicated that the silica support affects the isotopic composition of the product hydrogen gas (H2, D2, HD), similar to the effect of increasing the BD4– concentration (increasing HD) [34], signifying that some electron transfer from the silica [41] to the silver increases the negative charge on the Ag0-NPs. Given the new DFT results [39,40], one cannot prove that hydrides are present on the SiO2-Ag0-NPs as previously suggested [27]. According to the DFT results, during hydrolysis, the transfer of three H atoms from BH4– occurs on the Au0 surface and one H atom on the Ag0 surface. Although Ni and Co are more active and cheaper than gold, more information regarding the above reactions is available on Au. As the current work is focused on the support effect, we chose to work with gold NPs. Hence, checking whether gold nanoparticles behave similarly on the “inert” silica and reducible titania supports was interesting. To check whether the effects of the supports observed are general, we decided to study a second model reaction, the reduction of 4-NP, as this reaction definitely proceeds via a different mechanism. We also used the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction to draw a more general conclusion about the catalytic role of the support. The 4-NP reduction is a widely studied reaction with essential applications in catalysis and environmental remediation [42,43].

2. Results and Discussion

2.1. Characterizations of Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs

The transmission electron microscopy (TEM) images of gold nanoparticles, silica-supported gold NPs, and titania-supported gold nanoparticles are presented in Figure 1, and their corresponding Energy-dispersive X-ray spectroscopic analysis (EDX) spectra are shown in Figure 2. The average size of bare gold NPs was 4.2 nm, and those of supported gold NPs on silica and titania were 3.5 nm and 3.8 nm, respectively, as obtained from Figure S2. The EDX spectra results in Figure 2 confirmed the presence of Au in Au0-NPs, Si, O, and Au in SiO2-Au0-NPs, and Ti, O, and Au in TiO2-Au0-NPs. The Cu and C signals originated from the Cu grid. The Atomic % of each element is provided in Table S1. The UV-visible spectra of the Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs are provided in Figure S1.
Figure 3A represents the Powder X-ray diffraction analysis (PXRD) pattern obtained for bare gold NPs. The peaks at 38.2°, 44.4°, 64.6°, and 77.6° correspond to (111), (200), (220), and (311) planes of the face-centered cubic phase of Au [44]. Figure 3B depicts the PXRD pattern obtained for amorphous SiO2 and SiO2-Au0-NPs (JCPDS: 04-0784). The peak observed at 22° relates to amorphous silica [45], and the (111), (200), (220), and (311) planes of the face-centered cubic phase of Au are also observed [44]. Figure 3C shows the PXRD patterns obtained for bare titania and titania-supported gold nanoparticles. For titania, the diffraction peaks of (101), (004), (200), and (211) correspond to the crystal planes of the anatase phase (JCPDS file 21-1272) [46,47], and the diffraction peak of (121) crystal plane at 30.5°confirms the presence of the brookite phase (JCPDS file 29-1360) [48]. The diffraction peaks at (111), (200), (220), and (311) reflection planes at 2θ = 38.2°, 44.4°, 64.6°, and 77.6° show the presence of the face-centered cubic phase of Au [49]. In addition, for Au0-NPs and TiO2-Au0-NPs, a set of peaks at 2θ = 31.67°, 45.44°, 56.51°, 66.36°, and 75.30° were identified as NaCl (subproduct of the reaction) [50].

2.2. Isotopic Composition of the Hydrogen Product

Initially, the reactions of SiO2 (3.5 mM, results from a previous report [27]) and TiO2 (1.25 mM) with different NaBD4 concentrations were studied as controls. The results are summarized in Table 1. The average hydrogen composition (H2:HD:D2) evolved at different [BD4−] concentrations for TiO2 and SiO2 are 1.0:19.9:0.18 and 1.0:18.0:0.16, respectively. Table 1 demonstrates that the hydrogen formed contains ca. 48% of D atoms in the product in both cases. Note that the borohydride might reduce some TiIVO2 to (TiIII)2O3 [51].
Later, the reactions were performed in the presence of SiO2-Au0-NPs or TiO2-Au0-NPs ([Au] = 0.125 mM) and different concentrations of NaBD4; the results are summarized in Table 2 and Figure 4. The SiO2-Au0-NP results indicate that the products contained 45.3% D atoms, slightly lower than the blank experiments. However, the kinetic results (Figure 5B) suggest that the reaction occurred at the Au0 surfaces as the BHR rate increased with [Au0-NPs]. The TiO2-Au0-NP results indicate that the D atoms percentage slightly increases with [BD4−]. However, it is considerably lower than expected, meaning a considerable isotopic exchange. This decrease of [D] might be due to a partial, temporary reduction of the titania [51].
The results in Figure 4A,C,E indicate that the HD percentage in the hydrogen product increased with [BD4−]. However, whereas the HD percentage in the hydrogen product formed over SiO2-Au0-NPs was substantially higher than that formed over Au0-NPs, it was significantly lower over TiO2-Au0-NPs. Thus, the SiO2 and TiO2 supports affect the composition of the hydrogen product differently. This support effect was even more pronounced in the percentage of D2 in the evolved hydrogen (Figure 4B,D,F), where it decreased with [BD4−] over Au0- and SiO2-Au0-NPs, while it increased with [BD4−] over TiO2-Au0-NPs. These results indicate that the support’s nature dramatically affects the borohydride hydrolysis mechanism catalyzed by Au0-NPs. The observation that [D2] increased with [BD4−] on the TiO2 support does not fit either mechanism outlined by Reaction (1). A plausible mechanism explaining this observation is described by Reaction (7). Recall the mechanism in Reactions (7)–(10) was proposed on Au [40].

2.3. Kinetics of the BHR Catalysis over Au0-, SiO2-Au0- and TiO2-Au0-NPs

I.
The H2 formation kinetics were studied to answer the questions:
Is the H2 in the BH4− reaction with the SiO2-Au0- and TiO2-Au0-NPs solely formed at the surface of the Au0-NPs?
II.
Do the supports affect the borohydride hydrolysis rate differently?
The reactions of the gold-based catalysts with BH4− were conducted under an argon atmosphere; the rates were measured at different concentrations of Au0-NPs, SiO2-Au0-NPs (constant [SiO2]/[Au0]), TiO2-Au0-NPs (constant [TiO2]/[Au0]), and TiO2-NPs (Figure S6). The results are presented in Figure 5A–C, and the computed rate constants are provided in Figures S3–S6. NaBH4 was used for the kinetic study, as the isotope effect usually changes the rate constants of reactions and not their mechanisms; the catalytic trend should be the same as for NaBD4. The lines in Figure 5A–C are the first-order kinetic fits for the experimental data points in these graphs; the observed rate constants are presented in Figure 5D and summarized in Table 3.
The reaction rate increases with [NPs], with the Au0-NPs having a rate between those of SiO2-Au0-NPs and TiO2-Au0-NPs. This implies that the H2 evolution occurs on the Au0-NPs. As the observed rate constants of supported Au0-NPs are within an order of magnitude of those of Au0-NPs, it is reasonable to suppose that all the surface atoms in the NPs are involved. However, it is impossible to rule out that a gold atom in contact with the support has a unique contribution. The catalytic reactions over SiO2-Au0-NPs are much faster than over silica blank [27], and the rate constant of H2 evolution on Au0-NPs was even faster. On the other hand, the rate constant on TiO2-Au0-NPs was twice higher than that on Au0-NPs. The relatively significant intercepts in Figure 5D suggest equilibria processes. This is tentatively attributed to the BH4− adsorption on the Au0-NPs [40]. The order of BHR catalysis observed is TiO2-Au0-NPs > Au0-NPs > SiO2-Au0-NPs. Consequently, the SiO2 and TiO2 supports affect the H2 evolution in opposite directions. The SiO2 support had a similar effect on increasing BD4− concentration, likely due to donating electrons to the Au0-NPs. On the other hand, the reducible TiO2 induced a positive charge on the Au0-NPs. This conclusion aligns with a recent report using a completely different experimental approach [52].
To check whether this support effect also affects other Au0-NPs-catalyzed reductions, it was decided to study the kinetics of the 4-NP reduction.
UV-visible spectroscopy was employed to monitor the reduction of 4-nitrophenolate ion (λmax = 400 nm) to 4-aminophenol (λmax = 300 nm). The reduction process kinetics were measured every 0.050 min in the presence of SiO2, TiO2, TiO2-Au0, SiO2-Au0, and Au0 NPs. NaBH4 addition to the catalyst and 4-NP mixture resulted in a gradual decrease of the nitrophenolate peak at 400 nm and a simultaneous formation of the aminophenol peak at 300 nm (Figure 6 and Figure S7), indicating 4-NP conversion to 4-AP. The reduction using blank supports and NaBH4 did not catalyze the reduction process (Figure S7). TiO2-Au0-NPs catalyzed 4-NP reduction within 3.5 min after a short induction time, while the reduction was completed within 16 min in the presence of SiO2-Au0-NPs after approximately 3 min of induction time. The reduction of 4-NP in the presence of bare Au0-NPs was completed within 7 min. The kinetic traces of 4-NP reduction monitored at 400 nm are shown in Figure 7. The observed rate constants are 0.66, 0.49, and 0.16 min−1 for the catalysis on TiO2-Au0-NPs, Au0-NPs, and SiO2-Au0-NPs, respectively. The catalytic activity for 4-NP reduction on Au0-NPs falls within the range of activities exhibited by titania and silica-supported gold NPs.

3. Experimental Section

3.1. Synthesis of Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs

All the gold-based catalysts used were previously reported and thoroughly characterized; therefore, the synthesis and characterization are only briefly presented. Gold nanoparticles were prepared by adding an ice-cold solution of NaBH4 to an aqueous solution of HAuCl4 following the well-known Creighton’s procedure [53] that was improved by Zidki et al. to obtain more uniform Au0-NPs [16]. Monodispersed spherical SiO2-NPs were synthesized using Stöber’s method and functionalized with bridging 3-aminopropyltrimethoxysilane (APS) [54]. Colloidal TiO2 suspensions were prepared by hydrolyzing titanium tetrachloride (TiCl4) as developed by Rabani et al. [55,56]. and later improved by Sathiyan et al. [47]. The obtained SiO2 and TiO2 NPs were attached with AuIII, followed by NaBH4 reduction to form SiO2-Au0-NPs [24] and TiO2-Au0-NPs [57]. Detailed synthesis procedures and characterization methods are provided in ESI.

3.2. Reaction of SiO2-Au0-NPs and TiO2-Au0-NPs with NaBD4

Sealed vials containing argon-saturated SiO2-Au0-NPs (pH 9.0) or TiO2-Au0-NPs (pH 9.0) suspensions were treated with an equal volume of NaBD4 solution (Ar-saturated). After the reaction of NaBD4 was completed, mass spectrometry (MS, quadruple type gas analyzer, QMG 250 PrismaPro model with electron multiplier by Pfeiffer Vacuum GmbH, Asslar, Germany) gas analysis was performed. Blank reactions were also performed using bare SiO2 and TiO2 nanoparticles. The MS results are summarized in Table 1 and Table 2. The tables show the ratios of HD:H2 and D2:H2 at different NaBD4 concentrations.

3.3. Kinetic Measurements

Suspensions of SiO2-Au0-NPs, TiO2-Au0-NPs, and Au0-NPs were reacted with an equal volume of NaBH4 solution under Ar at room temperature. The reaction apparatus was connected to an open manometer. The height of the water in the manometer was measured with time to measure the kinetics of the reaction [27]. Different concentrations of SiO2-Au0-NPs (pH 9.0), TiO2-Au0-NPs (pH 9.0), and Au0-NPs (pH 9.0), along with their control blanks, SiO2-NPs, and TiO2-NPs, were studied with a constant NaBH4 concentration (0.50 mM).

3.4. Catalytic Reduction of 4-Nitrophenol

The 4-nitrophenol (4-NP) reduction to 4-aminophenol (4-AP) was carried out to compare the catalytic efficiency of silica- and titania-supported gold NPs and to assess the support effect on the gold NP activity as schematically presented in Scheme 1. The kinetic studies were performed through straightforward spectroscopic monitoring. A 0.50 mL of the supported gold catalyst (0.125 mM, pH 9.0) was mixed with 1.5 mL of 4-NP (0.20 mM) in a quartz cuvette. A 1.0 mL of freshly prepared NaBH4 solution (10 mM) was added to the mixture, followed by UV-visible spectra recording at specific intervals. The final concentrations were 0.020 mM gold, 0.10 mM 4-NP, and 3.0 mM NaBH4. The molar ratios between [BH4−]/[Au], [4-NP]/[Au], [BH4−]/[4-NP] were 160, 5, 33. A similar procedure was performed with unsupported Au0-NPs, silica, and titania. The reaction was monitored by UV-visible spectroscopy in the 200–600 nm wavelength range.

4. Conclusions

The findings of this study highlight the significant impact of TiO2 and SiO2 supports on the catalytic properties of Au0-NPs. The results indicate that SiO2 and TiO2, as the supports of Au0-NPs, reversely affect the borohydride hydrolysis kinetics and mechanism. The SiO2 support, which showed a similar effect to increasing [BD4−], likely donates electrons to the Au0-NPs. Conversely, the reducible TiO2 induces a positive charge on the Au0-NPs. These observations are further supported by the results of 4-NP reduction using the same gold catalysts. The catalytic activity of Au0-NPs for the reduction of 4-NP lies in a range between the activities of titania and silica-supported gold NPs, implying that TiO2-Au0-NPs are better catalysts for 4-NP reduction than SiO2-Au0-NPs and Au0-NPs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14090606/s1, Figure S1: UV-visible spectra of Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs; Figure S2: Size distribution curves of Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs; Figure S3: Kinetic plots of hydrogen formation in the hydrolysis of BH4– in the presence of Au0-NPs at pH 9.0; Figure S4: Kinetic plots of hydrogen formation in the hydrolysis of BH4– in the presence of SiO2-Au0-NPs at pH 9.0; Figure S5: Kinetic plots of hydrogen formation in the hydrolysis of BH4– in the presence of TiO2-Au0-NPs at pH 9.0; Figure S6: Kinetic plots of hydrogen formation in the hydrolysis of BH4– in the presence of TiO2-NPs at pH 9.0; Figure S7: Time-dependent UV-visible spectra of the catalytic reduction of 4-nitrophenol using SiO2-NPs and TiO2-NPs, at pH 9.0; Table S1: Atomic % of the elements obtained from EDX.

Author Contributions

Conceptualization, D.M. and T.Z.; Data curation, G.S.R., A.S. and K.S.; Formal analysis, G.S.R.; Funding acquisition, D.M. and T.Z.; Investigation, G.S.R. and A.S.; Methodology, G.S.R., A.S., K.S., D.M. and T.Z.; Project administration, T.Z.; Resources, D.M. and T.Z.; Supervision, D.M. and T.Z.; Validation, D.M. and T.Z.; Visualization, G.S.R.; Writing—original draft preparation, G.S.R.; Writing—review & editing, D.M. and T.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by the Pazy Foundation grant numbers ID126-2020 and RA1700000176. The APC was waived by the courtesy of MDPI.

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Acknowledgments

G.S.R. and A.S. thank Ariel University for their Ph.D. scholarships.

Conflicts of Interest

There are no conflicts to declare.

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Catalysts 14 00606 g001
Figure 1. TEM images of (A–C) Au0-NPs (4.2 nm), (D–F) SiO2-Au0-NPs (Au0 = 3.5 nm), and (G–I) TiO2-Au0-NPs (Au0 = 3.8 nm) at different scales.
Figure 1. TEM images of (A–C) Au0-NPs (4.2 nm), (D–F) SiO2-Au0-NPs (Au0 = 3.5 nm), and (G–I) TiO2-Au0-NPs (Au0 = 3.8 nm) at different scales.
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Catalysts 14 00606 g002
Figure 2. EDX spectra of Au0-NPs (A), SiO2-Au0-NPs (B), and TiO2-Au0-NPs (C).
Figure 2. EDX spectra of Au0-NPs (A), SiO2-Au0-NPs (B), and TiO2-Au0-NPs (C).
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Catalysts 14 00606 g003
Figure 3. PXRD of Au0-NPs (A), SiO2-Au0-NPs and SiO2 (B), and TiO2-Au0-NPs and TiO2 (C).
Figure 3. PXRD of Au0-NPs (A), SiO2-Au0-NPs and SiO2 (B), and TiO2-Au0-NPs and TiO2 (C).
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Catalysts 14 00606 g004
Figure 4. The correlations between %HD and %D2, as obtained for the NaBD4 reaction with bare Au0-NPs (A,B), SiO2-Au0-NPs (C,D), and TiO2-Au0-NPs (E,F).
Figure 4. The correlations between %HD and %D2, as obtained for the NaBD4 reaction with bare Au0-NPs (A,B), SiO2-Au0-NPs (C,D), and TiO2-Au0-NPs (E,F).
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Catalysts 14 00606 g005
Figure 5. Kinetic plots of hydrogen formation in the hydrolysis of BH4− in the presence of Au0-NPs (A), SiO2-Au0-NPs (B), TiO2-Au0-NPs (C) at pH 9.0 and the dependence of the observed rate constants on [Au0] (D). [BH4−] = 0.50 mM, pH 9.0, the ratios [Au0/TiO2] and [Au0/SiO2] were constant. Note [Au0] was the [AuIII] used to prepare the NPs, and [TiO2] was the [TiCl4] used to prepare NPs.
Figure 5. Kinetic plots of hydrogen formation in the hydrolysis of BH4− in the presence of Au0-NPs (A), SiO2-Au0-NPs (B), TiO2-Au0-NPs (C) at pH 9.0 and the dependence of the observed rate constants on [Au0] (D). [BH4−] = 0.50 mM, pH 9.0, the ratios [Au0/TiO2] and [Au0/SiO2] were constant. Note [Au0] was the [AuIII] used to prepare the NPs, and [TiO2] was the [TiCl4] used to prepare NPs.
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Catalysts 14 00606 g006
Figure 6. Time-dependent UV-visible spectra of the catalytic reduction of 4-nitrophenol using SiO2-Au0-NPs, TiO2-Au0-NPs, and bare Au0-NPs at pH 9.0. The time interval between the cycles was 0.050 min. The concentration of gold was 0.020 mM (in terms of the [AuIII] used to prepare the NPs). For better clarity, the absorption spectra shown are at intervals of 0.30 min for TiO2-Au0-NPs and 0.70 min for Au0-NPs and SiO2-Au0-NPs.
Figure 6. Time-dependent UV-visible spectra of the catalytic reduction of 4-nitrophenol using SiO2-Au0-NPs, TiO2-Au0-NPs, and bare Au0-NPs at pH 9.0. The time interval between the cycles was 0.050 min. The concentration of gold was 0.020 mM (in terms of the [AuIII] used to prepare the NPs). For better clarity, the absorption spectra shown are at intervals of 0.30 min for TiO2-Au0-NPs and 0.70 min for Au0-NPs and SiO2-Au0-NPs.
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Figure 7. Kinetic traces of 4-NP reduction monitored at 400 nm using SiO2-Au0-NPs, TiO2-Au0-NPs, and bare Au0-NPs measured every 0.050 min. The concentration of gold was 0.020 mM (in terms of the [AuIII] used to prepare the NPs).
Figure 7. Kinetic traces of 4-NP reduction monitored at 400 nm using SiO2-Au0-NPs, TiO2-Au0-NPs, and bare Au0-NPs measured every 0.050 min. The concentration of gold was 0.020 mM (in terms of the [AuIII] used to prepare the NPs).
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Catalysts 14 00606 sch001
Scheme 1. The catalytic reduction of 4-NP to 4-AP by NaBH4 on Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs catalysts.
Scheme 1. The catalytic reduction of 4-NP to 4-AP by NaBH4 on Au0-NPs, SiO2-Au0-NPs, and TiO2-Au0-NPs catalysts.
Catalysts 14 00606 sch001
Table 1. The hydrogen composition from the reaction of TiO2-NPs and SiO2-NPs with BD4−.
Table 1. The hydrogen composition from the reaction of TiO2-NPs and SiO2-NPs with BD4−.
[BD4−] [mM]%DH2HDD2
TiO2-NPs1.047.741.0017.640.15
2.048.081.0020.350.17
4.048.001.0019.470.17
8.048.231.0021.360.20
10.048.201.0020.800.21
SiO2-NPs [27]1.047.91.0018.70.17
10.048.11.0019.90.18
25.048.01.0019.50.17
50.047.91.0018.70.16
10046.91.0013.20.13
The results of SiO2-NPs reproduced with permission from reference [27]. The solution pH values were set to 9.0. [SiO2] = 3.5 × 10−3 M and [TiO2] = 1.25 × 10−3 M. Each value represents the average of at least five independent experiments. The error limit for the HD:H2 and D2:H2 ratios is ± 2%.
Table 2. The hydrogen composition from the reaction of Au0, SiO2-Au0-NPs, and TiO2-Au0-NPs with BD4−.
Table 2. The hydrogen composition from the reaction of Au0, SiO2-Au0-NPs, and TiO2-Au0-NPs with BD4−.
[BD4−] [mM]%DH2HDD2% HD% D2
Au0-NPs [34]1.041.11.02.650.6262.0614.51
2545.21.02.720.5963.1013.68
5046.01.03.340.6167.4712.32
7546.21.03.560.6168.8511.79
10045.91.03.490.6068.5611.78
SiO2-Au0-NPs1.043.31.03.700.3473.416.74
1046.21.05.400.4878.486.97
5046.21.05.900.4580.276.12
10045.61.05.700.3980.395.50
TiO2-Au0-NPs1.026.331.00.930.0646.733.01
1029.421.01.080.1049.544.58
5041.181.02.640.3067.007.61
7541.291.02.50.3365.278.62
10041.081.02.330.3563.319.51
The results of Au0-NPs reproduced with permission from reference [34]. The solution pH values were set to 9.0. [Au] = 1.25 × 10−1 mM. Each value represents the average of at least five independent experiments. The error limit for the HD:H2 and D2:H2 ratios is ± 2%.
Table 3. Observed rate constants of the NaBH4 hydrolysis over various NPs.
Table 3. Observed rate constants of the NaBH4 hydrolysis over various NPs.
SampleConcentration (M)Observed
Rate Constant
k min−1		Rate Constant
k·107 M−1 min−1
Au0-NPs[Au0] = 3.94 × 10−91.63
[Au0] = 1.75 × 10−91.4312
[Au0] = 4.37 × 10−101.20
SiO2-Au0-NPs[Au0] = 1.36 × 10−8
[SiO2] = 4.5 × 10−4		1.31
[Au0] = 6.05 × 10−9
[SiO2] = 2.0 × 10−4		1.181.4
[Au0] = 1.51 × 10−9
[SiO2] = 5.0 × 10−5		1.15
TiO2-Au0-NPs[Au0] = 1.83 × 10−9
[TiO2] = 1.00 × 10−5		1.22
[Au0] = 5.91 × 10−10
[TiO2] = 5.0 × 10−6		1.0325
[Au0] = 2.96 × 10−10
[TiO2] = 2.50 × 10−6		0.78
TiO21.00 × 10−50.02
2.50 × 10−60.02
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Rolly, G.S.; Sermiagin, A.; Sathiyan, K.; Meyerstein, D.; Zidki, T. The Contradicting Influences of Silica and Titania Supports on the Properties of Au0 Nanoparticles as Catalysts for Reductions by Borohydride. Catalysts 2024, 14, 606. https://doi.org/10.3390/catal14090606

AMA Style

Rolly GS, Sermiagin A, Sathiyan K, Meyerstein D, Zidki T. The Contradicting Influences of Silica and Titania Supports on the Properties of Au0 Nanoparticles as Catalysts for Reductions by Borohydride. Catalysts. 2024; 14(9):606. https://doi.org/10.3390/catal14090606

Chicago/Turabian Style

Rolly, Gifty Sara, Alina Sermiagin, Krishnamoorthy Sathiyan, Dan Meyerstein, and Tomer Zidki. 2024. "The Contradicting Influences of Silica and Titania Supports on the Properties of Au0 Nanoparticles as Catalysts for Reductions by Borohydride" Catalysts 14, no. 9: 606. https://doi.org/10.3390/catal14090606

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