Bifunctional catalysts, containing metal and acid sites, have been widely employed in fossil fuel hydrocracking and renewable biomass conversions [1-5]. The distance between the two types of sites plays a crucial role in the bifunctionality, which governs the catalytic performance including both activity and selectivity [6-8]. The maximum distance, beyond which catalytic activity drops, is the so-called intimacy criterion [6]. The most common way of probing the metal-acid proximity effect is to use a physical mixture of individual catalytic components [9, 10], such as Pt/SiO₂ and acidic zeolite, where the Pt metal is dispersed on non-acidic SiO₂, and the acid is present in the zeolite particles [11]. In this case, the metal-to-acid site distances are not precisely controlled, varying from the nanometer to even micrometer range. It has been suggested that catalytic performance could be improved by placing the metal and acid sites as close together as possible [12]. For instance, Ryoo et al. [13] reported that Pt nanoparticles (NPs) supported on the external surface of a zeolite particle with a well-controlled thickness allowed better control over distance. They showed that decreasing zeolite crystal thickness to nanosheets significantly improved the product selectivity to branched isomers in the hydroisomerization of n-heptane. Meanwhile, Martens and co-workers [14] recently argued that close proximity between metal and zeolite acid sites could be detrimental for cracking large hydrocarbon molecules (such as n-decane, and n-nonadecane).

Interest in the production of biofuels and chemicals from renewable biomass has been increasing [5]. However, the metal-acid proximity effect has been considerably less extensively explored in biomass conversions compared with fossil fuel hydrocracking [15]. Glycerol, an abundant and inexpensive by-product (~10 wt%) of biodiesel production [5, 10, 16], could be converted into value-added chemicals such as 1, 2-propanediol (1, 2-PD) and 1, 3-propanediol (1, 3-PD) via hydrogenolysis [17]. As shown in Scheme 1, glycerol hydrogenolysis involves sequential dehydration and hydrogenation steps on acid and metal sites, respectively [5, 17]. It has been proposed that the Lewis acid site attacks the terminal OH of glycerol to form acetol as an intermediate, while the Br nsted acid site attacks the internal OH to form 3-hydroxypropanal; next, hydrogenation of acetol or 3-hydroxypropanal takes place at a metal site to produce 1, 2-PD and 1, 3-PD, respectively [5, 17-19]. Therefore, the balance between acid and metal function is critical for effective hydrogenolysis.

Supported noble metal catalysts have been used for glycerol hydrogenolysis [19-26]. Adding acidic function to a metal catalyst could significantly promote the catalytic activity, as the dehydration step requires acid sites (Scheme 1). A number of studies have demonstrated that incorporating acid sites into either metal NPs [19, 23, 27] or supports [20], or even a simple physical mixture of solid acids and metal catalysts [10], could be effective in improving activity. However, to the best of our knowledge, the metal-acid proximity effect has not been explored.

In this work, we precisely deposited a porous Al₂O₃ overcoat onto a Pt/Al₂O₃ catalyst using atomic layer deposition (ALD) by taking advantage of its self-limiting surface reaction feature. Compared with the uncoated Pt/Al₂O₃ catalyst, the porous acidic Al₂O₃ overcoat deposited on the Pt improves the proximity between the Pt metal and alumina acid sites by increasing the area of the metal-acid interface, while keeping the overall catalyst acidity constant. This maintained acidity is expected as the overcoat is the same material as the support. This new strategy provides an ideal platform for investigating the metal-acid proximity effect on glycerol hydrogenolysis. Our results suggest that the close metal-acid proximity effect could promote catalyst activity approximately 2.8-fold, as well as improving selectivity to 1, 2-PD at high conversions.

The alumina supported Pt catalyst was prepared using the impregnation method [28]. In brief, 2.0 g of spherical gamma-alumina powder (Nanodur, Alfa Aesar, 99.5%) was added to an aqueous H₂PtCl₆·6H₂O(Sinopharm Chemical Reagent Co., Ltd.) solution (0.05 mol/L), and the mixture was stirred at room temperature for 24 h. The material was then collected by centrifugation and washed with deionized water several times. Following washing it was dried at 120 ℃ for 12 h. Finally, the dried material was further calcined under 10% O₂ in Ar (Nanjing Special Gases) at 400 ℃ for 2 h and then reduced under 10% H₂ in Ar (Nanjing Special Gases) at 300 ℃ for another 2 h to obtain the catalyst.

ALD was carried out on a viscous flow reactor (GEMSTAR-6™ Benchtop ALD, Arradiance) at a base pressure of 1.0 Torr. Ultra high purity N₂ (Nanjing Special Gases, 99.999%) was used as the carrier gas (200 mL/min). After loading the Pt/Al₂O₃ catalyst into the ALD reactor, Al₂O₃ ALD was performed by alternate exposure to trimethylaluminum (TMA, Sigma Aldrich, 99%) and deionized water at 200 ℃ for different numbers of cycles. The timing sequence was 9, 180, 15, and 180 s for TMA exposure, N₂ purge, water exposure and N₂ purge, respectively.

The morphologies of the catalysts were characterized using transmission electron microscopy (TEM) (JOEL-2010, University of Science and Technology of China). The Pt loading of the Pt/Al₂O₃ catalyst was determined to be 2% using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (University of Science and Technology of China).

The Pt dispersions of Pt/Al₂O₃ and alumina-coated Pt/Al₂O₃ catalysts were evaluated using the CO chemisorption method on an AutoChem™ Ⅱ 2920 Micromeritics instrument. After loading, the sample was first calcined under 10% O₂ in He at 300 ℃ for 2 h, followed by reduction under 10% H₂ in Ar at 200 ℃ for another 1 h. After cooling the sample to 50 ℃ in He, 10% CO in He was pulsed until saturation. All gases were provided by Nanjing Special Gases.

The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) CO chemisorption measurements were performed on a Nicolet iS10 spectrometer equipped with an MCT detector and a low temperature reaction cell (Praying Mantis Harrick). The sample was loaded into the cell and calcined under 10% O₂ in He at 300 ℃ for 2 h, followed by reduction under 10% H₂in He at 200 ℃ for another 1 h. After cooling the sample to room temperature in an atmosphere of He, a background spectrum was collected. 10% CO in He (20 mL/min) was then introduced into the cell. Finally, the sample was purged with He (20 mL/min) for 30 min to remove the gas phase CO. A DRIFTS spectrum was then collected at a resolution of 4 cm^-1.

The DRIFTS pyridine chemisorption measurements were also performed using the Nicolet iS10 spectrometer. The sample was loaded into the cell and calcined under 10% O₂ in He at 300 ℃ for 1 h. The cell was then cooled to room temperature under He and a background spectrum was collected. Following background collection, pyridine vapor was introduced into the cell, using He as a carrier, until the catalyst surface was saturated. Finally, the sample was purged with He (20 mL/min) for 45 min to remove gas phase and physisorbed pyridine. A DRIFTS spectrum was then collected at a resolution of 4 cm^-1.

Glycerol hydrogenolysis reaction was carried out in a 100-mL Parr autoclave at 220 ℃ under a hydrogen pressure of 8.0 MPa. For each reaction test, 40 g of 20 wt% glycerol (Sinopharm Chemical Reagent Co., Ltd.) aqueous solution was used. The amount of Pt/Al₂O₃ catalyst used was 450 mg, while the amount of alumina coated Pt/Al₂O₃ catalyst was adjusted to provide the same Pt content. After loading the substrate and catalyst, the autoclave was purged 5 times using He (99.999%) (Nanjing Special Gases, 99.999%) to remove any air. The autoclave was then charged with ultrapure H₂(Nanjing Special Gases, 99.999%). After the reactor was heated to 220 ℃, H₂ was pressurized to 8.0 MPa and the reaction started after turning on stirring at a speed of 700 r/min to remove any mass transfer contribution. Following reaction the reactor was quickly cooled to room temperature. The gas phase products were collected in a gasbag and the liquid phase products were separated from the catalyst by filtration. All products were analyzed using a Shimadzu GC-2014 gas chromatograph equipped with an Rtx-1 capillary column and an auto-injector. The products identified were: 1, 2-PD, 1, 3-PD, 1-propanol (1-PO) and ethylene glycol (EG). The carbon balance for this reaction was approximately 94.9%. The conversion of glycerol and product selectivity were calculated using the following equation:

Acetol hydrogenation reaction was carried out in a 100-mL Parr autoclave at 160 ℃ under a hydrogen pressure of 4.0 MPa. For each reaction test, 50 g of 2 wt% acetol (Sinopharm Chemical Reagent Co., Ltd.) aqueous solution was used. The amount of Pt/Al₂O₃ catalyst used was 75 mg, while the amount of 30AlPt/Al₂O₃ catalyst was adjusted to provide the same Pt content. Following loading of the substrate and catalyst, the autoclave was purged 5 times using He (99.999%) to remove any air. The autoclave was then charged with ultrapure H₂(99.999%). Next, the reactor was heated to 160 ℃; H₂ was then pressurized to 4.0 MPa and the reaction started after turning on stirring at a speed of 700 r/min to remove any mass transfer contribution. The products were analyzed using a Shimadzu GC-2014 gas chromatograph equipped with an Rtx-1 capillary column and an auto-injector. The only product identified was 1, 2-PD.

The conversion of acetol was calculated using the following equation:

A Pt/Al₂O₃ catalyst with a Pt particle size of approximately 7 nm was synthesized using the wet-impregnation method (Fig. 1(b) and (d)) [28]. This technique uses synthesis of relatively larger Pt NPs to amplify the difference of metal-acid proximity before and after alumina overcoating. The Pt loading of Pt/Al₂O₃ was determined to be 2 wt% by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Fig. 1(c) and (e) show the morphology of the Pt/Al₂O₃ sample coated with 30 cycles of alumina ALD (30Al/Pt/Al₂O₃). ALD is known to provide uniform and conformal deposition on high-surface area materials [29, 30]. In this case, the alumina overcoat uniformly encapsulated the Pt NPs with a thickness of ~3.6 nm, indicating a growth rate of ~0.12 nm per ALD cycle, similar to what has been reported in the literature [31]. The Pt particle size distribution data shown in the insets of Fig. 1(d) and (e) suggest that Pt particle size did not visibly change during the ALD process, which is in line with our previous results [32]. Dispersion measurements using CO indicated that the Al₂O₃ coating layer was porous, although the Pt dispersion largely decreased from 21% to 2.9%, 2.5% and 2.2%, for 10Al/Pt/Al₂O₃, 20Al/Pt/Al₂O₃, and 30Al/Pt/Al₂O₃, respectively (Table 1), consistent with our previous results [32]. On the other hand, DRIFTS measurements of pyridine chemisorption confirmed that only Lewis acid sites were presented in the Pt/Al₂O₃, 5Al/Pt/Al₂O₃, and 30Al/Pt/Al₂O₃ samples. This is indicated by the pyridine vibration band of the 8a mode at 1615 cm^-1 (Fig. 1(f)) [33], which is consistent with the literature [34]. It is worth noting that the alumina overcoat deposited by ALD is amorphous [35], which is different from the support which is γ-phase.

Hydrogenolysis of glycerol was conducted in a Parr batch reactor at 220 ℃ with a hydrogen pressure of 8.0 MPa. After reaction for 120 h, the uncoated Pt/Al₂O₃ catalyst showed a glycerol conversion of 60% and selectivity to 1, 2-PD of 45% (Fig. 2). 1-PO and EG were the major by-products, with selectivities of 45% and 8.3%, respectively. The selectivity to 1, 3-PD was only 1.5%. These results are expected given that there are only Lewis acid sites present on the alumina surface (Scheme 1 and Fig. 1(f)). After depositing 3 ALD cycles of alumina (3Al/Pt/Al₂O₃), the conversion increased to 76%, while the product selectivity did not change significantly. Further increasing the number of ALD cycles, continuously increased glycerol conversion, which reached 90% on 30Al/Pt/Al₂O₃; meanwhile, the 1, 2-PD selectivity also increased to 64%. In contrast, the 1-PO selectivity dropped to 27%, while the selectivity to EG and 1, 3-PD remained approximately constant. The data show that the presence of an alumina overcoat on Pt NPs considerably suppressed secondary hydrogenolysis of 1, 2-PD to 1-PO.

Catalytic performance of the Pt/Al₂O₃ and 30Al/Pt/Al₂O₃ catalysts was also compared after different reaction times. At low glycerol conversions (below 40%), the selectivity to 1, 2-PD on Pt/Al₂O₃ was ~65% (Fig. 3(a)). Increasing the conversion to 60% caused the selectivity to 1, 2-PD to decrease significantly to 45%. Meanwhile, the 1-PO selectivity increased from ~22% to 45%. The selectivity to other products showed negligible change throughout the entire reaction. On 30Al/Pt/Al₂O₃, the 1, 2-PD selectivity was ~76% at a glycerol conversion of 38% (Fig. 3(b)). Increasing the conversion to as high as 90% did not lead to a significant decrease in 1, 2-PD selectivity, which remained at 64%. The 1-PO selectivity was only 27% at 90% conversion. It is worth noting that glycerol conversion was less than 1% on blank Al₂O₃ after 24 h under the same conditions.

Dispersion measurements revealed that the Pt dispersion of Pt/Al₂O₃ decreased slightly to 15% from 21% after reaction for 48 h (Table 1). For 30Al/Pt/Al₂O₃, the Pt dispersion increased substantially from 2.2% to 11.2% after reaction for 12 h, and subsequently remained stable. The initial turnover frequencies (TOFs) of Pt/Al₂O₃ and 30Al/Pt/Al₂O₃ after reaction for 24 h were calculated according to the number of Pt surface atoms. We found that the TOF of 30Al/Pt/Al₂O₃ was ~341 h^-1, which was ~2.8 times higher than Pt/Al₂O₃ (122 h^-1) as shown in Fig. 3(c).

The sintering of Pt NPs in both Pt/Al₂O₃ and 30Al/Pt/Al₂O₃ samples was less pronounced after reaction for 120 h, which might be due to the relatively large Pt particle size (Fig. 4). We found that the alumina overcoat in the used 30Al/Pt/Al₂O₃ samples was hardly detected by TEM (Fig. 4(c)-(f)) when compared with the TEM observation of the fresh catalyst (Fig. 1(e)). This supports the Pt dispersion increase during reaction. Nonetheless, we noticed that the Pt dispersion of the used 30Al/Pt/Al₂O₃ sample was still considerably lower than that of the uncoated Pt/Al₂O₃ (Table 1). This suggests that there might be a small fraction of the Al₂O₃ overcoat remaining on the Pt NPs after subjection to the harsh reaction conditions.

DRIFTS CO chemisorption measurements were also carried out on the fresh and used Pt/Al₂O₃ and 30Al/Pt/Al₂O₃ samples. The fresh Pt/Al₂O₃ sample exhibited strong bands at 2093 and 2075 cm^-1, which are assigned to CO linearly-bonded to Pt sites associated with relatively larger and smaller Pt NPs, respectively (Fig. 5(a)). The weaker band at 1833 cm^-1 is assigned to bridge-bonded CO on Pt [36]. After reaction for 120 h, the linear CO peak shifted slightly to 2088 cm^-1, while the peak at 2075 cm^-1 was no longer detected, which indicates that smaller Pt NPs may have aggregated during the reaction, although this was not obvious in the TEM image at the resolution as shown in Fig. 4(e). For the fresh 30Al/Pt/Al₂O₃ sample, the CO chemisorption peak was barely distinguishable (Fig. 5(b)), consistent with the dispersion measurement (Table 1). After reaction for 12 h, the intensities of the CO peaks increased significantly, and the shapes of the CO chemisorption peaks were comparable to those of the peaks for the fresh uncoated Pt/Al₂O₃ sample. We also observed an additional peak around 1963 cm^-1. This new peak could be ascribed to bridge-bonded CO on Pt and Al atoms at the interface between the Pt NPs and the alumina overcoat [37-39]. As reaction time increased, the intensity of the peak at 1963 cm^-1 increased, which is likely due to the formation of a stronger Pt-alumina interaction. A weak peak at 1966 cm^-1 was also observed for the used Pt/Al₂O₃ sample (Fig. 5(a)). The much greater intensity of the peak at 1963 cm^-1 for 30Al/Pt/Al₂O₃ (120 h) confirmed that the area of the Pt-alumina interface was greater for 30Al/Pt/Al₂O₃ than for Pt/Al₂O₃. It also suggests that the interaction between the ALD alumina overcoat and Pt is strong, although some of the alumina overcoat became detached from the Pt NPs under the severe reaction conditions, as shown in the schematic illustration in Fig. 5(c).

To verify whether the leached alumina species in solution had a significant impact on the activity improvement, the 30Al/Pt/Al₂O₃ sample after reaction for 12 h was centrifuged and washed five times and then recycled under identical reaction conditions. As shown in Fig. 6, we found that the activity of the used catalyst did not change considerably, and only a slight decrease in the selectivity to 1, 2-PD was observed. The improvements in activity were therefore not due to the presence of leached alumina species in solution but to the Pt-alumina interface. This is not unexpected given that the glycerol conversion on blank alumina was negligible after 120 h (not shown).

To further understand the role of the alumina overcoat on the activity enhancement (Fig. 4(c) and (e)), the used Pt/Al₂O₃ (24 h) and 30Al/Pt/Al₂O₃ (24 h) samples were evaluated using acetol hydrogenation, since acetol is the likely reaction intermediate according to the literature [18]. As shown in Fig. 7, we found that the activity of 30Al/Pt/Al₂O₃ (24 h) was slightly lower than that of Pt/Al₂O₃ (24 h). However, both were approximately one order magnitude higher than the activities for the glycerol hydrogenolysis reaction, despite the reaction conditions (160 ℃ and 4.0 MPa hydrogen pressure) being much milder than in the glycerol hydrogenolysis case. These results strongly indicate the following. (1) The alumina overcoat on Pt NPs does not promote the acetol hydrogenation reaction, in line with our previous observation for ALD alumina coated Pd/Al₂O₃ catalyst for the selective hydrogenation of 1, 3-butadiene [32]. (2) The rate-determining step in the glycerol hydrogenolysis reaction is not the acetol hydrogenation reaction, but the dehydration step, again consistent with the literature where dehydration was suggested to be the rate-determining step, requiring relatively higher reaction temperatures than the following hydrogenation [40]. Therefore, the observed activity improvement resulting from the alumina overcoat in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. Since the overall Pt/Al₂O₃ catalyst acidity did not change considerably (Fig. 1(f)), the improved metal-acid proximity occasioned by the alumina overcoat is very likely responsible for the activity enhancement through the synergetic effect [12].

A Pt/Al₂O₃ catalyst with a smaller Pt particle size of ~2 nm was also synthesized. For glycerol hydrogenolysis, this sample showed selectivity to 1, 2-PD of approximately 78% at a glycerol conversion of 22%, which was considerably higher than that of the 7 nm Pt sample (Fig. 3(a)). This demonstrates that glycerol hydrogenolysis is a structure sensitive reaction, and decreasing Pt particle size suppresses the secondary hydrogenolysis of 1, 2-PD to 1-PO. This result is consistent with a previous study where the yield of 1-PO and degradation products increased on aggregated Pt catalyst in the 2^nd run of recycling testing [41]. Therefore, the increase in 1, 2-PD selectivity caused by the alumina overcoat might be due to a geometric effect, where the surface of the Pt NPs is divided into smaller enclaves by the porous alumina overcoat, approximately simulating a decrease of Pt particle size.

In this work, we have demonstrated that precisely controlled alumina overcoating of Pt/Al₂O₃ catalyst using ALD improved both activity and selectivity to 1, 2-PD in the aqueous-phase hydrogenolysis of glycerol. Our work provides solid evidence that close metal-acid proximity enhances bifunctionality, thus improving the catalytic activity. To the best of our knowledge, this is the first work to investigate close metal-acid proximity by applying an alumina overcoat, without varying the overall catalyst acidity. Finally, our work highlights a new method for investigating the metal-acid proximity effect in other catalytic reaction systems.