Effects of Surface Oxygen-Containing Groups of the Flowerlike Carbon

catalysts

Article Effects of Surface Oxygen-Containing Groups of the Flowerlike Carbon Nanosheets on Palladium Dispersion, Catalytic Activity and Stability in Hydrogenolytic Debenzylation of Tetraacetyldibenzylhexaazaisowurtzitane

Yun Chen 1, Xinlei Ding 2, Wenge Qiu 2,* , Jianwei Song 3, Junping Nan 2, Guangmei Bai 2 and Siping Pang 1

1 School of Materials Science & Technology, Beijing Institute of Technology, Beijing 100081, China; [email protected] (Y.C.); [email protected] (S.P.) 2 Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China; [email protected] (X.D.); [email protected] (J.N.); [email protected] (G.B.) 3 Qing Yang Chemical Industry Corporation, Liaoyang 111001, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-13521382103

Abstract: The inﬂuence of the surface chemical properties of the carbon support on the Pd dispersion,

activity and stability of Pd(OH)2/C catalyst for the hydrogenolytic debenzylation of tetraacetyldibenzylhexaazaisowurtzitane (TADB) was studied in detail. The ﬂowerlike nanosheet carbon material (NSC) was chosen as the pristine support, meanwhile chemical oxidation with nitric acid and physical Citation: Chen, Y.; Ding, X.; Qiu, W.; calcination at 600 ◦C treatments were used to modify its surface properties, which were denoted as Song, J.; Nan, J.; Bai, G.; Pang, S. NSCox-2 (treated with 20 wt% HNO3) and NSC-600, respectively. The three carbon supports and the Effects of Surface Oxygen-Containing corresponding catalysts of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 were characterized by scanning Groups of the Flowerlike Carbon electron microscope (SEM), transmission electron microscopy (TEM), nitrogen sorption isotherm Nanosheets on Palladium Dispersion, measurement (BET), powder X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectra Catalytic Activity and Stability in Hydrogenolytic Debenzylation of (XPS), temperature-programmed desorption (TPD), temperature-programmed reduction (H2-TPR), Tetraacetyldibenzylhexaazaisowurt- thermogravimetric analysis (TG), and element analysis. The debenzylation activities of Pd/NSC, zitane. Catalysts 2021, 11, 441. Pd/NSC-600, and Pd/NSCox-2, as well as the three catalysts after pre-reduction treatment were also https://doi.org/10.3390/ evaluated. It was found that the activity and stability of the Pd(OH)2/C catalysts in the debenzylation catal11040441 reaction highly depended on the content of surface oxygen-containing groups of the carbon support.

Academic Editor: Alfonso Grassi Keywords: palladium catalyst; hydrogenolytic debenzylation; surface oxygen-containing groups; palladium dispersion Received: 5 March 2021 Accepted: 25 March 2021 Published: 30 March 2021 1. Introduction Publisher’s Note: MDPI stays neutral Carbon materials have been widely used as support to fabricate the heterogeneous with regard to jurisdictional claims in catalysts due to its high stability, large tunable speciﬁc surface area, chemical inertness, published maps and institutional afﬁl- iations. easy tailorable surface chemical properties [1–3], especially for the precious metal catalysts. Palladium on carbon is an important class of catalyst, which can be used in several industrial chemical processes including hydrogenation, dehydrogenation, hydrogenolysis, hydro-dechlorination, nitroarenes reduction, and C–C coupling [4–6]. It is well known that the activity and selectivity of the Pd-based catalysts supported on carbons, either Copyright: © 2021 by the authors. Pd/C or Pd(OH)2/C, strongly depend on the preparation methods, the physicochemical Licensee MDPI, Basel, Switzerland. properties of Pd particles, such as size, morphology, and dispersion of Pd species, and the This article is an open access article nature and structure of carbon supports, involving porous structures and surface proper- distributed under the terms and conditions of the Creative Commons ties [2,7–9]. There are two main general routes for the preparation of Pd-based catalysts Attribution (CC BY) license (https:// on carbons: the ion exchange and the deposition-precipitation methods. In the case of creativecommons.org/licenses/by/ ion exchange, both anionic and cationic palladium precursors can be used [10–12], result- 4.0/). ing well dispersed Pd-based catalysts. In this process, the metal loading and dispersion

Catalysts 2021, 11, 441. https://doi.org/10.3390/catal11040441 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 441 2 of 17

highly depend on the surface properties of the carbon support and the metal precursor [2]. The deposition-precipitation method is a more extensively employed route to elaborate heterogeneous precious metal catalysts, in which a compulsory experimental procedure and a well decorated carbon support are desirable in order to get well-dispersed metal particles [13]. Several strategies have been reported to fabricate highly dispersed Pd/C catalysts through an effective control of the adsorption, deposition, and reduction process of palladium species, such as using homogeneous precipitation [14], simple liquid-phase precipitation-reduction [15], functional ion pre-adsorption on support [16], photochemical route [17], etc. Besides adjusting the preparation method, adding surfactant agents [18] or polymer molecules [19,20] during synthesis also had been employed to control the Pd particles size and dispersion. The textural and surface chemical properties of the carbon support also showed much more effects on the catalytic performance of precious metals supported on carbon. The textural properties of carbon support, such as specific surface area and porosity, influenced not only the Pd loading and dispersion [7,21], but also the diffusion of substrate and product molecules in the catalyst framework. Therefore, mesoporous carbon materials are supposed to be proper supports of palladium [22], particular for the large size substrate molecules [23,24]. The contribution of surface functional groups of carbon supports to the catalytic performance may reflect in many ways, such as affecting the size of Pd particles, the electronic state of palladium species, the stability of catalyst, and the adsorption of reactant molecules, so enormous efforts have been devoted toward modifying the surface chemical properties of carbon supports. It was found that the introduction of nitrogen atom [25–27] or other component, such as phosphomolybdic acid [28], could enhance chemical, electrical, and functional properties of the carbon surface and increase the interaction between the metal and carbon surface. The oxidation treatment was a convenient approach to introduce oxygen functional groups into the carbon support surface, which was also beneficial to improve the dispersion of active metal species and to enhance the stability of catalysts. Commonly used oxidant included nitric acid [29], mixtures of H2SO4/HNO3 [30], hydrogen peroxide [31], O3 [32], potassium permanganate [33], etc. Gu found that the pretreatment of coal-based commercial granular activated carbon using nitric acid at different temperature changed its surface groups and surface total acidity, resulting in catalysts with higher dispersion and activity on rosin disproportionation [34]. Tang reported that modification porous carbon spheres by UV-O3 increased their surface oxygen content and defects, leading to the improvement of the Pd loading and the activity of Pd-Ce/PCSs [32]. Benzyl group is one of the most commonly used protecting groups for O- and N-based functionalities and have been broadly employed in fine chemical synthesis and organic transformations. Palladium on carbon has been almost the first choice for hydrodeben- zylation reaction [35]. Although much attention has been paid to gain more insight into the influences of the support properties on the catalytic performance of palladium-carbon catalytic systems for cinnamaldehyde hydrogenation [7], furfural hydrogenation [36], cyclododecatriene hydrogenation [21], rosin disproportionation [34], liquid-phase hydro- dechlorination of chlorobenzene [5], etc. Few reports exist to clarify the correlation of the surface functional groups of support with the performance of Pd/C catalyst for the hydrogenolytic debenzylation reaction [37]. Therefore, an investigation of the catalytic performance of Pd-based hydrogenolytic debenzylation catalyst as a function of surface chemical properties of carbon support becomes desirable. As an outstanding representative of high energy density materials (HEDM), hexanitrohexaazaisowurtzitane (CL-20) has attracted much attention in the past decades. Hexabenzylhexaazaisowurtzitane (HBIW) has been the main precursor for the fabrication of CL-20 [38], but it cannot be nitrated directly to afford CL-20 due to its low stability of the cage framework. So the debenzylation and transformation of HBIW became the key step for the synthesis of CL-20. Usually, HBIW is firstly converted to TADB in the presence of acylation agent, and subsequently TADB can be further transformed to tetraacetylhex- Catalysts 2021, 11, x FOR PEER REVIEW 3 of 18

dium-carbon catalytic systems for cinnamaldehyde hydrogenation [7], furfural hydrogenation [36], cyclododecatriene hydrogenation [21], rosin disproportionation [34], liquid-phase hydro-dechlorination of chlorobenzene [5], etc. Few reports exist to clarify the correlation of the surface functional groups of support with the performance of Pd/C catalyst for the hydrogenolytic debenzylation reaction [37]. Therefore, an investigation of the catalytic performance of Pd-based hydrogenolytic debenzylation catalyst as a function of surface chemical properties of carbon support becomes desirable. As an outstanding representative of high energy density materials (HEDM), hexanitrohexaazaisowurtzitane (CL-20) has attracted much attention in the past decades. Hexabenzylhexaazaisowurtzitane (HBIW) has been the main precursor for the fabrication of CL-20 [38], but it cannot be nitrated directly to afford CL-20 due to its low stabil-

Catalysts 2021, 11, 441 ity of the cage framework. So the debenzylation and transformation of HBIW became3 of 17 the key step for the synthesis of CL-20. Usually, HBIW is firstly converted to TADB in the presence of acylation agent, and subsequently TADB can be further transformed to tetraacetylhexaazaisowurtzitane (TAIW), which can be nitrated easily to give CL-20 aazaisowurtzitane(Scheme 1) [39,40]. (TAIW),However, which the transformation can be nitrated of easily TADB to giveto TAIW CL-20 need (Schemeed a relative1)[ 39,40 in-]. However,tensive condition the transformation, including the of addition TADB to of TAIW acidicneeded cocatalyst a relative and a high intensive dose of condition, Pd cata- includinglyst, so TADB the addition was a proper of acidic substrate cocatalyst to distinguish and a high dosethe performance of Pd catalyst, of sothe TADB debenzyla- was a propertion catalysts. substrate to distinguish the performance of the debenzylation catalysts.

Scheme 1.1. Route for the synthesis of CL-20.CL-20.

InIn ourour previous workwork [[41],], aa flowerlikeflowerlike nanosheetnanosheet carboncarbon (NSC)(NSC) materialmaterial waswas provedproved toto bebe aa suitablesuitable palladium palladium catalyst catalyst support, support, which which showed showed an an evidently evidently hierarchical hierarchical porous po- structure.rous structure. Herein, Herein, the the chemical chemical treatment, treatment, oxidation oxidation using using various various concentrations concentrations ofof HNO , and a physical treatment, calcination at 600 ◦C under nitrogen atmosphere, were HNO3, and a physical treatment, calcination at 600 °C under nitrogen atmosphere, were usedused to adjust the the surface surface chemical chemical properties properties of of the the NSC NSC samples. samples. It was It was found found that that the theactivity activity and and stability stability of the of thecorresponding corresponding Pd Pdcatalyst catalyst in inthe the debenzylation debenzylation reaction reaction of oftetraacetyldibenzylhexaazaisowurtzitane tetraacetyldibenzylhexaazaisowurtzitane (TADB) (TADB) highly highly depended depended on on the the content of of surface oxygen-containingoxygen-containing groupsgroups ofof carboncarbon support.support. 2. Results and Discussion 2. Results and Discussion 2.1. Activities and Stabilities of the Catalysts in Hydrogenolytic Debenzylation of TADB 2.1. Activities and Stabilities of the Catalysts in Hydrogenolytic Debenzylation of TADB The catalytic activities of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 were evaluated by usingThe the catalytic hydrogenolytic activities debenzylation of Pd/NSC, of Pd/NSC TADB as-600 a probe, and Pd/NSC reaction.ox Considering-2 were evaluated the solu- by bilityusing ofthe TADB hydrogenolytic and the promotional debenzylation effect of acidicTADB mediumas a probe to the reaction. debenzylation Considering reaction, the aceticsolubility acid of was TADB chosen and as thea solvent. promotional The hydrogen effect of consumption acidic medium amounts to the as debenzylation a function of reactionreaction, times acetic are acid shown was inchosen Figure as1. Ita couldsolvent. be The seen hydrogen that the hydrogen consumpti consumptionon amounts rates as a overfunction Pd/NSC of reactionox-2 and times Pd/NSC are catalystsshown in were Fig significantlyure 1. It could faster be than seen that that of the Pd/NSC-600 hydrogen inconsumption the first 2 h, rates indicating over Pd/NSC that theox-2 Pd/NSC and Pd/NSCox-2 and catalysts Pd/NSC were had significantly higher activities faster thanthan Pd/NSC-600.that of Pd/NSC After-600 thatin the the first hydrogen 2 h, indicating consumption that the rates Pd/NSC of theox-2three and Pd/NSC catalysts haddecreased higher gradually.activities than The totalPd/NSC hydrogen-600. After consumption that the amounthydrogen of consumption Pd/NSCox-2 within rates 10of the h was three similar cat- toalysts that ofdecreased Pd/NSC, gradually. which was The higher total than hydrogen the data consumption of Pd/NSC-600. amount The activities of Pd/NSC of theox-2 threewithin catalysts 10 h was in similar order of to Pd/NSC that ofox-2 Pd/NSC,> Pd/NSC which > Pd/NSC-600 was higher in thethan whole the process,data of revealingPd/NSC-600. that The the pretreatmentsactivities of the of three carbon catalysts support was with in nitric order acid of Pd/NSC and calcinationox-2 > Pd/NSC at high > temperaturePd/NSC-600 under in the N whole2 flow couldprocess, improve revealing or inhibit that the the activitiespretreatments of the of corresponding carbon support catalysts,with nitric respectively. acid and These calcination results at were high consistent temperature with under the product N2 flow yield could and imp therove substrate or in- Catalysts 2021, 11, x FOR PEER REVIEW 4 of 18 conversionhibit the activities (Table S1). of the It can corresponding be found that catalysts, the activity respectively. of Pd/NSC Theseox-2 is results much better were thancon- thatsistent of thewith reported the product catalysts. yield and the substrate conversion (Table S1). It can be found that the activity of Pd/NSCox-2 is much better than that of the reported catalysts.

FigureFigure 1. 1.Hydrogen Hydrogen consumption consumption amounts amounts as as functions functions of of time time during during the the hydrogenolysis hydrogenolysis reaction reaction ofof TADB TADB with with fresh fresh catalysts catalysts ( a()a) and and re-used re-used catalysts catalysts (b (),b), respectively. respectively.

ItIt was was found found that that the the Pd Pd species species on on the the carbon carbon surface surface were were characterized characterized by by a a high high mobility.mobility. Pre-reductionPre-reduction mightmight leadlead toto thethe aggregationaggregation ofof PdPd particlesparticles ononcarbon carbonwith with a a consequent decrease in the Pd dispersion and the catalytic activity in the debenzylation reaction [24,42]. In order to understand the contribution of the surface oxygen-containing groups on carbon to the stability of Pd/C catalyst, the pre-reduction was performed on the three catalysts. The activities data of the resulted samples, Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R, are also given in Figure 1a. It could be seen that the hydrogen consumption amounts at a desired time over Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R appeared different degrees of decline comparing to the corresponding unreduced catalysts. However, the decrease in the activity of Pd/NSCox-2-R was much lower comparing to the other two, implying the high stability of Pd/NSCox-2 in the pre-reduction process. In addition, the stability of Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 was also evaluated by the recycling experiment in the debenzylation reaction of TADB under a similar condition. As shown in Figure 1b, the hydrogen consumption amounts of the re-used Pd/NSCox-2 and Pd/NSC catalysts were larger than that of the re-used Pd/NSC-600. Moreover, the reaction rate over Pd/NSCox-2 in the whole process was faster than over the other two. The total hydrogen consumption amount of the re-used Pd/NSCox-2 within 10 h was about 3.0 L, which was much higher than that of the re-used Pd/NSC-600 (0.9 L). In addition, the yield of product and the conversion of TADB over the re-used Pd/NSCox-2 were 75 and 83%, respectively (Table S1), which were significantly higher than the data of the re-used Pd/NSC (60 and 70%) and Pd/NSC-600 (10 and 15%), revealing the best stability of Pd/NSCox-2 in the hydrogenolytic debenzylation reaction. All these results indicated that the chemical nature of the surface groups exerted a considerable influence on the interaction of the Pd species with the carbon carrier [3]. In particular, the presence of relative high amounts of oxygen-containing groups enabled higher activities and stabilities of the catalysts. In addition, the activity declined trends of the three catalysts under three conditions: fresh one, after pre-reduction and the recovered one, were displayed in Figure 2. It could be seen that the pre-reduction treatment might cause the activity decreases of Pd/NSC and Pd/NSC-600 in some extent possibly due to the sintering of Pd particles and consequently the decrease in Pd dispersion in the reduction process. The activities of the re-used Pd/NSC and Pd/NSC-600 decreased further in the recycling experiments, particular for the Pd/NSC-600 catalyst, implying that there was one more factor leading to the catalyst deactivation, excepting the reduction effect of the hydrogenolytic debenzylation reaction itself. For Pd/NSCox-2, the pre-reduction treatment had little effect on its activity, however it decreased in the recycling experiment, further revealing the existence of other reason that caused the catalyst deactivation, although it was not clearly now.

Catalysts 2021, 11, 441 4 of 17

consequent decrease in the Pd dispersion and the catalytic activity in the debenzylation reaction [24,42]. In order to understand the contribution of the surface oxygen-containing groups on carbon to the stability of Pd/C catalyst, the pre-reduction was performed on the three catalysts. The activities data of the resulted samples, Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R, are also given in Figure1a. It could be seen that the hydrogen consumption amounts at a desired time over Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R appeared different degrees of decline comparing to the corresponding unreduced catalysts. However, the decrease in the activity of Pd/NSCox-2-R was much lower comparing to the other two, implying the high stability of Pd/NSCox-2 in the pre-reduction process. In addition, the stability of Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 was also evaluated by the recycling experiment in the debenzylation reaction of TADB under a similar condition. As shown in Figure1b, the hydrogen consumption amounts of the re-used Pd/NSC ox-2 and Pd/NSC catalysts were larger than that of the re-used Pd/NSC-600. Moreover, the reaction rate over Pd/NSCox-2 in the whole process was faster than over the other two. The total hydrogen consumption amount of the re-used Pd/NSCox-2 within 10 h was about 3.0 L, which was much higher than that of the re-used Pd/NSC-600 (0.9 L). In addition, the yield of product and the conversion of TADB over the re-used Pd/NSCox-2 were 75 and 83%, respectively (Table S1), which were signiﬁcantly higher than the data of the re-used Pd/NSC (60 and 70%) and Pd/NSC-600 (10 and 15%), revealing the best stability of Pd/NSCox-2 in the hydrogenolytic debenzylation reaction. All these results indicated that the chemical nature of the surface groups exerted a considerable inﬂuence on the interaction of the Pd species with the carbon carrier [3]. In particular, the presence of relative high amounts of oxygen-containing groups enabled higher activities and stabilities of the catalysts. In addition, the activity declined trends of the three catalysts under three conditions: fresh one, after pre-reduction and the recovered one, were displayed in Figure2. It could be seen that the pre-reduction treatment might cause the activity decreases of Pd/NSC and Pd/NSC-600 in some extent possibly due to the sintering of Pd particles and consequently the decrease in Pd dispersion in the reduction process. The activities of the re-used Pd/NSC and Pd/NSC-600 decreased further in the recycling experiments, particular for the Pd/NSC-600 catalyst, implying that there was one more factor leading to the catalyst deactivation, excepting the reduction effect of the hydrogenolytic debenzylation reaction itself. For Pd/NSC , the pre-reduction treatment had little effect on its activity, however Catalysts 2021, 11, x FOR PEER REVIEW ox-2 5 of 18 it decreased in the recycling experiment, further revealing the existence of other reason that caused the catalyst deactivation, although it was not clearly now.

FigureFigure 2.2. Yields of the product product (columns) (columns) and and conversions conversions (points) (points) of ofTADB TADB in inthe the hydrogenolysis hydrogenol- ysisreaction reaction over overvarious various catalysts. catalysts. Pd/NSC Pd/NSC-1,-1, Pd/NSC Pd/NSC-600-1,-600-1, and Pd/NSC andox- Pd/NSC2-1 presentedox-2-1 the presented re-used thecatalysts. re-used catalysts.

2.2. Characterization of the Carbon Supports

The morphology of the NSC, NSC-600, and NSCox-2 samples is showed in Figure 3, and similar flowerlike nanosheet structures were observed, indicating that the macrostructure of carbon supports did not change obviously during the chemical or physical treatment.

Figure 3. SEM images of NSC (a), NSC-600 (b), and NSCox-2 (c).

The N2 adsorption–desorption isotherms of the three carbon supports are shown in Figure 4. The specific surface area, and pore volume of these supports are listed in Table 1. The isotherms of NSC, NSC-600, and NSCox-2 samples showed a combination of type I and type IV curves with a type H3 hysteresis loop revealing the existence of the interparticle mesoporosity with the volume 0.9, 0.68, and 0.81 cm3/g, respectively. In addition, its pore size distribution (PSD) curve (Figure S1) showed several peaks in the range of 0.5 to 100 nm, further confirming the existence of mesoporous and macroporous structures from the accumulation of carbon nanosheets. The hysteresis loop shapes and the pore size distributions (Figure S1) of NSC-600 and NSCox-2 were similar to that of NSC, implying the similar textural properties of the carbon supports, so the effects of the textural properties of support on the performance of the corresponding catalysts could be diminish greatly. The surface area of NSC-600 was also similar to that of NSC (Table 1), while the data of NSCox-2 (510 m2/g) decreased in certain extent possible due to the destruction of a few flowerlike microspheres during the oxidation. The surface area order of the three carbon samples, NSC-600 ≈ NSC > NSCox-2, was different from the activity order of the corresponding catalysts, indicating that surface area of the carbon support was not the main reason affecting the Pd dispersion and the activity of the Pd-based catalyst on carbon.

Catalysts 2021, 11, x FOR PEER REVIEW 5 of 18

Figure 2. Yields of the product (columns) and conversions (points) of TADB in the hydrogenolysis reaction over various catalysts. Pd/NSC-1, Pd/NSC-600-1, and Pd/NSCox-2-1 presented the re-used Catalysts 2021, 11, 441 5 of 17 catalysts.

2.2. Characterization of the Carbon Supports

2.2. CharacterizationThe morphology of the of Carbonthe NSC, Supports NSC-600, and NSCox-2 samples is showed in Figure 3, and The similar morphology flowerlike of the nanosheet NSC, NSC-600, structures and were NSCox-2 observed,samples indicating is showed in that Figure the 3 macro-, and similarstructure ﬂowerlike of carbon nanosheet supports structures did not change were observed, obviously indicating during the that chemical the macrostructure or physical oftreat carbonment supports. did not change obviously during the chemical or physical treatment.

Figure 3. SEM images of NSC (a), NSC-600 (b), and NSCox-2 (c). Figure 3. SEM images of NSC (a), NSC-600 (b), and NSCox-2 (c).

TheThe NN22 adsorption–desorptionadsorption–desorption isotherms of of the the three three carbon carbon supports supports are are shown shown in inFig Figureure 4.4 The. The specific specific surface surface area, area, and and pore pore volume volume of these of these supports supports are listed are listed in Table in Table1. The1. isotherms The isotherms of NSC of, NSC,NSC- NSC-600,600, and NSC andox NSC-2 sampleox-2 ssamples showed showed a combination a combination of type I ofand type type I and IV typecurves IV with curves a type with H3 a type hysteresis H3 hysteresis loop revealing loop revealing the existence the existence of the ofin- theterparticle interparticle mesoporosity mesoporosity with withthe volume the volume 0.9, 0.68 0.9,, 0.68,and and0.81 0.81cm3/g, cm respectively3/g, respectively.. In addi- In addition,tion, its pore its pore size sizedistribution distribution (PSD) (PSD) curve curve (Figure (Figure S1) showed S1) showed several several peaks peaks in the in range the rangeof 0.5 of to 0.5 100 to 100 nm, nm, further further confirming confirming the the existence existence of of mesoporous and and macroporous macroporous structuresstructures from from the the accumulation accumulation of carbonof carbon nanosheets. nanosheets. The The hysteresis hysteresis loop loop shapes shapes and theand porethe poresize distributions size distributions (Figure (Fig S1)ure of S1) NSC-600 of NSC and-600 NSC andox-2 NSCwereox-2 similarwere similar to that to of that NSC, of implyingNSC, implying the similar the similar textural textural properties properties of the carbon of the supports, carbon supports so the effects, so the of effec the texturalts of the propertiestextural properties of support of on support the performance on the performance of the corresponding of the corresponding catalysts could catalysts be diminish could greatly.be diminish The surface greatly. area The of surface NSC-600 area was of NSC also- similar600 was to also that similar of NSC to (Table that of1), NSC while (Table the 2 data1), while of NSC theox-2 data(510 of m NSC/g)ox decreased-2 (510 m2/g) in certaindecreased extent in possiblecertain extent due to possible the destruction due to ofthe adestruction few flowerlike of a microspheresfew flowerlike during microspheres the oxidation. during The the surfaceoxidation area. The order surface of the area three or- carbonder of samples,the three NSC-600carbon samples,≈ NSC NSC > NSC-600ox-2 ≈, NSC was different> NSCox-2 from, was thedifferent activity from order the of activ- the Catalysts 2021, 11, x FOR PEER REVIEW 6 of 18 correspondingity order of the catalysts, corresponding indicating catalysts, that surface indicating area that of the surfac carbone area support of the was carbon not sup- the mainport reasonwas not affecting the main the reason Pd dispersion affecting andthe thePd dispersion activity of theand Pd-based the activity catalyst of the on Pd carbon.-based catalyst on carbon.

FigureFigure 4.4. NitrogenNitrogen adsorption adsorption/desorption/desorption isotherms isotherms of the of three the carbon three carbonsamples samplesand the corre- and the sponding catalysts. corresponding catalysts.

Table 1. Textural and structural properties of the carbon samples and the corresponding catalysts.

Sample SBET /m2 g−1 Smic /m2 g−1 Vmic/Vtot Vtot /cm3 g−1 NSC 595 280 0.19 0.90 NSC-600 600 364 0.30 0.68 NSCox-2 510 227 0.13 0.81 Pd/NSC 560 262 0.12 0.72 Pd/NSC-600 472 180 0.12 0.54 Pd/NSCox-2 485 167 0.10 0.68

Catalysts 2021, 11, 441 6 of 17

Table 1. Textural and structural properties of the carbon samples and the corresponding catalysts.

2 −1 2 −1 3 −1 Sample SBET /m g Smic /m g Vmic/Vtot Vtot /cm g NSC 595 280 0.19 0.90 NSC-600 600 364 0.30 0.68 NSCox-2 510 227 0.13 0.81 Pd/NSC 560 262 0.12 0.72 Pd/NSC-600 472 180 0.12 0.54 Pd/NSCox-2 485 167 0.10 0.68

In order to insight the phase structures of the carbon samples, XRD measurements were performed (Figure S2). The diffraction patterns of NSC, NSC-600, and NSCox-2 were similar. The broad peak at 23◦ and the weak peak at 44◦, corresponding to the (002) and the (100) diffractions of the graphitic domains, respectively, revealed the similar partial graphitic amorphous structure of the NSC, NSC-600, and NSCox-2. The Raman spectra of NSC, NSC-600, and NSCox-2 clearly displayed the characteristic D bands and G bands at 1340 and 1589 cm−1 (Figure S3), corresponding to the defects or disordered portions, and 2 the ordered sp carbon in the samples, respectively. The ID/IG ratio is often applied to measure the defects property of carbon materials [43]. It could be seen that the ID/IG ratios of the three carbon samples were comparable. The changes of surface functional groups after chemical and physical treatments were proved by the elemental analysis data of the samples of NSC, NSC-600 and NSCox-2 (Table S2). It was found that the hydrogen contents in the three samples were around 2%, and no nitrogen atoms were detected in NSC and NSC-600, but there was a little bit of nitrogen (1.3%) in NSCox-2, due to the interaction of nitrate ions or nitrogen oxides with the carbon matrix, which might give a positive effect on the Pd dispersion [25]. The oxygen content in NSCox-2 was much higher than that of the NSC sample due to the further oxidation of carbon surface by nitric acid, while the oxygen content in NSC-600 was lower than that of NSC, revealing the elimination of a few oxygen-containing groups in the calcination process, particular for the carboxylic group and anhydride group. The wetting angles of the carbon supports were also detected (Figure S4), which were in order of NSCox-2 < NSC < NSC-600, showing a significant difference in hydrophilicity of the carbon supports due to the variety of concentrations of the surface oxygen-containing groups. The high hydrophilicity of NSCox-2 may arise preferentially adsorption and nucleation of the Pd species on its surface and be profitable to improve the Pd dispersion. In addition, TG measurement was conducted to study the decomposition of oxygen-containing groups of NSC, NSC-600, and NSCox-2 under N2 atmosphere. The TG data showed that the pyrolysis of the three samples was a continuous process (Figure S5). The mass losses of NSCox-2, NSC, and NSC-600 from 100 ◦C up to 750 ◦C were 15.2, 6.2, and 3.5%, respectively. The weight losses of the three carbon supports were only slightly lower comparing to their total oxygen contents (Table S2), revealing that the mass losses could be mainly assigned to the decomposition of the oxygen-containing functional groups. TPD-technique was a method to characterize precisely the nature of oxygen-containing groups present on carbon material surface, and their amounts and thermal stability through the quantification of CO and CO2 emissions during temperature increase in inert atmosphere. So, the TPD measurement was performed to further detect the surface structures of the three carbon samples. The deconvolution method (using multiple Gaussian functions) was applied to the CO2 and CO profiles obtained for the three carbon samples, and the corresponding deconvolutions are shown in Figure5. Based on the literature results [ 44], the CO2 spectra of the NSC and NSCox-2 samples were decomposed into three peaks as fol- lows. The two peaks below 300 ◦C could be attributed to two kinds of carboxylic acids, and the peak centered at relative high temperature range (400–550 ◦C) was assigned to the carboxylic anhydrides. For the NSC-600 sample, only two weak peaks at 733 and 892 ◦C were detected, which were assigned to the decomposition of thermal stable lactone groups. The CO spectra of the NSC and NSCox-2 were also decomposed into three peaks, corresponding Catalysts 2021, 11, 441 7 of 17

to the decomposition of carboxylic anhydride (300–600 ◦C), phenols (510–750 ◦C) and carbonyl-quinones (600–900 ◦C), respectively [44]. For NSC-600, only carbonyl-quinones signals were observed. The amounts of oxygen-containing groups calculated from the surface areas of the deconvoluted CO2 and CO peaks are given in Tables2 and3, respectively. It clearly displayed that the 20% HNO3 oxidation led to signiﬁcantly increases of the concentrations of oxygen-containing groups, while the calcination treatment at 600 ◦C Catalysts 2021, 11, x FOR PEER REVIEW 8 of 18 resulted in the disappearance of most carboxylic acids, carboxylic anhydrides and phenol groups except the stable lactone and carbonyl-quinone groups.

Figure 5. Results of deconvolutions on CO2-TPD and CO-TPD profiles using a multiple Gaussian Figure 5. Results of deconvolutions on CO2-TPD and CO-TPD profiles using a multiple Gaussian function. function; (a) CO2-TPD profile of NSC-600, (b) CO-TPD profile of NSC-600, (c) CO2-TPD profile of NSC, (d) CO-TPD profile of NSC, (e) CO2-TPD profile of NSCox-2,(f) CO-TPD profile of NSCox-2. Table 2. Results of the deconvolution of CO2-TPD spectra using a multiple Gaussian function. Table 2. Results of the deconvolution of CO -TPD spectra using a multiple Gaussian function. Carboxylic-1 Carboxylic-2 2Anhydride Lactone Samples TM A Carboxylic-1TM A Carboxylic-2TM A AnhydrideTM A Lactone Samples (°C ) (μmol/g)T M (°C ) A(μmol/g)T M (°C ) A(μmol/g)T M(°C ) (μmol/g)A TM A (◦C) (µmol/g) (◦C) (µmol/g) (◦C)733 (µmol/g)234 (◦C) (µmol/g) NSC-600 ------892 34 733 234 NSC-600 ------NSC 210 356 315 614 525 627 - - 892 34 NSCox-2 188NSC 680 210272 356942 315427 6141293 525- 627- - - NSCox-2 188 680 272 942 427 1293 - - Table 3. Results of the deconvolution of CO-TPD spectra using a multiple Gaussian function. Anhydride Phenol Carbonyl-quinone Samples TM A TM A TM A (°C ) (μmol/g) (°C ) (μmol/g) (°C ) (μmol/g) 771 318 NSC-600 - - - - 920 160 NSC 509 322 613 562 762 386 NSCox-2 432 514 559 968 730 572

2.3. Characterization of the Catalysts

TEM images of Pd/NSC, Pd/NSC-600, Pd/NSCox-2 as well as the corresponding samples after pre-reduction treatment are given in Figure 6. As Figure 6a, 6i show, for the Pd/NSC and Pd/NSCox-2 catalysts, it was hardly to distinguish the most Pd nanopar-

Catalysts 2021, 11, 441 8 of 17

Table 3. Results of the deconvolution of CO-TPD spectra using a multiple Gaussian function.

Anhydride Phenol Carbonyl-Quinone Samples A A A T (◦C) T (◦C) T (◦C) M (µmol/g) M (µmol/g) M (µmol/g) 771 318 NSC-600 - - - - 920 160 NSC 509 322 613 562 762 386 NSCox-2 432 514 559 968 730 572

Catalysts 2021, 11, x FOR PEER REVIEW2.3. Characterization of the Catalysts 9 of 18

TEM images of Pd/NSC, Pd/NSC-600, Pd/NSCox-2 as well as the corresponding samples after pre-reduction treatment are given in Figure6. As Figure6a,i show, for the Pd/NSCticles in andthe low Pd/NSC magnifiedox-2 catalysts, TEM images, it was hardlyrepresenting to distinguish that most the of most the Pd Pd nanoparticles species were inuniformly the low magnified dispersed TEM on NSC images, and representingNSCox-2. On the that contrary, most of the forPd the species Pd/NSC were-600 uniformly sample, a dispersedfew large onPd NSC particles and NSC couldox-2 . be On observed the contrary, obviously for the ( Pd/NSC-600Figure 6e), implying sample, a the few low large Pd Pddispersion. particles couldThe statistical be observed particle obviously size distributio (Figure6e),ns implyingrevealed thethat low the Pdratio dispersion. of Pd particles The statisticalless than particle3 nm on sizePd/NSC distributionsox-2 was about revealed 75% that(Figure the 6k), ratio which of Pd was particles higher less than than the 3 data nm onof Pd/NSC Pd/NSCox-2 (65was%) aboutand Pd/NSC 75% (Figure-600 6 (55k),%). which The was average higher particle than the sizes data of of Pd/NSC Pd/NSC, (65%)Pd/NSC and-600 Pd/NSC-600, and Pd/NSC (55%).ox-2 were The average2.8, 5.0, particleand 2.0 sizesnm, of respectively Pd/NSC, Pd/NSC-600,(Figure 6), further and Pd/NSCdemonstratingox-2 were the 2.8, highest 5.0, and Pd dispersion2.0 nm, respectively on NSCox-2 (Figure. In addition,6), further the demonstrating Pd dispersity of thePd/NSC highest (42 Pd%), dispersion Pd/NSC-600 on NSC(23%ox-2), and. In Pd/NSC addition,ox-2 the (58 Pd%), dispersity was also determinedof Pd/NSC (42%), by CO Pd/NSC-600chemisorption (23%), method and (Table Pd/NSC S3),ox-2 which(58%), w wasas in also accordance determined with by the CO TEM chemisorption data. These methodresults indicated (Table S3), that which the waschemical in accordance oxidativewith treatment the TEM with data. nitric These acid resultswas beneficial indicated to thatthe the high chemical dispersion oxidative of palladium treatment with on carbon nitric acid due was to beneficial the introduction to the high of dispersion more oxy- ofgen palladium-containing on groups, carbon due which to the was introduction profitable to of en morehance oxygen-containing the interaction between groups, Pd which spe- wascies profitableand carbon to support enhance, but the the interaction physical between calcination Pd exhibited species and a negative carbon support, effect on but the themetal physical dispersity calcination due to exhibited the decomposition a negative of effect unstable on the carboxyl metalic dispersity acid and due anhydride to the decompositiongroups. of unstable carboxylic acid and anhydride groups.

FigureFigure 6. 6.TEM TEM images images and and the the corresponding corresponding histograms histograms of of particle particle size size distribution distribution of of the the Pd/NSC Pd/NSC (a,b), Pd/NSC-R (c,d), Pd/NSC-600 (e,f), Pd/NSC-600-R (g,h), Pd/NSCox-2 (i,j), and (a,b), Pd/NSC-R (c,d), Pd/NSC-600 (e,f), Pd/NSC-600-R (g,h), Pd/NSCox-2 (i,j), and Pd/NSCox-2-R Pd/NSCox-2-R (k,l) samples. (k,l) samples. Walker [45,46] and Derbyshire [47] emphasized that the surface chemical property of carbon had an important influence on metal dispersion. For precisely understanding the effects of various oxygen-containing groups on the Pd dispersions, we checked the possibility of a direct correlation between the amounts of various oxygen-containing groups and the Pd dispersions of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2. To reduce the occasionality of only three samples, the Pd/NSCox-1 and Pd/NSCox-3 samples were also checked (Figure S6, S7, Table S4 and S5). It was found that there were obvious correlations between the amounts of carboxyl group and anhydride group, as well as phenol group and the Pd dispersions of the catalysts (Figure S8), while no correlation was found between the amount of carbonyl-quinone and the Pd dispersions. The results indicated that the concentrations of carboxyl groups and anhydride groups, as well as phenol groups instead of carbonyl-quinone groups on the carbon surface had significant effect on the Pd dispersions and consequently on the activity of the catalysts. However, NSCox-3 had the highest concentration of oxygen-containing groups, but the Pd dispersi-

Catalysts 2021, 11, 441 9 of 17

Walker [45,46] and Derbyshire [47] emphasized that the surface chemical property of carbon had an important influence on metal dispersion. For precisely understanding the effects of various oxygen-containing groups on the Pd dispersions, we checked the possibility of a direct correlation between the amounts of various oxygen-containing groups and the Pd dispersions of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2. To reduce the occasionality of only three samples, the Pd/NSCox-1 and Pd/NSCox-3 samples were also checked (Figures S6 and S7, Tables S4 and S5). It was found that there were obvious correlations between the amounts of carboxyl group and anhydride group, as well as phenol group and the Pd dispersions of the catalysts (Figure S8), while no correlation was found between the amount of carbonyl-quinone and the Pd dispersions. The results indicated that the concentrations of carboxyl groups and anhydride groups, as well as phenol groups instead of carbonyl-quinone groups on the carbon surface had significant effect on the Pd dispersions and consequently on the activity of the catalysts. However, NSCox-3 had the highest concentration of oxygen-containing groups, but the Pd dispersity of Pd/NSCox-3 (45%) was similar to Pd/NSCox-1 (46%), and much lower than that of Pd/NSCox-2 (58%), implying that there were other factors that influenced the Pd dispersion. In other words, a proper concentration of oxygen-containing groups for an ideal carbon support was desirable. The TEM data of Pd/NSC-R, Pd/NSC-600-R, and Pd/NSCox-2-R showed that the pre- reduction treatment led to the growth of Pd particles in a certain extent (Figure6 , Table S6), but the variations of Pd particle size of the three samples were much different. For Pd/NSCox-2, the change of Pd particle size was inconspicuous, and there were still about 70% of Pd particles maintaining below 3 nm after reduction treatment (Figure6l ). On the contrary, for Pd/NSC-R and Pd/NSC-600-R catalysts, the Pd particle size and distribution changed obviously comparing to the fresh ones (Figure6b,d, or Figure6f,h). The stability of the three catalysts in a reduction condition was in order of Pd/NSC-600 < Pd/NSC < Pd/NSCox-2. In order to further investigate the change of Pd particle size and dispersion in the reduction condition, the recovered Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 catalysts were also characterized by TEM (Figure S9). For the recovered Pd/NSCox-2 catalyst, highly dispersed Pd particles with an average size of 3.2 nm still could be seen clearly, but obvious agglomerations of Pd particles were observed for the recovered Pd/NSC and Pd/NSC- 600 catalysts, and their average sizes of Pd particles increased to about 4.5 and 10.2 nm, respectively (Table S6). The TEM figures of the recovered Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R samples (Figure S9) and the Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 samples after the cycling experiment (Figure S10) indicated the further aggregation of Pd particles during the debenzylation reaction (Table S6). It further showed that Pd/NSCox-2 had a higher stability comparing to the two others. The results indicated that the high concentration of surface oxygen-containing groups of the carbon support was beneficial to improve the stability of the corresponding palladium-carbon catalyst. It has been known that the Pd catalysts with a smaller average particle size generally result in better catalytic performance [48]. Combining the above activity data, one can deduce that the Pd particle size and dispersion is a main factor influencing the debenzylation activity. Further analysis showed that there was a direct correlation between the Pd particle sizes and the debenzylation activity of the corresponding catalysts (Figure7). However, the result does not mean that this relationship is a strictly linear. It has been known that the catalytic activity of the noble metal nanoparticles can be understood by assuming the defective and strained nature of their surface structure. The physical study revealed that there was a greater degree of disorder in the atomic arrangement for smaller nanoparticles [49], so the detailed surface microstructure of the novel metal particles also had significant effects on its catalytic activity, particular for small nanoparticles or clusters. In addition, the influence of the nature of carrier also need pay attention as presented in above. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 18

ty of Pd/NSCox-3 (45%) was similar to Pd/NSCox-1 (46%), and much lower than that of Pd/NSCox-2 (58%), implying that there were other factors that influenced the Pd dispersion. In other words, a proper concentration of oxygen-containing groups for an ideal carbon support was desirable. The TEM data of Pd/NSC-R, Pd/NSC-600-R, and Pd/NSCox-2-R showed that the pre-reduction treatment led to the growth of Pd particles in a certain extent (Figure 6, Table S6), but the variations of Pd particle size of the three samples were much different. For Pd/NSCox-2, the change of Pd particle size was inconspicuous, and there were still about 70% of Pd particles maintaining below 3 nm after reduction treatment (Figure 6l). On the contrary, for Pd/NSC-R and Pd/NSC-600-R catalysts, the Pd particle size and distribution changed obviously comparing to the fresh ones (Figure 6b, 6d, or 6f, 6h). The stability of the three catalysts in a reduction condition was in order of Pd/NSC-600 < Pd/NSC < Pd/NSCox-2. In order to further investigate the change of Pd particle size and dispersion in the reduction condition, the recovered Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 catalysts were also characterized by TEM (Figure S9). For the recovered Pd/NSCox-2 catalyst, highly dispersed Pd particles with an average size of 3.2 nm still could be seen clearly, but obvious agglomerations of Pd particles were observed for the recovered Pd/NSC and Pd/NSC-600 catalysts, and their average sizes of Pd particles increased to about 4.5 and 10.2 nm, respectively (Table S6). The TEM figures of the recovered Pd/NSCox-2-R, Pd/NSC-R, and Pd/NSC-600-R samples (Figure S9) and the Pd/NSCox-2, Pd/NSC, and Pd/NSC-600 samples after the cycling experiment (Figure S10) indicated the further aggregation of Pd particles during the debenzylation reaction (Ta- ble S6). It further showed that Pd/NSCox-2 had a higher stability comparing to the two others. The results indicated that the high concentration of surface oxygen-containing groups of the carbon support was beneficial to improve the stability of the corresponding palladium-carbon catalyst. It has been known that the Pd catalysts with a smaller average particle size generally result in better catalytic performance [48]. Combining the above activity data, one can deduce that the Pd particle size and dispersion is a main factor influencing the debenzylation activity. Further analysis showed that there was a direct correlation between the Pd particle sizes and the debenzylation activity of the corresponding catalysts (Figure 7). However, the result does not mean that this relationship is a strictly linear. It has been known that the catalytic activity of the noble metal nanoparticles can be understood by assuming the defective and strained nature of their surface structure. The physical study revealed that there was a greater degree of disorder in the atomic arrangement for smaller nanoparticles [49], so the detailed surface microstructure of the Catalysts 2021, 11, 441 novel metal particles also had significant effects on its catalytic activity, particular10 of for 17 small nanoparticles or clusters. In addition, the influence of the nature of carrier also need pay attention as presented in above.

Figure 7. Correlation between the average Pd particle sizes of the three fresh, pre-reduction treated Catalysts 2021, 11, x FOR PEER REVIEW 11 of 18 and recovered catalysts with the product yields. The data were from Tables S1 and S6.

Comparing the XRD patterns of Pd/NSC, Pd/NSC-600 and Pd/NSCox-2 (Figure S11), Figure 7. Correlation betweenθ the average Pd particle size◦ s of the three fresh, pre-reduction treat- threeed and sharprecovered peaks catalysts at 2withof the ~40, product 46, yield ands. 68The datafor were the Pd/NSC-600from Tables S1 and catalyst S6. could be found, which were assigned to the (111), (200), and (220) diffractions of Pd0, respectively, demon- 0 stratingComparing the present the XRD of largepatterns Pd ofparticles Pd/NSC, Pd/NSC on it [50-600]. The and correspondingPd/NSCox-2 (Figure diffraction S11), peaks of Pd/NSCthree sharp were peaks much at 2θ weakerof ~40, 46, than and that68° for of Pd/NSC-600.the Pd/NSC-600 Nocatalyst obvious could signalbe found, of Pd0 could be detected,which were and assigned only two to the weak (111), and (200), wide and peaks (220) at diffractions around 34 of and Pd0, 43 respectively,◦ corresponding to the 0 carbondemonstrating support the were present observed of large Pd for particles Pd/NSC on it [,50 indicating]. The corresponding the high Pddiffraction dispersion and the peaks of Pd/NSC were much weaker than that of ox-2Pd/NSC-600. No obvious signal of Pd0 existencecould be detected, of PdO species. and only This two mightweak and be one wide more peaks reason at around why 34 the and Pd/NSC 43° corre-ox-2 catalyst ex- hibitedsponding high to the activity. carbon It support had been were reported observed that for thePd/NSC unreducedox-2, indicating Pd/C the showed high Pd a much higher activitydispersion than and the the corresponding existence of PdO reduced species. Pd/CThis might [37,51 be]. Theone more nitrogen reason adsorption/desorption why the isothermsPd/NSCox-2 ca fortalyst Pd/NSC, exhibited Pd/NSC-600, high activity. It and had Pd/NSCbeen reportedox-2 arethat shownthe unreduced in Figure Pd/C4 . The curves ofshowed the three a much catalysts higher activity were similarthan the tocorresponding their corresponding reduced Pd/C support, [37,51]. The revealing nitro- the similar texturalgen adsorption property,/desorption which isothermswas not the for key Pd/NSC, factor Pd/NSC inﬂuencing-600, and the Pd/NSC debenzylationox-2 are activity of shown in Figure 4. The curves of the three catalysts were similar to their corresponding the catalysts. support, revealing the similar textural property, which was not the key factor influencing theThe debenzylation high catalytic activity activity of the of catalyst the catalysts. is generally related to its reducibility. The TPR proﬁlesThe ofhigh the catalytic catalysts activity as a functionof the catalyst of reduction is generally temperature related to its are reducibility. presented The in Figure 8 . It is aTPR known profiles fact of that the catalysts bulk Pd as species a function can of be reduction reduced temperature by H2 at low are temperature presented in Fig- to form β-PdHx species,ure 8. It is which a known shows fact that a negative bulk Pd species signal can due be toreduced the hydrogen by H2 at low spillover temperature [52]. to An obviously negativeform β-PdH peakx species, was observed which shows at low a negative temperature signal due range to the of hydrogen 60 to 70 ◦ spilloverC for Pd/NSC, [52]. Pd/NSC- An obviously negative peak was observed at low temperature range of 60 to 70 °C for ◦ 600, and Pd/NSCox-2. Besides, a distinct positive peak centered at around 140 C was Pd/NSC, Pd/NSC-600, and Pd/NSCox-2. Besides, a distinct positive peak centered at obtained for Pd/NSC , suggesting the reduction of Pd2+→Pd0 and the good reducibility. around 140 °C was obtainedox-2 for Pd/NSCox-2, suggesting the reduction of Pd2+→Pd0 and 2+ Forthe good Pd/NSC reducibility. and Pd/NSC-600, For Pd/NSC and the Pd/NSC reduction-600, the peaks reduction of Pd peakswere of Pd inconspicuous.2+ were The ◦ peakinconspicuous. in the high The temperature peak in the high range temperature (>500 rangeC) was (>500 assigned °C) was to assigned the reduction to the of surface oxygen-functionalreduction of surface oxygen groups-functional (Table S7). groups (Table S7).

FigureFigure 8. 8. TPRTPR profiles proﬁles of the of various the various catalysts. catalysts.

The XPS analysis has often been used to determine the surface chemical composition and the bonding environment of various carbon materials. So XPS studies of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 were performed (Figure S12, Table S8). The C 1s spectra of the three catalysts are showed in Figure 9a. The signals were deconvoluted into four peaks referred to the aliphatic/aromatic carbon group (CHx, C-C/C=C) at 284.7 eV, carbons coupled with hydroxyl groups (C-OH) at 286.0 eV, carbonyl groups (C=O) at 287.0 eV and carboxylic groups, esters or lactones (-COOR) (288.8 eV) [53], respectively, demonstrating the existence of various oxygen-containing groups. The O 1s signal of the three catalysts could be fitted to four peaks centered at 531.0 eV (oxygen in quinone C=O), 532.9 eV (oxygen in OH, ether, ester, anhydride, and carboxyl), 534.5 eV (ether oxygen in ester and anhydride, as well as hydroxyl in carbonyl group (COOH)) and

Catalysts 2021, 11, 441 11 of 17

The XPS analysis has often been used to determine the surface chemical composition and the bonding environment of various carbon materials. So XPS studies of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 were performed (Figure S12, Table S8). The C 1s spectra of the three catalysts are showed in Figure9a. The signals were deconvoluted into four peaks referred to the aliphatic/aromatic carbon group (CHx, C-C/C=C) at 284.7 eV, carbons coupled with hydroxyl groups (C-OH) at 286.0 eV, carbonyl groups (C=O) at 287.0 eV and carboxylic groups, esters or lactones (-COOR) (288.8 eV) [53], respectively, demonstrating Catalysts 2021, 11, x FOR PEER REVIEWthe existence of various oxygen-containing groups. The O 1s signal of the three catalysts12 of 18

could be ﬁtted to four peaks centered at 531.0 eV (oxygen in quinone C=O), 532.9 eV (oxygen in OH, ether, ester, anhydride, and carboxyl), 534.5 eV (ether oxygen in ester and anhydride, as well as hydroxyl in carbonyl group (COOH)) and 536.5 eV (adsorbed 536.5 eV (adsorbed water) [54]. The surface atomic concentrations of C and O within water) [54]. The surface atomic concentrations of C and O within different functional different functional groups were determined using the Peak-Fit program (Table 4). It can groups were determined using the Peak-Fit program (Table4). It can be seen that the be seen that the concentrations of C and O coupled with the carboxylic groups were both concentrations of C and O coupled with the carboxylic groups were both in order of in order of Pd/NSC-600 < Pd/NSC < Pd/NSCox-2, which were consistent with the order of Pd/NSC-600 < Pd/NSC < Pd/NSC , which were consistent with the order of their Pd their Pd dispersion, demonstratingox-2 the relationship between the concentration of car- dispersion, demonstrating the relationship between the concentration of carboxylic group boxylic group and the Pd dispersion. This was consistent with the above analysis of TPD and the Pd dispersion. This was consistent with the above analysis of TPD data. The total data. The total surface oxygen contents and the O/C atomic ratios calculated by XPS data surface oxygen contents and the O/C atomic ratios calculated by XPS data were similar to were similar to the corresponding data determined by elemental analysis (Table S9). The the corresponding data determined by elemental analysis (Table S9). The XPS spectra for XPS spectra for the doublet Pd 3d5/2 and 3d3/2 of Pd/NSC, Pd/NSC-600 and Pd/NSCox-2 the doublet Pd 3d5/2 and 3d3/2 of Pd/NSC, Pd/NSC-600 and Pd/NSCox-2 are shown in Figureare shown9c. The in Figure Pd (3d) 9c. signal The couldPd (3d) be signal ﬁtted could to two be pairs fitted of doubleto two pairs peaks. of The double Pd 3d3/2 peaks. 0 peakThe Pd at 340.13d3/2 eVpeak and at Pd340.1 3d5/2 eV and peak Pd at 3d5/2 335.9 peak eV were at 335.9 assigned eV were to Pdassigned0 species, to andPd spe- the Pdcies, 3d3/2 and the peak Pd at 3d3/2 342.8 peak eV and at 342.8 Pd 3d5/2 eV and peak Pd at 3d5/2 337.8 peak eV were at 337.8 assigned eV were to Pd assigned2+ species, to 2+ 2+ 0 whichPd species, were similar which to were the literaturesimilar to data the [li55terature]. The ratios data of[55]. Pd The2+/Pd ratios0 for of the Pd three/Pd catalysts for the three catalysts were summarized in Table S8. The Pd/NSCox-2 sample presented the were summarized in Table S8. The Pd/NSCox-2 sample presented the highest content of 2+ Pdhighest2+ species content than of the Pd other species two than catalysts, the other possibly two duecatalysts, to the possibly interaction due of to the the Pd interac- atoms withtion of more the surfacePd atoms oxygen-containing with more surfac groupse oxygen-containing of carbon. groups of carbon.

Figure 9. Cont.

Catalysts 2021, 11, x FOR PEER REVIEW 12 of 18

536.5 eV (adsorbed water) [54]. The surface atomic concentrations of C and O within different functional groups were determined using the Peak-Fit program (Table 4). It can be seen that the concentrations of C and O coupled with the carboxylic groups were both in order of Pd/NSC-600 < Pd/NSC < Pd/NSCox-2, which were consistent with the order of their Pd dispersion, demonstrating the relationship between the concentration of carboxylic group and the Pd dispersion. This was consistent with the above analysis of TPD data. The total surface oxygen contents and the O/C atomic ratios calculated by XPS data were similar to the corresponding data determined by elemental analysis (Table S9). The XPS spectra for the doublet Pd 3d5/2 and 3d3/2 of Pd/NSC, Pd/NSC-600 and Pd/NSCox-2 are shown in Figure 9c. The Pd (3d) signal could be fitted to two pairs of double peaks. The Pd 3d3/2 peak at 340.1 eV and Pd 3d5/2 peak at 335.9 eV were assigned to Pd0 species, and the Pd 3d3/2 peak at 342.8 eV and Pd 3d5/2 peak at 337.8 eV were assigned to Pd2+ species, which were similar to the literature data [55]. The ratios of Pd2+/Pd0 for the three catalysts were summarized in Table S8. The Pd/NSCox-2 sample presented the highest content of Pd2+ species than the other two catalysts, possibly due to the interaction of the Pd atoms with more surface oxygen-containing groups of carbon.

Catalysts 2021, 11, 441 12 of 17

Figure 9. High-resolution XPS data of Pd/NSC-600, Pd/NSC, and Pd/NSCox-2 catalysts: C 1s (a), O 1s (b) and Pd 3d (c).

Table 4. Surface carbon and oxygen concentrations of the various catalysts.

Atomic Concentration (mol%) C 1s O 1s Sample C=C sp2 OH, COOH C-OH C=O COOH C=O COOHH O C-C sp3 N-O 2 284.7 286.0 287.0 288.8 531.0 532.9 534.5 536.5 Pd/NSC-600 70.2 12.1 9.7 8.0 16.9 67.8 11.2 4.1 Pd/NSC 67.9 11.4 10.1 10.6 15.4 66.2 16.2 1.7 Pd/NSCox-2 65.1 10.1 7.6 17.2 17.2 55.8 22.8 4.2

3. Experimental 3.1. Chemicals

DMF, Ac2O, PhBr, NaOH, HF(40%), CH3COOH and zinc acetate were all from Tianjin Fuchen Chemistry Co. Ltd. Glucose (AR 99.5%) was purchased from Aladdin Chem- istry Co. Ltd. PdCl2 (Pd, 59.5%) was purchased from Shanghai Jiuyue Chemical Co., Ltd. HCl (AR 98%) was purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. SiO2 was received from Chongqing Chemical & Pharmaceutical holding (Group) Company.

3.2. Preparation of the Carbon Support and the Reaction Substrate

TADB was synthesized in our own laboratory. Briefly, 100 mL of DMF, 50 mL of Ac2O, 0.9 mL of PhBr, 50 g of HBIW, which was prepared using the Nielsen’s method [38], and 1.68 g of 9% Pd(OH)2/C catalysts were placed into the reactor. Then, the system was exchanged with pure hydrogen. The reaction mixture was stirred for 15 h at 25 ◦C. The sample was received by filtering and washing with anhydrous ethanol. The flowerlike nanosheet carbon material (NSC) was prepared according to our previous work [41]. Briefly, glucose (1.8 g), zinc acetate (8.4 g) and SiO2 (Figure S13) (Zn/Si mass ratio = 4:1) were added into 25 mL of deionized water. Then, the mixture was put Teflon-lined stainless steel autoclave and heated at 180 ◦C for 24 h. After cooled to room temperature, the solid product was recovered by filtration, washed with deionized water and dried in an oven at ◦ ◦ 120 C for 4 h. Subsequently, the above solids were calcinated under N2 flow at 750 C for 2 h. The obtained black solids were added to 20 wt% HF solution under stirred for about 6 h and washed with water. Lastly, the porous carbon materials were collected and dried at 100 ◦C.

3.3. Treatment of the Carbon Supports The NSC sample was initially dispersed in nitric acid solution. Then, the mixture was heated at 50 ◦C for 2 h with magnetic agitation. After that, the NSC mixture was Catalysts 2021, 11, 441 13 of 17

ﬁltered and washed with abundant deionized water and dried in an oven at 100 ◦C. The received sample was denoted as NSCox-1, NSCox-2, and NSCox-3, respectively, when 10, 20, or 30 wt% nitric acid was used. NSC was directly calcinated under N2 ﬂow of 100 mL/min at 600 ◦C for 2 h with the heating rate of 10 ◦C/min in a quartz tube furnace. The collected sample was denoted as NSC-600.

3.4. Catalyst Preparation

The 5 wt% Pd(OH)2/C catalysts were prepared by deposition-precipitation method. Briefly, 4 g of as-prepared NSC support was dispersed in deionized water (40 mL) under magnetic stirring at room temperature. Then the PdCl2 (0.34 g) solution in diluted HCl was added. After stirred for another 1 h, the NaOH solution (5 wt%) was dropwise added into the mixture until the pH value of the suspension up to 10. The suspension was stirred for an additional 6 h. Then the mixture was filtered and washed with plenty of deionized H2O. The filter cake was dried in oven to give Pd/NSC catalyst. Pd/NSC-600, Pd/NSCox-1, Pd/NSCox-2, and Pd/NSCox-3 catalysts were prepared in the same manner using NSC-600, NSCox-1 NSCox-2, and NSCox-3 as the support instead of NSC, respectively. In addition, the above Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 catalysts were pre-reduced under ◦ 10% H2/Ar flow at 200 C for 2 h, which were denoted as Pd/NSC-R, Pd/NSC-600-R, and Pd/NSCox-2-R, respectively.

3.5. Catalytic Activity Tests The activities of the catalysts were investigated in the hydrogenolytic debenzylation of TADB. Firstly, 30 g of TADB, 1.26 g of catalyst, 120 mL of CH3COOH and 30 mL of H2O were put into a vessel, which was immediately purged three times with hydrogen. The ◦ reaction was gone under an atmospheric pressure of H2 at 45 C for 10 h with vigorously stirred. After that, the ﬁltrate was concentrated under vacuum, and the resulted white solid was washed with ethanol. Finally, the ﬁlter cake was dried in vacuum oven at 65 ◦C for 6 h.

3.6. Carbon Supports and Catalysts Characterization Physical adsorption was measured at 77 K on a Micrometrics, ASAP 2460 instrument (Micrometrics, Norcross, GA, USA). Prior to the measurements, the sample (about 100 mg) was degassed under vacuum at 150 ◦C for 6 h. The X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (Bruker/AXS D8 Advance, Ettlingen, Germany) operated at 40 kV and 30 mA using Cu Kα radiation (λ = 0.154 nm). The samples were scanned from 10 ◦ to 80 ◦ (2θ) with a step size of 0.02 ◦ and a scanning rate of 3.5 ◦/min. Raman spectra was collected at room temperature on a Renishaw 2000 spectro- graph with spectral resolution of 2 cm−1. A 532 nm laser was used as the excitation source and the power output was about 40 mW. The water contact angles of the samples were measured using theta optical tensiometer (Dataphysics-TP50, Filderstadt, Germany) and electro-optics with a closed-circuit television camera connected to a computer (Attension Theta software). The sample was placed on a sample stage and a droplet of distilled water (about 2 µL) was deposited on the surface of the sample. Each sample was measured thrice, and the average value was recorded. The surface chemical composition of the samples was analyzed by XPS (Thermo Fisher, ESCALAB 250 Xi, Waltham, MA, USA), using monochromatic Al Kradiation (1486.6 eV) operating at an accelerating power of 15 kW. Transmission electron microscopy (TEM) images were recorded over a JEM 2100 (JEOL, Tokyo, Japan) microscope, operated at an acceleration voltage of 200 KV and electric current of 20 mA. Scanning electron microscopy (SEM) images were measured using a JEOL JSM-35C instrument (JEOL, Tokyo, Japan). Before the test, the samples were sprayed with gold. Temperature-programmed desorption (TPD) was performed at a U-shaped tubular micro-reactor with 50 mg sample that was heated (10 ◦C/min) from room temperature to 950 ◦C under a helium stream (40 cm3/min) using Quantachrome ChemBet 3000 (Boynton Beach, FL, USA). The type and amounts of oxygen-containing groups were Catalysts 2021, 11, 441 14 of 17

determined by decomposition temperatures and the amounts of the decomposition products (CO and CO2) measured by on-line mass spectrometry. The temperature programmed reduction with H2 experiment (H2-TPR) was performed on a chemisorption analyzer (Micromeritics, AutoChem II 2920, Norcross, GA, USA). Firstly, 0.05 g of the catalyst was ◦ pretreated under O2 at 200 C for 30 min. After cooled to room temperature, the sample was purged by He until the baseline remained unchanged. The TPR profile was obtained ◦ by heating the sample from 30 to 750 C in a flow of 10% H2/He (30 mL/min) with a ◦ heating rate of 10 C/min. The H2 consumption was monitored by a thermal conductivity detector. Thermogravimetric analysis (TG) was carried out on a thermal analyzer TA Q600 (New Castle, DE, USA) with a heating rate of 5 ◦C/min in a nitrogen flow (100 mL/min). The products were analyzed using an Agilent Technologies 6100 (Waldbronn, Germany) high performance liquid chromatography (HPLC). The sample was prepared by dissolving the product (4 mg) in acetonitrile (5 mL), followed by filtration to remove particulates. The injection volume was 20 uL, and the mobile phase was 75% methanol in water at a 0.8 mL/min flow rate; a UV/vis detector operating at 230 nm was employed.

4. Conclusions In summary, the effects of surface chemical properties of the flowerlike nanosheet carbon materials (NSC) on the Pd dispersion and the activities and stabilities of the corresponding Pd(OH)2/C catalysts had been investigated. It was found that the Pd particle size and dispersion was a main factor influencing the debenzylation activity. The proper concentration of carboxyl group and anhydride, as well as phenol groups on the carbon surface could contribute to improve the hydrophilicity of the carbon support and enhanced the interaction between Pd species and the support, which consequently increased the Pd dispersion, activity and stability of the corresponding catalyst for the hydrogenolytic debenzylation reaction. This work may offer an efficient strategy to improve the activity and stability of the Pd-based catalyst on carbon.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/catal11040441/s1: Figure S1: Pore size distributions (PSD) of the three carbon supports and the corresponding catalysts; Figure S2: XRD patterns of the three carbon supports; Figure S3: Raman spectra of the carbon supports; Figure S4: Contact angles of the carbon supports; Figure S5: TG results for the carbon supports, which were measured under a nitrogen atmosphere with a heating rate of 5 ◦C/min; Figure S6: Results of deconvolutions on TPD proﬁles using a multiple Gaussian function; Figure S7: TEM images of the Pd/NSCox-1 (a), and Pd/NSCox-3 (c) samples, and the corresponding histograms of particle size distribution; Figure S8: Correlations between the Pd dispersion (determined by CO chemisorption method) and: (a) the amount of carboxyl group, and (b) the amount of anhydride group determined from the data of CO2-TPD spectra; (c) the amount of anhydride group, (d) the amount of phenol group, and (e) the amount of carbonyl- quinone group determined from the data of CO-TPD spectra; Figure S9: TEM images and the corresponding histograms of particle size distribution of Pd/NSC (a,b), Pd/NSC-R (c,d), Pd/NSC- 600 (e,f), Pd/NSC-600-R (g,h), Pd/NSCox-2 (i,j), and Pd/NSCox-2-R (k,l) samples after reaction of the hydrogenolysis debenzylation of TADB; Figure S10: TEM images and the corresponding histograms of particle size distribution of the Pd/NSC (a,b), Pd/NSCox-2 (c,d), and Pd/NSC-600 (e,f) catalysts after cycling experiment; Figure S11: X-ray diffraction patterns of the various catalysts; Figure S12: XPS survey spectrum of the Pd/NSC-600, Pd/NSC, and Pd/NSCox-2 catalysts; Figure S13: Nitrogen adsorption/desorption isotherm (a) and pore size distribution (b) of the SiO2 sample at 77K. Table S1: Catalytic performance of the catalysts in the hydrogenolysis reaction of TADB; Table S2: Total elementary composition and TG weight loss; Table S3: Dispersion of palladium on different carbon supports; Table S4: Results of the deconvolution of CO2-TPD spectra using a multiple Gaussian function; Table S5: Results of the deconvolution of CO-TPD spectra using a multiple Gaussian function; Table S6: The particle size of Pd nanoparticles calculated by TEM; Table S7:H2 consumptions of the various catalysts determined from the H2-TPR curves; Table S8: XPS binding energies (eV), surface atomic concentrations and atomic ratios of the Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 catalysts; Table S9: Surface atomic concentrations and atomic ratios of the Pd/NSC-600, Pd/NSC, Pd/NSCox-1, Pd/NSCox-2, and Pd/NSCox-3 catalysts. Catalysts 2021, 11, 441 15 of 17

Author Contributions: Conceptualization, Y.C., S.P., and W.Q.; Methodology, Y.C. and W.Q.; Formal Analysis, Y.C., G.B., and W.Q.; Investigation, Y.C., X.D., J.N., and W.Q.; Resources, W.Q.; Data Curation, Y.C., X.D., and W.Q.; Writing—Original Draft Preparation, Y.C. and W.Q.; Writing—Review and Editing, W.Q.; Visualization, J.S. and W.Q.; Supervision, S.P. and W.Q.; Funding Acquisition, W.Q. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (22075005). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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