Mott-Schottky Effect Leads to Alkynes Semi-Hydrogenation over Pd

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Mott-Schottky Effect Leads to Alkynes SemiHydrogenation over Pd-Nanocubes@N-Doped Carbon Xingxing Li, Yu Pan, Hong Yi, Jingcheng Hu, Dali Yang, Fengzhi Lv, Wendian Li, Jinping Zhou, Xiaojun Wu, Aiwen Lei, and Lina Zhang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Mott-Schottky Effect Leads to Alkynes SemiHydrogenation over Pd-Nanocubes@N-Doped Carbon Xingxing Li,†,§ Yu Pan,†,‡,§ Hong Yi,† Jingcheng Hu,† Dali Yang,† Fengzhi Lv,† Wendian Li,† Jinping Zhou,† Xiaojun Wu,*,‡ Aiwen Lei*,† and Lina Zhang*,† †College

of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China.

Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information & Quantum Technology, School of Chemistry and Materials Sciences, and CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, P. R. China. ‡

KEYWORDS: Visible-light photoredox catalysis; Palladium nanocubes; Nitrogen-doped carbon; Alkynes semihydrogenation; Mott-Schottky effect ABSTRACT: Improving the selectivity and keeping efficiency of catalysts is essential for industrial processes and remains a great challenge. Herein, we developed a facile route to synthesize Pd nanocubes (NCs) using Eosin Y as the photosensitizer under visible light. Subsequently, Pd NCs were uniformly loaded on N-doped carbon nanofibrous microspheres (NCM) from carbonated chitin microspheres. This Pd NCs@NCM exhibited high reactivity and selectivity in alkynes semi-hydrogenation. For example, the hydrogenation of phenylacetylene to styrene and 3-phenyl-2-propyn-1-ol to (Z)-cinnamyl alcohol were 12.9 and 18.3 times faster with Pd NCs@NCM than Lindlar catalyst. According to the Mott-Schottky effect, loading of Pd NCs on N-doped carbon constructed a rectifying contact and decreased the electron density of Pd NCs. Density functional theory (DFT) calculations suggested that high concentration of hole doped in Pd NCs weakened the interaction of alkenes on Pd (100) facet and prevented further hydrogenation for a long time, this period of durable time is very helpful to chemical manufacturing. Thus, Pd NCs@NCM maintained both high reactivity and selectivity comparing with surface modified catalysts. This work provides an alternative strategy to design the Mott-Schottky catalysts for selective hydrogenation reactions.

■ INTRODUCTION Heterogeneous catalysts have been developed in the chemical manufacturing,[1-3] in which Palladium is one of the widely used catalysts especially for hydrogenation due to its strong interactions with molecular hydrogen.[4] However, they are generally too active to tune their selectivity, such as the semi-hydrogenation of triple bonds to the desired products.[2] Poisoning, tuning the steric effects and electronic effects, and specific molecular recognition have been used for solving the selectivity of nanocatalysts.[2,5] Since 1952, Lindlar catalyst was one of the well-known heterogeneous catalysts (Pd/CaCO3 modified by using Pb-salts and quinoline as passivators) for alkynes semi-hydrogenation, which lies at the heart of industrial manufacture for pharmaceuticals, vitamins, agrochemicals, and fragrances.[5-9] While the selectivity improvement is often accompanied by the significant sacrifice of its activity, meanwhile, it also suffers the use of toxic lead salts and limited substrate scope.[7] Thus, improving of catalytic selectivity and keeping reasonable efficiency is essential for the successful industrial processes and remains a challenge. Some effort have been made by the inert surface and interfacial modification of metal catalysts: (1) surface decoration of metal catalysts by thiols, sulfur, or aminocontaining species;[6,7,10-18] and (2) constructing alloy or coreshell bimetallic catalysts, such as Pd-Pb, Pd-Cu, Pd@Ag, Au@CeO2, and Pd@DMSO-like matrix.[19-23] However, these

suffer from the use of additives or multistep fabrication processes, and the stability particularly under demanding conditions and the loss in overall catalytic activity should also be considered.[2] Recently, strong metal-support interaction (SMSI) has indicated a significant effect on the electronic properties of metal species, which further influenced their catalytic behavior.[2] From chemical manufacturing perspective, a facile supported metal catalyst without inert surface modification is much preferable for selective hydrogenation reactions. On one hand, constructing Pd1/Ni1 single-atom catalysts provided a useful way to improve selective hydrogenation of acetylene.[24,25] On the other hand, substantial progress has recently been made by using N-doped carbon as the substrate to load Pd/Au/Co/PdZn nanoparticles (NPs) for selective alkynes hydrogenation.[14,26-29] However, it remains unknown about how N-doped carbon tailors the charge state of metals and thus modulates their selectivity in alkynes semihydrogenation. It has become difficult to design an efficient supported catalyst with high selectivity fairly without guidance of the understanding fundamental processes. Therefore, to maintain progress in the field, a clear mechanism for designing catalysts for selective hydrogenation reactions is essential.

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As an excellent catalyst for alkynes semi-hydrogenation, it should not only possess a relative lower desorption barrier but also with higher barrier for further hydrogenation of alkenes.[30] Thus, facile construction of the Mott-Schottky catalysts, in which the electron density of active metals decreases to weaken the binding of alkenes by ideal semiconductive supports, would be alternative strategies for selective alkynes hydrogenation. In our laboratory, most intransigent chitin, an enticing biopolymer derived from the exoskeletons of crabs and shrimps,[31] has been dissolved in NaOH/urea aqueous solution with cooling to fabricate chitin microspheres consisted of nanofibers and N-doped carbon microspheres.[32-34] Owing to the stronger electronegative of N atom, the localized charge on the π-system of N-C bonds is much nearer to the N centers.[35] Meanwhile, the electron level structure of carbon materials could be adjustable by nitrogen dopant.[36] A worthwhile endeavor would be to use the carbonized chitin microsphere, namely N-doped carbon, as Pd support to modulate its catalytic performance in alkynes semihydrogenation. Here, we reported a facile route to synthesize Pd nanocubes (NCs) via visible-light photoredox catalysis. Coupling Pd NCs with N-doped carbon microsphere (NCM), Pd NCs@NCM exhibited high reactivity and selectivity in alkynes semihydrogenation, and offered the following advantages: (i) a facile fabrication process without surface modification; (ii) Construction of a rectifying contact by the Mott-Schottky effect to modulate its catalytic performance; (iii) Maintaining high dispersion and good stability of Pd NCs on the support. Next, the Pd NCs hole concentration dependence of hydrogen adsorption energy as well as the influence of both hole concentration and H coverage of Pd NCs on the adsorption energy of phenylacetylene and styrene were studied to understand the fundamental process by DFT. Eventually, Pd NCs@Cu2O nanocomposites, in which Cu2O is a typical ptype semiconductor, were designed to clarify the possibility of the Mott-Schottky catalysts for selective hydrogenation reactions. ■ RESULTS AND DISSCUSION It is noted that Pd catalysts generally require to achieve at an elevated temperature[37-40] or under strong ultra violet irradiation.[41-44] However, intrinsic temperature gradient with heating or the possible temperature elevation with the use of UV light in reaction medium were inevitably occurred.[41] As the activity of nanocatalysts is greatly enhanced by use of nanocrystals enclosed with specific facets,[39,40] therefore, an exquisite shape control of Pd nanocrystals under mild condition is highly desired.[41] Pd NCs enclosed with {100} facets were found to have an enhanced activity than that of nano-octahedrons with {111} facets.[37] Therefore, this method was applied for the synthesis of Pd NCs. Owing to the importance of both ascorbic acid (AA) and Br- for Pd NCs synthesis,[ 37-40] the combination of 10 mmol% Eosin Y and AA in the presence of Br- gave the yield of up to 99.5% (Table S1, entry 2). Furthermore, a series of control experiments were performed, indicating that the reaction yield was trace in the absence of a photocatalyst (Table S1, entry 1). And the yield without light irradiation was 16.5% (Table S1, entry 3). Consequently, this photoreduction process is promoted by photocatalysis. The constituent of each product

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was detected by X-ray powder diffraction (Figure S1), showing four peaks of Palladium (JCPDS No. 46-1043),[38] indicating that it consists of pure zero-valent palladium. Emission quenching experiments were conducted to gain insights into the mechanism of this process. AA displayed luminescence quenching of activated Eosin Y, indicating that the electron transfer of Eosin Y* could be proceeded by AA (Figure S2). Detailly, as shown in Scheme 1a, Eosin Y (EY) was excited to reactive excited states (EY*, E1/2red[EY*/EY·-] = +0.83 V vs. SCE)[45] under 3-W blue LEDs irradiation, which reductively quenched by mild reductants, to generate a strongly reduction state (EY·-, E1/2red[EY/EY·-] = -1.06 V vs. SCE),[46] finally reducing PdCl42- to Pd atoms (PdCl42-/Pd, 0.07 V vs. SCE).

Scheme 1 Schematic illustration of Pd NCs synthesis via facile visible-light photoredox catalysis (a) and fabrication of Pd NCs@NCM (b). Once Pd atoms formed and reached a critical concentration, the atoms could aggregate to form small Pd clusters with about 2-nm size within 10 min (Figure S3a).[41,47] After that, the nucleates began to undergo facet-directed growth in the presence of Br-, generating small irregular NCs with about 12nm size. Meanwhile, small NPs with the size of 2-3 nm also appeared around the irregular Pd NCs (Figure S3b), finally evolved into uniformly Pd NCs via Ostwald ripening processes.[39,40] On the basis of the inherently low surface diffusion rate of Pd adatoms at room temperature,[41] truncated and rounded corners Pd NCs with size of 23.5 ± 4.7, 16.7 ± 3.4, and 8.8 ± 1.1 nm were obtained by adding different amounts of KBr (Figure S4). The fabrication process of Pd NCs@N-doped carbon nanofibrous microspheres is illustrated in Scheme 1b. Firstly, chitin nanofibrous microspheres (CM) were fabricated by using a “bottom-up” nanofiber formation method in NaOH/urea aqueous solution with cooling.[33,34] Subsequently, the chitin microspheres were carbonized at 750 °C for 2 h at Ar atmosphere to obtain N-doped carbon microspheres consisted of nanofibers (NCM). The NCM microspheres were weaved with the nanofibers with mean width of 30-40 nm, and exhibited 3D interconnected framework structure without pore wall. The average diameter of microspheres was 47.6 ± 12.6 um (Figure S5). There were physically cross-linked nanofibers as good adhesion sites and hierarchical pores in the N-carbon microspheres, as shown in Scheme 1b, which were beneficial to the dispersion and adhesion of metal

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nanocatalysts. XRD pattern of NCM (Figure S6a) displays two broad peaks at 24.5 and 43.8°, indicating a disorder phase in the samples.[34] The NCM was used as the substrates to load 8.8-nm Pd NCs via physical mixing. The zeta potential of NCM changed from +36.2 to +0.3 mV when the pH increased from 2 to 10 (Figure S7a), confirming that its positive charged surface could interact fully with the negative charged Pd NCs (Figure S7b), anchoring the Pd NCs on the nanofibers of NCM. Clearly, Figure 1a-c shows Pd NCs as nanoclusters were uniformly dispersed on the outer layer nanofibers of NCM, consistent with the elemental maps of Pd elements in Figure 1d.

Figure 1 SEM and TEM images (a-c) and EDS mapping image (d) of C, N, and Pd elements of 8.8-nm Pd NCs@NCM, and its XPS spectra: Pd 3d (e) and N 1s (f). Furthermore, a weak peak at 40.1° appeared in Pd NCs@NCM (Figure S6a), corresponding to the (111) plane of Pd (JCPDS No. 46-1043),[38] and ICP results indicated the Pd content of 8.8-nm Pd NCs@NCM to be 4.44% (Table S2). The Pd 3d spectra of Pd NCs@NCM (Figure 1e) was fitted with two components, and the 3d5/2 peak located at a binding energy of 335.2 eV was the characteristic of metallic Pd0, whereas that at 337.0 eV was assigned to Pd2+ in the PdO state.[48] Moreover, 22.1% of Pd2+ existed in Pd NCs@NCM (Table S2), indicating that Pd NCs was partially oxidized when exposed to the air. The N 1s spectrum in Figure 1f can be divided into four peaks, pyridinic N (397.9 eV), pyrrolic N (398.6 eV), graphitic N (400.5 eV), and oxidized N (403.6 eV),[34] its corresponding content was is 21.6, 12.8, 59.0, and 6.6% (Table S3), respectively. Compared to NCM, the N 1s peaks of pyridinic N and pyrrolic N of Pd NCs@NCM shifted to a lower binding energy value, proving that more electrons of metallic Pd NCs were transferred to N-doped carbon support.[49-51] The isotherms and hysteresis loops of both NCM and Pd NCs@NCM (Figure S8) had a similar type and belong to a typical type I isotherm and type H3 loop, indicating the presence of the hierarchical porous architecture. The BET specific surface areas of NCM and Pd NCs@NCM were respectively 547.8 and 532.6 m2 g-1. All of the microspheres

displayed a meso-/macro-pore hierarchical structure and an extended continuous pore size distribution (1.4-115.3 nm). Encouraged by the Schottky effect at the Pd/N-C interface, Pd NCs@NCM was used for catalytic alkynes semihydrogenation. The ethanol (EtOH) was the most effective solvent in terms of both reactivity and selectivity, and the optimal loading of catalyst was 0.05 mol% (Table S4). When phenylacetylene was chosen as the model substance, as shown in Figure 2a and 2b, Pd NCs@NCM exhibited a selectivity of 95.3 % toward phenylacetylene at a conversion of 98.9% within 5 h. Meanwhile, the selectivity of phenylacetylene semi-hydrogenation to styrene was also observed at 86.9% even after full conversion over Pd NCs@NCM within 24 h, while phenylacetylene was almost further hydrogenated into phenylethane over Pd NCs in 24 h. The control group by using Pd NCs + NCM also displayed the same catalytic performance as Pd NCs@NCM, giving an excellent selectivity of 95.1 % toward phenylacetylene at a conversion of 88.9% within 5 h. The selectivity was maintained at 92.8% after 4 times’ reuse with very little decay in the activity (Figure 3a-b). The cubic morphologies and Pd 3d spectra of Pd NCs were found to be structurally stable, whereas some aggregation and morphological deformation occurred over Pd NCs without any substrate (Figure S9). Therefore, NCM as support played an important role on the improvement of stability, selectivity as well as homogeneous dispersion of Pd NCs.

Figure 2 Catalytic performance (a) and selectivity (b) of phenylacetylene semi-hydrogenation to styrene over Lindlar, 8.8-nm Pd NCs, 8.8-nm Pd NCs+NCM, and 8.8-nm Pd NCs@NCM. Schematic illustration of improvement of alkynes selectivity on Pd NCs@NCM (c). Reaction conditions: 0.75-mmol phenylacetylene and 0.05 mol% Pd in 4 mL EtOH, atmospheric H2 balloon, 20 °C. The Pd loading in mol% is the ratio of the mole of total Pd atoms to the mole of phenylacetylene. Pd NCs@NCM also exhibited an excellent selectivity of 93.1 % to (Z)-cinnamyl alcohol at a conversion of 97.7% within 2.5 h (Figure S10). In comparison, even when the conversion of 3-phenyl-2-propyn-1-ol reached 100% in 2 h,

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the selectivity to (Z)-cinnamyl alcohol was only 16.6% over Pd NCs. Meanwhile, the selectivity of 83.9% to (Z)-cinnamyl alcohol was observed after full conversion over Pd NCs@NCM within 4 h, while that of Pd NCs reduces to 10.4%. Thus, we speculated that such Pd NCs loaded N-doped carbon constructed a rectifying contact by the Mott-Schottky effect, and the electrons of Pd NCs were directly transferred to N-doped carbon (denoted by the red arrows in Figure 2c), leading to the lower electronic density of Pd NCs and the weaker binding of alkenes to prevent further hydrogenation. To elaborate the Mott-Schottky effect led selective alkynes hydrogenation, the control group by using Pd NCs + active carbon (AC) for both phenylacetylene and 3-phenyl-2-propyn1-ol semi-hydrogenation was also conducted, as shown in Figure 3c-d and S11. A selectivity of 65.1 % to styrene and 45.8 % to (Z)-cinnamyl alcohol occurred at a full conversion. Besides, the selectivity to styrene in 24 h and to (Z)-cinnamyl alcohol in 4 h reduce to 35.5% and 27.3%, respectively. As the comparable work function of metals and carbon materials, electrons can flow rather freely between the metals and carbon materials without rectification effect when the direct metalcarbon contact formed. Therefore, carbon materials were regarded as neutral supports, giving no crucial impart on the catalytic performance and selectivity of metals.[50]

Figure 3 Catalytic performance (a) and selectivity (b) of phenylacetylene semi-hydrogenation to styrene over 8.8-nm Pd NCs and 8.8-nm Pd NCs@NCM in 4 times’ reuse. Catalytic performance (c) and selectivity (d) of phenylacetylene semi-hydrogenation to styrene over 8.8-nm Pd NCs+AC and 8.8-nm Pd NCs@NCM. Ever though Lindlar catalyst showed well selectivity to both styrene and to (Z)-cinnamyl alcohol, its catalytic efficiency was very low with a phenylacetylene conversion of only 6.9% in 5 h and a 3-phenyl-2-propyn-1-ol conversion of only 20.1% in 2 h (Figure 2a and S12a). The specific rate of semihydrogenation of phenylacetylene to styrene and 3-phenyl-2propyn-1-ol to (Z)-cinnamyl alcohol over Pd NCs@NCM achieved up to 394.0 and 998.5 h-1 (Figure 4a and S13), respectively, which were 12.9 times and 18.3 times higher than that of the Lindlar (30.6 and 54.2 h-1). Moreover, the selectivity of diphenyl acetylene semi-hydrogenation to cisstilbene over Pd NCs@NCM was 95.4 % at a conversion of

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99.1% within 1.0 h, and no obvious decay in its selectivity was observed even after 9 h (Figure S14). Overall, Pd NCs@NCM exhibited much better comprehensive catalytic performance with high selectivity in alkynes semi-hydrogenation than Pd NCs, Pd NCs+AC, and Lindlar (Figure 4a and S12). Comprehensively considering H2 pressure, reaction temperature, loading amount and specific rate, Pd NCs@NCM maintained both high reactivity and selectivity comparing with surface modified catalysts. (Table S5). As the Mott-Schottky effect is good for improving selectivity in selective hydrogenation reactions, but accompanied by the sacrifice of reactivity. While the size effect on the selectivity and activity of Pd NPs with size decreasing is diametrically opposite.[52] That is, the role of Schottky effect and size effect on the catalyst performance interplays and competes each other. To make it clear, as the smallest size of Pd NCs is larger than 7.0 nm[37,38], Pd@AC (AC = active carbon) and Pd@NCM with smaller Pd NPs were in-situ synthesis by NaBH4-reduction of PdCl42-absorbed AC or NCM, the theoretical Pd content of two catalysts is about 4%. 16.7-nm and 23.5-nm Pd NCs@NCM were synthesized by the same method as 8.8-nm Pd NCs@NCM. Their TEM/SEM images and XPS spectra of Pd 3d were shown in Figure S15-S16 and Table S6, the average size of Pd NPs in Pd@AC and Pd@NCM is (3.5 ± 0.9) and (5.7 ± 2.4) nm, respectively. As shown in Table S7, Pd@AC exhibited an almost full conversion of phenylacetylene to ethylbenzene in 5.0 h. In comparison, Pd@NCM gave an excellent selectivity of 80.4% toward phenylacetylene at a full conversion in 5 h. Therefore, N-doped carbon carbonized at 750 ℃ could decrease the activity but increase the selectivity of Pd NPs, being in agreement with the results over 8.8-nm Pd NCs+AC and 8.8-nm Pd NCs@NCM (Figure 3c and 3d). 16.7 nm Pd NCs@NCM exhibited a selectivity of 97.3% toward phenylacetylene at a conversion of 66.3% within 5 h. The conversion of phenylacetylene over 23.5-nm Pd NCs@NCM was only up to 33.0% in 5 h with an excellent selectivity of 99.0%. As the work function of Pd nanoclusters decreases with particle size decreasing.[52] When smaller Pd NPs contacted with N-doped carbon, the difference of their work function enlarged, the number of electrons of smaller Pd NPs transferred to N-doped carbon was much more than the bigger, resulting in higher selectivity but lower reactivity. As shown in Table S7, with Pd size increasing, the selectivity of alkynes semi-hydrogenation increased but accompanied by the sacrifice of the activity. However, compared with 23.5 nm Pd NCs@NCM, the selectivity of phenylacetylene to styrene over 23.5 nm Pd NCs without any substrate was 47.9% with a full conversion in 5 h, the over-hydrogenation of alkynes was occurred without modulation of the Mott-Schottky effect. Hence, the Mott-Schottky effect at the Pd/N-C interface is the dominated role for improving the selectivity of Pd NPs. H2-TPD is one of very few available tools in surface science to investigate the metal cluster-hydrogen interaction, giving insights into how the substrates influence on the supported metal NPs.[53,54] From the H2-TPD spectra (Fig. 4b), a big peak centered at 556.3 K, which was close to the one at 580.4 K of the Lindlar catalyst, and a small peak appeared at 370.6 K in Pd NCs@NCM. However, Pd NCs@AC by using activated carbon as substrates showed two peaks centered at 556.3 and

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632.1 K. Compared with Pd NCs@NCM and Lindlar, the poor selectivity of Pd NCs@AC with an excellent conversion (Table S8) may be attributed to active β-H in palladium interstitial lattice sites,[55] whose desorption temperature was at 630.1 K. The gradual increasing signal above 823 K could be resulted from the decomposition of the OH groups of carbon materials.[53] The desorption amount of H2, which is the adsorption site of H2 per gram of the total Pd, decreased from 737.5 (Pd NCs@AC) to 298.6 cm2 gPd-1 (Pd NCs@NCM), the Lindlar was only 12.8 cm2 gPd-1 (Table S9). As heterocatalytic hydrogenations generally follow the Horiuti-Polanyi mechanism with a stepwise scheme involving the combination of co-adsorbed unsaturated hydrocarbon molecules and hydrogen atoms. Thus, an excellent catalyst for alkynes semihydrogenation should have higher barrier for the hydrogenation of alkenes than their desorption barrier.[28,29] As for Pd NCs@NCM, both its desorption temperature and the desorption amount of H2 decreased in contrast to Pd NCs@AC, namely, Pd NCs reduced the binding capacity of both hydrogen atoms and reactants because of their lower electron density. Nitrogen-dopant in NCM induced the electron of Pd NCs transferred to N-doped carbon, leading to decrease the activity but the significant increase of selectivity. In a sense, N-doped carbon is to Pd NCs@NCM what lead acetate/quinoline is to Lindlar.

Figure 4 Specific rate and selectivity of phenylacetylene semihydrogenation to styrene over Lindlar, 8.8-nm Pd NCs, and 8.8-nm Pd NCs@NCM (a). H2-TPD profiles of Lindlar, 8.8nm Pd NCs@AC, and 8.8-nm Pd NCs@NCM (b). Pd K-edge XANES spectra of 8.8-nm Pd NCs@AC and 8.8-nm Pd NCs@NCM (c). Schematic illustration of the Mott-Schottkytype contact of Pd NCs@NCM before and after contacting (d). To better understand the local environment of Pd in Pd NCs@NCM, X-ray absorption fine structure spectroscopy (XAFS) was performed, as shown in Figure 4c. The absorption edge for Pd NCs@NCM was upshifted with respect to that for Pd NCs@AC, suggesting a reduction of the electron density of Pd atoms in Pd NCs@NCM.[56] The extended XAFS spectrum in Figure S17 demonstrated that the coordination environment of Pd experienced no obvious change with either N-doped carbon or active carbon as the substrate. These results further confirmed the directional injection of Pd NCs electrons into N-doped carbon. It is note that the Mott-Schottky contact formed when a metal contacted with a semiconductor, which influences the electron transfer

between the metal and semiconductor.[50] The band gap of the carbon support is gradually enlarged by increasing the concentrations of nitrogen dopant, resulting in a valence band edge close to the Fermi level and thus a p-type semiconductive structure.[36,51] UPS results in Figure S18a and S19 showed that the work function (Φ) of 8.8-nm Pd NCs, NCM, and 8.8nm Pd NCs@NCM were determined to be 5.0, 5.6, and 5.5 eV, respectively. Therefore, when Pd NCs with smaller work function directly contacted with a p-type N-doped carbon, the electrons from Pd NCs would transfer to the N-doped carbon until their Fermi level reaches equilibrium (Figure 4d). This phenomenon is described as the Mott-Schottky effect in solidstate physics.[50] Such a Schottky barrier will obviously result in an electron redistribution at the interface of Pd and N-doped carbon and enrich the positive charges on the side of metallic Pd (Figure 4d). Namely, Pd NCs@NCM decreased the electronic density of Pd NCs by the Mott-Schottky effect, which led to selective alkynes hydrogenation.

Figure 5 The fermi level and the quantity of transfer charge of Pd (100) facet with two types of N-carbon (a), the adsorption energy of both phenylacetylene (PhA) and styrene (St) (b) as well as hydrogen adsorption energy (c) by increasing the hole concentration per Pd atom, the adsorption energy of both phenylacetylene and styrene (d) by increasing H coverage on Pd (100) facet. DFT calculations were used to elucidate the mechanism of the Mott-Schottky effect led selective alkynes semihydrogenation. Two p-type graphene doped with pyridinic N and pyrrolic N (Figure 5a, inset) were adopted as N-doped carbon model, respectively. The Fermi levels of pyridinic N (5.26 eV) and pyrrolic N (-5.36 eV) are deeper than that of Pd (100) facet (-5.22 eV), indicating that electrons transfer from Pd (100) facet to N-doped graphene. Both the calculated deformation charge density (Figure S20 in Supporting Information) and Bader charge analysis reveal that electrons are mainly transferred from the surface Pd atom to adjacent pyridinic N in N-doped graphene, and some electrons localize in the pyrrolic N in N-doped graphene. Therefore, Pd atoms are positively charged, agreeing well with the experiment results in Figure 4d. Next, the hole doping effect of Pd NCs on alkynes hydrogenation is considered. The hole concentration of Pd (100) facet correlated to the adsorption energy of both

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phenylacetylene (PhA) and styrene (St) was shown in Figure 5b and Table S10. With an increase of hole concentration in Pd NCs, the adsorption energies of both PhA and St increased slightly at first, then descended steeply. Meanwhile, the adsorption energy of PhA is larger than that of St, suggesting that only PhA can be adsorbed on Pd (100) facet at a high concentration of hole. Thus, relative high concentration of doped hole in Pd NCs would attenuate adsorption of St, and undermine the process of over-hydrogenation. This could be achieved by increasing pyridinic and pyrrolic N concentration of N-doped carbon.[36,57] Figure S21a-b showed the most stable adsorption configuration of PhA and St on Pd (100) facet, agreeing with previous theoretical study.[58] Note that the strong adsorption energy and the high coverage of hydrogen atoms on Pd surface can be contributed to the overhydrogenation of alkynes,[59,60] the hole doping effect of Pd (100) facet on hydrogen atom adsorption energy was also considered. As shown in Figure 5c and Table S11, the hydrogen adsorption energy prominently diminished with the increasing hole concentration of Pd NCs, which led to the decrease of the hydrogen coverage on Pd (100) facet and hinder the further hydrogenation of St. Furthermore, different degrees of hydrogen coverage on Pd surface would cause tremendous change in the adsorption energy of molecules with phenyl.[60] In Figure 5d and Table S12, adsorption energies of PhA and St were a function of the degree of hydrogen coverage on Pd (100) facet. The increasing hydrogen coverage overturned the adsorption energies of these two molecules when hydrogen coverage was larger than 0.50, namely St adsorbed on Pd (100) facet with nearly saturated hydrogen was hardly substituted by PhA and thus further hydrogenated to alkanes. As shown in Figure S21c-d, when two molecules adsorbed on hydrogen saturated Pd (100) facet, the C=C of St could lay parallel with Pd (100) facet, while the C≡C of PhA tilted at an angle away from Pd surface due to the steric hindrance with hydrogen atoms on the hollow site of Pd (100) facet. However, when hydrogen coverage was less than 0.50, the adsorption energies of PhA maintained larger than St, which indicated the incoming PhA could replace the St adsorbed on Pd (100) facet, and then hampered further hydrogenation consequently. Therefore, low hydrogen coverage on Pd surface also resulted in selective alkynes hydrogenation.The possible pathway of hydrogenation of phenylacetylene to ethyl benzene is generally considered to occur by a two-pathway mechanism (Figure S22a). One pathway involves the complete hydrogenation of the triple bond to a saturated single bond, the other one is the successive hydrogenation reactions involving styrene as the intermediate.[18] In this work, Pd NCs loaded on N-doped carbon exhibited selective hydrogenation of alkynes, indicating its hydrogenation pathway mainly occurred by the successive process. DFT results show that the hydrogen adsorption energy prominently diminished with the increasing hole concentration of Pd NCs led by the Mott-Schottky effect (Figure 5c), which led to hydrogen coverage of Pd (100) facet decreasing. When the hydrogen coverage was less than 0.50, the adsorption energies of PhA maintained larger than St, which consequently hampered further hydrogenation of alkenes. The competition experiment was conducted to verify the relative adsorption capacity of the two molecules on Pd (100) facet. Figure S23 show that the conversion curve of the

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reaction containing both 0.75-mmol phenylacetylene and 0.75mmol styrene is almost same as that containing only 0.75mmol phenylacetylene. Compared with the selectivity of 95.3% in only phenylacetylene within 5 h, the selectivity slightly decreased to 84.5%. Although the reaction rate of hydrogenation of styrene to phenylethane is 2.02 times higher than that of phenylacetylene to styrene (Figure S22b-d), Pd NCs@N-doped carbon still remained high selectivity in the reaction containing the equivalent styrene owing to the preferential adsorption of phenylacetylene molecules on the surface of Pd NPs. To clarify the universality of the Mott-Schottky catalysts for selective hydrogenation reactions, 16.7-nm Pd NCs@Cu2O NCs, in which Cu2O NCs were typical p-type semiconductors,[61] were designed for phenylacetylene semihydrogenation. Driven by electrostatic force, the negative charged Pd NCs were uniformly deposited on Cu2O NCs which changed +31.9 mV in aqueous solution (Figure S7b). Clearly, as shown in Figure S24, 16.4-nm Pd NCs were uniformly dispersed on the surface of Cu2O NCs. The Pd 3d spectra of Pd NCs@Cu2O (Figure S25) was fitted with two components, and the 3d5/2 peak located at a binding energy of 335.2 eV was the characteristic of metallic Pd0, whereas that at 336.6 eV was assigned to Pd2+ in the PdO state.[20] Moreover, 27.4% of Pd2+ existed in Pd NCs@Cu2O, indicating that Pd NCs was partially oxidized.

Figure 6 Catalytic performance (a) and selectivity (b) of phenylacetylene semi-hydrogenation to styrene over 16.7-nm Pd NCs@Cu2O. Schematic illustration of Mott-Schottky-type contact of Pd NCs@Cu2O: (i) Before contact, (ii) after contact, and (iii) irradiation under 3-W blue LED. Reaction conditions: 0.75 mmol phenylacetylene and 0.05 mmol% 16.7-nm Pd NCs@Cu2O in 4 mL EtOH, atmospheric H2 balloon, 20 °C. As shown in Figure 6a-b, 16.7-nm Pd NCs@Cu2O gave a conversion of 47.9% at 30 h in the dark, its selectivity to styrene always maintained at 100%. However, when irradiated by 3-W blue LED, Pd NCs@Cu2O exhibited an excellent selectivity of 91.0% to styrene at a conversion of 96.1% at 30 h. The work function (Φ) of 16.7-nm Pd NCs were 4.9 eV (Figure S18b), and that of Cu2O NCs ranged from 5.76 to

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6.24 eV (Figure S26). The band gap (Eg) of Cu2O NCs was estimated to 2.27 eV, their conduction band potential (ECB) and valence band potential (EVB) were calculated to be 4.20 and 6.47 eV vs. vacuum (Figure S27).[62] Therefore, when Pd NCs with smaller work function contact with p-type Cu2O, the electrons from Pd NCs will transfer to Cu2O and form a Schottky barrier at the interface of Pd and Cu2O, which enrich the positive charges on Pd NCs (Figure 6c(ii)). Owing to the larger work function of Cu2O than N-doped carbon (5.6 eV), the charge state of Pd NCs in Pd NCs@Cu2O was more positive than that of Pd NCs@NCM. This was the main reason for the lower catalytic activity of Pd NCs@Cu2O than Pd NCs@NCM with the comparable size of Pd NCs (Figure S28). Therefore, Cu2O decreased the catalytic performance of Pd NCs but resulted in high selectivity, whose role was the same as p-type semiconductive N-doped carbon. When irradiated under 3-W blue LED, in Figure 6c(iii), Cu2O was excited to generate hole-electron pairs. For small plasmonic cross-sections of Pd NCs, their weak surface plasmon response could most likely be activated by highintensity illumination.[63] Therefore, the partial photoexcited electrons from Cu2O transferred back to Pd NCs driven by Schottky junction at the interface of Pd and Cu2O, while the injection of Pd hot electrons into the Cu2O conduction band was weak under 3-W blue LED irradiation.[63,64] Thus, the transfer of electrons from Cu2O to Pd NCs dominated, and a new equilibrium between their Fermi level reached again,[64] the electrons density of Pd NCs under the irradiation of 3-W blue LED was relatively higher than that in the dark, giving rise to an excellent conversion of 96.1% with selectivity of 91.0 % to styrene. Therefore, to design Mott-Schottky catalysts for selective hydrogenation reactions is a promising and universal strategy, in which the electronic density of active metals decreases by proper semiconductive substrates. Meanwhile, it could rationally tailor the charge state of metals and thus modulate their catalytic performance in H2 activation as well as selective hydrogenation reactions by simple variation of the intensity of illumination.[4] ■ CONCLUSIONS In summary, a facile route to synthesize Pd NCs via visiblelight photoredox catalysis were successfully realized by using Eosin Y as the photosensitizer. Coupling Pd NCs with Ndoped carbon microsphere, Pd NCs@NCM exhibited both high reactivity and selectivity in alkynes semi-hydrogenation comparing with surface modified catalysts. Such Pd NCs loaded N-doped carbon constructed a rectifying contact by the Mott-Schottky effect at the Pd/N-C interface and decreased the electron density of Pd NCs. Density functional theory calculations suggested that hydrogen adsorption energies were prominently diminishing with the increasing hole concentration of Pd NCs, which led to decrease the hydrogen coverage of Pd (100) facet, weakening the binding of alkenes to prevent further hydrogenation for a long time, this period of durable time is very helpful to chemical manufacturing. This work opened up a new route to utilize N-doped carbon derived from chitin as support for the improvement of the stability, selectivity as well as homogeneous dispersion of metal nanocatalysts. Meanwhile, construction of Mott-Schottky catalysts provides a new dimension to design efficient catalysts for selective hydrogenation reactions.

■EXPERIMENTAL AND THEORETICAL METHODS Chemicals. Eosin Y (AR) and Ru(pby)3Cl2 (98%) were purchased from Aladdin Reagent Ltd. 9-Mesityl-10methylacridinium perchlorate (Acr+-Mes) were purchased from TCI (Shanghai) Development Co. Ltd. Alkynes and its corresponding hydrogenation products were purchased from Energy Chemical. And other chemicals were analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were used without further purification. Chitin powder was purchased from Golden-Shell Biochemical Co. Ltd. (Zhejiang, China) and purified.[33] Synthesis of 8.8-nm, 16.7-nm, and 23.5-nm Pd NCs. 2.0 mL of 20 mM Na2PdCl4 was added into a glass tube containing 2.0 mL of deionized water with 800-rpm stirring at room temperature. 24 mg of PVP (Mw≈55000) and 60 mg of KBr were added into the above solution. After 10 min, 0.4 mL of 6.9 mg/mL Eosin Y (10% mM) aqueous solution was injected. Then the reaction solution was degassed in flowing N2 atmosphere for 15 min. Finally, 24 mg of ascorbic acid was quickly added into the degassed solution and covered the lid on the glass tube. The glass tube was placed in water bath equipped with a recirculating cooling water system, and maintaining at 22 ± 3°C throughout the synthesis. After stirring for 5 min, the 3-W blue LED was turn on for 3 h with 800-rpm stirring. The final solution was collected via centrifugation at 20 000 rpm for 30 min, washed six times with water to remove excess PVP, and then dispersed in water for further use. The size of final Pd product is about 23.5 nm with a cubic morphology. For 8.8-nm Pd NCs, the procedure was kept the same except that 120 mg of KBr was added. As for the synthesis of 16.7-nm Pd NCs, the procedure was kept the same as 8.8-nm Pd NCs except that the aqueous solution was replaced by H2O/CH3CN with the volume ratio of 1:0.1. Synthesis of chitin microspheres (CM) and N-doped carbon microsphere (NCM). Nanofibrous chitin microspheres (CM) were fabricated by using a “bottom-up” fabrication method in NaOH/urea aqueous solution according to our previous work.[32,33] NCM was synthesized by direct carbonization of the as-prepared CM under 750 °C for 2 h in an Ar atmosphere with ramp rate of 3 °C/min. After naturally cooled down to room temperature, the black powders were collected and washed with deionized water, finally dried in vacuum drying oven at 60 °C for 24 h. Synthesis of 8.8-nm Pd NCs@NCM. The Pd NCs loaded NCM was synthesized by a facile physical blend. Driving by the electrostatic force, Pd NCs were thermodynamically deposited on the nanofibrous of NCM. Typically, 2.5 mL of 1.9 mg/mL 8.8-nm Pd NCs aqueous solution was dispersed in 50 mL of deionized water under a mild stirring for 10 min. 100 mg of NCM powder was added to the above solution with a mild stirring for 1 h. Finally, the samples were washed with deionized water by pumping filtration and then dried in vacuum drying oven at 60 °C for 24 h. Characterization. The morphology of the sample and EDS mapping were examined by a field-emission scanning electron microscope (FESEM, Zeiss SUPRA 55 Sapphire, Germany) by using an accelerating voltage of 5 kV. Transmission electron microscopic (TEM) images were obtained with a JEOL high-resolution transmission electron microscope operating at 200 kV TEM, Japan. The X-ray diffraction (XRD)

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pattern was recorded by X-ray powde diffraction (XRD, Rigaku Miniflex600 with Cu Kα radiation, λ=1.5406Å, Japan). X-ray photoelectron spectroscopy (XPS) and ultroviolet photoelectron spectroscopy (UPS) experiments were performed at a photoelectron spectrometer with Al Kα X-rays radiation as the X-ray source for excitation (XPS, Thermo Fisher scientific ESCALAB 250Xi, USA). The specific surface area and pore size distribution of catalysts were determined by a physical absorption analyzer accelerated surface area and porosimetry system (BET, Micromeritics ASAP 2020, USA). UV-Vis spectra of solutions were measured by a UV-Vis spectrophotometer (Agilent Cary 8454, USA). The fluorescence emission spectrum was recorded by using a PerkinElmer LS55 Fluorescence Spectrometer. The excitation wavelength is 445 nm. The Pd content of catalysts were determined with inductively coupled plasma (ICP) analyses (Thermo Elemental, IRIS Intrepid II XSP, USA) after completely dissolving in the concentrated HNO3 by refluxing at 105 ℃ for 12 h. TG curve was obtained by a thermogravimetric analysis (TA Instruments, TGA 550, USA). Zeta potential was measured three times by a laser particlesize analyzer (Malvern Zetasizer Nano ZS 90, USA). H2Temperature programmed desorption (H2-TPD) was performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. The catalysts were degassed at 398.15 K for 30 min, then cooling down in pure Ar. Adsorbents of H2 (20 mL min-1, 30 min) were introduced into the system at 283.15 K. Then, the system was purged with Ar (50 mL min-1, 10 min). The temperature ramped from 283.15 to 978.15 K with a rate of 20 K min-1 in Ar (50 mL min-1). The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Pd were recorded on beamline 01C1 at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The electron storage ring was operated at 1.5 GeV with a current of 360 mA in top-up injection mode. The Pd K-edge absorption spectra were recorded in fluorescence yield (FY) mode at room temperature using a Lytle detector. Catalytic activity. All reactions were performed in a stirred, glass tube (25 ml) fitted with a balloon containing 100% H2. Typically, 4.44 % 8.8-nm Pd NCs@NCM (0.9 mg, 0.05 mmol％ [Pd]) and a certain amount of biphenyl as an internal standard were added into the glass tube. Then the air in glass tube was replaced by 100% H2 three times and then sealed by an atmospheric balloon (1 bar) full of 100% H2. 4-mL ethanol and 0.75-mmol phenylacetylene were injected into the above glass tube by a micro-syringe at 20 ℃ with stirring (1000 rpm). The reaction proceeded, and samples were withdrawn at regular intervals, filtered, and analyzed. Conversion and selectivity of phenylacetylene and diphenylacetylene were determined by GC (GC, 7890B, Agilent) and that of 3-phenyl2-propyn-1-ol were determined by HPLC (HPLC, Alliance 2695-2998, Waters) with a C18 column (250 mm*4.6mm/5um, Spherisorb S5 ODS1) using biphenyl as an internal standard. The Pd loading in mol% is the ratio of the mole of total Pd atoms to the mole of phenylacetylene. Specific rate of the catalyst = (mole of phenylacetylene conversion) / (mole of total Pd atoms × reaction time).[52] Theoretical calculations. First-principle calculations were performed based on the density functional theory (DFT) using Vienna Ab-initio Simulation Package (VASP) with the

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projector augmented wave (PAW) method.[65,66] The electron exchange correlation for structure relaxations was described by Perdew-Burke-Ernzerhof functional (PBE) with the generalized gradient approximation (GGA).[67] The HeydScuseria-Ernzerhof hybrid functional (HSE06) was applied in order to obtain the accurate Fermi level of Pd (100) facet and N-doped graphene.[68] The cutoff energy of plane-wave basis was set as 400 eV and the criterion for Hellman-Feynman force convergence was 0.02 eV/Å. In our adsorption model, a 4x4 supercell of Pd (100) facet with three layers and 15 Å vacuum slab was adopted, and the atoms in bottom layer were fixed with bulk lattice constants. The Brillouin zone was sampled using 3×3×1 Monkhorst Pack mesh. The DFT-D3 method was employed to describe van der Waals interactions.[69] The amount of charge transfer between Pd (100) facet and N-doped graphene was obtained using Bader charge analysis.[70] Hole doping effect was achieved by modifying the NELECT parameter in the INCAR file. ■ ASSOCIATED CONTENT Supporting Information Pd NCs synthesis, characterization of the catalysts, catalytic studies, work function and electron level, and DFT data. The Supporting Information is available free of charge on the ACS Publications website. ■ AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] ORCID Lina Zhang: 0000-0003-3890-8690 Aiwen Lei: 0000-0001-8417-3061 Xiaojun Wu: 0000-0003-3606-1211 Author Contributions §X.L. and Y.P. contributed equally to the work. Notes The authors declare no competing financial interest ■ ACKNOWLEDGMENT This work was supported by the Major Program of National Natural Science Foundation of China (21334005, 21573204), the Major International (Regional) Joint Research Project of National (21620102004), and the National Natural Science Foundation of China (21390402, 21520102003), the 973 Program (2012CB725302), the MOST (2018YFA0208603, 2016YFA0200602), Strategic Priority Research Program of CAS (XDB01020300), the Fundamental Research Funds for the Central Universities. XAS study was performed at National Synchrotron Radiation Research Center, DFT calculations are supported by USTCSCC, SCCAS, and Shanghai Supercomputer Centers. ■ REFERENCES [1] Meemken, F.; Baiker, A. Recent progress in heterogeneous asymmetric hydrogenation of C=O and C=C bonds on supported noble metal catalysts. Chem. Rev. 2017, 117, 11522-11569.

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TOC Herein, we developed a facile route to synthesize Pd nanocubes (NCs) using Eosin Y as the photosensitizer under visible light. Subsequently, Pd NCs were uniformly loaded on N-doped carbon nanofibrous microspheres (NCM) from carbonated chitin microspheres. This Pd NCs@NCM exhibited high reactivity and selectivity in alkynes semi-hydrogenation. According to the Mott-Schottky effect, loading of Pd NCs on N-doped carbon constructed a rectifying contact and decreased the electron density of Pd NCs. Density functional theory calculations suggested that high concentration of hole doped in Pd NCs weakened the interaction of alkenes on Pd (100) facet and prevented further hydrogenation. Thus, Pd NCs@NCM maintained both high reactivity and selectivity comparing with surface modified catalysts. This work provides an alternative strategy to design the Mott-Schottky catalysts for selective hydrogenation reactions.

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