Atomically Dispersed Pd Sites on ZrO 2 Hybridized N-Doped Carbon for Efﬁcient Suzuki–Miyaura Reaction

: Researchers studying heterogeneous catalysis are intrigued by single-atom catalysts (SACs) due to their ultrahigh atomic utilization. However, only a few reports on SAC-catalyzed classical organic transformations are available. In this work, atomically dispersed Pd sites are conﬁned to a ZrO 2 hybridized N-doped carbon skeleton with a smart design. UiO-66-NH 2 is used to anchor Pd atoms by the coordination of the donor atoms including lone pairs of electrons and metal atoms. Subsequently, the in situ introduction of ZrO 2 doping is achieved using pyrolysis, which helps improve the catalytic performance by modulating the electronic state. The Pd@ZrO 2 /N–C catalyst obtained from the unique design exhibits a high yield (99%) in eco-friendly media with an extremely low noble metal dosage (0.03 mol% Pd) for the Suzuki reaction. Moreover, Pd@ZrO 2 /N–C remains highly active after being reused several times and possesses versatility in a variety of substrates. This strategy offers a feasible alternative to designing SACs with atomically dispersed noble metals for heterogeneous reactions.


Introduction
C-C coupling reactions are versatile methods to carry out organic syntheses, which arouse extensive attention from academic research and industrial applications [1][2][3][4].The palladium (Pd)-catalyzed Suzuki reaction is a potent technique for constructing C-C bonds [5][6][7][8].Pd complex-based homogeneous catalysts have extremely high catalytic performance.However, they are often hard to reuse due to their time-consuming and energy-intensive complex separation process [9][10][11].Such flaws can be fixed with heterogeneous catalysts decorated by Pd nanoparticles [12][13][14][15][16][17][18].However, as only a minority of noble metal atoms can become the catalytic sites, their metal utilization is lower compared with homogeneous catalysts [19,20].Consequently, it should be crucial to create heterogeneous catalysts with maximum atomic utilization from the economic and environmental perspectives.
Single-atom catalysts (SACs) are novel developments in heterogeneous catalysis [21,22], offering the advantages of homogeneous (maximum metal utilization) and heterogeneous (facile separation) catalysts together [23].However, the isolated single atoms in SACs tend to aggregation and deactivation because of their high surface energy [24,25].Presently, one of the most effective ways to deal with these challenges is to prepare N-doped carbon (N-C) supports [26][27][28] to confine metal atoms, in particular, metal-organic framework (MOF)-derived N-C carriers.
It has been shown that MOF-derived carbon materials are excellent supports for SACs [29][30][31][32][33].The most common technique for producing SACs is pyrolysis, which involves the high-temperature thermal decomposition of suitable precursors [34][35][36].In general, when metal nodes are atomically dispersed in MOFs, they may be pyrolyzed and reduced in situ to prepare transition metal SACs.However, such a method is hardly effective for noble metals, which are infrequently used as metal nodes during the preparation of MOFs [37].In addition, such catalysts are mostly confined to electrochemical catalysis [38][39][40].
Herein, a new composite material (Pd@ZrO 2 /N-C) consisting of Pd single atoms confined on zirconium dioxide (ZrO 2 ) hybridized with N-doped carbon is validated to be a promising catalyst for the Suzuki reaction.UiO-66-NH 2 , a widely used and thermally stable zirconium (Zr)-based MOF, is selected as the matrix for hosting Pd atoms.The free coordination -NH 2 functional groups, can adsorb metal precursors and further coordinate with the single noble metal atoms.Furthermore, ZrO 2 doping can be introduced in situ with the pyrolysis of the Zr-based MOF, which facilitates the regulation of the electronic state at single active sites, resulting in better catalytic performance.Specifically, Pd 2+ ions are adsorbed on UiO-66-NH 2 with hydrothermal synthesis.After pyrolysis, the Pd atoms are individually dispersed and well distributed on the ZrO 2 -hybridized support.Thanks to the distinctive atom-isolated structure and the electronic state regulation of single-atom active sites, Pd@ZrO 2/ N-C demonstrates superior catalytic activity and universality of various substrates for the Suzuki reaction in an aqueous solution.In addition, Pd@ZrO 2 /N-C can be reused with satisfactory catalytic performance.

Synthesis of Pd@ZrO 2 /N-C
The synthesis of Pd@ZrO 2 /N-C is depicted in Scheme 1. PdCl 2 is used as the metal precursor to adsorb Pd 2+ ions onto the skeleton of UiO-66-NH 2 using a self-assembly process.The corresponding complex resulting from the adsorption process is named PdCl 2 /UiO-66-NH 2 .Within this case, the uncoordinated -NH 2 can act as the Lewis base to immobilize the metal atom, thus the Pd 2+ ions can be coordinated with the nitrogen atoms for the formation of metal-N bonds.Following pyrolysis at 700 • C under a flowing nitrogen atmosphere, the Pd sites are immobilized on in situ ZrO 2 -hybridized nitrogendoped carbon to generate the Pd@ZrO 2 /N-C catalyst.It has been shown that MOF-derived carbon materials are excellent supports for SACs [29][30][31][32][33].The most common technique for producing SACs is pyrolysis, which involves the high-temperature thermal decomposition of suitable precursors [34][35][36].In general, when metal nodes are atomically dispersed in MOFs, they may be pyrolyzed and reduced in situ to prepare transition metal SACs.However, such a method is hardly effective for noble metals, which are infrequently used as metal nodes during the preparation of MOFs [37].In addition, such catalysts are mostly confined to electrochemical catalysis [38][39][40].
Herein, a new composite material (Pd@ZrO2/N-C) consisting of Pd single atoms confined on zirconium dioxide (ZrO2) hybridized with N-doped carbon is validated to be a promising catalyst for the Suzuki reaction.UiO-66-NH2, a widely used and thermally stable zirconium (Zr)-based MOF, is selected as the matrix for hosting Pd atoms.The free coordination -NH2 functional groups, can adsorb metal precursors and further coordinate with the single noble metal atoms.Furthermore, ZrO2 doping can be introduced in situ with the pyrolysis of the Zr-based MOF, which facilitates the regulation of the electronic state at single active sites, resulting in better catalytic performance.Specifically, Pd 2+ ions are adsorbed on UiO-66-NH2 with hydrothermal synthesis.After pyrolysis, the Pd atoms are individually dispersed and well distributed on the ZrO2-hybridized support.Thanks to the distinctive atom-isolated structure and the electronic state regulation of single-atom active sites, Pd@ZrO2/N-C demonstrates superior catalytic activity and universality of various substrates for the Suzuki reaction in an aqueous solution.In addition, Pd@ZrO2/N-C can be reused with satisfactory catalytic performance.

Synthesis of Pd@ZrO2/N-C
The synthesis of Pd@ZrO2/N-C is depicted in Scheme 1. PdCl2 is used as the metal precursor to adsorb Pd 2+ ions onto the skeleton of UiO-66-NH2 using a self-assembly process.The corresponding complex resulting from the adsorption process is named PdCl2/UiO-66-NH2.Within this case, the uncoordinated -NH2 can act as the Lewis base to immobilize the metal atom, thus the Pd 2+ ions can be coordinated with the nitrogen atoms for the formation of metal-N bonds.Following pyrolysis at 700 °C under a flowing nitrogen atmosphere, the Pd sites are immobilized on in situ ZrO2-hybridized nitrogendoped carbon to generate the Pd@ZrO2/N-C catalyst.Scheme 1. Schematic representation of the synthesis process for Pd@ZrO2/N-C.

Characterization of the Catalysts
Compared to pristine UiO-66-NH2, the color of the materials changes from yellow to gray after the addition of PdCl2, indicating strong electrostatic adsorption between the Pd 2+ ions and the -NH2 groups (Figure S1) [41].All prepared materials present the characteristic peaks at 2θ = 7.5° and 8.5° in X-ray diffraction (XRD) patterns (Figure S2a) Scheme 1. Schematic representation of the synthesis process for Pd@ZrO 2 /N-C.

Characterization of the Catalysts
Compared to pristine UiO-66-NH 2 , the color of the materials changes from yellow to gray after the addition of PdCl 2 , indicating strong electrostatic adsorption between the Pd 2+ ions and the -NH 2 groups (Figure S1) [41].All prepared materials present the characteristic peaks at 2θ = 7.5 • and 8.5 • in X-ray diffraction (XRD) patterns (Figure S2a) following self-assembly, which corresponds to the characteristic peaks of UiO-66.This is very consistent with earlier studies [42,43], demonstrating the preservation of MOF crystallinity and structure after the adsorption process.Fourier transform infrared (FT-IR) spectroscopy (Figure S3) is applied to further validate the interaction of the PdCl 2 precursor with UiO-66-NH 2 .The analysis implies that the scissoring vibration in N-H is responsible for the absorption peak at 1495 cm −1 , whereas the stretching vibration in C-N accounts for the peaks at 1098 as well as 1156 cm −1 [44].The decrease in peak intensity after Pd 2+ adsorption suggests that the -NH 2 groups may play a major part in the adsorption of Pd 2+ ions [45].After pyrolysis, it is clear from the XRD patterns of Pd@ZrO 2 /C and Pd@ZrO 2 /N-C that the characteristic reflection peaks of ZrO 2 appear (Figure S2b).Furthermore, the pattern of Pd@ZrO 2 /C shows the characteristic diffraction peaks belonging to Pd.The development in Pd nanoparticles is indicated by the peak at 40.1 • , which is associated with the (111) plane of Pd.In contrast, as for Pd@ZrO 2 /N-C, there are no diffraction peaks attributable to the Pd metal, excluding the aggregation and formation of large Pd nanoparticles.This provides proof that the uncoordinated -NH 2 groups have a crucial role in stabilizing the PdCl 2 precursors and further hindering their assembly during pyrolysis.The diffraction peaks in ZrO 2 vanish from the pattern of Pd/N-C (Figure S2b) following acid etching, verifying that the added HF solution successfully removed the ZrO 2 nanoparticles.
The morphology of the catalysts is analyzed using a series of tests.The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figures 1a and S4-S6) show that the samples retain their initial octahedral structure after adsorption and pyrolysis.The elemental mapping results (Figure 1b) verify that the elements are distributed uniformly in Pd@ZrO 2 /N-C.The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1c) of Pd@ZrO 2 /N-C displays the isolated Pd atoms (labeled with green circles).The high-resolution transmission electron microscopy (HRTEM) image of Pd@ZrO 2 /N-C (Figure S7a) demonstrates the distribution of ZrO 2 nanoparticles on the substrate, while the images of Pd/N-C (Figure S8) show that the prepared material retains the octahedral morphology and removes ZrO 2 .The Zr 3d XPS spectrum of Pd@ZrO2/N-C (Figure S11) gives the binding energy of Zr 3d5/2 (182.4 eV), which is inferior to the previously reported value (182.8 eV) [49].The downfield change of 0.4 eV of Zr in Pd@ZrO2/N-C indicates a notable electron transfer between the Zr center and Pd metal or the skeleton.Additionally, the binding energy of O 1s (Figure S12a) could give further insight into the coordination state of the materials.The peak at 530.2 eV (OM) arises from combinations of O connecting with metal atoms.The coordination states of the elements including C, N, Zr, O, and Pd are analyzed using X-ray photoelectron spectroscopy (XPS).The deconvolution in the C 1s peak of ZrO 2 /N-C leads to five components at 284.9, 285.8, 286.2, 287.0, and 288.9 eV, which can be assigned to C=C, C-N, C-O, N-C=O, and C=O bonds, respectively (Figure S9a) [46].C-N and N-C=O appear, which proves that N has been doped on the support.Contrasted with ZrO 2 /N-C, the amounts of those groups in Pd@ZrO 2 /N-C descend (Figure S9c) probably due to the generation of Pd-N bonds, which could be assisted with the state analysis of N. The N 1s spectrum for ZrO 2 /N-C (Figure S10a) displays the binding energy peaks at 398.5 eV of pyridinic N, 400.3 eV of pyrrolic N, and 401.3 eV of graphitic N [47].In comparison to ZrO 2 /N-C, the spectra of Pd@ZrO 2 /N-C and Pd/N-C display one new peak at 399.3 eV, which indicates that metal-nitrogen bonds are formed as a result of some N configuration coordinated to the metal sites (Figure S10b,c) [41].Furthermore, it can also be observed that the introduction of Pd metal increases the intensity of pyrrolic N as well as pyridinic N given that the π-conjugated structure can accept the p-electron from both varieties of N atoms, which are usually regarded as the coordination sites for the atomically dispersed Pd [48].
The Zr 3d XPS spectrum of Pd@ZrO 2 /N-C (Figure S11) gives the binding energy of Zr 3d 5/2 (182.4 eV), which is inferior to the previously reported value (182.8 eV) [49].The downfield change of 0.4 eV of Zr in Pd@ZrO 2 /N-C indicates a notable electron transfer between the Zr center and Pd metal or the skeleton.Additionally, the binding energy of O 1s (Figure S12a) could give further insight into the coordination state of the materials.The peak at 530.2 eV (O M ) arises from combinations of O connecting with metal atoms.Evidence for oxygen vacancies is shown by a peak at 531.8 eV (O V ).Other weakly bound oxygen species, such as H 2 O and hydroxyl, are responsible for the peak at 533.6 eV (O H ) [50].It can be shown that a significant increase in the intensity of O V appears after introducing Pd metal from the spectra of Pd@ZrO 2 /N-C and ZrO 2 /N-C (Figure 2a,b), indicating that additional oxygen vacancies are produced, which is further verified with Electron Paramagnetic Resonance (EPR).The results of EPR (Figure 2d) demonstrate a signal induced by oxygen vacancies formation at g = 2.003.The total spin number under quantitative conditions is from 8.372 × 10 16 to 9.909 × 10 16 when the Pd metal is introduced, indicating an augment of oxygen vacancies, which is identical to the XPS results.This is probably due to the generation of Pd-O by the electron-transfer process between Pd and ZrO 2 during the annealing in N 2 .The Pd content decreased from 3.27 mol% to 0.15 mol% (determined with XPS) after the removal of ZrO 2 , which also indicates that some Pd atoms are loaded on ZrO 2 with an established metal-support interaction.
The interaction is further verified with the change in specific surface area (SSA) and ZrO 2 particle.SSA is measured with Brunauer-Emmett-Teller (BET) at 42.4 m 2 /g for Pd@ZrO 2 /N-C as well as 8.9 m 2 /g ZrO 2 /N-C (Figure S13).The HRTEM images demonstrate that the size of ZrO 2 in the Pd@ZrO 2 /N-C catalyst (Figure S7b) is 3.4 nm on average, while it is 5.2 nm in ZrO 2 /N-C (Figure S14).These findings imply that the larger the size of the ZrO 2 nanoparticles, the smaller the SSA of the catalyst.It can also be assumed that the growth of ZrO 2 nanoparticles is responsible for the reduction in SSA.Conversely, it could be speculated that the existence of Pd atoms can suppress the particle growth of ZrO 2 during the pyrolysis treatment by converting the O-Zr species on the surface to the Pd-O species, which gives Pd@ZrO 2 /N-C a larger surface area.This provides more evidence for the interaction between Pd and ZrO 2 .After the removal of ZrO 2 with washing, there is only one fitting peak referring to O H in the O 1s spectrum of Pd/N-C (Figure S12b), which verifies the absence of metal-O bonds.The Pd 3d XPS spectrum (Figure S15a) reveals the two peaks associated with Pd 2+ 3d 3/2 (343.0 eV) and 3d 5/2 (337.9 eV) [51].As for Pd@ZrO 2 /C and Pd@ZrO 2 /N-C (Figures S15b and 2d), the Pd 3d signal is shielded by the Zr 3p signal.Nevertheless, inductively coupled plasma mass spectroscopic (ICP-MS) analysis reveals that the contents of Pd in Pd@ZrO 2 /C and Pd@ZrO 2 /N-C are 0.5 wt% and 0.3 wt%, respectively, confirming the existence of Pd in both of the samples.The interaction is further verified with the change in specific surface area (SSA) and ZrO2 particle.SSA is measured with Brunauer-Emmett-Teller (BET) at 42.4 m 2 /g for Pd@ZrO2/N-C as well as 8.9 m 2 /g ZrO2/N-C (Figure S13).The HRTEM images demonstrate that the size of ZrO2 in the Pd@ZrO2/N-C catalyst (Figure S7b) is 3.4 nm on average, while it is 5.2 nm in ZrO2/N-C (Figure S14).These findings imply that the larger the size of the ZrO2 nanoparticles, the smaller the SSA of the catalyst.It can also be assumed that the growth of ZrO2 nanoparticles is responsible for the reduction in SSA.Conversely, it could be speculated that the existence of Pd atoms can suppress the particle growth of ZrO2 during the pyrolysis treatment by converting the O-Zr species on the surface to the Pd-O species, which gives Pd@ZrO2/N-C a larger surface area.This provides more evidence for the interaction between Pd and ZrO2.After the removal of ZrO2 with washing, there is only one fitting peak referring to OH in the O 1s spectrum of Pd/N-C (Figure S12b), which verifies the absence of metal-O bonds.The Pd 3d XPS spectrum (Figure S15a) reveals the two peaks associated with Pd 2+ 3d3/2 (343.0 eV) and 3d5/2 (337.9 eV) [51].As for Pd@ZrO2/C and Pd@ZrO2/N-C (Figures S15b and 2d), the Pd 3d signal is shielded by the Zr 3p signal.Nevertheless, inductively coupled plasma mass spectroscopic (ICP-MS) analysis reveals that the contents of Pd in Pd@ZrO2/C and Pd@ZrO2/N-C are 0.5 wt% and 0.3 wt%, respectively, confirming the existence of Pd in both of the samples.The coordination states of Pd within Pd@ZrO 2 /N-C are further validated with X-ray absorption spectroscopy (XAS).The X-ray absorption near-edge structure (XANES) spectra suggest that the absorption threshold for Pd@ZrO 2 /N-C lies between Pd foil and PdO (Figure 3a).It shows that Pd is positively charged.The Fourier-transformed extended XAFS (FT-EXAFS) spectrum of Pd@ZrO 2 /N-C (Figure 3b) displays a single peak at 1.5 Å, which is in accordance with the Pd-O scattering pair [52].No Pd-Pd coordination features of the Pd foil reference are observed in Pd@ZrO 2 /N-C, proving that the Pd atoms exist in isolation.The local structure parameters are further obtained with least squares EXAFS fitting (Figures S16 and S17) and summarized in Table S1.The results reveal that the Pd atom may be anchored by three O or N atoms at a mean bond length of about 2.0 Å in Pd@ZrO 2 /N-C.The results of the wavelet transform (WT) are illustrated in Figure 3c.For Pd@ZrO 2 /N-C, there is one WT maximum at ~5 Å −1 , which coincides with the Pd-O bonding in the PdO plot.No additional signals can be identified, which is quite consistent with the findings of previous analyses.To sum up, the results presented above demonstrate that the Pd single-atom catalyst with ZrO 2 -hybridized N-C supports is successfully synthesized.
Pd@ZrO2/N-C, there is one WT maximum at ~5 Å −1 , which coincides with the Pd-O bonding in the PdO plot.No additional signals can be identified, which is quite consistent with the findings of previous analyses.To sum up, the results presented above demonstrate that the Pd single-atom catalyst with ZrO2-hybridized N-C supports is successfully synthesized.

Performance Evaluation of the Suzuki Reaction
For optimal reaction conditions, variables including reaction time, temperature, catalyst loadings, and bases are explored.To increase the solubility of the substrate, an equal volume of ethanol is mixed into water as a solvent.In a range of 2 h to 4 h, the yield increases from 61% to 99% (Table 1, entries 1-3).At this time, the turnover frequency (TOF) value reaches 825 h -1 , which is more efficient than most reported single-atom Pd catalysts applied to Suzuki coupling (Table S2).The yield raises with the reaction temperature acceleration (60 °C to 80 °C) from 50% to 99% (Table 1, entries 3-5).Considering that a further increase in the reaction temperature will reach the azeotropic point of the system and a prolonged reaction time will raise the operating cost, the suitable reaction time and reaction temperature are 4 h and 80 °C, respectively.The amount of the catalyst is also critical, so the effect of different amounts of catalyst (Pd content of the molar ratio of substrates) on the reaction is tested.Without the aid of the catalyst, the coupling product is negligible (Table 1, entry 12).However, the catalytic activity rises with the increasing amount of catalyst, reaching 99% yield with 0.03 mol% Pd (Table 1, entries 3, 6, and 7).

Performance Evaluation of the Suzuki Reaction
For optimal reaction conditions, variables including reaction time, temperature, catalyst loadings, and bases are explored.To increase the solubility of the substrate, an equal volume of ethanol is mixed into water as a solvent.In a range of 2 h to 4 h, the yield increases from 61% to 99% (Table 1, entries 1-3).At this time, the turnover frequency (TOF) value reaches 825 h -1 , which is more efficient than most reported single-atom Pd catalysts applied to Suzuki coupling (Table S2).The yield raises with the reaction temperature acceleration (60 • C to 80 • C) from 50% to 99% (Table 1, entries 3-5).Considering that a further increase in the reaction temperature will reach the azeotropic point of the system and a prolonged reaction time will raise the operating cost, the suitable reaction time and reaction temperature are 4 h and 80 • C, respectively.The amount of the catalyst is also critical, so the effect of different amounts of catalyst (Pd content of the molar ratio of substrates) on the reaction is tested.Without the aid of the catalyst, the coupling product is negligible (Table 1, entry 12).However, the catalytic activity rises with the increasing amount of catalyst, reaching 99% yield with 0.03 mol% Pd (Table 1, entries 3, 6, and 7).This may be due to the existence of more active sites.Considering that a further increase in catalyst dosage would be cost-prohibitive and that the catalytic yield has already reached a very high level, a Pd content of 0.03 mol% is determined to be the optimum catalyst amount.
Under these optimum reaction circumstances, the effects of four commonly used bases (K 2 CO 3 , Na 2 CO 3 , KOH, NaOH) on the reaction are investigated (Table 1, entries 3 and 8-10).The reaction yields involving such bases are all above 90%, which indicates that the effect of the base is insignificant.K 2 CO 3 is more appropriate for the reaction due to the best catalytic activity in its presence and its weak causticity.Furthermore, in the absence of the base, there are almost no products (Table 1, entry 11).The best reaction yield is obtained using a mixture of H 2 O and EtOH (1:1) solution containing a catalyst of Pd@ZrO 2 /N-C (0.03 mol% Pd) and a base of K 2 CO 3 at 80 • C for 4 h.Furthermore, no coupling product is observed using ZrO 2 /N-C as the catalyst for the control experiment.This demonstrates that the reaction is cannot take place without Pd.This may be due to the existence of more active sites.Considering that a further increase in catalyst dosage would be cost-prohibitive and that the catalytic yield has already reached a very high level, a Pd content of 0.03 mol% is determined to be the optimum catalyst amount.Under these optimum reaction circumstances, the effects of four commonly used bases (K2CO3, Na2CO3, KOH, NaOH) on the reaction are investigated (Table 1, entries 3 and 8-10).The reaction yields involving such bases are all above 90%, which indicates that the effect of the base is insignificant.K2CO3 is more appropriate for the reaction due to the best catalytic activity in its presence and its weak causticity.Furthermore, in the absence of the base, there are almost no products (Table 1, entry 11).The best reaction yield is obtained using a mixture of H2O and EtOH (1:1) solution containing a catalyst Pd@ZrO2/N-C (0.03 mol% Pd) and a base of K2CO3 at 80 °C for 4 h.Furthermore, no coupling product is observed using ZrO2/N-C as the catalyst for the control experiment.This demonstrates that the reaction is cannot take place without Pd.a Reaction condition: 0.5 mmol of iodobenzene, 0.6 mmol of phenylboronic acid, 1.5 mmol of the base, 2 mL of solvent (H2O:EtOH = 1:1), Pd@ZrO2/N-C (0.01 mol%-0.03mol%) at various temperatures (°C).b GC yield.
To prove the superiority of isolated atoms as well as the merits of the introduction of ZrO2 doping in the supports, other prepared catalysts are also evaluated.The Pd@ZrO2/C catalyst with the same Pd equivalent weight has a biphenyl yield of 79% under the same conditions (Figure 4a).Because Pd nanoparticles develop in Pd@ZrO2/C, it is less active than Pd@ZrO2/N-C.Specifically, Pd active sites in Pd@ZrO2/N-C are exposed on the surface, while those in Pd@ZrO2/C are occluded inside the Pd nanoparticles.The disclosure of Pd atoms enhances the atom's utilization coefficient and favors the catalytic performance.This demonstrates the superiority of SACs.Similarly, the Pd/N-C (the content of Pd is 1.25 wt% determined with XPS) catalyst obtained after the removal of ZrO2 exhibited poor catalytic performance with a yield of only 5%.This result demonstrates the positive effects of ZrO2 doping.The whole reaction pathway usually consists of three main steps (Figure 4b) [53,54].The catalyst has been shown to act as the donor of the charge within the oxidative addition process.The Zr ions can donate lone pair electrons to the Pd sites in Pd@ZrO2/N-C and boost the electron transfer [55], which enhances the catalyst's capacity to donate charges and makes the oxidative addition process more efficient.Subsequently, at the time of transmetallation and reductive elimination, the catalyst acts as the acceptor of the charge.Resulting from the electron transfer to the adjacent ZrO2, the Pd single-atom sites are positively charged, which facilitates the steps of transmetallation and a Reaction condition: 0.5 mmol of iodobenzene, 0.6 mmol of phenylboronic acid, 1.5 mmol of the base, 2 mL of solvent (H 2 O:EtOH = 1:1), Pd@ZrO 2 /N-C (0.01 mol%-0.03mol%) at various temperatures ( • C). b GC yield.
To prove the superiority of isolated atoms as well as the merits of the introduction of ZrO 2 doping in the supports, other prepared catalysts are also evaluated.The Pd@ZrO 2 /C catalyst with the same Pd equivalent weight has a biphenyl yield of 79% under the same conditions (Figure 4a).Because Pd nanoparticles develop in Pd@ZrO 2 /C, it is less active than Pd@ZrO 2 /N-C.Specifically, Pd active sites in Pd@ZrO 2 /N-C are exposed on the surface, while those in Pd@ZrO 2 /C are occluded inside the Pd nanoparticles.The disclosure of Pd atoms enhances the atom's utilization coefficient and favors the catalytic performance.This demonstrates the superiority of SACs.Similarly, the Pd/N-C (the content of Pd is 1.25 wt% determined with XPS) catalyst obtained after the removal of ZrO 2 exhibited poor catalytic performance with a yield of only 5%.This result demonstrates the positive effects of ZrO 2 doping.The whole reaction pathway usually consists of three main steps (Figure 4b) [53,54].The catalyst has been shown to act as the donor of the charge within the oxidative addition process.The Zr ions can donate lone pair electrons to the Pd sites in Pd@ZrO 2 /N-C and boost the electron transfer [55], which enhances the catalyst's capacity to donate charges and makes the oxidative addition process more efficient.Subsequently, at the time of transmetallation and reductive elimination, the catalyst acts as the acceptor of the charge.Resulting from the electron transfer to the adjacent ZrO 2 , the Pd single-atom sites are positively charged, which facilitates the steps of transmetallation and reductive elimination by acting as electron acceptors [56].Therefore, this strongly confirms the significant advantage of the ZrO 2 in the supports.
Catalysts 2023, 13, x FOR PEER REVIEW reductive elimination by acting as electron acceptors [56].Therefore, this strong firms the significant advantage of the ZrO2 in the supports.The recyclability of Pd@ZrO2/N-C is another evaluation index that should be assessed (Figure 4c).After the reaction, Pd@ZrO2/N-C is recovered with centrif and then dried after being washed with DI water.The dried powder is collected fo reaction cycle.The test repeats for three consecutive cycles.Pd@ZrO2/N-C is used f runs and the yield remains above 91%, showing no significant loss of catalytic ab evaluate the amount of Pd leached from Pd@ZrO2/N-C after each reaction cycle, t tion (10 mL) obtained by removing the Pd catalyst after the reaction is tested usi MS.The concentration of Pd in the supernatant is 0.024, 0.005, and 0.011 ppm, w equivalent to 1.59 mol%, 0.32 mol%, and 0.76 mol% of the Pd quantity in the c respectively.These results indicate that the leaching level is extremely low, which the catalytic activity could be maintained.
On the other hand, the universality of the catalysts with various substituted a ides and arylboronic acids is also tested.The use of various substrates with d groups obtains excellent isolated yields (Table 2).Pd@ZrO2/N-C is well tolerated tifarious groups on the substrates, including methyl (3b-3d), cyano (3e), nitro ( ether (3g).The reaction of aryl iodides obtains a higher yield than that of aryl br (3a) because of the relatively high activity of iodides.Steric hindrance also affects alytic reaction.For iodotoluene, the highest yield is obtained for para-substitution is superior to that of ortho-substituted and meta-substituted iodotoluene for the s action time (3b-3d).Overall, the catalyst exhibits superior catalytic performance a extensive variety of substrates.The recyclability of Pd@ZrO 2 /N-C is another evaluation index that should be further assessed (Figure 4c).After the reaction, Pd@ZrO 2 /N-C is recovered with centrifugation and then dried after being washed with DI water.The dried powder is collected for a new reaction cycle.The test repeats for three consecutive cycles.Pd@ZrO 2 /N-C is used for three runs and the yield remains above 91%, showing no significant loss of catalytic ability.To evaluate the amount of Pd leached from Pd@ZrO 2 /N-C after each reaction cycle, the solution (10 mL) obtained by removing the Pd catalyst after the reaction is tested using ICP-MS.The concentration of Pd in the supernatant is 0.024, 0.005, and 0.011 ppm, which are equivalent to 1.59 mol%, 0.32 mol%, and 0.76 mol% of the Pd quantity in the catalyst, respectively.These results indicate that the leaching level is extremely low, which is why the catalytic activity could be maintained.

Synthesis of PdCl2/UiO-66-NH2
PdCl2 (250 µL, 1mg/mL DMF), ZrCl4 (85.6 mg, 0.36 mmol), and H2BDC-NH2 (66.5 mg, 0.36 mmol) were added in 80 mL DMF mixed with 9.6 mL HAc in a 100 mL glass vial.A 100 mL Teflon-lined autoclave was used to pack the mixed solution after 30 min of sonication.Next, following sealing, the autoclave was put into a preheated oven.The oven was set to 120 °C, and the process took 24 h.Then, the product was centrifugated and washed 3 times with a mixed solution (MeOH:DMF = 1:4).The powder was collected for subsequent use after drying at 90 °C.

Synthesis of Pd@ZrO2/N-C
The powder was loaded into a tube furnace.Under a flowing nitrogen atmosphere, PdCl2/UiO-66-NH2 was heated to 700 °C at 5 °C/min and subsequently maintained at this condition for another 3 h.The black powder was collected once it cooled to room temperature for subsequent characterization and catalytic testing.For comparison, the ZrO2/N-C catalyst without Pd, the Pd@ZrO2/C catalyst without N doping, and the Pd/N-C catalyst without ZrO2 were also prepared.The supporting information describes the specific methods of synthesis.

Suzuki-Miyaura Cross-Coupling Reaction
Unless otherwise noted, the Suzuki reaction was carried out as follows: a typical procedure is to disperse the Pd@ZrO2/N-C catalyst (equivalent weight 0.03 mol% Pd) in a

Synthesis of PdCl2/UiO-66-NH2
PdCl2 (250 µL, 1mg/mL DMF), ZrCl4 (85.6 mg, 0.36 mmol), and H2BDC-NH2 (66.5 mg, 0.36 mmol) were added in 80 mL DMF mixed with 9.6 mL HAc in a 100 mL glass vial.A 100 mL Teflon-lined autoclave was used to pack the mixed solution after 30 min of sonication.Next, following sealing, the autoclave was put into a preheated oven.The oven was set to 120 °C, and the process took 24 h.Then, the product was centrifugated and washed 3 times with a mixed solution (MeOH:DMF = 1:4).The powder was collected for subsequent use after drying at 90 °C.

Synthesis of Pd@ZrO2/N-C
The powder was loaded into a tube furnace.Under a flowing nitrogen atmosphere, PdCl2/UiO-66-NH2 was heated to 700 °C at 5 °C/min and subsequently maintained at this condition for another 3 h.The black powder was collected once it cooled to room temperature for subsequent characterization and catalytic testing.For comparison, the ZrO2/N-C catalyst without Pd, the Pd@ZrO2/C catalyst without N doping, and the Pd/N-C catalyst without ZrO2 were also prepared.The supporting information describes the specific methods of synthesis.

Synthesis of PdCl2/UiO-66-NH2
PdCl2 (250 µL, 1mg/mL DMF), ZrCl4 (85.6 mg, 0.36 mmol), and H2BDC-NH2 (66.5 mg, 0.36 mmol) were added in 80 mL DMF mixed with 9.6 mL HAc in a 100 mL glass vial.A 100 mL Teflon-lined autoclave was used to pack the mixed solution after 30 min of sonication.Next, following sealing, the autoclave was put into a preheated oven.The oven was set to 120 °C, and the process took 24 h.Then, the product was centrifugated and washed 3 times with a mixed solution (MeOH:DMF = 1:4).The powder was collected for subsequent use after drying at 90 °C.

Synthesis of Pd@ZrO2/N-C
The powder was loaded into a tube furnace.Under a flowing nitrogen atmosphere, PdCl2/UiO-66-NH2 was heated to 700 °C at 5 °C/min and subsequently maintained at this condition for another 3 h.The black powder was collected once it cooled to room temperature for subsequent characterization and catalytic testing.For comparison, the ZrO2/N-C catalyst without Pd, the Pd@ZrO2/C catalyst without N doping, and the Pd/N-C catalyst without ZrO2 were also prepared.The supporting information describes the specific methods of synthesis.

Suzuki-Miyaura Cross-Coupling Reaction
Unless otherwise noted, the Suzuki reaction was carried out as follows: a typical procedure is to disperse the Pd@ZrO2/N-C catalyst (equivalent weight 0.03 mol% Pd) in a

Synthesis of PdCl 2 /UiO-66-NH 2
PdCl 2 (250 µL, 1mg/mL DMF), ZrCl 4 (85.6 mg, 0.36 mmol), and H 2 BDC-NH 2 (66.5 mg, 0.36 mmol) were added in 80 mL DMF mixed with 9.6 mL HAc in a 100 mL glass vial.A 100 mL Teflon-lined autoclave was used to pack the mixed solution after 30 min of sonication.Next, following sealing, the autoclave was put into a preheated oven.The oven was set to 120 • C, and the process took 24 h.Then, the product was centrifugated and washed 3 times with a mixed solution (MeOH:DMF = 1:4).The powder was collected for subsequent use after drying at 90 • C.

Synthesis of Pd@ZrO 2 /N-C
The powder was loaded into a tube furnace.Under a flowing nitrogen atmosphere, PdCl 2 /UiO-66-NH 2 was heated to 700 • C at 5 • C/min and subsequently maintained at this condition for another 3 h.The black powder was collected once it cooled to room temperature for subsequent characterization and catalytic testing.For comparison, the ZrO 2 /N-C catalyst without Pd, the Pd@ZrO 2 /C catalyst without N doping, and the Pd/N-C catalyst without ZrO 2 were also prepared.The supporting information describes the specific methods of synthesis.

Suzuki-Miyaura Cross-Coupling Reaction
Unless otherwise noted, the Suzuki reaction was carried out as follows: a typical procedure is to disperse the Pd@ZrO 2 /N-C catalyst (equivalent weight 0.03 mol% Pd) in a flask with 1 mL DI water as well as 1 mL ethanol.The solution was then supplemented with iodobenzene (103 mg, 0.5 mmol), K 2 CO 3 (209.4mg, 1.5 mmol), and phenylboronic acid (75 mg, 0.6 mmol).The catalytic reaction took place in the oil bath under constant stirring at a temperature of 80 • C under an air atmosphere.The reaction duration was 4 h.Following the reaction, ethyl acetate (2 mL × 3) was used to extract the corresponding products, which were then dried with magnesium sulfate anhydrous (MgSO 4 ) and analyzed using a gas chromatography (GC) system.

Conclusions
In conclusion, the single-atom catalyst named Pd@ZrO 2 /N-C is successfully synthesized using the in situ pyrolysis of UiO-66-NH 2 adsorbed with Pd 2+ ions.The anchoring of Pd atoms on ZrO 2 -hybridized N-doped C is confirmed with various characterization methods.Such an original structure design utilizes the interaction between isolated atom active sites and supports, resulting in synthesized materials with splendid catalytic performance.It is demonstrated that Pd@ZrO 2 /N-C exhibits excellent activity for the Suzuki reaction in an environmentally friendly solution with high yields, good universality, and no significant decline in catalytic performance after reuse.Overall, this catalyst benefits from the unique structural design based on MOF.This study offers novel perspectives on the design of SACs for efficient heterogeneous catalysis.

Figure 3 .
Figure 3. (a) XANES spectra of referenced materials and Pd@ZrO2/N-C for the Pd K-edge.(b) The k 3 -weighted Fourier transforms of Pd K-edge EXAFS spectra for Pd@ZrO2/N-C and referenced materials.(c) WT for Pd@ZrO2/N-C, Pd foil, and PdO, respectively.

Figure 3 .
Figure 3. (a) XANES spectra of referenced materials and Pd@ZrO 2 /N-C for the Pd K-edge.(b) The k 3 -weighted Fourier transforms of Pd K-edge EXAFS spectra for Pd@ZrO 2 /N-C and referenced materials.(c) WT for Pd@ZrO 2 /N-C, Pd foil, and PdO, respectively.

Figure 4 .
Figure 4. (a) Catalytic performance toward the Suzuki reaction over different catalysts.(b) S ics of the Suzuki reaction.(c) Recyclability of Pd@ZrO2/N-C for the Suzuki reaction.

Figure 4 .
Figure 4. (a) Catalytic performance toward the Suzuki reaction over different catalysts.(b) Schematics of the Suzuki reaction.(c) Recyclability of Pd@ZrO 2 /N-C for the Suzuki reaction.

Table 1 .
Suzuki reactions for a wide variety of conditions with Pd@ZrO 2 /N-C a .

Table 1 .
Suzuki reactions for a wide variety of conditions with Pd@ZrO2/N-C a .

Table 2 .
Suzuki reactions of various substituted aryl halides and arylboronic acids a .

Table 2 .
Suzuki reactions of various substituted aryl halides and arylboronic acids a .