Oxidation of 5-Hydroxymethylfurfural on Supported Ag, Au, Pd and Bimetallic Pd-Au Catalysts: Effect of the Support

: Oxidation of 5-hydroxymethylfurfural (HMF), a major feedstock derived from waste/fresh biomass, into 2,5-furandicarboxylic acid (FDCA) is an important transformation for the production of biodegradable plastics. Herein, we investigated the effect of the support (unmodified and modi- fied titania, commercial alumina, and untreated and treated Sibunit carbon) of mono- and bimetallic catalysts based on noble metals (Ag, Au, Pd) on selective HMF oxidation with molecular oxygen to FDCA under mild and basic reaction conditions. The higher selectivity to FDCA was obtained when metals were supported on Sibunit carbon (Cp). The order of noble metal in terms of catalyst selec- tivity was: Ag < Au < Pd < PdAu. Finally, FDCA production on the most efficient PdAu NPs catalysts supported on Sibunit depended on the treatment applied to this carbon support in the order: PdAu/Cp < PdAu/Cp-HNO 3 < PdAu/Cp-NH 4 OH. These bimetallic catalysts were characterized by nitrogen adsorption-desorption, inductively coupled plasma atomic emission spectroscopy, high resolution transmission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Hammet indicator method and X-ray photoelectron spectroscopy. The functionalization of Sibunit surface by HNO 3 and NH 4 OH led to a change in the contribution of the active states of Pd and Au due to promotion effect of N-doping and, as a consequence, to higher FDCA production. HMF ox- idation catalyzed by bimetallic catalysts is a structure sensitive reaction.


Introduction
The unavoidable predicted depletion of fossil resources determines the need to develop methods for using new and renewable sources. Nowadays, biomass, a valuable source of energy, along with biological and chemical raw materials, can be a real alternative to obtain biofuels and chemicals. Carbohydrates are the basis of biomass (up to 75%) [1]. Their dehydration into furan derivatives is one of the most intensively developed approaches to biomass transformation [2]. Cary 60 UV spectrophotometer. Then, suspensions of supports and catalysts were prepared in these solutions, and after the establishment of adsorption equilibrium and subsequent centrifugation and decantation, the optical density D1 was determined. To take into account the effect of the change in the pH of the medium and the dissolution of the sample on the optical density upon contact of the material with the solution, a suspension of the sorbent in distilled water was obtained. After 2 h, an indicator solution was added to the decantate and the optical density (D2) was measured. All determinations were carried out in cuvettes at a wavelength that corresponds to the maximum absorption of the indicator solution.
The content of active sites (qpKa, μmol/g) with a given pKa value was calculated using Equation (1): where Cind and Vind are the concentration and volume of the indicator, α1 and α2 are the mass of the sample when measuring D1 and D2. The "−" sign corresponds to the unidirectional change of D1 and D2 relative to D0. The "+" sign is for multidirectional [29]. Catalytic tests were carried out at 60 °C under 3 atm of oxygen in a semi-batch reactor with stirring at 1100 rpm. Typically, the catalyst sample was added in a HMF/Me (Me = Ag, Au, Pd, PdAu) ratio R = 200 mol/mol to 150 mL of 0.15 M HMF or Catalyst: HMF = (0.19-0.49):1 (wt/wt) and 0.3 M NaOH aqueous solution (distilled water), in a glass reactor equipped with heater, mechanical stirrer, gas supply system and thermometer. Small aliquots of the reacting mixture were taken after 15, 30, 60, 120, and in some cases, 180, 240 and 360 min, for monitoring the reaction progress. HMF and NaOH (Merck, Darmstadt, Germany) were used as reagents.
Reactants and products were periodically analyzed and quantified by high-performance liquid chromatography (HPLC, Agilent Technologies, 1220 Infinity, Santa Clara, CA, USA) using a column Alltech OA-10308 (L × i.d.: 300 mm × 7.8 mm, Fisher Scientific, Hampton, NH, USA) with ultraviolet (Varian 9050 UV, 210 nm) and refractive index (Waters RI) detectors. 0.4 mL/min of 0.1% aqueous solution of H3PO4 used as the eluent. The identification of compounds was achieved by calibration using reference commercial samples. The difference in concentration from the carbon mass balance was attributed to unknown products detected on HPLC chromatograms.
Conversion of HMF, selectivity and yield to products (5-hydroxymethyl-2-furancarboxylic acid (HFCA) and FDCA) and carbon balance, were calculated according to Equations (2)-(5), respectively: Conversion (% mol) = C moles of all products C moles of HMF × 100 (2) Selectivity of product (% mol) = C moles of product formed C moles of all products × 100 Yield of product (% mol) = C moles of product formed × С balance Initial C moles of HMF C balance (% mol) = 100 − Initial C moles of HMF − Final C of all products Initial C moles of HMF × 100 The carbon balance obtained in each catalytic reaction was always in the 95-100% range, with the exception of silver catalysts. In this reaction, the synthesis of FDCA, derived from the oxidation of both carbonyl and hydroxyl groups, proceeds through intermediate 5-hydroxymethyl-2-furancarboxylic acid (HFCA) formation, obtained by oxidation of the aldehyde group (Scheme 1). Scheme 1. Detected products from aqueous HMF oxidation over supported Au, Pd and PdAu catalysts under basic conditions.
The screening started with previously investigated catalysts: Au supported on unmodified and modified titania (MxOy/TiO2) materials used for aerobic liquid phase oxidation of different type of alcohols: n-octanol, glycerol and betulin, under mild conditions  [20][21][22][23]. Briefly, the best catalytic performance among Au catalysts supported on modified titania were reached with catalysts modified with lanthanum oxide (Au/La2O3/TiO2), regardless of the alcohol type studied [20][21][22][23]. In addition, recently obtained results in betulin oxidation using Au/Alumina catalysts revealed that Au/AlOOH-C catalyst is superior to Au/La2O3/TiO2 in both activity and selectivity [24].
However, catalytic systems based on titanium oxide ( Ag/CeO2/TiO2, the most active catalyst for betulin and n-octanol oxidation among Ag/MxOy/TiO2 (M = Ce, Fe, Mg) catalysts [26], was also examined for HMF oxidation (Table 1, Entry 11). This Ag catalyst was less active in terms of both conversion and FDCA formation. Only 36% of HMF was converted to HFCA and some unidentified byproducts were detected after 2 h of reaction. It explains the low carbon balance, comparable to the results in the blank experiments under basic conditions (Table 1, Entry 2). According to literature [12,30], these unknown substances are polymeric products (humins) formed from degradation of HMF, unstable in basic solution. Nevertheless, in the presence of an active catalyst, HMF can be rapidly converted to the oxidized intermediates because the high catalyzed reaction rate surpasses the degradation. Thus, it is important to design a highly active catalyst able to rapidly oxidize HMF before its thermal polymerization in basic solution, because no reaction occurred in the absence of NaOH (Table 1, Entry 1).
Many studies and reviews [14,31] have shown that Pd catalysts are more selective towards FDCA formation at low temperatures, while Au catalysts tend to be applied at high temperatures and base concentrations and much longer reaction time for higher performances. For this reason, Pd was loaded on the AlOOH-S, since Au on this support exhibited the highest performance to produce FDCA among Au-catalysts tested. 2.5-fold FDCA yield (50%) was achieved on Pd/AlOOH-S (Table 1, Entry 12), in comparison to its Au/AlOOH-S counterpart.
As the reached yields of FDCA (target product) were not satisfactory, the following step was to look for more effective supports for the noble metals.
There are a large number of examples indicating the higher activity of catalysts on carbon supports compared to oxides. Carbon supports are characterized by a high specific surface area, a developed porous space to transfer of reactants and reaction products, controlled chemical surface properties and chemical inertness, especially in the environment of strong acids and bases [32][33][34].
In this study, commercial graphite-like Siberian carbon with mesoporous structure, called Sibunit, obtained by matrix synthesis technology [34], was used for this purpose. Sibunit material, which combines the properties of graphite (strength characteristics, corrosion resistance) and active carbons (developed surface, etc.), is a suitable support for catalysts for liquid-phase oxidation processes [35][36][37].
Thus, single noble metals (Ag, Au and Pd) were deposited by a sol-immobilization method on Sibunit (Cp), and examined for HMF oxidation under the same conditions described above. The order of FDCA formation in this case was: Ag/Cp < Au/Cp < Pd/Cp ( Table 1, entries [13][14][15]. At the same time, in comparison to Ag/CeO2/TiO2, enhanced activity (98% of conversion) and selectivity (2% of FDCA) were observed for Ag/Cp, although carbon balance was still worse but at higher level (69%) than for Ag/CeO2/TiO2. Meanwhile, Au/Cp showed almost the same catalytic properties, when comparing to the most active among all gold catalysts, i.e., Au/AlOOH-S. Finally, slightly higher selectivity to FDCA (SFDCA = 55%) was obtained on Pd/Cp catalyst, in comparison to Pd/AlOOH-S Commentato [M19]: Revised according to table data, Please confirm format Please use consistent format (SFDCA = 50%). Nevertheless, it should be mentioned that the highest FDCA selectivity was achieved when metals (Ag, Au or Pd) were supported on Sibunit carbon.
It is well known that a well-designed binary noble metal system, with an optimal composition, can exhibit performance superior to those of the corresponding monometallic catalysts for many catalytic reactions, thus showing a synergistic effect. In addition, several studies [14,38,39] revealed that Au catalysts are more active for the oxidation of aldehyde groups, namely the first step of the oxidation reaction (HMF to HFCA), whereas Pd catalysts are more active for the oxidation of alcohol groups. Thus, the final step in finding the most selective catalyst for FDCA formation was the use of bimetallic systemsdepositing the combination of palladium and gold (since silver-based catalyst showed lower activity and did not successfully compete with HMF degradation) on the best support found for noble metals, i.e., Sibunit. As expected, FDCA selectivity (60%) was increased when PdAu/Cp was used ( Table 1, Entry 18), compared to both monometallic Pd/Cp (SFDCA = 55%) and Au/Cp (SFDCA = 21%).
Catalytic properties of nanodispersed metals deposited on carbon materials depend significantly on the support pretreatment. It is difficult to stabilize metal NPs on the carbon support due to its chemical inertness. Therefore, preliminary treatment of carbon is required before its interaction with precursors, in order to increase the carbon reactivity, i.e., to enhance the interaction between the active phase and the support [32][33][34]. For this reason, Sibunit support was treated with 20 wt.% solution of nitric acid and ammonium hydroxide (denoted as Cp-HNO3 and Cp-NH4OH, respectively) and the bimetallic catalysts obtained were investigated for HMF oxidation. As expected, after 2 h, the modified catalysts produced 5 and 10% FDCA more than the non-treated PdAu/Cp analogues (Table 1, entries 16 to 18). Moreover, this beneficial effect of both treatments can be clearly seen at the initial catalytic activity, i.e., after just 15 min of reaction 1.3-fold and 2.2-fold FDCA yield were obtained on PdAu/Cp-HNO3 and PdAu/Cp-NH4OH, respectively, in comparison to untreated PdAu/Cp (Table 2). On the basis of these promising results, we explored the performance of bimetallic catalysts with longer run times, up to 6 h. Thus, results show obvious enhancement of catalytic activity, as expected ( Table 2, Figures 1 and 2). In particular, the following order of support treatment was found in terms of FDCA formation (SFDCA, %): PdAu/Cp < PdAu/Cp-HNO3 < PdAu/Cp-NH4OH. As it should be noted, 100% conversion and 95-98% carbon balance were observed in all cases.

Commentato [M20]
: Please consistent format for explanaton " a " or " a -" Thus, the results obtained demonstrate a positive effect of the treatment of Sibunit, especially with NH4OH, for FDCA production. In this work the maximum FDCA yield (YFDCA = 76%) was obtained on PdAu/Cp-NH4OH after 6 h. At the same time, partial deactivation with run time was observed in case of PdAu/Cp-HNO3 catalyst (Figure 1).
In order to reveal the main factors responsible for the remarkable catalytic properties exhibited by these bimetallic systems and, notably, the effect of Sibunit treatment, a series of physicochemical studies were carried out for these dual noble metal samples.

Catalyst Characterization
According to the data presented in Table 3, the specific surface area (SBET) of Sibunit supports increased by 17% and 13%, as result of modification with HNO3 and NH4OH, respectively ( Table 3, entries 1-3). PdAu/Cp is characterized by a higher SBET (by 14%) and an increase in the pore volume (by 20%), when compared to the corresponding support. This is possibly due to the removal of the amorphous phase or the formation of additional defects on Sibunit under H2SO4 adding to immobilize metal nanoparticles on the support. At the same time, deposition of two metals on both modified supports did not lead to significant changes, which can be related with the removal of the amorphous phase or the formation of additional defects during treatment of Sibunite with HNO3 or NH4OH. N2 isotherms for all studied carbon-based materials are of type II, with H3 type hysteresis, indicating adsorption in multilayers, as in mesoporous materials with slit-like shaped pores ( Figure S1). Thus, in general, the textural properties of three catalysts do not differ significantly from each other ( Table 3, entries 4-6). Elemental analysis (Table 4) showed that deposition of Pd and Au on unmodified Sibunit was close to the nominal values; however, Pd and Au were loaded on both modified Sibunit samples in lower amounts: 17% and 40% less than the nominal values, respectively. This probably can be related to the changing of the point of zero charge (PZC) due to the modification of Sibunit, which led to incomplete deposition of the bimetallic colloid. Thus, total Pd-Au content in modified Sibunit samples was 24% less than on PdAu/Cp.  (Figure 3) showed no alteration in the phase composition or the support structure after bimetallic system loading. Besides that, no peaks related to Pd or Au were found, indicating either the small size of the Pd and Au particles or their x-ray amorphous structure. Diffraction lines belonging to the support (2θ = 25.7, 44.3, 54, 78) correspond to graphite with a hexagonal lattice, in agreement with the literature data [40,41].     XPS was used to study the electronic state of palladium, gold, oxygen, carbon and nitrogen in the studied bimetallic systems ( Figure 5).   Analysis of Pd3d spectra (Figure 5a-c) showed that palladium was present on the surface of all catalysts in three states: Pd 0 , Pd 2+ and Pd 4+ with binding energies (BE) (Pd3d5/2) 335.9-336.1, 337.7-337.8 and 338.7 eV, respectively [42][43][44][45]. The binding energies attributed in this study to Pd 0 , 335.9 and 336.0 eV, exceed the standard BE value characterizing the Pd 0 state (335.4 eV) by 0.5 and 0.6 eV, respectively. It shows the presence of highly dispersed metal particles on the surface of the studied samples, which shifts binding energies towards higher values up to 1 eV [46][47][48][49]. The contribution of the electronic states of palladium found on the surface of the investigated catalysts is presented in Table  5. As can be seen, the distribution among the various states of palladium was approximately the same for the three investigated catalysts. The total surface concentration of palladium was practically the same for all samples (Table 6).   (Figure 5d-f). The ratio between these states changed slightly upon Sibunit surface modification (Table 5). However, the largest contribution of monovalent gold was found for PdAu/Cp-NH4OH. At the same time, the surface concentration of gold was the same (0.3 at.%) for both modified catalysts, while the  (Table 6). These results of gold and palladium surface concentration were in line with the lower metal content after their deposition on treated Sibunit supports found by elemental analysis (Table 4). For all investigated catalysts, O1s peak can be decomposed into four peaks (Figure 5g-j): (i) BE 531.5-531.6 eV, associated with oxygen atoms that are part of carbonyl groups with (C=O); (ii) BE 532.5-532.7 eV, oxygen bonded to carbon atoms by a single bond with (C-O); (iii) BE 533.7-533.9 eV, in hydroxyl groups C-OH; and (iv) BE 535.0-535.1 eV in carboxyl groups and/or adsorbed water (O in H2O or COOH) [50][51][52]. The relative contribution of each type of oxygen is presented in Table 7. The main contribution is from oxygen bonded to carbon atoms by a single bond (50-59%). The contribution of oxygen of the hydroxyl and carbonyl groups varies from 17 to 28% and 11 to 18%, respectively. The amount of oxygen in the form of adsorbed water and/or carboxyl groups was approximately the same for all studied samples. The modified support samples had a slightly increased content of the surface oxygen concentration (3.9-4%) compared to the O1s content (2.9 at.%) in PdAu/Cp (Table 6).   (Table 8), the following conclusions can be drawn: the main contribution is made by C-C, which relative content varies from 72 to 78%; the carbon content in the oxygen-containing functional groups is in the range of 18-22%. Summarizing, modification of the Sibunit surface or the introduction of gold along with palladium has little effect on the contribution of the different carbon states. Additionally, not many changes in carbon surface concentration were observed for the modified samples in comparison to PdAu/Cp, since only a 1.1% reduction in surface carbon was observed for both pretreated catalysts (Table 6).  N1s spectrum was only analyzed for the modified Sibunit containing samples ( Figure  5n,o). Low surface nitrogen concentration complicated this peak interpretation for the unmodified sample. At the same time, for PdAu/Cp-NH4OH, nitrogen surface concentration was almost twofold higher than for PdAu/Cp-HNO3 ( of N into Sibunit supports during HNO3 and NH4OH treatment. Deconvolution of asymmetric N1s spectra identified that the incorporated N species were pyridinic (BE 397.8-398.4 eV), in aromatic amines -NH2 aniline and/or imines C=NH (BE 400.5 eV) or quaternary nitrogen (BE 401-402.6 eV) [56][57][58]. The calculated proportions of each state displayed in Table 9 show that both samples have predominance of aromatic amines. However, PdAu/Cp-NH4OH has higher relative contribution of aromatic nitrogen species, while PdAu/Cp-HNO3 is characterized by higher quaternary nitrogen contribution. Table 9. Effect of modification of support on contribution of different species of nitrogen calculated according to XPS. In general, from the XPS results on element surface content (Table 6), it can be concluded that bimetallic catalysts on pretreated Sibunit supports are different from the untreated catalyst, by slightly increased oxygen content and the presence of nitrogen, with the latter higher for PdAu/Cp-NH4OH. Surface concentration of Pd and Au was practically the same for all catalysts. Thus, the modification with nitric acid and ammonium hydroxide leads to the functionalization of the Sibunit surface with oxygen and nitrogen species, which can play an important role in modifying the carbon support properties. Such hetero-atoms can not only change the polarity and electronic properties of carbon materials, but also improve the reactivity of the catalysts [59][60][61][62]. Figure 6 and Table 10 show the distribution and concentration of acid-base sites on the surface of the supports and their corresponding catalysts, determined by the Hammett indicator method. The analysis of the results indicates the predominance of Brønsted acid and basic centers (BAC and BBC) on the surface of all studied samples. The concentration of Lewis acid and basic centers (LAC and LBC) did not change neither after the modification of Sibunit nor after the introduction of palladium and gold on all supports. The distribution of BAC and BBC also changed slightly when Sibunit was modified with NH4OH and/or after the deposition of a bimetallic system on Cp and Cp-NH4OH (Figure 6a,c). However, upon modification of Sibunit with HNO3 (Figure 6b), the concentration of BAC and BBC decreased by almost one half in comparison with the unmodified support. On the contrary, after the deposition of Pd and Au on Сp-HNO3, the number of these centers increased almost to the values found for PdAu/Cp and PdAu/Cp-NH4OH.  Thus, not many changes in acid-base properties were observed among the three bimetallic systems excepting the lower BBC concentration for PdAu/Cp-HNO3. This correlates with the XPS data, where both carbon and oxygen atoms from hydroxyl groups (C-OH, Tables 7 and 8) were also in lower amounts in comparison to PdAu/Cp and PdAu/Cp-NH4OH.

Catalyst
The obtained results of morphology, and the textural, structural, electronic and acidbase properties do not show much change for the studied bimetallic catalysts. However, if we compare the distinctive data on nitrogen content and relative contribution of divalent palladium and, notably, monovalent gold with selectivity to FDCA ( following correlation can be observed: the higher the nitrogen content, the larger the contribution of Pd 2+ and Au + , the active sites for the production of diacid according to some works [9,19,[63][64][65]. Namely, Chen et al. [9] found a structure-activity correlation-and demonstrated that the heteroatoms-doped supports enhance the fraction of surface Pd 2+ species, which plays a crucial role in reducing of the activation energies of both HMF conversion and FDCA formation. At the same time, the positively charged Au nanoparticles attract OH¯ ions, which results in an increase in dehydrogenation activity (last step for FDCA formation) [19,63]. Thus, the most active catalyst has the largest contribution of Pd 2+ and Au + and, most importantly, an increased amount of nitrogen heteroatoms, comparable to the gold content in the samples (≈0.3 at.%). The presence of N on the carbon surface can influence the oxidation state of active sites (Pd 2+ and Au + ). In turn, nitrogen-containing groups promoted basic properties, which also could favor diacid formation. However, a deeper study of these and new nitrogen-doped Sibunit noble metal-based catalysts is required for understanding the promoting effect of functionalization by nitrogen in detail. Moreover, analyzing the dependence of the catalytic activity on the average size of bimetallic nanoparticles, at first glance, there is no correlation between these parameters. At the same time, as shown in our previous work [24], the dependence of the catalytic activity, in particular TOF, on the average size of gold nanoparticles can be volcano type behavior, indicating the optimal size of gold nanoparticles required to achieve the highest TOF value. Analysis of the data allows suggesting that in the present study as in [66], a similar dependence of the catalytic activity on the average size of bimetallic nanoparticles can be observed. Further, the oxidation of HMF to FDCA is a structure-sensitive reaction requiring an optimal bimetallic nanoparticle size of about 4 nm.
Separately, it should be noted that the possible reasons for the observed deactivation for PdAu/Cp-HNO3 (Figure 1) is the agglomeration of nanoparticles, as well as the reduction of gold and palladium ions. Namely, acid pretreatment adversely affects the stabilization of Pd and Au NPs, which ultimately leads to a partial loss of activity. The same effect was observed by Donoeva et al. in [63]: the maximum agglomeration of gold particles was found for Au deposited on high surface area graphite treated with HNO3 vapor (Au/HSAG-ox) after 10 h of HMF oxidation reaction (from 4.3 to 10.3 nm), while the sample pretreated with gaseous NH3 at 600 °C (Au/HSAG-N) showed the least particle growth (from 3.2 to 5.9 nm). At the same time, reduction of Au + and Pd 2+ to metallic state could be the reason of the observed loss of activity due to the fact that the reverse transition of the metal to the ionic state is impossible for large nanoparticles. 4

. Conclusions
The efficiency of Ag, Au, Pd and bimetallic PdAu NPs supported on different materials (unmodified and modified titania, commercial alumina and untreated and treated Sibunit carbon support) was investigated for aerobic oxidation of 5-hydroxymethyl-2-furfural into high valuable 2,5-furandicarboxylic (FDCA) under mild conditions (T = 60 °C, 3 atm of O2, water as solvent and 2 equiv. NaOH).
The highest FDCA selectivity was obtained when the active metals were supported on Sibunit, while the order of catalyst selectivity in terms of noble metal was as follows: Ag < Au < Pd < PdAu. This is due to the low activity of silver, which cannot suppress HMF degradation to polymeric products (humins) occurring under basic conditions. The positive effect of the treatment of Sibunit with HNO3 or NH4OH for FDCA production with bimetallic PdAu nanoparticles was revealed. Thus, the highest FDCA yield in this work (76 mol%) was obtained on PdAu/Cp-NH4OH after 6 h.
The FDCA selectivity seems to be affected by the amount of nitrogen on carbon support surface, found after both treatments. The highest nitrogen content was detected on PdAu/Cp-NH4OH. These hetero-atoms can possibly influence the oxidation state of the active metals (Pd 2+ and Au + ), which are suggested to be the active sites for FDCA formation. Additionally, it was revealed that the oxidation of HMF towards FDCA is a structure-sensitive reaction, requiring an optimal PdAu NPs size of about 4 nm, obtained for PdAu/Cp-NH4OH.
Author Contributions: D.G. was responsible for preparation of supported Ag, Au, Pd, Pd-Au on Sibunit and Au/Alumina, performed catalytic tests on these materials and Hammett indicator method, interpreted XPS, XRD and TEM results and contributed to the writing; E.P. carried out catalysts preparation of Au/MexOy/TiO2, performed preliminary catalytic tests on these materials, and wrote the first draft of the paper; E.K. was responsible for methodology of catalytic tests with supported Ag, Au, Pd, Pd-Au on Sibunit materials and Au/Alumina, supervised those experiments, participated on the conceptualization and methodology of most characterization methods and contributed to the writing; S.A.C.C. was responsible for the XPS and BET analyses; M.S., A.V. and L.P. dealt with methodology of preliminary catalytic tests; M.S., A.V., L.P., S.A.C.C., N.B., V.C.C. and A.P. provided the means for the realization of this work and contributed to the supervision and paper revision. All authors read and approved the final manuscript. In this section, you should add the Institutional Review Board Statement and approval number, if relevant to your study. You might choose to exclude this statement if the study did not require ethical approval. Please note that the Editorial Office might ask you for further information. Please add "The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of NAME OF INSTI-TUTE (protocol code XXX and date of approval)." OR "Ethical review and approval were waived for this study, due to REA-SON (please provide a detailed justification)." OR "Not applicable" for studies not involving humans or animals.

Commentato [M36]:
Please add "Informed consent was obtained from all subjects involved in the study." OR "Patient consent was waived due to REASON (please provide a detailed justification)." OR "Not applicable" for studies not involving humans.