Ofelia Marilu Arias-Pinedo

Andy A Cardenas Riojas

Elena Pastor

Elvis O López

Geronimo Perez

Braulio S Archanjo

Miguel Ponce-Vargas

Gabriel Ángel Planes

Angélica María Baena-Moncada

Gabriel A Planes

Hierarchical Porous Carbon-PtPd Catalysts and Their Activity toward Oxygen Reduction Reaction

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The consumption of fossil fuels, such as reserves of oil, gas, and even easily extracted carbon [START_REF] Moriarty | Hydrogen's Role in an Uncertain Energy Future[END_REF][START_REF] Moriarty | What Is the Global Potential for Renewable Energy? Renew. Sustain[END_REF][START_REF] Patzek | A Global Coal Production Forecast with Multi-Hubbert Cycle Analysis[END_REF] has significantly raised greenhouse gas emissions causing a global average temperature increase of around 1 °C. [START_REF] Fawzy | Strategies for Mitigation of Climate Change: A Review[END_REF] Therefore, it is necessary to obtain efficient alternative sources of energy, such as fuel cells (FC), that use electrochemical processes for the conversion of chemical energy into electrical energy from a fuel oxidation reaction. [START_REF] Abdelkareem | Environmental Aspects of Fuel Cells: A Review[END_REF][START_REF] Das | Fuel cell hybrid electric vehicles: A review on power conditioning units and topologies[END_REF][START_REF] Wang | Materials, technological status, and fundamentals of PEM fuel cellsA review[END_REF] In addition, proton exchange membrane FC, direct methanol FC (DMFC), and alkaline FC have attracted increasing attention over the past decades due to their high power density and high efficiency in energy conversion. [START_REF] Acres | Recent Advances in Fuel Cell Technology and Its Applications[END_REF][START_REF] Dresselhaus | Alternative Energy Technologies[END_REF][START_REF] Lu | PtPd Porous Nanorods with Enhanced Electrocatalytic Activity and Durability for Oxygen Reduction Reaction[END_REF] However, the biggest challenge is still improving their cost and durability. A key factor in DMFC design is the choice of the catalysts employed as electrodes. [START_REF] Hou | Platinum-Group-Metal Catalysts for Proton Exchange Membrane Fuel Cells: From Catalyst Design to Electrode Structure Optimization[END_REF] Due to the catalytic properties of Pt-based nanomaterials, they are the most widely used electrocatalysts to accelerate the oxidation of the fuel at the anode and the reduction of oxygen at the cathode. [START_REF] Ren | Current Progress of Pt and Pt-Based Electrocatalysts Used for Fuel Cells[END_REF] The oxygen reduction reaction (ORR) is widely considered the most relevant reaction in FC, which must fulfill three key requirements: to be active, selective, and long lasting. [START_REF] Hou | Platinum-Group-Metal Catalysts for Proton Exchange Membrane Fuel Cells: From Catalyst Design to Electrode Structure Optimization[END_REF][START_REF] Zhang | A Review of Pt-Based Electrocatalysts for Oxygen Reduction Reaction[END_REF] However, the slow oxygen reduction kinetics at the cathode is a concern that still restricts the widespread commercialization of FC. [START_REF] Lu | PtPd Porous Nanorods with Enhanced Electrocatalytic Activity and Durability for Oxygen Reduction Reaction[END_REF] To decrease Pt load and prevent catalyst agglomeration, the noble metal is supported on porous carbon-based materials, where the support avoids the reduction of the active surface area, which is related to the catalytic performance and facilitates mass transport. [START_REF] Ren | Current Progress of Pt and Pt-Based Electrocatalysts Used for Fuel Cells[END_REF][START_REF] Li | Fabrication of Pt Nanoparticles on Nitrogen-Doped Carbon/Ni Nanofibers for Improved Hydrogen Evolution Activity[END_REF] Therefore, to optimize the catalytic activity, reduce the cost and increase the durability of Pt-based catalysts, new bi, tri, or multimetallic nano-catalysts are used. [START_REF] Lu | PtPd Porous Nanorods with Enhanced Electrocatalytic Activity and Durability for Oxygen Reduction Reaction[END_REF][START_REF] Liu | Enhanced Electrocatalytic Activity and Stability of Pd3V/C Nanoparticles with a Trace Amount of Pt Decoration for the Oxygen Reduction Reaction[END_REF][START_REF] Sun | Core/Shell Au/CuPt Nanoparticles and Their Dual Electrocatalysis for Both Reduction and Oxidation Reactions[END_REF] There is a vast amount of literature on Pd/Pt alloys due to their excellent catalytic activity, stability, and durability to be used in ORR compared to commercial Pt/C, and they can be easily synthesized in different structures and compositions. [START_REF] Ren | Current Progress of Pt and Pt-Based Electrocatalysts Used for Fuel Cells[END_REF][START_REF] Xiong | Tuning Surface Structure and Strain in Pd-Pt Core-Shell Nanocrystals for Enhanced Electrocatalytic Oxygen Reduction[END_REF] Pd/Pt nanosheets have specific mass catalytic activity 2.2 times higher than commercial Pt/C with a lower loss of mass activity (18.9 vs 50%) considering 10,000 cycles with the catalytic activity of 0.74 mA cm -2 at 0.9 V. [START_REF] Li | Carbon Nanotube-Polypyrrole Core-Shell Sponge and Its Application as Highly Compressible Supercapacitor Electrode[END_REF] Pd/Pt nanoflowers have a large surface area and lower density which provides more reaction sites reaching a specific activity of 0.70 mA cm -2 after 500 cycles with a loss of 1.6%, compared to 35.2% of the Pt/C in current density, [START_REF] Ren | Pd@Pt Core-Shell Nanoflowers as Efficient Catalyst Toward Methanol Oxidation[END_REF] and those that are supported on graphene oxide exhibit up to 70% catalytic activity (even up to 5 °C) compared to 57% of Pt/C. [START_REF] Xu | Ultra-Clean PtPd Nanoflowers Loaded on GO Supports with Enhanced Low-Temperature Electrocatalytic Activity for Fuel Cells in Harsh Environment[END_REF] Also, Pd/Pt nanocrystals supported on ionic polymer functionalized graphene exhibit improved ORR activity (specific activity: 1.89 mA cm -2 at 0.9 V RHE ) and greater durability with respect to commercial Pt/C (specific activity: 0.23 mA cm -2 at 0.9 V RHE ). [START_REF] Cho | Fine-Sized Pt Nanoparticles Dispersed on PdPt Bimetallic Nanocrystals with Non-Covalently Functionalized Graphene toward Synergistic Effects on the Oxygen Reduction Reaction[END_REF] Remarkably, Pd/Pt nanospheres supported on reduced graphene oxide (rGO-PtPd) exhibit a higher surface than Pt/C black (1.68:1), resulting in a greater specific activity of 0.54 versus 0.32 mA cm -2 . [START_REF] Esabattina | Fabrication of Bimetallic PtPd Alloy Nanospheres Supported on RGO Sheets for Superior Methanol Electro-Oxidation[END_REF] Another way to improve ORR is to support the catalysts in porous carbon, specifically, hierarchical porous carbons (HPCs). These promising materials present pores with sizes between 2 and 500 nm, with adjustable void spaces to avoid agglomeration and allow easy control of the metal catalyst load. The void spaces also promote the diffusion and migration of reactants and products inside the carbon matrix during the ORR reactions. [START_REF] Tan | Rational Design and Construction of Nanoporous Iron-and Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Li | Highly-Dispersed and High-Metal-Density Electrocatalysts on Carbon Supports for the Oxygen Reduction Reaction: From Nanoparticles to Atomic-Level Architectures[END_REF] For example, HPC materials doped with different elements such as Fe, N, and P exhibit an ORR behavior similar to commercial catalyst Pt/C, reaching current densities in the same order of magnitude, without using a noble metal. The catalytic efficiency of these materials is due to the combination of micro, meso, and macropores structures that promotes the oxygen transfer and diffusion to the active sites. [START_REF] Kone | Hierarchical Porous Carbon Doped with Iron/ Nitrogen/Sulfur for Efficient Oxygen Reduction Reaction[END_REF][START_REF] Wu | Phosphorus-Doped Hierarchical Porous Carbon as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Yasuda | Iron-Nitrogen-Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction[END_REF][START_REF] Sun | Nitrogen-Doped Hierarchically Structured Porous Carbon as a Bifunctional Electrode Material for Oxygen Reduction and Supercapacitor[END_REF] In the case of catalysts supported on carbon materials and HPC, results for Co and the bimetallic PtCo and PtPd are remarkable. These materials exhibit similar values of current densities relative to the commercial Pt/C, but with an increase in catalyst stability. [START_REF] Xue | Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three-Dimensional Ordered N-Doped Porous Carbon as an Efficient Catalyst for Oxygen Reduction Reaction[END_REF][START_REF] Pan | N Co-Doped Hierarchical Porous Carbon as Efficient Cathode Electrocatalyst and Its Impact on Microbial Community of Anode Biofilm in Microbial Fuel Cell[END_REF][START_REF] Lusi | Oxygen Reduction Reaction on Pd Nanoparticles Supported on Novel Mesoporous Carbon Materials[END_REF][START_REF] Venarusso | Carbon-Supported Metal Nanodendrites as Efficient, Stable Catalysts for the Oxygen Reduction Reaction[END_REF][START_REF] Li | Preparation of Self-Nitrogen-Doped Porous Carbon Nanofibers and Their Supported PtPd Alloy Catalysts for Oxygen Reduction Reaction[END_REF] In this work, PtPd nanoparticles supported on HPCs with different porous sizes are evaluated as ORR catalysts, given the still unclear relationships among pore dimensions, mass transport, and catalytics performance in FC cathodes. The catalyst characterization was conducted by surface and bulk techniques such as scanning electron microscopy-energydispersive X-ray spectroscopy (SEM-EDX), high-resolution transmission electron microscopy-EDX (HRTEM-EDX), Xray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), which are important tools in the characterization of PtPd composites. The influence of the surface electroactive area of PtPd nanoparticles and the HPC porous sizes in the ORR efficiency was evaluated by linear sweep voltammetry (LSV). Finally, a computational study in the framework of density functional theory (DFT) provides insights into the interaction between the bimetallic catalyst and a graphitic domain of HPC, as well as the interaction between the catalyst and the oxygen molecule.

METHODOLOGY

2.1. Synthesis of Hierarchical Porous Carbon. SiO 2 nanoparticles (SiO 2 -NPs) of different sizes (300, 400, and 500 nm) were used as templates for the synthesis of the HPCs. The synthesis of SiO 2 -NPs was carried out by the Stober method, [START_REF] Harris | The Base-Catalyzed Hydrolysis and Condensation Reactions of Dilute and Concentrated TEOS Solutions[END_REF] where a tetraethyl orthosilicate (TEOS) solution 0.4 mol L -1 in ethanolic medium (solution 1) was mixed with an ethanolic solution of ammonium hydroxide 0.4 mol L -1 and water 34 mol L -1 (solution 2). Both solutions were gently mixed at room temperature for 6 h. [START_REF] Wang | Fabrication of Twoand Three-Dimensional Silica Nanocolloidal Particle Arrays[END_REF] The obtained SiO 2 -NPs were washed with ethanol at pH ∼ 7. This process was repeated with ammonium hydroxide 1.2 and 3.04 mol L -1 in order to obtain SiO 2 -NPs of different sizes.

The resulting SiO 2 -NPs was thermally treated to form an interconnected opal at 1000 °C in the presence of oxygen. Then, SiO 2 opals were impregnated with the resorcinol/ formaldehyde resin. This resin is made up of a mixture of 1.0 g of resorcinol, 1.6 mL of formaldehyde, and 0.4 mL of 0.1 mol L -1 sodium carbonate as a catalyst. Then, a post-treatment was carried out at 100 °C. The samples were then placed in a furnace and carbonized at 850 °C under an inert atmosphere for 24 h. Finally, the SiO 2 -NP template was removed through treatment with 10% hydrofluoric acid in an ethanol/water medium (1:1). [START_REF] Baena-Moncada | A Novel Method to Produce a Hierarchical Porous Carbon as a Conductive Support of PtRu Particles. Effect on CO and Methanol Electrooxidation[END_REF] 2.2. Pt/Pd Catalysts Supported on Hierarchical Porous Carbon. For the synthesis of the PtPd/HPC catalysts, 100 mg of HPC with sizes of 300, 400, and 500 nm were weighted, and a mixture containing 348 μL of H 2 PtCl 6 and 77.33 mg of PdCl 2 in 50 mL of water was added. This mixture was left stirring for approximately 12 h. Then, the pH was adjusted to 5 by adding NaOH (final volume ∼ 70 mL) and stirring was continued for 6 h. The reduction was carried out using NaBH 4 (50 mg/50 mL of H 2 O) as the reducing agent, 1 mL was added every 3 min, and stirring was continued for about 24 h. Finally, the obtained catalyst was filtered, washed with distilled water, and dried.

2.3. Structural, Chemical, and Morphological Characterizations. The size of the synthesized silicon oxide nanoparticles was measured by the dynamic light scattering (DLS) technique with Particle Sizing Systems, Inc. The thermogravimetric Analysis-differential scanning calorimetry (TGA-DSC) study of carbon material was performed by PerkinElmer equipment, in a temperature range of 25-1000 °C in the presence of O 2 . Raman spectroscopy measurement was obtained using HORIBA Scientific equipment, with a laser length of 532 nm and a laser power of 10%.

Catalyst morphologies were obtained by SEM using a Carl Zeiss EVO 10-MA with an EDX micro-analyzer, Oxford-INCA.

Diffraction patterns were obtained by a PANalytical X'Pert diffractometer using a polychromatic X-ray of Cu Kα 1,2 and in Bragg-Brentano configuration, 2θ values ranging from 20 to 80°at a step size of 0.02°. The lattice parameter a and crystallite size were calculated from Rietveld refinements using FullProf software. The Pt/Pd crystallographic parameters (space group, atomic positions, and unit cell parameters) reported in the literature [START_REF] Darby | A Thermodynamic Study of Solid Pd-Pt Alloys[END_REF] were used to initiate the refinement of these three materials. A pseudo-Voigt profile function was used to model the Bragg reflection peak shapes. The background was manually selected using the WinPLOTR program. The maximum theoretical occupancies for Pt and Pd were fixed according to the appropriate Wyckoff positions sharing an occupancy of 1.

Analytical measurements of the functional ionic groups in the samples were made from Fourier transform infrared spectroscopy (FTIR), using a Shimadzu model IR-Prestige 21 equipment. Measurements were made by the transmission mode using the KBr powder. The measurement conditions were as follows: wavelength range between 400 and 4000 cm -1 , resolution of 4 cm -1, and scan of 150 times per sample.

XPS measurements for metal oxidation states of PtPd/HPC were carried out using a Thermo Scientific spectrometer with Al Kα 1,2 radiation. The high-energy resolution binding energy obtained at C 1s, O 1s, Pt 4f, and Pd 3d lines was 0.1 eV. Due to good material conductivity, the use of a flood gun was not necessary. In this way, the C 1s line at a binding energy of 284.7 ± 0.1 eV was used for energy calibration. For fitting the peaks of palladium and platinum, the difference between the 3d 5/2 and 3d 3/2 levels (5.26 eV) as well as that between 4f 7/2 and 4f 5/2 levels (3.33 eV) were considered.

Transmission electron microscopy (TEM) studies were conducted using a Fei Tecnai Spirit transmission electron ACS Omega microscope operating at 120 kV and a probe-corrected Fei Titan equipped with an energy-dispersive X-ray spectroscope operating at 300 kV on TEM and scanning TEM (STEM) modes. The samples were diluted in ethanol and sonicated for a while, and finally, a single drop was placed in a conventional TEM copper grid having a thin holey carbon film.

2.4. Electrochemical Characterization. The PtPd/HPC catalysts were prepared from an aqueous suspension containing 2 mg mL -1 of the catalyst in 15 μL of Nafion (5 wt %, Aldrich) and 500 μL of distilled water. An aliquot (10 μL) of the suspension was deposited on a glassy carbon disk (φ = 3 mm) in order to cover its entire surface and dried at room temperature under an Ar atmosphere.

Electrochemical measurements were carried out using an Autolab PGSTAT30 potentiostat-galvanostat, in a conventional three-electrode cell: a glassy carbon plate with a high surface area was used as an auxiliary electrode; a 3 mm diameter glassy carbon disk as the working electrode; a normal hydrogen electrode RHE as the reference electrode, and H 2 SO 4 0.5 mol L -1 is the supporting electrolyte. The electrochemical active areas of the catalysts were measured from the CO stripping by integration of the oxidation peak, assuming a load of 420 μC cm -2 . These electroactive areas were then used to calculate the current densities J (mA cm -2 ). CO was adsorbed at 0.2 V, bubbling this gas in the electrolyte for 15 min. The CO excess was removed from the solution by bubbling Ar gas for 20 min, keeping control of the potential. Finally, it was cycled between 0.05 and 1.0 V at 20 mV s -1 , and the current of the oxidation peak was integrated. The electroactive surface area (ESA) values were divided between the amount of catalyst present in the electrode according to EDX analysis in order to express them in m 2 g -1 for the sake of comparison.

Polarization curves for the electrochemical oxygen reduction were made by LSV using O 2 gas (air liquid; 99.995%) between 1 and 0.1 V at 2 mV s -1 at different rotation speeds (400, 600, 900, 1600, and 2500 rpm). Before each experiment, oxygen was bubbled for 20 minm, with H 2 SO 4 0.5 mol L -1 as the supporting electrolyte.

2.5. Computational Study. A computational study in the framework of density functional theory (DFT) has been conducted to estimate the magnitude of the interactions between the PtPd nanoparticles and a graphitic domain of the HPC, as well as the interactions between the catalyst and an oxygen molecule. The DFT calculations were carried out using the Gaussian16 software package, [START_REF] Frisch | Revision C. 01[END_REF][START_REF] Frisch | Revision a. 02[END_REF] tightening self-consistent field convergence thresholds (10 -10 a.u.). The M06-2X functional [START_REF] Zhao | The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Function[END_REF] and the 6-311G(d)/LANL2DZ basis set were employed for geometry optimizations without symmetry constraints, and the subsequent frequency calculations to ensure that the calculated geometry corresponds to energy minima. The bimetallic nanoparticle was modeled by using the most stable structure of a neutral Pt 2 Pd 2 cluster, as proposed by Alexandrova et al., [START_REF] Ha | Rutile-Deposited Pt-Pd Clusters: A Hypothesis Regarding the Stability at 50/50 Ratio[END_REF] which interacts with a graphitic domain of HPC, represented by a circumcoronene molecule (C 54 H 18 ). For the overall system, two configurations were considered that differ in the cluster atom (Pt or Pd) pointing to the oxygen molecule. The interaction energy is obtained by a fragments calculation, where the energies of selected fragments are subtracted from the overall energy. The solvent effects of water were included according to the polarizable continuum model [START_REF] Mennucci | Polarizable Continuum Model (PCM) Calculations of Solvent Effects on Optical Rotations of Chiral Molecules[END_REF][START_REF] Mennucci | Polarizable Continuum Model[END_REF][START_REF] Tomasi | Quantum Mechanical Continuum Solvation Models[END_REF] where the solute is placed in a cavity of molecular shape, and the solvent dielectric response is projected on the surface of such a cavity.

RESULTS AND DISCUSSION

SiO 2 -NPs were used as templates to obtain HPCs with different porous sizes. The synthesis mechanism consists of catalyzed hydrolysis of TEOS. The reaction starts with chemical hydrolysis and follows the condensation reaction, studied by Stober. [START_REF] Harris | The Base-Catalyzed Hydrolysis and Condensation Reactions of Dilute and Concentrated TEOS Solutions[END_REF][START_REF] Wang | Fabrication of Twoand Three-Dimensional Silica Nanocolloidal Particle Arrays[END_REF][START_REF] Baena-Moncada | Formic Acid Oxidation over Hierarchical Porous Carbon Containing PtPd Catalysts[END_REF][START_REF] Keefer | The Effect of Hydrolysis Conditions on the Structure and Growth of Silicate Polymers[END_REF] Si(OC H ) 4H OOH Si(OH) 4C H OH

+ + -V (1) Si(OH) OH SiO H O 4 2 2 ↓ + -V (2)
Figure S1 indicates the dispersion of SiO 2 -NP by the hydrodynamic diameter size and shows the average values of the SiO 2 -NP diameters (287, 371, and 425 nm, approximately). SEM images of the synthetized carbon obtained using SiO 2 -NP as the template are given in Figure S2. For the same magnification, it is observed an increment of the pore diameter as the size of the nanoparticle increases, showing an inverse replica of these templates. The hierarchical porous materials were named HPC-1, HPC-2, and HPC-3 according to SiO 2 -NP diameters 287, 371, and 425 nm, respectively.

Figure S3 presents the TGA-DSC and Raman spectroscopy analysis of the different HPCs (HPC-1, HPC-2, and HPC-3). The thermal analysis was performed in the presence of oxygen. A mass loss of HPC materials at a temperature below ∼150 °C is observed (Figure S3a,b), this mass loss can be attributed to the H 2 O molecules present in the environment and some volatile organic compounds. The DSC analysis reveals an endothermic process due to the decrease in the heat flow. Between ∼150 and 400 °C, the TGA analysis of the HPCs does not show a noticeable mass loss, except for the HPC-3. The DSC displays a small variation in endothermic heat flux at ∼350 °C. In the range of 400-600 °C, the total oxidation of the HPC occurred, and a notorious endothermic process with a large variation in heat flow due to oxidation is observed. At temperatures above 600 °C, with less than ∼5% mass of HPC remaining, an exothermic process is carried out with an increase in heat flux and ashes formation. [START_REF] Supan | Thermal Degradation of MWCNT/Polypropylene Nanocomposites: A Comparison of TGA and Laser Pulse Heating[END_REF] Figure S3c indicates the Raman measurement of HPC samples using a 532 nm laser. The intense band at 1600 cm -1 , represents the G band typical of sp 2 hybridization, and another weaker G′ band at 2695 cm -1 corresponds to the second-order of the G vibration. A D band at 1300 cm -1 was also observed along with a D′ + D band at 2900 cm -1 . The D band represents the sp 3 hybridization present in the HPC. The shifts of D and G bands (Figure S3c) with respect to the diamond and graphite crystalline structures reveal a disordered structure in the HPC samples. In addition, the weak G′ and D′ + D bands confirm the amorphous structure present in the 3D network of HPC samples. [START_REF] Dresselhaus | Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy[END_REF] The synthesis of PtPd/HPC was performed using NaBH 4 as the reducing agent, with a PtPd load of 20% in relation to the weight of carbon. By EDX analysis (Figure S4) it was determined that the three synthesized catalysts have a 1:1 ratio with respect to Pt/Pd, although the metallic load displays a variable distribution between 17 and 28.5% in relation to the weight of carbon (Table 1), being PtPd/HPC-1 the catalyst with the highest percent of 28.5%. This value is probably due to the remaining SiO 2 in HPC-1 that was dissolved during PtPd impregnation.

ACS Omega

The XRD patterns of the three samples and the Rietveld refinement results are observed in Figure 1 (right). The samples exhibit the (111), ( 200), ( 220), (311), and (222) planes that correspond to the face-centered cubic structure (FCC) of the Fm3̅ m space group. The Rietveld refinement of the XRD patterns was achieved by considering the Pt/Pd ratio obtained by EDX (Figure 1). The refinement determined that Pd 0 has a similar occupancy proportion in the Pt 0 unit cell, confirming a PtPd alloy, see Table 2. Besides, the lattice parameters of the samples were next to the unit cell of Pd 0 (a = 3.8902 Å) related to a microstrain caused by compression (ε ≈ 2.5 × 10 3 ) observed in all analyzed samples. Furthermore, it was possible to calculate the crystallite size of the materials using the Williamson-Hall method, where the results are in the order of 5 nm for all PtPd nanoparticles (PtPd-NP) (Table 2).

The synthesized PtPd/HPC samples were also characterized by XPS. Figure 2 shows the XPS spectrum at high-energy resolutions of the Pd 3d and Pt 4f lines for the 3 samples. [START_REF] Baranova | Formic Acid Electro-Oxidation on Carbon Supported PdxPt1-x (0≥ X≥ 1) Nanoparticles Synthesized via Modified Polyol Method[END_REF][START_REF] Ohashi | Electrochemical and Structural Characterization of Carbon-Supported Pt-Pd Bimetallic Electrocatalysts Prepared by Electroless Deposition[END_REF] The level of Pt 4f 7/2 revealed two adjusted peaks at ∼71.4 and ∼72.6 eV representing typical binding energies (BEs) of metallic (Pt 0 ) and II (Pt 2+ ) oxidation states, respectively. [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF][START_REF] Park | Carbon-Supported Pt-Ru Nanoparticles Prepared in Glyoxylate-Reduction System Promoting Precursor-Support Interaction[END_REF] The Pd 3d envelope also exhibits two binding energies (BEs) with Pd 3d 5/2 at ∼335.5 and ∼336.6 eV, which represent the oxidation state of Pd 0 and Pd 2+ , respectively [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF] (see Table 3). The atomic concentration relation at the surface for all elements observed by XPS spectra is presented in Table 3. The Pt 0 /Pd 0 ratio for the 3 samples shows a variation between 1.7 and 2.5 that can be attributed to the formation of oxides and other carbon species on the surface. The interface between the PtPd crystalline nanodomains and the HPC was also analyzed by XPS. Table 3 illustrates the presence of COOH-and COH-functional groups with high hydroxide contribution. [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF][START_REF] Lhoest | Combined Time-of-Flight Secondary Ion Mass Spectrometry and X-Ray Photoelectron Spectroscopy Study of the Surface Segregation of Poly (Methyl Methacrylate)(PMMA) in Bisphenol A Polycarbonate/ PMMA Blends[END_REF] The formation of Pt-OH (Pt 2+ oxidation state) and Pd-O (Pd 2+ oxidation state) [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF][START_REF] Tressaud | X-ray Photoelectron Spectroscopy of Palladium Fluorides[END_REF] suggests the possibility of OH-Pt-Pd-O-COOH chemical complexes at the surface of PtPd metallic or at the interface between PtPd and HPC structures. By FTIR, OC-OH-or OC-ions were not observed (Figure S5), which confirms the hypothesis that PtPd nanoparticles are linked with HPC through OH groups and oxides.

At the nanoscale, the three PtPd/HPC catalysts exhibit similar results with similar PtPd-NP size distribution and moderate aggregation in all samples. Figure 3a shows the TEM image of PtPd-NP on the HPC-2 sample. HRTEM (Figure 3b) images confirm the high crystallinity degree of PtPd-NP with sizes below 10 nm and an average size close to 4 nm, as given in the inset of Figure 3a. a Pt 0 lattice parameter, a = 3.9232 Å. Pd 0 lattice parameter, a = 3.8902 Å

ACS Omega

The EDX chemical analysis is presented in Figure 4. A PtPd alloy was formed, confirming the atomic concentration observed by the Rietveld refinement (Table 2), where, particles of a single element (Pt or Pd) were not found. Several particles of the HPC-2 sample were measured, and similar spectra were obtained. A representative spectrum is displayed in the inset of

ACS Omega

The CO stripping curves for the synthesized catalysts are presented in Figure 5a. The electrochemically active surface areas (ESA, expressed in m 2 g -1 ) are given in Table 4. PtPd/ HPC-3 presents the highest surface area of 49.6 m 2 g -1 , which suggests a better dispersion of PtPd-NP on the HPC-3 surface in contrast to PtPd/HPC-1 and PtPd/HPC-2 with ESA values of 38.8 and 40.2 m 2 g -1 , respectively. For ORR measurements in oxygen saturated solution of H 2 SO 4 0.5 mol L -1 , similar shapes are observed for PtPd/HPC-1 and PtPd/HPC-3, achieving a limited current at potentials more positive than PtPd/HPC-2 (Figures 5b andS10). Remarkably, materials do not follow the Koutecky-Levich approximation and both, the geometric surface area (0.0706 cm 2 ) and ESA values, are not appropriate for the calculation of the number of electrons involved in the process. [START_REF] Masa | Koutecky-Levich Analysis Applied to Nanoparticle Modified Rotating Disk Electrodes: Electrocatalysis or Misinterpretation[END_REF][START_REF] Wiberg | Levich Analysis and the Apparent Potential Dependency of the Levich B Factor[END_REF] The hierarchical nature of the carbonaceous material (see Supporting Information) makes this calculation difficult, resulting in n values (see Table S1) that are not in good agreement with the reported values of 4 electrons for PtPd catalyst. [START_REF] Cai | Effect of Heat Treatment on the Surface Structure of Pd@Pt-Ni Core-Shell Catalysts for the Oxygen Reduction Reaction[END_REF][START_REF] Tzorbatzoglou | Electrocatalytic Activity of Vulcan-XC-72 Supported Pd, Rh and PdxRhy toward HOR and ORR[END_REF][START_REF] Zhu | Facile Synthesis of Structurally Ordered Low-Pt-Loading Pd-Pt-Fe Nanoalloys with Enhanced Electrocatalytic Performance for Oxygen Reduction Reaction[END_REF] The main weakness is the lack of information about the real electroactive surface area (ESA) accessible for dissolved O 2 for a given condition. Even more, probably due to differences in carbon support pore size, the accessible surface (or ESA) depends on the support porosity.

Accordingly, in this work, for a suitable comparison, the polarization curves were represented by normalizing the current by PtPd mass (the active material), PtPd/HPC-2 being the catalyst with the highest value of -1285 A g -1 for the ORR at 1600 rpm, followed by PtPd/HPC-3 (-856 A g -1 ) and PtPd/HPC-1 (-789 A g -1 ) (Figure 5c).

In order to explain the use of PtPd mass for current normalization and the importance of the pore size, as well as the ESA, here, we assume that the ESA of PtPd catalysts is the main descriptor for catalysts performance. This hypothesis is reasonable and implies that current, as expressed in this text (A g -1 ), should be finally determined by the metal surface extension for a given mass of catalysts. Given that the ESA values (m 2 g -1 ) of PtPd catalysts follow the sequence: PtPd/ HPC-3 (49.6 m 2 g -1 ) > PtPd/HPC-2 (40.2 m 2 g -1 ) > PtPd/ HPC-1 (38.8 m 2 g -1 ), we could expect the same trend for the mass activity, that is, PtPd/HPC-3 > PtPd/HPC-2 > PtPd/ HPC-1, which is the same one we would obtain if only the pore size governed the activity. However, the results above discussed clearly show this order: PtPd/HPC-2 > PtPd/HPC-3 > PtPd/HPC-1, which, by itself, does not relieve the PtPd surface from a key role in the final result. The results interpretation, however, is not so straightforward. A tentative explanation could be given by assuming a compromise

ACS Omega

between effective diffusion lengths for dissolved oxygen in a given porous structure, and the neat PtPd catalysts load accessible in such portion of the catalytic layer. Under such a hypothesis, an improved mass transport support is also characterized by a less compact catalyst distribution.

Consequently, in such an improved mass transport support (as PtPd/HPC-3) a significant portion of catalyst particles would leave out of the active layer, diminishing the performance of the catalyst as a whole. On the other hand, in the most densely packed catalysts (i.e., PtPd/HPC-1) a sufficient number of metal nanoparticles are closer to the surface, but the diffusion length provided by the porous support is now excessively short. Additional evidence supporting this hypothesis is the unusual number of electrons that results from the RDE experiments (as stated above and shown in Table S1).

In contrast with commercial Pt/C catalysts (or others supported onto the "vulcan type" carbon nanoparticles), this work explores the use of highly porous, low-density carbon as support for PtPd nanoparticles. Contrasting with the former supports, here the primary open structure of the HPC prevents further agglomeration during film formation, assuring an open, free access porous structure.

Based on the state-of-the-art Pt or Pd based-catalysts supported on carbon materials, it is observed from Table 4 that the catalysts prepared in this work are in good agreement with the previous ones reported by other authors. A stability test was performed in the presence of oxygen by chronoamperometry at three different potentials: at the onset potential (at 0.9 V), after the onset (at 0.5 V), and in the diffusion-controlled region (at 0.1 V) (see Figure S11 in the Supporting Information) in H 2 SO 4 0.5 mol L -1 . First, a decrease in the current is observed after the application of 0.9 V from an initial potential of 1.0 V, remaining positive (anodic currents) as expected and stable after 4000 s until the 7000 s of the experiment. Cathodic currents, indicating that the reduction of oxygen takes place, are recorded at 0.5 and 0.1 V. At the former potential, after a first increase, the currents remain quite stable for PtPd/HPC-1 and PtPd/HPC-2 around -2.5 and -3.7 A g -1 , whereas some oscillations are recorded for PtPd/HPC-3 with a continuous increase in the signal (it seems that this electrode activates with the time) achieving a final value around -6.55 A g -1 . Finally, at 0.1 V, the negative currents achieve more cathodic values (final currents at 7000 s are -5.2, -8.2, and -10.6 A g -1 for PtPd/HPC-1, PtPd/HPC-2, and PtPd/HPC-3, respectively) but the behavior with the time is different. However, PtPd/HPC-1 is quite stable, the current for PtPd/HPC-2 continuously decreases while that for PtPd/HPC-3 increases. From these curves, it seems that the catalysts are quite stable without losing stability during 2 h. Accordingly, the enhancement of the catalyst stability when the catalyst was supported on porous carbons has been previously reported. [START_REF] Zhang | Facile Synthesis of N-Doped Porous Carbon Encapsulated Bimetallic PdCo as a Highly Active and Durable Electrocatalyst for Oxygen Reduction and Ethanol Oxidation[END_REF][START_REF] Carrera-Cerritos | Improved Pd Electro-Catalysis for Oxygen Reduction Reaction in Direct Methanol Fuel Cell by Reduced Graphene Oxide[END_REF][START_REF] Sun | Structural and Electrochemical Characterization of Ordered Mesoporous Carbon-Reduced Graphene Oxide Nanocomposites[END_REF][START_REF] Khan | Fabrication of Highly Stable and Efficient PtCu Alloy Nanoparticles on Highly Porous Carbon for Direct Methanol Fuel Cells[END_REF][START_REF] Varathan | Natural Aloe Vera Derived Pt Supported N-Doped Porous Carbon: A Highly Durable Cathode Catalyst of PEM Fuel Cell[END_REF] By considering a tetrahedral bimetallic cluster Pt 2 Pd 2 , according to the model proposed by Alexandrova et al., [START_REF] Ha | Rutile-Deposited Pt-Pd Clusters: A Hypothesis Regarding the Stability at 50/50 Ratio[END_REF] the interaction between the PtPd 1:1 nanoparticles with a graphitic domain of an HPC pore [START_REF] Long | In Situ Template Synthesis of Hierarchical Porous Carbon Used for High Performance Lithium-Sulfur Batteries[END_REF] and molecular oxygen was theoretically explored. Remarkably, this cluster corresponds to the minimum energy Pt/Pd tetramer calculated through a DFT approach and reflects the nanoparticle Pt/Pd ratio revealed by the EDX analysis for the three catalysts herein obtained (see Table 1). Two interaction situations were considered with molecular oxygen interacting through Pd (Figure 6a) or Pt (Figure 6b). The optimized structures of the three-fragments arrays are presented in Figure 6 where the cluster exhibits a distorted tetrahedral structure in both cases. The fragments calculation reveals a more stable situation when Pt 2 Pd 2 interacts with oxygen through Pt (Figure 6b), with more intense Pt 2 Pd 2 -C 54 H 18 (-24.8 kcal mol -1 ) and Pt 2 Pd 2 -O 2 (-142.4 kcal mol -1 ) contacts, in comparison to the Pd situation (-18.9 and -136.1 kcal mol -1 , respectively). The higher stability of the configuration favoring Pt-O 2 interactions is also evidenced by the shorter Pd-C interatomic distances involving the cluster and the circumcoronene molecule (2.53 and 3.15 Å), relative to the Pt-C distances in its counterpart (3.24 and 3.54 Å). On the other hand, the Pd-O (2.05 Å) and Pt-O (2.01 Å) distances are very similar and lower than the corresponding sum of van der Waals radii (Pt: 1.75 Å, Pd: 1.63 Å, and O: 1.52 Å) in line with a chemisorption process. These results suggest a more stable array when the catalyst is anchored to the HPC support through the palladium atoms, thus exposing the platinum atoms to the oxygen interaction that initiates ORR.

For the three synthetized catalysts, similar structural and surface characteristics are observed. All catalyst has a Pt/Pd ratio of 1:1 according to EDX, and from SEM and TEM measurements, the surface is composed of Pt 0 /Pt +2 and Pd 0 / Pd +2 (from XPS data) with a similar crystallite size of ∼5-6 nm (from XRD). Accordingly, we conclude that there are no substantial differences in PtPd-NP surface which would suggest different "inherent" catalytic activity.

Our results also indicate that there is no correlation between the neat catalyst surface and the current density obtained for all catalysts during ORR (Figure 5c and Table 4). In addition, PtPd/HPC-3 catalyst presents the highest surface/mass relationship due to the better PtPd-NP dispersion on the carbon surface (Table 4). Surprisingly, the highest surface area (m 2 g -1 ) of PtPd/HPC-3 would demand greater mass transfer, adversely affecting the observed j. The experimental data, however, reveals a relationship between the current density and the primary porous structure of the support.

The single most striking observation emerging from the data comparison is that not only the porous size of carbonaceous materials is a determinant for the ORR but also the PtPd catalysts load. Therefore, the results obtained in this work reveal that a well-developed pore structure (combination of macro-mesopores) promotes the catalyst dispersion, improving its electrochemical performance in the ORR process. This effect has been observed in our previous works where HPCs have been used to support NP of PtRu, PtPd, PtFe, and PtAu for applications in FC. [START_REF] Baena-Moncada | Formic Acid Oxidation over Hierarchical Porous Carbon Containing PtPd Catalysts[END_REF][START_REF] Baena-Moncada | Macroporous Carbon as Support for PtRu Catalysts[END_REF][START_REF] Baena-Moncada | Methanol Conversion Efficiency to CO2</Inf> on PtRu Nanoparticles Supported Catalysts, a DEMS Study[END_REF][START_REF] Cappellari | Catalytic Enhancement of Formic Acid Electro-Oxidation through Surface Modifications with Gold on Supported Pt Nanoparticles[END_REF][START_REF] Baena-Moncada | PtFe Catalysts Supported on Hierarchical Porous Carbon toward Oxygen Reduction Reaction in Microbial Fuel Cells[END_REF]

CONCLUSIONS

This work reports the synthesis of PtPd catalysts supported on HPC with different macropore sizes to be used for the study of the ORR. The CO stripping analysis shows that PtPd/HPC-3 exhibits the biggest ESA value of 49.6 m 2 g -1 , followed by PtPd/HPC-2 with 40.2 m 2 g -1 and PtPd/HPC-1 with 38.8 m 2 g -1 . For the three considered electrocatalysts, similar morphological and surface characteristics were observed by XRD, TEM, EDX, and XPS. The electrocatalysts were evaluated by electrochemical techniques such as cyclic voltammetry (CV) and linear scanning voltammetry for the ORR, obtaining the highest current density of -1285 A g -1 for the PtPd/HPC-2 catalyst in the oxygen saturated H 2 SO 4 0.5 mol L -1 solution. The computational DFT study offers a chemically intuitive picture of the preference of oxygen for the bimetallic cluster Pt atoms, as well as the more stable configuration when the cluster interacts with the HPC through the palladium atoms. The results of this study support the idea of a synergic effect between PtPd-NP and the porous supports, where the macropores on carbon material play a key role in catalyst dispersion and performance for the ORR. Consequently, PtPd-NP supported on HPCs can be considered good candidates as cathodes in FC.

■ ASSOCIATED CONTENT

* sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c01457. Physicochemical characterization of HPC supports: DLS analysis of the template, SEM analysis of HPCs, TGA-DSC, and Raman analysis; Physicochemical characterization of PtPd/HPC-X catalysts: EDX, TEM, and TEM-EDX analysis; and Electrochemical evaluation of PtPd/HPC-X during ORR: LSV curves, electrons number obtained by Koutecky-Levich equation, and stability test (PDF)

ACS Omega

Figure 1 .

 1 Figure 1. (Left) XRD patterns of (a) PtPd/HPC-1, (b) PtPd/HPC-2, and (c) PtPd/HPC-3 catalysts. (Right) Rietveld refinement using FullProf software of PtPd/HPC-1 and PtPd/HPC-3 samples.

Figure 4 .

 4 Figure 4. EDX maps were collected in areas of 20 × 20 nm 2 confirming alloy formation, where the elements Pt and Pd are homogeneously distributed (i.e., 50% Pt and 50% Pd). Other TEM and HRTEM measurements are presented in the Supporting Information, Figures S6-S9.

Figure 2 .

 2 Figure 2. High-energy resolution XPS spectra showing (a) Pd 3d and (b) Pt 4f peaks of the PtPd/HPC-1 sample, (c) Pd 3d and (d) Pt 4f peaks of the PtPd/HPC-2 sample, and (e) Pd 3d and (f) Pt 4f lines of the PtPd/HPC-3 sample.

Figure 3 .

 3 Figure 3. TEM images of (a) PtPd/HPC-2 and (b) HRTEM of PtPd-NP in HPC-2. The inset in (a) shows a representative particle diameter distribution of PtPd-NP in HPC-2 sample.

Figure 4 .

 4 Figure 4. STEM image and zoomed region where EDX maps were collected. The inset presents a representative EDX spectrum of PtPd-NP.

Figure 5 .

 5 Figure 5. a) CV during CO ad stripping, H 2 SO 4 0.5 mol L -1 ; E ad = 0.20 V; t ad = 600 s; υ = 20 mV s -1 . (b) LSVs in O 2 -saturated H 2 SO 4 0.5 mol L -1 , υ: 2 mV s -1 , at different rotary speeds (400-2500 rpm) for PtPd/HPC-3. (c) Comparison within all catalyst for ORR at ω: 1600 rpm and υ: 2 mV s -1 .

Figure 6 .

 6 Figure 6. Optimized structures of Pt 2 Pd 2 clusters interacting with a circumcoronene molecule, and oxygen through Pd (a) and Pt (b), at the M06-2X/6-311G(d)/LANL2DZ level of theory. Interatomic distances are expressed in Angstroms.

Table 1 .

 1 Pt/Pd Ratio, Pt-Pd to Carbon Mass Ratio (PtPd/ C %) by EDX Analysis

	catalyst	Pt/Pd ratio	Pt-Pd/C %
	PtPd/HPC-1	53:47	28.5
	PtPd/HPC-2	52:48	17.0
	PtPd/HPC-3	53:47	17.6

Table 2 .

 2 Microstructure Parameters of the PtPd/HPC Samples a

	sample	lattice parameter (Å) occupation crystallite size (nm)
	PtPd/HPC-1	3.904	Pt = 50%	5.4
			Pd = 50%
	PtPd/HPC-2	3.904	Pt = 50%	4.7
			Pd = 50%
	PtPd/HPC-3	3.894	Pt = 50%	5.9
			Pd = 50%

Table 3 .

 3 BE and Percentual Atomic Concentration (at. %) Obtained from XPS Data for the Three HPC Samples

		HPC-1		HPC-2		HPC-3
	level	BE (eV)	at. %	BE (eV)	at. %	BE (eV)	at. %	binding type
	C 1s	284.8	72.2	284.8	70.6	284.7	72.7	C-C/C-H x
		286.6	11.6	286.5	11.3	286.5	11.9	C-OH
		288.9	4.3	288.9	6.1	288.9	4.2	OC-OH
	O 1s	530.7	3.5	530.7	0.9	530.3	1.3	O-metal
		532.2	5.4	532.2	4.9	532.2	7.1	OH -
		533.3	0.6	533.7	4.1	533.7	2.0	OC-
	Pd 3d 5/2	335.5	0.6	335.4	0.6	335.6	0.2	Pd-Pd (Pd 0 )
		336.4	0.2	336.3	0.2	337.0	0.1	Pd-O (Pd 2+ )
	Pt 4f 7/2	71.4	1.4	71.2	1.0	71.5	0.5	Pt-Pt (Pt 0 )
		72.6	0.3	72.4	0.3	72.9	0.1	Pt-OH (Pt 2+ )

Table 4 .

 4 Reported ORR Values for Different Pt or Pd Based-Catalyst Supported on Carbon Materials during the LSV Measurements at 1600 rpm

	catalyst	catalyst description	ESA/m 2 g -1	onset potential/V E 1/2 /V	scan velocity/mV s -1	electrolyte	reference
	PtPd/f-p-CNF	PtPd alloy catalysts supported on self-nitrogen-doped	69	0.95	0.826	5	0.1 mol L -1	33
		porous carbon nanofibers					HClO 4
	PtPd/CKN	porous carbon-supported PtPd alloy nanoparticles		0.9	0.821	5	0.1 mol L -1	60
		derived from N-heterocyclic carbene bimetal complex					KOH
	Pt/HSAG300	Pt nanoparticles supported on high surface area graphite	26.69		0.8	5	0.5 mol L -1	61
		Timcal					H 2 SO 4
	Pt/OMC	Pt nanoparticles supported on ordered mesoporous	32.29		0.8	5	0.5 mol L -1	61
		carbon (OMC, ACS Materials)					H 2 SO 4
	PdCo@NPNCs PdCo bimetallic nanoparticles encapsulated on N-doped			0.914	5	0.1 mol L -1	62
		porous carbon nanocapsules					KOH
	Pt/C	commercial Pt/C (20 wt %)	46		0.89	10	0.1 mol L -1	32
							HClO 4
	Pt/DPC	Pt catalyst supported on doped porous carbon		0.9	0.83	1	0.1 mol L -1	63
		nanostructure					HClO 4
	PtPd/HPC-1	PtPd catalyst supported on HPC using SiO 2 -NP of 300 nm as the template.	38.8	0.94	0.851	2	0.5 mol L -1 H 2 SO 4	this work
	PtPd/HPC-2	PtPd catalyst supported on HPC using SiO 2 -NP of 400 nm as the template.	40.2	0.94	0.84	2	0.5 mol L -1 H 2 SO 4	this work
	PtPd/HPC-3	PtPd catalyst supported on HPC using SiO 2 -NP of 500 nm as the template.	49.6	0.92	0.845	2	0.5 mol L -1 H 2 SO 4	this work

https://doi.org/10.1021/acsomega.2c01457 ACS Omega 2022, 7, 20860-20871


Weiyudong Deng

Dario De Franceschi

Cédric Xiaoting Xu

Del Rio

Ling Shook

Zhekun Low

Robert A Zhou

Lili Spicer

Raoqiong Ren

Yimin Yang

Tian

Mengxiao Tian

Jiucheng Wu

Shuiqing Yang

Torsten Liang

Tao Wappler

Su

Low

Zhekun Zhou

Robert A Spicer

Lili Ren

Raoqiong Yang

Yimin Tian

Mengxiao Wu

Jiucheng Yang

Shuiqing Liang

Torsten Wappler
email: torsten.wappler@hlmd.de

Tao Su
email: sutao@xtbg.org.cn

Torsten Wappler

Tᴀxᴏᴅɪᴏɪᴅᴇᴀᴇ Endl Ex

K Koch

Taxodioxylon Hartig

Gothan

Plant-insect and -fungal interactions in Taxodium-like wood fossils from the Oligocene of southwestern China

Keywords: plant-herbivore interactions, wood fossil, Cupressaceae, Oligocene, climate change Stratigraphy: Lühe Formation. Geological age: Oligocene (Rupelian-Chattian boundary)

come   L'archive ouverte pluridisciplinaire

Introduction

Larvae of many arthropods, including Coleoptera, Hymenoptera, and Lepidoptera, spend their early life stage boring wood for nesting, self-protection, and food consumption [START_REF] Lieutier | Bark and Wood Boring Insects in Living Trees in Europe: a Synthesis[END_REF]. These arthropods are "architectural experts", which are capable of creating highly complex "pipeline systems" to cover all their life demands, ranging from tiny (approximately 1 mm diameter) tunnels to larger intersecting channels [START_REF] Zhang | Three-dimensional reconstruction of the galleries of several wood borer species[END_REF]. These borings cause permanent damage, affecting tree growth by compromising nutrition transport and defense strategies of the host plants, resulting in slower growth or even killing the plants [START_REF] Lieutier | Bark and Wood Boring Insects in Living Trees in Europe: a Synthesis[END_REF][START_REF] Hulcr | The sudden emergence of pathogenicity in insect-fungus symbioses threatens naive forest ecosystems[END_REF][START_REF] Villari | Progress and gaps in understanding mechanisms of ash tree resistance to emerald ash borer, a model for wood-boring insects that kill angiosperms[END_REF]. In addition to insects, decomposers like mites, springtails, and fungus also play essential roles in the later degradation of living or dead wood. Some decomposers initially parasitize wood, whereas some others are heavily influenced by the arthropod-mediated systems, growth depending on the excrements and the tunnels generated by the wood-borers [START_REF] Walter | Mites: Ecology, Evolution, and Behaviour[END_REF][START_REF] Stokland | Biodiversity in Dead Wood[END_REF]. The intricate interactions between woods, arthropods, and fungus are crucial to forest ecosystems, which help in recycling and stabilizing the basal parts of food webs [START_REF] Lonsdale | Wood-decaying fungi in the forest: conservation needs and management options[END_REF]. Some non-insect borers, like oribatid mites, had a very early origin in the Middle Devonian (Givetian; Norton et al., 1988). But it was not until the Late Devonian that the mite mediated borings were observed within Prototaxites [START_REF] Labandeira | Oribatid mites and the decomposition of plant tissues in Paleozoic coal-swamp forests[END_REF]. The mite coprolites and borings in seed ferns were found in the Late Carboniferous [START_REF] Stidd | Johnhallia lacunosa gen. et sp. n.: a new pteridosperm from the Middle Pennsylvanian of Indiana[END_REF]. These findings proved non-insect boring behavior formed before the Permian. Subsequent research has documented possible boring traces by beetles in the Middle Permian [START_REF] Naugolnykh | Possible traces of feeding by beetles in coniferophyte wood from the Kazanian of the Kama River basin[END_REF], and borings contain coprolite and fungal traces as evidence in the Late Permian [START_REF] Feng | Late Permian wood-borings reveal an intricate network of ecological relationships[END_REF][START_REF] Wei | Fungi-plant-arthropods interactions in a new conifer wood from the uppermost Permian of China reveal complex ecological relationships and trophic networks[END_REF], revealing symbiotic relationships between different arthropods and fungus in deep geological time. Borings from non-coleopteran insects are mostly reported from the Mesozoic, especially during the Cretaceous (e.g., [START_REF] Labandeira | Oribatid mites and the decomposition of plant tissues in Paleozoic coal-swamp forests[END_REF][START_REF] Labandeira | Why Did Terrestrial Insect Diversity Not Increase During the Angiosperm Radiation? Mid-Mesozoic, Plant-Associated Insect Lineages Harbor Clues[END_REF] J o u r n a l P r e -p r o o f Journal Pre-proof [START_REF] Mcloughlin | Synchrotron X-ray imaging reveals the three-dimensional architecture of beetle borings (Dekosichnus meniscatus) in Middle-Late Jurassic araucarian conifer wood from Argentina[END_REF], as numerous amber and wood fossil studies provide evidence of the upsurge of these arthropods, including wood wasps [START_REF] Ortega-Blanco | First record of anaxyelid woodwasps (Hymenoptera: Anaxyelidae) in Lower Cretaceous Spanish amber[END_REF], termites [START_REF] Francis | Termite borings in Early Cretaceous fossil wood, Isle of Wight, UK[END_REF], mayflies [START_REF] Moran | Attributes of the wood-boring trace fossil J o u r n a l P r e -p r o o f Journal Pre-proof Asthenopodichnium in the Late Cretaceous Wahweap Formation[END_REF], and soldier flies [START_REF] Han | New wood soldier flies from mid-Cretaceous Myanmar amber (Diptera, Stratiomyomorpha, Xylomyidae)[END_REF]. Compared to the other two eras, there are more damage records from angiosperm wood fossils [START_REF] Sutherland | Miocene petrified wood and associated borings and termite faecal pellets from Hukatere Peninsula, Kaipara Harbour, North Auckland, New Zealand[END_REF][START_REF] Greppi | Saproxylic arthropod borings in Nothofagoxylon woods from the Miocene of Patagonia[END_REF].

The wood fossils examined in this study were sampled from the Lühe coal mine in southwestern China. Previous research on the Lühe sediments has led to the discovery of numerous fossils, such as woods [START_REF] Yi | Late Miocene woods of Taxodiaceae from Yunnan, China[END_REF], leaves [START_REF] Ma | First evidence of Sequoia in the Miocene of Yunnan province, China[END_REF][START_REF] Linnemann | New U-Pb dates show a Paleogene origin for the modern Asian biodiversity hot spots[END_REF], and fruits [START_REF] Ding | An early Oligocene occurrence of the palaeoendemic genus Dipteronia (Sapindaceae) from Southwest China[END_REF]. Nevertheless, no fossil insects have been discovered so far from the succession. Thus, through the first Taxodium-like wood fossil and its internal boring traces, this work provides new insights into the sedimentary environment and herbivorous activities within the Oligocene Lühe flora.

Geological settings

The best known Lühe Basin sedimentary exposures are in the Lühe coal mine and Lühe town.

Lühe coal mine section (25.15°N, 101.35°E, Fig. 1A, B andC) is located near Chuxiong city, Yunnan Province, southwestern China. The second recognized outcrop in Lühe town (25.14°N,101.37°E, Fig. 1 D) is located 2.6 km southeast from the Lühe coal mine. This outcrop has yielded more leaf fossils than the coal mine section.

Based on an early geochronological study, the Lühe coal mine strata were identified as late Miocene and belonging to the Xiaolongtan Formation (Bureau of Geology and Mineral Resources of Yunnan Province, 1996). Hence a series of macro-fossil and pollen research adopted this age assignment. Subsequently, U/Pb dating of zircons within three primary volcanic ash layers in the Lühe town section gave an early Oligocene age (c. 32 ± 1 Ma, Linnemann et al., 2017). 40 Ar/ 39 Ar dating of primary ash layers from near the base of the Lühe coal mine section gave a similar age J o u r n a l P r e -p r o o f Journal Pre-proof (~33Ma, Li et al., 2020). Most of the coal mine section lies stratigraphically above the Lühe town section and the age is interpreted to span 35 to 26.5 Ma by magnetostratigraphy [START_REF] Li | Oligocene deformation of the Chuandian terrane in the SE margin of the Tibetan Plateau related to the extrusion of Indochina[END_REF]Fig. 1D). The fossil woods sampled in this study was preserved in the ~314 m lignite layer (Fig. 1E, "c2" marked), matched to the 380-410 m lithology map presented in Li et al. (2020) (Fig. 1E, "c1" marked). The result indicates the wood fossil-bearing layer is in Chron 10n near the Rupelian-Chattian boundary in the middle Oligocene.

Material and methods

The fossil wood is very brittle. The fossil surface and inner boring structures are partly coated or infilled with silica and calcite (or coarse permineralization). In contrast, freshly exposed surfaces reveal undigested wood components (attrinite over textinite, ulminite, suberinite ingredients, terms follow [START_REF] Kus | Coal petrological and xylotomical characterization of Miocene lignites and in-situ fossil tree stumps and trunks from Lusatia region, Germany: Palaeoenvironment and taphonomy assessment[END_REF], silica fillings, and carbonates (after acid treatment, Appendix A). A part of the fossil was embedded in epoxy resin before the preparation of thin sections following the usual technique used for petrography, on the three planes used for wood studies (transverse, radial longitudinal, and tangential longitudinal), mounted onto petrographic slides, and polished until the thickness allowed viewing anatomical details. They were then covered by a coverslip using epoxy. These slides were used for the wood description, studied using an Eclipse 80i Nikon microscope, and photographed using a Nikon D300 camera. Terminology of wood anatomy follows the IAWA (International Association of Wood Anatomists) Committee ( 2004).

Overviews of the damage traces and surrounding wood fragments were photographed by a Nikon D850 camera (Pictures taken by Xiaoting Xu and Weiyudong Deng). Examination of the inner boring traces was using the ZEISS Smart Zoom 5 system. Scans of the internal structure of the boring tunnels were made using by ZEISS Xradia 410 Versa system, and the 3D structures reconstruction were mediated by XM3D Viewer software (Reconstructed by Weiyudong Deng and Ting Tang). All these morphological observations were carried out in the Institutional Center for J o u r n a l P r e -p r o o f with dark contents more common in earlywood. Abrupt transition from earlywood to latewood.

Latewood with thin-to thick-walled tracheids (double wall larger than radial lumen diameter).

Radial longitudinal section -1-2 seriate bordered pits in radial walls (predominantly 2 seriate in earlywood), opposite more or less aligned vertically sometimes with crassulae separating pairs of opposite pits, spiral thickening absent. Ray cells with pits in horizontal and vertical thin walls.

Cross fields with 1-3 (up to 5) rounded oblique simple taxodioid (? but poorly preserved) pits, in 1 or 2 horizontal rows. Axial parenchyma with dark resinous contents, horizontal walls smooth or slightly nodular.

Tangential longitudinal section -Rays ( 6) 18 (28) cells high, mainly uniseriate, rarely biseriate.

Resin deposits present in axial parenchyma filling the cell lumina or forming separate bubbles.

Description of boring types

Four damage types were investigated from the fossil materials, namely LHIF 1-4 (Lühe wood ichnofossils 1-4). All borings were classified by shape, size, and embedded orientation. The potential host insects or other microorganisms were also inferred (see 5.2).

LHIF 1 (Fig. 4): These tunnels are the most common type in the fossil wood. The borings are oriented parallel, or obliquely at approximately 30-45°, to the longitudinal axis of the wood (Fig. 4F). Most of the borings are individually formed inside the wood, without terminal chambers. The length of the LHIF 1 boring type generally ranges from 15-22 mm, only a few specimens are shorter than 10 mm. The cross section varies from circular (1.2-2.2 mm in diameter) to elliptical (long axis: 1.8-2.5 mm, short axis: 1.2-1.5 mm) with reaction rim (Fig. 4B). Some of the tunnels are highly fragmented within the wood (Fig. 4A andD). Exceptionally, some tunnels were easy to separate (Fig. 4E andH), and were distinct by possessing a larger cross section (long axis: 2.0-4.0 mm, short axis: 1.2-2.2 mm), and were long (18-50 mm). The outer surface of the tunnels usually displayed regular parallel ring patterns wrapped by wood sawdust (Fig. 4A andD). Additionally, a helical architecture was also observed with ridges inclined at 30-45° to the tunnel axis (Fig. 4H).

J o u r n a l P r e -p r o o f

Journal Pre-proof LHIF 2 (Fig. 5A-I): Consists of a tunnel ending with a chamber underneath. The boring tunnel measures approximately 2-5 cm long and 0.8-2.2 mm in diameter, and is embedded perpendicular or slightly oblique to the wood longitudinal axis. The cross section of the tunnel exhibits various shapes from round, ellipse to polylobate (Fig. 5B-H; Appendix A), associated with a gradually increase in diameter from the tunnel cross section through into the chamber (Fig. 5B andC). The inner chamber was completely filled with quartz and calcite, appearing as a narrow oval or spindle shape, with a long axis of 32-55 mm, and a short axis of 5-11 mm (Fig. 5D-H).

LHIF 3 (Fig. 5J-N): This is the most complex boring type present in the fossil wood samples. This architecture comprises three elements: "I", "Y", or "L" -shaped (Fig. 5L-N) modules. The "I" module (Fig. 5L) is much shorter in length and diameter than in LHIF 1. The "Y" (Fig. 5M) module comprises three tunnels connected at 120° (Fig. 5K, with one side broken). The "L" module connects two vertically oriented tunnels, occasionally with one end being sealed (Fig. 5N). The "I", "Y", and "L" combined tunnel structures were relatively small and scattered abundantly within the fossil woods. However, unlike LHIF 1, the LHIF 3 structure can be easily detached from the wood substance. Most of the tunnels were incompletely preserved, and were 0.8-1.2 mm in diameter and shorter than 2 mm in length, making it hard to reconstruct the entire structure of the LHIF 3 type.

Every single boring has a smooth surface formed of carbonate, and quartz partially fills the inner part of the cross section. LHIF 4 (Fig. 6): The ichnofossil was present throughout the entire material of the tree base.

Here we used two samples: LHIF 4-1 (Fig. 6A) and LHIF 4-2 (Fig. 6F) to describe the traces. The wood piece of LHIF 4-1 is 11.2 × 3.8 cm long and covered with scoop-shaped and grey structures.

The majority of these structures exhibit an elongate ellipsoidal shape with 1.5-4 mm long axes, and approximately 1 mm in both depth and width (Fig. 6B andF, "s." marked). Some of the structures were preserved as cobblestone-like or ovoid shapes that were approximately 1 mm long and slightly shorter in depth/width (Fig. 6E). With few exceptions, these structures were aligned parallel to the longitudinal axis of the wood. The "Y1" side (radial section) of the LHIF 4-1 revealed the J o u r n a l P r e -p r o o f multilayered distribution of the damage structures within the tree stumps, like wafer biscuits (Fig. 6C). The "Y2" side of the LHIF 4-1 (Fig. 6D) is polyporous, with single or multiple structures situated in the spongy-like holes, some of the holes are empty. LHIF 4-2 wood piece measured 12.5 × 3.7 cm (Fig. 6F), and illustrates the anatomical structure of the ichno-traces. The "Y" side of LHIF 4-2 displayed more empty holes, and the "Z" side (tangential section) shows the wood matrix after removal of the damaged structures (Fig 6F, "Ep.1" and "Ep.2" show excised surfaces).

To uncover the inner content of tunnels, two samples (LHIF 1) were isolated from the fossil woods for CT scanning and mineralogical study (Fig. 4H; Fig. 7A;Appendix A). The CT scan revealed a quartz-filled inner structure without any frass or sawdust (Fig. 7F). The rough outer rim is clearly separated from the inner structure by a smooth and straight crystallization border, indicating multiple phases of mineralization (Fig. 7B-D). This result provides evidence that the screw-thread-shaped boring tunnel was formed before the mineralization but replaced latterly by quartz and carbonate. The second tunnel that was used for mineralogical study exhibited mainly colorless and clear crystals of α-quartz grains. Furthermore, the sample slightly bubbled during acid treatment at the base of the quartz crystals, which indicates the presence of carbonate minerals, such as calcite (Appendix A).

Discussion

Systematic affinity of the fossil wood

The studied fossil homoxylous conifer woods were partially permineralized. The predominantly uniseriate rays, the rectangular section of tracheids in transverse section, the opposite pits in radial walls, and the presence of parenchyma, all point to the material representing Cupressaceae (Taxodium, Sequoia, and Sequoiadendron) or Podocarpaceae (Retrophyllum; [START_REF] Greguss | Identification of living gymnosperms on the basis of xylotomy. Identification of living gymnosperms on the basis of xylotomy[END_REF].

J o u r n a l P r e -p r o o f

We used the IAWA Committee [START_REF] Richter | IAWA list of microscopic features for softwood identification[END_REF] code (40p 42p 45p 46p 72p 73p 74p 78p 80p 98p 104p 107p for this specimen) to interrogate the InsideWood ( 2004) database [START_REF] Wheeler | Inside Wood-A web resource for hardwood anatomy[END_REF]. These character states return the following compatible genera: Taxodium, Sequoia, and Sequoiadendron (Retrophyllum is not selected, as there is no description for this genus in the database).

Among Cupressaceae, modern Taxodium shows the most similar structure, in terms of the type of transverse end walls of the axial parenchyma cells, ray cells shape and density, and latewood cell wall thickness (characters 54 and 55), as well as the probable absence of secretory canals. The fossil description is therefore fully compatible with the extant genus Taxodium (Cupressaceae).

Regarding Retrophyllum, the wood of the species R. minor (Carrière) C.N. Page (= Podocarpus minor Parl. = "bois bouchon" NC, France) which is described and figured in [START_REF] Greguss | Identification of living gymnosperms on the basis of xylotomy. Identification of living gymnosperms on the basis of xylotomy[END_REF] and [START_REF] Carlquist | SEM studies of two riparian New-Caledonian conifers reveal air channels in stem wood; field observations[END_REF], seems also to have some similarity to the Lühe fossil. [START_REF] Carlquist | SEM studies of two riparian New-Caledonian conifers reveal air channels in stem wood; field observations[END_REF] mentions the presence of "air spaces" attesting to air channels that potentially connect roots to stems. These spaces are seen between ray cells and tracheids, but are not obvious, as checked on Retrophyllum minor wood (MNHN Wood collection / from Sorbonne University, number UPMC.239). The fossil wood presents some spaces that could correspond to the description given by [START_REF] Carlquist | SEM studies of two riparian New-Caledonian conifers reveal air channels in stem wood; field observations[END_REF], but with the current preservation, and the partly disrupted structure caused by pyrite, it is quite difficult to confirm the presence of this character in that it could be an artifact of preservation. According to the studied living sample of Retrophyllum minus, the character list for InsideWood DB for living softwood species should be 40p 43p 46p 72p 73p 74p 76p 80p 98p 103p 107p, which also corresponds to several species in Cupressaceae and Podocarpaceae, but is clearly different from the present fossil wood.

The fossil genus Taxodioxylon also fits with this description [see Hartig diagnosis (1848), and translation by [START_REF] Philippe | A key to morphogenera used for Mesozoic conifer-like woods[END_REF]]. But this genus includes many fossils from the Jurassic to Pleistocene, from diverse regions of the world, and corresponds (probably) to many extinct and living genera. According to [START_REF] Duan | Comparative chloroplast genomics of the genus Taxodium[END_REF], the concerned clade in Cupressaceae includes not J o u r n a l P r e -p r o o f only Taxodium but also Glyptostrobus, and Cryptomeria. The closest wood structure is found in Sequoia, which is at the base of Cupressaceae, together with Metasequoia, Taiwania, and Cunnighamia.

Previous studies reported the discovery of Taxodioxylon fossil woods from the Lühe coal mine section. [START_REF] Yi | Late Miocene woods of Taxodiaceae from Yunnan, China[END_REF] obtained four calcified wood specimens in the coal mine, two specimens (YNCN101 and YNCN104) were assigned to Taxodioxylon cryptomeripsoides Schonfeld 1953, and the others (YNCN102 and YNCN103) were assigned to Taxodioxylon cunninghamioides Watari 1948. There is another Taxodioxylon fossil from a Lower Cretaceous basin of Jixi, Heilongjiang Province, northeastern China, namely Taxodioxylon szei Yang and Zheng, which exhibits a striking similarity with extant Taxodium [START_REF] Yang | A new species of Taxodioxylon from the Lower Cretaceous of the Jixi Basin, eastern Heilongjiang, China[END_REF]. The distinct differences between the two samples in [START_REF] Yi | Late Miocene woods of Taxodiaceae from Yunnan, China[END_REF] are that Taxodioxylon cunninghamioides has occasional biseriate rays visible in tangential section, which is regarded as the identifying feature of modern Cuninghamia;

while Taxodioxylon cryptomeripsoides has mostly one row of bordered pits in radial walls, and rays that are 1-18 cells high. Both samples from [START_REF] Yi | Late Miocene woods of Taxodiaceae from Yunnan, China[END_REF] are preserved by calcification, which is distinctly different from the wood fossils in this study. The samples were not assigned to a particular species because the cross-field pits are not clearly observed. Previous studies have also shown Taxodium to have distinct nodular structures present on the end walls of axial parenchyma as seen in transverse section [START_REF] Dolezych | Taxonomie und Taphonomie von Koniferenhölzern und Cuticulae dispersae im 2. Lausitzer Flözhorizont (Miozän) des Senftenberger Reviers[END_REF][START_REF] Jiang | Atlas of Gymnosperms Woods of China[END_REF]. However, the "Atlas of Gymnosperms Woods of China" mentioned that "end walls of parenchyma cell smooth" in some species of Taxodium (Jiang et al., 2010, p.366), which is consistent with Taxodioxylon szei and the wood fossil in this study.

From a taphonomic perspective, the almost non-petrified (terms following Luczaj et al., 2008) fossil tree stumps were conspicuously different from the lignite surroundings. Well preserved nonpetrified woods (mummified, humified, non-mineralized, uncoalified, or others) have been found in many open-cast mines worldwide, the product of waterlogged anoxic preservation [START_REF] Erdei | The buried Miocene forest at Bükkábrány, Hungary[END_REF][START_REF] Luczaj | Comment on "Non-mineralized fossil wood[END_REF][START_REF] Mustoe | Non-mineralized fossil wood[END_REF][START_REF] Mustoe | Mineralogy of Eocene fossil wood from the "Blue Forest" locality, southwestern Wyoming, United States[END_REF][START_REF] Kus | Coal petrological and xylotomical characterization of Miocene lignites and in-situ fossil tree stumps and trunks from Lusatia region, Germany: Palaeoenvironment and taphonomy assessment[END_REF]. The lignite matrix J o u r n a l P r e -p r o o f surrounding the Lühe wood fossil is very similar to that described for the Meuro coal mine [START_REF] Kus | Coal petrological and xylotomical characterization of Miocene lignites and in-situ fossil tree stumps and trunks from Lusatia region, Germany: Palaeoenvironment and taphonomy assessment[END_REF], which was interpreted as a subtropical mixed deciduous conifer forest mire environment.

The quartz-filled boring tunnels within the wood also revealed a long-term waterlogged or highpressure environment during the sedimentation (Appendix A). Although diagenetic mineral replacement was evident, this was minimal and the largely unaltered wood, is vulnerable to modern fungus, mites, and other arthropods (Appendix A). The living habitat of two tree genera, i.e., Glyptostrobus and Taxodium, are described as "the only genera of Taxodiaceae to be amphibious" [START_REF] Kubitzki | The families and genera of vascular plants[END_REF], Volume I: Pteridophytes and Gymnosperms), consistent with the swamp-like environment inferred from the fossil setting.

In summary, we give Taxodioxylon sp., an affinity to extant Taxodiaceae (now in Cupressaceae) as it contains all listed characters of this family (Table1), the result is enough to study the plant-insect interactions. Considering the wood anatomy and inferred habitat, the Taxodioxylon sp. wood fossil in this study is most similar to extant Taxodium, but less so to Glyptostrobus. It shows little resemblance to Cryptomeria, Cunninghamia, Sequoia, Metasequoia, Taiwania, and the other listed fossil species (Table 1).

Parasitic arthropods and fungus

Modern Taxodiaceae wood hosts various consumers, including arthropods, fungus, and other decomposers [START_REF] Wood | Pests of the Native California Conifers[END_REF]Zhou et al., 2013). Arthropods ranging from Lepidoptera, Coleoptera, Hymenoptera, to Hemiptera are widely reported to exploit seeds, leaves, or the wood of Cupressaceae, but few of them are real wood borers [START_REF] Wood | Pests of the Native California Conifers[END_REF]. Lepidoptera, for example, mostly feed on needles and cones [START_REF] Chen | The species, geographic distribution and biological characteristics of pine caterpillar in China[END_REF][START_REF] Kruse | Archips goyeriana, n. sp. (Lepidoptera: Tortricidae) an important pest of baldcypress (Taxodiaceae) in Louisiana and Mississippi[END_REF], or bore into green branches [START_REF] Ohya | Rearing of the cypress bark moth, Epinotia granitalis (Butler) (Lepidoptera: Tortricidae) on artificial diets[END_REF], while wood boring families, including Cossidae [START_REF] Solomon | Frass characteristics for identifying insect borers (Lepidoptera: Cossidae and Sesiidae; Coleoptera: Cerambycidae) in living hardwoods[END_REF], Hepialidae [START_REF] Grehan | Life cycle of the wood-borer Aenetus virescens (Lepidoptera: Hepialidae). New Zeal[END_REF], and Sesiidae [START_REF] Solomon | Frass characteristics for identifying insect borers (Lepidoptera: Cossidae and Sesiidae; Coleoptera: Cerambycidae) in living hardwoods[END_REF][START_REF] Leskey | Evaluation of pheromone-based management strategies for dogwood borer (Lepidoptera: Sesiidae) in commercial apple orchards[END_REF], are seldom reported boring into conifer trees. The majority of the conifer wood borers are Coleoptera and Hymenoptera, including longhorn beetles (Cerambycidae), snout beetles (Curculionidae), jewel bugs (Buprestidae), and wood wasps (Siricidae) [START_REF] Wood | Pests of the Native California Conifers[END_REF][START_REF] Lieutier | Bark and Wood Boring Insects in Living Trees in Europe: a Synthesis[END_REF]. All these arthropods possess J o u r n a l P r e -p r o o f Journal Pre-proof different boring behaviors, which produce tunnels or chambers within the bark, sapwood, or even heartwood with a variety of architectures. Regarding the Taxodioxylon specimen described in this study, four types of damage traces have been identified, generated by arthropods and fungus (Figure 8, remarks: not all the arthropods in the figure are parasitic on Taxodiaceae, the figure only shows similar boring behaviors in arthropod genera).

For LHIF 1, there is no significant excreta (frass) and other content substances, making it difficult to identify the perpetrator definitively. The anatomical structure and the CT scan illustrate the demarcation between the tunnel mineral layer and the outer rim wall (Fig. 7B-D). The result indicates the parallel outer rim structures (Fig. 4H) represent the wall (Fig. 4H, with arrow) as originally built by arthropods. Subsequently, quartz mineralization filled the inner gallery space.

Most modern beetle and wood wasp mediated tunnels are embedded within the sapwood and are initially excavated vertically following tree rings (Fig. 8L), and then along fibers (Fig. 8B). Some extant insects produced the screw thread-like tunnel wall architecture, closely related to the tree rings (Fig. 8L) or wood fibers (Fig. 8A andB). For the oblique orientation of tunnels at 30~45° to the wood axis (Fig. 4F), a 30~45° helical arrangement of the tunnel rims was also observed, producing a screw thread appearance (Fig. 4H). There are also some exceptions where the tunnels did not follow these particular directions, but still display the screw thread structures (Fig. 8J). It is possible that secondary processes may have modified the original tunnel wall structure: the frass or sawdust caused by the original arthropods, possibly longicorn beetle or wood wasp larva, were subject to decomposition by mites, springtails, or fungus, which will result in a different gallery wall compared to that originally produced [START_REF] Fierke | Development and comparison of intensive and extensive sampling methods and preliminary within-tree population estimates of red oak borer (Coleoptera Cerambycidae) in the Ozark Mountains of Arkansas[END_REF]. Additionally, some tunnel surface structures (Fig. 4G andH) show clear modern (can be heated to charring) and mineralized (heating does not produce any changes) fungal structures, which indicates fungus had invaded the tunnels after the original bores were formed.

In regard to LHIF 1, we conclude that arthropods, attributable to the extant groups including Anobiidae, Cerambycidae, Curculionidae, Bostrichoidea, Siricidae, and Xylocopidae, are all J o u r n a l P r e -p r o o f possibly the primary insect borers [START_REF] Zhang | Three-dimensional reconstruction of the galleries of several wood borer species[END_REF]Fig. 8). These groups include parasitic genera on conifer trees, and most of them produce a similar boring tunnel size and orientation to LHIF 1 (Fig. 8A, B, G, I, andL). As most exhibit a screw thread pattern on the tunnel wall, it suggests arthropods of the Cerambycidae, like Semanotus, or Hylotrupes may be the potential borer, as their tunnels often show a similar architecture (Fig. 8A, B, andJ). Moreover, the Cerambycids may cover the pupal chambers with calcareous layers [START_REF] Kimoto | Exotic Forest Insect Guidebook[END_REF][START_REF] Greppi | Saproxylic arthropod borings in Nothofagoxylon woods from the Miocene of Patagonia[END_REF], although this is not obviously present on the fossil borings, the CT scan reveals the presence of an outer tunnel lining (possibly formed by calcareous materials or degraded organics) that is quite distinct from the inner (secondary) quartz layer (Fig. 7). Some tunnels of Sirex (Siricidae), although not showing distinct screw thread structures, are also consistent with the LHIF-1 (Fig. 8K &L).

LHIF 2 is particularly distinct in that it displays a chamber-like structure, with an interconnected tunnel (Fig. 5). The chamber is an expansion of the initial boring, which was created for pupal development. The tunnel connected to emergence hole for the adult (Fig. 5 A-I). Based on this boring feature, members of the Curculionidae, Cerambycidae, and Siricidae seem to be the most likely arthropods responsible for the borings [START_REF] Morgan | Bionomics of siricidae[END_REF][START_REF] Cope | Notes on the ecology of western Cerambycidae[END_REF]. One extant example for making a similar boring as seen in LHIF 2 is Eucryptorrhynchus (Curculionidae, Fig. 8H), where the newly hatched larva conducts their activities in the bark and later move deeper into the sapwood. The tunnels expand with the growth of the larva and end with the pupal chamber.

After the chamber is formed, the larva starts boring vertically and creates an emergence hole larger in cross section than the early-stage boring (Description translated from Zhang, 2017, Fig. 8E). The orientation of the emergence hole seen in the fossil coincides with the boring behavior of Eucryptorrhynchus (Fig. 8H). Regardless of the direction of the chamber formed (horizontal, vertical, or multi-direction) in modern wood, the emergence hole for the mature larva was always orientated vertically and situated on the top of the chamber (Fig. 5 A-I; Fig. 8E).

LHIF 3 comprises different shapes of modules that form intricate tunnel systems. These tunnels are shorter in length and smaller in diameter than LHIF 1. Furthermore, they display no J o u r n a l P r e -p r o o f specific orientation and have a smooth gallery wall. Abundant invasion holes of LHIF 3 were observed on the bark, representing a high density of arthropods present in this tree. The LHIF 3 appears to have been produced by small Coleoptera without any preference for boring orientation and the ability to build complex connected tunnels. Today cerambycid (longhorn), scolytid (bark), and buprestid (jewel or metallic wood-boring) beetles produce similar tunnels (Fig. 8A, C, D, andJ). Xylotrechus (Cerambycidae), for example (Fig. 8B andG), forms intersecting tunnels when there is a high density of insect activity in the same host plant, and track fiber direction, turning soon after the invasion hole and before the emergence hole (Fig. 8A). Xylotrechus shares similar characteristics to those of the fossils in terms of the tunnel shape, its location in the inner sapwood and abundant holes on the bark. However, most beetles, including Xylotrechus, are larger than those of the Buprestidae, and produce tunnels 1.5-3 times wider in cross section than are produced by the Buprestidae [START_REF] Zhang | Three-dimensional reconstruction of the galleries of several wood borer species[END_REF]. Moreover, the internal walls display screw thread structures, which contrast with the smooth linings of LHIF 3. Because of this, insects attributable to the Buprestidae are better candidates as producers of the fossil borings (Fig. 8D). The borings in the sapwood form a cross or bifurcate-shaped tunnels perpendicular or parallel to the long axis of the wood, and the tunnel size is comparable to those seen in LHIF-3 as most of the Buprestidae are relatively small. Because most of the tunnels are incomplete in LHIF-3, there is another possibility that these tunnels are just some branches of the main tunnel caused by Scolytinae (Fig. 8C, "M." and "S." marked; subfamily of Curculionidae).

It is difficult to determine whether LHIF 4 was caused by animal boring, or by fungal decomposition. This type most likely represents partially rotted wood (Fig. 8M) filled with carbonates during deposition. [START_REF] Genise | Fungus traces in wood: a rare bioerosional item[END_REF]2012) lists two similar ichnospecies belonging to the ichnogenus Asthenopodichnium Thenius, 1979: A. xylobiontum and A. lignorum. The ichnogenus is described as "small U-shaped structures in the wood", "pouch-like structures in wooden, organic substrates", or "as a mass of tongue filled structures, non-xylic material" [START_REF] Thenius | Lebensspuren von Ephemeropteren-Larven aus dem Jung-Tertiär des Wiener Beckens[END_REF][START_REF] Genise | Asthenopodichnium in fossil wood: Different trace makers as indicators of different terrestrial palaeoenvironments[END_REF]. These two types of ichnospecies generally follow the

J o u r n a l P r e -p r o o f

Journal Pre-proof description of Asthenopodichnium in terms of appearance, shape, and arrangement, but have distinctly different inducers and paleoenvironment backgrounds. Most studies have attributed A.

xylobiontum to typical mayfly nymphs (Ephemeroptera) boring traces [START_REF] Moran | Attributes of the wood-boring trace fossil J o u r n a l P r e -p r o o f Journal Pre-proof Asthenopodichnium in the Late Cretaceous Wahweap Formation[END_REF], likely from Polymitarcidae or Palingeniidae [START_REF] Kluge | The phylogenetic system of Ephemeroptera[END_REF]. Some have doubted that the traces were formed by fungi or animals at all [START_REF] Genise | Fungus traces in wood: a rare bioerosional item[END_REF][START_REF] Uchman | Recent freshwater wood borings from the Vistula River in Poland, 7th International Bioerosion Workshop[END_REF]. On the other hand, A. lignorum is believed to represent fungal mediated traces. [START_REF] Genise | Fungus traces in wood: a rare bioerosional item[END_REF] listed several fossil sites far from the influence of Polymitarcidae and Palingeniidae in geological time and noted inconsistencies between shapes produced by the larvae and the "U-shaped" tunnels in both fossil wood and sediments.

The architecture of LHIF 4 appears very similar to that described in the diagnosis for Asthenopodichnium, but displays some distinct characteristics in both size and color. The type of A.

xylobiontum caused by mayfly families is considered Asiatic in origin [START_REF] Genise | Asthenopodichnium in fossil wood: Different trace makers as indicators of different terrestrial palaeoenvironments[END_REF], and thus relevant to our samples. Nevertheless, the LHIF 4 ichnofossils are smaller, more rounded, and arranged more densely than A. xylobiontum. This covers the wood slide like a "wafer biscuit" (Fig. 6C). On isolating these structures, each presents with parallel stripes that may relate to wood fiber (Fig. 6F, "S." marked). Thus, compared to A. xylobiontum, the differences seen in LHIF 4 may possibly be because of the depositional condition of the wood fossil, as the samples in this study were not replaced by minerals, and unlike silica rinds, iron hydroxide, or sandstone casts, the preservation of the structures is mainly filled with quartz, calcite, or other carbonates, leading their white to grey color. Second, the borings may not have been caused by insects or other animals, but by fungus. Although some of the characteristics of LHIF 4 are also different to A. lignorum [START_REF] Genise | Asthenopodichnium in fossil wood: Different trace makers as indicators of different terrestrial palaeoenvironments[END_REF], the shapes and the arrangements are similar, and likely to be due to stem canker fungus as the structures are commonly observed in other wood fossils collected from the Lühe coal mine. The relationships between the different arthropods and fungi are not clear because of the wood preservation condition.

Paleoenvironment implication

The wood anatomy reveals strong seasonal contrasts in growth with sharp contrasts between early wood production and late wood, which in this Oligocene thermal regime at this latitude points to synchronous obligate shedding of leaves (i.e. the tree was deciduous) and the tree stump context shows it inhabited a swampy environment. Notably, the nearest living relative of the tree producing the wood lives this in kind of environment (e.g., Taxodium) and is deciduous. The paleoenvironment represented by the outcrop seems to have been a subtropical environment with a marked rainy season, forming a seasonally flooded swamp forest but with a distinct dryer season.

The inferred hosted arthropods in this study today mostly possess a wide distribution in tropical and temperate regions, constrained by the extant distribution of conifer trees. However, A. lignorum, the similar fungus related type to LHIF 4, inhabited humid to semi-humid environments, based on comparison with the modern terrestrial environment in Argentina [START_REF] Genise | Fungus traces in wood: a rare bioerosional item[END_REF][START_REF] Genise | Asthenopodichnium in fossil wood: Different trace makers as indicators of different terrestrial palaeoenvironments[END_REF]. The abundant consumer traces but low degradation condition, the quartz tunnel infill (formed after deposition), and the lignites, all indicate a waterlogged swamp-like environment during the deposition formation.

Besides the Taxodium-like wood fossil in this study, the discovery of other 4 types of Taxodiaceae (now in the Cupressaceae) fossils in the Oligocene Lühe succession [START_REF] Ma | First evidence of Sequoia in the Miocene of Yunnan province, China[END_REF][START_REF] Yi | Late Miocene woods of Taxodiaceae from Yunnan, China[END_REF], together with the Metasequoia that occurs in the Miocene Sanzhangtian and Maguan assemblages had both shown the existence of relict plants in southwestern China [START_REF] Ma | First evidence of Sequoia in the Miocene of Yunnan province, China[END_REF][START_REF] Zhang | Sequoia maguanensis, a new Miocene relative of the coast redwood, Sequoia sempervirens, from China: Implications for paleogeography and paleoclimate[END_REF][START_REF] Su | Uplift, climate and biotic changes at the Eocene-Oligocene transition in South-Eastern Tibet[END_REF]. [START_REF] Su | Uplift, climate and biotic changes at the Eocene-Oligocene transition in South-Eastern Tibet[END_REF] mentioned Metasequoia as an important forest element in much of the Northern Hemisphere after the Late Cretaceous, but subsequently retreated during the Neogene. Study of Sequoia also revealed a wide distribution during the Late Cretaceous in northern China, but is represented only by fossil records during the J o u r n a l P r e -p r o o f Journal Pre-proof Cenozoic in both the northern and eastern parts of China [START_REF] Zhang | Sequoia maguanensis, a new Miocene relative of the coast redwood, Sequoia sempervirens, from China: Implications for paleogeography and paleoclimate[END_REF]. This points to southwestern China as a refugium for the Cupressaceae during the Cenozoic. The intensification of monsoonal climate, characterized by increased seasonality of precipitation, may explain the absence of Sequoia and Metasequoia in modern forests of Yunnan [START_REF] Zhang | Sequoia maguanensis, a new Miocene relative of the coast redwood, Sequoia sempervirens, from China: Implications for paleogeography and paleoclimate[END_REF][START_REF] Su | Uplift, climate and biotic changes at the Eocene-Oligocene transition in South-Eastern Tibet[END_REF]. This is also the case for Cedrus in this region [START_REF] Su | The intensification of the East Asian winter monsoon contributed to the disappearance of Cedrus (Pinaceae) in southwestern China[END_REF].

Previous paleoenvironmental reconstructions based on macrofossil and palynological samples from the Lühe town section showed floristic differences between the early Oligocene flora and that seen there today [START_REF] Linnemann | New U-Pb dates show a Paleogene origin for the modern Asian biodiversity hot spots[END_REF]. [START_REF] Tang | Early Oligocene vegetation and climate of southwestern China inferred from palynology[END_REF] reconstructed a deciduous broadleaved mixed conifer and evergreen broad-leaved forest for Lühe in the Oligocene and suggested the vegetation was influenced by cold winter temperatures. The shift from deciduous mixed forest elements to the modern evergreen dominated forest indicates a temperature increase following the Yunnan-Guizhou Plateau uplift [START_REF] Tang | Early Oligocene vegetation and climate of southwestern China inferred from palynology[END_REF]. [START_REF] Tang | Early Oligocene vegetation and climate of southwestern China inferred from palynology[END_REF] also revealed no significant floristic changes during the Eocene-Oligocene transition (EOT), where no abrupt regional changes happened to plant community composition despite more global climatic changes recorded in deep sea sediments. For the coal mine section, biomarker analysis shows no marked cooling, which is similar to results from the town section (Lauretano et al., 2021, under review). Elsewhere in the nearby region, such as in Markam [START_REF] Su | Uplift, climate and biotic changes at the Eocene-Oligocene transition in South-Eastern Tibet[END_REF][START_REF] Deng | Sharp changes in plant diversity and plantherbivore interactions during the Eocene-Oligocene transition on the southeastern Qinghai-Tibetan Plateau[END_REF] and Jianchuan [START_REF] Wu | Paleoelevations in the Jianchuan Basin of the southeastern Tibetan Plateau based on stable isotope and pollen grain analyses[END_REF], interpretations of climate and vegetation changes across the EOT are complicated because of possible contemporaneous regional uplift. While it seems that heterogenous EOT climate changes took place worldwide, regional environment in southwestern China appears to have been relatively stable. Compared to the complex temperature differences across the region, the precipitation interpreted from most Neogene floras are strikingly similar, showing a pronounced higher annual precipitation compared to the modern environment [START_REF] Sun | Reconstructing Neogene vegetation and climates to infer tectonic uplift in western Yunnan, China[END_REF]. Most importantly, modern precipitation during the dry season is much less than was the case in the Oligocene [START_REF] Sun | Reconstructing Neogene vegetation and climates to infer tectonic uplift in western Yunnan, China[END_REF][START_REF] Tang | Early Oligocene vegetation and climate of southwestern China inferred from palynology[END_REF], which could explain the loss of the Cupressaceae and other relict plants here [START_REF] Huang | Distribution of Cenozoic plant relicts in China explained by drought in dry season[END_REF].

J o u r n a l P r e -p r o o f Journal Pre-proof

To conclude, the Taxodium-like deciduous Cupressaceae fossil wood in the coal mine, together with other reported fossil Cupressaceae genera in Yunnan Province, represent relict plants that once dominated the Northern Hemisphere during the Late Cretaceous and Paleogene. The Neogene retreat constrained their distribution in southwestern China, and the modern absence of the species in the fossil region related to further temperature and precipitation changes during the Neogene in southwestern China. From the early Oligocene onwards, colder winter temperatures led to coniferous-broad-leaved mixed forest in the region [START_REF] Tang | Early Oligocene vegetation and climate of southwestern China inferred from palynology[END_REF]. The relatively high precipitation during the growing season allowed the development of a swamp environment, guaranteeing the habitat of Taxodioxylon (Fig. 9A-F). The mass arthropod and fungi traces also prove evidence for abundant consumption and decomposition in the region. The tree base was buried within a waterlogged environment, making the main structure almost non-petrified and inner boring tunnels mineralized. The surrounding organic matter accumulated and decomposed as lignite, with higher rank sub-bituminous coals being restricted to the deeper parts of the Lühe mine (Fig. 9B). Over time, some original Cupressaceae and deciduous elements were replaced by the evergreen taxa, forming the modern evergreen broad-leaved dominated forests of the region (Fig. 9C). A tunnel connected boring chamber. "t" = tunnel, "eh" = emergence hole, "bc" = boring chamber.

Acknowledgments

(J) Multi-connection tunnels, forming a web structure in the wood. (K) Detail of photo J, "cp" = conjunction part. (L-N) Module of the multi-connection tunnels, including "I", "Y", and "L" shape. J o u r n a l P r e -p r o o f

Journal Pre-proof

  LHIF 1-4 indicates the woods represented within the Lühe floras encountered a variety of consumers both during life and afterward. Arthropods ranging from Cerambycidae, Curculionidae, Bostrichoidea, Buprestidae, and Siricidae are all possible damage inducers. At least four (Fig 4G, J o u r n a l P r e -p r o o f LHIF 4, and Appendix A) different ancient and extant fungi took part in the degradation progress.

Figure 1 (

 1 Figure 1 (A) Locality of the Lühe basin. (B) Geological context of Lühe coal mine and Lühe town

Figure 2 (Figure 3

 23 Figure2 (A-E) Different wood fossils preserved in Lühe coal mine section, A and B is embedded

  Figure 6 (A) LHIF 4-1, radial section. (B) Detail in of LHIF 4-1, calcite filled polyporous

Figure 8 (

 8 Figure 8 (A) Extant Xylotrechus grayii in Fraxinus pennsylvanica. "st" = screw thread structure.

Figure 9 (

 9 Figure 9 (A) Extent habitat of Taxodium distichum (Estimated landscape of Oligocene Lühe flora).

  Inner tunnel structure formed by quartz and calcite. (B) 10% HCL treated sample after 30 minutes reaction time, no obvious bubbles, main structure of mineral without any changes, evidencing quartz. (C) 10% HCL treated sample, some bubbles indicate the presence of calcite. (D) J o u r n a l P r e -p r o o f Journal Pre-proof Separated wood components and silica. (E-H) Modern decomposers separated out after the acid treatments (All modern samples can be burned and turned black after prolonged acid treatment). (I-L) Different types of tunnel cross sections, from mineral to sawdust formed gallery wall. I: LHIF 3, tunnel gallery with a smooth wall; J: LHIF 1, tunnel wall with regular rings; K: LHIF 1, tunnel wall with sawdust remains. L: LHIF 2, tunnels within the wood structure. (Unmarked scale bar = 1mm) J o u r n a l P r e -p r o o f

Table 1

 1 Anatomical structure comparison between different species from Cupressaceae.

Table 1

 1

We thank Vincent Rommevaux for the delicate work of wood section slide preparation; Anaïs Boura for providing original wood anatomy pictures of Retrophyllum; Hong Liu and Shuyin Li for providing extant Taxodium pictures images from South-Central University for Nationalities; Carlos D. Greppi for providing references. We thank the Institutional Center for Shared Technologies and Facilities, XTBG, for facilities accessed of micro-CT scan and technical support, particularly Ting Tang for the assistant on computerized tomography. This work is supported by the National Natural Science Foundation of China (Nos. 42072024, 41988101, 41922010); Second Tibetan Plateau

J o u r n a l P r e -p r o o f

The authors declared that they have no conflicts of interest to this work.

J o u r n a l P r e -p r o o f