F Kpeky

F Abed-Meraim
email: farid.abed-meraim@ensam.eu@www.lem3.fr

E M Daya

ANALYSIS OF PRIMAL AND DUAL VARIABLES IN STRUCTURAL SHAPE CONTROL BY PIEZOELECTRIC PATCHES USING SOLID-SHELL FINITE ELEMENTS

Keywords: Piezoelectric effect, Smart Structures, Finite elements, Solid-shell, Shape control

This paper presents an assessment of the performances of new piezoelectric solid-shell finite elements. Compared to conventional solid and shell elements, the solid-shell concept reveals to be very attractive, due to a number of well-established advantages and computational capabilities. This paper focuses on two element formulations, denoted SHB15E and SHB20E, which represent a quadratic prismatic solid-shell element and its hexahedral counterpart, respectively. The current analysis consists in an evaluation of primal and dual variables during the process of shape control of structures. The interest in this solid-shell approach is shown through a set of selective and representative plate and shell benchmark problems. The results obtained by the proposed formulations are compared with those given by state-of-the-art piezoelectric elements available in ABAQUS.

INTRODUCTION

Smart materials and structures are nowadays increasingly used in several areas of engineering. They can be found in automotive, aeronautics, aerospace, etc., where they are used for shape and vibration control [START_REF] Belouettar | Active control of nonlinear vibration of sandwich piezoelectric beams: A simplified approach[END_REF][START_REF] Nguyen | Voltage and evolutionary piezoelectric actuator design optimisation for static shape control of smart plate structures[END_REF][START_REF] Olsen | Electronic control of noise, vibration and reverberation[END_REF], or in auscultation of civil infrastructures. They also have medical applications, such as pacemakers, civil engineering usage, where they serve to monitor the state of health of civil infrastructures [START_REF] Duan | Applications of Piezoelectric Materials in Structural Health Monitoring and Repair: Selected Research Examples[END_REF][START_REF] Park | Impedance-based health monitoring of civil structural components[END_REF][START_REF] An | Piezoelectric transducers for assessing and monitoring civil infrastructures[END_REF], among many other fields. Piezoelectric materials, which represent a particular class of smart materials, have the ability to generate an electrical potential when subjected to mechanical loading. They also have the ability to deform under application of electrical potential. These properties provide them interest in several engineering domains, where they are generally used as very thin transducers, or as sensors and actuators.

The prediction of the behavior of such materials therefore becomes important for their proper implementation. One of the least costly ways to do this is via numerical modeling, and most commonly through the finite element method. Since the early works of Allik and Hughes [START_REF] Allik | Finite element method for piezoelectric vibration[END_REF], several tools have been proposed in the literature to model piezoelectric structures. A large variety of 2D and 3D piezoelectric finite elements have actually been developed [START_REF] Mindlin | Forced Thickness-Shear and Flexural Vibrations of Piezoelectric Crystal Plates[END_REF][START_REF] Plagianakos | Higher-order 2-D/3-D layerwise mechanics and finite elements for composite and sandwich composite plates with piezoelectric layers[END_REF][START_REF] Shiyekar | Higher order shear deformation effects on analysis of laminates with piezoelectric fibre reinforced composite actuators[END_REF]. Examples of these are the Classical Laminate Plate Theory (CLPT) models, the Carrera Unified Formulation (CUF) models [START_REF] Robaldo | A unified formulation for finite element analysis of piezoelectric adaptive plates[END_REF], among many others. Despite the availability of all of these models in the literature, worth noting is the lack of finite elements capable of low-cost modeling of structures combining thin and thick layers. Although, recently, authors such as Kulikov and Plotnikova [START_REF] Kulikov | Geometrically exact four-node piezoelectric solidshell element[END_REF][START_REF] Kulikov | The Use of 9-Parameter Shell Theory for Development of Exact Geometry 12-Node Quadrilateral Piezoelectric Laminated Solid-Shell Elements[END_REF], as well as Nestorović et al. [START_REF] Nestorović | Implementation of a user defined piezoelectric shell element for analysis of active structures[END_REF][START_REF] Nestorović | User defined finite element for modeling and analysis of active piezoelectric shell structures[END_REF], are stepping up to the proposal of solid-shell FE, the only alternative in commercial software packages like ABAQUS still remains the use of solid elements with refined meshes. To overcome these limitations, we have developed a family of piezoelectric finite elements using the solid-shell technology based on the works of Abed-Meraim et al. [START_REF] Abed-Meraim | New quadratic solid-shell elements and their evaluation on linear benchmark problems[END_REF][START_REF] Abed-Meraim | An improved assumed strain solid-shell element formulation with physical stabilization for geometric non-linear applications and elasticplastic stability analysis[END_REF]. These recently developed FE consist of hexahedral and prismatic solid-shell elements, using an inplane reduced-integration scheme and a special direction designated as the "thickness".

The main objective of this work is to assess the performance of the proposed elements through the calculation of primal and dual variables. To this end, the paper is organized as follows. Section 2 presents the coupled electromechanical constitutive equations as well as the discretized problem to be solved by the finite element method. Section 3 briefly outlines the formulation of the piezoelectric solid-shell elements. A set of selective and representative benchmark tests are conducted in Section 4, in order to assess the performance of the proposed piezoelectric solid-shell elements, for validation purposes. This paper is closed by Section 5, which summarizes the main contributions along with some concluding remarks.

CONSTITUTIVE EQUATIONS AND DISCRETIZATION OF THE PROBLEM

Electromechanical constitutive equations

Piezoelectric materials have the capability of generating electricity when subjected to mechanical loading (sensors). Conversely, they also have the ability to deform under electrical charging (actuators). These properties are described by the following coupled electromechanical equations:

T  = ⋅ -⋅  = ⋅ + ⋅  C e e σ ε ε E D E κ (1)
where σ and ε represent, respectively, the vector form of the stress and strain tensors; D and E denote the electric displacement and electric field vector, respectively; while C , e and κ stand for the elastic, piezoelectric and dielectric permittivity matrix, respectively.

The discretized forms { } ε and { } E for the strain tensor and the electric field vector are related, respectively, to the discretized displacement { } u and to the discretized electric potential { } φ , using the discrete gradient operators u     B and φ     B , as follows:

{ } { } { } { } u φ    =       = -     B B ε u E φ (2)
In the current contribution, the discrete gradient operators

u     B and φ    
B are obtained by finite element discretization for each of the proposed piezoelectric solid-shell formulations SHB15E and SHB20E, as will be shown in Section 3 (see [START_REF] Kpeky | Linear and quadratic solidshell finite elements SHB8PSE and SHB20E for the modeling of piezoelectric sandwich structures[END_REF][START_REF] Kpeky | New linear and quadratic piezoelectric solid-shell finite elements[END_REF] for more details).

Discretized problem

The variational principle pertaining to piezoelectric materials, which provides the governing equations for the associated boundary value problem, is described by the Hamilton principle [START_REF] Allik | Finite element method for piezoelectric vibration[END_REF]. In this weak form of equations of motion, the Lagrangian and the virtual work are appropriately adapted to include the electrical contributions, in addition to the more classical mechanical fields

v s p V V V S v s p V V S dv dv dv ds dv dv ds ρ δ δ δ δ δ δ δ δ δ - ⋅ - ⋅ + ⋅ + ⋅ + ⋅ = - ⋅ + ⋅ + ⋅ + ⋅ ∫ ∫ ∫ ∫ ∫ ∫ ∫  σ ε φ φ φ u u f u f u f u D E q q q (3)
where ρ is the material density; v q , s q and p q denote volume, surface and point charge, respectively; while v f , s f and p f represent volume, surface and point force, respectively.

The finite element discretization of the boundary value problem governed by Eq. ( 3) generally leads to the following system of discretized equations:

{ } { } { } { } { } { } { } uu uu u u φ φ φφ        + + =             + =       M K K K K  φ φ U U F U Q (4)
where all matrices and vectors involved in Eq. ( 4) are explicitly defined in Table 1.

Table 1: Explicit forms for the vectors and matrices resulting from electromechanical coupling

T uu u u V dv ρ =             ∫ M N N Mass matrix [ ] T uu u u V dv =             ∫ K B C B Stiffness matrix [ ] T V dv φφ φ φ = -             ∫ K B B κ Dielectric matrix [ ] ; T T u u u u T V dv φ φ φ φ = =                     ∫ K B e B K K Piezoelectric coupling matrix { } { } { } T T u u v s p V S dv ds = + +         ∫ ∫ F N f N f f Force vector { } { } { } T T v s p V S dv ds φ φ = - - -         ∫ ∫ Q N q N q q
Electrical charge vector

PIEZOELECTRIC SOLID-SHELL FINITE ELEMENT FORMULATIONS

The proposed quadratic piezoelectric solid-shell finite elements SHB15E and SHB20E are extensions of the quadratic prismatic and hexahedral solid-shell elements SHB15 and SHB20, which were developed in [START_REF] Abed-Meraim | New quadratic solid-shell elements and their evaluation on linear benchmark problems[END_REF], based on purely mechanical modeling. The starting point for these piezoelectric extensions is the addition of one piezoelectric degree of freedom to each node of their mechanical finite element counterparts. The outline of these formulations is given in the following sections.

Kinematics and interpolation

The piezoelectric solid-shell elements SHB15E and SHB20E denote a fifteen-node prismatic element and a twenty-node one, respectively. These elements have at each of their nodes three displacement degrees of freedom as well as one electric degree of freedom. Similar to their mechanical counterparts SHB15 and SHB20, a special direction is chosen, designated as the "thickness", normal to the mean plane of these elements. Also, an in-plane reduced-integration rule is adopted, with For the SHB15E and SHB20E elements, the spatial coordinates i x are related to the nodal coordinates iI x using quadratic shape functions, as follows:

( )

, , i iI I x x N ξ η ζ = ( 5
)
where i represents the spatial directions and ranges from 1 to 3; while I stands for the node number, which ranges from 1 to 15, for the SHB15E element, and from 1 to 20 for the SHB20E. Likewise, the displacement field i u and potential field φ are related to the nodal displacements iI u and nodal potentials I φ , respectively, using the shape functions

( ) ( ) , ,
, ,

u i iI I I I u u N N φ ξ η ζ φ φ ξ η ζ  =   =   (6)
Note that in Eqs. ( 5) and ( 6) above, the convention of implied summation over the repeated index I has been adopted.

Discrete gradient operators

For both elements SHB15E and SHB20E, the corresponding discrete gradient operators u     B and φ     B can be derived in the following compact form:

1 ,1 2 ,2 1 ,1 3 ,3 2 ,2 2 ,2 1 , 1 3 ,3 1 ,1 3 ,3 3 ,3 2 ,2
; where the expressions of T j b , , j h α and T α γ can be found in reference [START_REF] Abed-Meraim | New quadratic solid-shell elements and their evaluation on linear benchmark problems[END_REF], along with additional details on their derivation. Note again that, in Eq. [START_REF] Allik | Finite element method for piezoelectric vibration[END_REF] and in what follows, the convention of implied summation over the repeated index α is adopted, with α ranging from 1 to 11, for the SHB15E element, and from 1 to 16 for the SHB20E.

T T T T T T T T u T T T T T T T T T T T T T T T T h h h h h h h h h h h h α α α α α α φ α α α α α α α α α α α α α α α α α α   +   +     +   +     = = +   + +     +     + +     + +   0
Similar to the purely mechanics-based solid-shell elements SHB15 and SHB20 (see, e.g., [START_REF] Abed-Meraim | New quadratic solid-shell elements and their evaluation on linear benchmark problems[END_REF]), the benchmark tests performed with the piezoelectric solid-shell counterparts SHB15E and SHB20E did not reveal any particular locking and, accordingly, no specific enhanced assumed strain techniques have been applied to these quadratic solid-shell elements.

NUMERICAL TESTS AND DISCUSSIONS

In this Section, a set of representative plate and shell benchmark tests is conducted to evaluate the performance of the proposed piezoelectric solid-shell finite elements.

Shape control of square plate with piezoelectric patch models

One important advantage taken from the piezoelectric behavior is in the to the shape control of structures. In order to show the interest of solid-shell finite elements in this type of modeling, we consider a square aluminum plate of 200×200 mm 2 with a thickness of 8 mm. This plate is covered on both sides with a pair of localized PZT-5H patches, as shown in Fig. 2. Each patch has dimensions of 80×80 mm 2 with a thickness of 1 mm. The material parameters are reported in Table 2. With regard to loading conditions, the plate is subjected to a uniformly distributed load of 100 N.m -2 over its entire surface. A constant voltage is then supplied incrementally to the PZT-5H actuators, which are polarized in opposite directions, until the plate is flattened. Fig. 3 shows the magnitude of deflection of the composite plate under different applied input voltages. The results provided by the solid-shell elements SHB15E and SHB20E are reported in Tables 3 and4, where they are compared with the results given by the ABAQUS solid elements C3D15E and C3D20E. On the whole, it appears that fewer overall degrees of freedom (dof) are required for the proposed piezoelectric solidshell elements to achieve convergence, as compared to ABAQUS elements. P = 100 N.m 2 ; = 0 V P = 100 N.m 2 ; = 2 V Table 3: Primal and dual variables for the plate shape control test (P=100N.m 2 ; ϕ=0V)

C3D15E (20×20×3)×2 SHB15E (10×10×3)×2 C3D20E (20×20×3) SHB20E (10×10×3) Primal variables max (m) z U
8.111×10 -8 8.047×10 -8 8.078×10 -8 8.183×10 -8 Dual variables

min zz ε -9.965×10 -8 -1.100×10 -7 -9.920×10 -8 -1.001×10 -7 max zz ε 1.556×10 -7 1.780×10 -7 1.546×10 -7 1.832×10 -7 mn (Pa) xx σ -6.554×10 3 -6.133×10 3 -6.233×10 3 -6.752×10 3 max (Pa) xx σ 8.100×10 3 7.780×10 3 7.892×10 3 7.653×10 3 min -1 (V.m ) z E -9.229×10 1 -9.323×10 1 -9.240×10 1 -9.297×10 1 max -1 (V.m ) z E 9.229×10 1 9.323×10 1 9.240×10 1 9.292×10 2 min -2 ( ) C.m z D -2.961×10 -6 -3.094×10 -6 -2.950×10 -6 -3.026×10 -6 max -2 ( ) C.m z D 4.782×10 -6 4.713×10 -6 4.757×10 -6 4.782×10 -6
Table 4: Primal and dual variables for the plate shape control test (P=100N.m 2 ; ϕ=2V)

C3D15E (20×20×3)×2 SHB15E (10×10×3)×2 C3D20E (20×20×3) SHB20E (10×10×3) Primal variables max (m) z U 2.182×10 -
(V.m ) z E -2.315×10 3 -2.259×10 3 -2.289×10 3 -2.122×10 3 max -1 (V.m ) z E 2.284×10 3 2.274×10 3 2.277×10 3 2.181×10

Curved shell with piezoelectric patches

This benchmark test is inspired by the tests performed by Koconis et al. [START_REF] Koconis | Shape control of composite plates and shells with embedded actuators. II. Desired shape specified[END_REF][START_REF] Koconis | Shape control of composite plates and shells with embedded actuators. I. Voltages specified[END_REF], who proposed models for the shape control of shell structures. Here, we consider a simply supported curved shell, on top of which is bonded a localized PZT-5H patch with a thickness of 1 mm, as shown in Fig. 4. All other geometric parameters are also indicated in this figure .  A constant voltage 10 V φ = is then applied to the PZT-5H actuator, which is polarized in the thickness. Similar to the preview test, the results provided by the solid-shell elements SHB15E and SHB20E are compared with those given by the ABAQUS solid elements C3D15E and C3D20E. A convergence study is conducted on different variable fields in order to identify the minimum number of degrees of freedom required to achieve convergence for all variables. In Figs. 567, are presented the convergence curves and field maps for the tip displacement, the strain, and the electric flux, respectively. From this convergence analysis, it appears that fewer degrees of freedom are required for the proposed solid-shell finite elements to achieve convergence, as compared to ABAQUS elements. More specifically, less dof are required for the computation of primal variables, as compared to the computation of dual variables. Indeed, about 10,000 dof reveal sufficient to evaluate the displacement with an error margin smaller than 5%, whereas 10 times more dof are needed to obtain deformations and electric flux within the same margin of error. This test also allows us to emphasize some additional advantages of the solid-shell concept, compared to standard finite elements, especially in situations where the geometry is curved.

CONCLUSIONS

In this paper, a performance assessment of recently proposed piezoelectric solid-shell finite elements has been presented. The analysis consists in an evaluation of primal and dual variables under electromechanical loading conditions. These investigations are conducted in the context of shape control of structures. A set of selective and representative benchmark tests has been performed on plate and shell structures, on which piezoelectric patches are bonded. The obtained simulation results have been compared to their counterparts given by state-of-the-art finite elements available in ABAQUS. In all of the benchmark tests investigated, the proposed quadratic solid-shell elements have shown better performance, as compared to their ABAQUS counterparts, while systematically necessitating much less degrees of freedom for similar accuracy. It has also been revealed that particular attention should be paid to the modeling of electromechanical coupling in order to improve the efficient computation of dual variables.

  for the SHB20E (see, e.g., Fig.1, in the particular case of int 5 n = ).

Figure 1 :

 1 Figure 1: Schematic representation for the reference geometry of the SHB15E and SHB20E elements as well as for the location of their integration points in the case when the number of through-thickness integration points is int 5 n =

Figure 2 :

 2 Figure 2: Schematic representation of the square plate with piezoelectric patches meshed with SHB15E and SHB20E elements

Figure 3 :

 3 Figure 3: Shape of the plate for two different applied voltages

5 Figure 4 :

 54 Figure 4: Schematic representation of the curved shell with piezoelectric patch meshed with C3D20E

Figure 5 : 2 U

 52 Figure 5: (a) Convergence curves and (b) Field map of displacement 2 U for the actuated curved shell

Figure 6 :

 6 Figure 6: (a) Convergence curves and (b) Field map of strain 22 ε for the actuated curved shell

Figure 7 : 2 D

 72 Figure 7: (a) Convergence curves and (b) Field map of electric flux 2 D for the actuated curved shell

Table 2 :

 2 Material properties used for all simulations

		11	22			33
		12			13	23
	PZT-5H	44	55	66	2
		15	24
					2	2
		31	32			33
					8
		11	22			33


Simon Eibner

Franc ¸ois Broust
email: francois.broust@cirad.fr

Joël Blin

Anne Julbe

Catalytic effect of metal nitrate salts during pyrolysis of impregnated biomass ଝ

Keywords:

Catalytic pyrolysis is a promising way to improve bio-oil product quality. In this study, metal salts were directly impregnated in biomass to generate in situ catalysts and investigate their impact on pyrolysis products. Seven metals -Ce, Mn, Fe, Co, Ni, Cu and Zn -were selected and impregnated in eucalyptus using nitrate salts. A fixed-bed reactor, pre-heated at 500 • C and inerted with N 2 flow, was used for pyrolysis. Both gas and bio-oil compositions were analysed, paying particular attention to the production of anhydrosugars. The anhydrosugar yields were found to be strongly influenced by the presence of metal salt catalysts. In particular, both Zn and Co salts yielded more anhydrosugars in comparison with catalyst-free sample. Moreover, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2-one) was produced in higher amounts than levoglucosan which is commonly produced without any catalyst. Metals were found to remain in all chars and tended to form metal-based nanoparticles (e.g. Cu 0 , Ni 0 , ZnO) able to act as in situ catalysts during the pyrolysis process. It seems that those metal nanoparticles are closely related to LAC production. In parallel to metal cations, nitrates were also suspected to play a significant role during pyrolysis. The suspected impact of anions on levoglucosenone production is discussed. Concerning gas yields, the impregnated nitrate salts were found to strongly affect CO 2 production.

Introduction

Thermochemical processes are developed to convert lignocellulosic biomass into valuable products. Pyrolysis -thermochemical degradation of organic matter under an inert atmosphere -is investigated in order to obtain bio-oils and chars. Bio-oils contain a wide range of chemicals which can be of interest or can be used as fuels. However, bio-oils are known to be unstable over time and to be very sensitive to heat. Bio-oils also suffer from a low "lower heating value" (LHV) and a high viscosity [START_REF] Bridgwater | Review of fast pyrolysis of biomass and product upgrading[END_REF]. Thus, there is a consensus to say that catalysis is needed to improve the pyrolysis product quality. Transition metals are currently used as catalysts for the production of chemicals and fuels. For instance iron and cobalt are involved in the Fischer-Tropsch process although copper and zinc are rather used in methanol synthesis. Nickel catalysts are involved in hydrogenation/dehydrogenation processes. Different catalytic strategies were reported in the literature to improve the pyrolysis product quality. Most of the time, the catalysts are supported on alumino-silicate structures, like zeolites [START_REF] French | Catalytic pyrolysis of biomass for biofuels production[END_REF]. Catalysts can be mixed with the biomass within the reactor [START_REF] Samolada | Catalyst evaluation for catalytic biomass pyrolysis[END_REF], or placed in a fixed-bed at the outlet of the pyrolysis reactor [START_REF] Abu Bakar | Catalytic pyrolysis of rice husk for bio-oil production[END_REF], or used as a bed material in fluidised bed reactors [START_REF] Aho | Catalytic pyrolysis of woody biomass in a fluidized bed reactor: influence of the zeolite structure[END_REF]. Pyrolysis of metal salt impregnated biomass has also been considered. In this case, the catalyst is very close to the lignocellulosic structure and is supposed to act directly on the primary reactions. This approach is another promising catalytic strategy. A lot of studies reported that metals remain in chars after pyrolysis and that those metals can be recovered after char combustion [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF][START_REF] Font | Catalytic pyrolysis of almonds shells: influence of temperature and CoCl2 almond shell ratio[END_REF][START_REF] Auer | Carbons as supports for industrial precious metal catalysts[END_REF]. Thus, metal recycling from chars is easier than their recycling from alumino-silicate structures.

A large number of metal salts have been impregnated in biomass for comparing their catalytic performance. Lu et al. [START_REF] Lu | Selective fast pyrolysis of biomass impregnated with ZnCl2 to produce furfural: analytical Py-GC/MS study[END_REF] have compared the products derived either from the pyrolysis of ZnCl 2 impregnated biomass or from the pyrolysis followed by vapour catalysis with ZnCl 2 . Impregnation was found to favour the formation of water, furfural and anhydrosugars. Jakab et al. [START_REF] Jakab | Effect of slight chemical modification on the pyrolysis behavior of cellulose fibers[END_REF] nitrogen molar mass cellulose pyrolysis and found that it mainly act on anhydrosugar yields. CoCl 2 impregnation in almond shells was reported by Conesa et al. [START_REF] Conesa | Evolution of gases from the pyrolysis of modified almond shells: effects of impregnation with CoCl2[END_REF] for increasing furfural, H 2 and water yields. On the other hand, Khelfa et al. [START_REF] Khelfa | Investigations into the pyrolytic behaviour of birch wood and its main components: primary degradation mechanisms, additivity and metallic salt effects[END_REF] showed that impregnation of MnCl 2 and NiCl 2 has a negligible impact on lignin pyrolysis. Collard et al. [START_REF] Collard | Influence of impregnated metal on the pyrolysis conversion of biomass constituents[END_REF] showed that impregnation of cellulose with Fe(NO 3 ) 3 and Ni(NO 3 ) 2 catalyses both water and CO 2 production.

A wide variety of anions -chlorides [START_REF] Branca | Pyrolysis of corncobs catalyzed by zinc chloride for furfural production[END_REF][START_REF] Di Blasi | Influences of the chemicals state of alkaline compounds and the nature of alkali metal on wood pyrolysis[END_REF], nitrates [START_REF] Bru | Pyrolysis of metal impregnated biomass: an innovative catalytic way to produce gas fuel[END_REF][START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF], hydroxides [START_REF] Di Blasi | Influences of the chemicals state of alkaline compounds and the nature of alkali metal on wood pyrolysis[END_REF], carbonates [START_REF] Di Blasi | Influences of the chemicals state of alkaline compounds and the nature of alkali metal on wood pyrolysis[END_REF], acetates [START_REF] Jakab | Effect of slight chemical modification on the pyrolysis behavior of cellulose fibers[END_REF][START_REF] Di Blasi | Influences of the chemicals state of alkaline compounds and the nature of alkali metal on wood pyrolysis[END_REF], sulfates [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF][START_REF] Dobele | Application of catalysts for obtaining 1,6-anhydrosaccharides from cellulose and wood by fast pyrolysis[END_REF] and phosphates [START_REF] Dobele | Application of catalysts for obtaining 1,6-anhydrosaccharides from cellulose and wood by fast pyrolysis[END_REF] -has been considered in the literature dedicated to catalytic pyrolysis. It has been shown that anions have an impact on the pyrolysis products. Indeed, Conesa et al. [START_REF] Conesa | Evolution of gases from the pyrolysis of modified almond shells: effects of impregnation with CoCl2[END_REF] reported the formation of chloromethane when using CoCl 2 . Dobele et al. [START_REF] Dobele | Application of catalysts for obtaining 1,6-anhydrosaccharides from cellulose and wood by fast pyrolysis[END_REF] compared the role of Fe 2 (SO 4 ) 3 and Fe 3+ during cellulose pyrolysis, focusing on anhydrosugar formation. Sulfate anion seems to play the role of an acidic catalyst as it reduces levoglucosan yield while levoglucosenone -a less hydrated anhydrosugar -is favoured. Levoglucosenone was not detected in the presence of only Fe 3+ . Terakado et al. [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF] compared the performance of ZnCl 2 , Zn(NO 3 ) 2 and ZnSO 4 and revealed that nitrate salts were thermally degraded at lower temperature than sulfate and chloride salts. Moreover, Terakado and co-workers assumed that nitrates and sulfates evaporate as NO x and SO x respectively. Differences induced by the use of nitrate or sulfate salts on the primary degradation mechanisms of impregnated wood have been studied by Mayer et al. [START_REF] Mayer | Effect of sample preparation on the thermal degradation of metal-added biomass[END_REF]. However, no explanation was given to explain this difference. Some anions can also have a poisoning effect on catalyst. For instance, it has been shown that chlorides favour copper deactivation [START_REF] Twigg | Deactivation of supported copper metal catalysts for hydrogenation reactions[END_REF] upon heating.

The objective of this study was to investigate the pyrolysis of metal salt impregnated biomass. Indeed, the catalytic effect of cations on pyrolysis products remains unclear even if it seems that they promote especially anhydrosugar and gas formation. Seven metals -cerium, manganese, iron, cobalt, nickel, copper and zinc -were screened. Nitrates were chosen both to avoid catalyst poisoning and because they are easily removed at low temperatures during pyrolysis. Impregnation was carried out under vacuum, targeting constant molar contents of metals. Comparison of the pyrolysis product species and yields helped to better understand the catalytic effect of the impregnated metal salts by mainly focusing on both anhydrosugar and gas formation. The dispersion and the oxidation state of metal active sites in chars were investigated. Finally, a specific discussion is focused on nitrate evolution during pyrolysis and on its influence on product composition.

Materials and methods

Biomass and chemicals

Eucalyptus was selected for this study because of its low ash content which reduces potential undesirable catalytic effects during pyrolysis. Moreover eucalyptus is a fast-growing tree whose wood is commonly used as energy feedstock [START_REF] Van Den Broek | The energy crop growth model SILVA: description and application to eucalyptus plantations in Nicaragua[END_REF][START_REF] Buchholz | Modeling the profitability of power production from short-rotation woody crops in sub-Saharan Africa[END_REF]. Eucalyptus logs were purchased. Bark was removed. Logs were cut, grinded and sieved between 0.5 mm and 2 mm. Biomass was dried at least for 24 h at 105 • C before each experiment. The ultimate analysis of eucalyptus on a dry ash-free basis gave the following composition: C 47.9 wt.%; O 45.7 wt.%; H 6.1 wt.% and N 0.3 wt.%. The oxygen content was calculated by difference.

Ammonia, nitric acid and nitrate salts -

Zn(NO 3 ) 2 •6H 2 O, Co(NO 3 ) 2 •6H 2 O, Fe(NO 3 ) 3 •9H 2 O, Ni(NO 3 ) 2 •6H 2 O, Ce(NO 3 ) 3 •6H 2 O, Mn(NO 3 ) 2 •4H 2 O and Cu(NO 3 ) 2 •3H 2 O -
were purchased from Sigma Aldrich. Salt purity was at least 97%.

Metal salt impregnation under vacuum

Two adsorption-impregnation methods -either at atmospheric pressure or under vacuum -have been compared in our team and published by Richardson et al. [START_REF] Richardson | Catalytic investigation of in-situ generated Ni metal nanoparticles for tar conversion during biomass pyrolysis[END_REF]. Impregnation under vacuum was selected in the present work since it was found to reduce both the required solution volume and operation duration. 38 g of dried biomass were immersed in 205 mL of nitrate solution. Some salts such as zinc and copper nitrates were more easily impregnated than iron nitrate. Thus, calibration curves were previously determined for each solution in order to target 0.65 mmol of cations per gram of dried biomass. The corresponding concentrations of the impregnation solutions are reported in Table 1. The pH was considered as a key parameter as it acts on the biomass surface charges which are closely linked to the impregnation efficiency. In addition, a too low pH might degrade hemicelluloses and reduces the mass of organic matter to be valorised. Finally, low pH enhances the removal of ashes in biomass [START_REF] Collard | Influence of impregnated metal on the pyrolysis conversion of biomass constituents[END_REF], thus reducing at the same time undesirable catalytic effects. As a compromise, the pH was set at 1.9 by adding diluted nitric acid and/or ammonia. Biomass was maintained in the solution by a stainless grid, it was immersed in the salt solution during 25 min at 25 mbar and then 2 h at atmospheric pressure. After that, biomass was filtered and washed with 205 mL of demineralised water to remove unadsorbed cations. Then, the impregnated biomass was dried at 105 • C for at least 72 h. Metal concentrations were checked by ICP-MS.

A catalyst-free sample was also prepared for comparison purposes: biomass was immersed in 205 mL of demineralised water whose pH was adjusted at 1.9. The above-described protocol was also applied to this sample.

Pyrolysis reactor

Pyrolysis experiments were performed at 500 • C in a tubular fixed bed reactor already described by Collard et al. [START_REF] Collard | Influence of impregnated metal on the pyrolysis conversion of biomass constituents[END_REF] (Fig. 1). The carrier gas was N 2 at a flow rate of 41.7 NL/h. Oxygen content was lower than 0.1 mol% during pyrolysis. Biomass samples, first kept in a cooled chamber during pre-heating, were inserted for 12 min in the reactor with a stainless steel boat (length: 8 cm, mesh size: 200 m) connected to a stem. A resistive heater was placed at the end of the reactor to avoid early pyrolysis vapour condensation. The condensation train was composed of a condenser linked to a spherical flask immersed in a cooling bath connected to an electrostatic filter. Bio-oils were recovered in acetone. The temperature of the coolant was -15 • C. Both a glass ball trap and a silica gel trap were used to recover the remaining light pyrolysis vapours. The molecules adsorbed on the silica gel could not be recovered. Thus, the molecules in the glass ball trap were dissolved in acetone before GC-MS analysis and the composition was extrapolated to the silica gel trap. All the samples were stored at 4 • C in the dark in order to limit bio-oil ageing. Uncondensed gases were trapped in a sampling bag during pyrolysis and were analysed with a GC at the end of the experiment. Chars were recovered after cooling and were placed in a flask previously inerted with nitrogen before being weighed. Pyrolysis experiments were carried out at least twice in order to calculate mean values and to insure repeatability. The error bar extremities represent the minimum and maximum measured values.

Analysis

GC

After being collected in a sampling bag, gases were analysed with a GC (Agilent Varian CP-4900). N 2 , O 2 , H 2 , CO, CO 2 , CH 4 , C 2 H 4 and C 2 H 6 were analysed on two columns: Molsieve 5A and Poraplot Q. At least, five measurements were performed for each experiment in order to obtain average values. For each experiment, a good repeatability was obtained (error <0.1%). Gas mass could be calculated by comparison with nitrogen mass which was known.

Bio-oil analysis: GC-MS and Karl-Fischer

Bio-oils were analysed by both GC-MS and Karl-Fischer titration. The water yields in liquid samples were measured by Karl Fischer volumetric titration (Mettler Toledo). Comprehensive details on GC-MS analysis procedure have been previously published [START_REF] Collard | Influence of impregnated metal on the pyrolysis conversion of biomass constituents[END_REF][START_REF] Remón | Production of a hydrogen-rich gas from fast pyrolysis biooils: comparison between homogeneous and catalytic steam reforming routes[END_REF]. Bio-oils were recovered from the condenser in acetone. 2 mL samples of diluted bio-oils were filtered at 0.45 m. 1 mL of solution was placed in a vial. 0.1 mL of an internal standard solution containing deuterated phenol, toluene and phenantrene -was added to the vial. Samples were analysed by gas chromatography (Agilent 6890 -GC column: DB-1701 (14%-cyanopropyl-phenyl)-methylpolysiloxane) coupled with mass spectrum detector (Agilent 5975). Molecules were identified by comparison with the NIST database. About 70 molecules have been previously calibrated: the response factors were calculated for each of the related peaks. A solution containing known amounts of LAC and DGP, provided by the University of Bologna, was used for calibration purposes.

A typical chromatogram with labelled peaks is shown in Fig. 2. Table 2 presents a short list of the main molecules which were previously calibrated.

ICP-MS and ultimate analysis

Inductively coupled plasma mass spectroscopy and ultimate analysis were performed on the pyrolysis products to quantify their carbon, hydrogen, nitrogen and inorganic species contents.

For ICP-MS, about 500 mg of sample were required. The sample was ground at 500 m and immersed for 3 h in a boiling solution of aqua regia (hydrogen chloride and nitric acid, volumetric ratio 14:5). Solutions were filtered and diluted. Amounts of inorganic species were measured by ICP-MS.

For ultimate analysis, samples were finally burned at 960 • C in oxygen. Both CO 2 and H 2 O were trapped on a molecular sieve and NO x were reduced to N 2 thanks to a W-based catalyst. N 2 amounts were measured by a thermal conductivity detector (TCD). CO 2 and H 2 O were alternately desorbed to be quantified.

Knowing carbon, nitrogen, hydrogen and inorganic amounts in the samples, the oxygen content was calculated by difference.

Char analysis

Chars were analysed by TEM, XRD and XPS. For TEM analyses (TEM 1200EX2, 100 kV) the chars were ground and poured in a resin in order to be sliced (70 nm thick). Ground chars were also used for XRD analyses which were performed on a Philips X'Pert diffractometer using a copper radiation source (E K␣ (Cu) = 8050.9 eV). XPS analyses were carried out on 700 m × 300 m surfaces, using a Kratos Axis Ultra DLD spectrometer [START_REF] Auer | Carbons as supports for industrial precious metal catalysts[END_REF] (E K␣ (Al) = 1486.6 eV) and spectra were recorded with a take-off angle of 90 • .

Expression of product yields on a "catalyst-free basis"

In order to be able to compare the results, product yields have to be calculated on a "catalyst-free basis", which means that the mass of metals and nitrates (or nitrate derivatives) must be subtracted from the mass of impregnated biomass and related chars. The present subsection describes the assumptions and calculations made, respectively for metals and nitrates. Biomass samples were impregnated with the aim of inserting constant molar quantity of cations. It can be noticed that impregnation efficiency depends on the nature of the salt. Indeed, in spite of precautions taken during impregnation the iron, copper, cerium and cobalt contents in biomass measured by ICP-MS were lower than expected (Table 3). These discrepancies confirm that it is preferable to make systematic corrections, from the actual ICP-MS analyses. Additional, ICP-MS measurements (not reported) revealed that more than 95% of the metal remained in the chars after pyrolysis. Thus the mass of impregnated metal was finally deducted from both the mass of biomass and char.

High nitrogen contents -revealed by ultimate analyses -proved that nitrates were partially impregnated in biomass. However, the highest nitrate amounts were detected in biomass impregnated with trivalent salts -iron and cerium -while very low nitrate amount was impregnated in the copper-based sample. Interestingly, elemental analyses also revealed higher nitrogen amounts in impregnated chars in comparison with metal-free samples, which can be attributed to the presence of nitrate-derivatives in char. However, their actual form is not exactly known. The thermal degradation of metal nitrates in inert atmosphere has been broadly studied in the literature [START_REF] Strydom | The thermal decomposition of cerium(III) nitrate[END_REF][START_REF] Brockner | Thermal decomposition of nickel nitrate hexahydrate, Ni(NO3)2•6H2O, in comparison to Co(NO3)2•6H2O and Ca(NO3)2•4H2O[END_REF][START_REF] Ehrhardt | Thermal decomposition of cobalt nitrato compounds: preparation of anhydrous cobalt(II)nitrate and its characterisation by infrared and Raman spectra[END_REF][START_REF] Morozov | Thermal decomposition of Cu(NO3)2•3H2O at reduced pressures[END_REF][START_REF] Nikolic | Physico-chemical characterization of thermal decomposition course in zinc nitrate-copper nitrate hexahydrates[END_REF][START_REF] Wieczorek-Ciurowa | The thermal decomposition of Fe(NO3)3•9H2O[END_REF][START_REF] Maneva | The thermal dehydration, decomposition and kinetics of Mn(NO3)2•6H2O and its deuterated analogue[END_REF] and it was found that nitrates were removed as NO x between 150 and 300 • C. Similarly to the thermal decomposition of nitrocellulose -cellulose doped with nitric acid -it might be assumed that those NO x are mainly NO 2 [START_REF] Chin | Investigation of the decomposition mechanism and thermal stability of nitrocellulose/nitroglycerine based propellants by electron spin resonance[END_REF]. Since carbon-based materials are efficient adsorbents [START_REF] Serp | Carbon Materials for Catalysis[END_REF], the extra-nitrogen measured in chars might therefore originate from the partial adsorption of NO 2 on the carbonaceous material [START_REF] Jeguirim | Interaction mechanism of NO2 with carbon black: effect of surface oxygen complexes[END_REF] (Table 3). This assumption will be further discussed in Section 4.3.

Based on elemental analyses, the amount of impregnated nitrates and adsorbed NO 2 were calculated as follows (cf. Nomenclature):

%(NO 3 bio ) = (%(N bio ) -%(N cata-free bio )) × M NO 3 M N (1) %(NO 2 ads ) = (%(N char ) -%(N cata-free char )) × M NO 2 M N (2)
Finally, product yields were calculated on a catalyst-free basis as follows: Y (organic fraction) = m organic fraction

Y (gas) = m gas m bio × [1 -%(NO 3 bio ) -%(cation bio )] (3)
m bio × [1 -%(NO 3 bio ) -%(cation bio )] (4)
Y (water) = m water m bio × [1 -%(NO 3 bio ) -%(cation bio )] (5)
Y (chars) = m char × [1 -%(NO 2 ads )] m bio × [1 -%(NO 3 bio ) -%(cation bio )] (6)
The molar yields of each chemical species "a" -referring to CO 2 , CO, H 2 , CH 4 , C 2 H 4 or C 2 H 6 -were calculated, on a catalyst-free basis, as follows:

X(a) = m a m bio × [1 -%(NO 3 bio ) -%(cation bio )] × 1 M a ( 7
)
3. Results

Pyrolysis products derived from metal nitrate impregnated biomass

In this section, pyrolysis yields are presented and expressed on a "catalyst-free basis". As far as nitrate salts were supposed to catalyse mainly anhydrosugar and gas formation, the corresponding yields were thus discussed in more details.

Pyrolysis yields from impregnated biomass

Product yields -calculated on a "catalyst-free basis" -are reported in Fig. 3. Mass closure was higher than 94% for all experiments, thus confirming that major part of pyrolysis products was recovered. Metals were found to enhance char formation: by +20% on average, in comparison with a catalyst-free sample. Char yields were higher with cerium, manganese and zinc in comparison with nickel and copper. It was also noted that gas formation was significantly increased with metals: gas yield increased from +25% for Zn-sample, up to +55% for Ni-sample, in comparison with a catalyst-free sample. Interestingly, the lowest organic fraction yield and highest gas yield were obtained with nickel. This demonstrated that nickel is an active catalyst for cracking reactions. Water production was only slightly favoured by metals except in the case of copper (yielding a slightly lower water content).

Catalytic effects of metal salts on bio-oil compositions

Bio-oils are complex mixtures of organic molecules and water. 70 organic species -such as anhydrosugars, acids, furans, ketones, alcohols and phenol derivatives -were quantified in bio-oils; they represent more than 40% of water-free bio-oil, in agreement with typical bio-oil analysis [START_REF] Bayerbach | Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin). Part IV: Structure elucidation of oligomeric molecules[END_REF]. High yields of anhydrosugars, acids, furans, aldehydes and ketons were obtained whereas few phenol derivatives -phenols, guaiacols and syringols -were detected. Metals were found to have a huge catalytic effect on anhydrosugar yields.

Levoglucosan, LAC, DGP and levoglucosenone are the most classically reported anhydrosugars [START_REF] Furneaux | A novel hydroxylactone from the Lewis acid-catalyzed pyrolysis of cellulose[END_REF][START_REF] Zhu | Production of chemicals from selective fast pyrolysis of biomass[END_REF][START_REF] Fabbri | Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: an analytical study[END_REF] (Fig. 4). LAC -which is not in the NIST list -was identified by using the spectra published by Fabbri et al. [START_REF] Fabbri | Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: an analytical study[END_REF] High levoglucosan yield was obtained in the bio-oil derived from catalyst-free biomass (Fig. 5). Iron, zinc, cobalt, cerium, nickel and manganese were found to inhibit levoglucosan formationdecrease by 54% on average in comparison with a catalyst-free sample -while copper had no effect. LAC was highly favoured by zinc, cobalt, manganese, nickel, cerium and, to a lesser degree, iron. Very few LAC was produced with copper. Those results fit those of Furneaux et al. [START_REF] Furneaux | A novel hydroxylactone from the Lewis acid-catalyzed pyrolysis of cellulose[END_REF] who reported that LAC was catalytically produced by soaking cellulose with MnCl 2 , FeCl 3 , CoCl 2 , NiCl 2 and ZnCl 2 while it was not formed with CuCl 2 . This point will be discussed in Section 3.2.1. High LAC yield was also obtained by Jakab et al. [START_REF] Jakab | Effect of slight chemical modification on the pyrolysis behavior of cellulose fibers[END_REF] from zinc acetate impregnated cellulose, thus confirming the key role of the cation for enhancing LAC formation. Three times more DGP was produced in the presence of copper and zinc in comparison with catalyst-free samples. It has to be noticed that levoglucosenone was not detected in bio-oils, the possible reasons will be discussed in Section 4.4.

The highest yields of monomeric anhydrosugars were obtained with Zn(NO 3 ) 2 and Co(NO 3 ) 2 . Thus, these salts seemed to be very efficient towards cellulose depolymerisation. On the contrary, depolymerisation seemed to be inhibited by Fe(NO 3 ) 3 . The observed changes in LAC quantity -which contain an unusual lactone group -suggested that depolymerisation occurred differently in the presence of zinc, cobalt, nickel, manganese and cerium.

Impregnated nitrate salts have very low impact on acid and furan yields. In spite of high peak intensity (Fig. 2), furfural yields remain quite low -about 0.5 wt.% on a catalyst-free basis -whatever the catalyst. At first sight, this result does not fit those of Di Blasi et al. [START_REF] Di Blasi | Products and global weight loss rates of wood decomposition catalyzed by zinc chloride[END_REF] reporting a high catalytic effect of ZnCl 2 towards furfural formation from impregnated fir wood. This difference might be explained by the chemical differences between hemicelluloses from softwood and from hardwood and by the fact that a too high ZnCl 2 amount can reduce the catalytic effect [START_REF] Branca | Pyrolysis of corncobs catalyzed by zinc chloride for furfural production[END_REF][START_REF] Di Blasi | Products and global weight loss rates of wood decomposition catalyzed by zinc chloride[END_REF].

Gas yields

All the tested metal catalysts were found to favour gas formation (Fig. 3). Gas yields reported in Fig. 6 are given on a molar basis in order to help the comparison of light gas yields. It has to be noticed that CO 2 yields roughly doubled in the presence of all catalysts, especially with manganese, nickel and cerium. This important result fits those of Collard et al. [START_REF] Collard | Influence of impregnated metal on the pyrolysis conversion of biomass constituents[END_REF] who obtained a similar increase by impregnating Fe(NO 3 ) 3 and Ni(NO 3 ) 2 in biomass constituents at 600 • C. Surprisingly, ZnCl 2 and CoCl 2 salts were found to reduce CO 2 formation [START_REF] Font | Catalytic pyrolysis of almonds shells: influence of temperature and CoCl2 almond shell ratio[END_REF][START_REF] Branca | Pyrolysis of corncobs catalyzed by zinc chloride for furfural production[END_REF], thus suggesting that nitrates and chlorides play different roles on CO 2 formation. This will be discussed in Section 4.2.

Hydrogen yields were strongly enhanced by transition metals whereas very low amount of H 2 was produced with cerium. Whatever the considered transition metals, an increase of H 2 yield was associated to an increase of CO yield. Nickel was found to be the most efficient catalyst for producing both CO and H 2 . This fits the results of Richardson et al. [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF] who selected nickel to favour syngas formation. Whatever the selected catalyst, methane formation was found to slightly decrease in the presence of a catalyst.

Nanoparticle formation in chars

Metals -which have a catalytic effect on the formation of pyrolysis products -remain in chars after pyrolysis. Thus, TEM, XPS and XRD analyses were carried out on those chars to better understand their catalytic role during pyrolysis.

Presence of nanoparticles in chars

Nanoparticles were detected by TEM in all the chars derived from catalyst-impregnated biomass. Micrographs of chars containing nickel-, cerium-and copper-based nanoparticles are shown in Figs. 7-9 respectively. Nickel nanoparticles (Fig. 7) were very well dispersed in the char although cerium-based nanoparticles (Fig. 8) were not so uniformly distributed within the char matrix.

Very tiny nanoparticles were formed in all chars at 500 • C. Their mean diameters, measured on TEM micrographs, were close to 5 nm for cerium, cobalt, iron, manganese, nickel and zinc (Table 4). Similar dimensions were reported by Richardson et al. [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF] for nickel nanoparticles obtained at 500 • C. In the case of copper, a large particle size distribution was observed, ranging from 5 nm to 65 nm. The larger particles were formed by aggregation of small ones (doted circle in Fig. 9), in accordance with Twigg and Spencer [START_REF] Twigg | Deactivation of supported copper metal catalysts for hydrogenation reactions[END_REF] who claimed that copper more likely than iron, nickel and cobalt tends to aggregate when heated.

From a catalytic point of view, both the dispersion and size of active sites are key parameters. As copper nanoparticles were found to be sensitive to aggregation, a lower catalytic activity was expected for copper. Interestingly, copper was the only catalyst which did not enhance LAC formation. Thus, the low activity of copper catalyst towards LAC formation might be explained by its rapid deactivation -by aggregation -during pyrolysis. It confirms the results obtained by Fabbri et al. [START_REF] Fabbri | Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: an analytical study[END_REF] who emphasised that the nanosized characteristics are a key parameter to promote LAC production.

Metal oxidation state

Knowing the oxidation states of the catalytically active sites is of great importance to better understand the catalytic activity of the formed metal-based nanoparticles. Thus, both XRD and XPS were carried out in chars recovered after pyrolysis.

Crystallites were clearly detected by XRD in nickel and copperbased chars (Fig. 10), although no XRD signal was detected with others metals. We concluded that the nanoparticles observed in the cerium, manganese, iron, cobalt and zinc-based samples were amorphous. Both copper and nickel nanoparticles were in a metallic state (Cu 0 and Ni 0 ). Those results are in good accordance with those obtained by Terakado et al. [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF] who showed that mixture of nickel and copper nitrate salt with wood leads to the formation of metallic nickel and copper nanoparticles. Richardson et al. [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF] have shown that Ni(NO 3 ) 2 was first thermally degraded to NiO. Then the char might act as a reducing agent and lead to the reduction of NiO to Ni 0 between 400 • C and 500 • C. It is likely that Cu 0 nanoparticles were formed in the same way. H 2 and CO -produced in large quantity with both copper and nickel -might strengthen the reduction process.

XPS analyses have been performed on all the chars. XPS spectrum of the zinc-derived char is presented in Fig. 11. Very low XPS signal intensities were observed with chars containing cerium, manganese, iron, nickel, copper and cobalt. XPS analysis only provided information on the oxidation states of metals which were at the char surface. Thus, low XPS signal intensities might be due to the partial overlaying of metal by carbon deposit for mentioned chars. XPS spectrum of zinc derived chars (Fig. 11) fitted those obtained by Marrani et al. [START_REF] Marrani | Electrochemically deposited ZnO films: an XPS study on the evolution of their surface hydroxide and defect composition upon thermal annealing[END_REF] and confirmed the presence of ZnO on the char surface. Thus, zinc oxidation state is still Zn(II), revealing that char reduction effect did not occur in this case. It was not possible to determine the oxidation states of metals such as cerium, manganese, iron and cobalt in chars. However, regarding others works on the thermal degradation of metal nitrate salts in oxygen-free atmospheres, it might be considered that nanoparticles of CeO 2 [START_REF] Strydom | The thermal decomposition of cerium(III) nitrate[END_REF], MnO 2 [START_REF] Maneva | The thermal dehydration, decomposition and kinetics of Mn(NO3)2•6H2O and its deuterated analogue[END_REF], Fe 2 O 3 [START_REF] Wieczorek-Ciurowa | The thermal decomposition of Fe(NO3)3•9H2O[END_REF] and Co 3 O 4 [START_REF] Ehrhardt | Thermal decomposition of cobalt nitrato compounds: preparation of anhydrous cobalt(II)nitrate and its characterisation by infrared and Raman spectra[END_REF] were formed in chars. However, possible reduction of those oxides by chars was likely.

Discussion

It is well-known that cations have a catalytic effect on pyrolysis products, however anions were also suspected to play a role during pyrolysis. This section investigates nitrate behaviour and their potential role on CO 2 and levoglucosenone formation. The choice of an effective catalyst able to enhance LAC formation is also discussed. The high nitrogen amounts detected in chars were supposed to correspond to adsorbed NO 2 (Table 3). However, the low amount of adsorbed NO 2 in comparison with the nitrate content in biomass suggested that gaseous NO 2 or other nitrogen-containing species were produced. As molecules containing nitrogen such as pyridine were not detected in bio-oils, it was assumed that gaseous NO 2 was released during pyrolysis. Gaseous NO 2 yields were calculated using Eq. [START_REF] Auer | Carbons as supports for industrial precious metal catalysts[END_REF]. Results are reported in Table 5.

Y (NO 2 gas ) = %(NO 3 biomass ) × M NO 2 M NO 3 -%(NO 2 ads ) (8)
Very small amount of gaseous NO 2 was produced during pyrolysis of copper impregnated biomass and the highest NO 2 amounts were released from cerium and manganese-samples. The ironchars were found to adsorb the highest quantity of NO 2 .

Exogenous CO 2 formation

Gaseous NO 2 is known to be a strong oxidant. As a consequence, exogenous CO 2 might be formed through direct carbon oxidation already at 300 • C, as described by Jeguirim et al. [START_REF] Jeguirim | Interaction mechanism of NO2 with carbon black: effect of surface oxygen complexes[END_REF][START_REF] Jeguirim | Oxidation mechanism of carbon black by NO2: effect of water vapour[END_REF] (Eq. ( 9)).

C + 2NO 2 → CO 2 + 2NO (9)
In order to estimate the exogenous CO 2 yields, gaseous NO 2 was supposed to be totally converted to CO 2 (Eq. ( 10)). The estimated ratios of exogenous CO 2 are reported in Table 6.

Y (CO 2 exogenous ) = Y (NO 2 gas ) 2 × M CO 2 M NO 2 (10)
Those results demonstrate that nitrate degradation during pyrolysis might impact on CO 2 yields. Hence, CO 2 yields were probably slightly overestimated specifically for cerium, manganese, iron, nickel and to a lesser extent for cobalt and zinc. The CO 2 yield for copper was not altered because of the very low nitrate amount in the biomass. Although nitrates can be removed at lower temperatures than other anions, the use of nitrate salts probably affect gas yields.

NO 2 adsorption on chars

Ultimate analysis of chars revealed that chars derived from impregnated biomass contained more nitrogen than catalyst-free samples. Similar results were obtained by Terakado et al. [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF] when operating thermal degradation of metal nitrates doped biomass. Although the formation of gaseous NO 2 during nitrate thermal degradation is well documented [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF][START_REF] Chin | Investigation of the decomposition mechanism and thermal stability of nitrocellulose/nitroglycerine based propellants by electron spin resonance[END_REF], little is known about the nitrogen species retained in chars at high temperatures. As far as carbonaceous matrixes are efficient adsorbents [START_REF] Serp | Carbon Materials for Catalysis[END_REF], it can be assumed that extra-nitrogen in chars is due to NO 2 adsorption.

Although it has been shown that physisorbed NO 2 in chars was released at 110 • C [34], Zawadzki et al. [START_REF] Zawadzki | Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons[END_REF] demonstrated that NO 2 can be also chemisorbed on char surface and form different adsorbed species. Interestingly, although some species might desorb at 300 • C, others are very stable and might remain in chars at 500 • C. However, all the adsorbed species should decompose completely at 600 • C. Further work is needed to confirm these hypotheses.

Suspected role of anion on levoglucosenone formation

As reported in Section 3.1.2, levoglucosenone (anhydrosugar) was never detected in the bio-oils produced in this work. We can thus conclude that the nitrate salts (such as Fe(NO 3 ) 3 or Zn(NO 3 ) 2 ) do not catalyse its production. However, levoglucosenone formation was reported by Branca et al. [START_REF] Branca | Pyrolysis of corncobs catalyzed by zinc chloride for furfural production[END_REF], Klampfl et al. [START_REF] Klampfl | Investigations on the effect of metal ions on the products obtained from the pyrolysis of cellulose[END_REF] and Dobele et al. [START_REF] Dobele | Application of catalysts for obtaining 1,6-anhydrosaccharides from cellulose and wood by fast pyrolysis[END_REF] when using ZnCl 2 , FeCl 3 and Fe 2 (SO 4 ) 3 catalysts, respectively. This reveals that anions seem to play a major role in the formation of levoglucosenone. It is generally assumed that the thermal depolymerisation of cellulose yields anhydrosugars between 300 • C and 390 • C [START_REF] Collard | A review on pyrolysis of biomass constituents: mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin[END_REF]. Interestingly, nitrates are removed just before reaching this temperature while sulfate or chloride remain in biomass until 400 • C at least [START_REF] Terakado | Explosive degradation of woody biomass under the presence of metal nitrates[END_REF]. This could explain the role of both chlorides and sulfates towards levoglucosenone formation whereas nitrates are released before cellulose depolymerisation.

Selection of a relevant catalyst to enhance LAC formation

LAC (1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2-one) was first identified in bio-oils derived from metal chloride impregnated cellulose [START_REF] Furneaux | A novel hydroxylactone from the Lewis acid-catalyzed pyrolysis of cellulose[END_REF]. Two decades later, LAC was also obtained from cellulose mixed with aluminium titanate nanopowders [START_REF] Fabbri | Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: an analytical study[END_REF]. The nanosized characteristic of the catalyst was pointed out as a key factor to enhance its production. In the present work, analyses performed on chars confirmed the tendency of impregnated salts to form nanoparticles during pyrolysis, thus giving a possible way to understand and favour the production of LAC. Among the tested salts, zinc nitrate seemed to be the most active catalyst towards LAC formation. The highest catalytic activity of zinc acetate compared to calcium acetate has already been reported by Jakab et al. [START_REF] Jakab | Effect of slight chemical modification on the pyrolysis behavior of cellulose fibers[END_REF]. We should also emphasise that the selected catalyst should not deactivate at elevated temperature. Unfortunately the deactivation temperatures can hardly be determined for metal impregnated biomass. Although nitrate, chloride and acetate salts were found to favour LAC production [START_REF] Jakab | Effect of slight chemical modification on the pyrolysis behavior of cellulose fibers[END_REF][START_REF] Furneaux | A novel hydroxylactone from the Lewis acid-catalyzed pyrolysis of cellulose[END_REF] further work is needed to compare these salts in terms of catalytic activity and stability.

Conclusions

The objective of this study was to investigate the catalytic effect of salts impregnated in biomass paying attention to the potential role of nitrates on pyrolysis products. Seven salts -Ce(NO 3 ) 3 , Mn(NO 3 ) 2 , Fe(NO 3 ) 3 , Co(NO 3 ) 2 , Ni(NO 3 ) 2 , Cu(NO 3 ) 2 and Zn(NO 3 ) 2 -were successfully impregnated under vacuum to obtain constant molar amounts of cations. Impregnated biomass were pyrolysed under similar operating conditions. Mass balance closures were higher than 94%.

Huge changes were measured on anhydrosugar yields in biooils, depending on the selected catalyst: LAC production decreased in the following order: Zn > Co > Ni, Mn > Ce > Fe. However, only small LAC amounts were produced from copper impregnated biomass. TEM observations were performed on chars to explain the low activity of copper catalyst towards LAC formation. It was found that metal nanoparticles were formed in all the chars but that copper ones deactivate by aggregation.

Nitrate evolution during pyrolysis was investigated. Both the impregnated biomass and the derived chars were found to contain extra nitrogen due to nitrate impregnation. High nitrogen amounts in chars suggested that NO 2 , resulting from the thermal degradation of nitrates, was released and partially chemisorbed on chars even at elevated temperatures. A huge increase of CO 2 formation was evidenced for all catalysts compared to catalyst-free samples. Although gaseous NO 2 -which is a strong oxidant -might contribute to exogenous CO 2 formation even at 300 • C, only less than 15% of CO 2 was found to be exogenous.

To conclude, metal nitrate salts might be impregnated in biomass to control both anhydrosugar and CO 2 formation. Zinc and cobalt can be used to enhance LAC production while manganese, nickel and cerium are more suitable to catalyse CO 2 formation which might be a recommended pathway to promote de-oxygenation of bio-oils. More attention has to be paid on the possible impacts of anion during pyrolysis.

Fig. 1 .

 1 Fig. 1. Schematic diagram of the pyrolysis reactor. Adapted from Ref. [13].

Fig. 2 .

 2 Fig.2. Typical total ion chromatogram obtained by GC-MS analysis of the bio-oil produced from zinc salt-impregnated biomass.

Fig. 3 .

 3 Fig. 3. Comparison of pyrolysis product yields for a series of catalyst-impregnated biomass, in comparison with a catalyst-free sample. The error bar extremities represent the minimum and maximum values. Yields are mean values.

Fig. 4 .

 4 Fig. 4. From left to right: levoglucosan, 1,4:3,6-dianhydro-␣-d-glucopyranose (DGP), 1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC) and levoglucosenone. Adapted from Ref. [39].

Fig. 5 .

 5 Fig. 5. Comparison of levoglucosan, LAC and DGP yields (catalyst-free basis) for a series of catalyst-impregnated biomass, in comparison with a catalyst-free sample.

Fig. 6 .

 6 Fig. 6. Comparison of gas fraction compositions (gas molar yields) for a series of catalyst-impregnated biomass, in comparison with a catalyst-free sample. The error bar extremities represent the minimum and maximum values. Yields are mean values.

Fig. 7 .

 7 Fig. 7. TEM observation of nickel-based nanoparticles in char derived from biomass pyrolysis at 500 • C (full scale bar: 200 nm).

Fig. 8 .

 8 Fig. 8. TEM observation of cerium-based nanoparticles in char derived from biomass pyrolysis at 500 • C (full scale bar: 100 nm).

Fig. 9 .

 9 Fig. 9. TEM observation of copper nanoparticles in char derived from biomass pyrolysis at 500 • C. The doted circle evidences an agglomeration of nanoparticles (full scale bar: 200 nm).

Fig. 10 .

 10 Fig. 10. Comparison of XRD patterns for chars containing metals.

Fig. 11 .

 11 Fig. 11. XPS spectrum of the zinc-derived char.

  investigated the catalytic effect of zinc acetate on

	Nomenclature
	XRD	X-ray diffraction
	ICP-MS inductively coupled plasma mass spectrometry
	XPS	X-ray photoelectron spectroscopy
	TEM	transmission electron microscopy
	CHN	carbon, hydrogen, nitrogen elemental analysis
	GC-MS gas chromatography mass spectroscopy
	GC	micro gas chromatography
	NIST	National Institute of Standards and Technology
	PAH	polycylic aromatic hydrocarbons
	LAC	1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2-
		one
	DGP	1,4:3,6-dianhydro-␣-d-glucopyranose
	Y()	yield in weight percent on a catalyst-free basis
	X()	molar yield on a catalyst-free basis
	%()	amount in weight percent
	m	mass
	M	molar mass
	M N

Table 1

 1 Metal nitrate salt concentrations of impregnation solutions.

	Concentration (mol/L)

  mA and 15 kV). The monochromatic X-ray source was aluminium K ␣ line

	Intensity (arb. u.)
	7	10	13	16	19	22	25	28	31	34	37	40	43	46	49	52	55	58	61	64	67
									RetenƟon Ɵme (min)

Table 2

 2 List of the main molecules calibrated on GC-MS and associated peak number.

	Liquid organic fraction composition	Retention time	Peak
				(min)		number
	Anhydrosugars
	1,6-Anhydro-␤-d-glucopyranose		57.6		1
	(levoglucosan)
	1-Hydroxy, (1R)-3,6-		39.3		2
	dioxabicyclo[3.2.1]octan-2-one
	(LAC)
	1,4:3,6-Dianhydro-␣-d-		40.7		3
	glucopyranose
	(DGP)
	Aldehydes, ketones, acids and alcohols
	Glycolaldehyde		7.3		4
	1-Hydroxy-2-propanone		9.9		5
	3-Methyl-1,2-cyclopentanedione		28.2		6
	Acetic acid			8.7		7
	Other aldehydes, ketones, acids and
	alcohols a
	Furans
	Furfural			17.3		8
	5-Methylfurfural		24.9		9
	5-Hydroxymethylfurfural		43.4		10
	Other furans b
	Phenols
	Phenol			29.8		11
	Guaiacol (2-methoxyphenol)		30.3		12
	Syringol (2,6-dimethylphenol)		43.7		13
	Other phenols c
	PAH d
	Phenol,	2-methylphenol,	3-methylphenol,	4-methylphenol,	2,4-
	dimethyphenol, 3,4-dimethyphenol, hydroquinone, 2-methoxy-4-methylphenol,
	2-methoxy-4-ethylphenol, eugenol, isoeugenol, 2-methoxy-4-vinylphenol.

a Acetaldehyde, 2-cyclopenten-1-one, 3-methyl-2-cyclopenten-1-one, 1-hydroxy-butanone, formic acid, propionic acid, methanol.

b 2-Furanmethanol. c d Fluorene, naphthalene, phenantrene.

Table 3

 3 Metal content in biomass. Nitrogen amounts (wt.%) in both biomass and chars and their related amounts (wt.%) of nitrates and adsorbed nitrogen dioxide.

	Biomass

a Measured by elemental analysis. b Calculated.

  .

		120%
				Chars	Organic fracƟon	Water	Gases
		100%
	Yields (%mass.)	40% 60% 80%	36% 20% 14%	24% 22% 20%	27% 21% 19%	27% 19% 19%	28% 22% 19%	26% 21% 19%	22% 22% 22%	25% 21% 18%
		20%	26%	34%	31%	29%	31%	34%	29%	33%
		0%

gas yields on a nitrate salt free basis (mmol/g)

	0.21 2.23 1.53 0.60 0.14 Molar H2 0.17 1.93 3.02 0.46 0.09 0 1 2 3 4 5 6 7 8	0.52 1.67 3.07 0.42 0.07 CO CO2	0.62 1.77 2.88 0.53 0.09 CH4	0.76 2.20 2.55 0.49 0.11 C2H4 and C2H6 1.21 2.77 2.92 0.55 0.13	0.87 2.14 2.73 0.50 0.09	0.77 1.85 2.64 0.43 0.09

Table 4

 4 Mean diameter of metal-based nanoparticles measured directly on TEM micrographs.

	Metal	Cerium	Cobalt	Copper	Iron	Manganese	Nickel	Zinc
	Mean diameter of nanoparticles (nm)	4.1	6.5	[5-65]	4.5	5.6	6.2	4.5

Table 5

 5 Gaseous NO2 yields resulting from the pyrolysis of a series of nitrate metal saltimpregnated biomass, in comparison with a catalyst-free sample.

	Metal-based catalyst	Y(NO 2 (gas) ) (wt.%)
	Catalyst-free	-
	Cerium	3.74
	Cobalt	1.45
	Copper	0.17
	Iron	2.11
	Manganese	3.09
	Nickel	2.24
	Zinc	1.21

Table 6

 6 Estimated ratios of exogenous CO2 resulting from the pyrolysis of a series of nitrate metal salt-impregnated biomass, in comparison with a catalyst-free sample.

	Metal-based catalyst	Catalyst-free	Cerium	Manganese	Iron	Cobalt	Nickel	Copper	Zinc
	Y (CO 2 exogenous ) Y (CO 2 total )	(%)	-	15.0	11.6	8.6	6.4	8.3	0.7	5.3
	4.1. NO 2 formation from nitrates during pyrolysis

Acknowledgements

The authors sincerely acknowledge Danilo Malferrari and Daniele Fabbri (University of Bologna, Italy) for providing a calibrated solution containing anhydrosugars for calibration purposes.

The authors thank Eric Martin, J ér émy Valette, Ghislaine Volle, Nathalie Troalen, Ga ël Senecal (CIRAD), Martin Drobek and Didier Cot (IEM) for technical and administrative support.

A special acknowledgement is given to Aurélien Renard (LPCME) and Jean Yves Baliteau (SADEF) for XPS and ICP-MS analyses.

The authors warmly thank Bruno Gagnepain (ADEME) and Claude Mirodatos (IRCELyon) for their scientific advices.

The authors thank both the CIRAD and the ADEME for financial support.