Catalytic Investigation of in Situ Generated Ni Metal Nanoparticles for

Article pubs.acs.org/JPCC

Catalytic Investigation of in Situ Generated Ni Metal Nanoparticles for Tar Conversion during Biomass Pyrolysis Yohan Richardson,*,†,‡,∥ Julius Motuzas,‡,⊥ Anne Julbe,‡ Ghislaine Volle,† and Joel̈ Blin†,§ †

Biomass, Wood, Energy, Bioproducts Unit (BioWooEb), French Agricultural Research Centre for International Development (CIRAD), B-42/16, 73, Avenue J.-F. Breton, F-34398 Montpellier, Cedex 5, France ‡ European Membranes Institute (IEM), UMR 5635 CNRS-ENSCM-UM2, Université Montpellier 2 (CC 47), Place Eugene Bataillon, F-34095 Montpellier, Cedex 5, France § Biomass Energy and Biofuels Laboratory (LBEB), Joint Research Center Sustainable Energy and Habitat, International Institute for Water and Environmental Engineering (2iE Foundation), 01 BP 594 Ouagadougou 01, Burkina Faso S Supporting Information *

ABSTRACT: In order to promote process intensification in syngas production from biomass gasification, our research team has already considered the integration of transition metal-based nanocatalysts in the biomass feedstock through its impregnation with metal salt aqueous solutions. The purpose of this work is to provide new insights into the complex physicochemical and catalytic mechanisms involved in this catalytic pathway from nickel salt. Applying a primary vacuum during impregnation allowed the rate of nickel insertion to be optimized and the generation of strong interactions between the metal cations and the lignocellulosic matrix. During biomass pyrolysis, Ni0 nanoparticles (NPs) form in situ below 500 °C through carbothermal reduction and provide the active sites for adsorption of aromatic hydrocarbons and subsequent catalytic conversion. In order to test whether it was possible to improve the catalytic efficiency of Ni0 NPs by making them available right from the pyrolysis onset, some preformed Ni0 NPs were inserted into the biomass prior to pyrolysis. The in situ generated Ni0 NPs exhibit higher catalytic efficiency, particularly for aromatic tar conversion, than preformed Ni0 NPs. The high decrease in hard-to-destroy aromatic hydrocarbons formation during pyrolysis is of particular interest in the overall gasification process. The proposed catalytic strategy reveals promising for simplifying the cleaning up of the producer gas.

1. INTRODUCTION The production of syngas from biomass gasification draws increasing attention as an attractive and reliable route to produce biofuels, chemicals, and hydrogen along with heat and power, in the envisioned future biobased economy, considered as one of the key issues for sustainable development and green growth on a global scale.1−6 The gasification technology involves high temperature (generally in the range 600−900 °C or even higher) partial oxidation of biomass in the presence of a gasifying agent (for instance air, oxygen, steam, CO2, or mixtures of these components), resulting in the production of low-to-medium heating value fuel gases with significant amounts of CO, H2, CO2, and H2O along with tar, sulfur-, nitrogen-, and inorganic-based compounds and particulates as impurities. These impurities must be removed before the syngas can be used in an engine, turbine, or fuel cell for producing power or in a catalytic reactor for producing liquid fuels and chemicals. In the overall cost production of Fischer−Tropsch fuels and methanol, the biomass transformation into syngas has been estimated to account for 50−75%.7 The different stages of syngas production from biomass include biomass pretreatment (drying and grinding), gasification, gas cleaning up (removal of particulates, tar, and © XXXX American Chemical Society

inorganics), and gas conditioning (water gas shift, syngas reforming, and CO2 elimination). A considerable share of syngas production costs is assigned to the critical gas cleaning up stage.8 Process intensification is considered as an attractive approach in developing new processes enabling considerable reductions in both unit size and operating costs along with environmental footprint. Intensifying the processes involved in syngas production from biomass along with syngas catalytic conversion is gaining increasing attention as a promising pathway for substantially reducing the production costs of syngas, H2, Fischer-Trosch fuels, and other chemicals of interest derived from syngas.8 To that end, development and implementation of advanced gasification concepts and gas cleaning up technologies have been proposed and remain the subject of intense, highly interdisciplinary research within the scientific community working on catalysis, bioengineering, chemical reaction engineering, and process engineering.8−12 Three main approaches of process intensification in the biomass-derived syngas production Received: August 15, 2013 Revised: October 12, 2013

A

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Figure 1. Concept of integrated catalytic biomass gasification through catalyst/biomass integration strategy developed by our group. Step 1: catalyst biomass insertion into solid biomass. Step 2: catalytic pyrolysis of biomass. Step 3: in situ generation of catalytic active nanoparticles highly dispersed inside the solid fuel. Step 4: catalytic cracking/reforming of nascent tar over the freshly formed nanoparticles. Step 5: catalytic gasification of the nanocomposite char residue char. Step 6: recycling and reuse of the catalyst metal species maintained in the ashes.

the first chemical transformation of biomass that occurs in a gasification reactor and is thereby a key reaction step in the overall gasification process. Due to the complexity of biopolymers and their supramolecular arrangement in lignocellulosic biomass, the fundamental science of biomass pyrolysis remains poorly understood despite decades of study23,24 and remains the subject of intense investigations through both new experimental and modeling approaches.25,26 In the catalyst/ biomass integration strategy, the knowledge at nanoscale of the behavior of catalyst-derived transition metal species during impregnation, pyrolysis, gasification, and recycling is crucial for the development of our integrated catalytic biomass gasification concept. Our approach is thus to first study each of these reaction steps from fundamental material chemistry, catalysis, and biomass thermochemistry standpoints. In our previous works, we demonstrated an interest in incorporating a nickel salt precursor in the biomass feedstock for both limiting tar formation and substantially promoting H2 production during the first reaction stages of thermochemical conversion, i.e., during pyrolysis.27 Such a catalytic performance was attributed to in situ formation of crystalline nickel metal nanoparticles (Ni0 NPs) measuring 2−4 nm in diameter, dispersed inside the feedstock during the initial stages of the wood pyrolysis reaction.22 During impregnation, the numerous oxygenated groups present in the biomacromolecules act as adsorption sites for metal cations in an aqueous medium, leading to very wide dispersion of the metal precursor within the wood matrix. During wood pyrolysis, an amorphous NixOyHz phase is formed then reduced to nickel metal by carbon atoms at temperatures between 400 and 500 °C, leading to the formation of quasimonocrystalline Ni0 nanoparticles.22 Recent studies have been devoted to assess the effect on nickel salts on the pyrolysis mechanisms of the main constituents of biomass, i.e., cellulose, hemicelluloses, and lignin.28,29 According to Collard et al., nickel species are active on the primary pyrolysis reactions and catalyzed charring reactions in microcrystalline cellulose and in lignin, while promoting the depolymerization of amorphous xylan.28 On the other hand, Khelfa et al. conclude from their results that the nickel species do not catalyze the pyrolysis primary reactions but appreciably modify the emitted gas and vapors composition.29 The catalytic results reported can appear contradictory if some experimental details are not taken into account. A careful analysis of these studies seems to highlight that the catalytic effect of nickel species on biomass pyrolysis reactions depends on both the type of nickel-macromolecule

process are currently being considered and consist of (i) the incorporation of cleaning up multifunctional systems into existing bubbling fluidized bed gasification reactors,13,14 (ii) the implementation of specific highly reactive gasification media such as supercritical water15,16 or molten metal,17,18 and (iii) the development of new integrated catalytic gasification reactors and concepts.8 While the two first approaches seem to have only a limited potential for the reduction of a plant size/production capacity ratio, the third approach turn out to be more promising for dramatically decreasing the economically viable size compared to existing processes through the development of new high-efficiency, small-sized catalytic reactors. These intensification approaches consist of different strategies of catalyst integration that target both reduction of biomass residence time in the gasification reactor and simplification of downstream syngas treatment before its subsequent catalytic conversion. Two different strategies can be considered: (i) catalyst/reactor integration through the development of new catalytic microreactors or microstructured catalysts19−21 and (ii) catalyst/biomass integration through the development of a new concept of integrated catalytic biomass gasification.8 The catalyst/biomass integration strategy is developed by our group and lead to a new concept of integrated catalytic biomass gasification,8,22 illustrated in Figure 1. This concept consists of inserting the catalyst metal precursor into the biomass during an impregnation stage with aqueous solutions of transition metal salts, promoting wide dispersion of the precursor inside the lignocellulosic matrix. The catalytic active phase as metal or metal oxide nanoparticles is then in situ generated, inside the feedstock, during thermochemical conversion. As shown in Figure 1, this concept comprises different key reaction steps including (i) catalyst precursor insertion into solid biomass, (ii) catalytic pyrolysis of biomass and its main macromolecular constituents, (iii) in situ generation of catalytic active nanoparticles highly dispersed inside the solid fuel, (iv) catalytic conversion of nascent tar over the freshly formed nanoparticles, (v) catalytic gasification of the nanocomposite char residue, and (vi) recycling and reuse of the catalyst metal species maintained in the ashes. Each of these reaction steps requires a fundamental understanding in order to further develop a new high-efficiency, small-sized gasification process. Indeed, a fundamental understanding of the biomass thermochemical conversion processes at nanoscale is critical to develop new breakthrough conversion technologies leading to the design of the future intensified gasification processes. The pyrolysis step is B

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that of the nickel nitrate aqueous solutions (pH 6.5). The latter wood sample was called A0. Method B. A total of 50 g of beech wood chips previously dried at 105 °C for 1 h was impregnated with 250 mL of nickel nitrate solutions identical to those used for method A. The corresponding impregnated wood samples were called Bx, where x was the metal content in the wood determined by ICP-OES (expressed in mol kg−1 of dnf wood). The impregnation solution containing the wood chips was placed in a vacuum chamber. In order to keep the wood chips in the solution during depressurization, a wire mesh cover held down by a heavy object was placed over the wood chips before they were brought into contact with the impregnation solution. The vacuum chamber was pumped down to 7 mbar for 15 min, and then the mixture was left to stand at atmospheric pressure for 2 h. This procedure was adapted from a wood impregnation standard method using a solution containing a preservation product.30 The impregnated wood chips were then filtered, rinsed with 250 mL of deionized water, and then dried at 60 °C for 48 h. A blank sample prepared under the same conditions using a metal-free aqueous solution was called B0. Method C. The Ni0 NPs were synthesized by a microwaveassisted polyol method.31 The Ni0 NPs formed in 60 min at 240 °C under microwave irradiation in a closed vessel were isolated by magnetic separation and rinsed in acetone and then in deionized water, to remove traces of the solvents and organic additives used during synthesis. The rinsed Ni0 NPs were then isolated by centrifugation and dried at 150 °C for 3 h. A given quantity of the resulting Ni0 NPs was then redispersed in 50 mL of deionized water by ultrasound treatment for 15 min. Several nickel concentrations were used for the Ni0 NP aqueous suspensions: 0.004, 0.01, and 0.02 g mL−1. A total of 10 g of wood chips (screened and dried as in methods A and B) were then mixed with 50 mL of a Ni0 NP aqueous suspension and 50 mL of deionized water. In order to promote uniform deposition of the Ni0 NPs and their insertion into the structure of the lignocellulosic matrix, they were subjected to ultrasonic treatment for 30 min and then depressurization (water jet filter pump) for 15 min during wood chip impregnation. The resulting impregnated wood samples were then filtered on a Büchner funnel and dried at 105 °C for 1 h. These samples were called Cx (where x was the metal content in the wood determined by ICPOES, expressed in mol kg−1 of dnf wood). 2.2. Catalytic Tests. 2.2.1. Pyrolysis Experiments. The catalytic pyrolysis tests were carried out in a stainless steel, horizontal tube furnace type pyrolysis reactor (Pyrox VK65/550) equipped with modules enabling the recovery and analysis of pyrolysis products. The experimental devices have been described elsewhere.27,28 The reaction zone was continually swept by a N2 stream (Linde, 99.995% purity) with a flow rate of 40.04 NL.h−1 in order both to create an inert atmosphere and to draw the pyrolysis gas to the outlet. The N2 flow was regulated by a mass flow regulator (Brooks 5850S). The wood chips (around 6 g), previously dried at 105 °C for 1 h, were placed in a stainless steel boat (length 7 cm, mesh size 200 μm), attached to the end of a stainless steel arm used to position the sample in the hot zone of the reactor. Before entering the reactor, the wood sample was kept at ambient temperature in nitrogen in an air lock at the entrance to the tube furnace, cooled by water circulation. Once the furnace reached a temperature of 720 °C, the sample was rapidly positioned in the hot and isothermal zone of the reactor by quickly pushing the arm into the oven. Three thermocouples, placed at different heights in the bed formed by the wood chips in

interactions and pyrolysis conditions, such as pyrolysis temperature and heat flux density transferred to the biomass particles. The type of nickel−biomass interactions along with the nickel species evolution during biomass pyrolysis in relation to the extent of primary and secondary pyrolysis reactions are thus of great importance in the final catalytic results of biomass pyrolysis in the presence of a nickel salt as a catalyst precursor. The purpose of this current work was to provide a thorough fundamental understanding of the complex physicochemical and catalytic mechanisms involved in this catalytic pathway, which is not well understood yet and has yet to be widely studied in relation to the development of integrated catalytic biomass gasification processes. To that end, after optimizing biomass impregnation with aqueous nickel nitrate solutions, we first characterized the type of interactions existing between the nickel species and the lignocellulosic matrix. Then, Ni0 NPs previously synthesized by a microwave-assisted polyol method were inserted into the biomass prior to pyrolysis, with a view to improving the catalytic efficiency of the Ni0 NPs by making them directly available and active right from the start of pyrolysis. The catalytic efficiency of in situ generated Ni0 NPs and preformed Ni0 NPs was compared in terms of the yields and the compositions of condensable and gaseous products obtained in pyrolysis tests at 700 °C. Ex situ characterization of the catalyst at different stages of pyrolysis led us to propose some catalytic mechanisms.

2. EXPERIMENTAL METHODS 2.1. Biomass Sample Preparation. The biomass feedstock used was beech wood, conditioned in the form of chips screened at between 1.4 and 1.6 mm. The pretreated biomass samples were named according to the impregnation method used, as explained below. A first method of biomass impregnation with aqueous nickel nitrate solutions, for which procedure has already been described in detail,22 was used and called method A. A new impregnation method under reduced pressure was developed and is referred to as method B. In order to improve the catalytic efficiency of Ni0 NPs by making them directly available and active right from the start of pyrolysis, we attempted the direct insertion of Ni0 NPs beforehand synthesized into the biomass feedstock. This way of inserting Ni0 NPs into biomass is referred to as our method C below. Method A. A total of 50 g of beech wood chips previously dried at 105 °C for 1 h was impregnated with 500 mL of nickel nitrate solution prepared from Ni(NO3)2·6H2O (Sigma-Aldrich, 99% purity). Solutions of 0.1, 0.5, and 1 M nickel salt were used for the impregnation. The corresponding impregnated samples were called Ax, where x was the metal content in the wood determined by ICP-OES expressed in mol kg−1 of dried nickelfree (dnf) wood. In order to promote electrostatic interactions favoring nickel cation adsorption during wood impregnation, the pH of the impregnation solutions was adjusted to a value of about 6.5 by adding a few drops of concentrated ammonium hydroxide solution (Sigma-Aldrich, 28−30% NH3 basis), considering both the point of zero charge (pHZPC) of the wood chips and the nickel speciation in aqueous solution.22 Impregnation of the wood chips was then carried out at ambient temperature (25 °C), under atmospheric pressure with magnetic stirring. After a contact time of 72 h, the impregnated wood chips were filtered, rinsed with 500 mL of deionized water, and then dried at 60 °C for at least 48 h. A blank sample was prepared under the same conditions using a metal-free aqueous solution of similar pH to C

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pyrolysis oil produced determined by the gravimetric method and the mass of the initial wood sample.

the boat, were used to record temperature variations in the pyrolysis zone. Under these conditions, the average temperature of the solid at the end of pyrolysis (called final pyrolysis temperature) was around 700 °C. The pyrolysis gases formed then crossed a heat tracing zone heated by several heating resistors positioned inside and outside the furnace just before the condensation zone. Condensable vapors were trapped by cooling in a water-cooler followed by a flask placed in a bath thermostatically controlled at −45 °C. The aerosols not retained were precipitated on the collector electrode of an electrostatic filter operating at a voltage of 9 kV. Noncondensable gases were collected for 10 min in a sampling bag prior to analysis. After a residence time of 7 min in the hot zone of the reactor, the pyrolysis char obtained in the boat was cooled to ambient temperature under nitrogen in the zone cooled by water circulation, prior to being recovered and weighed. The liquid phase collected in the liquid condensation system was weighed and then recovered after rinsing the walls of the cooler and the electrostatic filter with a known quantity of isopropyl alcohol. The liquid samples diluted in isopropyl alcohol were stored at 5 °C in the dark. This procedure made it possible to calculate the mass yields for each of the pyrolysis products recovered (solid, liquid and gas) and establish mass balances (ratio of the mass of all the recovered products to the initial mass of dry wood). All of the mass yields for the pyrolysis products were calculated on the basis of dnf (“dried nickel-free”) wood mass, i.e., the initial mass of the dry wood sample corrected for the Ni mass determined by ICP-OES. The Ni species present in the wood were considered to be totally transferred to the solid phase (char) during pyrolysis, and the mass of the catalyst was therefore subtracted from the mass of the solid residue recovered after the test. This hypothesis was checked experimentally by several ICP-OES analyses in earlier work.22 The pyrolysis tests carried out with pristine wood samples and washed wood samples were considered as blank experiments on the basis of which the efficiency of the different catalysts was assessed. 2.2.2. Analysis of Pyrolysis Products. The main gases formed during pyrolysis and collected in the sampling bag were analyzed by gas chromatography (Micro-GC CP-4900). Gas productivity, expressed as the number of moles of gas formed per kilogram of dnf wood, was calculated from the molar fractions xj of each gas j, from the molar fraction of N2 (xN2), which served as the standard, from the known N2 flow rate (QN2 = 40.04 nL h−1 = 2.977 × 10−2 mol min−1) and from the sampling time (t = 10 min), according to formula 1: nj =

Y(H 2O) =

Y(AO) =

Y (Ci) =

2

(4)

mol of compound Ci formed 100 mass of sample in kg of dcf wood

(5)

The conversion rate for a given organic compound Ci was calculated from the yields obtained in the blank experiments with a reference sample, in accordance with formula 6: C(Ci) =

R Ci(reference) − R Ci R Ci(reference)

100 (6)

A positive value of C(Ci) could indicate a conversion and/or a reduction in the formation of compound Ci, whereas a negative value indicated that its conversion was enhanced in comparison with the reference sample considered (pristine wood sample or washed wood sample). 2.3. Physicochemical Characterization of the Solid Samples. 2.3.1. Wood Characterization after the Impregnation Step. The metal content (iron, nickel, and alkali metals) in the pristine wood and impregnated wood samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after mineralization of the sample. The type of interactions between the metal cations adsorbed and the lignocellulosic matrix in the impregnated wood samples was investigated by studying the vibrational and structural modifications in lignocellulosic matrix induced by metal cations adsorption, using FTIR spectroscopy and X-ray diffraction. The wood samples used for spectral characterizations and XRD were previously reduced to powder. This was done by grinding the wood chips in a cutting mill fitted with a grinding chamber maintained at 18 °C (IKA M20), then sieved at 200 μm, and dried at 105 °C for 1 h, just prior to analysis. FTIR spectroscopy was used to study the nature of the metal cation adsorption sites

(1)

where nN2 is the number of moles of N2 sampled, defined by formula 2: n N2 = Q N t

mass of anhydrous oil produced 100 mass of dnf wood

(3)

The tar composition of the pyrolysis oils diluted in isopropyl alcohol was studied by gas chromatography (Agilent Technologies CPG 6890N) coupled to a mass spectrometer (Agilent Technologies MD 5975). The chromatograph was equipped with a split/splitless injector. The column used was a DB1701 capillary column (14% cyanopropyl)-methylpolysiloxane (60 m × 0.25 mm × 0.25 μm, Agilent Technologies). The carrier gas used was helium at a constant flow rate of 1.9 mL/min. Quantitative analysis was carried out by internal standard calibration. The internal standards used were toluene D8 for the split method and phenanthrene D10 for the splitless method. The calibration curves were established beforehand with tar mixtures of known composition, prepared from dilution in isopropyl alcohols of pure compounds or commercial blends (PAH). Prior to analysis, the samples were filtered with a 0.45 μm filter (Millex-Gx). The internal standards were then added to the filtered samples and the mixture was analyzed. About fifty organic compounds were analyzed in this way. The organic compound yields (eq 5), expressed as the number of moles of organic compound Ci formed per kilogram of dnf wood, were determined from the organic compound content of the analyzed pyrolysis oils, from the dilution carried out during the pyrolysis test and from the mass of the wood sample.

xjn N2 x N2m

mass of H 2O produced 100 mass of dnf wood

(2)

and m is the mass of dnf wood in kg. The H2O content of the pyrolysis oils diluted in isopropyl alcohol was determined by Karl Fischer titration (Crison Compact Titrator) using the standard method ASTM E203-96 (volumetric method). The H2O content of the analyzed samples was used to calculate the mass yields of H2O (eq 3) and anhydrous pyrolysis oil (eq 4), taking into account the dilution carried out when the liquid phase was recovered, the mass of D

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Table 1. Influence of the Impregnation Method on the Quantity of Ni Adsorbed and the Rate of Ni Insertion into the Wood Feedstock impregnation method

pressure

method A

atmospheric pressure

method B

7 mbar then atmospheric pressure

contact time (h) 72

2.25

[Ni2+] (mol L−1) 0.1 0.5 1 0.1 0.5 1

solution volume (mL)

quantity of Ni introduced into the solution per kg of dried wood used for impregnation (mol kgwood−1)

quantity of Ni adsorbed (mol kgdnf wood−1)

rate of Ni insertion into wood (%)

500

1 5 10 0.5 2.5 5

0.070 0.288 0.348 0.077 0.295 0.471

7.0 5.8 3.5 15.4 11.8 9.4

250

referenced to the C−O component of C1s peak of carbon typically reported at 285 eV for wood samples.38 2.3.2. Ex Situ Characterizations of Nickel Species at Different Stages of Pyrolysis. In order to study the nickel species behavior and understand their catalytic effect during pyrolysis of wood samples impregnated by the different methods, some pyrolysis experiments at different final temperatures were carried out, applying the same procedure as that used in the catalysis tests, described in section 2.3.1. The wood samples were rapidly introduced into the hot zone of the reactor swept by a nitrogen stream. After a 7 min residence time in the hot zone of the reactor, the solid was cooled in a nitrogen atmosphere at ambient temperature, thereby protecting it from air oxidation at high temperature and making possible to characterize ex situ the metal at different stages of pyrolysis. The pyrolysis temperature studied ranged from 300 to 700 °C. The solid obtained was then ground in a mortar for the different characterizations described below. Wide-angle X-ray diffraction was used to detect the formation of crystalline metallic phases of nickel and study changes in their microstructure depending on the pyrolysis temperature. The diffraction patterns were recorded using the diffractometer described in section 2.3.1. The average diameter of the Ni0 crystallites, assumed spherical, was assessed by Scherrer’s formula36 from the full width at half-maximum of the diffraction peak 1 1 1 (2θ = 44.3°) and 2 0 0 (2θ = 41.5°). XPS spectroscopy was used to study nickel phase transformations and characterize the Ni0 NPs during wood pyrolysis. The spectrometer and analysis conditions were identical to those described in section 2.3.1. XPS spectra binding energy calibrations were referenced to the Ni2p3/2 peak of metal Ni, typically reported at 282.7 eV.39 For the Ni metal-free samples, the value of the C1s peak inferred from the XPS spectrum of the Ni metal-containing sample prepared at the closest pyrolysis temperature was used as reference. For a more accurate characterization of the Nin+ species, some reference compounds such as polycrystalline NiO (Sigma-Aldrich, 99% purity) and Ni(OH)2 (Sigma-Aldrich, 99% purity) were analyzed. For these samples, XPS spectra binding energy calibrations were referenced to the C1s peak of carbon contamination typically reported at 284.8 eV.40 Spectra were decoded using the curvefitting program with the subtraction of the Shirley background. The atomic compositions of the different elements were calculated using the transmission function of the electron energy analyzer and the theoretical sensitivity factors based on Scofield’s photoionization cross sections.41 The morphology of the chars obtained after pyrolysis was studied with a Hitachi S-4500 field emission scanning electron microscope (FESEM). The surfaces of the samples were metallized beforehand by a platinum deposit to prevent charge accumulation during analysis. The size of the Ni metal particles

inside the lignocellulosic matrix of the wood, giving rise to the formation of inner-sphere surface complexes. The spectral modifications induced by such interactions were studied by a fine comparison of normalized spectra and the crystallinity index of the cellulose in the different samples. The FTIR spectra were recorded in attenuated total reflection (ATR) mode with a Nexus Thermo Electron FTIR spectrometer equipped with a diamond ATR accessory (Golden Gate). The crystallinity index CIFTIR of the cellulose was determined using the intensity ratios of the bands at 1421 and 895 cm−1 considered respectively as a crystalline band (scissoring of the CH2 group) and an amorphous band (deformation of the C−H bond).32,33 Additional data on FTIR-ATR characterization can be found in the Supporting Information. In addition to the cellulose crystallinity index determined by FTIR-ATR, wide-angle X-ray diffraction was used to investigate structural modifications to the cellulose microfibrils induced by metal cation absorption. The crystallinity index CIXRD of the sample was determined by the method outlined by Ruland and Vonk34,35 which consists of subtracting the amorphous contribution from diffraction pattern using an amorphous standard. Commercial lignin powder (alkali, Aldrich 370959) was used as the amorphous standard. A scale factor is applied to the diffractogram of the amorphous material so that, after subtraction of the amorphous pattern from the original diffractogram, no part of the residual pattern contains a negative signal. After subtracting the diffractogram of the amorphous lignin from the diffractogram of the whole sample, the CIXRD was calculated by dividing the remaining diffractogram area due to crystalline cellulose by the total area of the original diffractogram. The Scherrer’s formula36 was used to determine the average thickness of the cellulose crystallites from the full width at halfmaximum of the diffraction peak of the plane (002) as suggested by Andersson et al.37 Due to the overlapping of the reflections (021) (2θ ≈ 20.5°) and (002) (2θ ≈ 22°), the (002) peak was fitted with a Gaussian model from 2θ = 21°. The diffraction patterns were recorded on a Philips X’Pert diffractometer in symmetrical reflection mode (Bragg−Brentano geometry θ/2θ), using Cu Kα radiation (λ = 0.154 nm), at a voltage of 40 kV and an intensity of 20 mA. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical environment of the nickel in the impregnated wood samples. Analyses were carried out with a Thermo Electron ESCALAB 250 spectrometer. High-resolution mode was used to detect the metal species present in all of the samples with very low atomic contents (