Palladium nanoparticles on modified cellulose as a novel catalyst for low temperature gas reactions

Palladium was incorporated into carboxymethylated cellulose fibers as a support, thereby becoming an efficient and stable catalyst for low temperature gas phase reaction. Thus, NO was used as test molecule of Greenhouse Gas to be catalytically reduced with hydrogen on an eco-friendly sustainable material containing palladium as the active site. Prior to the catalytic test, the catalysts were reduced with glucose as an eco-friendly reagent. The material characterization was performed by SEM–EDS, XRD, LRS, TGA and FTIR. The catalytic results showed that at 170 °C, NO conversion was 100% with a selectivity of 70% to nitrogen. While NOX species were completely converted into N2 at temperatures higher than 180 °C. The starting commercial dissolving pulp was also studied, but its performance resulted lower than the ones of functionalized fibers. The use of this strategy, i.e., the functionalization of cellulose fibers followed by in-situ formation of metallic nanoparticles, can be further applied for the design of a wide range of materials with interesting applications for gas and liquid phase reactions under mild conditions.

Abstract Palladium was incorporated into carboxymethylated cellulose fibers as a support, thereby becoming an efficient and stable catalyst for low temperature gas phase reaction. Thus, NO was used as test molecule of Greenhouse Gas to be catalytically reduced with hydrogen on an eco-friendly sustainable material containing palladium as the active site. Prior to the catalytic test, the catalysts were reduced with glucose as an eco-friendly reagent. The material characterization was performed by SEM-EDS, XRD, LRS, TGA and FTIR. The catalytic results showed that at 170°C, NO conversion was 100% with a selectivity of 70% to nitrogen. While NO X species were completely converted into N 2 at temperatures higher than 180°C. The starting commercial dissolving pulp was also studied, but its performance resulted lower than the ones of functionalized fibers. The use of this strategy, i.e., the functionalization of cellulose fibers followed by in-situ formation of metallic nanoparticles, can be further applied for the design of a wide range of materials with interesting applications for gas and liquid phase reactions under mild conditions.

Introduction
The degradation of the equilibrium of the environment caused by human activity and the consumption of natural resources for energy production and general inputs is a matter of global concern and growing importance (Letcher 2019;Qiao et al. 2019). The increase of the greenhouse gases (GHGs) concentration in the atmosphere straightly affects climatic parameters (planet global temperature, changes in rainfall patterns, extreme weather events, among others) (Arnell 1999;Jacob and Winner 2009;Kumar et al. 2018). Consequently, researchers and the scientific community stay alert looking for new and optimal strategies to control emissions from different productive activities. Nitrogen oxides (NO X ) emissions from the motor vehicle exhausts, and the combustion of coal, oil or natural gas, are among the main GHGs (Lammel and Graßl 1995;Hamada and Haneda 2012;Boningari and Smirniotis 2016).
The well-known NO x catalytic treatment of mobile or stationary sources involves the use of technologies and procedures that often lead to high-energy consumption and high equipment costs (Piumetti et al. 2015;Serrano et al. 2019). The optimization of these methodologies is sometimes associated with the modification of reactor devices; the synthesis of catalytic materials in which the cost of reagents is high as well as with the generation of harmful byproducts or wastes during the production stages (Praveena and Martin 2018).
In this context, the reduction of NO X employing hydrogen as reducing agent (H 2 -SCR) has recently been proposed as an effective and eco-friendly technology for the treatment of this gas and several active catalytic systems were reported (Fu et al. 2014;Wang et al. 2016;Resitoglu and Keskin 2017;Gioria et al. 2019a). Hence, we propose the synthesis of a novel catalytic material for the selective reduction of NO X to nitrogen, based on modified cellulose with metallic palladium nanoparticles as active sites.
Cellulose is the most abundant natural polymer, and it is present in most of the terrestrial biomass. Through several functionalization processes, it is possible to incorporate different functional groups into their polymeric chains. This allows changing the original material composition and structure, adapting it as a support for a wide variety of adsorbent and catalyst applications. Consequently, these physicochemical surface modifications of the cellulose also could give rise to the possibility of anchoring metallic entities on the structure. Therefore, the novelty of this work is that palladium is incorporated into the modified cellulose as an active phase, thereby becoming an efficient catalyst for the gas phase reaction.
Palladium was selected as the active metal owing to its high hydrogen spillover ability and its high activity for the H 2 and NO activation in the H 2 -SCR (YU et al. 2010;Patel and Sharma 2020). In addition, palladium is employed for numerous low temperature catalytic reactions, required to preserve the stability of the cellulosic fibers. In order to optimize the anchoring process of the metallic nanoparticles, a study of the cellulose derivatization with carboxymethyl group using different solvents was proposed (Dong and Hinestroza 2009;Song et al. 2012;Nishikata et al. 2014).
In this work, an effective catalyst for the mentioned reaction is presented, using modified cellulose as a novel support since it is an inexpensive and abundant biodegradable natural material.
Within the recent years, several researchers have reported the use of cellulose supporting palladium as active site. Mainly, hydrogenation and cross-coupling reactions in liquid phase were studied (Nishikata et al. 2014;Zheng et al. 2015;Li et al. 2017bLi et al. , 2018Gopiraman et al. 2018;Lu et al. 2018;Xiang et al. 2018;Islam et al. 2019;Seyednejhad et al. 2019;Dong et al. 2019;Kempasiddaiah et al. 2020;Yamada et al. 2020). However, and to the best of our knowledge, this is the first report where cellulose is employed as an effective catalytic support for the gas-phase reduction of NO, reaction of environmental concern.
Experimental Catalyst synthesis
It is well known that the cellulose functionalization with sodium monochloroacetate involves the replacement of the -OH groups in glucose residues by carboxyl groups. In heterogeneous conditions, this reaction takes place in two stages: mercerization and etherification (Togrul 2003).
The mercerization was carried out in a round flask under reflux at 25 8C during 90 min with magnetic stirring. The reaction mixture consisted of 3 g of cellulose fibers, 150 ml of a specific solvent (ethanol, isopropanol or deionized water) and 30 ml of a 30% w/w solution of sodium hydroxide (99% Cicarelli).
After that, and for the etherification step, 4.5 g of sodium monochloroacetate (99% Sigma-Aldrich) dissolved in 10 ml of distilled water were added, keeping the temperature at 308C during 1 h. Before filtration, the final suspension was neutralized (pH = 6-8) with 90% (v/v) acetic acid (Cicarelli).
The carboxymethylated cellulose (CMF) was washed several times with 80% ethanol, filtered and stored at 5-88C until further use. The degree of substitution (DS), defined as the number of the carboxymethyl groups in the molecular unit of the glucose residues, was determined by the standard method (ASTM D1439-03). The carboxyl content of pulp was measured according to the TAPPI T 237 cm-98 norm, while, the water solubility of carboxymethylated pulp was evaluated by the residual solid quantification (Shui et al. 2017). The supports were named CMFE, CMFIP and CMFW according to the solvent used in the mercerization stage (Scheme 1), CMF: carboxymethylated cellulose fibers; E, IP and W: ethanol, isopropanol and water respectively. In order to analyze the degree of substitution, the samples were compared to commercial carboxymethylcellulose (CMC, Sigma Aldrich 99%).

Incorporation of Pd on the cellulosic supports
In order to prepare the palladium-based catalyst, 100 mg of CMF fibers of carboxymethylated cellulose were sonicated during 30 min in 5 ml of 53 mM PdCl 2 solution (Vega & Camji). The pH was fixed to 7 with NaOH (Merck). After that, the suspension was heated in a water bath at 80°C during 1 h, filtered and washed with deionized water. The final Pd impregnated fibers (CMFE.Pd) were dried overnight at 80°C.
For the sake of comparison, palladium was also added onto the starting non-modified dissolving pump (DP) following the same protocol. The final material was named DP.Pd.
Following the principles of ''Green chemistry'', the use of hazardous and toxic reagents was avoided. Consequently, glucose was employed as a mild reducing agent. The reduction of the materials containing Pd (CMFE.Pd and DP.Pd) was carried out with an 8.3 mM glucose solution for 10 min at 60°C (Gioria et al. 2019b). After this step, the suspensions were filtered, washed several times with deionized water and dried overnight at 80°C. The final catalysts were named CMFE.Pd.Gluc and DP.Pd.Gluc.

Material characterization
The identification of the different crystalline species was done by X-ray diffraction analysis (XRD, Shimadzu XD-D1). In addition, temperature programmed XRD studies under reducing atmosphere were carried out. Using a H 2 flow of 50 ml min -1 and a heating rate of 5°C min -1 , XRD diffractograms were recorded at room temperature (before and after heating) and at 200°C.
Fourier Transform Infrared Spectroscopy (FTIR, Thermo-Mattson model Genesis II) and Laser Raman Spectroscopy (LSR, Horiba Labram HR) determined the nature of the functional groups. The macroscopic aspect at each stage of the synthesis was observed with an Olympus SZ51 optical stereo microscope.
The surface morphology of the material was studied using scanning electron microscopy (SEM benchtop Phenom ProX) and the chemical composition was analyzed by X-ray energy dispersion spectrometry (EDX), in mapping mode. In addition, Field emission scanning electron microscopy (FE-SEM) was employed at higher magnification in a ZEISS Gem-iniSEM500 NanoVP.
The thermogravimetric analysis (TGA-SDTA) was performed in a Mettler Toledo STAR analyzer with a TGA/SDTA851e module. A 70 lL alumina crucible containing 30-40 mg of sample was heated from 30°C to 800°C in air flow (50 ml.min -1 ). The heating rate was fixed at 10°C.min -1 .

Catalytic evaluation
The reduction of NO with H 2 was carried out in a quartz fixed bed reactor operating at atmospheric pressure, placed in a temperature-controlled furnace. The GHSV was fixed at 2500 h -1 , with a feed composition of 1000 ppm of NO and 2% H 2 using He for balancing and regulated by mass flow controllers. The temperature range evaluated was 30 up to 200°C with a heating rate of 10°C min -1 , in order to preserve the thermal stability of the fibers in agreement with the previous TGA analysis.
The concentrations of the reactor effluent gases (NO, NO 2 , N 2 O and NH 3 ) were monitored every 2 min with an on line FTIR Mattson Genesis II Scheme 1 Experimental conditions for the synthesis of carboxymethylated cellulose fibers (resolution of 4 cm -1 ) instrument equipped with a 15 cm path length gas IR cell with CaF 2 windows at room temperature. The N 2 production was calculated by atomic nitrogen balance. The NO conversion (C NO ) and the selectivity to N 2 (S N2 ) were calculated as follows (Eqs. 1 and 2):

Results
The cellulose functionalization with sodium monochloroacetate involves the partial replacement of the polymeric matrix -OH terminals by carboxyl groups. The substitution degree (DS) is a crucial factor that defines the water solubility of the carboxymethylated cellulose. It is established that the polymer is insoluble but swellable when DS values are below ca. 0.4 (Togrul 2003). On the other hand, in the carboxymethylation reaction, the role of the solvent is to provide the etherifying reagent accessibility into the reaction centres of the cellulosic chain. Consequently, the solvent polarity index (P) is an important property, as it affects the efficiency of this reaction: the bigger the polarity, the lower DS (P water = 1, P ethanol-= 0.655, P isop = 0.546) (Pushpamalar et al. 2006). The water solubility, degree of substitution (DS) and carboxyl group content (CG) obtained for the prepared fibers are given in Table 1. The degree of substitution (DS), defined as the number of the carboxymethyl groups in the molecular unit of the anhydroglucose units, was determined by the standard method (ASTM D1439-03). The carboxyl content of pulp was measured according to the TAPPI T 237 cm-98 norm. While, the water solubility of carboxymethylated pulp was evaluated by the residual solid quantification. The three cellulose etherification methods yielded the expected results, i.e. the less polar solvent brought about the best reaction efficiency and the highest water solubility compared to the other solvents (CMFIP).
The normalized FTIR spectra (Fig. 1) reveal the different degrees of substitution of the prepared functionalized fibers. The FTIR spectrum of the commercial pure CMC (DS = 0.8) presents two main bands at ca. 1600 and 1420 cm -1 in the range 1900-1350 cm -1 due to the carboxylate groups symmetric and asymmetric stretching vibrations (Biswal and Singh 2004;Riaz et al. 2018). These bands could be clearly observed in the CMFE and CMFIP. They could also be seen in the CMFW sample but with lower intensity compared with the CMC fiber. This conclusion could be corroborated with the absence of such bands in the spectrum of the nonmodified support (DP). It could be observed that when ethanol or isopropanol were used for the functionalization, similar results were obtained. Moreover, comparable degree of substitution and carboxyl group content were determined. In order to apply environmentally friendly protocols that follow the principles of ''Green Chemistry'', and taking into account that ethanol is cheaper and less toxic than isopropanol (Beach et al. 2009), the carboxymethylated fibers with ethanol were chosen for the anchoring of palladium nanoparticles. For the sake of comparison, the starting dissolving pump (DP) was also impregnated with palladium. Thereby, Figure S.1 shows the macroscopic aspect of the DP, CMFE and the impregnated DP.Pd and CMFE.Pd fibers.
The Scanning electron microscopy (SEM) micrographs are shown in Fig. 2. The presence of palladium on the matrix was detected by X-Ray spectroscopy (EDX) (Fig. 3). The chemical analysis evidenced 11% of Pd loading on the CMFE.Pd sample and 6% in the DP.Pd fibers. In order to verify these results, the palladium content was also analyzed by thermal gravimetric analysis; resulting 9.5% and 7.3% for CMFE.Pd and DP.Pd respectively (as it shows in Fig. 7).
The Raman spectra of CMFE.Pd and DP.Pd samples together with the CMFE support, are displayed in Fig. 4. Pd-Cl complex species, indicating the presence of partially oxidized palladium species could be observed in the palladium containing material. The signals in 251 cm -1 correspond to species of type Cl-Pd-Cl, while the signal at 650 cm -1 is characteristic of PdO (Baylet et al. 2011). On the other hand, the signals at 439, 581 and 648 cm -1 are characteristics of PdO X/ 2 Cl 2-X species. In addition, the bands at 330-380 and 893 cm -1 correspond to the cellulosic support (Agarwal 2019).
The X-ray diffractogram of the carboxymethylated fibers with ethanol (CMFE) is shown in Fig. 5a. It can be seen that crystallinity of the fibers changed from the cellulose I allomorph of the original commercial dissolving pulp SolucellÒ, after being partially carboxymethylated. This phenomenon is caused by the broadening or cleavage of hydrogen bonds by carboxymethyl substitution on the hydroxyl groups of cellulose (Yeasmin and Mondal 2015). The two broad peaks around 15°and 16.5°together with the intense one at 22.5°correspond to the (110), (110) and (200) planes of the type I cellulose structure, respectively (French 2014) (Cai and Kim 2010). After the functionalization, CMFE showed a broad peak at about 20°. This is in agreement with the structure of amorphous cellulose. Furthermore, the X-ray diffraction study detected the presence of metallic palladium in both samples, CMFE.Pd and CMFE.Pd.Gluc.
In addition, temperature programmed XRD studies under reducing atmosphere were carried out. Using a H 2 flow of 50 ml min -1 and a heating rate of 5°C min -1 , XRD diffractograms were recorded at room temperature (before and after heating) and at 200°C. For the CMFE.Pd.Gluc sample (after reduction in glucose solution), PdH x phase can be detected at room temperature, before heating and after cooling down (Fig. 5b). At 200°C, metallic fcc palladium is formed (JCPDS 03-065-6174). The mean crystallite size was determined using the Scherrer equation, indicating small crystallites of 20 nm.
After reducing treatment with glucose, the fibers were analyzed by Field emission scanning electron microscopy. For the non-functionalized fibers, isolated and non-uniform particles were observed (Fig. 6a-c). However, well-dispersed and uniform palladium nanoparticles were detected on the functionalized CMFE.Pd.Gluc fibers (Fig. 6d-f).
Carboxymethylated cellulose fibers have been reported as an economic and environmentally friendly adsorbent for different metallic ions in solution (Ashraf et al. 2014;Zhu et al. 2016;Ruan et al. 2016;Bediako et al. 2016;Li et al. 2017a;Rossi et al. 2018;Wang et al. 2019). This property can be related to different contributions: the incorporation of a carboxyl functional group and the modification of the crystalline structure of the cellulose. Equation (3) simplifies the interaction between the carboxyl functional group and divalent metallic ions. This interaction is stronger than in the case of hydroxyl groups in the cellulose, therefore the final metal loading is increased.
In addition, the mercerization step modifies the native cellulose (structure type I) in amorphous cellulose. The cellulose treatment with high concentrated NaOH solution conduces to the intra-and inter crystalline swelling, where the cleavage of hydrogen bonds increases the distance between cellulose chains. Therefore, the incorporation of the carboxymethyl groups and the adsorption of metallic ions could be improved (Wang et al. 2017). The crystallinity index of cellulose type I (CrI I ) of non-carboxymethylated fibers was calculated with the following equation (Eq. 4). The intensity data were measured from the XRD analysis shown in Fig. 5, a horizontal line was considered as the baseline from the lowest observed point.
As a result, no changes in the crystallinity of the DP based samples was observed (Table 2).
On the other hand, a possible reaction pathway between the cellulose and the metallic ions was proposed by Wu et al. (Wu et al. 2013). The electron-rich hydroxyl groups in the fibers act as a mild reducing agent forming PdNPs. In this step, hydroxyl groups are oxidized to aldehydes entities. Then, the rich-electronic density of these formed groups could act as an anchor centre for the PdNPs stabilization. In previous works, a similar behavior was observed when suspensions of AgNPs and PdNPs were prepared using glucose and starch (Giorello et al. 2018;Gioria et al. 2019bGioria et al. , 2020. Glucose was employed under the same mild reaction conditions reported in this work. For both Ag and Pd nanoparticles, the reducing role of the glucose was corroborated. Meanwhile starch was used as the stabilizer, anchoring the metallic nanoparticles into the polymeric chains. This analysis could explain the superior metal loading in the carboxymethylated fibers observed by TGA and EDX analysis. The incorporation of the carboxyl groups modifies the crystalline structure, allowing a better access of the palladium ions and their homogeneous distribution. Also, the incorporated carboxyl groups promote the stabilization of the formed PdNPs by electrostatic interactions. The modified structure by the addition of new carboxyl groups could allow a better palladium anion accessibility, which finally results in a homogeneous distribution of PdNPs. TGA and SDTA thermogravimetric analyses were carried out in order to study the thermal stability of the fibers (Fig. 7). The modified cellulose-based catalyst showed a thermal stability up to 300°C, which makes it competitive with traditional inorganic catalysts such as Pd/Al 2 O 3 , Pd/TiO 2 , Pd/Al 2 O 3 -TiO 2 , Pd/V 2 O 5 / TiO 2 /SBA-15 and Pd/V 2 O 5 /TiO 2 /MCM-41 applied under similar reaction conditions (Caravaggio et al. 2016;Wang et al. 2016;Borchers et al. 2021;Song et al. 2021). Figure 7.b shows the SDTA analysis. For the fibers containing palladium, it can be clearly observed a sharp and exothermic peak at 317°C. This could be related to the presence of the metallic entities that catalyzes the combustion of the fibers, as it was reported for similar palladium -cellulose based materials (Cai et al. 2009;Ruan et al. 2016;Shi et al. 2019;Dong et al. 2019). However, all fibers are stable up to 300°C. Thus, the reaction conditions assure the stability of the materials. Figure 8 shows the catalytic performance of both the CMFE.Pd.Gluc and DP.Pd.Gluc fibers. In the studied range of temperature, full conversion of NO was observed and only N 2 O and N 2 were formed as products. For CMFE.Pd.Gluc, NO is converted with a selectivity of 70% to nitrogen at 160°C. At temperatures higher than 180°C, NO X species are completely converted into nitrogen. Regarding the  catalytic behavior of the DP.Pd.Gluc, its performance is lower than for CMFE fibers, with selectivity to N 2 of 52% at 160°C, and 89% at 200°C, the highest temperature analyzed. In both catalyst, there were no evidence of NO 2 formation, which is in accordance with the reductive atmosphere. In addition, there was no evidence of ammonia formation, indicating the active and selective role of Pd towards the reduction of NO with hydrogen. Only N 2 and N 2 O were detected as reaction products. As a trend, it is observed that at low temperatures N 2 O appears as an intermediate specie.
While the temperature increases, the N 2 O concentration decreases, and N 2 increases. This would suggest that at lower temperature, NO is partially reduced to form N 2 O. When temperature increases, the intermediate entity is reduced to inert N 2 . This tendency is clearly evidenced for CMFE fibers. Meanwhile, for the DP fibers temperatures higher than 160°C are needed for a selectivity to N 2 higher than 50%.

Conclusions
Cellulose fibers were successfully functionalized with the partial replacement of hydroxyl groups by carboxymethyl groups: In this process, three different solvents were tested for the cellulose etherification step (isopropanol, water and ethanol). Ethanol was selected as the most convenient solvent due to its inherent features: it is non-toxic, inexpensive and environmentally friendly. Metallic palladium nanoparticles were formed on the surface of the functionalized and no-functionalized fibers by reducing Pd with glucose: After the impregnation procedure using Pd as precursor, it was possible to increase the metallic loading (7.3% to 9.5%) on the carboxymethylated fibers, and to form well-dispersed palladium metallic nanoparticles using glucose as a green and nonexpensive reducing agent.
A battery of techniques such as Raman laser spectroscopy, XRD and TP-XRD under H2 atmosphere, FTIR and SEM-EDX, conduced to corroborate the presence of palladium species on the polymeric structure and the nature of Pd entities: The higher palladium content occurred on the functionalized fibers (CMFE.Pd.Gluc). This advantage means that the modification of fibers with carboxymethyl groups involves an easier method for the increased of active sites on the cellulosic surface compared with the non-functionalized support.
The synthetized cellulosic materials with palladium were active for the reduction of NO with H 2 : The fibers with the higher palladium content (CMFE.Pd.Gluc) showed enhanced catalytic NO conversion to N 2 in comparison with the raw fibers. The full conversion of NO to inert nitrogen was completed even at 180 8C. Even more, the thermal stability up to 300°C assures the competitiveness of the metal-cellulosic fibers, compared with traditionally supported catalysts.
In addition, and to the best of our knowledge, the use of cellulose as support for the reduction of NO under mild conditions has never been reported. The use of this strategy can be further conceptualized for the design and development of a wide range of materials with interesting applications for gas and liquid phase reactions under mild conditions.
With this work, a hybrid metal-cellulosic material was synthesized with promising applications on the catalyst and adsorbent fields, such as the reduction of pollutants in water, cross-coupling reactions, and ion adsorption from polluted water, among others. Nevertheless, the challenge of improving the use of cellulose as promising material still guides our research.