Chia-Pin Liang

Jing Dong

Tim Ford

Rohith Reddy

Hamid Hosseiny

Hamid Farrokhi

Matthew Beatty

Kanwarpal Singh

Hany Osman

Barry Vuong

Grace Baldwin

Catriona Grant

Sarah Giddings

Michalina J Gora

Mireille Rosenberg

Norman Nishioka

Guillermo Tearney
email: gtearney@partners.org

Optical coherence tomography-guided laser marking with tethered capsule endomicroscopy in unsedated patients

Introduction

Tethered capsule endomicroscopy (TCE) is a new technology that implements optical coherence tomography (OCT) using a swallowable capsule that can be administered in unsedated patients [START_REF] Gora | Tethered capsule endomicroscopy enables less invasive imaging of gastrointestinal tract microstructure[END_REF][START_REF] Liang | Volumetric Mapping of Barrett's Esophagus and Dysplasia With en face Optical Coherence Tomography Tethered Capsule[END_REF]. Once swallowed, the device traverses the gastrointestinal (GI) tract, continuously collecting OCT images, creating a three-dimensional microscopic map of the esophagus in its entirety [START_REF] Gora | Tethered capsule endomicroscopy enables less invasive imaging of gastrointestinal tract microstructure[END_REF][START_REF] Liang | Volumetric Mapping of Barrett's Esophagus and Dysplasia With en face Optical Coherence Tomography Tethered Capsule[END_REF]. The procedure takes only 5-6 minutes and patients can return to their daily activities immediately after it is over [START_REF] Gora | Tethered capsule endomicroscopy: from bench to bedside at a primary care practice[END_REF][START_REF] Gora | Imaging the upper gastrointestinal tract in unsedated patients using tethered capsule endomicroscopy[END_REF]. Multiple studies have demonstrated the promise of using TCE for screening for esophageal diseases in patients [START_REF] Gora | Tethered capsule endomicroscopy enables less invasive imaging of gastrointestinal tract microstructure[END_REF][START_REF] Liang | Volumetric Mapping of Barrett's Esophagus and Dysplasia With en face Optical Coherence Tomography Tethered Capsule[END_REF][START_REF] Gora | Tethered capsule endomicroscopy: from bench to bedside at a primary care practice[END_REF][START_REF] Gora | Imaging the upper gastrointestinal tract in unsedated patients using tethered capsule endomicroscopy[END_REF][START_REF] Liang | Cycloid scanning for wide field optical coherence tomography endomicroscopy and angiography in vivo[END_REF][START_REF] Ahsen | Assessment of Barrett's esophagus and dysplasia with ultrahighspeed volumetric en face and cross-sectional optical coherence tomography[END_REF][START_REF] Li | Super-achromatic optical coherence tomography capsule for ultrahigh-resolution imaging of esophagus[END_REF], for other upper GI tract organs [START_REF] Gora | Tethered capsule endomicroscopy for microscopic imaging of the esophagus, stomach, and duodenum without sedation in humans (with video)[END_REF], and the use of esophageal TCE has been reported in a primary care clinic [START_REF] Gora | Tethered capsule endomicroscopy: from bench to bedside at a primary care practice[END_REF]. As opposed to endoscopy, TCE does not require sedation or a specialized setting and can be conducted by nurses or technicians. Following the procedure, the device can be disinfected an features of O Esophagus (B The next m diagnostic ac biopsies. Req whole organ m (VLE) [START_REF] Suter | Comprehensive microscopy of the esophagus in human patients with optical frequency domain imaging[END_REF][START_REF] Wolfsen | Safety and feasibility of volumetric laser endomicroscopy in patients with Barrett's esophagus (with videos)[END_REF] power laser l patient's esop these marked caused by pa [START_REF] Suter | Esophageal-guided biopsy with volumetric laser endomicroscopy and laser cautery marking: a pilot clinical study[END_REF][START_REF] Swager | Feasibility of laser marking in Barrett's esophagus with volumetric laser endomicroscopy: first-in-man pilot study[END_REF]. Stud marking is hi spatial targeti now being use Laser mar device in vivo balloon. Our p showed that exposure dura time required system that p artifact.

In addition into the capsu important ben (NURD) artif Here, we rep micromotor-b Fig. 1 over a distor nd reused ma CT-TCE poten BE), which here major step in th ccuracy agains quisite co-loca microscopy tec 1] and TCE. U light solves thi phagus that co sites are inher atient/device m dies using dri ighly effective ing error of ab ed clinically in rking in TCE o, as it is not previous laser laser marks w ation (Fig. 1). T d to generate a provided highe n, it has been ule has many a nefits of micro facts [START_REF] Liang | Ultrahigh speed en face OCT capsule for endoscopic imaging[END_REF] that c port the deve based laser mar A distal 2 beam and aim with the dista fiber within th size of 30-40 motor's shaft were focused diameter of th motor control study is rotocol is y subjects are recruited from patients with biopsy-proven BE, aged 18 and older, who are undergoing an upper endoscopy. After the capsule was swallowed by an unsedated subject, the operator started OCT imaging. Images were recorded while the capsule passively traveled down the esophagus. Once the OCT image indicated that the capsule had passed through the gastroesophageal junction and reached the stomach, the capsule was manually pulled back across the lower esophageal sphincter and through the esophagus using the tether. When a target of interest was identified during pull-back imaging, the tether was held in place, and the tether tick mark reading at the incisors was recorded. The operator subsequently stepped on the foot pedal and enabled the marking laser system by pressing the enable button. Then, the operator clicked on the target displayed on the screen and pressed the activation button to place the laser mark(s). One or two marks may be placed, depending on study protocols and user input prior to the procedure. After marking, the motor automatically resumed spinning to acquire post-marking images. Following a short period of post-marking imaging, the tether tick marks at the incisor were recorded again and imaging and pullback resumed. This process was repeated for the next region of interest (up to two marks per centimeter and a maximum of 6 marks per subject). Most laser markings were performed at the diseased area, which is typically at distal portion of the esophagus, near the gastroesophageal junction. After all targets in the subject were laser marked, the capsule was pulled out from the subject and a questionnaire regarding tolerability of the procedure was administered. Then, the subject was sedated and underwent standard of care endoscopy with biopsy. During the endoscopic procedure, the laser marks were identified and biopsies were acquired in between or adjacent to the laser marks. Histology from the biopsy samples were then correlated with the corresponding OCT images. When multiple biopsies were taken, the histology was further correlated with OCT data by the corresponding tether tick mark recorded at the incisors.

The targeting accuracy of TCE laser marking

We quantified the cross-sectional targeting accuracy in human subjects in vivo by subtracting the location of the selected target position in the pre-marking image from the location of hyper-reflective mark in the post-marking image. Pre-and post-marking images were first coregistered using anatomical landmarks (e.g. tissue fold, crypt, and glands etc.) shared by both images.

Results

1 Laser thermal injury tests on ex vivo swine esophagus

Using 800 mW of power at 1430/1450 nm, the exposure time between 1 and 4 seconds created cauterized marks that were visually identifiable (Fig. 7(A)). Longer exposure times created larger cauterized marks. Figure 7(B) shows a representative NBTC histology image at a marking site (800 mW, 1s). . (E) Histology from a biopsy taken between the two laser marks confirms that the tissue between the marks is squamous epithelium. Figure 12 shows an example of TCE laser marking guided biopsy of Barrett's esophagus, in vivo. The pre-marking OCT image (Fig. 12(A)) shows BE as tissue without squamous layering, heterogeneous backscattering, and an irregular mucosal surface. For this case, only one laser mark was applied on the BE region at the location denoted by the red line (Fig. 12(A)). After the laser mark was applied, the post-marking image showed a highly reflecting laser cautery mark near the intended location (Figs. 12(B), (C)). White light endoscopy performed after the TCE procedure demonstrates a clearly visible laser mark on the surface of salmon-colored esophageal mucosa (Fig. 12(D), red arrow). Histology from an endoscopic biopsy obtained adjacent to the mark (Fig. 12(E)) shows specialized intestinal metaplasia, the most common form of BE.

The accuracy of TCE laser marking in vivo

We were able to successfully co-register 9 pre-and post-marking images from the 5 subjects. After subtracting intent-to-mark and post-marking locations, the cross-sectional difference between the intended target and the actual marking positions was 0.95 +/-0.53 (standard deviation) mm.

Discussion

The field of TCE is growing with many studies now using tethered capsules for GI tract diagnosis [START_REF] Gora | Tethered capsule endomicroscopy enables less invasive imaging of gastrointestinal tract microstructure[END_REF][START_REF] Liang | Volumetric Mapping of Barrett's Esophagus and Dysplasia With en face Optical Coherence Tomography Tethered Capsule[END_REF][START_REF] Li | Super-achromatic optical coherence tomography capsule for ultrahigh-resolution imaging of esophagus[END_REF]. Since TCE does not have biopsy capability, it is essential that new technology become available to allow TCE images to be correlated to histopathology. Without histopathologic validation, this promising technology cannot be used for patient care. Laser marking using a tethered capsule is the most attractive way to conduct these validation studies and thus TCE laser marking technology presented in this study is a critical advancement. Moreover, we have demonstrated that using TCE laser marking to guide biopsy is feasible and safe in human subjects. After validation, the TCE device can be used independently for screening unsedated patients and for placing marks at lesions that may require follow-up during a subsequent endoscopy.

The main challenge that we encountered when conducting TCE-based laser marking was marking precision artifacts caused by motion of the capsule in the esophagus, which is greater than that of balloon-catheter VLE. By increasing the power to 800 mW and decreasing the laser exposure time to 1 second, we found that these artifacts can be overcome to produce tightly localized marks on the esophagus in living human subjects (Fig. 11(D) and 12(D)). Besides shortening the laser exposure time, we further mitigated this potential issue by performing laser marking only during pullback (Fig. 6). This procedural step allows us to fix the proximal end of the capsule to the leading edge of the peristaltic contraction. In addition, the size of targets, such as BE, dysplasia and adenocarcinoma are typically > few mm [START_REF] Cameron | Barrett's esophagus, high-grade dysplasia, and early adenocarcinoma: a pathological study[END_REF] and the accuracy of endoscopic biopsy is > 1 mm, which are roughly on the same scale as the ~1mm marking error at the dynamic gastroesophageal junction (GEJ) as shown in Fig. 12(A) and (B). Our overall cross-sectional targeting accuracy is 0.95 ± 0.53 mm (standard deviation from 5 subjects, 9 marks). Finally, since the hyper-reflective laser marks can be visualized in OCT images, we can correlate histology with post-marking OCT images for validating the diagnostic accuracy against histology. Given these considerations, the accuracy of TCE laser marking should be more than sufficient for most regions of interest in the esophagus.

In addition to optimizing the laser marking exposure parameters, we improved the device by incorporating a distal stepper micromotor in the capsule to scan the OCT beam. The A-line clock was used for synchronizing the A-line acquisition and motor rotation, so that a stationary target would not rotate and would remain stationary in the imaging window for high-precision marking. Once receiving the coordinate of target, the motor stopped at one of the 40 dwelling points that was closest to the target. The maximum discrepancy between the intended position and the actual dwelling position was half of the space interval between two neighboring dwelling points. Figure 10 shows the histogram of the size of space intervals. The maximum interval at the capsule's surface was 1 mm. Therefore, the maximum discrepancy between the intended position and the actual dwelling position (targeting error) was ~0.5 mm. Under same number of steps, the targeting accuracy scales inversely with diameter, as the diameter goes up, the accuracy goes down and vice versa. We have initially used 11-mm-diameter TCE devices as this is a standard set forth by predicate devices such as video capsule endoscopes. Unsedated TCE targeting accuracy is similar to sedated VLE targeting accuracy and thus both technologies may be used to guide targeted biopsy acquisition. The aim of this study was to demonstrate the feasibility of performing laser marking on unseated patients with TCE devices. In addition, since the stepper motor does not require an encoder for absolute position determination, the device can be less expensive and complex, which is consistent with the low-cost requirement of a capsule-based screening technology.

We used ex vivo animals to test our marking laser parameter set and froze those parameters for the clinical product. All subjects were tested under the same parameters. These parameters and the described clinical device satisfy the requirements for effective TCE-based laser marking in human subjects. For our exposure parameters (800 mW, 1430/1450 nm), the deposited laser energy, as determined by exposure duration (1-4 s), diffused predominantly in the lateral direction. From 1 to 4 seconds, the lateral injury width (Fig. 8(A)) increased by a factor of two, but the depth (Fig. 8(B)) only increased by ~20%. Therefore, it is not surprising that the laser thermal injury does not penetrate beyond muscularis mucosae even with a 4s exposure time (Fig. 8(C)). The extent of injury on the swine esophagus is comparable to that of standard biopsy forceps. For minimizing the motion artifact, we used a 1 s exposure time, which has a large 4x safety margin.

Results obtained in 5 subjects so far demonstrate that TCE laser marking can be a safe and effective way to correlate OCT TCE images to co-localized biopsies and corresponding histology. The missing mark (1 out of 12 attempts) was successfully identified in OCT image, but could not be identified by video endoscopy. Due to an uneven tissue surface, video endoscopy was not able to visualize 100% of the esophagus. In the future, we can increase the number of marks per target to avoid this potential issue. The co-localization capability is highly important now to validate the diagnosis of TCE images in histopathologic correlative studies. In addition, guided biopsy via TCE could also find similar utility to real-time targeting in VLE, enabling the sampling of aberrant tissue that is missed by random endoscopic biopsy, the current standard of care. TCE laser marking has advantages over balloon-based VLE targeting as the former can be conducted in unsedated patients at the point of care. Larger clinical studies using TCE laser marking should be conducted to validate TCE and determine its clinical utility for guiding biopsy.

Looking towards the future, it is also possible to consider a scheme where TCE diagnosis, targeting, and laser marking are performed automatically, based on advanced signal processing [START_REF] Swager | Quantitative attenuation analysis for identification of early Barrett's neoplasia in volumetric laser endomicroscopy[END_REF][START_REF] Ughi | Automated segmentation and characterization of esophageal wall in vivo by tethered capsule optical coherence tomography endomicroscopy[END_REF] or machine learning methods [START_REF] Tsai | Optical coherence tomography in gastroenterology: a review and future outlook[END_REF]. Such an advance could be facilitated by use of a pulsed Raman fiber laser [START_REF] Baac | High-energy pulsed Raman fiber laser for biological tissue coagulation[END_REF], coupled into the inner cladding of a double clad fiber, recently demonstrated for ablating tissue in real time while OCT imaging [START_REF] Beaudette | Laser tissue coagulation and concurrent optical coherence tomography through a double-clad fiber coupler[END_REF]. This strategy would also be consistent with the screening case use scenario of tethered capsule technologies, obviating expert image interpretation during the TCE procedure.

Conclusion

OCT-based tethered capsule endomicroscopy is an up and coming, minimally-invasive technique for upper GI tract microscopic screening in unsedated patients. Histologic validation is a critical step that is required for the adoption of TCE. Here, we have described a TCE laser marking system and a stepper micromotor, beam scanning capsule that allows TCE OCT images to be correlated to endoscopic biopsies taken from the patient. Animal and human data using this technology in vivo shows that TCE laser marking is an effective and safe method for making this correspondence. Beyond histopathologic validation, TCE laser marking also has the potential to become a useful tool for marking microscopically abnormal tissue found during an outpatient capsule screening procedure that can subsequently be biopsied endoscopically. The convenience and cost-profile of such a strategy could significantly improve the care of patients with BE and other gastrointestinal diseases.

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Fig. 11 .

 11 Fig. 11. TCE laser marking of a study subject's (a BE patient) normal esophagus. (A) Premarking OCT image of a region of the normal esophagus. The red lines indicate where the operator intends to place the laser marks. (B) OCT image of the study subject's esophagus after laser marking. Orange arrows demarcate the hyper-reflective signal from the cauterized laser marks. (C) Enlarged image of the marking sites, showing the OCT appearance of the laser marks (orange arrows) in greater detail. (D) Endoscopy image of the laser marks (red arrows). (E) Histology from a biopsy taken between the two laser marks confirms that the tissue between the marks is squamous epithelium.

Fig. 12 .

 12 Fig. 12. TCE laser marking of Barrett's esophagus in a study subject. A) Pre-marking OCT image of BE mucosa. The red line indicates where the operator intends to place the laser marks. (B) OCT image of the study subject's esophagus after laser marking. The orange arrow points to hyper-reflective signal from the cauterized laser mark. (C) Enlarged image of the marking site, showing the OCT appearance of the laser mark (orange arrow) in greater detail. (D) Endoscopy image of the laser mark (red arrow). (E) Histology from the biopsy taken adjacent to the laser mark, confirming that the marked tissue was BE.

Funding

National Institutes of Health grants (NIH R01CA184102, R01DK100569, R01EB022077); Remondi Family Foundation; the Mike and Sue Hazard Family; the MGH Research Scholars Program.

Disclosures

Massachusetts General Hospital has a licensing arrangement with NinePoint Medical. Dr. Tearney has the rights to receive royalties from this licensing arrangement. Dr. Tearney also consults for NinePoint Medical. Dr. Tearney receives sponsored research from Boston Scientific and iLumen Medical.


Katarzyna Świrk
email: katarzyna.swirk@umontpellier.fr

Gérard Delahay

Abdelali Zaki

Karim Adil

Amandine Cadiau
email: amandine.cadiau@umontpellier.fr

Facile modifications of HKUST-1 by V, Nb and Mn for low-temperature selective catalytic reduction of nitrogen oxides by NH 3

Keywords: Metal-organic frameworks, HKUST-1, manganese, NH 3 -SCR, NOx Highlights

HKUST-1 catalysts were impregnated with vanadium, niobium and manganese species and tested in NH 3 -SCR for NO removal. No reduction of NO by NH 3 was registered for the unmodified HKUST-1 and disappearance of NO was attributed to adsorption. After a pretreatment at 185°C, the best catalytic performance was found for HKUST-1-Mn with 100% and 80% conversion of NH 3 and NO, respectively at 185°C. For the first time, the H 2 O tolerance was examined on the modified HKUST-1 catalyst. The addition of water vapor resulted in decrease of NO and NH 3 conversions, which was immediately recovered when H 2 O feeding was stopped. The NO conversion dropped from 76% to 68% after 35 min of H 2 O addition.

Introduction

Air quality preservation is undoubtedly one of the major environmental issue facing by our modern society. The emissions of nitrogen oxides (NOx) are targeted to be reduced as they cause serious hazards to earth climate, biological ecosystems, and human health. Selective catalytic reduction of nitrogen oxides by NH 3 (NH 3 -SCR) is a well-known process with an efficient approach for removing air-polluting NOx [1]. At present, for stationary sources, the most common applied catalyst is V 2 O 5 -WO 3 (MoO 3 )/TiO 2 deposited on a ceramic monolith (vanadium plays the role of active phase). This material allows high conversion of NOx, however, its application is limited by relatively narrow temperature window (300-400°C) and toxicity of vanadium oxide [1,2]. This commercial catalyst is installed in the "high-dust" position, i.e., in contact with SO 2 , particles of fly ash and alkaline metals which impact its lifetime. The optimal placement of a catalyst would be at the "tail-end", behind the electrostatic precipitator and the flue-gas desulfurization unit. However, this position will require reheating of the flue gases, so that the commercial catalyst can work efficiently [3]. Therefore, finding a novel eco-friendly material with high NH 3 -SCR activity at temperatures lower than 250°C is a big challenge. Up to now, fruitful research has been made on the alternative oxides or zeolites materials containing Mn or Nb species, being active and stable in the NH 3 -SCR [4,5].

On the other hand, the use of Metal-Organic Frameworks (MOFs), a recent class of crystalline materials can offer an alternative to the inorganic compounds. MOFs are hybrid porous solids, built from organic linkers and inorganic nodes [6,7]. Their enhanced specific surface area, porosity and tunable topologies have already attracted much attention in different applications such as gas separation, adsorption, and catalysis [7][8][9][10][11][12]. Since copper-containing materials are a great alternative for NOx Herein, we proposed a facile modification of commercially available HKUST-1 by V, Nb and Mn salts. The catalysts were obtained by incipient wetness impregnation method assuming 2.5 wt.% of metal in the individual sample. The promotion with Nb and V is reported for the first time in this type of MOF materials. The vanadiumpromoted HKUST-1 catalyst was synthesized in order to compare the performance with the commercial catalyst containing V 2 O 5 as an active phase. The influence of the addition of a promoter (V, Nb, Mn) on the materials properties and the catalytic performance was also discussed in detail.

Experimental

Synthesis of catalysts

HKUST-1-x (x=V, Nb, Mn) catalysts were prepared by incipient wetness impregnation using ammonium monovanadate (Merck), ammonium niobate (V) oxalate hydrate (Sigma-Aldrich), and manganese (II) nitrate tetrahydrate (Sigma-Aldrich), respectively (as described in detail below). HKUST-1 powder was purchased from Sigma-Aldrich, under the commercial name of Basolite® C300. For the aqueous solutions, ultra-pure water was used. The assumed metal loading in the individual sample was 2.5 wt.%. After the impregnation step, the samples were sonicated for 20 min at 30°C, and subsequently dried overnight at 70°C. No washing was applied during the syntheses.

HKUST-1-V: 0.0589 mg of ammonium monovanadate was dissolved in ca. 3 ml of ultra-pure water. 1 g of commercial HKUST-1 was impregnated drop wisely with the so-prepared aqueous solution of the salt.

HKUST-1-Nb: 0.0836 mg of ammonium niobate (V) oxalate hydrate was dissolved in ca. 3 ml of ultra-pure water. 1 g of commercial HKUST-1 was impregnated drop wisely with the so-prepared aqueous solution of the salt.

HKUST-1-Mn: 0.1208 mg of manganese (II) nitrate tetrahydrate was dissolved in ca. 3 ml of ultra-pure water. 1 g of commercial HKUST-1 was impregnated drop wisely with the so-prepared aqueous solution of the salt.

Characterization methods

Powder X-ray diffraction analysis was carried out on a D8 Advance Bruker diffractometer equipped with Cu Kα radiation of λ=0.15406 nm. The diffraction patterns were obtained in an angle range of 2θ = 4-80° with a 0.02° step and an accumulation time of 1 s per step.

Textural properties of the studied HKUST-1 catalysts were determined by nitrogen adsorption-desorption at -196°C using a Micromeritics ASAP 2020 -surface area and porosity analyzer. Before the analysis, the samples were pretreated under vacuum at 150°C for 6 h. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area, and the Barrett-Joyner-Halenda (BJH) desorption method was applied to determine the volume of mesopores. The t-plot was used for calculation of microporous volume.

The morphology of metal-organic frameworks was studied by a scanning electron microscope Hitachi 4800S with an acceleration voltage of 5 kV. Energy Dispersive Spectroscopy (EDS) chemical analyses (in atomic %) were carried out on a FEI Quanta 200F (15 kV) equipment. The elemental analysis was performed with the aid of AZtec software.

Thermogravimetric analysis (TGA) and differential thermal analyses (DTA) were carried out by a PerkinElmer STA 6000 Simultaneous Thermal Analyzer. The sample (approximately 15 mg) was loaded into a crucible, and heated in a flow of air (60 cm 3 min -1 ) from 40°C to 900°C with a heating rate of 5 °C min -1 .

Temperature-programmed desorption of NH 3 (NH 3 -TPD) of as-synthesized catalysts was conducted on a Micromeritics AutoChem 2910 apparatus using a thermal conductivity detector (TCD) to monitor the NH 3 evolution. Firstly, the samples were heated from 40°C to 250°C with a heating rate 5°C min -1 . Thereafter, they were cooled down to 140°C, and NH 3 was adsorbed at 140°C for 30 min (a mixture 5 vol.% NH 3 /He, flow rate=30 cm 3 min -1 ). The physiosorbed ammonia was removed by flowing He (flow rate=30 cm 3 min -1 ) for next 30 min. Finally, ammonia desorption was carried out in helium (flow rate=30 cm 3 min -1 ) by increasing the temperature to 250°C with a heating rate of 2°C min -1 . The sample was kept for 60 min at 250°C.

Catalytic tests

Three types of NH 3 -SCR tests were performed: (i) temperature programmed surface reaction (TPSR) from 145°C to 250°C, with the pre-treatment at 250°C, (ii) isothermal tests at 185°C for 480 min, assuming either pre-treatment at 250°C, or 185°C, and (iii) isothermal test at 185°C, involving the presence of H 2 O in the reaction feed for 35 min.

The tests were performed in a U-shaped glass reactor with 0.075 g of a catalyst. For heating, an electric furnace was used. The experiments were conducted at atmospheric pressure with the total flow rate of 100 ml min -1 , corresponding to a weight hourly space velocity (WHSV) of 80,000 ml g -1 h -1 . Prior to the NH 3 -SCR tests, the catalyst was pre-treated in He from 40°C to either 250°C or 185°C with a heating rate of 5°C min -1 . In the first type of tests, heating rate of 2°C min -1 . The reactant gas mixture was adjusted to 1000 ppm NO, 1000 ppm NH 3 , 8 vol.% O 2 and balance with helium. In the H 2 O-resistance test, water was injected by circulating the helium flow for 35 min via a gas bubbler containing deionized water heated at 30 °C which corresponds to 3.5%

Results and discussion

The structure of the MOFs was confirmed by PXRD, SEM/EDS and N 2 adsorption/ desorption characterization methods. Fig. 1A presents PXRD diffractograms, where typical reflections of the commercial Basolite® C300 (here as HKUST-1) were observed at 2θ=6.7°, 9.5°, 11.7°, 13.5°, 17.4°, 19.3° for (200), ( 220), ( 222), (400), ( 333) and ( 440) planes [21][22][23]. For the HKUST-1-(V, Nb, Mn) the registered reflections did not change compared to the parent sample, proving the phase-purity of the as-modified materials. The absence of crystalline metal species suggests their well dispersion, as proven by SEM and EDS (Fig. 2; Fig. S2 andS3, ESI †). The reflections intensities of V-containing catalyst were, however, three times lower than for the other catalysts, pointing the loss of crystallinity in V-sample. This may be attributed to formation of some defect sites in the crystal lattice of .

Textural properties of the studied samples are listed in Table 1 (Fig. S1,ESI †). The values of specific surface area (S BET ) for HKUST-1 is in accordance to the data postulated by the chemical provider (Basolite® C300: 1500-2100 m 2 /g). For the modified samples, S BET and pore volume decreased as a consequence of a partial blockage of the cavities by V, Nb and Mn species.

Fig. 1 PXRD patterns of parent HKUST-1 and modified with V, Nb and Mn salts (A), and of the spent HKUST-1-Mn: pre-treated at 250°C and tested at 185°C for 480 min; pre-treated at 185°C and tested at 185°C for 480 min; and pre-treated at 185°C and tested at 185°C assuming H 2 O on/off step (B). The PXRD pattern of HKUST-1-V is multiplied by three in Fig. 1A. The PXRD pattern of HKUST-1-Mn pre-treated at 250°C is multiplied by six in Fig. 1B.

However, the impregnation of HKUST-1 led to the formation of new mesopores. Their existence can be also confirmed by the adsorption-desorption isotherms. For the modified samples, type-IV isotherms with type H4 hysteresis loop were observed, [25] whereas no hysteresis was registered for the parent sample (type-I isotherm). The coexistence of micro-and mesopores in HKUST-1 structure has been already reported for materials where mixed-ligand was assumed during a synthesis, [24,26,27] or impregnation with metal salts was used [28]. In the latter studies, the S BET decreased with the non-intraframework mesoporosity formation, alike to our results. 10 SEM images of HKUST-1 and HKUST-1-(V, Nb, Mn) samples are presented in Fig. 2, exposing mostly octahedron and some cuboctahedral crystal shape in agreement with the literature [21,23,29]. A magnification of V and Nb modified MOFs revealed a presence of rods, possibly arising from ammonium monovanadate and ammonium niobate, respectively, and covering the HKUST-1 crystals, leading to a blockage of the porous surface. The magnification of HKUST-1-Mn revealed aggregates with a laminar structure, perhaps as a part of manganese nitrate used during the synthesis (Fig. S2, ESI †). Moreover, Mn-catalyst showed numerous cracks on the crystals surface (Fig. 2; Fig. S2,ESI †). The EDS elemental mapping displayed the even distribution of vanadium, niobium, and manganese elements in the modified samples (Fig. S3, ESI †). The average C/Cu ratio was 9.8, 8.1, 12.6, 9.2 for the HKUST-1, HKUST-1-V, HKUST-1-Nb, HKUST-1-Mn, respectively (Table 1). The highest carbon content in HKUST-1-Nb can be attributed to Since HKUST-1 material exhibits Lewis-acidity, temperature programmed desorption of ammonia (NH 3 -TPD) was carried out to examine the catalyst. The materials were pre-treated in helium in the temperature range of 40-250°C to remove water molecules (Fig. S6, ESI †). Table 1 presents the calculated values of NH 3 desorption previously adsorbed at 140°C (from the profiles presented in Fig. S7A, ESI †), showing the positive effect of the V, Nb, and Mn-modifications. Total amounts of NH 3 desorbed were: HKUST-1 (965 µmol/g) < HKUST-1-V (1073 µmol/g) < HKUST-1-Nb (1324 µmol/g) < HKUST-1-Mn (1408 µmol/g). The obtained TPD-NH 3 desorption curve of the parent sample is similar to the one reported by Gao et al.[32] (Fig. S7B; ESI †). The open-metal sites, located on the interior of the pore wall, facilitate the NH 3 desorption in HKUST-1 [33]. For the modified samples, the desorption profiles were more intense, and a distinctive behavior was observed for HKUST-1-Mn. For the latter, a gradual increase of desorbed NH 3 was registered, slight increase at ca. 240°C for Nb-and V-catalysts was also recorded. (negative conversion values) (Fig. 3A). During this desorption phase, low NO conversion was registered (maximum 10%), suggesting a competitive adsorption/desorption of NH 3 and NO in HKUST-1, as no signals of possible NH 3 -SCR products (N 2 and/or H 2 O) were recorded (Fig. 3B). The modification with V led to the similar NH 3 adsorption behavior (Fig. 3C). A significant increase of NO conversion was recorded just after NH 3 desorption step (ca. 210°C), further reaching maximum of 45% at 250°C, and slightly decreasing in 15 min time period. Both main products were registered during NH 3 -SCR for this sample, i.e., N 2 and H 2 O. Also, CO 2 production was observed, as supported by the signal arising from carbon (Fig. 3D).

Since the trend of the ion current signals for both CO 2 and C is similar, the existence of N 2 O can be ruled out. The presence of carbon dioxide implies a certain damaging of the structure, i.e. a possible partial decomposition of catalyst. For HKUST-1-Nb catalyst a very similar performance to V-catalyst was observed (Fig. 3E). The former showed the maximum NO conversion of ca. 38%, which was progressively increasing in time-on-stream at 250°C. Meanwhile, promotion with Mn resulted in a considerably higher catalytic activity at 250°C with 80% and 100% of NO and NH 3 conversions, respectively (Fig. 3G). Conversions for both reactants were rapidly increasing as a function of temperature. In contrast to the other catalysts, no NH 3 desorption however, led to complete conversion of NH 3 , while NO was reduced in maximum 80%

to N 2 and H 2 O. This shows that temperature of pre-treatment determines the catalytic activity, as a partial degradation of the MOF structure negatively affects the NH 3 -SCR performance.

Since the existence of water vapor in exhaust gases in unavoidable, H 2 O-resistance was also examined for HKUST-1-Mn catalyst pre-treated at 185°C. As presented in Fig. 5A, the NO and NH 3 conversions rapidly reduced from 74% to 26% and from 78% to 37%, respectively, when H 2 O was introduced as a reactant for 35 min. When H 2 O was cut off, the conversions recovered quickly giving 68% for both NO and NH 3 after one hour of H 2 O-free reaction. This shows that the inhibiting water effect is reversible for HKUST-1-Mn. The obtained products were N 2 and H 2 O, whereas, during the H 2 Ofeeding, a slight formation of CO 2 was observed, indicating that water presence negatively affected the modified-MOF (Fig. 5B). In the framework of HKUST-1, four carboxylate groups per one copper forms a so-called paddle-wheel unit. A weak bond between Cu 2+ ions and the residual axial coordination site can be attracted by water molecule [9,33]. During the H 2 O-feeding, a competitive adsorption between H 2 O and NH 3 seems to be a plausible explanation [34,35].

  H 2 O vapor. The gas lines were heated to prevent condensation of water. The outlet gas composition was constantly monitored by vacuum stripping a fraction of the exit gas with a capillary towards the mass spectrometer (Pfeiffer Omnistar) equipped with Channeltron and Faraday detectors following the specific masses of: NO (m/z = 30), NH 3 (m/z = 15), N 2 (m/z = 28), H 2 O (m/z = 18), N 2 O (m/z = 44), CO 2 (m/z = 44), and C (m/z = 12).

Fig. 2

 2 Fig. 2 SEM images of HKUST-1 catalysts modified with V, Nb and Mn.

C 4 H 4

 4 NNbO 9 •xH 2 O used during the synthesis. The calculated C/Cu ratio for HKUST-1 is similar to one registered by SEM/EDS in the study of Li et al. [30]. The authors reported C/Cu of 8.4. Moreover, this ratio has been confirmed by XPS in the work of Yao et al. [9]. The latter authors found C/Cu for HKUST-1 and Mn-modified HKUST-1 in the range of 7.8 -8.3.Thermogravimetric analysis was carried out in order to determine the thermal stability (Fig.S4, ESI †). All samples revealed ca. 15% mass loss up to 150°C, which is assigned to the removal of water and guest molecules[23]. A second weight loss began at 282, 280, 277 and 275°C, respectively for HKUST-1, HKUST-1-V, HKUST-Nb, and HKUST-1-Mn catalysts, following a sudden mass reduction after a slight increase of the temperature (ca. 50°C). The latter resulted from the decomposition of BTC linker. The PXRD analysis of the residues, formed upon the thermal treatment at 900°C, exhibits the presence of CuO, CuNb 2 O 6 , and CuMn 2 O 4 for HKUST-1, HKUST-Nb, and HKUST-1-Mn, respectively (Fig.S5, ESI †). A lack of V-phases in HKUST-1-V sample arises from a sublimation of vanadium species at 900°C (Fig.S6, ESI †)[31].

Fig. 3

 3 Fig. 3 NH 3 -SCR catalytic results (left) and ion current of the products (right) for HKUST-1 (A, B), HKUST-1-V (C, D), HKUST-1-Nb (E, F), and HKUST-1-Mn (G, H). Reaction conditions: [NO] = [NH 3 ] = 1000 ppm, [O 2 ] = 8 vol.%, He balance, temperature range = 145-250°C, WHSV = 80,000 ml•g -1 •h -1 .

Fig. 5

 5 Fig. 5 NH 3 -SCR catalytic tests over HKUST-1-Mn at 185°C in the presence of H 2 O. Reaction conditions: [NO] = [NH 3 ] = 1000 ppm, [O 2 ] = 8 vol.%, 3.5 vol.% H 2 O, He balance, WHSV = 80,000 ml•g -1 •h -1 .

Table 1

 1 Textural properties, relative elemental composition and acid amount of HKUST-1 catalysts.

	Catalyst	S BET	V mic	V mes	C/Cu	Acidity
		[m 2 /g] a	[cm 3 /g] b	[cm 3 /g] c	[mol/mol] d	[µmol/g] e
	HKUST-1	1709	0.60	0.02	9.8	965
	HKUST-1-V	1248	0.43	0.05	8.1	1073
	HKUST-1-Nb	1043	0.36	0.06	12.6	1324
	HKUST-1-Mn	1315	0.44	0.13	9.2	1408
	HKUST-1-Mn f	335	0.09	0.35	n.m.	n.m.
	HKUST-1-Mn g	533	0.16	0.41	n.m.	n.m.
	HKUST-1-Mn h	465	0.15	0.37	n.m.	n.m.

a calculated from the Brunauer-Emmett-Teller (BET) equation; b determined by means of t-plot; c determined from the BJH desorption cumulative volume of pores; d average atomic ratio from SEM/EDS; e NH 3 desorption determined by the NH 3 -TPD; f, g pre-treated at 250°C and 185°C, NH 3 -SCR test at 185°C for 480 min, respectively; h pre-treated at 185°C, NH 3 -SCR test at 185°C assuming water on/off step; n.m. -not measured.

phenomenon was registered for Mn-sample. The same like for Nb and V modified MOFs, nitrogen and steam were recorded as products (Fig. 3H). Some CO 2 is detected, starting from 190°C. In order to prevent its production, we decided to carry out our next experiments at the highest possible (and carbon-free) temperature, i.e. 185°C. A stability test was performed for HKUST-1-Mn at 185°C for 480 min, evaluating pre-treatment either at 250°C (Fig. 4A), or 185°C (Fig. 4C). For the former, increasing conversions of NH 3 and NO were registered reaching almost 100% and 60%, respectively in the first 60 min. Furthermore, a remarkable decrease of both was observed, giving a steady value of ca. 45% in the next 230 min. This agrees with NO conversion obtained in TPSR test (cp. Fig. 3G). Moreover, only N 2 and H 2 O were recorded as products in this catalytic test (Figs. 4 B,D). The pre-treatment at 185°C,


Hassib Delahay

Nawel Jraba
email: naweljraba@hotmail.fr

Thabet Makhlouf

Gerard Delahay

Hassibtounsi

Prof Thabet Makhlouf

Catalytic activity of Cu/η-Al 2 O 3 catalysts prepared from aluminum scraps in the NH 3 -SCO and in the NH 3 -SCR of NO

Keywords: Cu/η-Al 2 O 3, NH 3 -SCR, NH 3 -SCO, H 2 -TPR, TEM 1

Catalytic activity of Cu/η-Al2O3 catalysts prepared from aluminum scraps in the NH3-SCO and in the NH3-SCR of NO

Nawel Jraba, Thabet Makhlouf, Gérard

Introduction

The selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen is considered a mature technology for the removal of NO from stationary sources [START_REF] Forzatti | Present status and perspectives in de-NOx SCR Catalysis[END_REF][START_REF] Usberti | Design of a highefficiency NH 3 -SCR reactor for stationary applications. A kinetic study of NH 3 oxidation and NH 3 -SCR over V-based catalysts[END_REF]. Moreover, this technology has been adapted for the modern diesel exhaust after treatment system using urea as the ammonia precursor (urea-SCR) [START_REF] Goldbach | Urea decomposition in selective catalytic reduction on V 2 O 5 /WO 3 /TiO 2 catalyst in diesel exhaust[END_REF][START_REF] Jung | NOx and N 2 O emissions over a Urea-SCR system containing both V 2 O 5 -WO 3 /TiO 2 and Cu-zeolite catalysts in a diesel engine[END_REF][START_REF] Kröcher | Selective catalytic reduction of NOx[END_REF]Nova and Tronkoni, 2014;[START_REF] Piumetti | Catalysis in Diesel engine NOx aftertreatment: a review[END_REF][START_REF] Yuan | Diesel engine SCR control: current development and future challenges[END_REF]. This system contains besides the urea delivery device, a catalyst for the selective catalytic reduction of NO with NH 3 (NH 3 -SCR) combined with an ammonia slip catalyst (ASC) for the selective catalytic oxidation of ammonia (NH 3 -SCO) [START_REF] Piumetti | Catalysis in Diesel engine NOx aftertreatment: a review[END_REF][START_REF] Walker | Future challenges and incoming solutions in emission control for heavy duty diesel vehicles[END_REF].

Vanadium-based catalysts (V-catalysts) V 2 O 5 -WO 3 /TiO 2 or V 2 O 5 -MoO 3 /TiO 2 are widely employed in stationary applications since the 1970s [START_REF] Lai | A perspective on the selective catalytic reduction (SCR) of NO with NH 3 by supported V 2 O 5 -WO 3 /TiO 2 catalysts[END_REF]. However, the major drawbacks bounded to the toxicity of vanadium and its weaker activity at low temperature limit the utilization of V-catalysts in automotive applications. Besides, some country regulations such as USA forbid the use of V-catalysts for automotive applications. Therefore, Cu/Fe-exchanged zeolites have been reported as alternative to vanadium-based catalysts because they are active and N 2 selective for the NH 3 -SCR (Boron et al., 2019;[START_REF] Villamaina | Effect of the NH 4 NO 3 Addition on the Low-T NH 3 -SCR Performances of Individual and Combined Fe-and Cu-Zeolite Catalysts[END_REF][START_REF] Xin | Zeolitic materials for DeNOx selective catalytic reduction[END_REF] and for the NH 3 -SCO (Jablonska, 2020). Among all the zeolite-catalysts, Cu/Fe-ZSM-5 and Fe/Cu-BEA are the most extensively investigated in the past 30 years [START_REF] Hamoud | Selective catalytic reduction of NOx over Cu-and Feexchanged zeolites and their mechanical mixture[END_REF][START_REF] Villamaina | Effect of the NH 4 NO 3 Addition on the Low-T NH 3 -SCR Performances of Individual and Combined Fe-and Cu-Zeolite Catalysts[END_REF][START_REF] Xin | Zeolitic materials for DeNOx selective catalytic reduction[END_REF]. Copper-based zeolites are usually more active in the lowtemperature (<350°C) range while iron-based zeolites are more active at higher temperatures (>350°C). Small-pore zeolite Cu-SSZ-13 has received great attention due to its higher activity and selectivity at low temperatures and improved hydrothermal stability for diesel vehicles. [START_REF] Gao | On the hydrothermal stability of Cu/SSZ-13 SCR[END_REF][START_REF] Lambert | Perspective on SCR NO x control for diesel vehicles[END_REF][START_REF] Shibata | NH 3 -SCR by monolithic Cu-ZSM-5 and Cu-AFX catalysts: Kinetic modeling and engine bench tests[END_REF] On the other hand, noble metals (Pt, Pd, and Rh) supported on metal oxides have been studied for the diesel exhaust after treatment system. Common metal oxides such as Al 2 O 3 , SiO 2 , CeO 2 , TiO 2 , and ZrO 2 are used as support materials for the diesel oxidation catalyst [START_REF] Usberti | Design of a highefficiency NH 3 -SCR reactor for stationary applications. A kinetic study of NH 3 oxidation and NH 3 -SCR over V-based catalysts[END_REF][START_REF] Sun | Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt/Al 2 O 3 and Pt/CeZrO 2 with Various Pt States[END_REF]. Nevertheless, γ-Al 2 O 3 due to its high surface area (100-200 m 2 /g) and its good thermal stability is preferred to all other metal oxides [START_REF] Kong | CuO-MoO 2 -CeO 2 yolkalbumen-shell catalyst supported on γ-Al 2 O 3 for denitration with resistance to SO 2[END_REF][START_REF] Panahi | Activity of γ-Al 2 O 3 -based Mn, Cu, and Co oxide nanocatalysts for selective catalytic reduction of nitric oxide with ammonia[END_REF]. For instance, Pt-γ-Al 2 O 3 is used in the diesel oxidation catalyst (DOC) and ammonia slip catalyst (ASC) (svintsitskiy et al., 2020). The DOC oxidizes CO, unburnt hydrocarbons and NO in the exhaust gas to CO 2, H 2 O and NO 2 , respectively. The ASC removed the excess of NH 3 by selective catalytic oxidation (SCO) with oxygen to N 2 and H 2 O. Pt-γ-Al 2 O 3 catalyst is considered to be the most active for ammonia oxidation below 300 °C than the other noble metals (Pd, and Rh). However, Pt-γ-Al 2 O 3 catalyst has high selectivity towards N 2 O and NO over 300°C [START_REF] Hansen | The Effect of Pt Particle Size on the Oxidation of CO, C3H 6 , and NO Over Pt/Al 2 O 3 for Diesel Exhaust Aftertreatment[END_REF]. The drawbacks of noble-metal catalysts motivate the vehicle manufacturers to reduce their content or substitute them with cheaper Mn/Cu-based oxides catalysts [START_REF] Damma | A review of low temperature NH 3 -SCR for removal of NOx[END_REF].

Cu based systems are discussed more comprehensively in the review of (Jablonska and Palkovits, 2016). Among Cu based systems, CuO/Al 2 O 3 catalyst has been proposed to substitute the noble metal-based emission control catalysts in the NH 3 -SCR [START_REF] Jeong | Selective catalytic reduction of NO by NH 3 over a bulk sulfated CuO/γ-Al 2 O 3 catalyst[END_REF][START_REF] Kwak | Size-dependent catalytic performance of CuO on γ-Al 2 O 3 : NO reduction versus NH 3 oxidation[END_REF][START_REF] Xie | Simultaneous removal of SO 2 and NOx from flue gas using a CuO/Al 2 O 3 catalyst sorbent: II. Promotion of SCR activity by SO 2 at high temperatures[END_REF] and NH 3 -SCO [START_REF] Gang | Selective low temperature NH 3 oxidation to N 2 on copper-based catalysts[END_REF]Jablonska et al., 2018;[START_REF] Usberti | Design of a highefficiency NH 3 -SCR reactor for stationary applications. A kinetic study of NH 3 oxidation and NH 3 -SCR over V-based catalysts[END_REF]Strom et al., 2018). In the early research of (Il'chenko and Ivanovna 1976), CuO was found as one of the most efficient oxide in selective ammonia oxidation into nitrogen and water vapour.

Further studies over preoxidised polycrystalline copper foil proved that CuO is an active phase for NH 3 -SCO [START_REF] Mayer | Comparison of oxidized polycrystalline copper foil with small deposited copper clusters in their behavior in ammonia oxidation: an investigation by means of in situ NEXAFS spectroscopy in the soft X-ray range[END_REF]. However, due to unsatisfying selectivity to N 2 , further studies concerning ammonia oxidation were carried out over CuO supported e.g. on Al 2 O 3 [START_REF] Jones | The selective oxidation of ammonia over alumina supported catalystsexperiments and modelling[END_REF]. It should be mentioned that all cited studies have used γ-Al 2 O 3 to support copper oxide. To our best knowledge, η-Al 2 O 3 has never been employed as support for the preparation of CuO/Al 2 O 3 catalysts for the NH 3 -SCR of NO and for the NH 3 -SCO. Gang et al., 1999 studied supported CuO/γ-Al 2 O 3 catalysts containing 5, 10 and 15 wt.% of copper . They found that the best catalytic results in the NH 3 -SCO were obtained for the sample loaded with 10 wt.% of Cu (90% of NH 3 conversion with 97% selectivity to N 2 at 300°C). This effect was explained by the formation of the surface CuAl 2 O 4 -like species, active in NH 3 -SCO, in the catalyst containing 10 wt.% of copper. [START_REF] Liang | The role of copper species on Cu/γ-Al 2 O 3 catalysts for NH 3 -SCO reaction[END_REF] obtained similar results for Cu(10%)/γ-Al 2 O 3 catalysts prepared by different copper precursors (nitrate, acetate and sulfate). They showed that a mixture of CuO and CuAl 2 O 4 species is formed on the various Cu(10%)/γ-Al 2 O 3 catalysts.

In our previous work [START_REF] Jraba | Valorization of aluminum chips into γ-Al 2 O 3 and η-Al 2 O 3 with high surface areas via the precipitation route[END_REF], we reported the preparation of γ-Al 2 O 3 and η-Al 2 O 3 with high surface areas using aluminum chips collected from metal manufacturing industry as starting materials. η-Al 2 O 3 and γ-Al 2 O 3 were obtained by calcination at 500 °C of bayerite (α-Al(OH) 3 ) and pseudo-boehmite, respectively. γ-Al 2 O 3 and η-Al 2 O 3 are considered to be the most important among other alumina's due to their high specific surface area (200-500 m 2 /g) and acidbase properties. The two alumina polytypes γ-Al 2 O 3 and η-Al 2 O 3 have the same nonstoichiometric spinel structure. η-Al 2 O 3 has a higher population of stronger Lewis acid sites and a slightly higher activity than γ-Al 2 O 3 as an acid catalyst [START_REF] Jraba | Valorization of aluminum chips into γ-Al 2 O 3 and η-Al 2 O 3 with high surface areas via the precipitation route[END_REF]. On one hand, γ-Al 2 O 3 counts for the most important industrial applications as adsorbents, catalysts and catalyst supports but on the other hand the application of η-Al 2 O 3 is rarely reported [START_REF] Peintinger | Quantum-chemical study of stable, meta-stable and high-pressure alumina polymorphs and aluminum hydroxides[END_REF]. The literature showed that η-Al 2 O 3 is mostly used as catalyst in the dehydration of methanol to Dimethyl Ether (DME) [START_REF] Osman | Silver-modified η-Al 2 O 3 catalyst for DME production[END_REF][START_REF] Osman | Kinetic investigation of η-Al 2 O 3 catalyst for dimethyl ether production[END_REF][START_REF] Hoyong | Surface Modification of η-Al 2 O 3 by SiO 2 Impregnation to Enhance Methanol Dehydration Activity[END_REF]. [START_REF] Osman | Silver-modified η-Al 2 O 3 catalyst for DME production[END_REF] showed that loading η-Al 2 O 3 with 10 wt.% of silver enhanced both surface Lewis acidity and the hydrophobicity of the carrier. Accordingly, they obtained a catalyst with high degree of activity and stability under steady-state conditions for the production of DME. [START_REF] Le | CO and CO 2 methanation over Ni catalysts supported on alumina with different crystalline phases[END_REF] investigated the methanation of CO and CO 2 using Ni catalysts supported by alumina

with different crystalline phases (α-Al 2 O 3 , θ-Al 2 O 3 , δ-Al 2 O 3 , η-Al 2 O 3 , γ-Al 2 O 3 , and κ-Al 2 O 3 ).
Among these catalysts, Ni/θ-Al 2 O 3 showed the highest activity for both CO and CO 2 methanation. [START_REF] Nikoofar | Nano alumina catalytic applications in organic transformations[END_REF] published a review which describes the various organic reactions promoted by nano alumina catalysts (η-Al 2 O 3 , γ-Al 2 O 3 , θ-Al 2 O 3 ,..) relevant up to 2017.

Herein, CuO/ η-Al 2 O 3 catalysts were prepared by impregnation/evaporation method using copper acetate as precursor. Then, these catalysts were tested for the first time in the reactions of catalysts were characterized by XRD, SEM, TEM, NH 3 -TPD, H 2 -TPR, UV-vis and N 2 adsorption-desorption techniques.

Experimental

Preparation of the catalysts

Five copper loaded η-alumina catalysts Cu(x)-Al 2 O 3 , with x theoretical copper loadings were prepared by wet impregnation/evaporation technique. In a flask containing 100 mL of distilled water, the desired amount of copper acetate Cu(CO 2 CH 3 ) 2 .H 2 O (Sigma Aldrich, ACS reagent, ≥ 98%) was added to obtain the copper contents of 1 wt.%, 2 wt.%, 3wt.%, 5 wt.% and 7.5 wt%. After the total dissolution of the copper acetate, a mass of 1.5 g of alumina η-Al 2 O 3 was added. The flask is then mounted on a rotary evaporator and the suspension is stirred for 4 hours at 80 ° C. After this step, the water was evaporated under reduced pressure for about 1 hour. Once dry, the solid was placed in an oven at 80 °C overnight and finally calcined at 500 °C under an air stream (2 °C/min) for 10 hours. In figure 1 are reported the photographs of the prepared catalysts. The textural properties, surface area and porosity of the support and the catalysts were determined from nitrogen adsorption-desorption isotherms measured at -196 °C using the "micrometrics Tristar Surface Area and Porosity analyzer". The sample (approximately 100 mg) was weighed exactly in a glass tube lined with an "insert" to reduce the void volume. Before all measurements, the samples were treated under high vacuum overnight at 150 °C.

H 2 -TPR profiles were carried out with an automated Micromeritics Autochem 2910 analyzer. Before H 2 -TPR measurements, samples (50 mg) were pretreated in a quartz U-tube reactor under 5%O 2 /He flow (30 cm 3 /min) at 550 °C (10 °C/min) for 30 min and then cooled under helium to 60 °C. The samples were then reduced from 60 °C to 800 °C (5 °C/min) under 3% H 2 /Ar atmosphere (30 cm 3 /min). The reduction gas H 2 /Ar, was passed after the reactor through a freezing trap (propan-2-ol + liquid nitrogen) kept at -80°C to remove the formed water.

Hydrogen consumption was monitored continuously by a thermal conductivity detector.

The ammonia desorption programmed as a function of the temperature (NH 3 -TPD) was carried out using the same H 2 -TPR equipment. A mass of 30 mg of catalyst is pretreated at 450 °C for 30 min, under air flow (30 cm 3 /min), then saturated with ammonia at 100 °C and purged with helium for 45 min. Following this adsorption, the physisorbed ammonia is removed by leaving the sample for 2 h at 100 °C, under a helium flow rate of 30 cm 3 /min. Finally, the temperature was raised to 550 °C (10 °C/min), under a helium flow rate of 30 cm 3 /min.

The Selective Catalytic Reduction of NO by NH 3 was carried out in a fixed-bed quartz flow reactor operating at atmospheric pressure. The catalyst (24 mg) was activated in-situ at 550°C for 1 hour under a flow of O 2 /He (20/80, v/v) and then cooled to 180°C. A feed mixture of 1000 ppm NO, 1000 ppm NH 3 , 8% O 2 in He and 3.5% H 2 O was then passed through the catalyst at a flow rate of 100 cm 3 /min (GHSV = 250000 cm 3 /g. h). The NH 3 -SCR was carried out on programmed temperature from 180 °C to 500 °C with the heating rate of 5 °C/min.

For the Selective Catalytic Oxidation of ammonia (NH 3 -SCO), the test was carried out on the catalysts already tested in the NH 3 -SCR of NO. At 550 °C, the NO flow is cut off and the NH 3 -SCO experiments were carried out adjusting He flow and by decreasing the temperature from 500 °C to 180 °C with the heating rate of 5 °C/ min.

The reactants and products were analysed by a quadruple mass spectrometer (Pfeiffer Omnistar) equipped with Channeltron and Faraday detectors (0-200 amu) following these characteristic masses: NO (30), N 2 (14, 28), N 2 O (28, 30, 44), NH 3 (15,17,18), O 2 (16, 32) and

H 2 O (17, 18).
The percentages of NO (X NO ) and NH 3 (X NH3 ) conversions were calculated on the basis of the differences in their concentrations measured before and after the catalyst bed.

Results and discussion

Characterization of the catalysts

The XRD patterns of the support η-Al 2 O 3 , CuO (Sigma Aldrich, ACS reagent 99.0 %) and 32. 6°, 35.6°, 38.8° and 48.8°, 58.3° and 61.5° (Liang et al., 2012). On the other hand, for the Cu(1)-Al 2 O 3 and Cu(2)-Al 2 O 3 catalysts, there is a slight decrease in the intensity of the peaks of the support and particularly the peak at 19.5 ° and the absence of the diffraction peaks of CuO. It appears that the copper species present in the Cu(1)-Al 2 O 3 and Cu (2)-Al 2 O 3 catalysts are small and well dispersed on the surface of the support. [START_REF] Friedman | Characterization of CuAl 2 O 3 catalysts[END_REF] showed that the saturation of the CuO/ γ-Al 2 O 3 catalyst surface a CuO monolayer occurs for a Cu content of about 4-5% by weight for every 100 m 2 /g of alumina. Beyond this threshold, crystalline CuO was observed. (S BET = 343 m 2 /g) and 60% with the higher content of copper 7.5% (S BET = 169 m 2 / g). Indeed, the XRD technique has shown that Cu(3)-Al 2 O 3 , Cu(5)-Al 2 O 3 and Cu(7.5)-Al 2 O 3 catalysts contain large CuO particles which block the porous structure of η-Al 2 O 3 . On the other hand, for Cu(1)-Al 2 O 3 and Cu (2)-Al 2 O 3 catalysts the copper species are well dispersed on the surface of the support and the decrease of S BET was moderate (only 16% for Cu(2)-Al 2 O 3 catalyst). On the other hand, we notice an increase in the pore volume up to a copper quantity of 2% wt. and then a decrease beyond this value. Actually, the pore volume of the support which was Vp = 0.295 cm 3 /g increases by about 30% (V p = 0.387 cm 3 /g) for 1% Cu and 40% (V p = 0.411cm 3 / g) for 2%

Cu. This result could explain the morphology of η-Al 2 O 3 and the formation of macropores as

shown by SEM technique. On the other hand, increasing the copper content from 3% to 7.5% induces a reduction in the pore volumes of the catalysts due to the sintering of η-Al 2 O 3 particles.

In figure 5 are reported the N 2 adsorption-desorption isotherms of η-Al 2 O 3 and Cu(x)-Al 2 O 3 catalysts. All adsorption isotherms are of type IV having hysteresis loops characteristics for mesoporous solids [START_REF] Petitto | Hydrothermal activation of silver supported alumina catalysts prepared by sol-gel method: Application to the selective catalytic reduction (SCR) of NOx by n-decane[END_REF]. Nevertheless, we note that the addition of copper to the η-Al 2 O 3 changes the hysteresis loop from H3 to H2(b) type. This behavior could reflect a change in the pore shape and distribution with the introduction of copper. Indeed, H3 type hysteresis loop indicates the presence of narrow slit-like pores particles with internal voids of irregular shape and broad size distribution but the H2(b) hysteresis loop type shows a narrow distribution of pore shape with a wide neck size distribution [START_REF] Cychosz | Progress in the physisorption characterization of nanoporous gas storage materials[END_REF]. Likewise, when the amount of copper increases there is a decrease in the adsorbed volume at low relative pressure (P/P°), indicating the decrease in microporosity and the increase in mesoporosity. was attributed according to [START_REF] Yan | Deactivation of Cu/ZSM-5 catalysts for lean NOx reduction: characterization of changes of Cu state and zeolite support[END_REF] to the reduction of well dispersed CuO clusters on the surface of the support. The second peak extending from 300 °C to 500 °C corresponds to the reduction of highly dispersed Cu 2+ cations in the structure of the alumina forming a surface spinel CuAl 2 O 4 type (Aguila et al., 2008;[START_REF] Il'chenko | Catalytic oxidation of ammonia[END_REF]. On the other hand, when the copper content was increased above 3 wt%, there are changes in the catalyst reduction profiles. Indeed, 2.

Cu(3)-Al 2 O 3 , Cu (
For Cu(3)-Al 2 O 3 , for example, the central peak was deconvoluted into three peaks. The first is located around 209°C with a relative surface area of around 11%, the second around 232°C (15%) and the last around 274°C. The presence of these peaks could be due to CuO aggregates with different sizes interacting differently with the support.

Evaluation of the catalytic activity of Cu(x)-Al 2 O 3 catalysts

Selective Catalytic Reduction of NO by NH 3

The prepared catalysts were tested in the NH 3 -SCR of NO in the presence of an excess of oxygen and of water vapor according to reaction (I):

4NO + 4NH 3 + O 2 → 4 N 2 + 6H 2 O (I)
In figures 9 and 10 are reported the NO conversion and NH 3 conversion of the prepared Cu(x)-Al 2 O 3 catalysts in the NH 3 -SCR of NO. The NO conversion increased initially with increasing temperature, then reached a maximum and decreased. The evolution of the NO conversion passing through a maximum reflects the existence of a competition between two reactions; the first concerning the reduction of NO and the second the oxidation of NH 3 by the oxygen present in the gas mixture. The competition between the two reactions is in favor of the oxidation of NH 3 at high temperatures which explains the decline in NO conversion. The decrease in the NO conversion is also accompanied with some formation of N 2 O according to the two the following reaction:

2NH 3 + 2O 2 → 2N 2 O + 3H 2 O (II)
It should be mentioned that Reaction III implied ammonium nitrate intermediate formed at temperature below 180°C and which starts to decompose above this temperature.

4NH 3 + 4NO + 3O 2 → 4N 2 O + 6H 2 O (III)
Cu( 1 showed that on 10 wt % CuO/γ-Al 2 O 3 , the NOx conversion is about 30% at 350°C and NH 3 reacts primarily with oxygen to produce NOx. However, on a 0.5 wt CuO/γAl 2 O 3 catalyst, NH 3 reacts with NO to form N 2 and the NOx conversion to N 2 was almost 80% at 450 °C. [START_REF] Jeong | Selective catalytic reduction of NO by NH 3 over a bulk sulfated CuO/γ-Al 2 O 3 catalyst[END_REF] carried out the NH 3 -SCR of NO over sulphated CuO/γ-Al 2 O 3 in a fixed-bed reactor (400 ppm NO, 400 ppm, 4% O 2 and the balanced N 2 ). They found that the optimum temperatures of the fresh and sulphated CuO/γ-Al 2 O 3 in the NH 3 -SCR of NO are 350 and 450 °C, respectively. Also, NO conversion over the sulphated catalyst is somewhat higher than that over the fresh catalyst.

The amount of N 2 O formation in NO reduction over the fresh and the bulk sulphated catalysts is below 10% of NO in the feed. °C. According to [START_REF] Gang | Selective low temperature NH 3 oxidation to N 2 on copper-based catalysts[END_REF] species is formed on the various Cu(10%)-γAl 2 O 3 catalysts. On the other hand, the dispersion and the nature of the copper species have a significant influence on the activity of the catalysts.

Indeed, the highly dispersed CuO nanoparticles on the support are responsible for the high activity of Cu(10%)-γAl 2 O 3 catalysts. [START_REF] Lenihan | The selective oxidation of ammonia using copper-based catalysts: The effects of water[END_REF] using lower levels of copper (Cu(3.4%)/γ-Al 2 O 3 ) found conversions of the order of 100% in NH 3 .

The nature of the copper precursor and the method of preparation were found to be determinants in the formation of active copper species in the NH 3 -SCO. For example, a sulphate precursor leads to the formation of CuAl 2 O 4 , whereas CuO of higher crystallinity is formed using an acetate compared to a nitrate precursor [START_REF] Jung | NOx and N 2 O emissions over a Urea-SCR system containing both V 2 O 5 -WO 3 /TiO 2 and Cu-zeolite catalysts in a diesel engine[END_REF]. However, the nature of the active species in NH 3 -SCO has not been fully verified yet. [START_REF] Gang | NH 3 oxidation to nitrogen and water at low temperatures using supported transition metal catalysts[END_REF] [START_REF] Walker | Future challenges and incoming solutions in emission control for heavy duty diesel vehicles[END_REF]Sazonova et al., 1996). This type of catalysts showed higher selectivity to N 2 , however, they need significantly higher operation temperatures as high as 300-500 °C than noble metal catalysts. For the three others Cu(x)-Al 2 O 3, selectivities towards N 2 are much lower. At 550 °C (Figure 16), 48%, 40%, 30% of selectivity towards NO were measured for (Cu(7.5)-Al 2 O 3 , Cu(5)-Al 2 O 3 , Cu(3)-Al 2 O 3 ) respectively.

The NH 3 -SCO method is an effective method for oxidizing NH 3 into N 2 . The overall selectivity into N 2 was close to 100% for Cu(1)-Al 2 O 3 and above 95% for Cu(2)-Al 2 O 3 over all the temperature while for the other three catalysts, N 2 selectivity remains as high as 95% only at temperature below 350°C, as reported by [START_REF] Jabłońska | Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour[END_REF][START_REF] Kröcher | Selective catalytic reduction of NOx[END_REF]. Highly dispersed CuOx favor moderate activity but N 2 selectivity up to 550 °C in NH 3 -SCO [START_REF] Chmielarz | Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides[END_REF]. In a similar work (Dong et al. 2013) showed that nitrogen gas was primarily formed by the direct dissociation of the NO produced by the oxidation of the adsorbed NH 3 [START_REF] Dong | Catalytic performance and characterization of copperbased Rare earth composite materials for selective catalytic oxidation reaction with simulated synthetic ammonia stream[END_REF]. Ethics approval and consent to participate "Not applicable.

Consent for publication "Not applicable".

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 1 :

 1 Figure 1: Colors of the prepared catalysts after copper loading and calcination.

  the prepared catalysts Cu(x)-Al 2 O 3 are shown in figure 2. The characteristic peaks at angles in 2θ 19.5°, 37.5°, 39.7°, 45.8°, 60.8° and 67.2° correspond to η-Al 2 O 3 phase having spinel lattice (JCPDS, No. 04-0875). The introduction of copper leads to the destruction of the structure of η-Al 2 O 3 for the catalysts with higher copper contents Cu(3)-Al 2 O 3 , Cu(5)-Al 2 O 3 and Cu (7.5)-Al 2 O 3 . the characteristic peaks of CuO (JCPDS, No. 80-0076) at the angles in 2θ

Figure 2 :Figure 3 :

 23 Figure 2: XRD patterns of η-Al 2 O 3 , CuO and the prepared catalysts Cu(x)-Al 2 O 3 .

Figure 5 :

 5 Figure 5: Adsorption-desorption isotherms of η-Al 2 O 3 and Cu(x)-Al 2 O 3 catalysts.

  5)-Al 2 O 3 and Cu(7.5)-Al 2 O 3 profiles consist only of a broad peak extending from 140-380°C.This reduction peak can be attributed to the reduction of CuO. In fact, Fierro et al., 1994 reported that the supported CuO particle reduction temperature range extends from 200 to 300°C depending on the type of support. The analysis of the deconvolution (not shown) of the reduction profiles of Cu(3)-Al 2 O 3 , Cu(5)-Al 2 O 3 and Cu(7.5)-Al 2 O 3 was reported in Table

  )-Al 2 O 3 and Cu(2)-Al 2 O 3 catalysts have similar NO and NH 3 conversion profiles up to 475 °C. Beyond this temperature, the NO conversion decreases for the Cu(2)-Al 2 O 3 catalyst, whereas it continues to increase for Cu(1)-Al 2 O 3 up to 500 °C where a maximum NO conversion is about 91%. If we look to the conversion of NO to N 2 , we notice that these two catalysts are almost selective towards N 2 (figure11), due to the fact that the oxidation of NH 3 is less favored due to the better dispersion of copper. The others catalysts, (Cu(3)-Al 2 O 3 , Cu(5)-Al 2 O 3 and Cu(7.5)-Al 2 O 3, display a volcano-shape curve of NO conversion as temperature increases while NH 3 conversion continue to increase with the temperature. For example (Fig.9), the catalyst Cu(7.5)-Al 2 O 3 has a maximum NO conversion of the order of 40% at 350 °C which is accompanied with small N 2 O production. Therefore, the large drop in NO conversion on this catalyst above 350°C is due to the NH 3 oxidation into NO. Moreover, above 425°C, no NO reduction by ammonia occurs since NO concentration in the outlet gas is superior at the NO concentration in the inlet gas. The behavior of these catalysts could be related to the presence of large CuO particles. One can conclude that the two catalysts Cu(1)-Al 2 O 3 and Cu (2)-Al 2 O 3 are the most efficient in the reduction of NO by NH 3 in the presence of 3.5% of water vapor. Nevertheless, the Cu(1)-Al 2 O 3 has a slightly better NO reduction behavior at high temperature. It has been found that the high NO conversion of these two catalysts can be related to the presence of small CuO clusters deposited on the surface of η-Al 2 O 3 alumina and to CuAl 2 O 4 surface spinel. We believe that small CuO clusters deposited on the surface and easily reduced at low temperatures are responsible for high temperature N 2 selectivity. In fact, according to the H 2 -TPR results, the quantity of these copper species is greater in the case of Cu(2)-Al 2 O 3 the and Cu(1)-Al 2 O 3 catalysts. Kwak et al., 2012 investigated the NH 3 -SCR of NO reaction under lean conditions on CuO-γ-Al 2 O 3 catalysts (350 ppm NO, 350 ppm NH 3 , 14% O 2 , 2% H 2 O, and the balance N 2 ). They

Figure 10 :

 10 Figure 10: NH 3 conversion profiles of the prepared Cu(x)-Al 2 O 3 catalysts in the NH 3 -SCR of

  claimed that the surface CuAl 2 O 4 spinel phase is responsible for the higher catalytic activity relative to CuO. A study conducted by[START_REF] Liang | The role of copper species on Cu/γ-Al 2 O 3 catalysts for NH 3 -SCO reaction[END_REF] has shown that a mixture of CuO and CuAl 2 O 4 phases is formed on the various Cu (10%) -γAl 2 O 3 catalysts. On the other hand, the dispersion and the nature of the copper species have a significant influence on the activity of the catalysts. Indeed, CuO nanoparticles highly dispersed on the support and easily reduced at low temperature are responsible for the high conversion of NH 3 .

Figure 12 :

 12 Figure 12: NH 3 oxidation profiles of Cu(x)-Al 2 O 3 catalyst.

Figure 13 :

 13 Figure 13: NO selectivity profiles obtained for NH 3 -SCO performed over Cu(x)-Al 2 O 3 catalysts.

Figure 14 :

 14 Figure 14: N 2 O selectivity profiles obtained for NH 3 -SCO performed over Cu(x)-Al 2 O 3 catalysts.

Figure 15 :

 15 Figure 15: N 2 selectivity profiles obtained for NH 3 -SCO performed over Cu(x)-Al 2 O 3 catalysts.

Table 1 :

 1 Textural parameters of the prepared catalysts Cu(x)-Al 2 O 3

	Sample	S BET (m 2 /g)	BJH Pore	BJH pore
			volume (cm 3 /g)	diameter (nm)
	η-Al 2 O 3	417	0.295	4.50
	Cu(1)-Al 2 O 3	343	0.387	4.79
	Cu(2)-Al 2 O 3	351	0.411	4.85
	Cu(3)-Al 2 O 3	226	0.319	4.27
	Cu(5)-Al 2 O 3	229	0.229	4.67
	Cu(7.5)-Al 2 O 3	169	0.165	4.69

Table 2 :

 2 Results of H

		Peak I	Peak II	Peak III
	Cu(3)-Al 2 O 3	209 (11%)	232 (15%)	274 (74%)
	Cu(5)-Al 2 O 3	_	220 (46%)	282 (54%)
	Cu(7.5)-Al 2 O 3	179 (11%)	222 (41%)	268 (48%)
	The nature and environment of copper species present in the prepared catalysts have been

2 -TPR profiles deconvolution of Cu(3)-Al 2 O 3 , Cu(5)-Al 2 O 3 et Cu(7.5)-Al 2 O 3 catalysts Catalysts Maximum reduction temperature T m (°C) studied by UV-vis spectroscopy. The UV-vis spectra of the carrier η-Al 2 O 3 and the Cu(x)-Al 2 O 3 catalysts are shown in figure 7. Generally, alumina is transparent in the UV-visible range.

  complete oxidation of NH 3 was obtained at 350 °C on Cu(10%)-γAl 2 O 3 catalyst with N 2 selectivity of 90%. Liang et al., 2012 obtained similar results for Cu(10%)-γAl 2 O 3 catalysts prepared by different copper precursors (nitrate, acetate and sulphate) and calcined at 500 °C and 600 °C. They showed that a mixture of CuO and CuAl 2 O 4

Availability of data and materials "Not applicable".

Funding "Not applicable".

Authors' contributions

Nawel Jraba: analyzed the data and write the complete paper.

HassibTounsi, Thabet Makhlouf and Gerard Delahay: gave this idea of work.

All authors read and approved the final manuscript.


come

Probabilistic resource limits using StatMemprof

Guillaume Munch-Maccagnoni * Inria, LS2N CNRS, Nantes, France June 4th, 2021

We present Memprof-limits1 , a probabilistic implementation of per-thread global memory limits, and per-thread allocation limits, for OCaml 4.12.

• Per-thread global memory limits let you limit the size the major heap can reach in specific parts of your program. • Per-thread allocation limits let you bound the execution of parts of the program measured in number of allocation, analogous to the same feature in Haskell. Allocation limits count allocations but not deallocations, and is therefore a measure of the work done which can be more suitable than execution time in addition to being more portable. Memprof-limits is probabilistic: it is based on the statistical memory profiler back-end Stat-Memprof (Gc.Memprof) added in OCaml 4.11. StatMemprof samples words in the OCaml heaps randomly, and runs custom callbacks at life events of these words: allocation, promotion, and deallocation. This can be used to implement memory profilers as libraries.

During integration into OCaml 4.11, the question arose of whether the callbacks should be allowed to raise exceptions. Such exceptions are asynchronous: they can arise at almost any location in the program, and for this reason are delicate to reason about. We argued at the time that raising from those callbacks could be useful to implement resource limits in addition to profilers, and we made the demonstration with a prototype that grew into a usable library.

The two main conceptual contributions are the following:

1. we provide Memprof-limits with a statistical analysis that the user can rely on to get guarantees about the enforcement of limits.2 2. we provide Memprof-limits with a guide on how to recover from asynchronous exceptions and other unexpected exceptions, thereby summarising practical knowledge acquired in OCaml by the Coq proof assistant, and also acquired in other programming languages such as Isabelle/ML-to my knowledge written for the first time. 3 The first part of the talk will focus on use-cases and usage of Memprof-limits, as well as current limitations. The second part of the talk will discuss the reasoning about programs in the presence of asynchronous exceptions; why Memprof-limits improves on the situation; and why the situation, although still imperfect, is likely to remain the same until more ambitious evolutions of the language are made possible.

https://gitlab.com/gadmm/memprof-limits

https://gitlab.com/gadmm/memprof-limits/-/blob/master/doc/statistical.md

https://gitlab.com/gadmm/memprof-limits/-/blob/master/doc/recovering.md


Salim Berrada

Marc Bescond

Nicolas Cavassilas

Laurent Raymond

Michel Lannoo

Carrier injection engineering in nanowire transistors via dopant and shape monitoring of the access regions

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

The continuous race to fulfill the International Technology Roadmap for Semiconductors (ITRS) requirements gave rise to ultra-scaled Metal-Oxide-Semiconductor Field-Effect-Transistors (MOSFETs) with channel length of only few nanometers. At this scale electrostatic control of the channel potential becomes difficult with conventional top gate geometry [1], leading to unsuitable short channel effects. Various novel MOSFET geometries [2] have been then proposed to circumvent these effects among which the gate-all-around NanoWire transistor (NWFET) represents an efficient solution [START_REF] Colinge | Technical Digest. of the International Electron Device Meeting (IEEE)[END_REF][START_REF] Nguyen | [END_REF][5]. NWFETs with channel length down to 5 nm have already been demonstrated experimentally [START_REF] Liow | Symposium on VLSI Technology[END_REF] and are thus considered as one of the most promising devices to reach the end of the ITRS [START_REF] Luisier | [END_REF]. In such narrow devices, the resistance of access regions (AR) may appear as the main contributor to the global resistance of the source-channel-drain system. In particular the shape of the doped source and drain reservoirs as well as the number and the positions of dopants in the AR can have a strong impact on the variability of the transistor [8,9]. At this scale, quantum tunneling and quantum confinement also play an important role [START_REF] Bescond | Technical Digest. of the International Electron Device Meeting (IEEE)[END_REF]. Sophisticated quantum transport simulations are then required to assess the impact of the AR and to optimize NWFET design. On the other hand, in most of the reported simulations charge resulting from dopant ionization is assumed to be continuously distributed over the whole source and drain regions. However, if one considers a NWFET with a square cross-section of 2×2 nm 2 , a concentration of 10 20 cm -3 corresponds on average to less than two dopants in a 4 nm long wire. Therefore, the continous doping assumption is no longer justified and the discrete nature of dopants should be taken into account. Ionized dopants give rise to an attractive Coulomb potential which creates quasi-localized states in the conduction band. Many investigations dealing with the presence of discrete dopants showed that the impurity level degrades the ON-current, while the sub-threshold behavior could be affected [8,[START_REF] Akhavan | [END_REF][12][START_REF] Georgiev | International Conference on Simulation of Semiconductor Processes and Devices[END_REF][START_REF] Asenov | [END_REF][15]. It is important to mention that these studies either used a mode-space approach, or did not take into account electron-phonon interaction when realspace representation was considered. Indeed, phonons are known to play an important role in the ON-state current degradation [START_REF] Anantram | Proc. IEEE[END_REF][START_REF] Rhyner | [END_REF] or in the broadening of quasi-localized states, thus modifying the carrier's lifetime [18].

Among the few studies dealing with discrete dopants, most of them considered NWFETs with constant cross-sections in source and drain regions. However, realistic AR are usually thicker than the channel region. It has been theoretically shown for double-gate MOSFETs that drain current versus gate to source voltage (I D -V GS ) characteristics are weakly affected by AR geometry when a continuous doping profile is assumed in those regions [19]. Our results first demonstrate that NWFETs exhibit similar behavior. However, the picture is quite different when a discrete doping profile is assumed. Indeed, Diarra et al. demonstrated that the impurity level in NW of radius R<3 nm is strongly affected by the dielectric confinement [20,21]. Consequently the energy of dopant level depends on the AR geometry.

The aim of this work is then to emphasize the importance of considering both realistic AR and discrete dopants when modeling ultra-scaled NWFETs. We will find an optimal configuration to enhance carrier injection when the dopant position is carefully chosen. Since it is possible nowadays to precisely place dopants in nanowires [22,23], these informations can be of a great interest to NWFET designers to minimize access resistance and boost the performance of the ultimate devices.

To tackle this problem, we developed a self-consistent Non-Equilibrium Green's Function (NEGF) solver that allows to consider AR with different shapes and geometries. The electrons of the six ∆-valleys of silicon conduction band are described using an effective mass hamiltonian in real-space representation [24]. The effective mass values have been fitted on full-band tight-binding calculations [25]. Electron-phonon interaction is included using the Self-Consistent Born Approximation (SCBA) [26]. Both intravalley and intervalley scattering are taken into account through the self-energy expressions given in reference [18].

The values of deformation potentials and interaction matrix elements, for both optical and acoustic phonons, can be found in reference [26]. Electron-electron interaction is treated in the Hartree approximation. One solves for the mean field given by Poisson equation selfconsistently with the charge resulting from NEGF calculations. In our model, each discrete dopant occupies the volume of the unit mesh (2×2×2 Å3 ) as shown in Ref. [18]. The resulting screening of this positive electron charge will give rise to the long range Coulomb potential of the impurity.

In order to capture only the influence of the AR, all considered devices in this study possess the same gated region. All transistors have then the same channel squared crosssection of 2×2 nm 2 , gate length L G = 8 nm and equivalent oxide thickness of 1 nm. Beyond the AR, we always consider external source and drain regions of 4 nm long, which are assumed to be continuously doped at N D = 10 20 cm -3 to ensure continuous injection into the AR. To illustrate the importance of discrete doping in realistic AR, we considered two NWFET structures that we shall call UNiform (UN) and Non-UNiform (NUN) (Fig. 1). The UN device has a uniform cross-section of 2×2 nm 2 from source to drain. The NUN device has a larger cross-section of 3.6×3.6 nm 2 in source and drain, and an AR with a non-uniform crosssection that linearly connects source and drain to the channel of 2×2 nm 2 . Both devices have AR of L AR = 4 nm represented in yellow. Figure 2 represents I D -V GS characteristics when continuous doping in AR is assumed for both devices. Logarithm curves (left scale) show a small change in sub-threshold voltage (V th ) of 14 meV and identical sub-threshold slope (S s-th ). Linear curves (right scale), after V th adjustment, show only a 6% degradation of I ON for the NUN device with respect to its UN counterpart. Therefore, we can conclude that the geometry has little impact when continuous doping is assumed for AR. This result is in accordance with the results previously obtained for double-gate devices [19]. We then replace the continuous doping of AR by a single impurity. For the NUN device we consider obtained I ON . Figure 6 shows the current spectra for the three devices at the bias points circled in Fig. 5b. The reference in energy is taken at the bottom of the first subband of each device at the source and the corresponding Fermi levels at the source (E F S ) are plotted in dashed lines. The maximum of injection occurs above E F S for W AR =3.6 nm and 2.8 nm, and below E F S for W AR =2.4 nm. This shows that the injection in this type of device is not governed by the position of E F S relatively to the first subband but rather by the energy of the impurity level in the AR of the source (indicated by vertical arrows). This behavior is explained on Figure 7 which shows LDOS and current spectrum along the source-drain axis for W AR =2.8 nm. We see the attractive impurity potentials in each AR as well as the subband mismatch resulting from the abrupt reduction of the cross-section in the channel. The current injection maximum clearly coincides with the impurity level energy in the source AR which promotes transport through the tunneling barrier resulting from the variation of cross-section. We also find that the more we confine the impurity, the lower is its impurity level energy relatively to the Fermi level of the source and the more efficiently it can promote the carrier transport. This is due to the so-called dielectric confinement in ultrascaled NWFETs [20,21]. Indeed, the mismatch of the dielectric constants of Si and SiO 2 creates an electric field at the interfaces which lowers the impurity level energy in narrow cross-section. Therefore, we can conclude that the higher I ON obtained for the NUN device in N-configuration -and the corresponding higher injection -is due to an enhanced injection through the impurity level thanks to the higher dielectric confinement of the impurity inside the narrowest cross-section.

In conclusion, we have used quantum simulations to show the importance of considering the discrete aspect of dopants in realistic ultra-scaled NWFETs. In real devices, the leads are thicker than the channel, leading to a subband mismatch that acts as a tunnel barrier for carriers crossing the access region that connects the leads to the channel. In this work, we demonstrate the importance of the impurity levels in enhancing injection by providing states inside this tunnel barrier. Thanks to the dielectric confinement, carrier injection is enhanced when the impurities are placed in the narrowest cross-section of the access region.

The low drive current being a major drawback of NWFETs scaling, this result could be of great help to experimentalists to boost the injection in ultra-scaled NWFETs.

FIG. 1 :

 1 FIG. 1: 3D view of the considered gate-all-around NWFETs. UN device (top panel) has a uniform cross-section of 2×2 nm 2 , while NUN device (down panel) has a non-uniform cross-section. A Channel width W Ch =2 nm (green) is assumed for all devices. Same gate and channel lengthes L G =L CH =8 nm are assumed for both UN and NUN. The gate insulator has an equivalent oxide thickness of 1 nm. Source and drain (red) are uniformly doped at N D = 10 20 cm -3 and are L S =L D = 4 nm long. Access regions are L AR = 4 nm long and represented in yellow. Square source and drain cross-sections for NUN have W S =W D =3.6 nm width. For all the simulations V DS =0.3 V and T=300 K.

FIG. 3 :

 3 FIG. 3: (a) View of the transverse section in the middle of the NUN device. Square source and drain (red) cross-sections for NUN have W S =W D =3.6 nm width and are uniformly doped at N D = 10 20 cm -3 . Each AR (yellow) contains one impurity. We consider 3 configurations labelled L (large), I (intermediate) and N (Narrowest) which correspond to the positions of one impurity of each side in cross-section of 3.2×3.2 nm 2 , 2.8×2.8 nm 2 and 2.4×2.4 nm 2 respectively. (b) Corresponding I D -V GS characteristics at V DS =0.3 V in both linear and logarithmic scales. For comparison, we plot I D -V GS for the UN L device where the impurity is placed exactly at the same position along the channel symmetry axis as NUN L.

FIG. 4 :

 4 FIG.4: Current spectra at source contact for the three NUN-configurations defined by circled points in Fig.3b. Reference in energy is taken at the Fermi level of the source (E F S =0 eV).

FIG. 5 :

 5 FIG. 5: (a) View of the transverse section in the middle of the devices with abrupt constrictions. Square source and drain (red) are 4 nm long uniformly doped at N D = 10 20 cm -3 . Each AR (yellow) contains one impurity at its center (red circle). The widths of AR for the three considered devices are W AR = 3.6 nm, 2.8 nm and 2.4 nm. Blue regions are 2 nm long spacer of intrinsic silicon to minimize the influence of the gate on the AR. (b) Corresponding I D -V GS characteristics in both logarithmic and linear scales.

FIG. 6 :

 6 FIG. 6: Current spectra at source contact for the circled points in Fig. 5b. Reference in energy (E= 0 eV) is taken at the bottom of the first subband for each device. For each configuration the corresponding Fermi levels at the source (E F S ) are represented by vertical dotted lines and the energy of the impurity levels of the source AR are represented by vertical arrows.

8 FIG. 7

 87 FIG. 7: a) LDOS and b) current spectrum corresponding to the circled point of Fig. 5b for W AR =2.8 nm. The horizontal dashed lines define the Fermi levels of source and drain while the white lines describe the profile of the first electronic subband. The current transmission through the first quasi-localized state of the source AR impurity is clearly visible.

Acknowledgement

This work is supported by the NOODLES contract (ANR-13-NANO-0009) funded by the ANR-French National Research Agency. This work was granted access to the HPC resources of Aix-Marseille Universit financed by the project Equip@Meso (ANR-10-EQPX-29-01) of the program d'Investissements d'Avenir supervised by the Agence Nationale pour la Recherche.

three configurations as depicted in Fig. 3a. Let's start by the comparison of the NUN Lconfiguration with a UN device in which the impurity is placed exactly at the same position along the channel symmetry axis. The corresponding I D -V GS plotted in Fig. 3b show that I ON in NUN L is 20% lower than in the UN L. This first result pinpoints the importance of having a discrete dopant description when a more realistic geometry of AR is considered.

On the other hand, subthreshold behavior is poorly affected by this change.

In order to see if the current degradation in NUN could be overcome, we now consider the I and N configurations as depicted in Fig. 3a. One can see in the corresponding I D -V GS (Fig. 3b) that the obtained I ON is all the better for impurities localized in the narrower cross-section, close to the channel. Fig. 4 shows the current spectra at source contact for the three NUN-configurations at given bias. We notice that we have a better injection at the source for the N-configuration, followed by I-configuration, while the worst injection is obtained for L-configuration. This surprising result shows that I ON is higher when the impurity is located just at the entrance of the channel. Besides the dopant position along the AR, carrier injection efficiency may also depend on the AR cross-section.

To understand this dependence, we considered three devices with abrupt constrictions defined by constant AR cross-sections of width's W AR of 3.6 nm, 2.8 nm and 2.4 nm (Fig. 5a). A spacer of 2 nm was added to the AR to prevent AR and gate overlap. One impurity is then placed in each AR at exactly the same position along the channel symmetry axis.

The corresponding I D -V GS characteristics for the three devices are shown in Fig. 5b.

Once again, we notice that the smaller is the cross-section of the AR, the better is the