Subscriber access provided by La Trobe University Library
Article
Ni in CNFs: highly active for nitrite hydrogenation Roger Brunet Espinosa, and Leon Lefferts ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01375 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 8, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Ni in CNFs: highly active for nitrite hydrogenation Roger Brunet Espinosa, Leon Lefferts * Catalytic Processes and Materials group, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands,
[email protected] *Corresponding author (Leon Lefferts)
1. Abstract: Nickel ‘hairy’ foam, consisting of carbon nano-fibers (CNFs) grown on the surface of nickel foam, were synthesized and tested for nitrite hydrogenation. Results showed that nickel ‘hairy’ foam is catalytically active in the absence of any noble metal, which is attributed to the formation of nickel particles with high carbon content during CNF growth. These C-doped nickel particles showed catalytic properties similar to noble metals, but easily deactivated as a result of oxidation treatments. This deactivation is partly attributed to nickel passivation which is reversible by reducing with H2 at room temperature in gas or liquid phase. In addition, oxidation treatment also caused partial removal of the carbon dissolved in the nickel particles, causing irreversible deactivation. Increasing severity of the oxidation treatment induced slower reactivation via reduction, as well as lower steady state activities after reactivation. This irreversible deactivation is attributed to the decreased concentration of dissolved carbon. Therefore, nickel ‘hairy’ foam is a promising hydrogenation catalyst provided it is protected against oxygen.
Keywords: nitrite hydrogenation, nickel, carbon, dissolved C, 'hairy' foam, carbon nano-fiber, passivation
2. Introduction: Three phase catalytic reactions (G-L-S) are becoming more important in the chemical industry, especially in the petrochemical, bulk and fine chemical processes [1-4]. Usually, in these G-L-S 1 ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 26
reactions, the efficiency of the chemical reaction is significantly influenced by external (gas-liquid and liquid-solid) and/or internal mass transport [5], and only partly determined by the intrinsic activity and selectivity of the catalyst. Existing technologies to perform these reactions are slurry and trickle/fixed bed reactors [6]. The small catalyst particles in slurry reactors (typically in the order of 50 μm) are usually separated from the product stream by filtration, elevating significantly the operation costs [1]. In addition, attrition of the small catalyst particles usually causes the formation of fines, further complicating the operation of the filtration. In trickle/fixed beds, the catalyst bodies have typical sizes larger than 1 mm to prevent excessive pressure drop. These large catalyst particles can easily induce internal mass transfer limitations, decreasing the overall rate. Additionally, maldistribution of the feed stream due to random packing of the catalyst particles can create channeling/by-passing of the feed and stagnant zones [1, 7-8].
Structured reactors are an attractive alternative for these conventional reactor configurations, consisting of an structured internal made of metal, ceramic or carbon [4]. These are characterized by high regularity, low pressure drop, easy catalyst recovery and high accessibility to the catalyst active phase [1, 7]. Monoliths are one of the most widely studied structured reactors due to their extensive use in the automotive industry and for environment applications [4, 8-9]. However, other structured reactors based on foams [10-14], cloths [15-16], wires [17], filters [18] and fibers [19] have also been explored.
Recently, solid foams have been attracting attention as structured packings since they can be used as catalyst support, heat exchanger and chemically inert packings [20]. Foams can be made from numerous materials comprising metals, ceramics, carbon and silicon carbide, mimicking the inverse structure of a packed bed made of dense spheres [21]. Although foams present similar specific surface areas as monoliths, they exhibit higher voidage (up to 97-98%) [1]. The main disadvantage is that the geometrical surface area of the solid foams is insufficient for direct application as catalyst support. Therefore, the surface of the foam has to be coated with a thin and highly-porous layer to
2 ACS Paragon Plus Environment
Page 3 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
increase the total surface area of the foam [21]. Covering the walls of the foam with an uniform washcoat is not trivial since the attachment of the coating is sometimes poor [21-22]. However, Cristiani et al. successfully managed to deposit f Ni/MgAl2O4 washcoats on FeCrAlloy foams [23]. An alternative to applying washcoats is growing carbon nano-fibers (CNFs) on metallic foams [11-12] or on e.g. carbon foams loaded with nickel, iron or cobalt nano-particles [10]. This type of catalyst is commonly known as ‘hairy’ foam, and benefits from the properties of the thin layers of entangled CNFs coated on the surface of the foam; SEM micrograph in Figure 1a present the morphology of nickel foam. The entangled CNFs present an open structure with high surface area (typically 100-200 m2/g) and large pore volumes (0.5-2 cm3/g), preventing internal mass transfer limitations during application as catalyst support [1, 10-11, 14, 24]. Additionally, CNFs present excellent mechanical stability, are chemically inert and their surface chemistry can be modified, making them very suitable for catalyst support applications. Importantly, CNFs grown on nickel foam are strongly attached to the foam, preventing catalyst loss [11].
Nitrite hydrogenation is a fast G-L-S reaction that can benefit from minimizing mass transfer limitations, by using Pd or Pt supported on metal foams with a thin layer of grown CNFs (metal ‘hairy’ foam) [1]. Generally, nitrate is an inorganic contaminant found in water. Nitrate can be easily converted to nitrite inside the human body,
creating
important health issues
like
methemoglobinemia (blue baby syndrome) or can form nitrosamines which are known to be carcinogenic [1, 25-26]. Catalytic hydrogenation using noble metals is a technique that was developed in 1989 to convert nitrate to, preferably, nitrogen or to ammonia as an unwanted byproduct. Nitrate is first converted to nitrite as the first intermediate product, which is rapidly further converted to the final products, nitrogen and ammonia. Many efforts have been directed to prevent the formation of ammonia [25, 27-28], whereas previous work in our group reported on the same starting form nitrite [11, 29]. However, these studies have in common that noble metal based catalysts are needed, except for catalysts containing metallic Fe, acting as a stoichiometric reducing agent [30-33]. Mikami et al. [34-37] reported nitrate hydrogenation with noble metal-free catalysts, 3 ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 26
proving that nickel is active for this reaction. However, results showed that nickel catalyst suffer from strong deactivation due to oxidation of the catalyst. This oxidation could be suppressed by addition of zirconia or platinum, greatly enhancing catalyst activity. Remarkably, these catalyst give significant nitrite selectivity in contrast to conventional bimetallic catalysts containing precious metals, which exhibit much higher activity for nitrite hydrogenation than for nitrate hydrogenation, resulting in very low nitrite selectivity. Despite the work of Mikami et al. on nitrate hydrongenation, to the best of our knowledge, no efficient noble metal-free catalysts are known for conversion of nitrite to nitrogen and ammonia.
The aim of this work is to synthesize a nickel catalyst for nitrite hydrogenation that does not require noble metals. A nickel ‘hairy’ foam catalyst was therefore synthesized without the addition of other metals, providing an alternative for conventional hydrogenation of nitrite with palladium or platinum catalysts. We also investigated the effect of carbon dissolved in the nickel on the catalyst performance via oxidation and reduction treatments.
3. Experimental 3.1.
Materials used:
Polycrystalline nickel foam (99% purity, Recemat) was used as a catalyst for the growth of CNFs. The foam consists of a three dimensional network of connected strands (figure 1 a). Nickel foam cylinders of 4.3 mm in diameter were cut from sheets with a size of 100x100 mm2 and 5 mm thickness using wire-cut Electrical Discharge Machining (AGIECUT CHALLENGE 2). The surface area per gram sample was estimated by Jarrah et al. [12] to be less than 1 m2/g. γ-Alumina spheres (Engelhard) with a total surface area of 250 m2/g were used as catalyst support for nickel. Ethylene (99.95% PRAXAIR), hydrogen and nitrogen (99.999% INDUGAS) were used for the growth of the CNFs on the nickel foam. Nickel nitrate hexahydrate (Merck), urea (Merck) and nitric acid (65%, Merck) were used for nickel deposition on γ-alumina. Palladium acetylacetonate (Alfa Aesar) and toluene 4 ACS Paragon Plus Environment
Page 5 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
(>99.9%, Merck) were used for depositing Pd on the nickel foam. Sodium nitrite (>99%, Merck) was used as nitrite source for the catalytic experiments.
3.2.
Sample preparation:
3.2.1. Nickel ‘Hairy’ foam: 3.2.1.1.
CNF growth
Nickel foam cylinders were used as received after being cut. A similar procedure as described in [24] was used to grow CNFs. In this work, 46 cylinders (1.7 g) were placed simultaneously in an inhouse-build quartz reactor of 50 mm inner diameter with a porous quartz plate to hold the nickel foam cylinders. The foam cylinders were first pre-treated at 700 °C for 1 h in static air followed by a reduction step of 2 h under a mixture of 20%H2 in N2 at 700 °C with a total flow rate of 100 ml/min. Next, the temperature was lowered to 440 °C under 80 ml/min of N2. The CNF growth was performed at 440 °C for 0.7 h with a mixture of 25% C2H4 in N2 (100 ml/min of total flow rate). The nickel foam cylinders with CNFs (nickel ‘hairy’ foam) were then cooled down to room temperature under N2 atmosphere.
Any loose CNFs were removed by applying a pressurized N2 flow. Almost no change in weight was observed (
