Direct Hysteresis Heating of Catalytically Active NiâCo Nanoparticles

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Direct hysteresis heating of catalytic active NiCo nanoparticles as steam reforming catalyst Peter Mølgaard Mortensen, Jakob Soland Engbæk, Søren Bastholm Vendelbo, Mikkel Fougt Hansen, and Martin Østberg Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02331 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Direct hysteresis heating of catalytic active Ni-Co nanoparticles as steam reforming catalyst Peter Mølgaard Mortensen1*, Jakob Soland Engbæk2, Søren Bastholm Vendelbo2, Mikkel Fougt Hansen3, and Martin Østberg1 1

Haldor Topsoe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark

2

Danish Technological Institute, Gregersensvej 1, DK-2630 Taastrup, Denmark

3

Department of Micro- and Nanotechnology, DTU Nanotech, Building 345B, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark

*Corresponding author. Tel.: +45 2552 9483; E-mail address: [email protected] (P.M. Mortensen).

Abstract We demonstrated proof-of-concept of a catalytic steam reforming flow reactor system heated only by supported magnetic nickel-cobalt nanoparticles in an oscillating magnetic field. The heat transfer was facilitated by the hysteresis heating in the nickel-cobalt nanoparticles alone. This produced a sufficient power input to equilibrate the reaction at above 780°C with more than 98% conversion of methane. The high conversion of methane indicated that Co-rich nanoparticles with a high Curie temperature provide sufficient heat to enable the endothermic reaction, with the catalytic activity facilitated by the Ni content in the nanoparticles. The magnetic hysteresis losses obtained from temperature-dependent hysteresis measurements were found to correlate well with the heat generation in the system. The direct heating of the catalytic system provides a fast heat transfer and thereby overcomes the heat transfer limitation of the industrial-scale steam reformer. This could consequently enable a more compact steam reformer design. Keywords: Catalysis; Electric heating; Ferromagnetism; Hysteresis; Induction; Steam reforming

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1

Introduction

Today, hydrogen is most efficiently produced in large chemical plants from fossil fuels by the steam reforming reaction. Here, natural gas (methane) and steam are converted to hydrogen and carbon monoxide according to the following reaction1-3: CH4+H2O ⇌ 3H2+CO (∆H° = 206.15 kJ/mol)

(1)

This reaction is always accompanied by the water gas shift reaction: CO+H2O ⇌ H2+CO2 (∆H° = -41.16 kJ/mol)

(2)

As the reforming reaction is endothermic, heat is required for the reaction to proceed. In order to achieve sufficient conversion of methane to hydrogen, the reactor is excessively heated to more than 850°C4. To enable transfer of energy to the reaction, a full-scale steam reforming plant typically uses a tubular reformer where many tubes are placed in a row (or parallel rows) in a furnace box. The reformer tubes are typically from 100 to 200 mm in outer diameter and 10 to 13 m in length5. The configuration of many tubes with a comparatively small diameter is used to optimize heat transfer, as this becomes the limiting factor of the primary reformer, especially near the exit of the reactor tubes1, 4. In conventional steam reforming plants, the heating takes place via convection and/or radiation. Heating by induction is a prospective technology that offers the possibility to deliver heat directly to the catalyst zone6. This may enable the design of new and more compact steam reforming reactors and additionally provides a fast heating mechanism that could potentially significantly shorten the start-up time for a reforming plant7. In induction heating, an oscillating magnetic field is applied to a medium susceptible to the magnetic field (the susceptor). The constant change in the magnetic field leads to heating by two principal mechanisms: hysteresis heating from a ferromagnetic material cycling through the magnetic hysteresis loop and Joule heating from eddy currents8-10. Previously, induction-heated catalysis was used in conjunction with magnetic nanoparticles in the pioneering work done by Ceykan et al.11 for the Suzuki–Miyaura and Heck coupling reactions in a liquid phase organic synthesis flow reactor. Chatterjee et al.12 demonstrated the use of induction heating for fast and isothermal heating of a microreactor system loaded with nickel ferrite micro-particles in the 80–100°C temperature range. Recently, Bordet et al.13 showed that iron carbide nanoparticles supported on SiRAlox were directly susceptible for induction heating, which they used to start the exothermic Sabatier reaction, converting carbon dioxide and hydrogen into methane and water. The work presented within this paper presents the first catalytic material, being hysteresis heated to a temperature of 800°C, a temperature high enough to run the indus-


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trially relevant endothermic steam reforming reaction, in a continuous flow reactor system where the heat of reaction is supplied using solely magnetic nanoparticles to receive the energy of an oscillating magnetic field. To achieve this, the catalyst would not only need to be catalytically active for steam reforming, but also be heatable by induction heating by either hysteresis heating or Joule heating. Materials with a catalytic activity for steam reforming have previously been investigated extensively. As a representative example, the density functional theory (DFT) work by Jones et al.14, showed the following tendency for steam reforming activity in the choice of active material Ru > Rh > Ni > Ir > Co > Pt ≈ Pd > Fe > Cu

(3)

To additionally make the catalyst susceptible to induction heating, it is favorable if the material is ferromagnetic (or ferrimagnetic). To heat the catalyst to the required operating temperature, the susceptible material needs to have a Curie temperature that is at least as high as the operating temperature6. Thus, the material selection is further limited to materials with high Curie temperatures. Most elements cannot maintain a magnetic moment on their own at temperatures above 100°C. Of pure elements, only Ni, Fe, and Co can be used at elevated temperatures, with Curie temperatures of 354°C, 770°C, and 1115°C, respectively15. The Curie temperature will additionally be influenced by the size of the crystal lattice of the ferromagnetic material, as small structures are more sensitive to fluctuations of the magnetic spins of the individual atoms16. In summary, a catalyst for induction-heated reforming should ideally fulfill the following criteria: •

Sufficient catalytic activity for the reforming reaction. -

•

Materials as Ru, Rh, Ni, and Ir are among the best to achieve high activity.

A Curie temperature ideally above 900°C. -

Only Co as a pure element can achieve this, but alloys with Co are also possible.

It seems evident that a catalyst for induction-heated steam reforming should at least contain some Co to enable inductive heating at high temperatures. However, the steam reforming activity of Co was previously shown to be significantly lower than typical steam reforming catalysts17. A candidate for induction-heated reforming could therefore be a system based on both Co and Ni. Alloys of Co and Ni have previously shown activity for reforming17. Consequently, we chose this system as a candidate for a ferromagnetic susceptor for induction-heated steam reforming.

2

Experimental

2.1

Catalyst synthesis and characterization procedures

A range of different Co—Ni-based catalysts were produced by sequential impregnation of a commercially used MgAl2O4 carrier (HT-80542) with calcinations in between each impregnation



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step. Co(NO3)2⋅6H2O (>98%, Fluka) was impregnated on the carrier initially and dried at 80°C overnight. The sample was calcined at 450°C for 1 h. Then, Ni(NO3)2⋅6H2O (≥97%, SigmaAldrich) was impregnated on the sample and it was dried at 80°C overnight. The sample was calcined at 450°C for 1 h and then reduced at 850°C in H2 for 4 h followed by passivation in 1% oxygen in nitrogen under ambient conditions. The catalysts were produced with loadings of 2– 11% cobalt and 2-13% nickel. Table 1 summarizes the catalysts used in this study. Table 1. Catalysts samples used in the current study. Cobalt and nickel content was quantified by ICP-OES and average particle size was quantified by XRD. Catalyst Ni [wt%] Co [wt%] dM [nm] Co/MgAl2O4 0 7.8 11 NiCo/MgAl2O4 6.3 1.8 10 NiCo/MgAl2O4 5.0 3.3 12 NiCo/MgAl2O4 3.8 5.1 12 NiCo/MgAl2O4 2.7 6.1 10 NiCo/MgAl2O4 1.7 6.5 10 NiCo/MgAl2O4 2.3 2.2 11 NiCo/MgAl2O4 4.3 4.1 12 NiCo/MgAl2O4 8.4 7.3 14 NiCo/MgAl2O4 11.4 10.7 17 NiCo/MgAl2O4 12.6 9.0 24

To simulate aging of the catalyst during prolonged operation under steam reforming conditions, the sample with 4.3 wt %/4.1 wt % NiCo/MgAl2O4 was aged at a H2O/H2 ratio of 2 at 750°C for 14 days and subsequently passivated in 1% oxygen in nitrogen under ambient conditions. Unless otherwise stated, this catalyst was chosen for thorough characterization. Characterization and reaction test were done on catalyst materials with varying Ni and Co content on the MgAl2O4 support. Despite differences in the exact system, characterization work done on one specific sample is considered generally indicative for this family of catalyst materials and therefore vast analysis of the many samples was not done. Elemental analysis of nickel and cobalt on the catalysts was performed using inductively coupled plasma atomic emission spectroscopy (ICP-OES) with an Agilent 720 ES ICP-OES instrument. For the analysis, the samples were crushed and melted together with potassium pyrosulfate. This was dissolved in a solution of water and HCl and subsequently analyzed by ICPOES. The instrument was calibrated with a certified nickel and cobalt standard.


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Powder X-ray diffraction (XRD) measurements were carried out using a PANalytical X'Pert PRO diffractometer in a Bragg–Brentano θ–θ geometry. A 125–300 µm sieved fraction of the catalyst was analyzed at a 5° < 2θ < 75° range using monochromatic Cu-Kα radiation (λ = 1.5418 Å). The average crystallite size was estimated using the Scherrer equation. The crystallite size analysis was carried out neglecting the lattice strain effects. The elemental composition and structure of the nanoparticles were investigated by scanning transmission electron microscopy (STEM). Samples were prepared by crushing a small amount of the sample in a mortar pestle in the presence of ethanol. The resulting suspension was ultrasonicated for one minute and a drop was placed onto a Cu-grid coated with continuous carbon. STEM maps were acquired using a FEI Talos F200X analytical STEM-microscope. The STEM maps were acquired with a spot size of 2.5 Å, a pixel dwell time of 1 µs per cycle, and a beam current of 0.7 nA. Several hundred cycles were accumulated to produce the maps. Aperture sizes of C1 at 2000 µm and C2 at 70 µm were used. A camera length of 125 cm was used, and a high-angle annular dark field (HAADF) detector was used in conjunction with Energydispersive X-ray spectroscopy (EDS). The minimum time for mapping was 10 minutes. The catalytic activity was quantified in a lab-scale plug flow reactor with an internal diameter of 8.5 mm with a thermopocket placed in the central axis for temperature measurement directly in the catalyst bed. The catalyst loading was varied to obtain sufficient conversion of methane for quantification of the activity while still not being too close to equilibrium. This resulted in loadings of 40–80 mg of catalyst in a sieve fraction of 125–300 µm mixed with 920 mg of spinel carrier in the same sieve fraction to achieve isothermal conditions in the catalyst bed. The activity was measured with a flow of 2 Nl/h CH4, 0.8 Nl/h H2, and 6.45 g/h of H2O at 450°C and 1.2 bar. The dry product gas was analyzed by GC-TCD/FID on an Agilent 7890 GC system. The experiments were performed under completely intrinsic conditions as quantified by Weisz-Prater and Mears criteria18, and additionally, the product gas was ensured to be far from equilibrium by controlling the catalyst loading. Magnetic properties were studied as function of temperature using a custom in-situ magnetometer at the Centre for Catalysis Research at the University of Cape Town16, 19. The magnetic setup makes use of a Weiss-extraction method to determine total magnetization of a catalyst under operando conditions (up to 50 bar and 700°C). The sample was placed into a ½” stainless steel reactor, which oscillates in a homogeneous magnetic field, the strength of which can be varied between -2 T and +2 T. Samples were heated from 50°C to 700°C under a constant flow of a hydrogen/argon mixture (ratio 1:1) at 2°C/min. After a short holding time at 700°C, the samples were cooled to 50°C at the same rate. The sample magnetization was measured during the



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experiment in an applied external magnetic field of 2 T. In addition, at 50°C before and after the temperature ramp, hysteresis measurements (sweeping of external field from -2 T to +2 T and back) were taken. Further studies of the magnetic properties were performed at the Technical University of Denmark in a LakeShore 7407 vibrating sample magnetometer (VSM) equipped with an oven (LakeShore 74034). Samples were mounted in a screw-cap boron nitride holder in an Ar atmosphere. Measurements were performed with a 120 Nml/min flow of 2% H2 in Ar at 1 bar in the topopen oven to prevent sample oxidation. Hysteresis measurements between ±Bmax for Bmax in a range between 5 mT and 50 mT were performed isothermally at temperatures between 100°C and 800°C in steps of 50°C. 2.2 Induction-heated steam reforming setup The induction-heated steam reforming bench-scale setup was supplied with methane and hydrogen (AGA quality 4.0), using Brooks digital flow controllers, type SLA5850. The demineralized water was supplied by a Knauer smartline HPLC pump, which was calibrated against a balance. The tubes leading to and leaving the reactor were heat traced to a temperature above 110°C. The reactor was a quartz tube insulated in the hot zone by Promat Promalight 1000R Ø40–Ø16 mm. The magnetic field was generated by a water-cooled copper coil, with nine windings, a diameter of 40 mm and a height of 80 mm. This coil was run by an UPT-S2 Ultraflex power supply capable of supplying a current of up to 300 A (40 mT magnetic field) at a frequency of 68 kHz. The reactor was supplied with methane, hydrogen, and steam as mentioned above, and water was removed from the output using an Armstrong 11 LD drain trap; this dry output gas was analyzed by GC-TCD/FID on an Agilent 7890B system. The power consumption was measured by an ABB EQ series Energy meter. The passivated catalyst was reduced in 10% hydrogen in nitrogen while heated by induction to roughly 400°C before start of the experiments. When the reaction temperature was reached, steam and then methane were fed to the reactor. 2.3

Calculations

The activity in the lab-scale intrinsic test setup was quantified as the rate of reaction ( ) calculated from the conversion of methane. The turnover frequency (TOF) was calculated as: TOF = ⋅


(4)

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where is Advogado’s number, and is the available metallic surface sites for the given cata-

lyst. was estimated on the basis of the dispersion of the metal, which was found from the av-

erage XRD particle size, as described by Borodzinski and Bonarowska20; Varied from 8.8⋅10-5 mol/g to 2.8⋅10-4 mol/g with a particle size from 10 nm to 24 nm in diameter. The approach to equilibrium of the steam reforming reaction was found by initially calculating the reaction quotient (): =

⋅ ⋅ ⋅

(5)

where is the molar fraction of compound and is the total pressure in bar. This was used to determine the equilibrium temperature ( ) at which the given reaction quotient is equal to the equilibrium constant: = ( ! )

(6)

where is the thermodynamic equilibrium constant of the steam reforming reaction. The conversion of methane was quantified from the carbon balance: #=

+ + +

(7)

where is the molar fraction of in the dry product gas. This expression is only valid when carbon monoxide and carbon dioxide are not present in the feed, as was the case for all presented measurements. The specific energy contribution %& in J/kg from hysteresis heating per sample mass was calculated as the area of the hysteresis measurement of the specific magnetization (magnetic moment per sample mass), σ in Am2/kg, vs. applied magnetic field (B): %& = ' (()) d)

(8)

In the induction-heated steam reforming experiments, the energy transferred to the gas was evaluated as the change in the enthalpy (+) of the gas given by the series expansion: 0

+ = , ⋅ - ./ d

0123


(9)


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where is the absolute temperature, 4

5

is the reference temperature of the thermodynamic

data, ./ is the molar specific heat capacity, and is the molar fraction of the th species. Data of

Barin21 was used in the calculations of the thermodynamic properties. Based on +, the energy transfer to the gas ( 67 ) was: 67 = +89: ⋅ ;89: − +=> ⋅ ;=>

(10)

where, ; denotes the flow in or out of the catalyst bed. The inlet temperature was measured upstream the catalyst bed, while the equilibrium temperature, calculated from the gas composition, was used as outlet temperature. The equilibrium temperature was chosen due to the general uncertainties on a direct measurement of temperature in the oscillating magnetic field. This expression considers only the energy transferred to the reaction gas and it thereby neglects energy loss to the surroundings, heat loss in the coil, and heat loss in the power supply.

3

Results and discussions

3.1

Catalytic activity

The catalyst activity was quantified using a standard activity measurement in a normal plug flow reactor setup heated by an external oven. To normalize for differences in loading and particle sizes, the activities are compared in Figure 1 in terms of turnover frequencies (TOF) of the fresh catalyst samples as function of the Ni loading on the catalyst samples. Initially, a sample impregnated with cobalt alone was tested (point with blue triangle of Figure 1), but this had limited activity, in agreement with expectations based on previous work17, 22. Instead, testing a series of catalyst with varying Ni/Co ratio showed that Ni was essential for the activity as the TOF increased with increasing Ni content in these.


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Figure 1. Activity of fresh NiCo catalysts normalized to TOF of a nickel-based reforming catalyst as a function of the Ni content in the sample. Series of variation in total loading and Ni/Co ratio are shown. The numbers next to the points are the molar Ni/Co ratio. Measurements at Steam/Carbon=4.1, H2O/H2=10, T=450°C, P=1.2 bar. Variation in the total loading was also found to have some effect on the TOF as shown in Figure 1, with a series of catalyst with a Ni/Co ratio close to 1. The highest TOF was found for a low total load because this produced a better dispersion of the active metals and therefore a lower particle size; for reference the catalyst with the lowest total loading of 4.5 wt% had an approximate average particle size of 11 nm while the catalyst with the highest total loading of 21.6 wt% had an approximate average particle size of 24 nm (see Table 1). As the reforming reaction is structure sensitive, a lower particle size will result in a larger fraction of exposed low-coordinated sites4. 3.2

Magnetic characterization

In-situ magnetometry was performed to evaluate the magnetic properties of the NiCo/MgAl2O4 samples at varying Ni/Co ratios. Also, pure MgAl2O4 was investigated, verifying that the support did not have any ferromagnetic characteristics and did not interfere with the measurements. During measurements, the passivated samples were heated in a reducing atmosphere. Consequently, the hysteresis loops measured after heating had a larger area than those measured prior to



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heating due to more metallic phase on the catalyst. The measurements after reduction were used to quantify the hysteresis loss according to Eq. (8). Generally, Wh was found to increase approximately linearly with the Co loading and to be essentially independent of the Ni loading for the investigated compositions (Figure 2). Consequently, Co must be the major contributor to the hysteresis loss, even at 50°C where the analysis was done and Ni is still ferromagnetic. The insert in Figure 2 shows a zoom-in on an example of a hysteresis curve for the 4.3 wt %/4.1 wt % NiCo/MgAl2O4 catalyst.

Figure 2. Hysteresis loss (±2T cycle) at 50°C of NiCo/MgAl2O4 catalysts as a function of Co loading. The dashed line is a linear fit. The numbers next to the points indicate Ni/Co molar ratio. The feed gas was 100 Nml/min H2 and 100 Nml/min Ar. The inset shows a zoom-in on the hysteresis loop measured for the 4.3 wt %/4.1 wt % NiCo/MgAl2O4 catalyst sample. Combined, the activity and magnetometer measurements establish a clear links between the activity of the catalyst and the Ni content, and between the hysteresis loss and the Co content in the samples. A suitable catalyst for induction heating would consequently need to contain both Ni and Co. A catalyst containing 9.0 wt % cobalt and 12.6 wt % nickel was synthesized and used for testing in an actual induction-heated steam reforming test rig. The hysteresis properties of this sample


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were quantified in a VSM as a function of temperature and maximum applied magnetic field, )?@6 in a slightly reducing atmosphere of 2% H2 in Ar. Figure 3 shows representative data for the hysteresis loss (%& ) vs. )?@6 at four selected temperatures. The data is fitted with a parabola of the

following expression: %& = A() )?@6 , where A() is a temperature-dependent constant. This

behavior can be interpreted as scaling a constant hysteresis loop shape on both axes with )?@6 ,

or assuming the imaginary component of the magnetic permeability is constant23. When )?@6 < 40 mT, the data is observed to follow this dependence, whereas the experimental data fall below the parabolic curve for larger fields.

Figure 3. The area of hysteresis curves as a function of the maximum applied magnetic field. The color indicates the temperature at which the curve was taken. The lines are parabola fits to the data. The measurement is done on a 12.6 wt %/9.0 wt % NiCo/MgAl2O4 catalyst in a 120 Nml/min flow of 2% hydrogen in argon at 1 bar. The inset shows the proportionality constant from the parabolas as a function of temperature. The crosses correspond to a fresh sample and the open circles correspond to the same sample after exposure to 800°C for 24 h. The inset in Figure 3 shows A() vs. T, and it is clearly observed that A() decreases monotonically with temperature and assumes a value near 20 J/(kg T2) at 800°C. Measurements for a fresh sample are shown as well as after exposure to 800°C for 24 h. A shoulder around a temperature of 350°C was observed in the first run, which corresponds to the Curie temperature of pure nickel. This shoulder disappeared after 24 h of heating at 800°C. Additionally, the shape



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of the curve altered significantly as the received power became higher at the lower temperatures, while it became lower at the highest temperatures. XRD analysis of the spent sample from the VSM measurements showed Ni-Co particles sizes of 32 nm, compared to 12 nm of the fresh sample (see Table 1). Consequently the changes in the hysteresis curve after exposure to the 800°C is a result of sintering of the nickel particles, which results in larger nanoparticles and probably also in a larger tendency for alloying between Ni and Co. Larger particles of alloyed NiCo have a larger hysteresis loss at lower temperatures, but alloying of Co with Ni also reduces the Curie temperature compared with Co, so that the hysteresis is reduced at high temperatures. This is in agreement with the phase diagram of a nickel—cobalt alloy, which reveals that the two metals will mix in any combination with no preferred ratios24. 3.3 Characterization of NiCo/MgAl2O4 In order to characterize the active phase of the catalyst, XRD analysis was performed initially, but it was not possible to determine the actual phase of Ni/Co on the catalyst as the XRD pattern of the pure elements is very similar. Instead, XRD was used only to estimate the average crystallite size in the samples. STEM was more suited to determine the phase composition. An artificially aged system of 4.3wt%/4.1wt% NiCo/MgAl2O4 was used for this analysis to represent the composition of the catalyst after operation under steam reforming conditions. STEM-EDS mapping showed a random and widely varying configuration of Ni and Co on the sample as summarized in Figure 4. The Ni and Co maps reveal that the two metals are largely present in the same regions as evidenced in the combined map (see Figure 4 b, c, and d). More than 20 different EDS spot analyses were made during the STEM investigation to analyze the composition of identified metal nanoparticles. Figure 4b shows the relative Ni and Co content from 22 different measurements. In the nanoparticles, the Ni/Co ratios spanning the entire range between pure Ni (spots 2 and 3) to pure Co (spots 5 and 6) with all compositions being represented. This corresponds well with the measurements shown in Figure 3 and suggests that the fresh catalyst could have a larger tendency for segregation between Co and Ni, but aging at 800°C results in sintering and formation of Ni/Co alloys.


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a) HAADF image

b) Combined map of c) Elemental map of d) Elemental map of Ni, Co, and O Co Ni

e) EDS spot analyses of Ni and Co content in nanoparticles Figure 4. Ni and Co content on EDS spot analysis from TEM images (a, b, c, and d as examples) of 4.3 wt %/4.1 wt % NiCo/MgAl2O4 sample. Points refer to specific analysis point. Image material for the spot analyses are found in the supplementary material. Overall, the final conclusion from the STEM analysis revealed that the Co and Ni composition did not follow a specific pattern. The combination of XRD and STEM analyses has revealed that practically any variation in the Ni/Co ratio of the metal nanoparticles will be present on the catalyst. This is in good agreement with the hysteresis loss data shown in Figure 3. 3.4

Induction-heated steam reforming

The combined characterization of the NiCo/MgAl2O4 catalyst revealed that the catalytic system: •

Was active for steam reforming (Figure 1)



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•

Was susceptible for a magnetic field (Figure 2–3)

•

Had a Curie temperature above 800°C (Figure 3)

All of these features were utilized when transferring the 12.6 wt %/9.0 wt % NiCo/MgAl2O4 catalyst to an induction-heated plug flow reactor system where the catalyst was heated directly and contact-free by an induction coil placed around the reactor in a series of power and total flow variations. The results of these experiments are summarized in Figure 5, showing the conversion of methane and the equilibrium temperature for the steam methane reforming reaction as a function of the input power to the induction heater, for experimental series with different total flows. These results verify the concept of performing steam methane reforming with induction heating as the only heat source where the nanoparticles displaying both catalytic and magnetic properties, as heating close to 800°C as well as a high conversion of methane were achieved. At the lowest flows (20 Nl/h, and 30 Nl/h), the methane conversion leveled out at high power output because almost complete conversion of methane was achieved. As an example, at a power input of 1600 W and a flow of 20 Nl/h, the conversion of methane was 98% giving an equilibrium temperature for the reforming reaction of 780°C. This also reveals that the nanoparticles had a temperature of at least 780°C. In this case the exit gas contained 70.2% of H2 and was consequently having a yield of 3.87 Nl H2 per Nl of CH4 in the feed (after subtracting the hydrogen in the feed gas).


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Figure 5. Conversion of methane and equilibrium temperature of the steam reforming reaction for a gas treated over a 12.6 wt %/9.0 wt % NiCo/MgAl2O4 catalyst as a function of power input to the induction heater and at different total flow rate series. Inlet temperature ≈ 200°C, Steam/Carbon = 2, yH2 = 11%, P = 1 bar. The lines are guides to the eye. The inset shows the equilibrium temperature vs. percentage of methane conversion based on the measurements points (small circles). Increasing the total flow resulted in a lower conversion at a given input power, as expected due to the larger gas flow being heated in the reactor. Noticeably, the system became limited by the catalytic activity as shown by the way the curves in Figure 5 level out at high input power due to insufficient residence time in the system. Consequently, heat transfer is not the problem but catalytic activity is. No significant degradation of the catalyst was found on the timescale of this test. The error on the point at 102 Nl/h at the highest power corresponds to two data points: one taken at the start of the measurement and one at the end after 43 h. Post-test characterization revealed that no carbon formation occurred during the test as the carbon content was quantified as < 500 ppm on the catalyst before the test and < 500 ppm after the test. However, some degree of sintering was



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observed during the test as the average particle size increased from ca. 24 to 52 nm, as quantified by XRD. This is expected for a MgAl2O4 supported catalytic system for steam reforming3, 25. The VSM measurements suggest that the hysteresis loss can be scaled with the maximum of the applied field squared. To compare the reactivity measurements, the power used to carry out the reaction was calculated using equation (10). This was then divided with the square of the magnetic field amplitude used to drive the reaction and the mass. This is shown as points in Figure 6 as a function of the average outlet gas temperature. The lines in Figure 6 indicate the received power calculated from the VSM measurements of Figure 3, assuming that the hysteresis loop is cycled at 68 kHz. The reactivity data, except at the lowest flow (20 Nl/h), fall on a line slightly below the heat-treated VSM data. Two factors can contribute to the fact that the power estimated from the reactivity measurements lie below that estimated from the VSM measurements. First, some of the received energy will be lost to the surroundings from the catalyst bed; this effect will be more dominant at low flows, so this may explain why the data at 20 N l/h are significant lower than the rest. Second, close to both inlet and outlet, the magnetic field will be smaller than in the center. Thus, the actual magnetic field experienced by the sample in the induction heating setup is lower than that estimated in the center of the coil used in the estimation. This results in an overestimation of the heating power based on the VSM measurements. Considering these errors, we find that reactivity data and VSM data are in agreement. Consequently, this reveals that the heating mechanism in the experiments presented in Figure 5 are coupled directly to the hysteresis curve and therefore also proves that it is the hysteresis loss of the magnetic/catalytic nanoparticles that supplies the heat of reaction for the system.


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Figure 6. Power used to convert methane in the reactor divided with the applied magnetic field squared and divided with the mass. The lines correspond to the data from Figure 3. Where the hysteresis loss is divided by the max field squared and mass as a function of temperature. 12.6 wt %/9.0 wt % NiCo/MgAl2O4. A frequency of 68 kHz was used. The results presented above demonstrate composite nanoparticles with a dual functionality – the ability to be inductively heated via their magnetic properties of Co and the catalytic conversion of methane due to their content of Ni. This direct inductive heating of catalytically active nanoparticles can be used to solve the general problem of industrial scale steam reforming, where heat transfer to the catalytic reaction is limited by the radiation, convection, and conduction mechanisms utilized in such a design. The direct heating of the catalyst material by induction will enable an immediate utilization of the heat in the endothermic reaction, and heat can consequently be utilized at a rate equivalent to the chemical reaction. In practice, this concept could open up for designing

a

steam

reforming

unit

which

will

be

significantly

smaller

in

size.

Additionally, the concept opens for much faster response in the system and heating, and thereby start-up, is also envisioned to be facilitated at a much smaller time-scale than in an industrial plant, where days typically are used to safely and slowly ramp up the temperature in the combustion chamber.



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Lastly, the induction heated reactor uses electricity over natural gas as heat source for the endothermic reaction. This means that a lower overall natural gas consumption can be realized on a steam reforming plant, which would mean the potential for designing more sustainable plants, if sustainably produced electricity can be used in the process.

4

Conclusion

Performing steam reforming on industrial scale is constrained by heat transfer from an external source to the catalyst bed, which practically makes heat transfer the limiting step and consequently leads to large chemical reactors. The present work demonstrated direct induction heating of catalyst nanoparticles in a chemical reactor to enable fast heat transfer at close to 800°C. Ni and Co was found as a suitable active phase for an induction-heated steam reforming catalyst. Mixing these two components on a Mg-Al spinel support resulted in a wide range of alloy compositions between Ni and Co, as visualized by STEM. Activity analysis revealed nickel as facilitating the catalytic activity for the steam reforming reaction. On the other hand, magnetometry revealed that cobalt brought ferromagnetic properties to the catalytic system quantifying that the Curie temperature of a NiCo/MgAl2O4 catalyst with 12.6 wt % Ni and 9.0 wt % Co was above 800°C and that the size of the hysteresis curve scaled with the cobalt loading. Testing the NiCo/MgAl2O4 in a laboratory reactor heated by induction alone proved that the catalyst could be heated to temperatures above 780°C by the oscillating magnetic field alone. The energy density received in the induction-heated steam reforming experiments correlated with the size of the hysteresis curve quantified by VSM, giving evidence for hysteresis heating being the primary heating mechanism and that consequently the Ni-Co nanoparticles were the direct susceptor of the magnetic field and thereby facilitating intimate contact between the catalytic active site and the heat source. The induction-heated catalysts both provided direct heating and catalytic activity to facilitate conversion of more than 90% of the methane at a corresponding equilibrium temperature of 700–800°C. Overall, the present work shows that it is possible to circumvent the heat transfer limitation of a traditional steam reforming reactor by induction heating and instead have direct heating of the catalyst bed. This offers fast response to the heat source and enables compact design of a reforming reactor.

5

Supporting Information

TEM images used for the EDS spot analysis of a NiCo/MgAl2O4 catalyst.


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6

Acknowledgments

The authors would like to thank The Danish Energy Agency for support via the EUDP program, grant number J.nr. 64013-0511. The authors wish to thank Cathrine Frandsen from DTU physics for fruitful discussions regarding this work.



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