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Catalytic Performance of Pd/CoO on SiC and ZrO Open Cell Foams for the Process Intensification of Methane Combustion in Lean Conditions Giuliana Ercolino, Pawe# Stelmachowski, and Stefania Specchia Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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Industrial & Engineering Chemistry Research

Catalytic Performance of Pd/Co3O4 on SiC and ZrO2 Open Cell Foams for the Process Intensification of Methane Combustion in Lean Conditions Giuliana Ercolino1, Paweł Stelmachowski1,2, Stefania Specchia1* 1

Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli

Abruzzi 24, 10129 Torino, Italy 2

Faculty of Chemistry, Jagiellonian University in Kraków, ul. Ingardena 3, 30-060 Kraków,

Poland

KEYWORDS Palladium; cobalt spinel; solution combustion synthesis; pressure drop; volumetric heat transfer coefficient; stability.

ABSTRACT

Ceramic open cells foams (OCF) are characterized by lower pressure drops, high geometric surface area, and enhanced radial convection in comparison with monolith-type supports. In this work, silicon carbide (SiC) and zirconia (Zir) OCF with different pore per inch (ppi) density were coated with 200 mg of Co3O4 by solution combustion synthesis and doped with 3 wt.% of

ACS Paragon Plus Environment

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Pd via wetness impregnation. Their catalytic activity was tested towards the lean oxidation of methane at different weight hourly space velocities. The Zir OCF with 30 ppi exhibits the best catalytic activity for all reacting conditions, followed by Zir 45 ppi, SiC 45 ppi, and SiC 30 ppi. The better performance of Zir OCF was rationalized considering their lower overall heat exchange coefficients, which favor the reaction heat removal by convection via the flue gases. A 200 h stability test on the best-structured catalyst demonstrated full methane conversion at a temperature below 400 °C. These results confirm the fundamental role of thermal conductivity of the structured catalysts’ support.

1. INTRODUCTION Carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons, usually known as greenhouse gasses (GHG) are responsible for the greenhouse effect,1–3 which lead to the increase of the earth’s temperature.3,4 The increase of temperature affects the socio-economic sector, ecological systems, and human’s life.2,5,6 GHG released due to human activities come from burning fossil fuels, industrial processes, transportation, agriculture facilities, and waste management processes.7–10 Methane is the most powerful GHG after carbon dioxide. In fact, methane contributes 14% to the overall GHG emissions.11 Methane emissions come from agriculture, energy, industry, and waste process sectors.1,5 The second contributor to the overall anthropogenic methane emissions is the energy sector (30%),7–9 in which the biggest contribution to methane emissions comes from NG systems (64%) and coal mines (22%).9,12–15 A characteristic feature of methane emissions due to human activities is its very low concentration,


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usually 1 vol.% or lower.16 Reducing methane emissions is beneficial for safety, climate change reasons, and for recover a source of wasted energy.13,14,17–20 So far, many different catalysts have been developed for methane combustion, mainly based on Pt21–24 and Pd,21,24–27 supported on metal oxides28–33 or perovskites.32,34–38 Moreover, these catalysts usually work in difficult reaction conditions: low temperature (less than 500-550 °C),24,39,40 low concentration of methane (0.5-1%),24,39,41 and a large excess of oxygen.24 Recently, the attention has been increasingly focused on the investigation of cobalt spinels based powdered catalysts. The spinel structure has some interesting physical and chemical properties.42,43 Indeed, the cobalt spinel based materials have high potential for use as catalysts,44,45 gas sensors,46 magnetic materials,47,48 pigments for ceramics,49 and electrochemical devices (rechargeable batteries, fuel cells, electrolyzers).50–52 Cobalt (II,III) oxide (Co3O4) and cobalt(II) oxide (CoO) particles are usually prepared by a wide assortment of wet chemical techniques, such as hydrothermal synthesis,53 spray pyrolysis,54 auto-combustion,55 coprecipitation,56 solubility-controlled synthesis,57 sol–gel,58 and combustion synthesis.30,56,59 Currently, among the various technologies adopted to mitigate methane emissions on a large scale,6–11,13 the attention is focused on open cells foams (OCF) as structured supports for catalysts to facilitate intensification of multiphase catalytic processes.60–68 OFC offer clear advantages compared to packed bed reactors, such as reduced pressure drops, high specific surface area, enhanced heat transfer, and high mechanical strength.69–75 These properties allow for increasing the space velocity, the effectiveness factor, and reducing the formation of dangerous hot-spots. Thus, from the industrial point of view, the use of OCF allows for process intensification in more compact reactors, with less energy consumed, higher product’s yield and selectivity, easier heat management and thermal stability.


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In this work, we investigated the use of silicon carbide and zirconia OCF as structured supports for a Pd/Co3O4 catalyst for the combustion of methane in lean conditions at low temperature. The selected catalyst, 3 wt.% Pd/Co3O4, was fully characterized and optimized based on our previous studies at a powder level. In particular, we investigated the synthesis technique (solution combustion synthesis, SCS, plus wetness impregnation, WI, or one-shot SCS),30 the effect of organic fuel used during the SCS (glycine or urea),56 and the best Pd doping level (optimized between 0.5 and 5 wt%).39 The applied operating conditions for the catalytic tests simulate the typical operating conditions of such applications as the abatement of fugitive methane from coal mine ventilation operations12,15 and of residual methane from methane-fed commercial vehicles.17,39 We determined the optimal reaction conditions for the complete oxidation of methane in lean conditions, and investigated the thermal properties of the structured catalysts, considering the different thermal conductivity of silicon carbide and zirconia materials.77,78 Finally, we investigated the stability operation conditions of the best-selected structured catalyst.

2. EXPERIMENTAL SECTION 2.1 Open cell foams and chemicals We purchased from Lanik s.r.o. (Czech Republic) ceramic OCF made of silicon carbide (Vukopor® S30 and S45) and zirconia (Vukopor® HT30 and HT45), each of 30 and 45 ppi (pore per inch), hereafter called SiC_30 and SiC_45, Zir_30 and Zir_45. The OCF are 9 mm in diameter and 30 mm long. Figure 1 shows the overview of the bare OCF. We purchased all the reagents from Sigma–Aldrich: Cobalt(II) nitrate hexahydrate Co(NO3)2·6H2O (≥98% purity), palladium(II) nitrate hydrate Pd(NO3)2·xH2O (≥99% purity), ammonia carbonate (NH4)2CO3


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(≥99% purity), glycine NH2CH2COOH (≥99% purity), and acetone CH3COCH3 (≥99.8% purity). We prepared the aqueous solutions using ultrapure water obtained from a Millipore Milli-Q system with a resistivity ≈ 18 MΩ cm. We purchased all the gases for reactivity tests from SIAD and used as received: pure methane, oxygen, and nitrogen (purity 99.999%).

Figure 1. The bare ceramic open cell foams used in this study: silicon carbide (SiC) and zirconia (Zir), with two different pore density values (30 and 45 ppi). 2.2 Preparation and physical characterization of the structured catalysts Before use, we cleaned all the OCF with the use of an ultrasonic bath in a solution of water/acetone (1:1), and dried at 250 °C for 30 min. Then, we deposited a thin layer of Co3O4 catalyst on OCF by the SCS method,30,37,59,79,80 according to our previous optimization.30,39 We prepared a 3 M solution of cobalt nitrate and glycine, with a cobalt nitrate/glycine stoichiometric ratio equal to 0.25.30,39 We immersed each OCF in the prepared solution for at least 5 min, and we removed the excess of the solution with compressed air. Then, we placed them in a furnace at 250 °C for 15 min to allow the SCS reaction initiating. We repeated this coating treatment several times to reach an amount of 200 mg of Co3O4 on each OCF. Specifically, considering the different geometry of the OCF, we repeated the procedure 10 times for 30 ppi OCF, and 5 times for 45 ppi OCF. Finally, we calcined all the OCF at 600 °C for 4 h in static air.


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After the deposition of Co3O4 as a carrier, we added 3 wt.% Pd (on the deposited Co3O4) to each OCF by WI using a solution of palladium nitrate.30,39 After each dipping, we dried each OCF at 140 °C for 10 min, and finally we calcined them in static air at 600 °C for 4 h. Considering our previous studies on the preparation technique of Pd/Co3O4 catalysts, the SCS plus WI or the oneshot SCS (where the amount of palladium is directly added to the starting precursors solution of organic fuel and nitrates),30 the two techniques provided catalytic materials with very similar catalytic activity. For the catalysts prepared by one-shot SCS, palladium is difficult to be detected because of its fine dispersion.30 Thus, for this work, we decided to use the combined SCS+WI to better monitor the amount of Pd deposited over the Co3O4, and to make sure that all Pd is available on the surface of the Co3O4. In our previous papers on Pd/Co3O4 at powder level,39,56 we verified experimentally that the real Pd loading was very close to the nominal one. Surely, a preparation technique in a unique step, such as the one-shot SCS, would be more suitable for an industrial point of view,59,80 and it will be considered as the next step of our research. We investigated the morphology and the homogeneity of the catalytic layer coated on all OCF by field-emission scanning electron microscopy (FESEM JEOL-JSM-6700F instrument). 2.2 Pressure drop measurements on the structured catalysts We measured the pressure drop across the coated OCF placed in a straight quartz tube reactor (10 mm I.D.) with a U-tube manometer connected upstream and downstream the OCF. We sent to the reactor a flow of nitrogen using a mass flow meter (Brooks), and varied the flow rate from 100 to 800 NmL min–1 at room temperature. We converted the height difference between the two columns of water of the U-tube manometer into pressure drop values via the Stevino equation.81


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We repeated the measurements several times on each coated OCF, and we calculated the theoretical pressure drop via the Forchheimer equation (1)62,67 for the comparison purposes.

(1) Forchheimer equation:

∆

=

, ∙∙()

+

, ∙∙()

According to Ergun and Orning,82 the values of α and β in the Forchheimer equation represent the viscous and the inertial or turbulent terms. In the case of OCF, they depend on their geometrical properties, in particular on the tortuosity, and they can be estimated experimentally.67,83,84 2.3 Heat transfer measurements on the structured catalysts We estimated the volumetric heat transfer coefficients, hv (W m–3 K–1), of the OCF at different superficial velocities and temperatures using the same straight quartz tube reactor used for the pressure drop measurements. The volumetric heat transfer coefficients take into account the internal heat exchange between the solid phase of the foam and the fluid: the heat transfer occurs between the fluid and the surface of the solid, and the solid network influences it. One has also to take into account that open cell foams are porous structures with irregular and tortuous pathways, and the strut is hollow, not solid.75,85,86 We used a PID-regulated electrical oven to maintain a constant temperature of 200, 400, and 600 °C for three collection campaigns. We placed two thermocouples in tight contact with the OCF to monitor the temperature drop over the structure (∆T = TIN – TOUT) during the measurements. We sent through the reactor a flow of nitrogen at four different velocities: 100, 200,400, and 800 NmL min–1. We collected the data at a constant temperature of the oven (TOVEN, recorded with a third thermocouple placed in the middle of the oven) to calculate the Reynolds (2) and Nusselt (3) numbers:


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(2) Reynolds number: =

(3) Nusselt number:

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∙

∙ ,

=

∙!

"#

= $ ∙ %

For Nu, the parameters C and m depend on the geometry of the OCF. To estimate C and m, we used the experimental correlation of Younis and Viskanta87, valid for ceramic foams with

&' ) (

ratio between 0.005 and 0.136 (foams with ppi values up to 70, and porosity higher than 83%) and Re between 5.1 and 564.87–89 !

!

(4) $ = 0.819 ∙ /1 − 7.33 ∙ 3 45 /1 − 7.33 ∙ 3 45 !

(5) 6 = 0.36 ∙ /1 + 15.5 ∙ 3 45 2.4 Methane temperature programmed oxidation (CH4-TPO) measurements on the structured catalysts For assessing the catalytic activity of palladium as active phase towards methane combustion, we tested first the catalytic activity of all the OCF coated only with 200 mg of Co3O4, then of the OCF coated with Pd/Co3O4. We performed the series of CH4-TPO measurements in the same lab-scale reactor used for heat transfer measurements. We wrapped the OCF in a vermiculite foil to avoid channeling of the gas flow while testing. For each OCF, we used the methane content of 0.5 and 1 vol.% in nitrogen, with a constant oxygen-to-methane molar ratio equal to 8 (lean conditions) and the WHSV equal to 30, 60, and 90 NL s−1 gcat−1, obtained by varying the reagents flow rate from 100 to 300 NmL min−1. For CH4-TPO measurements, first we increased the temperature of the oven up to 700 °C (10 °C min−1). When we reached full methane conversion


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in the steady-state conditions, we let the oven to cool down (5 °C min−1). We measured the concentration of the outlet gases during the cooling-down phase of the oven with an ABB analyzer (NDIR module Uras 14 for CO/CO2/CH4, paramagnetic module Magnos 106 for O2; water removed prior entering the analyzer in a condenser at 3 °C). We obtained the characteristic S-shaped graphs by plotting the methane conversion versus temperature. We repeated all measurements at least three times to assure their reproducibility, testing first the reacting mixture containing 1 vol.% methane at the three WHSV, then 0.5 vol.%, and 1 vol.% again to check any possible aging phenomena on the structured catalysts. 2.5 Stability measurements (CH4-TPO in TOS) on the best-selected structured catalyst Considering the good results obtained with the Zir_30 structured catalyst, we prepared a new Zir_30 coated in the same way as previously described, and we assessed the stability of this catalyst by maintaining it for 200 h of time on stream (TOS) at 400 °C. We fed the reactor with a reactive mixture containing 0.5 vol.% methane concentration at a WHSV equal to 30 NL s−1 gcat−1, according to the thermal cycling shown in Figure 2. At 0, 20, and 200 of TOS, we performed a series of CH4-TPO measurements, and analyzed the morphology of the structured catalyst by FESEM investigations. Moreover, we performed the elemental analysis of the Zir_30 after the stability test by Scanning Electron Microscope (SEM, Phenom XL, Phenom-World) equipped with energy dispersive spectroscopy (EDS). We mapped four different areas of the OCF, each with an overall surface of 537 µm (128×128 pixel) for O, Co, Pd, and Zr.


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Figure 2. Thermal cycling for stability tests: TOS vs time at 400 °C and CH4-TPO measurements (after 0, 20, and 200 h of TOS). Inlet methane concentration: 0.5 vol.%.

3. RESULTS AND DISCUSSION 3.1 Physical characterization of the structured catalysts Table 1 lists the textural properties of the four investigated OCF, based on the geometrical formula of the strut provided by Buciuman and Kraushaar-Czarnetzki.75 Zir-based OFC have slightly lower voidage and average pore dimensions, but higher surface area, compared to SiCbased ones. The 30 ppi OFC have larger pores compared to the respective 45 ppi ones.

Table 1. Textural properties of the four OCF investigated (SiC: silicon carbide; Zir: zirconia). @

Overall OCF dimensions, 9 : (, 9 mm x 30 mm. Overall OCF volume, ; = ? ∙ 3 4 ∙ (, 1909 mm3. Each OCF has been coated with 200 mg of Co3O4 and 3 wt.% of Pd. Formula of the strut from Buciuman and Kraushaar-Czarnetzki.75

Pore per inch

SiC_30

SiC_45

Zir_30

Zir_45

30

45

30

45


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Average pore dimension, dp / mm

1.63 ± 0.65

1.25 ± 0.52

1.3 ± 0.73

1 ± 0.5

Average strut thickness, ts / mm

0.42 ± 0.16

0.25 ± 0.1

0.47 ± 0.16

0.27 ± 0.10

Face diameter, df / mm

2.05

1.5

1.77

1.27

0.109

0.072

0.183

0.117

0.89

0.93

0.82

0.88

0.776

0.862

1.165

1.298

1481

1645

2224

2478

13.9

12.5

9.3

8.3

&A = &' + BC Foam relative density, ρr / DE,A =

DA

DCFGH!

BC = 2.59 ∙ J K &A

Voidage, ε / LA = 1 − DE Geometric surface area, Sag / mm–1 MNO =

P.Q !

∙ RDE

Calculated surface area, Sa / mm2 MN = ; ∙ MNO Catalyst loading / mg cm–2 $GFN! =

6SNT MN

Figure 3 shows FESEM pictures of SiC_30 and Zir_30 coated with 3% Pd/Co3O4 at different magnifications. FESEM pictures show the catalytic layer on SiC and Zir foams. At low magnification, both SiC_30 and Zir_30 appear well coated, with a uniform and thin layer of catalyst covering the entire geometric surface of the OCF. The catalyst layer is well anchored with an average thickness ranging from 30 to 50 µm, similar to what available on the literature for similarly structured catalysts obtained by SCS.75,81,90–92 The porous structure of the catalytic layer, noticeable at medium magnification, originates from a large amount of gases generated during the SCS reaction.79,80,93 At higher magnifications, the images point out the typical shape


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of Co3O4 spinel crystals, clusters of truncated octahedrons.30,42,56,94 We obtained this typical shape also on Pd/Co3O4 as a powder catalyst we synthesized by SCS+WI for methane combustion in lean conditions in our previous works.30,39,56 This is an indirect confirmation that we were able to deposit the desired catalytic phase, the Co3O4 spinel, on all the OFC via the SCS. Thus, the SCS is a suitable in situ technique for a fast and easy deposition of the desired catalyst on quite complex geometries as the OFC are.79–81

Figure 3. FESEM pictures of SiC_30 and Zir_30 coated with 3% Pd/Co3O4 (magnification: 70×, 25,000×, 100,000×, and 250,000× in the inset). 3.2 Pressure drop measurements on the structured catalysts Figure 4 shows the pressure drop values measured on all coated OFC by varying the nitrogen flow rate in the reactor. All OCF have very similar pressure drop values, in line with literature data for similar structures.61,67 On average, the Zir OCF have very similar or slightly lower pressure drop values than the SiC ones, especially increasing the flow rate. Different flow rates result in different superficial velocities for the OCF due to their different textural properties. In


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fact, the geometric surface areas of the Zir OCF are higher than that of the SiC ones with the same ppi, but the average dimension of the pores and the voidage are smaller (Table 1).

Figure 4. Pressure drop measurements and Forchheimer estimation vs the superficial velocity of the gas for all OCF coated with 3% Pd/Co3O4.

Table 2. Ergun and Orning82 values (α and β) used in the Forchheimer equation to evaluate the theoretical pressure drop across the coated OCF. Physical and thermal properties, C and m values of the coated OCF used to estimate = $ ∙ % according to Younis and Viskanta87 (eq. 3 valid for 0.005 U

&' ) U 0.136 and 5 U U 564). (

Forchheimer equation

Younis and Viskanta correlation87 C

m

W m–1 K–1

WX ) Y

-

-

0.09

0.477

0.0543

0.492

0.664

0.07

0.477

0.0416

0.569

0.592

α

β

λf

-

-

SiC_30

19.5

SiC_45

12.0


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Zir_30

6.0

0.01

0.02778

0.0433

0.560

0.601

Zir_45

4.5

0.0065

0.02778

0.0333

0.617

0.548

Coating the OCF with the same amount of catalyst, 3% Pd/Co3O4 (overall 206 mg of catalyst) results in a different thickness of the catalytic layer. In particular, the catalytic layer is slightly thicker for the SiC OCF than for the Zir ones. As a result, the pressure drops are similar, but at slightly different space velocities. For both types of OCF, the increase of the ppi value from 30 to 45 does not have an important effect on the pressure drop values. As expected, the theoretical values of the pressure drop calculated according to the Forchheimer equation perfectly suit the experimental values. The good matching is due to the introduction of the geometrical characteristics of the foams into the Forchheimer equation. Table 2 lists the values of α and β parameters, estimated from the experimental data for the OCF under study, which match with the values range suggested in the literature.67,83 3.3 Heat transfer measurements on the structured catalysts The stability of OCF plays a key role in maintaining the activity of the catalyst as high as possible to guarantee a long operative life of the entire system. Figure 5 shows the measured temperature drop, ∆T, at the three selected TOVEN, for all the coated OCF, for the different nitrogen flow rates. As a general trend, the ∆T decreases with the increase of the gas velocity. The ∆T values measured for the SiC OCF are slightly higher (from 5 to 8 °C) compared to the Zir ones (up to 3 °C) for all TOVEN values, because of the much higher thermal conductivity of SiC compared to Zir (see Table 2).


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Moreover, the Zir OCF have even a more flat profile with the increase of the gas velocity, especially at a low temperature. The ∆T is always higher for the 30 ppi Zir OCF compared to the 45 ppi one, and the contrary is valid for the SiC OCF.

Figure 5. ∆T (TIN – TOUT) temperature at various TOVEN for all OCF.

Figure 6A shows the Nusselt number as a function of the Reynolds number for the three selected TOVEN, calculated according to the correlation of Younis and Viskanta.87 Table 2 lists the physical and thermal properties of the OCF used to calculate Nu, and the estimated values of C and m based on our experimental data. As Re increases, Nu increases as well: increasing the turbulence of the gas flowing in the OFC, the convective heat increases. At equal nitrogen flow rate, the superficial velocity is higher for the Zir OCF because their pores diameter is lower than that of the respective SiC ones. Moreover, the number of pores influences the Nu dependence on the Re: the lower the ppi, the lower the geometric surface area, the higher the Nu. At low temperature, 200 °C, the Nu values are the highest for SiC_30, similar for Zir_30 and SiC_45, and lowest for Zir_45. Similar values of the Nu for Zir_30 and SiC_45 may originate from their similar average pore dimensions (Table 1). Increasing the temperature up to 600 °C results in the


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Nu values very similar for all the OCF, because of the increase of the turbulence and the decrease of the viscosity. These effects play a key role in the volumetric heat transfer coefficients (Figure 6B), which are much higher for the coated SiC OCF, even considering the different thermal conductivity of SiC and Zir OCF (Table 2). For both the OFC families, Zir and Sic: the lower the ppi (the higher the average pore diameter), the lower the geometric surface area, the lower the hv. The hv plotted in Figure 6B for SiC OFC are in line with those reported by Dietrich88 and Xia et al.89 According to our knowledge, no data related to Zir OFC are available in the literature.


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Figure 6. Nusselt numbers (A) and volumetric heat transfer coefficients (B) as a function of Reynolds number for all the coated OCF at various TOVEN.

3.4 Methane temperature programmed oxidation (CH4-TPO) measurements on the structured catalysts We tested the SiC and Zir OCF coated with Co3O4 and 3 wt.% Pd/Co3O4 towards catalytic methane combustion in lean conditions. We never detected carbon monoxide in the reactor effluent mixture, thanks to the high lambda value, and we estimated the standard deviation for the carbon balance for all measurements equal to ±3%. To evaluate the CH4-TPO results, we considered the light-off temperature (T10), the half-conversion temperature (T50), and the light-on temperature (T90). SiC and Zir OCF coated only with Co3O4 showed a limited and similar catalytic activity, with relatively high T10 and T50 (T50 ranges of 485-510 °C, 570-605 °C, and 610-670 °C for the three WHSV and two inlet methane concentrations investigated, see Figure S1 in the Supporting Information). The increase of the WHSV causes a general worsening of the catalytic activity, mainly due to the reduction of the contact time, which makes impossible reaching the full methane conversion at the intermediate and highest WHSV tested. Figure 7 shows the activity of all the 3 wt.% Pd/Co3O4 coated OCF at a methane inlet concentration of 0.5 to 1 vol.% and increasing the WHSV from 30 to 90 NL s−1 gcat−1. The presence of Pd over the Co3O4 coated OCF allows a huge reduction of the T10 and T50 of more than 200 °C (T50 ranges of 250-305 °C, 270-325 °C, and 310-375 °C for the three WHSV and


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two inlet methane concentrations investigated), compared to the OFC coated only with Co3O4. In particular, Pd doping makes possible the full combustion of methane also at the highest WHSV, especially on Zir OCF. These results regarding catalytic activity (T10 and T50) are even better that the results we obtained on the same 3 wt.% Pd/Co3O4 active phase tested as a powder catalyst for lean combustion of methane in the same conditions we used here for the structured catalysts.30,39 As a powder catalyst, Pd was present as PdO phase (evident both from Raman and XRD analyses published in our previous works).30,39 In each graph of Figure 7, we enlightened the differences in the light-off temperatures (T10). As a general trend, the catalytic activity decreases for all OCF as the WHSV increases, because the contact time between the catalyst and the reactive mixture decreases. Zir OCF exhibit better catalytic activity than the corresponding SiC ones, the Zir_30 being better than the Zir_45. In particular, both the Zir OCF can reach full methane conversion within 550 °C at the highest WHSV, and below 350 °C at the lowest WHSV. Instead, at higher WHSV the performance of both the SiC OCF decreases since they reach relevant methane conversion only at very high temperature (> 600 °C), mainly because of the reduction of the residence time. In fact, we checked the absence of both the internal and external mass tranport limitations via the WheelerWeizs criterion and Carberry number:95 [\]^ ∙!_`

(6) ZZ = a (7) $e = f

\]^ ∙=\]^ ,b

[\]^

h g ∙N ∙=\]^ ,i

∙

=

cd

U 0.1

=\]^ ,i =\]^ ,b =\]^ ,i

U

j.jk c

In all our experimental conditions (order of reaction n=1), WW and Ca numbers are well below the limit values for all the OCF (being the maximum values in the worst conditions equal to WW


18

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= 0.00013 and Ca = 0.0069 for SiC_30, and WW = 0.00024 and Ca = 0.0077 for Zir_30, at the highest WHSV). Thus, our experimental conditions are not affected by intra- or extra-particle mass transport. Interestingly, the analysis of the light-off temperatures reveals that the SiC OCF provide performance similar to the Zir ones at high WHSV (even slightly better for 1 vol.% methane inlet concentration), but then the performance worse when reaching the light-on temperature. The comparative analysis of the different inlet methane concentration shows that on average the reactive mixture with 0.5 vol.% reaches full methane conversion at a temperature slightly lower than the reactive mixture with 1 vol.%. Moreover, the differences between 0.5 and 1 vol.% are less accentuated for the SiC OCF compared to the Zir ones. To better enlighten these observations, we prepared a different version of Figure 7 in the Supporting Information by comparing together the curves at 0.5 or 1 vol.% methane concentration varying the WHSV (Figure S2), and by comparing together the curves at 30 or 60 or 90 WHSV varying the inlet methane concentration (Figure S3). The comparison includes the catalytic activity of all the OCF coated only with Co3O4 to highlight the positive effect of the presence of palladium better. From these results, we can point out that the best catalytic activity belongs to Zir_30 OCF, followed by Zir_45, SiC_45, and SiC_30. We can explain these results following the trends of the volumetric heat exchange coefficients plotted in Figure 6B: the Zir OCF have lower volumetric heat exchange coefficients, linked to their lower thermal conductivity. Thus, in these systems the convective heat removal via the flue gases is favored. Heat management is a fundamental aspect of adiabatic or quasi-adiabatic systems, like ours. Zir OCF maintain a stable thermal behavior (Figure 5), thus the heat of reaction is easily removed by convection, without influencing the thermodynamics of the exothermic reaction of combustion. Instead, for the SiC OCF the catalytic performance worse compared to those of Zir OCF especially at high


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temperature (close to the T90) for all WHSV. Noticeably, SiC OFC have comparable performance to Zir ones at low temperature (close to the T10, see inset in Figure 7), and when the WHSV increases. In these conditions, when the reaction is starting, the heat produced by the reaction is still negligible, because of the low conversion of methane. In this case, the higher thermal conductivity and higher volumetric heat exchange coefficients of SiC are helpful to retain the heat of reaction and consequently provide the necessary energy for boosting the ignition of the first reacting molecules on the surface of the OCF.96 In these conditions, SiC OCF properties favor the kinetic aspects more than they worsen the thermodynamic ones. The contrary is valid at a higher temperature, when almost all the inlet methane is burnt: SiC OCF depress the methane conversion because of the high volumetric heat exchange coefficients and thermal conductivity, which lead to the thermodynamic hindrance of the reaction. The same happens when the WHSV increases, that is the residence time decreases. Noticeably, Zir_30 and SiC_45, which are the best OCF per type of material, have comparable geometry regarding the average pore dimensions (1.3 vs 1.25 mm, Table 1), which in turns means comparable Nu numbers (Figure 6A). For a highly exothermic reaction conducted in adiabatic or quasi-adiabatic conditions, the full conversion of the reagents can be easily reached when the structured support has a low volumetric heat exchange coefficient and low thermal conductivity. The results reported here confirm the fundamental role of thermal conductivity of the OFC during a highly exothermic reaction, as in our case.96 Based on these considerations, in the case of process intensification for adiabatic systems, one could arrange an optimal catalytic system by placing first a catalytic SiC_45 OCF at the beginning of the reactor (to boost the ignition of the reaction, that is, the kinetics, especially at high WHSV), followed by a catalytic Zir_30 OCF to favor the reaction at


20

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higher temperatures (to enhance the thermodynamics). Interestingly, this possibility can also be inferred from the observation of the reactivity of the inlet methane mixtures on foams coated only with Co3O4 (Figures S1/S2/S3 in the Supporting Information), which have a poor catalytic activity, especially compared to those coated with Pd/Co3O4: the best results among all Co3O4 coated OCF belong to SiC supports. Thus, a synergetic effect by suitable mixing the two supports, SiC and Zir, and coating them with Pd/Co3O4 should allow obtaining a better performance.


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Figure 7. CH4-TPO for all the coated OCF at various methane inlet concentrations and WHSV. In the inserts: comparison of the T10.


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3.5 Stability measurements (CH4-TPO in TOS) on the best-selected structured catalyst Figure 8A shows the results of the stability tests, 200 h of TOS at 400 °C with an inlet methane concentration equal to 0.5 vol.%. At the beginning of TOS, methane conversion is equal to 100%, decreasing sharply to around 90% after few hours of TOS. After a stop at 20 h, to perform a full CH4-TPO test, the catalyst completely recovers its performance thanks to the thermal procedure, which forecasts a short permanence at 500 °C (Figure 2). Again, after few hours of TOS, methane conversion decreases to around 90%, and to 85% at around 50 h of TOS. After that, the performance of the catalyst remains more stable, and at the end of TOS methane conversion is equal to 82%. CH4-TPO tests after 0, 20, and 200 h of TOS, as illustrated in Figure 8B, enlighten that by “re-activating” the catalyst at 500 °C, the catalytic performance is recovered and the shift of the S-curves to higher temperatures is limited even after 200 h of TOS, which guarantee a full methane conversion at a temperature below 400 °C. Interestingly, the T10 at 0 and 20 h of TOS are almost the same, whereas the slopes of the S-curves at 20 and 200 h of TOS are the same, while being slightly lower compared to that of the fresh catalyst, indicating a change in the activation energy for the catalytic reaction. Considering our idea for the process intensification of adiabatic systems by joining a SiC with a Zir OCF, the double OCF catalyst should be able to sustain the catalytic reaction on prolonged TOS by lowering the T10 of the reaction. This effect should favor the light-off of the reaction, partially limiting the performance decay recorded from 20 to 200 h of TOS. FESEM images of the surface of the catalytic layer after 0/20/200 h of TOS (Figure 8A) show a variation in the morphology of the catalyst. In particular, the typical shapes of Co3O4 spinel


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crystals are preserved, but the morphology appears more corrugated compared to the fresh catalyst. Figure S4 in the Supporting Information shows more FESEM images of the Zir_30 OCF after 200 h of TOS, in comparison with the fresh structured catalyst. At low magnification (2,500× and 10,000×) the surface of the aged structured catalyst appears smoother, with lots of cracks mainly due to the long-time exposition at relatively high temperature. However, increasing the magnification to 100,000× and 250,000× reveals that the aged structured catalyst maintain the corrugated morphology, different to that of the fresh one. An apparent increase in the specific surface area due to the surface roughening may have a negative effect on the palladium dispersion, leading to its sintering, and an irreversible decrease in the activity. On the other hand, the surface roughening may enhance the adsorption of product molecules (or some other gases present in the feed) on the surface, leading to a reversible decrease in the activity, as evidenced by the regeneration after 20 h of TOS. In a working catalytic system, both effects will probably contribute to the evolution of the CH4 conversion efficiency.

Figure 8. Stability test of 3 wt.% Pd/Co3O4 on Zir_30: CH4 conversion as a function of TOS at a constant temperature of 400 °C with FESEM images at 100,000× (A) and CH4-TPO at 0/20/200 h of TOS, inlet methane concentration equal to 0.5 vol.% (B).


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To verify the chemical composition of the catalytic layer of Zir_30 after the stability test, we performed an elemental analysis by SEM/EDS, by checking four different areas of the OCF. Figure 9 shows a typical mapping of the central part of the OCF (the other areas, not reported here, look very similar). The catalytic layer appears very well distributed on the structure, and homogeneous in the elemental composition of all of the elements of the coating. Each single element (O, Co, and Pd) is homogeneously distributed, also in the other pictures not reported here. In particular, there is no evidence of Pd agglomeration, a sign of a good stability of the active phase. The average weight percentage of Pd is 3.75 %, which is in good agreement with the theoretical 3 % Pd over the Co3O4 (also considering the intrinsic error in the measurements). The atomic Pd/Co ratio ranges from 0.034 to 0.039. Moreover, the measurements reveal the presence of Zr, which is a clear sign that the thickness of the catalytic coating is very thin, less than 50 µm, in line with previous depositions by SCS on structured catalysts realized by our research group.35,37,79–81,90,91 This SEM/EDS analysis confirms that the SCS as a suitable technique to deposit homogeneous catalytic coatings on ceramic supports.

Figure 9. SEM/EDS mapping of 3 wt.% Pd/Co3O4 on Zir_30 after 200 h of TOS: overall and combined maps (537 µm, resolution 128×128 pixel).


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4. CONCLUSIONS We coated silicon carbide (SiC) and zirconia (Zir) open cells foams (OCF) with 3 wt.% Pd/Co3O4, by using the solution combustion synthesis method (for Co3O4) followed by the wetness impregnation (for Pd deposition). We characterized these coated OCF from the physical point of view, and evaluated their catalytic performance toward the combustion of methane at different methane inlet concentration (0.5 and 1 vol.%) and different WHSV (30, 60 and 90 Nl s– 1

gcat–1). The Zir based OCF catalyst, with the lowest average pore dimensions (Zir_30), exhibits

the best catalytic activity for all reacting conditions, followed by Zir_45, SiC_45, and SiC_30. The better performance of zirconia-based OCF can be explained considering their lowest volumetric heat exchange coefficients, which favor the reaction heat removal by convection via the flue gases. The thermal management is a fundamental aspect of adiabatic or quasi-adiabatic systems, where Zir OCF can maintain a stable thermal behavior. Noticeably, SiC OFC gain a comparable performance to Zir ones at low temperature, close to the T10, and when the WHSV increases. In these conditions, the higher thermal conductivity of SiC OCF is helpful to retain the heat of reaction for boosting the ignition of the first reacting molecules on the surface of the catalyst. Finally, the best coated Zir_30 was evaluated from the stability point of view with a test consisting of 200 h of time on stream (TOS) at 400 °C with an inlet methane concentration equal to 0.5 vol.%. At the end of TOS, methane conversion was equal to 82%. By “re-activating” the structured catalyst at 500 °C for a short time (20 min), its performance was recovered assuring a full methane conversion at a temperature below 400 °C. Based on the experimental considerations, an optimal system in case of process intensification for adiabatic or quasiadiabatic systems could be arranged by placing first a catalytic SiC_45 OCF at the beginning of


26

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the reactor, to favor the ignition of the reaction especially at high WHSV, followed by a catalytic Zir_30 OCF to favor the reaction at higher temperatures. These results confirm the important role of thermal conductivity of the structured catalysts support, which should be taken into account when choosing the most appropriate system for specific operation conditions.

ASSOCIATED CONTENT The following file (pdf) containing Figures S1, S2, S3, and S4 is available free of charge. Figure S1. CH4-TPO for all the OFC coated only with Co3O4 at 0.5/1% methane inlet concentrations and increasing WHSV (30/60/90 NL s−1 gcat−1). Figure S2. Comparison of CH4-TPO for all the OCF coated with 3 wt.% Pd/Co3O4 (A/B) and Co3O4 (C/D) and at fixed methane inlet concentrations and various WHSV. Figure S3. Comparison of CH4-TPO for all the OCF coated with 3 wt.% Pd/Co3O4 (A/B/C) and Co3O4 (D/E/F) at fixed WHSV and various methane inlet concentrations. Figure S4. FESEM images at various magnification: bare Zir_30 OCF; coated with 3 wt.% Pd/Co3O4 fresh and after 200 h of TOS.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +39.011.0904608. Fax: +39.011.0904699.


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ORCID Giuliana Ercolino: 0000-0002-4342-4546 Paweł Stelmachowski: 0000-0003-1126-8101 Stefania Specchia: 0000-0003-3882-3240 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The Italian Ministry of Education, University and Research supported this research through the project PRIN IFOAMS (“Intensification of catalytic processes for clean energy, low-emission transport and sustainable chemistry using open-cell FOAMS as novel advanced structured materials”, protocol n. PRIN-2010XFT2BB). The Italian Ministry of Foreign Affairs and the Polish Ministry of Science and Higher Education supported the exchange of researchers for this research through the Executive Programme for Scientific and Technological Cooperation CANALETTO (protocol n. M00478). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the following colleagues from the Politecnico di Torino: Mr Mauro Raimondo (FESEM measurements), Dr Giorgia Novajra and Prof. Chiara Vitale


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Brovarone (SEM/EDS measurements), and M.Sc. Cristian Di Carlantonio (help in the lab). Paweł Stelmachowski gratefully acknowledges the Politecnico di Torino for his Visiting Professorship.

ABBREVIATIONS & NOMENCLATURE a’

specific particle surface area, m–1

C

angular coefficient of Re (according to Younis and Viskanta)87

Ca

Carberry number

l=m^ ,n

methane concentration in bulk phase, mol m–3

l=m^ ,C

methane concentration at the surface of the catalyst particle, mol m–3

Cload

catalyst loading on OFC, mg cm–1

dcat

average particle size of the catalyst, m

dp

average pore dimension of OCF, mm

df

face diameter of OCF, mm

o=m^

methane effective diffusion coefficient, m2 s–1

FESEM

field emission scanning electron microscopy

GHG

greenhouse gasses

hv

volumetric heat transfer coefficient, W m–3 K–1

kg

extraparticle mass transfer coefficient, m s–1

L

length of OFC, m

m

exponent of Re (according to Younis and Viskanta)87

mcat

catalyst loading on OCF, mg cm–2

n

reaction order of methane combustion


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Nu

Nusselt number

OCF

open cell foam

ppi

pore per inch

=m^

observed volumetric reaction rate, mol mcat–3 s–1

Re

Reynolds number

Sa

calculated surface area of OCF, m2

Sag

geometric surface area of OCF, m–1

Sag,p

geometric surface area of the pore of OCF, m–1

SCS

solution combustion synthesis

SiC

silicon carbide (OCF)

ts

average strut thickness of OCF, mm

T10

temperature of 10% methane conversion (light-off temperature)

T50

temperature of 50% methane conversion (half-conversion temperature)

T90

temperature of 90% methane conversion (light-on temperature)

TOS

time on stream, h

TPO

temperature programmed oxidation

Vf

overall geometric volume of OCF, mm3

WHSV

weight hourly space velocity, NL s−1 gcat−1

WI

wetness impregnation

WW

Wheeler-Weisz number

Zir

zirconia (OCF)

Page 30 of 54

Greek letters α, β

geometric parameters of OCF (according to Ergun and Orning)82

∆P

pressure drop, Pa


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∆T

temperature drop (TIN – TOUT), °C

ε

voidage of OCF

λf

thermal conductivity, W m–1 K–1

µ

fluid dynamic viscosity, Pa s (1.78·10–5 kg m–1 s–1 for nitrogen)

ρ

density, kg m–3 (1.161 kg m–3 for nitrogen)

ρr

relative density

Φ

overall diameter of OCF, mm

ω

superficial velocity, m s–1

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Reforming. Catal. Today 2016, 273, 131.

For Table of Contents Only


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Figure 1. The bare ceramic open cell foams used in this study: silicon carbide (SiC) and zirconia (Zir), with two different pore density values (30 and 45 ppi). 71x47mm (300 x 300 DPI)



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Figure 2. Thermal cycling for stability tests: TOS vs time at 400 °C and CH4-TPO measurements (after 0, 20, and 200 h of TOS). Inlet methane concentration: 0.5 vol.%. 84x35mm (300 x 300 DPI)


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Figure 3. FESEM pictures of SiC_30 and Zir_30 coated with 3% Pd/Co3O4 (magnification: 70x, 25,000x, 100,000x, and 250,000x in the inset). 159x73mm (300 x 300 DPI)



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Figure 4. Pressure drop measurements and Forchheimer estimation vs the superficial velocity of the gas for all OCF coated with 3% Pd/Co3O4. 83x44mm (300 x 300 DPI)


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Figure 5. ∆T (TIN – TOUT) temperature at various TOVEN for all OCF. 88x56mm (300 x 300 DPI)



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Figure 6. Nusselt numbers (A) and volumetric heat transfer coefficients (B) as a function of Reynolds number for all the coated OCF at various TOVEN. 94x138mm (300 x 300 DPI)


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Figure 7. CH4-TPO for all the coated OCF at various methane inlet concentrations and WHSV. In the inserts: comparison of the T10. 157x212mm (300 x 300 DPI)



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Figure 8. Stability test of 3 wt.% Pd/Co3O4 on Zir_30: CH4 conversion as a function of TOS at a constant temperature of 400 °C with FESEM images at 100,000x (A) and CH4-TPO at 0/20/200 h of TOS, inlet methane concentration equal to 0.5 vol.% (B). 179x59mm (300 x 300 DPI)


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Figure 9. SEM/EDS mapping of 3 wt.% Pd/Co3O4 on Zir_30 after 200 h of TOS: overall and combined maps (537 µm, resolution 128x128 pixel). 80x59mm (300 x 300 DPI)



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Table of Contents 78x45mm (300 x 300 DPI)


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Co3O4 on SiC and ... - ACS Publications

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