CeO2 catalyst: Effect of daily start-up

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Kinetics, Catalysis, and Reaction Engineering

Methane steam reforming on Pt/CeO2 catalyst: Effect of daily start-up and shut-down on long term stability of catalyst Um-e-Salma Amjad, Carmen W. Moncada Quintero, Giuliana Ercolino, Cristina Italiano, Antonio Vita, and Stefania Specchia Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02436 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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Industrial & Engineering Chemistry Research

1 Methane steam reforming on Pt/CeO2 catalyst: Effect of daily start-up and shut-down on long term stability of catalyst

Um-e-Salma Amjad(1,2)*, Carmen W. Moncada Quintero(2), Giuliana Ercolino(2), Cristina Italiano(3), Antonio Vita(3), Stefania Specchia (2,3)*

(1)*

Department of Chemical Engineering, COMSATS University Islamabad Lahore Campus,

Defence Road, Off Raiwind Road, Lahore -54000, Pakistan

(2)

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

Abruzzi 24, 10129 Torino, Italy

(3)

CNR-ITAE “Nicola Giordano”, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy

*Tel: +92.42.111001007 ext 158 email: [email protected]

*Tel.:+39.011.0904608. Fax:+39.011.0904699. E-mail: [email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

2 ABSTRACT

An environmentally friendly Auxiliary Power Unit (APU) requires catalysts operating in cyclic mode, producing H2 at a steady rate with high CO2 selectivity and minimum CO clean-up. Here, the catalyst 1% Pt/CeO2 was synthesized using the method of solution combustion synthesis (SCS) method, and investigated towards methane steam reforming (MSR) reaction. Preliminary tests were performed by varying steam-to-carbon ratio (S/C), gas hourly space velocity (GHSV), and time-on-stream (TOS). The catalyst showed near equilibrium CH4 conversion and high H2 concentration (75% dry basis, N2-free) obtained at S/C = 2.8 and GHSV = 24,000 h–1. The catalytic activity was stable after 25 cycles of start-up and shut-down operations, equivalent to 150 h of TOS. XRD, TEM, and XPS techniques were used tp characterize either the fresh and aged catalyst after 150 h of TOS.

Keywords: Steam reforming; ceria supported Pt catalyst; thermodynamic analysis; stability.


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3 1. Introduction

Increase in global warming in recent years echoed a concern for reduction of harmful greenhouse gases (GHG) and carbon footprint. According to the European Environment Agency 1, transportation sector alone contributes to 25% of total GHG emissions and, specifically, road transport accounts for approximately 73%, because of

the use of

hydrocarbon-based fuels. The goal is to reduce GHG emissions to two thirds by 2050 1. Thus, alternative clean fuels are required to replace hydrocarbon fuels. In fact, thanks to the high energy level and zero emissions during combustion, hydrogen is a desirable candidate to replace conventional fuels 2–4.

H2 as a fuel can be incorporated as transportation fuel by replacing internal combustion engines (ICEs) with auxiliary power units (APUs) based on polymer electrolyte fuel cell (PEFC). APU devices consist of a reformer unit for fuels, a CO clean-up unit to reduce CO content in the reformate, and a fuel cell stack operating with hydrogen. Hydrogen fuel storage at high pressure is a challenge as it poses hazard to environmental and humans. However, onboard continuous hydrogen production for energy generation in PEMFC APU is a desirable option, using hydrocarbon fuels as hydrogen source. Nevertheless, the efficiency of an APU is directly linked with the hydrocarbon fuel used for hydrogen production and reduces following the order methane > gasoline > light diesel > heavy diesel 5,6. Catalytic methane steam reforming (MSR), despite being an endothermic reaction, is the most used among other technologies such as autothermal reforming, oxidative steam reforming, partial oxidation, and coal gasification 7, thanks



4 to the high energy efficiency, and reduced cost of hydrogen. An APU unit with a catalytic fuel reformer must provide a CO-free H2 stream at steady rate to PEFC under cyclic conditions of start-up and shut-down. The major requirement is the development of a self-regenerative catalyst for on-board production of H2 with high CO2 selectivity, resulting thus in reduced space for CO clean-up system, for efficient APU performance.

The shortcomings associated with hydrogen generation through conventional MSR Ni-based catalysts are carbon deposition on catalyst surface, sintering of metal particles, deactivation over a long period of time and failure to perform in cyclic conditions without any regeneration 8,9.

A prolonged use decreases porosity and Ni exposed to reactive gases, hence the catalyst

deactivates resulting in increased operatve pressure and temperature 8. Noble metal catalysts are a possible substitute compared to conventional Ni catalyst forMSR, due to their high efficiency at low temperature and pressure and at relatively low metal loading. A noble metal catalyst with only 1% loading

10

can outperform the 10% loading of Ni catalysts at low

temperature MSR 11–15.

Noble metals such as Rh, Pt, Ru, Pd, and Ir have been widely studied for MSR 16–21. The order of activity of these noble metals is still under debate 22: Till now, the most active are Rh and Ru, however Pt can be considered the most active in breaking the C–H bond during reforming reactions 23. In fact, according to De Souza et. al. 24, studied Pt in synergy with Ni/Al2O3 has a strong promotion effect on MSR, capable to reduce reforming temperature of 200-300 °C.


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5 The choice of the support for noble metal catalysts is important to guarantee a long-term usability. Usually, supports are of inert nature like Al2O3. However, for noble metal catalysts, supports like CeO2 are also studied because of its strong metal support interaction, oxygen storage capacity, reducibility (Ce4+/Ce3+), and soot resistance. These properties of CeO2 enhance the ability of noble metals in a reaction compared to supports with inert nature, specifically for reactions such as steam and CH4 dry reforming (MDR), water gas shift (WGS).. As an example, CeOx (with x = 2 or 1.5, able to provide lattice oxygen species to the noble metal) prevents carbon accumulation in MSR by accelerating the reaction of steam and adsorbed carbon at metal-oxide interface, allowing a fast conversion to gaseous products 25.

Many researchers investigated Pt/CeO2 catalysts for oxidative MSR reactions and CO oxidation26,27. For example, Mortola et. al.

28

studied MSR on Pt/CeO2-La2O3-Al2O3 and

discovered that the synergy between Pt and Ce forms Pt0/Ptδ+ and Ce4+/Ce3+ redox couples, resulting in increased CH4 conversion and reduced carbon deposition. In our previous work 29, we found that Pt/CeO2 catalyst, when used for MSR, showed high CO2 selectivity (~63%) and CO concentration of 13% in dry reformate.

In cyclic operation, catalysts usually show degradation and need regeneration after some timeon-stream (TOS). Many researchers have studied the cyclic daily start-up and shut-down of catalysts with and without regeneration 26,30–33. Li et. al. 30 studied the self-regenerative activity of Ni/Mg(Al)O at 900 °C under cyclic condition for 6 h: with steam purging of the catalyst in



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6 between the shut-down and start-up, they found a stable activity. Most of these studies are conducted away from equilibrium conditions, where methane conversion is low.

In the present work, we prepared a Pt catalyst supported on CeO2 (Pt/CeO2) by adopting the solutiom combustion synthesis (SCS) method. We fully characterized and tested this catalysts for MSR in long cyclic conditions near equilibrium conditions.. The purpose of ageing the catalyst near equilibrium conditions was to evaluate its performance at maximum conversion and observe steady H2 generation. In addition, we also performed a sensitivity analysis by comparing experimental results with simulated equilibrium concentrations, calculated with AspenPlusTM software 34.

2. Experimental 2.1. Catalyst synthesis

One-shot oxalyldihydrazide–nitrate solution combustion synthesis (SCS)

35,36

was used to

synthesize supported theoretical 1.1 wt.% Pt/CeO2 catalyst,, as detailed explained in the work of Pino et. al.

26.

Concisely, ceric ammonium nitrate (NH4)2Ce(NO3)6, chloroplatinic acid

H2PtCl6, and oxalyldihydrazide C2H6N4O2 were dissolved in water. Then, the dish containing the solution was placed into an oven at 350 °C. After an initial frothing and foaming, the solution boiled till complete dehydration. At this point, the remaining gel ignited reaching fast a flame temperature of ca. 1000 °C, producing thus a voluminous dispersed solid foam. A calcination process in static air at 800 °C for 3 h followed35.


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7 2.2. Catalyst characterization

Nitrogen adsorption/desorption isotherms at –196 °C, and in the relative pressure range of 0 to 1, were used to measure the specific surface area (SBET) of the catalyst via the Brunauer, Emmet, Teller (BET) method, (ASAP 2020 M Micromeritics Instrument). Before the analysis, 50 mg of powder was positioned in the cell and degassed with high vacuum at 350 °C. The same instrument was used for chemisorption analysis, to estimate the active metal dispersion on the support. First, we performed H2 saturation with an H2 flow (20 Ncm3 min−1 for 2 h at 350 °C). Then, we increased the temperature to 370 °C with an He flow (20 Ncm3 min−1 for 1.5 h). Then, we reduced the temperature to 20 °C, and we flow a mixture of 10% CO in He, injecting pulses of 500 Nµl each, till reaching constant outlet peaks. We determined the amount of adsorbed gas as the difference between the total injected volume and the residual escaped one. We calculated Pt metal dispersion on CeO2 surface using the following formula:

D%  100  S f 

V ads  M me V g  Fme

(1)

With Sf the stoichiometric factor equal to 1 (i.e., one CO molecule adsorbed on one Pt atom), Vads the total volume of CO chemisorbed (referred to the mass of CeO2 used for the analysis in Ncm3 g−1), Mme the atomic weight of Pt(195.08 g mol−1), Fme the total mass fraction of Pt on CeO2 (gme g−1 of carrier), and Vg the volume of one gas mole (22,414 cm3 at normal conditions).



8 A Philips X-Pert MPD X-ray diffractometer equipped with a Cu Kα radiation (40 kV and 30 mA) was used to collect X-ray diffraction (XRD) patternsin the 2θ range between 20° to 70° over 1 h. The PCPFWIN database was used to assign diffraction peaks.. The Scherrer’s equation, assuming a Gaussian shape of the peaks, was used to determine the particle size of CeO2.

A Micromeritics ChemiSorb 2750 instruments was used to carry out H2 temperatureprogrammed reduction (H2-TPR) analysis. Prior the analysis, the fresh catalyst was flown for 30 min at 200 °C with pure O2. Then, the catalyst was flown with a continuous flow of 5% H2 in Ar (30 Nml min−1), increasing the temperature from −80 to 950 °C at 20 °C min−1.. A thermal conductivity detector (TCD) was used to monitor H2 consumption.

Field Emission Scanning electron microscopy (FESEM Leo Supra 40 apparatus) and transmission electron microscopy (TEM Philips CM12) were used to examine tThe Pt particle size distribution and CeO2 morphology either at the fresh and aged status.

A Physical Electronics PHI 5800 (USA) multi-technique ESCA system (with monochromatic Al-Kα X-ray radiation) was used for X-ray photoelectron spectroscopy (XPS) measurements . The survey and narrow spectra were obtained under identical conditions and a charging correction with reference to C 1s at 284.6 eV, with a standard deviation of 0.3 eV, after degassing samples at 2 x10–10 Torr. The semi-quantitative atomic percentage compositions


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9 (deconvolution of the signals using Gauss-Lorentz equations, 70% Gaussian and 30% Lorentzian line shape, with a Shirley background) were obtained with Multipak 9.0 software.

2.3. Catalytic activity

Catalytic activity tests on the as prepared Pt/CeO2 catalyst for MSR were conducted in a fixedbed quartz micro-reactor (inner diameter of 4 mm) at atmospheric pressure. Two gas mass flow controllers (Bronkhorst EL Flow series) were used to inject the gases (N2, H2, CH4) into a vaporizer/mixer. Ultra pure water having resistivity higher than 18 MΩ cm was injected using a liquid mass flow controller (Bronkhorst LIQUI Flow series) into the vaporizer/mixer maintained at 130 °C. The gases and vapor mixture then passes through a heated pipe maintained at 130 °C to prevent condensation of steam. The quartz reactor is equipped with couple of three-way valves (each at entrance and exit of gases), to bypass the reactor and facilitate closure operation. Two quartz wool plugs block 300 mg of catalyst (diluted with 500 mg of SiO2, in size of 0.2–0.7 mm grains) in the middle of the quartz reactor, which is placed into a furnace heated with a PID temperature controller.. The inlet temperature of the catalytic bed is controlled with a K-type thermocouple inserted into the reactor.

Prior to MSR experiments, in-situ reduction of the catalyst is performed by flowing a 1:1 H2/N2 mixture (100 Nml min–1) at 200 °C for 1 h, followed by a purge of N2 flow (same flow rate value) till the desired reaction temperature. At this point, the reaction mixture is switched from N2 to CH4 and steam with an overall flow of 100 Nml min–1. As preliminary experiments, the



10 effect of gas hourly space velocity (GHSV: 24,000-33,000 h–1 at 1 bar) and steam-to-carbon ratio (S/C = 2.8-3.2) was evaluated. The relatively high S/C was on purpose chosen to simulate industrial MSR, where even higher S/C are used to limit as much as possible the effects of coke deposition on catalysts. To assure the repeatability of measurements, conducted with a Varian CP-3800 gas chromatograph (with a thermal conductivity detector, TCD, and two Molsieve 5 Å columns), all the performed tests regarding GHSV and S/C were repeated at least three times. The carbon balance was esteemed always within ± 5%. Prior entering the analyzer, the remaining water in reformate was condensed at 3 °C, thus, the calculated values denote dry gas composition.

Cyclic conditions, as reported in Fig. 1, were adopted to perform the stability test and observe the effect of ageing on the catalyst during TOS. The cycle consisted of daily start-up, reaction, and shut-down processes in 100 Nml min–1 of N2 flow (DSSinert). The start-up consisted in heating the catalyst bed from ambient temperature to working temperature using N2 flow (100 Nml min–1). Then, the flow was switched from N2 to the reaction mixture for 6 consecutive h of TOS. Finally, the reactor was cooled down to ambient temperature using N2 flow again.. On the contrary, for stability test under daily start-up and shut-down cycle in reaction environment (DSSrxn), the catalytic bed was heated and cooled in flow of reaction mixture (GHSV 24,000 h–1, S/C: 2.8, 100 Nml min–1) instead of N2.


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11

Figure 1. Schematic of daily start-up and shut down cycle of MSR reaction in inert environment ((DSSinert, red curve: heating/cooling in N2 flow) or in reaction environment (DSSrxn, blue curve: heating/cooling in reaction flow).

The experimental results concerning GHSV and S/C ratio were compared with their respective thermodynamic equilibrium values evaluated using AspenPlus™ based on the minimization of Gibbs free energy using Peng Robinson set equations 37,38

3. Results 3.1. Characterization of fresh catalyst

Table 1 lists the properties measured for the as prepared Pt/CeO2 catalyst. The catalyst shows a SBET of 14 m² g–1, which is a lower value compared to other ceria supported MSR catalysts available in the literature 21,39, but in the typical range of catalysts prepared by one-step SCS method

26,35.

Due to the low surface area (14 m² g-1) of the Pt/CeO2 catalyst, a reasonable



12 estimation of Pt dispersion on CeO2 can be made by CO chemisorption method40. The values obtained by CO chemisorption analysis (Table 1) show 12.7% metallic dispersion of Pt, indicating a fine metal particle dispersion on CeO2 support, with Pt particle size of 7.89 nm.

Table 1. Physical-chemical properties of the Pt/CeO2 catalyst.

Pt/CeO2 SBET [m2 g–1] (a)

14

Pt metal dispersion [%] (b)

12.7

Pt crystallite size [nm] (b)

7.89

Pt crystallite size [nm] (c)

5-7

Pt metal surface area [m2 g–1] (b)

0.313

CeO2 lattice parameter [nm] (d)

0.5414

CeO2 crystallite size [nm] (e)

16

CeO2 crystallite size [nm] (c)

20-60

(a) Calculated from BET adsorption/desorption N2 isotherms (b) Calculated from CO chemisorption analysis (c) Calculated from TEM analysis (d) Calculated from XRD analysis (d = h+k+1) (e) Calculated from Scherrer equation on Ce (1 1 1 ) reflections


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13 XRD patterns (Fig. 2a) have well-defined peaks, which can be assigned to fluorite oxide type face centered cubic phase of CeO2. The reflections of CeO2 (2θ = 28.54°, 33.08°, 47.47°, 56.33°, 59.08°, and 69.40°) are visible with a minor shift to lower degree (2θ ~ 0.02°) compared to pure CeO2 (JCPDS card # 81-0792). The related cell parameter of CeO2 was calculated as 5.415 Å, marginally higher than that of pure CeO2 (a = 5.411 Å). Probably, during SCS Pt ions entered CeO2 lattice, causing the shift of reflections to somewhat low angle and the increase of lattice parameter

26,27,41.

In addition to CeO2 reflections, a barely observable reflection

(2θ=39.765°, JCPDS card # 00-4-0802) is also present related to metallic Pt (see inset in Fig. 2a), which is usually existing when Pt/CeO2 t is synthesized by SCS

26,27,41,42.No

reflections

due to PtO are visible, indicating high metal dispersion, confirming CO chemisorption results (Table 1).

H2-TPR profile of Pt/CeO2 catalyst (Fig. 2b) displays a main low-temperature peak, at subzero temperature, due to the reduction of surface oxygen on CeO2, two relatively small peaks at intermediate temperature, and a high-temperature peak, related to the reduction of bulk oxygen on CeO2. This profile can be considered typical of well dispersed Pt nanoparticles over CeO2 support. In fact, as previously reported in the literature43,44, pure ceria prepared by SCS (not reported here) displays two main reduction peaks at 500 and 930 °C, respectively. The existence of Pt does not interfere with the high-temperature reduction peak, which remains at around 922 °C, indeed it causes huge alterations in the reduction zone at intermediate/low temperature. Specifically, the low-temperature peak moved to considerably lower temperature,



14 at around −10 °C, which can be attributed to the reduction of platinum oxide species welldispersed on CeO2. According to the literature 45, this peak could be linked to the subtraction of oxygen adjacent to platinum, via the spillover mechanism. Instead, the peaks in the region between 50 and 450 °C can be linked to the reduction of platinum oxide networking with CeO2 and the consequent establishment of a solid solution of Pt-Ce 27. However, the contribution of the reduction of CeO2 on the surface cannot be excluded.

Figure 2. (a) XRD patterns of as prepared Pt/CeO2: Miller indexes of pure CeO2 indicated (JCPDS card n. 81-0792); inset: enlargement at around 2 = 40° to highlight the presence of Pt (111) (JCPDS card n. 00-4-0802). (b) H2-TPR profile of Pt/CeO2: highlighted in gray * the reduction peaks of pure CeO2.

FESEM and TEM techniques were employed for investigation of surface morphology of the as prepared catalyst, as shown in Fig. 3. FESEM micrographs at low magnification (Fig 3a)


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15 show uniform porous structure and an integrated texture. Porous structure with macro-pores visible all over the catalyst surface are visible at higher magnification (Fig. 3b). No metal particles are detectable on the surface, indicating a fine metal dispersion, as further confirmation of XRD analysis.

TEM images on the other hand, show a medium-contrast micrograph due to comparable masses of CeO2 and Pt (Fig 3c/d). Well dispersed Pt particles on CeO2 surface can be observed. The particle size of CeO2 observed from TEM micrographs is about 20-60 nm, confirmimg XRD Scherrer calculations (Table 1), while Pt particle size ranges from 5 to 7 nm, confirming CO chemisorption results (Table 1). The high-magnification TEM image (Fig 3d) shows lattice fringes of CeO2 equal to 0.270 and 0.302 nm, slightly bigger compared to the interplanar distances of (200) and (111) of pure CeO2 46,47 Such a lattice expansion is due to Pt presence 48,

whose lattice fringes are 0.184 nm, consistent with the interplanar distance of (111) Pt 49,50.



16

Figure 3. Pt/CeO2 catalyst: (a,b) FESEM and (c,d) TEM images of the as prepared catalyst; (e,f) TEM images of the catalyst after 200 h TOS.

XPS analysis was carried out to clarify the metal-support interaction on the surface of the catalyst. Fig. 4a shows the overall surface spectrum of the catalyst, indicating presence of Ce,


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17 Pt, and O on the surface with no region corresponding to the presence of Cl. Table 2 summarizes the surface atomic composition of the as prepared catalyst at the fresh status. The catalyst has a low Pt/Ce atomic ratio, equal to 0.096, with a Pt surface atomic % of 1.47 (slightly higher compared to the theoretical value of 1.1%). Fig. 4b/c/d shows the high-resolution spectra of O 1s, Ce 3d, and Pt 4f, together with their peak deconvolutions.

Table 2. Elemental composition of the Pt/CeO2 catalysts before and after reaction (200 h TOS), evaluated by XPS analysis.

Element

Fresh catalyst (as prepared)

Aged catalyst (after 200 h of reaction)

C [at.%]

26.73

40.20

O [at.%]

56.56

50.27

Ce [at.%]

15.24

8.71

Pt [at.%]

1.47

0.82

Pt/Ce [-]

0.096

0.094

Ce3+/Ce4+ [-]

31.8

37.1

Pt0 [.%]

17.1

20.1

Pt2+ [%]

62.1

40.4

Pt-O-M [%]

20.8

39.5



18

Figure 4. XPS survey spectra of Pt/CeO2 catalyst: as prepared (a) and after 200 h TOS (e). Deconvolution of high-resolution spectra of O 1s (b/f), Pt 4f (c/g), and Ce 3d (d/h) for the as prepared (b/c/d) and aged (f/g/h) catalysts.


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19

The high-resolution profile of O 1s (Fig. 4b), exhibits four peaks, assigned to O–Ce(III), O– Pt, O–Ce(IV) and O–H, respectively 51. The existence of O–H specie can due towater adsorbed on CeO2 surface (i.e., chemisorbed oxygen) 52. The chemisorbed oxygen is 20% of the total oxygen present on the surface of fresh Pt/CeO2, and acts as the most reactive oxygen species 53.

The high-resolution Pt 4f spectrum (Fig. 4c) consisted of two doublets, Pt 4f7/2 and Pt 4f5/2. The Pt 4f7/2 was deconvoluted in three components with binding energy of 70.3, 70.9 and 72.3 eV, respectively, attributed to Pt0, Pt–O–M (M: oxygen vacancies or Ce), and Pt+2 species, respectively 51. The slight shift of Pt0 component of the catalyst towards low binding energy indicates a change in the electronic properties of Pt atoms. Such a variation could be linked with the increase of the electron density of d orbitals in Pt, because of the electron transfer from CeO2 to surface atoms of Pt 51, which is the well-known strong metal support interaction effect on catalysts 54. The total surface amount of Pt0 of the as prepared catalyst is roughly 17%.

The high-resolution spectrum of Ce 3d (Fig. 4d) could split into ten peaks, with labels used to identify Ce 3d peaks (“v” and “u” the spin−orbit coupling 3d3/2 and 3d5/2, respectively). Specifically, the peaks u, u′′, v, and v′′ represent Ce3+, while the other peaks (u′, u′′′, u′′′′, v′, v′′′, and v′′′′, respectively) cn be linked with Ce4+. The surface amount of Ce3+ can be estimated



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20 as 31.8 % (Table 2), according to a typical procedure 55. In fact, oxygen vacancies derive from the reaction:

𝐶𝑒4 + + 𝑂2 ― →4𝐶𝑒4 + +2𝑒 ― 𝜃 +0.5𝑂2→2𝐶𝑒2 + +2𝐶𝑒3 + + 𝜃 +0.5𝑂2

(2)

Where “θ” denotes an empty position. The higher the Ce3+ concentration , the more the oxygen vacancies available 26.

3.2.

MSR preliminary tests

3.2.1. Influence of gas hourly space velocity on activity of Pt/CeO2

We studied the effect of GHSV on the catalytic activity of Pt/CeO2 at S/C ratio of 3 and temperature between 400 and 750 °C, by increasing the GHSV from 24,000 to 33,000 h–1 (increase of the flow rate fed to the reactor, amount of catalyst constant at 0.3 g). Fig. 5 shows the results, compared to the equilibrium values. For both GHSV, CH4 conversion (χCH4, Fig. 5a) increased with increasing temperature. The catalyst reached near equilibrium conversion at 750 °C for both GHSV. At low temperature, the effect of GHSV is more pronounced, being χCH4 slightly higher for the higher GHSV. Instead, at high temperature χCH4 is slightly better for the lower GHSV. In fact, starting from 700 °C, the low GHSV (i.e., 24,000 h–1) favored χCH4 achieving near-equilibrium value (97% as compared to 93% at 33,000 h–1).


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21

Figure 5. Effect of GHSV on MSR over Pt/CeO2 catalyst: (a) CH4 conversion (χCH4), (b) CO2 selectivity (σCO2), (c) H2 dry outlet concentration (ζH2,do), (d) CO selectivity. Dotted lines represent equilibrium values. Reaction conditions: S/C = 3, p = 1 bar.

Concerning CO2 selectivity (σCO2, Fig. 5b), the values were always higher compared to equilibrium values, for both GHSV. They decreased with temperature raise, by remaining always above the equilibrium values. H2 dry outlet concentration (ζH2,do, Fig. 5c) and CO selectivity (Fig. 5d) values were always lower compared to the equilibrium values,



22 approaching them with temperature raise. The values at lower GHSV were always slightly higher compared to those at higher GHSV.

The Pt/CeO2 catalyst employed in this study resulted in high χCH4 at high GHSV 33,000 h–1 and relatively low temperature, 700 °C, compared to other noble metal based MSR catalysts a reported in the literature. In fact, Khani et. al. 17 conducted MSR on 3% Ru catalyst at 800 °C with only 80.6% χCH4 at GHSV 10,500 h–1, almost one third of the GHSV used in our reaction system. Kusakabe et al.

10

conducted MSR experiments on 3% Pt/CeZrO and found that the

catalyst reached only 72% χCH4 at 800 °C at GHSV almost 1.3 times higher than used in our system.

The slight loss of activity observed at higher GHSV can be associated to heat and mass transfer limitation effects because of the endothermic reaction together with short residence time of reactants on the surface of catalyst. It is interesting to observe that σCO2 of the catalyst exceeds the equilibrium values for both GHSV investigated (Fig. 5b). A combination of couple of reasons can explain this phenomenon: (i) WGS reaction at elevated temperature on Pt catalyst 55,56

and (ii) reaction of CO produced during MSR with surface oxygen of CeO2 support 56.

Since low residence time enhances the catalyst activity at 700 °C, GHSV of 24,000 h–1 was selected for further investigation on MSR.

3.2.2. Influence of steam-to-carbon molar ratio on activity of Pt/CeO2


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23 Figure 6 displays the influence of S/C on the performance of Pt/CeO2 at TSET = 700 °C and GHSV = 24,000 h–1, as χCH4, ζH2,do, σCO2, and H2/CO molar ratio, together with the respective equilibrium values. Results show that an increase in S/C ratio did not affect the performance of Pt/CeO2 in terms of χCH4. However, a slight decrease in ζH2,do (from 76% at S/C = 2.8 to 75% at S/C = 3.2) along with σCO2 (from 64% at S/C = 2.8 to 50 % at S/C = 3.2) was observed.

Figure 6. Effect of steam-to-carbon (S/C) molar ratio GHSV on MSR over Pt/CeO2 catalyst: (a) CH4 conversion (χCH4), CO2 selectivity (σCO2), and H2 dry outlet concentration (ζH2,do); (b)



24 H2/CO and H2/CH4,reacted molar ratios. Dotted lines represent equilibrium values . Reaction conditions: T = 700 °C, p = 1 bar, GHSV = 24,000 h–1.

As already mentioned before, CO2 concentration different from equilibrium values can be attributed to WGS reaction at high temperature on Pt catalyst 56. Addition of steam in reforming reaction prevents carbon/coke formation but simultaneously requires more energy. Hence, the S/C ratio equal to 2.8 was selected for subsequent investigation.

3.3.

MSR endurance test

Fig. 7 shows the performance of the as prepared Pt/CeO2 towards MSR at TSET = 700 °C, S/C = 2.8, and GHSV = 24,000 h–1 for 150 h of TOS. A total of 25 cycles of consecutive start-up and shut-down cycle of ca. 6 h each were performed as stability test. During the first 100 h of TOS, start-up and shut-down cycles were carried out in N2 environment, DSSinert, while for the rest of the time from, 101 to 150 h, start-up and shut-down cycles were carried out under reactive atmosphere, DSSrxn (according to the procedure in Fig. 1). The aim of the stability test was to scrutinize the ageing effect on catalyst, the formation of carbon/coke on the surface of the catalyst, and propose a model for MSR reaction on Pt/CeO2 catalyst based on experimental results.


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25

Figure 7. 150 h of time-on-stream (TOS) over Pt/CeO2 catalyst: (a) CH4 conversion (χCH4), CO2 selectivity (σCO2), and H2 dry outlet concentration (ζH2,do); (b) H2/CO and H2/CH4,reacted molar ratios and σCO2/σCO2 selectivity ratio. Reaction conditions: T = 700 °C, p = 1 bar, GHSV = 24,000 h–1, S/C = 2.8.

From Fig. 7, we observed a high stability of the catalyst during the first 100 h of TOS in DSSinert, with χCH4 almost constant at 98%, σCO2 above 59%, and an average ζH2,do of 76.5%, with H2/CO ratio varying between 6 and 11.5 and the CO2/CO selectivity ratio (σCO2/σCO)



Page 26 of 59

26 varying between 0.9 and 2.1. Due to this high stability observed during the first 100 h of TOS, the start-up and shut-down cycle scheme was changed to reaction environment (DSSrnx,) to observe the effect of a harsh environment on catalyst performance also at lower temperature. The catalyst remained highly stable also in the further 50 h, with χCH4 always higher than 96%, σCO2 above 51%, and an average ζH2,do of 76%, with H2/CO ratio varying between 6 and 10, σCO2/σCO varying between 1.0 and 1.9.

The stability of Pt/CeO2 observed during the overall 150 h of TOS is better compared to other MSR catalysts in the literature, which demonstrated reduced catalytic activity in the first 5 to 10 h of reaction

21,28,57,58.

For example, Zhai et al.

58

performed MSR stability test on Ni

catalysts with CH4 conversion higher than 90% in the first 10 h, slowly decreasing to 10% in the following 30 h, where deactivation was mainly due to sintering and coke deposition. Mortola et al.

28

performed MSR on Pt-based catalysts: the catalytic activity decreased

considerably in the first 24 h of reaction. Very similar results were obtained by Pino et al.

27

for Pt catalyst during partial oxidation of CH4, resulting in strong deactivation in 100 h of TOS.

Observing the behavior of our Pt/CeO2 catalyst during the overall 150 h of endurance test with start-up and shut-down cycles (Fig. 7), there is a frequent and extended oscillatory behavior of σCO2 (58.5 ± 10%) and, in lesser extent, of χCH4 (95.3 ± 4.1%) and ζH2,do (75.6 ± 4.3%), which can be associated to the recurring oxidation/reduction cycles of the CeO2 support59.


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27 To further investigate the high CO2 selectivity and its oscillations, we performed an in depth thermodynamic analysis on our reaction system. Table 3 lists the potential set of reactions occurring during MSR 60,61.

Table 3. Possible reactions pathways during MSR.

I

Reaction

Keqj

Reaction name

1.167 ∙ 1013exp ( ―26830 𝑇) MSR

R₁

𝐶𝐻4 + 𝐻2𝑂→𝐶𝑂 + 3𝐻2

R₂

𝐶𝑂 + 3𝐻2𝑂→𝐶𝑂2 + 𝐻2

R₃

𝐶𝐻4 + 2𝐻2𝑂→𝐶𝑂2 + 4𝐻2

2.063 ∙ 1011exp ( ―22430 𝑇) MSR

R₄

𝐶𝐻4 + 𝐶𝑂2→2𝐶𝑂 + 2𝐻2

6.607 ∙ 1014exp ( ―31230 𝑇) DRM

R₅

𝐶𝐻4 + 3𝐶𝑂2→4𝐶𝑂 + 2𝐻2𝑂

2.115 ∙ 1018exp ( ―40030 𝑇) DRM

R₆

𝐶𝐻4→𝐶 + 2𝐻2

R₇

2𝐶𝑂→𝐶 + 𝐶𝑂2

R₈

𝐶𝑂 + 𝐻2→𝐶 + 2𝐻2𝑂

3.214 ∙ 10 ―8exp ( ―16318 𝑇)

R₉

𝐶𝑂2 + 2𝐻2→𝐶 + 2𝐻2𝑂

1.775 ∙ 10 ―6exp (12002 𝑇)

R₁₀

𝐶𝐻4 + 2𝐶𝑂→3𝐶 + 2𝐻2𝑂

4.244 ∙ 10 ―10exp (22022 𝑇)

1.767 ∙ 10 ―2exp (4400 𝑇)

4.107 ∙ 105exp ( ―10614 𝑇)

WGS

Methane cracking (MC)

5.818 ∙ 10 ―10exp ( ―20634 𝑇) Boudouard reaction (BR)



Page 28 of 59

28 R₁₁

𝐶𝐻4 + 2𝐶𝑂2→2𝐶 + 2𝐻2𝑂

0.730 ∙ exp (1388 𝑇)

DRM

The thermodynamic analysis can reveal the direction of reaction according to the Vj ratio 60,62, defined as

𝑣

𝑉𝑗 =

(∏𝑖𝑝𝑖 𝑖)

(3)

𝐾𝑒𝑞𝑗

Where,

pi = partial pressure of gas species i [bar]

vij = stoichiometric coefficient of species i in reaction j

Keqj = thermodynamic equilibrium constant of reaction j

The criterion for the thermodynamic analysis is as follows:

i)

If Vj value 1: the reaction is expected to proceed in reverse direction.

In our reaction system, applying this thermodynamic analysis criterion to the product gas composition obtained at 700 °C during 150 h of reaction, the resulting thermodynamic analysis obtained can be divided into three categories as follows;

a) Vj value >1 for reaction R7-R11, therefore these reactions proceed in left direction if they proceed at all (not shown in Fig. 8)


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29 b) Vj value

CeO2 catalyst: Effect of daily start-up

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