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Effect of calcination temperature on steam reforming activity of Ni-based pyrochlore catalysts
Journal Pre-proof Effect of Calcination Temperature on Steam Reforming Activity of Ni-based Pyrochlore Catalysts Daniel J. Haynes, Dushyant Shekhawat, David Berry, Amitava Roy, James J. Spivey PII:
S1002-0721(19)30255-8
DOI:
https://doi.org/10.1016/j.jre.2019.07.015
Reference:
JRE 594
To appear in:
Journal of Rare Earths
Received Date: 29 March 2019 Revised Date:
8 July 2019
Accepted Date: 9 July 2019
Please cite this article as: Haynes DJ, Shekhawat D, Berry D, Roy A, Spivey JJ, Effect of Calcination Temperature on Steam Reforming Activity of Ni-based Pyrochlore Catalysts, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.07.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Effect of Calcination Temperature on Steam Reforming Activity of Ni-based Pyrochlore Catalysts Daniel J. Haynesa*, Dushyant Shekhawata, David Berrya, Amitava Royb, James J. Spiveyb
a
National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA b
Louisiana State University, Cain Department of Chemical Engineering, 3307 Patrick Taylor Hall, Baton Rouge, LA, 70803, USA
*Corresponding Author National Energy Technology Laboratory U.S. Department of Energy 3610 Collins Ferry Rd, Morgantown, WV 26057 Tel: 304-285-1355 Email_ [email protected]
Keywords: Nickel, Pyrochlore, Methane, Steam reforming, Catalyst, Lanthanum enrichment Foundation item: Project supported by The Louisiana Board of Regents (Grant # LEQSF(201617)-ENH-TR-07).
2
Abstract This work served as the second part of a study evaluating the effect of calcination temperature (700–1000°C) on Ni-based lanthanum zirconate pyrochlore catalysts for methane steam reforming. A previous study (Haynes et al. Ceram. Int. 2017 (43) 16744) provided a thorough characterization of the material properties for the catalysts used here, and this study focused on the evaluation of catalytic activity. The activity was assessed by two different experimental studies: the effect of reaction temperature using a temperature programmed surface reaction (TPSR), and the effect of reaction pressure. The results demonstrated a complex interaction between the Ni particles and surface LaOx species under the methane steam reforming conditions. Specifically, the material calcined at the lowest temperature (700 °C) possessed the highest activity and selectivity, which was attributed to smaller and more well-dispersed Ni particles on the surface, and, more importantly, a lesser degree La enrichment at the surface. All catalysts were deactivated by steam to NiO under all conditions tested, but at certain low reaction pressure (p=0.23 MPa) conditions the materials calcined at 700–900°C were able to completely recover equilibrium activity in-situ that was then robust and stable under both low and high reaction pressures (p=1.8 MPa) suggesting the formation of a synergistic relationship between Ni and La for syngas production. However, exposure of a fresh material to high reaction pressures led to a rapid and irreversible loss in both CH4 conversion and syngas selectivity whether in the fresh (no pretreatment), or pretreated (steam, H2 or Ar only at 800 °C) form for any catalyst. The mechanism for deactivation appeared to be due to the presence of LaOx species that became mobile, possibly by the formation of La-OH, and covered the active Ni particles and inhibited sites responsible for the CH4 decomposition.
3
1. Introduction The growing availability of cheap and abundant natural gas has been accompanied by an increased interest in its conversion into more useful and valuable fuels and chemicals. Given the favorable economics of readily available natural gas, near term non-energy uses of natural gas resources will undoubtedly be based on traditional reforming conversion methods. Of the available reforming technologies, steam methane reforming (SMR) will continue to play an important role, producing both synthesis gas and H2 gas for end products such as electrical power using fuel cells. Long-term reformer operation depends on a catalyst that must be not only active and selective for synthesis gas (H2 and CO), but also resistant to deactivation. Ni-based catalysts like Ni/Al2O3 and Ni/MgO are traditionally used for industrial steam reforming. However, exposure to the demanding reforming conditions results in the deactivation of the Ni catalyst through several well-known mechanisms, which include active surface area loss from thermal aging and metal agglomeration, and carbon formation [1-3]. Catalysts based on stable oxides such as pyrochlores are an attractive alternative to traditional catalysts [4-6]. These materials have demonstrated chemical and thermal stability under high temperatures [7]. The pyrochlore crystal structure can also be modified by the isomorphic substitution of a wide variety of different metals into the structure, which provides active sites for reactions such as steam reforming [8, 9]. Specifically, the substitution of the catalytically active metal into the lattice can provide stable and atomically dispersed crystallites that are more resistant to deactivation by carbon and sulfur compared to traditional supported metal catalysts [10]. These advantages may be used to improve the activity stability to Ni under steam reforming conditions, particularly by limiting the growth of whisker carbon. Previously, we characterized the effects of calcination temperature on the material properties of a 6 wt% Ni substituted lanthanum zirconate pyrochlore catalyst [11]. This paper is a continuation of that work. For this study, we report the effect of calcination temperature on the steam methane reforming activity of the same Ni substituted pyrochlores that were characterized in the previous study. The catalytic activity will be assessed in two different experimental studies: the effect of reaction temperature using a temperature programmed surface reaction (TPSR), and the effect of reaction pressure. 2. Experimental 2.1 Catalyst synthesis The pyrochlore catalysts used in this study were fabricated using a modified version of the Pechini method discussed elsewhere [11]. The precursor was then calcined to 4 different temperatures by 5 °C/min for 8 h. These materials will be designated as LSZN6-XXX, where XXX will indicate the calcination temperature (e.g. LSZN6-700 for the material calcined at 700 °C). 4
2.2 Catalyst characterization Phase analysis of powder samples was examined using a Panalytical (Malvern/Panalytical) X’pert Pro X-ray diffraction system, model number PW 3040 Pro. Jade v9.6 Materials Data Inc. was used for phase identification and pattern analysis, which included peak fitting, crystallite size, and lattice parameter determination. The Scherrer equation was used to estimate crystallite size. XAS (X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)) measurements of the various forms of the catalyst were performed at Louisiana State University’s synchrotron research facility, the J. Bennett Johnston, Sr., Center for Advanced Microstructures and Devices (CAMD), USA. Instrument information and analysis conditions can be found elsewhere [11]. 2.3 Catalytic reaction studies Catalyst testing was performed in a continuous-flow reactor (Autoclave Engineers, Model no. BTRS Jr). The dry gas products: H2, CO, CO2, CH4, and N2 were analyzed continuously by means of an online Thermo Onix mass spectrometer (Model no. Prima δb, a 200 a.m.u. scanning magnetic sector). Although water was a reactant and product in these experiments, it was not measured analytically. The conversion of methane for SMR was calculated by Eq. (1).
Conversion (%) =
(Moles CH 4 in − Moles CH 4 out ) Moles CH 4 in
(1)
2.3.1 Temperature programmed surface reaction (TPSR) Temperature programmed surface reaction studies (TPSR) were designed to assess the catalytic activity by showing hysteresis effects between the heating and cooling periods. The reactant flows, and conditions are provided in Table S1. Flows were run for 0.5 h at the start of the experiment to establish an initial baseline activity. The reaction temperature was then ramped from 650 to 900 °C at 2.5 °C/min and held at 900 °C for 0.5 h. After the isothermal hold, the reactor temperature was then cooled to 650 °C by 2.5 °C/min to determine any changes in activity. 2.3.2 SMR Activity screening studies The Ni-pyrochlore catalysts were also evaluated for steam reforming activity under more relevant flows rates, and reactant concentrations. The studies were also designed to evaluate the effect of pressure on catalyst activity. Two studies were performed with the exact same conditions, see SI Table S2, but with a different system pressure. Low-pressure studies were performed at the system pressure (0.23MPa), while the high-pressure experiments were performed at 1.8 MPa.
5
2.3.3 Post run carbon analysis After all reforming experiments, a burn-off was performed to quantify the amount of carbon deposited on the spent catalyst using mixture of air and Ar (12 vol% O2) at 800 °C. CO2 emission was measured by an online mass spectrometer and the quantity of carbon was determined from the plot of CO2 emitted as a function of time by integrating the area under the curve. 3. Results and discussion 3.1 Temperature Programmed Surface Reaction (TPSR) A temperature programmed surface reaction was performed to examine hysteresis effects on the catalyst activity between heating and cooling cycles. The H2, CO, and CO2 compositions, as well as CH4 conversion during the experiments are shown in Fig. 1(a–d) respectively for the LSZN6700, LSZN6-800, and LSZN6-900. A TPSR experiment was performed on the LSZN6-1000, but no conversion to syngas products was observed and is therefore not included in Figure 1 a-d. (a) 25 650 °C
Cool to 650 °C by 2.5 °C/min
Heat to 900 °C by 2.5 °C/min
Hold at 900 °C
Equilibrium
H2 composition / vol% dry gas
20
LSZN6-700 15
LSZN6-800
10
5
LSZN6-900 0 0
40
80
120 Time on stream / min
6
160
200
240
(b) 6 650 °C
Heat to 900 °C by 2.5 °C/min
Cool to 650 °C by 2.5 °C/min Hold at 900 °C
Equilibrium
CO composition / vol% dry gas
5
4 LSZN6-700
LSZN6-800 3
2
1
LSZN6-900
0 0
40
80
120 160 Time on stream / min
200
240
(c) 3.5 650 °C
Cool to 650 °C by 2.5 °C/min
Heat to 900 °C by 2.5 °C/min
Hold at 900 °C
CO2 composition / vol% dry gas
3
2.5
2 Equilibrium
1.5
1 LSZN6-700 0.5 LSZN6-800 LSZN6-900 0 0
40
80
120 160 Time on stream / min
7
200
240
(d) 100
Cool to 650 °C by 2.5 °C/min Hold at 900 °C
Equilibrium 650 °C
90 80
Heat to 900 °C by 2.5 °C/min
CH4 conversion / %
70
LSZN6-700 60
LSZN6-800
50 40 30 20 10 LSZN6-900 0 0
40
80
120 Time on stream / min
160
200
240
Fig. 1. TPSR results demonstrating the effect of calcination temperature on H2 production (a), CO production (b), CO2 production (c), and CH4 conversion (d). S/C=2.0, WHSV=128 000 scc/(gcat· h), Ar=79%, 0.10 MPa. The TPSR plots demonstrate the SMR activity of Ni in these materials is highly sensitive to both calcination and reaction temperature. The catalyst calcined at the lowest temperature produces the most active and selective to syngas. According to the XRD and XPS (see Table S3 for XPS data) [11], increasing the calcination temperature leads to a less homogenous material, with a progressively smaller amount of detectable Ni at the surface from both particle growth, and the formation of a La2ZrNiO6 perovskite phase in the bulk. Thus, the higher activity for the catalyst calcined at the lowest calcination temperature (LSZN6-700) can be attributed in part to smaller Ni particles, and more accessible Ni at the surface, which is consistent with other work [12]. However, the changes to the properties of the Ni do not fully explain the reason for the lower overall activity as the calcination temperature was increased. Another notable structural effect to the pyrochlores catalysts from exposure to higher calcination temperatures is an increasing degree of La enrichment at the surface (Table S3) [11, 13], which occurs primarily in our case due to low solubility of the Ni in the pyrochlore structure. Although Ni and Zr have a similar ionic radius (0.069 nm for Ni2+ vs. 0.072 nm Zr4+ in a 6-fold coordination) the valence difference between the two generates a large concentration of oxygen vacancies to maintain charge 8
neutrality when Ni is introduced into the structure, particularly at the loadings used for the materials in this study (i.e. Ni is about 28% of the atomic loading of the B-site). This charge compensation generates a large degree of lattice strain that becomes unstable due to the greater degree of ordering of the pyrochlore structure at higher temperatures (>700 °C). As a result, the Ni exsolves from the structure to the surface and grain boundary regions. Consequently, as these materials were synthesized assuming Ni would occupy a lattice position in the pyrochlore structure (i.e. La/(Ni+Zr) atomic ratio ~1) there becomes an excess amount of La present once Ni comes out of the structure, and similar to Ni, the excess La goes to the surface and grain boundaries regions leading to an enrichment at the surface. Comparing the extent of La enrichment to the catalytic behavior indicates that the lower activity and selectivity is not simply related to the particle size and/or amount of Ni, but also that the interaction between La and Ni at the surface also plays a significant role in the reforming activity of the Ni particles. This would also indicate that not only does increasing the calcination temperature produce a greater surface enrichment of La, it also creates a critical interaction between Ni and La which renders the Ni almost completely inactive during these experiments when the catalysts are calcined to temperatures ≥ 900 °C. Exposure to high reaction temperatures also detrimentally impacts the catalyst activity and stability. The LSZN6-700 and LSZN6-800 materials both show an increase in syngas selectivity from their initial levels at 650 °C as temperature is increased. However, operation beyond ~780 °C (75 minutes TOS) to 900 °C, and through the isothermal hold at 900 °C, results in a continuous decline in both syngas selectivity, and CH4 conversion. Decreasing the reaction temperature is decreased to 650 °C after the hold at 900 °C, the syngas product composition for all catalysts is much lower than the corresponding temperature during the heat up portion in Fig. 1(a–d). Within 220 minutes time on stream, the gaseous product composition of this catalysts approximates that of a blank reactor. It would be expected that the most obvious explanation for the decline in catalyst performance would be carbon formation, as this is a common and well-known problem for Ni reforming catalysts [2]. Surprisingly, there is very little oxidizable carbon on any of the catalysts, as shown from a post run burn-off in Table 1. Compared to this study, others have observed greater than 20 wt% carbon at comparable conditions without observable deactivation [14], so this could not be attributed to deactivation. Table 1. Carbon formed during the SMR TPSR experiments. Catalyst Carbon (gcarbon/gcatalyst) LSZN6-700
0.01
LSZN6-800
0.03
LSZN6-900
0.04
9
It was also considered that sintering could be the cause the deactivation. Under steam rich conditions, particle size growth would be expected according to a study by Sehested et al. [1]. However, the range of particle sizes observed for the spent LSZN6-700 material are still relatively small. For example, a study by Prasad et al. [15] have found Ni particles similar in size to those in the spent LSZN6-700 material to remain highly active and selective for syngas production. Additionally, activity loss due to sintering during the SMR shows a CH4 conversion which declines from initial values, but eventually reaches a steady level [15, 16]. Another, and more probable cause of the decline in activity is the oxidation of the Ni by steam. To confirm this behavior, an additional TPSR experiment was performed using the LSZN6-800 by stopping the H2O flow during reaction (step 4) and flowing only CH4 over the catalyst at 700 °C where the activity was almost completely deactivated. In doing so, the oxidized form of Ni should be reduced by CH4 back to the more active metallic Ni form resulting in higher levels of syngas once the steam is restarted. Resuming the steam flow at 700 °C (step 5) after a 10-minute exposure to only CH4 dose show (see Fig. 2) an immediate improvement to the activity, which would indicate the oxidation of the Ni to be the primary reason for deactivation. If carbon formation was responsible for the activity loss, the exposure to CH4 alone would not have increased activity. 2.
1.
25
3.
4. 5.
Composition / vol% dry gas
Isothermal hold @ 650 °C
20
15
H2
10
CH4
5 CO CO2
0 0
50
100
150 200 Time on stream / min
250
300
350
Fig. 2. TPSR experiment performed over LSZN6-800 material to validate Ni oxidation. Steps involved are 1) initiated ramp to 900 °C by 2.5 °C/min, 2) Isothermal hold at 900 °C, 3) started ramp by 2.5 °C/min to 700 °C, 4) stopped steam flow to catalyst at 700°C (reduction under CH4 started, 5) resumed steam flow. 10
Oxidation of Ni as a mechanism for deactivation is consistent with the results of a similar SMR experiment (S/C=1.0) by Pereniguez et al. [17] over a LaNiO3 perovskite catalyst. They observed a steep decline in activity during a similar temperature ramping experiment when the reaction temperature reached near 800 °C, which was comparable to this study (ca. ~780 °C for the pyrochlores). XANES results of their spent catalyst showed Ni had become oxidized by steam, and XPS results also showed La to enrich the surface and envelope the Ni as the material was reduced from the LaNiO3 to Ni/La2O3 under reaction conditions [17]. As the pyrochlores examined for this study also show an enrichment of La at the surface, this would suggest that the deleterious effects of Ni oxidation observed during the TPSR experiments could similarly be explained by the coverage of the Ni particles by the excess La at the surface. 3.2 SMR Activity Screening 3.2.1 Equilibrium Equilibrium product compositions for both conditions were determined by HSC Chemistry thermodynamic software [18], and the results are shown below in Table 2. Note that higher pressures inhibit the reforming rate by limiting the molar expansion of reactants into products, thereby leading to a reduction in the overall conversion of CH4. Table 2. Thermodynamic product distributions for the SMR conditions used in this study 800 °C, and S/C=2.0 as determined by HSC Chemistry [18]. Balance of compositions was inert gas. Equilibrium Composition (%) Product p=system (0.23 MPa) p=1.8 MPa H2
61.5
52.8
CO
15.1
10.4
CO2
4.0
5.4
CH4
0.1
7.8
3.2.2 Low Pressure Fig. 3 reveals the effect of calcination temperature on the activity during the low pressure SMR experiments only for the LSZN6-800 catalyst. Results for the same experiment performed over the 700, 900 and 1000 °C calcined materials are shown in the supplemental information (see Fig. S2 (a–c)). Upon introduction of the reactants, the conversion and selectivity are initially high for all catalysts but not stable. Further exposure to reactants produces a change in activity over each catalyst with the same general trend - a decline in selectivity to syngas and CH4 conversion, followed by an activity recovery.
11
70 H2
Composition / vol% dry gas
60 50 40 30 20
CO 10 CO2 CH4
0 0
5
10
15 Time on stream / h
20
25
30
Fig. 3. Experimental results from the SMR study at low pressure over the LSZN6-800 pyrochlore catalyst. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h).
The behavior shows an unusual trend compared to the literature [19-21] in which it is most often observed that Ni based catalysts have high initial activity, then either decline until the activity reaches a steady product yield, or deactivates completely. Fig. 3 suggests, however, that exposure to the reactive gases alters the catalyst surface structure in-situ to produce an improvement in activity, and there are several possible explanations as to why the materials exhibit this performance. Each of the materials calcined at 700–900 °C show via XRD to have a small amount of Ni that remains in the pyrochlore lattice [11]. In addition, the 900 and 1000 °C calcined materials showed some of the Ni to reside in the La2ZrNiO6 perovskite solid solution in the bulk. Under the reaction conditions, the Ni residing in the bulk forms of these materials (either pyrochlore, or perovskite) could exsolve to the surface as small and highly active particles. In fact, exsolution of catalytic metals from the lattice has been shown to improve catalytic activity for different reforming reactions [16, 22, 23] In our previous study [11], the XRD results indicated (by shift in lattice parameter value) the fresh LSZN6-700 had the highest amount of Ni in the pyrochlore structure of all materials used here, while the material calcined at 1000 °C had a negligible
12
amount. However as shown in Table 3, the lattice parameter for the LSZN6-700 material is identical to the fresh, which likely rules out this possibility. Table 3. Lattice parameters determined from a post-run low-P SMR reaction. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h). Material LSZN6-700
Fresh lattice constant (nm) Spent lattice constant (nm) 10.880
10.880
The effect of calcination temperature on the activity shows the trend in the magnitude of the increase of the initial decline, which can also be correlated to the La surface concentration. For these experiments, the higher surface La levels produce a greater activity loss due to an inhibition of the sites responsible for CH4 activation by steam. However, unlike the TPSR studies, all catalysts, apart from the LSZN6 1000, show the ability to recover stable, near equilibrium activity with no signs of deactivation for the remaining portion of the experiment. To confirm that oxidation was responsible for the activity loss during the decline in activity, XANES and EXAFS analysis were performed on three LSZN6-800 catalysts (see Fig. 4): a fresh catalyst, a sample that was run for 3h and then stopped at the greatest degree of activity loss (designated as partially deactivated catalyst), and a sample run for the entire 24 h experiment in which the activity fully recovered (designated as activated catalyst). The spectra (both XANES and EXAFS) was resolved using a linear combination fitting (LCF) to determine the relative amounts of Ni0 and NiO present in each sample (see Table 4). As expected, the fresh sample is completely oxidized and shows Ni to be present in the +2 state, entirely as NiO, within experimental detection limits. Meanwhile the XANES spectra for the partially deactivated and activated catalysts show a similar electronic structure, and thus reveal a similar degree of NiO present. Although NiO is present in both samples, their Ni K-edge EXAFS oscillations have a more pronounced difference than the XANES signals. This is confirmed by the LCF values derived from the EXAFS scans, and shows the oxygen present is interacting with the Ni differently in each sample. From Figure 3 it is seen that the activated catalyst shows near equilibrium activity. The EXAFS scan for this material shows a sharper amplitude for the main Ni oscillation that is comparable to the Ni0 foil, indicating the Ni is more metallic in nature. This would suggest that the oxygen, while present, was interacting with the Ni in a way that is not detectable by this technique and has a minimal effect on the metallic character of the Ni. Meanwhile, the partially deactivated catalyst reveals a dampening of the Fourier transform amplitude, which could be attributed to the presence of O within the Ni clusters affecting the interatomic distance and local structure of the Ni particles [24]. This would indicate the decline in activity of the Ni during this period could be attributed to the incorporation of oxygen within Ni particles, leading to a less active form of the catalyst. 13
Fig. 4. XANES spectra and EXAFS oscillations for three (3) LSZN6-800 samples. Table 4. Linear combination fitting results of the XANES and EXAFS spectra for the LSZN6800 samples. NiO
Fresh catalyst
Ni
XANES
EXAFS
XANES
EXAFS
1.00
1.00
0.00
0.00
Partially deactivated 0.06±0.01 0.12±0.01 0.94±0.01 0.88±0.01 Activated
0.08±0.01 0.0±0.02
0.92±0.01 1.0±0.02
The improvement in activity and stability under reaction conditions implies that exposure to the reactant gases, and or surface intermediates formed over the working catalyst, lead to structural modifications that enhance reforming rates. A study by Zhang et al. [25] observed an improvement in Ni activity over a La2O3 support during the CO2 reforming of CH4 at 750 °C, p=0.1 MPa. Regardless of pretreatment (H2 reduction, or oxidation), the catalyst had lower initial rates that increased with time under reaction conditions. They attributed the initial results (i.e. low activity) to the decoration of the Ni particles with LaOx species that originated from the support, and then the activity improvement to a new type of surface compound or synergetic sites between the Ni-La generated under reaction conditions at the interfacial area that are stable and active for reforming [25].
14
This complex behavior observed under reaction conditions is speculated to arise from structureactivity relationship which relies heavily on changes between the La and Ni at the surface of the Ni particles or the interfacial region [26]. It is certainly possible that these changes could be driven by mobile La species in-situ, present as free LaOHx species already at the surface and/or those generated under reducing conditions by the decomposition of the La2ZrNiO6 perovskite phase. A study by Garbujo et al. [27] observed La enrichment in a study of the effect of A-site dopants on perovskite materials used for three-way catalyst applications to be attributed to the formation of La- hydroxide species (La(OH)3 and LaOOH). This suggests that the exposure of the pyrochlore materials under elevated temperatures and steam concentrations might create mobile La-OH species that rapidly cover or interact at the Ni particle interface. Although this initially produces a disproportionate activity between steam activation and CH4 decomposition, at some point for all catalysts, the interaction became synergistic and enables more balanced surface reactions between decomposition and gasification of carbon. Carbon formation is observed (see Table 5) during reaction but did not appear to be significant and had no clear trend between the quantity of carbon formed and calcination temperature over the time frame of these studies. The amounts of carbon remained below 1.5% mass carbon/mass catalyst for the 24 h study, which is a low rate of formation. Table 5. Carbon formation determined from a burn-off after SMR studies at low pressure. 800 °C, 0.23 MPa, S/C=2.0. Catalyst
Carbon Formation (gcarbon/gcatalyst)
LSZN6-700
0.01
LSZN6-800
0.01
LSZN6-900
0.01
LSZN6-1000
0.02
3.2.2.1 Effect of Steam Pre-Treatment In view of the ability for the catalyst activity to be regenerated in-situ and its potential relationship to exposure to steam at high temperatures, a pre-treatment was performed to the enable the restructuring between the Ni and La prior to reaction by exposing a catalyst to steam only at 800 °C followed by a brief, 10 min, CH4 reduction at the same temperature. The 1000 °C calcined material was used for this study because it showed the highest activity loss, which was also irreversible. The material was treated under an Ar/steam mixture at 800 °C for 15 min, or 1 h, then reduced by CH4 prior to the start of the experiment at the same temperature.
The effects on CH4 conversion (Fig. 5) indicate the oxidation-reduction procedure leads to an improvement in the activity, resulting in increased CH4 conversion and selectivity to syngas (see 15
Fig. S3 for H2 composition results). While the catalysts each had the initial drop in activity, the progressively longer exposure to the steam led to a decline in activity that was less significant, and also a higher activity recovery at the end of the 24 h study. After 1h of pretreatment, the catalyst was able to achieve steady conversion and syngas production that is much closer to equilibrium compared to the fresh material with no pre-treatment. Studies by Xu et al. [28, 29] also noticed a steam pre-treatment over Ni3Al and Ni foils improved the reforming activity compared to the same untreated materials and attributed to activity improvement to the coarsening of the Ni particles. However, the coarsening was only found to improve activity to a steady level once the reaction was started after the pre-treatment and did not continually increase in-situ as it did for the pyrochlores in Fig. 5.
100 1 h under steam
90
CH4 conversion / %
80 70 60 50 15 min under steam 40 30 No pretreatment
20 10 0 0
5
10 Time on stream / h
15
20
Fig. 5. Effect of oxidation/reduction pretreatment on conversion of the LSZN6-1000. Steam/Ar was run prior to experiment for 15 min, or 1 h, followed by reduction by CH4 at 800 °C. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h). 3.2.3 High Pressure A high pressure SMR study, at p=1.8 MPa, using the same temperature, flow rates, and S/C ratio as set for the low-pressure tests, was performed directly after a short burn-off (20 min) on the same catalytic material used in low pressure study. The material was run for another 24h at high pressure to evaluate the catalytic stability and activity.
16
The activity of the 900 °C calcined material is shown in Fig. 6 and was representative for all calcination temperatures. Interestingly, no initial decline in activity was observed for any catalyst, and all are able to produce stable CH4 conversions with near equilibrium selectivity for the entire 24h experiment. This could be partially explained by the higher reforming rates at higher pressures, since steam reforming rate at these conditions is first order in CH4 [30]. However, a larger contribution to the stabilized activity was the conditioning that occurred while exposed to the low-pressure SMR conditions. It is further revealed that the active catalyst surface generated during the low-pressure experiment is not affected by the brief oxidation treatment during the burn-off and indicates that at least short exposures to air at high temperatures do not lead to a restructuring of the interaction between La and Ni that is detrimental to activity. Additionally, it would suggest that carbon (e.g. oxycarbonates [26]) likely does not play a role in the active, working catalyst structure.
60 H2
Composition / vol% dry gas
50
40
30
20 CO 10
CH4 CO2
0 0
5
10
15 Time on stream / h
20
25
30
Fig. 6. Experimental results for LSZN6-900 during the high pressure SMR experiment that was performed directly after a 24 h SMR experiment at low pressure experiment and short burn-off. 800 °C, 1.8 MPa, S/C=2.0, and WHSV= 50,000 scc/(gcat· h).
Carbon formation was again measured by a burn-off post reaction. Table 6 shows that the rate of the accumulation of carbon is low during these 24 h experiment high pressure studies. The estimated Ni crystallite sizes, also shown in Table 6, increase from the values over the low17
pressure study. Despite this growth, the activity remains stable near the values predicted by thermodynamic equilibrium for all catalysts. It is unclear whether these values would be constant over a longer-term run, and additional work for another study is currently underway to determine the exact mechanism causing the activity improvement as well and long-term stability. Table 6. Carbon formation over LSZN6 materials and estimated Ni crystallite sizes after high pressure SMR experiment. 800 °C, 1.8 MPa, S/C=2.0. Catalyst
Carbon Formation (gcarbon/gcatalyst)
Estimated Ni crystallite size (nm)
LSZN6-700
0.01
28
LSZN6-800
0.01
33
LSZN6-900
0.01
30
LSZN6-1000
0.04
39
Experimental studies were also performed at high pressure on all fresh calcined samples, i.e. not initially tested under low-pressure. Fig. 7 only shows the CH4 conversion activity for LSZN6800, which is representative for all catalysts used in this study at these conditions. An immediate and irreversible activity loss was observed for these materials as well, and the results are therefore not included. The continuous loss of activity indicates that the higher steam pressures produce a more rapid rate of deactivation by oxidation with steam compared to the lower steam pressure conditions. When exposed to the higher reaction pressures the adsorption of steam was found to be irreversible and limited the ability for any material to undergo the critical restructuring between the Ni and La to improve activity. This shows that there is an importance balance between the steam partial pressures, and the rate of surface restructuring to mitigate the deleterious effects of steam for the catalyst to recover its activity in-situ.
18
100 90 80
No pretreatment Reduction 10%H2/Ar
CH4 conversion / %
70
Ar only Steam/Ar mixture
60 50 40 30 20 10 0 0
5
10
15 Time on stream / h
20
25
30
Fig. 7. Effect of pretreatment on conversion of the LSZN6-800 catalyst. 800 °C, 1.8 MPa, S/C=2.0, and WHSV= 50,000 scc/(gcat· h).
3.2.4 Effect of pretreatment Three different pretreatments procedures, a 1h under steam, reduction under H2, and exposure to Ar- all conducted at 800 °C, were evaluated on the same catalyst formulation (LSZN6-800) to determine whether these conditions could produce the same structural changes to the catalysts as the low-pressure reforming studies and generate the highly active and stable Ni sites (Fig. 7). The same steam pretreatment procedure that was observed to improve the activity for the LSZN6-1000 under low-pressures (described in Section 3.2.2.1), did not have the same effect when the catalyst is exposed to high reaction pressures. The catalyst is immediately and irreversibly deactivated by steam, despite showing some activity improvement over the fresh catalyst. This indicates that the high reaction pressures overwhelm the catalyst and inhibit the ability for the catalyst to regenerate in-situ. A 4-hour reduction pretreatment under 20%H2/Ar at 800 °C has a negligible effect on activity, as the pre-reduced material demonstrates a nearly identical conversion profile under reaction 19
conditions compared to the untreated sample. Aging the catalyst under H2 prior to reaction is not able to structurally change the interaction between Ni and La to generate higher activity, and likely results in a loss of activity due to the presence of La(OH)3 species that are generated at the surface after reduction. A study by Silva et al. [31] showed La(OH)3 species formed via XRD of a LaNiO3 during an in-situ reduction at 700°C, and the generation of these species at the surface of the pyrochlore prior to reaction, rather than in-situ under the reducing conditions of the reaction for the fresh material, could explain the slightly faster deactivation rate for the reduced catalyst. Interestingly, however, aging the LSZN6-800 material for 4 h under Ar at 800 °C leads to an improvement in activity, as indicated by the higher conversion when exposed to elevated reaction pressures. The difference in catalytic behavior compared to the other two pretreatments may be due to a change of the interaction between Ni and La, as the inert heating leads to the dispersion of the Ni into the structure. As observed by Raman spectroscopy [11], the exsolution of Ni from the pyrochlore material produces an oxide lattice with a large concentration of defects. In the presence of air, the transport limitations from the diffusion of larger O anions may create diffusional resistances for the cations within the lattice [32]. However, under the inert environment, cation diffusion is hindered less by the movement of the O anions. The change in the estimated Ni particle size (measured by XRD) after the inert pretreatment, see Fig. S4, indicates a shrinkage as compared to the fresh material (11 nm vs. 16 nm). Despite the improvements in activity compared to the reduction treatment, the irreversible activity loss by steam remained an issue for this material, and none of the pre-treatments used were able to promote the desirable interaction between Ni and La which could achieve equilibrium predicted syngas yield under high reaction pressures. 4. Conclusions The effect of calcination temperature on the steam reforming activity of a 6 wt% Ni-substituted lanthanum zirconate pyrochlore has been evaluated. A temperature programmed surface reaction from 650–900 °C reveals a critical interaction between Ni and La that is highly dependent on calcination temperature and greatly reduces the activity of the Ni over the temperature range examined. Further activity screening at S/C=2.0, WHSV=50000 scc/(gcat· h), under low pressure (p=0.23 MPa) or high pressures (1.8 MPa), regardless of calcination temperature, showed all catalyst have a high initial activity, which declined quickly. However, under the high-pressure conditions, all catalysts were irreversibly deactivated by steam despite different pre-treatments, while activity recovery to near equilibrium values was possible if the calcination temperature was 900 °C or less. The activity improvement was related to the restructuring of the surface and formation of synergistic sites between La and Ni, which led to a surface with an improved balance between steam activation and CH4 decomposition. This restructuring proved to be resilient under short high temperature oxidation treatments, as high pressure SMR studies run directly after a short burn-off following the low-pressure study showed that catalysts calcined at all temperatures could be active and selective for equilibrium syngas yields. Post run burn-offs 20
after all studies showed very little carbon formation after any of the experiments, indicating carbon did not impact catalyst activity. 5. Acknowledgements The project was supported by The Louisiana Board of Regents (Grant # LEQSF(2016-17)-ENHTR-07).
6. References [1] Sehested J. Sintering of nickel steam-reforming catalysts. J. Catal. 2003, 217(2): 417. [2] Sehested J. Four challenges for nickel steam-reforming catalysts. Catal. Today. 2006, 111(1– 2): 103. [3] Sehested J, Gelten J A P, Helveg S. Sintering of nickel catalysts: Effects of time, atmosphere, temperature, nickel-carrier interactions, and dopants. Appl. Catal. A Gen. 2006, 309(2): 237. [4] Bussi J, Musso M, Quevedo A, Faccio R, Romero M. Structural and catalytic stability assessment of Ni-La-Sn ternary mixed oxides for hydrogen production by steam reforming of ethanol. Catal. Today. 2017, 296: 154. [5] Fang XZ, Zhang XH, Guo Y, Chen MM, Liu WM, Xu XL, Peng HG, et al. Highly active and stable Ni/Y2Zr2O7 catalysts for methane steam reforming: On the nature and effective preparation method of the pyrochlore support. Int. J. Hydrogen Energy. 2016, 41(26): 11141. [6] Kumar N, Roy A, Wang Z, L’Abbate E M, Haynes D, Shekhawat D, Spivey J J. Bireforming of methane on Ni-based pyrochlore catalyst. Appl. Catal. A Gen. 2016, 517: 211. [7] Zhang J, Guo XY, Jung Y-G, Li L, Knapp J. Lanthanum zirconate based thermal barrier coatings: A review. Surface and Coatings Technology. 2017, 323: 18. [8] Haynes D J, Berry D A, Shekhawat D, Spivey J J. Catalytic partial oxidation of n-tetradecane using Rh and Sr substituted pyrochlores: Effects of sulfur. Catal. Today. 2009, 145(1–2): 121. [9] Pakhare D, Shaw C, Haynes D, Shekhawat D, Spivey J. Effect of reaction temperature on activity of Pt- and Ru-substituted lanthanum zirconate pyrochlores (La2Zr2O7) for dry (CO2) reforming of methane (DRM). J. CO2 Util. 2013, 1(0): 37. [10] Haynes D J, Berry D A, Shekhawat D, Spivey J J. Catalytic partial oxidation of ntetradecane using pyrochlores: Effect of Rh and Sr substitution. Catal. Today. 2008, 136(3– 4): 206. [11] Haynes D J, Shekhawat D, Berry D A, Zondlo J, Roy A, Spivey J J. Characterization of calcination temperature on a Ni-substituted lanthanum-strontium-zirconate pyrochlore. Ceram. Int. 2017, 43(18): 16744. [12] Yang P, Li N, Teng JJ, Wu J, Ma H. Effect of template on catalytic performance of La0.7Ce0.3Ni0.7Fe0.3O3 for ethanol steam reforming reaction. J. Rare Earths. 2019, 37(6): 594.
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[13] Rida K, Peña M A, Sastre E, Martinez-Arias A. Effect of calcination temperature on structural properties and catalytic activity in oxidation reactions of LaNiO3 perovskite prepared by Pechini method. J. Rare Earths. 2012, 30(3): 210. [14] Song J H, Yoo S, Yoo J, Park S, Gim M Y, Kim T H, et al. Hydrogen production by steam reforming of ethanol over Ni/Al2O3-La2O3 xerogel catalysts. Mol. Catal. 2017, 434: 123. [15] Prasad D H, Ji H I, Kim H R, Son J W, Kim B K, Lee H W, et al. Effect of nickel nanoparticle sintering on methane reforming activity of Ni-CGO cermet anodes for internal steam reforming SOFCs. Appl. Catal. B Environ. 2011, 101(3): 531. [16] King D L, Strohm J J, Wang X, Roh H-S, Wang C, Chin Y-H, et al. Effect of nickel microstructure on methane steam-reforming activity of Ni–YSZ cermet anode catalyst. J. Catal. 2008, 258(2): 356. [17] Pereñíguez R, González-DelaCruz V M, Holgado J P, Caballero A. Synthesis and characterization of a LaNiO3 perovskite as precursor for methane reforming reactions catalysts. Appl. Catal. B Environ. 2010, 93(3–4): 346. [18] Roine A, Lamberg P, Mansikka-aho J, Bjorklund P, Kettala J-P, Talonen T. HSC Chemistry Outotec Research Oy, 2007. [19] Arregi A, Lopez G, Amutio M, Artetxe M, Barbarias I, Bilbao J, et al. Role of operating conditions in the catalyst deactivation in the in-line steam reforming of volatiles from biomass fast pyrolysis. Fuel. 2018, 216: 233. [20] Ochoa A, Barbarias I, Artetxe M, Gayubo A G, Olazar M, Bilbao J, et al. Deactivation dynamics of a Ni supported catalyst during the steam reforming of volatiles from waste polyethylene pyrolysis. Appl. Catal. B Environ. 2017, 209: 554. [21] Park S Y, Oh G, Kim K, Seo M W, Ra H W, Mun T Y, et al. Deactivation characteristics of Ni and Ru catalysts in tar steam reforming. Renewable Energy. 2017, 105: 76. [22] Thalinger R, Gocyla M, Heggen M, Dunin-Borkowski R, Grünbacher M, Stöger-Pollach M, et al. Ni–perovskite interaction and its structural and catalytic consequences in methane steam reforming and methanation reactions. J. Catal. 2016, 337: 26. [23] Park Y S, Kang M, Byeon P, Chung S-Y, Nakayama T, Ko T, et al. Fabrication of a regenerable Ni supported NiO-MgO catalyst for methane steam reforming by exsolution. J. Power Sources. 2018, 397: 318. [24] Anspoks A, Kuzmin A. Interpretation of the Ni K-edge EXAFS in nanocrystalline nickel oxide using molecular dynamics simulations. J. Non-Cryst. Solids. 2011, 357(14): 2604. [25] Zhang Z, Verykios X E. Carbon Dioxide Reforming of Methane to Synthesis Gas ofer Ni/La2O3 Catalysts. Appl. Catal. A Gen. 1996, 138: 109. [26] Batiot-Dupeyrat C, Gallego G A S, Mondragon F, Barrault J, Tatibouët J-M. CO2 reforming of methane over LaNiO3 as precursor material. Catal. Today. 2005, 107-108: 474. [27] Garbujo A, Pacella M, Natile M M, Guiotto M, Fabro J, Canu P, et al. On A-doping strategy for tuning the TWC catalytic performance of perovskite based catalysts. Appl. Catal. A Gen. 2017, 544: 94.
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[28] Xu Y, Harimoto T, Hirano T, Ohata H, Kunieda H, Hara Y, et al. Catalytic performance of a high-cell-density Ni honeycomb catalyst for methane steam reforming. Int. J. Hydrogen Energy. 2018, 43(33): 15975. [29] Xu Y, Ma Y, Demura M, Hirano T. Enhanced catalytic activity of Ni3Al foils towards methane steam reforming by water vapor and hydrogen pretreatments. Int. J. Hydrogen Energy. 2016, 41(18): 7352. [30] Wei J, Iglesia E. Structural requirements and reaction pathways in methane activation and chemical conversion catalyzed by rhodium. J. Catal. 2004, 225(1): 116. [31] Silva P P, Ferreira R A R, Noronha F B, Hori C E. Hydrogen production from steam and oxidative steam reforming of liquefied petroleum gas over cerium and strontium doped LaNiO3 catalysts. Catal. Today. 2017, 289: 211. [32] Rahaman MN. Ceramic Processing and Sintering. 2nd ed. Boca Raton, FL: Taylor and Francis Group, LLC; 2003:812.
Graphical Abstract
23
Activation in-situ 70
S/C=2.0, 800 °C; 0.23 or 1.8 MPa
60
Composition / vol% dry gas
Exposure to SMR reaction conditions
Fresh pyrochlore catalyst Lanthanum enriched surface
Ni
H2
50 40 30 20 CO 10 CO2 CH4
0 0
La2Zr2O7
5
10
15 Time on stream / h
20
25
30
Deactivation of Ni by steam
100 90 80
No pretreatment Reduction 10%H2/Ar
CH4 conversion / %
70
Ar only Steam/Ar mixture
60 50 40 30 20 10 0 0
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10
15 Time on stream / h
20
25
Ni-based pyrochlore catalysts showed a complex interaction between Ni and an enriched La layer under reaction conditions which led to stable, equilibrium syngas production, or irreversible deactivation, depending on the reaction conditions.
24
30
Highlights •
The substitution of Ni into La2Zr2O7 pyrochlore produces Ni particles on the surface.
•
The excess La increasingly enriches the surface at calcination temperatures above 700°C.
•
The steam reforming activity of the Ni is influenced by the interaction between Ni and La.
•
More surface La inhibits the CH4 decomposition activity leading to an oxidation of Ni.
•
Certain conditions can form synergistic Ni-La sites that are active and stable for equilibrium syngas production.
S1002-0721(19)30255-8
DOI:
https://doi.org/10.1016/j.jre.2019.07.015
Reference:
JRE 594
To appear in:
Journal of Rare Earths
Received Date: 29 March 2019 Revised Date:
8 July 2019
Accepted Date: 9 July 2019
Please cite this article as: Haynes DJ, Shekhawat D, Berry D, Roy A, Spivey JJ, Effect of Calcination Temperature on Steam Reforming Activity of Ni-based Pyrochlore Catalysts, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.07.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Effect of Calcination Temperature on Steam Reforming Activity of Ni-based Pyrochlore Catalysts Daniel J. Haynesa*, Dushyant Shekhawata, David Berrya, Amitava Royb, James J. Spiveyb
a
National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA b
Louisiana State University, Cain Department of Chemical Engineering, 3307 Patrick Taylor Hall, Baton Rouge, LA, 70803, USA
*Corresponding Author National Energy Technology Laboratory U.S. Department of Energy 3610 Collins Ferry Rd, Morgantown, WV 26057 Tel: 304-285-1355 Email_ [email protected]
Keywords: Nickel, Pyrochlore, Methane, Steam reforming, Catalyst, Lanthanum enrichment Foundation item: Project supported by The Louisiana Board of Regents (Grant # LEQSF(201617)-ENH-TR-07).
2
Abstract This work served as the second part of a study evaluating the effect of calcination temperature (700–1000°C) on Ni-based lanthanum zirconate pyrochlore catalysts for methane steam reforming. A previous study (Haynes et al. Ceram. Int. 2017 (43) 16744) provided a thorough characterization of the material properties for the catalysts used here, and this study focused on the evaluation of catalytic activity. The activity was assessed by two different experimental studies: the effect of reaction temperature using a temperature programmed surface reaction (TPSR), and the effect of reaction pressure. The results demonstrated a complex interaction between the Ni particles and surface LaOx species under the methane steam reforming conditions. Specifically, the material calcined at the lowest temperature (700 °C) possessed the highest activity and selectivity, which was attributed to smaller and more well-dispersed Ni particles on the surface, and, more importantly, a lesser degree La enrichment at the surface. All catalysts were deactivated by steam to NiO under all conditions tested, but at certain low reaction pressure (p=0.23 MPa) conditions the materials calcined at 700–900°C were able to completely recover equilibrium activity in-situ that was then robust and stable under both low and high reaction pressures (p=1.8 MPa) suggesting the formation of a synergistic relationship between Ni and La for syngas production. However, exposure of a fresh material to high reaction pressures led to a rapid and irreversible loss in both CH4 conversion and syngas selectivity whether in the fresh (no pretreatment), or pretreated (steam, H2 or Ar only at 800 °C) form for any catalyst. The mechanism for deactivation appeared to be due to the presence of LaOx species that became mobile, possibly by the formation of La-OH, and covered the active Ni particles and inhibited sites responsible for the CH4 decomposition.
3
1. Introduction The growing availability of cheap and abundant natural gas has been accompanied by an increased interest in its conversion into more useful and valuable fuels and chemicals. Given the favorable economics of readily available natural gas, near term non-energy uses of natural gas resources will undoubtedly be based on traditional reforming conversion methods. Of the available reforming technologies, steam methane reforming (SMR) will continue to play an important role, producing both synthesis gas and H2 gas for end products such as electrical power using fuel cells. Long-term reformer operation depends on a catalyst that must be not only active and selective for synthesis gas (H2 and CO), but also resistant to deactivation. Ni-based catalysts like Ni/Al2O3 and Ni/MgO are traditionally used for industrial steam reforming. However, exposure to the demanding reforming conditions results in the deactivation of the Ni catalyst through several well-known mechanisms, which include active surface area loss from thermal aging and metal agglomeration, and carbon formation [1-3]. Catalysts based on stable oxides such as pyrochlores are an attractive alternative to traditional catalysts [4-6]. These materials have demonstrated chemical and thermal stability under high temperatures [7]. The pyrochlore crystal structure can also be modified by the isomorphic substitution of a wide variety of different metals into the structure, which provides active sites for reactions such as steam reforming [8, 9]. Specifically, the substitution of the catalytically active metal into the lattice can provide stable and atomically dispersed crystallites that are more resistant to deactivation by carbon and sulfur compared to traditional supported metal catalysts [10]. These advantages may be used to improve the activity stability to Ni under steam reforming conditions, particularly by limiting the growth of whisker carbon. Previously, we characterized the effects of calcination temperature on the material properties of a 6 wt% Ni substituted lanthanum zirconate pyrochlore catalyst [11]. This paper is a continuation of that work. For this study, we report the effect of calcination temperature on the steam methane reforming activity of the same Ni substituted pyrochlores that were characterized in the previous study. The catalytic activity will be assessed in two different experimental studies: the effect of reaction temperature using a temperature programmed surface reaction (TPSR), and the effect of reaction pressure. 2. Experimental 2.1 Catalyst synthesis The pyrochlore catalysts used in this study were fabricated using a modified version of the Pechini method discussed elsewhere [11]. The precursor was then calcined to 4 different temperatures by 5 °C/min for 8 h. These materials will be designated as LSZN6-XXX, where XXX will indicate the calcination temperature (e.g. LSZN6-700 for the material calcined at 700 °C). 4
2.2 Catalyst characterization Phase analysis of powder samples was examined using a Panalytical (Malvern/Panalytical) X’pert Pro X-ray diffraction system, model number PW 3040 Pro. Jade v9.6 Materials Data Inc. was used for phase identification and pattern analysis, which included peak fitting, crystallite size, and lattice parameter determination. The Scherrer equation was used to estimate crystallite size. XAS (X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)) measurements of the various forms of the catalyst were performed at Louisiana State University’s synchrotron research facility, the J. Bennett Johnston, Sr., Center for Advanced Microstructures and Devices (CAMD), USA. Instrument information and analysis conditions can be found elsewhere [11]. 2.3 Catalytic reaction studies Catalyst testing was performed in a continuous-flow reactor (Autoclave Engineers, Model no. BTRS Jr). The dry gas products: H2, CO, CO2, CH4, and N2 were analyzed continuously by means of an online Thermo Onix mass spectrometer (Model no. Prima δb, a 200 a.m.u. scanning magnetic sector). Although water was a reactant and product in these experiments, it was not measured analytically. The conversion of methane for SMR was calculated by Eq. (1).
Conversion (%) =
(Moles CH 4 in − Moles CH 4 out ) Moles CH 4 in
(1)
2.3.1 Temperature programmed surface reaction (TPSR) Temperature programmed surface reaction studies (TPSR) were designed to assess the catalytic activity by showing hysteresis effects between the heating and cooling periods. The reactant flows, and conditions are provided in Table S1. Flows were run for 0.5 h at the start of the experiment to establish an initial baseline activity. The reaction temperature was then ramped from 650 to 900 °C at 2.5 °C/min and held at 900 °C for 0.5 h. After the isothermal hold, the reactor temperature was then cooled to 650 °C by 2.5 °C/min to determine any changes in activity. 2.3.2 SMR Activity screening studies The Ni-pyrochlore catalysts were also evaluated for steam reforming activity under more relevant flows rates, and reactant concentrations. The studies were also designed to evaluate the effect of pressure on catalyst activity. Two studies were performed with the exact same conditions, see SI Table S2, but with a different system pressure. Low-pressure studies were performed at the system pressure (0.23MPa), while the high-pressure experiments were performed at 1.8 MPa.
5
2.3.3 Post run carbon analysis After all reforming experiments, a burn-off was performed to quantify the amount of carbon deposited on the spent catalyst using mixture of air and Ar (12 vol% O2) at 800 °C. CO2 emission was measured by an online mass spectrometer and the quantity of carbon was determined from the plot of CO2 emitted as a function of time by integrating the area under the curve. 3. Results and discussion 3.1 Temperature Programmed Surface Reaction (TPSR) A temperature programmed surface reaction was performed to examine hysteresis effects on the catalyst activity between heating and cooling cycles. The H2, CO, and CO2 compositions, as well as CH4 conversion during the experiments are shown in Fig. 1(a–d) respectively for the LSZN6700, LSZN6-800, and LSZN6-900. A TPSR experiment was performed on the LSZN6-1000, but no conversion to syngas products was observed and is therefore not included in Figure 1 a-d. (a) 25 650 °C
Cool to 650 °C by 2.5 °C/min
Heat to 900 °C by 2.5 °C/min
Hold at 900 °C
Equilibrium
H2 composition / vol% dry gas
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LSZN6-700 15
LSZN6-800
10
5
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6
160
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(b) 6 650 °C
Heat to 900 °C by 2.5 °C/min
Cool to 650 °C by 2.5 °C/min Hold at 900 °C
Equilibrium
CO composition / vol% dry gas
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4 LSZN6-700
LSZN6-800 3
2
1
LSZN6-900
0 0
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(c) 3.5 650 °C
Cool to 650 °C by 2.5 °C/min
Heat to 900 °C by 2.5 °C/min
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2 Equilibrium
1.5
1 LSZN6-700 0.5 LSZN6-800 LSZN6-900 0 0
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80
120 160 Time on stream / min
7
200
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(d) 100
Cool to 650 °C by 2.5 °C/min Hold at 900 °C
Equilibrium 650 °C
90 80
Heat to 900 °C by 2.5 °C/min
CH4 conversion / %
70
LSZN6-700 60
LSZN6-800
50 40 30 20 10 LSZN6-900 0 0
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80
120 Time on stream / min
160
200
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Fig. 1. TPSR results demonstrating the effect of calcination temperature on H2 production (a), CO production (b), CO2 production (c), and CH4 conversion (d). S/C=2.0, WHSV=128 000 scc/(gcat· h), Ar=79%, 0.10 MPa. The TPSR plots demonstrate the SMR activity of Ni in these materials is highly sensitive to both calcination and reaction temperature. The catalyst calcined at the lowest temperature produces the most active and selective to syngas. According to the XRD and XPS (see Table S3 for XPS data) [11], increasing the calcination temperature leads to a less homogenous material, with a progressively smaller amount of detectable Ni at the surface from both particle growth, and the formation of a La2ZrNiO6 perovskite phase in the bulk. Thus, the higher activity for the catalyst calcined at the lowest calcination temperature (LSZN6-700) can be attributed in part to smaller Ni particles, and more accessible Ni at the surface, which is consistent with other work [12]. However, the changes to the properties of the Ni do not fully explain the reason for the lower overall activity as the calcination temperature was increased. Another notable structural effect to the pyrochlores catalysts from exposure to higher calcination temperatures is an increasing degree of La enrichment at the surface (Table S3) [11, 13], which occurs primarily in our case due to low solubility of the Ni in the pyrochlore structure. Although Ni and Zr have a similar ionic radius (0.069 nm for Ni2+ vs. 0.072 nm Zr4+ in a 6-fold coordination) the valence difference between the two generates a large concentration of oxygen vacancies to maintain charge 8
neutrality when Ni is introduced into the structure, particularly at the loadings used for the materials in this study (i.e. Ni is about 28% of the atomic loading of the B-site). This charge compensation generates a large degree of lattice strain that becomes unstable due to the greater degree of ordering of the pyrochlore structure at higher temperatures (>700 °C). As a result, the Ni exsolves from the structure to the surface and grain boundary regions. Consequently, as these materials were synthesized assuming Ni would occupy a lattice position in the pyrochlore structure (i.e. La/(Ni+Zr) atomic ratio ~1) there becomes an excess amount of La present once Ni comes out of the structure, and similar to Ni, the excess La goes to the surface and grain boundaries regions leading to an enrichment at the surface. Comparing the extent of La enrichment to the catalytic behavior indicates that the lower activity and selectivity is not simply related to the particle size and/or amount of Ni, but also that the interaction between La and Ni at the surface also plays a significant role in the reforming activity of the Ni particles. This would also indicate that not only does increasing the calcination temperature produce a greater surface enrichment of La, it also creates a critical interaction between Ni and La which renders the Ni almost completely inactive during these experiments when the catalysts are calcined to temperatures ≥ 900 °C. Exposure to high reaction temperatures also detrimentally impacts the catalyst activity and stability. The LSZN6-700 and LSZN6-800 materials both show an increase in syngas selectivity from their initial levels at 650 °C as temperature is increased. However, operation beyond ~780 °C (75 minutes TOS) to 900 °C, and through the isothermal hold at 900 °C, results in a continuous decline in both syngas selectivity, and CH4 conversion. Decreasing the reaction temperature is decreased to 650 °C after the hold at 900 °C, the syngas product composition for all catalysts is much lower than the corresponding temperature during the heat up portion in Fig. 1(a–d). Within 220 minutes time on stream, the gaseous product composition of this catalysts approximates that of a blank reactor. It would be expected that the most obvious explanation for the decline in catalyst performance would be carbon formation, as this is a common and well-known problem for Ni reforming catalysts [2]. Surprisingly, there is very little oxidizable carbon on any of the catalysts, as shown from a post run burn-off in Table 1. Compared to this study, others have observed greater than 20 wt% carbon at comparable conditions without observable deactivation [14], so this could not be attributed to deactivation. Table 1. Carbon formed during the SMR TPSR experiments. Catalyst Carbon (gcarbon/gcatalyst) LSZN6-700
0.01
LSZN6-800
0.03
LSZN6-900
0.04
9
It was also considered that sintering could be the cause the deactivation. Under steam rich conditions, particle size growth would be expected according to a study by Sehested et al. [1]. However, the range of particle sizes observed for the spent LSZN6-700 material are still relatively small. For example, a study by Prasad et al. [15] have found Ni particles similar in size to those in the spent LSZN6-700 material to remain highly active and selective for syngas production. Additionally, activity loss due to sintering during the SMR shows a CH4 conversion which declines from initial values, but eventually reaches a steady level [15, 16]. Another, and more probable cause of the decline in activity is the oxidation of the Ni by steam. To confirm this behavior, an additional TPSR experiment was performed using the LSZN6-800 by stopping the H2O flow during reaction (step 4) and flowing only CH4 over the catalyst at 700 °C where the activity was almost completely deactivated. In doing so, the oxidized form of Ni should be reduced by CH4 back to the more active metallic Ni form resulting in higher levels of syngas once the steam is restarted. Resuming the steam flow at 700 °C (step 5) after a 10-minute exposure to only CH4 dose show (see Fig. 2) an immediate improvement to the activity, which would indicate the oxidation of the Ni to be the primary reason for deactivation. If carbon formation was responsible for the activity loss, the exposure to CH4 alone would not have increased activity. 2.
1.
25
3.
4. 5.
Composition / vol% dry gas
Isothermal hold @ 650 °C
20
15
H2
10
CH4
5 CO CO2
0 0
50
100
150 200 Time on stream / min
250
300
350
Fig. 2. TPSR experiment performed over LSZN6-800 material to validate Ni oxidation. Steps involved are 1) initiated ramp to 900 °C by 2.5 °C/min, 2) Isothermal hold at 900 °C, 3) started ramp by 2.5 °C/min to 700 °C, 4) stopped steam flow to catalyst at 700°C (reduction under CH4 started, 5) resumed steam flow. 10
Oxidation of Ni as a mechanism for deactivation is consistent with the results of a similar SMR experiment (S/C=1.0) by Pereniguez et al. [17] over a LaNiO3 perovskite catalyst. They observed a steep decline in activity during a similar temperature ramping experiment when the reaction temperature reached near 800 °C, which was comparable to this study (ca. ~780 °C for the pyrochlores). XANES results of their spent catalyst showed Ni had become oxidized by steam, and XPS results also showed La to enrich the surface and envelope the Ni as the material was reduced from the LaNiO3 to Ni/La2O3 under reaction conditions [17]. As the pyrochlores examined for this study also show an enrichment of La at the surface, this would suggest that the deleterious effects of Ni oxidation observed during the TPSR experiments could similarly be explained by the coverage of the Ni particles by the excess La at the surface. 3.2 SMR Activity Screening 3.2.1 Equilibrium Equilibrium product compositions for both conditions were determined by HSC Chemistry thermodynamic software [18], and the results are shown below in Table 2. Note that higher pressures inhibit the reforming rate by limiting the molar expansion of reactants into products, thereby leading to a reduction in the overall conversion of CH4. Table 2. Thermodynamic product distributions for the SMR conditions used in this study 800 °C, and S/C=2.0 as determined by HSC Chemistry [18]. Balance of compositions was inert gas. Equilibrium Composition (%) Product p=system (0.23 MPa) p=1.8 MPa H2
61.5
52.8
CO
15.1
10.4
CO2
4.0
5.4
CH4
0.1
7.8
3.2.2 Low Pressure Fig. 3 reveals the effect of calcination temperature on the activity during the low pressure SMR experiments only for the LSZN6-800 catalyst. Results for the same experiment performed over the 700, 900 and 1000 °C calcined materials are shown in the supplemental information (see Fig. S2 (a–c)). Upon introduction of the reactants, the conversion and selectivity are initially high for all catalysts but not stable. Further exposure to reactants produces a change in activity over each catalyst with the same general trend - a decline in selectivity to syngas and CH4 conversion, followed by an activity recovery.
11
70 H2
Composition / vol% dry gas
60 50 40 30 20
CO 10 CO2 CH4
0 0
5
10
15 Time on stream / h
20
25
30
Fig. 3. Experimental results from the SMR study at low pressure over the LSZN6-800 pyrochlore catalyst. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h).
The behavior shows an unusual trend compared to the literature [19-21] in which it is most often observed that Ni based catalysts have high initial activity, then either decline until the activity reaches a steady product yield, or deactivates completely. Fig. 3 suggests, however, that exposure to the reactive gases alters the catalyst surface structure in-situ to produce an improvement in activity, and there are several possible explanations as to why the materials exhibit this performance. Each of the materials calcined at 700–900 °C show via XRD to have a small amount of Ni that remains in the pyrochlore lattice [11]. In addition, the 900 and 1000 °C calcined materials showed some of the Ni to reside in the La2ZrNiO6 perovskite solid solution in the bulk. Under the reaction conditions, the Ni residing in the bulk forms of these materials (either pyrochlore, or perovskite) could exsolve to the surface as small and highly active particles. In fact, exsolution of catalytic metals from the lattice has been shown to improve catalytic activity for different reforming reactions [16, 22, 23] In our previous study [11], the XRD results indicated (by shift in lattice parameter value) the fresh LSZN6-700 had the highest amount of Ni in the pyrochlore structure of all materials used here, while the material calcined at 1000 °C had a negligible
12
amount. However as shown in Table 3, the lattice parameter for the LSZN6-700 material is identical to the fresh, which likely rules out this possibility. Table 3. Lattice parameters determined from a post-run low-P SMR reaction. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h). Material LSZN6-700
Fresh lattice constant (nm) Spent lattice constant (nm) 10.880
10.880
The effect of calcination temperature on the activity shows the trend in the magnitude of the increase of the initial decline, which can also be correlated to the La surface concentration. For these experiments, the higher surface La levels produce a greater activity loss due to an inhibition of the sites responsible for CH4 activation by steam. However, unlike the TPSR studies, all catalysts, apart from the LSZN6 1000, show the ability to recover stable, near equilibrium activity with no signs of deactivation for the remaining portion of the experiment. To confirm that oxidation was responsible for the activity loss during the decline in activity, XANES and EXAFS analysis were performed on three LSZN6-800 catalysts (see Fig. 4): a fresh catalyst, a sample that was run for 3h and then stopped at the greatest degree of activity loss (designated as partially deactivated catalyst), and a sample run for the entire 24 h experiment in which the activity fully recovered (designated as activated catalyst). The spectra (both XANES and EXAFS) was resolved using a linear combination fitting (LCF) to determine the relative amounts of Ni0 and NiO present in each sample (see Table 4). As expected, the fresh sample is completely oxidized and shows Ni to be present in the +2 state, entirely as NiO, within experimental detection limits. Meanwhile the XANES spectra for the partially deactivated and activated catalysts show a similar electronic structure, and thus reveal a similar degree of NiO present. Although NiO is present in both samples, their Ni K-edge EXAFS oscillations have a more pronounced difference than the XANES signals. This is confirmed by the LCF values derived from the EXAFS scans, and shows the oxygen present is interacting with the Ni differently in each sample. From Figure 3 it is seen that the activated catalyst shows near equilibrium activity. The EXAFS scan for this material shows a sharper amplitude for the main Ni oscillation that is comparable to the Ni0 foil, indicating the Ni is more metallic in nature. This would suggest that the oxygen, while present, was interacting with the Ni in a way that is not detectable by this technique and has a minimal effect on the metallic character of the Ni. Meanwhile, the partially deactivated catalyst reveals a dampening of the Fourier transform amplitude, which could be attributed to the presence of O within the Ni clusters affecting the interatomic distance and local structure of the Ni particles [24]. This would indicate the decline in activity of the Ni during this period could be attributed to the incorporation of oxygen within Ni particles, leading to a less active form of the catalyst. 13
Fig. 4. XANES spectra and EXAFS oscillations for three (3) LSZN6-800 samples. Table 4. Linear combination fitting results of the XANES and EXAFS spectra for the LSZN6800 samples. NiO
Fresh catalyst
Ni
XANES
EXAFS
XANES
EXAFS
1.00
1.00
0.00
0.00
Partially deactivated 0.06±0.01 0.12±0.01 0.94±0.01 0.88±0.01 Activated
0.08±0.01 0.0±0.02
0.92±0.01 1.0±0.02
The improvement in activity and stability under reaction conditions implies that exposure to the reactant gases, and or surface intermediates formed over the working catalyst, lead to structural modifications that enhance reforming rates. A study by Zhang et al. [25] observed an improvement in Ni activity over a La2O3 support during the CO2 reforming of CH4 at 750 °C, p=0.1 MPa. Regardless of pretreatment (H2 reduction, or oxidation), the catalyst had lower initial rates that increased with time under reaction conditions. They attributed the initial results (i.e. low activity) to the decoration of the Ni particles with LaOx species that originated from the support, and then the activity improvement to a new type of surface compound or synergetic sites between the Ni-La generated under reaction conditions at the interfacial area that are stable and active for reforming [25].
14
This complex behavior observed under reaction conditions is speculated to arise from structureactivity relationship which relies heavily on changes between the La and Ni at the surface of the Ni particles or the interfacial region [26]. It is certainly possible that these changes could be driven by mobile La species in-situ, present as free LaOHx species already at the surface and/or those generated under reducing conditions by the decomposition of the La2ZrNiO6 perovskite phase. A study by Garbujo et al. [27] observed La enrichment in a study of the effect of A-site dopants on perovskite materials used for three-way catalyst applications to be attributed to the formation of La- hydroxide species (La(OH)3 and LaOOH). This suggests that the exposure of the pyrochlore materials under elevated temperatures and steam concentrations might create mobile La-OH species that rapidly cover or interact at the Ni particle interface. Although this initially produces a disproportionate activity between steam activation and CH4 decomposition, at some point for all catalysts, the interaction became synergistic and enables more balanced surface reactions between decomposition and gasification of carbon. Carbon formation is observed (see Table 5) during reaction but did not appear to be significant and had no clear trend between the quantity of carbon formed and calcination temperature over the time frame of these studies. The amounts of carbon remained below 1.5% mass carbon/mass catalyst for the 24 h study, which is a low rate of formation. Table 5. Carbon formation determined from a burn-off after SMR studies at low pressure. 800 °C, 0.23 MPa, S/C=2.0. Catalyst
Carbon Formation (gcarbon/gcatalyst)
LSZN6-700
0.01
LSZN6-800
0.01
LSZN6-900
0.01
LSZN6-1000
0.02
3.2.2.1 Effect of Steam Pre-Treatment In view of the ability for the catalyst activity to be regenerated in-situ and its potential relationship to exposure to steam at high temperatures, a pre-treatment was performed to the enable the restructuring between the Ni and La prior to reaction by exposing a catalyst to steam only at 800 °C followed by a brief, 10 min, CH4 reduction at the same temperature. The 1000 °C calcined material was used for this study because it showed the highest activity loss, which was also irreversible. The material was treated under an Ar/steam mixture at 800 °C for 15 min, or 1 h, then reduced by CH4 prior to the start of the experiment at the same temperature.
The effects on CH4 conversion (Fig. 5) indicate the oxidation-reduction procedure leads to an improvement in the activity, resulting in increased CH4 conversion and selectivity to syngas (see 15
Fig. S3 for H2 composition results). While the catalysts each had the initial drop in activity, the progressively longer exposure to the steam led to a decline in activity that was less significant, and also a higher activity recovery at the end of the 24 h study. After 1h of pretreatment, the catalyst was able to achieve steady conversion and syngas production that is much closer to equilibrium compared to the fresh material with no pre-treatment. Studies by Xu et al. [28, 29] also noticed a steam pre-treatment over Ni3Al and Ni foils improved the reforming activity compared to the same untreated materials and attributed to activity improvement to the coarsening of the Ni particles. However, the coarsening was only found to improve activity to a steady level once the reaction was started after the pre-treatment and did not continually increase in-situ as it did for the pyrochlores in Fig. 5.
100 1 h under steam
90
CH4 conversion / %
80 70 60 50 15 min under steam 40 30 No pretreatment
20 10 0 0
5
10 Time on stream / h
15
20
Fig. 5. Effect of oxidation/reduction pretreatment on conversion of the LSZN6-1000. Steam/Ar was run prior to experiment for 15 min, or 1 h, followed by reduction by CH4 at 800 °C. 800 °C, 0.23 MPa, S/C=2.0, and WHSV= 50000 scc/(gcat· h). 3.2.3 High Pressure A high pressure SMR study, at p=1.8 MPa, using the same temperature, flow rates, and S/C ratio as set for the low-pressure tests, was performed directly after a short burn-off (20 min) on the same catalytic material used in low pressure study. The material was run for another 24h at high pressure to evaluate the catalytic stability and activity.
16
The activity of the 900 °C calcined material is shown in Fig. 6 and was representative for all calcination temperatures. Interestingly, no initial decline in activity was observed for any catalyst, and all are able to produce stable CH4 conversions with near equilibrium selectivity for the entire 24h experiment. This could be partially explained by the higher reforming rates at higher pressures, since steam reforming rate at these conditions is first order in CH4 [30]. However, a larger contribution to the stabilized activity was the conditioning that occurred while exposed to the low-pressure SMR conditions. It is further revealed that the active catalyst surface generated during the low-pressure experiment is not affected by the brief oxidation treatment during the burn-off and indicates that at least short exposures to air at high temperatures do not lead to a restructuring of the interaction between La and Ni that is detrimental to activity. Additionally, it would suggest that carbon (e.g. oxycarbonates [26]) likely does not play a role in the active, working catalyst structure.
60 H2
Composition / vol% dry gas
50
40
30
20 CO 10
CH4 CO2
0 0
5
10
15 Time on stream / h
20
25
30
Fig. 6. Experimental results for LSZN6-900 during the high pressure SMR experiment that was performed directly after a 24 h SMR experiment at low pressure experiment and short burn-off. 800 °C, 1.8 MPa, S/C=2.0, and WHSV= 50,000 scc/(gcat· h).
Carbon formation was again measured by a burn-off post reaction. Table 6 shows that the rate of the accumulation of carbon is low during these 24 h experiment high pressure studies. The estimated Ni crystallite sizes, also shown in Table 6, increase from the values over the low17
pressure study. Despite this growth, the activity remains stable near the values predicted by thermodynamic equilibrium for all catalysts. It is unclear whether these values would be constant over a longer-term run, and additional work for another study is currently underway to determine the exact mechanism causing the activity improvement as well and long-term stability. Table 6. Carbon formation over LSZN6 materials and estimated Ni crystallite sizes after high pressure SMR experiment. 800 °C, 1.8 MPa, S/C=2.0. Catalyst
Carbon Formation (gcarbon/gcatalyst)
Estimated Ni crystallite size (nm)
LSZN6-700
0.01
28
LSZN6-800
0.01
33
LSZN6-900
0.01
30
LSZN6-1000
0.04
39
Experimental studies were also performed at high pressure on all fresh calcined samples, i.e. not initially tested under low-pressure. Fig. 7 only shows the CH4 conversion activity for LSZN6800, which is representative for all catalysts used in this study at these conditions. An immediate and irreversible activity loss was observed for these materials as well, and the results are therefore not included. The continuous loss of activity indicates that the higher steam pressures produce a more rapid rate of deactivation by oxidation with steam compared to the lower steam pressure conditions. When exposed to the higher reaction pressures the adsorption of steam was found to be irreversible and limited the ability for any material to undergo the critical restructuring between the Ni and La to improve activity. This shows that there is an importance balance between the steam partial pressures, and the rate of surface restructuring to mitigate the deleterious effects of steam for the catalyst to recover its activity in-situ.
18
100 90 80
No pretreatment Reduction 10%H2/Ar
CH4 conversion / %
70
Ar only Steam/Ar mixture
60 50 40 30 20 10 0 0
5
10
15 Time on stream / h
20
25
30
Fig. 7. Effect of pretreatment on conversion of the LSZN6-800 catalyst. 800 °C, 1.8 MPa, S/C=2.0, and WHSV= 50,000 scc/(gcat· h).
3.2.4 Effect of pretreatment Three different pretreatments procedures, a 1h under steam, reduction under H2, and exposure to Ar- all conducted at 800 °C, were evaluated on the same catalyst formulation (LSZN6-800) to determine whether these conditions could produce the same structural changes to the catalysts as the low-pressure reforming studies and generate the highly active and stable Ni sites (Fig. 7). The same steam pretreatment procedure that was observed to improve the activity for the LSZN6-1000 under low-pressures (described in Section 3.2.2.1), did not have the same effect when the catalyst is exposed to high reaction pressures. The catalyst is immediately and irreversibly deactivated by steam, despite showing some activity improvement over the fresh catalyst. This indicates that the high reaction pressures overwhelm the catalyst and inhibit the ability for the catalyst to regenerate in-situ. A 4-hour reduction pretreatment under 20%H2/Ar at 800 °C has a negligible effect on activity, as the pre-reduced material demonstrates a nearly identical conversion profile under reaction 19
conditions compared to the untreated sample. Aging the catalyst under H2 prior to reaction is not able to structurally change the interaction between Ni and La to generate higher activity, and likely results in a loss of activity due to the presence of La(OH)3 species that are generated at the surface after reduction. A study by Silva et al. [31] showed La(OH)3 species formed via XRD of a LaNiO3 during an in-situ reduction at 700°C, and the generation of these species at the surface of the pyrochlore prior to reaction, rather than in-situ under the reducing conditions of the reaction for the fresh material, could explain the slightly faster deactivation rate for the reduced catalyst. Interestingly, however, aging the LSZN6-800 material for 4 h under Ar at 800 °C leads to an improvement in activity, as indicated by the higher conversion when exposed to elevated reaction pressures. The difference in catalytic behavior compared to the other two pretreatments may be due to a change of the interaction between Ni and La, as the inert heating leads to the dispersion of the Ni into the structure. As observed by Raman spectroscopy [11], the exsolution of Ni from the pyrochlore material produces an oxide lattice with a large concentration of defects. In the presence of air, the transport limitations from the diffusion of larger O anions may create diffusional resistances for the cations within the lattice [32]. However, under the inert environment, cation diffusion is hindered less by the movement of the O anions. The change in the estimated Ni particle size (measured by XRD) after the inert pretreatment, see Fig. S4, indicates a shrinkage as compared to the fresh material (11 nm vs. 16 nm). Despite the improvements in activity compared to the reduction treatment, the irreversible activity loss by steam remained an issue for this material, and none of the pre-treatments used were able to promote the desirable interaction between Ni and La which could achieve equilibrium predicted syngas yield under high reaction pressures. 4. Conclusions The effect of calcination temperature on the steam reforming activity of a 6 wt% Ni-substituted lanthanum zirconate pyrochlore has been evaluated. A temperature programmed surface reaction from 650–900 °C reveals a critical interaction between Ni and La that is highly dependent on calcination temperature and greatly reduces the activity of the Ni over the temperature range examined. Further activity screening at S/C=2.0, WHSV=50000 scc/(gcat· h), under low pressure (p=0.23 MPa) or high pressures (1.8 MPa), regardless of calcination temperature, showed all catalyst have a high initial activity, which declined quickly. However, under the high-pressure conditions, all catalysts were irreversibly deactivated by steam despite different pre-treatments, while activity recovery to near equilibrium values was possible if the calcination temperature was 900 °C or less. The activity improvement was related to the restructuring of the surface and formation of synergistic sites between La and Ni, which led to a surface with an improved balance between steam activation and CH4 decomposition. This restructuring proved to be resilient under short high temperature oxidation treatments, as high pressure SMR studies run directly after a short burn-off following the low-pressure study showed that catalysts calcined at all temperatures could be active and selective for equilibrium syngas yields. Post run burn-offs 20
after all studies showed very little carbon formation after any of the experiments, indicating carbon did not impact catalyst activity. 5. Acknowledgements The project was supported by The Louisiana Board of Regents (Grant # LEQSF(2016-17)-ENHTR-07).
6. References [1] Sehested J. Sintering of nickel steam-reforming catalysts. J. Catal. 2003, 217(2): 417. [2] Sehested J. Four challenges for nickel steam-reforming catalysts. Catal. Today. 2006, 111(1– 2): 103. [3] Sehested J, Gelten J A P, Helveg S. Sintering of nickel catalysts: Effects of time, atmosphere, temperature, nickel-carrier interactions, and dopants. Appl. Catal. A Gen. 2006, 309(2): 237. [4] Bussi J, Musso M, Quevedo A, Faccio R, Romero M. Structural and catalytic stability assessment of Ni-La-Sn ternary mixed oxides for hydrogen production by steam reforming of ethanol. Catal. Today. 2017, 296: 154. [5] Fang XZ, Zhang XH, Guo Y, Chen MM, Liu WM, Xu XL, Peng HG, et al. Highly active and stable Ni/Y2Zr2O7 catalysts for methane steam reforming: On the nature and effective preparation method of the pyrochlore support. Int. J. Hydrogen Energy. 2016, 41(26): 11141. [6] Kumar N, Roy A, Wang Z, L’Abbate E M, Haynes D, Shekhawat D, Spivey J J. Bireforming of methane on Ni-based pyrochlore catalyst. Appl. Catal. A Gen. 2016, 517: 211. [7] Zhang J, Guo XY, Jung Y-G, Li L, Knapp J. Lanthanum zirconate based thermal barrier coatings: A review. Surface and Coatings Technology. 2017, 323: 18. [8] Haynes D J, Berry D A, Shekhawat D, Spivey J J. Catalytic partial oxidation of n-tetradecane using Rh and Sr substituted pyrochlores: Effects of sulfur. Catal. Today. 2009, 145(1–2): 121. [9] Pakhare D, Shaw C, Haynes D, Shekhawat D, Spivey J. Effect of reaction temperature on activity of Pt- and Ru-substituted lanthanum zirconate pyrochlores (La2Zr2O7) for dry (CO2) reforming of methane (DRM). J. CO2 Util. 2013, 1(0): 37. [10] Haynes D J, Berry D A, Shekhawat D, Spivey J J. Catalytic partial oxidation of ntetradecane using pyrochlores: Effect of Rh and Sr substitution. Catal. Today. 2008, 136(3– 4): 206. [11] Haynes D J, Shekhawat D, Berry D A, Zondlo J, Roy A, Spivey J J. Characterization of calcination temperature on a Ni-substituted lanthanum-strontium-zirconate pyrochlore. Ceram. Int. 2017, 43(18): 16744. [12] Yang P, Li N, Teng JJ, Wu J, Ma H. Effect of template on catalytic performance of La0.7Ce0.3Ni0.7Fe0.3O3 for ethanol steam reforming reaction. J. Rare Earths. 2019, 37(6): 594.
21
[13] Rida K, Peña M A, Sastre E, Martinez-Arias A. Effect of calcination temperature on structural properties and catalytic activity in oxidation reactions of LaNiO3 perovskite prepared by Pechini method. J. Rare Earths. 2012, 30(3): 210. [14] Song J H, Yoo S, Yoo J, Park S, Gim M Y, Kim T H, et al. Hydrogen production by steam reforming of ethanol over Ni/Al2O3-La2O3 xerogel catalysts. Mol. Catal. 2017, 434: 123. [15] Prasad D H, Ji H I, Kim H R, Son J W, Kim B K, Lee H W, et al. Effect of nickel nanoparticle sintering on methane reforming activity of Ni-CGO cermet anodes for internal steam reforming SOFCs. Appl. Catal. B Environ. 2011, 101(3): 531. [16] King D L, Strohm J J, Wang X, Roh H-S, Wang C, Chin Y-H, et al. Effect of nickel microstructure on methane steam-reforming activity of Ni–YSZ cermet anode catalyst. J. Catal. 2008, 258(2): 356. [17] Pereñíguez R, González-DelaCruz V M, Holgado J P, Caballero A. Synthesis and characterization of a LaNiO3 perovskite as precursor for methane reforming reactions catalysts. Appl. Catal. B Environ. 2010, 93(3–4): 346. [18] Roine A, Lamberg P, Mansikka-aho J, Bjorklund P, Kettala J-P, Talonen T. HSC Chemistry Outotec Research Oy, 2007. [19] Arregi A, Lopez G, Amutio M, Artetxe M, Barbarias I, Bilbao J, et al. Role of operating conditions in the catalyst deactivation in the in-line steam reforming of volatiles from biomass fast pyrolysis. Fuel. 2018, 216: 233. [20] Ochoa A, Barbarias I, Artetxe M, Gayubo A G, Olazar M, Bilbao J, et al. Deactivation dynamics of a Ni supported catalyst during the steam reforming of volatiles from waste polyethylene pyrolysis. Appl. Catal. B Environ. 2017, 209: 554. [21] Park S Y, Oh G, Kim K, Seo M W, Ra H W, Mun T Y, et al. Deactivation characteristics of Ni and Ru catalysts in tar steam reforming. Renewable Energy. 2017, 105: 76. [22] Thalinger R, Gocyla M, Heggen M, Dunin-Borkowski R, Grünbacher M, Stöger-Pollach M, et al. Ni–perovskite interaction and its structural and catalytic consequences in methane steam reforming and methanation reactions. J. Catal. 2016, 337: 26. [23] Park Y S, Kang M, Byeon P, Chung S-Y, Nakayama T, Ko T, et al. Fabrication of a regenerable Ni supported NiO-MgO catalyst for methane steam reforming by exsolution. J. Power Sources. 2018, 397: 318. [24] Anspoks A, Kuzmin A. Interpretation of the Ni K-edge EXAFS in nanocrystalline nickel oxide using molecular dynamics simulations. J. Non-Cryst. Solids. 2011, 357(14): 2604. [25] Zhang Z, Verykios X E. Carbon Dioxide Reforming of Methane to Synthesis Gas ofer Ni/La2O3 Catalysts. Appl. Catal. A Gen. 1996, 138: 109. [26] Batiot-Dupeyrat C, Gallego G A S, Mondragon F, Barrault J, Tatibouët J-M. CO2 reforming of methane over LaNiO3 as precursor material. Catal. Today. 2005, 107-108: 474. [27] Garbujo A, Pacella M, Natile M M, Guiotto M, Fabro J, Canu P, et al. On A-doping strategy for tuning the TWC catalytic performance of perovskite based catalysts. Appl. Catal. A Gen. 2017, 544: 94.
22
[28] Xu Y, Harimoto T, Hirano T, Ohata H, Kunieda H, Hara Y, et al. Catalytic performance of a high-cell-density Ni honeycomb catalyst for methane steam reforming. Int. J. Hydrogen Energy. 2018, 43(33): 15975. [29] Xu Y, Ma Y, Demura M, Hirano T. Enhanced catalytic activity of Ni3Al foils towards methane steam reforming by water vapor and hydrogen pretreatments. Int. J. Hydrogen Energy. 2016, 41(18): 7352. [30] Wei J, Iglesia E. Structural requirements and reaction pathways in methane activation and chemical conversion catalyzed by rhodium. J. Catal. 2004, 225(1): 116. [31] Silva P P, Ferreira R A R, Noronha F B, Hori C E. Hydrogen production from steam and oxidative steam reforming of liquefied petroleum gas over cerium and strontium doped LaNiO3 catalysts. Catal. Today. 2017, 289: 211. [32] Rahaman MN. Ceramic Processing and Sintering. 2nd ed. Boca Raton, FL: Taylor and Francis Group, LLC; 2003:812.
Graphical Abstract
23
Activation in-situ 70
S/C=2.0, 800 °C; 0.23 or 1.8 MPa
60
Composition / vol% dry gas
Exposure to SMR reaction conditions
Fresh pyrochlore catalyst Lanthanum enriched surface
Ni
H2
50 40 30 20 CO 10 CO2 CH4
0 0
La2Zr2O7
5
10
15 Time on stream / h
20
25
30
Deactivation of Ni by steam
100 90 80
No pretreatment Reduction 10%H2/Ar
CH4 conversion / %
70
Ar only Steam/Ar mixture
60 50 40 30 20 10 0 0
5
10
15 Time on stream / h
20
25
Ni-based pyrochlore catalysts showed a complex interaction between Ni and an enriched La layer under reaction conditions which led to stable, equilibrium syngas production, or irreversible deactivation, depending on the reaction conditions.
24
30
Highlights •
The substitution of Ni into La2Zr2O7 pyrochlore produces Ni particles on the surface.
•
The excess La increasingly enriches the surface at calcination temperatures above 700°C.
•
The steam reforming activity of the Ni is influenced by the interaction between Ni and La.
•
More surface La inhibits the CH4 decomposition activity leading to an oxidation of Ni.
•
Certain conditions can form synergistic Ni-La sites that are active and stable for equilibrium syngas production.