Effects of alkali and alkaline-earth metal dopants on magnesium oxide supported rare-earth oxide catalysts in the oxidative coupling of methane

Applied Catalysis A: General 528 (2016) 175–190

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Effects of alkali and alkaline-earth metal dopants on magnesium oxide supported rare-earth oxide catalysts in the oxidative coupling of methane Trenton W. Elkins, Samantha J. Roberts, Helena E. Hagelin-Weaver ∗ Department of Chemical Engineering, University of Florida, Gainesville, FL, 32611, USA

a r t i c l e

i n f o

Article history: Received 3 July 2016 Received in revised form 2 September 2016 Accepted 20 September 2016 Available online 22 September 2016 Keywords: Oxidative coupling of methane Magnesium oxide supported rare earth oxides Alkali and alkaline-earth metal dopants CO2 desorption XRD XPS

a b s t r a c t The oxidative coupling of methane (OCM) to ethane and ethylene was studied over alkali metal (Li and Na) and alkaline earth metal (Mg and Ca) doped rare earth oxides (Sm2 O3 , TbOx , PrOy and CeO2 ) catalysts supported on nanoparticle magnesium oxide (n-MgO). It was found that the Li-TbOx /n-MgO catalyst outperformed all other combinations of rare earth oxide and dopant, in terms of initial activity and selectivity, except at temperatures below 600 ◦ C where the undoped Sm2 O3 /n-MgO, Ca-Sm2 O3 /n-MgO and Ca-CeO2 /n-MgO catalysts resulted in higher C2+ yields. Li doping was most efficient in yielding C2+ products over all rare earth oxides, followed by doping with Na. These dopants result in the most basic sites on the surface according to CO2 desorption experiments. However, these sites are blocked at lower reaction temperatures, which can explain why most Li- and Na-doped catalysts are not active below 650 ◦ C. Below reaction temperatures of 650 ◦ C moderately basic sites, which bind CO2 up to ∼350 ◦ C, are important and correlate well with OCM activity. Li doping also resulted in the highest ethylene yields, which is a more desirable product than ethane, and the highest ethylene yield (9.7%) was obtained over the Li-CeO2 /n-MgO (at 800 ◦ C). However, Li-doped catalysts are not very stable, and the Na-TbOx /n-MgO catalyst therefore outperformed the Li-TbOx /n-MgO catalyst after only a few hours on stream. Removal of both Li and Na during reaction was also confirmed and is probably the reason for the loss in selectivity with time on stream. Tb2 O3 is the only TbOx phase observed with XRD on the Na- and Li-doped TbOx /nMgO catalysts after reaction and this is important since Tb2 O3 is likely more selective to C2+ products than TbO1.81 in the OCM reaction. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Research on the oxidative coupling of methane (OCM) has increased in intensity over the past decade due to the potential for converting natural gas directly to higher value chemicals, such as ethylene [1,2]. In the early OCM literature, a large focus was on surveying catalysts to find the highest C2+ selectivity, even at the compromise of overall C2+ yield [3,4–6]. The non-reducible rareearth oxides (REOs), in particular Sm2 O3 but also La2 O3 , were found to be the most active and selective single component OCM catalysts [4–9]. In contrast, reducible REOs, such as TbOx , CeO2 and PrOy (where 1.7 ≤ x,y ≤ 2.0) are more selective toward complete oxidation of methane rather than OCM [10,11]. The benefits of doping an oxide catalyst with an alkali metal in the OCM reaction were first discovered in the Li/MgO catalyst

∗ Corresponding author. E-mail address: [email protected]fl.edu (H.E. Hagelin-Weaver). http://dx.doi.org/10.1016/j.apcata.2016.09.011 0926-860X/© 2016 Elsevier B.V. All rights reserved.

[7,12,13]. Due to the high initial selectivity and activity of this system, Li/MgO was long the main comparison for any OCM study. However, while catalyst stability was rarely probed in the earlier studies, it has become a key component of the OCM literature, since the Li/MgO catalyst was found to deactivate rapidly during time on stream experiments due to Li volatility [14,15]. The high selectivity of the Li-MgO catalyst system motivated the development of other catalysts, utilizing the promoting effects of alkali metals, such as Li, Na, and K, on supports like MgO and CaO and on a number of rare earth oxides [10,11,16–22]. Alkaline-earth metal oxides have also been used favorably as dopants or supports for rare earth oxides in the OCM reaction [23–31]. These studies reveal that alkali and alkaline earth metal dopants can significantly increase the selectivity to C2 hydrocarbons in the OCM reaction, and in some cases also the CH4 conversion. However, stability is still an issue, particularly for alkali metal dopants. One of the most active and stable OCM catalyst systems to date is the SrO/La2 O3 or La-Sr/CaO catalyst [32–34], which results in yields comparable to those of Mn-Na2 WO4 /SiO2 , which is another highly active and stable OCM catalyst [35–44].

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It is generally well-accepted that the initial step in the OCM reaction is a hydrogen abstraction from CH4 over an active oxygen site [12,15]. Over many catalysts, this is likely the rate-determining step, and it is believed to occur over a basic oxygen site [15]. The resulting methyl radical is then either ejected into the gas phase to couple with another methyl radical and form C2 H6 , or it adsorbs on the surface and is oxidized further to CO and CO2 . Ethylene (C2 H4 ) is believed to form as a secondary product due to dehydrogenation of C2 H6 on the surface of the catalyst. The H-abstraction generates hydroxyl groups, which react to form water and generates an oxygen vacancy after desorption. To regenerate the active oxygen species, O2 from the gas phase must be activated. This is another potentially challenging step, and may be the limiting step over some catalysts [45,46]. Despite all of the studies on OCM catalysts [15,47], more research is needed to understand the nature of the active sites in these complex catalyst systems. Most reports in the OCM literature utilize the REOs as unsupported catalysts (i.e. single component catalyst) or as a ‘support’ for alkali or alkaline-earth dopants [5–11,16–19,23–30], although there are also studies where the REO is used as a dopant for alkaline earth oxides [48–50]. Despite the extensive research on alkali metals and alkaline-earth metals as dopants for REOs, not much research has been dedicated to the co-impregnation of these materials (REO plus dopant) onto a catalyst support, i.e. a three-component catalyst system [11,17–20,33,51,52], although some sequential preparations have been reported [53–55]. This is important, since it has been shown previously that supported rare-earth oxides using a fraction of the valuable material can surpass or at least compete with the activity of the pure component [20,56,57]. Our previous study revealed that very active and selective REO-based OCM catalysts can be prepared by co-impregnating a nanoparticle magnesium oxide (n-MgO) with lithium and terbia or samaria [57]. The most active and selective catalyst in that study was Li-TbOx /n-MgO, which outperformed both Sm2 O3 /n-MgO and Li-Sm2 O3 /n-MgO at temperatures as low as 650 ◦ C. While more stable than Li-Sm2 O3 /n-MgO (and Li/MgO), the Li-TbOx /n-MgO did exhibit some deactivation during a 30-h time on stream experiment. Therefore, the focus in the current study was to determine if other alkali or alkaline-earth dopants can yield highly active and selective REO/n-MgO catalysts that are more stable than the LiTbOx /n-MgO catalyst (and other Li-doped catalysts). As reducible rare earth oxides are not expected to be selective toward coupling of methane (they are more selective toward CO and CO2 ), it was decided to include CeO2 and PrOy in the investigation to determine if they behave similarly to doped TbOx , or if Li-doped TbOx /n-MgO is unique in giving a highly active and selective OCM catalyst. While some effects of alkali and alkaline earth metal dopants on REOs have been investigated previously, the studies are limited in that they do not report any characterizations apart from reaction data, or they investigate only a few dopants on a specific REO or the effects of limited dopants on different REOs [10,28–30,33,49,54,55]. No comprehensive investigation, including detailed characterizations, of alkali and alkaline earth metal dopants on MgO-supported Sm2 O3 , TbOx , PrOy , and CeO2 have been reported previously. Our study was also motivated by theoretical work which predicts diverse effects of dopants on oxide catalysts due to charge effects on a local scale [58–60].

2. Experimental 2.1. Catalyst preparation Catalysts were synthesized via an incipient wetness impregnation (IWI) technique onto a low surface area nanoparticle magnesium oxide support (n-MgO received from NanoScale

Materials Inc. [61]) using two consecutive loadings of an aqueous solution containing the nitrate salt precursors (99.9% REO, Alfa Aesar). In summary, an appropriate amount of REO and alkali/alkali-earth salt precursor was dissolved in a volume of DI water twice the volume of the pores of the n-MgO support. Half of the precursor solution was added slowly to the support under continuous stirring until the onset of incipient wetness. The catalyst was then dried at 80 ◦ C for two hours before the second loading of the solution was added to the catalysts. The catalyst was dried for another two hours at 80 ◦ C after the second impregnation before drying at 105 ◦ C overnight followed by calcination at 800 ◦ C for 4 h. The catalyst preparation for the undoped REO/n-MgO catalyst followed the same procedure, but only one impregnation step was needed. The weight percentages of the alkali/alkali-earth metals (2.5 wt%) and REO (20 wt%) were constant, resulting in catalyst compositions of 2.5/20/77.5 M-REO/n-MgO (where M represents the dopant, i.e, Li, Na, Ca, or Mg) and 20/80 REO/n-MgO, respectively, where REO = Sm2 O3 , TbOx , CeO2 or PrOy . 2.2. Catalyst characterization methods Brunauer-Emmett-Teller (BET) surface area measurements were performed on all catalysts using a 6-point isotherm on a Quantachrome NOVA 1200 instrument operating at liquid nitrogen temperatures. All correlation values were greater than 0.999. X-ray diffraction (XRD) patterns were collected on a X’Pert Powder analytical system using a Cu source (␭ = 0.154 nm) using a step size of 2␪ = 0.004. Catalysts were mounted on a glass slide using double-sided tape. Crystalline sizes were calculated using the Scherrer equation. Lattice parameter calculations were performed to analyze any unit cell contraction or expansion as a result of dopant incorporation into or metal cation reduction of the REO or MgO present in the catalyst. The lattice structures can be related to the Bragg equation (Eq. (1)) via the d-spacing (d) calculated and the Miller indices. n = 2d sin 

(1)

Both the REO and MgO crystallite volumes are present in the cubic structure which has the following relationship between the crystal’s planar values (h,k, and l), lattice axial distance (a) and the d-spacing (Eq. (2)). 1 h2 + k 2 + l 2 = 2 d a2

(2)

The reference values of a for each cubic REO or MgO are a = 4.22 Å for MgO (JCPDS#: 98-008-8058), a = 5.47 Å for PrO1.83 (JCPDS#: 00006-0329), a = 5.41 Å for CeO2 (JCPDS#: 98-062-1705), a = 5.29 Å for TbO1.81 (JCPDS#: 98-002-8916), and a = 10.92 Å for Sm2 O3 (JCPDS#: 98-064-7461). CO2 temperature programmed desorption (TPD) measurements were performed on a CHEMBET 3000 instrument. 250 mg of catalyst was loaded into a quartz u-tube and was outgassed under a He flow at 800 ◦ C for 30 min prior to CO2 exposure at 200 ◦ C for 30 min. After allowing physically absorbed CO2 to outgas at 200 ◦ C for 30 min, the catalyst was heated to 800 ◦ C at a rate of 10 ◦ C/min. Temperature measurements were recorded by a thermocouple inside the quartz u-tube (on the outlet side) and a TCD detector measured the amount of desorbed gas. XPS measurements were performed on a PerkinElmer 5100 XPS system equipped with an aluminum X-ray source. To minimize water adsorption on the catalysts, they were dried at 105 ◦ C overnight before the measurements. Survey scans used a time/step of 30 ms with 0.5 eV increments and a pass energy of 89.45 eV. Narrow scans used a time/step of 50 ms with 0.1 eV step sizes and a pass energy of 35.75 eV. A total of 10 scans were collected for surveys

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

while 25 scans were collected for narrow scans. Charge shift corrections were done by setting the C 1s peak at 284.6 eV. No signs of differential charging were detected on the fresh catalysts. Samples were prepared by placing a thin layer of catalysts on double-sided carbon conductive tape.

177

line earth metal-doped REO/n-MgO catalysts were subjected to methane coupling experiments and catalyst characterizations to determine the effects of the different dopants (Li, Na, Mg and Ca) on the REO/n-MgO catalyst system. 3.1. OCM activity measurement

2.3. OCM experiments Catalytic activity measurements were carried out in a quartz tube reactor with an inner diameter of 10 mm, using a previously reported system [56,57,62]. Each catalyst was pressed into a pellet using a Carver pellet press, then crushed and sieved to a size range of 180–250 ␮m. For loading the reactor, 0.4 g of the sieved catalyst was supported in the quartz reactor tube between two pieces of quartz wool. CH4 , O2 , and an internal standard (N2 ) were fed through the system at a rate of 160 standard cm3 (sccm) (with N2 constant at 23.2 sccm) using three mass flow controllers (MFCs), which results in a GHSV of 2400 h−1 . The CH4 :O2 feed ratio was held constant at 4:1 for all experiments. Under these conditions, there is no observable activity up to 800 ◦ C in a reactor tube containing only quartz wool. Furthermore, the reaction conditions were chosen to make sure that the O2 conversion is 100%, or close to 100%, at all times. We have shown previously that running at less than 100% O2 conversion is detrimental for the reaction, while the results are not changed significantly if the reaction is in a regime of 100% O2 conversion [63]. The catalytic activity and selectivity were measured as a function of temperature for each catalyst. Measurements started at 500 ◦ C and were then increased by 50 ◦ C increments until 800 ◦ C. The first measurement at 500 ◦ C was taken 15 min after reaching the reaction temperature to ensure a steady state measurement. Two of the best catalysts were monitored for 30 h on stream at the optimum operating temperature to observe the stability of the catalyst, as alkali metals (particularly lithium) have been shown to be volatile under OCM reaction conditions. As described previously [57], we report two methane conversions, one based on the overall methane converted (XCH4 , Eq. (3)) and one based on the methane converted to COx and C2+ products ∗ , Eq. (4)). The C (XCH 2+ selectivity is based on the COx and C2+ prod4 ∗ ucts only (Eq. (5)) and the C2+ yield is calculated using XCH (Eq. 4 (6)). XCH4 =

CH4,in − CH4,out CH4,in

(3)

∗ XCH =

[2 · (C2 H6 + C2 H4 ) + 3 · (C3 H8 ) + CO + CO2 ]out CH4,in

(4)

SC2+ =

sccm CH4 reacted to C2 and C3 products sccm CH4 reacted to COx , C2 , and C3 products

(5)

4

∗ YC2+ = XCH · SC2+ 4

(6)

3. Results and discussion Since the Li-TbOx /n-MgO catalyst outperformed the LiSm2 O3 /n-MgO catalyst both in terms of C2+ yield and stability, it was decided to investigate the effects of other dopants on TbOx /nMgO and Sm2 O3 /n-MgO catalysts to determine if Li is unique or if other alkali or alkaline earth metal dopants can yield similar activities and selectivities and at the same time exhibit a higher stability. Furthermore, as TbOx catalysts and other reducible REOs are not expected to be very selective OCM catalysts, CeO2 and PrOy were included in the study to determine if the results from LiTbOx /n-MgO are specific for TbOx or if other reducible rare earth oxides can be highly active and selective OCM catalysts with the use of dopants. Therefore, the prepared undoped and alkali or alka-

3.1.1. Doped Sm2 O3 /n-MgO catalysts The reaction results for the doped Sm2 O3 /n-MgO catalysts are shown in Fig. 1a. The undoped and alkaline-earth doped (Ca2+ and Mg2+) samaria catalysts exhibit a reasonable activity already at 500 ◦ C and exhibit a gradual increase in C2+ yield with temperature until the 650–700 ◦ C range. Thus, below 650 ◦ C these catalysts are more active and selective than the most active Li-doped Sm2 O3 /nMgO catalyst. Undoped Sm2 O3 /n-MgO exhibits a maximum C2+ yield of 11.1% at 650 ◦ C. Above 700 ◦ C the C2+ yield decreases for all catalysts mainly due to a loss in C2+ selectivity, although some catalysts also exhibit a slight drop in CH4 conversion, particularly at the highest temperature of the investigation, i.e. 800 ◦ C. The latter is likely due to a loss in active surface area at this high temperature. The Ca-Sm2 O3 /n-MgO catalyst exhibits a higher yield than the undoped Sm2 O3 /n-MgO catalyst in the whole range of temperatures investigated, and is, therefore, the Sm2 O3 /n-MgO catalyst with the highest C2+ yield below a temperature of 600 ◦ C. A maximum C2+ yield of 12.0% is obtained at a temperature of 700 ◦ C for this catalyst and at this temperature the C2 H4 /C2 H6 ratio is 1.2, which corresponds to a C2 H4 yield of 6.4% (Tables 1–3). Since the C2 H4 /C2 H6 ratio increases with temperature, the maximum C2 H4 yield obtained from the Ca-doped Sm2 O3 /n-MgO catalyst is 7.9% at 800 ◦ C and this is significantly higher than that obtained over the undoped Sm2 O3 /n-MgO catalyst (6.1% at 800 ◦ C, Table 3). Only above 700 ◦ C is there a benefit from adding Mg2+ to the Sm2 O3 /n-MgO catalyst. The maximum yield obtained from the MgSm2 O3 /n-MgO catalyst is 11.9% at a temperature of 750 ◦ C. Another benefit of adding Mg2+ to the Sm2 O3 /n-MgO catalyst is the higher C2 H4 /C2 H6 product ratio (1.5) at 750 ◦ C compared with undoped Sm2 O3 /n-MgO (which exhibits a C2 H4 /C2 H6 product ratio of 1.0 at the optimum conditions, Table 2). However, the C2 H4 /C2 H6 product ratios are lower than for the Ca-Sm2 O3 /n-MgO catalyst at temperatures above 700 ◦ C, so the maximum C2 H4 yield is lower (7.7%) over the Mg-Sm2 O3 /n-MgO catalyst (Table 3). The highest CH4 conversions and C2+ selectivities were observed over the Li-Sm2 O3 /n-MgO and Na-Sm2 O3 /n-MgO catalysts. However, compared with the alkaline-earth doped (Ca2+ and Mg2+) samaria catalysts, the alkali-doped catalysts show very little activity up to 600 ◦ C and are therefore less active than the undoped Sm2 O3 /n-MgO catalyst in this temperature range. A dramatic increase in activity and selectivity is observed at 650 ◦ C, which is the optimum reaction temperature for both Li- and Na-doped Sm2 O3 /n-MgO. The maximum yield for Li-Sm2 O3 /n-MgO is 15.1%, while that for Na-Sm2 O3 /n-MgO is 13.7%. In addition to the higher yield, the Li-doped catalyst also exhibits a higher C2 H4 /C2 H6 ratio than the Na-Sm2 O3 /n-MgO catalyst. At the optimum conditions (650 ◦ C) the ratios are 1.1 and 0.7, which corresponds to C2 H4 yields of 8.4 and 6.8% for the Li- and Na-doped Sm2 O3 /n-MgO catalysts, respectively. The C2 H4 /C2 H6 ratios increase with temperature, and reach values of 3.2 and 2.5 for the Li- and Na-doped samaria catalysts. However, since the activity and selectivity of the NaSm2 O3 /n-MgO is higher than for the Li-doped catalyst above 700 ◦ C, the maximum C2 H4 yield (8,6%) is obtained over the Na-Sm2 O3 /nMgO catalyst (Table 3). While the Li-Sm2 O3 /n-MgO exhibits one of the highest C2 H4 yields at 650 ◦ C (all other catalysts require temperatures on the order of 800 ◦ C for a C2 H4 yield of 8%), this catalyst is not very stable, as expected from a Li-doped catalyst. This is evident in a significant loss in activity and selectivity above 650 ◦ C for the Li-Sm2 O3 /n-MgO catalyst. While this could be a temperature effect,

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Sm2O3/n-MgO Catalysts

12 10 8 6 4 2

70 20

60 C2+ Selectivity [%]

14

C2+ Yield [%]

80

25

a) CH4 Conversion to COx and C2+ [%]

16

15 10 5

50 40 30 20 10

0

0

0 500 550 600 650 700 750 800

500 550 600 650 700 750 800

500 550 600 650 700 750 800

o

o

o

Temperature [ C]

Temperature [ C]

Temperature [ C]

TbOx/n-MgO Catalysts 16

C2+ Yield [%]

12 10 8 6 4 2

70 20

60 C2+ Selectivity [%]

b) CH4 Conversion to COx and C2+ [%]

14

80

25

15 10 5 0

500 550 600 650 700 750 800

500 550 600 650 700 750 800 o

o

Temperature [ C]

Temperature [ C]

CeO2/n-MgO Catalysts

10 8 6 4 2

70 20

60 C2+ Selectivity [%]

CH4 Conversion to COx and C2+ [%]

C2+ Yield [%]

80

25

12

20

0

o

14

30

500 550 600 650 700 750 800

Temperature [ C]

c)

40

10

0

16

50

15 10

50 40 30 20

5 10 0

0 500 550 600 650 700 750 800

0 500 550 600 650 700 750 800

500 550 600 650 700 750 800

o

o

Temperature [ C]

o

Temperature [ C]

Temperature [ C]

PrOy/n-MgO Catalysts 16

d)

C2+ Yield [%]

12 10 8 6 4 2

70 20

60 C2+ Selectivity [%]

CH4 Conversion to COx and C2+ [%]

14

80

25

15 10

0 500 550 600 650 700 750 800

5

o

40 30 20 10

0

0 500 550 600 650 700 750 800 o

Temperature [ C]

50

Temperature [ C]

500 550 600 650 700 750 800 o

Temperature [ C]

Fig. 1. OCM reaction results for undoped and doped rare earth oxide catalysts supported on nanoparticle magnesia (n-MgO). (a) Sm2 O3 /n-MgO, (b) TbOx /n-MgO, (c) CeO2 /nMgO, and (d) PrOy /n-MgO. Left: C2+ yield, Middle: CH4 conversion to COx and C2+ products, and Right: C2+ selectivity. Symbols for dopants are as follows: undoped catalyst: , n-MgO support:

, Mg2+ :

, Ca2+ :

, Na+ :

, Li+ :

.

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179

Table 1 Reaction data for the oxidative coupling of methane for bare support, undoped and doped REO/n-MgO catalysts at 700 ◦ C collected during temperature series studies. Catalyst Descriptiona

XCH4 [%]b

∗ XCH [%]c

SC2+ [%]d

YC2+ [%]e

SSA [m2 /g]

MgO

21.9

18.0

40.4

7.3

20.4

Sm2 O3 /n-MgO Li-Sm2 O3 /n-MgO Na-Sm2 O3 /n-MgO Ca-Sm2 O3 /n-MgO Mg-Sm2 O3 /n-MgO

24.3 24.4 25.5 25.4 24.4

21.3 21.1 23.7 22.2 21.7

52.3 62.5 57.8 54.3 54.2

11.1 13.2 13.7 12.0 11.7

26.0 2.7 5.5 23.6 15.4

TbOx /n-MgO Li-TbOx /n-MgO Na-TbOx /n-MgO Ca-TbOx /n-MgO Mg-TbOx /n-MgO

19.8 24.9 24.2 21.8 19.4

18.0 22.9 22.0 18.9 17.1

41.2 63.6 55.5 43.9 37.6

7.4 14.5 12.2 8.3 6.4

21.8 5.9 5.8 25.2 23.5

PrOy /n-MgO Li-PrOy /n-MgO Na-PrOy /n-MgO Ca-PrOy /n-MgO Mg-PrOy /n-MgO

19.0 25.6 22.0 21.8 17.8

17.1 21.3 19.6 18.9 16.1

36.0 60.6 48.4 43.9 31.5

6.1 12.9 9.5 8.3 5.1

24.8 2.1 4.3 19.8 21.9

CeO2 /n-MgO Li-CeO2 /n-MgO Na-CeO2 /n-MgO Ca-CeO2 /n-MgO Mg-CeO2 /n-MgO

16.7 22.1 24.4 22.5 16.8

14.1 17.5 21.8 20.2 14.5

15.3 54.9 56.0 48.1 18.6

2.2 9.6 12.2 9.7 2.7

24.5 3.4 6.8 19.8 16.4

a b c d e

4

REO loading is 20% by weight. Dopant loading is 2.5% by weight. Overall methane conversion. Calculated as (CH4in − CH4out )/CH4in . CH4 conversion to COx and C2 products only. Percent of CH4 converted to C2 products. ∗ Calculated as XCH · SC2+ . 4

Table 2 Product distribution at 700 ◦ C obtained over bare support, undoped and doped REO/n-MgO catalysts (collected during temperature series studies). Catalyst Descriptiona

Product Distribution [%] C2 H4

C2 H6

CO2

CO

C2 H4 /C2 H6 ratio

CO2 /CO ratio

MgO

23.2

17.3

54.8

4.7

1.3

11.7

Sm2 O3 /n-MgO Li-Sm2 O3 /n-MgO Na-Sm2 O3 /n-MgO Ca-Sm2 O3 /n-MgO Mg-Sm2 O3 /n-MgO

26.3 38.2 30.1 28.8 29.2

25.3 22.8 27.2 25.0 24.6

44.3 33.6 35.7 42.1 42.0

4.1 5.4 7.0 4.0 4.2

1.0 1.7 1.1 1.2 1.2

10.8 6.2 5.1 10.4 9.9

TbOx /n-MgO Li-TbOx /n-MgO Na-TbOx /n-MgO Ca-TbOx /n-MgO Mg-TbOx /n-MgO

16.4 36.8 25.1 19.1 13.2

24.7 25.5 30.1 24.5 22.3

57.8 36.2 44.1 55.6 62.9

1.2 1.6 0.8 0.9 1.6

0.7 1.4 0.8 0.8 0.6

48.0 22.6 56.5 59.2 39.0

PrOy /n-MgO Li-PrOy /n-MgO Na-PrOy /n-MgO Ca-PrOy /n-MgO Mg-PrOy /n-MgO

14.3 39.2 19.2 14.8 11.1

21.6 20.9 28.9 23.6 20.3

62.7 34.9 50.9 60.2 67.5

1.4 5.0 0.9 1.4 1.1

0.7 1.9 0.7 0.6 0.5

44.4 6.9 55.4 43.8 62.8

CeO2 /n-MgO Li-CeO2 /n-MgO Na-CeO2 /n-MgO Ca-CeO2 /n-MgO Mg-CeO2 /n-MgO

6.6 36.2 27.3 24.0 8.2

8.7 18.1 28.4 23.9 10.4

77.7 32.8 43.1 50.5 75.1

7.1 12.8 1.3 1.6 6.3

0.8 2.0 1.0 1.0 0.8

11.0 2.6 33.4 30.9 12.0

a

REO loading is 20% by weight. Dopant loading is 2.5% by weight.

it is more likely that it is due to prolonged time on stream. Li-doped catalysts are known to deactivate under OCM conditions [15], and at 700 ◦ C the catalysts have been in the reactor at OCM conditions for at least 4 h (6 h by the time of the measurements at 800 ◦ C). In contrast, the Na-Sm2 O3 /n-MgO catalyst is rather stable between 650 and 750 ◦ C, and can thus outperform the Li-Sm2 O3 /n-MgO at temperatures above 650 ◦ C. 3.1.2. Doped TbOx /n-MgO catalysts The reaction results for the doped TbOx /n-MgO catalysts are shown in Fig. 1b. The undoped TbOx /n-MgO yields a higher than

anticipated C2+ yield, as TbOx is expected to yield COx products over C2+ . While this is likely due to the fact that TbOx is supported on n-MgO, it is interesting to note that the TbOx /n-MgO is significantly more active and selective below 650 ◦ C than the bare n-MgO support. This synergistic effect suggests a strong interaction between the TbOx and n-MgO, and it appears that the MgO support alters the OCM properties of TbOx (or the TbOx alters the properties of the MgO). The catalyst preparation could have resulted in Tb-doped MgO or Mg-doped TbOx . Adding Mg2+ ions to the TbOx /n-MgO, significantly increases the C2+ selectivity, but decreases the CH4

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Table 3 Maximum C2 H4 yield obtained in the oxidative coupling of methane over the undoped and doped REO/n-MgO catalysts. Unless otherwise noted, the temperature which gives maximum C2 H4 yield is 800 ◦ C. Catalysta

CH4 Conversion to COx and C2+ [%]

C2+ Selectivity [%]

Yield [%]

C2 H4 Yield [%]

TbOx Li-TbOx Na-TbOx Ca-TbOx Mg-TbOx

18.9 21.7 22.1 19.7 18.6

43.7 58.0 54.9 46.8 43.1

8.3 12.6 12.1 9.3 8.0

5.3 8.8 8.0 6.4 3.4

Sm2 O3 Li-Sm2 O3 Na-Sm2 O3 Ca-Sm2 O3 Mg-Sm2 O3

19.7 20.3 21.4 21.3 21.4

46.3 55.0 56.7 50.9 51.2

9.1 11.2 12.1 10.8 11.0

6.1 8.5 8.6 7.9 7.7

CeO2 Na-CeO2 Li-CeO2 Ca-CeO2 Mg-CeO2

15.4 20.3 21.9 18.9 15.8

21.1 49.6 57.0 43.0 24.4

3.3 10.1 12.5 8.2 3.8

2.3 7.0 9.7 5.5 2.7

PrOy Li-PrOy b Na-PrOy Ca-PrOy Mg-PrOy

17.3 23.0 19.7 17.4 16.5

36.9 61.5 48.2 38.6 32.4

6.4 14.1 9.5 6.7 5.3

4.0 9.5 5.8 4.2 3.3

a b

REO loading is 20% by weight. Dopant loading is 2.5% by weight. Maximum C2 H4 yield was obtained at 750 ◦ C instead of 800 ◦ C.

conversion so that the C2+ yield remains the same below 650 ◦ C. At and above 650 ◦ C the activities and selectivities are similar for the Mg-TbOx /n-MgO, TbOx /n-MgO and n-MgO catalysts, The maximum C2+ yields are all the between 8.0 and 8,9% and are obtained at the highest temperature included in the study, i.e. 800 ◦ C. As for the Sm2 O3 /n-MgO catalyst, doping with Ca2+ results in a slight increase in both CH4 conversion and C2+ selectivity for the TbOx /n-MgO, although only above 550 ◦ C. The C2+ yield at 600 ◦ C is rather high at 8.6% and is fairly constant with increasing temperature for the Ca-doped TbOx /n-MgO catalyst, but the C2 H4 /C2 H6 product ratio increases from 0.6 at 600 ◦ C to 2.3 at 800 ◦ C, where a maximum C2+ yield of 9.3% is obtained. The maximum C2 H4 yield for the CaTbOx /n-MgO catalyst (6.4%) is obtained at 800 ◦ C, but it is lower than that obtained over the Ca-Sm2 O3 /n-MgO catalyst (8.6%) under the same conditions (Table 3). In contrast to the results over the Sm2 O3 /n-MgO catalysts, the Li-TbOx /n-MgO catalyst outperforms the Ca- and Mg-doped TbOx /n-MgO catalysts already at 550 ◦ C (over the Li-Sm2 O3 /n-MgO the high activity was not observed until 650 ◦ C). This is due to the highly active and selective Li-TbOx /n-MgO catalyst [57]. Sodium doping also results in a highly active OCM catalyst, but it is not as active as the Li-TbOx /n-MgO catalyst, and it requires a temperature of 600 ◦ C to outperform the undoped TbOx /n-MgO catalyst (Fig. 1b). As noted previously, the maximum yield over the Li-TbOx /n-MgO catalyst is 14.9% at 650 ◦ C. At these conditions, the C2 H4 /C2 H6 ratio is 1.1 resulting in a C2 H4 yield of 7.8%, while the maximum C2 H4 yield is 8.8% at 800 ◦ C (Table 3). For the Li-TbOx /n-MgO catalyst the overall C2+ yield decreases with increasing temperature above 650 ◦ C. In contrast, the yield is fairly constant or increases slightly with temperature for the Na-TbOx /n-MgO catalyst. The maximum yield over the Na-TbOx /n-MgO catalyst (12.7%) is therefore obtained at a higher temperature (750 ◦ C) compared with the Li-doped catalyst. As for the Li-doped Sm2 O3 /n-MgO catalyst, the decrease in C2+ yield with increasing temperature (above 650 ◦ C) for the Li-TbOx /n-MgO catalyst could be from deactivation due to prolonged time-on-stream (which would be expected to be more severe at the higher temperatures). The fact that the Na-TbOx /nMgO catalyst does not exhibit the same trend could indicate the Na-doped catalyst is more stable than the Li-TbOx /n-MgO catalyst. At higher temperatures and/or longer times on stream, the

Na-TbOx /n-MgO catalyst may therefore outperform the Li-TbOx /nMgO catalyst (vide infra). However, while the Na-doped catalyst may be more stable, it is not as efficient as Li in producing the desired C2 H4 product (Table 3). 3.1.3. Doped CeO2 /n-MgO catalysts As CeO2 is the most common “rare” earth oxide, it was decided to examine the effects of dopants also on ceria in the OCM reaction and determine if CeO2 had a similar behavior to TbOx with and without dopants. The reaction results for the doped ceria catalysts are presented in Fig. 1c. The CeO2 /n-MgO catalysts, collectively, were less active than the Sm2 O3 /n-MgO and TbOx /n-MgO catalysts. The undoped CeO2 /n-MgO catalyst reached a maximum C2+ yield of only 3.3% at 800 ◦ C. The main reason for the low yield is the significantly lower C2+ selectivity (21.1% maximum) compared with the undoped TbOx /n-MgO catalyst, although the CH4 conversion is also lower over this catalyst compared with the nMgO-supported Sm2 O3 and TbOx catalysts (Fig. 1a–c). A lower C2+ selectivity is expected from CeO2 compared with Sm2 O3 -based catalysts due to the deep oxidation behavior of ceria. However, since the C2+ yield obtained from undoped CeO2 /n-MgO (3.3% at 800 ◦ C) is significantly smaller than that obtained from the undoped TbOx n-MgO (8.3% at 800 ◦ C), it suggests that the interaction between CeO2 and MgO is different from the interactions between TbOx and MgO. The C2+ yield obtained from the CeO2 /n-MgO is significantly lower than that obtained from the bare n-MgO support (Fig. 1b and c), which likely indicates that CeO2 covers most of the support. This also suggests that TbOx is a better OCM catalyst than CeO2 . None of the CeO2 /n-MgO catalysts exhibited any activity at 500 ◦ C. Doping CeO2 /n-MgO with Mg2+ only results in a slight increase in C2+ selectivity. In contrast, adding Ca2+ to the CeO2 /nMgO catalyst dramatically improves the yield compared to the Mg2+ doped and undoped ceria catalysts, due to a significant increase in the C2+ selectivity as well as an increase in CH4 conversion. A maximum yield of 9.9% is observed over this catalyst at 650 ◦ C. In fact, the C2+ yield at 550 ◦ C (8.5%) is close to that obtained over the Ca-doped Sm2 O3 /n-MgO catalysts (10%). Thus by adding Ca2+ ions to a non-selective CeO2 /n-MgO catalysts, a catalyst which can compete with Ca-doped Sm2 O3 /n-MgO at low temperatures (550 ◦ C) is obtained. For comparison, the C2+ yield

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

over the Li-TbOx /n-MgO catalyst at 550 ◦ C is only 8.9%. While the C2 H4 /C2 H6 ratio is only 0.9 at 650 ◦ C where maximum conversion is obtained over the Ca-CeO2 /n-MgO catalyst, this ratio increases to 2.1 at 800 ◦ C which corresponds to a C2 H4 product yield of 5.5%. Doping the CeO2 /n-MgO catalyst with Na+ and Li+ ions yields more active and selective catalysts compared with Ca-doped CeO2 /n-MgO, at least above 600 ◦ C. While the Na- and Li-doped CeO2 /n-MgO catalysts have similar optimum yields, the light-off curves have very different shapes. The maximum yield for the NaCeO2 /n-MgO catalyst is 12.6% at 650 ◦ C, while the yield for the Li-CeO2 /n-MgO catalyst is 12.5% at 800 ◦ C and is still increasing with temperature. It appears that the Li-CeO2 /n-MgO catalyst is activated at a much higher temperature compared with both the Na-doped CeO2 /n-MgO and the Li-doped TbOx /n-MgO catalysts. Therefore, the Li-doped CeO2 /n-MgO catalyst has a very different behavior from the Li-TbOx /n-MgO catalyst, and Na+ is a better dopant than Li+ for CeO2 /n-MgO catalysts. However, since the C2 H4 product concentration increases with temperature, the C2 H4 /C2 H6 product ratio is 3.7 for the Li-doped CeO2 /n-MgO catalyst at the temperature of maximum C2+ yield (800 ◦ C), which corresponds to a C2 H4 product yield of 9.7% (Table 3). This is the highest ethylene yield observed in the study. The maximum C2 H4 product yield for the Na-CeO2 /n-MgO catalyst is only 8.0% (at 800 ◦ C). Therefore, more ethylene can be produced over the Li doped CeO2 /n-MgO compared with the Na-doped catalyst.

3.1.4. Doped PrOy /n-MgO Since the CeO2 /n-MgO and TbOx /n-MgO catalysts exhibited very different behavior both with and without dopants, another reducible oxide was included in the study, namely praseodymia (PrOy , where y is between 1.75 and 2). The subscript ‘y’ was chosen to differentiate the praseodymia phase from the terbia phase, TbOx . The reaction results for the doped and undoped PrOy /n-MgO catalysts are presented in Fig. 1d. The activity and C2+ selectivity obtained from the undoped PrOy /n-MgO catalyst are between those of TbOx /n-MgO and CeO2 /n-MgO catalyst, i.e. the C2+ yield obtained from the PrOy /n-MgO is lower than for TbOx /n-MgO but higher than for CeO2 /n-MgO. In contrast to the other REO/n-MgO catalysts, addition of Mg2+ ions to the PrOy /n-MgO catalyst results in a small but definite decrease in C2+ selectivity (and thus also C2+ yield) at all temperatures investigated. In addition to a lower C2+ selectivity, Mg2+ also results in a slight, but undesirable decrease in the C2 H4 /C2 H6 ratio and increase in the CO2 /CO ratio. Addition of Ca2+ to the PrOy /nMgO catalyst instead results in an insignificant increase in C2+ selectivity and thus also a slightly higher C2+ yield, but mainly at temperatures above 650 ◦ C. As expected from the results obtained over the other doped REO/n-MgO catalysts, addition of alkali metals, such as Na and Li, to the PrOy /n-MgO catalyst increases both activity and selectivity. Similar to the trends for Na- and Li-doped CeO2 /n-MgO catalysts, the Na-PrOy /n-MgO catalyst exhibits a higher yield at a lower temperature compared with the Li-PrOy /n-MgO catalyst. However, the Li-doped PrOy /n-MgO catalyst results in a significantly higher optimum C2+ yield compared with the Na-doped catalyst. This behavior and the shapes of the light-off curves for Li- and Na-doped PrOx /nMgO are similar to the Li- and Na-doped TbOx /n-MgO catalysts. However, the maximum C2+ yield for the Li-PrOy /n-MgO catalyst (14.1%) is not obtained until a temperature of 750 ◦ C and it is slightly lower than that obtained from the Li-TbOx /n-MgO catalyst (14.9%) at 650 ◦ C. Since the maximum yield is obtained at a higher temperature for the Li-PrOy /n-MgO catalyst, the C2 H4 /C2 H6 product ratio is higher (2.2) compared with the C2 H4 /C2 H6 ratio of 1.1 for the LiTbOx/n-MgO catalyst at 650 ◦ C. Consequently, a higher C2 H4 yield (9.5% at 750 and 800 ◦ C) can be obtained over the Li-PrOy /n-MgO

181

compared with the Li-TbOx/n-MgO catalyst (8.8% C2 H4 at 750 and 800 ◦ C). 3.1.5. Stability test of Na-TbOx /n-MgO While our previous 30-h stability test on the co-impregnated Li-TbOx /n-MgO catalyst revealed less deactivation than for an eight-hour run of a Li/MgO catalysts [57], it still lost roughly 30% of its initial activity over the 30 h. As the reaction results from the temperature study indicated that the Na-TbOx /n-MgO catalyst may be more stable than the Li-TbOx /n-MgO catalyst, it was also subjected to a 30-h stability test. The results are presented in Fig. 2 along with the data from the stability test on the Li-TbOx /n-MgO catalyst. While the Li-TbOx /n-MgO catalyst has a significantly higher initial activity than the Na-TbOx /n-MgO catalyst, the Li-doped catalysts suffers a higher rate of deactivation. Since the rate of deactivation is lower for the Na-doped catalyst, the two catalysts exhibit the same C2+ yield after six hours on stream. The deactivation rate for the Li-TbOx /n-MgO catalyst is lower after the initial four hours on stream, but it is still higher than the deactivation rate for the NaTbOx /n-MgO, which is fairly constant over the 30 h. Evidently, the Na-TbOx /n-MgO catalyst is more productive than the Li-TbOx /nMgO catalyst over 30 h. The decrease in yield with time on stream for the Li-TbOx /n-MgO catalyst is due to a significant loss in C2+ selectivity as well as a smaller decrease in CH4 conversion. In contrast, the loss in C2+ yield for the Na-TbOx /n-MgO catalyst appears to be due mainly to a slight loss in selectivity, as the conversion is rather constant during the 30 h. The maximum C2+ yield of 14.5% for the Li-TbOx /n-MgO has dropped to 10.1% after 30 h. For the NaTbOx /n-MgO catalyst, the maximum C2+ yield is 12.3%, but after 30 h the C2+ yield is still 11.3%. Thus, since the Na-TbOx /n-MgO catalyst is more stable it can outperform the Li-TbOx /n-MgO catalyst after only a few hours on stream. 3.2. Surface area measurements The BET surface area measurements of all fresh catalysts after calcination at 800 ◦ C for four hours are included in Table 1. The bare support surface area was also measured after calcination for comparison. As has been observed previously [57], addition of REOs to the support results in a slight increase in the surface area of the catalyst. Doping with alkaline-earth metals in most cases results in a small reduction in surface area compared with the undoped REO/n-MgO catalysts. In contrast, the alkali metals (both Na and Li) induce close to an order of magnitude decrease in catalyst surface area. Despite the drastic decrease in surface area, the CH4 conversion per unit mass of catalyst is the same or higher over the Liand Na-doped catalysts. This is an unusual behavior in catalysis, but is likely due to the fact that, after the initial C-H abstraction, the coupling of methyl radicals occurs in the gas phase. The drastic decrease in surface area is associated with a significant increase in selectivity, and this is consistent with the COx byproducts being formed via a surface reaction, while the coupling of methyl radicals takes place in the gas phase. The results are also consistent with the Na- and Li-doped catalysts being significantly more active per unit surface area compared with undoped and Mg- or Ca-doped REO/n-MgO catalysts. 3.3. CO2 TPD Since numerous previous studies have reported correlations between the number of strong basic sites and the C2+ yield in the OCM reaction [15,48,49,64–66], temperature programmed desorption (TPD) of CO2 is a commonly used characterization method for OCM catalyst. Therefore, CO2 TPD profiles were collected on all undoped, Ca- and Mg-doped REO/n-MgO catalysts after a CO2 exposure at 200 ◦ C. While our intent was to collect CO2 TPD profiles

182

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

a)

10

5

0

0

5

10

15

20

25

30

b)

60 CH4 Conversion, C2+ Selectivity [%]

C2+ Yield [%]

15

50 40 30 20 10 0

0

5

Time [hours]

10 15 20 Time [hours]

25

30

Fig. 2. 30 h stability test results for Li-TbOx /n-MgO and Na-TbOx /n-MgO. Reaction conditions: 700 ◦ C. (a): C2+ yield for Li-TbOx /n-MgO ( ) and Na-TbOx /n-MgO ( C2+ selectivity ( ) and methane conversion ( ) for Li-TbOx /n-MgO and C2+ selectivity ( ) and methane conversion ( ) for Na-TbOx /n-MgO.

of all our catalysts, the sodium- and lithium-doped catalysts, unfortunately, fused an alkali metal based material to the quartz reactor tubes used in the experiments. Therefore, TbOx was the only REO in which the alkali metal doped catalysts were subjected to CO2 TPDs in order to preserve the integrity of the equipment. It should be noted that while basic sites have been shown to be important in the OCM reaction, there is not always a straight correlation between the number or strength of basic sites and the C2+ yield [15,50]. This suggests that other properties are also important for OCM activity and selectivity, and the rate-determining step could be dependent on reaction conditions and specific catalyst composition. For example, the number and strength of acidic sites have been shown to affect the catalytic properties of OCM catalysts [26]. OCM catalysts can also be limited by regeneration of active oxygen sites, in which case the presence of defects sites could be more important than the initial presence of highly basic sites [45,46]. The TPD profiles for the undoped REO/n-MgO catalysts are dominated by a CO2 desorption peak located between 350 and 370 ◦ C, which is the same desorption temperature observed on the pure magnesia support (Fig. 3). Except for the CeO2 /n-MgO, all the REO catalysts have a higher CO2 uptake than the n-MgO support (Table 4). Thus, the TbOx /n-MgO, Sm2 O3 /n-MgO, and PrOy /n-MgO catalysts all have a higher number of basic sites compared with the n-MgO support. However, the most basic sites on the MgO support, which are located at higher desorption temperatures, a shoulder at 500 ◦ C and a minor CO2 desorption feature at 750 ◦ C, are smaller on the REO/n-MgO catalysts. The significantly lower CO2 uptake, and thus also smaller number of basic sites, on the CeO2 /n-MgO catalyst is consistent with its low C2+ yield. However, weak basic sites resulting in CO2 desorption below 500 ◦ C have been shown to be unimportant in the OCM reaction over supported alkaline earth doped REOs [54]. In contrast to previous observations, there appears to be a correlation between the intensity of the CO2 desorption peak at ∼360 ◦ C (and therefore the number of basic sites) and the activity (C2+ yield) in the OCM reaction over the undoped REO/n-MgO catalysts (Fig. 3 a and b). The most active catalyst, Sm2 O3 /n-MgO, has the highest number of basic sites (based on the area under the CO2 TPD curve, Table 4), followed by TbOx /nMgO and PrOy /n-MgO. The lowest CO2 adsorption is observed over the CeO2 /n-MgO catalyst, and the volume of CO2 adsorbed is even lower than the n-MgO support itself, which is consistent with the significantly lower OCM yield obtained from the CeO2 /n-MgO com-

) and (b)

Table 4 Volume of CO2 desorbed at different temperatures (after a CO2 adsorption at 200 ◦ C) calculated from the temperature programmed desorption data in Figs. 3a and 4. CO2 Desorption [␮L]a Catalyst

Low T

Int. T

High T

Total:

n-MgO (support) Sm2 O3 /n-MgO TbOx /n-MgO PrOy /n-MgO CeO2 /n-MgO

575 850 790 655 315

– – – – –

80 – 115 50 130

655 850 905 705 445

TbOx /n-MgO Mg-TbOx /n-MgO Ca-TbOx /n-MgO Na-TbOx /n-MgO Li-TbOx /n-MgO

790 715 530 295 –

– – 610 – 155

115 65 – 530 845

905 780 1140 825 1000

Ca-Sm2 O3 /n-MgO Ca-CeO2 /n-MgO Ca-TbOx /n-MgO Ca-PrOy /n-MgO

905 575 530 445

460 530 610 685

– – – –

1365 1105 1140 1130

a Volume of CO2 desorbed in different temperature intervals during the temperature programmed desorption experiment. The temperature regions are approximately: Low T: 200 ≤ T ≤ 500 ◦ C, Int. T: 500 < T ≤ 700 ◦ C, High T: T > 700 ◦ C.

pared with the other REO catalysts and the n-MgO support in this study. The n-MgO support also follows the trend between the number of basic sites and the C2+ yield, but only in a narrow temperature range around 600 ◦ C. This suggests that factors other than the basic sites resulting in CO2 desorption at 360 ◦ C are important for the nMgO support. It is possible that the bare MgO support is limited by regeneration of the active oxygen sites at low temperature, since it is expected to have fewer defect and no interfacial REO-MgO sites that could facilitate oxygen activation [45,46]. The higher activity at and above 750 ◦ C for the bare n-MgO support may be due to the more strongly basic sites on the n-MgO (the sites resulting in CO2 desorption at ∼750 ◦ C) compared with the REO/n-MgO catalysts. These strongly basic sites are likely blocked by CO2 at lower temperatures. The effect of dopants on the CO2 desorption from the TbOx /nMgO catalyst is shown in Fig. 4a. Compared with the undoped catalyst, all doped TbOx /n-MgO catalysts have stronger or a higher number of basic sites. The stronger basic sites are evident in CO2 desorption features at higher temperatures (above the ∼360 ◦ C feature). The TPD spectra obtained from the Mg-doped TbOx /n-MgO

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

Unpromoted REO/n-MgO

Unpromoted REO Catalysts 800

a)

Sm2O3/n-MgO PrOy/n-MgO n-MgO

500 400

CeO2/n-MgO 300

C2+ Yield [%]

o

600

Temperature [ C]

Intensity [Arbitrary Units]

700

TbOx/n-MgO

183

16

Sm2O3

14

TbOx

12

n-MgO

10

b)

PrOy CeO2

8 6 4 2

200 0

20

40

60

80

100

Time [min]

0 450 500 550 600 650 700 750 800 o

Temperature [ C]

Fig. 3. (a) CO2 desorption profiles as a function of temperature obtained from REO/n-MgO catalysts. (b) C2+ yield as a function of temperature obtained from the same REO/n-MgO catalysts as in (a). The C2+ yield in (b) is calculated from the C2+ selectivity and CH4 conversion in Fig. 1.

is similar in shape to the TPD spectra of n-MgO, but the number of basic sites (area under the CO2 -TPD curve) is similar to the TbOx /n-MgO catalyst and therefore higher than on the n-MgO support (Fig. 4a). As may be expected from the similar basic properties of the TbOx /n-MgO and Mg-TbOx /n-MgO catalysts, there is not a significant difference in OCM activity between these catalysts. This is true also for the other REO catalysts, i.e. there is not a significant difference in the TPD spectra (Fig. 4b) or the OCM activities (Fig. 1a–d) of the undoped and the Mg-doped REO/n-MgO catalysts. The only exception is Mg-PrOy /n-MgO catalyst, where Mg doping increases the number and the strength of the basic sites, while the activity and selectivity decreases. This again reveals that the number of basic sites is not the only important property in the OCM reaction. Furthermore, the benefits of MgO in these catalysts cannot be improved further by addition of more Mg ions during the catalyst preparation. After doping with Ca2+ ions there is a significant increase in the volume of CO2 adsorbed on the TbOx /n-MgO catalyst due to a new sharp feature at 630 ◦ C from more strongly adsorbed CO2 (Fig. 5a). Thus, calcium doping produces a large number of strong basic sites (stronger than those of the MgO support) on the catalyst surface compared with undoped TbOx /n-MgO. This CO2 desorption feature is consistent with decomposition of CaCO3 to CaO and CO2 [67]. While the more basic Ca-doped catalyst results in a slightly more productive catalyst than the TbOx /n-MgO and Mg-TbOx /n-MgO catalysts, the difference in C2+ yield is not as significant as may have been expected from the high intensity of the feature at 630 ◦ C, if these types of basic sites were important in the OCM reaction. This is true for all catalysts, i.e. all Ca-doped REO catalysts have a welldefined CO2 desorption feature around 630 ◦ C (Fig. 5a), but this only slightly improves the catalytic activity and selectivity to methane coupling (Fig. 1a–d). This could be the reason why studies involving CaO as a support do not show any or an inverse correlation between basic sites and activity or selectivity in the OCM reaction [22,49]. Only in the case of CeO2 -based catalysts, is the Ca-doped catalyst significantly more active compared with the undoped CeO2 /n-MgO catalyst (Fig. 1d). The Ca/CeO2 combination has been shown previously to be more effective than Mg/CeO2 and Sr/CeO2 catalyst combinations and this was attributed to an increase in electrophilic oxygen [30]. Plotting the C2+ yield as a function of Ca doped REO/nMgO catalysts (Fig. 5b) and comparing this with the in CO2 TPD data (Fig. 5a, Table 4), it appears that there is a trend, but it is not

the expected one where the catalysts with the strongest basic sites are more active and selective in OCM. The strongly basic sites associated with CaO do not seem to be important to OCM activity or selectivity over these catalysts. Instead, the C2+ yield increases as the number of weaker basic sites increases on the Ca-doped catalysts. The desorption feature at ∼360 ◦ C increased significantly with Ca-doping over the CeO2 /n-MgO catalyst, and this is likely the reason for the increase in OCM activity over this catalyst. Therefore, the trend observed on the undoped REO/n-MgO catalysts, i.e. the C2+ yield increases with the intensity of the CO2 desorption peak at ∼360 ◦ C, is the same over the Ca-doped REO/n-MgO catalysts (Fig. 5a and b). One explanation for why the very basic sites associated with CaO do not appear to be important in the OCM reaction could be that these sites are limited by either oxygen vacancy formation (needed to remove the hydroxyl groups after the hydrogen abstraction) or by regeneration of the reactive oxygen species from gas phase oxygen under the conditions of the experiments. Also, even though the same amount of CaO was added to each catalyst, and there is not a significant difference in specific surface area between the catalysts, the area under the CO2 desorption peak associated with CaO (Ca(CO3 ) decomposition) varies significantly between the Ca-doped REO/n-MgO catalysts. This suggests that the interaction strength between Ca-REO vary over these catalysts. Doping the TbOx /n-MgO catalyst with alkali metals results in the formation of considerably stronger basic sites compared with those obtained from doping with alkaline-earth metals (more specifically Ca). The Na-TbOx /n-MgO catalyst exhibits a broad CO2 desorption peak at 725 ◦ C, which is at a significantly higher desorption temperature than the feature at 630 ◦ C observed over the Ca-doped catalysts. The basic sites associated with the 725 ◦ C desorption feature appear to be important in the OCM reaction, as the CO2 desorption peak at ∼360 ◦ C is smaller on the Na-TbOx /n-MgO compared with the undoped TbOx /n-MgO catalyst, yet, the NaTbOx /n-MgO catalyst is more active and selective, at least at higher temperatures. The total number of the weaker basic sites has, therefore, decreased (the area under the ∼360 ◦ C peak is smaller). One reason for the smaller amount of CO2 desorbed from the Na-TbOx /n-MgO is the considerably lower surface area of this catalyst compared to the undoped and Ca- plus Mg-doped catalysts. However, it is evident that the strength of the basic sites on the NaTbOx /n-MgO is more important than the number of weak basic sites on these catalysts, since the Na-doped TbOx /n-MgO catalyst is more

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

Mg-Promoted REO/n-MgO

Doped TbOx/n-MgO

800

a)

800

b)

700

o

Na 500 400

Ca Mg

Mg-Sm2O3/n-MgO Mg-PrOy/n-MgO

600

o

600

Intensity [Arbitrary Units]

Li

Temperature [ C]

Intensity [Arbitrary Units]

700

Temperature [ C]

184

Mg-TbOx/n-MgO

500

n-MgO Mg-CeO2/n-MgO

400

300

300

n-MgO 200

200

Un

0

20

40

60

80

0

100

20

Time [min]

40

60

80

100

Time [min]

Fig. 4. CO2 desorption profiles as a function of time and temperature obtained from (a) undoped and doped TbOx /n-MgO catalysts and (b) Mg-doped REO/n-MgO catalysts. The CO2 desorption profile obtained from the n-MgO support was also included for comparison.

Ca-Promoted REO/n-MgO

Ca-Doped REO/n-MgO

800

14

700

12 o

600

Temperature [ C]

Intensity [Arbitrary Units]

Ca-Sm2O3

16

Ca-CeO2

500 400

Ca-TbOx

C2+ Yield [%]

a)

b)

Ca-Sm2O3 Ca-CeO2 Ca-TbOx Ca-PrOy

10 8 6 4

300 2

Ca-PrO2 0

20

40

60

80

200 100

Time [min]

0 500 550 600 650 700 750 800 o

Temperature [ C]

Fig. 5. (a) CO2 desorption profiles as a function of time and temperature obtained from Ca-REO/n-MgO catalysts (solid lines). Dashed lines are the CO2 desorption profiles for the same undoped REO/n-MgO catalyst. (b) C2+ yield as a function of temperature obtained from the Ca-REO/n-MgO catalysts in (a). The C2+ yield in (b) is calculated from the C2+ selectivity and CH4 conversion in Fig. 1.

active than the undoped and Ca-doped TbOx /n-MgO catalysts. Considering the low surface area of the Na-TbOx /n-MgO catalyst, this means that the Na-doped catalyst is significantly more active per unit surface area compared with the undoped or Ca-doped TbOx /nMgO catalyst. However, the Na-doped catalyst is not active at the lower temperatures investigated as part of this study. The higher temperature required for activation of this catalyst, is likely due to the very basic sites being blocked by CO2 below 600 ◦ C. Since CO2 always forms as a byproduct during reaction it must be removed to regenerate the active site. Active sites being blocked by CO2 is consistent with a recent DFT study revealing that adsorbed CO2 on La2 O3 is likely a poison to the reaction [68]. At 600 ◦ C a sufficient amount of CO2 has likely desorbed from the surface to generate free basic sites (due to the 10 ◦ C per minute ramp rate during the TPD experiment, the peak is at a higher temperature than would be observed at a lower ramp rate or during an isothermal experiment), and once the reaction is initiated the local temperature likely

increases to desorb more CO2 which can explain the sharp increase in activity between 550 and 600 ◦ C. The most active OCM catalyst of the ones investigated, LiTbOx /n-MgO, has a very different CO2 TPD profile compared to the other doped and undoped REO/n-MgO catalysts. This catalyst does not have a CO2 desorption peak at ∼360 ◦ C, instead it has a major feature at 800 ◦ C and a smaller feature at 560 ◦ C. Thus, this catalyst has a high number of strongly and very few weakly basic sites. As for the Na-doped catalyst, Li-doping is associated with a drastic decrease in surface area. This could explain why no CO2 feature from the support is observed. However, it appears more likely that lithium doping alters the nature of the magnesia basic sites and results in the formation of only strongly basic sites, rather than a complete absence of MgO support sites (particularly considering the CO2 TPD curve from the Na-TbOx /n-MgO, which also suffered from a drastic decrease in surface area but still had a small CO2 desorption peak at ∼360 ◦ C). Thus, the Li-TbOx /n-MgO is the cat-

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

alyst with the strongest basic sites, and it is also the most active OCM catalyst of those included in the investigation. Although the CO2 desorption data were not collected for all Na- and Li-doped REO catalyst (since both Na and Li comes off the surface during these measurements), it is expected, due to the similar trends in OCM activity and the same drastic decrease in specific surface areas (Table 1), that Na and Li dopant will yield qualitatively similar TPD spectra to the Na- and Li-doped TbOx /n-MgO, i.e. Na increases the basic strength (although the number of basic sites decreases due to a lower surface area), and the Li yields the strongest basic sites for each REO. The fact that the Li-doped catalysts are not active below 550 ◦ C (and in some cases not below 650 ◦ C) is likely because the strongly basic, active sites are blocked below those temperatures. It is interesting to note that the Li-doped TbOx /n-MgO catalyst, which binds CO2 strongly up to 750 ◦ C is active at 550 ◦ C. This may suggest that the basic sites yielding a CO2 desorption temperature around 550 ◦ C are sufficiently active to increase the temperature of the catalyst locally. The order of OCM activity of the doped TbOx catalysts, i.e.: TbOx /n-MgO ≈ Mg-TbOx /n-MgO < Ca-TbOx /n-MgO < NaTbOx /n-MgO < Li-TbOx /n-MgO, follows the trend in basic site strength obtained from the CO2 TPD data (Figs. 1b and 4a). However, there is not a straight-forward correlation between the basic strength or the number of basic sites and the catalytic activity and/or selectivity. Very strong basic sites (CO2 desorption features at and above 700 ◦ C) appear to be favorable in the OCM reaction, since Na- and Li-doped REO/n-MgO are the most active catalysts. Again, the fact that these catalysts are not active OCM catalysts below 600 ◦ C is likely because the highly active and strongly basic sites are blocked below this temperature. However, catalysts without very strong basic sites are also active, and they are active at lower temperatures, since the CO2 byproduct is not blocking the weaker basic sites. Therefore, below 600 ◦ C the weaker basic sites likely dominate the OCM activity, and can explain why the OCM activity of the non-doped REO/n-MgO correlates with the number of the weaker basic sites. Clearly, basic sites, or rather active oxygen sites, are important in the OCM reaction, as they are needed for the hydrogen abstraction step, and a large number of different oxides with varying properties are active in the OCM reaction [15,47]. However, the presence of basic sites is not the only requirement for a highly active OCM catalyst. This is very evident for the Ca-doped catalysts, as they have strong basic sites associated with CaO (resulting in a CO2 TPD feature at 620–630 ◦ C), yet there is no correlation between the intensity of the CaO-derived CO2 TPD feature and the catalytic activity or selectivity in the OCM reaction. There is likely a delicate balance between the basic properties of the active oxygen sites necessary for the hydrogen abstraction, the regeneration of these sites (water formation from the hydroxides, oxygen vacancy formation, i.e. water desorption, or regeneration of the active oxygen sites from gas phase O2 ), as well as CO2 poisoning on these complex OCM catalysts. 3.4. XRD The XRD patterns obtained from all fresh catalysts (after calcination at 800 ◦ C for four hours) and a few selected spent catalysts were collected in order to determine the crystalline phases present. The bare n-MgO support, after calcination at 800 ◦ C for four hours, was also subjected to XRD measurements to investigate the effect of depositing different rare-earth oxides on the MgO crystallite size. The bare n-MgO support after calcination at 800 ◦ C for 4 h has an average crystallite size of 20 nm determined by the Scherrer equation (Table 5). As reported previously, the main phases for the Sm2 O3 /n-MgO catalyst are cubic Sm2 O3 and cubic MgO with little or no impurities (Fig. 6a), and the terbia phase in the TbOx /n-MgO catalyst

185

Table 5 XRD crystallite sizes of the bare support, undoped and doped REO/n-MgO catalysts determined by the Scherrer equation. Catalyst Descriptiona

Crystallite size [nm] Sm2 O3

TbOx

CeO2

PrOy

MgO

MgO

–

–

–

–

20

Sm2 O3 /n-MgO Li-Sm2 O3 /n-MgO Na-Sm2 O3 /n-MgO Ca-Sm2 O3 /n-MgO Mg-Sm2 O3 /n-MgO

11 55 43 16 23

– – – – –

– – – – –

– – – – –

21 37 36 21 23

TbOx /n-MgO Li-TbOx /n-MgO Li-TbOx/n-MgOb Na-TbOx /n-MgO Na-TbOx /n-MgOb Ca-TbOx /n-MgO Mg-TbOx /n-MgO

– – – – – – –

12 16 39 27 43 15 14

– – – – – – –

– – – – – – –

24 35 41 39 42 25 24

CeO2 /n-MgO Li-CeO2 /n-MgO Na-CeO2 /n-MgO Ca-CeO2 /n-MgO Mg-CeO2 /n-MgO

– – – – –

– – – – –

14 68 55 19 19

– – – – –

28 37 39 27 30

PrOy /n-MgO Li-PrOy /n-MgO Na-PrOy /n-MgO Ca-PrOy /n-MgO Mg-PrOy /n-MgO

– – – – –

– – – – –

– – – – –

22 43 48 22 19

24 47 45 27 21

a b

REO loading is 20% by weight. Dopant loading is 2.5% by weight. Spent catalysts (8 h time on stream).

is consistent with a slightly reduced TbO1.81 phase [57]. The XRD pattern obtained from CeO2 /n-MgO reveals that the ceria phase is cubic in the fluorite structure. Similar to the TbOx /n-MgO catalyst, the PrOy in the PrOy /n-MgO catalyst is in a slightly reduced cubic phase, and the best match according to the XRD database is with a PrO1.83 phase (Fig. 6a). Impregnation of the n-MgO support with REO nitrate precursor results in a slight increase in MgO crystallite size after calcination (Table 5). The largest difference is observed for CeO2 , where the average MgO crystallite size increased from 20 to 28 nm after impregnation of cerium nitrate and subsequent calcination. The REO crystallite sizes of the fresh catalysts range from 11 nm for the Sm2 O3 to 22 nm for the PrOy phase (Table 5). The effects on the catalyst crystal structures from doping the REO/n-MgO catalysts with alkali and alkaline earth metals depend on the dopant and in some cases also on the specific REO. As several of the XRD patterns obtained from the doped REO/n-MgO are very similar to the undoped REO/n-MgO, only selected XRD patterns are presented in Figs. 6–8 (all XRD data are provided in Supporting information, Fig. S1). In general, it appears that the Ca and Mg dopants result in less crystalline catalysts compared with undoped or Li- and Na-doped catalysts. This is evident as a lower signal-to-noise in the XRD patterns obtained from Ca- and Mg-doped catalysts, and is shown for the Sm2 O3 /n-MgO catalysts in Fig. 6b. In contrast, doping with Na or Li yields significantly higher REO peak intensities (Fig. 6b), revealing more crystalline materials compared to the undoped catalysts. According to the Scherrer equation, both the REO and the MgO particle sizes are larger on the Li- and Na-doped catalysts compared to the undoped and the Mg- and Ca-doped REO/n-MgO (Table 5). In most cases, the average REO crystallite size has increased from the original 10–20 nm to 40–60 nm. For the MgO support the average crystallite has increased from 20–25 nm to 35–45 nm after REO deposition (Table 5), except for the CeO2 /n-MgO catalyst which exhibits a smaller increase (from ∼28 to ∼38 nm) due to the larger initial particle sizes. The larger crystallite sizes after Li and Na doping are consistent with the lower BET surface areas observed on these cat-

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Undoped REO/n-MgO Catalysts

a)

Doped Sm2O3/n-MgO Catalysts

b)

c)

Me-Sm2O3/n-MgO

Li

Li

Na

Na

Intensity [Arbitrary Units]

Intensity [Arbitrary Units]

Pr

Ce

Tb

Ca Ca

Mg

Mg

Un

Sm 20

Un 30

40

50

60

70

80

25

30

35 40 2θ [degrees]

2θ [degrees]

45

50

28

30

32

34

2θ [degrees]

Fig. 6. XRD patterns obtained from (a) undoped REO/n-MgO catalysts, (b) doped Sm2 O3 /n-MgO catalysts, and (c) enlarged section of dashed region in (b). (夽) cubic MgO, (䊏) cubic Sm2 O3 , (䊉) cubic TbO1.81 , () cubic CeO2 , () cubic PrO1.83 , (䊐) monoclinic Sm2 O3 .

Li-Doped REO/n-MgO a)

b)

c) Mg

Mg

Ce

Tb

Ca

Na Li

Ca

Intensity [Arbitrary Units]

Intensity [Arbitrary Units]

Pr Intensity [Arbitrary Units]

MgO(002), M-TbOx/n-MgO

TbOx(111), M-TbOx/n-MgO

Na

Li Un

Un

Sm 20

30

40

50

60

28

2θ [degrees]

29

30

2θ [degrees]

31

n-MgO 42

43 2θ [degrees]

44

Fig. 7. (a) XRD pattern obtained from Li-doped REO/n-MgO catalysts. (夽) cubic MgO, (䊏) cubic Sm2 O3 , (䊉) cubic TbO1.81 , () cubic CeO2 , () cubic PrO1.83 , (䊐) monoclinic Sm2 O3 , (×) Li2 O2 , (♦) LiPrO2 , () hexagonal Li2 CO3 , () monoclinic Li2 CO3 . (b) TbOx (111) XRD peak obtained from doped TbOx /n-MgO catalysts. (c) MgO(002) XRD peak obtained from doped TbOx /n-MgO catalysts. Dashed lines in (b) and (c) mark the positions of the peaks from the undoped (Un) TbOx /n-MgO catalyst. The red dotted line marks the position of the MgO(002) peak obtained from the bare n-MgO support. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

alysts compared with the undoped and Ca- or Mg-doped catalysts (Table 1). Significant particle growth was observed with TEM after Li doping in a previous publication [57], and it appears that both Li and Na cause particle growth for all REO/n-MgO catalysts (consistent with the BET data). Apparently, the increase in activity (per unit surface area) with addition of Li- and Na-dopants more than compensates for the negative effects that a lower surface area would be expected to have on the methane conversion (Fig. 1a–d). A lower specific surface area is also expected to increase the selectivity as the contribution from undesirable surface reactions will be lower. In addition to the general trends discussed above, some REOs exhibit features specific to the dopant used. As mentioned, most doped REO/n-MgO catalysts only exhibit peaks from the cubic REO and the MgO, but there are a few exceptions. For doped Sm2 O3 /n-

MgO both Li and Na, and to some extent also Mg and Ca, induce a higher concentration of monoclinic Sm2 O3 [JCDPS 98-004-2696], although the cubic Sm2 O3 phase is still dominant in all cases (Fig. 6b and c). In fact, there may be a small amount of monoclinic Sm2 O3 present also on the undoped Sm2 O3 /n-MgO, but the signals are close to the noise level for this catalyst (Fig. 6c). The crystal structure of Sm2 O3 could be important, since the CH4 conversion is higher on cubic compared with monoclinic Sm2 O3 [69]. The C2 selectivity is also lower over monoclinic compared with cubic Sm2 O3 [11], but it is possible that the addition of dopants have a larger effect on the selectivity in this case than the crystal structure of Sm2 O3 . As indicated previously [57], peaks associated with Li2 CO3 [JCPDS 0726635] are reasonably consistent with the peaks observed in the XRD pattern obtained from Li-TbOx /n-MgO. However, the peaks are also

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

Na-TbOx/n-MgO

Li-TbOx/n-MgO

b) Intensity [Arbitrary Units]

a) Intensity [Arbitrary Units]

187

Spent

Fresh

Spent

Fresh 20

30

40

50

20

60

30

2θ [degrees]

40 50 2θ [degrees]

60

Fig. 8. XRD pattern obtained from fresh and spent (a) Li-doped and (b) Na-doped TbOx /n-MgO catalysts. (夽) cubic MgO, (䊉) cubic TbO1.81 , () Tb2 O3 , (×) Li2 O2 .

consistent with a Li2 O2 phase [JCPDS 98-015-2183], and the peak match is higher for the Li2 O2 phase according to the HIGH Score Plus software on the instrument. Still, the presence of Li2 CO3 cannot be excluded, particularly since carbonates are known to be prevalent on the Li-TbOx /n-MgO [57]. Hexagonal as well as monoclinic Li2 CO3 phases [JCPDS 98-009-6486 and 98-006-6942, respectively] are both evident in the XRD pattern obtained from Li-PrOy /n-MgO (Fig. 7), and appear to match up better than for the XRD pattern obtained from Li-TbOx /n-MgO. In addition, LiPrO2 [JCPDS 00-0190726] is observed in the XRD pattern obtained from Li-PrOy /n-MgO. The formation of this phase can explain why the PrO1.83 (111) peak in the XRD pattern obtained from Li-PrOy /n-MgO has a lower intensity compared to all other PrOy /n-MgO catalysts. In fact, the formation of LiTbO2 [JCPDS 98-002-1013] would explain why the TbO1.81 (111) peak in the pattern obtained from Li-TbOx /n-MgO has a significantly lower intensity compared to all other TbOx /n-MgO catalysts. While the Li2 CO3 and LiPrO2 phases are easily distinguishable in the XRD pattern obtained from Li-PrOy /n-MgO, the LiTbO2 , Li2 CO3 , and Li2 O2 all have very similar 2␪ positions and are therefore difficult to unambiguously assign and more than one, or all of them, could be present on the Li-TbOx /n-MgO catalyst. To investigate the effects of doping on the structure of the REOs and the MgO, the most intense peaks REO (111) and MgO (002) were examined in detail. Since several of the trends are similar, only selected XRD patterns are presented in Fig. 7b and c (all XRD peaks are available in Supporting information, Figs. S2 and S3). Compared to the n-MgO support, the MgO(002) peaks obtained from the doped and undoped REO/n-MgO catalysts have all shifted to higher 2␪ values, which suggests that the REOs distort the MgO matrix and cause a slight contraction of the lattice. Since all REO and dopant cations in the study are larger than Mg2+ , the contraction is likely due to charge effects from ions entering or interacting with the MgO lattice. However, unit cell volume calculations for the MgO phase reveal that the lattice contractions are not significant (Table 6). In an attempt to determine if added dopants enter the REO lattice, unit cell volumes were calculated also for the REO(111) peaks (Table 6). Doping with Li, Na, Mg, and Ca does cause shifts in the REO(111) peaks, but the shifts are dependent on the dopant and the REO. In general, it appears that the peak shifts follow the same trend for both the REO(111) and MgO(200) peaks (see Figs. 7b and c plus S2 and S3 in Supporting information). However, in most cases no significant differences in the unit cell volumes were observed revealing no definitive evidence of dopant incorporation (Table 6). Only for the Ca- and Na-doped Sm2 O3 /n-MgO and the Li-TbOx /n-MgO catalysts do the unit cell calculations reveal signif-

Table 6 Unit cell volume calculations from XRD patterns. REO/n-MgO Catalysta

MgO Volume (Å3 )

REO Volume (Å3 )

n-MgO Support

74.99

– Sm2 O3

Sm2 O3 Li-Sm2 O3 Na-Sm2 O3 Ca-Sm2 O3 Mg-Sm2 O3

74.64 74.60 74.40 74.21 74.58

1305.03 1304.06 1295.51 1286.39 1301.08 TbOx

TbOx Li-TbOx , fresh Li-TbOx , spent Na-TbOx , fresh Na-TbOx , spent Ca-TbOx Mg-TbOx

74.61 74.75 74.13 74.50 74.35 74.28 74.60

147.80 143.39 152.22 146.85 153.09 146.43 148.19 PrOy

PrOy Li-PrOy Na-PrOy Ca-PrOy Mg-PrOy

74.44 73.91 74.41 74.51 74.74

162.19 163.07 162.53 162.14 163.32 CeO2

CeO2 Li-CeO2 Na-CeO2 Ca-CeO2 Mg-CeO2

74.41 74.34 73.75 74.22 74.06

157.46 157.23 156.42 157.29 156.63

a

REO loading is 20% by weight. Dopant loading is 2.5% by weight.

icant changes in the REO lattice (Table 6). This further supports the strong interactions observed between Li and TbOx in our previous study [57]. The fact that this is not observed in the Li-Sm2 O3 /n-MgO may explain why this catalyst is significantly less stable compared with the Li-TbOx /n-MgO catalyst. After exposure to reaction conditions the average TbOx particle sizes are larger on both the Li-TbOx /n-MgO and Na-TbOx /n-MgO catalysts, and there is a slight increase also in the n-MgO particle size (Fig. 8 and Table 5). If the Li-TbOx /n-MgO contains two Tb-containing compounds, TbO1.81 and LiTbO2 , this would explain the small peaks due to TbO1.81 on the fresh catalyst. On the spent Li-TbOx /n-MgO only the peaks due to TbOx can be detected, and they are significantly more intense compared with the fresh catalyst. This behavior would be expected if LiTbO2 is converted to a TbOx compound. However, as mentioned, the peaks on the fresh

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

TbOx/n-MgO Catalysts

Ca-TbOx Mg-TbOx

O 2s

Li-Tb

Na 2p

Na 2s

Na-TbOx

Intensity [Arbitrary Units]

Na-TbOx

Intensity [Arbitrary Units]

O KLL

Na 1s

Li-TbOx

c)

Li 2s Mg 2p

Tb 3d

Li-TbOx

Mg 2s

b)

a)

Intensity [Arbitrary Units]

TbOx/n-MgO Catalysts

TbOx/n-MgO Catalysts Tb 4d

188

Ca-TbOx

Mg-TbOx

Na-Tb

Ca-Tb

Mg-Tb

TbOx

TbOx

1300

1200

1100

1000

Binding Energy (eV)

900

Tb

200

100 Binding Energy (eV)

0

534

532 530 528 Binding Energy [eV]

526

Fig. 9. XPS spectra obtained from undoped and doped TbOx /n-MgO catalysts. (a) XPS survey spectra from 1300 to 840 eV, (b) XPS survey spectra from 205 to 0 eV, and (c) high resolution XPS spectra of the O 1s region.

catalyst could also be due to Li2 O2 or Li2 CO3 . The peaks due to TbO1.81 on the fresh catalysts have shifted to slightly lower 2␪ values for both the spent Li- and Na-doped TbOx /n-MgO catalysts. This together with the appearance of additional XRD peaks, reveal the formation of Tb2 O3 on the spent catalysts (as identified previously [57]). Lattice calculations indicate an increase in the TbO1.81 unit cell, which is consistent with the reduction from Tb4+ to Tb3+ , since the Tb4+ ion is smaller (1.02 Å) than the Tb3+ ion (1.18 Å). The reduction from TbO1.81 to Tb2 O3 could be very important in the reaction, as Tb2 O3 would be expected to be a more active OCM catalyst than for example TbO1.81 or TbO2 . Since the TbOx crystallites on the Naand Li-doped catalysts are about the same size (∼40 nm) after 8 h under OCM conditions, variations in TbOx particle growth do not explain the differences in deactivation rate between the Na- and Li-doped catalysts (Table 5).

3.5. XPS XPS data was collected on the doped TbOx /n-MgO catalyst to further investigate the effects of dopants on this catalyst. Data for the Li-TbOx /n-MgO and TbOx /n-MgO XPS spectra was previously collected [57], and are included in this study to allow comparisons with the other doped catalysts. Unfortunately, it was quickly discovered that XPS analysis of these catalysts is challenging, at best. The XPS survey spectra are, of course, dominated by peaks from magnesium and oxygen and are not very informative if the entire region is displayed (see Fig. S4 in Supporting information). Only if narrow regions from the survey are presented can differences between catalysts be observed (Fig. 9a and b). Quantitative analysis is complicated by overlapping peaks and low signal-to-noise (i.e. low concentrations and small atomic sensitivity factors). Therefore, only qualitative differences between the spectra obtained from the various catalysts will be discussed. The region at high binding energy, just below the Mg 1s peak down to approximately 850 eV, includes the Tb 3d, Na 1s (if present) and oxygen Auger (O KLL) peaks (Fig. 9a). It is evident that this is a region with a lot of secondary electrons which increases the baseline level, and the Tb 3d peaks are therefore superimposed on a noisy background. From the surveys it is evident that the Tb signal is lower on the Mg-doped versus the undoped TbOx /n-MgO catalyst, which suggests that the added Mg covers some of the TbOx . A large Na 1s peak is observed on the Na-TbOx /n-MgO catalyst (as

expected), and it appears that a small amount of Na is present also on the Li-doped TbOx /n-MgO catalyst, but the peak is very close to the noise level. In the low binding energy region from 105 down to 0 eV, the Mg 2p and 2s peaks at 50 and 88 eV dominate the spectra, but the O 2s peak is also evident at 23 eV (Fig. 9b). In addition to these peaks, a small Li 2s peak is visible at 55 eV in the spectrum obtained from Li-TbOx /n-MgO and the Na 2s and 2p peaks at 64 and 31 eV respectively are present in the spectrum obtained from Na-TbOx /nMgO. The Ca 3s and 3p peaks at 45 and 26 eV are not of sufficient intensity to be detected, and the more intense 2s and 2p peaks around 350 eV overlaps with the much more intense Mg KLL peaks (Fig. S4 in Supporting information). This region (Fig. 9b) reveals that the Tb:Mg ratio is higher on the doped versus undoped TbOx /nMgO catalysts, and this is particularly true for the Li- and Na-doped TbOx /n-MgO catalysts. High resolution spectra of the O 1s, C 1s, and Tb 3d5/2 XPS peaks were also collected to obtain more information about oxidation states of species on the surface of these catalysts. However, limited information can be obtained from the O 1s, C 1s and Tb 3d5/2 peaks, due to overlapping peaks and other complications. The C 1s region of the spectra does not provides any new information (Fig. S5a), and this region is also complicated by the fact that the sample is supported on carbon conductive tape (which means that there can be contributions to the spectra from the sample holder) and the Tb 3p3/2 peak at 285 eV overlaps the C 1s peak. It was determined previously that the Li-TbOx/n-MgO catalyst exhibits a higher amount of carbonate species at ∼289 eV in the near surface region compared to the undoped terbia catalyst [57]. Carbonates are also evident in the O 1s region at and above 531 eV (Fig. 9c). However, the O 1s peak due to carbonates overlaps with any contribution to the O 1s peak from hydroxyl groups, as well as from electrophilic surface oxygen (O22− ) [30]. On the fresh catalysts, the carbonates and hydroxyls are due to air exposure, which is minimized but cannot be avoided, during sample transfer. In addition to the O 1s peaks at and above 531 eV due to carbonates and hydroxyl groups (oxygen species OII ), the TbOx and MgO oxides result in an O 1s feature around 529.0 to 529.6 eV (oxygen species OI ). Therefore, there are at least four different species present on the surfaces of these catalysts, and deconvoluting the O 1s peak using peak fitting with more than two peaks, even though it has been done before [30], is rather arbitrary. In fact, the O 1s peak can be deconvoluted into only two

T.W. Elkins et al. / Applied Catalysis A: General 528 (2016) 175–190

Doped TbOx/n-MgO Catalysts C 1s

O 2s

Mg 2s

Mg 2p

Tb 4d

Intensity [Arbitrary Units]

Na Fresh

Na 2p

Na Spent

Na Spent

Na Fresh

Mg 2p

c)

b)

Intensity [Arbitrary Units]

O KLL

Na 1s

Tb 3d

Intensity [Arbitrary Units]

TbOx/n-MgO Catalysts

Doped TbOx/n-MgO Catalysts a)

189

Li-Tb Spent

Li 1s Li-Tb

Li Spent

Li Spent Li-Fresh

Tb

Li Fresh

1200 1100 1000 900 Binding Energy (eV)

300

250

200

150

100

50

0

Binding Energy (eV)

55

50

45

Binding Energy [eV]

Fig. 10. XPS spectra obtained from fresh and spent Li- and Na-doped TbOx /n-MgO catalysts. (a) XPS survey spectra from 1290 to 840 eV, (b) XPS survey spectra from 305 to 0 eV, and (c) high resolution XPS spectra of the Mg 2p region. Note: the spectral regions of the spent catalysts have been shifted by different amounts to line up with the main Mg 2p and O KLL peaks.

main features with a decent peak fit, the OI at ∼529.5 eV and the OII at 531 eV. This was done previously for Li doped and undoped TbOx /n-MgO [57]. As can be seen in Fig. 9c, the OII /OI ratio increases after doping, but, as may have been anticipated due to the complexity of the CO2 desorption data, there is not an observable trend which correlates with the strength or number of basic sites. This is of course due to the fact that the O 1s peak is composed of several oxygen-containing species. The Tb 3d5/2 peaks are broad and also provide limited information (Fig. S5b in Supporting information). It was shown in a previous study that lithium doping induces a one eV shift in BE of the Tb 3d5/2 peak [57], and it appears that this is the only dopant which causes a significant shift in the Tb 3d5/2 binding energy. As noted from the survey spectra, the Tb 3d5/2 peak intensity is lower for the Mg-doped TbOx /n-MgO, indicating a lower surface concentration. This is likely due to the Mg dopant covering some of the terbia on the surface, and can explain the slightly lower activity of this catalyst compared to the undoped TbOx /n-MgO. The most active catalysts, Na-TbOx /n-MgO and Li-TbOx /n-MgO, were subjected to XPS analysis also after reaction. Unfortunately, the catalysts after reaction suffer from differential charging, and it is therefore not possible to get accurate data on the oxidation states (as different parts of the spectra experience varying levels of charging). However, some qualitative information can be obtained. The surface concentration of terbia is lower for both catalysts after reaction, and the carbon content is higher (Fig. 10a and b). While some Tb could have been removed during reaction, it is likely that the higher carbon content is due to carbon deposition on the Tb species and this is why the surface concentration of terbia is lower on the spent catalysts. The high carbon concentration is the reason for the charging issues on the spent catalysts. It is also evident that both Na and Li are removed from the surface during reaction. The intense Na 1s peak obtained from the fresh catalyst is close to the noise level on the spent Na-TbOx /n-MgO catalyst in Fig. 10a. The Li 2s peak has a low intensity already on the fresh catalyst, and cannot be detected on the spent Li-TbOx /n-MgO catalyst (Fig. 10c). In fact, for the Li-TbOx /n-MgO catalyst the Li concentration is below the detection limit already after eight hours of reaction. Removal of Li during reaction is known to result in loss of OCM activity and selectivity [15], and it is possible that the loss of Na explains the decrease in C2+ yield with time on stream also for the Na-TbOx /n-MgO. As expected from the increase in TbOx particle size (Table 5), the nearsurface concentration of Tb is lower after reaction on both the Na-

and Li-doped TbOx /n-MgO catalysts. However, as mentioned, preferential carbon deposition on the terbia can also explain the lower Tb peak intensities after reaction. A loss in terbia surface concentration may be expected to affect the CH4 conversion more than the C2+ selectivity, while the opposite is true for the loss in Li and Na. Since the loss in CH4 conversion is smaller on the Na- versus Li-doped catalyst, it appears that the loss in TbOx surface concentration is not as serious as the loss in alkali metal concentration, which likely is the reason the C2+ selectivity decreases with time on stream. 4. Conclusion A number of rare earth oxides supported on MgO were prepared and tested for activity in the oxidative coupling of methane. Sm2 O3 was included as one of the most active single component OCM catalysts, and three reducible REOs (TbOx , PrOy and CeO2 ) were included to investigate the effects of alkali and alkaline-earth metal dopants on OCM activity and selectivity of these catalyst systems. Li and Na dopants give the highest C2+ yields for all the REOs, but they are in most cases not active until temperatures of 600 ◦ C and above. Therefore, below 600 ◦ C undoped Sm2 O3 /n-MgO, Ca-Sm2 O3 /n-MgO and Ca-CeO2 /n-MgO catalysts can significantly outperform the Li- and Na-doped catalysts. The combination of CaCeO2 is particularly interesting, as the Ca significantly improves the activity of the CeO2 /n-MgO catalyst, which is the worst performing catalyst of the ones under investigation. In addition to improving the activity and selectivity, another benefit of Li and Na doping is the significantly higher C2 H4 yield compared with the undoped and Caor Mg-doped catalysts. The maximum C2 H4 yields were obtained at 800 ◦ C (or at 750 ◦ C in the case of Li-PrOy /n-MgO) over the Li-doped catalysts, and the highest C2 H4 yield (9.7%) was obtained over the Li-doped CeO2 /n-MgO catalyst due to a very high C2 H4 /C2 H6 ratio (3.7). This is important, since C2 H4 is a more valued product than C2 H6 . It was shown that for the undoped REO/n-MgO catalysts, the OCM activity correlates well with the number of weakly basic sites (which give rise to a CO2 TPD feature around 360 ◦ C). The addition of Na and Li to the TbOx /n-MgO catalyst resulted in significantly stronger basic sites (CO2 TPD features above 700 ◦ C). If these sites are active in the OCM reaction, it would explain why the Li- and Na-doped catalysts are not active until higher tem-

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peratures since these sites would be blocked by CO2 byproduct at lower temperatures. XRD data obtained from the fresh catalysts reveal the presence of carbonates and/or Li2 O2 on the TbOx /n-MgO and PrOy /n-MgO catalysts, as well as a mixed LiPrO2 phase on the PrOy /n-MgO catalyst. After reaction, only Tb2 O3 is observed on the Li- and Na-doped TbOx /n-MgO catalysts, which could be important since Tb2 O3 would be expected to be a more active OCM catalyst (similar to Sm2 O3 ) compared with reducible oxides, such as TbO1.81 or CeO2 , which tend to overoxidize the reactant. XPS data reveal that the Tb/Mg ratio is higher on the Li- and Na-doped catalysts. This together with the formation of highly basic sites, and Tb2 O3 on the surface could be the reasons for the high activity and selectivity in the OCM reaction. The Li-TbOx /n-MgO catalyst is evidently unique in its high activity and selectivity at relatively low temperatures (650 ◦ C). No other combination of REO and dopant, of the ones included in the study, could outperform the Li-TbOx /n-MgO catalyst, at least not initially. However, the Na-TbOx /n-MgO catalyst is more stable and can therefore outperform the Li-doped catalyst after only a few hours on stream. Furthermore, the Li-CeO2 /n-MgO and Li-PrOy /n-MgO catalysts can produce more of the desired C2 H4 product. Investigating the effects of dopants on rare earth oxides is therefore important in the search for more stable, highly active and selective methane coupling catalysts. Acknowledgements The authors gratefully acknowledge NSF support (Chemistry grant numbers 1026712 and 1464765). Dr. Valentin Craciun, Dr. Paul Carpinone, and Dr. Eric Lambers are acknowledged for the analytical equipment training and technical assistance during the XRD and XPS measurements. These analyses were performed at the Major Analytical Instrumentation Center (MAIC). Samantha Roberts and Trenton Elkins gratefully acknowledge the University of Florida for their Graduate Student Fellowships. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.09. 011. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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