Results and Analysis

Saran x 3/16

Chapter 6 Chapter 6 Carbon Supported Catalysts

6.2 Results and Analysis

109

6.:3. The particle size of the Ni catalyst was smaller after sonication t.rf'atment, whf'n all other preparation parameters were the same.

'"

^{~ 3000}

~

2000

20

2Theta

.00

"

(c)

(b)

(a) (c) (b)

(a)

Figure 6.:3: X-ray diffraction patterns of Ni/fullerite sampks. (a) ful1erite frOl11 "esea.

(b) fullerite from MER, and (c) fullerite from MER with sonication used for catalyst preparation process.

--&- C₆₀,C₇₀ (1 st run) -fB- C_SO,C₇₀ (2nd run) 0.4 -S- C_SO,C₇₀ (3rd run)

~ Ni/C₆₀,C₇₀ (1st run) ... Ni/Cso,C70 (2nd run)

~0.3

~

<.) ___ 0.2 I

0.1

0.0

o 20

110

40 6Q

Pressure [baq ^{80 100}

Figure 6.4: Desorption isotherms of composition versus pressure at. 771\ for fullerite and Nijfullerite samples.

Adsorption isotherm measurements were also performed manually. The sample was first degassed at :200°(' and til(' manifold was evacuated. The reactor was valved off before H2 was introduced into the rest of tlw syst.elIl and the presnre was recorded.

A valve was opened to allow the sample to make coutact with

th,

and the pressure was recorded again after tequilibrium was achieved. This was one complete step of the adsorption measnrenwnt. The reactor was valved off and the pr(,SSl1r(' of the manifold was raised by introducing more hydrogen int.o the syst.elll. Tlwse steps w('re repeat.ed and t.he pressure before and after the valve was opened was recorded unt.il the pressure of the whole system reached about lOO bar. Some adsorption isot.herllls are shown in Figure 6.6.

6.2.2 Isotherlll Results

The results of desorption and adsorpt.ion isot.herms at 77 I\", :300 I\", alld!!)O I\

are summarized in Table 6.:2. From Table 6.:2, we see several t.reuds. The desorption capacit.v of the sample was lower than the adsorption cavacit.y of the same sample at the same isotherm condit.ion. By adding Ni part.icles 011 the sample, the hydrogen

0.10

0.08

~0.06 ~

~

0.04

0.02

20

111

__ Ni/fulierite (450K) ___ Ni/fulierite (300K, 1st run) ... Ni/fulierite (300K, 2nd run)

""""*""""" fullerite (300K)

40 60 80

Pressure [bar]

Figure 6 .. 5: Desorption isotherms of composition versus pressure at :~OOI~ and 4!)01~

respectively, for Ni/fullerite samples.

storage capacity of fullerit.e sample was increased. This was also observed for the DAC samples. The Ni particles may adsorb some hydrogen, but this can llOt. account.

for t.he tot.al increased capacity even by assuming complete coverage of hydrogcll molecules on the Ni particle surface. Assuming 5 wt.

%

of Ni particles are aggregat.ed as spheres of 20 nm diameter; the complete coverage of hydrogell on the Ni particle surface is equivalent to 0.01 wt.(J(, hydrogen storage capacity. We suggest that t.he hydrogen adsorbed and dissociated on tlw surface of Ni part.icle may migrate onto the surface of supporting carbon, which is called hydrogen :;pillol'rr.

The capacities of both Ni/fullerite and Ni/DAC samples at elevated temperature

Sample Hz Capacity Hz Capacit.y Hz Capacity 1(]77K (wt.%) 1·(tJ:300K (wt%)) 1(]4!)OK (wt(X)) abs. des. abs. des. abs. des.

OAt' 0.2 0.04 0.02

Ni/OAC

o

.~I 'r 0.4 0.12

fullerite 0.2-0.:3 0.06

Ni/fullerite 0.:3-0.4 0.1 0.08 0.2 0.1 L

Table 6.2: Hydrogen storage capacities of carbon samples with or without catalysts.

0.5

0.4

#0.3

...

... 3:

S:20.2 I

0.1

o

___ Ni/fullerite (300 K) ... Ni/fullerite (455 K)

112

-+-Darco acrivated carbon (300 K)

"""*""

Ni/OAC (300 K) ... Ni/OAC (455 K)

20 40 60

Pressure [bar]

⁸⁰

Figure 6.6: Adsorption isotherms of composition versus pressure at :JOOI--: and 4})OK respectively, for Darco activat.ed carbon, Ni/DAC, fullerite and Ni/fullerite samples.

(4})0 K) increased by a factor of two compared to those measured at amhient temper- ature. From the thermodynamics of adsorption, tht' fractional coveragt' of the sllrfacc increases with decreasing temperature and increasing pressure. However, t.he fullerit<, sample with Ni catalyst adsorbed more hydrogen at c!.1O K. This call be explained as the better performance and higher acti vi ty of Ni catalysts at high tem peral. ure.

6.2.3 Fourier Transfornl Infrared Spectronletry

Since the amollnt of hydrogen desorbed was less than adsorbed, some hydrogen molecules must have remained on the sample after the desorption. To determine tllf' mechanism of adsorption and check if there were any hydrogen carbon bonds formed, infrared spectrum were measured with a 860 l'vIaglla series FTIR spectrollle- ter. Infrared spectroscopy is a st.andard clwmical analysis tool to idf'nti fy molecular structures through molecula.r vibrations. The peaks around 2900 cm -1 are clue to (,H bond stretching.

To obtain the t.ransmission spectrum, samples were prepared into potassium bro- mide pellets. Approximat.ely l mg sample and :WO mg KGr were ground and mixpd

in the mortar and pressed into a transparent lwllet in a press. The pellf't. was thell placed in a pellet holder to obtain the spectrum. A background spectrum was also obtained from an empty pellet holder.

A strong absorption at 2900 cm-¹ was observpd for the as-recf'ived salllpks of both fullerite and Darco activated carbon. This indicatps that therf' lIlay he' sonw contamination and/or a portion of CH bonds ⁰¹¹ the surface of carholl samples. It ,vas report.ed that therf' are several types of hydrogen bOllds in coal, formed hy hydroxyls with various hydrogen-bonding acceptors [10, 11].

After isotherm measurement at ambif'ut temperature. the fullf'rite sample sliovv<'d a weaker absorption peak at 2900 ern-¹. The absorption intensity of as-prf'pared Ni/fullerite sample was also weaker than that of as-recf'ived sample. There is no sig- nificant absorption for the Ni/fullf'rite sample after isotherm at elevated temperaJurf'.

Before each hydrogf'n isotherm measnrenwnt. the sample was degassed at about :200°('. This process may have driven off the contamination. The reason for the disa.ppeara.nce of CH bonds after high temperature hyclrogf'n isotherm is unclear.