Investigation on Reverse Water-gas Shift over La 2 NiO 4 Catalyst by cw-Cavity Enhanced Absorption Spectroscopy during CH 4 /CO 2 Reforming B.S. Liu, Ling Li, C.T. Au 1, A.S.-C. Cheung Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China 1 Department of Chemistry and Centre for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKU 7101/02P).

Background CO 2 reforming : CH 4 + CO 2 → 2CO +2H 2 catalyst  Methane is available in huge quantity, besides burning, there are many way to utilize it. Coupling reaction: CH 4 + CH 4  C 2 H 6 + etc… Synthesis gas Steam reforming: CH 4 + H 2 O → CO + 3H 2 Synthesis gas Aromatization: nCH 4  C 6 H 6, C 10 H 8 + …

Background CH 4 + CO 2 → 2CO +2H 2 cat.  Numerous studies on CH 4 /CO 2 reforming reaction using Ni-based catalysts. CO 2 + H 2  CO + H 2 O Synthesis gas Problems: (i) Low yield ( ~55%) (ii) Carbon deposit (~after 1 hour) Our studies:  Reverse water-gas shift (RWGS) reaction over La 2 NiO 4 catalyst during CH 4 /CO 2 reforming by continuous wave cavity enhanced absorption spectroscopy (CEAS). Associated reaction:

Experimental Setup for CEAS

Experimental Conditions: Catalyst: Ni(NO 3 ) 2 · 6 H 2 O + HNO 3 + La 2 O 3  ’ La 2 NiO 4 ’ + · · · 50 mg ‘La 2 NiO 4 ’sol-gel-generated catalyst at >500 o C Laser system: Tunable diode laser: nm, 5 mW Linewidth: 5 MHz High reflective mirror: R >

Wavenumber (cm )  = H 2 O = CH 4  2000  s 1500  s 1000  s 500  s CEA spectra of CH 4 and H 2 O obtained at different pulsing rate (of CH 4 /CO 2 reaction. (Temperature = 700 o C) Results

HITRAN Data Base CH – H 2 O MoleculeAbsorption (cm -1 ) Vibrational transition Rotational  J =0 (Q branch) J = 5, K a = 0, K c = 5  J = 6, K a = 2, K c = 4

Wavenumber (cm )  = H 2 O = CH 4  2000  s 1500  s 1000  s 500  s CEA spectra of CH 4 and H 2 O obtained at different pulsing rate (of CH 4 /CO 2 reaction. (Temperature = 700 o C) Results

CEA spectra of CH 4 and H 2 O obtained at different time on stream. (reaction temperature =700 o C); fixed pulsing duration Wavenumber (cm ) CH 4 H 2 O   225 min 95 min 15 min 0 min After long reaction time, deactivation of catalyst

Results (1) The conversion of CO 2 over La 2 NiO 4 catalyst was nearly 8.0 % higher than that of methane, attributable to the contribution of RWGS reaction: CO 2 + H 2  CO + H 2 O  H = + 41 kJ/mol (2)The H 2 O formation in RWGS reaction increases as the temperature of the catalyst increases because of the formation of more H 2. CH 4 + CO 2 → 2CO + 2H 2 (More than one channel for the consumption of CO 2 )

High Resolution Transmission electron microscopy (HRTEM) images of as-prepared La 2 NiO 4 sol-gel-generated catalyst

HRTEM photographs of used La 2 NiO 4 catalyst for CH 4 /CO 2 reforming for 5 h under the condition of supersonic jet expansion; scale (a) 200 nm (b) 5 nm. (a)(b)

 Cavity enhanced absorption spectroscopy(CEAS) is an excellent technique for monitoring catalytic reaction in situ.  The reverse water-gas shift (RWGS) reaction promoted the conversion of CO 2 and decreased the partial pressure of hydrogen.  With high sensitivity, it is envisaged that chemical intermediates such as HCO, OH and CH 3 radicals produced in catalytic reaction could also be detected and studied. Conclusion and Future work

Schematic Diagram of Cavity Ring-down / Raman Shifter Experimental Setup

Cavity ring down spectrum of CH 4 /CO 2 reforming reaction at different temperature (1573 nm).