Catalytic Oxidation of Methane

Abstract

Natural gas fired engines provide high energy efficiency and low emissions of CO2, NOx, SOx and particle matter (PM). However, at the same time, the slip of unburnt CH4 is a severe concern due to the high greenhouse gas (GHG) potential of CH4. A catalyst that can be used under the exhaust gas conditions of natural gas engines, where both H2O and SO2 are present, is needed for CH4 emission abatement. Rh based catalysts supported on amorphous SiO2, γ-Al2O3, and zeolites with varied SiO2/Al2O3 ratios were prepared by an incipient wetness impregnation (IWI) method followed by calcination in air flow at 600 °C. The synthesized catalysts were characterized by X-ray Diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) techniques. A Rh ion exchanged zeolite catalyst was prepared for comparison and its Rh loading was determined by inductively coupled plasma optical emissions spectroscopy (ICP-OES). The performance of Rh catalysts was compared to that of the acknowledged Pd based catalysts for CH4 removal in both oxidation and steam reforming pathways. The activity of the catalysts was tested in a fixed-bed plug flow quartz reactor in powder form in 250-600 °C at a gas hourly space velocity (GHSV) of 150,000 Nml/(gcat·h) (STP: 0 °C, 1 atm). The influence of additional added H2O and SO2 was studied by varying the composition of the feed gas. The feed gas and the effluent gas were analyzed by a gas chromatograph (GC) and two IR analyzers. For CH4 oxidation, the comparison between Rh and Pd was carried out on SiO2, γ-Al2O3, and ZSM-5 zeolite with a SiO2/Al2O3 ratio of 280 in different reaction atmospheres. Additional added H2O, or/and SO2 inhibits the performance of both Rh and Pd catalysts with Rh/ZSM-5 catalyst shows better activity and stability in the presence of both H2O and SO2. It is able to achieve 79 % CH4 conversion on Rh/ZSM-5 catalyst at 500 °C in the presence of 5 vol.% H2O and 1 ppm SO2. The superior performance of Rh catalyst is attributed to an alleviating effect of H2O on the SO2 poisoning for the Rh catalyst while H2O causes additive deactivation to Pd catalyst when SO2 is present. The stability of Rh sulfate and Pd sulfate was studied by thermal gravimetric analysis (TGA) and temperature programmed desorption (TPD). The alleviating effect of H2O on SO2 poisoning for Rh-based catalysts is attributed to the formation of [Rh(H2O)x]2(SO4)3·yH2O, which yields a partial sulfur release below 400 °C. The influence of support material, Rh loading, operating temperature, SO2 concentration, and regeneration was further investigated for optimal catalyst composition and operating conditions. An Si-rich zeolite, ZSM-5 with a SiO2/Al2O3 ratio of 280, was found to be the best support having Rh mainly as nanoparticle sites on the zeolite which is similar to the bulk Rh sulfates that are able to release part of the sulfur at low temperature. While the acidic site rich zeolites result in atomic dispersion of Rh to single atom sites which is less active and less tolerant to the deactivation by H2O and sulfur than the nanoparticle sites. On ZSM-5(SiO2/Al2O3=280) zeolite, higher CH4 conversion was achieved by increasing Rh loading from 1 wt.% to 2 wt.% but further increase the Rh loading to 4 wt.% did not show significant improvement on CH4 conversion compared to the 2 wt.% catalyst. The CH4 removal efficiency of the 2 wt.% Rh/ZSM-5(SiO2/Al2O3=280) catalyst can be improved by operating at a higher temperature and a low SO2 concentration in the feed gas. The deactivation by SO2 was fitted by a Temkin Isotherm based on an assumption of a pure site-blocking poisoning by SO2. The adsorption energy at zero sulfur coverage was found to be 274.6 kJ/mol and is close to the estimated activation energy for decomposition of bulk rhodium sulfate from TGA experiment. The regeneration of the SO2 poisoned catalyst in SO2-free reaction gas, pure N2, and 2 vol.% H2 in N2 atmosphere was also studied. Partial activity recovery was achieved by removing SO2 from the feed gas and heating to 600 °C. Further regeneration in pure N2 and 2 vol.% H2 in N2 atmospheres has no improvement on the activity restoration. The deactivation of the regenerated catalyst by SO2 was further tested and found to be almost instantaneous. The fast deactivation upon addition of SO2 suggests that periodical regeneration is not a feasible strategy. Rh and Pd catalysts were tested for CH4 removal under steam reforming conditions (i.e. absence of oxygen) in the absence and presence of SO2. The reactions that take place under reforming conditions are CH4 steam reforming and water gas shift reaction. Rh catalysts show good activity in the absence of SO2, and the thermodynamic equilibrium conversion was achieved above 450 °C. The active Rh phase under reforming conditions is the reduced (metallic) Rh which predominately allows CO adsorption at the three-fold hollow sites at 25 °C as shown by the DRFITS studies. The influence of support on the activity under reforming conditions was investigated. Zeolite supported catalysts show lower activity and higher CO production than Rh on SiO2 and γ-Al2O3 catalysts below the equilibrium temperature (450 °C) due to the inhibition of CO on single Rh sites at the exchange sites of zeolites. The activity of Rh catalysts was stable under reforming conditions in the absence of SO2. Surprisingly, sustained oscillations in the catalytic activity occurred upon addition of SO2. The oscillation property is influenced by temperature, SO2 concentration, support material, and sintering history of the catalyst. The results suggest that the oscillation is due to rearrangement of the surface to an inactive phase driven by formation of surface rhodium sulfide and rearrangement to an active phase driven by removal of sulfide with H2O and SO2. CH4 can be converted to CO2, CO, and H2 in the oscillation mode. Above 475 °C, the average CH4 conversion reaches 50 % which is the highest conversion that can be achieved in an oscillation mode. As the conversion of CH4 under oxidation conditions in the presence of H2O and SO2 also oscillates but with a longer period of 24 h similar surface rearrangement may occur during long time methane oxidation. This thesis developed a Si-rich zeolite supported Rh catalyst that offers high activity and remarkable H2O and SO2 tolerance under conditions achievable in a lean-burn natural gas engine exhaust system upstream the turbocharger as a practically applicable solution to the CH4 emissions challenge for natural gas fired engines. The superior performance of the developed Rh catalyst is correlated to the ability of Rh bulk sulfate hydrate to release sulfur below 400 °C and the self-regeneration by the oscillations in the reforming pathway.