Study on the Deactivation Pathway of Supported Ni Catalyst in Fluidized-Bed Dry Reforming of Methane at High Space Velocity

Abstract

A 10% Ni/γ⁃Al2O3 catalyst was prepared for dry reforming of methane. Spherical γ⁃Al2O3 particles having an average particle size of 60 µm were used as the support and an iso-volume impregnation method was employed for the preparation. All activity-evaluation tests were conducted in a fluidized-bed quartz reactor at 0.1 MPa and variable temperatures and space velocities for 600 min. The results of the tests revealed that ① at an identical high space velocity the catalyst activity exhibited two different types of time-dependence with temperature as parameter: at 820 and 850 ℃ it reached stable after an initial decrease, and at 750 and 800 ℃ it decreased from the beginning to the end of the test; and ② at 750 ℃ the catalyst activity showed an obviously accelerated decrease with the increase of space velocity. On the other hand, thermogravimetric analysis of the spent catalysts revealed the following two facts: ① against our anticipation little carbon deposit formed in two spent samples that had experienced accelerated activity decreases at two increased high space velocities, and ② a relatively large amount of carbon deposit formed also unexpectedly in a spent sample that had provided a highly stable reforming activity at a decreased low space velocity. While these two facts strongly deny the possibility of carbon deposition leading to an accelerated activity decline at 750 °C and increased high space velocities, the effect of CO2 concentration in the feed on the activity stability was additionally tested to explore the factor responsible for the high space velocity-accelerated activity decline. The test results showed that increasing the CO2/CH4 ratio of the feed did not slow down as expected, but reversely, further accelerated the activity decrease. Meanwhile, the XRD and XPS analyses of the spent catalyst samples also confirmed that oxidative transformation of reduced Ni species in catalyst to oxidized NixO (x ≦ 1) species did occur in the reforming in the feed ratio of CO2/CH4 = 1.5. In summary, with all above results this study reaches, for the first time, the following conclusion. It was the gradual oxidative transformation of active Ni sites in 10%Ni/γ⁃Al2O3 catalyst to inactive Ni oxide species to cause a continuous decline of its dry reforming activity in fluidized-bed operation mode and at 750 °C and high space velocity condition.

Keywords: Dry reforming, Ni/γ⁃Al2O3, fluidized-bed, space velocity, deactivation pathway, Ni oxidation