Study on the deactivation pathway of supported Ni catalyst in fluidized-bed dry reforming of methane at high space velocity

• A 10%Ni/γ-Al 2 O 3 catalyst was prepared and tested for fluidized-bed dry reforming of methane. • While reaching finally stable at 850 ℃, the activity kept decreasing over 600 min at 750 ℃. • At 750 ℃ increasing space velocity and CO 2 /CH 4 ratio in feed accelerated the activity decrease. • Little carbon deposit formed in the activity decrease-accelerated spent catalyst samples. • Formation of Ni x O (x ≤ 1) in an activity decrease-accelerated spent catalyst sample was confirmed. • Oxidation of reduced Ni sites by CO 2 to inactive Ni x O species caused the activity to decrease. A 10 %Ni/γ⁃Al 2 O 3 catalyst was prepared for dry reforming of methane. Spherical γ⁃Al 2 O 3 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 at 750 ℃ the catalyst activity showed an obviously accelerated decrease trend with the increase of space velocity. Thermogravimetric analysis of the spent catalysts revealed, however, that little carbon deposit formed in those spent samples that had experienced faster decreases in activity at higher space velocities. While these two facts together deny the possibility of carbon deposition leading to the accelerated activity decline at 750 °C and high space velocities, increasing the CO 2 /CH 4 ratio of the feed was demonstrated to further accelerate the activity decrease. Meanwhile, XRD and XPS analyses of the spent catalyst samples revealed that oxidative transformation of metallic Ni in catalyst to Ni x O (x ≤ 1) species did occur in the reforming in the feed of CO 2 /CH 4 = 1.5. All these results together have strongly revealed, for the first time, a conclusion as follows: it was the gradual oxidative transformation of active Ni sites in 10 %Ni/γ⁃Al 2 O 3 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.