Structural Changes of Ni and Ni-Pt Methane Steam Reforming Catalysts During Activation, Reaction, and Deactivation Under Dynamic Reaction Conditions

Ni-based catalysts are the most widely used materials to produce H2 in large-scale methane steam reformers under stationary conditions. For domestic applications such as fuel cells, H2 production involves the exposure of the catalysts to more dynamic conditions due to the daily startup and shutdown operation mode, making Ni-based catalysts susceptible to oxidation and deactivation. In this context, we report a systematic investigation of the structural changes occurring for monometallic Ni/MgAlOx and bimetallic NiPt/MgAlOx catalysts during methane steam reforming under transient conditions, comprising catalyst activation, operation, and deactivation processes. Besides extensive catalytic tests, the samples prepared by incipient wetness impregnation were characterized by complementary methods, including N2-physisorption, X-ray diffraction, H2-temperature-programmed reduction, and electron microscopy. Next, the structure of the Ni and Pt species was monitored under reaction conditions using time and spatially resolved in situ/operando X-ray absorption spectroscopy. The results obtained show that before catalyst activation by H2-reduction, nickel diffuses into the support lattice and forms mixed oxides with magnesium. In the activated catalysts, Ni is present in the metallic state or alloyed with Pt. A clear beneficial effect of the noble metal addition was identified on both the activity and stability of the bimetallic NiPt/MgAlOx catalyst. In contrast, the pronounced oxidation and reincorporation of Ni into the support lattice were observed for the monometallic sample, and these catalyst deactivation effects are hindered in the bimetallic Ni-Pt catalyst. Overall, the outcome of our study not only helps in understanding the catalyst activation/deactivation processes at an atomic level but also provides the basis for the rational development of improved methane steam reforming catalysts.

A comparative study on the NOx storage and reduction performance of Pt/Ni1Mg2Al1Ox and Pt/Mn1Mg2Al1Ox catalysts

A series of Ni₁Mg₂Al₁O_x, Mn₁Mg₂Al₁O_x, 0.5Pt/Ni₁Mg₂Al₁O_x and 0.5Pt/Mn₁Mg₂Al₁O_x catalysts were carefully prepared and their NO_x storage and reduction (NSR) performance including NO_x oxidation efficiency (NOE), NO_x storage capacity (NSC), NO_x conversion rate (X_NO), N₂ selectivity (S_N2), N₂O selectivity (S_N2O) and NH₃ selectivity (S_NH3), was systematically investigated. A SO₂ resistance test was also performed in the presence of 100 ppm SO₂. The NOE and NSC experimental results revealed that the Ni₁Mg₂Al₁O_x catalyst possesses a higher NSC, while the Mn₁Mg₂Al₁O_x catalyst possesses a better NOE. With regard to X_NO, 0.5Pt/Ni₁Mg₂Al₁O_x presented higher results at 200 °C and 400 °C, while 0.5Pt/Mn₁Mg₂Al₁O_x obtained the highest result at 300 °C, which was more than 60% for both. In addition, compared to 0.5Pt/Ni₁Mg₂Al₁O_x, 0.5Pt/Mn₁Mg₂Al₁O_x exhibited a relatively higher S_N2 and lower S_N2O and S_NH3. The NO_x-TPD and H₂-TPSR results indicated that NO_x adsorbed on Ni₁Mg₂Al₁O_x and 0.5Pt/Ni₁Mg₂Al₁O_x is more stable, and that NH₃ can be formed in large amounts in a lower temperature range. Both Pt-containing catalysts presented a quite stable X_NO in ten cycles in the presence of 100 ppm SO₂, and their S_N2 can be remarkably enhanced to more than 80%, which could be attributed to the reactions of NH₃-SCR and SO₂ + NH₃. We believe this new insight can provide a new way of thinking for the development of NSR catalysts.

Seeded-growth preparation of high-performance Ni/MgAl2O4 catalysts for tar steam reforming

Preparing the high-performance catalyst by the novel seeded-growth strategy, which is green, simple and low-cost.