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Here we demonstrate that phosphorus (P) can affect both reforming and coking of alumina-supported nickel (10 wt% Ni with respect to alumina) at 750 °C and 1 atm. The P-containing catalysts were prepared by incipient wetness co-impregnation. Compared with the P-free nickel catalyst, the reforming activity is enhanced but the coke content decreases 8-fold at Ni:P atomic ratio of 4; the latter further reduces 55-fold at a ratio of 2, without a substantial decline in the reforming activity. Phosphorus not only improves nickel dispersion but also reduces the surface acidity and facilitates CO 2 activation. At high P loadings, a portion of nickel transforms into nickel oxide during reforming. Besides tuning the reforming and coking activity, phosphorus changes the behavior of carbon crystallization. In methane pyrolysis and dry reforming, filamentous carbon occurs on the P-free catalyst but it is absent on the P-containing catalyst. Image 1 • At a Ni:P ratio of 4, the reforming activity improves but coke content reduces 8-fold. • At a Ni:P ratio of 2, coking reduces 55-fold without dramatic decline in activity. • Phosphate promotes CO 2 splitting and makes the surface more basic. • Phosphorus affects the morphology of carbon deposited. [ABSTRACT FROM AUTHOR]