Dry reforming of methane is one of the most attractive reactions in industrial catalysis: it consumes two greenhouse gases, methane and carbon dioxide, and converts them into syngas, a mixture of hydrogen and carbon monoxide that serves as a feedstock for numerous downstream synthesis routes. Because the reaction is highly endothermic, with an enthalpy of +247 kJ per mole at 298 K, it is typically run at elevated temperatures to achieve high conversions of both reactants. That harsh thermal environment, however, is precisely what makes the reaction so difficult to sustain. Nickel-based catalysts, favored for their low cost and activity comparable to noble metals, suffer from severe sintering and carbon deposition at these temperatures, both of which lead to deactivation. A new open-access study published in Discover Catalysis on 12 August 2026 has now dissected exactly how the amount of nickel loaded onto a zirconia support determines whether a catalyst thrives or collapses under these conditions.
Researchers Tao Yang and Meng Zhang prepared a series of nickel-on-zirconia catalysts by conventional impregnation, targeting nominal nickel loadings of 1, 3, 5, and 10 weight percent, and labeled the resulting materials 1NZ, 3NZ, 5NZ, and 10NZ. Inductively coupled plasma optical emission spectrometry revealed that the actual bulk nickel loadings were slightly lower than the nominal values, at 0.89, 2.78, 4.92, and 9.66 weight percent respectively, a discrepancy attributed to slight loss of nickel species during preparation. By preparing and evaluating the entire series under strictly identical conditions, the team aimed to isolate the effect of nickel loading itself, something that is difficult to do when comparing catalysts reported across different laboratories with different preparation and testing protocols.
The characterization data revealed a consistent structural story. As nickel loading increased, the Scherrer-estimated NiO crystallite size in the fresh catalysts grew monotonically from 5.2 nanometers for 1NZ to 30.4 nanometers for 10NZ, passing through 15.8 nanometers for 3NZ and 18.9 nanometers for 5NZ. After reduction in hydrogen at 700 degrees Celsius, the metallic nickel crystallites followed the same trend, measuring 8.6, 14.1, 17.3, and 28.2 nanometers across the series. Nickel dispersion, measured by hydrogen chemisorption at 50 degrees Celsius assuming a 1:1 hydrogen-to-nickel stoichiometry, moved in exactly the opposite direction, falling continuously from 10.4 percent at the lowest loading to 4.1 percent at the highest. In other words, 10NZ contained the most nickel overall but exposed the smallest fraction of its nickel atoms, while 1NZ contained the least nickel but dispersed it most efficiently.
Temperature-programmed reduction added a crucial nuance. All four catalysts showed two reduction peaks: a lower-temperature peak between 352 and 392 degrees Celsius assigned to free or weakly interacting NiO species, and a higher-temperature peak between 414 and 446 degrees Celsius associated with NiO strongly bound to the zirconia support. With increasing loading, the weakly interacting fraction grew, with the low-temperature peak’s share of the total area rising from 54 to 74 percent. Total hydrogen consumption climbed dramatically from 15.2 to 121.1 millimoles per gram across the series, consistent with the rising nickel content. Yet the reduction degree fell from a complete 100 percent for 1NZ to just 74 percent for 10NZ, a decline the authors attribute to hydrogen diffusion limitations into the cores of large NiO particles and the formation of bulk-like nickel species that resist complete reduction. The intermediate 5NZ catalyst maintained a 90 percent reduction degree alongside a substantial 75.2 millimoles per gram of hydrogen consumption, striking a notable balance between reducible nickel content and reduction efficiency.
Catalytic testing was performed in a fixed-bed quartz reactor at 750 degrees Celsius and atmospheric pressure, with an equimolar methane-carbon dioxide feed at a weight hourly space velocity of 48,000 milliliters per gram per hour, over 600 minutes on stream. The initial methane and carbon dioxide conversions increased with nickel loading, and 10NZ, with its highest total nickel content, delivered the highest initial activity. But that advantage proved fleeting. 10NZ deactivated pronouncedly during the test, with a sharp decline in conversion, while 5NZ maintained relatively high conversions throughout the 600 minutes and deactivated far less severely. The two low-loading catalysts, 1NZ and 3NZ, showed lower initial activity because of insufficient nickel content, though their stability exceeded that of 10NZ. Notably, textural properties did not track performance: 10NZ actually possessed the highest specific surface area at 18.3 square meters per gram and the largest pore volume at 0.21 cubic centimeters per gram, yet was the least stable catalyst in the series.
Two efficiency metrics drove the central insight home. When methane conversion was normalized by the mass of nickel in each catalyst, the rate decreased with increasing loading, with 1NZ showing the highest value and 10NZ the lowest, the exact opposite of the apparent conversion trend. The intrinsic methane turnover frequency, measured at 500 degrees Celsius and normalized by the number of exposed nickel sites derived from chemisorption, followed the same descending order from 1NZ to 10NZ. This means that raising nickel loading reduced not only the fraction of nickel atoms actually exposed to reactants but also the average activity of the sites that remained chemisorption-accessible. The authors note that differences in nickel ensemble size or the fraction of interfacial nickel sites may contribute to this decline, although their data could not distinguish between these possibilities. The deactivation rate, calculated from the relative decrease in methane conversion between 40 and 600 minutes, followed the order 1NZ, then 3NZ, then 5NZ, then 10NZ, with the highest loading deactivating fastest.
Post-reaction characterization illuminated why. X-ray diffraction of the spent catalysts showed that metallic nickel crystallites grew during the reaction, from 8.6 to 10.8 nanometers for 1NZ, 14.1 to 15.0 for 3NZ, 17.3 to 18.9 for 5NZ, and 28.2 to 31.3 for 10NZ, with the largest absolute growth occurring at the highest loading. Because larger nickel crystallites favor methane dissociation and carbon formation, the initial loading effectively set the stage for deactivation. Raman spectroscopy detected the characteristic D band near 1340 and G band near 1570 wavenumbers in all spent samples, and the ratio of integrated intensities fell from 1.09 for 1NZ to 0.93 for 10NZ, indicating that deposited carbon became more graphitized and less defective at higher nickel contents. Thermogravimetric analysis quantified the carbon directly: weight loss upon oxidation in air rose from 2.9 percent for 1NZ to 6.0 percent for 3NZ, 9.1 percent for 5NZ, and 15.3 percent for 10NZ, confirming that carbon accumulation scales with nickel loading and matches the severity of activity loss.
The syngas composition told a complementary story. All catalysts produced hydrogen-to-carbon monoxide ratios below the stoichiometric value of 1.0, indicating that the reverse water-gas shift reaction, which consumes hydrogen and generates additional carbon monoxide, contributed significantly among the side reactions. 5NZ maintained a relatively high and stable ratio consistent with its stable conversions, whereas the ratio for 10NZ declined with time on stream as its reforming activity decayed and the relative contribution of the reverse water-gas shift reaction grew. 1NZ showed the lowest ratio of all, because its meager nickel loading limited the main reforming pathway and allowed the side reaction to dominate proportionally.
Taken together, the results establish that dry reforming performance over nickel-on-zirconia depends not on total nickel loading alone but on the balance among accessible nickel sites, reducibility, and resistance to deactivation. Low loading yields small crystallites, high dispersion, high nickel-normalized rates, high turnover frequencies, and minimal coking, but too few active sites to drive high apparent conversion. Excessive loading delivers strong initial activity but produces large particles, poor dispersion, incomplete reduction, lower site-specific activity, and rapid carbon-fueled decay. Under the tested conditions, the 4.92 weight percent catalyst, 5NZ, emerged as the best overall compromise, combining moderate crystallite size, 7.3 percent dispersion, 90 percent reduction degree, moderate carbon deposition, and substantially higher conversions than the low-loading catalysts while deactivating far less than 10NZ. The study offers a clear design principle for a reaction central to greenhouse gas utilization: the optimal catalyst is the one that uses its nickel most effectively, not the one that carries the most of it.
Subject of Research: Effect of nickel loading on active site utilization and deactivation of Ni/ZrO2 catalysts in dry reforming of methane
Article Title: Effects of nickel loading on active site utilization and deactivation of Ni/ZrO2 catalysts in dry reforming of methane
Article References: Yang, T., & Zhang, M. (2026). Effects of nickel loading on active site utilization and deactivation of Ni/ZrO2 catalysts in dry reforming of methane. Discover Catalysis, 3(1), Article 16. https://doi.org/10.1007/s44344-026-00047-6
Image Credits: AI Generated
DOI: 10.1007/s44344-026-00047-6
Keywords: dry reforming of methane, Ni/ZrO2 catalyst, nickel loading, catalyst deactivation, carbon deposition, Ni dispersion, syngas, H2-TPR, turnover frequency, zirconia support, sintering, catalyst characterization
News Source: Bethany Barker. (October 28, 2026). Nickel Loading Tipping Point Revealed for Methane Dry Reforming Catalysts. Scienmag.
