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Pd-promoted ternary oxides of the type Mn-Ce-Zr and Mn-Pr-Zr were characterized and evaluated for low temperature NOx storage applications such as diesel vehicle cold starts. While X-ray diffraction data were in all cases consistent with the formation of solid solutions, Mn/Zr and Mn/Ce(Pr) ratios found by XPS were consistently higher than the bulk values, indicative of an enrichment of Mn at the surface of the solids. Both sets of Pd-promoted mixed oxides showed remarkably high NOx storage efficiency in the range 80–160 °C, while a 1.8% Pd/Mn(27)-Ce(7)-Zr catalyst showed excellent NOx storage in simulated cold start experiments. Moreover, ramping the temperature to 370 °C in these experiments, simulating higher speed operation, resulted in near complete purging of stored NOx from the catalyst. NOx storage efficiency in isothermal storage experiments was found to improve with increasing Mn content for the 1%Pd/Mn(x)-Ce(7)-Zr series (x = 9, 18, 27 wt.%), DRIFTS measurements showing that relative to Ce-Zr mixed oxides, Mn incorporation favored NOx storage as nitrate. During temperature programmed desorption (TPD) two main desorption events were observed, corresponding to decomposition of nitrites (up to 200 °C), followed by loss of nitrates (⁓200–400 °C). Nitrates stored on Pd/Mn-Ce(Pr)-Zr mixed oxides desorbed during TPD at lower temperatures than for CeO2-ZrO2 mixed oxides, a finding attributed to the lower basicity of Mn compared to Ce. Hydrothermal aging of 1.8%Pd/Mn(27)-Ce(7)-Zr at 700 °C reduced NOx storage efficiency, although the catalyst was still able to store significant amounts of NOx. However, catalyst sulfation led to a large decrease in NOx storage efficiency and the efficiency could not be completely recovered with lean or rich desulfations at high temperatures.

Pd-promoted ternary oxides of the type Mn-Ce-Zr and Mn-Pr-Zr were characterized and evaluated for low temperature NOx storage applications such as diesel vehicle cold starts. While X-ray diffraction data were in all cases consistent with the formation of solid solutions, Mn/Zr and Mn/Ce(Pr) ratios found by XPS were consistently higher than the bulk values, indicative of an enrichment of Mn at the surface of the solids. Both sets of Pd-promoted mixed oxides showed remarkably high NOx storage efficiency in the range 80–160 °C, while a 1.8% Pd/Mn(27)-Ce(7)-Zr catalyst showed excellent NOx storage in simulated cold start experiments. Moreover, ramping the temperature to 370 °C in these experiments, simulating higher speed operation, resulted in near complete purging of stored NOx from the catalyst. NOx storage efficiency in isothermal storage experiments was found to improve with increasing Mn content for the 1%Pd/Mn(x)-Ce(7)-Zr series (x = 9, 18, 27 wt.%), DRIFTS measurements showing that relative to Ce-Zr mixed oxides, Mn incorporation favored NOx storage as nitrate. During temperature programmed desorption (TPD) two main desorption events were observed, corresponding to decomposition of nitrites (up to 200 °C), followed by loss of nitrates (⁓200–400 °C). Nitrates stored on Pd/Mn-Ce(Pr)-Zr mixed oxides desorbed during TPD at lower temperatures than for CeO2-ZrO2 mixed oxides, a finding attributed to the lower basicity of Mn compared to Ce. Hydrothermal aging of 1.8%Pd/Mn(27)-Ce(7)-Zr at 700 °C reduced NOx storage efficiency, although the catalyst was still able to store significant amounts of NOx. However, catalyst sulfation led to a large decrease in NOx storage efficiency and the efficiency could not be completely recovered with lean or rich desulfations at high temperatures.

Pt/CeO2-M2O3 and Pd/CeO2-M2O3 (M = La, Pr, Y, Sm, or Nd) were prepared by co-precipitation and impregnation and were investigated for potential passive NOx adsorber (PNA) use. During NOx storage at 120 °C, it was found that the amount of NOx stored as a function of time for Pt-promoted materials was higher than the Pd-promoted counterparts. For Pt/CeO2-M2O3 samples doped at the 5% level, NOx storage efficiency (NSE) followed the order Pr > Nd > Sm > Ce (undoped) > Y, La. Increasing dopant content from 5 to 20% decreased NSE in most cases, although in the case of Pr, NSE was increased. During subsequent NOx-temperature-programmed desorption (TPD), two NOx desorption events were apparent in all cases, the first occurring below 350 °C and the second occurring in the range 350–500 °C. Doping with Pr promoted the release of increased amounts of NOx below 350 °C compared to samples doped with other lanthanides; moreover, increasing the content of all doping metals except Pr shifted desorption peaks to higher temperatures, while the opposite trend was observed for Pr. Promotion with Pd was also examined, resulting in an increase of NOx desorption at low temperatures (<350 °C) relative to Pt. These results can be rationalized in terms of the ability of Pr to create vacancies in the CeO2 lattice, which facilitate NOx adsorption, and by the superior NO oxidation activity of Pt relative to Pd, which promotes NOx storage as nitrates possessing high thermal stability.

Pt/CeO2-M2O3 and Pd/CeO2-M2O3 (M = La, Pr, Y, Sm, or Nd) were prepared by co-precipitation and impregnation and were investigated for potential passive NOx adsorber (PNA) use. During NOx storage at 120 °C, it was found that the amount of NOx stored as a function of time for Pt-promoted materials was higher than the Pd-promoted counterparts. For Pt/CeO2-M2O3 samples doped at the 5% level, NOx storage efficiency (NSE) followed the order Pr > Nd > Sm > Ce (undoped) > Y, La. Increasing dopant content from 5 to 20% decreased NSE in most cases, although in the case of Pr, NSE was increased. During subsequent NOx-temperature-programmed desorption (TPD), two NOx desorption events were apparent in all cases, the first occurring below 350 °C and the second occurring in the range 350–500 °C. Doping with Pr promoted the release of increased amounts of NOx below 350 °C compared to samples doped with other lanthanides; moreover, increasing the content of all doping metals except Pr shifted desorption peaks to higher temperatures, while the opposite trend was observed for Pr. Promotion with Pd was also examined, resulting in an increase of NOx desorption at low temperatures (<350 °C) relative to Pt. These results can be rationalized in terms of the ability of Pr to create vacancies in the CeO2 lattice, which facilitate NOx adsorption, and by the superior NO oxidation activity of Pt relative to Pd, which promotes NOx storage as nitrates possessing high thermal stability.

The ability of Cu and Sn to promote the performance of a 20% Ni/Al2O3 catalyst in the deoxygenation of lipids to fuel-like hydrocarbons was investigated using model triglyceride and fatty acid feeds, as well as algal lipids. In the semi-batch deoxygenation of tristearin at 260 °C a pronounced promotional effect was observed, a 20% Ni-5% Cu/Al2O3 catalyst affording both higher conversion (97%) and selectivity to C10-C17 alkanes (99%) in comparison with unpromoted 20% Ni/Al2O3 (27% conversion and 87% selectivity to C10-C17). In the same reaction at 350 °C, a 20% Ni-1% Sn/Al2O3 catalyst afforded the best results, giving yields of C10-C17 and C17 of 97% and 55%, respectively, which contrasts with the corresponding values of 87 and 21% obtained over 20% Ni/Al2O3. Equally encouraging results were obtained in the semi-batch deoxygenation of stearic acid at 300 °C, in which the 20% Ni-5% Cu/Al2O3 catalyst afforded the highest yields of C10-C17 and C17. Experiments were also conducted at 260 °C in a fixed bed reactor using triolein − a model unsaturated triglyceride − as the feed. While both 20% Ni/Al2O3 and 20% Ni-5% Cu/Al2O3 achieved quantitative yields of diesel-like hydrocarbons at all reaction times sampled, the Cu-promoted catalyst exhibited higher selectivity to longer chain hydrocarbons, a phenomenon which was also observed in experiments involving algal lipids as the feed. Characterization of fresh and spent catalysts indicates that Cu enhances the reducibility of Ni and suppresses both cracking reactions and coke-induced deactivation.

The ability of Cu and Sn to promote the performance of a 20% Ni/Al2O3 catalyst in the deoxygenation of lipids to fuel-like hydrocarbons was investigated using model triglyceride and fatty acid feeds, as well as algal lipids. In the semi-batch deoxygenation of tristearin at 260 °C a pronounced promotional effect was observed, a 20% Ni-5% Cu/Al2O3 catalyst affording both higher conversion (97%) and selectivity to C10-C17 alkanes (99%) in comparison with unpromoted 20% Ni/Al2O3 (27% conversion and 87% selectivity to C10-C17). In the same reaction at 350 °C, a 20% Ni-1% Sn/Al2O3 catalyst afforded the best results, giving yields of C10-C17 and C17 of 97% and 55%, respectively, which contrasts with the corresponding values of 87 and 21% obtained over 20% Ni/Al2O3. Equally encouraging results were obtained in the semi-batch deoxygenation of stearic acid at 300 °C, in which the 20% Ni-5% Cu/Al2O3 catalyst afforded the highest yields of C10-C17 and C17. Experiments were also conducted at 260 °C in a fixed bed reactor using triolein − a model unsaturated triglyceride − as the feed. While both 20% Ni/Al2O3 and 20% Ni-5% Cu/Al2O3 achieved quantitative yields of diesel-like hydrocarbons at all reaction times sampled, the Cu-promoted catalyst exhibited higher selectivity to longer chain hydrocarbons, a phenomenon which was also observed in experiments involving algal lipids as the feed. Characterization of fresh and spent catalysts indicates that Cu enhances the reducibility of Ni and suppresses both cracking reactions and coke-induced deactivation.

The ability of Cu and Sn to promote the performance of a 20% Ni/Al2O3 catalyst in the deoxygenation of lipids to fuel-like hydrocarbons was investigated using model triglyceride and fatty acid feeds, as well as algal lipids. In the semi-batch deoxygenation of tristearin at 260 °C a pronounced promotional effect was observed, a 20% Ni-5% Cu/Al2O3 catalyst affording both higher conversion (97%) and selectivity to C10-C17 alkanes (99%) in comparison with unpromoted 20% Ni/Al2O3 (27% conversion and 87% selectivity to C10-C17). In the same reaction at 350 °C, a 20% Ni-1% Sn/Al2O3 catalyst afforded the best results, giving yields of C10-C17 and C17 of 97% and 55%, respectively, which contrasts with the corresponding values of 87 and 21% obtained over 20% Ni/Al2O3. Equally encouraging results were obtained in the semi-batch deoxygenation of stearic acid at 300 °C, in which the 20% Ni-5% Cu/Al2O3 catalyst afforded the highest yields of C10-C17 and C17. Experiments were also conducted at 260 °C in a fixed bed reactor using triolein − a model unsaturated triglyceride − as the feed. While both 20% Ni/Al2O3 and 20% Ni-5% Cu/Al2O3 achieved quantitative yields of diesel-like hydrocarbons at all reaction times sampled, the Cu-promoted catalyst exhibited higher selectivity to longer chain hydrocarbons, a phenomenon which was also observed in experiments involving algal lipids as the feed. Characterization of fresh and spent catalysts indicates that Cu enhances the reducibility of Ni and suppresses both cracking reactions and coke-induced deactivation.

The ability of Cu and Sn to promote the performance of a 20% Ni/Al2O3 catalyst in the deoxygenation of lipids to fuel-like hydrocarbons was investigated using model triglyceride and fatty acid feeds, as well as algal lipids. In the semi-batch deoxygenation of tristearin at 260 °C a pronounced promotional effect was observed, a 20% Ni-5% Cu/Al2O3 catalyst affording both higher conversion (97%) and selectivity to C10-C17 alkanes (99%) in comparison with unpromoted 20% Ni/Al2O3 (27% conversion and 87% selectivity to C10-C17). In the same reaction at 350 °C, a 20% Ni-1% Sn/Al2O3 catalyst afforded the best results, giving yields of C10-C17 and C17 of 97% and 55%, respectively, which contrasts with the corresponding values of 87 and 21% obtained over 20% Ni/Al2O3. Equally encouraging results were obtained in the semi-batch deoxygenation of stearic acid at 300 °C, in which the 20% Ni-5% Cu/Al2O3 catalyst afforded the highest yields of C10-C17 and C17. Experiments were also conducted at 260 °C in a fixed bed reactor using triolein − a model unsaturated triglyceride − as the feed. While both 20% Ni/Al2O3 and 20% Ni-5% Cu/Al2O3 achieved quantitative yields of diesel-like hydrocarbons at all reaction times sampled, the Cu-promoted catalyst exhibited higher selectivity to longer chain hydrocarbons, a phenomenon which was also observed in experiments involving algal lipids as the feed. Characterization of fresh and spent catalysts indicates that Cu enhances the reducibility of Ni and suppresses both cracking reactions and coke-induced deactivation.

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.