Background: Towards the goal of integrating CO2 capture using aqueous ammonia with its utilization for algae cultivation, Scenedesmus acutus (UTEX B72) was grown in 1100 L open raceway ponds using CO2 and NH3 supplied from gas cylinders. CO2/NH3 mole ratios of 7 and 10 were employed, the gas mixture acting as a surrogate for the output from a CO2 scrubbing system using aqueous ammonia. Results: Compared to Scenedesmus acutus grown in open ponds using gaseous CO2 and NaNO3 as the N-source, the ponds supplied with gaseous CO2 and NH3 displayed higher productivity at both CO2/NH3 ratios, with the higher ratio providing the best growth. Depending on the culturing conditions and CO2/NH3 ratio, CO2 utilization ranged up to 15.8% and NH3 utilization to 23.0%. These rather low values reflect the fact the high CO2/NH3 feed rate used, resulting in a substantial release of NH3 from the ORPs (similar to 45%). Conclusions: These findings demonstrate the suitability of gaseous NH3 as a N-source for microalgae cultivation, while highlighting the need for a control strategy that closely balances the CO2/NH3 supply with the algae growth rate. The produced algae biomass possessed a high protein and low ash content, rendering it particularly suitable for use as a bioplastic feedstock. (c) 2025 Society of Chemical Industry (SCI).

Three Pd/H-CHA samples were prepared containing 53.0 %, 10.8 % and 6.5 % paired Al sites at near fixed Si/Al ratio and similar Pd loading. According to H2 temperature-programmed reduction, Pd was present almost exclusively as isolated cations in the two samples containing the higher concentrations of paired Al sites, whereas in the other sample PdO was also present. Simulated lean cold start tests on the fresh samples conducted in a microflow reactor showed that the sample containing PdO stored the lowest amount of NOx. When tested with CO/H2, the sample containing 53.0 % paired Al sites showed significantly better storage capacity than the other samples and deactivated less rapidly upon sequential tests. Experiments using lean gasoline engine exhaust revealed similar trends. This study showed that a high concentration of paired Al sites in Pd/H-CHA is beneficial for NOx storage capacity, thermal durability, and minimizing deactivation in the presence of CO/H2.

Integrating CO2 scrubbing from flue gas with its utilization in algae cultivation represents a potential means of lowering the cost of CO2 capture. Towards this goal, this study sought to assess the feasibility of using gaseous a CO2/NH3 stream, derived from CO2 capture using aqueous ammonia, as a C- and N-source for algae cultivation. Scenedesmus acutus was cultured in 800 mL photobioreactors using gaseous CO2/NH3 in mole ratios varying from 7 to 18. Excellent growth of Scenedesmus acutus was observed, the average growth rate for CO2/NH3 = 10 of 0.171 ± 0.015 g/L·day exceeding that obtained using 1% CO2/N2 and urea as the N-source (0.099 ± 0.28 g/L·day). Under optimal growth conditions (CO2/NH3 mole ratio of 10), CO2 utilization ranged from 57 to 72%, while the NH3 utilization was >90%. The CO2/NH3 feed rate was also found to exert a significant effect on algae productivity, with excessive feed rates leading to the accumulation of NH3 in the culture at concentrations that were toxic to the algae. Consequently, to avoid the toxic effects of high NH3 concentrations (>2.0 mM), it proved necessary to balance the NH3 supply with the algae growth rate so that excessive NH3 accumulation was prevented. This indicates that for practical applications, a CO2/NH3 feed control strategy would be required that takes into account the ammonium ion concentration in solution and the pH so as to avoid significant concentrations of free NH3. Analysis of the harvested biomass revealed a high protein (≥ 47 wt%) and a low ash content (< 3.6 wt%), suggesting it would be well suited for use as animal feed or as a feedstock for the production of bioplastics.

The incomplete reduction and poor dispersion of Ni sites restrict the catalytic performance of Ni catalysts in the decarboxylation/decarbonylation (DCX/DCN) of triglycerides to fuel-like hydrocarbons. In this study, by employing Pd as promoter and/or using multifunctional supports, the activity of Ni catalysts in tristearin deoxygenation was greatly improved. Notably, 25%Ni/Al2O3 and 0.75%Pd/Al2O3 displayed conversion values <= 2%. In contrast, 25%Ni-0.75%Pd/Al2O3 afforded 100% conversion, 100% yield of diesel-like hydrocarbons and a selectivity to C17 - the main product of DCN/DCX - of 67%. The fact that the performance of the Ni-Pd catalyst is vastly superior to that of the corresponding monometallic formulations is clearly indicative of a promotion effect resulting from the interaction between the two metals. Pd addition promoted Ni reduction, thereby increasing the number of active sites. The effect of different supports (Al2O3, SiO2-Al2O3, ZrO2, and Ce0.8Pr0.2O2) of Ni-Pd catalysts were studied, and the use of SiO2-Al2O3 was observed to enhance catalyst performance by further promoting Ni reduction as well as through its high surface area and strong acidity. Moreover, using SiO2-Al2O3 as the support partially changes the deoxygenation pathway from DCX/DCN to hydrodeoxygenation, which is attributed to the ability of strong acid sites to catalyze the dehydrogenation of an alcohol intermediate to the corresponding alkene. (C) 2022 Elsevier Ltd. All rights reserved.

Current vehicles generate a large fraction of their total tailpipe emissions during the beginning of the cold start test. For a current 3.5L GTDI (gasoline turbocharged direct injection) production vehicle, the majority (>60%) of CO, HC, and NOx are emitted in the first 60 seconds before the three-way catalyst reaches operating temperature of 350°C. Clearly, a means of storing HC and NO during cold starts would have a meaningful, measurable impact on vehicle tailpipe emissions. Against this background, this project aimed to develop fundamental understanding of the chemistry of NO/HC adsorption and reaction in Pd-zeolites so as to facilitate the rational design of passive NOx adsorber (PNA) catalysts. Given that numerous publications exist concerning HC trapping by Pd-zeolites, emphasis was placed on identifying the factors controlling NOx adsorption in Pd-zeolites. The approach adopted combined both experimental and computational methods, which together allow a deeper understanding of the governing chemistry than the use of either method alone. The workflow began with Pd/H-CHA and Pd/H-BEA catalyst synthesis and characterization, in which the Si/Al ratio and extent of Al pairing were systematically varied. This was followed by catalyst evaluation using temperature-programed adsorption/desorption methods, as well as in situ diffuse reflectance UV-vis spectroscopy (DRUVS), X-ray absorption near edge spectroscopy (XANES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission IR spectroscopic measurements to probe the nature of the NO adsorption sites and the form of the adsorbed NO. In parallel, the adsorption of NO and other relevant species (H2O, CO, HCs) was studied by means of quantum chemical calculations in order to rationalize the experimental data and provide additional insights. Catalyst aging studies were also performed with the aim of elucidating the mechanism(s) of catalyst degradation. Finally, the insights gained in this project were applied to the preparation of an optimized PNA catalyst, the performance of which was validated using exhaust gas from an engine dynamometer. Key findings included the observation that the dominant mechanism of formation for ion-exchanged Z2Pd and Z[PdOH] species in Pd-CHA is solid-state ion-exchange, and Z2Pd species are likely the most thermodynamically stable ion-exchanged structure after air treatments (>400 °C). CO-DRIFTS showed the presence of Pd+-CO bands at low Pd loadings on Pd-BEA with increasing Pd2+ content at higher loadings. Reduction of ionic Pd and PdO species by H2 was found to generate large quantities of small, pore-confined Pd metal particles, H2-TPR experiments showing that Pd-CHA has a greater ability to maintain Pd in an ionic state than does Pd-BEA and achieves complete recovery of ionic Pd species by re-oxidation treatments regardless of the reductant employed. However, reduction in CO was shown to produce larger Pd metal particles than reduction in H2; moreover, a lower degree of Pd re-dispersion upon re-oxidation was also observed for CO-reduced Pd-BEA compared to the corresponding H2-reduced sample. Quantum mechanical calculations showed that ionically dispersed Pd species (Pd+ and Pd2+) are thermodynamically most favorable in Pd-CHA over a wide range of oxidizing and reducing conditions. Calculated Gibbs free energies for NO adsorption show that NO generally binds more strongly on Pd+ sites than on Pd2+ sites, suggesting that the former are responsible for the high-temperature desorption peak observed in TPD experiments, while the latter contribute to the observed low-temperature feature. When evaluated in simulated exhaust gas, the presence of reductants (C2H4, C3H6, CO/H2) improved the NOx storage performance of Pd/zeolite passive NOx adsorbers under lean conditions but not stoichiometric conditions and mitigated the inhibitory effects of water. Against this, repeated cold start tests showed progressive decline in NO storage capacity, which could be linked to facile reduction of Pd by CO and H2O. For CHA, increasing the percentage of paired Al sites in the zeolite was beneficial for improving robustness to repeated cold starts and improved thermal durability. However, the low intrinsic storage efficiency (at best 1 NO per Pd) and the continuing high cost of palladium, along with remaining durability challenges, create an unfavorable business case for implementing Pd/zeolite passive NOx adsorbers on vehicles at the current time.

The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NO x adsorbers were examined under various pretreatment conditions. Using in situ CO-diffuse reflectance infrared spectroscopy, Pd structures were characterized after 500 °C pretreatments in inert (Ar), water (1-2% H2O in Ar), oxidizing (air), and reducing (H2, CO) atmospheres. Two zeolites of similar Si/Al ratios but different framework topologies (Beta, CHA) were found to show different distributions of Pd species, depending on the reducing agent used. Reduction in H2 (500 °C; 10% H2 in Ar) followed by re-oxidation (500 °C; air) led to higher amounts of single-site Pd ions on Pd-CHA than Pd-Beta, whereas high-temperature reduction in CO (500 °C; 1000 ppm CO in Ar) followed by re-oxidation (500 °C; air) led to significant loss of ionic Pd on both Pd-CHA and Pd-Beta, albeit H2 temperature-programmed reduction and XPS experiments suggest that this phenomena may be limited to surface Pd. High-temperature treatments with water (500 °C; 1-2% H2O in Ar) are shown to form either Pd metal or PdO particles, with Pd-Beta being more susceptible to these effects than Pd-CHA. This work suggests that the effects of CO are especially problematic with respect to the durability of these materials in passive NO x adsorption applications, especially in the case of Beta zeolite.

The study investigated a synergistic effect of the hybrid Pt-Cd additives on a cobalt catalyst. Adding 1%Cd to 0.5%Pt-15%Co/Al2O3 led to 90% activity loss and significantly increased CH4 and CO2 selectivities. HR-TEM/STEM micrographs revealed that a fraction of Cd nanoparticles deposited on large Co particles, resulting in the severe deactivation. More interestingly, the hybrid Cd-Pt additives were found to have a synergistic effect on the secondary reactions of 1-olefins; each modifier counteracted the other individual additive's role for the formation of olefins. For example, Cd suppressed the 1-olefin hydrogenation ability of Pt and Pt suppressed the 1-olefin isomerization ability of Cd. The XPS, TEM and NH3-TPD studies revealed that the Pt-Cd dual additives provided unique electronic effect (e.g., Pt and Cd enhanced electronic backdonation and Pt blocked electron transfer), which improved 1-olefin selectivity and suppressed 2-olefin selectivity of the cobalt catalyst.

An effective model for describing NOx adsorption and desorption on a PtPd/CeO2-ZrO2 passive NOx adsorber is presented. The kinetic parameters are evaluated from the available experimental data obtained during NOx adsorption/desorption experiments including CO2 and H2O in the feed, performed at 80, 120 and 160 °C both in the presence and in the absence of reducing agents (CO or C2H4 ). The model describes the temperature dependence of the NOx adsorption rate and capacity, the impact of CO, and dynamics of the NOx desorption events. The model predicts formation of nitrites, nitrates, and additional storage enabled in the presence of CO. Thermal decomposition of the stored NOx species results in two main desorption peaks. Nitrites are desorbed at lower temperatures while nitrates are thermally more stable. The evolution of nitrite and nitrate species in the model corresponds with the measured DRIFTS spectra of the catalyst surface. The presence of CO significantly improves the rate of NOx adsorption and storage efficiency at low temperatures, most probably due to reduction of oxidic Pt and Pd nanoparticles. The developed model captures well the observed trends and can be utilized for simulations of PNA performance under real operating conditions.

Pd-promoted ZrO2 and WO3-ZrO2 (W-Zr) were investigated for low temperature NOx adsorption and release. Pd-promoted W-Zr exhibited high NOx storage efficiency at short storage times, subsequently releasing similar to 95% of the stored NOx upon thermal ramping to 350 degrees C. DRIFTS studies demonstrated that Pd increased nitrate formation relative to nitrite during NOx storage on both Pd-Zr and Pd-W-Zr. Moreover, Pd sites on Pd-W-Zr played a major role in NOx storage, the ad-species being readily removed by 350 degrees C. From NO- and CO-DRIFTS data, it is inferred that Pd on the acidic W-Zr support was present as mainly cationic species, and was therefore able to adsorb NO, whereas on ZrO2 Pd was not able to directly store NOx. Co-feeding CO with NO resulted in increased NOx storage capacity for Pd-W-Zr, which on the basis of DRIFTS measurements is attributed to the formation of Pd2+ (CO)(NO) complexes.

The effects of 1% of Cd, In and Sn additives on the physicochemical properties and Fischer-Tropsch synthesis (FTS) performance of a 15% Co/Al2O3 catalyst were investigated. The fresh and spent catalysts were characterized by BET, temperature programmed reduction (TPR), H2-chemisorption, NH3 temperature programmed desorption (TPD), X-ray absorption near edge spectroscopy (XANES), and X ray diffraction (XRD). The catalysts were tested in a 1 L continuously stirred tank reactor (CSTR) at 220 °C, 2.2 MPa, H2/CO = 2.1 and 20–55% CO conversion. Addition of 1% of Cd or In enhanced the reduction degree of 15%Co/Al2O3 by ~20%, while addition of 1% Sn slightly hindered it. All three additives adversely impacted Co dispersion by 22–32% by increasing apparent Co cluster size based on the H2-chemisorption measurements. However, the decreased Co active site density resulting from the additives did not result in a corresponding activity loss; instead, the additives decreased the activity of the Co catalysts to a much greater extent than expected, i.e., 82–93%. The additional detrimental effect on catalyst activity likely indicates that the Cd, In and Sn additives migrated to and covered active sites during reaction and/or provided an electronic effect. XANES results showed that oxides of the additives were present during the reaction, but that a fraction of metal was also likely present based on the TPR and reaction testing results. This is in contrast to typical promoters that become metallic at or below ~350 °C, such as noble metal promoters (e.g., Pt, Ru) and Group 11 promoters (e.g., Ag, Au) on Co catalysts in earlier studies. In the current work, all three additives remarkably increased CH4 and CO2 selectivities and decreased C5+ selectivity, with the Sn and In additives having a greater effect. Interestingly, the Cd, In, or Sn additives were found to influence hydrogenation and isomerization activities. At a similar conversion level (i.e., in the range of 40–50%), the additives significantly increased 2-C4 olefin content from 3.8 to 10.6% and n-C4 paraffin from 50 to 61% accompanied by decreases in 1-C4 olefin content from 48 to 30%. The Sn contributed the greatest impact on the secondary reactions of 1-olefins, followed by the In and Cd. NH3-TPD results suggest enhanced acid sites on cobalt catalysts resulting from the additives, which likely explains the change in selectivities for the different catalysts.

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 degrees 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 degrees 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 degrees C), followed by loss of nitrates (200-400 degrees C). Nitrates stored on Pd/MnCe(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 degrees 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- and Pd-promoted CexZr1-xO2 mixed oxides were characterized and investigated for:passive NOx adsorber applications. X-ray diffraction analysis revealed a phase transition from tetragonal to cubic with increasing cerium content in CexZr1-xO2, while H-2 and CO chemisorption data in all cases indicated average Pt and Pd particle sizes of close to 2 nm. 142 temperature programmed reduction. (THY) measurements revealed a shift of the Pt reduction peak to higher temperature with increasing Ce content, consistent with a corresponding increase hi the degree of Pt oxidation. According to niictoreactor data, doping Ce into the ZrO2 lattice resulted in a significant improvement in low temperature (80-160 degrees C) NOx storage efficiency. Diffuse reflectance infrared Fourier transform spectroscopy measurements on Pt/CexZr1-xO2 showed that as Ce content increased, relatively more nitrite species were generated during NOx storage. However, oxidation of nitrite to nitrate during subsequent NOx-TPD increasing the concentration of more thermally stable nitrate also =correlated with increased Ce content. The use of Pd as a promoter resulted in decreased NOx storage efficiency compared to Pt, although low-temperature NOx desorption behavior was improve& This is attributed to decreased formation of nitrate during NOx storage compared to that of Pt, as well as the lower activity of Pd for oxidation of nitrite to nitrate during subsequent NOx-TPD. To achieve more balanced NOx storage and desorption behavior, Ce0.2Zr0.8O2 was promoted with both Pt and Pd, resulting in superior overall. NOx performance relative to its Pt and Pd analogues. After hydrothermal aging at 750 degrees C for 16, h, the copromoted sample still maintained excellent NOx adsorptidn desorption performance.

CeO2 promoted with either 1 wt% Pt or 1 wt% Pd was evaluated as a model system for passive NO x adsorber applications. According to NO x storage experiments conducted over the temperature range 80–160 °C, promotion with Pt resulted in higher NO x storage efficiencies than for the Pd-promoted material. However, for NO x storage at 80 and 120 °C, the latter released more NO x below 350 °C during temperature-programmed desorption in relative and absolute terms, the Pt analog releasing most of its stored NO x above 350 °C. DRIFT spectra showed that the use of Pd leads to preferential adsorption of NO x in the form of nitrites, while predominately nitrate formation is observed over the Pt promoted material. Overall, the use of Pd promoted CeO2 is preferred for low temperature NO x storage and release due to its ability to store NO x as thermally labile nitrites.

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 degrees 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% of liquid product) in comparison with unpromoted 20% Ni/Al2O3 (27% conversion and 87% selectivity to C10-C17). In the same reaction at 350 degrees 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 degrees 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 degrees C in a fixed bed reactor using triolein - a model unsaturated triglyceride - as the feed. While both 20% Ni/Al2O3 and 20% Ni similar to 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. (C) 2016 Elsevier B.V. All rights reserved.

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 NO x adsorber (PNA) use. During NO x storage at 120 °C, it was found that the amount of NO x 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, NO x 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 NO x -temperature-programmed desorption (TPD), two NO x 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 NO x 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 NO x 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 NO x adsorption, and by the superior NO oxidation activity of Pt relative to Pd, which promotes NO x storage as nitrates possessing high thermal stability.

•NOx storage on Pt/Al2O3 and Pt/La-Al2O3 was studied in the temperature range 80–160°C.•La addition improved NOx storage but lowered NOx desorption efficiency below 250°C.•Catalyst pretreatment significantly impacted NOx storage and desorption behavior.•DRIFTS showed that nitrites and weakly bound nitrate species were initially removed during NOx-TPD.•Nitrites were also converted to more thermally stable nitrates during NOx-TPD.

Introduction The control of NOx emissions from lean-burn engines represents an on-going challenge to the automotive industry, particularly at the low exhaust temperatures associated with modern, fuelefficient engines. The ability to deploy urea-SCR at low operating temperatures is limited due to the slow rate of urea decomposition. Consequently, a system is required which combines low temperature NOx storage with subsequent NOx reduction. An attractive option which to date has been little explored is the use of a passive NOx adsorber (PNA) device in combination with a urea SCR catalyst. In this system, the PNA adsorbs NOx emitted from the engine during cold starts, and then releases the NOx at higher temperatures, e.g., above 200 °C. At this point the SCR catalyst is sufficiently warm to function efficiently. The first reference to such a concept appears to be a U.S. Patent granted to Ford in 2001 [1], in which γ-Al2O3 was claimed as the NOx adsorber, promoted with a platinum group metal. A recent presentation [2] made by Cummins showed that a PNA/urea SCR system was able to achieve lower emissions on a light duty (V8) truck than the corresponding urea SCR-only system. To acquire the basic knowledge required for the development of effective PNA materials, we performed an initial study using a simple Al2O3-based PNA.

Introduction In catalytic applications, ceria is frequently doped with rare earth elements in order to modify its electronic and/or structural properties. In the case that trivalent cations are incorporated into the CeO2 lattice, anion vacancies are created by a charge-compensating effect, resulting in an increase in oxygen-storage capacity (OSC). If the dopant has variable oxidation state, as is the case with praseodymium, the resulting vacancies are extremely mobile. This strongly influences the redox properties of the ceria by increasing both total and kinetic oxygen storage [1]. In view of the foregoing, we reasoned that the addition of praseodymium to ceria should be beneficial for NOx storage-reduction applications. Indeed, a study by Rohart and co-workers [2] found that doping 8 wt% Pr into a Ce-Zr mixed oxide resulted in a significant increase in the efficiency of NOx storage (for samples loaded with 1 wt% Pt), particularly at lower temperatures (200-350 oC). This increase was found to correlate with improved activity for NO oxidation over this temperature range. However, despite this promising result, there do not appear to have been any other reports concerning the properties of Ce-Pr mixed oxides in NOx storage-reduction (NSR). Therefore, in this study we studied model Pt/Ce0.9Pr0.1O2 NSR catalysts prepared via several different routes and, for comparison purposes, their Pt/CeO2 analogs.

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, H-2-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. (C) 2014 Elsevier B.V. All rights reserved.