ChemInformVolume 47, Issue 37 Reviews ChemInform Abstract: Forty Years of Catalysis by Ceria: A Success Story Robert W. McCabe, Robert W. McCabe Div. Chem. Bioeng. Environ. Transport Syst., Univ. Udine, I-33100 Udine, ItalySearch for more papers by this authorAlessandro Trovarelli, Alessandro Trovarelli Div. Chem. Bioeng. Environ. Transport Syst., Univ. Udine, I-33100 Udine, ItalySearch for more papers by this author Robert W. McCabe, Robert W. McCabe Div. Chem. Bioeng. Environ. Transport Syst., Univ. Udine, I-33100 Udine, ItalySearch for more papers by this authorAlessandro Trovarelli, Alessandro Trovarelli Div. Chem. Bioeng. Environ. Transport Syst., Univ. Udine, I-33100 Udine, ItalySearch for more papers by this author First published: 25 August 2016 https://doi.org/10.1002/chin.201637227AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume47, Issue37August, 2016 RelatedInformation

Thermal aging characteristics of model automotive Pd/Al2O3 catalyst materials were examined under simulated aging and light-off conditions as a function of aging temperature (up to 1000 degrees C) in both reducing and oxidizing gas mixtures, and in the presence and absence of steam. the catalysts were characterized by static H-2 chemisorption, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and both CO as well as propylene light-off experiments. Aging-induced changes in the Al2O3 support were modest and limited to slight loss of BET area and transition from predominantly gamma-phase (as received) to theta-phase (after 1000 degrees C aging). The main deactivation mode of aging was loss of Pd surface area via metallic Pd sintering, especially above the PdO decomposition temperature, as evidenced by higher light-off temperatures. Aging gas composition also impacted the Pd particle size, with more pronounced growth in rich gas (H-2) mixtures. In contrast, particle growth was minimal in lean (O-2) gas mixtures below the PdO decomposition temperature, and sintered Pd particles could be modestly re-dispersed by treating below the PdO decomposition temperatures in O-2 and in the absence of steam. The study suggests strategies for operating vehicles in ways that mitigate or reverse aging effects related to Pd particle growth. (C) 2014 Elsevier B.V. All rights reserved.

Four high-surface-area (50-60 m(2)/g) perovskite-based powder catalysts, Pd-doped LaFeO3, Pdimpregnated LaFeO3, Rh-doped CaTiO3, and Rh-impregnated CaTiO3, were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14h at 800 degrees C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts. (C) 2014 Elsevier B.V. All rights reserved.

In situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS), temperature-programmed desorption (TPD), and temperature-programmed surface reactions (TPSR) were employed to investigate the adsorption and reactive properties of Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts; these fully formulated washcoat cordierite monoliths were hydrothermally treated at 750 degrees C in the simulated feed gases. The intrinsic mechanism and reasons for the differences in NH3-SCR activity were proposed based on the characterization results. The in situ DRIFTS and TPD results showed that ammonia could adsorb on both the Lewis and Bronsted acidic sites on these two catalysts; the ammonia on the Bronsted acidic sites might be active in the NH3-SCR reaction. For the different NOx adsorption processes, the total NO desorption levels followed the following sequence: NO < NO + O-2 < NO2 = NO2 + O-2. The results confirm that the reaction pathways are totally different in the low and high temperature ranges on the Cu-SSZ-13 and Cu-SAPO-34 catalysts. In the low temperature range, the ammonium nitrates from the reaction between surface ammonia and nitrates are the key intermediates and are further reduced to form N-2 and H2O by the NO gas. In the high temperature range, the gas-phase NO2 gradually become more important in the NH3-SCR reaction. The activity tests indicated that the Cu-SAPO-34 catalyst had a relatively higher DeNO(x) performance than the Cu-SSZ-13 catalyst across the entire reaction temperature range, showing a double peak shapes with a dip point at approximately 390 degrees C. Cu-SAPO-34 might retain many surface nitrate species and did not produce much more NO2 gas than Cu-SSZ-13; this species hindered the SCR reaction at 390 degrees C. (C) 2014 Elsevier B.V. All rights reserved.

Temperature-programmed decomposition of PdO supported on high-surface-area alumina occurs in a two-step process starting near the temperature of bulk PdO decomposition, but slowing to near-zero rates as the bulk oxide decomposition temperature is traversed. Most of the oxide decomposes in a second step 40-60 degrees C above the bulk PdO decomposition temperature. We present evidence suggesting that a thin Pd shell forms initially on the surface of PdO particles and propose that the resulting core-shell structure is connected with the observed meta-stability. Both the experimental results and order-of-magnitude estimates indicate that neither small particle effects (metal-support interaction or surface energy) nor low oxygen permeability through the Pd shell explain the two-step process. We tentatively ascribe the higher-temperature PdO decomposition step to the contribution of strain energy to the Pd shell imparted by epitaxy with the underlying PdO lattice. (C) 2014 Elsevier B.V. All rights reserved.

A chromatographic adsorption unit was designed and built to study the adsorption of alkanes in zeolites for the hydrocarbon trap systems in three-way catalysts, to assist in the selection of optimal materials for this application. The experimental apparatus used a zeolite bed in place of the column in an ordinary gas chromatograph and could be accurately modeled to determine the adsorption equilibrium constants for simple alkanes in MFI zeolites. The adsorption of iso-pentane was studied in BEA zeolites with varying Si/Al 2 ratios, before and after ex situ zeolite aging simulating engine exhaust, and in the presence of water vapor. The elution times were shown to depend directly on the zeolite adsorption capacity. The primary effect of water was to decrease the iso-pentane adsorption capacity by partial filling of the zeolite pores through adsorption of water at acid sites. Some implications of this work for choosing the best materials for hydrocarbon trapping are discussed. © 2014 American Institute of Chemical Engineers AIChE J , 60: 2875–2881, 2014

Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts were fully formulated washcoat cordierite monoliths purchased from the major catalyst suppliers and were hydrothermally aged at 600, 750, and 850 °C in simulated exhaust gases containing water. Their catalytic activities were tested in selective catalytic reduction (SCR) of NOx with ammonia. The microstructure of the zeolite supports, identity of copper species, acidity and reactant adsorption ability were investigated in detail using various characterization methods. The results showed that hydrothermal treatment of Cu-SSZ-13 and Cu-SAPO-34 catalysts had a significant influence on the physicochemical properties and reactant adsorption abilities of the catalysts, which resulted in different catalytic performances in NH3-SCR. The hydrothermal treatment drastically decreased the surface area and pore volume of both the Cu-SSZ-13 and Cu-SAPO-34 catalysts. TEM results indicated that obvious aggregation of Cu-SSZ-13 and Cu-SAPO-34 occurred while some CuO particulates migrated from isolated Cu2+ species were formed during the aging test. Hydrothermal treatment of the Cu-SSZ-13 and Cu-SAPO-34 catalysts caused the migration of Cu2+, a decrease in acidity and dealumination. The change in the coordination environment of Cu2+ affected NOx adsorption and activation on the catalyst surface. Both the copper sites and the acidity might be the main factor for the NH3-SCR reaction on Cu-CHA catalyst.

This annual report describes recent progress on a collaborative project between scientists and engineers in the Institute for Integrated Catalysis at PNNL and at Ford Motor Company, involving investigations of laboratory- and engine-aged SCR catalysts, containing mainly base metal zeolites. These studies are leading to a better understanding of various aging factors that impact the long-term performance of SCR catalysts and improve the correlation between laboratory and engine aging, saving experimental time and cost. We are investigating SCR catalysts with reduced ammonia slip, increased low temperature activity, and increased product selectivity to N2. More recent recognition that high temperature performance, under regimes that sometimes cause deactivation, also needs to be improved is driving current work focused on catalyst materials modifications needed to achieve this enhanced performance. We are also studying materials effective for the temporary storage of HC species during the cold-start period. In particular, we examine the adsorption and desorption of various HC species produced during the combustion with different fuels (e.g., gasoline, E85, diesel) over potential HC adsorber materials, and measure the kinetic parameters to update Ford’s HC adsorption model.

NOx storage-reduction experiments were performed using a coupled LNT-SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu–zeolite SCR catalyst. Cycling experiments revealed that when a CO+H2+C3H6 mixture or C3H6 by itself was used as the reductant, the NOx conversion over the SCR catalyst exceeded the conversion of NH3 over the same catalyst. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NOx slip. Separate experiments, conducted under continuous flow and lean-rich cycling conditions, confirmed the ability of propene, as well as ethene, to function as a NOx reductant over the SCR catalyst. Cycling experiments also revealed that the SCR catalyst was able to store propene, such that NOx reduction by stored propene continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH3 and propene occured in the SCR catalyst, while under lean-rich cycling conditions the contributions of NH3 and C3H6 to NOx conversion were found to be essentially additive. These findings suggest that under actual driving conditions, NOx reduction by non-NH3 reductants (olefins and possibly other hydrocarbons) in the SCR catalyst can contribute to the mitigation of lean and rich phase NOx.

An overview is presented of laboratory and vehicle studies conducted at Ford Motor Company on the LNT+in situ SCR catalyst approach to diesel emissions control. Specifically, the paper focuses on results obtained with so-called “2nd-generation” systems developed by Ford's catalyst suppliers since 2007–2008. These systems are characterized by improvements in durability, performance and cost relative to their predecessors. Key features of the 2nd-generation LNT+in situ SCR technology include: (1) lower desulfation temperatures of the LNT, (2) lower LNT platinum group metal (PGM) content than required for LNT-only systems, (3) advanced SCR catalysts of the types used in commercial zeolite-based SCR applications, (4) synergetic coupling of the LNT and SCR catalyst performance to maintain high efficiency after aging, (5) a non-ammonia NOx reduction mechanism over the SCR catalyst in addition to the standard mechanism based on ammonia storage, and (6) improved hydrocarbon oxidation efficiency owing to the SCR catalyst. Taken together, these advances improve the prospect of meeting future emission regulations on light-duty diesel vehicles, but not without considerable challenge to both the aftertreatment system and control strategy for achieving simultaneous low NOx and non-methane hydrocarbon emissions while minimizing fuel consumption.

Degreened samples of monolithic catalysts containing Pd-only, Pt-only, Pt/Rh (5/0/1), and Pt/Pd/Rh (1/13/1 and 1/4/1) were reduced in rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature was increased from test to test, in order to assess the effects of the recent thermal-chemical history on the conversion of CH4, C3H8, and C3H6. A highly loaded Pt-only catalyst displayed relatively consistent HC conversion on the consecutive lean temperature ramps. However, the HC conversion of the other catalysts degraded from run to run as the maximum temperature on the previous test increased. For the Pd-only catalyst, this degradation was attributed to the increasing oxidation of metallic Pd to Pd oxide. While previous studies suggested that Pd oxide is more active than metallic Pd for CH4 and C3H8 conversion, the continued oxidation increasingly depleted the catalyst of metallic Pd sites, which may be involved in the dissociative adsorption of the HC. Alternatively, the continued oxidation may have decreased the phase boundary between Pd metal and Pd oxide, which has been reported to enhance CH4 combustion. When the Pd-only sample was exposed to ca. 800 °C under lean conditions, where Pd oxide is known to decompose, the CH4 conversion on the subsequent test improved dramatically and was similar to that after the rich treatment.

SpaciMS was employed to understand the factors influencing the selectivity of NOx reduction in two fully formulated LNT catalysts, both degreened and thermally aged. Both catalysts contained Pt, Rh, BaO and Al2O3, while one of them also contained La-stabilized CeO2. The amount of reductant required to fully regenerate each catalyst was first determined experimentally based on the OSC of the catalyst and the NOx storage capacity (NSC). In this way a correction was made for the change in catalyst OSC and NSC after aging, thereby eliminating these as factors which could affect catalyst selectivity to NH3. For both catalysts, aging resulted in an elongation of the NOx storage–reduction (NSR) zone due to a decrease in the concentration of NOx storage sites per unit catalyst length. In addition to decreased lean phase NOx storage efficiency, stretching of the NSR zone affected catalyst regeneration. Three main effects were identified, the first being an increase of the NOx “puff” that appeared during the onset of the rich front as it traversed the catalyst. Spatially, NOx release tracked the NSR zone, with the result that the NOx concentration peaked closer to the rear of the aged catalysts. Hence the probability that NOx could re-adsorb downstream of the reduction front and subsequently undergo reduction by NH3 (formed in the reduction front) was diminished, resulting in higher rich phase NOx slip. Second, the stretching of the NSR zone resulted in increased selectivity to NH3 due to the fact that less catalyst (corresponding to the OSC-only zone downstream of the NSR zone) was available to consume NH3 by either the NH3-NOx SCR reaction or the NH3-O2 reaction. Third, the loss of OSC and NOx storage sites, along with the decreased rate of NOx diffusion to Pt/Rh sites (as a result of Pt/Rh–Ba phase segregation), led to an increase in the rate of propagation of the reductant front after aging. This in turn resulted in increased H2:NOx ratios at the Pt/Rh sites and consequently increased selectivity to NH3.

The effect of regeneration conditions on NH3 formation in a fully formulated Pt–Rh/BaO/Al2O3 lean NOx trap catalyst was investigated. Experiments were performed on a bench flow reactor under simulated diesel exhaust conditions, employing NOx storage/reduction cycles. Using CO/H2 as the reductant, the selectivity of NOx reduction to NH3 increased with increasing regeneration time, reductant concentration and space velocity, and decreased with increasing amount of stored NOx and increasing temperature. At a given temperature the effect of these parameters on NH3 selectivity can be interpreted in terms of the local H2:NOx ratio at the precious metal sites and the extent to which NH3 is consumed in the reductant front as it propagates through the catalyst. However, selectivity to NH3 increased with increasing temperature (>300°C) during rich purging using C3H6 as the reducing agent. It was shown that NH3 selectivity was governed by the steam reforming activity of the catalyst, selectivity to NH3 increasing with increasing H2 generation. Experiments using a second catalyst to which ceria had been added as an OSC material confirmed these trends, although the presence of the ceria resulted in lower selectivity to NH3 when using H2 and/or CO as the reductant. After aging, the catalysts displayed increased selectivity to NH3; this is attributed in part to lengthening of the NOx storage-reduction zone, as demonstrated by SpaciMS data, and decreased OSC, resulting in decreased NH3 consumption by NOx and O2 downstream of the reductant front.