Rh Loading Research Articles

Photocatalytic Reforming of Ethanol in the Liquid Phase Using a Ternary Composite of Rh/TiO2/g-C3N4 as a Catalyst.

Photocatalytic reforming of ethanol provides an effective way to produce hydrogen energy using natural and nontoxic ethanol as raw material. Developing highly efficient catalysts is central to this field. Although traditional semiconductor/metal heterostructures (e.g., Rh/TiO2) can result in relatively high catalyst performance by promoting the separation of photoinduced hot carriers, it will still be highly promising to further improve the catalytic performance via a cost-effective and convenient method. In this study, we developed a highly efficient photocatalyst for ethanol reformation by preparing a ternary composite structure of Rh/TiO2/g-C3N4. Hydrogen is the main product, and the reaction rate could reach up to 27.5 mmol g-1 h-1, which is ∼1.41-fold higher than that of Rh/TiO2. The catalytic performance here is highly dependent on the wavelength of the light illumination. Moreover, the photocatalytic reforming of ethanol and production of hydrogen were also dependent on the Rh loading and g-C3N4:TiO2 ratio in Rh/TiO2/g-C3N4 composites as well as the ethanol content in the reaction system. The mechanism of the enhanced hydrogen production in Rh/TiO2/g-C3N4 is determined as the improvement in the separation of photoinduced hot carriers. This work provides an effective photocatalyst for ethanol reforming, largely expanding its application in the field of renewable energy and interface science.

Effect of Rh loading on the selectivity to methanol and ethanol in the hydrogenation of CO2 over the Rh/CeO2 catalyst

The capture and hydrogenation of CO2 into high-value chemicals such as alcohols is one of the important ways to reduce CO2 emission and achieve carbon resource recycling. In this work, the catalytic performance of Rh/CeO2 catalyst in the CO2 hydrogenation was investigated; with the help of various characterization methods including XRD, Raman, H2-TPR, CO2-TPD, CO-DRIFTS and XPS, the influence of Rh loading (0.1%–2.0%) on the catalytic activity of Rh/CeO2 and product selectivity in the CO2 hydrogenation was revealed. The results indicate that for the hydrogenation of CO2 at 250 °C and 3.0 MPa over the Rh/CeO2 catalysts, ethanol is the major product at a low Rh loading of 0.1%. With the increase of Rh loading, the conversion of CO2 increases, but accompanied by a decrease in the selectivity to ethanol; when the Rh loading reaches 2.0%, the main product turns to be methanol. It seems that the difference of various Rh/CeO2 catalysts with different Rh loadings in the product selectivity for the CO2 hydrogenation is ascribed to their difference in the structural and electronic properties of Rh; atomically dispersed Rh+ species favor the stabilization of CO* and its subsequent C–C coupling with CH3* to form ethanol, whereas metallic Rh clusters facilitate the hydrogenation of CO* to produce methanol.

Stereoselective Synthesis of anti-2,4-Disubstituted Tetrahydrofurans via a Pd-Catalyzed Hayashi-Heck Arylation and Rh-Catalyzed Hydroformylation Sequence.

A catalytic, two-step protocol for the expedient synthesis of anti-2,4-disubstituted tetrahydrofurans is described. In the first step, an enantioselective and regioselective Pd-catalyzed Hayashi-Heck arylation was developed using (R)-hexaMeOBiphep to generate 5-aryl-2,3-dihydrofurans. A subsequent Rh-catalyzed hydroformylation step proceeds at low Rh loading with high regio- and diastereoselectivity for the anti-2,4-disubstituted tetrahydrofuran isomer. Key to the development of the hydroformylation reaction was the utilization of either (R)-Me-i-Pr-INDOLphos or (R,R)-Ph-BPE to control the regioselectivity and provide the kinetic product isomer.

Rh, Co, and N-doped titanium mesh-based carbon nanosheet arrays for enhanced nitrite reduction reaction and ammonia generation

The electrocatalytic transformation of nitrite pollutants into valuable ammonia is crucial for the nitrogen cycle. However, the complex multi-reaction process necessitates the development of highly efficient and selective electrocatalysts. This study focuses on the creation of Rh-doped Co@NC anchored on a titanium mesh as an electrocatalyst to enhance the electrocatalytic nitrite reduction reaction (eNO2RR) for NH3 synthesis. By doping a trace amount of Rh within zeolitic imidazolate framework-67 nanosheets and subsequently pyrolyzing the material, uniformly distributed Rh and Co nanoparticles are anchored onto N-functionalized porous carbon nanosheet arrays. Consequently, the three-dimensional electrode enhances mass and electron transfer while providing abundant surface-exposed active sites for NH3 generation through eNO2RR. The Rh0.6 wt%–Co@NC/TM electrocatalyst, with a minimal Rh loading of 0.6 wt%, exhibits the highest Faradaic efficiency of 98.8 ± 0.7 % at −0.3 V, along with an NH3 yield of 1777.1 ± 7.0 μmol h−1 cm−2 at −0.7 V. Additionally, the electrocatalyst demonstrates exceptional durability over 20 consecutive cycles and remarkable environmental adaptability. Extensive experimental studies reveal that the superior performance of Rh0.6 wt%–Co@NC/TM arises from the synergistic catalysis between Co and Rh, which enhances NO2– adsorption and the production of active hydrogen for eNO2RR to NH3 synthesis. This research highlights that Rh incorporation facilitates the kinetics of eNO2RR and offers valuable insights for the development of efficient electrocatalysts towards NH3 synthesis.

Metal-assisted low-temperature cracking of n-hexane over Rh-encapsulated ZSM-5 catalysts

Naphtha cracking is the most common process for producing light olefins such as ethylene, propylene, and butene. This process consumes enormous amounts of energy, so decreasing the energy consumption and operating temperature is an urgent issue. To decrease the reaction temperature for naphtha cracking, we focused on the metal-assisted cracking reaction, in which paraffin is first dehydrogenated into the corresponding olefin on a metal catalyst, and the produced olefin is then decomposed into light olefins. To effectively realize metal-assisted cracking, we considered metal-encapsulated zeolite catalysts to be useful. In metal-encapsulated zeolite catalysts, the dehydrogenation reaction proceeds inside the zeolite particles, and the dehydrogenated intermediates can access the solid-acid sites frequently. In this study, Rh nanoparticle encapsulated ZSM-5 catalysts (Rh@ZSM-5) were employed for the metal-assisted cracking of n-hexane. Rh@ZSM-5 exhibited significantly high activity for n-hexane cracking below 450 °C owing to the metal encapsulation structure and close proximity between the metal and solid-acid sites. Furthermore, the effects of reaction conditions, reaction temperature, amounts of metal and solid-acid sites, and contact time on metal-assisted n-hexane cracking over the Rh@ZSM-5 catalysts were investigated. The highest light-olefin yield of 38.6 carbon mol% was achieved by the conversion of n-hexane over Rh@ZSM-5 using 0.3 wt% Rh loading, an Si/Al ratio of 100, temperature of 450 °C, reaction time of 0.5 h, and W/F of 2 h.

Efficient photocatalytic NADH regeneration with Rh-loaded Z-scheme mediator-free system

Nicotinamide adenine dinucleotide (NADH) regeneration is necessary for the sustainable application of enzymatic industry. The Rh-based complex [Cp*Rh(bpy)(H)]+ has been widely used as an important mediator in NADH regeneration systems, but it is limited by complexity and high cost. Here, a Z-scheme was constructed by loading Rh onto carbon nitride nanosheets/carbon nitride quantum dots (CN-CNQD). The resultant catalyst achieved a high yield of NADH in a mediator-free (M-free) system of 0.283 mmol L−1 g−1 min−1, which is 5.29 times that of pure CN. ADH enzyme introduction experiments confirmed that the enzyme active product 1,4-NADH could reach 34.21% selectivity in the M-free system. Mechanism research revealed that the heterojunction between CNs and CNQDs improved the NADH regeneration activity in the traditional M-involved system, while Rh loading was proved to optimize the yield and selectivity of 1,4-NADH in M-free system. The immobilized Rh shows more competitiveness than [Cp*Rh(bpy)(H)]+. This study contributes to the construction of an M-free system for further application in greener, lower-cost enzymatic processes.

Development and Characterization of Polymeric Composites Reinforced with Lignocellulosic Wastes for Packaging Applications

In this work, the effects of different fiber loadings on the mechanical properties of the composites at the sub-micron scale were studied through nanoindentation followed by physical characterization. The composites were prepared by incorporating different loadings of wheat straw, corn stalk, and rice husk in polypropylene copolymer using a melt processing method followed by thermal–hydraulic compression technique. Nanoindentation experiments in quasi-continuous stiffness mode were performed on the surfaces of produced composites to study the composites’ elastic modulus, hardness, and creep properties. The obtained results expressed the in-depth study of the micro- and macro-level structure and behavior of particle interactions. The findings demonstrated that observable shifts in composites’ hardness, elastic modulus, and creep rate had occurred. The WS-reinforced biocomposite sheet showed the highest elastic modulus of 1.09 and hardness of 0.11 GPa at 40 wt% loading in comparison to other loadings. An impact strength of 7.55 kJ/m2 was noted for the biocomposite at 40 wt% RH loading. In addition, optical microscopy, Fourier transform infrared spectroscopy, water absorption, thickness swelling, and Vicat softening point studies were conducted on biocomposite sheets to evaluate differences in physical, mechanical, and thermal properties. The outstanding mechanical performance of the newly developed composites makes them suitable for use as a biodegradable packaging material.

MIL-100(Fe)-derived catalysts for CO2 conversion via low- and high-temperature reverse water-gas shift reaction

Fe-derived catalysts were synthesized by the pyrolysis of MIL-100 (Fe) metal-organic framework (MOF) and evaluated in the reverse water-gas shift (RWGS) reaction. The addition of Rh as a dopant by in-situ incorporation during the synthesis and wet impregnation was also considered. Our characterization data showed that the main active phase was a mixture of α-Fe, Fe3C, and Fe3O4 in all the catalysts evaluated. Additionally, small Rh loading leads to a decrease in the particle size in the active phase. Despite all three catalysts showing commendable CO selectivity levels, the C@Fe* catalyst showed the most promising performance at a temperature below 500 °C, attributed to the in-situ incorporation of Rh during the synthesis. Overall, this work showcases a strategy for designing novel Fe MOF-derived catalysts for RWGS reaction, opening new research opportunities for CO2 utilization schemes.

Oxygen vacancy-engaged interfacial charge transfer modulation for upgrade Rh-catalyzed hydrogen and oxygen productions

Developing efficient modulation strategies to upgrade the catalytic activity and reusability of Rh-catalyzed hydrogen evolution from ammonia borane (AB) hydrolysis are definitely profitable but remains a grand challenge. Here, we develop a stepwise activation strategy to produce highly active and reusable Rh/CoFe2O4-SB-H2 with abundant oxygen vacancies and strong electronic metal-support interaction through stepwise reduction of Rh/CoFe2O4 precursor using sodium borohydride and H2 as the reducing agents. Under ultrasonic irradiation, Rh/CoFe2O4-SB-H2 with an ultralow Rh loading of 0.20 wt% can be utilized as an excellent catalyst for hydrogen production from room-temperature AB hydrolysis with a high turnover frequency (TOF) of 1894 min−1. The TOF value could be further promoted to 15,570 min−1 in the alkaline ultrasonic environment. The catalyst has a superior reusability with 75% maintaining activity of initial one in the 10th cycle. The strong electronic metal-support interaction, rich oxygen vacancies and ultrasound irradiation promote the oxidative cleavage of the O–H bonds in attracted H2O and thus account for high performance toward hydrogen production from AB. This catalyst can also be utilized as an active catalyst for oxygen generation from H2O2 decomposition. The developed strategies can be applied to upgrade the performance of other reducible metal oxides supported metal catalysts toward catalytic applications.

Enantioselective rhodium-catalyzed addition of arylboronic acids to N-heteroaryl ketones: synthesis of α-hydroxy acids.

The enantioselective addition of arylboronic acids to N-heteroaryl ketones provides a convenient access to chiral α-heteroaryl tertiary alcohols, yet addition reactions of this type have been challenging due to catalyst deactivation. In this report, an efficient rhodium-catalyzed addition of arylboronic acids to N-heteroaryl ketones is established, affording a variety of valuable α-heteroaryl alcohols with excellent functional group compatibility. The employment of the WingPhos ligand containing two anthryl groups is crucial for this transformation. In particular, a range of chiral benzoxazolyl-substituted tertiary alcohols were formed with excellent ee values and yields by employing a Rh loading as low as 0.3 mol%, which can serve as a practical protocol to furnish a series of chiral α-hydroxy acids after hydrolysis.

A novel and green synthesis of methylcyclohexane diisocyanate: Reaction properties, deactivation and regeneration of Rh/γ-Al2O3 catalyst in benzene ring selective hydrogenation

Methylcyclohexane diisocyanate (HTDI) is the core raw material for the synthesis of high-grade polyurethane. Aiming at the problems existing environmental pollution and safety risks in the phosgene process, a novel and green synthesis route of HTDI was proposed. We studied the catalytic hydrogenation of dimethyl toluene dicarbamate (TDC) benzene ring to methyl cyclohexyldicarbamate (HTDC) in this novel route. The effects of metal active components, supports and metal Rh loadings on the catalytic performance were investigated. It was found that the supported Rh/γ-Al2O3 catalyst had excellent catalytic activity and selectivity. The yield of HTDC was close to 100 % at 100 °C under 3 MPa hydrogen pressure. However, the catalyst was deactivated seriously after the reaction. The organic matter adsorbed on the catalyst surface and covered the active metal was the main reason for the catalyst deactivation. Fortunately, the deactivated catalyst could be regenerated by solvent washing and high temperature calcination.

Nature and Dynamic Evolution of Rh Single Atoms Trapped by CeO2 in CO Hydrogenation

Metal single-atom catalysts (SACs) have attracted extensive interest because of their maximum atom efficiency and potentially good performances. However, the understanding of the catalytic behavior and the stability of SACs in high-temperature catalytic reactions remains a major challenge, especially under reducing conditions. In this work, we investigated the nature and stability of Rh/CeO2 SACs with different Rh loadings (0.2–4 wt % Rh) during syngas conversion (260 °C, 2 MPa). Using CO-DRIFTS, we found that atomically dispersed Rh was dispersed on Rh/CeO2 catalysts with an Rh loading up to 3 wt %. The Rh/CeO2 catalysts were stable in H2 up to 350 °C, while Rh nanoclusters were formed in CO at 200 °C. During syngas conversion at 260 °C, the Rh single atoms (Rhiso) on a 0.5 wt % Rh/CeO2 SAC slowly agglomerated to form Rh nanoclusters (RhNC) and a Rhiso/RhNC ratio of ∼1:2.5 was obtained after ∼10 h time-on-stream run. Furthermore, the Rhiso/RhNC ratio did not change with further reaction, indicating that the reaction reached steady state after a 10 h run. After the catalysts reached steady state (∼10 h), the Rhiso/RhNC ratio on the spent Rh/CeO2 catalysts decreased with the increase in Rh loadings, while the reactivity increased first and then decreased. We observed an optimal value of Rhiso/RhNC ≈ 1:3 on the 1 wt % Rh/CeO2 catalyst, showing the maximum reaction rates of CO (rCO) and ethanol selectivity (SEtOH) of 85.8 μmol/s/gRh and 26.2%, respectively. This indicated that the combination of Rh single atoms and Rh nanoclusters is beneficial to the ethanol selectivity and reaction rate, which was confirmed by DFT simulations. Moreover, the ratios of Rhiso/RhNC on the spent catalysts under the steady state have good relationships with the CO conversion rate and product selectivity in the reaction.

Continuous synthesis of atomically dispersed Rh supported on MgAl2O4 using two‐stage microreactor

AbstractSingle‐atom catalysts with optimal atom utilization and outstanding activity have penetrated the frontier of heterogeneous catalysis. However, the large‐scale synthesis of this class of catalysts is still a bottleneck for their industrialization. Herein, we suggest a two‐stage micro‐dispersion approach to synthesize mesoporous MgAl2O4‐supported atomically dispersed Rh, which is more competitive than the batch method for boosting the uniform dispersion of Rh. By increasing the Rh loading, single‐atom catalysts (SACs, <0.05 wt%), single‐atom catalysts + nanoparticle catalysts (0.05–0.17 wt%), and nanoparticle catalyst (NPCs, 0.17–1.10 wt%) were obtained. For n‐octane steam reforming, the turnover frequency of the 0.01‐Rh‐MgAl2O4 was approximately 30 times that of the 1.10‐Rh‐MgAl2O4, while the Rh amount of the 0.01‐Rh‐MgAl2O4 was only 3% that of the 1.10‐Rh‐MgAl2O4 for the same fuel conversion. Under a high‐temperature (750°C) steam atmosphere for 15 h, the hydrogen formation rate only declined from 25.1 to 23.8 mol/mol‐C8H18.

Ultralow-Loading and High-Performing Ionic Liquid-Immobilizing Rhodium Single-Atom Catalysts for Hydroformylation.

We have developed a Keggin polyoxometalate (POM)-based ionic-liquid (IL)-immobilizing rhodium single-atom Rh catalyst (MTOA)5 [SiW11 O39 Rh] (MOTA=methyltrioctylammonium cation) that can afford exceptionally high catalytic activity for the hydroformylation of alkenes to produce aldehydes at an ultralow loading of Rh (ca. 3 ppm). For styrene hydroformylation, both the conversion and the yield of the aldehyde can reach almost 99 %, and a TOF as high as 9000 h-1 was obtained without using any phosphine ligand in the reaction process. Further characterization by FTIR, ICP and ESI-MS analysis revealed that the single Rh atom was incorporated in the lacunary POM anions. In particular, the bulky IL cation can play an additional role in stabilizing Rh species and thus prevent aggregation and leaching of Rh species. The IL catalyst was miscible with n-hexane at temperatures; this contributed to exceptionally high activity for hydroformylation even at ultra-low loading of IL catalyst.

Photocatalytic Hydrogen Production from Formic Acid Solution with Titanium Dioxide with the Aid of Simultaneous Rh Deposition

Photocatalytic hydrogen production was studied with a formic acid solution with titanium dioxide (TiO2) with the aid of simultaneous Rh deposition. The optimum conditions were as follows: Rh loading, 0.1 wt%; formic acid concentration, 1.0%; solution, pH 2.2; temperature, 50 °C. Under the optimum conditions, the photocatalytic hydrogen production with TiO2 by the simultaneous deposition of Rh was 5.0 mmol g−1, 12.2 mmol g−1 and 16.0 mmol g−1 after 1 h, 3 h and 5 h of irradiation time for black light, respectively. Rh/TiO2 photocatalysts were characterized by XRD, SEM, photoluminescence spectra, diffuse reflectance spectra and the BET surface area. The reaction mechanism of photocatalytic hydrogen production from formic acid by Rh/TiO2 was also proposed.

Continuous Room-Temperature Hydrogen Release from Liquid Organic Carriers in a Photocatalytic Packed-Bed Flow Reactor.

Despite the potential of hydrogen (H2) storage in liquid organic carriers to achieve carbon neutrality, the energy required for H2 release and the cost of catalyst recycling have hindered its large‐scale adoption. In response, a photo flow reactor packed with rhodium (Rh)/titania (TiO2) photocatalyst was reported for the continuous and selective acceptorless dehydrogenation of 1,2,3,4‐tetrahydroquinoline to H2 gas and quinoline under visible light irradiation at room temperature. The tradeoff between the reactor pressure drop and its photocatalytic surface area was resolved by selective in‐situ photodeposition of Rh in the photo flow reactor post‐packing on the outer surface of the TiO2 microparticles available to photon flux, thereby reducing the optimal Rh loading by 10 times compared to a batch reactor, while facilitating catalyst reuse and regeneration. An example of using quinoline as a hydrogen acceptor to lower the energy of the hydrogen production step was demonstrated via the water‐gas shift reaction.

Experimental and DFT studies on diesel-steam-reforming to hydrogen over a bimetallic Rh–Ni-based MgO-Al2O3 microsphere catalyst

Surface modification of 600-µm-sized monodisperse gamma-alumina microspheres into magnesium–aluminum spinel, and co-impregnation-based loading of Rh–Ni (0.46 wt% Rh, 9.03 wt% Ni) were performed to synthesize catalysts for diesel-steam-reforming to hydrogen. The intimate interactions between Rh and Ni, the formation of the Rh–Ni alloy, and the Rh-doping-induced decrease in the particle size of Ni were determined. The DFT calculations results revealed that Rh-doped Ni(111) established remarkably weak bonds with oxygen-containing intermediates, and the reaction barrier of the rate-limiting step (CH2* + O* → CH2O*) on Rh–Ni(111) (0.76 eV) was lower than those of Ni(111) (1.14 eV) and Rh(111) (1.56 eV). The bimetallic catalyst shows the enhanced activity and resistance of sintering and carbon deposition compared with those of Ni-based (9.33 wt% Ni) and Rh-based (0.47 wt% Rh) catalysts. The catalytic performance of Rh–Ni-based catalyst was superior to that of physically mixed Ni-based and Rh-based catalysts at similar active metal loadings, which further confirmed the synergistic effect of Rh–Ni.

Atomic layer deposition of Rh/ZnO nanostructures for anti-humidity detection of trimethylamine

The design of metal/semiconductor hybrid nanostructures is an important strategy for the development of efficient chemical gas sensors. In this paper, a highly sensitive gas-sensing material for trimethylamine (TMA) detection was designed by atomic layer deposition (ALD) of Rh onto ZnO flower-like nanostructures. The ZnO nanostructures consisted of porous nanosheets prepared by a hydrothermal method, and then functionalized with Rh catalysts to explore the sensing properties to TMA. The gas sensing investigations show that the loading of Rh has a remarkable influence on the performance of the sensors. The Rh/ZnO sensor with 10 cycles of Rh ALD showed the best response to TMA at 180 °C. A very high response of 11.3 was recorded when exposed to 10 ppm TMA, which is 3-times higher than that of pristine ZnO. Furthermore, the sensor also has a fast response-recovery and an excellent humidity resistance, as well as a low detection limit of 55 ppb, which suggest high potential for reliable detection of sub-ppm TMA.

Preparation and evaluation of A/ BaO‐MnO x catalysts (A: Rh, Pt, Pd, Ru) in lean methane catalytic combustion at low temperature

Lean methane catalytic combustion was studied over various noble metal catalysts supported on 10 wt% BaO-MnOx (BM), and the results demonstrated that the Rh/BM catalyst could present a dramatic change in the light-off temperature. The influence of the Rh loadings on the physicochemical features and catalytic activity and stability in CH4 combustion was evaluated in detail. The performance of the catalyst was evaluated using the temperatures (T50% and T90%) required for achieving methane conversion of 50% and 90%. The 1 wt% Rh/BM catalyst possessed the best performance with T50% = 279°C and T90% = 350°C and the light-off temperature of the 1wt%Rh/BM catalyst was about 50°C lower than that obtained for the 10wt% BaO-MnOx. The results indicated that the 1 wt% Rh/BM exhibited high stability even in the presence of various concentrations of CO2 and water vapor. Moreover, the influence of processing conditions such as pretreatment atmospheres, GHSV, and feed ratio was examined on the CH4 conversion. The 1 wt% Rh/BM catalyst was also utilized in the CO oxidation process, and the results presented a high activity at low temperature.

Promoting effect of Rh-impregnation on Ag/CeO2 catalyst for soot oxidation

In this study, the effect of Rh on the catalytic properties of CeO2 was examined using a series of Rh-impregnated catalysts with different Rh loadings (Rh(x)Ce, x = 1, 2, 3, 5 wt%). The Rh species were highly dispersed over CeO2, and Rh(x)Ce exhibited higher soot oxidation activity than CeO2. In particular, the H2-TPR and O2-TPD profiles of the prepared catalysts indicated that Rh(2)Ce outperformed all other Rh(x)Ce catalysts owing to its high reducibility and desorption of active oxygen species at low temperatures. Moreover, an excessive Rh loadings resulted in an increase in the Rh3+/Rh0 and a decrease in the surface oxygen reducibility of the Rh(x)Ce catalysts. Subsequently, the promoting effect of Rh on the catalytic properties of Ag/CeO2 was evaluated using a Rh-impregnated Ag/CeO2 catalyst (Rh(2)Ag(5)Ce). The Rh species were well-dispersed over Ag/CeO2, and Rh(2)Ag(5)Ce presented a higher catalytic activity than the Ag/CeO2 catalyst. XPS analysis revealed that the interaction between Rh and Ag caused changes in the electronic states of Rh and Ag. Moreover, the soot-TPR and Raman spectra data revealed that the Rh(2)Ag(5)Ce catalyst could use active oxygen species at a lower temperature and generate a higher amount of reactive oxygen species than all other analyzed catalysts. Furthermore, macroporous Rh(2)Ag(5)Ce exhibited excellent activity, which was ascribed to the high soot-catalyst contact efficiency.