An oxygen pool from YBaCo 4 O 7-based oxides for soot combustion †

Soot, often referred to as black carbon emitted from diesel engines, is not only a particulate matter pollutant but also a light-absorbing agent that may affect global climate, but can be effectively controlled using a catalytic diesel particulate filter (DPF). A new YBaCo4O7+δ-type oxygen storage material is reported as an effective catalyst for soot combustion. Isotopic isothermal reactions demonstrate the activation of gaseous oxygen and subsequent oxygen storage and reaction/desorption during an oxidation process. High activity and structural stability are achieved by the substitution of Co with Al and Ga to form YBaIJCo0.85Al0.075Ga0.075)4O7+δ. The specific rates at 300 °C of YBaCo4O7+δ and YBaIJCo0.85Al0.075Ga0.075)4O7+δ, normalized by surface areas, are an order of magnitude higher than those of CeO2-based oxides. This kind of oxygen-storage material acts as an oxygen pool, which ensures that the accumulated soot on a DPF can be promptly combusted.


Introduction
Soot, often referred to as black carbon, is not only a particulate matter pollutant but also a light-absorbing agent that may affect global climate. 1 Diesel engines are amongst the most abundant emission sources of soot that can be effectively controlled by a catalytic diesel particulate filter (DPF). 2 Commercial catalysts are composed of noble metals (Pt and Pd) supported on ceria-based oxides, 3 oxidizing NO into NO 2 , which is transported through a gas phase to soot aggregates where it oxidizes carbon while being reduced to NO. 4 A socalled continuously regenerated trap (CRT) overcomes the problem of poor contact between soot and a catalyst.However, the limited amounts of NO x present and the need to control NO x for Euro IV standards are not ideal situations to meet the requirements of the latest generation of diesel engines, leading to a drive to develop catalysts which can produce highly reactive oxygen species from gaseous O 2 molecules. 5,6][9][10][11][12] Unfortunately, the effective utilization of such active oxygen species is limited by the extent of contact between soot and a catalyst.This limitation could be overcome by producing a sufficiently high amount of active oxygen to ensure that, at least, a proportion reaches the soot particle surface. 54][15][16] The extraordinary oxygen storage capability of the YBaCo 4 O 7based oxides is due to the variable valence of Co ions between Co 2+ and Co 3+ .The limitation of YBaCo 4 O 7+δ is that it decomposes just above 600 °C in an oxygen-containing atmosphere, which limits its applications in catalytic combustion at elevated temperatures. 13Recently, Karppinen and colleagues identified that an YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ phase, where Co is co-substituted by Al and Ga is stable up to high temperatures under oxidizing conditions, 16 which creates the potential for YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ to become a more effective catalyst for catalytic combustion.
In this paper, YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ were studied from the perspective of catalysis for diesel soot combustion.Their oxygen storage performance allows them to create an oxygen pool supply of active oxygen which is created from a gaseous phase. 17Although YBaCo 4 O 7+δ has been reported as a robust catalyst for H 2 O 2 oxidation of cyclohexene in the liquid phase, 18 the present result is the first report on high-temperature oxidation reactions for this kind of non-stoichiometric transition metal oxide materials.
This journal is © The Royal Society of Chemistry 2016
X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/max-RC diffractometer employing Cu Kα radiation.Surface areas and pore size distributions were determined by N 2 adsorption-desorption at 77 K using a Micromeritics ASAP 2020 instrument after outgassing at 300 °C for 5 h prior to analysis.
Temperature programmed desorption of O 2 (O 2 -TPD) experiments were conducted in a fixed-bed flow reactor.A 150 mg sample was heated under a flow of high purity O 2 (30 ml min −1 ) at 300 °C for 1 h.After cooling to room temperature, high purity He was introduced.Desorption was started at a heating rate of 2 °C min −1 in He (30 ml min −1 ).The desorbed O 2 was monitored by a quadruple mass spectrometer (MS, OminiStar 200, Balzers).
Temperature programmed reduction with H 2 (H 2 -TPR) experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor H 2 consumption.A 50 mg sample was pretreated in situ at 300 °C for 1 h under a flow of O 2 and cooled to room temperature in the presence of O 2 .After purging with N 2 , TPR was conducted at 10 °C min −1 up to 700 °C under a 30 mL min −1 flow of 5 vol% H 2 in N 2 .To quantify the total amount of H 2 consumption, CuO was used as a calibration reference.
"Dynamic" OSC (DOSC) measurements with CO-O 2 pulses were carried out at 200-500 °C.CO (4% CO/1% Ar/He at 300 mL min −1 for 10 s) and O 2 (2% O 2 /1% Ar/He at 300 mL min −1 for 10 s) streams were pulsed alternately with at a frequency of 0.05 Hz.A DOSC value was obtained by integrating the CO 2 formed during one CO-O 2 cycle and was expressed as μmol of O per gram of catalyst (μmol [O] g −1 ).The concentration of CO 2 was determined using a MS.
Temperature programmed oxidation (TPO) reactions were conducted in a fixed bed micro reactor consisting of a quartz tube (6 mm i. d.).Printex-U from Degussa was used as the model soot.The soot was mixed with the catalyst in a weight ratio of 1 : 9 in an agate mortar for 30 min, which resulted in a tight contact between the soot and the catalyst.A 50 mg sample of the soot/catalyst mixture was pre-treated under a flow of He (50 mL min −1 ) at 200 °C for 30 min to remove adsorbed species.After cooling to room temperature, a gas flow with 5 vol% oxygen in He was introduced and then TPO was initiated at a heating rate of 5 °C min −1 until 880 °C.For pure soot combustion (non-catalytic), the catalyst was substituted by silica.CO and CO 2 concentrations in the effluent gas were monitored using an online gas chromatograph (GC) (SP-6890, Shandong Lunan Ruihong Chemical Instrument Corporation, China) fitted with a methanator.The ignition temperature for soot combustion was evaluated by the value of T 10 , which is defined as the temperature at which 10% of the soot is converted.The selectivity to CO 2 is defined as the percentage CO 2 outlet concentration divided by the sum of the CO 2 and CO outlet concentrations.
Isothermal reactions at 300 °C, at which a stable and low soot conversion (<15%) was achieved, were conducted within the kinetic regime.The reaction rate for soot combustion was obtained from the slope of the conversion lines with time.Specific rates normalized by BET surface areas and turnover frequency (TOF) 19 were used to characterize the activity for soot combustion.
An isotopic isothermal reaction was performed by switching the flowing gas from 1% 16 O 2 to 1% 18 O 2 diluted in Ar at 350 °C.50 mg of a mixture of the soot and catalyst in a tight contact mode was employed.The effluent gas from the reactor was continuously monitored by a MS for all of the isotopic molecules of CO 2 (at m/z = 44, 46 and 48).

Results and discussion
The fresh YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ were confirmed to be composed of a single phase (Fig. S1 †), similar to that of hexagonal LuBaAlZn 3 O 7 (JCPDS 40-4099). 13As the shifts in the XRD peaks for the separate substitution of Co by Al (r = 0.039 nm) and Ga (r = 0.047 nm) are opposed for each sample (Fig. S2 †), the simultaneous substitution has a slight effect on the cell volume parameters.Furthermore, both ionic radii of Al and Ga are smaller than the high spin Co 2+ ionic radius (r = 0.058 nm), thus Maignan et al. suggested that Al and Ga are substituted for Co 3+ . 20The BET surface areas are fairly low, in accordance with the hightemperature sintering preparation (Table S1 †) and the highly crystalline nature of the samples.

Catalysis Science & Technology Paper
In agreement with conclusions in the literature, 13,16 the O 2 -TPD profiles (Fig. 1) show that both YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ desorb a large amount of oxygen below 400 °C, corresponding to δ = 0.37 and δ = 0.34, respectively.The peak maximum of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ is 50 °C lower than that of YBaCo 4 O 7+δ , which suggests a promotion effect on O 2 desorption by doping, albeit with a slight decrease of the overall oxygen storage capability. 16Above 700 °C, further desorption of O 2 is observed, corresponding to the possible decomposition of the 114 structure. 13 A similar situation is observed in H 2 -TPR (Fig. S1 †).The 114 structure of YBaCo 4 O 7+δ is completely destroyed by H 2 in TPR to 700 °C (Fig. S1a and b †).In contrast, YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ was preserved with no formation of new oxide phases (Fig. S1c and d †).The structural stability of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ is vital to high-temperature redox reactions.As shown in Fig. 2, two peaks were observed for both YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ .The low-temperature TPR peak can be assigned to the removal of non-stoichiometric excess O accommodated within the lattice, and the values of which are slightly larger than those consumed by δ (Table 1).The 114 structure of YBaCo 4 O 7+δ is stable at this stage (Fig. S1a and b  Although H 2 -TPR and O 2 -TPD data may be useful in rapidly evaluating the potential OSC of the candidate materials, DOSC provides better simulation of instantaneous oscillations between lean (oxidizing) and rich (reducing) exhaust conditions during real operation and is therefore much more useful in the evaluation of the activity of OSC materials. 21ig. 3(a) shows the collected DOSC data and the corresponding transition curves at 320 °C of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ as an example, with alternate dynamic pulses of 4% CO/1% Ar/He (10 s) and 2% O 2 /1% Ar/He (10 s) under 0.05 Hz given in Fig. 3 The catalytic activity for soot combustion was first checked by TPO (Fig. 4a).YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ decrease from a T 10 value of 530 °C for non-catalytic combustion to 387 and 379 °C, respectively, confirming the catalytic effect of the YBaCo 4 O 7+δ -type material and the higher activity of the latter than the former.In terms of the selectivity towards CO 2 formation, the non-catalytic combustion is only 43.3%, while YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ yield nearly 100% CO 2 .After the TPO reactions, no phase decomposition occurs even for YBaCo 4 O 7+δ (Fig. S4 †), probably due to the high heating rate in 5 vol% oxygen in He. 23 Furthermore, the XRD peaks of YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 -Ga 0.075 ) 4 O 7+δ after TPO shift to higher angles, suggesting a lattice shrinkage, which confirms the participation of bulk oxygen.
The intrinsic activity was further demonstrated by kinetic rates at 300 °C, which can be obtained from the slope of the lines shown in Fig. 4b.0][21][22] This is significant because 300 °C is a relevant temperature for light diesel engines.This particularly high reaction rate can ensure that the accumulated soot on the DPF can be readily combusted, leading to a lower balance point temperature (BPT) at which the rate of soot oxidation is matched with the rate of soot accumulation. 24Furthermore, both the specific rate and TOF (Table 1) of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ are a little higher than that of  In order to explore the origin of the active oxygen, isotopic isothermal oxidation at 350 °C was performed (Fig. 5).Before switching from 16 O 2 to 18 O 2 (to the left of the dashed line), the main product was C 16 O 2 , confirming that the soot oxidation occurs with the bulk oxygen species.The concentration of C 16 O 2 for YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ is much larger than that for YBaCo 4 O 7+δ , which is again consistent with the O 2 -TPD, H 2 -TPR, DOSC, T 10 and specific rates.After switching from 16 O 2 to 18     provides an oxygen pool, which ensures that the accumulated soot on a DPF can be readily combusted.
†).The second peak corresponds to the reduction of bulk and surface Co 3+ of YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ , respectively.The substitution of Co 3+ by Al and Ga protects the structure from decomposition under reducing atmospheres (H 2 -TPR and O 2 -TPD). 20Furthermore, the lower TPR temperature of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ in comparison with that of YBaCo 4 O 7+δ coincides with the O 2 -TPD results.
O 2 (to the right of the dashed line), the sum of the products of C 16 O 18 O, C 18 O 2 and 16 O 18 O still possesses higher concentrations of YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ than that of YBaCo 4 O 7+δ .However, the concentration of C 16 O 2 decreased gradually to a very low value due to the depletion of 16 O 2 in the gaseous phase.Comparatively, for both YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ , the production of C 16 O 18 O increases and then reaches a maximum, while the C 18 O 2 production monotonically increases.This indicates that only the gaseous oxygen which has been activated by YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ can be used to oxidize the soot.In addition, the desorption of 16 O 18 O is detected, which suggests that the desorbed 16 O 18 O species do not interact with the soot to produce the product containing carbon, demonstrating that the intimate contact between the soot and catalysts is essential.Since the high DOSC can make up for the missing oxygen, the intrinsic activity of YBaCo 4 O 7based oxides is much more active than that of CeO 2 -based oxides.Conclusions In conclusion, a new YBaCo 4 O 7+δ -type oxygen storage material was used to catalyze soot combustion.Isotopic isothermal reactions demonstrate the activation of gaseous oxygen and subsequent oxygen storage and reaction/desorption during the oxidation process.Higher activity and structural stability are achieved by the substitution of Co with Al and Ga to form YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ .The specific rates at 300 °C of YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ , normalized by BET surface areas, are an order of magnitude larger than that of CeO 2 -based oxides.This type of oxygen-storage material

Fig. 4
Fig. 4 TPO patterns of CO x for soot combustion with O 2 over YBaCo 4 O 7+δ and YBaĲCo 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ (a); isothermal reactions for soot combustion at 300 °C within the kinetic regime (b) under tight contact conditions between the soot and catalysts.