Preparation of Cerium‐Bismuth Oxide Catalysts for Diesel Soot Oxidation Including Evaluation of an Automated Soot‐Catalyst Contact Mode

Abstract Cerium‐bismuth oxides have emerged as promising candidates for Diesel soot oxidation. The catalysts are synthesized via automated co‐precipitation methods. T50 values, where 50 % of soot is oxidized, and the dynamic oxygen storage capacity (OSCdyn) are used to compare the catalytic activity. The activity is measured by thermogravimetric methods. The synthesized catalysts are characterized through powder X‐ray diffraction (PXRD), Raman spectroscopy, and specific surface area (SBET) measurements. This work investigates the influence of the contact mode between soot and catalyst. The literature‐known manual contact modes “loose”, “tight”, and “wet” are compared with our developed automated contact mode, using a dual asymmetric centrifuge. The rotation speed rs and mixing time tM have been varied independently. Both factors influence the T50 value. A continuous transition from loose to tight contact mode with increasing rotation speed rs can be shown. Furthermore, the automated contact mode shows better reproducibility behavior compared to manual contact modes.


Introduction
The World Health Organization classifies Diesel soot as carcinogenic. [1] Diesel soot is suspected to cause lung or cardiovascular diseases. [2,3] Furthermore, Diesel soot influences the atmospheric visibility, soiling of buildings, the vegetation, or the climate due to its optical properties depending on particle size, shape, and composition. [4,5] Nevertheless, Diesel cars are popular because of more efficient engines, better fuel economy, and lower emissions of HC, CO, and CO 2 . Contrary to gasoline engines, Diesel engines emit higher quantities of NO x and the so-called soot. Both pollutants are restricted by vehicle emission standards. [6] Therefore, different exhaust gas post-treatment technologies are established. For example, Diesel particulate filters (DPF) remove soot from the exhaust gas stream. [7,8] Over time, the Diesel soot collected accumulates in the DPF. As a consequence, the backpressure of the exhaust gas post-treatment system rises and leads to higher fuel consumption because of filter regeneration processes or even to filter failure. [9,10] In the event that the pressure threshold of the DPF is reached, the DPF has to be regenerated. Mainly, there are two regeneration approaches: active and passive regeneration. [11] During the passive regeneration process, the accumulated soot is continuously oxidized by a catalytic chemical reaction utilizing residual oxygen in the off-gas stream. In this regeneration mode, fuel penalty is minimized and no additional heat of the exhaust gas stream is needed. The disadvantage of the passive regener-ation is the suitability, deactivation, and the catalysts costs. The catalysts used are most usually based on precious metals. [5,12,13] During the active regeneration process, there is a periodic combustion at temperatures of T = 550°C and higher of the accumulated soot in an oxidizing atmosphere. The required heat is generated by an electric heater, a flame-based burner, or microwave cavity. [14,15] Another possibility is the post-injection of fuel which is burnt in the Diesel oxidation catalyst to produce the heat for the Diesel soot combustion. [16] Catalyzed DPFs offer the possibility to lower the combustion temperature of soot to T = 400°C and overcome the problem of additional heat generation. [17] Different types of catalysts like spinels, [18][19][20][21] perovskites [22][23][24][25][26] or CeO 2 -based catalysts [27][28][29] are investigated for soot oxidation. In this paper, co-precipitated cerium-bismuth oxides are used as catalysts for the combustion of the model Diesel soot, the commercial carbon black P90. Wolf reported that Bi-doped CeO 2 is a promising candidate for the Diesel soot oxidation. [30] Cui et al. synthesized Bi-doped CeO 2 using reverse strike co-precipitation methods. The soot oxidation occurred at tight and loose contact modes. Doping the CeO 2 lattice improves the lattice oxygen mobility. The best activity was reached by a catalyst of Ce 0.5 Bi 0.5 O x composition with T 50 = 324°C at tight contact conditions. [31] Advantages of this system are the low costs and good availability of the precursors. In addition, the resulting catalysts are free of precious metals. [32] To test the catalytic activity in laboratory experiments instead of an engine test bench, the contact between soot and catalyst is crucial. There are several literature-known contact modes. Neeft et al. developed the "loose" and "tight" contact. The loose contact is determined by mixing soot and catalyst with a spatula for a preset time. The contact area between soot and catalyst is poor, resulting in high combustion temperatures. For the tight contact, soot and catalyst are milled in either an agate mortar or a ball mill for a preset time and, if necessary, rotation speed. This procedure increases the contact area between soot and catalyst. [33][34][35] Both contact modes involve manual steps that influence the repeatability of the measurements. To overcome this problem, Hensgen et al. developed the "wet" contact. The wet contact is an intimate contact and determined by dispersing soot and catalyst in a dispersion agent. Acetone has proven to be the best candidate as dispersion agent. Hensgen et al. used nanoscaled CeO 2 as catalyst with different types of model soot. In their study, a good measurement repeatability was achieved. [36] In the present paper, the loose, tight, and wet contact modes are compared using nano-scaled CeO 2 AdNanoCeria and co-precipitated CeO 2 and Bi 0.5 Ce 0.5 O x samples as catalysts. The catalysts are characterized using PXRD, Raman spectroscopy, XRF, S BET , and OSC dyn measurements. Furthermore, an automated contact mode with a dual asymmetric centrifuge is developed to exclude any manual influence using a commercial device. The T 50 values serve to compare the activities measured by an automated serial thermogravimetric analysis method.

Results and Discussion
Catalyst Characterization XRF measurements shown in Table 1 were performed to verify the nominal Bi and Ce percentages of the co-precipitated BiCeO samples (Bi : Ce = 1 : 1). The oxygen content is not included in the presented results because the detector does not allow the quantification of oxygen. The mass percentages of Bi and Ce of the BiCeO samples which are precipitated by the precipitants ammonium carbonate (AC) and oxalic acid (OA) differ not more than Δm% = 3 % between nominal and experimental received values. Table 2 presents the results of the specific surface area S BET and the dynamic oxygen storage capacity OSC dyn measurements. S BET was determined through N 2 physisorption experiments and by the multi-point BET method. The reference sample AdNanoCeria supplied by Evonik Degussa produced through flame spray pyrolysis has the highest S BET = 60 m 2 ·g À 1 and highest OSC dyn = 42 mmol O 2 �g À 1 cat compared to co-precipitated samples. The pure CeO 2 precipitated by AC solution has a lower S BET = 27 m 2 ·g À 1 as well as lower OSC dyn = 25 mmol O 2 �g À 1 cat compared to AdNanoCeria. In comparison, the pure CeO 2 precipitated by OA shows an even smaller specific surface with S BET = 4 m 2 ·g À 1 . On the other hand, the standard deviation of the OSC dyn relative to its absolute value of the CeO 2 _OA is very high with OSC dyn = 5 � 4 mmol O 2 �g À 1 cat and implies an inhomogeneous sample with respect to the distribution of oxygen vacancies and redox ability. A reverse trend was shown by the BiCeO samples. The BiCeO_AC has a smaller surface area of S BET = 8 m 2 ·g À 1 compared to BiCeO_OA. On the contrary, the BiCeO precipitated by OA has a S BET = 15 m 2 ·g À 1 , but the OSC dyn is higher for BiCeO_AC compared to BiCeO_OA (OSC dyn = 25 mmol O 2 �g À 1 cat versus OSC dyn = 6 mmol O 2 �g À 1 cat ). Figure 1 shows the PXRD patterns of the reference sample AdNanoCeria and the co-precipitated samples. In general, the PXRD patterns of the samples precipitated by OA reveal higher FWHM (full width at half maximum) of the reflections compared to the samples precipitated by AC, especially for the sample BiCeO_OA indicating lower crystallite size. The samples AdNa-noCeria, CeO 2 _AC and CeO 2 _OA crystallize in a cubic fluoritetype structure with space group Fm � 3m. Table 3 presents the results of Rietveld refinements.

PXRD and Raman Spectroscopy
Both BiCeO samples consist of more than one crystalline phase. According to Rietveld refinements, the BiCeO_OA sample consists of three different crystalline phases with exclusive Bi 3 + occupation on cation positions. These are the monoclinic α-Bi 2 O 3 phase with space group P2 1 /c, the tetragonal β-Bi 2 O 3 phase with space group P � 42 1 c, and the orthorhombic bismutite Bi 2 (CO 3 )O 2 phase with space group Imm2. For the BiCeO_AC sample, an amount of only m = 2.7 w% of bismutite is refined. Due to a detailed reflection profile analysis in the course of the Rietveld refinement, two cubic CeO 2 phases with different lattice parameters are identified in the BiCeO samples. One cubic fluorite-type phase is CeO 2 doped with Bi 3+ and the other phase is pure CeO 2 . This finding might be explained by the presence of domains of the CeO 2 phase with varying Bi 3 + content due to substitution of Ce 4 + by Bi 3 + . The Bi 3 + -doped CeO 2 phases in the samples BiCeO_ AC and BiCeO_OA have larger lattice parameters a = 5.436(2) Å and a = 5.4237(3) Å compared to pure AdNanoCeria with a = 5.4082(1) Å. The substitution of Ce 4+ by Bi 3 + is accompanied by oxygen lattice defect formation of Frenkel-type as indicated by Equation (1) (in Kröger-Vink notation): (1) Figure 2 shows the Raman spectra of the oxides used or prepared in this work. AdNanoCeria, CeO 2 _AC, and CeO 2 _OA all reveal the typical band at ν = 462 cm À 1 which is the triply degenerate F 2g mode that corresponds to a symmetric CeÀ O    [37,38] According to Schilling et al., two broad bands at ν = 510 and 570 cm À 1 can be assigned to oxygen vacancy formation of Frenkel-type Ce 3þ ðO 2À Þ 7 V �� O and Ce 4þ ðO 2À Þ 7 V �� O . [37] As these bands are missing in the samples specified above, this confirms that all CeO 2 samples have nearly stoichiometric composition. The Raman spectrum of BiCeO_AC shows bands at ν = 121, 161, 460, 514, 575, 1067, and 1148 cm À 1 . The bands at ν = 121, 161, and 1067 cm À 1 can be assigned to bismutite [39] and confirm the results of Rietveld refinements. The band at ν = 460 cm À 1 can again be assigned to the F 2g band of CeO 2 . The bands at ν = 514 cm À 1 and ν = 575 cm À 1 are broad and refer to the formation of the oxygen vacancies (see above) as a result of doping the CeO 2 lattice with Bi 3 + ions (see Equation (1)) in consistence with the results of the Rietveld refinement. The broad band at ν = 1148 cm À 1 is referenced to the adsorption of oxygen molecules, which are adsorbed at the surface during the ex situ measurements due to contact with air forming O 2À 2 groups at the surface by reaction with Ce 3 + ions. [37] The Raman spectrum of the BiCeO_OA sample shows some other bands.
These bands are observed at the frequencies ν = 126, 229, 314, 460, 512, 575, and 1152 cm À 1 . The bands at ν = 126, 229, 314 cm À 1 clearly refer to β-Bi 2 O 3 . Note that the band at ν = 460 cm À 1 can be assigned to both the F 2g mode of the CeO 2 lattice or β-Bi 2 O 3 . [40,41] Like in the Raman spectra for BiCeO_AC, there are the two broad bands at ν = 512 and 575 cm À 1 according to oxygen vacancies of the CeO 2 lattice. The band at ν = 1152 cm À 1 is again due to the vibration of O 2À 2 group. [37] According to Bao et al., the intensity ratio I 595 /I 464 of the bands at ν = 464 and 595 cm À 1 is to be calculated to get a better comparison of the relative concentration of the defect complexes of the BiCeO samples. [42] The intensity ratio I 595 /I 464 of BiCeO_AC is 0.68 (2) and that of BiCeO_OA amounts to 0.33 (2). Therefore, the amount of defects in BiCeO_AC is higher than in BiCeO_OA.

Repeatability Behaviors of Different Contacts
The activity measurements are conducted with a serial thermogravimetric and differential scanning calorimetry system (TGA/DSC). The repeatability behaviors of the loose, tight, wet contact modes, and our development, the automated contact were investigated with nano-scaled AdNanoCeria, CeO 2 _AC, and CeO 2 _OA as sample materials. The samples are calcined at T = 800°C to avoid any changes of the crystal structure during the reaction in a temperature range of T = 25-700°C and to improve the crystallinity of the samples. Piumetti et al. investigated the influence of different CeO 2 crystal structures. They determined that calcination temperatures above T > 500°C produces more active sites of the CeO 2 samples. [43] Hueso et al. showed that grinding their catalyst, a L 0.5 Sr 0.5 CoO 3À δ perovskite, and soot together can cause a partially reduction of Co 3 + to Co 2 + . [44] In the present investigation, we do not observe such an effect. Figure 3 shows the TGA curves measured for different contact modes between soot and catalyst with constant weight ratio of 1 : 4. AdNanoCeria60 was used for the activity measurements. Each gravimetric activity measurement was repeated five times. Considering Figure 3, loose and tight contact mode have problems of repeatability compared to wet and automated contact.
To compare the repeatability, the average and the standard deviation of the relative weight loss Δm/m 0 and σ(m/m 0 ) can be used. The value Δm/m 0 should be close to Δm/m 0 = 20 % (catalyst:soot = 4 : 1) and σ(m/m 0 ) should be as low as possible.
The obtained values are listed in Table 4. On the other hand, the T 50 value is a good indicator of contact tightness: as the soot combustion is only occurring at the direct contact area between soot and catalysts, lower T 50 values represent a more intimate contact between both components. The loose contact mode has the highest T 50 value of T 50 = 631 � 6°C. On the other side, the automated contact possesses the lowest T 50 value of T 50 = 436 � 2°C. The relative weight loss Δm/m 0 is in the same range of Δm/m 0 � 21.5 % for all contact modes. The loose contact has the highest standard deviation of the relative weight loss with σ(m/m 0 ) = 1.9 %. The 5 th repetition of this series (brown line) might be considered as an outlier, but is quite commonly observed for loose contact due to sample inhomogeneities (see below). This implicates that the loose contact has the lowest repeatability. The σ(m/m 0 ) of the tight, wet, and automated contact are below σ(m/m 0 ) = 1 %. These values indicate good repeatability behavior.
In contrast to AdNanoCeria, for example CeO 2 precipitated by AC reveals different properties, see Figure 4 and Table 5. The loose contact mode shows a poor repeatability behavior and the highest T 50 value with T 50 = 652 � 2°C. The average and deviation of the relative weight loss of soot is Δm/m 0 = 27 % and σ(m/m 0 ) = 12 %. Δm/m 0 is approximately 7 % higher than the theoretical relative weight loss of soot, indicating a rather inhomogeneous soot-catalyst mixture.
The wet contact mode seems to break into two different soot-catalyst mixtures, one mixture containing more and the other less soot. Compared to AdNanoCeria, CeO 2 _AC might have a broad or even bimodal particle size distribution and   during the wet contact preparation despite intense stirring, there is a separation of particles of different size and thus also of the amount of soot connected to the catalyst's fractions. The tight contact has the lowest T 50 value with T 50 = 621 � 1°C. The difference from the theoretical relative weight loss is quite low with Δm/m 0 = 18.5 � 0.1 %. The standard deviation is much smaller than 1 %, indicating good repeatability behavior as well as a homogeneous soot-catalyst mixture. The automated contact has a T 50 = 632 � 3°C, lying in between the loose and tight contact mode. As will be shown later, the automated mode offers the possibility to adjust the contact tightness by experimental parameters. The relative weight loss of soot is again quite close to the theoretical value with Δm/m 0 = 19 � 4 %. But the standard deviation of the relative weight loss is σ(m/m 0 ) = 4 % and thus higher than that of the tight contact mode, but better compared to the loose contact mode. As the automated contact is prepared by mixing soot and catalyst in a dual asymmetric centrifuge in polypropylene (PP) containers, which show different adhesion properties to the two components of the mixture, volume fractions of catalyst with soot-rich and -poor domains similar to those found for the wet contact mode might occur. In this case, also the sampling position (mixing container-wall near of close to center) has a certain influence on the sample properties. Table 6 presents the results of T 50 and Δm/m 0 measurements for the CeO 2 _OA sample. It can be seen that the loose and wet contact mode have the highest T 50 values with T 50,loose = 651 � 1°C and T 50,wet = 640 � 10°C. In this case, the  automated contact mode has the lowest T 50 = 625 � 3°C. With respect to repeatability behavior, the loose and wet contact mode have much higher standard deviations of the relative soot weight loss compared to the other two contact modes. This indicates homogeneous soot-catalyst mixtures and good repeatability behavior of tight and automated contact modes. All in all, the automated contact shows good repeatability behavior according to the relative weight loss of soot. The T 50 values achieved with the automated contact are in the range of the tight contact mode but the contact tightness can be adjusted by operational parameters as will be shown in the next chapter. It is noteworthy that the contact modes are influenced by specific properties of the samples and the fact that soot and catalyst have rather different surface polarities and energies. This makes the contact of the two components prone to demixing effects, additionally enforced by contact with accessories during the mixing process.

Further Investigation of the Automated Contact Mode
The automated contact was realized with a dual asymmetric centrifuge for a number of other samples, too. The investigation of operating conditions of the centrifuge are described in more detail in this section. The rotation speed rs was varied between rs = 300-3500 rpm in Δrs = 200 rpm steps and the mixing time was held constant at the two levels of t M = 60 s and t M = 300 s. The influence of rs at t M = 60 s was investigated for all samples. The results are shown in Figure 5 and Table 7.
Based on the TGA curves and T 50 values presented in Figure 5 and Table 7, there is a clear trend for all samples. With increasing rotation speed rs the TGA curves shift to lower temperatures and the T 50 values decrease. The sample CeO 2 _OA does not follow the trend of the other samples. Here, the problem is the low activity for soot oxidation due to both low S BET and low OSC dyn . The largest effect of increasing rs is seen for the AdNanoCeria60 sample in agreement with the by far highest S BET and highest OSC dyn within the samples investigated resulting in good activity for the soot oxidation reaction. For further explanation of the results, the particle size distributions measured by static light scattering were used, represented in Figure 6. The samples AdNanoCeria, CeO 2 _AC and BiCeO_AC possess a broad particle size distribution. Especially, in the range of hydrodynamic diameter d h below d h = 10 μm there are more particles available in these samples. For the samples precipitated by oxalic acid, the particles are more narrowly distributed around the maximum hydrodynamic diameter of d h � 11 μm and there are not many particles with sizes below d h = 10 μm. Table 7 shows the T 50 values at different rotation speeds. The highest difference of combustion temperature between the loose contact-like condition at rs = 300 rpm and the tight contact at rs = 3100 rpm is achieved by the nano-scaled AdNanoCeria60 with ΔT 50 = 213°C. The shift of T 50 values is greater for the samples precipitated by AC than for those precipitated by OA. The ΔT 50 values of BiCeO_AC and CeO 2 _AC are ΔT 50 = 57°C and ΔT 50 = 77°C, respectively. On the other hand, BiCeO_OA and CeO 2 _OA have a shift of T 50 values of only ΔT 50 = 10°C and ΔT 50 = 45°C to lower temperatures, respectively. Furthermore, the BiCeO_AC has, at the lowest rotation speed of rs = 300 rpm, the lowest T 50 value of T 50 = 539 � 1°C, indicating the best activity towards soot oxidation at loose contact conditions. In Figure 7, the T 50 values of the samples used in this work are compared at different rotation speeds rs. As already described above, the nano-scaled AdNanoCeria60 has the greatest shift of T 50 Figure 8 and

Conclusion
In this article, the characterization of co-precipitated CeO 2 and 1 : 1 mixed bismuth-ceria oxides by PXRD, Raman analysis, XRF, S BET , and OSC dyn and a comparison of different contact modes between catalyst and soot known in literature for the catalytic oxidation of soot by temperature programmed oxidation methods have been described. Additionally, an automated contact mode realized by an asymmetric dual centrifuge has been investigated.
According to PXRD and Raman analysis, the AdNanoCeria60 and the co-precipitated CeO 2 samples crystallize in the cubic fluorite-type CeO 2 structure without any oxygen defects, that is, without non-stoichiometry in the ceria lattice. The two parameters S BET and OSC dyn are very low for the co-precipitated CeO 2 samples. Therefore, the activity for soot oxidation is lower compared to the AdNanoCeria60 sample which has the highest S BET = 60 m 2 ·g À 1 and OSC dyn = 42 mmol O 2 �g À 1 cat and, consequently, the highest soot combustion activity of the catalysts investigated in this study. The results of PXRD and Raman analysis show that the BiCeO_OA samples consist of four crystalline phases, namely αand β-Bi 2 O 3 , bismutite Bi 2 (CO 3 )O 2 , and a ceria phase with spatially inhomogeneous distribution of Bi 3 + substitution for Ce 4 + accompanied by oxygen lattice defects in the CeO 2 phase. The latter was modelled by two distinct CeO 2 phases in the Rietveld refinements with different site occupations factors sof(Bi 3 + ). In contrast, the BiCeO_AC sample only reveals the Bi 3 + -doped CeO 2 phase together with a pure ceria phase and a small amount of bismutite. The OSC dyn of BiCeO_ AC is, with ΔOSC dyn = 19 mmol O 2 �g À 1 cat , higher than that of BiCeO_ OA. Therefore, BiCeO_AC has a higher activity for soot oxidation than BiCeO_OA. Additionally, the Raman spectroscopy data show that the mixed oxides have Frenkel defects. The intensity ratio I 595 /I 464 , which is proportional to the concentration of defects, is higher for the BiCeO_AC sample compared to the BiCeO_OA sample. Additionally, the particle size distribution measured by static light scattering is broad for the samples precipitated by AC with high availability of particles with hydrodynamic diameter below d h = 10 μm. The particles in the samples precipitated by OA are more narrowly distributed with a maximum hydrodynamic diameter d h = 11 μm. This could be the reason for the better OSC dyn and the better activity. The mixed oxides reveal better soot combustion activities compared to their pure CeO 2 samples recognizable by the lower T 50 values.
Regarding to the investigation of different contact modes between soot and catalyst for different catalysts, the loose and wet contact mode show for all samples a low repeatability behavior. In marked contrast, tight and automated contact modes show good repeatability behavior. The automated contact has the advantage that there is no manual influence and is therefore not depending on the experimentator. Building on these results, the automated contact was further investigated. The rotation speed rs and the mixing time t M were varied. The mixing time t M has a much lower effect on the T 50 values than the rotation speed rs.

Catalyst Preparation
The chemicals listed below were used without further preparation. AdNanoCeria60 supplied by Evonik-Degussa with lot no. PH36701 and S BET = 60 m 2 ·g À 1 is used as reference material. All syntheses performed are described in detail in. [32] Via co-precipitation, CeO 2 and the equimolar mixed oxide of cerium and bismuth are synthesized using the synthesis robot Accelerator SLT 106 from Chemspeed Technologies AG, Switzerland. Two precipitants are used: 1 mol (NH 4 ) 2 CO 3 (food grade, BASF, abbreviated as AC) and 0.6 mol C 2 H 2 O 4 · 2 H 2 O (99 %, ORG Laborchemie) dissolved in 1 L water (abbreviated as OA). The syntheses were performed in lined beakers with magnetic stirring bars. Using a thermostat from Huber Corp., the lined beakers were tempered to a temperature of T = 25°C. The control of the thermostat was implemented in the "ApplicationExecutor" software for the syn-  thesis robot. For the co-precipitation, 0.5 mol Ce(NO 3 ) 3 · 6 H 2 O and 0.5 mol Bi(NO 3 ) 3 · 5 H 2 O (99,5 %, Alfa Aesar) were dissolved in 1 L of 1 mol·L À 1 HNO 3 and transferred to beakers as reservoirs. With the help of the 4-needle head (4NH) of the synthesis robot, the volumes of the precursor solutions calculated were pipetted into lined beakers. For the mixed oxide Bi 0.5 Ce 0.5 O x , abbreviated as BiCeO, equimolar amounts of the nitrate solutions are used. The precursors were mixed by magnetic stirring and kept at T = 25°C. In different lined beakers, the two precipitants, which were dissolved in ultrapure water, were also brought to the temperature T = 25°C. The precipitants were transferred using the 4NH to the premixed and tempered nitrate solutions until the pH reached pH = 8 for AC and 3.5 times of the molar amount for OA was reached. The white to pale yellow precipitates were stirred for additional t = 0.5 h at T = 25°C. The precipitates were filtered off from the supernatants through a 3-fold parallel pressure filtration station at an overpressure of p = 1 bar after stirring. For filtration, Polyethersulfone membranes (Sartorius Stedim Biotech Corp.) with 0.1 μm mean pore size were used. The filtered-off samples were dried over night at room temperature, crushed and ground in an agate mortar until a fine powder was obtained and calcined at T = 800°C for t = 5 h. Table 9 summarizes the dispensing and aspirating speeds of the 4NH.

Contact between Soot and Catalyst
A weight ratio of 4 : 1 of catalyst and soot is used for all tests. The model soot used in this work is the carbon black P90 supplied by Evonik Degussa.

Loose Contact
Mixing soot and catalyst with a spatula for t = 60 s in a 5 mL glass vessel.

Tight Contact
Mixing soot and catalyst manually in an agate mortar for t = 300 s.

Wet Contact
Mixing soot and catalyst in acetone for t = 2.5 h in a 5 mL glass vessel under stirring. Afterwards, the suspension is dried for t = 48 h at T = 80°C in order to get rid of acetone.

Automated Contact
The automated contact was performed using a dual asymmetric centrifuge Speedmixer TM DAC 150 from Hauschild Corp. Soot and catalyst are mixed in cups PP5-6 mL and the holder for a single sample was used. Experiments are performed at mixing times t M = 60 s and 300 s. The rotation speed rs is varied in Δrs = 200 rpm steps between rs = 300 and 3500 rpm.

Activity Measurements
The activity measurements in means of T 50 values and dynamic oxygen storage capacity OSC dyn were investigated by automated serial thermogravimetric analyses coupled with heat flow differential scanning calorimetry (TGA-DSC 1 1600 from Mettler Toledo Corp.) according to Ref. [32]. Catalysts calcined at T = 800°C were used for activity measurements. The T 50 value is the temperature where 50 % of the soot mass is oxidized. To determine the T 50 values, approximately m = 10 mg of the catalyst-soot mixture in a 4 : 1 weight-ratio was placed in a corundum crucible of volume V = 70 μL and heated to a temperature of T = 700°C at a heating rate of r = 5°C·min À 1 with a synthetic air flow rate of _ V = 25 mL·min À 1 . The T 50 values were determined using the software Star e (Mettler Toledo Corp.).
The dynamic oxygen storage capacity OSC dyn is another significant property for soot oxidation catalysts. The OSC dyn was determined experimentally with the TGA-DSC 1 1600 from Mettler Toledo Corp. already described. Two heating and cooling cycles were performed in order to calculate the OSC dyn . Approximately m = 30 mg of the catalyst was placed in a V = 70 μL corundum crucible and heated to T = 700°C under nitrogen flow of _ V = 25 mL·min À 1 to get rid of adsorbed oxygen, water, or other compounds. The system was stabilized for t = 10 min. Afterwards, the system was cooled to T = 150°C under synthetic air flow of _ V = 25 mL·min À 1 for oxygen uptake and then stabilized for t = 10 min. This procedure was repeated and the weight loss of the second heating step was used for calculating the OSC dyn as mass difference of The formula symbols have the following meaning: m 150 � C O2 is the mass of catalyst at a temperature of T = 150°C after oxygen uptake, m 700 � C N2 is the mass of catalyst at T = 700°C after oxygen release, M O 2 is the molar mass of oxygen, and m cat is the buoyancy-corrected mass catalyst weighed in a V = 70 μL corundum crucible after desorption of compounds such as water. OSC dyn is given in mmol O 2 �g À 1 cat . The heating/cooling rate of r = 5°C·min À 1 was used for TGA-DSC measurements.

Catalyst Characterization
Powder X-Ray diffraction (PXRD) was performed on a Bruker D8 diffractometer with Co fine focus X-ray source (Ni filter, λ Kα = 1.79021 Å, Θ-Θ geometry, VDS, Lynxeye detector) for the coprecipitated samples. For the reference sample AdNanoCeria, the diffractometer STOE-Stadi P with Cu fine focus X-ray source (monochromator, λ Kα1 = 1.54056 Å, Θ-Θ geometry, MYTHEN2 R1 K detector) was used. The intensity data were collected in the 2θ range from 20-125°. Qualitative phase identification was achieved by diffraction pattern assignment according to ICDD (International Centre for Diffraction Data) data as specified. Crystalline phases were identified through Rietveld refinement using the software TOPAS version 4.2 according to Refs. [45][46][47], which was adapted for the cerium-bismuth mixed oxides. Instrumental parameters for the fundamental parameter TOPAS refinement have been determined by refining a LaB 6 diffraction pattern as reference sample.
Raman spectra were measured with an inVia Raman Microscope (Renishaw Corp.). A frequency-doubled Nd:YAG laser of the model RL473 C from Renishaw with a wavelength of λ = 532 nm and a UVDD-CCD-Array detector with a grid consisting of 1800 lines/mm was used. Normally, laser powers of 0.5 % of the maximal laser power P = 50 mW and a measuring time of t = 60 s were employed. The calcined samples were placed on a 96-well microplate with Vshaped bottom (Greiner Corp.). The operating status of the microscope was line focus. Each sample was measured at three different points.
The determination of the specific surface according to Brunauer, Emmett and Teller was measured on a NOVA touch LX4 surface area & pore size analyzer (Quantachrome Corp.). The evaluation was performed with the software TouchWin TM version 1.11 (Quantachrome Corp.). The specific surface was measured by N 2 adsorption-desorption isotherms at the temperature of liquid N 2 with T = À 196°C. In preparation for the measurements, the samples were degassed under high vacuum for t = 4 h at T = 250°C. The seven-point multi-point BET method was used to determine the specific surface S BET in a relative pressure range of 0.02-0.25.
To determine the mass fractions of Bi and Ce, Energy-dispersive Xray fluorescence (XRF) analyses were performed on a Fischerscope® X-RAY XAN® instrument and the evaluations with the software WinFTM® (Fischer Corp.). For the measurements, an excitation voltage of U = 30 keV and a measuring time of t = 60 s were used. The calcined catalysts were placed on top of a cellulose foil. Three repeated measurements were made at different locations on the sample and the composition average was taken. Calibration of the measurement data was performed using a standardless fundamental parameter method supplied by Fischer Corp. More details are described in Refs. [48][49][50][51].
Particle size distribution was measured by static light scattering performed on a CILAS 930 particle size analyzer. The range of hydrodynamic diameter was between d h = 0.2-500 μm. Approximately m = 0.5 g of the catalysts were suspended in the water container of the analyzer at T = 25°C and ambient pressure. In preparation of the particle size measurement, the samples dispersed in water were treated with ultrasound for t = 60 s to destroy agglomerates. This procedure was repeated for five times. and reviewed the article. All authors have read and agreed to the published version of the manuscript.