Hydrothermal synthesis and characterization of transition metal (Mn/Fe/Cu) co-doped cerium oxide-based nano-additives for potential use in the reduction of exhaust emission from spark ignition engines

The goal of this work was to synthesize new cerium oxide-based nano-additives to minimise emissions from spark ignition (SI) engines fueled with gasoline blends, such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx). To investigate the effect of transition metal dopants on their respective catalytic oxidation activity, nano-sized CeO2 catalysts co-doped with Mn, Fe, Cu and Ag ions were successfully produced by a simple hydrothermal technique. The synthesis of nano-catalysts with cubic fluorite geometry was confirmed by XRD data. The addition of transition metal ions to the CeO2 lattice increased the concentration of structural defects like oxygen vacancies and Ce3+ ions, which are advantageous for the catalytic oxidation reaction, as also supported by XAFS and RAMAN analysis. Further, nano-gasoline fuel emission parameters are measured and compared to straight gasoline fuel. The results demonstrated that harmful exhaust pollutants such as CO, HC and NOx were significantly reduced. The high surface area, better redox characteristics and presence of additional oxygen vacancy sites or Ce3+ ions have been linked to the improved catalytic performance of the synthesized catalyst.


Introduction
For centuries fossil fuels have played a critical role in the development of power engine applications. However, a gasoline-fuelled spark ignition (SI) engine emits harmful pollutants into the atmosphere such as CO, HC and NO x resulting in a variety of worldwide environmental problems such as greenhouse effects, acid rain and foggy weather. As a result, tight emission regulations are implemented to mitigate the effects of these polluting gases. Historically, great efforts have been made to reduce automotive engine emissions of these toxic exhaust gases using three key strategies: (1) internal engine alteration, (2) post-emission treatment of harmful exhaust gases and (3) fuel modication. The current inquiry acquires a gasoline modication strategy since it is simple and does not require extensive internal engine modication. 1 Recent research has focused on solid nanosized additions as helpful catalysts capable of greatly improving the characteristics of fuel. The addition of a trace amount of nanomaterials to hydrocarbon fuel results in more efficient combustion and lower emissions from automobile engines. 2,3 The inclusion of nano-additives such as metallic, non-metallic, oxygenates, organic and combinations has been demonstrated in diesel and biodiesel fuel mixes. The acquired results demonstrated an improvement in the physiochemical properties of the fuel, stabilisation of the fuel mixture, and an increase in the heat transfer rate of the fuel. Engine performance parameters also improve as hazardous exhaust emissions are reduced. [4][5][6][7][8] Due to their low cost, increased redox properties and resistance to sulphur poisoning, metal oxides are the most efficient catalysts as nano-additives in gasoline fuel. Among them, cerium oxide (CeO 2 ), a signicant rare earth metal oxide, has garnered increased attention in recent decades due to its numerous applications in the eld of catalysis, including threeway catalysis (TWC), dehydrogenation reactions, fuel cells, CO oxidation and water gas shi reactions. Cerium oxide's usefulness in pollution reduction and a variety of other sectors is attributed to its unique redox behaviour of oxygen storage and release under oxygen-rich and oxygen-decient situations, respectively. This distinguishing characteristic is commonly referred to as oxygen storage capacity (OSC). This feature of ceria is related to the generation of oxygen vacancies (V o ) and the low redox potential between Ce 4+ and Ce 3+ i.e., (1.7 V). [9][10][11][12][13] However, there are several limitations associated with ceria, in spite of the large number of applications it has. These include a small number of anionic defects, a high working temperature, instability at higher temperatures, and progressive agglomeration of its particles, which results in the eventual deactivation of its redox couple and the subsequent reduction of its optical transparency. 14 As a result, numerous solutions have been developed to improve ceria's thermal stability and electrical properties. Numerous studies are now being conducted on the doping of CeO 2 with transition metals. It demonstrates that including less expensive transition elements into the lattice structure of CeO 2 can enhance ceria's physiochemical properties such as heat stability, redox properties and oxygen vacancies, resulting in increased catalytic activity. 15,16 Mn, Fe, Co and Cu have received considerable attention due to their changeable oxidation state and better redox behaviour. Additionally, the strong synergetic interactions between the dopant metal oxide and the host CeO 2 lattice might considerably contribute to increased catalytic activity. As a result of the transition elements' signicant oxidative properties combined with oxygen storage materials such as CeO 2 , the resulting mixed oxide acts as an extremely efficient and inexpensive catalyst for oxidation reactions. [17][18][19][20] Recently, Jampaiah et al. asserted that the enhanced catalytic performance of copper and manganese doped cerium oxide is due to the production of more Ce 3+ species or oxygen vacancies (V o ) in the ceria lattice due to the dopant insertion. Furthermore, Yang et al. demonstrate that copper doped CeO 2 has improved catalytic activity due to increased surface oxygen defects in the cerium oxide lattice structure caused by the substitution of Ce 4+ ions with Cu dopant. 21 Thus, doping ceria with transition metal cations could result in the formation of Ce 3+ surface defects or oxygen vacancies (V o ), resulting in a signicant increase in the ratio of Ce 3+ /Ce 4+ species, which is required for the catalytic oxidation activity. Co-doping is the most effective method for increasing the catalytic performance of CeO 2 . Transition metals such as Mn, Fe, Cu have been chosen as possible co-dopants to improve the structural, surface, and redox properties of cerium oxide catalysts. [15][16][17][18][19][20][21][22] As a result, we synthesized transition metal co-doped CeO 2 catalysts, such as Mn/Fe-CeO 2 , Mn/Cu-CeO 2 , Fe/Cu-CeO 2 and Cu/Ag-CeO 2 by a hydrothermal approach and examined their efficacy in reducing exhaust emissions from spark ignition (SI) engines. The synthesised nano-catalysts were characterised using a variety of techniques, including X-ray diffraction (XRD), RAMAN spectroscopy, X-ray absorption studies (XAFS), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption ne structure (EXAFS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) and UV-diffuse reectance spectroscopy. Further, to the best of our knowledge, for the rst time we applied these materials as nano-catalysts in preventing exhaust emission from (SI) engines and found reduced emission of toxic exhaust gases when compared with pure gasoline fuel comprising no catalyst.

Material
A simple hydrothermal method was used to synthesize the transition metals co-doped ceria nano-catalysts. For synthesis, and AgNO 3 were employed as precursors of respective metals. All the chemicals purchased from sigma Aldrich and utilized without any process of purication.

Instrumentations and procedures
To obtain powder X-ray diffraction (pXRD) data on manufactured nano-catalysts, we used a source of Cu Ka (0.15418 nm) radiation at 298 K and a PAN analytical X'Pert. Raman measurements were made on Renishaw and the resulting Raman spectra were analysed in the (200-1000) cm À1 region utilizing a ViaTM Reex micro spectrometer. On the beamline of XAFS/XRF, Synchrotron-Light for Experimental Science and Applications in the Middle East, XAFS measurements were made (SESAME). SEM pictures were acquired using a Nova Nanosem 450 eld emission scanning electron microscope. Using an accelerating voltage of 10.0 kV, different resolutions were used to capture images from 1 mm, 2 mm, 100 nm and 200 nm. The surface area of samples was determined using a quanta chrome device and the N 2 adsorption-desorption isotherm (Version 11.04). The optical energy band gap values of nano-catalysts are calculated employing a PerkinElmer Lambda 35 UV-Vis spectrophotometer. Experimental details of XAFS and EXAFS measurements and t analysis are mentioned in ESI. †

Experimental procedure for exhaust emission analysis
In this analysis, the emission characteristic test has been carried out on a single cylinder four stroke air cooled SI engine with a cylindrical bore of 58 mm and stroke of 56.4 mm, as mentioned in Table 1. The emission parameters have been calculated at a constant speed of 1400 AE 50 rpm for different loads. The engine exhaust was directly discharged into a stainless-steel pipe without any kind of dilution. For each measured emission parameter, the experiment was carried out at least three times. Sampling data for each sample is analysed and collected aer the engine has been operated for at least 15 minutes. The whole procedure was carried out in two steps, rstly, preparing the nano-gasoline blends, lastly, measuring the exhaust emissions.

Experimental setup for emission measurement
In the present study, various experimental tests have been performed on SI engines using nano-gasoline fuel blends and pure gasoline to assess the engine emission characteristics. The dosing level of all synthesized nano-additives remains constant i.e., 20 ppm throughout measurements. The SI engine toxic exhaust emissions have been measured by using an exhaust gas analyser of E-Instrument Model 8500. Table 2 gives the speci-cations of exhaust gas emission analyser. This Model comprises ve gas emission analysers which are employed to calculate the emissions of O 2 , HC, CO 2 , CO, and NO x . For analysing different pollutant gases for instance CO, HC and NO x , the gas analyser was equipped with online measuring unit cells. The data analysing, processing and calculations were administered by analyser systems to measure oxygen, carbon dioxide and hydrocarbons as percentage volume while carbon monoxide and nitrogen oxide emissions were recorded in ppm.

Nano-catalysts preparation
The catalysts i.e., Mn/Fe-CeO 2 , Cu/Ag-CeO 2 , Fe/Cu-CeO 2 and Mn/Cu-CeO 2 were synthesized through facile hydrothermal methods. For the synthesis of Mn/Fe-CeO 2 , Ce (NO 3 ) 3 $6H 2 O (5.2 g) was mixed in deionized (DI) water (30 mL). Then the obtained solution stirred for one hour. Next, Mn (NO 3 ) 3 $4H 2 O (0.64 g) and Fe (NO 3 ) 3 $9H 2 O (1.0 g) were mixed independently in required amount of DI water and transferred simultaneously to the above solution of cerium nitrate and the resulting solution was vigorously stirred for two hours. Furthermore, dropwise addition of aqueous ammonium hydroxide solution to the above solution was carried out at room temperature with continuous stirring until the pH becomes 9. Now, the obtained mixture was later shied to 100 mL Teon lined stainless-steel autoclave sealed and thermally heated in a muffle furnace at 200 C for 24 hours. Aer heating, the autoclave was cooled down naturally to room temperature. The desired precipitates aerwards separated with ultracentrifugation and washed various times with DI water and later dried in an oven at 100 C for 12 hours. At last, the dried sample calcined at 500 C for 5 hours in a furnace to attain the nal product i.e.,

Characterization studies of nano-catalysts
The crystallinity and geometry of investigated samples have been characterized by XRD. The results of pure CeO 2 and transition metals co-doped ceria nano-additives are indicated in Fig. 1. XRD pattern of all nano-catalysts revealed intense and sharp peaks at 28.967 , 33.56 , 48.07 and 57.14 which conrm the presence of (111), (200), (220) and (311) planes respectively. These lattice planes are fairly attributed to the cubic uorite crystal lattice of cerium oxide and each peak is in accordance  with the standard reference pattern (JCPDS card number 34-0394). Within the sensitivity of XRD measurement, peaks related to the dopant metal oxide or other impurities are notably absent in the investigated XRD pattern. Fig. 1 also conrms that X-ray diffraction peaks of the samples are slightly shiing towards higher Bragg's angle with reference to pure CeO 2 . This is due to the shrinkage of ceria lattice structure as the ionic radii of dopants Mn x+ (Mn 4+ ¼ 0.53Å, Mn 3+ ¼ 0.65Å, Mn 2+ ¼ 0.83Å) and Fe (Fe 2+ ¼ 0.74Å; Fe 3+ ¼ 0.64Å) are smaller as compared to Ce 4+ (0.97Å). Each observation conrms the successful substitution of doped metal oxide into the crystal lattice of ceria to obtain the homogeneous solid solutions of CeO 2 . 11 Additionally, the XRD pattern of these materials displayed broader peaks than pure ceria, indicating that dopant ions act as a growth and crystallization inhibitor for ceria nanoparticles. To conrm this, the crystallite size of each sample was determined with the highly intense (111) peak of the cubic phase and the well-known Scherrer equation. The average crystallite size of pure CeO 2, Mn/Fe-CeO 2 , Cu/Ag-CeO 2 , Fe/Cu-CeO 2 and Mn/Cu-CeO 2 was calculated to be 8.98 nm, 8.48 nm, 8.84 nm, 6.19 nm and 5.46 nm respectively, which revealed the nanocrystalline nature of synthesized samples. The smallest particle size of Mn/ Cu-CeO 2 (5.46 nm) than ceria and other transition metal codoped ceria nano-catalysts suggests the co-doping of Mn/Cu ions assists the dispersion of cerium oxide and notably arrests the growth of ceria crystal, which might be the result of strong synergistic interaction among Ce and Mn/Cu oxide. 11 Raman analysis is a valuable approach which gives detail associated with the characterization of mixed metal oxides and presence of lattice defects or oxygen vacancies. The Raman spectra of materials are illustrated in Fig. 2. All synthesized materials show a prominent high intense peak for the F 2 g vibrational mode of CeO 2 . In the investigated Raman region, bands corresponding to dopant metal oxides were not   identied. This observation conrmed the formation of cerium oxide based solid solutions in line with the XRD results. 11 However, the shiing of F 2g peak position to lower wavenumber (peak position at 443-446 cm À1 ) was observed for transition metal co-doped ceria nano-catalyst as compared to pure CeO 2 i.e., 454 cm À1 indicating the defective lattice structure. 18,19 This redshi can be ascribed to variation of the M-O vibrational frequency, due to ionic radii difference among transition metal dopants and Ce 4+ ions. The shi also revealed the introduction of Ce 3+ species or oxygen vacancy sites in the cubic uorite structure of ceria in order to compensate for the charge imbalance in structure. Moreover, the low intensity peak (O v ) in case of Mn/Cu-CeO 2 at 540 cm À1 reveals the signicant number of oxygen vacancies hence leads to improved catalytic performance for oxidation reaction. [22][23][24][25] 3.2 Local coordination environment studies of nanocatalysts by XAFS XAFS spectroscopy is a synchrotron-based method which delivers valuable information about metal denite electronic structure and oxidation state through XANES and variations in the metal local atomic structure represented in EXAFS studies. In Fig. 3(a), the experimental Ce L 3 -edge XANES spectrum of cerium oxide exhibited four distinct A, B, C and D features that are in agreement with the literature reported cubic phase of CeO 2 . 26-28 The pre-edge peak A is attributed to the 2p 3/2 / 4f quadrupole transition that arises due to the 5d mixing with the 4f states. Moreover, due to the CeO 2 cubic lattice, the Ce 5d experiences crystal-eld splitting in e g and t 2g levels. Therefore, features B and C are assigned to the Ce 2p / 4f 1 5d e g -L and 2p / 4f 1 5d t 2g -L transitions, respectively, whereas L represents a 2p hole of oxygen ligand and 4f 1 is due to the transfer of an electron from an oxygen 2p orbital to the cerium 4f one. In this case the excitation from the Ce 2p to the 5d is accompanied by an electron excitation/transfer from the O 2p to the Ce 4f orbital, generating a hole in the valence band. The feature D is attributed to the transition from the Ce 2p to the 4f 0 5d state (with no f electrons). Nevertheless, the XANES spectra of d-transition metal ions (Cu, Fe and Mn) co-doped CeO 2 nanomaterials manifested similar four A, B, C and D features, suggesting the same CeO 2 cubic structure for these materials.
Alternatively, the Cu K-edge (8979 eV) XANES spectra of all the d-transition metals ions (Cu, Fe and Mn) co-doped CeO 2 nano-catalysts clearly illustrated the existence of prevailing Cu(II), resulting in similar broad low energy shoulder in the region below 8985 eV and intense rising edge from 8992 to 9002 eV (Fig. 3(a) right), characteristic of the Cu(II). The Fe Kedge (7112 eV) XANES spectra of the Fe/Cu-CeO 2 (B2 and C3) nano-catalysts displayed the weak 1s / 3d pre-edge peaks at $7114 eV (ref. 29) and rising edge peaks above 7128 eV (ESI Fig. S1 †), manifesting the existence of Fe(III) in octahedral (O h ) environment. Whereas, the Mn K-edge (6539 eV) XANES spectrum showed the presence of Mn 2/3+ in the Mn/Cu-CeO 2 (F2) nano-catalyst (ESI Fig. S1 †). It conrms the variable valence states of Mn(II and III) in Mn/Cu-CeO 2 sample and manifests the existence of Fe(III) and Cu(II) ions in Fe/Cu-CeO 2 and Cu/Ag-CeO 2 samples respectively. This provides an insight into the existence of facile redox behavior among Mn 3+ /Mn 2+ ions associated with the redox couple of Ce 4+ /Ce 3+ in ceria lattice hence leads to the production of increased oxygen vacancies in Mn/Cu-CeO 2 catalyst, also validated by Raman spectroscopy. 10 The improved oxygen vacancies in the sample might be catalytically benecial for minimizing toxic exhaust emission.
To investigate the local chemical environment around Ce ions, the Ce L 3 -edge EXAFS data of all the materials were quantitatively analysed and nonlinear best t was performed to their Fourier transformed k 3 -weighted experimental EXAFS signals, using Artemis program from the Demeter. [30][31][32] Fourier transforms of c(k) and their corresponding real components with best curve tting result exhibited the eight backscattering oxygen atoms in the rst coordination shell, suggesting CeO 2 cubic uorite structure with (225) space group for all the materials 33 as appear in the below Fig. 3(b). The spectral ts manifested that the bond distances of rst coordination shell for the (A1) Cu/Ag-CeO 2 , (B2) Fe/Cu-CeO 2 , (C3) Fe/Cu-CeO 2 and (F2) Mn/Cu-CeO 2 were slightly less than the reference CeO 2 , which might be due to slight distortion in the host CeO 2 lattice due to insertion of d-transition metal ions (Cu, Fe and Mn) dopants (Table 3). In addition, the structural disorder around the Ce is also increased in the Cu/Ag-CeO 2 (A1), Fe/Cu-CeO 2 (B2), Fe/Cu-CeO 2 (C3) and Mn/Cu-CeO 2 (F2) when compared to the reference CeO 2 , as demonstrated by their higher s 2 (Debye-Waller factors) values, suggesting defective local atomic structures for all these nano-catalysts. Nevertheless, the overall Ce L 3edge XAFS results suggest the existence of Ce sites between Ce 3+ (4f 1 L) and Ce 4+ (4f 0 ) character, with a hole (L) in the O 2p valence band. 26,27 Therefore, it validates the co-existence of Ce 4+ and Ce 3+ species on the surface of all cerium oxide-based nanoadditives. Previous studies specify the existence of reduced cerium cations (Ce 3+ ) to the production of oxygen vacancies in ceria lattice which play a key role in catalytic oxidation reaction. 11 In an effort to gain understanding on the local environment of Cu, Fe and Mn ions dopants in the CeO 2 host lattice, the XAFS analyses at the Cu K-edge (8979 eV), Fe K-edge (7112 eV) and Mn K-edge (6539 eV) respectively were employed for all the materials, as indicated in Fig. 3(c) and (ESI Fig. S1 †). The Cu K-edge Fourier transforms of c(k) of these nano-catalysts clearly demonstrated a dominant peak at z1.92Å, attributed to the Cu-Obond pair, suggesting existence of Cu 2+ incorporated in the CeO 2 cubic host lattice. Whereas, a small Cu-Ce contribution from Cu-[O x ]-Ce structures (R z 3.3Å), were also demonstrated, validating the existence of an interaction among copper and ceria, consistent with the previous literature reported EXAFS of copper-ceria. 34 So as to discover further the effect of Cu dopant ion incorporation on the local electronic structure of CeO 2 , the best t to the Cu K-edge c(k) was carried out in R-space from 1. BET surface area analysis of cerium oxide-based nanoadditives has been calculated by using nitrogen gas adsorption isotherm. Nitrogen adsorption-desorption isotherm of each sample shows the type IV hysteresis loop which conrms the mesoporous nature of the materials as illustrated in Fig. S4 (ESI †). The surface area of investigated samples has been measured and results are given in Table 5.
The above data illustrates that the transition metal codoping strategy of CeO 2 nano-catalyst clearly improves the exposed surface area of the catalysts. In general, smaller size particles provide a large surface area catalyst as compared to large size particles. Therefore, Mn/Cu-CeO 2 has highest surface area (91.733 m 2 g À1 ) indicating the introduction of Mn and Cu ions into CeO 2 lattice structure notably resists the crystallite growth, resulting in an enhancement in the surface area of nano-catalyst which is according to the XRD results. 18 Thus, the catalyst with large surface area facilitates more surface-active sites to the reactant molecule and hence promotes the reaction more benecially over its surface hence might be the best catalyst for exhaust emission reduction. 35 The effect of incorporation of multivalent transition metal cations into ceria lattice further investigated by using UV-DRS analysis which is achieved in the range of 200-1200 nm. Fig. 4 indicates the UV-DRS spectra of the synthesized samples. The Kubelka-Munk function equation has been utilized to calculate optical band gap (E g ) values of all the nano-catalysts. The Table 3 Derived fitting parameters of EXAFS, involving N: coordination number, R: mean coordination shell radius, s 2 : mean square relative displacement (MSRD) or Debye-Waller factor, S o 2 : amplitude reduction factor, E o : photoelectron energy and R factor : goodness of the fit for the Ce L3-edge (5723 eV) c(k) of CeO 2 and d-transition metal ions (Cu, Fe and Mn) co-doped CeO 2

Materials
Bond type N R(Å)  22 This enhances the ratio of Ce 3+ species that ultimately give rise to the production of localized energy states which are nearer towards conduction bands and thus lowering the energy band gap (E g ). These outcomes reveal that the substitution of transition metal codopants into ceria matrix signicantly improves the crystal structure and electronic characteristics of the CeO 2 . 36,37

Exhaust emission analysis
Carbon monoxide is a harmful pollutant gas and its emission must be minimized. It is formed as an intermediate species in the combustion reaction of hydrocarbon fuel therefore, its emission produced from incomplete combustion reaction. The CO emissions from automobiles mainly depend on the equivalence ratio. In fuel-lean mixtures there is an excess amount of oxygen available hence, CO oxidizes and forms carbon dioxide (CO 2 ). In fuel rich mixtures, there is deciency in the availability of oxygen, to fully combust the mixtures of fuel-air. As the concentration of oxygen is low to form CO 2 , the CO emission is enhanced. Fig. 5(a) shows the CO emissions for pure gasoline and nano-gasoline fuel blends. The graph clearly species the decrease in CO emissions to some extent compared to neat gasoline. Due to excellent oxygen storage and release capacity of ceria, nano-CeO 2 based additives act as an oxygen buffer in the gasoline fuel and supply excess oxygen for carbon and leads to the reduction of CO emissions, as revealed in eqn (1). 39 The oxygen vacancy defects of nano-ceria absorbed molecules of carbon monoxide (CO) to directly produce CO 2 or form the intermediate bidentate carbonate species, which oxidizes and may leads to the development of CO 2 as illustrated in eqn (2),   (2) Fig. 5(b) illustrates the variations in hydrocarbon emissions with respect to load. Hydrocarbon (HC) emission from nanogasoline fuel blends is much lower as compared to pure gasoline. Unburned hydrocarbon forms because of lack of sufficient oxygen content in combustion of fuel and added nano-additives provide excess oxygen to gasoline so as a result efficient combustion is achieved which ultimately reduces hydrocarbon emission, as mentioned in the following eqn (3). 12 NO x emission varies with reference to load as indicated in Fig. 5(c). It is understood from the graph that a lowering in NO x emission was evident aer the addition of nano-additive in gasoline as compared to base fuel. Owing to high thermal stability of ceria, cerous oxide (Ce 2 O 3 ) produced from the oxidation reaction of CO and HC still remains active aer increasing the initial combustion reaction and re-oxidized back to CeO 2 via the decline in the NO x emission level as described in eqn (4). Moreover, this decrease in NO x emissions can also be attributed to several other different reasons such as reduction in combustion temperature, peak pressure, reaction time and oxygen content. 38,39

Catalytic activity studies
3.4.1 Emission characteristics. We have evaluated the catalytic efficiency of transition metal co-doped ceria nanoadditives for exhasut emission reduction (CO, HC & NO x ) in gasoline fuel. For this purpose, various experimental tests have been carried out on SI engines using gasoline fuel blends and pure gasoline to assess engine emission parameters. The exhaust gas analyser (E-Instrumental Model 8500) processes, calculates and records the data of engine exhaust in order to determine the ppm or percentage of examined gases in each sample. The obtained experimental data is demonstrated as exhaust emission proles for CO, HC and NO x in Fig. 5(a)-(c) to investigate the catalytic performance of cerium oxide-based nano-additives.
In general, the oxidation of CO and HC occurs through the Mars-van Krevelen mechanism, 11 in which the lattice oxygen of ceria catalysts participates in the chemical reaction and is later supplied by molecular gas phase oxygen as indicated in eqn (1). It demonstrates that the mobility of lattice oxygen has a vital role in inuencing the catalytic oxidation activity. Previous research indicates that by generating homogeneous solid solutions with transition metal dopants, the catalytic performance of CeO 2 can be enhanced. This is accomplished by the strong synergistic interactions between the host CeO 2 and the dopant metal oxide, which improve the lattice oxygen mobility by enhancing the concentration of oxygen vacancies (V o ) in the synthesized sample. 12 In co-doped ceria samples, strong synergistic interactions between Ce and d-transition metal ion dopants (Mn, Fe and Cu) enhance Ce 4+ /Ce 3+ redox activity, which is critical for reducing high-valent cerium ions and releasing oxygen for oxidation reactions. As a result, the improved oxygen buffer capacity of CeO 2 based nano-additives in gasoline fuel permits simultaneous oxidation of CO and HC (eqn (2) and (3)) as well as a reduction in NO x emissions (eqn (4)), decreasing harmful exhaust gases, particularly in the stoichiometric ratio. It is obvious from Fig. 5(a)-(c) that Mn/Cu-CeO 2 nano-additive exhibits superior catalytic performance compared to other codoped ceria nano-additives. As explained above from XANES analysis, oxygen vacancies in Mn/Cu-CeO 2 sample not only formed due to the conversion of Ce 4+ to Ce 3+ but also the incorporation of Mn 3+ /Mn 2+ ions into lattice structure of ceria. As a result, the mobility and amount of oxygen vacancies on the surface of the ceria increases that plays a key role in the catalytic oxidation reaction. Thus, Mn/Cu-CeO 2 nano-additive is catalytically more active in gasoline fuel as compared to other nanocatalysts and considerably lowers the emissions of CO, HC and NO x . The higher catalytic activity of the nano-catalyst can be ascribed to the following factor, (1) the signicant amount of surface oxygen vacancies which is evident from XAFS, XANES, EXAFS, Raman and UV-DRS spectroscopy (2) the increased specic surface area of prepared samples combined with smaller crystallite sizes contributes to a greater number of surface-active sites, which play a vital role in improving the catalytic activity of the samples.

Conclusion
To summarize, Mn/Fe, Mn/Cu, Fe/Cu and Cu/Ag co-doped CeO 2 based nano-additives were successfully produced by facile hydrothermal method and investigated for exhaust emission reduction. XRD results conrm the formation of nanocrystalline solid solutions of ceria with cubic uorite geometry. UV-DRS analysis demonstrates that chemical co-doping dramatically reduces the band gap of CeO 2 , hence improving its redox characteristics. According to XAFS and Raman analysis, codoping of multivalent transition metal cations results in the formation of enhanced oxygen vacancies or Ce 3+ ions on the surface of doped CeO 2 , hence increasing the rate of oxygen transport for oxidation. All of the nano-additives based on cerium oxide were subjected to exhaust emission monitoring in a SI engine. The obtained results reveal that Mn/Cu-CeO 2 nanoadditive exhibits superior catalytic performance and signicantly reduces the harmful emission of CO, HC and NO x in comparison to pure gasoline. It can be attributed to smaller particle size, higher surface area, better redox properties and large number of oxygen vacancies or Ce 3+ ions evident from Raman, XAFS, XANES, EXAFS and UV-DRS spectroscopy. Finally, these ndings indicate that co-doping may be an effective strategy for developing highly active cerium oxide-