Background

Diesel engines are widely applied in the field of transportation, mining, and engineering machinery due to these advantages of low fuel consumption, high thermal efficiency, and good stability [1]. However, carbon monoxide (CO), unburned hydrocarbons (HCs), various oxides of nitrogen (NOx), and the particulate matter (PM) in diesel vehicle exhaust have caused a serious threat to ecological environment and human health [2, 3]. Furthermore, stringent environmental laws and regulations have driven recent advances in diesel emission control technologies. An integrated exhaust after-treatment system consisting of diesel oxidation catalyst (DOC), selective catalytic reduction (SCR), and catalyzed diesel particulate filter (DPF) has been widely used to purify diesel exhausts. The functions of the DOC in the after-treatment system are converting CO, HCs, and NO into CO2, H2O, and NO2, the NO2 being used as raw material for subsequent de-NOx reaction to promote SCR reaction. In addition, it could also oxidize the soluble organic fraction (SOF) to decrease PM emissions. HCs excessive emission will be caused owing to the incomplete combustion of HCs during the cold start of diesel vehicles. Therefore, the catalysts need to ignite rapidly at low temperatures [4]. Presently, noble metal catalysts (such as Pt, Pd, and Rh) supported on carbon materials or oxides (such as TiO2, Al2O3, CeO2, and ZrO2) are commercialized diesel oxidation catalysts with good performance for CO, NO, and HCs catalytic oxidation. However, there are drawbacks to commercialized catalysts, such as poor thermal stability, strong self-inhibition by CO, and high cost [5].

Ru and RuOx catalysts are widely applied in oxidizing CO [6], methane [7], and chlorobenzene [8]. Importantly, Ru catalysts have excellent low-temperature activity and poison resistance [8,9,10,11]. But Ru and RuOx are easily sintered, resulting in active sites’ exposure decreases [12]. Therefore, Ru catalysts should be supported on a carrier to prevent their sintering and improve catalytic activity.

TiO2 has been widely used to purify diesel exhausts. RuOx and rutile phase TiO2 have a similar lattice constant, and the rutile TiO2 in Ru/TiO2 catalysts plays an important role in stabilizing RuOx particles during calcination process in comparison with anatase-supported RuOx catalysts. Therefore, RuOx can be highly dispersed on the surface of TiO2. Furthermore, there is a synergistic effect between RuOx and TiO2, which is beneficial to improve the redox ability of Ru/TiO2 [13,14,15,16,17,18]. In order to further improve the thermal stability, dispersion of active components, and transformation of anatase to rutile phase, many studies have introduced Sn4+ into TiO2 to form SnxTi1−xO2 solid solution. Huang et al. [16] found that the introduction of Sn4+ into TiO2 lattice could improve the stability of the CuO/TixSn1−xO2 catalysts and dispersion of CuO. Bai et al .[17] indicated Sn4+ significantly improved the thermal stability of TiO2. Mehraz et al. [18] found doping Sn4+ promoted the phase transition of TiO2 from anatase to rutile.

Previous researches have focused on the preparation of diesel oxidation catalysts by co-precipitation method, sol-gel method, and solid-phase reaction [5, 6, 15, 19, 20]. Yang et al. [19] prepared the Pt/TiO2 catalysts via the co-precipitation method and found that the conversion of CO and C3H6 only reaches 50% at 232 °C. Li et al. [15] synthesized TiO2–SnO2 nano-composite by the sol-gel method and suggested that the conversion of TiO2–SnO2 to CO was 90% at 260 °C. Sharif et al. [6] prepared Ru/[Ca24Al28O64]4+(O2−)2 through solid-state reaction and showed that the conversion of Ru/[Ca24Al28O64]4+(O2−)2 to CO was only 82% at 240 °C due to lower dispersion of Ru. Therefore, there are critical challenges that remain in the low-temperature activity of diesel oxidation catalysts and a lot of efforts are still needed to remove CO and HCs caused in the diesel cold start. Furthermore, the current research [8, 16, 19, 21, 22] is mainly focused on the preparation of DOC catalysts by co-precipitation and sol-gel methods, which has a small grain size, but the samples have poor crystallinity and multiple crystal phases; furthermore, the subsequent heat treatment process of mixture by co-precipitation method is required. Hydrothermal treatment is adopted in the preparation process to avoid the traditionally followed calcination processes and the formation of hard aggregation of the catalysts, which could improve low-temperature catalytic activity [23]. However, there is a lack of systematic and comprehensive studies on the one-step hydrothermal method [24, 25].

Therefore, we reported that RuOx particles supported on the Sn4+-modified TiO2 by the one-step hydrothermal method were excellent CO and HC oxidation catalysts with promising low-temperature activity and stability. A series of SnxTi1−xO2 (x = 0, 0.33, 0.5, 0.67, 1) solid solution were prepared by the one-step hydrothermal method. Ru/SnxTi1−xO2 were then prepared by impregnation of the SnxTi1−xO2 with RuCl3 to oxidize CO and C3H8. The effect of hydrothermal temperatures, hydrothermal time, calcination temperatures, and the molar ratios of Sn/Ti of Ru/SnxTi1−xO2 catalysts were investigated in order to improve low-temperature activity and stability.

Method

Materials

Tin (IV) chloride pentahydrate (SnCl4·5H2O) was purchased from Guangdong Kehua Stock Corporation, tetrabutyl titanate (C16H36O4Ti) was purchased from Tianjin Kemiou Chemical Reagent Factory, and Ruthenium (III) chloride anhydrous, RuCl3, (37% Ru w/w) was purchased from Aladdin.

Preparation of Catalysts

SnxTi1−xO2 solid solution was prepared by the one-step hydrothermal method. The certain amounts of SnCl4·5H2O and C16H36O4Ti were dissolved in 200 mL of deionized water and 10 mL of anhydrous ethanol, respectively; then, C16H36O4Ti ethanol solution and SnCl4·5H2O aqueous solution were mixed while stirring at room temperature for 0.5 h. The homogeneous mixture was put in a 250-mL autoclave at 180 °C for 24 h. After that, the mixed solution was centrifuged washing with deionized water and ethanol several times until no residues of Cl−, and then was dried at 80 °C overnight in the oven. Subsequently, light yellow solid products were obtained, named SnxTi1−xO2. SnO2 and TiO2 were obtained by similar preparation methods, respectively.

Ru/SnxTi1−xO2 catalysts were prepared by impregnation of SnxTi1−xO2 with an aqueous solution including 1.0 wt.% of RuCl3. These samples were ultrasonic stirred for 2 h and dried at 80 °C for 12 h, and it was then calcined at 400 °C for 3 h (heating rate is 3 °C/min). The obtained powder was named Ru/SnxTi1−xO2.

Catalytical Performance

The activities of the catalysts were evaluated on a fixed bed quartz reactor with an electric heater. The simulative reactant gases contained a mixture of 3000 ppm CO, 600 ppm C3H8, 600 ppm NO, 50 ppm SO2, 7% O2, and N2 balance at a gas space velocity of 60,000 mL g−1 h−1. The gas flow rate was regulated by mass flow controllers. The temperature of fixed bed was tested by a 0.5-mm K-thermocouple which was placed in the middle of the center channels. The outlet CO and C3H8 were measured by a KM9106 flue gas analyzer (Kane International Limited, Britain). The conversion (X) of CO and C3H8 was calculated using the following equation:

$$ X=\frac{c_{\mathrm{in}}-{c}_{\mathrm{out}}}{c_{\mathrm{in}}}\times 100\% $$

where cin is the initial concentration of CO or C3H8 and cout is the instantaneous of CO or C3H8 at the reaction temperature; T50 is denoted as the low-temperature catalytic activity index.

Catalyst Characterization

X-ray diffraction (XRD) patterns of the samples were performed by power X-ray diffraction on a BRUKER D8 ADVANCE diffractometer equipped with a high-temperature chamber using Cu Kα radiation (0.15418 nm). The X-ray tube was operated at a source power of 40 kV × 40 mA.

The Brunauer-Emmett-Teller (BET) surface areas were tested by nitrogen adsorption at 77 K on a Micromeritics ASAP2020 adsorption apparatus; the specific surface area and pore distribution were calculated by the BET and BJH methods, respectively. These samples were degassed under vacuum at 300 °C for 4 h before each analysis.

Fourier transform infrared (FT-IR) spectroscopy was examined using a Nicolet is5 spectrometer at a spectral resolution of 4.0 cm−1. The powders were pressed into a self-supporting wafer (about 15 mg, 12 mm diameter). The wafer was pretreated with N2 at 300 °C for 1 h. After cooling to ambient temperature, the spectrum of samples was recorded.

Transmission electron microscopy (TEM) images of these samples were obtained by a Tecnai G2 F20 instrument at an acceleration voltage of 200 kV. The samples were ground, dispersed in ethanol, and deposited on carbon-coated copper grids prior to observation.

X-ray photoelectron spectroscopy (XPS) analysis was performed on a ESCALAB250Xi spectrometer, using monochromatic Al Kα radiation (1486.6 eV) at an accelerating power of 15 kW. The obtained sample spectra were corrected using C1s (284.6 eV) as the internal reference standard.

H2-temperature-programmed reduction (H2-TPR) experiments were performed in a quartz reactor connected to a thermal conductivity detector (TCD) with H2 (6.9% vol. %)-Ar mixture (30 mL/min) as reductant. Prior to the reaction, the sample (50 mg) was pretreated in N2 at 300 °C for 1 h and then cooled to room temperature. TPR started from room temperature to target temperature at a rate of 10 °C/min.

Temperature-programmed oxygen desorption (O2-TPD) experiments were carried out using the same device as H2-TPR. The spent catalyst (50 mg) was pretreated at 300 °C for 1 h under flowing Ar at 30 mL/min. Then, oxygen adsorption was conducted under an O2–Ar mixture (20% O2 vol. %) at 500 °C for 0.5 h. After cooling to room temperature, the system was purged in Ar (30 mL/min) for 1 h. After the treatment, the temperature was raised to target temperature (10 °C/min).

In situ infrared spectroscopy (IR) of CO adsorption was collected on a Nicolet 5700 FT-IR spectrometer at a spectral resolution of 4.0 cm−1. CO adsorption was performed by exposing a self-supporting wafer of catalyst (about 15 mg) and mounted in a commercial controlled environment chamber (HTC-3). The samples were exposed to a controlled stream of CO–Ar (10% of CO by volume) at a rate of 5.0 mL/min for 40 min. The spectra were recorded at various target temperatures at a rate of 10 °C/min from room temperature to 300 °C.

Results and Discussion

Catalytic Activity and Stability

Figure 1 shows the catalytic activities of CO and C3H8 oxidation on the Ru/SnxTi1−xO2 catalysts under the optimal preparation conditions (Fig. S1, S2 and S3) of hydrothermal temperature at 180 °C, hydrothermal time at 24 h, and calcination temperature at 400 °C. It can be seen that catalytic performances of Ru/SnxTi1−xO2 catalysts increased firstly and then tended to be stabilized with the increase of reaction temperature. When the molar ratio of Sn/Ti is 2/1, the T50 of Ru/Sn0.67Ti0.33O2 to oxidize CO and C3H8 is 180 °C and 320 °C, respectively, which is lower reaction temperature than other Sn/Ti molar ratios. The conversion of CO reached 90% at 240 °C, and the complete conversion of C3H8 could be achieved at 500 °C on the Ru/Sn0.67Ti0.33O2 catalyst. The catalytic performance for each sample was normalized with respect to the Ru atoms on the surface and expressed in terms of turnover frequency (TOF), as shown in Fig. 2. The TOF value for Ru/Sn0.67Ti0.33O2 was the highest among all the samples at any reaction temperature. It is attributed to the highly dispersed Ru on the surface of Sn0.67Ti0.33O2, and the active component Ru has a strong interaction with the carrier Sn0.67Ti0.33O2 [22, 26]. Sharif et al. [6] reported that the conversion of Ru/[Ca24Al28O64]4+(O2−)2 to CO was only 82% at 240 °C. Murayama et al. [27] reported that the conversion of Au/Nb2O5 and Au/SiO2 to CO was 55% and 38%, respectively, at 250 °C. Compared with other literatures [27, 28], when the molar ratio of Sn/Ti is 2/1, higher CO conversion can be achieved at lower temperature in this study. Okal et al. [29] reported that the T50 of CH4 oxidized by Ru/ZnAl2O4 catalysts was 480, 500, and 540 °C, when the loading of Ru was 0.5 wt.%, 1.0 wt.%, and 4.5 wt.%, respectively. Wilburn et al. [30] reported that the T50 of CH4 oxidation over 0.3Pd–0.7Pt/γ–Al2O3 catalyst was 360 °C. The catalytic activities of different catalysts for CO and C3H8 oxidation are shown in Table S1 and Table S2. Complete transformation of C3H8 can be achieved at a lower temperature in this work. The optimum molar ratio of Sn/Ti is 2/1, which is consistent with the activity of CO. From the above analysis, it can be concluded that the conversion of CO and C3H8 is greatly affected by the molar ratio of Sn/Ti. When the molar ratio of Sn/Ti is 2/1, the T50 of Ru/Sn0.67Ti0.33O2 to CO and C3H8 is 180 °C and 320 °C, respectively. When the reaction temperature is 240 °C, the conversion of CO can reach 90% and the complete conversion of C3H8 can be achieved when the reaction temperature is 500 °C.

Fig. 1
figure 1

Effect of different Sn/Ti molar ratios on Ru/SnxTi1−xO2 catalytic oxidation of CO (a) and C3H8 (b)

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Fig. 2
figure 2

TOF of reaction temperature for CO (a) and C3H8 (b) oxidation over various catalysts

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The stability of CO and C3H8 was investigated in Fig. 3, under hydrothermal temperature at 180 °C, hydrothermal time at 24 h, and calcination temperature at 400 °C (Fig. S1, S2 and S3). The conversion of CO reached 90% at 240 °C, and the complete conversion of C3H8 could be achieved at 500 °C. Interestingly, Ru/Sn0.67Ti0.33O2 catalyst is basically inactivated after a 12 h catalytic reaction; however, the activity of Ru/TiO2 and Ru/SnO2 catalysts decreased slightly with the increase of time when they oxidized CO. The phenomenon indicates that the formation of SnxTi1−xO2 solid solution can not only improve the activity of the catalysts, but also increase the stability. It is attributed that Ru is highly dispersed on the surface of Sn0.67Ti0.33O2; there is a strong interaction between the active component Ru and the carrier Sn0.67Ti0.33O2 [26].

Fig. 3
figure 3

The stability of Ru/SnxTi1−xO2 catalytic CO (a) and C3H8 (b)

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Catalyst Characterization

Texture Properties of SnxTi1−xO2 Supports and Ru/SnxTi1−xO2 Catalysts

Figure 4 shows the XRD patterns of both SnxTi1−xO2 solid solution and Ru/SnxTi1−xO2 catalysts. Typical peaks of anatase structure are observed in the TiO2 (25.78°) and Ru/TiO2 (25.67°) samples with grain sizes of about 4 nm and 5.5 nm (Table 1), respectively. A phase transition from anatase to rutile appeared with the introduction of Sn. The Ru diffraction peaks are not observed, indicating that Ru is highly dispersed on SnxTi1−xO2 surface or beyond the XRD detection limitation [31]. Furthermore, the diffraction peaks of SnxTi1−xO2 and Ru/SnxTi1−xO2 move gradually to lower angles with increasing Sn content, suggesting that the interplanar spacing d increases according to the Bragg equation, 2d sinθ = nλ. This is consistent with the increase in tetragonal lattice parameters (a and c) in Table 1, which is attributed to the substitution of larger ionic radius Sn4+ (0.071 nm) for Ti4+ (0.068 nm). The results suggest the Sn4+ has been successfully doped into the TiO2 lattice to form a uniform (–Sn4+–O–Ti4+–) solid solution while maintaining the rutile phase structure, which is in agreement with some previous studies [5, 18].

Fig. 4
figure 4

XRD patterns of SnxTi1−xO2 supports (a) and Ru/SnxTi1−xO2 catalysts (b)

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Table 1 Texture properties of SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts
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To determine the texture properties of samples, the N2 adsorption-desorption technique was used. The N2 adsorption-desorption isotherms and corresponding pore diameter distribution curves of these samples are shown in Fig. 5. The N2 adsorption-desorption isotherms of SnO2 distinctly belong to type II; others are classical type IV according to IUPAC classification and present a H2 complex hysteresis loop in a p/p0 range of 0.4–0.95, which is a common feature of mesoporous material (Fig. 5a, c) [17, 32]. The existence of these mesopores is an important reason for the large specific surface area of catalysts [33]. All of SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts exhibited a narrow distribution of small-sized pores (3–8 nm), especially the Sn0.67Ti0.33O2 support and Ru/Sn0.67Ti0.33O2 catalysts, with the pore diameter mainly uniformly distributed around 5 nm (Fig. 5b, d). This phenomenon suggested that an appropriate amount of Sn can weaken the diffusion coefficient of the catalytic surface and indirectly hinder the agglomeration of the crystallites [17].

Fig. 5
figure 5

N2 adsorption-desorption isotherms of SnxTi1−xO2 (a) and Ru/SnxTi1−xO2 (c) the pore size distribution of SnxTi1−xO2 (b) and Ru/SnxTi1−xO2 (d)

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The texture properties of SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts are listed in Table 1. The specific surface area and pore distribution were calculated by the BET and BJH method. Both the specific surface area and pore volume of Sn0.67Ti0.33O2 are 156.5 m2 g−1 and 0.17 cm3 g−1, respectively. But both specific surface area and pore volume of the Ru/Sn0.67Ti0.33O2 catalyst are decreased compared with the Sn0.67Ti0.33O2 support, which indicates that Ru loaded on the Sn0.67Ti0.33O2 surface. Moreover, the Ru/Sn0.67Ti0.33O2 catalyst is sintered and the open pore structure collapsed to form plugged pores during the high-temperature calcination process [31]. Nevertheless, Ru/Sn0.67Ti0.33O2 still maintains larger specific surface area (83.3 m2 g−1) and smaller pore diameter (5.3 nm) in comparison with other rutile samples such as Ru/Sn0.33Ti0.67O2, Ru/Sn0.5Ti0.5O2, and Ru/SnO2.

Figure 6 shows the FT-IR spectra of SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts. All the samples present similar vibration peaks at analogous wavenumber positions. The adsorption at around 3223.68 cm−1 is due to surface hydroxyl groups neighboring oxygen vacancy sites [34, 35]. The bands of 1501.86–1618.18 cm−1 belong to the angular vibration peak of water. The symmetrical stretching vibration peak of lattice oxygen appears at 1028.17 cm−1. The band of 527.27–681.2 cm−1 may be attributed to the stretching vibration peak of TiO2 or SnO2 [34]. Compared with SnxTi1−xO2 supports, Ru/SnxTi1−xO2 spectrum broadens, indicating that the active component Ru and support SnxTi1−xO2 have some interaction, resulting in the surface defects of catalysts [36, 37].

Fig. 6
figure 6

FT-IR spectra of the SnxTi1−xO2 supports (a) and Ru/SnxTi1−xO2 catalysts (b)

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Morphology of Catalysts

Low- and high-resolution TEM, HRTEM images, and the particle size distribution of Ru/SnxTi1−xO2 are exhibited in Fig. 7. Based on the observation of the TEM images presented in Fig. 7a, d, g, j, and m, we find that all samples are composed of well-defined particles with irregular shapes and disordered mesoporous structure, which is formed by the agglomeration of the nanoparticles [38]. Furthermore, it can be seen that the Ru/Sn0.67Ti0.33O2 sample has the highest degree of agglomeration because of the smallest grain size among these samples. From the HRTEM images (Fig. 7b, e, h, k, n), there is only one kind of lattice fringes with 0.327 nm, which is compatible with (110) plane of these samples. Besides, we find that the lattice fringes of TiO2 and SnO2 are not observed, which is attributed to Sn4+ having been successfully doped into the lattice of TiO2 to form a homogeneous SnxTi1−xO2 solid solution [39]. The results are consistent with XRD. The Ru particle size distribution (Fig. 7c, f, i, l, o) shows that the approximate sizes of Ru particles ranged from 3 to 20 nm. The introduction of Sn4+ could effectively decrease the sizes of Ru particles and achieve a higher dispersion on the SnxTi1−xO2 surface. Comparing with other samples, the Ru particle size distribution of Ru/Sn0.5Ti0.5O2 sample was wider (< 13 nm), which may be caused by the interaction between (–Sn4+–O–Ti4+–) species and Ru [26]. The Ru/Sn0.67Ti0.33O2 catalyst has better Ru dispersion and smaller particle size (5.49 nm) among all samples.

Fig. 7
figure 7

TEM, HRTEM images, and the particle size distribution of a, b, c Ru/SnO2; d, e, f Ru/Sn0.67Ti0.33O2; g, h, i Ru/Sn0.5Ti0.5O2; j, k, l Ru/Sn0.33Ti0.67O2; and m, n, o Ru/TiO2

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Surface Properties of Catalysts

To further determine the elementary states and surface composition, XPS analysis was carried out. Figure 8 shows the XPS spectra of Sn 3d, Ti 2p, O 1s, and Ru 3d for the SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts. The XPS binding energy values of the Sn 3d3/2 and Sn 3d5/2 are observed at 486.6–487.5 eV and 494.9–496.1 eV, respectively, which are characteristic of Sn4+ species in SnxTi1−xO2 supports or Ru/SnxTi1−xO2 catalysts. Interestingly, the binding energy of Sn 3d3/2 and Sn 3d5/2 shifted to higher values after the introduction of Sn4+, indicating some of the Sn4+ replace the Ti4+ sites and have a strong interaction with TiO2, which is in agreement with XRD. Also, the oxyen vacancies may be created by the lower valent Snδ+ [5]. Two peaks corresponding to Ti 2p3/2 and Ti 2p1/2 are observed at 458.7–459.9 eV and 464.3–465.8 eV in the XPS spectra of Ti 2p, suggesting that Ti4+ and Ti3+ existed in the samples, and the binding energy values of Ti 2p3/2 and Ti 2p1/2 shifted to higher binding energy values with the increase of Sn4+, further proving the existence of oxygen vacancies. It can be seen from Table 3 that the Sn/Ti molar ratio by XPS is observed to be slightly higher than theoretical calculation, indicating that Sn is enriched on the surface of catalysts, which leads to more oxygen vacancies. Because the electronegativity of Sn (1.96) is larger than that of Ti (1.62), in other words, the electron-capturing ability of Sn is stronger than that of Ti, which causes the redox equilibrium (Sn4++Ti3+ → Snδ++Ti4+) shifting to right [32].

Fig. 8
figure 8

XPS spectra (Sn3d (a, b), Ti2p (c, d), O1s (e, f) and Ru3d (g, h)) of SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts

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The high-resolution spectra of the O 1s ionization feature are numerically consistent with the Gaussian feature and deconvoluted into two peaks [5]. Higher binding energy (O’, 532.1 eV) is caused by chemisorbed oxygen that formed the (O2−, O−, or O22−) species. However, the O” (529.9 eV) is the characteristic peak of O2− on the surface of metal oxides. Interestingly, the binding energy of O 1s shifted to higher values after the addition of Sn4+.

Ru 3d spectra present Ru4+ and lower value Ruδ+. The signal of Ru 3d5/2 is often used to analyze the charge state of the Ru species, since another Ru 3d3/2 overlaps with C 1s at around 284.0 eV [40]. The binding energy of 282.0–283.5 eV is assigned to Ru 3d5/2, which corresponded to Ru4+. The lower binding energy at around 280.2–281.7 eV is attributed to lower state Ruδ+, and the Ruδ+ relative ratio in Ru/Sn0.67Ti0.33O2 reaches 53.9%, which is higher than other catalysts. It could be explained that the strong interaction between Sn0.67Ti0.33O2 and Ru caused a larger amount of surface reactive oxygen species [26].

XPS and EDS analyses are performed to determine the surface and bulk composition of the samples as shown in Table 2. Surface and bulk Ru analysis shows that Ru/Sn0.67Ti0.33O2 has the highest surface Ru (0.69 wt.%) and bulk Ru (0.40 wt.%) among all the catalysts, indicating that the active component Ru is more evenly distributed on the Sn0.67Ti0.33O2 support, and more Ru species enters the internal of Sn0.67Ti0.33O2 to form a strong interaction.

Table 2 The surface compositions by XPS and bulk compositions by EDS of the SnxTi1−xO2 supports and Ru/SnxTi1−xO2 catalysts
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In order to further investigate the reduction performance of the Ru/SnxTi1−xO2 catalysts, temperature-programmed reduction studies are performed (Fig. 9). The shapes of these H2-TPR profiles are almost identical. The reduction peaks of Ru/SnxTi1−xO2 are divided into two parts: the low-temperature reduction peaks 80–270 °C are associated to the lower state Ruδ+ reduced from RuO2 and a significant amount of Sn4+ which could be reduced to lower valent Snδ+ or can be attributed to the reduction of surface oxygen [41], while the high-temperature reduction peaks 600–640 °C are associated to Sn0 reduced from Snδ+ or the reduction of bulk oxygen of catalysts [26, 42], which is consistent with XPS results. The reduction temperature of Ru/SnxTi1−xO2 moves towards lower temperature, peaks broaden and H2 consumption increase with the addition of Sn, and hydrogen consumption from the H2-TPR measurements are shown in Table 3. The dispersion of active components on the surface of the samples has a significant effect on the reduction of surface oxygen, and hydrogen could be more easily activated with higher dispersion of Pd, resulting in the increase of H2 consumption [43]. Therefore, we can infer that the introduction of Sn significantly increased the dispersion of Ru on the carrier, which may have resulted from the formation of SnxTi1−xO2 solid solution. The results are in good agreement with XRD and TEM. Because the reduction of TiO2 is usually difficult to conduct at low temperature, there are no peaks of the TiO2 reduction observed during the H2-TPR from 50 to 800 °C [15]. Nevertheless, the Ru/Sn0.67Ti0.33O2 still exhibits a higher H2 consumption.

Fig. 9
figure 9

H2-TPR spectra of Ru/SnxTi1−xO2 catalysts

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Table 3 Hydrogen consumption of Ru/SnxTi1−xO2 catalysts
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The O2-TPD experiments (Fig. 10) of Ru/SnxTi1−xO2 samples are imposed to gain insight into the mobility of surface and lattice oxygen. The signal at low temperature (< 200 °C) is attracted by the desorption of surface chemisorbed oxygen (O2−, O22−, or O− species); the main peak centered at 280 °C or 500 °C which is attributed to the desorption of the structure oxygen species, and the peaks above 600 °C are assignable to the desorption of the lattice oxygen (O2−) species [44]. The incorporation of Sn increased the adsorbed oxygen species and shifted to a lower temperature [45]. The results indicate that the incorporation of Sn improved the oxygen activation ability of the Ru/SnxTi1−xO2 samples and the interaction between the carriers SnxTi1−xO2 and active component Ru [46, 47].

Fig. 10
figure 10

O2-TPD spectra of Ru/TiO2 (a), Ru/Sn0.33Ti0.67O2 (b), Ru/Sn0.5Ti0.5O2 (c), Ru/Sn0.67Ti0.33O2 (d), and Ru/SnO2 catalysis

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CO and/or O2 Interaction with these Samples

The in situ FI-IR spectra of CO adsorption are recorded to further investigate the effect of the ruthenium oxide species, as shown in Fig. 11. The band located at 2052 cm−1 is attributed to linear CO adsorbed on reduced Ru crystallites (Ruδ+–CO), the band at 2140 cm−1 and 2075 cm−1 can be assigned to two different types of multicarbonyl species on partially oxidized Ru sites (Run+(CO)x), and the band at 1765 cm−1 is attributed to (SnxTi1−xO2)Ru–CO species [48, 49]. The Ruδ+–CO adsorption peaks at room temperature indicate the presence of some lower state Ruδ+ species. This is in agreement with the XPS results. However, the desorption temperature of the Ruδ+–CO peak is related to the Sn/Ti ratio and temperature. As the temperature increases, the peak intensity enhances firstly and then decreases gradually. Simultaneously, the CO adsorption peak moves to a higher wave number (2052 cm−1 at 25 °C and 2060 cm−1 at higher temperatures). This red-shift indicates that Sn4+ has stronger electron-donating capability [50]. For the Ru/SnO2, Ru/Sn0.5Ti0.5O2, Ru/Sn0.33Ti0.67O2, and Ru/TiO2 samples, the CO maximum adsorption peak on Ruδ+ appears at about 200 °C and disappears basically at 300 °C. For the Ru/Sn0.67Ti0.33O2 sample, the CO maximum adsorption peak on Ruδ+ appears at about 200 °C, which can be observed clearly even at 300 °C. It can be concluded that Ruδ+ is much more stable in Ru/Sn0.67Ti0.33O2 sample, which can provide more CO adsorption sites than in the other samples.

Fig. 11
figure 11

In situ FI-IR spectra of the 10% CO/Ar interaction with a Ru/SnO2, b Ru/Sn0.67Ti0.33O2, c Ru/Sn0.5Ti0.5O2, d Ru/Sn0.33Ti0.67O2, e Ru/TiO2 at different temperatures

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Possible Reaction Mechanism over the Ru/SnxTi1−xO2 Catalysts

According to the characterizations mentioned above, a possible reaction mechanism of CO and C3H8 oxidation is proposed and schematized in Fig. 12. Based on the XPS results, electrons migrate between Ru and SnxTi1−xO2 solid solution; because the electronegativity of Ru (2.22) is larger than that of Ti (1.62) and Sn (1.96), the electrons will transfer from the SnxTi1−xO2 solid solution to Ru4+, in which lower state Ruδ+ will be generated. Meanwhile, –Ti4+–O–Sn4+– species are oxidized and more oxygen will be absorbed on the surface of SnxTi1−xO2 solid solution, which can provide oxygen to the oxidation reaction of CO and C3H8. At the same time, the by-products produced in the oxidation process will also be adsorbed on the surface of SnxTi1−xO2 solid solution, which will not deteriorate the activity of Ruδ+ species. It is also the reason for the high stability of the catalysts. Moreover, the lower state Ruδ+ species have more metal properties, which play a crucial role in the activation of CO and C3H8 [40]. Compared with Ru/TiO2 and Ru/SnO2, high dispersion of Ru on SnxTi1−xO2 solid solution is also an important cause for their excellent activity and stability. Based on O2-TPD analysis, O2 is first adsorbed on the surface of catalysts to form O2− species and CO and C3H8 adsorbed on Ruδ+ species react with O2− species to produce CO2 and H2O, which is a Langmuir-Hinshelwood mechanism.

Fig. 12
figure 12

Possible reaction mechanism of CO and C3H8 over Ru/SnxTi1−xO2

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Conclusions

A series of Ru/SnxTi1−xO2 catalysts were prepared by a one-step hydrothermal method for the catalytic oxidation of CO and C3H8. The preparation conditions of Ru/SnxTi1−xO2 catalysts were optimized for CO oxidation reaction. Ru/Sn0.67Ti0.33O2 catalyst shows best CO catalytic activity and stability at low temperature under the condition of hydrothermal temperature at 180 °C, hydrothermal time at 24 h, and calcination temperature at 400 °C.

The effects of different molar ratios of Sn/Ti on the catalytic properties of Ru/SnxTi1−xO2 catalysts for CO and C3H8 were investigated under the optimum preparation conditions. The results show that the Ru/Sn0.67Ti0.33O2 catalyst exhibits better low-temperature activity and stability. The conversion of CO reached 90% at 240 °C, and T50 of which keeps at 180 °C. The complete conversion of C3H8 could be achieved at 500 °C, and its T50 remains at 320 °C. The excellent catalytic activity of Ru/Sn0.67Ti0.33O2 catalyst is attributed to the factors listed as follows.

  1. (1)

    The successful incorporation of Sn4+ into the TiO2 lattice to replace Ti4+ forms a homogeneous solid solution (–Sn4+–O–Ti4+– species), which enhances the interaction between active component Ru and carrier SnxTi1−xO2. The crystal growth of the anatase phase can be inhibited by the introduction of Sn4+, which results in the presence of the rutile phase.

  2. (2)

    Ultrafine Ru nanoparticles (~ 5 nm) are highly dispersed on SnxTi1−xO2 support, suggesting that the introduction of Sn4+ could not only prevent grain agglomeration and induce a smaller grain size, but also produce more defects such as oxygen vacancies.

  3. (3)

    CO and C3H8 species can be absorbed on Ruδ+ sites; O2− is formed by the adsorption of O2 on the oxygen vacancies. The adsorbed CO and C3H8 react with O2− to produce CO2 and H2O.