Introduction

Production of synthesis gas (H2 and CO gas mixture) by methane (CH4) partial oxidation (MPO) is an important intermediate step in many existing energy conversion technologies, such as fuel cells and Fischer–Tropsch reaction [15]. Decreasing the reaction temperature and improving the reaction efficiency have been a great challenge to researchers. One of the solutions is to develop high-efficiency catalysts. Our group in their previous research [68] showed that treating a Pt/γ-Al2O3 catalyst at 500 °C in a humid reducing environment (H2O/H2) can significantly decrease the reaction temperature of catalytic CO preferential oxidation in H2. This is due to the decreased particle size, increased stability of Pt, and decreased CO chemisorption strength. In a subsequent research by others, a similar water treatment technique for reducing reaction temperature was also investigated and confirmed [9, 10].

In this work, we applied the same water treatment technique to Rh/γ-Al2O3 catalysts and investigated its catalytic effects on the MPO reaction. The reaction products were analyzed by mass spectrometry (MS), and the catalyst morphology was characterized by transmission electron microscopy (TEM). The results show that water-treated Rh/γ-Al2O3 catalyst can effectively decrease the size of Rh nanoparticles, and the water-treated Rh/γ-Al2O3 catalyst shows higher CO selectivity in the temperature range of 300–600 °C.

Experiment

Catalyst preparation

Catalyst standard treatment

1 wt% Rh/γ-Al2O3 catalyst was prepared by the incipient wetness method. Alumina (γ-Al2O3, 150 m2/g, acid type, ~0.25 mm, Alfa) was first calcined in air at 500 °C for 2 h to remove volatiles and then impregnated using an aqueous solution of Rh(NO3)2 (Alfa). The catalyst was dried in a fume hood overnight, followed by calcination in air at 500 °C for 2 h. The catalyst was finally reduced in H2 at 500 °C for 2 h and cooled down with N2 protection. This process is considered to be the standard treatment of the catalyst, as shown in Fig. 1.

Fig. 1
figure 1

Rh/γ-Al2O3 catalyst preparation process: standard treatment and water treatment

Catalyst water treatment

Water treatment begins after the standard treatment of the catalyst. The standard-treated catalyst (0.1 g) was first wetted with 5 mL deionized (DI) water, then reduced in H2 for 1 h at 500 °C at a heating rate of 10 °C/min, and finally cooled down to room temperature with N2 protection. This process is considered to be the water treatment of the catalyst, as shown in Fig. 1. γ-Al2O3 can be potentially converted into hydrated boehmite (AlOOH) by reacting with water, and the acidity and surface area could be decreased. However, this reaction needs high temperature and pressure [1113]. Overall, the water treatment process proposed in this study is safer to use in the case of γ-Al2O3.

Catalyst testing and characterization

Figure 2 shows a schematic of MPO system for testing the above catalysts. A Hiden Catlab Microreactor with a vertical quartz tube (6 mm I.D.) was used for the reaction. The gas flow rates were controlled by mass flow controllers (MFC), and the reaction temperature was controlled by a programmable controller. The catalyst (0.1 g) was fixed inside the vertical reactor with quartz fibers. The gas flow rates of CH4, O2, and Ar were controlled to 40, 20, and 20 ml/min, respectively. A mass spectrometer (Hiden QIC-20, MS) was used to analyze the reaction products, and Ar was used as an internal standard. A calcium sulfate (CaSO4) as a dryer was located between reactor and MS to remove the water from the products and therefore to prevent plugging of the MS capillary tube by water. The test conditions adopted were between room temperature and 800 °C at 1 atm, and data were collected every 50 °C from 300 to 800 °C, as the ignition temperature of MPO with a Rh/γ-Al2O3 catalyst is around 275 °C [14].

Fig. 2
figure 2

A schematic of methane partial oxidation system

The conversion of CH4 and the selectivities of CO and CO2 were calculated using the following formulas:

$${\text{CH}}_{4 }\;{\text{Conversion}}: \, \frac{{{\text{CO}}_{\text{out}} + {\text{CO}}_{{ 2 {\text{out}}}} }}{{{\text{CO}}_{\text{out}} + {\text{CO}}_{{ 2 {\text{out}}}} + {\text{CH}}_{{ 4 {\text{out}}}} }};$$
(1)
$${\text{CO Selectivity}}: \, \frac{{{\text{CO}}_{\text{out}} }}{{{\text{CO}}_{\text{out}} + {\text{CO}}_{{ 2 {\text{out}}}} + {\text{CH}}_{{ 4 {\text{out}}}} }};$$
(2)
$${\text{CO}}_{2}\;{\text{Selectivity}}: \, \frac{{{\text{CO}}_{{ 2\;{\text{out}}}} }}{{{\text{CO}}_{\text{out}} + {\text{CO}}_{{ 2 {\text{out}}}} + {\text{CH}}_{{ 4 {\text{out}}}} }}.$$
(3)

Transmission electron microscopy (TECNAI FEI 20, TEM) was used to characterize the size and morphology of the Rh nanoparticles on γ-Al2O3 support before the reaction. Each catalyst was ground in a mortar for 5 min and then dispersed in ethanol with ultrasonication. The dispersed particle was dropped onto a copper TEM grid with a carbon substrate and then dried for TEM characterization.

Results and discussion

Figure 3 shows the CH4 conversion, CO and CO2 selectivities of the standard-treated (“S”) and water-treated (“W”) Rh/γ-Al2O3 catalysts in the MPO reaction. CH4 conversion under both catalysts is similar in the temperature range of 300–800 °C. However, the water-treated catalyst shows higher CO selectivity and lower CO2 selectivity than the standard-treated catalyst between 300 and 600 °C. The selectivities of CO and CO2 of both catalysts are similar when the temperature is higher than 600 °C. This can be attributed to the MPO reaction reaching the thermodynamic equilibrium, and catalysts cannot change their reaction behavior. To confirm the repeatability of this phenomena, the catalytic reactions were tested in different reaction cycles of the same catalyst and different batches of standard/water-treated catalysts, and the results were found to be consistent.

Fig. 3
figure 3

The CH4 conversion, CO and CO2 selectivities of the standard-treated and water-treated 1 wt% Rh/γ-Al2O3 catalysts in methane partial oxidation reaction (S standard-treated catalyst, W water-treated catalyst)

The TEM images of the standard-treated and water-treated catalysts are shown in Figs. 4a and  5a, respectively. The dark parts denote Rh nanoparticles and the brighter parts γ-Al2O3 particles, and Rh has higher electron density than Al because of larger atomic number. The average size of the Rh nanoparticles in the water-treated catalyst is 4.7 nm (Fig. 5b), much smaller than the standard-treated catalyst wherein the average size is 8.1 nm (Fig. 4b).

Fig. 4
figure 4

Standard-treated 1 wt% Rh/γ-Al2O3 catalyst: (a) TEM image; (b) Rh nanoparticle size distribution

Fig. 5
figure 5

Water-treated 1 wt% Rh/γ-Al2O3 catalyst: (a) TEM image; (b) Rh nanoparticle size distribution

The increased selectivity of CO and the decreased selectivity of CO2 of the water-treated catalyst could be attributed to smaller Rh nanoparticles on the alumina support, as they could provide larger surface areas and increase the contact area with reactants, thus providing higher CO selectivity. Unpublished results [15] for Pt/γ-Al2O3 suggest that the chemisorption strength of CO on water-treated catalysts is significantly reduced, allowing more competitive adsorption of other reactants and products. These water treatment mechanisms require more investigation, but are thought to involve a type of metal–support interaction.

Conclusions

Both standard-treated and water-treated catalysts were tested for methane partial oxidation reaction. Water-treated catalyst shows higher CO and lower CO2 selectivity in the temperature range of 300–600 °C. The enhancement is attributed to the formation of well-dispersed smaller Rh nanoparticles after water treatment. Further research is needed to explore the mechanism of Rh nanoparticle size decreasing during the water treatment process.