Inductively Coupled Plasma with Atomic Emission Spectroscopy

The accurate content of Rhodium atoms in the nanoparticle solutions were always verified by the ICP-AES (Agilent, Varian 720-ES). For the measurement, samples have to be prepared by dissolving trace amount of Rh nanoparticle solution in aqua regia along with dilution.

3.4.6 GC-TCD

Gas Chromatography (GC)

A normal gas chromatography, includes at least four parts requiring constant optimization, which are samples, carrier gas (mobile phase), column (stationary phase) and detector. After injection and gasification of samples, ample elution of the tubing and loop of the catalytic system has to be done with inert carrier gas, such as N2, Ar, He inactive to the analytes, for bringing the gas samples through the capillary column.

The capillary column works based on the different strength of the interaction between sample molecules and inside stuffing, which results in the different retention times of the analytical signals. According to the retention time, the content of samples can be confirmed easily.

Capillary Column

According to the coating manner on the inner walls, three kinds of capillary columns are commonly used, PLOT, WCOT and SCOT (Figure 3.1). PLOT column packs a layer of porous solid support on the inner wall. Although it gives good performance in sieving gas molecules, plenty amount of polar molecules will retain inside the solid support and causes the issues in experiments. In contrast, WCOT is coated with a thin

34

layer of liquid stationary phase on the inner wall, giving a better separation and resolution. In 1979, a new kind of WCOT, support-coated open tubular column (SCOT) was released. SCOT is the one merging the advantages of POLT and WCOT. It has not only the special treated silica porous solid support but also the polyimide coated on the outer wall to enhance the physical strength, resulting in excellent resolution.

Figure 3.1 Schematic illustration of (a) wall-coated open tubular column (WCOT), (b) porous-layer open tubular column (PLOT), (c) support-coated open tubular column (SCOT)

Detector – Thermal Conductivity Detector (TCD)

The working principle of TCD is related to the composition of sampling gases. When reference flow, typically inert carrier gas like N2 or He, goes through the measurement channel and meanwhile alters the resistance (R4 in Figure 3.2). A difference in the resistance is kept constantly between R2 and R4 as the reference to that between R3 and R1 which is mainly induced by the analyte gases. In this principle, all samples can exhibit distinct thermal conductivity to that of the reference flow (N2 or He) and therefore become distinguishable in TCD detection which is non-destructive.

(a) (b) (c)

Capillary Column

Liquid Stationary Phase Porous Solid Support Porous Solid Support

Coated with Liquid Stationary Phase

35

Figure 3.2 Schematic interpretation of the TCD working principle.

Heterogeneous Catalysis System

The solid-gas heterogeneous catalysis system are divided into three parts, that is (a) sampling section where four mass flow controllers, a mixing chamber, and a checking valve is installed; (b) reaction section where a U-shaped stainless tube, a heating furnace, and a pressure controller are set; (c) analysis section where a 6-port loop injection valve, the GC-TCD system, and a PC are connected up for ultimate signal collection and data processing.

Figure 3.2. The scheme of the heterogeneous catalysis system. (a) sampling system, (b) reaction section, (c) analysis section.

(a) (b) (c)

36

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Shape-Controlled Rhodium (Rh) Nanoparticles

As mentioned in the experimental section, the synthetic procedure of Rh nanocatalysts is a one-pot method in which RhBr3, cetyltrimethylammonium bromide (CTAB) and formic acid are mixed in a glass sample vial sealed with a cap followed by heating at 90 °C in an oil bath for 18 hours. Formic acid is generally oxidized as CO2 to release electrons or decomposes into CO and H2, playing the dual roles of reducing and shaping agents.

In Figure 4.1, Rh tetrahedral (TD) with all surfaces concaved were obtained with moderate amounts of formic acid and CTAB. During the process of nanoparticle formation, partial formic acid would decompose into carbon monoxide and water (HCOOH  CO + H2O) when heating in the oil bath. CO molecules have been known a very strong shaping agent inhibiting the growth of 111 crystal faces of f.c.c. metals.

40,41

In addition, a tetrahedron is known a structure with four 111 facets and therefore the concaved tetrahedra are possibly the major products as a significant amount of CO molecules exist. Figure 4.2 is the TEM image of a single concaved Rh tetrahedron and its corresponding selected-area electron diffraction (SAED) pattern. Apparently, the concaved TD is a symmetric single crystalline. Figure 4.3 shows the 3d XPS spectrum and the fitting curves of the concaved Rh TDs. The peaks at 306 and 311 eV denote the 3d5/2 and 3d3/2 of Rh. After fitting, the concaved TDs are known to have the Rh2O3

37

composition, possibly due to oxide layers on the surfaces. It is a general phenomenon observed in the products made with aqueous synthesis. Besides, the peaks of RhBrx are also found because of the unreacted precursors. ⁴²

Figure 4.1. (a) Low-, (b) middle- and (c) high-magnification TEM images of Rh tetrahedral nanoparticles.

38

Figure 4.2. (a) TEM image and (b) the corresponding SAED pattern of a concaved Rh.

(c) Size-distribution histogram concaved tetrahedral with 100 particles counted.

Figure 4.3. 3d XPS fitting result before loading on oxide support.

10 12 14 16 18 20 22 24

39

4.1.1 The Role of Reducing Agent Reducing Agent

The control experiments that using methanol and formaldehyde instead of formic acid were done to understand the influence of reducing agents. In Figure 4.6a and b, the TEM images show that the Rh nanoparticles obtained with methanol are very tiny, around 2 nm with ill-defined shapes. In Figure 4.6c-e, the Rh products reduced by formaldehyde are the mixtures of cubic and irregular nanoparticles in which the nanocubes are dominant in yield. The difference in the results of Rh nanoparticles implies a critical factor controlling the morphology, which is the reducing ability of reducing agents.

Table 4.1 collects the standard Gibbs free energy of formation of selected chemicals.

43-45 Based on the information, the free energy of the oxidation of methanol, formaldehyde and formic acid could be calculated.

According to eq. 1 to 4, the formation of CO2 from the oxidation of methanol should be the most favored pathway during synthesis (∆G = −738.39 kJ/mol) while that of CO is the least (∆G = −6.94 kJ/mol). It is similar in the case of formaldehyde as shown in eq. 5 to 7. The free energy of formaldehyde oxidation to become CO2 is also the most favored (∆G = −541.89 kJ/mol). However, the free energy of formic acid oxidation to become CO2 (∆G = −12.95 kJ/mol) is harder than that of becoming CO (∆G = −33.01 kJ/mol). It means that a significant amount of CO would be generated during synthesis.

As mentioned, CO is known a shaping agent which inhibits the growth of 111 faces of f.c.c. metal. When methanol and formaldehyde are used, least CO gas is released to the reaction system and therefore the particle morphologies are not 111 face dominant.

The truth is confirmed by Figure 4.6 in which the particles obtained with methanol and formaldehyde are irregular and cubic, respectively. The methanol has strong reducing

40

ability; thus the rhodium precursor is quickly reduced to form tiny particles. The formaldehyde has relatively weak reducing ability but still release least CO during synthesis. Without the interference of CO chemical adsorption, the existing Br^─ ions dominate in the particle shaping which leads to 100 faceted nanocrystals.^46-49

Table 4.1. Standard Formation Gibbs Free Energy (ΔG) of the selected chemicals.

Standard Gibbs Free Energy (ΔG, kJ/mol) of Formation

H₂ O₂ H₂O CO CO₂ HCOH HCOOH CH₃OH

0 0 -237.13 -137.17 -394.36 -89.6 -361.35 -130.23

CH

₃

OH + O

₂

 HCOH + H

₂

O

41

Figure 4.4. (a) low-magnification and (b) middle-magnification TEM image using methanol as reducing agent. (c) low-magnification (d) middle-magnification and (e) HR-TEM image of a cubic Rh nanoparticle prepared with formaldehyde to replace formic acid. The lattice parameter (1.9 Å and 1.87 Å ) is referred to rhodium 200 faces.

(d)

(a) (b)

(c)

50 nm 20 nm

50 nm 10 nm

1.9 Å

1.87 Å

5 nm

(e)

361.35 -137.17 -237.13

HCOOH  CO

₂

+ H

₂

ΔG = -12.95

(kJ/mol)

ΔG = -33.01

(kJ/mol)

-394.36

HCOOH  CO + H

₂

O

361.35

Eq.(4.8)

Eq.(4.9)

42

Formic Acid Concentration

In previous section, we realized formic acid is the major factor to obtain concaved tetrahedra due to the release of CO. Accordingly, the concentration of formic acid is worth discussion. In Figure 4.7, very small tetrahedra sized 12.43 nm were obtained when the concentration is very low (0.005 M). Nevertheless, the reduction took relatively long time for completing synthesis (2 days). When the concentration is raised to 0.1 M, the tetrahedral skeletons with incomplete 111 faces are obtained. It clearly tells that the growth takes place on the edges and vertices of a small tetrahedral seed as the priority followed by the growth of the 111 crystal faces with increasing concentration of formic acid. The results of selective growth can be observed again when the concentration goes up to 0.4 M, in which the hierarchical nanostructures are obtained.³⁹.

Figure 4.7. TEM images of Rh nanocrystals prepared with (a) 0.05 M, (b) 0.1 M, (c) 0.2 M, (d) 0.3 M, (e) 0.4 M concentrations of formic acid. The small picture beside (e) is the scheme for hierarchical structure.

43

4.1.2 The Influence of Surfactant CTAB Concentration

In the synthesis, both of the from CTAB and CO released from formic acid have shaping ability on the particle morphology. Since the influence of CO has been discussed, that of Br^─ ions requires further investigation as well. For this purpose, the added CTAB concentration was varied from 0.005 M, 0.01 M, 0.025 M, 0.05 M to 0.075 M and their corresponding TEM results are shown in Figure 4.8. According to the TEM images, two phenomena, the shrinking size and the structure evolution of Rh nanoparticles, were observed. It has been known the variation of particle size mainly depended on the amount of CTA⁺ cations. ⁵⁰ In contrast, the structure evolution was closely related to the concentration of Br^─ ions. Figure 4.9 shows the zoom-in TEM images of those in Figure 4.8. In the extremely low CTAB concentration, the Rh nanoparticles formed as the excavated Rh nanostructures with mixed decahedra and tetrahedra. The result possibly arose from either [CTA⁺] or [Br^─]. To verify which factor dominated, the CTAC (0.01 M) was used as the capping agent instead of CTAB.

As shown in Figure 4.10, similar excavated nanostructures like those in Figure 4.8a and 4.9a were obtained. It is apparently different to the condition of 0.01 M CTAB in which the typical concaved Rh tetrahedral were got. This denotes that Br^─ ions have stronger binding energy than Cl^─ ions and it in fact has been validated and discussed in previous literatures.³⁸

Br^─ ions were also regarded the species inducing the formation of twin structures.

51,52 The fact can be easily confirmed by Figure 4.8b-e and 4.9b-e. The Rh nanostructures evolved from concaved TDs to twin particles through the mixtures of concaved TDs, bipyramids and twin particles with increasing [CTAB]. It turns out that the influence of Br^─ ions gradually got significant in the competition with CO

44

molecules when the [CTAB] is over 0.01 M. Overall, the two parts of CTAB, CTA⁺ and Br^─, have different influences on the particle growth. The CTA⁺ ions mostly provide size control. However, the Br^─ ions competing with the CO from formic acid would induce twin structures formation.

Figure 4.8. TEM images of Rh nanoparticles prepared with (a) 0.005 M, (b) 0.01 M, (c) 0.025 M, (d) 0.05 M (e) 0.075 M CTAB.

(a) (b) (c)

(d) (e)

50 nm 50 nm 50 nm

50 nm 50 nm

45

Figure 4.9. Magnified TEM images of Rh nanoparticles prepared with (a) 0.005M, (b) 0.01M, (c) 0.025M, (d) 0.05M. (e) 0.075M CTAB.

Figure 4.10. (a) low-, (b) middle- and (c) high-magnification TEM images of Rh nanoparticles prepared with 0.01M CTAC instead of CTAB.

(a) (b) (c)

(d) (e)

20 nm 20 nm

20 nm 20 nm

20 nm

(a) (b)

(c)

50 nm

20 nm 200 nm

46

4.1.3 Temperature effect

Ultimately, temperature is the last factor requires discussion. Heating a reaction means applying energy to trigger nucleation accompanied by growth. Thus, different temperatures usually lead to different results attributed to varied reduction rates of precursors. Figure 4.12 shows the TEM results of Rh nanocrystals prepared at different temperatures. In the condition of 75˚C, the products were irregular agglomerates formed in a very dilute concentration (Figure 4.12a). It tells that higher energy should be input. When the temperature was risen up to 80˚C, it was surprising that the octahedral nanoparticles formed instead of tetrahedra (Figure 4.12b). In contrast to the tetrahedral shape, an octahedron of f.c.c. metal is a more thermodynamically stable morphology which is commonly obtained. It comes from a very slow reduction rate of precursors and therefore least nucleation and slow growth. However, the concaved tetrahedral structures were always obtained when the temperature went over 80˚C (Figure 4.12c-e). They formed in similar size whatever the temperature was if being over the critical temperature. It indicates that the tetrahedra should be the kinetic products resulting from both the CO shaping and rapid growth. In conclusion, the influence of temperature is mainly on the growth kinetics. 75˚C is the lowest temperature limit to trigger nucleation and 80˚C is the critical temperature to the structure evolution.

47

Figure 4.12. TEM images of Rh nanoparticles synthesized at (a) 75 °C, (b) 80 °C, (c)

90 °C, (d) 100 °C and (e) 110 °C.

4.2 Test of Catalytic Performance

As mentioned in Chapter 3, the working principle of TCD is to distinguish the difference of resistance between the flows of analytes and reference. Nevertheless, TCD cannot tell the exact chemical composition of an analytes, while the number of compounds. Accordingly, peak identification must be carried out in advance for the following test on the catalytic performance of CO2 hydrogenation.

4.2.1 Spectrum Identification and Stability Test Spectrum Identification

In general, there are multiple products generated after a catalytic reaction. To accurately identify each product, two ways are typically utilized. The first is to search

50 nm

(a) (b) (c)

(d) (e)

50 nm 50 nm

50 nm 50 nm

48

for references from the database. However, if references are not fitting to the actual experiment conditions, building up reliable references using corresponding pure compounds become necessary. In addition, calibration lines of concentration vs signal abundance should be always carried out to quantify specific products.

Regarding to CO2 hydrogenation, the products can be miscellaneous. They typically include unreacted CO2, H2, methane, methanol, formic acid, and some further products generated based on the mentioned low-carbon compounds. In our experiment, the major product obtained with Rh nanocatalysts was methane. Helium (He) was used as the carrier gas for the whole catalytic system. To get correct signals with the TCD, He was also used as the reference gas, the same to the carrier gas, so that the He gas would not contribute any signal (black line in Figure 4.13). .

Figure 4.13 is the plot of signal abundance vs retention time of the reactants and products in the CO2 hydrogenation. The signal of H2 (red) is a set of positive and negative peaks, locating at the retention time of 3.258 and 3.345 min, because of the difference in conductivity to the carrier gas. ⁵³ The other references such as CO2 (blue), CO (pink) and CH4 (green) show up at the retention time of 5.33, 3.383 and 3.5 min, respectively. The dark blue curve shows the result after the CO2 hydrogenation. The inset graph shows more localized range of the intensity vs retention time plot from 3.1 to 3.7 nm. The curve can be deconvoluted as 1 to 4 area and well-fitted with H2 (1), CO (2), CH4 (3) and CO2 (4). Notably, the broad peak located in-between the range of 3.30 to 3.44 min is due to the overlap of the H2 and CO signals (Figure 4.14).

49

Figure 4.13. Intensity vs retention time plot for peak identification of CO2

hydrogenation. Different species are labeled by numbers.

Figure 4.14. (a) Peak deconvolution of the H2 and CO signals. (b) Localized peak fitting results in the range of 3.15 to 3.65 min.

3.5 4.0 4.5 5.0 5.5 6.0

3.30 3.32 3.34 3.36 3.38 3.40 3.42 3.44

Intensity (a.u.)

3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 Rentention time (min)

50

Stability Test

To confirm the stability of the CO2 gas flow, the stability test of the CO2, CO and CH4 gas inlet was carried out. Figure 4.15 shows the resulting plots in which the y-axis is the integration of signal areas and x-axis is the mass of gas flow. As the calibration lines, the R-square values of all are better than 0.99, indicating the quite stable gas flow.

Figure 4.15. Calibration lines for signal area vs gas flow mass of (a) CO2 (b) CO and (c) CH4. The R-square values of all reach 0.99.

51

4.2.2 Shape-dependent Catalytic Performance

In the synthetic section, we’ve investigated the tunability of the Rh nanoparticle morphology. It allows us to thoroughly study the shape-dependent catalytic performance of CO2 hydrogenation. We were especially interested in the selectivity of product formation based on the different structural platforms of Rh nanocatalysts. The phenomenon of shape-dependent selectivity in a catalytic reaction has been known in various cases. ^54-57 In our experiment, Rh concaved tetrahedra, excavated and twinned nanocrystals were used as catalysts for CO2 hydrogenation. In Figure 4.16 and Table 4.2, CO2 conversion is a function of temperature which shows the activities of the three kinds of Rh nanocatalysts. When the temperature was lower than 400°C, the conversions of all nanocatalysts were about 4%.

Once the temperature reaches 400°C, both the concaved Rh tetrahedra and excavated Rh nanocrystals had obvious rising in the conversion to 5.65 and 6.63%; however, only the twinned Rh nanocrystals did not. After the temperature was over 450°C, the activities of concaved tetrahedra and excavated nanocrystals went up over 10% but that of twinned nanocrystals merely increased to 5.94%.

Table 4.2. CO2 conversion value for each temperature.

CO

₂

Conversion (%)

52

Figure 4.16. CO2 conversion as a function of temperature (°C) of twinned nanoparticles (black bar), concaved tetrahedra (red bar) and excavated nanoparticles (blue bar).

Table 4.3 and Figure 4.17 collects the analyzed plots of CO2 conversion, CO and CH4 formation which show the selectivity of the products. For twinned nanocrystals, the CO2 hydrogenation did not take place when the temperature was lower than 400°C.

At 400°C, the products included very high ratio of CO and extremely low ratio of CH4. With the increasing temperature from 450 to 500°C, the ratio of CH4 gradually increased to 4.92 and 15.65 %. For excavated nanocrystals, it was not until 400°C the ratio of CH4 got improved over 5%. However, the ratio of CH4 dropped at 500°C, implying the possible poison or structure crack of the catalysts. For concaved tetrahedra, a clear selectivity in CH4 formation could be observed. When the reaction temperature

300

53

was going up from 400, 450 to 500°C, the ratio of the CH4 was increased from 13.15, 26.56 to 63.78%.

It has been known that rhodium 111 crystal faces typically have higher catalytic activity than other low-index (e.g. 100 or 110) crystal faces in hydrogenation reactions.^15,39,58 The conclusion corresponds to our results of CO2 hydrogenation in which the excavated nanocrystals and concaved tetrahedra always have better CO2

conversion than the twinned nanocrystals when the temperature is higher than 35%.

More importantly, if checking the selectivity of products, the concaved tetrahedra apparently generate the highest ratio of CH4 when the temperature is above 400°C. The high selectivity in CH4 is possibly due to the high-index concaved facets. According to Huang’s previous work, the high-index facets over the caves of the concaved tetrahedral were mostly 110 crystal faces. ⁵⁹ In contrast, both the twinned and excavated nanocrystals generate relatively high ratio of CO. It turns out that 111 and 100 crystal faces induce the preferential formation of CO.

Table 4.3. CO2 conversion, CH4 and CO selectivity value for each temperature.

Twin NPs Concave Td Excavate NPs

400 100 0 2.59 86.85 13.15 5.65 93.36 6.63 6.63

450 95.07 4.929 5.94 73.44 26.56 11.98 81.54 18.45 18.45

500 ^{84.34 15.65 5.61 36.21 63.78 12.57 95.06 4.93 4.93}

54

Figure 4.17. CO2 Conversion (blue bar), CH4 and CO formation (red and blue bar) as a function of temperature ( ° C) with the catalysts of (a) concaved tetrahedra, (b) excavated nanocrystals, and (c) twinned nanocrystals.

(a)

55

CONCLUSION

In summary, rhodium nanoparticles with tunable morphologies can be achieved with an one-pot aqueous synthesis. In the typical conditions, CTAB and formic acid were used as the capping agent and reducing agent, respectively. The formic acid could be partially decomposed with heat to release the CO, which was the shaping agent mainly slowing the growth of 111 crystal faces. By means of the CO, 111-faced nanocrystals were obtained as the major products, such as the concaved tetrahedra.

Tuning the amount of formic acid, the formation mechanism of concaved tetrahedra was known starting with the growth of tetrahedral seeds. More importantly, the particle morphology was tunable with the amount of CTAB. Low amount of CTAB facilitated the formation of excavated nanocrystals but high amount generated the twinned nanocrystals. It results from the competitive growth between 111 and 100 crystal faces which were induced by CO and bromide ions individually. To study the facet-dependent effect in catalysis, the concaved tetrahedra, excavated and twinned nanocrystals were loaded on the ZrO2 as the heterogeneous catalysts for CO2

hydrogenation. It turned out that the concaved tetrahedra led to the best selectivity in the CH4 formation, while the other two generated high ratio of CO in the products. The results confirmed that the existence of high-index, possibly 110 facets over the concaved tetrahedra brought the favored pathway of CH4 formation during the catalytic hydrogenation. This finding is believed a key to open the door leading to highly

hydrogenation. It turned out that the concaved tetrahedra led to the best selectivity in the CH4 formation, while the other two generated high ratio of CO in the products. The results confirmed that the existence of high-index, possibly 110 facets over the concaved tetrahedra brought the favored pathway of CH4 formation during the catalytic hydrogenation. This finding is believed a key to open the door leading to highly

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