Development of Novel Pyrolysis Technology Involving Chromium for the Measurement of D/H Ratios in n-Alkanes

Abstract

A new pyrolysis technology involving chromium is proposed for the determination of δD in alkanes based on the systematic analysis of reaction temperature, conversion rate, and reaction mechanism. Compared with the traditional high-temperature conversion (HTC) method, our findings suggest that chromium/high-temperature conversion (Cr/HTC) can improve the conversion rate of hydrocarbons and reduce the required pyrolysis temperature by up to 175 °C; meanwhile, the pyrolysis conversion rate of hydrocarbons increased by an average of 2.42% across the entire analyzed temperature range using the Cr/HTC method. Changes in the chromium wire itself were analyzed using X-ray photoelectron spectroscopy (XPS); this facilitated an understanding of the interaction mechanism between chromium and hydrocarbons and possible pathways of the catalytic pyrolysis process. The results show that chromium reacts with hydrocarbons, capturing carbon as chromium carbide (Cr₂C₃ and CrC₃) and releasing hydrogen in the form of H₂. As the reaction progresses, the resulting free carbon accumulates on the surface of the chromium wire or chromium carbide, resulting in a marked reduction in the Cr/C ratio; these findings provide reliable evidence for the further application of Cr/HTC technology.

1. Introduction

The first isotope ratio monitoring technique (gas chromatography–isotope ratio mass spectrometry; GC–IRMS) was introduced by Matthews and Hayes in 1978 [1], and commercial systems for ¹³C and ¹⁵N analysis of GC eluate became available in 1988 and 1994 [2], respectively. Subsequently, stable isotope techniques for other light elements, such as sulfur and hydrogen, have been developed. The substantial variations in D/H known to exist in nature have made hydrogen isotopes a desirable target for GC–IRMS analysis [3]. Research on the hydrogen isotope ratios of various natural samples is an essential tool for understanding the migration processes and evolution of hydrogen isotopes in many disciplines, including geochemistry [4,5,6,7,8,9], ecology [10], paleohydrology [11], climatology [12,13,14], and food science [15]; however, owing to practical difficulties, it was not until the end of the 20th century that a relatively accurate offline or online method for hydrogen isotope analysis was established.

Initially, hydrogen isotopes analysis of organic compounds required two offline conversion steps, combustion and reduction of water to analyte gas (H₂) using reducing metals such as zinc, uranium, magnesium, or tungsten. Subsequently, δD values were determined manually in dual-inlet mode using IRMS; these methods involve the direct pyrolytic conversion of organically bound hydrogen to H₂ via high-temperature conversion (HTC) at temperatures >1050 °C in an elemental analyzer (EA), or via combination with GC separation of mixtures and subsequent HTC of the target compounds at 1400–1450 °C [16]; however, these methods still have some limitations. The HTC-derived H₂ yields are incomplete due to the formation of hydrogen-containing byproducts. And the pyrolysis temperature is close to the design temperature, which affects the service life of the reaction apparatus.

In this study, a chromium catalytic pyrolysis approach is proposed for the determination of δD in alkanes based on the systematic analysis of reaction temperature and conversion rate. In addition, the catalyst characterization based on XPS analysis was carried out to ascertain the reaction mechanism. Combined with experimental results, we tried to provide reliable evidence for the further application of Cr/HTC technology.

2. Results and Discussion

2.1. Conversion Rate and δD Values of n-Alkanes with Changing Pyrolysis Temperature

The target compound was separated by the column and was first converted to H₂ by catalytic pyrolysis in the ceramic tube at the set temperature. The H₂ then flowed into the IRMS along with the carrier gas; however, the resulting effluent is a mixture of H₂ and the target compound when the conversion is incomplete.

Considering the mass range (1–80 amu) of the DELTA V Advantage instrument and the complexity and relative abundance of the ion fragments, n-hexane was employed to measure the conversion rate. Under the electron energy of 70 eV, n-hexane was primarily ionized and formed ion fragments with m/z of 15, 29, 43, 57, and 71 (Figure 1a) after entering the ion source. As shown in Figure 1b, the most abundant fragment ion after ionization of n-hexane was m/z 57, while a fragment ion showing moderate abundance was m/z 29. Therefore, the pyrolysis conversion rate of n-hexane was calculated according to the signal intensity of m/z 29. Based on the signal intensity of 0.5 µL n-hexane at 50 °C and a split ratio of 50:1, the percentage of the signal intensity of the effluent at each temperature setting could be calculated.

The conversion efficiency of n-hexane at different pyrolysis temperatures, using both the Cr/HTC and HTC methods, was investigated at temperatures ranging from 1200 to 1450 °C, at 25 °C intervals. As shown in Figure 2, the conversion efficiency increased gradually with an increase in pyrolysis temperature in both the HTC and Cr/HTC experiments. Using the HTC method, the conversion efficiency increased from 90.49% at 1100 °C to 99.69% at 1475 °C; using the Cr/HTC method, it increased from 97.59% at 1100 °C to 100% at 1475 °C. Notably, the conversion efficiency of n-hexane using the Cr/HTC method was higher than that using the HTC method at each temperature setting. Comparing the two methods at equivalent temperatures, the difference in conversion efficiency ranged from 0.31 to 7.11%, with an average of 2.42%. Furthermore, n-hexane had a maximum conversion efficiency of only 99.69% up to 1475 °C using the HTC method, while quantitative conversion (100%) of n-hexane was achieved at this temperature using the Cr/HTC method. The above results suggest that chromium wire can provide an excellent catalytic effect in improving the efficiency of the conversion of alkanes to H₂. Therefore, the novel Cr/HTC method proposed in this study may be superior to the traditional HTC method for the compound-specific hydrogen isotope analysis of n-alkane compounds.

Combined with the conversion efficiency study of n-hexane, the hydrogen isotope measurement of reference materials (RMs) using the two different pyrolysis methods was also investigated over a range of temperatures. The RMs, including USGS-67, USGS-68, and USGS-69, represent a wide range of hydrogen isotope values in natural abundance samples. In this investigation, absolute error (AE) was used to evaluate the accuracy and reliability of the hydrogen isotope analysis of RMs using the two methods.

The AEs values represent the deviation between the measured value and the reference value at the same pyrolysis temperature. As shown in

Table 1. Measured δD values and absolute errors of RMs using the HTC and Cr/HTC methods.

Series	Temperature (°C)	USGS-67		USGS-68		USGS-69
		δDmean (n = 3)	AE 1	δDmean (n = 3)	AE	δDmean (n = 3)	AE
HTC	1300	−145.3	20.9	6.7	16.9	306.4	−75.0
	1325	−149.1	17.1	3.3	13.5	346.7	−34.7
	1350	−150.5	15.7	0.2	10.4	355.9	−25.5
	1375	−161.6	4.6	−1.9	8.3	377.6	−3.8
	1400	−163.3	2.9	−6.9	3.3	379.6	−1.8
	1425	−169.4	−3.2	−10.3	−0.1	384.4	3.0
	1450	−167.9	−1.7	−10.0	0.2	378.8	−2.6
	1475	−163.4	2.8	−9.6	0.6	380.7	−0.7
Cr/HTC	1200	−154.7	11.5	31.5	−21.3	402.6	21.2
	1225	−163.7	2.5	−8.6	1.6	379.6	−1.8
	1250	−163.2	3.0	−11.2	−1.0	380.7	−0.7
	1275	−163.1	3.1	−11.5	−1.3	381.1	−0.3
	1300	−164.4	1.8	−9.8	0.4	379.2	−2.2
	1325	−162.5	3.7	−10.4	−0.2	379.8	−1.6
	1350	−163.7	2.5	−10.2	0.0	377.2	−4.2
	1375	−163.0	3.2	−9.6	0.6	376.5	−4.9
	1400	−162.6	3.6	−10.3	−0.1	383.1	1.7
	1425	−162.7	3.5	−9.9	0.3	378.2	−3.2
	1450	−163.2	3.0	−9.4	0.8	377.5	−3.9
	1475	−168.2	−2.0	−11.5	−1.3	379.5	−1.9

¹ AE = δD_{measured value} − δD_{reference value}. AE stands for absolute error.

, when the pyrolysis temperature was higher than 1400 °C, the hydrogen isotope composition of RMs measured using the traditional HTC method had high analytical precision and reliability. The corresponding AEs of USGS-67, USGS-68, and USGS-69 were −3.2 to 2.9‰, −0.1 to 3.3‰, and −2.6 to 3.0‰, respectively. When the pyrolysis temperature was lower than 1400 °C, as the temperature decreased, the measured hydrogen isotope ratios of USGS-67 and USGS-68 (that have negative values) gradually became heavier, while that of USGS-69 (that has a positive value) gradually became lighter; it indicates that only when the pyrolysis temperature is higher than 1400 °C can accurate hydrogen isotopic composition data be obtained in the traditional method. Using the Cr/HTC method, when the pyrolysis temperature was higher than 1225 °C, the AEs of USGS-67, USGS-68, and USGS-69 were −2.0 to 3.7‰, −1.3 to 0.8‰, and −4.9 to 1.7‰, respectively. Compared with the traditional HTC method, the experimental results show that the Cr/HTC method can not only reduce the pyrolysis temperature from 1400 to 1225 °C but also retains comparable precision and reliability. Thus, the development of chromium catalytic pyrolysis technology can be considered a promising new method for the δD analysis of n-alkanes.

2.2. Mechanism of Interaction between Chromium and Hydrocarbons

To further understand the mechanism of interaction between chromium and hydrocarbons, XPS testing was carried out on the chromium wire after the reaction. In this paper, the catalyst in the traditional method was a carbon membrane, while the novel approach employed chromium wire. Comparing the two materials, the carbon membrane only provides a reducing environment for hydrocarbon pyrolysis, while the chromium wire may be directly involved in the reaction. Hence, two sites were selected for XPS analysis according to the manufacturer’s guidelines, which indicated that the highest temperature region (T_max region) of the reactor is approximately 105 mm from the gas chromatograph.

When the HTC furnace reaches the set temperature, the temperature of the ceramic tube at different positions is variable. Specifically, the temperature increase moving from the tube ends to the center. Therefore, two different sites (Figure 3) were selected for our investigation of the catalytic mechanism; these represented the specimen obtained by the reaction when reaching the set temperature (site 1), and that obtained below the set temperature (site 2).

Figure 4 shows the XPS surveys and spectra of the chromium wire specimens obtained at different temperatures (Cr 2p binding energy was corrected to C 1s at 285.0 eV). As shown in Figure 4a,c, the surface of the specimens mainly comprised Cr, C, and O. The XPS spectrum of the Cr 2p energy region for site 1 (Figure 4b) contained four peaks, namely Cr 2p_3/2 (576.5 eV), Cr 2p_3/2 (578.7 eV), Cr 2p_1/2 (586.0 eV), and Cr 2p_1/2 (587.8 eV); these peaks are considered to result from Cr₂C₃ and CrC₃, which mainly originate from the reaction of chromium with hydrocarbons; meanwhile, the relative ratios of Cr/C and CrC₃/Cr₂C₃ in the site 1 sample were 0.2/99.8 and 13.62/35.73, respectively. In contrast, for sample site 2, obtained below the set temperature, the XPS spectrum of the Cr 2p energy region (Figure 4d) contained only two peaks, Cr 2p_3/2 (576.8 eV) and Cr 2p_1/2 (586.6 eV); these peaks are considered to result from Cr₂C₃; the relative ratio of Cr/C in the site 2 sample was 2.38/97.62.

The XPS results show that the catalytic pyrolysis of chromium is, in essence, a reaction between chromium and hydrocarbons, which then forms chromium carbide and releases hydrogen in the form of H₂ [17,18,19]. The results also showed that the structure of the reaction products was different at different temperatures. In the highest temperature zone of the tube, the reaction products were Cr₂C₃ and CrC₃, with Cr₂C₃ being dominant, while in the low-temperature region, only Cr₂C₃ formed. Therefore, the reaction processes of the traditional pyrolysis and chromium catalytic pyrolysis methods can be expressed by the following equations:

$${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}\to \frac{\mathrm{n}}{2}\mathrm{C}+\left(\mathrm{n}+1\right){\mathrm{H}}_2$$

(1)

$${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}+\frac{2\mathrm{n}}{3}\mathrm{Cr}\to \frac{\mathrm{n}}{3}{\mathrm{Cr}}_2{\mathrm{C}}_3+\left(\mathrm{n}+1\right){\mathrm{H}}_2$$

(2)

$${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}+\frac{\mathrm{n}}{3}\mathrm{Cr}\to \frac{\mathrm{n}}{3}{\mathrm{CrC}}_3+\left(\mathrm{n}+1\right){\mathrm{H}}_2$$

(3)

where Equation (1) represents the reaction process of traditional high-temperature pyrolysis, and Equations (2) and (3) describe the novel chromium catalytic pyrolysis reaction at high and low temperatures, respectively.

Based on the above results, we here propose a possible mechanism of novel chromium pyrolysis technology. In the initial phase of the reaction, chromium reacts with hydrocarbons to capture carbon and form chromium carbide, containing Cr₂C₃ and CrC₃ (Figure 4b,d), while releasing hydrogen in the form of H₂; this occurs in the highest temperature region of the reactor tube, and the reaction proceeds via Equations (2) and (3). In the lower temperature region of the reactor tube, the reaction proceeds via Equation (2) and produces only Cr₂C₃. As the reaction progresses, the resulting free carbon accumulates on the surface of the chromium wire or chromium carbide, resulting in a marked reduction in the Cr/C ratio, as shown in Figure 4a.

3. Materials and Methods

3.1. Materials

The hydrogen isotope reference materials, USGS-67 (δD = −166.2 ± 1.0‰), USGS-68 (δD = −10.2 ± 0.9‰), and USGS-69 (δD = +381.4 ± 3.5‰) were acquired from Reston Stable Isotope Laboratory United States Geological Survey (Reston, VA, USA). Helium gas (purity ≥ 99.99%) was used as the carrier gas, and cylinder H₂ (purity ≥ 99.995%) was used for δD calculation and IRMS system stability checks. Other materials, including chromium wire (purity ≥ 99.99%) and n-hexane (purity ≥ 99.0%) were purchased from Sigma–Aldrich (St. Louis, MO, USA).

3.2. Experimental Analysis of δD

The online determination of D/H isotope ratios was performed using a Trace 1310 gas chromatograph (Thermo Scientific, Milan, Italy) coupled with a Delta V Advantage (Thermo Scientific, Bremen, Germany) via a GC-III interface. The gas chromatograph was equipped with a fused silica capillary column (SE-54, 50 m × 0.53 mm × 2 μm, produced by the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China) with He gas as the carrier gas at a constant flow rate of 2 mL/min. The GC injector was maintained at 280 °C with a split ratio of 6:1. The initial GC oven temperature was 80 °C for 3 min, followed by an increase of 2 °C/min up to 300 °C, where this temperature was maintained for a further 20 min. In each run, the mixture of hydrocarbons separated by GC entered the high-temperature furnace, where the hydrocarbons were thermally and catalytically converted into H₂ in sequence. The purified H₂ was then transferred into the IRMS for δD analysis [3]. The H₃⁺ factor, an essential parameter for high-precision δD measurement, was determined daily using reference H₂ pulses with different signal intensities. The H₃⁺ factor was 7.3 ppm/nA with a variation of <0.2 ppm/nA.

The δD values are reported in per mil (‰) relative to the V-SMOW standard and expressed as follows:

$${\mathsf{\delta }\mathrm{D}=(\mathrm{R}}_{\mathrm{sample}}-{\mathrm{R}}_{\mathrm{reference}}{)/\mathrm{R}}_{\mathrm{reference}}\times 1000$$

(4)

where δD is the hydrogen isotope composition, and R_sample and R_reference are the isotope ratios of the sample and standard, respectively.

Pyrolysis and catalytic pyrolysis were performed in a ceramic tube (320 mm in length, 1.5 mm outer diameter, 0.5 mm inner diameter, Thermo Scientific, Milan, Italy). For comparative study, two different pre-processing methods were used for the pyrolysis of hydrocarbons before the experiment: (a) Conventional high-temperature conversion (HTC), whereby the ceramic tube was conditioned with a carbon coating following the manufacturer’s instructions (also refer to Cao et al. [20]); (b) Novel chromium wire/high-temperature conversion (Cr/HTC), whereby the reactor tube can be directly put into use after filling with twisted chromium wire [21]).

3.3. Experimental Analysis of the Catalytic Mechanism

The catalytic mechanism was studied via X-ray photoelectron spectroscopy (XPS) analysis of the distribution and relative content of elements in the chromium wires after the catalytic reaction. XPS analyses were carried out using an ESCALAB Xi+ spectrometer (Thermo Scientific, Brno, Czech Republic). An incident monochromated X-ray beam (Al Kα, 15 mA, 14 kV) was focused on a 700 × 300 μm area of the surface of a sample, orientated at 45° to the sample surface [22,23,24]. The electron energy analyzer (Thermo Scicentific, Brno, Czech Republic) was operated with a pass energy of 20 eV, enabling high-resolution spectra to be obtained. The energy precision of the spectrometer in our working conditions was 0.2 eV. The analyzer was located perpendicular to the sample surface. A step size of 0.02 eV was employed and each peak was scanned twice.

4. Conclusions

A novel pyrolysis approach involving chromium for the determination of δD in n-alkanes is proposed based on the systematic analysis of reaction temperature, conversion rate, and mechanism of interaction; this novel Cr/HTC method can substantially reduce the pyrolysis temperature of hydrogen isotope analysis, by up to 175 °C. Our results show that, using the standard HTC method, only at temperatures higher than 1400 °C are the hydrogen isotope test values of standard materials close to the reference values; here, the corresponding conversion rate of n-hexane is 98%. In contrast, the Cr/HTC method can achieve more accurate results at pyrolysis temperatures of 1225 °C and above, and the corresponding conversion rate of n-hexane is >99.5%.

In the Cr/HTC method, chromium wire was incorporated into the reactor system, which scavenged C from the target hydrocarbon, forming chromium carbide and releasing hydrogen in the form of H₂. The chromium catalytic pyrolysis process involves the reaction of chromium with the carbon in hydrocarbon compounds, forming a chromium carbide product dominated by Cr₂C₃.