Low‐Temperature Initiation Enhances HCN and NH₃ Formation From Methane and NO Over Pt/Al₂O₃

ABSTRACT

The low‐temperature activation of methane remains challenging due to its high C─H bond dissociation energy, yet it offers an attractive route for hydrogen cyanide (HCN) synthesis under milder conditions. We investigated HCN and NH₃ formation from CH₄ and NO over a commercially available Pt/Al₂O₃ catalyst, focusing on the influence of NO concentration, reaction temperature, and reaction‐start conditions. Increasing the NO concentration up to 4.5% significantly enhanced HCN production, achieving a maximum carbon‐based yield of 3.5%. Elevating the temperature further improved activity, with the highest HCN yield of 4.8% obtained at 475°C. Comparison between ramping and constant‐temperature conditions demonstrated that the initial reaction temperature strongly influenced product distribution, with ramping from 300°C enabling substantially higher CH₄ conversion and HCN formation. In situ Fourier Transform Infrared (FTIR) spectroscopy identified Pt‐CO, Pt‐CN, Pt‐NCO, and Al‐NCO species, among which Pt‐CO showed the strongest correlation with HCN production, whereas strongly adsorbed Pt‐CN behaved predominantly as an inhibiting spectator species. Temperature‐swing operation effectively promoted desorption of such inhibiting species, thereby enhancing HCN and NH₃ yield, a trend supported by in situ dispersive x‐ray absorption fine structure (DXAFS) analysis. These findings highlight the critical role of dynamically controlling surface intermediates in improving low‐temperature HCN synthesis.

Hydrogen cyanide (HCN) was formed from CH₄ and NO over Pt/Al₂O₃ at low temperatures. Increasing NO concentration and optimizing reaction‐start conditions significantly enhanced HCN yield. In situ FTIR and DXAFS measurements revealed that dynamically balancing Pt‑CO and Pt‑CN surface species, particularly their desorption under temperature‑swing operation, governed catalyst activity.

1. Introduction

The activation of the C─H bond in methane (CH₄) is intrinsically challenging due to its high bond dissociation energy [1]. As a result, reactions involving CH₄ often require elevated temperatures, as exemplified by the oxidative coupling of methane (OCM), where sophisticated heterogeneous catalyst design is essential to achieve adequate selectivity [2, 3]. Hydrogen cyanide (HCN) is an industrially important product synthesized from CH₄. HCN serves as a key precursor for adiponitrile, an essential intermediate in the manufacture of hexamethylenediamine, and consequently for nylon‐66 (PA66). Owing to its exceptional thermal and mechanical properties, PA66 is widely used in automotive components, engineering plastics, and textile fibers. In addition, HCN is utilized as a raw material in pharmaceutical manufacturing and in the extraction of precious metals, further contributing to its industrial demand.

Currently, HCN is produced by high‐temperature processes such as the Andrussov method, which employs CH₄, ammonia (NH₃), and oxygen (O₂), and the endothermic BMA (Blausäure aus Methan und Ammoniak) process, which uses CH₄ and NH₃. Both routes operate at temperatures exceeding 1000°C. HCN is also obtained as a byproduct of the Sohio process (ammoxidation of alkenes). These routes, however, require substantial energy input and impose strict operational constraints due to the extreme reaction temperatures. In recent years, increasing attention has been directed toward the low‐temperature synthesis of HCN. To overcome the intrinsic inertness of CH₄ under mild conditions, non‐equilibrium plasma systems as well as supported platinum or copper catalysts have been explored for the activation of CH₄ and NH₃, enabling HCN formation at significantly reduced temperatures [4, 5]. Moreover, direct HCN synthesis using nitrogen (N₂) as the nitrogen source instead of NH₃ has also been reported [6]. Related studies have also reported the formation of acetonitrile from methane and ammonia [7, 8]. On gallium nitride, surface CN species are generated and subsequently react with methane to produce acetonitrile. In addition, acetonitrile formation from methane and nitrogen has been observed on molybdenum carbide [9]. Although the required reaction temperature is relatively high (> 650°C), the study demonstrated that both methane and nitrogen can be activated under such conditions, and that C─N bond formation on the catalyst surface constitutes the rate‐limiting step.

We have discovered that using nitric oxide (NO) as an oxidant and commercially available alumina‐supported platinum as a catalyst enables C‐H bond cleavage and HCN generation even at low temperatures of 300°C [10]. At 400°C, HCN was stably produced with a carbon‐based yield of 1%. Furthermore, it was clarified that NH₃ was selectively obtained with a nitrogen‐based yield. In this reaction, CH₄ serves as the hydrogen source for NH₃ formation, and the overall process can be regarded as the reduction of NO to NH₃ with CH₄ acting as the reducing agent. We have previously demonstrated that the selectivity to HCN and NH₃ differs depending on the particle size of the platinum in the alumina‐supported platinum catalyst [11]. Larger platinum particles selectively yield HCN, while smaller particles enhance NH₃ selectively. Furthermore, the reactivity changes significantly depending on the type of support [12]. Using α‐alumina, which has fewer hydroxyl groups and a lower surface area, as the support results in higher HCN yield. Furthermore, simultaneous in situ x‐ray Absorption Fine Structure (XAFS) and Mass Spectrometry (MS) measurements, along with in situ FTIR measurements, revealed that Pt‐CN primarily existed as an adsorbate on the alumina‐supported platinum catalyst [13]. This adsorbate was well observed within the temperature range where HCN was formed and was considered an intermediate. However, since it was strongly adsorbed and was also observed on degraded catalysts at high temperatures, the Pt‐CN adsorbate is suggested to also act as an inhibitor.

In our previous studies, the influence of reaction temperature on catalytic activity was examined by cooling the catalyst bed to 300°C after hydrogen reduction, introducing CH₄ and NO to initiate the reaction, and subsequently increasing the temperature by 25°C increments. In the present study, we expand upon this approach by systematically varying the oxidant concentration beyond the conditions used previously and evaluating the resulting changes in reaction activity. This modification enables substantial improvements in both the HCN yield and its production rate. Furthermore, we compare catalytic performance between the previous procedure, where the reaction is initiated at 300°C, and a modified approach in which the reaction is started directly at a destination temperature without initial cooling. This comparison reveals that the reaction‐start temperature causes a significant influence on overall reactivity. To gain mechanistic insight into these differences, in situ FTIR measurements were conducted to elucidate the behavior of intermediates adsorbed on the catalyst surface.

2. Experimental

2.1. Materials

The platinum catalyst supported on (γ+θ)‐alumina (5 wt% Pt/Al₂O₃) was obtained from FUJIFILM Wako Pure Chemical and employed without further modification. Methane (99.999% purity), nitric oxide (11.0% in helium), and helium (99.9% purity) were supplied by Taiyo Nippon Sanso Co., Ltd. These were used for the reactivity study. Methane (39.9% in helium), nitric oxide (12.5% in helium), helium (99.99995% purity), and hydrogen (99.99999% purity) were supplied by Taiyo Nippon Sanso Co., Ltd. These were used for the FTIR study.

2.2. Characterization

The physicochemical properties of the catalysts were characterized, following the procedure described in our previous work [10, 13]. The crystal structure was determined by x‐ray diffraction (XRD, Ultima IV, Rigaku) with Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 20 mA. Diffraction patterns were collected at a scan rate of 5° min⁻¹ with a step width of 0.05° over a 2θ range of 10° to 80°. Transmission electron microscopy (TEM) images of the Pt/Al₂O₃ catalyst after the reaction were acquired using a JEM‐2100F (JEOL) electron microscope operated at 200 kV. The average size and size distribution of the Pt nanoparticles were determined by analyzing 134 particles using the DigitalMicrograph software package (Gatan). The quantity of surface platinum atoms was determined via CO titration at 50°C using a BELCAT II instrument (MicrotracBEL). Prior to titration, 50 mg of Pt/Al₂O₃ was reduced under a hydrogen flow at 400°C for 30 min, followed by cooling to 50°C under helium. Subsequently, five pulses of 10% CO in He were introduced over the pretreated catalyst at 50°C. The amount of adsorbed CO was calculated from the decrease in peak areas recorded by the thermal conductivity detector (TCD).

2.3. Reactivity Test

The reaction was carried out under ambient pressure in a fixed‐bed flow reactor. A 100 mg sample of Pt/Al₂O₃ catalyst was pelletized, crushed, and sieved to a particle size of 355–600 µm. Prior to each run, the catalyst was pretreated at 400°C for 1 h under a 50% H₂/He flow (100 mL min⁻¹). After pretreatment, the gas was switched to He, and the reactor was adjusted to the target temperature. Following a 30 min stabilization period, the reaction mixture containing 10.0% CH₄, 4.5% NO, and 85.5% He was introduced at a total flow rate of 100 mL min⁻¹. The reaction temperature varied from 300 to 500°C by 25°C increments.

Gaseous products were analyzed using two online gas chromatographs (GCs) and an FTIR spectrometer. Detected species included HCN, CO, CO₂, N₂O, N₂, NH₃, and H₂O, along with the reactants, CH₄ and NO. The first GC (Nexis GC‐2030, Shimadzu) equipped with a barrier discharge ionization detector (BID) and a capillary column (Restek, Rt‐U‐BOND, 30 m × 0.53 mm i.d. × 20 µm film) was used for HCN, CO₂, and H₂O analysis. The second GC (GC‐2014, Shimadzu) with a thermal conductivity detector (TCD) and a molecular sieve column (MS‐13X, 2 m × 3 mm i.d.) was employed for CH₄ and N₂. An online FTIR spectrometer (VIR‐200, JASCO) equipped with a triglycine sulfate (TGS) detector and a 10 cm gas cell was used to analyze NO, NO₂, N₂O, NH₃, and CO. FTIR spectra were recorded at 1 cm⁻¹ resolution over the range 7800–800 cm⁻¹. Quantification was based on calibration curves prepared using reference gases of known composition. Conversion, yield, and selectivity were calculated using the equations provided below, where n_i and n_{i ,0} represent the moles of species i at the reactor outlet and in the feed, respectively, and v_i denotes the number of carbon or nitrogen atoms in species i.

$$\mathrm{C}{\mathrm{H}}_4\mathrm{conversion}(\%)=\left(\frac{n_{\mathrm{C}{\mathrm{H}}_4,0}-n_{\mathrm{C}{\mathrm{H}}_4}}{n_{\mathrm{C}{\mathrm{H}}_4,0}}\right)\times 100$$

$$\mathrm{NO}\mathrm{conversion}(\%)=\left(\frac{n_{\mathrm{NO},0}-n_{\mathrm{NO}}}{n_{\mathrm{NO},0}}\right)\times 100$$

$$\mathrm{Yield}(\mathrm{carbon}\mathrm{based})(\%)=\frac{v_{\mathrm{i}}{n}_{\mathrm{i}}}{n_{\mathrm{C}{\mathrm{H}}_4,0}}\times 100$$

$$\mathrm{Yield}(\mathrm{nitrogen}\mathrm{based})(\%)=\frac{v_{\mathrm{i}}{n}_{\mathrm{i}}}{n_{\mathrm{NO},0}}\times 100$$

$$\mathrm{Selectivity}(\%)=\frac{v_{\mathrm{i}}{n}_{\mathrm{i}}}{\sum v_{\mathrm{i}}{n}_{\mathrm{i}}}\times 100$$

Note that when the CH₄ conversion was ≤5%, the decrease in CH₄ was minimal, and the uncertainty in gas chromatography analysis was significant. Therefore, under these conditions, CH₄ conversion was expressed based on the total carbon‐based product yields.

Turnover frequency (TOF) was calculated using the following equation based on surface platinum sites determined by CO chemisorption.

$$\mathrm{TOF}[{\mathrm{h}}^{-1}]=\frac{\mathrm{HCN}\mathrm{production}\mathrm{rate}[\mathrm{mmol}{\mathrm{h}}^{-1}]\times 1000}{(\mathrm{quantity}\mathrm{of}\mathrm{active}\mathrm{sites}[\mu \mathrm{mol}{\mathrm{g}}^{-1}])(\mathrm{catalyst}\mathrm{weight}[\mathrm{g}])}$$

Contact time was calculated by

$$\mathrm{Contact}\mathrm{time}[\mathrm{s}]=\frac{(\mathrm{quantity}\mathrm{of}\mathrm{active}\mathrm{sites}[\mu \mathrm{mol}{\mathrm{g}}^{-1}])(\mathrm{catalyst}\mathrm{weight}[\mathrm{g}])}{\mathrm{C}{\mathrm{H}}_4\mathrm{flow}\mathrm{rate}[\mu \mathrm{mol}{\mathrm{s}}^{-1}]}$$

2.4. In Situ FTIR Measurements

In situ FTIR measurements were conducted to monitor the evolution of adsorbed species on the catalyst surface during the reaction. A JASCO VIR‐200 spectrometer equipped with a mercury cadmium telluride (MCT) detector was employed. A self‐supporting disc (13 mm diameter) prepared from 66 mg of 5 wt% Pt/Al₂O₃ was mounted in an in situ stainless‐steel cell fitted with KBr windows, which were maintained at 20°C by circulating cooling water. In addition, 95% of the IR path inside the cell was filled with KBr rods to minimize the absorption by gaseous components. Methane (39.9% in He), nitric oxide (12.5% in helium), helium (99.99995% purity), and hydrogen (99.99999% purity) supplied by Taiyo Nippon Sanso Co., Ltd. were used to make the reaction gas mixtures. The relative gas composition, flow rate of the reactant mixture, and the temperature during catalyst pretreatment and reaction were identical to those used in the fixed‐bed reactor reactivity tests.

2.5. In Situ Dispersive XAFS Measurements

In situ dispersive XAFS (DXAFS) measurements were performed at the NW2A beamline of the Photon Factory, Institute of Materials Structure Science, High‐Energy Accelerator Research Organization (KEK‐IMSS‐PF, proposal no. 2023G650). A total of 100 mg of 5 wt% Pt/Al₂O₃ powder was pressed into a 10‐mm‐diameter disk and placed in an in situ reactor equipped with polyimide film (Kapton) windows cooled by circulating water. Pt L₃‐edge XAFS spectra were collected in transmission mode using a photodiode array (PDA) detector with a YAG phosphor (P46, Y₃Al₅O₁₂:Ce³⁺, 50 µm thickness). The in situ DXAFS measurements were conducted with an exposure time of 800 ms per spectrum over an energy range of 11,440–11,727 eV. The composition and flow rate of the reactant gas, as well as the temperature during catalyst pretreatment and reaction, were identical to those used in the fixed‐bed reactor activity tests. A Pt foil spectrum was employed for energy calibration. Data processing was carried out using Athena software (version 0.9.26) [14].

3. Results and Discussion

3.1. NO Concentration

The alumina‐supported platinum catalyst used in this study was identical to that previously reported for the same reaction [10, 13]. Pulse CO adsorption measurement indicated a capacity of 46 µmol g⁻¹. For the HCN synthesis reaction, larger platinum particle sizes were more favorable [11]. The support consisted primarily of γ‐alumina with a minor fraction of θ‐alumina. Due to the fine size of the platinum particles, XRD analysis exhibited only reflections attributable to the alumina support, with no detectable platinum peaks. The Brunauer‐Emmett‐Teller (BET) surface area was determined to be 99 m² g⁻¹.

In our previous studies, the CH₄:NO ratio was held constant to investigate the reaction mechanism. At that time, the CH₄:NO:He ratio was 13.4:1.8:84.8, and experiments were performed under relatively low NO concentrations [10, 11, 13]. In the present work, CH₄ was fixed at 10%, while NO concentration varied from 0.75 to 4.5% to evaluate its influence on the reaction (Figure 1). Among carbon‐containing products, CO₂ yield was the highest and increased nearly proportionally with NO concentration, leading to a corresponding increase in methane conversion. In contrast, NO conversion remained close to 100% across all concentration ranges examined. At the lowest NO concentration (0.75%), only CO₂ was obtained as a carbon‐containing product, leaving N₂ the main component on the nitrogen side. At NO concentrations of 1.5% or higher, the extent of complete oxidation decreased, while the formation of HCN increased. The HCN yield was highest at the highest NO concentration (4.5%), reaching 3.5% yield. Within the NO concentration range of 1.5–3.8%, the nitrogen‐containing products consisted of approximately 55% NH₃, 35% N₂, about 10% HCN, and < 5% N₂O. Across this range, the yields of NH₃, HCN, and N₂O decreased monotonically with increasing NO concentration. At the highest NO concentration (4.5%), N₂O formation and N₂ reduction were particularly pronounced.

FIGURE 1.

(a) Product yields (C‐based) and (b) product yields (N‐based) as a function of NO concentration. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄/NO/He = 10.0: X: 90.0−X, (X = 0.75−4.5), 450°C, and 0.1 MPa.

3.2. Reaction Temperature

NO concentration was fixed at 4.5%, and the reaction temperature was varied by 25°C increments from 300 to 500°C (Figure 2). At 300°C, only trace amounts of CH₄ and NO were converted, but the formation of HCN and NH₃ was confirmed (carbon‐based yield of HCN: 0.002%; nitrogen‐based yield of NH₃: 0.05%) (Figure 2a, b). As the temperature increased, the conversion of CH₄ and NO rose exponentially, and the yields of HCN and NH₃ exhibited similar trends. Arrhenius plots were constructed using the consumption rates of the reactants and the formation rates of the products, showing a good linear correlation up to 400°C (Figure 2c, d). The calculated apparent activation energies were 157 kJ mol⁻¹ for CH₄, 110 kJ mol⁻¹ for NO, 154 kJ mol⁻¹ for HCN, and 201 kJ mol⁻¹ for NH₃. The order of activation energies is consistent with the proposed reaction mechanism. NO exhibits the lowest activation energy and the highest reaction rate, whereas CH₄ has a higher activation energy than NO. NO first dissociates, generating active oxygen species on the platinum surface, which subsequently cleaves the C─H bond in methane. Comparing the activation energies of HCN and NH₃, HCN has a smaller value than NH₃, indicating that NH₃ forms after HCN, as confirmed by the contact time analysis (shown below). The highest HCN yield was obtained at 475°C, with a carbon‐based yield of 4.8% and a corresponding production rate of 12.8 mmol g⁻¹ h⁻¹. In a previous study using the same catalyst with a different reactant gas mixture (CH₄:NO = 13.4:1.8), the HCN yield and production rate were 3.2% and 11.4 mmol g⁻¹ h⁻¹, respectively [10]. Increasing the NO concentration, therefore, enhanced both the yield and the production rate. At 500°C, the HCN yield decreased slightly, and CO formation markedly increased. At temperatures between 300 and 350°C, most NO was converted to N₂O and N₂. The predominance of N₂O below 400°C is attributed to the high concentration of NO(ads), associatively adsorbed NO species, producing N₂O by the direct reaction between two neighboring NO(ads). From 375°C, the formation of NH₃ and HCN increased, and above 450°C, NH₃ became the major product.

FIGURE 2.

(a) CH₄ conversion and product yields (C‐based), (b) NO conversion and product yields (N‐based) as a function of reaction temperature, (c) Arrhenius plot obtained from reaction rates for CH₄ and NO, and (d) Arrhenius plot obtained from production rates for HCN and NH₃. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄/NO/He = 10.0: 4.5: 85.5, and 0.1 MPa.

It should be noted that the reaction temperature exceeded the temperature employed for the reduction pretreatment (400°C). Ideally, the reduction step should have been conducted at the highest temperature utilized during the reaction. Accordingly, an additional experiment was conducted in which hydrogen reduction was carried out at 500°C, followed by the identical reaction procedure. The CO uptake measured after hydrogen reduction at 500°C was 46 µmol g⁻¹, which is identical to the value obtained after reduction at 400°C. The reaction performance was likewise nearly identical to that observed for the sample reduced at 400°C (Figure S1, see Supporting Information). Furthermore, TEM analysis conducted after the reaction revealed an average particle size of 4.2 nm (Figure S2), identical to the pre‐reaction value of 4.2 nm [10], thereby indicating the absence of measurable particle growth under the reaction conditions. Taken together, these findings indicate that hydrogen reduction at 400°C and 500°C produces no appreciable difference in terms of active‐site density or catalytic behavior.

3.3. Contact Time Study

The NO concentration was fixed at 4.5%, and the reaction temperature was set at 475°C for conducting the contact time study. The inlet gas flow rate and the catalyst weight were varied to analyze the effect of the contact time (or its reciprocal, the space time) (Figure 3). The catalyst contact time was calculated by dividing the quantity of active sites, determined by CO chemisorption, by the methane flow rate. HCN was produced from CH₄ at a very short contact time (0.03 s) with a carbon‐based selectivity of 15%, although most of the CH₄ was converted to CO₂ via complete oxidation (Figure 3a). The CO₂ selectivity decreased slightly with increasing contact time up to 0.19 s, then increased again. In contrast, HCN selectivity increased up to a contact time of 0.19 s, reaching 24%, and then declined to 8% at 0.6 s. Furthermore, CO selectivity increased with contact time, reaching 23% at 0.6 s. Focusing on methane conversion, HCN and CO₂ were formed concurrently from the onset of the reaction, confirming CO₂ as the primary product. The formation of HCN at very short contact time suggests that it is also a primary product of methane; however, the trend in HCN selectivity indicates that HCN acts as a secondary product because the selectivity increased with contact time. The increase in CO selectivity as HCN selectivity declined confirms that CO is formed sequentially from HCN.

FIGURE 3.

(a) CH₄ conversion and product selectivities (C‐based) and (b) NO conversion and product selectivities (N‐based) as a function of contact time. Reaction conditions: 5 wt% Pt/Al₂O₃, CH₄/NO/He = 10.0: 4.5: 85.5, 475°C, and 0.1 MPa.

At short contact times, NO was converted predominantly to N₂ and N₂O, and these carbon‐free products decreased as the contact time increased (Figure 3b). During the early stage of the reaction, NO undergoes both dissociative and associative adsorption. Among the species formed by dissociative adsorption, atomically adsorbed nitrogen species (N(ads)) react with each other to produce N₂, while atomically adsorbed oxygen species (O(ads)) oxidize CH₄, generating CO₂ along with N₂. N₂O is formed from NO(ads) and N(ads) on the Pt surface. Thus, the formation of N₂O suggests that O(ads), or species derived from its reaction with CH₄ fragments, are present on the Pt surface. When HCN selectivity was evaluated on a nitrogen basis, it exhibited a characteristic trend of a secondary product: low selectivity at contact times shorter than 0.1 s, peaking at a contact time of 0.19 s, and then declining at longer contact times. In contrast, the NH₃ selectivity was 22% at the shortest contact time, indicating that NH₃ exhibits the characteristics of a primary product; however, it increased monotonically with contact time, indicating that it was largely attributable to tertiary products generated by sequential reactions from HCN.

Thus, this reaction is complex, and multiple pathways are likely involved in the formation of the target products, HCN and NH₃. However, the main pathways include NO dissociation, activation (or oxidation) of CH₄ by O(ads), formation of HCN, and subsequent formation of NH₃ and CO via the hydrolysis of HCN. In the hydrolysis of HCN, the reaction proceeds according to the equation HCN + 2H₂O → NH₃ + HCOOH, yielding NH₃ and formic acid. Formic acid subsequently reacts over a catalyst, and CO is produced in the dehydration reaction HCOOH → H₂O + CO, or CO₂ is produced in the dehydrogenation reaction HCOOH → H₂ + CO₂.

Scheme 1 presents a proposed simplified reaction pathway for the CH₄‐NO reaction over the Pt/Al₂O₃ catalyst, derived from the contact‐time analysis. In this pathway, water is produced together with CO₂ during the complete oxidation of methane. When the support possesses a high surface area and may have hydroxyl groups, it is inferred that hydrolysis of HCN occurs at the Pt‐support interfacial sites, leading to the formation of NH₃. In contrast, as demonstrated in the previous study on the influence of support properties on HCN formation [12], supports with low surface area and few hydroxyl groups exhibit significantly suppressed hydrolysis.

SCHEME 1.

Simplified reaction pathway proposed for the CH₄‐NO reaction over the Pt/Al₂O₃ catalyst, based on the results of the contact time study.

The turnover frequency (TOF) of the catalyst was evaluated in this study (Table 1). At the shortest contact time (0.03 s), the TOF calculated from the HCN production rate was the highest, reaching 1433 h⁻¹ (Table 1, entry 6). At this point, the CH₄ conversion was 7.2%, and the carbon‐based HCN yield was 1.1%. At a contact time of 0.19 s, the CH₄ conversion increased to 16.8%, the HCN yield reached 4.1%, and the TOF was 743 h⁻¹ (Table 1, entry 10). These results were compared with our previous studies [10, 12, 15] and with the results of Guo et al.’s literature using non‐thermal plasma [4, 5]. In the study by Guo et al., plasma combined with Pt‐ or Cu‐supported catalysts was employed to dissociate CH₄ and NH₃, which subsequently reacted to produce HCN with yields of 21% and 24%, respectively; however, the catalysts exhibited low TOFs (Table 1, entries 1 and 2) [4, 5]. In our reaction system, using the Pt catalyst under low NO concentration conditions, CH₄ conversion reached 6.8%, HCN yield was 3.2%, and TOF was 253 h⁻¹ (Table 1, entry 3) [10]. These results indicate substantial improvements in conversion, yield, and TOF by this study. Further enhancement of TOF was achieved by employing α‐Al₂O₃ as the support [12] and by alloying with Au [15] (Table 1, entries 4 and 5). Additional increases in catalytic activity are anticipated by applying similar reaction conditions to these catalysts in future work.

TABLE 1.

Comparison of TOF of the present study with previous works for low‐temperature HCN production using CH_4.

Entry	Catalyst	Temp. /°C	Gas composition	CH4 conv. /%	HCN yield /%	TOF (HCN) / h−1	Ref.
1	Plasma + Pt/TS‐1	400	CH4:NH3 = 1:2	26	21	18	[4]
2	Plasma + Cu/Silicate	400	CH4:NH3 = 1:2	30	24	10	[5]
3	Pt/(γ+θ)‐Al2O3	425	CH4:NO:He = 1:0.13:6.3	6.8	3.2	253	[10]
4	Pt/α‐Al2O3	400	CH4:NO:He = 1:0.13:6.3	8.8	3.0	375	[11]
5	AuPt/Al2O3	400	CH4:NO:He = 1:0.13:6.3	0.84	0.3	740	[15]
6	Pt/(γ+θ)‐Al2O3	475 (contact time = 0.03 s)	CH4:NO:He = 1:0.45:8.6	7.2	1.1	1433	This study
7	Pt/(γ+θ)‐Al2O3	475 (contact time = 0.19 s)	CH4:NO:He = 1:0.45:8.6	16.8	4.1	743	This study

3.4. Comparison Between Ramping and Constant Temperature Conditions

In our previous studies, after hydrogen reduction pretreatment, the temperature was lowered to 300°C [10, 11]. Following a helium purge, a reaction gas mixture containing CH₄ and NO was fed to initiate the reaction. The temperature was then increased by 25°C increments to evaluate the reaction activity. In the present work, two experiments were conducted to examine the effect of the initial reaction temperature on activity: one was starting at 300°C and assessing activity at the final temperature of 475°C after heating (hereafter referred to as the ramping temperature conditions) (Figure 4 the red line), and the other was introducing the reaction gas at 475°C from the outset of activity evaluation (constant temperature conditions) (Figure 4 the blue line). Additionally, in situ FTIR analysis was performed to investigate changes in adsorbed species on the catalyst surface. The temperature profiles employed are shown in Figure 4.

FIGURE 4.

Temperature profiles in experiments under ramping temperature conditions and constant temperature conditions. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 4.5: 85.5, 475°C, and 0.1 MPa.

Figure 5 shows the time‐dependent changes at a reaction temperature of 475°C. Figure 5a presents the results under ramping temperature conditions, while Figure 5b shows the results under the constant temperature conditions. The vertical axes (conversion and yield) in both graphs are presented using the same scale. The initial reaction temperature was found to have a significant effect on catalytic activity. Regarding the conversion, NO exhibited nearly complete conversion in both cases, whereas CH₄ conversion was 26% under the ramping temperature conditions but decreased markedly to 18% under the constant‐temperature conditions. For product distribution, under the ramping temperature conditions, the HCN yield started at 1.4% on a carbon basis and decreased over time, while the NH₃ yield remained high, ranging from 72% to 63%. In contrast, under constant‐temperature conditions, the HCN yield was initially nearly zero but increased over time, reaching 1.2% after 150 min, whereas the NH₃ yield was low, only 16% after 150 min. Under the constant‐temperature conditions, N₂ selectivity was as high as 75%. Furthermore, the yield of N₂O also increased over time. Thus, pronounced differences were observed between the two conditions. Under the ramping temperature conditions, CH₄ was effectively oxidized and selectively converted to HCN and NH₃. In contrast, under the constant‐temperature conditions, CH₄ conversion was low, and complete oxidation to CO₂ and N₂ was the predominant pathway; however, the yields of N₂O and HCN increased over time.

FIGURE 5.

Time courses of the formation of HCN and other products over the 5 wt% Pt/Al₂O₃ catalyst. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 4.5: 85.5, 475°C, and 0.1 MPa. (a‐1) CH₄ conversion and carbon‐based yields under ramping temperature conditions. (a‐2) NO conversion and nitrogen‐based yields under ramping temperature conditions. (b‐1) CH₄ conversion and carbon‐based yields under constant temperature conditions. (b‐2) NO conversion and nitrogen‐based yield under constant temperature conditions.

3.5. In Situ FTIR Analysis

In situ FTIR measurements were performed under the same reaction conditions as the reactivity tests (Figure 6). Under the ramping temperature conditions, measurements began at 300°C after introducing the reaction gas. Pt‐NO adsorption species were observed between 300 and 350°C. Above 400°C, this peak disappeared, and a broad band at 2300–2050 cm⁻¹, together with a distinct band at 1590 cm⁻¹, attributable to carbonate species on alumina [16], became evident. At 450 and 475°C, the band at 2300–2050 cm⁻¹ intensified, and two distinct peaks with shoulders appeared. The peak at 2237 cm⁻¹ was assigned to isocyanate species adsorbed on the alumina support (Al‐NCO)[16‐18], whereas the peak at 2142 cm⁻¹ was attributed to cyanide species adsorbed on Pt (Pt‐CN) [10, 17, 18]. A broad band in the 2300–2050 cm⁻¹ region was also observed under the constant‐temperature conditions, although its intensity and shape differed from those under ramping conditions. In both cases, changes in the bands within this region were observed over time at 475°C.

FIGURE 6.

In situ FTIR spectroscopy results for the reaction of CH₄ and NO on the 5 wt% Pt/Al₂O₃ catalyst (a) under ramping temperature conditions, and (b) under constant temperature conditions.

It is evident that the band between 2300 and 2050 cm⁻¹ comprises multiple components. Therefore, peak fitting was performed using Gaussian curves with equal width, resulting in four distinct components (Figure 7). The deconvolution revealed peaks at 2237 and 2142 cm⁻¹, as well as additional peaks at 2188 and 2085 cm⁻¹. This peak separation was possible for both the ramping and constant‐temperature conditions. The new peak at 2188 cm⁻¹ identified by Gaussian fitting is attributed to an isocyanate species adsorbed on Pt (Pt‐NCO) [18, 19], whereas the peak at 2085 cm⁻¹ is assigned to a linearly adsorbed CO species on reduced Pt [17, 19]. For these four components, the integrated area of each peak was calculated, and their variation over time was analyzed (Figure 8). In both the ramping and constant‐temperature cases, the cyanide species adsorbed on Pt (Pt‐CN) increased over time, whereas the Al‐NCO species decreased under both conditions. The isocyanate species adsorbed on Pt (Pt‐NCO) remained nearly constant throughout the reaction period. In contrast, during the initial reaction stage, the Pt‐CO species exhibited a decreasing trend under the ramping conditions, and an increasing trend under the constant‐temperature conditions (Figure 9). When these changes are compared with the time‐dependent variation in HCN formation within the catalytic activity profile (Figure 5), it becomes evident that only Pt‐CO species displayed behavior consistent with the increase and decrease in HCN. Our previous studies have suggested that Pt‐CN species act not only as reaction intermediates but also function as inhibitors when accumulated in excess, thereby reducing catalytic activity [13]. The present study revealed that there is no strong correlation between Pt‐CN species and HCN formation, suggesting the potential involvement of a new terminal Pt‐CO species. The Pt‐CO species observed at 2085 cm⁻¹ is attributed to CO adsorbed on the stabler Pt(111) crystal plane. Larger particle sizes are thought to promote the formation of this stable facet, consistent with our previous findings that catalysts with larger particle sizes exhibit higher HCN activity [11]. It is suggested that Pt‐CN species observed by FTIR measurement are strongly adsorbed and therefore act as spectator species, whereas the Pt‐CO species serve as intermediates for isocyanate or cyanide species. The adsorption amount of these Pt‐CO species correlates with the amount of HCN released as a product from the catalyst surface.

FIGURE 7.

Expanded view of in situ FTIR spectra for the reaction of CH₄ and NO on the 5 wt% Pt/Al₂O₃ catalyst (a) under ramping temperature conditions and (b) under constant temperature conditions.

FIGURE 8.

Time courses of peak areas attributed to four adsorbed species in the FTIR spectra during the reaction of CH₄ and NO on the 5 wt% Pt/Al₂O₃ at 475°C (a) under ramping temperature conditions and (b) under constant temperature conditions.

FIGURE 9.

Time courses of peak areas attributed to Pt‐CO species in the FTIR spectra during the reaction of CH₄ and NO on the 5 wt% Pt/Al₂O₃ at 475°C under ramping temperature conditions and constant temperature conditions.

3.6. Effect of Swinging Reaction Temperature From 300°C to 400°C

Although higher reaction temperatures resulted in increased CH₄ conversion and enhanced HCN and NH₃ yields, the HCN yield gradually decreased with time on stream. This decline is attributed to the formation of strongly adsorbed Pt‐CN species on the platinum active sites. To mitigate this deactivation behavior, experiments were conducted under swing‐temperature reaction conditions as follows: the reaction was initiated at 400°C, followed by cooling to 300°C over 15 min, and then reheating to 400°C for 10 min. This temperature cycle was repeated five times. The corresponding temperature profile is shown in Figure 10. For comparison, additional reaction tests were performed under isothermal conditions at 300°C and 400°C. Since the primary purpose here was to enable comparison with our previous work, the reaction temperature was set to 300–400°C instead of 475°C, and the CH₄:NO feed ratio was set to 10:1.5.

FIGURE 10.

Temperature profiles in experiments under swinging temperature conditions and constant temperature conditions. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 1.5: 88.5, and 0.1 MPa.

Figures 11 and 12 present the temporal profiles of reactant conversions and product yields. Figure 11 shows the CH₄ conversion together with the carbon‐based yields of HCN, CO, and CO₂, while Figure 12 displays the NO conversion and the nitrogen‐based yields of N₂O, NH₃, and N₂. It should be noted that CH₄, CO₂, and N₂ concentrations were measured every 30 min (or upon reaching 400°C during the temperature‐swing experiments) using gas chromatography, whereas HCN, NH₃, and other gaseous products were monitored every 2 min by FTIR spectroscopy.

FIGURE 11.

Time courses of the formation of HCN and other products over the 5 wt% Pt/Al₂O₃ catalyst under swing reaction temperature conditions and constant temperature conditions. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 1.5: 88.5, and 0.1 MPa. Carbon‐based conversion and product yields. (a) CH₄ conversion, (b) HCN yield, (c) CO yield, and (d) CO₂ yield.

FIGURE 12.

Time courses of the formation of NH₃ and other products over the 5 wt% Pt/Al₂O₃ catalyst under swing reaction temperature conditions and constant temperature conditions. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 1.5: 88.5, and 0.1 MPa. Nitrogen‐based conversion and product yields. (a) NO conversion, (b) N₂O yield, (c) NH₃ yield, and (d) N₂ yield.

The results obtained under the temperature‐swing conditions (between 300°C and 400°C) showed that the NO conversion and the yields of N₂ and N₂O generally fell between those observed at constant temperatures of 300°C and 400°C (Figure 12). In contrast, the CH₄ conversion and the yields of HCN, NH₃, CO, and CO₂ were markedly higher than the values measured under the isothermal 400°C condition, with particularly pronounced enhancements in HCN, NH₃, and CO formation (Figures 11 and 12). These observations suggest that the temperature increase promotes the desorption of adsorbed species, primarily Pt‐CN, from the catalyst surface as HCN, which subsequently undergoes further decomposition through sequential reactions.

3.7. In Situ DXAFS Analysis for Swinging Temperature From 300°C to 400°C

In situ DXAFS measurements were carried out under reaction conditions identical to those used in the reactivity tests to directly correlate catalyst structure with performance. Following reduction of the catalyst under a hydrogen flow at 400°C, the system was purged with helium, and the CH₄‐NO reactant mixture was introduced at 400°C. The reaction was allowed to proceed at this temperature for 3 min, after which the catalyst cell was cooled to 300°C and immediately reheated to 400°C. This temperature‐swing cycle was repeated five times. Using white X‐rays, each DXAFS spectrum was collected at 1 s intervals (with an exposure time of 800 ms), and the average of twenty consecutive measurements (i.e., 20 s) was taken for analysis.

During the pretreatment process, no discernible changes were observed in the XANES spectra either 1 h after hydrogen reduction or during the subsequent helium purge, confirming that Pt remained in a reduced state prior to the CH₄‐NO reaction (Figure S3). Figure 13 presents the in situ XANES spectra obtained during the temperature‐swing operation. A pronounced spectral change was observed immediately after introducing the reactant gas at 400°C, characterized by an increase in absorption above approximately 11,562 eV. This absorption continued to grow almost linearly during the first 3 min at 400°C (Figure 13a). Such evolution in the XANES features is consistent with our previous in situ XAFS observations and is attributed to the accumulation of adsorbed Pt‐CO and Pt‐CN species on the catalyst surface, rather than to oxidation of the Pt sites [10, 13]. Subsequently, the temperature was lowered to 300°C over 15 min (Figure 13b). During this cooling period, the XANES spectra continued to show an increase in absorption, and the absorption intensity reached its maximum at 300°C. When the temperature was then raised back to 400°C over 10 min (Figure 13c), the absorption initially decreased, indicating the desorption of surface species. However, after approximately 7 min of heating, corresponding to a temperature of around 370°C, the absorption began to increase again.

FIGURE 13.

Pt L₃‐edge XANES spectra during (a) initial stage of reaction at 400°C for 3 min, (b) lowering temperature to 300°C for 15 min, and (c) raising reaction temperature to 400°C for 10 min. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 1.5: 88.5, and 0.1 MPa.

These sequential increases and decreases in absorption reflect the dynamic behavior of adsorbed species on the platinum surface. Specifically, upon initial exposure to the CH₄‐NO mixture at 400°C, the formation of intermediates (primarily Pt‐CN and Pt‐CO) increased steadily. When the temperature was subsequently lowered, these adsorbed species remained on the surface and accumulated further, likely due to reduced rates of thermal desorption. Upon reheating, the adsorbates were partially removed from the platinum sites, consistent with the decrease in absorption. However, once the temperature exceeded ca. 370°C, the rate of formation of new adsorbed species again surpassed their desorption, leading to renewed growth in the absorption features. Thus, the XANES evolution captures the interplay between the accumulation and desorption of strongly bound Pt‐CN/CO species under temperature‐swing operation.

Previous studies have attributed the increase in absorption at 11,569 eV to the accumulation of Pt‐CN species [10, 13]. Based on this interpretation, Figure 14 shows the time‐dependent changes in the absorption at 11,569 eV obtained from the 20‐second average spectra, together with the corresponding measured temperature profile. As described above, the absorption continued to increase during the cooling period. Upon switching to the heating phase, the absorption initially decreased, indicating partial removal of the surface‐bound species, and then increased again once the temperature exceeded approximately 370°C. These findings further support the view that temperature‐swing operation modulates the balance between accumulation and desorption of strongly adsorbed intermediates on the platinum surface. In particular, the decrease in absorption during the early stage of heating suggests that Pt‐CN species desorb, consistent with the observed production of HCN and NH₃. Consequently, the temperature‐swing method facilitates periodic removal and subsequent reactive conversion of these surface species, leading to significantly higher overall HCN and NH₃ yields compared with the isothermal reaction.

FIGURE 14.

Changes of XAFS intensity at 11569 eV during swinging reaction temperature from 300°C to 400°C. Reaction conditions: 5 wt% Pt/Al₂O₃ (100 mg), CH₄ / NO / He = 10.0: 1.5: 88.5, and 0.1 MPa.

The observation that the temporal evolution of µT in the DXAFS measurements does not strictly coincide with the programmed temperature profiles is consistent with expectations derived from the reactivity test results. The transient decreases in µT observed at approximately 75 and 85 min, as well as the discontinuities in the data around 50 and 110 min, are most plausibly ascribed to intrinsic detector characteristics. Regarding the discontinuities at around 50 and 110 min, these arise simply from the absence of measurements during those intervals. A photodiode detector was employed in this study. Although DXAFS is inherently designed for measurements on the millisecond timescale, the present experiment required extended exposures on the order of several tens of minutes, which imposes considerable stress on the detector. Consequently, periodic interruptions of the x‐ray exposure were necessary to mitigate detector degradation. As shown in Figure 14, x‐ray exposure was suspended for approximately 5 min beginning at around 45 min and for approximately 10 min beginning at around 110 min, allowing the detector to recover. Such recovery intervals demonstrably improve detector performance and restore spectral quality. The temporary reductions in µT at approximately 75 and 85 min cannot be conclusively explained at this time. However, it is plausible that these deviations originated from compromised spectral quality, including background instability, during those specific measurement periods.

4. Conclusion

HCN formation from CH₄ and NO over Pt/Al₂O₃ catalyst was enhanced by higher NO concentrations and elevated temperatures, reaching a maximum yield of 4.8%. HCN was identified as a primary product, while NH₃ formed mainly through secondary reactions involving HCN. In situ FTIR and DXAFS showed that Pt‐CO, rather than strongly adsorbed Pt‐CN, correlates with HCN formation, and temperature‐swing operation effectively removed inhibiting surface species to boost performance. Controlling the accumulation and desorption of surface intermediates is therefore the key to improving low‐temperature HCN synthesis.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: chem71024‐sup‐0001‐SuppMat.docx.