Dependence on Under-Coordinated Pt at the Surface of Layered PtTe2 in the Decomposition of Methanol

doi:10.21203/rs.3.rs-2973986/v1

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Dependence on Under-Coordinated Pt at the Surface of Layered PtTe2 in the Decomposition of Methanol

https://doi.org/10.21203/rs.3.rs-2973986/v1

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published 22 Jan, 2024

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Decomposition of adsorbed methanol (CH₃OH) was activated on under-coordinated Pt (denoted as Pt_uc), produced by removing Te, at layered PtTe₂ surface. Both dehydrogenation and C-O bond scission occurred, resulting in the formation of intermediates CH_xO* (x = 2, 3; * denotes adsorbates) and CH_x* (x = 1, 2) and ultimately the production of gaseous molecular hydrogen, methane, water and formaldehyde. We observed a great reaction probability (> 90 %) on the Pt_uc sites and a catalytic selectivity dependent on the Pt_uc concentration. Approximately 60 % of the methanol decomposed to CH_xO* and 35 % to CH_x* at small Pt_uc concentrations (£ 10 %); nevertheless, with increased Pt_uc concentration (> 10 %), both the probability of decomposition to CH_xO* and the production of gaseous formaldehyde were selectively decreased. We propose the characteristic reactivity at small Pt_uc concentrations arises from both the triangular positioning and varied degrees of oxidation of surface Pt_uc.

Physical sciences/Chemistry/Surface chemistry/Surface spectroscopy

Physical sciences/Physics/Chemical physics

Physical sciences/Chemistry/Catalysis/Heterogeneous catalysis

Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films

Two-dimensional transition metal dichalcogenides (TMDs) have attracted considerable interest, owing to their distinctive electronic properties^1-8 and highly tunable surface reactivity.^9-16 TMDs, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been a goal in the design of ideal catalysts. Structurally perfect basal planes of TMDs are typically inert whereas their surface defects, such as unsaturated coordinative atoms at the surfaces or edges, become catalytically active centers. Many studies on MoS₂, one of the most developed TMDs, have demonstrated that by controlling the density of sulfur vacancies (active centers) at the MoS₂ surface, its reactivity toward varied reactions can be manipulated.^13-21 Pt-based catalysts, including single crystals, nanoclusters, alloys and electrodes, have been extensively investigated in both heterogeneous catalysis and electrocatalysis, because of their superior catalytic properties.^22-42 Nevertheless, the catalytic properties of Pt-based TMDs have been little explored,^{11, 43, 44}^,⁴⁵even though they could possess better chemical and structural stability than commercial Pt-based catalysts such as supported Pt nanoclusters. The present study has an aim to shed light on the catalytic properties of Pt-based TMDs and their potential as catalysts.

We studied the decomposition of methanol (methanol-d₄) on layered PtTe₂ with various surface-probe techniques under both ultrahigh-vacuum (UHV) and near-ambient-pressure (NAP) conditions. Methanol (CH₃OH) decomposition serves as the principal reaction in direct methanol fuel cells (DMFCs), a promising device that converts methanol efficiently to electricity,^46-52 and the liberation of hydrogen from the decomposition leads to viable generation of hydrogen, another clean source of energy.^{53, 54} PtTe₂, as a group-10 TMD material, draws much attention for its tunable bandgap, high charge mobility and ultrahigh air stability,^{1, 3, 5-7, 11, 43, 44} but its surface reactivity is little understood. We generated the surface defects on PtTe₂ and controlled their concentration with Ar ion (Ar⁺) bombardment. The surface structures of PtTe₂ were characterized using scanning tunneling microscopy (STM) and synchrotron-based photoelectron spectroscopy (PES), while the reactions, intermediates and products, were monitored using PES, near-ambient-pressure photoelectron spectroscopy (NAP-PES) and near-ambient-pressure mass spectroscopy (NAP-MS). We also performed calculations based on density functional theory (DFT) to explore the mechanisms in details.

The results showed that under-coordinated Pt (Pt_uc) at the PtTe₂ surface, corresponding to Te vacancies, served as active sites for the reaction. Both dehydrogenation and C-O bond scission occurred for adsorbed methanol on the Pt_uc sites, leading to the formation of CH_xO* (x = 2 and 3; * denotes adsorbates) and CH_x* (x = 1 and 2) as major intermediates and finally production of gaseous molecular hydrogen, methane, water and formaldehyde via various processes. Gaseous formaldehyde (CH₂O_(g)) was rarely observed from methanol decomposition on either Pt single crystals or supported Pt nanoclusters; meanwhile, CO* (or CO_(g)), a common product from Pt-based catalysts,^{22-26, 30, 33-36, 42} was not produced in the present reaction; the pathway of C-O bond scission in the present reaction accounted for a significant proportion (reflected on the considerable quantity of produced CH_x*), which also contrasts with its minor role on supported Pt clusters.³⁴ The catalytic nature of Pt_uc sites on PtTe₂ thus differs from that of typical Pt-based catalysts. Moreover, the reactivity depended notably on the Pt_uc concentration. At a small Pt_uc concentration (£ 10 %), methanol on the Pt_uc sites decomposed at a great probability (> 90 %); with the increased Pt_uc concentration (10 - 20 %), the probability decreased as the probability of decomposition to CH_xO* was selectively decreased, which was also reflected in the decreased production of CH₂O_(g). Nevertheless, in either case, the reaction probability (³ 80 %) exceeds those on Pt single crystals^{22, 25} and supported Pt clusters.³⁴ We propose, with the support of DFT modelling, that both structural and electronic effects play essential roles in determining the observed catalytic properties. These results suggest that a PtTe₂ surface with limited Pt_uc sites (Te vacancies) can serve as a superior catalyst for methanol decomposition; its catalytic selectivity can be controlled by using the Pt_uc concentration at the surface.

Structural characterization with STM and PES. We utilized STM to characterize the surface structures of layered PtTe₂ before and after the treatment of Ar⁺ bombardment. The as-cleaved surface was generally very flat and had few defects, as illustrated in Fig. 1a. Several types of intrinsic defects observed previously with STM,⁵⁵ such as Te and Pt vacancies, were also observed in the present sample (Figure S1). After Ar⁺ bombardment, the surface vacancies were evidently increased and some small clusters (white regions in Fig. 1b) were generated on the sample. Increasing either the bombardment time or the Ar⁺ kinetic energy roughened the surface and generated more structural variations (Figs. 1c, d). With the aim of correlating structures with reactivity, we chose a kinetic energy of 0.5 keV for incident Ar⁺ to control structural complexity — the vacancies were produced while the surface crystallinity (reflected in our RHEED measurements, Figure S2) was largely sustained at an appropriate Ar⁺ dosage. The high-resolution images and corresponding line profiles of the vacancies and clusters (Figs. 1e – 1h) revealed that the depth of the vacancies and the height of the clusters were about 0.1 and 0.15 nm, respectively. Accordingly, the vacancies were formed by the removal of the topmost Te atoms, i.e., Te vacancies, and the clusters likely corresponded to re-deposited atoms (one-atom high) after Ar⁺ bombardment. Our PES spectra shown below further characterized these surface defects.

Figure 2a compares the PES spectra of Pt 4f core level for the layered PtTe₂ as cleaved and bombarded by Ar⁺. The spectrum from as-cleaved PtTe₂ (upper panel of Fig. 2a) shows a doublet, Pt 4f_5/2 and Pt 4f_7/2, centered at the binding energies (BE) 75.9 and 72.6 eV respectively, which corresponds to intact Pt-Te chemical bonds in PtTe₂. Meanwhile, the PES spectrum from Ar⁺-bombarded PtTe₂ shows an additional feature at a BE slightly smaller than that of the main Pt feature, i.e., at 71.8 eV for Pt 4f_7/2 and 75.1 eV for Pt 4f_5/2 (indicated by red fitted curves in the bottom panel). The smaller BE for Pt 4f implies less oxidized Pt, corresponding to under-coordinated Pt (denoted as Pt_uc), at the PtTe₂ surface and therefore the removal of Te atoms by Ar⁺ bombardment, which agrees with the above Te vacancies observed with STM (Fig. 1). In contrast, the Te 4d signals were insensitive to the removal of neighboring Te atoms ― the line shape of Te 4d doublet altered little after Ar⁺ bombardment (Fig. 2b), since the surface Te is mainly bonded to the underlying Pt. Both the Pt 4f and Te 4d lines from Ar⁺-bombarded PtTe₂ shifted negatively (approximately 0.1 eV as shown in Figs. 2a,b), likely because the bombardment induced a band-bending effect. The re-deposited atoms contributed less to the Pt_uc signals because they could be Pt or Te and these atoms were present in small quantities. Our experiments show that the number or concentration of Pt_uc can be controlled with Ar⁺ dosage (measured by the sample current multiplied by sputtering time) and Ar⁺ kinetic energy. Figure 2c plots the ratio of the Pt_uc and total Pt signals (denoted as Pt_uc/Pt), measured by the integrated intensities of the red and black fitted curves (Fig. 2a), as a function of the Ar⁺ dosage. The Pt_uc/Pt ratio increased almost in a linear fashion with the Ar⁺ dosage, despite varied Ar⁺ kinetic energies. As about 90% of the Pt signals came from the top two PtTe₂ bilayers (according to the escape depth of the Pt 4f photoelectrons) and as Ar⁺ bombardment removed primarily the surface Te (Fig. 1), each Pt_uc/Pt ratio corresponds to a derivable concentration of surface Pt_uc or surface Te vacancies. Notably, the rate of increase of the Pt_uc/Pt ratio depended on the Ar⁺ kinetic energy, since the cross section of removing surface Te varied with the Ar⁺ kinetic energy. As the Ar⁺ at 0.5 keV exhibited the best controllability at small Pt_uc/Pt ratios and prevented structural complexity (Fig. 1), it was chosen to prepare different Pt_uc concentrations for our catalytic studies.

Methanol decomposition monitored with PES, NAP-PES and NAP-MS. The methanol reactions were characterized primarily with PES spectra; with the spectra, we monitored the evolution of surface species with temperature and Pt_uc concentrations. Figures 3a,b compare the C 1s spectra for methanol adsorbed on as-cleaved and Ar⁺-bombarded PtTe₂ at 145 K and annealed stepwise to selected temperatures. The dominant feature at 145 K on either surface, centered about 286.5 eV, is assigned to the C 1s line of monolayer methanol adsorbed on PtTe₂ (top in Figs. 3a,b), since multilayer methanol desorbed near 130 K.^{34, 56, 57} On the as-cleaved PtTe₂ surface, the methanol signals decreased with increased temperature and vanished at 200 K, reflecting the desorption of methanol (Fig. 3a). In contrast, on the PtTe₂ with a number of surface Pt_uc produced by Ar⁺ bombardment, new features grew at 283.8–285.1 eV above 160 K, at the expense of attenuating methanol feature at 286.5 eV (Fig. 3b). The results suggest that with increased temperature, a fraction of methanol desorbed whereas the other fraction decomposed and produced new carbon species. As the as-cleaved PtTe₂ has very limited surface defects, the contrasting results indicate that the structurally perfect basal plane of layered PtTe₂ is catalytically inert but the surface Pt_uc sites, the Te vacancy sites, are reactive toward methanol decomposition. The products at elevated temperature consisted of three carbon species: CH_xO* (x = 1–3), CH_x* (x = 1–3) and atomic carbon (C*), corresponding to the C 1s lines centered at 285.1,^58–64 283.8^65–67 and 284.2 eV (Figure S3), respectively. CO* was not expected because our adsorption experiments showed no CO adsorbed on such a defective PtTe₂ surface even at 145 K. Figures 3c,d exemplify the fits to the C 1s lines (at 180 and 260 K) with characteristic fitted curves representing C 1s signals from adsorbed monolayer methanol and the proposed products. CH_xO* (red curve in Figs. 3c,d) and CH_x* (blue) were the primary products at 160–260 K but C* became notable above 260 K, implying that further decomposition of CH_xO* and/or CH_x* occurred at elevated temperature. Figure 3e plots the integrated intensities of these fitted C 1s curves, used to measure the quantities of produced CH_xO*, CH_x* and C*, as a function of temperature. The produced CH_xO* (red circles) and CH_x* (blue) increased to maxima near 180 K and decreased at even higher temperature, whereas C* (purple) began to emerge above 200 K and became the major species at and above 300 K. The formation of CH_xO* and CH_x* indicates that dehydrogenation and C-O bond scission, the two competing processes of methanol decomposition,^{30, 34, 35, 68} were both catalytically activated at such low temperature on the surface Pt_uc sites. A fraction of CH_xO* and CH_x* must have desorbed at elevated temperature, since the quantity of the remaining C* was not comparable to that of the produced CH_xO* and CH_x* (Fig. 3e). Our NAP experiments presented below indicated that the desorbing carbon species consisted largely of CH₂O_(g) and CH_4(g).

To reveal how the catalytic properties of PtTe₂ surface vary with the concentration of surface Pt_uc, we plot the probabilities of the conversion of adsorbed monolayer methanol to CH_xO* (red circles) and CH_x* (blue) as a function of the concentration of surface Pt_uc, shown in Fig. 4a. The conversion probability was derived from the ratio of the C 1s intensities of CH_xO* (or CH_x*) to monolayer methanol (145 K); the former was measured at 180 K because the maximum quantity of CH_xO* (or CH_x*) was produced around 180 K; only monolayer methanol was considered because it was directly in contact with the PtTe₂ surface. The concentration of surface Pt_uc was estimated according to the measured Pt_uc/Pt ratios (Fig. 2), the limited escape depth of photoelectrons and the fact that the Ar⁺ bombardment removed primarily the surface Te; the percentage corresponds to the fraction of Pt_uc in the total amount of Pt in the top PtTe₂ bilayer. For CH_x* (blue circles), the conversion probability of monolayer methanol increased almost in linear proportion to the Pt_uc concentration, corroborating that Pt_uc served as reactive sites for the C-O bond scission. Nevertheless, the trend becomes complicated for CH_xO* (red). The conversion probability increased linearly at a Pt_uc concentration ≤ 10%, and became saturated (10% − 20%) or even decreased at a greater Pt_uc concentration (> 20%). A fraction of the Pt_uc sites at greater Pt_uc concentrations must have different structures and thus different catalytic properties although these structures were not resolved in the Pt 4f spectra; they could prefer a separate reaction pathway, in which the C-O bond scission was more facilitated.

The conversion probability of methanol adsorbed on the Pt_uc sites (denoted as methanol/Pt_uc) as a function of the Pt_uc concentration shows clearly the evolving catalytic selectivity (Fig. 4b). The fraction of methanol/Pt_uc in total monolayer methanol was estimated to be that of Pt_uc in total amount of Pt in the top PtTe₂ bilayer (the Pt_uc concentration), by assuming that monolayer methanol adsorbed uniformly on the PtTe₂ surface. As shown in Fig. 4b, the conversion probability of methanol/Pt_uc to CH_x* varied little with the Pt_uc concentration, remaining near 35%; in contrast, that to CH_xO* was approximately 60% at a Pt_uc concentration ≤ 10% but decreased continuously at a Pt_uc concentration > 10%. The dehydrogenation to CH_xO* was selectively obstructed at a Pt_uc concentration > 10%. Nevertheless, we note that the total reaction probability was remarkably great — about 95% at a Pt_uc concentration ≤ 10% and remaining greater than 80% even at a concentration near 20%. In either case, the reaction probability exceeded that for methanol on either Pt single-crystal surfaces (10%)^{22, 25} or supported Pt clusters (60–70%).³⁴ Additionally, the probability of conversion to CH_x* (35%) was also evidently greater than that on Pt clusters (< 10%);³⁴ the C-O bond scission pathway apparently played an important role in the present reaction.

The above experiments demonstrate that the catalytic reactivity of layered PtTe₂ is controllable by the surface Pt_uc under UHV conditions. To unveil their catalytic performance under “real-world” conditions,⁶⁹ we investigated methanol reactions on layered PtTe₂ near ambient pressure. Figure 5a exemplifies the NAP-PES spectra of C 1s core level from PtTe₂ bombarded by Ar⁺ (Pt_uc/Pt ratio = 0.10) and subsequently exposed stepwise to selected pressures of methanol at 300 K; 300 K was used because the desorption and further decomposition of products (intermediates) already occurred (Fig. 3). The as-bombarded PtTe₂ surface was free of carbon contamination, indicated by the absence of C 1s signals (the bottom of Fig. 5a); increasing methanol pressure to \({10}^{-4}\) mbar, a small C 1 s line arose around 284.2 eV and continued to grow with increased pressure; at \({10}^{-3}\) mbar and above, a shoulder centered about 283.0 eV (Figs. 5a,b) also grew. The former is assigned to C*, while the latter to CH_x*, both of which resulted from decomposed methanol on PtTe₂. The reactions must have occurred on the surface Pt_uc sites, as the experiments on as-cleaved PtTe₂ (with scarce surface Pt_uc), as a comparison, showed negligible C 1s signals. This observation agrees with that on the Ar⁺-bombarded PtTe₂ surface under UHV conditions, in which C* and CH_x* were remaining primary species on the surface at 300 K (Fig. 3e). These C 1s signals increased with methanol pressure, since more methanol decomposed on the PtTe₂ surface at greater methanol pressures. They decreased at 0.1 mbar (second from the top in Fig. 5a) as the photoelectrons were attenuated by the increased pressure; at such a great pressure, the C 1s feature resulting from gaseous methanol also appeared at 288.5 eV.³⁵ Notably, the ratio of CH_x* to C* signals increased with methanol pressure but altered little with the decreasing pressure from 0.1 to \({10}^{-7}\) mbar. The ratio was affected little by the pressure-induced signal attenuation but determined by the composition of carbon species on the surface. The fraction of CH_x* in total surface carbon species became greater at a greater pressure or C* concentration. We speculate that a greater C* concentration at the Pt_uc sites suppressed the dehydrogenation of CH_x* by altering the electronic properties of Pt_uc and/or adsorption configurations of CH_x*.

The corresponding gaseous products from the methanol reactions near ambient pressure were monitored with NAP-MS. Figure 5c exemplifies the observed main products, including D_2(g), D₂O_(g)/CD_4(g) and CD₂O_(g), from Ar⁺-bombarded PtTe₂ (Pt_uc/Pt ratio = 0.10) at 300 K exposed to methanol-d₄ (CD₃OD) at varied pressures. Instead of methanol, methanol-d₄ was used because these isotopic variants have similar chemical properties, such as the activation for desorption and decomposition (determined from their electronic structures), but methanol-d₄ gave clearer signals of molecular deuterium (D₂), an essential product to reveal the reaction mechanisms, in desorption experiments. These three desorbing species increased generally with methanol-d₄ pressure, consistent with the above NAP-PES experiments, as more methanol-d₄ interacted with the surface Pt_uc at a higher methanol-d₄ pressure. D_2(g) came from recombinative desorption of D*, while D* was produced from dehydrogenated CD₃OD*, CD_xO* and CD_x*; therefore, its formation as the major desorbing species reflects the essential role of dehydrogenation at different stages of methanol-d₄ decomposition. Both D₂O_(g) and CD_4(g) were possible products, reflecting the process of C-O bond scission, but they could not be resolved by our mass spectra. CD_4(g) was formed through CD_x* combining with D*, and D₂O_(g) through either O* or OD* combining with D*. The observed CD₂O_(g) suggests that CH_xO* decreased above 180 K (Fig. 3e) through not only C-O bond scission but also desorption as CH₂O_(g). The formation of CD₂O_(g) and D₂O_(g) also rationalizes scarce O* remaining on the surface, as evidenced by vanishing O 1s signals above 200 K in the UHV PES experiments as well as absent O 1s signals in the NAP-PES spectra (300 K). The absence of CO_(g) and CO* indicates that the dehydrogenation of methanol-d₄ (or methanol) to CO, a process typically observed on Pt clusters or single crystals,^{22–24, 26, 30, 33–36, 42} did not occur on the surface Pt_uc sites of PtTe₂. The observed C* accordingly originated from dehydrogenated CH_x*.

The gaseous products also confirm that the reactivity and selectivity of Pt_uc sites depend on the Pt_uc concentration. Figure 5d shows the gaseous products (at methanol pressure 10^− 2 mbar) as a function of the Pt_uc concentration. For a Pt_uc concentration < 20%, the products increased generally with the Pt_uc concentration, which is consistent with the trend shown in Fig. 4a. The production of CD₂O_(g) dropped dramatically when the Pt_uc concentration increased above 20%, in agreement with the decrease of CH_xO* (Fig. 4a). The produced D_2(g) also decreased, indicating the selectively suppressed dehydrogenation to CH_xO*. In contrast, CD_4(g)/D₂O_(g) decreased only slightly at a Pt_uc concentration > 20%, as CD_x* increased at 180 K (Fig. 4a); the proportion of CD_x* desorbing as CD_4(g) could vary with the Pt_uc concentration.

DFT modeling. Our first-principles DFT modelling aimed to elucidate the key mechanisms behind the observed reactions. We established a Te divacancy model by removing two adjacent Te atoms at the topmost layer in order to mimic a PtTe₂ surface with a small Pt_uc concentration (5–10%). A divacancy site has five Pt_uc: one is coordinated to four Te, corresponding to two missing Te-Pt bonds in the bilayer structure (denoted as Pt_uc2), and the other four are coordinated to five Te (Pt_uc1). We considered three main reaction processes in the methanol decomposition: dehydrogenation from either oxygen (red arrows) or carbon (green arrow), and C-O bond scission (deoxygenation and dihydroxylation; black arrows), illustrated in Fig. 6. The adsorption configurations and energies of methanol and its decomposition fragments are presented in Figure S4. The modelling shows that a methanol molecule adsorbs on a Pt_uc site with an O-Pt binding configuration and an adsorption energy − 1.02 eV, which is evidently stronger than that (− 0.41 eV) on pristine PtTe₂ basal plane (Figure S4). The surface Pt_uc therefore has the potential to serve as a reactive center on PtTe₂. The three main decomposition processes at varied stages at the divacancy sites were calculated and compared. For the first step of decomposition, the barrier for dehydrogenation from oxygen (0.66 eV) of CH₃OH* was significantly smaller than those for dehydrogenation from carbon (1.42 eV) and dehydroxylation (1.82 eV). The comparison suggests the preferential formation of methoxy (CH₃O*), agreeing well with the observed selectivity that methanol decomposed via a pathway to produce more CH_xO* than CH_x* at a small Pt_uc concentration (Fig. 4a). The great difference between the energy barriers for desorption and dehydrogenation to CH₃O* (1.02 vs. 0.66 eV) also explains the great conversion probability for methanol adsorbed on the Pt_uc site (Fig. 4b). CH₃O* is expected to undergo further dehydrogenation (with a barrier 1.08 eV) to formaldehyde (CH₂O*), because of the considerably greater activation energies for the two competing processes, namely desorption (2.58 eV) and deoxygenation (2.29 eV). CH₂O*, due to its planar structure with sp² hybridized carbons, has an adsorption energy (− 1.52 eV) smaller than those of the other intermediates, that impedes subsequent decomposition. Compared to the greater barriers for its C-O bond cleavage (3.33 eV) or further dehydrogenation (to CHO*, 1.56 eV), CH₂O* would prefer desorption (as CH₂O_(g)) to decomposition at elevated temperature, as observed in our NAP-MS experiments (Figs. 5c, d). Alternatively, if H* from the dehydrogenation is nearby (not yet desorbed as H_2(g)), then CH₂O* could also combine with H* to form CH₂OH* (with a small barrier 0.12 eV), which provides a feasible pathway for further reactions. CH₂OH* would decompose via dehydroxylation to CH₂* and OH*, instead of dehydrogenation to CHOH* and H*; the latter process does not occur because the inverse process (CHOH* + H* → CH₂OH*) has a negligible barrier (< 0.01 eV) and CH₂OH* has a lower total energy. As the barrier for the dehydroxylation (1.27 eV) of CH₂OH* is evidently smaller than those for the C-O bond scission of CH₃O* and CH₂O* (2.29 and 3.33 eV), the observed CH_x species resulted from decomposed CH₂OH*. As a result, the observed CH_xO* species in our PES spectra correspond to CH₃O* and CH₂O* and the CH_x species mainly to CH₂* and CH₁*. The CH₂* species may undergo either further dehydrogenation to yield C* or combination with H* to produce CH_4(g), as observed in NAP-PES spectra (Fig. 5a) and NAP-MS spectra (Figs. 5c,d) respectively. Details of the calculated energy barriers are provided in our Supplementary Information (Figures S4 – S17).

After the C-O bond scission of CH₂OH*, the produced OH* either diffuses away from the active sites or combines with H* to desorb as H₂O_(g) (Figs. 5c,d), so its “poison effect” is insignificant. The calculated activation energy for OH* to migrate to an intermediate adsorption site on Te at the edge of divacancy amounts to 0.95 eV; their diffusion barrier on the basal plane is even smaller (0.36 eV), as shown in Figure S18. These diffusion barriers are smaller than that for the C-O bond scission of CH₂OH*, so with the progress of methanol decomposition, the produced OH* can diffuse readily to other sites to prevent the active sites from obstruction. We should note that the present model is not applicable on the PtTe₂ surface with a Pt_uc concentration > 10%, for which the generation of CH_xO* was suppressed but that of CH_x was sustained (Fig. 4). At a great Pt_uc concentration, the new Pt_uc structures could have an raised barrier for dehydrogenation to CH_xO* but promote the C-O bond scission of CH₂OH*, or even open a new channel for the C-O bond scission to yield CH_x.

The above modelling shows that Pt_uc in this divacancy model activates the decomposition in a coordinative manner; the adsorbates (CH₃OH*, its decomposition intermediates and fragments) are bonded mostly to two or three Pt_uc in varied decomposition processes (Figures S4 –S17). Nevertheless, Pt_uc2 appears to be more active than Pt_uc1, indicated by that the processes are mostly centered around Pt_uc2 in spite of varied reaction pathways (Figures S4 –S17). The Pt_uc at the PtTe₂ surface are like separated Pt single atoms, triangularly positioned and oxidized to different extents dependent on their bonding to Te. The coordination (to Te) number of Pt in PtTe₂ determines its electronic properties and hence catalytic properties. To illuminate the effect of the coordination number of Pt_uc on the activity, a tri-vacancy model with a Pt_uc3 in the middle of the tri-vacancy site, which was also likely formed on the Ar⁺-bombarded PtTe₂ surface, was introduced to model the reactions. The results show that the adsorption energies of CH₃OH*, CH₃O* and CH₂O* on the tri-vacancy site were slightly increased by 0.05 ~ 0.10 eV, and the energy barrier for dehydrogenation of CH₃OH* to CH₃O* remained similar, whereas that of CH₃O* to CH₂O* was reduced by 0.12 eV (Figures S19 - S21). Consequently, the formation of CH₂O* on this tri-vacancy became more probable. Reducing the coordination to Te enhances the activity of Pt. The densities of states of Pt_uc1−3 near the Fermi level reflect the same trend. We compare the local density of states (LDOS) of Pt_uc1−3 at PtTe₂ surface to that of Pt at Pt single-crystal surface and find that with the decreased coordination number, the Pt_uc1−3 at PtTe₂ surface become more metallic (Figure S22) and their d-band centers shift toward higher energies, shown in Fig. 7, indicative of enhanced catalytic reactivity.^{70, 71} The LDOS of Pt_uc1−3 at PtTe₂ surface and Pt at Pt(111) surface are evidently different (Figure S22), accounting for their different catalytic behaviors. Previous studies indicate that the spatial distribution and orientation of frontier orbitals of single-atom catalysts are well correlated with adsorption and catalytic activities.⁷² We note that the spatial distributions of frontier orbitals (in the energy ranging from − 0.25 eV to the Fermi level) of Pt_uc at PtTe₂ surface expanded with decreased coordination number, as plotted in the insets of Fig. 7. The expansion reflects a promoted probability of wave-function overlap, which is required for adsorption and catalytic activities,⁷² so agrees with the enhanced reactivity indicated above. Thus, the characteristic reactivity of the defective PtTe₂ arises from not only the peculiar structural (geometric) effect — triangularly positioned Pt_uc, but also the electronic effect — differently oxidized Pt_uc.

With STM, PES, NAP-PES, NAP-MS and DFT calculations, we studied methanol reactions on layered PtTe₂ under both UHV and NAP (up to 0.1 mbar) conditions. A structurally perfect PtTe₂ surface was inactive whereas the surface with Pt_uc sites, which were formed by removing surface Te using controlled Ar⁺ bombardment, became reactive toward methanol decomposition. Adsorbed methanol on the Pt_uc sites began to decompose at approximately 160 K and yielded CH_xO* (x = 2 and 3) as the main intermediate; CH_xO* either desorbed as CH₂O_(g) or decomposed further, via the transient formation and subsequent C-O bond scission of CH₂OH*, to produce CH_x* (x = 1 and 2). The reaction ultimately produced gaseous hydrogen, methane, water and formaldehyde at elevated temperature. Moreover, catalytic selectivity was dependent on the Pt_uc concentration. At a small Pt_uc concentration (≤ 10%), a reaction probability exceeding 90% was observed for methanol adsorbed on the Pt_uc sites ― approximately 60% of the methanol decomposed to CH_xO* and 35% to CH_x* at 180 K. With further increased Pt_uc concentration (> 10%), the reaction probability decreased, that is, the probability of decomposition to CH_x* remained similar whereas that to CH_xO* decreased. A new Pt_uc structure must have emerged at a greater Pt_uc concentration (greater Ar⁺ dosage) and altered the preferential reaction pathway. The structure-dependent catalytic properties were also supported by the gaseous products from the reaction under NAP conditions. We argue that the Pt_uc at small Pt_uc concentrations activated the reaction processes like single-atom catalysts and in a coordinative manner; their triangular positioning and varied degrees of oxidation were responsible for the observed characteristic reactivity. The results suggest that the PtTe₂ surface can serve as a superior catalyst toward methanol decomposition, with advantages of tunable catalytic reactivity and selectivity via the surface Pt_uc concentration.

Sample preparation. Single-crystal PtTe₂ was synthesized by the self-flux method. High‐purity Pt (99.99%) foils and a Te ingot (99.9999%) were mixed in a ratio of 1:17 and sealed in a quartz tube under vacuum and heated to 1000°C. After that, it was slowly cooled to 500°C at a rate of -3 to -5°C. The excess tellurium was subsequently separated by centrifugation. The grown bulk crystal has a diameter near 8.0 mm; it was cleaved in situ before each experiment.

Characterization. The UHV experiments were conducted in UHV chambers with a base pressure in the regime of 10^− 10 Torr. Ar⁺ bombardment was performed with an acceleration energy of 0.5 -2.0 keV and under a pressure of 5 × 10^− 6 Torr; 0.5 keV was chosen for the sample preparation in catalytic studies. STM images were acquired at 77 K in constant-current mode using an electrochemically etched tungsten tip. The PES and NAP-PES experiments were conducted at the TLS BL09A2 and BL24A beamlines, respectively, at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan.^{73, 74} For the former, the incident photon beam (with a fixed energy 410 eV) was normal to the surface and photoelectrons were collected (Scienta R3000) at an angle of 58° from the surface normal; for the latter, the beam (with a fixed energy 380 eV) was incident 56° from the surface normal and the analyzer (SPECS NAP 150), equipped with a 4-stages differential pumping system, was placed normal to the surface. The energy resolution was estimated to be near 0.1 eV and the BE was referred to the Au 4f core level at 84.0 eV of an Au substrate placed beside the investigated PtTe₂ sample. All spectra presented here were normalized to the photon flux. The gaseous reaction products were measured with NAP-MS, comprised of a quadrupole mass spectrometer (Hiden HAL201RC) and a doubly-differentially pumping system with a stainless steel tubing inlet terminated with an aperture (a diameter 0.75 mm) 3 mm from the sample. We used a full range gauge mounted on the reaction chamber to calibrate the ion current at a selected m/z ratio to the partial pressure of the corresponding product. The presented partial pressures of the gaseous products were derived by subtracting the background, which was collected from as-cleaved PtTe₂ surface (inert toward methanol decomposition), under the corresponding methanol pressure.

DFT calculations. First-principles calculations were performed by using Vienna ab initio simulation package (VASP),^{75, 76} in the framework of DFT with the projector-augmented wave (PAW) method⁷⁷ and the Perdew–Burke–Ernzerhof functional⁷⁸ type generalized gradient approximation exchange-correlation functional. For the 1T-PtTe₂ calculations, the energy cutoff was 330 eV and the energy convergence criterion was 10^− 4 eV. A 12 × 12 × 1 Γ-centered k-point sampling was performed for the Brillouin zone integration until the relative energies converged to a few meV. The optimized lattice constant was 3.91 Å, in good agreement with a previous report of 3.9 Å.⁷⁹ We included weak van der Waals (vdW) interaction between adjacent two bilayers using the recently developed SCAN + rVV10⁸⁰ correction method that yielded excellent geometric and energetic results at a reasonable computational cost. The 1T-PtTe₂ layered structure was then modeled by a 6 × 6 supercell with three bilayers of 1T-PtTe₂. The bottom bilayer was fixed and the remaining two bilayers were allowed to relax. The adsorption energy (E_ads) for each adsorbate was evaluated by the following standard formula: E_ads = E_A+S – E_S – E_A, where E_A+S, E_S, and E_A denote the total energies of the adsorbed system, the clean PtTe₂ layered system, and the chemical potentials of the corresponding adsorbed species in a gas phase, respectively. The energy barriers were determined using the climbing image nudged elastic band (CINEB) method.⁸¹

Acknowledgement

The National Science and Technology Council in Taiwan provided support (Grants No. NSTC 110-2634-F-009-027, NSTC 110-2112-M-A49-013-MY3, NSTC 110-2112-M-A49-022-MY2, NSTC 109-2112-M-018- 008-MY3, and MOST 109-2112-M-008-030-MY2) for this work. This work is also partially supported by the “Center for the Semiconductor Technology Research” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan. CPU time at Taiwan’s National Center for High-performance Computing (NCHC) is greatly appreciated. We also thank Drs. Hung-Wei Shiu and An-Cheng Yang for their technical support.

Author contributions

M.-F. L., C.-L. L. and J.-P. C. conceived and supervised the project. J.-W. H. (and L.-H. K.) performed measurements of PES, NAP-PES, NAP-MS and RHEED, and analyzed the data. Y.-L. L., B.-H. L., C.-H. W. and Y.-J. H. assisted the synchrotron-based experiments. C.-N. K. and C.-S. L. prepared the TMDs samples. W.-H. C., Q.-Y. Z. and J.-Y. C. performed STM characterization. P.-H. C. and J.-P. C. performed DFT calculations. The text was initially composed by J.-W. H. and M.-F. L., with input from all coauthors. All authors further contributed to the discussion of the experimental work and the final version of the manuscript.

Competing interests

The authors declare no competing interests

Additional information

Supplementary information The online version contains supplementary material available at

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Dependence on Under-Coordinated Pt at the Surface of Layered PtTe2 in the Decomposition of Methanol

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