# Towards the Operational Window for Nitridic and Carbidic Palladium Nanoparticles for Directed Catalysis

Lucy G Costley-Wood, Khaled Mohammed, Marina Carravetta, Donato Decarolis, Alexandre Goguet, Apostolos Kordatos, Reza Vakili, Haresh Manyar, Erin Mcpake, Chris-Kriton Skylaris, et al.

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## To cite this version:

Lucy G Costley-Wood, Khaled Mohammed, Marina Carravetta, Donato Decarolis, Alexandre Goguet, et al. Towards the Operational Window for Nitridic and Carbidic Palladium Nanoparticles for Directed Catalysis. ChemCatChem, 2023, 15 (21), pp.e202300870. 10.1002/cctc.202300870. hal-05078017

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# Towards the Operational Window for Nitridic and Carbidic Palladium Nanoparticles for Directed Catalysis

Lucy G Costley-Wood,[a] Khaled Mohammed,[b] Marina Carravetta,[b] Donato Decarolis,[c, d]
Alexandre Goguet,[e] Apostolos Kordatos,[b] Reza Vakili,[e] Haresh Manyar,[e] Erin McPake,[a]
Chris-Kriton Skylaris,[b] Paul Thompson,[f] Emma K Gibson,[*a] and Peter P. Wells*[b, d]

The reactions under which interstitial structures of Pd form are
profoundly important and prevalent in catalysis; the formation
and stability of Pd hydride structures are well understood,
however, interstitial structures of the carbide and nitride are
relatively under explored. This work reports a systematic study
of the formation and stability of PdCₓ and PdNₓ at elevated
temperatures and different atmospheres using in situ Pd L₃
edge XANES spectroscopy. These studies were further comple-
mented by the application of ¹⁴N MAS-NMR experiments and
computational DFT investigations. The experiments confirmed

that PdCₓ was significantly more stable than PdNₓ; ¹⁴N MAS-
NMR provided direct confirmation on the formation of the
nitride, however, the XANES studies evidenced very limited
stability under the conditions employed. Moreover, the results
suggest that the formation of the nitride imparts some
structural changes that are not entirely reversible under the
conditions used in these experiments. This work provides
important insights into the stability of interstitial structures of
Pd and the conditions in which they could be employed for
directed catalytic processes.

Introduction

Supported Pd nanoparticles (NPs) underpin both existing and
emerging industrial catalytic processes; from exhaust emission
control technology[1,2] to directing the selective upgrading of
bioderived feedstocks[3-6] to value-added chemicals.[7-10] How-
ever, these supported Pd NPs are dynamic, and are known to
undergo significant changes under reaction conditions.[11-16] For
example, the formation of new bulk and subsurface phases,
which have profound consequences on the catalytic
performance.[17-21]

Palladium carbide can be formed on exposure to CO and some hydrocarbons at elevated temperatures.[12,14,15,22-24] Carbon containing feed molecules adsorb onto the palladium, disociate, and the carbon atoms are inserted into the FCC lattice of the reduced palladium particle.[12,25-28] DFT calculations by Garcia-Mota show carbide formation at surface step sites first.[29] Gradual dissolution of this carbon through the lattice initially causes a subsurface and eventually a bulk carbide like phase.[12,29] For alkyne semi-hydrogenation reactions, this PdCₓ phase increases selectivity to the alkene, by inhibiting over-hydrogenation to the alkane.[12,13,18,22,29] This effect on selectivity is multifaceted. Firstly, the top most layers prevent hydrogen from populating the subsurface.[13] In addition, the mobility of existing dissolved hydrogen through a carbide phase to the surface is decreased.[22,12] Finally, the carbide phase increases the energy barrier of adsorption of further hydrocarbons from the feed.[29] At low conversions, a surface poisoning effect of the alkyne is also responsible for high selectivities.[18] Some demonstrations of this improved selectivity include the semi-hydrogenation of acetylene, propyne and 1-pentyne.[12,22,28,29]

Palladium hydride is also formed under reactive conditions,
and has shown to influence selectivity.[13,15,22] In a similar manner
to the formation of the carbide, the hydride is also formed by

[a] L. G Costley-Wood, E. McPake, Dr. E. K Gibson
School of Chemistry
University of Glasgow
Glasgow, G12 8QQ (UK)
E-mail: emma.gibson@glasgow.ac.uk

[b] Dr. K. Mohammed, Dr. M. Carravetta, Dr. A. Kordatos, Prof. C.-K. Skylaris, Dr. P. P. Wells
School of Chemistry
University of Southampton
Southampton, SO17 1BJ (UK)
E-mail: p.p.wells@soton.ac.uk

[c] Dr. D. Decarolis
Cardiff Catalysis Institute
School of Chemistry
Cardiff University
Cardiff, CF10 3AT (UK)

[d] Dr. D. Decarolis, Dr. P. P. Wells
UK Catalysis Hub
Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell, Didcot, OX11 0FA (UK)

[e] Prof. A. Goguet, Dr. R. Vakili, Dr. H. Manyar
School of Chemistry and Chemical Engineering
Queen's University Belfast
Belfast, BT9 5AG (UK)

[f] P. Thompson
XMaS UK CRG Beamline
ESRF – The European Synchrotron
71, avenue des Martyrs, 38000 Grenoble (France)

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202300870

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This publication is part of a Special Collection on "Advanced Characteriza-
tion Techniques in Catalysis"

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© 2023 The Authors. ChemCatChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
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the adsorption, dissociation, and insertion of hydrogen from the feed gas into reduced Pd nanoparticles.[15] Formation of the hydride is not only a function of reaction mixture stoichiometry but can also be influenced by the prehistory of the sample as evidenced by Teschner.[15] Under the same conditions where the carbide phase can form, the hydride phase can be promoted by increasing the H₂ partial pressure in the feed.[13,22] In contrast to PdCₓ, the PdHₓ increases selectivity to the alkane by increasing the availability of dissolved hydrogen.[12-14,22,28] However, the presence of the hydride is not a requirement for total hydrogenation of the alkyne to occur.[18] Furthermore, hydrogen can be present in the subsurface of the Pd NP from surface incorporation of carbonaceous deposits which contain hydrogen, not only from the feed mixture.[14]

The formation of bulk phase crystallographic palladium nitride has been attempted as far back as 1952, but successful synthesis and characterization of the interstitial structure comes from much more recent work.[30] Dann used X-ray absorption spectroscopy (XAS) at the Pd K and L₃ edges to follow the formation of a palladium nitride phase on a Pd/Al₂O₃ catalyst.[31] The PdNₓ was formed *in situ* in a flow of NH₃ and O₂ in He, and also under NH₃ in He, at temperatures < 200 °C. Activity measurements by online mass spectroscopy (MS) showed that for the oxidation of ammonia, this PdNₓ phase is responsible for high N₂ selectivity.[31]

Following this, Decarolis performed spatial profiling of the same catalyst during ammonia oxidation, using *operando* XAS at the Pd K edge and online MS to look at the structure-activity relationship during the reaction.[32] Enhanced selectivity to N₂ was observed when the PdNₓ species was present, agreeing with the previous study. Under these conditions, multivariate curve resolution (MCR) analysis indicated a PdNₓ phase, but also a bulk Pd(0) phase, suggesting the nitride structure may be limited to the surface and subsurface regions.[32]

The main technique used to study the nitride has been XAS at either the Pd K or L₃ edges.[31,32] XPS has also proved successful, and work by Dann was able to identify reduced nitrogen arising from bulk metal nitride at a different energy to the surface adsorbed NH₃ species.[31] Another method able to distinguish adsorbed and bulk nitrogen is solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). Nitrogen has two isotopes which are NMR active. ¹⁴N has a natural abundance of 99.6% and an integer nuclear spin of 1, hence it has a nuclear quadrupolar interaction (NQI). Despite its low natural abundance, most NMR studies prefer ¹⁵N, by means of expensive isotopic labelling, or with the use of dynamic nuclear polarization to increase the signal intensity. This is because ¹⁵N is spin ¹/₂ and therefore is not subject to the significant line broadening often associated with the NQI, which is often a major drawback in organic solids. In our target materials, nitrogen can form nitrides on the surface or in interstitial positions of the Pd nanoparticle. These two scenarios will result in variation in chemical shift, δ, which may be moderate, and variation on the local symmetry about the nitrogen, which will affect the NQI significantly. While ¹⁵N NMR will only reveal chemical shift variations, ¹⁴N NMR is much more powerful in this context as it is likely to allow discrimination

between surface and interstitial nitrides. Moreover, metal nitrides are particularly amenable to ¹⁴N NMR investigation as they have been reported to be often associated with sharp, well resolved signals as the nitrides occupy highly symmetric sites.

Clearly, the formation of Pd interstitial structures plays an important role in the performance of Pd catalysts.[17-18,20,28] So far, this influence on selectivity is limited to reactions under which conditions these structures can form and are stable. There is potential however to take advantage of these structures further and look at their formation as a pre-treatment in other common catalyzed reactions. In this study, we have used *in situ* XAS at the Pd L₃ edge to observe the formation of each of the palladium hydride, carbide and nitride on a Pd/ Al₂O₃ catalyst. We have probed the stability of the carbide and nitride by exposing them to both inert and reducing atmos- pheres over a temperature range. Furthermore, solid state NMR calculations as well as MAS-NMR have been used to probe the nitrogen environment directly in Pd nitride, confirming the presence of interstitial nitrogen. Finally, we have applied Density Functional Theory (DFT) modelling to calculate the energy barriers for the decomposition of Pd nitride, via different mechanisms, and NMR electric field gradients.

## Results and Discussion

### Comparing interstitial structures

All experiments were performed using the same 1.5 wt % Pd on γ-alumina catalyst, that was prepared by standard incipient wetness impregnation of palladium nitrate, as described by Decarolis *et al.*[32] The preparation route yields well-dispersed particles with an average particle diameter of 2.1 nm.[31] Initially, the catalyst was exposed to three different sets of conditions to form palladium nitride, carbide and hydride, each on a fresh catalyst following a reductive pre-treatment; in brief, carbide was formed by exposure to C₂H₄/He at 200 °C, nitride was formed with NH₃/He at 100 °C, and hydride was produced by exposing the catalyst to H₂/He at 35 °C. These experiments were performed *in situ* using a dedicated sample environment developed by the BM28 team at the ESRF and the resultant spectra collected at the Pd L₃ edge.[33-34] The L₃ edge (3.2 keV) was chosen for this experiment, rather than the more accessible K edge (24.3 keV). This softer edge, involving an electronic transition from a 2p to 4d state, is more sensitive to interstitial structures, with clear differences to the position and intensity of the white line on their formation.[12,31] For the hydride, an additional peak is also present at 3182 eV.[12,18,28,31]

Figure 1a) shows the spectra of all species, to our knowl- edge this is the first time all three interstitial structures have been formed and characterised on one sample. All species show a shift in energy of the white line compared to the metallic palladium foil. The shift is + 0.63 eV for PdHₓ, +0.73 eV for PdNₓ and +0.92 eV for PdCₓ. This indicates structural changes to the palladium due to the incorporation of heter- oatoms into the metal lattice.[31] The hydride also shows the additional characteristic peak, generated by an antibonding
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 1.** Pd L3-edge XANES spectra of: a) Pd(0), PdHₓ, PdNₓ and PdCₓ, formed on a 1.5 wt% Pd/γ-Al₂O₃. b) PdNₓ as pre-reduced Pd(0), nitrided PdNₓ, and heated under H₂/He from 100–120 °C. c) Whiteline energy of species in figure d as a function of increasing temperature under He. Energy of assumed state at start, PdNₓ, and end, Pd(0), are included. d) PdNₓ as pre-reduced Pd(0), nitrided PdNₓ, and heated under He only from 100–180 °C. e) PdCₓ as pre-reduced Pd(0), carbided PdCₓ, then heated under H₂/He from 100–220 °C. f) PdCₓ as pre-reduced Pd(0), carbided PdCₓ, then heated under He only from 100–200 °C.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

Pd–H state formed from the overlap of a Pd *d*-orbital with a H
s-orbital.[28] Interestingly, the spectra for PdNₓ and PdCₓ are
almost identical at this energy. In the previous Pd L₃ inves-
tigation by Dann *et al.*, that assessed the evolving nature of Pd
NPs during selective ammonia oxidation, the white line
intensity of the nitride was greater than that reported here.[31]
However, in the earlier work the Pd nitride spectrum was
measured under ammonia oxidation conditions, and the
increased white line intensity is most likely a consequence of a
limited amount of oxygen adsorption. In fact, Decarolis
identified a small PdO component to the bulk nitride phase in
the work described above, where PdNₓ was formed under
identical conditions to this study.[32]

Formation of palladium hydride, PdH<sub>x</sub>

Formation of PdHₓ was achieved by cooling reduced Pd(0) to
35°C under hydrogen. Whilst not investigated in this experi-
ment, literature on the subject indicates the PdHₓ decomposi-
tion temperature lies around 75 °C.[35] Furthermore, Nag found a
correlation between structure, composition and decomposition
temperature. The temperature range was found to be 55–75 °C,
with temperature increasing as a result of both increasing Pd
crystallite size and stoichiometric amount of hydrogen.[36]

Formation and stability of palladium nitride, PdN<sub>x</sub>

To form palladium nitride, the catalyst was first reduced to Pd(0) in hydrogen, then exposed to 5% NH₃/He at 100°C for 1 h. To test the thermal stability, the gas was then switched to either diluted hydrogen or helium, and the temperature increased in 20°C increments. The spectra obtained are given in Figure 1b) for H₂/He and 1d) for He.

Under reducing conditions, the nitride phase is lost
immediately. The broadening of the white line is lost, however
a small residual energy shift of around 0.3 eV remains. The
stability is higher in helium, and the intensity and position of
the white line begin to shift back towards that of Pd(0) as
temperature is increased, indicating the decomposition to the
reduced metal. Initial decomposition and loss of lattice nitrogen
is rapid, and the shift towards Pd(0) is already substantial by
120 °C. The decomposition is more gradual as the temperature
is increased further, reaching a plateau from 160 °C, Figure 1c).

After decomposition of PdNₓ the position of the maximum
whiteline intensity did not return to the value associated with
Pd(0). This change was found for decomposition processes in
both H₂/He and He. These residual changes to the Pd NPs
following decomposition may indicate legacy effects, and
would be particularly important as Pd is often used as a catalyst
in processes where the conditions for nitride, carbide and/or
hydride formation can be experienced during operation.
However two separate previous studies have shown a metallic
Pd⁰ bond distance can be recovered following PdNₓ formation.
Decarolis et al. found that, whilst the XANES had features of the
PdNₓ, the EXAFS were consistent with Pd⁰, suggesting the

remaining nitride is limited to the surface region.[32] Similarly
Dann *et al.* were able to achieve reproducible metallic XAFS
spectra after *operando* catalytic investigations of the nitride by
following a reduction, oxidation then reduction pre-
treatment.[31] Therefore it is likely that the shift in whiteline
position of Pd⁰ observed here also relates to surface and
subsurface nitrides, rather than irreversible change to the NP
structure.

The potential influence of particle size warrants further discussion. The structural properties of PdHₓ, specifically the lattice size of the α and β hydride phases, depend on Pd NP size.[37,38] Size effects have also been shown for PdCₓ, although only inferred via effects on the conversion and activity of reactions proposed to proceed via *in situ* formation of a PdCₓ phase.[20] The studies by Tew *et al.*, where PdCₓ formation was observed *in situ* by XAS for samples with a range of Pd particle size, found no correlation.[12,18] Whilst no comprehensive study of size effects PdNₓ formation has yet been undertaken, successful PdNₓ formation was achieved by Dann *et al.* on both a Pd/γ-Al₂O₃ sample and Pd/zeolite-Y, with particle sizes 2.1 and 5.2 nm respectively.[31] Therefore within the small particle size range typically required for catalytic application, no significant particle size effects are expected.

Formation and stability of palladium carbide, PdC<sub>x</sub>

To form palladium carbide, a fresh sample of catalyst was
reduced, then exposed to 5% C₂H₄/He at 200°C, and cooled
under He. For the stability tests, again the gas was switched to
either diluted hydrogen or helium, and the temperature
increased in 20°C increments, to a higher maximum temper-
ature given the higher stability observed. The spectra obtained
are given in Figure 1e) for H₂/He and 1f) for He.

Under hydrogen there was an initial loss in whiteline
intensity at 100 °C, with the maximum of the whiteline intensity
shifting to lower energies from 140 °C. However, the data
remain broadly consistent with PdCₓ throughout, with no
indication of the complete decomposition of PdCₓ to Pd(0).
Under He, the features associated with PdCₓ were maintained
throughout the study, to the maximum temperature of 200 °C.
Ziemecki proposed a PdCₓ decomposition temperature of
>600 °C in an inert atmosphere, and of around 150 °C under
hydrogen or oxygen.[26] Furthermore, Tew found the palladium
carbide phase to be sufficiently stable that under high partial
pressures of hydrogen only partial decomposition was
observed.[28] The energy shift of the maximum white line
intensity, which is observed from 140 °C onwards in the
presence of hydrogen, is consistent with a partial decomposi-
tion of Pd carbide.

Stability comparison

The incorporation of boron into palladium as an interstitial
lattice structure has been considered previously for enhancing
the selectivity in alkyne hydrogenation, as an alternative to
Lindlar's catalyst.[21,39-43] As part of the that study, Chan investigated the stability of the PdBx phase as a function of temperature and gas atmosphere.[21] Whilst PdBx was formed and tested under different conditions on a different sample, the grouping of boron, carbon and nitrogen in the periodic table, and the conclusions that might be drawn from comparing the three, should not be ignored. Values were also found for palladium hydride under H₂/He from Nag.[36]

Under inert gas (He for the carbide and nitride, N₂ for the boride) the thermal stability of the three phases increases in the order PdNₓ\*, PdCₓ\*, PdBₓ\*, with decomposition temperatures of 120, 200 + and 400 + °C respectively (* indicates values originate from this experiment). The same trend occurs under hydrogen; stability increases in the order PdHₓ\*, PdNₓ\*, PdCₓ\*, PdBₓ\*, decomposition temperatures 75, ≤ 100, 140 and 200 + °C respectively.

Prior to this work, it was suggested that the stability difference between the boride and the carbide originates from the mixing between the d orbitals of the Pd lattice and the s-p orbitals of the heteroatom. A larger dopant atom would have a higher degree of orbital interaction. The atomic radius increases in the order H < N < C < B, and combining our results with literatures completes the set, giving an order of stability of PdNₓ < PdCₓ < PdBₓ. There are likely to be other factors which need to be considered when comparing these dopants, and the orbitals which overlap will not be equivalent for H as for B, C and N, but regardless, these results confirm a correlation between atomic size and stability of the bulk phases.

## Solid state NMR

¹⁴N MAS NMR was used to detect the nitrogen atoms within the supported Pd nanoparticles. The nitrides were formed following the same treatment steps as for the XANES experiment, but instead using 0.5% NH₃/He. Figure 2 shows a ¹⁴N NMR band at ~324.6 ppm for the nitrided samples. Nitrogen is spin-1, and a

<figure><img src="image_12.png" /><figcaption>**Figure 2.** Solid-state ¹⁴N NMR spectra (1 s delay) obtained for 1.5 wt% PdNₓ/Al₂O₃ and 1.5 wt% Pd/Al₂O₃ blank (with no nitride) measured under the same conditions. Spectra of ZrN with main band at 400 ppm and sidebands (denoted with \*) used as a reference for similar nitride systems. Vᵢ = 9 kHz for all spectra, and number of scans (NS) was 404k (PdN/Al₂O₃), 353k (Pd/Al₂O₃), and 16 (ZrN). Intensity scale is shown, and spectra are stacked for clarity.</figcaption></figure>

sharp signal can only be obtained for nitrogen in a highly symmetrical environment. Electric Field Gradient (EFG) calculations were performed through CASTEP, and NQI assignments agree with those stated above (Figure SI 1).

The NMR experiments on the Pd nitride particles show clearly just one significant signal. The width of the signal reflects small variation in local environment, mostly. There is no evidence of any spinning sidebands, suggesting this is largely originating from interstitial sites. Surface sites may be present in much smaller quantities, beyond detection under the experimental conditions considered here. This technique has been successfully used previously to confirm the integration of boron into a Pd lattice, PdBₓ, and to distinguish it from surface boron, using ¹¹B MAS NMR.[21]

## DFT modelling of PdNₓ formation and decomposition

The above absorption experiments demonstrate the lower stability of the palladium nitride phase compared to the carbide. To try and understand these results further, a DFT computational study was performed. The aim was to compare different mechanisms of Pd nitride decomposition, and hence find the lowest energy route to each.

For this study the truncated octahedral Pd₃₈ NP was used, which involves the most common facet [111] of Pd and finite size effects that cannot be captured by slab models. The energy of reaction between different configurations is calculated using the formula in Equation (1):

$$E_{\text{Reaction}} = E_{\text{Products}} - E_{\text{Reactants}} \quad (1)$$

where EProducts corresponds to the relaxed structure of products and EReactants to the relaxed structure of the reactants. For the PdNₓ decomposition in an inert atmosphere, the migration of interstitial N to bound surface N has a reaction energy (EReaction) of -84 kJmol⁻¹, with an activation energy (EAct) of 33.8 kJmol⁻¹, suggesting that N will eventually migrate and reside on the surface of the NP. Considering the formation of a surface N₂ molecule from two monoatomic nitrogen atoms, each nitrogen atom of the product will be bonded to one palladium atom whilst a predicted EReaction of -94.4 kJmol⁻¹ for the final N₂ configuration is calculated with an energy barrier EAct equal to 166.1 kJmol⁻¹. Therefore, the formation of molecular nitrogen from atomic nitrogen on the surface is energetically favoured. The energy of desorption of the N₂ molecule is 19.3 kJmol⁻¹ defined in Equation (2):

$$E_{\text{Desorption}} = -E_{\text{Pd38/N}_2} + (E_{\text{Pd38}} + E_{\text{N}_2}), \quad (2)$$

where EPd38/N₂ is the energy of the relaxed system of Pd₃₈ including one N₂ molecule, EPd38 is the energy of the perfect NP and EN₂ the energy of the N₂ molecule. The proposed mechanism for the decomposition of PdNₓ and subsequent formation of molecular nitrogen is displayed visually in Figures 3 and 4, with associated energy values in Table SI 1.
<figure><img src="image_13.png" /><figcaption>Figure 3. N₂ formation on the [111] facet of Pd₃₈ showing the (a) initial, b) transition, and c) final states. Green spheres correspond to Pd atoms and blue spheres to N atoms.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 4. Reaction profile for the formation of N₂ on the [111] facet of Pd₃₈.</figcaption></figure>

For the decomposition of PdNₓ in a reducing hydrogen atmosphere there is the alternative that, upon migration to the surface of the NP, atomic nitrogen could gradually bind with atomic hydrogen and desorb as an ammonia molecule. DFT calculations show that the formation of ammonia from surface chemisorbed nitrogen and hydrogen atoms is energetically favoured. The reaction is modelled through three separate steps, Equations (3)–(5):

$$N + H \rightarrow NH \quad (3)$$

$$NH + H \rightarrow NH_2 \quad (4)$$

$$NH_2 + H \rightarrow NH_3 \quad (5)$$

As shown in Figure 5, each separate reaction step is an exothermic process with the lowest Eₐct = 100.1 kJmol⁻¹ corre-

<figure><img src="image_15.png" /><figcaption>Figure 5. Reaction profile for the formation of NH, NH₂ and NH₃ on the [111] facet of Pd₃₈.</figcaption></figure>

sponding to the formation of NH. The associated values of the activation energies are in good agreement with previously reported results on the dehydrogenation of NH₃ on Pd (111) surface, however to our knowledge, the relevant mechanisms on Pd NPs have not yet been investigated.[44–45] Overall, the relaxed structures for each step of the reaction show that the minimum energy configurations for NH₂ and NH₃ are across the edges between [111] facets (Figure 6). Additionally, the bonding of the molecule with surface Pd atoms is reduced as the reaction proceeds along the reaction path showing that NH will bind with three Pd atoms (hollow), NH₂ will bind with two Pd atoms (bridge) and NH₃ with one Pd atom (top).[46] Again, the tabulated energy values corresponding to the each proposed mechanistic step are given in Table SI2.

Once NH₃ is formed we assume that it will be desorbed from the Pd₃₈ surface. The desorption energy for a single NH₃ molecule corresponds to 132.2 kJmol⁻¹ defined by Equation (6):

$$E_{\text{Desorption}} = -E_{\text{Pd}_{38}/\text{NH}_3} + (E_{\text{Pd}_{38}} + E_{\text{NH}_3}). \quad (6)$$

where Eₚd₃₈/ NH₃ is the energy of the relaxed system of Pd₃₈ including one NH₃ molecule, Eₚd₃₈ is the energy of the perfect NP and EₙH₃ the energy of the NH₃ molecule.

The complete energy of reaction for the formation of one NH₃ molecule on Pd₃₈ is –84.1 kJmol⁻¹ which is less exothermic than the associated value for N₂ formation (–94.4 kJmol⁻¹). However, the higher activation energy barrier for the formation of N₂ on the [111] facet is expected to limit the reaction process and it will be more favourable for Nitrogen to bind with Hydrogen to form NH. The calculated energy values for the

<figure><img src="image_16.png" /><figcaption>Figure 6. In order, initial, transition, and final states for the formation of (a)–(c) NH (d)–(f) NH₂ and (g)–(i) NH₃ on the [111] facet of Pd₃₈. Green spheres correspond to Pd atoms, blue spheres correspond to N atoms and white spheres to H atoms.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

nitride decomposition agree with the observations of the XANES experiment, that bulk Pd nitride is unstable.

## Conclusions

X-ray absorption spectroscopy experiments have been combined with solid state NMR and computational modelling in this benchmarking experiment on reaction selectivity controlling Pd species. These initial investigations are the first on a path towards utilizing activated species to improve selectivity in reactions where they do not spontaneously form.

PdCₓ, PdNₓ and PdHₓ have been characterized using soft X-rays. Whilst PdHₓ has a distinctive spectral feature, those arising in PdCₓ and PdNₓ are closely matched, suggesting the changes caused to the Pd lattice by their incorporation are very similar. XANES measurements showed PdCₓ to be the most stable as a function of temperature and atmosphere. PdNₓ however, decomposed immediately on exposure to hydrogen, and began to rapidly decompose under He from 100°C. Residual differences remained between the decomposed PdNₓ and Pd(0), suggesting irreversible structural changes. The presence of an NMR signal at 324.6 ppm using ¹⁴N confirms that nitrogen is incorporated into the bulk Pd lattice on formation of PdNₓ, not simply adsorbed to the Pd surface.

The decomposition of this nitride requires the breaking of Pd–N bonds and migration of single nitrogen atoms from bulk to surface. From there, we propose the nitrogen desorbs as molecular N₂ in inert atmospheres, and as either N₂ or NH₃ in the present of H₂ gas. Modelling of these mechanisms using DFT suggests a higher barrier to desorption of NH₃ than of N₂, however the activation energy for formation of N₂ is higher and so will likely limit this process.

## Experimental Section

**XANES.** Pd L₃ edge (3175 eV) XANES experiments were performed in fluorescence mode at the XMaS beamline on BM28 at the European Synchrotron Radiation Facility (ESRF). The beam was conditioned using an LN2 cooled Si(111) monochromator, a primary focussing toroidal platinum coated mirror with an angle of incidence of 2.5 mrad and two harmonic rejection mirrors, using stripes of Chromium and Silicon at 3.5 mrad angle of incidence. Both the fluorescence detector and incident beam monitor used were Ketek Silicon drift diodes, fed into a Mercury XIA digital signal processor. The incident beam monitor consists of an in vacuum silicon drift detector exposed to the scatter of the incident X-ray beam from a kapton foil.[33] The *in situ* reactor cell capable of both transmission and fluorescence measurements is described elsewhere (Figure S12).[34] The cell was mounted vertically in a helium filled stainless steel chamber to reduce absorption of X-rays by air.[34]

XANES data were acquired in step scans from 3155 to 3200 eV. XAS data was collected continuously during each experiment with each scan taking approximately 11 minutes. A reference Pd foil spectrum was acquired periodically during the course of the experiment. The spectra were merged, and analysed using Athena from the Demeter IFEFFIT package.[47]

**Synthesis.** A 1.5% Pd/γ-Al₂O₃ catalyst, prepared by incipient wetness impregnation, was used for all experiments. Prior to the formation of the carbide, nitride and hydride phase, samples were reduced by heating to 120 °C in 3% H₂/He at 25 ml/min for 30 minutes. To form the carbide, the reduced sample was heated to 200 °C in He and held for 10 minutes for temperature stabilisation. 5% C₂H₄/He was flowed at 40 ml/min for 30 min, sample was cooled to 100 °C under reaction gas then held under He. The signal:noise ratio for the carbide data is lower than for the nitride and hydride. The higher carbide formation temperature led to structural issues with the *in situ* cell, and disturbed the window for X-ray transmission. To form the nitride, the reduced sample was held under 40 mL/min 5% NH₃/He at 100 °C for 1 h. To form the hydride, the reduced sample was cooled to 35 °C in 3% H₂/He and held for 30 min. All ramp rates were 5°/min.

For the stability tests, the formed PdNₓ and PdCₓ were heated from 100°C in either He or 3% H₂/He in 20°C increments, allowing 10 min for temperature stabilisation at each temperature. Temperatures were increased until phase decomposition was observed to a maximum of 220°C.

**MAS NMR.** NMR experiments were performed on a Bruker Avance Neo widebore Spectrometer, on a 9.4 T wide-bore magnet, using a 5 mm PhoenixNMR probe, tuned to ¹⁴N in double resonance mode, with special-made plugins for this application. The scale was calibrated using ammonium chloride as indirect reference, with the ¹⁴N peak assigned to 39.3 ppm (compatible with liquid ammonia at 0 ppm).[48] Unless otherwise stated, NMR experiments on ¹⁴N consisted of 404000 acquisitions with a flip angle of 30 degrees and a pulse delay of 1 s. Each sample consisted of 95 mg of solid material packed in pencil-style zirconium oxide rotors.

**DFT simulations.** DFT calculations on the formation of NH₃ and N₂ have been performed with the linear-scaling DFT code ONETEP.[49] Radii of 9.0 Bohr were used for the localized non-orthogonal generalized Wannier functions (NGWFs). A kinetic energy cut-off of 800 eV has been applied for the p-sinc basis-set. The (PBE+D2) exchange correlation functional was used and core electrons were described by norm-conserving relativistic pseudopotentials. The electronic structure has been treated as metallic using the Ensemble-DFT (EDFT) approach with a Fermi-Dirac smearing of 0.1 eV.[50] Geometry optimizations have been performed through the BFGS algorithm. Activation energies have been investigated through the Linear Synchronous Transition / Quadratic Synchro- nous Transition (LSTQST) protocol.[51] Solid state NMR calculations have been performed through the plane-wave DFT code CASTEP.[52] Geometries have been initially relaxed in vacuum using EDFT with a Fermi-Dirac smearing of 0.1 eV whilst the EFG investigation towards the NQI for interstitial and surface PdNₓ have been performed thereafter. A kinetic cut-off of 800 eV has been applied within the PBE+D2 scheme with norm-conserving pseudopotentials.

**NMR simulations.** SIMPSON spin dynamics calculations were performed to simulate the effect of MAS on the observed signals using the NQI provided by CASTEP.[53,54] Simulations use 10 γ angles, 2000 α, β angles according to the REPULSION scheme, a spin rate of 9 kHz, a non-ideal excitation pulse and a dead time equal to the rotor period, a null chemical shift tensor.[55] The NQI parameters (anisotropy and asymmetry) were taken from the magres CASTEP output file.

The data that support the findings of this study are openly available via the University of Southampton DOI service at https://doi.org/10.5258/SOTON/D2749.
<figure><img src="image_18.png" /></figure>

Acknowledgements

XMaS is a UK national research facility supported by EPSRC. We
are grateful to all the beamline team staff on BM28 for their
support, particularly Paul Thompson. Beamtime award number
CH6009. Financial support is acknowledged to the EPSRC (EP/
V000691/1) and the UKCP consortium (EP/P022030/1) for access
to the ARCHER2 supercomputer. Also to the Materials and
Molecular Modelling Hub (EP/T022213/1) and for access to the
Young supercomputer, and the University of Southampton for
access to the Iridis5 supercomputer. Donato Decarolis and Peter
Wells acknowledges funded received through the UK Catalysis
Hub and the UK Catalysis Hub is kindly thanked for resources
and support provided via our membership of the UK Catalysis
Hub Consortium and funded by EPSRC grant: EP/R026815/1,
EP/R026939/1, EP/R026645/1 or EP/R027129/1.

Conflict of Interests

The authors declare no conflict of interest.

Keywords: DFT modelling · heterogeneous catalysts ·
nanoparticles · operando spectroscopy · palladium nitride

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Manuscript received: July 12, 2023

Revised manuscript received: August 23, 2023

Accepted manuscript online: September 7, 2023

Version of record online: October 6, 2023