Mechanistic Switch between Oxidative (Andrussow) and Nonoxidative

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Mechanistic Switch between Oxidative (Andrussow) and Nonoxidative (Degussa) Formation of HCN on Pt(111) by Density Functional Theory Jaime Gomez-Díaz and Nuria Lopez* Institute of Chemical Research of Catalonia, ICIQ, Avgda. Països Catalans 16, 43007 Tarragona, Spain ABSTRACT: We have investigated the full reaction path leading to the formation of HCN from ammonia and methane on Pt(111) by means of density functional theory. Industrially, the reaction can take place under oxidative or nonoxidative conditions, and thus, we have described the effect of oxygen in the reaction mechanism. DFT calculations show that the reaction network is very complex, including dehydrogenation steps, HxC-NHy (x = 0-2, y = 0-2) couplings, de/ hydrogenation of C-N-containing species, and isomerization. Under nonoxidative conditions (Degussa process), the main reaction path for C-N formation takes place through partially hydrogenated compounds, in particular, those coming from HxC þ NH2 (x = 0, 1) coupling with subsequent dehydrogenation of the resulting intermediate. Under oxygen-rich conditions (Andrussow), the main C-N bond formation step switches to HC þ N or C þ N. The mechanistic switch between oxidative and nonoxidative conditions is driven by the change in the relative stability of the intermediates, induced by the presence of oxygen and the lack of free surface H atoms. Our calculations clarify previous observations where different mechanisms were described.

1. INTRODUCTION HCN is a major chemical intermediate employed in the preparation of several polymers, such as nylon and acrylamides, and an ingredient of the primordial soup.1 Industrially, HCN is obtained mainly via the Andrussow process,2 where methane and ammonia are oxidized at high temperatures (∼1400 K) on a Ptcontaining catalyst (usually Pt90Rh10 gauzes). A second, nonoxidative process, the BMA-Degussa, is possible but requires even higher working temperatures due to its endothermicity.1 Under both conditions, the reaction is extremely fast, which poses difficulties to the experimental determination of the most likely reaction path of the intricate network summarized in Scheme 1. Andrussow and BMA-Degussa processes have been known for almost 75 years,1,2 but no consensus on the main reaction mechanism driving HCN formation has been reached. Both HCN synthetic procedures were originally developed and tested on pure Pt, although Rh was added later to the catalyst’s formulation.1 It has been reported that the addition of these small amounts of rhodium did not significantly change the temperature required for the onset of the methane oxidation.3 As in the Ostwald process (ammonia oxidation to NO), the role of Rh is speculated to be related to the stability of the catalyst. The key unanswered question is the way C-N coupling takes place: either from the atomic fragments, C þ N, or from partially hydrogenated moieties, CHx þ NHy. Schmidt et al.4-11 performed catalytic tests under almost industrial conditions and indicated that both mechanisms were possible. Ultra-highvacuum experiments inferred that the direct C þ N coupling was the main mechanism,12-15 whereas gas-phase experiments on ions16-18 pointed toward the preferential path through partially r 2011 American Chemical Society

hydrogenated intermediates. However, recent studies have assessed important differences between the chemistry of atoms and ions and that of the metal surfaces.19 Temporal analysis of products, TAP, studies20 have proposed HC þ N or C þ N as leading mechanisms in HCN formation, but the role of partially hydrogenated moieties was not clarified. Similar conclusions were obtained for a Rh catalyst.21 Very recent, comprehensive TAP studies on industrial alloy catalysts22,23 suggest the participation of absorbed oxygen species, and they propose as more likely CHx þ N or CHx þ NO reactions as main C-N formation steps. A different way to shed light on the reaction mechanism consists in the analysis of the adsorption of the reaction products. For HCN, its adsorption on Pt leads to the formation of CN fragments that were observed to be stable up to 700 K.24 As for the theoretical simulations, methane and ammonia dehydrogenations have been studied for different metals25 and the energy of different fragments has been found to depend on the adsorption energy of the central atom.26 Moreover, oxygen and hydroxyl groups are known to ease some dehydrogenation processes involving ammonia.27,28 In addition, previous analysis on the adsorption of HCN-derived fragments on Pt have been reported in the literature,29 and the direct C þ N coupling has been investigated on early transition metals for which linear relationships were analyzed in detail.30,31 Finally, the simplified reaction path for HCN formation was computationally studied on Rh only through the C þ N and HC þ N reaction paths.31-33 Received: September 29, 2010 Revised: February 2, 2011 Published: March 04, 2011 5667

dx.doi.org/10.1021/jp1093349 | J. Phys. Chem. C 2011, 115, 5667–5674

The Journal of Physical Chemistry C Scheme 1. Reaction Network Leading to the Formation of HCN on Pt(111) from Ammonia and Methane-Derived Intermediatesa

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Table 1. Reaction Energies, ΔE (in eV), and Activation Energies, Ea (in eV), for the Dehydrogenation of Methane and Ammoniaa reaction

ΔE

Ea

ΔE22

Ea22

ΔE24

Ea24

CH4 f CH3 þ H

0.04

0.73

-0.08

0.68

CH3 f CH2 þ H

0.30

0.89

0.19

0.82

-0.43

0.38

-0.58

0.14

CH f C þ H

0.94

1.51

0.75

1.53

NH3 f NH2 þ H NH2 f NH þ H

0.45 0.19

1.39 1.30

0.62 -0.01

1.38 1.30

0.53 -0.04

1.16 1.36

NH f N þ H

0.54

1.32

0.42

1.36

0.40

1.39

CH2 f CH þ H

a

Reference values from Michaelides and Hu25 and Offermans et al.27 are shown for comparison.

a

The desired product is indicated in red.

Figure 1. Schematic representation for the dehydrogenation reactions: initial, transition, and final states for (top) CH3, (center) CH2, and (bottom) C (equivalently, NH2, NH, and N). Turquoise small spheres represent hydrogen atoms, yellow spheres represent C (or N), and gray ones stand for the platinum atoms in the slab.

However, a deep theoretical analysis of the complex reaction network presented in Scheme 1 is still missing. In summary, experiments differ in pressure, temperature, and oxygen content; thus, it has been difficult to obtain a unified view on the reaction mechanism driving HCN formation, while theoretical simulations have not addressed the complete reaction path due to its complexity. The aim of the present work is thus to analyze the complete reaction described in Scheme 1 by means of density functional theory. To study such an intricate reaction network, we have taken several simplifications: (i) The model system contains only Pt; this is supported by its activity.34,35 (ii) Although severe reconstructions might take place along the

catalytic performance,23 the surface considered in the modeling has been the (111) facet, as this is the simplest way to address the full reaction path. (iii) Selectivity has not been addressed; thus, the presence of other possible minority products as CO or NOx has not been considered. These intermediates may decrease the selectivity22 but are not found in important amounts, and because our aim is to discover the main reaction path, the side paths leading to lower selectivity have not been taken into consideration. Although all the simplifications indicated above might sound too drastic, the computational effort required to fully determine all steps in Scheme 1 is very large and justifies the choice of our model.

2. COMPUTATIONAL DETAILS We have applied density functional theory, DFT, to slabs that represent the Pt(111) surface to determine the reaction network leading to HCN.36,37 To obtain the energy profiles, we have used the RPBE functional.38 The inner electrons have been replaced by PAW pseudopotentials,39 whereas the valence monoelectronic states have been expanded in plane waves with a cutoff energy of 400 eV. The supercell employed is a p(3 3). Thus, the coverage of different species corresponds to 0.11 ML, and the slabs contain four metal layers and eight equivalent vacuum layers. For these unit cells, the two uppermost layers have been allowed to relax in the z direction together with all adsorbate degrees of freedom. Given the asymmetric configuration, a dipole moment due to adsorption appears. This spurious dipole has been removed from the vacuum. The k-point sampling is Monkhorst-Pack-type and with a density of 5 5 1.40 To locate the transition states between two minima, the climbing image version of the nudged elastic band algorithm, CI-NEB, has been employed.41 In all cases, the transition-state structures located show a single imaginary frequency. As for the oxygenor hydroxyl-containing models, the employed coverage for the O-containing species was 0.11 ML, and the oxygen atoms were placed in fcc positions and the OH groups in on-top ones. The resulting water molecules are weakly adsorbed to the surface by about 0.5 eV. 3. RESULTS We will describe in the following the main features of the reaction path summarized in Scheme 1. First, the dehydrogenation steps of ammonia and methane are reported. In a second step, possible C-N bond formation routes leading to HxCNHy (x = 0-2 and y = 0-2) are described, as NH3 and CH3 were 5668

dx.doi.org/10.1021/jp1093349 |J. Phys. Chem. C 2011, 115, 5667–5674

The Journal of Physical Chemistry C

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Figure 2. Schematic representation for the bimolecular reactions: (left part) initial, transition, and final states for (top) C þ N, (center) C þ NH, and (bottom) C þ NH2; (middle part) initial, transition, and final states for (top) HC þ N, (center) HC þ NH, and (bottom) HC þ NH2; (right part) initial, transition, and final states for (top) H2C þ N, (center) H2C þ NH, and (bottom) H2C þ NH2. Turquoise small spheres represent hydrogen atoms; blue, nitrogen; yellow, carbon; and gray, platinum.

discarded as active species in the experiments.12 This can be understood in terms of the bond-order conservation theory, because the formation of a Y-XH3 bond would imply the partial loss of interaction with the surface at the transition step, and this will result in a large energy penalty. The following steps consist of the hydrogenation of CN or the dehydrogenation of HxCNHy intermediates. Finally, isomerization processes are investigated. 3.1. Methane and Ammonia Dehydrogenation under Nonoxidative Conditions. The schematic representation of the ammonia and methane decomposition reaction paths is shown in Figure 1 and Table 1. Methane is not chemisorbed on the Pt(111) surface. Experimental determinations found an adsorption energy of 0.17 eV, mainly due to van der Waals forces and a value for the dissociation barrier of about 0.60 eV.42 In our case, the functional employed does not include dispersion terms, and thus, the shallow molecular adsorption well is not described. This is a minor problem as the reaction takes place at more than 1000 K. Our estimated dissociation barrier for methane on Pt(111) is 0.7 eV, thus in good agreement with the abovementioned experimental value. However, the weak molecular interaction implies a rather short residence time at the reaction temperature, and thus, this process would be close to an EleyRideal mechanism. Therefore, it is a likely candidate for the high temperature needed in the Degussa process. Dissociation leads to a methyl group adsorbed on-top a Pt center and a H atom sitting in an fcc site. The reaction is close to thermoneutral. Ammonia is also adsorbed in an on-top position;43 our calculated binding energy is close to 0.6 eV. This is larger than previous reported values, but the differences arise from the different coverages employed in the calculations, 0.25 versus 0.11 ML. Thus, the starting points of the dissociation of ammonia and methyl are formally identical, both sitting on on-top sites. The dehydrogenation of methyl and ammonia proceeds via 0.89 and 1.39 eV barriers, and both reactions are endothermic, 0.30 (methyl) and 0.45 eV (ammonia). In the final state, the reaction products methylene and NH2, are sitting on a bridge site, whereas the H atom sits on a hollow site. NH2 and CH2 can dissociate further into NH and CH adsorbed on hollow sites, plus a H atom on fcc positions. The first reaction, NH2 f NH þ H, is endothermic by 0.19 eV and hindered by a 1.30 eV barrier, whereas the second, CH2 f CH þ H, is exothermic by 0.43 eV

Table 2. Reaction Energies, ΔE (in eV), and Activation Energies, Ea (in eV), for the Formation of C-N Bonds from HxC þ NHy (x, y = 0-2) Fragmentsa C-N coupling

ΔE

Ea

Eainf 2.25

C þ N f CN

-0.97

1.79

C þ NH f CNH

-1.33

2.05

1.98

C þ NH2 f CNH2

-1.22

1.03

1.07

HC þ N f HCN

-0.26

1.97

2.29

HC þ NH f HCNH

-0.81

1.65

1.99

0.07 0.50

0.70 1.82

1.24 1.96

HC þ NH2 f HCNH2 H2C þ N f H2CN H2C þ NH f H2CNH

-0.56

1.66

1.76

H2C þ NH2 f H2CNH2

-1.18

1.37

1.36

a inf Ea states for the barriers from infinitely separated moieties (i.e., isolated fragments calculated in the p(3 3) cell).

and hindered by a much smaller barrier, 0.38 eV. Finally, the dissociations into atomic N and C are endothermic processes, 0.94 and 0.54 eV, respectively, and the corresponding barriers are 1.51 and 1.32 eV for NH and CH, respectively. The common feature to all these reaction steps is that activation of the H atom leaving the molecule occurs over a top site. This is similar to other hydrogenation-dehydrogenation processes for small hydrocarbons and olefins.44 As can be noticed in Table 1, our results are in reasonable agreement with previous calculations for the dehydrogenation of methane and ammonia.25,27 3.2. C-N Coupling. The formation of C-N bonds producing HxC-NHy intermediates, where both x and y can range from 0 to 2, can be found in Figure 2 and Table 2. The simplest coupling starts when neighboring C and N atoms on the surface can form the C-N bond. This reaction takes place by the activation of N, and the barrier from neighboring fcc sites is 1.79 eV. The resulting CN fragment is placed C-down on a hollow site. Large C þ N barriers (>1.5 eV) are also observed for other couplings, such as HC þ N (to HCN), C þ NH (to CNH), HC þ NH (to HCNH), H2C þ N (to H2CN), and H2C þ NH (to H2CNH), which correspond to 1.97, 2.05, 1.65, 1.82, and 1.66 eV, respectively. The reaction paths can be described as follows. In the case of HC þ N, at the starting point, both fragments sit in neighboring sites and both moieties are activated toward 5669



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Table 3. Reaction Energies, ΔE (in eV), and Activation Energies, Ea (in eV), for the Hydrogenation/Dehydrogenation of HxCNHy Intermediates reaction H þ CN f HCN

Figure 3. Schematic representation for hydrogenation and dehydrogenation reactions: (top) H þ CN, (top-center) HCNH f HCN þ H, (bottom-center) HCNH f H þ CNH, and (bottom) HCNH2 f HCNH þ H. Dark small spheres represent hydrogen atoms, mediumsized spheres represent C (dark) or N (light), and large ones stand for the platinum atoms in the slab.

contiguous bridge sites. At the transition state, where the C-N distance is 1.956 Å. The final state is a tbt adsorbed HCN molecule. The reaction is weakly exothermic by 0.3 eV. The situation is similar for the HC þ NH; again, both moieties get activated to the bridge positions. In this case,the reaction is more exothermic, 0.8 eV,and the C-N distance at the transition state is 1.938 Å. Carbon reacts with NH following a similar pattern; the reaction ends up with a vertical C-down CNH species and is exothermic by more than 1 eV. At the transition state, the C-N distance is 2.050 Å. H2C can react with N and NH. In these cases, both C- and N-derived moieties are activated and the C-N distances at the transition state are 2.421 and 2.683 Å, respectively. The H2C þ N coupling is endothermic by 0.5 eV and forms H2CN parallel to the surface. This intermediate reorganizes to give an N-down species on a hollow site, gaining 0.81 eV. The H2C þ NH coupling is exothermic by 0.56 eV. Summarizing, in all the presented cases, to reach the transition state structure, the fragment showing the lowest valence moves toward the moiety that is more H-deficient, following the rules developed by Michaelides and Hu.30 When two fragments have the same valence, both are somehow activated at the transition state. To get activated, all the fragments choose the direction that implies reducing its coordination only by loosing one bond to the surface (C, N, NH, and CH from hollow toward the bridge and CH2, NH2 toward the top). Of all the nine possible HxC-NHy couplings, three of them are low-energy processes; see Table 2. These three reactions imply the attack of a carbon-derived moiety to NH2. The barriers

ΔE

Ea

-0.33

0.81

HCNH f HCN þ H

0.55

1.44

HCNH f H þ CNH

0.10

1.54

HCNH2 f HCNH þ H

0.09

0.89

HCNH2 f H þ CNH2

0.36

1.24

Figure 4. Schematic representation for the isomerization reactions: initial, transition, and final states for (top) CNH f HCN and (bottom) HCNH f CNH2. Same color code as in Figure 3.

starting from neighboring positions are 1.03, 0.70, and 1.37 eV, for HxC þ NH2, where x = 0, 1, and 2, respectively. If we analyze these three different reaction paths, we observe the following: The C þ NH2 reaction starts by C in hollow and NH2 in bridge positions. The transition-state structure shows a C-N distance of 2.554 Å, and the reaction is exothermic by more than 1 eV. The final state consists of a C-down, CNH2 fragment perpendicular to the surface. This species has been identified by vibrational spectroscopy.12 The HC þ NH2 path is thermoneutral; at the transition state, HC is placed on a bridge site and NH2 in an ontop position. The C-N distance is 2.240 Å. The final HCNH2 structure reveals the formation of a direct σ-like C-Pt bond without interaction of the NH2 with the surface; see Figure 2. Finally, the H2C þ NH2 reaction is also exothermic by 1 eV. The C-N distance at the transition state is 3.209 Å, and the final adsorbed structure resembles that of ethylene on Pd surfaces.45 3.3. Hydrogenation/Dehydrogenation HxCNHy Species. From the CN-containing species, either a single hydrogenation step, for CN, or dehydrogenation, for HxCNHy, are needed to produce HCN; see Figure 3 and Table 3. CN hydrogenation is hindered by a barrier of 0.81 eV. Starting from neighboring hollow positions, H is activated to an on-top position, while, simultaneously, the CN moiety rotates. The reaction is exothermic by 0.33 eV; see Table 3. We have also studied the following dehydrogenation paths: from HCNH2 to HCNH and CNH2, and both H cleavages starting from HCNH. These reaction paths, HCNH2 f CNH2 þ H and HCNH f CNH þ H, show barriers of 1.24 and 1.54 eV, respectively, and the reactions are only mildly endothermic, by 0.36 and 0.10 eV. The transition states resemble those from other de/hydrogenation steps, and the final CNH or CNH2 products are adsorbed perpendicular on the surface. From HCNH2, the cleavage to HCNH is very easy; the barrier is only 5670



Figure 5. Reaction barriers for C (or N) hydrogenations in the lateral panels in green (yellow) and HxC-NHy couplings (white and gray), and H2 recombination is indicated in black.

Figure 6. Relative stability of different methane fragments as a function of the presence of a clean surface or X = O or OH, 0.11 ML coverage considered in each of the calculations.

0.89 eV. The H splitting to HCN is more energetically demanding: the reaction is endothermic by 0.55 eV, and the barrier is 1.44 eV. Finally, HCN is formed on a tbt position, from which desorption is easy, 0.46 eV. 3.4. Isomerization. Several of the HxCNHy compounds can undergo isomerizations, as indicated in Scheme 1. In the present case, we have investigated two of these processes; see Figure 4. CNH and HCN are known to interconvert in the gas phase: The reaction is exothermic by 0.64 eV, and the barrier for interconversion starting from CNH is 1.40 eV. On the surface, CNH and HCN can be formed by different reaction steps. At a difference from the gas phase, on Pt(111), CNH is more stable than its isomer by 1.14 eV and its desorption to the gas phase is hindered by 1.60 eV. The CNH f HCN interconversion on the surface geometrically resembles that of the gas phase, but the barrier is much higher, 2.41 eV. This is due to the larger stability of the

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Figure 7. Relative stability of different ammonia fragments as a function of the presence of a clean surface or X = O or OH, 0.11 ML coverage considered in each of the calculations.

CNH structure on the surface. Therefore, provided that any CNH is formed, direct isomerization is unlikely and hydrogenation and dehydrogenation steps constitute a much more likely path for the interchange of these isomers. HCNH can also undergo interconversion to CNH2; the process is hindered by a barrier of 0.91 eV. The transition state shown in Figure 4 denotes that the structure resembles that of a dehydrogenation plus a rotation. Therefore, an active role of the surface as hydrogen acceptor and donor in the isomerization steps seems to be crucial. Similar effects have been described by Andersin et al.44 for the conversion of ethylene derivatives on Pd.

4. DISCUSSION Once we have analyzed the elementary steps, we consider the full reaction mechanism under nonoxidative or oxidative conditions. We have summarized all the barriers for all the C-N formation steps together with the C and N hydrogenation steps in Figure 5. With this data and the analysis of the intermediates in Figures 6 and 7, it is possible to analyze the reaction mechanisms under oxidative and nonoxidative conditions and clarify some of the previous experiments reported in the literature. We would like to stress that the employed model is quite simplified. This approach has been taken as the complete exploration of the full reaction path is highly computationally demanding. Our calculations thus allow a qualitative description of the reactivity and the most likely reaction paths under different conditions, but not a complete quantitative investigation on the role of structural defects or impurities, formation of byproducts, such as CO, carbon aggregates, and NOx, the role of the second metal (Rh) in the industrial catalyst, and the selectivity of both processes is beyond the scope of the present work. However, our computational results do present for the first time a comprehensive study of the reaction network and thus shall be employed as a starting point for more quantitative studies, even more, considering that, according to experimentalists, there is very little chance to carry out sensible mechanistic/kinetic investigations experimentally. 4.1. Nonoxidative Conditions. For the BMA-Degussa (no oxygen), dissociation (activation) of methane to methyl and methylene, and ammonia activation to NH2 are the following most energy-demanding steps. HC and NH2 are the intermediates 5671



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Figure 8. Schematic representation of the most likely reaction paths under different oxidative and nonoxidative conditions.

with lowest total binding energies, as shown in Figures 6 and 7. In addition, HC þ NH2 coupling is rather easy, less than 1 eV, and thus is an effective way to generate the C-N bond under nonoxidative conditions. From this intermediate, HCNH2 dehydrogenation is likely, the first barrier being quite low, about 0.9 eV, whereas the energetic requirements for other HxCNHy species dehydrogenation are more demanding, 1.4 eV, without considering the zero-point energy vibrational contributions (that will reduce this value by about 0.2 eV). Therefore, the calculated barrier is very close to that experimentally determined, Ea = 1.1-1.3 eV from the UHV experiments starting by ammonia and iodomethane activated by an e- beam.12 Thus, this set of reactions seems to be the leading route in the complex mechanism for HCN formation on Pt(111) under oxygen-lean conditions; see Figure 8. This is due to both the stability of HC and NH2 fragments on the surface and the low barrier for C-N formation from these moieties. In the complete route, that is, starting by methane and ammonia, the activation of methane is highly demanding due to the Eley-Rideal character of the process. It is very likely that this step is responsible for the high temperatures required for the process. 4.2. Oxidative Conditions. Under oxidative, Andrussow conditions, the relative contributions from different paths leading to HCN differ significantly. To start with, the relative balance between different intermediates on the surface is perturbed by the presence of oxygen. For instance, NH2 is no longer the most stable ammonia-derived intermediate, but NH or N (O or OH presence) instead; see Figure 7. The energy difference between NH2 þ H and N þ 3H is 0.84 eV more favorable to NH2 for nonoxidative conditions, whereas N is 0.73 eV more stable in the presence of OH. Dehydrogenations are oxygen-(hydroxyl)assisted, and for NH2 decomposition, this lowers the barrier from 1.39 to 0.55 (O) or 0.29 (OH) eV. As for methane, happens for methane as CH and C are the most common intermediates at high O coverages. Moreover, due to the presence of oxygen, fewer isolated H atoms are present on the surface due to the formation of OH and water. As a consequence, the contribution from HC þ N or direct C þ N coupling to the formation of HCN should increase with respect to other parallel routes; see Figure 8. Therefore, under Andrussow conditions, the recombination steps to form the CN bond (from either C þ N or HC þ N) are likely the most energetically demanding in the process as ammonia and methane decomposition are oxygen (OH)assisted.

Figure 9. Brønsted-Evans-Polanyi relationships for the dissociation of HxC-NHy intermediates. The corresponding linear fittings are HxC-NHy (y = 0, 1) r = 0.96, Ea = 1.01 ΔE þ 2.03 and Ea = 1.24 ΔE þ 0.98, r = 0.98 for the HxC-NH2 points.

4.3. General Discussion. These results showing different main C-N coupling steps depending on the reaction conditions conciliate previous experiments performed under very different environments. For instance, in different studies by the Schmidt group, the reaction was performed with5,7-9 or without6 O2 and were compared to discuss the reaction mechanism. In view of the results presented in the present work, although the whole list of reactions included in the mechanisms is very similar, their relative contributions to the rate are different as relative coverages are highly different. Another question regards the use of gas-phase ions to describe the reaction mechanism, as done in the group of Schwarz.16-18 There, the lack of the real ensembles present on a tridimensional particle, which are needed to obtain the most stable adsorption sites on the surface, compromises the extrapolation to complex surfaces. Indeed, on the metal ions, only fragments with a large number of H atoms were identified. With respect to the ultra-high-vacuum experiments performed in Trenary’s group,12-15 the comparison is not straightforward. This is due to the fact that these authors employed a high-energy e- beam that might disturb the relative coverage of different intermediates. Therefore, the resulting starting point for the HCN reaction might be different in the UHV experiments when compared to industrial conditions. Our results confirm the newest TAP experimental results that point out that the main path for HCN formation when some oxygen is present comes from HC þ N and C þ N couplings.22 Now, we turn to the product adsorption TPD experiments. When adsorbing HCN, the rupture of the H-C bond is the lowest energy-demanding process, and thus, CN is the most common intermediate on the surface when HCN is adsorbed at low temperatures and heated, in agreement with the experimental determinations.24 Therefore, the adsorption and formation of HCN (from CH4 and NH3) shows hysteresis as the CN appears in HCN adsorption, but it is unlikely in the HCN formation process. This complex behavior warns about the use of adsorption TPD studies of products when attempting to understand complex reaction networks, as those described in Scheme 1. A final aspect that has been reported in the literature is the appearance of Brønsted-Evans-Polanyi relationships46 for the formation-dissociation of the CN group.31 In Figure 9, 5672


The Journal of Physical Chemistry C we present the correlations between the dissociation energy of all the intermediates containing CN groups and the corresponding reaction energies with respect to the final-state position (i.e., dissociated moieties in neighboring sites). The figure clearly shows that HxCNHy dissociation follows two different regimes: the first corresponds to all the intermediates where y is either 0 or 1 and x = 0, 1, or 2, whereas the second groups all the barriers corresponding to y = 2. The differences in this behavior can be assigned to the different structure of the HxCNH2 intermediates where the NH2 fragments are not bound to the surface. 4.4. Comparison to Ammonia Oxidation. A final aspect that deserves a comment is the fact that HCN synthesis is closely associated with the Ostwald process. In this process, NH3 is oxidized with air over a Pt-based catalyst to produce NO, which is subsequently oxidized and then adsorbed in water to produce HNO3. In this case, the desired product is NO (usual yields = 93-98%) and the main byproducts are N2 and N2O. Indeed, all these species are present as well as byproducts in HCN production (HCN yields of 60-70% in Andrussow) under oxidative conditions. Therefore, a quantitative analysis of the selectivity of the process would necessarily concern the study of all these side routes that are common to ammonia oxidation. However, due to the high level of complexity introduced by the large number of side paths, this analysis is clearly beyond the scope of the present study. Still, a couple of parallelisms can be drawn between both processes. It is well known, for instance, that, for the Ostwald process conditions at low temperatures, N2 is the dominant product,47 but the selectivity to NO is a strong function of the O2/NH3 relation.48,49 Probably, high oxygen contents would favor the appearance of NO in the oxidative HCN process. In addition, possible surface restructuring that generates different oxygen species as those identified in previous studies for Pt under the Ostwald conditions would likely appear under Andrussow conditions.50

5. CONCLUSIONS In summary, the preferential route for HCN formation under BMA-Degussa conditions (oxygen-lean) on Pt(111) is the partial dehydrogenation of reactants and their coupling to form the CN bond. Dehydrogenation of the HCNH2 intermediate is the rate-limiting step in the recombination channel, the largest barrier being about 1.2 eV (ZPVE-corrected). In contrast, the presence of oxygen (Andrussow conditions) establishes a new balance on the most common species on the surface and induces both direct C þ N and HC þ N couplings as the leading terms for HCN formation. The latter reactions show a large energy barrier, about 2 eV. Therefore, the need for high temperatures in HCN synthesis is mainly controlled by the endothermicity of the reaction and the difficulties in reactant (methane) activation for the Degussa process and by the large energy demand of the recombination steps (HC þ N and C þ N) in the case of the Andrussow oxidation. In summary, the balance between common species on the surface and reaction barriers controls the leading reaction paths for the formation of C-N bonds on Pt. Our computational study shows how the complex nature of reaction networks, such as the one shown in Scheme 1, which might be too difficult to tackle experimentally both by their complex nature and by the high temperatures and short times involved, can benefit from detailed theoretical calculations. The deep analysis of the full reaction network can shed light on the

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different experimental results obtained under a wide variety of reaction conditions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank ICIQ, MICINN (Consolider Ingenio 2010 CSD2006-003, CTQ2009-07553/BQU), and the RES-BSC for computational resources. J.G.-D. thanks ICIQ for a predoctoral fellowship. We thank Ms. N. Vendrell for the translation of ref 2. ’ REFERENCES (1) Pesce, L. D., Ed. Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2001; Vol. 8, p 171. (2) Andrussow, L. Angew. Chem. 1935, 48, 0593. (3) Oh, S. H.; Mitchell, P. J. Appl. Catal., B 1994, 5, 165. (4) Waletzko, N.; Schmidt, L. D. AIChE J. 1988, 34, 1146. (5) Hasenberg, D.; Schmidt, L. D. J. Catal. 1987, 104, 441. (6) Hasenberg, D.; Schmidt, L. D. J. Catal. 1985, 91, 116. (7) Bodke, A. S.; Olschki, D. A.; Schmidt, L. D. Appl. Catal., A 2000, 201, 13. (8) Dietz, A. G.; Schmidt, L. D. Appl. Catal., A 1999, 180, 287. (9) Bharadwaj, S. S.; Schmidt, L. D. Ind. Eng. Chem. Res. 1996, 35, 1524. (10) Hickman, D. A.; Huff, M.; Schmidt, L. D. Ind. Eng. Chem. Res. 1993, 32, 809. (11) Williams, W. R.; Zhao, J.; Schmidt, L. D. AIChE J. 1991, 37, 641. (12) Herceg, E.; Trenary, M. J. Am. Chem. Soc. 2003, 125, 15758. (13) Herceg, E.; Trenary, M. J. Phys. Chem. B 2005, 109, 17560. (14) Deng, R. P.; Trenary, M. J. Phys. Chem. C 2007, 111, 17088. (15) Jentz, D.; Mills, P.; Celio, H.; Trenary, M. Surf. Sci. 1996, 368, 354. (16) Diefenbach, M.; Br€onstrup, M.; Aschi, D.; Schr€oder, D.; Schwarz, H. J. Am. Chem. Soc. 1999, 121, 10614. (17) Aschi, D.; Br€onstrup, M.; Diefenbach, M.; Harvey, J. N.; Schr€oder, D.; Schwarz, H. Angew. Chem., Int. Ed. 1998, 37, 829. (18) Koszinowski, K.; Schr€oder, D.; Schwarz, H. Angew. Chem., Int. Ed. 2004, 43, 121. (19) Garcia-Mota, M.; Cabello, N.; Maseras, F.; Echavarren, A. M.; Perez-Ramírez, J.; Lopez, N. ChemPhysChem 2008, 9, 1624. (20) Delagrange, S.; Schuurman, Y. Catal. Today 2007, 121, 204. (21) van Hardeveld, R. M.; van Santen, R. A.; Niemantsverdriet, J. W. J. Phys. Chem. B 1997, 101, 7901. (22) Kondratenko, V. A. Appl. Catal., A 2010, 381, 74. (23) Kondratenko, V. A.; Weinberg, G.; Pohl, D. S.; Su, D. S. Appl. Catal., A 2010, 381, 66. (24) Hagans, P. L.; Chorkendorff, I.; Yates, J. T. J. Phys. Chem. 1988, 92, 471. (25) Michaelides, A.; Hu, P. J. Am. Chem. Soc. 2000, 112, 9866. (26) Abild-Pedersen, F.; Greeley, J.; Studt, F.; Rossmeisl, J.; Munter, T. R.; Moses, P. G.; Skulason, E.; Bligaard, T.; Nørskov, J. K. Phys. Rev. Lett. 2007, 99, 016105. (27) Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Surf. Sci. 2006, 600, 1714. (28) Lopez, N.; Garcia-Mota, M.; Gomez-Diaz, J. J. Phys. Chem. C 2008, 112, 247. (29) Ford, D. C.; Xu, Y.; Mavrikakis, M. Surf. Sci. 2005, 587, 159. (30) Michaelides, A.; Hu, P. J. Chem. Phys. 2001, 114, 5792. (31) Crawford, P.; Hu, P. J. Chem. Phys. 2007, 126, 194706. (32) Novell-Leruth, G.; Valcarcel, A.; Perez-Ramírez, J.; Ricart, J. M. J. Phys. Chem. C 2007, 111, 860. 5673



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