{'fraction_non_alphanumeric': 0.029778335424508574, 'fraction_numerical': 0.015641990798828942, 'mean_word_length': 4.392873252142535, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 1, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Mechanisms of the gold-catalysed water-gas shift

Mechanisms proposed for the gold-catalysed Water-Gas Shift (WGS) are critically reviewed, and a complete reaction set and cycle involving activation of water on reduced support sites is considered; however, a modified route to CO2 formation by thermal decomposition of a carboxyl species results in a simpler and therefore more probable mechanism.


Introduction
The development of the industrial economy in the 19th Century required a source of a cheap and easily transportable energy form for heating, lighting and chemical processes. Passage of steam over a bed of coke, formed by carbonisation of coal, at red heat produced water-gas containing H 2 , CO and CO 2 [1]. Chemical reactions such as NH 3 synthesis and fat-hardening however need very pure H 2 , and while CO 2 is easily absorbed by water the CO has to be removed by the WGS:

CO + H 2 O = H 2 + CO 2
This is an endothermic reaction ( H o 298 = -41.2 kJ mol -1 ), so that for high conversion an active catalyst is needed to work below about 473 K. Currently on a large scale the reaction is conducted in two stages, the first at high temperature using an iron-chromium catalyst and the second at lower temperature with CuZn/Al 2 O 3 [1]. Interest in H 2 -powered fuel cells has led to efforts to use this reaction to lower the concentration of CO so as to avoid poisoning of the platinum electrodes, and early work [1,2] demonstrated that gold on reducible supports (CeO 2 , CeZrO 4 , and Fe 2 O 3 ) was even more active than CuZn/Al 2 O 3 at low temperatures. This article examines certain of the proposals for a reaction mechanism, and makes suggestions for a simple version that is capable of experimental test.

The redox mechanism
Reaction mechanisms have been widely discussed [1,2], and several basic forms have been considered. In the redox mechanism CO is oxidised by the support, which is thereby reduced, and is re-oxidised by water with the release of H 2 : this mechanism has been summarised in the case of ceria [3,4] [5] or perhaps even fully [6] by the two strongly reducing components of the water-gas system, but this requires the presence of gold [5] to chemisorb the CO or H 2 [7]. Hydrogen spillover from metals is very well documented [8], and it is significant that the best supports contain cations the common oxidation states of which differ by one (i.e. Ce 3+ /Ce 4+ , Fe 2+ / Fe 3+ ). It was established long ago [9] that reduction of oxides by spill over H atoms is only rapid when this is the case, and it is the case also with CO, the spillover of which was first demonstrated by comparing the activity of Pd/SnO 2 with that of Pd/SiO 2 for CO oxidation [10]. It is therefore not surprising to find that ZnO is a much worse support than CeO 2 for the WGS [1]. There is a widespread feeling that anion vacancies play a vital role not only in the WGS, but also in CO oxidation where they are thought to provide a means of chemisorbing the O 2 molecule as O 2 - [11,12]. -

The formate mechanism
The purely redox mechanism makes no allowance for CH x O y intermediates such as bicarbonate, formate or carboxyl, for which there is extensive IR evidence [13]. The formate group (HCO 2 ) has often been cited as the likely reactive species, being formed on the support by reaction of a support OH with CO emanating from either the gas phase or a gold particle [3].The latter seems more likely since it provides an otherwise lacking role for the metal. Wherever it is formed, making a C-H bond requires the breaking of the O-H bond, and the insertion of the CO molecule into it (see Scheme IA). To release CO 2 , the formate group may be oxidized by another OH species: The full reaction set for the formate mechanism is then written as The step describing the dissociation of the water molecule has to be repeated, because the OH s species is needed twice.

The carboxyl mechanism
The formate mechanism presents several difficulties that we will need to return to, but first we must note that there now appears to be increasing doubt that the formate species is indeed the essential intermediate. Quantitative DRIFTS has shown that only a minority of the CO 2 is formed in this way [14,15], and evidence is growing for the carboxyl group (-COOH) as the preferred alternative. It was in fact proposed as the first step in CO oxidation on both reducible [12] and ceramic oxides [16], and DFT calculations show that it is the likely intermediate on Au/CeO 2 [17] and Cu(111) [18]. Bearing in mind the polarities of the reacting species, its formation has a greater chemical logic than that of the formate group (see Scheme IB).
The last reaction set can equally well be written with carboxyl in place of formate, and we now return to examine two possible difficulties. The first concerns the mechanism of decomposing the carboxyl (or formate) group. As written, it requires the activation of a second water molecule, the first being needed to form it. It seems illogical to include a process that results in creating a water molecule, when it is in fact a reactant. The alternative is that carboxyl simply decomposes to give CO 2 [17].
Thermal decomposition of the Au-COOH species is reminiscent of the reaction of formic acid on metal catalysts, viz.

HCOOH CO 2 + H 2
This simple and easily monitored reaction was intensively studied in the 1950s and 1960s as a model to investigate the importance of electronic factors in catalysis by metals and alloys [19]. Supported gold catalysts (Au/SiO 2 , Au/Al 2 O 3 ) were found to be active between 373 and 573 K (see Table VII [19]), i.e. in the range where the catalysis of the WGS takes place. Activation energies were ~65 kJ mol -1 , and some of the values recorded for the shift reaction are in the range 45-55 kJ mol -1 [1], i.e. considerably higher than typical values for CO oxidation (~30 kJ mol -1 ). This reaction has long been neglected as being insufficiently photogenic, but it could be an easier method of scanning for WGS activity. The work performed on it reminds us that little work has been done using bimetallic catalysts of WGS; the success attending the use of palladium-gold catalysts for H 2 O 2 synthesis [20] should be a powerful incentive for examining gold alloys.

Activation of the water molecule
Yet a further problem with the last reaction set concerns the manner of activating the water molecule. The process as written requires there to be an anion vacancy close to the edge of the gold particle, so that an H atom can jump directly onto it. While the support surface may be largely reduced [6] and the supply of anion vacancies plentiful, we must remember that each vacancy has been formed by the loss of an O 2ion, and that two reduced support cations result, e.g.
The adsorption of water on these reduced centres might then proceed first as [ i.e. the CO is very weakly adsorbed and the water sites are almost fully saturated [22].

Criteria for selecting the reaction mechanism
The relative simplicity of the reactions set using decomposition of Au-COOH compared to that using its reaction with OH s is vividly demonstrated when the two sets are shown as catalytic cycles (see Schemes II and III); these are shown using the formalism employed previously for CO oxidation and PROX [23]. Although it would be possible for the second water molecule to be activated on the same site as the first, this is difficult to show in the cyclic format; the product water molecule in the more complex Scheme II is shown as being recycled, although it must ultimately appear as a product.
Consideration of these reaction cycles suggests a possible general principle to aid the selection of reaction mechanisms, namely, that the simplest mechanism is the most likely. Simplicity is achieved by choosing the route that involves the making and breaking of the smallest number of bonds, as the energy input needed is lower for the simpler mechanism. The merits of the most straightforward path have been invoked since William Occam deplored the unnecessary multiplication of entities in around 1300 AD; Einstein put a gloss on this when he said; "We must make things as simple as possible -but not simpler". Application of DFT is useful in identifying the route having the lowest energy barrier of those that are considered, but its success depends on making the proper choice. It sometimes seems that processes are treated that look to be improbable, while others that look better are ignored. An indication of the limits within which DFT must operate is provided by the statement [5] that it cannot treat the electronic structure of CeO 2 containing anion vacancies or other defects.

Comparison of the WGS with CO oxidation
It is instructive to compare and contrast the unit steps that are proposed above for the WGS with those put forward to explain CO oxidation on Au/Al 2 O 3 [15] and on reducible oxides [12], and to identify the cause of the much greater activity of gold catalysts for the latter. A common feature is that the CO molecule is activated by reaction with an OH group to form (probably) an Au-COOH species; in CO oxidation the ready availability of O 2 allows its oxidation to CO 2 and a peroxy group on reducible oxide supports, the latter [1,23]. In WGS however it has to decompose either by reaction with an OH species (Scheme II) or by itself (Scheme III) then oxidizing a further CO molecule at a necessarily higher temperature. Different roles are indicated for anion vacancies on the support, but although CO oxidation has been examined by gold on a great variety of oxides [12] a similarly catholic approach has not been adopted with the shift system, as such promising oxides as SnO 2 and V 2 O 5 have not yet received attention.

Mechanisms of deactivation
The deactivation of gold catalysts for WGS is generally attributed to a slow sintering of the gold particles [2,24], or to undesirable changes in its oxidation state or that of the support cations [6,25], but other work suggests that monodentate CO 3 is responsible, at least at high CO 2 concentrations [24]; the formate species has also been implicated [26]. How CO 3 might be formed is unclear, but it is said to decompose at higher temperature, when the stability is greater [24]; activity is restored by oxidation [26], again implicating CO 3 or HCO 2 -. The inclusion of a little O 2 (0.5%) reduces the deactivation rate of Au/CeO 2 [27]; although this might inhibit formation of HCO 2 -, which is sometimes classed as an 'unreactive spectator', it might equally well serve to maintain Au + species at the metalsupport interface, the existence of which may be necessary for the chemisorption of CO in a sufficiently strong state [17]. Although there are indications that very small gold particles [17], or even single atoms [4] can be active, the likely structure of particles active for WGS has not received the same attention as those performing CO oxidation. There the role of gold atoms at the periphery and in contact with the support, and perhaps positively charged, is often highlighted, and a clear distinction drawn between them and atoms above them. The existence of both is beneficial in CO oxidation, as shown by the superior activity of 'bilayer' particles; their importance in WGS is not yet established.

Conclusions
The WGS requires higher temperature than CO oxidation because of the difficulty of activating the water molecule and releasing its H 2 ; anion vacancies and reduced cations on the support are needed for this. The formation of CO 2 most likely occurs by thermal decomposition of a carboxyl (-COOH) species; formic acid (HCOOH) decomposes in a similar way, and might constitute an easy test reaction for WGS activity. The quality of simplicity is advanced as a helpful guide to selecting the most likely mechanism.
I am grateful to the late Dr David Thompson for his encouragement and assistance in the preparation of this paper.